A Virtual Synchronous Generator Control Method Considering the Dynamics of Speed Regulation and Excitation

Abstract

The present invention provides a virtual synchronous generator control method considering speed regulation and excitation dynamics, including: taking the output active power of the inverter as the input of the active-frequency control link to simulate the diesel engine speed regulation characteristics to obtain the output frequency of the inverter; taking the output reactive power of the inverter, the output voltage amplitude, and the d-axis and q-axis components of the output current as the inputs of the reactive-voltage control link to simulate the generator excitation characteristics to obtain the d-axis and q-axis reference voltage command values of the inverter, and using them as the inputs of the voltage-current double closed-loop, and obtaining the PWM control signal through SVPWM modulation; obtaining the phase difference and amplitude difference by using the vector relationship between the output voltage of the virtual synchronous generator and the voltage at the common point, obtaining the correction amounts of frequency and voltage through the PI link, and respectively superimposing them on the output frequency and output voltage amplitude reference values of the inverter to achieve smooth grid connection and disconnection switching; and performing matching design on the control parameters. The present invention improves the system reliability.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy power conversion, and particularly relates to a virtual synchronous generator control method considering speed regulation and excitation dynamics. Background Art

[0002] In an independent microgrid that is disconnected from the large power grid, such as a power supply system on an island or in a remote mountainous area, diesel generator sets are usually used as the main power supply. To reduce the consumption of petroleum resources and achieve the energy conservation and emission reduction goals of "carbon peak and carbon neutrality", the application of distributed renewable energy sources such as photovoltaic and wind energy in the microgrid has received extensive attention and development. Distributed energy sources are connected to the microgrid power supply system through power electronic converters and jointly supply power with traditional diesel generator sets.

[0003] Distributed power sources based on power electronic converters generally adopt four control schemes: constant power (PQ) control, constant voltage / constant frequency (Vf) control, droop control, and virtual synchronous generator control. Droop control realizes the equal sharing of active power and reactive power without an interconnection line by simulating the primary frequency regulation and primary voltage regulation characteristics of a synchronous generator set during parallel operation. Virtual synchronous generator control adds the simulation of the mechanical motion and electromagnetic characteristics of a synchronous generator on the basis of droop control. In an independent microgrid, an inverter adopts virtual synchronous generator control and is connected to the microgrid in the form of a voltage source, which can provide inertia for the system and participate in power regulation according to the set droop characteristics, avoiding problems such as complex control and generator reverse power during the traditional Vf and PQ switching control, enabling the inverter to be plug-and-play and operate autonomously and coordinately, and realizing friendly and stable grid connection. However, the influence of the speed regulation system and excitation system of diesel generators is not considered in traditional virtual synchronous generator control. During the process of suddenly adding or removing loads when operating in parallel with diesel generators, due to the differences in transient characteristics, the inverter and diesel generator based on traditional virtual synchronous generator control cannot achieve transient power equal sharing. During the transient process, the virtual synchronous generator bears most of the load, which may lead to overcurrent protection of the inverter, the inverter capacity cannot be fully utilized, and the economy is poor. Therefore, it is necessary to propose an improved virtual synchronous generator control strategy to solve the problem of uneven transient power sharing in an independent microgrid system where an inverter operates in parallel with a diesel generator and improve the transient operation performance of the system. Summary of the Invention

[0004] The purpose of the present invention is to solve the deficiencies existing in the above background art, provide a virtual synchronous generator control method considering speed regulation and excitation dynamics, solve the problem of serious power overshoot of the inverter caused by uneven transient power distribution in a traditional virtual synchronous generator and diesel generator parallel power supply system, effectively improve the transient operation characteristics of the independent microgrid, fully utilize the inverter capacity, and improve the system reliability and economy.

[0005] The technical solution adopted by the present invention is as follows: A virtual synchronous generator control method considering speed regulation and excitation dynamics, which is used for a power supply system where an inverter is connected in parallel with a diesel generator, includes the following steps:

[0006] In the per-unit system, the output active power of the inverter is used as the input of the active-frequency control link to obtain the output frequency of the inverter; the active-frequency control link is used to simulate the primary frequency regulation characteristics of the diesel generator, the speed regulation characteristics of the diesel engine, and the mechanical motion of the synchronous generator.

[0007] The output reactive power of the inverter, the output voltage amplitude, and the d-axis and q-axis components of the output current are used as the inputs of the reactive-voltage control link to obtain the d-axis and q-axis reference voltage command values of the inverter; the reactive-voltage control link is used to simulate the primary voltage regulation characteristics of the synchronous generator, the excitation dynamic characteristics, and the output impedance characteristics.

[0008] The d-axis and q-axis reference voltage command values of the inverter are used as the inputs of the voltage-current double closed-loop, and after SVPWM modulation, a PWM control signal is obtained to adjust the output voltage of the inverter.

