Tap Changer

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: EEX20204 W.A. Roussel

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Evaluation Of Tap ChangingEquipment For Applicability

Engineering Encyclopedia Electrical

Evaluation of Tap ChangingEquipment for Applicability

Saudi Aramco DeskTop Standards

CONTENTS PAGES

INTRODUCTION................................................................................................................ 1

Voltage Control ........................................................................................................ 1

NO-LOAD TAP CHANGERS: MAJOR COMPONENTS AND OPERATION................... 3

Operation .................................................................................................................. 3

Major Components.................................................................................................... 4

Load Tap Changers ................................................................................................... 9

Auxiliary Voltage Regulating Transformer (AVRT) ....................................... 9

POWER TRANSFORMER AUTOMATIC LOAD TAP CHANGER (ALTC):APPLICATIONS, SYSTEM COMPONENTS, OPERATING PROBLEMS,PREVENTIVE MEASURES...............................................................................................13

Purpose of ALTC Voltage Regulating ......................................................................13

Control System.........................................................................................................15

Parallel Operation.....................................................................................................16

Control Compartment...............................................................................................19

Mechanism Operation ...................................................................................22

Tap Selector Compartment.......................................................................................22

Tap Selector Operation.................................................................................23

LOAD TAP CHANGER (LTC) OPERATING METHODS ................................................24

Automatic Operation ................................................................................................24

Remote Operation ....................................................................................................27

Manual Operation....................................................................................................28

Parallel Operation.....................................................................................................29

GLOSSARY........................................................................................................................31



Saudi Aramco DeskTop Standards 1

INTRODUCTION

Voltage Control

The standards and recommendations for preferred voltage levels for primary and secondarydistribution systems, as well as for higher-voltage systems, have been evolving since the early1900’s. The latest listing of standard system voltages is ANSI C84.1-1982 “Voltage Ratings forElectrical Power Systems and Equipment at 60 Hz.”

This standard was formulated by both utilities and equipment manufacturers, and itsrecommendations are followed by both segments of the industry. Observance of this standardenables the utilities and manufacturers to work in harmony. This standard sets specific numericalvalves as standard voltage levels including single phase residential house voltage, three-phase lowvoltage distribution, as well as higher primary distribution voltages.

The standard designates two different ranges of voltages, Range A and Range B. A servicevoltage specifies that utility supply system be designed and operated so most service voltages arewithin the limits specified. Also, that the occurrence of service voltages outside these limits to beinfrequent. With the typical voltage drops between the service entrance and the points ofutilization, the utilization equipment is designed and rated to give fully satisfactory performancewithin Range A.

Range B service voltage includes voltages above and below Range that necessarily result frompractical design and operating conditions on supply or user systems. These conditions are limitedin extent, frequency, and duration. When they occur, connective measures should be undertakenwithin a reasonable time to improve voltages to meet, Range A requirements.

Utilization equipment is designed to give acceptable performance within Range B. The designand operating guidelines of all utilities is to provide service voltage to all customers at timeswithin the Range A limits.

An electrical power systems have several factors influencing the magnitude of its electricalparameters. Because of a need to maintain consistent electrical parameters it was determined thatthe voltage of the system would be the most logical to control. Usually there is voltage controlequipment in primary and secondary distribution substations also transmission substations. Thisequipment is usually a transformer or auto-transformer with no-load or load tap changingequipment. The application of the no-load or load tap changer can be on large powertransformers having automatic load tap changer assemblies affecting the total power system or onthe independent application of a particularly load by use of voltage regulating transformers.




The voltage of a system may be varied through the medium of adjusting taps on a tap changer ona power transformer or by some form of auxiliary voltage regulator which also may be a tappedtransformer or other type of variable voltage apparatus. The voltage may be varied, in steps or bystepless control. Experience has taught that voltage variation of transmission and distributionsystems can be carried out quite effectively in steps without creating objectionable disturbances onthe system.

All power and distribution transformers for use in Saudi Aramco electrical systems require a no-load or a load tap changer. The requirements of each individual electrical installation will dictatewhether a no-load or a load tap changer should be specified. One task of an electrical engineer isto evaluate no-load and load tap changer systems for applicability of use in Saudi Aramcoelectrical systems.

