* Synthesis and Control of Custom Power DEVCES for Enhancing Power Quality Problems.
By: Mr. Paisan Boonchiam (Thailand)
Supervisor: Dr. Nadarajah Mithulananthan (Chairperson), Dr. Weerakorn Ongsakul and Dr. Manukid Parnichkun
External: Prof. Dr. ir. R.W. De Doncker

Abstract

Custom power device is the applications of power electronic to improve electrical quality of power distribution system for sensitive load in industrial plants. Industries reporting production stops due to voltage disturbance, such as short interruptions, voltage sag/swell, voltage/current harmonics, voltage flicker, power factor correction. These industries include paper mills, semiconductors facilities and other industries with fully automated production. The objective of study is to use the custom power device for mitigating power quality problem created by disturbances and non-linear loads. The equipment studied in the dissertation exploit the control actions that can be taken by power electronic devices, which are much faster than speed of conventional equipment and protection systems. This dissertation presents three compensating types of custom power device based on connected voltage source converter topologies. The series-connected device, popularly referred to as DVR, has been proved suitable for the task of compensating voltage sag and swell and voltage regulation in power distribution systems. DSTATCOM is, the shunt connected device, connected to compensate the reactive power for improving harmonics, power factor corrections, and voltage fluctuation. UPQC is the hybrid-connected device that includes the performance of DVR and DSTATCOM.

The topology of voltage source converter for medium voltage grid is presented with multilevel topology. Three types of multilevel topology, namely diode-clamped, flying capacitor and series cased H-bridge have been investigated. The diode-clamped multilevel topology is chosen for custom power devices and can readily be deployed in practice when the system needed to support in short period time.

The modeling and control of custom power device are proposed with the dynamic decoupling control, feed forward control and improving the control performances in discrete time domain simulation. The control and system parameters are designed based on real parameters in power distribution system. The system model, that covers national standard, is used for testing and validating all concept and methodology.

Finally, the planning of high-power quality zone is discussed. The concept of a high-power quality or premium power zone is the zone that electric power inside is tightly regulated by custom power device. Moreover, the custom power devices, backed by distributed generator, will supply the most sensitive loads during the total line outages. In this dissertation, a configuration of the premium power zone is proposed and then various issues involved with the operation of the zone are discussed.

* Fuzzy Logic and Wide-area Control for Low Frequency Oscillation Damping in Power System.
By: Mr. Komkrit Prasertwong (Thailand)
Supervisor: Dr. Weerakorn O., Dr. N. Mithulananthan, Dr. Charles O.P. Marpaung and Dr. Poompat Saengudomlert
External: Dr. Ramesh Chand Bansal

Abstract

In interconnected power systems, there are hundreds of generating units with varying size, complexity and mechanical speed. These units are connected with load centers which are typically far from generating stations by long transmission lines. One of the major challenges in operation and control of such complex large networks is keeping all generating units at constant electrical speed or synchronism. Whenever power systems are subjected to small and sudden disturbances one or more of the generating units tend to exhibit oscillatory behavior. If the oscillations are not controlled, they would grow in amplitude, limit power transfer capability of transmission lines, induce stress in the system and trigger protection devices to function. Hence, oscillations threaten system security and hamper the efficient operation of the entire system. Low frequency oscillations have been discovered since 1960s during a trial interconnection of the Northwest Power Pool and the Southwest Power Pool in the Northern American power network. Afterwards, low frequency oscillations have been observed and reported in many places. Among various oscillatory problems, a low frequency, typically in the range of 0.1-0.7 Hz is considered as serious. This oscillation problem often referred to as interarea oscillation results from participation of several generating units from different areas. Low frequency oscillations in other frequency range are considered to be less serious and can be dealt with easily compared to the previous oscillation. The basic reason for oscillation problem is lack of damping on critical modes and many methods and control strategies have been proposed and devised for oscillation damping. This dissertation studies the use of fuzzy logic based controllers for oscillation damping in power systems and it is organized in three parts. In the first part, conventional and fuzzy logic based controllers are compared for oscillation damping at generator location. The controllers considered include, conventional Automatic Voltage Regulators (CAVR), conventional Power System Stabilizers (CPSS), Fuzzy logic based AVR (FLAVR), Fuzzy logic based PSS (FLPSS) and combinations of them. Performances of controller and combination of controllers are compared by comparing time domain simulations. One of the issues with Fuzzy logic controllers is that they cannot be modeled in state-space form for carrying out eigenvalue analysis to obtain critical eigenvalue and damping ratio. Hence, in the second part, the dissertation proposes a method to find out damping ratio on the critical mode to study fuzzy logic controller performance in more details using “envelop tracing”. The proposed method has been tested and validated in a system with conventional controller that can be modeled for both time domain and eigenvalue analyses. In the third part, application of wide-area control is used for oscillation damping. The controller used in the part is Static Var Compensator (SVC) and a new control input signal has been proposed to damp low frequency oscillation. Two-area test system that has been used for oscillation studies is used in this part to check the wide-area control with proposed control input signal. A number of cases has been tried to see the performance of proposed wide-area control.