CN113517724B - Method for suppressing voltage ripple on direct current side of alternating current-direct current hybrid micro-grid - Google Patents
Method for suppressing voltage ripple on direct current side of alternating current-direct current hybrid micro-grid 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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
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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
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/02—Arrangements for reducing harmonics or ripples
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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
Abstract
The invention belongs to the field of power systems, and particularly relates to a method for inhibiting voltage ripples on a direct current side of an alternating current-direct current hybrid micro-grid. Aiming at the problem of frequency doubling ripple at a direct current side 2 caused by the unbalanced voltage condition of an alternating current sub-network, the method provided by the invention cooperatively considers the alternating current side and the direct current side of the alternating current-direct current hybrid micro-grid, and inhibits the direct current voltage ripple at the alternating current side for one time, namely a negative sequence component compensation strategy is used; the direct current voltage ripple is secondarily restrained on the direct current side, namely a modified direct current active filter (DC-APF) is used, and the modified DC-APF uses a quasi-proportional resonant controller (QPR) to accurately track the direct current side 2 frequency doubling pulsation. Meanwhile, an alternating current-direct current hybrid micro-grid model system is built by using MATLAB/Simulink, and simulation results show that compared with the traditional method of simply controlling on the alternating current side or the direct current side, the suppression method can reduce the ripple content of the direct current voltage by about 4 times, so that the fluctuation of the bus voltage on the direct current side of the alternating current-direct current hybrid micro-grid can be effectively suppressed.
Description
Technical Field
The invention belongs to the field of power systems, and particularly relates to a method for inhibiting voltage ripples on a direct current side of an alternating current-direct current hybrid micro-grid.
Background
The alternating current-direct current hybrid micro-grid is considered as a development direction of a future smart grid because of the reliability, the flexibility and the capability of greatly absorbing renewable energy, and has own characteristics compared with an alternating current micro-grid and a direct current micro-grid, the direct current side voltage fluctuation of the alternating current-direct current hybrid micro-grid is a subject worth deep research and discussion, and the direct current side voltage fluctuation causes the direct current side voltage bus to lose stability, so that the stable operation of a load can be influenced, the protection action of the alternating current-direct current hybrid micro-grid is caused, and the normal operation of the power grids on the two sides of the alternating current and the direct current is influenced.
The voltage fluctuation on the direct current side of the alternating current-direct current hybrid micro-grid can be divided into disturbance type fluctuation and oscillation type fluctuation, wherein the main reasons for the disturbance type fluctuation are the fluctuation of loads on a direct current bus, the power fluctuation of a distributed power supply and the fluctuation of transmission power between the alternating current-direct current micro-grids, and the main reasons for the oscillation type fluctuation are the voltage unbalance on the alternating current side of the alternating current-direct current hybrid micro-grid, the power grid fault on the alternating current side, harmonic waves generated by nonlinear loads on the alternating current side and the cascade connection and the parallel connection of power electronic devices.
In an alternating current-direct current hybrid micro-grid, the voltage fluctuation of a direct current side bus is generally stabilized by increasing a direct current bus capacitor, the direct current bus capacitor generally adopts an electrolytic capacitor, but the large-scale and large-range use of the electrolytic capacitor is restricted by the large volume, the low power density and the short service life of the electrolytic capacitor. At present, voltage disturbance type fluctuation on a direct current side is developed on a large scale, mainly by accelerating disturbance detection and compensation speed of related power electronic equipment on direct current voltage fluctuation, a current feedforward control method, a power feedforward control method and a feedforward optimization control method are mainly provided, and a large number of observers are used in further research aiming at the control methods mainly for reducing the number of sensors and enhancing expandability of a microgrid.
At present, the oscillation type fluctuation of the voltage on the direct current side is not fully developed, the situation that the voltage on the direct current side of an alternating current sub-network is unbalanced frequently exists during normal operation, secondary ripples can be generated on the voltage on the direct current side of an alternating current and direct current hybrid micro-grid under the situation that the voltage on the direct current side is unbalanced, at present, researchers mainly solve the problem from the direct current side, and do not consider the suppression of the frequency multiplication fluctuation of the alternating current and direct current hybrid micro-grid 2 from the alternating current and direct current sides at the same time.
Disclosure of Invention
The invention provides a method for suppressing voltage ripples of a direct current side of an alternating current-direct current hybrid micro-grid, aiming at the problems that the alternating current sub-grid and a direct current sub-grid can transmit power with frequency multiplication of 2 under the condition of unbalanced voltage of the alternating current sub-grid of the alternating current-direct current hybrid micro-grid, and meanwhile, frequency multiplication ripple voltage can be generated on a direct current voltage bus, so that the stable operation of a load can be influenced, the protection action of the alternating current-direct current hybrid micro-grid can be caused, the normal operation of power grids on the two sides of alternating current and direct current can be influenced, and the like. The method is based on the synergistic consideration of the alternating current side and the direct current side of the alternating current-direct current hybrid micro-grid, the direct current voltage ripple is subjected to primary suppression (namely a negative sequence component compensation strategy) on the alternating current side, because the double frequency pulsation caused by the unbalanced alternating current micro-grid to the bus voltage of the direct current sub-grid is subjected to secondary suppression (namely an improved direct current active filter (DC-APF)) on the direct current side, the voltage stabilization of the direct current side is taken as a control target in the whole method, and the direct current voltage ripple of the alternating current-direct current hybrid micro-grid is suppressed.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention provides an alternating current-direct current hybrid microgrid model system which comprises: the system comprises a direct current sub-network, an alternating current converter DC/AC and a filter, wherein the alternating current sub-network is connected with the alternating current sub-network through the alternating current-direct current converter and the filter LCL; the alternating current sub-network comprises a system distributed power supply, a converter DC/AC, an alternating current bus and an alternating current load, wherein the system distributed power supply is connected to the alternating current bus through the DC/AC converter, and the alternating current load is connected to the alternating current bus.
