Virtual inertia control device and method for grid-connected converter based on frequency-locked loop

Abstract

The invention relates to the technical field of converter inertia control, in particular to a grid-connected converter virtual inertia control device and method based on a frequency-locked loop, and the method comprises the steps: introducing a three-phase frequency-locked loop and a frequency controller into a virtual inertia control module, and obtaining an active current reference value of a d-q axis and a reactive current reference value of the d-q axis according to the frequency change of a power grid; an improved MPC algorithm is introduced into a current loop, a future current track is predicted according to an active current reference value of a d-q axis and a reactive current reference value of the d-q axis, and a voltage control signal is generated; and the PWM module converts the voltage control signal into a switch control signal through pulse width modulation and outputs the switch control signal to the grid-connected converter. According to the invention, the induction power grid frequency change and the power reference signal are organically combined, the problem of frequency instability of a traditional phase-locked loop structure under a weak power grid condition is solved, and the problem that a single-phase frequency locker has a large number of ripples and generates negative effects on inertial response of a system is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of converter inertia control, in particular to a virtual inertia control device and method for a grid-connected converter based on a frequency-locked loop. Background Art

[0002] As global energy problems become increasingly serious, distributed generation technology has received widespread attention from all countries. New energy sources with renewable energy as the core are pollution-free, widely distributed and resource-rich, with many advantages and broad market prospects. However, an urgent issue that needs attention is that renewable energy is usually connected to the power system through power electronic converters, which does not provide any power system inertia. As the penetration rate of renewable energy continues to increase, the inertia generated by traditional synchronous generators has been significantly reduced. This will bring two challenges. The first is the increasing rate of change of frequency (Rate of Change of Frequency, RoCoF), which makes the generator set prone to protective tripping; the other is the large frequency deviation during severe frequency events. Excessive frequency deviation may lead to undesirable load shedding, cascading failures, and even large-scale power outages.

[0003] In order to solve the problem of insufficient inertia, a direct solution is to use the energy stored in the capacitor for inertia simulation. The capacitor voltage will change with the grid frequency to generate the required inertia. This method can avoid detecting the frequency derivative. However, it is only applicable to power converters with capacitors on the DC link and adjustable DC side voltage. Virtual Synchronous Generator (VSG) technology provides frequency and voltage support for microgrids by simulating the rotor inertia and droop characteristics of synchronous generators. It is a control method widely used in distributed power grid connection. VSG can not only compensate for the equivalent inertia of the system, but also reduce the frequency regulation pressure of microgrids. However, VSG does not increase the inertia of the power system to a large extent, and the embedded virtual inertia may introduce low-frequency active oscillations, which in turn affects the stability of the system and increases the risk of damage to the power electronic converter. Another way of inertia simulation is generalized droop control, which implements inertia by adding a low-pass filter in the active control loop. Similarly, it still has other stability problems under different types of interference.

[0004] In order to maintain or improve the frequency stability of the entire system, the control of the power electronic converter must be modified to adapt to the inertia of the system. In the prior art, the inertia control method adopted is to adopt a virtual inertia control strategy that detects frequency differentials, that is, a virtual inertia control strategy based on phase-locked loop (PLL) control is proposed to achieve system inertia enhancement. However, the differential operator will inevitably introduce high-frequency noise. Adding a first-order low-pass filter to alleviate it will affect the response speed of the virtual inertia. Later, a method based on a second-order generalized integrator single-phase frequency-locked loop (SOGI-FLL) to detect frequency differentials and adjust the active reference value to achieve virtual inertia enhancement was proposed. This method does not require frequency differential calculations, but there are a lot of ripples and have a negative impact on the inertial response of the system. Summary of the invention

[0005] The present invention provides a virtual inertia control device and method for a grid-connected converter based on a frequency-locked loop, which overcomes the shortcomings of the above-mentioned prior art and can effectively solve the problem that the existing method based on a second-order generalized integrator single-phase frequency-locked loop detects frequency differential and adjusts the active reference value to achieve virtual inertia control has a large amount of ripples and has a negative impact on the inertial response of the system.

