JP6599804B2 - Power conversion apparatus and control method thereof - Google Patents

Description

  The present invention relates to a power conversion device and a control method thereof.

  2. Description of the Related Art Conventionally, a power conversion device that links a distributed power source such as a solar cell to a system power source is known. If the power conversion device continues to operate independently without being disconnected from the system power supply at the time of a power failure of the system power supply, there is a possibility that a problem may arise from the viewpoint of ensuring safety and ensuring the reliability of power supply. For this reason, the conventional power converter device is provided with the function which detects an independent operation (for example, refer to patent documents 1). When the power conversion device detects an isolated operation, the power conversion device disconnects itself from the system power supply.

  Non-Patent Document 1, which defines a new interconnection regulation for PCS (power conditioning system) for PV (photovoltaic) systems, discloses a frequency feedback method with step injection as a common technique for performing active islanding detection. FIG. 7 is a block diagram showing a configuration of a conventional power conversion device 100 having a frequency feedback-equipped single operation detection function with step injection.

  The power converter (PCS) 100 is disposed between the distributed power source 2 and the system power source 3. Although not shown in the figure, between the power converter 100 and the system power supply 3, a pole transformer or a low-voltage distribution that lowers the high-voltage power sent from the high-voltage distribution line to the low-voltage distribution line and supplies it to the low-voltage distribution line. There is a load connected to the electric wire.

  The power conversion apparatus 100 includes an inverter unit 10 that converts DC power supplied from the distributed power source 2 into AC power. In addition, the power conversion device 100 includes a current control processing unit 11 that performs current control of the inverter unit 10. Furthermore, the power conversion device 100 includes a system frequency measurement unit 12, a frequency feedback unit 13, a reactive power step injection unit 14, and an isolated operation detection unit 15 so that the isolated operation can be detected.

  The system frequency measuring unit 12 measures the system frequency used for calculating the frequency deviation. The frequency feedback unit 13 calculates the reactive power to be injected from the system frequency deviation calculated by the moving average process, and prompts the frequency shift. The reactive power step injection unit 14 performs a process of step-injecting reactive power in order to promote a frequency shift under the condition that the frequency deviation is small even when the single operation occurs. It should be noted that the frequency deviation becomes minute when the output power of the power converter 100 and the power consumption of the load are balanced during the single operation of the power converter 100.

  The reactive power step injection unit 14 has a frequency deviation within a predetermined minute range (for example, within ± 0.01 Hz), and the amount of change in fundamental voltage or voltage harmonic distortion satisfies a predetermined condition. Determine that step injection is required. The fundamental wave voltage is a fundamental wave component included in the output voltage from the inverter unit 10. The voltage harmonic distortion is calculated based on harmonic components (secondary to seventh or higher) included in the output voltage from the inverter unit 10. The fundamental voltage and the harmonic voltage can be measured by a measurement circuit included in the reactive power step injection unit 14. The frequency feedback unit 13 adds the reactive power injection amount calculated by the reactive power step injection unit 14 to the reactive power injection amount calculated from the frequency deviation, and transmits the sum to the current control processing unit 11.

  The isolated operation detection unit 15 determines whether or not an isolated operation has occurred based on a change in the system frequency. The isolated operation detection unit 15 includes an active isolated operation determination unit and a passive isolated operation determination unit. The current control processing unit 11 performs current control of the inverter unit 10 so as to inject appropriate reactive power while performing synchronization processing based on the output of the system frequency measurement unit 12.

  Next, the independent operation detection operation of the frequency feedback method with step injection in the power conversion device 100 configured as described above will be described.

  In the power imbalance state between the power converter 100 and the load, the system frequency changes after a power failure of the system power supply 3. For this purpose, the reactive power is injected by the frequency feedback unit 13 and positive feedback is applied to the frequency change. As a result, the frequency change of the system frequency is increased to reach the isolated operation detection condition (preset), and the isolated operation is detected.

