Transformers Accessories and Support Equipment

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Transformer Accessories And Support Equipment

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramcoís employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise otherwise used in whole, or in part, without the the written permission permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : El Electrical File Reference: EEX20203

For additional information on this subject, contact W.A. Roussel

Engineering Encyclopedia

Electrical Transformer Accessories and Support Equipment

CONTENTS

PAGES

OPERATION OPERATION OF TEMPERA TEMPERATURE TURE INDICATI INDICATING NG DEVICES DEVICES ........................................... 1 Top Oil Temper Temperature ature Indica Indicator tor .................................................................................. 1 Hot Spot Temperature Temperature Indicator................................................................................ Indicator................................................................................ 3 Types of Hot Spot Indicatin Indicating g Circuits........................................................................ Circuits........................................................................ 3 Thermome Thermometer ter Heat Heat Well Type Type ........................................................................ 3 Bridge Bridge Type................................................................................................... 4 Overload Overload Indicato Indicatorr Relay..........................................................................................10 Relay..........................................................................................10 OPERATION OF LIQUID LEVEL INDICATORS (ADP-P-121, CHAPTER #8)...............11 Sight Sight Glass Glass ...............................................................................................................11 Dial Type Level Level Indicato Indicatorr .........................................................................................13 OPERATION OPERATION OF PRESSURE PRESSURE RELIEF RELIEF DEVICES DEVICES .............................................................15 Mechan Mechanica icall Relief.....................................................................................................15 Relief.....................................................................................................15 Diaphr Diaphragm agm Relief Relief......................................................................................................17 ......................................................................................................17 OPERATION OPERATION OF GAS DETECTOR RELAYS...................................................................20 OPERATION OF PRESSURE PRESSURE VACUUM INDICATORS AND BLEEDERS BLEEDERS ............ ...... ............ ........22 ..22 PressurePressure-Vacuum Vacuum Indicators.....................................................................................22 Indicators.....................................................................................22 Pressure Pressure-Vac -Vacuum uum Bleeder Bleeder ........................................................................................24 OPERATION OPERATION OF DEHYDRATI DEHYDRATING NG BREATHERS BREATHERS............................................................25 ............................................................25 Breather Breather Operation...................................................................................................25 Operation...................................................................................................25 TYPES OF BUSHINGS ON TRANSFORMERS TRANSFORMERS ................................................................27 Types Types of Bushings Bushings on Transfo Transformer rmers..........................................................................27 s..........................................................................27 Bushing Bushingss with with Draw-Th Draw-Throug rough h Cable Cable Leads Leads ...................................................27 Bushings Bushings with Solid Conductors Conductors ...................................................................28 OilOil-Fil Filled led Bushi Bushings ngs........................................................................................31 ........................................................................................31 Condense Condenserr Bushings......... Bushings......................................................................................31 .............................................................................31 Combin Combinati ation on Bushin Bushings gs ..................................................................................31 Basic Basic Bushing Bushing Constru Constructio ction......................................................................................36 n......................................................................................36 Saudi Aramco DeskTop Standards



Separable Separable Connectors Connectors (Dead Front Transformers Transformers)) ....................................................36 OIL FILTRATIO FILTRATION N AND VACUUM SYSTEMS SYSTEMS ................................................................38 Oil Filtra Filtratio tion n System Systemss ...............................................................................................38 Vacuum Vacuum Systems......................................................................................................38 Systems......................................................................................................38 GLOSSARY........................................................................................................................38

Saudi Aramco DeskTop Standards



OPERATION OF TEMPERATURE INDICATING DEVICES

This section gives a brief description of the function and operation of the following temperature indicating 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 temperature of a transformer. The indication is given by pointers that are rotated by an indicator shaft. Two pointers pointers normally normally are given: given: •

A whi white or or yell yellow ow one one to to sho show w top liqui quid tem tempe perratu ature

•

A red red one one to to sho show w the the maxi maximu mum m tem tempe pera ratu ture re reac reache hed d sin since ce the the las lastt res reset et

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. The bulb is is placed placed in a sensing sensing well that extends into the insulating insulating liquid at the top of the transformer. transformer. The liquid inside the sensing bulb expands and contracts as the temperature of the transformer liquid changes. changes. These changes changes are sent through through capillary capillary tubing tubing to a Bourdon tube. The Bourdon tube rotates the indicator indicator shaft inside inside the thermometer case. case. Indicator shaft motion moves the pointers and operates the switches. The two pointers are designed designed in a master-follower master-follower configuration. configuration. The top liquid temperature temperature pointer is the master and the maximum temperature reached pointer is the follower. This masterfollower configuration means that motion of the maximum temperature reached pointer follows the motion of the top liquid temperature pointer (as shown in Figure 1 below).


