JP6374213B2 - Power converter - Google Patents

Description

  The present invention relates to a power conversion device.

  Conventionally, a power converter connected to a commercial power system has been provided with an independent operation prevention function for the purpose of reliably disconnecting itself from the commercial power system in the event of a power failure in the commercial power system.

  As an example of the related art related to the above, Patent Document 1 by the applicant of the present application can be cited.

  Further, Non-Patent Document 1 that defines a new interconnection regulation of PCS (power conditioning system) for PV (photovoltaic) system proposes a frequency feedback function as a common technique for performing active islanding detection.

  The frequency feedback function is a function of injecting reactive power based on fluctuations occurring in the operating frequency of the PCS and promoting fluctuations in the operating frequency. When the PCS shifts to the single operation state, if the PCS output and the customer load are unbalanced, unintended fluctuations occur in the operating frequency of the PCS. At this time, if reactive power injection is performed by the frequency feedback function, fluctuations in the operating frequency can be promoted, so that it is possible to detect a single operation of the PCS by capturing this fluctuation.

Japanese Patent No. 3227480

Japan Electrical Manufacturers Association Standard JEM1498 (Established 2012.8.27)

  However, in the above-described prior art, even when the system frequency fluctuates even in a steady state that is not in an isolated operation state, a lot of reactive power may be output unnecessarily by the frequency feedback function (that is, the output current The frequency may become unstable.) In addition, if the feedback gain of the frequency feedback (gain that calculates the injection reactive power with respect to the frequency deviation) is suppressed to suppress the output of unnecessary reactive power, the divergence of the operating frequency during the single operation state is delayed. This is disadvantageous for detecting high-speed isolated operation.

  In view of the above problems, the present invention provides a power conversion device that can suppress unnecessary reactive power output in a steady state that is not in an isolated operation state, and that can detect an isolated operation at high speed in an isolated operation state. With the goal.

In order to achieve the above object, a power converter according to an aspect of the present invention is provided.
A power conversion unit that converts DC power into AC power and interconnects it with a commercial power system;
A system frequency measurement unit for measuring the system frequency;
An isolated operation detection unit that detects the isolated operation by monitoring the system frequency,
A frequency feedback unit that promotes a frequency change of the system frequency by injecting reactive power according to the deviation of the system frequency;
A determination unit for determining whether or not there is a sudden increase in voltage harmonic distortion;
A gain setting unit that switches and sets a feedback gain of the frequency feedback unit according to a determination result by the determination unit;
(First configuration).

  In the first configuration, the gain setting unit sets the first gain when the determination unit determines that there is a sudden increase in voltage harmonic distortion; otherwise, the gain setting unit sets the entire frequency deviation. A second gain that is equal to or lower than the first gain in the region and lower than the first gain in at least a part of the frequency deviation region may be set (second configuration).

  In the first or second configuration, the reactive power step injection unit that forcibly generates the deviation of the system frequency by step-injecting reactive power when a sudden increase in voltage harmonic distortion is detected is further provided. It is good also as providing (3rd structure).

  Further, in the third configuration, the determination condition of the voltage harmonic distortion rapid increase by the determination unit may be more relaxed than the determination condition of the voltage harmonic distortion rapid increase related to the reactive power step injection unit. (Fourth configuration).

  A distributed power source according to one aspect of the present invention includes a DC power source and any one of the first to fourth methods described above that converts DC power input from the DC power source into AC power and grid-connects to a commercial power system. And a power conversion device configured as described above.

  According to the power conversion device of the present invention, it is possible to suppress unnecessary reactive power output in a steady state that is not in an isolated operation state, and to detect an isolated operation at high speed in an isolated operation state.

It is a whole block diagram of PCS (power conditioner) concerning one embodiment of the present invention. It is a figure which shows the system frequency (period) measurement algorithm. It is an image figure of frequency deviation calculation. It is an update image figure of a moving average value. It is a figure which shows an example of the feedback gain of a frequency feedback function. It is a figure for demonstrating fundamental wave voltage fluctuation | variation. It is a figure for demonstrating the fluctuation | variation of voltage harmonic distortion. It is a figure which shows another example of the feedback gain of a frequency feedback function.

<First Embodiment>
An active islanding detection method, which is an islanding operation detection method capable of cooperating with a centralized interconnection, has been known recently. The outline is as follows. Characteristic functions include a function (frequency feedback function) that promotes frequency shift by calculating reactive power injected from system frequency deviation, and intentional in a state where the output power of the distributed power source and the power consumption of the system load are balanced. Is equipped with a function to generate frequency changes (reactive power step injection function), capable of high-speed detection of isolated operation, no unnecessary operation, no mutual interference with other systems, and small influence on system by active signal With features such as. Such an isolated operation detection method is referred to as an “active isolated operation detection method (frequency feedback method with step injection)”, and its contents will be described in detail.

