CN107241028B - A kind of inverter parallel droop control method based on electricity virtualization - Google Patents

Abstract

The invention belongs to power electronic control technology, and relates to a parallel droop control technology of inverters in an off-grid low-voltage microgrid. An inverter parallel droop control method based on power virtualization, comprising the following steps: Step 1: Sampling the voltage information output by each inverter at the outlet of each parallel inverter and current information Step 2, according to the preset droop control equation, combined with the active power P and reactive power Q output by each inverter, obtain the voltage reference value U _oref of the output voltage of each inverter; Step 3, for each inverter The branch where the inverter is located performs power virtualization; step 4, compare the current reference value _ILref corrected by the current inner loop in step 3 with the actual inductor current _IL flowing through the filter inductor to obtain a corresponding modulation signal; step 5. Compare the modulation signal in step 4 with the carrier signal to obtain the PWM trigger signal of each inverter; the PWM trigger signal drives the corresponding power switch device to output the target voltage.

Description

A parallel droop control method for inverters based on power virtualization

technical field

The invention belongs to the power electronic control technology, and relates to an inverter parallel droop control method based on power virtualization.

Background technique

As an effective supplement to the large power grid, the microgrid can realize off-grid and grid-connected operation, and has a bright development prospect in the future. In a microgrid, each distributed power source often supplies power to the load through an inverter. In the initial stage of the construction of the microgrid, the power supply capacity and load demand are both small, and sometimes only a single large-capacity inverter can meet the power demand of the system. capacity to meet system power requirements is impractical. Therefore, the parallel control technology of inverters has gradually become the focus of attention.

In the inverter parallel system of low-voltage microgrid, each inverter often adopts droop control, and for the resistive characteristics of low-voltage lines, resistive droop control is often the preferred choice. For the inverter parallel system, how to realize the accurate distribution of system power among the inverters is the key problem of research.

In practice, the line lengths of each inverter branch are often different, so the corresponding line impedances are also different, which will cause a difference between the equivalent connection impedances of the inverters, and the impedance difference is often the cause of the system power distribution. The main reason for the low precision. In order to change the equivalent connection impedance of the inverter, the traditional series virtual impedance method is often used, but when this method is adopted, an additional voltage loss link will be introduced, which will soften the external characteristics of the inverter output voltage, which will cause the system Deterioration of voltage quality.

SUMMARY OF THE INVENTION

The purpose of the present invention is to propose a parallel droop control method for inverters based on power virtualization. By adopting the power virtualization method, the parallel virtual resistance is equivalently introduced into the inverter branch, so as to change the inverter, etc. The purpose of effective connection impedance; combined with reasonable parameter settings, while realizing the accurate distribution of system power, it also takes into account the voltage quality of the system.

To achieve the above object, the present invention realizes through the following technical solutions:

An inverter parallel droop control method based on power virtualization, comprising the following steps:

Step 1, sample the voltage information output by each inverter at the outlet of each parallel inverter and current information According to the active power calculation formula and reactive power calculation formula, the active power P and reactive power Q supplied by each inverter to the common load are obtained;

Step 2, according to the preset droop control equation, combined with the active power P and reactive power Q output by each inverter, obtain the voltage reference value U _oref of the output voltage of each inverter. The voltage reference value U _orref is composed of the amplitude reference value U _ref and the frequency reference value f _ref , and the amplitude reference value U _ref and the frequency reference value f _ref are combined according to the formula U _oref =U _ref sin2πf _ref t for voltage synthesis, Obtain the voltage reference value U _oref of the output voltage of each inverter, where t is the time;

