JP3236986B2 - Power conversion system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power conversion system including a plurality of power converters connected to one power receiving end from a power system, such as a steel process line, each having an independent load.

[0002]

2. Description of the Related Art In plants such as steel processing lines,
A plurality of power converters may be used simultaneously. In this case, each AC-DC power converter (hereinafter, referred to as a converter) temporarily converts an AC power supply (system power supply) into a DC power supply, and drives a load such as an inverter using the DC power supply. Each converter is independently controlled, and controls the DC section and the input current.
When the converter is driven by PWM control (pulse width modulation control), harmonic current flows out to the grid. Conventional techniques for suppressing the harmonic current include the following. (1) Each converter is controlled as a converter synchronous PWM using a triangular wave carrier synchronized with a converter input voltage (input voltage command value of the converter) to reduce harmonics. (2) Using a triangular carrier synchronized with the power supply, each converter is driven as power supply synchronous PWM, and the phase of each triangular carrier is changed to reduce harmonics. (3) Suppress harmonics by adding a harmonic filter.

[0003]

SUMMARY OF THE INVENTION Converter Synchronous PWM
Is a PWM method suitable for suppressing harmonics in a power converter having a low carrier frequency. Since the triangular wave carrier including the phase is synchronized with the input voltage command of the converter, the content of the generated harmonic can be kept at a substantially constant low value. However, the following problems occur in actual use. In order to realize converter synchronous PWM, a PLL (Phase Locked Loop) is used.
p) and the like, and the converter input voltage greatly changes depending on the load condition (particularly, the phase with respect to the system power supply changes), so that there is a delay until the PLL follows and is synchronized. Furthermore, when a plurality of converters have independent loads, the phase of the generated harmonics (phase with respect to the system power supply) changes depending on the operating state of each converter. Depending on the load condition, there is a case where the two cancel each other, and another case where the number of the harmonics increases, and the content of the harmonics greatly changes. For this reason, it is difficult to grasp the generated harmonic, and it becomes difficult to suppress the harmonic at the power receiving end.

The power supply synchronous PWM generates a synchronous signal from a power supply, sends a common synchronous signal to each converter, and performs PWM control. In this case, since the triangular wave carrier is synchronized with the system power supply including the phase, by changing the phase (with respect to the system power supply) of each triangular wave carrier, it is possible to suppress harmonics at the power receiving end. In particular, it is effective in suppressing harmonic components near even harmonics (2fc, 4fc,...) Of the carrier frequency fc. However, when the phase of the converter input voltage changes (when the converter load changes), the harmonic content changes significantly (in this case, the amplitude changes more than the harmonic phase). Therefore, the amount of generation of harmonics also varies depending on the load state of the converter. In particular, this tendency is strong in a large-capacity device having a low carrier frequency. Further, the power supply synchronous PWM gives a common synchronous signal to a plurality of converters and changes the phase of each triangular wave carrier, so that a synchronous signal for cooperative operation is required. JP-A-6-351106
As described in the publication, when a plurality of converters are connected to one transformer, it is easy to share a synchronization signal, but as in a rolling plant, a number of converters are distributed and arranged. If so, cooperative driving becomes difficult.

[0005] When a large-capacity filter is connected to each converter, the size of the device is increased and the output is reduced. In a large-capacity converter, the frequency of the generated harmonics is low, and it is necessary to sufficiently consider anti-resonance when designing a filter, which makes the filter design itself extremely difficult.

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to suppress harmonics flowing out to a power supply system when a plurality of converters are arranged in a dispersed manner, to reduce the size of the device,
It is to provide a power conversion system suitable for achieving high efficiency.

