Power conversion device

Abstract

To sufficiently supply inertia even when an abnormality occurs in a power grid under output restrictions imposed by a grid operator in a grid-forming power conversion device.SOLUTION: A grid-forming power conversion device for interconnecting an asynchronous DC power source to an AC power grid, includes a main circuit for converting DC to AC, and a control circuit for controlling the main circuit as a grid-forming device. The control circuit includes a restriction command acquisition unit that acquires a restriction command Plim limiting AC power from an external source, a frequency detection unit that detects a fundamental frequency ω0 of the AC voltage of the AC power, and a restriction command correction unit A1 that corrects the restriction command according to the rate of increase or / and decrease of the fundamental frequency.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a power conversion device (inverter) connected to a power grid. [Background technology]

[0002] In recent years, distributed power sources such as solar power generation systems, especially those that are connected to the grid using inverters, have been introduced, and the proportion of rotary generators such as thermal power plants, which were previously the main source of power, is decreasing, which has brought to light the issue of a decrease in the system inertia force across the entire system, which was previously guaranteed by rotary generators.

[0003] To address this issue, a method has been proposed to compensate for the lack of inertia by adding virtual inertia to the inverter's behavior.In addition, in recent years, technologies such as grid-forming inverters (GFMs) that actively manipulate the grid frequency as shown in Patent Document 1 have been attracting attention, in contrast to conventional grid-following inverters (GFLs) that were operated to follow the grid frequency.

[0004] On the other hand, the spread of distributed power sources has also brought to light the issue that the supply-demand balance and the load factor of transmission lines cannot be adequately adjusted in the power grid using conventional power transmission and distribution systems.To address this issue, power grid operators have established rules, and systems are being put into operation in which, for example, grid operation servers send limiting commands to grid-connected inverters to reduce power generation, thereby forcibly adjusting output. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2024-16572 Summary of the Invention [Problem to be solved by the invention]

[0006] Currently, the requirements for grid connection in Japan are established for conventional grid-following inverters (GFLs), and the output adjustments based on the above-mentioned limiting directives are also applied to GFLs. However, in the future, grid-forming inverters (GFMs) may also be connected to the grid to address the shortage of grid inertia, and will also be subject to the above-mentioned limiting directives. Here, if the above-mentioned limiting directives are applied, the inverter output will be limited, which could result in a situation where sufficient inertia cannot be supplied, which could be an issue.

[0007] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a grid-forming power conversion device that can sufficiently supply inertia even when an abnormality occurs in the power grid under output restrictions imposed by a grid operator. [Means for solving the problem]

[0008] As a means for solving the above problems, there is a power conversion device of a system-forming type that connects an asynchronous DC power supply to an AC power system, the power conversion device comprising: a main circuit that converts DC and AC; and a control circuit that controls the main circuit as a system-forming type, the control circuit comprising: a limit command acquisition unit that acquires a limit command that limits AC power from the outside; a frequency detection unit that detects the fundamental frequency in the AC voltage of the AC power; and a limit command correction unit that corrects the limit command in accordance with the rate of increase and / or decrease of the fundamental frequency. [Effects of the Invention]

[0009] According to the present invention, in a grid-forming power conversion device, it is possible to supply sufficient inertia even if an abnormality occurs in the power grid under output restrictions imposed by a grid operator. [Brief explanation of the drawings]

[0010] [Figure 1] 1 shows an example of the configuration of a power conversion device according to a first embodiment of the present invention. [Figure 2] 3 shows an example of the configuration of a phase command generating unit according to the first embodiment of the present invention. [Figure 3]4 shows an example of the relationship between an active power command value, active power, and an angular frequency change amount command in the first embodiment of the present invention. [Figure 4] 3 shows an example of the configuration of a limit command correction unit A1 according to the first embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0011] 1 shows an example of the configuration of a power system using power conversion devices according to a first embodiment of the present invention. This power system includes two power conversion devices: a power conversion device 1A that connects a DC power supply 7A to a power grid 5, and a power conversion device 1B that connects a DC power supply 7B to the power grid 5. However, power conversion device 1A and power conversion device 1B have the same configuration.

