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Engineering, 2013, 5, 13-19 doi:10.4236/eng.2013.51b003 Published Online January 2013 (http://www.SciRP.org/journal/eng) Copyright © 2013 SciRes. ENG A Parallel Processing Uninterruptible Power Supply for Sudden Voltage Fluctuation for Power Management Sung-Hun Ko, Seon g-Ryong Lee Department of Control & Robot Engineering, Kunsan National University, Kunsan, Korea Email: merchin@ kunsan.ac.kr, srlee@kunsn a.ac.kr Received 2013 ABSTRACT This paper deals with a par allel pr ocess ing unint errup tible p ower sup ply (U PS) fo r sudden vo ltage f luctua tio n in power management to integrate power quality improvement, load voltage stabilization and UPS. To reduce the complexity, cost and number of power conversions, which results in higher efficiency, only one voltage-controlled voltage source inverter (VCVSI) is used. The VCVSI is connected in series on the DC battery side and in parallel on the AC grid side wit h a deco upling inductor. The system provides sinusoidal voltage at the fundamental value of 220V/60Hz for the lo a d during abnor mal utility power co nditions or grid failure. Also, the system can be operated to mitigate the harmonic cur- rent a nd volt age d emand from nonlinear loads and pr ovide voltage stab ilization for load s whe n sud den vol tage fluctua- tion occur, such as sag and swell. The experimental results confirm the system protects against outages cau sed by ab- normal utility power conditions and sudden voltage fluctuations and change. Keywords: Parallel P rocessing; UP S; Decoupling Inductor; Power Angle; VCVSI 1. Introduction Computers and automatic equipment are widely used in homes, offices, manufacturing, industrial and commer- cial applic ations. W ith the growth of infor matio n systems, internet data centers, on-line banking systems, life sup- port system, and other similar applications, uninterrupti- ble power supply (UPS) are being researched and devel- oped to improve the available power quality fo r nonli- near loads and to protect critical equipment cause from abnor mal utility po wer conditio ns, voltage transients, and voltage sag and swell [1-5]. According to the National Power Laboratory (NPL) power quality study [6], vol- tage sags and under-voltage account for the largest per- centage of these disturbances at 59.6% of the total. Vol- tage swells, or surge s, a nd o ver-voltages represent 28.9% of these disturbances, impulses account for 8.1%, and outages account for the o ther 3.4 %. This shows that loa d voltage stab ilization is the most impo rtant issue for power mana gement syst ems. Hence, it is generally expected that the UPS will per form the follo wi ng function s [7-9]. 1) Load voltage stabilization (±5% voltage regulation) in both normal and a bnor mal utilit y p ower condit ions; 2) Supply of clean and uninterrupted power to the loads; 3) Harmonic mitigation (THD < 5%) in both normal and abnormal utility power condition Due to the demand for UPS technologies, a considera- ble number of studies have b e en co nduc ted on hi gh p ow- er quality and voltage stabilization beyond the basic function. The on-line UPS using the double conversion principle was commonly used in the 19 70s. It consists o f a rectifier-charger, battery set and inverter. The charg- ing-rectifiers, charges the battery and supplies the inver- ter with DC power, and the inverter supplies the load with continuous, precisely regulated AC power. Whether or not the main power supply is normal, an on-line UPS always governs the output voltage without any transfer time and is more reliable. But har monic and hi gh energ y waste from on-line UPSs is becoming an increasing con- cern to utility engineers and operators of UPS systems. This system must use an extra power device for power factor correction in order to improve power quality. However, this increases the hardware and installation cost of the system and adds additional energy losses. The on-line UPSs dissipate too much power because incom- ing AC po wer must be first converted into DC po wer and then converted back to AC power. Typically, there is a 10% energy loss during this double conversion process [5,10-11]. The bi-directional VCVSI is able to transfer power flowing between the DC battery and the AC grid in both directions while in either battery charging mode or in- verting mode (UPS). The VCVSI provides load voltage stabilization, harmonic mitigation and UPS features [12,13]. A parallel processing UPS usi ng a bi-directional S.