JP2018133853A - Current detection method for inverter, current detection device for inverter, and active filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current detection method for an inverter capable of obtaining current values of three phases at a common time reference point even when using only one piece of current detection means.SOLUTION: The present invention relates to a current detection method for an inverter 11 comprising six switching elements UP, VP, WP, UN, VN and WN of a three-phase three-wire circuit. One piece of current detection means Rs is connected to an N-side common connection point of the switching elements. Current values that are obtained by the current detection means Rs, in different timing and of different phases are corrected into current values at a common time reference point.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an inverter current detection method, a current detection device using the same, and an active filter.

FIG. 11 shows a basic introduction example of the active filter 1. The active filter 1 is connected between a three-phase AC power source 2 and a harmonic generation load 3 that generates harmonics, and a load current detector 5 receives the load current i _L output from the harmonic generation load 3. detected, it extracts a harmonic component, by injecting a compensation current i _AF opposite phase of the harmonics on the power source 2 side, thereby reducing the harmonic current in the total current I _S (for example, Patent Reference 1).

  FIG. 12 shows a block circuit diagram of a conventional active filter 1. As shown in the figure, the active filter 1 is connected in parallel to a six-element inverter 6 of a three-phase three-wire circuit, a reactor ACL interposed on the input side of the inverter 6, and the inverter 6. The capacitor C and the control unit 7 that performs PWM control of the switching elements UP, VP, WP, UN, VN, and WN of the inverter 6 are provided.

Japanese Patent Laid-Open No. 6-113460 Japanese Patent Laid-Open No. 2-197295

  By the way, in order to cancel the harmonic component by the triangular wave comparison method PWM control of the active filter 1, a comparison value (reference value for ON / OFF control of each switching element) for outputting the opposite phase of the harmonic component is required. It becomes. An output current from the inverter is required to accurately generate a comparison value to be compared with the triangular wave. Therefore, conventionally, by providing a current sensor 8 in two phases of the three phases connected to the power supply 2 to detect the two-phase output current, and calculating the remaining one-phase output current from the two-phase output current, The output current of each of the three phases was obtained. However, since the current sensor 8 is expensive and large in size and requires a capacitor and a resistor incidental to the current sensor 8, there is a problem that the cost is increased and the size is increased.

  Thus, for example, as disclosed in Patent Document 2, it is conceivable to detect a three-phase current with one shunt resistor without using two current sensors 8. However, in the PWM control based on the triangular wave comparison, the current of each phase flows through the shunt resistor at different timings that fluctuate for each PWM cycle, and thus the current of each phase cannot be detected simultaneously. In addition, since current does not flow through the shunt resistor at the valley of the triangular wave that is the starting point (end point) of the PWM cycle or the apex of the triangular wave that is the intermediate point, the current in that section cannot be detected, and the comparison value is accurately obtained. There arises a problem that it cannot be generated.

  SUMMARY OF THE INVENTION In view of the above-described problems, the present invention has an object to provide a current detection method for an inverter that can obtain current values of three phases at a common time reference point even when only one current detection means is used. To do.

  The inverter current detection method according to the present invention is a current detection method for an inverter 11 including six switching elements UP, VP, WP, UN, VN, and WN of a three-phase three-circuit, and includes an N side of the switching element. One current detection means Rs is connected to the common connection point, and the current values of the different phases at different timings obtained by the current detection means Rs are corrected to the current values at the common time reference point. Features.

  The inverter 11 performs PWM control based on a triangular wave comparison, and averages current values at positions separated by the same time before and after the midpoint of the PWM cycle as a starting point. The current value is obtained.

  Further, the end point current of the current cycle is obtained from the midpoint current of the previous cycle and the midpoint current of the current cycle.

  The inverter current detection device according to the present invention is a current detection device for the inverter 11 including six switching elements UP, VP, WP, UN, VN, and WN of a three-phase three-circuit, and includes N of the switching elements. One current detection means Rs connected to the common connection point on the side, and the current values of different phases at different timings obtained by the current detection means Rs are corrected to current values at a common time reference point And a control unit 12.