[0007]

The object of the present invention is to detect a power supply phase of a power system for each power converter, generate a synchronization signal based on the detected value, and convert a triangular wave carrier to a power supply phase. By taking the synchronization and presetting the phase of the triangular wave carrier so as to cancel out the harmonics generated by each converter, and generating the triangular wave carrier,
Will be resolved. Thus, according to the present invention, the power supply system can be regarded as a synchronization signal, and harmonics at the receiving end can be reduced only by initializing the carrier phase of each power conversion device. In particular, there is no need to create a signal for cooperative operation between the power conversion devices, and since a filter for harmonics is not required, the size and efficiency of the device can be reduced.
Large capacity can be achieved. In addition, the above-mentioned problem is solved in a period (Δt) from a positive peak to a negative peak, which is a half cycle of the triangular wave carrier, from the instantaneous value of the command value of the power converter to the period. (Δ
The problem is solved by estimating the average value of t) and performing compensation using this value as a new command value. in this case,
In the present invention, since the amplitude of the harmonic is kept substantially constant by the voltage command compensation, the dependence on the load state is reduced, and the harmonic can be reduced to a certain value or less. Further, the above problem is solved by setting the phase of the triangular wave carrier preset in each power converter device to the phase angle of each triangular wave carrier determined based on the operation state of each power converter device. . In this case, according to the present invention, the cooperative operation between the power conversion devices can be dynamically performed, and the harmonic at the power receiving end can be minimized.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration of a power conversion system according to an embodiment of the present invention. In FIG. 1, 1 is a system power supply, 2 is a power receiving end of the present system, 3 is a power converter for one unit, 4 is a command value generator for giving an input voltage command of a converter, and 5 is a triangular wave generator for generating a triangular wave carrier. Vessel 5
2 and a PWM controller comprising a comparator 51 for comparing the output of the command value generator 4 with the triangular wave, 6 a carrier phase setting unit for setting the phase of the triangular wave carrier, 7 a converter main circuit 71 and a transformer 72 A converter section, 8 is a power supply phase detector for detecting the phase θe of the input voltage, and 9 is a power supply phase θ.
A phase command generator 10 for calculating the phase θs of the triangular wave carrier based on e and the carrier phase set value Φs1 is a load device for a converter such as an inverter or an AC motor.

Next, the operation of FIG. 1 will be described. The command value generator 4 is provided with the power required for the load device 10 and the converter 71.
A voltage command is calculated and output with the goal of setting the power factor of the unit to 1. On the other hand, the power supply phase detector 8 detects the power supply phase θe, and the phase command generator 9 generates a synchronizing signal based on the power supply phase θe.
e, the power supply phase θe and the phase setting value Φs1 of the triangular wave carrier of the carrier phase setting unit 6 are added,
The phase θs of the triangular wave carrier is calculated and output. The triangular wave generator 52 outputs a triangular wave carrier based on the phase command θs. The PWM controller 5 compares the output signal of the command value generator 4 with the triangular wave carrier of the triangular wave generator 52,
Create a PWM pulse. The switching element of the converter main circuit 71 performs a switching operation by the PWM pulse. As described above, in the present embodiment, each power converter 3 is provided with a function of detecting a power supply phase, a power supply synchronous PWM is realized in each power converter 3, and a triangular wave with respect to the power supply phase of each converter is provided. Set the carrier phase of Note that it is difficult to increase the switching frequency in a large-capacity converter due to the performance of the switching element. Therefore, a low frequency is usually used. In that case, synchronous PWM control is used in order to avoid a beat phenomenon due to harmonics.

Here, the name of the synchronous PWM has the following two meanings. One is that the ratio N (= fs / f1) between the frequency f1 of the fundamental wave (frequency of the power supply in the case of the converter) and the frequency fs of the triangular wave carrier is an integer (when three-phase alternating current is handled, In particular, the case where N is a multiple of 3 and an odd number is referred to as synchronous PWM, and the other is that, in addition to these conditions, the phase of the triangular wave carrier and the phase of the fundamental wave completely match. Synchronous PWM if
It may be called. In the case of synchronous PWM under the former condition,
The fundamental wave and the triangular wave carrier are operated with a certain phase difference, but if the latter condition is synchronous PWM,
The phase difference will be kept at zero. In the present embodiment, the former is defined and treated as synchronous PWM. The power supply synchronization PWM and the converter synchronization PWM are divided depending on whether the synchronization is performed with respect to the power supply or the input voltage of the converter (the voltage output from the command value generator 4).