-H. Ko, S.-R. Lee Copyright © 2013 SciRes. ENG 14 VCVSI has many advantages over an online UPS. For instance, in a parallel processing UPS the power is not taken via a separate rectifier to a DC bus in the same way as an online UPS. The power is fed directly, via a loss- less impedance decoupling inductor, to the battery by controlling the VCVSI phase angle. This means that a parallel processing UPS can offer many advantages over an online UPS, such as higher efficiency, no delay or break in supply load in changing from rectifier to invert- ing mo des, etc . In this paper, a parallel processing UPS using a bi-di- rectional VCVSI for sudden vo ltage fluctua tion in po wer management is presented. The main purposes of this system are to compensate the c urrent har mo nic a nd rea c- tive power demand from nonlinear lo a d s, to support the load voltage stabilization during sag and swell voltage fluctuations, a nd to sup ply clea n a nd uni nt er r upt ed po wer during abnormal utility power conditions at the point of installation for power distribution to critical loads. In order to verify the proposed system, computer simulation and experi mental results are also presented. 2. System Description and Anal ysis A typical configuration of the parallel processing UPS using a single-phase bi-directional VCVSI is shown in Figure 1. The system consists of a bi-directional VC VSI, a decoupling inductor, Xm, and a battery bank. The VCVSI is synchronized and connected to the grid through the decoupling inductor to prevent large power flows to or from the grid. T he VCVSI is connected to the battery, providing bi-directional power for rectification, charging and inversion (UPS) flow capability between the batter y and the AC si de . T he maint ena nce o f t he load voltage and power flow of the system is controlled by adjusting the amplitude and phase angle of the VCVSI output vol t age , with resp e c t to the grid volta ge. Figure 1. Sc hematic diagr am of a parallel processing UPS. The system operation has been divided into two modes, nor ma l and UPS. In normal mode, the gird supplies power to load without any power conversion. The VCVSI sup- plies or compensate the required reactive power demand of the load. The VCVSI produces a constant output vol- tage across the load by sinusoidal Pulse Width Modula- tion (PW M). T his allo ws the proposed system to mitigate the harmonic current and voltage demand of nonlinear loads and provide voltage stabilizatio n, whe n sudde n sag and swell voltage fluctuation occur, without power con- version losses. In the UPS mode, the system provides sinusoidal voltage at the fundamental value, 220V/60Hz, for the load during abnormal utility power conditions or grid failure. A VCVSI performs the same as a voltage source and maintains voltage support for the load in the absence of a grid. Figure 2 shows a simplified, equivalent schematic diagram of a parallel processing UPS using a VCVSI. In the following analysis, it is assumed that the output low pass L-C filters of VCVSI will filter out high-order har- monics generated by PWM. The decoupling inductor is an essential part of any VCVSI, as it makes power flow control possible. Assuming the maximum permissible voltage fluctua- tion in the gird voltage, Vg, is ±6% at 220V ± 13V and the load voltage, Vload = Vc, is kept at a cons tant 220V, then the decoupling inductor voltage, Vx, of Figure 2 can be expressed as (1) δ ∠−∠= cgx VVV 0 (1) where, Vg and Vc are the grid and the inverter funda- mental voltages, respectively. In steady state condition, the current flow from the grid through the decoupling inductor can be expressed as (2) x gm V IjX = (2) where, Ig is the fundamental gird current and Xm is the decoupling inductor impedance. Using Equation (1) and (2), the grid current can be rewritten as (3) Figure 2. The equivalent circuit diagram of a parallel processing UPS. S.-H. Ko, S.