Harmonics suppression control circuit for a PWM inverter

Abstract: In an apparatus for carrying out PWM control of an inverter having a filter connected to the a.c. output side thereof, pulse patterns adapted for determining switching timings for canceling a specified harmonic component, e.g., the fifth order harmonic component included in a voltage on the output side of the filter without exerting an influence on the output voltage fundamental wave, are stored in advance into a memory with respect to various vector quantities of the specified harmonic component to select, from a plurality of pulse patterns stored in the memory, each time a pulse pattern for reducing the specified harmonic component. The inverter is subjected to PWM control in accordance with the pulse pattern thus selected.

Description: BACKGROUND OF THE INVENTION Field of the Art The invention relates to a harmonic suppresion control circuit for a PWM inverter according to the preamble of claim 1. Prior Art PWM inverters which output constant voltage and constant frequency a.c. voltage are generally used as an uninterruptible power supply used when a failure in the system occurs, or the like. Inverters of this kind input d.c. power to convert it to an a.c. voltage having a predetermined voltage and a predetermined frequency to output such an a.c. voltage. This output a.c. voltage is ordinarily delivered to the load through the interconnecting transformer and the filter. The filter is composed of an a.c. filter reactor and an a.c. filter capacitor. On the input side of the inverter, a d.c. power source capable of having its output voltage adjusted is ordinarily provided. An adjustment of the output voltage of the inverter is made through the voltage adjustment of the d.c. power source. In that case, the inverter shares the role of the frequency adjustment and is controlled by a fixed gate signal corresponding to a predetermined phase reference so that the output voltage becomes equal to a sinusoidal waveform as far as possible. The filter on the output side of the inverter reduces harmonic components due to switching of power semiconductor elements constituting the inverter, and also serves to prevent overcurrent at the time of short-circuit failure on the side of the load. For the power semiconductor elements used in the inverter, there are many instances where transistors are used in the small or medium capacity inverter, and gate turn-off thyristors (GTO) are used in the large capacity inverter over several hundred KVA. In the case of GTO used in the large capacity inverter, it is impossible to operate it at a switching frequency as high as that of the transistor. In the case of an output frequency of 50 or 60 Hz, there are many instances where the number of switchings per cycle is less than 10. The number of switchings is limited in view of the switching speed and the loss of the element itself. The harmonics in the inverter output voltage are reduced by the filter provided on the side of the a.c. output of the inverter. Thus, an a.c. voltage similar to the sinusoidal wave is obtained from the output side of the filter. The a.c. voltage thus obtained is delivered to the load. When a d.c. load is connected through a rectifier on the output side of the filter, many harmonic components except for the fundamental sinusoidal wave are included in the a.c. output current of the inverter. Accordingly, harmonic components are also produced in the a.c. voltage depending on the magnitude of the load power. Such harmonic components can be reduced to some extent by the filter on the a.c. side, but voltage distortion may increase when the load power is large. Since the inverter cannot make such a corrective operation to reduce harmonics due to distorted power on the load side of the inverter in the case of carrying out operation when the number of pulses and the switching phase are fixed, when the load power becomes large, the voltage distortion will also become large. An overvoltage or undervoltage is momentarily produced on the side of the load, resulting in the possibility that equipment forming the load conducts an erroneous operation. In addition, in the case where another load or loads are connected differently from the rectifier load, an adverse influence or effect may be exerted on such load. SUMMARY OF THE INVENTION The document DE-A-32 41 828 discloses a method of supressing harmonics in the output voltage of an inverter, in which fifth and seventh order harmonics are suppressed. According to this document a pulse number of the PWM inverter is switched to nine pulses at a low frequency range, to five pulses at an intermediate frequency range and is switched on continuously at a high frequency range. Thus, it is intended to decrease harmonics of a PWM inverter output voltage at a low frequency range. A principal object of this invention is to provide an apparatus for controlling an inverter, which is capable of reducing distortion in the filter output voltage waveform which may be produced depending upon the kind of load. Another object of this invention is to provide an apparatus for controlling an inverter, which is capable of reducing a specified harmonic component or components of the filter output voltage. To achieve the above objects, the invention provides a harmonics suppression control circuit according to claim 1. BRIEF DESCRIPTION OF THE INVENTION In the