Conduct periodic intercourseFourier series Decomposition to obtain the component whose frequency is more than one integral multiple of the fundamental frequency.[5]
Harmonic refers to periodic nonSinusoidal AC flowconductFourier series The decomposition result is greater thanFundamental waveThe components of integral times of frequency, usually calledHigh order harmonicsThe fundamental wave refers to the component whose frequency is the same as the power frequency (50Hz).The interference of higher harmonics is the currentPower systemMedium impactPower qualityOne of“Public nuisance”It is urgent to take countermeasures.[1]
The harmonic problem of power system has attracted people's attention since the 1920s and 1930s.At that time in Germany, due to the use of static mercury arcconverterThe voltage and current waveforms are distorted.1945 JC. The paper on converter harmonics published by Read is a classic paper on harmonic research in the early stage.
definition
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Harmonic wave. Strictly speaking, harmonic wave means that the frequency contained in the current isFundamental waveIs an integral multiple of, which generally refers to the periodic non sinusoidal electricityFourier series Decomposition, the electricity generated by other currents greater than the fundamental frequency.In a broad sense, since the effective component of AC power grid is a single frequency of power frequency, any component different from the power frequency can be called harmonic. At this time, the meaning of the word "harmonic" has become somewhat inconsistent with the original meaning.It is because of the generalized concept of harmonics that we have“Fractional harmonic”、“Interharmonics”、“Subharmonics”And so on.
The frequency of harmonics must also be equal toFundamental waveThe integral multiple of the frequency of the fundamental wave is calledThird harmonicThe wave with a fundamental frequency of 5 times is called the fifth harmonic, and so on.No matter how many harmonics, they aresine wave。
Three phase of generatorwindingIt is difficult to achieve absolute symmetry in production. Due to the influence of production technologycoreIt is also difficult to achieve absolute uniformity. In addition to the stability of the generator and other reasons, some harmonics will be generated, but generally it is relatively small.
Power transformerIt is the main harmonic source in the process of power transmission and distribution. As the transformer design needs to considerEconomy, with iron coreMagnetization curveNonlinearSaturation state, so that theMagnetizing currentIt is a corner top type waveform, thus generating odd harmonics.High saturation degree of transformer core makesoperating pointIt deviates from the linear curve and generates large harmonic current, whose odd harmonic current proportion can reach that of the transformerRated currentMore than 0.5%.
1) RectificationThyristorEquipment.Because rectifier thyristor is widely used in switching power supply, electromechanical controlCharging deviceIn many aspects, it has brought quite a lot of harmonics to the power grid.According to statistics, the harmonics caused by rectifier equipment reach nearly 40% of all harmonics, which is a major source of harmonics.
2) Frequency conversion equipment.Motor, elevator, water pump, fan, etcelectromechanical equipmentFrequency conversion equipment commonly used inPhase controlIts harmonic components are complex. In addition to integer harmonic components, it also contains a certain number of fractional harmonic components. The power of frequency conversion equipment is generally large, and its wide application causes more and more harmonics to the power grid.
4) Household electrical equipment.stayAir conditioner, refrigerator, washing machineelectric fanIn electrical equipment containing windingsUnbalanced currentThe change of can also change the power waveform.In addition, computers, televisions, temperature controlled cookware, dimming lamps, etcVoltage regulationRectification function will also produce high-order odd harmonic components.These household appliances have also become a major source of harmonics.
Impedance is the resistance generated at a certain point in the distribution system at a specific frequency.The impedance depends on the transformer and theElectric equipment, and theCross-sectional areaAnd length.
Impedance coefficient
The impedance coefficient is the ratio of AF (carrier) impedance to 50Hz (fundamental) impedance.
The equipment in the power distribution system and their existing capacitance (cable, compensationcapacitorAnd inductance (transformer, reactance coil, etc.) to form a resonance circuit.The latter can be excited by system harmonicsresonance。One reason for harmonics in distribution system isTransformer coreThe characteristics of nonlinear magnetization.In this case, the main harmonic is 3 times;It has the same length as the single-phase component in all conductors, so it cannot be eliminated at the star point.