Step 3: Perform power virtualization on the branch where each inverter is located. The power virtualization is an improvement on the basis of the traditional inverter voltage and current double-loop control links. The specific process is as follows: The reference voltage U _oref of each inverter is used as the reference value of the voltage outer loop of the voltage and current double-loop control link of each inverter, and is compared with the actual voltage U _o at the outlet of each inverter. The voltage deviation after comparison After the signal U _oref - U _o passes through the proportional-integral controller of the voltage outer loop, the initial reference value I ^* _Lref of the current inner loop is obtained; after the virtual voltage proportional coefficient k _vir is multiplied by the actual voltage U _o at the outlet of the inverter, Obtain the virtual line voltage at both ends of the virtual line impedance, divide the virtual line voltage by the preset parallel virtual resistance R _vir to obtain the virtual current I _{vir of the virtual parallel resistance branch, the virtual current I vir and} the current initial reference of the current inner loop After the value I ^* _Lref is added, the current reference value I _Lref after the correction of the current inner loop is obtained;

Step 4: Compare the current reference value _ILref corrected by the current inner loop in Step 3 with the actual inductor current _IL flowing through the filter inductor, and subtract the current deviation signal _ILref - _IL by the current inner loop. After the proportional controller, the corresponding modulation signal is obtained;

Step 5: Compare the modulated signal in step 4 with the carrier signal to obtain the PWM trigger signal of each inverter; the PWM trigger signal drives the corresponding power switching device, so that each inverter outputs the target voltage.

In step 3, the virtual voltage proportional coefficient k _vir is obtained from the virtual voltage proportional relationship in the virtual circuit, that is, the value of k _vir can only be obtained after power virtualization, and power virtualization is based on the premise of current equivalent , the method changes the equivalent connection impedance of the inverter by correcting the actual output current of the inverter.

The beneficial effects of the present invention are: since the traditional series virtual impedance method cannot take into account the power distribution accuracy and voltage quality of the system at the same time, the power virtualization method proposed by the present invention uses control means to equivalently introduce parallel virtual resistance branches, and the combination is reasonable The parameter setting of the inverter can not only easily change the equivalent connection impedance of the inverter, but also does not increase the voltage loss of the system. While realizing the accurate distribution of system power, it can also take into account the voltage quality of the system.

Description of drawings

Figure 1 is a simplified circuit diagram of two inverters with the same capacity connected in parallel to supply power to the same load when the microgrid runs off-grid;

Figure 2 is a simplified control block diagram when a single inverter adopts droop control;

Fig. 3 is a schematic diagram of virtualizing electric power to some circuits in the rectangular dashed frame in Fig. 2;

Fig. 4 is the control block diagram of the inverter voltage and current double-loop control link after the parallel virtual resistance R _vir is equivalently introduced on the basis of the power virtualization method;

Figure 5 is the variation of the total equivalent virtual line impedance Z _p with the added parallel virtual resistance R _vir ;

Figure 6 is a Bode diagram of the internal impedance Z(s) of the inverter when the proportional coefficient k _ip in the current inner loop proportional controller changes in the voltage-current dual-loop control link of the inverter;

FIG. 7 is a Bode diagram of the internal impedance Z(s) of the inverter when the proportional coefficient k _vp in the voltage outer loop proportional-integral controller changes in the voltage-current dual-loop control link of the inverter;

8 is a Bode diagram of the internal impedance Z(s) of the inverter when the integral coefficient _kvi in the voltage outer loop proportional-integral controller changes in the voltage-current dual-loop control link of the inverter;

Fig. 9 is a Bode diagram of the impedance Z(s) in the inverter when the virtual voltage proportional coefficient k _vir changes in the voltage-current double-loop control link of the inverter;

Figure 10 is a Bode diagram of the voltage transfer function G(s) when all control parameters in the inverter voltage and current dual-loop control link are selected;

Figure 11 is a parallel system model in which two inverters with the same capacity are both filtered by an LC filter and jointly supply power to a common load;

Fig. 12 shows the variation of the effective value of the actual voltage at the outlet of the first inverter with time when the traditional series virtual impedance method and the power virtualizing method proposed by the present invention are respectively adopted;

Figure 13 shows the distribution of the active power and reactive power of the system between two parallel inverters using the traditional series virtual impedance method when a small series virtual resistance is added;

Figure 14 shows the distribution of the active power and reactive power of the system between two parallel inverters when a large series virtual resistance is added using the traditional series virtual impedance method;

FIG. 15 shows the distribution of the active power and reactive power of the system between two parallel inverters when the method for virtualizing electricity according to the present invention is adopted.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

