INVESTIGATION ON POWER QUALITY IMPROVEMENT USING UPQC

Abstract

The most intensive enlargement industry wise as key equipment for modern power distribution system are the equipments based on power electronic principles. These offers a vast range of advantages for power processing like flexible control, cost reduction, overall size optimization, etc. On the contrary, utilizing these devices gives rise to numerous problems like reactive power shortage and harmonics polluting the power distribution system. The need of excessive reactive power demand leads to poor power factor, bad voltage regulation and a surge in feeder losses with reduced active power flow capabilities of the distribution system. Additionally the situation worsens in the advent of nonlinear loads raising the bar for power quality issues on distribution system. The operation of non-linear equipments on distribution systems like transformers, induction machines, electric arc furnaces, welding equipment, fluorescent lamps (with magnetic ballasts), etc. are also responsible for generation of harmonics in electric power systems. Additionally various perturbations are reported which ranges from sub cycle duration to long term steady state condition leading to distorted waveform. Short disruption, voltage sag, swell, transients of both current and voltage, distorted harmonics and waveforms, voltage fluctuations , flicker, voltage unbalance are few listed interruptions. The severity of the situation lies when there is commence of waveform distortion, flicker, and voltage imbalance at the distribution system regulation of which becomes the prime concern. A slight deviation in magnitude of voltage from its prescribed limit, current and or frequency or waveform impurity results in a potent power quality problem. These power quality problems results into interruption to the normal operation of the electrical equipments that are connected to the distribution system. Modern equipment are highly responsive to the quality of voltage that is being supplied to them. Hence, by improving the power quality takes into account both healthy and efficient distribution system as well as reduced power losses in turn saving upon the cost. To protect the interest of utility, international agencies like IEC, IEEE have been developing various standards (IEC61000, IEEE 519-1992) for harmonic specifications for point of common coupling as well as individual equipment. In addition, these guidelines promote better practices in both power systems and equipment design hence helping to minimize the operational cost. Conventionally used power quality mitigating devices addresses to a single power quality problem at a single time. This truth lead researchers to develop dynamic and adjustable devices to mitigate multiple power quality problems. A promising approach to it is the Custom Power Devices (CDP’s). These devices amends most of the problems of distribution system due to which most of the existing mitigating devices are being replaced by the CDP’s reducing the cost as there are less number of overall switches involved. The family of CPDs includes distribution static synchronous compensator (DSTATCOM), dynamic i voltage restorer (DVR) and unified power quality conditioner (UPQC) which is used for compensating the power quality problems in the current and/or voltage waveforms. The typical custom power device is the distribution static compensator (DSTATCOM) connected in shunt at PCC. Its sole purpose is to diminish the power quality problem related to current on the distribution side. To achieve harmonic filtering, power factor correction and load balancing, the DSTATCOM injects current at the point of common coupling (PCC). An example of custom series compensation device is the dynamic voltage restorer (DVR). Its main motive is to provide protection to sensitive loads from voltage sag/ swell interruptions and harmonics in the supply side. The requisite voltage magnitude and phase angle is injected in series with the distribution feeder utilizing injection transformers and VSI accompanied by the dc-link voltage from the DC storage capacitor. A more modern and relatively versatile approach to the custom power devices is the unified power quality conditioner (UPQC). UPQC comprises of two inverters back to back in conjunction with a common dc-link. It attends to imperfections of both load current and supply voltage making it a substitute of both DSTATCOM and DVR combined. The present work portraits an extensive and elaborate literature survey of suggested topologies, control strategies of UPQC for the sole purpose of power quality improvement. The adequacy of UPQC with two-level inverter structures has been evaluated for various power quality problems. An extended studies has been performed to multilevel structure of UPQC credits to its unique design which allows a large plethora of high voltage and also reduces the device switching frequency. It is observed that a diode clamed inverter with all the six phases of back to back converter which are sharing a common dc-link is capable of amalgamating desired waveform from several levels of DC voltage. An alluring prospect