ct accuracy class
That is something you must know. It produces a current in its secondary which is proportional to the current in its primary. The logic behind this is to limit the worst-case system error when using the CT in a metering system with an inductive load having a power factor of 0.6. The transformer correction factor (TCF) is defined for current transformers in the IEEE C57.13 – 2008 standard pp 13-14 as follows.
For the Accu-CT, we offer two IEC 60044-1 grades. The higher accuracy grade “Opt C0.6” CTs meets the class 0.5 and 0.5 S (extended range) limits. This problem can be reduced by using thicker cables and CTs with lower secondary currents (1 A), both of which will produce less voltage drop between the CT and its metering devices. This means a CT with a burden rating of B-0.2 will maintain its stated accuracy with up to 0.2 Ω on the secondary circuit. On the other hand, if that condition is not true, then the transformer is inductive and gives a differential output.
For example, one secondary may be used for branch overcurrent protection, while a second winding may be used in a bus differential protective scheme, and a third winding used for power and current measurement.[1]. The limiting error shall not exceed the value tabulated below at rated frequency and burden from 50% to 100% of the rated burden.
A current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core.
In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter. The accuracy of a CT is affected by a number of factors including:
Each of these accuracy classes specifies the limit for the TCF as a percentage, so class 1.2 means the CT TCF must be within 1.2% of ideal at 100% of rated primary current.
However, in wattmeters, energy meters, and power factor meters, phase shift produces errors. While the physical principles are the same, the details of a "current" transformer compared with a "voltage" transformer will differ owing to different requirements of the application. First, let’s remind ourselves of the basics in a few sentences. This ratio can also be used to find the impedance or voltage on one side of the transformer, given the appropriate value at the other side. In some cases, the secondary impedance is referred to the primary side, and is found as ZS′ = N2ZP. A CT with a rating of 2.5L400 specifies with an output from its secondary winding of twenty times its rated secondary current (usually 5 A × 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent. If the actual output for a 500 amp input was 0.340 Vac (2% high), then the RCF would be: Multiplying the full-scale output of 0.340 Vac by 0.98038 yields the corrected full-scale output of 0.33333 Vac.
Ring-type current transformers are installed over a bus bar or an insulated cable and have only a low level of insulation on the secondary coil. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much higher but constrained within a very small range of the current transformer rating, typically 1.2 to 1.5 times rated current. Accuracy Class of Current Transformer (CT) is defined in Indian Standard IS 2705 Part-2 and Part-3.
Here, accuracy limit primary current is the value of primary current corresponding to, Composite Error of CT – Definition, Calculation & Formula. The most common cause of excess burden impedance is the conductor between the meter and the CT. Energizing a current transformer with an open circuit secondary is equivalent to energizing a voltage transformer (normal type) with a short circuit secondary. C100 CLASS For example, a C100 CT class means that the CT is capable of passing 20 × 5 A (CT secondary) = 100 A through a standard 1 Ω resistor with a current accuracy less than 10%. The burden in a CT metering electrical network is largely resistive impedance presented to its secondary winding. The C57.13 standard has different accuracy classes: Class 1.2, Class 0.6, and Class 0.3.
Both scenarios can be dangerous and damage the transformer.[1]. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the knee point is reached.
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