[0006] One of the technical solutions of the present invention is achieved by the following measures: a virtual inertia control device for a grid-connected converter based on a frequency-locked loop, comprising a virtual inertia control module, a current loop, and a PWM module;

[0007] A three-phase frequency-locked loop and a frequency controller are introduced into the virtual inertia control module to track the grid frequency, and the active current reference value of the dq axis and the reactive current reference value of the dq axis are obtained according to the frequency change of the grid;

[0008] An improved MPC algorithm is introduced into the current loop to predict the future current trajectory and generate a voltage control signal based on the active current reference value of the dq axis and the reactive current reference value of the dq axis.

[0009] The PWM module converts the voltage control signal into a switch control signal through pulse width modulation and outputs it to the grid-connected converter.

[0010] The following are further optimizations and / or improvements to the above technical solutions:

[0011] The virtual inertial control module includes:

[0012] A three-phase frequency-locked loop, obtained by expanding a single-phase frequency-locked loop based on a generalized second-order integrator, is used to track the grid frequency and generate phase angle and grid frequency change signals;

[0013] The frequency controller adjusts the frequency according to the frequency change signal of the power grid to obtain the corresponding power deviation;

[0014] A power calculation module includes a power calculation module, an active loop and a reactive loop. The power calculation module obtains active power and reactive power based on the grid output voltage and the grid output current. The active loop adjusts the active power based on the power deviation to obtain an active power reference value. The reactive loop obtains a reactive power reference value based on the reactive power.

[0015] The current reference calculation module obtains the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the active power reference value and the reactive power reference value.

[0016] The input-output transfer function of the above three-phase frequency-locked loop is as follows:

[0017]

[0018]

[0019]

[0020]

[0021] in, is the phase angle of the frequency-locked loop; ω g is the angular frequency of the power grid; ω is the angular frequency of the frequency-locked loop; V ga 、V gβ is the voltage amplitude of the three-phase electricity in the power grid.

[0022] The input signal of the generalized second-order integrator in the above three-phase frequency-locked loop is as follows:

[0023]

[0024] in, is the phase angle of the frequency-locked loop; ω g is the angular frequency of the power grid; ω is the angular frequency of the frequency-locked loop; V ga 、V gβ is the voltage amplitude of the three-phase electricity in the power grid.

[0025] The above frequency controller adjusts the frequency according to the frequency change signal of the power grid, and the corresponding power deviation is as follows:

[0026]

[0027]

[0028] Where, H is the inertia coefficient; is the differential value of the frequency change signal output by the frequency-locked loop; f ref is the frequency reference value.

[0029] The second technical solution of the present invention is achieved by the following measures: A virtual inertia enhancement control method for a grid-connected converter based on a frequency-locked loop, comprising:

[0030] The virtual inertia control module is used to track the grid frequency, analyze the frequency change of the grid, and obtain the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the frequency change of the grid;

[0031] The active current reference value of the dq axis and the reactive current reference value of the dq axis are input into the current loop, the future current trajectory is predicted based on the improved MPC algorithm, the active current reference value of the dq axis and the reactive current reference value of the dq axis are optimized, and the voltage control signal is generated;

[0032] The voltage control signal is pulse-width modulated by the PWM module to obtain a corresponding switch control signal, which is then output to the grid-connected converter to drive the grid-connected converter to output a corresponding voltage waveform.

