A CT under test try assumed to have proper polarity if instant present movement for major and supplementary present is actually other together. Picture: TestGuy.

The polarity scars on a CT specify the family member immediate directions for the currents. The polarity test proves the expected path of second CT recent (making) are appropriate for confirmed path of major existing (penetrating).

Taking care to observe best polarity is important whenever installing and connecting recent transformer to power metering and safety relays. In one immediate period, your primary existing are going into the main terminal the corresponding supplementary up-to-date needs to be making the similarly designated supplementary terminal.

A CT under examination try believed having appropriate polarity if instant current movement for primary and secondary active is actually opposing together. CT Polarity is crucial when CT’s are put along in single-phase or three-phase solutions.

Modern time CT examination machines can perform executing the ratio test immediately making use of a simplified examination lead create and certainly will show polarity as proper or wrong. CT polarity is validated manually with the use of a 9V power and analogue voltmeter with all the appropriate test process:

Markings on current transformers being sometimes misapplied of the factory. You’ll validate the polarity of a CT on the go with a 9V power. Photo: TestGuy.

CT Polarity Test Process

1. Disconnect all-power before screening and hook up the analogue voltmeter on additional terminal regarding the CT to-be examined. The good terminal in the meter try connected with terminal X1 with the CT even though the bad terminal was attached to X2.
2. Run an item of wire through the large region of the CT windows and immediately get in touch with the good end of the 9-volt electric battery to the H1 area (often marked with a mark) and unfavorable end with the H2 area. It is critical to avoid steady get in touch with, that’ll short-circuit battery pack.
3. If polarity are proper, the momentary-contact causes a tiny deflection inside analogue meter in positive course. In the event that deflection are negative, the polarity regarding the latest transformer try corrected. The terminals X1 and X2 have to be corrected while the examination can be carried out.

Note: Polarity isn’t vital whenever linking to ammeters and voltmeters. Polarity is essential only once linking to wattmeters, watt-hour meters, varmeters, and induction-type relays. To maintain polarity, the H1 section of the CT need to be toward the source of electricity; then your X1 additional terminal is the polarity link.

3. Excitation (Saturation) Test

When a CT is actually “soaked”, the magnetic course within the CT functions like a brief routine on the sign range. Most of the strength supplied by the primary winding is actually shunted away from the second winding and it is used build a magnetic area within the CT.

Saturation testing for an ongoing transformer identifies the rated knee aim against IEEE or IEC criteria, the point at which the transformer is no longer in a position to output existing in proportion to the given ratio.

Excitation assessments tend to be sang through the use of an AC voltage with the supplementary winding of this CT and enhancing the current in procedures until the CT is within saturation. The “Knee” aim depends upon watching a tiny current increase causing a sizable increase in existing.

The exam current is actually slowly 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 change period between typical process and saturation.

Excitation examinations were carried out by applying an AC current for the secondary winding associated with CT and increasing the voltage in tips until the CT is during saturation. Pic: TestGuy.

The excitation bend across details in which existing jumps up for limited increase of current; is essential for comparison of shape with released curves or close CT figure. The excitation test outcomes is weighed against printed maker’s information or earlier recordings to ascertain any deviations from formerly obtained figure.

IEEE describes the saturation as “the point where the tangent has reached 45 levels toward secondary exciting amperes”. Often referred to as “knee” point. This examination confirms that CT was of appropriate precision standing, has no shorted turns in CT without quick circuits exist during the biggest or supplementary windings pop over to the web-site in the CT under examination.

4. Insulation Weight Examination

The insulation between your recent transformer windings and windings to crushed should-be examined for dielectric energy while carrying out an extensive CT examination. Three tests were carried out to ascertain the condition in the insulation on the CT under test:

1. Biggest to additional: Checks the healthiness of the insulation between higher to low.
2. Major to ground: Checks the condition of the insulation between large to surface.
3. Second to surface: Checks the healthiness of the insulation between reduced to crushed.