JP2022080066A - Power conversion device - Google Patents

Abstract

To provide a highly reliable power conversion device having improved operation continuity performance.SOLUTION: A power conversion device in an embodiment includes a power converter, and a converter control unit. The power conversion device can convert alternating current and direct current. The power converter includes a switching element for enabling an AC terminal voltage on the AC side to be switched. The converter control unit gives an operation command to the switching element. The converter control unit has a current control unit. The current control unit controls the AC current by calculating a command value of the AC terminal voltage based on the detection values of the AC voltage and the AC current flowing through the power converter. The current control unit has a correction unit. The correction unit estimates an AC disturbance voltage based on the AC current and the characteristic of the impedance between the AC voltage and the power converter, and corrects the command value of the AC terminal voltage based on the estimated disturbance voltage. The detection of the AC current for calculating the estimated disturbance voltage in the correction unit is executed at a substantially higher speed than that of the detection of the AC voltage.SELECTED DRAWING: Figure 1

Description

Embodiments of the present invention relate to a power conversion device.

In recent years, as a power converter, a modular multilevel converter (hereinafter referred to as MMC: Modular Multilevel Converter) has been put into practical use. The MMC is a power converter that includes an arm unit including a plurality of unit converters connected in series, and can handle high voltage and large capacity by adding the outputsable voltage of each unit converter. The power converter is connected between, for example, an alternating current system and a direct current system, and converts electric power to each other.

If an abnormality such as an accident occurs in the AC system or DC system connected to the power converter, the power converter should continue to operate so as not to impair the stability of the voltage and frequency of the connected AC system. Alternatively, after the abnormality is resolved, it is required to restart at high speed. As a means for protecting the power converter from failure when an abnormality occurs, there is a method of stopping (gate blocking) the switching control of the semiconductor element.

By temporarily setting the gate block state as soon as possible after the occurrence of a system accident, it is possible to suppress the increase in the current flowing through the converter (converter current), but in reality, the gate block is detected from the accident detection. There are various delay times associated with detection and calculation, and it may not be possible to suppress the converter current within a predetermined range. If the transducer current exceeds the equipment capacity, the semiconductor element may fail. Further, even if the capacity of the device is not exceeded, by cutting off the excessive current, the energy is stored in the capacitor of the unit converter, and the capacitor voltage may rise. When the capacitor voltage rises and exceeds the overvoltage protection level, it is necessary to stop the operation for a long time until the voltage value is restored to the normal value by the discharge.

Japanese Unexamined Patent Publication No. 2017-138473

S. Nagai, H. N. Le, T. Nagano, K. Orikawa and J. Itoh, “Minimization of interconnected inductors for single-phase inverter with high-performance disturbance observer,” IPEMC-ECCE Asia 2016, pp. 3218-3225 (2016) -05)

An object to be solved by the present invention is to provide a highly reliable power conversion device having improved operation continuation performance.

The power converter of the embodiment includes a power converter and a converter control unit. The power converter can convert alternating current and direct current. The power converter includes a switching element that enables switching of the AC terminal voltage on the AC side. The converter control unit gives an operation command to the switching element. The converter control unit has a current control unit. The current control unit controls the alternating current by calculating the command value of the alternating current terminal voltage based on the detected values of the alternating current voltage and the alternating current flowing through the power converter. The current control unit has a correction unit. The correction unit estimates the AC disturbance voltage based on the characteristics of the AC current and the impedance between the AC voltage and the power converter, and corrects the command value of the AC terminal voltage based on the estimated disturbance voltage. The AC current detection for calculating the estimated disturbance voltage by the correction unit is executed substantially faster than the AC voltage detection.

The figure which shows an example of the structure of the power conversion apparatus 10 of 1st Embodiment. The figure which shows an example of the structure of the power converter 20 of 1st Embodiment. The figure which shows another example of the structure of the power converter 20 of 1st Embodiment. The figure which shows an example of the structure of the cell CL of 1st Embodiment. The figure which shows an example of the processing of the exchange information calculation unit 110 of 1st Embodiment. The figure which shows an example of the processing of the current control unit 120 of 1st Embodiment. The figure which shows an example of the operation of the power conversion apparatus 10 of 1st Embodiment. The figure which shows an example of the structure of the power conversion apparatus 10 of 2nd Embodiment. The figure which shows an example of the system accident detection part 170 of the 2nd Embodiment.

Hereinafter, the power conversion device 10 of the embodiment will be described with reference to the drawings.

(First Embodiment)
FIG. 1 is a diagram showing an example of the configuration of the power conversion device 10 of the first embodiment. The power conversion device 10 is provided at the interconnection point between the AC system and the DC system, and converts the AC power supplied by the AC system and the DC power supplied by the DC system. The AC system may be an AC power supply ACV or an AC load, and the DC system may be a DC power supply DCV or a DC load. The power converter 10 includes a power converter 20 and a converter control unit 100.

