Apparatus for controlling a power converter

Abstract: In an apparatus for controlling a power converter, which is adapted to control, in accordance with the PWM system, a voltage-type inverter for system interconnection interposed between the d.c. power source and the a.c. system to interchange power therebetween, a scheme is employed to switch voltage amplitude adjustment signals of respective phases for determining the amplitudes of respective phase output voltages of the inverter depending upon an output voltage phase signal for determining the phase of the output a.c. voltage to thereby provide a phase voltage amplitude, to therefore determine energization periods of respective controllable rectifier elements of the power converter on the basis of the phase voltage amplitude and the output voltage phase signal. Employment of such a scheme makes it possible to independently control the output voltage of the inverter every phase or with respect to both positive and negative sides of the phases, respectively.

Description: BACKGROUND OF THE INVENTION This invention relates to an apparatus for controlling a power converter, which is adapted to control, in accordance with the PWM system, a power converter for system interconnection interposed between the d.c. power source and the a.c. system to adjust interchange power therebetween, according to the preamble of claim 1. A controlling apparatus of this kind is known from the DE-A-31 51 318. Power converters for system interconnection of this kind may have an inverter operating mode used in supplying power from the d.c. power source to the a.c. system and a rectifier operating mode used in supplying power from the a.c. system to the d.c. power in a manner opposite to the above. When it is assumed that a voltage on the a.c. side of a power converter is V in , a voltage of the a.c. system is V sy , a reactance between the d.c. power source and the a.c. system is X, and a phase difference angle between the voltage V in and the voltage V sy is Φ (which is positive when the phase of the voltage V in leads the phase of the voltage V sy ), effective or active power P and reactive power Q processed in the power converter are respectively expressed as follows: P = {(V in ·V sy )/X}sinΦ Q = (V in ·V sy ·cosΦ - V sy ²)/X Where the power converter is operated in the rectifier operating mode, the amplitude of voltage V in is smaller than that of voltage V sy and the phase of voltage V in lags that of voltage V sy . Thus, the power converter dissipates lagging reactive power Q and transmits effective power P from the a.c. system to the d.c. power source. Moreover, as apparent from the above equations (1) and (2), even if the amplitude of voltage V in is smaller than that of voltage V sy , as long as the phase of voltage V in leads that of voltage V sy , the power converter dissipates lagging reactive power Q and transmits effective power P from the d.c. power source to the a.c. system. When the effective value of the voltage V in satisfies the condition expressed by the following equation with respect to the effective value of the voltage V sy , V in ≧ V sy /cosΦ the power converter operates as a capacitor and transmits effective power from the a.c. system to the d.c. power source. When the phase difference angle Φ is positive as apparent from the above equations (1) and (2) even in the case where the above equation (3) is satisfied, the power converter operates as a capacitor and transmits effective power from the d.c. power source to the a.c. system. When it is assumed that the voltage on the d.c. side of the power converter, i.e., the voltage of the d.c. power source is V dc , modulation factor in the PWM control of the power converter is M (0≦M≦1), and k is a constant, the voltage V in on the a.c. side of the power converter is expressed as follows: V in = k · V dc · M Substitution of the equation (4) into the equations (1) and (2) respectively gives: P = V sy (k·M·V dc /X)sinΦ and Q = {V sy (k·M·V dc ·cosΦ) - V sy ²}/X As apparent from the above equations (5) and (6), by controlling the modulation factor M and/or the phase difference angle Φ by the power converter, the effective power P and the reactive power Q can be adjusted. Meanwhile, the three phases are not necessarily balanced with each other in an ordinary a.c. system voltage. There are many instances where three phases are unbalanced, for example, in a form such that amplitudes are different from each other in respective R, S and T phases. Even in the case where the a.c. system voltage is unbalanced as stated above, the above-described power converter can interchange the effective and reactive powers between the a.c. system and the d.c. power source. However, because the three phases of the a.c. sytem voltage are unbalanced, the effective and reactive