Island Mode Operation
Island mode operation and load rejection
A refinery loses its grid connection. Its combined heat and power plant now has a few seconds to decide whether the refinery keeps running or goes dark. Getting it wrong means an uncontrolled shutdown: units tripped, product to the flare, a day of lost production and a mechanical inspection before restart.
We have implemented this transition and proved it on an operating plant.
What has to happen, and in what order
Detect the separation
Not "notice the frequency moving" — establish that the connection to the grid is gone, fast enough that the next steps still have time to act.
Transfer the governor
The turbine governor runs in droop while synchronised to a grid that sets the frequency. In an island there is no grid to follow: the machine itself sets the frequency, and the governor has to move to isochronous control. That transfer happens once, under load, and cannot be rehearsed on the running plant.
Shed load by priority
Generation left in the island is smaller than the load that was being served. Non-critical consumers go, in a sequence worked out in advance, fast enough that the frequency excursion stays inside what the turbine will tolerate. Which consumer is non-critical is a process question, not an electrical one, and getting the list wrong shuts down the refinery just as effectively as losing the plant.
Hold frequency and voltage
In the island the plant is the grid. Frequency and voltage control that were someone else's problem are now the control system's problem.
Resynchronise
When the grid returns, back onto it without a second disturbance.
Proven, not simulated
The algorithm was implemented in the DCS and tested on the running plant in two trials, before and after a plant overhaul, holding 13–14 MW in the island. Both trials were agreed in advance with the transmission system operator and the distribution system operator, because a deliberate separation affects the network on the other side of the breaker too.
Turbine acceptance testing on the same units included disconnection of the generator from the grid — the same event, under controlled conditions, with the machine held on house load.
Why it is rare
An automation contractor can write the logic. Running it on a plant that a refinery depends on requires someone the plant owner will let near the turbine, agreement from two network operators, and a load shedding list that someone has to be confident about. Most of that is not automation work.
Related
Primary and secondary frequency control, plant coordinated control and grid code compliance are covered under Power Generation. The plant's fuel and efficiency calculations are covered under Plant Performance.