Protection Relays - Part 6

Primary and Secondary Protection Schemes

Protection Relay systems are classified into two types.

Primary relaying Equipment and
Secondary relaying Equipment

The Primary relay protection equipment is the first line of defence. The secondary relay scheme comes in line when the primary relay system fails to act.

The Primary relay protection scheme can fail due to reasons such as

Failure of DC tripping voltage supply.
Failure of Current or voltage signal to the relays.
Failure of the Circuit Breaker.
Failure of the internal mechanism of the Protection Relays. 

The Secondary relay Protection scheme is intended to operate in the event of a failure of the primary supply. Hence, the secondary relay protection scheme should be totally independent of the primary. The current and voltage signals, the power supply of the relay, the output to the breaker should all be independent of the primary protection scheme. The secondary protection scheme has a time delay greater than that of the primary relay protection scheme. 

Non-directional Earth fault Protection using Residual Current Relay

The Residual Current based earth Fault relay works by measuring the vector sum of the three phase currents. 

Under healthy conditions, the vector sum of the three currents is zero. In the event of an earth fault, however, the fault current flows through the ground and hence, the vector sum of the currents is not equal to zero. This is known as the residual current. This current can be used to operate the earth fault relay.

The connection of the earth fault relay consists of three current transformers connected in parallel to each other. This kind of earth fault protection is also known as unrestricted earth fault protection.

The residual current protection is usually set to operate at around 10% of the nominal current. For fault currents lower than this value, as may be the case in high resistance grounded generators and transformers, the sensitive earth fault relay is used. This is because, the three current transformers used in the residual current protection may not be exactly identical in response, even if they are from the same manufacturer. Thus for very low setting, there is the risk of false operation of the relay due to errors in the current transformers. 

Since the sensitive earth fault relay uses one Core Balance Current Transformer instead of three individual current transformers, it can be set to lower values of earth fault current.

Voltage dependent overcurrent protection

Overcurrent protection is a crucial component of the generator protection scheme. Overcurrent protection is used to protection the generator against overloading. It is also used to isolate the generator in the event of a short circuit fault.

However, there is one issue to be considered when designing a protection for a generator. In the event of a short circuit, the fault current is very high for a few milliseconds after a fault. This heavy current causes the generator voltage to drop. This drop in voltage causes the current to decay. Therefore, a high overcurrent setting may not operate in the event of a short-circuit.

To solve this problem, voltage dependent overcurrent relays bias the overcurrent setting with the measured voltage. That is, at normal voltage, the overcurrent relay operates if the current exceeds the setpoint. However, if there is a voltage drop, the overcurrent setting also progressive decreases according to the biasing. Thus, at lower voltages, the current required to operate the relay is very low.

A variation of the voltage-dependent relay is the voltage controlled. This relay has an undervoltage setting and a overcurrent setting. The overcurrent setting is set at a value less than the rated current of the generator. For the relay to operate, both the undervoltage and the overcurrent need to occur at the same time. This can occur only at the instant of a short circuit.

Sensitive Earth Fault Protection

         The sensitive earth fault relay is a protective device that works by measuring the residual current across the three phases in a system. This is done using a Core Balance Current Transformer (CBCT).  In the ideal condition, the residual current will be zero as all the currents flow through the three wires and their magnetic fields cancel each other out.

         In the event of a fault, the residual current over the three phases will not be equal to zero as the current from the faulted phase flows through the earth.

         The sensitive earth fault protection is usually used in alternators and transformers with high resistance grounding. High resistance grounding restricts the earth fault current to less than 10A. High resistance grounding enables electrical systems to continue running when one of the phases is faulted. This prevents interruptions to the power supply. This kind of earthing system provides time to identify and isolate the fault.

         Once an earth fault occurs in the high resistance grounding system, an alarm needs to be generated and the fault needs to be traced. For this a reliable protection which detects earth faults even when the fault current is very low is necessary. Undetected earth faults in this system are dangerous as a second earth fault in another phase may result in a short-circuit. Conventional earth fault relays may not be accurate in detecting an earth fault at such low current values.

         The sensitive earth fault protection, as the name suggests, is a highly sensitive relay. It can sense currents as low as 0.2% of the CT secondary current.

         The sensitive earth fault relay may be configured to either generate an alarm or a trip signal.