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Introduction To Instrument Transformers
Aug 26, 2017

Introduction to instrument transformers

The transformer is a device that converts voltage or current in proportion. The function of the transformer is to standardize the high voltage or high current to standard low voltage (100V) or standard small current (5A or 1A, all rated) in order to realize the standardization of measuring instruments, protection equipment and automatic control equipment, miniaturization. Transformers can also be used to isolate high voltage systems to ensure personal and equipment safety.

Transformer principle

In the electricity supply line in the current and voltage vary greatly from the security to several tens of thousands of security have. In order to facilitate the secondary instrument measurement need to be converted to a more uniform current, the other line voltage is relatively high, such as direct measurement is very dangerous. Current transformers play a role in the flow and electrical isolation.

Earlier, most of the display instrument is a pointer of the current and voltage table, so the current transformer secondary current is mostly amperes (such as 5A, etc.). Now most of the electricity measurement of the word, and the computer sampling signal is generally mA level (0-5V, 4-20mA, etc.). Micro current transformer secondary current for the mA level, mainly from the big transformer and the bridge between the role of sampling.

Micro current transformer called "instrument with current transformer." ("Instrument Current Transformer" has a layer of meaning is used in the laboratory of multi-current than the precision current transformer, generally used to expand the instrument range.

Current Transformer Principle Circuit Diagram Micro current transformer and transformer similar is also based on the principle of electromagnetic induction work, the transformer is the voltage and micro current transformer transform is the current Bale. (Or primary winding, primary winding); winding N2 measurement instrument, known as the secondary winding (or secondary winding, secondary winding).

Micro current transformer primary winding current I1 and secondary winding I2 current ratio, called the actual current ratio K. Micro current transformer in the rated current when the current working current than the current transformer rated current ratio, said Kn.

Kn = I1n / I2n

Micro current transformers can be divided into two categories, measuring current transformers and protection with current transformers.

Measuring current transformers

Measuring current transformer with the main measurement instrument with the normal working conditions in the line, used to measure the current, voltage, power and so on.

Measurement of micro current transformer main requirements:

1, insulation is reliable, 2, high enough measurement accuracy, 3, when the measured line occurs when the high current transformer should be in the appropriate range of saturation (such as 500% of the rated current) to protect the measuring instrument.

Protection current transformer

The protection current transformer is mainly used in conjunction with the relay device to provide the signal to the relay device to cut off the fault circuit to protect the safety of the power supply system when the fault occurs such as short circuit overload. The working conditions of the micro current transformer for protection are completely different from those of the measuring transformer. The protective transformer is only effective when the current is several times larger than the normal current.

Protection of the main requirements of the transformer: 1, reliable insulation, 2, large enough accurate limit factor, 3, sufficient thermal stability and dynamic stability.

Protection of the transformer in the rated load to meet the requirements of the exact level of the maximum current is called the exact limit of the current limit. The exact limit factor is the rated accuracy limit of the primary current and the rated primary current ratio. When the current is large enough when the core will be saturated can not reflect the role of a current, accurate limit coefficient is that this feature. Protection Transformer Accuracy 5P, 10P, indicates the allowable error of 5%, 10% of the rated current at the rated exact limit.

Line current failure occurs when the impact of current and electromagnetic force, the protection of current transformers must bear. Secondary winding short circuit case, the current transformer in a second can withstand without damage to the primary current RMS, said the rated short-term thermal current. Secondary winding short circuit case, the current transformer can withstand without damage to the primary current peak, said the rated dynamic and stable current.

Protection current transformer is divided into: 1, overload protection current transformer, 2, differential protection current transformer, 3, ground protection current transformer (zero sequence current transformer).


Add: No.38 South Taoyuan Road, Yaozhuang Town,Jiashan County, Jiaxing City, Zhejiang Province, China
Contact person: Elva Zhuang&Sunny Ni
Tel:+86-573-84775555
Fax:+86-573-84776699
Phone:+86-13732570078
E-mail:sales9@great-relay.com
sunny.ni@great-relay.com