1. 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 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 !

2. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 2 Residual current I 0 and voltage U 0 FEEDER MEAS. INCOMER COMP. COIL I 0j I 0v U0 U0 Fault Point K ReRe RfRf U tres I Ctot Residual Current (kA) Residual Voltage x 10 2 (kV) I 0j (Faulty Feeder) I 0v (Healthy Feeder) U0 U0 Pulse width 400 – 800  s Pulse interval 5-300 ms Peak value ~0.1... 5 kA

3. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 3 Residual current and faulty phase voltage U PR FEEDER MEAS. INCOMER COMP. COIL I 0j I 0v Fault Point K ReRe RfRf U tres Residual Current (kA) Recovery Voltage x 10 2 (kV) I 0j (Faulty Feeder) I 0v (Healthy Feeder) U tres U PR Varying breakdown voltage U tres at fault point Resistor R e and K determines the increase of recovery voltage U PR U PR

4. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 4 Selectivity problems between feeder DE/F relay and station RO/V relay FEEDER COMP. COIL INCOMER I > 0 0 0 U > 0 MEAS. DE/FRO/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 operating time -> unselective operation resulting to unnecessary outage 2

5. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 5 Why conventional relaying does not work properly during IE/F fault ? IE/F detection in numerical relays depends mostly on: Actual waveform Filtered & sampled waveform Input filtering & sampling frequency U0U0 I 0 j Start delay 0 0 Start signal Length of the starting delay

6. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 6 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 U0U0 I0JI0J Operating sector, type 1 Operating sector, type 2 U0U0 I0JI0J I0VI0V Behaviour of I 0 and U 0 in respect to the operating sector Area for faulty feeder I 0 Area for healthy feeder I 0

7. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 7 New Detection Methods for IE/F (1) U0U0 I 0 j I 0v spike detection_v spike detection_j max_count_j max_count_v drop-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 I 0 -spikes of appropriate polarity Counter functions to minimise the risk of false operation Settable drop-off timer to prevent resetting between fault pulses

8. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 8 Operating criteria: - phase angle between U 0 and I 0 between certain limits - amplitudes of I 0 and U 0 above set values Settable drop-off timer to prevent resetting between fault pulses -120 60 120 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

9. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 9 IE/F field testing procedure in general U > 0 trip FAULTY FEEDER COMP. COIL INCOMER trip I > 0 0 R1 I > 0 0 R2 I > 0 0 R1 I > 0 0 R2 I > 0 0 R1 I > 0 0 R2 Analog and digital signals Oscilloscope HEALTHY FEEDER... HEALTHY FEEDER 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

10. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 10 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

11. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 11 Results from IE/F field testing 00.20.40.60.8 R2 start R2 trip R1 start R1 trip CB Time [msec] 00.20.40.60.8 -150 -100 -50 0 50 100 150 U 0 (t) [%] 00.20.40.60.8 -3 -2 0 1 2 3 I 0j (t)[kA] 00.20.40.60.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 I 0v (t)[kA ] 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

12. O. MÄKINEN FI Session 3 Block 3 Barcelona 12-15 May 2003 12 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