An inverter, control method, and related devices

Abstract

This application discloses an inverter, a control method, and related equipment. The inverter includes an inverter circuit, a filter, and an impedance compensation circuit. The AC side of the inverter circuit is connected to a first terminal of the filter, and the second terminal of the filter is used to connect to a load. The impedance compensation circuit includes a compensation inductor and a compensation resistor connected in series. In the event of a grid imbalance fault, the impedance compensation circuit is connected between the second terminal of the filter and the point of common coupling (PCC). The PCC is used to connect to the AC grid. In the event of a balanced three-phase grid, the second terminal of the filter is directly connected to the PCC. This technical solution can control the load voltage balance when a grid imbalance occurs.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to an inverter, control method, and related equipment. Background Technology

[0002] Inverters can enable grid-connected power generation from DC sources, such as in photovoltaic, energy storage, or photovoltaic-storage applications. While connected to the grid, inverters can also power local loads. However, when a fault occurs in the three-phase AC grid, it can lead to single-phase, two-phase, or three-phase voltage imbalances, resulting in load voltage imbalances and affecting the quality of power supply to the load. Summary of the Invention

[0003] In view of this, this application provides an inverter, a control method, and related equipment that can control the load voltage balance supplied to the load when there is an imbalance in the power grid.

[0004] This application provides an inverter, including: an inverter circuit, a filter, and an impedance compensation circuit; the AC side of the inverter circuit is connected to a first terminal of the filter, and the second terminal of the filter is used to connect a load; the impedance compensation circuit includes a compensation inductor and a compensation resistor connected in series; in the event of a grid imbalance fault, the impedance compensation circuit is connected between the second terminal of the filter and a point of common coupling; the point of common coupling is used to connect to the AC grid; in the event of a three-phase balanced grid, the second terminal of the filter is directly connected to the point of common coupling.

[0005] In one possible implementation, the impedance compensation circuit further includes a buffer capacitor and a buffer resistor; the buffer capacitor and the buffer resistor are connected in series and then connected in parallel across the series-connected compensation inductor and compensation resistor.

[0006] One possible implementation further includes: a controller; the impedance compensation circuit further includes a controllable switch; the controllable switch is connected in parallel across the series-connected compensation inductor and compensation resistor; the controller is used to control the controllable switch to open when an imbalance fault occurs in the power grid, and to control the controllable switch to close when the three phases of the power grid are balanced.

[0007] In one possible implementation, the inductance of the compensation inductor and the resistance of the compensation resistor are obtained from the grid-connected current of the inverter and the voltage difference between the load voltage of the inverter and the point of common coupling; the negative sequence component of the load voltage is 0.

[0008] One possible implementation further includes a controller; the controller is configured to obtain an inverter-side current reference value based on the load power, load voltage, load current and the voltage of the common connection point after the impedance compensation circuit is connected between the second terminal of the filter and the common connection point, and control the inverter circuit based on the inverter-side current reference value.

[0009] In one possible implementation, the controller is specifically configured to obtain a grid-connected current target value based on the load voltage, the voltage of the common coupling point, the inductance value of the compensation inductor, and the resistance value of the compensation resistor; obtain a load current target value based on the load power and the load voltage; and obtain an inverter-side current reference value from the load current target value and the grid-connected current target value.

[0010] In one possible implementation, the controller is specifically used to determine whether an imbalance fault has occurred in the power grid based on the load voltage or the grid-connected current.

[0011] This application also provides a control method for an inverter, the inverter comprising: an inverter circuit, a filter, and an impedance compensation circuit; the AC side of the inverter circuit is connected to a first terminal of the filter, and the second terminal of the filter is used to connect a load; the impedance compensation circuit comprises a compensation inductor and a compensation resistor connected in series; the method comprises: in the event of an imbalance fault in the power grid, controlling the impedance compensation circuit to connect between the second terminal of the filter and a point of common coupling; the point of common coupling is used to connect to the AC power grid; and in the event of a three-phase balance in the power grid, controlling the second terminal of the filter to be directly connected to the point of common coupling.

[0012] In one possible implementation, the impedance compensation circuit further includes a controllable switch connected in parallel across the series-connected compensation inductor and resistor; controlling the impedance compensation circuit to connect between the second terminal of the filter and the common connection point specifically includes: controlling the controllable switch to open, so that the impedance compensation circuit is connected between the second terminal of the filter and the common connection point; controlling the second terminal of the filter to be directly connected to the common connection point specifically includes: controlling the controllable switch to close, so that the second terminal of the filter is directly connected to the common connection point.

