CN113507143B - Hybrid microgrid IC self-adaptive control strategy based on improved VSG technology - Google Patents
Hybrid microgrid IC self-adaptive control strategy based on improved VSG technology Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J4/00—Circuit arrangements for mains or distribution networks not specified as ac or dc
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J5/00—Circuit arrangements for transfer of electric power between ac networks and dc networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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Abstract
The invention relates to a hybrid microgrid IC self-adaptive control strategy based on an improved VSG technology, which is characterized by comprising the following steps: the VSG control principle of the hybrid microgrid, the AC/DC hybrid microgrid self-adaptive virtual inertia control strategy and the self-adaptive VSG control algorithm process content improve the dynamic performance of the system when the load fluctuates by changing fixed inertia into self-adaptive inertia. Under two operation modes of system rectification and inversion, the inertia of the system is correspondingly increased according to the deviation degree when the frequency deviates from a rated value, and the inertia of the system is correspondingly reduced according to the regression degree when the frequency regresses the rated value. When the voltage waveform generates overshoot, the inertia of the system is correspondingly increased, and the overshoot of the voltage is reduced. The frequency waveform and the voltage waveform under the two operation modes are further optimized, and the effectiveness of the control strategy is verified.
Description
Technical Field
The invention relates to the technical field of alternating current and direct current hybrid micro-grids, in particular to a hybrid micro-grid Interface Converter (IC) self-adaptive control strategy based on an improved VSG technology.
Background
With the gradual depletion of traditional fossil energy, distributed energy is widely developed. The microgrid is an effective means for accepting the distributed power supply, and becomes an important way for grid-connected consumption of the distributed power supply. But the single micro-grid has the defects of limited working capacity, weak disturbance resistance and the like. Therefore, the alternating current-direct current hybrid micro-grid becomes an important component of a future intelligent power grid due to the characteristics of effectively avoiding the influence of the output uncertainty of the distributed power supply and the load randomness on the power system. However, although the VSG (Virtual synchronous generator) technology widely applied to the microgrid can solve the characteristic of inertia deficiency caused by the conventional droop control, the inertia in the conventional VSG control strategy is a fixed value, and the required inertia under different operating conditions is different, so that the control strategy ignores the rapidity of system adjustment while improving the system stability.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide a scientific and reasonable hybrid microgrid IC self-adaptive control strategy based on an improved VSG technology, which has strong applicability, and through the self-adaptive VSG control strategy, the frequency and voltage fluctuation in an AC/DC hybrid microgrid can be reduced, and the system stability is obviously enhanced.
The purpose of the invention is realized by the following technical scheme: a hybrid microgrid IC self-adaptive control strategy based on an improved VSG technology is characterized by comprising the following contents:
1. VSG control principle of hybrid microgrid
1) AC frequency-DC voltage control
Equation (1) is the equation of motion of the rotor of the virtual synchronous generator without considering the damping winding action:
in the formula: j is rotational inertia; p is m 、P e The output power and the electromagnetic power of the prime motor are respectively; omega, omega n The angular speed is AC angular speed and rated angular speed; delta is a power angle; k is a radical of ω For primary frequency modulation of ACA sag factor;
according to the formula (1), in the dynamic adjustment process of the alternating current and direct current micro-grid, the instantaneous power balance characteristic is the formula (2) -formula (3);
in the formula: delta P e 、ΔP dc The active power variation of the AC and DC subnets is respectively; u. of dc 、U dcn Actual direct current voltage and rated direct current voltage are respectively; c dc Is a direct current capacitor; k is a radical of u Is the direct current droop coefficient;
from the power balance perspective, when the load of the hybrid micro-grid increases or decreases, and after the load is adjusted by the interface converter, the active power variation of the ac and dc sub-networks is the same in a steady state, and the total active power variation in the hybrid micro-grid is borne in a balanced manner, that is:
ΔP e ＝ΔP dc (4)
substituting the formulas (2) and (3) into the formula (4) to obtain:
an alternating current frequency-direct current voltage control system is obtained according to the formula (5), in a virtual synchronous machine control strategy of an alternating current-direct current hybrid micro-grid interface converter, an alternating current frequency-direct current voltage control part simulates a rotor motion equation to adjust active power, and finally, an output signal of a control link is a virtual potential
