阅读说明：本技术 一种基于分层协调自适应虚拟电机策略的微网储能控制方法 (Microgrid energy storage control method based on hierarchical coordination self-adaptive virtual motor strategy ) 是由 姚玉永 李腾 张立臣 刘勇 穆勇 云飞 赵丹阳 王丽丽 王涛 徐怀铎 徐小华 于 2021-08-10 设计创作，主要内容包括：本发明涉及一种基于分层协调自适应虚拟电机策略的微网储能控制方法,涉及智能电网储能发电控制技术领域。技术方案是：首先是直流侧采用虚拟直流电机控制策略,模拟直流电机特性,增强系统惯性；其次是增设自适应参数控制环节；交流侧控制划分为两个层次来实现,即基本的虚拟同步电机控制和自适应参数环节设计；对于控制策略中的惯性本质进行分析,从原理出发,分析储和变换器本体惯性计算方法。本发明结构完整,交流侧直流侧控制策略互不影响,且简化了控制过程,提升了暂态性能,进一步降低波动幅值,缩短了达到稳定的时间,对频繁的负荷波动具有更强的适应性,对微网内储能系统的控制和保障交流频率、直流电压稳定运行具有指导意义。(The invention relates to a microgrid energy storage control method based on a hierarchical coordination self-adaptive virtual motor strategy, and relates to the technical field of intelligent power grid energy storage and power generation control. The technical scheme is as follows: firstly, a virtual direct current motor control strategy is adopted at a direct current side to simulate the characteristics of a direct current motor and enhance the inertia of the system; secondly, adding a self-adaptive parameter control link; the AC side control is realized by dividing into two layers, namely basic virtual synchronous motor control and adaptive parameter link design; and analyzing the inertia essence in the control strategy, and analyzing the storage and converter body inertia calculation method from the principle. The micro-grid internal energy storage system has a complete structure, alternating current side and direct current side control strategies do not influence each other, the control process is simplified, the transient performance is improved, the fluctuation amplitude is further reduced, the time for achieving stability is shortened, the micro-grid internal energy storage system has stronger adaptability to frequent load fluctuation, and the micro-grid internal energy storage system has guiding significance for controlling and guaranteeing the stable operation of alternating current frequency and direct current voltage.)

1. A microgrid energy storage control method based on a hierarchical coordination self-adaptive virtual motor strategy is characterized by comprising the following steps:

s1, firstly, the direct current side needs to ensure the voltage stability of the direct current bus, a virtual direct current motor control strategy is mainly adopted to simulate the characteristics of the direct current motor, the system inertia is enhanced, and the capacity of coping with load fluctuation and disturbance is improved; secondly, a self-adaptive parameter control link is added, inertia damping parameters can change along with transient fluctuation, and transient stability is further improved;

s2, the alternating current side control is realized by dividing into two layers, namely: designing basic virtual synchronous motor control and adaptive parameter links;

and S3, analyzing the inertia essence in the control strategy, analyzing the storage and converter body inertia calculation method based on the principle, and giving corresponding suggestions to realize reasonable utilization of resources.

2. The microgrid energy storage control method based on the hierarchical coordination self-adaptive virtual motor strategy according to claim 1, characterized in that in step S1, the direct current side mainly includes the following steps:

(1) firstly, introducing a direct-current side virtual direct-current motor control strategy, which comprises a voltage control link, a mechanical and rotor control link and a current inner ring control link, realizing the discharge control of an energy storage unit, stabilizing the direct-current bus voltage, enabling the direct-current bus voltage to be stabilized near a rated value, and ensuring that the direct-current bus voltage can be recovered to the rated value after fluctuation occurs;

(2) the control strategy of the virtual direct current motor is subjected to small-signal analysis, the influence of inertia and damping parameters on the stability of the system is analyzed when the load power changes, the change rule of the inertia and damping parameters in each stage of the transient process is obtained, the self-adaptive link of the virtual direct current motor is designed on the basis of bus voltage deviation, the inertia and damping parameters are enabled to better adapt to the load change, and compared with fixed parameter control, the amplitude of bus voltage fluctuation is further reduced, and the time for recovering the stability of the bus voltage fluctuation is shortened.