[0013] One possible implementation further includes: after the impedance compensation circuit is connected between the second terminal of the filter and the common connection point, obtaining an inverter-side current reference value based on the load power, load voltage, load current and the voltage of the common connection point, and controlling the inverter circuit based on the inverter-side current reference value.

[0014] One possible implementation, wherein obtaining the inverter-side current reference value based on the load power, load voltage, load current, and the voltage of the point of common coupling, specifically includes: obtaining a grid-connected current target value based on the load voltage, the voltage of the point of common coupling, the inductance value of the compensation inductor, and the resistance value of the compensation resistor; obtaining a load current target value based on the load power and the load voltage; and obtaining the inverter-side current reference value from the load current target value and the grid-connected current target value.

[0015] One possible implementation also includes: determining whether an imbalance fault has occurred in the power grid based on the load voltage or grid-connected current.

[0016] This application also provides a control device, including a processor and a memory, wherein the memory is used to store programs, instructions or code, and the processor is used to execute the programs, instructions or code in the memory to complete the inverter control method described above.

[0017] This application also provides a computer-readable storage medium storing a computer program, which is loaded by a processor to execute the inverter control method described above.

[0018] The inverter provided in this application embodiment has an impedance compensation circuit between the second terminal of the filter and the point of common coupling (PCC). The impedance compensation circuit includes a compensation inductor and a compensation resistor connected in series. When an imbalance fault occurs in the power grid, an active power compensation voltage is established across the compensation resistor, and a reactive power compensation voltage is established across the compensation inductor. This creates a voltage difference across the impedance compensation circuit, thereby compensating for the voltage difference between the load and the PCC, and achieving balanced control of the load voltage. Attached Figure Description

[0019] Figure 1 A schematic diagram of an inverter connected to the grid, provided for an embodiment of this application;

[0020] Figure 2 A schematic diagram illustrating voltage drop during power grid imbalance, provided as an embodiment of this application;

[0021] Figure 3 A schematic diagram of another inverter provided in the embodiments of this application;

[0022] Figure 4 A schematic diagram of yet another inverter provided in the embodiments of this application;

[0023] Figure 5 for Figure 4 A schematic diagram of the corresponding controllable switch closure;

[0024] Figure 6 This is a schematic diagram of a two-level inverter provided in an embodiment of this application;

[0025] Figure 7 A schematic diagram of a three-level inverter provided in an embodiment of this application;

[0026] Figure 8 A flowchart of an inverter control method provided in this application embodiment;

[0027] Figure 9 This is a schematic diagram of a control device provided in an embodiment of this application. Detailed Implementation

[0028] To enable those skilled in the art to better understand and implement the technical solutions provided in the embodiments of this application, the principle of inverter grid connection will be introduced below with reference to the accompanying drawings.

[0029] See Figure 1 The figure is a schematic diagram of an inverter connected to the grid according to an embodiment of this application.

[0030] The inverter provided in this application embodiment includes an inverter circuit 100 and a filter 200. The DC side of the inverter circuit 100 is used to connect to a DC source, and can also be connected to the output terminal of the preceding power circuit, for example, the input voltage of the inverter circuit 100 is Vin. This application embodiment does not make specific limitations. The first terminal of the filter 200 is connected to the AC side of the inverter circuit 100, and the second terminal of the filter 200 is connected to the common coupling point PCC. In addition, the grid-connected system may also include a local load 300 in addition to the inverter, and the local load 300 is connected to the second terminal of the filter 200.

[0031] The three phases of the power grid are connected to the three phases of the point of common coupling. Figure 1 The three-phase voltages of the power grid in the middle are e ga e gb and e gc . Figure 1 The three-phase inductor L ga L ga L ga and three-phase resistance r ga r gb r gc These are the equivalent inductance and equivalent resistance of the power grid, respectively.

[0032] The three-phase output voltages of inverter circuit 100 are V respectively a V b V c The three-phase output currents are i a i b i c .

[0033] The three-phase output voltages of filter 200 are u za uzb u zc The three-phase output currents are i oa i ob i oc .

[0034] The three-phase currents supplied by the inverter to the local load of 300 are i za i zb i zc The three-phase grid-connected currents of the inverter are i ga i gb i gc Therefore, i oa =i za +i ga i ob =i zb +i gb i oc =i zc +i gc .

[0035] For inverters under load, the quality of the grid-connected current and the load voltage is generally a primary concern. When the three-phase AC grid is operating normally, i.e., when the grid is balanced, the inverter under load will control the grid-connected current to reduce the total harmonic distortion (THD) while maintaining current balance. When the load is unbalanced or nonlinear, the inverter will improve the quality of the load voltage and balance it. When a fault occurs in the three-phase AC grid, it can lead to single-phase, two-phase, or three-phase amplitude imbalance. In this case, the inverter needs to use software control to balance the grid-connected current.

