Generator Islanding? 1

[lsuengineer]

What is Generator Islanding? I have heard several relay engineers talk about triping generators out and they always refer to islanding. Does anyone know what they are talking about?

 

[davidbeach]

Islanding is leaving a generator stranded with load but without any connection to the rest of the grid.

 

[rcw retired EE]

Imagine a power plant feeding a utility in a small town with a single transmission line to the larger utility system. Imagine a fault on the transmission line that opens its breakers leaving the power plant just feeding the small town. That generator is islanded on that town.

The problem is the generator does not know it has been islanded and assumes it is still feeding power into a large grid. The unbalance between generation and load will either cause frequency to increase (generation > connected load) or decrease. Protective relaying and turbine controls can either respond to the frequency change and try to keep the generator islanded or trip the unit, sending the town and the power plant black. Another problem occurs if the transmission line breakers do not have a means of resynchronizing the island with the main system. (Example drawn from a true event in Northern California, USA).

The power plant engineers challenge is to decide if the plant can be islanded (many can't) then decide how islanding is going to be detected, controlled and how the system will be returned to normal. When a plant goes to island mode, the governor must immediately switch from load (MW) control to frequency control (isochronous control or constant speed control).

From a protective relaying perspective, utilities usually require under/over frequency and voltage relaying at the power plant tie point to detect islanding and dump the generator off the system. The relays detect the generation-load mismatch by seeing the sudden frequency or voltage variations. In the above example, the town's load just happened to match the generator output (12 MW) so the generator operated islanded on the town for a few hours. No one knew until the boiler fan VFD's tripped due to the frequency drift. The operator dumped the generator breaker and was surprised to see all the lights in the plant and town go out.

 

[rmw]

The university I attended (and where I worked in the power house as a student worker) had portions of the campus that were islanded, and portions that were supplied by the grid. The frequency control of the islanded generator was two clocks on the control panel side by side, one on each system.

Hourly, an adjustment made was to tweak the frequency of the islanded system to keep that clock running consistent with the clock tied to the grid.

Once, after a boiler trip, and several unsuccessful attempts to restart the boiler, it ran out of steam and the turbine slowed down enough so that the lower frequencies were clearly visible in the plant lighting before the operator thought to trip the generator.

rmw

 

[ScottyUK]

Have a Google for "ROCOF relay" - abbreviation for "rate of change of frequency relay" and for "vector shift relay". These relays are typically used in the UK to detect loss of grid infeed to a system, and initiate islanding of the generator. If a generator does not island itself from a large system which has lost its grid infeed, the generator will usually be unable to support the load and will stall or trip on overload.


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