Hydel-Eletric Power PlantHEPP

M Salman BilalBSEE, MSEE, MSEM

Learning Objectives:Electrical Design Consideration

Electrical Design ParametersSLD of HEPP

Class Assignment

Submission Date: 1410 PST of 11th Nov 2010

Followed by Class Quiz/Viva

Refer Single Line DiagramHighlight the Electric Equipments

mentioned in 3rd Chapter on the SLD

3.3 Generator Connections and Neutral Earthlings

3.1 Design Considerations

3. GENERATORS AND EXCITATION SYSTEM

3.2 Ratings 3.2.1 Voltage. 3.2.2 Power Factor. 3.2.3 Short Circuit Ratio. 3.2.4 Synchronous and Runaway Speed. 3.2.5 Inertia.

3.4 Excitation System

3.5 Auxiliary Switchgear 3.5.1 Main Auxiliary Boards (11 kV) 3.5.2 Unit Auxiliary Supply Boards UASB (400 V). 3.5.3 Unit Auxiliary Boards UAB (400 V) 3.5.4 Common Services Boards CSB (400 V). 3.5.5 Essential Services Boards ESB (400 V) 3.5.6 Drainage & Dewatering Distribution Boards DWDB (400 V) 3.5.7 Substation Auxiliary Board SAB (400 V) 3.5.8 Lighting Distribution Board LDB (400 V) 3.5.9 Headwork’s Auxiliary Board HAB (11 kV) 3.5.10 Headworks Supply Board (0.4 kV) 3.5.11 Protection & Metering

3.6 Auxiliary Transformers 3.6.1 Main Auxiliary Transformers 3.6.2 Auxiliary Power Transformers 3.6.3 Rated Lighting Impulse Withstand Voltages

3.8 Control & Monitoring 3.8.1 General Control Philosophy 3.8.2 Generating Unit Control 3.8.3 Auxiliary Power 3.8.4 765 kV Substation 3.8.5 Control from NPCC (SCADA) 3.8.6 Gates

3.7 Miscellaneous Electrical Auxiliaries 3.7.1 General 3.7.2 Station Auxiliary Power Supply System 3.7.3 Standby Power Supply System 3.7.4 DC Supplies 3.7.5 Cables & Earthlings

3.1 Design Considerations

•generators will be three phase synchronous machines •meeting the requirements of the latest edition of IEC 60034 •designed taking into consideration the operating experience gained•Design will be for 30 years minimum operating lifetime.

•Vacuum-Pressure Impregnated (VPI) insulation is preferred as an almost universal standard except for units too large for available processing equipment. •Resin-rich or "loaded tape" insulation is still an option for the largest sizes. <Both the technologies are being used nowadays. For 20 kV Systems, resin rich insulation systems are not appropriate.>

•The stator insulation would be VPI. •In either case, the coil insulation shall be applied continuously throughout the coils with equal thickness to both the slot and end-turn portions. Stator and rotor insulation will be rated for Class F although temperature rises will be limited in operation to Class B values.•The coils will be protected against surface partial discharges (corona) outside the stator iron by a semi-conductive exterior layer and an installation method to obtain maximum contact with the stator core slot. •Methods for corona suppression would be specified.

The generators shall be designed to withstand all fault situations which can beexperienced during operation without any displacement of its windings or mechanicaldamage to any of its parts or to the generator foundations, such as •short circuit between two or three phases at its terminals, •faulty synchronization, •magnetic unbalance due to pole winding failure •and runaway conditions.

The generator shall be so designed that all repair works, maintenance and inspection ofthe generator and turbine parts may be done with a minimum of disassembly work.

The stator frame will be split into sections for transport and reassembly at site.

The stator core will be stacked at site in the erection bay without joints, ensuring good mechanical strength, low losses and circularity.