[0036] However, in an unbalanced power grid, when the local load is a normal load, the grid-connected current balance of the inverter can be achieved through software control.

[0037] However, when the local load is a sensitive load, the priority of load voltage control takes precedence over grid current control, and load voltage balance control should be prioritized. For example, ordinary loads can be fans, heaters, lighting equipment, or household appliances; sensitive loads can be uninterruptible power supplies, frequency converters, CNC equipment, or motor controllers. These are just examples; the specific configuration can be adjusted based on actual needs. However, when load voltage balance is the control objective, a significant three-phase voltage difference Δu will exist between the unbalanced grid and the load voltage. a , Δu b , Δu c ,Right now

[0038]

[0039] According to Kirchhoff's voltage law, the three-phase voltage difference must be borne by the grid impedance, such as... Figure 2 As shown, where Figure 2 (a) represents a single-phase power grid slippage scenario. Figure 2 (b) represents the two-phase slippage scenario of the power grid. Figure 2 (c) This represents the three-phase voltage drop scenario. In a strong grid, the grid impedance is relatively small and can be almost ignored in calculations. In this case, no matter how large the grid-connected current is, it cannot establish a voltage difference between the load and the point of common coupling (PCC). In a weak grid, the grid impedance is larger, and the three-phase voltage difference Δu can be established by the grid-connected current and the grid impedance, i.e.

[0040]

[0041] Among them, the differential term in the voltage difference The value is relatively small, and the three-phase voltage difference Δu a , Δu b , Δu c Active voltage is mainly supported by the resistance of the grid impedance.

[0042] However, the grid impedance cannot be too high, as this will reduce the stability of the inverter's grid connection. If the grid impedance is too low, the grid current needs to be increased to establish a voltage difference, which will cause excessive power loss across the grid impedance and reduce the inverter's grid connection efficiency.

[0043] Therefore, the inverter provided in this application embodiment adds an impedance compensation circuit between the load and the point of common coupling (PCC). When the power grid is balanced, this impedance compensation circuit can be left unconnected and thus does not function. When the power grid is unbalanced, the impedance compensation circuit needs to be connected to compensate for both active and reactive voltages, thereby achieving load voltage balance.

[0044] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0045] See Figure 3 This figure is a schematic diagram of another inverter provided in an embodiment of this application.

[0046] The inverter provided in this application includes: an inverter circuit 100, a filter 200, and an impedance compensation circuit.

[0047] The AC side of the inverter circuit 100 is connected to the first terminal of the filter 200, and the second terminal of the filter 200 is used to connect to the local load 300; the common coupling point PCC is used to connect to the AC power grid. The impedance compensation circuit includes a compensation inductor L connected in series. cs and compensation resistor R csIt should be understood that each of the three phases of the inverter is connected in series with a compensation inductor and a compensation resistor, and the inductance value of the compensation inductor and the resistance value of the compensation resistor are the same for each phase.

[0048] In the event of a power grid imbalance fault, the impedance compensation circuit is connected between the second terminal of filter 200 and the point of common coupling (PCC). When the three phases of the power grid are balanced, the second terminal of filter 200 is directly connected to the PCC. That is, when the three phases of the power grid are balanced, the impedance compensation circuit may not function and is not connected between the second terminal of filter 200 and the PCC.

[0049] The inverter provided in this application embodiment has an impedance compensation circuit connected between the local load 300 and the common connection point PCC.

[0050] When the inverter uses the grid-connected current (i ga i gb i gc When the control target is , the compensation resistor R cs With compensation inductor L cs The load is not connected to the common point of connection (PCC); for example, a compensation resistor R can be connected in series. cs With compensation inductor L cs A controllable switch S is connected in parallel across the two ends. When the controllable switch S is closed, the compensation resistor R is bypassed. cs With compensation inductor L cs The impedance compensation circuit is not connected to the inverter, therefore it will not affect the inverter control strategy.

[0051] The inverter provided in this application embodiment further includes: a controller (not shown in the figure); the impedance compensation circuit further includes a controllable switch S; the controllable switch S is connected in parallel across the compensation inductor Lcs and the compensation resistor Rcs connected in series.

[0052] The controller is used to open the controllable switch S when an imbalance fault occurs in the power grid, and to close the controllable switch S when the three phases of the power grid are balanced.

