Online monitoring method for power loop gain of grid-connected converter controlled by virtual synchronous machine

Abstract

The invention discloses an online monitoring method for power loop gain of a grid-connected converter controlled by a virtual synchronous machine, which comprises the following steps of: firstly monitoring response signals on two sides of a disturbance signal, then filtering the response signals on the two sides, and processing by an integral regulator, so that the frequency of the disturbance signal can be updated and the process is continuously repeated; and the bandwidth and the phase margin of the power loop are obtained. Compared with a frequency sweeping mode, the method has the advantages that the performance of the power loop of the grid-connected converter controlled by the VSG can be monitored only with small computing power, and therefore control parameters in the VSG can be flexibly adjusted according to dynamic performance requirements.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected converter control, and particularly relates to an online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous generator (VSG). Background Art

[0002] With the continuous increase in the penetration rate of wind power and photovoltaic power, power stations based on traditional synchronous generators will gradually be replaced by highly dispersed distributed micro-power sources, forming a new power system with new energy as the main body. Among them, new energy power generation units usually adopt a grid-following control strategy to access the power system, and the grid-following grid-connected inverter can be approximately described as a current source. However, the characteristics of low inertia and weak damping of the grid-following grid-connected inverter bring great challenges to the safe and stable operation of the new power system. At the same time, if all new energy power generation units in the new power system adopt the grid-following control strategy, there will be a lack of a voltage source that can construct the system voltage to provide active support in the system. As a result, the power grid dominated by traditional synchronous generators will be transformed into a weak power grid lacking inertia and damping, and the new power system will not be able to operate stably. Therefore, in order to improve the reliability of power supply in the new power system, the application of grid-forming control strategies is becoming increasingly widespread, so as to provide inertia, damping and active support for the new power system with new energy as the main body. VSG control is the most widely used grid-forming converter control technology.

[0003] VSG control is a converter control technology with the internal mechanism and external characteristics of a synchronous machine. VSG control uses the mathematical model of a traditional synchronous machine as the control core, so it has similar dynamic characteristics and synchronization mechanisms to traditional synchronous motors. The VSG-controlled converter automatically adjusts active and reactive power according to the changes in grid voltage and frequency, thereby participating in the regulation of grid voltage and frequency. At the same time, the VSG-controlled converter can also provide inertia and damping support to the grid like a traditional synchronous motor, improving the stability of the grid.

[0004] As a key interface device for new energy to access the power grid, the grid-connected converter is mutually coupled with the power grid through the grid impedance. Therefore, the frequent fluctuations of the grid impedance will directly cause the change of the inverter loop gain, thus affecting the stability of the inverter, specifically manifested as the decrease of the stability margin. At the same time, the dynamic performance of the control loop is also affected by the grid impedance, specifically manifested as the change of the bandwidth of the VSG. However, in order to achieve fast power response, generally, it is required that the VSG control has as high dynamic performance as possible, that is, a higher control bandwidth. The loop gain can truly reflect the control performance of the grid-connected converter under different grid strengths. Therefore, based on the online measurement results of the loop gain, the adaptive adjustment of the VSG control parameters can be realized to improve the adaptability of the grid-connected converter to the new power system with wide-range changes in strength.

[0005] To sum up, the design of the control loop of the grid-forming grid-connected converter with VSG control needs to make a good compromise between stability and dynamic performance. Therefore, it is necessary to monitor the actual loop gain of the VSG under different grid strengths to provide reliable information support for the design optimization and stability analysis of the VSG control loop. The online monitoring technology of loop gain has been developed for many years, but it is mostly applied to the DC / DC converter scenario and less applied to the converter with rich background harmonics. If the frequency of the disturbance signal is continuously changed to sweep the frequency within the required frequency band, a complete loop gain curve can be obtained. However, a complete measurement of the loop gain in the sweeping frequency manner means that a sufficient number of disturbance signals with different frequencies need to be injected in sequence, which often requires a large amount of computing power. Summary of the Invention

[0006] Aiming at the problem that the grid impedance cannot be ignored and fluctuates frequently in the current VSG control field, resulting in the deterioration of the power control loop performance, the present invention proposes an online monitoring method for the power loop gain of the grid-connected converter with virtual synchronous generator control to realize the online monitoring of the bandwidth and phase margin of the power loop of the grid-connected converter, so as to meet the requirements for loop gain information in terms of stability and dynamic performance design.