3.1 Design Considerations: Continued

3.1 Design Considerations: Continued

The thrust bearing shall carry the total load of the generator rotor, generator and turbine shaft and turbine runner, as well as the hydraulic thrust forces during operation, load rejection and runaway conditions. The upper guide bearing is considered essential to ensure that the rotor operates smoothly under all design conditions (short circuit, hydraulic transients, etc.) and is properly supported to safely withstand the seismic forces during the Maximum Credible Earthquake (MCE). Damage during MCE is permitted, but the unit must safely shut down. The unit must also withstand the Operational Basis Earthquake (OBE) with no damage, although the unit is allowed to trip. The thrust bearing design will include an automatic high pressure oil injection system, which will provide an oil film on thrust bearing pads in order to prevent damage to the bearing during starting and stopping of unit.

3.1 Design Considerations: Continued

A closed circuit air-cooling system with air/water heat exchangers will be specified. The surface air/water heat exchangers arranged around the periphery of the stator frame shall be mounted in a way that simplifies assembly, dismantling, maintenance and repairs.

Arrangement will be made that even with cooling coils equivalent to one complete heat exchanger choked (capacity factor) or out of service the cooling systemwill not be effected.

It shall be possible to maintain the maximum continuous output without the stator and rotor winding temperatures exceeding Class B permissible temperature.

The unit would never be run with one cooler fully disabled.

The generator housing will be an octagonal concrete structure. It will be sized toaccommodate the surface coolers, piping, main and neutral leads with sufficient spacefor circulation of cooling air and for a walkway around the generator stator which should be adequate for dismantling / installation of surface coolers and piping.

Class Activity: What are Insulation Classes? Download data and submit in the next lecture.

3.1 Design Considerations: Continued

The generator will have a mechanical braking system.An automatic carbon dioxide (CO ) fire protection system is foreseen for each generating unit.An air gap and vibration monitoring system for each of the (eight) turbine generator units is envisaged.For partial discharge monitoring system capable of monitoring the stator insulation system capacitive couplers from Iris Engineering or other alternative source, will beinstalled on the circuit rings.

The generator main parameters are shown in the Table 3.1 below for 540 MW Unit rating .

Table 3.1 Generator Preliminary DataUnit Rating Generator Main Parameters 540 MW•Rated Voltage [kV] 18 – 20 kV•Speed [RPM] 150•No. of pair of poles 20•Efficiency [%] 98.5•Power Factor 0.95•Short Circuit Ratio 1.07•Runaway Speed [RPM] 272•Line Charging Capacity [MVARs] 487•Rotor Dia [m] 10.27•Rotor Height [m] 3.59•Stator Outer Dia [m] 13.35•Rotor Weight [tons]•Rotor Weight = [(rpm/400)-0.4]x[(MW/37.5)0.8] x K•For Dasu units K = 80.•1000•GD2 [t-m2] Generator + Turbine 85000•WR2 [t-m2] Generator + Turbine 21260•Inertia Constant, H [kWs/kVA] 4.85•Mechanical Starting Time, Tm [s] 9.7•Stored Energy, E [MWs] 2757

Note: The generator parameters have been determined assuming a generator terminal voltage of 20 kV. The actual voltage selection, and hence the final parameters, will be left to the bidders.

3.2 RATINGS

•The data presented in Table 3.1 is preliminary and would need to be firmed up duringdetailed design stage.•Synchronous condenser operation of the generators is not foreseen initially. However, itmay be prudent to prepare several of the units with minimum piping provisions for thefuture eventuality of this requirement when more power stations are built on upper Indus River and system voltage control may become an issue.

3.2.1 VoltageThe generator terminal voltage is selected taking into consideration the ratings of commercially available connected equipment such as isolated phase bus and the generator circuit breaker.

There are only a few discrete choices of equipment that are commercially available. The equipment ratings are governed by international standards.The selection of rated generator voltage depends upon the benefits derived from overall reduction in losses and the generator design has main effect in deciding the voltage.

Experience has shown that for the generator design of a particular MVA rating to beeconomical, its terminal voltage shall be selected from the voltage ranges indicated in the Figure 3.1 on the next slide.

