AMforum Journal No. 6




Winding resistance of power transformer may take excessively long time to obtain stable results. Creating an instrument with two independent sources to charge primary winding and secondary winding separately will speed up this process. Special connecting arrangements provide for proper magnetic core saturation that makes stabilization of test results fast. Additional advantages of this arrangement are explained.




Winding resistance measurement of a large transformer is a complex process. We deal with inductance of a transformer that was designed for AC operation, and we test it using a DC voltage and current.  The inductance restricts current flow and creates three characteristic processes during the test: charging, stabilization, and discharging. Lets’ explain each one.


Charging is the process where a DC current is established through the winding, the time required for this process depends on the voltage rating of the transformer winding and applied DC test-voltage. It may be very long if testing using low voltage like 5-10V. Modern instruments apply 50-100V to speed–up this process. Our experience with 1000kV unit required 70 seconds for the charging process. Once the charging is done and the selected current is established, the stabilization starts. The time required for the current to stabilize depends on the L/R time constant.


Only when the test current is stable resistance can be measured correctly. To minimize this time constant we saturate the magnetic core using as high current as necessary to reach and exceed the knee-point on the hysteresis curve. Of course, the test-current should not exceed 10% of the rated current to avoid heating the winding. Stabilization can last even few hours for some particular situations. The worst ones are LV windings of large GSU transformers with low resistance and significant inductance. Speeding up this process is one of the main tasks of instrument designers.


Discharging is the last process after the measurement results are obtained, and requires safe discharging circuit to automatically discharge the energy stored in the magnetic core. This energy is proportional to the square of the test current and the winding inductance. Discharging usually lasts the same amount of time as the charging process.


Applying current through both HV and LV winding will speed up the process by saturating the magnetic core using high number of ampere-turns of the primary HV winding. We will use the term BOTH measurement for configuration where both HV and LV windings are charged during a test where only one, the other, or both resistances are measured.


Instrument with only one source can perform this by connecting HV and LV winding in series and running the same test current through the complete circuit. Our approach with two DC sources in the one instrument achieves this BOTH method with many advantages. Each one of the two independent sources will charge one winding, the HV and the LV, while these currents do not need to be the same value. It is desirable to have higher current output from the source connecting to the LV winding.




The instrument is equipped with test-lead connections for Kelvin method of measurement for both HV and LV windings. Depending on the winding configuration, a test may require 6, 7, or 8 cables. Number of voltage measurement channels used may differ depending on the required test procedures.


Two powerful DC sources in the instrument are configured such that one has higher voltage with lower current output, and that one is used for the HV winding measurements, while the other source has lower voltage and higher current output.


Two sources can be started either at the same time or one after another. Our field measurement proved that starting the HV winding charging, followed by charging of the LV winding is the optimal solution.  Each independent source is dedicated to one transformer side, either low voltage (LV) or high voltage (HV).


The instrument is also equipped with powerful discharging circuit and a set of relays that may accomplish any of the test configurations and even reverse the polarity of the applied voltage, as required for the demagnetization process. Multitude of additional input channels are used for various signals that are required such as: temperature, motor current, vibration, or anything else.




There are several interesting advantages in applying two sources for transformer testing that allow for easier and faster test performance.


1.  There is no need to short the primary HV winding when testing the secondary winding only (the LV winding) as the induced voltage on the primary is countered by primary DC source. When only one source is utilized for LV winding testing, the voltage induced during the charging process can be significant, especially at transformers with high turns ratio. For those situations, in order to avoid inadvertent discharge circuit activation, the HV winding would be shorted internally, inside the instrument.


2. The dynamic testing can take advantage of BOTH connections. While it is impractical to use saturation of the HV side when dynamically testing the OLTC on the LV side using only one source, due to the reaction of the current source – the two source option avoids this issue all together.


3. For the QuickYN test, explained in the section “Connections” below, magnetic flux is forced by two sources the way we want it to split between core legs. Advantage over single source is much faster stabilization due to forced equalizing of flux through the three core legs, while the single source requires natural flux distribution and equilibration that may take long time.


4. Two different modes of testing can be performed on transformers with OLTC. One is winding resistance measurement at each tap position, the other is a dynamic recording. We call the dynamic one the DVtest used to verify an OLTC performance during the transition from tap to tap. These two modes can be achieved using two procedures.


