CN110311375A - A kind of micro-capacitance sensor transient stability control method containing more virtual synchronous machines - Google Patents
A kind of micro-capacitance sensor transient stability control method containing more virtual synchronous machines Download PDFInfo
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Abstract
The present invention discloses a kind of micro-capacitance sensor transient stability control method containing more virtual synchronous machines, belongs to micro-capacitance sensor Transient Stability Analysis and control technology field.This method has fully considered virtual inertia, automatic virtual blocks, the sagging coefficient of virtual governor and the sagging coefficient of idle control ring, the energy function model of the micro-capacitance sensor containing more virtual synchronous machine parallel runnings is constructed, based on energy function come the stability of analysis judgment system.And use the self-adaptation control method based on energy function, adaptive adjustment in real time is carried out respectively to inertia virtual in VSG, automatic virtual blocks, the sagging coefficient of virtual governor and the sagging coefficient of idle control ring, system capacity after reducing the system failure, the domain of attraction of increase system, slow down the process that system capacity reaches transition energy, strive for effective time for failure removal, improves the transient stability of system.
Description
Technical Field
The invention relates to the technical field of micro-grid transient stability analysis and control, in particular to a micro-grid transient stability control method with multiple virtual synchronous machines.
Background
In recent years, depletion of fossil energy and exacerbation of environmental crisis have promoted rapid development of distributed power generation technology. Micro-grids are becoming increasingly popular in modern power networks as systems of Distributed Generation (DG), energy storage devices, power electronic converters, loads, and so on. According to the virtual synchronous machine control strategy (VSG), an electromechanical transient equation similar to that of a traditional synchronous generator SG is introduced into a control link of a power electronic converter, and the distributed power supply can simulate the external operation characteristics of the SG, such as active frequency modulation and reactive voltage regulation, and also has dynamic characteristics of inertia characteristics, damping characteristics and the like. When the micro-grid or the large-grid is disturbed or fails, the VSG can effectively adjust the frequency and the voltage and maintain the stability of the system. In addition, virtual inertia in an active control loop (APCM), a virtual damping coefficient, a droop coefficient in VG and a droop coefficient in a reactive control loop (QPCM) of the system can be flexibly adjusted according to actual needs to improve the stability of the system, and corresponding parameters of the traditional SG are fixed.
The existing transient stability analysis of the microgrid with the VSG is basically carried out according to two indexes of frequency change rate and frequency change amount after the system is disturbed, and the transient stability control is basically to improve the frequency and active power output of the system by adjusting two parameters of virtual inertia or virtual damping. Although convenient and direct, the analysis method is single, the analysis of the fault conditions such as voltage dip, short circuit and the like is slightly insufficient, and the change mechanism and the reaction change essence of the system cannot be explained only by the frequency. For the VSG, in addition to the virtual inertia and damping coefficients, the droop coefficient in VG and that of QPCM may also affect its transient stability. In addition, the transient stability analysis based on frequency needs time domain simulation to verify the effect of the VSG on the transient stability of the microgrid, the stability margin of the system is difficult to judge, the disturbed degree of the system cannot be quantitatively measured, and when the microgrid comprises a plurality of VSGs which are connected in parallel, the frequency deviation among different VSGs also easily influences the accuracy of the analysis.
Disclosure of Invention
In view of the above-mentioned deficiencies of the prior art, the present invention provides a method for controlling transient stability of a micro-grid including multiple virtual synchronous machines.
In order to solve the above technical problem, the technical solution adopted by the present invention is a method for controlling transient stability of a micro-grid including multiple virtual synchronous machines, the flow of which is shown in fig. 1, and the method includes the following steps:
step 1: constructing a power balance model of a grid-connected microgrid with a plurality of VSGs running in parallel, wherein the model comprises a VSG equivalent output power model, a connection line equivalent transmission power model and a power balance equation of the grid-connected microgrid;
step 1.1: constructing a VSG equivalent output power model, wherein the VSG equivalent output power model comprises a parameter virtual speed regulator VG, a virtual inertia and a virtual damping coefficient and comprises an active control module APCM and a reactive control module QPCM;
step 1.1.1: the active control module APCM is composed of VG and a virtual mechanical rotation module, and simulates the mechanical motion equations of a speed regulator and a rotor of the SG respectively, and the active control module APCM specifically comprises the following components:
wherein, Pei、PrefiAn active power reference value set for the output active power of the ith VSG (VSGi, i ═ 1, 2.., N) in the microgrid and the controller respectively;the virtual moment of inertia and the virtual damping coefficient are respectively VSGi and are used for simulating the rotor motion characteristic of SG; omegan、ωiRated angular velocity of VSGi and virtual rotor angular velocity of VSGi, ωni＝ωn-ωiRepresents the difference between the two; deltaniThe relative displacement angle of the VSGi virtual rotor is represented, and the phase angle difference between the output voltage and the rated voltage in the QPCM is also represented;andthe virtual mechanical power and the virtual droop coefficient of the VG output in VSGi are represented.
