Grid Simulator and Testing Device for Grid-Connected Inverter Stability Testing

Abstract

The present invention provides a grid simulator and a test device for the stability test of a grid-connected converter. Among them: The voltage-controlled converter uses a virtual impedance controller and a voltage controller to achieve the port voltage characteristics of the simulated grid, and the voltage-controlled converter has a relatively high switching frequency; the current-controlled converter uses a transient current controller and a current controller to absorb or generate the port load current of the simulated grid, and the current-controlled converter has a relatively low switching frequency; the AC sides of the two converters are connected in parallel and directly connected to the port of the simulated grid, or connected to the port of the simulated grid through a resistor-impedance network. The virtual impedance controller eliminates the differential operator through the Norton equivalent circuit and accurately reproduces the port impedance characteristics of the grid. The present invention can effectively simulate and shape the grid impedance characteristics in a wide frequency band, so as to conveniently realize the stability test of the grid-connected inverter under different grid impedance conditions.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and more particularly, to a grid simulator and a test device for grid-connected converter stability testing. Background Art

[0002] In recent years, with the large-scale application of new energy power generation, the proportion of grid-connected converters in the power grid has been increasing, and the stability of grid-connected converters in weak power grids has received more and more attention. Due to the modulation characteristics of different controllers in the converter, the grid-connected converter may generate resonances in three frequency bands. Among them, the frequency of low-frequency resonance is near the fundamental frequency, and the main influencing factors are low-bandwidth control systems such as phase-locked loops, power control, and DC voltage control; the frequency of medium-frequency resonance is in the range of several hundred hertz to several thousand hertz, and the main influencing factors are high-bandwidth control systems such as AC voltage control and AC current control; the frequency of high-frequency resonance is mainly around half of the switching frequency to the switching frequency, and the main influencing factor is the pulse width modulator.

[0003] To ensure the stability of the power grid, grid-connected converters need to be tested for stability before grid connection. The traditional stability test of grid-connected converters in weak power grids uses an AC voltage source in series with an actual inductor to simulate the characteristics of a weak power grid, and different weak power grids with different short-circuit ratios correspond to different inductor values. This test method has the following limitations:

[0004] High cost; this method requires different inductors to simulate weak power grids with different short-circuit ratios.

[0005] Low accuracy; an inductor is a non-linear device, and its value is different when flowing through different current values.

[0006] Poor flexibility; when testing different short-circuit ratios, the inductor needs to be continuously replaced.

[0007] After retrieval, it is found that:

[0008] The Chinese invention patent "A Detection Device and Test Method for the Anti-islanding Effect Protection Ability of Grid-connected Inverters" with the authorization announcement number CN103983880B includes a programmable DC power supply, a programmable AC power supply, a generator drive platform, a rectification module, an RLC adjustable load, a grid simulator, and an anti-islanding effect measurement and control platform. It is characterized in that: the programmable DC power supply is connected to the photovoltaic grid-connected inverter under test, and the programmable AC power supply is connected to the wind power grid-connected inverter under test either through the generator drive platform or directly. The photovoltaic grid-connected inverter under test and the wind power grid-connected inverter under test are connected to the RLC adjustable load and are also connected to the grid simulator through an AC contactor or directly connected to the grid. During the off-grid test, the anti-islanding effect measurement and control platform applies a trigger signal to the AC contactor, and an oscilloscope collects the voltage signal across the auxiliary contacts of the AC contactor as the off-grid trigger signal source. The present invention also provides a method for detecting the anti-islanding ability. The present invention has the following advantages: high test efficiency, simple operation; high measurement accuracy; and expanded detection ability range. However, the device and method still have the following problems:

[0009] 1. This technology cannot simulate any given grid impedance;

[0010] 2. This technology cannot be used for the stability test of grid-connected inverters.

[0011] Currently, no description or report of similar technologies to the present invention has been found, and no similar materials at home and abroad have been collected either. Summary of the Invention

[0012] In view of the above problems existing in the prior art, the present invention proposes a grid simulator and a test device for the stability test of grid-connected inverters.

[0013] According to one aspect of the present invention, there is provided a grid simulator for the stability test of grid-connected inverters, including a current-controlled converter, a voltage-controlled converter, a first DC power supply port, a second DC power supply port, an AC test port, a voltage converter control system, and a current converter control system; wherein:

[0014] The port of the current-controlled converter includes a DC input terminal and an AC output terminal, and the port of the voltage-controlled converter includes a DC input terminal and an AC output terminal;

[0015] The DC input terminal of the current-controlled converter is connected to the first DC power supply port, the DC input terminal of the voltage-controlled converter is connected to the second DC power supply port, and the first DC power supply port and the second DC power supply port are respectively connected to an external power supply;

[0016] After the AC output terminals of the current-controlled converter and the AC output terminals of the voltage-controlled converter are connected in parallel, they are connected to the AC test port; the AC test port outputs the grid characteristics and is connected to the external converter to be tested;

[0017] The voltage-controlled converter includes a high-switching-frequency converter and an impedance network B connected to each other, and is used to simulate the impedance of the medium-frequency power grid (simulated power grid) by controlling the voltage of the AC test port. Among them, the impedance network B is used to filter out voltage harmonics;

[0018] The current-controlled converter includes a low-switching-frequency converter and an impedance network A connected to each other, and is used to provide or absorb the power of the converter to be tested by sending or absorbing the fundamental low-frequency current of the AC test port; among them, the impedance network A is used to filter out current harmonics;