  In a state where the power conversion device 100 and the load are in a power balance, voltage harmonics are generated in the output from the inverter unit 10 due to the influence of the excitation current in the pole transformer after the power failure of the system power supply 3. For this purpose, the reactive power step injection unit 14 detects a change in the voltage harmonic distortion, and the reactive power is injected so as to reduce the frequency. Thereby, the frequency changes, and the frequency change is prompted by the frequency feedback unit 13. As a result, isolated operation is detected.

JP2015-220835A JP 2008-259400 A

Japan Electrical Manufacturers Association Standard JEM1498 (Established 2012.8.27)

  When the above-described frequency feedback type isolated operation detection function is provided, the frequency feedback unit 13 operates even when a disturbance occurs in the system power supply 3. At this time, if the system impedance is large, flicker (periodic voltage fluctuation) occurs in the system voltage due to injection of reactive power (see, for example, Patent Document 2). When voltage flicker occurs, there is a problem because an adverse effect such as flickering occurs on a device connected to the system power supply 3 such as a fluorescent lamp.

  In view of the above points, an object of the present invention is to provide a power conversion device and a control method thereof that can suppress the occurrence of voltage flicker due to the frequency feedback function.

  In order to achieve the above object, a power conversion device according to an aspect of the present invention is a power conversion device that links a distributed power source to a system power source, and converts the DC power supplied from the distributed power source into AC power. Information from the frequency feedback unit, an inverter unit for conversion, a system frequency measurement unit that measures system frequency, a frequency feedback unit that has a feedback function that promotes a frequency change of the system frequency according to the deviation of the system frequency, and A current control processing unit that performs current control of the inverter unit, a single operation detection unit that detects a single operation by monitoring the system frequency, and a gain of the feedback function temporarily And a first determination unit that determines whether or not to reduce automatically (first configuration).

  The power converter of the first configuration further includes a second determination unit that determines whether or not to end the gain reduction process when the gain of the feedback function is reduced (second configuration) ) Is preferred.

  In the second configuration, the current control processing unit performs current control so as to superimpose a predetermined harmonic component during gain reduction of the feedback function, and the second determination unit includes the harmonic component. It may be a configuration (third configuration) for determining whether or not to end the gain reduction process based on voltage harmonics appearing on the system side after superimposing.

  In the second or third configuration, when the second determination unit determines not to end the gain reduction process, the gain reduction process ends after a predetermined time has elapsed from the start of the gain reduction process. If the second determination unit determines that the gain reduction process is to be terminated, a configuration (fourth configuration) may be employed in which the gain reduction process is terminated even before the predetermined time has elapsed. .

  In any one of the first to fourth configurations, the first determination unit determines whether to temporarily reduce the gain of the feedback function based on a change in reactive power appearing on the grid side. (5th structure) may be sufficient.

  In the fifth configuration, when the reactive power that appears on the system side increases after exceeding the first threshold value, or the reactive power that appears on the system side is changed, May be a configuration (sixth configuration) that determines that the gain of the feedback function is to be temporarily reduced when the value becomes smaller after exceeding the second threshold and then increases.

  In order to achieve the above object, a method for controlling a power conversion device according to an aspect of the present invention includes a single operation detection that detects a single operation using a frequency feedback function that promotes a frequency change according to a deviation in system frequency. This is a control method for a power conversion device having a function, and has a configuration (seventh configuration) including a step of determining whether or not to temporarily reduce the gain of the feedback function.

  ADVANTAGE OF THE INVENTION According to this invention, the power converter device which can suppress generation | occurrence | production of the voltage flicker resulting from a frequency feedback function, and its control method can be provided.