Step 1.1.2: the reactive control module QPCM, which is for simulating the reactive-voltage regulation characteristics of the VSG, is specifically as follows:
wherein Q isei，QrefiThe reactive power reference value is respectively set for the output reactive power of the VSGi and the controller; ei、EnRespectively representing the voltage amplitude and the rated voltage amplitude of the VSGi output;the QPCM droop coefficient.
Step 1.2: constructing an equivalent transmission power model of a contact line:
wherein,respectively representing equivalent active and reactive transmission power of VSGi in the micro-grid connected with the grid;representing the transmission line impedance and the inductive reactance, respectively;
step 1.3: considering the system load condition, establishing a power balance equation of the grid-connected microgrid:
wherein, PLi、QLiRespectively representing the active power and the reactive power of a load connected with the microgrid;
step 2: the first-time integration method is utilized to construct a Lyapunov energy function of a grid-connected microgrid with VG, virtual inertia and virtual damping coefficient considered and with multiple VSGs running in parallel, and the total energy V in the microgrid system is calculatedTOT；
Step 2.1: constructing a kinetic energy total energy function V considering weight relationKE：
Step 2.1.1: setting weight factors for kinetic energy itemsWherein M iskiA threshold value of a kinetic energy function, i is 1,2, 3;
step 2.1.2: the method comprises the following steps of constructing kinetic energy caused by acceleration/deceleration of virtual rotors simulated by virtual inertia of all VSGs in the microgrid, and specifically comprising the following steps:
wherein M isk1As a function of kinetic energy VKE1A threshold value of (d);
step 2.1.3: the method comprises the following steps of constructing a system kinetic energy function consumed by a virtual damping coefficient and a droop coefficient of a virtual speed regulator, and specifically comprising the following steps:
wherein, tsIn order to be the time when the failure occurs,representing the angular frequency of rotation, M, at the point of stable equilibrium of the system after a faultk2As a function of kinetic energy VKE2A threshold value of (d);
step 2.1.4: the kinetic energy consumed in performing one adjustment of frequency and power is constructed as follows:
wherein, χ ═ d ω/dt, Mk3As a function of kinetic energy VKE3A threshold value of (d);
step 2.1.5: constructing a kinetic energy total energy function V according to the steps 2.1.1-2.14KE：
VKE＝(VKE1+VKE2+VKE3) (8)
Step 2.2: constructing a potential energy function V considering the weight relationPE：
Step 2.2.1: setting weight factors of different potential energy itemsWherein M ispiA threshold value of a potential energy function is defined as i ═ 1,2,3 and 4;
step 2.2.2: the method comprises the following steps of constructing potential energy functions caused by active reference power and active loads of all VSGs in the micro-grid system, and specifically comprising the following steps:
wherein M isp1As a function of potential energy VPE1A threshold value of (d);
step 2.2.3: constructing a potential energy function caused by reactive reference power and reactive load of the VSG, which comprises the following steps:
wherein,amplitude of the system voltage after a fault, Mp2As a function of potential energy VPE2A threshold value of (d);
step 2.2.4: constructing a potential energy function represented by active power transmitted on a connecting line between the VSG and a power grid, wherein the potential energy function is as follows:
wherein, is the phase angle of the system voltage after the fault, Mp3As a function of potential energy VPE3A threshold value of (d);
step 2.2.5: constructing a potential energy function represented by reactive power transmitted on a connecting line between the VSG and the power grid, wherein the potential energy function is as follows:
wherein M isp4As a function of potential energy VPE4A threshold value of (d);
step 2.2.6: according to the steps 2.2.1-2.2.5, a potential energy total energy function is constructed, which is as follows:
VPE＝VPE1+VPE2+VPE3+VPE4 (13)
step 2.3: calculating total energy function V of grid-connected microgridTOT：
VTOT＝VPE+VKE。 (14)
And step 3: setting a threshold value of an energy function according to the field requirement, and judging the running state of the system;
wherein M isAVEThreshold value of energy function, V, for stable operation of systemTOTIs the total energy in the microgrid system.