[0019] The voltage converter control system is connected to the voltage-controlled converter and includes a virtual impedance controller, a voltage controller, and a high-frequency pulse width modulator connected in sequence; among them, the input signal of the virtual impedance controller is used as the input signal of the voltage converter control system, the output signal of the virtual impedance controller is used as the input signal of the voltage controller, the output signal of the voltage controller is used as the input signal of the high-frequency pulse width modulator, and the output signal of the high-frequency pulse width modulator is used as the switching signal of the high-switching-frequency converter of the voltage-controlled converter; where:

[0020] The virtual impedance controller is used to describe the port impedance characteristics of the simulated power grid, and the series voltage source v sref and impedance Z ref in the weak grid model are equivalent to the Norton equivalent circuit, that is, the parallel current source v sref / Z ref and impedance Z ref , and the control object of the virtual impedance controller is the current i ref flowing through the parallel impedance Z z ;

[0021] The voltage controller is used to generate the modulation signal e v of the high-switching-frequency converter of the voltage-controlled converter, and the high-frequency pulse width modulator uses the modulation signal e v as the high-frequency switching signal corresponding to the high-switching-frequency converter;

[0022] The input signals of the virtual impedance controller include: the voltage signal v sref of the ideal voltage source in the simulated power grid, the sampled voltage signal v g of the AC test port, and the sampled current signal ig ; The output signals of the virtual impedance controller include an output reference signal and an output feedback signal;

[0023] The output reference signal and the output feedback signal of the virtual impedance controller are respectively the input reference signal i sref and the input feedback signal i s of the voltage controller. The output signal of the voltage controller is the input modulation signal e v ;

[0024] The input signal of the high-frequency pulse width modulator is the output signal e v of the voltage controller. The output signal of the high-frequency pulse width modulator is the switching signal of the high-switching-frequency converter;

[0025] The current converter control system is connected to the current-controlled converter and is used to absorb or emit the load current of the AC test port. It includes a transient current controller, a current controller, and a low-frequency pulse width modulator connected in sequence. Among them, the transient current controller is used to suppress the transient current flowing through the voltage-controlled converter when the current at the AC test port undergoes a transient mutation; the current controller is used to generate the switching signal of the low-switching-frequency converter; the low-frequency pulse width modulator provides the switching signal for the low-switching-frequency converter of the current-controlled converter. Among them:

[0026] The input signals of the transient current controller include an input reference signal and an input feedback signal. Among them, the input reference signal of the transient current controller is 0, and the input feedback signal of the transient current controller is the output current signal i g_v of the high-switching-frequency converter of the voltage-controlled converter;

[0027] The input signals of the current controller include an input reference signal and an input feedback signal. Among them, the current sampling signal i g of the AC test port and the output signal Δi g_i_ref of the transient current controller are subtracted to obtain the input reference signal of the current controller; the input feedback signal of the current controller is the current signal flowing through the impedance network A; the output signal of the current controller is the input modulation signal e i of the low-frequency pulse width modulator; the output signal of the low-frequency pulse width modulator is the switching signal of the low-switching-frequency converter.

[0028] Preferably, the high-switching-frequency converter adopts power semiconductor devices with high switching frequencies to achieve the high switching frequency of the voltage-controlled converter and provide high-frequency current. Among them, the ports of the high-switching-frequency converter include a DC terminal and an AC terminal. The DC terminal is connected to the second DC power supply port, and the AC terminal is connected to the impedance network B.

[0029] Preferably, the low-switching-frequency converter adopts power semiconductor devices with large voltage and current capacities to provide fundamental-frequency and low-frequency current. Among them, the ports of the low-switching-frequency converter include a DC terminal and an AC terminal. The DC terminal is connected to the first DC power supply port, and the AC terminal is connected to the impedance network A.

[0030] Preferably, the impedance network A adopts a current filtering structure; the impedance network B adopts a voltage filtering structure.

[0031] Preferably, the virtual impedance controller is used to describe the port impedance characteristics in the simulated power grid. Among them, the input signals of the virtual impedance controller are the voltage of the voltage source in the simulated power grid, the sampled voltage of the AC test port, and the sampled current of the AC test port; the output signal of the virtual impedance controller is the input signal of the voltage controller.

[0032] The virtual impedance controller includes two links: a given calculation link and a feedback calculation link. Among them, the input signal of the given calculation link is the given signal of the voltage source in the simulated power grid. After the voltage source given signal is divided by the impedance of the Norton equivalent circuit, a current source signal is obtained, and the current source signal is the output given signal; the input signals of the feedback calculation link are the sampled voltage signal of the AC test port and the sampled current signal of the AC test port. After the sampled voltage signal is divided by the impedance of the Norton equivalent circuit and added to the sampled current signal, a current source sampled signal is obtained, and the current source sampled signal is the output feedback signal.

[0033] Preferably, the parameters of the voltage controller are jointly determined by the given gain coefficient and the impedance of the Norton equivalent circuit (the simulated impedance); the parameters of the transient current controller and the current controller are independent of the parameters of the simulated power grid and only related to the parameters of the impedance network A, and can be designed independently, which simplifies the design process.

[0034] Preferably, the power grid simulator further includes any one or more of the impedance network C, the impedance network D, and the impedance network E. Among them:

[0035] The impedance network C is used to be connected between the DC input terminal of the current-controlled converter and the first DC power supply port.

[0036] The impedance network D is used to be connected between the DC input end of the voltage-controlled converter and the second DC power supply port;

[0037] The impedance network E is used to be connected between the parallel connection of the AC output end of the current-controlled converter and the AC output end of the voltage-controlled converter and the AC test port.

[0038] Preferably, the impedance network C and / or the impedance network D is used to block the common-mode current of the DC port.

