This the islanding problem. Recently, with more and

This section presents a broad comparison of various technical regulations enforced by modern GCs due to the penetration of PVPPs in different countries. The following set of common technical connection requirements requested by modern GCs will be surveyed.3.1 LVRT requirementsAs mentioned above, nowadays, PV power farms have become one of the most important renewable energy sources. In the past, when PV systems were poorly diffused and the new rules required by the distribution system operators were not in force, the focus was on the protection tripping when grid voltage variations occur as introduced previously.46, 47 The studies focused on the effects of power quality issues regarding GCPS and the protection of different topologies of PV inverter during disturbances. It requires the inverter to disconnect directly to avoid the islanding problem. Recently, with more and more PVPPs being built and the increasing penetration levels to the power system, the disconnection with grid during disturbances is no longer possible.48 This leads to the elaboration of specific technical requirements for the connections of PVPPs, usually as a part of GCs, which in this case are the LVRT requirements. The LVRT requirements are specified in the modern GCs. It requires the PVPPs to withstand grid voltage dip to a certain percentage of the nominal voltage (down to 0 in some GCs) for a specific duration. In this duration, PVPP should operate normally. After fault clearance, PVPPs must restore active and reactive power fast enough to the prefault value. In some countries, the PV system is required to operate like traditional synchronous generators where reactive current should be fed into the grid to support voltage stability.5, 49, 50Normally, the LVRT requirements are characterized with a voltage-vs-time graph as shown in Figure 2, which illustrates general LVRT requirements for GCPS. The PVPPs are required to work continuously in area A, which represents the nominal voltage at the connection point, that so-called point of common coupling (PCC). If the voltage is in area B, the PVPPs have to withstand voltage dip and remain connected to the system for a period of time (t0 ? t1). Otherwise, it has to be disconnected. In case the voltage at the connection point in area C is recovered to V1 within time t2 after fault occurrence, it is mandatory for PVPPs to remain under continuous operation without disconnection. The values of V0, V1, t1, and t2differ from one GC to another based on the standards and characteristics of the national grid.51