Converter impedance measurement method based on grid-side capacitor switching

Abstract

The invention provides a method for measuring the impedance of a converter based on grid-side capacitor switching. The method is applied to a new energy grid-connected system which at least comprises a direct-current constant-voltage source, a converter, a fling-cut switch, a capacitor and an alternating-current power grid. One end of the fling-cut switch is connected between the PCC and an alternating-current power grid, and the other end is grounded through a capacitor; the method comprises the following steps: determining a to-be-measured frequency point; when the converter is in a first working condition and a second working condition respectively, obtaining three-phase voltage and three-phase current responded by the PCC when a positive sequence current disturbance signal with the same frequency as a to-be-measured frequency point is injected, so as to calculate a corresponding positive sequence impedance value of the converter at the to-be-measured frequency point; wherein the first working condition is that the switching switch is switched on, and the converter stably operates in a scene of a specified working state; the second working condition is that the fling-cut switch is switched off, and the converter stably operates in a scene in a working state equivalent to the specified working state. According to the invention, the method can replace a conventional impedance measurement device for measurement, thereby achieving the purposes of reducing the cost and reducing the operation complexity.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for measuring impedance of a converter based on grid-side capacitor switching. Background Art

[0002] At present, the world is in a stage of rapid development of productivity, resulting in a continuous increase in global energy consumption. In the energy structure consumed by humans, traditional non-renewable resources still maintain a dominant position. With the development of the world, non-renewable energy will inevitably be depleted at an accelerated rate, and an energy crisis may be faced. In order to cope with the problem of accelerated consumption of non-renewable energy, various parts of the world have successively carried out energy transformation, vigorously developed renewable energy (ie, new energy), and continuously promoted the transformation of non-renewable energy to new energy. Among them, electric energy, as a clean and efficient energy, will become the main force in the development of new energy. In recent years, more and more electric energy converted from renewable energy such as wind energy and solar energy has been incorporated into the power system through power electronic equipment such as inverters, resulting in the current power system showing the characteristics of "high proportion of new energy and high proportion of power electronic equipment", referred to as "double high" characteristics. In the "double high" power system, the strength of the AC power grid is relatively weak, which will lead to broadband oscillation problems between new inverters and between the inverter and the power grid, affecting the stable operation of the new energy grid-connected system.

[0003] At present, the impedance method based on frequency domain is often used to analyze the new energy grid-connected system. The key to this method is to obtain the output impedance of the converter. In actual engineering, impedance measurement equipment is often used to measure the external characteristics of the converter port. Since there is no need to obtain the specific internal structure of the equipment, it is completely regarded as a "black box". However, impedance measurement equipment often requires huge investment and complex operation.

[0004] Therefore, it is necessary to propose a new method for obtaining the output impedance of the converter to replace the traditional impedance measurement equipment to measure the method, so as to achieve the purpose of reducing costs and operating complexity. Summary of the invention

[0005] The technical problem to be solved by the embodiments of the present invention is to provide a method for measuring the impedance of a converter based on grid-side capacitor switching, which can replace the measurement method using traditional impedance measurement equipment, thereby achieving the purpose of reducing costs and operation complexity.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a method for measuring the impedance of a converter based on grid-side capacitor switching, which is used in a new energy grid-connected system comprising at least a DC constant voltage source, a converter, a switching switch, a capacitor and an AC power grid; wherein the DC constant voltage source is connected to the converter and then connected to the AC power grid via a designated grid connection point PCC; one end of the switching switch is connected between the grid connection point PCC and the AC power grid, and the other end is grounded after passing through the capacitor; the method comprises the following steps:

[0007] Determine the frequency point to be tested;

[0008] When the converter is in the first operating condition and the second operating condition respectively, the three-phase voltage and three-phase current of the grid connection point PCC in response to the positive-sequence current disturbance signal with the same frequency as the frequency point to be measured are obtained to calculate the positive-sequence impedance value of the converter corresponding to the frequency point to be measured; wherein, the first operating condition is a scenario in which the converter stably operates in a specified working state when the switching switch is closed to connect the capacitor; and the second operating condition is a scenario in which the converter stably operates in a working state equal to the specified working state when the switching switch is opened to cut off the connection of the capacitor.

