Efficient Integration of Distributed Generation in Electricity Distribution Networks - Voltage Control and Network Design

Sammanfattning: Distributed generation (DG), i.e. generation connected to the low and medium voltage distribution network (DN), has been increasing a lot during recent years. Thus the traditional assumption of a unidirectional power flow and a voltage decrease along the distribution feeders is no longer valid in all operation conditions. Voltage control in these networks is often limited to the on-load tap changer at the high voltage/medium voltage substation. Thus keeping the voltage at the customer connection point, which is an important quality criterion for electricity supply, within the limits may become a challenge. Since most of the available voltage band is assigned to the voltage decrease caused by the load, only a small part is available for a voltage rise from DG power injection. To overcome this limitation, traditionally the network has to be reinforced, which is always a solution but quite expensive. Coordinated voltage control is introduced as an alternative to avoid or postpone network reinforcement. The proposed algorithm receives actual voltage measurements from electricity meters at the customer connection points. The voltage setpoint at the substation and the reactive and active power output of the DG units are then adjusted to keep the voltage within the limits. Thereby the voltage band is used more efficiently and as a last option, the active power output from the DG units may temporarily be limited and some energy spilled. The voltage control scheme has been verified by power flow simulations of an existing DN in Sweden using real time series for consumption, photovoltaics and wind generation. It turned out that the need for active power curtailment is low even for large DG penetration if applying coordinated voltage control. Next, a passive DN has been turned into an active DN by introducing coordinated voltage control in a field test. The main objective has been to test the effect of asynchronous measurements from electricity meters and DG units and the impact from the communication. Control with asynchronous measurement turned out to be possible and curtailment has been reduced considerably. As coordinated voltage control uses active power curtailment as a last option to keep the voltage within the limits, it is, especially for the DG developer, important to estimate, to what extent curtailment will be utilised. Based on this data DG developers have to decide, if they would prefer a more expensive connection, which is able to always transfer the maximum DG output, i.e. a firm connection, or if they prefer to accept some temporary restrictions, if it is at a lower cost and faster available. Power flow simulations could be used to determine the expected curtailment. They are exact but they require a lot of input data and are time consuming, especially for calculations over large time series. Therefore a 5-Step-Method, which is fast, simple-to-apply and needs only a reduced set of input data, has been developed. The 5-Step-Method can be applied to calculate the expected curtailment for a DG unit with a predefined nominal output at a given location. However, the method could also be applied to determine the maximum nominal DG output at a given location, if a predefined amount of curtailment can be accepted. To verify the 5-Step-Method, it is applied on DG connections in a generic test system. The obtained results are quite close to the ones from power flow calculations for the considered scenarios. The results for the expected curtailment calculated by the 5-Step-Method are however not conservative compared to power flow calculations, i.e. showing a larger amount of curtailment, for all scenarios. Finally the necessary steps for implementing coordinated voltage control and non firm DG connections are summarized both for distribution network operators and DG developers.