A multifunctional grid-connected inverter with active damper function

Abstract

The invention relates to a multifunctional grid-connected inverter with an active damper function. The multifunctional grid-connected inverter is connected in parallel with a plurality of grid-connected inverters at a common coupling point PCC and comprises a power output circuit and a control circuit. The power output circuit is composed of an energy storage type quasi-Z source inverter with an LCL filter. The control circuit comprises an A / D sampling module, a photovoltaic power control module, an output end current control module and a pulse width modulation module. The photovoltaic power control module realizes the control of photovoltaic power by tracking a photovoltaic voltage reference through photovoltaic voltage, and smoothes the photovoltaic power fluctuation by adjusting the charge and discharge state of an energy storage battery. The output end current control module obtains a harmonic reference current of a target frequency band, and generates a multifunctional inverter port reference current together with an output power reference current, and controls the inverter output current through a current control loop, so as to realize the active damper function of resonance suppression of an inverter parallel system under a weak power grid and the grid-connected power generation function of injecting power into a power grid.

Description

Technical Field

[0001] The invention belongs to the field of new energy grid-connected power generation, and in particular relates to a multifunctional grid-connected inverter having the function of an active damper. Background Art

[0002] As the global energy crisis and environmental problems become increasingly serious, new energy generation technologies such as photovoltaic power generation are gaining more and more attention. However, the output power of photovoltaic power generation is affected by light intensity, temperature, and shadow, and has the characteristics of randomness and intermittence. Therefore, photovoltaic grid-connected inverters need energy storage devices that can smooth the fluctuations of photovoltaic power generation. As an interface device between the distributed power generation system and the power grid, the grid-connected inverter plays an important role in converting DC power into high-quality AC power and transmitting it to the power grid. Therefore, the stable, safe and economical operation of the grid-connected inverter is of great significance to the entire distributed power generation system. As the proportion of power generation of distributed power generation systems in the power grid increases, the power grid gradually changes from strong to weak. Even if the grid-connected inverter is designed as a stable system in a strong grid, resonance is prone to occur in a multi-inverter parallel system under a weak grid due to factors such as inverter digital control delay and wide range changes in grid impedance. Therefore, it is of great significance to study the photovoltaic grid-connected inverter system with energy storage device with resonance suppression function under a weak grid.

[0003] At present, there are two main methods to suppress the resonance of the grid-connected inverter system under weak power grid. One is to start from the grid-connected inverter itself, by reducing the control delay and improving the inverter output characteristics. The other is to damp the resonance between the grid-connected inverter and the grid by connecting an additional damping device in parallel at the common coupling point (PCC) to improve the stability of the system. In addition, the energy storage device that balances the fluctuation of photovoltaic power generation usually requires a DC-DC converter to connect the energy storage battery to the photovoltaic grid-connected inverter system. Among the resonance suppression methods, the first method has poor versatility, and the control parameters of the grid-connected system need to be readjusted when the power grid changes. The second method requires an additional conversion device to achieve, and the system cost is relatively high and the volume is large. The energy storage device composed of a DC-DC converter and an energy storage battery in the photovoltaic grid-connected inverter system increases the system volume, cost and control complexity. Summary of the invention

[0004] The purpose of the present invention is to provide a multifunctional grid-connected inverter with the function of an active damper, which is beneficial to improving the stability of a photovoltaic grid-connected system under a weak power grid, smoothing photovoltaic power fluctuations, and reducing system volume, cost and control complexity.

[0005] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a multifunctional grid-connected inverter with active damper function, the multifunctional grid-connected inverter is connected in parallel with multiple grid-connected inverters at a common coupling point (PCC), the multifunctional grid-connected inverter includes a power output circuit and a control circuit, the power output circuit is mainly composed of an energy storage type quasi-Z source inverter with an LCL filter, and the control circuit includes an A / D sampling module, a photovoltaic power control module, an output current control module and a pulse width modulation module; the photovoltaic power control module realizes the control of photovoltaic power by tracking the photovoltaic voltage reference output by the photovoltaic maximum power point tracking (MPPT) algorithm unit, and smoothes the photovoltaic power fluctuation by adjusting the charging and discharging state of the energy storage battery in the multifunctional grid-connected inverter; the output current control module obtains the harmonic reference current of the target frequency band, and generates the multifunctional inverter port reference current together with the output power reference current, and controls the inverter output current through the current control loop, so as to realize the active damper function of the inverter parallel system resonance suppression under the weak power grid and the grid-connected power generation function of injecting power into the power grid.

[0006] Further, the power output circuit includes a photovoltaic cell array, a DC side filter capacitor, an impedance network with an energy storage battery, a three-phase inverter bridge and a three-phase LCL filter connected in sequence;

[0007] The impedance network with energy storage battery is that the energy storage battery is directly connected in parallel to the capacitor of the impedance network, without the need for a DC-DC converter, so as to balance the difference between the photovoltaic output power and the expected grid-connected output power; the impedance network is composed of a diode, an energy storage capacitor and an energy storage inductor, when the three-phase bridge arm is directly connected, the diode is turned off, the energy storage inductor is magnetized, and the energy storage capacitor is discharged; when the three-phase bridge arm is not directly connected, the diode is turned on, the energy storage inductor is demagnetized, and the energy storage capacitor is charged; the energy storage inductor of the impedance network is designed according to the requirements of energy storage inductor current ripple, maintaining the continuity of energy storage inductor current and avoiding resonance with the energy storage capacitor, and the energy storage capacitor of the impedance network is designed according to the requirements of energy storage capacitor voltage ripple;

[0008] The three-phase inverter bridge is a three-phase half-bridge inverter bridge composed of 6 switching tubes;

[0009] Each phase of the three-phase LCL filter consists of two inductors L f1 , L f2 and capacitor C f Composition: Inductor L connected to the inverter side f1 The inductor L connected to the PCC terminal on the grid side f2 Connected in series, the capacitor C f One end is connected to the same phase inductor L f1 and L f2 The nodes between the three-phase capacitors C fThe other ends of the LCL filter are connected together; the two inductors L of the LCL filter are designed according to the requirements of the filter inductor current ripple, port current conversion rate, resonant frequency and port current harmonic content. f1 , L f2 , design the filter capacitor C according to the filter reactive power requirements f .

