Analysis of Balanced and Unbalanced Distribution Systems to Reduce Neutral Currents

A critical problem that comes with distribution networks is unbalanced loading among the phases. Unbalanced loading causes unequal phase currents, which in turn give rise to deviation between the phase voltages, and neutral current in star connected systems. The system may thus become unreliable for the operation of various loads. The thermal limits of conductors could also get exceeded, causing accelerated aging. This may result in voltage fluctuations in any phase, which will hamper the performance of the load. Thus to compensate for it the load might draw in excessive current, this will lead to an unbalance in the system there by increasing the presence of neutral current in the system. A methodology to address unbalance is proposed in this work. Customers are classified into residential and non-residential. In this work, residential consumers are further considered as single-phase consumers having three phase connections, allowing them the freedom to connect the loads in any of the phases. Each residential consumer is modelled to have rephasing capability with momentary interruption in service, which is used for load balancing in the residential buses. Three different steps in varying sequences are proposed to mitigate neutral current. In step 1, balancing is performed to mitigate unbalance present in each bus and the unresolved unbalance of all the buses downstream to it. In step 2, reconfiguration is performed with minimisation of neutral current flow through the first branches. In step 3, renewable energy source based distributed generators (DG) are placed in locations with high unbalance to inject power into heavily loaded phases. These DGs are modelled to be capable of injecting different values of real and reactive power into the phases. These three steps are performed in different sequences, and the best solution is chosen based on the type of system. Finally, the reliability of the system is quantified in the context of failure rate and repair time, and momentary interruptions due to rephasing. The results are demonstrated using both balanced (IEEE 69 bus system) and unbalanced (IEEE 25 bus system) radial distribution systems.