Network impedance reachesMaximumFrequency of.In the parallel resonant circuit, the current components I L and I C are greater than the total current I.
The impedance level of the network reaches the minimum frequency.staySeries resonanceThe shunt voltages U L and U C in the circuit are greater than the total voltage U.
Series resonant circuit
By inductance（reactor）A circuit connected in series with a capacitor.
Motor and transformerMagnetic energySection, and forenergy exchangePurposivePower converterReactive power Q is required at such places.AndActive powerIn contrast, reactive power does not do work.The unit for measuring reactive power is Var orkvar。
The power supply department stipulates a minimumpower factor To avoid waste of electric energy.If the power factor of a factory is less than thisminimum valueIt pays for the reactive power.Otherwise, it should use capacitors to improve the power factor, which means that capacitors must be installed in parallel on electrical equipment.
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Harmonics are sine waves, each of which has a different frequency, amplitude and phase angle.
The harmonic frequency isFundamental waveThe integral multiple of frequency, according to the analysis principle of French mathematician M. Fourier, any repeated waveform can be decomposed into sine wave components containing fundamental frequency and a series of harmonics that are fundamental multiple.
According to different harmonic frequencies, it can be divided into:
The harmonics whose rated frequency is even times of the fundamental frequency are called“Even harmonics”, such as 2nd, 4th, 6th and 8th harmonics.
Generally speaking, odd harmonics cause more and greater harm than even harmonics.
In a balanced three-phase system, due toSymmetry relationEven harmonics have been eliminated, and only odd harmonics exist.For three-phase rectifier loadharmonic current It is 6n ± 1st harmonic, such as 5, 7, 11, 13, 17, 19, etc.
The frequency converter mainly produces 5th and 7th harmonics.
The component whose frequency is a non integral multiple of the fundamental wave is calledInterharmonicsSometimes, the interharmonics lower than the fundamental wave are called sub harmonics, which can be regarded as the interharmonics between DC and power frequency.Please refer toGB/T24337--2009.
2)Harmonic voltageIn many cases, it can make the sine wave more unsmooth, which not only leads to motor, transformercapacitorAnd other electrical equipmenthysteresisandEddy current lossIncrease, and makeInsulating materialThe electrical stress is increased.Harmonic current can makeCopper consumptionAs a result, motors and transformers will overheat locally under severe harmonic loadtemperature riseIncrease, thus acceleratingInsulation aging. Shorten the service life of electrical equipment such as transformers, waste increasingly valuable energy, and reducePower supply reliability。
3) Due to the motor, transformerPower capacitor, cables and other loads are constantly changing, which is very easy to match the large amount ofHarmonic sourceConstituting series or parallel connectionResonance condition, forming harmonic oscillation, generatingOvervoltageorOvercurrent, endangering the safe operation of motor, transformer and other loads and power system, and causing transmission and distribution accidents.
4) Grid harmonics will causeMeasuring instrument、Metering deviceError occurs, and correct indication and measurement cannot be achieved.Circuit breakerBreakharmonic content When the current is high, the breaking capacity of the circuit breaker will be greatly reduced, resulting in arc reignition, short circuit, and even circuit breaker explosion.
The method of restraining harmonic generated by frequency converter during operation is toHarmonic compensation, that is, increase harmonicsCompensation device, so that the input current becomessine wave。
2. Methods
Traditional harmonic compensation devices usually use LC tuningwave filterTo suppress harmonics, which can both suppress harmonics and compensateReactive power。deficienciesIts compensation characteristics are easily affected by the power grid impedance andrunning state Is easy to be connected in parallel with the systemresonance, resulting inHarmonic amplificationIt is easy to overload or even burn out the LC tuning filter.
On the other hand, LC tuned filter can only compensate harmonics of fixed frequency and the compensation effect is not ideal.However, due to the simple structure, low cost and easy setting of LC tuned filter, it is still widely used.[4]