[0009] Among them, through the formula Calculate the positive sequence impedance value Z of the converter corresponding to the frequency point to be measured 0+ ;in,

[0010] U p1 =U pa1 +aU pb1 +a 2 U pc1 , U p2 =U pa2 +aU pb2 +a 2 U pc2 ,I p1 =I pa1 +aI pb1 +a 2 I pc1 and I p2 =I pa2 +aI pb2 +a 2 I pc2 ;

[0011] (U pa1 ,U pb1 ,U pc1 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the positive sequence current disturbance signal injected into the converter in the first working condition; (I pa1 ,Ipb1 ,I pc1 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the first operating condition and the positive sequence current disturbance signal is injected;

[0012] (U pa2 ,U pb2 ,U pc2 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the positive sequence current disturbance signal injected into the converter in the second working condition; (I pa2 ,I pb2 ,I pc2 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the second operating condition and the positive sequence current disturbance signal is injected;

[0013]

[0014] Among them, the expression of the positive sequence current disturbance signal is: in,

[0015] I p is the amplitude, f p is the frequency, (Δi pa ,Δi pb ,Δi pc ) correspond to the three phases ABC respectively.

[0016] Wherein, the method further comprises:

[0017] When the converter is in the first operating condition and the second operating condition respectively, the three-phase voltage and three-phase current of the grid-connected point PCC in response to the negative-sequence current disturbance signal with the same frequency as the frequency point to be measured are obtained to calculate the negative-sequence impedance value of the converter corresponding to the frequency point to be measured.

[0018] Among them, through the formula Calculate the negative sequence impedance value Z of the converter corresponding to the frequency point to be measured 0- ;in,

[0019] U q1 =U qa1 +β 2 U qb1 +βU qc1 , U q2 =U qa2 +β 2 U qb2 +βU qc2 ,I q1=I qa1 +β 2 I qb1 +βI qc1 and I q2 =I qa2 +β 2 I qb2 +βI qc2 ;

[0020] (U qa1 ,U qb1 ,U qc1 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the negative sequence current disturbance signal injected into the converter in the first working condition; (I qa1 ,I qb1 ,I qc1 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the first operating condition and the negative sequence current disturbance signal is injected;

[0021] (U qa2 ,U qb2 ,U qc2 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the negative sequence current disturbance signal injected into the converter in the second working condition; (I qa2 ,I qb2 ,I qc2 ) is the three-phase current obtained after discrete Fourier transform is performed on the three-phase current that the grid connection point PCC responds to when the negative sequence current disturbance signal is injected into the converter in the second working condition;

[0022]

[0023] Among them, the expression of the negative sequence current disturbance signal is: in,

[0024] I q is the amplitude, f q is the frequency, (Δi qa ,Δi qb ,Δi qc ) correspond to the three phases ABC respectively.

[0025] Wherein, the DC constant voltage source is an energy storage capacitor of the new energy power grid.

[0026] Implementing the embodiments of the present invention has the following beneficial effects:

[0027] Compared with the measurement method of traditional impedance measurement equipment, the present invention aims to change the access scenario of the grid-side capacitor (such as the first operating condition and the second operating condition) by controlling the switching of the switching switch, and superimpose virtual positive and negative sequence disturbance current signals on the AC sampling signal of the converter in different scenarios, so as to measure the small signal voltage and current generated by the response of the grid connection point PCC (Point of Common Coupling), thereby calculating the positive and negative sequence impedance of the converter, making the measurement method more convenient and low-cost, and can achieve online measurement, which is very easy to promote. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, without paying creative labor, other drawings obtained based on these drawings still belong to the scope of the present invention.

[0029] Figure 1 A flow chart of a method for measuring impedance of a converter based on grid-side capacitor switching provided by an embodiment of the present invention;

[0030] Figure 2 A simplified circuit diagram of a new energy grid-connected system in a method for measuring converter impedance based on grid-side capacitor switching provided by an embodiment of the present invention;

[0031] Figure 3 A measurement logic structure diagram when a sampling signal is superimposed with a virtual current disturbance signal in a converter impedance measurement method based on grid-side capacitor switching provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings.

[0033] like Figure 1 As shown in the figure, a method for measuring impedance of a converter based on grid-side capacitor switching is proposed in an embodiment of the present invention, which is used to at least include a DC constant voltage source V dc (such as energy storage capacitors of new energy grids), converter VSC, switching switch K, capacitor C and AC grid u g On the new energy grid-connected system, such as Figure 2 shown in Figure 2 In the example, the DC constant voltage source V dc After connecting the converter VSC, it is connected to the AC grid through the designated grid connection point PCC. g One end of the switching switch K is connected to the grid connection point PCC and the AC grid u gThe other end is grounded after passing through a capacitor C; the method comprises the following steps:

[0034] Step S1, determining the frequency point to be measured;

[0035] Step S2, when the converter is in the first working condition and the second working condition respectively, the three-phase voltage and three-phase current of the grid connection point PCC when a positive sequence current disturbance signal with the same frequency as the frequency point to be measured is injected, so as to calculate the positive sequence impedance value of the converter corresponding to the frequency point to be measured; wherein, the first working condition is a scenario in which the converter stably operates in a specified working state when the switching switch is closed to connect the capacitor; and the second working condition is a scenario in which the converter stably operates in a working state equal to the specified working state when the switching switch is opened to cut off the connection of the capacitor;

[0036] Step S3, when the converter is in the first operating condition and the second operating condition respectively, the three-phase voltage and three-phase current of the grid connection point PCC in response to the negative-sequence current disturbance signal with the same frequency as the frequency point to be measured are obtained, so as to calculate the negative-sequence impedance value of the converter corresponding to the frequency point to be measured.