There are two types of transformer tap changer classifications:

• No-load tap changer

• Load tap changer

The major difference being that a no-load tap changer is not built to change taps while supplyingpower to a load and to do so would damage the transformer and tap changer while also causing asafety hazard.

The load tap changer is basically designed to operate while supplying power to a load andoperation is designed and constructed for this purpose.




NO-LOAD TAP CHANGERS: MAJOR COMPONENTS AND OPERATION

Operation

Often it is desirable to change, by a relatively small amount, the ratio of the transformer. Thismay be done to compensate for the voltage drop in the supply source or to supply a particularvoltage for the connected load.

Most transformers have a device or method to change the voltage rating of one or both of thetransformer windings. To change the relationship, all we need to do is to change the ratio of turnsbetween the winding. This is exactly what a no-tap changer does. It adds or subtracts turns in awinding. These taps do not improve voltage regulation but are only for changing general voltagelevel the transformer feeds.

On liquid-filled transformers this is typically done by turning a hand crank mounted on thetransformer. This hand crank operates an internal mechanism to add or subtract turns. On dry-type transformers you move wire jumpers between terminals to select a different tap.

We call this type a no-load tap changer, but a better name would be de-energized tap changer. Itis abbreviated as NLTC. This type of tap changer must be operated only when the transformer iscompletely de-energized.

CAUTION: Do not operate a no-load tap changer when the transformer is energized. Thetransformer must be completely isolated from all voltage sources. By operating a no-load tapchanger when the transformer is energized, you can cause damage to the transformer andmore importantly possible serious injury to yourself.




Major Components

No-load tap changers normally have two major assemblies: the tap changing mechanism and theoperating mechanism. A common wedge-type tap changing mechanism is shown in Figure 1.

Figure 1. Wedge-Type Tap Changing Mechanism

The tap leads from the transformer are connected to the circular group of nickel-plated copperrods. The rods are held together between two insulating heads. A wedge is found in the center ofthe assembly, and connected to a crankshaft. The wedge completes the winding circuit byconnecting two adjacent rods. A spring between the wedge and crankshaft gives high-pressureline contact between current-carrying components.




Two types of operating mechanisms are available. Most applications use a hex-head drivemechanism. The folding handle drive mechanisms are used when high driving torque is needed.Both types are shown in Figure 2.

A. Hex-Head Drive

B. Folding Handle Drive

Figure 2. Operating Mechanisms




The drive mechanism is normally found on the sidewall of the transformer. This mechanism isconnected to the crankshaft of the tap changer through an insulator. The mechanism is normallycovered with a cap. The cap gives a seal between the atmosphere and the operating mechanism.The cap is held on by 2 screws. One screw has an oversize head with a hole in it. The hole canbe fitted with a padlock to prevent access.

These types of no-tap changers will add and subtract turns in the primary winding of thetransformer. There are a total of 5 taps. The middle tap labeled Tap 3 will be the nominal voltagerating of the primary winding. There are 2 taps above and 2 taps below the nominal voltagerating. When the no-tap changer is at the nominal voltage tap, we call this the neutral tap orcenter tap. Each tap will change the voltage by 2.5% of the rated nominal voltage. This meansthere is a voltage range of + 5% and - 5% from the nominal voltage.

Figure 3 shows part of a transformer nameplate listing the no-load tap changer voltage ratings.This nameplate tells us the nominal voltage rating on Tap 3 of the primary winding is 13,800volts. Taps 1 and 2 are each 2.5% higher than tap 3 (13,800 x 0.025 = 345 volts). Taps 4 and 5are each 2.5% lower than Tap 3.




Figure 3. Typical Nameplate for Transformer with Off-Load Tap Changer




The last column tells us what part of the winding is in or out of the circuit. These numbers areshown on the winding drawing. For example, if the no-load tap changer is in position 3, the charttells us that the no-load tap changer connects numbers 3 to 6. This is done on all three primarywindings. By placing a jumper between points 3 and 6, all turns between those points are shortedout. Therefore, these turns are effectively removed from the winding and the turns ratio ischanged.

