Inverter stability optimization control method based on PLL link parameter adjustment
By adjusting the PLL parameters and optimizing the inverter control method, the stability problem of the grid-connected inverter was solved, the system's adaptability in weak grid environments was improved, low-frequency oscillations were reduced, and the stability of the power system and equipment safety were ensured.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SUQIAN POWER SUPPLY COMPANY OF JIANGSU PROVINCE POWER
- Filing Date
- 2022-07-29
- Publication Date
- 2026-05-15
AI Technical Summary
After grid-connected inverters are connected to traditional power systems, they suffer from stability problems such as small-disturbance synchronous instability, low-frequency oscillations, and subsynchronous oscillations. Traditional impedance analysis models cannot effectively reveal the dynamic coupling between different frequencies in the phase sequence and the strong coupling negative damping characteristics between the PLL and the current loop.
By adopting an inverter stability control method based on PLL link parameter adjustment, a synchronous dynamic mathematical expression for the phase-locked loop and its coupled interference is established, the dynamic control equation of the inverter is determined, and the loop characteristics are changed by adding a series transfer function H(s) to the PLL, thereby optimizing the PLL control parameter design.
It enhances the inverter's adaptability to weak grid environments, reduces losses caused by low-frequency oscillations, and ensures the stability of the power system and the safety of electrical equipment.
Smart Images
Figure CN115173487B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of inverter stability optimization control technology, specifically relating to an inverter stability optimization control method based on PLL link parameter adjustment. Background Technology
[0002] Grid-connected inverters are key interface devices between renewable energy devices and the power grid. With the rapid development of renewable energy power generation technologies and the widespread application of power electronic equipment, large-scale and diverse grid-connected inverters are being connected to traditional power systems, resulting in a clear trend towards power electronics in the power system. Unlike traditional power generation equipment, inverter devices have low inertia and fast response speeds, leading to complex stability issues when connected to traditional power systems.
[0003] Traditional impedance analysis models cannot clearly reveal the dynamic coupling between different frequencies in the phase sequence. The negative damping characteristics brought about by the strong coupling between the PLL and the current loop can cause small disturbance synchronous instability in the grid-connected system. The influence of different parameter designs and control loops may also bring about oscillations in a wide frequency range, such as low-frequency oscillations and subsynchronous oscillations, which threaten the stable operation of the system. Summary of the Invention
[0004] To address the aforementioned problems, this invention proposes an inverter stability optimization control method based on PLL link parameter adjustment.
[0005] To achieve the above-mentioned technical objectives and effects, the present invention provides the following technical solution: an inverter stability control optimization method based on PLL link parameter adjustment, comprising the following steps:
[0006] 1) Introduce inverters, grid impedance networks, etc. into the phase-locked loop in the form of coupled interference with port characteristics, and establish a synchronous dynamic mathematical expression for the phase-locked loop and its coupled interference.
[0007] 2) Determine the dynamic control equations of the inverter based on the coupling relationship of the PLL in the system;
[0008] 3) Based on the PLL dominant loop, the system stability is determined by the phase margin at the crossover frequency of the equation;
[0009] 4) By adding a series transfer function H(s) to the PLL to change the loop characteristics of the PLL, parameter design is carried out for the control optimization method of the PLL under different control modes of the inverter.
[0010] In step 1), the PLL is first modeled as the main loop, then the other control loops are introduced, and finally the inverter, grid impedance network, etc. are introduced into the phase-locked loop in the form of coupling interference with port characteristics.
[0011] The synchronous dynamic mathematical expression of the phase-locked loop and its coupling interference is obtained by frequency domain modeling in the dq coordinate system using complex transfer functions and space vectors, resulting in the dynamic equation of a single-machine grid-connected system.
[0012]
[0013]
[0014]
[0015] Where L F and C F These are the inverter-side inductors and capacitors of the filter, L g It is the grid-side inductance (grid-side inductance and line inductance are combined for simplified analysis), R g It is the grid-side resistance. It is the inverter voltage vector. It is the capacitor voltage vector. It is the grid-side current vector. ω is the inverter-side current vector, and ω is the rotational angular frequency of the control system's rotating coordinate system. It is the mains voltage vector (U) of the dq axis controller. * It represents the grid voltage amplitude, and δ is the d-axis of the controller's dq coordinate system intersecting with U. * (The included angle between them).
