A resonance point determination method for DC power distribution system

Abstract

The present invention relates to the technical field of direct current distribution systems, and discloses a method for determining the resonance point of a direct current distribution system. The line impedance between the source converter and the direct current bus in the direct current distribution system is prone to cause oscillation, and there are often multiple source converters in the direct current system, resulting in instability of the direct current distribution system. The present invention divides the resonant frequency in the system into three categories according to the interaction relationship between the line impedances of the source converters, namely: a low-frequency resonance point formed by the parallel connection of the LC filter inside the converter, a high-frequency resonance point formed by the parallel capacitor on the output side of the converter and the line impedance, and a resonance valley formed by a single converter and the line impedance. The present invention obtains the design boundary of the cut-off frequency of the load converter based on the relationship between the resonance peak and the input impedance of the load converter to guide the converter design.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current power distribution systems, and in particular to a resonance point determination method applicable to direct current power distribution systems. Background Art

[0002] With the rapid development of renewable energy such as photovoltaic and wind power and DC loads such as charging piles, electric vehicles, and data centers, DC distribution systems have gradually become popular. The system includes a large number of power electronic converters, such as photovoltaic grid-connected converters, energy storage converters, load converters, etc., and different types of sources and loads are connected to the DC bus through power electronic converters. However, in complex DC distribution systems, due to the interaction between converters, even if each converter can operate stably alone, multiple converters in series and parallel still have instability problems, resulting in low-frequency oscillations on the DC bus.

[0003] Current research usually ignores the line impedance between the converter and the DC bus. As the number of converters increases, the interaction between multiple converters and the line impedance easily causes multiple high-frequency resonance points. Existing research focuses on low-frequency resonance, and lacks an effective method for determining multiple resonance points in DC systems. Therefore, the determination method for the multiple resonance problem in DC systems is an issue that needs to be solved urgently. Summary of the invention

[0004] In view of the deficiencies mentioned in the above technical background, the purpose of the present invention is to propose a method for determining the resonance point suitable for a DC power distribution system. The present invention divides the multi-resonance points into three categories to address the multi-resonance problem caused by line impedance, and obtains corresponding expressions. According to the interaction relationship with the load converter, the stable boundary of the high-frequency resonance is obtained.

[0005] The present invention adopts the following technical solutions:

[0006] A method for determining a resonance point of a DC power distribution system, the method comprising the following steps:

[0007] Step 1, divide the multi-resonance frequency points into three types;

[0008] Step 2, find the parallel output impedance of the source converter corresponding to the three resonant frequencies;

[0009] Step 3: Based on the interaction between the peak values ​​of the multi-resonance points and the input impedance of the load converter, the boundary conditions for system stability and the design boundary of the cutoff frequency of the load converter are obtained to guide the design of the converter.

[0010] Furthermore, the three multi-resonance frequency point expressions in step 1 are respectively:

[0011] The first type: low-frequency resonant frequency f formed by parallel connection of LC filtersr1 ,

[0012]

[0013] The second method: Assume that L l1 <L l2 <… <L ln and R l1 <R l2 <… <R ln , in the high frequency band, The parallel capacitor C in the LC filter fi Interacting with the line impedance generates n-1 high-frequency resonant frequencies f r2_i (i=1,2…n-1),

[0014]

[0015] The third type: the line inductance L between the i-th LC filter and its busbar li Mutual resonance produces n resonance points f r3_i (i=1,2…n),

[0016]

[0017] Furthermore, the low-frequency resonance frequency f r1 The corresponding source converter parallel output impedance Z oS The peak value is:

[0018]

[0019] The high frequency resonance frequency f r2_i The corresponding source converter parallel output impedance Z oS The peak value is:

[0020]

[0021] The f r3_i The corresponding source converter output impedance Z oS The valley values ​​are:

[0022] Z oS_vi =R li .

[0023] Furthermore, in step 3, the input impedance of the load converter is:

[0024]

[0025] Among them, P oj (j=1,2,…,m) is the load power; T(s)=H v G v(s)G m (s)G vd_L (s) is the open loop gain of the load converter using voltage loop control;

[0026] At the cut-off frequency f c Within the range, |T(s)|>>1, so the parallel input impedance of m load converters is:

[0027]

[0028] According to the above formula, multiple load converters connected in parallel can be equivalent to one load converter.

[0029] Furthermore, the f r3_i It exhibits a valley characteristic and does not interact with the load converter, so only f r1 and f r2_i The impact of the interaction between the two resonant frequencies and the load converter on the system stability.

[0030] Furthermore, the f r1 and f r2_i The peak values ​​at the two resonant frequencies are Overlap occurs and the system becomes unstable:

[0031] ① When And the phase difference |φ(Z oS )-φ(Z inL )| is greater than 180°, at f r1 The instability problem of low-frequency oscillation occurs at f c Much larger than f r1 ;

[0032] ②When f r1 ≤f c ≤f r2_i , in the high frequency band, Z inL Shows inductive characteristics, even if Z inL With Z oS Overlapping occurs, and the phase difference |φ(Z oS )-φ(Z inL )| is always less than 180°, at f r2_i There is no oscillation problem.

