PQU diagram capabilities and operational needs for offshore HVDC converter stations

Therefore, provision of reactive power from the HVDC converter in steady state is only required to compensate for the capacitance of the array cable during energisation. As a result, the PQU requirements for the offshore converter can be reduced. In this paper we explore effect of different PQU requirements on the design of the offshore HVDC converter and provide recommendation on the possible PQU to adopt.

Through the load flow studies we explore what are the operational PQU requirements and propose three different PQU diagrams. These PQU diagrams cover different ranges starting with minimum requirement during normal operation and extending in Q and U domain. To illustrate effect of these different PQU diagrams on the design of the converter a 2GW +/525kV bipole HVDC is chosen for a case study. A high-level converter design is done to address each of the three proposed PQU diagrams. Key parameters of converter designs are assessed, and economic impact is evaluated. It was found that the primary impact is on the converter losses which are proportional to the value of the transformer converter side open circuit voltage and additional cost associated with cooling system.

It can be concluded that the overspecification, can lead not only to the increase in the initial cost, but also to the increase in the operational costs through the cost of losses and associated maintenance. Based on the analysis presented in this paper it is recommended to maintain the requirements in terms of the desired capability of the HVDC converter to the minimum required for the operation. In this regard, a PQU diagram for the future possible specification is proposed.