Neutral Earthing_Siemens

Neutral Grounding

Sector Energy PTI NC Theodor Connor

Copyright © Siemens AG 2008. All rights reserved.

Topics

Introduction Theoretical background Methods applied for neutral grounding Practical aspects

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

Page 1

01.2008

Th. Connor

PTD SE PTI

Significance of earth faults for network operation

December November October September August July June May

1 year

April

8760 hours

March February January

31 536 000 seconds

Method of neutral earthing has no influence on behaviour in normal operation For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

Page 2

01.2008

Th. Connor

PTD SE PTI

Kaleidoscope of methods available Isolated neutral KNOSPE

Earth fault compensation Solid earthing

NOSPE High-impedance neutral earthing

Arc suppression Low-impedance neutral earthing Floating neutral Effective earthing

Petersen coil

Resonant earthing

Short-time earthing For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

Page 3

01.2008

Th. Connor

PTD SE PTI

Statistical data of German networks Voltage level

10 kV

20 kV

110 kV

380 kV

111 000 km 60 000 km

17 000 km

Line lengths Overhead line

28 000 km

Cable

98 000 km

89 000 km

4 000 km

11

6

3

-

-

-

Amount of faults per 100 km and year

6

Isolated neutral

23 %

Earth fault compensation

60 %

95 %

78 %

8%

2%

22 %

Low-impedance neutral earthing

16 km

100 %

Source: VDEW 1993

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

Page 4

01.2008

Th. Connor

PTD SE PTI

Display methods for system values UL1

UL1

UL1(t) = ÛL1 sinωt t

Mathematical equations

t

Vector display

Time characteristic

UL1

Phase-related display

Positive sequence Negative sequence Zero sequence

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

Page 5

01.2008

Th. Connor

PTD SE PTI

The transient phase of an earth fault

Isolated neutral Discharge Charging oscillation

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

Page 6

01.2008

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Characteristics of transient phase

Frequency Discharge Charging oscillation

Overvoltage factor

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

kt =

3Û 2U

LE max

= 1.2...2.7...3.5

b

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

Page 7

01.2008

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PTD SE PTI

Permanent earth fault

ICE

Isolated neutral

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

Page 8

01.2008

Th. Connor

PTD SE PTI

Vector diagram for system with isolated neutral

UL1

UL1 = 0

UL2 Normal operation

UL2 Earth fault

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

Page 9

01.2008

Th. Connor

PTD SE PTI

Characteristics of power frequent voltage rise

Coefficient of earthing

ULE Max ZE = Ub

Earth-fault factor

cf =

DIN VDE 0111 IEC 71-1

3

ULE Max Ub

1 3

... 1

1 ...

3

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

Page 10

01.2008

Th. Connor

PTD SE PTI

Network calculation 3-phase short-circuit

~

IK =

UN 3⋅ Z

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

Page 11

01.2008

Th. Connor

PTD SE PTI

Network calculation Symmetrical components U1L1

Positive sequence

U1 = 3 (UL1+ a UL2+ a² UL3)

Negative sequence

U2 =

Zero sequence

U0 = 3 (UL1 + UL2 + UL3)

1

U1L2 UL1 U2 L1

U2 L3 UL3

1 (UL1+ a² UL2+ a UL3) 3

UL2

U0 L1

1

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

Page 12

01.2008

Th. Connor

PTD SE PTI

Symmetrical components Example

a2 UT a US UR

U1

a UT

a2 US UR

U2

US UR

U0 UT

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

Page 13

01.2008

Th. Connor

PTD SE PTI

Symmetrical components Example a2 UT

a US UR

U1

U2

U0 For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

Page 14

01.2008

Th. Connor

PTD SE PTI

Symmetrical components Line to ground fault I 1 = I R + I S ⋅ a + I T ⋅ a2 I 2 = I R + I S ⋅ a2 + I T ⋅ a I0 = IR + IS

