A New Topology for the Design and Control of Uninterruptible Power … · 2014-12-14 · general,...

International Journal of Electronic and Electrical Engineering. ISSN 0974-2174 Volume 5, Number 3 (2012), pp. 201-216 © International Research Publication House http://www.irphouse.com

A New Topology for the Design and Control of Uninterruptible Power Supply by Using Z – Source

Inverter

1T C Srinivasa Rao, 2D Shobha Rani and 3T C Subramanyam 4K Hanumantha Rao and 5Md Asif

1,2,4,5Department of Electrical and Electronics Engineering, Vardhaman College of Engineering, Hyderabad, A.P. India.

3 Department of Electrical and Electronics Engineering, School of Engineering, NNRESGI, Hyderabad, A.P. India.

[email protected], [email protected], [email protected] [email protected], [email protected]

Abstract

This paper presents a topology of uninterruptable power supply (UPS) by using a Z-source inverter, where a symmetrical LC network is employed to couple the main power Circuit of an inverter to a battery bank. With this new topology, The proposed UPS can maintain the desired a c output voltage At the significant voltage drop of the battery bank with high efficiency, low harmonics ,fast response ,and good steady-state performance in comparison with traditional UPS. The simulation and experimental results of a 3-kWUPS with the new topology confirm its validity. Keywords: Z source, UPS, Shoot through

INTRODUCTION: Application of such as major computer instillations, process control in chemical plants, safety monitor, general communication system, hospital intensive care etc where even a temporary power failure can cause a great deal of public inconvenience leading to large economic losses. For such critical loads, it is of paramount importance to provide uninterruptible power supply (UPS) system so as to maintain the continuity of supply in case of power outages. A common case is information loss caused by the utility shutdown in PCs. Nowadays; new company buildings usually have an uninterruptible power system

202 T C Srinivasa Rao et al

(UPS) that feeds the equipment of the whole building. However, in many other cases, each individual user has to connect a personal UPS to the computer in order to avoid an unexpected shutdown. There are many commercial products of this type but, in general, all of them are AC UPS –their output voltage is an AC voltage (sinusoidal, square, trapezoidal, etc.) that substitutes main voltage. It deals with the design of a power supply with internal DC ups that meets all the Advanced Technology extended (ATX) specifications [10] in order to be used as a PC power supply. ATX power supplies have six different output voltages, and all of them should be tightly regulated as shown in table 1. It should be noted that the size specifications are also the same. So that this prototype could fit into the standard chassis of a PC power supply.

Figure 1: Conventional scheme: AC UPS + AC/DC converter

THEORETICAL ANALYSIS TOPOLOGY SELECTION: The objective is to design a 200-W power supply with multiple outputs that can operate either from the AC mains or from a 12-V battery. This voltage value was selected for two main reasons –cost and safety. The power supply should operate from two very different input voltages: the main AC voltage and the 12V battery DC voltage. The option of using a very wide input voltage range converter has not even been considered because of the performance of these converters. Another important issue in this design is the modularity of the system. Although the objective is to comply with the ATX specifications shown in table-1, especially as for as size is concerned, it would be very interesting if we could select the value of the output voltages in order to use the same power supply for different applications. For this reason, the main output of the AC/DC converter is a 12-V output that can drive the total rated power (200W). The other outputs are post regulated from this main one (in our case, the main DC/DC converter is a half-bridge converter) by means of buck converters. Thus, any voltage combination can be obtained by selecting the appropriate post regulators.

