CPOL2 battery
5.8 CTRatioV (with voltage)
Use the CTRatioV test card to measure a current transformer’s ratio. To do so, from the 2kV AC output feed a voltage of up to 500V to the transformer’s secondary side.
The preferred method for CT ratio measurement is current injection using the CTRatio test card.
However, on CTs such as some GIS CTs or bushing CTs on power transformers where the primary current path is not accessible, the method described in this section is the only solution.
Both the amplitude of the injected test voltage as well as amplitude and phase angle of the voltage on the transformer’s primary side are measured. The measurements taken are the basis for the calculation of the actual ratio and the deviation from the nominal ratio.
To measure the CT ratio using the CTRatioV test card, connect the 2kV AC output to the CT’s secondary winding and the V2 AC input to the main conductors, for example, on a power transformer to the transformer’s bushings of different phases.
DANGER
Death or severe injury caused by high voltage or current
Feeding test voltage to a tap of a CT can cause life-threatening voltages on other taps and windings.
► Do not touch tapped windings.
► Make sure that no secondary windings are open.
P1/H1
S1/X1
P2/H2 S2/X2 CT
Burden
5.8.1 Test settings
Figure 5-21:CTRatioV test card with test results
Navigate to the parameter fields, and enter the values according to your test requirements:
Note:If the transformer’s knee point voltage is approximated or exceeded due to the transformer’s saturation, the measurement results are not correct anymore. If the knee point is extensively exceeded, the transformer can even be damaged.
Therefore, the knee point voltage should be known or measured beforehand.
Generally, to set the test voltage to 75% of the knee-point voltage is a good choice.
The frequency should be set 15 to 20 Hz off the mains frequency to avoid interferences by life systems in the neighborhood.
Iprim: nominal primary current Isec: nominal secondary current Vtest: secondary inception voltage
f: output frequency
Auto: select check box for automatic test (default), clear for manual test (refer to page 117).
5.8.2 Measurements
Selecting the check box Manual input lets you enter the value for Vprim manually, for example, when Vprim was measured with an external volt meter.
5.8.3 Automatic test vs. manual test 1. Option "Manual input" clear
Automatic test
► Press the I/O (test start/stop) push-button to start the test.
► Within 1 second the test voltage increases in a ramp characteristic from 0V to Vtest. Then Vtest is kept for a period of 2.4 seconds, and the measurements are taken. Next, the ramp decreases again within 0.3 seconds.
► The automatic test stops by itself, it can now be assessed.
► To repeat the test, first press Back to Top, if applicable, and then Clear Results.
Manual test
Clearing Auto lets you set the test voltage Vtest manually with the handwheel.
► Press the I/O (test start/stop) push-button to start the test. The focus is set onto the "V test:" entry field.
► Set the value of your choice either by turning the handwheel and/or by entering it with the numerical keys.
► A value set with the numerical keys needs to be fully entered and acknowledged by pressing either Enter or the handwheel before it is applied to the output. If needed, it can then be adjusted using the Vsec: actual output at the 2kV AC output that is injected to the CT’s secondary side
Vprim: measured voltage on CT’s primary side and phase angle ϕ relative to Vsec Iout measured current from Vsec output
Ratio: ratio Iprim /Isec, here 200:5.01341
1. The ratio value 5.0134A is calculated as follows:
Vprim act x (Iprim nom / Vsec act); here: 1.2536V x (200A / 50.010V) = 5.0134A
and deviation of current ratio in %. Formula:
((Kn x Vprim - Vsec) / Vsec) x 100%2
((200A / 5A * 1.2536V - 50.010V) / 50.010V) x 100% = +0.268%
2. Kn = rated transformation value, here: 200A / 5A
Polarity: displays the status of the polarity
OK= phaseVprim - phaseVsec = -45° < 0° < 45°
NOT OK = all other cases
► Stop the manual test by pressing the I/O (test start/stop) push-button. The test can now be assessed.
► To repeat the test, first press Back to Top, if applicable, and then Clear Results.
2. Option "Manual input" selected
Note:Select Manual input prior to starting the test. If the test card still contains results, clear them by pressing Clear Results.
When Manual input is selected, there is no phase available. Consequently, neither the polarity nor the phase angle nor cosϕ can be calculated.
Since an automatic test only lasts a very short period of time, and a manual measurement is rather difficult, an automatic test with a manual measurement is not useful. Therefore, this Reference Manual will not cover this subject.
Manual test
Clearing Auto lets you set the test voltage Vtest manually with the handwheel.
► Press the I/O (test start/stop) push-button to start the test. The focus is set onto the "V test:" entry field.
► Set the value of your choice either by turning the handwheel and/or by entering it with the numerical keys.
► A value set with the numerical keys needs to be fully entered and acknowledged by pressing either Enter or the handwheel before it is applied to the output. If needed, it can then be adjusted using the handwheel.
Values set by turning the handwheel are applied instantaneously.
► Vsec is measured and displayed.
► To save the measurement press Keep Results.
► Navigate to the "Vprim:" entry field
► Enter the measured Vprim value either by turning the handwheel or by entering this value with the numerical keys, and press Enter or the handwheel to acknowledge your setting.
► The ratio is calculated and displayed.
► To stop the manual test, press the I/O (test start/stop) push-button. The test can now be assessed.
► Alternatively it is possible to enter the manually measured value into the test card after the test was stopped (for example, if the measurement was carried out at a remote location).
► To repeat the test, first press Back to Top, if applicable, and then Clear Results.
To learn more about test assessment, refer to 10.1 "Test assessment" on page 268.