The principle of reducing harmonics at the power receiving end in this embodiment will be described with reference to FIGS.
FIG. 2 shows a case where two power converters 3 are connected to a power supply system. In converter 1, converter input voltage Vc is out of phase with power supply voltage E by the leakage inductance of transformer 72. Assuming that the phase difference of Vc with respect to E is Φ1, the relationship between the waveforms of E and Vc is shown in FIG.
become that way. FIG. 4 shows this relationship as a vector diagram. As is clear from FIG. 4, for example, when the input current i increases, the input ACL (AC reactor) voltage _VL increases, and the phase difference Φ1 increases.
Therefore, in order to keep the input power factor at 1 (to make E and i have the same phase), it is necessary to change the magnitudes of Φ1 and Vc in accordance with the magnitude of the input current i. . That is, the phase of the triangular wave carrier changes in synchronization with this. The same applies to converter 2. Accordingly, as shown in FIGS. 3B and 3C, the phase Φs of the triangular wave carrier with respect to E is respectively shown.
Are set to 0 degree and 90 degrees by the respective carrier phase setting units 6 of the converter 1 and the converter 2, the phase of the generated harmonic (particularly, the phase of the component near an even multiple of the carrier frequency) is changed by Φ1. Occurs with little dependence. In this case, the phases of the harmonics are opposite to each other, and when the two power converters 3 are used, the respective harmonic components cancel each other, and the harmonics at the power receiving end are reduced. Thus, in the present embodiment, the power conversion device 3
Is provided with a power supply phase detector 8 and a phase command generator 9 in each power conversion device 3, each of the power conversion devices 3 has a function of synchronizing with the power supply, and the phases of the power supply and the triangular wave carrier are provided. By first setting the angle by the carrier phase setting device 6 in each power conversion device 3,
In other words, by changing and setting each carrier phase with respect to the power supply, when a plurality of power converters 3 are used, harmonics at the power receiving end can be reduced.

In the converter synchronous PWM, since the command value (command value of the input voltage Vc) and the triangular wave carrier are synchronized, the harmonics of each converter can always be suppressed to a certain value or less. However, in the converter, since Vc greatly changes depending on the load state (input current) (particularly, phase Φ1 greatly changes), it is necessary to synchronize in accordance therewith. In a system having a large change in input current, a delay occurs until synchronization is achieved, and harmonics during this time have transient but various frequency components. When a single converter is used, the amount of generated harmonics is constant.However, when a plurality of converters are driven independently of each other, the harmonics at the power receiving end cause a change in the phase of each converter. Depending on the situation, there are cases where the two cancel each other out and cases where each other emphasizes each other. For this reason, it is difficult to suppress harmonics below a specific value.

Further, Japanese Patent Application Laid-Open No. 6-35 shown as a conventional example
In Japanese Patent Publication No. 1106, a synchronous signal is generated from a power supply, applied to a plurality of converters in common, and the phase of each triangular wave carrier is shifted to suppress harmonics at a power receiving end. In the conversion system,
Each converter is provided with a power phase detector 8 and a phase command generator 9, and each converter has a function of synchronizing with a power source. Therefore, it is not necessary to route a synchronization signal between the converters. If the phase angle of the carrier is initially set by the carrier phase setting device 6 in each converter, the harmonics at the receiving end can be reduced. Therefore, the independence of each converter is strong, and each converter is dispersed. This effect is large and effective for the installed system.

Next, FIG. 5 shows another embodiment of the present invention. The present embodiment is different from the system of FIG.
Is added. The operation of the command value compensator 11 is shown in FIG.
This will be described with reference to FIG. Here, first, a conventional PWM method is shown in FIG. In the conventional PWM method, the command value v
(T) * and triangular wave carrier et (t)
A pulse is being generated. The triangular wave comparison method means that the instantaneous value (height) of the command value is changed every half cycle of the triangular wave carrier (Δ
A sample (sampled at a point p in FIG. 6) is used at every t period) to convert the height of the command value into a pulse width. However, as shown in FIG. 6A, when the carrier frequency is low (when the frequency is reduced to about 10 times the frequency of the command value), the sample point p is not always a value representative of the Δt period. That is, the error due to PWM increases. As a result, the command value v (t) * and the triangular wave carrier et
Depending on the phase state of (t), the amount of generated harmonics greatly changes. Therefore, when the converters operate independently in the system of FIG. 1 when the carrier frequency is low, the amount of harmonics generated by each converter differs, and the amount of cancellation of the harmonics at the receiving end changes. In some cases, the effect of suppressing harmonics is weakened.

