Review: Uninterruptible Power Supply (UPS) system

Review: Uninterruptible Power Supply (UPS) system

Renewable and Sustainable Energy Reviews 58 (2016) 1395–1410 Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews jour...

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Renewable and Sustainable Energy Reviews 58 (2016) 1395–1410

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Review: Uninterruptible Power Supply (UPS) system Muhammad Aamir, Kafeel Ahmed Kalwar, Saad Mekhilefn Power Electronics and Renewable Energy Research Laboratory (PEARL), Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 2 August 2014 Received in revised form 7 September 2015 Accepted 18 December 2015

Uninterruptible Power Supplies (UPS) have reached a mature level by providing clean and uninterruptible power to the sensitive loads in all grid conditions. Generally UPS system provides regulated sinusoidal output voltage, with low total harmonics distortion (THD), and high input power factor irrespective of the changes in the grid voltage. This paper provides comprehensive review of UPS topologies, circuit configurations, and different control techniques used in the UPS system. A comparison based on the performance, size, cost, and efficiency of the system is presented. Different hybrid energy source UPS system and new generation UPS system for smart grid and micro-grid has been explained. Finally the paper describes performance evaluation of UPS system and explains different aspects that are to be considered for choosing a suitable UPS system by the user. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Uninterruptible Power Supply (UPS) Energy Storage Control Methods Topologies

Contents 1. 2.

3.

4.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1396 Classification of UPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1396 2.1. Offline UPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1396 2.2. Line Interactive UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1397 2.3. Online UPS System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1397 2.4. Power problems and UPS solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1397 Topology-Based Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1397 3.1. Conventional Transformer based UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1397 3.1.1. Single stage UPS system with trapezoidal AC supply [16] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 3.1.2. Three leg type converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 3.2. High Frequency Transformer Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 3.2.1. A UPS with 110-V/220-V Input Voltage and High-frequency Transformer Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 3.2.2. UPS System with BIFRED Converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1398 3.2.3. Two stage UPS with high power factor correction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 3.3. Transformer-less UPS System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 3.3.1. Four Leg Type Transformer-Less UPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 3.3.2. Non-isolated UPS topology with 110/220 V input–output voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 3.3.3. Z-Source Inverter Based UPS System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 3.3.4. Offline Transformer-less UPS System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1399 3.4. Comparison of transformer based and Transformer Less UPS system [38–42]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1400 Control Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1400 4.1. Single Loop Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1400 4.2. Multi-loop System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1401 4.2.1. Predictive Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1401 4.2.2. Repetitive control scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1401 4.2.3. Iterative Learning Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1402

Corresponding author. Tel.: þ 60 379676851. E-mail address: [email protected] (S. Mekhilef).

http://dx.doi.org/10.1016/j.rser.2015.12.335 1364-0321/& 2016 Elsevier Ltd. All rights reserved.

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4.3.

Non-linear Control Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Slide Mode control: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Adaptive Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Modern Control System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Uninterruptible Power Supplies with hybrid storage system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Fuel Cell/Batteries powered UPS system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Supercapacitor/Batteries powered UPS system [85–87]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Renewable energy Integrated UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Next Generation Uninterruptible Power Supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Selection Considerations of UPS system for specific application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Determining the need of UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Calculating the Power requirement of UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Selecting type of UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4. Select configuration of UPS system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5. Safety of UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6. Availability of UPS system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.7. Is selected UPS system affordable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Performance Evaluation of UPS System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Future Trends. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Uninterruptible power supply (UPS) system provides clean, conditioned, and uninterruptible power to the sensitive loads such as airlines computers, data centres, communication systems, and medicals support systems in hospitals etc. Generally the output of the UPS system must be regulated sinusoidal with low total harmonic distortion (THD), irrespective of the changes in the input voltage and abrupt changes in the load connected to the system [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. Broadly the UPS can be classified as the Static UPS system and Rotary UPS system. The static UPS system uses power electronics converters and inverters to process, store, and deliver power in grid failure, while Rotary UPS uses motors and generators for the same function. Sometime the combination of both static and rotary UPS system is used usually called hybrid UPS System [2,3]. Wide range of UPS systems is available in the market depending upon their ratings. The smaller units of only 300VA are used to provide back up to single computer, but the bigger unit of UPS may provide backup to entire building of several megawatts. To cope with the recent issues of global warming and greenhouse gas emission, the use of renewable energy resources is tremendously increasing. UPS system with photovoltaic power has also been introduced in [4,5] to utilize the solar energy for longer period of time. Commercially all the UPS system must fulfil the following requirements in order to maintain the output voltage [6,7].

1402 1402 1403 1403 1403 1404 1404 1404 1405 1405 1406 1406 1406 1406 1406 1406 1407 1407 1407 1408 1409 1409

This paper describes the classification of UPS system, different line faults, and the recommended UPS system for these faults. Also the state of the art high efficiency, low cost, and small size systems are covered and classified based on the circuit structure. Furthermore a comparison of a transformer based and transformer-less is described based on which a recommendation of a particular topology has been given depending upon the required power ratings.

2. Classification of UPS Depending on the topological configuration, the UPS system is classified as Offline UPS, Line interactive UPS, and Online UPS system [8–12]. 2.1. Offline UPS The offline UPS consists of a battery charger, a static switch, and an inverter as shown in Fig. 1. A filter and a surge suppressor are sometimes used at the output of the UPS to avoid line noise and disturbance before being supplied at the output of the UPS. During normal mode operation, a battery charger will charge the battery bank, and at the same time the load is being fed by the power from main AC line. The inverter is at the standby during this mode. When there is a power failure, the static switch connects the load to the inverter and the power is fed by the battery through the

1. Constant steady state RMS voltage for 2% variation in any parameter like temperature, load current, or battery voltage. 2. Maximum of 10% transient peak voltage deviation is allowed during both loading and unloading of the UPS system. 3. Voltage drop of not more than 5% of the rated voltage cannot be tolerated for more than 2 AC cycles. 4. Inverter output voltage with total Harmonic distortion (THD) of only 4% is allowed for all the load conditions. Fig. 1. Block Diagram of Offline UPS System.