[0009] The phase difference and amplitude difference between the output voltage of the inverter and the voltage at the common point pass through a PI link to obtain the correction amounts of frequency and voltage, and are respectively superimposed on the reference values of the output frequency and output voltage amplitude of the inverter to realize the tracking control of the output voltage of the inverter to the voltage at the common point.

[0010] According to the external command, smooth switching between grid-connected and off-grid is realized.

[0011] During the active-frequency control link, reactive-voltage control link, and smooth switching between grid-connected and off-grid, the corresponding control parameters are matched and designed to make the inverter and the parallel-operating diesel generator share the transient and steady-state power evenly.

[0012] In the above technical solution, the process of the active-frequency control link includes:

[0013] The output active power of the inverter is drooped through active-frequency droop to obtain the frequency reference value.

[0014] PI control and an equivalent first-order inertia link of the diesel engine speed regulation link are used to simulate the diesel engine speed regulation control link, so that the output frequency of the inverter tracks the frequency reference value, and the mechanical power of the virtual synchronous generator is obtained as the input of the rotor motion equation of the virtual synchronous generator.

[0015] The output frequency of the inverter is obtained through the rotor motion equation of the virtual synchronous generator, and after integration, the phase of the output voltage of the inverter is obtained.

[0016] In the above technical solution, the process of the reactive-voltage control link includes:

[0017] The reactive power output of the inverter is adjusted by reactive power droop control to obtain the reference value of the output voltage amplitude of the inverter;

[0018] PI control is used to make the voltage amplitude at the filter capacitor of the inverter output track the reference value of the output voltage amplitude, and a first-order inertia link equivalent to the excitation regulation system of the synchronous generator is introduced to obtain the stator excitation electromotive force;

[0019] The stator excitation electromotive force is substituted into the electromagnetic transient equation of the excitation winding and the stator voltage balance equation in the third-order model of the synchronous generator, so that the inverter simulates the excitation characteristics and output impedance characteristics of the generator, and the reference voltage command values of the d-axis and q-axis of the inverter are obtained.

[0020] In the above technical solution, the process of realizing smooth grid connection and disconnection according to external instructions includes:

[0021] After the inverter receives the grid connection instruction, the pre-synchronization control is enabled. After 1 s, it is judged whether the amplitude, phase and frequency of the output voltage and the voltage at the common connection point meet the closing conditions. After the conditions are met, a closing instruction is issued, and the pre-synchronization control is turned off after the closing is completed; during the grid-connected operation process, after receiving the off-grid instruction, a tripping instruction is sent to disconnect the grid-connected circuit breaker to complete the off-grid operation.

[0022] In the above technical solution, the matching design process of the control parameters of the active power-frequency control link includes: per-unit values are used in each control link, and the active power-frequency droop coefficient, PI parameters of the speed regulation link, equivalent first-order inertia time constant of the speed regulation link, virtual inertia time constant, and virtual damping in the active power-frequency control link are all kept consistent with the parallel-operated diesel generator.

[0023] In the above technical solution, the matching design process of the control parameters of the reactive power-voltage control link includes: in the per-unit value system, the reactive power-voltage droop coefficient, PI control parameters of the voltage regulation link, d-axis synchronous reactance, d-axis transient reactance, q-axis transient electromotive force, d-axis open-circuit transient time constant, stator winding resistance, and q-axis synchronous reactance in the reactive power-voltage control link are all kept consistent with the corresponding parameters of the parallel-operated diesel generator.

[0024] In the above technical solution, the matching design process of the control parameters of the grid connection and disconnection smooth switching process includes: a PI link is used in the grid connection pre-synchronization control to achieve zero-error control; in the per-unit value system, the output of the PI is limited, and the limit value is set according to 2% of the rated output voltage amplitude and frequency of the inverter.

[0025] In the above technical solution, the amplitude difference ΔU and phase difference Δθ between the output voltage of the inverter and the voltage at the common connection point are obtained ctrlThe process includes: obtaining the sine value sinΔθ and cosine value cosΔθ of the phase difference between the common point voltage and the inverter output voltage according to the vector relationship between the inverter output voltage and the common point voltage in the αβ stationary coordinate system; using the characteristic that the trigonometric function period is 2π to construct a variable that can approximately describe the phase difference between the inverter and the common point voltage as the phase difference Δθ between the inverter output voltage and the common point voltage ctrl :

[0026]

[0027] Among them, U α is the α-axis component of the inverter output voltage, and U β is the β-axis component of the inverter output voltage; U gα is the β-axis component of the common point voltage, and U gβ is the β-axis component of the common point voltage.

[0028] In the above technical solution, the frequency reference value ω is calculated by the following formula m :

[0029] ω m = ω 0 + m(P ref - P e )

[0030] In the formula, P ref is the given active power value of the virtual synchronous generator, which is given according to the rated active power of the inverter; P e is the active power output by the inverter, m is the active-frequency droop coefficient, and ω 0 is the rated angular velocity.

