CONVENTIONAL DIRECTIONAL OVERCURRENT & EARTH FAULT PROTECTION

- NOT ALWAYS STRAIGHTFORWARD

P J Hindle

PB Power Ltd, Newcastle, UK

BACKGROUND

Many important aspects concerning the application and limitations of directional overcurrent protection have been addressed in early protection papers and in many textbooks; one of the most reknowned being Sonnemann (1). Even so, apparently undocumented limitations have still been encountered. This class of protection is often applied as back up and so some performance limitations may lie hidden until a case of main protection failure exposes incorrect back-up relay responses.

Based on actual incidents and analysis of actual applications, this paper highlights three forms of protection limitation that do not appear to have received previous attention. In two cases, unwarranted utility supply interruptions occurred. In the third case, a common sub-transmission application, directional earth fault protection was seen to be ineffective for its intended purpose. In all cases, protection setting guidelines or possible relaying solutions are proposed, which would be attainable with modern numerical relay technology.

DIRECTIONAL EARTH FAULT PROTECTION

Incorrect Behaviour for Cross-Country Faults

Figure 1, depicts the circumstances for the unexpected behaviour of directional earth fault relays of a 33kV, resistance-earthed, ring system. This was in response to simultaneous, permanent, earth faults, on different phases and at two different locations.

Figure 1 Cross-Country Earth Fault Scenario

The relays employed conventional directional principles, based on the measured zero sequence

K G Chapman

NEDL, Newcastle, UK

voltage and current. In accordance with normal practice, the directional element characteristic angle was approximately zero degrees, in order to match the approximate angle of the resistor- dominated zero sequence source impedance. Relays 2 and 5 were set with mi,nimal delay since they are both at the bottom ends of the required time-grading sequences.

For relays 2 and 3 it is not easy to determine their expected behaviour, since earth faults had occurred in either direction out of Substation-X. The loss of supply to this substation was unavoidable. However, the trip issued by relay 5 was unexpected and it resulted in the unwarranted loss of supply to Substation-Y. Operators subsequently reclosed the relay 5 breaker, to restore supply to S/S-Y. Relay 4 then tripped for the 6-E fault.

To analyse and understand the reasons for the unexpected relay behaviour, it is helpful to consider the sequence component network diagram given in Figure 2. For simplicity, it was assumed that all line sections were of similar impedance, that the B-E fault was located at a fixed mid-position between SE-X and SE-Y. The A-E fault was located at n per-unit variable distance between SIS-S and SIS-X.

r

L

Figure 2 Sequence Component Diagram

Zero sequence directional relay elements compare the angle of zero sequence current in the relaying

Developments in Power System Protection, Conference Publication No.479 0 IEE 2001. 543

direction with the inverted zero sequence voltage, after it has been phase-shifted by the'relay characteristic angle setting.

Although the source relays 1 and 6 were not directional, since they were not required to be, it can be deduced from Figure 2 that the phase relationship between the signals for their zero sequence directional control would be heavily dominated by the source earthing resistance angle. Conversely, the phase relationship between the signals for relays 2, 3, 4 & 5 would be dominated by a complex combination of the positive and negative sequence network impedances, which are mainly inductive. Herein lies a problem for zero sequence directional relays; the phase relationship between the zero sequence signals for the cross-country earth fault does not correspond to relay characteristic angle setting.

Rddr . Anpbs ol Compamtor Slpn.1. Cr h r o S.qu.nu DEF M a y .I.m.nts . v o ~ ~ ~ p h . w . h m - o m . m .

s o

0 0 % 0 2 0 3 0 4 0 5 0 8 0 7 Od 0 9 1 P- Unn Losation ol APbs. Faun (n)

Figure 3 Relative Angles of Zero Sequence Relay signals

In Figure 3, the relative angles between the zero sequence directional comparator signals have been plotted against the per-unit location of the A-phase fault for the system summarised in Figure 2. Zero resistance faults have been considered. The criterion for directional element operation is a relative angle of less than

shown. It may not be feasible, however, to achieve the required low earth fault relay setting. The protection setting must offer stability with residual magnetising inrush currents during feeder energisation.

SIS-s SIS-x SIS-Y

\ 132kV133kV

Figure 5 Overcurrent and Earth Fault Relay Currents for Earth Fault with Multiple Earthing

Fault study analysis of actual DEF applications of this nature, has revealed that the phase overcurrent protection offers better back-up protection sensitivity to remote earth faults than the DEF protection. Thus the provision of the DEF protection was not cost effective in those applications.

