Research On Voltage Control In Active Distribution Networks Using A Dynamic Programming Aigorithm

In recent years, more and more attention has been paid to distributed generation (DG) because it is particularly environment-friendly, economy and flexible. Due to the capacity limitation, DG is generally accessed to a distribution grid which comes to an active distribution network. In such a network, because the power can be injected to the system along the distribution line in a distributed manner, the voltage will not be gradually decreased along the feeder. On the contrary, the voltage in the injection point will rise. Under this circumstance, the traditional and simple voltage control method in the distribution network is no longer valid.In some cases, if the distribution network fully accepts the active power of DG, it may cause the system voltage rises to excess the maximum, then we can appropriately reduce the DG’s active output to recover qualified voltage. Also, controlling the reactive power output of DG within a certain range can improve the overall voltage control ability in the distribution network. Many countries also have a rule that the DG should have the ability to regulate the active and reactive power output and encourage the DG actively participate in the voltage control in the grid.The paper studies the power flow calculation method in the radial active distribution networks. Firstly, considering the DG injecting point as PQ or PV node, a forward/backward sweep algorithm is used to accurately calculate the power flow. Secondly, an estimation formula is given about voltage variations caused by the power injected by DG. Then the influence and effect of DG to distribution network is analyzed from three aspects:voltage deviation, voltage fluctuation and DG access location. DG will bring difficulties to the traditional voltage control, but if its output is reasonable controlled, DG will also be a good way to resolve these difficult issues.Increase of the number of switching operation of the under-load tap changer (ULTC) and shunt capacitors (SC) will not only reduce the equipment’s life but also improve the operation and maintenance costs. And due to the control variable are discrete, steady-state voltage fluctuations problems and transient voltage oscillation will occur. To solve the problem, based on load and DG maximum active output forecasts for one day in advance, a new voltage control method using a dynamic programming algorithm is proposed, in which DG’s active and reactive output actively participates in voltage control, together with the ULTC and SC. The objective of the proposed method is to decrease the number of switching device operations, as well as to increase the net yield (net yield is defined as the DG’s active energy minus the loss energy in one day) in the distribution networks, while maintaining the grid voltage within the allowed range. Firstly, the differential evolution algorithm is used to obtain the feasible state which means the best output of DG when the status of ULTC and SC is determined, and then the objective function is transformed to a traditional "maximum distance" issue in dynamic programming. According to the feasible state in all times, the optimal value of the objective function is got using a dynamic programming algorithm. At the same time, the voltage control plan (the tap position of ULTC, the number of SC accessed to the network, the active and reactive output of DG) for one day in advance is obtained.The proposed control method is designed and implemented with MATLAB and the simulation cases demonstrate the effectiveness of the method.