IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 1361 A Reconfigurable Uninterruptible Power Supply System for Multiple Power Quality Applications Chia-Chou Yeh, Student Member, IEEE, and Madhav D. Manjrekar, Member, IEEE Abstract—A novel topology of a modular per-phase uninterruptible power supply (UPS) system based on reduced-switch-count configuration is proposed in this paper. The proposed power conversion methodology offers active front-end filtering capability which ensures unity input power factor (reactive power compensation) and low input total harmonic distortion (harmonic power compensation). In addition, this UPS system provides dynamic voltage sag compensation capability, which consequently eliminates any series transformer or dc-dc boost converter that is typically used in conjunction with traditional stand-alone UPS systems. Furthermore, it has the desirable characteristics of making seamless transition from normal to backup mode during power failures and vice versa, as well as providing controlled voltage charging at the dc bus link. The proposed UPS is also impervious to load variations, which enables it to operate under nonlinear load condition. Lastly, the circuit structure is conceived from a commonly used three-leg six-switch building block, thereby making it simple and cost-effective, and offering improved power conversion efficiency as compared to a conventional line-interactive or on-line UPS schemes. A complete set of simulation and sample experimental results based on a 1-kW test prototype of the proposed UPS system are presented in this paper to demonstrate its viability and efficacy. Index Terms—Active filter, power factor correction, power quality, reduced-switch-count, uninterruptible power supply (UPS), voltage sag. I. INTRODUCTION N RECENT years, extensive research has been dedicated towards the design of uninterruptible power supply (UPS) systems to provide clean, conditioned, and uninterruptible power to equipment in critical applications such as servers and storage systems, personal computers, medical equipment, telecommunication systems, industrial and commercial controls, etc. under essentially any normal or abnormal utility power conditions. In order to supply continuous power to the load in the absence of utility power, energy storage systems such as batteries or flywheels are incorporated in such UPS systems. Typically, power conversion is accomplished using static power electronic devices such as fast-switching high-current insulated gate bipolar transistors (IGBTs). Conventional UPS topologies can mainly be categorized into three different types: 1) off-line; 2) line-interactive; and 3) I Manuscript received April 4, 2006; revised October 7, 2006. Recommended for publication by Associate Editor P. Mattavelli. C.-C. Yeh is with the Department of Electrical and Computer Engineering, Marquette University, Milwaukee, WI 53233 USA (e-mail: chiachou.yeh@marquette.edu; cyeh@ieee.org). M. D. Manjrekar is with Danaher Power Solutions, Richmond, VA 23231 USA (e-mail: mmanjrekar@danaher-dps.com). Digital Object Identifier 10.1109/TPEL.2007.900486 Fig. 1. Simplified diagram of off-line UPS topology. on-line [1]. Off-line UPS, which is also sometimes referred to as line-preferred or passive-standby UPS, is usually used in low power applications with power ratings less than 2 kVA [1]–[3]. A typical off-line UPS, as depicted in Fig. 1, consists of a static bypass switch that connects the critical load directly to the unconditioned utility mains under normal condition while the battery is charged through the rectifier/charger. In the event of power failures, the static switch disconnects the mains, and the critical load gets fed from the backup battery through the inverter. The transfer switching time is specified within a quarter of line cycle and hence an interruption such as a voltage sag or voltage loss might occur at the load side before the backup power is delivered to the critical load. While the off-line UPS offers the advantages of simple design, low cost, small size, and high efficiency, the power supplied to the critical load is neither regulated nor conditioned. Therefore, the critical load is not protected from any voltage or frequency fluctuations along the utility power line. Accordingly, some power line filtering can be employed to remove large surges, spikes, sags, and other irregularities that are inherent in the utility power. However, no output voltage regulation and active power line conditioning, in terms of voltage correction, disturbance rejection, and reactive and harmonic compensations, is available at the utility side. In addition, other factors include performance issues with nonlinear loads and lack of isolation of the load from the utility mains. A line-interactive UPS offers an improved performance as compared to the off-line UPS and it is normally used in low to medium power applications [1]. There are two types of topologies offered for line-interactive UPS. The first and early topology [4]–[9], as shown in Fig. 2, consists of a series inductor between the utility mains and the critical load, and a bilateral converter in parallel with the critical load, acting as a battery charger