3. The microgrid energy storage control method based on the hierarchical coordination adaptive virtual motor strategy as claimed in claim 1 or 2, characterized in that in step S2, the ac side mainly includes the following steps:

(1) firstly, simulating the operating characteristics of a synchronous generator by using a self-adaptive virtual synchronous motor control strategy of an AC side bottom layer, wherein the self-adaptive virtual synchronous motor control strategy comprises a power calculation link, an active frequency link, a reactive voltage link, a mechanical torque link, an electromagnetic link, a voltage and current loop control link and a PWM (pulse width modulation) link so as to realize the quick response of distributed energy storage and realize power distribution and frequency stability;

(2) the virtual synchronous generator simulates the characteristics of the synchronous generator, and the transient process of the virtual synchronous generator is analyzed according to the power angle curve and the frequency oscillation curve of the synchronous generator; the dynamic response of a system is directly influenced due to different values of the rotational inertia and the damping, when the value of the rotational inertia is small, the system response is fast, but the frequency support is not obvious, when the value of the rotational inertia is overlarge, the adjusting time is greatly prolonged, but the frequency support effect is obvious, and the overlarge frequency deviation of the damping parameter is overlarge, so that the relation between the output power deviation of the alternating current side and the virtual inertia and the damping is established by analyzing the power angle curve and the angular frequency change curve of the alternating current side, the transient process can be more stable, and the fluctuation time is shortened.

4. The microgrid energy storage control method based on the hierarchical coordination self-adaptive virtual motor strategy as claimed in claim 3, wherein in step S3, the inertia analysis calculation mainly includes the following steps:

an inertia solving method of the energy storage unit is given by analyzing inertia solving methods of other things and combining an equivalent model of energy storage;

and calculating the frequency change rate and the voltage change rate of the initial stage of load fluctuation by combining models of the synchronous generator and the direct current motor and a method for calculating the rotational inertia respectively in a load shedding manner, and further calculating the inertia of the converter body.

5. The microgrid energy storage control method based on the hierarchical coordination self-adaptive virtual motor strategy as claimed in claim 4, wherein the specific content of the step (1) in the step S1 is as follows:

the control strategy of the VDCM is that a mechanical link and an armature loop part of the direct current generator are simulated and introduced into the control of a bidirectional DC/DC converter, so that the converter has inertia and damping similar to those of the direct current generator, the control link consists of a voltage loop, a VDCM link and a current loop, a reference value of output voltage at a direct current side is compared with actual output voltage, the current output by a proportional-integral link is multiplied by a given voltage value to obtain mechanical power of the VDCM, and further virtual mechanical torque can be obtained; and designing a VDCM control link, and finally obtaining a current reference value of the converter through the VDCM control link, and generating a control signal of the converter through a PI demodulator and PWM modulation to complete VDCM control.

6. The microgrid energy storage control method based on the hierarchical coordination self-adaptive virtual motor strategy according to claim 5, characterized in that the specific contents of the step (2) of S1 are as follows: and (3) carrying out small signal analysis of VDCM control according to a direct current side virtual direct current motor control strategy, and analyzing the influence of parameter change on the transient process in the transient process so as to obtain a self-adaptive control link.

7. The microgrid energy storage control method based on the hierarchical coordination self-adaptive virtual motor strategy according to claim 4, characterized in that the specific contents of step S2 are as follows: the virtual synchronous motor control simulates the characteristics of a traditional synchronous generator, and generates control signals through active-frequency, reactive-voltage, VSG links, a voltage current loop and PWM modulation, wherein the active frequency link is used for maintaining the frequency stability of the system, the reactive voltage link can adjust the voltage and the reactive power of a generator end, and the VSG link has certain capability of keeping the original state due to the existence of virtual inertia and a damping coefficient, so that a certain buffering effect is achieved, and the system stability is better guaranteed.

