Power-Shield: A Powerquality Enhancer for Current-Source Type of Nonlinear Loads

Abstract

Electrical power is the most convenient form of energy-source in terms of generation, transmission, utilization, and efficiency. It can be converted to any form which is necessary and useful to mankind. The process of conversion has undergone tremendous changes due to the advancements in the technology and the demand by the consumer. The efficiency, size, cost, and reliability in the conversion process are important. Power Electronics has been playing an important role in this process. However due to the switching action in the power-electronics-converters lot of harmonics are generated. These harmonics are injected into the power system and they spread across it, some times even getting amplified. This can affect the operation of other devices connected to the system, because such currents manifest as harmonic voltages across the power system network. The voltage disturbances – sag, swell, and switching transients at the load-point are also of serious concern to the load. The performance and the life of the electrical equipment suffer in general, while there are some equipment which are sensitive to the disturbances from the utility side. There are reports of heavy loss of revenue due to the failure of sensitive equipment. Thus the quality of Electrical Power provided by the utility and the permissible extent of pollution of the utility grid by the consumer became important. This has led to the development of the International Standards with the active participation of all the stakeholders – the utility, the consumer, the equipment manufacturer, the measuring instrument manufacturer, and the researchers – who play the role of defining the quantities which shall be/ can be compensated. The violation of these norms is likely to invite heavy penalties. One class of load drawing nonsinusoidal current from the source is the current-source type of nonlinear loads or current-stiff nonlinear loads. These loads are found in applications like – the Current-Source Inverter fed Induction Motor and Synchronous Motors, the DC Motor loads, the Permanent Magnet Synchronous Motors, the battery charging circuits in the constant-current charging mode, electro plating, high energymagnet applications, the superconducting magnetic energy storage systems, and plugin Electric vehicle battery chargers. While they draw the harmonic currents from the loads, they are also sensitive to disturbances from the source side. Considerable amount of work is done towards mitigating the problems due to such loads and to insulate them from the disturbances from the source. A thorough study is carried out regarding the sources of distortion, effects of distortion, power theory, applicable standards, and means for compensation – topology, compensating-signal extraction techniques, modeling, control, and switching techniques. The performance of different options available are evaluated. This led to the conceptualization of a multifunction device – Power-Shield that can shield the source and the load from each other as far as the unwanted disturbances and pollution are concerned. In this work the philosophy adopted is that of the mission in IEEE Std. 519:1992, revised in 2014, which prescribes that the utility is responsible for maintaining a good quality voltage and frequency, and that the consumer shall draw a near sinusoidal, unity power factor, and balanced current from the source. It is intended to make the load insensitive to the disturbances from the source side and offer itself as a balanced and linear load on the utility system. The custom power device Power-Shield is achieved by a judicious choice and combination of the topology and the control principle. The choice of the topology is validated through the extensive simulation study. The Power-Shield thus arrived at consists of a low power Parallel Active Filter, a Series Active Filter, a Parallel Passive Filter (a combination of tuned (to significant harmonics) filter and a Reactive Power Compensator), and a Commutation Reactance, in that sequence from the utility end to the load end. Also comprising this thesis are two novel, input-locked (in phase and amplitude) and synchronized algorithms for the extraction of the compensating quantities and a modified hysteresis switching technique based on the sensing of the voltage at the far-end (in relation to the injecting converter) of the injection inductor. The algorithms exhibit a very good transient response of around 60 ms, zero steady-state error, and excellent noise rejection capability. The novel switching principle proposed here for the sampled hysteresis control has the attraction of a lesser number of unipolar switchings per cycle xand a better control over the error band. A single-phase thyristor controlled rectifier feeding a separately excited DC motor as an example of a current-stiff load is assembled. Parallel Passive Filters comprising of capacitor for reactive power compensation and tuned harmonic filters for harmonic compensation are designed, implemented, and tested. A three-phase IGBT based inverter is fabricated to serve as a single-phase Parallel Active Filter and a half bridge Series Active Filter. It is tested up to 400 V, 15 A, and at a switching frequency of 32 kHz. The test results and analysis thereof are presented. It is found that the topology arrived at demonstrated the superior quality of filtering by rendering the Parallel Passive Filter more effective. The clean switching waveforms across the IGBTs at that voltage and frequency is a testimony to the quality of fabrication of the converter circuit.