Frequency and phase of;
2) virtual excitation control
The control strategy of the virtual synchronous generator simulates an excitation control system and is adjusted to be zeroPower, control link outputs virtual potential
The control equation is:
E＝E ref +k q (Q ref -Q) (6)
in the formula: e ref Is VSG no-load electromotive force; e is a virtual potential effective value; kq is a reactive-voltage droop coefficient; q ref Is a VSG reactive power reference value; q is the actual value of the VSG reactive power;
2. AC-DC hybrid microgrid self-adaptive virtual inertia control strategy
1) Adaptive virtual moment of inertia control strategy
When the system is disturbed, the frequency and the power can vibrate, the self-adaptive virtual rotational inertia control can improve the frequency response speed of the alternating current-direct current hybrid micro-grid at the initial stage of load change and limit the vibration degree in the frequency adjustment process of the system, and the expression is formula (7),
in the formula: j. the design is a square 0 The value of the rotational inertia of the system in a steady state is obtained; d ω/dt is the rate of change of the AC frequency; omega is the actual frequency of the alternating current system; omega n Is a rated alternating current frequency; k is a radical of 1 、k 2 The influence factors are respectively the influence factors of each item, and the dynamic characteristics of the system frequency can be changed by adjusting the influence factors; k is a decision threshold;
when | Δ ω (d ω/dt) & gtY<k, when the system judges that the variable quantity is in the set threshold range, the system rotational inertia takes a steady state value J 0 Frequent switching of parameters can be effectively avoided to keep the stable operation of the system; when the system judges that the variation reaches a set threshold, the moment of inertia J 0 Is an expression containing frequency change rate and frequency difference, when the two are in the same sign, the frequency is in a deviation state, the coefficient is positive, the moment of inertia is increased, and the value is greater than the fixed moment of inertia J 0 Suppressing frequency deviation; when the two are in opposite sign with each other,the frequency is in a regressive state, the coefficient is negative, the moment of inertia is reduced, and the value is smaller than the fixed moment of inertia J 0 Accelerating frequency regression;
2) adaptive virtual capacitance control strategy
A plurality of variables in the alternating current system and the direct current system have one-to-one correspondence, and an adaptive virtual capacitance control strategy similar to the form of an equation (7) is provided as an equation (8) through an analog reasoning method:
in the formula: c v Is a virtual capacitor; c v0 Is the virtual capacitance value of the system in steady state; dU dc The/dt is the change rate of the direct current bus voltage; k is a radical of formula 0 Setting a threshold value; u shape dc Is the actual dc bus voltage; u shape dcn Rated voltage for the direct current bus; k is a radical of 3 、k 4 Respectively are influence factors of each item;
the virtual capacitance value can reflect the inertia of the direct-current microgrid when the value is | delta u dc (du dc /dt)|<k 0 When the system judges that the variation is within the set threshold range, the virtual capacitance is equal to a fixed value C v0 Frequent parameter switching can be avoided, and normal operation of the system can be maintained; when the system judges that the variation reaches the set threshold k 0 Time, virtual capacitance C v No longer constant, is an expression containing the rate of change of the DC voltage and the difference between the DC voltage, and according to the formula (8), the virtual capacitor C v The voltage of the direct current bus is positively correlated with the voltage change of the direct current bus, when the actual voltage value is not equal to the rated voltage value, the virtual capacitor is increased, and the value of the virtual capacitor is larger than that of the fixed virtual capacitor C v0 The quality of the direct current voltage is improved, and the influence of larger fluctuation on the stable operation of the system is avoided;
3. adaptive VSG control algorithm flow
Firstly, inputting an initial value into a self-adaptive VSG control module, detecting the direct current voltage and the alternating current frequency in real time, calculating whether a judgment value exceeds a threshold value or not by calculating the difference value of an actual value and the initial value when a system generates load fluctuation, and finally outputting virtual inertia of a corresponding value,
the first step is as follows: inputting an initial value in an adaptive VSG control module;
the second step is that: respectively inputting and cutting loads on an alternating current side or a direct current side;
the third step: detecting the direct current voltage and the alternating current frequency in real time;
the fourth step: calculating the difference and derivative terms of voltage and frequency;
the fifth step: calculating whether the decision value exceeds a threshold value;
and a sixth step: if the judgment value is within the threshold value, the rotary inertia and the virtual capacitance are fixed values; if the judgment value exceeds the threshold range, the rotational inertia and the virtual capacitance are the provided adaptive values;
the seventh step: and outputting the obtained AC and DC inertia values.
