Multi-terminal Direct-Current Grids: Modeling, Analysis, and Control

Chapter 4Autonomous Power Sharing

4.1 Introduction

In a point-to-point HVDC link, typically one converter station controls the real power and the other station maintains the DC link voltage. As a natural extension, in an MTDC grid all converters except one could be operated in active power set point control and one converter station could be responsible for maintaining the DC voltage across the entire DC grid. The DC voltage controlling converter acts as the slack bus within the MTDC grid and deviates from an active power set point by the amount of loss that takes place within the MTDC grid. Due to a small percentage of loss in an MTDC grid this deviation is acceptable under normal operating conditions. However, if there is a converter outage the mismatch in active power is transferred to the DC voltage controlling converter. The DC voltage controlling converter either needs to increase or decrease its power depending on the nature of both the faulty converter and DC voltage controlling converter, i.e., rectifier or inverter. In an event of increase or decrease it may so happen that the DC voltage controlling converter needs to be loaded well beyond its rating (limited by the current controller limits mentioned in Section 2.4.1) or reverse the power direction that might have an impact on the surrounding AC systems. Therefore, following a converter outage in an MTDC grid, it is critical that the healthy converter stations share the power mismatch/burden in a desirable way.