A virtual oscillator control strategy for providing virtual inertia

Abstract

The present invention relates to the field of inverter control technology, and discloses a virtual oscillator control strategy that can provide virtual inertia. First, the measured value of the active power of the inverter is passed through a low-pass filter, and the power value after low-pass filtering is used to calculate the corresponding virtual feedback current; then the active power reference value is passed through a low-pass filter, and the power value after low-pass filtering is used to calculate the corresponding virtual reference current; then the virtual feedback current is subtracted from the virtual reference current to obtain a control error, and the control error is used as the input of the virtual oscillator to generate a voltage signal of the inverter. Finally, according to the mathematical model, the control parameters of the proposed virtual oscillator are designed to provide the required virtual inertia. The present invention can effectively solve the problem that inertial support cannot be provided in the existing virtual oscillator control, simplify the virtual oscillator parameter design, enhance the frequency stability of the system, and ensure the stable and reliable operation of the system performance.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and in particular to a virtual oscillator control strategy capable of providing virtual inertia. Background Art

[0002] With the increasing popularity of renewable energy power generation technology, inverter grid control can enhance the stability and resilience of power grid operation. Droop control and virtual synchronous machine (VSM) control are two common traditional grid control strategies. Virtual Oscillator Control (VOC) is a new type of grid control strategy, in which the inverter simulates the dynamic characteristics of a weak nonlinear oscillator for adjustment. Compared with droop control and virtual synchronous machine, VOC is a time domain control strategy that can provide good dynamic characteristics. When connected to the grid, it can achieve error-free tracking of power, and it can also be disconnected from the grid and operate in an island mode to supply power to the load. Multiple VOC inverters can achieve proportional load distribution.

[0003] However, existing VOC control still faces problems. First, its voltage dynamics and phase dynamics are tightly coupled, and the design parameters of VOC appear in both frequency and voltage dynamic equations. Therefore, the VOC parameter design method is very complicated, and compromises must be made between multiple performance indicators in the design, and it may not be possible to find a feasible design.

[0004] Secondly, the existing VOC cannot achieve virtual inertia. It cannot provide inertial support for the grid when connected to the grid. When operating independently, VOC cannot control the rate of change of frequency (ROCOF) and cannot provide power for ROCOF sensitive loads. Due to the lack of inertia, VOC and other grid-type controls (such as droop control and VSM) cannot achieve proportional power distribution during transient processes. Summary of the invention

[0005] The purpose of the present invention is to provide a virtual oscillator control strategy that can provide virtual inertia, aiming to solve the problem that traditional virtual oscillator control cannot provide virtual inertia and has complex design, realize virtual inertia, effectively control ROCOF, reduce the complexity of the controller, and facilitate application in actual systems. The technical solution is as follows:

[0006] A virtual oscillator control strategy for providing virtual inertia includes the following steps:

[0007] Step 1: Pass the measured active power value of the inverter through a low-pass filter, and calculate the corresponding virtual feedback current according to the power value after low-pass filtering;

[0008] Step 2: Pass the active power reference value through a low-pass filter, and calculate the corresponding virtual reference current according to the power value after low-pass filtering;

[0009] Step 3: Subtract the virtual feedback current from the virtual reference current to obtain a control error; use the control error as the input of the virtual oscillator to generate a voltage signal for the inverter;

[0010] Step 4: According to the differential equation describing the control strategy of the virtual oscillator, the control parameters of the virtual oscillator are designed to provide the required virtual inertia.

[0011] Furthermore, the step 1 is specifically as follows:

[0012] In a switching cycle T s The three-phase current i output by the inverter abc With three-phase voltage v abc The measured value P of the output active power and the measured value Q of the reactive power are obtained by calculation; the measured value P of the active power is passed through a low-pass filter to obtain the power value P after the measured value is filtered. f :

[0013]

[0014] In the formula, ω f is the low-pass filter bandwidth, s is the complex parameter of Laplace transform;

[0015] The power value P after the measured value is filtered f The virtual feedback current i is calculated with the reactive power measurement value Q α with i β :

[0016]

[0017] In the formula, v α With v β are the α and β components of the virtual oscillator voltage respectively.