Figure 3.1 for different generator ratings. Based on above considerations, a generator rated voltage selectable within a range of 18 - 20 kV is suggested at this stage pending further investigations at the design stage.

3.2.2 Power FactorIt is highly desirable that the generator be designed for a power factor at which it willoperate in order to improve system stability. In view of the fact that the Dasu power plant will be located remote relative to the load centres and will have long EHV transmission lines with inherent capacitances, a power factor of 0.95 is proposed for the generator.

The choice of this power factor has to be authenticated by system stability studies to beconducted at tender design stage.

3.2.3 Short Circuit Ratio (SCR)The short circuit ratio is the ratio of generator field current that produces rated open circuit voltage to the field current required to produce rated stator current when the generator terminals are short-circuited.

A higher value of short circuit ratio results in improved inherent stability of the machine. However, at the tender design stage, a system stability study is necessary to determine whether higher-than-normal short circuit ratio is required. Increasing the short circuit ratio above normal increases the machine size, the flywheel effect (WR2) and the machine costs, and decreases the efficiency and transient reactance of the generator. Figure 3.2 shows the expected price additions to thegenerator basic cost and reductions in efficiency when higher than normal short circuitratios are required.

SCR is also a measure of machine robustness and overall, the quantity of iron in the core. Manufacturers will want to provide SCR at values below one (SCR 0.8 to 0.85) because the machine will cost less to produce. As the machine owner, having the more robust machine serves in the favour of the Owner (SCR 1.0 to 1.1).Only extreme conditions would dictate going outside of these values.

A normal short circuit ratio of 1.07 corresponding to 0.95 power factor is being suggestedpending the outcome of the stability studies to be performed at design stage.

3.2.4 Synchronous and Runaway SpeedThe synchronous speed of the generator will be 150 rpm as determined by turbinehydraulic considerations and design net head. Runaway speed has been provisionally estimated at 272 rpm and that will be authenticated at detail design stage.

3.2.5 InertiaThe natural inertia is estimated at 85000 tm2. Inertia Constant, H [kWs/kVA] = 4.85 hasbeen assumed based on typical hydro units of large size. Based on this, the Stored Energy, E would be 2757 [MWs].

A detailed study including the effects of hydraulic transients, response to load variations, fault conditions, sudden full load rejection and system stability requirements will be undertaken during the detailed design stage.

These studies will form the basis for selecting the final value of inertia, inertia constant and stored energy.

3.3 Generator Connections & Neutral Earthing

The generator will be connected to the generator step-up transformer by means ofIsolated Phase Bus (IPB). The IPB will be fitted with branch connections to the unit auxiliary transformer, excitation transformer, voltage transformers and surge arrestors.

A generator circuit breaker with isolating and ground switches will also be fitted in the IPBconnections.

The IPB will be adequately rated thermally and dynamically. Natural or forced air-coolingcan be employed, as may be required. The IPB will be evaluated on the conductor andshield losses, efficiency of cooling, and effectiveness of reducing ingress of dust orMoisture (IP),etc.Natural air (self) cooled IPB up to 30 kA rating is available from various manufacturersand has been successfully used.

The neutral ends of the generator windings will be star-connected and high-resistanceearthed via a single phase neutral grounding transformer loaded with a suitable resistorto limit the stator earth fault current to about 10 A.The Generator Circuit Breaker would be specified based on the requirements of IEEEStd C37.013-1997 “IEEE Standard for AC High-Voltage Generator Circuit Breakers ratedon a Symmetrical Current Basis”.

Based on the selected unit size and generator terminal voltage, the estimated ratings ofthe IPB and the generator circuit breakers are depicted in Table 3.2.

The ratings depicted in the table are preliminary and are required to be firmed up duringdetailed design stage based on the generator and transformer ratings / parametersfinally selected. NOTE: Variety of generator CB & isolated bus ducts with vide range of ratings for use with large generators are available from various equipment manufacturers to absorb changes, if any, in the generator ratings in the detailed design stage.