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inverter. The switching time of the static switch is normally less than 10ms, which does not affect the normal computer load. The advantages of the offline UPS are low cost, simple design, and smaller size of the system. But the lack of real isolation from the load and the lack of voltage regulation are the main drawbacks of the offline UPS system. Also the performance of this system during non-linear load is also very poor. Offline UPS are suitable for smaller loads with rating of about 600 VA. 2.2. Line Interactive UPS system Line Interactive UPS consists of a static switch, bidirectional converter/inverter, and a battery bank as shown in Fig. 2. The bidirectional converter/inverter connects the battery bank to the load. During normal mode of operation, the main AC line supplies the power to the load and the bidirectional converter/inverter charges the battery. During the grid failure, the static switch disconnects the load from the main supply and the bidirectional converter/inverter supplies the power to the load. The line interactive UPS has the advantages of low cost, small size, and high efficiency. The only limitation is that it does not provide any voltage regulation during normal mode of operation. Generally the Line interactive UPS system is rated between 0.5 kVA and 5 kVA, and the efficiency of the system is normally greater than 97%, provided the main AC line is clean from any transients and spikes. 2.3. Online UPS System Online UPS consist of a rectifier, an inverter, and a static switch as shown in the Fig. 3. During normal mode of operation, the rectifier charges the batteries as well as maintains the constant DC link voltage. While the inverter converts the DC link voltage to the required AC in order to feed the load. During power failure, the Magnetic Contactor (MC) disconnects the AC line, but the inverter keeps supplying power to the load from the battery bank without any interruption. Thus the inverter keeps on operation in both the modes. The inverter supplies clean, and conditioned power to the

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load irrespective of the harmonics and variations in the grid voltage. The advantages of the online UPS include isolation of the load from the main line and almost negligible switching time. The major drawbacks of the online UPS are low efficiency, low power factor, and high total harmonic distortion (THD). All the commercial units of 5 KVA and above are commonly online UPS system. 2.4. Power problems and UPS solutions The power supplied by the grid is not always very clean and continuous. There may be some major faults in the system which leads to long interruptions and complete black out of the grid. Besides voltage swells and dropouts, voltage sag, harmonic distortion, etc. are other faults which are commonly encountered in the grid. Different UPS system provides protection against the specific faults as shown in the Table 1.

3. Topology-Based Classification UPS system can be classified on the basis of the topologies and circuit configuration. The UPS system may be transformer-based, transformer-less, or high frequency transformer based. These UPS system are developed in the variety of configurations in order to fulfil the exact requirement of the application. 3.1. Conventional Transformer based UPS system Fig. 4 shows the circuit diagram of conventional UPS system [13,14]. It consists of a rectifier, an inverter, line frequency transformers, and a bypass circuit. The rectifier converts the AC line voltage into DC link voltage in order to charge the battery bank and maintain a constant DC link voltage. The inverter converts the DC link voltage into the AC line voltage and provides the regulated sinusoidal voltage to the connected load. Two power frequency transformers are employed, one at the input side to step down the line voltage into low voltage of battery bank and the other at the output to step up the battery bank voltage as well as the operation Table 1 Grid Disturbance and UPS Classification. Sr. No UPS System

Time

Common line Faults

1

Off-line UPS

410 ms

2

Line Interactive UPS Online UPS System

Continuous

Line failure or long interruption, Voltage sags or dips, dynamic overvoltage Under voltage, Over voltage and voltage swell Transients, Harmonic distortion, Noise, frequency variations, Impulses

3 Fig. 2. Block Diagram of Line Interactive UPS system.

Fig. 3. Block Diagram of Online UPS system.

o 4 ms, Continuous and periodic

Fig. 4. Conventional UPS System.

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of bypass switch [15]. Such system has an advantage of providing galvanic isolation from the transients and spikes generated inside the line grid. Also they are more robust in operation and can be designed for high power applications. But since both the transformers are operated at line frequency, so the size and weight of the system is enormously increased and so is the cost of the system. Also most of the switches are connected to the low voltage battery bank. So high current is flowing through these switches; causing extra current stress in these switches. Hence the efficiency of such systems is very low. 3.1.1. Single stage UPS system with trapezoidal AC supply [16] Fig. 5 shows a single stage UPS system which generates a trapezoidal shape output voltage is specifically design for the optical fiber/coax cable hybrid networks. The circuit design of this UPS is almost similar to the conventional UPS system with the only difference of not using the power factor correction (PFC) circuit and smaller DC link capacitor used in the circuit. The trapezoidal shaped output voltage is synchronized with the input AC supply; hence smaller DC link capacitor is used to remove the current harmonics generated by the inverter. Since the line frequency transformer used in the system are more costly, and have high size and weight. Also the power factor of the system is considerable low because of the absence of the power factor correction circuit, thus this UPS system is not suitable for high power applications. 3.1.2. Three leg type converter UPS systems using three leg type converter also gain much popularity due to reduced number of active switches [17–19]. Fig.6 shows the circuit diagram of the UPS system proposed in [17]. In three leg type converter, the first leg and the common leg act as rectifier which also charges the battery bank. The third leg and the common leg act as an inverter. The switches of the common leg are controlled at the line frequency. By using this common leg, the number of switches is reduced, which increases the overall efficiency of the system. Two leakage frequency transformers are used both at the input and output of the converter to reduce the cost of the system. Though the number of switches are reduces but two low frequency transformer increase the size and weight of the

Fig. 5. Circuit diagram of Single Phase UPS system with trapezoidal AC Supply.

Fig. 6. Three Leg type converter proposed in [14].

system. Also the batteries connected to the DC bus are high in number; charging and discharging at the same time. Thus continuous overcharging may reduce the battery life. 3.2. High Frequency Transformer Isolation With the development in the semiconductor industry, fast switches and diodes are now available in the market with nearly ideal characteristics. Now the transformer can be used at high frequency with the advantages of reduced volume, inherent property of galvanic isolation, and improved efficiency of the system. Several UPS topologies with high frequency transformer have been introduced in [20–27]. Such UPS system has smaller size and weight as compare to the conventional UPS systems. But since high number of active switches is employed in such systems to operate the transformer at high frequency, it reduces the overall efficiency and increase the cost of the system. 3.2.1. A UPS with 110-V/220-V Input Voltage and High-frequency Transformer Isolation Fig. 7 shows a flexible UPS topology which can operate over a wide range of input voltage [20]. During normal mode of operation, the chopper converts the grid voltage into DC and delivers high frequency pulses to the primary of the high frequency transformer. The transformer steps down the rectified voltage in order to charge the batteries. During the power failure mode, the battery bank voltage is stepped up using boost converter and is applied to the inverter which can supply regulated output voltage. Although this topology has the advantages of small size and weight because of the high frequency transformer and can also provide galvanic isolation. But high number of active switches and extra power processing stage decrease the efficiency of the system and add complexity to the circuit. 3.2.2. UPS System with BIFRED Converter An improved UPS system using High frequency transformer is proposed in [21] as shown in the Fig. 8. In this topology, boost integrated flyback rectifier/energy storage DC–DC converter (BIFRED) has been used, which maintains the constant DC link voltage in order to feed it to the inverter and conventional

Fig. 7. UPS system proposed in [18].

