The overcurrent relays are very sensitive as these relays cannot differentiate between heavy loads and minor faults. The differential protection scheme (or differential relays) is used in a network to avoid this malfunction of relays.
The relays that operate when the phasor difference of two or more similar electrical quantities exceeds predefined value is known as differential relays.
Therefore, the current differential relay operates when the phasor difference of current enters and leaves the system to be protected.
Both currents have equal magnitude and are in phase in normal condition. But in an abnormal condition, both currents have different magnitude and are out of phase.
The relay is connected in such a way the difference of current entering and leaving passes through the operating coil.
Hence, if the current difference exceeds a preset value, the relay operates and sends a signal to a circuit breaker.
Most of the differential protection schemes use current to be measured. Therefore, this scheme is also known as the current differential scheme or Merz-price protection scheme.
The connection diagram of differential protection is shown in the figure below.
Here, two current transformers (CT) having the same ratio is connected on both sides of the system, or equipment needs to be protected.
The secondaries of both CTs are connected in series. Hence, they carry induced currents in the same direction.
The current flowing through the primary winding of a current transformer is the rated current (normal current).
In an external fault condition, the current flowing through primaries of both CT is I. As both CTs are identical, the secondaries of both CTs carry equal currents.
Therefore, there is no current difference between both CTs. And the current passes through the operating coil remains zero.
The current flow through the fault from both sides in an internal fault condition. And this current is not equal. Therefore, the current passes through the secondaries are not equal.
So, the difference of both currents passes through the relay coil (operating coil). And this current is high enough to operate relays.
- The current transformers are connected through the pilot cables. During the performance, the impedance of the pilot cable causes a slight difference between the currents at the ends of the section to be protected. Therefore, a sensitive relay can operate to a very small difference in the two currents; even there is no fault.
- Here we have assumed both current transformers are identical. But some constructional errors produce slight differences in both secondaries. And it may result in the malfunction of a relay.
- Under a severe fault condition, the current transformer may saturate and cause a difference in secondaries. The difference between the currents may approach the pick value, but it results in the inaccurate operation of the relay.
- Under a heavy current situation, the pilot cable capacitance may cause inaccurate operation of the relay.
Zone of Protection in Differential Relay
From above, we can say that the differential relay operates only during the internal fault. And it remains inoperative in case of external fault.
Therefore, the differential relays operate when a fault occurs on either side of the equipment. Or we can say, the differential relay operates when a fault occurs between the CTs.
The differential relay zone of protection is shown in the figure below.
Ideally, for a small internal fault, the differential relay must operate and not respond to the significant external fault.
But due to the non-ideal characteristics of various components, the ideal behaviour of differential protection is not possible, especially for severe external faults.
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