[0007] The technical solution for the present invention to solve the above technical problems is: designing an online monitoring method for the power loop gain of the grid-connected converter with virtual synchronous generator control, which is characterized in that the method specifically includes the following steps:

[0008] Step 1: Set the rated power of the grid-connected converter as P ref , and set the output power of the grid-connected converter as P e ; when the power grid system is operating normally, inject a disturbance signal p with a frequency of r and an amplitude of |p r | into P ref and P e in the VSG active power control loop.After the difference, where the perturbation signal p r As shown in Equation (2):

[0009]

[0010] is the set reference frequency; then the first response signals p before and after the injection point are collected simultaneously in and the second response signal p out ;

[0011] Step Two: The first response signal p in and the second response signal p out collected in Step One are respectively filtered by band - pass filters with variable frequencies to extract the response signals with the same frequency as the perturbation signal, and the first filtered signal pv in and the second filtered signal pv out are correspondingly obtained;

[0012] Step Three: The first response signal p in and the second response signal p out are respectively processed through the first filtered signal pv in and the second filtered signal pv out filtered by the band - pass filter as shown in Equations (4), (5), (6), and (7) to obtain the processed signals;

[0013]

[0014] where |pv in | and |pv out | are respectively the amplitudes of pv in and pv out , and x in and x out are respectively the phases of pv in and pv out ;

[0015] The processed signals include a DC component and a signal with a frequency four times that of the VSG bandwidth. The quadruple - frequency component is filtered out by a low - pass filter, and then the DC component of the response signal is subjected to polar coordinate conversion to obtain the amplitude and phase of the corresponding response signal;

[0016] Step Four: The amplitudes of the first response signal p in and the second response signal p out obtained in Step Three are subtracted, and the difference is sent into an integral regulator. The output value of the integral regulator is added to the reference frequency to obtain the frequency value of the perturbation signal after primary frequency modulation;

[0017] Step 5: Use the frequency value after primary frequency modulation of the disturbance signal obtained in Step 4 as the new frequency of the disturbance signal, and execute the processes of Steps 1 to 5 according to the new frequency value, and repeat continuously until the difference in the amplitudes obtained in Step 3 between the first response signal and the second response signal is 0. At this time, the frequency of the disturbance signal is the bandwidth of the grid-connected converter power loop gain; at the same time, subtract the phases of the first response signal and the second response signal at this time to obtain the phase margin of the grid-connected converter power loop gain, realizing the online monitoring of the bandwidth and phase margin in the grid-connected converter power loop gain.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The online monitoring method for the grid-connected converter power loop gain controlled by the virtual synchronous generator designed in the present invention first monitors the response signals on both sides of the disturbance signal, then filters the response signals on both sides, and then through the processing of the integral regulator, the frequency of the disturbance signal can be updated, and the process is repeated continuously until the bandwidth and phase margin of the power loop are obtained. Compared with the frequency sweep method, the advantage of the present invention is that only a small amount of computing power is required to monitor the performance of the grid-connected converter power loop controlled by the VSG, so as to flexibly adjust the control parameters in the VSG according to the dynamic performance requirements. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of the online monitoring principle of the grid-connected converter loop gain controlled by the virtual synchronous generator.

[0020] Figure 2 It is the active power when the grid-connected converter controlled by the virtual synchronous generator operates normally in an embodiment.

[0021] Figure 3 It is a schematic diagram of the disturbance signal application position and response signal extraction position of the online monitoring method for the grid-connected converter power loop gain controlled by the virtual synchronous generator of the present invention.

[0022] Figure 4 It is a schematic diagram of the principle and process of filtering the response signal through a band-pass filter in an embodiment of the online monitoring method for the grid-connected converter power loop gain controlled by the virtual synchronous generator of the present invention.

[0023] Figure 5 It is a schematic diagram of the response signal amplitude and phase calculation method in an embodiment of the online monitoring method for the grid-connected converter power loop gain controlled by the virtual synchronous generator of the present invention.

[0024] Figure 6 It is a schematic diagram of the frequency adjustment method in an embodiment of the online monitoring method for the grid-connected converter power loop gain controlled by the virtual synchronous generator of the present invention.

[0025] Figure 7The bandwidth result obtained by the online monitoring method of the power loop gain of the grid-connected converter using the virtual synchronous machine control of the present invention for one embodiment.

[0026] Figure 8 The phase margin result obtained by the online monitoring method of the power loop gain of the grid-connected converter using the virtual synchronous machine control of the present invention for one embodiment.

[0027] Figure 1 Where:

[0028] P ref : VSG power reference value;

[0029] P e : VSG output power;

[0030] D p : Damping coefficient;

[0031] J: Moment of inertia coefficient;

[0032] ω n : Grid rated angular frequency. Detailed implementation manners

[0033] To enable those skilled in the art to better understand the solution of the present invention, the technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0034] The principle of the grid-connected converter with virtual synchronous machine control is as Figure 1 shown, where the VSG active power control loop is a closed-loop control, which includes a damping link D p , an inertia link J and an inverter body power constraint link. The method of the present invention is to monitor the bandwidth and phase angle margin information of the above control links.

