Direct duty ratio VSG control method based on power loop output

Abstract

The invention discloses a direct duty ratio VSG control method based on power loop output, and the method comprises the following steps: sampling an output voltage Uo and an output current Io of a VSG, and carrying out the calculation to obtain an active power P and a reactive power Q; constructing a three-phase voltage reference value Uref through a power loop according to the calculated active power P and reactive power Q; constructing a three-phase current Iref based on the Uref and the power value; on the basis, two groups of three-phase duty ratio functions are obtained through calculation according to Uref and Iref, and a group of duty ratio signals are directly selected to generate PWM control signals according to the relation between VSG output voltage and reference voltage. According to the method, the duty ratio signal is directly generated by adopting the independent reference values of the three-phase voltage and the three-phase current, the method has the advantages of fast dynamic response, small oscillation, strong anti-interference capability and strong robustness, and the stability and the reliability of the new energy grid-forming inverter under different working conditions and power grid intensities are enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy grid-connected control, and in particular to a direct duty cycle VSG control method based on power loop output. Background Art

[0002] With the rapid development of renewable energy power generation, Virtual Synchronous Generator (VSG), as a new type of grid-connected inverter control technology, has been gradually applied to microgrids and large-scale renewable energy grid connection.

[0003] When VSG is connected to the grid, disturbances such as changes in active power command values, sudden load changes, and changes in grid voltage and frequency will cause overshoot and oscillation in the system's output power and frequency. Therefore, in order to enable VSG to achieve high-quality and reliable operation under various working conditions, the robustness of VSG must be enhanced. At present, the most common VSG control method is voltage and current dual closed-loop control. The current inner loop of this control method can realize the current limiting function in the event of a fault, and the voltage outer loop can ensure that the output voltage tracks the reference voltage without static error under various load conditions. Although the dual closed-loop VSG system has strong stability under weak power grids, in strong power grids, due to the higher grid short-circuit ratio and lower grid impedance, there is a problem of poor system adaptability and easy oscillation instability.

[0004] In order to enhance the stability of the grid-type converter under a strong power grid, an existing technology proposes to add an additional virtual impedance link outside the voltage and current double closed loop to increase the output impedance of the converter. This method can alleviate the oscillation problem caused by the excessive strength of the power grid to a certain extent, but there are problems of frequency response lag and increased complexity of the control system. In another existing technology solution, the inner loop of the grid-type converter is controlled by a single voltage loop. This solution directly generates a modulation wave through an integral link, thereby ensuring that the amplitude or effective value of the output voltage can track its reference value without static error. This method does not need to add a virtual impedance link, and the control method is simple, but it is difficult to simultaneously ensure stability and dynamic performance for frequently fluctuating or unstable power grid environments. In addition, due to the lack of control over the output current, when a power grid failure occurs, this method requires the addition of an additional current limiting loop to prevent the converter from overcurrent. Summary of the invention

[0005] In order to solve the problems of slow response speed, complex parameter design, insufficient dynamic performance and inability to adapt to strong power grids in existing VSG control, the present invention proposes a direct duty cycle VSG control method based on power loop output to solve the above problems.

[0006] The present application discloses a direct duty cycle VSG control method based on power loop output, comprising the following steps:

[0007] S1. Get the sampled VSG output voltage U o and sampling VSG output current I o , and calculate the active power P and reactive power Q;

[0008] S2, the active power reference value P ref And active power P is input into the active power loop, and the reactive power reference value Q ref and reactive power Q are input into the reactive power loop, and the output phase of the active power loop and the output voltage amplitude of the reactive power loop are synthesized to obtain a three-phase synthetic voltage;

[0009] S3, take the three-phase composite voltage obtained by S2 as the three-phase voltage reference value U ref , and according to the three-phase voltage reference value U ref And the power value is calculated to get the three-phase current I ref ;

[0010] S4, according to the three-phase voltage reference value U ref and three-phase current I ref Calculate and obtain two sets of three-phase duty cycle functions D H and D L ;

[0011] S5, sample the VSG output voltage U o The three-phase voltage reference value U obtained by S3 ref The comparison is performed and two sets of three-phase duty cycle functions D are obtained in S4 according to the comparison results. H and D L Select a group to generate PWM control signal d PWM .

