The capacity of the current transformer is mainly based on the secondary use of the current transformer; Generally speaking, the secondary line is long, and the required capacity is larger; Secondary line; The capacity of the current transformer is generally 5VA-50VA, for short lines;

The selection of current transformer capacity should be combined with actual requirements, not the larger the larger; To consider whether it is installed on the distribution cabinet, it is necessary to look at the measuring unit (kilowatt-hour meter or comprehensive; It is suggested to comprehensively consider the following three aspects: 1, according to the size of the load current selection ratio, generally in accordance with 60-8; 2, the measurement of the transformer is selected with high accuracy.

The capacity of the current transformer is mainly determined by the size of the secondary load used by the current transformer, which is mainly related to the length and load of the secondary connection.

Generally speaking, the secondary line is long, and the required capacity is larger; The secondary line is short, and the capacity is optionally smaller.

The capacity of the current transformer is generally 5VA-50VA, 5VA can be selected for short lines, 20VA or 30VA is generally a little longer, and the special case can be larger.

The selection of the capacity of the current transformer should be combined with the actual requirements, not the larger the better, only when the selected secondary capacity is close to the actual secondary load, the accuracy of the current transformer is higher, and the capacity is too large or too small will affect the measurement accuracy.

Consider is installed on the power distribution cabinet, it depends on the measuring unit (meter or integrated protection device) and the distance between the transformer, if the measuring unit is in the distance of the integrated control room, then generally choose 20VA or 30VA, if the measuring device is also installed on the power distribution cabinet, then choose 5VA or 10VA can meet the requirements.

It is suggested to consider the following three aspects:

1, according to the size of the load current to choose the ratio, generally in accordance with 60-80 % the rated current selection is ideal;

2. The measurement transformer is selected with higher accuracy (0.5 grade is enough), and the measurement can be lower;

3, according to the layout of the distribution cabinet to choose the core type or ordinary transformer, it is strongly recommended to use the ordinary type, the fixed support problem of the core type has been done less reliable, if the layout is really small, it has to use the core type; In addition, please note the following points:

1. The current transformer with multiple secondary windings must firmly connect the idle secondary terminal with a copper core wire;

2, remember that it is strictly prohibited to install protection elements such as insurance and air switch on the secondary side of the current transformer;

3, must be in the power outage to work on the current transformer, do not disassemble, install the current transformer;

4, it is best not to take the load when the first live, even if the wrong line caused by the harm will be much less;

5, the current transformer cracking, discoloration, deformation, heating and other phenomena immediately cut off the power, do not carry. Calculation and selection of secondary capacity of current transformer

1.  Introduction

The current transformer plays an important role in the power system. The working principle of the current transformer is similar to that of a transformer. It changes the large current into a small current according to a certain proportion, provides the current used by various instruments and relay protection, and isolates the secondary system from the high voltage. It not only ensures the safety of people and equipment, but also simplifies and standardizes the manufacturing of instruments and relays, and improves economic benefits.

The rated primary current of the current transformer varies according to the normal current of different circuits, but the rated secondary current of the current transformer is standardized, and there are only two kinds of 1A and 5A. This paper calculates the secondary capacity required by the current transformer for measurement and maintenance at different transmission distances.

2.  Calculation of secondary load of current transformer

The load of the current transformer usually consists of two parts: one is the connected measuring instrument or protection device; The other part is the connecting wire. When calculating the load of the current transformer, attention should be paid to the impedance conversion coefficient under different wiring modes and fault conditions.

The secondary load of the current transformer can be expressed in terms of impedance Z2(Ω) or capacity S(VA). The relationship between the two is

S=I2*I2*Z2

When the secondary current of the current transformer is 5A, S=25 Z2

When the secondary current of the CT is 1A, S=Z2

The secondary load rating (S) of the current transformer can be selected according to the need of 5, 10, 15, 20, 25, 30, 40, 50, 60, 80, 100VA.

2.1 Load calculation of current transformer for measurement.

Generally, the phase difference between the impedance can be negative in engineering calculation, and the secondary load Z2 can be calculated according to the following formula

Z2=Kcj.zkZcj+Klx.zkZlx+Zc

Where: Zcj——- Impedance of meter coil (Ω)

Zlx——- one-way impedance (Ω) of the connecting wire, generally can ignore the reactance, only calculate the resistance.

