Single-phase vs three-phase

The difference between single-phase and three-phase reclosing is that in single-phase reclos- ing, tripping and reclosing is performed only to the faulted phase. In single-phase reclosing, electrical power can be transfered through the non-faulted phases while the faulted phase is

open. This leads to decrement is rotor-angle drift rate between the synchronous machines and increases the stability margin of the system. The only major problem is affection of the ground protection of the system due to circulation of the ground currents.

During fault clearance in a single-phase reclosing, due to capacitive an also inductive cou- pling of the phases, there is an induced voltage in the de-energized line. The magnitude of this voltage depends on the mentioned capacitance values and therefore, on the transmission line’s configuration and physical dimensions. This voltage may causes the arc to stay on for a longer time period while the faulted line is isolated. In this case, the arc is called the secondary arc.

The main purpose of considering the reclosing time-delay is to give enough time to the sec- ondary arc to be extinguished and therefore, to avoid reclosing-onto-fault and arc restriking. The amount of this time-delay, called the dead-time, is chosen based on the system analysis. One of the differences between single-phase and three-phase reclosing schemes is the differ- ence in the dead-time which is considerably shorter in three-phase reclosing.

In some cases of single-phase reclosing, the amount of the induced voltage on the secondary arc is really huge which can make the arc extinction time very long or even infinity. In such cases, one practical method for decrement of the arc extinction time is to compensate the line capacitances and therefore, to reduce the amount of the induced voltage. This is basically done using for-legged shunt reactor, i.e., a three-phase reactor with a neutral reactor. Another possible method is to close and reopen the grounding switches installed on the faulted phase in a fast manner.

In some special power system substation designing methodologies, single-phase tripping/re- closing and three-phase tripping/reclosing are considered independent and non-related. In such designs, it is the role of the protection system ro recognize the fault type, i.e., single-phase or multi-phase and also to introduce the faulted phase(s). In general, three-phase tripping (with no reclosing) are applicable when:

- A single-phase-to-ground fault is followed by an unsuccessful reclosure, - There is a multi-phase fault,

- There is an abnormal delay in performance of single-phase reclosure task while the fault is recognized as single-phase-to-ground,

- More phases become involved in the fault while the main faulted phase is isolated.

2.4

Protection System Operation During Reclosing

When the reclosing task is considered for the protection system, the sequence of operations can be listed as 1- single-phase isolation of the faulted phase and 2- reclosing of the open phase. If the reclosing process is successful, it means the isolated phase is back to operation, success- fully, and the normal performance of the power system is expected afterwards. However, if the reclosing is not successful, the next action is either restarting the process by isolating the faulted phase for the second time or refusing the reclosing for the second time by three-phase tripping of the breakers.

Sequence of operation of the protection system during a successful reclosing is shown in Figure 2.1 [74]. As obsrved, there is a delay for operation after the fault instant, called oper- ating time which represents the fault detection delay. Afterwards, another delay is considered, intentionally, called dead time. This time duration consists of 1- opening time, i.e., the time needed for the contacts of faulted phase to separate, 2- arcing time in which the secondary arc fed by the charge of the line and also by the non-faulted phases becomes extinguished and 3- waiting time for reclosing. After the dead time, reclosing command is issued and after a delay which is for the closing time of contacts, the system becomes energized and goes to normal operating mode. The typical value for the dead time setting varies between half a second to one second [3].

Figure 2.1: Sequence of operation of the protective relay during a successful reclosing [74].

Reclosing can be unsuccessful due to permanent fault inception of reclosing before the secondary arc extinction. If the reclosing is unsuccessful, the sequence of operations is more complicated. As shown in Figure 2.2, the sequence of an unsuccessful reclosing is similar to the successful one until the first reclosing. Then, as the fault is not cleared yet, the whole procedure may become reset by starting from the beginning and energizing the trip coil and trying the whole process for couple of times. If any of the reclosing trials are successful, the system can go back to normal operation. However, the number of trials is limited and finally, the last action will be to issue the three-phase trip signal and to de-energizing the whole line.