transformers accessories and support equipment

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcoís employees.Any material contained in this document which is not already in the publicdomain may not be copied, reproduced, sold, given, or disclosed to thirdparties, or otherwise used in whole, or in part, without the written permissionof the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX20203 W.A. Roussel

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Transformer Accessories And Support Equipment

Engineering Encyclopedia Electrical

Transformer Accessories and Support Equipment

Saudi Aramco DeskTop Standards

CONTENTS PAGES

OPERATION OF TEMPERATURE INDICATING DEVICES ........................................... 1

Top Oil Temperature Indicator .................................................................................. 1

Hot Spot Temperature Indicator................................................................................ 3

Types of Hot Spot Indicating Circuits........................................................................ 3

Thermometer Heat Well Type........................................................................ 3

Bridge Type................................................................................................... 4

Overload Indicator Relay..........................................................................................10

OPERATION OF LIQUID LEVEL INDICATORS (ADP-P-121, CHAPTER #8)...............11

Sight Glass ...............................................................................................................11

Dial Type Level Indicator .........................................................................................13

OPERATION OF PRESSURE RELIEF DEVICES.............................................................15

Mechanical Relief .....................................................................................................15

Diaphragm Relief......................................................................................................17

OPERATION OF GAS DETECTOR RELAYS...................................................................20

OPERATION OF PRESSURE VACUUM INDICATORS AND BLEEDERS ....................22

Pressure-Vacuum Indicators .....................................................................................22

Pressure-Vacuum Bleeder ........................................................................................24

OPERATION OF DEHYDRATING BREATHERS............................................................25

Breather Operation...................................................................................................25

TYPES OF BUSHINGS ON TRANSFORMERS ................................................................27

Types of Bushings on Transformers..........................................................................27

Bushings with Draw-Through Cable Leads ...................................................27

Bushings with Solid Conductors ...................................................................28

Oil-Filled Bushings........................................................................................31

Condenser Bushings......................................................................................31

Combination Bushings ..................................................................................31

Basic Bushing Construction......................................................................................36



Saudi Aramco DeskTop Standards

Separable Connectors (Dead Front Transformers) ....................................................36

OIL FILTRATION AND VACUUM SYSTEMS ................................................................38

Oil Filtration Systems ...............................................................................................38

Vacuum Systems......................................................................................................38

GLOSSARY........................................................................................................................38



Saudi Aramco DeskTop Standards 1

OPERATION OF TEMPERATURE INDICATING DEVICES

This section gives a brief description of the function and operation of the following temperatureindicating devices:

• Top Oil Temperature Indicator

• Hot Spot Temperature Indicator

• Hot Spot Indicating Circuit

• Overload Indicator Relay

Top Oil Temperature Indicator

Figure 1 shows a typical top oil indicator. This type of indicator shows the top liquid temperatureof a transformer. The indication is given by pointers that are rotated by an indicator shaft. Twopointers normally are given:

• A white or yellow one to show top liquid temperature

• A red one to show the maximum temperature reached since the last reset

These devices also are available with internal switches, operated by cams on the indicator shaft.The switches are used to operate cooling equipment or to actuate high temperature alarms.

The temperature sensing element consists of a temperature-sensitive bulb filled with liquid. Thebulb is placed in a sensing well that extends into the insulating liquid at the top of the transformer.The liquid inside the sensing bulb expands and contracts as the temperature of the transformerliquid changes. These changes are sent through capillary tubing to a Bourdon tube. The Bourdontube rotates the indicator shaft inside the thermometer case. Indicator shaft motion moves thepointers and operates the switches.

The two pointers are designed in a master-follower configuration. The top liquid temperaturepointer is the master and the maximum temperature reached pointer is the follower. This master-follower configuration means that motion of the maximum temperature reached pointer followsthe motion of the top liquid temperature pointer (as shown in Figure 1 below).




