O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 1 What is an intermittent earth fault (IE/F)? Insulation Conductor Screen Insulation break-down

1O. MÄKINEN FI Session 3 Block 3

Barcelona 12-15 May 2003

What is an intermittent earth fault (IE/F)?

InsulationConductor

Screen

Insulationbreak-down

• Characterised as series of cable insulation break-downs

• Originates from insulation deterioration

• Extinguishes itself when fault current crosses zero

In spite of complicated deterioration processes the resulting fault pattern is usually very same alike !



Residual current I0 and voltage U0

FEEDER FEEDER MEAS.

INCOMER

COMP. COIL

I0j I0v U0

FaultPoint

KRe

RfUtres

ICtot

50 100 150 200 250 300 350-0.4

-0.3

-0.2

-0.1

0

0.1

50 100 150 200 250 300 350-0.4

-0.3

-0.2

-0.1

0

0.1

Res

idu

al C

urr

ent

(kA

)R

esid

ual

Vo

ltag

e x

102

(kV

)

I0j

(Faulty Feeder)

I0v

(Healthy Feeder)

U0Pulse width400 – 800 s

Pulse interval5-300 ms

Peak value~0.1 ... 5 kA



Residual current and faulty phase voltage UPR

FEEDER FEEDER MEAS.

INCOMER

COMP. COIL

I0j I0v

FaultPoint

KRe

RfUtres

50 100 150 200 250 300 350-0.4

-0.3

-0.2

-0.1

0

0.1

Res

idu

al C

urr

ent

(kA

)R

eco

very

Vo

ltag

e x

102

(kV

)

I0j

(Faulty Feeder)

I0v

(Healthy Feeder)

Utres

50 100 150 200 250 300 350-0.4

-0.3

-0.2

-0.1

0

0.1

Utres UPR

Varying breakdownvoltage Utres at fault point

Resistor Re and K determines the increaseof recovery voltage UPR

UPR



Selectivity problems between feeder DE/F relay and station RO/V relay

FEEDER FEEDER FEEDER

COMP. COIL

INCOMER

I >0 I >0 I >0 U >0

MEAS.

DE/F RO/V

1

1. Intermittent E/F occurs• No detection (or delayed operation) by the DE/F relay• RO/V relay starts normally

trip

2. RO/V relay trips incoming CB according to set operatingtime-> unselective operation resulting to unnecessary outage

2



Why conventional relaying does not work properly during IE/F fault ?

• IE/F detection in numerical relays depends mostly on:

0.015 0.02 0.025 0.03-0.1

0

0.1

0.2

0.3

0.4

0.5

Actual waveform

Filtered & sampled waveform

• Input filtering & sampling frequency

U0

I0 j

Start delay0

0Start signal

• Length of the starting delay



Why conventional relaying does not work properly during IE/F fault ?

• IE/F detection in numerical relays depends mostly on:

Problems to perform satisfactorily most likely to be faced !!

• Shape of the operating sector

U0

I0J

Operating sector,type 1

Operating sector,type 2

U0

I0J

I0V

• Behaviour of I0 and U0 in respect to the operating sector

Area for faulty feeder I0

Area for healthyfeeder I0



New Detection Methods for IE/F (1)

U0

I0 j

I0v

spike detection_v

spike detection_j

max_count_j

max_count_vdrop-off time_j

operate time delay_j

start signal_j

0

0

0

0

counter_pos

counter_neg

• Spike Detection Method

• Based on detection of I0-spikes of appropriate polarity

• Counter functions to minimise the risk of false operation

• Settable drop-off timer to prevent resetting between fault pulses



• Operating criteria: - phase angle between U0

and I0 between certain limits - amplitudes of I0 and U0

above set values

• Settable drop-off timer to prevent resetting between fault pulses

-120

60120

0-60

drop-off time_j

operate time delay_j

start signal_j (internal)

0

0

0

j

v

extended operating sector

start signal_j 0

New Detection Methods for IE/F (2)

• Phase Angle Criterion



IE/F field testing procedure in general

U >0

trip trip trip

FAULTYFEEDER

COMP. COIL

INCOMER

trip

I >0 I >0

R1

I >0 I >0

R2

I >0 I >0

R1

I >0 I >0

R2

I >0 I >0

R1

I >0 I >0

R2

Analog and digital signals

Oscilloscope

HEALTHYFEEDER . . .

HEALTHYFEEDER

• Different relay types in use in each feeder (R1 and R2)

• Fault point was arranged by drilling a hole through the cable insulation

• The hole was filled with water

• Energisation of the faulty feeder initiates the intermittent fault

• Relevant analog and digital signal were recorded

10 kV



Network parameters varied during IE/F tests

• Capacitive earth fault current 60 – 140 A

• Degree of compensation 0.5 – 1.3

• Current rating of the earthing resistor in parallel of the coil 2, 10 or 20 A

• Fault distance from the station 20 – 400 m

• The variation of the network parameters on the relay response

and on the fault characteristics were investigated



Results from IE/F field testing

0 0.2 0.4 0.6 0.8

R2 start R2 trip R1 start R1 trip CB

Time [msec]

0 0.2 0.4 0.6 0.8-150-100-50050100150

U0 (t

) [%

]

0 0.2 0.4 0.6 0.8-3-2-10 123

I 0j (t

)[k

A]

0 0.2 0.4 0.6 0.8-0.6-0.4-0.20

0.20.40.6

I 0v(t

)[k

A]

• In the faulty feeder both detection methods gave out the trip signals correctly

• A few false starts occurred in the healthy feeder

• False starts occurred when the network was heavily under or overcompensated and the feeder in question had a high capacitive current contribution

• Detection algorithms still need some improvements to prevent false starts



IE/F Conclusions

• Intermittent earth faults have been investigated by theoretical studies, simulations and field tests

• Field test results have been used to verify simulation models and actual relay response, and results seem to be very promising although some improvements are still needed

• Simulation models to be used more in further algorithm development work

• Conventional feeder DE/F relays have difficulties to detect this fault type• Dedicated protection functionality is needed

• New detection methods have been developed and implemented in new numerical feeder terminals

Documents

O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 1 What is an intermittent earth fault (IE/F)? Insulation Conductor Screen Insulation break-down