Fig. 8. UPS system with BIFRED Converter.

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bidirectional converter is used to charge and discharge the battery. The circuit provides excellent power factor correction, and with high frequency transformer the size of the system is reduced considerable. But the battery bank voltage of the circuit will be increased significantly if the system is design for 220 V grid voltage. 3.2.3. Two stage UPS with high power factor correction A two stage UPS as shown in the Fig. 9 is proposed in [23]. The first stage consists of an integrated battery charger which utilizes the flyback converter for charging the batteries, maintaining high power factor, and providing high frequency isolation. The second stage consists of boost inverter which supplies the regulated output voltage. Since the flyback converter may operate in discontinuous conduction mode, so the proposed topology is not suitable for high power applications. 3.3. Transformer-less UPS System Nowadays with the development of advanced microcontrollers, and advancement in the power electronics, transformer-less UPS are getting popularity in the market. These UPS are less costly, highly efficient, and most importantly smaller in size than the transformer-based UPS. But the transformer-less UPS still has also some major limitation which needs to be addressed. This type of UPS is more likely to be effected by the transients and spikes caused by miscellaneous devices connected to the main utility grid [28]. The battery bank in transformer-less UPS is very high to achieve high DC link voltage, which increases the battery cost and lower the reliability of the system [29,30]. 3.3.1. Four Leg Type Transformer-Less UPS Four leg type transformer-less online UPS system has been proposed in [29]. The four leg type converter act as a rectifier, battery charger/discharger, and an inverter as shown in the Fig.10.

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The common leg is switched at a line frequency while the rectifier, the battery charger/discharger, and the inverter are switched at their respective PWM signals. Since a bidirectional converter has been used, that charges the battery during normal mode and discharges the battery during the power failure mode. So the system has been operated without transformer, and the battery bank is reduced to 192 V. 3.3.2. Non-isolated UPS topology with 110/220 V input–output voltage Another non-isolated online topology is proposed in [31] as shown in the Fig.11. This UPS system can be operated at two different voltage levels and can also provide two output of 110 V.The proposed UPS topology consist of a battery charger, three level boost rectifier, and a double half bridge inverter. The double half bridge inverter generates two independent 110 V AC output voltages. An autotransformer is used at the input of the system to enable the operation at 110 V. The DC link voltage in this topology is about 450 V and nine batteries connected in series form the battery bank, which is still very high. 3.3.3. Z-Source Inverter Based UPS System Another transformer-less UPS topology has been proposed in [32–36] which utilizes a Z-source inverter. No dedicated boost converter has been used to step up the battery bank voltage as the Z source inverter combines the two stages of power conversions (DC–DC Step up converter and DC–AC inverter) into a single power conversion stage. Also a dual loop control scheme has been used to increase the transient response time of the system. No dead time in the PWM signal is required to prevent the switches of the same leg turning ON at the same time. Thus the distortion in the AC output voltage is reduced considerable. Thirty batteries connected in series provide the 360 V DC voltage at the input of the DC voltage. So the battery bank is very high, and is only feasible for high power applications.(Fig. 12) 3.3.4. Offline Transformer-less UPS System Transformer-less topologies has been proposed in Offline UPS system as well. The system studied in [37] has been designed with reduced number of components which reduces the cost of the system. During charging mode, the uncontrolled rectifier converts

Fig. 11. Non-isolated UPS system. Fig. 9. UPS system proposed in [23].

Fig. 10. Circuit Diagram of Four Leg type converter.

Fig. 12. Z-Source inverter UPS System.

Smaller 360 V

Medium 108 V

3KVA 4 90% Two Stages

-

Both 110 Vac & 220Vac 220 Vac 0.9 2.6 KVA 86% Auto-transformer

Smaller 0.99

220 Vac 3 KVA 96%

192 V

Low power Applications Smaller 110 Vac 0.99 o500 VA 84%

48 V

Used for smaller loads smaller high o1 KVA Less than 90%

48 V

Low power applications Medium 0.7 2 KV 86%

110 Vac as well as 220 Vac 110 Vac

0.99 3 KVA 87%

96 V

High

110 Vac 0.9

220 Vac

110 V/220 Vac 0.8  0.9

Less than 90% Available with ratings greater than 750KVA 85% 1 KVA

192 V

High

From single computer to large data centres with load in MVA Design for hybrid fiber/coax networks For High power applications, Very High

Considerable high Power Application Medium And High Power Application High Power Application Non-isolated UPS with 110/220 V input – output voltage [31] Z Source Inverter Based UPS System [32]

In single loop control scheme, the output voltage feedback loop is used to provide the well regulated output voltage [43]. The output voltage is compared with the reference signal to generate

two & Power Frequency Transformers Single Stage UPS system with trapezoidal AC Three & a Power Frequency supply [16] Transformer Three Leg Type Converter UPS system [17] three & leakage Transformers A UPS with 110/220-V Input Voltage and Three & a High Frequency High-Frequency Transformer Isolation [20] Transformer An on-line UPS system with electric isolation Three & a High Frequency using BIFRED converter [21] Transformer Two stage UPS with high power factor corTwo & High Frequency rection [23] Transformer Transformer-less Online UPS System [29] Three Stages

4.1. Single Loop Control

UPS topology

The control strategy is the most important part of all UPS systems. Parameters like THD of the output voltage, dynamic response to the transients and spikes; power factor correction, voltage and current regulation etc. are all dependent on the control strategy applied in the UPS system. Nowadays many modern control techniques have been proposed to provide regulated output voltage in all the circumstances. Broadly the control techniques can be classified as single control and multi loop control schemes

Table 2 Comparison of different UPS system Configurations.