Further, the establishment of the mathematical model of the converter specifically comprises the following steps:
and 2, converting the three-phase abc coordinate system in the step 1 into a dq rotation coordinate system by using park transformation: wherein i 1A 、i 1B 、i 1C Conversion to i in dq coordinate system 1d 、i 1q ；i 2A 、i 2B 、i 2C Conversion to i in dq coordinate system 2d 、i 2q ；U A 、U B 、U C Converted to U under dq coordinate system d 、U q ；U cA 、U cB 、U cC Conversion to U in dq coordinate system cd 、U cq ；U gA 、U gB 、U gC Conversion to U in dq coordinate system gd 、U gq ；L 1A ＝L 1B ＝L 1C ＝L 1 ；L 2A ＝L 2B ＝L 2C ＝L 2 ；C A ＝C B ＝C C = C; w is the AC sub-network voltage angular frequency;
the invention also provides a method for inhibiting the direct-current voltage ripple of the alternating-current and direct-current hybrid microgrid, which comprises the following steps:
and 2, secondarily suppressing the direct-current voltage ripple on the direct-current side, namely suppressing by using an improved direct-current active filter.
Further, in the step 1, the method for performing primary suppression on the dc voltage ripple on the ac side specifically includes the following steps:
(1) Decomposing the alternating-current sub-network voltage and the alternating-current sub-network current into positive and negative sequence components under an alpha beta coordinate system by using an all-pass filter method, and converting the positive and negative sequence components under the alpha beta coordinate system into positive and negative sequence components under a dq coordinate system;
(2) The alternating current sub-network voltage and the merged alternating current sub-network current are controlled by restraining power fluctuation, positive and negative sequence component reference values of the merged alternating current sub-network current under a dq axis are calculated by combining positive and negative sequence components of the dq axis of the alternating current sub-network voltage, and a direct current voltage outer ring is used for generating a power reference value through PI control;
(3) The positive sequence component of the alternating current sub-network voltage and the current merged into the alternating current sub-network is controlled by PI double closed loops to obtain a corresponding positive sequence dq axis component, the negative sequence component is controlled by the same PI double closed loops to obtain a corresponding negative sequence dq axis component, the negative sequence component is superposed into a control loop according to a control method of the positive sequence component, and the negative sequence component flowing into the alternating current sub-network by a converter is inhibited by SVPWM modulation, so that 2-frequency multiplication pulsation caused by the unbalance of the alternating current sub-network on the side of the direct current sub-network is inhibited.
Further, the method for performing secondary suppression on the dc voltage ripple at the dc side in step 2 specifically includes the following steps: and transferring the power fluctuated by the direct-current bus into the direct-current active filter or transferring the power in the direct-current active filter into the direct-current bus by using the improved direct-current active filter on the direct-current sub-network side of the converter so as to maintain the stability of the direct-current bus.
Further, the specific calculation process of step (1) is as follows:
under the condition of unbalanced voltage of the alternating-current sub-network of the alternating-current and direct-current hybrid micro-grid, the harmonic component is not considered, and the three-phase voltage of the alternating-current sub-network can be expressed as follows:
wherein U is gA 、U gB 、U gC For three-phase voltage, U, of AC sub-network + 、U - Positive and negative sequence component amplitudes of the ac sub-network voltage respectively,
further, the ac sub-network positive and negative sequence voltages can be written as follows:
for convenience of analysis, let θ =0 °, the positive and negative sequence components of the ac sub-network voltage in the α β stationary coordinate system can be obtained by performing clark transformation on equations (6) and (7):
wherein
To be handed overThe positive sequence component of the current sub-grid voltage in the alpha beta coordinate system,
is the negative sequence component of the AC sub-network voltage under the alpha beta coordinate system.
Under the condition that the voltage of the alternating-current sub-network of the hybrid micro-grid is unbalanced, the voltage of the alternating-current sub-network and the current merged into the alternating-current sub-network are as follows under an alpha beta static coordinate system:
wherein
To incorporate the positive sequence component of the ac sub-network current in the alpha beta coordinate system,
to incorporate the negative sequence component of the alternating sub-network current in the α β coordinate system.
