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An antenna-based radio frequency (RF) partial discharge (PD) measurement system has been investigated in the substation environment. This research concentrates on the complex propagation of the RF signals, the different responses of the PD defects and the effect of the equipment metallic structure on identifying a PD location using the RF PD measurement. A high voltage CT-busbar structure is used for experimental investigations as it is widely applied in a substation.

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This thesis investigates the performance of a medium-voltage (MV) 12-pulse controlled rectifier in conjunction with a tapped front-end transformer used with a shunt active power filter (APF) for ac-side compensation. A series APF based on a magnetic amplifier is used for dc-side compensation. The series APF is coupled with the shunt APF to a common controlled dc-bus. forming an ac/dc unified power controller. An operating power locus is introduced that combines symmetrical and asymmetrical firing of the two constituent 6-pulse converters. It provides the ' lowest' total harmonic distortion (THO) with a maximum reactive power of 0.5 pu. Even with the proposed power locus, a harmonic compensation technique is mandatory. Hence a shunt APF is connected to the secondary taps of the front-end, star/star/delta, transformer to improve the input current THD and power factor. The APF with predictive current control and synchronous d-q frame base control arc used. To avoid time delay effects, the actual APF inverter transfer function is u.sed to formulate an open loop control strategy, used to achieve better harmonic cancellation than previously investigated techniques. To reduce the active filter current, a fixed shunt capacitor bank is connected to the transformer secondary taps to offset the proposed power locus and consequently, to minimize VAr compensation required of the APF. A series APF with a magnetic amplifier is used as a voltage harmonic compensator on the deside. It improves the output voltage ripple factor (RF) and reduces the de-link smoothing inductance requirement. Coupling both compensation techniques improves the supply current TI-ID, produces a near unity power factor, and the output voltage RF is reduced. An auxiliary dc-voltage source for the series APF is avoided by coupling the two compensators back-to-back. MATLAi3 simulation results for the MY system are experimental1y verified with a scaled low-voltage prototype system.

Recent developments in power electronic switching devices and microprocessors have led to power electronic systems with high efficiency and reliability. Today, high performance and low cost are the requirements to be met for most practical systems. This work describes the development of real-time based Uninterruptible Power Supply systems (UPS) which can operate on-line and off-line. Synchronous operation is an inevitable condition in off-line UPS systems. Here, back-synchronisation after a power failure is achieved automatically with the pre-set values. A new modified regular sampled symmetric pulse width modulation method has been developed, it has many advantages especially in low frequency applications. High switching frequency (18 kHz) reduces significantly the filtration and noise problems in UPS systems. Phase delay associated with transformer, filter, load, and control algorithm might cause drastic power failures in by-pass switches, thus to overcome this problem, phase shift control algorithms are developed. PID control algorithm is simplified for real-time on-line operation at high switching frequency. The complete system is based on a single Intel 80C196KC microcontroller chip and the developed software has the flexibility for further development. The developed system has been tested in the laboratory under linear load condition and the results are given. The test results are found to be satisfactory for most load conditions.

Electrical power quality has grown from obscurity to a major issue due to the wide use of non-linear power electronic loads in many applications. Passive filters have been extensively used to prevent harmonics from entering the power system. However, there are many disadvantages and hence active power filters provide an alternative solution to the passive filters. Active power filters have the advantages over the passive filters such as low cost, smaller physical size and are able to compensate for randomly varying harmonic currents. In this thesis, Shunt active power filters have been employed for harmonic current and reactive power compensation for typical non-linear loads. The characteristics of active power filters have been discussed and the operation modes of both single-phase and three-phase active power filters have been documented. The proposed APF system is based on voltage-source current-controlled PWM converter topology, and uses two separate controllers, namely current control loop (CCL) and voltage control loop (VCL). CCL deals with the supply current using a PI controller for current harmonic reduction by injecting the required amount of harmonic current components with equal in magnitude but opposite in polarity to the utility to cancel the harmonic currents produced by the non-linear power electronics load. The VCL is based on sliding mode (SLMC) control, it served two purposes: it regulates and maintains the DC capacitor voltage at a constant level and provides the feedback information for the reference current. The design of the proposed active power filter system has been carried out using ORCAD PSPICE simulation software package. The contribution of this research work is in-depth investigation of the shunt active power filter for supply current harmonics reduction and reactive power compensation, the current waveform is maintained sinusoidal and in phase with the supply voltage for a given non-linear load so as to improve the power factor. The current information is extracted using the supply current detection method and the voltage source current controlled PWM converter is employed to extract the voltage information required for the APF action. The simulation and practical results are discussed and conclusion drawn.

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