neutral earthing

Copyright © Siemens AG 2008. All rights reserved.

Sector Energy PTI NCTheodor Connor

Neutral Grounding

01.2008For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

PTD SE PTITh. Connor

Topics

Introduction

Theoretical background

Methods applied for neutral grounding

Practical aspects



Significance of earth faults for network operation

JanuaryFebruary

MarchApril

MayJune

JulyAugust

SeptemberOctober

NovemberDecember

1

8760

31 536 000

year

hours

seconds

Method of neutral earthing has no influence on behaviour in normal operation



Kaleidoscope of methods available

Isolated neutral

Earth fault compensation

High-impedance neutral earthing

Low-impedance neutral earthing

Solid earthing

Effective earthing

Short-time earthing

Floating neutral

Arc suppression

Resonant earthing

NOSPE

KNOSPE

Petersen coil



Statistical data of German networks

Voltage level 10 kV 20 kV 110 kV 380 kV

Line lengths

Overhead line 28 000 km 111 000 km 60 000 km 17 000 km

Cable 98 000 km 89 000 km 4 000 km 16 km

Amount of faults per 100 km and year 6 11 6 3

Isolated neutral 23 % - - -

Earth fault compensation 60 % 95 % 78 % -

Low-impedance neutral earthing 8 % 2 % 22 % 100 %

Source: VDEW 1993



Display methods for system values

Positive sequenceNegative sequence

Zero sequence

UL1(t) = ÛL1 sinωtUL1

t

Vector display

Mathematicalequations

t

UL1

Time characteristic

Symmetrical components

Phase-related display

UL1



The transient phase of an earth fault

Isolated neutral

DischargeCharging oscillation



Frequency Discharge Charging oscillation

500 Hz . . . 100 000 Hz 100 Hz . . . 4 000 Hz

Overvoltage factor kt = b

LE

UÛ2

3 max = 1.2...2.7...3.5

Characteristics of transient phase



Permanent earth fault

Isolated neutral

ICE



Vector diagram for system with isolated neutral

Normal operation

UL1

UL2

Earth fault

UL1 = 0

UL2



Characteristics of power frequent voltage rise

Coefficient of ZE = ... 1earthing

Earth-fault factor cf = 1 ... 3

13

ULE Max

Ub

Ub

ULE Max

3DIN VDE 0111IEC 71-1



Network calculation3-phase short-circuit

~

IKN =

3 ZU

⋅



Network calculationSymmetrical components

UL1

UL2UL3

U1L1

U1L2

U2 L1

U2 L3

U0 L1

Positivesequence U1 = (UL1+ a UL2+ a² UL3)

Negativesequence U2 = (UL1+ a² UL2+ a UL3)

Zerosequence U0 = (UL1 + UL2 + UL3)

13

13

13



Symmetrical componentsExample

U1

a US

a2 UT

UR

U2

U0UR

UR

a2 US

a UT

UT

US



Symmetrical componentsExample

U1

a US

a2 UT

UR

U2

U0



Symmetrical componentsLine to ground fault

T

S

R

UR = 0

IS = 0

21 aIaIII TSR ⋅+⋅+=

aIaIII TSR ⋅+⋅+= 22

TSR IIII ++=0

~

Boundary requirements at the fault location

IS = IT = 0

UR = 0

Positive sequence

Negative sequence

Zero sequence

Boundary requirements at the fault location are fulfilled by series connection of component systems



Fault current calculationSystem with isolated neutral

~Positive sequence

Negative sequence

Zero sequence

IFN

1 2 0

= 3Z Z Z

⋅+ +

U



Isolated neutral

ICEICE - capacitive earth fault current

Transient overvoltages kt ≤ 3.0Voltage rise of the fault-free phases cf ≤ ICE = f (Type of lines, voltage level, size of network)Operation under permanent earth fault possible