The maximum temperature reached pointer only follows the top liquid temperature pointer whenthe oil temperature is rising. The maximum temperature reached pointer continues to indicate thehighest oil temperature that has occurred since the last reset, even when the top liquidtemperature pointer has moved to a lower temperature. The maximum temperature reachedpointer can be reset to the present oil temperature being indicated by the top liquid temperaturepointer by pressing the reset button.

Figure 1. Top Oil Temperature Indicator




Hot Spot Temperature Indicator

Hot spot temperature indicators give an indication that is equivalent to the transformer windinghot-spot. These indicators also have provisions to automatically control transformer cooling fansand winding high temperature alarms. Figure 2A shows a sectional view of a transformer with amounted hot spot indicator with flexible capillary tube.

The internal winding hot-spot temperature of the transformer winding is determined by the load itcarries, its thermal characteristics, and the temperature of its cooling liquid and the ambienttemperature around the transformer. Through use of the relationship between these factors, aheater is designed to duplicate the winding hot-spot rise over top liquid, when supplied with acurrent proportional to that in the winding. The heater is placed around a well in the top liquid;the temperature rise from the heater is added to the top oil liquid temperature. An equivalent ofthe winding hot-spot temperature can be measured through insertion of the temperature sensitiveelement of an indicator or detector in this well.

Types of Hot Spot Indicating Circuits

• Thermometer heat well type

• Bridge type

Thermometer Heat Well Type

The thermometer heated well type hot spot indicator is applied chiefly to large power transformerwhere the money loss would be relatively large in case of damage due to excessive temperature inthe windings. This device gives the operator an indication of the temperature of the windingrather than that of the oil. This type hot spot indicator has an assembly utilizing a bourdon gage,calibrated in degrees centigrade, connected to a bulb by a capillary tube (same as top oil indicatorFigure 1). The thermometer bulb and a heating coil are assembled in a well located in the hottestportion of the insulating oil near the top of the transformer. See Figure 2A. In most cases thethermometer dial or indicator is mounted on the transformer case, but it can be placed a limiteddistance away from the transformer. The maximum distance is fixed by the practical length of thetube connecting the gage mechanism and the thermometer bulb. The current in the winding of thepower transformer passes through the primary winding of a current transformer, as shown inFigures 2B and 2C thus causing a current to flow in its secondary, proportional to that in theprimary. This secondary current flows through a heating coil which encases the temperatureindicator bulb, thus generating heat in the bulb proportional to that developed in the transformerwindings. This adds an increment of temperature to the thermometer bulb which is equal to thewinding hot spot rise above the hottest oil temperature.




The temperature of the bulb is at all times proportional to the temperature of the main transformerwindings, therefore the instrument will indicate the hottest spot temperature of the windings.

Switches are normally provided in this instrument to operate forced cooling control and alarmcircuits.

Figure 2D shows a basic electrical diagram of a thermometer type “hot-spot” indicating circuit.The diagram shows the connection of a “hot-spot” Current Transformer (CT) to a thermal relay(device 49-1) and to a Resistance Temperature Detector (RTD) for remote electrical indicationsor control.

Heating coil #1 (HC1) is used to heat the Bourdon tube with ambient oil temperature. Theresulting indication represents the “hot-spot” temperature. The RTD that is heated by heating coil#3 (HC3) is connected to a bridge to give remote temperatures. In this case, the 49-1 device isused to control fans and/or pumps in the cooling circuit.

Bridge Type

This type used only on power transformers. This is the most accurate indication of the windingtemperature of a large transformer. A thermometer is not used and the scheme is purely onelectrical one. For this reason the indicating dial can be placed on a switchboard at anyreasonable distance from the transformer.

This device consist of a current transformer (CT) that is energized from the current in the maintransformer winding. A heat-insulated coil is included in the secondary circuit of this CT. Theamount of heat generated in this coil is proportional to the heat generated in the main transformerwinding. Anon-conductive resistance embedded in the heating coil forms the fourth leg of awheatstone bridge circuit, see Figure 2E. The bridge, completed by means of resistances outsideof the transformer, derives its voltage from a direct-current source. A voltmeter is connectedbetween two points, normally of equal potential.

