A method for improving transient stability of active power scheduling of grid-connected inverter under weak grid

Abstract

The application discloses a method for improving active scheduling transient stability of a grid-constructing inverter under a weak power grid, and relates to the technical fields of grid-connected inverter control and power electronics. The method comprises scheduling transient stability improvement coefficient calculation, grid-constructing inverter active power instruction value generation and grid-constructing inverter modulation wave calculation. The grid-constructing inverter active power instruction value is generated by a grid-constructing inverter active power initial instruction value through a scheduling transient stability improvement coefficient calculation formula and a scheduling transient stability improvement calculation formula, so that the transient stability of the grid-constructing inverter is ensured and the active power fast response is realized under the weak power grid. The method is simple to implement and does not need to increase additional equipment.

Description

Technical Field

[0001] This invention relates to the field of grid-connected inverter control and power electronics technology, specifically to a method for improving the transient stability of active power dispatch of grid-connected inverters in weak power grids. Background Technology

[0002] As the penetration rate of new energy sources continues to increase, the power grid is gradually exhibiting characteristics of a weak grid at the end of the grid. This means that the equivalent grid impedance faced by new energy power plants is gradually increasing, leading to prominent voltage stability issues. In recent years, the scale of large-scale new energy bases has gradually increased, while the proportion of traditional synchronous machines has gradually decreased, resulting in more apparent frequency stability problems. When system frequency fluctuates, energy storage is needed to promptly provide active power to address frequency issues, which places certain demands on the control of grid-connected inverters. However, the problem of a weak grid with a large equivalent grid impedance still exists in large-scale new energy bases. Under weak grid conditions, when grid-connected inverters experience rapid active power step changes, transient synchronization problems may arise due to their simulated inertia characteristics.

[0003] Regarding the transient synchronization stability problem of grid-forming inverters during active power dispatch, there have been relevant studies. In the paper "Transient stability based power climbing control of grid-forming inverter" published at the "2022 IEEE International Power Electronics and Application Conference and Exposition (PEAC)", the causes of transient synchronization instability of grid-forming inverters during active power climbing were studied, and a variable step size climbing method was proposed to improve the transient synchronization stability of grid-forming inverters. However, this restricts the response speed of grid-forming inverters when the system has an active power deficit. Summary of the Invention

[0004] The technical problems this invention aims to solve are transient synchronization issues that exist in existing technologies when grid-connected inverters experience rapid active power step changes under weak power grid conditions, and the limitation on the response speed of grid-connected inverters when the system experiences active power deficits. This invention proposes a method to improve the transient stability of active power dispatch for grid-connected inverters under weak power grid conditions. By passing the active power of the grid-connected inverter through a pre-filter, this invention reshapes the active power characteristics of the inverter, avoiding the transient synchronization stability problem caused by sudden increases in active power commands in weak power grid scenarios, while simultaneously improving the active power ramp-up speed.

[0005] The objective of this invention is achieved as follows.

[0006] A method for improving the transient stability of active power dispatch for grid-connected inverters in weak power grids is disclosed. The power system applying this method includes grid-connected inverters, branch transmission line impedances, grid impedances, and the power grid connected in series in sequence. The steps of the improvement method are as follows: Step 1. Sample the AC side voltage u of the grid-connected inverter. GFMa ,u GFMb ,u GFMc And grid-type inverter AC side current i GFMa i GFMb i GFMc The AC side voltage u of the dq axis of the grid-type inverter is obtained through coordinate transformation. GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq ; Step 2, based on the AC side voltage u of the grid-connected inverter GFMa ,u GFMb ,u GFMc The AC side voltage amplitude U is obtained from the formula for calculating the AC side voltage amplitude of a grid-connected inverter. GFM The scheduling transient stability lift coefficient k is obtained through the formula for calculating the scheduling transient stability lift coefficient. improved ; Step 3: Given the initial active power command value P of the grid-connected inverter. refinit The active power command value P of the grid-type inverter is obtained by the scheduling transient stability improvement calculation formula. ref According to the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq Active power command value P of grid-connected inverter ref The voltage command U of the grid-connected inverter is obtained through the voltage command calculation equation. refGFM ; Step 4, based on the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq , grid-connected inverter dq axis AC side current i GFMd i GFMq And grid-type inverter voltage command U refGFM The dq-axis current command i of the grid-connected inverter is obtained through the voltage control loop equation of the grid-connected inverter. GFMdref i GFMqref Then, the dq-axis modulation wave e of the grid-type inverter is obtained through the current control loop equation of the grid-type inverter. GFMd ,e GFMq Then, through coordinate transformation, the modulation wave e of the grid-type inverter is obtained. GFMa ,eGFMb ,e GFMc ; Modulation wave e of grid-type inverter GFMa ,e GFMb ,e GFMc The switching signals of the power devices in the grid-type inverter are generated by SPWM modulation to control the turn-on and turn-off of the power devices in the grid-type inverter.