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

+ IT T IS = 0 S

Positive sequence

~

R UR = 0

Negative sequence Boundary requirements at the fault location IS = IT = 0 UR = 0

Zero sequence

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

Page 15

01.2008

Th. Connor

PTD SE PTI

Fault current calculation System with isolated neutral

Positive sequence ~

Negative sequence

Zero sequence

IF =

3 ⋅ UN Z1 + Z2 + Z0

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

Page 16

01.2008

Th. Connor

PTD SE PTI

Isolated neutral

ICE ICE - capacitive earth fault current

ICE = 3 UN⋅ ω ⋅ C0 Isolated neutral

y y y y

Transient overvoltages kt ≤ 3.0 Voltage rise of the fault-free phases cf ≤ 3 ICE = f (Type of lines, voltage level, size of network) Operation under permanent earth fault possible

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

Page 17

01.2008

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PTD SE PTI

Estimation of capacitive earth fault current Line type

Voltage level

Capacitive earth fault current ICE

Overhead lines 110 kV 20 kV 10 kV

0.36 0.06 0.03

A/km A/km A/km

Cables Oil cable XLPE cable

110 kV 110 kV

13.3 ... 36.1 A/km 7.5 ... 11.4 A/km

Lead-sheath cable XLPE cable

20 kV 20 kV

2.2 ... 5.3 A/km 1.6 ... 4.4 A/km

Lead-sheath cable XLPE cable

10 kV 10 kV

1.2 ... 2.7 A/km 1.2 ... 3.3 A/km

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

Page 18

01.2008

Th. Connor

PTD SE PTI

Intermittent earth fault

-0,5

First earth fault

Recovery

Second earth fault

Ice (t2) = 0 For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

Page 19

01.2008

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PTD SE PTI

Intermittent earth fault

First earth fault Recovery Second earth fault

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

Page 20

01.2008

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PTD SE PTI

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 Trigger:

About 2nd harmonic

Switching events

About 0.5 nominal frequency (subharmonic) For internal use only. / Copyright © Siemens AG 2008. All rights reserved.

Page 21

01.2008

Th. Connor

PTD SE PTI

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

Page 22

01.2008

Th. Connor

PTD SE PTI

Earth fault compensation

Petersen coil

Ires

Ires - Residual current

Earth fault compensation

• Transient overvoltages kt ≤ 2.7 • Voltage rise of the fault-free phases cf ≤ 3 • Ires = f (Detuning, Harmonics, quality factor of the coil ) • Transient earth faults • Operation under permanent earth fault possible • Extinction limit

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

Page 23

01.2008

Th. Connor

PTD SE PTI

Fault current calculation System with earth fault compensation

Positive sequence

~

Negative sequence

Zero sequence

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

Page 24

01.2008

Th. Connor

PTD SE PTI

Estimation of residual current

Line type Overhead lines

Voltage level 110 kV 20 kV 10 kV

Residual current Ires approx. 5 % of ICE approx. 8 % of ICE approx. 8 % of ICE i.e. 8 A per 100 A coil current