[0016] In step 2), the inverter dynamic control equations that reflect the PLL coupling relationship are divided into the control of the output voltage of the inner current loop and the reactive power control of the inverter output.
[0017] The inner current loop determines the inverter's output voltage, and its dynamic equation is:
[0018]
[0019] In the formula, f C-PI (s)=K C-P +K C-I / s is the transfer function of the PI regulator in the current control loop. It is the current reference vector provided by the power control loop, f UF (s)=K UF / (T UF s+1) represents the first-order filter transfer function in voltage feedforward.
[0020] The reactive power control of the inverter includes three different reactive power control methods: PQ control, PV control, and droop control. These three control methods are implemented using a unified reactive power control equation, as follows:
[0021] I qr =fQ-PI (s)×[k Q (Q o -Q r )-k QU (U co -U cd )] (16)
[0023] In the formula, f Q-PI (s)=K Q-P +K Q-I / s is the transfer function of the PI regulator in the reactive power control loop, k Q It is the reactive power coefficient, k QU It is the droop coefficient, Q o It is reactive power, Q r It is the reactive power reference value, U cd It is the amplitude of the capacitor voltage, U co This is a reference value for the capacitor voltage amplitude.
[0024] When setting k Q =1 and k QU When k = 0, this control loop is equivalent to constant reactive power control in PQ control; when k is set... Q =0 and k QU When k ≠ 0, this control loop is equivalent to constant AC voltage control in PV control; when k is set... Q =1 and k QU When the value is not equal to 0, the reactive power control loop can realize reactive-voltage control based on droop control.
[0025] The criterion for system stability in step 3) is as follows:
[0026] If and only if 1 / f PLL (s) equation and f δ (s) equation in 1 / f PLL (s) and f δ When the amplitudes of L(s) are equal, that is, when the phase difference at the gain crossover frequency of the open-loop transfer function L(s) of the system is less than 180°, the inverter system is stable; otherwise, it is unstable.
[0027] Where f PLL (s) is based on the PLL transfer function equation, f δ L(s) is the voltage output equation formed after angle input, where L(s) = f δ (s)×f PLL (s).
[0028] In step 4), the dynamic equation of the PLL after adding a series transfer function H(s) to the PLL is:
[0029]
[0030] In the formula, f L-PI (s)=K L-P +K L-I / s is the transfer function of the PI controller in the PLL, θ is the output phase of the PLL, and ω is the angular frequency of the PLL.
[0031] In step 4), when the inverter uses PQ control, a lead-lag element is added to change 1 / f. PLL Given the phase characteristics of equation (s), H(s) is as follows:
[0032]
[0033] In the above formula, T F and T B These are the lead time constant and the lag time constant, respectively.
[0034] The maximum phase lag that can be provided is as follows:
[0035]
[0036] At the same time, the phase lag frequency point at this time can be obtained as:
[0037]
[0038] When the inverter uses PV control or droop control, the control optimization method for the PLL is as follows:
[0039] Adding a second-order notch filter increases 1 / f PLL The amplitude of equation (s) at the resonant frequency is given by H(s) as follows:
[0040]
[0041] In the formula, ε1 and ε2 are two damping ratio constants, and ω L This is the notch filter frequency, used to increase 1 / f PLL The amplitude of equation (s) at the resonant frequency, let ω L with f δ If the resonant frequencies of the (s) functions are equal, then the amplitude of 1 / H(s) at that frequency can be calculated as follows:
[0042] |1 / H(jω L )|=ε2 / ε1 (twenty two)
[0044] This invention enhances the adaptability of the grid-connected system to changes in a weak grid environment, and is of great significance for large-scale access of new energy sources, reducing losses caused by low-frequency oscillations of inverters, ensuring the stability of the power system, and protecting the safety of electrical equipment. Attached Figure Description
[0045] Figure 1 This is a flowchart of an inverter stability control optimization method based on PLL link parameter adjustment.
[0046] Figure 2 This is a block diagram of a grid-connected inverter based on a PLL.
[0047] Figure 3 This is the PLL synchronous master circuit diagram for the inverter.
[0048] Figure 4 This is a diagram of the PLL structure. Detailed Implementation
[0049] The invention will now be further described with reference to the accompanying drawings.
[0050] This invention proposes an inverter stability control optimization method based on PLL link parameter adjustment, the overall process of which is as follows: Figure 1 As shown, it includes the following steps:
[0051] 1) Introduce inverters, grid impedance networks, etc. into the phase-locked loop in the form of coupled interference with port characteristics, and establish a synchronous dynamic mathematical expression for the phase-locked loop and its coupled interference.