[0012] Preferably, the calculation formula of active power in S1 is:

[0013] P = 1.5 (U od I od +U oq I oq );

[0014] The calculation formula of the reactive power in S1 is:

[0015] Q=1.5(U oq I od -U od I oq );

[0016] Among them, U od To sample the VSG output voltage U o The d-axis component in the dq coordinate system, U oq To sample the VSG output voltage Uo The q-axis component in the dq coordinate system, I od To sample the VSG output current I o The d-axis component in the dq coordinate system, I oq To sample the VSG output current I o The q-axis component in the dq coordinate system.

[0017] Preferably, the active power loop in S2 is obtained by the synchronous generator rotor motion equation:

[0018]

[0019] Among them, J is the moment of inertia, D is the damping coefficient, ω is the steady-state fundamental angular frequency, ω1 is the current output angular frequency of VSG, and integrating ω1 gives the output phase θ of the active power loop.

[0020] Preferably, the reactive power loop in S2 is obtained by the droop equation:

[0021] U=U N +K q (Q ref -Q);

[0022] Among them, U is the output voltage amplitude, U N is the rated voltage, K q is the reactive power droop coefficient.

[0023] Preferably, the three-phase composite voltage in S2 is synthesized as follows:

[0024]

[0025] Among them, U ref is the three-phase composite voltage, U is the output voltage amplitude of the reactive power loop, and θ is the output phase of the active power loop.

[0026] Preferably, the three-phase current I in S3 ref The calculation formula is as follows:

[0027]

[0028] Among them, U d is the three-phase voltage reference value U ref The d-axis component in the dq coordinate system, U q is the three-phase voltage reference value U ref The q-axis component in the dq coordinate system, t is the time, I dref is the three-phase current I ref The d-axis component in the dq coordinate system, Iqref is the three-phase current I ref The q-axis component in the dq coordinate system, Irefa is the A phase output current reference value, I refb is the B phase output current reference value, I refc It is the output current reference value of phase C.

[0029] Preferably, the calculation formulas of the two groups of three-phase duty cycle functions in S4 are as follows:

[0030]

[0031] in,

[0032]

[0033] L is the inverter filter inductor, C is the inverter filter capacitor, U dc is the DC side voltage, U refa is the A phase output voltage reference value, U refb is the B phase output voltage reference value, U refc is the C phase output voltage reference value, K H and K L is the duty cycle coefficient, K H >1,K L <1, and K H -1=1-K L .

[0034] Preferably, S5 comprises the following steps:

[0035] In each switching cycle, the VSG output voltage U is sampled. o The three-phase voltage reference value U obtained by S3 ref Make comparisons;

[0036] WhenU o ref When the duty cycle coefficient K is selected H The corresponding duty cycle function is D H ; when U o ≥U ref When the duty cycle coefficient K is selected L The corresponding duty cycle function is D L ;

[0037] The selected duty cycle function is compared with the carrier to generate a PWM control signal which is sent to the power device for VSG control.

[0038] Beneficial effects of the present invention:

[0039] ​(1) Simplified control architecture: The present invention replaces the traditional voltage-current dual closed-loop structure, reduces the number of control links, makes the system design and implementation process simpler, makes parameter design simpler, reduces the possibility of system failure, and simplifies subsequent maintenance and debugging work.

[0040] (2) Fast dynamic response: The present invention has a fast adjustment mechanism that can quickly adapt to grid changes under transient conditions, effectively reducing power and frequency overshoot caused by load mutations or grid disturbances, and ensuring stable operation of the system under various operating conditions.

[0041] (3) Reducing power oscillation: The present invention effectively reduces power oscillation under strong and weak grid conditions, improves the stability of the overall system, is crucial to ensuring efficient grid connection of renewable energy, and helps to improve the operation quality of the grid.

[0042] (4) Enhanced flexibility and adaptability: The present invention can flexibly respond to a variety of operating conditions, including load changes, grid failures and voltage drops, etc., and enhances the system's anti-interference ability, making it more practical and reliable in the application of new energy access to the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A schematic diagram of the structure of a grid-type converter according to an embodiment of the present invention;

[0044] Figure 2 Flow chart of a direct duty cycle VSG control method based on power loop output according to an embodiment of the present invention;

[0045] Figure 3 It is a power waveform comparison diagram and a frequency waveform comparison diagram of the embodiment of the present invention and the traditional VSG+dual closed-loop control and single voltage loop control under weak power grid condition 1;

