Peter Jones, Sector Manager Renewables ABB PT (UK), outlines the key technical issues relating to the AC grid connection of offshore wind farms and explains how reactive power control via SVC (Static VAr Compensation) can enhance system stability and reliability
With very large offshore windfarm arrays about to become more commonplace, new challenges are being placed on the transmission system operators to maintain system stability and limit dynamic voltage variations.
SYSTEM CONNECTION
In the past, wind turbine units typically had a small power output rating when compared to the strength of the connecting electrical network, so a simple control system that disconnected the wind farms whenever a network disturbance occurred was sufficient.
With the larger windfarms presently being planned, this design philosophy becomes questionable. Windfarm connections must be designed so that the wind turbines are capable of continuous, uninterrupted operation during the protection clearance times for the faulted, adjacent, network components (‘ride-through capability’).
STABILITY AND RELIABILITY
For large wind farms there are a number of stability and reliability issues that need to be addressed during the design stages.
Conventional induction generator units and doubly-fed induction generator (DFIG) wind turbines may disconnect from the transmission system for low voltage conditions caused by system faults more quickly than conventional existing synchronous generator power plants.
An induction generator has the potential to over-speed beyond its pullout torque, at which point the machine races away and disconnects from the grid. For DFIG induction generators, there are issues relating to the control and protection of the voltage on the converter DC bus that can lead to the tripping of the unit, which a conventional synchronous generator could normally endure.
If these issues are not addressed at an early planning stage, the performance of the windfarm may be in violation of system security, planning and availability criteria resulting in a requirement for an increase in spinning reserve. In more serious situations it may lead to the grid system experiencing a cascading power outage.
VOLTAGE CONTROL
Reactive power control is necessary to address these network stability and reliability issues. With synchronous generators, reactive power control is achieved by means of the exciter system. However, this is not possible for basic induction generators. Instead, a Static VAr Compensator (SVC) positioned at the grid connection point can act as a central exciter system but with the advantage that reactive power can be controlled even when no power is generated.
The transmission systems to which offshore windfarms may be connected are usually designed to distribute power from the main grid to remote customers. These remote systems are, in many cases, weak and a change in power flow direction will affect voltage levels.
Mechanically switched capacitor banks (MSC) are often used to deal with voltage level problems. However, power production, and thus reactive power consumption, in windfarms varies with wind speed. The resulting frequent switching of MSC deteriorates power quality and decreases the lifetime of the MSC. An SVC, with continuously variable susceptance, offers a cost efficient alternative to several small MSC units.
Several phenomena associated with power produced from wind introduce voltage flicker on the connecting node generator start and stop, wind speed variations, and tower shadow effects. This flicker has a detrimental effect upon other components connected to the grid causing complaints from power consumers. By connecting an SVC at the grid connection points, this flicker can be mitigated.
SVC IMPLEMENTATION
SVCs are available in two different versions. The first SVC approach is based on conventional capacitor banks together with parallel thyristor-controlled inductive branches, which consume the excess of reactive power generated by the capacitor bank. This type of equipment can be directly connected to the intermediate voltage bus, which interconnects the wind farms (up to 36kV). When needed, it is also possible to connect the SVC to the high-voltage network via a dedicated transformer.
The second alternative implementation of the SVC makes use of a power electronic voltage source converter (VSC). The converter utilises semiconductors having turn-off capability. The converter can inject or consume reactive power to or from the bus where it is connected. This application of VSC technology is usually referred to as STATCOM (Static Compensator). This alternative has the benefits of a smaller footprint, as large air-cored inductors are not used. Another advantage stems from the fact that a smaller parallel capacitor bank can be used, as the converter itself may contribute reactive power.
By combining the two types of schemes, a cost-effective dynamic compensator can be achieved, rated for a high dynamic yield during a short time and a lower yield for steady-state operation.
SUMMARY
The UK has generally benefited from a stable and reliable transmission grid system based on traditional sources of generation. SVC technology will have an increasingly vital role to play in ensuring that networks with large amounts of windfarm connections remain resilient and that small scale local network faults do not escalate into more serious widespread transmission outages.