Protective Relaying Concepts

 

A zone of protection in electrical system protection refers to the area or segment of an electrical power system that is protected by a particular protective relay. The protective relay is designed to detect abnormal conditions, such as overcurrent, overvoltage, underfrequency, or faults, within its designated zone of protection and send a signal to the circuit breaker to isolate the faulty section of the system.


Related Article: Fundamentals of Protective Relaying


The concept of the zone of protection is critical for ensuring the reliability and safety of the power system. By limiting the protected area, the protective relay can quickly and accurately detect and isolate faults, thus minimizing the impact on the power system and preventing the fault from spreading to other parts of the network.

Example of Zones of Protection for 3 Circuit Breakers


In power systems, all power system elements must be encompassed by at least one zone.

  • The more important elements must be included in at least two zones. 
  • Zones must overlap to prevent any element from being unprotected.
  • The overlap must be finite but small to minimize the likelihood of a fault inside this region. 
  • A zone boundary is usually defined by a Current Transformer and a Circuit Breaker.
  • The Current Transformer provides the ability to detect a fault inside the zone.
  • The Circuit Breakers provide the ability to isolate the fault.


Depending on the type of protection, the zone of protection can be either primary or backup protection. Primary protection is designed to provide fast and selective protection within a specific zone, while backup protection is designed to provide additional protection in case the primary protection fails. Overall, the proper selection and coordination of protection relays and zones of protection are essential for ensuring the reliability and safety of the power system.


Closed and Open Zones

In electrical power system protection, the terms "closed "and "open" zones of protection refer to the different methods of defining the boundaries of a protective relay's zone of protection. 


Closed and Open Zone


A "closed" zone of protection means that the boundaries of the protected zone are determined by the physical layout of the power system. For example, a protective relay may be installed at the end of a transmission line, and its zone of protection would cover the entire length of the line up to the next protective device. This is a closed zone because the boundaries are defined by the physical limits of the transmission line, and there is no flexibility in changing the zone's size or location.


Related Article: Protection Relays in Power System


In contrast, an "open" zone of protection means that the boundaries of the protected zone are not determined solely by the physical layout of the power system but rather by the settings of the protective relay. The zone of protection can be set to cover any part of the system, including multiple transmission lines or parts of a substation. An open zone provides greater flexibility in designing the protection scheme and allows for adjustments to be made based on changes in the power system configuration or operation.


Primary and Back Up Protection Zone

Primary protection is the first line of defense against faults in the power system. It is designed to provide fast and selective protection within a specific zone to quickly detect and isolate the fault before it causes damage to the equipment or poses a risk to the system's stability. Primary protection is typically applied to the most critical parts of the power system, such as generators, transformers, and transmission lines. The boundaries of the primary protection zone are typically closed and defined by the physical layout of the power system.


Zoning Example

Backup protection is a secondary layer of protection that provides additional protection in case the primary protection fails to detect and isolate the fault. Backup protection is designed to cover a wider area than primary protection and is usually applied to less critical parts of the power system. The boundaries of the backup protection zone can be open or closed, depending on the type of protection scheme. 

Primary and Backup protection in the same location 


In some cases, the backup protection system may open more circuit breakers than necessary to clear the fault, which can cause additional disruptions to the power system. Backup protection systems are also used to provide primary protection when the usual primary equipment is out of service. Overall, the proper coordination and setting of primary and backup protection systems are crucial to ensure that the power system is protected in the event of a fault or abnormal condition.


Primary and backup protection in different locations 


The coordination between primary and backup protection is crucial to ensure that the protection scheme is reliable, selective, and fast. The protective relays must be properly coordinated to ensure that the backup protection does not trip unnecessarily or cause any delays in clearing the fault. The coordination also includes setting the time delays and thresholds of the primary and backup protection to avoid conflicts and ensure that the protective scheme is both effective and efficient.


Example 1: Local Backup Protection


For fault at F1: Suppose that breaker J operate, and breaker H failed to operate. The breakers G and I must act as local backup protection. 


Example 2: Local Backup


For fault at F2: Suppose that breakers P and Q operate, and breaker M failed to operate. The breaker L must operate as local backup protection and I should be tripped by communication signal (Transfer Trip)


 Remote Backup Protection

Remote backup protection is a type of backup protection that is designed to provide additional protection for a specific zone in the power system from a remote location. This type of backup protection is typically used for transmission lines or areas where there is a need for extra protection against faults or abnormal conditions that cannot be adequately covered by the primary protection system.


Remote Backup Protection

Remote backup protection works by detecting faults in the protected zone using a communication link, such as a pilot wire or fiber optic cable, to send signals to a remote protective relay. The remote protective relay then initiates the backup protection system to clear the fault, typically by tripping the appropriate circuit breakers. 


Example 3: Remote Backup


For fault at F1: Suppose that breaker J operate and breaker H failed to operate. The  circuit breakers E, F, L and M must operate as Remote backup protection.


The remote backup protection systems also have some limitations. They are typically slower to operate than local backup protection systems due to the communication delays between the protection equipment and the remote protective relay. Remote backup protection systems also require careful coordination and setting to avoid any conflicts with the primary protection system and ensure proper selectivity and performance. 

As power systems continue to grow and become more complex, it is important to maintain and improve the protection schemes to ensure the safe and efficient operation of the power system. The proper selection, coordination, and setting of protection relays and zones of protection are essential for ensuring the reliability and safety of the power system. Both primary and backup protection systems play a critical role in detecting and isolating faults to prevent damage to equipment and maintain power system stability. 

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