Siemens APT transformer - the use, principle and selection of current transformer

The principle of the current transformer is based on the principle of electromagnetic induction. A current transformer is composed of a closed core and windings. Its primary side winding turns are very small, stringed in the line of the current that needs to be measured, so it often has all the current of the line flowing through, the number of turns of the secondary side winding is more, connected in series in the measuring instrument and the protection loop, when the current transformer is working, its secondary side loop is always closed, so the impedance of the series coil of the measuring instrument and the protection loop is very small, and the working state of the current transformer is close to short circuit. The current transformer converts the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.

If the current transformer ratio is determined purely from the rated current, the current transformer ratio can be selected as 1.3-2 times of the rated current; for the current transformer used for electric energy metering, the transformation ratio can be selected a little smaller, such as 1.3 times, or even a little smaller, and the current transformer with "S" class can be selected for the place where the load change is relatively large and there is often a small load; For the current transformer used for protection, the change ratio can be selected to be a little larger, such as 2 times, or even larger.

Current transformer accuracy selection and calibration:

The so-called accuracy refers to the maximum error when the primary current is the rated value within the specified secondary load range. For different measuring instruments, current transformers with different accuracy should be selected. Principles of accuracy selection:

The accuracy of the current transformer for billing and metering is 0.1-0.5;

The ammeter used to monitor the load current in each incoming and outgoing circuit should be selected as a 1.0-3.0 current transformer.

Proper use of current transformers:

1) The wiring of the current transformer should comply with the principle of series connection: that is, the primary winding resistance should be connected in series with the tested circuit, and the secondary winding resistance should be connected in series with all instrument loads;

2) According to the size of the measured current, select the appropriate change, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the high voltage of the primary side from entering the secondary low voltage side once the insulation is damaged, causing personal and equipment accidents;

3) Open circuits are absolutely not allowed on the secondary side

4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault recording devices, current transformers with 2-8 secondary winding are set up in generators, transformers, outgoing lines, bus segmented circuit breakers, bus tie circuit breakers, bypass circuit breakers and other circuits. For high-current grounding systems, it is generally configured as three-phase; For low-current grounding systems, they can be configured as two- or three-phase depending on the specific requirements.

Siemens APT transformer - the use, principle and selection of current transformer

Siemens APT transformer - the use, principle and selection of current transformer

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