CN110829489B - Estimation method for disturbance-free signal injection weak power grid and series compensation power grid - Google Patents
Estimation method for disturbance-free signal injection weak power grid and series compensation power grid Download PDFInfo
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Abstract
The invention discloses a disturbance-free signal injection weak power grid and series compensation power grid estimation method. According to the method, disturbance signals do not need to be injected, the weak power grid and the series compensation power grid are estimated only by detecting the voltage and the current of the PCC points, and then the method is used for self-adaptive control of the inverter, stability of a grid-connected system is improved, and meanwhile cost is not increased.
Description
Technical Field
The invention belongs to the field of power grid quality analysis and signal processing, and particularly relates to a disturbance-free signal injection weak power grid and series compensation power grid estimation method.
Background
With the fact that new energy power generation such as photovoltaic power generation and wind power generation occupies larger and larger proportion in a power system and the fact that the local consumption capacity of the new energy power generation is limited, long-distance power transmission is needed, line inductance is increased, and in order to improve power transmission capacity, a series compensation capacitor can be connected into a power transmission line, so that the power grid is in a weak power grid state and a series compensation power grid state.
When the power grid is switched between a weak power grid state and a series compensation power grid state, for an inverter which cannot estimate the power grid state, a grid-connected system has the phenomenon of insufficient stability margin, and finally grid-connected current is diverged to cause the shutdown protection of the inverter. Therefore, the estimation of the weak power grid and the series compensation power grid has very important significance for the stable operation of the inverter.
Currently, there are many academic papers on estimation of weak grid and series compensation grid, for example:
1. the title is a power grid impedance estimation method based on a complex filter and non-characteristic subharmonic injection, which is an article on pages 2796-2801 of No. 10 of 2013 in the technical field of power grids. The method for estimating the power grid impedance by injecting the non-characteristic subharmonic and extracting the injection harmonic response by the complex filter is adopted, but the working condition that the stability margin of the original system is low and disturbance cannot be injected is not considered.
2. An article entitled "Grid impedance detection and adaptive control of converters", a.ghanem, m.rashed, m.Sumner, m.a.Elsayes, and I.I.I.Mansy, IET Power Electronics, 2017:1279 [ "inverter island detection and adaptive control based on Grid impedance estimation", "IET institute of Electrical and Electronics, published in 2017 network") estimates the Grid impedance using the circuit equation when the switch tube is turned on and off by sampling the switching frequency level voltage and current, but requires high precision sensor sampling, is not beneficial to engineering applications, and weak Grid inductance and series compensation Grid capacitance are both imaginary parts in the impedance expression, and are mixed together, so that the weak Grid and series compensation Grid cannot be estimated.
3. The invention discloses a series compensation sensor tester and a test method which are disclosed in 2019, 10, 8 and 8 of Chinese patent document (publication No. CN 110308353A). The invention provides the series compensation sensor tester and the test method.
In view of the above documents, the prior art has the following disadvantages:
1. the existing disturbance injection-based weak grid impedance estimation method does not consider the working condition that the original system contains harmonic waves and disturbance can not be injected, and needs to research the estimation of a weak grid and a series compensation grid when disturbance injection is not carried out;
2. in the existing method for estimating the impedance of the weak power grid by using the self harmonic wave of the system, the inductive reactance of the weak power grid and the capacitive reactance of the series compensation power grid are both imaginary parts in an impedance expression, so that the weak power grid and the series compensation power grid cannot be estimated;
3. in the existing research on the estimation aspect of the series compensation power grid, external independent detection equipment and series compensation sensors on lines are adopted, so that the cost is increased, and meanwhile, whether the series compensation is connected in a power transmission line or not can only be determined, and the weak power grid cannot be estimated.
Disclosure of Invention
The invention provides a disturbance signal injection-free weak power grid and series compensation power grid estimation method, which is used for estimating a weak power grid and a series compensation power grid only by detecting the voltage and the current of a PCC point without injecting disturbance signals, and further is used for self-adaptive control of an inverter, the stability of a grid-connected system is improved, and meanwhile, the cost is not increased.