4. Control Techniques

Properties

Efficiency

Power Ratings

Nowadays the transformer-based UPS system has been subjugated by the transformer-less UPS system because of its small size, light weight, and high efficiency. These UPS system offers highly compact and cost effective design for low power applications without using any bulky power transformer. Table 2 shows the comparison of different UPS configurations. But which one is the most suitable UPS system according to the required circumstances? The answer is very complex. It requires understanding of the basic topologies and the particular requirement for the specific application. In selecting a UPS system, there is always a trade-off among certain features of the system, and the selectors always have to go for the features which are important for that specific application. Transformer based UPS system isolates the load from the faults generated in the main supply. In fact the transformer itself acts like a barrier and avert all the transients and spikes propagating to the DC bus from the main supply and vice versa. Also the transformer based UPS are more reliable and robust in operation with high Mean Time Before Failure (MTBF). However the transformer based UPS are more expensive than transformer-less UPS system. Without input and output transformer, the cost of the transformer-less UPS system can be reduced to 30% or even more. Also the size and weight can be reduced to 50% in transformer-less UPS system as shown in the Table 2. Moreover the efficiency of the transformer-less UPS is about 10% greater than the transformer-based UPS system of the same specification due to the absence of the power frequency transformer.

System Specification

3.4. Comparison of transformer based and Transformer Less UPS system [38–42]

Power Factor

Battery bank

Size & Weight

the line AC into DC link voltage, while the boost converter performs the PFC (Power Factor Correction) and the buck converter step down the voltage in order to charge the battery. During power failure, the boost converter steps up the battery bank voltage and the inverter converts it into the AC voltage to provide it to the load terminals.(Fig. 13)

Conventional Transformer Based UPS

Fig. 13. Offline Transformer-less UPS system.

12 V  360 V 80 V

Applications

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Number of Power stages

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an error that is compensated by suitable controller. Though this system is simple to design and quiet inexpensive but its performance is poor in complex loading condition such as unbalanced and nonlinear loads. 4.2. Multi-loop System Multi-loop control schemes are more suitable in order to get better performance. They are more robust and flexible in control, even in non-linear and unbalanced system [44,45]. A conventional multi-loop control scheme has been shown in the Fig. 14. In this control scheme, different parameters are used as a feedback to the controllers like filter inductor/capacitor current or output current and voltage. The outer loop uses output voltage as feedback signal; while the inner loop uses inductor or capacitor output filter current as the feedback signal. The feedback signal is compared with the reference signal to generate an error, which is compensated by the suitable compensator to achieve stable output. Similarly the output of the voltage loop is the reference for the current loop. Hence both the voltage and current stability is achieved using multi-loop system. Different high performance controllers have been developed by employing multi-loop feedback control scheme which provide excellent performance [46–51]. Such as dead beat control[47,52–54], Model Predictive control [48,55,56], Iterative learning control [49] etc. 4.2.1. Predictive Control Predictive control technique has emerged as promising control technique for power inverters. Predictive control uses the system model to predict the future behaviour of the controller variables and respond according to predefined optimization criteria. Predictive control concepts are easy to understand and can handle the system with many constraints and non-linearity’s. The predictive control can be classified in Deadbeat control and Model Predictive control. The detail of each controller is as follows. 4.2.1.1. Deadbeat Control. Deadbeat control scheme is one of the most popular schemes for UPS system [47]. In deadbeat control, the reference voltage is calculated during each sampling period using system model parameters, and is applied to follow the reference value in the next sampling instant. It offers fastest transient response because all the closed loop poles are placed near zero. This results in minimum settling time as few sampling steps are required. However the dead beat control is very complex and is highly sensitive to parameter variations, loading uncertainties, and steady state error. Moreover, performance of the deadbeat control also reduces due to unpredicted sources of disturbance, such as dead-times, dc-link voltage fluctuations, and so

Fig. 14. Multi-loop control Scheme.

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on, since there is no inherent integral action in the control structure. Fig. 15 shows a deadbeat control scheme for UPS system. A state observer is used to compute the delay while the load current is estimated using the disturbance observer. Any disturbance in the system is compensated by the state observer. In Deadbeat control the load current at time k is different to the reference current iref. This error is used for calculation the reference voltage Voref, which is applied to the load at time k. Ideally at time k þ 1, the load current will be equal to the reference current. This method gives better performance by reducing control sensitivity to model uncertainties, parameter mismatches, and noise on sensed variables 4.2.1.2. Model Predictive Control [55,57]:. Model Predictive Control (MPC) is considered as one of the important advancement in the process control engineering. Model Predictive control also knows as receding horizon control, provides high performance and stability in the control of UPS system. MPC is very flexible control in which different system constrains e.g current and voltage limitation, switching states, and non-linearity can be included in the optimization of the controller. A cost function is usually formulated considering different variables and weighting factor. A Switching state is selected in order to minimize the cost function and applied in the next switching state. Fig. 16 shows the common model predictive control for the inverter of UPS system. The load current measured at instant K is used as input to the predictive model which derives the value of the current for the next sampling time, for each switching state of the inverter. At each instant K, cost function over a finite horizon of length N is minimized. The cost function can be derived by equation iðxðkÞ; uðkÞÞ ¼ F ðxðK þN ÞÞ þ

Kþ N1 X

LðxðlÞ; uðlÞÞ

ð1Þ

l¼K

where F and L are the weighting functions which predicts the system behaviour e.g difference between the current reference and predicted value. The optimization of the moving horizon is performed at each sampling step i.e. at time K þ 1, the system state x(K þ1) is measured and the horizon is shifted to next step, where another optimization has been performed. 4.2.2. Repetitive control scheme Repetitive control scheme has widely been used for the rejection of periodic disturbance in a dynamic system [46,58,59]. This scheme is based on the multiple feedback loops, with time delay unit which results in eliminating the periodic errors efficiently. But the limitations of this system include slow dynamic response, large memory requirements, and poor performance in nonperiodic disturbance. Repetitive control has been introduced for the control of inverter with non-linear load. The steady state performance of the repetitive control is quite good but the dynamic response is not satisfactory because of long delay time between input and output. Therefore, repetitive control is normally incorporated with other feedback controller with fast dynamic response.   In repetitive controller, a periodic signal generator zN 1 1 has been added in the closed loop system for exact tracking a

Fig. 15. Deadbeat Control for UPS system.

Fig. 16. Model Predictive control for UPS system.