Step 4, if the system runs stably, executing the step 2 to continue calculating;
and 5: if the system fails, calculating the critical energy V of the system by adopting a dominant unstable balance point methodcr；
Step 5.1: calculating the time from the system fault track to the fault clearing time and the exit point, specifically as follows:
step 5.2: taking the exit point as an initial value, and calculating the minimum gradient point of the gradient system after the fault by adopting a Gear method, wherein the method specifically comprises the following steps:
wherein,is the minimum gradient point;
step 5.3: at the point of minimum gradientAs an initial value, solving a power balance equation of the system to obtain a stable balance point and a dominant unstable balance point of the system, which are specifically as follows:
step 5.4: calculating critical energy of the microgrid system according to state quantity at dominant unstable balance point of the system
Wherein,respectively setting a stable balance point and an unstable balance point of VSGi in the microgrid after the system fault;respectively representing the virtual inertia, the virtual damping coefficient, the VG droop coefficient and the QPCM droop coefficient of VSGi during the stable operation of the microgrid;representing the angular frequency of rotation, the phase angle and the amplitude of the voltage, χ ═ d ω/dt, t, respectively, at the point of stable equilibrium of the system after a faultsIs the time of occurrence of the failure.
Step 6: starting a self-adaptive control method of VSG virtual inertia, a virtual damping coefficient, a VG droop coefficient and a QPCM droop coefficient based on an energy function;
step 6.1: the VSG virtual inertia self-adaptive control method based on the energy function comprises the following steps:
wherein,respectively the virtual inertia and the maximum value of the virtual inertia, M, of VSGi when the micro-grid operates stablyAVEIs a threshold value as a function of the total energy.
Step 6.2: the self-adaptive control method of the VSG virtual damping coefficient based on the energy function comprises the following steps:
wherein,respectively representAnd when the micro-grid stably runs, the virtual damping coefficient of the VSGi and the maximum value of the virtual damping coefficient.
Step 6.3: designing an adaptive control method of a VG droop coefficient based on an energy function:
wherein,and respectively representing the VG droop coefficient and the VG droop coefficient minimum value of VSGi in the stable operation of the microgrid.
Step 6.4: the self-adaptive control method for designing the QPCM droop coefficient based on the energy function comprises the following steps:
in the formula,and respectively representing the maximum value of the QPCM droop coefficient and the QPCM droop coefficient of the VSGi during the stable operation of the micro-grid.
And 7: judging whether the fault is cleared;
and 8: step 2 is executed to calculate the total energy V in the microgrid system at the moment of fault clearancecl；
And step 9: total energy V in microgrid system at fault clearing timeclAnd critical energy VcrAnd comparing, analyzing and judging the stability of the microgrid.
Adopt the produced beneficial effect of above-mentioned technical scheme to lie in:
1. in the invention, the virtual inertia, the virtual damping, the droop coefficient of the virtual speed regulator and the droop coefficient of the reactive control loop are considered, and the energy function model of the microgrid with a plurality of virtual synchronous machines running in parallel is constructed, so that the influence of disturbance on the microgrid system can be analyzed, and the change of system energy after a fault occurs can be reflected.
2. According to the invention, the stability of the system is analyzed and judged based on the energy function, when the system fails, the change of the system energy is far greater than the change of the frequency, so that the detection and calculation are convenient, the sensitivity to the failure is high, the timely and effective control can be favorably carried out at the initial stage of the failure, the damage of the failure to the system is reduced, and the transient stability of the system is improved.
3. According to the invention, a self-adaptive control method of 4 VSG parameters based on an energy function is adopted for the first time in a microgrid with a plurality of virtual synchronous machines running in parallel, the self-adaptive control method of the 4 parameters is respectively designed according to the influence of virtual inertia, virtual damping, a virtual speed regulator droop coefficient and a reactive control ring droop coefficient in the VSG on system energy, the system energy after system failure is reduced through real-time self-adaptive adjustment of the VSG parameters, the attraction domain of the system is increased, the process that the system energy reaches critical energy is slowed down, effective time is strived for fault removal, and the transient stability of the system is improved.
4. The transient stability of the micro-grid with a plurality of virtual synchronous machines running in parallel is analyzed by using the transient energy after the fault, so that the transient stability of the system can be judged, and the stability margin of the system can be quantitatively measured.
