POWER QUALITY IMPROVEMENT WITH SHUNT ACTIVE POWER FILTER

Abstract

In recent years, there has been increased use of power electronic-based systems in industry as well as among residential consumers for controlling AC and DC powers so as to increase automation, controllability and energy efficiency. Major applications of these power electronic systems lies in static converters, adjustable speed drives, traction systems, arc furnaces, flexible AC transmission systems (FACTS), high voltage direct current (HVDC) system, switch mode power supplies (SMPS), computers and programmable logic controllers (PLC's), etc. These power electronic controllers provide precise control of the process and serve as energy conditioners among various powerlevel consumers. In addition to the numerous advantages, these power electronic-based systems suffer from the problem of drawing harmonics (non-sinusoidal currents) and reactive components of current from the source, and offer highly nonlinear characteristics. The injected harmonics are responsible for the distortion of the voltage and current wave shapes which affect the power quality. In addition, voltage source harmonics producing loads inject voltage harmonics into the system. This result in non-sinusoidal voltage in the system, which affects other linear loads connected to the same point of common coupling (PCC). Further, poor power quality is a matter of concern not only in three-phase, threewire system but also in three-phase, four-wire system. Moreover, asymmetrical distribution of large number of single-phase loads, ranging from few watts to MWs results in voltage and current imbalance in the electrical power system. They are also responsible for excessive neutral current in three-phase, four-wire distribution system. Non-sinusoidal and unbalanced voltage and/or current and excessive reactive power demand have several adverse impacts on utilities and the loads connected to it. These impacts include increased rms value of the input current, pulsating torques in rotating machines, increased losses in transformers and cables, disturbance to other consumers, failure of reactive power compensating capacitors, neutral burning, malfunctioning of the protective devices, blowing of fuses, interference in nearby communication network, poor power factor and low system efficiency, etc. In concerning with above problems of poor power quality an effective compensation solution with defined harmonic limit is required to improve the power quality. Many electrical regulatory commissions and standards such as IEEE 519-1992, IEC 61000-3-2, etc. recommend voltage and current distortion limits for consumers and utilities at various power levels. These guidelines are supportive in promoting better practices in planning and erection of both power systems and nonlinear equipment. Quantitative assessment of harmonics, their magnitude and detailed analysis is essential for design, development and placement of various compensating devices to mitigate the power quality problems. Therefore, in the beginning of the thesis, a harmonic survey has been conducted at different residential, commercial and industrial loads to know the existing level of harmonics present in the system. Various voltage and current waveforms of some of the commonly used loads and their harmonic spectrum have been recorded using power quality analyzer. The total harmonic distortion (THD) is used as an index to identify the effects of different nonlinear loads on voltage and current waveforms. Important findings during the survey are outlined. Passive filters, active power filters and their combinations have been used with conventional converters in order to improve the power quality. Conventionally, passive filters (L-C filters) have been used to improve power quality. Though passive compensation is a simple approach, but they have several drawbacks such as inability to provide dynamic compensation, bulky size, cost, resonance problem, separate filters for each harmonics, etc. Shortcomings of passive compensation and increased concern of power quality has attracted wide attention of researchers in the design, analysis and implementation of various custom power devices with robust control and ease implementation using high speed processing devices for three-phase, three-wire and three-phase, four-wire supply system. The active power filters (APFs) is one of the widely used compensation device to mitigate power quality problems by drawing or supply suitable compensating signal from or to the utility so that voltage and current (AC side quantities) can be maintained sinusoidal. It acts as a power conditioning device which provides cluster of multiple functions. APF topologies are reported in both current controlled voltage source converter and current controlled current source converter. However, the voltage source converter gives higher efficiency, lower cost, lower size and simple structure as compared to current source converters. With the remarkable development and advances in power semiconductor devices and integrated digital control circuits, various topologies of the APFs are developed for the compensation of voltage and current harmonics, reactive power, neutral current, unbalance, voltage-flicker, etc. In this connection, a literature survey on various topologies, control techniques and applications of APFs has been done and a comprehensive report is presented. Among them, shunt APFs are proved to be effective, to absorb current harmonics, compensation of reactive power and current imbalance at the customer end. Therefore, in