powers with three phases being as a lump under condition where amplitudes of the output a.c. voltage are different from each other in the three phases take predetermined values, respectively. In this case, when the degree of imbalance becomes large, the output current of the converter of a specified phase will increase and therefore a current over an allowed current will flow in controllable rectifier elements of the specified phase. As a result, it becomes impossible to continue the operation of the converter. In addition to the above-described inconvenience, there is another inconvenience described below. Since there essentially exists unevenness in the characteristics of respective controllable rectifier elements even if the energization periods of respective controllable rectifier elements of the converter are determined, energization is not necessarily conducted as required by a designated energization period, so that there may be produced unevennesses between energization periods of respective controllable rectifier elements. Such unevennesses between energization periods of respective controllable rectifier elements cause the current on the a.c. side of the converter to produce d.c. components. The d.c. components mentioned above cause d.c. polarized magnetization in the interconnecting transformer ordinarily provided between the converter and the a.c. system, thus allowing the interconnecting transformer to over-heat, or otherwise extremely distorting an output current of the converter. When the occasion demands, there may occur a circumstance where a large peak current based on strain exceeds an allowed current value of the controllable rectifier element. Such circumstances make the operation of the converter impossible. SUMMARY OF THE INVENTION Therefore, an object of this invention is to provide an apparatus for controlling a power converter interposed between the d.c. power source and the a.c. system to interchange power therebetween, which is capable of eliminating the above-described overcurrent or d.c. polarized magnetization based on imbalance of the a.c. system voltage or unevenness between element characteristics. In accordance with this invention, there is provided an apparatus for controlling a power converter comprised of the features of claim 1. Preferred embodiments of the invention are defined in the dependent subclaims. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a block diagram showing an embodiment of an apparatus for controlling a power converter according to this invention; Fig. 2 is a connection diagram showing the detailed construction of the converter shown in Fig. 1; and Fig. 3 is a time chart for explaining the function of the control apparatus shown in Fig. 1. DESCRIPTION OF THE PREFERRED EMBODIMENTS In Fig. 1, a voltage controlled type inverter 11 generally used for system interconnection as a power converter controlled by a control apparatus according to this invention is shown. By the inverter 11 and d.c. capacitor 12 connected on the d.c. input side thereof, an inverter main circuit 10 is constituted. A d.c. power source 20 is interconnected with an a.c. system 50 through the inverter main circuit 10, an interconnecting reactor 30 and an interconnecting transformer 40. Fig. 2 shows the circuit construction of the voltage controlled inverter 11 constituting the inverter main circuit 10. This circuit arrangement is provided with a pair of positive and negative d.c. terminals PT and NT, and three-phase a.c. terminals R, S and T. The inverter 11 is composed of six controllable rectifier elements GU, GV, GW, GX, GY and GZ which are three-phase bridge connected, and six free-wheeling diodes DU, DV, DW, DX, DY and DZ connected in inverse-parallel with these controllable rectifier elements, respectively. Such controllable rectifier elements GU to GZ are each comprised of an element of the self-quenching type, such as, for example, a gate turn off thyristor (GTO). The interchange of power in the system construction of Fig. 1 is controlled by the inverter 11. For controlling such interchange power, the voltage of the a.c. system 50, i.e., system voltage V sy is detected by a potential transformer (which will be referred to as "PT" hereinafter) 61, the voltage of the d.c. power source 20, i.e., d.c. voltage V dc is detected by a d.c. voltage detector (which will be referred to as "VD" hereinafter), and output a.c. currents I r , I s and I t of respective phases of the inverter 11 are detected by a current transformer (which will be referred to as a "CT" hereinafter) 63. In order to form a control signal for adjusting an effective power P and a reactive power Q flowing into the a.c. system 50, a power control unit 8 is provided. The power control unit 8 calculates a modulation factor signal M f for voltage