  The active filter of the present invention is connected to an inverter 11 having six switching elements UP, VP, WP, UN, VN, and WN of a three-phase three-circuit and a common connection point on the N side of the switching element. Each of the current detection means Rs, and a control unit 12 that corrects the current values of the different phases at different timings obtained by the current detection means Rs to current values at a common time reference point. Features.

  In an inverter having six switching elements of a three-phase three-circuit, if one current detection means is connected to the common connection point on the N side of the switching element, the detected time (timing) is different, but the three-phase A two-phase current value is obtained. Therefore, according to the current detection method of the inverter of the present invention, the current value of the two phases is corrected to the current value at the common time reference point (for example, the apex of the triangular wave), so that the common time is also obtained for the remaining one phase. The current value at the reference point is obtained (because the sum of the currents of the three phases becomes 0), and even with one current detection means, the current values of the three phases at the common time reference point Can be obtained.

  A triangular wave that cannot be directly detected by the current detection means if the current value at the midpoint of the PWM cycle is obtained by averaging the current values at the same time before and after the midpoint of the PWM cycle. The current value at the apex of can be obtained.

  Further, if the current at the end point of the current cycle is obtained from the midpoint current of the previous cycle and the midpoint current of the current cycle, the current value at the trough of the triangular wave that becomes the start point (endpoint) of the PWM cycle can be obtained. .

  According to the inverter current detection device of the present invention, the two-phase current values are corrected to the current values at the common time reference point (for example, the apex of the triangular wave), so that the common time reference point is also obtained for the remaining one phase. (Because the sum of the currents of the three phases becomes 0), and even with one current detection means, the current values of the three phases at the common time reference point are obtained. be able to. Therefore, it is not necessary to provide two current detection means such as a current sensor, and the cost and size can be reduced.

  According to the active filter of the present invention, by correcting the current value of two phases to the current value at the common time reference point (for example, the apex of the triangular wave), the current at the common time reference point is also obtained for the remaining one phase. Value (because the sum of the currents of the three phases becomes 0), and even with one current detection means, the current values of the three phases at the common time reference point can be obtained. . Therefore, the comparison value can be generated with high accuracy based on the current value, so that the reverse phase can be output with high accuracy and the harmonics can be effectively canceled out. Further, the cost and size can be reduced as compared with the case where two current detection means such as a current sensor are provided.

It is a block circuit diagram of the active filter in the embodiment of the present invention. It is a figure which shows the triangular wave and comparison value in embodiment of this invention. It is a figure which shows the PWM pulse and switch mode in embodiment of this invention. It is a figure which shows the electric current change of each phase and shunt resistance in embodiment of this invention. It is a figure which shows the electric current path | route in mode E0 (E0P) in embodiment of this invention. It is a figure which shows the electric current path | route in mode E0 (E0N) in embodiment of this invention. It is a figure which shows the electric current path in mode E1 in embodiment of this invention. It is a figure which shows the current pathway in mode E2 in embodiment of this invention. It is a figure which shows the current polarity of the switching element in each switch mode in embodiment of this invention. It is a figure which shows prediction of the end point current in embodiment of this invention. It is a circuit diagram which shows the basic use condition of an active filter. It is a block circuit diagram of the active filter of a prior art example.

  The present invention uses a single current detection means at the N terminal of the inverter to detect currents of different phases at different timings, and detects them at a common time reference point (for example, the start point (end point) or intermediate point of the PWM cycle). Etc.), a current detection device including such a control unit, and an active filter that outputs an antiphase of a harmonic component based on a current value at a common time reference point. Hereinafter, the details of the present invention will be described based on the active filter.

FIG. 1 shows a block circuit diagram of the active filter 10. As shown in the figure, the active filter 10 includes a six-element inverter 11 for each phase, and an input side of the inverter 11 and a three-phase three-wire power supply system Va, Vb, Vc that outputs current. The current detecting means (shunt resistor) Rs connected to the common reactor (common connection point) on the N side of the three-phase switching elements UP, UN, VP, VN, WP, and WN of the three-phase switching elements of the inverter 11 And the capacitor C connected in parallel between the positive electrode side of the inverter 11 and the lower end of the shunt resistor Rs, and the switching elements UP to WN of the inverter 11 are PWMed to detect the load current and cancel the harmonic components. The control unit 12 is configured to control, and an operational amplifier 13 that calculates a voltage from both ends of the shunt resistor Rs and inputs an output signal to the control unit 12. The current detection device includes a current detection means Rs, a control unit 12, and an operational amplifier 13. The switching element UP~WN is, for example, using the IGBT (I nsulated G ate B ipolar T ransistor). For convenience, the direction of the current is positive in the direction of the arrow in FIG.

