Harm of frequency converter and its countermeasures

First, the inverter application problems

In the field of industrial speed control transmission, compared with the traditional mechanical speed control, speed control using the inverter has many advantages and is widely used. However, due to the switching characteristics of the inverter circuit of the frequency conversion power supply, a typical Non-linear loads, variable frequency power supplies are often operated in the field simultaneously with other devices, such as computers and sensors, which are often installed in close proximity to each other and may interact with each other.

Therefore, the power electronics represented by inverter power supply is one of the most important harmonic sources in public power network, which has an important impact on power quality in power system. Power system harmonic is defined as Fourier series decomposition of periodic non-sinusoidal power, in addition to get the same frequency with the fundamental frequency of the power grid, but also to get a series of components larger than the fundamental frequency of the power grid, this part of the power is called harmonics. The ratio of harmonic frequency to fundamental frequency (n = fn / f1) is called the harmonic order. Non-integer harmonics, sometimes known as non-harmonics or fractional harmonics, sometimes exist in the grid. Harmonic is actually a kind of interference, so that the power grid is "polluted" and the power quality is deteriorated. Mainly in the field of electrical engineering harmonic generation, transmission, measurement, hazards and suppression, the frequency range is generally 2 ≤ n ≤ 40.

Second, the harmonic generation process

Electrical equipment that injects harmonic currents into the utility grid or produces 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 equipment with power electronics, AC controllers and arc furnaces, induction furnaces, fluorescent lamps, transformers and the like.

The fundamental cause of harmonics is due to non-linear load. When the current flows through the load, and the applied voltage is not a linear relationship, the formation of non-sinusoidal current, resulting in harmonics.

The harmonic frequency is an integral multiple of the fundamental frequency. According to the M. Fourier analysis principle of the French mathematician, any repetitive waveform can be decomposed into a sine wave containing fundamental frequencies and a series of harmonics based on fundamental multiples Component. Harmonic is a sine wave, each harmonic has a different frequency, amplitude and phase angle. Harmonics can be divided into even and odd harmonics. In a balanced three-phase system, the even harmonics have been eliminated due to the symmetry. Only odd harmonics exist. The odd harmonics cause more harm than even harmonics.

Industrial enterprises in our country are also using more and more harmonic electrical equipment, such as thyristor-powered DC hoist, AC-AC drive, rolling mill DC drive, thyristor cascade fan pump and smelting electric arc furnace Wait. The current drawn by these devices is non-sinusoidal and their 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 has nothing to do with the grid parameters, it can be regarded as a constant current source. The number of harmonics generated by various thyristor circuits is related to the form of the circuit, which is called the characteristic harmonic of the circuit. In addition to the characteristics of harmonics, the three-phase voltage imbalance in the trigger pulse asymmetrical or unstable state, the circuit will produce non-characteristic harmonics. Harmonic analysis and calculation of the most significant is the characteristic harmonic, if 5,7,11,13 times and so on. If the DC current ripple is larger, then the 5th harmonic amplitude will increase, the rest of the harmonic amplitude will be reduced.

When multiple harmonic sources are connected to the grid, the sum of the harmonic components of the harmonics of each harmonic source will be smaller than the sum of the components. Transformer magnetizing current contains 3,5,7 and other harmonic components. As the transformer primary and secondary windings in a group of angled connection, provides the third harmonic 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 enter the grid.

Third, harmonic hazards

For the power system, the main hazards of power harmonics have the following aspects:

(1) increase the additional loss for the input, supply and consumption of electrical equipment, overheating the temperature of the equipment, reducing the utilization of the equipment and economic benefits:

(2) Impact 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 within the resonance region near the network resonance point, dielectric breakdown will occur on the power transmission lines and power cable lines.

(3) The impact of power harmonics on the transformer:

Harmonic voltage increases the hysteresis loss of the transformer, eddy current loss and insulation of the electric field, the presence of harmonic current increases the copper loss. For transformers with asymmetric loads, the harmonic component of the field current is greatly increased.

(4) Effect of power harmonics on power capacitors:

When the voltage containing power harmonics is applied to both ends of the capacitor, the harmonic current is superposed on the fundamental wave of the capacitor due to the small harmonic impedance of the capacitor. As a result, the capacitor current increases, the temperature increases, and the service life shortens, causing the capacitor to pass The load even explodes, while harmonics may also cause harmonics of power harmonics in the power grid together with the capacitors, aggravating the failure.

(5) Impact on the reliability of relay protection and automatic device work:

Especially for electromagnetic relays , power harmonics often cause relay protection and automatic device malfunction or refuse to move, so that the loss of selectivity, reliability, easy to cause system accidents, a serious threat to the safe operation of power systems.

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

Odd low-frequency harmonic current with large amplitude flowing through the power line will generate interference voltage in the communication line near the power line when it interferes with the magnetic field coupling, thus interfering with the operation of the communication system and affecting the clarity of the communication line call. Even at the extreme , It also threatens the security of communications equipment and personnel.