The power converter 20 mutually converts AC power and DC power based on the control of the converter control unit 100. The power converter 20 is a circuit configured by using a self-extinguishing switching element such as an IGBT or MOSFET. In the first embodiment, the power converter 20 is a modular multi-level converter (hereinafter, MMC: Modular Multilevel Converter). An interconnection inductor L _tr is provided between the power converter 20 and the grid interconnection point P1. The interconnection inductor L _tr may be partially or wholly replaced by a leak reactance of a reactor or a transformer.

The converter control unit 100 includes an AC information calculation unit 110, a current control unit 120, an overvoltage determination unit 150, an overcurrent determination unit 140, and a gate command generation unit 160. The converter control unit 100 has, for example, an AC information calculation unit 110, a current control unit 120, and an overvoltage by executing a program (software) stored in a storage unit (not shown) by a hardware processor such as a CPU. The determination unit 150, the overcurrent determination unit 140, and the gate command generation unit 160 are realized as functional units. Further, some or all of these components may be realized by hardware such as LSI, ASIC, FPGA, GPU, or may be realized by the cooperation of software and hardware.

The AC information calculation unit 110 uses the detection values of the grid interconnection point voltages Vsr, Vss, and Vst to calculate the variables Vsd and Vsq on the rotation coordinate axis synchronized with the grid voltage detection phase theta.

Although not shown, the current control unit 120 calculates variables Isd and Isq on the rotation coordinate axis synchronized with the system voltage detection phase theta using the detected values of the AC currents Isr, Iss, and Ist. The current control unit 120 performs general proportional integral control or the like on the grid interconnection point voltages Vsd, Vsq, AC currents Isd, and Isq, and is a voltage command for controlling the AC currents Isr, Iss, and Ist to predetermined values. The values Vr *, Vs *, and Vt * are output. The current control unit 120 further includes a disturbance observer 130 that estimates the disturbance voltage based on the detected values of the alternating currents Isr, Iss, and Ist (variable-transformed Isd, Isq) and cancels the influence thereof.

The overcurrent determination unit 140 is in advance so that the detected values of the arm currents Ipr, Inr, Ips, Inr, Ipr, Inr and the AC currents Isr, Iss, and Ist inside the power converter do not exceed the equipment withstand capacity of the power converter 20. When the set threshold value is compared with the arm currents Ipr, Inr, ... And, for example, absolute values of the AC currents Isr, Iss, and Ist, and the threshold value is detected, a valid overcurrent detection signal OC is generated. Output to unit 160.

The overvoltage determination unit 150 compares a preset threshold value so that the capacitor voltage Vc1 inside the power converter, ..., Does not exceed the equipment withstand capacity of the power converter 20, and the capacitor voltage Vc1, ... If detected, a valid overvoltage detection signal OV is output to the gate command generation unit 160.

The gate command generation unit 160 sets the voltage command values Vr *, Vs *, Vt * output by the current control unit 120 as the AC terminal voltage output command value, and the DC terminal voltage output command value (constant or DC current) (not shown). The gate command gtp, gtn, ... Is given to the switching element inside the power converter so that each voltage command value is output in a pseudo manner to the AC terminal and the DC terminal. And output. Further, when a valid overvoltage detection signal OV or overcurrent detection signal OC is input, the gate command generation unit 160 stops switching control, so that all gate commands are set to zero. Further, in the following description, the parameters expressed in the s region by Laplace transforming the voltage and current values are also used. In that case, the grid interconnection point voltage is Vs (s), the AC current is Is (s), the converter AC terminal voltage is V (s), and the converter AC terminal voltage command value is V * (s).

Next, an example of the configuration of the power converter 20 will be described. FIG. 2 is a diagram showing an example of the configuration of the power converter 20 of the first embodiment. As shown in FIG. 2, the power converter 20 includes a plurality of leg LGs between the positive electrode of the DC system (terminal P shown) and the negative electrode of the DC system (terminal N shown).

The number of leg LGs corresponds to, for example, the number of phases of AC power supplied by the AC system. In the present embodiment, the AC system supplies three phases of AC power: a first phase (R phase shown in the figure), a second phase (S phase shown in the figure), and a third phase (T phase shown in the figure). Therefore, the power converter 20 includes a leg LGr corresponding to the R phase, a leg LGs corresponding to the S phase, and a leg LGt corresponding to the T phase. In the following description, when the leg LGr, the leg LGs, and the leg LGt are not distinguished from each other, they are collectively referred to as "leg LG".