[0007] Preferably, the formula for calculating the AC side voltage amplitude of the grid-type inverter in step 2 is: .

[0008] Preferably, the formula for calculating the scheduling transient stability improvement coefficient in step 2 is:

[0009] Among them, X g ω0 is the equivalent grid impedance, and ω0 is the grid's rated angular frequency.

[0010] Preferably, the scheduling transient stability boost calculation formula in step 3 is:

[0011] Among them, J GFM D is the inertia constant of the grid-connected inverter. GFM T1 is the damping constant of the grid-connected inverter, T2 is the first time constant, T2 is the second time constant, and s is the Laplace operator.

[0012] Preferably, the voltage command calculation equation for the grid-type inverter in step 3 is as follows:

[0013] Where, k QGFM U0 is the reactive power loop coefficient of the grid-connected inverter, and U0 is the rated AC voltage of the grid-connected inverter.

[0014] Preferably, the voltage control loop equation for the grid-type inverter in step 4 is:

[0015]

[0016] Among them, K pVC_GFM K is the proportional control coefficient of the PI regulator in the voltage control loop of a grid-connected inverter. iVC_GFM Z is the integral regulation coefficient of the PI regulator in the voltage control loop of the grid-connected inverter. vir s represents the virtual impedance of the grid-connected inverter, and s is the Laplace operator.

[0017] Preferably, the current control loop equation for the grid-type inverter in step 4 is:

[0018]

[0019] Among them, K pCC_GFM K is the proportional control coefficient of the PI regulator in the current control loop of the grid-connected inverter. iCC_GFM is the integral regulation coefficient of the PI regulator in the current control loop of the grid-connected inverter, and s is the Laplace operator.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention considers the transient synchronization stability problem caused by virtual inertia when the active power command of a grid-connected inverter jumps under weak power grid conditions, and proposes a transient stability improvement method based on pre-filtering, thereby realizing the stable operation of the grid-connected inverter under weak power grid conditions; 2. Compared with traditional control, the present invention can improve the active power response speed and realize the rapid active power frequency support of grid-type inverters in weak power grids. Attached Figure Description

[0021] Figure 1 This is a power system topology diagram used in the embodiments of the present invention.

[0022] Figure 2 This is a flowchart illustrating the implementation of the control method of the present invention.

[0023] Figure 3 The active power waveform of the grid-type inverter without using the method of the present invention is shown.

[0024] Figure 4 The active power waveform of the grid-type inverter when using the method of the present invention is shown. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings.

[0026] The power system topology used in the embodiments of this invention is as follows: Figure 1 As shown. By Figure 1 It can be seen that the power system using this method includes grid-type inverters, branch transmission line impedances, grid impedances, and the grid connected in sequence.

[0027] from Figure 1 As can be seen, the grid-type inverter includes a DC-side power supply, a three-phase half-bridge inverter circuit, and an LC filter. The LC filter includes a filter inductor, a filter capacitor, and a damping resistor.

[0028] exist Figure 1Above, 10 is the DC power supply, 20 is the three-phase half-bridge inverter circuit, 30 is the LC filter, 40 is the grid-type inverter, 50 is the branch transmission line impedance, and 60 is the grid impedance. dc For DC voltage, L f For the filter inductor, C f For the filter capacitor, R d For damping resistance, L t and R t L represents the resistance and reactance in the branch transmission line impedance. g and R g These are the resistance and reactance in the power grid impedance.

[0029] In this embodiment, the main circuit parameters of the grid-type inverter are: DC voltage V dc The rated voltage is 1200V, the rated output line voltage is 600V / 50Hz, and the rated power is 3.125MW. The inductive component of the equivalent mains impedance is 3.3336 × 10⁻⁶. -4 H.

[0030] Figure 2 This is a flowchart of the control method of the present invention. Figure 2 Therefore, this invention provides a method for improving the transient stability of active power dispatch for grid-type inverters under weak power grid conditions. In this method, the active power command value of the grid-type inverter is generated from the initial active power command value of the grid-type inverter using the dispatch transient stability improvement coefficient calculation formula and the dispatch transient stability improvement formula.

[0031] The control method comprises the following steps: Step 1. Sample the AC side voltage u of the grid-connected inverter. GFMa ,u GFMb ,u GFMc And grid-type inverter AC side current i GFMa i GFMb i GFMc The AC side voltage u of the dq axis of the grid-type inverter is obtained through coordinate transformation. GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq .

[0032] Step 2, based on the AC side voltage u of the grid-connected inverter GFMa ,u GFMb ,u GFMc The AC side voltage amplitude U is obtained from the formula for calculating the AC side voltage amplitude of a grid-connected inverter. GFM The scheduling transient stability lift coefficient k is obtained through the formula for calculating the scheduling transient stability lift coefficient. improved .

[0033] In this embodiment, the formula for calculating the AC side voltage amplitude of the grid-type inverter in step 2 is: .

[0034] The formula for calculating the scheduling transient stability boost coefficient is:

[0035] Among them, X g ω0 is the equivalent grid impedance, and ω0 is the grid's rated angular frequency.

[0036] Step 3: Given the initial active power command value P of the grid-connected inverter. refinit The active power command value P of the grid-type inverter is obtained by the scheduling transient stability improvement calculation formula. ref According to the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq The voltage command U of the grid-connected inverter is obtained through the voltage command calculation equation. refGFM .

[0037] In this embodiment, the scheduling transient stability boost calculation formula in step 3 is:

[0038] Among them, J GFM D is the inertia constant of the grid-connected inverter. GFM T1 is the damping constant of the grid-connected inverter, T2 is the first time constant, T2 is the second time constant, and s is the Laplace operator.

[0039] The voltage command calculation equation for the grid-type inverter is as follows:

[0040] Where, k QGFM U0 is the reactive power loop coefficient of the grid-connected inverter, and U0 is the rated AC voltage of the grid-connected inverter.

[0041] Step 4, based on the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq , grid-connected inverter dq axis AC side current i GFMd i GFMq And grid-type inverter voltage command U refGFM The dq-axis current command i of the grid-connected inverter is obtained through the voltage control loop equation of the grid-connected inverter. GFMdref i GFMqref Then, the dq-axis modulation wave e of the grid-type inverter is obtained through the current control loop equation of the grid-type inverter.GFMd ,e GFMq Then, through coordinate transformation, the modulation wave e of the grid-type inverter is obtained. GFMa ,e GFMb ,e GFMc .

[0042] Modulation wave e of grid-type inverter GFMa ,e GFMb ,e GFMc The switching signals of the power devices in the grid-type inverter are generated by SPWM modulation to control the turn-on and turn-off of the power devices in the grid-type inverter.

[0043] In this embodiment, the voltage control loop equation of the grid-type inverter in step 4 is:

[0044]

[0045] Among them, K pVC_GFM K is the proportional control coefficient of the PI regulator in the voltage control loop of a grid-connected inverter. iVC_GFM Z is the integral regulation coefficient of the PI regulator in the voltage control loop of the grid-connected inverter. vir s represents the virtual impedance of the grid-connected inverter, and s is the Laplace operator.

[0046] The current control loop equation for the grid-type inverter is:

[0047]

[0048] Among them, K pCC_GFM K is the proportional control coefficient of the PI regulator in the current control loop of the grid-connected inverter. iCC_GFM This refers to the integral regulation coefficient of the PI regulator in the current control loop of a grid-connected inverter.

[0049] The two coordinate transformation equations in this embodiment are as follows: In step 1, the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq The transformation equation is:

[0050]

[0051]

[0052]

[0053] Grid-type inverter phase θ GFM The calculation equation is as follows:

[0054] Among them, J GFM D is the inertia constant of the grid-connected inverter. GFM P is the damping constant of the grid-connected inverter, ω0 is the rated angular frequency of the grid, and s is the Laplace operator; ref’ This is the active power command value for the grid-connected inverter obtained in the previous cycle; The AC side dq-axis voltage of the grid-connected inverter obtained in the previous cycle is calculated as follows: , In the formula, u GFMd’ ,u GFMq’ i represents the AC side dq-axis voltage of the grid-connected inverter obtained in the previous cycle. GFMd’ i GFMq’ This refers to the dq-axis current on the AC side of the grid-connected inverter obtained in the previous cycle.

[0055] The modulation wave e of the grid-type inverter in step 4 GFMa ,e GFMb ,e GFMc The transformation equation is:

[0056]

[0057] .

[0058] In this embodiment, X g =0.1047Ω, ω0=314.1593, P refinit =3.125MW, T1=0.05, T2=0.02, J GFM =62.8319, D GFM =316.7893, k QGFM =7.8384×10 -6 U0 = 489.9V, K pVC_GFM =5,K iVC_GFM =2000, Z vir =0,K pCC_GFM =0.15, K iCC_GFM =8.

[0059] To demonstrate the beneficial effects of the present invention, MATLAB / Simulink simulations were performed on the invention.

