CN112583050A - Control method and system for multi-VSG inverter loop current suppression and fault handling - Google Patents
Control method and system for multi-VSG inverter loop current suppression and fault handling Download PDFInfo
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- H—ELECTRICITY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention provides a control method and a system for loop current suppression and fault treatment of a multi-VSG inverter, which comprise the following steps: when grid-connected switching is performed by off-grid: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met; when steady state operation after grid connection: if the load has asymmetric faults, the current type inverter and the VSG inverter which finishes the power parameter setting are connected in parallel and are connected into a power grid, and unbalanced current is compensated; wherein the power parameter of each VSG inverter is set based on the rated capacity of each VSG inverter. The multi-VSG inverter manufacturing method provided by the invention not only effectively ensures that the multi-VSG inverter can realize power distribution through parameter setting, but also can ensure the synchronization with a power grid in the pre-synchronization process, effectively inhibits grid-connected impact current, improves the power quality and realizes seamless switching between operation modes. And the problem of circulating currents between multiple VSG inverters is eliminated.
Description
Technical Field
The invention belongs to the technical field of grid-connected power generation of an intelligent power grid, and particularly relates to a control method and a control system for circulation suppression and fault treatment of a multi-VSG inverter.
Background
In the microgrid, two operation modes of grid connection and grid disconnection (isolated island) are the key for embodying the microgrid technology and economic advantages, so that a seamless and smooth switching technology is the key technology for ensuring the stable transition of the microgrid between the two operation modes. The Virtual Synchronous Generator (VSG) technology can freely realize the process of converting from an island to grid connection by simulating the operation mechanism of a synchronous generator, thereby omitting the fussy conversion of converting V/F into P/Q, having an inertia link, better realizing friendly access of a distributed power supply and improving the stability of a power system.
Although the VSG inverter already has a certain amplitude and phase compensation capability compared to a common inverter, its main circuit is still composed of fragile power electronics, and its over-voltage and over-current resistance capability is weak. If the VSG inverter is still connected to the grid by using the conventional synchronous generator to pull the synchronous generator into the grid, or an emergency situation in which the load is asymmetric may occur after the VSG inverter is disconnected from the grid or connected to the grid, the inrush current generated at the moment of connecting the VSG inverter to the grid may cause serious damage to the switching devices and the magnetic elements of the VSG inverter. And the traditional grid-connected inverter needs 2 times of coordinate transformation in the presynchronization link, and 5 PI links ensure the smooth realization of presynchronization, which is tedious and time-consuming.
In the process of switching from off-grid to grid-connected of a multi-inverter, particularly in the moment of switching from off-grid operation to grid-connected operation, or under emergency conditions such as sudden load change and the like, large impact current exists, so that grid-connected current and PCC point voltage are distorted, the power quality is reduced, and damage to electronic devices is serious, so that the problem of presynchronization is not only required to be considered during switching, but also serious circulation problem exists when a plurality of VSGs form a microgrid, and presynchronization grid-connected control, presynchronization grid-connected control under off-grid load faults, power distribution and circulation suppression of the multi-VSG inverter are particularly important.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a control method for loop current suppression and fault handling of a multi-VSG inverter, which is improved by comprising the following steps:
when grid-connected switching is performed by off-grid: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met;
when steady state operation after grid connection: if the load has asymmetric faults, connecting a current type inverter and the VSG inverter after the power parameter setting is completed in parallel and merging the current type inverter and the VSG inverter into a power grid, and compensating unbalanced current;
wherein the power parameter of each VSG inverter is set based on the rated capacity of each VSG inverter.
In a first preferred aspect of the present invention, the improvement wherein the setting of the power parameter of each VSG inverter based on the rated capacity of each VSG inverter comprises:
acquiring rated capacity of each VSG inverter;
setting power parameters for each VSG inverter according to the rated capacity proportion of each VSG inverter;
the power parameters comprise a virtual reactance and a reactive voltage droop coefficient which are inversely proportional to rated capacity, and an active frequency droop coefficient, a virtual rotational inertia and a virtual damping coefficient which are directly proportional to the rated capacity.
The second preferred technical solution provided by the present invention is improved in that, for each VSG inverter that completes the power parameter setting, a pre-synchronization control compensation method is adopted to compensate each VSG inverter respectively until the grid-connection requirement is met, and the method includes:
performing pre-synchronization control to compensate the angular frequency and voltage of each VSG inverter which finishes the power parameter setting;
respectively judging whether each VSG inverter reaches grid-connected requirements after pre-synchronous control: and if so, merging the VSG inverter which meets the grid-connected condition into the power grid, otherwise, continuing to perform presynchronization control until the grid-connected requirement is met.
In a third preferred embodiment, the improvement of the present invention is that the pre-synchronization control for compensating the angular frequency and the voltage of the VSG inverter includes:
acquiring a voltage phase angle of a power grid by adopting a phase-locked loop;
carrying out dq conversion on the output voltage of the VSG inverter by taking the phase angle as a reference to obtain a d-axis component and a q-axis component;
inputting the d-axis component into a PI regulator to obtain an angular frequency regulating quantity, and inputting the deviation between the q-axis component and the three-phase power grid voltage amplitude into the PI regulator to obtain an amplitude regulating quantity;
and adding the angular frequency regulating quantity reference angular frequency of the island mode to obtain the reference angular frequency of a pre-synchronization link, and adding the amplitude regulating quantity and the island mode reference voltage amplitude to obtain the reference voltage amplitude of the pre-synchronization link.