under normal condition and an inverter supply backup power from the battery to the load in the event of power 0885-8993/$25.00 © 2007 IEEE 1362 Fig. 2. Simplified diagram of line-interactive UPS topology with single converter structure. Fig. 3. Simplified diagram of line-interactive UPS topology with two converter structures. outage. This type of UPS is also sometimes referred to as parallel-processing UPS which involves only one power stage. The early development of the line-interactive UPS given in [4] and [5] offers the capability of input current harmonic suppression, but very little reactive power compensation. Furthermore, the topology does not provide voltage sag compensation (output voltage regulation) capability and a tri-port high leakage inductance transformer is incorporated between the mains and the load for isolation purpose. An improved version of such line-interactive UPS without any tri-port high leakage transformer has been proposed in [6]–[9] where the UPS has the ability to suppress input current harmonics. However, the performance attributes in terms of input power factor correction and output voltage regulation in these schemes reported in [6]–[9] appear to be limited. An alternative type of line-interactive UPS, which has received considerable attention in recent years, is known as the series-parallel compensated line-interactive UPS or the so-called “delta-conversion” UPS [10]–[16]. As illustrated in Fig. 3, this type of UPS consists of two power conversion stages, one converter in series with the utility mains through a series transformer and the other in parallel with the load. During the normal operation, the utility mains supplies power directly to the load while the battery is charged through the parallel converter. When the mains fail, the static switch opens to disconnect the load from the mains, and the battery maintains the continuity of power to the load through the parallel converter. Besides functioning as a battery charger, the parallel power converter also facilitates the input current harmonic suppression and power factor correction. The complementary IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 Fig. 4. Simplified diagram of on-line UPS topology. series power converter acts as a voltage regulator to regulate the output voltage in the event of utility voltage sag or swell. This UPS configuration allows an independent control of the output voltage, input power factor correction, and input harmonic power compensation, thus providing series-parallel active power line conditioning capabilities [10]–[16]. Even though this line-interactive UPS scheme consists of two power conversion stages, its efficiency is relatively high due to the fact that the rating of the series power converter is typically 10%–20% of the overall UPS rating, since its purpose is to compensate for the voltage difference between the mains and the load [11]. Some of the drawbacks associated with such UPS are lack of effective isolation between the load and the utility mains, complex control algorithm, as well as the need for the series transformer which could be bulky, heavy, and expensive. The on-line UPS, which is also sometimes referred to as inverter-preferred or double-conversion UPS, has evolved into a dominant candidate for high power and high voltage applications in industrial and manufacturing plants [1]. This is due to its ability to supply conditioned and regulated power to the critical load, as well as its seamless transition from normal to backup mode and vice versa, and its decoupling capability of the utility and the load under power outage. A typical on-line UPS functional block topology is shown in Fig. 4. It mainly consists of a rectifier/charger that converts the ac input supply voltage into unregulated dc voltage for the inverter and the battery charging, a battery that supplies backup energy in the event of an utility power outage, an inverter that converts the unregulated dc voltage from the rectifier (or the battery) into regulated and filtered ac voltage for the load, and a static bypass switch that transfers the load to the ac input supply without any interruption in the supply of power in the event of power conditioner failure. A number of new and improved on-line UPS systems have been reported in the past to provide active power line conditioning capability in order to suppress input current harmonics and to realize close to unity input power factor [17]–[23]. However, when the utility is not functioning in its full operating condition or if there is utility voltage sag, the UPS is unable to supply full power to the load. Hence, a dc-dc boost converter in series with the inverter is required to stabilize the output voltage [17]–[20]. Accordingly, the addition of dc-dc boost converter to the UPS results in increased cost and footprint of the overall system. Furthermore, a typical on-line UPS requires two power conversion stages as depicted in Fig. 4. Both power converters YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS Fig. 5. Single-phase half-bridge inverter. 