8. The microgrid energy storage control method based on the hierarchical coordination self-adaptive virtual motor strategy as claimed in claim 7, characterized in that the concrete contents of the self-adaptive control link in step (2) in S2 are as follows: the virtual synchronous generator simulates the characteristics of the synchronous generator, the transient process of the virtual synchronous generator is analyzed according to the power angle curve and the frequency oscillation curve of the synchronous generator, the relation between the power deviation and the virtual inertia and the damping is established, the power is relatively stable, the power difference is easier to obtain, the fluctuation is less, and the frequency fluctuation is relatively smooth.

Technical Field

The invention relates to a microgrid energy storage control method based on a hierarchical coordination self-adaptive virtual motor strategy, and relates to the technical field of intelligent power grid energy storage and power generation control.

Background

The wide application of a large amount of distributed energy storage and new energy power generation changes the original power grid structure and brings new challenges to the power system. Smart grids are developing vigorously as a novel power supply and utilization mode. With the innovative development of the economic society, the types and the applications of distributed energy storage and loads are more and more, and a pure alternating-current microgrid cannot meet the existing requirements, so that the alternating-current and direct-current microgrid is considered as an economic and reliable new choice. The alternating current-direct current micro-grid can be connected with a power grid to operate as a novel power supply mode, and can also be disconnected from the power grid in isolated island operation when a fault occurs. Meanwhile, the alternating current side and the direct current side have independent operation capacity, and energy interaction is carried out through the power electronic converter.

The control strategy of the alternating-current side inverter mostly adopts droop control, but the droop control lacks inertia and cannot meet the requirements of supporting frequency and voltage. Therefore, in recent years, virtual synchronous machine control for simulating motor characteristics is widely researched, and a virtual synchronous machine control strategy can improve the inertia of a power grid so as to improve the stability of the system. However, in the face of complicated and variable load fluctuation, a control strategy with fixed parameters cannot achieve a better control effect. The control strategy of the direct current side converter is that droop control is too much, inertia is also lacked, load fluctuation and other disturbances cannot be better coped with, meanwhile, droop control is poor control, and due to the existence of droop resistance, when load fluctuation occurs, direct current bus voltage cannot be stabilized at a rated value, voltage deviation can occur, and system stability is not facilitated. The newly proposed virtual direct current motor control strategy is similar to the virtual synchronous generator strategy, and the problem that fixed parameters cannot better cope with load disturbance also exists.

Meanwhile, most of the existing researches mainly aim at the control research of an independent alternating current side virtual synchronous motor or the control research of an independent direct current side virtual direct current motor, and the direct current micro-grid energy storage virtual motor control of a complete system is lacked. Meanwhile, the relation between the inertia of the energy storage body and the inertia of the system is not considered, the inertia nature is not analyzed, and a specific method for limiting the inertia amplitude is provided. Therefore, the research on the microgrid energy storage layered distributed control based on the self-adaptive virtual motor strategy can better cope with the load fluctuation while ensuring the stability of the system, and the realization of flexible plug and play of the energy storage unit is very necessary.

Disclosure of Invention

The invention aims to provide a microgrid energy storage control method based on a hierarchical coordination self-adaptive virtual motor strategy, which adopts an alternating-current side hierarchical distributed virtual synchronous motor control strategy, comprises the control of a bottom self-adaptive virtual synchronous generator, a smooth transient process, the shortening of time for reaching a steady state and the realization of primary frequency modulation; the direct current side adopts self-adaptive virtual direct current motor control to ensure that the voltage of a direct current bus is stable and better copes with transient fluctuation; and finally, the inertia nature is fully analyzed, and a calculation method and corresponding suggestions are given by combining the energy storage and the inertia of the converter body, so that the problems in the background technology are solved.

