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Why does the frequency on a power grid increase when generation exceeds load?
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Mar15-12, 06:40 PM
I can help with the VERY basics.
There are some simple control systems at play and they interact.
Voltage is controlled by an electronic voltage regulator on each generator.
Turbine speed is controlled by a fast, high gain governor that will drive steam valves (hence power being generated) full travel over about a 3% speed change. Some are 5%.
To prevent a system collapse on loss of a generating unit, Load shedding relays in the system disconnect customers when frequency decays to certain frequencies, we had three stages i dont remember exactly but like 59.6, 59.5 and 59.4 sound familiar.
After a short time below ~58 hz turbines start disconnecting themselves to protect plant equipment.
Around 56 hz some nukes trip the reactor because the pumps need to run faster than that to assure cooling flow..
there's a lot going on.
Imagine now, in an overload you need more generation but if frquency falls low enough you trip your big generators possibly cascading into a blackout. Too much power flowing into a region that's deficient in generation wil trip transmission lines.....
In case of over-generation, the turbines would run down to zero power at about 61.8 hz for a 3% governor, 63 hz for a 5%.. Around 66 hz the turbine will trip itself offline to protect against centrifugal force.
Your basic answer is what gerbi said, power imbalance accelerates the machine (whole grid) in accordance with its inertia.
It's really that simple. But the interactions get interesting - soo many permutations.
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