Drawings
FIG. 1 is a flowchart of a method for controlling transient stability of a micro-grid including multiple virtual synchronizers according to the present invention;
FIG. 2 is a system diagram and a control structure diagram of a virtual synchronous machine according to an embodiment of the present invention;
FIG. 3 is an equivalent circuit diagram of an embodiment of the present invention;
FIG. 4 is a diagram of energy variation for a microgrid system according to an embodiment of the present invention;
FIG. 5 is a schematic diagram illustrating adaptive control changes of a virtual inertia, a virtual damping coefficient, a VG droop coefficient, and a QPCM droop coefficient of a VSG according to an energy function in an embodiment of the present invention;
fig. 6 is a schematic diagram of actual values of the virtual inertia, the virtual damping coefficient, the VG droop coefficient, and the QPCM droop coefficient of the VSG according to the embodiment of the present invention.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
In this embodiment, a grid-connected microgrid including three virtual synchronous machines operating in parallel is used for analysis, and a system structure diagram and a control block diagram of a VSG of the embodiment are shown in fig. 2. The three VSGs are operated in parallel to supply power to the load and are connected with a power grid through a connecting line, an equivalent circuit diagram is shown in figure 3, the three VSGs have the same rated capacity, the active rated power is 6kW, the reactive rated power is 1kVA, the rated voltage is 380V, the rated angular speed is 314rad/s, and the VSG and the power grid jointly supply power to the load (20kW 4 kVA).
Step 1: constructing a power balance model of a grid-connected microgrid with three VSGs running in parallel, wherein the model comprises a VSG equivalent output power model, a connection line equivalent transmission power model and a power balance equation of the grid-connected microgrid;
step 1.1: constructing a VSG equivalent output power model, wherein the VSG equivalent output power model comprises a parameter virtual speed regulator VG, a virtual inertia and a virtual damping coefficient and comprises an active control module APCM and a reactive control module QPCM;
step 1.1.1: the active control module APCM is composed of VG and a virtual mechanical rotation module, and simulates the mechanical motion equations of a speed regulator and a rotor of the SG respectively, and the active control module APCM specifically comprises the following components:
wherein, Pei、PrefiAn active power reference value set for the output active power of the ith VSG (VSGi, i is 1,2 and 3) in the micro-grid and the controller respectively;are respectively asThe virtual moment of inertia and the virtual damping coefficient of the VSGi are used for simulating the rotor motion characteristic of the SG; omegan、ωiRated angular velocity of VSGi and virtual rotor angular velocity of VSGi, ωni＝ωn-ωiRepresents the difference between the two; deltaniThe relative displacement angle of the VSGi virtual rotor is represented, and the phase angle difference between the output voltage and the rated voltage in the QPCM is also represented;andthe virtual mechanical power and the virtual droop coefficient of the VG output in VSGi are represented. In this embodiment, three VSGs have the same rated capacity and have the same active power reference value PrefiAre all 6kW and the rated angular velocity omegan＝314rad/s；
Step 1.1.2: the reactive control module QPCM, which is for simulating the reactive-voltage regulation characteristics of the VSG, is specifically as follows:
wherein Q isei，QrefiThe reactive power reference value is respectively set for the output reactive power of the VSGi and the controller; ei、EnRespectively representing the voltage amplitude and the rated voltage amplitude of the VSGi output;the QPCM droop coefficient. Reactive power reference value Q of three VSGs in this embodimentrefiAll of which are 1kVA, rated voltage amplitude E of the microgridn＝380V；
Step 1.2: constructing an equivalent transmission power model of a contact line:
wherein,respectively representing equivalent active and reactive transmission power of VSGi in the micro-grid connected with the grid;representing the transmission line impedance and the inductive reactance, respectively; impedance and inductive reactance of transmission line in this embodiment0.64 omega/km and 0.10 omega/km respectively, amplitude E of rated voltage of the microgridn380V, phase angle deltan＝0°；
Step 1.3: considering the system load condition, establishing a power balance equation of the grid-connected microgrid:
wherein, PLi、QLiRespectively representing the active power and the reactive power of a load connected with the microgrid; active power P of load connected to microgrid in this embodimentLiReactive power QLi20kW and 4kVA respectively;
step 2: the first-time integration method is utilized to construct a Lyapunov energy function of a grid-connected microgrid with VG, virtual inertia and virtual damping coefficient considered and with multiple VSGs running in parallel, and the total energy V in the microgrid system is calculatedTOT；
