Hydro Generators: Braking Systems

[21121956]

Hello everybody:

In some hydroelectric plants, AC generators have two braking systems: a pneumatic (used as principal) and an electrical braking (used as backup).

It is obvious the purpose of specifying an electrical braking system as backup but, what approach is followed in the power plants where is available only the traditional pneumatic braking system? This implies also ask what approach is followed in the power plants where both systems are specified?

The advantages and / or disadvantages of having one system or the two seem obvious, but it might not be.

I appreciate your comments.

By the way, Happy New Year!


El que no puede andar, se sienta.

 

[crshears]

Hey there 21121956, Prospero Año 2015!

By "electric braking" I'm ASSuming you mean completely separate electro-mechanical or electro-hydraulic back-up braking and NOT dynamic or regenerative braking...

None of the hydraulic plants I worked in had such mechanically discrete back-up brakes, although one of them had a last-ditch means of providing emergency braking upon loss of pressurized air by injecting hydraulic oil into the pneumatic system. The reason this was considered last-ditch was that only the preclusion of wiping bearings [so as to avoid extended unit overhauls] was considered sufficiently important that it warranted the need to tear down / rebuild / flush the braking system clean post-event with a shorter and less invasive outage. The imperative to not wipe bearings was so great that if braking a unit to rest clearly wasn't succeeeding the operators were to release the brakes and re-accelerate the unit to sufficient rotative speed to maintain an oil wedge in the bearings.

In that same plant, some of the units were equipped with jacking oil / oil lift pumps; for these generating units the possibility of wiping bearings was completely eliminated [provided the lift pumps didn't fail, of course], and the only remaining need for a braking system was to bring the unit to rest to establish an outage condition.

Most typically, the only reason a unit could not be brought to rest was if there was débris stuck between the wicket gates [if that had not already caused a shear pin to snap], and less commonly due to not being able to establish enough wicket gate 'pinch' due to linkages having gone, or having been forced out of, adjustment, or because of loss of actuating medium pressure.

Although I never worked in them, a few of our company's hydraulic plants had units that incorporated dynamic braking as part of the shutdown sequence, the rationale being that the dynamic braking system would perform the lioness' share of slowing the unit down, with the mechanical braking system configured to be applied at some pre-determined point during the deceleration process when the dynamic braking had lost a proportion of its effectiveness. As a consequence the brake shoes didn't heat up or wear nearly as badly, so they lasted longer. The overriding concern was to balance the desire to bring the unit to rest quickly with not overheating the brakes, hence the delicate balancing act.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[21121956]

Hello everybody:

Thanks CR for your insights.

The generators are equipped with both electrical and pneumatic brakes. Normally, mechanical braking is frequently used to stop quickly the hydro-generator, to bring a turbine at a standstill. These braking functions on a principle of mechanical friction which slows down a rotating mass of the generator.

In case of peak-load and pump-storage power plants where frequent unit starts and stops are present, and where a normal time required for stopping the generator is long, the mechanical braking, due to its characteristic, does not represent entirely the most convenient solution. In such cases it is better to use electric braking.

The electrical brake and pneumatic brake are typically applied at 50% and 5% of designed speed or below respectively.

In some arrangements, in case of failure of the electrical brake or electrical faults, the pneumatic brake shall be only applied at 30% of designed speed or below.

Once again, thank you CR.

El que no puede andar, se sienta.