[0039] Preferably, the impedance network E adopts an AC common-mode inductor or a three-phase transformer to block the common-mode current in the AC loop; wherein, the side of the three-phase transformer connected to the AC test port adopts a Δ connection mode.

[0040] According to another aspect of the present invention, a grid-connected converter stability test device is provided, which is implemented by using the grid simulator described in any one of the above; wherein: the current-controlled converter and the voltage-controlled converter of the grid simulator are respectively used as the two converters of the test device, the voltage converter control system and the current converter control system of the grid simulator are respectively used as the two controllers of the test device, the DC power supply port is connected to an external power supply, and the AC test port is connected to the device under test.

[0041] Due to the adoption of the above technical solutions, compared with the prior art, the present invention has at least one of the following beneficial effects:

[0042] The grid simulator and the test device for grid-connected converter stability test provided by the present invention adopt the virtual impedance technology, and the virtual impedance controller eliminates the differential operator through the Norton equivalent circuit, accurately reproducing the port impedance characteristics of the power grid.

[0043] The grid simulator and the test device for grid-connected converter stability test provided by the present invention can effectively simulate and shape the power grid impedance characteristics in a wide frequency band, so as to conveniently realize the stability test of the grid-connected inverter under different power grid impedance conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, objects and advantages of the present invention will become more obvious:

[0045] Figure 1 It is a schematic structural diagram of a grid simulator for grid-connected converter stability test in an embodiment of the present invention;

[0046] Figure 2Schematic diagram of the three-phase DC / AC power electronic converter topology of a low switching frequency converter in a preferred embodiment of the present invention;

[0047] Figure 3 Schematic diagram of the three-phase DC / AC power electronic converter topology of a high switching frequency converter in a preferred embodiment of the present invention;

[0048] Figure 4 Schematic diagram of the topology of a passive resistive impedance network A connected to the AC side of a low switching frequency converter in a preferred embodiment of the present invention;

[0049] Figure 5 Schematic diagram of the topology of a passive resistive impedance network B connected to the AC side of a high switching frequency converter in a preferred embodiment of the present invention;

[0050] Figure 6 Schematic diagram of the structure of the simulated power grid in a specific application example of the present invention;

[0051] Figure 7 Schematic diagram of the Norton equivalent circuit structure of the simulated power grid in a specific application example of the present invention;

[0052] Figure 8 Schematic diagram of the virtual impedance control link and the virtual current control link in a specific application example of the present invention;

[0053] Figure 9 Schematic diagram of the transient current control link in a specific application example of the present invention;

[0054] Figure 10 Schematic diagram of the current control link in a specific application example of the present invention;

[0055] Figure 11 Schematic diagram of the topology of a passive resistive impedance network C connected to the DC side of a low switching frequency converter in a preferred embodiment of the present invention;

[0056] Figure 12 Schematic diagram of the topology of a passive resistive impedance network D connected to the DC side of a high switching frequency converter in a preferred embodiment of the present invention;

[0057] Figure 13 Schematic diagram of the topology of a passive resistive impedance network E for connecting the AC side of a voltage-controlled converter, the AC side of a current-controlled converter, and the input side of an AC test port in a preferred embodiment of the present invention;

[0058] Figure 14This is a schematic diagram of the topology of another passive impedance network E for connecting the AC side of a voltage-controlled converter, the AC side of a current-controlled converter, and the input side of an AC test port in a preferred embodiment of the present invention.

[0059] In the figure: 1 - low-switching-frequency converter; 2 - high-switching-frequency converter; 3 - impedance network A; 4 - impedance network B; 5 - current converter control system; 6 - transient current controller; 7 - virtual impedance controller; 8 - voltage controller; 9 - voltage converter control system; 10 - first DC power supply port; 11 - AC test port; 12 - DC power supply terminal of the low-switching-frequency converter; 13 - AC terminal of the low-switching-frequency converter; 14 - DC power supply terminal of the high-switching-frequency converter; 15 - AC terminal of the high-switching-frequency converter; 16 - simulated power grid; 17 - Norton equivalent circuit of the simulated power grid; 18 - input terminal of impedance network A; 19 - output terminal of impedance network A; 20 - input terminal of impedance network B; 21 - output terminal of impedance network B; 22 - current controller; 23 - low-frequency pulse width modulator; 24 - high-frequency pulse width modulator; 25 - second DC power supply port; 26 - current-controlled converter; 27 - voltage-controlled converter; 28 - impedance network C; 29 - impedance network D; 30 - impedance network E; 31 - DC input terminal of impedance network C; 32 - DC output terminal of impedance network C; 33 - DC input terminal of impedance network D; 34 - DC output terminal of impedance network D; 35 - AC input terminal of impedance network E; 36 - AC output terminal of impedance network E. Detailed implementation manners

[0060] The following is a detailed description of the embodiments of the present invention: These embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention.

[0061] Figure 1 This is a schematic diagram of the structure of a grid simulator for medium-frequency stability testing of a grid-connected converter provided in an embodiment of the present invention.

