Volt Reg Apps10

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GRID

Technical Institute

On/Under Load Tap ChangingTransformer

Voltage Regulating Control


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On/Under Load Tap Changing Transformer Voltage Regulating Control2

Contents

On Load Tap Changing Transformers

Voltage Regulation

Time Delays

Grading Between Voltage Levels

Line Drop Compensation

Parallel Operation

Distributed Generation

Supplementary Control Functions


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Voltage RegulationPurpose

Electrical plant is designed to operate within finite

voltage limits

To control the system voltage transformers with tap

changers are commonly usedOff Load manually changed

On Load automatically adjusted

Tap changers physically alter the transformer ratio and

hence the voltage

Automatic voltage regulating relay is used to control the

tap changer


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Voltage RegulationPerformance Criteria

Two Mutually Exclusive Parameters

voltage quality

number of tap changes

Voltage Quality

customer focused quality measurement

percentage of time outside deadband or statuary limits

length of voltage disturbance

Number of Tap Changes

supplier focused quality measurement

directly relates to cost of ownership of the tap changer


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On Load Tap Changing Transformers


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On Load Tap Changing TransformersTypical Supply Network


7/53On/Under Load Tap Changing Transformer Voltage Regulating Control7

On Load Tap Changing TransformersTap-Outs and Windings

Tap-outs are normally on the HV winding

Tap-outs can be at the Line End or Neutral End

Causes variations in the impedance of the transformer

Transformer impedance calculated at the centre tap position



On Load Tap Changing TransformersDevelopment of the Tap Changer

Developed in the late 1920s

Two primary considerations

The load current must not be interrupted

No windings should be short-circuited during transitions

Early tap changers were reactors, could be continuously

rated useful when mechanisms were unreliable

Resistor transition tap changers now used, reliable

stored energy drive mechanisms

Typical Operation ~75ms



On Load Tap Changing TransformersDouble Resistor Flag Sequence

1

2

3

4

5

6

7

8+

R R

Selector Compartment

No load switching takes place here.

'In Tank' double compartment tapchangers, have selectors immersed

in the main transformer tank.

'External' double compartment tap

changers, have selectors external to

the main transformer tank andseparate from diverters, but can

share oil with the main transformer

tank.

Diverter Compartment

Oil contamination takes place here.Oil kept separate from selectors

and main transformer tank.

All load switching takes place here.

Double Compartment Tap Changer Single Compartment Tap Changer

Oil is shared between selectors and

diverters.

Oil is not shared between the

single compartment and the maintransformer tank.

The diverter may be a vacuum

switch to prevent oil contamination.

Double compartment tap changersare usually found on large

distribution and transmissiontransformers.

Single compartment tap changers

are usually found on distribution

transformers.

All switching takes place in the same

compartment.



Voltage Regulation



Voltage RegulationBasic Operation

Controller monitors secondary voltage

voltage setting - nominal voltage level

voltage deadband - permissible range

Initial Time delay used to filtertransient voltage fluctuations

definite time

inversely proportional to the

deviation from the voltage

setting



Control a voltage

regulating

transformer (on-load

tap changer)

Maintain system

voltage within set

limits

Issue tap up/down

commands

Time

Voltage

Vs

+ dVs

- dVs

Basic Regulating Requirements



Voltage RegulationExample Operation

+ Voltage

Deadband

- Voltage

Deadband

Voltage

Setting

Time Delay

t

V

Voltage out

of deadband

Voltage restored

to within deadband



Voltage RegulationAlgorithms

There are different voltage regulation algorithms

Basic On/Off Controller

Simple time delayed with immediate reset

Hysteresis on the Voltage Limits

Using different pick-up and drop-off quantities

Proportional Resetting Initial Delay Timers

The timer resets proportionally

Complex

Standard deviation measures

Performance criteria functions



Time Delays



DeadbandVoltageV



Initial DelayInter-tap Delay

Time

Tap pulse duration - 0 to 5s (MVGC fixed at 1s)

Multiple Tap Change SequenceInter-Tap Delay - Definite Time

VoltageDeviation

Vs

dVs

The timer is reset after each operation to invoke initial timer



VoltageDeviation

Vs

dVs

Initial Delay (Inverse Time)

Multiple Tap Change SequenceInter-Tap Delay - Inverse Time

Time

The timer is reset after each operation to invoke initial timer



Initial Time DelaysTypical Setting Ranges

Initial Time Delay Setting Ranges

0 to 300s

Inter-tap Delay Setting Ranges

0 to 120s (typically 5 10s)