When the low-voltage microgrid runs off-grid, take two inverters with the same capacity connected in parallel to supply power to the same load as an example. Its simplified circuit is shown in Figure 1. Among them, Z _total is the common load impedance; Z _n ∠θ _n =R _n +jX _n (n represents the inverter number, n=1 or 2) is the equivalent connection impedance of the nth inverter (Z _{n ∠θn} includes the internal impedance and line impedance of the nth inverter); U _{n ∠φun} is the actual voltage at the outlet of the nth inverter; I _{on ∠φin} is the nth inverter Output load current; U _z ∠ 0° is the common load terminal voltage.

The complex power S _n output by the nth inverter is:

P _n in formula (1) is the active power output by the nth inverter, Q _n in formula (1) is the reactive power output by the nth inverter, and the nth inverter can be obtained from formula (1) The active power P _n and reactive power Q _n output by each inverter are:

Still taking the nth inverter branch as an example, since the resistance component of the line impedance is dominant in the low-voltage microgrid, θ _n ≈ 0°; in addition, the voltage phase angle φ of the output voltage of the nth inverter is _un is very small, then sinφ _un ≈φ _un . Therefore, the active power P _n and reactive power Q _n output by the nth inverter can be expressed as:

It can be seen that when the equivalent connection impedance of the nth inverter is resistive, that is, when Z _n ∠θ _n ≈ _{R n} , the active power P _n and reactive power Q _n output by the nth inverter will be related to the voltage amplitude U _n and the voltage phase angle φ _un respectively, therefore, the resistive droop control of active power-voltage amplitude and reactive power-voltage phase angle can be used, but considering the initial output voltage of the inverter It is difficult to obtain the phase angle, so for reactive power, the reactive power-frequency droop method can be used.

Figure 2 is a simplified control block diagram of the system when a single inverter adopts droop control, where L and C are the filter inductance and filter capacitor of the LC filter respectively, U _dc is the DC bus voltage, and U _inv is the inverter terminal voltage , U _o is the actual voltage at the inverter outlet, U _z is the load terminal voltage, _IL is the inductor current, I _C is the capacitor current, I _o is the load current, and Z _load is the equivalent load of a single inverter impedance, Z _line is the line impedance.

In Figure 2, the inverter can generate the reference value of the voltage outer loop of the voltage and current double-loop control link according to the droop control equation, and the voltage and current double-loop control link adjusts the actual voltage at the outlet of the inverter, thereby realizing the inverter. The active power and reactive power output by the device are controlled.

The voltage and current double-loop control link of the inverter is generally composed of a voltage outer loop and a current inner loop. The voltage outer loop adopts a proportional-integral controller, which can realize the precise control of the output voltage of the inverter, and the current inner loop adopts a proportional controller, which can control the output voltage of the inverter. The resonance peak of the LC filter is suppressed, and the power virtualization method of the present invention is essentially realized by improving the voltage and current double-loop control links of the inverter.

The specific steps when adopting the power virtualization method of the present invention are as follows:

Step 1: Sampling the output voltage information and current information at the outlet of each inverter, and calculate the active power and reactive power supplied by each inverter to the common load;

Step 2: According to the preset power droop characteristic equation of each inverter, combined with the active power P and reactive power Q output by each inverter, the voltage amplitude reference of the actual voltage at the outlet of each inverter is obtained. value and frequency reference value.

The corresponding power droop characteristic equation is:

U _ref =U ⁰ -m _P P (6)

f _ref =f ⁰ +m _Q Q (7)

where U _ref is the amplitude reference value of the actual voltage at the inverter outlet, f _ref is the frequency reference value of the actual voltage at the inverter outlet, and U ⁰ is the voltage of the actual voltage at the inverter outlet under no-load conditions Amplitude, f ⁰ is the frequency of the actual voltage at the inverter outlet under no-load conditions, m _P is the droop coefficient of active power, and m _Q is the droop coefficient of reactive power.

