Dc Resistance Test; Principle - Raytech mini-ATOS Instruction Manual

4.2 DC Resistance Test

4.2.1 Principle

The winding DC resistance test is one of the most important tests. Many power transformer faults
such as shorted turns, tap changer problems and loose connections can be detected.
Figure 4 represents the summarized winding resistance measurement circuit of the mini-ATOS. It
utilizes the 4-wire measuring technique for each measurement. 2 wires to inject current and 2 wires
to measure voltage. The test measuring cables and clamps are Kelvin type which means that the
user does not have to hook up separately additional cables to the transformer for measuring.
Figure 4: Summarized schematic of mini-ATOS winding resistance measuring circuit
The mini-ATOS has two power supplies with the same output power which can be used to
measure both high voltage side and low voltage side winding simultaneously.
The maximum output voltage and current of each power supply are different to provide optimal
output for each side. One power supply has more output voltage which is suitable for the high
voltage side with a bigger resistance and the other one has more output current which is suitable
for the low voltage side with a smaller resistance.
During the measurement, because of the high inductivity, the core and winding must be charged
and only after charging, stable and accurate measuring is possible. The following equation
represents the relation between the current injected into the winding, ��(��), and the voltage drop
value, ��(��), across the winding. �� is the winding resistance value that is aimed to be measured
and �� is the equivalent value of the inductivity.
����(��)
��(��) = ��. �� ( �� ) + ��.
����
The voltage drop value consists of two parts; the first part is the voltage drop because of the
resistance, and the second part is the voltage drop because of the inductivity. After charging, the
second part tends to zero and then the Resistance value can be calculated:
����(��) ��
→ 0 ⇒ �� ( �� ) = ��. �� ( �� ) ⇒ �� =
���� ��
90297-0.22 Instruction Manual mini-ATOS Page 13 of 92