Abstract
The future direct current (DC) grids will require additional control functions on voltage source converters (VSC) in order to ensure stability and integrity of DC grids under wide range of disturbances. This study proposes a 3-level cascaded control topology for all the VSC and DC/DC converters in DC grids. The inner control level regulates local current which prevents converter overload. The middle control level uses fast proportional integral feedback control of local DC voltage on each terminal which is essential for the grid stability. The hard limits (suggested ±5%) on voltage reference will ensure that DC voltage at all terminals is kept within narrow band under all contingencies. At the highest level, each station follows power reference which is received from the dispatcher. It is proposed to locate voltage droop power reference adjustment at a central dispatcher, to maintain average DC voltage in the grid and to ensure optimal power flow in the grid. This slow control function has minimal impact on stability. Performance of the proposed control is tested on PSCAD/EMTDC model of the CIGRE B4 DC grid test system. A number of severe outages are simulated and both steady-state variables and transient responses are observed and compared against conventional droop control method. The comparison verifies superior performance of the proposed control topology.
Highlights
High voltage direct current (HVDC) transmission based on voltage source converters (VSC) has become accepted technology in many projects worldwide
A 3-Level control topology for the VSC and DC/DC converters is proposed as the generic VSC control in DC grids
The fast proportional integral based DC voltage control at each terminal is the essence of this approach enabling excellent stability and tight DC voltage control at all terminals
Summary
High voltage direct current (HVDC) transmission based on voltage source converters (VSC) has become accepted technology in many projects worldwide. DC voltage droop feedback control method has been extensively studied for DC grids and in general for parallel connected converters like in micro grids [5,6,7,8,9,10] This is a very good approach since local power order at each converter is moderated in response to DC grid situation which is detected through DC voltage variation. The method is robust, since only local feedback signals are used for control at each terminal and no grid-wide communication is required This method resembles the widely used frequency droop feedback with generators in AC systems. The significant advantage of this method is the use of DC voltage control and the use of identical control topology at each DC grid terminal This 2-level control method still uses fast droop feedback (VDC-IDC) at each. We will use detailed model of the recently proposed CIGRE (B4.57 and B4.58) DC grid test system [13] and evaluate performance for a range of most serious outages
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