In the use of voltage transformers and current transformers, it is said that voltage transformers and current transformers are only the difference in turns ratio, meaning that the difference is not large, voltage transformers and current transformers can be used interchangeably. But is this really true?
The voltage transformer is strictly similar to a transformer, except that its secondary voltage is fixed at 100V.
From the circuit model point of view, it is true that only the åŒ is different. But in reality, the difference is still a lot
The winding operating current of the voltage transformer is small compared to the current transformer, so the wire diameter is different. The voltage transformer acts as a current transformer and the rated current is very small.
The current transformer is relatively low voltage transformer, the winding working voltage is very small, so even if the ratio is the same, the number of turns is much less. If the current transformer is used as a voltage transformer, the rated voltage will be very small.
Even if the ratio is the same, the current or voltage is small and can be used, which is very uneconomical.
Current transformers, that's very different. The following briefly describes the characteristics of the current transformer:
Let's look at the picture below:
Let us see Figure 1:
The impedance of the primary winding of the current transformer is much smaller than the load impedance, so the influence on the current of the primary system is negligible. Therefore, the current in the primary winding of the transformer is considered to be the true primary system current.
We know that the ratio of the secondary winding current of the transformer to the primary winding current is equal to the turns ratio of the primary and secondary windings. Therefore, we can estimate the magnitude of the system current by the magnitude of the secondary winding current.
The impedance of the secondary winding of the current transformer is large, but the load impedance of the secondary circuit is small, so the current transformer is equivalent to a converter that is close to short-circuit operation.
Let's look at Figure 2 and Figure 3:
The transfer characteristics of the current transformer have a nonlinear characteristic: when the primary current is larger, the deviation between the actual transfer characteristic and the ideal transfer characteristic is larger, and the error is also larger.
Note that this is related to the nonlinear characteristics of the core.
In addition, the secondary side of the current transformer is similar to a current source with a nonlinear internal impedance, and the ratio of the current source current to the primary current is always equal to the ratio.
Since we can only measure the current I2 after the internal impedance shunt, the larger the load impedance, the more the internal impedance shunting component will be, and the larger the error.
We all know that the voltage of the constant current source is determined by the external resistance. For Figure 3, if the external impedance Z2 is larger, the voltage on it will be higher. In particular, if the secondary circuit of the current transformer is open, the current transformer outputs the highest voltage, which will seriously harm personal safety. Therefore, the secondary circuit of the current transformer must be grounded.
The error curve of the current transformer is called the 10% error curve and is a very important indicator of the current transformer.
It can be seen that the current transformer and the voltage transformer are not interchangeable and must not be replaced with each other.
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