CN105098804B - The control method and device of the three phase unbalance current of virtual synchronous generator - Google Patents
The control method and device of the three phase unbalance current of virtual synchronous generator Download PDFInfo
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Abstract
The invention discloses a kind of control method and device of the three phase unbalance current of virtual synchronous generator, wherein, method comprises the following steps：Establish VSG equation of rotor motion and the virtual excitation equations of VSG；Unbalanced line voltage is decomposed into positive sequence, negative phase-sequence, residual voltage and corresponding phase-sequence current, the VSG active power exported and reactive power are decomposed to analyze the imbalance problem run into；The reference instruction of dq shaft currents is calculated according to the three-phase voltage signal of output；The reference instruction of electric current is converted into by PWM voltage modulation signals by the current inner loop of designed, designed, and virtual synchronous generator operation is controlled by PWM voltage modulation signals, so that the output three-phase current of virtual synchronous generator reaches balance.The control method of the embodiment of the present invention can make VSG still to export three-phase equilibrium electric current in power network imbalance of three-phase voltage, better ensure that the stability of operation of power networks, not only securely and reliably, and also it is simple and convenient.
Description
Technical Field
The invention relates to the technical field of power control, in particular to a method and a device for controlling three-phase unbalanced current of a virtual synchronous generator.
Background
The output of the distributed power supply is generally direct current or non-power frequency alternating current, so that the distributed power supply needs to be connected with a power grid through a grid-connected inverter. Because the traditional grid-connected inverter belongs to static equipment and cannot provide inertia and damping support for a power grid, the traditional grid-connected inverter cannot participate in power grid regulation, and the operation stability of the power grid is bound to be seriously threatened along with the increase of the permeability of a distributed power supply.
In recent years, grid-connected inverter control based on a Virtual Synchronous Generator (VSG) has been attracting attention. The basic idea of the VSG is to simulate the characteristics of a synchronous generator, and control an inverter by utilizing a rotor motion equation and excitation regulation characteristics simulating the synchronous generator, so that the inverter has the characteristics similar to those of the synchronous generator, and the purpose of providing inertia and damping support for a power grid is achieved.
In practice, the grid voltage is susceptible to the influence of factors such as load unbalance, short-circuit fault, non-full-phase operation and the like, so that the phenomenon of three-phase unbalance occurs, and under the condition, the problems of current overload, power oscillation, current unbalance and the like can occur in the VSG, so that the safe and stable operation of the VSG is threatened. Therefore, the control research of the VSG under the three-phase unbalanced network voltage is considered to be of great significance. Currently, the concern and research on the problem are little at home and abroad, and the simulation of the traditional inverter control is mainly taken as the main point. However, the conventional inverter control method directly obtains the current inner loop reference command through power outer loop calculation, the VSG needs to convert the power command into a voltage reference value for control after simulating the operation principle of the synchronous generator, and the input and output interfaces, the control mechanism and the like of the VSG are different, so the conventional control method cannot be directly used for VSG unbalanced operation control.
Disclosure of Invention
The present invention is directed to solving, at least to some extent, one of the technical problems in the related art described above.
Therefore, an object of the present invention is to provide a method for controlling three-phase unbalanced current of a virtual synchronous generator, which can enable a VSG to output three-phase balanced current when three-phase voltages of a power grid are unbalanced, and is simple to implement.
Another object of the present invention is to provide a control device for three-phase unbalanced current of a virtual synchronous generator.
In order to achieve the above object, an embodiment of the present invention provides a method for controlling three-phase unbalanced current of a virtual synchronous generator, including the following steps: establishing a rotor motion equation of VSG simulating the mechanical characteristics of the synchronous generator and a VSG virtual excitation equation of the electromagnetic characteristics; the method comprises the steps of decomposing the voltage of a power grid into positive sequence voltage, negative sequence voltage, positive sequence current and negative sequence current, and decomposing active power and reactive power output by VSG into average components and fluctuation components according to the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current so as to analyze the problems of power fluctuation and three-phase current unbalance of a virtual synchronous generator facing the operation surface when the voltage of the power grid is unbalanced; calculating a reference command of the dq axis current according to the three-phase voltage signal so that the average component is equal to a set value of the virtual synchronous generator and suppresses the negative sequence current; and converting the reference instruction of the current into a PWM (Pulse Width Modulation) voltage Modulation signal, and controlling the virtual synchronous generator to operate through the PWM voltage Modulation signal, so that the output three-phase current of the virtual synchronous generator is balanced.
According to the control method of the three-phase unbalanced current of the virtual synchronous generator provided by the embodiment of the invention, the grid voltage is decomposed into the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current, the problems of power fluctuation and three-phase current unbalance faced by the virtual synchronous generator when the grid voltage is unbalanced are analyzed according to the relation of the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current, the average component is equal to the set value of the virtual synchronous generator and the negative sequence current is inhibited by calculating the reference instruction of the dq shaft current, and finally the three-phase current of the virtual synchronous generator is balanced by converting the reference instruction into the PWM voltage modulation signal.
In addition, the control method for three-phase unbalanced current of the virtual synchronous generator according to the above embodiment of the present invention may further have the following additional technical features:
further, in one embodiment of the present invention, the VSG rotor equation of motion is:
wherein J is the virtual moment of inertia, ω is the VSG virtual angular velocity, TmAnd TeRespectively, virtual mechanical torque and electromagnetic torque, D is damping coefficient, omega0Is a reference value for the synchronous angular velocity.
Further, in an embodiment of the present invention, the VSG virtual excitation equation is:
wherein M isfIs the maximum value of mutual inductance between the rotor and a phase of the stator, ifFor the magnitude of the exciting current, K is the inertia coefficient of the excitation regulation, Q is the reactive power value set by VSG, QeAnd the value is the reactive power value of the grid-connected port.
Further, in one embodiment of the present invention, the reference command for the dq-axis current is calculated according to the following formula:
wherein,the dq component of the positive sequence current, L and R are the total inductance and total resistance from the inverter to the grid, s is the differential operator, ω is the VSG virtual angular velocity,is the difference of the q-axis component of the inverter output voltage and the q-axis component of the grid positive sequence voltage,is the difference between the d-axis component of the inverter output voltage and the d-axis component of the grid positive sequence voltage.
Further, in an embodiment of the present invention, the 2 nd harmonic voltage generated in the decomposition process is filtered by a trap to obtain the positive sequence voltage through decomposition.
