CN110198055A - Based on the microgrid bi-directional inverter control method of virtual synchronous machine and stability analysis - Google Patents
Based on the microgrid bi-directional inverter control method of virtual synchronous machine and stability analysis 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
- H02J3/00—Circuit arrangements for ac mains or ac 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
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
- H02J3/46—Controlling of the sharing of output between the generators, converters, or transformers
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Abstract
It is a kind of based on the microgrid bi-directional inverter control method of virtual synchronous machine and stability analysis, increase integrator by damping link in equation of rotor motion, feedback compensation angular frequency deviation value realizes the indifference control that subnet frequency is exchanged under microgrid off-network mode.In view of the transmitted in both directions characteristic of inverter, by changing virtual machine active power reference value given way, control target is switched to DC voltage from a-c cycle in real time, realizes the stability contorting of DC bus-bar voltage.By establishing power inner ring and voltage/frequency outer ring small-signal model, the transmission function under frequency inverter control and DC voltage control mode is sought respectively, and carried out stability analysis.The present invention can be good at realizing the non differential regulation of off-network mode lower frequency and the stability contorting of DC voltage.
Description
Technical Field
The invention relates to an alternating current-direct current hybrid microgrid bidirectional converter. In particular to a microgrid bidirectional converter control method based on a virtual synchronous machine and stability analysis.
Background
With the increasing environmental and energy problems in the world, various distributed power sources (photovoltaic, wind power, etc.) are highly regarded by relevant scholars. The microgrid was originally proposed by professor r.h.lasseter and can provide an effective way for distributed access to a power distribution network. With the increase of the variety and the number of the distributed power supplies and the popularization of direct current loads, the structure of the power distribution network is complex and various, and the alternating current microgrid cannot completely meet the increasing power supply requirement. In order to ensure the efficient utilization of new energy and renewable energy and better meet the diversified power requirements of users, the alternating-current and direct-current hybrid micro-grid is produced. The alternating current-direct current hybrid microgrid has the advantages of an alternating current microgrid and a direct current microgrid, and belongs to the research focus at present. The alternating current region and the direct current region are connected through the microgrid bidirectional converter, and play an important role in maintaining power balance in the alternating current-direct current hybrid microgrid.
However, more and more new energy sources are incorporated into the power grid, so that the penetration rate of various distributed power sources in the power system is higher and higher, and the corresponding traditional synchronous generator power sources are gradually reduced in the whole power system. The distributed power supply with the power electronic device as an interface lacks inertia and damping of a traditional motor, and when power fluctuation or fault occurs in a system, rapid fluctuation of the power grid frequency is difficult to suppress. The appearance of a Virtual Synchronous Generator (VSG) control technology enables power electronic devices to simulate the characteristics of a synchronous generator, namely, the VSG control technology has the characteristics of inertia, damping and the like, effectively solves the problem of insufficient inertia damping of a power system due to the access of a distributed power supply, and has attracted extensive attention in recent years. When the alternating current-direct current hybrid micro-grid operates in an off-grid mode, the system voltage and frequency should be kept stable, and a small transient process should be kept in the off-grid switching process. However, the traditional virtual synchronous machine control has frequency deviation and belongs to poor regulation.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a microgrid bidirectional converter control method based on a virtual synchronous machine and stability analysis, which can realize the frequency deviation-free adjustment of a microgrid alternating current bus.
The technical scheme adopted by the invention is as follows: a microgrid bidirectional converter control method based on a virtual synchronizer is characterized in that an integration link is connected in parallel with a rotor motion equation damping link in the control of the existing virtual synchronizer, the rotor motion equation is changed from a first-order equation to a second-order equation, and the integration link isWherein s is Laplace operator, KiThe integral coefficient of the integral element isDetermining a value range, wherein ξ represents the damping ratio of the closed-loop transfer function of the active power loop, D is the damping coefficient, and XsIs the total output reactance of the microgrid bidirectional converter after the introduction of the virtual impedance, E0Representing the output potential U of the microgrid bidirectional converter under the condition of steady-state operatione0The output port voltage, omega, of the microgrid bidirectional converter under the condition of steady-state operation0And J is the rotational inertia of the synchronous generator.
The second-order equation of the rotor motion is as follows:
wherein, KiIs the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, omega0Is the synchronous angular velocity of the power grid, omega is the actual angular velocity of the power grid, d is a differential operator, t is time, PmVirtual mechanical active power P representing output of micro-grid bidirectional convertereAnd active power output by the microgrid bidirectional converter is represented, D is a damping coefficient, and s is a Laplace operator.
