Protective Relays in Power Systems
RELAY PANEL IN SUBSTATION
The capital investment involved in a power system for the generation, transmission, and distribution of electrical power is so great that proper precautions must be taken to ensure the equipment operates efficiently and remains protected from faults. Protective relays and relaying systems are used to operate the correct circuit breakers to disconnect only the faulty equipment as quickly as possible. This minimizes damage and ensures system stability.
Although protective equipment constitutes only 4 to 5% of the total system cost, it plays a vital role in maintaining reliable supply. Shunt faults are the most severe failures, characterized by increased current, reduced voltage, power factor, and frequency. Protective relays do not prevent faults but act after a fault has occurred. Only specific relays like the Buchholz relay (gas detector relay) can anticipate faults under certain conditions.
Types of Protection
Backup protection operates only when primary protection fails. Reasons for primary failure include:
- Failure of protective relays (mechanical/electrical)
- Failure of circuit breakers
- DC tripping voltage failure
- Loss of current/voltage to relays
Backup relays should be independent and ideally located at different stations. They also act as primary relays when primary equipment is under maintenance.
Definitions
- Relay: An automatic device that senses an abnormal condition and closes its contacts to trip the circuit breaker, isolating the faulty section.
- Pick up Level: Minimum actuating current/voltage that triggers relay operation.
- Reset Level: The level below which the relay returns to normal state.
- Operating Time: Time between actuating quantity crossing pickup value and contact closure.
- Reset Time: Time between actuating quantity dropping below reset value and contact returning to normal.
- Primary Relays: Relays connected directly to the protected circuit.
- Secondary Relays: Relays connected through CTs/PTs.
- Auxiliary Relays: Assist main relays and may include time delay or instantaneous operation.
- Reach: Maximum impedance (or distance) within which the relay operates.
- Underreach: Tendency to not operate at impedance less than the set value.
- Overreach: Tendency to operate beyond the set impedance value.
Characteristics of Protective Relays
A protective relay must satisfy four essential characteristics:
- Reliability
- Selectivity
- Speed
- Sensitivity
1. Reliability
Reliability is the most basic requirement of a protective relay. The relay must operate when it is required to. There are various components that contribute to the functioning of a relay, and each plays a vital role in ensuring its operation. For instance, the absence of suitable current and voltage transformers can result in unreliable relay performance.
Since protective relays remain idle most of the time in a power system, proper and periodic maintenance becomes essential to ensure their reliable operation when a fault occurs. Inherent reliability is primarily a function of good design and long-standing experience.
Inherent reliability can be partly achieved by:
- Simplicity and robustness in construction
- High contact pressure
- Dust-free enclosures
- Good contact material
- Good workmanship
- Careful maintenance
2. Selectivity
Selectivity is the ability of the relay to isolate only the faulty part of the system without affecting the healthy part. It is achieved by:
- Unit System Protection – responds only to faults in its own zone (e.g., differential protection).
- Non-Unit System Protection – achieved by time or current grading among different relays in different locations.
3. Speed
A protective relay must operate neither too fast nor too slow. High-speed operation avoids prolonged fault duration, which helps maintain system stability. However, if it's too fast, it may react to transient faults, leading to unnecessary isolation. The goal is to minimize fault clearing time while ensuring dependable operation.
4. Sensitivity
Sensitivity refers to the ability of the relay to respond to the smallest fault current or voltage. It is usually expressed in terms of the minimum volt-amperes (VA) needed for the relay to operate reliably under actual system conditions. High sensitivity ensures early detection and quick isolation of even minor faults.