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Power Quality Improvement For Critical/Unbalanced Loads Using UPQC Keerti Kulkarni PG Scholar, Department of Electrical and Electronics SDM College of Engineering and Technology, Dhavalgiri, Dharwad-580002. Karnataka. India Email:[email protected] Abstract— This paper presents control strategy of three phase four wire UPQC (unified power quality conditioner) to improve power quality. A case study at the hospital is taken for the study of power quality issues. To improve upon certain problems observed, one of the customer power devices called UPQC is mentioned. UPQC consist of combine series active filter and shunt active filter. The former compensates voltage harmonics and the latter will compensate harmonic currents of non-linear load. The proposed UPQC system can improve the power quality at the point of common coupling on distribution system under unbalanced load conditions. The 3P4W UPQC systems control strategy to balance the unbalanced load currents is presented in this paper. The neutral current that may flow towards transformer neutral point is compensated by using a four-leg voltage source inverter topology for shunt part. Hence, the series transformer neutral will be at virtual zero potential. A simulation study of the proposed topology has been carried out using MATLAB/SIMULINK and the results are presented. Keywords—Power quality, Total harmonic distortion (THD), Unbalanced loads, neutral current, filters, UPQC. I. INTRODUCTION The healthcare environment comprises of various combination of sensitive electronic loads and other commercial loads. Hence maintaining quality power is essential and critical. Power quality is the term used for overall voltage quality, current quality and nature of output waveform which directly or indirectly affect the loads as well as source [1]. Power quality problems may arise due to non-linear loads, injection of harmonics, and interaction between medical equipments. Common sources of power quality problems found in hospitals include: inadequate wiring and grounding, high wattage operating equipment, testing of emergency generators, physical plant renovation [1]. An attempt was made to identify the power quality levels to assess the quality of service received from the utility and locates the areas within the hospital where problems arising from instability exists or may develop in the near future. Unbalanced load currents are common and important problem in 3P4W distribution system. Some of the custom power devices have come for the rescue. To provide a balance, distortions free, and constant magnitude Vinay Janardhan Shetty Lecturer, Department of Electrical and Electronics KLS Gogte Institute of Technology, Belgaum-580008. Karnataka. India Email:[email protected] power to sensitive loads and also to restrict the harmonic, unbalance by the load, UPQC is one of the best solutions [3]. The definition of power quality given in the IEEE std 1100 is Power quality is the concept of powering and grounding sensitive equipment in a matter that is suitable to the operation of that equipment. Power quality is the combination of voltage quality and current quality. Thus power quality is concerned with the deviations of voltage and/or current from the ideal. Voltage disturbances originate in the power network and potentially affect the customers, whereas current disturbances originate with a customer and potentially affect the network. Description of the hospital system, methodology and observations of the system is discussed in section II. UPQC configuration and control algorithm is discussed in section III. This is followed by simulation results and conclusion in section IV. II. DESCRIPTION OF THE SYSTEM The hospital complex under study has incoming supply of 11kV from the utility. Transformer with Δ-Υ connection and solidly grounded is used to buck the voltage to 440 volts. The critical medical loads are: X-ray, CT, MRI, ICUs, operation theatres which contains surgical suits etc. The hospital under study has the following types of power supply. Normal Supply (NS) is a direct utility supply (HESCOM) for non-essential areas. Essential Supply (ES) is used for areas of medical importance that are not critical to patients in the case of supply interruption. The essential supply is backed up by an emergency generator (Diesel generator). Uninterruptible Power Supply (UPS)/stabilizers is used for operating theatres, patient monitoring and other equipment that is important to the well being and safety of patients. IEEE std-519 establishes harmonic limits on voltage as 5% for total harmonic distortion and 3% of the fundamental voltage for any single harmonic. For hospitals and airports Voltage THD is less than 3% [7]. To define current distortion limits, IEEE std-519 uses a short circuit ratio to establish a customer’s size and