The fundamentals of protection relay co-ordination and time/current grading principles | EEP (2024)

The importance of overcurrent protection

Transmission and distribution systems are exposed to overcurrent flow into their elements. In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment.

Possible causes for overcurrent include short circuits, excessive load,transformer inrush current, motor starting, incorrect design, or a ground fault.

Therefore, fornormal system conditions, some tools such as demand – side management, load shedding, and softmotor starting can be applied to avoid overloads.

In addition, distribution systems are equippedwith protective relays that initiate action to enable switching equipment to respond only toabnormal system conditions. The relay is connected to the circuit to be protected via CTs and VTsaccording to the required protection function.

In order for the relay to operate, it needs to be energized. This energy can be provided by battery sets (mostly) or by the monitored circuit itself.

This article deals with co-ordination between protection relays in general andprinciples of Time/Current gradingused to achieve correct relay co-ordination.

1. Co-ordination procedure
2. Principles of Time/Current grading
   1. Discrimination by Time
   2. Discrimination by Current
   3. Discrimination by both Time and Current

1. Co-ordination procedure

Correct overcurrent relay application requires knowledge of the fault current that can flow in each part of the network. Since large-scale tests are normally impracticable, system analysis must be used.

The data required for a relay setting study are:

1. Single-line diagram of the power system involved,showing the type and rating of the protection devicesand their associated current transformers.
2. The impedances in ohms, per cent or per unit, of allpower transformers, rotating machine and feedercircuits.
3. The maximum and minimum values of short circuitcurrents that are expected to flow through eachprotection device.
4. The maximum load current through protection devices.
5. The starting current requirements of motors and thestarting and locked rotor/stalling times of inductionmotors.
6. The transformer inrush, thermal withstand and damagecharacteristics.
7. Decrement curves showing the rate of decay of the faultcurrent supplied by the generators.
8. Performance curves of the current transformers.

The relay settings are first determined to give the shortestoperating times at maximum fault levels and then checked tosee if operation will also be satisfactory at the minimum faultcurrent expected.

It is always advisable to plot the curves ofrelays and other protection devices, such as fuses, that are to operate in series, on a common scale. It is usually moreconvenient to use a scale corresponding to the currentexpected at the lowest voltage base, or to use the predominantvoltage base.

The alternatives are a common MVA base or aseparate current scale for each system voltage.

The basic rules for correct relay co-ordination can generally bestated as follows:

RULE #1

Whenever possible, use relays with the same operatingcharacteristic in series with each other.

RULE #2

Make sure that the relay farthest from the source hascurrent settings equal to or less than the relays behindit, that is, that the primary current required to operatethe relay in front is always equal to or less than theprimary current required to operate the relay behind it.

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2. Principles of Time/Current grading

Among the various possible methods used to achieve correct relay co-ordination are those using either time or overcurrent, or a combination of both. The common aim of all three methods is to give correct discrimination.

That is to say, each one must isolate only the faulty section of the power system network, leaving the rest of the system undisturbed.

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2.1 Discrimination by Time

In this method, an appropriate time setting is given to each of the relays controlling the circuit breakers in a power system to ensure that the breaker nearest to the fault opens first.

A simple radial distribution system is shown in Figure 1, to illustrate the principle.

Overcurrent protection is provided at B, C, D and E, that is, atthe infeed end of each section of the power system.

Eachprotection unit comprises a definite-time delay overcurrentrelay in which the operation of the current sensitive elementsimply initiates the time delay element. Provided the setting ofthe current element is below the fault current value, thiselement plays no part in the achievement of discrimination.

For this reason, the relay is sometimes described as an‘independent definite-time delay relay’, since its operating timeis for practical purposes independent of the level ofovercurrent.

It is the time delay element, therefore, which provides themeans of discrimination. The relay at B is set at the shortesttime delay possible to allow the fuse to blow for a fault at A onthe secondary side of the transformer. After the time delay hasexpired, the relay output contact closes to trip the circuitbreaker. The relay at C has a time delay setting equal to t₁seconds, and similarly for the relays at D and E.

If a fault occurs at F, the relay at B will operate in t secondsand the subsequent operation of the circuit breaker at B willclear the fault before the relays at C, D and E have time tooperate.

The time interval t₁ between each relay time settingmust be long enough to ensure that the upstream relays donot operate before the circuit breaker at the fault location hastripped and cleared the fault.