accompanying drawings: Fig. 1 is a block diagram showing an embodiment of an apparatus for controlling an inverter according to this invention; Fig. 2 shows a fundamental output voltage waveform of the inverter shown in Fig. 1; Fig. 3 is a diagram explaining the pulse pattern in the embodiment of this invention; Fig. 4 is a diagram explaining switching patterns of respective elements of the inverter; and Fig. 5 is a diagram explaining a pulse pattern in another embodiment of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the main circuit of Fig. 1, the d.c. power delivered from a d.c. power source 1 is introduced to a three-phase PWM inverter 4 through a chopper 2 and a d.c. filter capacitor 3 and then converted to a.c. power by the inverter 4. The output a.c. power of the inverter 4 is delivered to a load 8 through an interconnecting three-phase transformer 5 and an a.c. filter 6. The a.c. filter 6 is composed of a filter reactor 6a and a filter capacitor 7a. In the case of carrying out, e.g., five switchings per cycle (= 2π) for the switching mode of the inverter 4, the inverter R, S and T output voltages and the R-S phase line output voltage are as shown in Fig. 2. In the case of the line output voltage, five pulses are produced per half cycle. Harmonic components are reduced from such pulse-like voltage by the filter 6 on the a.c. side. As a result, an a.c. voltage similar to a sinusoidal wave is output from the filter 6. In the section of the control apparatus, an output a.c. voltage V ac of the a.c. filter 6 is detected as an a.c. voltage detection signal V acd by a voltage transformer 9, a rectifier 10 and a filter 11 connected to the output side of the a.c. filter 6. A deviation between the a.c. voltage detection signal V acd thus obtained and an output voltage reference V ref is obtained by an adder 12. The chopper 2 is controlled through a voltage control circuit 13 so that the deviation becomes equal to zero. Thus, the output voltage of the chopper 2, i.e., the input voltage of the inverter 4 is controlled. In order to give a phase reference ϑ of the fundamental wave for firing of the inverter 4, a phase reference generator 14 is provided. For the purpose of computing harmonic components included in the output a.c. voltage V ac detected by the voltage transformer 9, e.g., a sine component V 5d and cosine component V 5q with the above-mentioned phase reference ϑ being a reference of the fifth order harmonic component V₅, a harmonic component computer 17 is provided. Correction values V 5yy and V 5xx for cancelling these sine and cosine components V 5d and V 5q by waveform control are computed by a correction value computer 18. A pulse pattern memory 16 stores therein, in the form of a Table, pulse patterns for cancelling sine and cosine components of various vector quantities of the harmonic component without exerting an influence on the fundamental wave component. The pulse pattern memory 16 delivers to a gate pattern generator 15, PWM patterns ϑ ₀ to ϑ ₄ most suitable for canceling the sine and cosine components V 5d and V 5q with the correction values V 5yy and V 5xx from the correction value computer 18 that forms an index. The gate pattern generator 15 carries out on-off control of the inverter 4 with the phase reference ϑ from the phase reference generator 14 being as a reference in accordance with the PWM pattern phases ϑ ₀ to ϑ ₄. A pulse pattern table constructed as shown in Fig. 3 is stored in the pulse pattern memory 16. Information of phases ϑ ₀ to ϑ ₄ is stored on individual coordinate points in Fig. 3. These phases ϑ ₀ to ϑ ₄ can be calculated in accordance with the following equation: where V₁ is the quantity proportional to the fundmental wave component. Values obtained by the equation (1) are theoretical values. In practice, such a Table is prepared by taking into consideration the minimum switching width of an element determined by the rating value of the switching element of the inverter, and the like. By having such information relating to phases as a Table, the inverter can output the fifth order harmonic component for correction suitable for canceling the fifth order harmonic component included in the filter output voltage within the range of the information existing in the Table. The fifth order harmonic component of the a.c. voltage V ac output from the a.c. filter 6 is computed by the harmonic component computer 17. When it is assumed that the phase of the fundamental wave component of the output a.c. voltage V ac is ϑ, the instantaneous value of the output voltage is V, and the fundamental wave component of the output voltage is V₁, the fifth order harmonic component of the output a.c. voltage V ac can be calculated from an output voltage corresponding to one phase in accordance with the equation (2). The information relating to the fundamental wave phase ϑ of the output a.c. voltage V ac is received from the phase reference generator 14. The computation expressed by the equation (2) shows the case where the fifth order harmonic component is represented by components having phases different from each other by 90 degrees. The correction value computer 18 computes correction values ΔV 5x and ΔV 5y allowing the output voltage to produce opposite components to the fifth order components V 5d and V 5q of the output voltage obtained from the harmonic component computer 17. These correction values