CT selection parameters refer to the current transformer model and the main parameters.
I. Current transformer model?
The first letter: L-current transformer
The second letter: A-through-wall; Z-pillar; M-busbar; D-single-turn-through; V -structural inverted; J-zero sequence
ground detection; W-anti-fouling; R-winding exposed type
The third letter: Z-epoxy resin casting type; C -porcelain insulation; Q-gas insulation medium; W-specialized with microcomputer protection
Fourth letter: B-band protection level; C-differential protection; D-D level; Q -strengthened; J-strengthened ZG
Fifth number: voltage level Product serial number
II, the main technical requirements
1, rated capacity: rated secondary current through the secondary rated load consumed in the apparent power. Rated capacity can be expressed in terms of apparent power V.A, also can be expressed in terms of secondary rated load impedance Ω.
2, a rated current: the current transformer primary winding through the current load current. Used for power system current transformer primary rated current for 5 ~ 25000A, used for test equipment precision current transformer for 0.1 ~ 50000A.
Current transformer can be a rated current long-term operation, load current exceeds the value of the rated current is called overloading, the current transformer long-term overloading operation, will burn out the windings or reduce the service life.
3, secondary rated current: allowed to pass through the secondary winding of the current transformer primary induction current.
4, rated current ratio (ratio): primary rated current and secondary rated current ratio.
5, rated voltage: the primary winding can withstand long-term maximum voltage to ground (RMS value in kV units), should not be less than the rated phase voltage of the connected line. Current transformer rated voltage is divided into 0.5, 3, 6, 10, 35, 110, 220, 330, 500kV and other voltage levels.
6, 10% times: in the specified secondary load and any power factor, the current transformer current error of -10%, the primary current to its rated value of the times. 10% times is related to the technical indicators of relay protection.
7, accuracy level: indicates the level of error of the transformer itself. Current transformer accuracy level is divided into 0.001 ~ 1 multiple levels, compared with the original accuracy improved greatly. Used in power plants, substations, power units distribution control panel on the electrical instrumentation generally use 0.5 or 0.2 level; for equipment, line relay protection is generally not less than 1 level.
8, than the difference: the error of the transformer includes the ratio of the difference and the angle of the difference between the two parts. Ratio error is referred to as the ratio difference, generally expressed by the symbol f, which is equal to the actual secondary current and converted to the secondary side of the difference between the primary current, and converted to the secondary side of the ratio of the primary current, expressed as a percentage.
9, angular difference: phase angle error is referred to as angular difference, generally expressed by the symbol δ, which is rotated 180 ° after the secondary current vector and the primary current vector of the phase difference between the vectors. Provides that the secondary current vector exceeds the primary current vector δ for positive values, and vice versa for negative values, with minutes (') for the unit of calculation.
10, thermal stability and dynamic stability times: power system failure, the current transformer is caused by the short-circuit current due to the thermal and electrodynamic effects of the huge current, the current transformer should be able to withstand the ability not to be damaged, the ability to withstand the thermal stability and dynamic stability times.
Thermal stability times refers to the thermal stability of the current 1s does not cause the current transformer heating over the permissible limit of current and current transformer rated current ratio. Dynamic stability times is the current transformer can withstand the maximum current instantaneous value of its rated current ratio.
Expanded information:
1, ratio
Current transformer ratio should be selected according to the primary load calculation current IC. Current transformer primary side rated current standard ratio (such as 20, 30, 40, 50, 75) and other specifications, the secondary side of the rated current is usually 1A or 5A. 2 × a / C, which indicates that the same product has two kinds of current ratios, by changing the product's connecting piece wiring to achieve, when connected in series, the current ratio of a / c, when connected in parallel, the current ratio of 2 × a / C.
Generally
Generally speaking, the current ratio of the current transformer for metering should be selected so that its primary rated current I1n is not less than the load current (i.e., calculated IC) in the line. If the load current in the line is calculated as 350A, the current transformer ratio should be selected 400/5. Protection of the current transformer to ensure its accuracy requirements, the ratio can be selected a little larger.
2, accuracy level
Accuracy level of current transformer should be selected and calibrated according to the requirements of measurement accuracy. Accuracy level selection principles: billing metering current transformer accuracy level is not less than 0.5 level; used to monitor the load current size of the inlet and outlet circuits of the ammeter should be selected 1.0-3.0 level current transformer.
In order to ensure that the accuracy of the error does not exceed the specified value, generally also calibrate the current transformer secondary load (volt-ampere), the secondary load of the transformer S2 is not greater than the rated load S2n, the accuracy of the selected in order to be guaranteed. Accuracy calibration formula: S2 ≤ S2n.
Baidu Encyclopedia - Current Transformer