After the voltage synthesis of f _ref and U _ref , the reference voltage of the actual voltage U _o at the inverter outlet is:

U _orref =U _ref sin2πf _ref t (8)

In step 3, power virtualization is performed on some circuits in the rectangular dotted box in FIG. 2, and the corresponding power virtualization process is shown in FIG. 3. FIG. In Figure 3, the left side of the arrow is the actual circuit, the right side of the arrow is the corresponding virtual circuit, is the virtual line impedance, to flow through the virtual current, is an infinite virtual resistance, flow through the virtual current, is the dummy load impedance, to flow through the virtual current.

When the load suddenly changes, the load current I _o can better reflect the change of the output power of the inverter compared with the small-scale change of the actual voltage U _o at the inverter outlet. Therefore, the power virtualization is based on the virtual current It is equal to the actual current, that is, it is carried out under the constraint of current equivalent value.

Under the constraint of equal value of current, when the actual impedance Z _line and Z _load change, will press respectively relationship changes; at the same time, due to The resistance value of is infinite, then are all zero, The purpose of power virtualization in Figure 3 is to establish the corresponding relationship between the corresponding voltages, currents and components between the two circuits, so as to introduce the relevant virtual voltages and virtual currents into the control of the actual inverter.

Using the power virtualization method, the voltage and current double-loop control link of the inverter is improved, and the parallel virtual resistance R _vir branch is equivalently introduced. The control block diagram of the improved voltage and current double-loop control link is shown in Figure 4, where, The solid line is the control block diagram when the relevant virtual voltage and virtual current are combined with the actual voltage and actual current before the parallel virtual resistance R _vir is introduced, and the dotted line is the introduced parallel virtual resistance link. G _v (s) is the voltage outer-loop controller of the inverter voltage and current dual-loop control link, G _i (s) is the current inner-loop controller of the inverter voltage and current dual-loop control link, where G _v (s) = k _vp +k _vi /s, _Gi (s)=k _ip k _PWM , k _ip is the current proportional coefficient of the current inner loop controller, k _vp is the voltage proportional coefficient of the voltage outer loop controller, k _vi is the voltage outer loop controller The voltage integral coefficient of the loop controller, k _PWM is the equivalent gain of the inverter, k _vir is the virtual voltage proportional coefficient, I _vir is the virtual current flowing through the branch of the parallel virtual resistance R _vir , and I ^* _Lref is The reference value of , I ^* _Lref is also the current initial reference value of the current inner loop, I _Lref is the reference value of I _C + I _o , I _Lref is also the current reference value I _Lref after the correction of the current inner loop, U _orref is the value of U _o Reference.

In Figure 4, U _oref is used as the reference value of the voltage outer loop of the inverter voltage and current double-loop control link, which is compared with the actual voltage U _o at the inverter outlet, and the compared voltage deviation signal U _oref -U _o is processed by the voltage After the proportional-integral controller of the outer loop, the initial current reference value I ^* _Lref of the inner loop is obtained;

Combined with Fig. 2, Fig. 3 and Fig. 4 for joint analysis, before the parallel virtual resistance R _vir is not introduced, because It can be considered that the voltage outer loop deviation signal U _oref - U _o is generated by G _v (s) for The reference value of , at this time I ^* _Lref = _ILref . For different branches, the line impedance Z _line may be different, so the load current I _o in each inverter branch may not be equal, and the corresponding Also different.

When the parallel virtual resistance R _vir shown by the dotted line is introduced in Fig. 4, the virtual line impedance is the virtual voltage proportional coefficient k _vir The ratio of the virtual voltage at both ends to the actual voltage U _o at the inverter outlet, which is equivalent to the virtual line impedance in the virtual circuit A virtual resistor with an impedance value of R _vir is connected in parallel with the two ends of the Replaced with R _vir , and the virtual current I _vir flowing through R _vir will forcibly correct the reference value I ^* _Lref of the actual current I _L flowing through the filter inductor, so that I _Lref =I ^* _Lref +I _vir , at this time, It is equivalent to indirectly changing the equivalent connection impedance of the inverter.