In order to realize the above content, the invention adopts the following technical scheme:

a microgrid energy storage control method based on a hierarchical coordination self-adaptive virtual motor strategy comprises the following steps:

s1, firstly, the direct current side needs to ensure the voltage stability of the direct current bus, a virtual direct current motor control strategy is mainly adopted to simulate the characteristics of the direct current motor, the system inertia is enhanced, and the capacity of coping with load fluctuation and disturbance is improved; secondly, a self-adaptive parameter control link is added, inertia damping parameters can change along with transient fluctuation, and transient stability is further improved;

s2, the alternating current side control is realized by dividing into two layers, namely: designing basic virtual synchronous motor control and adaptive parameter links;

and S3, analyzing the inertia essence in the control strategy, analyzing the storage and converter body inertia calculation method based on the principle, and giving corresponding suggestions to realize reasonable utilization of resources.

Preferably, in step S1, the dc side mainly includes the steps of:

(1) firstly, introducing a direct-current side virtual direct-current motor control strategy, which comprises a voltage control link, a mechanical and rotor control link and a current inner ring control link, realizing the discharge control of an energy storage unit, stabilizing the direct-current bus voltage, enabling the direct-current bus voltage to be stabilized near a rated value, and ensuring that the direct-current bus voltage can be recovered to the rated value after fluctuation occurs;

(2) the control strategy of the virtual direct current motor is subjected to small-signal analysis, the influence of inertia and damping parameters on the stability of the system is analyzed when the load power changes, the change rule of the inertia and damping parameters in each stage of the transient process is obtained, the self-adaptive link of the virtual direct current motor is designed on the basis of bus voltage deviation, the inertia and damping parameters are enabled to better adapt to the load change, and compared with fixed parameter control, the amplitude of bus voltage fluctuation is further reduced, and the time for recovering the stability of the bus voltage fluctuation is shortened.

Preferably, in step S2, the alternating current side mainly includes the steps of:

(1) firstly, simulating the operating characteristics of a synchronous generator by using a self-adaptive virtual synchronous motor control strategy of an AC side bottom layer, wherein the self-adaptive virtual synchronous motor control strategy comprises a power calculation link, an active frequency link, a reactive voltage link, a mechanical torque link, an electromagnetic link, a voltage and current loop control link and a PWM (pulse width modulation) link so as to realize the quick response of distributed energy storage and realize power distribution and frequency stability;

(2) the virtual synchronous generator simulates the characteristics of the synchronous generator, and the transient process of the virtual synchronous generator is analyzed according to the power angle curve and the frequency oscillation curve of the synchronous generator; the dynamic response of a system is directly influenced due to different values of the rotational inertia and the damping, when the value of the rotational inertia is small, the system response is fast, but the frequency support is not obvious, when the value of the rotational inertia is overlarge, the adjusting time is greatly prolonged, but the frequency support effect is obvious, and the overlarge frequency deviation of the damping parameter is overlarge, so that the relation between the output power deviation of the alternating current side and the virtual inertia and the damping is established by analyzing the power angle curve and the angular frequency change curve of the alternating current side, the transient process can be more stable, and the fluctuation time is shortened.

Preferably, in step S3, the inertia analysis calculation mainly includes the steps of:

(1) at present, although a virtual inertia maximum value evaluation method is considered in a small part of research, the relation between the inertia of the energy storage unit and the inertia of the converter is not considered, and an inertia calculation method of the energy storage unit is provided by analyzing inertia calculation methods of other things and combining an equivalent model of energy storage;

(2) and calculating the frequency change rate and the voltage change rate of the initial stage of load fluctuation by combining models of the synchronous generator and the direct current motor and a method for calculating the rotational inertia respectively in a load shedding manner, and further calculating the inertia of the converter body.

Further, the specific content of the "dc-side virtual dc motor control strategy" in step (1) in S1 is as follows:

the control strategy of the VDCM is that a mechanical link and an armature loop part of the direct current generator are simulated and introduced into the control of a bidirectional DC/DC converter, so that the converter has inertia and damping similar to those of the direct current generator, the control link consists of a voltage loop, a VDCM link and a current loop, a reference value of output voltage at a direct current side is compared with actual output voltage, the current output by a proportional-integral link is multiplied by a given voltage value to obtain mechanical power of the VDCM, and further virtual mechanical torque can be obtained; and designing a VDCM control link, and finally obtaining a current reference value of the converter through the VDCM control link, and generating a control signal of the converter through a PI demodulator and PWM modulation to complete VDCM control.