The fourth preferred technical solution provided by the present invention is improved in that the determining whether the VSG inverter meets the grid-connection requirement includes:
carrying out dq conversion on the output voltage of the VSG inverter to obtain d-axis output voltage and q-axis output voltage;
judging whether the absolute value of the q-axis output voltage is smaller than a q-axis threshold value or not, and judging whether the error between the d-axis output voltage and the peak value of the power grid phase voltage is smaller than a d-axis threshold value or not;
and when the two judgment results are yes, the VSG inverter meets the grid-connected requirement, otherwise, the VSG inverter does not meet the grid-connected requirement.
The fifth preferred technical solution provided by the present invention is improved in that after the grid connection requirement is met, the method further comprises:
and (4) zero setting the compensation quantity of the angular frequency and the voltage of the VSG inverter after the VSG inverter is incorporated into the power grid.
In a sixth preferred aspect of the present invention, the improvement wherein the current source inverter and the VSG inverter after completion of the power parameter setting are connected in parallel to a grid to compensate for an unbalanced current includes:
connecting a current-type inverter and the VSG inverter after the power parameter setting is completed in parallel and merging the current-type inverter and the VSG inverter into a power grid;
decomposing the load current into a balanced active current, a balanced reactive current, an unbalanced current and a null current component by adopting a conservative power theory;
and compensating the unbalanced current by controlling the output current of the current type inverter to be equal to the unbalanced current and the air current component in the load current.
In a seventh preferred embodiment, the improvement further comprises:
when off-grid operation and asymmetric failure of the load occurs: and aiming at each VSG inverter which finishes the power parameter setting, a negative sequence control method is adopted to control each VSG inverter respectively, so that the unbalance degree of the output voltage when the off-grid load is in an asymmetric fault is reduced.
In an eighth preferred embodiment, the improvement of the VSG inverter controlled by a negative sequence control method includes:
converting three-phase voltage and three-phase current of the VSG inverter under a three-phase static coordinate system into two-phase voltage and current under a two-phase static coordinate system;
performing positive-negative sequence separation on the two-phase voltage and current by using a second-order generalized integrator to obtain positive-sequence components and negative-sequence components of the two-phase voltage and current;
converting the positive sequence component and the negative sequence component of the two-phase voltage and current from a two-phase static coordinate system to a two-phase rotating coordinate system to respectively obtain a positive-sequence dq-axis voltage and current and a negative-sequence dq-axis voltage and current;
and controlling the voltage and the current of the dq axis of the positive sequence, and setting the voltage of the dq axis of the negative sequence to zero and controlling to realize that the output voltage of the inverter only contains a positive sequence component.
Based on the same inventive concept, the invention also provides a control system for loop current suppression and fault handling of the multi-VSG inverter, and the improvement is that the control system comprises: the system comprises a grid-connected switching module and a grid-connected fault handling module;
the grid-connected fault handling module is used for, when the grid-connected fault handling module runs in a steady state mode: if the load has asymmetric faults, connecting a current type inverter and the VSG inverter after the power parameter setting is completed in parallel and merging the current type inverter and the VSG inverter into a power grid, and compensating unbalanced current;
wherein the power parameter of each VSG inverter is set based on the rated capacity of each VSG inverter.
In a ninth preferred embodiment, the present invention further comprises a parameter setting module for setting power parameters of each VSG inverter, wherein the parameter setting module comprises: a capacity acquisition unit and a parameter setting unit;
the capacity acquisition unit is used for acquiring the rated capacity of each VSG inverter;
the parameter setting unit is used for setting power parameters for each VSG inverter according to the rated capacity proportion of each VSG inverter;
the power parameters comprise a virtual reactance and a reactive voltage droop coefficient which are inversely proportional to rated capacity, and an active frequency droop coefficient, a virtual rotational inertia and a virtual damping coefficient which are directly proportional to the rated capacity.
In a tenth preferred technical solution provided by the present invention, the improvement is that the grid-connection switching module includes: the device comprises a pre-synchronization control unit and a grid connection judging unit;
the pre-synchronization control unit is used for performing pre-synchronization control to compensate the angular frequency and the voltage of each VSG inverter which finishes the power parameter setting;
the grid connection judging unit is used for respectively judging whether each VSG inverter reaches the grid connection requirement after pre-synchronous control: and if so, merging the VSG inverter which meets the grid-connected condition into the power grid, otherwise, continuing to perform presynchronization control until the grid-connected requirement is met.
In an eleventh preferred embodiment, the grid-connected fault handling module includes: the current type inverter network access unit, the current decomposition unit and the current compensation unit;
the current-type inverter network access unit is used for connecting the current-type inverter and the VSG inverter after the power parameter setting is finished in parallel and merging the current-type inverter and the VSG inverter into a power grid;
the current decomposition unit is used for decomposing the load current into a balanced active current, a balanced reactive current, an unbalanced current and a null current component by adopting a conservative power theory;
the current compensation unit is used for compensating the unbalanced current by controlling the output current of the current type inverter to be equal to the unbalanced current and the air current component in the load current.
Compared with the closest prior art, the invention has the following beneficial effects:
the invention provides a control method and a system for loop current suppression and fault treatment of a multi-VSG inverter, which comprise the following steps: setting power parameters of each VSG inverter based on the rated capacity of each VSG inverter; when grid-connected switching is performed by off-grid: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met; when steady state operation after grid connection: and if the load has an asymmetric fault, connecting the current-type inverter and the VSG inverter after the power parameter setting in parallel into the power grid, and compensating the unbalanced current. The multi-VSG inverter manufacturing method provided by the invention not only effectively ensures that the multi-VSG inverter can realize power distribution through parameter setting, but also can ensure the synchronization with a power grid in the pre-synchronization process, effectively inhibits grid-connected impact current, improves the power quality and realizes seamless switching between operation modes. And the problem of circulating currents between multiple VSG inverters is eliminated.