Now let’s look at what happens when the no-tap changer is in position 1. The table tells us points4 and 5 are connected. The winding diagram shows us that all turns are in the circuit. Rememberthe winding turns ratio and voltage ratio are the same. The turns ratio is equal to:

Number of Primary Turns (Np)/Number of Secondary Turns (N

s)

If the number of primary turns increase, then the turns ratio increases. This also changes thevoltage ratio.

The opposite happens when you move to tap position 5. Points 2 and 7 are connected. Thismeans that all turns between 2 and 7 are jumpered. Using the above formula, this lowers the turnsratio and voltage ratio.

The winding is always tapped in the center. This prevents the no-tap changer leads andmechanism from being exposed to voltage surges and high terminal voltages.

All power and distribution transformers should have a no-load tap changer. This will allow you toadjust or match the primary winding to the actual primary voltage. All transformers should bespecified with a no-load tap changer with the standard + 2.5% taps - 2 up and 2 down.

Example 1:

You have selected a 1000 kVA transformer with a primary voltage of 4160 volts. Calculate thetap values for the standard five taps.

Answer:

Nominal voltage is 4160 volts. This is Tap 3.2.5% of 4160 = 0.025 x 4160 = 104 volts.Tap 2 = 4160 + 104 = 4264 voltsTap 1 = 4264 + 104 = 4368 voltsTap 4 = 4160 - 104 = 4056 voltsTap 5 = 4056 - 104 = 3952 volts




Load Tap Changers

When voltage regulation is required during loading conditions, there are two type of devices used,each with a different application.

The two main types of variable load voltage control are:

• Auxiliary voltage regulating transformer

• Power transformer with automatic load tap changer

Auxiliary Voltage Regulating Transformer (AVRT)

An AVRT is used to maintain voltage of a circuit close-in at the load. These can be used onfeeders up to 34.5 kV with 2 MVA output. It is essentially a special application of an auto-transformer. The voltage in the portion of the winding in an auto-transformer, which is added orsubtracted, is made variable, so the outgoing voltage may thus be kept approximately at the ratedvalue. There are two types of voltage regulators commonly used, and the principle of operationof both is the same as for the auto-transformer, which are:

• Induction voltage regulator

• Step-type voltage regulator

Inductor Voltage Regulators. The primary (high voltage) winding is mounted so that it canrotate on the axis of the secondary winding. The voltage induced in the secondary or serieswinding depends on the position of the primary winding. The primary winding thus can be placedso that the voltage induced in the series winding will add or subtract from the input line voltage.When the load current is small, the primary will be rotated in one direction, thus lowering orbucking the line voltage. When load current is larger, the primary winding will be rotated in theopposite direction, thus boosting it. The turning of the primary coil is usually controlled by meansof a voltage sensitive relay connected in the output side of the circuit. This type of voltageregulator is not recommended for use by Saudi Aramco but is used by SCECO in someapplications.




Step-Type Voltage Regulator. The connection of this type of regulator in the circuit is the sameas that of the induction voltage regulator.

The only type of automatic voltage regulator that Saudi Aramco uses is the step-voltage type.This type of regulator automatically controls the voltage on feeders up to 34.5 kV with athroughput up to 2.5 MVA. These regulators are available for both single- and three-phaseinstallations.

A typical step-voltage regulator is shown in Figure 4. This unit involves a shunt winding, a serieswinding and a bridging reactor or preventive autotransformer. Taps, each representing essentially1-1/4% voltage, are affixed to the series winding and brought to a specially designed dial switch.(See Figure 5a.) The main transformer comprises the shunt winding and the series winding. Theseries winding most often is rated at 10% voltage of the shunt winding. Usually eight taps on theseries winding are brought to a dial-switch assembly as individual contacts, with the voltagedifference between contacts being 1-1/4% voltage.

