[ThE4-3] Fig. 1. Conventional UPS system

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1 [ThE4-3] 8th International Conference on Power Electronics - ECCE Asia May 30-June 3, 2011, The Shilla Jeju, Korea Non-isolated Single Phase Uninterruptible Power Supply (UPS) System Muhammad Aamir and Hee-Jun Kim Dept. of Electronics, Electrical, Control, and Instrumentation Engineering, Hanyang University, Korea Abstract-- In this paper, a high performance single phase transformer-less online uninterruptable power supply (UPS) is proposed. The use of the bidirectional buck-boost converter with high conversion ratio not only reduces the number of batteries but also ensures a transformer-less system. The rectifier has capability of power factor correction and provides regulated DC link voltage whereas the inverter provides a regulated sinusoidal output voltage to the load. In order to control the transient effect, efficient control scheme is adopted in the system. The overall efficiency of the system is improved with significant reduction in size and weight of the system due to decrease number of batteries. Qualitative analysis and experimental results obtained with a 500VA prototype shows a normal efficiency of over 94% and an input power factor of over 99%. Fig. 1. Conventional UPS system Index Terms Transformer-less, Uninterruptable power supply (UPS), Bidirectional Converter, DC Link Voltage, Inverter I. INTRODUCTION Uninterruptible power supply (UPS) are used to supply clean, conditioned, and uninterruptible power to equipment in critical applications under any normal or abnormal utility power condition. Generally, UPS system requires the regulated sinusoidal output voltage with low total harmonics distortion (THD) that is independent from the changes in the input voltage or the load conditions [1]. Besides, low transients response time from online mode to battery powered mode and vice versa, unity power factor, high reliability, high efficiency, low cost, low weight, and small size, etc. are other essential considerations in the UPS system. The development of the UPS system can mainly be classified as online, line-interactive, and offline. The problem associated with the line interactive and offline UPS is the fact that the critical load is supplied with power directly from the utility which is neither regulated nor conditioned. Therefore online UPS system is preferred due to wide tolerance of the input voltage, precise regulation of the output voltage, and reliability of the system [2]-[3]. A conventional UPS system consists of a PFC rectifier, a battery bank, an inverter, isolation transformers, and a bypass circuit as shown in Fig. 1. The rectifier converts the input line voltage into the DC link voltage and simultaneously acts as a charger for the battery. On the other hand, the inverter converts the DC link voltage into the output sinusoidal voltage. An isolation transformer at the input side is used to reduce the number of batteries, Fig. 2. High-frequency isolation UPS system and other transformer at the output side is used for the operation of the bypass circuit [4]. However, since the transformers are operated at the grid frequency, it results in the increase in size and cost of the system. Other topologies were proposed as a solution to overcome this problem by using transformer at high frequency as shown in the Fig. 2. Although it helps in reducing the size and weight of the system, but it results in an increase number of active switches affecting the efficiency and reliability of the system [5]-[7]. A three leg-type converter has also been proposed in [8]-[10]. A common leg for both the PWM rectifier and the PWM inverter results in reducing the power loss in the system. But the drawbacks caused by the transformer are still there. Transformer-less UPS, incorporating bidirectional converter has attracted special interest due to its high efficiency and small weight and volume of the system. But it still has some disadvantages as the switches of rectifier and DC-DC converter are directly expose to DC link voltage so the transformer-less UPS is more susceptible to interference from spikes and transients caused by load [11]. Also, a high battery bank is required to achieve high DC link voltage, which leads to the increase in the storage battery cost and lower reliability [12] /11/$ IEEE

