RU2532252C2 - Direct frequency converter with natural commutation control method - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: invention is related to the field of electric engineering and power electronics and can be used for manufacturing of three-phase alternating current generating systems for aircraft. In such systems a synchronous generator with variable shaft speed serves as a primary source with unstable parameters of the input energy. Function of qualitative indications provision for the generated electric energy is rest on the direct frequency converter with natural commutation and output low-frequency power filter. The suggested method lies in generation of control pulses by thyristors when reference signals are equal to the control three-phase signals, generation of three sinusoidal signals shifted per 120°, generation of a signal, which is equal to the average value of the generated sinusoidal signals and the received signal is scaled, and each control signal is generated as the sum of the scaled signal and the respective sinusoidal signal.

EFFECT: suggested method allows obtaining of the technical result that lies in reduction of the synchronous generator weight due to increase in input power factor by the direct frequency converter.

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Description

The invention relates to the field of electrical engineering and power electronics and can be used in the construction of three-phase alternating current electric power generation systems for aircraft, in which static converters of electrical energy are used to achieve high-quality output energy indicators. The primary sources with unstable input energy parameters in such systems is a synchronous generator with a variable shaft speed. The function of ensuring quality indicators of the generated electric energy is assigned to the direct frequency converter with natural switching and the output power low-pass filter.

в этом соотношении S_сг - установленная мощность синхронного генератора (масса синхронного генератора пропорциональна этой величине), P_н - активная мощность нагрузки, η_cn - коэффициент полезного действия преобразователя. Поэтому возникает задача повышения величины коэффициента мощности непосредственного преобразователя частоты с естественной коммутацией.For the indicated application of generation systems, an important indicator is the mass of all elements of the system, during the design of which it is necessary to strive to reduce it. The mass of a synchronous generator when working with a direct frequency converter with natural switching is largely determined by the value of the input power factor of the converter (χ _cg ). Really $$S_{\mathrm{from}g}=\frac{P_n}{{\eta }_{cn}\cdot {\chi }_{cn}},$$

in this ratio, S _cg is the installed power of the synchronous generator (the mass of the synchronous generator is proportional to this value), P _n is the active load power, η _cn is the efficiency of the converter. Therefore, the problem arises of increasing the power factor of a direct frequency converter with natural switching.

A known method of controlling a direct frequency converter with natural switching [G. Zhemerov Direct coupled thyristor frequency converters. M., Energy, 1977], which consists in the fact that they form reference signals and three-phase rectangular control signals; at the moments of equality of these signals, thyristor control pulses are generated.

This method has the disadvantage that in the spectrum of the output voltage of the direct frequency converter with natural switching in this case there are large harmonic components with frequencies that are multiples of the frequency of the control signals. This leads to a significant increase in the mass and dimensions of the output power low-pass filter.

In addition, there is a known method of controlling a direct frequency converter with natural switching as part of a system consisting of a synchronous generator, a direct frequency converter with direct connection and an output power low-pass filter [Juji L., Pelly B. Power semiconductor frequency converters: Theory, characteristics, application . Per. from English - M .: Energoatomizdat, 1983. - 400 p., Ill.], Which is the prototype of the present invention, which consists in the fact that the control pulses of the thyristors are formed when the reference signals are equal to the three-phase control signals of a sinusoidal shape.

The disadvantage of this method is the relatively small value of the input power factor of the direct frequency converter with natural switching and, as a result, the high mass of the synchronous generator.

The objective of the invention is to reduce the mass of the electric power generation system by reducing the mass of the synchronous generator, which is achieved by increasing the input power factor of the direct frequency converter with natural switching.

This is achieved by the fact that in the known control method, three sinusoidal signals shifted by 120 ° are generated, a signal is generated, which at each moment of time is equal to the average value of the generated sinusoidal signals, the received signal is scaled and each control signal is generated as the sum obtained by scaling the signal and corresponding sinusoidal signal.

Figure 1 presents the structural diagram of a device that implements the proposed method of controlling a direct frequency converter with direct connection and natural switching.

The block diagram of the device includes: a reference signal generator (1), the number of reference signals is equal to the number of thyristors of a direct frequency converter with natural switching, a three-phase sinusoidal control signal generator (2), an average value allocation circuit (3), a scaling unit (4), an adder ( 5) and a comparison circuit (6), their number is equal to the number of thyristors of a direct frequency converter with natural switching.

The outputs of the reference signal generators (1) are connected to the second inputs of the comparison circuits (6). The outputs of the control three-phase sinusoidal signal generator (2) are connected to the inputs of the average value allocation circuit (3) and the second inputs of the adders (5). The output of the average value allocation circuit (3) is connected to the first input of the scaling unit (4), whose output is connected to the first inputs of the comparison circuit (6). The output signals of the comparison circuits (6) are then fed to the thyristors.

Blocks operate as follows. The reference signal generator (1) generates linear or cosine reference signals with an amplitude value of U _{on max} synchronized with the voltage of the synchronous generator, the number of these signals is equal to the number of thyristors in a direct frequency converter with direct coupling. The generator of the control three-phase sinusoidal signals (2) generates three sinusoidal signals (figure 2)

where U _ym is the amplitude value of the control signal, Ω is the cyclic frequency of the control signals.