[0062] As Figure 1 shown, the grid simulator for medium-frequency stability testing of a grid-connected converter provided in this embodiment may include: a current-controlled converter, a voltage-controlled converter, a first DC power supply port, a second DC power supply port, an AC test port, a voltage converter control system, and a current converter control system; wherein:

[0063] The ports of the current-controlled converter include a DC input terminal and an AC output terminal, and the ports of the voltage-controlled converter include a DC input terminal and an AC output terminal;

[0064] The DC input terminal of the current-controlled converter is connected to the first DC power supply port, and the DC input terminal of the voltage-controlled converter is connected to the second DC power supply port. The first DC power supply port and the second DC power supply port are respectively connected to an external power supply;

[0065] After the AC output terminals of the current-controlled converter and the voltage-controlled converter are connected in parallel, they are connected to the AC test port; the AC test port outputs the grid characteristics and is connected to the external converter to be tested;

[0066] The voltage-controlled converter includes a high-switching-frequency converter and an impedance network B connected to each other, which is used to simulate the impedance of the medium-frequency power grid (simulated power grid) by controlling the voltage of the AC test port. Among them, the impedance network B is used to filter out voltage harmonics;

[0067] The current-controlled converter includes a low-switching-frequency converter and an impedance network A connected to each other, which is used to provide or absorb the power of the converter to be tested by emitting or absorbing the fundamental low-frequency current of the AC test port; among them, the impedance network A is used to filter out current harmonics;

[0068] The voltage converter control system is connected to the voltage-controlled converter, and includes a virtual impedance controller, a voltage controller, and a high-frequency pulse width modulator connected in sequence; among them, the input signal of the virtual impedance controller is used as the input signal of the voltage converter control system, the output signal of the virtual impedance controller is used as the input signal of the voltage controller, the output signal of the voltage controller is used as the input signal of the high-frequency pulse width modulator, and the output signal of the high-frequency pulse width modulator is used as the switching signal of the high-switching-frequency converter of the voltage-controlled converter; where:

[0069] The virtual impedance controller is used to describe the port impedance characteristics of the simulated power grid, and the series voltage source v sref and impedance Z ref in the weak grid model are equivalent to the Norton equivalent circuit, that is, the parallel current source v sref / Z ref and impedance Z ref . The control object of the virtual impedance controller is the current i ref flowing through the parallel impedance Z z ;

[0070] The voltage controller is used to generate the modulation signal e v of the high-switching-frequency converter of the voltage-controlled converter. The high-frequency pulse width modulator uses the modulation signal e vAs the high-frequency switching signal corresponding to the high switching frequency converter;

[0071] The input signals of the virtual impedance controller include: the voltage signal v of the ideal voltage source in the simulated power grid sref , the sampled voltage signal v of the AC test port g and the sampled current signal i of the AC test port; g The output signals of the virtual impedance controller include an output reference signal and an output feedback signal;

[0072] The output reference signal and the output feedback signal of the virtual impedance controller are respectively the input reference signal i sref and the input feedback signal i s of the voltage controller. The output signal of the voltage controller is the input modulation signal e v of the high-frequency pulse width modulator;

[0073] The input signal of the high-frequency pulse width modulator is the output signal e v of the voltage controller, and the output signal of the high-frequency pulse width modulator is the switching signal of the high switching frequency converter;

[0074] The current converter control system is connected to the current-controlled converter and is used to absorb or emit the load current of the AC test port. It includes a transient current controller, a current controller, and a low-frequency pulse width modulator connected in sequence. Among them, the transient current controller is used to suppress the transient current flowing through the voltage-controlled converter when the current at the AC test port undergoes a transient mutation; the current controller is used to generate the switching signal of the low switching frequency converter; the low-frequency pulse width modulator provides the switching signal for the low switching frequency converter of the current-controlled converter. Among them:

[0075] The input signals of the transient current controller include an input reference signal and an input feedback signal. Among them, the input reference signal of the transient current controller is 0, and the input feedback signal of the transient current controller is the output current signal i g_v of the high switching frequency converter of the voltage-controlled converter;

[0076] The input signals of the current controller include an input reference signal and an input feedback signal. Among them, the current sampling signal i g of the AC test port is subtracted from the output signal Δi g_i_ref of the transient current controller to obtain the input reference signal of the current controller; the input feedback signal of the current controller is the current signal flowing through the impedance network A; the output signal of the current controller is the input modulation signal e i of the low-frequency pulse width modulator; the output signal of the low-frequency pulse width modulator is the switching signal of the low switching frequency converter.

[0077] The technical solution provided by the above-mentioned embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings and a preferred embodiment.

[0078] A preferred embodiment of the present invention provides a grid simulator for grid-connected converter stability testing. The grid simulator includes: a current-controlled converter, a voltage-controlled converter, a first DC power supply port, an impedance network C, a second DC power supply port, an impedance network D, an AC test port, an impedance network E, a voltage converter control system, and a current converter control system. Among them:

[0079] The impedance network C and the impedance network D are optional impedance networks. The impedance network C and the impedance network D are used to filter the common-mode current of the DC port, and a common-mode inductor is often used. Both the impedance network C and the impedance network D have a DC input terminal and a DC output terminal. Among them, the DC input terminal of the impedance network C is connected to the first DC power supply port, and the DC output terminal of the impedance network C is connected to the current-controlled converter; the DC input terminal of the impedance network D is connected to the second DC power supply port, and the DC output terminal of the impedance network D is connected to the voltage-controlled converter.

[0080] The impedance network E has an AC input terminal and an AC output terminal. Among them, the AC input terminal of the impedance network E is connected to the AC sides of the current-type converter and the voltage-type converter, and the AC output terminal of the impedance network E is connected to the AC test port. The impedance network E is used to filter the common-mode current in the AC loop, and a common-mode inductor or a three-phase transformer is often used. Among them, the side of the three-phase transformer connected to the AC test port adopts a Δ connection method.

[0081] The voltage-controlled converter is used to simulate the port voltage of the grid at the circuit level, and the current-controlled converter is used to absorb or generate the power of the AC test port at the circuit level.

