Abstract

DC-AC Converters (inverters) are frequently employed as interfaces of distributed power sources and energy storage units to ac distribution grids. The approach of operating them as a voltage source with droop based control loops and using locally measured quantities offers an effective way to control the amount of active and reactive power they provide/absorb. In this way, fluctuating renewable energy sources, such as photovoltaic (PV) and wind, can help with power balancing, while grid forming units can better share load variations without dedicated communication channels. Besides, it can allow a smooth transition of a micro-grid from the grid-tie to the autonomous mode in case of a grid fault. However, the dynamic response and steady state operation of a system with droop controlled inverters depends quite a bit on systems parameters, such as feeder impedances, as well as on the droop characteristics of the other units, what is not usually known.
This work focuses on the analysis of the performance of droop controlled inverters operating in various conditions. First, a 10 kVA three-phase inverter with a dq (vector) voltage control loop and active power (P) vs. frequency (f) and reactive power (Q) vs. grid voltage magnitude (V) droop characteristics is designed. Then, its behavior when operating connected to a stiff grid is investigated. Time domain simulations with SIMULINK and the technique of root locus, for which a small signal model is derived, are used to observe how the droop factors, frequency of the low pass filters used in the power measurements and feeder impedance affect the dynamic response. Next, the operation of two grid forming inverters in an autonomous micro-grid is considered. Again, the performance of the system is investigated with time domain simulations and root locus. The need for a virtual impedance loop as a means for allowing large droop factors to be used along with feeders with small inductances is observed and the effectiveness of this technique is demonstrated. The adverse impact of the conventional virtual impedance loop on the load voltage regulation is observed and an alternative implementation that minimizes this problem is proposed and its effectiveness is demonstrated. Finally, an autonomous micro-grid consisting of an inverter and a diesel engine generator set (genset) is studied. Time domain simulations are used to show that when the speed of response, in terms of power, of two grid forming units is very different, the smallest one can be overloaded. An approach for slowing down the fastest unit is proposed to minimize this issue.