In the process of transformer design, I believe that many friends have encountered the same problem, that is, the transformer no-load closing problem. So what kind of problem will lead to transformer no-load trip? Below, I will start from the cause of the closing of the gate, analyze what are the reasons for the closing of the gate, and give advice.

Transformer steady state operation, I0 (no-load current) is very small, large transformers and even less than 1% times the rated current; but in the no-load closing, the transformer suddenly access to the grid, this instantaneous may have a large impact current, also known as the excitation inrush current. Transformer excitation inrush current is generally 8 to 10 times the rated current. Therefore, when the transformer is closed at no load, due to the action of the relay protection may not be able to close the situation, analyzed as follows.

Inrush current characteristics:

1, the transformer will produce excitation inrush current when feeding and closing, which is the inherent characteristics of the transformer, the existence of this phenomenon is due to the magnetic through the saturation and remanent magnetism caused by. The size of the excitation inrush is related to the initial phase angle of the voltage at the time of closing, when the initial phase angle of the voltage at the moment of closing α = 0, the excitation inrush, when the initial phase angle of the voltage at the moment of closing α = π/2, the excitation inrush is small, so the general solution is to send a few more times to let the closing moment to avoid the zero point, it can be sent on.

2, due to the presence of transformer resistance and line resistance, the closing shock current will gradually attenuate, generally small transformer can reach steady state after a few weeks, the capacity of the transformer needs to go through a dozen or even dozens of weeks before it may reach steady state.

Transformer no-load drop causes:

1, with the development of transformer technology, silicon steel sheet using high permeability oriented grain material, greatly reducing the no-load loss, but also make the excitation inrush current increased to 10 ~ 14 times the rated current.

2, the increase in grid capacity and the shortening of power supply lines to increase the excitation current.

3, the power supply department in the relay protection setting is too conservative.

Transformer no-load dropout solution:

1, according to the field experience is generally overcurrent protection action, overcurrent protection should be set according to avoid over the line on the distribution substation of the excitation current to set. Transformer excitation current is generally 8 to 14 times the rated current. When the transformer capacity is large, the excitation inrush current is also large. Due to the slow decay of the excitation inrush current of large transformers, it can also cause the overcurrent protection action. This will need to adjust the action time limit of overcurrent protection to avoid the transformer’s excitation inrush, (Suggestion: Because the 10kV line is mostly the last level of protection, overcurrent action time limit can generally be adjusted to 0.3s, this section of the time limit is permissible).

2, the current local grid voltage is generally high, usually 10.5KV, the transformer will be tapped on the file (voltage file), can greatly improve the excitation inrush.

3, if the high-voltage fast-break protection action, should adjust the limit value of the fast-break protection, fast-break protection limit value must escape from the excitation current of its own transformer.

4, most of the users in the transformer no-load can not close the gate, often suspect that there are quality problems with the transformer, this problem can be confirmed by the following methods, the transformer will be symmetrically accessed on the low-voltage side of the two 2000W (two in series between each phase) of the resistor wire as a load, and then from the high-voltage side of the power supply. If, the transformer can close the gate normally, it shows that the transformer quality is not a problem, you can find other reasons to solve the problem of closing the gate.

5, if the high-voltage zero-sequence protection action, because the initial phase angle of the three-phase voltage is different, three-phase current is seriously unbalanced, according to the principle of zero-sequence protection action, zero-sequence protection will be implemented action, the solution is to adjust the zero-sequence protection of the setting time.

Above from three aspects to the transformer no-load closing solution provides the possibility. I hope that the transformer no-load closing difficulties of friends can be gained.