Excitation Transformer Sizing

[luisbitron]

Can any one help me. I am preparing a calculation sheet for sizing an Exitation transformer, so far I have this
Generator Data:
Rated output: Sgn = 108 MVA
Rated voltage: Un = 13.8 kV
Rated frequency: f = 60 Hz
Rated speed: n = 276.9 rpm

Preliminary Assumed Excitation Data:
Excitation power: Pfn = 540 kW | 0.5% Sn
Rated excitation voltage: Ufn = 320 V | adopted value
Rated excitation current: Ifn = 1687.5 A | Pfn/Ufn

Maximum excitation voltage: Ufmax = 640 V | 2.0*Ufn
Maximum excitation current: Ifmax = 1856.3 A | 1.1*Ifn

Corresponding values on the ac-side:
Uac = 547.4 V | 1.05*1.1*[?/(3?2)]*Ufmax
Iac = 1515.6 A | ?(2/3)*Ifmax
S = 1436.9 kVA | ?3*Uac*Iac

Excitation Transformer
Rated output: Sn = 1500 kVA | adopted value
Rated voltage at LV side: Uac = 550 V | adopted value
Rated current at LV side: Iac = 1574.6 A | Sn/(?3*Uac)
Rated current at HV side: In = 62.8 A | Sn/(?3*Un)
Rated CT ratio at HV side: 100/5 A | adopted value


My question is if there is a method to calculate the Rated excitation voltage, normally this value is given by the manufacturer. But I need to now hoy they calculate this value.

Thanks

 

[3I0]

The power rating of an excitation transformer is based on the steady state power requirements, plus derating factors for the high harmonic load. The static excitation systems I have been exposed to have a 6-pulse front end on them, so present a lot of 5th/7th/etc harmonics.

The voltage rating is based on the positive/negative ceiling voltage requirements set forth by your relevant grid codes. Ceiling voltages can be calculated if you want to get into power electronics theory for the thyristor bridge, taking into account the firing angles and transformer impedance. For a unit this large I would recommend working with the excitation system/transformer vendor to ensure you don't have any surprises.