RU2172057C2 - Voltage inverter having low higher harmonic content - Google Patents

Abstract

FIELD: converter engineering; independent voltage and frequency control of multiphase load such as induction-motor stator winding. SUBSTANCE: inverter has its voltage and frequency setting elements; logic device; power switches unit; multiphase generator whose input functions also as output of inverter frequency setting element and its nine outputs are connected to first nine inputs of logic device; reference-voltage generator; inverting amplifier whose input is output of inverter voltage setting element; two comparators whose first inputs are connected, respectively, to output of voltage setting element and to that of inverting amplifier, its second inputs being interconnected and connected to reference-voltage generator output and its outputs, to tenth and eleventh inputs of logic device; three outputs of the latter function as inputs of dead zone shaper; six outputs of the latter are connected to six control inputs of load power switches unit. Novelty is introduction of third, fourth, fifth, and sixth comparators whose first inputs are connected to output of reference-voltage generator and outputs, to additional inputs of logic device; two scaling amplifiers with gain factors of 0.732 and 0.268 whose inputs are connected to output of inverter voltage setting element and outputs, to second inputs of third and fifth comparators, and to newly introduced second and third inverting amplifiers, respectively, outputs of these amplifiers being connected to second inputs of fourth and sixth comparators, respectively. EFFECT: improved output characteristics of inverter (essentially reduced higher-harmonic content of its output voltage). 10 dwg

Description

 The invention relates to a converter technique and can be used to independently control the frequency and voltage of a multiphase load, for example, a stator winding of an induction motor.

 Known inverter voltage with a low content of higher harmonics [1], containing a control system and two or more cascade-connected separate inverters through the secondary windings of their transformers (Fig. 1). The voltage on the load in this case is the geometric sum of the voltages of the individual inverters and can be continuously adjusted from zero to the maximum value by shifting the opening phases of the individual inverters, and the frequency of the output voltage depends on the switching frequency of the switching devices.

 The disadvantages of such a device include its complexity, the practical impossibility of obtaining a high-quality form of low-frequency voltages due to the need to increase the dimensions of transformers, a large number of power switching devices, especially for multiphase loads.

Known m-phase voltage inverter with a lower content of higher harmonics [2], containing a control signal generation unit (BFSU), a key control unit (BPC) and a frequency converter with direct coupling (PCNS), the output of which is connected to the load (Fig. 2) . BFSU is used to generate m phase shifted by 2 π / m radians (where m is the number of load phases) step voltage signals. The magnitude of the steps and their number n at the half-cycle of the output frequency are chosen in such a way that the largest possible harmonic row is absent in the voltage spectrum. BFSU contains a multiphase generator, a logic device, an inverting amplifier, a resistive voltage divider, and m identical formers F1 ... Фm, each of which contains n keys Cl1 ... Cln and an adder Σ. Each combination of signals at the output of the multiphase generator determines the number i of the interval on the period of the generated control signal (i = 1 ... 2n). In accordance with this number, the logical device includes the desired key in each driver for transmitting the required voltage levels from the outputs of the resistive voltage divider to the inputs of the adders. The values of all the steps in the generated control signal at the output of the adders are proportional to the given signal U _ass , therefore, the ratios between them, and therefore the signal shape, remain unchanged. For the example of FIG. 2 shows a circuit generating a control signal with the number of steps n = 6.

 The key control unit contains a reference voltage generator, which is sections of a sinusoidal supply network, m comparators K1 ... Km, at the output of which, in accordance with the principle of vertical control, key control pulses are generated at the moments of equality of the corresponding control signal and the reference voltage, and a distribution amplifier pulses, which converts the output signals of the comparators to the level necessary to control the power switches, and taking into account the signal of the current sensor, DT distributes them between the anode second gating groups and cathode power unit FCDC each phase.

 The voltage at the load phases Z1 ... Zm is composed of sections of the sinusoidal voltage of the network in accordance with the control signal generated at the output of the formers F1 ... Фm.

 The main disadvantage of this device is the fact that the effect of the reduced content of higher harmonics in a specially formed control signal decreases, because when using the power part of the frequency converter with direct coupling, the load voltage is added from sections of the sinusoid of the supply network, as a result of which again higher harmonics are added, the frequency of which is a multiple of the frequency of the mains.

 Another disadvantage of the device is its increased complexity associated with the sequential conversion of the reference signal first into bipolar step control signals and only then into key control pulses.

