CN110850175A - Direct current micro-grid line impedance detection method and device based on low-frequency current injection - Google Patents
Direct current micro-grid line impedance detection method and device based on low-frequency current injection Download PDFInfo
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- CN110850175A CN110850175A CN201911137235.4A CN201911137235A CN110850175A CN 110850175 A CN110850175 A CN 110850175A CN 201911137235 A CN201911137235 A CN 201911137235A CN 110850175 A CN110850175 A CN 110850175A
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Abstract
The invention discloses a low-frequency current injection-based direct current micro-grid line impedance detection method and a low-frequency current injection-based direct current micro-grid line impedance detection device, which belong to the technical field of direct current micro-grid operation control, wherein in the method, an alternating current signal with specific frequency is selected, and the current signal is injected into an inductive current of an energy storage converter; after the current disturbance signal is injected, measuring the input voltage and the input current of the energy storage unit converter; calculating to obtain line impedance between the voltage of the direct current bus and the energy storage unit according to the relation between the voltage and the current in the circuit principle; the device includes: the device considers the line impedance of each energy storage unit, can be applied to direct current micro-grids with various structures, and has the advantages of simple structure, high economy and high line impedance detection efficiency.
Description
Technical Field
The invention relates to the technical field of operation control of a direct-current microgrid, in particular to a method and a device for detecting line impedance of the direct-current microgrid based on low-frequency current injection.
Background
In recent years, the micro-grid technology becomes an important direction for the research of a smart grid, and as a new branch of the micro-grid research, the direct-current networking technology has the advantages of being few in energy conversion times, simple in structure, free of considering frequency stability, reactive fluctuation and the like, and compared with the alternating-current networking technology, the direct-current networking technology can more efficiently receive renewable energy, and is widely concerned in the fields of airplanes, ships and the like.
The self-stable operation of the direct current micro-grid needs to meet two important indexes: (1) maintaining the voltage of the direct current bus stable; (2) the reasonable power distribution of the distributed power supply and the dynamic stability of the power equalizing process are realized, and the control strategies adopted by the prior art when a plurality of micro sources are operated in parallel mainly comprise master-slave control and peer-to-peer control. The master-slave control strategy designates one of the micro-source converters as a master module and adopts voltage control, the other converters are slave modules and adopt current control, the strategy has higher requirements on the master module, and once the master module has problems, the whole system loses control; distributed micro-sources in the peer-to-peer control strategy are provided with respective underlying controllers, communication is not needed between the distributed micro-sources, and the micro-grid system is high in reliability and low in cost. The existing peer-to-peer control strategy mostly adopts droop control, and documents of An improved droop control method for DC microprocessor based on low bandwidth communication with DC bus voltage estimation and enhanced current sharing access adjust droop coefficients by using a method of improving An initial voltage value of a converter, so that the voltage adjustment of a direct current bus can be realized, but the load current sharing effect is poor. In order to solve the problem that the current sharing precision is not high due to the fact that line impedances are not consistent, a droop control method based on virtual impedance is adopted at present, and the difficulty is that a droop coefficient is not easy to determine; in an improved current load distribution control strategy applicable to a direct-current micro-grid, an auxiliary controller shares the current of each micro-source through communication, calculates the current required to be provided by each micro-source, and uses the information to adjust a droop coefficient in the controller, so that a micro-source direct-current converter has the same output impedance to realize load current sharing, but the method needs mutual communication among the micro-sources, thereby reducing the reliability of the system; the document 'impedance detection of direct current micro-grid circuit based on single pulse injection' adopts an alternating current small signal injection method to obtain the load sharing condition of the current converter, so as to adjust the output voltage value of each distributed micro-source, and the method is complex in practical application and has high requirement on hardware; in the document "a decentralized dc control Method for a Low-Voltage dc microgrid", a line impedance value is predicted in a grid-connected mode, and the influence of the line impedance is offset by using a compensated virtual impedance, however, the microgrid cannot operate in an off-grid mode during initial operation, and the operation mode is limited. Therefore, in order to effectively improve the droop control efficiency of the direct-current microgrid and the operation stability of the direct-current microgrid, accurate detection of the line impedance value becomes the key of the droop control technology.
