VOLTAGE SOURCE INVERTER BASED STATIC COMPENSATOR FOR CURRENT HARMONIC MITIGATION AND POWER FACTOR IMPROVEMENT OF THREE-PHASE SIX PULSE CONVERTERS
Mansoor A. Soomro
Department of Electrical Engineering, MUET, Sindh, Pakistan mansoor.soomro@faculty.muet.edu.pk
Shoaib A. Dayo
Department of Electrical Engineering, ISRA University, Sindh, Pakistan shoaib.dayo@isra.edu.pk
Arsalan Ansari
Department of Electronics Engineering, DUET, Sindh, Pakistan arsalan.07el@gmail.com
Amir M. Soomro
Department of Electrical Engineering, MUET, Sindh, Pakistan amir.soomro@faculty.muet.edu.pk
ABSTRACT
Power quality has wide impacts on the reliability of connected power system network. Due to recurring advancement in power electronic converters, though the fast switching operation and dual conversion is achieved but at the same time power quality problems have also risen. Harmonics is one of the dominant power quality problem which has not only caused the distortion in the supply voltage and supply current but it has also affected the power factor of the connected network. The aim of this work is to investigate the undesirable effects of current harmonics and consequent reduction in the power factor in the six pulse bridge multiplication converters and mitigate harmonic distortion in it at an accepted level through VSI based STATCOM. In order to achieve the aforementioned objectives, MATLAB / SIMULINK simulation model of VSI based STATCOM has been developed with three-phase six pulse bridge multiplication converters. The behavior of current harmonics and power factor improvement in three phase six pulse multiplication converters has been analyzed before and after the implementation of VSI based STATCOM.
The results of simulation model are validated by considering IEEE 519-1992 and IEEE 519-2014 harmonic standards.
Keywords: Power Quality, Harmonics Elimination, Power Factor Improvement, VSI based Statcom.
1. Introduction
Electrical power system consists of majority of inductive loads. Nowadays, electrical industry deals with power electronics based devices which can produce sufficient reactive power. Transformers and motors are used in access to power system for working which makes phase difference between electrical variables (voltage and current) which results in behaving the system power factor less than unity. Power electronic converters such as (GTO, IGBT, MOSFET) and also excessively used in various non-linear devices such as electric arc furnace and therefore results in the poor quality of power supply [1]. Electric arc furnaces operation produces voltage flicker and harmonics known as fundamental problems related to quality of power supply. The purpose of the power quality is to maintain the electrical wave shape of grid system voltage bus at rated magnitude of voltage and frequency. Electrical arc furnace is fed by AC supply system either single-phase or three-phase system with high magnitude. Arc is produced from electrodes by a high current which generate the heat up to the temperature if 3000’C used for melting and refining purpose. Arc furnaces have time-varying characteristics with active and reactive power fluctuating consumptions. Arc furnaces consumes more electrical current when voltage is decreased in order to maintain the same torque,
that’s why input power to the furnace is exposed to sensitive voltage variations [2-3]. Most of the inductive loads in the low voltage AC system use single-phase rectifier or three-phase rectifiers. These converters have non-linear characteristics (voltage and current are not in phase) give dangerous effects on input of utility mains and also on load side. Apart from arc furnaces, the high frequency switching converters are used in wind energy generation in compliance with power quality standards. Since, wind power is based on AC power, therefore it is vulnerable to variation in supply frequency. As a matter of fact, they can inject harmonics to the grid side creating major power quality issues [4].
The major effect of harmonics is producing power loss in the electrical equipment causes failure of protective devices, improper reading in the measurement of metering devices, interference with telecommunication devices and lines. The harmonics cane be appearing in even and odd form. Odd harmonics produces heating and insulation problems in the electrical equipment [5-6]. Induction motors, transformers and reactors are inductive loads. Induction motors are widely used because of their low cost, robustness and dependability and reliability. Three-phase induction motors are used as prime moves for most of machines which are used in industrial and mining applications [7-8]. At the residential level, the harmonics are not properly measured, so smart meters are used that collects the data from utility consumers and measure all power quality parameters, which are mentioned by the electrical power producers. Smart meters collect different types of data which consists of power factor, frequency, voltage and electrical energy consumptions by utility consumers and the power grid [9]. AC and DC loads are used in the various electrical power consumers such as, power electronic drives applications, telecom industry.
Parallel-connected switch mode power supplies are used in telecom industry for supporting DC voltage bus. These devices draw non-linear current from utility suppliers and causes poor power factor, less efficiency and more power losses [10]. Passive filters were conventionally used to reduce the current harmonics which were generated due to the operation of non-linear loads [11]. Passive filters are affected from overloading and more harmonics current is flow from filters which produced short comings of current and resonance conditions [4-12]. Active power filters are also used for mitigation of harmonics and compensation of reactive power in power system. These filters are made by the combination of passive elements (capacitors and inductors) and power electronic switching converters. Active filters are not suitable for high power applications because of high switching frequency of pulse width modulation converter (PWM) and also because of high ratings [5-13]. Active power filter is can be connected such a way that parallel and series resonance is mitigated by producing the voltage which has same magnitude to the current harmonics [14]. Hybrid power filters are made by the active and passive filters. These filters are used harmonics compensations and voltage regulations [15]. STATCOM made up by power electronics based converters can be interfaced with electrical power system in order to eliminate the problems related to quality of power supply, improve the voltage profile and makes the power system stable [16].
