CN110556861B - Switching control method applied to island mode and grid-connected mode of micro power grid system - Google Patents
Switching control method applied to island mode and grid-connected mode of micro power grid system Download PDFInfo
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Abstract
The invention relates to the field of micro power grids, in particular to a switching control method applied to an island mode and a grid-connected mode of a micro power grid system, which comprises the following steps: each voltage source inverter calculates a reference signal V thereof according to an island control strategy when in an island moderef.IAnd calculating a reference signal V according to a grid-connected control strategy in a grid-connected moderef.GAnd according to Vref.IAnd Vref.GControlling the output voltage thereof; the central control unit is responsible for sending a mode switching instruction; in the process of switching from an island mode to a grid-connected mode, a central control unit realizes synchronization of alternating-current bus voltage and grid voltage by adopting a public grid voltage phase tracking and effective value tracking control strategy, and then each voltage source inverter receives an instruction and switches from the island control strategy to the grid-connected control strategy, so that seamless switching is realized; in the process of switching from the grid-connected mode to the island mode, each voltage source inverter receives an instruction and then switches from the grid-connected control strategy to the island control strategy, and therefore seamless switching is achieved.
Description
Technical Field
The invention relates to the field of micro power grids, in particular to a switching control method applied to an island mode and a grid-connected mode of a micro power grid system.
Background
With the rising price of fossil fuels such as petroleum and coal, various countries in the world are adopting various measures to deal with the challenges brought by the energy crisis. At present, the construction of new efficient energy systems is actively promoted all over the world, wherein distributed power generation, renewable energy and microgrid technologies are one of the hot spots of the current research.
The distributed power generation technology can integrate various renewable energy sources such as solar energy, wind energy and the like and various energy storage units, and the renewable energy sources and the various energy storage units are connected to a public bus through power electronic equipment such as an inverter and the like to form a micro power grid system, so that the high-efficiency utilization of energy sources is realized, and high-quality electric energy is provided for users. The system combines a distributed power generation unit, a distributed energy storage unit, a user load and a control system, and the whole micro power grid system is regarded as a load of a public power grid, so that the micro power grid system can be connected with the public power grid in a grid-connected mode (grid-connected operation mode), and can also be disconnected from the power grid to operate alone when the public power grid is abnormal or needs to be overhauled (island operation mode). When the public power grid fails due to electrical faults and the like, the micro power grid passively enters an island mode, namely a passive island operation mode; when the micro-grid receives the instruction of a superior control system due to the reasons that the public power grid needs debugging, scheduling and the like, the micro-grid actively cuts off the electrical connection with the public power grid, and the micro-grid is called as an active island operation mode.
Traditionally, when a microgrid is in a grid-tied mode, its common bus is directly connected to the grid, and the distributed units typically operate in a current source mode to achieve accurate power flow control. In an island mode, a common bus is disconnected from a power grid, a plurality of distributed units are generally required to work in a voltage source mode to ensure the quality of bus voltage and the stability and redundancy of a system, and a voltage source parallel operation control strategy is also required to be introduced to accurately share load power of the plurality of voltage source distributed units and eliminate circulation current among the voltage source distributed units.
In this case, when the microgrid switches between a grid-connected mode and an island mode, the control mode of the distributed unit is also switched between a current source and a voltage source. In the dynamic process, the switching of the control strategy can cause the discontinuity of the driving signal of the switching tube at the switching moment, and the problems of impact, oscillation, drop, overshoot, frequency runaway and the like of the bus voltage or current during the switching action are easily caused.
Disclosure of Invention
In order to solve the problems, the invention provides a switching control method applied to an island mode and a grid-connected mode of a micro power grid system, so as to realize seamless switching between the grid-connected mode and the island mode.
The switching control method applied to the island mode and the grid-connected mode of the micro power grid system comprises the following steps: the switching control method comprises the following steps that the power generation units are respectively connected with an alternating current bus through voltage source inverters, the alternating current bus is connected with a public power grid through a mode switching switch, the central control unit is connected with controllers of the voltage source inverters and is simultaneously responsible for monitoring the public power grid and the alternating current bus and controlling the mode switching switch to be switched on and off, and the switching control method comprises the following steps:
when the voltage source inverter works in an island mode, each voltage source inverter calculates respective reference signal V according to an island control strategyref.IAnd based on the reference signal Vref.IControlling respective output voltages;
when the grid-connected inverter works in a grid-connected mode, each voltage source inverter calculates respective reference signal V according to a grid-connected control strategyref.GAnd based on the reference signal Vref.GControlling respective output voltages;
in the process of switching from an island mode to a grid-connected mode, the central control unit calculates bias power P by adopting a public power grid voltage phase tracking control strategy and a public power grid voltage effective value tracking control strategybiasAnd QbiasAnd transmitting it to each voltage source inverter; each voltage source inverter is according to PbiasAnd QbiasSimultaneously, regulating respective reference signal V by adopting an island control strategyref.ISynchronizing the AC bus voltage of the micro-grid with the grid voltage; when the AC bus voltage is synchronous with the grid voltage, the central control unit sends a switching instruction to each voltage source inverter, the mode switch is closed at the same time, connection between the micro grid and the public grid is achieved, each voltage source inverter receives the instruction and then switches from an island control strategy to a grid-connected control strategy, and respective reference signals V are calculatedref.IInstead of calculating respective reference signals Vref.GControlling the output voltage thereof;
in the process of switching from a grid-connected mode to an island mode, the central control unit sends a switching instruction to each voltage source inverter, and simultaneously switches off the mode switch to realize the disconnection of the micro power grid and the public power grid, and each voltage source inverter receives the instruction and then switches from a grid-connected control strategy to an island control strategyBy calculating respective reference signals Vref.GInstead of calculating respective reference signals Vref.IAnd controlling the output voltage.
