Correction Factor for Generator (Kg)

Dear Vishnu! You could have asked me directly. There are two reasons:

i) Whenever you specify a DG set, the DG set manufacturer - who would normally be the franchisee of a Diesel Engine Manufacturer - would offer his standard diesel engine but would give you a choice of various makes of alternators. The price of the DG Set would depend upon your choice of the alternator. Now, say, if you are from India, the DG Set manufacturer would give you a choice of either a Marathon Electric make alternator or a Stamford (UK) make alternator or a Leroy Somer make alternator or a Kirloskar Electric or Jyothi Make alternator. You may weigh the pros & cons of each of the above makes of alternators and decide your alternator. Now, if you select a Marathon Electric Alternator, which is US made, the design rated generator voltage (UrG) is 380V. If you choose a Leroy Somer alternator, it would be 400V; for Stamford (UK), it would be 420V & for Kirloskar Electric or Jyothi, it is 415V. Now, whichever alternator you choose, the moment it reaches your site in India, you would tune the AVR of the alternator to deliver 415V. Wouldn't you? Thus, you would have meddled with the design rated generator voltage. Now, after all, the design impedance parameters of an alternator are based on the design rated generator voltage. But, the moment you meddle with the alternator voltage, the impedance parameters would also change. So, for a design generator voltage 'UrG', the generator resistance was 'Rg'; thus, for the readjusted system nominal voltage 'Un', it would be{ (Rg/UrG) x Un} or ( (Un/UrG) x Rg}; so also for 'Xg'. This is the reason for the term (Un/UrG) in the correction factor (Kg) formula.

ii) Then, you might ask, if 'UrG' & 'Un' are same, like in the case of Kirloskar Electric or Jyothi Alternators, in the above case in India, should we not consider any correction factor, as (Un/UrG) is '1'?.

Now, the generator resistance itself is told as a fraction of the generator's subtransient reactance (x"d). During the sub-transient period the alternator is generating sub-transient emf and thus the impedance parameters are given by the alternator manufacturer for this sub-transient emf. The sub-transient emf of the alternator is the rated generator voltage PLUS teh subtransient reactive voltage drop within the alternator (i.e.) UrG (1 + x"d Sin phi); This is the denominator in the correction factor formula. And, of course, you know that "C" is the voltage factor, which is 1.05 for LV and 1.10 for HV.

Hope you understand.