[0010] Furthermore, the A / D sampling module includes a three-phase PCC voltage sampling unit, a filter capacitor current sampling unit, an inverter side three-phase filter current sampling unit, a photovoltaic voltage sampling unit, a photovoltaic current sampling unit, an energy storage capacitor voltage sampling unit and an energy storage battery current sampling unit.

[0011] Furthermore, the photovoltaic power control module includes a photovoltaic maximum power point tracking (MPPT) algorithm unit and a photovoltaic voltage regulator; the sampled photovoltaic voltage U PV and current i PV Send it to the MPPT algorithm unit to get the photovoltaic maximum power point voltage U PV *, and the photovoltaic voltage is combined with the photovoltaic voltage to generate a direct duty cycle D0 through the photovoltaic voltage regulator and sent to the pulse width modulation module to achieve photovoltaic power output control; the transfer function of the photovoltaic voltage regulator module is G PV (s) is:

[0012]

[0013] Where k p For photovoltaic voltage regulator module G PV (s) proportionality coefficient, k i For photovoltaic voltage regulator module G PV The integration coefficient of (s).

[0014] Furthermore, the output current control module includes a harmonic reference current generation unit, a phase-locked loop unit, an output power reference current generation unit, a current regulator and a capacitor current feedback unit to achieve the function of suppressing the resonance of the grid-connected inverter system under a weak power grid and injecting power into the power grid; the harmonic reference current generation unit is composed of a harmonic detection unit G HD (s), a virtual resistance generating unit and a virtual impedance compensation function unit G ca (s) composition; the three-phase PCC voltage is obtained by abc / αβ Clarke coordinate transformation u pcc_α 、u pcc_β , and through the harmonic detection unit G HD (s) Detect the αβ-axis component u of the harmonic voltage in the target frequency band of the PCC voltage pcch_α 、u pcch_β and through the virtual resistor R V and virtual impedance compensation function G ca(s) Obtain the αβ-axis component i of the harmonic reference current in the target frequency band hα *、i hβ *; Virtual resistance R V The virtual resistance generating unit is adaptively obtained according to the target resonance component to be suppressed, so as to realize the active damper function of suppressing the system resonance;

[0015] The harmonic detection unit G HD (s) is used to filter out the harmonics other than the target resonant component, and its transfer function is:

[0016]

[0017] Where ω1 is the angular frequency of the fundamental wave, h is the harmonic order outside the target resonant component frequency band, and Q is the harmonic detection unit G HD The quality factor of (s);

[0018] The virtual impedance compensation function G ca (s) for improving the virtual impedance performance of a multifunctional grid-connected inverter having a source damper function, so that the virtual impedance can present the characteristics of a resistor within the current loop bandwidth of the multifunctional inverter, thereby improving the stability of the grid-connected inverter system, wherein the current loop bandwidth is greater than the target resonant frequency; G ca The transfer function of (s) is:

[0019]

[0020] Where T b and T a is the virtual impedance compensation function G ca (s) compensation coefficient; when the equivalent virtual resistance of the multifunctional grid-connected inverter presents a positive resistance characteristic and the system stability margin is small, the hysteresis compensation, i.e., T a >T b ; If the active damper equivalent virtual resistance presents a positive resistance characteristic and the system stability margin meets the system requirements, the compensation function is 1, that is, T a =T b ; When the active damper equivalent virtual resistance presents negative resistance characteristics, the lead compensation, i.e. T a <T b ;

[0021] The phase-locked loop unit is used to obtain the phase θ of the three-phase PCC voltage g , angular velocity ω g , d-axis voltage component u pcc_d and the q-axis voltage component u pcc_q ;

[0022] The output power reference current generating unit generates the output active power and reactive power reference current i by the power instruction generating unit. d *、i q *, and obtain the αβ axis component i of the output power reference current through dq / αβ Park inverse coordinate transformation α *、i β *; the output power reference current i α *、i β *With harmonic reference current i hα *、i hβ * is subtracted to obtain the output reference current of the multifunctional grid-connected inverter and compared with H i2 times the αβ-axis component i of the inverter-side inductor current α 、i β Subtract the current regulator G i (s), minus H i1 times the αβ axis component i of the filter capacitor current cf_α 、i cf_β , get the αβ axis component u of the inverter bridge output voltage modulation signal α *、u β *;

[0023] The current regulator G i The transfer function of (s) is:

[0024]

[0025] Where k pi For the current regulator G i (s) proportionality coefficient, ω c For the current regulator G i The bandwidth of (s), k r For the current regulator G i (s) is the resonance coefficient, and h is the low-frequency harmonic order other than the target resonance component;

[0026] Finally, the photovoltaic power control module outputs the direct duty cycle signal D0 and the energy storage capacitor voltage signal u c1 The inverter bridge output voltage modulation signal u generated by the output current control module α *、u β *Input to the pulse width modulation module, and obtain the driving signal of the six switch tubes S1~S6 that control the three-phase half-bridge bridge arm of the energy storage type quasi-Z source inverter through modulation.