In the calculation process, only one phase-locked loop is needed, only 2 PI links are needed, and the frequency compensation link is directly connected to the VSG rotor, so that the calculation amount is greatly reduced, and the grid-connected algorithm is simple, quick and effective. The method not only reduces current impact, realizes power distribution, but also effectively inhibits the problems of current circulation and fault treatment at the moment of grid connection.
The invention not only can realize the correct distribution of power, but also can effectively inhibit the problems of seamless switching of grid connection and grid connection, instantaneous circulation, harmonic wave and the like under the emergency condition after the grid connection and the grid connection.
Drawings
Fig. 1 is a schematic flow chart of a control method for loop current suppression and fault handling of a multi-VSG inverter according to the present invention;
fig. 2 is a schematic diagram of a pre-synchronization flow in a control method for loop current suppression and fault handling of a multi-VSG inverter according to the present invention;
FIG. 3 is a diagram of a main circuit structure in which two VSG inverters are connected in parallel after a pre-synchronization strategy is added;
fig. 4 is a block diagram of pre-synchronization control in a control method for loop current suppression and fault handling of a multi-VSG inverter according to the present invention;
FIG. 5 is a block diagram of an improved VSG control incorporating a negative sequence control element according to the present invention;
fig. 6 is a schematic diagram of phase current waveforms of two VSGs connected in parallel with a microgrid bus without adding a pre-synchronization control strategy;
fig. 7 is a schematic diagram of an active/reactive power waveform output by the VSG1 without adding the pre-synchronization control strategy;
fig. 8 is a schematic diagram of an active/reactive power waveform output by the VSG2 without adding the pre-synchronization control strategy;
fig. 9 is a schematic diagram of a phase current waveform of a micro grid bus connected in parallel with a presynchronized VSG 1;
fig. 10 is a schematic diagram of phase current waveforms of a microgrid bus connected in parallel with a presynchronized VSG 2;
FIG. 11 is a schematic diagram of the VSG1 output active/reactive power waveform with presynchronization added;
FIG. 12 is a schematic diagram of the VSG2 output active/reactive power waveform with presynchronization added;
FIG. 13 is a schematic diagram of a circulating current waveform between two inverters before and after grid connection without pre-synchronization control;
FIG. 14 is a schematic diagram of a circulating current waveform between two inverters before and after pre-synchronization control is added to grid connection;
FIG. 15 is a schematic diagram of grid-connected voltage and current waveforms before and after a grid-connected current compensation scheme is adopted under an asymmetric load;
fig. 16 is a schematic diagram of a basic structure of a control system for loop current suppression and fault handling of a multi-VSG inverter according to the present invention;
fig. 17 is a detailed structural schematic diagram of a control system for loop current suppression and fault handling of a multi-VSG inverter according to the present invention.
Detailed Description
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
Example 1:
fig. 1 shows a flow diagram of a control method for loop current suppression and fault handling of a multi-VSG inverter, which includes:
step 1: when grid-connected switching is performed by off-grid: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met;
step 2: when steady state operation after grid connection: if the load has asymmetric faults, the current type inverter and the VSG inverter which finishes the power parameter setting are connected in parallel and are connected into a power grid, and unbalanced current is compensated;
wherein the power parameter of each VSG inverter is set based on the rated capacity of each VSG inverter.
The invention aims to solve the problems of large impact current, circulation or fault in the process that a plurality of inverters with different capacities are connected from an off-grid state to a grid state in an emergency, and how to quickly and accurately realize synchronization with the voltage of a power grid and realize power distribution, circulation inhibition and fault treatment.
The following specifically describes embodiments of the present invention.
Aiming at a microgrid system formed by multiple VSG grid-connected inverters, on the basis of realizing power distribution by configuring proper VSG power parameters, the invention extracts the voltage phase of a power grid by utilizing a phase-locked loop (PLL) technology and waits for a pre-synchronization enabling signal of grid connection. After the signal arrives, dq conversion is carried out on the voltage of the microgrid bus by taking the voltage phase of the power grid as a reference, the error between the q-axis output value and zero and the error between the d-axis output value and the voltage amplitude of the power grid when each VSG inverter is off the grid are continuously detected, and if the two errors are judged to be smaller than a preset threshold value, the voltage deviation of two sides of a PCC point is small enough, so that the frequency and the amplitude which need to be compensated are rapidly given, a grid-connected switch is closed, and grid connection is realized. The influence caused by the asymmetric working condition of the load is considered before and after grid connection, and a current type inverter which runs in parallel with the VSG inverter is added to compensate the unbalanced current component of the load current, so that the grid connection current imbalance and the distortion of the PCC point voltage are effectively inhibited.
The number of phase-locked loops and the number of PI control links of the adopted control strategy are both less than that of a traditional inverter pre-synchronization strategy, and the frequency compensation link is directly compensated into a rotor equation of the VSG, so that seamless switching from off-grid to grid-connected of the multi-VSG inverter can be quickly, accurately and simply realized even under the condition of load asymmetric faults, the circulating current in a pre-synchronization instant micro-grid is well restrained, and the power quality is improved.
The control method for multi-VSG inverter loop current suppression and fault treatment comprises the following specific processes:
step 101, establishing a virtual synchronous generator model
The virtual synchronous generator model includes an electronic electromagnetic equation, a torque and frequency equation, a motion equation, and a virtual excitation equation of the VSG. The form of these equations is well known and not described in this application.
Step 102, VSG off-grid and on-grid switching process analysis
In the analysis process, when switching from grid connection to grid disconnection is found, the VSG inverter does not have an obvious transient process in the operation mode switching process, but generates a large impact current in the process from grid disconnection to grid connection of the VSG inverter, even causes grid connection failure, and the maximum instantaneous deviation can reach two times of the grid voltage amplitude, namely 2U. And also causes distortion of voltage and deterioration of power quality.
