CN107134806B - Decoupling current droop control method for parallel inverters in micro-grid - Google Patents
Decoupling current droop control method for parallel inverters in micro-grid Download PDFInfo
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Abstract
The invention discloses a decoupling current droop control method for parallel inverters in a microgrid, and belongs to the field of coordination control of the parallel inverters in the microgrid. The method is different from the traditional droop control method, the output power of an inverter does not need to be calculated, but the current vector is subjected to rotation transformation, so that a virtual current vector which is in the same phase with the voltage vector is generated, and the droop relation between the voltage and the virtual current is established under a synchronous rotation coordinate system. Through reasonable design parameters, the droop relation can naturally form the control characteristic of virtual impedance, and the reduction of the current sharing effect caused by different line impedances is compensated. In addition, the voltage phase-locked loop based on the proportional regulator is adopted to generate the reference value of the voltage frequency, and compared with the traditional phase-locked loop based on the proportional integral regulator, the phase-locked loop in the method can ensure that the parallel inverters synchronously run with equal identity, and the redundancy is kept. The invention can realize accurate load current sharing among the inverters.
Description
Technical Field
The invention belongs to the field of coordination control of parallel inverters in a microgrid, and particularly relates to a decoupling current droop control method of parallel inverters in the microgrid.
Background
With the increasing environmental and energy crisis, the concept of the microgrid has gained more and more attention and applications. A microgrid is an energy system that combines distributed power sources and interconnected loads. In most cases, the microgrid is connected to the bus bars via a power electronic interface, such as an inverter. Therefore, coordination control among the parallel inverters is one of the key factors for realizing stable and efficient operation of the microgrid.
Due to the geographical dispersion of distributed power sources, it is often not feasible to use communication lines to communicate signals between parallel power sources, which increases cost and decreases system reliability. In this case, the application of droop control can achieve power sharing between the parallel power sources without the use of communication lines.
However, conventional droop control also suffers from drawbacks such as instability due to coupling between power controls, poor equipartition characteristics of reactive power, and deviations in frequency and voltage amplitude. In addition, the conventional droop control relies on the calculation of the output power of the inverter, and the calculated instantaneous power needs to be low-pass filtered to eliminate harmonics and noise therein, obtain the average power of the fundamental wave, and generate smooth and stable voltage and frequency reference values. However, the limited bandwidth of the low pass filter can slow down the dynamic response of the system and even cause stability problems for the entire system.
In order to solve the above problem, a droop control method aiming at instantaneous current control rather than power control has been a hot point of research in recent years. Although the document proposes to share the current by using communication lines, this method realizes the current sharing among a plurality of inverters, however, the use of communication lines increases the cost and is also susceptible to noise interference. There is also a document that proposes a method of controlling the peak value of current using a current resonator, and although this method can also achieve accurate current sharing, this method requires real-time adjustment of the resonant frequency of the resonant controller, which does not improve the dynamic response of the system. Recent documents also propose a novel current droop control strategy by using an intrinsic droop mechanism between the direct-axis component and the quadrature-axis component of the output current and the amplitude and the phase of the voltage, however, the method requires that each inverter has similar hardware parameters and controller parameter design, and the practical feasibility is not strong. In a synchronous rotating coordinate system, a droop relation between voltage and current is established to realize current sharing, but the synchronous mechanism of the method is established based on a master-slave control structure, so that the redundancy of system operation is reduced.
Disclosure of Invention
In order to overcome the defects in the prior art and improve the dynamic response and the system stability of droop control, the invention provides a decoupling current droop control method for parallel inverters in a microgrid, which can realize accurate load current sharing among the inverters.
