ct accuracy class
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. CTs are specified by their current ratio from primary to secondary. 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. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window. Low-voltage single ratio metering current transformers are either a ring type or plastic molded case. Current transformers, especially those intended for high voltage substation service, may have multiple taps on their secondary windings, providing several ratios in the same device. Can Protection CT be used as Metering CT & Vice Versa? [5], Bar-type current transformers have terminals for source and load connections of the primary circuit, and the body of the current transformer provides insulation between the primary circuit and ground. Special application metering CTs are a special category in which it is desired that the CT should accurately measure the current from 1% to 120% of the rated current. The primary circuit is largely unaffected by the insertion of the CT. 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. The transformer correction factor (TCF) is defined for current transformers in the IEEE C57.13 – 2008 standard pp 13-14 as follows. Since the major insulation of a live-tank current transformer is not exposed to the heat of the primary conductors, insulation life and thermal stability is improved.[1]. 5, 10 and 15 in these classes are the maximum composite error corresponding to accuracy limit primary current. [1], The knee-point voltage of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. These specification diagrams show accuracy parallelograms on a grid incorporating magnitude and phase angle error scales at the CT's rated burden. Often, multiple CTs are installed as a "stack" for various uses. Ultimately, depending on client requirements, there are two main standards to which current transformers are designed. [2] For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. However, in wattmeters, energy meters, and power factor meters, phase shift produces errors. If the burden resistance is much less than inductive impedance of the secondary winding at the measurement frequency then the current in the secondary tracks the primary current and the transformer provides a current output that is proportional to the measured current. In practice, this is impossible, but, at normal power frequencies, phase shifts of a few tenths of a degree are achievable, while simpler CTs may have phase shifts up to six degrees. 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. The secondary of a current transformer should not be disconnected from its burden while current is in the primary, as the secondary will attempt to continue driving current into an effective infinite impedance up to its insulation break-down voltage and thus compromise operator safety. First, let’s remind ourselves of the basics in a few sentences. Energizing a current transformer with an open circuit secondary is equivalent to energizing a voltage transformer (normal type) with a short circuit secondary. Current transformers are the current-sensing units of the power system and are used at generating stations, electrical substations, and in industrial and commercial electric power distribution. Current transformers used to sense ground fault (zero sequence) currents, such as in a three-phase installation, may have three primary conductors passed through the ring. CCS does not provide RCF values for our CTs, although the RCF may be computed from the “Measured Accuracy” provided on the Accu-CT Certificate of Calibration. It shall be noted that phase displacement error is not defined, Accuracy Class of Protection Current Transformer (CT) is defined by the maximum permissible Composite Error at the rated accuracy limit primary current followed by letter “P” (meaning protection). Figure 1 C400 1200:5 CT 24 kA, Burden 2+j3.464 Figure 2 C400 1200:5 CT 24 kA, Burden 4-Ohm Resistive TABLE 1 EQUIVALENT CT ACCURACY RATINGS IEEE C57.13 IEC 60044-1 C100 25 VA 5P 20 C200 50 VA 5P 20 C400 100 VA 5P 20 For the 4000:5 CT, the secondary impedance can be found as ZS = NZP = 800ZP, and the secondary voltage can be found as VS = NVP = 800VP. Here, accuracy limit primary current is the value of primary current corresponding to Accuracy Limit Factor (ALF). Let us now discuss the accuracy class of Protection Class and Metering Class CT individually. For power and energy measurement, the errors are considered to be negligible at unity power factor but become more significant as the power factor approaches zero. It shall be noted that phase displacement error is not defined for these accuracy class. Current transformers may be installed inside switchgear or in apparatus bushings, but very often free-standing outdoor current transformers are used. Where a metal shield is present in the cable jacket, it must be terminated so no net sheath current passes through the ring, to ensure accuracy. The accuracy class of protection class and metering class CTs are defined by Composite Error and Current Error & Phase Angle Error respectively. The current transformer presents a negligible load to the primary circuit.[1]. [4] The introduction of electronic power and energy meters has allowed current phase error to be calibrated out. For example, on name plate of CT, you may find 30 VA, 5P10 or 30/5P10. The higher accuracy grade “Opt C0.6” meets the IEEE C57.13 accuracy class 0.6 limits as well as tighter accuracy and phase angle limits not required by C57.13. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right). A high-voltage current transformer may have several secondary windings with the same primary, to allow for separate metering and protection circuits, or for connection to different types of protective devices. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. 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. Let us now have a look at errors specified for these accuracy classes: The limiting error shall not exceed the value tabulated below at rated frequency and burden from 25% to 100% of the rated burden. Live tank CTs are useful because the primary conductor is short, which gives better stability and a higher short-circuit current rating. Each class has an allowable maximum phase error for a specified load impedance. Transformer used to scale alternating current, used as sensor for AC power, "IEC 60050 - International Electrotechnical Vocabulary - Details for IEV number 321-02-34: "knee point voltage, "Measurement Errors Due to CT Phase Shift – Continental Control Systems", "CT Phase Angle Correction – Continental Control Systems", https://en.wikipedia.org/w/index.php?title=Current_transformer&oldid=985841849, Short description is different from Wikidata, Creative Commons Attribution-ShareAlike License, Capacitive coupling between primary and secondary, This page was last edited on 28 October 2020, at 08:03. Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. The standard accuracy classes for protection current transformer are 5P, 10P and 15P. The current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary.


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