[0033] The following are further optimizations and / or improvements to the above technical solutions:

[0034] The above-mentioned virtual inertia control module is used to track the grid frequency, analyze the frequency change of the grid, and obtain the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the frequency change of the grid, including:

[0035] The three-phase frequency-locked loop is used to track the grid frequency and generate phase angle and grid frequency change signals;

[0036] The frequency controller introduces the inertia coefficient and adjusts the frequency reference value according to the frequency change signal of the power grid to obtain the corresponding power deviation;

[0037]

[0038]

[0039] Where, H is the inertia coefficient; is the differential value of the frequency change signal output by the three-phase frequency-locked loop; f ref is the frequency reference value;

[0040] The power calculation module obtains active power and reactive power based on the grid output voltage and the grid output current;

[0041] The active loop obtains the power deviation and active power, adjusts the active power using the power deviation, and obtains the active power reference value;

[0042] The reactive power loop obtains a reactive power reference value based on the reactive power;

[0043] The current reference calculation module obtains the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the active power reference value and the reactive power reference value.

[0044] The present invention introduces a three-phase frequency-locked loop and a current loop based on an improved MPC. By organically combining frequency tracking, induction of grid frequency changes and power reference signals, the frequency instability problem of the traditional phase-locked loop structure under weak grid conditions is solved. At the same time, the problem that a large amount of ripple exists in a single-phase frequency locker and has a negative impact on the inertial response of the system is solved, providing support for ensuring the stable operation of the grid. The present invention also has robustness and adaptability during dynamic frequency changes. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Attached Figure 1 Schematic diagram of a power control loop with a frequency controller in the present invention.

[0046] Attached Figure 2 It is a schematic diagram of the structure of a single-phase phase-locked loop in the present invention.

[0047] Attached Figure 3 It is a schematic diagram of the structure of a single-phase frequency-locked loop in the present invention.

[0048] Attached Figure 4 It is a structural schematic diagram of the second-order generalized integrator in the present invention.

[0049] Attached Figure 5 Schematic diagram of the zero-pole distribution of the single-phase frequency-locked loop in the present invention.

[0050] Attached Figure 6 It is a schematic diagram of the device structure of the present invention.

[0051] Attached Figure 7 It is a schematic diagram of the structure of the three-phase frequency-locked loop in the present invention.

[0052] Attached Figure 8 It is a schematic diagram for comparing the dynamic responses of the frequency derivatives of the two controllers in the present invention.

[0053] Attached Fig. 9 It is a structural schematic diagram corresponding to the calculation of the active current reference value in the present invention.

[0054] Attached Fig.10 It is a schematic diagram comparing the control effects of MPC and PI in the present invention.

[0055] Attached Fig.11 It is a schematic diagram of the method flow of the present invention.

[0056] Attached Fig.12 It is the frequency response simulation waveform diagram of the present invention.

[0057] Attached Fig.13This is a power tracking response simulation diagram of the present invention.

[0058] Attached Fig.14 It is a frequency derivative response simulation diagram of the present invention. DETAILED DESCRIPTION

[0059] The present invention is not limited by the following embodiments, and specific implementation methods can be determined based on the technical solution of the present invention and actual conditions.

[0060] As global energy and environmental issues are increasingly valued, the application of new energy in power systems has received more and more attention from various countries. New energy with renewable energy as the core is pollution-free, widely distributed and rich in resources, and has many advantages and broad market prospects. Renewable energy is usually connected to the power system through power electronic converters. Compared with traditional synchronous power electronic converters, the inertia provided is lower. As the penetration rate of renewable energy continues to increase, the overall inertia of the power system is significantly reduced. This will lead to a decrease in the anti-disturbance ability of the power system, poor frequency stability of the system, and a shorter response time for the system, which is easy to cause generator cascade tripping and low-frequency load reduction. In severe cases, frequency collapse or even large-scale regional power outages may occur.

[0061] In order to improve the frequency stability of the grid-connected converter system, the control strategy of the grid-connected converter must be modified to adapt to the inertia of the power system.

[0062] For grid-connected converter systems, traditional control methods have mostly used dual closed-loop control. The voltage or current on the DC side is controlled by controlling the output current on the AC side. The dual closed-loop control usually uses the inductor current on the AC side as the inner control loop, and tracks the current command given by the outer control loop to improve the dynamic and steady-state performance of the system; the outer loop is mainly determined according to the control target.