Initial Time Delays also ensure time grading between

voltage levels



Grading Between Voltage Levels



Grading Between Voltage LevelsCauses of Voltage Deviations

Grid Supply

Load

Load

A

CB

Line

T3T2

T1

Load

Voltage deviations occur for three reasons

Change in downstream load

Change in upstream supply voltage

Embedded generation

di l l



Grading Between Voltage LevelsTime Grading

Grid Supply

Load

Load

A

C

BT3

T2

T1

Load

Discrimination to allow higher voltage to operate first

T1 time delay < T2 time delay < T3 time delay (initial)

Otherwise hunting can occur between voltage levels

Can be compromised by embedded/distributed generation


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Li D C ti


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Line Drop CompensationRegulating the Voltage at a Remote Point

Tap Changer

Time Delay

VoltageDeadband

Voltage SettingAVC Relay

Supply

Load

VT

CT

+-

Bus

X R

Feeder

Voltage Drop


)(FEEDERFEEDERLOADSLOAD

jXRIVV

)( AVCAVCAVCSBUS jXRIVV

Simulates voltage drop of the line

Artificially boosts transformer voltage at times of high loading

Li D C ti


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Line Drop CompensationRelay Implementation

Vr Vxl

Remote

Voltage

VREM

Load

IL

Bus Voltage VBUS = VREM + Vr + Vxl

Calculate the Voltage Drop at nominal load

Bus

Voltage

VB

Li D C ti


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Line Drop CompensationPhasor Diagram

IL

VBUS

-ILX

-ILR

VREM Voltage Setting

The relay calculates and

regulates the VREM magnitude

ILX required for power factor

The measured

bus voltage


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Parallel Operation

P ll l O ti


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Parallel OperationTransformer Operating in Parallel

Transformers in parallel can cause Circulating Currents

Circulating Current affects LDC causing tap changer

runaway

Circulating Current are damaging to Transformer

Therefore special control techniques are required

Paralleling using Master-Follower Control

Paralleling using Circulating Current Control

Paralleling using Negative Reactance Control

P ll l O ti


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Parallel OperationMaster - Follower

One controller designated the master

All other controllers (followers) follow the master

Requires that

Identical transformers / tap changers

Locality of equipment

Security of communications is critical

Circulating current should be monitored

Parallel Operation


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Parallel OperationCirculating Currents due to Tap Disparity

2IL

T1

T2

Ic

IL-Ic

IL+Ic

If T1 is on a higher

tap than T2

Current seen by

#1 is IL + IC

Current seen by

#2 is IL - IC

#2

#1

Parallel Operation


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Parallel OperationCirculating Current Effect on LDC

2IL

T1

T2

Ic

Vxl Vr

IL-Ic

IL+Ic

IC is measured in LDC circuit but

is not present in the feeder

#2

#1

Parallel Operation


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Parallel OperationLDC on T1

IL

VBUS

-ILX

-ILR

VREM

ICIL + IC (Volts High)

VREG

-IcR

-IcX

The regulated voltage VREG

contains a -VC component

VREG will be regulated up to VREM,

therefore increasing the over

voltage even further

Voltage Setting

The regulatedvoltage

The measured

bus voltage

Parallel Operation


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Parallel OperationLDC on T2

IL

VBUS

-ILX

-ILR

VREM

IC

IL - IC (Volts

Low)

VREG

-ICR

-ICX

The regulated voltage VREG

contains a +VC component

VREG will be regulated down to VREM,

therefore reducing the under

voltage even further

The measured

bus voltage

Voltage Setting

The regulated

voltage

Parallel Operation


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Parallel OperationCirculating Current Control

Pilot Method (when tap changers are matched)

Extract the circulating current

Apply to a compensation circuit, derive a proportional

voltage magnitude

Feed this derived VC back into the measuring circuit to

compensate for the circulating current effect

Circulating Current Control


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Circulating Current ControlPilot Method

T1

T2

VC

VC

T1>T2

Ic

jXt

jXt

IL+IC

IL-IC

IC

-IC

IL

IL

V1C = ICXt

V2C = -ICXt

2IL



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Circulating Current ControlCirculating Current Compensation on T1

IL

VBUS

-ILX

-ILR

VREM


VREG

Voltage Setting

VC

The voltage VREG is compensated

by VC

VREG will be regulated correctly

reducing the over voltage

The measured

bus voltage

The regulated

voltage

-IcR

-IcX

VREG



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Circulating Current ControlCirculating Current Compensation on T2

The voltage VREM is compensated

by VC


increasing the under voltage

IL

VBUS

-ILX

-ILR

IC

IL - IC (Volts

Low)

