DC electric springs for DC microgrids

Abstract

In the revolutionary development of information and communication technologies, DC microgrids have been considered as promising grid systems to host the renewable generations and electronic loads. However, the intermittent renewable generations and converter-interfaced electronic loads can cause voltage instability issues in the form of (i) DC bus voltage variations, (ii) harmonic voltages and (iii) voltage flickers. This hinders the sustainable operations of the interfaced DC appliances. Research to date has not been able to provide a viable solution that can maintain the stability of the DC bus voltages via using only a relatively small energy storage capacity.

This thesis aims to investigate the application of electric springs in addressing the aforementioned bus voltage instability issues in DC microgrids and the main contributions of this thesis can be summarized as follows:

1. A comparative study on the series and shunt type DC electric springs is provided. Here, the circuit operation principles and small-signal models of both types of DC electric springs are discussed. The design of appropriate controllers for dealing with the regulation of the DC bus voltage, the compensation of the harmonic voltage, and for providing fault-ride-through support is discussed. The performances of both types of DC electric springs in these applications are compared experimentally and through simulations.

2. A new configuration of storage system comprising the photovoltaic (PV) panels, a series DC electric spring (series ES) and a non-critical load is proposed to reduce the battery storage capacity of DC microgrids that have substantial PV installations. This arrangement forms a PV-embedded series DC electric spring (PVES). An optimization method considering the minimization of electricity bills of the DC microgrids is proposed to size the storage capacity and to determine the rating of the PV that are connected to the series ES. Experimental works and simulation studies are conducted to show that the PVES can tackle the intermittency of the solar power with a reduced storage capacity.

3. A new circuit topology of the hybrid DC electric spring is proposed to decouple the DC power and AC harmonic power. The hybrid DC electric spring can divert the AC current to the ground and retain the function of manipulating noncritical load for DC voltage regulation. The immediate benefits of this hybrid DC electric spring are the reduction of storage capacity and a prolonged lifetime of the battery. Both the operating principle and the mathematical model of the proposed circuit topology are discussed in this thesis.

The results drawn from this thesis have provided strong evidences which show that the DC electric springs are cost-effective technologies because of their independency of communication networks, distinctive focus on achieving local bus voltage control using input feedforward and input control loop, complementary engagement with noncritical loads to automatically achieve power fluctuation control, and inherent advantage of reducing the storage requirement for balancing power supplies and demands. For this reason, it can be concluded that the DC electric springs is a highly promising type of real-time demand side management technology for the emerging DC microgrids.