Distribution Grid Planning Considering Smart Grid Technologies

Connection of distributed generation (DG) units, energy storages, and new loads into low voltage (LV) grids leads to a constrained operation of the grids. Consequently, voltage limits violation and overloading of grid assets occur more frequently. To tackle these issues, distribution system operators (DSO) still employ traditional techniques in planning distribution grids to increase grid hosting capacity (HC). This implies a huge investment for the distribution grid operator. Nonetheless, smart grid technologies can be considered as alternative options for the DSO. Two methods, centralized voltage control (CVC) and decentralized voltage control (DVC) are compared technically and economically as an alternative to the traditional approach aimed at increasing grid HC. INTRODUCTION Due to increased installation of DGs in LV grids, DSO increase grid HC by means of traditional grid reinforcement [1]. This paper presents how smart grid technologies can be considered as planning alternatives to traditional approaches for solving voltage problems in the LV grid. This could eventually delay the investment associated with traditional grid planning. In this paper the concepts of CVC and DVC are presented. Both can be considered as alternative approaches to the traditional planning approach. An improved algorithm has been developed by the authors in [2, 3] to coordinate voltage control among smart grid technologies. The DVC strategy will be presented in this paper. Finally, the results of CVC and DVC strategies will be technically and economically discussed and compared with each other as well as with traditional grid planning. VOLTAGE CONTROL STRATEGIES Application of an OLTC transformer in MV/LV substations is suggested in a lot of research for regulating voltage in LV grids. Furthermore, DG units can also support voltage in the LV grid in terms of reactive power support. Hence, both technologies contribute to a flexible operation of distribution grids. Traditionally, MV/LV transformers were operated at off-load tap and DG units were operated at constant power factor of one. DG units can contribute to voltage control by managing their active power ‘curtailment’ and reactive power. According to the German grid code, DG units connected to the grid should provide voltage support [4]. These methods are already implemented in the operation, however, are not yet integrated in distribution grid planning. There are two main strategies for voltage control, centralized voltage control (CVC) and decentralized voltage control (DVC) as shown in Figure 1. Figure 1 Voltage control strategies Figure 1.a represents a CVC strategy. In this strategy there is a bidirectional flow of information between the control center, distributed measurement and control asset over the grid. The state of the grid is measured and transmitted via a communication medium to the control center. In the control center the set points for each of the relevant grid elements are calculated and sent to the control units for execution. In the DVC strategy, there is no or very limited communication over the grid assets as shown in Figure 1.b. The set points for the control assets are estimated and executed based on the local measurements. The algorithm for the CVC strategy considered for the compression in this paper has been developed by the authors in [2, 3]. This paper focuses on the DVC strategy in detail. Decentralized voltage control strategy In this section different applicable methods for DVC are described. Much focus is given to describing different methods with respect to DG units participating in voltage control, mainly curtailment and reactive power management such as cos φ(P) and Q(V). The voltage control by the OLTC transformer in the secondary station MV/LV is based on the common approach. This is to MV