**Abstract**

This chapter describes a new method for operating the offshore wind farm (OWF) with diode rectifier unit (DRU)-high-voltage direct current (HVDC) (offshore side), where a medium voltage (MV) submarine cable is in parallel operation with DRU-HVDC link. In order to avoid uncontrolled current flow through the MV submarine cable, a phase shifting transformer (PST) is applied on the onshore side of the MV submarine cable. The application of PST is to ensure the smooth blackstart and stable operation of the OWF and DRU-HVDC link. Both static and dynamic behaviors of the proposed method are presented in this chapter and the simulation results validate the proposed method.

**Keywords:** Offshore wind farm, phase shifting transformer, blackstart, diode rectifier unit (DRU), HVDC

### **1. Introduction**

Since the wind energy is renewable and environmental natural resource, the utilization of wind power plant increased quickly. In the future, the development of wind power utilization will focus on large offshore wind farms (OWFs) [1–4].

Especially, many planned OWFs become larger and more distant from the onshore grid. Conventional HVAC transmission is not flexible and limited due to large charging currents of the submarine cables. With the development of power electronics, OWFs with voltage source converter (VSC)-high-voltage direct current (HVDC) grid connection become more popular. The VSC-HVDC technology is based on insulated gate bipolar transistors (IGBTs), and it offers significant advantages over the thyristor-based line-commutated converter-HVDC (LCC-HVDC) technology. VSC-HVDC converters can be used to supply weak grids, offer blackstart capability, and can provide decoupled active and reactive power controls [1–4].

Due to the continuous increase of the OWF capacity, the capacity of offshore converter stations should also be increased. This could result in a larger dimension and heavier offshore converter stations. In order to reduce the costs of offshore

VSC-HVDC converter station and offshore platform, the diode rectifier unit (DRU) concept for offshore converter stations is proposed in [5–7], as shown in **Figure 1**.
