An Investigation into the Ancillaries, Parallel Operation of Machines with Voltage Source Inverters, and Development of Switched Capacitor Converters for a Microgrid

Abstract

With the increase in energy demand worldwide, there is a push towards integrating distributed generation (DG) in parallel with the utility grid at the distribution level. The DGs are integrated into the electrical network either through power-converters or rotating machines. Several such DGs and loads form a microgrid that can operate either in grid-connected or islanded mode. There are several technical challenges which need to overcome for the reliable operation of such microgrid. In this thesis, the author tries to propose some relevant solutions for the enhancement of microgrid operation.

The thesis is broadly divided into three-part. The first part is about the ancillaries for the testing and performance enhancement of the microgrid. When connected to the utility, the DG needs to follow certain standards demanded by the utility like voltage ride-through, voltage support and fault ride-through. Also, power-converter based DG, when connected to a weak grid, exhibits stability issue. A virtual impedance-based grid emulator is proposed in this thesis to test whether DG meets these requirement specifications before commissioning. With the proposed grid emulator, the DG performance under various scenarios like voltage sag, voltage harmonics and weak grid can be analysed. The other main requirement from the utility is that the connecting DG must possess anti-islanding capabilities. A parallel RLC resonant load per the DG rating is required to test the effectiveness of the anti-islanding algorithm. A power-converter-based RLC load emulator is proposed to test both rotating machine and power converter-based DGs' anti-islanding capability.

A significant portion of the loads in a microgrid is a single phase in nature. The distributive nature of these loads among the three phases results in an unbalanced flow of current. An active phase router is proposed for dynamically switching single-phase loads between the phases to minimise the unbalance in line current. The switching instance is selected to minimise the transients seen by the load. In combination with a power converter-based sequence compensator, the phase router can perfectly balance the phase currents.

The second part of the thesis is about the parallel operation of voltage source inverter (VSI) and rotating machines-based DG. When two of them are operated in parallel, there is a poor transient power-sharing between them because of the machine's higher transient impedance. A control method is proposed in this thesis for the parallel operation of a doubly-fed induction generator (DFIG) and VSI. In the proposed control, the DFIG is operated as a current-controlled voltage source, and thereby the issue with the transient load sharing is mitigated.

Apart from the passive loads, the microgrid can have active loads like active front-end (AFE) converters which behaves like a constant power load. A linearised state-space model of the complete system is developed to study the interaction between such loads and rotating machine-based DG. The dependency of the different controller parameters of the DG and the AFE converter on the stability is analysed through eigenvalue analysis. The developed analysis can further be used for co-design of the control of the DG and AFE converter.

The third part of the thesis is a collaborative work on the switched capacitor (SC) converter for Photovoltaic/Battery grid interface, between Indian Institute of Science Bangalore and the Loughborough University, UK as part of the Joint UK-India Clean Energy initiative (JUICE). A new switched capacitor topology is devised for high-power high-voltage application. The proposed topology has the advantages in terms of capacitor volt-ampere (VA) rating, active switch voltage rating and zero current switching. Two variants of the proposed SC converter topology are also presented with a minimum number of resonant inductors.