Smart loads based on “electric spring” concept

Abstract

The integration of renewable energy sources into the grid has been significantly increasing in recent years. This increase in renewable energy sources is creating new challenges for electrical engineers because power quality problems due to these distributed generation sources are also on the rise. The main power quality problems include over-voltages and under-voltages, frequency fluctuations and also increased harmonics in the electrical grid. This project shall only focus of the voltage fluctuation problems because the variation in active and reactive power being injected into the grid due to the intermittent nature of RES causes variations in the voltage drop along the power line. In literature, one can find several compensators which help mitigate these problems such as STATCOMS, static var compensators and energy storage system. However, each of these has various operational limitations. STATCOMS and static var compensators can only supply reactive power into the grid. On the other hand, the energy storage systems can supply both reactive and active power compensation but are expensive and bulky. Smart loads with electric spring concept (ES) are a new emerging technology which is aimed to help solve power quality problems in the electrical networks. Loads present in electrical networks can be categorised broadly under two main categories: ‘critical loads’ and ‘non-critical loads’. Non-critical are defined as electrical loads that can withstand a range of supply voltage variations without affecting the operation or performance of the same equipment. On the other hand, loads that does not support voltage variations over a wide range are defined as ‘critical loads’. For the latter, regulation of the supply voltage is critical to ensure that the electrical equipment does not sustain any damage during normal operation. There are three types of ES which can be found in literature. The first type uses just a DC-link capacitor as a storage system, the second type uses a battery while in the third type the energy storage is replaced by a bidirectional a.c.-to-d.c. converter. The ES can supply both reactive and active power compensation while reducing the need for distributed energy storage in the electrical network. This project serves as a proof of the concept of the operation of the ES based smart loads in the LV distributed networks. An ES with energy storage was designed, modelled and simulated in Simulink/PLECS in order to verify its effectiveness in maintaining voltage stability across critical loads. The model considers various scenarios in order to verify thoroughly the effectiveness of the considered ES under different operating conditions. From the simulations which were performed, it was concluded that the ES is an effective in regulating the voltage across the critical load even though it has some limitations. Following the simulations of the ES, the hardware interface necessary to implement the ES was also designed and tested. Results are also given of the testing of the circuitry which was implemented during this project. However, the construction of the lab-base prototype was considered to be beyond the scope pf this project.