[0053] When the power grid experiences any of the single-phase, two-phase, or three-phase imbalances, the inverter's load becomes a sensitive load, requiring load voltage balance as the primary control objective. In this situation, the controllable switch S is open, and the compensation resistor R... cs With compensation inductor L cs Connect to the inverter. Compensation resistor R cs Establish an active power compensation voltage and compensate inductor L. cs A reactive power compensation voltage is established on the load, and a voltage difference is established on the impedance compensation circuit to compensate for the voltage difference between the load and the point of common coupling (PCC), thereby achieving balanced control of the load voltage.

[0054] See Figure 4 This figure is a schematic diagram of another inverter provided in an embodiment of this application.

[0055] In the inverter provided in this application embodiment, the impedance compensation circuit further includes a buffer capacitor C. s and buffer resistor R s .

[0056] Buffer capacitor C s and buffer resistor R s After being connected in series, it is connected in parallel to the series compensation inductor L. cs and compensation resistor R cs The two ends.

[0057] This application includes a buffer capacitor C. s and buffer resistor R s The purpose is to suppress voltage surges. In the compensation resistor R... cs With compensation inductor L cs At the moment of connection, the compensation inductor L cs The sudden change in current on the compensation inductor L cs A large voltage surge will occur. At this time, the buffer capacitor Cs is connected to the momentary short-circuit effect, so that the compensating inductor L... cs The current is diverted to the buffer resistor Rs, thereby suppressing voltage surges.

[0058] It should be understood that each of the three phases of the inverter is connected in series with a buffer capacitor C. s and buffer resistor R s And the buffer capacitance C of each phase s The capacitance values ​​are equal, and the buffer resistor R of each phase is... s The resistance values ​​are also equal.

[0059] See Figure 5 The image is Figure 4 A schematic diagram of the corresponding controllable switch closure.

[0060] from Figure 5 As can be seen from this, when the controllable switch S is closed, the compensation resistor R... cs With compensation inductor L cs Bypassed, buffer capacitor C s and buffer resistor R s They were also bypassed and were not connected to the inverter's output.

[0061] The selection principles for each parameter in the impedance compensation circuit are described below. For ease of description, the local load will be referred to simply as the load in the following examples.

[0062] Step 1: Sample the voltage at the point of common coupling (PCC), which is approximately equal to the grid voltage.

[0063] Based on the current unbalanced operating conditions of the power grid, the voltage u at the point of common coupling (PCC) can be sampled. PCCa u PCCb u PCCc If the equivalent inductance and equivalent resistance of the power grid are ignored, the voltage at the point of common coupling (PCC) is approximately equal to the grid voltage e. ga e gb e gc Using abc / dq to represent u in a three-phase rotating coordinate system PCCa u PCCb u PCCc Perform a coordinate transformation to obtain the PCC voltage u in the positive-sequence two-phase rotating coordinate system. PCCdp u PCCqp The PCC voltage u in the negative sequence two-phase rotating coordinate system PCCdn u PCCqn The positive-sequence DC PCC voltage u on the d-axis is obtained by filtering out the coupled second harmonic components. PCCd_p With q-axis positive sequence DC PCC voltage u PCCq_p , and the negative sequence DC PCC voltage u on the d-axis PCCd_n With q-axis negative sequence DC PCC voltage u PCCq_n .

[0064] Step 2: Sample to obtain the load voltage and grid current.

[0065] Follow the steps to pair the load voltage u za u zb u zc With grid current i ga i gb i gc The load voltage u is obtained by performing positive and negative sequence abc / dq transformation. zd u zq With grid current i gd i gq And perform positive and negative sequence separation to obtain the d-axis positive sequence DC load voltage u. zd_p With q-axis positive sequence DC load voltage u zq_p d-axis negative sequence DC load voltage u zd_n With q-axis negative sequence DC load voltage u zq_n d-axis positive sequence DC grid-connected current i gd_p With the positive sequence DC grid current i along the q-axis gq_p d-axis negative sequence DC grid current i gd_n With q-axis negative sequence DC grid current i gd_n .

[0066] Step 3: Based on the voltage difference between the load voltage and the point of common coupling (PCC) and the grid current, obtain the conditions that the compensation impedance in the impedance compensation circuit needs to meet.