Converting the alternating sub-network voltage and the current merged into the alternating sub-network from the alpha beta coordinate system to the dq rotating coordinate system as follows:
in the formula (I), the compound is shown in the specification,
wherein the content of the first and second substances,
for the ac sub-network voltage and the positive sequence component incorporated into the ac sub-network current in the dq coordinate system,
for the AC sub-network voltage and the negative sequence component incorporated in the AC sub-network current in dq coordinate system, e jwt Is a twiddle factor.
The specific calculation process of (2) is as follows:
the complex power available from the instantaneous power is:
the combined vertical type (11) and the combined vertical type (12) are solved to obtain the transmission active power P and the reactive power Q of the alternating current sub-network and the direct current sub-network of the alternating current and direct current hybrid micro-grid as follows:
wherein, P 0 And Q 0 Respectively an active and a reactive DC component, P 2(1) And P 2(2) Is the active frequency-doubling sine-cosine component amplitude, Q 2(1) And Q 2(2) For the reactive double frequency sine and cosine component amplitude, each component is calculated as follows:
in order to control the condition that the alternating-current sub-network voltage of the alternating-current and direct-current hybrid micro-grid is unbalanced, because a converter adopts a grid voltage fixed vector control mode, the method comprises the following steps
Then there are:
with the objective of active power suppression, there are:
according to equation 16, the reference value of the positive and negative sequence currents in the dq axis coordinate system is:
wherein P is 0ref As active power reference value, Q 0ref In order to be the reference value for the reactive power,
in order to access the d-axis reference current of the positive sequence of the alternating-current microgrid,
in order to access the q-axis reference current of the positive sequence of the alternating-current microgrid,
for accessing the negative sequence d-axis reference current of the alternating-current microgrid,
in order to access the negative sequence q-axis reference current of the alternating-current microgrid,
set dq synchronous rotation coordinate and AC sub-network voltage U g And synchronizing, wherein the active power and the reactive power transmitted by the converter according to the instantaneous power theory are as follows:
because of the use of voltage vector control, U gq =0, then equation 18 can be simplified to:
if the active power input by the direct current side of the converter is as follows: p = U dc I dc Irrespective of the loss of the converter itself, there are
DC side voltage U of converter dc And the output current I of the inverter gd In direct proportion, the active power P and the current I gd Also becomeIs proportional, so that the output current I can be controlled gd To control the output active power P and the DC side voltage U dc (ii) a Therefore, a direct-current voltage outer ring is introduced, direct-current voltage closed-loop control is formed through PI regulation, and the output quantity is the active power reference value P 0ref Setting the power factor of the converter to 1, the reference value Q of reactive power 0ref ＝0。
The improved direct current active filter is a quasi-proportional resonant controller which can better track double frequency pulsation by changing a traditional proportional controller.
The method for inhibiting the direct-current voltage ripple twice specifically comprises the following steps:
firstly, the actual DC bus voltage U is measured dc The voltage obtained by the low-pass filter and the actual DC bus voltage U dc Obtaining a direct current bus voltage ripple component delta U by difference dc ；
DC bus voltage ripple component delta U dc Obtaining a direct current bus capacitor ripple wave reference current i through a quasi-proportional resonant controller according to the value after zero comparison c1ref ；
DC active filter compensation capacitor C 2 Voltage U of dc2 Compensating capacitor C of DC active filter 2 After the reference voltage is compared, the reference voltage is regulated by a proportional-integral controller to obtain a compensation capacitor reference current i of the direct current active filter c2ref ；
Ripple reference current i of bus capacitor c1ref And compensating capacitor reference current i of DC active filter c2ref Sum and inductor current i L And the difference is regulated by a proportional-integral controller and then is compared with a PWM carrier to be used as a Q1 and Q2 complementary modulation signal to control the on and off of Q1 and Q2, so that whether the power fluctuating by the direct current bus is transferred into a direct current active filter or the power in the direct current active filter is transferred into the direct current bus to maintain the stability of the direct current bus is determined, and the direct current ripple is restrained.
Compared with the prior art, the invention has the following advantages:
(1) According to simulation results, the negative sequence component compensation method is used for primary suppression of the direct current ripple, the improved DC-APF is used for secondary suppression of the direct current ripple, the coordinated operation of the suppression of the alternating current ripple and the direct current ripple is realized, according to analysis results, the direct current ripple is reduced from 10V voltage fluctuation to 0.5V voltage fluctuation, active power fluctuation is reduced from 3.29kW to about 0.38KW, and the effectiveness and the feasibility of the suppression method are verified.
(2) The method only starts from the control layer, does not increase primary equipment, and can greatly reduce the treatment cost of the direct current ripple waves.
(3) The quasi-proportional resonant controller (QPR) is selected, so that the advantage of high gain of the proportional resonant controller (PR) at the resonant frequency is kept, the bandwidth with high gain is widened, and a good tracking effect can be achieved when the DC voltage ripple deviates.
Drawings
FIG. 1 is a diagram of an AC/DC hybrid microgrid model system.
Fig. 2 is a diagram of an inverter model according to the present invention.
Fig. 3 is a block diagram of inverter control according to the present invention.
FIG. 4 is a schematic diagram of an all-pass filter extracting positive and negative sequence components.
