What is a Current Transformer?
A current transformer, often known as a CT, is a type of electrical instrument transformer that is used to step down high currents in electrical circuits to a lower current that can be measured for the purposes of current protection and measurement.
A primary winding, a core, and a secondary winding are the components of a current transformer. However, some transformers, especially current transformers, utilize an air core. The characteristics of a “current” transformer comparison to a “voltage” transformer will vary despite the fact that the fundamental physical principles are the same due to various application needs. A correct ratio between the currents in its primary & secondary circuits must be maintained by a current transformer across a certain range.
Testing of Current Transformer
The six fundamental tests performed on the CT- current transformers are
- Insulation Resistance Test
- Winding Resistance Test ( @ 75’C )
- Polarity Test
- Ratio Test
- Magnetizing Current Test
- Burden Test
1). Insulation Resistance Test
An insulation tester is used to measure the insulation resistance at a voltage of one thousand volts direct current (1000 VDC) for a maximum of ten seconds per core or line. The core insulation is measured against ground, and then each core is measured against every other core in the transformer. In order to perform this, the secondary grounding that is on the cores of the transformer must be removed.
Insulation testing on the CT’S – current transformers are typically carried out at a voltage of one thousand volts direct current. Connecting H1 and H2 together will create a short in the primary winding of the CT that will be tested. Next, create a short in the CT’s secondary winding that will be tested by connecting X1 to X2-X5 together. Take away the neutral ground, and separate the CT of any burdens that are associated with it.
2). Winding Resistance Test ( @ 75’C )
The winding resistance of a current transformer can be calculated by dividing voltage drop across the winding, which is determined using a dc millivoltmeter, by the dc current that is passed through the winding. After the winding resistance test is finished, the CT needs to be demagnetized so that it can be used.
When determining the actual condition, status, and level of accuracy of a CT, one of the most important measurements to take is the DC winding resistance test. The winding resistance of a CT will fluctuate with time based on the age of the specimen, how it is used, the external conditions, and the loading effect.
3). Polarity Test
The polarity of a CT is established by two factors: the direction (clockwise or counterclockwise) in which the coils are twisted around the transformer core, and the manner in which the leads are taken out of the CT casing.
4). Ratio Test
The CT ratio is the primary current input to the secondary current output at the full load. When 600A pass through the primary, a 600:5 CT produces 5 A of secondary current.
If primary current changes, secondary current output changes. 300 amps through a 600A primary produces 2.5A of secondary current.
Unlike voltage or power transformers, current transformers have one or few turns in their primary winding. A single flat turn, a coil of heavy-duty wire wrapped around the core, or a conductor or bus bar through a central hole can be used for this primary winding.
5). Magnetizing Current Test
The magnetic path in a “saturated” CT short circuit the transmission line. The CT’s magnetic field is created by redirecting most of the primary winding’s energy away from secondary winding.
The rated knee point of a current transformer is determined by saturation testing against IEEE or IEC standards, at which time it can no longer output current in the proportion to its prescribed ratio.
Excitation testing begin by supplying an AC voltage to the CT’s secondary winding and increasing it in increments until saturation. A moderate voltage increase causes a significant current increase, revealing the “Knee” point.
De-magnetize the CT by gradually decreasing the test voltage to zero. The test results are analysed using a logarithmic graph and taken from the transition time between the normal operation & the saturation.
The IEEE defines saturation as “the point where the tangent is at 45 degrees to the secondary exciting amperes”. Also called “knee” point. This test ensures that the CT has enough precision, no shorted turns, and no short circuits in its primary or secondary windings.
6). Burden Test
The maximum load (measured in VA) that may be placed on the CT secondary is referred to as the burden test for CT. The weight of the responsibility can be stated in two different ways. It is possible to express the load as the overall impedance of the circuit measured in ohms, the total volt-amperes (VA), and the power factor at a certain value of current, voltage, or frequency.