[0067] From the voltage difference between the load voltage and the point of common coupling (PCC) voltage, and the vector relationship between the grid current and the compensation impedance, we can obtain:

[0068] Δu=i g ·(R cs +jX cs (3)

[0069] Among them, X cs To compensate for the reactance, the relationship between the compensating reactance and the compensating inductance is X. cs =ω g L cs ω g Let be the grid angular frequency. From equation (3), the relationship between the negative-sequence DC load voltage, the negative-sequence DC PCC voltage, and the negative-sequence DC grid-connected current can be obtained as follows:

[0070]

[0071] Simplifying equation (4) yields:

[0072]

[0073] Since the control objective is to balance the load voltage, the negative sequence component of the load voltage is 0, i.e., u zd_n =0, u zq_n =0, so the compensation resistor R can be obtained from equation (6). cs With compensating reactance X cs The relational expression satisfies:

[0074] [-u PCCd_n ·i gq_n +u PCCq_n ·i gd_n ]·R cs =[-u PCCd_n ·i gd_n +u PCCq_n ·i gq_n ]·X cs (6)

[0075] Step 4: Design the parameters of the impedance compensation circuit.

[0076] Compensation resistor R cs With compensating reactance X cs The inverter's power requirements must be met. The total active power output of the inverter is P. outT The total reactive power output is Q. outT The active power consumed by the load is P. outZ The reactive power consumed is Q. outZ The active power of the inverter connected to the grid is P.outG The reactive power is Q outG The active power consumed on the compensation impedance is set to P. outCS The reactive power is Q outCS According to the power balance theorem, we can obtain:

[0077]

[0078] Meanwhile, the active power consumed on the compensation impedance is P outCS The reactive power is Q outCS It can be determined by the grid-connected current i ga i gb i gc and compensation resistor R cs With compensating reactance X cs To obtain, that is

[0079]

[0080] Transforming to the dq coordinate system yields:

[0081]

[0082] From equation (9), we can obtain that the compensation resistor Rcs and the compensation reactance Xcs should satisfy:

[0083]

[0084] The parameters in the impedance compensation circuit can be selected within the constraints of equations (6) and (10) above, based on actual needs such as the cost of the actual resistor and inductor, to choose a suitable compensation resistor R. cs With compensating reactance X cs .

[0085] The parameters of the impedance compensation circuit have been determined above. The following describes the inverter provided in the embodiment of this application with reference to the accompanying drawings. It is equipped with an impedance compensation circuit and achieves balance by controlling the grid current when the grid is unbalanced.

[0086] See Figure 6 The figure is a schematic diagram of a two-level inverter provided in an embodiment of this application.

[0087] When the power grid is unbalanced, and the inverter aims to balance the load voltage, the controllable switch S in the impedance compensation circuit is open, and the compensation resistor R... cs With compensation inductor L csThe connection is between the load and the point of common coupling (PCC). To better illustrate the function of the impedance compensation circuit, this application uses a three-phase two-level inverter as an example for analysis. This application does not specifically limit the specific topology of the three-phase two-level inverter; the control strategy provided in this application's embodiments can be used in all cases.

[0088] Figure 6 The inverter shown is a photovoltaic-powered two-level inverter with a photovoltaic DC source. It is a full-bridge controllable inverter, and the filter topology uses an inductor-capacitor-inductor (LCL) configuration, where the first inductor is L. f1 The second group of inductors is L f2 It also includes the filter capacitor C f It should be understood that filters can also be other topologies, such as inductor-capacitor LC configurations. Let's take a linear load as an example, with three-phase linear loads having Z... a Z b and Z c .

[0089] According to the positive and negative sequence separation method, the inverter side current, load current, grid-connected current, load voltage and the voltage at the point of common coupling are all separated into positive and negative sequences.

[0090] Specifically, for the three-phase inverter side current i Lfa i Lfb i Lfc Three-phase load current i za i zb i zc Three-phase grid-connected current i ga i gb i gc Three-phase load voltage u za u zb u zc voltage u at the three-phase common coupling point PCC PCCa u PCCb u PCCc Both positive and negative sequence separations are performed to obtain the positive sequence DC component i of the inverter-side current. Ld_p i Lq_p negative sequence DC component i Ld_n i Lq_n ; positive sequence DC component of load current i zd_p i zq_p negative sequence DC component i zd_n i zq_n ; Positive sequence DC component of grid-connected current i gd_p i gq_p negative sequence DC component i gd_n i gq_n ; positive sequence DC component of load voltage u zd_p uzq_p negative sequence DC component u zd_n u zq_n d-axis positive sequence DC PCC voltage u PCCd_p q-axis positive sequence DC PCC voltage u PCCq_p d-axis negative sequence DC PCC voltage u PCCd_n With q-axis negative sequence DC PCC voltage u PCCq_n .