[0018] Furthermore, the step 2 is specifically as follows:

[0019] Given active power reference value P * and reactive power reference value Q * , the active power reference value P * The power value after the reference value is filtered is obtained through a low-pass filter

[0020]

[0021] Power value after using reference value filtering and reactive power reference value Q * , calculate the virtual reference current and

[0022]

[0023] Furthermore, the step 3 is specifically as follows:

[0024] Subtract the virtual feedback current from the virtual reference current to get the control error e α With e β :

[0025]

[0026] In the formula, i α with i β are the α and β components of the virtual feedback current, and are the α and β components of the virtual reference current respectively; the error signal As the input of an Andronov–Hopf type virtual oscillator,

[0027]

[0028] Where ξ is the oscillator speed control parameter, η is the virtual oscillator feedback control parameter, V n is the effective value of the system voltage level, ω n is the system power frequency, ||v αβ || is the voltage vector The modulus of R(φ) is the rotation matrix used to adapt to the power grid with different types of impedance, expressed as:

[0029]

[0030] For inductive networks, take φ = 90°; for resistive networks, take φ = 0°;

[0031] The virtual oscillator voltage v of the Andronov–Hopf type virtual oscillator is α With v β As the reference value of the inverter output voltage.

[0032] Furthermore, the step 4 is specifically as follows:

[0033] The differential equation describing the control strategy of the virtual oscillator is expressed as:

[0034]

[0035] According to formula (8), the dynamic equation of the virtual oscillator frequency ω is:

[0036]

[0037] In the formula, ω f is the cutoff frequency of the low-pass filter used in step 1 and step 2; V represents the effective value of the virtual oscillator voltage.

[0038] According to formula (8), the dynamic equation of the virtual oscillator voltage effective value V can be obtained as follows:

[0039]

[0040] Let the differential terms in the ω dynamic equation (9) and the V dynamic equation (10) be 0, and inversely solve ω and V. Then, under steady-state conditions, the virtual oscillator frequency ω and the virtual oscillator voltage effective value V satisfy:

[0041]

[0042] The three design parameters of the virtual oscillator control strategy are the oscillator speed control parameter ξ, the virtual oscillator feedback control parameter η and ω. f , the specific design method is:

[0043] Step a: According to equation (11), select the virtual oscillator feedback control parameter η based on the steady-state frequency:

[0044] Assuming the output reaches the rated active power P rated When the virtual oscillator steady-state frequency is ω MIN , and considering the steady-state condition, the virtual oscillator voltage effective value V satisfies V≈V n ,but

[0045]

[0046] Step b: Based on the steady-state voltage V equation, select the oscillator speed control parameter ξ according to the steady-state voltage:

[0047] Assume that the output reaches the rated reactive power Q rated When the virtual oscillator steady-state voltage is V MIN ,but

[0048]

[0049] Step c: Select ω based on the required virtual inertia J f , and considering the steady-state condition, the virtual oscillator voltage effective value V satisfies V≈V n ,but

[0050]

[0051] The beneficial effects of the present invention are as follows: a virtual oscillator control strategy applied to a three-phase inverter proposed in the present invention can effectively solve the problem that the traditional virtual oscillator control strategy is complex in design and cannot provide virtual inertia; the virtual oscillator control strategy proposed in the present invention can provide virtual inertia, effectively control ROCOF, reduce the complexity of the controller, facilitate application in actual systems, provide inertial support for the power grid, and enhance the frequency stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The structure diagram of a three-phase inverter controlled by a virtual oscillator that can provide virtual inertia.

[0053] Figure 2 The inverter circuit diagram is a single inverter adopting the inverter of the present invention.

[0054] Figure 3 The output characteristics of a single inverter adopting the present invention when it is connected to the grid.

[0055] Figure 4 The output characteristics of a single inverter adopting the present invention when switched from grid-connected to island operation.

[0056] Figure 5 The output characteristics of a single inverter adopting the present invention when operating in an isolated island mode.

[0057] Figure 6 The following are circuit diagrams of two inverters using the present invention.

[0058] Figure 7 The output characteristics (ω) of two inverters using the present invention when operating in an isolated island f =62.8rad / s).

[0059] Figure 8 The output characteristics (ω) of two inverters using the present invention when operating in an isolated island f =18.8rad / s).

