CN115833166A - Resonance protection design method of grid-connected inverter - Google Patents
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Abstract
The invention relates to a resonance protection design method of a grid-connected inverter, wherein the output of the inverter is connected with a grid-connected alternating current contactor after being filtered by LC, the alternating current contactor is closed and connected with the grid through a grid-side inductor L, and the resonance frequency range of the inverter is determined according to the LCL resonance principle; according to the resonance frequency range of the inverter, setting the sampling cut-off frequency of the output voltage of the inverter and the voltage on the side of the power grid; respectively designing an inverter output voltage sampling circuit and a power grid side voltage sampling circuit according to the sampling cut-off frequency; reading instantaneous inverter output voltage sampling and instantaneous power grid side voltage sampling during the grid-connected operation of the inverter through a designed sampling circuit; and calculating an instantaneous voltage difference value to trigger the resonance protection of the inverter. The resonance protection of the inverter can be realized without adding an additional voltage sampling circuit, a resonance detection circuit/equipment and a resonance suppression circuit, the damage of electrical equipment and the excessive harmonic loss of a power grid are avoided, and the practicability is high.
Description
Technical Field
The invention relates to a power electronic technology, in particular to a resonance protection design method of a grid-connected inverter.
Background
The output LC or LCL filter of the new energy grid-connected inverter is connected into a power grid through a transformer, or a plurality of inverters are connected in parallel, so that impedance of a public grid-connected connection point is changed, and the resonance problem of a grid-connected system is caused. The method brings great influence on the reliability and the electric energy quality of a large-scale new energy grid-connected system, and brings great challenge to the safe operation of a power distribution network and user electrical equipment.
The existing resonance protection schemes are all the suppression and prevention measures of resonance. For example, when the inverter is connected to the grid, a matched transformer is selected; an alternating current filter is arranged on the output side of the inverter to filter out resonance; a resonance detection device is added, and when resonance occurs, the output filter capacitor and the inductor of the grid-connected inverter are switched, so that the capacitive and inductive parameters of the grid-connected system of the inverter are changed, and the resonance area of the system is avoided; increasing the loop loss, connecting an actual resistor in series in the output filter or adopting a virtual impedance algorithm to increase the damping effect of the system, inhibiting the system resonance, and the like. The methods all affect the low-frequency transfer characteristic of the system, the additionally added electrical equipment increases the cost of the system and the complexity of the design, and when the operation condition of the external power grid changes, the originally matched parameters are not always suitable.
If the inverter grid-connected system resonates due to improper selection of the mutual inductor, the control parameters and the like or changes of operation conditions of grid-connected points, the resonant voltage does not reach a protective threshold value of overhigh voltage of the inverter, the inverter cannot detect abnormality and cannot cut off the abnormality in time, harmonic loss of a power grid is further increased, detection and communication equipment are interfered, and even potential safety hazards of electrical equipment are caused.
Therefore, when the inverter grid-connected system resonates, the inverter should stop transmitting the harmonic of the resonant frequency to the grid in time, and how to provide a resonant protection scheme which can quickly detect the resonance, is simple and easy to implement and has low cost is a technical problem to be solved by technical personnel in the field.
Disclosure of Invention
Aiming at the problem of improving the power quality of a power grid, the resonance protection design method of the grid-connected inverter is provided, the resonance protection of the inverter can be realized without adding an additional voltage sampling circuit, a resonance detection circuit/equipment and a resonance suppression circuit, the damage of electrical equipment and excessive power grid harmonic loss are avoided, and the method has strong practicability.