adjustment corresponding to the modulation factor M, and a phase difference angle signal Φ c corresponding to a phase difference angle Φ in accordance with the above equations (5) and (6) for the purpose of allowing the inverter 11 to interchange predetermined effective and reactive powers P and Q on the basis of the system voltage V sy (phase voltages V r , V s , V t ) and the d.c. voltage V dc respectively detected by PT 61 and VD 62. Further, the power control unit 8 calculates correction modulation factor signals M r , M s and M t of respective phases for adjusting output voltage amplitudes of respective phases on the basis of respective phase alternative currents I r , I s and I t detected by the CT 63 or respective phase voltages V r , V s and V t of the system. Thus, the power control unit 8 sends the respective values obtained by calculation to the firing control unit 100. For example, for the purpose of suppressing d.c. polarized magnetization of the interconnecting transformer 40, d.c. components included in the alternative currents I r , I s and I t of respective phases are detected, whereby correction modulation factor signals M r , M s and M t are determined so as to suppress such d.c. components. In this case, alternative currents I r , I s and I t are respectively integrated per cycle of the a.c. voltage to assume these three integrated values as correction modulation factor signals M r , M s and M t , respectively. In the case of using a single three-phase transformer in place of three single-phase transformers as the interconnecting transformer, because the relationship of I r + I s + I t = 0 holds, one of three correction modulation factor signals may be set to a fixed value. Furthermore, in order to balance output currents of the inverter under the condition where the three-phase voltages V r , V s and V t of the system are unbalanced, it is sufficient to output differences between the system voltage V sy and respective phase voltages V r , V s and V t as correction modulation factor signals M r , M s and M t , respectively. The firing control unit 100 is provided with a synchronous detector circuit 110, a voltage phase signal generator circuit 120, a firing signal generator circuit 130, and a circuit 140 for producing a voltage amplitude signal per each phase (hereinafter simply referred to as a phase voltage amplitude signal generator circuit 140). The firing signal generator circuit 130 is comprised of a triangular wave generator circuit 131, a firing timing determination circuit 132, and a firing phase determination circuit 133. The triangular wave generator circuit 131 is a circuit to generate triangular wave signals TRI with 60 degrees being its period as shown in Fig. 3. The energization periods of respective controllable rectifier elements GU, GV, GW, GX, GY, and GZ are determined with the triangular wave signal TRI being as a reference. The synchronous detector circuit 110 obtains a system voltage V sy from PT 61 to detect the phase of the system voltage, thus to output a system phase signal SCLK. The voltage phase signal generator circuit 120 receives a phase difference angle signal Φ c from the power control unit 8 to send, to the firing signal generator circuit 130, an output voltage phase signal ϑ comprised of a digital signal such that a phase difference angle between the output voltage of the inverter 11 and the voltage of the a.c. system 50 becomes equal to Φ every time it receives the system phase signal SCLK from the synchronous generator circuit 110. The output voltage phase signal ϑ consists of a phase-judgement phase signal ϑ h indicating a period of 360 degrees every 60 degrees with the R-phase output voltage being used as a reference, and a 60° period phase signal ϑ j repeating from zero count to a predetermined count value every period of 60 degrees. This 60° period phase signal ϑ j may be prepared by a binary counter circuit (not shown). In this case, when, e.g., an 11 bit binary counter circuit is used, the predetermined count value mentioned above becomes equal to 2¹¹ - 1 = 2047 counts. In this example, the predetermined count value is set to 2047. The phase judgement phase signal ϑ h may be prepared by a hexanary counter circuit repeating from 0 up to 5, and is assumed to have a relationship of Table 1 shown below between the phase-judgement phase signal ϑ h and the phase α of the inverter R-phase output voltage. The output voltage phase signal ϑ consists of a phase-judgement phase signal ϑ h and 60° period phase signal ϑ j , and is in such a form to express the phase α of the R-phase output voltage of the inverter as a digital signal. The phase voltage amplitude signal generator circuit 140 inputs a phase-judgement phase signal ϑ h from the voltage phase signal generator circuit 120 to output a voltage