Fig. 5 is a voltage orientation vector diagram.
Fig. 6 is a negative sequence component compensation control schematic diagram.
Fig. 7 is a bode diagram of QPR when Kr =3,kp =1,10,100.
Fig. 8 is a QPR bode plot for Kp =1,kr =3,30,300.
Fig. 9 is a model diagram of a dc active filter.
Fig. 10 is a control block diagram of the dc active filter.
Fig. 11 is a dc voltage waveform diagram of simulation case 1.
Fig. 12 is a diagram of dc ripple FFT analysis for simulation case 1.
Fig. 13 is a diagram of an active power waveform of simulation scenario 1.
Fig. 14 is a dc voltage waveform diagram of simulation case 2.
Fig. 15 is a graph of the active power waveform of simulation scenario 2.
Fig. 16 is a dc voltage waveform diagram of simulation case 3.
Fig. 17 is a graph of the active power waveform of simulation scenario 3.
Fig. 18 is a dc voltage waveform diagram of simulation case 4.
Fig. 19 is a graph of an active power waveform for simulation scenario 4.
Fig. 20 simulates an FFT analysis plot of the integrated ac sub-network current for case 4.
Detailed Description
The technical solution in the embodiments of the present invention will be specifically and specifically described below with reference to the embodiments of the present invention and the accompanying drawings. It should be noted that variations and modifications can be made by those skilled in the art without departing from the principle of the present invention, and these should also be construed as falling within the scope of the present invention.
Example 1
An alternating current-direct current hybrid microgrid model system (fig. 1): the system comprises a direct current sub-network, an alternating current-direct current converter DC/AC and a filter, wherein the alternating current sub-network is connected with the alternating current sub-network through the alternating current-direct current converter DC/AC and the filter LCL; the direct-current sub-network comprises a distributed power supply, a converter DC/DC, a direct-current load, a direct-current active filter and a direct-current bus, the system distributed power supply is connected to the direct-current bus through the DC/DC converter, and the direct-current load and the direct-current active filter are connected to the direct-current bus; the alternating current sub-network comprises a system distributed power supply, a DC/AC, an alternating current bus and an alternating current load, wherein the system distributed power supply is connected to the alternating current bus through a DC/AC converter, and the alternating current load is connected to the alternating current bus.
Example 2
Establishment of mathematical model of current converter
The converter AC side dynamic equation is established according to the converter model in FIG. 2 as follows:
the three-phase abc coordinate system is converted to a dq rotation coordinate system by using park transformation for the dynamic equation. Wherein i 1A 、i 1B 、i 1C Conversion to i in dq coordinate system 1d 、i 1q ；i 2A 、i 2B 、i 2C Conversion to i in dq coordinate system 2d 、i 2q ；U A 、U B 、U C Conversion to U in dq coordinate system d 、U q ；U cA 、U cB 、U cC Conversion to U in dq coordinate system cd 、U cq ；U gA 、U gB 、U gC Converted to U under dq coordinate system gd 、U gq ；L 1A ＝L 1B ＝L 1C ＝L 1 ；L 2A ＝L 2B ＝L 2C ＝L 2 ；C A ＝C B ＝C C = C; w is the AC sub-network voltage angular frequency;
fig. 3 is a converter control block diagram.
Example 3
Method for suppressing direct-current voltage ripple of alternating-current and direct-current hybrid microgrid
1. The method specifically comprises the following steps of inhibiting the DC voltage ripple at the AC side for one time:
the specific principle of the all-pass filter method is as follows: first, the unbalanced component X is shifted to 90 DEG
In the formula
Multiplying the unbalanced component X by rho and adding or subtracting the unbalanced component X and the unbalanced component to obtain a positive sequence component and a negative sequence component, namely:
use of all-pass filter in simulation process
Instead of-p. The principle structure is shown in figure 4.
According to the principle of the all-pass filter method, under the condition that the alternating current and direct current hybrid microgrid alternating current sub-network voltage is unbalanced (no harmonic component is considered), the three-phase voltage of the alternating current sub-network can be expressed as follows:
wherein U is + 、U - Positive and negative sequence component amplitudes of the ac sub-network voltage respectively,
further, the ac sub-network positive and negative sequence voltages can be written as follows:
for convenience of analysis, let θ =0 °, the positive and negative sequence components of the ac sub-network voltage in the α β stationary coordinate system can be obtained by performing clark transformation on equations (6) and (7):
under the condition that the voltage of the alternating-current sub-network of the hybrid micro-grid is unbalanced, the voltage and the current of the alternating-current sub-network in the alpha beta static coordinate system are as follows:
converting the voltage and the current of the alternating current sub-network from an alpha beta coordinate system to a dq rotating coordinate system as follows:
in the formula (I), the compound is shown in the specification,
the complex power available from the instantaneous power is:
wherein ". Sup." represents a complex conjugate.