The invention is realized by the following technical scheme:
the invention discloses a decoupling current droop control method for parallel inverters in a microgrid, which comprises the following steps of:
1) in order to improve the redundancy and the reliability of a system, each inverter is equally controlled to be a voltage source by the same control strategy;
2) each inverter acquires voltage phase information at two ends of a capacitor through a voltage phase-locked loop, acquires a direct-axis component and a quadrature-axis component of a voltage vector, and generates a reference value of voltage frequency;
the voltage phase-locked loop adopts a proportion (P) regulator, and the phase-locked loop based on the proportion (P) regulator can ensure that a frequency reference value is close to a fundamental frequency (50Hz) and all inverters are kept synchronous by matching with the voltage regulator;
3) each inverter acquires phase information of current through a current phase-locked loop to acquire a direct-axis component and a quadrature-axis component of a current vector;
4) performing rotation transformation on the current vector according to the voltage phase information and the current phase information acquired in the step 2) and the step 3) to obtain a virtual current vector, wherein the virtual current vector and the voltage vector have the same direction;
5) in a synchronous rotating coordinate system, establishing a droop relation between the virtual current vector in the step 4) and the voltage vector in the step 2) to achieve the effect of current sharing, wherein the droop relation is used for generating reference values of a direct axis component and a quadrature axis component of the voltage, and the reference values are input into a voltage current regulator to generate a PWM modulation signal.
Preferably, the dummy current in step 4) comprises a direct-axis component i'dAnd quadrature component i'qStraight-axis component i'dAnd quadrature component i'qCalculated from the following formula:
where δ is a phase angle difference between the output voltage and the output current calculated by the voltage-current phase locked loop, idAnd iqRespectively, a direct-axis component and a quadrature-axis component of the current in the synchronous rotating coordinate system.
Preferably, the calculation formula of the droop relationship established in step 5) is as follows:
wherein v isd *And vq *Respectively are reference values of a direct-axis component and a quadrature-axis component of the voltage under a synchronous rotating coordinate system; v. ofd0And vq0Are each vd *And vq *A nominal value of (d); m and n are defined as positive values and are gains for droop control; i'dAnd i'qRespectively a direct-axis component and a quadrature-axis component of the virtual current under the synchronous rotating coordinate system; i'd0And i'q0Are each i'dAnd i'qOf the target value of (c).
More preferably, the control formula is obtained from the virtual current calculation formula and the droop relation calculation formula as follows:
selecting m and n as the same value, i.e. considering as introducing a virtual impedance in series with the line impedance, the value of which is:
Rv＝m·cosδ；
wherein R isvAnd LvRespectively a virtual resistance value and a virtual inductance value; omegabaseIs the fundamental frequency.
Preferably, the design time is chosen to be large enough in m to satisfy:
Wherein R islineAnd LlineThe resistance and inductance of the transmission line impedance, respectively, m satisfying this condition causes the virtual impedance to be much larger than the line impedance, so that in contrast the line impedance is negligible and each inverter will generate the same voltage reference value vd *And vq *Therefore, as long as the control parameters of the voltage and current droop relational expression of each inverter are ensured to be the same, accurate current sharing among the inverters connected in parallel can be realized.
Preferably, a proportional integral regulator is employed in the current phase locked loop.
Compared with the prior art, the invention has the following beneficial technical effects:
the invention discloses a decoupling current droop control method of a parallel inverter in a microgrid, which is different from the traditional droop control method. Secondly, through reasonable design parameters, the droop relation can naturally form the control characteristic of virtual impedance, and the reduction of the current sharing effect caused by different line impedances is compensated. Finally, a voltage phase-locked loop based on a proportional regulator is adopted in the method to generate a reference value of the voltage frequency, and compared with the traditional phase-locked loop based on a proportional integral regulator, the phase-locked loop in the method can ensure that the parallel inverters synchronously run with equal identity and the redundancy is kept. Therefore, the method does not depend on communication among the inverters, has quicker dynamic response and good system stability compared with the traditional droop control, can still realize current sharing among the inverters connected in parallel under the condition of different line resistances, and provides good reference value for engineering application.
