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Unsymmetrical faults, such as single‐line‐to‐ground faults, on DER connected distribution lines, can cause Transient Overvoltages (TOVs) [27]. TOVs may also be caused by sudden connection or tripping of DERs, tripping of downstream feeders or loads, large and sudden fluctuations of DER power output due to cloud passage/wind flow changes [27]. The magnitude of TOV is, of course, dependent upon the connection scheme of the coupling transformers (start‐star, star‐delta, etc.) [42]. These TOVs can potentially have a damaging impact on other equipment connected to the distribution line.

The magnitude of TOV is seen to increase with the number of DER connections or increasing power output from the DERs. For this reason, utilities often impose a restriction on the number of DERs to be connected to their distribution feeders based on this criterion.

For the same study system in Section 1.2.3, Figure 1.12 displays the TOV in phase voltages when a single line to ground fault of six cycles occurs at the load end, during nighttime. This represents a low loading condition. This TOV exceeds the utility specified limit of 1.3 pu in the worst case [42].

Transient overvoltages are evaluated for nighttime and three solar farm output cases (0, 50, and 98% solar farm output) during daytime. The maximum wind power that can be integrated into the grid without violating TOV limit of 1.3 pu in the grid is 11.1 MW for Case 1, 5.4 MW for Case 2, and 2.1 MW for Case 3. During nighttime, when the load is lowest, the maximum wind power output permissible is only 3.96 MW. It is noted that the TOV criterion restricts the DER connectivity even more than the steady‐state voltage criterion.

Figure 1.12 TOV in PCC phase voltages for fault at load end.

In addition, there are other overvoltage issues such as rapid voltage change (transformer energization), load rejection overvoltage [43], and ground fault overvoltage [44].

The above TOV situation can be avoided by using transformers of different configurations which provide an effective grounding on the feeder side. One of the mechanisms of limiting TOVs is the use of grounding transformers or a fast ground overvoltage protection strategy [21] to prevent supply of power to the loads on an ungrounded system after a fault or unplanned switching event. Recently, SVCs have been employed to mitigate TOVs in wind farms [45].