2. STATIC SYNCHRONOUS COMPENSATOR (STATCOM)
STATCOM based technique is used to inject the current from voltage source inverter to the grid in such a way that supply side current harmonics are eliminated and make the phase angle in-phase with supply voltage and source current [17]. The injected current from the voltage source inverter based STATCOM will eliminate the harmonics from load side and induction generator current, and also improve the power factor in the wind induction generators [18]. STATCOM is divided in to the two types: current source type bridge circuit (CSI), and voltage source type bridge circuit (VSI). The function of DC capacitor in the STATCOM provides the stable voltage support to the inverter circuit. Inverter basically consists of power electronic based switching devices (IGBT or GTO) to convert DC voltage from capacitor side to the stable three-phase AC voltage. The function of combined transformer and reactor to filter out the harmonics from the output of inverter and makes the waveforms similar to sine wave as shown in fig.01 [19-20]. The following equation (1) is applied to determine quantitatively the magnitude of inverter side DC capacitor in the STATCOM and is given by;
(1)
VS = Source voltage (Peak value) IL = Load side current
T = Time period
VCMAX = Pre-set upper capacitor voltage VDC = Voltage across Capacitor
The voltage across DC capacitor is can be calculated by Equation (2):
(2)
FIG. 1. The Schematic diagram of STATCOM
3. RESULTS AND DISCUSSIONS
In order to analyze the performance of VSI based STATCOM, the MATLAB simulation was done with three-phase six pulse converters as shown in Fig. 2. In this process, Simulation model is followed by comparing the characteristics of supply current waveform with and without the application of VSI based STATCOM.
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FIG. 2. MATLAB/ Simulink model of VSI based STATCOM with three-phase six pulse converters
The Table 1 demonstrates the model parameters for the simulation model.
Table 1. Electrical parameters for simulation model [5].
3.1 Assessment of Current Harmonics and power factor without STATCOM
The supply current waveform as a function of time both in single phase and three phase waveform is shown in Fig. 3 and Fig. 4. These results are taken without Voltage Source Inverter (VSI) based STATCOM. In these waveforms, the distorted wave gives the evidence of presence of current harmonics abominable for the load side applications.
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FIG. 3. Single phase Supply current without STATCOM
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FIG. 4. Three-phase Supply current without STATCOM
In the following spectrograph, the overall THD value of supply current harmonics is abnormally high i.e.
24.16% as shown in Fig. 5. This excessively large value is not significantly used for soft operated and sensitive electrical and electronic devices like microprocessor based control units, computers, televisions, CCTV cameras etc.
Single Phase Voltage at source side 220 Vrms
Source side inductance 1.601 Mh
Rectifier face inductance 23.01 mH
Load capacitor 50.0 µF
Load resistor 78.0 Ω
DC capacitor STATCOM 4.5 mF
Coupling Transformer 50.0 KVA, 50.0 Hz, 400/400V
1mH, 0.005 Ω
Filter Reactor (R, L) 11.0 Ω, 15.0 mH
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FIG. 5. Spectrograph of Supply current Harmonics without STATCOM
Similarly, the result of corresponding power factor is recorded without STATCOM and it is observed that its value is low i.e. 0.7782. It further needs improvement for increasing the reliability and efficiency of the electric system.
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FIG.6. Power factor measured value without STATCOM 3.2 Assessment of Harmonics and power factor with STATCOM
The supply current Harmonic waveform both in single phase and three phase with the application of VSI based STATCOM is shown in Fig. 7 and Fig. 8. It can be clearly judged the sinusoidal wave shape of supply current which indicate the remarkable reduction in the current harmonics with the use of appropriate VSI based STATCOM technique. It reflects the mitigation of current harmonics at accepted level and therefore proper functioning of the electrical system.
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FIG. 7. Single Phase Supply current waveform with STATCOM
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FIG. 8. Three-phase Supply current waveform with STATCOM
The following spectrograph is demonstrating the simulation results with the application of VSI based STATCOM technique as shows in Fig. 9. The results obtained show an incredible decrement in current harmonics waveform from previous results i.e. 3.17%. This value is safe in accordance with IEEE 519-1992 and IEEE 519-2014 harmonic control standards. The following spectrograph shows that fifth, seventh harmonic have THD values of supply current harmonics which are less than the 5% shown in Fig.
9.
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FIG. 9. Supply current spectrum with STATCOM
Fig. 10 clearly shows the power factor result of three-phase six pulse converters after the implementation of VSI based STATCOM which is obtained as 0.975. The obtained result shows that power factor is improved and subsequently the reactive power is reduced. In this way, it reduces not only the recurring electrical losses in the system but it also increase the reliability and efficiency of the power supply.
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FIG.10. Power factor measured value with STATCOM
4. CONCLUSION
Undoubtedly, the performance of electrical supplies need not involve current harmonics in order to avoid any malfunctioning in the power system. In this respect, the sincere efforts is applied through careful circuit modelling in MATLAB simulation tool in order to reduce the supply current waveform of six pulse three phase converters at an accepted IEEE 519-1992 and 519-2014 harmonic control standards when used with VSI based STATCOM. Similarly, power factor of this converter with the aforementioned technique needs to be high for better performance.
ACKNOWLEDGEMENT
Foremost, Authors would like to acknowledge with a heartfelt gratitude to the department of Electrical Engineering, Mehran University of Engineering and Technology, Sindh, Pakistan, for strong motivation towards modern day research and providing the learning environment and necessary facilities to complete this research work. Furthermore, authors are thankful to the department of Electrical Engineering ISRA University Hyderabad, Sindh, Pakistan, for cordial support and appreciation during research work.
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