CN118017542A - Virtual oscillator control strategy capable of providing virtual inertia - Google Patents
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Abstract
The invention relates to the technical field of inverter control, and discloses a virtual oscillator control strategy capable of providing virtual inertia. Firstly, an inverter active power measured value passes through a low-pass filter, and a corresponding virtual feedback current is obtained by calculating a power value after low-pass filtering; the active power reference value is passed through a low-pass filter, and the corresponding virtual reference current is obtained by calculating the power value after low-pass filtering; and then, the virtual feedback current and the virtual reference current are subjected to difference to obtain a control error, and the control error is taken as the input of the virtual oscillator to generate a voltage signal of the inverter. And finally, designing control parameters of the provided virtual oscillator according to the mathematical model to provide the required virtual inertia. The invention can effectively solve the problem that the inertial support cannot be provided in the control of the existing virtual oscillator, simplify the parameter design of the virtual oscillator, enhance the frequency stability of the system and ensure the stable and reliable operation of the system performance.
Description
Technical Field
The invention relates to the technical field of inverter control, in particular to a virtual oscillator control strategy capable of providing virtual inertia.
Background
With the increasing popularization of new energy power generation technology, the inverter grid-structured control can enhance the running stability and toughness of a power grid. Droop control and virtual synchro machine (Virtual Synchronous MACHINE VSM) control are two common conventional mesh control strategies. Virtual oscillator control (Virtual Oscillator Control VOC) is a novel mesh control strategy in which the inverter simulates the dynamics of a weak nonlinear oscillator to adjust. Compared with droop control and a virtual synchronous machine, the VOC is a time domain control strategy, can provide good dynamic characteristics, can realize error-free tracking of power during grid connection, can also break away from a power grid, supplies power to loads through island operation, and can realize equal-proportion distribution of the loads among a plurality of VOC inverters.
Existing VOC control then still faces problems. First, its voltage and phase dynamics are tightly coupled, and the design parameters of the VOC appear in both frequency and voltage dynamics equations. Therefore, in the case of VOC parameter design methods that are very complex, compromises must be made among multiple performance metrics in the design, and a viable design may not be found.
And secondly, the existing VOC cannot realize virtual inertia. It cannot provide inertial support for the grid during grid-tie operation. The VOC also cannot control the rate of frequency change (Rate of Change of Frequency, rocf) when operating independently, and cannot power rocf sensitive loads. VOC and other grid-type controls (droop control and VSM) cannot reach equal proportions of power distribution during transients due to lack of inertia.
Disclosure of Invention
The invention aims to provide a virtual oscillator control strategy capable of providing virtual inertia, and aims to solve the problems that the traditional virtual oscillator control cannot provide virtual inertia and is complex in design, realize virtual inertia, effectively control ROCOF, reduce the complexity of a controller and facilitate application in an actual system. The technical proposal is as follows:
a virtual oscillator control strategy that provides virtual inertia, comprising the steps of:
step 1: the inverter active power measured value is passed through a low-pass filter, and corresponding virtual feedback current is obtained according to the power value after low-pass filtering;
step 2: the active power reference value passes through a low-pass filter, and corresponding virtual reference current is obtained through calculation according to the power value after low-pass filtering;
Step 3: the virtual feedback current and the virtual reference current are subjected to difference to obtain a control error; taking the control error as the input of the virtual oscillator, and generating a voltage signal of the inverter;
step 4: and designing control parameters of the virtual oscillator according to a differential equation describing a control strategy of the virtual oscillator, and providing required virtual inertia.
Further, the step 1 specifically includes:
In a switching period T s, measuring three-phase current i abc and three-phase voltage v abc output by the inverter, and calculating to obtain an output active power measured value P and a reactive power measured value Q; the active power measurement P is passed through a low pass filter to obtain a measurement filtered power value P f:
wherein omega f is the bandwidth of the low-pass filter, and s is the complex parameter of the Laplacian transformation;
The virtual feedback currents i α and i β are calculated by measuring the filtered power value P f and the reactive power measurement value Q:
Where v α and v β are the α and β components, respectively, of the virtual oscillator voltage.
