In electrical engineering, quantities with large values need to be calculated relatively frequently. Current transformers are used to solve this problem, the purpose and operating principle of which allow some measurements to be carried out. For this reason, the device’s primary winding is attached sequentially to an alternating current circuit, the frequency of which must be determined. The primary and secondary windings have a certain proportion between the currents. All such transformers are highly accurate. Their design includes two or more secondary windings which are connected to protective devices, measuring instruments and metering devices.
What is a Current Transformer?
A current transformer is an electrical device that is used to increase or decrease the alternating current supplied to it.Current transformers provide where the secondary current used for calculation is equal to the electrical network’s main current. Inclusion in the primary winding circuit is performed in series with current lead. The secondary winding in the form of measuring instruments and various relays is connected to any load. There is a proportional relation, referring to the number of turns between the currents of both windings. Insulation between the windings in high voltage transformer systems is based on the maximum operating voltage. As a rule, one of the ends of the secondary winding is grounded, so the winding and earth potentials will be approximately equivalent.
All current transformers are designed to perform two main functions: measurement and protection. In some devices, both functions can be combined.
Measuring transformers forward the information received to related measuring instruments. They are mounted in high voltage circuits into which the measurement instruments can not be directly connected. Therefore, the connection of ammeters, counters, current windings of power meters and other metering devices is only in the secondary winding of the transformer. As a result, the transformer converts alternating current, even of very high value, into alternating current with indicators best suited for the use of conventional measuring instruments. At the same time, separation of measuring instruments from high voltage circuits is maintained and there is improved electrical health of maintenance personnel.
Protective transformer devices mainly forward the measurement information received to the control and safety devices. Using protective transformers, an alternating current of any value is converted to alternating current with the most appropriate value, providing the relay protection devices with the most power.
What is the purpose of the current transformer?
Current transformers belong to the group of special auxiliary devices used in the AC circuits together with different measuring devices and relays. Such transformers have the main function of converting any current value to the most convenient values for measuring, providing power to disconnect devices, and relay windings. Maintenance workers are adequately secured from electrical shock because of the insulation of the equipment.
Measuring current transformers are designed for high voltage electrical circuits where the direct connection of measuring instruments is not feasible. The main purpose is to relay the electric current data obtained to measuring devices connected to the secondary winding.
Transformers have an important function in controlling the state of the electrical current in the circuit they are attached to. Constant network checks, presence and grounding conditions are conducted when connecting to the power relay. When the current reaches the emergency value, it triggers security which switches off all the equipment used.
What is the working principle of Current Transformer?
The current transformers operating principle is based on the law of electromagnetic induction. With a certain number of turns, the voltage from the external network is supplied to the primary power winding and overcomes its total resistance. That results in the appearance of a magnetic flux trapped by the magnetic circuit around the coil. Which is perpendicular to the current direction . Due to this, there will be minimal loss of electric current during conversion. Flux also varies depending on the type of magnetic material.
The electromotive force stimulates the magnetic flux at the intersection of the secondary winding switches, located perpendicularly. A current occurs under the control of EMF which is required to resolve coil impedance and output load. At the secondary winding source, simultaneously, a voltage drop is observed.
Current Transformer Parameters and specifications:
Each current transformer has individual parameters and technical characteristics that determine the scope of these devices.
1. Current Rating.
It allows the apparatus to work without overheating for a long time. There is a significant margin for heating in such transformers and normal operation with overloads of up to 20 percent is possible.
2. Calculated Voltage.
The reliability would ensure that the transformer works normally. It is this indicator that affects the quality of insulation between the windings, one of which is grounded under high voltage and the other.
3. Transformer Ratio.
In the primary and secondary windings, it describes the relationship between currents and is defined by a special formula. Due to some losses during the process, the actual value may vary from the nominal.
4. Current Error.
It happens under the influence of magnetization current within a transformer. Just by this fact, the absolute value of the primary and secondary current varies between themselves. The current magnetization produces a magnetic flux in the core. The current transformer error also increases with its rise.