Performance of Industrial Network improved by Low Resistance Neutral Treatment

1Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Performance of Industrial Network improved by Low Resistance Neutral Treatment

Ralf Heitbreder

Volkswagen AG

Germany

Steffen Schmidt

Siemens AG

Germany

2Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Structure of 6 kV system 110 kV

6 kV switchgear(with circuit breakers)

0.4 kV busbar 6 kV unit substation (with load interrupter switches and HV fuses)

3Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

IC=100A

transient earth fault relay

IC=100A

transient earth fault relay

Initial state (isolated neutral)

- unreliable operation of transient earth fault relays - high overvoltages (transient and power frequent)- high fault current over a long time

virtually every earth fault results in multi-pole fault- high voltage dips in 0.4 kV system in case of multi-pole 6 kV fault- man power needed for fault location and switch off

4Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Steps of Investigation

• Measurements for data acquisition– Zero sequence impedance; Reduction factor of cable shield

– Earthing and touch Voltages

• Calculation of earth fault currents

• Comparison of variants of neutral treatment– Effects on operation; Risk of fault propagation

– Devices for fault detection and protection

– Neutral devices

– Compliance with touch voltage criteria

• Installation of proposed variant of neutral treatment

• Commissioning and earth fault test

5Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Measurements for data acquisitionItestCurrent source

A VItest U

6 kV cableNYFG(b)Y

V

remote stationfeeding station

V

measurement of earthingand touch voltages

A Isheath

A Iearth

measurement of earthingand touch voltages

Results:

• Z0 1,5 /km + j 1,5 /km

• rE = 0,60 ... 0,80

• UE 20 V/kA

• UB 10 V/kA

Results:

• Z0 1,5 /km + j 1,5 /km

• rE = 0,60 ... 0,80

• UE 20 V/kA

• UB 10 V/kA

6Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Improved operation with isolated neutral

+ no interruption of supply + no cost for neutral devices

- high fault current over a long time risk of multi-pole fault - high overvoltages (transient and power frequent) risk of double fault- new capacitive directional earth fault relays necessary- man power needed for fault location and switch off

IC=100A

capacitive earth fault relayIE> = 50 A

IC=100A

capacitive earth fault relayIE> = 50 A

7Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Operation with earth fault compensation

+ no interruption of supply+ small earth fault current low risk of multi-pole fault

- high overvoltages (transient and power frequent) risk of double fault- new wattmetric earth fault relays and window-type transformers

necessary- high costs for neutral earthing transformer and arc suppression coil - man power needed for fault location and switch off

IRest<10A

wattmetric earth fault relaywindow-type current transformer

ICoil = 140 AIRest<10A

wattmetric earth fault relaywindow-type current transformer

ICoil = 140 A

8Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Low impedance earthing 2000 A

+ fast fault clearing low risk of multi-pole fault+ no transient overvoltages low risk of double fault+ existing overcurrent-time relays with phase fault detection can be used

- setting of phase fault detection must changed to -smaller starting currents (to detect earth faults) and -higher delay times (to prevent tripping by inrush currents)

- voltage dips of up to 10 % in 0.4 kV system in case of 6 kV earth faults- unselective or no tripping of HV fuses

Ik1,max=2kAIk1,min=0,77kA

overcurrent-time relay(phase fault detection)t» = 0,3 s

Ik1,max=2kAIk1,min=0,77kA

overcurrent-time relay(phase fault detection)t» = 0,3 s

9Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Low impedance earthing 500 A

+ fast fault clearing and low fault current low risk of multi-pole fault+ no transient overvoltages low risk of double fault+ low voltage dips in 0.4 kV system in case of 6 kV earth faults

- new overcurrent-time relays with earth fault detection necessary

- unselective or no tripping of HV fuses

Ik1,max=500AIk1,min=310A

overcurrent-time relay(earth fault detection)IE> = 200A; tE> = 0,3 s

Ik1,max=500AIk1,min=310A

overcurrent-time relay(earth fault detection)IE> = 200A; tE> = 0,3 s

10Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Comparison of Investment Costs

Isolated neutral

Earth fault compensation

Low impedance earthingIk1 = 2000 A

Low impedance earthingIk1 = 500 A

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

cost for protection devices

costs for neutral device

11Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Equipment used for low impedance earthing

Neutral earthing transformer

Neutral resistor

12Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Results of the short-circuit test

R

6 kV0,4 kV

voltages 6 kV system

voltages 400 V system

earth fault current

13Schmidt DE Session 2 – Block 1 – Paper 17

Barcelona 12-15 May 2003

Operational experience

• low impedance earthing (Ik1 = 500 A) is in operation for more than 2 years

• 2 earth fault were selectively cleared by protection relays

• no unselective tripping in response to earth fault starting

• no sensible voltage dips in 0.4 kV system during 6 kV earth faults