The invention discloses a hybrid microgrid IC self-adaptive control strategy based on an improved VSG technology, which comprises the following steps: according to the VSG control principle of the hybrid microgrid, the AC/DC hybrid microgrid self-adaptive virtual inertia control strategy and the self-adaptive VSG control algorithm process, when the load changes, the system can prevent the AC/DC hybrid microgrid from generating impact disturbance when the load changes by automatically adjusting the inertia of the AC power grid and the DC power grid, improve the phenomenon that the voltage and the AC frequency of a DC bus suddenly rise or fall, and provide guarantee for the safe and stable operation of the system; the method comprises the steps that a judgment value, a threshold value, fixed inertia and adaptive inertia are respectively represented in an adaptive control module through actual direct-current voltage and alternating-current frequency which are input in real time, and finally, when frequency and voltage changes are smaller than the threshold value, the system inertia is a fixed value and when the frequency and voltage changes are larger than the threshold value, the system inertia is an adaptive value through an if Function in an MATLAB Function module; under two operation modes of system rectification and inversion, correspondingly increasing the system inertia according to the deviation degree when the frequency deviates from a rated value, and correspondingly reducing the system inertia according to the regression degree when the frequency regresses the rated value; when the voltage waveform generates overshoot, the system inertia is correspondingly increased, and the overshoot of the voltage is reduced; the frequency waveform and the voltage waveform in the two operation modes are further optimized, and the effectiveness of the control strategy is verified. Has the advantages of scientific and reasonable structure and strong applicability.
Drawings
FIG. 1 is a block diagram of AC frequency-DC voltage control;
FIG. 2 is a virtual excitation control block diagram;
FIG. 3 is a flow chart of adaptive virtual inertia control;
FIG. 4 is a block diagram of an adaptive virtual synchronous generator control;
FIG. 5 is a waveform diagram of an inversion mode AC frequency simulation;
FIG. 6 is a waveform of an inversion mode moment of inertia simulation;
FIG. 7 is a waveform diagram of DC voltage simulation in the inversion mode;
FIG. 8 is a diagram of an inversion mode virtual capacitor simulation waveform;
FIG. 9 is a graph of a rectified mode AC frequency simulation waveform;
FIG. 10 is a plot of a commutation pattern moment of inertia simulation waveform;
FIG. 11 is a waveform diagram of a rectified mode DC voltage simulation;
FIG. 12 is a diagram of a commutation pattern virtual capacitance simulation waveform.
Detailed Description
The practice of the present invention will be further illustrated, but not limited, by the following examples and drawings. The invention relates to a hybrid microgrid IC self-adaptive control strategy based on an improved VSG technology, which comprises the following contents:
1. VSG control principle of hybrid microgrid
1) AC frequency-DC voltage control
Equation (1) is the equation of motion of the rotor of the virtual synchronous generator without considering the damping winding action:
in the formula: j is moment of inertia; p m 、P e The output power and the electromagnetic power of the prime motor are respectively; omega, omega n Alternating current angular velocity and rated angular velocity respectively; delta is a power angle; k is a radical of ω Is an AC primary frequency modulation droop coefficient;
according to the formula (1), during the dynamic adjustment process of the alternating current and direct current micro-grids, the instantaneous power balance characteristics are respectively shown as the formulas (2) and (3).
In the formula: delta P e 、ΔP dc The active power variation of the AC and DC sub-networks respectively; u. of dc 、U dcn Actual direct current voltage and rated direct current voltage are respectively; c dc Is a direct current capacitor; k is a radical of u Is the dc droop coefficient.