The terminals of a center-tapped preventive autotransformer are able to transition (slide) betweenthe dial switch contacts in a manner which avoids momentary loss of load. As one fingeradvances to the next step, arcing results because of the inductive nature of the reactor, but loadcurrent is maintained (see Figure 5b) through the finger which does not part contact. When theparting finger remakes on the adjacent contact, a bridging condition is established (see Figure 5c).A circulating current is established through the prevent autotransformer (PA), or a center tappedbridging reactor, and load potential is seen to be average potential of the taps being bridged.

To minimize the arc duration, a quick-break mechanism accelerates the moving contacts. Therapid separation of the contacts and the use of contact tips composed of a tungsten-carbon alloymitigate the attendant ablation of contact material. (See Figure 5).

A reversing switch permits the polarity of the series winding to be reversed relative to the shuntwinding, thereby accommodating plus and minus regulation with the same series winding.

A voltage transformer and current transformer are used to provide signals necessary for thecontrol to perform its function.

Dielectric protection of the series winding is afforded by the bypass arrestor. A surge propagatedon the line will be shunted past the regulator. Lightning arrestors, often provided at both thesource and the load terminals, similarly protect the regulator from overvoltage surge conditions.




Saudi Aramco uses regulators to overcome the voltage drop on long feeders where the operationof feeder transformer tap changers may cause excessive and unacceptable voltage rises at thetransformer end of the feeder. Saudi Aramco may also install a regulator on an existing feederwhere the regulation of an existing transformer with deenergized taps has become excessive. Thedecision to install a regulator in these situations should be based on an economic comparison of allalternatives. These alternatives include replacing the existing transformer with one that has a loadtap changer, installing power factor correction capacitors, etc.

1

L3

52

SL

Bypass Arrestor

Series Winding

Preventive Autotransformer

Current Transformer

Load

Source

Lightning Arrestor

Shunt Winding

Contacts on Dial Switch

Voltage Transformer

Regulator Tank

Control Panel Enclosure

Reversing Switch

Figure 4. Wiring Diagram of a Typicao Distribution Step-VoltageRegulator Showing Both External and Internal Connections.

Preventive Autotransformer Shown on a NonBridging Position.(1) Bypass Switch; (2) Source Switch; (3) Load Switch




L

Series Winding

Dial Switch

Reversing Switch

Preventive Autotransformer

Shunt Winding

S

( a )

( b )

L

Dial Switch Motion

Arc

( c )

Ι CIRCL

Figure 5. Operations of the Internal Mechanisms of the Regulator




POWER TRANSFORMER AUTOMATIC LOAD TAP CHANGER (ALTC):APPLICATIONS, SYSTEM COMPONENTS, OPERATING PROBLEMS, PREVENTIVEMEASURES

The ALTC type of transformer voltage regulating device is used more for a system approach thanfor effects at just equipment loading. ALTC transformers are most often found in electricalsubstations. These substations are usually at least primary distribution system or largersubtransmission or transmission systems. This type of equipment is associated with the supply oflarge amounts of power. The connection of this type of regulating device in the circuit is thesame as that of the induction voltage regulator. The changes in voltage are accomplished byvarying the ratio of transformation by changing the number of turns in the primary by means ofvarious taps.

The tap-changing device, because of it arcing during operation is located in a separatecompartment of oil to prevent contamination of main tank oil. Such ALTC transformers may beused to regulate voltage over a plus 10 percent to minus 10 percent range, and may have thirty-sixsteps incorporated in the tap changing mechanism. ALTC transformers are available for mostmedium voltage and power ratings.

They are normally only used on the following Saudi Aramco transformers:

• Power transformers stepping down from 69 kV or above.

• Power transformers interconnecting two systems, where power flow can be ineither direction.

These are the normal applications because the added cost, complexity, and maintenance ofautomatic load tap changers is not justifiable for the others. They can be used in other places inexceptional cases. An example would be an installation requiring load voltage control.

Purpose of ALTC Voltage Regulating

Voltage regulating equipment is installed to carry out one or more of the following functions:

• Maintain a constant secondary voltage with a variable primary voltage.

• Control secondary voltage with a fixed primary voltage.

• Control the flow of reactive power between two generating systems or branches ofloop circuits.