[0082] The components of the voltage controller converter include: a high-switching-frequency converter and an impedance network B.

[0083] The semiconductor devices of the high-switching-frequency converter adopt high-switching-frequency devices (for example, devices with a switching frequency higher than 100 kHz). After passing through the impedance network D, the positive and negative ports on the DC side of the high-switching-frequency converter are respectively connected to the positive and negative poles of the second DC power supply port, or directly connected to the positive and negative poles of the second DC power supply port. The AC output terminal of the high-switching-frequency converter is connected to the first end of the impedance network B.

[0084] One end of the impedance network B is connected to the AC end of the high-switching-frequency converter, and the other end is connected to the AC test port after passing through the impedance network E, or directly connected to the AC test port.

[0085] The components of the current-controlled converter include: a low-switching-frequency converter and an impedance network A.

[0086] The semiconductor devices of the low-switching-frequency converter use high-power devices (such as devices with a switching frequency lower than 20 kHz). After passing through the impedance network C, the positive and negative ports of the DC side of the low-switching-frequency converter are respectively connected to the positive and negative poles of the first DC power supply port, or directly connected to the positive and negative poles of the first DC power supply port. The AC output terminal of the low-switching-frequency converter is connected to the first end of the impedance network A.

[0087] One end of the impedance network A is connected to the AC terminal of the low-switching-frequency converter, and the other end is connected to the AC test port after passing through the impedance network E, or directly connected to the AC test port.

[0088] Among the impedance network A and the impedance network B, the impedance network B is used to filter voltage harmonics; the impedance network A is used to filter current harmonics.

[0089] The AC test port, the output terminal is connected to the converter to be tested.

[0090] The virtual impedance controller is used to describe the port impedance characteristics of the power grid to be simulated; according to the current i g of the AC test port, the voltage v g of the AC test port, simulate the voltage source voltage v sref in the power grid and the line impedance Z ref in the simulated power grid, and generate the input signal of the voltage controller.

[0091] Specifically, according to the Norton principle of the circuit, the power grid to be simulated is equivalent to a Norton equivalent circuit, and the equivalent circuit is a current source in parallel with an impedance. The current of the current source in the equivalent circuit is the voltage source voltage signal v sref in the power grid to be simulated divided by the power grid impedance Z ref , and the impedance in the equivalent circuit is equal to the power grid impedance.

[0092] The port voltage characteristics of the power grid to be simulated are converted into the current characteristics of the current source in the Norton equivalent circuit. The given value i sref of the current source is the current signal v sref / Z ref of the current source in the Norton equivalent circuit, and the feedback signal of the current source is the current v gref / Z ref flowing through the impedance plus the sampled current i g of the AC test port.

[0093] A voltage controller, which takes the output signal of a virtual impedance controller as an input, generates a pulse width modulation signal for a high-switching-frequency converter, and controls the device switches of the high-switching-frequency converter to simulate the voltage characteristics of a given grid port.

[0094] A voltage controller with an input signal being a virtual current reference signal i zref subtracts a virtual current feedback signal i z . The voltage controller adopts a PI controller, and the proportional control coefficient and the integral control coefficient are determined by three parameters, namely a gain coefficient K VCL , an inductance L in the simulated grid impedance ref and a resistance R in the simulated grid impedance ref . Among them, the proportional control coefficient is the gain coefficient multiplied by the inductance in the simulated grid impedance, and the integral control coefficient is the gain coefficient multiplied by the resistance in the simulated grid impedance.

[0095] A high-frequency pulse width modulator, which is used to generate a switching signal for a high-switching-frequency converter, and has a relatively high switching frequency.

[0096] A transient current controller, which is used to suppress the transient current flowing through a voltage-controlled converter during the current mutation process at an AC test port.

[0097] A transient current controller with a reference input signal of 0, a feedback input signal being the current flowing through a resistor-capacitor network B, and an output signal added to the sampled current signal at the AC test port to obtain a reference signal for the current controller. The transient current controller adopts a proportional controller.

[0098] A current controller, which is used to convert a current reference signal into a device switching signal for a low-switching-frequency converter, so as to achieve power control of the low-switching-frequency converter.

[0099] A current controller takes the sum of an AC test port current i g and the output Δi of the transient current controller g_i_ref as a reference signal, takes the output current i of the low-switching-frequency converter g_i as a feedback signal, generates a pulse width modulation signal for the low-switching-frequency converter, and controls the device switches of the low-switching-frequency converter to absorb most of the power at the AC test port.

[0100] The input port of the current controller is the input port of the current controller, and the output port of the current controller is connected to the input port of the low-frequency pulse width modulator. The control frequency of the current controller is the fundamental frequency and low frequencies.

[0101] A low-frequency pulse width modulator, which is used to generate a switching signal for a low-switching-frequency converter, and has a relatively low switching frequency.

[0102] Specifically:

[0103] As Figure 1 shown, the components of the power grid simulator proposed in the above preferred embodiment of the present invention include: a current-controlled converter 26, a voltage-controlled converter 27, a DC power supply port 10, an impedance network 28, an impedance network 29, an impedance network 30, a DC power supply port 25, an AC test port 11, a virtual impedance controller 7, a voltage controller 8, a transient current controller 6, and a current controller 22. It should be noted that Figure 1 auxiliary circuits and software modules are omitted. Adding conventional circuit modules to the technical solution provided in the above preferred embodiment of the present invention also belongs to the essential content of the present invention.

[0104] The current-controlled converter 26 includes a low-switching-frequency converter 1 and an impedance network 3.