[0037] The specific process is as follows: in step S1, a plurality of frequency points to be measured are defined to form a frequency set, and one of the frequency points to be measured is randomly selected from the frequency set to perform transformer impedance measurement.

[0038] In step S2, the first step is: first, close the switching switch K to connect the capacitor C, so that the converter VSC stably operates in the specified working state scenario, that is, the converter VSC is in the first working condition, and then inject a positive-sequence disturbance current signal with the same frequency as the frequency point to be measured into the current sampling signal of the grid connection point PCC, measure it through the voltage and current sensors, and record the three-phase voltage and three-phase current that the grid connection point PCC responds to when the positive-sequence current disturbance signal is injected. Secondly, the three-phase voltage and three-phase current measured and recorded above are all subjected to discrete Fourier transformation to realize the discrete Fourier transformation of the three-phase voltage and three-phase current that the grid connection point PCC responds to when the positive-sequence current disturbance signal is injected when the converter VSC is in the first working condition, and obtain the three-phase voltage (U pa1 ,U pb1 ,U pc1 ) and three-phase current (I pa1 ,I pb1 ,I pc1 ). Then, through the formula U p1 =U pa1 +aU pb1 +a 2 U pc1 and I p1 =Ipa1 +aI pb1 +a 2 I pc1 , and the positive sequence voltage U under the corresponding first operating condition is obtained p1 and positive sequence current I p1 .in,

[0039] It should be noted that the positive sequence current disturbance signal Δi p The expression is Among them, I p is the amplitude, f p is the frequency, (Δi pa ,Δi pb ,Δi pc ) correspond to the three phases ABC respectively.

[0040] At this time, if Figure 3 As shown in the figure, the measurement logic structure diagram when a virtual current disturbance signal (such as a positive sequence current disturbance signal) is superimposed on the sampling signal, and the controller is regarded as a completely unknown black box, where Δi abc For the stack

[0041] The added disturbance signal, i abc It is the sampling current signal after superimposing the positive sequence current disturbance signal.

[0042] Step 2: First, disconnect the switching switch K to cut off the connection of the capacitor C, so that the converter VSC converter can stably operate in a scenario of a working state equal to the specified working state in the first working condition, that is, the converter VSC is in the second working condition, and then inject a positive-sequence disturbance current signal with the same frequency as the frequency point to be measured into the current sampling signal of the grid connection point PCC (that is, the positive-sequence disturbance current signal equivalent to that injected in the first working condition), measure it through voltage and current sensors, and record the three-phase voltage and three-phase current to which the grid connection point PCC responds when the above-mentioned positive-sequence current disturbance signal is injected. Secondly, the three-phase voltage and three-phase current measured and recorded above are subjected to discrete Fourier transform to realize discrete Fourier transform of the three-phase voltage and three-phase current to which the grid connection point PCC responds when the above-mentioned positive-sequence current disturbance signal is injected when the converter VSC is in the second working condition, and obtain the three-phase voltage (U pa2 ,U pb2 ,U pc2 ) and three-phase current (I pa2 ,I pb2 ,I pc2 ). Then, through the formula U p2 =U pa2 +aU pb2 +a 2 U pc2 and I p2 =Ipa2 +aI pb2 +a 2 I pc2 , and the positive sequence voltage U under the corresponding second operating condition is obtained p2 and positive sequence current I p2 .

[0043] Finally, through the formula Calculate the positive sequence impedance Z of the converter VSC corresponding to the frequency point to be measured 0+ It should be noted that the above formula is a vector calculation formula, and the parameters involved in the calculation include amplitude and phase angle.