From the power balance perspective, when the load of the hybrid micro-grid increases or decreases, and after the load is adjusted by the interface converter, the active power variation of the ac and dc sub-networks is the same in a steady state, and the total active power variation in the hybrid micro-grid is borne in a balanced manner, that is:
ΔP e ＝ΔP dc (4)
substituting the formulas (2) and (3) into the formula (4) to obtain:
according to the formula (5), a corresponding diagram 1 is obtained, namely an alternating current frequency-direct current voltage control block diagram, and as can be seen from the diagram 1, in the control strategy of the virtual synchronous machine of the alternating current-direct current hybrid microgrid interface converter, the alternating current frequency-direct current voltage control part simulates a rotor motion equation so as to adjust active power. The final output signal of the control link is a virtual potential
Frequency and phase.
2) Virtual excitation control
The control strategy of the virtual synchronous generator simulates an excitation control system, adjusts the reactive power and controls the link to output the virtual potential
The control equation is as follows:
E＝E ref +k q (Q ref -Q) (6)
in the formula: e ref Is VSG no-load electromotive force; e is the virtual potential effective value; kq is a reactive-voltage droop coefficient; q ref Is a VSG reactive power reference value; q is the actual value of the VSG reactive power. The corresponding fig. 2 can be obtained from equation (6), which is a virtual excitation control block diagram.
2. AC-DC hybrid microgrid self-adaptive virtual inertia control strategy
1) Adaptive virtual moment of inertia control strategy
When the system is disturbed, the frequency and power will generate a large oscillation process. The self-adaptive virtual rotational inertia control scheme provided by the invention can improve the frequency response speed of the alternating current-direct current hybrid micro-grid at the initial stage of load change and limit the oscillation degree of the system in the frequency adjustment process as shown in the formula (7).
In the formula: j. the design is a square 0 The value of the rotational inertia of the system in a steady state is obtained; d ω/dt is the rate of change of the AC frequency; omega is the actual frequency of the alternating current system; omega n Is a rated alternating current frequency; k is a radical of 1 、k 2 The dynamic characteristics of the system frequency can be changed by adjusting the influence factors of each item; k is a decision threshold.
When | Δ ω (d ω/dt) & gtY<k, when the system judges that the variable quantity is in the set threshold range, the system rotational inertia takes a steady state value J 0 Frequent parameter switching can be effectively avoided to maintain the systemAnd (5) stable operation. When the system judges that the variation reaches a set threshold, the moment of inertia J 0 Is an expression containing frequency change rate and frequency difference, when the two are in the same sign, the frequency is in a deviation state, the coefficient is positive, the moment of inertia is increased, and the value is greater than the fixed moment of inertia J 0 Suppressing frequency deviation; when the two have different signs, the frequency is in a regression state, the coefficient is negative, the rotational inertia is reduced, and the value is smaller than the fixed rotational inertia J 0 And accelerating frequency regression.
2) Adaptive virtual capacitance control strategy
A plurality of variables in an alternating current system and a direct current system have one-to-one correspondence, and an adaptive virtual capacitance control strategy similar to the form of an equation (7) is provided by an analogy reasoning method as follows:
in the formula: c v Is a virtual capacitor; c v0 Is the virtual capacitance value of the system in steady state; dU dc The/dt is the change rate of the direct current bus voltage; k is a radical of 0 Setting a threshold value; u shape dc Is the actual dc bus voltage; u shape dcn Rated voltage for the direct current bus; k is a radical of formula 3 、k 4 Respectively, the influence factors of each item.
The virtual capacitance value can reflect the inertia of the direct-current micro-grid. When | Δ u dc (du dc /dt)|<k 0 When the system judges that the variation is within the set threshold range, the virtual capacitance is equal to a fixed value C v0 The frequent switching of parameters can be avoided, and the normal operation of the system can be kept. When the system judges that the variation reaches the set threshold k 0 Time, virtual capacitance C v No longer constant, is an expression containing the rate of change of the dc voltage and the difference between the dc voltages. According to equation (8), a virtual capacitance C v The voltage of the direct current bus is positively correlated with the voltage change of the direct current bus, when the actual voltage value is not equal to the rated voltage value, the virtual capacitor is increased, and the value of the virtual capacitor is larger than that of the fixed virtual capacitor C v0 The quality of the DC voltage is improved, and the generation of large fluctuation is avoidedAffecting the stable operation of the system.