Further, the step 2 specifically includes:
giving an active power reference value P * and a reactive power reference value Q *, and obtaining a power value after the active power reference value P * is filtered by a low-pass filter
Power value filtered using reference valueAnd a reactive power reference value Q *, calculating to obtain a virtual reference current/>And (3) with
Further, the step 3 specifically includes:
The virtual feedback current is differenced with the virtual reference current to obtain control errors e α and e β
Where i α and i p are the alpha and beta components of the virtual feedback current respectively,And/>Alpha and beta components of the virtual reference current, respectively; error signal/>As input to Andronov-Hopf virtual oscillators, i.e
Wherein ζ is the oscillator speed control parameter, η is the virtual oscillator feedback control parameter, V n is the voltage class effective value of the system, ω n is the system power frequency, and iiv αβ is the voltage vectorR (phi) is a rotation matrix for adapting to the grid with different types of impedance, expressed as:
for an inductive network, take Φ=90°; for a resistive network, take Φ=0°;
virtual oscillator voltages v α and v β of the outputs of the Andronov-Hopf type virtual oscillator are taken as reference values of the inverter output voltages.
Further, the step 4 specifically includes:
the differential equation describing the virtual oscillator control strategy is expressed as:
The virtual oscillator frequency ω dynamic equation is obtained according to equation (8):
Wherein ω f is the cut-off frequency of the low-pass filter used in step 1 and step 2; v represents the virtual oscillator voltage effective value.
The virtual oscillator voltage effective value vbdynamic equation is obtained according to equation (8):
Let the differential term in omega dynamic equation (9) and V dynamic equation (10) be 0, solve omega and V, then under steady state working condition, virtual oscillator frequency omega and virtual oscillator voltage effective value V satisfy respectively:
The three design parameters of the virtual oscillator control strategy are respectively an oscillator speed control parameter xi, a virtual oscillator feedback control parameter eta and omega f, and the specific design method is as follows:
step a: according to equation (11), a virtual oscillator feedback control parameter η is selected based on the steady state frequency:
Assuming that the steady-state frequency of the virtual oscillator is omega MIN when the output reaches the rated active power P rated, and considering that the effective value V of the voltage of the virtual oscillator meets V (approximately equal to V n) under the steady-state working condition, then
Step b: based on the steady-state voltage V equation, an oscillator speed control parameter ζ is selected according to the steady-state voltage:
assuming that the virtual oscillator steady-state voltage is V MIN when the output reaches the rated reactive power Q rated
Step c: omega f is selected based on the required virtual inertia J, and the virtual oscillator voltage effective value V satisfies V approximately equal to V n under the steady-state working condition
The beneficial effects of the invention are as follows: the virtual oscillator control strategy applied to the three-phase inverter can effectively solve the problems that the traditional virtual oscillator control strategy is complex in design and cannot provide virtual inertia; the virtual oscillator control strategy provided by the invention can provide virtual inertia, effectively control ROCOF, reduce the complexity of a controller, be convenient for application in an actual system, provide inertial support for a power grid and enhance the frequency stability of the system.
Drawings
Fig. 1 is a block diagram of a three-phase inverter that can provide virtual oscillator control of virtual inertia.
Fig. 2 is a circuit diagram of an inverter employing the present invention alone.
Fig. 3 shows output characteristics of the inverter according to the present invention when the inverter is operated in a grid-connected mode.
Fig. 4 shows output characteristics of a single inverter of the present invention when switching to island operation.
Fig. 5 is an output characteristic of an inverter island operation using the present invention alone.
Fig. 6 is a circuit diagram of two inverters employing the present invention.
Fig. 7 shows the output characteristics (ω f =62.8 rad/s) of two inverter islands operating with the present invention.
Fig. 8 shows the output characteristics (ω f =18.8 rad/s) of two inverter islands operating with the present invention.
Fig. 9 shows the output characteristics (ω f =6.28 rad/s) of two inverter islands operating with the present invention.
Detailed Description
The invention will now be described in further detail with reference to the drawings and to specific examples.
