Auto-reclosing in transmission lines refers to the automatic reopening of breaker contacts after a fault has caused the system to be momentarily isolated. This feature is designed to help quickly restore power after a temporary or semi-permanent fault, which are types of faults that are of short duration and often result in power outages.
Faults on transmission lines can be broadly categorized into three types: transient, semi-permanent, and permanent. Transient faults are the most common type of fault and are typically caused by lightning strikes or other temporary disruptions. Semi-permanent faults are similar to transient faults but tend to last longer. Permanent faults, on the other hand, are caused by serious issues such as equipment failure or damage, and require manual intervention to fix.
Auto-reclosing is designed to quickly clear temporary and semi-permanent faults, which account for around 70-80% of the faults that occur on overhead transmission lines. When a fault is detected, the breaker initiates a tripping operation, which isolates the faulty portion of the line. Then the breaker performs a reclosing operation, which restores power to the line. If the fault is temporary or semi-permanent, it will be cleared in these attempts, and the power will remain restored.
However, if the fault is permanent, the auto-reclosing mechanism will not be able to clear it. In this case, the breaker will trip and reopen multiple times, but will not be able to restore power. After a certain number of attempts, the auto-reclosing feature will stop trying to clear the fault, and the breaker contacts will remain open until a manual reclosing is performed. This manual reclosing is done when the permanent fault is cleared.
The use of auto-reclosing can enhance stability margins and overall system stability by quickly restoring power after temporary faults, but it can be detrimental if used in the presence of a permanent fault, since it will prolong the outage and may cause additional damage. Thus it is implemented selectively, only for temporary and semi-permanent faults and not for permanent faults.
Types of Auto-reclosing Schemes:
Auto-reclosing schemes in transmission lines are classified based on the number of tripping and reclosing operations that are performed. The different types of auto-reclosing schemes that are employed in transmission lines include:
Single-shot auto-reclosing:
Single-shot auto-reclosing refers to the process of automatically re-closing a circuit breaker, but only once. The re-closure is typically done as soon as possible after a fault occurs, in order to minimize the time that the line is de-energized. This type of auto-reclosing scheme is typically used in high voltage (EHV) transmission lines, where there is a likelihood of temporary or permanent faults but not semi-permanent faults.
A temporary fault is a type of power system fault that is expected to be cleared quickly, often within one re-closure operation. For example, a temporary fault might be caused by a tree branch falling on a power line. When a temporary fault occurs, the circuit breaker will trip and de-energize the line. But as soon as the fault is cleared, the single-shot auto-reclosing mechanism will re-close the circuit breaker and re-energize the line.
A permanent fault, on the other hand, is a fault that is not expected to be cleared and will require repair or replacement of the equipment. When a permanent fault occurs, the circuit breaker will trip and de-energize the line. Unlike with temporary faults, the single-shot auto-reclosing mechanism will not attempt to re-close the circuit breaker in this case, as there would be no point in doing so.
Semi-permanent faults refer to faults that stay for some time and cleared by other means.
However, in case of EHV transmission lines, such faults are less frequent due to the height at which the transmission system is installed.
In the case of protection of EHV lines at both ends, both the circuit breakers must be tripped and reclosed simultaneously in order to ensure that the line remains de-energized for a specified duration. This is important to ensure that the line remains disconnected and de-energized until the fault is cleared, and to ensure that the line is not re-energized while the fault is still present.
This increases the safety of the workers and protection of the equipment.
Multi-shot auto-reclosing:
Multi-shot auto-reclosing refers to the process of automatically re-closing a circuit breaker multiple times within a predetermined sequence. This type of auto-reclosing scheme is typically used for low and medium voltage transmission lines, where there is a likelihood of semi-permanent faults, which are faults that persist for some time and can be cleared after multiple re-closing operations.
When a semi-permanent fault occurs, the circuit breaker will trip and de-energize the line. The first re-closure is typically done as soon as possible, in order to minimize the time that the line is de-energized. If the fault is not cleared by the first re-closure, the multi-shot auto-reclosing mechanism will attempt to re-close the circuit breaker again after a few seconds, and again after 1 or 2 minutes if the fault is still present.