[0063] Compared with the DC side control strategy that takes DC voltage and current as the control target, the AC side control strategy focuses more on the power and frequency received from the grid side. Its structure is similar to PQ control. Its control strategy is based on power control and adds multiple control strategies such as frequency controller. Figure 1 shown.

[0064] The frequency controller has similar functions to the frequency regulator of the traditional synchronous generator in providing frequency regulation. Its characteristics can quickly respond to frequency changes, achieve better frequency tracking, and handle rapidly changing power. In addition, with appropriate control parameters, the frequency controller can optimize the system frequency and effectively manage and regulate the stable system power.

[0065] Among them, a phase-locked loop (PLL) is provided. In order to ensure the synchronization between the output and the power grid, the grid-connected converter often adopts the phase-locked loop synchronization technology. The phase-locked loop technology can achieve accurate phase and frequency matching between the converter and the power grid, allowing the converter to efficiently and safely transmit electrical energy to the power grid. However, the control strategy based on the phase-locked loop has certain limitations in the problem of frequency oscillation, especially in weak grid conditions. This oscillation is mainly caused by the coupling effect between the dynamic response of the phase-locked loop and the grid frequency disturbance. When the grid short-circuit ratio (SCR) is low, the phase-locked loop has a lag in tracking the grid voltage phase, resulting in unstable output frequency response of the converter. In addition, when dealing with frequency disturbances, the traditional phase-locked loop structure lacks the ability to respond quickly to frequency changes due to the limitations of its frequency locking mechanism, which further aggravates the frequency oscillation. This phenomenon may significantly affect the stability and power quality of the system in the power system. The structure of the above-mentioned phase-locked loop is shown in the attached figure. Figure 2 shown.

[0066] Therefore, a single-phase frequency-locked loop can be introduced in the grid-connected system. The single-phase frequency-locked loop and the phase-locked loop (PLL) have their own advantages in synchronous control. The single-phase frequency-locked loop focuses on frequency tracking and can adapt to frequency changes more quickly, especially showing higher robustness in dynamic environments and low signal-to-noise ratios. In contrast, the phase-locked loop focuses more on phase matching, but is susceptible to frequency disturbances and noise environments. The design of the frequency-locked loop simplifies the system and reduces costs. It has more advantages in frequency change response, noise resistance and system simplification, and is suitable for application scenarios with fast startup and frequent frequency changes.

[0067] The frequency-locked loop is usually implemented by a second-order generalized integrator (SOGI). The role of the second-order generalized integrator is particularly reflected in its strong adaptability to grid frequency and phase changes. It can effectively suppress the influence of harmonics and noise in the grid, thereby generating an accurate orthogonal output signal. The structure of the single-phase frequency-locked loop is shown in the attached figure. Figure 3 As shown in the attached figure, the second-order generalized integrator Figure 4 shown.

[0068] Based on the SOGI framework, the input-output transfer function of the single-phase frequency-locked loop can be expressed as:

[0069]

[0070]

[0071] Since the angular frequency of the frequency-locked loop is close to but not equal to the angular frequency of the power grid, the amplitude and phase angle of the frequency-locked loop output transfer function can be obtained respectively:

[0072]

[0073] Therefore, the output of the frequency locked loop can be expressed as:

[0074]

[0075]

[0076] Furthermore, the input signal of the integrator can be obtained:

[0077]

[0078] Substituting it into the integrator, the closed-loop transfer function of the angular frequency is:

[0079]

[0080] From the attached Figure 5 It can be seen that the closed-loop system has only one pole in the left half plane.

[0081] Therefore, under this design, it is easy to Figure 5 The frequency derivative is obtained, and the stability of the system is also guaranteed.