Fig. 1 is a schematic diagram of a three-phase inverter according to the present invention, where V dc is dc voltage, V b,abc is three-phase grid voltage, V abc is inverter output three-phase voltage, L c is a grid-side inductance, r c is a grid-side resistance, i abc is inverter output three-phase current, P * and Q * are active power and reactive power reference values, respectively, and ω n is fundamental angular frequency.
The virtual oscillator control strategy capable of providing virtual inertia provided by the invention is as follows:
Step 1: and (3) passing the inverter active power measured value through a low-pass filter, and calculating the corresponding virtual feedback current according to the low-pass filtered power value.
In a switching period T s, three-phase current i abc and three-phase voltage v abc output by the inverter are measured, and an output active power measurement value P and a reactive power measurement value Q are obtained through calculation. The active power measurement P is passed through a low pass filter to obtain a measurement filtered power value P f:
wherein omega f is the bandwidth of the low-pass filter, and s is the complex parameter of the Laplacian transformation;
Using P f and Q, feedback currents i α and i β were obtained by the following calculation:
Where v α and v β are the α and β components, respectively, of the virtual oscillator voltage.
Step 2: and the active power reference value passes through a low-pass filter, and the corresponding virtual reference current is obtained by calculation according to the power value after the low-pass filtering.
In the same way, a reference current is calculated from a given active power reference value P * and reactive power reference value Q * And/>The active power reference value P * is passed through a low-pass filter to obtain the power value/>, after the reference value is filtered
UsingWith Q *, a reference current/>, is obtained by the following calculationAnd/>
Step 3: the virtual feedback current and the virtual reference current are subjected to difference to obtain a control error; the control error is used as an input of the virtual oscillator to generate a voltage signal of the inverter.
The virtual feedback current is differenced with the virtual reference current to obtain control errors e α and e β
Error signalAs input to Andronov-Hopf virtual oscillators, i.e
Wherein ζ is the oscillator speed control parameter, η is the virtual oscillator feedback control parameter, V n is the voltage class effective value of the system, ω n is the system power frequency, and iiv αβ is the voltage vectorR (phi) is the rotation matrix for adapting to the grid with different types of impedances:
For an inductive network, take Φ=90°; for a resistive network, let phi=0°.
The outputs v α and v β of the Andronov-Hopf type virtual oscillator are the reference values of the output voltage of the inverter. In the invention, v α and v β can be used as modulation voltage to be directly input into the PWM module or used as reference value to be input into the voltage inner loop.
Step 4: and designing control parameters of the virtual oscillator according to a differential equation describing a control strategy of the virtual oscillator, and providing required virtual inertia.
The differential equation describing the virtual oscillator control strategy is expressed as:
Taking the phi=90°, the above formulas are combined to obtain
In a polar coordinate system, virtual oscillator voltage effective value V may be expressed as
The phase angle theta of the voltage is
Dynamic satisfaction of the voltage phase angle θ according to equations (24) and (26)
Considering that the effective value V of the virtual oscillator voltage has small change, V can be considered as unchanged when the frequency characteristic is studied, and the dynamic equation of the virtual oscillator frequency omega can be obtained by the simultaneous equations (27), (16), (18)
Where ω f is the cut-off frequency of the low-pass filter used in step 1 and step 2.
According to the formulas (24) and (25), the virtual oscillator voltage effective value V dynamic equation is obtained
As can be seen from the above equation, the frequency dynamic equation of the proposed virtual oscillator control strategy is a first order differential equation. Virtual inertia may also be provided, similar to the equations of a conventional droop control and Virtual Synchro Machine (VSM) containing virtual inertia. Wherein ω f determines the magnitude of the virtual inertia. The voltage dynamic equation of the proposed virtual oscillator control strategy is then the same as the conventional virtual oscillator control strategy.
Let the differential term in ω dynamic equation (28) and V dynamic equation (29) be 0, solve ω and V, then under steady state working condition, the virtual oscillator frequency ω and virtual oscillator voltage effective value V satisfy respectively:
it can be seen that in steady state conditions, the proposed virtual oscillator control strategy also exhibits droop control characteristics, with frequency droop characteristics being linear and voltage droop characteristics being non-linear.