Further, the specific content of the "dc-side adaptive virtual dc motor control strategy" in step (2) of S1 is as follows:

and (3) carrying out small signal analysis of VDCM control according to a direct current side virtual direct current motor control strategy, and analyzing the influence of parameter change on the transient process in the transient process so as to obtain a self-adaptive control link.

Further, the specific content of the "bottom layer adaptive virtual synchronous motor control strategy" in step S2 is as follows:

the virtual synchronous motor control simulates the characteristics of a traditional synchronous generator, and generates control signals through active-frequency, reactive-voltage, VSG links, a voltage current loop and PWM modulation, wherein the active frequency link is used for maintaining the frequency stability of the system, the reactive voltage link can adjust the voltage and the reactive power of a generator end, and the VSG link has certain capability of keeping the original state due to the existence of virtual inertia and a damping coefficient, so that a certain buffering effect is achieved, and the system stability is better guaranteed.

Further, the specific content of the adaptive control link of the "bottom layer adaptive virtual synchronous motor control strategy" in step (2) of S2 is as follows:

the virtual synchronous generator simulates the characteristics of the synchronous generator, the transient process of the virtual synchronous generator is analyzed according to the power angle curve and the frequency oscillation curve of the synchronous generator, the relation between the power deviation and the virtual inertia and the damping is established, the power is relatively stable, the power difference is easier to obtain, the fluctuation is less, and the frequency fluctuation is relatively smooth.

The method utilizes the alternating current side and the direct current side to respectively adopt the virtual synchronous motor and the direct current motor to control and increase the system inertia, improves the disturbance rejection capability, analyzes a power angle curve and an angular frequency oscillation curve, and designs a virtual synchronous generator self-adaptive parameter link on the basis of power deviation. The direct current side analyzes the bus voltage fluctuation process, a virtual direct current motor self-adaption link is designed on the basis of bus voltage deviation, the inertia essence of the energy storage body and the converter is analyzed, and the inertia amplitude is limited.

The invention has the beneficial effects that: the structure is more complete, alternating current side direct current side control strategy does not influence each other, and the self-adaptation control link compares with traditional self-adaptation control, has simplified the control process, has promoted the transient state performance, further reduces the fluctuation amplitude, has shortened and has reached stable time, has stronger adaptability to frequent load fluctuation, has guiding significance to the control of energy storage system in the microgrid and guarantee alternating current frequency, direct current voltage steady operation.

Drawings

FIG. 1 is a control framework for an embodiment of the present invention;

FIG. 2 is a power angle curve of the synchronous motor of the present invention;

FIG. 3 is an angular frequency oscillation curve of the synchronous motor of the present invention;

FIG. 4 is a comparison graph of the adaptive virtual synchronous machine strategy simulation of the present invention;

FIG. 5 is a VDCM control small signal model according to the present invention;

FIG. 6 is a comparison graph of the control strategy simulation of the adaptive virtual DC motor of the present invention;

FIG. 7 illustrates the initial phase change of the load shedding DC bus voltage of the present invention;

FIG. 8 shows the initial phase change of the frequency of the load dump AC side according to the present invention.

Detailed Description

The present invention will be described in further detail below with reference to the accompanying drawings and examples.

Control strategy for micro-grid energy storage virtual motor

The microgrid energy storage virtual motor control can be divided into two parts, wherein the microgrid energy storage virtual motor control comprises an alternating current side virtual synchronous motor and a direct current side virtual direct current motor, and control strategies of the two parts are not influenced by each other. The distributed energy storage is connected with a three-phase load and a power grid through a DC/DC converter and a DC/AC converter, grid connection and isolated island operation are achieved through switching of a PCC switch, and a specific control system framework is shown in fig. 1.