[0082] Although the above-mentioned single-phase frequency-locked loop focuses on frequency tracking and can adapt to frequency changes more quickly, it omits the ripple term, which will have a negative impact on the inertial response of the system. Therefore, the present invention expands the single-phase frequency-locked loop into a three-phase frequency-locked loop, which is not only required by the three-phase converter itself, but also eliminates the ripple term existing in the single-phase frequency-locked loop.

[0083] The present invention is further described in detail with reference to the embodiments and the accompanying drawings:

[0084] Embodiment 1: As attached Figure 6 As shown, an embodiment of the present invention discloses a virtual inertia control device for a grid-connected converter based on a frequency-locked loop, comprising a virtual inertia control module, a current loop, and a PWM module;

[0085] A three-phase frequency-locked loop and a frequency controller are introduced into the virtual inertia control module to track the grid frequency, and the active current reference value of the dq axis and the reactive current reference value of the dq axis are obtained according to the frequency change of the grid;

[0086] In this embodiment, the virtual inertia control module includes:

[0087] The three-phase frequency-locked loop is obtained by expanding the single-phase frequency-locked loop based on the generalized second-order integrator, and is used to track the grid frequency and generate the phase angle θ and the frequency change signal f of the grid;

[0088] The above three-phase frequency locked loop is as shown in the attached Figure 7 As shown, the three-phase frequency-locked loop is obtained by expanding the single-phase frequency-locked loop based on the generalized second-order integrator;

[0089] In a three-phase converter, the transformation matrix from the three-phase coordinate system to the stationary coordinate system is as follows:

[0090]

[0091] The three-phase voltage can be expressed as:

[0092]

[0093] Further get the voltage in the stationary coordinate system:

[0094]

[0095] Its transfer function can be obtained:

[0096]

[0097]

[0098]

[0099]

[0100] Approximately, the integrator input signal can be derived as:

[0101]

[0102] In the controllers based on PLL and FLL, the simulation of RoCoF stepping from 0 Hz / s to 0.2 Hz / s is carried out respectively. Figure 8 The dynamic responses of the frequency derivatives of the two controllers are given. In order to compare the differences between the two controllers, the RoCoF response is plotted, where the blue line is the frequency derivative waveform change diagram of the phase-locked loop, the red line is the frequency derivative waveform change diagram of the frequency-locked loop, and the specific area in the waveform on the right is magnified.

[0103] As can be seen from the figure, the phase-locked loop has high-frequency oscillations during the change, while the frequency-locked loop presents the correct change results, which shows the feasibility of using a three-phase frequency-locked loop.

[0104] The frequency controller adjusts the frequency according to the frequency change signal of the power grid to obtain the corresponding power deviation ΔP m ;

[0105] As attached Fig. 9 As shown, the inertia coefficient is introduced into the frequency controller, and the frequency controller obtains the corresponding power deviation ΔP m As shown below:

[0106]

[0107]

[0108] Where, H is the inertia coefficient; is the differential value of the frequency change signal output by the frequency-locked loop; f ref is the frequency reference value.

[0109] When tuning the above inertia coefficient, as the H value increases, the system response becomes smoother, the oscillation amplitude decreases, and the system gradually tends to a stable state, but the response speed will become slower accordingly. Therefore, in order to obtain the best dynamic response, it is necessary to make a trade-off between RoCoF and response speed and select the required inertia coefficient H.

[0110] The power calculation module includes a power calculation module, an active loop and a reactive loop. The power calculation module obtains active power p and reactive power Q based on the grid output voltage and grid output current. The active loop is based on the power deviation ΔP m The active power p is adjusted to obtain an active power reference value, and the reactive power loop obtains a reactive power reference value based on the reactive power;

[0111] The above is as attached Fig. 9 As shown, the active loop adjusts the active power based on the power deviation to obtain the active power reference value, and the reactive loop can be a PI controller or directly set the reactive power reference value.