Step 103, selection of presynchronization control strategy of single VSG inverter
A VSG inverter pre-synchronization control strategy based on PLL is designed on the basis of step 102, and on the basis of extracting the voltage phase of a power grid by using the PLL method, when the amplitude, the frequency and the phase of the output voltage of the VSG inverter in the pre-synchronization stage are consistent with those of the power grid, a grid-connected switch is closed, so that the control strategy avoids the defect caused by direct grid connection.
Step 104, generating grid-connected signals of single VSG grid-connected inverter
After step 103 is completed, that is, after the pre-synchronization enable signal of the grid connection arrives, the frequency compensation link Δ ω and the amplitude compensation link Δ U start to operate, and fig. 2 is a specific flow:
acquiring a power grid voltage phase through a phase-locked loop, when a presynchronization enabling signal arrives, operating a VSG algorithm by using an algorithm added with presynchronization control, continuously detecting whether a grid-connected requirement is met, and if the presynchronization enabling signal does not reach the grid-connected requirement, continuing to perform presynchronization adjustment; if so, the grid-connected switch is closed and the compensation amount is set to zero.
By continuously detecting the q-axis output voltage U of the inverter after dq conversionqIs less than the q-axis threshold uqermsD axis output voltage UdWhether the error of the peak value of the grid phase voltage is smaller than a d-axis threshold value udermsIf both the two conditions are met, the voltage deviation of the two sides of the PCC points is small enough, and the synchronization of the voltage of the output end of the VSG inverter and the voltage of the power grid is basically realized, so that the grid connection can be carried out. At the moment, the grid-connected switch is closed, and finally seamless switching from grid-off to grid-connection of the single VSG inverter is achieved.
Step 105, power distribution and parameter configuration of multiple VSG inverters
For the condition that VSG inverters with different capacities in a microgrid operate in parallel, power distribution is one of the keys for realizing stable operation of the system no matter in an off-grid or grid-connected operation mode. Therefore, the power allocation and the parameter configuration thereof are firstly carried out, and then the design of the presynchronization control strategy is carried out. Here, two VSG inverters are explained as an example. A specific embodiment thereof can be shown by means of fig. 3.
Fig. 3 is a diagram of a main circuit structure in which two VSG inverters are operated in parallel after a pre-synchronization control strategy is added, and the diagram includes a main circuit topology and a control circuit. The main circuit topology adopts a typical LC three-phase bridge circuit, and the circuit topology structure can work in two operation modes of off-grid operation and on-grid operation and realizes the communication between an external power system and a power supply network through a Point of Common Coupling (PCC). When the PCC is disconnected with the main network, the system can automatically operate in an off-grid mode, and uninterrupted power supply for loads in the microgrid is ensured. The control circuit part comprises a power calculation module, a VSG algorithm module and a voltage and current double closed-loop control module. The power calculation module can sample active power and reactive power output by the inverter in real time; the VSG algorithm module enables the output characteristic of the inverter to be similar to that of a synchronous generator; the voltage and current double closed-loop control can further improve the stability of the system. Adding virtual in presynchronization processFrequency compensation value delta omega in wire framei(i-1, 2) and the voltage amplitude compensation value DeltaUi(i is 1,2), the pre-synchronization of the two inverters can be realized.
Step 105-a, neglecting line impedance, and obtaining the active power and the reactive power output by each VSG inverter terminal as follows:
wherein, UgFor grid voltage amplitude, EiThe amplitude of the output voltage of the ith VSG inverter; deltaiOutputting a voltage phase angle for the ith VSG inverter; ziIs the equivalent impedance between the ith VSG inverter and the power grid.
And 105-b, ensuring that the electromagnetic power is equal to the mechanical power in a stable state in the speed regulator process. Therefore, the electromagnetic power P of the ith VSG inverteriAnd excitation electromotive force EiCan be expressed as: pi＝Prefi+kωi(ωref-ωmi)，Ei＝Eref+kqi(Qrefi-Qi). Wherein k isωiRepresents the reactive voltage droop coefficient, k of the ith VSG inverterqiRepresents the active frequency droop coefficient, P, of the ith VSG inverterrefiRepresents the given active power reference value, omega, of the ith VSG inverterrefRepresenting a given angular frequency reference value, ω, of the VSG invertermiReal angular frequency device representing the operation of the ith VSG inverter, ErefRepresenting a given voltage amplitude reference, Q, of a VSG inverter grid-connected pointrefiA reference value representing the reactive power given by the ith VSG inverter.
Step 105-c, setting the rated capacity S of two VSG invertersi *When the operation is stable, the two excitation electromotive forces are equal, and the angles of the two grid-connected inverters are also equal, that is, E1＝E2，δ1＝δ2Then, the ratio of the available active power is: z2＝NZ1。
Step 105-d, considering the distribution of the reactive power again, when the reactive power satisfies kq2＝kq1N, the VSG inverters may be assigned according to the rated capacity ratio.
Step 105-e, in order to make the VSG inverters running in parallel have better dynamic response characteristics, the variation of the active power of the two inverters is considered to be respectively delta P1And Δ P2The following can be obtained: when VSG inverters with different capacities are operated in parallel, parameters are set so that virtual reactance X of each inverteriReactive voltage droop coefficient kωiInversely proportional to capacity; active frequency droop coefficient kqiVirtual rotation inertia JiVirtual damping coefficient DiAnd capacity Si *Proportional ratio, namely, the power distribution can be realized, and the dynamic characteristic is better when disturbance occurs. The specific relation is as follows:
step 106, control strategy for pre-synchronization, circulation suppression and fault handling of multi-VSG inverter in emergency
On the basis of realizing the distribution of the power of the parallel operation of a plurality of VSGs in the step 105, the operation control of the system is divided into the following three working conditions for processing:
the first condition is as follows: under normal conditions, when multiple VSGs are operated in parallel to perform off-grid-connection switching, frequency compensation Δ ω and amplitude compensation Δ U are performed respectively according to the output voltage setting of each VSG inverter during off-grid connection, as shown in a dashed line frame in fig. 3, and the specific calculation modes of Δ ω and Δ U are shown in fig. 4. By ensuring that the public connection PCC point voltage does not generate larger deviation with the power grid voltage due to the instantaneous amplitude difference and the phase difference of the output voltage of each VSG inverter, the impact current is effectively restrained, and the seamless switching from the off-grid to the grid-connected of a plurality of inverters is realized. And the proposed control strategy effectively suppresses the circulating current problem between multiple inverters.
