Why are there no even harmonics in three phase power supply?

Due to the symmetry relationship, the even harmonic has been eliminated, only the odd harmonic exists, for the three-phase rectifier load, the harmonic current that occurs is 6n 1st harmonic, for example, etc., the frequency converter mainly produces the second harmonic.

The fundamental reason for harmonic generation in the power system is due to the nonlinear load, when the current flows through the load and the added voltage is not linear to form a non-sinusoidal current, that is, there are harmonics in the circuit, harmonics can be distinguished into even and odd, the first number is the odd harmonic, and the equal is the even harmonic, such as when the fundamental wave is 50Hz, the second harmonic is L00Hz, and the third harmonic is 150Hz.

1. Why does the three-symmetrical power system not contain even harmonics?

In "Power System Harmonics: Basic Principles, Analysis Methods and Filter Design", by George of Austria and translated by Xu Zheng.

p8 is described as follows:

Among the characteristics of harmonics in the power system, there is a half-wave symmetry, which is characterized by f(t t 2) = -f (t), no DC component and even harmonics (2, 4, 6,..Offset. This feature allows us to ignore even harmonics in the power system.

Because the power system is made up of bidirectionally symmetrical elements, the voltages and currents generated by these elements have half-wave symmetry.

2. How to analyze the third harmonic when the transformer is connected by Y-shaped connection and triangle?

In "Evaluation and Regulation of Harmonics in Public Power Grid", edited by Xu Yao.

P171 has the following description: the delta-connected transformer winding blocks the flow of multiple harmonics of zero sequence 3, and it acts like a bidirectional filter, blocking harmonics on both the supply side and the load side.

1 If the generator is triangular wiring, then there is no 3rd harmonic in its no-load voltage;

2. If the generator is loaded, and there is a device that generates a large 3rd harmonic in the load, then there may be 3rd harmonic in the outlet voltage when the generator is loaded;

3. If the generator is connected to the power supply system after being boosted by the transformer, and the transformer has a winding with triangular wiring, then there is no 3rd harmonic in the generator outlet voltage.

Generally speaking, the generator will not produce the 3rd harmonic significantly during operation, but if the generator has stator loop asymmetry, the odd harmonic will be obviously generated.

The loads under the generator are mostly single-phase loads, and these loads generate a large number of third harmonics during operation, so when you measure the position of the generator, you will measure a large number of third harmonics.

2.The generator has an asymmetrical stator circuit, so there will be a high third harmonic.

The generator has a high ability to connect with three-phase unbalanced loads with stars, avoiding the resulting circulation, and also eliminating part of the third harmonic.

For the three-phase alternator, due to the problem of the pole structure, the magnetic field is not a sine wave change, and the induced electromotive force caused by it is not a sine wave, but the waveform distortion of the no-load voltage is less than 4%, so the alternator has harmonics even without load, whether there are only 3 harmonics, it is not necessarily.

Harmonic is a waveform analysis method for the voltage distortion of a generator, which divides the waveform into a sine wave at the harmonic frequency to calculate.

Generators are available in both star and triangle connections, mainly for different applications in voltage levels and load distribution.

Has your problem been solved? Now we encounter the same problem, the even harmonic content is large, and the odd order is less.

First of all, the system structure should be clarified, and external interference may also cause large even harmonics, so it is recommended to eliminate them one by one.

See if the resonance is causing the even harmonic to amplify.

Let's analyze the specific situation on a case-by-case basis.

The rectifier filter class is indeed a large odd harmonic.

If the transformer is closed, there is a large second harmonic.

1 If the generator is triangular wiring, then there is no 3rd harmonic in its no-load voltage;

2. If the generator is loaded, and there is a device that generates a large 3rd harmonic in the load, then there may be 3rd harmonic in the outlet voltage when the generator is loaded;

3. If the generator is connected to the power supply system after being boosted by the transformer, and the transformer has a winding with triangular wiring, then there is no 3rd harmonic in the generator outlet voltage.

This is a serious pollution of the power grid, which refers to the large harmonic interference in the power supply grid. From the analysis of the time you said, the main thing is that the power grid is in a peak period of electricity consumption, for example, the power supply network of your factory shares the main line with many factories or civil electricity, causing harmonic interference in many lines to be connected to the main line of power supply and transmitted to your factory. Or maybe the place you're testing has a lot of electromagnetic interference.

Finding the source of the pollution can be solved by the power supply department.

If you are very different from what they tested, you try to test it with a better oscilloscope, provided that the equipment is good, only then can you more strongly convince the power supply department of what you say.

There's only one way! Good luck!

This is obviously the 35kV power supply contaminated with harmonics. The power supply department can be notified to find the source of the pollution.

From the point of view of the time period when harmonics appear, the equipment that generates harmonics mainly uses valley electricity, which should be the equipment of large households with high energy consumption.

Add UPS, power frequency UPS with filter can solve the above problems.