The art of fault clearance in transmission systems: The logic of main and backup relays

The Art of Fault Clearance Protection

The protection and fault clearance requires great attention. In terms of fault clearance protection, we categorize the relays into main protection relays and backup protection relays. The main protection relay is installed at all primary equipment, and it is essential that the relay for the equipment experiencing the fault operates with the highest speed.

The primary function of fault clearance relays is to minimize equipment damage and isolate the faulty component from the rest of the power system.

Consequently, it is essential for fault clearance relays to operate at high speeds to promptly address faults, while also ensuring sensitivity and selectivity to reduce the impact of the fault.

Table of Contents:

1. Where and Why are Fault Clearance Relays Used?
2. What’s Required of Protective Relays to Operate Properly?
3. Critical Factors Affecting the Operation of Relays:
   1. Protective Zone (Performance Selectivity)
   2. Arrangement of Current Transformers (CTs)
   3. Main Relay Protection
   4. Backup Relay Protection:
      1. Why is Backup Protection Required?
      2. Classification of Backup Relays:
         1. Local Backup Relay
         2. Remote Backup Relay
         3. Circuit Breaker Failure Protection (CBF)
         4. Bus Coupler Sequential Splitting
      3. Circuit Breaker Failure Protection (CBF)
      4. Bus Coupler in Double-Bus Arrangement Substations
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1. Where and why are fault clearance relays used?

Figure 1 depicts the function of the fault clearance relays and special protection schemes. The red crosses “X” in this figure indicate the prevention of fault extension through the proper functioning of the protection relays.

In the event that a protection relay fails to function or a circuit breaker does not operate, backup relays will engage to mitigate the risk of fault propagation.

The criteria for implementing special protection schemes are dependent upon the configuration of the system and/or the operational criteria of the system.

Figure 1 Functional Diagram of Protection Relays

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2. What’s Required of Protective Relays to Operate Properly?

Here are the three main points about the protective relays’ requirements:

1. Fault clearance performance in respect of:
   a) Speed;
   b) Selectivity;
   c) Sensitivity;
2. Reliability; and
3. Cost

The high-speed operation of the protective relays protects the power system against excessive damage and instability. Protection relays with high selectivity permit little faulty-zone separation. Sensitivity refers to the ability to identify the fault at its lowest level.

There are three techniques to increasing the reliability of protection:

1. High security
2. Duplication and Redundancy
3. Self-diagnostic functions

A relay’s reliability can be defined as the assurance that it will function successfully at the time that it is expected to. The capacity of the protection relay to not run when it is not required to operate is what constitutes security by default. Even in the event that one of the relays fails, the dependability of fault clearance will be improved through the utilization of redundant and duplicated equipment.

It is necessary to take into account the cost in relation to the level of redundancy, the utilization of backup, and the entire life cycle cost over the duration of the substation’s operation.

In general, numerical type relays have high performance self-diagnostic functions and remote alarms that have the potential to significantly reduce their Mean Time to Repair (MTTR). This is in contrast to earlier technologies, which may have caused the device to remain in a non-operational state until its next scheduled test, or even worse, until the next time a fault occurs and it fails to operate correctly.

In the process of introducing a new protection system, these many factors are investigated, and the specifications for each system are determined in accordance with the requirements of the power system.

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3. Critical Factors Affecting the Operation of Protection Relay

Not only is it required to select protection devices that have the proper inherent performance, but it is also necessary to select the protection scheme while taking into consideration the requirements of the power system. These criteria include the significance of the system that needs to be protected as well as the constraints that the system places on its operation.

In order to successfully integrate the protective relay into a power system, it is necessary to consider a number of performance factors, including sensitivity, operation time, and selectivity, as depicted in Figure 2.

Figure 2 – Consideration Factors in Applying Protection Scheme

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3.1 Protective Zone (Performance Selectivity)

The protective zone is a critical factor that defines the selectivity of performance in which a protection relay is necessary. Unit protection establishes a protective zone based on the positioning of the current transformers (CTs) to identify faults exclusively inside the prescribed area between the CT locations, in accordance with the sensitivity of the settings.

Non-unit protection, omitting directional comparison, includes a zone that varies based on the setting values and is not limited by the positions of other current transformers.

Conversely, when a fault arises in equipment outside the protective zone, it may remain unaddressed or need an extended duration for the fault to develop into the protective zone before being resolved.

A fundamental premise of protection is that a minimum of two distinct devices must be able to detect any malfunction inside the power supply.

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3.2 Arrangement of Current Transformers (CTs)

Figures 3, 4 and 5 illustrate three types of CT arrangements. This illustration depicts the configuration of current transformers for line protection and busbar protection as a case study. The third configuration is typically regarded as the optimal layout, as it eliminates any dead zone for fault detection between the current transformers and the circuit breaker, unlike the initial two configurations.

Nonetheless, each must be evaluated concerning the type of current transformer to be utilized (e.g., bushing or post) or the available substation space.