Preferably, the reference signal V output by each voltage source inverter is calculated according to an islanding control strategyref.IThe method comprises the following steps:
when operating in island mode, each voltage source inverter is controlled by the formula:
calculating to obtain omega of each control perioda.kAnd Va.k；
Wherein n represents the number of the voltage source inverters, a represents any one voltage source inverter, and the value range of the voltage source inverter is 1 to n; k represents any one control period, and k-1 represents the last control period; k is a radical ofaRepresenting the proportion of load power shared by each voltage source inverter in the parallel mode, and simultaneously satisfying the formula
The relationship of (1); omegaa.kRepresenting the angular frequency of the reference voltage of the kth control period of the a-th station voltage source inverter; va.kAn effective value of a reference voltage representing the kth control period of the a-th station voltage source inverter, when k is equal to 0, ωa.kAnd Va.kAre respectively equal to omegarAnd VrWherein ω isrAnd VrRespectively, the rated angular frequency and the effective value of the output voltage of the inverter; po.a.k-1The active power actually output in a control period of the a-th station voltage source inverter is represented; qo.a.k-1The reactive power actually output in one control period on the a th station voltage source inverter is represented; pref.a.k-1The active power of the reference output of one control period on the a th station voltage source inverter is represented; qref.a.k-1The reactive power of the reference output of one control period on the a th station voltage source inverter is represented; p in each control period in parallel moderef.a.kAnd Qref.a.kPassing through formula
Calculating to obtain; pbias.k-1Representing the previous control period, and sending a bias active power instruction to each voltage source inverter by the central control unit so as to synchronize the frequency and the phase of the alternating-current bus voltage and the public power grid voltage; qbias.k-1Representing the last control period, and sending a bias reactive power instruction to each voltage source inverter by the central control unit so as to synchronize the effective values of the alternating-current bus voltage and the public power grid voltage; m isω.aA droop coefficient representing an angular frequency of the a-th station voltage source inverter output voltage; n isv.aA droop coefficient representing an effective value of the output voltage of the a-th station voltage source inverter;
each voltage source inverter according to omegaa.kAnd Va.kBy the formula
Calculating to obtain reference signals V in the control periodref.I。
Preferably, the grid-connected control strategy similar to the island control strategy is adopted to calculate the reference signal V output by each voltage source inverterref.GThe method comprises the following steps:
when the grid-connected inverter works in a grid-connected mode, each voltage source inverter adopts a formula:
calculating to obtain omega of each control perioda.kAnd Va.k；
Wherein n represents the number of the voltage source inverters, a represents any one voltage source inverter, and the value range of the voltage source inverter is 1 to n; k represents any one control period, and k-1 represents the last control period; omegaa.kRepresenting the angular frequency of the reference voltage of the kth control period of the a-th station voltage source inverter; va.kThe effective value of the reference voltage of the kth control period of the ith station voltage source inverter is represented; when k is equal to 0, ωa.kAnd Va.kAre respectively equal to omegarAnd VrWherein ω isrAnd VrRespectively, the rated angular frequency and the effective value of the output voltage of the inverter; m isω.aA droop coefficient representing an angular frequency of the a-th station voltage source inverter output voltage; n isv.aA droop coefficient representing an effective value of the output voltage of the a-th station voltage source inverter; po.a.k-1The active power actually output in a control period of the a-th station voltage source inverter is represented; qo.a.k-1The reactive power actually output in one control period on the a th station voltage source inverter is represented; pref.a.k-1The active power of the reference output of one control period on the a th station voltage source inverter is represented; qref.a.k-1The reactive power of the reference output of one control period on the a th station voltage source inverter is represented; pref.a.k-1、Qref.a.k-1The value of (a) is determined according to the working state of the voltage source inverter or specified according to the running state of the microgrid;
each voltage source inverter according to omegaa.kAnd Va.kBy the formula
Calculating to obtain reference signals V in the control periodref.G。
Preferably, the public power grid voltage phase tracking control strategy includes:
the central control unit monitors the working state of the public power grid;
if the working state of the public power grid is judged to be abnormal, the central control unit commands the bias power P for correcting the frequency difference between the alternating-current bus voltage and the public power grid voltageωAnd a bias power P for correcting the phase difference between the AC bus voltage and the public network voltageθIs equal to 0, by the equation Pbias＝Pω+PθCalculating PbiasLet PbiasEqual to 0, and P isbiasIs sent to each voltage source inverter, let Pbias.k-1Is equal to the latest received Pbias；
If the working state of the public power grid is judged to be normal, the central control unit obtains the voltage angular frequency omega of the public power grid according to the obtained voltage angular frequency omegagBy the formula ωdif＝ωg-ωrCalculate ωgAnd the reference angular frequency omegarDifference ω therebetweendifBy the formula Pω＝kω·ωdifCalculating PωA value of (a), wherein kωIs a set proportionality coefficient; according to the obtained grid voltage phase thetagridAnd bus voltage phase thetabusBy the formula θdif＝θbus-θgridCalculating the difference theta between the twodifIf theta is greater than thetadifGreater than or equal to the set phase deviation value thetasetThen let PbiasEqual to the maximum bias active power Pset(ii) a If theta is greater than thetadifLess than-thetasetThen let PbiasIs equal to-Pset(ii) a If theta is greater than thetadifIs less than thetasetThen by the formula Pθ＝GPI·ωdifCalculating PθA value of (b), wherein GPIRepresenting a proportional integral calculation function while ensuring PθIs not more than P in absolute valueset(ii) a According to PωAnd PθBy the formula Pbias＝Pω+PθCalculating Pbias；
The central control unit calculates PbiasTo each voltage source inverter;
each voltage source inverter receiving PbiasAfter information, P in the island control strategy control equation is orderedbias.k-1Is equal to the latest received PbiasRealize the pair omegaa.kSo as to realize the phase difference adjustment of the alternating current bus and the public power grid voltage.
