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Side harmonics of general-purpose frequency converters and Their Countermeasures

I. Introduction

with the increasing popularity and application of frequency converters, it is a pity that their proportion in the total power load has become larger and larger. Most of them are AC-DC-AC frequency converters with rated voltage of three-phase 380V (hereinafter referred to as frequency converters in this paper). The resulting side harmonic problem has also attracted more and more attention of frequency converter users and power supply departments

this paper will briefly introduce the generation mechanism of harmonic at the side of frequency converter and some common harmonic suppression technologies, and then focus on recommending a practical harmonic calculation method. The user can calculate the harmonic current generated when using the frequency converter according to the method recommended in this paper, so as to decide which countermeasure should be adopted to make the whole electric drive system meet the relevant national standards

generation mechanism of second and side harmonics

1 The generation of harmonic current

because the rectification part of the frequency converter is generally three-phase full wave uncontrollable rectification, the DC circuit uses large capacitance as the filter. In this way, although the side input voltage waveform of the frequency converter is basically a sine wave, the input current is a pulse charging current, which contains rich harmonics. Its waveform is shown in Figure 1

Figure 1 input voltage and current waveform of the frequency converter

the current waveform at the frequency converter side is jointly determined by the total equivalent impedance of the line and the voltage at both ends of the main capacitor, and is affected by the parameters of the diode rectifier itself. In addition, its current size and waveform are closely related to the DC side voltage, and the DC side voltage will fluctuate with the load change. Therefore, it is difficult to quantitatively calculate the converter side current through analytical expression, and it is not practical in engineering. In general analysis, a simplified approximate method can be used to calculate

2. The relationship between harmonic current and line resistance take RGM ⑵ 00 as an example. The greater the total line impedance on the

side, the smoother the input current and the smaller the harmonic current. Therefore, DC or AC reactors are often used to increase the line impedance, so as to improve the input current waveform

after adding the reactor, the peak of the input current becomes smaller and the conduction time of the diode becomes longer, so the harmonic content of the side current of the frequency converter can be reduced

both DC reactor and AC reactor can be used to suppress harmonics, but they have their own characteristics. Take reactor accessories fr-bel (DC) and fr-bal (AC) of Mitsubishi inverter as examples to illustrate the similarities and differences between them. See Table 1 for its application results. Table 1 Comparison of application effects of AC reactor and DC reactor

3 Relationship between side current waveform and DC side voltage

when the load of frequency converter changes, it will affect the DC side voltage. Only when the rectifier voltage is greater than the voltage (ED) at both ends of the main capacitor, the rectifier will have input current. Therefore, the DC voltage will determine the conductive width of the diode rectifier. Table 2 shows the relationship between the input current waveform of the frequency converter and the DC voltage. Table 2 Relationship between input waveform of frequency converter and DC voltage

in Table 2, waveform coefficient and peak coefficient represent the distortion degree of input current, and the input power factor of frequency converter is defined as the ratio of total input power and apparent power. Because the fundamental phase of input voltage and current are basically the same, ignoring the influence of three-phase imbalance, we can get

λ L ≈ ili/il

that is, the power factor is approximately equal to the ratio of fundamental current to total current. Therefore, table 2 can also reflect the relationship between DC side voltage and power factor

III. countermeasures to suppress high-order harmonics

1 Relevant national standard

the content of even harmonic and triple harmonic in the input current of frequency converter is very small, which is generally far lower than the national standard. Therefore, this paper mainly takes the analysis of 5, 7, 11, 15, 17, 19 harmonic current in the input current as an example. According to the national standard gbt14549-93 "public power harmonics of power quality" and gb12668.3-2003 "electromagnetic compatibility standard and its specific test methods for variable speed electrical drive system products", the harmonic current limit of the common connection point (PCC) is related to the ratio of the short-circuit current of the power supply and the maximum fundamental load current. The allowable values of each harmonic current under the reference short-circuit capacity are shown in Table 3. Table 3 allowable values of harmonic currents under reference short-circuit capacity

where the relationship between reference short-circuit capacity (SJ) and voltage is 0.38kv ~ 10MVA; 6kV10kV～100MVA。

the limit value of harmonic current used in this paper is the allowable value of each harmonic current under the reference short-circuit capacity specified in gbt14549 ~ 93, while the index given in Appendix B of gb12668.3 ~ 2003 is the limit value (%) of each harmonic current under different RSC. The two can be converted to each other, and users can choose by themselves according to their actual situation