[0112] The current reference calculation module obtains the active current reference value i of the dq axis according to the active power reference value and the reactive power reference value. dref and the reactive current reference value i of the dq axis qref .

[0113] The improved MPC algorithm is introduced into the current loop, according to the active current reference value i of the dq axis dref and the reactive current reference value i of the dq axis qref , predict the future current trajectory and generate voltage control signals;

[0114] Since the traditional current loop control based on PI controller has the problems of complex parameter setting and slow dynamic response, an improved MPC algorithm is introduced into the current loop in this embodiment, as shown in the attached figure. Fig.10 As shown in the figure, when the current reference undergoes a step of 1A, the control effects of the current loop using MPC and PI are compared. It can be seen that the dynamic response of the improved MPC algorithm predictive control (that is, the model predictive control in the figure) is faster. In practical applications, the PI controller often also includes other parameter influences such as the voltage loops on both sides and the system frequency control, which makes the dynamic response of the system slower. Therefore, the introduction of the improved MPC algorithm has significant advantages over the parameter design and dynamic performance of the PI controller.

[0115] The PWM module converts the voltage control signal into a switch control signal through pulse width modulation and outputs it to the grid-connected converter.

[0116] It should also be noted that the above-mentioned Figure 6 In the system, the main circuit consists of grid-connected converter, output filter, load, line impedance and virtual synchronous generator VSG.

[0117] The present invention discloses an inertia enhancement control strategy based on a frequency locked loop suitable for a grid-connected converter, introduces a three-phase frequency locked loop and a current loop based on an improved MPC, and solves the frequency instability problem of a traditional phase-locked loop structure under weak grid conditions by organically combining frequency tracking, induction grid frequency changes and a power reference signal. At the same time, it solves the problem that a single-phase frequency locked device has a large amount of ripples and has a negative impact on the inertial response of the system, thereby providing support for ensuring the stable operation of the grid.

[0118] Embodiment 2: As attached Fig.11 As shown, the embodiment of the present invention discloses a virtual inertia enhancement control method for a grid-connected converter based on a frequency-locked loop, comprising:

[0119] Step S110, using the virtual inertia control module to track the grid frequency, analyze the grid frequency change, and obtain the active current reference value i of the dq axis according to the grid frequency change. dref and the reactive current reference value i of the dq axis qref ;

[0120] Step S120: the active current reference value i of the dq axis is dref and the reactive current reference value i of the dq axis qref In the input current loop, the future current trajectory is predicted based on the improved MPC algorithm, and the active current reference value i of the dq axis is dref and the reactive current reference value i of the dq axis qref Perform optimization and generate a voltage control signal;

[0121] Step S130: The voltage control signal is pulse-width modulated by the PWM module to obtain a corresponding switch control signal, and outputs the signal to the grid-connected converter to drive the grid-connected converter to output a corresponding voltage waveform.

[0122] In step S110, the virtual inertia control module is used to track the grid frequency, analyze the frequency change of the grid, and obtain the active current reference value i of the dq axis according to the frequency change of the grid. dref and the reactive current reference value i of the dq axis qref ,include:

[0123] The three-phase frequency-locked loop is used to track the grid frequency and generate a phase angle θ and a grid frequency change signal f;

[0124] The frequency controller introduces the inertia coefficient and adjusts the frequency reference value f according to the frequency change signal f of the power grid. ref Make adjustments to obtain corresponding power deviations;

[0125]

[0126]

[0127] Where, H is the inertia coefficient; is the differential value of the frequency change signal output by the three-phase frequency-locked loop; f ref is the frequency reference value;

[0128] The power calculation module obtains active power p and reactive power Q based on the grid output voltage and grid output current;

[0129] Active loop obtains power deviation ΔP m and active power p, using the power deviation ΔP m Adjust the active power p to obtain an active power reference value;

[0130] The reactive power loop obtains a reactive power reference value based on the reactive power;

[0131] The current reference calculation module obtains the active current reference value i of the dq axis according to the active power reference value and the reactive power reference value. dref and the reactive current reference value i of the dq axis qref .