1363 Fig. 7. Single-phase half-bridge active rectifier/inverter. to offer the characteristics of either an “on-line” or “line-interactive” UPS. In other words, the UPS can either act as a direct voltage feeder (“line-interactive” feature) or an output voltage regulator (“on-line” feature) during normal operation. This can be realized through modifying the control algorithm of the PWM switching scheme. In this paper, the simulation and sample experimental results based on a 1-kW test prototype are presented to demonstrate the viability and efficacy of the proposed UPS system. It may be noted that the results presented here are only pertained to the operation as a direct voltage feeder (“line-interactive” feature) during normal condition. Nevertheless, the control algorithm for the operation as an output voltage regulator (“on-line” feature) will be addressed here in this paper. Fig. 6. Single-phase half-bridge diode rectifier/inverter. are required to operate at full power rating of the UPS. This also results in a lower operation efficiency and higher system cost as compared to the line-interactive UPS. In this paper, a new and low-cost modular per-phase UPS based on reduced-switch-count topology is proposed. The proposed topology offers improved power conversion efficiency, low cost, light weight, and it has a simple circuit structure owing to an employment of commonly used building block. In addition, this novel UPS system provides active front-end filtering capability which results in unity input power factor (reactive power compensation) and low input total harmonic distortion (harmonic power compensation). Moreover, this UPS system offers desirable characteristics of making seamless transition from normal to backup mode during power failures and vice versa, as well as the ability to control the dc bus link voltage. The proposed UPS can also function as a dynamic voltage sag compensator (voltage restorer) which ensures output voltage stabilization when the utility is experiencing voltage sag or dip. Hence, this unique feature eliminates any dc-dc boost converter used in conjunction with the traditional stand-alone on-line UPS system. Also, this UPS system is impervious to load variations, which enables it to operate under nonlinear load condition. Due to its unique configuration, the proposed UPS has the ability II. THE PROPOSED UPS SYSTEM TOPOLOGY To illustrate the development of the proposed UPS system, let us begin with a simple single-phase half-bridge inverter, the circuit topology of which is shown in Fig. 5. It can be seen that this topology operates by causing the load current to flow directly through the dc capacitors. As the switches are turned ON and OFF to control the level of the output voltage, the current changes from one capacitor to the other. Depending on the direction of the current flow, energy is either stored in or extracted from the capacitors. To further improve on the dc bus charging, a half-bridge diode rectifier is included in the same circuit topology, as given in Fig. 6. As may be observed in Fig. 6, the circuit topology offers only limited dc bus charging with a diode rectifier. Besides that, one would need an alternative path for current flow from the dc bus capacitors during the backup mode, that is when the input mains are isolated from the system. To mitigate this issue, a half-bridge active rectifier is used in place of the diode rectifier, as shown in Fig. 7. Such an active rectifier provides active input current shaping as well as input power factor control. Meanwhile, the dc bus charging can also be controlled to provide the necessary energy required for the specified boost. More importantly, this scheme now provides alternative path for current flow when the input mains are isolated from the system. However, as shown in Fig. 7, the dc bus capacitors are always in the load current path, and hence they experience significant charge fluctuations. In order to reduce the dc 1364 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 Fig. 10. Normal mode of operation. Fig. 8. Proposed UPS topology. III. MODES OF OPERATIONS The proposed UPS system can function in three different modes of operations, namely the normal mode, the backup mode, and the voltage sag compensation mode. A. Normal Mode Fig. 9. Complete topology of proposed UPS system. bus voltage variations, an additional leg, namely the “source” leg, is included in the model, as shown in Fig. 8. In this proposed UPS topology, there are three different modes of operations, namely: 1) normal mode; 2) backup (battery) mode; and (3) voltage sag compensation (voltage restoration) mode. The details of these modes of operations are described in the following section. A complete UPS topology is depicted in Fig. 9. The battery is introduced to provide backup energy storage in the event of power outage, as well as to provide additional voltage when the utility is experiencing voltage sag. The battery switch is made up of a contactor and a SCR. During normal mode, the SCR is opened and the contactor is closed which allows the battery to be charged up by the active rectifier comprising of the “source” and “neutral” legs. During the backup mode, the “source leg” transistor switches are turned-OFF, while the contactor is opened and the SCR is closed, which enables the battery to act as a dc supply