-R. Lee Copyright © 2013 SciRes. ENG 15 0 cos sin g cgc c gm mm V VVV V Ij jXX X δδ δ ∠− ∠− = =−− (3) Using the per unit values Sba se= V2base/Zbase, Vbase=Vc and Zbase = Xm, where Vbase, Zbase and Sbase are the base voltage, impedance, and apparent power values, respec- tively. The apparent power of the grid, inverter and de- coupling inductor are given by: 2 sin(cos ) gpugpugpugpugpu gpu SPjQVj VV δδ =+=−+ − (4) sin(cos 1) cpu cpucpugpugpu SPjQVjV δδ =+=− +− (5) 2 (2cos 1) xpuxpugpu gpu SjQj VV δ ==−+ (6) where Sgpu, Scpu and Sxpu are per unit values of the grid, inverter and decoupling inductor’s apparent power re- spectively, and Vgpu is the per unit value of the grid vol- tage The phase angle could be both lagging or leading, pro- viding either active power flow from the grid to the VCVSI, or vice versa. In the lagging phase case, active power flows from the grid toward the load. The higher phase angle results in more active power supply to the load or inver ter fro m the gr id. It means t hat t he pro pose d system can be providing bi-directional power flow, both charging and discharging, between the battery and the AC side by controlling the phase angle. Therefore, in charging mode, the active power flow from the grid is expressed as (7) gload bat PP P= + (7) where, Pg and P load are active power the grid and the load, respectively, Pbat is charging power flow to the batte ry. In the charging mode, power angle can be calculated as (8) 1 () sin loadbat m gc P PX VV δ − + = − (8) From equation (8), when the required charging power is increased, the phase angle is increased. When the bat- tery is fully charged, the phase angle is decreased. 3. Simulation and Experimental Results 3.1. Simulation Results A PSim simulator was used to verify the operation of the proposed system. The operation characteristics are ana- lyzed in the normal mode and the UPS mode. Nonlinear and variation load conditions for the simulation are 1KVA load capacity. Table 1 illustrates the simulation condition and para meter values. The simulation was conducted to evaluate the perfor- mance of the proposed system in the presence of differ- ent operation modes. Vg, Vc and Vload are the voltage waveforms of the grid, inverter and load, and Ig, Ic and Iload are current waveform of the grid, inverter and load, respectively. Pload is the active power waveform of the load. The waveform of the normal operation mode at nonli near loa d and a crest factor of 3 is sho wn in Figure. 3 Figure 3 indicates that the VCVSI can mitigate the harmonic curre nt of a nonlinear l oad demand and t he gr id supplies the remaining acti ve po wer. In normal mode, the VCVSI prevents any lower order harmonics from being injected into the grid. The harmonic spectrum of the load voltage, grid current, load current and VCVSI current with the nonlinea r lo ad, as shown in Figure 4. As shown Figure 4, t he reactive power associated with low order harmonics from the nonlinear load of ITHD: 62.7% could be supplied by the VCVSI. It is shown that the proposed system satisfies THD, grid ITHD: 1.1%, requirements of voltage and current for the full range of the load. The waveform of the charging mode under nonlinear load conditions is shown in Figure 4. Pg and Pc are the active power waveforms of the grid and the VCVSI, re- spectively. Table 1. System parameters and specif ication s . Parameter Value Pa r ameter Value Vac 220[Vrms] Vdc(battery) 200V Frequency 60Hz Fsw 10kHz Full load 1 KV A Lm 42mH δmax 20° Trans former 1:2 Figure 3. Waveforms of normal mode at nonlinear load condition. (a) Voltage of grid and inverter. (b) Current of grid and lo ad. ( c) Inverter curr ent. (d) Load active power. S.-H. Ko, S.