Operating Strategies Of Grid-connected PV Station During Grid Faults

With the development of renewable energy technologies, the capacity of photovoltaic systemsintegrating into the power grid has been increasing. As new electricity generators, PV stations and theiroperating strategies exert tremendous influence on the power system into which they connect. In orderto guarantee safer and more stable operation for the power grid, it is necessary for PV stations to makereasonable operating decisions (whether low-voltage-ride-through (LVRT) or anti-islanding) whenevera fault occurs in the connected grid. This thesis aims to analyze the application and technicalrequirements of islanding protection and low-voltage-ride-through (LVRT) technologies, and toinvestigate the operating strategies of PV stations during grid faults.Considering the islanding protection of PV stations, two commonly used islanding detection method(the active-frequency-drift method and the Q-f droop curve based method) are introduced. Theprinciples of detection are illustrated and the detecting effectiveness and the impacts on power qualityare investigated. Based on the AFD method, it has been shown that under the condition of loads withlarge quality factor, a relatively large value of chopping coefficient is required to ensure successfuldetection, which would cause considerable THD for the grid current. The active-frequency-driftmethod with positive feedback (AFDPF), on the other hand, does not have such a problem. Withsufficient positive feedback coefficient, the nondetection zone could be eliminated without severedeterioration of power quality. As for the Q-f droop curve based method, it has been demonstrated thatthe detecting rapidity is related to the reactive power disturbing coefficient as well as the quality factorof the load. A severer reactive disturbance would promise a quicker confirmation, but the impact onpower quality would be severer. Meanwhile, large amount of reactive disturbance would be required ifthe load has a larger value of the quality factor, which also means slower confirmation accordingly.Additionally, the detection effectiveness of these two methods in multi-inverter systems are verified bysimulation.Dealing with the LVRT of PV stations, the power injection method and its effect on voltage supportduring balanced fault are investigated. Theoretical analysis and simulation results show that powerinjection of PV station during grid faults can support the voltage in the PCC (point of commoncoupling) to some extent, among which the effect is more considerable when the PV penetration is high.To obtain the maximum voltage support, the active and reactive power injected by the PV invertersshould be distributed proportional to the resistance and the inductance of the grid impedance. (P/Q=R/X). This conclusion can provide some guidance for the power injection of PV station duringgrid faults.The islanding protection and LVRT of PV stations have their own applications: small PV stations takethe priority of islanding protection while large PV stations are supposed to take the LVRT as their priorchoice. However, for median stations connected with the utility, there is no consensus yet about theoperating strategies during grid faults. For the safe operation of the power grid, a coordinated strategybased on calculation of PV penetration level is proposed. The penetration level can be obtained bycalculation of output capacity of PV systems and the equivalent impedance between the grid and thePCC.Moreover, for large-scale PV stations, additional reactive facilities are demanded. The simulationanalyses in this thesis demonstrate that the additional facility can help support the voltage in the PCCduring grid faults. The STATCOM is able to raise the voltage in the PCC effectively, thus serves toimprove the LVRT ability.