In a power system, relays are used to detect abnormal conditions such as faults in the electrical circuits. A fault in the power system can cause damage to equipment and disrupt the supply of power to a large portion of the system. Therefore, it is important to detect faults as quickly as possible in order to minimize the damage caused by the fault and to restore power quickly.
When a fault occurs, the protection scheme employed in the power system should be able to quickly isolate the faulted portion of the system and disconnect the supply of power to it. This is done by using relays that are sensitive to abnormal conditions and can quickly detect and respond to faults. The relays are typically connected to a circuit breaker, which can quickly open and disconnect the supply of power to the faulted portion of the system.
It's important to note that the protection scheme should also be selective, meaning that it should be able to distinguish between different types of faults, such as a fault on the transmission line or a fault in a distribution system. This allows the system to isolate only the affected portion of the system and avoid disconnecting the supply of power to healthy circuits and equipment.
In addition, the protection scheme should also be able to quickly clear the fault, meaning that it should be able to quickly restore power to healthy portions of the system after the fault has been isolated. This can be done by using relays that can quickly re-close the circuit breaker and restore the supply of power to healthy portions of the system once the fault has been cleared.
There are two types of protection schemes employed in power system networks.
- Primary or main protection, and
- Back-up protection.
1. Primary or main protection:
The primary or main protection is the first line of defense in a power system's protection scheme. It is designed to immediately sense and respond to faults in order to isolate the faulty portion of the system from the healthy portion.
The power system is divided into various zones of protection, and for each zone, there is a specific protective scheme. When a fault occurs in a protected zone, it is the duty of the primary or main relays to detect the fault and take action to isolate the faulty element.
Primary protection is characterized by its speed and instantaneous action. When a fault occurs, the primary protection relays quickly detect the fault and immediately take action to isolate the faulty portion of the system. This is done by tripping a circuit breaker which opens the contacts and disconnects the supply of power to the faulty portion of the system.
The primary protection is designed to be selective, meaning that it should only isolate the faulty portion of the system and avoid disconnecting the supply of power to healthy circuits and equipment. This is done by using relays that are sensitive to different types of faults and can quickly detect and respond to the specific type of fault that has occurred.
One of the main advantages of primary protection is that it can quickly clear faults and restore power to the healthy portion of the system. This is done by re-closing the circuit breaker after the fault has been cleared and restoring the supply of power to the healthy portion of the system.
2. Back-up protection:
Back-up protection is an additional layer of protection that is provided in case the primary protection fails to operate. The primary protection may fail due to a variety of reasons such as circuit breaker failure, tripping mechanism failure, current or voltage supply failure to the relay, failure of current transformers (CT) or potential transformers (PT), or the main protective relay failure.
Back-up protection operates independently of the factors that cause primary protection to fail. It is designed to sense faults immediately, but it should not operate instantaneously. It must only operate when primary protection fails to operate.
When the primary protection fails, the backup protection takes over and isolates the faulty section from the healthy one, to minimize the damage caused by the fault and restore power quickly.
In case the main protection is made inoperative for maintenance, testing, etc., the back-up protection acts as the primary protection. The back-up protection is usually provided for short circuits only, for economic reasons and technical facts. This is because short circuits are the most common type of fault in power systems and have the most severe consequences.
It's worth noting that providing backup protection for other types of faults such as earth faults, overvoltages, and overcurrents can be more complex and costly.
Different types of backup protection are listed below:
- Remote back-up protection
- Relay back-up protection
- Bus back-up protection
- Centrally coordinated beak-up protection.
1. Remote back-up protection:
Remote back-up protection is a type of backup protection where the primary and backup protections are located at different stations. This means that the primary protection is located at one station, and the backup protection is located at a different station.
This type of protection is most widely used for the protection of transmission lines, which are long stretches of power lines that transmit electricity over long distances. Transmission lines are critical components of the power system and need to be protected from faults to minimize damage and quickly restore power.
The primary protection is typically located at the substation closest to the transmission line, while the backup protection is located at a distant substation. This allows for a more comprehensive protection of the transmission line, as the primary protection can detect and respond to faults that occur close to the substation, while the backup protection can detect and respond to faults that occur further away from the substation.