Figure 6 is a typical example of an automatic load tap changers installation. Figure 6 shows thatthe primary windings of the transformers in bank number one and in bank number two are eachsupplied with a source of incoming line power. The transformers supply power through twooutput breakers to various load circuits. The primary side of each bank is also equipped with anautomatic load tap changer (ALTC). The automatic load tap changer functions to maintain 13.8kV to the downstream load circuits when the incoming line power varies. The automatic load tapchangers will also function to control the flow of reactive power between bank 1 and bank 2(shown on Figure 6) when the two banks are operated in parallel.

Typical Automatic Load Tap Changer InstallationFigure 6




Control System

A voltage regulator control circuit has four major components:

• Sensing circuit

• Reference circuit

• Comparison circuit

• Control drive

These four components are shown in Figure 7. Figure 7 is a basic block diagram of a voltageregulator control circuit.

Figure 7. Control System Block Diagram




This circuit has three functions:

• Monitor the output voltage and/or current of the transformer.

• Find out if they are in the allowable range.

• Adjust the output of the transformer if needed.These functions are carried out by the four major components. The sensing circuit monitors thetransformer’s output. This output is reduced to an equivalent value that is sent to the comparisoncircuit.

The reference circuit produces an adjustable signal that is also sent to the comparison circuit.Once the reference signal is set to the needed value, it gives a stable signal to compare with theactual value.

The comparison circuit compares the actual value to the reference value. This produces an errorsignal. The error signal is sent to the control device.

The control device gives the interface between the control circuit and the voltage regulating unit.The control device output causes the voltage regulating unit to adjust the transformer’s output. Itwill adjust the output up or down, based on the error signal produced.

Parallel Operation

Operation of two voltage regulators, in parallel, can result in unsatisfactory load distribution andhigh circulating currents. These operational problems are explained below in reference toFigure 8.




Installation Using Parallel TransformersFigure 8




For purposes of explanation, assume that the HV tie switch is open, main breakers #1 and #2 areclosed. The regulator for Transformer No. 1 is maintaining 12,688 volts, and that the regulatorfor Transformer No. 2 is maintaining 13,000 volts. If the tie breaker is closed to place bothtransformers #1 and #2 in parallel, the following sequence of events would occur:

• Transformer No. 2 picks up load from Transformer No. 1 plus its normal load.

• The line drop compensator for Transformer No. 2 sees a larger voltage drop.

• The regulator for Transformer No. 2 operates to raise transformer output.

• At the same time, the load on Transformer No. 1 is lowering.

• The line drop compensator for Transformer No. 1 sees a smaller voltage drop.

• The regulator for Transformer No. 1 operates to lower transformer output.

• This process continues until Transformer No. 2 is in full boost and TransformerNo. 1 is in full buck.

• Transformer No. 2 will be in an extreme overload condition.

• High circulating currents will be flowing.

• Transformer No. 2 would be over heating.

If transformers are to be operated in parallel, the control system must prevent overloading andhigh circulating currents. The preferred control scheme is the “current balance” type. Thisscheme uses current transformers. The other acceptable control schemes are the“master/follower”, “odd/even”, and “reverse reactance” types.




LOAD TAP CHANGERS: CONTROL COMPARTMENT, DRIVE MECHANISM ANDTAP SELECTOR

Load tap changers can be divided into three major sections:

• Control compartment.

• Drive mechanism.

• Tap selector compartment.

Control Compartment

A typical tap changer control compartment contains the following components:

• Operating panel with control power circuit breaker.

• Position indicator.

• Operation counter.

• Automatic static control equipment.

• Optional auxiliary equipment.

A typical tap changer control compartment is shown in Figure 9.




Figure 9. Typical Tap Changer Control Compartment




Drive Mechanism

The drive mechanism is the physical interface between the automatic controls and the tap selectorswitches. A typical drive mechanism is shown in Figure 10.

A. Drive Mechanism (Assembled)

B. Drive Mechanism (Exploded View)

Figure 10. Typical Drive Mechanism




Mechanism Operation

The load tap changer (LTC) is driven by a 115V, 60 Hz, single-phase, capacitor-start, capacitor-run motor. Positive stopping of the drive is done using dynamic braking.