3

Isolated neutralICE N 0= 3U C⋅ ⋅ω



Estimation of capacitive earth fault current

Line type Voltage level Capacitive earth fault current ICE

Overhead lines110 kV 0.36 A/km

20 kV 0.06 A/km10 kV 0.03 A/km

Cables

Oil cable 110 kV 13.3 ... 36.1 A/kmXLPE cable 110 kV 7.5 ... 11.4 A/km

Lead-sheath cable 20 kV 2.2 ... 5.3 A/kmXLPE cable 20 kV 1.6 ... 4.4 A/km

Lead-sheath cable 10 kV 1.2 ... 2.7 A/kmXLPE cable 10 kV 1.2 ... 3.3 A/km



Intermittent earth fault

First earth fault Second earth faultRecovery

-0,5

Ice (t2) = 0



Intermittent earth fault

First earth fault

Second earth fault

Recovery



Ferro resonances

Prerequisite:

Circuit with non linear characteristic e.g. saturation on reactance

Principle:

Driving voltage UN

From operational point 1 to point 2

But operational point 2 not stable

Current reversal to operational point 3

Types:

Nominal frequency, 3rd harmonic

About 2nd harmonic

About 0.5 nominal frequency (subharmonic)

Trigger:

Switching events



Ferro resonances

Example in three phase system:

Stable ferro resonance of 2nd harmonic level

No disturbance of phase to phase voltage URS

Possible configurations:

Single phase voltage transformer with bus bar or short cable

Energising reactor via short cable




Petersencoil Ires


• Transient overvoltages kt ≤ 2.7• Voltage rise of the fault-free phases cf ≤ • Ires = f (Detuning, Harmonics, quality factor of the coil )• Transient earth faults• Operation under permanent earth fault possible• Extinction limit

3

Ires - Residual current



Fault current calculationSystem with earth fault compensation

~Positive sequence

Negative sequence

Zero sequence



Estimation of residual current

Line type Voltage level Residual current Ires

Overhead lines 110 kV approx. 5 % of ICE20 kV approx. 8 % of ICE10 kV approx. 8 % of ICE

i.e. 8 A per 100 A coil current

Cables 110 kV approx. 2 % of ICE20 kV approx. 3 % of ICE10 kV approx. 3 % of ICE

i.e. 3 A per 100 A coil current



Reappearing of phase voltage

Exact tuning v = 0

Tuning v = -10



Solid earthing

Transient overvoltages kt < 1.8Voltage rise of the fault-free phases cf ≤ 1.4Ik1 = f (Short-circuit capacity, zero-sequence impedance) Ik1 ~ Ik3

Selective fault clearing

Ik1 – short-circuit currentIk1

Solid earthing



Fault current calculation Solid earthing

~Positive sequence

Negative sequence

Zero sequence

I Ik1 0N

1 2 0

= 3 = 3 UZ Z Z

⋅+ +



Vector diagram for system with solid earthing

UL1

Normal operation 1-phase short circuit

UL1 = 0

UL2UL2



Examples for zero-sequence impedances of lines

Line type Voltage level Zero sequence Zero sequenceresistance R'0 reactance X'0

Overhead lines

110 kV 0.4 ... 0.2 Ω /km 1.0 ... 1.5 Ω /km20 kV 0.6 ... 0.2 Ω /km 1.5 Ω /km10 kV 0.6 ... 0.2 Ω /km 1.6 Ω /km

Cables

Oil cable 110 kV 0.8 ... 0.4 Ω /km 0.4 ... 0.2 Ω /km XLPE cable 110 kV 0.6 ... 0.5 Ω /km 0.3 ... 0.2 Ω /km

Lead-sheath cable 20 kV 0.6 ... 0.4 Ω /km 0.4 ... 0.3 Ω /km XLPE cable 20 kV 0.6 ... 0.4 Ω /km 0.3 ... 0.2 Ω /km

Lead-sheath cable 10 kV 2.0 ... 1.1 Ω /km 0.5 ... 0.2 Ω /km XLPE cable 10 kV 3.2 ... 1.4 Ω /km 0.9 ... 0.3 Ω /km




Transient overvoltages for earthing via resistor kt ≤ 1.8 Transient overvoltages for earthing via reactor kt ≤ 2.7 Voltage rise of the fault-free phases cf ≤Ik1 = f (Neutral impedance, fault location) Ik1 << Ik3

