In the measuring large current in a do circuit current and voltage is measured by using low range ammeter with a suitable shunt. For measuring high voltage high resistance is used in parallel.



For AC circuit measurements specially for high voltage measuriment the simple instruments are not suitable as these are designed for low voltage. Also a high rating instrument are not suitable on the economic point of view.

The measurement of these quantities is done with special device known as instrument transformers. These transformers steps down current and voltage in definite proportion. This reduced quantity is measured with the help of low range ammeters and voltmeters and are then changed to actual value by knowing the transformation ratio of the transformer.

Advantages Of Instrument Transformers:

The following are the advantages of instrument transformers over the shunts and multipliers etc.

1.The readings of instrument transformers does not depends upon their constants such as resistance, capacitance and inductance etc.

2.The secondary winding of CT &VT shows a current and voltage respectively of such a low value which falls in the range of measuring instrument can be done easily.

3.With the use of instrument transformer the measuring circuit is isolated from power circuit.

4.With the standardization of CT & PT's secondary winding it is possible to standardize instrument around the rating and so it becomes easy for manufacture to have a great reduction in costs of instrument transformer.

Types of instrument transformer:

Two types of instrument transformer are there.

1.current transformer (CT)

2.pottential transformer (PT/VT)

CURRENT TRANSFORMER:

Current transformer is an instrument transformer in which the secondary current in normal conditions of use is substantially proportional to thee primary current and differs in phase from it by an angle which is approximately zero for an appropriate direction of connections. It should be noted that in case of CT and PT that if one stteped down the another will get stteped up.

All current transformers operate on the principal of ampere turn balance in a closed magnetic circuit. The CT consists of a primary winding of Np turns and secondary of Ns turns carrying current Ip and Is respectively. A component of the primary ampere turns is utilized in magnetising the core with the result that ampere turns available for transformation is the vector differ of primary and exciting ampere turns the exciting ampere turns give rise to error in CT.

CT CORES:

CT cores are of two types:

* LAMINATED CORE

* SPIRAL WOUND CORE

Laminated core:

The laminated cores are made from T, U, L, E and I stampings. The strips of such a shape are cut and arranged in a struck to form complete core cruciform core is not popular in case of CT's.

The laminated cores are convenient for making CT's of the type in which primaries are also wound.

Spiral wound core:

The CT's of high grade are having spiral wound core. The core is made by ring shapeings stacked in cylindrical form. The secondary winding is wound on the core. The conductor carrying the current acts like primary winding and is made passing through the hole.

The spiral wound cores may be of two different types one so to have a stack of ring shaped stamping and another is to use stamping of spiral form. The cores of such a type are called torodial cores. These cores are most popular these days because it ensures that the flux path is always along the grains of grain oriented material and so minimum reluctance is there.

Core Material:

The requirements of core material for CT's are

1.Low reluctance

2.Low core loss

3.Highly permeable

4.Small retentively

5.Easy availability

6.Low cost

The high permeability nickel iron cores are used for high precision CT's.

The materials used for making CT cores are.

1.Mumetal (70% Ni)

2.Permandur (50% Co, 50% Fe)

3.Hipernik (50% Ni, 50% Fe)

All the materials have their own advantages and disadvantages. The material is chosen on the basic of the requirement of CT's.

TYPE OF CT:

Irrespective of the types of CT due to core material some more types of CT's are popular on the basic of their use and winding.

1. CT Wound Type:

It is type of CT in which the primary winding of more than one full turns wound on the core. The wound types CT's are used for measuring current in a circuit of high voltage where a very precise measurement is not essential.

2. CT Bar Type:

It is CT in which the primary winding consists of a bar of suitable size and material forming an integral part of the CT. the current line on which we have to measure current is connected to the bar. This type of CT is suitable to measure high currents.

3. CT Liquid Immersed:

Ordinary CT'S are of dry type they do not require and oil or cooling medium other than natural air for its operation.

But a special class of C's has been developed which require use of oil or other suitable liquid of suitable characteristics as insulating and closing medium. These types of CT's are used in switchyards and substations for the purpose of measurement and for connecting relays etc. for protection of the device.

4. Hermetically sealed CT:

It is a liquid immersed CT which is sealed and does not communicate with atmospheric air. A casing of suitable material is made which does not allowed the air to be in touch with CT.

5. Ring type CT:

It is type of CT which has an opening in the center to accommodate a primary conductor through it. In these types of CT's the core is wound only for secondary winding. The conductor passing through CT itself acts as primary winding. These are the most popular type of CT's. Because of their easy availability and ready to use property.

6. Multicore CT:

It is CT having more than one secondary core and winding with a common primary winding.

The different cores of such a CT are used to connect different relays and protective devises with it. Such as one core is used for measuring current other is connected with differential protection relay.

7. Multi Ratio CT:

It is type of CT in which more than one ratio is obtainable by reconnection, tappings etc. in primary or secondary winding. This is the advantage of this type of CT on others. It can be used for measuring a wide range of currents according to the measuring conditions. Ordinary CT's are manufactories to measure a particular value of current and for a higher value of current they are not suitable. But the multi ratio CT's can be used for measuring a long rang of current by changing the turn's ratio.

Lalit Bansal