Another embodiment of the present invention provides a control apparatus for three-phase unbalanced current of a virtual synchronous generator, including: the building module is used for building a rotor motion equation of VSG simulating the mechanical characteristics of the synchronous generator and a VSG virtual excitation equation of the electromagnetic characteristics; the decomposition module is used for decomposing the power grid voltage into positive sequence voltage, negative sequence voltage, positive sequence current and negative sequence current, and decomposing active power and reactive power output by the VSG into average components and fluctuation components according to the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current so as to analyze the power fluctuation and three-phase current unbalance problems faced by the virtual synchronous generator when the power grid voltage is unbalanced; the calculation module is used for calculating a reference instruction of the dq axis current according to the three-phase voltage signal so that the average component is equal to a set value of the virtual synchronous generator and the negative sequence current is restrained; and the control module is used for converting the reference instruction of the current into a PWM (pulse-width modulation) voltage modulation signal and controlling the virtual synchronous generator to operate through the PWM voltage modulation signal, so that the three-phase current of the virtual synchronous generator is balanced.
According to the control device for the three-phase unbalanced current of the virtual synchronous generator, provided by the embodiment of the invention, the power grid voltage is decomposed into the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current, so that the problems of power fluctuation and three-phase current unbalance faced by the virtual synchronous generator when the power grid voltage is unbalanced are analyzed, the average component is equal to the set value of the virtual synchronous generator and the negative sequence current is inhibited by calculating the reference instruction of the dq-axis current, and finally, the three-phase current of the virtual synchronous generator is balanced by converting the reference instruction into the PWM voltage modulation signal, so that the three-phase balanced current can be still output even if the VSG is unbalanced in the three-phase voltage of the power grid, and the operation stability of the power grid is better ensured.
In addition, the control device for three-phase unbalanced current of the virtual synchronous generator according to the above embodiment of the present invention may further have the following additional technical features: a
Further, in one embodiment of the present invention, the VSG rotor equation of motion is:
wherein J is the virtual moment of inertia, ω is the VSG virtual angular velocity, TmAnd TeRespectively, virtual mechanical torque and electromagnetic torque, D is damping coefficient, omega0Is a reference value for the synchronous angular velocity.
Further, in an embodiment of the present invention, the VSG virtual excitation equation is:
wherein M isfIs the maximum value of mutual inductance between the rotor and a phase of the stator, ifFor the magnitude of the exciting current, K is the inertia coefficient of the excitation regulation, Q is the reactive power value set by VSG, QeAnd the value is the reactive power value of the grid-connected port.
Further, in one embodiment of the present invention, the reference command for the dq-axis current is calculated according to the following formula:
wherein,the dq component of the positive sequence current, L and R are the total inductance and total resistance from the inverter to the grid, s is the differential operator, ω is the VSG virtual angular velocity,is the difference of the q-axis component of the inverter output voltage and the q-axis component of the grid positive sequence voltage,for outputting voltage to inverterIs compared to the d-axis component of the grid positive sequence voltage.
Further, in an embodiment of the present invention, the decomposition module is further configured to filter, by a trap, 2 th harmonic voltages generated during the decomposition process, so as to decompose the positive sequence voltage.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
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The above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
fig. 1 is a flowchart of a control method of three-phase unbalanced current of a virtual synchronous generator according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a VSG basic topology connection according to one embodiment of the present invention;
FIG. 3 is a schematic diagram of the structure of the current inner loop control according to one embodiment of the present invention;
FIG. 4 is a schematic diagram of a VSG control with added negative-sequence current suppression, according to one embodiment of the present invention;
FIG. 5 is a schematic diagram of a current output waveform under VSG control in a single-phase short circuit in the prior art;
FIG. 6 is a schematic diagram of a VSG controlled current output waveform during a single-phase short circuit according to an embodiment of the present invention;
FIG. 7 is a graph illustrating a comparison of output power under VSG control for a single phase short circuit according to the prior art and according to one embodiment of the present invention; and
fig. 8 is a schematic structural diagram of a control device for three-phase currents of a virtual synchronous generator according to an embodiment of the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
In the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally connected; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
In the present invention, unless otherwise expressly stated or limited, "above" or "below" a first feature means that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact with each other via another feature therebetween. Also, the first feature being "on," "above" and "over" the second feature includes the first feature being directly on and obliquely above the second feature, or merely indicating that the first feature is at a higher level than the second feature. A first feature being "under," "below," and "beneath" a second feature includes the first feature being directly above and obliquely above the second feature, or simply meaning that the first feature is at a lesser level than the second feature.
The following describes a method and an apparatus for controlling three-phase unbalanced current of a virtual synchronous generator according to an embodiment of the present invention with reference to the drawings, and first, a method for controlling three-phase unbalanced current of a virtual synchronous generator according to an embodiment of the present invention will be described with reference to the drawings. Referring to fig. 1, the control method includes the steps of:
s101, establishing a rotor motion equation of VSG simulating the mechanical characteristics of the synchronous generator and a VSG virtual excitation equation simulating the electromagnetic characteristics of the VSG.
Further, in one embodiment of the present invention, the VSG rotor equations of motion are:
wherein J is the virtual moment of inertia, ω is the VSG virtual angular velocity, TmAnd TeRespectively, virtual mechanical torque and electromagnetic torque, D is damping coefficient, omega0Is a reference value for the synchronous angular velocity.
Further, in one embodiment of the present invention, the VSG virtual excitation equation is:
wherein M isfIs the maximum value of mutual inductance between the rotor and a phase of the stator, ifFor the magnitude of the exciting current, K is the inertia coefficient of the excitation regulation, Q is the reactive power value set by VSG, QeAnd the value is the reactive power value of the grid-connected port.
Specifically, firstly, a basic topology structure corresponding to the virtual synchronous generator technology is determined: the distributed power generation system can comprise 5 subsystems of an inverter, an LCL filter, grid-connected port power calculation, VSG control algorithm calculation and SVPWM (Space vector pulse Width Modulation). The Virtual Synchronous Generator (VSG) technology is that an inverter has the characteristics similar to those of a synchronous generator by simulating the mechanical characteristics and the electromagnetic characteristics of the synchronous generator, so that the aim of providing inertia support and damping support for a power grid is fulfilled.
The rotor motion equation of the mechanical characteristic of the simulated synchronous generator is as follows:
in the formula: j is virtual rotational inertia, and the rotational inertia of the rotor of the synchronous generator is simulated; omega is VSG virtual angular velocity; t ismAnd TeVirtual mechanical torque and electromagnetic torque respectively; d is a damping coefficient, and the damping of the generator rotor is simulated; omega0The reference value for synchronous angular velocity is typically 314 rad/s. Solving the formula (1) to obtain a virtual angular velocity omega, and obtaining a rotor angle theta through integrationo。
Further, the amplitude of the three-phase output voltage of the virtual synchronous generator is as follows:
Vo＝Mfifω(2)
Mfis the maximum value of mutual inductance between the rotor and a phase stator, ifIs the magnitude of the excitation current.
Further, a VSG virtual excitation equation that models electromagnetic characteristics:
and K is the inertia coefficient of excitation regulation.
S102, the power grid voltage is decomposed into positive sequence voltage, negative sequence voltage, positive sequence current and negative sequence current, and active power and reactive power output by the VSG are decomposed into average components and fluctuation components according to the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current so as to analyze the power fluctuation and three-phase current unbalance problems of the virtual synchronous generator facing the operation surface when the power grid voltage is unbalanced.