[0027] Furthermore, the power instruction generation unit adopts different control methods according to the system control needs; according to the upper system scheduling instruction, the multifunctional grid-connected inverter outputs active power and reactive power reference current i d *、iq *, or generate active power and reactive power reference current i according to the inverter's own application function needs d *、i q *; When the external characteristics of the multifunctional grid-connected inverter are required to have similar rotational inertia, damping and droop characteristics as synchronous generators to maintain grid voltage stability, the control method of the current-controlled VSG is adopted, and the VSG grid-connected output active power reference P is obtained according to the angular velocity and amplitude of the VSG output terminal voltage, i.e., the PCC voltage. out * and reactive power reference Q out *, then through P out * = 1.5 (u pcc_d i d *+u pcc_q i q *), Q out * = 1.5 (u pcc_d i q *-u pcc_q i d *) to obtain the output active power and reactive power reference current i d *、i q *; When the multifunctional grid-connected inverter is required to control the charging and discharging current of the energy storage battery, the inverter grid-connected output active power reference current i is obtained through the energy storage battery current loop control d *, that is, the energy storage battery reference current I B * and the energy storage battery current i B Subtract the current regulator G of the energy storage battery iB (s) and then the inverter grid-connected output active power reference current i is obtained d *, reactive power reference current i q *Set to 0.

[0028] Furthermore, the active power-frequency control equation of the current-controlled VSG is:

[0029]

[0030] Where P m is the mechanical power of VSG; P ref is the VSG reference active power; K ω is the active power droop coefficient; ω g is the PCC voltage angular velocity, which is also the actual angular velocity of VSG; ω0 is the rated angular velocity; D is the damping coefficient; J is the moment of inertia; P out * is the VSG output active power reference; θ g is the PCC voltage phase, which is also the VSG output power angle;

[0031] The reactive power-voltage control equation of the current-controlled VSG is:

[0032] U pcc =U pcc0 +K Q (Q ref -Q out *)

[0033] In the formula, Q out * is the VSG output reactive power reference; Q ref is the VSG reference reactive power; K Q is the reactive power droop coefficient; U pcc0 is the given PCC voltage amplitude reference, which is also the VSG no-load electromotive force; U pcc is the PCC voltage amplitude, which is also the output voltage amplitude of VSG.

[0034] The transfer function G of the energy storage battery current regulator iB (s) is:

[0035]

[0036] In the formula, k pB G is the current regulator of the energy storage battery iB (s) proportionality coefficient, k iB G is the current regulator of the energy storage battery iB The integration coefficient of (s).

[0037] The energy storage type quasi-Z source inverter includes all voltage type quasi-Z source grid-connected inverters that can connect energy storage batteries in parallel to energy storage capacitors. The implementation of the power instruction generation unit is not limited to the method proposed above, and also includes other methods that can generate active power and reactive power reference current i of the multifunctional grid-connected inverter output. d *、i q The control method of the current-controlled VSG is not limited to the method proposed above, but also includes various other control methods of the current-controlled VSG obtained by transforming the formula.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The present invention improves the stability of the grid-connected inverter parallel system under a weak power grid by using a multifunctional grid-connected inverter that also has the function of an active damper. There is no need to change the control method and software and hardware parameters of other inverters in the inverter parallel system, and the present invention has strong versatility.

[0040] (2) The present invention not only improves the stability of the grid-connected inverter parallel system under weak power grid, but also realizes the photovoltaic grid-connected power generation function and the photovoltaic power fluctuation smoothing function.

[0041] (3) The present invention integrates the active damper function and the grid-connected inverter function into one grid-connected inverter, thereby saving costs, reducing volume, and improving system efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The circuit structure diagram of the multifunctional grid-connected inverter with active damper function and the photovoltaic grid-connected inverter parallel system in the weak power grid where the multifunctional grid-connected inverter is located according to the embodiment of the present invention.

[0043] Figure 2 Schematic diagram of the structure of the control circuit of the multifunctional grid-connected inverter having the function of a source damper in an embodiment of the present invention.

[0044] Figure 3 The present invention provides a circuit topology and control strategy for a multifunctional grid-connected inverter having a source damper function in an embodiment of the present invention.

[0045] Figure 4 : is a photovoltaic voltage loop control block diagram of a photovoltaic power control module in an embodiment of the present invention. PV is the equivalent resistance of the photovoltaic cell array.

[0046] Figure 5 : is an equivalent circuit diagram of the multifunctional grid-connected inverter in the embodiment of the present invention in the direct-connection state and the non-direct-connection state. c are the parasitic resistances of the impedance network energy storage inductor and energy storage capacitor, i PN It is the instantaneous value of the inverter bridge bus current when the multifunctional grid-connected inverter is in non-through working state.

[0047] Figure 6 is an equivalent single-phase system mathematical model of a multifunctional grid-connected inverter with active damper function that adopts inverter-side inductor current feedback control when three-phase is balanced in the embodiment of the present invention. * (s) is the inverter output reference current signal generated by the power command generation unit, i1(s) is the inverter side inductor current signal, i2(s) is the grid side inductor current signal, i cf (s) is the filter capacitor current signal, u cf (s) is the voltage signal of the filter capacitor, u pcc (s) is the PCC voltage signal.

[0048] Figure 7 It is an equivalent single-phase simplified mathematical model of a multifunctional grid-connected inverter with a source damper function that adopts inverter-side inductor current feedback control when three-phase is balanced in an embodiment of the present invention.