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

Figure 1. Top Oil Temperature Indicator


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Hot Spot Temperature Indicator

Hot spot temperature indicators give an indication that is equivalent to the transformer winding hot-spot. These indicators also have provisions to automatically control transformer cooling fans and winding high temperature alarms. Figure 2A shows a sectional view of a transformer with a mounted hot spot indicator with flexible capillary tube. The internal winding hot-spot temperature of the transformer winding is determined by the load it carries, its thermal characteristics, and the temperature of its cooling liquid and the ambient temperature around the transformer. Through use of the relationship between these factors, a heater is designed to duplicate the winding hot-spot rise over top liquid, when supplied with a current 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 of the winding hot-spot temperature can be measured through insertion of the temperature sensitive element 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 transformer where the money loss would be relatively large in case of damage due to excessive temperature in the windings. This device gives the operator an indication of the temperature of the winding rather 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 indicator Figure 1). The thermometer bulb and a heating coil are assembled in a well located in the hottest portion of the insulating oil near the top of the transformer. See Figure 2A. In most cases the thermometer dial or indicator is mounted on the transformer case, but it can be placed a limited distance away from the transformer. The maximum distance is fixed by the practical length of the tube connecting the gage mechanism and the thermometer bulb. The current in the winding of the power transformer passes through the primary winding of a current transformer, as shown in Figures 2B and 2C thus causing a current to flow in its secondary, proportional to that in the primary. This secondary current flows through a heating coil which encases the temperature indicator bulb, thus generating heat in the bulb proportional to that developed in the transformer windings. This adds an increment of temperature to the thermometer bulb which is equal to the winding hot spot rise above the hottest oil temperature.


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The temperature of the bulb is at all times proportional to the temperature of the main transformer windings, 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 alarm circuits. 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 indications or control. Heating coil #1 (HC1) is used to heat the Bourdon tube with ambient oil temperature. The resulting 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 is used 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 winding temperature of a large transformer. A thermometer is not used and the scheme is purely on electrical one. For this reason the indicating dial can be placed on a switchboard at any reasonable distance from the transformer. This device consist of a current transformer (CT) that is energized from the current in the main transformer winding. A heat-insulated coil is included in the secondary circuit of this CT. The amount of heat generated in this coil is proportional to the heat generated in the main transformer winding. Anon-conductive resistance embedded in the heating coil forms the fourth leg of a wheatstone bridge circuit, see Figure 2E. The bridge, completed by means of resistances outside of the transformer, derives its voltage from a direct-current source. A voltmeter is connected between two points, normally of equal potential. When a current flows in the power transformer windings, a current proportional to the power current flows in the CT. This latter current raises the temperature of the heating coil, which in turn increases the temperature of the noninductive resistance. The bridge therefore becomes unbalanced and the amount of unbalanced is indicated by the voltmeter. There is a definite relation between this current and the variation of temperature, and the voltmeter scale is calibrated to read the temperature of the heating coil directly in degrees Celsius. The wheatstone bridge circuit is balanced when normal current is flowing in the transformer. Any unbalance of the bridge will be an indication of the temperature of the heating coil, which in turn is proportional to the temperature of the main windings.


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


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


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


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Figure 2D. “Hot-Spot” Indicating Circuit


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Figure 2E. Hot-Spot Temperature Indicator Utilizes Wheatstone Bridge Method to Determine Transformer Temperature Saudi Aramco DeskTop Standards

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Overload Indicator Relay

Figure 3 shows an overload indicator relay. This type of relay gives a visual indication of the transformer 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 placed in 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 maximum indicator (or “drag hand”) that is resettable with the reset knob.

Figure 3. Overload Indicator Relay


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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 if they 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’s standard arrangement. They are not the preferred method for level indication. The sight glass is one of the most basic indicating devices. Figure 4 is a basic diagram of a typical sight glass used for level indication. The sight glass operates as follows: •

The liquid in the tank exerts a pressure in the downward direction that is proportional to the volume of liquid in the tank.

•

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

•

The increase or decrease in pressure exerted is felt at the bottom sight glass connection.

•

When the pressure exerted increases, liquid from the tank will flow through the bottom 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 the opposite direction through the bottom sight glass connection, causing the liquid level to decrease in the sight glass.


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Figure 4. Basic Sight Glass


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Dial Type Level Indicator

The dial type indicator is preferred for use on Saudi Aramco transformers. Figure 5 shows a typical 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 motion of the indicator shaft. The micro switch can be connected to a remote alarm panel through use of the electrical leads to provide high and/or low liquid level alarms.