Further, in one embodiment of the present invention, the 2 nd harmonic voltage generated in the decomposition process is filtered by the trap to obtain the positive sequence voltage.
Specifically, the analysis of operational problems that may occur with a VSG when the grid voltage is unbalanced: the voltage of the power grid is decomposed into positive sequence, negative sequence and zero sequence components, and the positive sequence voltage, the negative sequence voltage and the zero sequence voltage respectively act to generate positive sequence current, negative sequence current and zero sequence current. Because there is no neutral line in the VSG, zero sequence current cannot flow, so the output current is only decomposed into two components of positive sequence and negative sequence, which are respectively marked as i+,i-。
The instantaneous active power output by the VSG can be expressed as:
in the formula v+,v-,v0Three sequential components of the VSG port voltage are provided, where the sequential component labels all represent three phase voltage vectors. "·" denotes the inner product of the three-phase vector. The three-phase components of the zero-sequence voltage are identical and the inner product with symmetrical positive-sequence and negative-sequence currents is always zero, i.e. v0·i+＝v0·i-When the ratio is 0, the formula (4) can be simplified as follows:
p＝v+·i++v-·i-+v+·i-+v-·i+(5)
expressed as a sine function:
V+、V-the amplitudes of the positive sequence voltage and the negative sequence voltage are respectively; i is+、I-The amplitudes of the positive sequence current and the negative sequence current respectively;is the phase angle difference between the a-phase positive sequence voltage and the a-phase positive sequence current, i.e.The rest of the phase angle differences and so on.
As can be seen from the formula (6), the positive sequence voltage and current and the negative sequence voltage and current can generate constant active power; positive sequence voltage and negative sequence current, positive sequence current and negative sequence voltage can produce the active power that fluctuates at 2 times power frequency. Thus, the active power can be decomposed into an average componentAnd a fluctuating componentThe expressions of the two are:
in the same way, expressions for the average and fluctuating components of the reactive power can be derived:
in the formula: "x" represents a cross product of the vector.
According to the formulas (7) and (8), if the three phases of the control current are balanced, the negative sequence voltage of the power grid is equal toThe positive sequence current action still generates power fluctuation; on the other hand, if the fluctuation component of the control output power is 0, the negative-sequence current cannot be 0. Therefore, the three-phase current balance and the power constancy cannot be achieved at the same time. However, in actual engineering, the average component of the output power is set to be equal to the power set value P in current balance*,Q*And the positive sequence voltage and negative sequence current acting terms in the fluctuation component are eliminated, so that the effect of reducing active and reactive fluctuation can be achieved.
And S103, calculating a reference command of the dq axis current according to the three-phase voltage signals so that the average component is equal to the set value of the virtual synchronous generator and the negative sequence current is restrained.
Further, in one embodiment of the present invention, the reference command for the dq-axis current is calculated according to the following formula:
wherein,the dq component of the positive sequence current, L and R are the total inductance and total resistance from the inverter to the grid, s is the differential operator, ω is the VSG virtual angular velocity,is the difference of the q-axis component of the inverter output voltage and the q-axis component of the grid positive sequence voltage,is the difference between the d-axis component of the inverter output voltage and the d-axis component of the grid positive sequence voltage.
In particular, three-phase voltage signals v output by a VSG algorithmoAnd calculating to obtain a reference instruction of the dq-axis current, and enabling the average component of the power output to be the same as the VSG set value while achieving the effect of inhibiting the negative sequence current.
Referring to fig. 2, if the positive sequence circuit is analyzed while ignoring the effect of the capacitor branch in the circuit shown in fig. 2, the relationship between the positive sequence voltage and the positive sequence current in the dq coordinate system is:
in the formula: v. ofod,voqIs the VSG algorithm output voThe dq component of (1);andis the dq component of the grid positive sequence voltage and current; l and R are the total inductance and total resistance from the inverter to the grid; s is a differential operator. Due to voOnly the positive sequence component and can therefore be used directly for the calculation.
From the formulas (1) and (3), VSG can be obtained by adjusting voThe active power and the reactive power are automatically controlled to be the same as the set values, so that the power can be the same as the set values as long as the calculated relationship between the current and the voltage is consistent with the real circuit. To eliminate the negative sequence current, the positive sequence current in the equation (9) can be taken as the current reference value of the output, and the expression is:
in the formula:
because the VSG has closed-loop control of a voltage outer ring, current calculation does not need to completely simulate an actual circuit, and effective control can be realized as long as the basic relation of voltage and current dq axis components is represented to ensure that the positive and negative relations are not reversed. Using the final value theorem for the step response for the second order system of (10) and (Ls + R), the current command calculation can be simplified to:
further, the above calculation process requires a positive sequence component of the grid voltage, and therefore the positive and negative sequence components of the grid voltage are separated.
In the dq coordinate system, the expression of the grid voltage decomposed into positive and negative sequence components is as follows:
in the formula:
therefore, in the dq coordinate system, the original three-phase alternating-current positive sequence voltage can be changed into direct-current voltage, and the original three-phase alternating-current negative sequence voltage can be changed into 2-order harmonic voltage, so that the 2-order harmonic voltage can be filtered by the wave trap, and the effect of separating the positive sequence voltage is achieved. The transfer function of the trap may be:
in the formula: omeganThe method is characterized in that the notch angular frequency is designed to be 2 times of power frequency angular frequency; q is a quality factor. It should be noted that Q cannot be made too small, otherwise the filtering effect would be poor; q should not be too large, otherwise the trap response is too slow, deteriorating the dynamic effect.
And S104, converting the reference instruction of the current into a PWM (pulse-width modulation) voltage modulation signal, and controlling the virtual synchronous generator to operate through the PWM voltage modulation signal, so that the three-phase current of the virtual synchronous generator is balanced.
Specifically, the original VSG voltage output is connected to the current reference command input required by the current inner loop through the current command calculation module. The current is generally controlled under a dq rotating coordinate system, and a feedforward decoupling control strategy is adopted, and a control block diagram of the control strategy is shown in fig. 3. In the figure id,iqAnd ed,eqActual values obtained by converting the measured values of the VSG port current and the grid voltage through abc/dq coordinates are respectively obtained;is a dq-axis current reference command;is a modulated wave signal of the inverter output voltage. Fig. 3 can implement a differential-free control of the VSG port current due to the presence of the PI controller.
