instrument transformation

Instrument transformers are a special type of transformers, these transformers are defined as the transformer intended to power measuring instruments, meters, relays and other similar devices.

In measuring a large current in a do circuit, the current and voltage are measured using a low range ammeter with a suitable tap. To measure high voltage, high resistance is used in parallel.

For AC circuit measurements, especially high voltage measurements, simple instruments are not suitable as they are designed for low voltage. Furthermore, a highly rated instrument is not economically appropriate.

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

Advantages of instrument transformers:

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

1. Instrument transformers readings do not depend on their constants such as resistance, capacitance and inductance etc.

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

3. With the use of an instrument transformer, the measurement circuit is isolated from the power circuit.

4. With the standardization of the secondary winding of CT and PT, it is possible to standardize the instrument around the rating, and thus it is easy for manufacturing to have a great reduction in instrument transformer cost.

Instrument transformer types:

There are two types of instrument transformers.

1. current transformer (CT)

2. potential transformer (PT/VT)

CURRENT TRANSFORMER:

The current transformer is an instrument transformer in which the secondary current under normal use conditions is substantially proportional to the primary current and differs in phase from it by an angle that is approximately zero for a proper direction of the connections. It should be noted that in the case of CT and PT, if one is removed, the other will be increased.

All current transformers work on the principle of balancing amperes in a closed magnetic circuit. The CT consists of a primary winding of Np turns and a secondary of Ns turns that carry current Ip and Is respectively. A component of the primary amp turns is used to magnetize the core with the result that the amp turns available for transformation is the different vector from the primary and exciting amp turns. Exciting amp turns result in a CT error.

CT CORE:

TC cores are of two types:

* LAMINATED CORE

* SPIRAL WOUND CORE

Laminated core:

Laminated cores are made from T, U, L, E and I stampings. Strips of such shape are cut and arranged in one punch to form a full core cruciform core which is not popular in the case of TCs.

Laminated cores are convenient for making CTs of the type in which the primaries are also wound.

Spiral Wound Core:

High grade CTs have a spirally wound core. The core is made up of cylindrical stacked ring shapes. The secondary winding is wound on the core. The current carrying conductor acts as a primary winding and is passed through the hole.

Spiral wound cores can be of two different types, one to have a ring-shaped stamping stack and one to use spiral-shaped stamping. Nuclei of this type are called torodial nuclei. These cores are most popular these days because they ensure that the flow path is always along the grains of the grain-oriented material, and therefore there is minimal reluctance.

core material:

Core material requirements for CT are

1. Low reluctance

2. Low core loss

3. Highly permeable

4. Small with retention

5.Easy availability

6. Low cost

High permeability nickel iron cores are used for high precision CT.

The materials used for the manufacture of CT cores are.

1. Mumetal (70% Ni)

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

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

All materials have their own advantages and disadvantages. The material is chosen on the basis of the CT requirements.

CT TYPE:

Regardless of CT types due to core material, some more CT types are popular based on their usage and winding.

1. Type of wound by CT:

It is a type of CT in which the primary winding of more than one full turn is wound on the core. Winding type CTs are used to measure current in a high voltage circuit where a very accurate measurement is not essential.

2. CT bar type:

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

3.CT liquid immersed:

Ordinary CT’s are of the dry type and do not require oil or cooling medium other than natural air for their operation.

But a special class of C’s has been developed which requires the use of oil or other suitable liquid of suitable characteristics as an insulating and sealing medium.

These types of CTs are used in switchyards and substations for metering purposes and for connecting relays, etc. for device protection.

4. Hermetically sealed CT:

It is a liquid-immersed CT that is sealed and does not communicate with atmospheric air. A casing is made of suitable material that does not allow air to come into contact with the TC.

5. CT Ring Type:

It is a type of CT that has an opening in the center to accommodate a primary conductor through it. In these types of CTs, the core is wound for the secondary winding only. The conductor passing through the CT itself acts as the primary winding. These are the most popular type of CT. For its easy availability and ready-to-use property.

6. Multicore CT:

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

Different cores of such a CT are used to connect different relays and protective coins with it. For example, one core is used to measure current, the other is connected with a differential protection relay.

7. Multi-ratio CT:

It is a type of TC in which more than one relationship can be obtained through reconnection, taps, etc. in primary or secondary winding. This is the advantage of this type of CT over others. It can be used to measure a wide range of currents depending on the measurement conditions. Ordinary CTs are factory for measuring a particular current value and for higher current value they are not suitable. But multi-ratio CTs can be used to measure a long range of current by changing the turn ratio.