Step 4, compare the current reference value _ILref in step 3 with the actual current _IL flowing through the filter inductor, and the current deviation signal _ILref - _IL obtained after the comparison is passed through the proportional controller of the current inner loop to obtain the corresponding value. the modulated signal;

In the inverter parallel system, if equal parallel virtual resistances R _vir are introduced into each inverter branch with different line impedance Z _line , it is easy to know through analysis that when the parallel virtual resistance R _vir is sufficiently small, according to the virtual parallel According to the characteristics of the circuit, it can be considered that the total impedance Z _p of the virtual line of each inverter branch is close to the same, its value is about R _vir , and it is still resistive, which meets the impedance condition of resistive droop control; at the same time, each inverter The actual load current I _o in the inverter branch will be close to the same due to the above-mentioned forced correction, thereby reducing the difference between the equivalent connection impedances of the inverters.

Considering that the resistance in the low-voltage line is dominant, set the line resistance as R _line , then |Z _line |≈R _line , corresponding to the virtual circuit with When the parallel virtual resistance R _vir is added, the change of the total virtual line impedance Z _p of the inverter branch with the added parallel virtual resistance R _vir is shown in Figure 5, where 0.1Ω, 0.15Ω, 0.3Ω and 0.5Ω, respectively, It can be seen that when the parallel virtual resistance R _vir takes 0.1 times the minimum line resistance in each line, that is, 0.01Ω, the Z _p of each inverter branch is nearly equal, and both are about 0.01Ω. Therefore, the parallel virtual resistance The value of R _vir can be selected as 0.1 times the minimum line resistance in each inverter branch.

In Fig. 4, the most important thing is how to determine the value of the virtual voltage proportional coefficient k _vir . In order to simplify the control, k _vir in Fig. 4 can take a constant value, and its value can be roughly calculated according to formula (9):

Taking the circuit in Figure 1 as an example, if it is known that the equivalent impedance of the common load is Z _total , then ideally, the equivalent load impedance borne by each inverter is Z _load =2Z _total , corresponding to each inverter. virtual branch has Thus, k _vir in each inverter branch can be calculated according to equation (9).

However, in practice, the load may change, and k _vir in equation (9) should change accordingly. If a constant k _vir continues to be used, it will not be able to match the impedance ratio relationship in the virtual circuit after the load changes, which will eventually cause the system power A drop in allocation accuracy. For this problem, adaptive k _vir can be used to solve it.

Equation (10) is the calculation formula of adaptive k _vir , wherein U _oref is the reference value of U _o in Fig. 4, and its value is adjusted in real time with P and Q in each inverter droop control equation. When the load changes, k _vir will change accordingly, and when the system enters a steady state, it can be considered that U _oref =U _o , at this time, k _vir U _oref =k _vir U _o =I _o Z _p =I _vir R _vir , therefore, using formula (10) to determine the value of k _vir can realize the real-time accurate control of the system. Note that there is a complex number operation in equation (10), but because φ _i is small, and the change of R _vir to the phase angle is also very limited, in order to simplify the control, k _vir can be calculated as the amplitude value.

From Figure 4, the inverter output voltage can be obtained as:

△=LCR _vir s ³ +k _ip k _PWM CR _vir s ² +(k _ip k _PWM k _vp R _vir +R _vir -k _ip k _PWM k _vir )s+k _ip k _PWM k _vi R _vir (12)

Equation (11) can be written as

U _o (s)=G(s)U _oref (s)-Z(s)I _o (s) (13)

in

Z(s) is the internal impedance of the inverter, but since Z _p in equation (10) has already taken the line impedance Z _line into account, it is equivalent to integrating the line impedance into the internal impedance of the inverter through control means. Therefore, Z(s) can also be considered as the equivalent connection impedance of the inverter. It can be seen from equation (15) that after selecting a sufficiently small R _vir , Z(s) is also related to other control parameters of the inverter. In order to achieve resistive droop control, Z(s) should be used at power frequency. It is resistive. Figures 6 to 9 are Bode plots of Z(s) when k _ip , k _vp , k _vi and k _vir change, respectively.

