Automatic Reclosing Scheme in Transmission Systems

Automatic Reclosing Scheme for Transmission Systems

The automatic reclosing system is a series-connected network designed to reduce operating costs and enhance network reliability. Extra-high voltage (EHV) transmission lines are used to transmit large amounts of power, on the order of thousands of megawatts (MW), and therefore should not be interrupted at all costs. Although faults on these overhead lines are common, the power transmitted through them should not be interrupted for long periods due to temporary or permanent faults.

Transient faults such as fallen trees, lightning strikes, or bird contact on overhead lines can clear themselves automatically without requiring corrective action. In contrast, permanent faults—such as conductor breakage or insulator failure—cannot be restored quickly, and during such events, automatic reclosing is ineffective. When manual reclosing is used, the operator must reset the relay and close the circuit breaker. If the fault is transient, the line remains stable after the second closure; however, if the fault persists, the protection system trips the circuit again and classifies it as a permanent fault. During transient faults, manual reclosing introduces significant delays.

Given that EHV transmission lines carry large amounts of power, any operational delay can lead to substantial system losses from both cost and stability perspectives. To avoid such delays caused by manual intervention, automatic reclosing schemes are introduced into EHV transmission systems, eliminating unnecessary human-induced delays. Reclosers help manage these faults by dividing the network into smaller segments (sectionalizers), as shown in the figure. Reclosers are programmed to automatically perform the reset process, enabling a more robust service restoration approach. As a result, supply availability is increased.

Main Purposes of Automatic Reclosing Systems:

1.Reduce power supply interruptions to consumers

2.Improve supply continuity

3.Reduce substation visits

Transmission line faults can be categorized into three types:

1.Transient faults: These are short-duration (temporary) faults. For example, a lightning strike on a transmission line causes overvoltage, which is suppressed by various devices within a very short time and then clears automatically. Transient faults account for approximately 80% to 90% of overhead transmission line faults.

2.Semi-permanent faults: These faults persist for one or more arc cycles. For instance, a tree contacting a live phase conductor creates a ground arc. The arc persists for several seconds until the tree is burned away, after which the fault clears automatically. This type of fault occurs in 5% to 8% of cases.

3.Permanent faults: These result from conductor breakage, insulator failure, or any electrical equipment malfunction, causing a permanent fault on the transmission line. Restoration is not possible until the damaged components are replaced or repaired.

The recovery time for the first two types of faults can be significantly shortened using automatic reclosing schemes. An automatic reclosing system includes high-speed operating contacts and solid dielectric insulation materials, along with vacuum interrupters for current interruption and arc extinction, and advanced current and voltage sensing devices. In an automatic reclosing scheme, if the first attempt fails to clear the fault, two or three reclosing attempts are made until the fault is cleared. If the fault persists, the system permanently opens the circuit breaker. A specified time delay can be applied to the automatic reclosing system to allow semi-permanent faults to clear from the circuit.

Factors Affecting Automatic Reclosing Schemes

Key factors influencing the selection of dead-time in reclosing include recovery time and the number of reclosing attempts. The factors affecting the choice of system dead-time are as follows:

1.System stability and synchronism

2.Load type

3.Circuit breaker (CB) characteristics

4.Deionization time of the fault path

5.Protection relay reset time

For high-speed reclosing, synchronism check is not required at the time of reclosing. However, for delayed reclosing, synchronism must be checked before reclosing, typically achieved using a synchronism relay.