How to Select the Right CT
Q: When is a CT necessary?
A: A current transformer (CT) is necessary if you need to measure the current, energy, frequency, and other parameters of the power system and when one or both of the following conditions apply:
(1) The voltage of your power system is above 240 V.
(2) The current of your power system is greater than 100 amps.
Q: How do I select the right CT?
A: If any particular specification has been requested, please follow it when choosing your
product. In areas where no specification has been requested, please refer to guidelines below.
Primary Current ( I1 ) and Secondary Current ( I2 )
• I1 should be the lowest standard value above 125 % of the maximum primary load current.
• I2 should be 5 A in most cases. However, when the circuit of the secondary side is longer than found in average situations, it is suggested, in order to reduce power loss (i.e. the output VA of the CT), that the secondary current be 1 A. When the secondary current is 1 A, the burden is reduced to 1/25 of that when I2 is 5 A.
Burden
• For a revenue CT, the burden should be IEC 5 ~ 15 VA or IEEE B.01 ~ B.05.
• For a metering CT, choose a burden VA equal to, or greater than, 125 % of the total
combined VA of instruments connected to the secondary side.
• For a protective CT, please contact the manufacturer for assistance.
Accuracy
• For a revenue CT, the accuracy should be at least IEC 0.2 or 0.5 CL or IEEE 0.3 or 0.6 CL.
• For a metering CT, it should be at least IEC 1.0 or 3.0 CL or IEEE 0.6 or 1.2 CL.
• For a protective CT, please contact the manufacturer for assistance, Rated Frequency
Choose the same rated frequency as that of the power system.
Outdoor Type or Indoor Type
Choose according to the location where the CT will be used. Note that an outdoor CT can also be used indoors.
Terminal Type, Window Type, or Wound Type
• A terminal type CT comes with a primary conductor. Ordinarily, a terminal type CT is chosen in most circumstances where one needs a medium-voltage CT with a primary current of 3000 A or lower.
• A window type CT does not come with a primary conductor. This type of CT is easier to install, can withstand a greater thermal overcurrent, and reduces failure rate. Usage scenarios of a window type CT are as follows:
(1) When a low-voltage CT with a primary current of 100 A or greater is needed, a window type CT is usually chosen.
(2) When a medium-voltage CT with a primary current of 300 A or greater is needed, a window type CT may be chosen.
(3) When a medium-voltage CT with a primary current greater than 3000 A is needed, a window type CT is usually chosen.
However, when a greater output (VA), a higher accuracy level, or a greater inside diameter of the CT window is required of a CT, please consult with the manufacturer - as well as the selection table provided by them - to ensure output and accuracy requirements can be met.
• Sometimes, a low-voltage CT with a primary current lower than 100 A may be called for, together with the requirement of a large output, a high accuracy, or a great inside diameter of the CT window. To meet this demand, a window type CT with a primary current of 120 A, 150 A, or 200 A can be selected and be wound with a primary conductor (with more than one turn). This solution is sometimes called a wound type CT.
Insulation Material
If any indoor or outdoor CT will be used in a location of extreme conditions, such as a desert, the Frigid Zones, or high altitudes (above 1000 meters), special requirements will apply -please contact the manufacturer for information. In other situations, please follow these suggestions:
• When choosing an outdoor CT, select either resin or oil as insulation material.
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