A day ahead electricity storage flexibility prediction for peak shaving.

More and more renewable electricity sources have been integrated in the energy grid in recent years. This is a positive development from an ecological point of view but it also brings new challenges for the electricity grid. One of the problems is the peak load resulting from abundant photovoltaics (PV) or wind power generation. In these peak situations the energy grid is used near to its limits or even overloaded. However, although these bottleneck situations are only temporary (the sun only shines during the day and wind is also only blowing at certain times), a stable electricity grid needs to be dimensioned for such worst case scenarios. These scenarios are occurring in the distribution grid at times with almost no demand and a high renewable power sources. In order to overcome the need for reinforcement for these temporary situations, the distribution grid provider requires a smart way to reduce or shift the energy peaks over time. One of the possible options for this is an electricity storage. The electricity storage can buffer power peaks caused by the renewable power producer. In situations of increasing power flows through e.g. a ransformer, the storage can start charging as soon as a certain threshold is reached. In this way the power peak at the transformer is limited to this threshold. The storage can be discharged when the power flow decreases below the threshold. In this master thesis a method is presented that predicts when and to what extent such a storage is used within the distribution grid for peak shaving. Hereby we limit our focus to renewable generation from PVs. We develop a regression based forecast for the PV generation and the power flow in the grid at the grid transformer for the next day. The used regression forecast method is tailored to forecasts in non-stable weather regions like in Germany or the Netherlands. To increase the accuracy of the forecast a fitting method is added that calculates a separate regression function for specific time intervals in order to adjust to the present situation in the grid and the actual PV generation. We show that it is possible to forecast the state of charge (SoC) in the storage a day ahead quite accurately. As the results show that the storage is not used all the time, an interesting follow up question is to investigate if at certain times a certain amount of storage capacity can be given for use to a third party. For this, it is necessary to know how much capacity has to be used to balance the grid and at which time the storage inverter can not be used to its full potential, because that would endanger the grid. These two constraints are called ”grid requirements”. The term ”grid requirements” represents the capacity constraints of the storage itself and the power constraints of the storage power inverter. Based on the known grid requirements, the unused capacity can be given to a third party. This unused resources are called flexibility. In practice it is important that for the use of flexibility by a third party strict boundaries are predicted and imposed. They have to ensure that the use of the flexibility by a third party does not put the grid in danger. In order to make the communication about flexibilities between the Distribution System Operator (DSO) and third parties easier, a so-called traffic light concept was published by the Germany DSO union BDEW. In this concept a manner of prioritisation of grid situations is given. It introduced three phases and coordinated the use of the flexibility. We incorporate the specification of the boundaries on the flexibility of the use of a storage by a third party in this traffic light concept. To test the developed methods, a specific case of the German DSOWestnetz GmbH is used. This specific situation occurred in the area of Wettringen, Germany, where a temporary reinforcement was necessary in order to reduce the power peaks of PV generators. A regular 10-kV cable could have been used as reinforcement to overcome the voltage problems, but due to other grid reinforcements this cable would have been needed only for five years and after that it would be obsolete. Furthermore, load issues on transformers (30/10 and 10/0.4kV) would not have been reduced by the cable. Westnetz decided to invest in storage for this situation instead and to evaluate the economic and technical benefits of a temporary energy storage instead of introducing an extra 10-kV cable.