Case two: when a plurality of VSGs are connected in parallel and operated off-grid, the emergency situation of asymmetric load occurs, the scheme introduces a negative sequence control link in the control loop of the traditional VSG inverter, and the improved control loopThe structure of the VSG control is shown in fig. 5. Inverter output voltage U under three-phase static coordinate systemabcAnd an output current iabcBy coordinate transformation to U in two-phase stationary coordinate systemαβAnd iαβAnd converting positive and negative sequence components of the voltage and current after positive and negative sequence separation by using a second-order generalized integrator (SOGI) from a two-phase static coordinate system to a two-phase rotating coordinate system, controlling the positive sequence voltage and current and the negative sequence voltage and current respectively and independently, setting given values of negative sequence voltages of d and q axes in a negative sequence control loop to be 0, and controlling to realize that only the positive sequence component is contained in the output voltage of the VSG inverter. When the VSG inverter system is in a stable operation state after a negative sequence control link is introduced, the pre-synchronization enabling signal arrives, and respective frequency compensation delta omega and amplitude compensation delta U are given to the output voltage of each VSG inverter when the VSG inverter is disconnected from the grid according to the description of the situation I, so that the impact current can be effectively inhibited, and the seamless switching from the disconnection of the plurality of inverters to the grid connection is realized.
Case three: on the basis of realizing the seamless switching of the multiple VSGs in parallel operation from grid connection, the system is already in a stable operation state, at the moment, if a load asymmetric emergency working condition occurs, a current type inverter which operates in parallel with the VSG inverter is added, the load current is decomposed into a balanced active current, a balanced reactive current, an unbalanced current and an empty current component by adopting a Conservative Power Theory (CPT), and the output current of the current type inverter is controlled to be equal to the unbalanced current and the empty current component in the load current, so that the unbalanced current and the empty current component in the load current are compensated. The external characteristics of the load after reaching the steady state can be represented as a three-phase balanced load, and the structure diagram is shown in fig. 3, so that the grid-connected current imbalance and the distortion of the PCC point voltage are effectively inhibited.
Fig. 4 is a diagram of a presynchronization control, and particularly shows a calculation manner of the frequency and voltage amplitude compensation amount in fig. 3. The designed presynchronization control strategy mainly comprises a frequency compensation link delta omega and an amplitude compensation link delta U. Three-phase network voltage ugabcOutputting the grid voltage phase angle theta through a phase-locked loopgOutput voltage u of VSG inverterabcAt thetagD-axis component U is obtained by dq transformation as referencedAnd q-axis component Uq. Will UqIs set to 0, UqThe deviation from 0 is sent to a PI regulator, and the output regulating quantity delta omega and the island mode reference angular frequency omega are output*Adding them together to form the reference angular frequency omega of the presynchronization linkm. And d-axis component UdThe reference value is three-phase power grid voltage amplitude Ugm，UdAnd UgmThe deviation is sent to a PI regulator, and the output regulating quantity delta U is superposed with the island mode reference voltage amplitude E to obtain a reference voltage amplitude UrefAnd finally, the reference phase and the reference phase are combined together to form a three-phase reference voltage, so that pre-synchronization control is realized.
FIG. 5 is a block diagram of an improved VSG control incorporating a negative sequence control element, in which the inverter output voltage U is in a three-phase stationary coordinate systemabcAnd an output current iabcBy coordinate transformation to U in two-phase stationary coordinate systemαβAnd iαβPositive and negative sequence components of the voltage and current are converted from a two-phase static coordinate system to a two-phase rotating coordinate system after positive and negative sequence separation by a second-order generalized integrator (SOGI), the positive sequence voltage and current and the negative sequence voltage and current are independently controlled at the moment, given values of negative sequence voltages of d and q axes in a negative sequence control loop are set to be 0, and the output voltage of the VSG inverter is controlled to only contain the positive sequence components, so that the unbalance degree of the output voltage when the off-grid load is in asymmetric fault is reduced.
Example 2:
an embodiment of a specific application of the control method for loop current suppression and fault handling of the multi-VSG inverter is given below. The control method is adopted to carry out simulation analysis and verification on the parallel operation of two VSGs with different capacities. The VSG1 capacity is selected to be 20kW +10kvar, the VSG2 capacity is selected to be 10kW +5kvar, the capacity ratio of the two is 2:1, and the local load shared by the two is 18kW +15 kvar. Fig. 6 to 8 are diagrams of phase current waveforms of the microgrid bus and output active/reactive power waveforms of the VSG1 and the VSG2, respectively. According to simulation results, in a microgrid system with multiple VSGs connected in parallel, grid connection can be achieved without any pre-synchronization control strategy, however, bus current of the microgrid has large impact and is approximately 8 times of normal grid connection current, meanwhile, output power of two inverters can generate huge peaks and oscillations, and the quality of electric energy is reduced.