The virtual synchronous motor and the virtual direct current motor control improve the inertia of the system by simulating the characteristics of the motor. The virtual synchronous motor control strategy mainly comprises a power calculation link, an active frequency link, a reactive voltage link, a mechanical torque link, an electromagnetic link, a voltage and current loop control link and a PWM (pulse width modulation) link.

The rotor motion equation and the electromagnetic equation controlled by the virtual synchronous generator are shown in the formula (1).

In the formula: omega₀、ω_NRespectively output angular frequency and rated angular frequency; t is_m、T_e、T_DRespectively virtual mechanical, electromagnetic and damping torques; p_refFor a given reference power, P is the active output power, J, D is the virtual inertia and damping, respectively. u. of_abc、i_abcTerminal voltage and output current, e, of the virtual synchronous machine, respectively_abcIs the synchronous generator potential. L, R is the equivalent synchronous inductance and resistance.

The active frequency link is used for maintaining the frequency stability of the system and plays a certain buffering role. The active frequency link equation is as follows:

in the formula: p_mFor mechanical power, T_mIs a mechanical torque, P_refGiven value of active power, k_fIs the frequency modulation coefficient, f_refIs a frequency given value, and f is the system output frequency.

The reactive voltage link can adjust the voltage and the reactive power of the generator end, and the expression of the link is as follows:

△E＝U_ref+k_q(Q_ref-Q)-E_m (3)

in the formula: e_mIs no-load potential, k_qTo adjust the coefficient of reactive power, Q_refGiven value of reactive power, Q is the system output reactive power, U_refThe effective value of the voltage at the machine end.

The control strategy of the VDCM is to introduce the mechanical link and armature loop parts of the DC generator into the control of the bi-directional DC/DC converter by simulating them, so that the converter has similar inertia and damping as the DC generator.

The mechanical equation and the armature circuit equation are obtained according to the direct current generator model as follows:

wherein J is rotational inertia, D is damping coefficient, and T_mFor mechanical torque, T_eAs electromagnetic torque, ω_NActual angular velocity and nominal angular velocity. P_eIs electromagnetic power.

The armature loop equation is:

c in formula (5)_TAnd psi are torque coefficient and magnetic flux, respectively; e_aIs armature electromotive force, R_aIs armature resistance, I_aThe armature current, U, is the generator terminal voltage.

The VDCM control link consists of a voltage loop, a VDCM link and a current loop, and the direct current side outputs a voltage reference value U_refAnd the actual output voltage U_dcComparing the current output by the proportional-integral link with the given voltage valueMultiplying to obtain the mechanical power P of VDCM_mFurther, a virtual mechanical torque T can be obtained_m. According to the foregoing formula; designing a VDCM control link, and finally obtaining a current reference value I of the converter through the VDCM control link_refAnd generating a control signal of the converter through a PI demodulator and PWM modulation to complete VDCM control.

Adaptive virtual motor control

1. Adaptive virtual synchronous generator design

In the self-adaptive link, when the distributed energy storage is in an island operation condition and load fluctuation occurs, the values of virtual inertia and damping are different, the effect of the dynamic frequency adjustment process is different, when the virtual inertia is larger, the supporting effect on the frequency is more obvious, but the time for the frequency to reach a stable state is longer; the larger the damping parameter is, the more obvious the buffering effect on the frequency change is, and when the frequency changes, the damping effect is realized to a certain extent. The relationship between the angular frequency change and the angular frequency change rate and the relationship between the virtual inertia and the damping parameter are derived according to the formula (1) as shown in the following formula:

according to the formula, when the D parameter is constant, the larger the J parameter is, the smaller the angular speed change rate is; when J is fixed, the larger the D parameter is, the smaller the frequency deviation is; when J, D parameters are all large, the frequency deviation is smaller, the angular speed change rate is smaller, and in order to ensure the stability of the system, the inhibition is carried out by adjusting J, D parametersAnd changes in Δ ω.