[0060] Fig. 9 The output characteristics (ω) of two inverters using the present invention when operating in an isolated island f =6.28rad / s). DETAILED DESCRIPTION

[0061] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0062] Figure 1 is a structural diagram of a three-phase inverter based on the present invention, wherein V dc is the DC voltage, v b,abc is the three-phase grid voltage, v abc The inverter outputs three-phase voltage, Lc are the grid-side inductance, r c are the grid side resistance, i abc The inverter outputs three-phase current, P * and Q * are the reference values ​​of active power and reactive power respectively, ω n is the fundamental angular frequency.

[0063] The virtual oscillator control strategy that can provide virtual inertia provided by the present invention is specifically as follows:

[0064] Step 1: Pass the measured active power value of the inverter through a low-pass filter, and calculate the corresponding virtual feedback current according to the power value after the low-pass filtering.

[0065] In a switching cycle T s The three-phase current i output by the inverter abc With three-phase voltage v abc The output active power measurement value P and reactive power measurement value Q are obtained by calculation. The active power measurement value P is passed through a low-pass filter to obtain the power value P after the measurement value is filtered. f :

[0066]

[0067] In the formula, ω f is the low-pass filter bandwidth, s is the complex parameter of Laplace transform;

[0068] Using P f and Q, the feedback current i is obtained by the following calculation α with i β :

[0069]

[0070] In the formula, v α With v β are the α and β components of the virtual oscillator voltage respectively.

[0071] Step 2: Pass the active power reference value through a low-pass filter, and calculate the corresponding virtual reference current according to the power value after the low-pass filtering.

[0072] In the same way, according to the given active power reference value P * and reactive power reference value Q * Calculate the reference current and The active power reference value P * The power value after the reference value is filtered is obtained through a low-pass filter

[0073]

[0074] use With Q * , the reference current is obtained by the following calculation and

[0075]

[0076] Step 3: Subtract the virtual feedback current from the virtual reference current to obtain a control error; use the control error as the input of the virtual oscillator to generate a voltage signal for the inverter.

[0077] Subtract the virtual feedback current from the virtual reference current to get the control error e α With e β :

[0078]

[0079] The error signal As the input of an Andronov–Hopf type virtual oscillator,

[0080]

[0081] Where ξ is the oscillator speed control parameter, η is the virtual oscillator feedback control parameter, V n is the effective value of the system voltage level, ω n is the system power frequency, ||v αβ || is the voltage vector The modulus of , R(φ) is the rotation matrix used to adapt to the power grid with different types of impedance:

[0082]

[0083] For inductive networks, take φ=90°; for resistive networks, take φ=0°.

[0084] The output v of the Andronov–Hopf type virtual oscillator α With v β That is, the reference value of the inverter output voltage. In the present invention, v α With v β It can be directly input into the PWM module as a modulation voltage, or it can be input into the voltage inner loop as a reference value.

[0085] Step 4: According to the differential equation describing the control strategy of the virtual oscillator, the control parameters of the virtual oscillator are designed to provide the required virtual inertia.

[0086] The differential equation describing the control strategy of the virtual oscillator is expressed as:

[0087]

[0088] Taking φ=90°, combining the above equations, we can get

[0089]

[0090] In the polar coordinate system, the virtual oscillator voltage effective value V can be expressed as

[0091]

[0092] The voltage phase angle θ is

[0093]

[0094] According to equations (24) and (26), the dynamics of the voltage phase angle θ satisfies

[0095]

[0096] Considering that the effective value V of the virtual oscillator voltage changes very little, V can be considered unchanged when studying the frequency characteristics. Combining equations (27), (16), and (18), the dynamic equation of the virtual oscillator frequency ω can be obtained as follows:

[0097]

[0098] In the formula, ω f is the cutoff frequency of the low-pass filter used in steps 1 and 2.

[0099] According to equations (24) and (25), the dynamic equation of the virtual oscillator voltage effective value V can be obtained as follows:

[0100]

[0101] It can be seen from the above equation that the frequency dynamic equation of the proposed virtual oscillator control strategy is a first-order differential equation. Similar to the equations of the traditional droop control with virtual inertia and virtual synchronous machine (VSM), virtual inertia can also be provided. f The voltage dynamic equation of the proposed virtual oscillator control strategy is the same as that of the traditional virtual oscillator control strategy.