  By the way, in order to cancel the harmonic component of the load current, it is necessary to detect the load current by the load current detection unit, extract the harmonic component, and output the opposite phase of the harmonic component from the inverter 11 using this as a control target. There is. Specifically, since one end of the reactor ACL is connected to the system voltage (power supply system), the current flowing at the start of the PWM cycle of the PWM circuit (not shown) that controls the inverter 11 and the end of the PWM cycle A voltage at which the reactor ACL obtains the above current increase / decrease (difference) during one PWM cycle due to the difference between the command currents that should sometimes flow (in FIG. 10, control target c−current cycle start point b) is output from the inverter 11. There is a need to.

  For this purpose, first, on / off control of the switching elements UP to WN is performed by comparison with a triangular wave having a height of 0 to 0.5 and a period of 1, thereby comparing values (Ta, Tb, Tc) necessary for controlling the inverter 11 Is generated. Note that on / off control of the switching elements UP to WN is performed according to a command from the control unit 12. This comparison value (comparison value determined at the start point of PWM) does not change until the end point of the PWM cycle. In the meantime, it is assumed that fluctuations in the inductances of the system voltages Va, Vb, Vc, the capacitor voltage Vdc, and the reactor ACL can be ignored. The comparison values (Ta, Tb, Tc) are expressed by the following formula.

Ta = 1/2 * (− La * Δia / Ts + Va) / Vdc (1)
Tb = 1/2 * (− Lb * Δib / Ts + Vb) / Vdc (2)
Tc = 1/2 * (− Lc * Δic / Ts + Vc) / Vdc (3)
However,
Va, Vb, Vc: System voltage (Va + Vb + Vc = 0)
Vdc: capacitor voltage of capacitor C Ts: PWM period La, Lb, Lc: inductance of reactor ACL of each phase Δia, Δib, Δic: current change of each phase = end point current−start point current (control target c− in FIG. 10) Start point of current cycle b)

  Next, Ta, Tb, and Tc added with offsets so that the values are evenly within the range of 0 to 0.5 are set as T1, T2, and T3 in ascending order. Although there are six types of actual magnitude correlation combinations, here, a case where Ta <Tb <Tc will be described as a representative example. In this representative example, T1 = Ta + α, T2 = Tb + α, and T3 = Tc + α. α is an offset amount having a relationship of T3 = 0.5−T1.

  Along with the rearrangement, the phase corresponding to T1 is referred to as # 1 phase, the phase corresponding to T2 is referred to as # 2 phase, and the phase corresponding to T3 is referred to as # 3 phase. The phase current of the # 1 phase is i1, the phase current of the # 2 phase is i2, and the phase current of the # 3 phase is i3. This is obtained by rearranging the three-phase alternating currents ia, ib, and ic in the same manner as Ta, Tb, and Tc. In this representative example, ia = i1, ib = i2, and ic = i3.

  FIG. 2 shows the comparison between T1, T2, T3 and the triangular wave, and FIG. 3 shows the generated pulses. In FIG. 3, Pls-a is a PWM pulse in the # 1 phase, Pls-b is a PWM pulse in the # 2 phase, and Pls-c is a PWM pulse in the # 3 phase.

