A grid-connected converter, a grid-connected system and a control method

Abstract

The present application discloses a grid-connected converter and a control method, including: a power conversion circuit, a filter circuit, and a controller; the DC side of the power conversion circuit is used to connect to a DC source, the AC side of the power conversion circuit is connected to the first end of the filter circuit, and the second end of the filter circuit is used to connect to a grid connection point; the controller is configured to, when in a weak grid, obtain an active power compensation command through a first proportional-integral regulator for the difference between the rated angular frequency and the angular frequency of the grid connection point, and obtain a reactive power compensation command through a second proportional-integral regulator for the difference between the rated voltage and the voltage of the grid connection point; obtain a frequency reference value according to the active power compensation command and an active power reference value, obtain a voltage reference value according to the reactive power compensation command and a reactive power reference value, generate a drive signal for the power conversion circuit according to the frequency reference value and the voltage reference value, and control the power conversion circuit. This technical solution can achieve fast grid connection under a weak grid.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and particularly to a grid-connected inverter, a grid-connected system and a control method. Background Art

[0002] With the gradual development of the energy structure towards cleaner energy, clean energy such as wind energy and solar energy is being widely used, and the installed capacity continues to increase, changing the original power grid system. In new energy, the grid-connected inverter serves as the interface between new energy power generation and the power grid.

[0003] The grid-connected inverter generally adopts a current source control technology based on the phase-locked loop synchronization principle to achieve grid voltage tracking and power transmission. However, with the increase in the scale of new energy grid connection and the increase in the transmission distance, the grid connection point of the inverter gradually shows a weak grid state, which affects the dynamic tracking performance of the phase-locked loop of the inverter, thereby reducing the stability of current control and power control. Summary of the Invention

[0004] In view of this, the present application provides a grid-connected inverter, a grid-connected system and a control method, which can achieve fast grid connection under a weak grid.

[0005] The present application provides a grid-connected inverter, including: a power conversion circuit, a filter circuit and a controller;

[0006] The DC side of the power conversion circuit is used to connect to a DC source, the AC side of the power conversion circuit is connected to the first end of the filter circuit, and the second end of the filter circuit is used to connect to the grid connection point;

[0007] The controller is used to, when in a weak grid, obtain an active power compensation command through a first proportional-integral regulator for the difference between the rated angular frequency and the angular frequency of the grid connection point, and obtain a reactive power compensation command through a second proportional-integral regulator for the difference between the rated voltage and the voltage of the grid connection point; obtain a frequency reference value according to the active power compensation command and the active power reference value, obtain a voltage reference value according to the reactive power compensation command and the reactive power reference value, generate a drive signal for the power conversion circuit according to the frequency reference value and the voltage reference value, and control the power conversion circuit.

[0008] Preferably, the controller obtains a frequency reference value according to the active power compensation command and the active power reference value, specifically including:

[0009] Superimpose differential control on the difference between the active power compensation command and the active power reference value to obtain an adjusted active power command, and perform frequency loop control on the active power command to obtain a frequency reference value.

[0010] Preferably, the controller obtains a voltage reference value according to the reactive power compensation command and the reactive power reference value, specifically including:

[0011] When in a weak grid, perform proportional control on the difference between the reactive power compensation command and the reactive power set value to obtain the voltage set value;

[0012] When in a strong grid, perform proportional-integral control on the difference between the reactive power compensation command and the reactive power set value to obtain the voltage set value.

[0013] Preferably, the controller is further configured to, when in a strong grid, obtain an active power compensation command through a first proportional regulator for the difference between the rated angular frequency and the angular frequency of the grid connection point, and obtain a reactive power compensation command through a second proportional regulator for the difference between the rated voltage and the voltage of the grid connection point.

[0014] Preferably, the controller generates a driving signal for the power conversion circuit according to the frequency set value and the voltage set value, specifically including:

[0015] Obtain a direct-axis voltage set value and a quadrature-axis voltage set value according to the voltage set value, obtain a direct-axis voltage and a quadrature-axis voltage from the grid connection point voltage; obtain a direct-axis current and a quadrature-axis current from the grid connection point current;

[0016] Perform proportional-integral control on the difference between the direct-axis voltage set value and the direct-axis voltage to obtain a direct-axis current set value; perform proportional-integral control on the difference between the quadrature-axis voltage set value and the quadrature-axis voltage to obtain a quadrature-axis current set value;

[0017] Superimpose a first power feedforward value on the direct-axis current set value and then compare it with the direct-axis current to obtain a first comparison result, and perform proportional-integral adjustment on the first comparison result to obtain a direct-axis modulation voltage value;

[0018] Superimpose a second power feedforward value on the quadrature-axis current set value and then compare it with the quadrature-axis current to obtain a second comparison result, and perform proportional-integral adjustment on the second comparison result to obtain a quadrature-axis modulation voltage value;

[0019] Generate a driving signal for the power conversion circuit according to the direct-axis modulation voltage value, the quadrature-axis modulation voltage value, the frequency set value, and the voltage set value.

[0020] Preferably, the first power feedforward value is obtained according to the given active power and the given reactive power, and the second power feedforward value is obtained according to the given active power and the given reactive power.

