2292 Winding Resistance Meter0 pages

OPTIMISED TOOL FOR THE MEASUREMENT OF WINDING

RESISTANCES ON POWER TRANSFORMERS

Marc Muller1

1Haefely Test AG, Basel, Switzerland

*Email: mueller.marc@haefely.com

Abstract: The topic of this paper is the analysis of the fundamentals of the winding resistance measurement

and their application for the development of an optimized tool. The ultimate goal is to create a portable

device, which measures a complete transformer the fastest and easiest way possible. The influence of the

charging voltage and the measuring current on the measuring time has to be analyzed. Another important

issue is the stabilization time on low ohmic delta windings. Further, testing time can be reduced by

introducing a demagnetization feature, which eliminates the need of applying high voltage AC after a DC

resistance test. Finally, the implementation of the optimized tool is presented with its efficient connection

scheme and the multi-channel architecture.

1 INTRODUCTION

Fast, efficient and accurate measurement of

winding resistances on large power transformers

creates several difficulties.

Long charging and discharging times, unstable

values on closed delta winding systems due to

long stabilization times, inaccurate temperature

measurements for resistance correction, residual

magnetism and its unwanted effects, inefficient

connection and disconnection of the measuring

equipment are just some of the difficulties to deal

with.

The paper describes an integrated, mobile

instrument developed to speed up the stabilisation

time when supplying DC to a transformer winding

by an intelligent magnetic-flux optimised charging.

With example diagrams of various transformers the

charging and stabilisation effects are shown.

After applying DC to a transformer, the core

remains magnetized. This can cause problems for

further measurements or reconnecting the

transformer to the grid. Thus, an integrated, low

voltage demagnetization function to bring a power

transformer into a defined, demagnetised state will

be shown.

Typical problems arising when connecting a

measuring system to a power transformer have

been solved. In particular these problems are time

consumption and faulty connections, which have

been solved by a fully integrated connection set

and a multiplexing circuit. Error possibilities in

setting up the measurement equipment have to be

faced and eliminated by offering state-of-the-art

graphical, self-explanatory user interfaces with

online information about all related values and

conditions in an easy and well-arranged way.

2 SUPPLY VOLTAGE AND CURRENT

2.1 General measuring principle

Figure 1 shows a schematic of a common winding

resistance measurement technique.

Figure 1: Winding resistance measuring principle

using a DC power supply

The instrument consists of a programmable power

supply, which is normally operated in constant

current mode. It is used to supply a user-defined

current into the DUT. Additionally, there is a

voltage and current measurement unit. Thus, the

resistance is calculated according equation (1):

(1)

2.2

Magnetizing inductivity LM

Also, Equation 1 shows the main difference

between a normal and a winding resistance

measurement. There is a large inductance in

series with the resistance of interest. This

inductance is typically in the range of L0 = 0.1 H to

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