[0049] Figure 8 for Figure 1 Norton equivalent circuit diagram of the inverter parallel system.invt (s) is the equivalent current source of n parallel inverters, Z o (s) is the port impedance of n parallel inverters, Z g (s) is the grid impedance, u g (s) is the grid voltage, i s (s) is the equivalent current source of the multifunctional grid-connected inverter with active damper function, Z p (s) is the original port impedance of the multifunctional grid-connected inverter with active damper function, Z v (s) is the virtual impedance of the multifunctional grid-connected inverter which also has the function of source damper.

[0050] Fig. 9 for Figure 8 The simplified equivalent circuit diagram of ov (s) is the parallel impedance of the inverter parallel system, that is, the impedance after the original port impedance of the inverter parallel system and the virtual impedance of the multifunctional grid-connected inverter which also has the function of source damper are connected in parallel. DETAILED DESCRIPTION

[0051] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0052] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0053] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0054] The present embodiment provides a multifunctional grid-connected inverter having an active damper function. The multifunctional grid-connected inverter is connected in parallel with a plurality of grid-connected inverters at a common coupling point (PCC). The multifunctional grid-connected inverter comprises a power output circuit and a control circuit. The power output circuit is mainly composed of an energy storage type quasi-Z source inverter with an LCL filter. The control circuit comprises an A / D sampling module, a photovoltaic power control module, an output end current control module and a pulse width modulation module. The photovoltaic power control module realizes photovoltaic power control by tracking the photovoltaic voltage reference output by a photovoltaic maximum power point tracking (MPPT) algorithm unit, and smoothes photovoltaic power fluctuations by adjusting the charge and discharge state of the energy storage battery in the multifunctional grid-connected inverter. The output end current control module obtains the harmonic reference current of the target frequency band, and generates the multifunctional inverter port reference current together with the output power reference current, and controls the inverter output current through a current control loop, thereby realizing the active damper function of the inverter parallel system resonance suppression under a weak power grid and the grid-connected power generation function of injecting power into the power grid.

[0055] The circuit structure diagram of the multifunctional grid-connected inverter with active damper function in this embodiment and the circuit structure diagram of the photovoltaic grid-connected inverter parallel system in the weak power grid where it is located are as follows: Figure 1 The multifunctional grid-connected inverter with source damper function is composed of an energy storage type quasi-Z source grid-connected inverter, including a photovoltaic cell array, a DC side filter capacitor, an impedance network with an energy storage battery, a three-phase inverter bridge, and a three-phase LCL filter connected in sequence.

[0056] The structural schematic diagram of the multifunctional grid-connected inverter control circuit with the function of source damper in this embodiment is as follows: Figure 2 As shown, it includes an A / D sampling module, a photovoltaic power control module, an output current control module and a pulse width modulation module. In order to suppress the resonance of the LCL filter, an active damping method of filter capacitor current feedback is adopted. The three functions of the multifunctional grid-connected inverter with active damper in this embodiment are realized by the control circuit, namely, suppressing the resonance of the parallel system of grid-connected inverters under weak power grid, injecting power into the power grid and smoothing the fluctuation of photovoltaic power.

[0057] The circuit topology and control strategy block diagram of the multifunctional grid-connected inverter with active damper function in this embodiment are shown in FIG. Figure 3 As shown. The circuit topology consists of a photovoltaic cell array, a DC side filter capacitor C i , with energy storage battery U BImpedance network, three-phase inverter bridge, three-phase LCL filter. The impedance network consists of diode D1, energy storage capacitors C1, C2 and energy storage inductors L1, L2. When the three-phase bridge arm is directly connected, the diode is turned off, the energy storage inductor is magnetized, and the energy storage capacitor is discharged; when the three-phase bridge arm is not directly connected, the diode is turned on, the energy storage inductor is demagnetized, and the energy storage capacitor is charged. With energy storage battery U B The impedance network is the energy storage battery U B It is directly connected in parallel to the capacitor C1 of the impedance network to balance the difference between the photovoltaic output power and the expected grid-connected output power, providing hardware conditions for smoothing photovoltaic power fluctuations. The three-phase inverter bridge is a three-phase half-bridge inverter bridge composed of 6 switching tubes. Each phase LCL filter of the inverter bridge consists of two inductors L f1 , L f2 and capacitor C f Connect the inductor L on the inverter side. f1 The inductor L connected to the PCC terminal on the grid side f2 Series connection, three-phase capacitor C f One end is connected to the same phase inductor L f1 and L f2 The nodes between the three-phase capacitors C f The other end of the control circuit includes an A / D sampling module, a photovoltaic power control module, an output current control module and a pulse width modulation module.

[0058] The three-phase PCC voltage (u pcca 、u pccb 、u pccc )、Three-phase filter capacitor current (i cfa 、i cfb 、i cfc ), inverter side three-phase filter current (i a1 、i b1 、i c1 ), photovoltaic voltage U PV , photovoltaic current i PV , energy storage capacitor voltage u c1 In order to suppress the resonance of the LCL filter, an active damping method using filter capacitor current feedback is used.

[0059] The three-phase PCC voltage u obtained by A / D sampling pcca 、u pccb 、u pccc , the phase θ of the three-phase PCC voltage is obtained through the phase-locked loop unit g , angular velocity ω g , d-axis voltage component u pcc_d and the q-axis voltage component u pcc_q .

[0060] At the same time, the three-phase PCC terminal voltage upcca 、u pccb 、u pccc , generate u through abc / αβ Clarke coordinate transformation pcc_α 、u pcc_β , and then through the harmonic detection unit G HD (s) Detect the αβ-axis component u of the harmonic voltage in the target frequency band of the PCC voltage pcch_α 、u pcch_β and through the virtual resistor R V and virtual impedance compensation function G ca (s) Obtain the αβ-axis component i of the harmonic reference current in the target frequency band hα *、i hβ *.