In the embodiment of the present invention, the VSG in the embodiment of the present invention implements a basic control method by simulating the electromagnetic and mechanical characteristics of a synchronous generator, and then analyzes two possible operation problems of the VSG control method by decomposing the grid voltage into a positive sequence voltage, a negative sequence voltage, a zero sequence voltage and a corresponding sequence current when the grid voltage is unbalanced: the power fluctuation and the three-phase current are unbalanced, and then the three-phase voltage signal v output by the VSG algorithm is outputoAnd finally, because the output current of the VSG is indirectly determined by the output voltage, a current inner loop controller is designed to convert the obtained current reference instruction into a PWM voltage modulation signal. The embodiment of the invention does not depend on the type of voltage unbalance when inhibiting the negative sequence current, can have better performance under the conditions of voltage balance and various unbalance types, and has simpler design and easy engineering realization.
Specifically, the method for controlling the three-phase unbalanced current of the virtual synchronous generator provided by the embodiment of the invention has the following advantages:
1. the VSG can be effectively controlled to output three-phase balance current when the voltage of the power grid is unbalanced, and the effects of reducing active and reactive oscillation, restraining power transient impact and reducing fault current amplitude are achieved.
2. The method is independent of the unbalanced type of the power grid voltage and does not influence the performance of the VSG in normal operation.
3. The control structure is simple, and engineering realization is easy.
In summary, the embodiment of the invention can effectively ensure that the VSG outputs three-phase balanced current, and is easy for engineering implementation.
The following describes in detail a control method for three-phase unbalanced current according to an embodiment of the present invention with a specific embodiment. In one embodiment of the invention, the embodiment of the invention comprises the following steps:
1) determining a basic topological structure corresponding to the virtual synchronous generator technology: the distributed power generation system comprises an inverter, an LCL filter, grid-connected port power calculation, VSG control algorithm calculation and 5 subsystems of SVPWM modulation. The Virtual Synchronous Generator (VSG) technology is that an inverter has the characteristics similar to those of a synchronous generator by simulating the mechanical characteristics and the electromagnetic characteristics of the synchronous generator, so that the aim of providing inertia support and damping support for a power grid is fulfilled. Setting the inverter as a three-phase three-bridge arm bus-free inverter, wherein the direct-current voltage is 700V, the effective value of the alternating-current line voltage is 381V, the switching frequency of the inverter is 10kHz, and the L of the filter inductorS,RS2mH and 0.1 Ω, filter capacitance C40 μ F, line parameter LlS,RlSFor 2mH and 0.1 Ω the inverter is given an active set of 10kW and a reactive set of 5 kvar.
The motion equation of the rotor simulating the mechanical characteristics of the synchronous generator is as follows:
in the formula: j is virtual rotational inertia, and the rotational inertia of the rotor of the synchronous generator is simulated (selected according to actual engineering); omega is VSG virtual angular velocity; t ismAnd TeVirtual mechanical torque and electromagnetic torque respectively; d is a damping coefficient (a value is selected according to actual requirements), and the damping of the generator rotor is simulated; omega0The reference value for synchronous angular velocity is typically 314 rad/s. Solving the formula (1) to obtain a virtual angular velocity omega, and obtaining a rotor angle theta through integrationo。
Virtual synchronous generator three-phase output voltage amplitude:
Vo＝Mfifω (2)
Mfis the maximum value (selected according to actual requirements) of mutual inductance between the rotor and the stator of one phase ifIs the magnitude of the excitation current.
VSG virtual excitation equation for simulating electromagnetic characteristics:
and K is an inertia coefficient (selected according to actual requirements) of excitation adjustment.
2) Analyzing possible operational problems of the VSG when the grid voltage is unbalanced: the voltage of the power grid is decomposed into positive sequence, negative sequence and zero sequence components, and the positive sequence voltage, the negative sequence voltage and the zero sequence voltage respectively act to generate positive sequence current, negative sequence current and zero sequence current. Because there is no neutral line in the VSG, zero sequence current cannot flow, so the output current is only decomposed into two components of positive sequence and negative sequence, which are respectively marked as i+,i-。
The instantaneous active power output by the VSG can be expressed as:
in the formula v+,v-,v0Three sequential components of the VSG port voltage are provided, where the sequential component labels all represent three phase voltage vectors. "·" denotes the inner product of the three-phase vector. The three-phase components of the zero-sequence voltage are identical and the inner product with symmetrical positive-sequence and negative-sequence currents is always zero, i.e. v0·i+＝v0·i-When the ratio is 0, the formula (4) can be simplified as follows:
p＝v+·i++v-·i-+v+·i-+v-·i+(5)
expressed as a sine function:
V+、V-the amplitudes of the positive sequence voltage and the negative sequence voltage are respectively; i is+、I-The amplitudes of the positive sequence current and the negative sequence current respectively;is the phase angle difference between the a-phase positive sequence voltage and the a-phase positive sequence current, i.e.The rest of the phase angle differences and so on.
As can be seen from the formula (6), the positive sequence voltage and current and the negative sequence voltage and current can generate constant active power; positive sequence voltage and negative sequence current, positive sequence current and negative sequence voltage can produce the active power that fluctuates at 2 times power frequency. Thus, the active power can be decomposed into an average componentAnd a fluctuating componentThe expressions of the two are:
in the same way, expressions for the average and fluctuating components of the reactive power can be derived:
in the formula: "x" represents a cross product of the vector.
According to the formulas (7) and (8), if the three phases of the control current are balanced, the action of the negative sequence voltage and the positive sequence current of the power grid still generates power fluctuation; on the other hand, if the fluctuation component of the control output power is 0, the negative-sequence current cannot be 0. Therefore, the three-phase current balance and the power constancy cannot be achieved at the same time. However, in actual engineering, the average component of the output power is set to be equal to the power set value P in current balance*,Q*And the positive sequence voltage and negative sequence current acting terms in the fluctuation component are eliminated, so that the effect of reducing active and reactive fluctuation can be achieved.
3) Calculating a design current instruction: three-phase voltage signal v output by VSG algorithmoAnd calculating to obtain a reference instruction of the dq-axis current, and enabling the average component of the power output to be the same as the VSG set value while achieving the effect of inhibiting the negative sequence current.
3-1) calculation method: in the circuit shown in fig. 2, the positive sequence circuit is analyzed by neglecting the effect of the capacitor branch, and then the relationship between the positive sequence voltage and the positive sequence current in the dq coordinate system is:
in the formula: v. ofod,voqIs the VSG algorithm output voThe dq component of (1);andis the dq component of the grid positive sequence voltage and current; l and R are the total inductance and total resistance from the inverter to the grid; s is a differential operator. Due to voOnly the positive sequence component and can therefore be used directly for the calculation.
From the formulas (1) and (3), VSG can be obtained by adjusting voThe active power and the reactive power are automatically controlled to be the same as the set values, so that the power can be the same as the set values as long as the calculated relationship between the current and the voltage is consistent with the real circuit. To eliminate the negative sequence current, the positive sequence current in the equation (9) can be taken as the current reference value of the output, and the expression is:
in the formula:
because the VSG has closed-loop control of a voltage outer ring, current calculation does not need to completely simulate an actual circuit, and effective control can be realized as long as the basic relation of voltage and current dq axis components is represented to ensure that the positive and negative relations are not reversed. Using the final value theorem for the step response for the second order system of (10) and (Ls + R), the current command calculation can be simplified to:
3-2) positive sequence voltage extraction: the above calculation process requires a positive sequence component of the grid voltage, and therefore the positive and negative sequence components of the grid voltage are separated.