The block diagram which shows the structure of the power converter device which concerns on 1st Embodiment of this invention. The block diagram which shows the detailed structure of the 1st determination part with which the power converter device which concerns on 1st Embodiment of this invention is provided. The block diagram which shows the detailed structure of the 2nd determination part with which the power converter device which concerns on 1st Embodiment of this invention is provided. The power converter which concerns on 1st Embodiment of this invention WHEREIN: The flowchart which shows the operation example when it determines with the possibility that the voltage flicker has generate | occur | produced by disturbance The block diagram which shows the structure of the power converter device which concerns on 2nd Embodiment of this invention. The schematic diagram for demonstrating the power converter device which concerns on 3rd Embodiment of this invention. The block diagram which shows the structure of the conventional power converter device provided with the independent operation detection function of the frequency feedback system with step injection

Hereinafter, the power converter concerning the embodiment of the present invention and its control method are explained in detail.
<First Embodiment>

  FIG. 1 is a block diagram showing a configuration of a power conversion device 1 according to the first embodiment of the present invention. The power conversion device 1 according to the first embodiment has a frequency feedback independent operation detection function with step injection, and is generally the same as the configuration of the conventional power conversion device 100 shown in FIG. Parts having the same functions as those of the conventional power conversion apparatus 100 are denoted by the same reference numerals as those in FIG. 7, and descriptions of contents determined to be unnecessary are omitted.

  The power converter 1 connects the distributed power source 2 to the system power source 3. Examples of the distributed power source include a natural energy power generation means such as a solar cell, a fuel cell, and a storage battery, and any of them may be used. The inverter unit 10 converts DC power supplied from the distributed power source 2 into AC power. The system frequency measuring unit 12 measures the system frequency.

  The frequency feedback unit 13 has a feedback function that promotes the frequency change of the system frequency in accordance with the deviation of the system frequency obtained by the system frequency measurement unit 12. The reactive power step injection unit 14 performs a process for step-injecting reactive power so as to force a system frequency deviation under a predetermined condition. The isolated operation detection unit 15 monitors the system frequency and detects isolated operation. The current control processing unit 16 performs current control of the inverter unit 10 so as to inject reactive power based on information from the frequency feedback unit 13. The current control processing unit 16 has a function that the current control processing unit 11 of the conventional power conversion device 100 does not have. Details of this point will be described later.

  By providing the above configuration, the power conversion device 1 can detect an isolated operation by an active method (frequency feedback method with step injection), as in the past. Moreover, the power converter device 1 of this embodiment employs a configuration that can suppress voltage flicker that occurs due to the isolated operation detection function by the frequency feedback method, and this point is different from the conventional configuration. This will be described below.

  As illustrated in FIG. 1, the power conversion device 1 includes a first determination unit 17 that determines whether to temporarily reduce the gain of the feedback function. FIG. 2 is a block diagram illustrating a detailed configuration of the first determination unit 17 included in the power conversion device 1 according to the first embodiment of the present invention. As shown in FIG. 2, the first determination unit 17 includes a current / voltage detection unit 171, a reactive power calculation unit 172, and a flicker determination unit 173.

  The current / voltage detection unit 171 is provided between the output side of the inverter unit 10 and the system power supply 3. Specifically, the current / voltage detection unit 171 includes a current detection circuit and a voltage detection circuit, and can detect a current and a voltage appearing on the system side. The reactive power calculation unit 172 acquires the current and voltage detected by the current / voltage detection unit 171 and calculates reactive power. The reactive power calculation unit 172 always calculates reactive power at predetermined intervals. The reactive power calculation unit 172 calculates the average value of reactive power retroactively from the present to a certain period in the past. The reactive power calculation unit 172 outputs the acquired reactive power value and the average value of the reactive power to the flicker determination unit 173.

  The flicker determination unit 173 determines the possibility of occurrence of voltage flicker based on the change in reactive power appearing on the system side. Specifically, the flicker determination unit 173 determines that there is a possibility that voltage flicker has occurred when the following first condition or second condition is satisfied. The first condition is a condition assuming a case where the reactive power is positive, and the second condition is a condition assuming a case where the reactive power is negative. The first condition is that the current reactive power value becomes larger than the first threshold value set in advance with respect to the average value of reactive power at the time point, and then the current reactive power value is reduced. Is the case. The second condition is such that the current reactive power value becomes smaller than a preset second threshold with respect to the average value of reactive power at that time, and then the current reactive power value increases. Is the case. The first threshold value and the second threshold value may be the same value or different values.