Disclosure of Invention
According to the problems in the prior art, the invention discloses a direct current micro-grid line impedance detection method based on low-frequency current injection, which is applied to an energy storage unit in a charging mode, a converter of the method adopts current single-loop control, and the method comprises the following specific steps:
s1: selecting an alternating current signal with a specific frequency, and injecting the current signal into an inductive current of the energy storage converter;
s2: after the current disturbance signal is injected, measuring the input voltage and the input current of the energy storage unit converter;
s3: according to the relation between voltage and current in the circuit principle, the line impedance between the voltage of the direct current bus and the energy storage unit is calculated, and the voltage-current relation satisfies the following formula:
Ubus＝Uin+rlineIin(1)
in the formula (1), UbusIndicating DC bus voltage, UinRepresenting the input voltage, I, of the energy storage unitinRepresenting the input current, r, of the energy storage unitlineRepresenting the line impedance;
the droop resistance value of the dc converter control loop is compensated by using the line impedance value obtained in step S3.
Further: the alternating current signal adopts sinusoidal current with the frequency of 50 Hz.
A DC microgrid line impedance detection device based on low-frequency current injection comprises:
the disturbance current injection unit selects a current signal with a specific frequency and injects the current signal into an inductive current of the energy storage unit converter;
the voltage and current measuring unit is used for measuring input voltage and input current of the converter of the energy storage unit after the current disturbance signal with the specific frequency is injected;
the line impedance calculation unit is used for calculating and obtaining a line impedance value r between the direct current bus and the energy storage unit by measuring the input voltage and the input current of the energy storage unit converterlineThe voltage and current need to satisfy the following equation:
in the formula (2), the reaction mixture is,
showing the fluctuation amount of the DC bus voltage after the disturbance current is injected,
Showing the input voltage fluctuation amount of the energy storage unit after the disturbance current is injected,
Representing the input current fluctuation amount of the energy storage unit after the disturbance current is injected;
in the formula (3), rlineRepresenting the calculated line impedance value.
Furthermore, the line impedance detection device comprises a line impedance calculation unit, the direct current bus voltage of the unit is hardly influenced by the injection of the 50Hz current signal,
the value is close to zero.
By adopting the technical scheme, the impedance detection method and the impedance detection device for the direct-current micro-grid circuit based on low-frequency current injection provided by the invention have the advantages that the injected alternating current frequency selected based on the impedance frequency characteristic has small influence on the stability of a system, the load sharing effect when the direct-current converters are connected in parallel and share current can be improved, and the stable operation of the system is facilitated; the device considers the line impedance of each energy storage unit, can be applied to the direct current micro-grid of various structures, and has the advantages of simple structure, high economy and high line impedance detection efficiency.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments described in the present application, and other drawings can be obtained by those skilled in the art without creative efforts.
FIG. 1 is a partial block diagram of a DC microgrid;
FIG. 2 is a frequency characteristic diagram of the closed loop output impedance ZCo(s) of the DC bus pre-stage converter;
FIG. 3 is a flow chart of the method of the present invention;
FIG. 4 is a schematic diagram of the DC microgrid line impedance detection apparatus of the present invention;
fig. 5 is a simulation result of line impedance measurement.
In the figure: 1. the device comprises a first energy storage converter, a second energy storage converter, a third energy storage converter, a first line impedance, a second line impedance, a third line impedance, a disturbance current injection unit, a voltage and current measurement unit, a third line impedance, a second line impedance, a third line impedance, a disturbance current injection unit, a voltage and current measurement unit and a line impedance calculation unit, wherein the first energy storage converter is 2, the second energy storage converter is 3, the.