VREGVoltage Setting

VC

VREM

The measured

bus voltage

The regulated

voltage

VREG

-ICX

-ICR

Alternative Connections for


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Ic

IL+Ic IL-Ic

Ic

IL IL

IcRs Rs

IL

+Ic -Ic

2IL

Requires the use of swamping resistors (Rs)

Alternative Connections forLDC Circuits (Parallel)

Alternative Connections for


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Ic

IL+Ic

IL-Ic2IL 2IL

IL

+Ic -Ic

IL+Ic IL+Ic

ICTICT

2IL

Requires the possible use of interposing CTs (ICTs)

Alternative Connections forLDC Circuits (Series)

Parallel Operation


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Parallel OperationNegative Reactance Control

Reverse Reactance Control (non matched tap changers

or different sources)

Utilises the reactance compensation of the Line Drop

Compensation circuit

Reverses reactance and feeds back into compensation

circuit

Lack of specific feeder reactance in LDC results in

susceptibility to power factor

Negative Reactance Control


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Negative Reactance ControlCompensation on T1

IL

VBUS

VREM


VREG

Voltage Setting

ILXt

-ILR-ICRICXt

The measured

bus voltage

The regulated

voltage

VREM

VREG

-ILX

-ILR-IcR

-IcX

The reactance of LDC is reversed


reducing the over voltage



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Negative Reactance ControlCompensation on T2

IL

VBUS

-ILX

-ILR

VREM

IC

VREG

-ICR

-ICX

Voltage Setting

ILXt

-ILR-ICR

ICXt

The reactance of LDC is reversed


increasing the under voltageIL - IC (Volts

Low)

The measured

bus voltage

The regulatedvoltage

VREM

VREG



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Negative Reactance ControlUnity Power Factor

IL

Error

VBUSVREG

ILXt

VX = ILXt

VR = 0

Small error present at

unity power factor



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Negative Reactance ControlNon - Unity Power Factor

IL

VBUSVREG

ILXt

VX = ILXt

VR = IL(Xt.tan) Error enlarges with a lower

power factor, unless R is

specified

ILXt.tan



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Negative Reactance ControlLDC and Low Power Factor

ZA2390

IL

VBUS

ILXt

-ILR

VREM

-ILX

-ILR

VREG

IL(Xt+X).tan

Error

VX = ILXt

VR = -ILR + IL(X+Xt).tan Addition of LDC results in

greater error



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Negative Reactance ControlLDC and Low Power Factor Correction

IL

VBUS

ILXt

-ILR

VREM

-ILX

-ILR

VREG

IL(Xt+X).tan

VX = ILXt

VR = -ILRcos - IL(Xt+X).sin Corrects for low power

factor

VR

ILR

IL(Xt+X).tanVR

(IL(Xt+X).tan).cos = IL(X+Xt).sin

VREG


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Distributed Generationand LDC circuits

Distributed generation can cause reverse power flow

through transformer bus Effects LDC circuit and hence voltage regulation

Bus

Voltage

VB

Load A

Load B

Load C

G

When:

G < Load A no significant problem (if design practice followed)

G > Load A potential for low voltage levels due to reduced observed load

G > Load A, B and C LDC scheme will not operate correctly due to reverse power



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Distributed Generationand Cancellation CT

Cancellation CT can be used to maintain direction load

seen by control relayMaintains voltage during high load periods

VB Load A

Load B

Load C

G

Now When:

G < Load A no significant problem (if design practice followed)

G > Load A IG summed into IB therefore control relay sees all load current

G > Load A, B and C LDC scheme will not operate correctly due to reverse power

IB

IG



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Distributed Generationand Negative Reactance Schemes

Distributed generation can cause a change in power factor

This will effect Negative/Reverse Reactance paralleling

schemes which require a constant and specified power

factor

Circulating current minimisation schemes are preferential


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pp y

Load Shedding

voltage setting override ( 10%) Over Current / Under Voltage Supervision

inhibit control during fault conditions

Excessive Circulating Current

protects against parallel control failure

Under Current Inhibit

stops operations when low forward or reverse power

Reverse Current Operation

Auto/Manual/Remote Operations

Measurements



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pp yTap Changer Supervision

Tap Changer Monitoring

tap changer confirmation response (time limited)

voltage monitoring

voltage change occurs

in the correct direction

of sufficient magnitude

operations with no initiating signal

Tap Change Operations Counter

alarms at settable threshold

frequency alarms

excessive operations per time period

(day / month / maintenance period)

Volt Reg Apps10

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