[0091] Because the impedance compensation circuit includes three-phase symmetrical impedances, the load voltage can be balanced after adding the voltage of the impedance compensation circuit to the voltage at the unbalanced point of common coupling (PCC). However, the balanced three-phase load voltage will be higher than the original three-phase rated voltage. The target value for the balanced positive-sequence load voltage is set to u. zd_pref u zq_pref Therefore, in the ascending sequence loop, by u zd_pref u zq_pref with u PCCd_p u PCCq_p The vector relationship allows us to obtain the target value i of the positive sequence grid-connected current after load voltage balancing. gd_pref i gq_pref for:

[0092]

[0093] The rated active power P of the load outZ With rated reactive power Q outZ Decomposed into positive-sequence load active power P outZ_p With positive sequence load reactive power Q outZ_p Then by u zd_pref u zq_pref The target value of the positive sequence load current i can be obtained after balancing the load voltage. zd_pref i zq_pref for:

[0094]

[0095] From equations (11) and (12), the target value of the positive-sequence inverter side current i after load voltage balancing can be obtained. Ld_pref i Lq_pref for:

[0096]

[0097] In the negative sequence loop, since the control objective is load voltage balance, the negative sequence component of the balanced load voltage is 0. Therefore, u PCCd_n u PCCq_n The target value of the negative sequence grid-connected current i after load voltage balancing can be obtained. gd_nref i gq_nref for:

[0098]

[0099] The rated active power P of the load outZ With rated reactive power Q outZ Decomposed into negative sequence load active power P outZ_n With negative sequence load reactive power Q outZ_n The target value of the negative sequence load current i can be obtained from the actual load conditions. zd_nref i zq_nref This leads to the target value i of the negative sequence inverter side current after balancing the load voltage. Ld_nref i Lq_nref for:

[0100]

[0101] Finally, in the positive sequence loop, the inverter-side current flows at i Ld_pref i Lq_pref Design a positive-sequence control loop for the target, where the inverter-side current in the negative-sequence loop is i Ld_nref i Lq_nref A negative-sequence control loop is designed to target the desired outcome. Finally, the negative-sequence output is superimposed on the positive-sequence output to obtain the modulation voltage of the modulation circuit. This enables control of the positive-sequence and negative-sequence components of the inverter-side current, and subsequently, the compensation resistor R... cs Establish positive-sequence active power compensation voltage and negative-sequence active power compensation voltage on the compensation inductor L. cs Positive-sequence reactive power compensation voltage and negative-sequence reactive power compensation voltage are established to achieve balanced control of load voltage.

[0102] Figure 6 The two-level inverter shown does not specifically limit the type of load; it can be a linear load or a non-linear load. Figure 6 This example only uses a linear load; the controller's control strategy is also applicable to nonlinear loads.

[0103] In an unbalanced power grid, when an inverter is connected to a sensitive load, it is necessary to prioritize balancing the load voltage. This is achieved through a compensation resistor R. cs Establish an active power compensation voltage on the compensation inductor L cs A reactive power compensation voltage is established to compensate for the voltage difference between the load and the point of common coupling (PCC), thereby achieving load voltage balance control. The inverter provided in this application embodiment can balance the load voltage under grid-connected conditions and is applicable to various inverter topologies and different types of loads.

[0104] The inverter provided in this application generally has a low grid impedance. When balancing the load voltage, due to the presence of the impedance compensation circuit, a small grid-connected current can be used to establish a voltage difference across the impedance compensation circuit to compensate for the load voltage, without needing to increase the grid-connected current excessively. Therefore, the inverter does not need to output excessive active power, improving the grid-connected efficiency of the inverter; at the same time, there is no need to redesign the grid impedance, which does not affect the stability of the entire inverter; the grid impedance does not consume excessive power, and excessive heat is not generated, ensuring the safety of the inverter's grid-connected operation.

[0105] The inverter provided in this application embodiment is described for the purpose of balancing load voltage.

[0106] See Figure 7 The figure is a schematic diagram of a three-level inverter provided in an embodiment of this application.

[0107] Figure 7 and Figure 6 The difference lies in the inverter topology and load type; everything else is the same, and the similarities will not be repeated here.

[0108] Figure 7 The inverter shown is a three-level inverter with a photovoltaic local load. The three-level inverter is described using a midpoint clamped three-level inverter as an example. It should be understood that other types of three-level inverters can also be used. This application does not limit the specific implementation of the embodiment.

[0109] The load can be a nonlinear load, including a three-phase uncontrolled rectifier bridge, inductor Lz, capacitor Cz, and resistor Rz. When the control objective is to balance the load voltage, the controller's control strategy is different from... Figure 6 The corresponding implementation is the same, that is, through the compensation resistor R cs Establish positive-sequence active power compensation voltage and negative-sequence active power compensation voltage on the compensation inductor L. cs The positive-sequence reactive power compensation voltage and the negative-sequence reactive power compensation voltage are established to achieve balanced control of the load voltage, which will not be elaborated here.

