DC Network for Interconnection

If multi-terminal and medium voltage direct current (MVDC) grids were possible, wind turbines can be interconnected as shown in Figure 3.5. In comparison with the DC star network as described in Section 3.1.3, the total cable length can be reduced with interconnection. Offshore HVDC Converter Station To onshore substation and AC grid Wind turbine generators AC-DC DC-DC MVDC LV

Figure 3.5: MVDC network with parallel interconnections between WTs.

A DC-DC chopper or a buck converter is simple, efficient and cheap in design but has a limited DC voltage gain between conversion. DC-DC converters with higher voltage gains for serving a large wind farm can be very large in size. The space required in offshore platforms adds to further structural costs. The MVDC network voltage needs to be converted to AC for transformer stepping and HV isolation and then converted again to HVDC for onward transmission to shore. Compared with an HVDC connected AC collection topology, there is another converter stage in the line of the power flow contributing to loss and overall cost of the system [117]. The AC transformer at the intermediate stage of the DC-DC converter is used to step up the voltage and can operate at the medium frequency range around 500 Hz.

3.2. Wind Turbine Interconnection 69 The peak flux is lower at higher frequencies, therefore the core can be considerably smaller and lighter than traditional 50/60 Hz AC transformer solutions [118].

Although the DC cables are more efficient and cheaper than AC cables for distri- bution and transmission, the main challenges associated with DC networks are the loss and cost of converters, and limited DC fault protection. As discussed in Sec- tion 2.1.8), DC circuit breakers (CB) are currently very expensive and do no react quick enough [119]. The high costs of DC CB devices makes it difficult to justify their practical use. One of the most cost effective method to isolate the fault in the DC network is to de-energise the entire DC system first and then re-energise it once the fault is cleared, but this can be very disruptive for the power flow. If the power from the WT has nowhere to go, energy storage or a resistor load bank would be necessary to store or dump the surplus energy offshore. A sudden disconnection of a large source of wind generation will disrupt the grid system unnecessarily. Fast on- shore spinning reserves and ancillary services are expensive, and if they are required to keep the grid stable, it will inadvertently increase the overall cost of energy.

Using Diode Rectifiers

Elliot in [120] has studied a simplified DC network approach that requires minimum power electronics. Power from permanent magnet synchronous generators (PMSG) can be converted to DC using robust passive diode rectification instead of a VSC at the WT. The speed of all the PMSGs in a multi-terminal cluster network is regulated by the DC side voltage. Due to the nature of diode rectifiers, the 6th order harmonic component is present in the machine torque and in the DC side current. As the rotor speed of adjacent WTs vary, this creates a ‘beat’ on the aggregated currents. The study was based on the standard three-phase PMSG and rectifier bridge. This topology does not have to keep with the three-phase standards because the generators are decoupled from the main power system. A multi-phase PMSG, for example with at least six phases up to 60◦ apart, should exhibit higher frequency harmonics in the machine torque and current, which is easier to filter and should have less impact on the mechanics of the machine and the DC network voltage. PMSGs are the only generators suitable for this type of rectifier topology

because a supply for the magnetising currents is not required. Although the overall system is simple and reliable, the permanent magnets are expensive due to restricted global supply (as discussed in Section 2.2.5).

DC Series Interconnection

If the DC side of each converter is connected in series, as shown in Figure 3.6, the sum of the voltages can be enough for direct HVDC transmission without an expensive offshore platform. This topology has potential cost benefits as the cable layout is simple and the large offshore HVDC converter substation and platform structure are not necessary [102]. The onshore side HVDC converter station can be either VSC-based or even thyristor-based LCC, which is more cost effective, efficient and feasible because the onshore AC grid is strong. The larger filter, converter, and switch yard required on the onshore side not a problem because the onshore space is not as expensive compared to the offshore. More than one set of series WT can exist in parallel, sharing the same long distance HVDC cable.

+HVDC –HVDC To onshore HVDC converter station and AC grid Wind turbine generators

Figure 3.6: WT interconnected at the DC side in series, which then builds up to the HVDC voltage.

Major practical challenges for the series DC connection topology include en- suring there is even voltage sharing between the terminals of each WT when they are generating uneven power, and requiring expensive high voltage insulation and clearance with respect to ground. A transformer with XLPE insulated windings between the AC-DC converter and the WT generator isolates the HVDC potential from the generator. The insulation requirement is reduced for WTs closer to the ground point. If a WT device is out of service, a by-pass switch shorting its DC

3.2. Wind Turbine Interconnection 71 terminals ensures continuity of power flow for the rest of the system [121]. The one- quadrant DC-DC buck converter ensures the continuity of current in the HVDC during operation [122].

A ring interconnection is not possible with a DC series topology as with AC or DC parallel ring interconnection, which can cover a fault in one of the interconnections and still keep all the WTs connected. If one cable interconnection in the DC series connection fails, the entire string of WTs in that series is lost because the circuit is cut. The cable fault has the longest downtime in the offshore system, therefore a cable failure can be very disruptive if no redundancy is in place. Adding effective redundancy in the DC series case significantly increases the cost because all the connection has to be at least doubled up. Little is also known about DC fault current protection in practise for the topology.