Zc——- Contact resistance (Ω), generally 0.05~0.1(Ω).

Kcj.zk—- Impedance conversion factor of the meter

Klx.zk—- Impedance conversion factor of the connecting wire

The impedance conversion coefficients of various connections of the current transformer used for measurement are shown in Table 1 below:

2.2 Current transformer for protection.

Generally in the engineering calculation can be negative slightly reactance, secondary load Z2 can be calculated according to the following formula

Z2=Kj.zkZj+Kcx.zkZlx+Zc

Where: Zj——- Impedance of relay current coil (Ω)

Zlx——– One-way resistance (Ω) of the connecting wire.

Zc ——- Contact resistance (Ω), generally 0.05~0.1(Ω).

Kj.zk—– impedance conversion factor of relay

Kcx.zk——- Impedance conversion factor of the connecting wire

The impedance conversion coefficients of the protection current transformer under different short circuit types in various wiring modes are shown in Table 2 below

3 Calculation result of secondary load of current transformer

3.1 Calculation Conditions

This paper only considers the currently commonly used microcomputer protection measurement and control devices and multi-functional electronic metering devices. At present, the power of the measurement and protection devices produced by major integrated self-protection manufacturers is not more than 1VA/ phase. Generally, three-phase star connection or two-phase star connection is adopted. The cable length is 50 meters, 100 meters, 150 meters and 200 meters respectively.

3.2 Calculation result of secondary load of current transformer for measurement

Under the conditions of 3.1 calculation, the calculation results of the secondary load of the measuring current transformer are shown in Table 3:

Sl in the table is the cross-sectional area of the cable.

According to the requirements of 10.1.5 of Technical Regulations for Design of Electrical Measurement and Energy Metering Devices (DL/T 5137-2001), the load (including measuring instruments, energy metering devices and connecting wires, etc.) connected to the secondary winding of the current transformer should ensure that the actual secondary load is within the rated secondary load range of 25%~100%. As can be seen from Table 3, When the secondary current of the current transformer is 1A, the secondary capacity of 10VA can be selected and 2.5mm is used

22 section of the cable transmission distance can reach at least 200 meters, and when the current transformer secondary current is 5A, if the 2.5 mm section of the cable for transmission, then choose the secondary capacity of 30VA, the transmission distance can not reach 100 meters, so according to the requirements of the regulations should not be used 2.5 mm section

If the required transmission distance is L≤100 meters, a current transformer with a secondary capacity of 30VA can be selected. When the transmission distance is 100 meters ≤L≤200 meters, a current transformer with a secondary capacity of 60VA can be selected. If the transmission cable section is increased, such as the cable with a 6mm cross-section, the current transformer with a secondary capacity of 60VA can be selected. As long as L≤100 meters, the secondary capacity is only 30VA, and when 100 meters ≤L≤200 meters, the secondary capacity is only 40VA.

3.3 Calculation result of secondary load of current transformer for protection

In 3.1 calculation conditions, according to the impedance conversion coefficient is larger (except for two-phase phase phase connection) current transformer two-phase star connection, short-circuit type is Y,d transformer two-phase short-circuit calculation, the result is the other short-circuit method is larger, can be used as the current transformer secondary load calculation result.

How is the capacity of CT calculated, how is the accuracy of protection level and measurement level expressed, and what is the meaning

The capacity of the current transformer is generally Sn=I square *R, and the unit VA protection level is expressed by P, 10P,5P, and the measuring level is 0.2 and 0.5. The capacity of the constant voltage transformer is generally expressed by Sn=U square /R, and the unit VA protection level is expressed by P, and the unit VA protection level is expressed by P, and the measuring level is 0.2 and 0.5

Calculation and selection of secondary capacity of current transformer

Calculation diagram of secondary capacity of current transformer 1 Calculation diagram of secondary capacity of current transformer 2 Figure 3

         

See under what voltage: three-phase 220 load, KW 3.5 amps (KW/3.5A); Commonly used 380 load, a kilowatt two amps (KW/2A), low voltage 660 load, kilowatt one point two amps (KW/1.2A); 1,140 volts, 67 per kilowatt point (KW/0.67A); High voltage three thousand volt load; Four kilowatt-amps (4KW/1A)

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