A New Topology for the Design and Control 203

Table. No. 1 : ATX voltage tolerance specifications

Rail voltage Tolerance +5V DC +5% to -5% -5V DC +5% to -5% +12 V DC +5% to -5% -12 V DC +5% to -5% +3.3 V DC +5% to -5% +5 V VSB +5% to -5%

Bearing in mind that a PC needs quite low voltage to operate (5 and 3.3v), the buck converter designed for this purpose was a synchronous buck converter in order to improve the efficiency as much as possible. Although the use of synchronous MOSFET is more expensive than the use of conventional Scotty diode, the efficiency improvement reduces the cost of the heat sink needed and enlarges the autonomy of the UPS. Moreover, as the input voltage of the buck converter is not too high (12V), the cost of the control and drive circuit can be further reduced. A special effort was made to design a simple, cost effective, and discrete drive circuit avoiding the use of either pulse transformer or expensive commercial drivers. It should be mentioned that the buck output voltage is trim able from around 9-3V. Two of these post regulators can be incorporated in to the main board and, therefore, the power supply can have up to three different power rails –two of them with a selectable output voltage. It should also be mentioned that two more outputs (-12V and -5V) are also obtained from main converter. In this case, due to their low power ratings, the -12V output is obtained from the main transformer and regulated by means of cross regulation methods coupling both output inductors in the same core. The -5V output is obtained from the -12V output by means of a linear regulator connected in cascade. NEED OF UPS: A sudden loss of power will disrupt most business operations, it is not only total mains failure or ‘block outs’ which can trigger divesting effects. Many electrical loads, for example computer systems, are equally susceptible to power sags, brown-outs, power spikes and surges, noise and radio frequency interference, and supply frequency changes. Such loads are often referred to as ‘critical loads’ , partly because their continuous operation is fundamental to the functioning of the business, and also because they require a more stable and reliable power source than that generally offered by the utility mains supply in order to guarantee their correct function. CRITICAL LOAD APPLICATION: The numbers and types of load falling into the ‘critical’ category are rapidly expanding as an ever increasing range of microprocessor-based equipment enters both the industrial and commercial marketplaces. This is typified by the growth of on-line transaction processing and E-commerce where 24 hour trading demands absolute power quality with zero downtime.


Among typical loads are: Computer –e.g. data processing and control systems. Industrial process equipment- e.g. precision manufacturing. Medical equipment- e.g. life support and monitoring systems. Point of sales (POS) terminals- e.g. retailing environment. On-line business transactions- e.g. internet shopping.

The effects of an inadequate supply to critical load can include:

Cessation of the business process- i.e. a total inability to trade and /or communicate

Expensive hardware failure including component damage- i.e. due to power sags, spikes etc,

Production loss due to incorrect operation of manufacturing process and possible production equipment damage.

POWER PROBLEMS:

Fig.2 Power problems Spikes: Spikes are short duration rapid voltage transitions superimposed on the mains waveform. Electrical noise: Common mode noise is a result of disturbances between the supply lines and earth. Normal mode noise is the result of disturbances between line-to-line and line-to-neutral and can be caused by lightning strikes, load switching, cable faults, and nearby radio frequency equipment etc. Surges: Surges are sustained voltage increases above the normal mains value that last for more than one cycle. They typically appear after a large load is switched off or following load switching at substations. Sags: Sags are drops in the mains supply that can last for several cycles. Sags are usually result of switching on large load.


Harmonics: Harmonics are generally caused by non-linear loads which pull current from main supply in large peaks. Harmonics cause a disproportionate rise in current, resulting in increased temperature which can cause component failure, general equipment overheating etc. Brownouts: Brownouts are identical to sags but have a much longer duration and are generally more serious. They are caused when the mains supply is unable to cope with the present load demand. INVESTIGATION OF THE PROBLEM : A new topology of the UPS is proposed by using a Z-source inverter. With this new topology, the proposed UPS offers the following advantages over the traditional UPSs:

i. the dc/dc booster and the inverter have been combined into one single-stage power conversion.

ii. the distortion of the ac output-voltage waveform is reduced in the absence of dead time in the PWM signals.

iii. the system has achieved fast transient response and good steady state performance by adopting dual-loop control.