[0031] In the above technical solution, the electromagnetic transient equation of the excitation winding and the stator voltage balance equation in the third-order model of the synchronous generator are as follows:

[0032]

[0033] Each variable in the formula is a virtual parameter of the virtual synchronous generator, where E f is the stator excitation electromotive force, X d is the d-axis synchronous reactance, X' d is the d-axis transient reactance, E' q is the q-axis transient electromotive force, T d ' 0 is the d-axis open-circuit transient time constant, R a is the stator winding resistance, X q is the q-axis synchronous reactance, E q is the q-axis no-load electromotive force, U dref is the d-axis voltage reference value, U qrefis the q-axis voltage reference value; I od is the d-axis component of the inverter output current in the rotating coordinate system, I oq is the q-axis component of the inverter output current in the rotating coordinate system.

[0034] The beneficial effects of the present invention are as follows: A virtual synchronous generator control strategy considering speed regulation and excitation dynamics is proposed. By introducing a diesel generator speed regulation and excitation simulation link into the traditional virtual synchronous generator control, the virtual synchronous generator has the same speed regulation characteristics as the diesel generator, effectively solving the problem of serious overshoot of the inverter power caused by uneven transient power distribution when the new energy access inverter is connected in parallel with the diesel generator, realizing the transient and steady-state power sharing between the inverter and the diesel generator, avoiding the overcurrent problem of the inverter during the sudden load addition process, making full use of the inverter capacity, and improving the reliability and economy of the microgrid. The present invention adds a simulated diesel generator speed regulation link to the traditional virtual synchronous generator active-frequency control, enabling the virtual synchronous generator to have the same speed regulation characteristics as the diesel generator. The present invention introduces the electromagnetic transient equation of the excitation winding and the stator voltage balance equation in the simple and third-order synchronous generator model that can take into account the excitation system dynamics, enabling the inverter to simulate the excitation characteristics and output impedance characteristics of the generator, and solving the problem of inconsistent transient voltage regulation characteristics between the inverter controlled by the traditional virtual synchronous generator and the diesel generator. The present invention makes all control parameters in the active-frequency control link consistent with those of the networked diesel generator to ensure the consistency of the speed regulation systems of the virtual synchronous generator and the diesel generator. The present invention makes all control parameters in the reactive-voltage control link consistent with the corresponding parameters of the networked diesel generator to ensure better consistency between the voltage regulation systems of the inverter and the diesel generator. The method of obtaining the amplitude difference and phase difference between the inverter output voltage and the common point voltage in the grid-connected and off-grid smooth switching control method of the present invention not only does not require a phase-locked loop but also can eliminate the influence of the 2π jump of the phase difference when obtaining the phase difference through the phase-locked loop. The present invention limits the PI output in the grid-connected and off-grid smooth switching control to prevent large deviations in the output voltage amplitude and frequency during the pre-synchronization adjustment process. Description of the Drawings

[0035] Figure 1 is the schematic diagram of the virtual synchronous generator control strategy of the present invention;

[0036] Figure 2 is the main circuit topology diagram of the independent microgrid in the embodiment;

[0037] Figure 3 is the control block diagram of the traditional virtual synchronous generator;

[0038] Figure 4 is the control block diagram of the virtual synchronous generator of the present invention;

[0039] Figure 5 It is the speed regulation and excitation control block diagram of the diesel generator in the embodiment;

[0040] Figure 6a It is the voltage vector diagram in the αβ coordinate system of the pre-synchronization control method based on voltage vector operation;

[0041] Figure 6b It is the schematic diagram of the phase difference of the pre-synchronization control method based on voltage vector operation;

[0042] Figure 7a It is the output active power change curve of the inverter and the diesel generator based on the traditional virtual synchronous generator control under the set working conditions in the embodiment;

[0043] Figure 7b It is the output reactive power change curve of the inverter and the diesel generator based on the traditional virtual synchronous generator control under the set working conditions in the embodiment;

[0044] Figure 8a It is the output active power change curve of the inverter and the diesel generator based on the improved virtual synchronous generator control of the present invention under the set working conditions in the embodiment;

[0045] Figure 8b It is the output reactive power change curve of the inverter and the diesel generator based on the improved virtual synchronous generator control of the present invention under the set working conditions in the embodiment. Detailed implementation manners

[0046] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, which is convenient for clearly understanding the present invention, but they do not constitute a limitation to the present invention.