The motor (1) drives the geneva pinion (2) through two spur gear reductions (3, 4). The genevapinion must make three revolutions per tap change for a 32-step LTC. This pinion makes sixrevolutions per tap change for a 16-step LTC. The geneva pinion motion is sent to a six-toothgeneva gear (5). The geneva gear drives a universal driveshaft (6) through a pair of miter gears(7, 8). The motion of the universal driveshaft changes the position of the tap selector.

Tap Selector Compartment

The tap selector compartment is a separate, oil-filled compartment. It is isolated from thetransformer by an oil-tight insulating panel. A typical compartment is shown in Figure 11.

Figure 11. Typical Tap Selector Compartment

The main component, inside the tap selector compartment, is the tap selector switch. The type oftap selector switch shown in Figure 11 is an arcing tap switch. Figure 12 shows a detailed viewof the switch.




Figure 12. Arcing Tap Switch

Tap Selector Operation

Refer to Figure 12. Universal drive shaft motion is sent to the horizontal drive shaft through twomiter gears. Horizontal driveshaft motion is sent to the scroll cam. This causes the scroll cam torotate. Every 180-degrees of motion operates one of two roller plates. Roller plate motioncauses the movable arcing contacts to move from one stationary arcing contact to the adjacentone. The reversing switch changes the winding connections to raise or lower regulation.




LOAD TAP CHANGER (LTC) OPERATING METHODS

The different methods of operating LTCs are best described through use of diagrams ofthe control circuits. A typical transformer tap changer control circuit is shown in Figure 13.Figure 13A shows the automatic voltage regulation scheme and Figure 13B shows the drivemotor control circuit.

Automatic Operation

Refer to Figure 13A and Figure 13B, which show a typical transformer tap changer control circuitschematic. The components shown in Figure 13A and 13B are identified through use of standarddevice function numbers and standard schematic symbology. This discussion of automaticoperation only covers the essential components.

Automatic operation is the normal operating mode. Automatic operation is started by placing theswitches in Figure 13B in the following positions; Auto-Manual switch in Auto, and the Local-Remote switch in Local. These positions allow for automatic drive motor control circuit responseto changes in line voltage. As shown in 13A, line voltage changes are felt by the static voltagesensor (device 90) through the auxiliary power transformer and the VRR supply potentialtransformer. When the difference between line voltage and desired voltage reaches a preset limit,the static voltage sensor automatically actuates the time delay raise (TR) or time delay lower

(TL) relay as applicable. If the TR relay actuates, it closes the contacts in the drive motor

control circuit if the TL relay actuates, it closes the contacts in the drive motor control

circuit. When or closes, the respective motor contactor (84R or 84L) actuates. If the

84R contactor actuates, it closes contacts and and opens

contact . If the 84L contactor actuates, it closes contacts and

and opens contact . These series of contacts energize the proper motor (84) coil for

moving the tap changer up or down to the next position and align the braking circuit foroperation.




A. Automatic Voltage Regulation Scheme

Figure 13(A). Typical Transformer Tap Changer Control Circuit




B. Drive Motor Control CircuitFigure 13(B). Typical Transformer Tap Changer Control Circuit




When the tap changer starts to move, cam operated switch closes to seal-in the motor contactor

(84R or 84L). When the tap changer reaches the next position, cam operated switch , opens,

deenergizing the associated motor contactor (84R or 84L). When the motor contactor

deenergizes, all of the seven associated contacts are repositioned. When or returns to their

normally closed position, braking contactor 84D actuates and closes contacts and . When

contacts and close, the drive motor is stopped through use of dynamic braking. This prevents

further motion of the tap changer. Contact will open after a short time delay (several seconds)

and deenergize the braking contactor. When the braking contactor deenergizes, the six 84D

contacts return to their normally open position. The circuit is now back to its original condition,

and ready for the next sequence of automatic operation.

