Over-current and Earth Fault Protection *****part 4

Over-current and Earth Fault Protection *****part 4

Discrimination by both time and current
3Discrimination by both time and current
Each of the two methods described so farhas a fundamentaldisadvantage. In the case of discrimination by time alone, the disadvantage is due to the factthat the more severe faults are cleared in the longest operating time.Discrimination by current can only be applied where there is appreciableimpedance between the two circuit breakers concerned.

It is because of the limitations imposedby the independent use of either time or current co-ordination that theinverse time over current relay characteristic has evolved. With this characteristic, the time of operation is inversely proportionalto the fault current leveland the actual characteristic is a function of both 'time' and 'current' settings.
The advantage of this method of relay
Co-ordination may be best illustrated by the system shown in(Fig.23) which is identical tothat shown in (Fig.21) except that typical system parameters have been added.
In order to carry out a system analysis, before a relay co-ordination study of the system shown in (Fig. 23), it is necessary to refer all thesystem impedances to a commonbase and thus, using 10 MVA as the reference base, we have:

4MVA transformer percentage impedance on10MVA base=7X (10/4) =17.5%
11 kV cable between B and A percentage impedance on10 MVA base

= (0.04 X 100 X 10) / 112=0.33%

11 kV cable between C and B percentageimpedance on 10 MVA base

= (0.24 X 100 X10) /112 =1.98%

30 MVA transformer percentage impedance on 10 MVA base

=22.5 X 10 / 30 =7.5 %

132 kV overhead line percentage impedanceon10 MVA base

= (6.2x100x10)/ 1322 =0.36%

132 kV source percentage impedance on 10 MVAbase

= (100 x 10) /3500=0.29%

The graph in (Fig.23) illustrates the use of 'discriminationcurves', which are an important aid to satisfactory protection co-ordination.In this example, a voltage base of 3.3kV has been chosen and the first curve plotted is that of the 200 A fuse,which is assumed to protect the largest outgoing 3.3kV circuit. Once theoperating characteristic of the highest rated 3.3kV fuse has been plotted, thegrading of the over current relays at the various sub-stations of the radial system is carriedout as follows:

Substation B
CT ratio 250/5A Relay over currentcharacteristic assumed to be extremely inverse, as for the type CDG 14 relay. This relay must discriminate with the 200Afuse at fault levels up to:

(10 x 100) /(17.5+0.33+1.98+7.5+0.36+0.29) = 35.7 MVA

That is, 6260 A at 3.3kV or 1880 A at 11kV. The operating characteristics of the CDG 14 relay show that at a plugsetting of 100%, that is, 250 A and 4.76 MVA at 11 kV, and at a time multipliersetting of 0.2, suitable discrimination with the 200 A fuse is achieved.

SubstationC
CT ratio 500/5A Relay over currentcharacteristic assumed to be extremely inverse, as for the type CDG 14 relay. This relay must discriminate with the relayin substation
B at fault levels up to:

(10 X 100) / (1.98 +7.5 +0.36+0.29) = 98.7MVA

That is, 17,280 A at 3.3kV or 5180 A at11 kV. The operating characteristics of the CDG 14 relay show that at a plugsetting of 100%, that is, 500 A and 9.52 MVA at 11 kV, and at a time multipliersetting of 0.7, suitable discrimination with the relay at substation B isachieved.
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SubstationD
CT ratio 150/1A Relay over currentcharacteristic assumed to be extremely inverse, as for the type CDG 14 relay. This relay must discriminate with the relayin substation C at fault levels up to

(10 X 100) / (7.5 + 0.36 +0.29) = 123 MVA

That is, 21,500 A at 3.3kV or 538 A at132 kV. The operating characteristics of the CDG 14 relay show that at a plugsetting of 100%, that is, 150 A and 34.2 MVA at 132 kV and at a time multipliersetting of 0.25, suitable discrimination with the relay at substation C isachieved.

SubstationE
CT ratio 500/1 A Relay over current characteristicassumed to be extremely inverse,as for the type CDG 14 relay. This relay must discriminate with the relay
in substation D at fault levels up to:

(10 x 100) / (0.36+0.29) = 1540 MVA

That is, 270,000 A at 3.3kV or 6750 A at 132 kV. The operating characteristics of the CDG 14relay show that at a plug setting of 100%, that is, 500 A and 114 MVA at 132kV, and at a time multiplier setting of 0.9, suitable discrimination with therelay at sub-station D is achieved.
A comparison between the relay operatingtimes shown in (Fig.21) and the times obtained from the discrimination curves of (Fig. 23) at the maximum fault level revealssignificant differences. These differences can be summarized as follows:

Relay	Fault level (MVA)	Time from Fig.12 (seconds)	Time from Fig.14 (seconds)
B	98.7	0.25	0.07
C	123	0.65	0.33
D	1540	1.05	0.07
E	3500	1.45	0.25

These figures show that for faults closeto the relaying points the inverse time characteristic can achieve appreciablereductions in fault clearance times.
Even for faults at the remote ends ofthe protected sections, reductions in fault clearance times are still obtained,as shown by the following table:

Relay	Fault level (MVA)	Time from Fig.14 (seconds)
B	35.7	0.17
C	98.7	0.42
D	123	0.86
E	1540	0.39

To finalize the co-ordination study itis instructive to assess theaverage operating time for each extremely inverse over current relay at its maximum and minimumfault levels, and to compare these with the operating time shown in (Fig.21)for the definite time over current relay.

