Restricted Earth fault Protection in
Transformers & Generators
1.0 General :
Transformers and generators are voltage sources. They are traditionally protected by an Over current + Earth fault relay , normally mounted in the breaker panel. This is shown in fig. 1. It should be noted that this protection alone is not adequate.
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It should be noted that the fault currents in Zone A is limited by the impedance of the equipments in the zone – for transformers and generators it is very low – the fault currents can rise very fast and damage the equipment. Consequently REF protection is of utmost importance for generators and transformers.
2.0 Why is this called Restricted E/F ?
The name Restricted is derived – since the objective of the protection is to detect the earth fault in the specific zone , restricted to the zone, starting from the breaker to the machine terminals. In case of a generator, the machine terminal is the neutral point. In case of a transformer , the machine terminal becomes the star point of either the primary or secondary winding or both. In case of delta winding, it is the winding itself.
3.0 How can we detect internal earth faults ?
It is well established that the sum of currents at the beginning of the zone A should be equal to the sum of currents exiting the zone. Two sets of CTs are
used to derive these sum of currents at the inlet and exit. A fault in the zone will result in a difference in current .
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An over current relay is used to measure the difference in the sum of these currents.
Please refer to fig.2 where a typical REF scheme shown . The REF relay is connected between P & S . It will pick up if there is enough voltage across P-S to drive the pick up current through the relay.
There are two current sources in the CT secondary circuit PQRSTU :
a) the current produces by CT X ( loop RSPQ)
b) the current produced by CT Y ( loop UPST)
Under normal conditions, vectorial sum of the currents of these two CTs will be equal and opposite in the branch P-S (ie) the resultant current in the branch PS will be zero. In this situation , the currents of CT X and CT Y will circulate in the loop PQRSTU. The voltage across P-S will be zero.
When there is an earth fault within the zone A, the currents of X & Y CTs will not be equal – a small difference of current will flow in the branch P-S. This current will result in a voltage across P-S – since the current flows through the relay impedance. If this voltage is adequate to operate the relay , the relay will pick up – thus detecting a fault within the zone.
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4.0 What if a fault occurs outside the zone ?
For a fault outside the zone, the currents through CTs X & Y will be the same – the resultant current through P-S will still be zero and the relay will not not pick up. In this case the fault has to be cleared by another O/C + E/F relay connected near the breaker.
5.0 Why is a Stabilising resistor required in REF scheme ?
We have said in the sections 3 & 4 that the REF relay will detect a fault within a zone restricted between two CTs and it will not detect a fault out side the zone defined by the CTs. This is true, only for ideal conditions- where the two CTs are perfectly matched. Following mis matches will occur under practical situations in the field :
a) the CT secondary impedances may not be equal
b) the lead wires connecting the CT secondaries to the relay may not have equal resistances
c) the CTs may have different ratio error and phase angle error – due to this , the secondary cuurents will not be equal even if the primary currents are same..
d) the CTs may have different saturation characteristics – this will cause small difference in the secondary currents for a same primary current
The cumulative effect all the above, can make the relay trip even when there is a full load current flowing in the primaries – though the primary side currents are same, the secondary side currents need not be the same – a voltage sufficient to trip the REF relay may develop across P-S and hence the relay will trip.
To make the relay insensitive to this voltage produced by CT mismatch, a resistor is added in series with the relay. Once this resistor is added, the relay will need a voltage which is higher than the voltage produced by the CT mismatch. This resistor is called stabilizing resistor – this is an important component in REF scheme –since this ensures stability in the scheme by avoiding spurious tripping.
6.0 What is knee point voltage ?
It should be noted that actual input crcuit to trip mechanism (consisting of the
relay + stabilizing resistor) has become a high impedance circuit. If the relay
has to trip, the CT secondaries should produce sufficiently high enough
voltage to activate the relay, after allowing for the drop across the stabilizing
resistor.
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To ensure the CTs produce enough voltage, an additional specification – the
Knee poit voltage - is included for the CTs used for REF protection .
Knee point voltage (KPV)is defined as the point on the magnetizing curve (of
the material used for the CT core) where the core will need 50% increase in
the magnetizing force (ampere turns) to cause a 10% increse in the flux
density.(voltage build up across secondary). In effect, KPV defines the end of
the linear portion of the BH curve . Higher the KPV, larger is the linear zone
and better will be secondary output for higher fault currents. Higher the KPV,
better are the chances of a high impedance relay trip.
7.0 How can we calculate the value of the stabilizing resistor and KPV ?
The value of stabilizing resistor and KPV will depend on the following parameters which are unique to a given feeder:
a) Impedance of the CT secondaries
b) Lead wire resistance between the CT secondaries and the REF relay
c) The impedance of the REF relay - this can vary with respect to pick up setting . We have to consider the relay impedance at the pick up setting being contemplated for the feeder.
d) The maximum fault current which can occur on the CT secondary side – CT should not saturate under this maximum fault conditions. If the CT saturates, it will offer an alternate path for the resultant current – and the REF relay may not trip.
A detailed method is provided in annexure 1, for calculation of stabilizing resistor and KPV
8.0 What are L&T solutions for REF protections ?
L&T manufactures a high impedance Over curret relay – which is ideally suited for the REF protection of generators and transformers. Typical schemes are shown in fig. 3 &4.
L&T offers a unique feature in the REF relay SC14S – in additon to instantaneous trip, user can select a definite time delay of either 100 millisec or 200 millisec. In case of small transformers & generators ( upto 5 MVA), the feeder trips during breaker closing. This mainly is due to the large inrush current causing momentary difference in CT secondary currents due to mismatch in saturation characteristics. A 100 millisec time delay will help in this case.
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Fig.4 shows a typical scheme for REF protection for transformers. It should be noted that transformers will need two REF schemes – one on the primary side and the other on the secondary side.
For the transformer primary side , which is usually delta connected, following are envisaged in the REF scheme :
a) 3 nos. phase CTs
b) 1 no. Relay SC14S
d) 1 no. Stabilising resistor
For the transformer secondary side, which is usually star connected, following are envisaged in the REF scheme :
a) 3 nos. phase CTs
b) 1 no. neutral CT
b) 1 no. Relay SC14S
d) 1 no. Stabilising resistor
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