The main disadvantage of this method of discrimination is thatthe longest fault clearance time occurs for faults in the sectionclosest to the power source, where the fault level (MVA) ishighest.

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2.2 Discrimination by Current

Discrimination by current relies on the fact that the faultcurrent varies with the position of the fault because of thedifference in impedance values between the source and thefault.

Hence, typically, the relays controlling the various circuitbreakers are set to operate at suitably tapered values of currentsuch that only the relay nearest to the fault trips its breaker.

Figure 2 illustrates the method.

For a fault at F1, the system short-circuit current is given by:

I = 6350 / (Z_S + Z_L1) A

where:

• Z_S = source impedance = 11² / 250 = 0.485Ω
• Z_L1 = cable impedance between C and B = 0.24Ω

Hence,

I = 6350 / 0.725 = 8800 A

So, a relay controlling the circuit breaker at C and set tooperate at a fault current of 8800A would in theory protect thewhole of the cable section between C and B.

Points affecting this method

However, thereare two important practical points that affect this method ofco-ordination:

Point #1 – It is not practical to distinguish between a fault at F₁and a fault at F₂, since the distance between thesepoints may be only a few metres, corresponding to achange in fault current of approximately 0.1%.

Point #2 – In practice, there would be variations in the source fault level, typically from 250MVA to 130MVA.

At this lower fault level the fault current would not exceed 6800A, even for a cable fault close to C. A relay set at 8800A would not protect any part of the cable section concerned.

Discrimination by current is therefore not a practicalproposition for correct grading between the circuit breakers atC and B. However, the problem changes appreciably whenthere is significant impedance between the two circuit breakersconcerned.

Consider the grading required between the circuitbreakers at C and A in Figure 2. Assuming a fault at F₄, theshort-circuit current is given by:

I = 6350 / (Z_S + Z_L1 + Z_L2 + Z_T)

where:

• Z_S = source impedance = 11² / 250 = 0.485Ω
• Z_L1 = cable impedance between C and B = 0.24Ω
• Z_L2 = cable impedance between B and 4MVA transformer =0.04Ω
• Z_T = transformer impedance = 0.07× (11²/4) = 2.12Ω

Hence,

I = 6350 / 2.885 = 2200 A

For this reason, a relay controlling the circuit breaker at B andset to operate at a current of 2200A plus a safety marginwould not operate for a fault at F₄ and would thusdiscriminate with the relay at A.

Assuming a safety margin of20% to allow for relay errors and a further 10% for variations inthe system impedance values, it is reasonable to choose a relaysetting of 1.3 x 2200A, that is, 2860A, for the relay at B.

Now,assuming a fault at F₃, at the end of the 11kV cable feedingthe 4MVA transformer, the short-circuit current is given by:

I = 6350 /(Z_S + Z_L1 + Z_L2)

Thus, assuming a 250MVA source fault level:
I = 6350 / (0.485 + 0.24 + 0.04) = 8300A

Alternatively, assuming a source fault level of 130MVA:
I = 6350 / (0.93 + 0.214 + 0.04) = 5250A

For either value of source level, the relay at B would operate correctly for faults anywhere on the 11kV cable feeding the transformer.

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2.3 Discrimination by both Time and Current

Each of the two methods described above has a fundamentaldisadvantage. In the case of discrimination by time alone, thedisadvantage is due to the fact that the more severe faults are cleared in the longest operating time.

On the other hand,discrimination by current can be applied only where there isappreciable impedance between the two circuit breakersconcerned.

It is because of the limitations imposed by the independent useof either time or current co-ordination that the inverse timeovercurrent relay characteristic has evolved.

With this characteristic, the time of operation is inversely proportional tothe fault current level and the actual characteristic is a functionof both ‘time’ and ‘current’ settings.

Figure 3 shows thecharacteristics of two relays given different current/timesettings.

For a large variation in fault current between the twoends of the feeder, faster operating times can be achieved bythe relays nearest to the source, where the fault level is thehighest.

So, by using both functions the disadvantages of grading by time or current aloneare overcome!

Variations of current/time tripping characteristics of IDMT relays will be discussed in some of the coming technical articles.

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References //

• Network Protection & Automation Guide by Alstom Grid
• The Basics Of Overcurrent Protection – Seminar Paper by Genc Baruti