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

Chapter 5Frequency Support

5.1 Introduction

Wind turbine generators (WTGs) are usually interfaced with the electric power grids through power electronic converters. Due to fast power reference control of the converters, WTGs do not inherently contribute to the system inertia unlike conventional synchronous power plants. In future, WTGs are likely to replace a significant fraction of the conventional power plant fleet which could reduce the overall system inertia drastically. Moreover, asynchronous interconnections through DC links act as “fire breaks” contributing further to the reduction in overall system inertia. With growing penetration of both asynchronous generation and interconnection, it is envisaged that the inertia of future systems could become low. This would result in unacceptably high rate of change and absolute variation in system frequency following a supply–demand imbalance condition.

To resolve this problem, mechanisms for extracting inertial support from WTGs have been proposed and demonstrated [73, 74]. These methods rely on detected changes in grid frequency to control the power reference of the WTGs. For instance, if the grid frequency goes down, the wind turbines are made to slow down so that they can release the stored kinetic energy in the blades in order to provide the inertial support. The mechanism is relatively straightforward for onshore wind farms and offshore wind farms with AC connection to the onshore AC system. For remote offshore wind farms ...