The combined vertical type (11) and the combined vertical type (12) are solved to obtain that the active power and the reactive power transmitted by the alternating current sub-network and the direct current sub-network of the alternating current and direct current hybrid micro-grid are as follows:
wherein, P 0 And Q 0 Respectively, an active and a reactive DC component, P 2(1) And P 2(2) Is the active double frequency sine and cosine component amplitude, Q 2(1) And Q 2(2) For the reactive double frequency sine and cosine component amplitude, the component calculation method is as follows:
in order to control the AC sub-network voltage unbalance condition of the AC/DC hybrid micro-grid, the converter adopts a grid voltage fixed vector control mode, so that the method comprises the following steps
Then there are:
with the objective of suppressing active power, there are:
the reference values of the positive and negative sequence currents in the dq axis coordinate system can be obtained according to equation 16:
wherein P is 0ref As active power reference value, Q 0ref In order to be the reference value for the reactive power,
for accessing the positive sequence d-axis reference current of the alternating-current microgrid,
in order to access the positive sequence q-axis reference current of the alternating-current microgrid,
for accessing the negative sequence d-axis reference current of the alternating-current microgrid,
the negative-sequence q-axis reference current is accessed to the alternating-current microgrid.
Because the current at the AC side of the converter often contains abundant harmonic waves with different frequencies in the actual operation process, the PLL adopts directionallyAnd the grid voltage fundamental wave is used for orientation, so that the dynamic response of the phase-locked loop is improved. Set dq synchronous rotation coordinate and AC sub-network voltage U g And the active power and the reactive power transmitted by the converter according to the instantaneous power theory are as follows:
u because of the use of voltage vector control gq =0, then equation 18 can be simplified to:
if the active power input by the direct current side of the converter is as follows: p = U dc I dc Regardless of the loss of the converter itself, there are
DC side voltage U of converter dc And the output current I of the inverter gd In direct proportion, the active power P and the current I gd Is also proportional, so that the output current I can be controlled gd To control the output active power P and the DC side voltage U dc 。
Therefore, a direct-current voltage outer ring is introduced, direct-current voltage closed-loop control is formed through PI regulation, and the output quantity is the active power reference value P 0ref Setting the power factor of the converter to 1, the reference value Q of reactive power 0ref ＝0。
And 3, carrying out PI double closed-loop control on the positive sequence component of the alternating current sub-network voltage and the current merged into the alternating current sub-network to obtain a corresponding positive sequence dq axis component, carrying out the same PI double closed-loop control on the negative sequence component to obtain a corresponding negative sequence dq axis component, superposing the negative sequence component into a control loop according to a control method of the positive sequence component, and inhibiting the negative sequence component of a current converter flowing into the alternating current sub-network through SVPWM (space vector pulse width modulation), thereby inhibiting 2-frequency multiplication pulsation caused on the direct current sub-network side due to the unbalance of the alternating current sub-network. The specific control schematic diagram is shown in fig. 6.
2. The secondary suppression of the direct-current voltage ripple specifically comprises the following steps: the improved direct current active filter is used for transferring power fluctuated by the direct current bus into the direct current active filter or transferring power in the direct current active filter into the direct current bus so as to maintain stability of the direct current bus. The improved direct current active filter is a quasi-proportional resonant controller (QPR) which can better track double frequency pulsation by changing a traditional proportional controller (PI).
The quasi-proportional resonant controller (QPR) has a transfer function of:
wherein K is p Is a proportionality coefficient, K R Is the resonance coefficient, w 0 At a resonant angular frequency, w c Is the cut-off frequency.
Fig. 7 is a bode plot of QPR when Kr =3,kp =1,10,100, and it can be seen that the gain of QPR at the resonance point is larger as Kp is smaller at a certain time of Kr, and fig. 8 is a bode plot of QPR when Kp =1,kr =3,30,300, and it can be seen that the gain of QPR at the resonance point is larger as Kr is larger at a certain time of Kp. Therefore, the simulation analysis of the present invention adopts QPR parameters as follows: k p ＝1，K r ＝300，w c ＝8，w 0 ＝200πrad/s。
FIG. 9 is a schematic diagram of a DC active filter, U for DC bus voltage dc C for DC bus capacitance 1 Showing that the DC active filter is used for compensating the capacitor C 2 The Insulated Gate Bipolar Transistor (IGBT) Q1 and the diode D2 constitute a step-down chopper (buck) circuit, and the Insulated Gate Bipolar Transistor (IGBT) Q2 and the diode D1 constitute a step-up chopper (boost) circuit. Instantaneous value U of DC bus voltage dc With given value U of DC bus voltage dcref Comparing to determine the operation mode of DC-APF when U is higher than dc ＞U dcref When the DC-APF works in buck mode, the fluctuating power of the direct current bus will be transferred to the compensating capacitor C of the direct current active filter 2 Performing the following steps; when U is turned dc ≤U dcref And the DC-APF works in a boost mode, and the compensation capacitor of the DC active filter can compensate the missing fluctuating power of the DC bus to maintain the stability of the DC bus voltage.