2 Fig. 3. Configuration of the proposed transformer-less online UPS system Fig. 4. Simplified block diagram of the modes of operation of UPS system According to the analysis of the drawback related to the aforementioned UPS topologies, a feasible transformerless UPS system is proposed in this paper. Fig. 3 shows the proposed UPS topology, which consist of PFC boost rectifier, a cascaded bidirectional buck-boost converter, and an inverter. It is important to accentuate the fact that the use of bidirectional converter reduces the number of batteries considerable. Thus for low power applications, the proposed system is more feasible with high efficiency and reduced size, and weight. With normal DC link voltage, the bidirectional converter is operated as a buck converter and act as a battery charger. When the input power failure occurs, the DC link voltage decreases abruptly, and now the bidirectional converter acts as boost converter and starts discharging the battery. In this way, the operation mode of the buck-boost converter is changed by the DC link voltage, thus the transient effect of the output voltage can be minimized. The major drawback of the transformerless UPS is the susceptibility to the interference from the spikes and transients caused by assorted devices connected to the utility grid. To overcome this problem, a control technique is employed which limits the excessive current and quickly recover the output voltage under transients and impulsive load. Under this technique, mode of operation of bidirectional converter is changed by DC link voltage according to the shutdown or restoration of the grid power. Experimental results of a 500VA prototype show the performance of the proposed UPS system. The experimental results obtain the conversion efficiency and the input power factor up to 94% and 99% respectively. The main advantages of the proposed system are high power factor correction by the rectifier and efficient control of the DC link voltage which results in reduction of the transient effect cause by the output voltage. But the most important is the utilization of bidirectional converter which provides high efficiency and considerable reduction in the number of the batteries. This paper is organized as follows: the proposed topology is explained in section II. The control technique is described in section III. Design considerations are presented in section IV, followed by experimental results in section V. Section VI is the conclusion. II. TOPOLOGY DESCRIPTION The proposed UPS is shown in Fig. 3. It is composed of the following parts: An isolated boost rectifier comprising of rectifier diodes D 1 -D 4 and traditional boost converter consisting of switch Sr, Diode Dr, Capacitor Cr and Inductor Lr; Cascaded Bidirectional Buck-boost Converter comprising of switch S 1 -S 3, Diodes D 5 -D 6, and inductors L 1, L 2 ; a full-bridge voltage source inverter comprising of the switches S 5 -S 8 ; and the output filter formed by inductors L f1, L f2 and capacitor Co. A. Modes of Operation: The operation of the proposed UPS can be divided into two modes, as shown in the Fig. 4, the grid mode or the normal mode, and the battery powered mode. Grid Mode: When there is no power failure or the utility power is at least 80% of its rated operating condition, the Grid Mode is active. During this mode the rectifier, the

3 Fig. 5. Circuit Diagram of bidirectional DC-DC converter (b) (a) (c) Fig. 6 (a) Boost Mode of operation of bidirectional Converter (b) When S 2 is ON (c) When S 2 is OFF Battery Powered Mode: In case of the instantaneous decrease in the DC link voltage due to AC power failure or abrupt decrease in the input voltage, the battery powered mode activates. During this mode the magnetic contactor (MC) is opened, and the rectifier is disabled. The batteries provide the required power to the load. The bidirectional converter acts as a boost converter and (a) converts the battery voltage to DC link voltage. The inverter maintains the AC output voltage during a specified battery discharge time. (b) (c) Fig. 7 (a) Buck mode of operation of bidirectional Converter (b) S 1 & S 3 is ON, (c) S 1 & S 3 is OFF bidirectional converter, and the inverter are in operation. The rectifier converts the input AC voltage to DC link voltage. The bidirectional converter operates as a buck converter and acts as a charger for the battery bank. The inverter provides sinusoidal output voltage. B. Bidirectional Converter: The bidirectional converter is shown in the Fig. 5. The proposed converter has two boost stages, thus high conversion ratio can be achieved which results in reduced battery bank. The control of the switches S 1 -S 3 depicts the operational mode of the bidirectional converter. During the battery powered mode of operation of the UPS system, the bidirectional converter operates as a boost converter. The power will be delivered from low voltage side (battery bank) to high voltage side (DC link voltage) by controlling the duty cycle D of the switch S 2. During this period, the switches S 1 and S 3 are off. The circuit diagram is given in Fig. 6 and the voltage conversion ratio is given by Eq. 2 (1) (2) Since both the step up stages are controlled by a single switch S 2, so it will suffer high switching losses. Thus the efficiency of the system will decrease. To overcome this problem, some suitable snubber circuit is placed in the converter structure to control the switching-on and switching-off losses [13]-[14]. In the proposed topology RCD passive snubber circuit is used to reduce the switching loss of the S 2 as shown in Fig. 8. This snubber consists of capacitance, resistance, and diode

