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Power Quality Improvement through Unified power quality conditioner (UPQC) Mohammed Nasser TANDJAOUI, A. KECHICH, C. BENOUDJAFER, M. HABAB∗ Abstract The quality of the power electric is characterized by the measurement and the analysis of the electric disturbances which make it possible to understand the origin of the disturbances, to evaluate their impact on the sensitive equipment, and thus to find and choose the most suitable solution either economically or technically. This paper deals with Unified Power Quality Conditioners (UPQC’s), which aim at the integration of series and shunt inverters which can compensate the current and voltage distortion correctly. The results of simulation in MATLAB/SIMULINK software show appropriate operation and the capability of the proposed system to improve the power quality at the point of installation on power distribution systems or industrial power systems. Keywords: power quality, voltage sags, UPQC, harmonic, active power filter 1. Introduction The term “power quality” (PQ) has gained significant attention in the past few years [1]. The quality of electric power concerns all the stakeholders of the energy field is they managers of networks, suppliers, or electricity consumers. It has become a subject of great interest these last years, primarily for reasons such as the urgent economic requirements, the generalization of the equipment sensitive to the disturbances, and the opening of the electricity market. Though electricity production is rather reliable, the quality of this product is not so in the distribution systems because of the many nonlinear loads that significantly affects the purity of the waveform of the voltages and the cause the loss of the power supply. This produces many problems of energy quality due to the significant loads. Such a situation has encouraged researchers to find means of increasing the quality of energy supply [2]. The most problems known the quality of ∗ Mohammed Nasser TANDJAOUI: University of Bechar, Department of Technology, Route de Kenadsa, BP.417, Bechar 08000, Algeria, e-mail: [email protected] A. KECHICH: University of Bechar, Department of Technology, Route de Kenadsa, BP.417, Bechar 08000, Algeri, e-mail: [email protected], C. BENOUDJAFER: University of Bechar, Department of Technology, Route de Kenadsa, BP.417, Bechar 08000, Algeria, M. HABAB: University of Bechar, Department of Technology, Route de Kenadsa, BP.417, Bechar 08000, Algeria electric are in voltage and current disturbances. There are many different methods to improve the power quality, but the use of a custom Power device is considered to be the most efficient method. The concept of custom Power was introduced by N.G. Hingorani in 1995. Like Flexible AC Transmission Systems (FACTS) for transmission systems, the term custom power pertains to the use of power electronics controllers in a distribution system, especially, to deal with various power quality problems [3, 4]. Each of Custom Power devices has its own benefits and limitations. The advancement in the semiconductor device technology has made it possible to realize most of the power electronics based devices/prototypes at commercial platform. The development of power electronic technology makes it possible to realize many kinds of Flexible Alternating Current Transmission Systems devices to obtain high quality electric energy and enhance the control over power system [1]. This paper deals with the most effective type of these devices is considered to be the Unified Power Quality Conditioners (UPQC) which aim at the integration of series active and shunt active filters. The main purpose of a UPQC is to compensate for supply voltage flicker/imbalance, reactive power, negativesequence current, and harmonics. 40 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2 In other words, the UPQC has the capability of improving power quality at the point of installation on power distribution systems or industrial power systems. 