In the dq coordinate system, the expression of the grid voltage decomposed into positive and negative sequence components is as follows:
in the formula:
therefore, in the dq coordinate system, the original three-phase alternating-current positive sequence voltage can be changed into direct-current voltage, and the original three-phase alternating-current negative sequence voltage can be changed into 2-order harmonic voltage, so that the 2-order harmonic voltage can be filtered by the wave trap, and the effect of separating the positive sequence voltage is achieved. The transfer function of the trap is:
in the formula: omeganThe method is characterized in that the notch angular frequency is designed to be 2 times of power frequency angular frequency; q is a quality factor. It should be noted that Q cannot be made too small, otherwise the filtering effect would be poor; q should not be too large, otherwise the trap response is too slow, deteriorating the dynamic effect.
4) Designing a current inner ring to convert a current instruction into a voltage instruction: referring to fig. 4, the original VSG voltage output is connected to the current reference command input required by the current inner loop through the current command calculation module. The current is generally controlled under a dq rotating coordinate system, and a feedforward decoupling control strategy is adopted, and a control block diagram of the control strategy is shown in fig. 3. In the figure id,iqAnd ed,eqActual values obtained by converting the measured values of the VSG port current and the grid voltage through abc/dq coordinates are respectively obtained;is a dq-axis current reference command;is a modulated wave signal of the inverter output voltage. Fig. 3 can realize no-difference control of VSG port current due to existence of PI controllerAnd (5) preparing.
Further, fig. 5 shows a current output waveform under VSG control when a single-phase short circuit occurs in the prior art. Fig. 6 shows the current output waveform under VSG control in the embodiment of the present invention when there is a single-phase short circuit. Wherein the abscissa is time in seconds and the ordinate is current in amperes. It can be seen that the three-phase current is not balanced when the present invention is not used, and the current is balanced when the present invention is used. Fig. 7 is a comparison of output power for the two cases described above in the case of a single-phase short circuit. Wherein, the abscissa is time, the unit is second, the ordinate is active power and reactive power respectively, the unit is watt and lack. When the VSG stabilizing device is not used, the adjusting time is long, the amplitude is large, transient overshoot can be reduced, and the VSG stabilizing operation capability is improved.
According to the control method of the three-phase current of the virtual synchronous generator provided by the embodiment of the invention, the power grid voltage is decomposed into the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current, so that the problems of power fluctuation and three-phase current unbalance of the virtual synchronous generator facing the operation when the power grid voltage is unbalanced are analyzed, the average component is equal to the set value of the virtual synchronous generator by calculating the reference instruction of the dq shaft current, and finally, the three-phase current of the virtual synchronous generator is balanced by converting the reference instruction into the PWM voltage modulation signal, so that the three-phase balanced current can be still output even if the VSG is unbalanced in the three-phase voltage of the power grid, and the operation stability of the power grid is better ensured, and the control method is safe, reliable, simple and convenient.
Next, a control apparatus of three-phase unbalanced current of a virtual synchronous generator proposed according to an embodiment of the present invention is described below with reference to the accompanying drawings. Referring to fig. 8, the control device 10 includes: a build module 100, a decompose module 200, a compute module 300, and a control module 400.
The building module 100 is used for building a rotor motion equation of the VSG simulating the mechanical characteristics of the synchronous generator and a VSG virtual excitation equation of the electromagnetic characteristics. The decomposition module 200 is configured to decompose a grid voltage into a positive sequence voltage, a negative sequence voltage, a positive sequence current and a negative sequence current, and decompose active power and reactive power output by the VSG into an average component and a fluctuation component according to the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current, so as to analyze power fluctuation and three-phase current imbalance problems faced by the virtual synchronous generator when the grid is unbalanced. The calculation module 300 is configured to calculate a reference command of the dq-axis current from the three-phase voltage signals, so that an average component is equal to a set value of the virtual synchronous generator and suppresses the negative-sequence current. The control module 400 is configured to convert the reference command of the current into a PWM voltage modulation signal, and control the virtual synchronous generator to operate according to the PWM voltage modulation signal, so that the three-phase current of the virtual synchronous generator is balanced. The control device 10 of the embodiment of the invention adds a control method for designing a current inner loop and calculating current to restrain the operation problem caused by the unbalance of the three-phase voltage of the power grid on the basis of the traditional virtual synchronous generator control technology, so that the VSG can still output the three-phase balanced current when the three-phase voltage of the power grid is unbalanced.
Further, in one embodiment of the present invention, the VSG rotor equations of motion are:
wherein J is the virtual moment of inertia, ω is the VSG virtual angular velocity, TmAnd TeRespectively, virtual mechanical torque and electromagnetic torque, D is damping coefficient, omega0Is a reference value for the synchronous angular velocity.
Further, in one embodiment of the present invention, the VSG virtual excitation equation is:
wherein M isfIs the maximum value of mutual inductance between the rotor and a phase of the stator, ifFor the magnitude of the exciting current, K is the inertia coefficient of the excitation regulationQ is a value of reactive power set for VSG, QeAnd the value is the reactive power value of the grid-connected port.
Further, in one embodiment of the present invention, the reference command for the dq-axis current is calculated according to the following formula:
wherein,the dq component of the positive sequence current, L and R are the total inductance and total resistance from the inverter to the grid, s is the differential operator, ω is the VSG virtual angular velocity,is the difference of the q-axis component of the inverter output voltage and the q-axis component of the grid positive sequence voltage,is the difference between the d-axis component of the inverter output voltage and the d-axis component of the grid positive sequence voltage.
Further, in an embodiment of the present invention, the decomposition module 200 is further configured to filter out the 2 nd harmonic voltage generated in the decomposition process by the trap to obtain the positive sequence voltage through decomposition.
Specifically, firstly, a basic topology structure corresponding to the virtual synchronous generator technology is determined: the distributed power generation system can comprise an inverter, an LCL filter, grid-connected port power calculation, VSG control algorithm calculation and 5 subsystems of SVPWM modulation. The Virtual Synchronous Generator (VSG) technology is that an inverter has the characteristics similar to those of a synchronous generator by simulating the mechanical characteristics and the electromagnetic characteristics of the synchronous generator, so that the aim of providing inertia support and damping support for a power grid is fulfilled.
