Improving power system voltage regulation: a capacitive ladder-based power electronic AC voltage regulator
This thesis proposes a new circuit topology for a capacitive ladder-based electronic voltage regulator (CL-EVR) which is a substitute for an electromechanical tap changer in power transformers. The proposed CL-EVR employs a combination of capacitors and TRIACs to generate voltage steps that can be added or subtracted from the grid voltage to achieve voltage regulation. In comparison to traditional on-load tap changers (OLTC), the CL-EVR is much faster and also eliminates arcing while increasing the speed of the voltage regulation, thereby potentially increasing the operating lifetime of transformers and reducing failures. Other advantages over conventional OLTCs are the elimination of di/dt limiting reactors and numerous tap changer contact leads to a potentially lower cost. The thesis begins with an examination of traditional approaches to voltage regulation such as electromechanical and solid-state tap changing transformers. Improved models for the change-over switch are introduced and used to design a better change-over switch. The focus then shifts to the pièce de résistance which is an entirely new topology. A converter-based on-load tap changer (COLTC) is proposed to be used as a substitute for the traditional tap changers. The proposed COLTC improves the transient behaviour, arcing, and response time as compared to the traditional tap changers. The proposed design includes a new failure detection method that detects a failed TRIAC switch and isolates it from the circuit not only to prevent further damage to the other CL-EVR elements but also to continue the voltage regulation, albeit with a slightly reduced capacity. To ascertain its performance and stability over the operating range, the thesis conducts a comprehensive theoretical analysis encompassing the steady state and transient performance of a simplified system consisting of the CL-EVR connected to resistive or inductive loads. A series of Electromagnetic Transient (EMT) simulations are conducted in order to investigate the CL-EVRS operation over a wider operating range and to fine-tune the component values and to coordinate the various control loops and to select the control system parameters. EMT simulation is also used to ensure the successful operation of the failure detection and protection strategy. Finally, a single-phase leg rated at 33 kVA of the full three-phase 100 kVA design was prototyped in the laboratory and successfully tested under inductive and resistive loads. The results prove that the proposed CL-EVR can apply different voltage levels properly without significant overshoot during the transition from one level to the other.
Tap changers, Power transformers, Voltage source converters, Power Electronics, Power Systems, Transient study, Change-over switch