[0021] This application provides a grid-connected system, including at least one of the above grid-connected converters; when multiple grid-connected converters are included, the AC sides of the multiple grid-connected converters are all connected to the grid connection point.

[0022] This application also provides a control method for a grid-connected converter, including:

[0023] When in a weak power grid, the difference between the rated angular frequency and the angular frequency of the grid connection point is passed through a first proportional-integral regulator to obtain an active power compensation command;

[0024] The difference between the rated voltage and the voltage of the grid connection point is passed through a second proportional-integral regulator to obtain a reactive power compensation command;

[0025] A frequency reference value is obtained according to the active power compensation command and the active power reference value, and a voltage reference value is obtained according to the reactive power compensation command and the reactive power reference value;

[0026] A drive signal for the power conversion circuit is generated according to the frequency reference value and the voltage reference value to control the power conversion circuit.

[0027] Preferably, obtaining a frequency reference value according to the active power compensation command and the active power reference value specifically includes:

[0028] Differential control is superimposed on the difference between the active power compensation command and the active power reference value to obtain an adjusted active power command, and frequency loop control is performed on the active power command to obtain a frequency reference value.

[0029] Preferably, obtaining a voltage reference value according to the reactive power compensation command and the reactive power reference value specifically includes:

[0030] When in a weak power grid, proportional control is performed on the difference between the reactive power compensation command and the reactive power reference value to obtain the voltage reference value;

[0031] When in a strong power grid, proportional-integral control is performed on the difference between the reactive power compensation command and the reactive power reference value to obtain the voltage reference value.

[0032] Preferably, it further includes:

[0033] When in a strong power grid, the difference between the rated angular frequency and the angular frequency of the grid connection point is passed through a first proportional regulator to obtain an active power compensation command, and the difference between the rated voltage and the voltage of the grid connection point is passed through a second proportional regulator to obtain a reactive power compensation command.

[0034] Preferably, generating a drive signal for the power conversion circuit according to the frequency reference value and the voltage reference value specifically includes:

[0035] A direct-axis voltage reference value and a quadrature-axis voltage reference value are obtained according to the voltage reference value, a direct-axis voltage and a quadrature-axis voltage are obtained from the grid connection point voltage; a direct-axis current and a quadrature-axis current are obtained from the grid connection point current;

[0036] Perform proportional-integral control on the difference between the direct-axis voltage set value and the direct-axis voltage to obtain the direct-axis current set value; perform proportional-integral control on the difference between the quadrature-axis voltage set value and the quadrature-axis voltage to obtain the quadrature-axis current set value;

[0037] Superimpose a first power feedforward value on the direct-axis current set value and then compare it with the direct-axis current to obtain a first comparison result, and perform proportional-integral regulation on the first comparison result to obtain the direct-axis modulation voltage value;

[0038] Superimpose a second power feedforward value on the quadrature-axis current set value and then compare it with the quadrature-axis current to obtain a second comparison result, and perform proportional-integral regulation on the second comparison result to obtain the quadrature-axis modulation voltage value;

[0039] Generate the drive signal for the power conversion circuit according to the direct-axis modulation voltage value, the quadrature-axis modulation voltage value, the frequency set value, and the voltage set value.

[0040] Preferably, the first power feedforward value is obtained according to the given active power and the given reactive power, and the second power feedforward value is obtained according to the given active power and the given reactive power.

[0041] Thus, the present application has the following beneficial effects:

[0042] For the grid-connected converter provided by the embodiment of the present application, when in a weak grid, proportional-integral modulation is adopted for the control of the angular frequency, and proportional-integral modulation is also adopted for the control of the voltage. Only using proportional regulation cannot achieve the function of stabilizing the system. By using the integral modulation link, static errors can be eliminated, thereby playing a dynamic regulation role. The integral can also play a certain frequency deviation compensation function. Therefore, the grid-connected converter provided by the embodiment of the present application can achieve fast grid connection under a weak grid, especially for the grid-connected power generation scenario of new energy, where the grid exhibits weak grid characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of a grid-connected converter of the present application;

[0044] Figure 2 It is a schematic diagram of another grid-connected converter provided by the embodiment of the present application;

[0045] Figure 3 It is a control schematic diagram of the power compensation link provided by the embodiment of the present application;

[0046] Figure 4 It is a control schematic diagram of the power compensation loop and the virtual synchronous generator control loop provided by the embodiment of the present application;

[0047] Figure 5A control schematic diagram of a voltage loop and a current loop provided by an embodiment of the present application;

[0048] Figure 6 A schematic diagram of a grid-connected system provided by an embodiment of the present application;

[0049] Figure 7 A flowchart of a control method for a grid-connected converter provided by an embodiment of the present application. Specific embodiments

[0050] To enable those skilled in the art to better understand and implement the technical solutions provided by the embodiments of the present application, the grid-connected converter will be introduced below with reference to the accompanying drawings.

[0051] The grid-connected converter provided by the embodiments of the present application does not specifically limit the application scenario. For example, it can be a photovoltaic power generation scenario, a wind power generation scenario, a energy storage scenario, etc.

[0052] See Figure 1 , this figure is a schematic diagram of a grid-connected converter of the present application.