One is called step by step and requires double number of steps compared to number of tap positions. Steps alternate between resistance measurements and dynamic recording. Following a resistance measurement on a position 1, the dynamic test would record a graph of transition between taps 1 and 2. These two steps are then repeated until all the positions are measured.


The other procedure is called the Continuous test. It is a faster test as the process records the entire time a dynamic graph as the tap changer switches between positions – form one extreme position to the other.




This section will deal with different winding configurations and vector groups. Also, specific test procedures for non-routine tests such as heat run or synchronization are examined.


a. Regular winding resistance testing can be performed three ways: only the HV, only the LV or testing BOTH sides. If testing in the BOTH sides mode we make the following arrangements for each winding configuration:


When testing only the HV side the process is straight forward, while testing the LV side requires certain modifications. Issue of voltage induced on the primary side when testing only the LV side is avoided as explained above, but the HV side has to be charged although not measured.


Charging with reduced voltage is another option that provides certain advantages, and makes charging process somewhat slower but may speed up the stabilization process. As lower charging voltage and the spike it creates when applied induces smaller effect in the other winding, this procedure is applied whenever problem of induced voltage being too high is detected.


b. Heat run testing issues – following IEC or IEEE standards user can apply the standard or alternative procedure for heat run test. The standard procedure requires two sources to measure two hot spots at two windings, the HV and LV. The alternative one is using only one source. Connection of the standard procedure with two sources is shown in the figure below, as provided by the IEEE standard. The HV side is charged through the middle phase and the return is through the outer phases connected in parallel (for YN connection). The resistance is measured on the winding of the middle leg.


The LV side connected in delta/triangle is charged and measured at the middle leg only, using its own independent source. 


c. Synchronization test

Having several tap changers (2 or 3) on the same drive mechanism, would require verification of their synchronization. The current is injected in all three phases from one source. The test side is the one where the OLTC is located and the current will distribute through the phases in accordance with the ohmic resistance of the three phase windings. The flux induced in the core applying current through all three phases will have no return path, except in case of a five legged cores, and will return through the air as leakage flux. This automatically minimizes inductance and makes core saturated for a perfect dynamic graph recording.


d. The QuickYN test is a great time saving function for transformers with on load tap changers (OLTC). This method measures all the phase resistances at the same time.  Having units with 33 positions will require 99 regular measurements. Using QuickYN procedure only 33 tests are performed, and 2/3 of the testing time is saved.

Connection can be established with one source or with both current sources. Using one source, current applied through the middle phase returns through the other two. Having neutral accessible in the star/wye connection (that is the reason for YN in the name QuickYN) provides measurement point to test the voltage drop on each phase winding.

 The other option is applying two currents, one through each of the outer phases and returning through the middle one. Again, voltage measurement is obtained on all 3 phases using accessible neutral point. Having three voltages and three currents measured, the resistance of all three phase windings can be calculated. Advantage of the two sources is faster initial stabilization time.


e. Dynamic testing

Tap changers are positioned usually in the LV winding in the US configurations and elsewhere they are in the HV winding. For that reason we have developed two different methods for dynamic recording of OLTC performance depending on the position of the OLTC. While the recording of tap changers positioned in the HV side requires only one source charging the HV winding, the LV side is different. Here we charge both HV and LV winding and record the current through the LV side only. The HV serves as saturation charge and even a small amount of current in the order of couple of amperes is sufficient to flow through the HV winding.


f. Testing autotransformers

A special configuration of an autotransformer where HV and LV windings are in fact electrically connected, as common and series windings, makes test procedure somewhat different. The first source is applying current through the common and series winding. The other is charging only the common winding. The instrument uses the sum of the two currents to calculate the resistance.


g. Demagnetization can be performed from the HV or from the LV side. Of course less current is required for the process performed from the HV side. However, for the Dyn configuration it may be desirable to do it from the LV side, as each phase and core leg is demagnetized separately. DV Power instrument can perform this process from either transformer side using proprietary algorithm.



Two sources instrument developed by DV Power solves many issues and provides several important benefits for a winding resistance static and dynamic testing.



Manuscript received on July 19, 2017 – final version accepted for publication TBD