to the inverter comprising of the “load” and “neutral” legs. During the voltage sag compensation mode, the “neutral” leg transistor switches are turned-OFF, while the contactor is opened and the SCR is closed, which allows the “source” and “load” legs to act as a voltage restorer to supply the additional voltage from the battery to the load in the event of utility sag. The bypass static switch is included in case of power conditioner faults, which will switch the load directly to the utility. During the normal mode, that is under the condition of which there is no power failure or the utility is at least 90% of its rated operating condition, the “source” leg and the “neutral” leg operate as an active rectifier (and battery charger), while the “source” leg and the “load” leg synchronize to operate as a direct voltage feeder (a “line-interactive” feature) from the utility to the load, as shown in Fig. 10. In order to obtain sinusoidal input current waveform and unity input power factor, current-regulated pulse-width modulation (CRPWM) control scheme is employed here [24], [25], the block diagram of which is given in Fig. 11. As shown in the figure, there are two feedback control loops, namely the outer dc bus voltage control loop and the inner current control loop. The , is sensed and compared with a reference dc bus voltage, dc voltage, , and the error is fed to a proportional-integral (PI) controller. The output of the PI controller is the amplitude , which is then multiplied by of the reference utility current, a unity waveform, having the same phase angle as the utility voltage, , to ensure unity input power factor. In other words, the unity waveform is the normalized utility voltage, , which can be realized using the normalization method as delineated in Fig. 12, [26]. Next, this reference utility current, , is compared with the measured utility current, , and the error is again conditioned by a PI controller to produce a control . This control voltage, , is then compared with voltage, , to synthesize the a fixed frequency triangular waveform, desired PWM switching patterns for the transistor switches of Fig. 10. During the normal mode, the CRPWM control scheme is applied to the “source” and “neutral” legs, while the PWM switching patterns of the “load” leg is synchronized with that of the “source” leg. B. Backup Mode In the backup (battery) mode, that is when there is a power outage or when the utility is less than 50% of its rated condition, YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS 1365 Fig. 11. Control schemes for various modes of operations. Fig. 12. Normalization method. the “source” leg transistor switches are turned-OFF, thus preventing backward power feed into the utility, while the “load” leg and the “neutral” leg operate as an inverter which supplies power from the battery to the load, as depicted in Fig. 13. During this mode, the CRPWM scheme cannot be used since the utility is isolated, which results in a zero utility current. Hence, sinusoidal PWM with unipolar voltage switching scheme is utilized to synthesize the switching sequences of the “load” and “neutral” legs, see Fig. 11. The desired reference load voltage, , is , to produce compared with the same triangular waveform, the PWM switching patterns. The unipolar voltage switching is preferred over bipolar voltage switching due to its three-level PWM output voltages as well as lower harmonic contents, hence lesser ripples, in the output voltage waveform. With this control operation for the backup mode, a regulated sinusoidal output voltage with low total harmonic distortion is delivered. C. Voltage Sag Compensation Mode During the voltage sag compensation (voltage restoration) mode, that is when the utility is operating between 50% and 90% of its rated condition, the “source” leg and the “load” leg operate Fig. 13. Backup (battery) mode of operation. as a voltage restorer which supplies additional power from the battery to compensate for the utility voltage sag, as shown in Fig. 14. Meanwhile, the “neutral” leg transistor switches are turned-OFF. In this voltage sag compensation mode, a refer, in phase synchronization with the utility ence voltage, 1366 Fig. 14. Voltage sag compensation mode of operation. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 (a) Fig. 15. Simplified equivalent circuit diagram under voltage sag compensation mode. (b) Fig. 17. (a) Utility input voltage and current waveforms under linear load condition. (b) Load voltage and current waveforms under linear load condition. Fig. 16. Control strategy to demonstrate output voltage regulation (“on-line” feature). voltage, is realized by multiplying the amount of sag voltage, , with the normalized utility voltage waveform obtained using the normalization method given in Fig. 12. Accordingly, , is used to synthesize the this resulting reference voltage, desired PWM switching patterns, see Fig. 11. The sinusoidal PWM control scheme is applied to the “source” and “load” legs, while no switching is performed on the “neutral” leg. The voltage supplied from the battery, in this case 0.2 , in addition to the voltage from the sag utility, in this case 0.8 , ensures desired full voltage of 1.0 to