Drawings
FIG. 1 is a general control block diagram of the control method proposed by the present invention;
FIG. 2 is a diagram of a decoupling relationship between voltage vectors and current vectors in a synchronous rotating coordinate system;
FIG. 3 is a simulation waveform diagram of direct-axis component, quadrature-axis component and frequency of current under two control methods when load changes;
wherein, (a1), (a2), and (a3) are waveform diagrams of a direct-axis component, a quadrature-axis component, and a frequency of the current under the conventional droop control method, respectively; (b1) (b2) and (b3) are waveform diagrams of direct-axis component, quadrature-axis component and frequency of the current under the control method proposed by the invention respectively;
FIG. 4 is a simulated waveform diagram of the direct-axis component, quadrature-axis component and frequency of the current under two control methods when two inverters are in parallel connection;
wherein, (a1), (a2), and (a3) are waveform diagrams of a direct-axis component, a quadrature-axis component, and a frequency of the current under the conventional droop control method, respectively; (b1) (b2) and (b3) are waveform diagrams of direct-axis component, quadrature-axis component and frequency of the current under the control method proposed by the invention respectively;
FIG. 5 is a waveform diagram of an experiment of output power of two parallel inverters according to the control method of the present invention;
wherein, (a) is a waveform diagram of output active power; (b) is a waveform diagram of output reactive power;
FIG. 6 is a waveform diagram of steady-state voltage and current test output by two parallel inverters before and after load change under the control method proposed in the present invention;
wherein, (a) is a waveform diagram before load change; (b) is a waveform diagram after load change.
Detailed Description
The present invention will now be described in further detail with reference to specific examples, which are intended to be illustrative, but not limiting, of the invention.
Referring to fig. 1 and 2, the invention provides a decoupling current droop control method for parallel inverters in a microgrid, and aims to solve the problems of current sharing effect, dynamic response and system stability of the traditional droop control inverter.
The method comprises the following concrete implementation steps:
1) the alternating-current microgrid comprises N inverters connected in parallel, and each inverter is controlled to be a voltage source according to the same current droop control strategy.
2) Each inverter acquires phase information of voltage at two ends of a capacitor through a voltage phase-locked loop, acquires direct-axis components and quadrature-axis components of voltage vectors, and generates reference values of voltage frequency; a proportional (P) regulator is employed in the voltage phase locked loop to ensure that the frequency reference is near the fundamental frequency (50Hz) and to maintain synchronization between the inverters.
3) Each inverter acquires phase information of current through a current phase-locked loop to acquire a direct-axis component and a quadrature-axis component of a current vector; a proportional-integral (PI) regulator is employed in the current phase-locked loop.
4) Performing rotation transformation on the current vector according to the voltage phase and the current phase acquired in the step 2) and the step 3) to obtain a virtual current vector, wherein the virtual current vector and the voltage vector have the same direction, and the direct component i 'of the virtual current'dAnd quadrature component i'qCalculated from the following formula:
where δ is a phase angle difference between the output voltage and the output current calculated by the voltage-current phase locked loop, idAnd iqRespectively, a direct-axis component and a quadrature-axis component of the current in the synchronous rotating coordinate system.
5) In a synchronous rotating coordinate system, establishing a droop relation between the virtual current vector in the step 4) and the voltage vector, and generating reference values of a direct-axis component and a quadrature-axis component of the voltage so as to achieve the effect of current sharing. The specific control of each inverter is as follows:
wherein v isd *And vq *Respectively are reference values of a direct-axis component and a quadrature-axis component of the voltage in the synchronous rotating coordinate system; v. ofd0And vq0Are each vd *And vq *A nominal value of (d); m and n are defined as positive and are gains for droop control; i'dAnd i'qRespectively a direct-axis component and a quadrature-axis component of the virtual current under the synchronous rotating coordinate system; i'd0And i'q0Are each i'dAnd i'qOf the target value of (c).
6) Combining step 4) and step 5), the complete control formula is as follows:
when designing, m and n are selected to be the same value, namely, a virtual impedance connected with the line impedance in series is introduced, and the value is
Rv＝m·cosδ；
Wherein R isvAnd LvRespectively a virtual resistance value and a virtual inductance value; omegabaseIs the fundamental frequency.