It can be seen from Figure 6 that when k _ip changes in a large range, Z(s) is basically resistive in the industrial frequency band _. k _ip is not conducive to system stability, therefore, k _ip is selected as 0.31.

In Figure 7, when k _vp increases, the resistive frequency band of Z(s) near the power frequency will become wider, but too large k _vp will make the high frequency band of Z(s) tend to be inductive, which is not conducive to high frequency Therefore, the value of k _vp should not be too large; in addition, too small k _vp will increase the amplitude of Z(s), thereby increasing the voltage loss of the system. Therefore, k _vp is selected as 1.06.

In Figure 8, the smaller k _vi is, the wider the resistive frequency band of Z(s) is near the power frequency, but if k _vi is too small, the voltage tracking accuracy of the system will deteriorate, so k _vi is selected to be 0.5.

When the adaptive virtual voltage proportional coefficient k _vir is used, k _vir will change with the change of load. It can be seen from Figure 9 that the resistive frequency band and amplitude of Z(s) are basically not affected by k _vir , so , the adaptive k _vir is still feasible.

When all control parameters are selected, it can be seen from Figure 10 that the amplitude error and phase angle error of the voltage transfer function G(s) are approximately zero near the power frequency, which meets the design requirements.

Step 5: Compare the modulated signal generated in step 4 with the carrier signal to obtain the PWM trigger signal of the inverter; the PWM trigger signal drives the corresponding power switching device, so that the inverter outputs the target voltage.

In order to verify the effectiveness of the power virtualization method of the present invention, a parallel system model in which two inverters with the same capacity are both filtered by an LC filter and jointly supply power to the common load as shown in Figure 11 is built in MATLAB. . Among them, Z _linei is the line impedance of the branch where the ith inverter is located, Z _total is the common load impedance, U _invi is the terminal voltage of the ith inverter (i=1 or 2), and U _oi is the ith inverter is the actual voltage at the outlet of the inverter, U _z is the common load terminal voltage, Li and C _i are the filter inductance and filter capacitor of the LC filter in the branch of the _ith inverter, respectively, and I _Li is the flow through L The inductor current of _i , I _Ci is the capacitor current flowing through C _i , and I _oi is the load current output by the ith inverter. In the simulation, Z _line1 = 0.1+j0.013Ω, Z _line2 = 0.17+j0.022Ω, and at the same time, for the convenience of comparison, the traditional series virtual impedance method and the power virtualization method described in the present invention (the introduced parallel virtual resistance R _vir = 0.01Ω), the influence of the active power-voltage amplitude droop characteristic on the output voltage of each inverter should be reduced. Therefore, when setting the active power droop coefficient m _P , select a smaller value.

Between 0 and 1s, the common load Z _total =6.661+j1.998Ω;

The load suddenly increases at 1s, and the common load is merged into 16.754+j11.178Ω.

The traditional series virtual impedance method and the power virtualizing method of the present invention are respectively used for simulation comparison, and the results are shown in Fig. 12-Fig. 15 .

Figure 12 shows the change of the effective value of the actual voltage at the outlet of the first inverter with time when the two methods are used respectively. Curve 1 is the actual voltage at the outlet of the first inverter when the power virtualization method is adopted. The change of the effective value of the voltage with time, the curves 2 and 3 are when the traditional series virtual impedance method is used, when a small series virtual resistance is added and a large series virtual resistance is added, respectively, when the output of the first inverter is RMS value of the actual voltage as a function of time. It can be seen that curves 1, 2, and 3 all decrease to varying degrees after a sudden load increase, but the voltage drop of curve 1 is smaller than that of curves 2 and 3.

Figure 13 shows the distribution of the active power and reactive power of the system between two parallel inverters when a small series virtual resistance is added. It can be seen that when a small series virtual resistance is added, the difference between the equivalent connection impedances of the inverters is still obvious, so the distribution accuracy of the system power is low.

Figure 14 shows the distribution of the active power and reactive power of the system between two parallel inverters when a large series virtual resistance is added. It can be seen that when a larger series virtual resistance is added, the difference between the equivalent connection impedances of the inverters is reduced, so the distribution accuracy of the system power is improved.