[0012] The power conversion devices 1A and 1B each include a main circuit 2, a control circuit 3, and a filter circuit 4. The main circuit 2 is a power conversion circuit (e.g., a three-phase full-bridge inverter) that converts DC power to AC power, and the AC terminal of the main circuit 2 is connected to a power grid 5 and an AC load 6 via a filter circuit 4. The two power conversion devices 1A and 1B are each connected in parallel to the power grid 5. The power grid 5 is, for example, a 50 Hz or 60 Hz, three-phase 200 V power grid, and the AC load 6 is, for example, a consumer-owned device such as an air conditioner. Note that the AC load 6 is illustrated as a single AC load, but in reality, multiple AC loads may be connected. The filter circuit 4 is composed of an inductor and an X capacitor. A relay 51 is installed at the interconnection point with the power grid 5 so that the power conversion devices 1A and 1B and the AC load 6 can be disconnected from the power grid 5.

[0013] The power conversion devices 1A and 1B are configured to be able to communicate with an externally provided general controller 80 via wired or wireless communication. Furthermore, the general controller 80 is configured to be able to communicate with, for example, a system operation server 82 that operates the power system 5, via a wide area communication network 81.

[0014] The general controller 80 may have a control function for a power generation company using a power generation device such as a renewable energy power generation device to manage and adjust the power generation output at the connection point to the power grid 5. Alternatively, the general controller 80 may have a control function for a power consumer using an AC load 6 to adjust the received power and create economic or environmental value. Furthermore, the general controller 80 may have a control function for a power consumer to provide a comprehensive power adjustment service, including the operation of the power conversion devices 1A and 1B and the AC load 6, as a service to various power markets established by the operator of the power grid 5, and receive remuneration from the market. Since the power conversion devices 1A and 1B have the same configuration, only the power conversion device 1A will be described below.

[0015] The DC end of the main circuit 2 of the power conversion device 1A is connected to a DC power source 7A. The DC power source 7A may be a device capable of generating DC electromotive force, such as a solar panel or a fuel cell stack. The DC power source 7A may also be connected to a secondary battery, such as a lithium-ion battery, a nickel-metal hydride battery, or a lead-acid battery, or a power storage device capable of storing energy, such as a capacitor, compressed air storage, or a flywheel, and a device that converts that energy into DC electrical energy. A DC voltage detector 24 and a DC current detector 25 are installed on the power circuit of the DC power source 7A, and the detected DC voltage Vdc and DC current Idc are input as signals to the control circuit 3.

[0016] A voltage meter 21 is installed on the power system 5 (AC load 6) side of the filter circuit 4, and a current meter 22 is installed between the filter circuit 4 and the main circuit 2. The control circuit 3 obtains the AC voltage measurement value Vout and the reference angular frequency (system frequency) ω0 from the voltage meter 21, and obtains the AC current measurement value Iout from the current meter 22, as signals.

[0017] The control circuit 3 includes a power calculation unit 31, an active power command generation unit 32, a phase command generation unit 33, a reactive power command generation unit 34, a reactive power control (AQR) unit 35, a main circuit control unit 36, and a limit command acquisition unit 37.

[0018] The power calculation unit 31 calculates the active power Pout (in this embodiment, the power flow that charges the DC power supply 7 is considered positive) and the reactive power Qout (in this embodiment, the lead is considered positive) output by the power conversion device 1A from the AC voltage measurement value Vout and the AC current measurement value Iout, and detects the reference angular frequency (system frequency) ω0 from the AC voltage measurement value Vout.

