Modeling and Simulation of the Controllable Network Transformers

ABSTRACT: The work presented in this thesis looked into the working principle of the Controllable Network Transformer in controlling the flow of power through tie lines. It has always been difficult to control the power flowing through the tie lines that connect two areas. This causes a lot of stress on the power grid and makes it weak as time goes on. There had been a number of ways of controlling this flow of power and this has proven to achieve a limited amount of control. The previous method had made use of devices such as the Load tap Changing Transformers and Phase Shifting Transformers. The controllable transformer is introduced as a simple device that was realized by augmenting a fraction of a centre-tapped or a Linear Tap Changing transformer (LTC) with a small bidirectional low-rated AC-AC converter. The dual virtual quadrature sources (DVQS) scheme was also developed and used as against the conventional Pulse Width Modulation. This DVQS makes use of the Even Harmonic Modulation techniques (EHM) by injecting a series of voltage sources between the two buses to control both the magnitude and direction of the flow of power in the model. This thesis discussed the basics, modeling and theoretical analyses of the CNT extensively. The derived equations were then compared with results obtained from simulations using MATLAB simulink. The Simulations of the CNT Model was discussed extensively in Chapter Three. During the simulations, each elements of the model were tested to see their contributions to the model. It was also observed that there are huge amount of third harmonics in the circuit. These are unwanted, hence a third-harmonic trap was designed to reduce or remove the unwanted third harmonics in the circuit. The effect of the DC component K0 of the reference voltage, the second harmonic amplitude K2 and the phase angle φ on the Power output were also considered. The bidirectional control property of the CNT on the power flow was also considered. Finally, we also developed the Variable Structural System control of the CNT. This enabled us to analyze the full working principle of the CNT and the effect of the duty cycle d on the overall performance of the CNT model. The advantages and the shortcomings of the CNT as compared to the other power flow controller were also analyzed and discussed under the discussion section in Chapter 3. Keywords: Controllable Network Transformer (CNT), Dual Virtual Quadrature Scheme (DVQS), Even harmonic modulation (EHM), Smart grid, Power flow control. ……………………………………………………………………………………………………………………………………………………………………………………………………………………