The rotor motion equation of the mechanical characteristic of the simulated synchronous generator is as follows:
in the formula: j is virtual rotational inertia, and the rotational inertia of the rotor of the synchronous generator is simulated; omega is VSG virtual angular velocity; t ismAnd TeVirtual mechanical torque and electromagnetic torque respectively; d is a damping coefficient, and the damping of the generator rotor is simulated; omega0The reference value for synchronous angular velocity is typically 314 rad/s. Solving the formula (1) to obtain a virtual angular velocity omega, and obtaining a rotor angle theta through integrationo。
Further, the amplitude of the three-phase output voltage of the virtual synchronous generator is as follows:
Vo＝Mfifω (2)
Mfis the maximum value of mutual inductance between the rotor and a phase stator, ifIs the magnitude of the excitation current.
Further, a VSG virtual excitation equation that models electromagnetic characteristics:
and K is the inertia coefficient of excitation regulation.
Further, the analysis of operational problems that may occur with the VSG when the grid voltage is unbalanced: the voltage of the power grid is decomposed into positive sequence, negative sequence and zero sequence components, and the positive sequence voltage, the negative sequence voltage and the zero sequence voltage respectively act to generate positive sequence current, negative sequence current and zero sequence current. Because there is no neutral line in the VSG, zero sequence current cannot flow, so the output current is only decomposed into two components of positive sequence and negative sequence, which are respectively marked as i+,i-。
The instantaneous active power output by the VSG can be expressed as:
in the formula v+,v-,v0Three sequential components of the VSG port voltage are provided, where the sequential component labels all represent three phase voltage vectors. "·" denotes the inner product of the three-phase vector. The three-phase components of the zero-sequence voltage are identical and the inner product with symmetrical positive-sequence and negative-sequence currents is always zero, i.e. v0·i+＝v0·i-When the ratio is 0, the formula (4) can be simplified as follows:
p＝v+·i++v-·i-+v+·i-+v-·i+(5)
expressed as a sine function:
V+、V-the amplitudes of the positive sequence voltage and the negative sequence voltage are respectively; i is+、I-The amplitudes of the positive sequence current and the negative sequence current respectively;is the phase angle difference between the a-phase positive sequence voltage and the a-phase positive sequence current, i.e.The rest of the phase angle differences and so on.
As can be seen from the formula (6), the positive sequence voltage and current and the negative sequence voltage and current can generate constant active power; positive sequence voltage and negative sequence current, positive sequence current and negative sequence voltage can produce the active power that fluctuates at 2 times power frequency. Thus, the active power can be decomposed into an average componentAnd a fluctuating componentThe expressions of the two are:
in the same way, expressions for the average and fluctuating components of the reactive power can be derived:
in the formula: "x" represents a cross product of the vector.
According to the formulas (7) and (8), if the three phases of the control current are balanced, the action of the negative sequence voltage and the positive sequence current of the power grid still generates power fluctuation; on the other hand, if the fluctuation component of the control output power is 0, the negative-sequence current cannot be 0. Therefore, the three-phase current balance and the power constancy cannot be achieved at the same time. However, in actual engineering, the average component of the output power is set to be equal to the power set value P in current balance*,Q*And the positive sequence voltage and negative sequence current acting terms in the fluctuation component are eliminated, so that the effect of reducing active and reactive fluctuation can be achieved.
Further, three-phase voltage signals v output by the VSG algorithmoAnd calculating to obtain a reference instruction of the dq-axis current, and enabling the average component of the power output to be the same as the VSG set value while achieving the effect of inhibiting the negative sequence current.
Referring to fig. 2, if the positive sequence circuit is analyzed while ignoring the effect of the capacitor branch in the circuit shown in fig. 2, the relationship between the positive sequence voltage and the positive sequence current in the dq coordinate system is:
in the formula: v. ofod,voqIs the VSG algorithm output voThe dq component of (1);andis the dq component of the grid positive sequence voltage and current; l and R are the total inductance and total resistance from the inverter to the grid; s is a differential operator. Due to voOnly the positive sequence component and can therefore be used directly for the calculation.
From the formulas (1) and (3), VSG can be obtained by adjusting voThe active power and the reactive power are automatically controlled to be the same as the set values, so that the power can be the same as the set values as long as the calculated relationship between the current and the voltage is consistent with the real circuit. To eliminate the negative sequence current, the positive sequence current in the equation (9) can be taken as the current reference value of the output, and the expression is:
in the formula:
because the VSG has closed-loop control of a voltage outer ring, current calculation does not need to completely simulate an actual circuit, and effective control can be realized as long as the basic relation of voltage and current dq axis components is represented to ensure that the positive and negative relations are not reversed. Using the final value theorem for the step response for the second order system of (10) and (Ls + R), the current command calculation can be simplified to:
further, the above calculation process requires a positive sequence component of the grid voltage, and therefore the positive and negative sequence components of the grid voltage are separated.
In the dq coordinate system, the expression of the grid voltage decomposed into positive and negative sequence components is as follows:
in the formula:
therefore, in the dq coordinate system, the original three-phase alternating-current positive sequence voltage can be changed into direct-current voltage, and the original three-phase alternating-current negative sequence voltage can be changed into 2-order harmonic voltage, so that the 2-order harmonic voltage can be filtered by the wave trap, and the effect of separating the positive sequence voltage is achieved. The transfer function of the trap may be:
in the formula: omeganThe method is characterized in that the notch angular frequency is designed to be 2 times of power frequency angular frequency; q is a quality factor. It should be noted that Q cannot be made too small, otherwise the filtering effect would be poor; q should not be too large, otherwise the trap response is too slow, deteriorating the dynamic effect.
Further, the original VSG voltage output is connected to the current reference command input required by the current inner loop through the current command calculation module. The current is generally controlled under a dq rotating coordinate system, and a feedforward decoupling control strategy is adopted, and a control block diagram of the control strategy is shown in fig. 3. In the figure id,iqAnd ed,eqActual values obtained by converting the measured values of the VSG port current and the grid voltage through abc/dq coordinates are respectively obtained;is a dq-axis current reference command;is a modulation signal of the output voltage of the inverterNumber (n). Fig. 3 can implement a differential-free control of the VSG port current due to the presence of the PI controller.
In the embodiment of the present invention, the VSG in the embodiment of the present invention implements a basic control method by simulating the electromagnetic and mechanical characteristics of a synchronous generator, and then analyzes two possible operation problems of the VSG control method by decomposing the grid voltage into a positive sequence voltage, a negative sequence voltage, a zero sequence voltage and a corresponding sequence current when the grid voltage is unbalanced: the power fluctuation and the three-phase current are unbalanced, and then the three-phase voltage signal v output by the VSG algorithm is outputoAnd finally, because the output current of the VSG is indirectly determined by the output voltage, a current inner loop controller is designed to convert the obtained current reference instruction into a PWM voltage modulation signal. The embodiment of the invention does not depend on the type of voltage unbalance when inhibiting the negative sequence current, can have better performance under the conditions of voltage balance and various unbalance types, and has simpler design and easy engineering realization.