The closest in technical essence to the claimed invention is a voltage inverter selected as a prototype [3] (Fig. 3), containing a power switch block 1, made in the form of three identical racks of two series-connected switches, powered by a constant voltage, which is connected to the load 2 inverter generator "dead" zones 3, the logic device 4, whose outputs 5, 6, 7 are connected to the inputs of driver "dead" zones 3, 8 multiphase generator having an input coupled to the output frequency f _backside setpoint 9 Inver torus, and its nine outputs 10 ... 18 are connected to the first nine inputs of the logic device 4, the reference voltage generator 19, an inverting amplifier 20, the input of which is the output U _{back of the} voltage regulator 21 of the inverter, two comparators 22 and 23, the first inputs of which are connected respectively, with the output of the voltage regulator 21 and the output of the inverting amplifier 20, the second inputs are combined and connected to the output of the reference voltage generator 19, and the outputs are connected to the tenth 24 and eleventh 25 inputs of the logic device 4. Note that in some ryh cases, reference voltage generator 19 and voltage setting unit 21 may have control inputs and controlled by an auxiliary device 26, frequency control reference signal as a function of f _butt 9 setpoint frequency.

Формирователь "мертвых" зон 3 формирует три прямых 30, 31, 32 и три инверсных 33, 34, 35 сигнала управления стойками блока силовых ключей 1 так, что между прямым и инверсным сигналами каждой стойки существует небольшая задержка ("мертвое" время) для исключения возможности одновременного включения верхнего и нижнего ключей стойки. Для примера на фиг. 6 приведены диаграммы сигналов 5, 30, 35, управляющих стойкой фазы А. Аналогичные диаграммы имеют группы сигналов 6, 31, 34 и 7, 32, 33, управляющие стойками фаз В и C соответственно. Формируемое напряжение 36 фазы А показано на фиг. 4. Аналогичные напряжения, сдвинутые на 120 эл. градусов, образуются в фазах В и C.The principle of operation of the device is illustrated by diagrams in FIG. 4, FIG. 5, FIG. 6. The multiphase generator 8 produces three shifted by 120 el. degrees of the signal 10, 11, 12 of the meander type, as well as a sequence of six shifted by 60 el. degrees of rectangular pulses 13 ... 18 with a duration of 60 el. degrees and a frequency equal to the specified frequency f _{back of the} inverter. The reference voltage generator 19 generates a bipolar sawtooth voltage 27 of the carrier frequency f _op (Fig. 5), which is compared at the inputs of the comparators 22 and 23, respectively, with direct 28 and inverse 29 voltage setting signals U _{back of the} inverter. The outputs of comparators 22 and 23, on which signals 24 and 25 are present, are connected by a logic device 4 to its outputs 5, 6, 7 at time instants determined by the state of signals 10 ... 18 in accordance with the following logical equations:

The shaper of the “dead” zones 3 generates three direct 30, 31, 32 and three inverse 33, 34, 35 signals of control of the racks of the power key unit 1 so that there is a slight delay (dead time) between the direct and inverse signals of each rack (dead time) to exclude the ability to simultaneously enable the upper and lower rack keys. For the example of FIG. Figure 6 shows diagrams of signals 5, 30, 35 that control the phase A rack. Similar diagrams have signal groups 6, 31, 34, and 7, 32, 33 that control the phases B and C racks, respectively. The generated phase A voltage 36 is shown in FIG. 4. Similar voltages shifted by 120 el. degrees are formed in phases B and C.

где U1_н - действующее значение первой гармоники напряжения на нагрузке;
U_дн - действующее значение напряжения на нагрузке.The main disadvantage of this device is the high content of higher harmonics in the phase voltage curve of the inverter and, as a result, the increased distortion coefficient characterizing the degree to which the voltage curve at the load is removed from the sinusoidal:

where U1 _n is the effective value of the first harmonic of the voltage at the load;
U _days - the actual value of the voltage at the load.

где U_и - напряжение в звене постоянного тока. Из (2) видно, что амплитуды наиболее выраженных пятой и седьмой гармоник составляют соответственно 20% и 14,3% от амплитуды первой гармоники.So, for the maximum voltage reference signal 28, 29, equal to the amplitude of the sawtooth voltage 27, the expansion in the Fourier series of the phase voltage 36 gives the following spectral composition:

where U _and is the voltage in the DC link. From (2) it is seen that the amplitudes of the most pronounced fifth and seventh harmonics are 20% and 14.3% of the amplitude of the first harmonic, respectively.

коэффициент искажения по (1) с учетом (2) и (3):

что является достаточно большой величиной.The effective value of such a step voltage is

distortion coefficient according to (1) taking into account (2) and (3):

which is a fairly large value.

 The purpose of the invention is to obtain better output characteristics of the inverter, expressed in a significant reduction in the level of higher harmonics in its output voltage.