Remote Operation

Remote operation allows control of the LTC through use of a remote momentary raise-lowerswitch. Remote operation is started by placing the Local-Remote switch in the Remote position,and the Auto-Manual switch in the Manual position. The LTC can now be operated electricallyby placing the remote momentary Raise-Lower switch in either the raise or lower position. Whenthe remote momentary Raise-Lower switch is in the raise or lower position, the associated motorcontactor (84R or 84L) will actuate. The resultant circuit operation is the same as that discussedpreviously under automatic operation, with one exception. If the operator does not release theremote momentary Raise-Lower switch, the tap changer will continue stepping the voltage up ordown until one of the cam operated limit switches or open.




Manual Operation

Manual operation of the LTC can mean two different things:

• Manual operation of the LTC with the electrical control circuit while in the manualmode.

• Manual operation of the LTC with the handcrank.

Manual operation of the LTC with the electrical control circuit is started by placing the Local-Remote switch in the Local position and the Auto-Manual switch in the Manual position. TheLTC can now be operated in manual, through use of the electrical control circuit, by placing theLocal Momentary Raise-Lower switch in either the raise or lower position. When the LocalMomentary Raise-Lower switch is in the raise or lower position, the associated motor contactor(84R or 84L) will actuate. The resultant circuit operation is the same as that discussed previouslyunder remote operation.

Manual operation of the LTC with the handcrank does not require the use of any of the electricalcontrol circuit components and, therefore, the handcrank is not shown in Figure 13. Operatingthe LTC with the manual handcrank instead of the drive motor (84) is normally done only duringmaintenance. Manual handcrank operation with the transformer energized is not recommended.Such operations should be done only in an extreme emergency.

Manual handcrank operation is done by deenergizing the electrical control circuit. The handcrankis then removed from its holder and placed on the handcrank shaft in the drive box. Thehandcrank shaft was shown previously in Figure 10 Item #6. Turning the handcrank after it isplaced on the handcrank shaft will manually reposition the tap changer.




Parallel Operation

Previous discussions pointed out more problems with operating transformers in parallel. Theyalso pointed out the control schemes that can be used to compensate for these problems. Thepreferred scheme for Saudi Aramco installations is the “current balance”. An example of thisparalleling control scheme is shown in Figure 14, with all of the components identified. Onlythose components that are vital to the following explanation will be discussed.

Figure 14. Typical Paralleling Control Scheme




In this scheme, load current flows through the line drop compensator CT (LDC CTs) of Banks 1and 2. The secondary current of the LDC splits into two components:

• The load component that flows through the load auxiliary current transformer(LCT) and the compensator auxiliary current transformer (CCT).

• The circulating current component.

When the tap changers are on the same step, circulating current will be minimal. When the tapchangers are on different steps, circulating current will flow from the bank on a higher step to thebank on a lower step. In the high bank, the circulating current through the LDC flows in theopposite direction to the load component of current through the LDC. The resultant current tothe voltage regulating relay (90) will be less, and the tap changer will lower. In the low bank, thecirculating current through the LDC flows in the same direction as the load component of currentthrough the LDC. Thus, the resultant current to the voltage regulating relay (90) will be more,and the tap changer will rise.

With one incoming breaker open, the in-service bank will carry the total bus load. The 152acontact will be open, breaking the circulating current path. The 152b contact will be closed,shorting out the secondaries of LCT1 and LCT2. The current will split through the balance leg,so each line drop compensator sees half of the load, preventing both regulators from moving.

With both banks in service, and the bus tie breaker (24) open, the 24a contacts will be open,interrupting the circulating current path and the balance leg. The 24b contacts will be closed,shorting out the secondaries of LCT1, LCT2, CCT1, and CCT2. Each bank is now set up tooperate independently, with correct load tap changer control.




GLOSSARY

load tap changer An attachment or auxiliary to a transformer whose primary function is toprovide a major change in voltage between input and output.

regulator A step (or line) voltage regulator is a device whose only function is tomaintain a voltage within specified limits. In an emergency, the regulatorcan be bypassed or jumpered out because both input and output voltagesare in the same class.

reactive power The energy that is stored and transferred between the inductive andcapacitive elements of a system. Reactive power does not perform usefulwork.

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