The technical scheme of the invention is as follows: a resonance protection design method of a grid-connected inverter specifically comprises the following steps:
s1: the output of the inverter is connected with a grid-connected alternating current contactor after being filtered by LC, the alternating current contactor is closed and connected with a grid through a grid side inductor L, and the resonant frequency range of the inverter is determined according to the LCL resonance principle;
s2: according to the resonance frequency range of the inverter, the sampling cut-off frequency f of the output voltage of the inverter and the grid side voltage is set uInvc 、f uGrid ；
S3: according to the sampling cut-off frequency f uInvc 、f uGrid Respectively designing an inverter output voltage sampling circuit and a power grid side voltage sampling circuit;
s4: reading the instantaneous inverter output voltage sample u during the grid-connected operation of the inverter by the sampling circuit designed in the step S3 fInv And instantaneous grid side voltage sampling u fGrid ；
S5: calculating instantaneous voltage difference value delta u = | u fInv -u fGrid And triggering the inverter resonance protection according to the instantaneous voltage difference.
Further, the step S2 samplesCut-off frequency f uInvc 、f uGrid At least one frequency being greater than the LCL standard resonant waveform frequency f r And f is uGrid >f uInvc >f rmin ，f rmin Is the minimum value in the resonant frequency range of the inverter.
Further, the design method of the sampling circuit in the step S3 includes: according to the sampling cut-off frequency f uInvc 、f uGrid Calculating the sampling time constant tau of the output voltage of the inverter 1 ＝1/(2πf uInv ) And grid side voltage sampling time constant tau 2 ＝1/(2πf uGrid ) Then according to τ 1 ＝R 1 C 1 And τ 2 ＝R 2 C 2 Selecting an inverter output voltage sampling resistor R 1 Sampling filter capacitor C 1 And grid side voltage sampling resistor R 2 Sampling filter capacitor C 2 。
Further, the inverter resonance protection method in step S5: if the instantaneous voltage difference value delta u is larger than or equal to the maximum difference value cMaxVolDiff, the inverter protection action is executed after the delay time t1, and the inverter stops running and the inverter resonance protection contactor fails; if the delta u is smaller than the maximum difference value cMaxVolDiff and a contactor fault exists, the contactor fault is cleared after the delay time t2, and the inverter is restarted to operate; if Δ u < maximum difference cMaxVolDiff and there is no contactor fault, the inverter operates normally.
Further, the maximum difference value cMaxVolDiff is larger than the allowable error of the two ends of the alternating current contactor when the inverter operates normally.
Further, the maximum difference value cMaxVolDiff setting method: if the parasitic resistance of the AC contactor is Rd when the AC contactor is closed, the maximum current peak value of the inverter flowing through the AC contactor during normal operation is cInvCurrPeakMax, and the amplitude margin of the detection resonance within 30V is cMaxVolDiff = cInvCurrPeakMax Rd + cMax.
Furthermore, the delay time t1 is at least larger than one power frequency period and smaller than two power frequency periods, so that the inverter is prevented from triggering resonance protection action when the inverter normally operates.
The utility model provides a resonance protection system of grid-connected inverter, the inverter output connects grid-connected AC contactor after passing through LC filtering, and AC contactor closes and is incorporated into the power networks through net side inductance L, includes inverter output voltage sampling circuit and electric wire netting side voltage sampling circuit, two sampling circuit are used for sampling grid-connected AC contactor closed instantaneous both ends voltage, and two sampling circuit sampling voltage value send inverter controller for inverter resonance protection judges.
Preferably, the two sampling circuits omit a sampling frequency delay link, and are both simplified to transfer functions of a first-order low-pass filter:
wherein tau is a time constant, R is a sampling filter resistance, C is a sampling filter capacitance, and K m The time constant tau is determined according to the output voltage of the inverter and the sampling cut-off frequency of the voltage on the power grid side, and the sampling cut-off frequency of the output voltage of the inverter and the voltage on the power grid side is set according to the resonance frequency range of the inverter.
The application of the resonant protection design method of the grid-connected inverter is used for photovoltaic grid-connected resonant protection in a distributed power generation system.
The invention has the beneficial effects that: according to the resonance protection design method of the grid-connected inverter, the cut-off frequency of voltage sampling at two ends of the grid-connected alternating current contactor is designed according to the resonance principle, the sampling circuit is designed according to the sampling cut-off frequency, and whether grid connection is carried out or not is judged by utilizing the voltage difference of the voltage sampling at the two ends of the grid-connected alternating current contactor, so that the purposes of resonance detection and protection are achieved.