 This goal is achieved by the fact that in the voltage inverter containing the voltage regulator and the frequency inverter, a logic device, a power switch block, a multiphase generator, the input of which is the output of the frequency inverter, and nine of its outputs are connected to the first nine inputs of the logic device, the reference generator voltage, an inverting amplifier, the input of which is the output of the inverter voltage setter, two comparators, the first inputs of which are connected respectively to the output of the voltage setter the output and the inverting amplifier, the second inputs are combined and connected to the output of the reference voltage generator, and the outputs are connected to the tenth and eleventh inputs of the logic device, the three outputs of which are inputs of the shaper of the "dead" zones, and the six outputs of the latter are connected to six control inputs of the power unit keys, supplying the load, according to the invention additionally introduced the third, fourth, fifth and sixth comparators, the first inputs of which are connected to the output of the reference voltage generator, and the outputs with additional inputs of the logic device, two scaling amplifiers with transmission coefficients 0.732 and 0.268, connected by inputs to the output of the inverter voltage adjuster, and outputs, respectively, with the second inputs of the third and fifth comparators and the inputs of the additionally introduced second and third inverting amplifiers, whose outputs are connected to the second the inputs of the fourth and sixth comparators, respectively. Note that in some cases, the voltage adjuster and the reference voltage generator may have control inputs and be controlled by an auxiliary control device as a function of the frequency reference signal from the frequency adjuster.

 An advantage of the present invention is the fact that due to the introduction of two additional voltage dividers, inverting amplifiers and four comparators, two additional levels are formed in the step curve of the inverter output phase voltage. By appropriate selection of the transmission coefficients of the scaling amplifiers 0.732 and 0.268, it is possible to obtain a significant decrease in the content of higher harmonics in the output voltage of the inverter and, therefore, a decrease in the distortion coefficient of its shape.

 In FIG. 7 shows a diagram of the device, and in FIG. 8, 9, 10 are signal diagrams of its main nodes.

 A voltage inverter with a lower content of higher harmonics (Fig. 7) contains a power switch block 1 made in the form of three identical racks of two switches connected in series, powered by a constant voltage, which is connected to the load 2 of the inverter, a shaper of "dead" zones 3, a logic device 4, the outputs of which 5, 6, 7 are connected to the inputs of the shaper of the “dead” zones 3, a multiphase generator 8, the input of which is connected to the output of the frequency inverter 9 of the inverter, and nine of its outputs 10 ... 18 are connected to the first nine inputs of the logical device 4, the reference voltage generator 19, the first inverting amplifier 20, the input of which is connected to the output of the voltage regulator 21 of the inverter, the first and second comparators 22 and 23, the first inputs of which are connected respectively to the output of the voltage regulator 21 and the output of the inverting amplifier 20, and the outputs are connected to the tenth (24) and eleventh (25) inputs of the logic device 4, two scaling amplifiers 26 and 27 with transmission coefficients 0.732 and 0.268, respectively, the inputs of which are connected to the output of the voltage regulator 21, third, fourth the fifth, sixth and sixth comparators 28, 29, 30, 31, and the first inputs of the comparators 28 and 30 are connected respectively to the outputs of the scaling amplifiers 26 and 27, and the first inputs of the comparators 29 and 31 are connected to the outputs of the scaling amplifiers 26 and 27 through the second and third inverting amplifiers 32 and 33, respectively, the second inputs of all comparators are combined and connected to the output of the reference voltage generator 19, and the outputs of the comparators 28 ... 31 are connected respectively to the inputs 34 ... 37 of the logical device 4.

 Note that in some cases, the voltage adjuster 21 and the reference voltage generator 19 may have control inputs and be controlled by the auxiliary control device 38 as a function of the frequency reference signal from the frequency adjuster 9.

 The device operates as follows.

Multiphase generator 8 produces three shifted by 120 el. degrees of the signal 10, 11, 12 of the meander type (Fig. 8), as well as a sequence of six shifted by 60 el. degrees of rectangular pulses 13 ... 18 with a duration of 60 el. degrees and a frequency equal to the specified frequency f _{back of the} inverter. The reference voltage generator 19 generates a bipolar sawtooth voltage 39 of the carrier frequency f _op (Fig. 9), which is compared at the inputs of the comparators 22 and 23, respectively, with direct 40 and inverse 41 voltage setting signals U _{back of the} inverter. The reliability obtained at the outputs of the comparators 22 and 23 of the signals 24 and 25 determines the amplitude value U _{max of the} output voltage of the inverter. The level of the second stage of the output voltage is formed similarly by comparators 28 and 29 by comparing the sawtooth voltage 39 of the carrier frequency f _{op of the} generator 19 with direct 42 and inverse 43 signals of the scaling amplifier 26 with a transmission coefficient K = 0.732 at their inputs. The reliability of the signals 34 and 35 obtained at the outputs of the comparators 28 and 29 determines the level of 0.732 U _max of the inverter output voltage. The level of the third voltage level 0.268 U _max is determined by the duty cycle of signals 36 and 37 at the outputs of comparators 30 and 31, comparing the signals of the reference voltage 39 of the generator 19 with direct 44 and inverse 45 signals of the scaling amplifier 27 with the transmission coefficient K = 0.268.