[0053] The grid-connected converter includes a power circuit 100 and also includes a filtering circuit. The embodiments of the present application do not specifically limit the specific form of the filtering capacitor. Figure 1 Taking the filtering circuit including a filtering inductor L and a filtering capacitor C as an example.

[0054] The DC side of the power circuit 100 is connected to a DC source. The embodiments of the present application do not specifically limit the form of the DC source. For example, it can be from a battery, or it can also be from photovoltaic or wind power. Figure 1 Taking the DC source as a battery BAT as an example for introduction. The AC side of the grid-connected converter is connected to a grid connection point Vg, and the grid connection point Vg is connected to the power grid through a circuit breaker QF. It should be understood that when the power grid is three-phase, the grid-connected converter is three-phase.

[0055] Figure 1 The equivalent inductance of the line in [[ ]] is represented by Lm, and the equivalent resistance of the line is represented by Rm. I represents a current sensor, and the three-phase current can be obtained.

[0056] For the convenience of understanding, several parameters and transformation formulas in grid connection control will be introduced below. The transformation formulas are existing formulas commonly used in control.

[0057] Active power calculation formula: P = Va * Ia + Vb * Ib + Vc * Ic.

[0058] Reactive power calculation formula: Q = (Vbc * Ia + Vca * Ib + Vab * Ic) / 1.732.

[0059] Active power, Q - reactive power, Va, Vb, and Vc are the phase voltages at the grid connection point, Ia, Ib, and Ic are the phase currents at the grid connection point, and Vab, Vbc, and Vca are the line voltages at the grid connection point.

[0060] Clark transformation:

[0061] Park transformation:

[0062] When more and more new energy is connected to the grid for power generation, the grid is prone to the state of a weak grid. To evaluate whether the grid is a strong grid or a weak grid, it can be judged by the short - circuit ratio (SCR).

[0063] SCR refers to the ratio of the system short - circuit capacity to the equipment capacity. When the short - circuit ratio is large, it indicates that the equipment is connected to a strong system, and the switching of the equipment has a relatively small impact on the system. The short - circuit capacity is numerically equal to the system admittance value under the unit voltage condition, that is, the reciprocal of the Thevenin equivalent impedance of the system. The larger the short - circuit capacity, the smaller the Thevenin equivalent resistance of the system, and the switching of loads, shunt capacitors or reactors will not cause large changes in the voltage amplitude. Therefore, the system is relatively strong.

[0064] For easy understanding, the physical meanings of all parameters appearing in the embodiments of this application are introduced below.

[0065] Vg: Grid connection point voltage; Rm: Line equivalent resistance; ω0: Rated angular frequency; ω g : Grid connection point angular frequency; U n : Rated voltage; k p1 : Active power compensation ratio coefficient; k i1 : Active power compensation integral coefficient; k p2 : Reactive power compensation ratio coefficient; k i2 : Reactive power compensation integral coefficient; P m : Active power compensation value; Q m : Reactive power compensation value; P E : Active power command issued by EMS; Q E : Reactive power command issued by EMS; P r : Total active power given value; Q r : Total reactive power given value; P: Active power of the converter; Q: Reactive power of the converter; k1: Power loop differential coefficient; k p3 : Reactive power - voltage loop proportional coefficient; k i3 : Reactive power - voltage loop integral coefficient; J: Inertia coefficient; D: Damping coefficient; θ: Calculated phase angle given value; U: Calculated voltage given value; U dr : d - axis voltage given value; U qr : q - axis voltage given value; U d: d-axis voltage feedback value; U q : q-axis voltage feedback value; I dr : Direct-axis current reference value; I qr : Quadrature-axis current reference value; I d : Direct-axis current; I q : Quadrature-axis current; K d : Power feedforward coefficient; K q : Power feedforward coefficient; U d * : d-axis modulated voltage value; U q * : q-axis modulated voltage value; U α * : α-axis modulated voltage value; U β * : β-axis modulated voltage value.

[0066] To make the above objects, features, and advantages of the present application more obvious and understandable, the following further describes the embodiments of the present application in detail with reference to the accompanying drawings and specific embodiments.

[0067] See Figure 2 , which is a schematic diagram of another grid-connected converter provided by the embodiment of the present application.

[0068] The grid-connected converter provided by the embodiment of the present application includes: a power conversion circuit 100, a filter circuit 200, and a controller 300.

[0069] The DC side of the power conversion circuit 100 is used to connect to a DC source, the AC side of the power conversion circuit 200 is connected to the first end of the filter circuit 200, and the second end of the filter circuit 200 is used to connect to the grid connection point Vg.

[0070] The controller 300 is configured to, when in a weak grid, obtain an active power compensation command through a first proportional-integral regulator for the difference between the rated angular frequency and the angular frequency of the grid connection point, and obtain a reactive power compensation command through a second proportional-integral regulator for the difference between the rated voltage and the voltage of the grid connection point; obtain a frequency reference value according to the active power compensation command and the active power reference value, obtain a voltage reference value according to the reactive power compensation command and the reactive power reference value, generate a drive signal for the power conversion circuit according to the frequency reference value and the voltage reference value, and control the power conversion circuit 100.

[0071] The control of the grid-connected converter generally adopts the following four-loop control: power compensation loop, virtual synchronous generator control loop, voltage loop, and current loop.