  FIG. 1 is an overall block diagram of a power conditioner (hereinafter referred to as a PCS [power conditioning system]) employing the above active islanding system according to an embodiment of the present invention. The PCS 100 (an example of a power conversion device) of this configuration example includes a system frequency measurement unit 110, a frequency feedback unit 120, a reactive power step injection unit 130, an isolated operation detection unit 140, a current control processing unit 150, an inverter Part 160.

  The system frequency measurement unit 110 is a measurement unit that measures a system frequency used for calculating a frequency deviation, and includes a frequency detection circuit 111, a frequency measurement processing unit 112, and a phase difference measurement synchronization processing unit 113. FIG. 2 is a diagram showing an algorithm for measuring the system frequency (cycle).

  The frequency detection circuit 111 generates a square wave signal synchronized with the system voltage and outputs the square wave signal to the frequency measurement processing unit 112 (see the upper part of FIG. 2). The frequency detection circuit 111 is implemented as a hardware unit.

  The frequency measurement processing unit 112 monitors the square wave signal input from the frequency detection circuit 111 and measures the periodic data (frequency data) of the system voltage (see the middle part of FIG. 2). More specifically, the frequency measurement processing unit 112 calculates the intermediate value when counting from the falling edge to the rising edge of the square wave signal and the intermediate value when counting from the next falling edge to the rising edge. Is obtained as period data (frequency data). The frequency measurement processing unit 112 has sufficient resolution (for example, 2.5 MHz or more (400 ns or less)) for measuring the system frequency, and the frequency measurement processing unit 112 is implemented as a software unit.

  The phase difference measurement synchronization processing unit 113 synchronizes the period data (frequency data) measured by the frequency measurement processing unit 112 with the rising edge of the square wave signal (see the lower part of FIG. 2). The phase difference measurement synchronization processing unit 113 is implemented as a software unit.

  The frequency feedback unit 120 is a control unit that calculates the reactive power to be injected from the frequency deviation (periodic deviation) of the system frequency calculated by the moving average process, and promotes the frequency shift. 2 moving average calculation part 122, frequency deviation calculation part 123, reactive power injection amount calculation part 124, and addition part 125 are included.

  The first moving average calculation unit 121 calculates a first moving average value of the system frequency (system cycle). The first moving average calculation unit 121 is implemented as a software unit.

  The second moving average calculation unit 122 calculates a second moving average value of the system frequency (system cycle). The second moving average calculation unit 122 is implemented as a software unit.

  The frequency deviation calculation unit 123 calculates the frequency deviation (period deviation) of the system frequency from the difference between the first moving average value and the second moving average value. The frequency deviation calculation unit 123 is implemented as a software unit.

  FIG. 3 is an image diagram of frequency deviation calculation, and FIG. 4 is an update image diagram of a moving average value. Period data used for calculating the frequency deviation is updated for each period of the system voltage. The moving average value used for the calculation of the frequency deviation is updated every time t1 (for example, t1 = 5 ms), and the calculation of the frequency deviation itself is also performed every time t1. The time t1 is uniform without depending on the frequency (50 Hz / 60 Hz) of the commercial power system. As the first moving average value (most recent cycle), a moving average value for time t2 (for example, t2 = 40 ms) is used with the acquisition timing of the latest cycle as an end point. On the other hand, as the second moving average value (past cycle), the moving average value for the time t4 (for example, t4 = 80 ms) with the timing retroactive from the acquisition timing of the latest cycle by the time t3 (for example, t3 = 200 ms) as the end point. Is used.

  The reactive power injection amount calculation unit 124 calculates reactive power to be injected according to the frequency deviation (periodic deviation) calculated by the frequency deviation calculation unit 123. The reactive power injection amount calculation unit 124 is implemented as a software unit.

  FIG. 5 is a diagram showing frequency deviation-injection reactive power characteristics. As shown in this figure, the reactive power injection amount calculation unit 124 has a frequency deviation of ± f [Hz] (for example, f = 0.01 Hz (± 4.0 μs @ 50 Hz, ± 2.8 μs @ 60 Hz) as a boundary). The reactive power calculation gain is changed (first stage gain and second stage gain) The upper and lower limits of the reactive power to be injected are set to ± A [pu] (for example, A = 0.25). Here, pu [per unit] is a symbol used to express the ratio to the reference value (rated capacity) in the unit method, for example, when the reference value (rated capacity) is 4 kW. ± 0.25 pu = ± 1 kW (± 1 kVar for reactive power).