[0047] As Figure 1 shown, the present invention provides a virtual synchronous generator control method considering speed regulation and excitation dynamics, which is used for a power supply system where an inverter is connected in parallel with a diesel generator, and includes the following steps:

[0048] Collect the voltage at the output filter capacitor of the inverter, the grid-side inductor current, and the common point voltage; through coordinate transformation and calculation, obtain the d and q axis components of the output voltage and output current in the dq rotating coordinate system required for control implementation in the per-unit system, the α and β axis components of the output voltage and the common point voltage in the αβ stationary coordinate system, the output voltage amplitude, the output active power, and the output reactive power;

[0049] In the per-unit system, use the output active power of the inverter as the input of the active-power - frequency control link to obtain the output frequency of the inverter; the active-power - frequency control link is used to simulate the primary frequency regulation characteristics of the diesel generator, the speed regulation characteristics of the diesel engine, and the mechanical motion of the synchronous generator;

[0050] The output reactive power of the inverter, the amplitude of the output voltage, and the d-axis and q-axis components of the output current are used as the input quantities of the reactive power-voltage control link to obtain the reference voltage command values of the d-axis and q-axis of the inverter; the reactive power-voltage control link is used to simulate the primary voltage regulation characteristics, excitation dynamic characteristics, and output impedance characteristics of a synchronous generator.

[0051] The reference voltage command values of the d-axis and q-axis of the inverter are used as the input quantities of the voltage-current double closed-loop, and after SVPWM modulation, a PWM control signal is obtained to regulate the output voltage of the inverter.

[0052] The phase difference and amplitude difference between the inverter output voltage and the common point voltage pass through a PI link to obtain the correction amounts of frequency and voltage, and are respectively superimposed on the reference values of the output frequency and output voltage amplitude of the inverter to achieve the tracking control of the inverter output voltage to the common point voltage.

[0053] Achieve smooth switching between grid connection and off-grid according to external instructions.

[0054] In the process of active power-frequency control link, reactive power-voltage control link, and smooth grid connection and off-grid switching, the corresponding control parameters are matched and designed to make the inverter share the transient and steady-state power equally with the parallel-operated diesel generator.

[0055] The present invention also provides a virtual synchronous generator control system for a power supply system in which an inverter is connected in parallel with a diesel generator, including: an information acquisition module, an active power-frequency control module, a reactive power-voltage control module, a PWM control module, a tracking control module, a grid connection smooth switching module, and a control parameter design module; wherein,

[0056] The information acquisition module is used to collect the voltage at the output filter capacitor of the inverter, the grid-side inductor current, and the common point voltage; through coordinate transformation and calculation, the d-axis and q-axis components of the output voltage and output current in the dq rotating coordinate system required for control implementation in the per-unit system, the α-axis and β-axis components of the output voltage and common point voltage in the αβ stationary coordinate system, the output voltage amplitude, the output active power, and the output reactive power are obtained.

[0057] The active power-frequency control module is used to use the output active power of the inverter as the input quantity of the active power-frequency control link to obtain the output frequency of the inverter; the active power-frequency control link is used to simulate the primary frequency regulation characteristics of the diesel generator, the speed regulation characteristics of the diesel engine, and the mechanical motion of the synchronous generator.

[0058] The reactive - voltage control module uses the output reactive power of the inverter, the output voltage amplitude, and the d - and q - axis components of the output current as the input quantities of the reactive - voltage control link to obtain the reference voltage command values of the d - and q - axes of the inverter; the reactive - voltage control link is used to simulate the primary voltage regulation characteristics, excitation dynamic characteristics, and output impedance characteristics of a synchronous generator.

[0059] The PWM control module uses the reference voltage command values of the d - and q - axes of the inverter as the input quantities of the voltage - current double - closed - loop, and obtains the PWM control signal through SVPWM modulation to regulate the output voltage of the inverter.

[0060] The grid - connection smooth - switching module is used to achieve smooth switching between grid - connection and off - grid according to external commands.

[0061] The control - parameter design module is used to match and design the corresponding control parameters during the active - frequency control link, reactive - voltage control link, and grid - connection / off - grid smooth - switching process, so that the inverter and the parallel - operating diesel generator achieve transient and steady - state power sharing.

[0062] The present invention is used for a power supply system in which an inverter is connected in parallel with a diesel generator, such as Figure 2 the independent micro - grid system shown. The system includes a photovoltaic array unit connected to the DC bus through the DC / DC converter; an energy - storage unit connected to the DC bus; a DC bus connected to the AC bus through the DC / AC inverter; a diesel - generating set connected to the AC bus; a load unit connected to the AC bus; the specific parameters of the relevant units are:

[0063] The electrical parameters of the diesel generator are: rated capacity 380 kVA, power factor 0.8, rated line voltage 390 V, rated frequency 50 Hz, inertia time constant H = 0.8 s, damping coefficient D = 0.1, armature resistance Ra = 0.051 pu, d - axis synchronous reactance X d = 2.4 pu, d - axis transient reactance X' d = 0.1075 pu, d - axis sub - transient reactance X d " = 0.072 pu, d - axis open - circuit transient time constant T d ' 0 = 0.4912 s, d - axis open - circuit sub - transient time constant T d " 0 = 0.001493 s, q - axis synchronous reactance X q = 1.92 pu, q - axis sub - transient reactance X q " = 0.0868 pu, q - axis open - circuit sub - transient time constant T d " 0 = 0.02212 s.

