indoor & outdoor substations, an introduction

Indoor and outdoor substations - Overview

Types of substationsElements of substations

Classifying criterion of substationsMedium voltage switchgear assemblies & CBs

Outdoor circuit breakersOutdoor disconnect switches

Indoor & outdoor instrument transformersProtection, PLCs & SCADA

Lightning arrestersCables, cable/bus ducts & control wires

Communication protocolsStandards

Main functions of substations:The main functions of a substation are: the transfer of power in a controlled manner as well as to make it possible to perform the necessary switching operations in the grid (energizing and de-energizing of equipment and lines) and provide the necessary monitoring, protection and control of the circuits under its control and supervision.A substation is a high-voltage electric system facility. It is used to switch generators, equipment, and circuits or lines in and out of a system. It is also used to change AC voltages from one level to another, and/or change alternating current to direct current or direct current to alternating current. Some substations are small with little more than a transformer and associated switches. Others are very large with several transformers and dozens of switches and other equipment.


Types of substations: Transmission Terminal Transformer Distribution Unit Collector

Transmission substations:A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches (and or circuit breakers) that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control devices such as capacitors, reactors or Static VARs and equipment such as phase shifting transformers to control power flow between two adjacent power systems.Terminal substations:A terminal substation is a facility that forms a strategic node point in an interconnected electricity transmission system.A terminal substation fulfills either or both roles: 1) It provides a connection point where transmission lines of the same voltage may be joined to enable an electricity supply to be established to a new demand center or centers (for example, the transmission line voltage is 230 KV and the transformer station has the 230 KV disconnect switch plus the transformer rated 230/27.6 KV, where 27.6 KV can be the sub-transmission voltage level or the distribution level), or to achieve a greater degree of interconnection within the existing system (in which it comes closer to a non-transformation transmission substation). It is a bulk supply point in the electrical grid, where it may serve a significant area within the metropolitan area and/or some country areas.2) It is a transformation point where lower voltages are produced to supply the metropolitan transmission system.Transformer substations:A transformer substation is a point where the transmission voltage level is stepped down to the sub-transmission voltage level. The latter voltage is then either used to feed a distribution substation to further reduce the voltage level to the distribution level or itself used as an input to distribution transformers (eg. 27.6 KV/ 600 V or 208 V) i.e. power is tapped from the sub-transmission line for use in an industrial facility along the way, otherwise, the power goes to a distribution substation. . Thus the major components in such a station will be: one or two high voltage disconnect switches, one or two power transformers, one or two medium voltage switchgear lineups with their breakers, instrument transformers, relays, communication and control networks.Distribution Substation:Distribution substations are located near to the end-users. Distribution substation transformers change the transmission or sub-transmission voltage to lower levels. Typical distribution voltage is 4,160Y/2400 volts. From here the power is distributed to industrial, commercial, and residential customers through distribution transformers, pad mounted, overhead pole mounted, vault installed, the secondary of which is 600/347 V or 120/208 V.Unit substations:A unit substation would typically consist of a load break switch with a set of power or current limiting fuses, in series with it ,connected to the high voltage winding of a distribution (or a power transformer), the low voltage winding of the transformer would be connected to the main circuit breaker plus the feeder circuit breakers, motor contactors plus disconnect switch and fuses, or load break switches in the switchgear lineup. Within the lineup, there would be the utility metering compartment with the current and voltage transformers approved for utility meter application as well as the user instrument transformers, meters, protection and control.

Collector substation:In distributed generation projects such as a wind farm, a collector substation may be required. It somewhat resembles a distribution substation although power flow is in the opposite direction, from many wind turbines up into the transmission grid. Usually for economy of construction the collector system operates around 35 kV, and the collector substation steps up voltage to a transmission voltage for the grid.

Distribution substation, outdoor Unit substation, indoor, double-ended

Transmission sub, GIS


Elements of substations: Primary breaking devices Transformer and its secondary switching device Switchgear lineup Instrument transformers Relays Meters & instruments Transducers & SCADA Cables & bus ducts Control & communication wires/cables


Classifying criterion: Primary voltage Secondary voltage Location Transformer type Primary breaking device type Secondary switching device type

Substations can be classified based on their primary voltage (whether it is in the transmission level eg. 500 KV, the subtransmission level, 27.6 KV or the distribution level 4.16 KV ), secondary voltage (120/ 208 or 600 V the utilization voltage level or any other higher voltage), location of installation (whether it is installed totally indoor or outdoor or partially inside and the rest outside eg. transformer stations), transformer type (liquid fill, dry or epoxy resin coated windings), primary breaking device (whether it is a disconnect switch with a series breaker, or load break switch with fuses in series) & secondary switching device types (switches and fuses or breakers).Substations can be classified broadly, into air insulated or gas (mainly SF6) insulated ones. They can also be identified by the configuration of their main bus, ie single bus bar system, sectionalized single bus bar system, double bus bar/double breaker, ring bus system and breaker and a half system.Air-insulated substations:Air-insulated switchgear (AIS) is the most commonly used type of equipment for substations. It offers flexibility in terms of equipment configuration, as well as comparatively low installation costs. Substations using AIS can be found in transmission networks of all sizes, all over the world. Gas-insulated substations:Gas insulated switchgear (GIS) incorporates all of the same functional elements as air insulated switchgear, but condenses them inside a sealed housing that occupies one tenth of the space. GIS is ideal for use in harsh environments, such as corrosive, salty air or extreme temperatures. Its is also used when real estate is scarce, this means that GIS can be located in dense urban areas where space is limited and aesthetic issues are important. Single bus bar:It is the least complicated system. It is used mostly in smaller switchgear with single line feeding. The availability rate is almost similar to that for the line.This arrangement involves one main bus with all circuits connected directly to the bus. When properly protected by relaying, a single failure to the main bus or any circuit section between its circuit breaker and the main bus will cause an outage of the entire system.

In addition, maintenance of devices on this system requires the de-energizing of the line connected to the device. Maintenance of the bus would require the outage of the total system, use of standby generation, or switching to adjacent station, if available. Since the single bus arrangement is low in reliability, it is not recommended for heavily loaded substations or substations having a high availability requirement. Reliability of this arrangement can be improved by the addition of a bus tiebreaker to minimize the effect of a main bus failure.Sectionalized single bus bar (H-configuration):Sectionalized single bus is used for smaller distribution substations. With 2 incoming lines and 2 transformers, the probability that power is available on the MV bus is very high. In general, for a distribution substation a sectionalized single bus has better performance than a conventional double bus bar system.Main and transfer bus configuration:This scheme is arranged with all circuits connected between a main (operating) bus and a transfer bus (also referred to as an inspection bus). Some arrangements include a bus tie breaker that is connected between both buses with no circuits connected to it. Since all circuits are connected to the single, main bus, reliability of this system is not very high. However, with the transfer bus available during maintenance, de-energizing of the circuit can be avoided. Some systems are operated with the transfer bus normally de-energized. When maintenance work is necessary, the transfer bus is energized by either closing the tie breaker, or when a tie breaker is not installed, closing the switches connected to the transfer bus. With these switches closed, the breaker to be maintained can be opened along with its isolation switches. Then the breaker is taken out of service. The circuit breaker remaining in service will now be connected to both circuits through the transfer bus. This way, both circuits remain energized during maintenance. Since each circuit may have a different circuit configuration, special relay settings may be used when operating in this abnormal arrangement.

Ring bus:With ring bus each load is fed from 2 directions. The availability performance is very good. The disadvantages are: it is more complicated, it uses more space and may affect the layout.In this scheme, as indicated by the name, all breakers are arranged in a ring with circuits tapped between breakers. For a failure on a circuit, the two adjacent breakers will trip without affecting the rest of the system. Similarly, a single bus failure will only affect the adjacent breakers and allow the rest of the system to remain energized. However, a breaker failure or breakers that fail to trip will require adjacent breakers to be tripped to isolate the fault. Maintenance on a circuit breaker in this scheme can be accomplished without interrupting any circuit, including the two circuits adjacent to the breaker being maintained. Breaker and a half:Breaker and a half system is used for bigger transmission and primary distribution substations. The availability rate is high as each load is normally fed from two directions. One disadvantage is that if one bus bar is out of service, the two loads (transformers) are connected to the other bus via one CB only.The breaker-and-a-half scheme can be developed from a ring bus arrangement as the number of circuits increases. In this scheme, each circuit is between two circuit breakers, and there are two main buses. The failure of a circuit will trip the two adjacent breakers and not interrupt any other circuit. With the three breaker arrangement for each bay, a center breaker failure will cause the loss of the two adjacent circuits. However, a breaker failure of the breaker adjacent to the bus will only interrupt one circuit.