amplitude signal per phase denoted by M p (hereinafter simply referred to as a phase voltage amplitude signal M p ) to the firing signal generator circuit 130 at the time when the phase-judgement phase signal ϑ h changes. The relationship between the phase voltage amplitude signal M p output from the phase voltage signal generator circuit 140 at the time when the phase-judgement phase signal ϑ h varies and the phase-judgement phase signal ϑ h is shown in Table 2. The phase voltage amplitude signal M p maintains the value when the phase-judgement phase signal ϑ h changes last as long as the phase-judgement phase signal ϑ h does not change. The phase voltage amplitude signal M p is varied as shown in Table 3 every 60 degrees of the phase of the R-phase output voltage of the inverter. The triangular wave generator circuit 131 within the firing signal generator circuit 130 inputs a 60° period phase signal ϑ j from the voltage phase signal generator circuit 120 to output a triangular wave signal TRI to the firing timing determination circuit 132. For the triangular wave signal TRI, triangular waves each corresponding to an isosceles triangle with the phase angle of 30 degrees of the 60° period as a center are repeatedly output every 60° periods of the R-phase output voltage of the inverter 11, i.e., at periods of 60°, as shown in Fig. 3. Namely, from the time of 0 counts of the 60° period phase signal ϑ j up to the time of 1023 counts thereof, the triangular wave signal TRI has a positive gradient corresponding to those count values, and from the time of 1023 counts of the 60° period phase signal ϑ j up to the time of 2047 counts thereof, the triangular wave signal TRI has a negative gradient corresponding to from 1023 down to 0 counts. The firing timing determination circuit 132 determines the timing for controlling the energization periods of respective controllable rectifier elements of the inverter 11 on the basis of the phase voltage amplitude signal M p from the phase voltage signal generator circuit 140 and the triangular wave signal TRI from the triangular wave generator circuit 131 in accordance with the time sequence shown in Fig. 3. A method of generating timing signals for determining energization periods of the controllable rectifier elements GY and GV and the controllable rectifier elements GU and GX of the inverter 11 carried out when the R-phase output voltage phase α of the inverter 11 is at time periods of 0°≦α<60° and 60°≦α<120°, i.e., the phase-judgement phase signal ϑ h represents "0" and "1" will be described with reference to Fig. 3. When the inverter R-phase output voltage phase α of Fig. 3 is at a time period of 0°≦α<60°, i.e., the phase-judgement phase signal ϑ h represents "0", as is apparent from Table 3, signal M y is output as the phase voltage amplitude signal M p . The signal M y and the triangular wave signal TRI are compared with each other to output a signal TIM indicating the times a₁ and a₂ at which both signals are equal to each other. The firing-phase determination circuit 133 determines the energization periods on the basis of the signal TIM obtained from the firing timing determination circuit 132 at the times of a₁ and a₂ and the output voltage phase signal ϑ from the voltage phase signal generator circuit 120 so that the controllable rectifier elements GU and GW are energized and the controllable rectifier elements GX and GZ are not energized because controllable rectifier elements except for the controllable rectifier elements GV and GY of controllable rectifier elements of the inverter 11 are such that phase-judgement signal ϑ h of the digital phase signal ϑ represents "0". When the inverter R-phase output voltage phase α is at a time period of 60≦α<120°, as is apparent from Table 3, signal M u is output as the phase voltage amplitude signal M p . The signal M u and the triangular wave signal TRI are compared with each other to output a signal TIM indicating the times b₁ and b₂ at which both signals are equal to each other to the firing-phase determination circuit 133. The firing-phase determination circuit 133 determines, as shown in Table 3, energization periods except for the controllable rectifier elements GU and GX of controllable rectifier elements of the inverter 11 on the basis of the signal TIM obtained from the firing timing determination circuit 132 at the times of b₁ and b₂ and the output voltage phase signal ϑ from the voltage phase signal generator circuit 120. The firing-phase determination circuit 133 determines respective energization periods so that the controllable rectifier elements GV and GW are not energized and the controllable rectifier elements GY and GZ are energized because controllable rectifier elements except for the controllable rectifier elements GV and GY of controllable