A phase of the three phases of AC power supplied by the AC system is connected to the leg LG. The leg LG may be connected to a phase via a transformer, if desired. Specifically, the R phase is connected to the leg LGr, the S phase is connected to the leg LGs, and the T phase is connected to the leg LGt. In the following description, the connection point between the leg LGr and the R phase is described as the connection point CPr, the connection point between the leg LGs and the S phase is described as the connection point CPs, and the connection between the leg LGt and the T phase is described. The point is referred to as a connection point CPt. In the following description, when the connection point CPr, the connection point CPs, and the connection point CPt are not distinguished from each other, they are simply described as the connection point CP. Further, in the following description, the portion having the same potential as the terminal P of the DC voltage output by the power converter 20 is also described as the terminal P of the leg LG, and the portion having the same potential as the terminal N of the DC voltage is described. It is also described as the terminal N of the leg LG. Each leg LG has a similar configuration to each other. In the following description, "r" is added to the end of the code for the configuration related to the leg LGr, "s" is added to the end of the code for the configuration related to the leg LGs, and "s" is added to the end of the code for the configuration related to the leg LGt. , Add "t" to the end of the code. Further, when it is not distinguished from each other which leg LG has the configuration, "r", "s", or "t" is omitted. Hereinafter, the leg LGr will be described on behalf of each leg LG.

The leg LGr includes two sets of n cell CL groups (cells CL1-1r to CL1-nr shown in the figure and cells CL2-1r to CL2-nr) and a plurality of reactors RT (reactors RT1r and RT2r shown in the figure). To prepare for. Here, n is a natural number. The cell CL is, for example, a half-bridge circuit, and the details of its configuration will be described later. Here, the cell CL group from the terminal P of the leg LG to the connection point of each phase is also described as a positive arm unit. Further, the cell CL group between the connection point of each phase and the terminal N of the leg LG is also described as a negative arm unit.

Cells CL1-1r to CL1-nr are connected in series to the positive arm unit of the leg LGr from the terminal P side toward the connection point CPr side in the order described, and these are connected to the connection point CPr via the reactor RT1r. Be connected. Further, cells CL2-1r to CL2-nr are connected in series to the negative arm unit of the leg LGr from the connection point CPr side toward the terminal N side in the order described, and these are connected via the reactor RT2r. Connected to CPr.

The leg LGr has a current detector (not shown) that detects a positive arm current (not shown) flowing from the connection point CP to the terminal P, and a current detector (not shown) that flows from the terminal N to the connection point CP. A current detector (not shown) for detecting the negative arm current (R-phase negative side current Inr (not shown)) may be provided. The AC current Isr may be detected directly by providing a current detector at the AC side terminal separately, or it may be indirectly detected by calculating from the difference between the detected positive arm current and the negative arm current Ipr-Inr. May be good.

The interconnection inductor Ltr in FIG. 1 is an equivalent interconnection impedance that collectively represents the effective inductance for the AC currents Isr, Iss, and Ist of each phase. It is 0.5 times the inductance value of the reactor RT of. That is, assuming that the inductance value of the reactor RT is L, Ltr = L / 2 [H]. Further, when a reactor or a transformer having an inductance L'is connected to the AC terminal side, Ltr = L'+ L / 2 [H]. If the power converter 20 is not an MMC and has a configuration like a general two-level converter that does not have a reactor on the arm, the inductance value of the reactor simply connected to the AC terminal or the transformer leakage reactance will be Ltr. Match.

Next, another example of the configuration of the power converter 20 will be described. FIG. 3 is a diagram showing another example of the configuration of the power converter 20 of the first embodiment. In FIG. 3, the reactor RT of FIG. 2 is replaced with a transformer having a special winding structure having a leakage reactance that replaces the function of the reactor. The positive arm unit and the negative arm unit are connected to each other via the first and second windings of a transformer having leakage reactance, and further, a third winding electrically isolated from these windings. It is connected to the AC system etc. via.

For the inductance value (equivalent interconnection impedance) corresponding to the interconnection inductor Ltr in FIG. 1, the total short-circuit inductance between the terminals of the first and second windings is L, and between the terminals of the third and first windings. If both the short-circuit inductance of the above and the short-circuit inductance between the terminals of the third and second windings are L'(almost equal values), it can be calculated by Ltr = L'-L / 4 [H].

Next, the configuration of the cell CL will be described. FIG. 4 is a diagram showing an example of the configuration of the cell CL of the first embodiment. As described above, the cell CL is, for example, a half-bridge circuit. As shown in FIG. 4, the cell CL includes, for example, a plurality of switching elements Q (switching elements Q1 to Q2 in the figure), a number of diodes D (diodes D1 to D2 in the figure) corresponding to the switching element Q, and a capacitor C. And prepare. The switching element Q is, for example, an isolated gate bipolar transistor (hereinafter referred to as an IGBT: Integrated Gate Bipolar Transistor). However, the switching element Q is not limited to the IGBT. The switching element Q may be any self-extinguishing switching element capable of realizing the function of a converter or an inverter. In this embodiment, a case where the switching element Q is an IGBT will be described.

The switching element Q1 and the switching element Q2 are connected in series with each other. The switching element Q1 and the switching element Q2 and the capacitor C are connected in parallel with each other. Each switching element Q and the diode D are connected in parallel with each other. Specifically, the switching element Q1 and the diode D1 are connected in parallel with each other, and the switching element Q2 and the diode D2 are connected in parallel with each other.

