Droop Control Method for the Parallel Operation of Online Uninterruptible Power Systems using Resistive Output Impedance

Abstract

In this paper, a novel wireless load sharing controller for parallel connected online UPS inverters is proposed. As opposed to the conventional droop method, the proposed method achieves stable steady-state frequency and phase and an good dynamic response is obtained. A virtual output impedance is proposed in order to reduce its line impedance impact and to properly share nonlinear loads. Experimental results are presented from two 6-kVA UPS inverters controlled by DSP boards, showing the feasibility of the proposed approach.

Energy Management

“…The paper presents a smallsignal analysis for parallel-connected inverters in stand-alone AC power systems. The control approaches have an inherent trade-off between voltage regulation and power sharing (Guerrero, Berbel et al 2006). The signal injection technique proposed by (A. Tuladhar 1998;Tuladhar 2000) is not dependent in the plant parameters and can share reactive power even if the VSCs have not perfectly matched output inductors by having each VSC inject a non-60-Hz signal and use it as a means of sharing a common load with other VSCs on the network.…”

“…Furthermore, using this approach it is not possible to connect with the high level voltage which is using the regular conventional droop functions. In (Guerrero J.M, García de Vicuña et al 2003;Guerrero, Vicuña et al 2004;Guerrero, Berbel et al 2006;Guerrero 2005) the authors focus on the transient behaviour of parallel connected UPS inverters, they claim that damping and oscillatory phenomena of phase shift difference between the paralleled inverters could cause instabilities, and a large transient circulating current that can overload and damage the paralleled inverters. To overcome this they proposed using a method called "droop/boost" control scheme which adds integral-derivative terms to the droop function.…”

“…However, adding external communication can be considered as a drawback. Such communications increase the complexity and reduce the reliability, since the power balance and the system stability rely on these signals (Guerrero, Berbel et al 2006). In (Glauser, Keller et al 2000;Chen, Kang et al 2004) a communication bus is used in addition to the conventional droop, it has to trigger all inverters to measure their load sharing parameters at the same line period, this is used to correct the load sharing calculation.…”

“…In (Karlsson, Björnstedt et al 2005;Guerrero, Berbel et al 2006) the method selected here is to modify the droop functions of the source converters so that the regular droop functions are used in the steady-state case and opposite droops are used in transients, see …”

Bounded droop controller for parallel operation of inverters

“…Additionally, the inverter output impedance plays a key role in accurate load sharing, since inverters equipped with the conventional droop control are required to have the same per-unit output impedance [7]. Therefore, recently, several control designs have been proposed in order to achieve accurate power sharing among the inverters [10], [8], [19], [17], [16], [26].…”