Preferably, the common grid voltage effective value tracking control strategy comprises:
the central control unit monitors the working state of the public power grid; if the working state of the public power grid is judged to be normal, the voltage of the micro power grid is made to track the reference V*Is equal to the effective value V of the public network voltagerms.g(ii) a If the working state of the public power grid is abnormal, the voltage of the micro power grid is enabled to track the reference V*Rated reference value V equal to output voltage of voltage source inverterr；
The central control unit obtains the communication motherLine voltage effective value Urms.bBy the formula Vdif＝Urms.b-V*Calculating the difference V of effective values of voltagedifIf V isdifGreater than or equal to the set effective value deviation value VsetThen Q is assertedbiasEqual to maximum bias reactive power Qset(ii) a If VdifLess than equal-VsetThen Q is assertedbiasIs equal to-Qset(ii) a If VdifIs less than VsetThen, the equation Q is passedbias＝GPI·VdifCalculating PθA value of (b), wherein GPIRepresenting a proportional integral calculation function while ensuring QbiasIs not more than Qset；
The central control unit calculates QbiasTo each voltage source inverter;
each voltage source inverter receiving QbiasAfter information, enabling Q in an island control strategy control equationbias.k-1Equal to the latest received QbiasRealize to Va.kAnd (4) correcting so as to realize amplitude difference adjustment of the alternating current bus and the public power grid voltage.
Preferably, the first and second liquid crystal materials are,
the central control unit obtains the voltage V of the public power grid according to the detectiongridAnd AC bus voltage UbusObtaining the phase theta of the public power grid voltage through a phase-locked loop algorithmgridEffective value Vrms.gAnd frequency fgridAnd phase theta of AC bus voltagebusEffective value Urms.bAnd frequency fbus；
The central control unit calculates the phase difference theta between the AC bus voltage and the public power grid voltagedifSum effective value difference VdifWhen the phase difference between the AC bus voltage and the public network voltage is thetadifLess than a predetermined value thetasAnd the effective value difference V between the AC bus voltage and the public power grid voltagedifLess than a predetermined value VsThe method is characterized in that the synchronization of the alternating-current bus voltage and the public power grid voltage is realized, and the phase tracking control strategy of the public power grid voltage and the effectiveness of the public power grid voltage are simultaneously expressedAnd finishing the execution of the value tracking control strategy.
The invention has the beneficial effects that:
(1) each inverter in the micro power grid system adopts a voltage source inverter, and each voltage source inverter controls the output voltage of each voltage source inverter based on a voltage reference signal in a grid-connected mode and an island mode, so that the transition process that an inverter control strategy needs to be switched between working modes of a current source and a voltage source in the traditional mode switching control method is eliminated, and seamless switching is more favorably realized;
(2) a grid-connected control strategy and an island control strategy of a micro power grid system both adopt an improved voltage source inverter control equation based on a droop method to calculate a reference signal V of each voltage source inverterref.GAnd Vref.IMeanwhile, the control equations of the two are very similar, so that the violent change of the reference signal is not caused during mode switching, the continuity of the reference signal is effectively ensured, and the seamless switching is more favorably realized;
(3) in the process of switching the micro power grid system from the grid-connected mode to the island mode, due to the similarity of the grid-connected control strategy and the island control strategy, the voltage source inverter can be directly switched from the grid-connected control strategy to the island control strategy, and a transition process is not needed in the switching process, so that the time needed in the micro power grid mode switching process is effectively shortened.
(4) In the process of switching the micro power grid system from the island mode to the grid-connected mode, after the synchronization of the alternating current bus voltage and the public power grid voltage is completed, due to the similarity of the grid-connected control strategy and the island control strategy, the voltage source inverter can be directly switched from the island control strategy to the grid-connected control strategy, and the transition process is not needed in the switching process, so that the time needed by the micro power grid mode switching process is effectively shortened;
(5) when the micro power grid system operates in an island mode, a public power grid voltage phase and effective value tracking control strategy can be operated in advance without waiting for a switching instruction, and the synchronization of the alternating current bus voltage and the public power grid voltage is completed in advance when the public power grid is in a normal state. When the micro power grid receives an instruction for switching from an island mode to a grid-connected mode, mode switching can be directly carried out, so that the synchronization process of alternating current bus voltage and power grid voltage is saved, and the time required by the micro power grid mode switching process is effectively shortened.
Drawings
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
FIG. 1 is a schematic structural diagram of a microgrid system in an embodiment of the present invention;
FIG. 2 is a schematic structural diagram of a control system in the microgrid system according to an embodiment of the present invention;
FIG. 3 is a schematic overall flow chart of a mode switching control method for the microgrid system in the embodiment of the present invention;
FIG. 4 is a schematic flow chart of switching from a grid-connected mode to an island mode in the embodiment of the present invention;
FIG. 5 is a schematic flow chart of switching from an island mode to a grid-connected mode in the embodiment of the present invention;
FIG. 6 is a schematic flow chart of a phase tracking control strategy for the voltage of the public power grid according to an embodiment of the present invention;
fig. 7 is a schematic flow chart of a common grid voltage effective value tracking control strategy in the embodiment of the present invention.
Detailed Description
The technical solutions of the present invention will be further described below with reference to the accompanying drawings, but the present invention is not limited to these embodiments.
The basic idea of this embodiment is that each inverter of the microgrid in the grid-connected mode and the island mode adopts a similar droop-based improved voltage source control equation for calculating the respective reference signal VrefI.e. Vref.GAnd Vref.IAnd using the same voltage-based reference signal VrefThe output voltage control strategy controls the output voltage of the control strategy, and meanwhile, the reference signal V is ensured in the mutual switching process between an island mode and a grid-connected moderefThe micro grid-connected/isolated island working mode can be seamlessly switched.
The structure of the microgrid system is shown in figure 1The method comprises the following steps: a plurality of distributed power generating units So.aThe power generation units are respectively connected with a voltage source inverter Vo.aImpedance Lw.aIs connected with an AC bus through an impedance LgMode transfer switch STS and public power grid VgridConnecting, while the AC bus also outputs a connecting load Zload。
Meanwhile, on the basis of a communication network, a controller of each voltage source inverter in the microgrid system and a central control unit of the microgrid form an integral control system, and the central control unit is responsible for monitoring the states of the public power grid and the alternating current bus and controlling the on and off of a static transfer switch STS, as shown in FIG. 2. The control method is realized by a central control unit in the micro power grid system and the controllers of all the voltage source inverters.