  The first condition is that the current reactive power value is larger than an added value obtained by adding a preset first threshold value to the average value of reactive power at the time, and then the current reactive power value is equal to or lower than the previous added value. It may be the case. The second condition is that the current reactive power value is smaller than a subtraction value obtained by subtracting a preset second threshold value from the average value of reactive power at the time, and then the current reactive power value is equal to or greater than the previous subtraction value. It may be the case.

  The first condition and the second condition may be configured such that the average value of reactive power is not used. In this case, the first condition may be a case where the current reactive power value becomes larger than a preset first threshold value, and then the current reactive power value becomes smaller. In addition, the second condition may be a case where the current reactive power value becomes smaller than a preset second threshold value and then the current reactive power value becomes larger. However, in the case of this method, a threshold table is prepared so that the threshold can be changed according to the output in consideration that the magnitude of the reactive power in the steady state may vary depending on the output of the power conversion device 1. It is preferable to do this.

  If it is determined that there is a possibility that voltage flicker has occurred, the information is transmitted to the current control processing unit 16, and the gain of the frequency feedback function of the frequency feedback unit 13 is temporarily reduced. If the flicker determination unit 173 does not determine that there is a possibility that voltage flicker has occurred, the gain of the frequency feedback function of the frequency feedback unit 13 is maintained. That is, the determination of the possibility of occurrence of voltage flicker means determining whether or not to temporarily reduce the gain of the feedback function. Note that the reduction of the gain of the feedback function includes the stop of the feedback function (gain is zero).

  The current control processing unit 16 that has been notified that voltage flicker may have occurred, for example, simultaneously superimposes a predetermined harmonic component for a predetermined period (for example, one cycle) simultaneously with a decrease in the gain of the feedback function. Perform current control. The injection of the harmonic component is repeatedly performed, for example, in a predetermined cycle, but may be performed only once depending on circumstances. The harmonics to be superimposed may be, for example, a single order harmonic, or may be a combination of a plurality of orders of harmonics. In some cases, the harmonics to be superimposed may be inter-order harmonics.

  As illustrated in FIG. 1, the power conversion device 1 includes a second determination unit 18 that determines whether or not to end the gain reduction process when the gain of the feedback function is reduced. FIG. 3 is a block diagram illustrating a detailed configuration of the second determination unit 18 included in the power conversion device 1 according to the first embodiment of the present invention. As shown in FIG. 3, the second determination unit 18 includes a harmonic voltage measurement unit 181, a harmonic distortion calculation unit 182, and an isolated operation possibility determination unit 183.

  The harmonic voltage measurement unit 181 is provided between the output side of the inverter unit 10 and the system power supply 3. The harmonic voltage measuring unit 181 measures the harmonic voltage that appears on the system side. The harmonic distortion calculation unit 182 calculates a change amount (or change rate) of the total harmonic distortion (THD). For example, a value obtained by subtracting the average value of three total harmonic distortions from three cycles before to five cycles before the most recent total harmonic distortion is the amount of change in the total harmonic distortion. Note that the harmonic voltage measurement unit 181 and the harmonic distortion calculation unit 182 may share those provided in the reactive power step injection unit 14 in some cases. Further, the harmonic distortion calculation unit 182 replaces the total harmonic distortion with, for example, harmonic distortion of each order, harmonic distortion between orders, or some harmonic distortion of a plurality of orders. It is also possible to calculate the amount of change by calculating the total value.

  The isolated operation possibility determination unit 183 is based on the total harmonic distortion (more specifically, the change amount or rate of change of the total harmonic distortion) that appears on the system side after the harmonic component is superimposed by the current control processing unit 16. Determine the possibility of isolated operation. Note that it is preferable that the determination period and the harmonic current injection period in the current control processing unit 16 be the same period.