Step 2.1: constructing a kinetic energy total energy function V considering weight relationKE：
Step 2.1.1: setting weight factors for kinetic energy itemsWherein M iskiA threshold value of a kinetic energy function, i is 1,2, 3;
step 2.1.2: the method comprises the following steps of constructing kinetic energy caused by acceleration/deceleration of virtual rotors simulated by virtual inertia of all VSGs in the microgrid, and specifically comprising the following steps:
wherein M isk1＝140；
Step 2.1.3: the method comprises the following steps of constructing a system kinetic energy function consumed by a virtual damping coefficient and a droop coefficient of a virtual speed regulator, and specifically comprising the following steps:
wherein, ts＝6s，Mk2＝170，Representing the rotational angular frequency at the stable equilibrium point of the system after the fault;
step 2.1.4: the kinetic energy consumed in performing one adjustment of frequency and power is constructed as follows:
wherein, χ ═ d ω/dt, Mk3＝500；
Step 2.1.5: constructing a kinetic energy total energy function V according to the steps 2.1.1-2.14KE：
VKE＝(VKE1+VKE2+VKE3) (8)
Step 2.2: constructing a potential energy function V considering the weight relationPE：
Step 2.2.1: setting weight factors of different potential energy itemsWherein M ispiA threshold value of a potential energy function is defined as i ═ 1,2,3 and 4;
step 2.2.2: the method comprises the following steps of constructing potential energy functions caused by active reference power and active loads of all VSGs in the micro-grid system, and specifically comprising the following steps:
wherein M isP1＝1.8×103；
Step 2.2.3: constructing a potential energy function caused by reactive reference power and reactive load of the VSG, which comprises the following steps:
wherein,amplitude of the system voltage after a fault, MP2＝5×103；
Step 2.2.4: constructing a potential energy function represented by active power transmitted on a connecting line between the VSG and a power grid, wherein the potential energy function is as follows:
wherein, is the phase angle of the system voltage after the fault, MP3＝1.12×103；
Step 2.2.5: constructing a potential energy function represented by reactive power transmitted on a connecting line between the VSG and the power grid, wherein the potential energy function is as follows:
wherein M isP4＝7.33×103；
Step 2.2.6: according to the steps 2.2.1-2.2.5, a potential energy total energy function is constructed, which is as follows:
VPE＝VPE1+VPE2+VPE3+VPE4 (13)
step 2.3: calculating total energy function V of grid-connected microgridTOT：
VTOT＝VPE+VKE。 (14)
In the embodiment, when a plurality of VSGs running in parallel are designed to stably run for 6s, a short-circuit fault occurs at a grid-connected position, and the fault is removed at 6.1 s. When the micro-grid operates stably, the total energy of the micro-grid is 1kJ, and after a fault occurs, the change of the total energy is shown in fig. 4.
And step 3: setting a threshold value of an energy function according to the field requirement, and judging the running state of the system;
wherein M isAVEThreshold value of energy function, V, for stable operation of systemTOTIs the total energy in the microgrid system.
In order to make the system more sensitive to faults in this embodiment, a threshold M for the total energy function is setAVE＝VTOT1kJ, in the period from 5.8s to 6s in fig. 4, the total energy of the system is calculated to be less than or equal to 1kJ, and the system is judged to be in a stable state;
step 4, if the system runs stably, executing the step 2 to continue calculating;
calculating the total energy V of the system when the time is 6sTOTWhen the voltage is more than 1kJ, the system can be judged to have faults according to the method.
And 5: if the system fails, calculating the critical energy V of the system by adopting a dominant unstable balance point methodcr；
Step 5.1: calculating the time from the system fault track to the fault clearing time and the exit point, specifically as follows:
step 5.2: taking the exit point as an initial value, and calculating the minimum gradient point of the gradient system after the fault by adopting a Gear method, wherein the method specifically comprises the following steps:
wherein,is the minimum gradient point;
step 5.3: at the point of minimum gradientAs an initial value, solving a power balance equation of the system to obtain a stable balance point and a dominant unstable balance point of the system, which are specifically as follows:
step 5.4: calculating critical energy V of the microgrid system according to state quantity at dominant unstable balance point of the systemcr：
Wherein,respectively setting a stable balance point and an unstable balance point of VSGi in the microgrid after the system fault;respectively representing the virtual inertia, the virtual damping coefficient, the VG droop coefficient and the QPCM droop coefficient of VSGi during the stable operation of the microgrid;representing the angular frequency of rotation, the phase angle and the amplitude of the voltage, χ ═ d ω/dt, t, respectively, at the point of stable equilibrium of the system after a faultsIs the time of occurrence of the failure.