[0035] After the difference between P ref and P e in the VSG active control loop, a sinusoidal small perturbation signal p r with a controllable frequency is injected. After the perturbation signal passes through the active power closed-loop control loop, different response signals will be generated at different positions in the control loop. Through derivation, the relationship between the corresponding response signals before and after the injection point and the loop gain can be obtained as:

[0036]

[0037] Therefore, this method extracts the corresponding response signals p in and p out with corresponding frequencies before and after the injection point respectively. When the amplitudes of the two response signals are equal, the frequency of the perturbation signal is the bandwidth of the VSG control loop, and the phase difference between the two response signals is the phase angle margin of the VSG control loop.

[0038] The principle of Equation (1) can be found in the literature: A. Khodamoradi, G. Liu, P. Mattavelli, T. Caldognetto and P. Magnone, "Analysis of an Online Stability Monitoring Approach for DC Microgrid Power Converters," in IEEE Transactions on Power Electronics, vol. 34, no. 5, pp. 4794 - 4806, May 2019.

[0039] The present invention designs an online monitoring method for the power loop gain of a grid - connected converter with virtual synchronous generator control. The method specifically includes the following steps:

[0040] Step 1: Set the rated power of the grid - connected converter as P ref , in this embodiment, P ref is 100 kW; set the output power of the grid - connected converter as P e ; the operating conditions of the grid - connected converter are as Figure 3 shown, and online monitoring of the loop gain can be carried out at the 100 - kW operating point without affecting the normal operation of the power grid system; when the power grid system is operating normally, a disturbance signal p with a frequency of r and an amplitude of |p r | is injected into the difference between P ref and P e in the active power control loop of the VSG. Among them, the disturbance signal p r is as shown in Equation (2):

[0041]

[0042] is the set reference frequency; then, the first response signal p in before (closer to the power closed - loop side is defined as before) and after the injection point and the second response signal p out are collected simultaneously.

[0043] Step 2: The control loop contains periodic signals with multiple frequencies, as Figure 4 shown. The first response signal p in and the second response signal p out collected in Step 1 are respectively filtered using a band - pass filter (BPF) with variable frequency to extract the response signals with the same frequency as the disturbance signal, and the first filtered signal pv in and the second filtered signal pvout In the BPF structure, the output signal is p v , select an appropriate constant m to achieve the best response effect. Generally, the value range is [0.1, 0.2]. In this embodiment, m takes 0.2; the corresponding transfer function is shown in Equation (3). After BPF filtering, only the periodic signal with an angular frequency of is retained, and the periodic signals with other frequencies are filtered out.

[0044]

[0045] In the formula, s is the complex frequency.

[0046] Step 3: As Figure 5 shown, to calculate the amplitudes and phases of the first response signal p in and the second response signal p out , the first response signal p in and the second response signal p out are respectively passed through the first filtered signal pv in and the second filtered signal pv out obtained after being filtered by the BPF and processed as shown in Equations (4), (5), (6), and (7) to obtain the processed signals;

[0047]

[0048] where |pv in | and |pv out | are the amplitudes of pv in and pv out respectively, and x in and x out are the phases of pv in and pv out respectively.

[0049] The processed signals include a DC component and a signal with a frequency four times that of the VSG bandwidth. The quadruple-frequency component is filtered out by a low-pass filter (LPF). The bandwidth of the low-pass filter must be less than the quadruple frequency. Here, 0.5 Hz is selected. The DC component contains the amplitude and phase information of the response signal. The DC component of the response signal is subjected to polar coordinate conversion to obtain the amplitude and phase of the corresponding response signal.

[0050] Step 4: As Figure 6 shown, perform frequency adjustment on the first response signal p in and the second response signal p outSubtract the amplitudes obtained in step three, and send the difference into the integral regulator. The integral coefficient in the integral regulator is selected as 0.003, and the reference frequency is set. Because of the inertia and damping in the VSG, the control bandwidth is generally not very large. Here, the reference frequency is selected as 3 Hz. Then, set the amplitude of the disturbance signal. Here, 2.5% of the rated power is selected. Add the output value of the integral regulator to the reference frequency to obtain the frequency value after the first frequency modulation of the disturbance signal.

[0051] Step five: Use the frequency value after the first frequency modulation of the disturbance signal obtained in step four as the new frequency of the disturbance signal, and execute the process from step one to step five according to the new frequency value, and repeat continuously until the difference between the amplitudes of the first response signal and the second response signal obtained in step three is 0. At this time, the frequency of the disturbance signal is the bandwidth of the power loop gain of the grid-connected converter; at the same time, subtract the phases of the first response signal and the second response signal at this time to obtain the phase margin of the power loop gain of the grid-connected converter, and realize the online monitoring of the bandwidth and phase margin in the power loop gain of the grid-connected converter.