Further, the value of m is selected to be large enough to satisfy the requirement during design
Wherein R islineAnd LlineResistance and inductance values of the transmission line impedance, respectively;
the design can make the value of the virtual impedance far larger than that of the line impedance, so that compared with the negligible value of the line impedance, each inverter can generate the same voltage reference value vd *And vq *Therefore, as long as the control parameters of the voltage and current droop relational expression of each inverter in claim 3 are ensured to be the same, accurate current sharing among the parallel inverters can be realized.
Specifically, the simulation model of the invention is composed of a system formed by connecting two identical inverters in parallel. And in order to clearly compare with the traditional droop control, a simulation model of the traditional droop control is also established for simulation. Simulation parameters see Table 1, where Zline1And Zline2Respectively, the line impedances of the two inverters.
TABLE 1 simulation parameters
The two sets of simulations are respectively current sharing effect comparison and dynamic response speed comparison of the two control methods, and simulation results are respectively shown in fig. 3 and fig. 4. In fig. 3 and 4, (a1), (a2), (a3) are graphs of a direct-axis component, a quadrature-axis component and a frequency waveform of current under the conventional droop control method, respectively; (b1) the (b2) and (b3) are respectively a direct-axis component, a quadrature-axis component and a frequency waveform diagram of the current under the control method provided by the invention. In fig. 3, the active load increases in the 1 st second, and it can be seen from the simulation waveform that even under the condition of different line impedances, the control method proposed in the present invention has better current sharing effect than the traditional droop control; in fig. 4, at the 1.45 th second, two inverters start to operate in parallel, and it can be seen from the simulation waveforms that the control method proposed in the present invention has faster dynamic response speed and more stable transient response compared to the conventional droop control method. Simulation results prove that the method has a more accurate current sharing effect and a faster and more stable dynamic response speed.
The experimental platform is formed by connecting two inverters (MWINV-9R144) with the same model in parallel. See Table 2 for experimental parameters, wherein Zline1And Zline2Respectively, the line impedances of the two inverters.
TABLE 2 Experimental parameters
| Parameter(s) | Numerical value | Parameter(s) | Numerical value |
| m | 2V/A | n | 2V/A |
| vd0 | 200V | vq0 | 0V |
| i'd0 | 0A | i'q0 | 0A |
| Zline1 | 1.0Ω+2.8mH | Zline2 | 2.8mH |
Fig. 5 and 6 show experimental waveforms of the control method according to the present invention, i.e. the output power waveform and the steady-state voltage-current waveform of the control method according to the present invention. In fig. 5, (a) is a waveform diagram of the active power output by the parallel inverters; (b) outputting a waveform diagram of reactive power for the parallel inverters; in fig. 6, (a) a graph of voltage and current waveforms output by parallel inverters before load change; (b) the voltage and current waveform diagram of the parallel inverter output after load change. In the experiment shown in fig. 5, the reactive load is reduced at the 21 st second, and the active power and the reactive power output by the inverter are accurately and evenly divided as can be seen from the experimental waveforms; as can also be seen from fig. 6, before and after the load is switched, the parallel inverter using the method of the present invention has good current sharing characteristics. Therefore, experiments prove that the method has a good current sharing effect.
In summary, the invention provides a decoupling current droop control method in a synchronous rotating coordinate system. In order to verify the feasibility of the control method, a simulation model with two inverters connected in parallel is built in simulation software PSCAD, and an experimental platform is built by using the two MWINV-9R144 inverters for hardware verification. Simulation and experiment results prove that compared with the traditional droop control method, the method disclosed by the invention can still realize accurate equalization of the output current of each inverter under the condition of different line impedances, and has faster dynamic response and good system stability. The method is correct and reliable, and provides a good reference value for engineering application.