Medium voltage switchgear assemblies & circuit breakers: Different types of switchgear enclosures Different types of circuit breakers Interrupting medium Operating mechanisms

Types of switchgear assemblies:There are few common types of assemblies that cover almost any application. A broad classification, according to the location of the switchgear assemblies, is whether the gear is installed indoor or outdoor. A further classification for the indoor gear is standard ventilated indoor, indoor with drip hood, indoor for location in sprinklers area, indoor in hazardous location, arc- proof (type a, b or c) and indoor in corrosive environment. Outdoor switchgear can also be further classified as whether it has an isle or not (walk-in vs. non-walk-in types) with or without a working area. Another method of classifying a gear is whether it is a bottom or top entry, the power and control cables are entering from the bottom or the top of the gear. They can also be classified according to their rated voltage (class) into medium voltage (m.v.) & low voltage (l.v.). The m.v. switchgear can contain as switching/ interrupting device(s) a circuit breaker (c.b.), a switch/fuse combination, a contactor/fuse combination (for motor switching). The l.v. assemblies may have switches (fused or unfused) or breakers in series with contactors (it is a common combination for motor switching). Circuit breaker compartments configurations are of the non-draw-out construction, or the draw-out ones. If special requirements for the gear are needed the special property can be used as a criteria for defining the gear, for example if dust tight enclosure is required due to the presence of dust, fine or course, in the atmosphere, this gear is designated as dust tight. For switchgear assemblies, as heat is generated from the high current flowing in the bus bars & connections, a dust tight enclosure is almost impossible thus a dust proof one is used, instead. The major standards that govern the design, manufacture & testing of these assemblies are: CSA, ANSI, IEC, EEMAC, NEMA & UL. Types of circuit breakers:Most of the codes define a circuit breaker (low or medium voltage) as a device designed to open and close a circuit automatically at a predetermined overcurrent values (short circuit or overload) without injury to itself (when properly applied within its rating). Low voltage power (air-magnetic) circuit breakers are found mainly in low voltage switchgear assemblies (line-ups) and sometimes in switchboards. Medium voltage circuit breakers are found in indoor (MV) switchgear line-ups as well as outdoor ones; they are found, also, mounted on pedestals & installed outdoors without he metallic enclosure. Circuit breakers are used to connect & disconnect power to all types of loads

found in a distribution system as well as to provide the necessary isolation of the faulty portion in an electrical system (under fault conditions).Moulded case circuit breakers:Most of the codes define a circuit breaker as a device designed to open and close a circuit automatically at a predetermined overcurrent values (short circuit or overload), without injury to itself, when properly applied within its rating. A moulded case circuit breaker irrelevant of the manufacturer comprises of the following parts: the molded case (frame), operating mechanism, arc extinguishers, contacts, trip elements and the terminals (connectors: mechanical/ compression lugs or solid bar). The function of the moulded case is to provide an insulated housing to assemble and mount all of the circuit breaker components. The cases are moulded from phenolic material. It provides ruggedness and high dielectric strength to the breaker in a compact design. Maximum current, voltage, and interrupting current determine the size and strength of the moulded case circuit breaker. Different manufacturers build breakers for the same rating with different physical sizes i.e. non-interchangeable. The faceplate gives all the important data such as: catalogue number, serial number of breaker, interrupting ratings at different voltage classes, standards registration numbers i.e. for the CSA or UL, calibration temperature for thermal magnetic units, lugs data, electrical accessories, date of manufacturing. The function of the operating mechanism is to provide a means of opening and closing the breaker. It is of the quick-make, quick-break mechanism. These breakers are also trip free i.e. if the breaker operating-handle is pushed and hold in the on position and there is a fault in the system (or the trip test button is pushed, if provided) the breaker would trip open & stay open. There are three distinct positions for the handle to settle in; they are the ON, OFF & TRIP (which is midway between the ON and OFF) positions. The breaker has to be reset after tripping by pushing the handle all the way to the OFF position and than pushing it to the ON position. The function of the arc extinguisher is to confine, divide and extinguish the arc drawn between the breaker’s contacts each time the breaker interrupts.

Oil circuit breakers:They are simple in construction. The major parts of a minimum oil c.b., excluding the poles, are the base frame and the drive which is constructed as a stored energy opening and closing mechanism (the operating mechanism). The opening spring of the stored energy mechanism is charged automatically during the closing action. The closing spring is charged either by means of an electric motor (is built into the drive housing) or by means of a removable crank. The pole constitutes of insulating cylinder, arc chamber and fixed, guiding & moving contacts. It also has the gas expansion chamber, terminals, oil sump, oil draining and oil filling plugs as well as the oil level indicator.Air magnetic circuit breakers::The basic characteristics for air magnetic circuit breakers are maximum rationalization and constructional simplicity. The most significant features are: extremely long electrical life, energy required for operation is slightly higher than the other techniques, maximum personal safety and simplicity of inspection, the exclusion of any possibility of over-voltages due to the fairly high de-ionization time constant of the breaking medium air, (i.e. a mixture of nitrogen, oxygen and copper vapour).The major components of such breakers are: the poles, the arc chutes, the base frame, and the operating mechanism.The operating mechanism construction and parts are similar to that of the minimum oil CB. The poles include: the fixed and moving arcing contacts, the fixed and moving main contacts, epoxy resin bushings, moving isolating contacts (main disconnects), pneumatic blow nozzles & the connections to the coils arc chute.The arc chutes contain: the blowout coils, the arc splitter plates, the arc runners & supporting insulating plates (the magnet pole pieces). Vacuum circuit breakers:The most significant characteristics for vacuum circuit breakers are: reduced overall dimensions and weight, long electrical life & low energy requirement for operation. In a vacuum system, pressure is maintained below atmospheric pressure. Pressure is measured in terms of mm of Hg (mercury). One mm of Hg is known as one torr. The standard atmospheric pressure at 0° is 760 mm of mercury. It is now possible to obtain pressures as low as 10-8 torr. In a vacuum, the current

growth cannot take place prior to breakdown due to formation of electron avalanches. However, if it could be possible to liberate gas in the vacuum by some means, the discharge can take place. In the vacuum arc the neutral atoms, ions and electrons do not come from the medium in which the arc is drawn, rather, they are obtained from the electrodes (contacts) themselves through the evaporation of their surfaces. The major parts of a vacuum CB are: the bottle supports, the bottles, shown in the above figure (which include: the fixed contact with fixed stem, the moving contact with moving stem, the bellows, the metallic arcing chamber at 10-8 torr of vacuum, the insulators, the mechanical coupling to the operating mechanism, the operating rod, the contact force spring, the operating crank and operating lever) & the operating mechanism (in its mechanism housing). The operating mechanism includes: the electric spring charging motor, the breaker shaft, the closing spring, the opening spring, ratchet gear, tensioning shaft, coupling rod and any other auxiliaries required like shunt trip, close release, auxiliary switches, etc. SF6 circuit breakers: The different types of SF6 breakers are: the two-pressure interrupter, puffer interrupter, the self-blast interrupter & magnetic (rotary arc type) interrupter. The basic idea of the two-pressure interrupters is the use of a second high-pressure reservoir that is isolated (separated) from the low-pressure reservoir by a blast valve. During breaker operation, the opening of the blast valve is synchronized with the opening of the contacts; thus, a blast of cool SF6 gas is pushed into the arcing chamber to cool the arc. The main disadvantages of this type of interrupters are: its bigger physical size, the larger amounts of SF6 gas required, the additional compressor system & the use of heaters to prevent the transform of the gas into liquid.The puffer type of interrupters uses a piston to compress SF6 gas through a nozzle arranged in such a manner as to exchange (at a high rate) the dielectric medium in the region of the arc. As the ionized gas has the ability to capture free electrons, has high thermal conductivity and has high insulating qualities, the ionized gas can quickly regain its insulating characteristics near current zero. The self-blast type of interrupters uses the arc energy to heat the gas and increase its pressure. The gas is then allowed to expand. With this expansion, the arc extinguishing process takes place in a manner quite similar to that of the puffer interrupter. In the magnetic type of interrupter, the arc plasma is moved by magnetic forces into a new region of fresh SF6, (rather t

han moving the SF6 into the arc plasma region). The higher the current being interrupted, the higher the force of the magnetic field will be. The interrupting characteristics depend on the rate at which the arc plasma encounters fresh SF6. This is a function of the current being interrupted.The main components of an SF6 CB are: the supports, the interrupters or poles (which consist of cylindrical insulating envelopes, moving and fixed arcing and main contacts, sliding contacts, upper and lower terminals, blow nozzles, operating and insulating connection shafts or rods, activated alumina filter, pressure switches and charging valve/plug, the operating mechanism (which includes the enclosure, the charging motor, the closing and opening springs, closing cam and latch, the tripping latch, any auxiliaries like auxiliary switches, operating push buttons, operation counters and breaker contact indicators). Properties of SF6: The SF6 gas is colourless, odourless and non-toxic. SF6 is an electronegative gas, which means that it has a high affinity for electrons. Whenever the electron collides with the neutral gas molecule, it is absorbed to form a negative ion, the movement of which is much slower than the free electron. It also has excellent dielectric properties, arc quenching capability and good thermal/chemical characteristics. The dielectric strength is attributed to the large collision cross-section of its molecules and the many elastic collision mechanisms that allow an efficient slowing down of free electrons. The gas not only possesses a good dielectric strength, but it also has the unique property of fast recombination, after the source energizing the spark is removed.