When a current flows in the power transformer windings, a current proportional to the powercurrent flows in the CT. This latter current raises the temperature of the heating coil, which inturn increases the temperature of the noninductive resistance. The bridge therefore becomesunbalanced and the amount of unbalanced is indicated by the voltmeter. There is a definiterelation between this current and the variation of temperature, and the voltmeter scale iscalibrated to read the temperature of the heating coil directly in degrees Celsius. The wheatstonebridge circuit is balanced when normal current is flowing in the transformer. Any unbalance ofthe bridge will be an indication of the temperature of the heating coil, which in turn is proportionalto the temperature of the main windings.




Figure 2A . Sectional View Of Transformer Showing MountingOf Hot Spot Indicator With Flexible Capillary Tube




Figure 2B. Connection Diagram for Current Transformer and Heating Coil(Used with Winding Temperature Indicator).




Figure 2C. Thermal (or Winding Temperature) Relay Uses a Heating Elementto Duplicate Effects of Current in Transformer




Figure 2D. “Hot-Spot” Indicating Circuit




Figure 2E. Hot-Spot Temperature Indicator Utilizes Wheatstone Bridge Methodto Determine Transformer Temperature




Overload Indicator Relay

Figure 3 shows an overload indicator relay. This type of relay gives a visual indication of thetransformer temperature. An overload indicator relay consists of an indicator unit with a dial,reset knob, switches, and a temperature-detecting bulb. The temperature-detecting bulb is placedin direct contact with the low-voltage power lead, and is insulated with a piece of ceramic tubing.There are two switches located inside the indicator units, one for fan control and one for alarm.The switches are factory calibrated and not adjustable. The dial has a resettable maximumindicator (or “drag hand”) that is resettable with the reset knob.

Figure 3. Overload Indicator Relay




OPERATION OF LIQUID LEVEL INDICATORS (ADP-P-121, CHAPTER #8)

Saudi Aramco uses two types of devices to give level indication on liquid-immersed transformers:

• Sight glass

• Dial-type indicator

Liquid-level indicators are preferred but not required on transformers smaller than 2.5 MVA ifthey are not part of the normal equipment supplied by the vendor (per ADP-P-121, Chapter #8,Section 2.2.1).

Sight Glass

Sight glasses are acceptable on small transformers if these sight glasses are part of the vendor’sstandard arrangement. They are not the preferred method for level indication. The sight glass isone of the most basic indicating devices. Figure 4 is a basic diagram of a typical sight glass usedfor level indication.

The sight glass operates as follows:

• The liquid in the tank exerts a pressure in the downward direction that isproportional to the volume of liquid in the tank.

• The pressure exerted increases as the liquid level increases, and the pressureexerted decreases as the liquid level decreases.

• The increase or decrease in pressure exerted is felt at the bottom sight glassconnection.

• When the pressure exerted increases, liquid from the tank will flow through thebottom sight glass connection, causing the liquid level to increase in the sight glass.

• When the pressure exerted decreases, liquid from the sight glass flows in theopposite direction through the bottom sight glass connection, causing the liquidlevel to decrease in the sight glass.




Figure 4. Basic Sight Glass




Dial Type Level Indicator

The dial type indicator is preferred for use on Saudi Aramco transformers. Figure 5 shows atypical dial type magnetic liquid level indicator.

This level indicating device operates as follows:

• The float moves up and down as level varies.

• The float causes the float arm to rotate.

• As the float arm rotates, the driving magnet rotates.

• The driving magnet causes the driven magnet to rotate.

• The driven magnet is attached to the indicator shaft, which will also rotate.

• The indicator shaft repositions the indicating pointer.

Most dial type liquid level indicators also contain a micro switch that is operated through motionof the indicator shaft. The micro switch can be connected to a remote alarm panel through use ofthe electrical leads to provide high and/or low liquid level alarms.