And the central control unit of the micro power grid is responsible for monitoring the states of the alternating current bus and the public power grid. In an island mode, if the grid is detected to be normal, the central control unit executes a phase and effective value tracking control strategy of alternating current bus voltage to public grid voltage, issues switching instructions from an island to a grid-connected control strategy to each voltage source inverter after the bus voltage and the grid voltage are synchronous, and controls the action of the static transfer switch STS to realize mode switching from the island to the grid-connected. In the grid-connected mode, if the voltage of the power grid is detected to be abnormal, the central control unit issues a switching instruction of a grid-connected to island control strategy to each voltage source inverter, and controls the action of the static transfer switch STS to realize the mode switching from grid connection to island. After receiving a switching instruction from a grid-connected mode to an island mode or from the island mode to the grid-connected mode, each voltage source inverter controller switches a control strategy from the grid-connected mode to the island mode or from the island mode to the grid-connected mode according to the instruction.
Based on the above conception, the present invention provides a switching control method applied to an island mode and a grid-connected mode of a micro grid system, as shown in fig. 3, including:
s1: when the voltage source inverter works in an island mode, each voltage source inverter calculates respective basis according to an island control strategyQuasi signal Vref.IAnd based on the reference signal Vref.IControlling respective output voltages;
s2: when the grid-connected inverter works in a grid-connected mode, each voltage source inverter calculates respective reference signal V according to a grid-connected control strategyref.GAnd based on the reference signal Vref.GControlling respective output voltages;
s3: in the process of switching from an island mode to a grid-connected mode, the central control unit calculates bias power P by adopting a public power grid voltage phase tracking control strategy and a public power grid voltage effective value tracking control strategybiasAnd QbiasAnd transmitting it to each voltage source inverter; each voltage source inverter is according to PbiasAnd QbiasSimultaneously, regulating respective reference signal V by adopting an island control strategyref.ISynchronizing the AC bus voltage of the micro-grid with the grid voltage; when the AC bus voltage is synchronous with the grid voltage, the central control unit sends a switching instruction to each voltage source inverter, the mode switch is closed at the same time, connection between the micro grid and the public grid is achieved, each voltage source inverter receives the instruction and then switches from an island control strategy to a grid-connected control strategy, and respective reference signals V are calculatedref.IInstead of calculating respective reference signals Vref.GThe output voltage is controlled and the switching process is ended. At the moment, the alternating-current bus voltage and the power grid voltage are synchronous, and the similarity between an island control strategy and a grid-connected control strategy can ensure the V before and after switchingref.IAnd Vref.GApproximately equal, ensuring the continuity of the switching process.
S4: in the process of switching from a grid-connected mode to an island mode, the central control unit sends a switching instruction to each voltage source inverter, and simultaneously switches off the mode switch to realize the disconnection of the micro-grid and the public grid, each voltage source inverter receives the instruction and then switches from a grid-connected control strategy to an island control strategy, and respective reference signals V are calculatedref.GInstead of calculating respective reference signals Vref.IThe output voltage is controlled and the switching process is ended. Due to the similarity of the island control strategy and the grid-connected control strategy, the method can ensure that the control strategy is before switchingRear Vref.GAnd Vref.IApproximately equal, ensuring the continuity of the switching process.
Specifically, a reference signal V output by each voltage source inverter is calculated according to an island control strategyref.IThe method comprises the following steps:
when operating in island mode, each voltage source inverter is controlled by the formula:
calculating to obtain omega of each control perioda.kAnd Va.k；
Wherein n represents the number of the voltage source inverters, a represents any one voltage source inverter, and the value range of the voltage source inverter is 1 to n; k represents any one control period, and k-1 represents the last control period; k is a radical ofaRepresenting the proportion of load power shared by each voltage source inverter in the parallel mode, and simultaneously satisfying the formula
The relationship of (1); omegaa.kRepresenting the angular frequency of the reference voltage of the kth control period of the a-th station voltage source inverter; va.kAn effective value of a reference voltage representing the kth control period of the a-th station voltage source inverter, when k is equal to 0, ωa.kAnd Va.kAre respectively equal to omegarAnd VrWherein ω isrAnd VrRespectively, the rated angular frequency and the effective value of the output voltage of the inverter; po.a.k-1The active power actually output in a control period of the a-th station voltage source inverter is represented; qo.a.k-1The reactive power actually output in one control period on the a th station voltage source inverter is represented; pref.a.k-1The active power of the reference output of one control period on the a th station voltage source inverter is represented; qref.a.k-1The reactive power of the reference output of one control period on the a th station voltage source inverter is represented; p in each control period in parallel moderef.a.kAnd Qref.a.kPassing through formula
Calculating to obtain; pbias.k-1Representing the previous control period, and sending a bias active power instruction to each voltage source inverter by the central control unit so as to synchronize the frequency and the phase of the alternating-current bus voltage and the public power grid voltage; qbias.k-1Representing the last control period, and sending a bias reactive power instruction to each voltage source inverter by the central control unit so as to synchronize the effective values of the alternating-current bus voltage and the public power grid voltage; m isω.aA droop coefficient representing an angular frequency of the a-th station voltage source inverter output voltage; n isv.aA droop coefficient representing an effective value of the output voltage of the a-th station voltage source inverter;
each voltage source inverter according to omegaa.kAnd Va.kBy the formula
Calculating to obtain reference signals V in the control periodref.I。
Specifically, the reference signal V output by each voltage source inverter is calculated by adopting a grid-connected control strategy similar to the island control strategyref.GThe method comprises the following steps:
when the grid-connected inverter works in a grid-connected mode, each voltage source inverter adopts a formula:
calculating to obtain omega of each control perioda.kAnd Va.k；
Wherein n represents the number of the voltage source inverters, a represents any one voltage source inverter, and the value range of the voltage source inverter is 1 to n; k represents any one control period, and k-1 represents the last control period; omegaa.kRepresenting the angular frequency of the reference voltage of the kth control period of the a-th station voltage source inverter; va.kThe effective value of the reference voltage of the kth control period of the ith station voltage source inverter is represented; when k is equal to 0, ωa.kAnd Va.kAre respectively equal to omegarAnd VrWherein ω isrAnd VrRespectively, the rated angular frequency and the effective value of the output voltage of the inverter; m isω.aRepresenting the output of the a-th station voltage source inverterDroop coefficient of angular frequency of voltage; n isv.aA droop coefficient representing an effective value of the output voltage of the a-th station voltage source inverter; po.a.k-1The active power actually output in a control period of the a-th station voltage source inverter is represented; qo.a.k-1The reactive power actually output in one control period on the a th station voltage source inverter is represented; pref.a.k-1The active power of the reference output of one control period on the a th station voltage source inverter is represented; qref.a.k-1The reactive power of the reference output of one control period on the a th station voltage source inverter is represented; pref.a.k-1、Qref.a.k-1The value of (a) is determined according to the working state of the voltage source inverter or specified according to the running state of the microgrid;
each voltage source inverter according to omegaa.kAnd Va.kBy the formula
Calculating to obtain reference signals V in the control periodref.G。
In the embodiment, the island control strategy and the grid-connected control strategy both adopt improved control equations based on the traditional droop method to calculate respective reference signals Vref.IAnd Vref.G. By the formula:
it can be seen that the two formulas are very similar, and the switching process of the grid-connected/isolated island mode can be converted into a reference signal Vref.I/Vref.GWhile a similar grid/island control strategy can be adopted to ensure Vref.I/Vref.GContinuity in mode switching.
