Protection Relays - Part 1

Types of Relays

Electric Protection Relays are vital components in the protection scheme. Protection Relays protect an equipment such as a Transformer or a Generator from internal and external faults such as overvoltage, overcurrent, earth fault, etc.

Relays have been in existence since the early years of Electrical Engineering. Relays can be configured for instantaneous operation or delayed operation. 
There are three broad categories of relays based on the principle of functioning. 

They are Electro-Mechanical Relays, Numerical Relays and Solid State Relays

Electro-mechanical Relays

Electro-mechanical Relays are the oldest type of relays. These relays are simple in construction. They are easy to adjust. These Relays consist of a disc, usually made of aluminium, which rotates when there is a fault. The rotation occurs due to the presence of eddy currents caused by current and voltage coils. 
When a fault occurs, the rotating disc rotates and closes the alarm contacts which generate the alarm. If the fault is severe or persistent, it closes the tripping contacts which issue the tripping command to the circuit breaker. 
The disadvantages of these relays is that the values tend to drift over time, due to the effects of heat, vibration and aging on the relay components. These relays are gradually being replaced by numerical and solid State Relays

Numerical Relays

These relays are electronic relays. They do not have moving parts. In the Numerical Relay, the analog values are converted into numbers. The alarm and the trip values are also fed into the relay and stored as digital values.   Numerical relays are also called digital relays. The microprocessor monitors the field values and generates the alarm or the trip command. Numerical Relays are programmable. The behaviour and the characteristics or these relays can be programmed. Numerical Relays are also multifunctional which means that the same relay can be used for overvoltage as well as overcurrent protection. 
Modern Numerical Relays can also communicate with protocols such as Ethernet, RS 485, etc. They can store historical data of trends and events. This feature will be helpful in analyzing a fault condition or a blackout. Timestamping also enables sequential registering of events.

Static Relays

Static Relays are analog relays. In static relays, the voltage or the current from the field is converted into rectified voltages and currents. These values are then processed by means of op-amps, transistors, etc and the output signal is generated. 

Common Terms in Relay Protection

Common Terms in Relay Protection are

Pick Up Value

Pick up Value of relay refers to the value above which the relay will generate the alarm or the trip.

Operating Time

The Operating Time is the time which is allowed to elapse after the pick up value has been exceeded before the output is given. 

Reset Value

Once the relay operates, it has to be reset. The relay resets after the measured value falls beyond a certain value. This value is below the pick up value in case of functions such as overvoltage or overcurrent. In case of functions such as undervoltage or underfrequency, this value is above the pick up value.

Reset Time delay

Reset Time delay is the time taken by the relay to reset after the reset value has been reached.

Reach of the relay

This is a term used in distance protection. The distance Relay operates when there is fault in a cable.   Reach of the relay refers to the distance till which the relay can sense the fault. 

VA Burden in Protection Relaying

Protection Relays serve to protect equipment and circuits from abnormalities such as overcurrent, overvoltage, overloading and under reactance. The input to the protection Relays are the current and the voltage of the system. Using these two basic parameters, the relays are able to calculate a host of values such as kW, kVAr, pf, etc.
Thus, every protection relays needs an input from the Potential Transformer or the current transformer or both. When the relay is connected to the circuit of an instrument transformer, it becomes a load. The Potential or Current Transformer acts as the source.  
When designing the Protection Scheme, we must ensure that the Potential Transformer or the Current Transformer does not get overloaded. This is done by adding the VA burden of each protection relay in the system. Every Relay will have the VA burden mentioned in the manual. The total VA burden imposed by all the relays should be calculated. The VA capacity of the instrument transformer should be greater than this. 
While designing a system, the instrument transformer should have an excess capacity of 10% of the present load. This is an allowance for future relays which may be added to the system. 
If the transformer is overloaded, the voltage and current signals will not be accurate. 
The wires used in the protection system should be of sufficient thickness so as not to added unnecessary burden on the transformers.