[0052] 2) Determine the dynamic control equations of the inverter based on the coupling relationship of the PLL in the system;
[0053] 3) Based on the PLL dominant loop, the system stability is determined by the phase margin at the crossover frequency of the equation;
[0054] 4) By adding a series transfer function H(s) to the PLL to change the loop characteristics of the PLL, parameter design is carried out for the control optimization method of the PLL under different control modes of the inverter system.
[0055] Specifically, in step 1), the PLL is first modeled as the main loop, then the other control loops are introduced, and finally the inverter, grid impedance network, etc. are introduced into the phase-locked loop in the form of coupling interference with port characteristics.
[0056] The synchronous dynamic mathematical expression of the phase-locked loop and its coupling interference is obtained by frequency domain modeling in the dq coordinate system using complex transfer functions and space vectors, resulting in the dynamic equation of a single-machine grid-connected system.
[0057]
[0058]
[0059]
[0060] Where L F and C F These are the inverter-side inductors and capacitors of the filter, L g It is the grid-side inductance (grid-side inductance and line inductance are combined for simplified analysis), R g It is the grid-side resistance. It is the inverter voltage vector. It is the capacitor voltage vector. It is the grid-side current vector. ω is the inverter-side current vector, and ω is the rotational angular frequency of the control system's rotating coordinate system. It is the mains voltage vector (U) of the dq axis controller. * It represents the grid voltage amplitude, and δ is the d-axis of the controller's dq coordinate system intersecting with U. * (The included angle between them).
[0061] In step 2), the inverter dynamic control equations that reflect the PLL coupling relationship are divided into the control of the output voltage of the inner current loop and the reactive power control of the inverter output.
[0062] The inner current loop determines the inverter's output voltage, and its dynamic equation is:
[0063]
[0064] In the formula, f C-PI (s)=K C-P +K C-I / s is the transfer function of the PI regulator in the current control loop. It is the current reference vector provided by the power control loop, f UF (s)=K UF / (T UF s+1) represents the first-order filter transfer function in voltage feedforward.
[0065] The active power control loop that determines the active current reference value is as follows:
[0066] I dr =f P-PI (s)×(P r -P o (5)
[0067] In the formula, f P-PI (s)=K P-P +K P-I / s is the transfer function of the active-loop PI regulator, P r It is the active power reference value, P o The output active power of the inverter can be expressed by equation (6):
[0068] P o =U cd Id +U cq I q (6)
[0069] The reactive power control of the inverter includes three different reactive power control methods: PQ control, PV control, and droop control. These three control methods are implemented using a unified reactive power control equation, as follows:
[0070] I qr =f Q-PI (s)×[k Q (Q o -Q r )-k QU (U co -U cd (7)
[0071] In the formula, f Q-PI (s)=K Q-P +K Q-I / s is the transfer function of the PI regulator in the reactive power control loop, k Q It is the reactive power coefficient, k QU It is the droop coefficient, Q o It is reactive power, Q r It is the reactive power reference value, U cd It is the amplitude of the capacitor voltage, U co This is a reference value for the capacitor voltage amplitude.
[0072] When setting k Q =1 and k QU When k = 0, this control loop is equivalent to constant reactive power control in PQ control; when k is set... Q =0 and k QU When k ≠ 0, this control loop is equivalent to constant AC voltage control in PV control; when k is set... Q =1 and k QU When the value is not equal to 0, the reactive power control loop can realize reactive-voltage control based on droop control.
[0073] Combining equations (1) and (4), we can obtain:
[0074]
[0075] in
[0076]
[0077] The criterion for system stability in step 3) is as follows:
[0078] If and only if 1 / f PLL (s) equation and f δ (s) equation in 1 / fPLL (s) and f δ When the amplitudes of L(s) are equal, that is, when the phase difference at the gain crossover frequency of the open-loop transfer function L(s) of the system is less than 180°, the inverter system is stable; otherwise, it is unstable.
[0079] Where f PLL (s) is based on the PLL transfer function equation, f δ L(s) is the voltage output equation formed after angle input, where L(s) = f δ (s)×f PLL (s).
[0080] By dynamically partitioning the system into 1 / f PLL (s) equation and f δ The two parts of the equation (s) allow for a more intuitive analysis of the interaction between the rest of the inverter system and the PLL, and further analysis of the instability mechanism and improvements to the PLL.
