Instrument Transformers - CT and PT

Instrument Transformer
Instrument transformers are typically used to operate instruments from high voltage lines or high current circuits, safely isolating measurement and control circuitry from the high voltages or currents. The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit. Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used without affecting the primary circuitry.

Current transformer

A current transformer (CT) is a series connected measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary. Current transformers are commonly used in metering and protective relays in the electrical power industry.

Current transformers are often constructed by passing a single primary turn  through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, a 1000:1 CT provides an output current of 1 amperes when 1000 amperes flow through the primary winding. Standard secondary current ratings are 5 amperes or 1 ampere, compatible with standard measuring instruments. The secondary winding can be single ratio or have several tap points to provide a range of ratios. Care must be taken to make sure the secondary winding is not disconnected from its low-impedance load while current flows in the primary, as this may produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer.

Voltage transformer or potential transformer

Voltage transformers (VT), also called potential transformers (PT), are a parallel connected type of instrument transformer, used for metering and protection in high-voltage circuits or phasor phase shift isolation. They are designed to present negligible load to the supply being measured and to have an accurate voltage ratio to enable accurate metering. A potential transformer may have several secondary windings on the same core as a primary winding, for use in different metering or protection circuits. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal.

There are three primary types of voltage transformers : electromagnetic, capacitor, and optical. The electromagnetic voltage transformer is a wire-wound transformer. The capacitor voltage transformer uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic VT. An optical voltage transformer exploits the electrical properties of optical materials. Measurement of high voltages is possible by the potential transformers.

Types of Transformers - Shell and Core type Transformer

Core type & Shell type transformer

In core type transformer it has two vertical legs or limbs with two horizontal sections named yoke. Core is rectangular in shape with a common magnetic circuit. Cylindrical coils (HV & LV) are placed on both the limbs.

Shell type transformer: It has a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. Double magnetic circuit is present. 

Dry-Type or Liquid-cooled Transformer

Oil Cooled & Dry Type Transformer:

In oil cooled transformer the cooling medium is transformer oil whereas the dry type transformer is air cooled.

Dry type transformer never uses any insulating liquid where its winding with core be immerged. Rather windings with core are kept within a sealed tank that is pressurized with air.

The dry type transformer is of two types. They are

A-   Cast Resin Dry Type Transformer ( CRT)

B-   Vacuum pressure Impregnated Transformer ( VPI)

Transformers change voltages from one level to another. Most commonly, that change involves very high power line transmission voltages (500 kV) being reduced to the much lower levels used in heavy industry (as much as 30 kV) and households (120-240 V). Dry type, or air-cooled, transformers accomplish this function so safely and efficiently that they are commonly used for indoor applications where other transformer types are considered too risky.

Basically, there are two distinct types of transformers: Liquid insulated and cooled (liquid-filled type) and non liquid insulated, air or air/gas cooled (dry type). Also, there are sub categories of each main type.

For liquid-filled transformers, the cooling medium can be conventional mineral oil. There are also wet type transformers using less flammable liquids, such as high fire point hydrocarbons and silicones.

Comparision:

Dry-type transformers with ratings exceeding 5MVA are available, but the vast majority of the higher-capacity transformers are liquid-filled. For outdoor applications, wet-type transformers are the predominate choice.

Liquid-filled transformers are normally more efficient than dry-types, and they usually have a longer life expectancy. Also, liquid is a more efficient cooling medium in reducing hot spot temperatures in the coils. In addition, liquid-filled units have a better overload capability.

Dry Type transformers use air as the cooling medium. Oil Type transformers are considered a potential fire and safety hazard. Oil Type transformers require the development and maintenance of reliable fire safety and extinction procedures.

Oil Type transformers may require periodic sampling of the oil and more exhaustive maintenance procedures.

Dry Type Transformers can be located closer to the load unlike oil transformers which require special location and civil construction for safety reasons. Locating the transformers near the loads may lead to savings in cable costs and reduced electrical losses.

Transformer Types | Step Up Transformer, Step Down Transformer

The input winding to a transfonner is called the primary winding. The output winding is called the secondary winding. If there are more turns of wire on the primary than on the secondary, the output voltage will be lower than the input voltage. This is called step down transformer.

If there are more turns of wire on the secondary than on the primary, the output voltage will higher than the input voltage. This is called step up transformer.

Transformer Types

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are given below

1- Step Up Transformer

    2-Step Down Transformer

     3- Three phae Transformer

       4- Single Phase Transformer

  5- Core Type Transformer

  6- Shell Type Transformer

           7- Oil Cooled Type Transformer

 8-  Dry Type Transformer

9-  Outdoor Transfomer

10-   Indoor Transformer

      11- Instrument Transformer

12-    Power Transformer

         13-  Distribution Transformer

What is transformer? Definition and Working Principle of Transformer

                                                           Transformer Basics

A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Electromagnetic induction produces an electromotive force within a conductor which is exposed to time varying magnetic fields. Transformers are used to increase or decrease the alternating voltages in electric power applications.

Electrical power transformer is a static device which transforms electrical energy from one circuit to another without any direct electrical connection and with the help of mutual induction between two windings. It transforms power from one circuit to another without changing its frequency but may be in different voltage level.

One of the main reasons that we use alternating AC voltages and currents in our homes and workplace’s is that AC supplies can be easily generated at a convenient voltage, transformed (hence the name transformer) into much higher voltages and then distributed around the country using a national grid of pylons and cables over very long distances.

The reason for transforming the voltage to a much higher level is that higher distribution voltages implies lower currents for the same power and therefore lower I²R losses along the networked grid of cables. These higher AC transmission voltages and currents can then be reduced to a much lower, safer and usable voltage level where it can be used to supply electrical equipment in our homes and workplaces, and all this is possible thanks to the basic Voltage Transformer.

The Voltage Transformer can be thought of as an electrical component rather than an electronic component. A transformer basically is very simple static electro-magnetic passive electrical device that works on the principle of Faraday’s law of induction by converting electrical energy from one value to another.

The transformer does this by linking together two or more electrical circuits using a common oscillating magnetic circuit which is produced by the transformer itself. A transformer operates on the principals of “electromagnetic induction”, in the form of  Mutual Induction.

Mutual induction is the process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. Then we can say that transformers work in the “magnetic domain”, and transformers get their name from the fact that they “transform” one voltage or current level into another.

Transformers are capable of either increasing or decreasing the voltage and current levels of their supply, without modifying its frequency, or the amount of electrical power being transferred from one winding to another via the magnetic circuit.