Outdoor circuit breakers: Live tank Dead tank Operating mechanism Ratings

Outdoor high voltage circuit breakers are classified into live tank and dead tank (the interrupters are enclosed in a grounded metal enclosure). Based on the method of disconnecting the circuit breaker from the high voltage incoming line (or bus), it can be categorized into DCB (disconnecting circuit breaker) or WCB (withdrawable circuit breaker). These breakers can be gang operated (the 3 poles closes and opens together), or each pole has its operating mechanism, thus each pole closes and opens independently of the others. These breakers in general are installed outdoor on pedestals. The operating mechanisms can be any of the following pneumatic, hydraulic- operated mechanisms or spring-operated mechanisms. The main requirement of the operating mechanism is to open and close the contacts of the circuit breaker within a specified time. The operating mechanism shall provide the following consecutive functions: charging and storing of energy, release of energy, transmission of energy and operation of the contacts. In addition, an operating mechanism shall provide control and signaling interface to a network’s control and protection system. The interrupting media was air-blast or minimum oil while now it is mainly SF6 (more compact). The ratings for high voltage circuit breakers are: 170 to 800 KV for the maximum rated voltage, 3150 to 4000 A for the rated current, 40 to 63 KA for the maximum rated breaking current. The major components of the live tank circuit breakers are: one or more breaking units (interrupters), support insulators, one or more operating mechanisms and support structure (stand or pedestal). Other components that may be found connected to the circuit breakers are grading capacitors and preinsertion resistors.Circuit breakers undergo a series of tests & inspection procedures during their lifetimes starting from conception all the way until they are replaced. These tests can be classified into type (design), production, field and conformance.The purpose of the design (type) tests, is to confirm the adequacy of the design of a particular type of CB, it is going to operate satisfactorily under actual & practical conditions. The tests are performed in order to assure that the breaker is capable of safe operation under the following conditions/circumstances: rated maximum voltage, rated voltage factor, rated frequency, rated transient recovery voltage, rated interrupting time, rated permissible tripping delay, rated re-closing time, load current switching, rated capacitor switching current, rated line closing surge

factor, out-of-phase switching current tests, shunt reactors, rated excitation current switching, rated control voltage current. Also, rated continuous current-carrying capacity (thermal testing) tests are performed.The production tests will be conducted on each assembled unit to check for good workmanship and no errors in parts used. They will include, where applicable: nameplate checks,resistors, heaters and coil checks, control and secondary wiring checks, clearance and mechanical adjustment checks, mechanical operations, stored energy system tests, electrical resistance of current path, breaker contacts timing tests, low frequency withstand voltage tests on major insulation components and control/secondary wiring. Tests after delivery are performed to assure that no damage has been inflicted on the breakers during shipment. Field tests are divided into commissioning/start-up to ensure that the breaker is in good condition and is suitable for energization and routine maintenance ones that are conducted on the breaker at specific intervals during its lifetime. Conformance tests are certain type tests that are performed on certain breakers in a group of breakers as agreed upon by the purchaser & the manufacturer to re-prove the conformance of the design with the applicable standards.


Outdoor disconnect switches Ratings Components Material

Outdoor disconnect switches (can neither break fault currents nor switch load currents) can be classified based on their voltage ratings, mounting and break type (eg. vertical, single-break side, center, double-break side, knee, pantograph). Basically, outdoor disconnect switches can either be medium voltage (like those used in overhead distribution systems mounted on poles) or high voltage found in substations (to isolate the transformer or the bus).Mounting of medium voltage overhead switches can be classified into: upright, vertical, tiered outboard, in line (mid span openers), triangular or pole top. Center Break:In the center-break design, the two arms rotate and the disconnector opens in the center. It is the most commonly used high voltage disconnector. The center-break requires an increased inter-phase distance.Double-Side Break:The double-side break design features three insulators. The end insulators are fixed while the center one pivots and provides two breaks in series. It requires a minimal inter-phase distance.Knee-Type:The knee-type has two fixed and one moving insulator and, thanks to its folding-arm design, requires a limited overhead clearance.Pantograph and Semi-Pantograph:The pantograph and semi-pantograph disconnectors feature one fixed and one rotating insulator.They are usually used to connect the two busbars of double-deck substations. Placed diagonally to the axis of the busbars and feeder, they offer a very clear thus safe arrangement and space-saving solutions (compared to the standard center break disconnectors).Aluminum Vertical Break:It has three insulator, vertical break outdoor air disconnect switch constructed primarily of aluminum. Operation is accomplished by rotation of the rear insulator. It allows for minimum phase spacing. It has a welded lamination design and a reverse loop contacts. It is an outdoor, group-operated vertical opening three insulator, air disconnect switch with

the rear insulator rotating during operation. It is usually available in ratings from 7.5kV through 230kV nominal (8.25kV through 242kV maximum) and 1200 through 3000 continuous amperes.It is applied in three-phase line or substation applications such as transformer or line disconnecting, breaker isolating, bypassing or bus sectionalizing. It is controlled by either manual control or by motor operator.Contacts: They are of the reverse loop, silver-to-silver jaw contacts. The contact fingers, fabricated from hard drawn copper, are silver metallized and electro-tin plated.The male contact, also of hard drawn copper, has a brazed silver overlay and is electro-tin plated. These methods and materials used (in the application of silver) provide for the surfaces of differing hardness with anti-galling properties with the aim of minimizing wear over the years of operation.Blade:The blade is constructed of heat-treated aluminum tubing with heat-treated aluminum castings welded to hinge and jaw ends.Blade Mechanism:The blade mechanism toggles in the closed position locking the switch closed. In the open position, the blade toggles over center. Blade rotation and opening are provided by a simple mechanism designed to operate in adverse conditions. The mechanism is insulated to prevent any welding during fault conditions.Main Switch Bearings:In general, the switch bearings consist of stainless steel balls, utilizing stainless steel bearing races with galvanized ductile iron housings and rotors. The bearings are sealed. The switch bearings are supplied with adjusted stops.Switch Bases:Hot dipped structural steel channel is used for the construction of the switch base. Leveling studs are provided on the bearings for insulator alignment.

Insulators:The switch is designed to accommodate commercially available insulators with three, five or seven inch bolt circle.Operating Mechanisms:The switch is operated with a swing handle, gearbox or motor operator. In general, the control arrangements are factory designed and customized to the structure, with all linkage components factory cut to the required dimensions.The switch may come with any of the following: grounding switches (through 100kA momentary), auxiliary switches (12 contact decks are usually standard), standard arcing horns (installed on all group operated switches), outriggers (custom designed) and connectors.These switches have a rated voltage between 170 KV and 800 KV, a rated current up to 4000 A, a rated short time withstand current of 31.5 to 63 KA (3 sec.), a BIL of between 860 to 2100 + 455 KV (across isolating distance) and 750 to 2100 KV to ground.