The method comprises the steps that a given mode of virtual mechanical active power in the control of a virtual synchronizer is changed from unidirectional selection to bidirectional selection, specifically, when an alternating current sub-network has power shortage, the transmission power of a micro-grid bidirectional converter is positive, and at the moment, the control target is the frequency of the alternating current sub-network; when the power shortage occurs in the direct current sub-network, the transmission power of the micro-grid bidirectional converter is negative, and the control target is the direct current bus voltage.
The alternating current sub-network frequency formula is as follows:
Pm＝Pref+kf(fref-f)
wherein, PmVirtual mechanical active power P representing output of micro-grid bidirectional converterrefAnd frefActive power reference value and system frequency reference value, k, respectively output by the microgrid bidirectional converterfIs a frequency adjustment coefficient;
the direct current bus voltage formula is as follows:
wherein U isdcRepresenting the DC bus voltage, Udc_refRepresenting the reference value, k, of the DC bus voltagepuAnd kpiAnd respectively representing a proportional coefficient and an integral coefficient of the direct current bus voltage PI regulator.
The stability analysis of the microgrid bidirectional converter control method based on the virtual synchronous machine is carried out by establishing a small-signal model of the output power of the microgrid bidirectional converter and carrying out the stability analysis of the microgrid bidirectional converter control method, and the stability analysis method comprises the following specific steps:
1) active power P output by rotor motion second order equation and microgrid bidirectional convertereAnd reactive power QeAnd output potential equation of microgrid bidirectional converterThe variable in (1) is expressed as the sum of the steady-state quantity and the disturbance quantity, i.e.
Wherein, KqExpressing the output potential differential coefficient of the microgrid bidirectional converter, and E expressing the microgrid bidirectional converterOutput potential, QeAnd QrefRespectively outputting reactive power and outputting reference value of reactive power, U, for the bidirectional converter of the microgrideAnd UenRated values of output port voltage and output port voltage of the microgrid bidirectional converter respectively, DqIs a voltage regulation factor; e0The output potential of the microgrid bidirectional converter under the condition of steady-state operation is represented,the output potential disturbance quantity of the microgrid bidirectional converter is represented, wherein delta is a power angle and delta0For the power angle in the case of steady state operation,for power angle disturbance, Pe0And Qe0Active power and reactive power output by the microgrid bidirectional converter under the condition of steady-state operation are respectively,andactive power disturbance quantity and reactive power disturbance quantity, P, respectively output by the microgrid bidirectional convertermVirtual mechanical active power P representing output of micro-grid bidirectional converterm0The virtual mechanical active power output by the microgrid bidirectional converter under the condition of steady-state operation is represented,the disturbance quantity of virtual mechanical active power output by the microgrid bidirectional converter is represented, wherein omega is the actual angular speed of a power grid0In order to synchronize the angular speed of the grid,disturbance variable, U, of the actual angular velocity of the gride0The output port voltage of the microgrid bidirectional converter under the condition of steady-state operation,and the voltage disturbance quantity of the output port of the microgrid bidirectional converter is obtained.
Eliminating disturbance quantity secondary components and steady state quantity to obtain a small signal model of the output power of the microgrid bidirectional converter:
in the formula: x is equivalent output reactance of the microgrid bidirectional converter, D is damping coefficient, J is rotational inertia of the synchronous generator, and KiThe integral coefficient is the integral coefficient of an integral link;
when calculating the small-signal model of the output power of the microgrid bidirectional converter, setting:
Ue＝E，sinδ0＝δ0，cosδ0＝1；
2) the coupling of an active ring and a reactive ring is not considered, and a closed-loop transfer function formula of the active power ring is obtained through a small-signal model of the output power of the microgrid bidirectional converter as follows:
wherein G(s) is an active power loop closed loop transfer function,andrespectively an active power disturbance quantity output by the microgrid bidirectional converter in a complex frequency domain and an output virtual mechanical active power disturbance quantity, XsThe total output reactance of the microgrid bidirectional converter after the virtual impedance is introduced, and s represents a Laplace operator; k is a proportionality constant,ωnand ξ, which respectively represent the natural oscillation angular frequency and the damping ratio of the closed-loop transfer function of the active power loop, are obtained by the following formula:
3) according to the closed-loop transfer function of the active power loop and the alternating-current sub-network frequency formula, the open-loop transfer function of the micro-grid bidirectional converter under the alternating-current sub-network frequency is obtained and expressed as follows:
in the formula, Gf(s) is an open loop transfer function, k, of the microgrid bidirectional converter under the condition that the control target is the frequency of the alternating current sub-networkfIs a frequency adjustment factor.