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reference signal. The repetitive controller eliminates all the harmonics below the Nyquist frequency by introducing infinite gain at the harmonic frequency [60]. A repetitive control system is shown in Fig. 17 The feedback control and repetitive control are complementary. The transfer function of repetitive control is given as kg Z  N1 kg Z N 3 Gf ð Z Þ Gf ð Z Þ ¼ N Gg ð z Þ ¼  N 1z Z 1

ð2Þ

where kg is the control gain and Gf ðZ Þ is a low-pass filter. The conventional feedback controller offers fast response and robustness. However the feedback controller has no memory. Hence if there is any imperfection, it will keep repeating in all subsequent cycles. Similarly the repetitive controller stored pervious information in memory, and ensures steady-state zero error tracking by repetitive learning. But the zero error tracking took longer time. Hence the repetitive control scheme together with feedback controller ensures fast dynamic response of feedback controller and the high precision tracking ability of repetitive controller [61].

If (4) is satisfied for all ω, then monotonic decay of the tracking error to zero will take place over successive cycles, and stable operation will be achieved. Table 3 shows the comparison between different multi-loop control schemes. 4.3. Non-linear Control Schemes Non-linear controllers are more robust in operation, show good performance result as compared to linear controllers. However the implementation of this system is very complex. The most common non-linear control system is slide mode control and adaptive control for the UPS inverter control.

4.2.3. Iterative Learning Scheme In Iterative Learning Control (ILS), the control command is adjusted at each iteration, thus converging to zero tracking error. The ILS aims to accomplish this result without the knowledge of the system. The system is examined at each cycle and is adjusted for the next repetition. But the design procedure of the ILS is very complex. ILS can be used to eliminate tracking error caused by the periodic disturbance. The updated rule for ILC is given by

4.3.1. Slide Mode control: For non-linear load, Slide Mode Control (SMC) [62,63] strategy has gained special interest. SMC has been widely implemented in the power inverters because of its effective performance against non-linear system with uncertainties. A major feature of the SMC is its robustness, good dynamic response, stability against nonlinear loading conditions, and easy implementation. But the SMC has the disadvantages of inheriting the chattering phenomena i.e the undesirable oscillation with finite frequency and amplitude which leads to low control accuracy and high heat losses in the system [64]. A SMC has been presented in Fig. 18. SMC changes the dynamics of the system by employing the discontinuous control signal that forces the system to slide along the system normal behaviour [65]. The linear sliding surface function for the UPS inverter can be expressed as (6)

ui þ 1 ðzÞ ¼ ui ðzÞ þ k∅ðzÞei ðzÞ

S ¼ λx1 þ x2 λ 4 0

ð3Þ

Where ui ðzÞ is the Z transform of the command that is given to the system at repetition I, k is the learning gain and ∅ is the designed controller transfer function. While ei is the z-transform of the racking error at repletion i. ei þ 1 ðzÞ ¼ ðð1  k∅ðzÞPðzÞÞei ðzÞ ¼ T f ðzÞei ðzÞ

ð4Þ

where Tf is the transfer function between the two consecutive repetitions. The error component at a particular frequency will decay over successive repetition if   ð5Þ j1  k∅ ejwT PðejwT Þj o 1

ð6Þ

where λ is a real constant and x1 is the voltage error, and its derivative is given by x2 ¼ x_ 1 . For the dynamic behaviour the (6) will be S ¼ λx1 þ x_ 1 ¼ 0

ð7Þ

The objective of the control in (6) is to drive the trajectory of the system from any initial condition x(0) to the sliding surface S (x)¼0. This trajectory is maintained at the sliding surface, and consequently directs the system towards the steady state condition. The slide mode control law for the inverter is given by

Fig. 17. Repetitive control for the UPS system.

Fig. 18. Slide Mode Control scheme.

Table 3 Comparison of Different Multi-loop Control Schemes. Control Scheme

Dead Beat Control [47] Model Predictive Control [55] Repetitive Control [46] Iterative Learning Controller [49] Neural Network Control [50] B-spline Network (BSN) Control [51]

Features Controller Used

Response Time

Performance

Sensor

ADMC401 TMS320C6713 TMS320FS40 TMS320F240/MPC8240 Analog Circuit DS1104

0.5 ms Slow Slow Slow Fast, 7.55 us Fast, 7.78 us

Not good for Non-linear loads Good Excellent Excellent Good Excellent

Output Output Output Output Output Output

Complexity Voltage, Inductive current voltage, Filter current Voltage voltage Voltage Voltage

Complex Simple Complex Complex Complex Simple

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equation 1 S ¼ λðVout  Vref Þ þ ðiC  iref Þ C

ð8Þ

The control law need to be tuned very precisely in order to achieve a trade-off between the tracking precision and robustness to the uncontrolled dynamics. The chattering phenomenon in the SMC is eliminated by using smoothed control law in narrow boundary layer. The smoothed control law applied to the pulse width modulator results in the fixed switching frequency of the inverter. The control scheme shows excellent performance with THD less than 1.7% for non-linear loads. 4.3.2. Adaptive Control Adaptive control is another robust control scheme which automatically adjusts to the structural and environmental uncertainties. It does not need a priori information about the uncertain parameters rather the system characteristics are obtained on-line, while the system is operating. The adaptive control provides high performance with excellent voltage regulation for both unbalance and non-linear loads. Also it provides fast transient behaviour, small steady-state error, and low THD under sudden load change. However the computation complexity of adaptive control is very high. Fig. 19 shows the block diagram of adaptive control law implemented in the inverter of the UPS system. A linear optimal load current observer is designed to accurately estimate load current. The load current observer is asymptotically stable in operation. The load current information is forwarded to adaptive control law. For deriving the adaptive control law, the control input are derived using both the compensated control terms and the feedback control term. vi ¼ uf f þ ufb

ð9Þ

Applying the adaptive control law leads to equation U ff ¼

4 X

mi p i þ v L

ð10Þ

i

where ‘m’ is the adaptive gain incorporate in the compensated control term vi given in Eq. (9). 4.4. Modern Control System Recently many state of the art control scheme has been proposed in [30,66–70]. Any advance control scheme should have the features of less execution time, low cost, high flexibility, and excellent performance with all the loading conditions. According to [67], selecting of a suitable control scheme depends on the optimum trade-off between cost, complexity, and waveform quality required for a particular application. A multi loop SPWM control scheme has been proposed in [66] as shown in the Fig. 20. The outer voltage loop controls the fundamental frequency components and the steady state RMS value of the output voltage using PI compensator. The first inner loop acts as voltage feed forward loop and provide fast transient response. The second inner loop compares the AC output to a reference created by the

Fig. 19. Adaptive control for the UPS Inverter.