[0110] Figure 7 The three-level inverter shown does not specifically limit the type of load; it can be a non-linear load or a linear load. Figure 7 This example only uses a nonlinear load; the controller's control strategy is also applicable to linear loads.

[0111] Based on the inverter provided in the above embodiments, this application also provides a control method for the inverter, which will be described in detail below with reference to the accompanying drawings.

[0112] See Figure 8 The figure is a flowchart of a control method for an inverter provided in an embodiment of this application.

[0113] The inverter control method provided in this application includes an inverter circuit, a filter, and an impedance compensation circuit. The AC side of the inverter circuit is connected to the first terminal of the filter, and the second terminal of the filter is used to connect to the load. The impedance compensation circuit includes a compensation inductor and a compensation resistor connected in series.

[0114] The method includes:

[0115] S801: In the event of a power grid imbalance fault, the control impedance compensation circuit is connected between the second terminal of the filter and the point of common coupling; the point of common coupling is used to connect to the AC power grid.

[0116] S802: When the three phases of the power grid are balanced, the second end of the control filter is directly connected to the point of common coupling.

[0117] The inverter control method provided in this application establishes an active compensation voltage on the compensation resistor and a reactive compensation voltage on the compensation inductor when balancing the load voltage, thereby establishing a voltage difference on the impedance compensation circuit, realizing compensation for the voltage difference between the load and the point of common coupling (PCC), and thus achieving balanced control of the load voltage.

[0118] One possible implementation includes a controllable switch connected in parallel across the series-connected compensation inductor and resistor. Controlling the connection of the impedance compensation circuit between the second terminal of the filter and the common connection point involves: opening the controllable switch to allow the impedance compensation circuit to connect between the second terminal of the filter and the common connection point; and directly connecting the second terminal of the filter to the common connection point by closing the controllable switch.

[0119] One possible implementation also includes: after the impedance compensation circuit is connected between the second end of the filter and the common connection point, obtaining the inverter-side current reference value based on the load power, load voltage, load current and the voltage of the common connection point, and controlling the inverter circuit based on the inverter-side current reference value.

[0120] One possible implementation involves obtaining an inverter-side current reference value based on load power, load voltage, load current, and the voltage at the point of common coupling. Specifically, this includes: obtaining a grid-connected current target value based on the load voltage, the voltage at the point of common coupling, the inductance of the compensation inductor, and the resistance of the compensation resistor; obtaining a load current target value based on the load power and load voltage; and obtaining an inverter-side current reference value from the load current target value and the grid-connected current target value.

[0121] One possible implementation method also includes: determining whether an imbalance fault has occurred in the power grid based on the load voltage or grid-connected current.

[0122] In one possible implementation, see Figure 9 The figure is a schematic diagram of a control device provided in an embodiment of this application.

[0123] The control device may include a memory 1011 and a processor 1012. The processor 1012 may be connected to the power converter and can drive the switches in the various power conversion circuits of the power converter. For example... Figure 9 As shown, the memory can be random access memory (RAM), flash memory, read-only memory (ROM), EPROM, non-volatile read-only memory (Electronic Programmable ROM), registers, hard disks, removable disks, etc.

[0124] The memory 1011 can store computer instructions. When the computer instructions stored in the memory 1011 are executed by the processor 1012, the processor 1012 can be used to execute the control method of the inverter. The memory 1011 can also store data, such as preset ranges, preset thresholds, and other information involved in the above embodiments.

[0125] In the above embodiments, implementation can be achieved, in whole or in part, through software, hardware, firmware, or any combination thereof. When implemented in software, it can be implemented, in whole or in part, as a computer program product. A computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the flow or function according to the embodiments of this application is generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted from one computer-readable storage medium to another. For example, computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium that a computer can access or a data storage device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape) or a semiconductor medium (e.g., solid-state disk (SSD)).

[0126] This application also provides a readable storage medium for storing the methods provided in the above embodiments. Examples include random access memory (RAM), flash memory, read-only memory (ROM), EPROM, non-volatile read-only memory (EPROM), registers, hard disks, removable disks, or any other form of storage medium in the art.

[0127] It should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. Regarding the methods disclosed in the embodiments, since they correspond to the product embodiments disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to in the description of the product embodiments.