схемы выделения средних значений (3) каждый момент времени равен среднему значению управляющих трехфазных синусоидальных сигналов (фиг.2) в соответствии с соотношениемOutput signal $$({\stackrel{⌢}{U}}_{midl})$$

the scheme of allocation of average values (3) each time instant is equal to the average value of the control three-phase sinusoidal signals (figure 2) in accordance with the ratio

$${\stackrel{⌢}{U}}_{midl}=Midl\left\{U_{y\sin A},U_{y\sin B},U_{y\sin C}\right\}.(\mathrm{one})$$

The output signal of the scaling unit (U _midl ) is determined by the ratio of the form

$$U_{midl}=\frac{\mathrm{one}}{2}{\stackrel{⌢}{U}}_{midl}.(2)$$

As follows from figure 2 and relations (1), (2), the signal U _midl changes with a frequency of 3Ω and contains harmonics of the order of (2k-1) Ω, k = 1,2,3, ...

The output signals of the adder (5) are determined by the ratio

$$U_{y\mathrm{to}\mathrm{about}mj}=U_{y\sin j}+U_{midl},j=A,B,C.(3)$$

The waveform of the output signal of the adder for j = A is shown in Fig.3.

раза [Берестов В.М., Харитонов С.А. Анализ синусоидальной ШИМ с натуральной выборкой (методический аспект). Технiчна електродинамiка. Тематический выпуск. Силова електронiка та енергоефективнiсть. Частина 2. Киiв, 2002. - с.31-37].With such a control signal, the linear range of variation of the amplitude value U _ym expands, while its maximum value can be equal to $$U_{ym\max }=\frac{2}{\sqrt{3}}{U}_{on\max }=\mathrm{1,155}{U}_{on\max }$$

times [Berestov V.M., Kharitonov S.A. Analysis of sinusoidal PWM with natural sampling (methodological aspect). Technical electrodynamics. Thematic issue. Power electronics and energy efficiency. Chastina 2. Kiiv, 2002. - p.31-37].

In this case, the fundamental harmonic of the output voltage of the direct frequency converter with natural switching increases (E _{n (1)} ). In accordance with GOST 19880-74, the power factor is defined as the ratio of active power to apparent power, neglecting active losses in the frequency converter and assuming that the load is symmetrical, the expression for the input power factor of the direct frequency converter takes the form

$$\chi cn=\frac{P}{S}=\frac{3E_{n\left(\mathrm{one}\right)}{I}_{n\left(\mathrm{one}\right)}\cos {\phi }_{n\left(\mathrm{one}\right)}}{{\displaystyle \sum _{i=\mathrm{one}}^6{E}_{\mathrm{one}i}}{I}_{\mathrm{one}i}}(6)$$

where: E _{n (1)} , I _{n (1)} is the effective value of the fundamental harmonics of the phase voltage and current at the output of the frequency converter, ϕ _{n (1)} is the phase shift between the output current and voltage of the frequency converter, E _1i , I _1i are values of phase voltages and currents of the synchronous generator.

As follows from relation (4), the input power factor of the direct frequency converter increases.

.The calculation results are shown in figure 4, which shows how the relative values of the effective values of the main harmonics of the voltages at the output of the frequency converter when using the prototype method (indicated by "Sin") and in the inventive method (indicated by "Midl") and their relationships as a function of modulation depth $$\left(M=\frac{U_{ym}}{U_{on\max }}\right)$$

.

, происходит уменьшение установленной мощности генератора на 7.8÷42.7% относительно установленной мощности синхронного генератора в случае применения способа прототипа.As follows from figb in the proposed system, the effective value of the fundamental harmonic of the voltage at the load increases by 8.5-44.5%. The same percentage increases the input power factor of the direct frequency converter (χ _cn ) in accordance with relation (1). And, because $$S_{\mathrm{from}g}=\frac{P_n}{{\eta }_{cn}\cdot {\chi }_{cn}}$$

, there is a decrease in the installed power of the generator by 7.8 ÷ 42.7% relative to the installed power of the synchronous generator in the case of applying the prototype method.

The presence of harmonics of the zero sequence order (2k-1) Ω in the signals U _ykjj , k = 1, 2, 3, ... will lead to their appearance in the output voltages of the direct frequency converter, however, if the load does not have a neutral wire, then the voltage at the load these harmonics will be absent.

Thus, the proposed method for controlling a direct frequency converter with natural switching allows to reduce the mass of the synchronous generator by increasing the input power factor of the direct frequency converter.

Claims (1)

A method of controlling a direct frequency converter with natural switching, which consists in the formation of thyristor control pulses at the moments of equality of the reference signals with control three-phase signals, characterized in that they form three sinusoidal signals shifted by 120 °, form a signal that at each moment of time is equal to the average value of the generated sinusoidal signals, the resulting signal is scaled and each control signal is formed as the sum obtained by scaled I signal and a corresponding sine wave.

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