The cell CL includes a positive electrode terminal connected to the terminal P side of the leg LG and a negative electrode terminal connected to the terminal N side. The positive electrode terminal of the cell CL is connected to the connection point between the switching element Q1 and the switching element Q2, and the negative electrode terminal of the cell CL is connected to the emitter terminal of the switching element Q2. In the following description, the voltage generated between the positive electrode terminal and the negative electrode terminal of the cell CL will be referred to as the cell voltage Vo.

Each switching element Q is provided with a switching terminal (not shown) for switching on / off of the switching element Q. The switching terminal is connected to the converter control unit 100, and a control signal is input. Specifically, the first gate signal gtp is input to the switching element Q1 as a control signal, and the second gate signal gtn is input to the switching element Q2 as a control signal. By switching each switching element Q on or off based on the control signal, the capacitor C included in the cell CL is charged or discharged. Further, the cell CL is provided with a voltage detector (not shown) that detects the capacitor voltage Vc, which is the voltage of the capacitor C.

When the control signal for turning on the switching element Q is expressed as "1" and the control signal for turning off the switching element Q is expressed as "0", the cell voltage Vo is when (gtp, gtn) = (1, 0). , The capacitor voltage Vc, and when (gtp, gtn) = (0, 1), it becomes 0 [V]. By switching the switching element Q included in each leg LG in this way, a multi-level waveform can be generated.

It is prohibited to set the switching element Q to (gtp, gtn) = (1, 1) because the capacitor C is short-circuited. Further, in order to prevent the state of the switching element Q from transiently becoming (gtp, gtn) = (1, 1) at the time of switching, the switching element Q is usually transiently (gtp, gtp,) for a very short time. It is controlled to the state (dead time) of gtn) = (0,0). Further, when the switching control of the switching element Q is stopped, it is realized by fixing it in the state of (gtp, gtn) = (0,0). Stopping the switching control of all the switching elements Q of the power converter 20 is called a gate block, and the state is called a gate block state.

Next, the processing of the AC information calculation unit 110 will be described. FIG. 5 is a diagram showing an example of processing of the AC information calculation unit 110 of the first embodiment. As shown in FIG. 5, the AC information calculation unit 110 includes a conversion unit 111, a PI calculation unit 112, an addition unit 113, and an oscillator 114 as functional units.

The conversion unit 111 acquires information indicating the grid interconnection point voltage (R-phase voltage Vsr, S-phase voltage Vss, and T-phase voltage Vst) detected by the voltage detector. The conversion unit 111 converts (calculates) the acquired R-phase voltage Vsr, S-phase voltage Vss, and T-phase voltage Vst into AC system active voltage Vsd and AC system invalid voltage Vsq using the equation (1). .. The AC system voltage phase theta is a value output by the oscillator 114 described later, and is a value indicating the voltage phase of a reference phase (R phase in this example) of the AC system.

The PI calculation unit 112 is a frequency difference (hereinafter, frequency) between the frequency of the AC system voltage interconnected by the power converter 20 and the reference AC system frequency fs0 based on the AC system invalid voltage Vsq converted by the conversion unit 111. The difference Δfpll) is calculated. The frequency difference Δfpll takes a positive value when the frequency of the AC system voltage is higher than the reference AC system frequency fs0, and takes a negative value when the frequency is lower than the reference AC system frequency fs0. The reference AC system frequency fs0 is the rated frequency of the interconnected AC system, and is, for example, a constant of 50 [Hz] or 60 [Hz]. The frequency difference Δfpll continues to increase or decrease until the calculated value of the AC system invalid voltage Vsq input to the PI calculation unit 112 becomes zero, and the value of the difference between the actual AC system frequency and the reference AC system frequency fs0. Converges to.

The addition unit 113 adds the frequency difference Δfpll calculated by the PI calculation unit 112 to the reference AC system frequency fs0. In the following description, the frequency obtained by adding the frequency difference Δfpll to the reference AC system frequency fs0 will be referred to as the AC frequency fpll.

The oscillator 114 outputs an AC system voltage phase theta that repeats and monotonically increases from a minimum value of 0 to a maximum value of 2π based on the frequency of the AC frequency fpll calculated by the addition unit 113. As described above, the AC system voltage phase theta is used for conversion of the AC system active voltage Vsd and the AC system invalid voltage Vsq of the conversion unit 111, and for AC current control. In AC current control, when non-interference current control based on variables on general rotational coordinates is applied, AC system voltage phase theta is used for rotational coordinate conversion for voltage / current values or inverse conversion (fixed coordinate conversion). Is used. By the above processing, the AC information calculation unit 110 obtains the AC system voltage phase theta by repeating the calculation of the AC system voltage phase theta so that the calculated value of the AC system invalid voltage Vsq in the conversion unit 111 becomes zero. ..

Next, an example of the processing of the current control unit 120 will be described using the parameters in the s region, the control block, and the circuit model. FIG. 6 is a diagram showing an example of processing of the current control unit 120 of the first embodiment. The processing of the current control unit 120 is performed based on, for example, the voltage / current detection value and the command value converted into variables on the rotation coordinate axis. The current control unit 120 detects the AC current Is (s) via the AC current detection filter (including delay) Ffb (s), and obtains the deviation from the AC current command value Is * (s). Next, the current control unit 120 obtains the control operation amount Vacr * (s) by multiplying the obtained deviation by the current control gain Gacr (s). The current control gain Gacr (s) has, for example, the characteristics of a general PI controller.