3.4 Excitation System

A Static Excitation System with a Digital Automatic Voltage Regulator is envisaged forthe state-of-art for generators. Excitation power shall be taken from the generator itself, through a branch-off from the generator terminal bus bars and supplied to the excitation rectifier via the excitation transformer.

The excitation transformer will be installed in a self-supported steel plate cubicle toachieve personnel safety. The excitation transformer shall be of dry insulated type using non-flammable Class B insulating material.

Embedded temperature detectors (Pt-100) for monitoring winding temperatures will be included. However, Single phase excitation transformers can also be provided to maintain the iso-phase construction used on the units.

The excitation rectifier envisaged will be of solid-state type with controlled silicon powerthyristor for both polarities. It will be capable of reversing its output voltage to obtain fastresponse in case of load rejection and unit over speed. Each rectifier branch will consist of at least two parallel thyristor, so that one thyristor can be removed during operation (redundant bridges). The number of thyristor bridges supplied will be one complete thyristor bridge more than the required number of bridges to achieve rated capability (n+1), but not less than two complete bridges.

The rated continuous output of the excitation rectifier will correspond to not less than theexcitation power required for continuous operation of the generator at rated output andpower factor and 105% of rated voltage. The excitation system would require a dynamicresponse range typically 150% to 200% of the continuous rating.

The excitation rectifier will preferably be of the self-ventilated type. However, if forced ventilation is offered, redundancy of the cooling system must be provided to avoid shutdown of the generator in the event of breakdown of the fan motors. Cooling fans must be self powered from the excitation transformer so that loss of station service does not cause excitation failures.

De-excitation during normal shutdown of the unit will be performed by opening of thefield circuit breaker. Simultaneously, the AVR shall trigger all thyristors simultaneously tofully open state, thereby providing a "free-wheeling" circuit for the field current. This is theone way to do this. Other technically equal methods will also be explored.

The field suppression system will consist of voltage-dependent resistors, dimensioned towithstand the excessive field currents resulting from fault conditions. Tripping of the field circuit breaker will instantaneously put the field suppression system into operation. Many modern excitation system supplied today do not include a field breaker, only a defibrillating circuit (crowbar circuit) to de-excite the main field. This option may be studied at the detailed design stage.

An over voltage protection against induced over voltages in the field circuit will beincluded.The generator is envisaged to have a state-of-the-art static excitation system with adigital Automatic Voltage Regulator (AVR). The AVR shall be equipped with fully redundant controllers with automatic and manual channels with auto-followers to track position of the digital controller that is in control to provide bump less, two-way transfers between controllers and manual-auto control. Part of the redundancy scheme requires redundant voltage transformers on the generator isolated phase bus duct.

Over- and under-excitation limiters will be included. The under-excitation limit shall match the static and dynamic stability curves for the generator. Volts per Hertz limiter will also be included.

The AVR shall include software functions for a power system stabilizer (PSS) unit withadjustable parameters. The supplier will be required to perform modeling and fieldtesting to correctly determine setting to cause the voltage regulator response to optimizeon frequency instead of voltage.

The excitation system shall have built-in protection and supervision equipment. All fault signals shall be displayed on the AVR front panel.

The entire excitation system is foreseen to be totally self sufficient, in that it self excitesthe generator, provides all of the required power supplies for cooling and thyristorcontrol, etc from the secondary of the excitation transformer.

External power is supplied in the form of dc control voltage, field flashing source, and power supply for cubicle lighting and power sockets.

The system is foreseen to have a touch screen operator interface for local control. The equipment will provide input transducers for all generator quantities and therefore will display all unit quantities in digital format.

The digital AVR will interface directly to the digital control system (DCS) for the station. All power and control circuits will use circuit breakers or mini-circuit breakers for protection and disconnection means. Fuses will not be allowed.

3.5 Auxiliary SwitchgearThe Unit and Station Auxiliary Supply System configuration showing auxiliary switchgearis depicted in Figure 3.3.

