ON
Recently, the use of critical loads such as computers, medical equipment, process controllers, and data processing systems has steadily increased. These critical loads are sensitive to voltage variations and other power quality related problems. Uninterruptible Power Supply (UPS) systems that can supply reliable power to critical loads during power quality events are one of the most popular solutions used in dealing with power quality problems [1]. UPS systems are used in parallel configuration frequently to increase total power capacity of the system or to secure a higher reliability at critical loads [2] [3]. During parallel operation, the load sharing control to maintain the current balance is critical. The load sharing is very sensitive to differences between each UPS module such as amplitude/phase difference, line impedance, and output LC filter. Various methods of parallel operation to increase the total capacity rating and reliability have been researched, including power deviation compensation, voltage/frequency droop method and instantaneous modulation control [4] [5] [6]. However, in these methods, the output voltages of all the modules in parallel need to be synchronized exactly in frequency, phase and amplitude to guarantee identical equality of load sharing. Otherwise, the output current may contain reactive circular components, that is, circulating currents. This can result in decrease of system capability, malfunction and damage to the parallel operation system [7]. Thus, parallel operation schemes are difficult to implement and provide
expansion of the system. Another disadvantage is that these kinds of parallel operation methods could not be applied to UPS systems with different ratings. In this paper, a current sharing control strategy is introduced to solve some of the existing problems with conventional parallel operation. The system consists of a master and a slave UPS. The master UPS provides CVCF (constant voltage, constant frequency) output. The slave UPS added in parallel operation tracks and follows the current reference acquired from the load current. If the slave UPS regulates the output voltage simultaneously with the master UPS without applying adequate controls that were mentioned previously, a circulation current will be generated due to the difference in amplitude and phase difference of the voltages of the master and slave UPS systems. However, this problem can be solved, if the slave UPS performs only current sharing control. This will allow parallel operation of UPS systems with different ratings. Values of parameters such as passive filters (L, C) for UPS systems are not identical for different rating. This affects current sharing and it is difficult to achieve precise current sharing between systems that are operated in parallel. To solve this problem a phase control algorithm is proposed If the master UPS fails during operation, the slave UPS can also control the output voltage by changing the operation mode from a current control to a voltage control within the operating limits of the slave UPS. Therefore, a higher reliability can be guaranteed. In this paper, general problems that need to be considered for parallel operation of UPS systems with different ratings are also presented. The solution is discussed in detail. II. GENERAL PROBLEMS OF PARALLEL OPERATION
A Master and Slave Control Strategy for Parallel Operation of Three-Phase UPS Systems with Different Ratings
Woo-Cheol Lee, Taeck-Ki Lee Sang-Hoon Lee, Kyung-Hwan Kim, Dong-Seok Hyun Department of Electrical Engineering Department of Electrical Engineering HanKyong National University HanYang University AnSung, Kyonggi-do, Korea Seoul, Korea woocheol@hnu.hankyong.ac.kr sanghoon@ihanyang.ac.kr Abstract— Parallel operation of UPS system has been used to increase power capacity of the system or to secure reliable supply of power to critical loads. During parallel operation, load sharing control to maintain the current balance is critical for reliable operation, since load sharing is very sensitive to differences in components of each module such as amplitude /phase difference, line impedance, and output LC filters. To solve these problems various control algorithms have been researched. However, these methods cannot be applied to UPS systems with different ratings. For this case, master and slave control algorithms for parallel operation is adequate. If the ratings of UPS systems are different, the value of passive LC filters will be different, and it will affect current sharing. This paper presents the general problems associated with parallel operation of UPS systems, and control strategy for parallel operation with different ratings. The validity of the proposed control strategy is investigated through simulation and experiment with two UPS systems. Keywords-component; Uninterruptible Power Supply (UPS), Parallel Operation, Load-sharing Control In-Young Suh Power&Industrial Systems R&D Center Hyosung Corporation Seoul, Korea iysuh@