The polarity marks on a CT employ the family member instantaneous directions from the currents. The polarity test proves your predicted way of additional CT recent (making) was proper for certain movement of major present (penetrating).
Taking good care to see right polarity is very important when installing and connecting latest transformer to energy metering and defensive relays. At the same quick of the time, that the major recent is going into the main terminal the corresponding second active must certanly be leaving the likewise marked second terminal.
A CT under examination try believed for proper polarity if immediate recent course for major and supplementary present is face-to-face to each other 
. CT Polarity is important when CT’s are increasingly being utilized along in single-phase or three-phase programs.
Modern day CT examination devices is capable of performing the ratio test immediately utilizing a simplified test lead set-up and can show polarity as proper or wrong. CT polarity is actually confirmed manually through the use of a 9V battery pack and analog voltmeter utilizing the soon after examination therapy:
Marks on recent transformers are sometimes misapplied of the manufacturer. You can easily examine the polarity of a CT on the go with a 9V power supply. Photo: TestGuy.
CT Polarity Test Process
- Disconnect all-power in advance of evaluation and connect the analog voltmeter on the secondary terminal with the CT as tested. The positive terminal on the meter was linked to terminal X1 of this CT as the bad terminal is linked to X2.
- Operate a bit of cable through the high area of the CT windows and shortly get in touch with the good
9-volt electric battery to your H1 side (occasionally marked with a mark) as well as the bad end to your H2 part. It is important to abstain from constant get in touch with, which will short-circuit the battery. - If polarity are correct, the momentary contact trigger a little deflection in the analog meter inside the positive course. In the event that deflection is actually negative, the polarity of current transformer try corrected. The terminals X1 and X2 have to be corrected plus the examination can be executed.
Note: Polarity is not vital whenever hooking up to ammeters and voltmeters. Polarity is essential only if hooking up to wattmeters, watt-hour meters, varmeters, and induction-type relays. To steadfastly keep up polarity, the H1 area of the CT must certanly be toward the source of electricity; then your X1 supplementary terminal will be the polarity connection.
3. Excitation (Saturation) Test
When a CT try “over loaded”, the magnetic road in the CT operates like a short circuit about indication range. Most of the electricity given by the main winding is shunted from the supplementary winding and is put develop a magnetic industry within the CT.
Saturation screening for an existing transformer recognizes the rated knee aim against IEEE or IEC expectations, the point at which the transformer is no longer capable output latest equal in porportion to their specified ratio.
Excitation reports are performed by making use of an AC voltage to your secondary winding with the CT and enhancing the current in strategies until the CT is actually saturation. The “Knee” point is determined by watching limited voltage enhance triggering big escalation in current.
The exam voltage is actually gradually decreased to zero to dea€?magnetize the CT. The test email address details are plotted on a logarithmic (loga€?log) chart and examined according to the changeover duration between typical procedure and saturation.
Excitation tests were performed by making use of an AC current towards the second winding in the CT and improving the voltage in measures before the CT is during saturation. Image: TestGuy.
The excitation contour all over guidelines in which current leaps up for a little boost of current; is extremely important for comparison of shape with posted curves or similar CT figure. The excitation test outcomes must certanly be compared with released maker’s information or past recordings to determine any deviations from formerly acquired shape.
IEEE describes the saturation as “the point whereby the tangent has reached 45 levels towards additional interesting amperes”. Referred to as “knee” point. This test verifies your CT is of appropriate precision standing, doesn’t have shorted turns within the CT with no small circuits are present inside the primary or second windings of CT under test.
4. Insulation Resistance Test
The insulation involving the existing transformer windings and windings to crushed must be examined for dielectric power while carrying out a comprehensive CT examination. Three assessments include performed to look for the state regarding the insulation associated with the CT under test:
- Primary to secondary: Checks the condition of the insulation between higher to reasonable.
- Primary to soil: Checks the condition of the insulation between highest to soil.
- Second to soil: Checks the healthiness of the insulation between low to floor.