**3.2 FC wind turbine model**

The typical configuration of the FC wind turbine is given in **Figure 6**. Usually, the generator could be a multi-pole synchronous generator designed for low speed, and this allows for gearless design. The generator can either be electrically excited or permanent magnet synchronous generator. For allowing variable speed operation, the synchronous generator is connected to the grid through two full power converters

**Figure 5.** *Two-level VSC circuit.*

**Figure 6.** *Structure of FC.*

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

(LSC and RSC), where they convert the variable frequency output power of the generator to AC power with grid frequency [1–3].

#### **3.3 DFIG and FC controller**

The controls of both DFIG and FC wind turbines are achieved by controlling LSC and the RSC utilizing vector control techniques [1–3].

Vector control allows decoupled control of both active and reactive power. RSC is used to control the active and reactive powers delivered to the grid. LSC is used to maintain the DC bus voltage regardless of the magnitude and direction of the rotor power. The reactive power controllability of the LSC is always applied to reinforce faultride-through (FRT) capability and provides grid voltage/reactive power control [1–3].

#### **3.4 Onshore VSC-HVDC converter model**

In this chapter, the onshore VSC-HVDC converter applies the modular multilevel converter (MMC) topology, and it is illustrated in **Figure 7** [15]. Each converter phase consists of upper and low multi-valve units. Each multi-valve unit has a modular structure with series-connected sub-modules (SMs). Each SM contains a capacitor and two IGBTs/diodes as illustrated in **Figure 8** [15]. This chapter is concentrated on the operation of OWF and DRU, and the half-bridge SMs are applied [15].

**Figure 7.** *Detailed MMC topology.*

#### **3.5 Onshore VSC-HVDC controller**

The onshore MMC injects the active power transmitted by the offshore DRU to the onshore AC grid while maintaining the DC voltage at desirable level. In addition, it supports the onshore AC grid voltage in steady state operation and during faults. It uses a vector control [1–3, 15].

The frame of the onshore VSC-HVDC is shown in **Figure 9** [15].

#### **3.6 DRU model**

As described in [5], the DRU combines a transformer with a diode rectifier and DC smoothing reactors in a common tank filled with synthetic ester. In this chapter, the 6 pulse DRU is considered [16].

All the system is modeled in PowerFactory [15]. The main system components, e.g. offshore DRU, DC cable, onshore VSC-HVDC, 33-kV MVAC submarine cable and PST, are shown in **Figure 10**, where the OWF is operating with nominal power.

### **4. Simulation results**

The system illustrated in **Figure 3** is simulated in this section. Both static and dynamic behaviors of the proposed method are considered.

#### **4.1 System parameter**

#### *4.1.1 33-kV cable parameter*

The ABB 33-kV submarine cable is applied in this simulation and the parameters are given in **Table 1** [15].

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

#### **Figure 9.**

*Control frame of the onshore VSC-HVDC converter.*

#### *4.1.2 PST parameter*

A standard transformer model is modified to enable the PST function. The parameters are given in **Table 2** [15]. For simulating the energization, saturation is also considered.

#### *4.1.3 Wind turbine settings*

The OWF consists of 20 strings, and 10 FC wind turbines (*SN* ¼ 5*:*6 MVA, *PN* ¼ 5 MW) are equipped on each string. One string structure is given in **Figure 11a**. The total OWF capacity is 1 GW [15].

#### *4.1.4 Static operation*

Firstly, the static operation of the proposed method is considered. OWFs operating with 20% and 100% of nominal power are selected for the demonstration of the proposed approach.

#### *4.1.4.1 OWF operating with 20% of the nominal power*

**Figure 11a** shows the power flow of one string in the OWF, where the active power of each wind turbine is 20% of its nominal power (1 MW). The wind turbines

#### *Wind Turbines - Advances and Challenges in Design, Manufacture and Operation*

#### **Figure 10.**

*DRU and VSC-HVDC with nominal power output of OWF.*


#### **Table 1.**

*33-kV submarine cable parameter.*

on the string are connected by 33-kV submarine cable and the cable length (between two wind turbines) is 1.5 km.

Power control for the 33-kV cable between onshore and offshore:


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