In island mode, the traditional droop method cannot accurately suppress circulating reactive power between inverters, and can generate steady-state deviation of output voltage frequency and amplitude which vary with load. In the embodiment, the island control strategy can effectively inhibit the circulation active power and the circulation reactive power between the inverters in the island mode, improve the dynamic performance of the system, improve the reliability of the system, simultaneously eliminate the steady-state errors of the frequency and the amplitude of the output voltage of each inverter unit, and realize that each inverter unit equally divides the common load power according to the proportion of the power capacity of each inverter unit in the parallel system.
In a grid-connected mode, the traditional droop method cannot track changes of the voltage frequency and the amplitude of a power grid, so that active power and reactive power output by a grid-connected inverter cannot be accurately controlled. In the embodiment, the grid-connected control strategy can effectively track changes of the voltage frequency and the amplitude of the power grid, so that the active power and the reactive power output by the grid-connected inverter can be accurately controlled, the dynamic performance of the system is improved, and the reliability of the system is improved.
In an embodiment, as shown in fig. 4, a control method adopted by the microgrid system to switch from a grid-connected mode to an island mode is as follows:
step 1: and the central control unit sends an off-grid signal for switching from a grid-connected mode to an island mode to the controller of each voltage source inverter, and appoints to perform mode switching after a specific time. After receiving the off-grid signal, a controller of the voltage source inverter prepares to perform mode switching after a specific time;
step 2: after the specific time comes, the central control unit sends a control signal to turn off the static transfer switch STS, so that the micro power grid is separated from the public power grid. Meanwhile, the controller of each voltage source inverter uses the voltage reference calculation formula of the voltage source inverter to form a control equation of a grid-connected mode, namely
Control equations for switching to island mode, i.e.
And step 3: and ending the control flow of switching from the grid-connected mode to the island mode.
In an embodiment, as shown in fig. 5, a control method adopted by the micro grid to switch from the island mode to the grid-connected mode is as follows:
step 1: the central control unit detects the voltage V of the power gridgridAnd AC bus voltage UbusObtaining the phase theta of the power grid voltage through a phase-locked loop algorithmgridEffective value Vrms.gAnd frequency fgridAnd the phase θ of the AC bus voltagebusEffective value Urms.bAnd frequency fbusAnd entering step 2;
step 2: frequency f of the central control unit via the mains voltagegridAnd a valid value Vrms.gJudging the state of the power grid, executing a control strategy of public power grid voltage phase tracking and public power grid voltage effective value tracking, and then sending bias active power P to controllers of all voltage source invertersbiasAnd bias reactive power QbiasInstructions; after receiving the instruction, the controller of the voltage source inverter updates the island mode control equation
Middle Pbias.k-1And Qbias.k-1A value of (d);
and step 3: the central control unit judges according to the state of the public power grid, if the voltage state of the power grid is normal, the step 4 is carried out, otherwise, the step 1 is carried out;
and 4, step 4: the central control unit calculates the phase difference theta between the AC bus voltage and the power grid voltagedifDifference of sum effective value Vdif. If theta is greater than thetadifAnd VdifAre respectively smaller than a value theta preset by the central control unitsAnd VsIf the alternating current bus voltage and the power grid voltage are considered to be synchronously completed, the step 5 is carried out; if the conditions cannot be met at the same time, the synchronization is considered to be not completed, and the step 1 is returned;
and 5: and the central control unit sends a grid-connected signal for switching from an island mode to a grid-connected mode to the controller of each voltage source inverter, and appoints to perform mode switching after a specific time. After receiving a grid-connected signal, a voltage source inverter controller prepares to switch modes after a specific time;
step 6: after the specific time comes, the central control unit sends a control signal to close the static transfer switch, so that the connection between the micro power grid and the public power grid is realized. Meanwhile, the controller of each voltage source inverter uses the voltage reference calculation formula of the inverter to be the control equation of the island mode, namely
Control equations for switching to grid-tie mode, i.e.
And 7: and ending the control flow of switching from the island mode to the grid-connected mode.
In addition, in the control strategy for switching the micro-grid from the island mode to the grid-connected mode, the invention further adopts or combines a public grid voltage phase tracking control strategy and a public grid voltage effective value tracking control strategy.