When the transmission power of the microgrid bidirectional converter is negative, the active power transmitted by the microgrid bidirectional converter is as follows:
in the formula, ReqIs an equivalent resistance, U, of the DC sub-networkdcIs a DC bus voltage, Pe1The active power output when the transmission power of the microgrid bidirectional converter is negative is transmitted, C is a direct-current side capacitor, and d is a differential operator. Considering small signal interference, the active power/dc voltage transfer function is expressed as follows:
in the formula, GP-U(s) is an active power/direct-current voltage transfer function when the transmission power of the microgrid bidirectional converter is negative,andthe direct-current bus voltage disturbance quantity and the active power disturbance quantity output when the transmission power of the microgrid bidirectional converter is negative under the condition of steady-state operation are respectively.
According to the closed-loop transfer function of the active power loop, the direct-current bus voltage formula and the active power/direct-current voltage transfer function when the transmission power of the microgrid bidirectional converter is negative, the lower open-loop transfer function of the microgrid bidirectional converter control target which is the direct-current bus voltage is obtained and expressed as follows:
wherein: gU(s) is an open loop transfer function, k, of the microgrid bidirectional converter under the condition that the control target is the direct-current bus voltagepuAnd kpiAnd respectively representing a proportional coefficient and an integral coefficient of the direct current bus voltage PI regulator.
4) And performing stability analysis on the lower open-loop transfer function of the frequency of the alternating-current sub-network and the voltage of the direct-current bus according to the control target of the micro-grid bidirectional converter. The analysis comprises the following steps:
drawing parameters J, D, K by using a root locus method according to the open-loop transfer function of the microgrid bidirectional converter control target, namely the frequency of the alternating-current sub-network and the voltage of the direct-current busiWhen the variation is carried out, the variation track of the closed-loop transfer function pole on the s plane is analyzed to obtain:
the larger the rotary inertia J is, the closer the pole of the closed-loop transfer function is to the virtual axis, the more unstable the system is, the larger the rotary inertia is, the larger the overshoot and the adjustment time are caused, so that the power oscillation is caused, and the more difficult the system is to be stable; along with the increase of the damping coefficient, the stability of the system is enhanced; along with the increase of the integral coefficient of the integral link, the pole of the closed loop is far away from the gradually real axis, the overshoot is increased, but the pole is always kept on the left side of the virtual axis, and the system stability is unchanged.
When the control target of the microgrid bidirectional converter is direct-current bus voltage, the open-loop transfer function is composed of a proportional link, a first-order differential link, an integral link, a second-order oscillation link and a first-order inertia link. Cut-off frequency f for keeping the system stable and having good dynamic characteristicscIs located in the middle frequency band and satisfies kpi/kpu＜fc＜ωn。
The virtual synchronizer-based microgrid bidirectional converter control method and the stability analysis realize the non-deviation adjustment of the frequency of the microgrid alternating current bus, the correctness of the microgrid alternating current bus is proved by theoretical derivation, the inertia of a microgrid system is increased, and the quick fluctuation of the system frequency can be responded when the load fluctuates or fails. And in consideration of the bidirectional transmission characteristic of the converter, the virtual mechanical active power given mode is changed, and the real-time switching of the control target of the alternating current frequency and the direct current voltage is realized.
Drawings
Fig. 1 is a control block diagram of a first example of a microgrid bidirectional converter control method based on a virtual synchronous machine;
fig. 2 is a control block diagram of a second example of the microgrid bidirectional converter control method based on the virtual synchronous machine;
FIG. 3 is a main circuit equivalent circuit in the present invention;
FIG. 4 is a small signal model of the active reactive power loop of the present invention;
FIG. 5 is a control block diagram in the frequency control mode according to the present invention;
FIG. 6a is a closed loop transfer function pole change trajectory when the rotational inertia of the synchronous generator changes;
FIG. 6b is a trace of the change of the pole of the closed loop transfer function when the damping coefficient is changed;
FIG. 6c is a trace of the change of the pole of the closed loop transfer function when the integral coefficient of the integral element changes;
FIG. 7 is a simplified schematic diagram of a system in a DC bus voltage control mode according to the present invention;
FIG. 8 is a control block diagram of the DC bus voltage control mode of the present invention;
FIG. 9 is an AC frequency waveform under the frequency control mode of the conventional VSG control and method in an embodiment of the present invention;
FIG. 10 is an AC bus voltage waveform in the frequency control mode in an embodiment of the present invention;
FIG. 11 is a DC bus voltage waveform in the DC bus voltage control mode according to an embodiment of the present invention;
fig. 12 is an ac frequency waveform in the dc bus voltage control mode according to the embodiment of the present invention.