2. Harmonic content in different system configurations

according to the data provided by Mitsubishi Electric, when using diode three-phase bridge rectifier frequency converter, the harmonic content in different configurations is shown in Table 4. Table 4 table of harmonic current content without active interruption and continuous unloading when the experimental force unloading reaches the limit sometimes

IV. calculation method of harmonic current

1 Calculation steps

as mentioned above, it is difficult to calculate the harmonic current of the frequency converter directly through the analytical formula. A calculation method is recommended below for your reference

Step 1: calculate the allowable harmonic current of each order according to the national standard and the short-circuit capacity of the actual transformer. The specific formula is

ih=igb (sr/sj)

where: IH is the allowable limit of harmonic current of each order; IGB partially replaces foreign imported products; Meet the functional protection requirements of heat, biochemistry, static electricity, radiation, etc; The performance of high temperature filtration and water filtration products meets the requirements of various utilization fields; Geotechnical materials meet the construction requirements of complex geological environment as the limit value of each harmonic current under the reference short-circuit capacity; SR is the actual short-circuit capacity, MVA; SJ is the reference short-circuit capacity, 10MVA at 380V

the allowable harmonic current injected into electricity by each user at the same common connection point is distributed according to the ratio of the user's agreed capacity or maximum load capacity at that point to the capacity of its power supply equipment. If we simply use the arithmetic sum of harmonic currents, the results are often too conservative, which will cause a waste of resources. It is recommended to use the pseudo square summation method, that is,

ihi=ih (si/st) 1/a

, where: Si is the user's power consumption protocol capacity or maximum load capacity, MVA; ST is the capacity of power supply equipment, MVA; IHI is the allowable value of each harmonic current after conversion; A is the phase superposition coefficient, which is taken according to table 5. The phase superposition coefficient of each harmonic is shown in Table 5. Table 5 phase superposition coefficient of each harmonic

step 2: conversion of rated current

i'e=ie × (0.38 standard voltage)

where: I'e is the converted rated current; Ie is the rated current of the frequency converter

step 3: determine the size of each harmonic current according to table 4 and the circuit form of the frequency converter, and compare it with the result of step 1 to judge whether it conforms to the national standard. The calculation formula is as follows:

ih=i'e × Harmonic content (%) × Load rate

if it does not meet the national standard, other countermeasures should be taken, such as using reactors, adding harmonic compensation equipment, etc

2. Example analysis

the short-circuit capacity of the power supply system (10kV) is 10MVA, and the total power supply capacity is 1mva. The protocol capacity of a user is 0.5mva. If a user only uses one frequency converter, its model is fr-ak, and the load rate is 80%, analyze whether its harmonic current meets the national standard. The calculation process is as follows

(1) according to ih=igb (sr/sj) and table 3, it is easy to get the converted limit value, as shown in Table 6. Table 6 converted limit value

then calculate the limit value of each harmonic current corresponding to the user according to the formula ihi=ih (si/st) 1/a and table 5, such as i5=2 × (0.51) 1/1.2=1.12

calculate the limit value of each harmonic current respectively, and get the maximum allowable harmonic current of the user, as shown in Table 7

table 7 maximum allowable harmonic current

(2) check the frequency converter manual to get the rated current of the frequency converter: ie=86 (a)

converted to 10kV side:

i'e =ie × (0.3810) =3.27 (a)

(3) when there is no reactor, there is

i5=3.27 × 65% × 80%=1.7 (a)

when using DC reactor (DCL), there is

i '5=3.27 × 30% × 80% =0.78 (a)

similarly, the data in Table 8 can be calculated

comparing table 7 and table 8, it can be found that the 5th and 7th harmonics exceed the standard when the DC reactor is not used. If DC reactor is used, the harmonic standard can be met

v. conclusion

in addition to the use of reactors mentioned above, the commonly used harmonic suppression measures in practical applications mainly include 12 phase rectification and the use of controllable rectification. However, since these circuit topologies are rarely used in general-purpose converters, we will not describe them in detail. Harmonic problem has always been a major technical obstacle to be solved in the development of frequency converters. In the past two years, some commercial low-voltage inverter products with new circuit topology have begun to appear, such as three-level inverter, matrix inverter (MC), etc. It is believed that with the reduction of cost and the solution of some technical problems, the problem of harmonic at the side of the frequency converter is expected to be effectively solved in the next 5 to 10 years, and the frequency converter will also become a real "green power supply". (end)

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