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Although the ac fault current provided by photovoltaic (PV) power plant is limited as a weak-infeed power or a electric load, the voltage and current directions are different from conventional synchronous source, since the current is highly controlled by the PV inverter. This paper investigates the measured impedance and the distance directional element of PV side line protections. The impedance trajectory are drawn and compared with conventional power source. Simulation result shows that the directional comparison teleprotections may miss operate under a line internal fault, since the measured impedance drops into the negative direction area, even though the weak-infeed tripping function is activated.

This paper investigates a typical 220kV GIS circuit breaker re-ignition fault in practical systems. All types of protection, such as transmission line protection, circuit break failure protection, bus-bar protection and transform protection, coordinate in one fault case. The protection behavior is analyzed. By reproducing the fault process, the coordination among line protections, circuit break failure protections, bus bar protections and transformer neutral point gap protections are studied. The evidence for protection configurations and settings is provided. According to the comparisons between the fault data from the fault recorder and from the busbar protection device, the bus bar differential current is calculated. Analysis result from different current transformers (CTs) verified the fault locations.

The contribution of doubly fed induction generator (DFIG) to ac system short-circuit current can not be ignored with large wind power integrations. The traditional methods, which neglect the influence of DFIG or take DFIG as a load, are inevitably to introduce error in short-circuit current calculations. In this paper, analytical formula of DFIG short-circuit current under voltage dips is derived based on a mathematical model. The sequence network models of DFIG with different slip speed are developed and can be implemented in short-circuit calculation program. Simulations in Matlab/Simulink verify the accuracy of the proposed models of DFIG. It is demonstrated that DFIG with larger slip we can adopt the constant impedance model of induction motor, while the model with transient potential and transient reactance is suitable for the DFIG with smaller slip.

This paper discusses the coordination of arm overcurrent protective functions, which are configured both in valve controls and DC protections of modular multilevel converter based HVDC (MMC-HVDC). Since the parameter mismatching in valve control and DC protection may lead to unexpected block, the ac protection coordination diagram is used to analyze the setting and time delay of arm overcurrent protections. A temporary blocking function in valve control is introduced and is configured properly with other protection stages in DC protection device. Simulation results based on a detailed PSCAD model prove the proposed protection coordination method can avoid the protection malfunction during ac fault low voltage ride through (LVRT). Meanwhile the protection logic guarantees the safety of power electronic devices during dc fault by fast triggering the shunted thyristor.