MODELING, CONTROL AND APPLICATION OF ACTIVE POWER FILTERS TO IMPROVE POWER QUALITY

Abstract

Electrical energy has not only become a basic need for human existence but also has become the backbone for our economic development. The demand for electrical energy is expected to increase as long as world population increases and people continue to demand a higher standard of living. However, many developing countries are not able to increase their generation capacities in the desired rate and proportion and thus, a large portion of demand remains unserved. The ever increasing load demand and scarcity of electric power have attracted the attention of power engineers/researchers to evolve the methods for effective utilization of electrical energy. Such evolutions have increased the use of power electronics devices and controllers in manifold over last two decades for more efficient operation of the electrical equipments and to transfer more power through the power networks. Although these power electronics devices/controllers operate on the sinusoidal voltage, they inject harmonics and increase reactive power demand in power system networks. The injected harmonics are responsible for the distortion of the voltage and current wave shapes. Different sources of harmonics include static converters, adjustable speed drives, switching power supplies, cycloconverters, magnetizing current of transformer, arc furnaces, welding machines, etc. A major portion of total current harmonics is generated by nonlinear loads and is termed as customer generated current harmonics. Further, asymmetrical distribution of large number of single-phase loads results in voltage and current imbalance in the three-phase system. Moreover, the single-phase loads like personal computer (PC), fluorescent light, printer, xerox machine, television, etc. are also responsible for excessive neutral current in three-phase four wire (3P4W) distribution system. Any significant deviation in the magnitude of the voltage, current and frequency or their waveform purity may result in a potential power quality problem. Nonsinusoidal, unbalanced voltage and/or current and excessive reactive power have several adverse impact on utilities and the connected loads. These impacts include increase in overall system losses, malfunctioning of the protective devices, demolition of reactive power compensating capacitors and create dangerous operating conditions by means of parallel resonance between the supply system reactance and the power factor improvement capacitors. Hence, an effective elimination of these harmonics, balancing of system and improvement of power factor are essential for the utilities and end users. Study of harmonics, their causes and effects, and analysis is essential for design, development and placement of various devices to mitigate the power quality problems. Therefore, in this thesis, a broad survey of harmonic generated loads, which mainly consist of large number of residential, commercial and industrial loads, is conducted. The voltage and current waveforms of some commonly used loads and their harmonic spectrums are recorded. The total harmonic distortion (THD) is used as an index to identify the effects of different nonlinear loads on voltage and current waveforms. Discussions on the generation of harmonics resulting from application of different nonlinear loads and other related important findings during the survey are outlined. Some most commonly used residential and commercial nonlinear loads are simulated and validated experimentally to verify their characteristics whether they behave like currenttype or voltage-type harmonic source. To meet the harmonic mitigation requirements, passive as well as active power filters (APFs) have been in vogue. Classically, shunt passive filters consisting of tuned LC components were used to suppress the harmonics. However, these filters suffer from the problem of bulkiness, high cost and fixed compensation. These can also excite resonance phenomena with the system parameters. Moreover, to filter out all harmonics, one needs as many tuned filters as the number of harmonics to be eliminated. In contrast, modern APFs are superior in filtering performance, smaller in size and more flexible in application as compared to traditional passive filters. At present, APFs are becoming affordable due to reduction in cost of power semiconductor devices and integrated digital control circuits. In addition, APF also acts as a power conditioning device which provides cluster of multiple functions such as harmonic compensation, reactive power control, damping out harmonics caused by resonance, load balancing, and their combinations. APF topologies are reported in both current controlled voltage source converter and current controlled current source converter. The voltage source converter gives higher overall efficiency, lower cost, lower size and simple control structure as compared to current source converters. Therefore, in this thesis voltage source converter topology is considered for implementing the APF control algorithms. The quality and performance of APF as compensator depends mainly on three considerations, the design of the power converter, the method implemented to generate the reference command and the