**1.2 Technical challenges and possible solutions for DRU-HVDC link**

Compared to the well-controlled VSC-HVDC, the DRU is a passive device without any controllability, and therefore it cannot provide the AC reference voltage for the OWF [8, 9]. Furthermore, in order to meet the power quality and reactive power requirements on the point of common coupling (PCC), filters and reactive compensations should be equipped on the DRU [10].

In order to deal with the above-mentioned challenges, different solutions were proposed:

1.First solution: since the DRU has not the blackstart capability, an additional 33 kV or 66 kV medium voltage (MV) AC cable is suggested in [5] for the startup of OWFs, as shown in **Figure 1** [5]. However, during the startup of OWFs, there

**Figure 1.** *OWF with DRU-HVDC.*

*Offshore Wind Farm Grid Connection with Diode Rectifier Unit HVDC and Phase Shifting… DOI: http://dx.doi.org/10.5772/intechopen.103111*

**Figure 2.** *Hybrid topology on the offshore side.*

could be a period where the DRU-HVDC and medium voltage AC (MVAC) cables are in parallel operation. Large current on the MVAC cable could occur because of the uncontrolled DRU operation. After the startup of the OWFs, the MVAC cable will be disconnected and part of the offshore wind turbine will take over the control of the OWF [5, 9], e.g. grid forming controls (because the DRU is a passive rectifier without any control capability). Therefore, modifications on the wind turbine controllers (from grid following to grid forming) are necessary. Furthermore, the coordinated control of the wind turbines in OWF is necessary [9].

2. Second solution: in [6], a hybrid topology with parallel operation of DRU and VSC is suggested for controlling the voltage and frequency of the offshore grid, as shown in **Figure 2**. However, considering the dimension and costs of the offshore VSC converter station, the cost-effectiveness could be significantly reduced [11]. Therefore, the first solution is concentrated in this chapter.

#### **1.3 Main objective of this chapter**

The main objective of this chapter is to ensure the stable operation and improvement of the dynamic behavior of OWF with DRU-HVDC grid connection. Therefore, a new method is applied, where the phase shifting transformer (PST) is used on the onshore side of the MVAC cable [12].

This chapter is organized as follows: following the introduction, the proposed system structure, startup procedure, wind turbine and HVDC models are introduced in Section 5. In Section 6, both static and dynamic behaviors are analyzed. Finally, brief conclusions are deduced.

### **2. System structure and operation**

#### **2.1 Proposed system structure**

The proposed method is shown in **Figure 3**, where the PST is applied on the onshore side of MVAC cable. The main objective of PST is for the smooth startup and stable operation of the OWF [12–14].

Similar to the method proposed in [5], the MVAC cable is used for the startup of OWF. Due to the application of PST, the MVAC cable and DRU-HVDC link could be operated in parallel (no additional switching procedure necessary).

**Figure 3.** *Proposed system structure.*
