Transformer energisation is one of the most important switching events during the connection of an offshore wind development.
When a transformer is first connected to the network, it can draw a large magnetising current. This transient may cause a voltage dip at the point of connection.
The effect is particularly important on weak networks, where the same current can create a more noticeable voltage response.
What causes transformer inrush current?
Transformer inrush is a product of the core magnetising the moment the unit connects to the network. Two factors decide how severe it is: how the transformer itself is built, and precisely where in the AC cycle the breaker makes contact.
If the breaker closes at an unfavourable moment, the core can enter saturation. The resulting current may reach several times the transformer’s rated current before gradually decaying.
The paper notes that large power transformer inrush can reach five to ten times rated current.
The transformer may be designed to withstand this current. The network around it still experiences the effect.
From inrush current to voltage dip
High inrush current can pull down the voltage at the point of connection. If the disturbance is too large or continues for too long, it may exceed the applicable grid requirements. Other customers connected to the same system could also experience the voltage change.
The risk increases where the network is weak.
Multiple transformers create another consideration. Energising one transformer can influence units that are already connected, producing sympathetic inrush. This may extend the duration of the voltage dip even where the initial magnitude does not increase significantly.
Controlling breaker closing time
Point-on-Wave switching controls the point in the voltage waveform at which the circuit breaker closes. By selecting an appropriate closing angle, the transformer can be energised with less core saturation. This reduces magnetising inrush and limits the resulting voltage disturbance.
The timing needs to be accurate.
At Viking Wind Farm, Point-on-Wave switching was installed on each of the 103 turbine transformers. Breaker closing was controlled within a narrow timing window to support energisation on Shetland’s weak network.
The paper reports that the transformers were energised without excessive transients or unacceptable voltage depression.
Point-on-Wave switching at project scale
Point-on-Wave control can be applied to individual turbine transformers or larger units at an offshore or onshore substation. Its suitability depends on the transformer, the circuit breaker and the network requirements.
Detailed studies are needed to establish the target closing angles, and the same studies can indicate where reactor compensation elsewhere in the design may need adjusting. Japanese EPC contractors already supplying substation switchgear are well placed to integrate this relay technology directly, without resorting to more expensive alternatives.
Commissioning then confirms that the controller and breaker operate as intended. The solution therefore combines study work, control equipment and site validation.
Other approaches to inrush mitigation
Pre-insertion resistors may also be used to limit transformer inrush. The breaker initially closes through a resistor, which reduces the current during the first cycles of energisation. The resistor is then bypassed.
A staged commissioning sequence offers another option. Fewer transformers can be energised at one time, allowing the network to recover between operations.
The correct approach should be selected from the study results. Some connections may tolerate uncontrolled energisation. Others may require precise control to remain within the permitted voltage envelope.
Using simulation to select the solution
EMT simulation can model transformer saturation and test a range of breaker closing conditions, comparing uncontrolled switching against Point-on-Wave control. The output is a quantified worst-case voltage dip that can be checked directly against the applicable standard, whether that is the UK’s percentage limit or the tighter flicker requirement used in Japan.
This provides evidence for equipment selection and the commissioning plan. The objective is to understand the event before the first energisation takes place on site.
Read the complete technical white paper here for further detail on transformer inrush, sympathetic inrush and Point-on-Wave switching.