[0019] The active power command generation unit 32 calculates an active power command Pref and issues the calculated active power command to the phase command generation unit 33. If the DC power source 7 is a solar power generation panel, the active power command generation unit 32 may be equipped with a maximum power point tracking calculation that maximizes the power generation amount according to the amount of solar radiation at any given time using information on the DC voltage Vdc at both ends of the panel and the DC current Idc flowing therethrough, and the power near the maximum power point may be used as the active power command Pref. In another example, instead of searching for the maximum power point by tracking, an algorithm that estimates the maximum power point may be installed, and the active power command Pref may be generated based on the estimated maximum power point. Furthermore, if the DC power source 7 is a power storage device, the active power command Pref may be a command value for charge / discharge power obtained via communication from, for example, an external general controller 80. Furthermore, the active power command Pref may be a value corrected to protect the power storage device from overcharging or over-discharging, taking into account the storage rate of the power storage device. Note that the power conversion device 1A may be provided with information input means (not shown) and the active power command Pref may be generated from information obtained from this information input means.

[0020] The phase command generating unit 33 generates a phase command θref from the active power Pout and fundamental frequency ω calculated by the power calculating unit 31, the active power command Pref output by the active power command generating unit 32, and a limit command Plim, which will be described later. Details will be described later.

[0021] The reactive power command generation unit 34 generates a reactive power command Qref by multiplying the deviation V0 - Vout between the rated voltage V0 (e.g., 200V) and the measured AC voltage value Vout by a proportional gain Kq (Kq > 0). Then, the reactive power control (AQR) unit 35 generates a voltage command Vref by performing proportional-integral control on the deviation Qref - Qout between this reactive power command Qref and the reactive power Qout calculated by the power calculation unit 31. The generated voltage command Vref is output to the main circuit control unit 36.

[0022] The main circuit control unit 36 generates a gate signal 23 for the switching element of the main circuit 2 by performing pulse width modulation calculation (PWM) using the phase command θref and the voltage command Vref, and drives the main circuit 2.

[0023] Fig. 2 shows a configuration example of the phase command generation unit 33, and Fig. 3 shows an example of the relationship between the active power command value, active power, and angular frequency change amount command.

[0024] In the phase command generation unit 33, first, the active power command Pref and the active power Pout are input to the subtractor 331 to obtain the deviation Pout - Pref. This is input to the proportional controller 332 with a gain Kp to calculate the angular frequency change amount command Δωref’. Illustrated, it is as shown in Fig. 3, and the angular frequency change amount command Δωref’ is determined by the loop control with the deviation of the active power input. The slope of the loop characteristic of this active power - angular frequency change amount command is the gain Kp, and is expressed by the following mathematical formula.

[0025]

Equation

[0026] When Pout < Pref, Δωref’ < 0, and the active power Pout is increased (increase in charging power or decrease in discharging power) by decreasing the angular frequency. Also, when Pout > Pref, Δωref’ > 0, and the active power Pout is decreased (decrease in charging power or increase in discharging power).

[0027] The angular frequency change command Δωref′ is input to a virtual inertia imparting filter 333. The virtual inertia imparting filter 333 is, for example, a first-order lead-lag filter. Its time constant is determined using the unit inertia constant and unit damping constant of the synchronous generator, and outputs an angular frequency change command Δωref in which the inertia of the synchronous generator is virtually imparted to Δωref′. This virtual inertia imparting filter 333 allows Δωref to change gradually even when the deviation Pout-Pref changes stepwise, thereby achieving an angular frequency change with inertia. This inertia imparting angular frequency change command Δωref and a reference angular frequency ω0 (in this embodiment, the reference frequency is 50 Hz and 2π×50 rad / s) are input to an adder 334 to obtain an angular frequency command ωref. The angular frequency command ωref is then input to an integrator 335 for integration, thereby generating a phase command θref. The generated phase command θref is output to the main circuit control unit 36.

[0028] As a result, power conversion device 1A and power conversion device 1B can be operated in a grid-connected state with virtual inertia. Note that power conversion device 1A and power conversion device 1B are driven as voltage sources using phase command θref and voltage command Vref, and therefore can be operated independently not only in a grid-connected state but also when power grid 5 is disconnected.

