Differential Relay

Differential Relay

• Differential protection is based on the fact that any fault within an electrical equipment would cause the current entering it, to be different, from that leaving it.

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• We can compare the two currents either in magnitude or in phase or both and issue a trip output if the difference exceeds a predetermined set value.

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• This method of detecting faults is very attractive when both ends of the apparatus are physically located near each other.
• A typical situation, where this is true, is in the case of a transformer, a generator or a busbar.
• In the case of transmission lines, the ends are too far apart for conventional differential relaying to be directly applied.

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Differential Relay

• Applications:- Transformer (1 MVA), Generator or a busbar.
• In the case of transmission lines, the ends are too far apart for conventional differential relaying to be directly applied.

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Differential Relay

• The Vars on either side are equal and thus if CTs on either side is proplerly choosen then the difference in current is almost zero and kit will not operate for external faults.
• The CTs on either side are connected to form circulating current system.

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Simple Differential Protection or Circulating current differential protection.

Simple Differential relay under normal conditions

The currents transformed by the two CTs, being equal in magnitude as well as in phase, just circulate on the secondary side.

There is no tendency for the current to spill into the over-current relay. The over-current relay connected in the spill path is wired to trip the two circuit breakers on either side of the equipment being protected.

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Simple Differential relay during external fault or Through faults

Simple Differential relay during Internal fault

If, min = (CT ratio) (Plug setting of the OC relay) = nI ps

Simple Differential relay Double end fed system

Zone of protection

Problems in Circulating current differential protection.

• Polarity of CTs
• Protected equipment in switchyard and relay in control room
• Control wiring required to be done (pilot wires).
• Current I1 and I2 as circulating pilot wire.
• Voltage drop from left of R and right of R should be equal for zero spill current

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Problems in Circulating current differential protection.

• In practice, the characteristics of two CTs never coincide even if they are purchased from same supplier. This may cause spill current through relay even when there is no internal fault. If the spill current exceeds the pickup current then undesirable operation occurs.
• It is large for heavy external fault

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Actual behaviour of simple differential scheme

CTs are subject to ratio and phase angle errors. Both these errors depend upon the burden on the CTs, which in turn depends on the lead lengths and the impedance of the relay coil. CT saturation and Unequal lead length

Through fault stability and stability ratio

As the ‘through fault’ current goes on increasing, various imperfections of the CTs get magnified. This causes the spill current to build up.

When relay trips it is said to have lost stability

Through fault stability and stability ratio

The minimum internal fault current required for the scheme to operate, correctly in this case, is decided by pick-up value of the over-current relay in the spill path. To signify the spread between the minimum internal fault current at which the scheme operates and the maximum ‘through fault’ current beyond which the scheme (mal)operates, we define a term called stability ratio.

The higher the stability ratio, the better is the ability of the system to discriminate between external and internal faults. The stability ratio can be improved by improving the match between the two CTs.

Equivalent Circuit of CT

Differential scheme considering CT equivalent circuit

Since the magnetizing currents of the two CTs will generally vary widely, there is a substantial spill current during ‘through fault’ conditions.

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This results into loss of stability and maloperation of the simple differential scheme.

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Thus, the simple differential scheme, which looks attractively simple, cannot be used in practice without further modifications.

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Why not simple Differential scheme?

Transformers:- Close match cannot be obtained

Currents on the two sides of the transformer are, in general, different, the ratios of transformation of the CTs are also different.

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Their designs are therefore different, making it impossible to get a close match between their characteristics. This explains why the spill current goes on increasing as the ‘through fault’ current increases.

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Busbars:- Subjected to very high fault current on secondary will magnify the difference between CTs.

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Percentage Differential Relay

Percentage Differential Relay

Spill current must be greater than a definite percentage of the ‘through fault’ current for the relay to operate. Hence, the name percentage differential relay.

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The slope of the relay is customarily expressed as a percentage. Thus, a slope of 0.4 is expressed as 40% slope.

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Percentage Differential Relay

The percentage differential relay does not have a fixed pick-up value. The relay automatically adapts its pick-up value to the ‘through fault’ current. As the ‘through fault’ current goes on increasing, we are in effect asking the relay to take it easy, by introducing a restraining torque proportional to the circulating current.

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‘Through fault’ stability and the stability ratio of the percentage differential relay is substantially better than that of the simple differential relay.

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Percentage Differential Relay

The restraining winding is also known as the biasing winding because we bias the relay towards restraint. The slope of the characteristic is also known as percentage bias.

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For Internal Fault

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Thus, during internal faults the spill current will be two times the circulating current, giving a slope of 2, which is expressed as 200%.

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Percentage Differential Relay

The ratio of minimum internal fault current below which the scheme will not respond and the maximum ‘through fault’ current above which the scheme will maloperate is stability factor

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The percentage differential relay can be made more immune to maloperation on ‘through fault’ by increasing the slope of the characteristic.

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Relay setting

• Basic or sensitivity setting:- It is the value of the operating coil above which the relay operate. It is termed as percentage of rated current of relay.
• Bias setting:- Ratio of minimum current through operating coil to cause operation to average restraining current.

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• I1-I2/(I1+I2)/2

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Block diagram of percentage differential relay

The relay has two settings: the slope setting and the minimum pick-up setting.

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The slope is adjusted by changing the tapping on the restraining coil.

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The minimum pick-up is adjusted by changing the tension of the restraining spring.

Earth Leakage Protection

• Many times because of insulation failure the chassis of the equipment becomes live.
• This causes a leakage of current to earth from the chassis as the chassis is always connected to earth.
• However, the leakage current may be too small for an over-current relay to operate. This poses danger to the personnel who come in contact with the chassis.

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Earth Leakage Protection

• A special type of differential relay known as the earth leakage relay or current balance relay can easily detect such faults. In case the chassis of the equipment is not earthed, the relay will not trip because of leakage.
• However, as soon a person whose body is in contact with earth, touches the chassis, a path to earth becomes available, and assuming that the leakage current is of sufficient magnitude, the OC relay trips. The person will, of course, receive an electric shock before the circuit is tripped out.

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Earth Leakage Protection of single phase load

Earth Leakage Protection of three phase load