Detailed Description
In order to make the technical solutions and advantages of the present invention clearer, the following describes the technical solutions in the embodiments of the present invention clearly and completely with reference to the drawings in the embodiments of the present invention:
fig. 1 is a partial structure diagram of a direct current microgrid adopted by the invention, an energy storage battery of the microgrid is connected to a direct current bus through a direct current converter, the microgrid mainly comprises a first energy storage converter 1, a second energy storage converter 2, a third energy storage converter 3, a first line impedance 4, a second line impedance 5 and a third line impedance 6, the energy storage converter 1 works in a constant voltage mode to ensure the voltage of the direct current bus to be stable, the energy storage converter 2 and the energy storage converter 3 work in a constant current mode to realize the load sharing of a system, the first energy storage converter 1 is connected with the direct current bus through the first line impedance 4, the second energy storage converter 2 is connected with the direct current bus through the second line impedance 5, and the third energy storage converter 3 is connected with the direct current bus through the second line impedance 6.
Taking the energy storage converter 1 as an example for modeling, obtaining a transfer function Gvd(s) of converter control-output, a transfer function Gvi(s) of inductor current-output, an open-loop output impedance Zo(s) and a double closed-loop control down-converter output impedance ZCo(s) as follows respectively:
wherein Upv1 and Udc1 are input and output voltages of the converter,
IL1, Idc1 are converter inductor current and output current, d is reference voltage1For the duty cycle of the converter, 1/VM is a PWM transfer function, and the frequency of the injected alternating current is selected according to a frequency characteristic diagram of the closed-loop output impedance ZCo(s) of the system.
FIG. 2 is a frequency characteristic diagram of the closed loop output impedance ZCo(s) of the DC bus pre-stage converter, and it can be seen that the response amplitude is gradually increased with the increase of frequency, but not more than-20 dB; in a wider frequency range, the output impedance value is very small, and the low-frequency and high-frequency interference suppression method has good suppression capability, namely the influence of small load disturbance bus voltage is not large. Based on the above analysis, the present invention selects to inject 50Hz ac current signal, the gain at 50Hz in fig. 4 is about-76.9 dB, the output impedance of the converter is very small, about 0.14m Ω, i.e. the influence of load disturbance on the bus voltage is very small.
Fig. 3 is a flow chart of the method of the present invention, which includes injecting a low-frequency ac signal into an inductive current, detecting fluctuation information of an input voltage and an input current of a converter, extracting a voltage and a current of a specific frequency by fourier transform, obtaining a line impedance value, and compensating the line impedance value to a droop resistor by using the calculated line impedance, so as to eliminate the influence of the line impedance on the system, wherein the line impedance detection method specifically includes the following steps:
s1: selecting an alternating current signal with a specific frequency, and then injecting the current signal into an inductive current of the energy storage converter;
s2: after the current disturbance signal is injected, measuring the input voltage and the input current of the energy storage unit converter;
s3: calculating the line impedance r between the DC bus voltage and the energy storage unit according to the relationship between the voltage and the current in the circuit principle and the voltage and current parameters measured in the step S2lineThe voltage-current relationship satisfies the following equation:
Ubus＝Uin+rlineIin(5)
in the formula of UbusIndicating DC bus voltage, UinRepresenting the input voltage, I, of the energy storage unitinWhich is indicative of the input current of the energy storage unit,
the droop resistance value of the dc converter control loop is compensated by using the line impedance value obtained in step S3.
Based on the line impedance detection method and combined with a specific structure of a direct current microgrid, a direct current microgrid line impedance detection device is invented, and fig. 4 is a schematic diagram of the direct current microgrid line impedance detection device of the invention; the device includes:
the disturbance current injection unit 7 is used for selecting a current signal with a specific frequency and injecting the current signal into the inductive current of the energy storage unit converter;
the voltage and current measuring unit 8 is used for measuring the input voltage and the input current of the converter of the energy storage unit after the current disturbance signal with the specific frequency is injected;
the line impedance calculation unit 9 is used for calculating and obtaining a line impedance value r between the direct current bus and the energy storage unit by measuring the input voltage and the input current of the energy storage unit converterlineThe voltage and current need to satisfy the following equation:
in the above formula, the first and second carbon atoms are,
showing the fluctuation amount of the DC bus voltage after the disturbance current is injected,Showing the input voltage fluctuation amount of the energy storage unit after the disturbance current is injected,
And the input current fluctuation quantity of the energy storage unit after the disturbance current is injected is shown.