Drawings
FIG. 1 is a partial flow chart of the resonance protection algorithm of the present invention
FIG. 2 is a schematic diagram of an inverter phase A resonant circuit of the present invention;
FIG. 3 is a diagram of a model inverter resonant circuit of the present invention;
FIG. 4a is a graph showing the amplitude-frequency response of the transfer functions of the present invention in the formula (1) and the formula (2);
FIG. 4b is a phase-frequency response graph of the transfer functions of the present invention in formula (1) and formula (2);
FIG. 5a is a graph of the amplitude-frequency response of equation (3) below for different sampling cut-off frequencies in accordance with the present invention;
FIG. 5b is a phase frequency response plot of equation (3) below for different sampling cut-off frequencies in accordance with the present invention;
FIG. 6a is a graph of the amplitude frequency response of the inverter voltage samples at the resonant frequency of the present invention;
FIG. 6b is a phase frequency response plot of the inversion voltage sampling at the resonant frequency of the present invention;
FIG. 7 is a schematic diagram of a resonant waveform of the present invention.
Detailed Description
The invention is described in detail below with reference to the figures and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the present invention, and a detailed implementation manner and a specific operation process are given, but the scope of the present invention is not limited to the following embodiments.
The embodiment of the application discloses a resonance protection design method applied to a grid-connected inverter, the design method is applied to photovoltaic grid-connected occasions in a distributed power generation system, is suitable for various inverter structure design schemes, and comprises the following steps as shown in figure 1:
s1: the output of the inverter is connected with a grid-connected alternating current contactor after being filtered by LC, the alternating current contactor is closed and connected with a grid through a grid side inductor L, and the resonant frequency range of the inverter is determined according to the LCL resonance principle;
s2: setting the sampling cut-off frequency f of the inverter output voltage and the grid side voltage according to the resonance frequency range of the inverter uInvc 、f uGrid ；
S3: according to the sampling cut-off frequency f uInvc 、f uGrid Respectively designing an inverter output voltage hardware sampling circuit and a power grid side voltage sampling circuit;
s4: reading the instantaneous inverter output voltage sample u during the grid-connected operation of the inverter by the sampling circuit designed in the step S3 fInv And instantaneous grid side voltage sampling u fGrid ；
S5: computingInstantaneous voltage difference Δ u = | u fInv -u fGrid If the delta u is larger than or equal to the maximum difference value cMaxVolDiff, the inverter protection action is executed after a period of time t1 is delayed, the inverter stops running and outputs, and the fault of the alternating current contactor is reported; if Δ u<If the maximum difference value cMaxVolDiff is met and a contactor has a fault, the fault of the contactor is cleared after a period of time t2 is delayed, and the inverter is restarted to operate; if Δ u<And if the maximum difference value cMaxVolDiff is not provided with a contactor fault, the inverter operates normally.
In the step S1, according to engineering design experience, the resonant frequency of the LCL of the grid-connected inverter is generally within the range of 1 kHz-5 kHz;
in step S2, the sampling cut-off frequency f uInvc 、f uGrid At least one frequency being greater than the LCL standard resonant waveform frequency f r And f is uGrid >f uInvc >f rmin ＝1kHz，f rmin Is the minimum value in the resonance frequency range of the inverter;
in step S3, the design of the sampling circuit is carried out according to the sampling cut-off frequency f uInvc 、f uGrid Calculating the sampling time constant tau of the output voltage of the inverter 1 ＝1/(2πf uInv ) And grid side voltage sampling time constant tau 2 ＝1/(2πf uGrid ) Then according to τ 1 ＝R 1 C 1 And τ 2 ＝R 2 C 2 Selecting proper inverter output voltage sampling resistor R 1 Sampling filter capacitor C 1 And grid side voltage sampling resistor R 2 Sampling filter capacitor C 2 ；
In step S5, in order to avoid the inverter triggering a resonance protection action when the inverter is operating normally, the resonance detection delay time t1 is at least greater than one power frequency cycle for 20ms and less than two power frequency cycles for 40ms; meanwhile, the maximum difference value cMaxVolDiff is larger than the allowable error of two ends of the alternating current contactor when the inverter normally operates. If the parasitic resistance is Rd when the AC contactor is closed, the maximum current peak value cInvCurrPeakMax flowing through the AC contactor when the inverter normally operates, and the amplitude margin cMarg of the detection resonance within 30V is reserved, then the maximum difference cMaxVolDiff = cInvCurrPeakMax Rd + cMarg.
