Transformer Design - IEC Vs ANSI/IEEE

  

TRANSFORMERS DESIGN PARAMETERS
IEC	ANSI/IEEE
For transformers up to 10 MVA, values of rated power roughly follow the R-10 series [3] given in ISO 3 (1997), preferred numbers : (…100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, etc.)	Reference ANSI / NEMA C57 standard for power and distribution transformers, Standard ratings for different cooling methods OA / FA 55 / 650 C, and for standard impedance.
Commonly International practice is to use Sealed tank transformers up to 1600 kVA and Conservator type for larger transformers	American typical common practice in Petroleum & Chemical (P&C) applications is to use sealed tank transformers up to 15-20 MVA and conservator type for larger transformers.
Shall be capable of sustaining a maximum of 105% rated Volts/Hz continuously without damage	Shall sustain 110% Volts/Hz at no-load and shall be able to deliver rated power at 110% of the rated secondary voltage at a power factor of at least 80% without overheating.
IEC gives no requirement for direction of power flow.	ANSI standards state that transformers shall be for step-down operation unless otherwise specified.
Transformers with a specified nominal impedance Z ³ 10% allowable tolerance ±  7.5% Z < 10% allowable tolerance ± 10%	Allowable tolerance for the impedance of liquid filled transformers is ± 7%.
Number of shots of short-circuit withstand current is 3	Number of shots of short-circuit withstand current is 6 (first two shots are fully offset).
IEC 60076-5 (kVA) Category I -              25-2500 Category II -            2501 – 100 000 Category III -           > 100 000	ANSI/IEEE C57.12.01(kVA) Category I -           15-500 Category II -          501-5000 Category III -         5001-30 000
Each phase of transformer is subjected to a total 3 tests – in the position corresponding to high voltage,  Principal, Lower voltage tapping.	Each Phase of transformer shall be subjected to a total 6 tests – 4 times of symmetrical current and 2 times of asymmetrical current.
Duration of each test shall be 0.5 sec for category I. Duration of each test shall be 0.25 Sec for category II and III.	Duration of each test shall be 0.25 sec.

HIGHEST VOLTAGE Um (kV)	SHORT CIRCUIT APPARENT POWER
	IEC	ANSI/IEEE
7.2; 12; 17.5 and 24	500	500
36	1 000	1 500
52 and 72.5	3 000	5 000
100 and 123	6 000	15 000

CONDITION	IEC 60076-1	ANSI/IEEE C57.12.00
Ambient Temperature	40 deg. C	40 deg. C
24 h average Temperature	-	30 deg. C
Monthly Average Temperature	30 deg. C	-
Yearly average Temperature	20 deg. C	-