AVR on Load Tap Changer

On/Under Load Tap Changing Transformer Automatic Voltage Regulating Control Relay

Contents

X On Load Tap Changing Transformers X Voltage Regulation X Time Delays X Grading Between Voltage Levels X Line Drop Compensation X Parallel Operation

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Voltage Regulation Purpose

X Electrical plant is designed to operate within finite voltage limits X To control the system voltage transformers with tap changers are commonly used

Š Off Load – manually changed Š On Load – automatically adjusted X Tap changers physically alter the transformer ratio and hence the voltage X Automatic voltage regulating relay is used to control the tap changer

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Voltage Regulation Performance Criteria

X Two Mutually Exclusive Parameters

Š voltage quality Š number of tap changes X Voltage Quality

Š customer focused quality measurement Š percentage of time outside deadband or statuary limits Š length of voltage disturbance X Number of Tap Changes

Š supplier focused quality measurement Š directly relates to cost of ownership of the tap changer 5

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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On Load Tap Changing Transformers

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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On Load Tap Changing Transformers Typical Supply Network

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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On Load Tap Changing Transformers Tap-Outs and Windings

XTap-outs can be in the HV or LV winding XTap-outs can be at the Line End or Neutral End XCause a variation in the impedance of the transformer

ŠTransformer impedance calculated at the centre tap position

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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On Load Tap Changing Transformers Development of the Tap Changer XDeveloped in the late 1920s XTwo primary considerations

ŠThe load current must not be interrupted ŠNo windings should be short-circuited during transitions XEarly tap changers were reactors, could be continuously rated – useful when mechanisms were unreliable XResistor transition tap changers now used, reliable stored energy drive mechanisms

ŠTypical Operation ~75ms 9

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Voltage Regulation

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Voltage Regulation Basic Operation X Controller monitors secondary voltage

Š voltage setting - nominal voltage level Š voltage deadband - permissible range X Initial Time delay used to filter transient voltage fluctuations

Š definite time Š inversely proportional to the deviation from the voltage setting

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Voltage Regulation Example Operation

V Voltage out of deadband

Time Delay

+ Voltage Deadband Voltage Setting - Voltage Deadband

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

t

Voltage restored to within deadband

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Time Delays

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Time Delays Initial Time Delays X Initial time delay used to filter transient voltage fluctuations

Š definite time Š inversely proportional to the deviation from the voltage setting Operating Time

Definite Time Characteristic

Inverse Time Characteristic

Voltage VBand+

VSetting

VBand-

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Multiple Tap Change Sequence Inter-Tap Delay - Definite Time

Voltage Deviation Initial Delay

Initial Delay (Definite Time)

+dVs Vs

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

Time

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Multiple Tap Change Sequence Inter-Tap Delay - Inverse Time

Voltage Deviation

Initial Delay (Inverse Time)

dVs Vs

Time

X The timer is reset after each operation to invoke initial timer

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Initial Time Delays Typical Setting Ranges

X Initial Time Delay Setting Ranges

Š MVGC - 0 to 120s Š KVGC - 0 to 300s X Inter-tap Delay Setting Ranges

Š MVGC - 0 to 10s Š KVGC - 0 to 120s X Initial Time Delays also ensure time grading between voltage levels

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Grading Between Voltage Levels

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Grading Between Voltage Levels Causes of Voltage Deviations Load T1 Line

Grid Supply A

T2

B Load

T3

C Load

X Voltage deviations occur for two reasons

Š Change in downstream load Š Change in upstream supply voltage

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Grading Between Voltage Levels Time Grading Load Load T1

Load

Grid Supply

T2

A T3

B C

X Discrimination to allow higher voltage to operate first

Š T1 time delay < T2 time delay < T3 time delay (initial) X Otherwise hunting can occur between voltage levels

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Line Drop Compensation

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Line Drop Compensation Regulating the Voltage at a Remote Point X Simulates voltage drop of the line X Artificially boosts transformer voltage at times of high loading Bus

Supply Feeder

CT

Tap Changer

Load X R Voltage Drop

VT

Time Delay

Voltage Deadband AVC Relay

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+ -

Line Drop Compensation

VLOAD = VS − I LOAD(RFEEDER + jX FEEDER) VBUS = VS − I AVC (RAVC + jX AVC )