Further, the line impedance detection device comprises a line impedance calculation unit, the direct current bus voltage of the unit is hardly influenced by the injection of the 50Hz current signal,
the value is close to zero.
The line impedance detection device has the specific implementation process that a low-frequency sinusoidal signal is injected into the inductive current, the input voltage and the current value of the converter after the signal is injected are measured,
the input voltage is the input voltage after the low-frequency alternating current signal is injected;
for the injected input current, the response signal is transformed from the time domain signal to the frequency domain signal by Fourier Transform (FT), thereby obtaining a voltage response frequency domain and a current response frequency domain ofAnd
the response voltage and current signals can be written as follows:
in the formulaIs an amplitude spectrum; φ u (ω) and φ i (ω) are phase spectra. In this method, the inductive impedance is negligible, so the line impedance expression is obtained as follows:
in order to verify the effectiveness of the invention, simulation verification is carried out through MATLAB/Simulink software, FIG. 5 is a simulation result diagram of line impedance measurement, which respectively shows the measured values of three line impedances, the data of the first two periods cannot represent the real line impedance value because the calculation process of the line impedance value is not completed, the obtained impedance value after 1.04s is more accurate, and the line impedance gamma isline1About 0.1501 omega, line impedance gammaline2About 0.2502 omega, line impedance gammaline3About 0.3503 omega, it can be seen that the measurement accuracy is relatively high.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.
Claims (3)
1. A direct current micro-grid line impedance detection method based on low-frequency current injection is applied to an energy storage unit in a charging mode, and a converter of the method adopts current single-loop control, and is characterized in that: the method comprises the following specific steps:
s1: selecting an alternating current signal with a specific frequency, and injecting the current signal into an inductive current of the energy storage converter;
s2: after the current disturbance signal is injected, measuring the input voltage and the input current of the energy storage unit converter;
s3: according to the relation between voltage and current in the circuit principle, the line impedance between the voltage of the direct current bus and the energy storage unit is calculated, and the voltage-current relation satisfies the following formula:
Ubus＝Uin+rlineIin(1)
in the formula (1), UbusIndicating DC bus voltage, UinRepresenting the input voltage, I, of the energy storage unitinRepresenting the input current, r, of the energy storage unitlineRepresenting the line impedance;
the droop resistance value of the dc converter control loop is compensated by using the line impedance value obtained in step S3.
2. The method for detecting the impedance of the direct-current microgrid circuit based on low-frequency current injection of claim 1 is further characterized in that: the alternating current signal adopts sinusoidal current with the frequency of 50 Hz.
3. The utility model provides a little electric wire netting line impedance detection device of direct current based on low frequency current injection which characterized in that: the method comprises the following steps:
the disturbance current injection unit selects a current signal with a specific frequency and injects the current signal into an inductive current of the energy storage unit converter;
the voltage and current measuring unit is used for measuring input voltage and input current of the converter of the energy storage unit after the current disturbance signal with the specific frequency is injected;
the line impedance calculation unit is used for calculating and obtaining a line impedance value r between the direct current bus and the energy storage unit by measuring the input voltage and the input current of the energy storage unit converterlineThe voltage and current need to satisfy the following equation:
in the formula (2), the reaction mixture is,
showing the fluctuation amount of the DC bus voltage after the disturbance current is injected,
Showing the input voltage fluctuation amount of the energy storage unit after the disturbance current is injected,Representing the input current fluctuation amount of the energy storage unit after the disturbance current is injected;
in the formula (3), rlineRepresenting the calculated line impedance value.
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