TYPES OF UPS: I) Offline System: The Off-line UPS offers the bare bones power protection of basic surge protection and battery backup. Through this type of UPS your equipment is connected directly to incoming utility power with the same voltage transient clamping devices used in a common surge protected plug strip connected across the power line. When the incoming utility voltage falls below a predetermined level the UPS turns on its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS then mechanically switches the connected equipment on to its DC-AC inverter output. The switch over time is stated by most manufacturers as being less than 4 milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it takes the UPS to detect the lost utility voltage. When selecting this type of an UPS, be aware that your computer equipment, as well as most electronic equipment is designed for use in the United States. As such it was designed to operate from a 230 volt, 50 Hertz (Hz), sine wave utility source. Most Off-line UPS products on the market today only provide a sine wave output to your equipment when operating normally from the utility line. When they switch to their internal DC-AC inverter they may only provide a square wave, modified square wave or quasi-sine wave, not a pure sine wave. In many cases your equipment may appear to operate normally on these waveforms, but over time may be damaged by them. If you decide only minimal protection is needed, an off-line UPS offers, it is always best to select an UPS that states it has an inverter with a true sine wave output. You should also be aware that most off-line UPS units will not be capable of accepting additional battery packs for extended battery operation. To keep the cost down and prevent overheating, their inverters are designed to only operate as long as the internal battery capacity allows.


Fig. 3 Topologies of UPS. (a) DC/AC inverter + transformer. (b) DC/DC booster + DC/AC inverter. (c) Z-source inverter The line-interactive UPS: The Line-interactive UPS offers the same bare bones surge protection and battery back-up as the offline, except it has the added feature of minimal voltage regulation while the UPS is operating from the utility source. This UPS design came about due to the off-line UPS’s inability to provide an acceptable output voltage to the connected equipment during “brown-out” conditions. A “brown-out” happens when the utility voltage remains excessively low for a sustained period. Under these conditions the off-line UPS would go to battery operation and if the brown-out was sustained long enough, the UPS battery would become fully discharged, turn the power off to the connected equipment and not be able to be turned back on until the utility voltage returned to normal. To prevent this from happening a voltage regulating transformer was added, hence the term line-interactive was born. This feature really does help as low voltage utility conditions are common. The down side


for this design, most of the units available have to switch to battery momentarily when making transformer voltage adjustments and this can be a bit annoying in a quiet home office on a bad power day.

Fig .4 Off Line UPS

Fig.5 Line-Interactive UPS


Fig .6 on -Line UPS Again when selecting a Line-interactive UPS it is always best to select a model with a true sine wave output. Several manufacturers have models available that will accept extended battery packs to provide additional battery runtime. This type of UPS typically costs more than the off-line type, but is worth the additional cost. III) Online System: The On-line UPS provides the highest level of power protection for the serious home office user. It does typically cost more, but like all electronic equipment today the cost is coming down as the technology advances. The true advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and your sensitive electronic equipment. While the off-line and line-interactive design leaves the equipment connected directly to the utility power with minimal surge protection, the On-line UPS provides an electronic layer of insulation from power quality problems ATX form motherboards became increasingly popular because of their advantages over older AT motherboards. AT-style computer cases had a power button that was directly connected to the system computer power supply (PSU). The general configuration was a double-pole latching mains voltage switch with the four pins connected to wires from a four-core cable. The wires were either soldered to the power button (making it difficult to replace the power supply if it failed) or blade receptacles were used. First the incoming AC utility voltage is passed through surge protected rectifier stage where it is converter to a Direct Current (DC) and is heavily filtered by large