Time t when fault occurs in the present embodimentsWhen the time is equal to 6s,calculating to obtain the critical energy V of the systemcr＝3.25kJ。
Step 6: starting a self-adaptive control method of VSG virtual inertia, a virtual damping coefficient, a VG droop coefficient and a QPCM droop coefficient based on an energy function;
step 6.1: the VSG virtual inertia self-adaptive control method based on the energy function comprises the following steps:
wherein the maximum value of virtual inertia of VSGi when the micro-grid is stably operatedThreshold value M of energy functionAVE＝1kJ；
Step 6.2: the self-adaptive control method of the VSG virtual damping coefficient based on the energy function comprises the following steps:
wherein, when the micro-grid operates stably, the maximum value of the virtual damping coefficient of VSGi
Step 6.3: designing an adaptive control method of a VG droop coefficient based on an energy function:
wherein, VG droop coefficient minimum value of VSGi during stable operation of micro-grid
Step 6.4: the self-adaptive control method for designing the QPCM droop coefficient based on the energy function comprises the following steps:
wherein, when the micro-grid is in stable operation, the maximum value of QPCM droop coefficient of VSGi
In this embodiment, the adaptive control changes of the virtual inertia, the virtual damping coefficient, the VG droop coefficient, and the QPCM droop coefficient of the VSG according to the energy function are shown in fig. 5, and specific values thereof are shown in fig. 6.
And 7: fault clearance at 6.1 s;
and 8: step 2 is executed to calculate the total energy V in the microgrid system at the moment of fault clearancecl；
And step 9: total energy V in microgrid system at fault clearing timeclAnd critical energy VcrAnd comparing, analyzing and judging the stability of the microgrid.
In this embodiment, due to the application of the adaptive control method, the total energy V in the microgrid system after the fault is clearedclBelow critical energy VcrAccording to the method, the system stability can be judged.
As shown in fig. 4, if there is no adaptive control of the virtual inertia, the virtual damping coefficient, the VG droop coefficient, and the QPCM droop coefficient, the energy of the system will exceed the system critical energy V at about 6.08scrThe method can judge the system instability, at the moment, if the fault is not removed in time, huge damage is certainly brought to the microgrid system, and after the fault is removed in 6.1s, the total energy of the system is reduced to the critical energy V in 6.12scrIn this case, the system can be judged to be stable. After the adaptive control of the virtual inertia, the virtual damping coefficient, the VG droop coefficient and the QPCM droop coefficient is added, the total energy of the system in the fault period is relatively small, the growth trend is slower than that without the adaptive control, effective time is strived for fault removal, and when the fault is removed in 6.1sTotal energy of the system VTOTDoes not exceed the critical energy VcrThe system has no instability state, and good transient stability is maintained.
Claims (9)
1. A micro-grid transient stability control method containing multiple virtual synchronous machines is characterized by comprising the following steps:
step 1: constructing a power balance model of a grid-connected microgrid with a plurality of VSGs running in parallel, wherein the model comprises a VSG equivalent output power model, a connection line equivalent transmission power model and a power balance equation of the grid-connected microgrid;
step 2: first-time integration method is utilized to construct Lyapunov energy function V of grid-connected microgrid with VG, virtual inertia and virtual damping coefficient considered and multiple VSGs running in parallelTOTCalculating the total energy in the microgrid system;
and step 3: setting a threshold value of an energy function according to the field requirement, and judging the running state of the system;
step 4, if the system runs stably, executing the step 2 to continue calculating;
and 5: if the system fails, calculating the critical energy V of the system by adopting a dominant unstable balance point methodcr；
Step 6: starting a self-adaptive control method of VSG virtual inertia, a virtual damping coefficient, a VG droop coefficient and a QPCM droop coefficient based on an energy function;
and 7: judging whether the fault is cleared;
and 8: step 2 is executed to calculate the total energy V in the microgrid system at the moment of fault clearancecl；
And step 9: total energy V in microgrid system at fault clearing timeclAnd critical energy VcrAnd comparing, analyzing and judging the stability of the microgrid.
2. The method according to claim 1, wherein the method comprises: the VSG equivalent output power model in the step 1 comprises a parameter virtual speed regulator VG, a virtual inertia and a virtual damping coefficient and comprises an active control module APCM and a reactive control module QPCM;
the active control module APCM is composed of VG and a virtual mechanical rotation module, and simulates the mechanical motion equations of a speed regulator and a rotor of the SG respectively, and the active control module APCM specifically comprises the following components:
wherein, Pei、PrefiAn active power reference value set for the output active power of the ith VSG (VSGi, i ═ 1, 2.., N) in the microgrid and the controller respectively;the virtual moment of inertia and the virtual damping coefficient are respectively VSGi and are used for simulating the rotor motion characteristic of SG; omegan、ωiRated angular velocity of VSGi and virtual rotor angular velocity of VSGi, ωni＝ωn-ωiRepresents the difference between the two; deltaniThe relative displacement angle of the VSGi virtual rotor is represented, and the phase angle difference between the output voltage and the rated voltage in the QPCM is also represented;andrepresenting the virtual mechanical power and the virtual droop coefficient of the VG output in the VSGi;
the reactive power control module QPCM simulates the reactive-voltage regulation characteristics of the VSG, as follows:
wherein Q isei，QrefiThe reactive power reference value is respectively set for the output reactive power of the VSGi and the controller; ei、EnRespectively representing the voltage amplitude and the rated voltage amplitude of the VSGi output；The QPCM droop coefficient.
