Tracking control system and method for minimum collection leakage current of photovoltaic direct current system

Abstract

The invention discloses a tracking control system and method for the minimum collection leakage current of a photovoltaic direct current system, and the method comprises the steps: carrying out the uniform phase shift of a carrier wave of each direct current converter to an optimal phase shift angle 2pi / N when N direct current converters with the same switching frequency are collected in parallel; when N direct-current converters with different switching frequencies are connected in parallel and collected, the direct-current converters with the same switching frequency are divided into one group, then the collected leakage current amplitude at the switching frequencies of the different direct-current converter groups and the expected minimum collected leakage current amplitude are subjected to PI adjustment, and after the optimal phase shifting angles of the different converter groups are obtained, uniform phase shifting is carried out; and tracking control of the minimum collection leakage current of the system is realized. The method suppresses the collection leakage current of the PV-LVDC system as much as possible, achieves the minimum collection leakage current of the system, improves the operation safety and stability of the system, and guarantees the normal operation of the system.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics and power network control, and mainly relates to a tracking control system and method for the minimum aggregated leakage current of a photovoltaic DC system. Background Art

[0002] With the access of distributed photovoltaics, energy storage, etc. to the power grid, the generated direct current needs to be converted into alternating current for transmission. Most existing industrial and residential loads use direct current, and the alternating current needs to be converted into direct current again on the distribution side. Too many conversion links greatly reduce the power grid efficiency and power quality, and increase the complexity of the system. Against this background, the low-voltage direct current (LVDC) power supply system has developed rapidly due to its advantages such as high efficiency, high power quality, high power supply reliability, low line cost, and easy maintenance.

[0003] The photovoltaic access low-voltage direct current (PV-LVDC) system generally uses a non-isolated DC converter. However, due to the lack of a transformer in the non-isolated DC converter, there is a lack of electrical isolation between the photovoltaic panels and the DC bus. The photovoltaic panels have parasitic capacitance to the ground, and the high-frequency switching actions of the power devices in the non-isolated DC converter may generate high-frequency time-varying voltages acting on the parasitic capacitance, thereby generating a leakage current i cm (also known as common-mode current or ground current) in the common-mode (CM) loop composed of the parasitic capacitance of the battery panel, the non-isolated DC converter, the DC grid, and the ground. With the increase in photovoltaic installed capacity, while the electric energy accumulates, the leakage current generated by each non-isolated DC converter in the PV-LVDC system will also aggregate at the DC bus, thus forming a system-level aggregated leakage current i cm_collected . The existence of the aggregated leakage current will cause distortion of the DC bus current, affect the power quality, and even endanger the safety of personnel and equipment, affecting the safe and stable operation of the PV-LVDC system. Summary of the Invention

[0004] The present invention precisely addresses the problems existing in the prior art and provides a tracking control system and method for the minimum collective leakage current in a photovoltaic DC system. When N DC converters with the same switching frequency are connected in parallel, the carrier waves of each DC converter are uniformly phase-shifted to the optimal phase-shift angle of 2π / N. When N DC converters with different switching frequencies are connected in parallel, the DC converters with the same switching frequency are first grouped together. Then, the collective leakage current amplitude at the switching frequencies of different DC converter groups and the expected minimum collective leakage current amplitude are adjusted through PI control. After obtaining the optimal phase-shift angles of different converter groups, uniform phase-shifting is performed to achieve the tracking control of the minimum collective leakage current in the system. The method of the present invention suppresses the collective leakage current of the PV-LVDC system as much as possible, realizes the minimum collective leakage current of the system, improves the operation safety and stability of the system, and ensures the normal operation of the system.

[0005] To achieve the above object, the technical solution adopted by the present invention is: a tracking control system for the minimum collective leakage current in a photovoltaic DC system, which is used in a PV-LVDC system. When N DC converters with the same switching frequency are connected in parallel, the carrier waves of each DC converter are uniformly phase-shifted to the optimal phase-shift angle of 2π / N. When N DC converters with different switching frequencies are connected in parallel, the DC converters with the same switching frequency are first grouped together. Then, the collective leakage current amplitude at the switching frequencies of different DC converter groups and the expected minimum collective leakage current amplitude are adjusted through PI control. After obtaining the optimal phase-shift angles of different converter groups, uniform phase-shifting is performed to achieve the tracking control of the minimum collective leakage current in the system.