4 Fig. 8. Circuit Diagram of Snubber During the switching-off transients, the diode Ds connected the capacitor Cs in parallel to the switch. The resistor Rs limits the discharge capacitor current. On the other hand, the Rs must confirm that the capacitor will be completely discharged during the next interval ON. Thus it will prevent extra voltage stress on the switch. During Grid mode of operation of the UPS system, the converter will operate as buck converter. The power will be delivered from high voltage side (DC link voltage) to the low voltage side (battery Bank) by controlling the switches S1, S3 simultaneously. During this period the switch S 2 will be off. The equivalent circuit is shown in Fig. 7. The voltage conversion ratio is given by Eq. 3 (3) Two PWM control IC TL494 with voltage sensor at high voltage side (DC link voltage), and current sensor like ACS715 at low voltage side (battery bank) are adopted to achieve the objective of feedback a control. A conventional proportional integral (PI) control is used and the corresponding PI gain is chosen to obtain the best dynamic changes in experiments. C. Boost Rectifier: During Grid mode of operation of the UPS system, the boost rectifier converts the AC grid voltage to the DC link voltage. So a boost power factor correction (PFC) regulator has been used as a solution to suppress current harmonics, achieve unity power factor and utilize full line power. The boost regulator input current must be forced or programmed to be proportional to the input voltage waveform for power factor correction. Boost power corrector circuit is shown in Fig. 9.There is a diode bridge ahead of the inductor of boost converter to rectify the AC input voltage. The output capacitor must be rated to handle the second harmonics ripple current as well as the high frequency ripple current from switch of the boost converter. Since an active power factor corrector must control both the input current and the output voltage, so a conventional technique of average current mode control is implemented by using well known PWM controller UC3854[15]-[16]. D. Inverter A voltage source full bridge inverter is used at the output of the boost stage to convert the DC link voltage to sinusoidal output voltage. The circuit is shown in the Fig. 10. In order to control the output voltage, a sinusoidal PWM control with unipolar voltage switching is applied. Fig. 9. Circuit Diagram of Boost Rectifier Fig. 10. Circuit Diagram of Voltage Source Inverter At the output of the inverter, the LC filter is employed to obtain the regulated sinusoidal output voltage for the load. III. CONTROL STRATEGY In order to improve the performance of the system, the fast detection technique of the input voltage is required in order to decrease the transient effect of the system. Thus an efficient control technique of the DClink voltage is employed in the proposed system [17]. Fig. 11 shows the condition of the input power according to the variation in the DC link voltage. Due to the failure in the input power, the DC link voltage decreases instantly. When the DC link voltage reaches the starting voltage of the Battery powered mode V-start, the magnetic contactor (MC) is opened and the bidirectional converter operates as a boost converter. The battery voltage is step up to the DC link voltage. The DC link voltage is regulated to the output voltage of the bidirectional converter V-discharge. The rectifier is disabled by MC due to loss of the input power. Upon the restoration of the input power, the rectifier comes again to its operation condition, and the bidirectional converter operates as buck converter. The DC-link voltage steps down to the battery voltage in order to charge the battery. The rectifier provides regulated DC link voltage to the inverter as well as the bidirectional converter. During this transition period between charging and discharging mode, the capacitors connected to the DC link bus are selected such to provide sufficient energy to the inverter till the battery bank or the rectifier is connected. Since the operation of the bidirectional converter is changed by the DC link voltage, the power required for the load is supplied by either input power or the battery power.

