B. Formulas to determine eddy losses and total losses where the transformer load losses (PLL) are to be determined as follows:
PLL=PDC(1+K(PEC))
where:
PDC = the total I2R losses
K= the K-factor rating at the transformer (4, 9, 13, 20, 30, 40 or 50)
PEC= assumed eddy current losses calculated as follows:
for transformers rated
300 KVA or less, and
For transformers rated
more than 300 KVA
in which:
PAC = the impedance loss
C=0.7 for transformers having a turn ratio greater than 4:1 and having one or more windings with a current rating greater than 1000 amperes, or C=0.6 for all other transformers
PDC-I =the I2R losses for the inner winding
The impedance losses and the I2R losses shall be determined in accordance with the Test Code for Dry Type Distribution and Power Transformers, ANSI/IEEE C57.12.91-1979.
As Stated in ANSI/IEEE C57.110-1986
Harmonic load currents may be accompanied by DC components in the load current which are frequently caused by the loss of a diode in a rectifier circuit. A dc component of load current will increase the transformer core loss slightly, and may increase the magnetizing current and audible sound level .
Relatively small dc components (up to the RMS magnitude of the transformer excitation current at rated voltage) are expected to have no significant effect on the load carrying capability of a transformer determined by this recommended practice. Higher dc load current components may adversely affect transformer capability and must be corrected by the user.
Harmonic currents flowing through transformer leakage impedance and through system impedance may also produce some small harmonic distortion in the voltage waveform at the transformer terminals. Such voltage harmonics may cause extra harmonic losses in the transformer core. However, operating experience has indicated that core temperature rise usually will not be the limiting parameter for determination of safe magnitudes of nonsinusoidal load currents.
Harmonic load currents may be accompanied by DC components in the load current which are frequently caused by the loss of a diode in a rectifier circuit. A dc component of load current will increase the transformer core loss slightly, and may increase the magnetizing current and audible sound level .
Relatively small dc components (up to the RMS magnitude of the transformer excitation current at rated voltage) are expected to have no significant effect on the load carrying capability of a transformer determined by this recommended practice. Higher dc load current components may adversely affect transformer capability and must be corrected by the user.
Harmonic currents flowing through transformer leakage impedance and through system impedance may also produce some small harmonic distortion in the voltage waveform at the transformer terminals. Such voltage harmonics may cause extra harmonic losses in the transformer core. However, operating experience has indicated that core temperature rise usually will not be the limiting parameter for determination of safe magnitudes of nonsinusoidal load currents.
The grounded shield between the primary and secondary windings is called an electrostatic shield. This shield does not perform any function with regard to harmonic current or voltage distortion wave forms. However the shield is extremely valuable in protecting sensitive equipment from common-mode electrical noise and transients generated on the line side of the transformer.
The ratio of the common mode noise attenuation (CMA) on the input to that of the output of the transformer is expressed in decibels as shown in Equation 3. An isolation transformer with an electrostatic shield can have a ratio of input noise voltage (VIN) to output noise voltage (VOUT) within the range of 10:1 to 1000:1 or even higher.
