Dynamic Resistance Measurement (Drm) - Raytech mini-ATOS Instruction Manual

4.6 Dynamic Resistance Measurement (DRM)

The Onload Tap Changer plays a critical role in numerous power transformers, as it is responsible
for regulating the output voltage by selecting different winding taps while the transformer is
operating under load. Due to the presence of moving components, tap changer failures often
contribute to the malfunction of power transformers.
To address this issue, the mini-ATOS offers a comprehensive solution by effectively testing the
dynamic resistance of the tap changer with a remarkable level of accuracy. With a fast-sampling
rate of 10ksps, the mini-ATOS ensures precise measurements. Additionally, as a valuable
diagnostic tool for OLTC, it allows for the measurement of the tap changer's motor drive current
(Please see 4.7) when used in conjunction with an optional current clamp.
In order to analyse the switching process of onload tap changer, the mini-ATOS injects a desired
DC current and record it during each transition. The mini-ATOS can perform DRM on resistive type
OLTCs. Transition resistors are used in an onload tap changer to limit the circulating current in
transformer winding during tap transitions. Because of these transition resistors, there is a current
drop during switching process which can be investigated by DRM test.
Typical DRM curve of a resistive type OLTC can be seen in Figure 6. The mini-ATOS calculate the
parameters listed in Table 3 for each DRM curve.
Parameter Unit
Tdrm [ms]
Idrop [%]
[A/s] or
Slope
[%/ms]
90297-0.24 Instruction Manual mini-ATOS
Table 3: DRM parameters for each transition
Time when transition resistors are carrying current
Maximum current drop during transition
The rate of the current drop in A/sec or %/ms at the moment of
Preliminary
Description
breaking the first tap
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