3. The method according to claim 1, wherein the method comprises: in the step 1, an equivalent transmission power model of the interconnection line is constructed, and a calculation formula is as follows:
wherein,respectively representing equivalent active and reactive transmission power of VSGi in the micro-grid connected with the grid;representing the transmission line impedance and the inductive reactance, respectively; deltaniRepresenting the phase angle difference between the rated voltage of the power grid and the output voltage of the VSG; ei、EnRepresenting the voltage magnitude of the VSGi output and the nominal voltage magnitude, respectively.
4. The method according to claim 1, wherein the method comprises: in the step 1, the power balance equation of the grid-connected microgrid considers the system load condition, and the following equation is constructed:
wherein, Prefi、QrefiThe active power reference value and the reactive power reference value are respectively set for the controller; omegan、ωiA nominal angular velocity of VSGi and a virtual rotor angular velocity of VSGi, respectively; ei、EnRespectively representing the voltage amplitude and rating of the VSGi outputA voltage amplitude;respectively representing the impedance and the inductive reactance of the communication line;virtual moment of inertia and virtual damping coefficient of VSGi respectively; deltaniRepresenting the phase angle difference between the rated voltage of the power grid and the VSGi output voltage; pLi、QLiRespectively representing the active power and the reactive power of a load connected with the microgrid;a virtual droop coefficient representing the VG output in VSGi;the QPCM droop coefficient.
5. The method according to claim 1, wherein the method comprises: in the step 2, a first integration method is utilized to construct a Lyapunov energy function V of a grid-connected microgrid with a plurality of VSGs running in parallel and considering VG, virtual inertia and virtual damping coefficientTOTThe process of (2) is as follows:
step 2.1: constructing a kinetic energy function V taking into account a weight relationshipKE；
Step 2.1.1: setting weight factors for kinetic energy itemsWherein M iskiA threshold value of a kinetic energy function, i is 1,2, 3;
step 2.1.2: the method comprises the following steps of constructing kinetic energy caused by acceleration/deceleration of virtual rotors simulated by virtual inertia of all VSGs in the microgrid, and specifically comprising the following steps:
wherein,virtual moment of inertia, ω, for VSGinRated angular velocity, ω, of VSGiniIs the difference between the rated angular velocity of VSGi and the virtual rotor angular velocity of VSGi;
step 2.1.3: the method comprises the following steps of constructing a system kinetic energy function consumed by a virtual damping coefficient and a droop coefficient of a virtual speed regulator, and specifically comprising the following steps:
wherein, tsIn order to be the time when the failure occurs,indicating the rotational angular frequency at the stable equilibrium point of the system after a fault,for the virtual damping coefficient of VSGi,a virtual droop coefficient representing the VG output in VSGi;
step 2.1.4: the kinetic energy consumed in performing one adjustment of frequency and power is constructed as follows:
wherein χ ═ d ω/dt;
step 2.1.5: constructing a kinetic energy total energy function V according to the steps 2.1.1-2.14KE：
VKE＝(VKE1+VKE2+VKE3) (8)
Step 2.2: constructing a potential energy function V considering the weight relationPE；
Step 2.2.1: setting weight factors of different potential energy itemsWherein M ispiA threshold value of a potential energy function is defined as i ═ 1,2,3 and 4;
step 2.2.2: the method comprises the following steps of constructing potential energy functions caused by active reference power and active loads of all VSGs in the micro-grid system, and specifically comprising the following steps:
wherein, deltaniRepresenting the phase angle difference, P, between the rated voltage of the grid and the VSGi output voltagerefiActive power reference value, P, set for the controllerLiActive power for a load connected to the microgrid;
step 2.2.3: constructing a potential energy function caused by reactive reference power and reactive load of the VSG, which comprises the following steps:
wherein,amplitude of the system voltage after a fault, QrefiReactive power reference value, Q, set for the controllerLiReactive power of loads connected to the microgrid, EniThe difference value of the rated voltage of the power grid and the rated voltage of the ith VSG in the micro-grid is obtained;
step 2.2.4: constructing a potential energy function represented by active power transmitted on a connecting line between the VSG and a power grid, wherein the potential energy function is as follows:
wherein, is the phase angle of the system voltage after the fault,respectively representing the impedance and inductive reactance of the interconnection line, EiThe voltage amplitude output for VSGi;
step 2.2.5: constructing a potential energy function represented by reactive power transmitted on a connecting line between the VSG and the power grid, wherein the potential energy function is as follows:
wherein,for QPCM sag factor, EnA rated voltage amplitude of VSGi;
step 2.2.6: according to the steps 2.2.1-2.2.5, a potential energy total energy function is constructed, which is as follows:
VPE＝VPE1+VPE2+VPE3+VPE4 (13)
step 2.3: calculating total energy function V of grid-connected microgridTOT：
VTOT＝VPE+VKE。 (14)
6. The method according to claim 1, wherein the method for determining the operating state of the system in step 3 is as follows:
wherein M isAVEThreshold value of energy function, V, for stable operation of systemTOTThe total energy in the microgrid system calculated for step 2 in claim 1.
