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How to deal with the unstable output voltage of the transformer, sometimes high and sometimes low?

The output voltage of the transformer will be high and low. What causes it?

The voltage on the primary side of the transformer fluctuates greatly, and the load on the secondary side of the transformer fluctuates greatly. When the load is greater than the capacity of the transformer, the voltage on the secondary side will decrease. When the load falls within the range that the transformer capacity can withstand, the voltage will again Will rise.

   First of all, let’s analyze the internal factors. There is a high-power electrical appliance at the load of the inverter, which causes the voltage to drop instantaneously. When the high-power motor is started, the voltage returns to normal. This is the most common cause. Analysis of external factors, a high-voltage line has multiple transformers in use, and the voltage drop is usually close to the end of the high-voltage power consumption. The increase in the power consumption of the current transformers will definitely affect the voltage of the transformers at the end. The high and low voltage is basically caused by the increase in power consumption, so don't care too much, just adjust the internal power relay protection of the enterprise.

Ignoring the problems of the transformer itself, assuming that the transformer is an ideal transformer (the so-called ideal transformer means that the permeability is infinite and there is no loss in the core), it can be obtained that there is a numerical formula U1/U2= between the voltage at the output terminal of the transformer and the input voltage on the high voltage side. N1/N2


If N1/N2 can be regarded as a fixed value if it is not a load regulating transformer, then the output voltage of the transformer is only proportional to the input voltage on the high voltage side. The possible reasons for the fluctuation of the transformer output voltage are:

   1. The high voltage side voltage is unstable.

2. The load of the transformer fluctuates greatly. When the other parameters of the transformer remain unchanged, the load becomes larger, that is, the current flowing through the secondary side of the transformer becomes larger, and the current flowing through the high voltage side of I1/I2=N2/N1 will also change. The power loss in the high voltage transmission circuit is proportional to the square of the current. When the terminal voltage is not increased on the high voltage side, the load current becomes larger, and the voltage drop generated on the cable in the transmission circuit △U=I*R is also Will become bigger.

   3. There are large equipments in the nearby power grid that are starting or stopping because of the impact of the large current on the power grid.

   Knowledge supplement:

  How to adjust transformer output voltage

  General distribution transformers are mostly no-load voltage regulation, and there are 3 gears for regulation. The number of turns of the transformer winding is changed by changing the position of the moving contact of the tap changer, thereby changing the output voltage. Commonly used distribution transformers have a primary voltage of 10 kV and a secondary output voltage of 0.4 kV. The first gear position of the distribution transformer tap switch is 10.5 kV, the second gear position is 10 kV, and the third gear position is 9.5 kV. Generally, it should be in the second gear position.


The specific steps for adjusting the tap changer are:

   1. Power off first. After disconnecting the load on the low-voltage side of the distribution transformer, use an insulating rod to open the high-voltage side drop-out fuse, and then take necessary safety measures. Unscrew the protective cover of the tap changer on the transformer and place the positioning pin in the neutral position.

   2. When adjusting the gear, adjust the tap switch to the corresponding position according to the output voltage. The basic principle of adjusting the tap switch is:

   When the output voltage of the transformer is lower than the allowable value, adjust the tap switch position from Ⅰ to Ⅱ, or from Ⅱ to Ⅲ;

   When the transformer output voltage is higher than the allowable value, adjust the tap switch position from Ⅲ gear to Ⅱ gear, or Ⅱ gear to Ⅰ gear.

   3. After adjusting the gear, use a DC bridge to measure the DC resistance of each phase winding, and check whether the DC resistance between each winding is balanced. If the resistance value difference between the phases is greater than 2%, it must be re-adjusted, otherwise, after operation, the moving and static contacts will heat up or even discharge due to poor contact and damage the transformer.


Knkong Electric

Date:2020-04-17 09:40

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