[0046] Figure 4 It is a power waveform comparison diagram and a frequency waveform comparison diagram of the embodiment of the present invention and the traditional VSG+dual closed-loop control and single voltage loop control under weak power grid condition 2;

[0047] Figure 5 It is a power waveform comparison diagram and a frequency waveform comparison diagram of the embodiment of the present invention and the traditional VSG+dual closed-loop control and single voltage loop control under weak power grid condition 3;

[0048] Figure 6 This is a comparison diagram of the VSG output power waveform under a strong power grid between the embodiment of the present invention and the traditional VSG+dual closed-loop control;

[0049] Figure 7 It is a comparison diagram of the voltage and current waveforms of the VSG output under a strong power grid between the embodiment of the present invention and the traditional VSG+dual closed-loop control;

[0050] Figure 8 It is a frequency waveform comparison diagram of the embodiment of the present invention and traditional VSG+virtual impedance control and single voltage loop control under two working conditions of strong power grid. DETAILED DESCRIPTION

[0051] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples.

[0052] The present application discloses a direct duty cycle VSG control method based on power loop output, which is applied to Figure 1 The common grid-type converter shown in the figure is L f is the filter inductor, C f is the filter capacitor, PCC is the VSG grid connection point, L g is the grid-side inductance.

[0053] The method flow is as follows Figure 2 As shown, the specific steps include:

[0054] S1. Get the sampled VSG output voltage U o and sampling VSG output current I o , and calculate the active power P and reactive power Q.

[0055] The calculation formula for active power is:

[0056] P = 1.5 (U od I od +U oq I oq );

[0057] The calculation formula for reactive power is:

[0058] Q=1.5(U oq I od -U od I oq );

[0059] Among them, U od To sample the VSG output voltage U o The d-axis component in the dq coordinate system, U oq To sample the VSG output voltage U o The q-axis component in the dq coordinate system, I od To sample the VSG output current I o The d-axis component in the dq coordinate system, I oq To sample the VSG output current I o The q-axis component in the dq coordinate system.

[0060] S2, the active power reference value Pref And active power P is input into the active power loop, and the reactive power reference value Q ref The active power loop and reactive power Q are input into the reactive power loop, and the output phase of the active power loop and the output voltage amplitude of the reactive power loop are synthesized to obtain a three-phase synthetic voltage.

[0061] The active power loop is obtained from the synchronous generator rotor motion equation:

[0062]

[0063] Among them, J is the moment of inertia, D is the damping coefficient, ω is the steady-state fundamental angular frequency, ω1 is the current output angular frequency of VSG, and integrating ω1 gives the output phase θ of the active power loop.

[0064] The reactive power loop is obtained from the droop equation:

[0065] U=U N +K q (Q ref -Q);

[0066] Among them, U is the output voltage amplitude, U N is the rated voltage, K q is the reactive power droop coefficient.

[0067] The synthesis method of the three-phase composite voltage is as follows:

[0068]

[0069] Among them, U ref is the three-phase composite voltage, U is the output voltage amplitude of the reactive power loop, and θ is the output phase of the active power loop.

[0070] S3, take the three-phase composite voltage obtained by S2 as the three-phase voltage reference value U ref , and according to the three-phase voltage reference value U ref And the power value is calculated to get the three-phase current I ref .

[0071] Three-phase current I ref The calculation formula is as follows:

[0072]

[0073] Among them, U d is the three-phase voltage reference value U ref The d-axis component in the dq coordinate system, U q is the three-phase voltage reference value U ref The q-axis component in the dq coordinate system, t is the time, I dref is the three-phase current I refThe d-axis component in the dq coordinate system, I qref is the three-phase current I ref The q-axis component in the dq coordinate system, I refa is the A phase output current reference value, I refb is the B phase output current reference value, I refc It is the output current reference value of phase C.

[0074] S4, according to the three-phase voltage reference value U ref and three-phase current I ref Calculate and obtain two sets of three-phase duty cycle functions D H and D L :

[0075]

[0076] in,

[0077]

[0078] L is the inverter filter inductor, U dc is the DC side voltage, U refa is the A phase output voltage reference value, U refb is the B phase output voltage reference value, U refc is the C phase output voltage reference value, K H and K L is the duty cycle coefficient, K H >1,K L <1, and K H -1=1-K L .

