Instrument Transformer CT and PT - Weikewei - Professional FAE Guidance
For largealternating currentThe measurementWe use current transformers.On the other hand, in order to measure heightAC voltage,We used itVoltage Transformer.We collectively refer to these current transformers and voltage transformers asInstrument Transformer.Not only for measurement purposes, we also use transformers for electrical protection purposes.
Application of Transformer
Current transformers reduce the current of power circuits for measurement and protection purposes.on the other hand,For certain measurement and protection purposes, voltageTransformer willReduce the voltage of the power system.In fact, the amount of current flowing through the high-voltage power grid is very large.Therefore, there is no universal measuring instrument that can directly measure this high current.The same applies to the voltage of high-voltage power systems.Although theoretically, we can design instruments that can measure these huge currents and voltages without lowering them.But in reality, these tools have become large, complex, and expensive.Therefore, by using transformers, we can use standard rated measuring instruments to measure relatively high voltages and currents.Another advantage of using transformers is safety.During the measurement process, there is no need for anyone to come into contact with the high voltage and high current of the system.Therefore, it is safe for them.In electrical protection, transformers play a crucial role in overcurrent, undervoltage, ground faults, and many other types of protection schemes.
Basic Circuit Diagram
Advantages of transformers
Previously, the secondary rated current of current transformers was 1A or 5A.Currently, there are only 1A available.In modern times, we rarely use 5A current transformers.Since the secondary current is 1A in all current transformer levels, we can use an ammeter with a rated current of 1A to measure different current levels.
VoltageTransformerThe standard secondary rated voltageFor 110V.Therefore, using the same 110V rated voltmeter, we can measure various high voltages.
Instrument transformers help to keep measuring instruments away from live high voltage and high current power systems.In this case, we use a longer secondary lead from the secondary iron core of the instrument transformer to the remote measuring instrument.
A single instrument transformer can operate many instruments at once.
These consume very little power from the system.
By using a current transformer, it is possible toWithout disconnecting the linemeasureThe routealternating current.For example, clip on instruments are such instruments.
Disadvantages of transformers
Because it's a transformerTransformers cannot handle direct current
The burden of instrument transformers
The burden is the VA rating of the transformer.More clearly, it is the rated volt ampere value at the rated secondary voltage.This is the maximum allowable load on the secondary winding, below which the transformer will cause limited errors.Instrument transformers cannot be error free.But each of them has an allowed error limit.Therefore, the burden is the load, under which the transformer will cause errors within its limit range.
If the secondary induced voltage of the device is V. The impedance of the secondary instrument and relay circuit (including the impedance of the secondary winding) is Z. The secondary burden is V2/Z.
Similarly, if the current passing through the secondary circuit is I.
Transformation ratio of instrument transformer
The transformation ratio is the ratio of the primary signal to the secondary signal.For current transformers
For voltage transformers,