[0061] Virtual resistance R V The virtual resistance generating unit is adaptively obtained according to the target resonance component to be suppressed, so as to realize the active damper function of suppressing the system resonance.

[0062] Harmonic detection unit G HD (s) is used to filter out the harmonics other than the target resonant component, and its transfer function is:

[0063]

[0064] Where ω1 is the angular frequency of the fundamental wave, h is the harmonic order outside the target resonant component frequency band, and Q is the harmonic detection unit G HD The quality factor of (s).

[0065] Virtual impedance compensation function G ca (s) is used to improve the virtual impedance performance of the multifunctional grid-connected inverter with the function of source damper, so that the virtual impedance can present the characteristics of resistance within the current loop bandwidth of the multifunctional inverter, thereby improving the stability of the grid-connected inverter system, wherein the current loop bandwidth is greater than the target resonant frequency. G ca The transfer function of (s) is:

[0066]

[0067] Where T b and T a is the virtual impedance compensation function G ca When the equivalent virtual resistance of the multifunctional grid-connected inverter presents a positive resistance characteristic and the system stability margin is small, the hysteresis compensation, i.e., T a >T b ; If the active damper equivalent virtual resistance presents a positive resistance characteristic and the system stability margin meets the system requirements, the compensation function is 1, that is, T a =T b; When the active damper equivalent virtual resistance presents negative resistance characteristics, the lead compensation, i.e. T a <T b .

[0068] The output power reference current generation unit generates the output active power and reactive power reference current i from the power command generation unit. d *、i q *, and obtain the output power reference current i by dq / αβ Park inverse coordinate transformation α *、i β *. Output power reference current i α *、i β *With harmonic reference current i hα *、i hβ * is subtracted to obtain the output reference current of the multifunctional grid-connected inverter and compared with H i2 times the αβ-axis component i of the inverter-side inductor current α 、i β Subtract the current regulator G i (s), minus H i1 times the αβ axis component i of the filter capacitor current cf_α 、i cf_β , get the αβ axis component u of the inverter bridge output voltage modulation signal α *、u β *.

[0069] Current Regulator G i The transfer function of (s) is:

[0070]

[0071] In the formula, k pi For the current regulator G i (s) proportionality coefficient, ω c For the current regulator G i The bandwidth of (s), k r For the current regulator G i (s) is the resonance coefficient, and h is the low-frequency harmonic order other than the target resonance component.

[0072] In this embodiment, the power command generation unit adopts a current-controlled VSG to maintain the grid voltage stability. The inverter grid-connected output active power reference P is obtained according to the angular velocity and amplitude of the VSG output terminal voltage, i.e., the PCC voltage. out * and reactive power reference Q out *, then through P out * = 1.5 (u pcc_d i d *+u pcc_q i q *), Qout * = 1.5 (u pcc_d i q *-u pcc_q i d *) to obtain the output active power and reactive power reference current i d *、i q *.

[0073] The active power-frequency control equation of the current-controlled VSG is:

[0074]

[0075] Where P m is the mechanical power of VSG; P ref is the VSG reference active power; K ω is the active power droop coefficient; ω g is the PCC voltage angular velocity, which is also the actual angular velocity of VSG; ω0 is the rated angular velocity; D is the damping coefficient; J is the moment of inertia; P out * is the VSG output active power reference; θ g is the PCC voltage phase, which is also the VSG output power angle.

[0076] The reactive power-voltage control equation of the current-controlled VSG is:

[0077] U pcc =U pcc0 +K Q (Q ref -Q out *) (5)

[0078] In the formula, Q out * is the VSG output reactive power reference; Q ref is the VSG reference reactive power; K Q is the reactive power droop coefficient; U pcc0 is the given PCC voltage amplitude reference, which is also the VSG no-load electromotive force; U pcc is the PCC voltage amplitude, which is also the output voltage amplitude of VSG.

[0079] The photovoltaic power control module includes a photovoltaic maximum power point tracking (MPPT) algorithm unit and a photovoltaic voltage regulator. The sampled photovoltaic voltage U PV and current i PV Send it to the MPPT algorithm unit to get the photovoltaic voltage reference value U PV * and with the photovoltaic voltage U PV Through the photovoltaic voltage regulator G PV (s) Generates the direct duty cycle D0 and sends it to the pulse width modulation module. Figure 4The photovoltaic voltage loop control block diagram of the photovoltaic power control module is given. PV is the equivalent resistance of the photovoltaic cell array; i ci is the input filter capacitor C i The photovoltaic cell array is equivalent to a resistor R PV With voltage source U s Series composition;

[0080] G iL1_d0 (s) is the transient value of the shoot-through duty cycle d0 to i L1 The transfer function of G PV (s) is the transfer function of the photovoltaic voltage regulator module.

[0081]

[0082] In the formula, k p For photovoltaic voltage regulator module G PV (s) proportionality coefficient, k i For photovoltaic voltage regulator module G PV By adjusting the photovoltaic voltage regulator G PV (s) parameters to achieve the stability of the photovoltaic voltage loop.

[0083] G iL1_d0 (s) is obtained by establishing the state equation of the system through the state space averaging method. The specific process is as follows:

[0084] Considering the influence of parasitic parameters of energy storage components on the system, the parasitic resistance of energy storage components, r and r, are introduced in modeling. c are the parasitic resistances of the energy storage inductor and capacitor respectively. L1 i L2 u c1 u c2 ] T is the state variable; u=[U PV i PN i B ] T is the input variable. PN It is the instantaneous value of the inverter bridge bus current when the inverter is in non-through working state.