of investigation would be integration of multilevel diode clamped inverter to UPQC. There are a myriad of features offered by UPQC-ML on deciding the type of control to be used while compensating source voltage (sag, unbalanced voltage, voltage harmonics, current harmonics or reactive power). It also does the same compensating load current playing an important role in the control scheme of back to back inverters. UPQC-3L showcases a much elevated performance in comparison to UPQC-2L as it showcases superior compensating characteristics against distortion in the system. The improvement in the percentage value of THD is also observed for UPQC-3L configuration in comparison to UPQC-2L. Additionally an improved quantity of compensation is seen in with various power quality disturbances for UPQC-3L than UPQC-2L. A simplified model predictive control for UPQC-2L is proposed, in which predictive voltage control for series converter to maintain a constant value of load voltage during voltage disturbances and predictive current control for shunt converter to maintain source current free from distortions is used without need of multivariable complex mathematical ii evaluation. The sensed voltage and current signals of source and load are used to derive the future predicted values of source current and load voltage from the discrete state space model of UPQC. The appropriate switching state is selected and applied to the converters based on the minimization of the cost function, which is selected as the square of the difference between reference and predicted values. Independent generation of reference control signals for both shunt and series converters are performed. The two reference signals are injection voltage by the series transformer into the system, considered for series converter and injectable compensation current by the shunt converter against load distortions, entitled as control reference for shunt converter. In similar way the approach is extended to UPQC-3L configuration with diode clamped multilevel inverter structure. The cost function is modified as the dc-link voltage balancing is mandatory for three-level structure along with series injecting voltage and shunt compensating current signals. The work also focused on the reducing dc-link voltage rating of UPQC. The thorough literature has been studied on this issue. The requirement of dc-link voltage for shunt and series active filter, for UPQC are different. These fluctuation in values lead to a provocative task to assign a common dc-link of pertinent rating to achieve adequate shunt and series compensation. Conventionally, to achieve legitimate compensation the shunt filter requires higher dc-link voltage in comparison to the series active filter thus to fulfill this criterion, researchers have been left with no preference other than to choose common dc-link voltage on the basis of shunt active filter prerequisites. This leads to over rating of the series active filters as the requisites are less in comparison to shunt active filter. Thus, literature studies have revealed UPQC topologies with elevated dc-link voltage. Hence the voltage source inverters (VSIs) turn out to be bulky due to high value of dc-link capacitor. Additionally, the switch rating has to be chosen with increased value of voltage and current in return the entire cost and size upsurges. It has been observed in literatures, that few attempts have been initiated to minimize the inverter capacity by lowering the storage capacity of dc-link voltage by introducing hybrid APFs, thus intensifying system reliability. The usage of IGBTs as switching devices in the VSI of active filters cutbacks on the rating of active filter elements in hybrid filters with sensibly elevated rating. As a result it operates at very high frequency contributing to fast response and decrement in size of ripple filter passive elements and size of dc bus capacitor. A combination of shunt passive power filter (PPF) and series APF constitutes the series hybrid APF (SeHAPF). SHAF is gaining popularity due to its reduced capacity and versatile usage. An example to testify the theory would be a hybrid filter as unification of active series filter (5%) and passive series filter (20%) utilizing only 20% rating of load in case of voltage fed loads. Industrial investigation and dedicated research is directed toward series active filters as they are more popular than shunt counterpart due to its simple configuration and iii operating procedures. The shunt hybrid active power filters (SHAPFs) are series connection of passive and active filters for ease of operation at a conducive voltage and current in conjunction to high rating of active filter up to 60%-80% of the load. Unified power quality conditioner in comparison to series and shunt active filters receives less attention by the researchers by implementing hybrid structures with passive filters. A hybrid UPQC with a branch of passive filters attached to it, tuned specifically with 3rd, 5th, and 7th order harmonics has been proposed by L.H. Zhou et al. This model reduces the capacity of dc-link of shunt converter compared to conventional UPQC. The present