Reactive power compensating apparatus with higher harmonic supressing function

Abstract: Higher harmonic power components (P h , Q h ) and fundamental wave reactive power components (Q o ) are derived from a load voltage (V S ) and current (i L ), using theory (instantaneous p-q theory) regarding instantaneous real power and instantaneous imaginary power. Gains are then obtained which make the higher harmonic power components (P h , Q h ) and fundamental wave reactive power components (Q o ) not exceed predetermined limit values. The higher harmonic power components (P h , Q h ) and fundamental wave reactive power components (Q o ) are respectively multiplied by the gains to obtain limited higher harmonic power components (P h ', Q h ') and fundamental wave reactive power components (Q o '). An output current command is generated in accordance with the limited higher harmonic power components (P h ', Q h ') and fundamental wave reactive power components (Q o '). The output current command is supplied to an inverter which in turn supplies a compensation current (i c ) to the power system.

Description: The present invention relates to a reactive power compensating apparatus having a higher harmonic suppressing function. In PESC'88 RECORD 19th Annual IEEE POWER ELECTRONICS SPECIALISTS CONFERENCE, April 11-14, 1988, KYOTO, JAPAN, pages 1174-1179, edited by M. Takeda et al.: "Harmonic current and reactive power compensation with an active filter" is recited. This paper deals with the control of a static converter to inject in a circuit a current which compensates the reactive power and the harmonics. The current command value of the converter is calculated by a control circuit comprising: a current calculating means which calculates the instantaneous active and reactive currents in the p-q-coordinates; a means which calculates fundamentals and harmonics of the active and reactive currents; and a parameter selection device which selects parameters obtained from the calculating means to a three-phase converter. In the parameter selection cicuit, signals within the range of the circuits and the different signals cannot exceed the values corresponding to a high limit of that range. The signals are therefore limited to those high limit values. Moreover, the tree-phase converter calculates current command values to control the static converter. In IEEE Transactions on Industry Applications, vol. IA-22, No. 3 May/June 1986, New York US; pages 460-464, edited by H. Akagi et al.: "Control Strategy of Active Power Filters Using Multiple Voltage-Source PWM Converters" is disclosed. In the subject matter apparatus of this document, the instantaneous real power and the instantaneous reactive power are calculated from the respective currents and voltages. BACKGROUND OF THE INVENTION There has been already proposed a compensating apparatus for compensating reactive current components and higher harmonic components of a load current of a power system by connecting an inverter to the power system to supply a compensation current thereto. Fig. 1 shows a current command calculating apparatus in such a compensating apparatus, used with an inverter for supplying a compensation current to a power system. In Fig. 1, in accordance with a load current i L of a power system detected by a current detector (not shown) and a system voltage V S , i.e., in accordance with a so-called instantaneous p-q theory, a PQ calculation circuit 3 as a power vector calculation circuit calculates an instantaneous real power P and an instantaneous imaginary power Q. The instantaneous p-q theory is described in detail in "Generalized Theory of Instantaneous Reacitve Power and its Application" by Hirofumi AGKAGI, et al. Papers B of the Institute of Electrical Engineers of Japan, Vol. 103, No. 7, pp. 483 to 490 (1983). A power calculation circuit 4 derives components to be compensated, from an output of the PQ calcuation circuit 3. The power calculation circuit 4 derives higher harmonic active power components P h , fundamental wave reactive power components Q o , and higher harmonic reactive power components Q h from the instantaneous real power P and instantaneous imaginary power Q. In accordance with the derived components P h , Q o , and Q h , and the system voltage V S , an inverse PQ calculation circuit 6 as an inverse calculation circuit calculates a current command i C * which is expressed by i C * = P h 2 + (Q o + Q h )² / V s An absolute value limiter 7 clamps the current command i C * to such a magnitude that makes an output of a compensation current supplying inverter not exceed its rated value. The clamped current command i C *, is supplied to the inverter. Since the absolute value limiter 7 shown in Fig. 7 clamps the i C * to such a current command i C *′ that makes an inverter output current not exceed its rated value, there occurs wave saturation, i.e., distortion caused by wave clamping, posing the problem that the current command i C *′ includes higher harmonics. Fig. 2 is a block diagram showing another conventional current command calculating apparatus. Instead of the absolute value limiter 7 shown in Fig. 1, a peak detector 16, inverse function calculation circuit 17c, and multiplier 19 are used to configure