**Table 2.**

*PST parameter.*

operating on its natural charging reactive power (9.4 MVar), as illustrated in **Figure 11b**.

As illustrated in the power flow result, all node voltages are in nominal range.

#### *4.1.4.2 OWF operating on nominal power output*

**Figure 12a** shows the power flow of one string in the OWF, where each wind turbine generates nominal power (5 MW).

As illustrated in **Figure 12b**, the offshore grid station can control the reactive power on the 33-kV onshore-offshore cable with its charging reactive power (9.4 MVar, shown in **Figure 12b**). Similar to the results in Section 6.1.4.1, the total active power generated by the OWF is transferred by the DRU-HVDC link.

It is clear that all the node voltages are in the nominal range.

#### *4.1.4.3 Startup of OWF and HVDC system*

Since the dynamic behaviors during the startup of the PST and 33-kV onshoreoffshore cable were discussed in [13], this chapter is concentrating on the startup of OWF wind turbines and DRU.

#### *4.1.4.4 Startup of OWF*

The wind turbines of the OWF are started with 20% of the rated power (1 MW). At 0.5 s, the first string of the OWF is switched on. From 1 s to 5.5 s, the wind turbines on this string are started sequentially.

At 6 s, the second string of the OWF is switched on and from 6 s to 10 s, and the wind turbines on the second string are started.

In order to demonstrate the capability of OWF park controller (offshore grid station) [15], the reactive power of the 33-kV cable between onshore and offshore is set to 14 MVar, as shown in **Figure 13**.

**Figure 11.** *OWF operates with 20% of nominal power. (a) One string of OWF and (b) OWF with DRU.*

After starting the first string (1–5.5 s), since the wind turbines on the first string are working on 20% of their nominal power, the total reactive power capability of the first string (10 wind turbines) is not enough to maintain the regulated reactive power on the 33-kV cable (14 MVar).

After the startup of the second string, the reactive power of the 33-kV cable can be controlled effectively.

During the startup of the OWF, it is obvious that there are oscillations on the reactive power. This is mainly due to the parameter of the onshore-offshore 33-kV cable and the saturation parameters of the PST.

#### *4.1.4.5 Startup of OWF transformer and 150-kV submarine cable*

After starting the OWF wind turbine, the OWF transformers (illustrated in **Figure 11b**) are started at 12 s. Then two 150-kV submarine cables (illustrated in **Figure 11b**) are switched on at 15.5 s.

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

**Figure 12.** *OWF operates with nominal power. (a) One string of OWF and (b) OWF with DRU.*

**Figure 13.** *Reactive power on the 33-kV cable (offshore side).*

**Figures 14** and **15** show the current and the voltage of the OWF transformer. The transformer inrush current is limited effectively by means of the controllable switch of the circuit breaker (switching on at voltage maximum) [15] and the voltage is also in the nominal range.