The presynchronization control strategy proposed by the patent is introduced into the multi-machine parallel system. Fig. 9 to 12 are graphs of the microgrid bus phase current waveform and the output active/reactive power waveforms of the VSG1 and the VSG2 after the presynchronization control is added. According to simulation results, the VSG1 and the VSG2 can distribute power according to the capacity ratio of an inverter according to the local load requirement in the off-grid operation mode, and provide energy for the power grid and the load according to the power reference value in the grid-connected operation mode, so that the correctness of the power distribution control strategy is proved. After the pre-synchronization control strategy is added, the impact current of the microgrid bus is effectively inhibited, the peak and the oscillation of the power are weakened to a great extent, and seamless switching from off-grid to grid-connected of a plurality of VSG inverters with different capacities can be realized. By observing the microgrid bus phase current after the presynchronization is added and the waveform diagrams 9-12 of the VSG1 and the VSG2 output power, and comparing with the waveform diagrams 6-8 without the presynchronization control strategy, it can be seen that the impact current is reduced from about 440A to about 80A, and the power spike and oscillation are suppressed, thus the correctness and effectiveness of the control strategy can be seen.
The ring current suppression ratio after the compensation link is added is shown in fig. 13-14, the ring current is reduced from 80A to 12A, which shows that the control strategy can also effectively suppress the ring current generated between inverters at the moment of grid connection.
When the load has an asymmetric fault, another current-mode inverter is added to compensate the nonlinear current, as shown in fig. 15. Load asymmetry fault occurs at 0.2s, current and voltage are distorted at the moment, a current compensation scheme is added at 0.7s, the scheme can effectively inhibit distortion and unbalance degree of grid-connected voltage and current, total harmonic distortion THD of the grid-connected voltage is reduced to 3.26% from 6.85%, the total harmonic distortion THD of the grid-connected current is reduced to 0.66% from 4.57%, and grid-connected power quality is remarkably improved. Simulation results show that the method can effectively inhibit distortion of grid-connected voltage and current, obviously reduce unbalance degree of three-phase grid-connected current, effectively reduce grid-connected voltage and current THD and improve grid-connected electric energy quality.
Example 3:
based on the same inventive concept, the invention also provides a control system for multi-VSG inverter loop current suppression and fault processing, and the principle of solving the technical problems of the equipment is similar to the control method for multi-VSG inverter loop current suppression and fault processing, so repeated details are not repeated.
The basic structure of the system is shown in fig. 16, and comprises: the system comprises a grid-connected switching module and a grid-connected fault handling module;
wherein the content of the first and second substances,
and the grid-connected switching module is used for switching the grid connection from the off-grid mode: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met;
the grid-connected fault handling module is used for, when the grid-connected steady-state operation is carried out: if the load has asymmetric faults, the current type inverter and the VSG inverter which finishes the power parameter setting are connected in parallel and are connected into a power grid, and unbalanced current is compensated;
wherein the power parameter of each VSG inverter is set based on the rated capacity of each VSG inverter.
Fig. 17 shows a detailed structure of a control system for loop current suppression and fault handling of a multi-VSG inverter.
Wherein, this system still includes the parameter setting module that is used for setting up the power parameter of each VSG inverter, and the parameter setting module includes: a capacity acquisition unit and a parameter setting unit;
a capacity acquisition unit for acquiring the rated capacity of each VSG inverter;
the parameter setting unit is used for setting power parameters for each VSG inverter according to the rated capacity proportion of each VSG inverter;
the power parameters comprise a virtual reactance and a reactive voltage droop coefficient which are inversely proportional to the rated capacity, and an active frequency droop coefficient, a virtual rotational inertia and a virtual damping coefficient which are directly proportional to the rated capacity.
Wherein, the switching module that is incorporated into the power networks includes: the device comprises a pre-synchronization control unit and a grid connection judging unit;
the pre-synchronization control unit is used for performing pre-synchronization control to compensate the angular frequency and the voltage of each VSG inverter which finishes the power parameter setting;
and the grid connection judging unit is used for respectively judging whether each VSG inverter reaches the grid connection requirement after pre-synchronous control: and if so, merging the VSG inverter which meets the grid-connected condition into the power grid, otherwise, continuing to perform presynchronization control until the grid-connected requirement is met.
Wherein, the presynchronization control unit includes: the device comprises a phase angle acquisition subunit, a first dq transformation subunit, an adjustment quantum unit and a reference value subunit;
the phase angle acquisition subunit is used for acquiring a voltage phase angle of the power grid by adopting a phase-locked loop;
the first dq conversion subunit is used for carrying out dq conversion on the output voltage of the VSG inverter by taking a phase angle as a reference so as to obtain a d-axis component and a q-axis component;
the adjusting quantum unit is used for inputting the d-axis component into the PI adjuster to obtain an angular frequency adjusting quantity, and inputting the deviation between the q-axis component and the three-phase power grid voltage amplitude into the PI adjuster to obtain an amplitude adjusting quantity;
and the reference value subunit is used for adding the angular frequency regulating quantity reference angular frequency of the island mode to obtain the reference angular frequency of the pre-synchronization link, and adding the amplitude regulating quantity and the island mode reference voltage amplitude to obtain the reference voltage amplitude of the pre-synchronization link.
Wherein, the judgement unit that is incorporated into the power networks includes: the system comprises a second dq conversion subunit, a dq judgment subunit and a grid connection judgment subunit;
the second dq conversion subunit is used for carrying out dq conversion on the output voltage of the VSG inverter to obtain a d-axis output voltage and a q-axis output voltage;
the dq judging subunit is used for judging whether the absolute value of the q-axis output voltage is smaller than a q-axis threshold value or not and judging whether the error between the d-axis output voltage and the peak value of the grid phase voltage is smaller than a d-axis threshold value or not;
and the grid connection judgment subunit is used for judging that the VSG inverter meets the grid connection requirement when the two judgment results are yes, and otherwise, the VSG inverter does not meet the grid connection requirement.
