A positive-sequence current baseddirectional relaying approach for CCVTsubsidence transient condition

With the increasing connection of controllable devices to isolated community microgrid, an economic operation model of isolated community microgrid based on the temperature regulation characteristics of temperature controlling devices composed of wind turbine, micro-gas turbine, energy storage battery and heat pump is proposed. With full consideration of various economic costs, including fuel cost, start-stop cost, energy storage battery depletion expense and penalty for wind curtailment, the model is solved by hybrid particle swarm optimization (HPSO) algorithm. The optimal output of the micro-sources and total operating cost of the system in the scheduling cycle are also obtained. The case study demonstrates that temperature adjustment of temperature controlling devices can adjust the power load indirectly and increase the schedulability of the isolated community microgrid, and reduce the operating cost of the microgrid.     

Analysis of lightning-caused ferroresonance in Capacitor Voltage Transformer (CVT)

Power networks contain capacitances and inductances that can saturate, presenting opportunities for ferroresonance to occur. Most power equipment is designed through linear theory but ferromagnetic materials are highly non-linear and when they resonate, the non-linearity produces currents and voltages that are larger than usual. Ferroresonance is a complex, non-linear electrical phenomenon. It can cause dielectric and thermal problems through overvoltage, an intrinsic phenomenon present in all networks. A network dynamic response to lightning and switching will be energy storage and release. The transfer of energy will propagate an overvoltage through the network and damage substation equipment when lightning strikes near the substation. Application of conventional mathematics is inappropriate to ferroresonance study in which actual events are simulated. Lightning strikes that occurred near a substation and that led to explosion of CVTs have been reported, so this study investigates the effect of a lightning strike on a tower with a 132 kV Capacitor Voltage Transformer (CVT). Alternative Transient Program (ATP) was used for the simulation which duplicated the lightning-strike effect that causes a CVT to explode.     

Short circuit fault analysis in a grid connected DFIG based wind energy system with active crowbar protection circuit for ridethrough capability and power quality improvement

In power industry, improvement of the short circuit Fault-ride through (FRT) capability of grid integrated Doubly Fed Induction Generators (DFIGs) for the wind power system is an important issue. In this paper an Active Crowbar Protection (ACB_P) system is proposed to enhance the Fault-ride through (FRT) capability of DFIG so as to improve the power quality of the system. The protection scheme proposed here is designed with a capacitor in series with the resistor unlike the conventional Crowbar (CB) having only resistors. The importance of ACB_P is to maintain the connection of DFIG with the grid during fault conditions to provide uninterruptable power supply to the loads. The major functions of the capacitor in the protection circuit are to eliminate the ripples generated in the rotor current and to protect the converters as well as the DC-link capacitor. The main objectives of the proposed approach are: minimisation of magnitude of rotor fault currents, maintenance of constant DC-link voltage, reduction in crowbar operation time to avoid disconnection between the DFIG and the Rotor side converter (RSC), improvement in the short circuit response of terminal voltage and enhancement in the dynamic responses of the DFIG. These objectives are achieved through the incorporation of ACB_P scheme between the rotor of the DFIG and RSC. The proposed scheme is validated on different types of fault conditions and it is observed that there is significant improvement in the objectives. Simulation results are carried out on a 1.7 MVA DFIG based WECS under different types of short circuit faults in MATLAB/Simulation and functionality of the proposed scheme is verified.