An investigation into the electric spring technologies

Abstract

The deployment of renewable energy sources (RESs) could relive the grid from the heavy reliance on fossil fuel for power generation. On the other hand, the increasing penetration of stochastic RESs poses new challenges for the grid. Recent report and survey indicate that when RESs reach a certain level of the total power generation, the stability of the power grid could be interrupted. Maintaining the balance of power supply and demand is the elementary rule of keeping the grid stable. This task has become increasingly challenging when the share of RESs keeps escalating. Supply-side management is one pillar of the solution in taming the intermittency of RESs. Power limiting and droop control are typical examples of supply regulation to provide frequency and voltage support. An alternative approach is to use storage facilities in keep the supply-to-demand balance at the expense of high price and limited capacity. It is notable that the upgrade of power grid into a smart version spurs the renovation of all sectors from supply to demand. Under this trend, traditional control paradigm of “supply following demand” in the grid is gradually being supplemented by “demand following supply”, In the face of climbing share of RESs, such a change become especially necessary. “Demand-side management (DSM)” techniques such as load scheduling, direct load control, real-time pricing, and demand bidding have been proposed. While these DSM methods provide forecasts and information for the utilities to plan their power generation, they do not offer automatic power balancing function in an instantaneous manner. With a response time in the order of milliseconds, the electric spring (ES) has recently been introduced as a fast automatic DSM technology in addressing various stability and power quality issues including voltage and frequency instability, power quality improvement, and power imbalance. Building on these achieved advancements, a comprehensive investigation into the ES technologies is conducted to cover the new controls, topologies, and applications of the ES. In chapter 1, the mainstream technologies in smart grid are surveyed, and the state of the art of ES technologies is reviewed. In chapter 2, the ancillary services of the ES-2 (the ES with storage devices) for reactive power compensation and harmonic reduction are looked into. This function of power factor correction can be enacted on top of the voltage regulation function of the ES. The proposed input current control enables the ES to improve the quality of demand without interfering the upstream system. In chapter 3, a novel 3-ph ES-2 topology is suggested for the reduction of power imbalance. The proposed topology and its control can make use of the adaptive capacity of noncritical loads toward balancing the power of critical loads whilst retaining the useful function of voltage regulation. In chapter 4, the decomposed controller is suggested to replace the genetic algorithm (GA) based controller to improve the control response and simplify the implementation structure. The new scheme integrates the two favorable functions of the 3-ph ES-2 into a single controller and allows them to be operated individually or collectively. The controller shows a good performance in partitioning the ES capacity into fulfilling multiple control targets. The coordination of distributed ESs is investigated in chapter 5. Droop controls are enforced to ensure the good compliance of operating mode among multiple ESs. The active role that the ES plays in enhancing the dynamic stability of microgrids has been successfully demonstrated. The back-to-back (B2B) topology of the ES proposed in chapter 6 expands the operating range of ES for a great amount. The bidirectional power flow enabled by the B2B configuration could reduce the size of DC capacitors, omit the requirement of energy storage devices, and improve the dynamic response of the ES. More importantly, the primary function of voltage regulation of the ES is notably enhanced, especially the ability to suppress the mains voltage when supply is over demand.