Selection of Energizing Quantities for Sensitive Ground Fault Protection of MV Electric Power Networks

Effective detection and clearing of ground faults in MV networks particularly under high fault resistance is difficult to achieve due to too low values of both voltage and current being used as threshold for fault indication. It must be noted that MV networks employed in mining industry operate with different neutral point arrangement i.e. isolated, compensated and ineffectively grounded via selected resistor as well. Many scientific works were performed if about as detection, modeling and measurement as well as methodology of restoring power supply such failures [1]. Therefore after many years of experience authors have found that one of the possibility to overcome this problem can be application of argument difference detection as a criteria. Thus the argument difference between 1 st and 2 nd


Introduction
Effective detection and clearing of ground faults in MV networks particularly under high fault resistance is difficult to achieve due to too low values of both voltage and current being used as threshold for fault indication.
It must be noted that MV networks employed in mining industry operate with different neutral point arrangement i.e. isolated, compensated and ineffectively grounded via selected resistor as well.
Many scientific works were performed if about as detection, modeling and measurement as well as methodology of restoring power supply such failures [1].Therefore after many years of experience authors have found that one of the possibility to overcome this problem can be application of argument difference detection as a criteria.Thus the argument difference between 1 st and 2 nd harmonic of both zero sequence voltage (U 0 ) and current (I 0L ) for healthy and faulty lines were found to be convenient.However, it requires application of a suitable both voltage and current measuring units.
Paper presents and discusses simulation results of possible variations of selected quantities for different network and fault conditions.The influence of leakage (ground fault) resistance and line loading were taken under consideration.On the basis of the investigated results conclusions on reliable operation of the new developed ground fault protection are formulated.

Principle of operation of the sensitive ground fault protection
A simplified scheme of MV network with sensitive ground fault protection is presented in Fig. 1.It's operation is based on detection of 1 st and 2 nd harmonic arguments in both zero sequence voltage (U 0 ) and current (I 0L ).Since 2 nd harmonics does not exist under normal state of the network operation therefore the injection unit (UW) has been developed and applied what is described in more details in [2,3].
Filtering of zero sequence value of both voltage (arising between neutral point and ground during ground faults) and currents of protected lines is carried out through Rogowski coils based measuring units [4].
Considering high sensitivity of developed protection a Hall sensor based unit was taken into account and tested.
The UW unit injects current harmonics into the protected network at a moment of the ground fault occurrence.The currents and voltages selected harmonics being measured are than extracted using FU H , FI H units.At the next step the phase comparator unit (PC) is delivered with the arguments of filtered harmonics so that it could provide initiation signal based on selected harmonics arguments difference between the protected lines.The harmonics argument differences (obtained from PC) can be further used for signalizing of the decrease in the leakage resistance as well as for tripping faulty line.

Analysis of selected indication quantities
Respective analysis has been performed for considering content of harmonics and possibility of application of their relations when used as efficient energizing quantities for the sensitive ground fault protection for MV networks especially under the increased value of fault resistance (leakage resistance).In Fig. 2 and 3 are presented (as an example) selected secondary current wave forms I" BL1 provided by current transformer PP L1 installed at the beginning of the protected line in phase B (faulty one), and currents from the neutral conductor (I 0z , I" 0z ) respectively.The simulation results obtained for unloaded as well as loaded network (I load =200A ) are respectively presented in Fig. 2 and Fig. 3.There was also found that content of higher harmonics in selected signals for protection depends on the leakage resistance value R p .With the increase of the R p value their contribution becomes more significant (Fig. 8) what is considered as a good for the protection reliability.On the basis of comparison of higher harmonics content (Fig. 9) in the zero sequent current of both faulty and healthy lines it is clear that using above mentioned comparison as the criteria one can also discriminate the faulty feeder successfully.Efficiency of operation of the developed sensitive protection under ground faults can be deduced from Fig. 10 and 11 when compare the argument differences for various R p value.It is seen that employment of mutual differences values results in detection of the grounding up to about 10kΩ successfully.On the basis of theoretical results it was found that in such case the comparison of arguments value for selected electrical quantities (zero sequence voltage and current) when injected even harmonics under the ground faults can be used as a choice.It is recommended to apply the diode with resistor in series (as current limiter) between the transformer neutral point and grounding electrode.

Fig. 1 .
Fig.1.Simplified scheme of MV network with sensitive ground fault protection.UW-even harmonics injection system, FU H ,FI Hsecond voltage and current harmonics filters, FU 0 ,FI 0 -zero sequence voltage and current filters respectively, ZL1,ZL2detection measuring units, PP L1 ,PP 0 -current measuring transformers, I 0Z ,I 0S -contribution due to injection and ground admittances of healthy line into the short circuit current, PC-phase comparator

Fig. 2 .
Fig. 2. Primary and secondary current waveforms in neutral conductor (I 0z , I'' 0z ) and secondary in faulty phase (I'' BL1 ) under one phase metallic ground fault (R p = 0), (R z = 100 grounding resistance of neutral point) for unloaded line

Fig. 3 .
Fig. 3.Primary and secondary current waveforms in neutral conductor (I 0z , I'' 0z ) and secondary in faulty phase (I'' BL1 ) under one phase metallic ground fault (R p = 0), (R z = 100 grounding resistance of neutral point) for loaded line As one can observe the I" BL1 wave gets smoother for the loaded network and harmonics visible in the phase current (without the load) are significantly smaller.It was found that the load inflicts the content of harmonics of the fault current what has been presented in Fig. 4.

Fig. 4 .
Fig. 4. Harmonics content in current I 0z under metallic ground fault (R p = 0), for loaded line The zero sequence voltage is also dependent on the ground fault resistance value as indicated in Fig 5.

Fig. 8 .
Fig. 8.Comparison of secondary current harmonics content in neutral connector under metallic R p = 0 and resistive R p = 1000 ground fault respectively

Fig. 9 .
Fig. 9. Comparison of harmonics content in zero sequence current at output of faulty (L1) and healthy (L2) line filter under metallic R p = 0 ground fault

Fig. 10 .Fig. 11 .
Fig. 10.Argument difference between zero sequence voltage U 0 and zero sequence currents (I 0L2 ) for healthy and (I 0L1 ) faulty line respectively with leakage resistance R p value