100 Percent Stator Earth Fault Protection Siemens

Stator Earth Fault Protection 100 % Protection Range Dr. Hans-Joachim Herrmann Power Transmission and Distribution

Content

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Basics of stator earth fault protection Fault locations Disturbances from the high voltage side  

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100%-stator-earth-fault protection with 20-Hz-injection method Basic idea Practical design Numerical measuring technique Frequency operating range    

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Design of the protection schemes

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Examples of real practical failures

Basics of Stator Earth Fault Protection Fault Location Region of earth faults

Earth fault locations in the stator

U0 Low displacement voltage at earth faults close to the star point

Basics of Stator Earth Fault Protection Disturbances from the High Voltage Side

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An additional load resistor limits the disturbance influence The design of the load resistor determines the protective range (80% or 90 % or 95%)

100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Goal and Basic Idea Goal

Basic idea

max. 200 V

20 Hz RE I

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Protection principle should cover 100 % protection range Different measuring principles

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With injection of a 20-Hzvoltage into the star point a e.m.f. is available in case of an earth fault

100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Practical Design

Two connections are possible

The protection measures the injected 20-Hz-voltage and current

100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Numerical Measuring Technique Replica: i sSEF R E

i SEF

i SEF

C E

= Ground cap stator circu

V SEF

1) Filtering V SEF

= Fault resist

20Hz

R B

C E

R E

2) Calculation of the real part Fourierfilter (20Hz)

3) Impedance replica of fault

V 20 I 20

= V r + jV i

= I r + jI i

R e

  I  20    =  V   20 

V r . I r

+ V i . V 2r + V 2i

4) Calculatio of fault resistor

100%-Stator-Earth-Fault Protection with 20-Hz-Injection Method – Relay Settings (secondary values)

Two Stages Backup overcurrent Supervision Calibration

Primary fault resistances: 1 to 3 kΩ Trip stage 5 to 8 kΩ Alarm stage

100%-Stator-Earth-Fault Protection with 20-HzInjection Method – Frequency Operating Range Stand still

0Hz

Rated frequency

10Hz

40Hz

50Hz

70Hz

100%-Stator earth fault protection with 20 Hz injection

90% Stator earth fault protection with displacement voltage   

Protection uses a frequency tracking for all protection functions 20-Hz-generator is activated before starting the generator Due to disturbances from zero sequence system during starting (e.q. static frequency converter) or braking of the generator an internal blocking avoids an overfunction of the injection method

Design of the Protection Schemes Application used in Three Gorges 100% stator earth fault protection

Option: 90% stator earth fault protection with calculated U0

90% stator earth fault protection

Design of the Protection Schemes Redundancy of 100%-Stator Earth Fault Protection Group1

Group2

90% U0 calculated 90% U0 calculated

UL

UL

7UM62

UE

7UM62

100% SEF-1 IEE1

UE IEE1 Neutral transformer 

100% SEF-2

Commissioning Primary Tests on a Generator (stand still and running)

Phase to ground voltage

Displacement voltage

Ground current

The secondary signals are very small The capacitive current is visible. This disturbance current will be eliminated by the algorithm.

Real Practical Failures External insulation test equipment failed

Real Practical Failures Calculated Secondary Fault Resistance 200

200

150

REm

k 

  m    h    O SES100Aus k    n    i    R

100 86

Drop off

GAnr k 

60

Pickup

50

0

0 100

200

300

400 k .ta t in ms

100

Resistance Trip Pickup

500

600

700 687.5

Real Practical Failures Earth Fault in the Cable of Static Frequency Converter

SFC

Starting of the generator with static frequency converter

G Permanently pickup by 100%-stator earth fault protection

Real Practical Failures Earth Fault in the Cable of Static Frequency Converter Fault Record frequency appr. 1,7 Hz

Input signals of the 100% SEF

Real Practical Failures Earth Fault in the Cable of Static Frequency Converter Calculated Secondary Fault Resistance 100

100

80

REm k  − 10

  m    h    OSES100Aus k    n    i    R GAnr k 

57

60

48

Drop off Pickup

40

20

0

0

0

100

99.92

200

300

400 k⋅ ta t in ms

Resistance Trip Pickup

500

600

700 686.95