Detailed Description
The following describes in detail a microgrid bidirectional converter control method and stability analysis based on a virtual synchronous machine according to the present invention with reference to embodiments and drawings.
In the alternating current-direct current hybrid microgrid, a microgrid bidirectional converter coordinates power distribution of an alternating current sub-network and a direct current sub-network and plays a role in maintaining alternating current bus frequency and direct current bus voltage. The traditional virtual synchronous machine control simulates the steady-state droop characteristic and the transient inertia and damping of the synchronous generator and has steady-state and electromechanical dynamic characteristics similar to those of the synchronous generator. However, in the off-grid operation mode of the microgrid, the traditional virtual synchronous machine has frequency deviation and belongs to poor regulation;
when the frequency fluctuation is caused by sudden load increase in an alternating current sub-network in the alternating current and direct current hybrid micro-network, the distributed power supplies and the micro-network bidirectional converter adjust respective output to maintain power balance in the sub-network. In the control of the traditional virtual synchronous machine, the active frequency link can only realize one-time adjustment when the load changes, and the frequency deviation can influence the normal operation of part of the load in the microgrid. When the load suddenly increases, the mechanical power and the electromagnetic power deviate, and the motion equation of the rotor of the synchronous generator can be transformed into:
solving the first order linear differential equation to obtain:
in the formula, KiIs the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, omega0The method is characterized in that the synchronous angular velocity of the power grid is adopted, omega is the actual angular velocity of the power grid, D is a differential operator, t is time, D is a damping coefficient, s is a Laplace operator, delta P is the difference value of virtual mechanical active power and active power output by the microgrid bidirectional converter, and C is a constant and is determined by the initial condition of the state.
The angular frequency after the load sudden increase is composed of two parts, namely a steady-state componentAnd transient componentsThe transient component finally decays to zero with a decay time constant ofTherefore, under the control of the traditional VSG, the final steady-state value of the angular frequency isBelonging to the regulation with difference. The frequency offset depends on the power deviation and the damping coefficient, and the steady-state frequency offset can be reduced by properly increasing the damping coefficient.
The invention discloses a virtual synchronizer-based microgrid bidirectional converter control method, which is characterized in that an integral link is connected in parallel with a rotor motion equation damping link in the control of the existing virtual synchronizer as shown in figure 1, wherein the integral link isWherein s is Laplace operator, KiThe integral coefficient of the integral element isDetermining a value range, wherein ξ represents the damping ratio of the closed-loop transfer function of the active power loop, D is the damping coefficient, and XsIs the total output reactance of the microgrid bidirectional converter after the introduction of the virtual impedance, E0Representing the output potential U of the microgrid bidirectional converter under the condition of steady-state operatione0The output port voltage, omega, of the microgrid bidirectional converter under the condition of steady-state operation0And J is the rotational inertia of the synchronous generator.
So as to change the rotor motion equation from a first order equation to a second order equation, wherein the second order equation of the rotor motion is as follows:
wherein, KiIs the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, omega0Synchronizing angular speed for a power gridDegree, omega is the actual angular velocity of the grid, d is the differential operator, t is time, PmVirtual mechanical active power P representing output of micro-grid bidirectional convertereAnd active power output by the microgrid bidirectional converter is represented, D is a damping coefficient, and s is a Laplace operator.
The above formula is arranged into a standard form of a second-order constant coefficient differential equation:
in order to keep the system running stably, parameters are set so that the solution of the standard form of the rotor motion equation meets the following form:
wherein: t is time, C1And C2Initial values, T, representing angular frequency components 1 and 2 in the transient case1And T2Representing the decay time constants, ω, of angular frequency components 1 and 2, respectively, in the transient case*(t) represents the steady state value after the angular frequency component has decayed in the transient case. When the load suddenly increases, the frequency suddenly changes and tends to be stable. Solving equation (5) yields the frequency expression under steady state conditions as:
ω＝ω*(t)＝ω0 (6)
therefore, under the microgrid off-grid operation mode, through derivation of a second-order synchronous generator rotor motion equation under the transient condition, control of the microgrid off-grid operation mode is proved to belong to no-difference regulation, and non-deviation control of frequency under the microgrid off-grid mode is achieved.
