Governor Gate Timing

[esee135]

Hydro generator question, since gate timing is set for different opening and closing rates, is the governor speed response to a SYSTEM disturbance significantly favored in the underfrequency case more than the overfrequency case? I wonder if an IEEE paper has been written on this?

 

[wbd]

The speed of the governor closing the wicket gates is timed to not overly stress the penstock. System disturbances have no bearing. If the unit is tripped for any reason, the gates closing time will be set so that the penstock can withstand the shock of the water.
Opening rates are dependent on the response of the governor system to an open command.

 

[esee135]

Understood, in our low head plants we tune the closing rate to avoid excessive scroll case pressure, synonymous to the penstock collapse situation that you're refering to.

In the case of the mechanical governor, I was wondering if the closing rate which is a physical motion limitation (maximumum distributing valve opening in the main servo close direction), cause a decrease in the overfrequency response, as compared to the underfrequency case since the distributing valve in the open position usually has more travel?

 

[wbd]

I'm not entirely clear on what you are asking about.

If the unit is tied into the grid, the frequency is set by the grid. No matter how fast or slow the governor operates the wicket gates, the only affect will be how fast or slow the loading is.

With the unit not tied to the grid, the speed of the unit will be set by where the gate position is. Obviously, faster gate operation will cause the unit to change speed faster.

 

[tmoritz]

As was stated earlier, closure rate is determined by the penstock/scroll case allowable pressure. Too fast and water hammer can cause a failure.

As for opening rate, this is a function of the torque available and the inertia of the machine. If you could open the gates instantaneously the unit would still ramp up to speed almost linearly due to the inertial constraints. The problem comes in as you approach sync speed. Now you have to control overshoot. Wicket gate opening rates are generally established so the system is over damped to prevent overshoot.

Realize, the limiting factor in unit control generally isn't gate speed it's the characteristic time of the unit as a whole due to inertia. There's also fluid dynamic limitations, close or open the gates too quickly and you'll probably notice more cavitation damage at your wicket gates, stay vanes, and turbine blades/buckets.



Tom Moritz
Mechanical Engineer
US Bureau of Reclamation