ΔV 5x and ΔV 5y can be expressed using the equation (3) as follows: In the equation (3), f is an output frequency (Hz), L is an inductance of the a.c. filter 6, and C is a capacitance (F) of the a.c. filter 6. "The correction value computer 18 has the function such that it calculates ΔV 5x , ΔV 5y by equation (3) from V 5d , V 5q and reserves the former V 5xx , V 5yy , as V 5x , V 5y and then calculates and outputs V 5xx , V 5yy by the equation (4) on the basis of the calculated ΔV 5x , ΔV 5y and the reserved V 5x , V 5y ." From these correction values ΔV 5x and ΔV 5y and the preceding correction values V 5x , and V 5y , new correction values V 5xx and V 5yy for eliminating the fifth order harmonic component of the output voltage waveform are computed as follows: V 5xx = V 5x + ΔV 5x V 5yy = V 5y + ΔV 5y (4). Because information storage points are arranged at intervals, only discrete values can be taken as the fifth order harmonic component which can be output from the inverter 4. Accordingly, table coordinates closest to the values V 5x and V 5yy are assumed as new values V 5x and V 5yy , respectively. From the new correction values V 5xx and V 5y obtained here, switching phase angles ϑ₁ to ϑ₄ corresponding thereto are obtained from the phase Table. The switching phase angles thus obtained are delivered to the inverter 4. Thus, the inverter 4 operates in accordance with the switching patterns as shown in Fig. 4 on the basis of the phase angles. In the case of a load in which a current that does not have a sinusoidal waveform flows, such as, for example, a rectifier load, harmonic distortion is produced in the output voltage due to the harmonic current flowing in the load. Harmonics of a multiple of 3 among those harmonics and harmonics of other phases cancel each other out in the case of the three-phase output inverter, so that such harmonics of a multiple of 3 hardly appear on the line voltage. In addition, harmonic components of higher frequencies are filtered by the a.c. filter. Accordingly, for harmonics appearing on the output voltage, the fifth order component thereof is the largest. By reducing such a harmonic component, the overall voltage distortion factor of the output voltage can be considerably decreased. In the above-described embodiment, the inverter 4 is controlled so that the fifth harmonic component included at the highest frequency in the output voltage is reduced, thus making it possible to provide an apparatus having an overall distortion factor smaller than that with the conventional inverter even in the case of a load producing many harmonics, such as, for example, a rectifier load. Another embodiment will be now described. In the preceding embodiment, attention was paid to the fifth order harmonic of the output voltage as a specified harmonic. In an inverter having a greater number of pulses, it is also possible to reduce harmonics of multiple orders at the same time in accordance with this invention. For example, in the case of reducing the fifth and seventh order harmonics, it is sufficient to compute the magnitudes and the phases of respective harmonics to move the pulse pattern in a direction to reduce those magnitudes. The seventh order harmonic component can be calculated by the following computational equation proportionately with the case of the above equation (2) relating to the fifth harmonic component: The fifth order harmonic component is calculated from equation (2) and the seventh order harmonic component is calculated from equation (5). Thus, such a pulse pattern to apply a correction voltage for canceling these harmonic components to the output voltage is given. For example, when it is assumed that the pattern position for reducing the fifth order harmonic component is the point A of Fig. 5, the seventh order harmonic component has a two-dimensional arrangement with the point A as a center. As a whole, a four-dimensional pattern Table is provided. Because of high computation speed and large capacity of the memory in processors for computation recently available, complicated computation and/or storage of a large quantity of data have become easier. Accordingly, control in consideration of multiple harmonic orders as in the embodiment just mentioned above can be realized relatively easily. In accordance with this embodiment, an output voltage having a lesser number of harmonics of the fifth and seventh orders is obtained on the output side of the a.c. filter 6. Thus, this embodiment makes it possible to deliver an a.c. power of high quality having a distortion smaller than that in the first-mentioned embodiment. While it has been described that the inverter outputs an a.c. voltage of constant voltage and constant frequency, an inverter operative to output an a.c. voltage of variable voltage and variable frequency may also obtain advantages similar to those mentioned above. As described above, this invention is directed to a control apparatus for an inverter equipment having a lesser number of pulses, wherein the control apparatus is constructed to detect an a.c. output voltage to compute a specified frequency component thereof to select a pulse pattern so that the specified frequency component is reduced to control the inverter, thereby making it possible to deliver to the load an a.c. power of high quality having an output voltage distortion smaller than that of conventional inverter equipment.

