Impacts of DFIG and Type IV WTG’ Capacity and the Location of Placement on IEEE 13 Node Radial Test Feeder Positive, Negative and Zero Sequence Reactance in Short Circuits

With the ever increasing number and capacities of the distributed generations (DGs)penetrating the conventional radial electrical power distribution networks, there is need for a detailed assessment on the impacts the DGs have on the distribution network’s sequence impedances during fault conditions. The conventional distribution networks are designed to extract power from the transmission network and distribute it to the loads. The distribution networks were not designed to have DGs directly connected into them hence their power flow is unidirectional from the main utility grid to the loads. When balanced, un-faulted and in normal operating state, the power system’s voltages and currents can be evaluated and determined with utmost simplicity. When the power systems is balanced and un-faulted, the line and phase for both the voltages and the currents are of equal magnitudes and displaced by 120º from each other, however, when the power system network is unbalanced and faulted, the magnitudes of the phase voltages and currents are not equal and are displaced by angles more or less than 120º, hence evaluating and determining the network quantities can be difficult under the unbalanced and faulted operating conditions.

During a fault, a simplified approach to the evaluation of the network voltages and currents can be achieved by use of symmetrical components theory which proposes that for any set of unbalanced currents occurring in an electrical network, a set of three balanced currents namely the positive sequence currents, the negative sequence currents and the zero sequence currents can be developed and used to simplify the solution to the unbalanced phase voltages and currents. The flow of the three set of symmetrical currents gives rise to a set of three symmetrical voltages namely the positive, negative and zero sequence voltages. The impedance offered by each power system equipment to the flow of the symmetrical sequence currents are respectively referred to as the positive sequence impedances, the negative sequence impedances and the zero sequence impedances.

Wind turbine generators (WTGs) are one of the most common renewable energy dependent DG technologies largely integrated into the distribution networks. These WTGs are interfaced into the distribution networks either through induction or synchronous machines with the two generator technologies broadly classified as the doubly fed induction generator (DFIG) and the Type IV WTG technologies. An important aspect of the DFIG and the Type IV WTG studies is to evaluate their impacts on the positive, negative and zero sequence impedances of a distribution network under different short circuit conditions. For purposes of this paper, the IEEE 13 node radial test feeder was modelled for the short circuit study in electrical transient analysis program (ETAP) software. The short circuit study was then performed on the radial test feeder firstly without WTGs connected and secondly with DFIGs and Type IV WTGs interchangeably connected at NODE650, NODE632, NODE671 and NODE680 of the radial test feeder. This paper presents a detailed investigation on the impacts the DFIG and the Type IV WTG with their capacities being increased from 1MW to 3MW have on the positive, the negative and the zero sequence reactance of NODE650, NODE632, NODE671 and NODE652 of the IEEE 13 node radial test feeder during a short circuit.