There is a disadvantage in a low-impedance restricted earth fault scheme. Suppose, due to any fault, any of the line CTs comes under saturation. A spill current starts flowing through the earth fault relay. As a result, the relay may initiate mal-tripping to the transformer circuit breakers. Although the fault is not related to the transformer itself.
In other words, we can say that due to any external fault that is in the downstream feeder, the restricted earth fault relay may operate because of line CT saturation of the phase where that external fault occurs. This creates an unnecessary mal-tripping of the relay. To overcome this disadvantage, we use the high-impedance restricted earth fault scheme.
Circuit Diagram of High Impedance REF
Here, all four CTs, meaning three line CTs and one neutral CT, are connected in parallel. A current-operated relay, along with a series stabilizing resistor, is connected across the CT secondaries, as shown. Also, we connect one Metrosil across the CT secondary.
Working of High Impedance REF
In normal conditions, or during an external fault, the vector sum of three-phase currents (R, Y, B) equals the neutral current. As a result, currents in the secondary circuit are balanced. Hence, no operating voltage is developed across the relay.
That means the voltage appearing across the line CT secondary terminals and the voltage appearing across the neutral CT secondary terminals remain equal and do not oppose each other. As a result, the secondary current circulates smoothly through the CT secondary loop without developing any voltage across the relay. Therefore, the relay does not operate.
Slight CT Mismatching
However, no matter how perfectly we use PS-class matching CTs, there is always a possibility of a small error. Even a slight mismatch due to CT characteristics, installation tolerance, wiring resistance, or fitting errors among the four CTs can create a small difference in secondary voltages. Because of this mismatch, a small spill current may always flow through the 64 relay.
That is why we must apply a minimum pickup current setting for the 64 relay. When the current exceeds the minimum set current, the relay operates. Otherwise, even a small CT mismatch could activate the restricted earth fault (REF) protection.
CT Saturation
Now consider a situation where there is a heavy through fault (external fault), and during that condition, one of the phase CTs becomes saturated. When a CT saturates, it produces a distorted and relatively reduced secondary current. Physically, on the primary side, the vector sum of the three line currents still equals the neutral current. But on the CT secondary side, due to one saturated CT, the sum of the three-phase CT secondary currents will not match the neutral CT secondary current.
This happens because the neutral CT secondary current is the true reflection of the actual neutral current of the transformer, which is the real vector sum of all three-phase currents (including the faulted one). But the sum of line CT secondary currents cannot match it, because one CT is saturated and cannot reproduce the correct current waveform. Therefore, due to this current difference, a voltage develops across the relay circuit. This voltage produces a spill current through the relay. As a result, the 64 relay may operate even though the fault is external.
Current Setting for High Impedance REF
To avoid false operations, we must keep the relay current setting higher than the estimated spill current caused by CT mismatch and saturation during through faults. Normally, we keep the setting around 10% to 20% of the rated current.
Internal Faults
However, during an internal fault inside the transformer, the imbalance becomes very large. In that case, the spill current through the relay becomes sufficiently high. Hence, the relay operates to disconnect the transformer by tripping the breakers.
Stabilizing Resistor
We can control the current through the relay for a developed voltage by inserting a high resistance (stabilizing resistor) in series with the relay. If we increase the resistance, the same voltage causes a reduced spill current through the relay. Therefore, during slight mismatches or CT saturation, the current through the relay cannot easily cross the relay pickup value. Consequently, the REF protection scheme becomes stable.
Metrosil
Another important issue in high-impedance REF is that during severe through faults, the CT secondary circuit may develop a very high voltage. Possibly, it approaches nearer the knee-point voltage of the CT. In some cases, this knee-point voltage may even be in the kilovolt range. Such a high voltage creates heavy stress on the relay coil, CT secondary insulation, and the overall secondary wiring insulation system. This may damage the relay or the CT secondary circuit insulation.
To protect the circuit from these dangerous high voltages, we connect a device called Metrosil across the CT secondary circuit. Metrosil is actually a non-linear resistor.
Under normal voltage conditions, Metrosil has a high resistance, so it does not affect the circuit. But once the voltage rises above the specified limit, its resistance drops sharply, thereby clamping the voltage. So, it prevents the voltage from rising further. In simple words, Metrosil protects the relay and CT secondary circuit by limiting the excessive voltage during severe fault conditions.
So, the stabilizing resistor ensures stability by limiting spill current, and the Metrosil ensures safety by limiting excessive secondary voltage.