[0033] ③When f c ≥f r2_i ,like If the opposite is true, the system will have multiple oscillation frequencies, so f r2_i Greater than the boundary frequency f r2_ib When , there is no high-frequency oscillation in the system, and the stability boundary of the system in the high-frequency band is

[0034]

[0035] Where Δ=(R l(n-i) +R l(n-i+1) )V2 bus / P o -R l(n-i) R l(n-i+1) .

[0036] Beneficial effects:

[0037] (1) The proposed method for determining the resonance point of a DC power distribution system takes into account the influence of line impedance when performing two-port small signal modeling, classifies different types of resonance points, improves the accuracy of the model, and fills the gap in multi-resonance analysis of DC systems.

[0038] (2) The present invention obtains the boundary conditions for system stability and the design boundary of the cutoff frequency of the load converter through the interaction between the peak values ​​of multiple resonance points and the input impedance of the load converter, thereby guiding the design of the converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative work.

[0040] Figure 1 It is a DC power distribution system topology with n source converters and m load converters operating in parallel;

[0041] Figure 2 is the Z with multiple resonance peaks oS With Z inL Bode plot of

[0042] Figure 3 This is the simulation diagram of the main waveforms of the DC power distribution system after considering the line impedance;

[0043] Figure 4 It is the Fourier analysis diagram of DC bus voltage.

[0044] Explanation of symbols in the figure: Z oSi is the output impedance of the i-th LC filter; Z inLj is the input impedance of the jth load converter; R fi , L fi and C fi They are parasitic resistance, inductance and capacitance respectively; R li and L li are line resistance and inductance respectively; R L is the load resistance; V ojis the output voltage of the jth load converter; V ref is the output voltage reference value; G v (s) and G m (s) are the transfer functions of the voltage loop PI controller and the PWM generator respectively; H v is the sampling factor. DETAILED DESCRIPTION

[0045] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] The technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific implementation methods.

[0047] A method for determining a resonance point of a DC power distribution system, characterized in that the method comprises the following steps:

[0048] Step 1, divide the multi-resonance frequency points into three types;

[0049] Step 2, find the parallel output impedance of the source converter corresponding to the three resonant frequencies;

[0050] like Figure 1 As shown in Figure 1, the typical topology of a multi-source DC power distribution system includes n source converters and m load converters. Since the output impedance of the source converter exhibits an LC characteristic, the source converter is replaced by an LC filter for simplified analysis.

[0051] Secondary loop gain T of DC power distribution system m It can be expressed as

[0052]

[0053] Among them, Z oS is the parallel impedance of the LC filter output impedance; Z inL is the parallel impedance of the load converter input impedance; V bus is the DC bus voltage;

[0054] The output impedance of the i-th LC filter is

[0055]

[0056] Z oS With Z inL The Bode plot is as follows Figure 2As shown in the figure, in the low frequency band, the output impedance of the LC filter presents an inductive characteristic, and the line inductance L li Much smaller than L fi Therefore, there is a low-frequency resonance frequency f formed by a parallel LC filter r1 .

[0057]

[0058] f r1 The corresponding resonance peak is

[0059]

[0060] In the high frequency band, the frequency is much greater than f r1 , LC filter exhibits capacitance characteristics. Assuming L l1 <L l2 <… <L ln and R l1 <R l2 <… <R ln , the interaction between the LC filter and the line impedance leads to n-1 high-frequency resonant frequencies

[0061]

[0062] The corresponding source converter parallel output impedance Z oS The peak value is

[0063]

[0064] A single LC filter and its line inductance L between the busbars li Mutual resonance produces n resonance points f r3_i like

[0065]

[0066] f r3_i Although resonance occurs at , the amplitude is small, showing the characteristics of valley value. Therefore, the corresponding source converter output impedance Z oS The valley value is

[0067] Z oS_vi =R li (8)

[0068] Step 3: Based on the interaction between the peak values ​​of the multi-resonance points and the input impedance of the load converter, the boundary conditions for system stability and the design boundary of the cutoff frequency of the load converter are obtained to guide the design of the converter.

[0069] According to the above analysis, there are three resonant frequencies in the system, but the resonant frequency shown in formula (7) presents a valley characteristic and will not interact with the load converter. Therefore, it is only necessary to study the influence of the interaction between the two resonant frequencies of formulas (3) and (5) and the load converter on the system stability.

[0070] The input impedance of the load converter is

[0071]

[0072] Among them, P oj (j=1,2,…,m) is the load power; T(s)=H v G v (s)G m (s)G vd_L (s) is the open-loop gain of the load converter using voltage loop control.

[0073] In the cut-off frequency range, |T(s)|>>1, so the parallel input impedance of m load converters is

[0074]

[0075] According to formula (10), multiple load converters connected in parallel can be equivalent to one load converter.