Indoor & outdoor instrument transformers: Definitions Applications Components

Instrument transformers can be located indoor or outdoor.. the secondary current of current transformers is usually either 5 A or 1 A, the secondary voltage of the voltage transformer is 120 or 69 V. Instrument transformers can also be classified based on the system voltage they are installed on, low voltage (up to 1000 V), medium voltage (over 1 KV and up to 72 KV and high voltage (above 72 KV).Some useful definitions:Rated voltage: the rated voltage is the maximum voltage (phase-phase), expressed in kV rms, of the system for which the equipment is intended. It is also known as maximum system voltage.Rated insulation level: the combination of voltage values which characterize the insulation of an instrument transformer with regard to its capability to withstand dielectric stresses. The rated value given is valid for altitudes ≤1000 m above sea level. A correction factor is introduced for higher altitudes as the external dielectric strength is reduced due to the lower density of the air.Lightning impulse test: the lightning impulse test is performed with a standardized wave shape – 1.2/50 μs – for simulation of lightning overvoltage.Rated Power Frequency Withstand Voltage test: this test is to show that the apparatus can withstand overvoltages at the power frequency. The Rated Power Frequency Withstand voltage indicates the required withstand voltage. The value is expressed in kV rms.Rated SIWL ( Switching Impulse Withstand Level) test: for voltages ≥300 kV the power-frequency voltage test is partly replaced by the switching impulse test. The wave shape 250/2500 μs simulates switching over-voltage. The rated SIWL indicates the required withstand level phase-to-earth (phase-to-ground), between phases and across open contacts. The value is expressed in kV as a peak value.Rated Chopped Wave Impulse Withstand voltage, phase-to-earth: the rated chopped wave impulse withstand level at 2 μs and 3 μs respectively, indicates the required withstand level phase-to-earth (phase-to-ground).Rated frequency: the rated (power) frequency is the nominal frequency of the system expressed in Hz, which the instrument transformer is designed to operate in. Standard frequencies are 50 Hz and 60 Hz.

Other frequencies, such as 16 2/3 Hz and 25 Hz might be applicable for some railway applications.Ambient temperature: Average 24 hours ambient temperature above the standardized +35 °C influences the thermal design of the transformers.Creepage distance: the creepage distance is defined as the shortest distance along the surface of an insulator between high voltage and ground. The creepage distance is defined by mm (total creepage distance) and mm/kV (creepage distance in relation to the highest system voltage).Pollution level: environmental conditions, with respect to pollution, are sometimes categorized in pollution levels. Five pollution levels are described in IEC 60815-1. There is a relation between each pollution level and a corresponding minimum nominal specific creepage distance.Wind load: the specified wind loads for instrument transformers intended for outdoor normal conditions are based on a wind speed of 34 m/s.Current transformers:Instrument transformers are covered in ANSI standard C57.13 and CSA CAN3 - C.13 as well as in IEC 60185 & 60186. Current transformers come in a few forms & shapes to provide for the space limitations in certain switchgear designs and to provide the required high accuracy for others. Current transformers can be classified broadly into bar, window, bushing and wound (for higher accuracy). Current transformers for protection application (rather than metering) do not require a high accuracy at rated current as much as they require their operation without saturation at high fault currents. For example, a C.T rated 2.5 L 200 will have a 2.5 % error at 20 times its rated secondary current times the burden connected to the secondary. If the rated secondary current is 5 then the maximum burden to be connected is 2 ohm (200 = 20 times 5 times 2), if rated secondary is 1 then the burden can be as high as 10 ohm. The value of 200 is the knee voltage (the maximum voltage supplied to the burden, by the CT, without being driven into saturation) on the secondary exciting voltage vs. excitation current curve (of C.T). The other important information that are required about a C.T. are: the secondary resistance, cross section area of the core and the saturation flux density for the

silicone iron grade used for the core so that the C.T. can be checked for its proper operation under the given conditions. Current transformers are subjected to heavy primary currents for short periods of time during faults. Mechanical damage may be caused by magnetic forces in the windings due to the first cycle (and subsequent ones) peak fault current, and is proportional to the peak current squared. Thermal damage to the transformer insulation (and maybe its copper) may occur due to heating of the winding due to the integrated effective value of the fault current over the time period until fault removal. (Note: the same discussion is applicable when sizing cables and cable supports). Partial discharge in instrument transformers are obtained by testing at the factory and the acceptable level varies depending on the requirement of the end user, though the common practice limit (a maximum value) is given as 50 pico-coulomb. A measure of the condition of the insulating material is the dielectric loss that is represented by the dissipation or power factor. An ideal insulation material should have a very low power factor close to 0. Polarity of instrument transformers is important with differential protection schemes. Polarity markings designate the relative instantaneous directions of currents in the transformer leads. A high and a low voltage (or current) leads have the same polarity at a given instant if the current enters the high voltage lead and leaves the low voltage lead at this instant (or vice versa), giving the effect as though the two leads belong to the same continuous circuit. The polarity of transformers can be additive (H1 & X1 are diagonally opposite) or subtractive (H1 & X1 are adjacent). A.C. or D.C. polarity tests can be performed at site to determine the polarity of unmarked transformers. Potential (voltage) transformers: This type of instrument transformers provides isolation between the high voltage circuit and the relaying/control/metering circuit. It, also, provides a standard low voltage signal irrelevant of the voltage class of the high voltage system. In relay circuits, PTs are used in voltage-restrained circuits, voltage protection and distance relays. The burden of the loads on the secondary of the P.T. winding should not exceed the maximum designated to the PT. Each PT is rated for thermal withstandability, at different ambient temperatures. According to the CSA standards,

there are three groups of potential transformers, they define the rated voltage of the P.T. and what should be the system voltage to which this P.T. is connected. They, also, define the rated overvoltage factor and the duration of the overvoltage for each group.The major components of an outdoor current transformer are: paper insulated u-shaped primary winding (single or multiple conductor), the core (made of nickel alloy for metering, as mentioned before to provide higher accuracy with low saturation levels, while for relaying it is made from high-grade grain oriented steel laminations), insulated (double-enamel) copper wire, oil filling unit, quartz filling, secondary terminal box, tank & insulator (aluminum & porcelain or silicone rubber), expansion vessel, primary terminal, ground terminal and oil level indicator.The major components of an outdoor potential transformer are: the multi-layer (double-enamelled wire) primary winding with inter-layer paper insulation & press-board, windings' ends metal shields, the double enamelled wire secondary and tertiary windings insulated from the core, additional terminals for other ratios (taps), the transformer core, the lower section of the transformer (aluminum tank in which the winding and core are placed), O-ring gaskets (sealing system), porcelain or silicone rubber insulator, oil level indicator, secondary terminals box, primary terminal, quartz filling, oil, neutral end terminal and ground connection.A capacitor voltage transformer consists of a Capacitor Voltage Divider (CVD) and aninductive Intermediate Voltage Transformer (IVT). The capacitor voltage divider has: expansion system, capacitor elements, intermediate voltage bushing, primary terminal (flat 4-hole Al-pad), low voltage terminal (for carrier frequency use). The electromagnetic unit (or IVT) has: oil level glass, compensating reactor, ferro-resonance damping circuit, primary and secondary windings, gas cushion, terminal box and core.


Protection, PLCs & SCADA: Overview of protection schemes Principle of operation of PLCs Introduction to SCADA

Every system is subjected to short circuits (S.C.) and ground faults (or any other type of fault), which should be removed quickly to maintain stability of the system and continuity of service to non-faulty portion of the grid. The over-current (O/C) relay is the most commonly used relay for S.C. protection. A short circuit on an electric system is always accompanied by a corresponding voltage dip (an overload will cause a moderate voltage drop). A voltage restrained or voltage-controlled O/C relay is able to distinguish between overload (O/L) and fault conditions. The O/C relay can incorporate an arc flash detectors (light sensors) to detect the light produced by an arc-flash event. For the relay to trip under arc-flash condition it also has to detect an overcurrent condition, simultaneously. When both conditions are met, the relay sends a trip signal to the circuit breaker in as fast as 2 ms. This fast tripping significantly reduces the damage causing energy released by the arc-flash event.The different types of protective relays are:Directional overcurrent: consists of a typical O/C unit and a directional unit, which are combined to operate jointly, for a pre-determined phase angle and magnitude of current. Such a relay operates only for current flowing to a fault in one direction and will be insensitive to current flowing in the opposite direction.Directional power relays: comes in single or three phase versions and they work on the wattmeter principle. The contacts (in electromechanical construction), movable and fixed, get in contact at a predetermined value of power. For example, it could be used for directional overpower; it operates if excess energy flows out of an industrial plant into the utility. It can also be used to sense an under-power condition and separate two sources operating in parallel (to avoid the 2 sources from running out of synchronism that may eventually lead to the collapse of both sources). Differential relays: the basic principle of operation for such relays is the continuous comparison of two or more current quantities. When a fault occurs, the resulting differential current will cause the relay to operate. Differential protection schemes for generators, motors,

