Forcing Ratio is one of the most important parameters when designing an excitation system. Forcing ratio and Vp has not to be confused.
Let’s begin with a few definitions:
Eef_base: this is the field voltage at the AVR output when the generator is at rated voltage in open circuit on the air gap line.
Eef_rated: this is the field voltage at the AVR output when the generator is at rated MW and rated MVAR. The ratio between Eef_base and Eef_rated depends on the characteristics of the synchronous machine (Xd and Xq).
Eef_forcing: this is the maximum field voltage at the AVR output. The maximum field voltage depends on the excitation system power supply (voltage level and number of phases).
The Vp:
In the math model of the Basler DECS range (DECS-100, DECS-200, DECS-200N, DECS-250, DECS-300 and DECS-400), the Vp is the ratio between Eef_base and Eef_forcing.
Common value for Vp: 4 to 8.
The forcing (initial response):
The forcing is the ratio between Eef_rated and Eef_forcing.
Common value for the forcing: 1.5 to 3. For grid connected machines, the NER require a forcing of 1.6 or more for indirect machines (such as brushless) and a forcing ratio of 2.3 or more for direct machines (static excitation systems).
The forcing ratio also have a large effect on the capability of a generator to recover from remote fault and keep synchronism.
If you need more details about the forcing ratio & Vp, don’t hesitate to contact us.