[0064] The speed regulation and excitation control block diagram of the diesel generator is as shown in Figure 5 , and the control parameters are as follows: In speed regulation control, P ref = 0.8, the active droop coefficient m = 0.0125, the proportional parameter K p_GOV of the PI link = 10, the integral parameter K i_GOV of the PI link = 10, and the inertia time constant of the equivalent inertia link of the diesel engine and the speed regulation mechanism is T d = 0.05 s; In the excitation control link, Q ref = 0.6, the reactive - voltage droop coefficient n = 0.033, the proportional parameter K p_AVR of the excitation control = 2, the integral parameter K i_AVR of the excitation control = 5, and the inertia time constant T e of the excitation system = 0.05 s.

[0065] The main circuit parameters of the inverter are as follows: The rated capacity of the inverter is 380 kVA, the rated output line voltage of the inverter is 390 V, the rated output phase current of the inverter is 562.56 A, the filter inductor L 1 on the inverter bridge arm side = 200 μH, the filter inductor L 2 on the inverter grid side = 100 μH, and the filter capacitor C of the inverter = 360 μF.

[0066] The inverter adopts the Figure 4 virtual synchronous generator control strategy considering speed regulation and excitation dynamics. The present invention includes an active - frequency control link for simulating the speed regulation characteristics of the diesel engine, a reactive - voltage control link for simulating the excitation characteristics of the generator, a smooth switching control strategy for grid connection and disconnection, and a control parameter matching design method, which specifically includes the following steps:

[0067] Step 1, collect the voltages (U a , U b , U c ) at the filter capacitor of the inverter output, the grid - side inductor currents (i a2 , i b2 , i c2 ), and the common - point voltages (U ga , U gb , U gc ); Through coordinate transformation and calculation, obtain the d - axis and q - axis components of the output voltage and current of the inverter required for control implementation in the dq rotating coordinate system of the per - unit value system (U d , U q ; I od , I oq ), and the α - axis and β - axis components of the output voltage and the common - point voltage in the αβ stationary coordinate system (U α , U β ; U gα , U gβ), the output voltage amplitude V m , the output active power P e , the output reactive power Q e .

[0068] Step 2. Use the output active power P of the inverter e as the input of the active - frequency control link to obtain the output frequency ω of the inverter. The active - frequency control link simulates the primary frequency regulation characteristics of a diesel generator, the speed regulation characteristics of a diesel engine, and the mechanical motion of a synchronous generator. Among them, in the active - frequency control link, per - unit values are used for each control link in the improved virtual synchronous generator control. In the active - frequency loop, the active droop coefficient m, the PI parameters of the speed regulation link, the equivalent first - order inertia time constant T d , the virtual inertia time constant H, and the virtual damping D are all kept consistent with the networked diesel generators to ensure the consistency of the speed regulation systems of the virtual synchronous generator and the diesel generator.

[0069] Furthermore, in the active - frequency control link of the virtual synchronous generator, an additional simulation of the diesel engine speed regulation link is added to the traditional active - frequency control of the virtual synchronous generator, enabling the virtual synchronous generator to have the same speed regulation characteristics as the diesel generator. Specifically as follows:

[0070] Obtain the frequency reference value ω through active droop m :

[0071] ω m = ω 0 + m(P ref - P e )

[0072] Preferably, P ref = 0.8 pu, which is the given value of the active power of the virtual synchronous generator, given according to the rated power of the inverter and is a constant; P e is the output active power of the inverter; m = 0.0125, which is the active - frequency droop coefficient; ω 0 = 1, which is the rated angular velocity of the virtual synchronous motor.

[0073] Then, use PI control and the equivalent first - order inertia link of the diesel engine speed regulation system to simulate the diesel engine speed regulation control link. The proportional parameter K ωp = 10, the integral parameter K ωi = 10, and the equivalent inertia link time constant T d = 0.05 s, which is set the same as the diesel generator speed regulation control, to obtain the mechanical power P m of the virtual synchronous generator as the input of the rotor motion equation of the virtual synchronous generator:

[0074]

[0075] Among them, H is the virtual inertia time constant, D is the virtual damping coefficient. Preferably, H and D are set consistent with the response parameters of the diesel generator, that is, H = 0.8 s, D = 0.1, ω is the mechanical angular velocity, and P e is the active power output of the inverter, and P e is calculated as follows:

[0076] P e = 1.5(U d I od + U q I oq )

[0077] U d and U q are the d-axis and q-axis components of the inverter output voltage in the rotating coordinate system respectively; I od and I oq are the d-axis and q-axis components of the inverter output current in the rotating coordinate system respectively;

[0078] The output frequency of the inverter is obtained through the rotor motion equation, and its integral can obtain the phase of the output voltage.