Figure 5. Dial Type Magnetic Liquid Level Indicator




OPERATION OF PRESSURE RELIEF DEVICES

Oil-filled transformers need some type of pressure relief device to prevent transformer tankrupture, if an over pressurization occurs. Saudi Aramco uses two common types of pressurerelief devices:

• Mechanical relief

• Diaphragm relief

Mechanical Relief

Figure 6 shows the two operating positions of a typical mechanical relief device: reset positionand venting position. This device is mechanical, self-sealing, and self-resetting and is used toprotect the tank against excessive internal pressures, such as the pressures that accompany arcs inthe insulating liquid. The device can be mounted directly to the top of the transformer tank or tothe top of an adapter which is directly mounted to the top of the transformer tank. Figure 6shows the device mounted through use of an adapter.

The device also has a visual-type indicator (flag or semaphore) and an alarm assembly to providea visual and audible indication that the device has operated. The visual type indicator consists of aplastic flag that normally rests on top of the relief cover in a horizontal position.

The bottom side of the relief cover is used as the means of sensing transformer tank pressure andtripping this mechanical relief device. The gas pressure inside of the transformer tank istransmitted to the bottom side of the relief cover through the hollow shaft guide assembly. Whenthe tank pressure becomes excessive, the relief cover and the operating shaft move upward whichvents the excessive gas pressure. The movement of the relief cover and operating shaft alsopushes the indicating flag into a vertical position and actuates the alarm. The indicating flag mustbe reset to provide visual and audible indications of subsequent relief device operations.

The alarm circuits should always be installed so there is a means to remotely trip the transformerprimary breaker off-line if the pressure relief device operates. This pressure relief device shouldbe tested during the installation testing phase of commissioning and should also be testedperiodically as part of a preventive maintenance program. The pressure relief and reset pressurevalues should be recorded and compared to the new installation values. The alarm for theindicating pressure relief should also be tested at the same time the pressure relief device is tested.




Figure 6. Mechanical Relief Device




Diaphragm Relief

Figure 7 shows a typical diaphragm relief device that automatically resets. This device is mountedon the transformer tank and operates to vent the tank if over pressurization occurs.

The diaphragm relief device is equipped with a lightweight plastic semaphore that rests on thediaphragm before operation. When the diaphragm rises during operation, it lifts the semaphoreinto view and shows that the relief device has operated. Alarm contacts can be supplied to showthat the unit has operated.

This semaphore button can (if requested in purchase specifications) have a yellow plastic bananashape device that sets over the top of this button. When pressure relief by the diaphragm occursthe button pushes the banana samaphore into a vertical position where it stays because of springtension until it is reset. The size and length of the banana semaphore allows it to be seen betterfrom ground level. Some transformer designs make it impossible to see the button semaphore byitself from ground level.

Figure 7. Diaphragm Relief Device (Sealed Position)




Figure 8 shows the same device with an alarm mounted on the side of the cover. The alarmdevice has a double throw microswitch. The switch actuating lever is extended and rests againstthe edge of the diaphragm for the untripped position of the relief device. When the switchactuating lever is in this extended position, a set of normally closed contacts inside the alarmdevice are held open to disable the alarm circuit. When the relief device operates, movement ofthe diaphragm releases the switch actuating lever which allows the lever to retract inside theswitch reset lever. When the switch actuating lever is in this retracted position, the contactsinside the alarm device return to their normally closed condition and actuate the alarm. The alarmmust be manually reset by pressing inward on the switch reset lever following a trip of thediaphragm relief device.

When the pressure in the tank rises to the tripping pressure, the diaphragm lifts slightly,exhausting into the space between the outer ring and the body of the relief device. This exhaustallows the tank pressure to spread over the entire diaphragm area, which is twice the area of thecenter section. This spread of pressure causes the device to open rapidly and to remain open untilthe pressure in the tank falls well below the tripping pressure. This differential between trippingpressure and closing pressure ensures that positive sealing occurs upon closure.

The relief device resets itself and reseals when the pressure in the gas space has fallen to aboutone-half the tripping pressure. A relief device that reshuts before the pressure being relievedreturns to normal (zero for this relief device) is said to have blowdown. Moisture is preventedfrom entering the transformer tank because the diaphragm relief device is designed withblowdown. Blowdown prevents the entry of moisture by shutting the relief device while thepressure in the transformer tank is still higher than atmospheric pressure.

