techniques for the control of steady state voltage in … · with regulated 11kv busbar 0 1,000...

© Electric Power Consulting Pty Ltd

1

Techniques for the Control of Steady State Voltage in MV and LV Networks

by

Dr Robert Barr, Electric Power Consulting Pty Ltd

Prof. Vic Gosbell, University of Wollongong

Mr Chis Halliday, Electrical Consulting & Training

Energy 21C Sydney, November 2007


2

Introduction• Many Australian LV sites have poor LV steady state voltage

performance– outside 230V-2% to 230V+10% range

• Adverse impacts on customer equipment– performance and life

• In most cases steady state voltage performance can be improved with the application of simple principles and procedures

• This paper is a “how to” guide for distributors

• Achieve alignment with the Australian Steady State Voltage Standard that is currently under development


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The Ideal LV Supply 230V +10% - 2%


4

Well Controlled LV Supply

Voltage Variation with Time

210215220225230235240245250255260

Tues

day

Wed

nesd

ay

Thur

sday

Frid

ay

Sat

urda

y

Sun

day

Mon

day

RM

S Vo

lts Regulated230V+10%230V-2%


5

Well Controlled LV SupplyLV Customer Supply

with Regulated 11kV Busbar

0

1,000

2,000

3,000

4,000

5,000

6,000

220

222

224

226

228

230

232

234

236

238

240

242

244

246

248

250

252

254

256

258

260

Voltage - 1 volt bins

Freq

uenc

y of

Occ

urre

nce

30 second readings

253.

0V225.

4V

Nom

inal

230

V


6

11kV Feeder System

Zone Substation Regulated 11kV Busbar


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Typical Network Arrangement

66kV

11kV

66kV

230/400V

On Load Tap Changing Transformer66kV (+7x1.5% - 14x1.5%)/11kV

Off Load Tap Change Transformer

tap "1" 11kV+5% / 433Vtap "2" 11kV+2.5% / 433Vtap "3" 11kV / 433Vtap "4" 11kV-2.5% / 433Vtap "5" 11kV-5% / 433V


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OLTC BehaviourVoltage

11kV +1.5%

11kV

11kV -1.5%

Time

Float voltage

Upper limit

Lower limit

Delay (~30 seconds)

Tap change


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Typical 11kV Feeder11kV Voltage Profile

00.10.20.30.40.50.60.70.80.9

11.1

-5 0 5 10 15

11kV km length

PU V

olta

ge

66kV no load66kV full load11kV no load11kV full load

A B C D E


10


00.10.20.30.40.50.60.70.80.9

11.1

-5 0 5 10 15

11kV km length

PU V

olta

ge


A B C D E


11


0.7

0.8

0.9

1

1.1

-5 0 5 10 15

11kV km length

PU V

olta

ge


A B C DE

Point of 11kV Regulation


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Light Load Conditions

259.5252.8246.5240.5234.812E

260.0253.3247.0241.0235.29D

261.3254.6248.2242.1236.46C

263.3256.5250.1244.0238.23B

267.1260.2253.7247.6241.70A

tap 5tap 4tap 3tap 2tap 1

Light Load Voltage with 11kV±1.5% at regulated 11kV busbar

km from ZS

Location


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Full Load Conditions

237.0231.0225.2219.7214.512E

238.5232.4226.6221.1215.89D

242.2236.0230.1224.5219.16C

248.1241.8235.7230.0224.53B

259.2252.6246.2240.2234.50A

tap 5tap 4tap 3tap 2tap 1

Full Load Voltagekm from ZS

Location


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Line Drop Compensation11kV Voltage Profile - with LDC

0.7

0.8

0.9

1

1.1

-5 0 5 10 15

11kV km length

PU V

olta

ge


A B C D E

Point of 11kV LDC Control

Point of 11kV Regulation


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LDC – Tap Selection11kV Voltage Profile - with LDC