Specifically, the method for controlling the three-phase unbalanced current of the virtual synchronous generator provided by the embodiment of the invention has the following advantages:
1. the VSG can be effectively controlled to output three-phase balance current when the voltage of the power grid is unbalanced, and the effects of reducing active and reactive oscillation, restraining power transient impact and reducing fault current amplitude are achieved.
2. The method is independent of the unbalanced type of the power grid voltage and does not influence the performance of the VSG in normal operation.
3. The control structure is simple, and engineering realization is easy.
In summary, the embodiment of the invention can effectively ensure that the VSG outputs three-phase balanced current, and is easy for engineering implementation.
It should be noted that the specific implementation manner of the apparatus according to the embodiment of the present invention is similar to the specific implementation manner of the method portion, and details are not described here in order to reduce redundancy.
According to the control device for the three-phase unbalanced current of the virtual synchronous generator, provided by the embodiment of the invention, the power grid voltage is decomposed into the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current, so that the problems of power fluctuation and three-phase current unbalance faced by the virtual synchronous generator when the power grid voltage is unbalanced are analyzed, the average component is equal to the set value of the virtual synchronous generator and the negative sequence current is inhibited by calculating the reference instruction of the dq-axis current, and finally, the three-phase current of the virtual synchronous generator is balanced by converting the reference instruction into the PWM voltage modulation signal, so that the three-phase balanced current can be still output even if the VSG is unbalanced in the three-phase voltage of the power grid, and the operation stability of the power grid is better ensured.
Any process or method descriptions in flow charts or otherwise described herein may be understood as representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or steps of the process, and alternate implementations are included within the scope of the preferred embodiment of the present invention in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present invention.
The logic and/or steps represented in the flowcharts or otherwise described herein, e.g., an ordered listing of executable instructions that can be considered to implement logical functions, can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. For the purposes of this description, a "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic device) having one or more wires, a portable computer diskette (magnetic device), a Random Access Memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, the computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory.
It should be understood that portions of the present invention may be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, the various steps or methods may be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, any one or combination of the following techniques, which are known in the art, may be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application specific integrated circuit having an appropriate combinational logic gate circuit, a Programmable Gate Array (PGA), a Field Programmable Gate Array (FPGA), or the like.
It will be understood by those skilled in the art that all or part of the steps carried by the method for implementing the above embodiments may be implemented by hardware related to instructions of a program, which may be stored in a computer readable storage medium, and when the program is executed, the program includes one or a combination of the steps of the method embodiments.
In addition, functional units in the embodiments of the present invention may be integrated into one processing module, or each unit may exist alone physically, or two or more units are integrated into one module. The integrated module can be realized in a hardware mode, and can also be realized in a software functional module mode. The integrated module, if implemented in the form of a software functional module and sold or used as a stand-alone product, may also be stored in a computer readable storage medium.
The storage medium mentioned above may be a read-only memory, a magnetic or optical disk, etc.
In the description herein, references to the description of the term "one embodiment," "some embodiments," "an example," "a specific example," or "some examples," etc., mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the schematic representations of the terms used above do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention, and that variations, modifications, substitutions and alterations can be made in the above embodiments by those of ordinary skill in the art without departing from the principle and spirit of the present invention.
Claims (8)
1. A method for controlling three-phase unbalanced current of a virtual synchronous generator is characterized by comprising the following steps:
establishing a VSG rotor motion equation simulating the mechanical characteristics of the synchronous generator and a VSG virtual excitation equation simulating the electromagnetic characteristics;
the method comprises the steps of decomposing the voltage of a power grid into positive sequence voltage and negative sequence voltage to respectively act to generate positive sequence current and negative sequence current, and decomposing active power and reactive power output by a VSG into average component and fluctuation component according to the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current so as to analyze the power fluctuation and three-phase current unbalance problems faced by the operation of a virtual synchronous generator when the voltage of the power grid is unbalanced;
calculating a reference instruction of the dq axis current according to the a, b and c three-phase voltage signals output by the VSG so that the average component is equal to a set value of the virtual synchronous generator and the negative sequence current is restrained; converting the reference instruction of the current into a PWM voltage modulation signal, and controlling the virtual synchronous generator to operate through the PWM voltage modulation signal so as to balance the three-phase current of the virtual synchronous generator, wherein the reference instruction of the dq-axis current is calculated according to the following formula:
<mrow> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <msubsup> <mi>i</mi> <mi>d</mi> <mo>+</mo> </msubsup> </mtd> </mtr> <mtr> <mtd> <msubsup> <mi>i</mi> <mi>q</mi> <mo>+</mo> </msubsup> </mtd> </mtr> </mtable> </mfenced> <mo>=</mo> <mfrac> <mn>1</mn> <mrow> <msup> <mi>R</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mrow> <mo>(</mo> <mi>&omega;</mi> <mi>L</mi> <mo>)</mo> </mrow> <mn>2</mn> </msup> </mrow> </mfrac> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <mrow> <msubsup> <mi>R&Delta;v</mi> <mi>d</mi> <mo>+</mo> </msubsup> <mo>+</mo> <msubsup> <mi>&omega;L&Delta;v</mi> <mi>q</mi> <mo>+</mo> </msubsup> </mrow> </mtd> </mtr> <mtr> <mtd> <mrow> <msubsup> <mi>R&Delta;v</mi> <mi>q</mi> <mo>+</mo> </msubsup> <mo>-</mo> <msubsup> <mi>&omega;L&Delta;v</mi> <mi>d</mi> <mo>+</mo> </msubsup> </mrow> </mtd> </mtr> </mtable> </mfenced> <mo>,</mo> </mrow>
wherein,the dq component of the positive sequence current, L and R are the total inductance and total resistance from the inverter to the grid, ω is the VSG virtual angular velocity,is the difference of the q-axis component of the inverter output voltage and the q-axis component of the grid positive sequence voltage,is the difference between the d-axis component of the inverter output voltage and the d-axis component of the grid positive sequence voltage.
2. The method of controlling three-phase unbalanced current of a virtual synchronous generator of claim 1, wherein the VSG rotor equation of motion is:
<mrow> <mi>J</mi> <mfrac> <mrow> <mi>d</mi> <mi>&omega;</mi> </mrow> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <msub> <mi>T</mi> <mi>m</mi> </msub> <mo>-</mo> <msub> <mi>T</mi> <mi>e</mi> </msub> <mo>-</mo> <mi>D</mi> <mrow> <mo>(</mo> <mi>&omega;</mi> <mo>-</mo> <msub> <mi>&omega;</mi> <mn>0</mn> </msub> <mo>)</mo> </mrow> <mo>,</mo> </mrow>
wherein J is the virtual moment of inertia, ω is the VSG virtual angular velocity, TmAnd TeRespectively, virtual mechanical torque and electromagnetic torque, D is damping coefficient, omega0Is a reference value for the synchronous angular velocity.