Compared with the conventional design method, the design method provided by the invention can realize resonance protection without adding an additional voltage sensor, so that the flexibility of hardware circuit design is increased, and meanwhile, the method can also be used for detecting whether the alternating current contactor is abnormal or not.
It should be noted that, in order to illustrate the difference between the design method of the present invention and the conventional design method, the following analysis process is performed by taking the inverter a-phase output circuit shown in fig. 1 as an example:
in fig. 2, the inverter output is filtered by LC and then connected to the ac contactor, which is closed and connected to the grid through the grid side inductor L. u. u oa For the inverter output midpoint voltage, L f And C f For outputting filter inductance and filter capacitance, L, to the inverter g Is a net-side equivalent inductance u Inv Is the voltage of the sampling point at the inverter end of the contactor (i.e. the filter capacitor C) f Voltage across u), u Grid The voltage of the sampling point on the electric network side of the contactor is shown.
Neglecting the influence of parasitic resistance, the equivalent LCL resonant circuit model of the inverter in the s-domain shown in FIG. 3 can be obtained from FIG. 2.
If the inverter outputs the filter inductance L f Capacitor C f And network side inductance L g To form LCL resonance, as can be seen from FIG. 2, the voltage u across the output filter capacitor of the inverter Inv (s) to inverter output midpoint voltage u o The open loop transfer function of(s) is:
neglecting the sampling frequency delay link, the voltage sampling link can be simplified as the transfer function of a first-order low-pass filter:
wherein tau is a time constant, R is a sampling filter resistance, C is a sampling filter capacitance, and K m Is the amplification factor.
Grid side voltage sampling u fGrid (s) and inverter output voltage sample u fInv (s) to inverter midpoint voltage u o The open loop transfer function of(s) is:
in the formula, R 1 And C 1 Resistors and capacitors, R, for sampling and filtering the output voltage of the inverter, respectively 2 And C 2 And the resistance and the capacitance are respectively used for sampling and filtering the voltage at the power grid side.
If L is f ＝0.1mH，C f ＝501uF，L g ＝0.014mH，K m =1.42, inverter output voltage u Inv The sampling cut-off frequency is 2kHz, and the voltage u at the side of the power grid Grid When the sampling cut-off frequency is 5kHz, fig. 4a and 4b are bode diagrams of the formula (1) and the formula (2), the bode diagrams include an amplitude-frequency response curve diagram and a phase-frequency response curve diagram, the curve 1 is the bode diagram of the formula (1), and the curve 2 is the sampling cut-off frequency f of the voltage at the side of the power grid uGridc Is a Bode diagram of the equation (2) at 5kHz, and the curve 3 is the sampling cut-off frequency f of the inversion output voltage uInvc Is a Bode diagram of equation (2) at 2 kHz.
From fig. 4a, 4b, the resonance frequency f of the LCL can be seen r Is 2.15kHz. Resonant frequency f r At f uGridc And f uInvc In f r At frequency point u Grid The phase angle margin of the sample is-22.2eg Inv The phase angle margin of the sample is-45.6 deg.