The three design parameters of the virtual oscillator control strategy are an oscillator speed control parameter zeta, a virtual oscillator feedback control parameter eta and omega f respectively;
The specific design method comprises the following steps:
Step a: according to equation (30), a virtual oscillator feedback control parameter η is selected based on the steady state frequency:
Assuming that the steady-state frequency of the virtual oscillator is omega MIN when the output reaches the rated active power P rated, and considering that the effective value V of the voltage of the virtual oscillator meets V (approximately equal to V n) under the steady-state working condition, then
Step b: according to equation (31), the oscillator speed control parameter ζ is selected based on the steady state voltage:
assuming that the virtual oscillator steady-state voltage is V MIN when the output reaches the rated reactive power Q rated
Step c: omega f is selected based on the required virtual inertia J, and the virtual oscillator voltage effective value V satisfies V approximately equal to V n under the steady-state working condition
By way of specific example, the validity of the proposed virtual oscillator control strategy is verified.
The main circuit parameters shown in fig. 2 are as follows: v dc＝75V,Lc＝2mH,rc＝85mΩ,Vb＝39V,ωn = 314rad/s, resistance value R 1 = 15Ω for load 1. The proposed virtual oscillator control strategy parameters are as follows: ζ=4, η=20, ω f =62.8 rad/s.
Fig. 3 shows output characteristics of the three-phase inverter in the grid-connected operation. The output current is a three-phase sine wave, and the output power can accurately follow a given power reference value of 500W. Therefore, the provided virtual oscillator control strategy can realize stable grid-connected operation and realize error-free tracking of the active power reference value.
Fig. 4 is an output characteristic of the three-phase inverter at the time of grid-connected switching to island operation. In the switching process, the output voltage of the inverter is a stable three-phase sine wave, and automatic switching between grid connection and island operation can be realized.
Fig. 5 is an output characteristic of the three-phase inverter in island operation. During island operation, the output voltage of the inverter is a stable three-phase sine wave, and the inverter provides electric energy for local loads.
The main circuit parameters shown in fig. 6 are as follows ：Vdc1＝Vdc2＝75V,Lc1＝Lc1＝2mH,rc1＝rc2＝85mΩ,ωn＝314rad/s, the resistance value R 1 =15Ω of load 1, the resistance value R 1 =15Ω of load 2. The proposed virtual oscillator control strategy parameters are as follows: ζ=4, η=20.
Fig. 7 is an output characteristic (ω f =62.8 rad/s) of the load conversion of two three-phase inverters in island operation. At the initial moment, only the load 1 is connected into the circuit, the output currents of the two inverters are the same, and the loads are distributed in equal proportion. After the load 2 is connected into the circuit, the two inverters immediately increase the same output current to supply electric energy to the load 1 and the load 2 simultaneously. Meanwhile, the output currents of the two inverters are the same, and the loads are distributed in equal proportion. At the moment of switching in load 2, the system rocf is-14.94 Hz/s.
Fig. 8 is an output characteristic (ω f =18.8 rad/s) of the load conversion of two three-phase inverters in island operation. The output currents of the two inverters are the same, and the loads are distributed in equal proportion. As the virtual inertia is increased, the system rocf is-4.92 Hz/s at the moment of load 2 access.
Fig. 9 is an output characteristic (ω f =6.28 rad/s) of the load conversion of two three-phase inverters in island operation. The output currents of the two inverters are the same, and the loads are distributed in equal proportion. As the virtual inertia is increased, the system rocf is-1.78 Hz/s at the moment of load 2 access. Therefore, the provided virtual oscillator control strategy can effectively control the system ROCOF, provide inertia support for the system and enhance the frequency stability of the system.