2. General UPQC Unified power quality conditioner (UPQC) is the powerful tool to settle the power quality problem. The general configuration of the UPQC is shown in Figure 1. Figure 1. General configuration of the UPQC With ideal compensation, the voltage at PCC is the fundamental positive sequence sinusoidal voltage of the power source side and the load is equal to a resistance. The currents of the source are sinusoidal current and the phase angles of them are the same as the fundamental voltage in phase respectively. The UPQC is installed in order to protect a sensitive load from all disturbances. It is a combination of series and shunt active filters, two active filters have different functions and they are consists of two voltage source inverters implemented with Insulated gate Bipolar Transistors (IGBTs) and connected back to back, sharing a common DC link [5]. The series active filter suppresses and isolates voltage-based distortions but the shunt active filter cancels current-based distortions. The series inverter is connected through transformers in series between the source and the common connection point, acts as a controlled voltage source maintaining the load voltage sinusoidal and at desired constant voltage level. The other inverter is connected in parallel in load side with the common connection point through transformers, It acts of helps in compensating load harmonic current, reactive current and maintain the dc link voltage at constant level [5, 6]. The single phase equivalent circuit for a UPQC is shown in Figure 2. Figure 2. Equivalent Circuit of a UPQC The source voltage, terminal voltage at PCC and load voltage are denoted by Vs, Vt and VL respectively. The source and load currents are denoted by is and iL, respectively [5]. The voltage injected by series APF is denoted by Vsr, where as the current injected by shunt APF is denoted by ish. The series PWM converter is modeled as voltage source, while the shunt PWM converter as current source. Harmonic generating load is modeled as generic current source [6]. The high-pass filter impedance of the shunt PWM converter referred to the primary is Z1− Sh = RSh + jωLSh (1) ZSh in series with the network reactor impedance referred to the secondary of the series single-phase transformer is given by Z 2 − Sh = Z Sh + Z S (2) Zp-sr parallel with the high-pass filter impedance of the series PWM converter is given by Z p − Sr = Z 2 Sh Z Sr Z ( R + jωLSr ) = 2 Sh Sr Z 2 Sh + Z Sr Z 2 Sh + ( RSr + jωLSr ) (3) The impedance of the series PWM converter referred to the primary is Z Sr = R Sr + j ω L Sr (4) The total impedance ZT connected to the power supply, when there is no load is Z T = Z S + Z Sr + Z p − Sr (5) 3. The UPQC control strategy The proposed control strategy is aimed to generate reference signals for both shunt ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2 and series APFs of UPQC. In the following section, an approach based on SRF (Synchronous reference frame) theory combined of extended p-q theory is used to get reference signals for the series and shunt APFs. One of advantages of a d-q domain derivation of reference signals lies in easier signal filtration, since the 50Hz components are transferred into DC. Reference voltages for the series active filter can be determined based almost on the same procedure [7]. The shunt active power filter rating mainly depends on the compensating provided the current generated by nonlinear load and the reactive power of the system need. It acts as a controlled current generator that compensated the load current to force the source currents drained from the network to be sinusoidal, balanced and in phase with the positive-sequence system voltages. ⎤ ⎥ = ⎥⎦ 2 3 The instantaneous reactive power (p-q) theory is used to control of shunt APF in real time [8]. In this theory, the instantaneous three-phase currents are transformed to d-q coordinates as shown in equation (6). ⎡1 / 2 1 / 2 ⎤ ⎡V a 1/ 2 ⎢ ⎥ cos( θ − 2 π / 3 ) cos( θ − 4 π / 3 ) ⎥ ⎢⎢ V b ⎢ cos θ ⎢ − sin θ − sin( θ − 2 π / 3 ) − sin( θ − 4 π / 3 ) ⎥ ⎢ V ⎣ ⎦⎣ c Where θ is the instantaneous supply voltage angle given by t θ = θ0 + ∫ ωtdt (7) 0 Currents in rotating frame can be decomposed in DC (50 Hz) and AC (harmonic, sub harmonic or inter-harmonic) component: − The shunt APF reference current signal generation block diagram is shown in Figure 3. Figure 3. The control system of the PAPF 3.1. Shunt control strategy ⎡ id ⎢ ⎢⎣ i q ~ − 41 ⎤ ⎥ ⎥ ⎥⎦ (6) compensation voltage that synthesized by the PWM converter and inserted in series with the supply voltage, to force the voltage of PCC to become sinusoidal and balanced. The proposed series APF reference voltage signal generation algorithm is shown in figure 4. ~ id = id + id , iq = iq + iq (8) where id corresponds to the reactive and iq to the active power component. 