two winding transformer banks and buses are common in industrial plants. In sub-transmission and distribution levels, differential protection is used with power transformers and for bus protection. It protects against abnormalities within a zone and should be insensitive to faults outside this zone (through faults) as well as during over-excitation periods or during energization (starting) conditions.Ground fault relaying: it can have any of the following configurations: residually connected, zero sequence (vectorial summation) or neutral relaying (direct sensing). For residually connected sensing method, three current sensors are installed, one per phase. The secondary windings of the sensors are connected to the input of the O/C relay. When there is a ground fault, the unbalanced current will flow into the relay (indicating the fault or tripping the breaker feeding the circuit having the fault). In zero sequence method, a single window-type current transformer is mounted in such a way to encircle all three-phase conductors of incoming or outgoing circuits. For 3-phase, 4-wire circuits, the neutral is also run through the sensor and the secondary of this sensor is connected to an O/C relay.For direct sensing method, a current transformer is in the neutral grounding circuit (neutral of system to ground point) of which its output (secondary) terminals are connected to the input of a single phase O/C relay. The synchro-check relay: it is used to verify that two alternating current circuits are within the desired limits of frequency, voltage and phase angle, in order to permit or allow them to be connected & operated in parallel.The synchronizing relay: it monitors two separate systems that are to be paralleled, initiating switching when the following three conditions are met: the voltage difference of the two systems as well as the frequency difference are within the pre-determined range (setting) & the phase angle, between the two systems’ voltages, is zero (taking into consideration the operating time of the switching devices).Pilot wire relays: they operate on the principle of comparing the conditions at the terminals of the protected line. The relays will operate if the comparison indicates a fault internally on the

line; they are insensitive to external faults. This scheme is used when tie lines have to be protected, either between the industrial system and the utility system or between major load centers within the industrial plant. It is also used to protect transmission lines against permanent faults through he use of PLC (power line carriers) and wave traps.Voltage relays: they can be classified according to the cause of their operation (i.e., over-voltage, under-voltage or both, voltage unbalance, reverse phase voltage or excessive negative sequence voltage).Under/over-voltage relays are found in the following circuits: capacitor switching control, a.c. & d.c. over-voltage protection for generators, automatic transfer of power sources (supplies), load shedding on U/V & U/V protection for motors.An example for the application of voltage unbalance (comparing of two sources) relay is its use in conjunction with the voltage-restrained relays. When the P.T. (connected to the voltage restraint coil in the O/C relay) fuse blows, this is seen as a fault by the voltage-restrained relay. The use of balance relays can block the operation of the O/C voltage-restrained relay thus avoiding a nuisance indication or tripping.Reverse phase voltage relays are used to detect reverse connections in three phase circuits, feeding motors, generators or transformers.Negative sequence voltage relays are used to detect single-phase conditions (and protect against such conditions). As long as the sensing P.T. is on the load side of the opened phase, the faulty condition will be sensed & the relay will operate.Negative sequence over-current relays: they are used against single-phase conditions (for single-phase protection). The location of the current transformer (sensor) with respect to the opened point (fault) is insignificant. Distance relays: they come in the following types (designs): the MHO type, impedance, reactance, MHO or admittance, OHM or angle impedance, offset MHO, modified impedance, complex characteristics type, elliptical characteristics and quadrilateral type. The measured voltage & current are expressed in terms of impedance (the ratio of the measured values).

The impedance can represent the equivalent impedance of a generator or large synchronous motor or a transmission line. The MHO relay is used to detect the loss of field of synchronous generators as well as motors.Frequency relays: they sense under or over frequency conditions during system disturbances. They are applied & used to sense & protect against the following conditions:1)To selectively drop the load, based on the frequency, in order to restore normal system stability. 2)Splitting up a grid by opening tie lines to prevent complete system collapse.3)For the generators and auxiliaries protection, when frequency supervision can prevent turbines and drive damages 4)For isolating small systems having their own generation from the main system.Temperature sensitive relays: they, usually, operate in conjunction with temperature detecting devices. These devices can be classified into either RTDs (resistance temperature detectors) or thermocouples. They are located in the equipment to be protected (embedded in the stator winding or the bearings of the motor or generator). The temperature detectors can have 10, 100 or 120 ohm and is connected in a bridge configuration, with the temperature sensitive relay connected diagonally across the bridge. Replica-type temperature relays have their operating characteristics closely matching the heating curve of the general purpose motor curves (in the light and medium overload zones), thus they are used for overload protection of motors in the medium voltage range.Multifunction relays: These relays are microprocessor-based and provide more than one protection function and even some indications , logs (data storage, )and metered data. They can be classified into feeder protection units, induction motors protection and synchronous motors protection units. They have the provision of being interrogated and adjusted remotely through their communication ports and the local area network they are connected to.

A brief introduction of programmable logic controllers (PLC):PLCs are used in almost every industry in order to control the process and theoperation of equipment. Despite the vast range of configurations and levels ofsophistication that certain applications demand, there are some basic elementscommon to all programmable logic controllers (PLCs).A PLC is a solid state device that works on conventional microprocessor computerprinciples. The microprocessor is programmed to respond as a programmablecontroller that continuously and sequentially performs certain functions. PLCsreceive input from a variety of switches and sensors, make decisions based on inputstatus plus the program logic and, finally, write outputs to affect equipment control.The control function that a PLC system performs has three basic elements: the sense,the decision and the control. Input from the field is generated by devices like pushbuttons limit switches, relay contacts, selector switches, sensors and other types ofswitches. Outputs are sent to the field to control motor starters, circuit breakers, valveactuators, solenoids, and relays. The decisions a processor makes are executed bymanipulating the registers that reside in the PLC processor (a register is a smallportion of memory that can be used by the processor to store different kinds ofinformation).This manipulation does not occur unless a user developed control program is storedin the processor memory. The control program is developed using ladder diagramprogramming or any other programming language (instruction list, grafcet, sequentialfunction charts, structured text & functional block diagram). In the ladder diagram, all inputs (I/Ps) are shown as contact symbols and all outputs (O/Ps) as coil symbols along with an associatednumber that is the address of the field device or for internal use. These addressnumbers reference either the location of the external I/P & O/P connections to thePLC or the internal relay equivalent address within the processor. The major modulesthat build a PLC are: the processor, the different types of the input/output modules,the process control module, the stepper motor controller, the different types of

interface modules, the racks and the peripheral devices which constitute: the loader/monitor,process control station, CRT programmers, hand held programmers, tape loader, laptops, netbooks.An introduction to SCADA (supervisory control and data acquisition) systems:A typical SCADA system will include, but not limited to, hardware (man/machineinterface including work stations, terminals, remote terminal units), software and acommunication medium.The hardware: system will have the mini or micro-computer (CPU), the memory, thehard disk drive, the floppy disk drives, the CD-ROM / DVD drive, the colour(monitors) terminals (2 or 3), the event logger, the data logger, the printer, the rackswith the digital and analog interfaces (inputs and outputs). Remote terminal units (RTUs) areusually microprocessor based and are responsible for controlling data acquisition.They offer local control, as they receive and execute commands from the CPU. Thecommunication protocol can be of the open or proprietary one (more about protocolslater). Certain RTUs provide for the event recording facility. In this case, timesynchronization of the RTUs is provided so that indications or occurrences arestamped with the correct time. Apart from the CPU and disk drives, the masterstation equipment may include the following: colour terminals, loggers for printinglogs and reports, printers for printing hard copies of any screen picture or tabulardisplay, mimic panel to provide a synoptic overview of the network, a large screen(video display unit) either connected directly to the CPU or in parallel with theterminals.The software: will include the proper operating system and real time database. Anysoftware should provide for a flexible program development environment and asecure/efficient run time environment. In general, a real time data base will containall system data in a tree like structure. It is divided into several levels (calledhierarchies) that consist of several components. One component contains various

information termed attributes and these constitute a detailed description of the component. An analog point can have any number of attributes: name, value, limits,..etcTypically, pictures in a SCADA system can be classified into: the index picture, system picture, alarm table, telemetry system picture, hardware configuration picture, event table, stream routing picture, trending pictures, signal list, archiving picture, calculations picture, operator's notebook, complete installation picture, overview picture, process overview, tabular summary of the process picture, process pictures of remote stations with real time data andprofile pictures (not all systems will have all these pictures). The system is controlledfrom the master though cursor positioning, function keys and alphanumeric keys. Theoperator is, usually, guided through input sequences. Commands to system objectsare entered via the operator's console using the appropriate station diagram picture.The modes of operation can be classified into: operator mode, programming mode, training mode and special mode. Facilities are provided for acknowledging and deleting alarms.The trending facility allows the user to generate graphic displays showingthe historical trend of measurand data. Regularly sampling the values and states of measurands in the database, and storing them in history files for a predefined retention period may be a useful feature of any SCADA system. Reports can be generated from the SCADA system. The report consists of data that will eventually be presented to the user in a predetermined tabular format. Most system reports are scheduled to be, cyclically, produced from the historical database. The major data & information obtained from a SCADA regarding the power network are: indications, measurands, levels and pulses.


