What does harmonic current mean
Harmonic current is a collective term for the sinusoidal components whose frequency is an integer multiple of the original periodic current frequency. A harmonic current with a frequency equal to k times the frequency of the original periodic current is called a k-th harmonic current, and all harmonic currents with a frequency greater than 1 are collectively referred to as higher-order harmonic currents.
A periodic signal can be decomposed into a linear superposition of DC component c0 and sine signals of different frequencies through Fourier transform: f (t)=c0+∑ _ (m=1) ^ ∞ ▒ [c_m sin (m ω T+ φ_ m) ]
Among them, c_msin (m ω T+ φ_ m) The expression for the m-th harmonic, where cm represents the amplitude of the m-th harmonic and its angular frequency is m ω， The initial phase is φ m. Its effective value is cm/√ 2.
When m=1, c1sin（ ω T+ φ_ 1) The expression for the fundamental component, whose angular frequency is ω，The initial phase is φ The root mean square value c1/√ 2 is called the fundamental effective value.
ω/ 2 π is the frequency of the fundamental component, which is called the fundamental frequency. The frequency of the fundamental component is equal to the frequency of the AC signal. The frequency of the m-th harmonic is an integer multiple (m times) of the fundamental frequency.
The definition of harmonics in power supply systems is the Fourier series decomposition of periodic non sinusoidal electrical quantities. In addition to obtaining components that are the same as the fundamental frequency of the power grid, a series of components that are greater than the fundamental frequency of the power grid are also obtained. This part of the electrical quantity is called harmonics. The ratio of harmonic frequency to fundamental frequency (n=fn/f1) is called the harmonic order. Sometimes there are non integer harmonics in the power grid, known as non harmonics or fractional harmonics.
Harmonics are actually a type of interference that pollutes the power grid. The field of electrical technology mainly studies the occurrence, transmission, measurement, harm, and suppression of harmonics, with a frequency range generally of 2 ≤ n ≤ 40.
2. Causes of harmonic current generation
The fundamental reason for the generation of harmonic currents is due to nonlinear loads.
When the current flows through the load and does not have a linear relationship with theapplied voltage, it forms a non sinusoidal current, resulting in harmonics. The suppression of harmonics in the power system is how to reduce or eliminate the harmonic current injected into the system, in order to control the harmonic voltage within the specified value.
There are four main measures to suppress harmonic current:
(1) Reduce the harmonic content of the harmonic source. That is to take measures on the harmonic source to minimize the generation of rabbit harmonics. This method is more proactive, can improve the quality of the power grid, and can greatly save the cost of eliminating harmonic effects.
(2) Adopting pulse width modulation (PWM) method. By using Pulse Width Modulation (PWM) technology, the DC voltage is modulated into a series of AC voltage pulses of equal amplitude but not equal width within the required frequency cycle. This method can greatly suppress the generation of harmonics.
(3) Absorb harmonic current at the harmonic source. This type of method is an effective way to suppress existing harmonics, which is currently the most widely used method for suppressing harmonics in the power system.
(4) Improve the power supply system and environment. For the power supply system, the generation of harmonics is inevitable, but measures such as increasing the short-circuit capacity of the power supply system, improving the voltage level of the power supply system, increasing the capacity of power supply equipment, and maintaining three-phase load balance as much as possible can all improve the ability of the power grid to resist harmonics. Choosing a reasonable power supply voltage and maintaining three-phase voltage balance as much as possible can effectively reduce the impact of harmonics on
the power grid. The harmonic source is supplied by a larger capacity power supply point or a higher voltage grid, and the ability to withstand harmonics will increase. Powering the harmonic source load through dedicated lines reduces the impact of harmonics on other loads, and also helps to concentrate the suppression and elimination of higher-order harmonics.
3. Hazards of harmonic currents
In the past thirty to forty years, the rapid development of various power electronic devices has led to increasingly severe harmonic pollution in the public power grid. Various faults and accidents caused by harmonics have also occurred continuously, and the severity of harmonic hazards has attracted high attention from people. The harm of harmonics to public power grids and other systems can be roughly summarized in the following aspects.
(1) Harmonics cause additional harmonic losses in the components of the public power grid, reducing the efficiency of power generation, transmission, and consumption equipment. When a large number of third-order harmonics flow through the neutral line, itcan cause the line to overheat or even cause a fire.
(2) Harmonics affect the normal operation of various electrical equipment. The impact of harmonics on motors not only causes additional losses, but also generates mechanical vibration, noise, and overvoltage, causing severe local overheating of transformers. Harmonics can cause overheating, insulation aging, shortened lifespan, and even damage to capacitors, cables, and other equipment.
(3) Harmonics can cause local parallel resonance and series resonance in the public power grid, thereby amplifying harmonics, greatly increasing the harm of (1) and (2) above, and even causing serious accidents.
(4) Harmonics can cause misoperation of relay protection and automatic devices, and can lead to inaccurate measurement of electrical measuring instruments.
(5) Harmonics can cause interference to adjacent communication systems, and in light cases, noise can be generated, reducing communication quality; Severe cases result in the loss of accommodation, making the communication system unable to function properly