If the fault persists even after three re-closing operations, it is considered to be a permanent fault and will require manual intervention. In this case, the staff will have to detect and repair the fault before the circuit breaker can be re-closed.
Since the Multi-shot auto-reclosing scheme involves multiple tripping and re-closing operations, it can cause wear and tear on the circuit breaker. Vacuum circuit breakers, which are relatively maintenance-free, are well-suited for this type of scheme.
In summary, Multi-shot auto-reclosing is used in the case of low and medium voltage transmission lines with voltage of 33 kV or below. This scheme can clear semi-permanent faults and increase the availability of supply by providing re-closure in stages. However, it increases the wear and tear of the circuit breaker and hence vacuum circuit breaker is mostly preferred.
Single-phase or single-pole auto-reclosing:
Single-phase or single-pole auto-reclosing refers to the ability of a circuit breaker to automatically re-close one pole or phase of a circuit breaker, using a specialized mechanism for single-phase-to-ground faults. In this scheme, each pole of the circuit breaker has its own tripping and closing mechanism, allowing for more precise fault detection and isolation.
For the implementation of this auto-reclosing scheme, a complex relaying scheme is required to detect the faulty phase. During multi-phase faults, the relaying scheme trips all three phases, and auto-reclosing will be done for all three phases. However, during single-phase-to-ground faults, the relaying scheme will trip only the faulty phase, allowing the other two phases to remain energized.
This type of auto-reclosing scheme is typically used for interconnectors, where two different power systems are being interconnected by a single tie line. During a single-phase-to-ground fault, if only the faulty phase is tripped, then the synchronizing power can still flow through the two healthy phases, minimizing the impact of the fault on the power system.
However, Single-phase auto-reclosing scheme has some drawbacks. One of the main drawbacks is the high cost associated with the design and implementation of the complex relaying scheme. Additionally, the scheme suffers from a problem of large deionization time, which is the time it takes for the arc to be fully extinguished. This is due to the fact that when only the faulty phase is isolated, the capacitive coupling between the healthy phases and the faulty phase tends to maintain the arc, resulting in a longer deionization time.
In summary, Single-phase auto-reclosing is a scheme that is used when a tie line connects two power systems, it allows isolating the faulty phase while keeping the other phases energized, reducing the impact of faults. However, it is expensive due to the complexity in design and the problem of large deionization time.
Three-phase auto-reclosing:
Three-phase auto-reclosing refers to the process of automatically re-closing all three phases of a circuit breaker at the same time. This type of auto-reclosing scheme is simpler and less expensive than the single-phase auto-reclosing scheme because it does not require a complex relaying scheme to detect the faulty phase. Instead, it simply relies on the detection of a fault and the subsequent tripping of all three phases.
For the implementation of this auto-reclosing scheme, a simple relaying scheme is required to detect the fault and trip all three phases. Additionally, the design of the circuit breaker is also simpler as it is not required to have individual tripping and closing mechanisms for each pole.
Because the relaying scheme does not have to detect the faulty phase, and the design of the circuit breaker is simpler, the cost of this auto-reclosing scheme is generally lower than that of a single-phase auto-reclosing scheme. Furthermore, three-phase auto-reclosing has the additional advantage of having less deionization time. Since all three phases are tripped simultaneously, there is less capacitive coupling between the healthy phases and the faulty phase, which results in less time required for the arc to be fully extinguished.
In summary, Three-phase auto-reclosing refers to a scheme where all three phases are tripped and reclosed together. It is a simple and cost-effective scheme that requires a simple relaying scheme, and is less expensive than single-phase auto-reclosing. It also has less deionization time compared to single-phase auto-reclosing.
Delayed auto-reclosing:
Delayed auto-reclosing refers to the process of automatically re-closing a circuit breaker after an intentional time delay. This time delay is generally longer than the time delay used in high-speed auto-reclosing schemes, which reduces the problems associated with deionization and power swings. In this scheme, all three phases are tripped and re-closed simultaneously.