[0072] First, the improvement of the power compensation loop of the grid-connected converter provided by the embodiment of the present application is introduced below.

[0073] To facilitate the understanding of the technical solution provided by the embodiments of the present application, the following will be described in conjunction with the control schematic diagram of the power compensation link.

[0074] See Figure 3 , which is the control schematic diagram of the power compensation link provided by the embodiments of the present application.

[0075] The power compensation loop includes an active power compensation loop and a reactive power compensation loop.

[0076] For the active power compensation link, the first switch S1 and the second switch S2 are a proportional control switch and an integral control switch respectively. Under a weak power grid, both the first switch S1 and the second switch S2 are closed. Under a strong power grid, the first switch S1 is closed and the second switch S2 is open. It should be understood that S1 and S2 are not actual switches, but are only used for illustrating the control principle.

[0077] The controller is also used to obtain the active power compensation command through the first proportional regulator for the difference between the rated angular frequency and the angular frequency of the grid connection point under a strong power grid. k p1 is the proportional coefficient of the first proportional regulator. The reactive power compensation command is obtained through the second proportional regulator for the difference between the rated voltage and the voltage of the grid connection point. k p2 is the proportional coefficient of the second proportional regulator.

[0078] Under a weak power grid, the difference between the rated angular frequency ω0 and the angular frequency ωg of the grid connection point is used to obtain the compensation power command Pm through a proportional-integral regulator. Pm=(k p1 +ki1*s)*(ω0 - ωg), where ki1 is the integral coefficient of the active power compensation loop. The specific values of k p1 and ki1 are not specifically limited in the embodiments of the present application. For example, they can be selected according to the power of the grid-connected system. Under a strong power grid, k p1 can take a larger value. Under a weak power grid, k p1 can take a smaller value. The limiter 1 limits the output of the integral to prevent the integral value from being too large.

[0079] Under a strong power grid, when the grid angular frequency deviates from the rated angular frequency, increasing or decreasing the active power of the grid-connected converter has little impact on the power grid (has little impact on the grid frequency), and the grid angular frequency deviation remains basically unchanged (or changes little). If the integral link is engaged (S2 is closed), due to the existence of the deviation, the integral will keep increasing until it reaches the integral limit value, and the integral saturation loses its regulating function. The proportional link can quickly achieve the primary frequency modulation function. When the grid frequency is lower than the rated frequency (primary frequency modulation), the change in the output power of the converter is kp1*(ω0 - ωg).

[0080] Under a weak power grid, when the grid angular frequency deviates from the rated angular frequency, the converter increasing or decreasing the active power will have an impact on the power grid (a greater impact on the grid frequency). The grid angular frequency will change with the power. If only the proportional link is enabled (S1 closed), when the grid connection point angular frequency is lower than the rated angular frequency, the greater the deviation, the greater the output power. The frequency F and the active power P exhibit a droop characteristic (P-F droop). Eventually, an equilibrium point of power and frequency will be reached, and the grid connection point frequency will be lower than the rated frequency, with the frequency deviation always existing. When the load is heavy, the frequency deviation will increase further. If the integral link is also enabled (S2 closed), due to the existence of the deviation, the integral value will keep increasing, and the output power will increase. The increase in the output power will cause the grid connection point frequency to rise, eventually reaching near the rated angular frequency of the power grid. The integral eliminates the static error and plays a dynamic regulation role. The integral can perform a certain frequency deviation compensation function. The limiter 1 module can use dynamic limiting, and adjust the limit value in real time according to the grid connection point frequency, realizing the function that the greater the deviation of the grid connection point frequency from the rated frequency, the larger the range of limiter 1.

[0081] The above describes the active power compensation link. Now, the reactive power compensation link will be introduced. Under a weak power grid, the third switch S3 and the fourth switch S4 are both closed. Under a strong power grid, the third switch S3 is closed and the fourth switch S4 is open. It should be understood that neither S3 nor S4 is an actual switch, and they are only schematically shown in the figure for introducing the control principle. The difference between the rated voltage Un and the grid-connected voltage Vg passes through the proportional-integral regulator to obtain the reactive power compensation command Qm, Qm = (k p2 +k i2 *s)*(U0 - Vg). Its principle is the same as that of the active power compensation link and will not be elaborated here.

[0082] The above describes the working principle of the power compensation loop. Now, the improvement of the grid-connected converter for the virtual synchronous generator control loop provided by the embodiment of the present application will be introduced. The virtual synchronous generator control loop includes an active frequency loop and a reactive voltage loop.

[0083] Refer to Figure 4 , which is the control principle diagram of the power compensation loop and the virtual synchronous generator control loop provided by the embodiment of the present application.

[0084] When the voltage source converters are operating in a network, the grid connection point angular frequency ωg is determined by multiple machines. As can be seen from Figure 4 , the P-ω characteristic of the virtual synchronous generator is jointly determined by the power and the rotor motion equation. The P-ω characteristic is:

[0085]

[0086] As can be seen from Equation 1, the frequency deviation (ω0 - ωg), the power deviation (P EThe transfer functions between (-P) and the output frequency ω all belong to the first-order inertia link, where Js is the moment of inertia and D is the damping coefficient.