The method specifically comprises the following steps: firstly, the actual DC bus voltage U is dc The voltage obtained by the low-pass filter and the actual DC bus voltage U dc Obtaining a direct current bus voltage ripple component delta U by difference dc ，
DC bus voltage ripple component delta U dc Obtaining the reference current i of the direct current bus capacitor ripple through QPR according to the value after being compared with zero c1ref
DC active filter compensation capacitor C 2 Voltage U of dc2 Compensating capacitor C of DC active filter 2 After the reference voltage is compared, the compensation capacitor reference current i is obtained after PI regulation c2ref
Ripple the reference current i of the bus capacitor c1ref And compensating capacitor reference current i of DC active filter c2ref Sum and inductor current i L And comparing the difference with a PWM carrier after PI regulation to serve as a Q1 and Q2 complementary modulation signal to control the on and off of Q1 and Q2, so that whether the power fluctuating by the direct current bus is transferred into the direct current active filter or the power in the direct current active filter is transferred into the direct current bus to maintain the stability of the direct current bus, and the direct current ripple is restrained. The specific control schematic diagram is shown in fig. 10.
Simulation analysis
An alternating current-direct current hybrid microgrid simulation model system is built on an MATLAB/SIMULINK simulation test platform, the schematic diagram of the model system is shown in figure 1, and the parameters of the simulation model are shown in the following table:
TABLE 1 model System principal parameters
The ripple factor calculation formula used in the analysis herein is:
u in formula (20) up Is the peak value of the DC voltage wave, U low Direct currentValley of voltage, U average Is the average value of the direct current voltage, and delta U is the ripple factor.
In simulation case 1, the three-phase voltage of the ac sub-network is set to be the a-phase voltage at 0.5 s: 0.9pu, B phase voltage: 0.8pu and C phase voltage are 0.45pu, and the active power transmitted by the converter is 24.5kW (the three-phase unbalanced voltage and the active power are set in the following simulation situations, and the active power of the converter is a negative value when the converter works in a rectification mode, otherwise, the active power is a positive value). The system does not adopt any suppression strategy, the simulation waveform is shown in fig. 11, the direct current voltage fluctuation amplitude is about 10V without any suppression strategy, the direct current ripple coefficient delta U is 1.43%, and the active power fluctuation amplitude is 3.29kW when the alternating current sub-network voltage is unbalanced as shown in fig. 13. Fig. 12 shows that the dc voltage ripple is mainly 2 frequency-doubled ripple when the ac sub-network is unbalanced, and the dc voltage frequency-doubled ripple is coupled to the ac side of the converter through the converter, and 3 times of harmonic is generated on the ac side, so that the distortion of the current merged into the ac sub-network affects the power quality of the system, and the total harmonic distortion THD =9.96% of the current merged into the ac sub-network can be shown by performing fourier analysis on the current merged into the ac sub-network.
In simulation case 2, the system adopts the improved DC-APF to suppress the DC voltage double frequency ripple and compares the DC voltage double frequency ripple with the conventional DC-APF, and starts the DC-APF adopting PI control and QPR control to suppress the DC voltage at 0.7s, as shown in fig. 14, when the DC-APF adopts PI control, the DC voltage fluctuation amplitude is reduced from 10V to about 6V, and the ripple coefficient δ U is reduced to 0.86%; when the DC-APF adopts QPR control, the direct-current voltage fluctuation amplitude is reduced to about 2V from 10V, and the ripple factor delta U is reduced to 0.29%; FIG. 15 is a comparison graph of active power waveforms of the conventional DC-APF and the improved DC-APF, and it can be seen that the active power fluctuation increases from 3.29kW to about 4.12kW when the conventional DC-APF and the improved DC-APF are both used.
In simulation case 3, the converter uses a negative sequence compensation suppression strategy to compensate the unbalanced ac sub-network voltage, incorporate the negative sequence component of the ac sub-network current to suppress the 2-fold ripple of the active power, and thereby suppress the dc voltage ripple. From fig. 13, it can be seen that the negative sequence component compensation suppression strategy is adopted at 0.5s, and from fig. 16, it can be seen that the dc voltage fluctuation amplitude is reduced from 10V to about 3V, and the dc ripple coefficient δ U is reduced to 0.43%; from FIG. 17, it can be seen that the active power fluctuation of the negative sequence component compensation suppression strategy is reduced from 3.29kW to about 0.36kW at 0.5 s.
In simulation case 4, in combination with the advantages that the DC-APF improved in simulation case 2 can effectively suppress voltage ripples and the negative sequence component compensation suppression strategy in simulation case 3 can effectively suppress active power and can stabilize active power fluctuations, an ac/DC cooperative suppression strategy is proposed, that is, the negative sequence component compensation suppression strategy is used on the ac side of the converter as the primary suppression of DC voltage ripples, and the improved DC-APF is used on the DC side of the system as the secondary suppression strategy of DC ripples to secondarily suppress DC voltage and active power simulation waveforms of the DC ripples as shown in fig. 18 and 19: starting a negative sequence component compensation suppression strategy at 0.5s to serve as primary suppression of direct-current voltage ripples, reducing the fluctuation amplitude of the direct-current voltage from 10V to 3V, and reducing the fluctuation of active power from 3.29kW to 0.36kW; the improved DC-APF is used as the secondary suppression of the direct-current voltage ripple at 0.7s, the fluctuation amplitude of the direct-current voltage is reduced to about 0.5V from 3V, the ripple coefficient delta U is reduced to 0.07 percent, the direct-current voltage is basically consistent with the alternating-current sub-network when balanced, the active power fluctuation is about 0.38kW, and the active power fluctuation can be well suppressed; from fig. 20, it can be known that the Total Harmonic Distortion (THD) of the current flowing into the ac sub-network from 9.96% to 1.28% after the ac/dc cooperative suppression strategy is used.
