Winding Resistance Measurements Using the 2293

Winding resistance measurements taken on transformers are essential for type, routine, and field testing. The measurements provide information of conductor losses, winding temperature at the end of a temperature test cycle, and provide a simple diagnostic tool for assessing possible damage in the field.

Transformers are subject to thermal and mechanical stresses, foreign particle ingress, and variations in temperature and humidity throughout their service lifetime. Winding resistance measurements can help reveal severe mismatches or opens in the windings. Transformers with mechanical tap changers should also have those tap changes checked during a winding resistance test. Regardless of configuration, measurements are usually made phase to phase using DC current. The readings are compared to each other, and these are acceptable if they are all within 1% of each other. 

Resistance is calculated according to the principle described in the figure below. The reading is only correct after the current has been stabilized (dI/dt = 0).

Advantages of the 2293 Winding Analyzer:

The Haefely 2293 is an automatic winding analyzer that is optimized to perform routine and maintenance tests on three phase power and distribution transformers. The 2293 can perform resistance measurements on all types of objects, but has been specially designed to measure resistance on highly inductive objects. Highly inductive objects require complete core magnetization to achieve a stable value.

Easy Test Setup:

The 2293 features simple, one-time connections for both three phase and single-phase measurements. Once connected, the 2293 can conduct all measurements in all phases on both low voltage and high voltage windings, drastically reducing the measuring time.

Simultaneous Winding Magnetization:

The 2293 was the first instrument on the market with Simultaneous Winding Magnetization (SWM). SWM utilizes the 2293’s three programmable power supplies to simultaneously inject current into both the high voltage and low voltage side of the transformer to speed up measuring time. Stable measurements can even be conducted on large power transformers with delta windings on the low voltage side. 

Intuitive Programmable Power Supplies:

The 2293’s three programmable power supplies can operate in constant current or constant voltage mode. Further, these power supplies operate in two quadrants. This means that a power supply can act as an active load which can be used to quickly discharge the current from high inductive DUTs. To determine the resistance of the DUT the 2293 uses two voltage and two current measuring channels. This way the unit can measure two different resistances with individual currents at the same time. 

According to international standards, the maximum test current should not be higher than 10% (IEC) or 15% (ANSI) of the nominal winding current to avoid heating up the winding and therefore giving non-stable readings. Using a measuring current above these thresholds can cause the transformer to breakdown. 

The 2293 has a maximum measuring DC current of 32 A. Normally, 32 A is not necessary, except when very low resistances (< 0.5 Ω) have to be measured. It is recommended to use 16 A at a maximum, which then allows two windings of the transformer to be measured simultaneously while using SWM, drastically cutting down on measuring time.

“Automated” and “Classic” Operating Modes:

“Automated mode” is used for measuring resistance on power transformers. The 2293 is equipped with an internal relay matrix that performs the tests on all phases of a three-phase transformer without any reconnection.

“Classic mode” allows the 2293 to work as a standard low resistance measuring bridge using the Kelvin method. Classic mode is used when taking resistance measurements on objects that aren’t transformers.

Automatic Demagnetization Function:

When a transformer is disconnected from the power grid or after a DC current has been applied during a winding resistance measurement, the transformer core will likely have some remnant magnetization. This can generate high over currents when the transformer is placed back into service, known as transformer inrush current. Magnetized cores can also cause errors in subsequent measurements, such as sweep frequency response analysis or turns ratio testing. The 2293 includes a fully automatic demagnetization function, which performs the correct core demagnetization all in a matter of seconds.

Automatic Temperature Correction:

Winding resistance is highly dependent on temperature and must be corrected according to the winding material used (copper or aluminum). The 2293 has 6 temperature channels with automatic resistance correction. The number of channels can be extended up to 30 by using the optional temperature extension box (2293/TEMPEXT), which can be used with either the temperature probe for liquids (2293/TEMP1) or the magnetic temperature probe (2293/TEMP2).

Easy Operation with Touchscreen Interface:

The 2293 is equipped with a robust yet user-friendly interface that simplifies the steps needed to get accurate results. The 2293’s software experience has been refined over many years of continuous customer and market feedback. All inputs are done via the 7-inch color touchscreen.  The touch screen is resistive, so unlike the screen on your smartphone, you can use a stylus or gloved hand. All important information about the transformer under test can be entered into the equipment and this configuration of the DUT can be saved and labeled for future testing.

Simple Data Handling & Test Reports:

All measurements are stored in the unit and can be exported via integrated USB port or printed out with the built-in printer. Remote control of the 2293 is also possible with an external laptop by using the built-in Ethernet interface.

Multipurpose Measuring Device with Additional Capabilities:

Not only can the 2293 be used to accurately measure winding resistance, additional measuring techniques can be supplied with the unit, which include:

• turns ratio
• tap changer dynamic resistance
• short circuit impedance
• magnetic balance
• heat run (heat rise and cooling curve)

All tests can be conducted on all windings without any reconnections, except for the short circuit impedance test.

Combine with the MIDAS micro 2883 for a Complete Transformer Test Solution

The MIDAS micro 2883 is capable of measuring capacitance and power factor / tan delta up to 12 kV. Combination of the 2293 and 2883 provides a pair of compact, lightweight, and rugged test instruments for a complete transformer test test solution. The two instruments have compatible file formats to allow data exchange. With this solution a complete transformer can be tested by a single operator in a fraction of the time compared to traditional equipment.

If you would like to learn more, feel free to contact one of our sales managers at 

+1 (703) 365-2330 or hvsales@hvtechnologies.com.