What is the current transformer?
The insulation on the bar-type primary windings is the nickelized paper tube or a epoxy resin directly moulded on the bar. Such bar type transformer is an integral part of the current transformer. The core and secondary windings are the same in bar type transformer.
The large alternating current which cannot be sensed or passed through the normal ammeter and current coils of wattmeter's, energy meters can easily be measured by use of current transformer along with normal low range instruments. The current flowing through the high-voltage transmission lines is easily measured by the current transformers.
A transformer is a device that consists of two windings called primary and secondary windings. It transfers energy from the primary winding side to the secondary winding side with a suitable change in the level of current or voltage.
The primary winding of the current transformer is connected in series with the high current carrying circuit and the secondary winding is connected to a measuring or relaying device for accurate measurement of current for energy accounting or protective purposes.
The current transformer is a device that is used to convert higher-value current to lower-value current. The current transformer is used to convert high voltage current to low voltage current which can easily be measured by an ammeter. In a current transformer current in the primary is proportional to the current in the secondary windings. the current transformer is used for measuring high voltage current because metering instruments like ammeters have the difficulty of insulations. the current transformer is used to measure a high voltage current up to 100 amperes.
Principle of current transformer:
The current transformer is basically a step-up transformer i.e stepping up the voltage from primary to secondary. thus current reduced from primary to secondary. so from the current point of view, current transformers step down transformer, stepping down the current value considerably from primary to secondary.
Construction of current transformer:
The core of the current transformer is made from the lamination of silicon steel. In current transformers, for making core permalloy or mumetal are used for achieving higher accuracy.
The primary winding of the current transformer carries a high voltage current which is to be measured and the primary winding of the current transformer is connected in series with a high voltage main circuit. The secondary windings of the current transformer carry current which is to be easily measured. The secondary winding is connected to the current coil of the ammeter. The current transformer steps down the secondary current for secondary protection and primary current monitoring. By using a current transformer, the high voltage current is converted to the low voltage current, and the secondary low voltage current is correlated to the primary high voltage current.
The current transformer has a primary coil with one or more turns of heavy cross-sectional area. In some current transformers, a bar carrying a high current is known as the primary winding. The primary winding of the current transformer is connected in series with the line carrying a high current.
The current transformer has a secondary coil with a large number of turns of fine wire with a small cross-sectional area. the secondary winding of the transformer is rated for 5A, 1A, 0.1A. The current transformer primary coil is rated from 10A to 3000A or more. The secondary winding is connected to the coil of the normal range ammeter. The load impedance of the current transformer is very low. So, current transformers are operating in short circuits.
The ratio of primary current to secondary current is known as the current transformer ratio.
Burden of Current Transformer:
The value of the load which is connected to the secondary winding of the current transformer is known as burden of the current transformer. The burden of the current transformer is expressed in Volt-amperes (VA).
Type of current transformer and its constructions:
There are two types of current transformers.
1. Wound-type current transformer.
2. Bar-type current transformer.
1. Wound-type current transformer
In wound type transformer, the primary is wound for more than a full turn, on the core of the current transformer as shown in fig.
In a low voltage wound type current transformer, the secondary winding is wound on the Bakelite former. The heavy primary winding is directly wound on the top of the secondary winding with suitable insulation in between primary and secondary windings. Otherwise, the primary winding wound completely separately, taped with suitable insulating material and assembled with the secondary windings on the core.
The current transformer can be ring type or window type. The core of the wound-type transformer is made from nickel-iron alloy or oriented electrical steel. Before installing the secondary winding on the core it is insulated with help of end collars and circumferential wraps of pressboards. these pressboards made from epoxy material provide additional insulation and protection to the winding from damage due to the sharp corners. The wound type of transformers are used for measuring the current from 1A to 100A.
2. Bar type current transformer:
In this type of current transformer, the primary is nothing but a bar of suitable size as shown in fig.
In the current transformer, laminations' stampings have a larger cross-sectional area than other ordinary transformers. Due to the larger cross-sectional area of stampings, the reluctance of the interleaved corners of laminations is remained as low as possible. due to this corresponding magnetizing current remains small. The primary and secondary windings are placed close to each other to reduce leakage reactance. The external diameter of the tube is kept large to avoid the corona effect.
The windings of the current transformer are designed so that without any damage, windings can withstand short circuit current forces which may occur due to a short circuit in the circuit in which the current transformer is inserted.
The tape and varnish insulation material are used for small line voltages. The oil-immersed or compound-filled current transformers are used for line voltage above 7kV.
Advantages of current transformer:
1. The current transformer can measure high current easily.
2. The current transformer has high electrical isolation.
3. The construction has a simple construction.
4. The current transformer has easy maintenance and installation.
5. The current transformer is used as a controlling device to operate protective devices such as relays and pilot lights.
Applications of current transformer
1. The current transformer is used for converting high voltage current to low voltage current and measuring the current flowing through the high voltage lines.
2. The current transformer has high accuracy which is used to measure power in lines.
3. The current transformer is used in revenue metering applications.
Why secondary winding of the C.T. should not be open-circuited?
Secondary windings of the current transformer mustn't be open-circuited. Either the secondary winding of the current transformer is shorted or connected in series with the low resistance coil of the wattmeter or ammeter. If the secondary winding of the current transformer remains open, the secondary current becomes zero and secondary ampere turns which is generally opposed to primary ampere turns become zero.
Due to the secondary ampere turn zero, counter m.m.f becomes zero and unopposed primary m.m.f (ampere-turns) produce high flux in the core of the current transformer. This causes excessive losses in the core and heats the core beyond the limits and heavy e.m.fs are produced in primary and secondary sides. This causes damage to the insulation of windings. this is very dangerous from the operator's point of view also.
It is very important to ground the current transformer's secondary side to avoid the danger of shock to the operator. Never open the secondary winding of the current transformer when the primary winding of the current transformer is energized. This is also causing damage to the insulation of the current transformer and also accuracy loss in the current transformer.
Most of the current transformer has a short circuit link or a switch at the secondary side of the current transformer. when the primary is energized, the short circuit link must be closed so that there is no danger of open circuit secondary.