[0087] When the virtual synchronous generator (VSG) operates in the power control (PQ) mode, it can output active power and reactive power according to the power command. The dynamic and steady-state characteristics of the active power are determined by the values of the virtual inertia and the damping coefficient. If the virtual inertia of the VSG is large, the frequency support ability of the VSG is strong. When there is a step change in the power reference value, there will be a dynamic oscillation process at the stable operating point. When the grid-connected system includes multiple grid-connected converters connected in parallel at the grid connection point, more complex power oscillations and overshoot processes will occur during the operation of multiple grid-connected converters, causing a large current impact on the grid-connected converters. Moreover, the grid-connected converters do not have the same overload capacity as synchronous motors, and power overshoot can lead to overcurrent protection of the grid-connected converters. If the damping coefficient is increased, the power oscillation can be eliminated and the power overshoot can be reduced, but the system response speed slows down and the steady-state power error increases, and the control performance is limited.

[0088] To suppress power oscillation and overshoot, the controller obtains the frequency reference value based on the active power compensation command and the active power reference value, specifically including: superimposing differential control on the difference between the active power compensation command and the active power reference value to obtain the adjusted active power command, and performing frequency loop control on the active power command to obtain the frequency reference value. That is, a differential control term k1*s is superimposed on the power deviation, and the differential only acts on the dynamic change, increasing the system damping, reducing power overshoot and oscillation. The differential control term has no effect in the steady state.

[0089] The P-ω characteristic becomes:

[0090]

[0091] The reactive power control Q-U characteristic of the virtual synchronous generator is determined by Figure 4 It can be seen that:

[0092] U = [(U n -V g )*(k P2 +k i2 *s)+Q E -Q]*(k P3 +k i3 *s)+Un Formula 3

[0093] The controller obtains the voltage reference value based on the reactive power compensation command and the reactive power reference value, specifically including: in a weak grid, performing proportional control on the difference between the reactive power compensation command and the reactive power reference value to obtain the voltage reference value; in a strong grid, performing proportional-integral control on the difference between the reactive power compensation command and the reactive power reference value to obtain the voltage reference value. The following is a detailed introduction in combination with the figure.

[0094] Under a weak power grid, the fifth switch S5 is closed and the sixth switch S6 is open, with only the proportional link. It should be understood that neither S3 nor S4 is an actual switch, and they are only shown in the figure for introducing the control principle. The Q-U exhibits a droop characteristic, that is:

[0095] U = (Q r -Q) * kp3 + Un Formula 4

[0096] Under a strong power grid, both S5 and S6 are closed, and both the proportional and integral links come into play; under a weak power grid, S5 is closed and S6 is open, with only the proportional link in effect.

[0097] Under a weak power grid, if both S5 and S6 are closed and multiple grid-connected converters operate in a network, when responding to the reactive power command Q E issued by the Energy Management System (EMS), since the voltage of the weak power grid is jointly determined by multiple grid-connected converters, as can be seen from Formula 3, due to the existence of the integral term k i3 *s, the deviation of the reactive power will cause the voltage U to increase continuously, and the voltage U will keep accumulating, resulting in the voltage U deviating too much from the normal operating range. If S6 is open and there is no effect of the integral term, the control exhibits the Q-U droop characteristic, and there will be an equilibrium point between the voltage and the reactive power, and the situation of voltage exceeding the limit will not occur. It should be understood that Figure 4 the voltage U here is the voltage set value of the voltage control loop.

[0098] Under a strong power grid, the grid voltage is relatively stable, and it is not easy for the grid-connected converter to change the grid voltage. If S6 is open, the control exhibits the droop characteristic. As can be seen from Formula 4, there is a certain static error between the reactive power command Qr and the converter reactive power Q, and the command cannot be completely tracked. Therefore, under a strong power grid, an integral link is required to eliminate the static error of reactive power tracking, that is, both S5 and S6 are closed under a strong power grid.

[0099] The working principles of the voltage control loop and the current control loop provided by the embodiments of the present application are introduced below in conjunction with the accompanying drawings. It should be understood that the output result of the voltage control loop is used as the set value of the current control loop.

[0100] See Figure 5 , which is a control schematic diagram of a voltage loop and a current loop provided by the embodiments of the present application.

[0101] For the grid-connected converter provided by the embodiments of the present application, due to the strength of the power grid connected to the grid connection point, it will affect the line impedance, and further the line impedance will affect the generation of the voltage of the virtual synchronous generator. The output voltage of the virtual synchronous generator plus the voltage drop of the line impedance is equal to the grid voltage. Moreover, the inductive or resistive nature of the line impedance will affect the angle of the power grid, and there is a vector relationship.