The grid-connected switching module also comprises a zero setting unit;
and the zero setting unit is used for setting the compensation quantity of the angular frequency and the voltage of the VSG inverter after the VSG inverter is incorporated into the power grid to zero.
Wherein, grid-connected fault handles the module and includes: the current type inverter network access unit, the current decomposition unit and the current compensation unit;
the current-type inverter network access unit is used for connecting the current-type inverter and the VSG inverter which finishes the power parameter setting in parallel and then merging the current-type inverter and the VSG inverter into a power grid;
the current decomposition unit is used for decomposing the load current into a balanced active current, a balanced reactive current, an unbalanced current and a null current component by adopting a conservative power theory;
and the current compensation unit is used for compensating the unbalanced current by controlling the output current of the current type inverter to be equal to the unbalanced current and the air current component in the load current.
The control system for the multi-VSG inverter loop current suppression and fault processing further comprises an off-grid fault handling module;
an off-grid fault handling module for, when an off-grid operation occurs and an asymmetric fault of a load occurs: and aiming at each VSG inverter which finishes the power parameter setting, a negative sequence control method is adopted to control each VSG inverter respectively, so that the unbalance degree of the output voltage when the off-grid load is in an asymmetric fault is reduced.
Wherein, off-grid fault handling module includes: the device comprises a first conversion unit, a positive-negative separation unit, a second conversion unit and a positive-negative sequence control unit;
the VSG inverter comprises a first conversion unit, a second conversion unit and a control unit, wherein the first conversion unit is used for converting three-phase voltage and three-phase current of the VSG inverter under a three-phase static coordinate system into two-phase voltage and two-phase current under a two-phase static coordinate system;
the positive-negative separation unit is used for performing positive-negative sequence separation on the two-phase voltage and current by adopting a second-order generalized integrator to obtain a positive sequence component and a negative sequence component of the two-phase voltage and current;
the second conversion unit is used for converting the positive sequence component and the negative sequence component of the two-phase voltage and current from the two-phase static coordinate system to the two-phase rotating coordinate system to respectively obtain the dq-axis voltage and current of a positive sequence and the dq-axis voltage and current of a negative sequence;
and the positive and negative sequence control unit is used for controlling the dq axis voltage and current of the positive sequence, setting the dq axis voltage of the negative sequence to zero and controlling, and realizing that the output voltage of the inverter only contains a positive sequence component.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
It should be noted that the above-mentioned embodiments are only for illustrating the technical solutions of the present application and not for limiting the scope of protection thereof, and although the present application is described in detail with reference to the above-mentioned embodiments, those skilled in the art should understand that after reading the present application, they can make various changes, modifications or equivalents to the specific embodiments of the application, but these changes, modifications or equivalents are all within the scope of protection of the claims to be filed.
Claims (13)
1. A control method for multi-VSG inverter circulating current suppression and fault handling is characterized by comprising the following steps:
when grid-connected switching is performed by off-grid: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met;
when steady state operation after grid connection: if the load has asymmetric faults, connecting a current type inverter and the VSG inverter after the power parameter setting is completed in parallel and merging the current type inverter and the VSG inverter into a power grid, and compensating unbalanced current;
the power parameter of each VSG inverter is preset based on the rated capacity of each VSG inverter.
2. The method of claim 1, wherein setting the power parameter for each VSG inverter based on a rated capacity of each VSG inverter comprises:
acquiring rated capacity of each VSG inverter;
setting power parameters for each VSG inverter according to the rated capacity proportion of each VSG inverter;
the power parameters comprise a virtual reactance and a reactive voltage droop coefficient which are inversely proportional to rated capacity, and an active frequency droop coefficient, a virtual rotational inertia and a virtual damping coefficient which are directly proportional to the rated capacity.
3. The method of claim 1, wherein the step of compensating each VSG inverter by using a pre-synchronization control compensation method until a grid-connection requirement is met for each VSG inverter that completes the power parameter setting comprises:
performing pre-synchronization control to compensate the angular frequency and voltage of each VSG inverter which finishes the power parameter setting;
respectively judging whether each VSG inverter reaches grid-connected requirements after pre-synchronous control: and if so, merging the VSG inverter which meets the grid-connected condition into the power grid, otherwise, continuing to perform presynchronization control until the grid-connected requirement is met.
4. The method of claim 3, wherein the performing pre-synchronization control to compensate for angular frequency and voltage of the VSG inverter comprises:
acquiring a voltage phase angle of a power grid by adopting a phase-locked loop;
carrying out dq conversion on the output voltage of the VSG inverter by taking the phase angle as a reference to obtain a d-axis component and a q-axis component;
inputting the d-axis component into a PI regulator to obtain an angular frequency regulating quantity, and inputting the deviation between the q-axis component and the three-phase power grid voltage amplitude into the PI regulator to obtain an amplitude regulating quantity;
and adding the angular frequency regulating quantity reference angular frequency of the island mode to obtain the reference angular frequency of a pre-synchronization link, and adding the amplitude regulating quantity and the island mode reference voltage amplitude to obtain the reference voltage amplitude of the pre-synchronization link.
5. The method of claim 3, wherein determining whether the VSG inverter meets grid-tie requirements comprises:
carrying out dq conversion on the output voltage of the VSG inverter to obtain d-axis output voltage and q-axis output voltage;
judging whether the absolute value of the q-axis output voltage is smaller than a q-axis threshold value or not, and judging whether the error between the d-axis output voltage and the peak value of the power grid phase voltage is smaller than a d-axis threshold value or not;
and when the two judgment results are yes, the VSG inverter meets the grid-connected requirement, otherwise, the VSG inverter does not meet the grid-connected requirement.