a so-called auto-gain-control circuit. The current command i C * outputted from the inverse PQ calculation circuit 6 is supplied to the multiplier 19 as one input thereof, and is also supplied to the peak detector 16. The peak detector 16 detects and holds the peak value of the current command i C *. The inverse function calculation circuit 17c calculates a gain k if the peak value held by the peak detector 16 exceeds a rated value of the inverts. The gain k is determined through an inverse function calculation so that the current command does not exceed the inverter rated value. The calculated gain k is supplied to the multiplier 19 as the other input thereof. The multiplier 19 multiplies the current command i C * by the gain k to output the current command i C *′ to the inverter. In this auto-gain-control circuit shown in Fig. 2, higher harmonic components and fundamental wave reactive power components are uniformly limited. Therefore, even if either the higher harmonic components or the fundamental wave reactive power components exceed a limit value, both the components are limited, thereby posing the problem that the performance of the inverter cannot be effectively used. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems. It is an object of the present invention to provide a power compensating apparatus capable of suppressing an output current of a compensating inverter to within a rated value without generating higher harmonics otherwise caused by clamping a current command, and capable of effectively using the performance of the inverter, and limiting the current command value of the current without limiting directly this value. This object is solved according to independent claims 1 and 3, respectively. Preferred embodiments are listed in the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 shows a conventional current command calculating apparatus having an absolute value limiter; Fig. 2 shows another conventional current value command calculating apparatus having an auto-gain-control circuit; Fig. 3 shows a power compensating apparatus having a current command calculating apparatus according to an embodiment of the present invention; Fig. 4 is a block diagram showing the current command calculating apparatus of the power compensating apparatus shown in Fig. 3; and Fig. 5 is a block diagram showing a limiter different from that in the current command calculating apparatus of the power compensating apparatus shown in Fig. 3. EMBODIMENTS OF THE INVENTION Fig. 3 shows the structure of an embodiment according to the present invention. In Fig. 3, like elements to those shown in Fig. 1 are represented by using identical reference numerals. Referring to Fig. 3, a load current is supplied from a power source 9 to a load 11 via a power system bus 10. On the load side of the power system bus 10, a system voltage V S is detected by a voltage detector 12, and a load current i L is detected by a current detector 13. In accordance with the load current i L detected by the current detector 13 and the system voltage VV S detected by the voltage detector 12, a current command calculating apparatus 1 calculates a current command i C *′ for a compensation current i C . The compensation current i C generated, in accordance with the current command, by an inverter 2 having a capacitor 2C connected between d.c. terminals, is supplied via a transformer 14 to the power system bus 10 in order to compensate for fundamental reactive current components, higher harmonic active current components, and higher harmonic reactive current components, respectively contained in the load current i L . The current command calculating apparatus 1 has a PQ calculation circuit 3, power calculation circuit 4, inverse PQ calculation circuit 6, and absolute value limiter 7, similar to a conventional apparatus. A different point from a conventional apparatus is that a limiter 5 is connected between the power calculation circuit 4 and an inverse PQ calculation circuit 6. This limiter 5 limits the magnitudes of higher harmonic components P h and Q h and fundamental wave power components Q o derived by the power calculation circuit 4 to predetermined limit values. The detailed structure of the limiter 5 is shown in Fig. 4. Referring to Fig. 4, a vector absolute value calculation circuit 15a receives the higher harmonic components P h and Q h and calculates a higher harmonic power W h which is given by W h = P h 2 + Q h 2 A peak detector 16 is connected to the output of the vector absolute value calculation circuit 15a, the peak detector 16 detecting and holding the peak value of the higher harmonic power W h . An inverse function calculation circuit 17a is connected to the output of the peak detector 16. The inverse function calculation circuit 17a calculates a gain k₁ for limiting the higher harmonic power W h to a limit value h max if the power W h is judged as exceeding the limit value h max . There is also provided an absolute value limiter 18 which receives the fundamental wave reactive power components Q o , and sets a gain k₂ for limiting the value of the fundamental wave reactive power components Q o to a limit value Q omax . There are also provided three multipliers 19a, 19b, and 19c which receive the