The protection provided by the protective relay equipment can be categorised into two types:
- Primary protection
- Backup protection
In this post, we'll dive into an essential topic for electrical engineers: Power System Protection. Understanding how Primary and Backup Protection systems function is key for ensuring the stability and reliability of power systems. These protective relays play a vital role in identifying and isolating faults to prevent system failures, which are commonly addressed in JE and AE exams.
- In the event of failure or non-availability of the Primary Protection, some other means of ensuring that the fault is isolated must be provided.
- These secondary systems are referred to as backup protection. Backup protection may be considered as either being local or remote.
- Local backup protection is achieved by the protection that detects an un-cleared primary system fault at its own location and which then trips its own circuit breakers, e.g. Time-Graded Overcurrent Relays.
- Primary protection (Main protection) is the essential protection provided for protecting an equivalent/machine or a part of the power system. As a precautionary measure, additional protection is generally provided and is called ‘Backup Protection’.
Primary Protection:
- Below is the power system protection scheme which is designed to protect the power system parts and components, each line is associated with an overcurrent relay that protects the lines from faults.
- If a fault happens on any line, it will be cleared by its relay and circuit breaker. This is called primary or main protection and acts as a first-line defender.
- The service record of primary relaying is very high with well over 90% of all operations being correct. But this is not always the case, sometimes faults are not illuminated by the primary or main protection system i.e. circuit breaker and relay system, because of trouble within the relays, circuit breakers or wiring system in different conditions.
- In those conditions, the Secondary or backup protection system does the required job.
Cause of failure of Primary Protection: Primary protection may fail due to the following reasons
- Failure of DC supply to the tripping Circuit
- Failure to relay operating current or voltage
- Failure of circuit breaker tripping mechanism
- Failure of the main protective relay operation
- Failure in the wiring of the relaying system
- Failure of CTs or PTs operation
Secondary Protection:
- Backup protection is very important for stable and reliable power systems we know, it is not possible to design a 100% secure and efficient system because there are possibilities of failure in the connected CTs, PTs, circuit breakers etc. in power system. If it happens, then it will destroy our whole switching system.
- If the primary protection operation falls into trouble, then secondary protection disconnects the faulty part from the system.
- Moreover, when we disconnect primary protection for testing or maintenance purposes, then secondary or backup protection will act as primary protection. In the above fig, relay “X” (1 Sec time setting) provides backup protection for each of the four connected lines to the main bus.
- In addition, a larger part is disconnected than when the primary relaying functions correctly.
- Therefore, greater emphasis should be placed on the better maintenance of primary relaying which is economical.
Types of Secondary or backup protection
- Relay Backup Protection
- Breaker Backup Protection
- Remote Backup Protection
- Centrally Co-ordinate Backup Protection
Difference between Primary and Backup Protection
Aspect | Primary Protection | Backup Protection |
Purpose | First line of defense to detect and isolate faults immediately. | Secondary defense, activated only if primary protection fails. |
Activation Time | Operates with minimal delay (instantaneous or very fast). | Operates after a time delay to allow primary protection to act. |
Location | Installed close to the specific equipment or system part it protects. | Can be local (same area) or remote (different area), protecting a broader zone. |
Coverage | Provides protection to a specific section or component of the power system. | Provides coverage to a larger section of the power system. |
Reliability | Highly reliable, as it is the main protection system. | Acts as a redundancy, stepping in when primary protection fails. |
Example | Differential relay protecting a transformer or generator. | Time-graded overcurrent relay providing backup for multiple lines. |
Economic Impact | Limits damage and system downtime, making it cost-effective. | Disconnects a larger portion of the system, which can cause more disruption. |
Failure Response | Clears the fault quickly to prevent equipment damage. | Clears the fault if primary protection doesn’t act, though more slowly. |
Maintenance Role | Requires regular testing and maintenance to ensure high reliability. | Steps in when primary protection is offline for maintenance or testing. |
Risk of Operation | Protects the system effectively with minimal impact on surrounding components. | May lead to unnecessary disconnection of non-faulty areas, but ensures fault clearance. |
FAQs: Primary and Backup Protection in Power Systems
1. What is the difference between primary and backup protection in power systems?
Primary protection is the first line of defense that detects and isolates faults almost immediately, while backup protection serves as a secondary measure, activated only if the primary protection fails or is unavailable.
2. Why is primary protection important in power systems?
Primary protection is essential for safeguarding power system components from faults. It operates quickly to clear faults and prevent damage, making it critical for the stability and reliability of the system.
3. What is backup protection in electrical power systems?
Backup protection is a secondary defense mechanism that steps in when primary protection fails to isolate a fault. It operates with a delay and ensures that the fault is cleared even if the primary system is non-functional.
4. What causes primary protection to fail?
Primary protection may fail due to various reasons, such as failure of the DC supply to the tripping circuit, relay malfunction, wiring issues, or a failure in current transformers (CTs) or potential transformers (PTs).
5. How does backup protection work in case of primary protection failure?
Backup protection operates by detecting faults that the primary protection missed. It disconnects the faulty part from the system, often disconnecting a larger portion of the network to ensure the fault is cleared.
6. What are the types of backup protection?
Backup protection can be classified into four types:
- Relay Backup Protection
- Breaker Backup Protection
- Remote Backup Protection
- Centrally Coordinated Backup Protection
Each type ensures the system remains operational even if one protection method fails.
7. What is the role of time-graded overcurrent relays in backup protection?
Time-graded overcurrent relays are a type of backup protection that delays operation to allow the primary protection system to act first. If the primary system fails, the overcurrent relay steps in to isolate the fault.
8. Why is the maintenance of primary protection critical?
Regular maintenance of primary protection systems ensures they function efficiently and prevent faults from spreading. Well-maintained primary protection minimizes the need for backup protection and reduces system downtime.
9. Can backup protection act as primary protection?
Yes, backup protection can temporarily act as primary protection when the primary system is offline for testing or maintenance, ensuring continuous protection for the power system.
10. How do primary and backup protection contribute to the reliability of power systems?
Both primary and backup protection systems are vital for ensuring the reliability and stability of power systems. While primary protection provides immediate fault clearance, backup protection ensures fault isolation if the primary system fails.