potential influence on the voltage distortion of the system. TABLE 1: MAXIMUM VOLTAGE HARMONIC DISTORTION For hospitals/airports) General system Dedicated system 10% 20% 50% 3% 5% 10% notch depth % THD voltage The gate pulses required for converter are generated by the comparison of a fundamental voltage reference signal with a high-frequency triangular waveform [6]. Shunt active filter will reduce the current harmonics and provide reactive power demand to the load. It has to control the dc link voltage also which is provided between these two filters. Usually hysteresis band current controller is employed for pulse generation. TABLE 2: MAXIMUM CURRENT HARMONIC DISTORTION (ISC/IL) % THD < 20 5% 20-50 8% 50<100 12% 100-1000 15% > 1000 20% Fig.1 Equivalent circuit diagram of upqc The equations that govern the Fig 1 are: Where Isc is maximum short circuit current at PCC and IL is maximum load current at PCC. For study purpose, few medical loads and few miscellaneous loads were taken for measurement of electrical parameters like, % voltage THD, % current THD, current in each phase, Vrms. The analysis of the system highlighted some aspects. They were high THD and unbalanced load currents. These both contribute to high neutral current which will flow back to transformer neutral resulting in excess heating of the neutral cable. As the THD increases, the neutral current also increases. To reduce the high THD, a passive filter, or an active filter can be used locally. K-rated transformer can be used. Sharing of neutral wires is to be reduced, or over sizing of neutral wires is to be done. Installation of custom power device like, Unified Power Flow Conditioner which will tackle almost all the power quality problems can be thought of in the near future. This is discussed in detail in the following section. III. UPQC The UPQC consists of two three phase inverters connected in cascade in such a manner that series active filter is connected in series with the supply voltage through a transformer shunt active filter is connected in parallel with the load. The series converter acts as a voltage source to mitigate voltage distortions. It is used to eliminate supply voltage flickers or imbalance from the load terminal voltage and forces the shunt branch to absorb current harmonics generated by the nonlinear load. As shown in Fig.1 Voltage injected by series APF must be equal to the difference between the supply voltage and the ideal load voltage [3]. Control of the series converter output voltage is usually performed using sinusoidal pulse-width modulation (SPWM). Vs: Voltage at power supply VSR: Series-APF for voltage compensation, VL: Load voltage and ISH: Shunt-APF for current and VSR compensation. The source voltage can be expressed as Vs + VSR = VL (1) The current provided by the shunt APF, is the difference between the input source current and the load current ISH=IS-IL (2) A. Control strategy for series active filter The control strategy for series active filter is based on unit vector template. Vsa, Vsb, Vsc are the supply side voltages. As shown in Fig 2, the injected voltages by the series active filter are Vinja, Vinjb, Vinjc. These injected voltages will cancel out the distortions present in the supply voltages and thereby making the voltages at load side sinusoidal and maintaining magnitude. Phase locked loop (PLL) is used to achieve synchronization with the supply voltage. The supply voltages are sensed and fed to PLL. This will generate two quadrature unit vectors viz., sinωt and cosωt. The in-phase components from these unit vectors are used to compute the in-phase supply voltages which are 1200 displaced. This is given by the equation 1 Ua Ub - 1/2 Uc - 1/2 3 2 3 2 0 sin cos (3) To get reference voltages, the unit vectors Ua, Ub, Uc as obtained above will be multiplied by the desired peak value at the PCC voltage. V * la Ua V * lb V* Ub lm V * lc Uc (4) The reference voltages obtained above and sensed load voltages Vla, Vlb, Vlc are given to hysteresis controller. The output of the hysteresis is given to the gate signals of the series active filter, and thereby the voltage is injected through injection transformer. Instantaneous reactive power is: qlabc= Vlabc_α*ilabc_α-Vlabc_β*ilabc_β (8) Reactive power is taken as zero because utility should not provide load reactive power. As the load is unbalanced, each phase may not carry equal currents, and therefore the power too. In order to balance the power from utility side, total active power is redistributed among the three phases. Hence power per phase is Ps/phase= {( Pla+ Plb + Plc )/ 3} (9) The reference currents are obtained by the inverse of equation (6) as below: i * sa _ Vla _ Vla _ ps / phase pdc / phase i * sb _ = inv Vlb _ Vla _ * 0 (10) Pdc/phase is the constant dc-link voltage to overcome the UPQC losses. Hence reference current for phase a is: = …… . ……………… (11) Fig 2: Control scheme for series active filter[4] The reference neutral current can be obtained by adding all the sensed load currents. B. Control strategy for shunt active filter The control scheme for the shunt active filter as shown in fig.3 is based on P-Q theory (Instantaneous reactive power theory). According to this theory, a single phase system can be defined as a pseudo-two phase system by giving 900 lead or lag i.e, each phase voltage and current of the original three phase system can be considered as three independent two phase systems. These two phase system can be represented in α-β coordinates. The actual load current and load voltages are considered as α-axis quantities, whereas lead/lag voltages and currents are considered as β-axis co-ordinates. The reference voltages extracted for series active filter are used instead of actual load voltages, as P-Q theory gives poor results under unbalanced load voltages. Equations governing this theory: iln=ila+ilb+ilc (12) and compensating neutral current is ish_n=-iln (13) Using PI controller, the dc link is maintained. The dc link voltage is given by ………….. (14) For phase a, the load voltage and current in α-β coordinates are: Vla _ Vlm sin( t ) Vlb _ = Vlm cos(t ) ila _ ilb _ = ………… (5) il a (t l ) ilb((t l ) / 2) ………(6) Similarly for phase b, with 1200 phase shift and for phase c, 1200 phase shift, equations are formed. Instantaneous active power is: plabc= Vlabc_α*ilabc_α+Vlabc_β*ilabc_β …………… (7) Fig 3: Control scheme for shunt active filter [5] I. SIMULATION AND RESULTS The proposed control scheme has been simulated in MATLAB/SIMULINK. The system is modeled in SIMULINK with system frequency 50 Hz; taking unbalance load condition prevalent in the hospital is as below: Selected signal: 25 cycles. FFT window (in red): 2 cycles 100 50 0 -50 -100 0.3 0.35 0.4 0.45 0.5 Time (s) Fundamental (50Hz) = 113.1 , THD= 0.85% 0.6 Mag (% of Fundamental) 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 Harmonic order 12 14 16 18 20 Fig 6: balanced source current and THD with UPQC Fig 7: non zero neutral current flowing towards transformer neutral without UPQC. Fig 4: SIMULINK model of UPQC Fig 8: compensating neutral current provided by UPQC Selected signal: 50 cycles. FFT window (in red): 2 cycles 50 0 Fig 9: reduced neutral current towards transformer with UPQC -50 0.8 0.85 0.9 0.95 1 Time (s) II. CONCLUSION Fundamental (50Hz) = 53.58 , THD= 8.24% 7 Mag (% of Fundamental) 6 5 4 3 2 1 0 0 2 4 6 8 10 Harmonic order 12 14 16 Fig 5: unbalanced source current and THD without UPQC 18 20 From the results we can observe that, source current THD without UPQC is 8.24%, while with UPQC, it has reduced considerably to 0.89%, which is within IEEE-519 standards mentioned during the case study of hospital analysis of power quality. Albeit the load is unbalanced, the source current is not getting affected with UPQC. Another observation is reduction of neutral current with the aid of UPQC. The UPQC provides a compensating neutral current in order to negate the neutral current, and thereby reducing the unnecessary effect on neutral cable and transformer. A custom power device like UPQC can be installed at the PCC panel in order to overcome the problems encountered in the hospital. ACKNOWLEDGMENT The authors wish to acknowledge the constant support from the SDM hospital administration, where the study was undertaken. REFERENCES [1] Keerti Kulkarni, (2014), “Power Quality Issues In Healthcare Centre”, “International Journal of Current Engineering and Technology”, Vol.4, No.3. [2] Yash Pal, A. Swarup, and Bhim Singh, (2012), “A Novel Control Strategy of Three-phase, Four-wire UPQC for power Quality Improvement”, Journal of Electrical Engineering & Technology Vol. 7, No. 1, pp. 1~8. [3] Vinod Khadkikar, A. Chandra, A. O. Barry, T. D. Nguyen (2005), “Steady State Power Flow Analysis of Unified Power Quality Conditioner (UPQC)”, IEEE, 0-7803-9419-4/05. [4] Yash Pal, A. Swarup, Bhim Singh, (2011), “A control strategy based on UTT and I CosΦ theory of three-phase, four wire UPQC for power quality improvement”, International Journal of Engineering, Science and Technology, Vol. 3, No. 1, 2011, pp. 33-40. [5] Swapnil Y. Kamble , Madhukar M. Waware, (2013), “Unified Power Quality Conditioner for Power Quality Improvement with Advanced Control Strategy”, IEEE, 978-14673-5090-7/13 [6] Simarjeet Kaur, (2012), “Investigation on the role of upqc for power quality improvement of distribution network with foc induction motor”, Thesis submitted in partial fulfillment of the requirements for the award of degree of Master of Engineering In Power Systems & Electric Drives, Thapar University, Patiala. [7] Tony Hoevenaars, Kurt LeDoux, P.E. Matt Colosino, (2003) Interpreting IEEE Std 519 and Meeting its Harmonic Limits in VFD Applications. Copyright Material IEEE Paper No. PCIC-2003-15.