[0006] As an improvement of the present invention, when N DC converters with the same switching frequency are connected in parallel, after uniform phase-shifting, the i-th DC converter is phase-shifted At this time, the vector sum of the leakage currents of each DC converter in the system is 0.

[0007] As another improvement of the present invention, when N DC converters with different switching frequencies are connected in parallel, if there are k DC converters with a switching frequency of ω ck of the DC converter, l DC converters with a switching frequency of ω cl of the DC converter,..., m DC converters with a switching frequency of ω cm of the DC converter, then after uniform phase-shifting, each converter in the DC converter group with a switching frequency of ω ck is uniformly phase-shifted Each converter in the DC converter group with a switching frequency of ω cl is uniformly phase-shifted ..., each converter in the DC converter group with a switching frequency of ω cm is uniformly phase-shifted At this time, the vector sum of the leakage currents of each DC converter in the system is 0.

[0008] To achieve the above object, the technical solution adopted by the present invention is also as follows: a tracking control method for the minimum aggregated leakage current of a photovoltaic DC system, including the following steps:

[0009] S1: Group each DC converter according to the switching frequency, and the DC converters with the same switching frequency are divided into one group;

[0010] S2: Measure the system aggregated leakage current i cm_collected in real time;

[0011] S3: Use a band-pass filter to extract the aggregated leakage current waveforms i cmf_collected_ωck 、i cmf_collected_ωcl 、……、i cmf_collected_ωcm at the switching frequencies of different DC converter groups;

[0012] S4: Use a Fourier analysis module to calculate the amplitudes I cmf_collected_ωck 、I cmf_collected_ωcl 、……、I cmf_collected_ωcm of the aggregated leakage current at the switching frequencies of different DC converter groups;

[0013] S5: Compare the obtained amplitudes I cmf_collected_ωck 、I cmf_collected_ωcl 、……、I cmf_collected_ωcm of the aggregated leakage current at the switching frequencies of different DC converter groups with the expected minimum aggregated leakage current amplitude, and obtain the optimal phase shift angle θ cb_ωck 、θ cb_ωcl 、……、θ cb_ωcm required for each converter group after PI adjustment;

[0014] S6: Input the optimal phase shift angles θ cb_ωck 、θ cb_ωcl 、……、θ cb_ωcm obtained in step S5 into the control links of different DC converter groups respectively, uniformly phase-shift each DC converter in the system, and realize the tracking control of the minimum aggregated leakage current of the system.

[0015] As an improvement of the present invention, in the step S2, the system aggregated leakage current is:

[0016]

[0017] where θ ci 、θ cj 、θ ch are the phase shift angles of each DC converter in the DC converter groups with switching frequencies of ω ck 、ω cl 、ω cm respectively.

[0018] As another improvement of the present invention, after the step S5 is uniformly transposed, the aggregated leakage current in the system is specifically as follows:

[0019]

[0020] Compared with the prior art, the present invention has the following beneficial effects: The present invention proposes a tracking control system and method for the minimum aggregated leakage current of a photovoltaic DC system, which is applicable to PV-LVDC systems with multiple DC converters in parallel with different conditions. The method of the present invention is easy to operate and has remarkable effects, so as to achieve the minimum aggregated leakage current of the system, improve the operation safety and stability of the system, ensure the normal operation of the system, and suppress the aggregated leakage current of the PV-LVDC system as much as possible. Brief Description of the Drawings

[0021] Figure 1 It is a schematic diagram of the parallel aggregation of multiple DC converters in the PV-LVDC system applicable to the system of the present invention;

[0022] Figure 2 It is a schematic diagram of the minimum aggregated leakage current tracking control system of the present invention;

[0023] Figure 3 It is a schematic diagram of the phase shift control of a certain DC converter group in the parallel system of multiple DC converters of the present invention;

[0024] Figure 4 It is a schematic diagram of the simulation model of the minimum aggregated leakage current tracking control strategy with the same converter switching frequency in the system of Test Example 1 of the present invention;