Lightning arresters: Types Application Testing

Lightning arresters can be classified accordingly: distribution (heavy, normal and light duty), riser pole, intermediate or station. Lightning arresters are found in overhead & underground distribution systems, substations, switchgear assemblies and motor controllers. Based on the internals of lightning arresters, they can be categorized accordingly: expulsion, nonlinear resistor with gaps or silicone-carbide gapped and gapless metal oxide lightning arresters.Lightning arresters (L.A.s) are selected, based on the following: continuous system voltage, temporary overvoltages, switching surges (more often considered for transmission voltages of 345KV and higher, capacitor banks, and cable applications), lightning surges, system configuration (grounded or ungrounded/effectively ungrounded). The major components of a typical station type L.A. are: porcelain insulator,sealing cover, venting duct, sealing ring, spring, desiccant bag, ZnO (zinc-oxide) blocks, copper sheet, and flange coverThe characteristics & testing of arresters: The major parameters that define a lightning arrester are: MCOV (maximum continuous operating voltage), duty cycle, maximum energy capability, maximum discharge current, discharge voltages to currents relationship. The service conditions to which the arresters are subjected to can be classified into standard and non-standard and these conditions are defined in CSA 233 & ANSI C62. The following are considered standard service conditions: ambient temperature between -50 to 40°C, altitude not exceeding 1,800 meter (6,000 ft.), nominal power system frequency of 50 or 60 HZ, the voltage ratings and overvoltage capability of the arrester should not be exceeded at any time under all system operating conditions (normal and under fault). On the other hand, non-standard service conditions are for: altitudes in excess of 1,800 meter, temperatures outside the range previously mentioned, exposure to excessive contamination, damaging fumes, abnormal vibration and when system operating conditions are expected to exceed the capability of the arrester. The routine tests that are performed on all gapless arresters are: peak values of arrester currents (total and resistive) when the voltage applied to the arrester is equal to the MCOV, the

rated voltage and a reference voltage at a stated ambient temperature, discharge voltage measurement at the rated discharge current, RIV (radio interference voltage) when the arrester is subjected to 1.1 MCOV. The design tests are: insulation withstand, discharge voltage vs. current characteristics, surge current withstand, line and rectangular wave discharge, contamination, internal RIV and pressure relief. The conformance tests include the routine tests, plus thermal stability on an agreed upon quantity of arresters. One comment worthwhile mentioning here is that the level of voltage at which the intermediate arrestors are tested to is higher than distribution for impulse, 60 HZ RMS dry (1 min.) and wet (10 sec.). The routine tests that are performed on gapped arresters (i.e. with integral series gaps) are the dry and wet power frequency sparkover test. The design tests are: voltage withstand, power frequency sparkover, impulse sparkover, discharge voltage characteristics, discharge current withstand, duty cycle test, internal ionization, pressure relief, pollution. The conformance tests are the routine tests plus the impulse sparkover and discharge voltage to be performed on an agreed upon number of arresters. Failure of arrester: can be attributed to any of the following: moisture leakage, contamination, overvoltages including switching and resonance, surges of excessive magnitude and duration.Detecting of arrester failure in the field can be accomplished in any of the following ways: Leakage Current: is a good symptom of the condition of the arrester. High leakage currents indicate the internal deterioration of the arrester; this leads to an increase of the temperature of the arrester. A temperature rise of 10 - 20°C can be detected by infrared thermography or infrared thermometer remote temperature sensing. Insulation Resistance: arresters with large leakage currents will demonstrate a lower insulation value when tested with a 2.5 KV megger. Thus, testing will indicate a defect or an arrester with deteriorated internal resistance.


Cables, bus ducts & control wires: Cables types Bus-ducts designs Control wires ratings

Current is transferred from one point to the other through a conducting material (copper or aluminum). Depending on its installation it can be insulated or bare conductor. The conductor can be flexible (as in cables) with different designs to offer different degrees of flexibility or solid like in bus ducts and isolated phase bus (IPB). In bus dusts, all 3 phases of the circuit are enclosed in one metal enclosure with their buses epoxy insulated and supported every a couple of feet. In general, this design is used in industrial installations as well as in the medium voltage circuits in substations (eg. in a transformer substation, between the secondary winding of the power transformer and the indoor switchgear line-up). In industrial applications, the solid bus can be replaced by 3 single phase (conductors) insulated cables while maintaining the same metallic enclosure surrounding the cable.IPB is a method of construction for circuits carrying very large currents, typically between a generator and its step-up transformer in a steam or large hydroelectric power plant. Each phase current is carried on a separate conductor, enclosed in a separate grounded metal housing. Conductors are usually hollow aluminum tubes or aluminum bars, supported within the housing on porcelain or polymer insulators. The metal housings are electrically connected so that induced current, nearly of the magnitude of the phase current, can flow through the housing, in the opposite direction from the phase current. The magnetic field produced by this current nearly exactly cancels the magnetic field produced by the phase current, so there is almost no external magnetic field produced. Isolated-phase bus is made in ratings from 3000 amperes to 45,000 amperes, and rated for voltages from 5000 volts up to about 35,000 volts. In the larger current ratings, dry air is forced through the enclosures and within the tubular conductors to cool the conductors.

Underground cables are classified according to their voltage class i.e. .6 KV, 15 KV, 25 KV, 35KV,... The sub-classifications function of the voltage class are: the insulation type (solid type, oil filled), number of conductors per cable (1, 2, 3 or 4), insulation thickness (100 % or 133 % of the rated voltage), neutral size for cables with phase and neutral conductors (full vs. 1/3 phase capacity), neutral conductor shape (concentric vs. solid or stranded round), jacket type (sleeved, encapsulated or cable in conduit). Cables are either directly buried underground or run in conduits encased in concrete, in this latter configuration, manholes are designed and constructed across the cable path.They consist of three essential parts: the conductor for transmitting electrical power, the insulation medium required to insulate the conductor from direct contact with earth or other objects and the external protection cover to protect against mechanical damage, chemical or electrochemical attack. Copper and aluminum conductors are found in underground distribution cables. The conductor can be solid or stranded. The stranding class indicates the degree of conductor flexibility (AA, A, B, C, D - AA being the most stiff and D the most flexible). The strand is made up of a number of wires. The wires in a stranded conductor are twisted together to form lays. The successive layers are usually stranded in opposite direction. The stranded conductor construction is more flexible than solid conductors. Another advantage of stranded cables is that breaking and kinking of conductor in the dielectric are eliminated. In general, the cables size l/0 to 4/0 will have 18 or 19 strands, 250 to 500 MCM 35 to 37 strands, 600 to 1000 MCM 58 to 61 strands and 1250 MCM 91 strands. Cable size can be given, for example, 19/0.1, where the first is the number of strands and the second is the diameter of each strand in mm (or in AWG). Stranded conductors can be classified into concentric, compressed or compact. The main dimensions that define a cable are: the aluminum or copper conductor cross section area, the diameter over the insulation, the diameter over the insulation screen, the diameter over the concentric neutral or tape (if applicable) and the diameter over the jacket. The jacket thickness may be given or can be calculated. Examples for the standards that govern the material, construction and testing of cross linked thermosetting- polyethylene insulated wires and cables are: ICEA (insulated

Cables Engineers Association) S-66-524, CSA standard 22.2 No. 0.3, Ontario Hydro specification M405. The parameters that affect the selection of a cable are: rated voltage, maximum operating voltages, insulation level (impulse withstand voltage), short-cut circuit current (transient value and steady state value, root mean square, referred to a specific duration), transmitted power at rated voltage, layout and profile of the complete cable alignment, method of laying, thermal conditions, site conditions.