rectifier elements of the inverter 11 are such that the phase-judgement phase signal ϑ h of the output voltage phase signal ϑ represents "1". Also when the inverter R-phase output voltage phase α is at respective 60° periods except for 0°≦α<60° or 60°≦α<120°, the phase voltage amplitude signal M p of Table 3 takes respective values of M z , M v , M x and M w in dependence upon the phase-judgement phase signal ϑ h . Thus, energization periods of respective controllable rectifier elements of the inverter 11 are determined. The difference between the control system of this invention and the conventional PWM control system resides in that the modulation factor M corresponding to the phase voltage amplitude signal M p of Fig. 3 is altered independent of the phase of the output voltage of the inverter 11 in the latter system. In this invention, by changing the phase voltage amplitude signal M p so that it becomes equal to signals M y , M u , M z , M v , M x and M w every phase angle of 60° of the inverter R-phase output voltage, it is possible to control the amplitudes of the phase output voltages of the inverter 11 in regard to respective phases every positive or negative side thereof. The controlled phase-voltages of the output voltages of the inverter 11, are also described together in Table 3. As described above, the control apparatus of Fig. 1 can control the amplitudes of the respective phase output voltages of the inverter 11 with regard to respective phases every positive or negative side of the amplitudes. Thus, even in the case where there is unevenness in the characteristics of the respective controllable rectifier elements of the inverter 11, this control apparatus can eliminate d.c. components produced in the output voltage energy respective phase. Accordingly, this makes it possible to allow currents flowing in respective controllable rectifier elements to be balanced to prevent d.c. polarized magnetization of the inter-connecting transformer 40, resulting in stable operation of the inverter 11. In the above-described embodiment, changes in the phase voltage amplitude signal M p based on changes in the phase-judgement phase signal ϑ h are determined in accordance with Table 2. However, such a determination may be conducted as shown in Table 4. In this case, the inverter output voltage will be controlled per each phase. The relationship between the phase-voltage controlled and the phase voltage amplitude signal M p of the output voltage of the inverter 11 is as shown in the Table 5. By effecting control in this way, even in the case where the voltage of the a.c. system 50 is unbalanced per phase, it is possible to individually control the output voltage of the inverter 11 per phase. Accordingly, control such that an output current of a specified phase of the inverter 11 is not caused to flow above an allowed current value of the element is conducted, thereby making it possible to stably operate the inverter 11. In the apparatus of Fig. 1, a scheme is employed to output the modulation factor signal M f , and correction modulation factor signals M r , M s and M t per phase from the power control unit 8 to the phase voltage amplitude signal generator circuit 140. In addition, a scheme may be employed to output the phase voltage amplitude signals M y , M u , M z , M v , M x and M w of Table 3 or the phase voltage amplitude signals M r1 , M s1 and M t1 of Table 5 which are varied by the phase-judgement phase signal ϑ h from the power control apparatus 8 to the phase voltage amplitude signal generator circuit 140. In this case, the phase voltage amplitude signal generator circuit 140 outputs, by the phase-judgement phase signal ϑ h , these phase voltage amplitude signals M y , M u , M z , M v , M x and M w or the phase voltage amplitude signals M r1 , M s1 and M t1 , to the firing signal generator circuit 130. As described above, the apparatus for controlling a power converter according to this invention is constructed to obtain a voltage amplitude adjustment signal for determining the voltage amplitudes per each phase of an a.c. voltage generated in the power converter to switch voltage amplitude adjustment signals every respective phase in accordance with an output voltage phase signal from the output voltage phase signal generator circuit to produce a phase voltage amplitude signal to determine energization periods of respective controllable rectifier elements constituting the power converter from the phase voltage amplitude signal and the output voltage phase signal, to thereby control the output voltage of the power converter per phase. Accordingly, this apparatus can correct imbalance between respective phases of the output alternative currents of the power converter, or suppress d.c. components included in output currents of respective phases.