Fig. 15 shows the distribution of the active power and reactive power of the system between two parallel inverters when the method for virtualizing electric power according to the present invention is adopted. It can be seen that since the virtual voltage proportional coefficient k _vir can satisfy the virtual impedance ratio relationship in each inverter branch in real time, Z _p does not change significantly before and after the sudden load increase, and due to the introduced parallel virtual resistance R _vir is small enough to make the equivalent connection impedance of each inverter close to the same, and it is still resistive, which not only satisfies the power decoupling droop control, but also realizes the accurate distribution of system power.

Different from the traditional series virtual impedance method, the power virtualization method proposed by the present invention: under the constraint of the current equivalent value, from the perspective of control, equivalently introducing a parallel virtual resistance into each parallel inverter branch, which can The equivalent connection impedance of each parallel inverter is conveniently changed, and no additional voltage loss is introduced, which not only realizes the accurate distribution of system power, but also takes into account the voltage quality of the system.

Claims (2)

1. The inverter parallel droop control method based on electric quantity virtualization is characterized by comprising the following steps of:

step 1, sampling voltage information output by each inverter at the outlet of each parallel inverterAnd current informationAccording to active workObtaining active power P and reactive power Q supplied to a public load by each inverter through a rate calculation formula and a reactive power calculation formula;

step 2, according to a preset droop control equation, combining the active power P and the reactive power Q output by each inverter to obtain a voltage reference value U of the output voltage of each inverter_orefSaid voltage reference value U_orefFrom a magnitude reference value U_refAnd a frequency reference value f_refComposition of reference value of amplitude value U_refAnd a frequency reference value f_refAccording to the formula U_oref＝U_refsin2πf_reft, synthesizing the voltage to obtain a voltage reference value U of the output voltage of each inverter_orefWherein t is time;

step 3, performing electric quantity virtualization on the branch where each inverter is located, wherein the electric quantity virtualization is improved on the basis of a traditional inverter voltage and current double-loop control link, and the specific process is as follows: the reference voltage U of each inverter generated in the step 2 is compared_orefAs the voltage outer ring reference value of the voltage and current double-ring control link of each inverter and the actual voltage U at the outlet of each inverter_oComparing the voltage deviation signals U_oref－U_oAfter passing through a proportional-integral controller of the voltage outer ring, obtaining a current initial reference value I of the current inner ring^* _Lref(ii) a Virtual voltage scaling factor k_virMultiplied by the actual voltage U at the inverter outlet_oThen, the virtual line voltage at two ends of the virtual line impedance is obtained, and the virtual line voltage is divided by the preset parallel virtual resistor R_virObtaining the virtual current I of the virtual parallel resistance branch circuit_virVirtual current I_virInitial reference value of current I with current inner ring^* _LrefAfter addition, a current reference value I after current inner loop correction is obtained_Lref；

Step 4, correcting the current reference value I of the current inner loop in the step 3_LrefWith the actual inductor current I flowing through the filter inductor_LComparing the current deviation signals I and subtracting the current deviation signals I_Lref－I_LRatio through current inner loopAfter the example controller, obtaining a corresponding modulation signal;

step 5, comparing the modulation signal in the step 4 with a carrier signal to obtain a PWM (pulse width modulation) trigger signal of each inverter; the PWM trigger signal drives the corresponding power switch device to enable each inverter to output the target voltage.

2. The inverter parallel droop control method based on electric quantity virtualization according to claim 1, wherein the calculation formulas of the active power P and the reactive power Q are as follows:

wherein,is the output current of the inverterConjugation of (1);

the droop control equation is as follows:

U_ref＝U⁰-m_PP

f_ref＝f⁰+m_QQ

wherein, U_refIs the amplitude reference value, f_refIs a frequency reference value, U⁰Is the voltage amplitude of the output voltage of the inverter during no-load operation, f⁰Is the frequency, m, of the output voltage of the inverter during no-load operation_PIs the droop coefficient of active power, m_QIs the droop coefficient of the reactive power.