Claims (6)
1. A decoupling current droop control method for parallel inverters in a microgrid is characterized by comprising the following steps:
1) in an alternating-current microgrid, N inverters which are connected in parallel are included, and each inverter is equally controlled as a voltage source by the same control strategy;
2) each inverter acquires voltage phase information at two ends of a capacitor through a voltage phase-locked loop, acquires a direct-axis component and a quadrature-axis component of a voltage vector, and generates a reference value of voltage frequency; wherein, a proportional regulator is adopted in the voltage phase-locked loop, so that the frequency reference value is near the fundamental frequency and all inverters are kept synchronous;
the capacitor is a capacitor of an LC filter at the output side of the inverter;
3) each inverter acquires phase information of current through a current phase-locked loop to acquire a direct-axis component and a quadrature-axis component of a current vector;
4) performing rotation transformation on the current vector according to the voltage phase information and the current phase information acquired in the step 2) and the step 3) to obtain a virtual current vector, wherein the virtual current vector and the voltage vector have the same direction;
5) in a synchronous rotating coordinate system, establishing a droop relation between the virtual current vector in the step 4) and the voltage vector in the step 2), wherein the droop relation is used for generating a reference value of a direct-axis component and a quadrature-axis component of the voltage, and the reference value is input into a voltage current regulator to generate a PWM modulation signal.
2. The method for controlling the droop of the decoupling current of the parallel inverters in the microgrid according to claim 1, wherein the virtual current in the step 4) comprises a direct-axis componenti' d And quadrature componenti' q Component of direct axisi' d And quadrature componenti' q Calculated from the following formula:
wherein the content of the first and second substances,δis the phase angle difference between the output voltage and the output current calculated by the voltage-current phase-locked loop,i d andi q respectively, a direct-axis component and a quadrature-axis component of the current in the synchronous rotating coordinate system.
3. The method for controlling the droop of the decoupling current of the parallel inverters in the microgrid according to claim 2, wherein the droop relation established in the step 5) is calculated by the following formula:
；
；
wherein the content of the first and second substances,v d * andv q * respectively are reference values of a direct-axis component and a quadrature-axis component of the voltage under a synchronous rotating coordinate system;v d0andv q0are respectivelyv d * Andv q * a nominal value of (d);mandndefined as a positive value, is the gain of droop control;i' d andi' q respectively a direct-axis component and a quadrature-axis component of the virtual current under the synchronous rotating coordinate system;i' d0andi' q0are respectivelyi' d Andi' q of the target value of (c).
4. The method for controlling the droop of the decoupling current of the parallel inverters in the microgrid according to claim 3, wherein the control formula is obtained according to the virtual current calculation formula and the droop relation calculation formula as follows:
selectingmAndnis the same value, i.e. it is considered to introduce a virtual impedance in series with the line impedance, whose value is:
wherein the content of the first and second substances,R v andL v respectively a virtual resistance value and a virtual inductance value;ω base is the fundamental frequency.
5. The method for controlling the droop of the decoupling current of the parallel inverters in the microgrid according to claim 4, characterized in that the selected value is large enoughmSuch that it satisfies:
wherein the content of the first and second substances,R line andL line the resistance value and the inductance value of the transmission line impedance are respectively, m meets the condition that the value of the virtual impedance is far larger than that of the line impedance, the line impedance value can be ignored, and each inverter generates the same voltage reference valuev d * Andv q * 。
6. the method of controlling droop on decoupled current of parallel inverters in microgrid of any of claim 1 ~ 5, characterized in that proportional-integral regulator is used in current phase locked loop.
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Effective date of registration: 20230717 Address after: Room 101, 1st Floor, South Cross, Tianhong Park, No. 25 Biyuan 1st Road, High tech Zone, Xi'an City, Shaanxi Province, 710199 Patentee after: Xi'an Singularity Energy Co.,Ltd. Address before: Beilin District Xianning West Road 710049, Shaanxi city of Xi'an province No. 28 Patentee before: XI'AN JIAOTONG University |