In configuration depicted in Figure 3, a current transformer for busbar protection and a current transformer for line protection are positioned on the line side of the breaker. The busbar protection will activate for a fault occurring between the current transformer and the circuit breaker, which is effectively a line fault, resulting in a section of the busbar being de-energized.

Nevertheless, since the issue is not entirely resolved by the busbar protection, it will also require involvement from the remote backup protection.

Figure 3 – CT Arrangement: CTs are installed on the line side from the CB

In arrangement depicted in Figure 4, current transformers for busbar protection and current transformers for power line protection are positioned on the busbar side of the circuit breaker. In this configuration, when a fault occurs between the current transformer (CT) and the circuit breaker (CB), initially a busbar fault, the line protection will operate, resulting in the line being out of operation.

Nonetheless, as the fault is not entirely cleared by the line protection, it must additionally be cleared by the remote backup protection or locally by the CBF, if implemented.

Figure 4 – CT Arrangement: CTs are installed on the bus side from the CB

In arrangement depicted in Figure 5, the current transformers (CTs) are positioned on both sides of the circuit breaker (CB); specifically, the CT for line protection is installed on the busbar side of the CB, while the CT for busbar protection is installed on the line side of the CB.

Both the line protection and the busbar protection will operate in response to the fault occurring between the CT and the CB, resulting in the high-speed clearance of the fault depicted in the above Figures 3 and 4.

Figure 5 – CT Arrangement: CTs are Line installed on both sides of the CB

Where:

• LP stands for Line Protection, and
• BP stands for Busbar Protection

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3.3 Main Relay Protection

The main relay protection is typically installed for each equipment unit, including transmission lines, busbars, transformers, and so forth. Figure 6 illustrates the protective zones established by various main protection relays installed to protect each segment of the power system. The protective zone of the main relay illustrated in this figure applies to the scenario where current transformers are installed on both sides of the breaker.

In the event of a fault within the power system, it is essential for the main protection mechanism nearest to the fault to operate faster than other protections. This rapid response is crucial to limit the portion of the power system that needs to be disconnected in order to clear the fault. The protection system should operate the circuit breakers at the boundary of the protection zone to isolate the fault while minimizing impact on the overall power system.

Given that the protection zones need to overlap, it is essential to evaluate the method of achieving selectivity to prevent both zones from tripping simultaneously, as illustrated in Figure 6.

The redundant system addresses the potential shortcomings of the relay’s internal fault detection mechanism, which may fail to identify faults due to variations in characteristics or algorithms, physical malfunctions of the relay, or an open circuit in the trip circuit leading to the breaker.

This configuration ensures reliable protective operation across various scenarios, except in the event of a malfunction in the breaker mechanism itself, in which case Circuit Breaker Fail protection is implemented as detailed in the next section on Backup Protection.

Duplicate protection is consequently implemented through:

• 2 independent CT cores at the same location
• 2 VT signals on independent circuits from the VT although may be derived from the same VT core due to the difficulty of duplicating VT posts
• 2 relays of different operating principles or vendors – e.g. distance and differential, two distance from different vendors or two differential relays from different vendors
• 2 independent trip coils in a common circuit breaker

Figure 6 – Protective zone for main protection in case of installation of CTs on both sides of the breaker (click to zoom)

* Dotted square means “Protective Zone” for Main Protection

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3.4 Backup Relay Protection

3.4.1 Why is Backup Protection Required?

The backup protection is crucial for clearing a fault and preventing its escalation, particularly in scenarios where the main protection, circuit breaker, or voltage/current transformers have not operated as intended. Consequently, the backup protection MUST function in the following manner:

1. In case of the occurrence of a fault within the protective zone if the main protection operation is blocked.
2. In case of failure in operation of the main protection due to some reason (failure/settings/characteristic etc.).
3. In case of the occurrence of a particular type of fault that the main protection cannot detect, for example, the transverse differential protection can’t detect a simultaneous phase-to-earth fault on both lines because there is no zero differential current.
4. In case of erroneous inputs to the relay because of the VT transients, or the CT saturation or failure, or failure of the breaker.
5. In case of a fault on busbar without busbar protection.

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3.4.2 Classification of Backup Relays

Figure 7 illustrates four types of backup relays.

Figure 7 – Classification of backup relays

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3.4.2.1 Local Backup Relay

The local backup relay is installed within the same substation and operates when the main relay fails to operate to a fault condition. The operating time of these relays is typically slower than that of the main protection, which may be explained by varying operating characteristics or the grading between the relays.

An example is an instantaneous overcurrent relay with local backup provided by utilizing an inverse time overcurrent relay. This relay may be connected to the same current transformer (CT) or on a different circuit, such as the incomer to the substation, using different CTs. This configuration allows both types of relays to detect the fault, with one designed to operate faster.

This also differs from the application of Circuit Breaker Fail protection outlined below.

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3.4.2.2 Remote Backup Relay

The remote backup relay is usually installed at the remote substation mainly to protect its own power substation and the associated power line. Additionally, it is designed to detect faults that would typically be cleared by the local protection relays. The remote back up relay will therefore have a slower operating time for faults in the local substation but will operate if the local protection system (relay and breaker) fail to clear the fault.