[0085] Depend on Figure 5 (a) The equivalent circuit in the direct-flow state, the energy storage inductor voltage and energy storage capacitor current during the direct-flow period are obtained as follows:

[0086]

[0087] From formula (7), we get Figure 3 The state equation of the multifunctional grid-connected inverter shown in the direct-flow state is:

[0088]

[0089] in,

[0090]

[0091] Depend on Figure 5 (b) The equivalent circuit in the non-shoot-through state, the energy storage inductor voltage and energy storage capacitor current during the non-shoot-through period are obtained as follows:

[0092]

[0093] From formula (9), we get Figure 3 The state equation of the multifunctional grid-connected inverter shown in the non-straight-through state is:

[0094]

[0095] in,

[0096]

[0097] Then the average state equation of the multifunctional grid-connected inverter in one cycle is:

[0098]

[0099] Among them, A=d0A1+(1-d0)A2, B=d0B1+(1-d0)B2.

[0100] From formula (11), we can see that when Ax+Bu=0, the static operating point of the system is X=[I L1 I L2 U c1 U c2 ] T , U=[U PV I PN I B ] T .

[0101] After introducing small signal disturbance into equation (11), the small signal state equation of the system is:

[0102]

[0103] After performing the Lapse transformation on equation (12), we can obtain:

[0104] x(s)=(sE-A) -1 Bu(s)+(sE-A) -1 [(A1-A2)X+(B1-B2)U]d0(s) (13)

[0105] In the formula, E is a 4th-order unit matrix. The transient value of the direct duty cycle d0 to i can be obtained by formula (13): L1 The transfer function G iL1_d0 (s).

[0106] Finally, the photovoltaic power control module outputs the direct duty cycle signal D0 and the energy storage capacitor voltage signal u c1 The inverter bridge output voltage modulation signal u generated by the output current control module α *、u β *Input to the pulse width modulation module, and obtain the driving signal of the six switch tubes S1~S6 that control the three-phase half-bridge bridge arm of the energy storage type quasi-Z source inverter through modulation.

[0107] Next, the current loop stability principle of the multifunctional grid-connected inverter with active damper is explained.

[0108] When the three-phase system is in equilibrium, the stability of the system can be simplified to a single-phase system for analysis in the stationary αβ coordinate system. Figure 3 The corresponding mathematical model using inverter side inductor current feedback control is as follows: Figure 6 shown. Figure 7 Given Figure 6 The simplified model of . Among them, the equivalent transfer function G x1 (s), G x2 The expression of (s) is:

[0109]

[0110]

[0111] In the formula, k pwm is the transfer function from the modulation wave to the inverter bridge output voltage, G i (s) is the current loop regulator, G d (s) is the 1.5-beat digital control delay introduced by digital control, L f1 is the inverter side inductance, L f2 is the grid-side inductance, C f is the output filter capacitor, H i1 is the capacitor current sampling coefficient, H i2 is the sampling coefficient of the inductor current on the inverter side, and s is a complex frequency domain variable.

[0112] Current loop gain T of multifunctional grid-connected inverter with active damper function d The expression of (s) is:

[0113]

[0114] Then the expression of the port current of the multifunctional grid-connected inverter with active damper function is:

[0115]

[0116] In the formula, i s (s) is the equivalent current source of the multifunctional grid-connected inverter with active damper function, Z p (s) is the original port impedance of the multifunctional grid-connected inverter with active damper function, Z v (s) is the virtual impedance of the multifunctional grid-connected inverter with the function of source damper, and their expressions are:

[0117]

[0118]

[0119]

[0120] From equation (17), it can be seen that the multifunctional grid-connected inverter with active damper function creates a virtual impedance Z at the PCC through the control circuit. v (s) to realize the active damper function of grid-connected inverter resonance suppression. Then we get Figure 1 Norton equivalent circuit of the grid-connected inverter parallel system, such as Figure 8 As shown. Among them, i invt (s) is the equivalent current source of n parallel inverters, Z o (s) is the port impedance of n parallel inverters, Z g (s) is the grid impedance, u g (s) is the grid voltage.

[0121] The simplified diagram of the Norton equivalent circuit of the inverter parallel system is as follows: Fig. 9 As shown, the parallel impedance Z of the inverter parallel system ov (s) is:

[0122] Z ov (s) = Z o (s)||Z p (s)||Z v (s) (21)

[0123] According to control theory, in order to ensure the stability of the multi-inverter system under weak power grid, (1) each inverter needs to be designed to be stable in strong power grid, and (2) Z needs to be g (s) / Z ov (s) satisfies the Nyquist stability criterion, that is, when Z g (s) and Z ov (s) interception, at the interception frequency f i The phase margin PM at the position is 180°-[∠Zg (j2πf i )-∠Z ov (j2πf i )]>0°. In the analysis, the worst case of grid impedance is considered, that is, it is regarded as pure inductance, so it is only necessary to make the parallel impedance Z of the inverter parallel system ov (s) A phase greater than -90° can ensure the stability of the inverter parallel system.

[0124] Next, the function of the multifunctional grid-connected inverter with active damper to smooth photovoltaic power fluctuations is explained.

[0125] Figure 5 The equivalent circuit diagram of the multifunctional grid-connected inverter in the direct-connection and non-direct-connection states shown in the figure, ignoring the parasitic resistance, and based on the impedance network energy storage inductor voltage volt-second product balance, the average value of the energy storage capacitor voltage U c1 , U c2 for:

[0126]

[0127]

[0128] Then the inverter DC bus voltage amplitude U can be obtained PN And the phase voltage peak value E when using a simple boost modulation strategy (M = 1-D0) m They are:

[0129]

[0130]

[0131] Where M is the modulation index.