[0052] In this embodiment, a simulation model is established with a 100kW VSG-controlled three-level inverter as the object. Based on the Matlab / simnlink simulation platform, the effectiveness of the above-mentioned VSG monitoring method is verified. As Figure 7 shown, this figure is the online monitoring result of the VSG loop gain bandwidth. From 0 s to 0.4 s is the reference frequency, with a value of 3 Hz. From 0.4 s to 10 s is the bandwidth when the grid strength SCR = 10 (R / X = 1 / 10), with a value of 5.9 Hz. At 10 s, the grid strength changes suddenly. From 10 s to 20 s is the bandwidth when the grid strength SCR = 8 (R / X = 1 / 10), with a value of 4.1 Hz.

[0053] As shown in 8, this figure is the online monitoring result of the VSG loop gain phase margin. The value from 0 s to 0.4 s is 0. Dynamic frequency search starts at 0.4 s. From 0.4 s to 10 s is the phase margin when the grid strength SCR = 10 (R / X = 1 / 10), with a value of 84°. At 10 s, the grid strength changes suddenly. From 10 s to 20 s is the phase margin when the grid strength SCR = 8 (R / X = 1 / 10), with a value of 87°.

[0054] The above operation results can verify that under the VSG control, the proposed scheme can online monitor the actual loop gain according to the change of grid strength, and this method is feasible.

[0055] The above has made an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification or equivalent replacement that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

[0056] What is not described in this invention applies to the prior art.

Claims

1. Online monitoring method for power loop gain of grid-connected converter controlled by virtual synchronous machine, characterized in that, The method specifically includes the following steps: Step 1: Set the rated power of the grid-connected converter as P ref , and set the output power of the grid-connected converter as P e ; When the power grid system is operating normally, a disturbance signal p with a frequency of r and an amplitude of |p r | is injected into the difference between P ref and P e in the active power control loop of the VSG. The disturbance signal p r is shown in Equation (2) as follows: is the set reference frequency; then simultaneously collect the first response signal p before and after the injection point in and the second response signal p out ; Step 2: The first response signal p collected in Step 1 in and the second response signal p out are respectively filtered by a band-pass filter with variable frequency to extract the response signals with the same frequency as the disturbance signal, and the first filtered signal pv in and the second filtered signal pv out are correspondingly obtained; Step 3: The first response signal p in and the second response signal p out are respectively passed through a first filtered signal pv in and a second filtered signal pv out which are obtained by filtering through a band-pass filter, and are processed as shown in equations (4), (5), (6), and (7) to obtain a processed signal; where |pv in | and |pv out | are the amplitudes of pv in and pv out respectively, and |x in | and |x out | are the phases of pv in and pv out respectively. The processed signal includes a DC component and a signal with a frequency four times that of the VSG bandwidth. The quadruple-frequency component is filtered out by a low-pass filter, and then the DC component of the response signal is subjected to polar coordinate transformation to obtain the amplitude and phase of the corresponding response signal. Step 4: Subtract the amplitudes of the first response signal p in and the second response signal p out obtained in Step 3, send the difference value to an integral regulator, add the output value of the integral regulator to the reference frequency to obtain the frequency value after the primary frequency modulation of the disturbance signal; Step 5: Use the frequency value of the disturbance signal after primary frequency modulation obtained in Step 4 as the new frequency of the disturbance signal, and execute the processes of Steps 1 to 5 according to the new frequency value, and repeat continuously until the difference in the amplitudes obtained in Step 3 between the first response signal and the second response signal is 0. At this time, the frequency of the disturbance signal is the bandwidth of the power loop gain of the grid-connected converter; at the same time, subtract the phases of the first response signal and the second response signal at this time to obtain the phase margin of the power loop gain of the grid-connected converter, realizing the online monitoring of the bandwidth and phase margin in the power loop gain of the grid-connected converter.

2. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to claim 1, characterized in that, In Step 1, the reference frequency is 3 Hz.

3. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to claim 1, characterized in that In Step 1, the amplitude of the disturbance signal is 2.5% of the rated power of the grid-connected converter.

4. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to claim 1, characterized in that In Step 2, the value range of m of the band-pass filter is [0.1, 0.2].

5. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to claim 1, wherein In Step 2, the transfer function of the band-pass filter is shown in Equation (3): In the formula, s is the complex frequency, and m is a set constant.

6. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to any one of claims 4 or 5, characterized in that m is 0.

2.

7. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to claim 1, characterized in that In Step 3, the bandwidth of the low-pass filter is 0.5 Hz.

8. The online monitoring method for the power loop gain of a grid-connected converter controlled by a virtual synchronous machine according to claim 1, characterized in that In Step 4, the integral coefficient in the integral regulator is selected as 0.003.

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