capacitors. This tier removes line noise, high voltage transients, harmonic distortion and all 50/60 Hertz frequency related problems. The capacitors also act as an energy storage reservoir giving the UPS the ability to “ride-through” momentary power interruptions. The battery is also connected to this tier and takes over as the energy source in the event of a utility loss. This makes the transition between utility and battery power seamless, without an interruption. The filtered DC is sent into the next tier, a voltage regulator stage. In the regulator stage the DC voltage is tightly regulated and fed to a second set of storage capacitors. The regulator stage gives the UPS its ability to sustain a constant output even during sustained brown-out or low line conditions. The additional stored energy in the second set of capacitors yields even more ride-through time. The regulated DC voltage is next fed to the Inverter stage where a totally new 50/60 Hertz, true AC sine wave output power is made. This tier gives the UPS a new, clean output with superior voltage and frequency regulation ready for connection to any sensitive equipment. The On-line UPS can give the home office user other benefits like frequency conversion for operating equipment designed for a 50 Hertz utility source, or the reverse. The continuous duty inverter also allows for the connection of large extended battery packs, giving the home office user battery run times in excess of 4 hours. Many On-line UPS models offer a feature called Input Power Factor Correction. COMPARISON OF AC UPS AND DC UPS: In the beginning electricity meant direct current, DC .Soon it was found that alternating current, ac, had some important advantages – case of changing the voltage level by means of transformers, less wear and tear of switches (caused by arcing) and so on. With the advent of computers the loads became less forgiving of interruptions. The immediate solution was to include battery-backed inverters to feed them. That was the birth of the alternating current uninterruptible power supply, the AC UPS. In comparison the direct current uninterruptible power supply, the DC UPS, offers the unsurpassed opportunity of simple parallel redundancy and direct contact between the load and the backup battery. Besides the obvious advantages of vastly increased reliability the DC UPS also excels in energy conservation and economy, simply by being simple, straightforward and avoiding unnecessary conversion steps. The DC UPS is very simple in implementation and operation. The only parameter which requires management and supervision, is the voltage. This concept provides direct connection of the battery to the load, which is a great advantage for reliable service. In comparison, the AC UPS is far more complex and intricate to operate. All of the parameters. SOLUTION OF PROBLEM Z-source inverter: Z-Source inverters have recently been proposed as an alternative power conversion concept as they have both voltage buck and boost capabilities. These inverters use a unique impedance network, coupled between the power source


and converter circuit, to provide both voltage buck and boost properties, which cannot be achieved with conventional voltage-source and current-source inverters. The recently presented z source inverter has additional zero vectors: shoot-through switching states that are forbidden in the traditional V-source inverter. For the traditional V-source inverter, both switches of any phase leg can never be gated on at the same time or a short circuit (shoot through) would occur and destroy the inverter. The new Z-source inverter advantageously utilizes the shoot through states to boost the DC bus voltage by gating on both upper and lower switches of a phase leg. Therefore the Z-source inverter can boost voltage and produce a desired output voltage that is greater than the available dc bus voltage. In addition, the reliability of the inverter is greatly improved because the shoot through can no longer destroys the circuit. Thus it provides a low-cost, reliable, and high efficiency single stage structure for buck and boost power conversion. This paper is to investigate and develop a new inverter topology, the Z-source inverter for FCVs. This section discusses a comprehensive comparison of the Z-source inverter versus the two existing inverter topologies, performed using a 50-kW (max) fuel cell stack as the prime energy source and a 34-kW Solectria AC55 induction motor as the traction drive motor. The comparison results show that the Z-source inverter can increase conversion efficiency by 1% over a wide load range, extend CPSR by 1.55 times, and minimize the switching device power rating (SDPR), a cost indicator, by 15%. Simulation models and results will be reported to verify the comparison.

Fig .7 Equivalent circuit of the Z-Source Inverter. (a) Non shoot-through state. (b) Shoot-through state.


Table.2. Switching states and Vector representations of the Z-Source Inverter

CONTROL METHODS FOR Z SOURCE INVERTER: Simple Control: The simple control uses two straight lines to control the shoot-through states, as shown. When the triangular waveform is greater than the upper envelope, Vp, or lower than the bottom envelope, Vn, the circuit turns into shoot-through state. Otherwise it operates just as traditional carrier-based PWM. This method is very straightforward; however, the resulting voltage stress across the device is relatively high because some traditional zero states are not utilized. Maximum Boost Control: To fully utilize the zero states so as to minimize the voltage stress across the device, maximum boost control turns all traditional zero states into shoot-through state, as shown in Figure.