[0060] Figure 3The waveform of the active power of the grid-connected inverter without using the method of this invention is shown. Figure 3 It can be seen that at 50 seconds, the active power command of the grid-connected inverter jumps, and due to the existence of virtual inertia, the power angle loses synchronization, and the system becomes unstable. Figure 4 The image shows the active power waveform of a grid-type inverter using the method of this invention. Figure 4 It can be seen that, after adopting the method of the present invention, the power angle remains stable during the active power step process, and the power response speed is significantly accelerated.

Claims

1. A method for improving the transient stability of active power dispatch of a grid-type inverter in a weak power grid, wherein the power system applying this method includes a grid-type inverter, branch transmission line impedance, grid impedance, and power grid connected in series in sequence; characterized in that, The steps of the lifting method are as follows: Step 1. Sample the AC side voltage u of the grid-connected inverter. GFMa ,u GFMb ,u GFMc And grid-type inverter AC side current i GFMa i GFMb i GFMc The AC side voltage u of the dq axis of the grid-type inverter is obtained through coordinate transformation. GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq ; Step 2, based on the AC side voltage u of the grid-connected inverter GFMa ,u GFMb ,u GFMc The AC side voltage amplitude U is obtained from the formula for calculating the AC side voltage amplitude of a grid-connected inverter. GFM The scheduling transient stability lift coefficient k is obtained through the formula for calculating the scheduling transient stability lift coefficient. improved ; Step 3: Given the initial active power command value P of the grid-connected inverter. refinit The active power command value P of the grid-type inverter is obtained by the scheduling transient stability improvement calculation formula. ref According to the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq The AC side current i of the dq axis of the grid-connected inverter GFMd i GFMq Active power command value P of grid-connected inverter ref The voltage command U of the grid-connected inverter is obtained through the voltage command calculation equation. refGFM ; Step 4, based on the AC side voltage u of the dq axis of the grid-connected inverter GFMd ,u GFMq , grid-connected inverter dq axis AC side current i GFMd i GFMq And grid-type inverter voltage command U refGFM The dq-axis current command i of the grid-connected inverter is obtained through the voltage control loop equation of the grid-connected inverter. GFMdref i GFMqref Then, the dq-axis modulation wave e of the grid-type inverter is obtained through the current control loop equation of the grid-type inverter. GFMd ,e GFMq Then, through coordinate transformation, the modulation wave e of the grid-type inverter is obtained. GFMa ,e GFMb ,e GFMc ; Modulation wave e of grid-type inverter GFMa ,e GFMb ,e GFMc The switching signals of the power devices in the grid-type inverter are generated by SPWM modulation to control the turn-on and turn-off of the power devices in the grid-type inverter.

2. The method for improving the transient stability of active power dispatch of a grid-type inverter under weak power grid conditions according to claim 1, characterized in that, The formula for calculating the AC side voltage amplitude of the grid-connected inverter in step 2 is as follows: 。 3. The method for improving the transient stability of active power dispatch of a grid-type inverter under weak power grid conditions according to claim 1, characterized in that, The formula for calculating the scheduling transient stability boost coefficient in step 2 is: Among them, X g ω0 is the equivalent grid impedance, and ω0 is the grid's rated angular frequency.

4. The method for improving the transient stability of active power dispatch of grid-type inverters under weak power grids according to claim 1, characterized in that, The formula for calculating the scheduling transient stability boost in step 3 is: Among them, J GFM D is the inertia constant of the grid-connected inverter. GFM T1 is the damping constant of the grid-connected inverter, T2 is the first time constant, T2 is the second time constant, and s is the Laplace operator.

5. The method for improving the transient stability of active power dispatch of a grid-type inverter under weak power grid conditions according to claim 1, characterized in that, The voltage command calculation equation for the grid-type inverter in step 3 is as follows: Where, k QGFM U0 is the reactive power loop coefficient of the grid-connected inverter, and U0 is the rated AC voltage of the grid-connected inverter.

6. The method for improving the transient stability of active power dispatch of grid-type inverters under weak power grids according to claim 1, characterized in that, The voltage control loop equation for the grid-connected inverter described in step 4 is as follows: Among them, K pVC_GFM K is the proportional control coefficient of the PI regulator in the voltage control loop of a grid-connected inverter. iVC_GFM Z is the integral regulation coefficient of the PI regulator in the voltage control loop of the grid-connected inverter. vir s represents the virtual impedance of the grid-connected inverter, and s is the Laplace operator.

7. The method for improving the transient stability of active power dispatch of a grid-type inverter under weak power grid conditions according to claim 1, characterized in that, The current control loop equation for the grid-type inverter described in step 4 is as follows: Among them, K pCC_GFM K is the proportional control coefficient of the PI regulator in the current control loop of the grid-connected inverter. iCC_GFM is the integral regulation coefficient of the PI regulator in the current control loop of the grid-connected inverter, and s is the Laplace operator.

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