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Instrument transformers are special types of transformers.These transformers are defined as transformers intended to supply measuring instruments, meters, relays and other similar devices.
When measuring large currents in a DO circuit, the current and voltage are measured using a low range ammeter with a suitable shunt. A high resistance parallel is used to measure high voltage.
Simple instruments are not suitable for AC circuit measurements especially for measuring high voltages as these are designed for low voltages. Also high rating tool is not suitable from financial point of view.
These quantities are measured by a special device known as an instrument transformer. These transformers step down the current and voltage by a certain amount. This reduction is measured with the help of low range ammeter and voltmeter and then converted to actual value by knowing the transformation ratio of the transformer.
Advantages of Instrument Transformers:
Instrument transformers have the following advantages over shunts and multipliers etc.
1. The readings of instrument transformers do not depend on their constants like resistance, capacitance and inductance etc.
2. The secondary windings of CT and VT show current and voltage respectively of such low value that the measurement falls within the range of the instrument.
3. The measurement circuit is isolated from the power circuit by the use of an instrument transformer.
4. Standardization of secondary windings of CT and PT makes it possible to standardize the instrument around the rating and thus greatly reduces the cost of instrument transformers making it easier for manufacture.
Types of Instrument Transformers:
There are two types of instrument transformers.
1. Current Transformer (CT)
2. Potential Transformer (PT/VT)
A current transformer is an instrument transformer in which the secondary current under normal use conditions is proportional to its primary current and varies in phase by an angle approximately zero for the correct direction of connection. It should be noted that in case of CT and PT that if one goes down the other will go up.
All current transformers work on the principle of ampere-turn balance in a closed magnetic circuit. A CT has a primary of Np turns and a secondary of Ns turns with currents Ip and Is respectively. A component of the primary ampere turns is used to magnetize the core and the resulting ampere turns are vectors of primary and exciting ampere turns are different and the exciting ampere turns cause errors in the CT.
CT cores are of two types:
* Laminated core
* Spiral wound core
Laminated cores are made from T, U, L, E and I stampings. Strips of such shape are cut and arranged in strokes to form the entire core cruciform core is not popular with CT.
Laminated cores are convenient for making CTs in which the primaries are also wound.
Spiral Wound Core:
A high quality CT has spiral wound cores. The core is formed by ring shaping stacked in cylindrical form. The secondary winding is wound on the core. The current carrying conductor acts as the primary winding and passes through the hole.
Spiral wound cores can be of two different types one is a ring shaped stamping stack and the other is using a spiral shaped stamper. Such types of cores are called toroidal cores. These cores are the most popular these days as it ensures that the flow path is always along the grain of the material and hence there is minimal reluctance.
CT requires key materials
1. Less reluctance
2. Less core loss
3. Highly permeable
5. Easy availability
6. Low cost
High permeability nickel iron cores are used for high precision CT.
CT is the material used to make the core.
1. Mumetal (70% Ni)
2. Parmandur (50% Ca, 50% Fe)
3. Hipernik (50% Ni, 50% Fe)
All materials have their own advantages and disadvantages. Materials are selected according to CT requirements.
Type of CT:
Regardless of the types of whistles due to the main material, some other types of whistles are popular based on the basics of their use and winding.
1. Type of CT lesion:
It is a type of CT in which more than one full turn of primary winding results in a wound on the core. CT is used to measure current in high voltage circuits in wound types where highly accurate measurement is not required.
2. CT Bar Types:
It is a CT in which the primary winding consists of a suitable sized bar and material forming an integral part of the CT. The current line on which we want to measure the current is connected to the strip. This type of CT is suitable for measuring high currents.
3. Dissolve CT Liquid:
Common CT’S are dry type and do not require oil or cooling medium for its operation without natural air.
But a special class of C has been developed which requires the use of oil or other suitable liquid of suitable characteristics as insulating and closing medium.
CTs of this type are used in switchyards and substations for measurement purposes and for connecting relays etc. for protection of machinery.
4. Hermetically Sealed CT:
It is a liquid immersed CT that is sealed and does not interact with ambient air. A cover is made of suitable material which does not allow air to come in contact with the CT.
5. Ring type CT:
This type of CT has an opening in the center through which the primary conductor fits. In these types of CTs the core is wound only for the secondary winding. The conductor passing through the CT itself acts as the primary winding. These are the most popular types of CT. Due to their easy availability and ready to use property.
6. Multicore CT:
It is a CT that has multiple secondary cores and a winding with a common primary winding.
Different cores of such a CT are used to connect various relays and protective devices to it. As one is used to measure the core current the other is connected with the differential protection relay.
7. Multi Ratio CT:
It is a type of CT in which multiple ratios can be obtained by reconnection, tapping, etc. in the primary or secondary winding. This type of CT has an advantage over others. It can be used to measure a wide range of currents depending on the measurement conditions. Normal CTs are manufactured to measure a specific value of current and are not suitable for higher values of current. But a multi-ratio CT can be used to measure long range current by varying the turns ratio.
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