In order to solve these problems, a command value compensator 11 is added in this embodiment. Command value compensator 11
6A, as shown in FIG. 6B, the command value v (t) * is averaged every Δt period, and the value is used as a new command value v (t), which is compared with the triangular wave carrier et (t). . The average value v (t) of the command values is estimated and calculated using the instantaneous value of the original command value v (t) *. As a result, the triangular wave carrier et
The change in the amount of harmonic generation due to the phase difference between (t) and the command value v (t) * is reduced, and the harmonics generated by each converter cancel out to almost the same level regardless of the conditions.

FIG. 7 shows, by simulation, a change in the harmonic current Ih (effective value of all harmonic components) when the phase angle Φs of the triangular wave carrier et (t) with respect to the command value v (t) * is changed. The results are shown (in the case of a converter alone, when N = 9). In the case where there is no command value compensation indicated by the broken line in FIG. 7 (conventional method), it can be seen that the worst case may increase the harmonics up to about 1.5 times. However, as shown by the solid line in FIG.
It can be seen that there is almost no dependence of harmonics on s. As described above, in the present embodiment, the amplitude of the harmonic is kept substantially constant by the voltage command compensation, so that the dependence on the load state is reduced, and even when the carrier frequency is low, the harmonic generation amount is reduced. Since it can be suppressed to a substantially constant value, the harmonics at the power receiving end can be suppressed to a substantially constant value or less even if each converter is driven independently.

FIG. 8 shows another embodiment of the present invention. In the embodiments shown in FIGS. 1 and 5, each converter is driven independently, and the carrier phase Φs is initialized, thereby suppressing harmonics at the power receiving end. However, if you want to specifically suppress certain harmonics, or
If it is desired to further suppress the harmonics to the limit, it is effective to use the embodiment of FIG. In the present embodiment, the carrier phase Φs is changed in real time according to the load state of each converter, and cooperative operation between the converters is performed. In FIG. 8, reference numeral 12 denotes input currents i1 and i of each converter.
The current detector 13 detects the input current of each converter in order to suppress harmonics at the power receiving end 2, and supplies an appropriate carrier phase command Φs (Φs1, Φs2, Φs3 to φsn).

FIG. 9 shows an algorithm representing the operation of the harmonic suppression device 13. First, the input currents i1 to in of each converter are read, and the values of x (x1 to xn) representing the operating state of each converter are calculated from the current values. As shown in FIG. 4, x is x = tanΦ1, and can be obtained as x = ωLi / E (that is, if input current i is known, it can be calculated from power supply voltage E and inductance L). When x is a positive value, power is being supplied to the load (power running), when negative, the regenerative state is indicated, and zero indicates no load state. Next, the carrier phase angle Φ
Temporarily set s1 to Φsn to an appropriate value (for example, half of a plurality of converters is set to 0 degrees and half is set to 90 degrees). Next, the harmonic Hn generated by each converter is estimated from these values and the values of x1 to xn. The harmonic Hn is a quantity representing a ratio of generation of a harmonic with respect to the DC voltage of the converter, and corresponds to a current harmonic generated by the converter. 10 and 11,
The Hn values of the 17th and 5th sine components are shown.
Hn is, as shown in these figures, a function of Φs and x,
Calculate in advance and make a table. The harmonic at the receiving end is generated as the sum of Hn of each converter. The value of x is determined by the operating state of the converter.
Can be arbitrarily selected, so that the total harmonic amount (sum of Hn) can be minimized using Φs. To this end, after setting Φs, the sum of Hn (harmonics at the receiving end) is calculated, the amount of harmonic generation is determined, and if the value is not good, Φs is set again and the sum of Hn is calculated. Repeat the calculation of calculating, and find the minimum point.

