Abstract
Ultracapacitors (UCs), with their features of high power density and high current charge-discharge, have become the best choice for dynamic power compensation to improve the stability of microgrids and are increasingly being applied in microgrids. This paper presents the control of an energy storage system (ESS) based on ultracapacitors in the context of grid-connected microgrids. The ESS is composed of DC/AC and DC/DC converters tied by a dc link. An improved dynamic model for the ESS is proposed. Based on the proposed model a Proportional-Integral-Resonant (PIR) DC link voltage controller is proposed to maintain the DC link voltage through the charging-discharging control of ultracapacitors, capable of working properly under all operating conditions. An extra double frequency component is injected into the UC current by a R controller to dynamically compensate for DC instantaneous power and double frequency AC instantaneous power due to unbalanced grid conditions and disturbances. This feature maintains the DC link voltage constant under unbalanced conditions and increases the degrees of freedom of the DC/AC converter and thus facilitates the application of UCs in microgrids. Simulation and experimental results verify the effectiveness of the proposed control strategy.
Highlights
Microgrids, containing a large number of distributed power sources, such as solar power, wind power and others, are devoted to the key technology of connecting distributed power to the grid [1].To increase the stability of the system, making it more immune to perturbations, such as changes in the loading conditions or changes in the electric energy production due to environmental variability, energy storage devices are configured and used, which has become an extensively and commonly employed solution
Reference [5] proposed a sliding model control algorithm to control the power exchange between UCs and grid-connected inverters, the compensation effect of which depends on the power instructions issued by the micro-grid control center (MGCC) and the DC link voltage was controlled by the GSC and not the Energy Storage Side Converter (ESSC) (UC)
Equations (8) and (9) describe the dynamics of the DC and AC components of the DC link voltage respectively. po0 in Equation (8) is given by Equation (3), while poc and pos in Equation (9) are given by Equation (4). pL indicates the effect of other possibly existing branches’ power on the DC link voltage. po0 and pL represent the disturbance power. It can be known from Equation (8) that the DC component of the UC current iuc0 can be taken as the controlled variable of the DC component of the DC link voltage y0
Summary
Microgrids, containing a large number of distributed power sources, such as solar power, wind power and others, are devoted to the key technology of connecting distributed power to the grid [1]. Reference [5] proposed a sliding model control algorithm to control the power exchange between UCs and grid-connected inverters, the compensation effect of which depends on the power instructions issued by the micro-grid control center (MGCC) and the DC link voltage was controlled by the GSC and not the ESSC (UC). The DC link voltage control algorithms proposed in reference [7,8,9,10] are similar They all use grid-connected inverters to control the DC link voltage and utilize the injected negative sequence current to directly control the double frequency instantaneous active power to zero. Based on the established model the paper proposes an improved compensation control strategy for the ESS of a UC in the context of microgrid applications.
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