  In FIG. 5, the characteristic is indicated by a solid line and a broken line because the gain can be switched. This will be described in detail later.

  The addition unit 125 adds the reactive power injection amount calculated by the reactive power injection amount calculation unit 124 and the reactive power step injection amount calculated by the step injection amount calculation unit 136 described later, and performs current control processing. Transmitted to the unit 150. The adding unit 125 is implemented as a software unit.

  The reactive power step injection unit 130 performs step injection of reactive power in order to promote frequency shift under the condition that the output power of the PCS 100 and the power consumption of the load device are balanced and the deviation of the system frequency is small during the single operation of the PCS 100. The fundamental wave voltage measurement circuit 131, the harmonic voltage measurement circuit 132, the fundamental wave voltage calculation unit 133, the harmonic distortion calculation unit 134, the step injection generation condition determination unit 135, and the step injection amount. A calculation unit 136.

  The fundamental wave voltage measurement circuit 131 measures a fundamental wave component included in the output current of the PCS 100 as a fundamental wave voltage. The fundamental wave voltage measurement circuit 131 is implemented as a hardware unit.

  The harmonic voltage measurement circuit 132 measures a harmonic component included in the output current of the PCS 100 as a harmonic voltage. The harmonic voltage measurement circuit 132 is implemented as a hardware unit.

  The fundamental wave voltage calculation unit 133 calculates the amount of change in the fundamental wave voltage. The fundamental wave voltage calculation unit 133 is implemented as a software unit.

  The harmonic distortion calculation unit 134 calculates the amount of change in voltage harmonic distortion. For the calculation of the voltage harmonic distortion, second-order to seventh-order or higher harmonics are used. The harmonic distortion calculation unit 134 is implemented as a software unit.

  The step injection generation condition determination unit 135 determines whether or not the step injection generation condition is satisfied according to the outputs of the fundamental voltage calculation unit 133 and the harmonic distortion calculation unit 134. More specifically, the step injection generation condition determination unit 135 has a frequency deviation within a predetermined minute range (for example, within ± 0.01 Hz) and a change amount of the fundamental voltage or voltage harmonic distortion is predetermined. When the above condition is satisfied, it is determined that the need for step injection has occurred. Step injection generation condition determination unit 133 is implemented as a software unit.

  FIG. 6 is a diagram for explaining a first condition (fundamental voltage fluctuation) for occurrence of step injection. In the example of FIG. 6, Mi indicates the fundamental wave voltage i cycles before the latest, and Mavr indicates the average value of three from 3 cycles before to 5 cycles before. For example, the fundamental wave voltage calculation unit 133 calculates a change amount (M0−Mavr) of the fundamental wave voltage, and the step injection generation condition determination unit 135 calculates the fundamental wave voltage when the change amount exceeds a predetermined threshold. It is determined that fluctuation has occurred.

  FIG. 7 is a diagram for explaining a second condition (voltage harmonic distortion fluctuation) for occurrence of step injection. In the example of FIG. 7, Ni represents the second to seventh total harmonic distortion THD [total harmonic distortion] i cycles before the latest, N0 is the latest value, and Navr is 3 cycles to 5 cycles before. The average value of up to three is shown. For example, the harmonic distortion calculation unit 134 calculates a change amount (N0−Navr) of the voltage harmonic distortion, and the step injection generation condition determination unit 135 calculates the voltage harmonic when the change amount exceeds a predetermined threshold. It is determined that the distortion has increased rapidly.

  The step injection amount calculation unit 136 calculates the step injection amount of reactive power according to the determination result of the step injection generation condition determination unit 135. The injection time is set to a predetermined number of cycles or less (for example, 3 cycles or less). A predetermined upper limit value (for example, 0.1 p.u.) is determined for the injection amount. The reactive power is injected in the direction of delaying the current phase as viewed from the PCS 100 (the direction of decreasing the frequency). The reactive power step injection is performed within a half cycle of the system frequency (period) after the above-described condition is satisfied. The step injection amount calculation unit 136 is implemented as a software unit.

  The isolated operation detection unit 140 is a control unit that determines whether or not an isolated operation has occurred based on a change in system frequency, and includes an active isolated operation detection unit 141 and a passive isolated operation detection unit 142.

  The current control processing unit 150 performs current control of the inverter unit 160 so as to inject appropriate reactive power while performing synchronization processing based on the output of the system frequency measurement unit 110.