The virtual synchronous generator simulates the characteristics of the synchronous generator, and the transient process of the virtual synchronous generator is analyzed according to the power angle curve and the frequency oscillation curve of the synchronous generator, wherein fig. 2 is a power angle curve, and fig. 3 is an angular frequency oscillation curve.

From the analysis of fig. 2, when the output power of the system is reduced from the point a to the point b, the output power of the system is smaller than the given power value, the frequency of the system is increased, and when the output power of the system is larger than the given power value, the frequency is decreased; and so on. The actual output power of the system and the given power determine the regulation state of the frequency. The oscillation process of the virtual synchronous machine is consistent with the process. Dividing a transient process into 4 intervals as shown in FIG. 3, wherein in the interval t1-t2, the angular frequency of the rotor is greater than the rated angular speed, and the angular speed is increased continuously, at the moment, the change rate of the angular speed is increasedThe difference Δ ω of the angular velocity gradually increases from the minimum value to the maximum value gradually decreasing from the maximum value to 0, for which it is necessary to use larger J and D parameters to prevent them from becoming too large. In the interval t2-t3, the angular frequency of the rotor is still larger than the rated angular speed, but the angular speed begins to be gradually reduced, and at the moment, the change rate of the angular speedThe difference Δ ω of the angular velocity is gradually reduced from 0 to the maximum negative value, and for this purpose, a smaller J parameter and a larger parameter are required to accelerate the recovery process. t3-t4 is similar to t1-t2, and t4-t5 is similar to t2-t 3. The principle of J, D parameter selection under different conditions can be obtained in conclusion. Specific changes are shown in the following table:

table 1 selection principle of virtual inertia and damping parameter

Considering that most researches show inertia parameters as functions of angular frequency change rate, on one hand, in the mode, a differentiation link changes frequently, so that frequency fluctuation is obvious, and secondly, most researches do not consider damping parameter self-adaption, although a transient process is more stable, the time for reaching a steady state is longer. Therefore, by analyzing the transient process, the relation between the power deviation and the virtual inertia and the damping is designed and established, the power is relatively stable, the power difference is easier to obtain, and the frequency fluctuation is gentle with less fluctuation. The adaptive control strategy is designed as follows:

when the angular frequency change rate is less than 1.5, the system is considered to be in a stable state, the virtual inertia and the damping are equal to initial values, and when the angular frequency change rate is more than 1.5, the product of the angular frequency deviation and the change rate is judged to be positive and negative, so that the corresponding virtual inertia and the damping are changed according to the power deviation.

Fig. 4 is a comparison graph of control effects of the ac side virtual motor fixed parameter, the adaptive virtual inertia control, the conventional adaptive virtual inertia and damping control and the ac side control strategy, and the VSM fixed parameter, the adaptive inertia and damping control are obtained through simulation. When the actual output power is equal to the rated power, the frequency is stabilized at the power frequency. At 1s, sudden load increase occurs, primary frequency modulation is started, the frequency drops to be stabilized at 49.95Hz, and at 2s, the sudden load increase is cut off, and the frequency increase is recovered to the rated frequency. Through comparison, when the adaptive inertia control strategy is adopted, although the descending process is more stable, the time for reaching the steady state is longer, namely 1.3s is stable, but the fixed parameter control is stable around 1.1 s. For the self-adaptive inertia and damping control, the frequency drop process is relatively more stable, the adjustment time is almost equal to the fixed parameter by 1.1s, and the frequency fluctuation range is reduced. In the traditional frequency differential-based adaptive inertia damping control, frequency fluctuation is obvious due to the existence of a differential link; compared with the traditional self-adaptive inertia and damping control, the control strategy has the advantages of small frequency ripple and higher stability.