When the comparison value is smaller than the triangular wave, the P-side switching elements UP, VP, and WP are turned on, and the N-side switching elements UN, VN, and WN are turned off. On the contrary, the P-side switching elements UP, VP, and WP are turned off, and the N-side switching elements UN, VN, and WN are turned on.
That is, the following situation occurs.
Triangle wave> Comparison value → P side: On N side: Off Comparison value> Triangle wave → P side: Off N side: On

As for the magnitude relationship between the triangular wave and the comparison values T1, T2, and T3, the following four states appear in order.
(1) E0N: The comparison value of all phases is larger than the triangular wave.
At this time, the switching elements UP, VP and WP on the P side of all phases are turned off, and the switching elements UN, VN and WN on the N side are turned on. At this time, the output of the inverter 11 is 0 V as in E0P described later.
(2) E1: Only T1 is smaller than the triangular wave, and T2 and T3 are larger than the triangular wave.
At this time, in the # 1 phase, the P-side switching element UP is on, and the N-side switching element UN is off. In the # 2 and # 3 phases, the P-side switching elements VP and WP are off, and the N-side switching elements VN and WN are kept on.

(3) E2: When T1 and T2 are smaller than the triangular wave and T3 is larger than the triangular wave.
At this time, in the # 1 phase and the # 2 phase, the P-side switching elements UP and VP are on, and the N-side switching elements UN and VN are off. In the # 3 phase, the P-side switching element WP is off and the N-side switching element WN is kept on.
(4) E0P: When the comparison value of all phases is smaller than the triangular wave.
At this time, the switching elements UP, VP and WP on the P side of all phases are turned on, and the switching elements UN, VN and WN on the N side are turned off. At this time, similarly to the above E0N, the output of the inverter 11 is 0V.

As shown in FIG. 3, the mode changes in the following order within one PWM cycle.
Start point → E0N → E1 → E2 → E0P → vertex → E0P → E2 → E1 → E0N → end point Note that E0N and E0P are classified as E0 because the voltage of the inverter 11 is 0V and the current change rate is equal.

The start / end / continuation time of each mode is as follows. (Unit: Percentage of PWM1 period)
Start End Period 1st E0N 0 T1 T1
1st E1 T1 T2 T2-T1
1st E2 T2 T3 T3-T2
1st E0P T3 0.5 T1
2nd E0P 0.5 1-T3 T1
2nd E2 1-T3 1-T2 T3-T2
2nd E1 1-T2 1-T1 T2-T1
2nd E0N 1-T1 1 T1
Total period = 1.0 ∵T3 = 0.5−T1

  5 to 8 show states of the respective modes and current paths. FIG. 5 shows the mode E0, where E0P and triangular wave> comparison value (T1, T2, T3). The control unit 12 controls the three switching elements UP, VP, and WP on the P side to be all on and the three switching elements UN, VN, and WN to the N side are all off. At this time, no current flows through the shunt resistor Rs (is = 0).

  FIG. 6 shows the mode E0, where E0N and comparison values (T1, T2, T3)> comparison values. The control unit 12 controls the three N-side switching elements UN, VN, and WN to be all on and the P-side three switching elements UP, VP, and WP to be all off. At this time, no current flows through the shunt resistor Rs (is = 0).

  FIG. 7 shows mode E1, in which one phase switching element UP of the P side switching elements is ON (VP and WP are OFF), and the other two phase switching elements VN and WN are ON (N side switching elements are ON ( 7 is E1 (E1U) in the U phase, the switching elements UP, VN, WN are on, the switching elements UN, VP, WP are off, and the shunt resistor Rs has a U phase current ( is = ib + ic = −ia = −i1) flows in the opposite direction.

  FIG. 8 shows the mode E2, in which the switching element WN among the three switching elements on the N side is ON (UN and VN are OFF), and the other two-phase switching elements UP and VP among the three switching elements on the P side are 8 is E2 (E2W) in the W phase, the switching elements UP, VP, WN are on, the switching elements UN, VN, WP are off, and the shunt resistor Rs has W A phase current (is = ic = i3) flows.

  Thus, it can be seen that no current flows through the shunt resistor Rs in the mode E0, the current -i1 flows through the shunt resistor Rs in the mode E1, and the current + i3 flows through the shunt resistor Rs in the mode E2. As described above, even if there is only one shunt resistor, it is possible to detect a phase current of two phases out of the three phases by selecting and detecting the timing, although the detected timing is different.

  FIG. 9 shows the P-side switching elements UP, VP, WP of each phase (U-phase, V-phase, W-phase) and the N-side switching elements UN, VN, WN in the switch mode (E0, E1, E2). The current polarity and the current is flowing through the shunt resistor Rs are shown.

