Offshore wind developments rely on long HVAC export cables to transfer power from the offshore network to the onshore grid. As cable length increases, its electrical characteristics become increasingly important.
Long cables introduce substantial capacitance. This can affect voltage during light-load operation and create interactions with transformers or shunt reactors. These effects need to be understood as part of the overall grid connection design.
Ferranti voltage rise
When a long AC cable is lightly loaded or open at its remote end, the receiving-end voltage can rise above the sending-end voltage. This is known as the Ferranti effect.
The cable capacitance draws charging current, which causes the voltage profile to rise along the length of the circuit. The effect becomes more significant as cable length increases and may be amplified by a weak source network.
This is particularly relevant during testing, when the cable may be run open-circuited or lightly loaded, before it carries its full operating load.
The paper gives the example of an open 60 km export cable reaching approximately 1.1 per unit at the offshore substation. Without mitigation, the voltage could move outside the permitted range.
Using shunt reactors to control voltage
Shunt reactors absorb the reactive charging current produced by the cable. They are commonly installed at the cable ends, with the exact arrangement depending on the project. The reactor reduces the Ferranti voltage rise and controls the reactive power exchanged with the network.
The compensation level needs careful selection.
A fully compensated cable and reactor arrangement may create a condition known as zero-missing. The inductive and capacitive currents cancel, leaving a DC component without normal current zero crossings. This can prevent a circuit breaker from interrupting current as expected.
Energisation sequence matters
Cable and reactor switching therefore needs to be planned. One option is to energise the cable and reactor at different times. Another is to use less than full compensation during energisation so that an AC current component remains. Controlled switching may also be considered.
The appropriate sequence should be tested through simulation before commissioning. This allows engineers to examine transient overvoltages, reactor inrush and circuit-breaker current.
Harmonic resonance
The combination of cable capacitance and system inductance can create resonant frequencies. Transformers and shunt reactors add further inductance to the network. Together, these components form an LC circuit.
If a resonant frequency aligns with harmonic currents already present in the system, voltage distortion may be amplified.
The paper notes that a 50 km cable could resonate near a lower-order harmonic such as the third or fifth. Where the network or wind turbines produce current at the same frequency, distortion at the point of connection may increase.
Managing resonance risk
A harmonic study can identify the resonant points of the proposed network. The model should include the export cable, transformers and reactor arrangement. It also needs to consider the impedance of the wider system.
Where the study identifies a potential exceedance, filtering or damping may be required. A C-type filter is one possible approach discussed in the paper. Adjusting reactor sizes may also move the resonance away from a problematic frequency.
Long HVAC cables should therefore be assessed as active electrical components within the connection.
Download the full technical white paper here for a wider discussion of Ferranti voltage rise, zero-missing and harmonic resonance.
