Generator MVA Rating 2

[CuriousElectron]

Hi Gents,
For a Hydro generator, I see that typically nominal MVA rating of the generator corresponding to 60 degree Celsius rise is shown on facility single-line drawing. Generators that have been rewound in the past 30+ years typically specify 60 deg. C and 80 deg. C temp. rise rated stator winding insulation. Does anyone know why we typically see the 60 deg. C MVA rating shown on the station prints? This appears to be the convention in the Utility. Is this because the generators are typically operated at 60 deg. C temp. rise and it is not desirable to exceed this rating in accordance with the unit capability curve?
Thanks for feedback.
EE

 

[waross]

If you raise the temperature rise from 60 deg. to 80 deg the maximum allowable current will be higher. More Amps means a higher MVA.
Rating at 60 deg. provides a common base for MVA ratings.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

CuriousElectron: Think about when most of the large-scale hydro units were first built (1920s through 1950s). Back then, the modern Class F and Class H materials were not around for insulating. That meant the windings had to stay below the breakdown ratings for Class B material (or in some of the REALLY old ones, Class A). Class B has an absolute TOTAL temperature of 130 C. Take away 40 C for a "maximum ambient" condition, and another 25 C for the "winding hot spot" (since we can't measure the hottest spot in the coil directly), and we're down to 65 C. Thus the limitation to 60 C maximum allowable rise, with a few degrees margin since designerse weren't necessarily calculating everything quite so rigorously.

Modern insulation systems use Class F and H materials, so they can have higher rises and higher total temperatures. As an added bonus, they also tend to use up somewhat less space, so we can squeeze in a bit more conductor material as well. This leads to being able to handle more current - which, at the same voltage level, means more power (MVA) production capability.

Converting energy to motion for more than half a century

 

[dpc]

There are other factors to consider in addition to the insulation rating of the windings. Bearings may run hotter at higher loads. In addition the excitation system must be capable of providing the additional current. And of curse, the turbine, gates, penstocks, surge tanks, etc must be able to operate at the increased water flow required.

 

[waross]

Would some of your factors apply more to MW than to MVA, David?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

Would some of your factors apply more to MW than to MVA..

You're right on the mechanical side, but if the generator was re-rated up to 80 MVA, unless the power factor on the nameplate was changed, it would generally be assumed to be capable of operating at 80 MW at unity pf.

 

[waross]

Thanks for the reply. Your points are valid.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!