5 (9) where Po in the apparent power of the inverter. The filter capacitor values C fi can be calculated considering the resonant frequency of the filter as shown in the Eq.10 (10) Fig. 11. Control of the DC link voltage A. Preliminary Specifications The design specifications of the proposed UPS are shown in the Table 1. The switching frequency for all the stages is assumed to be f S = 20 KHz B. Design procedure of Bidirectional Converter The values of the inductor L 1 and L 2 should be selected as such that it work for both the buck and boost mode of operation of the bidirectional converter for the specified duty ratio. To calculate the values of the inductor L 1 and L 2 for the bidirectional converter, select either buck or boost mode of operation of the converter [18]. where, Ts is the switching time period, R is the load resistance, and v o is the output ripple voltage. C. Design procedure for Boost Rectifier The boost inductance and output capacitor of the rectifier can be obtained according to [16], IV. D where, t is the hold time, V o(m is the minimum voltage the load will operate at. D. Design procedure of the inverter The filter inductance is obtained from the inductor voltage equation. The design considers purely resistive load, and the angle of the fundamental input voltage across the LC filter is = t = /2 [19]. Substituting the design parameters in Eq. 8 DEIGN CONSIDERATION where V b is the DC link voltage, h / is the hysteresis band and fs is the switching frequency. h / can neither be too small, in that case the inductor size will be large, nor too big because the ripple current will be so high. Therefore h / is chosen to be 50% of the rated load current at the highest outpu voltage. min) (8) (4) (5) (6) (7) V. EXPERIMENTAL RESULTS To verify the theoretical analysis of the proposed system, a 500VA prototype is built in the laboratory as shown in Fig. 19. The specifications of the proposed system are shown in Table I whereas the experimental parameters of different parts of the system is shown in Table II-IV. The battery bank of the system is reduced to 36V in the proposed configuration which is considerablee less than 192V in [12] and 108V in [19]. Thus the size of the system is significantly reduced. Fig. 12 shows the inductor current waveform during the boost mode of operation of the DC-DC converter whereas Fig. 13 showss the inductors waveforms during buck mode of operation of the DC-DC converter. Fig. 14 shows the waveform of drain to source voltage of the switch S 2 without snubber. Notice the high voltage spike in V DS which cause voltage stress on the switch S 2 resulting in switching loss. By using the snubber, the switching loss is reducedd efficiently as shown in the drain to source voltage and current waveform of S 2 in Fig. 14. The drain to source voltage and current waveform of the switches S 1 & S 3 is shown in the Fig. 15 and Fig. 16 respectively. Fig. 17 shows the current waveform of the inductor Lr of the rectifier. The waveform of the output voltage and current is shown in Fig. 18. It can be seen that a high quality sinusoidal voltage waveform is supplied by the inverter to the load, with a high power factor and low THD. Table I Developed UPS Specifications Input voltage Vi = 220Vac Output Voltage V O = 220Vac DC Link Voltage V d = 350V Grid Frequency fr = 60Hz Output Power Capacity So = 500VA Output frequency f O = 60Hz Output Power factor 0.9 Number of batteries (in series) 3(12V) Table II Experimental Parameters of UPS Bidirectional Converter Boost Inductors L 1 = 164uH L 2 =384uH Output Electrolyte Capacitors C 1 = 1000uF C 2 = 2 330uF Diode D 5, D 6 FFH 60UP40S Switch S 1, S 2, & S 3 IPW60R045CS Controller TL494

