Protection Current Transformer 1

Protection Current Transformer

Protection Current Transformers are designed to measure the actual currents in power systems and to produce proportional currents in their secondary windings which are isolated from the main power circuit. Satisfactory operation of protective relays can depend on accurate representation of currents ranging from small leakage currents to very high over currents. This requires protective CT to be linear and operate below its saturation point. This CT will be connected to protection device or relays. Manufacture of protection devices will normally specify the classification for the protection current transformer intended to operate the particular device. For Space and economy reasons, designers should avoid over specifying protection CT.

Protection CT is usually defined in terms of composite errors and accuracy limit factor (ALF).

Accuracy Class

5P and 10P, as indicated in the chart.

Instrument Security Factor

Standard ALF is 5, 10, 15, 20 & 30, but it is burden dependent. These numbers indicate factor of primary current up to which composite error will be achieved. Although the use of low induction high loss silicon steel can achieve the require ALF with an over-size core, but the sensitivity is low due to its higher excitation current.

Phase Fault Stability

Well matched Protection CT and operating below saturation will deliver no current to the earth relay, since the 3 phase currents sum to zero. If they are unmatched (having different magnetization curves), a 'spill' current will arise which may trip the relay. This can be more severe in 3rd Harmonics rich system as they are additive. Accurate operation of the relay can be severely affected by inaccuracy or uneven saturation of its associated CT.

Inter Win protection current transformer is manufactured with annealed core selected in sets of 3 or 4 with tight inductance tolerance. They were wound and tested for its accuracy class. Turn to turn and layer to layer insulation strength tested at 3.0KV. Induction similarity is also tested. This will ensure protection CT packed in sets will have near identical performance for the same specification.

Proof & testing for composite error and ALF compliance can be a challenge. Traditional method may impose danger to the test engineer and the product itself. Inter Win uses software simulation method which is the common method used by utilities company to test their CT at site.

PX Class (class X) protection current transformer

For used in balanced protection system where sensitivity and stability levels are required. Typically these information are required.
1. Turns ratio, 2. Min. knee point voltage. 3. Max. excitation current at the rated min. knee point voltage. 4. Maximum secondary winding resistance.

Sensitivity & Stability: CT Knee point voltage (Vkp) and Ie (magnetizing current) determine the proper operation of the CT. The use of low loss high permeability gain oriented silicon steel is required. All Inter Win PX class CT will be supplied with inhouse comprehensive test report.
Graph below shows a typical excitation curve of a protection CT. Its knee point voltage and magnetizing current clearly mark and indicated.

PVC Insulated Ring Transformer

Encased Protection Transformer

Current Transformer Ordering

The CT ratio, rated burden, accuracy class and security factor are required. Custom specification can be manufactured upon request. Accuracy and other properties of CT is burden dependent. DO NOT OVER SIZE.
e.g. 400/5A, 7.5-1.5VA C0.5, FS5 (for ratio = 400/5A, 7.5 to 1.5VA, class 0.5, FS =<5)
other information: ID=?mm min, OD=?mm max, Ht=?mm if non-standard.

Factory Inspection Report

Inter Win will provide factory inspection for both metering and protection CT produced. This request must be made known when ordering. Fees may be required.

Installation Note

Low voltage current transformer can be mounted on: 1. Current bus-bar(s), 2. Solid cable or multiple cables. 3. Plates.
It can be mounted in any position, horizontal or vertical or tilted. It should be securely fasten with holders or mounting bolt and nuts.
* Current transformer must be connected to its load at all times. Unused CT secondary terminals must be shorted at all times. Open circuited CT secondary will overheat. It also induced dangerously high voltage which can be fatal to life and damage to equipment.

Transport & Handling

Precision CT accuracy can be damaged due to impact. Care should be taken against impact and drops. Current transformer is a non-repairable product. No spare parts will be provided.