  The inverter unit 160 is a power conversion unit that converts DC power supplied from a DC power source (for example, a PV panel) into AC power and connects it to the commercial power system PW.

  Although FIG. 1 shows an example in which the above-described components are divided into a hardware part (other than the CPU) and a software part (CPU), the division boundary is not limited to this.

  Here, the PCS 100 further includes a harmonic voltage measurement circuit 171 and a harmonic distortion calculation unit 172. The harmonic voltage measurement circuit 171 measures a harmonic component included in the output current of the PCS 100 as a harmonic voltage. The harmonic voltage measurement circuit 171 is implemented as a hardware unit.

  The harmonic distortion calculation unit 172 calculates the amount of change in voltage harmonic distortion. For the calculation of the voltage harmonic distortion, second-order to seventh-order or higher harmonics are used. The harmonic distortion calculation unit 172 is implemented as a software unit. As shown in FIG. 7, the harmonic distortion calculation unit 172, for example, changes in voltage harmonic distortion (N0−Navr) (N0: latest total harmonic distortion THD, Navr: 5 cycles from 3 cycles before. The previous three THD average values) are calculated.

  Then, the reactive power injection amount calculation unit 124 switches the reactive power calculation gain (feedback gain) according to the amount of change in the voltage harmonic distortion calculated by the harmonic distortion calculation unit 172. More specifically, when the amount of change in voltage harmonic distortion exceeds a predetermined threshold, the feedback gain is set to the gain (first gain) indicated by the solid line in the frequency deviation-injection reactive power characteristic shown in FIG. Set. This gain setting is performed only for several cycles.

  On the other hand, when the amount of change in voltage harmonic distortion is equal to or less than the predetermined threshold, the feedback gain is set to the gain (second gain) indicated by the broken line in FIG. The gain indicated by the broken line is equal to or lower than the gain indicated by the solid line in the entire region of the frequency deviation, but is lower than the gain indicated by the solid line in a part of the frequency deviation region from the minus side to the plus side (the broken line). 1st stage gain and 2nd stage gain).

  Here, when the commercial power system PW fails and the PCS 100 is in a single operation state, the harmonic current generated by a pole transformer (not shown) located between the PCS 100 and the commercial power system PW is the impedance of the load device. Since it flows toward the impedance of the transformer on the pole, voltage harmonics are generated at the inverter output.

  When the PCS 100 enters the single operation state and the voltage harmonic distortion rapidly increases, the amount of change in the voltage harmonic distortion calculated by the harmonic distortion calculation unit 172 exceeds a predetermined threshold value. A feedback gain is set to a gain of 1 (solid line in FIG. 5). At this time, if the PCS output and the power consumption of the load device are unbalanced, the system frequency changes. Therefore, the frequency feedback unit 120 injects reactive power based on the frequency deviation and the set first gain to promote the frequency shift. Then, the active islanding detection unit 141 detects islanding by detecting frequency divergence. Since the first gain is set high, a large amount of reactive power is injected, frequency divergence is promoted, and the isolated operation can be detected at high speed.

  Further, since voltage harmonics are not generated in the inverter output at the time of grid connection with the commercial power system PW, the amount of change in voltage harmonic distortion calculated by the harmonic distortion calculation unit 172 is below a predetermined threshold value. Thus, the reactive power injection calculating unit 124 sets the feedback gain to the second gain (broken line in FIG. 5). At this time, when fluctuation occurs in the system frequency, reactive power is injected based on the frequency deviation and the second gain. However, since the second gain is set low, unnecessary reactive power injection is performed. Can be suppressed.

  As described above, according to this embodiment, it is possible to suppress unnecessary reactive power output in a steady state that is not in an isolated operation state, and to detect an isolated operation at a high speed in an isolated operation state.

  In addition, when the PCS output and the power consumption of the load device are balanced in the single operation state, the change in the system frequency becomes small, but the voltage harmonic distortion increases rapidly, so that the harmonic distortion calculation unit 134 calculates it. The amount of change in voltage harmonic distortion exceeds a predetermined threshold. Accordingly, the reactive power is step-injected by the step injection amount calculation unit 136 according to the determination result of the step injection generation condition determination unit 135. At this time, the reactive power injection amount calculation unit 124 sets the feedback gain to the first gain by detecting the sudden increase in voltage harmonic distortion. Therefore, frequency divergence of the system frequency is promoted, and high-speed isolated operation can be detected.