3. The method for controlling three-phase unbalanced current of a virtual synchronous generator according to claim 1, wherein the VSG virtual excitation equation is:
<mrow> <mi>K</mi> <mfrac> <mrow> <mi>d</mi> <mrow> <mo>(</mo> <msub> <mi>M</mi> <mi>f</mi> </msub> <msub> <mi>i</mi> <mi>f</mi> </msub> <mo>)</mo> </mrow> </mrow> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mi>Q</mi> <mo>*</mo> <mo>-</mo> <msub> <mi>Q</mi> <mi>e</mi> </msub> <mo>,</mo> </mrow>
wherein M isfMaximum value of mutual inductance between rotor and phase stator, ifFor the magnitude of the exciting current, K is the inertia coefficient of the excitation regulation, Q is the reactive power value set by VSG, QeAnd the value is the reactive power value of the grid-connected port.
4. The method for controlling three-phase unbalanced current of a virtual synchronous generator as claimed in claim 1, wherein 2 harmonic voltages generated during the decomposition process are filtered by a trap filter to obtain the positive sequence voltage through decomposition.
5. A control apparatus for three-phase unbalanced current of a virtual synchronous generator, comprising:
the building module is used for building a VSG rotor motion equation simulating the mechanical characteristics of the synchronous generator and a VSG virtual excitation equation simulating the electromagnetic characteristics;
the decomposition module is used for decomposing the power grid voltage into positive sequence voltage and negative sequence voltage to respectively act to generate positive sequence current and negative sequence current, and decomposing active power and reactive power output by the VSG into average component and fluctuation component according to the positive sequence voltage, the negative sequence voltage, the positive sequence current and the negative sequence current so as to analyze the power fluctuation and three-phase current unbalance problems faced by the operation of the virtual synchronous generator when the power grid voltage is unbalanced;
the calculation module is used for calculating a reference instruction of the dq axis current according to the three-phase voltage signals a, b and c output by the VSG so that the average component is equal to a set value of the virtual synchronous generator and the negative sequence current is restrained, wherein the reference instruction of the dq axis current is calculated according to the following formula:
<mrow> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <msubsup> <mi>i</mi> <mi>d</mi> <mo>+</mo> </msubsup> </mtd> </mtr> <mtr> <mtd> <msubsup> <mi>i</mi> <mi>q</mi> <mo>+</mo> </msubsup> </mtd> </mtr> </mtable> </mfenced> <mo>=</mo> <mfrac> <mn>1</mn> <mrow> <msup> <mi>R</mi> <mn>2</mn> </msup> <mo>+</mo> <msup> <mrow> <mo>(</mo> <mi>&omega;</mi> <mi>L</mi> <mo>)</mo> </mrow> <mn>2</mn> </msup> </mrow> </mfrac> <mfenced open = "[" close = "]"> <mtable> <mtr> <mtd> <mrow> <msubsup> <mi>R&Delta;v</mi> <mi>d</mi> <mo>+</mo> </msubsup> <mo>+</mo> <msubsup> <mi>&omega;L&Delta;v</mi> <mi>q</mi> <mo>+</mo> </msubsup> </mrow> </mtd> </mtr> <mtr> <mtd> <mrow> <msubsup> <mi>R&Delta;v</mi> <mi>q</mi> <mo>+</mo> </msubsup> <mo>-</mo> <msubsup> <mi>&omega;L&Delta;v</mi> <mi>d</mi> <mo>+</mo> </msubsup> </mrow> </mtd> </mtr> </mtable> </mfenced> <mo>,</mo> </mrow>
wherein,the dq component of the positive sequence current, L and R are the total inductance and total resistance from the inverter to the grid, ω is the VSG virtual angular velocity,is the difference of the q-axis component of the inverter output voltage and the q-axis component of the grid positive sequence voltage,the difference value of the d-axis component of the output voltage of the inverter and the d-axis component of the positive sequence voltage of the power grid is obtained; and
and the control module is used for converting the reference instruction of the current into a PWM (pulse-width modulation) voltage modulation signal and controlling the virtual synchronous generator to operate through the PWM voltage modulation signal, so that the three-phase current of the virtual synchronous generator is balanced.
6. The apparatus for controlling three-phase unbalanced current of a virtual synchronous generator of claim 5, wherein the VSG rotor equation of motion is:
<mrow> <mi>J</mi> <mfrac> <mrow> <mi>d</mi> <mi>&omega;</mi> </mrow> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <msub> <mi>T</mi> <mi>m</mi> </msub> <mo>-</mo> <msub> <mi>T</mi> <mi>e</mi> </msub> <mo>-</mo> <mi>D</mi> <mrow> <mo>(</mo> <mi>&omega;</mi> <mo>-</mo> <msub> <mi>&omega;</mi> <mn>0</mn> </msub> <mo>)</mo> </mrow> <mo>,</mo> </mrow>
wherein J is the virtual moment of inertia, ω is the VSG virtual angular velocity, TmAnd TeRespectively, virtual mechanical torque and electromagnetic torque, D is damping coefficient, omega0Is a reference value for the synchronous angular velocity.
7. The apparatus for controlling three-phase unbalanced current of a virtual synchronous generator according to claim 5, wherein the VSG virtual excitation equation is:
<mrow> <mi>K</mi> <mfrac> <mrow> <mi>d</mi> <mrow> <mo>(</mo> <msub> <mi>M</mi> <mi>f</mi> </msub> <msub> <mi>i</mi> <mi>f</mi> </msub> <mo>)</mo> </mrow> </mrow> <mrow> <mi>d</mi> <mi>t</mi> </mrow> </mfrac> <mo>=</mo> <mi>Q</mi> <mo>*</mo> <mo>-</mo> <msub> <mi>Q</mi> <mi>e</mi> </msub> <mo>,</mo> </mrow>
wherein M isfMaximum value of mutual inductance between rotor and phase stator, ifFor the magnitude of the exciting current, K is the inertia coefficient of the excitation regulation, Q is the reactive power value set by VSG, QeAnd the value is the reactive power value of the grid-connected port.
8. The apparatus for controlling three-phase unbalanced current of a virtual synchronous generator as claimed in claim 5, wherein the decomposition module is further configured to filter out 2 nd harmonic voltage generated in the decomposition process by a trap filter to obtain the positive sequence voltage through decomposition.
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| CN107171346B (en) * | 2017-06-14 | 2019-10-01 | 上海电力学院 | Unbalance voltage control method based on virtual synchronous generator |
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| CN110176787B (en) * | 2019-06-28 | 2021-07-16 | 广东志成冠军集团有限公司 | Load virtual synchronous machine control device and method with harmonic compensation function |
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| CN111668859B (en) * | 2020-06-24 | 2023-07-25 | 西安许继电力电子技术有限公司 | Virtual synchronous generator control method and system with unbalanced load |
| CN112187088B (en) * | 2020-09-08 | 2022-06-28 | 上海正泰电源系统有限公司 | Virtual synchronous machine-based unbalanced load control method |
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