[0029] In recent years, with the increasing adoption of distributed power sources, such as photovoltaic power generation systems, problems have become apparent in that conventional power transmission and distribution systems are unable to adequately adjust the supply-demand balance in the power system and the load factor of the transmission line. To address these problems, a system has been put into operation, based on rules established by the power system operator, in which a system operation server 82 or the like transmits a limit command to the overall controller 80 and the power conversion device 1A to reduce the amount of power generated, thereby forcibly adjusting the output of the power conversion device 1A. To comply with this system, a limit command acquisition unit 37 provided in the control circuit 3 inputs a limit command Plim acquired from the overall controller 80 via communication to the phase command generation unit 33, and the phase command generation unit 33 limits the active power Pout based on the limit command Plim.

[0030] As shown in Figure 2, specific limit calculation methods include applying upper and lower limiter processing L1 to the active power command Pref, applying upper and lower limiter processing L2 downstream of subtractor 331, and applying upper and lower limiter processing L3 downstream of proportional controller 332. The upper limit of upper and lower limiter processing L1 may be set to Plim or a value equivalent thereto. The absolute value |Pout-Plim| of the difference between the active power Pout and the limit command Plim is calculated via subtractor A2 and absolute value calculator A3, and the positive and negative signs of this absolute value are assigned to the upper and lower limits of upper and lower limiter processing L2. The upper and lower limits of upper and lower limiter processing L3 may be set to values ​​obtained by multiplying the upper and lower limits set in upper and lower limiter L2 by a value equivalent to the control gain of proportional controller 322. Here, three types of upper and lower limiter processing L1, L2, and L3 are shown, but by applying at least one of the limiter processing, the manipulated variable for active power control can be adjusted and the power generation amount of the power conversion device 1A can be guided in the direction of reduction.

[0031] Because the power conversion device 1A can operate as a voltage source having virtual inertia, even when, for example, a large generator connected to the interconnected power grid 5 outside the interconnection point drops due to an accident or the like and the inertial force decreases, it is expected that the power conversion device 1A will supply inertia and suppress a drop in the frequency of the power grid 5. However, when the above-mentioned limit command Plim is applied, the output of the power conversion device 1A is limited, which may result in a state in which sufficient inertia cannot be supplied.

[0032] Therefore, in this embodiment, the phase command generating unit 33 of the power conversion device 1A is provided with a limit command correcting unit A1 that detects various types of faults in the power system 5 based on the reference angular frequency (system frequency) ω0 and temporarily corrects the limit command Plim.

[0033] FIG. 4 shows a detailed example of the limit command correction unit A1, in which the correction calculation of the limit command Plim is indicated by a correction calculation unit A13. The correction calculation unit A13 may include, for example, an adder A131 that adds a predetermined signal Plim1 to the limit command Plim to temporarily correct the value of Plim to a larger value (Plim+Plim1). In another example, an upper / lower limiter process A132 is provided to temporarily correct the lower limit value to the larger value Plim1. In yet another example, a switch A133 may be provided to forcibly switch the limit command Plim to another larger value Plim1. Note that Plim1 may not be a fixed value, but may be generated so that it can be temporarily switched to a larger value. For example, a switch A130 may be provided to switch the signal Plim1 between zero and the rated output power value Pmax of the power conversion device 1 based on a switching signal Fsig.

[0034] FIG. 4 also shows a frequency anomaly detection unit A12 that receives a reference angular frequency ω0 as an input and calculates a switching signal Fsig. For example, a delay unit A120 is provided to delay the reference angular frequency ω0 by a predetermined time, and the difference (ω0 - ωd) between the instantaneous value of the reference angular frequency ω0 and the output ωd of the delay unit A120 is calculated as the frequency change rate sω. A frequency determination unit A122 or A123 may be provided to determine that a grid fault accompanied by a frequency anomaly has occurred when the frequency change rate sω exceeds a predetermined threshold Fg, and change the switching signal Fsig from 0 to 1, for example. For example, if the frequency change rate sω falls below the threshold Fg, the frequency determination unit A122 determines that a frequency drop has occurred due to the tripping of a large generator in the power grid, and sends a switching signal Fsig to the correction calculation unit A13 so that the power conversion device 1A can supply more inertia. Conversely, the frequency determination unit A123 detects a significant frequency increase as a grid fault. In this embodiment, the frequency abnormality detection unit A12 shown in FIG. 4 is used as an example of a calculation algorithm for the switching signal Fsig, but any other method may be used as long as it is capable of estimating an abnormality in the power system 5 from the trend of the reference angular frequency ω0.

