Barry Rawn

Coordinated Active Power-Dependent Voltage Regulation in Distribution Grids With PV Systems

High penetrations of photovoltaic (PV) systems in distribution grids have brought about new challenges such as reverse power flow and voltage rise. One of the proposed remedies for voltage rise is reactive power contribution by PV systems. Recent German Grid Codes (GGC) introduce an active power dependent (APD) standard characteristic curve, Q(P), for inverter-coupled distributed generators. This study utilizes the voltage sensitivity matrix and quasi-static analysis in order to locally and systematically develop a coordinated Q(P) characteristic for each PV system along a feeder. The main aim of this paper is to evaluate the technical performance of different aspects of proposed Q(P) characteristics. In fact, the proposed method is a systematic approach to set parameters in the GGC Q(P) characteristic. In the proposed APD method the reactive power is determined based on the local feed-in active power of each PV system. However, the local voltage is also indirectly taken into account. Therefore, this method regulates the voltage in order to keep it under the upper steady-state voltage limit. Moreover, several variants of the proposed method are considered and implemented in a simple grid and a complex utility grid. The results demonstrate the voltage-regulation advantages of the proposed method in contrast to the GGC standard characteristic.

A Market-Based Transmission Planning for HVDC Grid—Case Study of the North Sea

Summary form only given. There is significant interest in building HVDC transmission to carry out transnational power exchange and deliver cheaper electricity from renewable energy sources which are located far from the load centers. This paper presents a market based approach to solve a long-term TEP for meshed VSC-HVDC grids that connect regional markets. This is in general a nonlinear, non-convex large-scale optimization problem with high computational burden, partly due to the many combinations of wind and load that become possible. We developed a two-step iterative algorithm that first selects a subset of operating hours using a clustering technique, and then seeks to maximize the social welfare of all regions and minimize the investment capital of transmission infrastructure subject to technical and economic constraints. The outcome of the optimization is an optimal grid design with a topology and transmission capacities that results in congestion revenue paying off investment by the end the projects economic lifetime. Approximations are made to allow an analytical solution to the problem and demonstrate that an HVDC pricing mechanism can be consistent with an AC counterpart. The model is used to investigate development of the offshore grid in the North Sea. Simulation results are interpreted in economic terms and show the effectiveness of our proposed two-step approach.

Technology and Topology Optimization for Multizonal Transmission Systems

This paper presents a method to optimize equipment investments in multizonal transmission systems, considering spatial properties of the areas of focus. Together with a probabilistic technique for assessing nodal injection capability, the method in the paper completes the methodology of a long term transmission system planning tool. Transmission topology, line route and technology are optimized through iterative application of linear integer programming and Dijkstras shortest path algorithm. By optimizing the transmission route, the spatial properties of the area of focus are taken into account, which in turn can significantly influence the installation costs of transmission equipment. The optimization method considers both AC and DC technology and takes the N-1 security criterion into account.

Trading Energy Yield for Frequency Regulation: Optimal Control of Kinetic Energy in Wind Farms

The burden on conventional units to regulate the system frequency increases if they are replaced due to wind farms. This paper explores up to which time scales the rotating kinetic energy in wind turbines can smooth frequency variations and assist with the regulation task. To this end, a comparison is made between a standard wind turbine controller and optimal control of wind turbines, respectively derived from causal time-domain simulations and an optimization algorithm that allows predicting. The latter algorithm is used to give a benchmark for the smoothing potential, shown by plotting the Pareto efficiency of the normalized standard deviation of frequency variability versus a normalized measure of the energy yield. Results indicate that smoothing comes with an energy loss that is determined by the energy content of power imbalances. It is shown that a wind share of 20%, within the instantaneous generation mix, can absorb frequency variations on timescales up to 100 sec while the energy loss is limited to only 2%. A higher share of wind power aggravates frequency variability. Nevertheless, in such circumstances the potential of rotating kinetic energy in wind farms increases.

A market-based transmission planning for HVDC grid—case study of the North Sea

There is significant interest in building HVDC transmission to carry out transnational power exchange and deliver cheaper electricity from renewable energy sources which are located far from the load centers. This paper presents a market-based approach to solve a long-term TEP for meshed VSC-HVDC grids that connect regional markets. This is in general a nonlinear, non-convex large-scale optimization problem with high computational burden, partly due to the many combinations of wind and load that become possible. We developed a two-step iterative algorithm that first selects a subset of operating hours using a clustering technique, and then seeks to maximize the social welfare of all regions and minimize the investment capital of transmission infrastructure subject to technical and economic constraints. The outcome of the optimization is an optimal grid design with a topology and transmission capacities that results in congestion revenue paying off investment by the end the projects economic lifetime. Approximations are made to allow an analytical solution to the problem and demonstrate that an HVDC pricing mechanism can be consistent with an AC counterpart. The model is used to investigate development of the offshore grid in the North Sea. Simulation results are interpreted in economic terms and show the effectiveness of our proposed two-step approach.

Power flow control and its effect on flow-based transmission cost allocation

In a transmission system with HVDC lines or other power flow controlling devices, it is possible to control the active power flow in the lines and indirectly also in other parts of the system. The active control of the power flow through a grid can affect flow-based transmission cost allocation methods as they are based on the DC load flow model. This study investigates the effect of such power flow control on flow-based transmission cost allocation methods. Particularly, three cost allocation schemes: postage stamp, marginal participation and Aumann-Shapley, are compared in a transmission system with power flow control, using HVDC as power flow controlling device, and without power flow control. The results are presented for a 14 bus test system.

Optimization of transmission technology and routes for Pan-European electricity Highways considering spatial aspects

An algorithm to determine optimal transmission routes and to perform technology selection for transmission system expansion is developed and presented in this paper. The aim of the optimization is to minimize investment and installation costs of grid expansion, taking into account spatial properties. The optimization is performed by formulating possible expansion options around a weighted graph and using a modified A* shortest path algorithm to find the optimal solution. The algorithm delivers optimal locations routes and determines the best technology along the route. The algorithm is applied to possible future pan European electricity highways. The implications of different installation areas on transmission routes and total costs are discussed.