Frequency converter high-order harmonic interference and countermeasures - News - Global IC Trade Starts Here.

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1, the generation of higher harmonics

Generally, the low-voltage inverter with a voltage of 380~500V adopts the "AC-DC-AC" line, and it is a voltage type. Its input has a rectifier circuit, and the output uses an IGBT inverter circuit, so the input voltage is sinusoidal. Waveform, current is a non-sinusoidal waveform; the output voltage is a non-sinusoidal (stepped) waveform, and the current is approximately sinusoidal (SPWM modulation).

In short, high-order harmonics are generated regardless of whether the input and output are non-sinusoidal. In particular, the output has a large impact. When the inverter capacity is ≤10% of the power transformer capacity, the impact is not significant. When the capacity exceeds the above value, the influence of higher harmonics cannot be ignored, and certain anti-interference measures must be adopted.

According to the harmonic analysis, the integer multiple harmonic components of 3 times and 3 are not included in the inverter circuit. In a three-phase symmetrical system, the integer multiple of 3 harmonics can be eliminated by itself, and can be ignored or even harmonics. The percentages at which each harmonic peak can be reached are shown in the following table:

2, the harm of higher harmonics

2.1 Distort the power supply voltage, reduce the voltage quality, and increase the line loss;

2.2 The motor is heated (current harmonics increase copper consumption, voltage harmonics increase iron consumption), efficiency decreases, COSφ decreases;

2.3 Increase the vibration of the motor, the speed is unstable, and the vibration is generated;

2.4 Increase motor noise;

2.5 Higher harmonics and wire capacitance resonance generate overvoltage, which jeopardizes the insulation strength and reduces the withstand voltage, resulting in overvoltage breakdown (more dangerous when the line is long);

2.6 overheating the capacitor, increasing losses, resulting in electrical breakdown or thermal breakdown;

2.7 Increase the unbalance of the three-phase input current of the circuit (the line current can be 50% difference at the maximum);

2.8 Interfere with the normal operation of the computer system, make the electronic equipment work unstable, even when it is serious, it may not work or the setting parameters are too large, which will affect the normal work.

3. Countermeasures for higher harmonics

3.1 From the perspective of design and manufacture: use IGBT power components, space voltage vector control, multi-phase superposition, such as six-phase, twelve-phase, multiple phase shift, select reasonable parameter values ​​in the modulation process. It is generally good to have high grade, famous brand and products adopting new technology.

3.2 From the installation angle: use the incoming AC reactor, the output line adopts DC reactor or sine filter; do not share the ground wire, separate the power supply (inverter, the victim device is separately powered); the interference-prone device uses the isolation inductor The power supply and the incoming line of the inverter are shielded and grounded, and separated by a certain distance; the inlet and outlet wires are grounded and grounded; the output uses a four-core cable (one core grounding), the motor casing is grounded, and the inverter is grounded separately; Insulated power transformer (neutral point is not grounded); shorten the line length; power line and signal line are laid separately to avoid crossover. When it cannot be avoided, it must be vertically crossed, and it must not be laid equally; signal line shield is not connected to motor or frequency conversion The ground of the device should be connected to the common end of the control line; if necessary, a zero-sequence reactor, a surge absorber, a surge suppressor, an input suppression reactor, and a stranded wiring can be used.

It can also reduce the carrier frequency of the inverter to eliminate the effects of interference. Generally, the frequency reduction interference will decrease, but the noise may be larger and the current waveform smoothness is worse. The specific can be determined according to the on-site commissioning, and a dedicated variable frequency motor must be used when necessary.

In short, after adopting the above countermeasures, the interference of higher harmonics can be basically eliminated or the influence of higher harmonics can be greatly attenuated. Many of the above measures can only be selected, depending on the specific conditions and conditions of the site.

4, the standard of higher harmonics

The specific provisions of the Chinese national standard GB12668-90 are as follows:

Voltage distortion ≤ 10%, odd harmonic ≤ 5%, even harmonic ≤ 2%, short time (less than 30 seconds) ≤ 10%. With the United States, Britain, Germany, Japan, Australia, the regulations are slightly larger, mainly considering the national conditions.

5, the limit of higher harmonics

According to the actual operating experience, the higher harmonics are generally in the following ranges:

Motor: 10~20%, electronic switch: malfunction when >10%;

Instrument: voltage distortion <10%, current distortion <10%, error <1%;

Computer: More than 5% will be disturbed.

6. High-order harmonic problems of high-voltage inverters

The high-voltage inverter refers to the inverter of 3KV, 6KV, 10KV voltage level and power of 300~10000KW. Because of its high voltage, high power, harmonic problems should be paid more attention. Once there is interference, the impact is even greater and the damage is more serious. Therefore, in 1992, the United States formulated the IEEE95 standard to stipulate various indicators of high-voltage inverters. However, some manufacturers have met or exceeded the following parameters, as follows:

6.1 Using pulse frequency conversion, it can meet the most stringent requirements of voltage and current distortion of power supply departments in various countries without adding any harmonic filters;

6.2 Using multiplexed pulse width modulation technology, the output waveform is a perfect sine wave;

6.3 COSφ>0.95;

6.4 Frequency converter efficiency is 98%, system 96.5%;

6.5 noise <75dB;

6.6 Frequency accuracy <±0.5%;

6.7 Torque ripple 0.1% (0 ~ 500Hz);

6.8 Perfect harmonic-free total current distortion of 0.8% and total voltage distortion of 1.2%.