As shown in fig. 6, the phase tracking control strategy flow of the voltage of the public power grid is as follows:
step 1: the central control unit obtains the voltage V of the power gridgridAngular frequency of (omega)gPhase thetagridAnd a valid value Vrms.gInformation, judging the state information of the public power grid, and entering the step 2 if the state of the public power grid is abnormal; if the public power grid state is normal, entering step 3;
step 2: the central control unit commands PωAnd PθEqual to 0 and go to step 5; pbiasNamely the bias active power P transmitted to each voltage source inverter controller by the central control unitbiasWherein the active power P is biasedωFor correcting the frequency difference between the AC bus voltage and the public power grid voltage, and biasing the active power PθFor correcting quadraturePhase difference of the current bus voltage and the public power grid voltage;
and step 3: the central control unit obtains the angular frequency omega of the public power grid voltage according to the obtained voltagegBy the formula ωdif＝ωg-ωrCalculate ωgAnd the reference angular frequency omegarDifference ω therebetweendifThen by the formula Pω＝kω·ωdifCalculating PωThen step 4 is entered; wherein k isωIs a preset proportionality coefficient;
and 4, step 4: the central control unit obtains the phase theta of the power grid voltage according to the obtained phase thetagridAnd bus voltage phase thetabusBy the formula θdif＝θbus-θgridCalculating the difference theta between the twodif. If theta is greater than thetadifGreater than or equal to the phase deviation value theta set by the central control unitsetThen let PbiasEqual to the maximum bias active power Pset(ii) a If theta is greater than thetadifLess than-thetasetThen let PbiasIs equal to-Pset(ii) a If theta is greater than thetadifIs less than thetasetThen by the formula Pθ＝GPI·ωdifCalculating PθThe value of (c). Wherein G isPIRepresenting a proportional integral calculation function while ensuring PθIs not more than P in absolute valuesetTo ensure the stability of the control system;
and 5: the central control unit is based on PωAnd PθBy the formula Pbias＝Pω+PθCalculating PbiasAnd is combined with PbiasThe value of (d) is communicated to the controller of each voltage source inverter;
step 6: the controller of each voltage source inverter receives P sent by the central control unitbiasAfter the information, the control equation is updated
Middle Pbias.k-1Value of (1), i.e. order Pbias.k-1Is equal to the latest received PbiasThereby realizing the pair omegaa.kIs further corrected, therebyThe frequency difference and the phase difference between the alternating-current bus voltage and the public power grid voltage are adjusted;
and 7: and returning to the main program to prepare for entering the next control period.
As shown in fig. 7, the flow of the tracking control strategy for the effective value of the voltage of the public power grid is as follows:
step 1: the central control unit judges according to the acquired public power grid state information, and if the public power grid state is normal, the central control unit makes the voltage of the micro power grid track the reference V*Equal to the effective value V of the network voltagerms.gIf the public power grid is abnormal, the voltage of the micro power grid is made to track the reference V*Rated reference value V equal to output voltage of voltage source inverterr；
Step 2: the central control unit obtains the effective value U of the alternating-current bus voltage according to the obtained effective value Urms.bBy the formula Vdif＝Urms.b-V*Calculating the difference V of effective values of voltagedif. If VdifGreater than or equal to the effective value deviation value V set by the central control unitsetThen Q is assertedbiasEqual to maximum bias reactive power Qset(ii) a If VdifIs less than or equal to-VsetThen Q is assertedbiasIs equal to-Qset(ii) a If VdifIs less than VsetThen, the equation Q is passedbias＝GPI·VdifCalculating PθThe value of (c). Wherein G isPIRepresenting a proportional integral calculation function while ensuring QbiasIs not more than QsetTo ensure the stability of the system;
and step 3: the central control unit calculates QbiasThe value of (d) is communicated to the controller of each voltage source inverter;
and 4, step 4: the controller of each voltage source inverter receives Q transmitted by the central control unitbiasAfter the information, the control equation is updated
Middle Qbias.k-1Value of (1), i.e. Qbias.k-1Equal to the latest receivedQbiasThereby realizing the pair Va.kAnd further realizing the adjustment of the effective value difference between the alternating-current bus voltage and the public power grid voltage.
And 5: and returning to the main program to prepare for entering the next control period.
In one embodiment, the central control unit obtains the voltage V of the public power grid according to the detectiongridAnd AC bus voltage UbusThe synchronization of the alternating-current bus voltage and the public power grid voltage and the execution conditions of a public power grid voltage phase tracking control strategy and a public power grid voltage effective value tracking control strategy are verified.
The method comprises the following steps:
the central control unit obtains the voltage V of the public power grid according to the detectiongridAnd AC bus voltage UbusObtaining the phase theta of the public power grid voltage through a phase-locked loop algorithmgridEffective value Vrms.gAnd frequency fgridAnd phase theta of AC bus voltagebusEffective value Urms.bAnd frequency fbus；
The central control unit calculates the phase difference theta between the AC bus voltage and the public power grid voltagedifSum effective value difference VdifWhen the phase difference between the AC bus voltage and the public network voltage is thetadifLess than a predetermined value thetasAnd the effective value difference V between the AC bus voltage and the public power grid voltagedifLess than a predetermined value VsAnd meanwhile, the synchronization of the alternating-current bus voltage and the public power grid voltage is realized, and the execution completion of the public power grid voltage phase tracking control strategy and the public power grid voltage effective value tracking control strategy is represented.
Compared with the traditional switching method between the grid-connected mode and the island mode, the switching control method of the island mode and the grid-connected mode of the micro grid system provided by the invention has the following advantages:
(1) each inverter in the micro power grid system adopts a voltage source inverter, and each voltage source inverter controls the output voltage of each voltage source inverter based on a voltage reference signal in a grid-connected mode and an island mode, so that the transition process that an inverter control strategy needs to be switched between working modes of a current source and a voltage source in the traditional mode switching control method is eliminated, and seamless switching is more favorably realized;
(2) a grid-connected control strategy and an island control strategy of a micro power grid system both adopt an improved voltage source inverter control equation based on a droop method to calculate a reference signal V of each voltage source inverterref.GAnd Vref.IMeanwhile, the control equations of the two are very similar, so that the violent change of the reference signal is not caused during mode switching, the continuity of the reference signal is effectively ensured, and the seamless switching is more favorably realized;
(3) in the process of switching the micro power grid system from the grid-connected mode to the island mode, due to the similarity of the grid-connected control strategy and the island control strategy, the voltage source inverter can be directly switched from the grid-connected control strategy to the island control strategy, and a transition process is not needed in the switching process, so that the time needed in the micro power grid mode switching process is effectively shortened.
