In order to control the flooding of harmonics, purify the waveform of the power supply and distribution system, and improve the power quality and efficiency of the power supply and distribution system, the country has promulgated a national standard for power quality: "Power Quality-Harmonics in Public Grid" (GB/T14549-93) "Allowable deviation of power supply voltage" (GB12325-90); "Allowable unbalance of three-phase voltage" (GB/T15543-95); "Allowable deviation of power system frequency" (GB/T15945-95); "Allowable voltage fluctuation and Flicker" (GB12326-90).
According to the characteristics of pulp and paper enterprises, there are many different methods to suppress the harm caused by harmonics.
1. All kinds of high-power non-linear electrical equipment transformers are powered by the distribution network with large short-circuit capacity.
2. The following measures should be taken for high-power converter equipment:
(1) Increase the number of phases on the secondary side of the rectifier transformer and increase the number of rectifier pulses. The high-order harmonic current is closely related to the number of rectified phases, that is, as the number of phases increases, the very low order of the high-order harmonic becomes higher, and the harmonic current amplitude becomes smaller. Generally, SCR rectifiers are mostly 6-phase. In order to reduce the high-order harmonic current, 12-phase or 36-phase can be used instead. When 12-phase rectification is used, the high-order harmonic current only accounts for about 1% of the full current, and the harmfulness is greatly reduced.
(2) When two or more rectifier transformers are powered by the same busbar, the primary windings of the rectifier transformer can be alternately connected into Y-shaped and △-shaped respectively, which can make the 5th and 7th harmonics cancel each other, but only Considering the influence of the 11th and 13th harmonics, due to the high frequency and small wave amplitude, the harmfulness is reduced.
(3) Set up multiple rectifiers with the same number of phases so that the secondary side of the rectifier transformer has an appropriate phase angle difference.
3. Choose a three-phase transformer with delta-connected windings.
When the transformer has delta-connected windings, it provides a zero-sequence current path including the third harmonic current and its odd multiples of harmonics, so that these harmonic currents are short-circuited in the delta winding, so that the transformer feeds the power distribution network The harmonic short-circuit content is significantly reduced.
4. Connect a set of reactors in series in the compensation capacitor loop.
Before Xc is added, the resistance is omitted, the harmonic voltage at the harmonic source In bus is: Un=Xsn•In; after the compensation capacitor is connected in parallel, the input harmonic reactance of the harmonic source is: harmonic voltage at this time , The harmonic current Un,Isn>In injected into the system, that is, the parallel capacitor makes the harmonics of the system amplified. If Xsn-Xcn=0 corresponding to a certain order of harmonics, that is, harmonics occur, and the harmonic currents and voltages tend to be very large. In order to get rid of this resonance point, reactors are usually connected in series in the capacitor branch. The inductive reactance value should be selected so that the total reactance of the capacitor circuit is inductive reactance instead of capacitive reactance under any harmonics that may be generated. The possibility of generating harmonics is fundamentally eliminated.
5. The frequency converter adopts AC reactor for incoming line, and DC reactor or sine filter for outgoing line.
Do not share the ground wire and separate the power supply (the frequency converter and the interfered equipment supply power separately); the equipment that is susceptible to interference is powered by an isolated inductor; the outlet and inlet wires of the frequency converter are shielded and grounded, and separated by a certain distance; incoming and outgoing wires Wear a metal pipe and ground; output a four-core cable (one core is grounded), the motor shell is grounded, and the inverter is grounded separately; use an insulated power transformer (the neutral point is not grounded); shorten the line length.
6. Laying of power lines and communication lines.
Keep communication lines away from power lines as much as possible. Power lines and signal lines should be laid separately to avoid crossing. If unavoidable, they should be crossed vertically and cannot be laid in parallel. The shielding layer of signal lines should not be connected to the ground of the motor or inverter, but should be connected to the control line. Common end; zero-sequence reactor, surge absorber, surge suppressor, input suppression reactor can be used when it should; use twisted wire wiring.
Seven, set the passive filter.
The passive filter uses the resonance principle of the circuit, that is, when the resonance of a certain harmonic occurs, the device forms a low-impedance path for the harmonic to achieve the purpose of filtering. In terms of structure, it is made up of power capacitors, reactors and resistors through a proper combination. It is connected in parallel with the harmonic source during operation. In addition to filtering, it also takes into account the needs of reactive power compensation. The passive filter has a simple structure, low cost, low operating cost, and has a significant effect in absorbing higher harmonics. However, due to its structural principle, there are also some insurmountable shortcomings in its application:
(1) The single-tuned filter that suppresses lower harmonics is only effective for the harmonics at the tuning point, but has no obvious effect on the harmonics that deviate from the tuning point. In actual engineering design, design investment is considered, and it is impossible to increase The filter method is solved.
(2) When the harmonic current in the system increases, the passive filter may be overloaded and even damage the equipment. Moreover, the filtering effect varies with the operating conditions of the system. When the system impedance and frequency fluctuate, the filtering effect becomes worse.
(3) When the system impedance and frequency change, parallel resonance may occur with the system, making the device unable to operate, and even the entire filtering system cannot operate normally.
(4) In addition, an important passive filter can only filter one kind of harmonic component. If you filter different harmonics, you must connect different filters.
8. Set up the active filter.
Active power filter (APF) is a new type of power electronic device for dynamically suppressing harmonics and compensating reactive power. It can compensate for harmonics with varying sizes and frequencies as well as varying reactive power. The reason why it is called Active, as the name suggests, the device needs to provide power. Its application can overcome the shortcomings of traditional harmonic suppression and reactive power compensation methods such as LC filters (traditional only fixed compensation), realizes dynamic tracking compensation, and can compensate for harmonics Wave and reactive power compensation; three-phase circuit instantaneous reactive power theory is the main basic theory for the development of APF; APF has two types of parallel type and series type, the former is mostly used; parallel active filters are mainly used to control current harmonics, and series The active filter is mainly to deal with the problems caused by voltage harmonics. Compared with the passive filter, the active filter has a better treatment effect, and can mainly filter out multiple and high-order harmonics at the same time without causing resonance. The main problem is the high cost.
In short, the influence of harmonics is a cumulative effect, such as insulation aging, which is not obvious at the beginning of operation, but it will be revealed as the operation time continues, resulting in a shortened maintenance period and a significant increase in the operating cost of electrical equipment. Especially the impact on the DCS communication system cannot be ignored. To solve the harmonic problem in the pulp and paper power distribution system, it is necessary to formulate reasonable and effective measures to suppress harmonics from the early stage of the project.