this thesis design and development of a three-phase shunt APF based on current controlled voltage source PWM converter has been investigated for the compensation of harmonics and reactive power. The quality and performance of the APF depends on the topology of APF, methods of estimation of compensating signal by appropriate control algorithm, supply voltage distortions, and the modulation techniques used to generate switching signals for PWM converter. This also covers minimization of calculation and conversion steps involved in control scheme to make it efficient and simple. A number of algorithms in time as well as frequency domain have been proposed to estimate the compensating current in the literature. Most of the research work is being centered on it. However, one category of control algorithms is based on sensing harmonics and reactive power requirement of loads, which is quite complex and difficult to implement. Another approach of algorithms is based on sensing line currents and part of reference current is estimated by regulating the dc-bus voltage. This category of algorithms is simple and easy to implement. Direct and indirect current control techniques are established in view of switching ripples, load perturbation response, % THD is supply current, losses, number of current sensors count, and computational time etc. These current control techniques are based on the method for calculating the reference current signal for the PWM converter. In view of above, the efforts have been made to analyze, design and develop high performance control technique for shunt APF, to improve the power quality under sinusoidal and distorted supply voltage. The proposed algorithm is based on average power theory by sensing line currents and does not require to sense harmonics or var requirement of the load. The control is simple, thereby enhancing the system reliability, which is applicable for three-phase, three-wire as well as three-phase, four-wire systems. The method calculate the peak value of the supply current which consists two main components of current, viz., average component of real power and loss component of current which indirectly feeds losses in the converter. Average power is calculated by averaging instantaneous power over one sixth of a cycle. The loss component of APF is obtained by comparing actual capacitor voltage and reference capacitor voltage. This in turn is used to obtain the peak current component of fundamental load current. The threephase reference source currents are obtained by multiplying this peak value of current component with unit template of supply voltage. Moreover, in most industrial environment, utility voltage is distorted and it is observed that the performance of the APF is deteriorated under distorted voltage conditions. Sometimes, the generated voltage waveforms are sinusoidal but there are many devices that distort the mains voltage, and these distortions are propagated all over the network. In view of this, a fundamental tuned filter (FTF) is developed to derive fundamental signal from distorted supply voltage without using phase-locked loop (PLL) or low/high pass filter. Easy implementation of FTF for selected frequency value using high speed processing device like digital signal processors (DSPs), reduce the cost and complexity of a real-time system. The scheme makes the control easier. Complete design aspects of power and control circuit parameters are described, and system performance is investigated to validate the algorithm. The comparison of actual source current with the reference source current is used to generate error signal which is process by carrier less hysteresis based current controller to obtain the switching signals. The overall simulation model of shunt APF with control algorithm is developed in MATLAB® and Simulink® environment. C28X IQmath library of optimized blocks and chip support block of Texas instruments C2000™ library are used for the implementation purpose. Detailed simulation results are presented to investigate the steady state as well as transient performance of shunt APF under sinusoidal and distorted supply voltage condition. The performance of the active filter is tested on an experimental prototype, in which TMS320F28335 floating-point DSP has been used for computation and control purpose. Control scheme is tested for different types of nonlinear load. The simulated and experimental results demonstrate the effectiveness of the proposed controller. In many commercial and industrial applications power is distributed through a three-phase, four-wire system. Load unbalance as well as single-phase nonlinear loads, leads to excessive neutral current, which can cause overload to power feeders and transformers, voltage distortion, and common mode noise. Performance of a three-phase, three-wire APF in such cases is not found to be satisfactory. Three single-phase units can be connected between each phase and neutral, but it is cumbersome and expensive. In place of three-phase converters, four-wire converters are proposed in the literature. The four-wire converters can take either of two forms; four-leg converter with one capacitor, and three-leg converter with split capacitor topology. Due to the better controllability and simplicity with reduced dc-link capacitor rating, a three-phase, four-wire shunt APF based on four switching-leg topology is developed and investigated in a non-ideal supply voltage distribution system. The control scheme is based on sensing supply voltage, load and supply current. Balanced and sinusoidal