[0102] Let the differential terms in the ω dynamic equation (28) and the V dynamic equation (29) be 0, and inversely solve ω and V. Then, under steady-state conditions, the virtual oscillator frequency ω and the virtual oscillator voltage effective value V satisfy:

[0103]

[0104] It can be seen that in the steady state, the proposed virtual oscillator control strategy also exhibits the characteristics of droop control, its frequency droop characteristic is linear, and its voltage droop characteristic is nonlinear.

[0105] The three design parameters of the virtual oscillator control strategy are the oscillator speed control parameter ξ, the virtual oscillator feedback control parameter η and ω. f ;

[0106] The specific design method is:

[0107] Step a: According to equation (30), select the virtual oscillator feedback control parameter η based on the steady-state frequency:

[0108] Assuming the output reaches the rated active power P rated When the virtual oscillator steady-state frequency is ω MIN , and considering the steady-state condition, the virtual oscillator voltage effective value V satisfies V≈V n ,but

[0109]

[0110] Step b: According to equation (31), select the oscillator speed control parameter ξ based on the steady-state voltage:

[0111] Assume that the output reaches the rated reactive power W rated When the virtual oscillator steady-state voltage is V MIN ,but

[0112]

[0113] Step c: Select ω based on the required virtual inertia J f , and considering the steady-state condition, the virtual oscillator voltage effective value V satisfies V≈V n ,but

[0114]

[0115] The effectiveness of the proposed virtual oscillator control strategy is verified through a specific example.

[0116] Figure 2 The main circuit parameters shown are as follows: V dc =75V, L c =2mH, r c =85mΩ, V b =39V,ω n =314rad / s, the resistance value of load 1 R1 = 15Ω. The parameters of the proposed virtual oscillator control strategy are as follows: ξ = 4, η = 20, ω f =62.8rad / s.

[0117] Figure 3 The output characteristics of the three-phase inverter in grid-connected operation. Its output current is a three-phase sine wave, and its output power can accurately follow the given power reference value of 500W. It can be seen that the proposed virtual oscillator control strategy can achieve stable grid-connected operation and error-free tracking of the active power reference value.

[0118] Figure 4 The output characteristics of the three-phase inverter when switching from grid-connected to island operation. During the switching process, the inverter output voltage is a stable three-phase sine wave, which can automatically switch between grid-connected and island operation.

[0119] Figure 5 The output characteristics of the three-phase inverter in island operation. When operating in an island, the inverter output voltage is a stable three-phase sine wave, providing power for local loads.

[0120] Figure 6 The main circuit parameters shown are as follows: V dc =V dc2 =75V, L c1 =L c1 =2mH, r c1 =r c2 =85mΩ,ω n =314rad / s, the resistance value of load 1 R1=15Ω, the resistance value of load 2 R1=15Ω. The parameters of the proposed virtual oscillator control strategy are as follows: ξ=4, η=20.

[0121] Figure 7 The output characteristics of two three-phase inverters in island operation load transformation (ω f =62.8rad / s). At the initial moment, only load 1 is connected to the circuit, and the output currents of the two inverters are the same, and the loads are distributed in equal proportion. After load 2 is connected to the circuit, the two inverters immediately increase the same output current to provide power to load 1 and load 2 at the same time. At the same time, the output currents of the two inverters are the same, and the loads are distributed in equal proportion. At the moment when load 2 is connected, the system ROCOF is -14.94Hz / s.

[0122] Figure 8 The output characteristics of two three-phase inverters in island operation load transformation (ω f =18.8rad / s). The two inverters have the same output current and distribute the load in equal proportion. Due to the increase in virtual inertia, the system ROCOF is -4.92Hz / s at the moment of connecting to load 2.

[0123] Fig. 9 The output characteristics of two three-phase inverters in island operation load transformation (ω f=6.28rad / s). The two inverters have the same output current and distribute the load in equal proportion. Due to the increase in virtual inertia, the system ROCOF is -1.78Hz / s at the moment of connecting to load 2. It can be seen that the proposed virtual oscillator control strategy can effectively control the system ROCOF, provide inertia support for the system, and enhance the system frequency stability.

