BS IEC 60076-21:2011 pdf – Power transformers Part 21: Standard requirements, terminology,and test code for step-voltage regulators.
The impedance voltage of a oltage regulator is the voltage required to circuLate rated current through one of two specified windings when the other winding is short circutted while in a specified tap position. Impedance voltage is usually expressed in per unit or in percent of the rated voltage of the windtng across which the voltage is applied and measured. The impedance voltage comprises a resistive component and a reactive component The resistive component of the impedance voltage, called the resistance drop. is in phase with the current and corresponds to the load losses, The reactive component of the impedance voltage, called the n’acsance drop, is in quadrature with the current and corresponds to the leakage-tins linkage’s of the windings. The impedance voltage is the phasor sum of the two components. The impedance voltage is measured during the load loss test by measuring the voltage required to circulate rated current in the windings. The measured voltage is the impedance voltage at the temperature of the test and the power loss dissipated within the voltage regulator is equal to the load losses at the temperature of the test and at rated load. The impedance voltage and the load losses are corrected to a reference temperature using the equations specified in
The maximum impedance voltage of a step.-voltage regulator generally will he less than 0.6% of the rated voltage, stated on the circuit ILVA base. Maximum impedance occurs at vanous tap positions depending on the design type and rating. Impedance is minimal in the neutral position. The impedance voltage will vary with tap position and may he somewhat higher for a two-core design (series translhrmcr).
The impedance kVA is the product of the impedance voltage across the energized winding in kilovolts times the winding current in amperes. The ratio of the load losses in kilowatts at the temperature of test to the impedance kVA at the temperature of test is the load loss power factor of the voltage regulator during the test and is used for correction of phase-angle error as specified in 8.4.7.
8.5.2 Factors affecting the values of load losses and impedance voltage
The magnitudes of the load losses and the impedance voltage will vary depending on the voltage regulator tap position. These changes are due to the changes in the magnitudes of winding currents and associated leakage-flux linkages, as well as changes in stray liux and accompanying stray losses. In addition, several other factors affect the values of load losses and impedM ‘evoltage of a voltage regulator. Considerations of these factors, partly. explain variations in load loss values and impedance voltage for the same voltage regulator under dtli’erent test conditions, as well as variations between load loss values and impedance voltage of different voltage regulators of the same design. These factors are dLscussed in tt5.21 through
11.523. Design
The design-related factors include conductor material, conductor dimensions, winding design. winding arrangement. sltietding design. and selection ot’ structural materials. Process
The process-related factors that impact the values of load losses and impedance voltage are tlse dimensional tolerances of conductor materials, the final dinwnsions of completed windings. phase assemblies, metallic pans exposed to stray flux, and variations in properties of conductor material and other metallic parts. Temperature
Load loss values are also a function of termperaturc. The I’R component of the load losses increases withtemperature,whereas the stray loss component decreases with temperature. Procedures for correcting theload losses and impedance voltage to the standard reference temperature are described in
8.5.3 Tests for measuring load loss and impedance voltage Preparation
The following preparatory requirements shall be satisfied for accurate lest results:
)To dctcrmine the temperature of the windings with sufficient accuracy, the following conditions
shall be met, except as stated in the NOTE below:
l)The temperature of the insulating liquid has stabilized,and the difference between top and
bottom fluid tempcratures docs not exceed 5 C.
2)The temperature of the windings shall be taken immediately before and after the load losses
and impedance voltage test in a manner similar to that described in 8.1.1.The average shall betaken as the true temperaturc.
3The difference in winding temperature before and after the test shall not exceed 5 C.
NOTE—For voltage regulators, where it may not be practical to wait for thermal equilibrium, the methodused to determinc the winding temperature shall take into consideration the lack of thermal cquilibrium andthe effect of ohmic heating of the winding conductors by load current during the test. The method used can bevcrified by staging a repeatcd measurcrment of the load losses and impedance voltage at a latcr time whenconditions 1y through condition 3) are met.