A New Communication-Free Grid Frequency and AC Voltage Control of Hybrid LCC-VSC-HVDC Systems for Offshore Wind Farm Integration

We propose frequency and voltage control method for a hybrid high-voltage direct current (HVDC) system for integrating an offshore wind farm into transmission networks. Conventionally, DC voltage level is maintained as constant regardless of the wind velocity and the grid frequency, so AC voltage fluctuates with active power variation in a hybrid HVDC system due to the innate reactive power absorption of inverter side line-commutated converter (LCC). However, in the proposed method, the DC voltage changes according to the wind velocity. Furthermore, DC voltage is simultaneously regulated to participate in frequency control in a communication-free manner. These two characteristics can be achieved by using five droop control methods proposed here. Additionally, by determining droop constants using the proposed determination methods, AC voltage can be maintained as constant during active power fluctuations. Therefore, using the proposed method, an offshore wind farm can be stably connected to mainland transmission networks and successfully used as frequency supporting resources, because voltage characteristics of the AC network are not destabilized. A small-signal state-space (SS) model of the target system was also developed to investigate stability of the proposed control. The utility of the proposed method is demonstrated by case studies using PSCAD and SS models.

I. Introduction

Offshore wind farms (OWFs) are considered to be a promising future energy source [1]. When integrating OWFs into conventional transmission networks, high-voltage direct-current (HVDC) systems have been shown to offer technical, economic, and environmental advantages over AC counterparts [2]. Three important HVDC systems are available: line-commutated converter (LCC), voltage source converter (VSC), and hybrid LCC-VSC (called a hybrid for readability) HVDC systems [3]. Generally, an LCC HVDC system is economically superior due to its lower installation cost and conversion losses. In offshore installation, however, a VSC is cheaper because of its small footprint [4]–[7]. Therefore, hybrid HVDC systems featuring a VSC at the rectifier side and an LCC at the grid side are the best option for an offshore wind farm integration. Hybrid HVDC technologies have gained attention in recent years due to several ongoing projects in China: the Baihetan–Jiangsu kV system [8] and the Wudongde–Yunnan kV system [9].