Adaptive autoreclosure to increase system stability and reduce stress to circuit breakers

Automatic reclosure is a key element in the concept of self-healing grids. According to statistics, a large amount of faults in transmission and distribution networks are temporary faults. These faults disappear a certain time after deenergization of the faulted sections of the network. Automatic reclosure is used to recover the original status of the network without any human interaction. Automatic reclosure can be done as a three pole autoreclosure or a single pole autoreclosure. During a three pole autoreclosure the line will be de-energized completely for all three phases even for a single phase to ground fault. A single pole autoreclosure only de-energizes the faulted phase of a single phase to ground fault. Single pole autoreclosure increases the system stability because during the single pole dead time energy can be transmitted through the two healthy phases. Another advantage is that a single pole autoreclosure does not require a synchrocheck before reclosing because both ends of the line are still synchronized through the two healthy phases. If the voltage transformers are located on the line side of the circuit breaker, autoreclosure with an adaptive dead time can be used. In this case autoreclosure with a fixed dead time is executed at one end of the line only. The other end only gives a close command to the breaker if the voltage measurements indicate that the line was successfully re-energized from the remote end. Otherwise if the fault persists the circuit breaker at this position is not reclosed and is not unnecessarily stressed as a result. Single pole autoreclosure can lead to the occurrence of a secondary arc. During the single pole dead time capacitive and inductive coupling induces a voltage into the open phase conductor and feeds the secondary arc. The success of the single pole autoreclosure depends on the extinction of this secondary arc. This paper reviews methods to detect the extinction of a secondary arc by monitoring the voltages based on a set of real faults that occured at a German TSO. If the secondary arc disappears, an auto reclose command will be issued. Thereby the dead time can be reduced. If a permanent fault occurs, auto reclosure will be blocked and a three pole trip will be initiated to reduce stress to the circuit breaker. Under extreme weather conditions line swinging can cause an increasing number of phase to phase faults. These faults are mostly flash-arcs between two wires. This paper also describes an approach used in Germany, Poland and Austria to clear such phase to phase faults without ground by the means of a single pole autoreclosure. Also in this case voltage measurements during the single pole dead time can predict whether or not a reclosure will be successful.