And in consideration of the bidirectional transmission characteristic of the microgrid bidirectional converter, the virtual mechanical active power reference value setting mode is changed, and the real-time switching of the alternating current frequency and direct current bus voltage control target is realized. The direct-current voltage control strategy in the direct-current micro-grid generally adopts active/voltage droop control, but has the problem of voltage deviation. The active controller shown in fig. 1 introduces a frequency deviation to calculate virtual machine active power, and adjusts an angular frequency to control active power output. Similar to the non-deviation control of the frequency in the alternating-current sub-network, when the load fluctuation occurs in the direct-current sub-network, the control quantity is changed into the direct-current bus voltage, the voltage deviation is sent to the PI regulator to calculate to obtain the virtual mechanical active power, and the stable control of the direct-current bus voltage is achieved. According to the invention, the power of the microgrid bidirectional converter is positive when the microgrid bidirectional converter flows from the direct-current sub-network to the alternating-current sub-network.
As shown in fig. 2, the method changes the given mode of virtual mechanical active power in the virtual synchronous machine control from unidirectional selection to bidirectional selection, specifically, when the ac sub-network has power shortage, the transmission power of the bidirectional converter of the microgrid is positive, and the control target is the ac sub-network frequency; when the power shortage occurs in the direct current sub-network, the transmission power of the micro-grid bidirectional converter is negative, and the control target is the direct current bus voltage. Wherein,
the alternating current sub-network frequency formula is as follows:
Pm＝Pref+kf(fref-f) (7)
wherein P ismVirtual mechanical active power P representing output of micro-grid bidirectional converterrefAnd frefActive power reference value and system frequency reference value, k, respectively output by the microgrid bidirectional converterfIs a frequency adjustment coefficient;
the direct current bus voltage formula is as follows:
wherein U isdcRepresenting the DC bus voltage, Udc_refRepresenting the reference value, k, of the DC bus voltagepuAnd kpiAnd respectively representing a proportional coefficient and an integral coefficient of the direct current bus voltage PI regulator.
The stability analysis of the microgrid bidirectional converter control method based on the virtual synchronous machine is to perform stability analysis on the microgrid bidirectional converter control method by establishing a small-signal model of the output power of the microgrid bidirectional converter, and specifically comprises the following steps:
step 1)
In FIG. 3, ReAnd LeThe equivalent output resistance and reactance of the virtual synchronous machine, the equivalent output impedance Z of the virtual synchronous machine is:
Z＝Re+jωLe≈jX (9)
the neutral point voltage phasor of the bridge arm of the microgrid bidirectional converter is represented as E ∠ delta, and the port voltage of the alternating-current sub-network is Ue∠ 0, the active power and the reactive power output by the microgrid bidirectional converter are as follows:
active power P output by rotor motion second order equation and microgrid bidirectional convertereAnd reactive power QeAnd output potential equation of microgrid bidirectional converterThe variable in (1) is expressed as the sum of the steady-state quantity and the disturbance quantity, i.e.
Wherein, KqExpressing the output potential differential coefficient of the microgrid bidirectional converter, E expressing the output potential of the microgrid bidirectional converter, QeAnd QrefAre respectively a microgrid pairOutputting reactive power to the converter and outputting a reference value of reactive power, UeAnd UenRated values of output port voltage and output port voltage of the microgrid bidirectional converter respectively, DqIs a voltage regulation factor; e0The output potential of the microgrid bidirectional converter under the condition of steady-state operation is represented,the output potential disturbance quantity of the microgrid bidirectional converter is represented, wherein delta is a power angle and delta0For the power angle in the case of steady state operation,for power angle disturbance, Pe0And Qe0Active power and reactive power output by the microgrid bidirectional converter under the condition of steady-state operation are respectively,andactive power disturbance quantity and reactive power disturbance quantity, P, respectively output by the microgrid bidirectional convertermVirtual mechanical active power P representing output of micro-grid bidirectional converterm0The virtual mechanical active power output by the microgrid bidirectional converter under the condition of steady-state operation is represented,the disturbance quantity of virtual mechanical active power output by the microgrid bidirectional converter is represented, wherein omega is the actual angular speed of a power grid0In order to synchronize the angular speed of the grid,disturbance variable, U, of the actual angular velocity of the gride0The output port voltage of the microgrid bidirectional converter under the condition of steady-state operation,bidirectional switching for microgridAnd voltage disturbance quantity of an output port of the current transformer.
Eliminating disturbance quantity secondary components and steady state quantity to obtain a small signal model of the output power of the microgrid bidirectional converter:
in the formula: x is equivalent output reactance of the microgrid bidirectional converter, D is damping coefficient, J is rotational inertia of the synchronous generator, and KiThe integral coefficient is the integral coefficient of an integral link;
when calculating the small-signal model of the output power of the microgrid bidirectional converter, setting:
Ue＝E，sinδ0＝δ0，cosδ0＝1；
and performing Laplace transform on the linearized equation to obtain a transfer block diagram of the active ring and reactive ring small signal model, as shown in FIG. 4.