The current control unit 120 further detects the grid interconnection point voltage Vs (s) via the AC voltage detection filter (including delay) Fff (s), and from Fff (s) Vs (s) to Vacr'* (s). ) Is subtracted to obtain the converter AC terminal voltage command value V * (s). Note that Vacr'* (s) is a control operation amount obtained by subtracting the estimated disturbance voltage (output value) Edis'(s) from Vacr * (s). Here, by using the detected value of the grid interconnection point voltage Vs (s) for the calculation to the converter AC terminal voltage command value V * (s), the fluctuation of the grid interconnection point voltage is feed-forward compensated. Equivalent to being. The converter AC terminal voltage V (s) is output according to the value of V * (s), but in reality, V * (s) is multiplied by the gate generator and switching control characteristics Hpwm (s). Is output.

After that, a voltage obtained by adding the disturbance voltage Vdis (s) to the difference between the grid interconnection point voltage Vs (s) and the converter AC terminal voltage V (s) was applied to the interconnection inductor Ltr, and the applied voltage was adjusted. AC current Is (s) flows.

Here, the calculation of the estimated disturbance voltage (output value) Edis'(s) will be described. The disturbance observer 130 obtains an estimated disturbance voltage (calculated value) Edis (s) using the following equation (2).

However, Fobs (s) represents an observer filter, and ltr represents a nominal inductance value of the interconnection inductor Ltr. The observer filter is, for example, a low frequency pass filter that removes noise. The calculation of the estimated disturbance voltage (calculated value) Edis (s) is not limited to the configuration of the equation (2) and FIG. For example, a specific observer filter characteristic formula may be substituted into Fobs (s), and the calculation formula and the control block may be equivalently converted based on the characteristic formula.

Edis (s) ≈ Edis'(s), that is, the calculated value of the estimated disturbance voltage and the output value are almost equal, and when Ltr≈ltr and Ffb (s) Hpwm (s) ≈ 1 are further calculated, the estimated disturbance is calculated. The voltage (calculated value) Edis (s) is expressed by the following equation (3).

The disturbance observer 130 estimates and estimates the error due to the delay included in the Fff (s) of the grid interconnection point voltage Vs (s) detection and the disturbance voltage Vdis (s) using the equation (3), and estimates Edis (s). By subtracting s) from the control operation amount Voltage * (s) in advance, these effects are canceled out. However, this cancellation detects the AC current Is (s) used in the estimation calculation with a delay time smaller than the grid interconnection point voltage Vs (s), and Ffb (s) Hpwm (s) ≈1 as described above. It works effectively when the equation (3) is established when it can be regarded as.

When the grid interconnection point voltage fluctuates abruptly due to a grid accident, the grid interconnection point voltage Vs (s) is detected and feed-forward compensated by the current control unit 120 as in the first embodiment. , The influence of the disturbance such as the sudden change in the system voltage can be suppressed to a certain extent. However, if the detection speed of the grid interconnection point voltage Vs (s) is low, the sudden fluctuation cannot be immediately reflected, and the effect of feed-forward compensation is reduced. Therefore, by estimating the disturbance based on the AC current Is (s) detected faster than the grid interconnection point voltage Vs (s), it responds to the fluctuation of the grid interconnection point voltage at high speed and cancels the influence. be able to.

Therefore, in order to effectively operate the disturbance voltage estimation calculation based on the AC current detection in the control configuration in which the grid interconnection point voltage is detected and the feed forward is compensated for the current control as in the first embodiment. It is necessary to make the substantial delay time of AC current detection shorter than the substantial delay time of the grid interconnection point voltage, that is, to detect the AC current faster than the grid interconnection point voltage. The substantial delay time is a delay time evaluated by comprehensively adding the dead time associated with detection sampling and signal communication transmission and the settling delay time (time constant) by the filter. Performing detection substantially faster means configuring the detection system so as to reduce the substantial delay time.

The current control unit 120 further outputs the value Edis'(s) obtained by subjecting the calculated estimated disturbance voltage Edis (s) to the dead zone and the limiter processing by the estimated disturbance calculation, and finally outputs the control operation amount. It is a value to be subtracted from Vacr * (s). The dead zone processing is a processing in which Edis ′ (s) = 0 when the absolute value of the calculated Edis (s) is smaller than a preset threshold value. As a result, compensation is enabled only when a system accident occurs, the system voltage suddenly fluctuates, and the estimated disturbance voltage Edis (s) becomes large. The reason for providing the dead zone processing is that in steady normal operation where abnormalities such as system accidents do not occur, compensation by estimated disturbance voltage continues stable normal operation depending on the state of system impedance and load. This is because it may hinder the operation. When the dead band processing is not provided, for example, the compensation control by the estimated disturbance voltage may interfere with the impedance of the system and the operation of other devices, the signal in the specific frequency band may be amplified, and harmonics may be excessively generated. Since the harmonics adversely affect the equipment connected to the system, it is necessary to stop the power conversion device 10 depending on the generation level. Alternatively, the alternating current is likely to be distorted while the operating condition is being restored immediately after the system accident is restored, but if the estimated disturbance voltage is output as it is at that time, the distortion will be excessively amplified and the operation may become unstable. be. By setting a dead zone so that it does not react to a minute disturbance voltage, such an unstable phenomenon can be prevented.