Claims

1. A virtual oscillator control strategy that can provide virtual inertia, characterized in that: The following steps are involved: Step 1: Pass the measured active power value of the inverter through a low-pass filter, and calculate the corresponding virtual feedback current according to the power value after low-pass filtering; Step 2: Pass the active power reference value through a low-pass filter, and calculate the corresponding virtual reference current according to the power value after low-pass filtering; Step 3: Subtract the virtual feedback current from the virtual reference current to obtain a control error; use the control error as the input of the virtual oscillator to generate a voltage signal for the inverter; Step 4: According to the differential equation describing the control strategy of the virtual oscillator, the control parameters of the virtual oscillator are designed to provide the required virtual inertia; The step 4 is specifically as follows: The differential equation describing the control strategy of the virtual oscillator is expressed as: According to formula (8), the dynamic equation of the virtual oscillator frequency ω is: In the formula, ω f is the cut-off frequency of the low-pass filter used in step 1 and step 2; V represents the effective value of the virtual oscillator voltage; According to formula (8), the dynamic equation of the virtual oscillator voltage effective value V is: Let the differential terms in the dynamic equation (9) of the virtual oscillator frequency ω and the dynamic equation (10) of the virtual oscillator voltage effective value V be 0, and inversely solve ω and V. Then, under steady-state conditions, the virtual oscillator frequency ω and the virtual oscillator voltage effective value V respectively satisfy: The three design parameters of the virtual oscillator control strategy are the oscillator speed control parameter ξ, the virtual oscillator feedback control parameter η and ω. f , the specific design method is: Step a: According to equation (11), select the virtual oscillator feedback control parameter η based on the steady-state frequency: Assuming the output reaches the rated active power P rated When the virtual oscillator steady-state frequency is ω MIN , and considering the steady-state condition, the virtual oscillator voltage effective value V satisfies V≈V n ,but Step b: According to equation (12), select the oscillator speed control parameter ξ based on the steady-state voltage: Assume that the output reaches the rated reactive power Q rated When the virtual oscillator steady-state voltage is V MIN ,but Step c: Select ω based on the required virtual inertia J f , and considering the steady-state condition, the virtual oscillator voltage effective value V satisfies V≈V n ,but 2. The virtual oscillator control strategy capable of providing virtual inertia according to claim 1, characterized in that: The step 1 is specifically as follows: In a switching cycle T s The three-phase current i output by the inverter abc With three-phase voltage v abc The measured value P of the output active power and the measured value Q of the reactive power are obtained by calculation; the measured value P of the active power is passed through a low-pass filter to obtain the power value P after the measured value is filtered. f : In the formula, ω f is the low-pass filter bandwidth, s is the complex parameter of Laplace transform; The power value P after the measured value is filtered f The virtual feedback current i is calculated with the reactive power measurement value Q α with i β : In the formula, v α With v β are the α and β components of the virtual oscillator voltage respectively.

3. The virtual oscillator control strategy capable of providing virtual inertia according to claim 2, characterized in that: The step 2 is specifically as follows: Given active power reference value P * and reactive power reference value Q * , the active power reference value P * The power value after the reference value is filtered is obtained through a low-pass filter Power value after using reference value filtering and reactive power reference value Q * , calculate the virtual reference current and 4. The virtual oscillator control strategy capable of providing virtual inertia according to claim 3, characterized in that: The step 3 is specifically as follows: Subtract the virtual feedback current from the virtual reference current to get the control error e α With e β : In the formula, i α with i β are the α and β components of the virtual feedback current, and are the α and β components of the virtual reference current respectively; The error signal As the input of an Andronov–Hopf type virtual oscillator, Where ξ is the oscillator speed control parameter, η is the virtual oscillator feedback control parameter, V n is the effective value of the system voltage level, ω n is the system power frequency, ||v αβ || is the voltage vector The modulus of R(φ) is the rotation matrix used to adapt to the power grid with different types of impedance, expressed as: For inductive networks, take φ = 90°; for resistive networks, take φ = 0°; The virtual oscillator voltage v of the Andronov–Hopf type virtual oscillator is α With v β As the reference value of the inverter output voltage.

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