be supplied to the load in order to stabilize the output voltage at one per unit rated utility voltage, as illustrated in Fig. 15. It will be seen from the results that the load voltage and current waveforms regulate seamlessly when the utility sag occurs, as well as when the utility restores to its healthy 100% condition. It is evident that the control methods of Fig. 11 for various modes of operations are simple and can be easily implemented using a digital microprocessor. With this proposed UPS topology, one does not need any series transformer used in conjunction with line-interactive UPS [10]–[16] or any dc-dc boost converter used in conjunction with on-line UPS [17]–[20] when compensating for the voltage sag. Furthermore, the active rectifier (or charger) not only controls the dc bus voltage charging, but also provides active front-end filtering and power factor correction capabilities. In order to control the proposed UPS as an output voltage regulator (“on-line” feature) during normal mode, the control strategy employs a modified sinusoidal PWM scheme, as demonstrated in Fig. 16. The “load” and “neutral” legs operate as an inverter capable of generating three-level waveform at the output. Therefore, a conventional unipolar PWM strategy is adopted which employs positive and negative polarity sine waves (thin dotted sinusoids) to the “load” and “neutral” legs as reference commands. If the same reference as in the “neutral” leg is fed to the “source” leg (thin dotted sinusoids), then there is no net voltage between the “source” and “neutral” legs, thereby producing no contribution at the utility front end because of this inverter modulation. On the other hand, a conventional current regulated PWM scheme is adopted to YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS Fig. 18. UPS switches from normal to backup mode at t = 0:5 s under linear load condition. (a) Utility input current. (b) Load voltage and current. synthesize a three-level rectifier input voltage as a set of two opposite polarity waveforms at the “source” and “neutral” legs (thick solid sinusoids). Again, if the same reference command as the “neutral” leg is applied to the “load” leg (thick solid sinusoids), then there is no interference in the inverter output because of modulation of the “source” and “neutral” legs. This implies that the rectifier and inverter operation are decoupled from each other. Accordingly, adding together the rectifier and inverter reference commands for each leg will produce the resultant reference command that is required to synthesize the PWM switching patterns for each leg, as shown in Fig. 16. As mentioned earlier, the results presented here are only pertained to the operation as a direct voltage feeder depicted in Fig. 10. IV. SIMULATION RESULTS The proposed modular per-phase UPS system is simulated in Matlab-Simulink under linear and non-linear load conditions for various modes of operations. The specifications of the system are as follows: kW; • Output power, ; • Utility voltage, • DC link voltage, V; kHz. • Switching frequency, A. Normal Mode Under Linear Load Condition Here, the utility input voltage and current waveforms in normal mode under resistive (linear) load condition are shown 1367 Fig. 19. UPS switches from backup to normal mode at t = 0:5 s under linear load condition. (a) Utility input current. (b) Load voltage and current. in Fig. 17(a). It can be seen that the utility input current waveform is close to sinusoidal with a total harmonic distortion (THD) of 3.54% and has a unity input power factor. The load voltage and current waveforms under the same condition are shown in Fig. 17(b). Again, both the voltage and current waveforms are close to sinusoidal with each having a THD of 0.22%. Hence, the system performance demonstrates the active input current shaping and input power factor control of the proposed UPS system. B. Backup Mode Under Linear Load Condition In the event of power outage or utility voltage sag of more than 50%, the UPS switches immediately from normal to backup (battery) mode. Fig. 18(a) shows the utility input current waveform and Fig. 18(b) shows the load voltage and current waveforms, in the case of a power outage at s. It can be observed that the load voltage and current continue to regulate with only slight interruptions at the point where the power failure occurs. The same seamless transition can be seen in Fig. 19(a) and (b) where the UPS switches from backup to s. normal mode at C. Normal Mode under Non-Linear Load Condition Here, the UPS is operating in the normal mode under diode-rectifier (non-linear) load condition. Fig. 20(a) depicts the utility input voltage and current waveforms. As may be 1368 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 (a) Fig. 20. (a) Utility input voltage and current waveforms under non-linear load condition. (b) Load voltage and current waveforms under non-linear load condition. observed therein, the utility input current is close to sinusoidal with a THD of 6.23% and has a unity input power factor. Meanwhile, the load voltage and current waveforms under the same condition are shown in Fig. 20(b). Accordingly, the proposed UPS is impervious to load variations