Cambio de sistema de referencia y ensayo de calibración

Experimental method of trial switching is commonly used in a traditional fault manage- ment as described in chapter 3.5. The trial switching method is utilized to locate the faulty disconnector zone if there are not enough variables to deduce the exact fault location or the fault location inference results more than one possible location. The principle of the trial switching is to divide the faulty feeder using disconnectors and close the substation circuit breaker against the suspected fault. If the circuit breaker trips, the fault is located

between the CB and opened disconnector, otherwise the fault is located behind the opened disconnector. By these means, procedure is repeated until the fault is located to a single disconnector zone. After the fault is located and isolated with remote-controlled operations, operator continues the trial switching by dispatching field crews to operate manual disconnectors. [12] The effectivity of an automatic trial switching method relies on the number of RCDs along the feeder and the reliability of the communication [39]. In a bi-section method, faulted feeder is divided into approximately two equally sized zones. Division of the feeder can be based on the length of the feeder, the number of customers or type of the line sections. For example, a rural part consisting of overhead lines, in which fault probability is higher, can be separated from the section in a town plan area. After the disconnector is opened, the circuit breaker in the primary substation is closed against the suspected fault. If the circuit breaker stays closed, the fault is lo- cated behind the opened disconnector. The operator continues to divide the feeder into smaller sections until the fault is located between two remote controlled disconnectors. [12] A simplified example of bi-section method is presented in Figure 22.

Figure 22. Bi-section method in simplified feeder

1. Circuit breaker trips due to permanent fault and the feeder is de-energized 2. Feeder is bisected using remote-controlled disconnector A and the circuit breaker

stays closed. Fault is located behind the RCD A.

3. Suspected fault area is bisected using remote-controlled disconnector B and RCD A is closed.

4. Circuit breaker trips and fault can be located between RCDs A and B. Supply can be restored via backup connection from the end of the feeder.

5. Remote-controlled disconnector C dividing zone between A and B is opened. Circuit breaker is closed.

6. Circuit breaker trips and fault can be located between RCDs A and C.

7. Remote-controlled disconnector A is opened, and fault is isolated for operator to handle. Supply can be restored to RCD zone C – B using the backup connection. Another method for trial switching is so called zone-by-zone rolling or step-by-step method. The basic principle of the zone-by-zone rolling is to energize the feeder one disconnector zone at a time. Usually all the RCDs of the faulted feeder are opened and then closed after another until the fault has been located. Likewise, in the bi-section method, the circuit breaker is closed against the suspected fault and the CB tripping indicates the faulty zone. Simplified zone-by-zone rolling method in straightforward feeder branch is described in the Figure 23.

Figure 23. Zone-by-zone rolling method

1. Circuit breaker trips due to permanent fault and the feeder is de-energized. RCDs in the branch are opened.

2. – 6. Remote controlled disconnectors are closed one by one towards down- stream until the upstream RCD of faulty zone is closed.

8. Faulty zone is isolated, and supply can be restored to feeder upstream from the substation and feeder downstream using open backup connection.

Disadvantage of the zone-by-zone rolling method is its long execution time in case of a long branching feeder including multiple disconnector zones. On the other hand, number of unnecessary tripping of the circuit breaker can be reduced. [12] While the bi-section method is faster but can include more network straining tripping of the circuit breaker, one possible solution is to combine these two methods. The first coarse sequence is performed using the bi-section method, if there are RCDs available in the faulty zone. After the feeder bisection, the zone-by-zone method is applied until the fault is located to the one RCD zone. [39]

There are certain disadvantages in the trial switching method. In case of a short circuit fault, resupplying the faulty feeder against the fault causes thermal stress in the conduc- tors and the circuit breaker. This is most harmful in the cabled network due to slow cool- ing time of the conductors. Therefore, number of trials must be limited or even disabled in some of the feeders. [41] Subsequent trial switching against the earth fault causes voltage strain between healthy phases and the ground. A single-phase earth fault can thereby develop into a cross-country earth fault and increase the extent of the faulty area. Overvoltage can also cause damage to network insulation and customer devices. [41] Also, the electrical safety of the working personnel along the network must be considered in the utilization of automatic trial switching. Operation of the trial switching sequence must be blocked to prevent re-energization when the field crew is operating in the faulty feeder near energized network equipment. [28]