This pressure relief device should be tested during the installation testing phase of commissioningand should also be tested periodically as part of a preventive maintenance program. The pressurerelief and reset pressure values should be recorded and compared to the new installation values.The alarms indicating pressure relief should also be tested at the same time the pressure reliefdevice is tested.




Figure 8. Diaphragm Relief With Alarm Device (Venting Position)




OPERATION OF GAS DETECTOR RELAYS

Figure 9 shows an internal and external view of a gas detector relay. Large power transformersuse gas detector relays to warn of approaching faults. Gas generated by an incipient fault will riseto the center of the domed transformer cover and will pass through tubing to the gas detectorrelay.

The accumulated gas forces the oil down in the normally full relay chamber. When the oil level inthe relay chamber lowers, the float located inside the relay chamber also lowers. When the floatlowers, the float arm moves downward on the float end and upward on the gear end. This floatarm movement causes the indicator shaft to rotate in the counterclockwise direction. When theindicator shaft rotates, the pointer shaft also rotates because the two shafts are connected througha magnetic coupling. Pointer shaft rotation repositions the pointer and operates the alarm switch.

The liquid level gauge (gradiated in cubic centimeters) show the gas volume (or volume ofdisplaced oil). The pointer shaft operates the alarm switch when 200cc of gas have accumulatedin the relay chamber.




Figure 9. Gas Detector Relay




OPERATION OF PRESSURE VACUUM INDICATORS AND BLEEDERS

Saudi Aramco uses pressure vacuum indicators and bleeders on sealed gas, cushion-typetransformers when these accessories are recommended by the manufacturer. This section gives abrief explanation of how pressure vacuum indicators and bleeders operate.

Pressure-Vacuum Indicators

Pressure-vacuum indicators show leaks in transformer tank seals. Figure 10 shows an internal andan external view of a pressure-vacuum gauge that is calibrated from -10 to +10 psig. Thepressure vacuum gauge is a bourdon-tube type gauge. The gauge operates as follows:

• Transformer tank pressure enters the Bourdon-tube through the socket.

• The end of the Bourdon-tube connected to the adjustable linkage moves up on apressure increase or down on a pressure decrease.

• The adjustable linkage moves in the same direction as the Bourdon-tube, whichcauses the lever to rotate around the pivot point.

• The segment gear rotates in the same direction as the lever, which causes thepinion gear to rotate.

• The pinion gear repositions the pointer.

Gauge readings vary with transformer temperature change and normally show a positive pressure.The gauge may show a vacuum in the tank when the transformer is deenergized or is operatingunder light or no-load conditions in a low ambient temperature. Any change in temperature,without a subsequent change in the pressure-vacuum reading, is an indication of a leak in thetransformer seal and should be checked.




Figure 10. Pressure Vacuum Indicator




Pressure-Vacuum Bleeder

Pressure-vacuum bleeders are used to release excessive pressure and vacuum from a transformer.Figure 11 shows a typical pressure-vacuum bleeder. The bleeder is connected to the gasexpansion space of sealed tank transformers. The settings for a particular gas or oil reliefapplication are unique and are stamped on the device.

The bleeder has diaphragm-operated pressure and vacuum bleeder valves that exhaust gas or thatdraw in outside air through the vent.

Since the vacuum bleeder valve only functions during extremely low temperature and loadingconditions, the moisture and oxygen content of the small amount of air drawn into the transformermay be considered negligible.

Figure 11. Pressure-Vacuum Bleeder




OPERATION OF DEHYDRATING BREATHERS

Saudi Aramco requires the use of dehydrating breathers on the air side of diaphragm sealedconservators. The section that follows contains a brief explanation of how these breathersoperate.