2. Adaptive virtual DC motor design

According to the virtual dc motor control strategy, a small signal model of VDCM control can be obtained as shown in fig. 5:

wherein the energy storage output power under the control of the VDCM can be expressed as:

according to the formula, the power deviation and the direct current bus deviation are 0 in the initial stage, when power fluctuation occurs, the direct current bus voltage fluctuates, the variation of the power is increased from 0, and if the bus voltage deviation is smaller, J, D parameters are required to be smaller, so that the bus voltage deviation is smaller when the power fluctuation occurs. Therefore, when power fluctuation occurs to cause bus voltage drop, the J, D parameter is smaller, so that the drop amplitude is reduced. In the bus voltage recovery process, the power variation is kept unchanged, the bus voltage deviation is gradually reduced from the maximum value to 0, and at the moment, J, D is required to be gradually reduced to the initial value by a larger parameter to ensure the stability of the recovery process.

TABLE 2 principle of variation of inertia and damping parameters of virtual DC motor

In summary, the adjustment of J and D during load variation is shown in equation (10):

and if the voltage deviation of the direct current bus is less than 0.5V, determining that the stable state is achieved, and setting the virtual inertia and the damping as initial values. When the deviation exceeds the threshold value, the running state is determined according to the positive and negative of the product of the voltage deviation and the voltage change, when the product is larger than 0, the J, D parameter is reduced, and in the recovery stage, the product is negative, the running is carried out by the larger parameter, and the initial value is recovered as the deviation is smaller and smaller.

Fig. 6 is a comparison graph of virtual dc motor adaptation versus fixed parameter control, for VDCM control, the dc bus voltage fluctuates when power fluctuations occur due to load surges. Under fixed parameters, the bus voltage drop is about 6V, and the time to reach a steady state is about 0.6s, but the bus voltage fluctuation under the control of the self-adaptive VDCM is 4.8V, and the time to reach the steady state is 0.4 s. In summary, compared with the traditional fixed parameter and adaptive control, the method has lower fluctuation amplitude and faster regulation speed.

Third, virtual inertia amplitude analysis

At present, a great deal of research only provides a corresponding control strategy and a self-adaptive control strategy, but rarely provides a selection principle of the virtual inertia parameter amplitude. Although a virtual inertia maximum value evaluation method is considered in a small part of research, the relation between the inertia of the energy storage unit and the inertia of the converter is not considered. The consideration is still incomplete, so that the self inertia of the stored energy and the inertia of the converter are fully considered in the design, the amplitude limit of the set virtual inertia is comprehensively considered, and the resources are reasonably utilized.

Inertia exists as an inherent property, characterized by the ability to maintain an original state of motion. For the control of the energy storage battery, the energy storage battery supplies energy to a three-phase load through a converter, the inertia comprises an energy storage battery body inertia and a converter body inertia, and the current energy storage battery can be equivalently expressed as follows:

C_eqis an equivalent capacitance value, S_NIs the battery capacity, U_CN、U_NFor rating the voltage, omega, of the battery_NIs the nominal angular velocity of 314 rad/s.

The equivalent moment of inertia for AC/DC and DC/DC converters can then be characterized by the following equation.

The equivalent inertia of the two can be calculated according to a load shedding mode, wherein the load shedding is set to be 2kW, and the frequency change rate and voltage change rate curves are observed at the initial stage of load shedding, and are substituted into the formula to further obtain corresponding rotation inertia. The direct current side and alternating current side load shedding process simulation diagrams are shown in figures 7 and 8. After load change occurs to the direct current bus voltage, due to inertia, the direct current bus voltage is kept for a period of time, the voltage change rate slope is approximately 0, and therefore the DC/DC converter J is approximately an infinite value. And the alternating current side obtains an equivalent inertia value of 15.5879 through calculation. The energy storage element of converter body includes virtual reactance, filtering reactance and return circuit reactance, and the inertial support effect that its provided is limited, and this defect is remedied in the energy saving of energy storage ring, has increased direct current bus capacitance value in other words, and the electric capacity that the battery provided is great, ensures sufficient inertial support. The super capacitor provides a small capacitance support, does not have the inertia support capability of a long time scale, and is not suitable for being set as inertia support energy storage. Finally, the smaller inertia of the three is selected as amplitude limit, and resources are fully utilized.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.