3.5.1 Main Auxiliary Boards (11 kV)Four (4) Main Auxiliary Boards (MAB-1 thru 4) connected to their respective MainAuxiliary Transformers are provided. The transformers and the MABs will be directly connected with the isolated phase bus of the relevant generating unit.

MAB-1 & 2 will be able to be interconnected to one another via bus coupler circuit breakers, although during normal operation the two boards will operate independently with the bus coupler in normally open (NO) position. MAB 3 & 4 will have similar arrangement.

MAB-1 & 4 feed the following switchboards in the station:- The unit auxiliary supply boards UASB-1 & 2;- The common services boards CSB-1 & 2 ;- The essential services boards ESB-1 & 2;- The substation auxiliary board SAB;-The main lighting distribution boards LDB-1 & 2

MAB-2 & 3 also feed all the above boards with the exception of SAB and additionally feed head-works auxiliary board.Main auxiliary boards (MAB) are proposed to be located in the respective unit galleries.

3.5.2 Unit Auxiliary Supply Boards UASB (400 V)

There are two (2) unit auxiliary supply boards (UASB-1 & 2), each with two bus sectionsinterconnected via a bus tie circuit breaker.

Each of the bus sections is supplied from the respective MAB Main Auxiliary Boards via related unit auxiliary transformers. Each bus section of the unit auxiliary supply board is connected to unit auxiliary boards (UAB) of two units.

During normal operation, the bus tie circuit breaker remains open and closesautomatically when the incoming supply of any of the two bus sections fails.

The unit auxiliary supply boards will be located one each in the respective galleries ofUnit No. 3 and Unit No. 6 so as to minimize the cable lengths.

3.5.3 Unit Auxiliary Boards UAB (400 V)

There is a dedicated unit auxiliary board UAB for each generating unit fed from the respective unit auxiliary supply board (UASB) as described above.The principal design of the UAB is a double-ended motor control centre (MCC) withmanually operated main and tie breakers. One bus is designated the essential bus which carries the loads required to be able to start a generator (i.e. only one main governor oil pump, the cooling water pump and thrust bearing oil injection pump etc.) and the balance of the loads that are not essential for starting of a main unit are placed on the normal bus. During emergency start conditions, a diesel generator is configured to supply only the critical loads. All motor starters for the unit auxiliary equipment will be located in the UAB rather than providing local starters.

In this way, the control wiring is eliminated, the motor starter control designs are common regardless of the starter size, the starting equipment will be of the same manufacturer for the entire station minimizing spare parts requirements, and the equipment will be contained in draw-out cubicles so they may be removed for servicing. MCC wiring standards will be defined at the time of detailed design. NOTE: Motor controllers and feeders are all to be of the circuit breaker controlled typerather than fuse type. Control circuit protection will be via miniature circuit breakers.The unit auxiliary boards are proposed to be located in the respective unit galleries.

3.5.4 Common Services Boards CSB (400 V)

There are two common services boards (CSB-1 & 2). The CSBs are identical in construction and features to the UASB and are similarly supplied from the Main Auxiliary Boards. The CSBs provide power to major, non-unit electrical loads in the station including those of drainage, dewatering and lighting systems, which are supplied through their respective distribution boards. The majority of these loads appear in the erection/service bay of the powerhouse. Like the UAB, the loads are divided into essential and normal loads. The essential loads are ventilation, elevators and battery chargers etc.The common services boards are proposed to be located in the erection bay.

3.5.5 Essential Services Boards ESB (400 V)

There are two essential services boards (ESB -1 & 2). The ESBs are identical in construction and features to the CSBs and are similarly supplied from the Main Auxiliary Boards.

The ESBs provide power to major unit and non-unit electrical loads in the stationincluding those of lighting systems, which in turn are supplied through their respectivedistribution boards.

The unit essential loads are the main governor oil pump, the cooling water pump and thrust bearing oil injection pump whereas other common essential loads include ventilation, elevators and battery chargers etc.

The essential services boards are proposed to be located at end of the powerhouseopposite the erection bay.