[0079] Step three. Take the inverter reactive power Q e , the output voltage amplitude V m , the d-axis and q-axis components of the output current (I d , I q ) as the input quantities of the reactive-voltage control link, and obtain the d-axis and q-axis reference voltage command values of the inverter. The reactive-voltage control link simulates the primary voltage regulation characteristics, excitation dynamic characteristics, and output impedance characteristics of the synchronous generator. Among them, in the per-unit system of the reactive-voltage control link, the reactive droop coefficient n in the reactive-voltage loop, the PI control parameters of the voltage regulation link, X d , X' d , E' q , T d ', 0 , R a , X q all need to be consistent with the corresponding parameters of the networked diesel generator to ensure the consistency of the inverter and the diesel generator voltage regulation system.

[0080] Furthermore, the inverter reactive-voltage control link is as follows:

[0081] The reference value E ref of the output voltage amplitude is obtained through reactive droop regulation:

[0082] E ref = E 0 + E Q = E0 +n(Q ref -Q e )

[0083] Wherein, E 0 is the no-load electromotive force of the virtual synchronous generator, and is the given rated voltage; n is the reactive power-voltage droop coefficient; Q ref is the given value of the reactive power of the virtual synchronous generator, which is given according to the rated active power of the inverter; Q e is the reactive power currently output by the inverter.

[0084] More preferably, E 0 =1; n = 0.033; Q ref =0.6, Q e is the average reactive power output by the inverter, and its calculation method is:

[0085] Q e =1.5(U q I od -U d I oq )

[0086] Simulating the excitation control of the synchronous generator, using PI control to make the output voltage amplitude track E ref , and obtaining the stator excitation electromotive force Ef through the first-order inertia link equivalent to the excitation link.

[0087] More preferably, the proportional parameter K Ep and the integral parameter K Ei of the PI link are consistent with the corresponding parameters in the excitation system inertia time constant and the diesel generator excitation control, that is, K Ep =2, K Ei =5, T e =0.05s; By introducing the third-order model of the synchronous generator, the virtual synchronous generator can more accurately simulate the excitation characteristics of the generator and solve the problem of uneven transient power sharing.

[0088] Substituting the stator excitation electromotive force of E f into the excitation winding electromagnetic transient equation and the stator voltage balance equation in the third-order model of the synchronous generator, so that the inverter simulates the excitation characteristics and output impedance characteristics of the generator, and obtains the d-axis and q-axis reference voltage command values of the inverter, specifically as follows:

[0089]

[0090] Each variable in the formula is the virtual parameter of the inverter obtained by the virtual synchronous generator algorithm, where E d is the d-axis no-load electromotive force, X d is the d-axis synchronous reactance, X' dis the d-axis transient reactance, E' q is the q-axis transient electromotive force, T d ' 0 is the d-axis open-circuit transient time constant, R a is the stator winding resistance, X q is the q-axis synchronous reactance, U dref 、U qref are the d-axis and q-axis voltage reference values respectively.

[0091] More preferably, R a 、X d 、X' d 、T d ' 0 、X q are all consistent with the electrical parameters of the diesel generator, R a =0.051pu, X d =2.4pu, X' d =0.1075pu, T d ' 0 =0.4912s, X q =1.92pu.

[0092] Step Four. Take the d-axis and q-axis reference voltage command values U dref 、U qref of the inverter as the input quantities of the voltage-current double closed-loop, and then obtain the PWM control signal through SVPWM modulation, and further adjust the output voltage of the inverter to maintain the stability of the microgrid voltage and frequency.

[0093] More preferably, the parameters of the voltage-current double closed-loop are: the proportional coefficient K Up of the voltage loop is 3, the integral coefficient K Ui is 30, the proportional coefficient K Ip of the current loop is 1, and the K Ii parameter is 10.

[0094] Step Five. The on-grid and off-grid smooth switching control strategy, according to the α and β axis components of the output voltage of the inverter in the αβ stationary coordinate system and the common point voltage in the per-unit value system (U α , U β ; U gα , U gβ ), calculate the phase difference and amplitude difference between the output voltage of the inverter and the common point voltage. The phase difference and amplitude difference pass through the PI link to obtain the correction amounts of frequency and voltage, and are respectively superimposed on the output frequency ω of the active-frequency control link and the voltage amplitude reference value E ref of the reactive-voltage control link, realize the tracking control of the output voltage of the inverter to the common point voltage, and combine the on-grid and off-grid smooth switching control logic to realize the on-grid and off-grid smooth switching control.

[0095] Among them, for the design of the PI parameters and limit values in the grid-connected and off-grid smooth switching control, a PI link is adopted in the grid-connected pre-synchronization control to achieve zero-error control, so as to ensure that the output voltage of the virtual synchronous generator is consistent with the amplitude and phase of the grid-connected point voltage; to prevent large deviations in the output voltage amplitude and frequency during the pre-synchronization regulation process, it is necessary to limit the PI output, and the limit values are set according to 2% of the voltage amplitude and the rated frequency.