work proposes an analytic method to control the dc-link voltage of hybrid UPQC. In majority of the UPQC based power quality conditioner, load reactive power compensation is performed by shunt APF. It is so because the utilization factor of the shunt APF is much elevated in comparison to series APF when utilized in steady state operation and is heavily influenced by load reactive power needs. To achieve lower dc-link voltage, the compensation burden on shunt APF rating should be decreased. This can be made possible by adopting phage angle control (PAC). This method enables the sharing of reactive power between series and shunt APFs by introducing a power angle difference between source and load voltage, maintaining the magnitude of voltages for both the APFs equal. The present work makes an attempt to maintain the magnitude of voltage on the dc-link as low as possible by application of PAC to hybrid UPQC which enables to achieve further reduction in dc-link voltage in comparison to conventional UPQC. The algorithm proposed in this work, indirectly identifies the range of minimum possible dc-link voltage for a given load power factor with suitable power angle between source and load voltage. A comparative study is performed for VA loadings and utilization of power electronic converters of the designed hybrid UPQC under different conditions to the traditional UPQC. Additionally, a generalized algorithm is proposed to evaluate optimal dc-link voltage over a percentage range of voltage sag/swell combinations. Thus, the proposed algorithm gives the best fixed minimum dc-link voltage corresponding to within the range fixed by the algorithm based on the compensation level. In this work the maximum dc-link voltages are evaluated for a range of ±10% of system sag/swell index till 50%, at a given phase angle 𝛿𝛿 varying from 00 to 450. The analysis of minimum dc-link voltage requirement is simulated in MATLAB Simulink platform for three cases of 𝑘𝑘: no voltage sag/swell, 20% sag and swell with the combination of load lagging power factor of 0.7 and also validated experimentally. The feasibility of inclusion of Thyristor Controlled Impedance (TCZ) based PPF in hybrid UPQC is also accessed allowing for a broader range of loading reactive power from lagging to leading power factor. The TCZ-PPF enables to further minimize the dc-link voltage of hybrid UPQC in comparison to the PPF based hybrid UPQC over wide range of compensation. In an Ideal case, the dc-link voltage requirement is zero inside the iv compensating angle and vice-versa for outside the range of TCZ-PPF. Additionally, the minimum dc-link voltage is also evaluated for the compensation of current harmonics that are injected by the presence of nonlinear loads apart from the reactive power compensation. Three cases have been accessed to analyse the performance of the proposed topology: border point, inside the TCZ-PPF reactive power compensation range and outside the range. To accomplish minimum requirement of minimum dc-link voltage outside the compensating range, PAC control has been implemented. In addition to that, the influence of voltage sag on the dc-link voltage requirement is also scrutinized for three cases of loading reactive power. The study focuses on the reduction of cost of UPQC by downsizing the number of components. Due to its easy adaptability, an increase in research interest has taken place in which prime focus has been to improve the performance, efficiency and reduction of size and cost. Numerous topologies have come to lamplight in an effort for reduction of size and cost whilst maintaining its performance and efficiency. A reduction in the number of components can lead to a reduction in the cost of a UPQC. The major component of UPQC is series injection transformer. A Transformer-less series injection (TLSI) UPQC is an example of component reduction of UPQC. The size is so selected that it prevents problems occurring due to magnetization current demand from erratic voltage compensation and possible saturation of transformer due to DC biasing. This Thesis introduces TLSI-UPQC with its construction and explains its mode of operation. The topology integrates UPQC directly with in-coming distribution transformer averting the requirement of series injection transformer. This topology takes advantage of its location, a UPQC places in later to the distribution transformer towards the loads. The leverage provided by the topology is cost reduction and minimizing the requisite of bulky series injection transformers. This design cannot be implemented in single-phase loads due to the absence of physical neutral point. An alternative is to provide an additional forth leg in the shunt converter. Proper control strategy is designed enabling virtual neutral point for transformers on the distribution end and neutral current paths for the unbalanced loads. The proposed topology is simulated in MATLAB Simulink platform and the results are validated through an experimental prototype, which is developed in the laboratory. The experimental results show the validity and effectiveness of the proposed topology in accordance with the simulation results.