[0132] From formula (22), we can see that assuming U B unchanged, when U PV PV * When U is increased by reducing D0 PV , to achieve photovoltaic voltage U PV Track U PV *, thus realizing photovoltaic power output control. Similarly, when U PV >U PV * When U is increased, U can be reduced by increasing D0. PV , to achieve photovoltaic voltage U PV Track U PV *, thus realizing photovoltaic power output control. Photovoltaic output power P PV , Energy storage battery power P B And the grid-connected output power P​out satisfy:

[0133] P PV -P out +P B =0 (26)

[0134] According to formula (26), if P PV <P out , then P B >0, the battery is discharged. When the battery power reaches the minimum limit, the discharge stops and the grid-connected output power P is changed. out The reference makes P out =P PV , the battery is no longer discharged; if P PV >P out , then P B <0, battery charging, when the battery is fully charged, adjust the photovoltaic reference voltage U PV *, so that the photovoltaic output power is equal to the grid-connected output power, the battery is no longer charged; if P PV =P out , then P B =0, the battery will be in a state of neither charging nor discharging.

[0135] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A multifunctional grid-connected inverter with active damper function, characterized in that: The multifunctional grid-connected inverter is connected in parallel with a plurality of grid-connected inverters at a common coupling point PCC. The multifunctional grid-connected inverter includes a power output circuit and a control circuit. The power output circuit is mainly composed of an energy storage type quasi-Z source inverter with an LCL filter. The control circuit includes an A / D sampling module, a photovoltaic power control module, an output current control module and a pulse width modulation module. The photovoltaic power control module realizes the control of photovoltaic power by tracking the photovoltaic voltage reference through the photovoltaic voltage, and smoothes the photovoltaic power fluctuation by adjusting the charging and discharging state of the energy storage battery in the multifunctional grid-connected inverter. The output current control module obtains the harmonic reference current of the target frequency band, and generates the multifunctional inverter port reference current together with the output power reference current, and controls the inverter output current through the current control loop, so as to realize the active damper function of the inverter parallel system resonance suppression under the weak power grid and the grid-connected power generation function of injecting power into the power grid. The power output circuit includes a photovoltaic cell array, a DC side filter capacitor, an impedance network with an energy storage battery, a three-phase inverter bridge and a three-phase LCL filter connected in sequence; The impedance network with energy storage battery is that the energy storage battery is directly connected in parallel to the capacitor of the impedance network, without the need for a DC-DC converter, so as to balance the difference between the photovoltaic output power and the expected grid-connected output power; the impedance network is composed of a diode, an energy storage capacitor and an energy storage inductor, when the three-phase bridge arm is directly connected, the diode is turned off, the energy storage inductor is magnetized, and the energy storage capacitor is discharged; when the three-phase bridge arm is not directly connected, the diode is turned on, the energy storage inductor is demagnetized, and the energy storage capacitor is charged; the energy storage inductor of the impedance network is designed according to the requirements of energy storage inductor current ripple, maintaining the continuity of energy storage inductor current and avoiding resonance with the energy storage capacitor, and the energy storage capacitor of the impedance network is designed according to the requirements of energy storage capacitor voltage ripple; The three-phase inverter bridge is a three-phase half-bridge inverter bridge composed of 6 switching tubes; Each phase of the three-phase LCL filter consists of two inductors L f1 , L f2 and capacitor C f Composition: Inductor L connected to the inverter side f1 The inductor L connected to the PCC terminal on the grid side f2 Connected in series, the capacitor C f One end is connected to the same phase inductor L f1 and L f2 The nodes between the three-phase capacitors C f The other end of the LCL filter is connected together; the two inductors L of the LCL filter are designed according to the requirements of the filter inductor current ripple, port current conversion rate, resonant frequency and port current harmonic content. f1 , L f2 , design the filter capacitor C according to the filter reactive power requirements f ; The A / D sampling module includes a three-phase PCC voltage sampling unit, a filter capacitor current sampling unit, an inverter side three-phase filter current sampling unit, a photovoltaic voltage sampling unit, a photovoltaic current sampling unit, an energy storage capacitor voltage sampling unit and an energy storage battery current sampling unit; The photovoltaic power control module includes a photovoltaic maximum power point tracking MPPT algorithm unit and a photovoltaic voltage regulator; the sampled photovoltaic voltage U PV and current i PV Send it to the MPPT algorithm unit to get the photovoltaic maximum power point voltage U PV *, and the photovoltaic voltage is combined with the photovoltaic voltage to generate a direct duty cycle D0 through the photovoltaic voltage regulator and sent to the pulse width modulation module to achieve photovoltaic power output control; the transfer function of the photovoltaic voltage regulator module is G PV (s) is: In the formula, k p For photovoltaic voltage regulator module G PV (s) proportionality coefficient, k i For photovoltaic voltage regulator module G PV (s) integration coefficient; The output current control module includes a harmonic reference current generation unit, a phase-locked loop unit, an output power reference current generation unit, a current regulator and a capacitor current feedback unit to achieve the function of suppressing the resonance of the grid-connected inverter system under a weak power grid and injecting power into the grid; the harmonic reference current generation unit is composed of a harmonic detection unit G HD (s), a virtual resistance generating unit and a virtual impedance compensation function unit G ca (s) composition; the three-phase PCC voltage is obtained by abc / αβ Clarke coordinate transformation u pcc_α 、u pcc_β , and through the harmonic detection unit G HD (s) Detect the αβ-axis component u of the harmonic voltage in the target