MEASUREMENT OF INDIPENDENCE VOLTAGE AND LOAD LOSSES
   
 
PURPOSE OF THE MEASUREMENT
 

This measurement is carried out to determine the load-losses of the transformer and the impedance voltage at rated frequency and rated current. The measurement are made separately for each winding pair (e.g., the pairs 1-2, 1-3 for a three-winding transformer) and if required on the extreme tapping also.
   
 

APPARATUS AND MEASURING CIRCUIT
The measurement is performed by means of volt-ammeter method.
 


Fig. 3.1
 Circuit for impedance and load-losses measurement.
G1 supply regulator, T1 transformer to be tested, T3 current transformers, P1 wattmeters, P2 ammeters (r.m.s. value), P3 voltmeter (r.m.s. value).
 

Current is generally supplied to the H.V. winding and the L.V. winding is short-circuited.
   
 
PERFORMANCE OF THE MEASUREMENT
 

The voltage of the supply regulator is raised until the current has attained the required value (25...100% of the rated current according to the standard). In order to increase the accurancy readings will be taken at several current values near the required level. If a winding in the pair to be measured is equipped with an off-circuit or on-load tap-changer, the measurement is carried out on the principal tapping and if required on the extreme tapping also. The readings have to be taken as quickly as possible, because the windings tend to warm up due to the current and the loss value obtained in the measurement are accordingly too high.

If the transformer has more than two windings all winding pairs are measured separately.
   
 
RESULTS
 

If the measuring current Im deviates from the rates current IN, the power Pkm and the voltage Ukm at rated current are obtained by applying corrections to the value Pc and Uc relating to the measuring current. The corrections are made as follows:
 
(3.1)
   
(3.2)
  Mean values are calculated of the values corrected to the rated current and the mean values are used in the following. According to the standards the measured value of the resistances and losses shall be corrected to a reference temperature of 75 °C for transformers having A-E-B insulation class and of 120 °C for transormers having F-H or C insulation class. The transformer is at ambient temperature when the measurement are carried out, and the loss values are correced to the reference temperature according to the standards as follows.
   
  The windings resistances values are corrected at the same reference temperature of the losses in AC:
 
(3.3)
   
(3.4)
  = measuring temperature of the losses in AC
= measuring temperature of the resistance
  km = 235 for copper
km = 225 for aluminium
  The d.c. losses P0m at the measuring temperature are calculated using the resistance value R1m and R2m:
 
(3.5)
   
  The additional losses Pam at the measuring temperature are
 
(3.6)
   
 

Here Pkm the measured power, corrected to the rated current according to Equation (3.1)

The short circuit impedance Zkm and resistance Rkm at the measuring temperature are

 
(3.7)
   
(3.8)
  Ukm is the measured short-circuit voltage corrected according to Equation (3.2); UN is the rated voltage and SN the rated power. The short circuit reactance Xk does not depend on the losses and Xk is the same at the measuring temperature () and the reference temperature (75°C or 120°C), hence
   
 
(3.9)
   
  When the losses are corrected to 75 °C or 120 °C, it is assumed that the d.c. losses vary directly with resistance and the additional losses inversely with resistance. The losses corrected to 75 °C are obtained as follows:
   
 
(3.10)
   
  while the losses corrected to 120 °C are obtained as follows:
   
 
(3-11)
  = 235 °C for Copper - 225 °C for Aluminium
   
  Now the short circuit resistance Rkc and the short circuit impedance temperature can be determined:
 
(3.12)
   
(3.13)
   
 
RESULT
  The report indicates the following values corrected to reference temperature and relating to the principal tapping and if required, to the extreme tappings also.
 
   

d.c. losses P0c

 (PDC)
additional losses Pac (PA)
load losses Pkc (PK)
short circuit resistance Rkc (RX)
short circuit resistance Xkc (XK)
short circuit resistance Zkc (ZK)