As can be seen from FIG. 4, there is coupling between the active loop and the reactive loop, but the gain of the coupled branches contains δ0An item. Under the normal operation condition of the microgrid, the power angle value is very small. The invention realizes the approximate decoupling control of the active loop and the reactive loop by introducing a virtual impedance control algorithm. Synchronous generator potential E, output stator current I and terminal voltage UeThe relationship is as follows:
in the formula, LvA virtual inductance, X, corresponding to a virtual reactancesIs the inverter output reactance after the introduction of the virtual impedance.
The introduced virtual impedance is equivalent to a virtual inductor which is connected with the output end of the converter in series, so that the output impedance of the converter is increased, the gain of the coupling branch is greatly reduced, and the approximate decoupling of an active loop and a reactive loop is realized.
2) The coupling of an active ring and a reactive ring is not considered, and a closed-loop transfer function formula of the active power ring is obtained through a small-signal model of the output power of the microgrid bidirectional converter as follows:
wherein G(s) is an active power loop closed loop transfer function,andrespectively an active power disturbance quantity output by the microgrid bidirectional converter in a complex frequency domain and an output virtual mechanical active power disturbance quantity, XsThe total output reactance of the microgrid bidirectional converter after the virtual impedance is introduced, and s represents a Laplace operator; k is a proportionality constant,ωnand ξ, which respectively represent the natural oscillation angular frequency and the damping ratio of the closed-loop transfer function of the active power loop, are obtained by the following formula:
3) according to the closed-loop transfer function of the active power loop and the ac sub-network frequency formula, a control block diagram in the frequency control mode shown in fig. 5 can be obtained, and the open-loop transfer function in the frequency control mode is obtained as follows:
under normal conditions, the direct current sub-network side stably operates according to the upper-layer optimized power. In the transient state situation, the alternating current sub-network injects or absorbs active power to the direct current sub-network through the AC/DC bidirectional converter. For simplicity of design, the DC sub-network can be equivalent to a constant resistance element R on the DC buseqAnd the resistance value of the resistor depends on the active power injected into the direct current sub-network side.
As can be seen from fig. 7, when the internal active power loss of the converter is neglected, the active power transmitted by the converter is
In the formula, ReqIs an equivalent resistance, U, of the DC sub-networkdcIs a DC bus voltage, Pe1The active power output when the transmission power of the microgrid bidirectional converter is negative is transmitted, C is a direct-current side capacitor, and d is a differential operator. Considering small signal interference, the active power/dc voltage transfer function is as follows:
in the formula, GP-U(s) is an active power/direct-current voltage transfer function when the transmission power of the microgrid bidirectional converter is negative,andthe direct-current bus voltage disturbance quantity and the active power disturbance quantity output when the transmission power of the microgrid bidirectional converter is negative under the condition of steady-state operation are respectively. According to the closed loop transfer function of the active power loop and the DC bus voltageThe active power/dc voltage transfer function when the transmission power of the bidirectional inverter for microgrid and formula is negative can be obtained from the control block diagram in the frequency control mode shown in fig. 8, and the open-loop transfer function in the dc bus voltage control mode is obtained as:
wherein: gU(s) is an open loop transfer function, k, of the microgrid bidirectional converter under the condition that the control target is the direct-current bus voltagepuAnd kpiAnd respectively representing a proportional coefficient and an integral coefficient of the direct current bus voltage PI regulator.
4) And performing stability analysis on the lower open-loop transfer function of the frequency of the alternating-current sub-network and the voltage of the direct-current bus according to the control target of the micro-grid bidirectional converter. The analysis comprises the following steps:
drawing parameters J, D, K by using a root locus method according to the open-loop transfer function of the microgrid bidirectional converter control target, namely the frequency of the alternating-current sub-network and the voltage of the direct-current busiWhen the variation is carried out, the variation track of the closed-loop transfer function pole on the s plane is analyzed to obtain:
as can be seen from fig. 6(a), the larger the moment of inertia J, the closer the pole of the closed-loop transfer function is to the virtual axis, the more unstable the system is, and the larger the moment of inertia is, the larger the overshoot and the adjustment time are caused, thereby causing power oscillation, and the less stable the system is; as can be seen from fig. 6(b) (c), as the damping coefficient increases, the system stability increases; along with the increase of the integral coefficient of the integral link, the pole of the closed loop is far away from the gradually real axis, the overshoot is increased, but the pole is always kept on the left side of the virtual axis, and the system stability is unchanged.