  Next, a method for correcting current values of different phases at different timings obtained by the above method to a common time reference point will be described.

  The current output by the comparison value has the same current change rate and the same time width in the same mode (E0, E1, E2), and the order of the modes is reversed in the first half and the second half of the PWM cycle. Therefore, as shown in FIG. 4, the current waveform of each phase is point-symmetric about the PWM midpoint. Therefore, if we focus on an arbitrary period of the triangular wave, the start point is called the start point, the end of one period is called the end point, and the temporal intermediate point is called the mid point. The current value at the midpoint of the PWM cycle can be obtained by averaging the current values at positions separated by the same time before and after.

Specifically, the current −i1 at the middle point (horizontal axis 0.5 in FIG. 4) is obtained by averaging the currents at two points E1 that are the same time (t1) before and after the middle point. .
Time from the first start point t11 = T1 to T2 = 0.5-t1
Time from the second start point t12 = 1−T2−1−T1 = 0.5 + t1

Moreover, the current + i3 at the middle point (horizontal axis 0.5 in FIG. 4) is obtained by averaging the currents at two points E2 that are the same time (t2) apart from the middle point.
Time from the first start point t21 = T2 to T3 = 0.5-t2
Time from the second start point t22 = 1−T3−1−T2 = 0.5 + t2

  The current of the other phase of the midpoint is given by i1 + i2 + i3 = 0 to i2 = − (i1 + i3). Then, when the original rearrangement is restored to correspond to the actual phase, a value based on the actual measurement of the midpoint of the corresponding ia, ib, ic is obtained. Note that such calculation is performed in the control unit 12.

  Subsequently, based on the current value at the midpoint (the current value at the midpoint of the previous period and the current value at the midpoint of the current period) obtained by the above method, the end point current (c ′ in FIG. 10) ) Will be described. The end point current of the current cycle obtained here is a current value at the trough of the triangular wave that is the start point of the next cycle, and the current change Δia of each phase used when calculating the comparison values Ta, Tb, Tc, Used as the starting point of Δib and Δic.

FIG. 10 shows the current change in any of the three phases. here,
a is the midpoint of the previous cycle (value based on actual measurement)
b is the end point after correction of the previous cycle = start point of the current cycle c is the end point of the current cycle (control target)
d is the midpoint of the current cycle (planned)
d ′ is the midpoint of the current period obtained by the above method (value based on actual measurement)
Is shown.

  In the current cycle, in order to set the current value at the end of the current cycle to the control target c, the offset α is added to T1, T2, T3 (comparison values Ta, Tb, Tc from Δi = c−b, and they are arranged in ascending order. (Replaced) is being calculated and is being executed. Therefore, originally, the control target c is equal to the actual end point current c ′. However, in reality, the end point current c 'is slightly off the control target c due to various errors. Therefore, correction is performed using the current value d 'at the midpoint of the current cycle, and the end point current c' is predicted (corrected) with high accuracy.

The following is an example of the method.
From the midpoint current value d ′, the end point current c ′ is
c ′ = c + (d′−d) * k
Guess (assum) that

FIG. 10a shows the case where the starting point b is correct and there is an error in Δi. As shown in the figure, since b = b ′ (b ′ is an actual current value at the start point of the current cycle), the error at the end point is twice the error at the midpoint. Therefore,
c ′ = c + (d′−d) * 2 (k = 2)
It can be expressed as.

FIG. 10b shows a case where Δi has no error and the start point b has an error. As shown in the figure, since b → d → c and b ′ → d ′ → c ′ are parallel, the error at the end point has the same value as the middle point. Therefore,
c ′ = c + (d′−d) * 1 (k = 1)
It can be expressed as.

FIG. 10c shows a case where both the start points b and Δi have errors. Assuming that the middle point a of the previous cycle is correct in actual measurement and assuming that the same error occurs between a → b ′, b ′ → d, and d ′ → c ′, the error at the end point is the error at the middle point. 1.5 times. Therefore,
c ′ = c + (d′−d) * 1.5 (k = 1.5)
It can be expressed as.
Note that the value of k may be appropriately changed according to the characteristics of the device used.

