An investigation of electric spring technology for smart grid applications

Abstract

With the increasing trend of renewable energy penetration in modern power systems, many nations in the world are establishing smart grids to tackle the uncertainty of renewable power inputs. Electric Spring (ES) technology is an emerging demand-side management (DSM) method which can provide the grid with some ancillary services (e.g., voltage regulation, frequency regulation, power quality improvement, etc.). ESs transfer some non-critical loads into smart loads (SLs) with adjustable power consumption. In this thesis, the ES topology evolvement from ES-1, ES-2 to ES-B2B and hybrid ES are reviewed in the first section. The diverse control schemes for single ES as well as for ESs group and the system-level studies of massive distributed ESs in the power grid are also inclusively recapped. An in-depth investigation on the ES control and its valuable smart grid applications is then conducted, and progress have been made in the following three aspects: 1. In the ES control aspect, a uniform controller, on which three fixed-interior-angle control strategies can be executed, is proposed in chapter 3. The operating curves as well as the distinctive characteristics of the SLs under these three fixed-interior-angle control strategies are further analyzed. Experimental results show that the proposed uniform controller could be programmed to perform three fixed-interior-angle control strategies successfully, and the operating curves under these fixed-interior-angle control strategies are verified. 2. In the aspect of using ES for voltage regulation, an investigation into using the ESs for overvoltage prevention in distribution LV networks with high PV penetration is reported in chapter 4. It is revealed that due to the uncertainty of surplus PV generation and the corresponding reverse power flow, the overvoltage issues may happen at many nodes of the LV networks. The ES-B2Bs is combined with the storage-type electric water heater to form the smart thermal loads. The dynamic consensus protocol is used to coordinate different smart thermal loads units. A hierarchical collaboration model of using BESSs as a back-up is also established in chapter 3. The simulation results based on Hong Kong Sha Luo Bay LV network verifies the effectiveness of the proposed SL-plus-BESS system. The following comparison case study between the proposal and pure BESSs program shows the economic benefits of introducing ESs for overvoltage prevention. 3. In the aspect of using ES for frequency regulation, besides traditional primary frequency response service, the SLs are enabled to provide virtual inertia to the power system in this thesis. In chapter 5, the control scheme for ES-B2B to emulate inertia is presented and the mathematic expression of how much virtual inertia can be provided from SLs is derived. The case study in the CIGRE microgrid shows that significant virtual inertia could be obtained from SLs by only allowing the load voltages to fluctuate within 0.95~1.05 times the nominal value. The proposed uniform controller in this thesis paves the way for the research of the ES with multiple switchable functions. The investigation also fills in the blanks of using ESs for overvoltage prevention and virtual inertia purpose. Future research can focus on the economic benefits of ES providing multiple services.