[0132] Example 3: In order to verify the improvement of the present invention, simulation tests and experimental tests of power, frequency and other responses were designed, wherein the main circuit parameters are designed as shown in Table 1.

[0133] Table 1

[0134]

[0135] In order to evaluate the improvement of the frequency-locked loop on the system performance, the frequency response of the present invention was tested under a 4% load jump condition, as shown in the attached figure. Fig.12As shown, the valley value of the frequency response of the traditional method at a 4% load jump is about 48.81Hz, while after adopting the present invention, the valley value of the frequency response curve is about 49.84Hz, which is about 20% higher than that of the traditional method. It should be noted that load jumps will cause instantaneous voltage fluctuations, thereby affecting the quality of electricity. Especially when there are a large number of sensitive devices in the power system, even short-term voltage fluctuations may cause equipment misoperation or damage. In addition, rapid changes in load will also cause frequency fluctuations in the power system. In more serious cases, load jumps may also trigger misoperation of protection devices, thereby threatening the safe and stable operation of the power grid. Generally, load jumps of more than 3% can be considered to cause significant interference to the system.

[0136] In order to evaluate the improvement of the frequency-locked loop on system performance, the power tracking response of the present invention was tested under a 4% load jump. Fig.13 A power tracking response simulation diagram is provided for this embodiment. The grid-side output power has a change value of 0% due to comparison with itself, while the active power of the converter in the charging mode of the present invention responds quickly and returns to normal after a jump of about 2%, proving that the grid-connected converter has good power tracking performance when the load changes.

[0137] In order to evaluate the improvement of the frequency-locked loop on the system performance, the frequency derivative response of the present invention is tested under a 4% load jump. Fig.14 It is a frequency derivative response simulation diagram of the embodiment. The RoCoF represented by the frequency derivative is an important performance indicator for evaluating frequency regulation. A high RoCoF level, such as 1 Hz / s, may cause the protection relay to trip, causing greater interference to the power system. The present invention estimates that the RoCoF can be increased by 25%, and the value can be further increased in the presence of a larger inertia coefficient H. Compared with the traditional method, the system using the frequency-locked loop control shows better frequency stability and smaller response delay. Here, the design of the inertia coefficient depends on the inertia requirements and the inertia of the existing power system. The difference between the required inertia and the existing inertia is the target value of H.

[0138] In summary, the present invention significantly improves the frequency response characteristics of the system, solves the frequency instability problem of the traditional phase-locked loop structure under weak power grid conditions, and solves the problem that a large amount of ripple exists in the single-phase frequency lock and has a negative impact on the inertial response of the system, providing support for ensuring the stable operation of the power grid and demonstrating its robustness and adaptability in the process of dynamic frequency changes.

[0139] The above technical features constitute the best embodiment of the present invention, which has strong adaptability and best implementation effect. Non-essential technical features can be added or reduced according to actual needs to meet the requirements of different situations.

Claims

1. A virtual inertia control device for a grid-connected converter based on a frequency-locked loop, characterized in that: Including virtual inertia control module, current loop, PWM module; A three-phase frequency-locked loop and a frequency controller are introduced into the virtual inertia control module to track the grid frequency, and the active current reference value of the dq axis and the reactive current reference value of the dq axis are obtained according to the frequency change of the grid; An improved MPC algorithm is introduced into the current loop to predict the future current trajectory and generate a voltage control signal based on the active current reference value of the dq axis and the reactive current reference value of the dq axis. The PWM module converts the voltage control signal into a switch control signal through pulse width modulation and outputs it to the grid-connected converter.

