The polarity scars on a CT designate the general instantaneous directions associated with currents. The polarity examination demonstrates that predicted way of second CT recent (leaving) try proper for certain path of major present (entering).
Taking good care to observe right polarity is essential whenever installing and linking present transformer to power metering and protective relays. In one quick of the time, that the biggest latest was entering the primary terminal the corresponding additional present need making the similarly marked secondary terminal.
A CT under examination is presumed to possess proper polarity if instant latest movement for biggest and supplementary active is actually other to escort Plano each other. CT Polarity is very important when CT’s are made use of along in single-phase or three-phase programs.
Most advanced time CT examination machines can perform doing the ratio examination automatically utilizing a simplified examination lead build and will display polarity as proper or wrong. CT polarity are verified manually with the use of a 9V electric battery and analogue voltmeter aided by the following test therapy:
Markings on current transformers have already been periodically misapplied of the manufacturing plant. You can easily verify the polarity of a CT in that particular niche with a 9V electric battery. Pic: TestGuy.
CT Polarity Test Therapy
- Disconnect all power just before screening and hook the analog voltmeter toward additional terminal with the CT getting examined. The positive terminal regarding the meter is actually attached to terminal X1 from the CT although the bad terminal are connected with X2.
- Operated some cable through the highest region of the CT windows and immediately make contact with the positive end of the 9-volt power into the H1 side (occasionally designated with a mark) while the bad conclusion into the H2 part. You will need to eliminate constant call, that will short circuit the battery.
- If polarity is actually appropriate, the momentary-contact trigger a tiny deflection within the analogue meter into the positive path. If the deflection is actually adverse, the polarity with the present transformer is stopped. The terminals X1 and X2 have to be corrected and the examination can be carried out.
Note: Polarity isn’t vital whenever hooking up to ammeters and voltmeters. Polarity is important only when linking to wattmeters, watt-hour yards, varmeters, and induction-type relays. To maintain polarity, the H1 area of the CT must certanly be toward the origin of electricity; then the X1 supplementary terminal could be the polarity hookup.
3. Excitation (Saturation) Test
When a CT are “over loaded”, the magnetic course in the CT functions like a brief routine regarding indication range. Most of the stamina furnished by the main winding is actually shunted out of the additional winding and is used make a magnetic industry inside CT.
Saturation evaluating for a present transformer recognizes the rated leg point against IEEE or IEC criteria, the point where the transformer no longer is capable output present equal in porportion to the specified proportion.
Excitation studies are performed by making use of an AC voltage to the secondary winding regarding the CT and increasing the current in procedures before the CT is within saturation. The “Knee” aim will depend on watching a small voltage enhance creating big upsurge in recent.
The exam voltage try gradually diminished to zero to dea€?magnetize the CT. The test answers are plotted on a logarithmic (loga€?log) chart and examined in line with the change duration between normal operation and saturation.
Excitation examinations were carried out by applying an AC current into the second winding from the CT and improving the voltage in measures before CT is during saturation. Photograph: TestGuy.
The excitation contour across the guidelines in which latest jumps up for a small boost of current; is very important for review of curves with released curves or close CT curves. The excitation test results must in contrast to circulated manufacturer’s information or earlier recordings to ascertain any deviations from earlier obtained curves.
IEEE describes the saturation as “the point where the tangent is located at 45 levels into supplementary interesting amperes”. Also called “knee” aim. This examination confirms that the CT was of proper reliability score, does not have any shorted changes when you look at the CT no small circuits can be found inside main or secondary windings with the CT under examination.
4. Insulation Resistance Test
The insulation involving the latest transformer windings and windings to ground should-be checked for dielectric power while doing an extensive CT examination. Three studies are performed to look for the problem of insulation from the CT under test:
- Biggest to additional: Checks the healthiness of the insulation between higher to lower.
- Primary to surface: Checks the health of the insulation between high to ground.
- Additional to soil: Checks the health of the insulation between reasonable to crushed.