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This study presents a study on the time when the short-circuit fault current in an MV distribution network will cross the zero point in the presence of DC offset affected by the local synchronous and/or induction machines. The motivation of the study is to examine using thyristors to bypass a series compensator in the network while the subsequent turning-off of the thyristors depends on the zero-crossing of the current. One of the main uncertainties is the DC offset in the fault current due to the machines connected locally, which may defer the thyristor turning-off. This possibility has been examined in this study and the results demonstrate the parameters of the machines that have most influence on the time when the fault current will pass the zero point. It is shown that machines with higher unit ratings usually cause more delays in comparison with smaller units.

To control power flow and manage fault level in meshed MV networks, back-to-back voltage source converters (B2B-VSCs) are being used. However, their high cost and relatively low efficiency are of concerns. Partially rated series compensators, such as SSSCs or UPFCs, are desired but come with the challenge of protecting the device during grid faults. Their potential of use has been limited in comparison with the fully rated back-to-back converters. This paper proposes a new system topology including thyristor crowbars and a varistor to protect the SSSC in an MV network and improve the reliability and flexibility of the network operation. Using the proposed method, the time required for isolating the series compensator from the grid is reduced from at least 20 ms, corresponding to the interruption time of conventional circuit breakers, down to 3 μs in the worst case in addition to the grid fault detection delay. The performance is evaluated by simulation. A small-scale single-phase prototype operating at 230 V/16 A is tested in order to demonstrate the concept.

The main requirements of the wide-area monitoring system is to acquire the real-time measurement of state variables that can be provided by phasor measurement units (PMUs). The optimal PMU placement is the leading object of this paper, which is presented as an integer linear programming problem. The zero injection buses (ZIBs) and conventional measurements (CMs) enhance the system observability. In this paper, a novel integrated model is presented to consider the effects of the ZIBs and CMs for PMU placement. The proposed model considers limitations caused by ZIBs and CMs that have not been studied earlier. The single branch and single PMU outage, as two common contingencies, are also separately and simultaneously taken into account. The proposed method is applied to five different IEEE test systems (14, 30, 39, 57, and 118 bus) in order to demonstrate its effectiveness. The obtained results are compared with that of the other studies.