Further, the limiter process is a process of limiting the absolute value of the output amount to a threshold value when the calculated absolute value of Edis (s) is larger than a preset threshold value. This prevents, for example, the AC terminal voltage command value V * (s) including Edis'(s) from exceeding the maximum output voltage of the power converter 20. Therefore, the absolute value threshold value of the limiter is required to be equal to or less than the maximum output voltage of the power converter 20, and the absolute value threshold value of the dead zone is further required to be equal to or less than the absolute value threshold value of the limiter. The absolute value threshold value of the dead zone is a value larger than the estimated disturbance voltage calculated during steady normal operation in which an abnormality such as a system accident has not occurred.

Next, the operation of the power conversion device 10 will be described. FIG. 7 is a diagram showing an example of the operation of the power conversion device 10 of the first embodiment. When a grid accident occurs at time T0, the grid interconnection point voltage drops sharply. The AC system active voltage Vsd also drops sharply. On the other hand, the detection value of the AC system active voltage Vsd obtained by the AC information calculation unit 110 in FIG. 5 reflects the voltage drop after the delay time associated with the detection / calculation processing.

The maximum value Ismax (absolute value) of the alternating current rises immediately after the grid interconnection point voltage drops. This is because a large potential difference occurs between the grid interconnection point voltage and the AC terminal voltage of the converter. Since the current control unit 120 detects the AC current at a higher speed than the grid interconnection point voltage, the estimated disturbance voltage Edis in the decreasing direction can be immediately obtained based on the increase in the AC current detection value. Since the AC current is changing rapidly, the estimated disturbance voltage Edis exceeds the dead zone threshold ± α, and from that time T1, a valid estimated disturbance voltage Edis'is output and the AC terminal voltage command value V * is corrected. do.

In this way, the AC terminal voltage follows the actual grid interconnection point voltage drop in a shorter time than the drop in the grid interconnection point voltage is reflected in the control by the detection and calculation processing of the AC grid active voltage Vsd. By correcting so as to lower the command value, the potential difference between the grid interconnection point voltage and the AC terminal voltage of the converter can be reduced, and a further increase in the AC current can be suppressed.

If the correction by the estimated disturbance voltage is not performed, or if the AC current detection speed is slower than the grid interconnection point voltage detection speed, the period from T0 to T2, which corresponds to the detection delay of the grid interconnection point voltage, The AC terminal voltage still does not drop from the state before the system accident occurred, and the AC current continues to rise. Once the AC current increases, even if the power converter 20 is subsequently put into the gate block state and the AC current is cut off, the increased current energy is charged to the capacitors of each cell, leading to operation stop due to overvoltage detection OV. The risk is high. By correcting with the estimated disturbance voltage and suppressing the current rise, even if the switching control is continued as it is or if the gate block state is temporarily set, the operation stop due to overcurrent or overvoltage detection is prevented and the system accident. It is possible to quickly restart the operation after removal.

Since the estimated disturbance voltage is further subjected to the limiter processing of the threshold value ± β, it is prevented that the estimated disturbance voltage becomes excessive and the AC terminal voltage command value exceeds the output voltage of the converter. There is. Further, the calculated value Edis of the estimated disturbance voltage includes a vibration component based on the steady distortion of the AC current even in the period before the occurrence of the system accident T0, but it is smaller than the disturbance voltage at the time of the occurrence of the system accident. Since it is less than the dead zone threshold ± α, it is not included in Edis'and is not reflected in the control. This makes it possible to prevent the harmonics from being amplified during normal operation.

According to the first embodiment described above, the power conversion device 10 capable of converting AC and DC includes a power converter 20 including a switching element capable of switching the AC terminal voltage on the AC side, and the switching. The converter control unit 100 includes a converter control unit 100 that gives an operation command to the element, and the converter control unit 100 has the AC terminal voltage based on the detected values of the AC voltage and the AC current flowing through the power converter 20. A current control unit 120 that controls the AC current by calculating a command value is provided, and the current control unit 120 determines the characteristics of the impedance between the AC current, the AC voltage, and the power converter. A correction unit for estimating the disturbance voltage of the AC based on the estimation and correcting the command value of the AC terminal voltage based on the estimated disturbance voltage is provided, and the correction unit detects the AC current for calculating the estimated disturbance voltage. Is executed substantially faster than the detection of the AC voltage. As a result, the followability of the AC terminal voltage of the converter is improved by estimating and canceling the disturbance due to the drop in the grid interconnection point voltage at high speed. As a result, the increase in the alternating current flowing from the system to the converter can be suppressed in a very short time, and the risk of the converter operation stop due to overcurrent / overvoltage can be reduced. Further, by calculating the estimated disturbance voltage based on the current detection value having a high detection speed, even when the voltage fluctuation feed forward compensation by the grid interconnection point voltage detection is applied, the disturbance suppression compensation at high speed can be effectively performed.

