Reactive calculation feedforward-based low-short-circuit-ratio grid-connected operation control method for grid-following type three-phase grid-connected inverter
By introducing reactive computing feedforward control into the three-phase grid-connected inverter with the grid, the problem of slow response speed when the inverter is large, and stable grid-connected operation under low short-circuit ratio is achieved. Especially in high power situations, the system instability is effectively avoided.
Patent Information
- Application Number
- CN202411313480.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-09-20
- Publication Date
- 2024-12-27
AI Technical Summary
When the inverter power is large, the system switching frequency is low and the bandwidth of the internal ring of the current is insufficient, resulting in slow response speed of the grid connection point voltage and risk of instability. Especially in the low short-circuit ratio, traditional methods are difficult to effectively support the system stability and net.
The reactive support current value is calculated by using the three-phase grid-connected inverter control method based on reactive power calculation feedforward. The reactive support current value is calculated through the grid-side inductance estimation link and the reactive support calculation link, and applied to the AC voltage loop. In this paper, feedforward control is realized to enhance the support for the voltage of the connection point.
It effectively avoids the risk of instability of the system under low short-circuit ratio and high-power conditions, and realizes stable grid-connected operation when inverter power fluctuates and grid capacity changes.
Smart Images
Figure CN119209707A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a low short-circuit ratio grid-connected operation control method for a grid-following three-phase grid-connected inverter, and in particular to a low short-circuit ratio grid-connected operation control method for a grid-following three-phase grid-connected inverter based on reactive power calculation feedforward. Background Art
[0002] In distributed power generation systems, grid-connected inverters have been widely used. With the large-scale grid connection of distributed power generation systems, the short-circuit ratio of local grid connection points will continue to decrease, which poses a challenge to the stable grid connection of grid-connected inverters.
[0003] At present, in the field of grid-connected inverter control, the traditional method uses the AC voltage loop as the outer loop of the q-axis current loop to support the grid voltage at the grid connection point, thereby achieving stable grid connection under low short-circuit ratio conditions. There are also methods that use the reactive power loop or the reactive voltage loop as the outer loop of the q-axis current loop.
[0004] Although the above method can achieve stable grid connection in most cases, when the inverter power is large, the switching frequency of the system is relatively low, the bandwidth of the inner current loop is low, and the bandwidth of the q-axis outer loop will be further reduced. The response speed of the grid connection point voltage support is slow, and there is a risk of instability caused by the slow response speed of reactive power support when the system state fluctuates. Summary of the invention
[0005] The purpose of the present invention is to provide a low short-circuit ratio grid-connected operation control method for a grid-following three-phase grid-connected inverter based on reactive calculation feedforward, so as to achieve stable grid connection under low short-circuit ratio grid-connected conditions, especially when the inverter power is large, it can effectively avoid instability.
[0006] The invention discloses a low short-circuit ratio grid-connected operation control method for a grid-connected three-phase grid-connected inverter based on reactive power calculation feedforward. The implementation topology of the method includes a three-phase grid-connected inverter main circuit, a low-pass filter, a voltage and current detection circuit, and a controller. The controller includes a phase sequence separation link, a phase-locked loop PLL link for extracting frequency phase information from three-phase components in an ABC three-phase stationary coordinate system, a positive sequence PARK transformation link, a negative sequence PARK transformation link, a negative sequence current low-pass filter link, a voltage feedforward low-pass filter link, a DC voltage and current PI link, a positive sequence PARK inverse transformation link, a voltage balancing control link, a voltage feedforward control coefficient, a grid-side inductance estimation link, a reactive power support calculation link, an AC voltage loop and reactive power calculation feedforward control link, and an SPWM link. In the controller, on the basis of the traditional three-phase grid-connected inverter control method, a grid-side inductance estimation link, a reactive power support calculation link, and an AC voltage loop and reactive power calculation feedforward control link are added. The grid-side inductance estimation link is based on the d-axis voltage u of the grid connection point. d , grid connection point q-axis voltage u qAnd the positive sequence d-axis current at the grid connection point Positive sequence q-axis current Calculate the estimated grid-side inductance value L gcal The reactive support calculation stage is based on the grid-connected power P of new energy and the estimated value of grid-side inductance L gcal , and AC voltage loop reference value Calculate the reactive support value I qcal , will I qcal The input AC voltage loop and reactive power calculation feedforward control link are used to determine the positive sequence q-axis current inner loop reference value.