[0025] Figure 5 It is a schematic diagram of the change of the aggregated leakage current of the PV-LVDC system with the same converter switching frequency in Test Example 1 of the present invention;

[0026] Figure 6 It is a schematic diagram of the FFT analysis of the aggregated leakage current of the PV-LVDC system with the same converter switching frequency in Test Example 1 of the present invention;

[0027] Figure 7 It is a schematic diagram of the change of the phase shift angle θ of the PV-LVDC system with the same converter switching frequency in Test Example 1 of the present invention;

[0028] Figure 8 It is a schematic diagram of the simulation model of the implementation of the minimum aggregated leakage current tracking control method with different converter switching frequencies in the system of Test Example 2 of the present invention;

[0029] Figure 9 It is a schematic diagram of the change of the aggregated leakage current of the PV-LVDC system with different converter switching frequencies in Test Example 2 of the present invention;

[0030] Figure 10 Schematic diagram of FFT analysis of the aggregated leakage current of the PV-LVDC system with different switching frequencies of the DC converter in Test Example 2 of the present invention;

[0031] Figure 11 Schematic diagram of the change of the phase shift angle θ of the PV-LVDC system with different switching frequencies of the DC converter in Test Example 2 of the present invention. Detailed implementation manners

[0032] The present invention will be further clarified below in conjunction with the accompanying drawings and specific implementation manners. It should be understood that the following specific implementation manners are only used to illustrate the present invention and not to limit the scope of the present invention.

[0033] Embodiment 1

[0034] A minimum aggregated leakage current tracking control system applicable to a photovoltaic access low-voltage DC system is used in a PV-LVDC system. As shown in, there are multiple DC converters connected in parallel and aggregated in the PV-LVDC system. The structure and working principle of this system are as shown in and, specifically as follows: Figure 1 When N DC converters with the same switching frequency are connected in parallel and aggregated, the carriers of each DC converter are evenly phase-shifted to the optimal phase shift angle of 2π / N, that is, the i-th DC converter is phase-shifted by Figure 2 and Figure 3 As shown, it can make the vector sum of the leakage currents of each DC converter in the system be 0, that is,

[0035] When N DC converters with different switching frequencies are connected in parallel and aggregated, first divide the DC converters with the same switching frequency into a group, and then adjust the aggregated leakage current amplitude at the switching frequency of different DC converter groups and the expected minimum aggregated leakage current amplitude through PI regulation. After obtaining the optimal phase shift angle of different converter groups, perform uniform phase shift to achieve the minimum aggregated leakage current tracking control of the system. It can make the vector sum of the leakage currents of each DC converter in the system be 0, that is,

[0036]

[0037] Assume that among the N DC converters in a certain PV-LVDC system, there are k DC converters with a switching frequency of ω

[0038] , l DC converters with a switching frequency of ω ck ,......, m DC converters with a switching frequency of ω cl , and assume that the other parameters of the N DC converters are the same, that is, the leakage current amplitude of each converter is the same as A1. Divide the DC converters with the same switching frequency into a group. At this time, the aggregated leakage current of the system is: cm The aggregated leakage current of the system is:

[0039]

[0040] where θ ci , θ cj , θ ch are respectively the phase-shift angles of each DC converter in the DC converter group with switching frequencies ω ck , ω cl , ω cm .

[0041] For each DC converter in each DC converter group, uniform phase-shifting is performed according to the optimal phase-shift angle of the group, that is, for each converter in the DC converter group with switching frequency ω ck uniform phase-shifting is performed for each converter in the DC converter group For each converter in the DC converter group with switching frequency ω cl uniform phase-shifting is performed for each converter in the DC converter group ……, for each converter in the DC converter group with switching frequency ω cm uniform phase-shifting is performed for each converter in the DC converter group At this time, the aggregated leakage current in the system can be expressed as

[0042]

[0043] In summary, the system of this embodiment takes the PV-LVDC system as the research object, studies its aggregated leakage current suppression strategy, discloses a minimum aggregated leakage current tracking control system applicable to the PV-LVDC system, realizes the minimum aggregated leakage current of the system, and ensures the normal operation of the system.