  In this way, even in the case of current detection using a single shunt resistor, the current value at the end point (triangular wave valley) of the current cycle can be accurately obtained from the current value at the midpoint based on the actual measurement. it can. Therefore, if a comparison value is generated based on this current value, the antiphase can be output with high accuracy and the harmonics can be canceled with high accuracy. That is, in a current output inverter that controls the current of the reactor ACL, such as an active filter, when the inductance of the reactor ACL is relatively small, the current change within the PWM period is large, so that a current value at a common time reference point is obtained. Inability to obtain the current value of the trough of the triangular wave, which is the base point of control, makes it impossible to obtain a comparison value with high accuracy, leading to a decrease in the accuracy of the output current, but such problems can be solved It is.

  Further, in the present invention, since there is only one current detecting means, it is possible to reduce the cost and size. In the above embodiment, the shunt resistor Rs is used as the current detecting means. However, since the output voltage of the shunt resistor Rs is very small, amplification and an appropriate offset for inputting to the A / D converter of the microcomputer are performed. In order to make the setting, the operational amplifier 13 is generally required, and although not shown, a capacitor and a resistor attached to the operational amplifier are also required. However, both are small and small, and it is also possible to use a shunt resistor that is used for overcurrent detection of IPM (Intelligent Power Module). Therefore, the number of parts is reduced as a whole, and the size of the board can be reduced.

  Although specific embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the present invention. For example, the current detection means is not limited to a shunt resistor, and various known devices such as a current sensor may be used. Further, it is not always necessary to provide an operational amplifier. In the above embodiment, the control unit 12 performs calculations when calculating the comparison value, the midpoint current, and the end point current. However, a calculation means is separately provided inside or outside the apparatus, and the calculation result is controlled there. You may make it tell to the part 12.

  Further, the method for obtaining the end point current is not limited to the above method, and various methods can be adopted. For example, the line connecting the midpoint current of the previous cycle and the current cycle may be simply extended to the end point, and this may be assumed as the end point current, or the end point current of the previous cycle (start point current of the current cycle) may be taken into account. The accuracy may be improved. In short, the end point current may be obtained based on the midpoint current at two points of the previous period and the current period. In any case, since the value based on the actual measurement is used, the end point current of the current cycle can be obtained with high accuracy.

DESCRIPTION OF SYMBOLS 1 Active filter 2 Power supply 3 Harmonic generation load 5 Load current detection part 6 Inverter circuit 7 Control part 8 Current sensor 10 Active filter 11 Inverter 12 Control part 13 Operational amplifier UP, VP, WP, UN, VN, WN Switching element i _L Load current i _AF compensation current i _S total current

Claims (5)

A current detection method for an inverter (11) having six switching elements (UP, VP, WP, UN, VN, WN) of a three-phase three-circuit,
One current detection means (Rs) is connected to the common connection point on the N side of the switching element,
An inverter current detection method, comprising: correcting current values of different phases at different timings obtained by the current detection means (Rs) to current values at a common time reference point. The inverter (11) is subjected to PWM control by triangular wave comparison,
The current detection method for an inverter according to claim 1, wherein the current value at the midpoint of the PWM cycle is obtained by averaging current values at positions separated by the same time before and after the midpoint of the PWM cycle. The inverter current detection method according to claim 2, wherein an end point current of the current cycle is obtained from a midpoint current of the previous cycle and a midpoint current of the current cycle. A current detection device for an inverter (11) comprising six switching elements (UP, VP, WP, UN, VN, WN) of a three-phase three-circuit,
One current detection means (Rs) connected to a common connection point on the N side of the switching element;
A control unit (12) for correcting current values of different phases at different timings to current values at a common time reference point obtained by the current detection means (Rs), Inverter current detector. An inverter (11) having six switching elements (UP, VP, WP, UN, VN, WN) of a three-phase three-circuit;
One current detection means (Rs) connected to a common connection point on the N side of the switching element;
A control unit (12) for correcting current values of different phases at different timings to current values at a common time reference point obtained by the current detection means (Rs), Active filter.

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