Relay	Fault level (Max./Min MVA)	Time from Fig.14 (seconds) (Max./ Min)	Average time (seconds)
B	98.7/35.7	0.07/0.17	0.12
C	123/98.7	0.33/0.42	0.375
D	1540/123	0.07/0.86	0.465
E	3500/1540	0.25/0.39	0.32

This comparison clearly shows that whenthere is a large variation in fault level all along the system network theoverall performance of the inverse time over current relay is far superior tothat of the definite over current relay.

4 GRADING MARGIN
The time interval between the operationsof two adjacent relays depends upon a number of factors:
1. The fault current interrupting time of the circuitbreaker.
2. The overshoot time of the relay.
3. Errors.
4. Final margin on completion of operation.

A. Circuitbreaker interrupting time
The circuit breaker interrupting thefault must have completely interrupted the current before the discriminatingrelay ceases to be energized.

B. Overshoot
When the relay is de-energized,operation may continue for a little longer until any stored energy has beendissipated. For example, an induction disc relay will have stored kineticenergy in the motion of the disc; static relay circuits may have energy storedin capacitors. Relay design is directed to minimizing and absorbing theseenergies, but some allowance is usually necessary.
The overshoot time is not the actualtime during which some forward operation takes place, but the time which wouldhave been required by the relay if still energized to achieve the same amountof operational advance.

C. Errors
All measuring devices such as relays andcurrent transformers are subject to some degree of error. The operating timecharacteristic of either or both relays involved in the grading may have apositive or negative error, as may the current transformers, which can havephase and ratio errors due to the exciting current required to magnetize their core. This does not, however, apply to independentdefinite time delay over current relays.
Relay grading and setting is carried outassuming the accuracy of thecalibration curves published by manufacturers, but since some error is to be expected, some tolerance must be allowed.

D. Finalmargin
After the above allowances have beenmade, the discriminating relay must just fail to complete its operation. Someextra allowance, or safety margin, is required to ensure that a satisfactorycontact gap (or equivalent) remains.

E. Recommendedtime
The total amount to be allowed to coverthe above items depends on the operating speed of the circuit breakers and the relay performance. Atone time 0.5s was a normal grading margin. With fastermodern circuit breakers and lower relay overshoot times 0.4s is reasonable,while under the best possible conditions 0.35s may be feasible.
In some instances, however, rather thanusing a fixed grading margin, it is better to adopt a fixed time value, toallow for the operating time of the circuit breaker and relay overshoot, and toadd to it a variable time value that takes into account the relay errors, theCT errors and the safety margin.

A value of 0.25s is chosen for the fixedtime value, made up of 0.1 s for the fault current interrupting time of thecircuit breaker, 0.05s for the relay over-shoot time and 0.1 s for the safetymargin. Considering next the variable time values required, it is first assumed that each inverse timeover current relay complies withError Class E7^.5 defined as normal British practice in BS 142:1966.

The normal limits of error for an E7.5relay are ±7.5% but allowance should also be made for the effects oftemperature, frequency, and departure from reference setting. A practical approximation is to assume atotal effective error of 2 x7.5, that is, 15%, this to apply to the relay nearest to the fault, which shallbe considered to be slow.
To this total effective error for therelay a further 10%should be added for the overall current transformer error. Hence, for the time interval t'required between inverse time over current relays it is proposed to adopt theequation:

t' = 0.25t + 0.25 seconds

Where t = nominal operating time ofrelay nearer to the fault.

As far as the independent definite timedelay over-current relaysare concerned, it is assumed that these comply with Error Class El 0, defined as normal Britishpractice in BS 142:1966. The normal limits of error for an El 0 relay are ±10%, but allowance should also be made for the effects of temperature, voltage,frequency and departure from reference setting. A practical approximation is to assume a total effective error of 2 x 10, that is,20%, this to apply to the relay, nearest to the fault, which shall beconsidered to be slow. However, unlike the inverse time over current relay, itis not necessary to add a further error for the current transformers. Hence,for the time interval t' required between independent definite time delay overcurrent relays, it is proposed to adopt the equation:
t' = 0.2t + 0.25 seconds
Where t = nominal operating time ofrelay nearest to the fault.

v STANDARD I.D.M.T. OVER CURRENT RELAY (TYPE CDG 11)
Limits of accuracy have been consideredby various national committees and (Fig.24) shows a typical example of thelimits set by the British Standards Institution specification BS 142:1966 for the standard inverse definite minimum time overcurrent relay.
The discriminating curves shown in(Fig.25) illustrate the application of such a relay to a sectioned radialfeeder; it will be seen that with the assumed relay settings and the tolerancesallowed in BS 142:1966 the permissible grading margin between the over current relays at each section breaker is approximately 0.5s. With the increase in system fault current it isdesirable to shorten the clearance timefor faults near the power source, in order to minimize damage. It is thereforenecessary to reduce the time errors, which are in this situationdisproportionately large when compared with the clearance time of moderncircuit breakers; this can only be achievedby improving the limits of accuracy,pick-up and overshoot
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NOTE: Theallowance error in operating time should not be less than 100ms
All this must be obtained without detriment tothe general performance of the relay; in other words, there must be noreduction in the operating torque or weakening of the damper magnets or contactpressures, and the construction must remain simple with the minimum number ofmoving parts. While these requirements present considerable difficulties inmanufacture, owing to variations in materials and practical tolerances, theprogress made in the GEC Measurements relays has made it possible todiscriminate more closely by reducing the margin between both the current andthe time setting of the relays on adjacent breakers.
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These relays will thus enable thetime setting of the relay nearest the power source to be reduced, or,alternatively, make it possible to increase the number
of breakers in series without increasing the time setting of the relays
at the power source.

» Over-current and Earth Fault Protection *****part 1
» Over-current and Earth Fault Protection *****part 2
» Over-current and Earth Fault Protection *****part 3
» Basic of protection system *****part 2
» Basic of protection system *****part 3