what will happen to a synchronous generator if it operates at lower (or higher voltage) than 5% of its rated voltage over a long period?

IEC 60034-1, chapter 7.1 gives the frequency and voltage limits during operation of rotating electrical machines. Operating zones A and B are defined. E.g. generators that run within a voltage range of +/- 5% and frequency range +/- 2% of the ideal operating point operate in zone A. Zone B lies within +/-8% voltage and +3% and -5% of the ideal operating point.

It is stated that:

"A machine shall be capable of performing its primary function within zone B, but may exhibit greater deviations from its performance at rated voltage and frequency than in zone A. Temperature rises may be higher than at rated voltage and frequency and most likely will be higher than those in zone A. Extended operation at the perimeter of zone B is not recommended."

I am specifically interested in the physical reason for the voltage limitations in synchronous generators. E.g. what will happen to a synchronous generator if it operates at lower (or higher voltage) than 5% of its rated voltage over a long period? What is the reason for these narrow voltage limitations?

Thank you so much for your help!

 

Dear,

If you operates a generator at a higher voltage than it was designed, you are aging the isolation (isolation aging faster at higher voltage).
If you operates at a lower voltage, you increase the current for the same load, then you are heating the generator and aging the insulation too.

It is supposed that the generator life is calculated and will be longer if you operates inside the limits.

 

krigl,

My response is going to be very simple, but is nonetheless probably one of the most important considerations.

Running a synchronous generator at high terminal voltages (relative to a nominal bus voltage) requires lots of field current. This produces excess heat and expansion in the generator rotor and windings.

There's also the effects of volts/Hz on the generator which have to be considered based on generator construction.

Running a synchronous generator at low terminal voltages (relative to a nominal bus voltage) results in stator end turn heating.

The reactive capability curve ("D curve") for every synchronous generator has three distinct lines--all of which reflect the limits of the ability of the generator to withstand the effects of excess heating due to three distinct causes (high rotor current; high stator current; high end turn current). Generators are rated primarily based on their ability to be cooled--to withstand the effect of heating and to sufficiently continuously remove heat to maintain insulation integrity and prolong generator life without the need for more frequent rewinds and maintenance.

Hope this helps! There's lots written about the effects of exceeding reactive capability curves, as well as the effects of exceeding volts/Hz ratings. Use your preferred Internet search engine to research both.