-R. Lee Copyright © 2013 SciRes. ENG 16 As indicated in Figure 5, the grid s hould be the active p ower supply of 500W required from the load when in normal mode, suc h as Figure 3. If the required charging power input to the battery is increased, the proposed sys- tem controls the phase angle between the grid and the VCVSI. The increased phase angle results in more active power supply from the grid, 500W to 1KW, to the load and VCVSI. The harmonic spectrum of the load voltage, grid current, load current and VCVSI current with the nonlinear load during charging operation is shown in Figure 6. It shows t hat t he proposed system can mitigate low order harmonics of the load voltage and current in order to meet IEEE standards in the presence of nonli- near loads in both normal and charging modes. Figure 4. The harmonic spectrum analysis when normal mode. (a) Load voltage (b) Grid current. (c) Load current. (d) Load active power. Figure 5. Waveforms of charging operation mode at nonli- near load condition. (a) Voltage of grid and inverter. (b) Current of grid and load. (c) Inverter current. (d) Active power of gird, invert er and load. Figure 6. The harmonic spectrum analysis when the charg- ing operation mode. (a) Load voltage. (b) Grid current. (c) Load current. ( d) Inverter curr ent. Figure 7. The operation waveforms of voltage stabilization when voltage sag. (a) Grid voltage. (b) Load voltage. (c) Current of grid and load. (d) Inverter current. (e) Load active. Figure 7 and 8 shows the performance of the proposed system during voltage stabilization in the presence of nonlinear load conditions. In this simulation, the grid voltage is changed from its nominal value of 220V to 132V, during voltage sag, and to 308, during voltage swell . Figure 7 shows that the si mulation results during no rmal mode, wher e the grid vo ltage c hange s fro m RMS value of 220V to 132V for period 12 at 20ms and, re- turning to 220V. In Figure 8, the grid voltage changes form an RMS value of 220V to 308V for the same situa- tion shown in Figure 7. The system mitigates the har- monic current of a nonlinear load demand and the grid supplies the remaining active power, as in Figure 3, during a sudden voltage change. The proposed system picks up the load rapidly after sudden voltage changes. S.-H. Ko, S.-R. Lee Copyright © 2013 SciRes. ENG 17 The load voltage is maintained at 220V, ±1.2% vol- tage regulation, and is unaffected by the sudden voltage sag and voltage swell. This means that the propo sed sys- tem meets the IEEE standard of le ss than 5% of THD a nd voltage regulation within ±5%, for the whole control range Figure 9 shows that the simulation results in the UPS mode during nonlinear load conditions. It assumes that at 30ms the gri d fai ls a nd the VCVSI has to supply the load. As shown, before grid failure the system supplies the reactive power demanded by the nonlinear load and ra- pidly picks up the load after grid failure. Figure 8. The operation waveforms of voltage stabilization when voltage swell. (a) Grid voltage. (b) Load voltage. (c) Current of grid and load. (d) Inverter current. (e) Load active power. Figure 9. The operation waveforms of voltage stabilization when voltage sag. (a) Grid voltage. (b) Load voltage. (c) Current of grid and load. (d) Inverter current. (e) Load active. 3.2. Experimental Results In order to confirm the above mentioned simulation re- sults pertaining to system performance, the prototype proposed system(1KVA) for the experiment was con- structed to be identical to the simulation parameters and the operating condition with the simulation. The scope and power analyzers were used to record the information for further evaluative comparisons of the analytical and simulation results. The digital scope (TDS3054B) was used to capture the following results. The experimental waveform of the normal mode at nonlinear load condition is shown in Figure 10. In Fig- ure 10, Ch 1 and Ch2 are the voltage waveforms of the grid and inverter, and Ch 3 and Ch4 are the current waveforms of the load and grid, respectively. Figure 10 indicates that the VCVSI can mitigate the harmonic cur- rent of a nonlinear load demand and the grid supplie s the remaining active power. In normal mode, the VCVSI prevents any lower order harmonics from being injected into the grid. This results support the simulation results (Figure 3). Figure 11 presents the current harmonic spectrum of the gird current and load current from zero to 5[kHz]. The Tektronix (TDS3054B) digital scope already has a FFT module . As s ho wn Figure 12, all the reactive power associated with low order harmonics from the nonlinear load (load current THD: 6 3.2%) could be supplied by t he VCVSI. It is s ho wn t hat t he p r o p ose d syst e m ca n ac hieve satisfies THD (grid current THD:3.1%) requirements of voltage and c urre nt fo r the full range o f the l oad. Voltage stabilization would be one of the most impor- tant requirements in power management systems. The following tests were carried out in order to confirm the performance of the proposed system in stabilizing the Figure 10. Experimental results of the normal operation mode at nonlinear load condition. S.