The table included in Figure 5, highlights the fact that negative sequence overcurrent protection would be 5 times more sensitive to earth faults than zero sequence overcurrent with a similar pick-up current setting. Thus a negative sequence relay appears to offer some advantage. One disadvantage would be the possible relay response to unbalanced LV system faults. This could be prevented, if the directional control element for negative sequence overcurrent protection is based on zero sequence signals and an appropriate zero sequence voltage threshold setting is applied.

DIRECTIONAL OVERCURRENT PROTECTION

A common application of directional phase overcurrent relays is at the receiving ends of parallel feeders, as illustrated in Figure 6. The directional protection offers discriminative main or back-up protection for the faulted feeder. To attain the necessary co-ordination, the directional relay current pick-up setting should be less than the non-directional relay settings. Since the directional relays are set to look away from the load, they are typically set below the feeder load current.

Well-known texts do not acknowledge that setting directional overcurrent protection too far below

maximum feeder load current can compromise protection security during power system source faults. One text (2) only mentions that relay thermal withstand limitations must be taken into account, since such limitations did exist with electromechanical, induction-disc relays. Reference is also made to a practice of setting "standard inverse", dependent-time relays down to 50% of normal load current for plain parallel feeders - and with a very low time multiplier setting. A lack of awareness of the load current issue is confirmed through reference to one North American text book (3), which positively implies that there is no load current setting constraint.

Figure 6 Parallel Feeder Overcurrent Protection

The behaviour of directional phase overcurrent protection with unbalanced fault currents was well covered many years ago (I), but a proper consideration of unbalanced load currents does not appear to exist. Modern numerical relays are not subject to the electromechanical relay thermal constraints which previously prohibited the use of very low settings in relation to load current. The analysis of one 66kV power system failure, due to unwarranted simultaneous operation of parallel feeder directional protection (4), highlighted the fact that the relay settings were too low in relation to load current. This was coupled with the fact that an independent operating time delay had been set to less than source fault clearance time.

Figure 7 illustrates the distribution of receiving end load current for one circuit of a parallel feeder in the presence of a 8-C fault in the source network. Figure 8 illustrates the relationship between receiving end voltage and current vectors.

Usend) ~ +I (receive) A ..

& Figure 7 Distribution of Plain Feeder Load Current for a Source Phase-Phase Fault

From Figure 9, which considers quadrature polarised directional control elements, it can be seen that the C-phase element will be free to

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operate with load current during the 8-C phase source fault.

V V,

T T

Figure 8 Plain Feeder Receiving End Vectors

RESTRAIN OPERATE F Figure 9 Directional Responses Plain Feeder

With the aid of Figures 10 to 12, the foregoing analysis is repeated for a Dynl 1 transformer feeder. It can be seen that there would be oDeration of the C-Phase directional element.

w L JDOCJ Figure 10 Distribution of Transformer Feeder Load Current for a Source Phase-Phase Fault

V,=0.866 p.u. 3

/

I, = 0.8664 J Figure 11 Transformer Feeder Receiving End Vectors

With reference to Figure 8 and Figure 11, the folldwing pick-up current setting rules are applicable for the security of receiving-end directional protection of parallel feeders, when the relay operating times may be less than source fault clearance times:

Plain Feeders: > 50% Prefault Load Transformer Feeders: > 87% Prefault Load

One relay manufacturer specifically offers two-out- of-three phase tripping logic to eliminate such constraints

~vpoL.,v~b.L xva.b OPERATE OPERATE

. I, 4 RESTRAIN RESTRAIN

Figure 12 Directional Responses Transformer Feeder

CONCLUSIONS

It cannot be assumed that directional relays will maintain directional discrimination for all power system fault conditions. Some well-known imitations were documented many years ago. Additional reliability weaknesses and limitations of directional protection have been highlighted in this paper for cases of cross-country earth faults on resistance earthed ring systems, for earth faults on multiple earthed power systems and in relation to load current of parallel feeders. These apparently undocumented problems were highlighted following unwarranted power supply failures or during fault analysis.

It has been suggested that the highlighted problems could be overcome through the deployment of alternative relaying techniques or through special setting considerations. The suggested use of negative sequence relaying techniques is particularly compatible with the capabilities of modern numerical relay platforms and algorithms and they would be worthy of consideration by relay manufacturers.

REFERENCES

1. Sonnemann W K, A Study of Directional Element Connections for Phase Relays,

2. GEC Measurements, Protective Relay Applications Guide, 3rd edition, 1987, section 9.19. 3. IEEE, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, first edition, 1986, section 5.5.2.1.

4. Hindle P J, Wright J W, Lloyd G, Directional Overcurrent Protection - New Technology Reveals New Load Limits - Southern African Protection Conference, Johannesburq - October 1998.

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ISBN: 0-471-85687-8

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