Claims (7)
1. A method for suppressing voltage ripples on a direct current side of an alternating current-direct current hybrid micro-grid is characterized by comprising the following steps:
step 1, carrying out primary suppression on a direct-current voltage ripple at an alternating-current side, namely using a negative sequence component compensation strategy;
step 2, performing secondary suppression on the direct current voltage ripple at the direct current side, namely performing suppression by using an improved direct current active filter; the method for performing primary suppression on the direct-current voltage ripple at the alternating-current side in the step 1 specifically comprises the following steps:
step 1.1, decomposing the alternating current sub-network voltage and the current merged into the alternating current sub-network into positive and negative sequence components under an alpha beta coordinate system by using an all-pass filter method, and converting the positive and negative sequence components under the alpha beta coordinate system into positive and negative sequence components under a dq coordinate system;
step 1.2, the alternating current sub-network voltage and the current merged into the alternating current sub-network are used for restraining power fluctuation as a control target, positive and negative sequence component reference values of the current merged into the alternating current sub-network under a dq axis are calculated by combining positive and negative sequence components of the voltage dq axis of the alternating current sub-network, and a power reference value is generated by using a direct current voltage outer ring through PI control;
and step 1.3, carrying out PI double closed-loop control on the positive sequence components of the alternating current sub-network voltage and the alternating current sub-network current to obtain corresponding positive sequence dq axis components, carrying out the same PI double closed-loop control on the negative sequence components to obtain corresponding negative sequence dq axis components, superposing the negative sequence components into a control loop according to a control method of the positive sequence components, and inhibiting the negative sequence components of a current converter flowing into the alternating current sub-network through SVPWM modulation, thereby inhibiting 2-frequency-multiplication pulsation caused at the direct current sub-network side due to the imbalance of the alternating current sub-network.
2. The method for suppressing the voltage ripple on the direct current side of the alternating current-direct current hybrid micro-grid according to claim 1, wherein the method comprises the following steps: the specific calculation process of the step 1.1 is as follows:
under the condition that the alternating-current and direct-current hybrid microgrid alternating-current sub-network is unbalanced in voltage, the harmonic component is not considered, and the three-phase voltage of the alternating-current sub-network can be expressed as follows:
wherein, U gA 、U gB 、U gC For three-phase voltage, U, of AC sub-network + 、U - Positive and negative sequence component amplitudes of the ac sub-network voltage respectively,
further, the ac sub-network positive and negative sequence voltages can be written as follows:
wherein
Being the positive sequence component of the ac sub-network voltage,
being the negative-sequence component of the ac sub-network voltage,
for convenience of analysis, let θ =0 °, and the positive and negative sequence components of the ac sub-grid voltage in the α β stationary coordinate system can be obtained by performing clark transformation on equations (6) and (7):
wherein
Is a positive sequence component of the AC sub-network voltage under an alpha beta coordinate system,
is the negative sequence component of the AC sub-network voltage under the alpha beta coordinate system,
under the condition that the voltage of the hybrid microgrid alternating-current sub-network is unbalanced, the alternating-current sub-network voltage and the current merged into the alternating-current sub-network are as follows under an alpha beta static coordinate system:
wherein
To incorporate the positive sequence component of the ac sub-network current in the alpha beta coordinate system,
to incorporate the negative sequence component of the ac sub-network current in the alpha beta coordinate system,
converting the alternating sub-network voltage and the current merged into the alternating sub-network from the alpha beta coordinate system to the dq rotating coordinate system as follows:
in the formula (I), the compound is shown in the specification,
for the ac sub-network voltage and the positive sequence component incorporated in the ac sub-network current in the dq coordinate system,
for the negative-sequence component of the AC sub-network voltage and of the current, e, incorporated into the AC sub-network in the dq coordinate system jwt Is a twiddle factor.