[0035] Furthermore, one example of the invention disclosed in the above-mentioned specification of the present application can also be expressed as follows.

[0036] <Part 1> A system-forming power conversion device that connects an asynchronous DC power supply to an AC power system, A main circuit for converting DC and AC, and a control circuit for controlling the main circuit as a system-forming type, The control circuit a limit command acquisition unit that acquires a limit command to limit AC power from an external source; a frequency detection unit that detects a fundamental frequency in the AC voltage of the AC power; a limit command correction unit that corrects the limit command according to the rate of increase and / or decrease of the fundamental wave frequency; A power conversion device comprising:

[0037] <Part 2> In <Part 1>, The power conversion device is characterized in that the limit command correction unit corrects the limit command so that the power sent to the power grid increases when the rate of decrease of the fundamental wave frequency exceeds a threshold.

[0038] <Part 3> In <1> or <2>, The control circuit an active power command generating unit that generates an active power command from the DC voltage and DC current of the asynchronous DC power supply; a power calculation unit that calculates effective power from the voltage and current of the AC power and detects a fundamental frequency; a phase command generating unit that generates a phase command from the active power command, the active power, the fundamental frequency, and the limit command, the phase command generation unit corrects the phase command by correcting an active power command or an active power using the corrected limit command; A power conversion device characterized in that the main circuit is controlled using a corrected phase command.

[0039] <Part 4> In <Part 3>, The control circuit The inverter includes a reactive power command generating unit, a reactive power control unit, and a main circuit control unit, the power calculation unit calculates reactive power from the voltage and current of the AC power, the reactive power command generation unit generates a reactive power command from a voltage of the AC power, the reactive power control unit generates a voltage command from the reactive power command and the reactive power; The power conversion device is characterized in that the main circuit control unit generates a PWM signal for controlling the main circuit from the phase command and the voltage command. [Explanation of symbols]

[0040] 1A...power conversion device, 1B...power conversion device, 2...main circuit, 3...control circuit, 31...power calculation unit, 32...active power command generation unit, 33...phase command generation unit, 34...reactive power command generation unit, 35...reactive power control (AQR) unit, 36...main circuit control unit, 4...filter circuit, 5...power system, 6...AC load, 7A...DC power supply, 7B...DC power supply, 80...overall controller, 82...system operation server

Claims

1. A system-forming power conversion device that connects an asynchronous DC power supply to an AC power system, A main circuit for converting DC and AC, and a control circuit for controlling the main circuit as a system-forming type, The control circuit a limit command acquisition unit that acquires a limit command to limit AC power from an external source; a frequency detection unit that detects a fundamental frequency in the AC voltage of the AC power; a limit command correction unit that corrects the limit command according to the rate of increase and / or decrease of the fundamental wave frequency; A power conversion device comprising:

2. In claim 1, The power conversion device is characterized in that the limit command correction unit corrects the limit command so as to increase the power sent to the power grid when the rate of decrease of the fundamental wave frequency exceeds a threshold value.

3. In claim 1 or 2, The control circuit an active power command generating unit that generates an active power command from the DC voltage and DC current of the asynchronous DC power supply; a power calculation unit that calculates effective power from the voltage and current of the AC power and detects a fundamental frequency; a phase command generating unit that generates a phase command from the active power command, the active power, the fundamental frequency, and the limit command, the phase command generation unit corrects the phase command by correcting an active power command or an active power using the corrected limit command; A power conversion device characterized in that the main circuit is controlled using a corrected phase command.

4. In claim 3, The control circuit The inverter includes a reactive power command generating unit, a reactive power control unit, and a main circuit control unit, the power calculation unit calculates reactive power from the voltage and current of the AC power, the reactive power command generation unit generates a reactive power command from a voltage of the AC power, the reactive power control unit generates a voltage command from the reactive power command and the reactive power; The power conversion device is characterized in that the main circuit control unit generates a PWM signal for controlling the main circuit from the phase command and the voltage command.

Images