3.2. Series control strategy The series active power filter is to compensate the voltage disturbance provided in the source side. It handles no active or reactive power, it generates the ⎡Vd ⎢ ⎢⎣ V q ⎤ ⎥= ⎥⎦ Figure 4. The control system of the SAPF In equation (1), supply voltages vSabc are transformed to d-q-0 coordinates. ⎡1 / 2 1 / 2 ⎤ ⎡ia ⎤ 1/ 2 ⎥ 2⎢ cos θ cos( θ − 2 π / 3 ) cos( θ − 4 π / 3 ) ⎥ ⎢⎢ ib ⎥⎥ ⎢ 3⎢ ⎥⎢ ⎥ ⎣ − sin θ − sin( θ − 2 π / 3 ) − sin( θ − 4 π / 3 ) ⎦ ⎣ ic ⎦ (9) The voltage in d axes (ud) given in (2) consists of average and oscillating 42 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2 components of source voltages (vSd and ~vSd). The average voltage vSd is calculated by using second order LPF (low pass filter). − ~ Vd = Vd + Vd (10) These produced three-phase load reference voltages are compared with load line voltages and errors are then processed by sinusoidal PWM controller to generate the required switching signals for series APF IGBT switches [8]. 3.3. The DC voltage regulator: In compensation process, the DC side voltage will change because UPQC compensates the voltage /current disturbance, the active /reactive power and the losses of switches, etc. Also, shunt inverter control undertakes the duty of (stabilizing) DC link voltage during series inverter operation to compensate voltage distortions. DC link capacitor voltage controlling loop is used here by applying PI (proportional integrator) controller [9]. The DC voltage regulator shown in Figure 5 is used to generate a control signal to keep the voltage be a constant. The three phase terminal voltages, three phase load voltages, three phase source currents and the DC link voltage are sensed and used to generate the switching patterns for shunt and series APFs, whereas, an R-L load with uncontrolled diode rectifier is considered as a sensitive load to be protected. The simulation results are shown in the figure 6, figure 7 and figure 8. Circuit parameters used in simulation are located in table 1. Table 1. Parameters for the power circuit of UPQC Source Phase Voltage (rms) Frequency DC Link voltage Shunt inverter Inductance(Lf) Switching Frequency Series inverter Inductance (Ls) Series inverter Capacitance (Cs) Series inverter Resistance (Rs) Load Resistance (RL) Load Inductance (LL) 220 v 50 Hz 850 v 0.001 H 12 kHz 0.6e-3 H 220e-6 F 30e-3 Ω 3.34 Ω 60e-3 H The maximum simulation time is regulated on 500 msec. Shunt inverter starts to operate at time started and series inverter starts at 150 msec. Figure 6 shows the performance of series APF of the UPQC in the system which supply a linear load R-L. Figure 5. The DC voltage regulator It forces the shunt active filter to draw additional active current from the network. The study of the regulation of the continuous voltage at the boundaries of the storage capacity showed that a compromise must be done between filtering and the speed in the control of this voltage. For that, the studied regulator, proportional integrator (PI) is more suited to assure an optimal filtering characteristic and an optimal cost. 4. Simulation results: The performance of each topology of UPQC under the steady state conditions have been verified by computer simulation in the MATLAB/ SIMULINK environment, when it is evaluated in terms of voltage sags and voltage / current harmonics mitigation. Figure 6. Waveforms of voltage source (Vs), voltage load (VL) and injected voltage by a series APF (Vinj) During the voltage sag condition in an interval of the time from 0.15 s to 0.35 s, the series APF is providing the required power of ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2 43 the load to correct this fault by injecting in phase compensating voltage (50 %) equals to the difference between the reference load voltage and source voltage through a series coupling transformer in order to obtain a stable, proper and effective load voltage to protect. In the second case, the proposed system developed by supplies a non-linear load with a source voltage purely sinusoidal. At start time of simulation, the shunt APF is put into the operation when the load current is disturbed by the current harmonics produced by the diode rectifier. Simulation results shown in Figure 7 demonstrate the effectiveness of the developed system for the control of shunt APF. Figure 8. Waveforms of voltage/current of source, load and compensating of UPQC Figure 7. Waveforms of current source (Is), current load (IL) and injected current by a shunt APF (Iinj) In figure 8, one will analyze the robustness in terms of speed and precision of the UPQC compensating for the voltage sags applied in an interval of the time from 0.05 s to 0.25 s and the voltage harmonics presented in an interval of the time from 0.30 s to 