[0007] In the proposed grid-side inductance estimation link, the grid-side inductance estimation value L gcal The specific formula is:
[0008]
[0009] in
[0010]
[0011]
[0012] In addition, U g is the ideal grid voltage amplitude, is the positive sequence d-axis current at the grid connection point, is the positive sequence q-axis current at the grid connection point, u d is the d-axis voltage at the grid connection point, L gcal is the estimated value of grid-side inductance.
[0013] If the grid capacity changes due to the connection or disconnection of equipment, it can be equivalent to a sudden change in the grid equivalent inductance. At the moment of the sudden change, use L' gcal is the estimated value of the equivalent inductance on the grid side at the moment of fluctuation,
[0014] L' gcal =L gcal +ΔL g
[0015] in
[0016]
[0017] △L g is the grid-side inductance fluctuation value, △u d , △u q They are the instantaneous fluctuation values of the d-axis and q-axis voltages at the grid connection point respectively.
[0018] In the reactive support calculation link proposed, the reactive current value I required to be generated by a single inverter for reactive support is qcal The specific formula is:
[0019]
[0020] in
[0021]
[0022]
[0023] Among them U Ld is the d-axis voltage on the grid-side equivalent inductance, U L is the total voltage on the grid-side equivalent inductance, L gcal is the estimated value of the grid-side inductance obtained through the grid-side inductance estimation link, P is the inverter power, is the AC voltage loop reference value, I qcal It is the reactive current value that a single inverter needs to generate for reactive support, and f is the fundamental frequency. K iq is the reactive current feed-forward coefficient.
[0024] The AC voltage loop and reactive power calculation feedforward control link described in the present invention is to apply the feedforward coefficient to the reactive current calculation value obtained after the grid-side inductance estimation and reactive power support calculation and then superimpose it on the output result of the d-axis voltage loop of the grid connection point.
[0025] The grid-connected operation method of the grid-following three-phase grid-connected inverter under low short-circuit ratio conditions based on reactive power calculation feedforward of the present invention realizes stable grid-connected operation under low short-circuit ratio conditions, and can effectively avoid instability in the case of high-power inverters.
[0026] The present invention is applicable to a three-phase grid-connected inverter which uses semiconductor devices such as MOSFET and IGBT as power switches, uses a positive and negative sequence separation method, and utilizes synchronous rotating coordinate transformation to control positive and negative sequence currents through PI. The present invention can be applicable to various grid-connected energy systems such as solar energy, wind power, and fuel cell power generation. BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Figure 1 Permanent magnet synchronous direct drive wind turbine grid-connected system topology;
[0028] Figure 2 This is the main circuit diagram of the grid-connected inverter of the present invention;
[0029] Figure 3 It is the overall control block diagram of the controller of the present invention;
[0030] Figure 4 To cope with the control results of power fluctuations using traditional control methods;
[0031] Figure 5 To show the control results of the proposed method to cope with power fluctuations;
[0032] Figure 6 To show the control results of the proposed method to cope with power fluctuation and short-circuit ratio mutation; DETAILED DESCRIPTION
[0033] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific examples.
[0034] According to a specific example of the present invention, Figure 1 The figure shows the topology of the grid-connected system of a permanent magnet synchronous direct-drive wind turbine generator, which includes a permanent magnet synchronous direct-drive wind turbine 1, a machine-side LC low-pass filter 2, a machine-side converter 3, a DC bus capacitor 4, a grid-side converter 5, a grid-side LC low-pass filter 6, a power grid and its equivalent inductor 7.
[0035] Figure 2 Shown is Figure 1 A specific main circuit diagram example of a grid-connected inverter in FIG. It includes a DC bus capacitor 4, a grid-side converter 5 which is an IGBT-based I-type three-level grid-side converter circuit, a grid-side LC low-pass filter 6, a power grid and its equivalent inductor 7, a voltage and current detection circuit 8, and a controller 9.
[0036] According to a specific example of the present invention, the controller 9 is as follows Figure 3 As shown, it includes a phase sequence separation link 901, a phase-locked loop PLL link 902 for extracting frequency phase information from the three-phase components in the ABC three-phase stationary coordinate system, a positive sequence PARK transformation link 903, a negative sequence PARK transformation link 904, a negative sequence current low-pass filter link 907 and a voltage feedforward low-pass filter link 909, a DC voltage PI controller 905 and a current PI controller 908, a positive sequence PARK inverse transformation link 911, a voltage balancing control link 912, a voltage feedforward control coefficient 910, a grid-side inductance estimation link 913, a reactive support calculation link 914, an AC voltage loop and reactive calculation feedforward control link 906 and an SPWM link 915.