One of the main advantages of remote back-up protection is that it operates independently of the factors that cause primary protection to fail. This means that even if the primary protection fails to operate, the backup protection can still detect and respond to faults, minimizing the damage caused by the fault and quickly restoring power.
2. Relay back-up protection:
Relay back-up protection is a type of protection where both primary and backup protections are provided for the same circuit breaker. In this type of protection, both the primary and backup protections are located at the same station, and they are both connected to the same circuit breaker.
In case the primary protection fails to operate, the backup protection takes over and trips the breaker without delay. This is done by using separate trip coils for the same circuit breaker. The trip coils are connected to the primary and backup protections respectively, and they are designed to trip the circuit breaker when they receive a signal from the protection relays.
The principles of operation of both primary and backup protection are different. The primary protection is designed to detect and respond to faults quickly and take action to isolate the faulty portion of the system. The backup protection, on the other hand, is designed to take over when the primary protection fails and trip the breaker without delay.
This type of backup is provided where remote backup is not possible, for example, in substations or other locations where there is no remote station to provide backup protection.
3. Bus back-up protection:
Bus back-up protection is a type of protection that is used to protect a bus-bar system, which is a common point of connection for a number of circuit breakers. A bus-bar system is used to distribute power to multiple circuits, and it is a critical component of the power system that needs to be protected from faults.
When a fault occurs on a system and the circuit breaker fails to trip, it is called a bus fault. A bus fault occurs when a short circuit occurs on the bus bar, and it can cause damage to equipment and disrupt the supply of power to multiple circuits.
To protect the bus-bar system from bus faults, bus back-up protection is provided with an appropriate time delay. The protection scheme is designed to detect bus faults and trip all the circuit breakers connected to the bus in case of a bus fault. This is done by using a relay that is connected to the bus and is designed to detect bus faults and send a trip signal to all the circuit breakers connected to the bus.
The time delay is provided to ensure that the protection scheme only trips the circuit breakers in case of a bus fault, and not in case of other types of faults that may occur on the system. The time delay allows the protection scheme to differentiate between a bus fault and other types of faults and ensure that only the circuit breakers connected to the bus are opened in case of a bus fault.
4. Centrally coordinated beak-up protection:
Centrally coordinated back-up protection is a type of protection that is used for systems that have central control. In this type of protection, the main protection is located at different stations, and the backup protection for all stations is located at the central control center.
This type of protection is the coordination of protective relaying equipment, high-frequency carrier current equipment, and digital computers. The central control center is responsible for monitoring the load flow frequency and taking action in case of any abnormal condition. This can be done by using sensors and relays that are connected to the different stations and are designed to detect abnormal conditions and send signals to the central control center.
The central control center can then use the information from the sensors and relays to take the necessary action. For example, if a fault occurs on a transmission line, the central control center can use the information from the sensors and relays to quickly isolate the faulted portion of the system and restore power to healthy portions of the system.
The centrally coordinated back-up protection is useful for large power systems where the main protection is located at different stations and the backup protection is located at the central control center. It allows for a more comprehensive protection of the system, as the central control center can monitor the load flow frequency and take action in case of any abnormal condition.
Example:
In the given scenario, remote backup protection is provided by a small time graded relay, as shown in the figure. Let F be the fault that occurs on relay R4. The relay R4 operates the circuit breaker at D and isolates the faulty section. This is the main protection for the system, and it is designed to detect and respond to faults quickly and take action to isolate the faulty portion of the system.
However, if the circuit breaker D fails to operate, the faulty section would be isolated by the operation of the relay R3 at C. This is the backup protection for the system, and it is designed to take over when the primary protection fails and isolate the faulty section.
The use of the backup protection depends on the economics and technical considerations. The primary protection is designed to detect and respond to faults quickly, but it may be more expensive and complex to implement. The backup protection is usually slower than the primary protection, for economic reasons. This is because providing a faster backup protection would require additional equipment and increase the cost of the system.
Therefore, the backup protection is usually designed to be slower than the primary protection, but it is still fast enough to isolate the faulty section and minimize the damage caused by the fault. The backup protection is also designed to be reliable, so that it can take over and isolate the faulty section even if the primary protection fails.