3.5.6 Drainage and Dewatering Distribution Boards DRDB, DWDB (400 V)

The 400 Volt DRDB & DWDB are also identical in construction and features to the UASBs. However, their service is limited to the drainage and dewatering pumps and their controls. Independent feeders from the CSB 1 & 2 to each of the DRDB & DWDB are provided.

Dual, non-contact analogue sensors are foreseen for level sensing in each sump. The actual levels upon which the control is to be accomplished can then be accomplished within the digital controller.

Additional remote input/output (I/O) signals are required for sensing of drainage and dewatering valve positions. Status and operation will be displayed graphically with digital information supplied on status (position of valves, water levels, etc).

3.5.7 Substation Auxiliary Board SAB (400 V)

The 400 Volt SAB is similar in construction and features to the UASB. SAB provides the required power to the GIS and the main power transformer auxiliaries.

Principal loads are stored energy mechanisms for circuit breakers, cooling pumps for the transformers and electric heaters as may be required.

GIS hall and transformer gallery normal lighting loads will also be supplied from this board.The substation auxiliary board SAB is proposed to be located in the GIS room.

3.5.8 Lighting Distribution Board LDB (400 V)

Two (2) 400 volt LDBs will be provided that are identical in construction and features tothe UASB.

Each LDB is supplied from the CSB & ESB.

Being an underground powerhouse, lighting is a critical function. The emergency light system will be powered from the station batteries, and has limited duration and limited light output.

When the time comes to restart the station and/or trouble shoot the problem, the first thing needed is normal lighting. Being directly supplied from the essential and common buses of ESB & CSB, this configuration minimizes the potential path interruptions and makes the lighting easier to restore if lost. The diesel generators located at head-works back up essential buses.

The main lighting distribution boards are proposed to be located adjacent to the commonservices boards.

3.5.9 Headworks Auxiliary Board HAB (11 kV)

The 11 kV Headwork’s Auxiliary Board (HAB) is supplied from the MAB 2 & 3. The HAB has two 11 kV bus sections interconnected through a bus tie CB.The two bus sections supply their dedicated 2000 KVA headwork’s auxiliarytransformers to feed the 400 V headwork’s supply switchgear. The bus tie remains openduring normal operating conditions <NO> & closes automatically if either one of the twoincomer circuit breaker trips due to fault conditions.

HAB is proposed to be located in the local control building for the gates.HAB is connected to two diesel generator sets of each about 2000 KVA, which startautomatically in case of loss of complete power supply. These diesel generator sets will be located close to the local control room.Each diesel generator shall be adequate capacity to:- be able to start auxiliaries for one unit start-up.- Operate the station drainage pumps- Operate the powerhouse HVAC system- Operate the access tunnel lighting system- Operate the powerhouse emergency lighting system- Operate one dewatering pumpTo be able do this job, each diesel generator will be a large unit, estimated at around2000 KVA. The actual size of these units will be selected during detailed design.

3.5.10 Headwork’s Supply Board (0.4 kV)The 400 Volt board at the headwork’s supplies the spillway, the intake gates, lighting, forany required outlet works, access tunnel lighting, and required external ventilation etc.

3.5.11 Protection and MeteringProtection and metering equipment is foreseen at three locations. •The first group is with each generating unit. •The second group is with the underground GIS and •The third group is with the surface AIS substation. Protection relays are foreseen as numerical relays in rack mounts that directly interface with the distributed control system (DCS) for indication of both tripping and alarm signals. The primary outputs are wired to lockout relays in the same switchboard for high speed direct tripping. All trip circuits are directly wired to the device being tripped. Relay protection will be conservatively applied with appropriate reserve protection. If possible, the reserve protection will be a second identical protective device that is fed by separate instrument transformers. Metering devices will be included as part of the DCS.

High accuracy measurement circuits will independently measure and record information for a particular energy measurement. The same devices will be able to place the recorded information on the DCS bus for access via the MMI (man-machine interface). VT secondary protection will be in the form of MCB. Fuses are to be avoided to the extent possible.