due to almost sinusoidal input current and unity input power factor. D. Backup Mode Under Non-Linear Load Condition The proposed UPS is operating under non-linear load condition in the backup mode of operation. The utility input current waveform, and the load voltage and current waveforms are illustrated in Fig. 21(a) and (b), respectively, in the event of power failure at s. It can be seen from the figure that it takes approximately 0.1 s for the load waveforms to stabilize when s. the UPS switches from normal to backup mode at The same scenario can also be observed from Fig. 22(a) and (b), where the UPS switches from backup to normal mode at s. E. Voltage Sag Compensation Mode under Linear Load Condition When the UPS experiences a utility voltage sag of 10% to 50% of its full condition, it immediately switches to the voltage sag compensation (voltage restoration) mode. The utility input voltage waveform under linear load condition in the event of Fig. 21. UPS switches from normal to backup mode at t = 0:5 s under nonlinear load condition. (a) Utility input current. (b) Load voltage and current. s is depicted in Fig. 23(a). 30% utility voltage sag at Meanwhile, the load voltage and current waveforms are shown in Fig. 23(b). Notice from the figure that the load waveforms are restored immediately without any discrepancy when the voltage sag occurs. When the utility is recovered to its healthy 100% s, the load waveforms regulate seamcondition at lessly which demonstrates the dynamic performance of the UPS system. V. EXPERIMENTAL SETUP AND RESULTS The performance of the proposed modular per-phase UPS is verified experimentally from a 1-kW laboratory prototype rated , as illustrated in Figs. 24 at a nominal input voltage of 120 and 25. The control method of Fig. 11 was implemented using Analog Devices ADMC401 DSP board. The PWM switching frequency was set at 10 kHz. The software was programmed in assembly language to achieve the maximum speed and to reduce the program overhead. The backup energy storage system of Fig. 25 consists of several sets of dc capacitor banks capable of producing dc voltage up to 700 V. The utility input voltage and current waveforms in normal mode under resistive (linear) load condition are shown in Fig. 26(a). It can be observed from the figure that the input current waveform is close to sinusoidal and has a unity input power factor. The same conclusion can also be stated for the load voltage and current waveforms under the YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS 1369 (a) (b) Fig. 22. UPS switches from backup to normal mode at t = 0:5 s under nonlinear load condition. (a) Utility input current. (b) Load voltage and current. same operating condition, as given in Fig. 26(b). This demonstrates that the proposed UPS can suppress the input current harmonics as well as the reactive power. The input voltage and current waveforms, as well as the load voltage and current waveforms in normal mode under diode-rectifier (non-linear) load condition are given in Fig. 27(a) and (b), respectively. Again, the input current waveform is close to sinusoidal and has a near unity input power factor. This shows that the proposed UPS is immune to non-linear load while still maintaining a near sinusoidal input current waveform and unity input power factor. Note that these experimental results in Figs. 26 and 27 are in good agreement with the simulation results presented earlier in Figs. 17 and 20, respectively. Meanwhile, the utility input current as well as the load voltage and current waveforms when the UPS switches from normal to backup mode are depicted in Fig. 28. Notice therein that the load voltage and current continue to regulate even when the utility current is zero. When the utility current is finally restored, the load voltage and current again regulate seamlessly with only slight interruptions, as shown in Fig. 29. Evidently from both the simulation and experimental results, the proposed UPS clearly demonstrates its superior performance for multiple power quality applications. In light of its simple circuit configuration and reduced active switching devices, the proposed UPS offers a low cost design and higher power conversion efficiency as compared to the line-interactive UPS systems Fig. 23. (a) Utility input voltage waveform (linear load). 30% utility voltage sag at t = 0:4s. (b) Load voltage and current waveforms (linear load). Fig. 24. UPS laboratory prototype. presented in [10]–[16]. It also eliminates the need of a low frequency series transformer which is bulky, heavy, and expensive. Comparing the proposed UPS with the on-line UPS systems presented in [17]–[20], the distinct difference is the elimination of a dc-dc boost converter used for voltage sag compensation purpose. In addition, comparing the proposed per-phase system with a typical single phase on-line UPS [21] as shown in Fig. 30, the proposed system only requires six active switches as compared to eight active switches shown in Fig. 30. Reducing the number of active switches not only reduces the system cost, it 1370 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 Fig. 25. Backup energy storage system. (a) (a) (b) Fig. 27. (a) Normal mode—Utility