(4) In the process of switching the micro power grid system from the island mode to the grid-connected mode, after the synchronization of the alternating current bus voltage and the public power grid voltage is completed, due to the similarity of the grid-connected control strategy and the island control strategy, the voltage source inverter can be directly switched from the island control strategy to the grid-connected control strategy, and the transition process is not needed in the switching process, so that the time needed by the micro power grid mode switching process is effectively shortened;
(5) when the micro power grid system operates in an island mode, a public power grid voltage phase and effective value tracking control strategy can be operated in advance without waiting for a switching instruction, and the synchronization of the alternating current bus voltage and the public power grid voltage is completed in advance when the public power grid is in a normal state. When the micro power grid receives an instruction for switching from an island mode to a grid-connected mode, mode switching can be directly carried out, so that the synchronization process of alternating current bus voltage and power grid voltage is saved, and the time required by the micro power grid mode switching process is effectively shortened.
In a word, the switching control method for the isolated island mode and the grid-connected mode of the micro power grid system provided by the invention avoids the problem that an inverter needs to be switched between a current source working mode and a voltage source working mode in the traditional method, eliminates the transition process in the mode switching process, can greatly shorten the time required by the mode switching process, effectively inhibits the problem of out-of-control of alternating current bus voltage or current in the mode switching process, is more favorable for realizing the seamless switching of the grid-connected/isolated island working mode of the micro power grid system, has important significance for the stable working of the micro power grid, and has important application value.
Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.
Claims (5)
1. The switching control method applied to the island mode and the grid-connected mode of the micro power grid system is characterized in that the micro power grid system comprises the following steps: the switching control method comprises the following steps that the power generation units are respectively connected with an alternating current bus through voltage source inverters, the alternating current bus is connected with a public power grid through a mode switching switch, the central control unit is connected with controllers of the voltage source inverters and is responsible for monitoring the states of the public power grid and the alternating current bus and controlling the mode switching switch to be switched on and off, and the switching control method comprises the following steps:
when the voltage source inverter works in an island mode, each voltage source inverter calculates respective reference signal V according to an island control strategyref.IAnd based on the reference signal Vref.IControlling respective output voltages;
when the grid-connected inverter works in a grid-connected mode, each voltage source inverter calculates respective reference signal V according to a grid-connected control strategyref.GAnd based on the reference signal Vref.GControlling respective output voltages;
in the process of switching from an island mode to a grid-connected mode, the central control unit calculates bias power P by adopting a public power grid voltage phase tracking control strategy and a public power grid voltage effective value tracking control strategybiasAnd QbiasAnd transmit it to eachA voltage source inverter; each voltage source inverter is according to PbiasAnd QbiasSimultaneously, regulating respective reference signal V by adopting an island control strategyref.ISynchronizing the AC bus voltage of the micro-grid with the grid voltage; when the AC bus voltage is synchronous with the grid voltage, the central control unit sends a switching instruction to each voltage source inverter, the mode switch is closed at the same time, connection between the micro grid and the public grid is achieved, each voltage source inverter receives the instruction and then switches from an island control strategy to a grid-connected control strategy, and respective reference signals V are calculatedref.IInstead of calculating respective reference signals Vref.GControlling the output voltage thereof;
in the process of switching from a grid-connected mode to an island mode, the central control unit sends a switching instruction to each voltage source inverter, and simultaneously switches off the mode switch to realize the disconnection of the micro-grid and the public grid, each voltage source inverter receives the instruction and then switches from a grid-connected control strategy to an island control strategy, and respective reference signals V are calculatedref.GInstead of calculating respective reference signals Vref.IControlling the output voltage thereof;
when the grid-connected inverter works in a grid-connected mode, each voltage source inverter adopts a formula:
calculating to obtain omega of each control perioda.kAnd Va.k；
Wherein n represents the number of the voltage source inverters, a represents any one voltage source inverter, and the value range of the voltage source inverter is 1 to n; k represents any one control period, and k-1 represents the last control period; omegaa.kRepresenting the angular frequency of the reference voltage of the kth control period of the a-th station voltage source inverter; va.kThe effective value of the reference voltage of the kth control period of the ith station voltage source inverter is represented; when k is equal to 0, ωa.kAnd Va.kAre respectively equal to omegarAnd VrWherein ω isrAnd VrRespectively, the rated angular frequency and the effective value of the output voltage of the inverter; m isω.aRepresenting angular frequency of output voltage of a-th station voltage source inverterA sag factor; n isv.aA droop coefficient representing an effective value of the output voltage of the a-th station voltage source inverter; po.a.k-1The active power actually output in a control period of the a-th station voltage source inverter is represented; qo.a.k-1The reactive power actually output in one control period on the a th station voltage source inverter is represented; pref.a.k-1The active power of the reference output of one control period on the a th station voltage source inverter is represented; qref.a.k-1The reactive power of the reference output of one control period on the a th station voltage source inverter is represented; pref.a.k-1、Qref.a.k-1The value of (a) is determined according to the working state of the voltage source inverter or specified according to the running state of the microgrid;
each voltage source inverter according to omegaa.kAnd Va.kBy the formula
Calculating to obtain reference signals V in the control periodref.G。
2. The switching control method applied to the micro grid system in the island mode and the grid-connected mode according to claim 1, wherein the reference signal V output by each voltage source inverter is calculated according to an island control strategyref.IThe method comprises the following steps:
when operating in island mode, each voltage source inverter is controlled by the formula:
calculating to obtain omega of each control perioda.kAnd Va.k；
