Harmonic control and reactive power compensation in power supply systems - News - Global IC Trade Starts Here Free

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1 Harmonic Fourier series decomposition of periodic non-sinusoidal electric power, in addition to obtaining the same component as the fundamental frequency of the power grid, also obtain a series of components larger than the fundamental frequency of the power grid. 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. Harmonics are actually an amount of interference that causes the grid to be "contaminated." Its frequency range is generally 2 ≤ n ≤ 40.
2 Harmonic sources Electrical equipment that injects harmonic currents into the utility grid or generates harmonic voltages on the utility grid is called a harmonic source. Electrical equipment with nonlinear characteristics is the main source of harmonics. For the specific power supply environment of Tianjin Port, the main harmonic source is the AC-DC-AC and frequency control terminal. Mechanically, 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 harmonic analysis and calculations are characteristic harmonics, such as 5, 7, 11, 13 and so on.
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.
3 Harmonic transmission For multi-voltage grade grids, the harmonic characteristics are that the harmonic current flows from the low-voltage side to the high-voltage side, and its size is basically independent of the high-voltage side parameters, which can be regarded as a constant current source. The harmonic voltage is transmitted from the high voltage side to the low voltage side and can be regarded as a constant voltage source. In the harmonic analysis, the harmonic equivalent circuit of the power grid is constructed according to this principle.
3.1 Frequency characteristics of grid components In the harmonic frequency range, due to the effects of eddy currents and leakage magnetic fields, the harmonic parameters of grid components should take into account the long-line effect, that is, the equivalent resistance R of transformers and conductors increases with increasing frequency, equivalent inductance L decreases as the frequency increases. The capacitance C of cables, wires and capacitors does not substantially keep constant with frequency. The relationship between load impedance and frequency varies with load.
3.2 Grid equivalent circuit The grid can be composed of the harmonic parameters Rn, In and Cn of the components of the grid to form an equivalent network. The equivalent circuit diagram of a three-phase symmetrical grid is usually expressed in a single phase. The node admittance matrix Yn at each frequency is calculated according to the equivalent circuit, the impedance Zn is obtained, and the harmonic voltage Un=ZnIn is calculated.
4 Harmonic limit To keep the grid harmonic voltage below the allowable value, the amount of harmonic current injected into the grid by the harmonic source must be limited. Most industrialized countries have successively formulated standards or regulations for harmonic management of power grids. Harmonic management standards are based on the principle of electromagnetic compatibility, that is, in a common electromagnetic environment, electrical equipment can work normally without excessive interference with the environment.
In 1993, China promulgated the national standard “Power Quality: Utility Grid Harmonics” (GB/T14549-93), which limits the harmonics of power systems, and specifies the harmonic voltage limits of public utilities and users to inject harmonic currents into the utility grid. Allowable value.
5 Harmonic hazard Harmonics increase the heat loss of electrical equipment, interfere with its function and even cause malfunction. In addition, harmonics can cause frequency coupling interference to the information system.
5.1 Motor Harmonic Voltage The harmonic current generated on the short-circuit impedance of the motor together with the negative-sequence fundamental current I of the motor causes additional heat loss to the device and is easily developed into an interference torque when the motor is started. The sum of the rms value of the harmonic current and the negative sequence fundamental current is generally not to be greater than 5% to 10% of the rated current Ie of the motor.
5.2 Capacitor Harmonics can cause overcurrent heating of the capacitor. The relevant regulations stipulate that the capacitor's long-term operating current must not exceed 1.3 times the rated current (Ic = CUn). Capacitors or filter capacitors located near harmonic sources are typically fabricated at higher current rms values.
5.3 The harmonic voltage of the electronic device can cause a trigger error in the thyristor trigger device and even cause equipment failure. Harmonics can also have an adverse effect on the grid's audio control system and computer.
5.4 Communication system The coupling of inductance and capacitance between wires below 2.5 kHz increases approximately linearly with frequency, especially the higher harmonics will cause interference to communication and information processing equipment.
6 Harmonic suppression increases the ripple number of the three-phase bridge circuit from 6 to 12, eliminating the 5th and 7th harmonics. Connect multiple harmonic sources to the same bus, and use the mutual compensation of harmonics to reduce the harmonic content of the power grid.
When the amount of harmonics exceeds the allowable value of the regulation or the grid has resonances in the harmonic range, a single tuned filter is typically set to absorb the characteristic harmonics. For harmonics of 13 times and above, a high-pass filter can be set. The filter loop also absorbs the original harmonics of the grid and can cause overload. Overloading is generally avoided by adjusting the rate of detuning, reducing the quality factor, or controlling the current value by means of additional electronics. Capacitors can form harmonic blocking loops through series reactors to prevent capacitor harmonic overload. The series resonant frequency is typically set below 250 Hz.
7 Reactive power compensation in the case of harmonics in the power grid 7.1 Compensation for the original converter load When the grid is connected to a harmonic source load (such as a converter, etc.), the compensation capacitor cannot be directly connected to the grid because the capacitor A parallel resonant loop is formed with the grid impedance. When estimating the resonant frequency, Vr=F(Qcl/Sk") can be calculated according to the short-circuit power Sk" of the grid and the fundamental compensation capacity Qc1 of the capacitor.
At the 5th harmonic frequency, the grid has resonance, and the parallel impedance Xp is greatly increased. After the 5th harmonic current from the harmonic source flows into the resonant tank, a high harmonic voltage is generated, and the harmonic voltage is superimposed on the fundamental wave. The voltage causes distortion of the voltage waveform. The balance current flowing between the grid and the capacitor can reach several times the current generated by the harmonic source, that is, the harmonic amplification. At this time, the transformer and the capacitor are subjected to a load greater than normal, especially the capacitor, and the long-term operation is in an overload state. Accelerate insulation aging and even breakdown the explosion. The parallel resonant frequency can be pre-calculated according to the grid impedance and the capacitor capacitive reactance, and the capacitor capacity configuration can be adjusted to keep the parallel resonant frequency and the characteristic harmonic frequency at a certain distance to avoid harmonic amplification. However, the actual grid impedance is not constant, and it is constantly changing. It is difficult to completely avoid resonance, especially when the capacitor group is adjusted and operated.
When it is necessary to compensate the grid with harmonic source equipment, technical measures must be taken to move the parallel resonance point to a safe position, and the most reliable method has been proved to be a series reactor in the capacitor circuit.
7.2 Capacitor Circuit String Reactance Capacitor The string reactance forms a series resonant circuit that exhibits a very low impedance (in theory, 0) at the resonant frequency. If the series resonant frequency is consistent with the grid characteristic harmonic frequency, it becomes a pure filtering loop. If only a small amount of harmonics are absorbed, it is called a detuning filter loop.
The main purpose of the harmonic loss loop is to prevent harmonic amplification, and the filtering effect is not large. The loop series resonant frequency is usually lower than the lowest secondary characteristic harmonic frequency of the power grid, that is, set to 3.8 to 4.2 times the fundamental frequency.