frequency band of the PCC voltage pcch_α 、u pcch_β and through the virtual resistor R V and virtual impedance compensation function G ca (s) Obtain the αβ-axis component i of the harmonic reference current in the target frequency band hα *、i hβ *; Virtual resistance R V The virtual resistance generating unit is adaptively obtained according to the target resonance component to be suppressed, so as to realize the active damper function of suppressing the system resonance; The harmonic detection unit G HD (s) is used to filter out the harmonics other than the target resonant component, and its transfer function is: Where ω1 is the angular frequency of the fundamental wave, h is the harmonic order outside the target resonant component frequency band, and Q is the harmonic detection unit G HD The quality factor of (s); The virtual impedance compensation function G ca (s) for improving the virtual impedance performance of a multifunctional grid-connected inverter having a source damper function, so that the virtual impedance can present the characteristics of a resistor within the current loop bandwidth of the multifunctional inverter, thereby improving the stability of the grid-connected inverter system, wherein the current loop bandwidth is greater than the target resonant frequency; G ca The transfer function of (s) is: Where, T b and T a is the virtual impedance compensation function G ca (s) compensation coefficient; when the equivalent virtual resistance of the multifunctional grid-connected inverter presents a positive resistance characteristic and the system stability margin is small, the hysteresis compensation, i.e., T a >T b ; If the active damper equivalent virtual resistance presents a positive resistance characteristic and the system stability margin meets the system requirements, the compensation function is 1, that is, T a =T b ; When the active damper equivalent virtual resistance presents negative resistance characteristics, the lead compensation, i.e. T a <T b ; The phase-locked loop unit is used to obtain the phase θ of the three-phase PCC voltage g , angular velocity ω g , d-axis voltage component u pcc_d and the q-axis voltage component u pcc_q ; The output power reference current generating unit generates the output active power and reactive power reference current i by the power instruction generating unit. d *、i q *, and obtain the αβ axis component i of the output power reference current through dq / αβ Park inverse coordinate transformation α *、i β *; the output power reference current i α *、i β *With harmonic reference current i hα *、i hβ * is subtracted to obtain the output reference current of the multifunctional grid-connected inverter and compared with H i2 times the αβ-axis component i of the inverter-side inductor current α 、i β Subtract the current regulator G i (s), minus H i1 times the αβ axis component i of the filter capacitor current cf_α 、i cf_β , get the αβ axis component u of the inverter bridge output voltage modulation signal α *、u β *; The current regulator G i The transfer function of (s) is: In the formula, k pi For the current regulator G i (s) proportionality coefficient, ω c For the current regulator G i The bandwidth of (s), k r For the current regulator G i (s) is the resonance coefficient, and h is the number of low-frequency harmonics other than the target resonance component; Finally, the photovoltaic power control module outputs the direct duty cycle signal D0 and the energy storage capacitor voltage signal u c1 The inverter bridge output voltage modulation signal u generated by the output current control module α *、u β * Input to the pulse width modulation module, and obtain the driving signal of the six switch tubes S1 to S6 of the three-phase half-bridge bridge arm of the energy storage quasi-Z source inverter through modulation; The power instruction generation unit adopts different control methods according to the system control needs; according to the upper system scheduling instruction, the multifunctional grid-connected inverter outputs active power and reactive power reference current i d *、i q *, or generate active power and reactive power reference current i according to the inverter's own application function needs d *、i q *; When the external characteristics of the multifunctional grid-connected inverter are required to have similar rotational inertia, damping and droop characteristics as synchronous generators to maintain grid voltage stability, a current-controlled virtual synchronous generator VSG control method is adopted to obtain the inverter grid-connected output active power reference P according to the angular velocity and amplitude of the VSG output terminal voltage, i.e., the PCC voltage. out * and reactive power reference Q out *, then through P out * = 1.5 (u pcc_d i d *+u pcc_q i q *), Q out * = 1.5 (u pcc_d i q *-u pcc_q i d *) to obtain the output active power and reactive power reference current i d *、i q *; When the multifunctional grid-connected inverter is required to control the charging and discharging current of the energy storage battery, the inverter grid-connected output active power reference current i is obtained through the energy storage battery current loop control d *, that is, the energy storage battery reference current I B * and the energy storage battery current i B Subtract the current regulator G of the energy storage battery iB (s) and then the inverter grid-connected output active power reference current i is obtained d *, reactive power reference current i q * Set to 0; The active power-frequency control equation of the current-controlled VSG is: Where P m is the mechanical power of VSG; P ref is the VSG reference active power; K ω is the active power droop coefficient; ω g is the PCC voltage angular velocity, which is also the actual angular velocity of the VSG; ω0 is the rated angular velocity; D is the damping coefficient; J is the moment of inertia; P out * is the VSG output active power reference; θ g is the PCC voltage phase, which is also the VSG output power angle; The reactive power-voltage control equation of the current-controlled VSG is: U pcc =U pcc0 +K Q (Q ref -Q out *) In the formula, Q out * is the VSG output reactive power reference; Q ref is the VSG reference reactive power; K Q is the reactive power droop coefficient; U pcc0 is the given PCC voltage amplitude reference, which is also the VSG no-load electromotive force; U pcc is the PCC voltage amplitude, which is also the output voltage amplitude of VSG. The transfer function G of the energy storage battery current regulator iB (s) is: In the formula, k pB G is the current regulator of the energy storage battery iB (s) proportionality coefficient, k iB G is the current regulator of the energy storage battery iB The integration coefficient of (s).

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