Voltage Setting

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Line Drop Compensation Relay Implementation X Calculate the Voltage Drop at nominal load

Vr

Vxl

Load IL

Bus Voltage VB

Remote Voltage VREM

Bus Voltage VBUS = VREM + Vr + Vxl 23

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Line Drop Compensation Phasor Diagram

-ILX -ILR

The measured bus voltage

VBUS VREM

IL

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Voltage Setting

X The relay calculates and regulates the VREM magnitude

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Line Drop Compensation Typical Setting Ranges

XReactive and Resistive Compensation, Vr and Vxl

ŠMVGC - 0 to 48V ŠKVGC - 0 to 50V

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Transformers Parallel Operation

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation Transformer Operating in Parallel X Transformers in parallel can cause Circulating Currents

ŠCirculating Current affects LDC causing tap changer runaway

ŠCirculating Current are damaging to Transformer X Therefore special control techniques are required

ŠParalleling using Master-Follower Control ŠParalleling using Negative Reactance Control ŠParalleling using Circulating Current Control 27

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation Master - Follower

X One controller designated the master X All other controllers (followers) follow the master X Requires that

Š Identical transformers / tap changers Š Locality of equipment Š Security of communications is critical X Circulating current should be monitored

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation Circulating Currents due to Tap Disparity

#2

T2

X If T1 is on a higher tap than T2

IL-Ic

2IL Ic

T1

IL+Ic

X Current seen by #1 is IL + IC X Current seen by #2 is IL - IC

#1

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation Circulating Current Effect on LDC

#2

T2

IL-Ic

Vxl Ic

T1

Vr

2IL

IL+Ic #1

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X IC is measured in LDC circuit but is not present in the feeder

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation LDC on T1 -ILX -IcR

The measured bus voltage

The regulated voltage

-ILR

-IcX VREM VBUS VREG IL

IC IL + IC (Volts High)

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Voltage Setting

X The regulated voltage VREG contains a -VC component X VREG will be regulated up to VREM, therefore increasing the over voltage even further

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation LDC on T2 -ILX -ILR

The measured bus voltage

-ICX -ICR

VREM

The regulated voltage

VBUS VREG

Voltage Setting

IL IC IL - IC (Volts Low)

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X The regulated voltage VREG contains a +VC component X VREG will be regulated down to VREM, therefore reducing the under voltage even further

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Parallel Operation Negative Reactance Control

X Reverse Reactance Control (non matched tap changers or different sources)

ŠUtilises the reactance compensation of the Line Drop Compensation circuit

ŠReverses reactance and feeds back into compensation circuit

ŠLack of specific feeder reactance in LDC results in susceptibility to power factor

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Negative Reactance Control Compensation on T1 The regulated voltage

ICXt

-ICR

ILXt

-ILX

-ILR VBUS

VREG

-IcR

-ILR

-IcX V REM

VREM

The measured bus voltage Voltage Setting

VREG

IL

IC IL + IC (Volts High)

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X The reactance of LDC is reversed X VREG will be regulated correctly reducing the over voltage

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Negative Reactance Control Compensation on T2 ILXt

-ILX

-ILR-I R C

The measured bus voltage

VREM The regulated voltage

-ILR

-ICR

ICXt VBUS

VREG

VREG

VREM IL

IC IL - IC (Volts Low)

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-ICX

Voltage Setting

X The reactance of LDC is reversed X VREG will be regulated correctly increasing the under voltage

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Supplementary Control Functions AVR Relaying

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Supplementary Control Functions X Load Shedding

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

Š inhibit control during fault conditions X Excessive Circulating Current

Š protects against parallel control failure X Under Current Inhibit

Š stops operations when low forward or reverse power X Reverse Current Operation X Auto/Manual/Remote Operations X Measurements

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On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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Supplementary Control Functions Tap Changer Supervision X Tap Changer Monitoring

Š tap changer confirmation response (time limited) Š voltage monitoring z

voltage change occurs

z

in the correct direction

z

of sufficient magnitude

Š operations with no initiating signal X Tap Change Operations Counter

Š alarms at settable threshold Š frequency alarms z

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excessive operations per time period (day / month / maintenance period)

On/Under Load Tap Changing Transformers – Voltage Regulating Control Relays

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