Abstract:The growing importance of renewable generation connected to distribution grids requires an increased coordination between transmission system operators (TSOs) and distribution system operators (DSOs) for reactive power management. This work proposes a practical and effective interaction method based on sequential optimizations to evaluate the reactive flexibility potential of distribution networks and to dispatch them along with traditional synchronous generators, keeping to a minimum the information exchange. A modular optimal power flow (OPF) tool featuring multi-objective optimization is developed for this purpose. The proposed method is evaluated for a model of a real German 110 kV grid with 1.6 GW of installed wind power capacity and a reduced order model of the surrounding transmission system. Simulations show the benefit of involving wind farms in reactive power support reducing losses both at distribution and transmission level. Different types of setpoints are investigated, showing the feasibility for the DSO to fulfill also individual voltage and reactive power targets over multiple connection points. Finally, some suggestions are presented to achieve a fair coordination, combining both TSO and DSO requirements.

This work presents an optimal reactive power management strategy for the operation of a transmission connected distribution grid with high share of wind power. Main control objective is minimizing reactive power exchange with the overlaying transmission grid. For this purpose, a mixed integer non-linear optimal power flow (MINLP-OPF) problem is formulated utilizing reactive power capabilities of wind farms and transformer tap-changer positions whilst respecting voltage limitations. Loss minimization and flat voltage profile are possible secondary sequential optimization objectives. The proposed control is evaluated for a real German 110-kV distribution grid with 1.6 GW installed wind power and yearly time series. Throughout a year, reactive power exchange with the transmission grid can be reduced by 96.8% while minimizing the increase in active power losses to 11.1%. Choosing voltage profile as secondary objective, reactive power exchange is reduced by 96.5% while quadratic deviation from nominal voltage is reduced by 30.8%

In this work a multi-objective model predictive control for reactive power management in transmission connected distribution grids with high share of wind power is presented. The proposed control utilizes reactive power capabilities of wind farms and tap-changer positions in order to improve distribution grid operation. Control signals namely tap-changer positions and reactive power set-points are smoothed over the forecast horizon. Further possible optimization objectives are power loss reduction, voltage profile smoothing and complying with reactive power exchange limits with the transmission grid. A mixed-integer non-linear optimal power flow problem (MINLP-OPF) is formulated incorporating grid operation limits. The performance is evaluated on a real German 110-kV distribution grid with 1.6 GW wind power for one year. With the proposed control, reactive power exchange within allowable limits is increased from 58.3% to 94.5%, compared to a reference operation where on ly tap-changer positions are utilized for loss reduction with a single time-step optimization

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The article at hand gives an overview of a newly developed method and tool to flexibly and efficiently remediate different kinds of congestion in the power grid. The presented approach consists of three main steps: Building the network model, identifying the technical factors influencing the congestion, formulating and solving an optimization problem to remediate the congestion by changing the active power set points of power plants. Within the optimization approaches for technically and economically optimized redispatch are presented. The paper focuses on the development of a solution considering both, technical and economic, objectives to find an optimized solution. The problem formulation is explained, implemented and finally tested in a study case using the well-known IEEE 39 bus system. Results expose the special characteristics of the different obtained optimizations. Especially the implemented normalization approach combining technical and economic aspects shows promising results with low costs and a low amount of redispatched power as well

The so-called short-term voltage stability is typical of nonlinear problems arising in dynamic performance of electric power systems. This paper reports a simulation result on load margin for short-term voltage stability of the IEEE 9-bus system connected to a multiterminal DC system based on voltage-source converter (VSC). The simulation shows that the introduction of AC voltage control in VSC increases the load margin under a step change in load on the AC system, thereby showing a clear benefit of the AC voltage control for enhancing the short-term voltage stability without harnessing the AC system.