higher harmonic components P h and Q h , and fundamental wave reactive power components Q o as their one inputs, and the gains k₁, k₁, and k₂ obtained from the inverse function calculation circuit 17a and absolute value limiter 18 as their other inputs. Higher harmonic components P h ′ and Q h ′ outputted from the multipliers are supplied to the inverse calculation circuit 6. The operation of the embodiment apparatus constructed as above will be described below. The power calculation circuit 4 calculates the higher harmonic components P h and Q h and fundamental wave reactive power components Q o . The vector absolute value calculation circuit 15a calculates the magnitude of the higher harmonic power W h . The peak detector 16 detects and holds the peak value of the higher harmonic power. The inverse function calculation circuit 17a calculates the gain k₁ for limiting the higher harmonic power W h to the limit value h max if the higher harmonic power W h is judged, from the peak value of the higher harmonic power W h held by the peak detector 16, as the higher harmonic power h exceeds the limit value h max . The absolute value limiter 18 receives the fundamental wave reactive power components Q o and sets the gain k₂ for limiting the magnitude of the fundamental wave reactive power components Q o to the limit value Q omax if it exceeds the limit value Q omax . The multipliers 19a, 19b, and 19c multiply the higher harmonic components P h by the gain k₁, the higher harmonic components Q h by the gain k₁, and the fundamental wave reactive power components Q o by the gain k₂, to obtain the higher harmonic components P h ′ and Q h ′ and fundamental wave reactive power components Q o ′ which are then supplied to the inverse PQ calculation circuit 6. The limit value h max for the higher harmonic power and the limit value h omax for the fundamental wave reactive power components are properly determined from the rated value of a compensation inverter, the load condition, and the like. The limit values may not be necessarily set to fixed values, but they may be dynamically changed in accordance with the load condition and running condition. In the above manner, the higher harmonic components P h ′ and Q h ′ as the higher harmonic power command and the fundamental wave reactive power components Q o ′ as the fundamental wave reactive power command can be maintained equal to or lower than the limit values. By using the higher harmonic components P h ′ and Q h ′ and fundamental wave reactive power components Q o ′ obtained by the limiter 5, the inverse PQ calculation circuit calculates the output current command i C * which is given by i C * = (P h ′) 2 + (Q o ′ + Q h ′)² / V s The calculated output current command i C * considers the limit values for the higher harmonic components and fundamental wave reactive power components, independently from each other. Therefore, it can be said theoretically that if the output current i C of the inverter 2 is controlled by the output current command i C * calculated by the equation (3), and if the sum of the upper limits of the higher harmonic power command value and the fundamental wave reactive power command value are set to within the apparatus rated values, then the output current i C will not exceed the apparatus rated value. In the embodiment shown in Fig. 3, the absolute value limiter 7 is provided between the inverse PQ calculation circuit 6 and inverter 2. This absolute value limiter 7 is not necessarily required, but it is provided as a backup means of the limiter 5 for any trouble of the limiter 5. The limiter 7 operates in a similar manner to the limiter 7 shown in Fig. 1 so that it outputs an output current command i C *′ to limit the output current i C of the inverter 2. Fig. 5 shows another structure of the limiter 5. This limiter 5 does not process the higher harmonic power components and fundamental wave reactive power components separately, but a vector absolute value calculation circuit 15b calculates the higher harmonic power components W C to be compensated, in accordance with the higher harmonic components P h and Q h and the fundamental wave reactive power components Q o . The higher harmonic power components W C is given by W C = P h 2 + (Q o + Q h )² The peak value of the higher harmonic power components is detected and held by the peak detector 16. An inverse function calculation circuit 17b calculates a common gain k₃ for limiting the inverter output power to a value equal to or smaller than a limit value. The gain k₃ is supplied to the multipliers 19a, 19b, and 19c as common one inputs thereof. The limiter 5 constructed as shown in Fig. 5 can also maintain the output current command i C *′ to a value equal to or smaller than a predetermined value. As appreciated from the foregoing description of the present invention, the output current command does not include higher harmonic components otherwise caused by clamping the current command, and in addition it is possible to suppress an output compensation current of the inverter to within a rated value. Furthermore, by limiting the higher harmonic components and the fundamental wave reactive components separately (Fig. 4), the performance of the apparatus can be effectively used.