[0037] The specific formula of the proposed grid-side inductance estimation link 913 is:
[0038]
[0039] in
[0040]
[0041]
[0042] In addition, U g is the ideal grid voltage amplitude, is the positive sequence d-axis current at the grid connection point, is the positive sequence q-axis current at the grid connection point, u dis the d-axis voltage at the grid connection point, L gcal is the estimated value of grid-side inductance.
[0043] If the grid capacity changes due to the connection or disconnection of equipment, it can be equivalent to a sudden change in the grid equivalent inductance. At the moment of the sudden change, there is
[0044] L' gcal =L gcal +ΔL g
[0045]
[0046] Where △L g is the grid-side inductance fluctuation value, △u d , △u q are the instantaneous fluctuation values of the d-axis and q-axis voltages at the grid connection point, L' gcal It is the estimated value of equivalent inductance on the grid side at the moment of fluctuation.
[0047] The specific formula of the proposed reactive support calculation link 914 is:
[0048]
[0049] in
[0050]
[0051]
[0052] Among them U Ld is the d-axis voltage on the grid-side equivalent inductance, U L is the total voltage on the grid-side equivalent inductance, L gcal is the estimated value of the grid-side inductance obtained by the grid-side inductance estimation link 913, P is the inverter power, is the AC voltage loop reference value, I qcal It is the reactive current value that a single inverter needs to generate for reactive support, and f is the fundamental frequency. K iq is the reactive current feed-forward coefficient. In this example, K iq =0.75.
[0053] The high-power grid-connected operation method of the grid-connected three-phase grid-connected inverter based on reactive calculation feedforward control under low short-circuit ratio working conditions of the present invention is: the controller 9 sends the positive sequence voltage component to the PLL phase-locked loop 902 to extract the frequency phase information after the grid-connected point voltage on the three-phase inverter side passes through the phase sequence separation link 901; the grid-connected point voltage is subjected to positive sequence PARK transformation 903 using the phase information and is subjected to low-pass filtering 909 and then multiplied by the voltage feedforward coefficient 910 as a voltage feedforward signal. In this example, the voltage feedforward coefficient K is taken. u=0.9; the three-phase inverter side inductor current is subjected to positive sequence PARK transformation 903 as the positive sequence d-axis and positive sequence q-axis current inner loop feedback signals, the positive sequence d-axis current inner loop reference value is determined by the DC voltage PI controller 905, and the positive sequence q-axis current inner loop reference value is determined by the proposed AC voltage loop and reactive power calculation feedforward control link 906, Figure 3 In the example shown, the output of the DC voltage PI controller 905 is also passed through a limiting link to obtain a positive sequence d-axis current inner loop reference value, where I max is the set maximum output current; the three-phase inverter side inductor current is subjected to negative-sequence PARK transformation 904 and passed through the negative-sequence current low-pass filter link 907 as the negative-sequence d-axis and negative-sequence q-axis current loop feedback signals, and the negative-sequence d-axis and q-axis current inner loop reference values are 0; the positive and negative sequence currents pass through the current PI controller 908, i.e. the current inner loop, to obtain the PI control signals of the dq axes, the PI control signals of the dq axes are superimposed on each other and then superimposed on the feedforward signal of the dq axes, and then subjected to positive-sequence PARK inverse transformation 911, the obtained three-phase control signal is superimposed on the voltage-sharing control signal of the voltage-sharing control link 912, and then passes through the SPWM link 915 to obtain the grid-side inverter drive signal.
[0054] The proposed AC voltage loop and reactive power calculation feedforward control link is specifically as follows: the grid-side inductance is estimated 913 according to the relevant voltage and current quantities, the reactive support is calculated 914 based on the wind turbine power generation and the estimated value of the grid-side inductance and the AC voltage loop reference value, and the output result of the AC voltage loop PI controller is superimposed on the reactive current calculation value of the reactive current feedforward coefficient as the positive-sequence q-axis current inner loop reference value.
[0055] Taking the grid-connected operation of wind turbines with power fluctuations under low short-circuit ratio conditions (SCR=1.654) as an example, Figure 4 This is the control result using the traditional AC voltage loop. It can be seen that the system becomes unstable when the fan power steps from 0.9pu to 1.0pu; Figure 5 The control results of the proposed method show that the system can still maintain stable operation when the fan power increases from 0.9pu to 1.0pu.
[0056] Figure 6 The control effect of the proposed method in dealing with the sudden change of short-circuit ratio. It can be seen that the system can operate at rated power when SCR=1.378, and can still maintain stable operation after SCR suddenly changes to 1.181.