The object of the invention is thus achieved. The invention provides a disturbance-free signal injection weak grid and series compensation grid estimation method.A main circuit topological structure of a grid-connected inverter applying the method comprises a direct-current side voltage source, a three-phase full-bridge inverter circuit, a three-phase LC filter, three-phase line impedance and a three-phase grid, wherein the direct-current side voltage source is connected with the three-phase full-bridge inverter circuit, and the three-phase full-bridge inverter circuit is connected with the three-phase line impedance through the three-phase LC filter and then is connected into the three-phase grid;
the estimation method comprises the following steps:
Step (ii) of4, calculating initial phase differences of the harmonic voltage phase and the harmonic current phase of the N frequency points x selected in the step 3, and marking any one of the initial phase differences as an initial phase difference Delt _ Phax：
When Mag _ UALxIf k is greater than Std _ U, Delt _ Phax＝Pha_UALx-Pha_IALx；
When Mag _ IALxIf k is greater than Std _ I, Delt _ Phax＝Pha_UALx-Pha_IALx；
When Mag _ UALxk.times.Std _ U and Mag _ IAL ≦ k.times.Std _ U andxnot more than kXStd _ I, then Delt _ Phax＝0°；
Wherein k is a proportionality coefficient;
When Delt _ PhaxIs greater than 180 DEG, Delt _ Pha'x＝Delt_Phax-360°；
When Delt _ Phax< -180 deg., then Delt _ Pha'x＝Delt_Phax+360°；
When the angle is less than or equal to minus 180 degrees and less than PhaxLess than or equal to 180 degrees, then Delt _ Pha'x＝Delt_Phax；
in the state 2, if the condition of the state 1 is not satisfied, the absolute value | Delt _ Pha 'of the phase difference is output from N'xFinding out absolute values | Delt _ Pha' of all output phase differences of 45 DEG or more in |.xL and outputting the phase difference Delt _ Pha'xAnd after summing, taking an average value to obtain an average phase difference y:
when y is more than 0 degrees, the grid is a weak grid;
when y is 0 degrees, a strong current network is formed;
and when y is less than 0 degree, the series compensation power grid is established.
Compared with the prior art, the invention has the beneficial effects that:
1. according to the disturbance-free signal injection weak power grid and series compensation power grid estimation method, the weak power grid and the series compensation power grid can be accurately estimated by detecting the voltage and current of the PCC point, and then the method is used for self-adaptive control of an inverter and improves the stability of a grid-connected system;
2. according to the method, the weak power grid and the series compensation power grid are estimated by analyzing the characteristics of the voltage and current amplitude phase angle of the PCC point and calculating the phase difference of the harmonic voltage and current, so that the problem that the weak power grid and the series compensation power grid are difficult to estimate when the original system cannot inject disturbance is solved;
3. the disturbance-free signal injection weak power grid and series compensation power grid estimation method only improves the algorithm of the existing power electronic converter system, does not need to add extra power electronic equipment, reduces power consumption and saves cost.
Drawings
Fig. 1 is a topological structure diagram of a main circuit of a grid-connected inverter applying the method.
Fig. 2 is a waveform of an estimation result of switching a strong power grid into a weak power grid and then into a series compensation power grid according to an embodiment of the present invention.
Fig. 3 is an estimation result waveform of the series compensation power grid switched to the weak power grid first and then to the strong power grid according to the embodiment of the present invention.
Fig. 4 is an estimation result waveform of switching the strong power grid into the series compensation power grid and then into the strong power grid according to the embodiment of the present invention.
Detailed Description
The present embodiment will be described in detail below with reference to the accompanying drawings.
Fig. 1 is a topology structure diagram of a main circuit of a grid-connected inverter to which the present invention is applied, and as can be seen, the topology structure includes a dc-side voltage source 10, a three-phase full-bridge inverter circuit 20, a three-phase LC filter 30, a three-phase line impedance 40, and a three-phase grid 50.
The direct current side voltage source 10 is connected with a three-phase full-bridge inverter circuit 20, and the three-phase full-bridge inverter circuit 20 is connected with a three-phase line impedance 40 through a three-phase LC filter 30 and then is connected to a three-phase power grid 50. In FIG. 1, VdcIs a DC side voltage source 10, LfIs a bridge arm side inductance, C, of a three-phase LC filter 30fA filter capacitor of the three-phase LC filter 30, R a passive damping resistor of the three-phase LC filter 30, RgIs the resistance, L, in the three-phase line impedance 40gIs the inductance, C, in the three-phase line impedance 40gGrid is the capacitance in the three-phase line impedance 40 and is the three-phase Grid 50.