[0081] In step 4), the dynamic equation of the PLL after adding a series transfer function H(s) to the PLL is:
[0082]
[0083] In the formula, f L-PI (s)=K L-P +K L-I / s is the transfer function of the PI controller in the PLL, θ is the output phase of the PLL, and ω is the angular frequency of the PLL. 1 / f PLL The properties of the (s) equation are affected by the addition of this transfer function.
[0084] In step 4), when the inverter uses PQ control, a lead-lag element is added to change 1 / f. PLL Given the phase characteristics of equation (s), H(s) is as follows:
[0085]
[0086] In the above formula, T F and T B These are the lead time constant and the lag time constant, respectively.
[0087] The maximum phase lag that can be provided is as follows:
[0088]
[0089] At the same time, the phase lag frequency point at this time can be obtained as:
[0090]
[0091] When designing a PLL, you can choose to obtain f through modeling analysis. B The gain crossover frequency, or the oscillation frequency obtained by performing an FFT on the actual waveform, is used as the gain crossover frequency. The expected phase adjustment value is then...
[0092] When the inverter uses PV control or droop control, the control optimization method for the PLL is as follows:
[0093] Adding a second-order notch filter increases 1 / f PLL The amplitude of equation (s) at the resonant frequency is given by H(s) as follows:
[0094]
[0095] In the formula, ε1 and ε2 are two damping ratio constants, and ω L This is the notch filter frequency, used to increase 1 / f PLL The amplitude of equation (s) at the resonant frequency, let ω L with f δ If the resonant frequencies of the (s) functions are equal, then the amplitude of 1 / H(s) at that frequency can be calculated as follows:
[0096] |1 / H(jω L )|=ε2 / ε1 (14)
[0098] Where 1 / H(s) is at ω L The amplitude at ω is determined by ε2 / ε1, while 1 / H(s) at ω L The amplitude in the vicinity is influenced by the magnitude of ε2. 1 / H(s) at ω L The amplitude in the vicinity increases with increasing ε2, so increasing the value of ε2 can improve the robustness of the system's resonant frequency shift. However, the negative impact is due to the increase of 1 / f in ε2. PLL (s) equation in ω L The phase at frequencies below a certain frequency will also increase, reducing the overall phase margin of the system. Therefore, parameter selection needs to consider both the system's phase margin and robustness.
[0099] This invention enhances the adaptability of the grid-connected system to changes in a weak grid environment, and is of great significance for large-scale access of new energy sources, reducing losses caused by low-frequency oscillations of inverters, ensuring the stability of the power system, and protecting the safety of electrical equipment.
[0100] Simulation verification
[0101] To verify the reliability and effectiveness of this invention, a simulation model was built in MATLAB / Simulink to analyze the inverter stability control optimization method based on PLL link parameter adjustment. Tables 1 and 2 show the simulation results of the inverter operating state under different control modes.
[0102] Table 1 Inverter operating status under PQ control mode
[0103]
[0104] Table 2 Inverter operating status under PV (droop) control mode
[0105]
[0106] In Table 1, the time constant for inverter voltage feedforward is taken as one time step T. UF =0.01s and PQ control is used. The time constant of the inverter voltage feedforward in Table 2 is taken as one beat T. UF =0.0015s and the inverter uses PV control or droop control. Table 1 shows that when H(s) = 1, i.e., without PLL optimized control, the single-unit grid-connected system is unstable. However, after adding a lead-lag element to improve (T... F =0.02s, T B =0.004s) The system reached a steady state with a fast response speed; Table 2 shows that after the PLL control loop was optimized by adding a second-order notch filter (ε1=0.005, ε2=2.5), the system could also be brought from an unstable state to a steady state, which proves that adding the H(s) optimized control link to the PLL dominant control loop can effectively improve the system's synchronous stability.
[0107] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.