[0044] Embodiment 2

[0045] A tracking control method for the minimum aggregated leakage current of a photovoltaic DC system, comprising the following steps:

[0046] Step S1: Group each DC converter according to the switching frequency, and the DC converters with the same switching frequency are grouped into one group;

[0047] Step S2: Measure the aggregated leakage current i cm_collected of the system in real time;

[0048] Step S3: Use a band-pass filter to extract the aggregated leakage current waveform i cmf_collected_ωc at the switching frequencies of different DC converter groups;

[0049] Step S4: Use a Fourier analysis module to calculate the amplitude I cmf_collected_ωc of the aggregated leakage current i cmf_collected_ωc at the switching frequencies of different DC converter groups;

[0050] Step S5: The obtained amplitudes I cmf_collected_ωcCompare with the expected minimum aggregated leakage current amplitude, and the optimal phase shift angle θ required for each converter group can be obtained after PI regulation. cb_ωc ;

[0051] Step S6: Input the optimal phase shift angle θ obtained in Step S5 cb_ωc into the control links of different DC converter groups respectively, and uniformly shift the phases of each DC converter in the system to achieve the tracking control of the minimum aggregated leakage current of the system.

[0052] For an N - number of DC converters with the same or different switching frequencies in the PV - LVDC system, the tracking control method of the minimum aggregated leakage current is as follows:

[0053] (1) The switching frequencies of the N DC converters in the PV - LVDC system are the same

[0054] Step 1: For the parallel - aggregated system of N DC converters with the same switching frequency, define that each DC converter uniformly shifts its phase by 0°, θ, 2θ, ……, (N - 1)θ in sequence. First, set the initial value of the phase shift angle θ to 0. At this time, the phases of each DC converter in the system are the same, and the aggregated leakage current is the largest;

[0055] Step 2: Then, measure the aggregated leakage current i of the system in real - time cm_collected ;

[0056] Step 3: Use a band - pass filter to extract the waveform of the aggregated leakage current i at the switching frequency of the system cmf_collected ;

[0057] Step 4: Use the Fourier analysis (FFT) module to calculate the amplitude I of the aggregated leakage current at the switching frequency cmf_collected , since the aggregated leakage current i at the switching frequency cmf_collected is a sine - wave signal, so the optimal phase shift angle θ required for the system cannot be directly obtained through the PI link using this signal cb ;

[0058] Step 5: After PI regulation of I cmf_collected and the expected minimum aggregated leakage current amplitude, the optimal phase shift angle θ required for this PV - LVDC system can be obtained cb ;

[0059] Step 6: Input the obtained θ cb into the control link of the PV - LVDC system with multiple parallel - connected DC converters, and uniformly shift the phases of each converter in the system, then the tracking control of the minimum aggregated leakage current of the system can be achieved.

[0060] (2) The switching frequencies of the N DC converters in the PV - LVDC system are different

[0061] Assume that among the N DC converters, k have a switching frequency of ω ckDC converters, one with a switching frequency of ω cl DC converters, ……, m with a switching frequency of ω cm DC converters;

[0062] Step 1: First, group each DC converter according to its switching frequency. DC converters with the same switching frequency are grouped together;

[0063] Step 2: Define that each DC converter in each DC converter group is evenly phase-shifted in sequence, that is, the DC converter group with a switching frequency of ω ck is evenly phase-shifted by 0°, θ ωck , 2θ ωck , ……, (k - 1)θ ωck ; The DC converter group with a switching frequency of ω cl is evenly phase-shifted by 0°, θ ωcl , 2θ ωcl , ……, (l - 1)θ ωcl ; ……; The DC converter group with a switching frequency of ω cm is evenly phase-shifted by 0°, θ ωcm , 2θ ωcm , ……, (m - 1)θ ωcm , and set the initial values of the phase-shift angles θ ωck , θ ωcl , ……, θ ωcm to 0. At this time, the phases of the converters in each DC converter group of the system are the same, and the aggregated leakage current is the largest;

[0064] Step 3: Then, measure the aggregated leakage current i of the system in real time cm_collected ;

