1. The influence of ambient temperature on the error of the transformer
(1) The influence of the temperature characteristics of magnetic permeability on the error.
The temperature characteristics of the magnetic permeability of the core will affect the error of the transformer.
The Curie temperature of cold-rolled silicon steel sheet is about 1000℃.
When the temperature changes by 25℃, the magnetic permeability changes by about 0.3%, and the excitation admittance changes by 0.3%, which may affect 1/30 of the whole unit.
The Curie temperature of iron-nickel alloy is about 340℃.
When the temperature changes by 25℃, the magnetic permeability changes by about 5%, and the excitation admittance changes by 5%, which may affect 112 whole units.
It can be seen that for current transformers made of silicon steel sheet core and iron-nickel alloy core, the influence of temperature characteristics on the error is not large.
In 1971, the United States developed an amorphous alloy composed of iron, boron, silicon and carbon, namely metallic glass, with core loss as low as 0.17W/kg.
In the 1980s, a small amount of copper and rare earth were added to the iron-based amorphous matrix.
After crystallization and annealing at an appropriate temperature, a soft magnetic alloy with ultrafine grains (diameter of about 10nm) with excellent performance was obtained, which is called microcrystalline material.
The magnetic properties of amorphous and microcrystalline materials are close to those of iron-nickel alloy materials, and the manufacturing cost is only 1/5 of that of iron-nickel alloy.
They have been used to manufacture precision transformers and power transformers with high performance requirements.
Amorphous and microcrystalline materials do not follow the ferromagnetic equation of crystals, and the effect of temperature on errors needs to be measured by experimental methods.
Experimental data show that the magnetic permeability of most amorphous and microcrystalline materials decreases significantly at low temperatures, and transformers made of these materials are not suitable for use in northern regions or outdoors.
(2) The effect of copper's resistance-temperature characteristics on errors.
Another effect of temperature on current and voltage transformers is to change the resistance value of the winding.
The temperature coefficient of copper resistance is 0.004/℃. When the temperature changes by 25℃, the resistance changes by 10%.
The theoretical error of the transformer changes by 10%, which may affect one unit.
Therefore, at the critical point of the error, attention should be paid to the adverse effect of temperature on the error of the transformer.
(3) The influence of the temperature characteristics of the component on the error.
Capacitive voltage transformers use film paper or full film capacitors.
A temperature change of 25℃ may cause the capacitance to change by 0.25%~0.5%.
The same material is used to make the high-voltage arm and low-voltage arm capacitors during design.
Through compensation, the error change can be reduced by one order of magnitude, equivalent to 1~2 units.
The damper is composed of a capacitor tuned to 50Hz and an inductor in parallel.
The capacitance of the capacitor generally has a negative temperature coefficient, and the inductance of the inductor has a positive temperature coefficient.
The influence of the temperature characteristics of the damper on the error needs to be determined through experiments.
Dampers produced by different manufacturers will have great differences due to different materials, structures and processes.
Test results show that the temperature coefficient of the damper can affect the error by as much as 0.3%, so it must be taken seriously.
The compensating reactor of the capacitive voltage transformer needs to be adjusted to resonate with the voltage divider capacitor.
This inductor uses an iron core with an air gap. The size of the air gap is affected by temperature.
The change in the size of the air gap can affect the inductance by 10-orders.
Due to the increase in detuning, the impedance of the primary circuit of the medium-voltage transformer increases, and the copper resistance of the inductor coil affects the Q value of the inductor coil, which will have an impact of 1~2 integer units on the error of the capacitive voltage transformer.