This type of auto-reclosing scheme is typically used for interconnectors, where two different power systems are being interconnected by multiple tie lines. The reason for this is that, in such systems, there is less likelihood of losing synchronism or deviation of the phase angle between the two power systems.
The reason for this is that the longer time delay between the tripping of the circuit breaker and the re-closing allows the power system to stabilize and reduces the chances of power swings. Additionally, the longer deionization time also reduces the chances of problems caused by the re-establishment of the arc after the circuit breaker is re-closed.
This scheme also eliminates the chances of losing synchronism between the two power systems which might happen if all three phases are tripped simultaneously and re-closed with high speed as it would cause a deviation of the phase angle of both the power system voltages.
In summary, Delayed auto-reclosing refers to a scheme where all three phases are tripped and reclosed together, but after an intentional time delay. This delay is longer than that of high-speed auto-reclosing and eliminates issues such as deionization and power swings. It is mainly used in interconnectors where two power systems are interconnected through multiple tie lines, which minimizes the chances of losing synchronism and deviation of phase angle of both power systems.
Advantages of Auto-recloser:
Auto-reclosing circuit breakers have several advantages that make them an important component in power systems. Some of the key advantages include:
- Stability margins: Auto-reclosing circuit breakers enhance the stability margins of a power system by automatically re-closing the circuit breaker after a fault occurs. This minimizes the impact of temporary faults on the power system, which helps to maintain stability and prevent cascading failures.
- Reliability: Auto-reclosing circuit breakers improve the reliability of a power system by automatically re-closing the circuit breaker after a fault occurs. This reduces the need for manual intervention, which can lead to more consistent and reliable power delivery.
- Reduced outages: Auto-reclosing circuit breakers reduce the duration of outages caused by temporary faults. They minimize the time that the power system is de-energized, which helps to keep power disruptions to a minimum.
- Remote locations: Auto-reclosing circuit breakers provide a greater advantage in remote locations. They eliminate the need for operators to physically go to the site to re-close the circuit breaker after a fault occurs, which can be difficult or impossible in remote locations.
- Radial feeders: Auto-reclosing circuit breakers can be particularly useful in the case of radial feeders, where the continuity of supply is difficult to maintain under fault conditions. By automatically re-closing the circuit breaker after a fault occurs, auto-reclosing circuit breakers can help to reduce the duration of power disruptions and maintain continuity of supply.
In summary, Auto-reclosing circuit breakers are an important component of power systems, as they enhance the stability margins, improve reliability, reduce the duration of outages, and can be particularly useful in remote locations. Additionally, in radial feeders, it helps to maintain the continuity of supply under fault conditions.
Limitations of Auto-recloser:
Auto-reclosing circuit breakers, while providing many benefits, also have some limitations. Some of the key limitations include:
- Permanent faults: Auto-reclosing circuit breakers are not suitable for permanent faults. A permanent fault is a fault that persists even after the circuit breaker is re-closed multiple times. In this case, manual intervention is needed to detect and repair the fault before the circuit breaker can be re-closed.
- Reclose lockout: Auto-reclosing circuit breakers can get locked after a pre-defined number of trips, which is known as a reclose lockout. This is a safety feature that is intended to prevent the circuit breaker from repeatedly attempting to re-close when there is a permanent fault present. In order to unlock the auto-recloser, a manual reset is required by service personnel. It has been standardized by the manufacture that the auto-recloser goes into the lock-out stage after 4 trips.
- Defective element: If a defective auto-recloser element is present, it can affect the entire function of the auto-recloser. This can lead to an inability to automatically re-close the circuit breaker, or an increased likelihood of a reclose lockout. This requires the defective element to be replaced and the whole auto-recloser to be maintained by the service personnel.
In summary, Auto-reclosing circuit breakers have limitations in their ability to handle permanent faults, may get locked after pre-defined trips, this is called reclose lockout, and defective elements may affect the entire function of the auto-recloser. These limitations have to be considered in the design and maintenance of the power system.