[0105] The low-switching-frequency converter 1 is used to absorb or generate the power of the AC test port at the circuit level, and can adopt any DC / AC topology structure including but not limited to a three-phase two-level structure. The semiconductor device can be selected from but not limited to high-power fully controlled or semi-controlled power devices such as Si IGBT. The three-phase two-level structure adopted by the low-switching-frequency converter 1 is as Figure 2 shown.

[0106] The impedance network A is a circuit structure composed of one or more of passive devices such as a resistor R, an inductor L, and a capacitor C, including at least one set of input terminals and output terminals, and is used to cooperate with the low-switching-frequency converter to absorb or generate the power of the AC test terminal and reduce the high-order harmonics of the current at the AC test terminal in the system; the passive impedance network adopts a circuit topology structure including but not limited to Figure 4 and so on.

[0107] The voltage-controlled converter 27 includes a high-switching-frequency converter 2 and an impedance network 4.

[0108] The high-switching-frequency converter 2 is used to simulate the port voltage of a weak power grid at the circuit level, and can adopt any DC / AC topology structure including but not limited to a three-phase two-level structure. The semiconductor device can be selected from but not limited to high-switching-frequency fully controlled or semi-controlled power devices such as SiC MOSFET. The three-phase two-level structure adopted by the high-switching-frequency converter 2 is as Figure 3 shown.

[0109] The impedance network B is a circuit structure composed of one or more of passive devices such as a resistor R, an inductor L, and a capacitor C, including at least one set of input terminals and output terminals, and is used to cooperate with the high-switching-frequency converter to shape the voltage at the AC test terminal and reduce the high-order harmonics of the voltage at the AC test terminal in the system; the passive impedance network adopts a circuit topology structure including but not limited toFigure 5 circuit topologies including

[0110] The impedance network C is a circuit structure formed by one or more of passive devices such as resistors R and inductors L, including at least one set of DC input terminals and DC output terminals for suppressing common-mode current; the passive impedance network adopts circuit topologies including but not limited to Figure 11 those including

[0111] The impedance network D is a circuit structure formed by one or more of passive devices such as resistors R and inductors L, including at least one set of DC input terminals and DC output terminals for suppressing common-mode current; the passive impedance network adopts circuit topologies including but not limited to Figure 12 those including

[0112] The impedance network E is a circuit structure formed by one or more of passive devices such as resistors R, inductors L, and transformers, including at least one set of AC input terminals and AC output terminals for suppressing common-mode current; the passive impedance network adopts circuit topologies including but not limited to Figure 13 and Figure 14 those including

[0113] The first DC power supply port 10 includes a positive terminal and a negative terminal, and is connected to the positive and negative poles of a low-switching-frequency converter or the impedance network C. The power supply methods include but are not limited to DC voltage sources, rectifier circuits, etc.

[0114] The second DC power supply port 25 includes a positive terminal and a negative terminal, and is connected to the positive and negative poles of a high-switching-frequency converter or the impedance network D. The power supply methods include but are not limited to DC voltage sources, rectifier circuits, etc.

[0115] The first DC power supply port 10 and the second DC power supply port 25 can be connected to the same power supply or different power supplies.

[0116] The AC test port 11 has its input terminal directly connected to the AC output terminals of a voltage-controlled converter and a current-controlled converter, or connected to the AC output terminals of a voltage-controlled converter and a current-controlled converter through the impedance network E; the output terminal is connected to the converter to be tested.

[0117] The following will use Figure 1 , Figure 6 and Figure 7 The circuit structure schematic diagrams shown as specific application examples to further describe the implementation process of the technical solutions provided in the above embodiments of the present invention. This specific application example is for the implementation solution of a grid simulator in the abc stationary coordinate system.

[0118] The control system of the power grid simulator includes the control system of the voltage-controlled converter and the control system of the current-controlled converter.

[0119] Specifically, the control system of the voltage-controlled converter mainly includes a virtual impedance controller 7, a voltage controller 8, and a high-frequency pulse width modulator 22, where:

[0120] In the first step, by giving the model of the simulated power grid, the Figure 6 voltage signal v of the ideal voltage source in the simulated power grid is obtained sref and the series power grid impedance Z ref ; through the sampling circuit, the current signal i at the AC test port is detected g ;

[0121] In the second step, Figure 6 the voltage signal at the output port can be sorted out as:

[0122] v gref = v sref - i g Z ref (1)

[0123] In the third step, the Figure 6 circuit is equivalently processed into a Figure 7 circuit, where the current signal of the current source is v sref / Z ref , and the parallel impedance is Z ref . Through Figure 7 , the given current signal of the ideal current source can be obtained as:

[0124]

[0125] From this, the given signal i of the virtual impedance controller as shown in Figure 8 can be obtained: sref The given value of the current flowing through the impedance is the given signal v of the current source Figure 7 / Z sref / Z ref .

[0126] In the fourth step, through the sampling circuit, the voltage signal v at the AC test port is detected g and the current signal i g , and the actual current signal flowing through the current source in Figure 7 is obtained as

[0127]

[0128] From this, the actual feedback signal i of the virtual impedance controller as shown in Figure 8 can be obtained: the voltage signal v at the AC test port z : the voltage signal v at the AC test port gDivided by the parallel impedance Z ref , and then added to the current signal i g of the AC test port.

[0129] Step 5, subtract the given current signal flowing through the Figure 7 impedance from the actual feedback current signal to obtain the input signal of the voltage controller.