  In some cases, the degree of sudden increase in voltage harmonic distortion may be weak during the single operation state. In this case, the reactive power step injection unit 130 cannot detect the rapid increase in voltage harmonic distortion, and the reactive power step injection function is disabled. Become. However, since the threshold value for harmonic determination in the reactive power injection amount calculation unit 124 is set lower than the threshold value in the step injection generation condition determination unit 135, the reactive power injection amount calculation unit 124 uses the above case. However, a sudden increase in voltage harmonic distortion can be detected, and the feedback gain is set to the first gain. The higher first gain promotes the divergence of the frequency and enables high-speed isolated operation detection.

  If the PCS 100 adopts such an active islanding detection method, the PCS 100 that has entered the islanding state can be stopped at high speed without delay (for example, within 0.2 s, which is the specified value after the islanding operation). It becomes.

Second Embodiment
In the first embodiment, the feedback gain set by the reactive power injection amount calculation unit 124 is exemplified by the feedback gain shown in FIG. 5, but the feedback gain may take various forms.

  For example, as shown in FIG. 8, a part of the second gain (broken line) may overlap the first stage part of the first gain (solid line). In FIG. 8, the second gain is extended from the overlapped linear portion to both ends until the first gain limit value (± A [pu]) is crossed (+ f1 to + f2 and −f1 to + f1). -F2 region). Therefore, the extended portion of the second gain is a portion whose value is lower than that of the first gain.

  As another example, a part of the feedback gain may be configured in a curved shape. For example, in FIG. 5, the first-stage gain and the second-stage gain may be configured in a curved shape.

<Third Embodiment>
In the first embodiment, the reactive power injection amount calculation unit 124 determines the voltage harmonic distortion rapid increase by lowering the threshold for the calculated amount of change in voltage harmonic distortion than the step injection generation condition determination unit 135. The conditions are relaxed. Such determination conditions can be relaxed by other methods.

  For example, the threshold value for determination is the same for the step injection generation condition determination unit 135 and the reactive power injection amount calculation unit 124. Then, in the harmonic distortion calculation unit 134, for example, as shown in FIG. 7, the difference between the latest total harmonic distortion THD (N0) and the THD average value (Navr) from 3 cycles before to 5 cycles before is obtained. Thus, the amount of change in voltage harmonic distortion is calculated. Further, the harmonic distortion calculation unit 172 calculates the difference between the latest total harmonic distortion THD (N0) and the THD average value from 6 cycles before to 8 cycles before (that is, the past from 3 cycles before to 5 cycles before). The amount of change in voltage harmonic distortion is calculated by taking That is, the determination condition of the voltage harmonic distortion rapid increase is relaxed by lengthening the period during which the voltage harmonic distortion change amount is taken even with the same threshold.

  As mentioned above, although embodiment of this invention was described, if it is in the range of the meaning of this invention, embodiment may be variously deformed.

  For example, the present invention can be widely applied to a distributed power source other than a distributed power source composed of a solar cell module (an example of a DC power source) and a PCS (using a natural energy power generation means other than a solar cell, a fuel cell, and a storage battery). Distributed power supply etc.).

100 PCS
DESCRIPTION OF SYMBOLS 110 System frequency measurement part 120 Frequency feedback part 130 Reactive power step injection part 140 Independent operation detection part 150 Current control processing part 160 Inverter part 171 Harmonic voltage measurement circuit 172 Harmonic distortion calculation part PW Commercial power system

Claims (3)

A power conversion unit that converts DC power into AC power and interconnects it with a commercial power system;
A system frequency measurement unit for measuring the system frequency;
An isolated operation detection unit that detects the isolated operation by monitoring the system frequency,
A frequency feedback unit that promotes a frequency change of the system frequency by injecting reactive power according to the deviation of the system frequency;
A determination unit for determining whether or not there is a sudden increase in voltage harmonic distortion;
A gain setting unit that switches and sets a feedback gain of the frequency feedback unit according to a determination result by the determination unit;
A reactive power step injection unit that forcibly generates a deviation of the system frequency by step injection of reactive power when a sudden increase in voltage harmonic distortion is detected, and
The determination condition of the voltage harmonic distortion rapid increase by the determination unit is a power converter that is relaxed than the detection condition of the voltage harmonic distortion rapid increase related to the reactive power step injection unit .   The gain setting unit sets a first gain when the determination unit determines that there has been a sudden increase in voltage harmonic distortion; otherwise, the gain setting unit sets the first gain below the first gain in the entire frequency deviation region. The power converter according to claim 1, wherein a second gain that is lower than the first gain is set in at least a part of the frequency deviation region. A DC power source, characterized in that it comprises a power converter according to claim 1 or claim 2 to system interconnection to the commercial power system by converting DC power into AC power inputted from the DC power supply Distributed power supply.

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