[0065] Step 4: Use a band-pass filter to extract the aggregated leakage current waveforms i at the switching frequencies of different DC converter groups cmf_collected_ωck , i cmf_collected_ωcl , ……, i cmf_collected_ωcm ;

[0066] Step 4: Use a Fourier analysis (FFT) module to calculate the amplitudes I of the aggregated leakage currents at the switching frequencies of different DC converter groups cmf_collected_ωck , I cmf_collected_ωcl , ……, I cmf_collected_ωcm ;

[0067] Step 5: After PI regulation of the obtained amplitudes of the aggregated leakage currents at the switching frequencies of different DC converter groups and the expected minimum aggregated leakage current amplitude, the optimal phase-shift angles θ cb_ωck , θ cb_ωcl , ……, θ cb_ωcm for different converter groups can be obtained;

[0068] Step 6: The obtained optimal phase-shift angles θcb_ωck , θ cb_ωcl , ……, θ cb_ωcm By separately inputting the control links of different DC converter groups and uniformly phase-shifting each DC converter in the system, the minimum collective leakage current tracking control of the system can be achieved.

[0069] Test Example 1

[0070] Based on MATLAB / Simulink, a PV-LVDC system with seven parallel-connected DC converters having the same switching frequency of 20 kHz is built. The simulation model of its minimum collective leakage current tracking control system is as Figure 4 shown. Under the adjustment of the minimum collective leakage current tracking control system, the change of the collective leakage current in the system is as Figure 5 shown. The result of FFT analysis of its waveform is as Figure 6 shown. The change of the phase-shift angle output by the minimum collective leakage current tracking control system is as Figure 7 shown.

[0071] It can be seen from Figures 5 - 7 that within 0 - 0.01 s, the DC converters in the system have not been uniformly phase-shifted, and the phase-shift angles of each converter are the same. At this time, the collective leakage current in the system is the largest. It can be known from the FFT analysis that the amplitude of the collective leakage current at the switching frequency is 12.38 A. After the system reaches stable operation at 0.01 s, the minimum collective leakage current tracking control system starts to uniformly phase-shift each DC converter in the system. The minimum collective leakage current tracking control system responds quickly and reaches a stable state at about 0.03 s, and correctly outputs the required optimal phase-shift angle θ cb = 2π / 7 ≈ 51.43°. After uniform phase-shifting, the collective leakage current at the switching frequency of the system is only 0.023 A, which is reduced by 99.81% compared with that before uniform phase-shifting.

[0072] Test Example 2

[0073] Based on MATLAB / Simulink, a PV-LVDC system with seven parallel-connected DC converters is built. Four of the DC converters have a switching frequency of 20 kHz, and three of the DC converters have a switching frequency of 40 kHz. The simulation model of its minimum collective leakage current tracking control system is as Figure 8 shown. Under the adjustment of the minimum collective leakage current tracking control system, the change of the collective leakage current in the system is as Figure 9 shown. The result of FFT analysis of its waveform is as Figure 10 shown. The change of the phase-shift angle output by the minimum collective leakage current tracking control system is as Figure 11 shown.

[0074] It can be seen from Figures 9 - 11It can be seen that within 0 to 0.01 s, the DC converters in the system have not yet undergone uniform phase shift, and the phase shift angles of each converter are the same. At this time, the aggregated leakage current of the system is the largest. According to the FFT analysis, the amplitude of the aggregated leakage current at 20 kHz is about 7.10 A, and the amplitude of the aggregated leakage current at 40 kHz is about 1.63 A. After the system operates stably at 0.01 s, uniform phase shift is performed on each DC converter group in the system, and the minimum aggregated leakage current tracking control system responds quickly and reaches a stable state at about 0.04 s. Moreover, both converter groups correctly output the required optimal phase shift angle θ cb , where the optimal phase shift angle of the DC converter group with a switching frequency of 20 kHz is θ cb_20kHz = 2π / 4 ≈ 90°, and the optimal phase shift angle of the DC converter group with a switching frequency of 40 kHz is θ cb_40kHz = 2π / 3 ≈ 120°. After uniform phase shift, the aggregated leakage current of the system at 20 kHz is only about 0.049 A, which is reduced by 99.31% compared with that before uniform phase shift; the aggregated leakage current at 40 kHz is only about 0.012 A, which is reduced by 99.26% compared with that before uniform phase shift.