component of supply voltage is derived using symmetrical component theory and FTF control algorithm, respectively, so that source currents can be made as sinusoidal. The control is tested to compensate neutral current along with harmonic and reactive current compensation and load balancing in three-phase, four-wire distribution systems with different load conditions and diverse cases of supply voltages. Various simulation results are presented to validate the scheme and verified experimentally using TMS320F28335 DSP. The results show that the proposed algorithm is capable of meeting the requirements of IEEE 519 standards. Generally, conventional PI controller is used to regulate the DC link voltage for the estimation of part of reference currents in control scheme developed for APF. However, PI controller requires precise mathematical model of the system, which is some times difficult to obtain or may not be available due to nonlinear nature of the system. Under such cases controller fails to perform satisfactorily due to nonlinearity, parameter variations, load disturbances, etc. Recently, fuzzy logic is an alternate approach to handle this type of problem and generate a good deal of interest in control architecture of many power electronic applications due to its distinct advantages of robustness against parameter variation, customization, etc. Therefore, a fuzzy logic based control scheme is developed, which is based on the simple linguistic variables and offer more freedom of design. A fuzzy logic based load controller gives nonlinear control with fast response and virtually no overshoot. Simulation studies have been carried out to study the performance of the fuzzy controlled shunt APF for compensation of current harmonics, reactive power and input current balancing, in three-phase, three-wire and three-phase, four-wire systems. Simulation results are presented during transients and steady states, verified experimentally, and compared with the conventional PI controller. Recently, artificial neural network (ANN) in power electronics area due to its selfadapting and rapid calculation characteristics, allows control to handle high non-linearity and uncertainties in a nonlinear system. Neural network gives required output by proper on-line and off-line training based on different learning rules. Adaptive neural network techniques like ADALINE have been used in active power filtering in various ways by different researchers. However, the use of ANN in distorted supply voltage case is less explored. In view of this, an indirect current controlled and ANN based shunt APF is developed and analyzed to compensate current harmonics and reactive power under nonlinear load and distorted supply voltage conditions. Direct and indirect current techniques are presented based on reference current signal generation. The proposed compensation process is based on sensing of source current (indirect current control) instead of filter current, an approach different from conventional techniques for eliminating of switching ripples. The instantaneous reactive (p-q) theory is used to find the real and reactive powers of the load. The advantage of p-q theory is that real and reactive powers associated with fundamental components are DC quantities. These quantities can be extracted using LPF. The average capacitor voltage is compared with reference voltage and error signal is processed through PI controller to get the loss component of APF which is added to real power component of load power to derive reference source currents. The performance of conventional p-q theory is improved by using ADALINE network under distorted mains voltage. Fundamental frequency signal extraction is done using ADALINE network from distorted voltage signal. ADALINE gives simple, robust and accurate design for selected frequency extraction. Various simulations are carried out to study the performance of proposed ADALINE design. Simulated response has been validated with a laboratory prototype using similar platform of TMS320F28335 DSP and tested on different non-linear loads. A comparative study is conducted to validate the effectiveness of the indirect current control over direct current control, and neural network performance. To summarize, first of all, a harmonic survey is carried out at the commercial and industrial loads, to know the existing level of harmonic pollution present in the system and future trends. Various voltage and current waveforms have been recorded. A control algorithm for shunt APF is then designed developed and investigated for power quality improvement, by compensating harmonics, reactive power requirement of the nonlinear loads, and input current balancing in three-phase, three-wire and three-phase, four-wire systems. Implementation of average power theory and fundamental tuned filter using IQmath library of Texas Instruments C2000™ Library is explored and used. Simulation and experimental results are obtained under different loading conditions, to investigate the steady state and transient performance. Results have been presented for different cases of supply voltage. A fuzzy logic controlled system is developed to improve the transient performance of APF and also compared with the conventional PI controller. An ADALINE network and indirect current control based control algorithm using p-q theory is proposed to eliminate current harmonics and reactive power compensation under sinusoidal and distorted supply voltage. Its viability is ascertained by simulation results and verified experimentally for different loading conditions.