[0102] A controller that generates a drive signal for a power conversion circuit based on a frequency setpoint and a voltage setpoint, specifically including:

[0103] Obtain a direct-axis voltage setpoint and a quadrature-axis voltage setpoint from the voltage setpoint, and obtain the direct-axis voltage and the quadrature-axis voltage from the grid connection point voltage; obtain the direct-axis current and the quadrature-axis current from the grid connection point current;

[0104] Perform proportional-integral control on the difference between the direct-axis voltage setpoint and the direct-axis voltage to obtain a direct-axis current setpoint; perform proportional-integral control on the difference between the quadrature-axis voltage setpoint and the quadrature-axis voltage to obtain a quadrature-axis current setpoint;

[0105] Superimpose a first power feedforward value on the direct-axis current setpoint and then compare it with the direct-axis current to obtain a first comparison result, and perform proportional-integral adjustment on the first comparison result to obtain a direct-axis modulation voltage value;

[0106] Superimpose a second power feedforward value on the quadrature-axis current setpoint and then compare it with the quadrature-axis current to obtain a second comparison result, and perform proportional-integral adjustment on the second comparison result to obtain a quadrature-axis modulation voltage value;

[0107] Generate a drive signal for the power conversion circuit based on the direct-axis modulation voltage value, the quadrature-axis modulation voltage value, the frequency setpoint, and the voltage setpoint.

[0108] The first power feedforward value is obtained based on the given active power and the given reactive power, and the second power feedforward value is obtained based on the given active power and the given reactive power.

[0109] Through Figure 4 the output voltage setpoint U and the frequency setpoint (phase angle θ), the direct-axis voltage setpoint U Figure 5 can be obtained dr = U, and the quadrature-axis voltage setpoint U qr = 0. The grid connection point three-phase voltage Vg is transformed through Clark and Park inverse transformation to obtain the direct-axis voltage U d , the quadrature-axis voltage Uq, and the difference between the d-axis and q-axis voltage setpoints and the voltage feedback is output through the voltage loop PI controller as I dr , I qr , I dr and I qr , respectively, as the direct-axis current setpoint and the quadrature-axis current setpoint. The actual three-phase current I of the grid-connected converter is transformed through Clark and Park inverse transformation to obtain the direct-axis current I d , the quadrature-axis current I q .

[0110] To increase the current response speed, power decoupling feedforward control is added. Specifically, as shown in Figure 5 , a power feedforward term K d *P is added to the d-axis current loop setpointr -K q *Q r , add a power feedforward term K to the given value on the q-axis current loop q *P r +K d *Q r , where K d and K q are feedforward coefficients. The impact of the grid line impedance on the feedforward method varies greatly. When the line impedance (Lm and Rm, the impedance from the grid connection point to the grid) is resistive, K d is zero and K q is a non-zero number; when the line impedance is inductive, K d is a non-zero number and K q is zero; when the line impedance is resistive-inductive, K d is a non-zero number and K q is a non-zero number. The current loop control outputs the d-axis and q-axis voltage signals U d * and U q * through a PI controller. The voltage signals U d * and U q * are transformed through the Park transformation to obtain the voltage modulation signals U α * and U β *. The voltage modulation signals generate space vector pulse width modulation (SVPWM, Space Vector Pulse Width Modulation) SVPWM drive signals to drive the switching tubes in the power conversion circuit.

[0111] The grid-connected inverter provided by the embodiments of the present application has been improved in the control of the current loop. In order to improve the response speed of current control, power decoupling feedforward control is added, that is, the total reactive power given value and the total active power given value are superimposed on the control of the quadrature-axis current and the direct-axis current. And the power decoupling feedforward control includes, in addition to the total reactive power given value and the total active power given value, coefficients affected by the line impedance of the grid. Therefore, the present application performs power feedforward considering the line impedance, which can better improve the current response speed.

[0112] Based on the grid-connected inverter provided in the above embodiments, the embodiments of the present application also provide a grid-connected system, which will be introduced in detail below with reference to the accompanying drawings.

[0113] See Figure 6 , which is a schematic diagram of a grid-connected system provided by the embodiments of the present application.

[0114] The grid-connected system provided by the embodiment of the present application includes at least one grid-connected converter introduced above; there can be multiple grid-connected converters. When there are multiple grid-connected converters, for example, from the first grid-connected converter 10 to the nth grid-connected converter 1n, the AC sides of the first grid-connected converter 10 to the nth grid-connected converter 1n are all connected to the grid connection point Vg.

[0115] It should be understood that each grid-connected converter in the grid-connected system can be controlled by referring to the manner introduced in the above embodiments. However, when multiple grid-connected converters are connected in parallel, the total active power set value and total reactive power set value of the system need to be distributed among the multiple grid-connected converters.

[0116] The grid-connected system provided by the embodiment of the present application can operate quickly and stably under both weak grid and strong grid conditions.

[0117] Based on the grid-connected converter and grid-connected system provided in the above embodiments, the embodiment of the present application also provides a control method for the grid-connected converter, which will be introduced in detail below with reference to the drawings.

[0118] See Figure 7 , this figure is a flowchart of a control method for a grid-connected converter provided by the embodiment of the present application.

[0119] The control method for the grid-connected converter provided by the embodiment of the present application includes:

[0120] S701: When in a weak grid, obtain an active power compensation command for the difference between the rated angular frequency and the angular frequency of the grid connection point through a first proportional-integral regulator;

[0121] S702: Obtain a reactive power compensation command for the difference between the rated voltage and the voltage of the grid connection point through a second proportional-integral regulator;

[0122] S703: Obtain a frequency set value according to the active power compensation command and the active power set value, and obtain a voltage set value according to the reactive power compensation command and the reactive power set value;

[0123] S704: Generate a driving signal for the power conversion circuit according to the frequency set value and the voltage set value, and control the power conversion circuit.