It is commonly accepted that this remote backup protection is completely autonomous in its operation from the local protection. This means that the backup protection does not require any signals from the local substation in order to operate or prevent operation.

A distance scheme or an overcurrent scheme is widely used as backup protection, which can detect an internal fault by inputting the electrical quantities from its own remote end in order to determine the slow or non operation of the main protection at the local end.

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3.4.2.3 Circuit Breaker Failure protection (CBF)

Circuit Breaker Failure protection (CBF) is installed to ensure local backup functionality in the event that a circuit breaker does not operate when a trip command is received. CBF presumes that the local protection has functioned as intended and has made an effort to operate the circuit breaker; however, the circuit breaker has malfunctioned, potentially due to an open circuit between the relay and the trip coil or a failure within the circuit breaker mechanism itself.

The details of CBF protection are outlined in the following section.

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3.4.2.4 Bus coupler sequential splitting

Bus coupler sequential splitting involves the operation of a bus-coupler splitting relay to isolate the bus-tie, effectively separating a faulty busbar from a healthy one. This action is crucial when a main protection relay or breaker fails to operate, or in the event of a severe fault. This reduces the blackout range, thereby lessening the impact of a system fault and enhancing the stability of the power system.

A splitting relay must function with greater speed than other backup relays, ensuring the system is isolated effectively. This prevents the fault’s impact from propagating to a higher voltage class system or other operational sections of the substation. It is occasionally installed within a transformer or at an interconnection point.

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3.4.3 Circuit Breaker Failure Protection (CBF)

When the protection relay operates as intended for a fault, yet the breaker fails to open, the power system could experience instability because the fault remains until additional slower or delayed backup protection detects the issue and operates. The circuit breaker failure protection (CBF) operates by activating the relay nearest to the fault, which subsequently triggers a trip to its circuit breaker.

The trip signal is used to initiate the CBF such that if the fault current is not cleared within a short time, the circuit breaker is assumed to have failed to operate either due to an open circuit in the trip circuit from the relay to the trip coil, or failure of the circuit breaker mechanism itself. The CBF will then initiate tripping of circuit breakers on adjacent lines in the substation or busbar, and possibly the remote end, which whilst increasing the extent of power system outage, will minimise the consequential damage of sustained or slow clearing faults.

The CBF is typically intended to operate through the same tripping circuits utilized by the busbar protection, as it is inherently set up to trip for adjacent faults. Consequently, the CBF indicates that the fault should be regarded as a bus protection trip. While this may result in the tripping of an entire bus section, it will effectively prevent the extensive operation of remote backup protection, which could lead to an even larger blackout.

Figures 8 and 9 illustrate the fault clearance method, comparing scenarios with and without CBF protection, specifically when the breaker CBB12 fails to open in response to the fault in Line BC1.

Figure 8 illustrates the power system’s behavior after a circuit breaker failure, occurring in the absence of breaker failure protection at the B substation. In this scenario, when the C end of Line BC1 is tripped by the main protection relay while the breaker at the B end remains closed, all remote backup protection relays at the distant ends of the lines linked to the B substation will operate.

Substation B will experience a blackout as a result of the breakers at Substation A opening for remote backup purposes.

Figure 8 – Effect of installation of breaker failure protection: Fault clearance by remote backup relays

Figure 9 shows the power system after the same fault but with the breaker failure protection at the B substation. The main protection at substation B and C operates as in Figure 8 with correct operation at the C end of Line BC1. In this case, the breaker failure protection recognizes as non-operation of the breaker at B-end due to the continuation of the fault current.

The breaker failure protection gives the information about the continuation of the fault to the busbar protection at substation B, and the breakers at the lines connected to the breaker at the B busbar and the bus-tie will open as minimize the blackout area.

Figure 9 – Effect of installation of breaker failure protection: Fault clearance by breaker failure protection

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3.4.4 Bus Coupler in Double-Bus Arrangement Substations

The application of bus coupler sequential splitting is typically used in double-bus arrangement substations. It is installed for the purpose of allowing continued operation of the healthy busbar by tripping the bus tie CB.

This isolates the faulty busbar in order to avoid a blackout of the whole substation when:

1. a line CB fails to operate for a line fault, or
2. the busbar differential protection fails for a fault on one of the two bus bars, or
3. when there is a fault on a busbar or section of it (blind spot) that is not equipped with busbar differential protection.

For short lines, it is essential to evaluate the reach of the distance relay to guarantee proper coordination (Refer to Figures 10 and 11). Depending on the substation, system topology, and generation scenario, satisfying all these potentially conflicting requirements can present significant challenges.

Figure 10 – Effect of installation of bus coupler sequential splitting: Fault clearance by remote backup relays

Figure 11 – Effect of installation of bus coupler sequential splitting: Fault clearance by system splitting relay

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Source: Protection Relay Coordination by Working Group B5.19