Fig .8 Sketch map of simple control.


Fig .9 Sketch map of maximum boost control. Indeed, turning all zero states into shoot-through state can minimize the voltage stress; however, doing so also causes a shoot-through duty ratio varying in a line cycle, which causes inductor current ripple. This will require high inductance for low-frequency or variable-frequency applications. Reducing the voltage stress under a desired voltage gain now becomes important to the control of Z source inverter. As analysed above, the voltage gain and the voltage stress , therefore, to minimize the voltage stress for any given voltage gain, we have to minimize B and maximize M, with the restriction of that their product is the desired value. On the other hand, we should maximize B for any given modulation index to achieve the maximum voltage gain. Consequently, we have to make the shoot through duty ratio as large as possible. Maximum Constant Boost Control: The sketch map of maximum constant boost control is shown in Figure. This method achieves maximum boost while keeping the shoot-through duty ratio always constant; thus it results in no line frequency current ripple through the inductors. The sketch map of maximum constant boost control with third harmonic injection is shown in Figure (b). With this method, the inverter can buck and boost the voltage from zero to any desired value smoothly within the limit of the device voltage.


Fig .10. Sketch map of maximum constant boost control.

Fig .11. Z-Source Inverter for the proposed UPS.


When the Z-source inverter is working in nonshoot-through states during time interval T1, the diode D is on, and the H-bridge inverter can be considered as a current source iin. Consequently, the equivalent circuit of the Z-source inverter at nonshoot-through states is shown, and its voltage equations are

When the Z-source inverter is working in shoot-through states during time interval T0, where T0 = Ts − T1, and Ts is the switching period, the diode D is off, and the H-bridge inverter can be considered as a short circuit. As a result, the equivalent circuit of the Z-source inverter at shoot-through states is shown , and its voltage equations are

RESULTS AND CONCLUSION: In this paper, a new topology of the UPS with the Z-source inverter has been presented. Compared with traditional UPSs, the proposed UPS shows the strong regulation capability to maintain the desired ac output voltage at 50% voltage sag of the battery bank with high efficiency, low harmonics, fast response, and good steady-state performance. All these advantages were verified by simulation and experimental results of a 3-kW UPS with the new topology.


Fig.12 Simulation Results : a) Traditional UPS when the battery bank voltage declines by 20%. (b) Proposed UPS when the battery bank voltage declines by 20%. (c) Proposed UPS when the battery bank voltage declines by 50%. REFERENCES :

[1] R. Krishnan and S. Srinivasan, “Topologies for uninterruptible power supplies,” in Proc. IEEE Int. Symp. Ind. Electron., Budapest, Hungary, 1993, pp. 122–127.

[2] M. S. Racine, J. D. Parham, and M. H. Rashid, “An overview of uninterruptible power supplies,” in Proc. 37th Annu. North Amer. Power Symp., Oct. 23–25, 2005, pp. 159–164.

[3] H. Pinheiro and P. Jain, “Comparison of UPS topologies based on high frequency transformers for powering the emerging hybrid fiber–coaxial networks,” in Proc. 21st Int. Telecommun. Energy Conf., Copenhagen, Denmark, 1999, pp. 2–11.

[4] J. M. Guerrero, L. Garcia de Vicuna, J. Matas, M. Castilla, and J. Miret, “Output impedance design of parallel-connected UPS inverters with wireless load-sharing control,” IEEE Trans. Ind. Electron., vol. 52, no. 4, pp. 1126–1135, Aug. 2005.

[5] A. Fernandez, J. Sebastian, M. M. Hernando, J. A. Martin-Ramos, and J. Corral, “Multiple output AC/DC converter with an internal DC UPS,” IEEE Trans. Ind. Electron., vol. 53, no. 1, pp. 296–304, Feb. 2006.

[6] J.M. Guerrero, L. Garcia de Vicuna, and J. Uceda, “Uninterruptible power supply systems provide protection,” IEEE Ind. Electron. Mag., vol. 1, no. 1, pp. 28–38, Spring 2007.


