Uncontrolled Reactive Power Flow Due to Local Control of Distributed Generators

The central issue of this paper is the analysis of the reactive power behaviour in the presence of distributed generation and the outline of a method to control and dynamically optimize the reactive power flow. Results show that the local control of the decentralized reactive power produces an uncontrolled reactive power flow in the high voltage grid and changes the static behaviour of the load seen from it. Under these conditions, the coordinated operation of the medium and high voltage grid is essential to maintain efficient and safe operation of the power system and to facilitate further DG integration. The implementation of the Volt/var secondary control in high and medium voltage levels upgraded with the relevant static and dynamic constraints maintains the reactive power flow and the voltage in both voltage levels within acceptable limits and opens the door for the dynamic optimization of the grid. INTRODUCTION Distributed Generation (DG) penetration is increasing continuously in all voltage levels of distribution networks. In many cases, the production capacity dispersed on it has reached a critical mass (i.e. more than 50% of the total production capacity). Under these conditions a notable uncontrolled reactive power flow in high voltage grid have been observed /1/. Many projects have been implemented to study the DG integration however, the effects of the Volt/var interdependencies between high and medium voltage grid were not apparent and consequently have hardly been investigated. There are two reasons why this interdependency was not in focus: firstly, each research project is normally restricted to an individual model region, which means to only one HV/MV intersection point. Figure 1 a) shows a typical model region embedded within the power grid and the corresponding uncontrolled reactive power Q that is injected from the medium voltage grid (MVG) into the high voltage grid (HVG) through the supplying transformer. Secondly, the DG share on the model region was limited. These two factors have limited the uncontrolled reactive power flow in a minimum and as a result, the impact on the HVG were not visible. However, in the case of a large scale DG penetration, it is observed that from each HV/MV intersection point a notable uncontrolled reactive power is injected into the HVG, Figure 2b). Under those conditions, the Volt/var interdependencies between HVG and MVG become apparent and dangerous /1/. This paper shows some of the results from the research project ZUQDE (Central Volt/var Control in presence of decentralized generation) /2/, from another perspective: the interdependencies of Volt-vars. Firstly the global and local effect of the vars injection in MVG on the voltage profile of a feeder is investigated. The effect of the inverters PQ diagram (which connects the PV and wind energy resources to the grid) on the uncontrolled reactive power flow is examined in detail. Secondly, the impact of the local control behavior Q(U), implemented at the PV facilities at the low voltage grid (LVG) on the uncontrolled reactive power and on the load-voltage behavior of the feeder lumped load is investigated carefully. Finally, to resolve the adverse effects of DGs integration, a method to control the reactive power flow through the HV/MV transformers is outlined. VARS FLOW BY MEANS OF MVG The ZUQDE project was performed in Salzburg, Austria, /2-4/. Within the project framework the voltage was controlled automatically and the network operation was being dynamically optimized in real time. Automatic voltage control was realized based on the distribution system state estimator (DSSE) and the Volt/var control (VVC). Both DSSE and VVC were integrated in the Fig. 1. General overview of the power grid: a) uncontrolled reactive power in the case of a typical model region; b) uncontrolled reactive power in the case of a large DG share on the whole power grid. HV VHV