Selective fault clearing

Ik1 – 1-phase short-circuit current

Ik1

3




Fault current calculation Low-impedance neutral earthing

~Positive sequence

Negative sequence

Zero sequence

I Ik1 0N

1 2 0

= 3 = 3 UZ Z Z

⋅+ +




Transient overvoltages kt ≤ 2.7

Voltage rise of the fault-free phases cf ≤

IF = IR ≥ ICE

Selective fault clearing possible

32

CE

2

R + II

IF


IF - Earth fault current



Fault current calculationSystem with high-impedance neutral earthing

~Positive sequence

Negative sequence

Zero sequence



Practical aspects

Methods applied

Criteria for comparison

High voltage systems

Medium voltage systemsIsolated neutralResonant groundingLow impedance neutral grounding



Neutral earthing practice in Germany

380/220 kV Interconnected systems NE IK1 ≤ IK3

110 kV Regional systems

City system (cable) EFC (Bavaria NG)

NE with limitation

IK1 = 5 ... 10 kA 20 kV Regional & urban network

Urban cable network EFC or EFC with SE

NE

IK1 = 1.5 ... 2 kA 10 kV Small network

Large network

Urban cable network

IS

EFC or EFC with SE

NE

ICE = 20 ... 100 A

ICE ≤ 500 A

IK1 = 1 ... 2 kA Industry

5 ... 20 kV Small systems

Large systems IS

EFC ICE = 5 ... 30 A

ICE ≤ 300 A

NE = Low-impedance neutral earthing IS = Isolated neutral SE = Short time earthing EFC = Earth fault compensation



Criteria for comparison of methods

– Voltage stress

– Current stress

Destruction at the fault locationPotential riseInductive interference; EMC

– Operational aspects

Fault detectionFault localisationAmount of switching requiredFail safe situation

– Investment costs

– Development in the future



Earth fault transients and voltage rise

Isolated neutralEarth fault compensation

ICE = 100 A


Ik1 = 1.5 kA Neutral resistor

Fault duration min ... h

Fault duration ms



Fault current and voltage rise during earth fault

0 2 4 6 8 10 ∞0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8 3

Solid

Effective neutralearthing

Low-impedanceneutral earthing

Isolated neutral



Z0 / Z1

U / ULE

IK1 / IK3

Voltages of the fault-free phasesU / ULE

Fault currentIK1 / IK3



Fault current and voltage rise during earth fault

N Neutral earthing with voltage rise below 1.4

S Neutral earthing with voltage rise exceeding 1.4

W Low impedance neutral earthing

K Earth fault compensation

I Isolated neutral



Neutral earthing in high voltage networks

IK1 ≥ IK3

IK1< IK3

IK1< IK3



Required protection for transmission systems

Low-impedance neutral earthing (110 kV ... 380 kV)

Overhead lineDistance protection, Overcurrent protection (Back-up)1-phase auto reclosure

CableDifferential protection, Distance protection ,Overcurrent protection (Back-up)

Earth fault compensation (110 kV)

Overhead lineTransient earth fault relay

Cable-



Operational aspectsSystem with isolated neutral

3I0A

3I0B

3I0C

3I0D

3I0E

ICE



Determination of zero sequence voltage

Open delta winding

L1L2L3

UL1 + UL2 + UL3 = 3 · U0

3

100V3

100V3

UN3

58 V Normal operation100 V Earth fault

100 V Earth fault

U0



Determination of earth fault currentHolmgreen

L1

L2

L3

IL1 + IL2 + IL3= 3 · I0 Relay



Operational aspectsSystem with earth fault compensation

3I0A

3I0B

3I0C

3I0D

3I0E

Ires

Icoil

Icoil



Petersen coil details

Type of coilFixed coil

Tapped coil 1:3

Plunger coil 1:10

Plunger coil with controller

Connection point to network neutral• Feeding transformer

• Neutral busbar

• Earthing transformer

Location of coil in the networkCentral compensation

Distributed coils in the network



Earth fault compensationTypes of Petersen coil

Plunger coils for medium voltage

Fixed coil for 110 kV



Tuning of Petersen coil

Earth fault currentin case of fault

Voltage of neutral pointin normal operation

Overhead line

Cable

under-compensated over-compensated tuning

IE

Uneutral



Double earth fault current distribution

L3L2

L1

L3L2

L1

L3L2

L1

3IEE ≤ IK2 = IK32



Directional earth fault relay

Capacitivedirectional earth fault relay

Wattmetricdirectional earth fault relay

U0

ICE

U0

ICE

Ires

Icoil



Required protection relays for earth fault detection

Signalling relay for zero sequence voltage

Central earth fault detection relay

Directional earth fault relay

Overcurrent relay with directional earth fault relayDistance relay with directional earth fault relay

