A dynamic D-curve (capability curve) was recently built for an older 160 mVA generator. Dispatched, it's in spinning reserve at ~30 mW most of the time. At low loads it is often above (lagging PF) the 0.85 PF upper limit of the manufacturer's D-curve. Nobody saw this before the dynamic D-curve was built. The generator, although almost 50 years old, is in good shape. There is a shorted turn in the far end of the rotor, but the stator appears to be fine. Vibration is not a problem. It is an asphaltic mica, H2-cooled circle-bar-W and has not been rewound. It has never faulted or overheated. Should VARs be kept inside the PF lines of the D-curve?

 

Road Surfer... the data you provided indicates that max design kVAr (lagging) is about 136 (omitting any deliberate margin of error). Then, if max spinning reserve load is limited to 30MW, the machine produces about 139MVA. The corresponding armature current is about 87% of design, which provides an ample safety margin.

However, the question to resolve is... what are the current magnitudes, and their duration excursions, above the D-curve limit? Remember, damage is cumulative. Thus, to obtain a definitive limit, integration of the equation (I^2) x Time is necessary!

Regards, Phil Corso (cepsicon [at] aol [dot] com}

 

Thanks for your comments, Phil. The I^2*T comment got me thinking about V-curves, relating generator current to exciter current. At low loads the PU current is so low that heating is not an issue. I mistakenly took the PF lines in the D-curve as some kind of secondary limit. The only purpose of the PF lines in the D-curve are to show which region of the capability curve the operating point is in. Operating points at any PF won't be of concern until they exceed the D-curve limits. Thanks again for making me think and answering my question. If I could attach a picture to a post you likely would have answered my question right away.