Saturday, June 15, 2019

Current transformer

What is the current transformer function?

The function of the current transformer is to feed the measuring instruments with a small current that is proportional to the current current in the circuit. The second side current should preferably be under 5 amperes in normal conditions.

Known as Turns Ratio, with standard values, for example: 100: 5 - 200: 5 - 300: 5, until we reach the maximum value in practice is 3000: 1.
The current transformer is connected directly to the circuit to measure its current.

Installation of the power adapter: The current transformer (C.T) consists of:
1. The heart is made of silicon steel or made of alloy of iron and nickel, and represents the magnetic circle.
2 - A primary file is a large clip connector, consisting of one or more rolls and connected directly with the line that will be installed the current transformer and hence came the label of the transformer in the Series Transformer and if the rated transformer is high, the primary file The expression of a straight conductor passes through the magnetic circle.
3 - Secondary file of a large number of twists made of insulated wire, damage to the heart, the primary file is isolated from the secondary file with an insulating material depending on the operating voltage, the greater the voltage increased insulation.
Current transformers:
Found three types of current transformers are:
1. Window-type C.T or Ring-type C.T or Through-type C.T
Consists of a heart in the form of a cylindrical ring made of iron strips, the secondary coil is wrapped on the heart - while the cable or the line of money through the aperture of the current transformer is the primary file
2. Bar-Type C.T
This type contains a solid bar which is the primary coil of the current transformer and is connected directly to the line or cable to which the current transformer is installed. This type is subject to the stresses produced by the high currents.
3. C-Type Wound-Type Transformer
It consists of a heart of iron filings, a primary file and a secondary file. The primary file is usually one or more parts of a connector with a large clip. It is connected directly to the circuit to be installed. This type sometimes contains a dual ratio, The converter has two primary files that are connected respectively or parallel according to the conversion ratio.

Why should the two terminals be shortened when not connected to a load?
In the current transformer, the initial current value (passing through the cable, line or bars, etc.) is determined according to the network conditions and no secondary current is in its value (unlike the voltage transformer). Ie, the primary stream is independent of the converter's conditions, including the conditions of its secondary circuit. Most of the primary current produces the magnetic flux at the heart of the transformer, which generates an electric impulse in secondary files. That is, the primary stream represents (mostly) the magnetization current. The load current (in secondary) produces the opposite magnetic flood of the primary flood, limiting the flow and thus the voltage on both ends of the secondary coil. If the secondary circuit of a current transformer is not connected to the load while remaining open, the secondary current is eliminated and the counter-effect of the large magnetic flood produced by the primary current of high (or very high) value is absent. The voltage difference between the two ends of the secondary (open) increases to very high levels that may reach a level that causes significant risks to both the transformer and the person handling it or to the equipment containing the transformer or adjacent to it. The iron heart of the transformer is also affected by the very high value of the magnetic flood due to its extreme saturation as well as high levels of heat generated by eddy currents and magnetic underdevelopment.
What is the term BURDEN ??
BURDEN is a term that expresses the carrying of measurement and protection circuits connected to the secondary transformer. The BURDEN is measured in Volt Ampere and not in Ohm, but they are equal and can be deduced one by the other. For example, the current transformer, which has a 12.5 VVA beam and its normal current in the secondary, is equal to 5A.
 ZB = P / I ^ 2 = 12.5 / 25 = 0.5.
And is always written on the Name Plate as a number such as 15 VA or 30 VA and always mentions next to the Accuracy number such as 5P20 if the Department of prevention or example Cl 0.5 if the measurement circle
The lower the protective device on the current transformer the better, because in this case we will not need a high-voltage transformer. This is one of the many advantages of digital shielding devices because they represent low Burden compared to traditional types of devices (Electromagnetic or static relays).

What is the difference between the current transformer used in the measurement of current transformers used in the protection?
The function of current transformers for measuring devices is to measure current in normal pregnancies
Current transformers for protection devices are used to measure current during faults
Thus, the magnetization curves of these transformers depend on their function. The measurement transformers are characterized by two linear equations with relatively high accuracy at small currents. The curve loop (KNEE POINT) of these transformers is low, ie, the iron heart of these transformers is malfunctioned.
The protection transformers are characterized by non-linear grants at the relatively small currents and linear curve at high currents. The KNEE POINT of these transformers is high, because the iron heart of these transformers does not occur in case of malfunction.
What are the main characteristics of the current transformer?
The most important quantities that should be characterized accurately with the current transformer are the following:
Rated Burden is the capacity in volts of ampere that can be loaded on current or voltage transformers permanently. The error value of the current and the angle of the face shall be within the permissible limits according to the accuracy level of the transformers.
The most popular values ​​are: 2.5, 5, 7.5, 10, 15, 30 VA
2. Maximum current current: The maximum current is often referred to as the primary side.
Short circuit current & duration time: The secondary current is often referred to. The maximum duration is usually the maximum current of between half a second and three seconds.
Rated current: often 1, 2, or 5 amp. Often, if the distance between the transformer and the protection device exceeds 30 meters, we use a secondary current equal to 1A.
5. Turns ratio: the maximum is 3000/1, and of course there are smaller values.
6 - Class: It is one of the most important and famous values ​​described by the converter.
The classes used in the protection only are 10P, 5P, in addition to the Class X, which is used with differential protection devices in accordance with the British BS. Class 5P is often used with Instantenous OC relays, while Class 10P is used with overcurrent devices of the type known as IDMT. In other words, the higher the accuracy required, the smaller the class used, and the applications that do not require high precision are limited to Class X
7- Accuracy limit factor ALF: Its value expresses the maximum current passing through the primary side without affecting the accuracy of reading in the secondary side, in other words the maximum current passes in the primary before the heart is saturated, and distort the secondary stream.

For example, if a transformer has a conversion ratio of 200/1 and the value of ALF = 5, it means that the maximum value of the primary current without distortion of the secondary current is equal to 200 * 5 = 1000.


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