input voltage and current waveforms (nonlinear load). (voltage-50 V/div., current-10 A/div.). (b) Normal mode—Load voltage and current waveforms (non-linear load). (voltage-50 V/div., current-10 A/div.). (b) Fig. 26. (a) Normal mode—Utility input voltage and current waveforms (linear load). (voltage-50 V/div., current-10 A/div.). (b) Normal mode—Load voltage and current waveforms (linear load). (voltage-50 V/div., current-10 A/div.). Fig. 28. Transition from normal to backup mode under linear load condition. Utility current, load current, and load voltage. (voltage: 100 V/div., current: 10 A/div.). also enhances the reliability and lowers the size and weight of the overall system. Another desirable feature of the proposed UPS is the use of a common neutral connection between the input and output, thus eliminating the need of an isolation transformer as is used in the system of Fig. 30. The UPS systems presented in [22] and [23] are also based on the reduced-switchcount configuration for single-phase UPS. Although the UPS systems presented in [22] and [23] only require five and four active switches, respectively, in the topologies, these structures are not capable of providing utility voltage sag compensation capability. For three-phase operation, a three-phase topology comprising of three per-phase proposed UPS is essential, as YEH AND MANJREKAR: RECONFIGURABLE UPS SYSTEM FOR MULTIPLE POWER QUALITY APPLICATIONS Fig. 29. Transition from backup to normal mode under linear load condition. Utility current, load current, and load voltage. (voltage: 100 V/div., current: 10 A/div.). 1371 can function as a voltage sag compensator when a utility voltage sag occurs. Meanwhile, the proposed UPS has the ability of active front-end filtering, as well as seamless transition from normal to backup mode and vice versa. It also provides grid isolation without any backward power feed into the utility in the event of power failure, hence eliminates the needs of circuit breaker or static switch. More importantly, this UPS is simple in design at a lower cost as compared to a conventional line-interactive or on-line UPS owing to the reduced number of power electronic switching devices. Accordingly, it can be implemented using a standard commercially available three-leg six-switch building block. Finally, a 1-kW UPS prototype was built and tested, and the experimental results are in good agreement with the simulation results which demonstrate the effectiveness and soundness of the proposed UPS system. ACKNOWLEDGMENT The authors would like to thank Dr. J. Kikuchi and E. F. Buck at Eaton Corporation for their support during the experimental work. REFERENCES Fig. 30. A typical single-phase on-line UPS system. Fig. 31. A three-phase schematic of proposed UPS system based on TRINIT topology. illustrated in Fig. 31. This is referred to as Rectifier Inverter Neutral Inter- ) topology. (Triple VI. CONCLUSION A novel topology of a modular per-phase UPS system based on reduced-switch-count configuration has been proposed in this paper. Due to its unique configuration, the proposed UPS [1] S. Karve, “Three of a kind [UPS Topologies, IEC Standard],” Proc. Inst. Elect. Eng. Review, vol. 46, no. 2, pp. 27–31, Mar. 2000. [2] S. Martínez, M. Castro, R. Antoranz, and F. Aldana, “Off-line uninterruptible power supply with zero transfer time using integrated magnetics,” IEEE Trans. Ind. Electron., vol. 36, no. 3, pp. 441–445, Aug. 1989. [3] M. T. Tsai and C. H. Liu, “Design and implementation of a cost-effective quasi line-interactive UPS with novel topology,” IEEE Trans. Power Electron., vol. 18, no. 4, pp. 1002–1011, Jul. 2003. [4] G. J. Smollinger and W. J. Raddi, “Reverse energy transfer through an ac line synchronized pulse width modulated sine wave inverter,” in Proc. IEEE Int. Telecommunication Energy Conf., 1981, pp. 126–131, Intelec 81. [5] W. J. Raddi and R. W. Johnson, “An utility interactive PWM sine wave inverter configured as a high efficiency UPS,” in Proc. IEEE Int. Telecommunication Energy Conf., 1982, pp. 42–48, Intelec 82. [6] T. Kawabata, T. Miyashita, N. Sashida, and Y. Yamamoto, “Three phase parallel processing UPS using multi-functional inverter,” in Proc. IEEE Industry Applications Soc. Annu. Meeting Conf., Oct. 1989, vol. 1, pp. 982–987. [7] J. C. Wu and H. L. Jou, “A new UPS scheme provides harmonic suppression and input power factor correction,” IEEE Trans. Ind. Electron., vol. 42, no. 6, pp. 629–635, Dec. 1995. [8] G. Joos, Y. Lin, P. D. Ziogas, and J. F. Lindsay, “An online UPS with improved input-output characteristics,” in Proc. IEEE Applied Power Electronics Conf. and Expo., Feb. 1992, pp. 598–605. [9] Y. Lin, G. Joos, and J. F. Lindsay, “Performance analysis of parallelprocessing UPS systems,” in Proc. IEEE Applied Power Electronics Conf. and Expo., Mar. 1993, pp. 533–539. [10] S. Rathmann and H. A. Warner, “New generation UPS technology, the delta conversion principle,” in Conf. Proc. 31st IEEE Industry Applications Soc. Annu. Meeting, Oct. 1996, vol. 4, pp. 2389–2395. [11] F. Kamran and T. G. Habetler, “A novel on-line UPS with universal filtering capabilities,” IEEE Trans. Power Electron., vol. 13, no. 3, pp. 410–418, May 1998. [12] S. J. Jeon and G. H. Cho, “A series-parallel compensated uninterruptible power supply with sinusoidal input current and sinusoidal output voltage,” in Proc. 28th IEEE Power Electronics Specialists Conf., Jun. 1997, vol. 1, pp. 297–303. [13] S. A. O. da Silva, P. F. Donoso-Garcia, P. C. Cortizo, and P. F. Seixas, “A three-phase line-interactive UPS system implementation with series-parallel active power-line conditioning capabilities,” IEEE Trans. Ind. Appl., vol. 38, no. 6, pp. 1581–1590, Nov./Dec. 2002. [14] B. H. Kwon, J. H. Choi, and T. W. Kim, “Improved single-phase line-interactive UPS,” IEEE Trans. Ind. Electron., vol. 48, no. 4, pp. 804–811, Aug. 2001. 