State calculations for under/over excited synchronous generators

[constantins]

I am currently analysing the process model code for a synchronous generator simulation. The model calculates the internal EMF of the machine under normal operating conditions (i.e. operating within its capability curves) as a function of the Automatic Voltage Regulator's (AVR) internal setpoint (set to 22kV for normal operation). Reactive power is calculated using the standard text book formula involving torque angle.

Once the generator's operating point falls outside its described capability curves, however, these calculations change to some abberation I do not fully understand. Briefly, Qn and EMF is calclated as:

Qn = Qg + Qo/0.0125
En = Eo - 0.0125(Qo - Qn)

Where En is the current EMF, Eo was the EMF when the machine became over/under excited, Qn is the current reactive power, Qo was the reactive power when the machine became over/under excited, and Qg is the grid reactive power. (!?)

My question is: does the generator's internal EMF and reactive power have to be calculated differently when the the generator is operated outside of its capability? Does the AVR have no effect on a generator operating outside of it's capability? If so, what are these calculations and what are the underlying reasons for this.

Many thanks
Constantin

 

[jbartos]

Suggestions marked by ///\\\:
Qn = Qg + Qo/0.0125
En = Eo - 0.0125(Qo - Qn)
///Normally, engineering equations must have their dimensional fit. It appears that you have voltage and power in the equation with constants that need clarifications. Else, these would be ill posed relationships.\\My question is: does the generator's internal EMF and reactive power have to be calculated differently when the generator is operated outside of its capability?
///The internal EMF and Reactive Power outside of the generator capability can be almost anything since the generator is not rated for such operation on continuous basis.\\Does the AVR have no effect on a generator operating outside of it's capability?
///The generator is supposed to trip its load outside of its capability on continuous basis. When the given load is thrown off, the machine being disconnected from the infinite busbar but the speed and excitation remaining unchanged. The regulation is the voltage rise at the terminals when the load is thrown off. The rise is the scalar difference between Eint and Vterm and the regulation is defined as
e=(Eint-Vterm)/Vterm in per units (p.u.). The regulation depends on the load and power factor.\\ If so, what are these calculations and what are the underlying reasons for this.
///As mentioned above and in Reference, for example:
M. G. Say, "Alternating Current Machines," John Wiley & Sons, 1978, Chapter 10 "Synchronous Machines: Theory and Performance."\\