• Burden class/saturation class
• Rating factor
• External electromagnetic fields
• Temperature and physical conditions, and
• The selected tap
The IEC 60044-1 standard sets out various accuracy classes. The classes are 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The class designation is an approximate measure of the CT’s accuracy. For an example; the ratio error of a Class 1 CT is 1% at rated current. The ratio error of Class 0.5 CT is 0.5% or less. Each class also has an allowable maximum phase error for specified load impedance. Current transformers used for protective relays will have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults.
The load, or burden, in a CT metering circuit is the impedance presented to its secondary winding. This impedance will be largely resistive. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA and 60 VA. ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0, and B-4.0. This means a CT with a burden rating of B-0.2 can tolerate up to 0.2 ohms of impedance in the metering circuit before its output current is no longer a fixed ratio of the primary current. Switch-blocks, meters and intermediate conductors will all contribute to the burden of a current measurement circuit. The conductor between the meter and the CT will typically be the source of excess burden.
The knee-point of a CT is the magnitude of the secondary voltage after which the output current ceases to follow linearly the input current. Or, in other words, the one-to-one relationship between input and output is no longer within declared accuracy. When testing, a voltage is applied across the secondary terminals and magnetizing current will increase in proportion to the applied voltage up until the knee point. The knee-point is defined as the point at which an increase of applied voltage of 10% results in an increase in magnetizing current of 50%.
Figure 1: CT Knee-point
The knee-point voltage is less applicable for metering CTs since their accuracy is generally much tighter and constrained within a very small bandwidth of the current transformer rating. Knee-point, however, is very important to protection CTs since they are exposed to as much as 20 times the rated current before saturation occurs during faults.
Protec Equipment Resources offers a vast inventory of microprocessor-based current transformer test sets from Vanguard Instruments. The Vanguard EZCT series of CT test sets are designed specifically for CT testing and will greatly increase productivity and save time during the commissioning and maintenance testing. These sets will perform CT excitation, ratio, polarity, and phase angle deviation tests. Depending on the model selected, insulation resistance and winding resistance of the CT secondary windings plus secondary burden can be measured.
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Editor’s Note: This post was originally published October, 2011 and has been updated freshness, accuracy and comprehensiveness.