  If the gain of the feedback function is reduced when there is a possibility of isolated operation, the detection of isolated operation is delayed. For this reason, when it is determined that there is a possibility of isolated operation, the gain reduction of the feedback function is terminated, and the normal feedback function is returned to detect the isolated operation. On the other hand, when there is no possibility of independent operation, the gain of the feedback function is reduced for a predetermined period. That is, the determination of the possibility of isolated operation means determining whether or not to end the gain reduction of the feedback function.

  FIG. 4 is a flowchart illustrating an operation example when it is determined that there is a possibility that voltage flicker has occurred due to disturbance in the power conversion device 1 according to the first embodiment of the present invention. As described above, the determination of the possibility of occurrence of voltage flicker is performed by the first determination unit 17 by monitoring how the reactive power changes on the system side.

  If it is determined that voltage flicker may have occurred, the flicker possibility flag is turned on (step S1). When the flicker possibility flag is turned on, the current control processing unit 16 reduces the gain related to the feedback processing and processes information input from the frequency feedback unit 13. That is, the gain of the feedback function by the frequency feedback unit 13 is reduced (step S2). At this time, the frequency feedback function may be stopped by setting the gain to zero.

  Further, the current control processing unit 16 starts injecting current harmonics simultaneously with a decrease in gain related to feedback processing (step S3). When the injection of the current harmonics is started, the second determination unit 18 (more specifically, the isolated operation possibility determination unit 183) determines that the amount of change in the total harmonic distortion (THD) of the voltage is a predetermined threshold value (the first threshold value). 3 (threshold value 3) or more is confirmed (step S4).

  When switching to a single operation while the feedback function is stopped, the harmonic voltage fluctuates in response to the injection of current harmonics, so that the amount of change in total harmonic distortion is equal to or greater than a predetermined threshold (Yes in step S4). . For this reason, the second determination unit 18 determines that there is a possibility of isolated operation, and the flicker possibility flag is turned off (step S5).

  When the flicker possibility flag is turned off, the current control processing unit 16 restores the gain related to the reduced feedback processing, and injects reactive power according to the information input from the frequency feedback unit 13. That is, the feedback function by the frequency feedback unit 13 is returned to the normal state (step S6). At the same time as the feedback function is returned to the normal state, the current harmonic injection process is terminated (step S7). By returning the feedback function to the normal state, it is possible to accurately detect the isolated operation.

  On the other hand, if the total harmonic distortion does not exceed the predetermined threshold value in step S4 (No in step S4), it is determined that there is no possibility of an independent operation. If the predetermined time has not elapsed since the start of the gain reduction of the feedback function (No in step S8), the process returns to step S5, and the possibility of independent operation is determined again. If a predetermined time has elapsed since the start of the gain reduction of the feedback function (Yes in step S8), the flicker possibility flag is turned off (step S5), and then the above-described processing is performed.

  Note that the predetermined time in step S8 is not particularly limited, but is, for example, about 100 milliseconds. When this amount of time has elapsed, there is a high possibility that the voltage flicker will be settled. In addition, the feedback function can be returned to the normal state in a relatively short time, and the isolated operation can be detected appropriately.

  As described above, according to the power conversion device 1 of the first embodiment, even when a disturbance occurs in the system, the occurrence of voltage flicker is suppressed by temporarily reducing the gain of the frequency feedback function. be able to. In addition, even when the gain of the frequency feedback function is lowered, a determination unit that determines the possibility of isolated operation is provided, and the gain of the frequency feedback function is returned to the normal state with an appropriate timing. Driving can be detected appropriately.

Hereinafter, different embodiments will be described. The description of the different embodiments will be focused on the parts different from the first embodiment.
Second Embodiment

  In the first embodiment, when the possibility of occurrence of voltage flicker is detected, the current control processing unit 16 performs a gain reduction process of the feedback function. However, this is only an example.

