Problems in Variable Frequency Power Supply Applications
In the field of industrial speed control and transmission, variable frequency power supply (VFD) speed regulation has many advantages over traditional mechanical speed regulation and is widely used. However, due to the switching characteristics of the inverter circuit of a VFD, it forms a typical nonlinear load on its power supply. VFDs often operate simultaneously with other equipment in the field, such as computers and sensors, which are often installed close together, potentially causing mutual interference.
Therefore, power electronic devices, represented by VFDs, are one of the most significant harmonic sources in the public power grid, significantly impacting power quality in the power system. The definition of power system harmonics is the Fourier series decomposition of periodic non-sinusoidal electrical quantities. Besides the component with the same frequency as the grid's fundamental frequency, a series of components with frequencies higher than the grid's fundamental frequency are also obtained; these components are called harmonics. The ratio of the harmonic frequency to the fundamental frequency (n=fn/f1) is called the harmonic order. Sometimes, non-integer multiples of harmonics also exist in the power grid, called non-harmonics or fractional harmonics. Harmonics are essentially a type of interference that "pollutes" the power grid and degrades power quality. The field of electrical engineering primarily studies the generation, transmission, measurement, hazards, and suppression of harmonics, with a typical frequency range of 2 ≤ n ≤ 40.
The Generation Process of Harmonics
Electrical equipment that injects harmonic current into the public power grid or generates harmonic voltage on the public power grid is called a harmonic source. Electrical equipment with nonlinear characteristics is a major harmonic source, such as converters with power electronic devices, AC controllers, electric arc furnaces, induction furnaces, fluorescent lamps, and transformers.
The fundamental cause of harmonic generation is nonlinear loads. When current flows through a load, its relationship with the applied voltage is not linear, forming a non-sinusoidal current, thus generating harmonics.
Harmonic frequencies are integer multiples of the fundamental frequency. According to the analysis principle of the French mathematician Fourier, any repeating waveform can be decomposed into sinusoidal components containing the fundamental frequency and a series of harmonics that are multiples of the fundamental frequency. Harmonics are sinusoidal waves, each with a different frequency, amplitude, and phase angle. Harmonics can be classified into even-order and odd-order harmonics. In a balanced three-phase system, due to symmetry, even-order harmonics are eliminated, leaving only odd-order harmonics. Odd-order harmonics cause more and greater harm than even-order harmonics.
Increasingly, Chinese industrial enterprises use electrical equipment that generates harmonics, such as DC power-driven hoists powered by thyristor circuits, AC-AC frequency converters, DC drives for rolling mills, thyristor-cascade speed-regulating fans and pumps, and electric arc furnaces in smelting. The current drawn by these devices is non-sinusoidal, and its harmonic components distort the system's sinusoidal voltage. The amount of harmonic current depends on the characteristics and operating conditions of the harmonic source equipment itself, and is independent of the grid parameters; therefore, it can be considered a constant current source. The harmonic orders generated by various thyristor circuits are related to their circuit configuration and are called the characteristic harmonics of that circuit. Besides characteristic harmonics, the above circuits will also generate non-characteristic harmonics under conditions of three-phase voltage imbalance, asymmetrical trigger pulses, or unstable operation.
Characteristic harmonics are the most meaningful for harmonic analysis and calculation, such as the 5th, 7th, 11th, and 13th harmonics. If the DC-side current ripple is large, the amplitude of the 5th harmonic will increase, while the amplitudes of the other harmonics will decrease. When the power grid has multiple harmonic sources, because the phases of the same harmonic current components from different sources are different, their sum will be less than the arithmetic sum of the individual components. The transformer excitation current contains 3rd, 5th, and 7th harmonic components. Since one set of windings in the primary and secondary windings of the transformer is always delta-connected, it provides a path for the 3rd harmonic, so the 3rd harmonic current does not flow into the power grid. However, when the excitation currents of each phase are unbalanced, a residual component of the 3rd harmonic (up to 20%) can enter the power grid.
Harmful Effects
For power systems, the hazards of power harmonics mainly manifest in the following ways:
1. Increased additional losses in transmission, supply, and consumption equipment, leading to overheating and reduced equipment utilization and economic efficiency.
2. Impact of power harmonics on transmission lines: Harmonic currents increase energy losses in transmission lines. When the harmonic frequency injected into the grid is located in the resonance region near the network resonance point, it can cause insulation breakdown in transmission lines and power cables.
3. Impact of power harmonics on transformers: The presence of harmonic voltage increases hysteresis losses, eddy current losses, and the electric field strength of the insulation in transformers. The presence of harmonic current increases copper losses. For transformers with asymmetrical loads, it significantly increases the harmonic component of the excitation current.
4. Impact of Power Harmonics on Power Capacitors: When a voltage containing power harmonics is applied across a capacitor, the capacitor's impedance to harmonics is very low. The harmonic current superimposed on the capacitor's fundamental frequency, increasing the capacitor current, raising its temperature, shortening its lifespan, and potentially causing overload or even explosion. Furthermore, harmonics can cause power harmonic resonance in the power grid, exacerbating faults.
5. Impact on the Reliability of Relay Protection and Automatic Devices: Especially for electromagnetic relays, power harmonics often cause relay protection and automatic devices to malfunction or fail to operate, resulting in loss of selectivity, reduced reliability, and increased risk of system accidents, seriously threatening the safe operation of the power system.
6. Interference with Communication Systems: When large-amplitude odd-order low-frequency harmonic currents flowing through power lines are coupled through a magnetic field, they can generate interference voltages in nearby communication lines, interfering with communication system operation, affecting call clarity, and in extreme cases, threatening the safety of communication equipment and personnel.
7. Impact on Electrical Equipment: Power harmonics can cause image distortion and fluctuations in screen brightness in televisions and computers, and can overheat internal components, leading to errors in computer and data processing systems, and even serious damage to the equipment.
Furthermore, power harmonics can adversely affect the inaccurate readings of measuring and metering instruments and rectifier devices, making them a major public nuisance affecting power quality in current power systems.
Harmonic Mitigation
To mitigate harmonic problems and suppress radiated interference and power supply system interference, techniques such as shielding, isolation, grounding, and filtering can be employed.
The main measures for harmonic mitigation include: increasing system short-circuit capacity; raising the power supply voltage level; increasing the pulsation number of converter devices; improving system operation methods; and installing AC filters, all of which can reduce harmonic components in the system. AC filters are divided into passive filters and active filters. An active filter is a dynamic filtering device that injects compensating harmonic current into the system to offset the harmonic current generated by nonlinear loads. It can quickly and dynamically track and compensate for changing harmonics, and its compensation characteristics are not affected by system impedance. Its structure is relatively complex, resulting in significant operating losses and high equipment costs.
While compensating for harmonics, it also injects new harmonics. Passive filters (also known as LC filters) utilize the LC resonance principle to artificially create a series resonant branch, providing a path with extremely low impedance for the main harmonics to be filtered, preventing them from being injected into the power grid. LC filters have a simple structure and a significant harmonic absorption effect; however, they only have a good compensation effect on harmonics at their natural frequencies; and their compensation characteristics are greatly affected by the grid impedance. At specific frequencies, parallel resonance or series resonance may occur between the grid impedance and the LC filter.
In summary, reactive power compensation and harmonic mitigation technologies are one of the effective means to alleviate the contradiction between power supply and demand and improve power quality for the present and for a considerable period to come. After widespread application, they can bring huge economic and social benefits to the country and users. Controlling the harmonics generated by frequency converters to a minimum achieves scientific and rational power use, suppresses grid pollution, and improves power quality.