FIGS. 5a and 5b are diagrams of the Bott of formula (3) with the resonant frequency f r A slight decrease compared to formula (1). Sampling the output voltage of the inverter and the grid-side voltage at the resonant frequency f r There is a difference in both amplitude and phase angle, fig. 6a, 6b are resonant frequencies f r And (4) processing a baud graph of the output voltage of the inverter. In FIGS. 6a and 6b,. DELTA.A Max To be at a resonant frequency f r The maximum difference value of the sampling amplitude of the inverter output voltage at the two ends of the AC contactor and the sampling amplitude of the voltage at the side of the power grid,about 20dB;
to be at a resonant frequency f r The maximum difference between the inverter output voltage sampling at the two ends of the ac contactor and the grid side voltage sampling phase angle is about 40.8deg.
Assuming that the inverter grid-connected voltage is 630V, the LCL resonance waveform is a standard sine wave, the resonance voltage is 20% of the grid voltage, the maximum output current peak cInvCurrPeakMax of the inverter is 2000A, the parasitic resistance Rd when the alternating-current contactor is closed is 0.02 omega, the amplitude margin cMax for detecting resonance is 20V, the resonance detection delay time is 25ms, the waveform is 630V sqrt (2) = 0.2=178.19V (630V sqrt (2) is the effective value of the line voltage multiplied by sqrt (2), and is the grid peak voltage), and the frequency is f r The maximum difference cMaxVolDiff = (2000 x 0.02+ 20) V =40V (shown in fig. 7), u is shown in fig. 6a, 6b Grid And u Inv The difference of resonance on amplitude gain can be ignored, the phase angle difference is 40.8 degrees, and then the grid side voltage sampling value u fGrid And inverter output voltage sample value u fInv The difference of instantaneous values can reach 178.19V × 0.275=49V>cMaxVolDiff =40V, and after the delay time of the inverter is 25ms, the resonance protection is triggered, the output is stopped, and the damage of the electrical equipment is avoided.
To sum up, when the ac contactor is closed, if the inverter normally operates at a frequency of 50Hz, the voltage u sampled at both ends of the contactor fInv And u fGrid Similarly, if the system resonates at an LCL, the different sampling cut-off frequencies cause a large difference in instantaneous values of the voltages sampled at the resonant frequencies. Therefore, whether the system resonates or not can be judged through the instantaneous voltage difference between the two ends of the alternating current contactor, and when the instantaneous voltage difference between the two ends exceeds the limit value cMaxVolDiff, the inverter performs protection action and reports the fault of the alternating current contactor.
According to the invention, the cut-off frequency of the sampling circuit of the voltages (the output voltage of the inverter and the grid voltage) at the two ends of the alternating current contactor is designed according to the resonance principle, so that different sampling voltage values at the two ends of the contactor are obtained when a system resonates, and the condition of resonance protection action is achieved by using the overrun of the instantaneous voltage difference of the two. The output voltage of the inverter and the power grid voltage need to be sampled according to the requirements of a control algorithm, so that the resonance protection of the inverter can be realized without adding an additional voltage sampling circuit and resonance detection equipment, and the damage of electrical equipment and excessive power grid harmonic loss are avoided.
The resonance protection design of the invention has been verified through experiments.
Finally, it is further noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The above description of the embodiments is only intended to facilitate the understanding of the method of the invention and its core idea, and not to limit it. It should be noted that, for those skilled in the art, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall into the protection scope of the present invention.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (10)
1. A resonance protection design method of a grid-connected inverter is characterized by comprising the following steps:
s1: the output of the inverter is connected with a grid-connected alternating current contactor after being filtered by LC, the alternating current contactor is closed and connected with a grid through a grid side inductor L, and the resonant frequency range of the inverter is determined according to the LCL resonance principle;
s2: setting the sampling cut-off frequency f of the inverter output voltage and the grid side voltage according to the resonance frequency range of the inverter uInvc 、f uGrid ；
S3: according to the sampling cut-off frequency f uInvc 、f uGrid Respectively designing an inverter output voltage sampling circuit and a power grid side voltage sampling circuit;
s4: reading the instantaneous inverter output voltage sample u during the grid-connected operation of the inverter by the sampling circuit designed in the step S3 fInv And instantaneous grid side voltage sampling u fGrid ；
S5: calculating instantaneous voltage difference value delta u = | u = fInv -u fGrid And triggering the resonance protection of the inverter according to the instantaneous voltage difference value.