Breather Operation

Figure 12 shows a typical dehydrating breather assembly. This breather has two diaphragm- typerelief valves that are factory set at 1, 5, or 8 psi. One valve permits in-breathing, and one permitsout-breathing only when the internal pressure of the transformer exceeds the setpoints. Withrelief valves, the breather regulator protects the dehydrating material from humid atmospheres byeliminating breathing for pressure changes between the relief valve setpoints.

The dehydrating breather assembly is connected to the transformer through a pipe fitting. Whenthe pressure inside the transformer increases, air travels from the transformer into the breathingpipe, and then travels out of the holes in the breathing pipe which pressurizes the dehydratingchamber. When the pressure rises to the setpoint of the outbreathing relief valve, the valve opensto relieve the pressure. When the relief valve opens, the air passes directly down the breathingpipe, and bypasses the dehydrating material. Direct exhaust to the atmosphere minimizes thecontamination of the dehydrating material by oil vapors from the transformer.

When the pressure inside the transformer decreases, air is drawn from inside the dehydratingchamber, through the holes in the breathing pipe, and into the transformer. The withdrawal of airfrom the dehydrating chamber causes the pressure to drop inside the dehydrating chamber. Whenthe pressure drops to the setpoint of the inbreathing relief valve, the valve opens and allowsoutside air to pass through the valve and the perforated screen into the bottom of the dehydratingchamber. The air then flows up through the dehydrating chamber that is filled with dehydratingmaterial, into the holes in the breathing pipe, and then into the transformer. The air that reachesthe transformer must pass through the dehydrating material, which removes most of the moisture.

Upper and lower windows are provided in the dehydrating chamber to allow the monitoring ofthe color of the dehydrating material. The dehydrating material changes colors as the materialabsorbs moisture. The dehydrating material must be replaced when the color of the dehydratingmaterial that is seen through the upper window has changed. A drain plug and a filler plug areprovided to allow easy replacement of the dehydrating material.




Figure 12. Dehydrating Breather Assembly




TYPES OF BUSHINGS ON TRANSFORMERS

This section describes the following:

• Types of Bushings on Transformers

• Basic bushing construction

• Separable connectors (Dead Front Transformers)

Types of Bushings on Transformers

The electrical power circuits must be insulated where they enter the tank. In addition, thisentrance must be oil-tight and weatherproof. This is the purpose of bushings. It is usuallycomposed of an outer porcelain body for rigid insulation required. At higher voltages, additionalinsulation in the form of oil and molded paper is used within this porcelain. The four types ofbushings used on transformers as main lead entrances are as follows:

• Bushing draw through cable leads.

• Bushing solid conductors.

• Condenser bushings.

• Oil-filled bushings.

Bushings with Draw-Through Cable Leads

This type of transformer bushings usually consist of high grade porcelain cylinders through whichthe windings leads pass and terminate to the top metallic terminal cap of the bushings. The outersurface may be plain or have a series of corrugations or skirts to increase the surface leakage path(creepage distance) to the metal case.

Figure 13 shows an assembled view and a cross-sectional view of a bushing with draw-throughcable leads. These bushings are considered low current bushings because bushings with draw-through cable leads are generally rated for a maximum current of 600 amperes and used formedium and high voltage.




Bushings with draw-through cable leads are supplied with a detachable terminal stud to permit theuse of the transformer winding lead as the bushing conductor. The transformer winding lead isbrazed to the detachable terminal stud, drawn up through the core of bushing porcelain, and thenattached with a terminal cap to the top of the bushing. The terminal cap provides the connectionpoint for the incoming high voltage power. The completed bushing assembly fits into a hole in thetop or side of the transformer enclosure and is held in place with a mounting flange and bolts.

Bushings with Solid Conductors

This type of bushing is usually constructed very similar to the draw through type bushing but hasa solid conductor in the center of the bushing.

Figure 14 shows an assembled view and a cross-sectional view of a bushing with a solidconductor. These bushings are considered high current bushings because bushings with solidconductors are generally rated for a maximum current of 5000 amperes and used for medium andhigh voltage.