(Second embodiment)
Next, the configuration of the power conversion device 10 of the second embodiment will be described. FIG. 8 is a diagram showing an example of the configuration of the power conversion device 10 of the second embodiment. The converter control unit 100 further has a system accident detection unit 170 as a functional unit. The system accident detection unit 170 detects an abnormality in the system voltage using the AC system active voltage Vsd, and outputs an accident detection signal FLT.

The current control unit 120 that has received the effective system accident detection signal FLT has the characteristics of either one or both of the AC current detection filter Ffb (s) and the grid interconnection point voltage detection filter Fff (s) in FIG. To substantially nullify the sensitivity reduction by the filter or mitigate the amount of sensitivity reduction. Relaxing the amount of sensitivity reduction corresponds to shortening the equivalent delay time of the filter and speeding up the detection.

The characteristics of the filter include, for example, the characteristics of a low-pass filter, are set to reduce detection noise and the like, and to ensure steady stability during normal operation. When a valid accident detection signal FLT is input, the sensitivity reduction is substantially disabled by disabling the filter processing and outputting the input signal as it is, or by increasing the cutoff frequency of the low frequency pass filter. Or reduce the amount of sensitivity reduction. When the cutoff frequency of the low frequency pass filter is increased to a high frequency, the attenuation factor of the filter is reduced from the low frequency region, and the amount of sensitivity reduction is relaxed.

Next, the configuration of the system accident detection unit 170 of the power conversion device 10 will be described. FIG. 9 is a diagram showing an example of the system accident detection unit 170 of the second embodiment. The system accident detection unit 170 includes a comparator 171 and a comparator 172, an OR operation unit 173, and a timer 174.

In the case of three-phase equilibrium, for example, the AC system effective voltage Vsd corresponds to the amplitude absolute value of the AC system voltage in the system interconnection point voltages Vsd and Vsq on the rotation coordinate axis synchronized with the system voltage detection phase theta. In the normal state where there is no abnormality such as a ground fault in the AC system, the instantaneous value of Vsd becomes a substantially constant value within the normal voltage fluctuation range. On the other hand, for example, when three phases of an AC system have a ground fault at the same time and a three-phase equilibrium accident occurs, Vsd decreases according to the voltage decrease rate and deviates from the normal range. Further, for example, when any one or two phases of the AC system have a ground fault and the three-phase equilibrium is lost, Vsd becomes oscillating and deviates from the normal range.

The comparator 171 compares the AC system effective voltage Vsd with the AC system voltage threshold value Vth_H. The comparator 171 outputs a valid upper limit excess signal OV when Vsd> Vth_H. The comparator 172 compares the AC system active voltage Vsd with the AC system undervoltage threshold Vth_L. The comparator 172 outputs a lower limit exceeding signal UV effective when Vsd <Vth_L. The range between Vth_L and Vth_H is an example of a predetermined range of the absolute value of the AC voltage.

The OR calculation unit 173 calculates the OR between the upper limit excess signal OV output by the comparator 171 and the lower limit excess signal UV output by the comparator 172. The OR unit 173 outputs a valid system voltage abnormality signal ERR when the upper limit excess signal OV or the lower limit excess signal UV is valid. That is, when the absolute value of the AC system active voltage Vsd is not within the predetermined range, the effective system voltage abnormality signal ERR is output.

When the valid system voltage abnormality signal ERR is input from the disjunction calculation unit 173, the timer 174 outputs a valid accident detection signal FLT during the period Tt. The period Tt is set to, for example, the period until the AC circuit breaker disconnects the accident line and the AC system voltage is restored. In a general AC system protection system, this period is about several times the AC system voltage cycle.

Further, in the second embodiment, the system voltage abnormality signal ERR is switched by comparing the AC system active voltage Vsd with the threshold value. For example, the AC system active voltage Vsd and the AC system invalid voltage based on the equation (4) are switched. The combined voltage vector value Vsdq of Vsq may be calculated and compared with the threshold value, and almost the same result can be obtained.

According to the second embodiment described above, by substantially disabling the sensitivity reduction of the detection filter when the system voltage is abnormal or relaxing the sensitivity reduction amount, for example, the performance of the grid interconnection point voltage feed forward compensation. Can be improved. Further, by reflecting the AC current detected at a higher speed in the disturbance voltage estimation calculation, the compensation effect by the estimated disturbance voltage can be enhanced. As a result, the followability of the AC terminal voltage of the converter when the system voltage is abnormal can be improved, the increase of the AC current flowing into the converter can be suppressed, and the risk of inability to continue operation due to overcurrent / overvoltage can be further reduced.