Fig. 20. Modern Control Scheme for UPS system.

main loop, and an error signal is generated. This will correct the phase shift and improve the quality of the waveform. In the last, a current loop is added which provides overload protection and current limiting. Thus the outer voltage loop provides voltage regulation while the inner loop maintains THD, with excellent stability of the system. For linear loads, the controller achieve 0.3% THD while for non-linear loads, it achieve 3% THD. This control scheme can easily be implemented with less complexity and high response time of 10ms for any load. Another robust tracking control of UPS Inverter has been proposed in [71]. The proposed controller utilizes state feedback and integral controller of the tracking error. The gain of the controller is designed keeping in view the parameters of uncertainties. Thus the controller shows fast tracking performance in uncertain conditions of the load. Also a one step ahead predictor has been introduced to reduce the time delay caused by computation and space vector modulation. Luenberger type observer is used to make the prediction of the inductor current, capacitor voltage and unmeasured disturbance. The prediction gain is calculated using Kalman filter and robust stat predictor. The proposed controller shows good performance under linear and non-linear loads with very low total harmonics distortion. In [70] a multiple resonant control scheme has been proposed which provides very robust control framework to the UPS systems. In this scheme, output feedback controller has been designed using Linear Matrix Inequality (LMI) constraints. Thus efficient control parameters are derives to achieve a good compromised between transient performance, sinusoidal reference tracking and harmonics rejection. Multiple resonant controller attenuates the periodic disturbance efficiently resulting in low THD of the UPS output voltage. Table 4 shows the comparison of the modern control schemes used in the UPS systems.

5. Uninterruptible Power Supplies with hybrid storage system Uninterruptible power supplies with batteries as storage source provides good performance during grid interruption and blackout by suppling instant backup energy. However batteries cannot provide backup for a very long period of time and have limited charge/discharge cycles. Also batteries contain toxic heavy metals such as cadmium, mercury, and lead which may cause serious environmental problems. Recently other methods of energy storage such as fuel cells, super-capacitor, and their combinations have gained popularity. The power sharing between these energy storage devices is a promising solution for improving system performance due to their dynamic behaviour and long life. Fig. 21 shows options of back-up power and their energy capacity. Many

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Table 4 Comparison of Modern Control Schemes. Ref Controller

[66] [70] SPWM Controller Multiple Resonant Controller

[72] Synchronous Ref. Frame Voltage Control

[65] Fix Switch Frequency Slide Mode Control

[71] Robust Tracking Controller

THD(L) THD(NL) Transient (ms) Complexity

1.1% 3.8% 60 Medium

0.2% 1.68% 1.0 Complex

1.1% 1.7% 0.5 medium

1.3% 5.5% Medium

2.7% 16 Complex

Fig. 21. Options of Backup power system.

reviews have focused on the composition, features, and performance of different energy storage systems [73–77]. In this portion brief review has been presented to highlight the performance of different energy systems in UPS applications. 5.1. Fuel Cell/Batteries powered UPS system A UPS system with hybrid energy source has been presented in the [78–82]. In this system, fuel cell and battery bank is combined as such to ensure that there is sufficient energy available to provide backup to the external load. When there is interruption in the utility grid, the hydrogen gas will be supplied to the pack of fuel cell. Since fuel cell required some time to develop the required voltage hence cannot provide instant backup energy. To overcome this problem rechargeable battery or super-capacitor can be employed to respond quickly to the external load. A block diagram of the hybrid UPS system is illustrated in Fig. 22 [24]. The Fuel cell is the main source of energy. Batteries and supercapacitor act as secondary source of energy. Fuel cell is linked to DC-Bus through the DC–DC converter while all other sources are linked to the common DC-Bus through bidirectional converter. The bidirectional converter acts as battery charger during grid mode and discharger during backup mode of operation. The DC-Bus supply energy to the connect load through inverter. The circuit diagram of the hybrid energy storage UPS system is shown in Fig. 23. A conventional boost converter is used to step up the fuel cell voltage to DC-link voltage. Bidirectional converter charges the battery/supercapacitor during grid mode (buck operation) and discharges the battery/supercapacitor during backup mode (boost mode), in order to provide stable supply to the DC-Bus. Different bidirectional converters have been presented in literature with both isolated and non-isolated topologies [83,84]. Bidirectional converter is selected depending on the conversion ratio, efficiency, and reliability of the system. H-bridge inverter is used to deliver regulated output AC voltage to the load. A fuel cell powered UPS system provides stable operation during utility interruption. Fuel cell is excellent replacement to the conventional UPS energy sources in near future. Supercapacitor module is incorporated to overcome transients such as instantaneous power fluctuations, slow dynamics of the fuel pre-processor and overload conditions. However the fuel cell technology still faces drawbacks such as high cost, slow response time, and sensitivity to low frequency ripples.

Fig. 22. Block Diagram of hybrid energy storage UPS system.

5.2. Supercapacitor/Batteries powered UPS system [85–87] Normally batteries of UPS system provide backup power for 5– 15 min until the generator starts and provides backup to the load. Supercapacitor provides backup for 5–15 s. Hence in application where the long backup is not required, supercapacitor can replace the battery storage system. Nowadays hybrid system of battery for higher energy and supercapacitor for higher power are combined to provide more reliable and high power. Table 5 shows the comparison of supercapacitor with lead-acid battery. Introducing supercapacitor in parallel reduces the stress on the battery. Supercapacitor supply power during transient demand of power while batteries supplies during smooth demand of power. However the cost of supercapacitor are still very high and further research and investigation is required to balance the cost and performance of the supercapacitor based UPS system. 5.3. Renewable energy Integrated UPS system Since global warming and greenhouse effect has reached to its threatening level, renewable energy is the only option for future energy requirement. Photovoltaic (PV) and wind energy are the most promising solution to supply energy in isolated areas. Uninterruptible power supplies with renewable energy resources connected with the utility grid provide more reliable and quality power to the connected load [88–90]. UPS with PV system is shown in the Fig. 24. The PV module is connected to the system through the DC–DC converter while the batteries and supercapacitor are connected to the DC-Bus using bidirectional

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Fig. 23. Circuit Diagram of Hybrid Storage UPS system. Table 5 Comparison of Lead-Acid Battery and Supercapacitor. Properties

Lead acid battery

Supercapacitor

Specific energy density Specific power density Cycle life Charger/discharger efficiency Fast Charge time Discharge time

10–100 Wh/kg o1000 W/Kg 1000 70–85% 1–5 H 0.3–3 h

1–10 Wh/kg o 10,000 W/Kg 85–98% 0.3–30 s 0.3–30 s 0.3–30 s

Fig. 25. Block Diagram of Intelligent UPS system.