Bushings with solid conductors normally contain a solid rod as the conductor for current ratingsup to 2000 amperes. Hollow-core conductors are normally used in bushings with current ratingsabove 2000 amperes. Having a larger surface area than the solid rod conductor, the hollow-core conductor is able to carry more current.

Bushings with solid conductors have a threaded terminal stud attached to the top and the bottomof the solid conductor. The threaded terminal at the top of the bushing provides the connectionpoint for the incoming high voltage power. A threaded blade is screwed on to the bottomthreaded terminal to provide a connection point for the transformer winding lead. The completedbushing assembly fits into a hole in the top of the transformer enclosure and is held in place with amounting flange and bolts.




Figure 13. Bushing with Draw-Through Cable Leads




Figure 14. Bushing with Hollow-Core Conductor




Oil-Filled Bushings

The oil-filled type bushing has a central conducting rod or tube which serves to strengthen theconstruction of the bushing. Through this tube the winding conductor passes. Around this is aseries of insulating barriers held apart by spacers. The barriers and spacers are enclosed in askirted porcelain shell which is filled with oil. (See Figure 15.) This type of bushing is fortransformers operating at voltage up to 161,000 volts. There are oil level indicators or gauges atthe top of the bushing.

Condenser Bushings

The condenser type is similar to the oil-filled bushing except that the central rod is wound withalternating layers of insulation and tin foil. This results in a path from the conductor to the caseconsisting of a series of condensers. The layers are so designed as to provide an approximatelyequal voltage drop between each two condensers. In some types the entire bushing is enclosed ina skirted porcelain shell. Other types have only the exposed part of the bushing so enclosed. SeeFigure 16 for condenser bushing without porcelain shell. (See Figure 16.)

Combination Bushings

Bushing manufacturers may use combinations of these four types of bushings. The oil-filled andcondenser bushings are used in combination a great deal (see Figure 17). The extra high voltagebushings are good example of this type device. Figure 17 illustrates the various parts ofconstruction typical for a high voltage bushing.




Figure 15. Oill-Filled Bushing




Figure 16. Core of Condenser Type Bushing Without Porcelain




Figure 17. Typical Oil-Filled and Condenser Type Bushing for 66kV Transformer




Basic Bushing Construction

Figure 18 shows the basic bushing construction requirements.

Transformer bushings for use on outdoor Saudi Aramco transformers must be made from wetprocess glazed porcelain in a petticoat fashion. Bushings are made in a petticoat fashion toincrease the creepage distance (the shortest distance measured on the surface of the insulatorbetween parts of different polarities). Saudi Aramco requires a minimum creepage distance of 40mm per kV line-to-line of nominal system voltage. Refer to Figure 18 for locating creepagedistance related to a bushing.

Bushings for outdoor applications also must have threaded studs. The conductors used may becopper or aluminum. Also, outdoor bushings for nominal system voltages of 69 kV and abovemust have a capacitor tap for testing.

Saudi Aramco also requires all bushings of the same voltage class to be interchangeable. Thissimplifies the process of holding spares.

Separable Connectors (Dead Front Transformers)

Saudi Aramco prefers the use of separable connectors on dead front pad-mounted transformers.Separable connectors perform the same function for pad-mounted transformers as the bushingsperform for other types of transformers. The additional feature provided by separable connectorsis the ease with which the connection can be broken.

Separable connectors consist of three major components:

• The bushing well

• The loadbreak insert

• The loadbreak elbow




Figure 18. Basic Bushing Construction




Figure 19 shows a separable connector for a dead front pad-mounted transformer. The bushingwell is inserted through a hole in the high-voltage compartment of the transformer. The bushingwell is made of porcelain and has a conductor that runs through the center. One end of theconductor is used as a male contact, and the other end of the conductor has a lug to provide aconnection point for the high voltage winding lead.

The loadbreak insert screws into the bushing well. The loadbreak insert is made of plastic and hasa conductor that runs through the center. Both ends of the conductor have female contacts.