0.75 s. The UPQC through the transformer of the series active filter injects the compensating voltage necessary to satisfy the request voltage of the load. It is noted that at the moment t=0.3s, the UPQC through the active filter series starts to correct the voltage harmonics, by injecting compensating voltage have forms of well synchronized waves and in opposition of phase with the voltage of the source. The shunt APF injects a leading compensating current and supplies the load, this quantity of current injecting is a high current to grid, a part of which is consumed to feed the load and else is injected to grid to mitigate the current disturbance and making the input power close to unity. The shunt APF controller acts immediately forcing DC link voltage to settle down at new steady state value i.e. at 850 V. 44 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, Vol. 61 (2013), Nr. 2 While series APF is providing the required real power to the load, the shunt APF is maintaining the DC link voltage at constant level such that the series APF can provide the needed real power to the load. To maintain the DC link voltage at constant level the source delivered more current. 5. Conclusion The work presented the capability of UPQC lies within the scope of the search for new solutions for the improvement of the power quality in the electric supply network. As the UPQC is a combination of series and shunt active filters, two active filters have different functions. It takes advantages of these filters (series APF and shunt APF) to compensate the distortions of both source voltages and load currents. UPQC has a complicated structure that uses several elements working together, that’s why we need rigorous choice of its parameters. The parallel active filter aimed to compensate for the harmonic, reactive and unbalanced interference currents. The active filter series its objective was the compensation of the harmonic disturbing voltage, and of the voltage sags. Finally, the UPQC was proposed as a general solution of the compensation of all the disturbances due to voltage or/and current. The obtained results of the simulations show that the UPQC is one of FACTS equipment able to compensate all disturbances of voltage and/or current with a great efficiency. References [1] Benachaiba C., Abdelkhalek O., Dib S., Allali M. and Dib D., “The Unified Power Quality Conditioner (UPQC): The Principle, Control and Application”, in CIGE’10, 03-04 Nov. 2010, Bechar, Algeria. [2] Tandjaoui M.N., Benachaiba C., Abdelkhalek O., Doumbia M.L., Mouloudi Y., “Sensitive Loads Voltage Improvement Using Dynamic Voltage Restorer”, in Explore IEEE, 17-19 July 2011, Bandung, Indonesia. [3] Tandjaoui M.N., Benachaiba C., Abdelkhalek O. and Doumbia M.L., “Mitigation of voltage sags/swells unbalanced in low voltage distribution systems”, in IJSAT, (ISSN 22218386), pp.46-51, Vol. 1 No 6, Aug. 2011. [4] Tandjaoui M.N., Benachaiba C. and Abdelkhalek O., “Role of DVR in Power Quality Enhancement”, in CIAM'2011, November 22-24, 2011, Oran, Algeria. [5] Khadkikar V., Chandra A., Barry A.O. and Nguyen T.D., “Steady State Power Flow Analysis of Unified Power Quality Conditioner (UPQC)”, in Explore IEEE, 2005. [6] Benachaiba C., Abdelkhalek O., Dib S., Haidas M., “Optimization of Parameters of the Unified Power Quality Conditioner using Genetic Algorithm Method”, in ITC, ISSN 1392 – 124X, Vol.36, No.2, 2007. [7] Benachaiba C., Ferdi B., Dib S., Rahli M., “Impacts of Short-Circuit Power on Hysteresis Control of UPQC”, in EJSR, ISSN 1450216X, , Vol.37 No.4, pp.525-534, 2009. [8] Kesler M., Ozdemir E., “A Novel Control Method for Unified Power Quality Conditioner (UPQC) Under Non-Ideal Mains Voltage and Unbalanced Load Conditions”, in Explore IEEE, 2010. [9] Siahi M., Najafi M., Hoseynpoor M., Ebrahimi R., “Design and Simulation of UPQC to Improve Power Quality and Transfer Power of photovoltaic array to grid”, in AJBAS, ISSN 1991-8178, 5(3): 662-673, 2011. Biography Mohammed Nasser TANDJAOUI received the state engineer degree in Electric Engineering in 2005 from the University of Sciences and Technology of Oran (USTO). He was Magister in electric engineering in 2009 from university of Bechar, Algeria. He currently was holding the post of Assistant maitre in university of bechar. He was preparing a Doctorate of improvement of the quality of energy electric in a wind network by the integration of FACTS systems. His research area interests are power electronics, FACTS, HVDC, power quality issues, renewable energy and energy storage