Differential relay + Back-up overcurrent relay with directional earth fault relay

Transient earth fault relay



Determination of residual currentWindow type current transformer

L1

3 · I0 = IL1 + IL2 + IL3

L2 L3

I Earth

I SheathI Fault



Transient earth fault relay

L1

L2

L3

E

t [ms]



Transient earth fault relayFault indication survey



Increase of residual current

3I0ZA

3I0ZB 3I0ZA

IAdditionalIres

IAdditional

+

3I0ZA



Pulsating residual current

IPulse

IPulseIres +

Icoil

ICE



Detection by 5th harmonic

Ires

ICoil

ICE



Combined methodEarth fault compensation with phase grounding

3I0A

3I0B

3I0C

3I0D

3I0E

Ires

Icoil

Icoil




3I0B

3I0C

3I0D

3I0E

Ires

Icoil

3I0A

Icoil 3I0




L3L2

L1

L3L2

L1

L3L2

L1

Load current

Induced loop current



Combined methodResidual current compensation

3I0A

3I0B

3I0C

3I0D

3I0E

Ires = 0

Icoil

Icoil

Ires

~~



Earth fault trial in 20 kV network

Harmonic analysis of residual current

0.0

0.4

0.8

1.2

1.6

0 1 2 3 4 5 6 7 8 9 10 12 14

1 pu

f/fg

Residual current

3I0 in the feeding cable

0.40 0.45 0.50 0.55 t/sec.

0

150 2

0.40 0.45 0.50 0.55 t/sec.

0

150 2System circuit

3I0 in A

3I0 in A

110/20kV



Earth fault trial in compensated networkVoltages and current at the fault location

20 kV

110 kV

t [ms]

Ires [A]

t [ms]

t [ms]

U [kV]

Ires

Ires [A]



Operational aspectsSystem with low-impedance neutral earthing

3I0 = IK1

Ineutral

IK1

IK1 >> ICE

Fault current indicator



Comparison of neutral resistor and reactor

Neutral resistor Neutral reactor

t

UR

US

UT

IR

t

UR

US

UT

IR



Short circuit trialVoltages at medium- and low-voltage level

R

10 kV

0.4 kV

U [kV]

t [ms]

U [V]

t [ms]



Low-impedance neutral earthingDesign types of neutral resistors



Low-impedance neutral earthingConnection possibilities for a neutral resistor

110 kV

10 (20) kV

110 kV

10 (20) kV

110 kV

10 (20) kV



Low-impedance neutral earthingDesign types of earthing transformers

Earthing transformer for 6 kV

Earthing transformer with increased zero sequence impedance for 10 kV



Required protection for low-impedance neutral earthing

3phase protection equipment necessary

• Overcurrent relaysensitive earth fault detection 0.1 IN

• Distance protectionOvercurrent pick-up

Earth fault for loop selectionImpedance pick-up

in meshed networks

Transformer differential protectionSuppression of neutral current



Test circuit for measurements

Remote station Feeding station Current source

Test voltage

Active powerTestcurrent

Sheathcurrent

Earthcurrent

Z0

UE

V

Uind



Combined methodShort term neutral earthing for detection

IK1

Ires/ IK1/ Ires


Short term earthing for detection tSE < tprotection



Combined methodReverse short term neutral earthing

IK1

IK1/ Ires


Short term earthing for detection tSE < tprotection



Combined methodShort term neutral earthing for trip

IK1

Ires/ IK1


Short term earthing for trip tSE > tprotection



Comparison of neutral earthing schemesFrance - Germany

Cable system Overhead line system

France FranceGermany Germany

Ik1 max = 1000A

Ik1 max = 2000A

Ik1 max = 300A

Trip

neutral earthing

Documents

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