The loadbreak elbow is inserted into the loadbreak insert and held in place by metal springcompression. The loadbreak elbow is also made of plastic. The loadbreak elbow comes with acompression fitting that is crimped on to the incoming high-voltage power lead. The other end ofthe compression fitting is threaded such that a conducting pin can be screwed into thecompression fitting. The end of the conducting pin forms a male contact that is inserted into thefemale contact in the loadbreak insert to complete the high-voltage electrical connection.

Figure 19. Separable Connector for a Dead Front Pad-mounted Transformer




OIL FILTRATION AND VACUUM SYSTEMS

This section briefly describes the functions, components, and operation of the following systems:

• Oil filtration systems

• Vacuum systems

Oil Filtration Systems

Oil filtration involves the removal of water and solid particles from transformer oil through use ofseveral types of filters, centrifuges, and vacuum dehydrators.

The filters include various kinds of cartridge-type filters and the common filter press. These filtersare capable of removing water, carbon, particulate matter, and sludge that is in suspension.Because the ability of a filter to remove water depends on the dryness of the filter, it is essentialthat steps be taken to make certain that the filter is dry before the filtration is started.

The centrifuge is particularly useful when there are large quantities of water or othercontamination is present. In general, it is better at removing large concentrations, but cannotremove contaminants as well as filters can.

In some situations, it is advantageous to use a combination centrifuge and filter press. The oil isfirst passed through the centrifuge to remove the large quantities of contamination, and thenthrough the filter to remove any remaining contamination. This makes use of the best qualities ofthe two systems.

The vacuum dehydrator provides an efficient means of reducing the water content of transformeroil. The vacuum dehydrator operates on the principle that lowering the pressure of the oil (e.g.exposing the oil to a vacuum) will result in a lowering of the boiling point of the water in the oil.If a heat source is applied at this new lower pressure, the water will quickly evaporate out of theoil. The vacuum dehydrator sprays the transformer oil into a heated vacuum chamber where thewater quickly evaporates.




Vacuum Systems

The use of vacuum has increased the efficiency in removing moisture from transformer windinginsulation, and air voids. As a vacuum is drawn on the main transformer tank, any moisturecontained in the tank will evaporate, and be removed by the vacuum pump as a vapor. With theaddition of an external heat source, a greater amount of moisture can be removed because of ahigher evaporation rate and saturation level for hot air.

Figure 20 shows a typical vacuum pump hookup. This particular configuration is used forvacuum filling. It also can be used for moisture removal, but special testing is needed to find thelevel of dryness, like dew point or power factor readings.

Figure 20. Piping Arrangement for Vacuum Filling




Vacuum filling operations are performed as follows:

• The vacuum pump is started. It causes a vacuum to be drawn on the maintransformer tank through a valve and piping arrangement that is connected to thetop of the main tank.

• After the vacuum recommended by the transformer manufacturer has been reachedaccording to the vacuum gauge on the main tank, the top filter valve can beopened. The opening of the top filter valve will allow oil to flow from the oilsupply, through the oil filter and top filter valve, and into the main transformertank.

• The oil level in the transformer tank can be monitored during fill operationsthrough use of the tygon tube oil sight glass that is connected to the drain valve onthe main transformer tank.




GLOSSARY

AA Ambient air or self-cooled, ventilated dry type transformer.

AA/FA Ambient air and forced-air-cooled, ventilated dry type transformer.

actuate To put into mechanical or electrical action or motion.

ANV Ambient or self-cooled, non-ventilated dry type transformer.

FA Forced-air cooled.

FOA Forced-oil-air cooled, liquid-immersed transformer.

incipient fault A fault that is just beginning to come into being.

load growth factor The estimated amount of additional electrical load that may be added toan installation over time.

OA Open air or self-cooled, liquid-immersed transformer.

OA kVA rating The power rating of a transformer when forced cooling is not in use oropen air or self cooled base rating.

setpoint The numerical value at which a protective device or control deviceshould operate.

transformers accessories and support equipment

Documents

spot temperature indicator

type level indicator

pressure vacuum indicators

pressurevacuum indicators

oil temperature indicator

pressure relief devices

indicator relay

technical material