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Figure 5. Dial Type Magnetic Liquid Level Indicator


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OPERATION OF PRESSURE RELIEF DEVICES

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

Mechanical relief

•

Diaphragm relief

Mechanical Relief

Figure 6 shows the two operating positions of a typical mechanical relief device: reset position and venting position. This device is mechanical, self-sealing, and self-resetting and is used to protect the tank against excessive internal pressures, such as the pressures that accompany arcs in the insulating liquid. The device can be mounted directly to the top of the transformer tank or to the top of an adapter which is directly mounted to the top of the transformer tank. Figure 6 shows the device mounted through use of an adapter. The device also has a visual-type indicator (flag or semaphore) and an alarm assembly to provide a visual and audible indication that the device has operated. The visual type indicator consists of a plastic 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 and tripping this mechanical relief device. The gas pressure inside of the transformer tank is transmitted to the bottom side of the relief cover through the hollow shaft guide assembly. When the tank pressure becomes excessive, the relief cover and the operating shaft move upward which vents the excessive gas pressure. The movement of the relief cover and operating shaft also pushes the indicating flag into a vertical position and actuates the alarm. The indicating flag must be 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 transformer primary breaker off-line if the pressure relief device operates. This pressure relief device should be tested during the installation testing phase of commissioning and should also be tested periodically as part of a preventive maintenance program. The pressure relief and reset pressure values should be recorded and compared to the new installation values. The alarm for the indicating pressure relief should also be tested at the same time the pressure relief device is tested.


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Figure 6. Mechanical Relief Device Saudi Aramco DeskTop Standards

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Diaphragm Relief

Figure 7 shows a typical diaphragm relief device that automatically resets. This device is mounted on 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 the diaphragm before operation. When the diaphragm rises during operation, it lifts the semaphore into view and shows that the relief device has operated. Alarm contacts can be supplied to show that the unit has operated. This semaphore button can (if requested in purchase specifications) have a yellow plastic banana shape device that sets over the top of this button. When pressure relief by the diaphragm occurs the button pushes the banana samaphore into a vertical position where it stays because of spring tension until it is reset. The size and length of the banana semaphore allows it to be seen better from ground level. Some transformer designs make it impossible to see the button semaphore by itself from ground level.

Figure 7. Diaphragm Relief Device (Sealed Position)


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Figure 8 shows the same device with an alarm mounted on the side of the cover. The alarm device has a double throw microswitch. The switch actuating lever is extended and rests against the edge of the diaphragm for the untripped position of the relief device. When the switch actuating lever is in this extended position, a set of normally closed contacts inside the alarm device are held open to disable the alarm circuit. When the relief device operates, movement of the diaphragm releases the switch actuating lever which allows the lever to retract inside the switch reset lever. When the switch actuating lever is in this retracted position, the contacts inside the alarm device return to their normally closed condition and actuate the alarm. The alarm must be manually reset by pressing inward on the switch reset lever following a trip of the diaphragm 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 exhaust allows the tank pressure to spread over the entire diaphragm area, which is twice the area of the center section. This spread of pressure causes the device to open rapidly and to remain open until the pressure in the tank falls well below the tripping pressure. This differential between tripping pressure 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 about one-half the tripping pressure. A relief device that reshuts before the pressure being relieved returns to normal (zero for this relief device) is said to have blowdown. Moisture is prevented from entering the transformer tank because the diaphragm relief device is designed with blowdown. Blowdown prevents the entry of moisture by shutting the relief device while the pressure in the transformer tank is still higher than atmospheric pressure. This pressure relief device should be tested during the installation testing phase of commissioning and should also be tested periodically as part of a preventive maintenance program. The pressure relief 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 relief device is tested.


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


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OPERATION OF GAS DETECTOR RELAYS

Figure 9 shows an internal and external view of a gas detector relay. Large power transformers use gas detector relays to warn of approaching faults. Gas generated by an incipient fault will rise to the center of the domed transformer cover and will pass through tubing to the gas detector relay. The accumulated gas forces the oil down in the normally full relay chamber. When the oil level in the relay chamber lowers, the float located inside the relay chamber also lowers. When the float lowers, the float arm moves downward on the float end and upward on the gear end. This float arm movement causes the indicator shaft to rotate in the counterclockwise direction. When the indicator shaft rotates, the pointer shaft also rotates because the two shafts are connected through a 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 of displaced oil). The pointer shaft operates the alarm switch when 200cc of gas have accumulated in the relay chamber.