According to at least one embodiment described above, it is possible to provide a highly reliable power conversion device having improved operation continuation performance.

(Other variants)
Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and variations thereof are included in the scope of the invention described in the claims and the equivalent scope thereof, as are included in the scope and gist of the invention.

10 Power converter 20 Power converter 100 Converter control unit 110 AC information calculation unit 120 Current control unit 130 Disturbance observer 140 Overcurrent determination unit 150 Overvoltage determination unit 160 Gate command generation unit 170 System accident detection unit

Claims (9)

It is a power conversion device that can convert between alternating current and direct current.
A power converter including a switching element capable of switching the AC terminal voltage on the AC side, and
A converter control unit that gives an operation command to the switching element,
Equipped with
The converter control unit controls the AC current by calculating a command value of the AC terminal voltage based on the detected values of the AC voltage and the AC current flowing through the power converter. Prepare,
The current control unit estimates the AC disturbance voltage based on the characteristics of the AC current, the AC voltage, and the impedance between the power converter, and commands the AC terminal voltage based on the estimated disturbance voltage. Equipped with a correction unit that corrects the value
The detection of the alternating current for calculating the estimated disturbance voltage by the correction unit is executed substantially faster than the detection of the voltage of the alternating current.
Power converter. The correction unit sets the impedance characteristics between the AC voltage and the power converter into a value obtained by multiplying the AC current and a value based on the difference between the AC current and the command value of the AC current. Calculate the estimated disturbance voltage based on the
The power conversion device according to claim 1. The correction unit includes correction value adjusting means for selectively outputting an effective correction value that is not substantially zero according to the state of the AC voltage or the AC current.
The power conversion device according to claim 1 or 2. The correction value adjusting means sets the correction value to zero when the absolute value of the estimated disturbance voltage is less than the first threshold value, and sets the estimated disturbance voltage when the absolute value of the estimated disturbance voltage is equal to or more than the first threshold value. Executes the processing of the dead zone as the correction value,
The power conversion device according to claim 3. The correction value adjusting means limits the absolute value of the correction value to the second threshold value when the absolute value of the estimated disturbance voltage exceeds the second threshold value, and the absolute value of the estimated disturbance voltage becomes the second threshold value. In the following cases, the limiter process using the estimated disturbance voltage as the correction value is executed.
The power conversion device according to claim 3 or 4. The current control unit includes a sensitivity reduction unit that reduces and detects the sensitivity of the AC voltage or the harmonic component of the AC current in a predetermined frequency range.
The sensitivity reducing unit substantially invalidates the reduction of the sensitivity when the absolute value of the AC voltage is not within a predetermined range, or alleviates the amount of the sensitivity reduction.
The power conversion device according to any one of claims 1 to 5. The sensitivity reducing unit executes a process including a low frequency pass filter having a cutoff frequency capable of attenuating harmonic components in a predetermined frequency range of the AC voltage or the AC current.
The sensitivity reduction unit relaxes the sensitivity reduction amount by adjusting the cutoff frequency to a higher frequency than the original state.
The power conversion device according to claim 6. The power converter
A first arm unit in which a plurality of unit converters including a capacitor whose charge and discharge can be switched by a switching element are connected in series, and
A second arm unit in which a plurality of the unit converters are connected in series is provided.
A first series circuit configured by further connecting a reactor in series at one end of the first arm unit or at an arbitrary position between the unit converters, and one end of the second arm unit or the unit. A second series circuit configured by further connecting a reactor in series at an arbitrary position between the converters, and a terminal connected to each other as a terminal connected to the AC voltage are used.
The correction unit is obtained by adding an inductance value of substantially 1/2 of the inductance value of the reactor to an impedance value between the AC voltage and the power converter excluding the two reactors. The estimated disturbance voltage is calculated based on the value obtained by multiplying the AC current by the characteristic corresponding to the equivalent interconnection impedance and the value based on the difference between the AC current and the command value of the AC current.
The power conversion device according to any one of claims 1 to 7. The power converter
A first arm unit in which a plurality of unit converters including a capacitor whose charge and discharge can be switched by a switching element are connected in series, and
A second arm unit in which a plurality of the unit converters are connected in series is provided.
The first arm unit and the second arm unit are connected to each other via a first winding and a second winding of a transformer having a leakage reactance to each other.
A third winding, which is electrically isolated from the first winding and the second winding, is connected to the AC voltage.
The correction unit is based on the short-circuit inductance value between the terminals of the third winding and the first winding, or the short-circuit inductance value between the terminals of the third winding and the second winding. The AC current has characteristics corresponding to the equivalent interconnection impedance obtained by subtracting an inductance value of substantially 1/4 of the short-circuit inductance value between the terminals of the first winding and the second winding. The estimated disturbance voltage is calculated based on the value multiplied by and the value based on the difference between the AC current and the command value of the AC current.
The power conversion device according to any one of claims 1 to 7.

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