[0096] Further, the grid-connected and off-grid smooth switching control method specifically includes:

[0097] 1) Obtain the sine value sinΔθ, cosine value cosΔθ of the phase difference between the grid-connected point voltage and the output voltage and the voltage amplitude difference ΔU according to the relationship between the inverter output voltage and the grid-connected point voltage in the αβ stationary coordinate system; utilize the characteristic that the trigonometric function period is 2π to construct a variable Δθ that can approximately describe the phase difference between the inverter output voltage and the grid-connected point voltage ctrl , Δθ PLL is the phase difference obtained through the phase-locked loop, and Δθ ctrl can more accurately describe the actual phase difference and can solve the problem of phase difference jump obtained through the phase-locked loop.

[0098]

[0099]

[0100] Among them, U α , U β are the α-axis and β-axis components of the inverter output voltage respectively; U gα , U gβ are the α-axis and β-axis components of the grid-connected point voltage under the two-phase stationary coordinate system respectively.

[0101] 2) The correction amounts of frequency and voltage amplitude are obtained after Δθ ctrl and ΔU pass through the PI link and the limit link respectively, and the correction amounts are superimposed on the output frequency ω of the active-frequency control link and the voltage amplitude reference value E ref of the reactive-voltage control link respectively.

[0102] Preferably, the proportional parameter K p_θ of the PI link in the phase pre-synchronization control is 0.5, the integral parameter K i_θ is 0.1, and the upper and lower limit values of the limit link are 0.02 and -0.02 respectively; the proportional parameter K p_U of the PI link in the amplitude pre-synchronization control is 0.4, the integral parameter K i_U is 0.1, and the upper and lower limit values of the limit link are 0.02 and -0.02 respectively.

[0103] 3) Grid-connected and off-grid smooth switching logic: after the inverter receives the grid-connected command, it enables the pre-synchronization control. After 1 second, it determines whether the amplitude, phase, and frequency of the output voltage and the common point voltage meet the closing conditions. If the conditions are met, it issues a closing command. After the circuit breaker S1 is closed, the pre-synchronization control is turned off. During the grid-connected operation, after receiving the off-grid command, it sends an opening command to disconnect the circuit breaker S1 to complete the off-grid operation.

[0104] The present invention also provides a computer-readable storage medium, on which is stored a virtual synchronous generator control method program that takes into account speed regulation and excitation dynamics. When the virtual synchronous generator control method program that takes into account speed regulation and excitation dynamics is executed by a processor, the steps of the virtual synchronous generator control method that takes into account speed regulation and excitation dynamics as described in the above technical solution are implemented.

[0105] The above-mentioned independent microgrid system operates under the following working conditions: 0-10s diesel generator runs at full load with 304kW+j228kvar load, at 9s the inverter receives the grid connection command and performs pre-synchronization, at 10s the closing switch is closed, the inverter and diesel generator are networked and operated, at 25s the load of 304kW+j228kvar is suddenly added, at 35s the load of 304kW+j228kvar is suddenly unloaded, at 45s the inverter is cut off, Figure 7a , Figure 7b and Figure 8a , Figure 8b They are the active and reactive waveforms of all working conditions respectively. The results show that the inverter based on improved virtual synchronous generator control can achieve smooth switching between grid and off-grid, and can achieve transient steady-state power sharing when networked with diesel generators. The proposed control strategy is correct and effective.

[0106] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. A virtual synchronous generator control method considering speed regulation and excitation dynamics, characterized in that: For a power supply system where an inverter is connected in parallel with a diesel generator, it includes the following steps: In the per-unit system, the output active power of the inverter is used as the input of the active-power - frequency control link to obtain the output frequency of the inverter; the active-power - frequency control link is used to simulate the primary frequency regulation characteristics of the diesel generator, the speed regulation characteristics of the diesel engine, and the mechanical motion of the synchronous generator; The output reactive power of the inverter, the output voltage amplitude, and the d-axis and q-axis components of the output current are used as the inputs of the reactive-power - voltage control link to obtain the d-axis and q-axis reference voltage command values of the inverter; the reactive-power - voltage control link is used to simulate the primary voltage regulation characteristics of the synchronous generator, the excitation dynamic characteristics, and the output impedance characteristics; The d-axis and q-axis reference voltage command values of the inverter are used as the inputs of the voltage-current double closed-loop, and after SVPWM modulation, a PWM control signal is obtained to regulate the output voltage of the inverter; The phase difference and amplitude difference between the output voltage of the inverter and the voltage at the common connection point pass through a PI link to obtain the correction amounts of frequency and voltage, and are respectively superimposed on the output frequency reference value and the output voltage amplitude reference value of the inverter to achieve the tracking control of the output voltage of the inverter to the voltage at the common connection point; According to external commands, smooth switching between grid-connected and off-grid is achieved; During the processes of the active-power - frequency control link, the reactive-power - voltage control link, and the smooth switching between grid-connected and off-grid, corresponding control parameters are matched and designed to make the inverter and the parallel-operating diesel generator share the transient and steady-state power equally.

