An investigation into the use of electric springs for smart grid applications

Abstract

With consideration of environmental protection, traditional fossil fuel power plants and nuclear fission power plants is progressively replaced by environmental friendly renewable energy generations. However, the output power of the renewable energy sources vary from time to time. The system operator is difficult to determine the instantaneous amount of power generation to balance the load demand with the traditional power system control paradigm. The imbalance of power generation and load demand causes the frequency to fluctuate. On the other hand, there is lack of voltage regulators along the distribution network in the traditional power system. If the active and reactive power flowing through the distribution line varies intensively because of the intermittent nature of renewable energy, the supply voltage experienced by customers also fluctuates dramatically. To mitigate the voltage and frequency fluctuations, the existing centralized control scheme has to be changed into a new control paradigm which should include but not limited to these features, 1) the demand following the generation facilitation; 2) local voltage regulation capability; 3) decentralized control; and 4) distributed devices. Various demand response management schemes have been proposed such as 1) Scheduling of demand tasks; 2) Use of energy storage; 3) Real-time pricing; and 4) Direct control of smart loads. In 2012, a novel “Electric Spring” (ES) concept was proposed. It is a proven technology for stabilizing smart grid with high penetration of intermittent renewable energy sources. With this technology, traditional electric appliances can be converted as a smart load to 1) provide mains voltage support; 2) store electric energy; 3) damp electric oscillation; and 4) accomplish demand to follow generation. To further investigate the characteristic of ES in demand side load management with various kinds of non-critical load such as resistive, resistive-inductive, resistive-capacitive and constant power loads, mathematical derivations and experiments are used to study the characteristic of ES. The results obtained provide useful information for the future ES development. While many grid connected renewable energy generators are commissioned, more remedies should be implemented simultaneously. The use of the ES concept in supply side management is demonstrated in a grid tie photovoltaic inverter system. A coordinated control scheme is proposed to automatically select either the voltage control or the frequency control methods depending on the instantaneous grid conditions. Grid tie inverter embedded with ES control method provides a straightaway solution to lessen the voltage and frequency fluctuation in a microgrid. The effectiveness of the proposed control scheme is explicitly investigated by time domain computer simulation using Matlab/Simulink.