[0079] S5, sample the VSG output voltage U o The three-phase voltage reference value U obtained by S3 ref The comparison is performed, and according to the comparison result, one group is selected from the two groups of three-phase duty cycle functions obtained in S4 to generate a PWM control signal d PWM .

[0080] In each switching cycle, the VSG output voltage U is sampled. o The three-phase voltage reference value U obtained by S3 ref For comparison. o ref When the duty cycle coefficient K is selected H The corresponding duty cycle function is D H ; when U o ≥U ref When the duty cycle coefficient K is selected L The corresponding duty cycle function is D L ​The PWM control signal is generated by comparing the selected duty cycle function with the carrier and sent to Figure 1 The power devices in the dashed box are shown to be VSG controlled.

[0081] In a specific embodiment, in order to verify the superiority of the control method proposed in the embodiment of the present application in terms of dynamic performance compared to the traditional control method, a detailed description is given below through simulation.

[0082] Working condition 1: Change the active power command from 10kW to 20kW in 2s and recover in 3s.

[0083] Working condition 2: The load changes from 10kW to 20kW in 2s and recovers in 3s.

[0084] Working condition 3: The grid voltage drops to 0.7pu in 2s and recovers in 3s.

[0085] Table 1 Simulation parameters

[0086]

[0087]

[0088] The simulation parameters are set according to Table 1, the load is 10kW, and the simulation time is 4s to verify the dynamic response characteristics of the embodiment of the present application. Under the same parameters, the simulations of the traditional VSG+voltage and current dual closed-loop control, the single voltage loop control and the control method of the present invention are respectively run.

[0089] Get as Figure 3 The power waveform and frequency waveform under working condition 1 are shown in FIG. Figure 4 The power waveform and frequency waveform under working condition 2 are shown in FIG. Figure 5 The power waveform and frequency waveform under working condition 3 are shown.

[0090] The above comparison of three control methods is carried out under various working conditions. Figure 3-Figure 5 It can be seen from the waveform that the present invention can reduce the power and frequency response overshoot under this series of working conditions, greatly improving the dynamic performance, and verifying the superior dynamic response characteristics of the present invention.

[0091] In a specific embodiment, in order to verify the adaptability of the control method proposed in the embodiment of the present application under a strong power grid, a detailed description is given below through simulation.

[0092] The simulation parameters such as grid-side inductance, moment of inertia and damping coefficient are set according to Table 2, and the other simulation parameters are the same as Table 1. The simulation compares the traditional VSG+ double closed-loop control and the present invention. By selecting a low inductance value to simulate a strong grid (L g =300μH(SCR≈18)).

[0093] Table 2 Simulation parameters

[0094] parameter Numeric parameter Numeric Grid side inductor 300μH Active power reference value 20kW Moment of Inertia 0.4 Reactive power reference value 0var Damping coefficient 15 Reactive power droop coefficient 10

[0095] Get as Figure 6 The VSG output power waveform under strong power grid is shown in Figure 7 The VSG output voltage and current waveforms are shown in the figure. Figure 6 Waveform, under the traditional VSG+dual closed-loop control strategy, the output power has subsynchronous oscillation, and the VSG loses stability. However, under the control strategy of the present invention, the output power can still follow the power command well and will not oscillate due to the strong power grid. Figure 7 Waveform, under the traditional VSG+double closed-loop control strategy, the voltage and current waveforms also undergo sub-synchronous oscillations. The voltage and current under the control strategy of the present invention are output stably, which reflects the adaptability of the present invention under strong power grids.

[0096] In a specific embodiment, in order to verify the superiority of the control method proposed in the embodiment of the present application compared with the prior art under a strong power grid, a detailed description is given below through simulation.

[0097] Under a strong power grid, the present invention and traditional VSG+virtual impedance control and single voltage loop control are simulated under the following two working conditions to observe the frequency waveforms.

[0098] Working condition 4: Change the active power command from 10kW to 20kW in 2s and recover in 3s.

[0099] Working condition 5: The load changes from 10kW to 20kW in 2s and recovers in 3s.

[0100] get Figure 8 The frequency waveforms of the present invention, traditional VSG+virtual impedance control, and single voltage loop control under two working conditions under strong power grid. Figure 8 It can be seen from the waveform that the frequency oscillation of the present invention under working condition 4 and working condition 5 is smaller than that of the other two control methods, and the same is true for the adjustment time. This means that even under a strong power grid, the present invention has good dynamic response characteristics compared to the prior art.