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Figure 9. Gas Detector Relay


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OPERATION OF PRESSURE VACUUM INDICATORS AND BLEEDERS

Saudi Aramco uses pressure vacuum indicators and bleeders on sealed gas, cushion-type transformers when these accessories are recommended by the manufacturer. This section gives a brief 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 and an external view of a pressure-vacuum gauge that is calibrated from -10 to +10 psig. The pressure 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 a pressure increase or down on a pressure decrease.

•

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

•

The segment gear rotates in the same direction as the lever, which causes the pinion 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 operating under 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 the transformer seal and should be checked.


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Figure 10. Pressure Vacuum Indicator


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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 gas expansion space of sealed tank transformers. The settings for a particular gas or oil relief application are unique and are stamped on the device. The bleeder has diaphragm-operated pressure and vacuum bleeder valves that exhaust gas or that draw in outside air through the vent. Since the vacuum bleeder valve only functions during extremely low temperature and loading conditions, the moisture and oxygen content of the small amount of air drawn into the transformer may be considered negligible.

Figure 11. Pressure-Vacuum Bleeder


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OPERATION OF DEHYDRATING BREATHERS

Saudi Aramco requires the use of dehydrating breathers on the air side of diaphragm sealed conservators. The section that follows contains a brief explanation of how these breathers operate. Breather Operation

Figure 12 shows a typical dehydrating breather assembly. This breather has two diaphragm- type relief valves that are factory set at 1, 5, or 8 psi. One valve permits in-breathing, and one permits out-breathing only when the internal pressure of the transformer exceeds the setpoints. With relief valves, the breather regulator protects the dehydrating material from humid atmospheres by eliminating breathing for pressure changes between the relief valve setpoints. The dehydrating breather assembly is connected to the transformer through a pipe fitting. When the pressure inside the transformer increases, air travels from the transformer into the breathing pipe, and then travels out of the holes in the breathing pipe which pressurizes the dehydrating chamber. When the pressure rises to the setpoint of the outbreathing relief valve, the valve opens to relieve the pressure. When the relief valve opens, the air passes directly down the breathing pipe, and bypasses the dehydrating material. Direct exhaust to the atmosphere minimizes the contamination of the dehydrating material by oil vapors from the transformer. When the pressure inside the transformer decreases, air is drawn from inside the dehydrating chamber, through the holes in the breathing pipe, and into the transformer. The withdrawal of air from the dehydrating chamber causes the pressure to drop inside the dehydrating chamber. When the pressure drops to the setpoint of the inbreathing relief valve, the valve opens and allows outside air to pass through the valve and the perforated screen into the bottom of the dehydrating chamber. The air then flows up through the dehydrating chamber that is filled with dehydrating material, into the holes in the breathing pipe, and then into the transformer. The air that reaches the 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 of the color of the dehydrating material. The dehydrating material changes colors as the material absorbs moisture. The dehydrating material must be replaced when the color of the dehydrating material that is seen through the upper window has changed. A drain plug and a filler plug are provided to allow easy replacement of the dehydrating material.


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Figure 12. Dehydrating Breather Assembly Saudi Aramco DeskTop Standards

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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, this entrance must be oil-tight and weatherproof. This is the purpose of bushings. It is usually composed of an outer porcelain body for rigid insulation required. At higher voltages, additional insulation in the form of oil and molded paper is used within this porcelain. The four types of bushings 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 which the windings leads pass and terminate to the top metallic terminal cap of the bushings. The outer surface 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-through cable leads. These bushings are considered low current bushings because bushings with drawthrough cable leads are generally rated for a maximum current of 600 amperes and used for medium and high voltage.


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Bushings with draw-through cable leads are supplied with a detachable terminal stud to permit the use of the transformer winding lead as the bushing conductor. The transformer winding lead is brazed to the detachable terminal stud, drawn up through the core of bushing porcelain, and then attached with a terminal cap to the top of the bushing. The terminal cap provides the connection point for the incoming high voltage power. The completed bushing assembly fits into a hole in the top 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 has a solid conductor in the center of the bushing. Figure 14 shows an assembled view and a cross-sectional view of a bushing with a solid conductor. These bushings are considered high current bushings because bushings with solid conductors are generally rated for a maximum current of 5000 amperes and used for medium and high voltage. Bushings with solid conductors normally contain a solid rod as the conductor for current ratings up to 2000 amperes. Hollow-core conductors are normally used in bushings with current ratings above 2000 amperes. Having a larger surface area than the solid rod conductor, the hollowcore conductor is able to carry more current. Bushings with solid conductors have a threaded terminal stud attached to the top and the bottom of the solid conductor. The threaded terminal at the top of the bushing provides the connection point for the incoming high voltage power. A threaded blade is screwed on to the bottom threaded terminal to provide a connection point for the transformer winding lead. The completed bushing assembly fits into a hole in the top of the transformer enclosure and is held in place with a mounting flange and bolts.