[0076] When the peak value of the resonant frequency in equations (3) and (5) is equal to Overlap occurs and the system becomes unstable:

[0077] when And the phase difference |φ(Z oS )-φ(Z inL )| is greater than 180°, at f r1 The instability problem of low-frequency oscillation occurs at f c Much larger than f r1 .

[0078] When f r1 ≤f c ≤f r2_i , in the high frequency band, Z inL Shows inductive characteristics, even if Z inL With Z oS Overlapping occurs, and the phase difference |φ(Z oS )-φ(Z inL )| is always less than 180°, at f r2_i There is no oscillation problem.

[0079] When f c ≥f r2_i ,like If the opposite is true, the system will have multiple oscillation frequencies, so fr2_i Greater than the boundary frequency f r2_ib When the system does not have high-frequency oscillation

[0080]

[0081] Where Δ=(R l(n-i) +R l(n-i+1) )V2 bus / P o -R l(n-i) R l(n-i+1) .

[0082] Application examples:

[0083] The following takes a simple DC power distribution system as an example and combines simulation results to verify the application effect of the technical solution of the present invention.

[0084] The simulation parameters are as follows:

[0085] Table 1: Main simulation parameters

[0086]

[0087] Figure 3 The main working waveform simulation diagram of the DC power distribution system with two LC filters and load converters running in parallel after considering the line impedance is shown in Table 1. It can be seen from the figure that there are obviously two oscillation frequencies in the DC bus. Figure 4 From the Fourier analysis in , we can see that the two oscillation frequencies in the DC bus voltage are 440 Hz and 1760 Hz, which are consistent with the judgment results of 459 Hz and 1678 Hz in equations (3) and (5).

[0088] In summary, the resonance point determination method applicable to a DC power distribution system of the present invention can accurately evaluate the multi-oscillation frequency on the DC bus through a resonance model.

[0089] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A method for determining a resonance point of a DC power distribution system, characterized in that: The method comprises the following steps: Step 1, divide the multi-resonance frequency points into three types; Step 2, find the parallel output impedance of the source converter corresponding to the three resonant frequencies; Step 3: According to the interaction between the peak values ​​of the multi-resonance points and the input impedance of the load converter, the boundary conditions for system stability and the design boundary of the cut-off frequency of the load converter are obtained to guide the design of the converter; The three multi-resonance frequency point expressions in step 1 are: The first type: low-frequency resonant frequency f formed by parallel connection of LC filters r1 , The second method: Assume that L l1 <L l2 <… <L ln and R l1 <R l2 <… <R ln , in the high frequency band, The parallel capacitor C in the LC filter fi Interacting with the line impedance generates n-1 high-frequency resonant frequencies f r2_i , i=1,2…n-1, The third type: the line inductance L between the i-th LC filter and its busbar li Mutual resonance produces n resonance points f r3_i , i=1,2…n, The low frequency resonance frequency f r1 The corresponding source converter parallel output impedance Z oS The peak value is: The high frequency resonance frequency f r2_i The corresponding source converter parallel output impedance Z oS The peak value is: The f r3_i The corresponding source converter output impedance Z oS The valley values ​​are: Z oS_vi =R li 。 2. A resonance point determination method applicable to a DC power distribution system according to claim 1, characterized in that: The input impedance of the load converter in step 3 is: Among them, P oj , j=1,2,…,m is the load power; T(s)=H v G v (s)G m (s)G vd_L (s) is the open loop gain of the load converter using voltage loop control; At the cut-off frequency f c Within the range, |T(s)|>>1, so the parallel input impedance of m load converters is: According to the above formula, multiple load converters connected in parallel are equivalent to one load converter.

3. A resonance point determination method applicable to a DC power distribution system according to claim 2, characterized in that: The f r3_i It exhibits a valley characteristic and does not interact with the load converter, so only f r1 and f r2_i The impact of the interaction between the two resonant frequencies and the load converter on the system stability.

4. A resonance point determination method applicable to a DC power distribution system according to claim 3, characterized in that: The f r1 and f r2_i The peak values ​​at the two resonant frequencies are Overlap occurs and the system becomes unstable: ① When And the phase difference |φ(Z oS )-φ(Z inL )| is greater than 180°, at f r1 The instability problem of low-frequency oscillation occurs at f c Much larger than f r1 ; ②When f r1 ≤f c ≤f r2_i , in the high frequency band, Z inL Shows inductive characteristics, even if Z inL With Z oS Overlapping occurs, and the phase difference |φ(Z oS )-φ(Z inL )| is always less than 180°, at f r2_i There is no oscillation problem. ③When f c >f r2_i ,like If the opposite is true, the system will have multiple oscillation frequencies, so f r2_i Greater than the boundary frequency f r2_ib When , there is no high-frequency oscillation in the system, and the stability boundary of the system in the high-frequency band is in,

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