How to prepare the specification of a 80/100MVA ONAN/ONAF transformer

Tap changing gear
i) Type
ii) Provided on
iii) Tap range
iv) Step Voltage
v) Rated voltage of on load tap changer
vi) Rated current of on load Tap changer
Over voltage operating capability and duration

Max flux density in any part of core and yoke at rated MVA ,normal voltage(Tesla)
Current density of HV /LV Windings
Insulation Levels for windings
a) 1.2 /50 micro – second wave shape impulse withstand (kVp)
b) Separate source withstand voltage (kV rms)
c) Power frequency voltage withstand (kVp)
Type of winding insulation
a) HV
b) LV winding
System short circuit level and duration for which the transformer shall be capable to withstand thermal and dynamic stresses (kA rms /Sec)
Noise level at rated voltage and frequency
Permissible temperature rise over ambient temp of 50 DC
i) Of top oil measured by thermometer
ii) Of winding measured by resistance
Minimum clearance in air (mm)
a) Phase to Phase
b) Phase to ground
c) Phase to Phase
d) Phase to ground
Terminals :
a) HV winding Line end
b) HV winding Neutral end
c) LV Winding
Bushing current rating
Max Radio Interference voltage level at 1 MHZ & 1.1 times max rms phase to ground voltage for HV Terminal Connector
Cooling Equipments
a) Number of Banks
b) No of Pumps
c) No of fans
Insulation level of bushings
a) Lightning impulse withstand (kVP)
b) Switching impulse withstand (kVP)
c) 1 Minute power frequency withstand voltage (kV rms)
d) Creepage distance (mm)
Bushing current transformers for purchasers use

(for restricted E/F protection)
i) Type
ii) Quantity
i) No of Cores
iii) Current Ratio (A/A): (A/A)
ii) Turn ratio
iv) Knee Point Voltage
v) Accuracy class
iii) Location for mounting
HV Neutral side CT for owners use
iv) Type
v) Quantity
vi) No of Cores
vii) Current Ratios (A/A)
viii) Turn ratio
ix) Knee point voltage
x) Class of accuracy
xi) Location for mounting
Bushing CT on LV side for load measurement
i) Type
ii) Current ratio
iii) Burden in VA
iv) Accuracy class
v) No of cores
Maximum acceptable (without any positive tolerance) no load loss (Iron Loss )at rated voltage and at rated frequency at 75 deg C in KW
Maximum acceptable (without any positive tolerance) load loss including cooler loss because of power consumption by the cooler fans ,motor pump sets etc at rated current and at rated frequency at 75 deg C in KW
Maximum acceptable (without any positive tolerance) Auxiliary Loss in KW

How to achieve all those.Pls guide me in the calculation part
asked 9/24/2011

1 Answer

Each of these is a reasonably involved calculation (or bit of research). Many of the parameters (fault ratings for example) will be a result of computer modelling of the system. Impulse and insulation levels are from standards and depend on the insulation coordination you are trying to achieve. CT parameters are calculated dependant on the system, relays and other devices. The IEC and BS transformer standards give guidance on specifying transformers (and for an 80/100 MVA transformer I think it is worth having a good understanding of these).

I think at this stage (for the project) someone should be using software to model the system, understanding the relevant standards, understanding the operating requirements of the transformer (and associated switchgear, tap-change control, etc). This all goes together to form the requirements of the specification. If you company is regularly involved in power system design, the would like also have existing specifications which could be modified for your case.
answered 9/26/2011 Steven McFadyen 246
Steven McFadyen

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