When the number of converters is small, Φs can be set without performing such repetitive calculations. A specific method of setting Φs will be described. As shown in FIG. 2, there are two converters,
Consider suppressing the seventh harmonic (FIG. 10). now,
Converter 1 is set to φs1 = 90 degrees and x = 0.
It is assumed that driving is performed in state A of 1. Further, it is assumed that converter 2 is driven in the state of x = −0.1. In order for the harmonics generated by the converters 1 and 2 to cancel each other at the power receiving end, it is preferable that the signs of the harmonic components of the two are opposite and their absolute values are close. In FIG. 10, since it is C that is equal to the size of A, B is equal to Φs2.
When the angle is set to 180 degrees rather than 0 degrees, the 17th-order component is suppressed. Therefore, in this state, the harmonic suppression device 1
3 is Φs1 = 90 degrees for converters 1 and 2;
A carrier phase command of 2 = 180 degrees is output. Thereby, harmonic components are minimized. Next, while converter 2 remains in the state of x = −0.1, converter 1 sets x = −0.1.
It is assumed that the state has changed to the state of 0.1 (the harmonic generation amount changes to A '). This time, since B is closer to the size of A ', harmonics of this component are suppressed when Φs2 = 0. Therefore, a command value of Φs2 = 0 is output to converter 2. Thereby, harmonic components are minimized. As described above, by appropriately switching Φs, it is possible to suppress a specific harmonic at the power receiving end, and further suppress the harmonic to the limit. In addition to the 17th harmonic, the fifth harmonic shown in FIG.
The same can be done for the second harmonic and other harmonic components. Suppressing specific harmonics has effects such as preventing anti-resonance when a filter or the like is inserted at the power receiving end. It is also possible to calculate in advance how the amount of harmonic generation at the receiving end changes for all operating states of a plurality of converters, and to provide a carrier phase that minimizes the entirety of the total. is there.

[0020]

As described above, according to the present invention,
In a plant in which a plurality of power converters are dispersedly arranged, each power converter has a function of synchronizing with a power source, and the phase angle of the power source and the triangular wave carrier is initially set in each power converter. By setting
There is no need to route synchronization signals between power converters as in the past, and it is possible to reduce harmonics at the receiving end,
In particular, the present invention has a strong independence of each power converter, and is effective for a system in which each power converter is installed in a distributed manner. Further, according to the present invention, since it is not necessary to connect a filter for suppressing harmonics,
The size of the device can be reduced. Further, since the carrier frequency of the converter can be set low, switching loss can be reduced, and high efficiency and large capacity of the device can be achieved. Further, according to the present invention, the amplitude of the harmonic is kept substantially constant by the voltage command compensation, so that the dependence on the load state is reduced. Also, even when the carrier frequency is low, the amount of generation of the harmonic is substantially reduced. Because it can be suppressed to a constant value, even if each converter is driven independently,
Harmonics at the receiving end can be suppressed to a substantially constant level. Also, according to the present invention, by appropriately switching the carrier phase command output to each power conversion device, it is possible to suppress a specific harmonic at the power receiving end, and further suppress the harmonic to the limit. . By suppressing specific harmonics, when it is necessary to insert a filter or the like at the power receiving end, effects such as prevention of anti-resonance can be obtained. In addition, by calculating in advance how the amount of generated harmonics at the receiving end changes for all operating states of the plurality of power converters, and by giving a carrier phase that always minimizes the whole. , Harmonics at the receiving end can be minimized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a power converter system showing one embodiment of the present invention.

FIG. 2 is a configuration diagram when two converters are used in the present invention.

FIG. 3 is a diagram illustrating a relationship among a power supply voltage, an input voltage of a converter, and a triangular wave carrier.

FIG. 4 is a vector diagram showing an input voltage of a converter.

FIG. 5 is a configuration diagram of another embodiment of the present invention.

FIG. 6 is a diagram illustrating the principle of a command value compensator according to another embodiment of the present invention.

FIG. 7 is a diagram comparing the amount of harmonic generation between the present invention and the conventional method.

FIG. 8 is a configuration diagram of another embodiment of the present invention.

FIG. 9 is an algorithm showing an operation of a harmonic suppression device according to another embodiment of the present invention.