Claims: A reactive power compensating apparatus having a higher harmonic suppressing function, comprising: power vector calculating means (3) for calculating an instantaneous real power (P) and an instantaneous imaginary power (Q) in accordance with a voltage and current of a load connected to a power system; power calculating means (4) for deriving higher harmonic active and reactive power components (P h , Q h ), and fundamental wave reactive power component (Q o ) from said intanstaneous real power (P) and instantaneous imaginary power (Q) calculated by said power vector calculating means (3); a limiter (5) for deriving limited higher harmonic active and reactive power components (P h ′, Q h ′) and fundamental wave reactive power component (Q o ) from said higher harmonic active and reactive power components (P h , Q h ) and fundamental wave reactive power component (Q o ) derived by said power calculating means (4), by using different gains (k₁, k₂) for the higher harmonic active and reactive power components and the fundamental wave reactive power component; inverse power vector calculating means (6) for calculating an output current command value (i c *) in accordance with said higher harmonic active and reactive power components (P h ′, Q h ′) and fundamental wave reactive power component (Q o ′) and said voltage of said load; and an inverter (2) for supplying an output current (i c ) controlled by said output current command value (i c *) to said power system as a compensation current. An apparatus according to claim 1, wherein said limiter (5) comprises: vector absolute value calculating means (15a) for calculating a higher harmonic power from said higher harmonic active and reactive power components (P h , Q h ) and derived by said power calculating means (4); peak detecting means (16) for detecting and holding the peak value of said higher harmonic power (W h ) calculated by said vector absolute value calculating means (15a); inverse function calculating means (17a) for calculating a first gain (k₁) for said higher harmonic active and reactive power components (P h , Q h ) for limiting said peak value of said harmonic power (W h ) to a value equal to or smaller than a predetermined value (h max ); means (19a, 19b) for calculating said limited higher harmonic active and reactive components (P h ′, Q h ′) by multiplying said higher harmonic active and reactive power components (P h , Q h ) by said first gain (k₁); an absolute value limiter (18) for setting a second gain (k₂) for limiting said fundamental wave reactive power components (Q o ) derived by said power calculating means (4) to a predetermined value (Q max ); and means (19c) for calculating said limited fundamental wave reactive power components (Q o ′) by multiplying said fundamental wave reactive power components (Q o ) by said second gain (k₂). A reactive power compensating apparatus having a higher harmonic suppressing function, comprising: power vector calculating means (3) for calculating an instantaneous real power (P) and an instantaneous imaginary power (Q) in accordance with a voltage and current of a load connected to a power system; power calculating means (4) for deriving higher harmonic active and reactive power components (P h , Q h ), and fundamental wave reactive power component (Q o ) from said intanstaneous real power (P) and instantaneous imaginary power (Q) calculated by said power vector calculating means (3); a limiter (5) for deriving limited higher harmonic active and reactive power components (P h ′, Q h ′) and fundamental wave reactive power component (Q o ) from said higher harmonic active and reactive power components (P h , Q h ) and fundamental wave reactive power component (Q o ) derived by said power calculating means (4), by using a same gain (k₃) for the higher active and reactive power components and the fundamental wave reactive power component; inverse power vector calculating means (6) for calculating an output current command value (i c *) in accordance with said higher harmonic active and reactive power components (P h ′, Q h ′) and fundamental wave reactive power component (Q o ′) and said voltage of said load; and an inverter (2) for supplying an output current (i c ) controlled by said output current command value (i c *) to said power system as a compensation current. An apparatus according to claim 3, wherein said limiter (5) comprises: vector absolute value calculating means (15b) for calculating a compensation power in accordance with said higher harmonic active power components (P h , Q h ) fundamental wave reactive power components (Q o ), and higher harmonic reactive power component (Q h ) derived by said power calculating means (4); peak detecting means (16) for detecting and holding the peak value of said compensation power calculated by said vector absolute value calculating means (15b); inverse function calculating means (17b) for calculating a gain (k₃) for limiting said peak value of said compensation power (W c ) to a value equal to or smaller than a predetermined value; and means (19a, 19b, 19c) for calculating said limited higher harmonic active and reactive power components (P h ′, Q h ′), and fundamental wave reactive power component (Q o ) by multiplying said higher harmonic active and reactive power components (P h , Q h ) and fundamental reactive power component (Q o ) by said gain (k₃).

Inventors: KAWAGUCHI AKIRA, TANOUE YOSHIROU, KAWAGUCHI, AKIRA, TANOUE, YOSHIROU
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H02J3
IPC codes: H02M1, G05F1, H02J3