Abstract
HVDC grids have been made practical with the introduction of the voltage source converter (VSC). The development of VSC technology has been, and continues to be rapid, with new converter topologies and new manufacturers entering the market. For the power transmission industry to fully exploit the benefits a ‘multi-vendor’ supply chain is considered essential. However, unlike AC grids, there is at present no standardised grid code for HVDC grids and interoperability of equipment and systems, purchased from many different and competing suppliers, is a major concern. Compatibility of control between converters supplied by different manufacturers has been considered by several groups such as CENELEC TC8X – WG6. The initial findings were that there was little reason to suggest that restrictions should be placed on which control strategy should be employed. However recent work uncovered an adverse interaction between converters with differing operating modes. This study illustrates how multiple operating points in the control characteristics could occur and show why such operation is undesirable and how it can be avoided by careful scheme design.
Highlights
With the introduction of the voltage source converter (VSC) into the field of HVDC power transmission the full advantages of multi-terminal HVDC systems or grids may, at last, be realised
Examples of different control methods are described below. (The convention used here is; power flow from the AC system to the DC grid the converter is IMPORTING and DC current flowing from the converter into the DC grid is defined as positive DC current; power flow from the DC grid to the AC system the converter is EXPORTING and DC current flowing from the DC grid into the converter is defined as negative DC current)
The use of droop control characteristics for converters has previously been shown to be a stable method of controlling power flow in HVDC grids
Summary
With the introduction of the voltage source converter (VSC) into the field of HVDC power transmission the full advantages of multi-terminal HVDC systems or grids may, at last, be realised. It has often been stated [1,2,3] that for large power transmission grids to become a reality the supply of the equipment to that grid should ideally be multi-vendor. At the present time each manufacturer has created their own control concept and it is possible, likely, that DC grids will develop with converters employing differing control characteristics. Cardiff University, in collaboration with Alstom Grid, undertook a study on HVDC grid control The studies involved both digital simulation (PSCAD/EMTDC) and physical modelling using the University’s four-terminal VSC simulator. This interaction was due to multiple cross-overs between the different VSC converter control systems using different control characteristics
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