[0124] For the control method of the grid-connected converter provided by the embodiment of the present application, when in a weak grid, proportional-integral modulation is adopted for the control of the angular frequency, and proportional-integral modulation is also adopted for the control of the voltage. Only using proportional regulation cannot achieve the role of stabilizing the system. By using the integral modulation link, static errors can be eliminated, thereby playing a dynamic regulation role. The integral can also play a certain frequency deviation compensation function. Therefore, the grid-connected converter provided by the embodiment of the present application can achieve rapid grid connection under a weak grid, especially for the grid-connected power generation scenario of new energy, where the grid exhibits weak grid characteristics.

[0125] A possible implementation method is to obtain a frequency reference value based on an active power compensation command and an active power reference value, which specifically includes: superimposing a differential control on the difference between the active power compensation command and the active power reference value to obtain an adjusted active power command, and performing a frequency loop control on the active power command to obtain the frequency reference value.

[0126] A possible implementation method is to obtain a voltage reference value based on a reactive power compensation command and a reactive power reference value, which specifically includes: in a weak power grid, performing a proportional control on the difference between the reactive power compensation command and the reactive power reference value to obtain the voltage reference value; in a strong power grid, performing a proportional-integral control on the difference between the reactive power compensation command and the reactive power reference value to obtain the voltage reference value.

[0127] A possible implementation method further includes: in a strong power grid, obtaining an active power compensation command through a first proportional regulator for the difference between the rated angular frequency and the angular frequency of the grid connection point, and obtaining a reactive power compensation command through a second proportional regulator for the difference between the rated voltage and the voltage of the grid connection point.

[0128] A possible implementation method is to generate a drive signal for a power conversion circuit based on the frequency reference value and the voltage reference value, which specifically includes: obtaining a direct-axis voltage reference value and a quadrature-axis voltage reference value according to the voltage reference value, obtaining the direct-axis voltage and the quadrature-axis voltage from the grid connection point voltage; obtaining the direct-axis current and the quadrature-axis current from the grid connection point current; performing a proportional-integral control on the difference between the direct-axis voltage reference value and the direct-axis voltage to obtain a direct-axis current reference value; performing a proportional-integral control on the difference between the quadrature-axis voltage reference value and the quadrature-axis voltage to obtain a quadrature-axis current reference value; superimposing a first power feedforward value on the direct-axis current reference value and comparing it with the direct-axis current to obtain a first comparison result, and performing a proportional-integral regulation on the first comparison result to obtain a direct-axis modulation voltage value; superimposing a second power feedforward value on the quadrature-axis current reference value and comparing it with the quadrature-axis current to obtain a second comparison result, and performing a proportional-integral regulation on the second comparison result to obtain a quadrature-axis modulation voltage value; generating a drive signal for the power conversion circuit according to the direct-axis modulation voltage value, the quadrature-axis modulation voltage value, the frequency reference value, and the voltage reference value.

[0129] A possible implementation method is that the first power feedforward value is obtained based on the given active power and the given reactive power, and the second power feedforward value is obtained based on the given active power and the given reactive power.

[0130] The control method provided by the embodiments of the present application improves the control of the current loop. To improve the response speed of current control, power decoupling feedforward control is added, that is, the total reactive power set value and the total active power set value are superimposed on the control of the quadrature-axis current and the direct-axis current. Moreover, in addition to the total reactive power set value and the total active power set value, the power decoupling feedforward control also includes a coefficient affected by the line impedance of the power grid. Therefore, the present application performs power feedforward considering the line impedance, which can better improve the current response speed.

[0131] It should be noted that the various embodiments in this specification are described in a progressive manner, and the key point of each embodiment is to illustrate the differences from other embodiments. The same or similar parts among the embodiments can be referred to each other. For the methods disclosed in the embodiments, since they correspond to the product embodiments disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the description of the product embodiments.

[0132] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A grid-connected inverter, characterized in that, Comprising: A power conversion circuit, a filtering circuit and a controller; The DC side of the power conversion circuit is used to connect to a DC source, the AC side of the power conversion circuit is connected to the first end of the filtering circuit, and the second end of the filtering circuit is used to connect to the grid connection point; The controller is configured to, when in a weak grid, obtain an active power compensation command for the difference between the rated angular frequency and the angular frequency of the grid connection point through a first proportional-integral regulator, and obtain a reactive power compensation command for the difference between the rated voltage and the voltage of the grid connection point through a second proportional-integral regulator; obtain a frequency set value according to the active power compensation command and the active power set value, obtain a voltage set value according to the reactive power compensation command and the reactive power set value, generate a drive signal for the power conversion circuit according to the frequency set value and the voltage set value, and control the power conversion circuit.

2. The grid-connected inverter according to claim 1, wherein The controller obtains a frequency set value according to the active power compensation command and the active power set value, specifically including: Superimposing differential control on the difference between the active power compensation command and the active power set value to obtain an adjusted active power command, and performing frequency loop control on the active power command to obtain a frequency set value.

3. The grid-connected inverter according to claim 1, wherein The controller obtains a voltage set value according to the reactive power compensation command and the reactive power set value, specifically including: When in a weak grid, performing proportional control on the difference between the reactive power compensation command and the reactive power set value to obtain the voltage set value; When in a strong grid, performing proportional-integral control on the difference between the reactive power compensation command and the reactive power set value to obtain the voltage set value.

