Rationalization of DC Power Transfer Limits for VSC Transmission

Earlier work on calculating maximum power curves (MPCs) and maximum available power (MAPs) for VSC converters was not consistent with the way it is done for LCC converters. This paper attempts to use an approach consistent with LCCs by using a Thévenin voltage that is calculated to give 1.0 pu terminal voltage at rated power and unity power factor for the VSC. The MAP values calculated using this approach are considerably larger than those calculated using earlier approaches. The paper also discusses how the MAP is reduced when voltage limits for the VSC´s internal voltage are considered. The paper also identifies problems with this consistent approach, as the Thévenin voltage at low SCR can be significantly greater than 1.0 pu. In the LCC this apparent voltage is adjusted by tap changer operation, and if a similar action is taken for the VSC, an unreasonably large tap ranger range would have to be assumed for a low SCR. As an alternative, the paper considers an ac network where some reactive power is locally supplied by shunt capacitors, which makes a Thévenin voltage close to 1.0 pu possible, and recommends that such a network be used when calculating the MPCs and MAP for a VSC converter.