FIG. 5 is a block diagram showing a configuration of the power conversion device 4 according to the second embodiment of the present invention. In the power conversion device 4 of the second embodiment, when the first determination unit 17 determines that there is a possibility that voltage flicker has occurred, the first determination unit 17 transmits the information to the frequency feedback unit 13. Thereby, the frequency feedback part 13 performs the gain reduction process of a frequency feedback function.
<Third Embodiment>

  In the first embodiment, a configuration in which a change in reactive power is monitored and a possibility of occurrence of voltage flicker is detected. However, this is only an example.

  FIG. 6 is a diagram showing frequency deviation-injection reactive power characteristics. As shown in FIG. 6, the frequency feedback unit 13 changes the reactive power calculation gain (first gain and second gain) with a frequency deviation of ± f [Hz] (for example, f = 0.01 Hz) as a boundary. When voltage flicker occurs, a phenomenon occurs in which the reactive power that becomes the second gain is injected and then the first gain is returned (or the second gain in the reverse direction). In the third embodiment, the possibility of occurrence of voltage flicker is determined using this phenomenon (gain switching). If the gain once decreased, it is determined that there is a possibility that voltage flicker has occurred.

In addition, the detection of the possibility of voltage flicker may be configured using a voltage frequency instead of the method using reactive power described above. Alternatively, it may be determined that there is a possibility of voltage flicker when a periodic variation in reactive power or voltage frequency is detected.
<Others>

  The configurations of the embodiment and the modification described above are merely examples of the present invention. The configuration of the embodiment and the modification may be changed as appropriate without departing from the technical idea of the present invention. In addition, a plurality of embodiments and modifications may be implemented in combination within a possible range.

DESCRIPTION OF SYMBOLS 1, 4 Power converter 2 Distributed type power supply 3 System power supply 10 Inverter part 12 System frequency measurement part 13 Frequency feedback part 15 Independent operation detection part 16 Current control process part 17 1st determination part 18 2nd determination part

Claims (5)

A power converter for connecting a distributed power source to a system power source,
An inverter unit for converting DC power supplied from the distributed power source into AC power;
A system frequency measurement unit for measuring the system frequency;
A frequency feedback unit having a feedback function that promotes a frequency change of the system frequency according to the deviation of the system frequency;
A current control processing unit that performs current control of the inverter unit so as to inject reactive power based on information from the frequency feedback unit;
An isolated operation detection unit that detects the isolated operation by monitoring the system frequency,
If there is a possibility of voltage flicker, a first determination unit determines that temporarily lowers the gain of the feedback function,
Equipped with a,
The current control processing unit performs current control to superimpose a predetermined harmonic component during gain reduction of the feedback function,
A power converter , further comprising: a second determination unit that determines whether or not to end the gain reduction process based on a voltage harmonic that appears on the system side after superimposing the harmonic component . If the second determination unit determines not to end the gain reduction process, the gain reduction process is ended after a predetermined time has elapsed from the start of the gain reduction process,
2. The power conversion device according to claim 1 , wherein when the second determination unit determines to end the gain reduction process, the gain reduction process is ended even before the predetermined time has elapsed. . The power converter according to claim 1 or 2 , wherein the first determination unit determines the possibility of occurrence of the voltage flicker based on a change in reactive power appearing on the grid side. The first determination unit is configured such that the reactive power appearing on the system side increases after exceeding the first threshold value and then decreases, or the reactive power appearing on the system side exceeds the second threshold value. The power converter according to claim 3 , wherein the voltage flicker is determined to have occurred when the voltage flickers in a direction that increases after the voltage decreases. A method for controlling a power converter having an isolated operation detection function that detects an isolated operation by using a frequency feedback function that promotes a frequency change according to a deviation of the system frequency,
If there is a possibility of voltage flicker, determining temporarily reduces the gain of the feedback function,
Superimposing a predetermined harmonic component during gain reduction of the feedback function;
Determining whether to end the gain reduction process based on voltage harmonics appearing on the system side after superimposing the harmonic components;
The control method of the power converter device characterized by the above-mentioned.

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