2. The virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: The process of the active-power - frequency control link includes: The output active power of the inverter is drooped through active-power - frequency to obtain the frequency reference value; PI control and an equivalent first-order inertia link of the diesel engine speed regulation link are used to simulate the diesel engine speed regulation control link, so that the output frequency of the inverter tracks the frequency reference value, and the mechanical power of the virtual synchronous generator is obtained as the input of the virtual synchronous generator rotor motion equation; The output frequency of the inverter is obtained through the virtual synchronous generator rotor motion equation, and after integration, the phase of the output voltage of the inverter is obtained.

3. The virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: The process of the reactive-power - voltage control link includes: The output reactive power of the inverter is regulated through reactive-power - droop to obtain the output voltage amplitude reference value of the inverter; PI control is used to make the voltage amplitude at the output filter capacitor of the inverter track the output voltage amplitude reference value, and an equivalent first-order inertia link of the synchronous generator excitation regulation system is introduced to obtain the stator excitation electromotive force; The stator excitation electromotive force is substituted into the excitation winding electromagnetic transient equation and the stator voltage balance equation in the synchronous generator third-order model, so that the inverter simulates the generator excitation characteristics and output impedance characteristics, and the d-axis and q-axis reference voltage command values of the inverter are obtained.

4. The virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: The process of realizing grid-connected and off-grid smooth switching according to external instructions includes: After the inverter receives the grid-connected instruction, it enables pre-synchronization control. After 1 s, it judges whether the amplitudes, phases, and frequencies of the output voltage and the common point voltage meet the closing conditions. After meeting the conditions, it issues a closing instruction and closes the pre-synchronization control after the closing is completed. During the grid-connected operation process, after receiving the off-grid instruction, it sends a tripping instruction to disconnect the grid-connected breaker to complete the off-grid operation.

5. A virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: The matching design process of the control parameters of the active-power - frequency control link includes: Each control link uses per-unit values. In the active-power - frequency control link, the active-power - frequency droop coefficient, the PI parameters of the speed regulation link, the equivalent first-order inertia time constant of the speed regulation link, the virtual inertia time constant, and the virtual damping are all kept consistent with those of the diesel generator operating in parallel.

6. A virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: The matching design process of the control parameters of the reactive-power - voltage control link includes: In the per-unit value system, the reactive-power - voltage droop coefficient, the PI control parameters of the voltage regulation link, the d-axis synchronous reactance, the d-axis transient reactance, the q-axis transient electromotive force, the d-axis open-circuit transient time constant, the stator winding resistance, and the q-axis synchronous reactance in the reactive-power - voltage control link are all kept consistent with the corresponding parameters of the diesel generator operating in parallel.

7. A virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: The matching design process of the control parameters of the grid-connected and off-grid smooth switching process includes: A PI link is used to achieve zero-error control in the grid-connected pre-synchronization control; In the per-unit value system, the output of the PI is limited, and the limit value is set according to 2% of the rated output voltage amplitude and frequency of the inverter.

8. A virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 1, characterized in that: Obtain the amplitude difference ΔU and phase difference Δθ between the inverter output voltage and the common point voltage ctrl The process includes: obtaining the sine value sinΔθ and cosine value cosΔθ of the phase difference between the common point voltage and the inverter output voltage according to the vector relationship between the inverter output voltage and the common point voltage in the αβ stationary coordinate system; using the characteristic that the trigonometric function period is 2π, constructing a variable that can approximately describe the phase difference between the inverter and the common point voltage as the phase difference Δθ between the inverter output voltage and the common point voltage ctrl : Among them, U α is the α-axis component of the inverter output voltage, and U β is the β-axis component of the inverter output voltage; U gα is the β-axis component of the common point voltage, and U gβ is the β-axis component of the common point voltage.

9. A virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 2, characterized in that: The frequency reference value ω is calculated by the following formula m :[[]] ω m = ω 0 + m(P ref - P e ) where, P ref is the active power reference value of the virtual synchronous generator, which is given according to the rated active power of the inverter; P e is the active power output by the inverter, m is the active-frequency droop coefficient, and ω 0 is the rated angular velocity.

10. A virtual synchronous generator control method considering speed regulation and excitation dynamics according to claim 3, characterized in that: The electromagnetic transient equation of the excitation winding and the stator voltage balance equation in the three-order model of the synchronous generator are as follows: In the formula, each variable is a virtual parameter of the virtual synchronous generator, where E f is the stator excitation electromotive force, X d is the d-axis synchronous reactance, X' d is the d-axis transient reactance, E' q is the q-axis transient electromotive force, T d ' 0 is the d-axis open-circuit transient time constant, R a is the stator winding resistance, X q is the q-axis synchronous reactance, E q is the q-axis no-load electromotive force, U dref is the d-axis voltage reference value, U qref is the q-axis voltage reference value; I od is the d-axis component of the inverter output current in the rotating coordinate system, I oq is the q-axis component of the inverter output current in the rotating coordinate system.

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