Cables

110 kV 20 kV 10 kV

approx. 2 % of ICE approx. 3 % of ICE approx. 3 % of ICE

i.e. 3 A per 100 A coil current

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

Page 25

01.2008

Th. Connor

PTD SE PTI

Reappearing of phase voltage

Exact tuning v = 0 Tuning v = -10

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

Page 26

01.2008

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Solid earthing

Ik1 Ik1 – short-circuit current Solid earthing

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

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

Page 27

01.2008

Th. Connor

PTD SE PTI

Fault current calculation Solid earthing

Positive sequence

~

Negative sequence

Zero sequence

I k1 = 3I0 =

3 ⋅ UN Z1 + Z2 + Z0

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

Page 28

01.2008

Th. Connor

PTD SE PTI

Vector diagram for system with solid earthing

UL1

UL1 = 0

UL2

UL2 Normal operation

1-phase short circuit

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

Page 29

01.2008

Th. Connor

PTD SE PTI

Examples for zero-sequence impedances of lines Line type

Voltage level Zero sequence Zero sequence resistance R'0 reactance X'0

Overhead lines 110 kV 20 kV 10 kV

0.4 ... 0.2 Ω /km 0.6 ... 0.2 Ω /km 0.6 ... 0.2 Ω /km

1.0 ... 1.5 Ω /km 1.5 Ω /km 1.6 Ω /km

110 kV 110 kV

0.8 ... 0.4 Ω /km 0.6 ... 0.5 Ω /km

0.4 ... 0.2 Ω /km 0.3 ... 0.2 Ω /km

Lead-sheath cable XLPE cable

20 kV 20 kV

0.6 ... 0.4 Ω /km 0.6 ... 0.4 Ω /km

0.4 ... 0.3 Ω /km 0.3 ... 0.2 Ω /km

Lead-sheath cable XLPE cable

10 kV 10 kV

2.0 ... 1.1 Ω /km 3.2 ... 1.4 Ω /km

0.5 ... 0.2 Ω /km 0.9 ... 0.3 Ω /km

Cables Oil cable XLPE cable

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

Page 30

01.2008

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PTD SE PTI

Low-impedance neutral earthing

Ik1 Ik1 – 1-phase short-circuit current Low-impedance neutral earthing

y Transient overvoltages for earthing via resistor kt ≤ 1.8 y Transient overvoltages for earthing via reactor kt ≤ 2.7 y Voltage rise of the fault-free phases cf ≤ 3 y Ik1 = f (Neutral impedance, fault location) Ik1 > ICE Fault current indicator

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

Page 65

01.2008

Th. Connor

PTD SE PTI

Comparison of neutral resistor and reactor

UR

UR

US

US

UT

UT

IR

IR t

t

Neutral resistor

Neutral reactor

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

Page 66

01.2008

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Short circuit trial Voltages at medium- and low-voltage level U [kV]

t [ms]

10 kV

U [V]

R

0.4 kV t [ms]

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

Page 67

01.2008

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PTD SE PTI

Low-impedance neutral earthing Design types of neutral resistors

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

Page 68

01.2008

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Low-impedance neutral earthing Connection possibilities for a neutral resistor

110 kV

110 kV

110 kV

10 (20) kV

10 (20) kV

10 (20) kV

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

Page 69

01.2008

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Low-impedance neutral earthing Design types of earthing transformers

Earthing transformer for 6 kV

Earthing transformer with increased zero sequence impedance for 10 kV

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

Page 70

01.2008

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Required protection for low-impedance neutral earthing

y 3phase protection equipment necessary • Overcurrent relay sensitive earth fault detection 0.1 IN • Distance protection Overcurrent pick-up Earth fault for loop selection Impedance pick-up in meshed networks y Transformer differential protection Suppression of neutral current

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

Page 71

01.2008

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Test circuit for measurements

Remote station

Current source

Feeding station

Test voltage Active power Test current

UE V

Sheath current

Earth current

Uind

Z0

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

Page 72

01.2008

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PTD SE PTI

Combined method Short term neutral earthing for detection

IK1

Fault current indicator

Ires/ IK1/ Ires

Short term earthing for detection

tSE < tprotection

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

Page 73

01.2008

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PTD SE PTI

Combined method Reverse short term neutral earthing

IK1

Fault current indicator

IK1/ Ires

Short term earthing for detection

tSE < tprotection

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

Page 74

01.2008

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PTD SE PTI

Combined method Short term neutral earthing for trip

IK1

Fault current indicator

Ires/ IK1

Short term earthing for trip

tSE > tprotection

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

Page 75

01.2008

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PTD SE PTI

Comparison of neutral earthing schemes France - Germany

Cable system France

Overhead line system

Germany

Ik1 max = 1000A

France

Germany

Ik1 max = 300A

Ik1 max = 2000A

Trip

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

Page 76

01.2008

Th. Connor

PTD SE PTI