2. The method for designing the resonance protection of the grid-connected inverter according to claim 1, wherein the step S2 samples a cutoff frequency f uInvc 、f uGrid At least one frequency being greater than the LCL standard resonant waveform frequency f r And f is uGrid >f uInvc >f rmin ，f rmin Is the minimum value in the resonant frequency range of the inverter.
3. The method for designing resonance protection of a grid-connected inverter according to claim 1 or 2, wherein the method for designing a sampling circuit in step S3 comprises: according to the sampling cut-off frequency f uInvc 、f uGrid Calculating the sampling time constant tau of the output voltage of the inverter 1 ＝1/(2πf uInv ) And grid side voltage sampling time constant tau 2 ＝1/(2πf uGrid ) Then according to τ 1 ＝R 1 C 1 And τ 2 ＝R 2 C 2 Selecting an inverter output voltage sampling resistor R 1 Sampling filter capacitor C 1 And grid side voltage sampling resistor R 2 Sampling filter capacitor C 2 。
4. The grid-connected inverter resonance protection design method according to claim 3, wherein the inverter resonance protection method in the step S5 comprises: if the instantaneous voltage difference value delta u is larger than or equal to the maximum difference value cMaxVolDiff, the inverter protection action is executed after the delay time t1, and the inverter stops running and the inverter resonance protection contactor fails; if the delta u is smaller than the maximum difference value cMaxVolDiff and a contactor fault exists, the contactor fault is cleared after the delay time t2, and the inverter is restarted to operate; if Δ u < maximum difference cMaxVolDiff and there is no contactor fault, the inverter operates normally.
5. The resonant protection design method of the grid-connected inverter as claimed in claim 4, wherein the maximum difference value cMaxVolDiff is larger than the allowable error at two ends of the ac contactor when the inverter is in normal operation.
6. The grid-connected inverter resonance protection design method according to claim 4 or 5, wherein the maximum difference value cMaxVolDiff setting method: if the parasitic resistance of the AC contactor is Rd when the AC contactor is closed, the maximum current peak value of the inverter flowing through the AC contactor during normal operation is cInvCurrPeakMax, and the amplitude margin of the detection resonance within 30V is cMax, the maximum difference value is obtained
cMaxVolDiff＝cInvCurrPeakMax*Rd+cMag。
7. The grid-connected inverter resonance protection design method according to claim 4, wherein the delay time t1 is at least larger than one power frequency cycle and smaller than two power frequency cycles, so that the inverter is prevented from triggering resonance protection action when the inverter normally operates.
8. The resonance protection system of the grid-connected inverter is characterized by comprising an inverter output voltage sampling circuit and a grid side voltage sampling circuit, wherein the two sampling circuits are used for sampling the closed instantaneous voltages at two ends of the grid-connected AC contactor, and the sampled voltage values of the two sampling circuits are sent to an inverter controller for judging the resonance protection of the inverter.
9. The grid-connected inverter resonance protection system according to claim 5, wherein the two sampling circuits omit a sampling frequency delay link, and are both simplified to a transfer function of a first-order low-pass filter:
wherein tau is a time constant, R is a sampling filter resistance, C is a sampling filter capacitance, and K m The time constant tau is determined according to the sampling cut-off frequency of the inverter output voltage and the grid side voltage, and the sampling cut-off frequency of the inverter output voltage and the grid side voltage is set according to the resonance frequency range of the inverter.
10. The application of the resonant protection design method of the grid-connected inverter is characterized in that the resonant protection design method is used for photovoltaic grid-connected resonant protection in a distributed power generation system.
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