2. The virtual inertia control device for grid-connected converter based on frequency-locked loop according to claim 1, characterized in that: The virtual inertia control module comprises: A three-phase frequency-locked loop, obtained by expanding a single-phase frequency-locked loop based on a generalized second-order integrator, is used to track the grid frequency and generate phase angle and grid frequency change signals; The frequency controller adjusts the frequency according to the frequency change signal of the power grid to obtain the corresponding power deviation; A power calculation module includes a power calculation module, an active loop and a reactive loop. The power calculation module obtains active power and reactive power based on the grid output voltage and the grid output current. The active loop adjusts the active power based on the power deviation to obtain an active power reference value. The reactive loop obtains a reactive power reference value based on the reactive power. The current reference calculation module obtains the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the active power reference value and the reactive power reference value.

3. The virtual inertia control device for grid-connected converter based on frequency-locked loop according to claim 2, characterized in that: The input-output transfer function of the three-phase frequency-locked loop is as follows: in, is the phase angle of the frequency-locked loop; ω g is the angular frequency of the power grid; ω is the angular frequency of the frequency-locked loop; V ga 、V gβ is the voltage amplitude of the three-phase electricity in the power grid.

4. The virtual inertia control device for grid-connected converter based on frequency-locked loop according to claim 2 or 3, characterized in that: The input signal of the generalized second-order integrator in the three-phase frequency-locked loop is as follows: in, is the phase angle of the frequency-locked loop; ω g is the angular frequency of the power grid; ω is the angular frequency of the frequency-locked loop; V ga 、V gβ is the voltage amplitude of the three-phase electricity in the power grid.

5. The virtual inertia control device for grid-connected converter based on frequency-locked loop according to claim 2 or 3, characterized in that: The frequency controller performs frequency adjustment according to the frequency change signal of the power grid, and obtains the corresponding power deviation as shown below: Where H is the inertia coefficient; is the differential value of the frequency change signal output by the frequency-locked loop; f ref is the frequency reference value.

6. The virtual inertia control device for grid-connected converter based on frequency-locked loop according to claim 4, characterized in that: The frequency controller performs frequency adjustment according to the frequency change signal of the power grid, and obtains the corresponding power deviation as shown below: Where H is the inertia coefficient; is the differential value of the frequency change signal output by the three-phase frequency-locked loop; f ref is the frequency reference value.

7. A method for virtual inertia enhancement control of a grid-connected converter based on a frequency-locked loop according to any one of claims 1 to 6, characterized in that: include: The virtual inertia control module is used to track the grid frequency, analyze the frequency change of the grid, and obtain the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the frequency change of the grid; The active current reference value of the dq axis and the reactive current reference value of the dq axis are input into the current loop, the future current trajectory is predicted based on the improved MPC algorithm, the active current reference value of the dq axis and the reactive current reference value of the dq axis are optimized, and the voltage control signal is generated; The voltage control signal is pulse-width modulated by the PWM module to obtain a corresponding switch control signal, which is then output to the grid-connected converter to drive the grid-connected converter to output a corresponding voltage waveform.

8. The virtual inertia control method for grid-connected converter based on frequency-locked loop according to claim 7, characterized in that: The method of using the virtual inertia control module to track the grid frequency, analyze the frequency change of the grid, and obtain the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the frequency change of the grid includes: The three-phase frequency-locked loop is used to track the grid frequency and generate phase angle and grid frequency change signals; The frequency controller introduces the inertia coefficient and adjusts the frequency reference value according to the frequency change signal of the power grid to obtain the corresponding power deviation; Where H is the inertia coefficient; is the differential value of the frequency change signal output by the three-phase frequency-locked loop; f ref is the frequency reference value; The power calculation module obtains active power and reactive power based on the grid output voltage and the grid output current; The active loop obtains the power deviation and active power, adjusts the active power using the power deviation, and obtains the active power reference value; The reactive power loop obtains a reactive power reference value based on the reactive power; The current reference calculation module obtains the active current reference value of the dq axis and the reactive current reference value of the dq axis according to the active power reference value and the reactive power reference value.

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