modulation techniques used to follow the current template. Reference command generation is the most important task and has a major role in active power filtering. Therefore, most of the research work is centered on it. A number of algorithms in time as well as frequency domains are proposed to estimate the reference compensating current. One category of control algorithms, based on sensing of harmonics and reactive power requirement of loads, is complex and difficult to implement. Another category of algorithms is based on sensing of line current and estimation of the reference current by regulating the dc-bus voltage. This category of algorithms is simple and easy to implement. The transient response of the APF during compensation of rapidly varying nonlinear loads is very important. The dc-bus capacitor voltage of active filter is the function of load current, and hence, it is significantly affected by variation in load current. The smooth operation of filter during change of load needs variation of the dc-bus voltage within the prescribed limits. Conventionally, proportional integral (PI) controller is employed for this purpose taking the deviation of the dc-bus capacitor voltage from its reference value as an input. However, PI controllers, generally, require a precise mathematical model of the system to determine the control parameters, which is difficult to obtain due to nonlinear nature of the system. Moreover, in most industrial environment, utility voltage is often distorted and it is observed that the performance of the APF is deteriorated under distorted voltage conditions. The current is further distorted due to distortion in the utility voltage and these distortions are named as utility or source generated current harmonics. In this work, the efforts have been made to analyze, design, and develop high performance control techniques for shunt APF. In order to improve system response and to simplify dc-bus controller, two indirect current control techniques are developed. The dynamic response of APF is simulated under MATLAB/Simulink environment and then, experimentally validated using TMS320F2812 digital signal processor under abrupt variation in load. The proposed dc-bus voltage controller is based on the dc-bus capacitor energy. In the developed dc-bus controller, only gain Kpe is to be adjusted for the given system to regulate overshoot and minimizing steady-state error. The detailed modeling and simulation verifications are presented to prove the capabilities of the proposed dcbus voltage controller over conventional dc-bus controller. In the first method of indirect control, conductance emulation based scalar technique is proposed to compensate only customer generated current harmonics and reactive power. If the utility voltage is sinusoidal, the shunt APF compensates total current harmonics, and hence, the compensated source current is also sinusoidal. However, if the utility voltage is distorted, it allows similar level of distortion as available in utility voltage in the compensated source current. This results in the same shape of the compensated source current as that of the source voltage. Thus, this algorithm attributes the responsibilities of the customer as well as utility at the point of common coupling (PCC). Second method of indirect control technique is suitable for distorted source voltage conditions, and hence, compensates customer as well as utility generated current harmonics. The proposed methods do not need to sense load currents and compensating currents, and hence, less number of sensors are required for their implementation. The simulated and experimental results under sudden application and removal of loads demonstrate the robustness of the proposed controller. A procedure for selection of power circuit parameter of APF is also presented in the thesis. It is observed from the simulated results that the performance of the indirect control techniques are suitable for distorted utility voltage and nonlinear load conditions. However, under unbalance and fluctuation in supply voltage, the response of the indirect methods is not adequate. This is due to the fact that the influence of source voltage significantly affects the dc-bus voltage, and hence, the designing needs the consideration of source voltage variation. This makes the overall design of the dc-bus controller complex. Further, the performance of the indirect current control is largely affected in case of unbalanced single-phase loads in 3P4W supply system. In view of above, a direct control technique, suitable for distorted, unbalanced, and fluctuating source voltage condition, is also developed and its performance is simulated and validated experimentally for three-phase three-wire (3P3W) distribution system. The control is also tested to compensate neutral current along with harmonic and reactive current compensation and load balancing in 3P4W distribution systems. In order to make the source currents sinusoidal, the positive sequence components of the source voltage are deduced in the proposed direct control method. The simulated results under transient and steady-state condition for 3P3W and 3P4W systems are presented to validate the effectiveness of the proposed technique. APFs topologies alone are very