The main circuit parameters in this embodiment are: voltage V at DC sidedc800V, 380V/50Hz of rated output line voltage of the inverter, 100kW of rated power of the inverter and L of filter inductancef0.56mH, filter capacitance CfAnd the passive damping resistor r is 270uF/0.3 omega, and the line inductance L is high in the power gridg0.1mH, line resistance R in weak gridgInductor Lg0.091 omega/2.07 mH, and line resistance R in series compensation of the power gridgInductor LgCapacitor CgIt is 0.091 omega/2.07 mH/16 mF.
The estimation method comprises the following steps:
In this embodiment, 25 analysis frequency points, that is, N is 25, are selected. The difference between the frequencies at each analysis frequency point is 1HZ, i.e. the frequency at frequency point 1 is 1HZ, and the frequency at frequency point 25 is 25 HZ.
When Mag _ UALxIf k is greater than Std _ U, Delt _ Phax＝Pha_UALx-Pha_IALx；
When Mag _ IALxIf k is greater than Std _ I, Delt _ Phax＝Pha_UALx-Pha_IALx；
When Mag _ UALxk.times.Std _ U and Mag _ IAL ≦ k.times.Std _ U andxnot more than kXStd _ I, then Delt _ Phax＝0°；
Where k is a proportionality coefficient, and in the present embodiment, k is 0.01.
When Delt _ PhaxIs greater than 180 DEG, Delt _ Pha'x＝Delt_Phax-360°；
When Delt _ Phax< -180 deg., then Delt _ Pha'x＝Delt_Phax+360°；
When the angle is less than or equal to minus 180 degrees and less than PhaxLess than or equal to 180 degrees, then Delt _ Pha'x＝Delt_Phax。
in the state 2, if the condition of the state 1 is not satisfied, the absolute value | Delt _ Pha 'of the phase difference is output from N'xFinding out absolute values | Delt _ Pha' of all output phase differences of 45 DEG or more in |.xL and outputting the phase difference Delt _ Pha'xAnd after summing, taking an average value to obtain an average phase difference y:
when y is more than 0 degrees, the grid is a weak grid;
when y is 0 degrees, a strong current network is formed;
and when y is less than 0 degree, the series compensation power grid is established.
Fig. 2 is an estimation result waveform of the embodiment of the invention, in which a strong power grid is first cut into a weak power grid and then cut into a series compensation power grid. As can be seen from the graph, the strong grid is switched to the weak grid at 2.2s, the voltage-current phase difference changes from 0 to a positive value, and the weak grid is switched to the series compensation grid at 5.1s, the voltage-current phase difference changes from a positive value to a negative value.
Fig. 3 is an estimation result waveform of the series compensation power grid switched to the weak power grid first and then to the strong power grid according to the embodiment of the present invention. As can be seen from the graph, the series compensation grid is switched to the weak grid at 2.2s, the voltage-current phase difference changes from a negative value to a positive value after 1s of transition time, and the weak grid is switched to the strong grid at 5.2s, and the voltage-current phase difference changes from a positive value to 0 after 1s of transition time.
Fig. 4 is an estimation result waveform of switching the strong power grid into the series compensation power grid and then into the strong power grid according to the embodiment of the present invention. As can be seen from the graph, the voltage-current phase difference changes from 0 to a negative value when the strong power grid is switched to the series compensation grid at 2.2s, and changes from a negative value to 0 when the series compensation grid is switched to the strong power grid at 5.1s and the voltage-current phase difference changes from a negative value to 0 after 1s of transition time.
Fig. 2, fig. 3 and fig. 4 prove that the weak grid and the series compensation grid can be estimated accurately by the method.
