• Document: Real-time Implementation of Optimal Power Flow Calculator for HVDC Grids
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21, rue d’Artois, F-75008 PARIS 365 LUND 2015 http : //www.cigre.org Real-time Implementation of Optimal Power Flow Calculator for HVDC Grids Muhammad Hassan Fidai*, Davood Babazadeh*, Jonathan Hanning†, Tomas X. Larsson†, Lars Nordström* * Industrial Information & Control Systems, School of Electrical Engineering, KTH- Royal Institute of Technology, SE-100 44, Stockholm, Sweden † DC Grids Simulation Center, ABB AB, 721 83 Västerås, Sweden E-mail: fidai@kth.se, davood@kth.se, jonathan.hanning@se.abb.com, tomas.x.larsson@se.abb.com, larsn@ics.kth.se SUMMARY The aim of the paper is to present the centralized architecture for power balancing management in an HVDC, High Voltage Direct Current, grid connecting different AC areas with high penetration of variable energy resources. Such a centralized high level DC Supervisory Control (DCSC) that functions in slower time scale compared to outer level controller has been evaluated in a real time co-simulation test-bed. The test platform includes OPAL-RT’s eMEGAsim real time simulator to model the power system, the ABB’s industrial HVDC controller (MACH), real time communication simulator OPNET to model the communication network and finally the DCSC application which is implemented on a Linux machine. The DCSC consists of a network topology manager to identify the grid configuration and employs an Optimal Power Flow (OPF) calculator based on interior point optimization method to determine the set-point values for all HVDC stations in a grid. The OPF calculator takes into account the DC voltage, converter and DC line constraints. The performance of the DC supervisory control has been tested for various test cases for a 7-terminal HVDC grid. Test cases include I) Variable power generation from wind farms, II) Station disconnection and III) DC grid islanding. Besides, the proper sampling rate has been chosen and justified to show the benefit of frequent updating of set-point compared to letting the DC droop control scheme take over the mismatch in the system. The results of different test cases show that a DCSC can improve the power extraction from wind farms by updating the set-points following any change in the system. Using a 3.2 GHz machine, it approximately takes 15 ms for the DCSC to converge to a proper solution and send the updated set-points. KEYWORDS Optimal power flow, Real-time simulations, Supervisory control, VSC-HVDC grid. 1 INTRODUCTION Integration of renewable resources such as remote solar or wind farms and electric power trading between neighbouring countries lead to new requirements on the development of the transmission grids. Since AC grid expansion is limited by legislations issues, HVDC technology with its diverse benefits compared to AC is being considered as appropriate alternative solution. HVDC technology is being used to transmit power over long distances and to connect different AC systems for past six decades. Line Commutated Converters (LCC) and Voltage Source Converters (VSC) are the two HVDC technologies currently available. The latter has significant advantages over LCC when it comes to integration of off- shore wind energy [1] or to provide reactive power support to the connecting AC system [2]. A VSC-HVDC grid has been proposed in the literature [3, 4, 5, 6] to overcome the aforementioned challenges. Such an HVDC grid can be augmented or integrated within single or different AC systems. Due to the versatile nature of future applications, different solutions have been suggested for building HVDC grids. Its various aspects with the technological and economical perspectives have been addressed in the literature which includes but are not limited to different grid topologies, protection schemes and detailed control strategies [7, 8]. Various control schemes have been presented, where some advocate distributed control strategy [7], others address the problem with a centralized control scheme [9] [10]. The developed HVDC grid can be either embedded inside one AC grid or connecting several AC areas. Variable power injection from wind farms, varying active and reactive power requirements of the connecting AC systems and changes in the grid topologies due to faults are the major challenges in the balanced, stable and reliable operation of the HVDC grid. In both the distributed and centralized architectures, a separate DC supervisory control can be proposed to control the HVDC grids using the interfacing information from the AC SCADA(s) (Supervisory Control And Data Acquisition). The supervisory control is supposed to calculate the OPF in order to run the system in the most optimal situation. Based on the architecture, the required information, the boundaries of the

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