When the self-adaptive VSG control strategy provided by the invention is adopted, when the load changes, the inertia of the alternating current power grid and the direct current power grid is automatically adjusted by the system through the formulas (7) and (8), so that the impact disturbance of the alternating current-direct current hybrid micro-grid to the load when the load changes is prevented, the phenomenon that the voltage and the alternating current frequency of the direct current bus suddenly rise or fall is improved, and the safe and stable operation of the system is guaranteed. Fig. 4 is a block diagram of an adaptive virtual synchronous generator control. The method comprises the steps of respectively representing a judgment value, a threshold value, fixed inertia and adaptive inertia in an adaptive control module through real-time input actual direct current voltage and alternating current frequency, and finally realizing that the system inertia is a fixed value when frequency and voltage changes are smaller than the threshold value and is an adaptive value when the frequency and voltage changes are larger than the threshold value through an if Function in an MATLAB Function module. As can be seen from the simulation results shown in fig. 5-12, in both the rectification and inversion operation modes of the system, the system inertia is correspondingly increased according to the deviation degree when the frequency deviates from the rated value, and is correspondingly decreased according to the regression degree when the frequency regresses the rated value. When the voltage waveform generates overshoot, the inertia of the system is correspondingly increased, and the overshoot of the voltage is reduced.
Example (b):
fig. 4 is a control block diagram of a simulation system for verifying the algorithm provided by the present invention, and a corresponding MATLAB simulation model is built according to the above method, and the control parameters are shown in table 1.
TABLE 1 simulation System parameters
(1) Inversion mode
In the initial state, the alternating current frequency and the direct current voltage operate at rated values, and the transmission power of the interface converter is zero. When t is 3s, the alternating current side inputs 10kW load; when t is 4.5s, the load of 10kW is cut off on the AC side. After the load is increased, the direct current voltage is reduced by 5V, the alternating current frequency is reduced by 0.2Hz, the alternating current micro-grid and the direct current micro-grid respectively bear 5kW of alternating current load increment, 5kW of active power flows from the direct current micro-grid to the alternating current micro-grid in the interface converter, and at the moment, the interface converter works in an inversion mode. Fig. 5-8 are simulation waveforms comparing the ac/dc hybrid microgrid adaptive VSG control with the conventional VSG control in the inversion mode.
Fig. 5 and 6 show the frequency deviation time, the regression time and the corresponding rotational inertia under two control strategies. In a traditional VSG control strategy, the rotational inertia is a fixed value, the inertia and the response speed are constant, and the deviation time and the regression time are both 0.86s when a system switches loads. When the adaptive VSG control strategy presented herein is employed, the system has a shorter regression time of 0.69s and a longer departure time of 1.07s as compared to the conventional VSG control strategy. I.e. in fig. 6, the frequency is greater than the fixed moment of inertia J 0 Self-adaptive moment of inertia [1.5-2.16 ]]Slowly off-rating; to be less than a fixed moment of inertia J 0 Self-adaptive moment of inertia of 0.52-1.5]And rapidly returning to the rated value.
Fig. 7 and 8 show the dc voltage overshoot and the corresponding virtual capacitance under two control strategies. In a traditional VSG control strategy, a virtual capacitor is a fixed value, inertia is small, overshoot is large, and after sudden change of a load, the overshoot of direct-current voltage is 7.4V. When the self-adaptive VSG control strategy provided by the text is adopted, compared with the traditional VSG control strategy, the overshoot of the direct current bus voltage is small when the system switches the load, and the overshoot is respectively 4.1V and 4.2V. That is, in FIG. 8, the DC voltage is larger than the fixed virtual capacitor C v0 Adaptive virtual capacitance of [0.18-0.281 ]]Slowly off-rating.
(2) Commutation pattern
In the initial state, the alternating current frequency and the direct current voltage operate at rated values, and the transmission power of the interface converter is zero. When t is 3s, the direct current side inputs 10kW load; the load of 10kW is cut off on the direct current side of 4.5 s. After the load is increased, the direct current voltage is reduced by 5V, the alternating current frequency is reduced by 0.2Hz, the alternating current micro-grid and the direct current micro-grid respectively bear 5kW of direct current load increment, 5kW of active power flows from the alternating current micro-grid to the direct current micro-grid in the interface converter, and at the moment, the interface converter works in a rectification mode. Fig. 9-12 are simulation waveforms comparing the ac/dc hybrid microgrid adaptive VSG control with the conventional VSG control in the rectification mode.
