Some aspects of electric springs for smart grid applications

Abstract

Power distribution system is a highly critical infrastructure for modern power grid. In recent years, with the large scale integration of intermittent renewable energy generation which causes instability in the power grid, the power quality problems in distribution system have earned widespread attention.

In the beginning of this thesis, the major power quality problems in distribution system have been investigated, and the modern technologies which are used to improve system power quality are analyzed. The survey indicates that in future smart grid, in order to maintain the power system stability, using supervised power consumption to match the power supply via demand side management technologies will become the main trend.

A novel concept of smart load technology termed as Electric Spring (ES) is illustrated and evaluated in the rest parts of this thesis. The ES is able to provide dynamic compensation to maintain the system stability. The effectiveness of ES on voltage regulation is successfully demonstrated by hardware experiments. In order to implement ES in complex distribution networks, a software model of ES is designed in Matlab/Simulink for further research. The performance of ES on voltage regulation is firstly evaluated by comparing ES with the STATic COMpensator (STATCOM) which provides traditional centralized voltage control. Two case studies which include an IEEE 13-bus test feeder system and a part of the distribution network in Sha Lo Wan Bay are selected to evaluate their voltage regulation capability. The simulation results turn out that, with the voltage control in a distributed manner, a group of ESs achieves better total voltage regulation than STATCOM with less overall reactive power compensation.

Furthermore, after replacing capacitors on the DC link of the ES with batteries, both the active and reactive power compensation can be realized by the ES. Several new control strategies of the ES which offer more functions on power quality improvement are derived. In chapter 4, a new adaptive smart load which is composed by a large-scale ice-thermal storage system and an ES is introduced. With the help of the ES, the proposed adaptive smart load can damp the voltage oscillation, reduce the frequency variation, and shave the peak demand. Based on practical data, a typical commercial building energy model with the adaptive smart load has been successfully used to demonstrate the advantageous features using computer simulation. In chapter 5, a multi-objective control strategy for ES is proposed. The proposed ES can reduce the three-phase power imbalance and regulate the mains voltage. There are two operating modes for the ES of this version, one has higher priority on voltage regulation, while the other has higher priority on power balancing. This special feature offers more options to system operators. The performance of the proposed ES control strategy is evaluated by applying it in a building model with severe power imbalance and voltage fluctuation. The simulation results confirm the effectiveness of this multi-objective ES control strategy.

These case studies proved that the ES can be an effective solution on power quality improvement for future smart grid.