[0057] Aiming at the problem of grid instability that may occur when a grid-following three-phase grid-connected inverter is operating at high power in low short-circuit ratio conditions due to fluctuations in renewable energy power generation or changes in grid-side equipment, the present invention proposes a method for high-power grid-connected operation of a grid-following three-phase grid-connected inverter under low short-circuit ratio conditions based on reactive power calculation feedforward control.
Claims
1. A method for controlling low short-circuit ratio grid-connected operation of a three-phase grid-connected inverter based on reactive power calculation feedforward, characterized in that: On the basis of the traditional control link of the three-phase grid-connected inverter, the grid-side inductance estimation value and reactive support calculation value are calculated according to the grid-connected point voltage, grid-connected point current and AC voltage loop reference value, and the obtained reactive support calculation value is superimposed on the AC voltage loop output result to achieve stable grid-connected operation of the grid-connected three-phase grid-connected inverter in response to power fluctuations or SCR fluctuations under low short-circuit ratio conditions.
2. According to the low short-circuit ratio grid-connected operation control method of a grid-connected three-phase grid-connected inverter based on reactive power calculation feedforward according to claim 1, the implementation topology of the method comprises a permanent magnet synchronous direct-drive wind turbine (1), a machine-side LC low-pass filter (2), a machine-side converter (3), a DC bus capacitor (4), a grid-side converter (5), a grid-side LC low-pass filter (6), a power grid and its equivalent inductance (7), a voltage and current detection circuit (8), and a controller (9), characterized in that: The controller (9) comprises a phase sequence separation link (901), a phase-locked loop PLL link (902) for extracting frequency phase information from three-phase components in an ABC three-phase stationary coordinate system, a positive sequence PARK conversion link (903), a negative sequence PARK conversion link (904), a negative sequence current low-pass filter link (907), a voltage feedforward low-pass filter link (909), a DC voltage PI controller (905), a current PI controller (908), a positive sequence PARK inverse conversion link (911), a voltage balancing control link (912), a voltage feedforward control coefficient (910), a grid-side inductance estimation link (913), a reactive power support calculation link (914), an AC voltage loop and reactive power calculation feedforward control link (906) and an SPWM link (915), wherein the grid-side inductance estimation link (913) is based on the d-axis voltage u of the grid connection point. d , grid connection point q-axis voltage u q And the positive sequence d-axis current at the grid connection point Positive sequence q-axis current Calculate the estimated grid-side inductance value L gcal The reactive support calculation link (914) is based on the grid-connected power P of new energy and the estimated value of grid-side inductance L gcal , and AC voltage loop reference value Calculate the reactive support value I qcal , will I qcal The input AC voltage loop and reactive power calculation feedforward control link (906) are used to determine the positive sequence q-axis current inner loop reference value.
3. The method for controlling low short-circuit ratio grid-connected operation of a three-phase grid-connected inverter based on reactive power calculation feedforward according to claim 2 is characterized in that: In the grid-side inductance estimation step (913), the grid-side inductance estimation value L gcal for in In addition, U g is the ideal grid voltage amplitude, is the positive sequence d-axis current at the grid connection point, is the positive sequence q-axis current at the grid connection point, u d is the d-axis voltage at the grid connection point, f is the fundamental frequency, L gcal is the estimated value of grid-side inductance.
4. The method for controlling low short-circuit ratio grid-connected operation of a three-phase grid-connected inverter based on reactive power calculation feedforward according to claim 3 is characterized in that: The grid capacity changes due to the connection or disconnection of equipment on the grid side, which is equivalent to a sudden change in the equivalent inductance of the grid side. At the moment of the sudden change, L' gcal is the estimated value of the equivalent inductance on the grid side at the moment of fluctuation, It gcal =L gcal +ΔL g in △L g is the grid-side inductance fluctuation value, △u d , △u q They are the instantaneous fluctuation values of the d-axis and q-axis voltages at the grid connection point respectively.
5. The method for controlling low short-circuit ratio grid-connected operation of a three-phase grid-connected inverter based on reactive power calculation feedforward according to claim 2, characterized in that: In the reactive support calculation link (914), a single inverter takes into account the reactive current value I required to be generated by the reactive support. qcal for: in Among them U Ld is the d-axis voltage on the grid-side equivalent inductance, U L is the total voltage on the grid-side equivalent inductance, L gcal is the estimated value of the grid-side inductance obtained through the grid-side inductance estimation link (913), P is the grid-connected power of the new energy, is the reference value of the AC voltage loop, f is the fundamental frequency, K iq is the reactive current feed-forward coefficient.