Fig. 24. UPS system with PV energy Source.

converter. The inverter supply AC voltage to the connected load. When there is a surplus energy available, it is stored in the connected battery bank and supercapacitor. Under the conditions, when load demand exceeds the generation or during night time, the stored energy is utilized to fulfil the requirements of the load. Supercapacitor is added in the system to provide fast dynamic regulation of the power. UPS with PV system helps in peak shearing, smoothing out load fluctuations, and making up for intermittent variation in renewable energy sources so as to make an efficient energy management in integrated systems [91].

accommodation of DG sources. Fig. 25 shows the block diagram of the next generation UPS system. The UPS system has high frequency converter which allows the parallel connection of the batteries with other Distributive generation (DG) system to the smart grid. Hence the proposed UPS system in addition to traditional operation can also realize the cyclic use of electrical power between the power grid and storage system. Due to its modular structure, it can be applied to motor drive, auxiliary power supplies for hybrid electrical vehicles and DG system. Line interactive UPS systems proposed in [98,99] are designed for microgrid. The proposed system helped in improving the reliability, economy, and efficiency of the microgrid. Power can be exported to the grid when the tariffs are advantageous. Hence the UPS system can share power with in the microgrids in parallel with other DG Units. Multiple energy sources, multiple storages, and a highly reliable power conversion system work together to guarantee the uninterruptible power supply. But the idea of intelligent UPS system still needs extensive research in order to realize the concept in smart grid and micro grid.

6. Next Generation Uninterruptible Power Supply In recent years, the concept of smart grid is getting famous and is considered as the next generation power grid [92–95]. Electricity generation using sustainable energy is environmental friendly and can be added to the smart grid. The distributive generating system provides standby power during grid interruption and load sharing during peak hours, thus it helps in cost reduction and reliable power delivery. In fact the concept of distributive generation system falls into the category of the UPS system. An intelligent UPS system has been presented in [96,97] for the smart grid which is energy saving, reliable and flexile for

7. Selection Considerations of UPS system for specific application With so many choices of the UPS system available to choose from, which is the most suitable for your circumstances? Selecting a particular type and configuration of UPS depends upon the following factors. (1) Power requirement, (2) Power factor, (3) Cost, (4) Size and Weight, (5) Grid environment, (6) Reliability, (7) Protection, (8) Required level of Power quality, and (9) Size of the battery bank. The Process of selecting a UPS system consists of seven steps. (1) Determining the need of UPS system, (2) Calculating the Power

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requirement of UPS system, (3) Selecting type of UPS system, (4) Select configuration of UPS system, (5) Safety of UPS system, (6) Availability of UPS system, (7) Is selected UPS system affordable. Fig. 26 shows the flow chart for selection of a UPS system for particular application. 7.1. Determining the need of UPS system The selection of UPS system is strongly dependent on the application for which the system is chosen. Applications such as hospital life support and medical equipment, military equipment’s, and communication devices cannot tolerate any power interruption even for very short period of time. UPS system required for such application should provide backup until the utility grid power is restored. For data centres, the backup may require for only few moments until the devices are shutdown. Surveying the history of commercial outages by power companies can better provide statistics about selection of UPS system.

7.4. Select configuration of UPS system The level of protection and the power requirement of the load determine the type and configuration of the UPS system. Transformer-based UPS are more suitable for high power application. They are more suitable to provide protection even in the polluted grid environment to more sensitive equipment like medical equipment’s and data centres because of the galvanic isolation. While the transformer-less UPS systems are cheaper, with smaller size, and are suitable for low power applications. They are more suitable for circumstances, where the grid supply is less polluted. Similarly the complex control system also escalates the cost of the UPS system. So the choice of the UPS system is determined by the balance between the performance and cost, taking into consideration the acceptable level of other factors which best suits the circumstances. Table 7 correlates the properties of UPS system with the different topologies of UPS system. 7.5. Safety of UPS system

7.2. Calculating the Power requirement of UPS system Specification such as load kVA, power factor, inrush current, load voltage, number of phases and frequency, and backup time is required to determine the size of the UPS system. Load kVA can be calculated as (V  I)/1000 for single phase and (1.73  V  I)/1000 for three phase system. The load power factor should be in the range of 0.7–1 depending upon the load. Battery sizing depends on the size of the load and duration of backup requirement until the critical load is safely shutdown. Ampere-hour (AH) capacities decrease as the rate of discharge increases. For varying load, the summation of the ampere-hour of each load gives the approximate size of the battery.

Safety should be given up most importance before selecting any UPS system. For safety it is preferred to follow the codes and standards adopted by government and commercial agencies. Battery bank needs special care and safety for its sound operation [100]. 7.6. Availability of UPS system Since UPS system provides back up to critical load without any interruption [101]. The availability and reliability of the system is very crucial. The availability of the UPS system is defined by equation MTBF MTBF þMTTR

7.3. Selecting type of UPS system

AUPS ¼

Selection of UPS system depends upon the power quality, protection, efficiency, volumetric size, and weight of the system. The details discussion of different type of UPS system has been done in Section 2. Table 6 summarizes the features of different type of UPS system.

where AUPS is the availability of the UPS system, MTBF is the Mean Time Before Failure, and MTTR is the Mean Time To Repair. Reliability of the UPS system can be improved by introducing the redundant units in parallel where each one is capable of sharing the connected load [102,103].

ð11Þ

Fig. 26. Flow diagram for selecting the UPS system.

Table 6 Comparison of Different Type UPS system.