Claims: An apparatus for controlling a power converter (11), which is adapted to control, in accordance with a PWM control system, the power converter (11) for system interconnection interposed between a d.c. power source (20) and an a.c. power system (50) to interchange power therebetween, comprising a synchronous detector (110), a voltage phase-signal generator (120), a phase voltage amplitude signal generator (140), and a firing signal generator (130) determining energization periods of respective controllable rectifier elements constituing the power converter, characterized in that a reactor (30) is interposed between the power converter (11) and the a.c. power system (50); the synchronous detector (110) detects a phase of an a.c. system voltage to output it as a synchronous signal (SCLK); the voltage phase-signal generator (120) determines a phase of an output a.c. voltage of the power converter (11) from a phase difference angle signal (⊘ c ) for determination of a phase difference between the a.c. system voltage and the output a.c. voltage of the power converter (11) and the synchronous signal (SCLK) provided by the synchronous detector (110) to output it as an output voltage phase signal (Θ); the phase voltage amplitude signal generator (140) respo nsive to voltage amplitude adjustment signals (M f , M r , M s , M t ) determines an amplitude per phase of the output a.c. voltage to switch voltage amplitude adustment signals (M f , M r , M s , M t ) of at least two phases in accordance with the output voltage phase signal (Θ) from the voltage phase-signal generator (120) to output a signal obtained by switching as a phase voltage amplitude signal (M p ); and the firing signal generator (130) generates a triangular wave signal (TRI) on the basis of the output voltage phase signal (Θ), and determines the energization periods of respective controllable rectifier elements of said power converter (11) by comparing said triangular wave signal (TRI) and said phase voltage amplitude signal (M p ). A control apparatus as set forth in claim 1, characterized in that the firing signal generator (130) comprises: a triangular wave generator (131) for generating the triangular wave signal (TRI) having a period of 60 degrees on the basis of the output voltage phase signal (Θ) from said voltage phase signal generator (120); a firing timing determination circuit (132) for forming a timing signal (TIM) for determining the energization periods of respective controllable rectifier elements of said power converter (11) on the basis of said triangular wave signal (TRI) from said triangular wave generator (131) means and said phase voltage amplitude signal (M p ) from said phase voltage amplitude singal generator means (140); and a firing phase determination circuit (133) for determing a controllable rectifier element or elements to be fired on the basis of said output voltage phase signal (Θ) from said voltage phase signal generator (120) and said timing signal (TIM) from said firing timing determination circuit (132). A control apparatus as set forth in claim 1 or 2, characterized in that the output voltage phase signal from said voltage phase signal generator means (120) consists of a phase-judgement phase signal (ϑ h ) indicating a period of 360 degrees of said output a.c. voltage from said power converter (11) every period of 60 degrees with a specified phase thereof being as a reference, and a 60 degrees period phase signal (ϑ j ) repeating the operation for counting up from zero to a predetermined value to further count down from said predetermined value to zero every period of 60 degrees, said phase-judgement phase signal (ϑ h ) being delivered to said phase voltage amplitude generator means (140) , said 60 degrees period phase signal (ϑ j ) being delivered to said firing signal generator means (130). A control apparatus as set forth in one of the preceding claims, characterized in that a first voltage detector (61) detects a voltage of said a.c. system (50); a second voltage detector (62) detects a voltage of said d.c. power source (20); a current detector (63) detects an output current of said power converter (11); and a power control unit (8) forms said phase difference angle signal ( c ) and said voltage amplitude adjustment signal (M f , M r , M s , M t ) on the basis of a voltage of said a.c. system (50), a voltage of said d.c. power source (20) and said output current of said power converter (11) which are detected by said respective detectors. A control apparatus as set forth in one of the preceding claims, characterized in that the phase voltage amplitude signal generator (140) outputs a signal for independently controlling amplitudes of respective phase output voltages of the power converter (11) with respect to both positive and negative sides to said respective controllable rectifier elements of said power converter, as a phase voltage amplitude signal (M p ). A control apparatus as set forth in one of the preceding claims, wherein said phase voltage amplitude signal generator (140) outputs a signal for controlling the amplitude of respective phase output voltages of said power converter every respective phase, to said respective controllable rectifier elements of said power converter as a phase voltage amplitude signal.

Inventors: HIROSE SYUN-ICHI, KAWAKAMI TAKEICHI, KUDOR TOSHIAKI, HIROSE, SYUN-ICHI, KAWAKAMI, TAKEICHI, KUDOR, TOSHIAKI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1988
Main classification: H02M7
IPC codes: H02J3, H02J5, H02M7