Title of Thesis

Power Flow Control Strategy at the Load Bus in the presence of Dispersed Generation

Author(s)

Aamir Hanif

Abstract
Due to increased proliferation of sensitive loads in the distribution network, voltage sags, swells and phase jumps mitigation has become the focus of power quality (PQ) research. This reduces its adverse economical impact. Equipment sensitivity curve shows that small duration outage of one-cycle is tolerable for sensitive loads. Therefore, the control system should be capable for voltage sag detection and mitigation in less than 20 ms at 50 Hz.
There are various custom power devices for protecting industrial processes against voltage sags. Due to simple control, fast response and fewer transients; dynamic voltage restorer (DVR) is investigated in this research work. Several voltage-sourced converter (VSC) control strategies for the DVR are proposed to maintain constant load voltage for smooth and clean power delivery to the load. A new leading series voltage injection (LSVI) technique is presented as a first strategy for the mitigation of sag. A mathematical model of LSVI by a DVR for voltage sag mitigation has been developed. Simulation results have been obtained to validate the developed model. Conventionally, distributed generation (DG) source is connected in parallel with utility to share load and export its excess power to utility. A new concept of power export through series connection of DG supported DVR is presented, whose power export capability is managed by magnitude of
injected voltage as well as its phase-angle instead of the current. However, the approach for mitigating voltage sag using LSVI scheme is suitable for those locations where the phase-shift in the voltage will not cause any problem.
A feed-forward VSC control strategy for the DVR is then presented, based on unipolar SPWM switching strategy. This is a novel but a simple control strategy in which the VSC not only converts DC to AC but also mitigates voltage sags, swells and phase jumps in the utility voltage, acting as power flow controller. Comparison of instantaneous utility voltage and generated three-phase reference voltage is carried out to detect sags, swells and phase jumps in the utility voltage. It then generates appropriate switching signals for the VSC of DVR to mitigate them.With the help of proposed control scheme of DVR, the dynamic capability of DVR increases the sensitive load penetration in the distribution system. LSVI control uses leading series voltage injection technique whereas a feed-forward method of compensation uses in-phase injection to regulate the load voltage. The magnitude of injected DVR voltage is minimal in in-phase injection
compensation strategy. However, the presented power export concept is valid only for the LSVI scheme.
The simulation for the presented control strategies have been performed using SimPowerSystems™ tool box of MATLAB®. The proposed control strategies detect utility disturbances and mitigate them within half-cycle against a target of one-cycle. These strategies succeed in keeping load voltage THD limit within 5%.

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2.1 Introduction
2.2 Issues with Distribution System Power Flow
2.3 Power Flow Control Techniques
2.4 DG and Distribution System Power Flow
2.5 Summary

3.1 Introduction
3.2 Recent Developments
3.3 Summary

4.1 Introduction
4.2 DVR Components
4.3 DVR Operation
4.4 DVR Compensation Strategies and their Limitations
4.5 DVRs System Topologies
4.6 Summary

5.1 Introduction
5.2 Proposed Mathematical Model for Leading Voltage Injection
5.3 Power Export
5.4 Simulation Results
5.5 Feed-forward Control Strategy for the VSC of DVR
5.6 Simulation Results
5.7 Cost Benefit Analysis
5.8 Summary

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