Fundamentals of Protective Relaying

 

Fundamentals of Protective Relaying | Photo: Alstom

The purpose of an electrical power system is to generate and supply electrical energy to consumers. The system should be designed to deliver this energy both reliably and economically. Frequent or prolonged power outages result in severe disruption to the normal routine of modern society, which is demanding ever-increasing reliability and security of supply. As the requirements of reliability and economy are largely opposed, power system design is inevitably a compromise. 

While it is true that engineers can provide a well-designed power system, however, faults will always occur on a power system, and these faults may represent a risk to life and/or property.

The provision of adequate protection to detect and disconnect elements of the power system in the event of fault is therefore an integral part of power system design. Only by doing this can the objectives of the power system be met and the investment protected.

Protection Equipment

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system. 

1. The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. 
2. The electrical quantities, which may change under fault conditions, are voltage, current, frequency and phase angle. 
3. Having detected the fault, the relay operates to close the trip circuit of the breaker. 
4. This results in the opening of the breaker and the disconnection of the faulty circuit.

Typical Protection Relay Circuit

The definitions that follow are generally used in relation to power system protection:

• Protection System: a complete arrangement of protection equipment and other devices required to achieve a specified function based on a protection principle (IEC 60255-20) 
• Protection Equipment: a collection of protection devices (relays, fuses, etc.). Excluded are devices such as Current Transformers (CTs), Circuit Breakers (CBs) and contactors 
• Protection Scheme: a collection of protection equipment providing a defined function and including all equipment required to make the scheme work (i.e. relays, CTs, CBs, batteries, etc. 

In order to fulfill the requirements of protection with the optimum speed for the many different configurations, operating conditions and construction features of power systems, it has been necessary to develop many types of relay that respond to various functions of the power system quantities. 

For example, simple observation of the fault current magnitude may be sufficient in some cases but measurement of power or impedance may be necessary in others. Relays frequently measure complex functions of the system quantities, which may only be readily expressible by mathematical or graphical means. 

Relays may be classified according to the technology used: 

• electromechanical 
• static 
• digital
• numerical 

Zones of Protection

To limit the extent of the power system that is disconnected when a fault occurs, protection is arranged in zones. Ideally, the zones of protection should overlap so that no part of the power system is left unprotected. 

Sample Zones of Protection

Basic Requirements of Protective Relaying

Selectivity - It is the ability of the protective system to select correctly that part of the system in trouble and disconnect the faulty part without disturbing the rest of the system. A well-designed and efficient relay system should be selective by exactly detecting the point at which the fault occurs and causing the opening of the circuit breakers closest to the fault with minimum or no damage to the system. 

Sensitivity - It is the ability of the relay system to operate with a low value of actuating quantity. 

• The sensitivity of a relay is a function of the volt-amperes input to the coil of the relay necessary to cause its operation. 
• The smaller the volt-ampere input required to cause relay operation, the more sensitive is the relay. 
• Thus, a 1 VA relay is more sensitive than a 3 VA relay. It is desirable that the relay system should be sensitive so that it operates with low values of volt-ampere input. 

Reliability - It is the ability of the relay system to operate under the predetermined conditions. Without reliability, the protection would be rendered largely ineffective and could even become a liability. 

Simplicity - The relaying system should be simple so that it can be easily maintained. Reliability is closely related to simplicity. The simpler the protection scheme, the greater will be its reliability. 

Economy - The most important factor in the choice of a particular protection scheme is the economic aspect. Sometimes it is economically unjustified to use an ideal scheme of protection and a compromise method has to be adopted. 

• As a rule, the protective gear should not cost more than 5% of total cost. However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission line etc.), economic considerations are often subordinated to reliability.

Protective Relay Input Devices

Read: Instrument Transformers in Power System Protection

 

Protective Relay Output Devices

Contact Systems

Relays may be fitted with a variety of contact systems for providing electrical outputs for tripping and remote indication purposes. The most common types encountered are as 

follows: 

• Self-reset: The contacts remain in the operated condition only while the controlling quantity is applied, returning to their original condition when it is removed. 
• Hand or electrical reset: These contacts remain in the operated condition after the controlling quantity has been removed.

Contact Types | Source: Alstom

Operation Indicators

Protection systems are invariably provided with indicating devices, called ‘flags’, or ‘targets’, as a guide for operations personnel. Not every relay will have one, as indicators are arranged to operate only if a trip operation is initiated. Indicators, with very few exceptions, are bi-stable devices, and 

may be either mechanical or electrical. A mechanical indicator consists of a small shutter that is released by the protection relay movement to expose the indicator pattern. 

Electrical indicators may be simple attracted armature elements, where operation of the armature releases a shutter to expose an indicator as above, or indicator lights (usually light emitting diodes). For the latter, some kind of memory circuit is provided to ensure that the indicator remains lit after the initiating event has passed. 

The introduction of numerical relays has greatly increased the number of LED indicators (including tri-state LEDs) to enhance the indicative information available to the operator. In addition, LCD text or graphical displays, which mimic the electrical system provide more in-depth information to the 

operator.

Tripping Circuits

Series Sealing

The coil of the series contactor carries the trip current initiated by the protection relay, and the contactor closes a contact in parallel with the protection relay contact. This closure relieves the protection relay contact of further duty and keeps the tripping circuit securely closed, even if chatter occurs at the main contact. The total tripping time is not affected, and the indicator does not operate until current is actually flowing through the trip coil.

Shunt Reinforcing

Here the sensitive contacts are arranged to trip the circuit breaker and simultaneously to energise the auxiliary unit, which then reinforces the contact that is energising the trip coil. Two contacts are required on the protection relay, since it is not permissible to energise the trip coil and the reinforcing contactor in parallel. If this were done, and more than one protection relay were connected to trip the same circuit breaker, all the auxiliary relays would be energised in parallel for each relay operation and the indication would be confused. The duplicate main contacts are frequently provided as a three-point arrangement to reduce the number of contact fingers.

Shunt reinforcement with sealing 

This is a development of the shunt reinforcing circuit to make it applicable to situations where there is a possibility of contact bounce for any reason. Using the shunt reinforcing system under these circumstances would result in chattering on the auxiliary unit, and the possible burning out of the contacts, not only of the sensitive element but also of the auxiliary unit. The chattering would end only when the circuit breaker had finally tripped. 

The effect of contact bounce is countered by means of a further contact on the auxiliary unit connected as a retaining contact. This means that provision must be made for releasing the sealing circuit when tripping is complete; this is a disadvantage, because it is sometimes inconvenient to find a suitable contact to use for this purpose.