When the control target of the microgrid bidirectional converter is direct-current bus voltage, the open-loop transfer function is composed of a proportional link, a first-order differential link, an integral link, a second-order oscillation link and a first-order inertia link. Cut-off frequency f for keeping the system stable and having good dynamic characteristicscIs located inFrequency band, and satisfy kpi/kpu＜fc＜ωn。
Examples are given below:
the invention discloses a virtual synchronous machine-based control method for an alternating current-direct current hybrid micro-grid bidirectional converter, which is applied to an alternating current-direct current hybrid micro-grid and realizes deviation-free control of alternating current frequency and stable control of direct current bus voltage by virtue of the micro-grid bidirectional converter. Under normal conditions, the alternating current-direct current hybrid microgrid stably operates, and the transmission power of the microgrid bidirectional converter is zero. When the load on the alternating current side suddenly increases, the frequency decreases, the control system detects the frequency deviation, a virtual mechanical active power signal is generated through calculation and sent to the inertial damping link, and the frequency is adjusted without deviation through the feedback compensation of the integral link. When the load suddenly increases at the direct current side, the voltage of the direct current bus is reduced, the system switches the control mode, and the virtual mechanical active power signal is generated through the direct current voltage deviation signal to maintain the stability of the voltage of the direct current bus.
In order to verify the effectiveness of the invention, an alternating current-direct current hybrid microgrid is established, wherein the voltage level of an alternating current bus is 380V (line voltage), the voltage of a direct current bus is 800V, a bidirectional converter filtering device of the microgrid selects an LC filter, circuit parameters are designed to be 0.6H for inductance L and 20 muF for capacitance C, and control circuit parameters are designed to be 2kg.m for moment of inertia J2Damping coefficient D is 50, integral coefficient Ki＝1000。
During off-grid operation, at the initial moment, the micro-grid bidirectional converter works in a frequency control mode, active power is transmitted by 80kW, and the system stably operates. The input load of the AC sub-network is 20kW at 0.5s, and the load is 40kW when the AC sub-network is cut off at 1 s. Fig. 9 shows frequency variation waveforms in the frequency control mode of the conventional VSG control and the present method. When the converter is controlled by the traditional virtual synchronous machine, the initial frequency is stabilized at 49.92Hz, the frequency is reduced and stabilized at 49.87Hz when the frequency is 0.5s, and the frequency is increased and stabilized to about 49.96Hz when the frequency is 1s through micro-overshoot. When the frequency control mode of the method is adopted, the initial frequency is stabilized at 50Hz, and the frequency can be quickly recovered and maintained at the rated frequency after the short-time change when the frequency is 0.5s and 1 s. The comparison shows that the frequency change can be reduced by both the two, the function of providing inertial support for the system is achieved, and the dynamic response is basically consistent. But under the action of the frequency control mode, the method can realize the non-deviation adjustment of the frequency. When the frequency deviation exceeds the threshold value, the frequency control mode of the method is adopted to recover the frequency of the microgrid to a rated value, and the frequency stability of independent operation of the system is improved. Fig. 10 shows the ac bus voltage amplitude waveform in the frequency control mode of the method. When the load suddenly increases for 0.5s, the voltage amplitude of the alternating current bus is recovered to be stable through transient oscillation, and when the load suddenly decreases for 1s, the voltage amplitude is slightly reduced but is kept stable at about 311V all the time.
When the microgrid bidirectional converter works in a direct-current bus voltage control mode, active power of 180kW is transmitted, and the control purpose of the converter is to keep the direct-current bus voltage stable. Suppose that at 0.5s the dc sub-network load suddenly increases by 50 kW. Fig. 11 shows a dc bus voltage waveform in the dc bus voltage control mode, and fig. 12 shows an ac bus frequency waveform in the dc bus voltage control mode. The voltage was initially stabilized at 800V rating and at 0.5s the dc bus voltage suddenly dropped to around 780V and gradually returned to near rating. The method can keep the stability of the direct-current bus voltage in the direct-current bus voltage control mode. When the load suddenly increases, the frequency of the alternating current bus bar slightly increases, but the frequency value is basically stabilized at 50 Hz.
Claims (6)
1. A microgrid bidirectional converter control method based on a virtual synchronizer is characterized in that an integration link is connected in parallel with a rotor motion equation damping link in the control of the existing virtual synchronizer, the rotor motion equation is changed from a first-order equation to a second-order equation, and the integration link isWherein s is Laplace operator, KiIntegration system as integration elementIs counted byDetermining a value range, wherein ξ represents the damping ratio of the closed-loop transfer function of the active power loop, D is the damping coefficient, and XsIs the total output reactance of the microgrid bidirectional converter after the introduction of the virtual impedance, E0Representing the output potential U of the microgrid bidirectional converter under the condition of steady-state operatione0The output port voltage, omega, of the microgrid bidirectional converter under the condition of steady-state operation0And J is the rotational inertia of the synchronous generator.
