IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING IEEJ Trans 2010; 5: 539–547 Published online in Wiley Online Library (wileyonlinelibrary.com). DOI:10.1002/tee.20570 Paper Phase Load Balancing in Distribution Power System Using Discrete Passive Compensator Nguyen Xuan Tung ∗a , Non-member Goro Fujita ∗ , Member Kazuhiro Horikoshi ∗∗ , Member This paper describes a new proposal to deal with the unbalanced phase loading phenomenon in power distribution systems. Discrete switched passive shunt compensators such as reactors or capacitor banks are considered as the means to compensate for the imbalance. The discrete switched passive compensator offers advantages in term of installation cost and simplifies the maintenance process. A new algorithm is developed to calculate the size of discrete compensators, and this algorithm incorporates the unbalanced power flow calculation module and the optimal compensator sizing module. In addition, the algorithm is written in MATLAB language and tested on an actual three phase-three wire distribution feeder. Extensive tests have validated the effectiveness of the proposal and shown that this proposal can be a useful tool for any electrical utilities.  2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. Keywords: phase loading imbalance, discrete optimization, passive compensation, distribution system Received 13 March 2009; Revised 16 July 2009 1. Introduction In power system, voltages (and currents) are expected to be sinusoidal and equal in magnitude, with the individual phases being 120 ◦ apart. However, the utilities usually experience imbal- ance in both voltage and current. The nature of the imbalance includes unequal magnitude and phase angle deviation among phases. A major cause of voltage and current imbalances is that loads are not uniformly spread among the three phases and load peaks are noncoincident due to the diversity of load categories. Additional causes of power system imbalances can be asymmet- rical distribution of feeder impedances possibly caused by incom- plete transposition of feeder lines. The influence of imbalance voltage and current on power sys- tem has been well investigated in literature. Annette et al. [1] and Ochoa et al. [2] concluded that unbalanced voltages and currents can result in adverse effects on equipment and on the power sys- tem; for example, a small imbalance in the phase voltages can cause a disproportionately larger imbalance in the phase currents. Under unbalanced conditions, the power system will incur more losses and heating effects. The effect of voltage imbalance can also be severe on equipment such as induction motors, power electronic converters, and adjustable speed drives. Many mitigation techniques have been developed to deal with the imbalance phenomenon in distribution system. Generally, those solutions can be divided into two categories: • Rearrange feeders or redistribute the loads in such a way that the system becomes more balanced [3–5]. However, the utilities usually cannot afford too many load swappings due to a Correspondence to: Nguyen Xuan Tung, Power System Laboratory (G. Fujita Laboratory), Department of Engineering, Shibaura Institute of Tech- nology, 3-7-5, Toyosu, Koto-ku, Tokyo 135-8548, Japan. E-mail: tunghtd@yahoo.com ∗ Shibaura Institute of Technology, 3-7-5, Toyosu, Koto-ku, Tokyo 135-8548, Japan ∗∗ Tohoku Electric Power Co., Inc., 7-2-1, Nakayama, Aoba-ku, Sendai, Miyagi 981-0952, Japan the long-time interruption to customers and the cost of labor to implement those operations. • Install compensators (power quality conditioners) to compen- sate for any imbalance quantities [6–9]. They seem to be the best possible solutions but another concern raised by the utili- ties is the capital cost of these solutions. This paper focuses on the second mitigation technique and will establish an appropriate solution in terms of capital cost. For imbal- ance compensation, an active compensator such as distribution static compensator (DSTATCOM) is widely introduced in the lit- erature. Power electronic solutions are elegant, but they involve higher costs. Cost has been the major factor in limiting the appli- cation of power electronic solution at the power distribution level. Based on that fact, solutions that involve passive compensators such as switched-capacitor or switched-reactor banks seem to be the most cost-effective solution and these will be investigated in this paper. Moreover, a new algorithm is developed to calculate the size of the compensators. This algorithm includes an imbalance power flow calculation module and a discrete optimal compensator siz- ing module. The objective of this algorithm is to find the optimal size of compensators so as to minimize the unbalanced power flow through the main feeder. The algorithm allows the user to spec- ify the type and the maximum number of available taps for each compensator. In addition, a detailed model of the feeder, including mutual coupling effect, the type of loads, and time-varying load patterns, is also considered in the algorithm in order to suggest the compensator’s switching patterns over time. Finally, the algorithm is written in MATLAB language and tested on the data retrieved from an actual three-phase, three-wire distribution feeder to verify its effectiveness. 2. Compensation Principle Figure 1 shows the general unbalanced three-phase load fed from a three-phase, three-wire source. Load and compensator are connected in the delta form; therefore, the zero sequence component will equal to zero.  2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.