Claims: A harmonics suppression control circuit for a PWM inverter having a filter (6) including an inductance (6a) and a capacitor (7a) connected to the a.c. output of the inverter (4), characterized in that: voltage detector means (9) detect the output voltage of the filter (6); harmonic component computation means (17) compute a specified harmonic component included in the voltage detected by the voltage detector means (9); memory means (16) store pulse timing patterns adapted to determine switching timings for superimposing the specified harmonic component on the output voltage of the inverter (4) without exerting an influence on an output voltage fundamental wave with respect to various vector quantities of the specified harmonic component; correction value computation means (18) compute a correcting harmonic component for superimposing on the output voltage of the inverter (4) to cancel the specified harmonic component and selects, from a plurality of patterns stored in the memory means (16), a pulse timing pattern by means of the correcting harmonic component; and gate pattern generator means (15) carries out PWM control of the inverter in accordance with the pulse pattern selected by the correction value computation means (18). A harmonics suppression control circuit as set forth in claim 1 characterized in that phase reference generator means (14) delivers a reference phase to the harmonic component computation means (17) and to the gate pattern generator means (15). A harmonics suppression control circuit as set forth in claims 1 or 2 characterized in that the harmonic component computation means (17) is operative to compute the fifth order harmonic component included in the voltage detected by the voltage detector means (9), and the correction value computation means (18) is operative to select a pulse pattern such that the quantity of the fifth order harmonic component becomes small. A harmonics suppression control circuit as set forth in claim 1 characterized in that: harmonic component computation means (17) computes specified first and second harmonic components included in the voltage detected by the voltage detector means (9), and computes a correcting harmonic component for superimposing or the output voltage of the inverter (4) to cancel the specified first and second harmonic components; and memory means (16) stores first pulse timing pattern adapted to determine switching timings for superimposing the correcting harmonic component on the output voltage of the inverter (4) without exerting an influence on an output voltage fundamental wave of the inverter (4) with respect to various vector quantities of the specified first harmonic component, and second pulse timing pattern adapted to determine switching timings for superimposing the correcting harmonic component on the output voltage of the inverter (4) without exerting an influence on an output voltage fundamental wave of the inverter (4) with respect to various vector quantities of the specified second harmonic component. A harmonics suppression control circuit as set forth in claims 4 characterized in that phase reference generator means (14) delivers a reference phase to the harmonic component computation means (17) and to the gate pattern generator means (15). A harmonics suppression control circuit as set forth in claim 4 or 5 characterized in that the harmonic component computation means (17) is operative to compute harmonic components of the fifth and seventh orders included in the voltage detected by the voltage detector means (9), and the correction value computation means (18) is operative to select a pulse pattern such that the quantities of the fifth and seventh order harmonic components become small.

Inventors: NAKAJIMA KIHEI, SATO SHINJI, NAKAJIMA, KIHEI, SATO, SHINJI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H02M7
IPC codes: H02M7