The following Terms are used in protective relaying:
1. Protective Relay
2. Relay Time
3. Breaker Time
4. Fault Clearing Time
5. Pick up
6. Pick up value
7. Drop Out or Reset value
8. Time Delay
9. Sealing Relay or holding Relay
10. Current Setting
11. Plug setting multiplier (P.S.M)
12. Time/P.S.M. curve
13. Time-setting multiplier
14. Trip Circuit
15. Earth Fault
16. Phase Fault
17. Protective Scheme
18. Protective System
19. Unit Protection
20. Reach
1. Protective Relay:
A protective relay is a device that is used to protect electrical equipment from damage or failure. It is designed to detect abnormal conditions, such as a power surge or a short circuit, and respond by opening or closing electrical contacts.
When the actuating quantity, such as the current or voltage in a circuit, reaches a certain preset value, the relay will close its contacts and initiate a trip circuit or an alarm circuit. This helps to prevent further damage or failure to the electrical equipment.
2. Relay Time:
Relay time is the amount of time it takes for the relay to respond to a fault after it has occurred. This is the time between the instant of fault occurrence and the instant of closure of relay contacts.
3. Breaker Time:
Breaker time is the amount of time it takes for a circuit breaker to open or close its contacts after a fault has been detected. This is the time between the instant at circuit breaker operates and opens the contacts, to the instant of extinguishing the arc completely.
4. Fault Clearing Time:
Fault clearing time is the total time required between the instant of fault and the instant of final arc interruption in the circuit breaker. It is the sum of the relay time and circuit breaker time.
5. Pick up:
When a relay moves from the 'OFF' position to 'ON' position, it is said to be picked up.
6. Pick up value:
The minimum value of an actuating quantity at which relay starts operating is called pickup value. The actuating quantity can be current in the relay coil and the pickup value of current is indicated along with the relay.
7. Drop Out or Reset value:
A relay is said to be dropout or reset when it comes back to original position, i.e. when relay contacts open from its closed position. The value of an actuating quantity, such as current or voltage, below which the relay resets is called reset value of that relay.
8. Time Delay:
The time taken by relay to operate after it has sensed the fault is called time delay of relay. Some relays are instantaneous while in some relays intentionally a time delay is provided.
9. Sealing Relay or holding Relay:
Sealing relays or holding relays are auxiliary relays that are used to perform the duties of a protective relay after it has closed its contacts. These relays are designed to be more durable and can handle the heavy loads associated with tripping and time lag functions. They are used to ensure that the protective relay is not damaged or worn out by performing these duties.
10. Current Setting:
Current Setting is the process of adjusting the minimum value of current at which a relay will operate. This is done by adjusting the number of turns on the relay coil, which can be achieved by using tappings on the relay coil. These tappings are brought out to a plug bridge, as shown in Fig.
The tap values are expressed as a percentage of the full-load rating of the current transformer (C.T.) that is associated with the relay. This allows the relay to be adjusted to match the specific needs of the electrical system it is protecting.
For example, if the current transformer has a rated secondary current of 10A, and the current setting is 150%, the pickup current of the relay will be 1.5 x 10 = 15A. This means that the relay will operate when the current in the relay coil is greater than or equal to 15A.
In simpler terms, current setting is a way to set the sensitivity of the protective relay. It allows you to adjust the minimum current level that the relay will respond to. This is a very important feature as it allows you to set the relay to trip at the right time, avoiding false tripping or not tripping when a real fault occurs.
11. Plug setting multiplier (P.S.M):
Plug Setting Multiplier (P.S.M.) is a measure of the sensitivity of a protective relay. It is the ratio of the actual fault current in the relay coil to the pickup current of the relay. It is mathematically expressed as the actual fault current divided by the pickup current.
For example, if the actual fault current in the relay coil is 30A and the pickup current of the relay is 15A, the P.S.M. would be 2. This means that the fault current is 2 times the pickup current of the relay.
The P.S.M. is an important factor to consider when setting the relay's sensitivity. If the P.S.M. is too low, the relay may not trip even when a fault occurs. On the other hand, if the P.S.M. is too high, the relay may trip unnecessarily, resulting in false tripping. Therefore, the P.S.M. should be set to a value that is appropriate for the specific electrical system being protected.