[0075] It should be noted that the above content only illustrates the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. For those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements all fall within the protection scope of the claims of the present invention.

Claims

1. A tracking control system for the minimum aggregated leakage current of a photovoltaic DC system, which is used on a PV-LVDC system, characterized in that : When N DC converters with the same switching frequency are connected in parallel, the carrier waves of each DC converter are evenly phase-shifted to the optimal phase-shift angle of 2π / N; when N DC converters with different switching frequencies are connected in parallel, the DC converters with the same switching frequency are first grouped together, and then the amplitudes of the converging leakage currents at the switching frequencies of different DC converter groups and the expected minimum converging leakage current amplitude are adjusted by PI to obtain the optimal phase-shift angles of different converter groups, and then evenly phase-shifted to achieve the tracking control of the minimum converging leakage current of the system.

2. The tracking control system for the minimum aggregated leakage current of a photovoltaic DC system according to claim 1, wherein: When N DC converters with the same switching frequency are connected in parallel and aggregated, after uniform phase shifting, the phase shift of the i-th DC converter is At this time, the sum of the leakage current vectors of each DC converter in the system is 0.

3. The tracking control system for the minimum aggregated leakage current of a photovoltaic DC system according to claim 1, characterized in that: When N DC converters with different switching frequencies are connected in parallel, if there are k DC converters with a switching frequency of ω ck , l DC converters with a switching frequency of ω cl , ……, m DC converters with a switching frequency of ω cm , then after uniform phase shifting, each converter in the DC converter group with a switching frequency of ω ck is uniformly phase shifted by Each converter in the DC converter group with a switching frequency of ω cl is uniformly phase shifted by Each converter in the DC converter group with a switching frequency of ω cm is uniformly phase shifted by At this time, the sum of the leakage current vectors of each DC converter in the system is 0.

4. A tracking control method for the minimum aggregated leakage current of a photovoltaic DC system, using the system as described in claim 1, characterized in that, It includes the following steps: S1: Group each DC converter according to its switching frequency, and the DC converters with the same switching frequency are grouped together; S2: Measure the system aggregated leakage current i cm_collected in real time; S3: Use a band-pass filter to extract the aggregated leakage current waveforms \(i_{\langle0000008\rangle}\), \(i_{\langle0000009\rangle}\), ……, \(i_{\langle0000010\rangle}\); cmf_collected_ωck 、i cmf_collected_ωcl 、……、i cmf_collected_ωcm ; S4: Use the Fourier analysis module to calculate the magnitudes I cmf_collected_ωck , I cmf_collected_ωcl , ……, I cmf_collected_ωcm of the aggregated leakage current at the switching frequencies of different DC converter groups; cmf_collected_ωck 、I cmf_collected_ωcl 、……、I cmf_collected_ωcm ; S5: Aggregate the magnitudes of the leakage currents I cmf_collected_ωck 、I cmf_collected_ωcl 、……、I cmf_collected_ωcm at the switching frequencies of the obtained different DC converter groups with the expected minimum aggregate leakage current magnitude I cmf_collected_ωc for comparison. After PI regulation, obtain the optimal phase-shift angles θ cb_ωck 、θ cb_ωcl 、……、θ cb_ωcm required for each converter group; S6: Input the optimal phase shift angles θ cb_ωck , θ cb_ωcl , ……, θ cb_ωcm into the control links of different DC converter groups respectively, uniformly shift the phases of each DC converter in the system, and achieve the minimum collection leakage current tracking control of the system.

5. The tracking control method for the minimum aggregated leakage current of a photovoltaic DC system according to claim 4, characterized in that: In step S2, the converging leakage current of the system is: where θ ci 、θ cj 、θ ch are the phase-shift angles of each DC converter in the DC converter group with switching frequencies ω ck 、ω cl 、ω cm respectively.

6. The tracking control system for the minimum collection leakage current of a photovoltaic DC system according to claim 5, characterized in that: After the uniform phase shift in step S5, the converging leakage current in the system is specifically:

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