3. The method for suppressing the voltage ripple on the direct current side of the alternating current-direct current hybrid micro-grid according to claim 2, wherein the method comprises the following steps: the specific calculation process of the step 1.2 is as follows:
the complex power available from the instantaneous power is:
wherein "+" represents a complex conjugate number,
the combined vertical type (11) and (12) are solved, and the active power P and the reactive power Q transmitted by the alternating current sub-network and the direct current sub-network of the alternating current and direct current hybrid micro-grid are as follows:
wherein, P 0 And Q 0 Respectively an active and a reactive DC component, P 2(1) And P 2(2) Is the active frequency-doubling sine-cosine component amplitude, Q 2(1) And Q 2(2) For the reactive double frequency sine and cosine component amplitude, the component calculation method is as follows:
in order to control the AC sub-network voltage unbalance condition of the AC/DC hybrid micro-grid, the converter adopts a grid voltage fixed vector control mode, so that the method comprises the following steps
Then there are:
with the objective of active power suppression, there are:
the reference values of the positive and negative sequence currents in the dq axis coordinate system can be obtained according to equation 16:
wherein P is 0ref As active power reference value, Q 0ref In order to be the reference value for the reactive power,
for accessing the positive sequence d-axis reference current of the alternating-current microgrid,
in order to access the positive sequence q-axis reference current of the alternating-current microgrid,
for accessing the negative sequence d-axis reference current of the alternating-current microgrid,
in order to access the negative sequence q-axis reference current of the alternating-current microgrid,
set dq synchronous rotation coordinate and AC sub-network voltage U g And synchronizing, wherein the active power and the reactive power transmitted by the converter according to the instantaneous power theory are as follows:
since voltage constant vector control is adopted, U gq =0, then equation 18 can be simplified to:
if the active power input by the direct current side of the converter is as follows: p = U dc I dc Regardless of the loss of the converter itself, there are
DC side voltage U of converter dc And the output current I of the inverter gd Proportional to the active power P and the current I gd Is also proportional, so that the output current I can be controlled gd To control the output of active powerPower P and DC side voltage U dc (ii) a Therefore, a direct-current voltage outer ring is introduced, direct-current voltage closed-loop control is formed through PI regulation, and the output quantity is the active power reference value P 0ref Setting the power factor of the converter to 1, the reference value Q of reactive power 0ref ＝0。
4. The method for suppressing the voltage ripple on the dc side of the ac/dc hybrid microgrid according to claim 1, characterized in that: the method for performing secondary suppression on the dc voltage ripple at the dc side in step 2 specifically includes the following steps: and (3) transferring the power fluctuated by the direct current bus into the direct current active filter or transferring the power in the direct current active filter into the direct current bus by using the improved direct current active filter on the direct current sub-network side of the converter so as to maintain the stability of the direct current bus.
5. The method for suppressing the voltage ripple on the direct current side of the alternating current-direct current hybrid micro-grid according to claim 4, wherein the method comprises the following steps: the improved direct current active filter is a quasi-proportional resonant controller which can better track double frequency pulsation by changing a traditional proportional controller.
6. The method for suppressing the voltage ripple on the direct current side of the alternating current-direct current hybrid microgrid according to claim 4, characterized in that: the method for inhibiting the direct-current voltage ripple twice specifically comprises the following steps:
firstly, the actual DC bus voltage U is measured dc The voltage obtained by the low-pass filter and the actual DC bus voltage U dc Obtaining a direct current bus voltage ripple component delta U by difference dc ；
DC bus voltage ripple component delta U dc The value after being compared with zero is used for obtaining the ripple wave reference current i of the direct current bus capacitor through a quasi-proportional resonant controller c1ref ；
DC active filter compensation capacitor C 2 Voltage U of dc2 Compensating capacitor C of DC active filter 2 The reference voltage is compared and then regulated by a proportional-integral controller to obtain a direct current active sourceFilter compensation capacitor reference current i c2ref ；
Ripple the reference current i of the bus capacitor c1ref And compensating capacitor reference current i of DC active filter c2ref Sum and inductor current i L And the difference is regulated by a proportional-integral controller and then is compared with a PWM carrier to be used as a Q1 and Q2 complementary modulation signal to control the on and off of Q1 and Q2, so that whether the power fluctuating by the direct current bus is transferred into a direct current active filter or the power in the direct current active filter is transferred into the direct current bus to maintain the stability of the direct current bus is determined, and the direct current ripple is restrained.
7. The utility model provides a little electric wire netting model system is mixed to alternating current-direct current which characterized in that: the system comprises a direct current sub-network, an alternating current converter DC/AC and a filter, wherein the direct current sub-network is connected with the alternating current sub-network through the alternating current/direct current converter and the filter LCL; the alternating current sub-network comprises a system distributed power supply, a DC/AC, an alternating current bus and an alternating current load, wherein the system distributed power supply is connected to the alternating current bus through a DC/AC converter, and the alternating current load is connected to the alternating current bus;
the establishment of the mathematical model of the DC/AC converter specifically comprises the following steps:
step 1, establishing a dynamic equation of three phases at the AC side of the converter:
and 2, converting the three-phase abc coordinate system in the step 1 into a dq rotation coordinate system by using park transformation: wherein i 1A 、i 1B 、i 1C Conversion to i in dq coordinate system 1d 、i 1q ；i 2A 、i 2B 、i 2C Conversion to i in dq coordinate system 2d 、i 2q ；U A 、U B 、U C Conversion to U in dq coordinate system d 、U q ；U cA 、U cB 、U cC Conversion to U in dq coordinate system cd 、U cq ；U gA 、U gB 、U gC Conversion to U in dq coordinate system gd 、U gq ；L 1A ＝L 1B ＝L 1C ＝L 1 ；L 2A ＝L 2B ＝L 2C ＝L 2 ；C A ＝C B ＝C C = C; w is the AC sub-network voltage angular frequency; obtaining:
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