Harmonic hazards of VFD (Variable-frequency Drive) and solutions

1. Problems in the application of VFD (Variable-frequency Drive)

In the field of industrial speed control transmission, compared with the traditional mechanical speed control, VFD (Variable-frequency Drive) speed control has many advantages and is widely used. However, due to the switching characteristics of the inverter circuit of the variable frequency power supply, the power supply Forming a typical nonlinear load, the variable frequency power supply usually runs simultaneously with other equipment in the field, such as computers and sensors. These equipment are often installed very close, which may cause mutual influence. Therefore, the power electronic device represented by variable frequency power supply is one of the most important harmonic sources in the public grid, which has an important impact on the power quality in the power system. The definition of harmonics in the power supply system is to perform Fourier series decomposition on periodic non-sinusoidal power. In addition to obtaining the same component as the fundamental frequency of the power grid, a series of components greater than the fundamental frequency of the power grid are also obtained. This part of the power is called harmonics. The ratio of the harmonic frequency to the fundamental frequency (n=fn/f1) is called the harmonic order. Sometimes there are non-integer harmonics in the power grid, which are called non-harmonics or fractional harmonics. Harmonics are actually a kind of interference, which makes the power grid "polluted" and the power quality drops. The field of electrotechnical technology mainly studies the generation, transmission, measurement, harm and suppression of harmonics, and its frequency range is generally 2≤n≤40.

Second, the generation process of harmonics

Electrical equipment that injects harmonic current into the public grid or generates harmonic voltage on the public grid is called a harmonic source. Electrical equipment with nonlinear characteristics is the main harmonic source, such as converter equipment with power electronic devices, AC controllers and electric arc furnaces, induction furnaces, fluorescent lamps, transformers, etc.

The root cause of harmonic generation is due to non-linear loads. When the current flows through the load, it does not have a linear relationship with the applied voltage, forming a non-sinusoidal current, thereby generating harmonics.

The harmonic frequency is an integral multiple of the fundamental frequency. According to the analysis principle of French mathematician Fourier (M.Fourier), any repeated waveform can be decomposed into a sine wave containing the fundamental frequency and a series of harmonics that are multiples of the fundamental portion. Harmonics are sine waves, each with a different frequency, amplitude, and phase angle. Harmonics can be divided into even and odd harmonics. In a balanced three-phase system, due to the symmetrical relationship, even harmonics have been eliminated, only odd harmonics exist, and odd harmonics cause more harm than even harmonics. Chinese industrial enterprises are also increasingly using electrical equipment that generates harmonics, such as DC hoists powered by thyristor circuits, AC-AC frequency conversion devices, DC drives for rolling mills, fans and water pumps with thyristor cascade speed regulation, and smelting electric arc furnaces wait. The current drawn by these devices is non-sinusoidal, and its harmonic components distort the sinusoidal voltage of the system. The amount of harmonic current depends on the characteristics and working conditions of the harmonic source equipment itself, and has nothing to do with the grid parameters, so it can be regarded as a constant current source. The harmonic times generated by various thyristor circuits are related to their circuit forms, which are called the characteristic harmonics of the circuit. In addition to the characteristic harmonics, the above circuit will also generate non-characteristic harmonics when the three-phase voltage is unbalanced, the trigger pulse is asymmetrical, or the working state is not stable. The most meaningful harmonic analysis and calculation is the characteristic harmonics, such as 5th, 7th, 11th, 13th and so on. If the current ripple on the DC side is large, the amplitude of the 5th harmonic will increase, and the amplitude of the other harmonics will decrease. When there are multiple harmonic sources connected to the power grid, since the phases of the same harmonic current components of each harmonic source are different, their sum will be less than the arithmetic sum of each component. The transformer excitation current contains 3, 5, 7 and other harmonic components. Since there is always a group of the primary and secondary windings of the transformer that is connected in a delta form, which provides a path for the 3rd harmonic, the 3rd harmonic current does not flow into the grid. However, when the excitation current of each phase is unbalanced, the residual component of the third harmonic (up to 20%) can enter the grid.

3. Harmonic hazard

For the power system, the harm of power harmonics mainly manifests in the following aspects:

(1) Increase the additional loss of transmission, power supply and power consumption equipment, overheat the temperature of the equipment, reduce the utilization rate and economic benefits of the equipment:

(2) The impact of power harmonics on transmission lines:

Harmonic currents increase the power loss of transmission lines. When the harmonic frequency injected into the power grid is located in the resonance area near the network resonance point, it will cause insulation breakdown to the transmission line and power cable line.

(3) The influence of power harmonics on transformers:

The existence of harmonic voltage increases the hysteresis loss, eddy current loss and insulation electric field strength of the transformer, and the existence of harmonic current increases copper loss. For transformers with asymmetrical loads, the harmonic components of the excitation current will be greatly increased.

(4) The influence of power harmonics on power capacitors:

When the voltage containing power harmonics is applied to both ends of the capacitor, since the capacitor has a very small impedance to the power harmonics, the harmonic current is superimposed on the fundamental wave of the capacitor, causing the capacitor current to increase, the temperature to rise, and the life to be shortened, causing the capacitor to overheat. The load may even explode, and at the same time, the harmonic may also cause power harmonic resonance together with the capacitor in the power grid, which will aggravate the fault.

(5) Affect the working reliability of relay protection and automatic devices:

Especially for electromagnetic relays, power harmonics often cause relay protection and automatic devices to malfunction or refuse to operate, making their actions lose selectivity, reduce reliability, easily cause system accidents, and seriously threaten the safe operation of power systems.

(6) Interference with the work of the communication system:

When the odd-order low-frequency harmonic current with large amplitude flowing on the power line is coupled by the magnetic field, it will generate interference voltage in the communication line adjacent to the power line, interfere with the work of the communication system, and affect the clarity of the communication line. In some cases, it will also threaten the safety of communication equipment and personnel.

(7) Impact on electrical equipment:

Power harmonics will distort the graphics of TVs and computers, and the brightness of the screen will fluctuate, and the temperature of the components inside the machine will be overheated, causing errors in the computer and data processing system, and even seriously damaging the machine.