Control wires: The three most commonly used types have as insulating material TBS, SIS or teflon. The wire sizes vary, and the size selected should provide the minimum possible burden on the C.T. or P.T (as well as reasonably economical). For C.T. sizes AWG 8, 10 and 12 are common; for P.T. sizes AWG 12 and 14 are not unusual. The ratings of TBS vs. SIS is: temperature rating: 90oC vs. 90oC, voltage rating 600 V vs. 600 V, solid or stranded for both from #14 to 2, insulation thickness for both is the same though the material of TBS is PVC (with cotton braid) and for SIS is XLPE (filled). Wires with teflon insulation have higher temperature rating.Terminal blocks: Terminal blocks come in many shapes and forms to suit the specific needs of the user. Just to list a few types, they can be any of the following: standard feed through terminals, C.T. double clamp, sliding link, knife disconnect terminals, ground, neutral/disconnect & miniature feed-through terminals. They are rated up to 750 volt, up to about 130 amperes. Certain designs come up to 3-tier for dense wiring applications. The material of the insulating material can be black phenolic, melamine or polyamide.


Communication & protocols: Introduction to data communication Industrial oriented communication protocols Utility oriented communication protocols

Introduction to data communications concepts:In analog and digital communication, analog corresponds to continuous and digital to discrete. The common three contexts that these terms are used in are: data, signalling and transmission. Data are entities that convey meaning. Information is the interpretation of those data. Signals are electric or electromagnetic encoding of data. Signalling is the act of making possible the propagation of the signal along some suitable medium. Transmission is the communication of data (between 2 points) by the propagation & processing of signals. Analog data takes on continuous values on some interval, voice and video are examples of continuously varying patterns of intensity. Another example is data collected by sensors like temperature, pressure, currents, voltages (continuous valued). Digital data take on discrete values like integers, on/off switches, circuit breaker auxiliary contacts and relay contacts. Analog signal is continuously varying electromagnetic wave that may be transmitted over a variety of media (function of frequency): wire (twisted pair or coaxial), fiber optic cable, space propagation or line of sight (microwave, infrared, laser). A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium. Digital signalling is less susceptible to noise interference and is cheaper. The principal draw back is that digital signals suffer more in attenuation than analog signals. Attenuation can lead to loss of information contained in the propagated signal. Analog data are a function of time and occupy a limited frequency spectrum. For example, voice data have frequency components in the range 20 Hz to 20 KHz. Most of the speech energy is in a much narrower range (spectrum of voice signals is 300 to 3400 Hz). Digital data can also be represented by analog signals through the use of modem. The modem converts a series of binary (two valued) voltage pulses into an analog signal (by modulating a carrier frequency). Modems represent digital data in the voice spectrum to allow the propagation over ordinary voice grade telephone lines. At the other end of the line a modem demodulates the signal to recover the original data. On the other hand, analog data can be represented by digital signals. The device that performs this function is known as a codec. It

takes an analog signal that directly represents the voice data and approximates that signal by a bit stream. At the other end of the line, the bit stream is used to reconstruct the analog data. Analog transmission is a means of transmitting analog signals without regard to their content: the signals may represent analog data (voice) or digital data (data that pass through a modem). The analog transmission system includes amplifiers (for longer distances) that boost the energy in the signal. Amplifiers boost the noise component. For analog data (eg. voice), a bit of distortion can be tolerated. For digital data, cascaded amplifiers will introduce errors. Digital transmission is concerned with the content of the signal. Digital signal can be transmitted on only a limited distance before attenuation endangers the integrity of the data. Repeaters are used to alleviate this problem. They receive the digital signal and after recovering the pattern of 1 and 0, retransmit a new signal. The same technique may be used with analog signals and digital data. The transmission system has at appropriate spaced points retransmission devices (rather than amplifiers). With repeaters, noise is non-cumulative, as these devices recover the digital data from the analog signal and generate a new analog signal.

Industrial oriented communication protocols:Fieldbus: Fieldbus Networks are a special form of local area network dedicated to applications in the field of data acquisition and the control of sensors and actuators in machines or on the factory floor. In general, its protocols are organized into 5 layers: physical layer (layer 1, OSI model), link layer (2), network (3), transport (4) and application (7). Fieldbus Networks typically operate on low cost twisted pair cables. Process automation (including electrical power distribution systems) requires continuous regulatory control. Oil and gas processing, pulp & paper, power generation & distribution and chemical processes utilize scalar measurement devices and continuous modulated control. Process-related networks include FOUNDATION Fieldbus, PROFIBUS PA and HART. Factory automation involves fast-moving machinery and very quick action by discrete I/O devices. Automotive, bottling, packaging and other assembly-line manufacturing are predominantly controlled using discrete logic and sensors. AS-Interface (AS-I), DeviceNet, ControlNet and PROFIBUS DP are common in these manufacturing processes. For example, the PROFIBUS would have the following properties:1) It is a Fieldbus network designed for deterministic communication between computers and PLCs. 2) Based on a real-time capable asynchronous token bus principle, it defines multi-master and master-slave communication relations, with cyclic or acyclic access, allowing transfer rates of up to 500 kbit/s. 3) The physical layer 1 (2-wire RS-485), the data link layer 2, and the application layer are all standardized. It distinguishes between confirmed and unconfirmed services, allowing process communication, broadcast and multitasking.The protocols of the FOUNDATION fieldbus communication network are organized into 3 layers, namely, physical, data link and application (which includes fieldbus message specification & fieldbus access sublayer). The IEC fieldbus solves pending communication tasks by using 2 bus systems, the slow (safe), 31.25 KB/sec, and the fast higher level bus (1 to 2.5 MB/sec). The FOUNDATION fieldbus assigns all functions and device data to 3 different types of blocks: resource, transducer & function.

Modbus: It defines a message structure that the connected devices to the communication network will recognize and use. It describes the process that the devices on the network (eg. controllers, RTUs,…) use to request access to another device, how it will respond to requests from the other devices, and how errors will be detected and reported.It establishes a common format for the layout and contents of message fields.The Modbus protocol provides the internal standard for parsing messages. During communications on a Modbus network, the protocol determines how each device will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the device will construct the reply message and send it using Modbus protocol. The 2 transmission modes of the Modbus protocol are ASCII & RTU. Users select the desired mode, along with the serial port communication parameters (baud rate, parity mode,…etc), during configuration of each device (in the outstations as well as in the central control station). The mode and serial parameters must be the same for all devices on a Modbus network. It defines the bit contents of message fields transmitted serially on those networks. It determines how information will be packed into the message fields and decoded. On other networks (like MAP) Modbus messages are placed into frames that are not related to serial transmission. In either of the two serial transmission modes (ASCII or RTU), a Modbus messageis placed by the transmitting device into a frame that has a known beginning andending point. This allows receiving devices to begin at the start of the message, read the address portion and determine which device is addressed (or all devices, if the message is broadcast), and to know when the message is completed. Partial messages can be detected and errors can be set as a result. On other networks (like MAP), the network protocol handles the framing of messages with beginning and end delimiters that are specific to the network. Those protocols, also, handle delivery to the destination device, making the Modbus address field imbedded in the message unnecessary for the actual transmission (the Modbus address is converted to a network node address and routing path by the originating device or its network adapter.)

Utility oriented communication protocols:DNP 3.0: DNP (distributed network protocol) is an open protocol originally developed by Harris Canada for application in both SCADA & distributed automation systems. It can perform the following functions:1)Request & respond with multiple data types in single messages.2)Segment messages into multiple frames to assure acceptable error detection & recovery.3)Include only changed data in response messages.4)Assigns priorities to data items & request data items (periodically) based on their priority.5)Respond without request – unsolicited (report by exception). .6)Support time synchronization & a standard time format.7)Allow multiple masters & peer-to-peer operations.8)Allow user definable objects (binary input & output, counter, analog input & output, time, class and device) including file transfer.UCA 2.0: It is organized according to the 7-layer OSI (open systems interconnection) reference model. The top 3 layers are directly concerned with the actual application messages being sent between the stations. The bottom 4 layers are concerned with the method used to transport these messages between the stations.With devices that may not have the processing/memory capabilities to support the 7-layer profile, as well as for low bandwidth environments, the reduced profile model is used. Such profile has layers 3 to 6 (including their functionalities) protocols eliminated. It is equivalent to IEC 60870 enhanced performance architecture (EPA) model. The exchange of real-time data acquisition & control information within the utility industry breaks down into 2 classes which are the access to data in data bases (SCADA & EMS) and access to end devices (circuit breakers, meters, RTUs, relays,..). All real time data acquisition & control application make use of the application layer standard ISO/IEC 9506: Manufacturing message specification (MMS). The MMS services & data representation allows both classes of applications to be supported (with different data formats & interpretation). UCA supports models of real-time database (SCADA