[7] P. K. Jain, J. R. Espinoza, and H. Jin, “Performance of a single-stage UPS system for single-phase trapezoidal-shaped AC-voltage supplies,” IEEE Trans. Power Electron., vol. 13, no. 5, pp. 912–923, Sep. 1998.

[8] C.G.C. Brancol, C. M. T. Cruz, R. P. Torrico-Bascope, F. L. M. Antunes, and L. H. S. C. Barreto, “A transformerless single phase on-line UPS with 110V/220V input output voltage,” in Proc. 21st Annu. IEEE Conf. Expo.Appl. Power Electron., Mar. 19–23, 2006, pp. 348–354.

[9] C.-H. Lai and Y.-Y. Tzou, “DSP-embedded UPS controller for high performance single-phase on-line UPS systems,” in Proc. 28th IEEE Annu. Conf. Ind. Electron. Soc., Nov. 5–8, 2002, vol. 1, pp. 268–273.

[10] F.Z.Peng, “Z-source inverter,” IEEE Trans. Ind. Appl., vol. 39, no. 2,pp. 504–510, Mar./Apr. 2003.

[11] M. Shen, A. Joseph, J. Wang, F. Z. Peng, and D. J. Adams, “Comparison of traditional inverters and Z-source inverter,” in Proc. IEEE 36th Power Electron. Spec. Conf., Sep. 11–14, 2005, pp. 1692–1698.

[12] F. Z. Peng, X. M. Yuan, X. P. Fang, and Z.M. Qian, “Z-source inverter for adjustable speed drives,” IEEE Power Electron Lett., vol. 1, no. 2, pp. 33–35, Jun. 2003.

[13] K. Holland, M. Shen, and F. Z. Peng, “Z-source inverter control for traction drive of fuel cell–battery hybrid vehicles,” in Proc. 40th Annu. Meeting Ind. Appl., Oct. 2–6, 2005, vol. 3, pp. 1651–1656.

[14] A. Kawamura, R. Chuarayapratip, and T. Haneysoshi, “Deadbeat control of PWM inverter with modified pulse patterns for uninterruptible power supply,” IEEE Trans. Ind. Electron., vol. 35, no. 2, pp. 295–300, May 1988.

[15] M. J. Ryan, W. E. Brumsickle, and R. D. Lorenz, “Control topology options for single-phase UPS inverters,” IEEE Trans. Ind. Appl., vol. 33, no. 2, pp. 493–501, Mar./Apr. 1997.

[16] P. Mattavelli, “An improved deadbeat control for UPS using disturbance observers,” IEEE Trans. Ind. Electron., vol. 52, no. 1, pp. 206–212, Feb. 2005.

[17] A. Von Jouanne, P. N. Enjeti, and D. J. Lucas, “DSP control of high power UPS systems feeding nonlinear loads,” IEEE Trans. Ind. Electron., vol. 43, no. 1, pp. 121–125, Feb. 1996.

[18] C. J. Gajanayake, D. M. Vilathgamuwa, and P. C. Loh, “Development of a comprehensive model and a multi loop controller for Z-source inverter DG systems,” IEEE Trans. Ind. Electron., vol. 54, no. 4, pp. 2352–2359, Aug. 2007.

[19] G. Escobar, A. A. Valdez, J. Leyva-Ramos, and P. Mattavelli, “Repetitive based controller for a UPS inverter to compensate unbalance and harmonic distortion,” IEEE Trans. Ind. Electron., vol. 54, no. 1, pp. 504–510, Feb. 2007.

[20] P. C. Loh, D. M. Vilathgamuwa, Y. S. Lai, G. T. Chua, and Y. W. Li, “Pulse-width modulation of Z-source inverters,” in Conf. Rec. 39th IEEE IAS Annu. Meeting, Oct. 3–7, 2004, vol. 1, pp. 148–155.

A New Topology for the Design and Control of Uninterruptible Power … · 2014-12-14 · general,...

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