Fig. 9 and 10 show the frequency deviation time, the regression time and the corresponding rotational inertia under two control strategies. In a traditional VSG control strategy, the rotational inertia is a fixed value, the inertia and the response speed are constant, and the deviation time and the return time are both 0.85s when a system switches loads. When the adaptive VSG control strategy provided by the invention is adopted, compared with the traditional VSG control strategy, the system has shorter regression time of 0.67s and longer deviation time of 1.12 s. That is, in FIG. 10, the frequency is greater than the fixed moment of inertia J 0 Self-adaptive moment of inertia [1.5-2.18 ]]Slowly off-rating; to be less than a fixed moment of inertia J 0 Self-adaptive moment of inertia [0.53-1.5 ]]And rapidly returning to the rated value.
Fig. 11 and 12 show dc voltage overshoot and corresponding virtual capacitance magnitudes under two control strategies. In a traditional VSG control strategy, a virtual capacitor is a fixed value, inertia is small, overshoot is large, and after sudden change of a load, the overshoot of a direct-current voltage is 6.2V. When the self-adaptive VSG control strategy provided by the invention is adopted, compared with the traditional VSG control strategy, the overshoot of the direct current bus voltage is smaller and is respectively 2.9V and 3.1V when the system switches the load. That is, in FIG. 12, the DC voltage is larger than the fixed virtual capacitance C v0 Adaptive virtual capacitance of [0.18-0.282 ]]Slowly off-rating.
Compared with the traditional VSG control, the AC/DC hybrid micro-grid controlled by the self-adaptive VSG has better frequency and voltage fluctuation inhibition performance no matter the micro-grid runs in a rectification mode or an inversion mode, the running stability of a system is improved, and the flexibility of the self-adaptive virtual synchronous machine technology in the aspect of control is reflected.
The detailed description of the present invention is merely exemplary in nature and is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and modifications and variations which will be apparent to those skilled in the art are intended to be included within the scope of the invention.
Claims (1)
1. A hybrid microgrid IC self-adaptive control strategy based on an improved VSG technology is characterized by comprising the following contents:
VSG control principle of hybrid microgrid
1) AC frequency-DC voltage control
Equation (1) is the equation of motion of the rotor of the virtual synchronous generator without considering the damping winding action:
in the formula: j is rotational inertia; p m 、P e The output power and the electromagnetic power of the prime motor are respectively; omega, omega n The angular speed is AC angular speed and rated angular speed; delta is a power angle; k is a radical of ω Is an AC primary frequency modulation droop coefficient;
according to the formula (1), in the dynamic adjustment process of the alternating current and direct current micro-grid, the instantaneous power balance characteristic is the formula (2) -formula (3);
in the formula: delta P e 、ΔP dc The active power variation of the AC and DC sub-networks respectively; u. of dc 、U dcn Actual direct current voltage and rated direct current voltage are respectively; c dc Is a direct current capacitor; k is a radical of u Is the direct current droop coefficient;
from the power balance perspective, when the load of the hybrid micro-grid increases or decreases, after the load is adjusted by the IC, i.e., the interface converter, in a steady state, the active power variation of the ac and dc sub-networks is the same, and the total active power variation in the hybrid micro-grid is assumed in a balanced manner, i.e.:
ΔP e ＝ΔP dc (4)
substituting the formulas (2) and (3) into the formula (4) to obtain:
obtaining control correlation of alternating current frequency-direct current voltage according to the formula (5), wherein in a virtual synchronous machine control strategy of the alternating current-direct current hybrid microgrid interface converter, an alternating current frequency-direct current voltage control part simulates a rotor motion equation to adjust active power, and finally, an output signal of a control link is virtual potential
Frequency and phase of;
2) virtual excitation control
The control strategy of the virtual synchronous generator simulates an excitation control system, adjusts the reactive power and controls the link to output the virtual potential
The control equation is:
E＝E ref +k q (Q ref -Q) (6)
in the formula: e ref Is VSG no-load electromotive force; e is the virtual potential effective value; kq is a reactive-voltage droop coefficient; q ref Is a VSG reactive power reference value; q is the actual value of the VSG reactive power;
AC/DC hybrid microgrid self-adaptive virtual inertia control strategy
1) Adaptive virtual moment of inertia control strategy