[0130] The voltage controller adopts a proportional-integral controller, and the controller coefficients are jointly determined by the given control gain K VCL and the impedance Z ref in the simulated power grid. Among them: the coefficient of the proportional controller is the given control gain K VCL multiplied by the inductance L ref of the power grid impedance, and the absorption of the integral controller is the given control gain K VCL multiplied by the resistance R ref of the power grid impedance. Add the outputs of the proportional controller and the integral controller to obtain the output signal e v .

[0131] Step 6, obtain the switching signal of the switching tube by passing the modulation signal e v output by the voltage controller 8 through the high-frequency modulation module 22.

[0132] Specifically, the control system of the low switching frequency converter mainly consists of a transient current controller 6, a current controller 22, and a low-frequency pulse width modulator 23, where:

[0133] Step 1, through the sampling circuit, obtain the current i g_v at the output port of the impedance network 4, and input it to the transient current controller as a feedback control signal. The given control signal of the transient current controller is 0.

[0134] Step 2, as Figure 9 shown, the transient current controller adopts a proportional controller, and the output signal is Δi g_i_ref .

[0135] Step 3, through the sampling circuit, obtain the current i g at the AC test port, and add the output of the transient current controller and the current i g at the AC test port to obtain the input given signal of the current controller 5 as Figure 10 shown;

[0136] Step 4, through the sampling circuit, obtain the current i g_i at the output port of the impedance network 3. The current i g_i at the output port of the impedance network 3 is the feedback input signal of the current controller 5 as Figure 10 shown;

[0137] Step 5: The current controller 22 adopts a proportional-resonant controller to achieve zero-static error control, and the resonant frequency of the resonant controller in the proportional-resonant controller is the fundamental frequency of the system;

[0138] Step 6: The output of the current controller 22 is a modulation signal e i , and the modulation signal e i generates the switching signals of the switching devices in the low-switching-frequency converter through the low-frequency modulation link 23;

[0139] An embodiment of the present invention provides a grid-connected converter stability test device, and this performance test device is implemented by using any one of the above grid simulators in the above embodiments of the present invention; wherein: the current-controlled converter and the voltage-controlled converter of the grid simulator are respectively used as the two converters of the test device, and the voltage converter control system and the current converter control system of the grid simulator are respectively used as the two controllers of the test device.

[0140] For the grid simulator and the test device for grid-connected converter stability test provided in the above embodiments of the present invention, the voltage-controlled converter adopts a virtual impedance controller and a voltage controller to implement the port voltage characteristics of the simulated power grid, and the switching frequency of the voltage-controlled converter is relatively high; the current-controlled converter adopts a transient current controller and a current controller to absorb or generate the port load current of the simulated power grid, and the switching frequency of the current-controlled converter is relatively low; the AC sides of the two converters are connected in parallel and directly connected to the port of the simulated power grid, or connected to the port of the simulated power grid through a resistor-capacitor network. For the grid simulator and the test device for grid-connected converter stability test provided in the embodiments of the present invention, the virtual impedance controller eliminates the differential operator through the Norton equivalent circuit and accurately reproduces the port impedance characteristics of the power grid; it can effectively simulate and shape the power grid impedance characteristics in a wide frequency band, thereby conveniently realizing the stability test of the grid-connected inverter under different power grid impedance conditions.

[0141] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which does not affect the essence of the present invention.

Claims

1. A grid simulator for grid-connected converter stability testing, characterized in that, It includes a current-controlled converter, a voltage-controlled converter, a first DC power supply port, a second DC power supply port, an AC test port, a voltage converter control system, and a current converter control system; wherein: The ports of the current-controlled converter include a DC input terminal and an AC output terminal, and the ports of the voltage-controlled converter include a DC input terminal and an AC output terminal; The DC input terminal of the current-controlled converter is connected to the first DC power supply port, the DC input terminal of the voltage-controlled converter is connected to the second DC power supply port, and the first DC power supply port and the second DC power supply port are respectively connected to an external power supply; After the AC output terminals of the current-controlled converter and the voltage-controlled converter are connected in parallel, they are connected to the AC test port; the AC test port outputs the grid characteristics and is connected to an external converter to be tested; The voltage-controlled converter includes a high-switching-frequency converter and an impedance network B connected to each other, and is used to simulate the impedance of the medium-frequency power grid to be simulated by controlling the voltage of the AC test port. Among them, the impedance network B is used to filter out voltage harmonics; The current-controlled converter includes a low-switching-frequency converter and an impedance network A connected to each other, and is used to provide or absorb the power of the converter to be tested by emitting or absorbing the fundamental low-frequency current of the AC test port; among them, the impedance network A is used to filter out current harmonics; The voltage converter control system is connected to the voltage-controlled converter and includes a virtual impedance controller, a voltage controller, and a high-frequency pulse width modulator connected in sequence; wherein, the input signal of the virtual impedance controller is used as the input signal of the voltage converter control system, the output signal of the virtual impedance controller is used as the input signal of the voltage controller, the output signal of the voltage controller is used as the input signal of the high-frequency pulse width modulator, and the output signal of the high-frequency pulse width modulator is used as the switching signal of the high-switching-frequency converter of the voltage-controlled converter; wherein: The virtual impedance controller is used to describe the port impedance characteristics of the simulated power grid, and the series voltage source v in the weak power grid model sref and impedance Z ref are equivalent to a Norton equivalent circuit, that is, a parallel current source v sref / Z ref and impedance Z ref . The control object of the virtual impedance controller is the current i ref flowing through the parallel impedance Z z in the Norton equivalent circuit; The voltage controller is used to generate the modulation signal e of the high-switching-frequency converter of the voltage-controlled converter v , and the high-frequency pulse-width modulator uses the modulation signal e v as the high-frequency switching signal corresponding to the high-switching-frequency converter; The input signals of the virtual impedance controller include: the voltage signal v of the ideal voltage source in the simulated power grid sref , the sampled voltage signal v of the AC test port g and the sampled current signal i of the AC test port g ; The output signals of the virtual impedance controller include an output given signal and an output feedback signal; The output reference signal and the output feedback signal of the virtual impedance controller are respectively the input reference signal i sref and the input feedback signal i s of the voltage controller. The output signal of the voltage controller is the input modulation signal e v ; The input signal of the high-frequency pulse width modulator is the output signal e of the voltage controller v , and the output signal of the high-frequency pulse width modulator is the switching signal of the high-switching-frequency converter; The current converter control system is connected to the current-controlled converter and is used to absorb or emit the load current of the AC test port. It includes a transient current controller, a current controller, and a low-frequency pulse width modulator connected in sequence; wherein, the transient current controller is used to suppress the transient current flowing through the voltage-controlled converter when the current of the AC test port undergoes a transient mutation; the current controller is used to generate the switching signal of the low-switching-frequency converter; the low-frequency pulse width modulator provides the switching signal for the low-switching-frequency converter of the current-controlled converter; wherein: The input signals of the transient current controller include an input reference signal and an input feedback signal; among them, the input reference signal of the transient current controller is 0, and the input feedback signal of the transient current controller is the output current signal i of the high-switching-frequency converter of the voltage-controlled converter g_v ; The input signals of the current controller include an input reference signal and an input feedback signal; among them, the current sampling signal i of the AC test port g is subtracted from the output signal Δi of the transient current controller g_i_ref to obtain the input reference signal of the current controller; the input feedback signal of the current controller is the current signal flowing through the impedance network A; the output signal of the current controller is the input modulation signal e of the low-frequency pulse width modulator i ; the output signal of the low-frequency pulse width modulator is the switching signal of the low switching frequency converter.

