The offshore grid is inevitable for the integration of offshore wind energy either it is trade driven or trade unconstrained [90]. There are various technical challenges associated with the formation of offshore grids that has to be addressed and solved. The three challenges which are identified in this section are analyzed in this thesis. They arise because of no direct AC connection exists between the offshore grid and the main onshore AC grids and the governing principle of operational control do not apply directly as on the onshore AC network.
System Integration and Power Flow Control: The future offshore grid will not serve a single purpose. It will cover several application such as integration of offshore wind energy, provides interconnections for power balancing, international trade, bootstraps etc [10]. However, the unique offshore grid infrastructure cannot be built at once rather it will grow organically with time from simple initial phase to fully functional integrated network. The main challenge is to adopt the approach of building the system that can be expanded with minimum modification both from the prospective of control and network infrastructure. The future offshore grid must evolve from the network currently exists in the North and Baltic sea.
After the definition of a suitable network architecture, the next step is to establish a mechanism of power flow control for the integrated AC and DC network. Although the power electronics devices provide the flexibility and sufficient control over the power flow, suitable control schemes and optimization algorithms are required for both AC and DC network operation especially considering the power sharing constraints of TSOs and long term network stability.
1.4 Offshore Grid Challenges
Dynamics and Stability: The dynamics and stability of the network are the major issues on which the successful operation of the offshore grid highly rely. The offshore AC network control analogue to onshore network requires the operation of inverters in parallel. The control schemes for inverters connected in parallel have been introduced but so far is applied for small scale micro-grid or island network [69]. The principle of controlling offshore AC grid using frequency and voltage droop scheme in inverters makes power balancing phenomena similar to the network with synchronous machines [71]. However, the dynamic response of the offshore AC grid is different as compared to onshore grid due to the absence of inertia and fast response of power electronics. The droop gain analysis along with the inverter voltage and current control performance is the key aspect that needs to be address for the control design of offshore AC grids.
The main difference between island network and offshore AC grid is the cable capacitive effects. The micro-grid or island network is assumed more resistive while the offshore AC grid is more capacitive. The inverters in the offshore AC grids are acting like reference machines or slack sources which absorb the network power by controlling voltages. The rise in the voltage set-point due to voltage droop scheme as the function of absorb reactive power creates a chain reaction between network reactive power and voltages. This effect is significant in offshore AC grid due to the high cable capacitance which could produce the long term voltage instability [91, 92]. Thus, it is crucial to determine the criteria for the selection of voltage droop gain to keep system stable while keeping the characteristic of reactive power distribution by the VSCs.
Fault Behavior: The large wind energy generation must not be discon- nected to ensure the onshore grid stability. It is desirable to have the same characteristics for the future offshore grid as for onshore grid regarding reliability and availability. The use of DC circuit breaker is imperative to ensure the selectivity in the MTDC network [65]. The DC circuit breaker technology is at an early stage of development however it is expected to be available at the final stage of offshore grid development.
for the offshore AC network that are either connected to a single or multiple onshore grids using point-to-point VSC-HVDC system. Although the VSC capability will not be the constraint of short circuit current level within the offshore AC network due to the contribution of fault by the multiple wind power plants, the short circuit current control characteristic of the inverters is the main concern for the successful operation of the network. The offshore inverter must also have current control to ensure the fast response against faults and to be able to manipulate the short circuit current characteristic directly [22]. Although the wind power curtailment requirements due to the over frequency in the network could be derived from the onshore grid codes, the offshore AC network frequency behavior and its operational characteristics still must to be analyzed as the inertia is very low as compared to onshore network [77]. A well coordinated frequency control system is still needed considering the dynamic and stability limitation imposed by offshore inverters.