The formula for Plug Setting Multiplier (P.S.M.) is:
P.S.M. = Actual Fault Current / Pickup Current
Where:
Actual Fault Current is the amount of current that is present in the relay coil at the time of a fault.
Pickup Current is the minimum value of current at which the relay will operate.
12. Time/P.S.M. curve:
A Time/P.S.M. curve is a graph that shows the relationship between the time taken for a relay to operate and the Plug Setting Multiplier (P.S.M.). The P.S.M. is the ratio of the actual fault current to the pickup current of the relay.
A typical Time/P.S.M. curve for a relay is shown in Fig. It can be seen that for low values of overcurrents, the operating time of the relay varies inversely with the current. This means that as the current increases, the operating time decreases. However, as the current increases and approaches 20 times its rated value, the operating time becomes almost constant.
This type of characteristic is necessary to ensure discrimination on very high fault currents flowing through healthy parts of the system. It allows the relay to quickly trip on high fault currents while allowing more time to clear faults on lower currents.
Using the Time/P.S.M. curve and a time-setting multiplier, the actual time of operation of a relay can be calculated. For example, if the P.S.M. is 10 and the time corresponding to that P.S.M. on the curve is 4 seconds, as shown in Fig. 2. Multiplying this by a time-setting multiplier, the actual time of operation can be obtained. This is useful in adjusting the relay's time delay to match the specific needs of the electrical system it is protecting.
13. Time-setting multiplier:
A time-setting multiplier is a feature on a relay that allows the user to adjust the time delay before the relay operates. It is a dial or knob that is calibrated from 0 to 1 in steps of 0.05, as shown in Fig.
The time-setting multiplier can be used in conjunction with the Time/P.S.M. curve to determine the actual time of operation for a relay. The Time/P.S.M. curve shows the time delay for a given P.S.M. or actual fault current, and the time-setting multiplier allows the user to adjust the time delay for specific needs.
For example, if the P.S.M. is 10 and the time corresponding to that P.S.M. on the curve is 4 seconds, as shown in Fig. 2. And the time-setting multiplier is selected as 0.2. Then the actual time of operation can be calculated as:
Actual time of operation = time in seconds x time-setting multiplier
= 4 x 0.2 = 0.8 seconds
So, the time-setting multiplier allows the user to adjust the time delay based on the specific needs of the electrical system that the relay is protecting. It is an important tool for fine-tuning the protection of the system and ensuring that the relay operates correctly in the event of a fault.
14. Trip Circuit:
A trip circuit is a system that controls the opening operation of a circuit breaker. It typically consists of a trip coil, relay contacts, auxiliary switch, and battery supply. When a fault occurs, the trip circuit sends a signal to the circuit breaker to open, disconnecting the faulty section of the electrical system.
15. Earth Fault:
An earth fault is a type of electrical fault that involves the earth. Examples of earth faults include single line to ground fault and double line to ground fault.
16. Phase Fault:
A phase fault is a type of electrical fault that does not involve the earth. An example of a phase fault is a line to line fault.
17. Protective Scheme:
A protective scheme is a combination of various protective systems that cover a particular protective zone for a specific piece of equipment. For example, a generator may be protected by overcurrent, differential, and earth fault protective systems. The combination of all of these systems is called the generator protective scheme.
18. Protective System:
A protective system is a combination of circuit breakers, trip circuits, CTs, and other protective relaying equipment. Its purpose is to detect faults and disconnect the faulty section of the electrical system to prevent damage to equipment and ensure safety.
19. Unit Protection:
A unit protection is a protective system in which the protection zone is clearly defined by the CT boundaries. Such systems work for internal faults only, meaning it will only trip the breaker if the fault is within the defined zone.
20. Reach:
The reach of a protective system is the limiting distance within which the system responds to faults. If the protective system operates beyond the set distance, it is called over-reach, while failure of the distance relay within the set distance is called under-reach. It's important to have the correct reach setting to ensure the protective system works as intended and to avoid over-reach or under-reach.