Stabilization of constant-power loads by passive impedance damping

Abstract

This paper addresses stability problems of power systems with actively controlled loads that exhibit constant-power behavior. Instability occurs in such systems due to the negative incremental resistance of the constant-power loads (CPL). Existing approaches to stabilizing such systems require modification of the source and/or the load control characteristics, or isolating the CPL from the rest of the system by additional active devices, which are difficult to implement and often conflict with other system requirements such as control bandwidth, size, weight, and cost. In this work, we propose passive damping as a general method to stabilize power systems with CPL. Using a representative system model consisting of a voltage source, an LC filter, and an ideal CPL, we demonstrate that a CPL system can always be stabilized by a simple passive damping circuit added to one of the filter elements, no matter how the original system behaves. Three different damping methods are considered and for each analytical models are developed to define their parameters required for stabilizing the system. The different damping methods are also compared in terms of their stabilization capabilities and impact on other system performances such as filter attenuation. Time-and frequency-domain measurements from an experimental system are presented to validate the proposed methods.

Constant Power Loads (CPL) with Microgrids: Problem Definition, Stability Analysis and Compensation Techniques

“…It will be expensive when passive elements are added, especially capacitors. This method in [39,59] is valid for small disturbances [60]. …”

“…The power electronic converters tends to have CPL properties if them output voltage is bigger than voltage reference (V c ) [21,22,30,[41][42][43]; in other word, the converter behaves as resistive load at start-up but as a CPL when it exceeds the value of voltage reference (V c ) [30]. The load converters (second stage of multiconverter) tend to display the properties of a constant power load when control is applied tightly [22,36,37], which means a converter has a fast response and low output ripple [37]; therefore, the converter behaves as the CPL with their control bandwidth [39]. Constant power loads (CPL) means that the output power of the convertor is constant, i.e., the output voltage drops when small variations occur in the current value (increasing current amount), which means that when neglecting the dissipated power into the circuit, the output power and input power are equal [22,36].…”

“…Passive damping is a simple method to increase the damping of a system by adding passive elements; which are contain resistance (R), inductor (L) and capacitor (C), to the input filter of the system, as described in [39,59]. The authors introduced three methods for passive damping by adding RC and RL in parallel with input filter and RL in series.…”

“…In [60], the method used is adding only R in parallel with the L-filter. As in [39], the method proposed here to stabilise the system is minimisation of the peak output impedance of the filter by adding a passive damping (passive elements) circuit to the LC filter. The stability of the CPL system requires the impedance ratio Z d /Z 1 to meet the Nyquist stability criterion, or alternatively, the characteristic polynomial of (1/(1 + Z d /Z 1 )) to meet the Routh-Hurwitz criterion.…”

Effect of Control Method on Impedance-Based Interactions in a Buck Converter

“…The set of open-loop transfer functions of the DDR-controlled buck converter can be given according to (12) applying the statespace averaging method [23], [31]. The validity of the analytic models in (12) is verified, e.g., in [31, pp.…”