[0044] In step S3, the first step is: first, close the switching switch K to connect the capacitor C, so that the converter VSC stably operates in the specified working state scenario, that is, the converter VSC is in the first working condition, and then inject a negative-sequence disturbance current signal with the same frequency as the frequency point to be measured into the current sampling signal of the grid connection point PCC, measure it through the voltage and current sensors, and record the three-phase voltage and three-phase current of the grid connection point PCC when the above-mentioned negative-sequence current disturbance signal is injected. Secondly, the three-phase voltage and three-phase current measured and recorded above are all subjected to discrete Fourier transformation to realize the discrete Fourier transformation of the three-phase voltage and three-phase current of the grid connection point PCC when the converter VSC is in the first working condition and the above-mentioned negative-sequence current disturbance signal is injected, and obtain the corresponding transformed three-phase voltage (U qa1 ,U qb1 ,U qc1 ) and three-phase current (I qa1 ,I qb1 ,I qc1 ). Then, through the formula U q1 =U qa1 +β 2 U qb1 +βU qc1 and I q1 =I qa1 +β 2 I qb1 +βI qc1 , and the negative sequence voltage U under the corresponding first operating condition is obtained q1 and negative sequence current I q1 .in,

[0045] It should be noted that the negative sequence current disturbance signal Δi q The expression is Among them, I q is the amplitude, f q is the frequency, (Δi qa ,Δi qb ,Δi qc) correspond to the three phases ABC respectively.

[0046] Step 2: First, disconnect the switching switch K to cut off the connection of the capacitor C, so that the converter VSC converter stably operates in a scenario of a working state equal to the specified working state in the first working condition, that is, the converter VSC is in the second working condition, and then inject a negative-sequence disturbance current signal with the same frequency as the frequency point to be measured into the current sampling signal of the grid connection point PCC (that is, the negative-sequence disturbance current signal equivalent to that injected in the first working condition), measure it through the voltage and current sensors, and record the three-phase voltage and three-phase current to which the grid connection point PCC responds when the above-mentioned negative-sequence current disturbance signal is injected. Secondly, the three-phase voltage and three-phase current measured and recorded above are subjected to discrete Fourier transform to realize discrete Fourier transform of the three-phase voltage and three-phase current to which the grid connection point PCC responds when the above-mentioned negative-sequence current disturbance signal is injected when the converter VSC is in the second working condition, and obtain the three-phase voltage (U qa2 ,U qb2 ,U qc2 ) and three-phase current (I qa2 ,I qb2 ,I qc2 ). Then, through the formula U q2 =U qa2 +β 2 U qb2 +βU qc2 and I q2 =I qa2 +β 2 I qb2 +βI qc2 , and the negative sequence voltage U under the corresponding second operating condition is obtained q2 and negative sequence current I q2 .

[0047] Finally, through the formula Calculate the negative sequence impedance Z of the converter VSC corresponding to the frequency point to be measured 0- It should be noted that the above formula is a vector calculation formula, and the parameters involved in the calculation include amplitude and phase angle.

[0048] In summary, by analogy, steps S1 to S3 are repeated to complete the measurement of the positive and negative sequence impedance values ​​of the converter VSC at all the frequency points to be measured.

[0049] based on Figure 2 , the working principle of a method for measuring impedance of a converter based on grid-side capacitor switching in an embodiment of the present invention is further described as follows:

[0050] Assume the reference current of the converter VSC is I dref , the current closed-loop transfer function of the converter VSC is expressed as G i(s), then i0=I dref G i (s).

[0051] according to Figure 2 , you can list:

[0052]

[0053] u PCC (s) = Z g (s)i g (s)+u g (s)(2);

[0054] After injecting the disturbance Δi into the sampled signal, I dref For I dref -Δi, the new relationship after superposition is:

[0055]

[0056] u PCC1 (s) = Z g (s)i g1 (s)+u g (s)(4);

[0057] By changing the capacitance on the grid side and superimposing the same disturbance Δi, we get:

[0058]

[0059] u PCC2 (s) = Z g '(s)i g2 (s)+u g (s)(6);

[0060] Subtracting equation (3) from equation (5) yields:

[0061]

[0062] Right now:

[0063]

[0064] It can be concluded that by carrying out two identical disturbance experiments at the same frequency point under different grid impedances, the grid-connected impedance of the converter can be calculated by equation (8).

[0065] Implementing the embodiments of the present invention has the following beneficial effects:

[0066] Compared with the measurement method of traditional impedance measurement equipment, the present invention aims to change the access scenario of the grid-side capacitor (such as the first operating condition and the second operating condition) by controlling the switching of the switching switch, and superimpose virtual positive and negative sequence disturbance current signals on the AC sampling signal of the converter in different scenarios, so as to measure the small signal voltage and current generated by the PCC response of the grid connection point, thereby calculating the positive and negative sequence impedance of the converter, making the measurement method more convenient and low-cost, and can achieve online measurement, which is very easy to promote.

