These days, electrical energy has been transformed as one of the human’s basic needs. Almost all of our activities as humans at home and the places we work depend a lot on the continuity, stability, and security of the electrification.

As the most populated area in Indonesia, Java-Madura-Bali consumes more than 80 percents of Indonesia total energy needs. No wonder if the existence of Java-Madura-Bali electrical system is very essential for the economic development in Indonesia. Considering the function of 500kV Java-Bali’s interconnection system as the backbone of Java-Madura-Bali electrical supplying, the related studies are very important to support the operation of the system. In addition, referring to the statistical data showing the electrification ratio in the area that reaches 63.5 % and the load increment of 6% per year, those studies are urgent enough especially in designing the system expansion. One of those important studies is power flow calculation.

Normally, power flow calculation of a power system is done under the assumption that the condition of the system is perfectly balance on its three phases. This kind of calculation would be valid if the transmission lines are well transposed, the power on the load buses are evenly distributed, and the terminal voltage of each generator are maintained balance. This assumption really simplifies the analysis of the system. From the symmetrical components of the phases, we can just use the positive sequence in the calculation and ignore the zero and negative sequences. This will make the system easily modeled as a one phase system.

But if the unbalance exists, there are cases in which the unbalance can’t be completely ignored. Some of the effects of system unbalance are: the overheating in rotor of motor caused by the negative sequence current, and the protection relay maloperations caused by the zero sequence current.

It can bee seen that in designing phase of a system, it is necessary to involve the system unbalance. Unfortunately, the transmission lines of 500kV Java-Bali system aren’t transposed adequately. So even though the power on load buses and the generator outputs are maintained balance, the system modeling as a single phase system is far from enough.

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