Offline UPS System Line Interactive UPS system Online UPS system

Practical Power Range

Voltage Conditioning

Cost per VA

Efficiency

Inverter always operating

0–0.5 0.5–5 5–5000

Low Design implementation High

low Medium Medium

Very High Very high Medium

No Design Dependent Yes

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Table 7 Selection of UPS System. Recommended Type of UPS Topology

Power Level (KVA)

Grid Condition

Load Condition

Capital Cost

Weight& Volume

Power Quality

Efficiency

Transformer Based High Frequency Transformer-less Transformer Based High Frequency Transformer-less Transformer Based High Frequency

1–10

Polluted Less Polluted Clean Polluted Less Polluted Clean Polluted Clean

Variable

High Medium Low High Medium Low Very high High

High Medium Small High Medium Small High Medium

Average Good Excellent Average Good Excellent Average Average

Low High High Medium High High Low Medium

10–50

50–200

Almost Fix Variable Variable Fix Variable Fix

Table 8 Typical 3-Ø UPS System Specification by ANSI/IEEE 446–1987 [105]. Input (Rectifier/charger) Voltage Power factor Frequency Harmonic Contents in Current Start-up current Limiting

208 V or 480 V,7 l0%, 3-phase Minimum 0.8 at rated load 50 or 60 Hz, t5% 10% (5% preferred)

Maximum 25% of full-load current (energizing rectifier transformer with inverter at no load) Steady State Current limit Adjustable, with two standard settings:1) For utility power, 125% rated load, 2) For emergency power, 100% rated load plus 5 kVA Output (Inverter) Voltage 208 V or 480 V, 3-phase, 3- or 4-wire Regulation 1) 72% for balanced load, 2) 7 3% for 20% unbalanced load (100% or 80%) Line Drop Compensation 0 to 5%, adjustable Transient Response 1) 75% for loss or return of ac input power, 2) 7 8% for 50% load step, 3) 7 10% for bypass or return from bypass Harmonic content of 4% total, 3% any single harmonic voltage Frequency 50 or 60 Hz Current capability 1) Overload 125% for 10 m and 150% for 10 s Fault clearing 150% to 300% for 10 cycles, maximum limited for self-protection DC Link (Battery) Battery type Recharge time Energy storage capacity

Lead-acid or nickel-cadmium (NICAD) 10 times discharge time Sized to requirement (normally 15 min)

7.7. Is selected UPS system affordable The cost of UPS system varies depending upon the type, configuration, protection, backup time, and extra additional features of the system. Mostly sophisticated system having automatic monitoring, fast switching, and control functions require additional components, adding to the complexity and cost of the system. Cost analysis can only be done when the first six steps are completed.

8. Performance Evaluation of UPS System Several features are essential to analyse the performance of the UPS system. These features include input power factor, THD of the output voltage, transient response time and transfer time from utility grid mode to backup mode and vice versa. Hence the performance indicator of the UPS system is proposed to be according to the specified standards. Table 8 shows the specifications of a 3Ø UPS system define by IEEE standard ANSI/IEEE 446–1987. Experimental results of a 2 kVA laboratory prototype of online UPS system have been presented to demonstrate clearly the performance of UPS system. The input voltage and current of the UPS

Fig. 27. Input Voltage and Current waveform.

system is shown in the Fig. 27. The power factor of the input current is close to unity with minimum THD. According to the IEEE standard ANSI/IEEE 446–1987, minimum power factor is 0.8 at the rated load and harmonics content less than 5% is preferred for the input rectifier of the UPS system. The output voltage and current of the inverter of UPS system should be well regulated, with THD less than 5% for both nonlinear and unbalanced load. In addition, the inverter should exhibit excellent response against sudden change in the load. Fig. 28 shows output voltage and current of the inverter of UPS system where the THD is less than 3% for both the linear and non-linear load well below according to the standard (IEEE standard 4% total & 3% for each harmonic). Fig. 29 shows the step response of the inverter for change of load from 0 to 100% and vice versa. Fig. 30 shows the dynamic deviation of the output voltage for the addition and removal of the step linear load. The dynamic behaviour of the controller should not exceed the classification 1 of IEC62040-3 standard The transfer time from grid mode to backup powered mode is very important to evaluate the performance of UPS system. Online UPS system has negligible transfer time, while line-interactive and offline UPS system inherits some transfer time during transition switching of UPS operating modes. Fig. 31 and Fig. 32 show the experimental waveform of the step change from grid mode to backup mode and vice versa.

9. Future Trends With the development in the advanced microcontrollers and fast switching devices, ever most capable UPS systems have been proposed with high performance, greater efficiency, and more importantly at lower costs. Transformer-based UPS systems have been developed to a mature level and are now available even in megawatt range, utilized for varying applications. Besides, a considerable development has been made in transformer-less UPS

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Fig. 28. Output Voltage and Current for linear load and non-linear load.

Fig. 29. Experimental waveform of step change from 0% to 100% and 100% to 0.

systems. New trends are heading towards the development of efficient control techniques to ensure fast transient and dynamic response, low THD, good voltage regulation, and stability against load variations. Further improvement in the UPS technology in terms of replacing the conventional storage with fuel cells will be a real boost for UPS in low power applications [5,104]. Fuel cells have high specific energy, high reliability, and are environment friendly as compared to conventional storage systems. UPS systems that use the fuel cell in combination with the super-capacitor are also not that extensively investigated. Recognizing the advantages of the hybrid system, we can expect more advanced UPS system, with added functionality and better performance.

Fig. 30. Output Dynamic performance of the UPS Inverter.

Fig. 31. Transition from Normal to Battery Powered Mode. Input Voltage Vin and Current Iin, Output Voltage Vout and Current Iout.

10. Conclusion In this paper, a review of UPS systems has been presented to explain the various configurations, control strategies, and comparisons of important UPS topologies. A topological classification of the UPS system has been discussed with their performance, efficiency, advantages, and disadvantages. Comparative analysis of different systems and their control schemes have been presented to provide useful information, which helps in easy selection of control scheme for a particular application. Model predictive control shows excellent performance for the control of the inverter. Non-linear control schemes can also be adopted for non-linear loads though their implementation is complex. Hybrid energy sources UPS systems and their application in smart grid bring new direction for research and development in this field. Power sharing between different energy storage devices adds dynamic stability and reliability to the performance of UPS system. Depending upon the grid environment, power rating, volumetric size, and backup time a suitable topology can be selected. Performance evaluation

M. Aamir et al. / Renewable and Sustainable Energy Reviews 58 (2016) 1395–1410

Fig. 32. Transition from Battery power mode to Normal mode, Input Voltage Vin and Current Iin, Output Voltage Vout and Current Iout.

of the UPS system has been presented and important features for selection of UPS system have been explained such as input power factor, output voltage, step response, and transient time for change of modes (Grid and Backup). This review will be very useful reference for the researchers, designers, users, and manufacturers working on the UPS system.

Acknowledgment The authors would like to acknowledge the financial support from University of Malaya. This research was carried under the High Impact Research Grant (UM.C/HIR/MOHE/ENG/24)scheme and Bright Spark Unit, University of Malaya.

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