Modular-based Power Quality Enhancement Technologies


Student thesis: Doctoral Thesis

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Award date22 Jan 2018


One of the emerging trends in electrical industry is an integration of distributed generation (DG) systems into the distribution network. It allows more clean and renewable energy resources connected to the grid at the point of coupling. However, high penetration of DG systems gives challenges of controlling the power flow among various devices in the distribution network and raising the difficulty of maintaining a high power quality level. This thesis presents the use of power electronics systems to enhance the power quality of the network with DG systems, including power flow control among DG systems, voltage restoration and voltage regulation on the distribution side, and unified power quality conditioning.

Firstly, a modular and scalable series compensation technique for enhancing controllability and flexibility in power transmission in distributed power systems is presented. The concept is based on paralleling multiple static synchronous series compensators through daisy-chained transformers to perform reactive power compensation, and thus control the amount and direction of the power flow over the transmission link. Each compensator unit is under an autonomous control for regulating its output voltage. Its output current is coupled to two adjacent compensators through two of the daisy-chained transformers, so that the transmission current is shared among the parallel-connected compensators.

Secondly, the modular and scalable technique is extended to regulate and stabilize the supply voltage at the customer side. A modular and scalable voltage-regulation structure for enhancing service continuity and flexibly changing the system power rating is presented. The structure is based on paralleling multiple series-voltage compensators through daisy-chained transformers to regulate the load voltage. The output voltage of each compensator is controlled locally, while the output current of each compensator is coupled to two adjacent compensators through two of the daisy-chained transformers. The load current can be shared near-equally among the compensators through the transformer structure.

Thirdly, a mathematical model to describe the static and dynamic characteristics of a capacitor-supported series-voltage-compensator-based (SVC-based) voltage regulator for driving different load types is formulated. In addition, a phase jump technique and a gain scheduling scheme that can reduce the load voltage fluctuation under different load-type and load-value disturbances are presented. The technique can effectively reduce the load voltage fluctuation and the settling time under a sudden change of the load type from inductive to capacitive, and vice versa.

Finally, a single-phase transformer-less unified power quality conditioner (TL-UPQC) is presented. It can deal with both voltage- and current-type power quality issues. Apart from having no isolation transformer, the proposed structure utilizes four switching devices only, forming two half-bridge voltage-source inverters - one connected in parallel with the load and another one connected in series with the AC mains. The two inverters share the same DC link. The parallel inverter, which is controlled by a hysteresis current controller, is used to shape the current drawn from the AC mains and regulate the DC-link voltage. The series inverter, which is controlled by a boundary controller with second-order switching surface, is used to regulate the steady-state load voltage and provide voltage sag / swell ride-through. A DC-link capacitor voltage balancing control that coordinates the operations of the hysteresis and boundary controllers is designed.