Claims (5)
1. A virtual oscillator control strategy that provides virtual inertia, comprising the steps of:
step 1: the inverter active power measured value is passed through a low-pass filter, and corresponding virtual feedback current is obtained according to the power value after low-pass filtering;
step 2: the active power reference value passes through a low-pass filter, and corresponding virtual reference current is obtained through calculation according to the power value after low-pass filtering;
Step 3: the virtual feedback current and the virtual reference current are subjected to difference to obtain a control error; taking the control error as the input of the virtual oscillator, and generating a voltage signal of the inverter;
step 4: and designing control parameters of the virtual oscillator according to a differential equation describing a control strategy of the virtual oscillator, and providing required virtual inertia.
2. The virtual oscillator control strategy for providing virtual inertia according to claim 1, wherein the step 1 is specifically:
In a switching period T s, measuring three-phase current i abc and three-phase voltage v abc output by the inverter, and calculating to obtain an output active power measured value P and a reactive power measured value Q; the active power measurement P is passed through a low pass filter to obtain a measurement filtered power value P f:
wherein omega f is the bandwidth of the low-pass filter, and s is the complex parameter of the Laplacian transformation;
The virtual feedback currents i α and i β are calculated by measuring the filtered power value P f and the reactive power measurement value Q:
Where v α and v β are the α and β components, respectively, of the virtual oscillator voltage.
3. The virtual oscillator control strategy for providing virtual inertia according to claim 2, wherein step 2 is specifically:
giving an active power reference value P * and a reactive power reference value Q *, and obtaining a power value after the active power reference value P * is filtered by a low-pass filter
Power value filtered using reference valueAnd a reactive power reference value Q *, calculating to obtain a virtual reference current/>And/>
4. A virtual oscillator control strategy providing virtual inertia according to claim 3, wherein step 3 is specifically:
The virtual feedback current is differenced with the virtual reference current to obtain control errors e α and e β
Where i α and i β are the alpha and beta components of the virtual feedback current respectively,And/>Alpha and beta components of the virtual reference current, respectively;
error signal As input to Andronov-Hopf virtual oscillators, i.e
Wherein ζ is the oscillator speed control parameter, η is the virtual oscillator feedback control parameter, V n is the voltage class effective value of the system, ω n is the system power frequency, V αβ is the voltage vectorR (phi) is a rotation matrix for adapting to the grid with different types of impedance, expressed as:
for an inductive network, take Φ=90°; for a resistive network, take Φ=0°;
virtual oscillator voltages v α and v β of the outputs of the Andronov-Hopf type virtual oscillator are taken as reference values of the inverter output voltages.
5. The virtual oscillator control strategy that provides virtual inertia of claim 4, wherein step 4 is specifically:
the differential equation describing the virtual oscillator control strategy is expressed as:
the virtual oscillator frequency ω dynamic equation is derived according to equation (8) as:
wherein ω f is the cut-off frequency of the low-pass filter used in step 1 and step 2; v represents the virtual oscillator voltage effective value;
The virtual oscillator voltage effective value V dynamic equation is obtained according to the formula (8):
and enabling the differential terms in the dynamic equation (9) of the virtual oscillator frequency omega and the dynamic equation (10) of the virtual oscillator voltage effective value V to be 0, and solving omega and V reversely, wherein under the steady-state working condition, the virtual oscillator frequency omega and the virtual oscillator voltage effective value V respectively meet the following conditions:
The three design parameters of the virtual oscillator control strategy are respectively an oscillator speed control parameter xi, a virtual oscillator feedback control parameter eta and omega f, and the specific design method is as follows:
step a: according to equation (11), a virtual oscillator feedback control parameter η is selected based on the steady state frequency:
Assuming that the steady-state frequency of the virtual oscillator is omega MIN when the output reaches the rated active power P rated, and considering that the effective value V of the voltage of the virtual oscillator meets V (approximately equal to V n) under the steady-state working condition, then
Step b: according to equation (12), an oscillator speed control parameter ζ is selected based on the steady state voltage:
assuming that the virtual oscillator steady-state voltage is V MIN when the output reaches the rated reactive power Q rated
Step c: omega f is selected based on the required virtual inertia J, and the virtual oscillator voltage effective value V satisfies V approximately equal to V n under the steady-state working condition
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