2. The grid simulator for grid-connected converter stability testing according to claim 1, characterized in that, The high-switching-frequency converter uses a power semiconductor device that can operate at a high switching frequency to achieve the high switching frequency of the voltage-controlled converter and provide high-frequency current; among them, the ports of the high-switching-frequency converter include a DC end and an AC end, the DC end is connected to the second DC power supply port, and the AC end is connected to the impedance network B.

3. The grid simulator for grid-connected converter stability testing according to claim 1, characterized in that, The low-switching-frequency converter uses power semiconductor devices capable of withstanding large voltage and current capacities to provide fundamental-frequency and low-frequency currents. Among them, the ports of the low-switching-frequency converter include a DC terminal and an AC terminal. The DC terminal is connected to the first DC power supply port, and the AC terminal is connected to the impedance network A.

4. The grid simulator for grid-connected converter stability testing according to claim 1, characterized in that, The impedance network A adopts a current filtering structure; the impedance network B adopts a voltage filtering structure.

5. The grid simulator for grid-connected converter stability testing according to claim 1, characterized in that, The virtual impedance controller is used to describe the port impedance characteristics in the simulated power grid. Among them, the input signals of the virtual impedance controller are the voltage of the voltage source in the simulated power grid, the sampled voltage of the AC test port, and the sampled current of the AC test port; the output signal of the virtual impedance controller is the input signal of the voltage controller. The virtual impedance controller includes two links: a given calculation link and a feedback calculation link. Among them, the input signal of the given calculation link is the given signal of the voltage source in the simulated power grid. After the voltage source given signal is divided by the impedance of the Norton equivalent circuit, a current source signal is obtained, and the current source signal is the output given signal; the input signals of the feedback calculation link are the sampled voltage signal of the AC test port and the sampled current signal of the AC test port. After the sampled voltage signal is divided by the impedance of the Norton equivalent circuit and added to the sampled current signal, a current source sampled signal is obtained, and the current source sampled signal is the output feedback signal.

6. The grid simulator for grid-connected converter stability testing according to claim 1, characterized in that, The parameters of the voltage controller are jointly determined by the given gain coefficient and the impedance of the Norton equivalent circuit to ensure that the bandwidth of the simulated impedance remains unchanged; the parameters of the transient current controller and the current controller are independent of the parameters of the simulated power grid and are only related to the parameters of the impedance network A.

7. The grid simulator for grid-connected converter stability testing according to any one of claims 1-6, characterized in that, The power grid simulator further includes any one or more of an impedance network C, an impedance network D, and an impedance network E; among them: The impedance network C is used to be connected between the DC input terminal of the current-controlled converter and the first DC power supply port. The impedance network D is used to be connected between the DC input terminal of the voltage-controlled converter and the second DC power supply port. The impedance network E is used to be connected between the parallel connection of the AC output terminal of the current-controlled converter and the AC output terminal of the voltage-controlled converter and the AC test port.

8. The grid simulator for grid-connected converter stability testing according to claim 7, characterized in that, The impedance network C and / or the impedance network D is used to block the common-mode current of the DC port.

9. The grid simulator for grid-connected converter stability test according to claim 7, characterized in that The impedance network E adopts an AC common-mode inductor or a three-phase transformer to block the common-mode current in the AC loop; among them, the side of the three-phase transformer connected to the AC test port adopts a Δ connection method.

10. A grid-connected converter stability test device, characterized in that It is implemented by using the power grid simulator described in any one of claims 1-9; among them: the current-controlled converter and the voltage-controlled converter of the power grid simulator are respectively used as the two converters of the test device, the voltage converter control system and the current converter control system of the power grid simulator are respectively used as the two controllers of the test device, the DC power supply port is connected to an external power supply, and the AC test port is connected to the device under test.

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