Claims (1)
1. A method for estimating a weak grid and a series compensation grid without disturbance signal injection is characterized in that a main circuit topological structure of a grid-connected inverter applying the method comprises a direct-current side voltage source (10), a three-phase full-bridge inverter circuit (20), a three-phase LC filter (30), a three-phase line impedance (40) and the three-phase grid (50), wherein the direct-current side voltage source (10) is connected with the three-phase full-bridge inverter circuit (20), and the three-phase full-bridge inverter circuit (20) is connected with the three-phase line impedance (40) through the three-phase LC filter (30) and then is connected with the three-phase grid (50);
the method is characterized by comprising the following steps:
step 1, sampling the voltage of the capacitor output end of the three-phase LC filter (30) and recording the voltage as the voltage U of a PCC pointPCC_A，UPCC_B，UPCC_CThe current flowing through the three-phase line impedance (40) is sampled and recorded as the PCC point current IPCC_A，IPCC_B，IPCC_C；
Step 2, the PCC point voltage U acquired in the step 1 is processedPCC_A，UPCC_B，UPCC_CAnd PCC point current IPCC_A，IPCC_B，IPCC_CAt the fundamental frequency point pair UPCC_AAnd IPCC_APerforming Fourier transform to obtain an A-phase fundamental wave voltage amplitude Std _ U and an A-phase fundamental wave current amplitude Std _ I;
step 3, selecting N frequency points except the fundamental frequency point as analysis frequency points, and marking any one of the N frequency points as a frequency point x, wherein x is a serial number of the analysis frequency points arranged according to the frequency, and x is 1, 2. Then, x pairs U at N frequency pointsPCC_AAnd IPCC_AThe following parameters were obtained by performing fourier transform: n A-phase harmonic voltage amplitudes are recorded as the A-phase harmonic voltage amplitude Mag _ UALxN A-phase harmonic voltage phases and marking any one of the N A-phase harmonic voltage phases as an A-phase harmonic voltage phase Pha _ UALxN A-phase harmonic current amplitudes and marking any one of the N A-phase harmonic current amplitudes as an A-phase harmonic current amplitude Mag _ IALxAnd N A-phase harmonic current phases and any one of them is taken as the A-phase harmonic current phase Pha _ IALx；
Step 4, calculating initial phase differences of the harmonic voltage phase and the harmonic current phase of the N frequency points x selected in the step 3, and marking any one of the initial phase differences as an initial phase difference Delt _ Phax：
When Mag _ UALxIf k is greater than Std _ U, Delt _ Phax＝Pha_UALx-Pha_IALx；
When Mag _ IALxIf k is greater than Std _ I, Delt _ Phax＝Pha_UALx-Pha_IALx；
When Mag _ UALxk.times.Std _ U and Mag _ IAL ≦ k.times.Std _ U andxnot more than kXStd _ I, then Delt _ Phax＝0°；
Wherein k is a proportionality coefficient;
step 5, the N initial phase differences Delt _ Pha obtained in the step 4xN output phase differences are obtained according to the following rule and any one of the N output phase differences is expressed as an output phase difference Delt _ Pha'x：
When Delt _ PhaxIs greater than 180 DEG, Delt _ Pha'x＝Delt_Phax-360°；
When Delt _ Phax< -180 deg., then Delt _ Pha'x＝Delt_Phax+360°；
When the angle is less than or equal to minus 180 degrees and less than PhaxLess than or equal to 180 degrees, then Delt _ Pha'x＝Delt_Phax；
Step 6, the N output phase differences Delt _ Pha 'obtained in the step 5 are respectively compared'xObtaining absolute values to obtain N absolute values of output phase differences | Delt _ Pha'xAnd then compares the output phase difference absolute value | Delt _ Pha 'one by one'xAnd | and 45 degrees, and judging the power grid state:
state 1, N output phase difference absolute values | Delt _ Pha'xIf the phase difference y is equal to 0 degree, the electric network is formed;
in the state 2, if the condition of the state 1 is not satisfied, the absolute value | Delt _ Pha 'of the phase difference is output from N'xFinding out absolute values | Delt _ Pha' of all output phase differences of 45 DEG or more in |.xL and outputting the phase difference Delt _ Pha'xAnd after summing, taking an average value to obtain an average phase difference y:
when y is more than 0 degrees, the grid is a weak grid;
when y is 0 degrees, a strong current network is formed;
and when y is less than 0 degree, the series compensation power grid is established.
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