-H. Ko, S.-R. Lee Copyright © 2013 SciRes. ENG 18 load voltage. Figure 12 shows that the experimental re- sults during normal mode, where the grid voltage changes from RMS value of 220V to 160V fo r 0.5second and, returning to 220V. The proposed system can miti- gates the harmonic current of a nonlinear load demand and the grid supplies the remaining active power, as in Figure 10, during a sudden voltage change. As shown Figure 12, the load voltage is maintained at 220V, less than ±2% voltage regulation, and is unaffected by the sudden voltage change. It is shown that the proposed system can maintain the load voltage regardless of changes in the grid voltage. Figure 13 shows that the experimental results in the UPS mode during nonlinear load conditions. In Figure 13, Ch1 and Ch2 are the vol- tage waveforms of the load inverter and load, and Ch3 and Ch4 are the current waveforms of the load and grid, respectively. (a) (b) Figure 11. The harmoni c spec tru m anal ysis o f the proposed system at nonlinear load condition. (a) Grid current. (b) Load current. Figure 12. Experimental results of the proposed system when sudden voltage cha nges. Figure 13. 14 Experimental results of the UPS operation mode for state an transient response at nonlin e ar load con- dition. As shown Figure 13, the proposed system can supply and mai ntain the pure sinusoidal voltage waveform when the gird fails in the presence of nonlinear loads. And also VCVSI can mitigate low order harmonics during the whole control range. These results also comply with si- mulation results (Figure 7 and Figure 9). Figure 14 presents the voltage harmonic spectrum of the load vol- tage from zero to 5[kHz]. 4. Conclusion This paper addressed parallel processing UPS using a single phase bi-directional VCVSI for sudden voltage fluctuation in power management. To reduce the com- plexity, cost and number of power conversions and im- prove efficiency, only one VCVSI was used. It was demonstrated that the system performs well in either the normal or UPS mode of operation. In the normal mode, S.-H. Ko, S.-R. Lee Copyright © 2013 SciRes. ENG 19 Figure 14. The harmoni c spec tru m anal ysis o f the proposed sys tem when UPS operation mo de . the VCVSI operates to compensate the current harmonic and the reactive power demand of nonlinear or variable loads. The system controlled and provided voltage stabi- lization for the load when sudden voltage sag and swell occurred. In the UPS mode, the proposed system sup- plied sinusoidal voltage at the fundamental value for the load during an abnormal utility power condition. It was shown that the proposed system protected the system from outages by an abnormal utility power conditions and sudden voltage fluctuations and changes. The de- tailed simulation and experimental results verified the performance syste m theor y an d its capacity to satisfy the IEEE recommended requirements for electric power quality. 5. Acknowledgements This work was supported by the New & Renewable Energy (No. 20123021020010) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowled ge Economy. REFERENCES [1] H. Deng, R. Oruganti, and D. Srinivasan, “Modeling and Control of Single-Phase UPS Inverter: A Survey”, 2005 PEDS Conf. Power el ectro ni cs and Dri ves S ystems, vol. 2, pp. 848-853, Nov. 2005. [2] S. Karve, “Three of a Kind[UPS topologies, IEC stan- dard],” IEE Revi ew , vol. 48, no. 2, pp. 27-31, March. [3] B. H. Kwon, J.H. Choi, and T.W. Kim, “Improved Sin- gle-Phase Line-Interactive UPS,” IEEE Trans. Ind. Elec- tron., vol. 48, no. 4, Aug. 2001. [4] W. J. Ho, J.B. Lio and W.S. Feng, “Economic UPS structure with phase-controlled battery charger and in- put-power-factor improvement,” IEE Proc. Eletr. Power Appl., vol. 144, no. 4, pp. 221-226, July 1997. [5] J. C. Wu and H.K. 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