Incorporation of on-load tap changer transformers in low-voltage network planning

The introduction of distributed generation in the low-voltage (LV) network can generate bi-directional power flows and thus voltage increases instead of decreases from consumers along the feeder towards the substation. The new generation installed at the consumer premises may induce voltage problems while the loading of the cables is still under nominal values. Conventionally possible resulting voltage violations are solved by reinforcing the network, however smart grid alternatives like voltage control in the LV network can also alleviate the network problems. LV-networks are traditionally designed with medium to low voltage transformers equipped with off-line tap changers. The addition of an on-load tap changer (OLTC) for voltage control can decrease the voltage violations in the network, however this needs to be considered within the optimisation method applied for the planning of the LV-network. In this paper the smart grid alternative of installing an OLTC in this optimisation has been performed. By assessing different OLTC control strategies under conditions with increasing distributed generation over many types of networks, the effectiveness of the OLTC becomes apparent. The OLTC is included in the optimization problem formulation by the introduction of additional voltage constraints and relaxing the constraints in the form of a penalty function. When the introduction of an OLTC is more efficient rather than the conventional strengthening of the network is demonstrated with a case study on the impacts of distributed generation.