effective to compensate harmonics and other power quality related problems. However, high cost of the APFs is the main hindrance for their adoption in the distribution system for utility point of view. The hybrid filters, the combination of one or more passive and active filters provide cost effective and efficient solution particularly for high power nonlinear loads. In hybrid filters, conventionally, the single-tuned passive filters (STPFs) were used in conjunction with APF for reducing the overall rating of active filter. However, the STPFs are bulky and expensive, cause more power loss and require more number of switchgears for their operation. An attempt, therefore, has been made in this research work on the analysis, design and development of a new topology and control of hybrid filter for reducing the rating of shunt APF. The three-tuned passive filters (TTPFs) are suggested instead of STPFs. The main objective deals with the design and selection of TTPF parameters. A schematic design and selection procedure for TTPF parameters is developed and outlined in this thesis. In the proposed control strategy of hybrid filter, the most dominating harmonics of lower order are suppressed by the TTPFs and remaining higher order harmonics are compensated by APF. Apart from selective harmonic suppression, the TTPFs are also used for fixed reactive power compensation. The overall control scheme also takes care of change in reactive power demand due to change in load. This additional reactive power demand is compensated by APF. The overall system of proposed hybrid filter is developed in Matlab/Simulink environment and then, cosimulated using TMS320F2812 digital signal processors through processor-in-loop (PIL). The co-simulation is carried out for transient as well as steady-state conditions under distorted and unbalanced supply. The simulated responses for APF alone, APF in conjunction with STPFs and APF combined with TTPFs are presented to show the reduction in rating of APF along with harmonic and reactive current compensation and load balancing. Despite of above mentioned tasks such as harmonic elimination, reactive power compensation, load balancing, and neutral current elimination, APFs can also be used to interface the non-conventional energy sources, such as photovoltaic (PV), wind, fuelcells, etc., with the grid. In this case, APF is used to perform bidirectional active power exchange with the grid. PV generation is one of the most promising and foremost nonconventional energy sources. The compensation functions, if implemented through a bidirectional converter, would lead to increase in efficiency and better utilization of the Grid-interactive PV systems. This may result in considerable savings in cost and size of the equipments. An attempt has been made in this thesis on the modeling, control and cosimulation of a Grid-interactive PV system with active filter functions. The performance of the proposed Grid-interactive PV system in conjunction with APF is analyzed for active and reactive power control under distorted grid voltage conditions. The simplified circuit of a PV array with maximum power point tracking is modeled in MATLAB/Simulink. The developed model is directly connected to dc-side of the voltage source inverter used for grid-PV interaction. The proposed work also deals with the development of a controller which either controls PV power or works as an APF or performs both the functions simultaneously. The injected active and reactive powers are regulated by changing the phase and amplitude, respectively, of the output voltage of the inverter under varying solar insolation and temperature conditions. The simulation model of proposed system is developed in MATLAB/Simulink environment and then, co-simulated using TMS320F2812 DSP through PIL under rapidly changing atmospheric and load conditions to confirm its effectiveness and robustness. Following is the summary of major endeavors of the author in the thesis work: 1.To identify the existing level of harmonic distortion present in the supply system and its upcoming trends, an extensive survey on the voltage and current harmonics measurement for a variety of nonlinear loads is conducted. 2. Design, analysis and control of source current sensing based two indirect current control techniques for APF are performed. Simulation followed by experimental study is carried out to investigate the performance of the APF for varying non-linear loads. 3. Design, analysis and development of APF based on direct current control technique is carried out for 3P3W and 3P4W distribution systems. Exhaustive simulations under MATLAB/Simulink environment are carried out to investigate the performance of the APF during transient as well as steadystate conditions. The control strategy is also verified experimentally by developing a laboratory prototype. 4. Design and development of a new topology for hybrid active power filter is presented. The overall system is modeled in MATLAB/Simulink environment and then co-simulated in TMS320F2812 DSP through PIL. 5. Modeling, analysis and control of Grid-interactive photovoltaic system with APF functions are performed. The overall system is modeled in MATLAB/Simulink environment and then co-simulated in TMS320F2812 DSP through PIL.