6 Tabble III Experimenttal Parameteers of UPS Booost Rectifier Rectifier Dioode Boost Inducttor Output Electtrolyte Capacittor Switch Sr Diode Dr Controller S15W WB60 Lr=2335uH Co=4770uf IPW600R045CS IXY DSEI60-12A D UC38854 Taable IV I Expeerimental parameters of Inverter Filter Induuctor Filter Cappacitor Switches S5-S8 Controllerr Lf1= Lf2 = 2.8mH 2 Co= 2 1uF F 11N80C3 dspic30f VGS VGS il1 il1 il2 il2 w off Boost modde of Fig. 12. Exxperimental waveform bidirectional converter (il1 div, VGS: 10V V/div, L, il2: 10A/d Time: 20us/ddiv) Fig. 13. Experim mental wavefforms of Bu uck mode off bidirrectional connverter (il1:110a/div, il2:2 2A/div, VGS: 10V V/div, Time: 200us/div) VDS2 VGS ids2 (a). VDS2Withhout Snubber((VDS2: 20V/divv, Time 20us//div) (b) VDS2With Snuubber (VDS2: 500V/div, ids2: 10A/div 1 Timee: 20uss/div) Fig. 14. Waaveform of Draain to Source voltage of sw witch S2 of bidiirectional Con nverter VDSS3 VDS1 ids3 Fig. 15. Voltage and Currrent waveform m of Switch S3 of bidirectional converter (V VDS3: 200V/diiv, ids3: 10A A/div, Time: 10us/ddiv) ids1 waveform of Switch S1 off Fig. 16. Voltage and Current w bidirrectional Connverter (VDS1: 200V/div, ids1: 20A/div,, Tim me: 20us/div)

7 V GS V O I O i Lr Fig. 17. Experimental Waveform of the Rectifier (V GS : 10V/div, i Lr : 2A/div, Time: 20us/div) Fig. 18. Output Voltage and Current of the inverter for linear load (V O : 100V/div, I O : 2A/div, Time: 5ms/div) Fig. 19. Laboratory prototype of the propose system VI. CO ONCLUSIONS In this paper a transformer-less online UPS system is proposed. Utilizing the Bidirectional cascaded converter, the battery bank of the system is significantly reduced. The bidirectional act as boost converter during battery powered mode and act as a buck converter during grid mode of operation in orderr to charge the battery. Thus the size, weight and cost of the system is reduces and the overall efficiency of the system is improved. The rectifier provided regulated DC link voltage with high power factor correction. The inverter stage also provided sinusoidal output voltage to the load. The control technique of the DC link voltage improved the dynamic response of the output voltage by reducing the transient and spikes from the output voltage. The experimental results confirmed the effectiveness of the proposed system with a high efficiency of 94% and low THD of 3%. ACKNOWLEDGMENT This research work is sponsored by Higher Education Commission (HEC), Govt. Of Pakistan under the scholarship program titled: MS Level Training in Korean Universities/ /Industry, REFERENCES [1] J. M. Guerrero, L. G. Vicuna, and J. Uceda, Uninterruptible power supply systemss provide protection, IEEE Ind. Electron. Mag., vol. 1, no. 1, pp , Spring [2] S. B. Bekiarov and A. Emadi, Three of a kind [UPS topologies, IEC standard], IEE rev., vol. 46, no. 2, pp , Mar [3] S. B. Bekiarov, A. Emadi, Uninterruptible Power Supplies: Classification, Operation, Dynamics, and Control, 7th IEEE Applied Electron Conf Exp APEC, vol. 1, pp , Aug [4] F. Botteron and H.Pinheiro, A three-phase UPS thatt complies with the standard IEC , IEEE trans. Ind. Electron., vol. 54, no. 4, pp , Aug [5] R. P. Torrico-Bascope, D. S. Oliveira, Jr., C. G. C. Branco, F. L. M. Antunes, and C. M. T. Cruz, A High Frequency Transformer Isolation 110V/220V Input Voltage UPS System, in Proc. IEEE Appl. Power electronics. Conf.,, vol. 1, pp Mar [6] R. Krishnan, Design and development of a high frequency on-line uninterruptible power supply, in Proc. IEEE Ind. Electron. Control Instr. Conf.., 1995, vol. 1, pp [7] K. Hirachi, K. Nishimura, J. Yoshitsugu, Y. Arai, A. Chibani, and M. Nakaoka, A high frequency linked single phase UPS with power factor correction scheme in Proc. IEEE ISIE, 1997, vol. 2, pp [8] S. J. Chiang, T.S. Lee, and J. M. Chang, Design and implementationn of a single phase three-arm rectifierr inverter, Proc. Inst. Electr. Eng. - Electr. Power Appl., vol. 147, no. 5, pp , Sep [9] N. Hirao, T. satonaga, T. Uematus, T. Kohama, T. Ninomiya, and M. Shoyma, Analytical considerations on power loss in a three-arm-type uninterruptible power supply, in Proc. IEEE PESC, 1998, vol2, pp [10] I. Ando, I. Takahashi. Y. Tanaka, and M. Ikchara, Development of a high efficiency UPS having active filter ability composed of a three arms bridge, in Proc. IEEE IECON, 1997, vol. 2, pp

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