0.7

0.8

0.9

1

1.1

-5 0 5 10 15

11kV km length

PU V

olta

ge

A B C D E

Tap selection governed by

light load curve

Tap selectiongoverned by

full load curve


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Tap Selection with LDC

248.5242.2236.1230.4224.912E full load255.7249.1242.9236.9231.312E light load

259.8253.1246.8240.8235.13B full load257.4250.8244.5238.6232.93B light load

271.0264.1257.5251.2245.20A full load263.2256.4250.0243.9238.10A light loadtap 5tap 4tap 3tap 2tap 1

Voltage - 11kV with LDCkm from ZS

Location


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Voltage Signature 1Tap setting - too much boost

Source impedance - low

0

1,000

2,000

3,000

4,000

5,000

6,00022

022

2

224

226

228

230

232

234

236

238

240

242

244

246

248

250

252

254

256

258

260


Freq

uenc

y of

Occ

urre

nce

30 second readings

253.

0V

225.

4V

Nom

inal

230

V


18

Voltage Signature 2Tap setting - too little boost

Source impedance - low

0

1,000

2,000

3,000

4,000

5,000

6,000

220

222

224

226

228

230

232

234

236

238

240

242

244

246

248

250

252

254

256

258

260


Freq

uenc

y of

Occ

urre

nce

30 second readings

253.

0V225.

4V


19

Voltage Signature 3Tap setting - too little boostSource impedance - high

0

200

400

600

800

1,000

1,200

1,400

1,600

220

222

224

226

228

230

232

234

236

238

240

242

244

246

248

250

252

254

256

258

260


Freq

uenc

y of

Occ

urre

nce

30 second readings

253.

0V225.

4V


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11kV Tapping Zones

Tap 2 zone

Tap 3 zone

Tap 4 zone


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Best Practice Procedures• Electricity distributors need clear voltage control objectives and processes

• OLTC transformer float, bandwidth, time delay method, time delay and LDC settings need to be carefully selected, documented and implemented

• Consider time delay methods - fixed, inverse or integrated:

– tap changer operations and maintenance– restores voltage levels quicker in response to large load changes e.g. due to a feeder

trip– consider fast tap changer response – capacitor switching– grading with upstream OLTC transformer operation (hunting)

• Light load and full load MV distribution profiles (11kV & 22kV) need to be modelled on a regular basis – say every 2 years

– particularly where LDC is used – volts can creep up with load growth


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Best Practice Procedures• Based on VRR settings & voltage profile

– allocate a tap setting for every distribution transformer via tapping zone plans

• Field staff – implement distribution tap setting at every maintenance or other

opportunity– understand practices– information and training sessions needed

• VRR and tap settings– No unauthorised adjustment– Voltage regulating relays need to be carefully set and tested on a regular

basis

• Monitoring of network voltages– essential to ensure proper control is being achieved


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Operational Issues• Alternate switching configurations

– can adequate voltage levels be maintained?– isolate supply if voltage cannot be maintained between predetermined limits – be careful of LDC impacts with load transfers

• OLTC transformer operation– be prepared for possible VRR maloperation– if necessary limit OLTC range to prevent extreme under or over voltage conditions

• Distribution transformers issues– inconsistent taping ratios– incompatible tapping ranges– different tap names– be prepared to scrap incompatible transformers

• Embedded generation and switched field capacitor banks– Requires special consideration on the voltage profiles and voltage control strategies

• Routine monitoring of strong and weak sites


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Conclusions

• There are considerable customer benefits to be gained by improving voltage control in many distribution networks

– extended equipment life– better equipment performance– Compliance with standards

• Achieving voltage control:– is not technically difficult - no rocket science required– clear objectives– requires discipline and good engineering systems– requires both office staff and field staff cooperation and coordination – optimises use of network assets– can defer the need for capital expenditure– is an essential part of good network planning– can be achieved at low cost

techniques for the control of steady state voltage in … · with regulated 11kv busbar 0 1,000...

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