Формирователь "мертвых" зон 3 формирует три прямых 46, 47, 48 и три инверсных 49, 50, 51 сигнала управления стойками блока силовых ключей 1 инвертора так, что между прямым и инверсным сигналами каждой стойки существует небольшая задержка ("мертвое" время) для исключения возможности одновременного включения верхнего и нижнего ключей стойки. Для примера на фиг. 10 приведены диаграммы сигналов 5, 46, 51, управляющих стойкой фазы А. Аналогичные диаграммы имеют группы сигналов 6, 47, 50 и 7, 48, 49, управляющие стойками фаз В и С. Среднее значение сформированного выходного напряжения фазы А 52 показано на фиг. 8. Аналогичные напряжения, сдвинутые на 120 эл. градусов, образуются в фазах В и C.The signals 24, 25 and 34 ... 37 received at the output of the comparators are transmitted by the logic device 4 to its outputs 5, 6, 7 at time instants determined by the state of the signals 10 ... 18 in accordance with the following logical equations:

The “dead” zone 3 generator generates three straight lines 46, 47, 48 and three inverse 49, 50, 51 control signals of the racks of the power switch block 1 of the inverter so that there is a slight delay (dead time) between the direct and inverse signals of each rack (dead time) eliminating the possibility of simultaneously turning on the upper and lower rack keys. For the example of FIG. 10 shows diagrams of signals 5, 46, 51 that control phase A racks. Similar diagrams have signal groups 6, 47, 50 and 7, 48, 49 that control phase B and C racks. The average value of the generated output voltage of phase A 52 is shown in FIG. . 8. Similar voltages shifted by 120 el. degrees are formed in phases B and C.

где U_и - напряжение в звене постоянного тока инвертора;
α₀ - отношение величины сигнала 40 задания напряжения к амплитуде пилообразного напряжения ШИМ;.The expansion in a Fourier series of the phase voltage 52 gives the following spectral composition:

where U _and - voltage in the DC link of the inverter;
α ₀ - the ratio of the magnitude of the voltage setting signal 40 to the amplitude of the sawtooth PWM voltage ;.

 k = 12n ± 1 is the harmonic number (n = 1,2,3 ...).

 Comparison of (5) and (2) shows a significant decrease in the content of higher harmonics in the output voltage of the inverter. Consequently, the inverter output phase voltage has six levels, and the choice of transmission coefficients of the scaling amplifiers 0.732 and 0.268 ensures that harmonics with numbers 3k, 6k-1 and 6k + 1, where k = 1.3, are excluded from the inverter output voltage curve in addition to even harmonics 5 ... The distortion coefficient calculated according to (1) taking into account (5) is 0.1505, which is almost half that of the prototype (0.2953).

Sources of information
1. Yu. K. Rozanov. The basics of power electronics. - M .: Energoatomizdat, 1992, 296 p.

 2. G.G. Zhemerov. Direct coupled thyristor frequency converters. - M .: Energy, 1977, 280 p.

 3. Kalashnikov, S. O. Krivitsky, I.I. Epstein. Autonomous inverter control systems. - M .: Energy, 1974, 105 p.

Claims (1)

 A voltage inverter with a reduced content of higher harmonics, comprising a voltage regulator and an inverter frequency adjuster, a logic device, a power switch unit, a multiphase generator, the input of which is the output of the inverter frequency adjuster, and nine of its outputs are connected to the first nine inputs of the logic device, a reference voltage generator, an inverting amplifier, the input of which is the output of the inverter voltage regulator, two comparators, the first inputs of which are connected respectively to the output of the voltage regulator voltage and the output of the inverting amplifier, the second inputs are combined and connected to the output of the reference voltage generator, and the outputs are connected to the tenth and eleventh inputs of the logic device, three outputs of which are inputs of the shaper of the "dead" zones, and six outputs of the latter are connected to six control inputs of the power unit keys supplying the load, characterized in that it introduced the third, fourth, fifth and sixth comparators, the first inputs of which are connected to the output of the reference voltage generator, and the outputs - with additional actual inputs of the logic device, two scaling amplifiers with transmission coefficients 0.732 and 0.268, connected by inputs to the output of the inverter voltage adjuster, and outputs, respectively, with the second inputs of the third and fifth comparators and the inputs of the additionally introduced second and third inverting amplifiers, whose outputs are connected to the second inputs respectively, the fourth and sixth comparators

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