4. The grid-connected converter according to any one of claims 1 to 3, characterized in that The controller is further configured to, when in a strong grid, obtain an active power compensation command for the difference between the rated angular frequency and the angular frequency of the grid connection point through a first proportional regulator, and obtain a reactive power compensation command for the difference between the rated voltage and the voltage of the grid connection point through a second proportional regulator.

5. The grid-connected inverter according to any one of claims 1 to 3, characterized in that The controller generates a drive signal for the power conversion circuit according to the frequency set value and the voltage set value, specifically including: Obtaining a direct-axis voltage set value and a quadrature-axis voltage set value according to the voltage set value, obtaining a direct-axis voltage and a quadrature-axis voltage from the grid connection point voltage; obtaining a direct-axis current and a quadrature-axis current from the grid connection point current; Performing proportional-integral control on the difference between the direct-axis voltage set value and the direct-axis voltage to obtain a direct-axis current set value; performing proportional-integral control on the difference between the quadrature-axis voltage set value and the quadrature-axis voltage to obtain a quadrature-axis current set value; Superimposing a first power feedforward value on the direct-axis current set value and then comparing it with the direct-axis current to obtain a first comparison result, and performing proportional-integral regulation on the first comparison result to obtain a direct-axis modulation voltage value; Superimposing a second power feedforward value on the quadrature-axis current set value and then comparing it with the quadrature-axis current to obtain a second comparison result, and performing proportional-integral regulation on the second comparison result to obtain a quadrature-axis modulation voltage value; Generating a drive signal for the power conversion circuit according to the direct-axis modulation voltage value, the quadrature-axis modulation voltage value, the frequency set value and the voltage set value.

6. The grid-connected inverter according to claim 5, characterized in that, The first power feedforward value is obtained according to the given active power and the given reactive power, and the second power feedforward value is obtained according to the given active power and the given reactive power.

7. A grid-connected system, characterized in that, including at least one grid-connected converter according to any one of claims 1-6; when a plurality of the grid-connected converters are included, the AC sides of the plurality of grid-connected converters are all connected to the grid connection point.

8. A control method for a grid-connected converter, characterized in that, Comprising: in a weak grid, obtaining an active power compensation command for the difference between the rated angular frequency and the angular frequency of the grid connection point through a first proportional-integral regulator; obtaining a reactive power compensation command for the difference between the rated voltage and the voltage of the grid connection point through a second proportional-integral regulator; obtaining a frequency set value according to the active power compensation command and the active power set value, and obtaining a voltage set value according to the reactive power compensation command and the reactive power set value; generating a driving signal for the power conversion circuit in the grid-connected converter according to the frequency set value and the voltage set value, and controlling the power conversion circuit.

9. The control method according to claim 8, wherein Obtaining a frequency set value according to the active power compensation command and the active power set value specifically includes: superimposing differential control on the difference between the active power compensation command and the active power set value to obtain an adjusted active power command, and performing frequency loop control on the active power command to obtain a frequency set value.

10. The control method according to claim 8, characterized in that Obtaining a voltage set value according to the reactive power compensation command and the reactive power set value specifically includes: in a weak grid, performing proportional control on the difference between the reactive power compensation command and the reactive power set value to obtain the voltage set value; in a strong grid, performing proportional-integral control on the difference between the reactive power compensation command and the reactive power set value to obtain the voltage set value.

11. The control method according to any one of claims 8 to 10, characterized in that, Further comprising: in a strong grid, obtaining an active power compensation command for the difference between the rated angular frequency and the angular frequency of the grid connection point through a first proportional regulator, and obtaining a reactive power compensation command for the difference between the rated voltage and the voltage of the grid connection point through a second proportional regulator.

12. The control method according to any one of claims 8-10, characterized in that, Generating the driving signal for the power conversion circuit according to the frequency set value and the voltage set value specifically includes: obtaining a direct-axis voltage set value and a quadrature-axis voltage set value according to the voltage set value, obtaining a direct-axis voltage and a quadrature-axis voltage from the grid connection point voltage; obtaining a direct-axis current and a quadrature-axis current from the grid connection point current; performing proportional-integral control on the difference between the direct-axis voltage set value and the direct-axis voltage to obtain a direct-axis current set value; performing proportional-integral control on the difference between the quadrature-axis voltage set value and the quadrature-axis voltage to obtain a quadrature-axis current set value; superimposing a first power feedforward value on the direct-axis current set value and then comparing it with the direct-axis current to obtain a first comparison result, and performing proportional-integral regulation on the first comparison result to obtain a direct-axis modulation voltage value; superimposing a second power feedforward value on the quadrature-axis current set value and then comparing it with the quadrature-axis current to obtain a second comparison result, and performing proportional-integral regulation on the second comparison result to obtain a quadrature-axis modulation voltage value; generating the driving signal for the power conversion circuit according to the direct-axis modulation voltage value, the quadrature-axis modulation voltage value, the frequency set value and the voltage set value.

13. The control method according to claim 12, wherein The first power feedforward value is obtained according to the given active power and the given reactive power, and the second power feedforward value is obtained according to the given active power and the given reactive power.

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