[0067] A person skilled in the art can understand that all or part of the steps in the above-mentioned embodiment method can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium, such as ROM / RAM, disk, CD-ROM, etc.

[0068] The above disclosure is only the preferred embodiment of the present invention, which certainly cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for measuring converter impedance based on grid-side capacitor switching, characterized in that: Used in a new energy grid-connected system comprising at least a DC constant voltage source, a converter, a switching switch, a capacitor and an AC grid; wherein the DC constant voltage source is connected to the converter and then connected to the AC grid via a designated grid connection point PCC; one end of the switching switch is connected between the grid connection point PCC and the AC grid, and the other end is connected to ground via the capacitor; the method comprises the following steps: Determine the frequency point to be tested; When the converter is in the first operating condition and the second operating condition respectively, the three-phase voltage and three-phase current of the grid connection point PCC in response to the positive-sequence current disturbance signal with the same frequency as the frequency point to be measured are obtained to calculate the positive-sequence impedance value of the converter corresponding to the frequency point to be measured; wherein, the first operating condition is a scenario in which the converter stably operates in a specified working state when the switching switch is closed to connect the capacitor; and the second operating condition is a scenario in which the converter stably operates in a working state equal to the specified working state when the switching switch is opened to cut off the connection of the capacitor.

2. The method for measuring impedance of a converter based on grid-side capacitor switching according to claim 1, characterized in that: By formula Calculate the positive sequence impedance value Z of the converter corresponding to the frequency point to be measured 0+ ; in, U p1 = U pa1 + aU pb1 + a 2 U pc1 、U p2 = U pa2 + aU pb2 + a 2 U pc2 、I p1 = I pa1 + aI pb1 + a 2 I pc1 及I p2 = I pa2 + aI pb2 + a 2 I pc2 ; (U pa1 ,U pb1 ,U pc1 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the positive sequence current disturbance signal injected into the converter in the first working condition; (I pa1 ,I pb1 ,I pc1 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the first operating condition and the positive sequence current disturbance signal is injected; (U pa2 ,U pb2 ,U pc2 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the positive sequence current disturbance signal injected into the converter in the second working condition; (I pa2 ,I pb2 ,I pc2 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the second operating condition and the positive sequence current disturbance signal is injected; 3. The method for measuring impedance of a converter based on grid-side capacitor switching according to claim 2, characterized in that: The expression of the positive sequence current disturbance signal is: in, I p is the amplitude, f p is the frequency, (Δi pa ,Δi pb ,Δi pc ) correspond to the three phases ABC respectively.

4. The method for measuring impedance of a converter based on grid-side capacitor switching according to claim 1, characterized in that: The method further comprises: When the converter is in the first operating condition and the second operating condition respectively, the three-phase voltage and three-phase current of the grid-connected point PCC in response to the negative-sequence current disturbance signal with the same frequency as the frequency point to be measured are obtained to calculate the negative-sequence impedance value of the converter corresponding to the frequency point to be measured.

5. The method for measuring impedance of a current transformer based on grid-side capacitor switching according to claim 4, characterized in that: By formula Calculate the negative sequence impedance value Z of the converter corresponding to the frequency point to be measured 0- ; in, U q1 = U qa1 + β 2 U qb1 + βU qc1 、U q2 = U qa2 + β 2 U qb2 + βU qc2 、I q1 = I qa1 + β 2 I qb1 + βI qc1 and I q2 =I qa2 +β 2 I qb2 +βI qc2 ; (U qa1 ,U qb1 ,U qc1 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the negative sequence current disturbance signal injected into the converter in the first working condition; (I qa1 ,I qb1 ,I qc1 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the first operating condition and the negative sequence current disturbance signal is injected; (U qa2 ,U qb2 ,U qc2 ) is the three-phase voltage obtained after discrete Fourier transform is performed on the three-phase voltage obtained when the grid connection point PCC responds to the negative sequence current disturbance signal injected into the converter in the second working condition; (I qa2 ,I qb2 ,I qc2 ) is the three-phase current obtained after performing discrete Fourier transformation on the three-phase current that the grid connection point PCC responds to when the converter is in the second operating condition and the negative sequence current disturbance signal is injected; 6. The method for measuring impedance of a current transformer based on grid-side capacitor switching according to claim 5, characterized in that: The expression of the negative sequence current disturbance signal is: in, I q is the amplitude, f q is the frequency, (Δi qa ,Δi qb ,Δi qc ) correspond to the three phases ABC respectively.

7. The method for measuring impedance of a converter based on grid-side capacitor switching according to claim 6, characterized in that: The DC constant voltage source is an energy storage capacitor of the new energy power grid.

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