When the system is disturbed, the frequency and the power can vibrate, the self-adaptive virtual rotational inertia control can improve the frequency response speed of the alternating current-direct current hybrid micro-grid at the initial stage of load change and limit the vibration degree in the frequency adjustment process of the system, and the expression is formula (7),
in the formula: j. the design is a square 0 The value of the rotational inertia of the system in a steady state is obtained; d ω/dt is the rate of change of the AC frequency; omega is the actual frequency of the alternating current system; omega n Is a rated alternating current frequency; k is a radical of 1 、k 2 The dynamic characteristics of the system frequency can be changed by adjusting the influence factors of each item; k is a decision threshold;
when | Δ ω (d ω/dt) & gtY<k, wherein Δ ω ═ ω - ω n When the system judges that the variable quantity is in the set threshold value range, the system rotational inertia takes a steady state value J 0 Frequent switching of parameters can be effectively avoided to keep the stable operation of the system; when the system judges that the variation reaches a set threshold, the moment of inertia J 0 Is an expression containing frequency change rate and frequency difference, when the two are in the same sign, the frequency is in a deviation state, the coefficient is positive, the moment of inertia is increased, and the value is greater than the fixed moment of inertia J 0 Suppressing frequency deviation; when the two have different signs, the frequency is in a regression state, the coefficient is negative, the rotational inertia is reduced, and the value is smaller than the fixed rotational inertia J 0 Accelerating frequency regression;
2) adaptive virtual capacitance control strategy
A plurality of variables in the alternating current system and the direct current system have one-to-one correspondence, and an adaptive virtual capacitance control strategy similar to the form of an equation (7) is provided as an equation (8) through an analog reasoning method:
in the formula: c v Is a virtual capacitor; c v0 Is the virtual capacitance value of the system in steady state; dU dc The/dt is the change rate of the direct current bus voltage; k is a radical of 0 Setting a threshold value; u shape dc Is the actual dc bus voltage; u shape dcn Rated voltage for the direct current bus; k is a radical of 3 、k 4 Respectively the influence factors of each item;
the virtual capacitance value can reflect the inertia of the direct-current microgrid when the value is | delta u dc (du dc /dt)|<k 0 Wherein Δ u dc ＝u dc -u dcn When the system judges that the variation is within the set threshold range, the virtual capacitance is equal to a fixed value C v0 The frequent switching of parameters can be avoided, and the normal operation of the system can be kept; when the system judges that the variation reaches the set threshold k 0 Time, virtual capacitance C v No longer constant, is an expression containing the rate of change of the DC voltage and the difference between the DC voltage, and according to the formula (8), the virtual capacitor C v The voltage of the direct current bus is positively correlated with the voltage change of the direct current bus, when the actual voltage value is not equal to the rated voltage value, the virtual capacitor is increased, and the value of the virtual capacitor is larger than that of the fixed virtual capacitor C v0 The quality of the direct current voltage is improved, and the influence of larger fluctuation on the stable operation of the system is avoided;
third, adaptive VSG control algorithm flow
Firstly, inputting an initial value in a self-adaptive VSG control module, detecting the direct current voltage and the alternating current frequency in real time, calculating whether a judgment value exceeds a threshold value or not by calculating the difference value of an actual value and the initial value when a system generates load fluctuation, and finally outputting virtual inertia of a corresponding numerical value,
the first step is as follows: inputting an initial value in an adaptive VSG control module;
the second step: respectively inputting and cutting loads on an alternating current side or a direct current side;
the third step: detecting the direct current voltage and the alternating current frequency in real time;
the fourth step: calculating the difference and derivative terms of voltage and frequency;
the fifth step: calculating whether the judgment value exceeds a threshold value;
and a sixth step: if the judgment value is within the threshold value, the rotary inertia and the virtual capacitance value are fixed values; if the judgment value exceeds the threshold range, the rotational inertia and the virtual capacitance are the provided adaptive values;
the seventh step: and outputting the obtained AC and DC inertia values.
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