[0101] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of protection of the present invention is defined by the attached claims and their equivalents.

Claims

1. A direct duty cycle VSG control method based on power loop output, characterized in that: The following steps are involved: S1. Get the sampled VSG output voltage U o and sampling VSG output current I o , and calculate the active power P and reactive power Q; S2, the active power reference value P ref And active power P is input into the active power loop, and the reactive power reference value Q ref and reactive power Q are input into the reactive power loop, and the output phase of the active power loop and the output voltage amplitude of the reactive power loop are synthesized to obtain a three-phase synthetic voltage; S3, take the three-phase composite voltage obtained by S2 as the three-phase voltage reference value U ref , and according to the three-phase voltage reference value U ref And the power value is calculated to get the three-phase current I ref ; S4, according to the three-phase voltage reference value U ref and three-phase current I ref Calculate and obtain two sets of three-phase duty cycle functions D H and D L ; S5, sample the VSG output voltage U o The three-phase voltage reference value U obtained by S3 ref The comparison is performed and two sets of three-phase duty cycle functions D are obtained in S4 according to the comparison results. H and D L Select a group to generate PWM control signal d PWM .

2. The direct duty cycle VSG control method based on power loop output according to claim 1, characterized in that: The calculation formula of the active power in S1 is: P=1.5(U od AND od +U oq AND oq ); The calculation formula of the reactive power in S1 is: Q=1.5(U oq I od -U od I oq ); Among them, U od To sample the VSG output voltage U o The d-axis component in the dq coordinate system, U oq To sample the VSG output voltage U o The q-axis component in the dq coordinate system, I od To sample the VSG output current I o The d-axis component in the dq coordinate system, I oq To sample the VSG output current I o The q-axis component in the dq coordinate system.

3. The direct duty cycle VSG control method based on power loop output according to claim 2, characterized in that: The active power loop in S2 is obtained from the synchronous generator rotor motion equation: Among them, J is the moment of inertia, D is the damping coefficient, ω is the steady-state fundamental angular frequency, ω1 is the current output angular frequency of VSG, and integrating ω1 gives the output phase θ of the active power loop.

4. The direct duty cycle VSG control method based on power loop output according to claim 3 is characterized in that: The reactive power loop in S2 is obtained by the droop equation: U=U N +K q (Q ref -Q); Among them, U is the output voltage amplitude, U N is the rated voltage, K q is the reactive power droop coefficient.

5. The direct duty cycle VSG control method based on power loop output according to claim 4, characterized in that: The synthesis method of the three-phase composite voltage in S2 is as follows: Among them, U ref is the three-phase composite voltage, U is the output voltage amplitude of the reactive power loop, and θ is the output phase of the active power loop.

6. The direct duty cycle VSG control method based on power loop output according to claim 5, characterized in that: The three-phase current I in S3 ref The calculation formula is as follows: Among them, U d is the three-phase voltage reference value U ref The d-axis component in the dq coordinate system, U q is the three-phase voltage reference value U ref The q-axis component in the dq coordinate system, t is the time, I dref is the three-phase current I ref The d-axis component in the dq coordinate system, Iqref is the three-phase current I ref The q-axis component in the dq coordinate system, I refa is the A phase output current reference value, I refb is the B phase output current reference value, I refc It is the output current reference value of phase C.

7. The direct duty cycle VSG control method based on power loop output according to claim 6, characterized in that: The calculation formulas of the two groups of three-phase duty cycle functions in S4 are as follows: in, L is the inverter filter inductor, C is the inverter filter capacitor, U dc is the DC side voltage, U refa is the A phase output voltage reference value, U refb is the B phase output voltage reference value, U refc is the C phase output voltage reference value, K H and K L is the duty cycle coefficient, K H >1,K L <1, and K H -1=1-K L .

8. The direct duty cycle VSG control method based on power loop output according to claim 7, characterized in that: The S5 comprises the following steps: In each switching cycle, the VSG output voltage U is sampled. o The three-phase voltage reference value U obtained by S3 ref Make comparisons; When U o ref When the duty cycle coefficient K is selected H The corresponding duty cycle function is D H ; when U o ≥U ref When the duty cycle coefficient K is selected L The corresponding duty cycle function is D L ;​ The selected duty cycle function is compared with the carrier to generate a PWM control signal which is sent to the power device for VSG control.

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