elements) as well as a variety of scheduling and other data models through the use of TASE (Telecontrol Application Service Element) 2. TASE is also known as ICCP (Inter-Control Center Communication Protocol). TASE.2 is defined by the standards 60870-6-503/802/702 developed for use with MMS. For UCA real-time device access, detailed device object models have been developed that identify the set of variables, algorithms..etc. required to support the basis functionality of each device class (voltage regulator, tap changer,…).IEC 60870: The IEC 60870-5 protocols are organized into 3 layers as mentioned previously (EPA) rather than the OSI 7-layer model. The layers are physical (plus the physical interface), the link (plus the link interface) & the application. In general, he automating (for remote metering, indication & control) portion of a distribution system can be classified broadly into central station (in the control room including the terminal & PCs) and outstations (RTUs, transducers, communication devices, antennas). Using the EPA, each station performs its own local application processes. For example the central station would use the keyboard & monitor (or any other human-machine interface) to manage the SCADA database (measurands, alarms, levels,..etc.). On the other hand, the outstations (RTUs) would have application processes to scan, read & store the local values, indications & levels (eg. current, voltage, breaker position, tap changer position) as well to control the local devices & equipment under its supervision & control. Communication between the application processes (in the central station & the outstations) is performed according to the (communication) protocol. The protocol is mainly concerned with standardizing provisions so that different suppliers of distribution systems automation (telemetry & telecontrol) would supply the components that are compatible & inter-operable. The IEC standard defines 2 sets of provisions for the application protocol, which are the application functions & the application service data units (ASDU). The ASDU is part of the APDU (application protocol data unit). The serial message seen outside of the stations has a nested structure that has The ASDU is part of the APDU, and the LPDU (link protocol data unit) includes the APDU plus the LPCI. The LPCI is divided into header & trailer bits or characters. The general format of the ASDU would have: type identification (of length = 1 octet), variable structure qualifier (1 octet), cause of

transmission (fixed, 1 or 2 octet), common address (fixed, 1 or 2 octet), information object address (fixed, 1, 2 or 3 octets), set of information elements (as defined in the type identifier field) and time tag of information object (as defined in the type identification field). The application functions are: station initialization, data acquisition by polling, cyclic data transmission, acquisition of events, general interrogation, clock synchronization, command transmission, transmission of integrated totals, parameter loading, test procedure, simple file transfer & acquisition of transmission time delay.


National & international standards: ANSI & IEEE IEC CSA

The standards that govern the components manufacturing, the assembly and testing of the transformers are either of CSA (Canadian Standards Association), ANSI (American National Standards Institute) or IEC (International Electrotechnical Commission) and may be CEA (Canadian Electrical Association). For the dry type transformers the CSA standards are: CSA - C9 & C9.1 that cover "the dry type transformers" design, testing and loading. The CSA-C2 & C88 cover the single phase & three phase distribution transformers (ONAN, Oil Natural Air Natural & LNAN, Liquid Natural Air Natural) and power transformers & reactors. The ANSI C57 (a series of standards) covers dry & liquid filled transformers design, testing and loading. IEC 60076-1 Power transformers - General.IEC 60076-2 Power transformers -Temperature rise.IEC 60076-3 Power transformers - Insulation levels, dielectric tests and external clearances in air.IEC 60076-5 Power transformers - Ability to withstand short circuit.IEC 60076-10 Power transformers - Determination of sound levels.IEC 60137 Insulated bushings for alternating voltages above 1000 V.IEC 60214 On-load tap-changers.IEC 60354 Loading guide for oil-immersed power transformersIEC 60529 Degrees of protection provided by enclosuresIEC 61639 Direct connection between power transformers and gas-insulated metal-enclosedswitchgear for rated voltages 72.5 kV and above.IEC TS 60859 Cable connections for gas-insulated metal-enclosed switchgear for rated voltages 72.5 kV and above:- Fluid-filled and extruded insulation cables- Fluid-filled and dry type cable-terminationsNEMA TR1 Transformers, regulators and reactors [for audible sound levels]Oil/SF6 bushings shall be designed in accordance with the requirements of IEC 61639, IEC 60137,

IEC 60076 and IEC 62271.On load tap changers shall comply with IEC 60214, IEC 60354.The disconnecting chamber shall be capable of withstanding on site the cable high voltage test level in accordance with IEC 60055 and IEC 60141 as appropriateThe transformer oil shall comply with the requirements of IEC 60296.

The standards that govern the design, manufacturing, assembly & testing of surge (lightning) arresters: are:IEC 60099: The series of standards covering surge arresters entitled: Surge arresters, example for the parts it includes: Part 4: Metal-oxide surge arresters without gaps for ac systems, Part 5: Selection and application recommendations & Part 6: Surge arresters containing both series & parallel gapped structures – rated 52 KV & less. IEC 61109: Composite insulators for ac overhead lines with a nominal voltage greater than 1000 V – Definitions, test methods & acceptance criteriaANSI/IEEE 62.11: Standard for metal-oxide surge arresters for alternating power circuits (greater than 1 KV)NEMA LA 1: Surge arrestersCSA C233: Non-linear resistor (valve) type lightning arresters for ac systems.IEEE Std 605-1998, “Guide for Design of Substation Rigid-Bus Structures”The standards that govern the design, manufacturing, assembly & testing of capacitors and their protection are:IEC 831: Shunt capacitors (of the self-healing type) for a.c. systems having a rated voltage up to & including 660 V.IEC 871: Shunt capacitors for a.c. power systems having rated voltage above 660 V.NEMA CP-1: Shunt capacitors.IEEE std # 18: Shunt power capacitors.IEEE std # 1036: Guide for application of shunt power capacitors.ANSI C37.99: Guide for the protection of shunt capacitor banks.

The standards that govern the design, manufacturing, assembly & testing of insulators are:IEC 60168: Tests on indoor & outdoor post insulators of ceramic materials or glass for systems with nominal voltages greater than 1 KV.IEC 60273: Characteristics of indoor & outdoor post insulators for systems with nominal voltages greater than 1 KV.IEC 60305: Insulators for overhead lines with a nominal voltage above 1 KV, ceramic or glass insulator units for ac systems – Characteristics of insulator units of the cap & pin type. IEC 60383-1: Insulators for overhead lines with a nominal voltage above 1 KV. Part 1 Ceramic or glass insulator for ac systems – definitions, test methods & acceptance criteria.IEC 60383-2: Insulators for overhead lines with a nominal voltage above 1 KV. Part 2 Insulator strings & insulator sets for ac systems – definitions, test methods & acceptance criteria. IEC 60660: Insulators – tests on indoor post insulators of organic material for systems with nominal voltages greater than 1 KV up to but not including 300 KV.IEC 60720: Characteristics of line post insulators.IEC 60815: Guide for the selection of insulators in respect of polluted conditions.IEC 61109: Composite insulators for ac overhead lines with a nominal voltage greater than 1 KV – definitions, test methods & acceptance criteria.IEC 61211: Insulators of ceramic material or glass for overhead lines with a nominal voltage greater than 1 KV – puncture test.IEC 61952: Insulators for overhead lines – composite line post insulators for ac with a nominal voltage greater than 1 KV.

ANSI C29.2: American National Standard for Insulators—Wet-Process Porcelain and Toughened Glass—Suspension Type.ANSI C29.12: American National Standard for Insulators—Composite Suspension Type.ANSI C29.13: American National Standard for Insulators—Composite-Distribution Deadend Type.ANSI C29.17: American National Standard for Insulators—Composite-Line Post Type.ANSI C29.18: American National Standard for Insulators—Composite-Distribution Line Post Type.CSA C411.1: AC Suspension Insulators.CSA C411.4: Composite Suspension Insulators for Transmission Applications.CSA C1325: Insulators for Overhead Lines With a Nominal Voltage Above 1000 V.

With GIS, Circuit breakers shall be designed to IEC 62271 - 203 and fully tested in accordance with IEC 62271- 100, IEC 60694 and IEC 62771-110.The GIS disconnect switches shall be constructed and fully tested in accordance with the requirements of IEC 62271-102, IEC60694, IEC61128, IEC62271 - 203, and IEC 60265. Earth switches shall comply with IEC 62271-102 IEC60694 and IEC 62271 – 203. The current transformers shall comply with IEC 60044-1 & 6. Voltage transformers shall comply with the requirements of IEC 60044-2 and IEC 60186. IEEE Standard 738-1993 “IEEE Standard for Calculating the Current-Temperature of Bare Overhead Conductors”

indoor & outdoor substations, an introduction

Documents