7. The method according to claim 1, wherein the critical energy V of the system is calculated by using a dominant unstable equilibrium point method in the step 5crThe process of (2) is as follows:
step 5.1: calculating the time from the system fault track to the fault clearing time and the exit point, specifically as follows:
wherein,virtual moment of inertia and virtual damping coefficient, P, of VSGi, respectivelyrefiActive power reference value, omega, set for the controllern、ωiRated angular velocity of VSGi and virtual rotor angular velocity of VSGi, ωni＝ωn-ωiRepresents the difference between the two, PeiFor the output active power, delta, of the ith VSG in the microgridiIs the phase angle of the rated voltage of the ith station VSG in the microgrid,a virtual droop coefficient representing the VG output in VSGi;
step 5.2: taking the exit point as an initial value, and calculating the minimum gradient point of the gradient system after the fault by adopting a Gear method, wherein the method specifically comprises the following steps:
wherein,is the minimum gradient point;
step 5.3: at the point of minimum gradientAs an initial value, solving a power balance equation of the system to obtain a stable balance point and a dominant unstable balance point of the system, which are specifically as follows:
wherein, deltaniRepresenting the phase angle difference between the rated voltage of the grid and the VSGi output voltage,respectively representing the impedance and the inductive reactance, P, of the interconnection lineLiActive power of loads connected to the microgrid, Ei、EnRespectively representing the voltage amplitude and the rated voltage amplitude of the VSGi output;
step 5.4: calculating critical energy V of the microgrid system according to state quantity at dominant unstable balance point of the systemcr：
Wherein,respectively setting a stable balance point and an unstable balance point of VSGi in the microgrid after the system fault;respectively representing the virtual inertia, the virtual damping coefficient, the VG droop coefficient and the QPCM droop coefficient of VSGi during the stable operation of the microgrid;respectively indicate after failureAngular frequency of rotation, phase angle and amplitude of voltage, χ ═ d ω/dt, t at stable equilibrium point of the systemsIs the time of occurrence of the failure.
8. The method for controlling the transient stability of the microgrid with multiple virtual synchronous machines according to claim 1, characterized in that the process of the energy function-based adaptive control method for the virtual inertia of the VSG, the virtual damping coefficient, the VG droop coefficient and the QPCM droop coefficient in step 6 is as follows:
step 6.1: the VSG virtual inertia self-adaptive control method based on the energy function comprises the following steps:
wherein,respectively the virtual inertia and the maximum value of the virtual inertia, M, of VSGi when the micro-grid operates stablyAVEA threshold value that is a function of total energy;
step 6.2: the self-adaptive control method of the VSG virtual damping coefficient based on the energy function comprises the following steps:
wherein,respectively representing the virtual damping coefficient and the maximum value of the virtual damping coefficient of VSGi when the micro-grid is in stable operation;
step 6.3: designing an adaptive control method of a VG droop coefficient based on an energy function:
wherein,respectively representing the VG droop coefficient and the VG droop coefficient minimum value of VSGi during the stable operation of the microgrid;
step 6.4: the self-adaptive control method for designing the QPCM droop coefficient based on the energy function is as follows:
in the formula,and respectively representing the maximum value of the QPCM droop coefficient and the QPCM droop coefficient of the VSGi during the stable operation of the micro-grid.
9. The method according to claim 1, wherein the step 9 is to remove the total energy V in the microgrid at the fault clearing timeclAnd critical energy VcrComparing, analyzing and judging the stability of the microgrid as follows:
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