Harmonic hazards and solutions for variable frequency power supplies - Solutions - Huaqiang

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First, the problem of variable frequency power supply application In the field of industrial speed control transmission, compared with the traditional mechanical speed regulation, there are many advantages to using variable frequency power supply speed regulation, the application is very extensive, but due to the switching characteristics of the inverter power supply inverter circuit, The power supply forms a typical non-linear load. The variable frequency power supply is usually operated in parallel with other equipment, such as computers and sensors. These devices are often installed close together, which may cause mutual influence. Therefore, the power electronic device represented by the variable frequency power supply is one of the most important harmonic sources in the public power grid, and it has an important influence on the power quality in the power system. The harmonics of the power supply system are defined as the Fourier series decomposition of the periodic non-sinusoidal electric energy. In addition to obtaining the same component as the fundamental frequency of the power grid, a series of components larger than the fundamental frequency of the power grid are obtained. This part of the electric quantity is called harmonic. The ratio of the harmonic frequency to the fundamental frequency (n = fn / f1) is called the harmonic order. Non-integer harmonics are sometimes found in the power grid, called non-harmonics or fractional harmonics. Harmonics are actually a kind of interference, which causes the power grid to be "contaminated" and the quality of power is degraded. The field of electrical engineering mainly studies the occurrence, transmission, measurement, hazard and suppression of harmonics, and its frequency range is generally 2≤n≤40.
2. Harmonic Generation The electrical equipment that injects harmonic current into the utility grid or generates harmonic voltages on the utility grid is called a harmonic source. Electrical equipment with non-linear characteristics is the main source of harmonics, such as converter devices with power electronics, AC controllers and electric arc furnaces, induction furnaces, fluorescent lamps, transformers, and the like.
The root cause of harmonic generation is due to nonlinear loads. When current flows through the load, it does not have a linear relationship with the applied voltage, and a non-sinusoidal current is formed, thereby generating harmonics.
The harmonic frequency is an integral multiple of the fundamental frequency. According to the French mathematician M. Fourier's analysis principle, any repeated waveform can be decomposed into a sine wave containing the fundamental frequency and a series of harmonics of the fundamental multiple. Component. Harmonics are sinusoidal 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, even harmonics have been eliminated due to the symmetrical relationship, and only odd harmonics exist, and the odd harmonics cause more damage than even harmonics. Chinese industrial enterprises are also increasingly using electrical equipment that generates harmonics, such as DC hoists for thyristor circuit power supply, AC-AC inverters, DC drives for rolling mills, fan pumps for 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 of the harmonic source device itself and its operating conditions, and is independent of the grid parameters, so it can be regarded as a constant current source. The number of harmonics generated by various thyristor circuits is related to their circuit form and is called the characteristic harmonic of the circuit. In addition to the characteristic harmonics, the above circuit also produces non-characteristic harmonics when the three-phase voltage is unbalanced and the trigger pulse is asymmetrical or unstable. The most significant of the harmonic analysis and calculations are characteristic harmonics, if 5, 7, 11, 13 etc. If the DC side current ripple is large, the 5th harmonic amplitude will increase, and the remaining harmonic amplitudes will decrease. When the power grid is connected to multiple harmonic sources, the sum of the harmonic current components of each harmonic source is different, and the sum will be smaller than the arithmetic sum of the components. The transformer excitation current contains 3, 5, 7 and other harmonic components. Since there is always a set of angled connection in the original secondary winding of the transformer, the third harmonic is provided with a path, so the third harmonic current does not flow into the grid. However, when the excitation current of each phase is unbalanced, the residual component of the 3rd harmonic (up to 20%) can be entered into the grid.
Third, harmonic hazard For the power system, the main dangers of power harmonics are as follows:
(1) Increase the additional loss of transmission, supply and use of equipment, overheating the equipment, reducing equipment utilization and economic benefits:
(2) The influence of power harmonics on transmission lines:
Harmonic currents increase the power loss of the transmission line. When the harmonic frequency injected into the grid is located in the resonance region near the resonance point of the network, insulation breakdown occurs on the transmission line and the power cable line.
(3) The influence of power harmonics on the transformer:
The presence of harmonic voltage increases the hysteresis loss, eddy current loss and the electric field strength of the transformer. The presence of harmonic current increases the copper loss. For transformers with asymmetric loads, the harmonic components of the field current are greatly increased.
(4) The effect of power harmonics on power capacitors:
When a voltage containing power harmonics is applied across the capacitor, the harmonic impedance of the capacitor is small, and the harmonic current is superimposed on the fundamental wave of the capacitor, so that the capacitor current becomes larger, the temperature rises, and the life is shortened, causing the capacitor to pass. The load may even explode, and the harmonics may also cause power harmonic resonance in the grid together with the capacitor, which will aggravate the fault.
(5) Affect the reliability of relay protection and automatic devices:
Especially for electromagnetic relays, power harmonics often cause relay protection and automatic device malfunction or refusal, causing their operation to lose selectivity, reduce reliability, easily cause system accidents, and seriously threaten the safe operation of the power system.
(6) Interference with the operation of the communication system:
When the odd-frequency low-frequency harmonic current flowing through the power line is coupled by the magnetic field, the interference voltage is generated in the communication line adjacent to the power line, which interferes with the work of the communication system and affects the clarity of the communication line, even at the extreme. In this case, it also threatens the safety of communication equipment and personnel.
(7) Impact on electrical equipment:
Power harmonics can cause distortion of the graphics of TVs and computers, fluctuations in picture brightness, and overheating of components in the machine, causing errors in computers and data processing systems, and even damaging the machine.
In addition, power harmonics can also adversely affect the indications of measurement and measurement instruments and rectifiers. It has become a major public hazard affecting power quality in current power systems.
Fourth, harmonic control to control harmonic problems, to suppress radiation interference and power system interference, can take shielding, isolation, grounding and filtering and other technical means. The main measures to control harmonics are: increase the system short-circuit capacity; increase the supply voltage level; increase the pulsation number of the converter; improve the system operation mode, set the AC filter, etc., can reduce the harmonic components in the system. The AC filter is further divided into a passive filter and an active filter. An active filter is an active filtering device that injects a harmonic current into the system to cancel the harmonic current generated by the nonlinear load. It provides fast dynamic tracking compensation for varying harmonics and the compensation characteristics are unaffected by system impedance. The structure is relatively complicated, the running loss is large, and the equipment cost is high; while compensating harmonics, new harmonics are also injected. Passive filters (also known as LC filters) use the principle of LC resonance to artificially create a series resonant branch that provides a very low impedance path for the main harmonics to be filtered, so that it is not injected into the grid. The LC filter has a simple structure and obvious harmonic absorption effect; however, it only has a good compensation effect on the harmonics of the natural frequency; and the compensation characteristic is greatly affected by the impedance of the power grid. At a specific frequency, the impedance of the grid and the LC filter Parallel resonance or series resonance may occur between them.
V. Review of reactive power compensation, harmonic control technology is one of the effective means to alleviate the contradiction between power supply and demand and improve power supply quality in the current and long period of time. After extensive application and promotion, it can be a national And the user brings huge economic benefits and good social benefits. The harmonics generated by the variable frequency power supply are controlled within a minimum range to achieve scientific and rational use of electricity, suppress grid pollution, and improve power quality.