6. The method of claim 3, after meeting grid-connection requirements, further comprising:
and (4) zero setting the compensation quantity of the angular frequency and the voltage of the VSG inverter after the VSG inverter is incorporated into the power grid.
7. The method of claim 1, wherein the coupling a current mode inverter to the VSG inverter after completion of power parameter setting in parallel to a power grid to compensate for unbalanced current comprises:
connecting a current-type inverter and the VSG inverter after the power parameter setting is completed in parallel and merging the current-type inverter and the VSG inverter into a power grid;
decomposing the load current into a balanced active current, a balanced reactive current, an unbalanced current and a null current component by adopting a conservative power theory;
and compensating the unbalanced current by controlling the output current of the current type inverter to be equal to the unbalanced current and the air current component in the load current.
8. The method of claim 1, further comprising:
when off-grid operation and asymmetric failure of the load occurs: and aiming at each VSG inverter which finishes the power parameter setting, a negative sequence control method is adopted to control each VSG inverter respectively, so that the unbalance degree of the output voltage when the off-grid load is in an asymmetric fault is reduced.
9. The method of claim 8, wherein the controlling the VSG inverter using a negative sequence control method comprises:
converting three-phase voltage and three-phase current of the VSG inverter under a three-phase static coordinate system into two-phase voltage and current under a two-phase static coordinate system;
performing positive-negative sequence separation on the two-phase voltage and current by using a second-order generalized integrator to obtain positive-sequence components and negative-sequence components of the two-phase voltage and current;
converting the positive sequence component and the negative sequence component of the two-phase voltage and current from a two-phase static coordinate system to a two-phase rotating coordinate system to respectively obtain a positive-sequence dq-axis voltage and current and a negative-sequence dq-axis voltage and current;
and controlling the voltage and the current of the dq axis of the positive sequence, and setting the voltage of the dq axis of the negative sequence to zero and controlling to realize that the output voltage of the inverter only contains a positive sequence component.
10. A control system for multi-VSG inverter circulating current suppression and fault handling, comprising: the system comprises a grid-connected switching module and a grid-connected fault handling module;
the grid-connected switching module is used for switching the grid connection from the off-grid mode: aiming at each VSG inverter which finishes the power parameter setting, respectively compensating each VSG inverter by adopting a pre-synchronous control compensation method until the grid-connected requirement is met;
the grid-connected fault handling module is used for, when the grid-connected fault handling module runs in a steady state mode: if the load has asymmetric faults, connecting a current type inverter and the VSG inverter after the power parameter setting is completed in parallel and merging the current type inverter and the VSG inverter into a power grid, and compensating unbalanced current;
wherein the power parameter of each VSG inverter is set based on the rated capacity of each VSG inverter.
11. The system of claim 10, further comprising a parameter setting module for setting power parameters of each VSG inverter, the parameter setting module comprising: a capacity acquisition unit and a parameter setting unit;
the capacity acquisition unit is used for acquiring the rated capacity of each VSG inverter;
the parameter setting unit is used for setting power parameters for each VSG inverter according to the rated capacity proportion of each VSG inverter;
the power parameters comprise a virtual reactance and a reactive voltage droop coefficient which are inversely proportional to rated capacity, and an active frequency droop coefficient, a virtual rotational inertia and a virtual damping coefficient which are directly proportional to the rated capacity.
12. The system of claim 10, wherein the grid tie switching module comprises: the device comprises a pre-synchronization control unit and a grid connection judging unit;
the pre-synchronization control unit is used for performing pre-synchronization control to compensate the angular frequency and the voltage of each VSG inverter which finishes the power parameter setting;
the grid connection judging unit is used for respectively judging whether each VSG inverter reaches the grid connection requirement after pre-synchronous control: and if so, merging the VSG inverter which meets the grid-connected condition into the power grid, otherwise, continuing to perform presynchronization control until the grid-connected requirement is met.
13. The system of claim 10, wherein the grid-tie fault handling module comprises: the current type inverter network access unit, the current decomposition unit and the current compensation unit;
the current-type inverter network access unit is used for connecting the current-type inverter and the VSG inverter after the power parameter setting is finished in parallel and merging the current-type inverter and the VSG inverter into a power grid;
the current decomposition unit is used for decomposing the load current into a balanced active current, a balanced reactive current, an unbalanced current and a null current component by adopting a conservative power theory;
the current compensation unit is used for compensating the unbalanced current by controlling the output current of the current type inverter to be equal to the unbalanced current and the air current component in the load current.
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Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
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| CN114172189A (en) * | 2021-11-04 | 2022-03-11 | 江苏大学 | Control method of virtual synchronous generator under unbalanced power grid |
| CN116014693A (en) * | 2023-02-01 | 2023-04-25 | 中国电力科学研究院有限公司 | Method and system for inhibiting asymmetric fault current of GFM-VSC grid-connected system |
| CN116155270A (en) * | 2023-03-01 | 2023-05-23 | 南通大学 | Method for adjusting three-phase voltage phase-locked loop by improving MSTOGI structure and nonlinear PI |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN114172189A (en) * | 2021-11-04 | 2022-03-11 | 江苏大学 | Control method of virtual synchronous generator under unbalanced power grid |
| CN114172189B (en) * | 2021-11-04 | 2023-12-15 | 江苏大学 | Control method of virtual synchronous generator under unbalanced power grid |
| CN116014693A (en) * | 2023-02-01 | 2023-04-25 | 中国电力科学研究院有限公司 | Method and system for inhibiting asymmetric fault current of GFM-VSC grid-connected system |
| CN116155270A (en) * | 2023-03-01 | 2023-05-23 | 南通大学 | Method for adjusting three-phase voltage phase-locked loop by improving MSTOGI structure and nonlinear PI |
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