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Figure 13. Bushing with Draw-Through Cable Leads


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Figure 14. Bushing with Hollow-Core Conductor


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Oil-Filled Bushings

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

The condenser type is similar to the oil-filled bushing except that the central rod is wound with alternating layers of insulation and tin foil. This results in a path from the conductor to the case consisting of a series of condensers. The layers are so designed as to provide an approximately equal voltage drop between each two condensers. In some types the entire bushing is enclosed in a skirted porcelain shell. Other types have only the exposed part of the bushing so enclosed. See Figure 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 and condenser bushings are used in combination a great deal (see Figure 17). The extra high voltage bushings are good example of this type device. Figure 17 illustrates the various parts of construction typical for a high voltage bushing.


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Figure 15. Oill-Filled Bushing


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Figure 16. Core of Condenser Type Bushing Without Porcelain


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Figure 17. Typical Oil-Filled and Condenser Type Bushing for 66kV Transformer Saudi Aramco DeskTop Standards

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Basic Bushing Construction

Figure 18 shows the basic bushing construction requirements. Transformer bushings for use on outdoor Saudi Aramco transformers must be made from wet process glazed porcelain in a petticoat fashion. Bushings are made in a petticoat fashion to increase the creepage distance (the shortest distance measured on the surface of the insulator between parts of different polarities). Saudi Aramco requires a minimum creepage distance of 40 mm per kV line-to-line of nominal system voltage. Refer to Figure 18 for locating creepage distance related to a bushing. Bushings for outdoor applications also must have threaded studs. The conductors used may be copper or aluminum. Also, outdoor bushings for nominal system voltages of 69 kV and above must have a capacitor tap for testing. Saudi Aramco also requires all bushings of the same voltage class to be interchangeable. This simplifies 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 bushings perform for other types of transformers. The additional feature provided by separable connectors is 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


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Figure 18. Basic Bushing Construction


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Figure 19 shows a separable connector for a dead front pad-mounted transformer. The bushing well is inserted through a hole in the high-voltage compartment of the transformer. The bushing well is made of porcelain and has a conductor that runs through the center. One end of the conductor is used as a male contact, and the other end of the conductor has a lug to provide a connection point for the high voltage winding lead. The loadbreak insert screws into the bushing well. The loadbreak insert is made of plastic and has a 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 spring compression. The loadbreak elbow is also made of plastic. The loadbreak elbow comes with a compression fitting that is crimped on to the incoming high-voltage power lead. The other end of the compression fitting is threaded such that a conducting pin can be screwed into the compression fitting. The end of the conducting pin forms a male contact that is inserted into the female contact in the loadbreak insert to complete the high-voltage electrical connection.

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


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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 of several types of filters, centrifuges, and vacuum dehydrators. The filters include various kinds of cartridge-type filters and the common filter press. These filters are 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 essential that 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 other contamination is present. In general, it is better at removing large concentrations, but cannot remove contaminants as well as filters can. In some situations, it is advantageous to use a combination centrifuge and filter press. The oil is first passed through the centrifuge to remove the large quantities of contamination, and then through the filter to remove any remaining contamination. This makes use of the best qualities of the two systems. The vacuum dehydrator provides an efficient means of reducing the water content of transformer oil. 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 the oil. The vacuum dehydrator sprays the transformer oil into a heated vacuum chamber where the water quickly evaporates.


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Vacuum Systems

The use of vacuum has increased the efficiency in removing moisture from transformer winding insulation, and air voids. As a vacuum is drawn on the main transformer tank, any moisture contained in the tank will evaporate, and be removed by the vacuum pump as a vapor. With the addition of an external heat source, a greater amount of moisture can be removed because of a higher evaporation rate and saturation level for hot air. Figure 20 shows a typical vacuum pump hookup. This particular configuration is used for vacuum filling. It also can be used for moisture removal, but special testing is needed to find the level of dryness, like dew point or power factor readings.

Figure 20. Piping Arrangement for Vacuum Filling


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Vacuum filling operations are performed as follows: •

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

•

After the vacuum recommended by the transformer manufacturer has been reached according to the vacuum gauge on the main tank, the top filter valve can be opened. The opening of the top filter valve will allow oil to flow from the oil supply, through the oil filter and top filter valve, and into the main transformer tank.

•

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


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Transformers Accessories and Support Equipment

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