Wherein n represents the number of the voltage source inverters, a represents any one voltage source inverter, and the value range of the voltage source inverter is 1 to n; k represents any one control period, and k-1 represents the last control period; k is a radical ofaRepresenting the proportion of load power shared by each voltage source inverter in the parallel mode, and simultaneously satisfying the formula
The relationship of (1); omegaa.kRepresenting the angular frequency of the reference voltage of the kth control period of the a-th station voltage source inverter; va.kAn effective value of a reference voltage representing the kth control period of the a-th station voltage source inverter, when k is equal to 0, ωa.kAnd Va.kAre respectively equal to omegarAnd VrWherein ω isrAnd VrRespectively, the rated angular frequency and the effective value of the output voltage of the inverter; po.a.k-1The active power actually output in a control period of the a-th station voltage source inverter is represented; qo.a.k-1The reactive power actually output in one control period on the a th station voltage source inverter is represented; pref.a.k-1The active power of the reference output of one control period on the a th station voltage source inverter is represented; qref.a.k-1The reactive power of the reference output of one control period on the a th station voltage source inverter is represented; p in each control period in parallel moderef.a.kAnd Qref.a.kPassing through formula
Calculating to obtain; pbias.k-1Representing the previous control period, and sending a bias active power instruction to each voltage source inverter by the central control unit so as to synchronize the frequency and the phase of the alternating-current bus voltage and the public power grid voltage; qbias.k-1Representing the last control period, and sending a bias reactive power instruction to each voltage source inverter by the central control unit so as to synchronize the effective values of the alternating-current bus voltage and the public power grid voltage; m isω.aA droop coefficient representing an angular frequency of the a-th station voltage source inverter output voltage; n isv.aA droop coefficient representing an effective value of the output voltage of the a-th station voltage source inverter;
each voltage source inverter according to omegaa.kAnd Va.kBy the formula
Calculating to obtain reference signals V in the control periodref.I。
3. The switching control method applied to the micro grid system in the island mode and the grid-connected mode according to claim 2, wherein the public grid voltage phase tracking control strategy comprises:
the central control unit monitors the working state of the public power grid;
if the working state of the public power grid is judged to be abnormal, the central control unit commands a bias active power P for correcting the frequency difference between the alternating-current bus voltage and the public power grid voltageωAnd a bias active power P for correcting the phase difference between the AC bus voltage and the public power network voltageθIs equal to 0, by the equation Pbias＝Pω+PθCalculating PbiasLet PbiasEqual to 0, and P isbiasIs sent to the voltage source inverter to order Pbias.k-1Is equal to the latest received Pbias；
If the working state of the public power grid is judged to be normal, the central control unit obtains the voltage angular frequency omega of the public power grid according to the obtained voltage angular frequency omegagBy the formula ωdif＝ωg-ωrCalculate ωgAnd the reference angular frequency omegarDifference ω therebetweendifBy the formula Pω＝kω·ωdifCalculating PωA value of (a), wherein kωIs a set proportionality coefficient; according to the obtained grid voltage phase thetagridAnd bus voltage phase thetabusBy the formula θdif＝θbus-θgridCalculating the difference theta between the twodifIf theta is greater than thetadifGreater than or equal to the set phase deviation value thetasetThen let PbiasEqual to the maximum bias active power Pset(ii) a If theta is greater than thetadifLess than-thetasetThen let PbiasIs equal to-Pset(ii) a If theta is greater than thetadifIs less than thetasetThen by the formula Pθ＝GPI·ωdifCalculating PθA value of (b), wherein GPIRepresenting a proportional integral calculation function while ensuring PθIs not more than P in absolute valueset(ii) a Root of herbaceous plantAccording to PωAnd PθBy the formula Pbias＝Pω+PθCalculating Pbias；
The central control unit calculates PbiasTo each voltage source inverter;
each voltage source inverter receiving PbiasAfter information, P in the island control strategy control equation is orderedbias.k-1Is equal to the latest received PbiasRealize the pair omegaa.kSo as to realize the phase difference adjustment of the alternating current bus and the public power grid voltage.
4. The switching control method applied to the micro grid system in the island mode and the grid-connected mode according to claim 1, wherein the common grid voltage effective value tracking control strategy comprises:
the central control unit monitors the working state of the public power grid, and if the working state of the public power grid is judged to be normal, the voltage of the micro power grid is enabled to track the reference V*Is equal to the effective value V of the public network voltagerms.g(ii) a If the working state of the public power grid is judged to be abnormal, the voltage of the micro power grid is made to track the reference V*Rated reference value V equal to output voltage of voltage source inverterr；
The central control unit obtains the effective value U of the alternating-current bus voltage according to the obtained effective value Urms.bBy the formula Vdif＝Urms.b-V*Calculating the difference V of effective values of voltagedifIf V isdifGreater than or equal to the set effective value deviation value VsetThen Q is assertedbiasEqual to maximum bias reactive power Qset(ii) a If VdifIs less than or equal to-VsetThen Q is assertedbiasIs equal to-Qset(ii) a If VdifIs less than VsetThen, the equation Q is passedbias＝GPI·VdifCalculating PθA value of (b), wherein GPIRepresenting a proportional integral calculation function while ensuring QbiasIs not more than Qset；
The central control unit calculates QbiasTo each voltage source inverter;
each voltage source inverter receiving QbiasAfter information, enabling Q in an island control strategy control equationbias.k-1Equal to the latest received QbiasRealize to Va.kAnd (4) correcting so as to realize amplitude difference adjustment of the alternating current bus and the public power grid voltage.
5. The switching control method applied to the micro grid system in the island mode and the grid-connected mode according to claim 1,
the central control unit obtains the voltage V of the public power grid according to the detectiongridAnd AC bus voltage UbusObtaining the phase theta of the public power grid voltage through a phase-locked loop algorithmgridEffective value Vrms.gAnd frequency fgridAnd phase theta of AC bus voltagebusEffective value Urms.bAnd frequency fbus；
The central control unit calculates the phase difference theta between the AC bus voltage and the public power grid voltagedifSum effective value difference VdifWhen the phase difference between the AC bus voltage and the public network voltage is thetadifLess than a predetermined value thetasAnd the effective value difference V between the AC bus voltage and the public power grid voltagedifLess than a predetermined value VsAnd meanwhile, the synchronization of the alternating-current bus voltage and the public power grid voltage is realized, and the execution completion of the public power grid voltage phase tracking control strategy and the public power grid voltage effective value tracking control strategy is represented.
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