[0128] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An inverter, characterized in that, include: Inverter circuit, filter and impedance compensation circuit; The AC side of the inverter circuit is connected to the first terminal of the filter, and the second terminal of the filter is used to connect to the load; the impedance compensation circuit includes a compensation inductor and a compensation resistor connected in series. In the event of an imbalance fault in the power grid, the impedance compensation circuit is connected between the second terminal of the filter and the point of common coupling; the point of common coupling is used to connect to the AC power grid; when the three phases of the power grid are balanced, the second terminal of the filter is directly connected to the point of common coupling.

2. The inverter according to claim 1, characterized in that, The impedance compensation circuit also includes a buffer capacitor and a buffer resistor. The buffer capacitor and the buffer resistor are connected in series and then connected in parallel across the two ends of the series-connected compensation inductor and compensation resistor.

3. The inverter according to claim 1 or 2, characterized in that, Also includes: Controller; The impedance compensation circuit also includes a controllable switch; the controllable switch is connected in parallel across the series-connected compensation inductor and compensation resistor. The controller is used to control the controllable switch to open when an imbalance fault occurs in the power grid, and to control the controllable switch to close when the three phases of the power grid are in balance.

4. The inverter according to claim 1 or 2, characterized in that, The inductance of the compensation inductor and the resistance of the compensation resistor are obtained from the grid-connected current of the inverter and the voltage difference between the load voltage of the inverter and the point of common coupling; the negative sequence component of the load voltage is 0.

5. The inverter according to any one of claims 1-4, characterized in that, It also includes the controller; The controller is configured to, after the impedance compensation circuit is connected between the second terminal of the filter and the common connection point, obtain an inverter-side current reference value based on the load power, load voltage, load current and the voltage of the common connection point, and control the inverter circuit based on the inverter-side current reference value.

6. The inverter according to claim 5, characterized in that, The controller is specifically configured to obtain a target value for the grid-connected current based on the load voltage, the voltage of the common connection point, the inductance value of the compensation inductor, and the resistance value of the compensation resistor; and to obtain a target value for the load current based on the load power and the load voltage. The inverter-side current reference value is obtained from the load current target value and the grid-connected current target value.

7. The inverter according to claim 3, characterized in that, The controller is specifically used to determine whether an imbalance fault has occurred in the power grid based on the load voltage or grid-connected current.

8. A control method for an inverter, characterized in that, The inverter includes an inverter circuit, a filter, and an impedance compensation circuit; the AC side of the inverter circuit is connected to the first terminal of the filter, and the second terminal of the filter is used to connect to the load; the impedance compensation circuit includes a compensation inductor and a compensation resistor connected in series. The method includes: In the event of a power grid imbalance fault, the impedance compensation circuit is connected between the second terminal of the filter and the common coupling point; the common coupling point is used to connect to the AC power grid. When the three phases of the power grid are balanced, the second terminal of the filter is directly connected to the common connection point.

9. The control method according to claim 8, characterized in that, The impedance compensation circuit also includes a controllable switch, which is connected in parallel across the series-connected compensation inductor and compensation resistor. Controlling the impedance compensation circuit to be connected between the second terminal of the filter and the common connection point specifically includes: controlling the controllable switch to open, so that the impedance compensation circuit is connected between the second terminal of the filter and the common connection point; Controlling the second end of the filter to be directly connected to the common connection point specifically includes: controlling the controllable switch to close so that the second end of the filter is directly connected to the common connection point.

10. The control method according to claim 8 or 9, characterized in that, Also includes: After the impedance compensation circuit is connected between the second terminal of the filter and the common connection point, an inverter-side current reference value is obtained based on the load power, load voltage, load current and the voltage of the common connection point, and the inverter circuit is controlled based on the inverter-side current reference value.

11. The control method according to claim 10, characterized in that, The process of obtaining the inverter-side current reference value based on the load power, load voltage, load current, and the voltage at the point of common coupling specifically includes: The target value of the grid-connected current is obtained based on the load voltage, the voltage of the common connection point, the inductance value of the compensation inductor, and the resistance value of the compensation resistor; The target value of the load current is obtained based on the load power and the load voltage; The inverter-side current reference value is obtained from the load current target value and the grid-connected current target value.

12. The control method according to any one of claims 8-11, characterized in that, Also includes: Determine whether an imbalance fault has occurred in the power grid based on the load voltage or grid-connected current.

13. A control device, characterized in that, It includes a processor and a memory, the memory being used to store programs, instructions, or code, and the processor being used to execute the programs, instructions, or code in the memory to perform the control method of the inverter as described in any one of claims 8-12.

14. A computer-readable storage medium, characterized in that, The system contains a computer program that is loaded by a processor to execute the inverter control method as described in any one of claims 8-12.

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