FIG. 10 is a diagram showing a load state of a converter and a generation amount of a 17th harmonic.

FIG. 11 is a diagram showing a load state of a converter and a generation amount of a fifth harmonic.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 system power supply 2 receiving end 3 power converter 4 command value generator 5 PWM controller 51 comparator 52 triangular wave generator 6 carrier phase setting unit 7 converter unit 71 converter main circuit 72 transformer 8 power supply phase detector 9 phase command generation 10 Load device 11 Command value compensator 13 Harmonic suppression device

──────────────────────────────────────────────────続 き Continuing on the front page (72) Takashi Ikun 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Power & Electricity Development Division, Hitachi, Ltd. (72) Masahiro Tobiyo Omika-cho, Hitachi City, Ibaraki Prefecture 5-2-1, Hitachi, Ltd. Omika Plant (56) References JP-A-7-200084 (JP, A) JP-A-7-59351 (JP, A) JP-A-5-227757 (JP, A) JP-A-5-184156 (JP, A) JP-A-7-322629 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H02M 7/17 H02J 3/38 H02M 3 / 28 H02M 7/155 H02M 7/48

Claims (7)

(57) [Claims] 1. A power system comprising: a power system; one power receiving end of the power system; and a plurality of independent power converters connected to the power receiving end, wherein each of the power converters issues a command value for giving a command. A generator, a triangular carrier generator that generates a triangular carrier, and a PWM (pulse width modulation) that compares the command value with the triangular carrier and outputs a PWM pulse.
In a power conversion system having a controller and a converter unit, and controlling a switching element of the converter unit by the PWM pulse, each of the power conversion devices detects a power supply phase of the power system, , And synchronizes the triangular wave carrier with the power supply phase, and changes the phase of the triangular wave carrier for each of the power converters so as to cancel the harmonics generated by each of the converter units. Characterized in that the power conversion system generates a triangular wave carrier in advance. 2. An average value of command values of a power converter in a period (Δt) from a positive peak to a negative peak, which is a half cycle of a triangular wave carrier, or a period from a negative peak to a positive peak. Is estimated from the instantaneous value of the command value, and the value is used to compensate the command value of the power converter. 3. The phase of each triangular wave carrier determined according to claim 1 or 2, wherein a phase of a triangular wave carrier preset for each of said power converters is determined based on an operation state of each power converter. A power conversion system characterized by having a corner. 4. A power system, a power receiving end of the power system, and a plurality of independent power converters connected to the power receiving end, wherein each of the power converters issues a command value for giving a command. A generator, a triangular carrier generator that generates a triangular carrier, and a PWM (pulse width modulation) that compares the command value with the triangular carrier and outputs a PWM pulse.
In a power conversion system having a controller and a converter section, and controlling a switching element of the converter section by the PWM pulse, a means for detecting a power phase of the power system in each of the power conversion devices; Carrier phase setting means for presetting the phase of the triangular wave carrier so as to cancel harmonics generated by each converter section; and phase command generation for calculating the phase of the triangular wave carrier based on the power supply phase and the carrier phase set value. Means for generating a triangular wave carrier based on the calculated carrier phase. 5. The power converter according to claim 4, further comprising a command value compensating means for compensating a command value issued from a command value generator, wherein the command value compensating means has a positive cycle corresponding to a half cycle of the triangular wave carrier. Negative peak to negative peak, or
An average value of command values of the command value generator during a period (Δt) from a negative peak to a positive peak is estimated from an instantaneous value of the command value, and the estimated value is used as a compensation value. Power conversion system. 6. A harmonic control unit according to claim 4 or 5, further comprising: a harmonic control unit for giving a phase command to a carrier phase setting unit of each of the power converters.
A power conversion system, wherein a phase angle of a triangular wave carrier is determined based on an operation state of each of the power conversion devices. 7. The harmonic control unit according to claim 6, wherein the harmonic control unit reads an input current of each of the converter units and calculates a value representing an operation state of each of the converter units.
Power conversion characterized by setting a carrier phase angle, estimating a harmonic generated by each of the converter units based on the calculated value and the set phase angle, calculating and determining a harmonic generation amount at a power receiving end. system.