Claims
1. An inverter stability control optimization method based on PLL link parameter adjustment, characterized in that, Includes the following steps: 1) Introduce the inverter and grid impedance network into the phase-locked loop in the form of coupled interference with port characteristics, and establish a synchronous dynamic mathematical expression for the phase-locked loop and its coupled interference; 2) Determine the dynamic control equations of the inverter based on the coupling relationship of the PLL in the system; 3) Based on the PLL dominant loop, the system stability is determined by the phase margin at the crossover frequency of the equation; 4) By adding a series transfer function H(s) to the PLL to change the loop characteristics of the PLL, parameter design is carried out for the control optimization method of the PLL under different control modes of the inverter. The reactive power control of the inverter includes three different types: PQ control, PV control, or droop control. When the inverter uses PQ control... ， T F and T B These are the lead time constant and the lag time constant, respectively; When the inverter uses PV control or droop control ， In the formula, and Let be two damping ratio constants. This is the notch filter frequency.
2. The inverter stability control optimization method based on PLL link parameter adjustment according to claim 1, characterized in that: In step 1), the PLL is first modeled as the main loop, then the other control loops are introduced, and finally the inverter and grid impedance network are introduced into the phase-locked loop in the form of coupling interference of port characteristics. The synchronous dynamic mathematical expression of the phase-locked loop and its coupling interference is obtained by frequency domain modeling in the dq coordinate system using complex transfer functions and space vectors, resulting in the dynamic equation of a single-machine grid-connected system. ， ， ， Where L F and C F These are the inverter-side inductors and capacitors of the filter, L g It is a grid-side inductor. It is the grid-side resistance. It is the inverter voltage vector. It is the capacitor voltage vector. It is the grid-side current vector. ω is the inverter-side current vector, and ω is the rotational angular frequency of the control system's rotating coordinate system. It is the mains voltage vector of the dq axis controller. It represents the grid voltage amplitude, and δ represents the d-axis of the controller's dq coordinate system. The angle between them.
3. The inverter stability control optimization method based on PLL link parameter adjustment according to claim 1, characterized in that, Step 2) describes the inverter dynamic control equations that reflect the PLL coupling relationship. These equations are divided into the control of the inner current loop output voltage and the control of the reactive power output of the inverter. The inner current loop determines the inverter's output voltage, and its dynamic equation is: ， In the formula, It is the transfer function of the PI regulator in the current control loop. It is the current reference vector provided by the power control loop. This represents the transfer function of a first-order filter in a voltage feedforward circuit. The reactive power control method of the inverter is implemented using a unified reactive power control equation, as follows: ， In the formula, =K Q-P +K Q-I / s is the transfer function of the PI regulator in the reactive power control loop. It is the reactive power coefficient. It is the droop coefficient. It is reactive power. This is a reactive power reference value. It is the amplitude of the capacitor voltage. This is a reference value for the capacitor voltage amplitude; When setting =1 and When =0, this control loop is equivalent to constant reactive power control in PQ control; when set =0 and When the value is 0, this control loop is equivalent to constant AC voltage control in PV control; when set... =1 and When the value is 0, the reactive power control loop can realize reactive-voltage control based on droop control.
4. The inverter stability control optimization method based on PLL link parameter adjustment according to claim 1, characterized in that, The criterion for system stability in step 3) is as follows: If and only if 1 / f PLL (s) equation and f δ (s) equation in 1 / f PLL (s) and f δ (s) Where the amplitudes are equal, that is, at the gain crossover frequency of the open-loop transfer function L(s) of the system, the phase difference is less than 180°. ◦ When the inverter system is stable, it is stable; otherwise, it is unstable. in It is based on the transfer function equation of PLL. The voltage output equation formed after angle input. .
5. The inverter stability control optimization method based on PLL link parameter adjustment according to claim 2, characterized in that, In step 4), the dynamic equation of the PLL after adding a series transfer function H(s) to the PLL is: ， In the formula, This is the transfer function of the PI controller in the PLL, where θ is the output phase of the PLL, ω is the angular frequency of the PLL, and U... q ( s ) is the q-axis voltage s Domain representation.
6. The inverter stability control optimization method based on PLL link parameter adjustment according to claim 3, characterized in that, In step 4), when the inverter uses PQ control, a lead-lag element is added to change 1 / f. PLL The phase characteristics of the (s) equation, from which the maximum phase lag can be provided are as follows: ， At the same time, the phase lag frequency point at this time can be obtained as: ， When the inverter uses PV control or droop control, the control optimization method for the PLL is as follows: Adding a second-order notch filter increases 1 / f PLL The amplitude of equation (s) at the resonant frequency, in order to increase 1 / f PLL Let the amplitude of equation (s) at the resonant frequency be... with f δ If the resonant frequencies of the (s) functions are equal, then the amplitude of 1 / H(s) at that frequency can be calculated as follows: 。