1372 [15] H.-L. Jou, J.-C. Wu, C. Tsai, K.-D. Wu, and M.-S. Huang, “Novel line-interactive uninterruptible power supply,” Proc. Inst. Elect. Eng. Electric Power Applications, vol. 151, no. 3, pp. 359–364, May 2004. [16] F. Barrero, S. Martinez, F. Yeves, F. Mur, and P. M. Martinez, “Universal and reconfigurable to UPS active power filter for line conditioning,” IEEE Trans. Power Del., vol. 18, no. 1, pp. 283–290, Jan. 2003. [17] N. Vázquez, C. Aguilar, J. Arau, R. O. Cáceres, I. Barbi, and J. A. Gallegos, “A novel uninterruptible power supply system with active power factor correction,” IEEE Trans. Power Electron., vol. 17, no. 3, pp. 405–412, May 2002. [18] V. M. Pacheco, L. C. de Freitas, J. B. Vieira, Jr., A. A. Pereira, E. A. A. Coelho, and V. J. Farias, “An online no-break with power factor correction and output voltage stabilization,” IEEE Trans. Power Electron., vol. 20, no. 5, pp. 1109–1117, Sep. 2005. [19] J. Y. Lee, Y. M. Chang, and F. Y. Liu, “A new UPS topology employing a PFC boost rectifier cascaded high-frequency tri-port converter,” IEEE Trans. Ind. Electron., vol. 46, no. 4, pp. 803–813, Aug. 1999. [20] M. A. de Rooij, J. A. Ferreira, and J. D. van Wyk, “A novel unity power factor low-EMI uninterruptible power supply,” IEEE Trans. Ind. Appl., vol. 34, no. 4, pp. 870–877, Aug. 1998. [21] T.-J. Liang and J.-L. Shyu, “Improved DSP-controlled online UPS system with high real output power,” Proc. Inst. Elect. Eng. Electric Power Applications, vol. 151, no. 1, pp. 121–127, Jan. 2004. [22] S. B. Bekiarov, A. Nasiri, A. , and A. Emadi, “A new reduced parts on-line single-phase UPS system,” in Conf. Proc. 29th IEEE Industrial Electronics Soc. Conf., Nov. 2003, vol. 1, pp. 688–693. [23] D. M. Divan, “A new topology for single phase UPS systems,” in Conf. Proc. IEEE Industry Applications Soc. Annu. Meeting, Oct. 1989, vol. 1, pp. 931–936. [24] D. M. Brod and D. W. Novotny, “Current control of VSI-PWM inverters,” IEEE Trans. Ind. Appl., vol. IA-21, no. 4, pp. 562–570, Jun. 1985. [25] N. R. Zargari and G. Joós, “Performance investigation of a current controlled voltage-regulated PWM rectifier in rotating and stationary frames,” IEEE Trans. Ind. Electron., vol. 42, no. 4, pp. 396–401, Aug. 1995. [26] M. D. Manjrekar, P. K. Steimer, and T. A. Lipo, “Hybrid multilevel power conversion system: A competitive solution for high-power applications,” IEEE Trans. Ind. Appl., vol. 36, no. 3, pp. 834–841, May/Jun. 2000. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 4, JULY 2007 Chia-Chou Yeh (S’99) was born in Taipei, Taiwan, in 1978. He received the B.S. and M.S. degrees in electrical engineering from Marquette University, Milwaukee, WI, in 2000 and 2003, respectively, where he is currently pursuing the Ph.D. degree in electrical engineering. From 2003 to 2007, he was with the Innovation Center, Eaton Corporation, Milwaukee, WI, as a Power Electronics Intern, where he was involved in the research, development, and testing of uninterruptible power supply (UPS) systems and motor soft-starters. His current research interests include electric machines and adjustable speed drives, power electronics, modeling and control of electric machines, as well as condition monitoring and fault diagnosis of electric machinery drive systems, and their corresponding fault tolerant and mitigation strategies. Mr. Yeh is a member of Sigma Xi, Eta Kappa Nu, Tau Beta Pi, and Pi Mu Epsilon. Madhav D. Manjrekar (S’96–M’99) received the B.E. degree from the Government College of Engineering, University of Pune, India, the M. Tech. Degree from the Center of Electronic Design and Technology, Indian Institute of Science, India, the M.S. degree from Montana State University, Bozeman, and the Ph.D. degree at the University of Wisconsin, Madison, in 1993, 1995, 1997, and 1999, respectively. From 1999 to 2002, he was with the Technology and Innovation Center of ABB as a Principal Engineer, where he focused on the design, development, and testing of high power electronics for variable speed drives, sag correctors, and other power quality systems. From 2002 to 2005, he was with Eaton Corporation, as a Power Electronics Specialist and then as a Manager, where he was a Lead Technical Consultant for power quality portfolio development. Since January 2006, he has been with Danaher Power Solutions, Richmond, VA, as a Director of Engineering. His research interests are modeling, design, and control of power conversion systems, utility applications of power electronics, adjustable-speed and torque drives, and nonlinear dynamics and controls.