2. The virtual synchronous machine-based microgrid bidirectional converter control method according to claim 1, characterized in that the rotor motion second-order equation is as follows:
wherein, KiIs the integral coefficient of the integral link, J is the moment of inertia of the synchronous generator, omega0Is the synchronous angular velocity of the power grid, omega is the actual angular velocity of the power grid, d is a differential operator, t is time, PmVirtual mechanical active power P representing output of micro-grid bidirectional convertereAnd active power output by the microgrid bidirectional converter is represented, D is a damping coefficient, and s is a Laplace operator.
3. The virtual synchronous machine-based microgrid bidirectional converter control method is characterized in that a given mode of virtual mechanical active power in the virtual synchronous machine control is changed from unidirectional selection to bidirectional selection, specifically, when an alternating current subnet has power shortage, the transmission power of the microgrid bidirectional converter is positive, and the control target is the frequency of the alternating current subnet; when the power shortage occurs in the direct current sub-network, the transmission power of the micro-grid bidirectional converter is negative, and the control target is the direct current bus voltage.
4. The virtual synchronous machine-based microgrid bidirectional converter control method according to claim 3, characterized in that the alternating current sub-network frequency formula is as follows:
Pm＝Pref+kf(fref-f)
wherein, PmVirtual mechanical active power P representing output of micro-grid bidirectional converterrefAnd frefActive power reference value and system frequency reference value, k, respectively output by the microgrid bidirectional converterfIs a frequency adjustment coefficient;
the direct current bus voltage formula is as follows:
wherein U isdcRepresenting the DC bus voltage, Udc_refRepresenting the reference value, k, of the DC bus voltagepuAnd kpiAnd respectively representing a proportional coefficient and an integral coefficient of the direct current bus voltage PI regulator.
5. The stability analysis method for the microgrid bidirectional converter control method based on the virtual synchronous machine as claimed in claim 1 is characterized in that the stability analysis is performed on the microgrid bidirectional converter control method by establishing a small signal model of the output power of the microgrid bidirectional converter, and the method specifically comprises the following steps:
step 1) obtaining a rotor motion second-order equation and active power P output by a microgrid bidirectional convertereAnd reactive power QeAnd output potential equation of microgrid bidirectional converterThe variable in (1) is expressed as the sum of the steady-state quantity and the disturbance quantity, i.e.
Wherein, KqExpressing the output potential differential coefficient of the microgrid bidirectional converter, E expressing the output potential of the microgrid bidirectional converter, QeAnd QrefRespectively outputting reactive power and outputting reference value of reactive power, U, for the bidirectional converter of the microgrideAnd UenRated values of output port voltage and output port voltage of the microgrid bidirectional converter respectively, DqIs a voltage regulation factor; e0The output potential of the microgrid bidirectional converter under the condition of steady-state operation is represented,the output potential disturbance quantity of the microgrid bidirectional converter is represented, wherein delta is a power angle and delta0For the power angle in the case of steady state operation,for power angle disturbance, Pe0And Qe0Active power and reactive power output by the microgrid bidirectional converter under the condition of steady-state operation are respectively,andactive power disturbance quantity and reactive power disturbance quantity, P, respectively output by the microgrid bidirectional convertermVirtual mechanical active power P representing output of micro-grid bidirectional converterm0The virtual mechanical active power output by the microgrid bidirectional converter under the condition of steady-state operation is represented,the disturbance quantity of virtual mechanical active power output by the microgrid bidirectional converter is represented, wherein omega is the actual angular speed of a power grid0In order to synchronize the angular speed of the grid,disturbance variable, U, of the actual angular velocity of the gride0The output port voltage of the microgrid bidirectional converter under the condition of steady-state operation,and the voltage disturbance quantity of the output port of the microgrid bidirectional converter is obtained.
Eliminating disturbance quantity secondary components and steady state quantity to obtain a small signal model of the output power of the microgrid bidirectional converter:
in the formula: x is equivalent output reactance of the microgrid bidirectional converter, D is damping coefficient, J is rotational inertia of the synchronous generator, and KiThe integral coefficient is the integral coefficient of an integral link;
when calculating the small-signal model of the output power of the microgrid bidirectional converter, setting:
Ue＝E，sinδ0＝δ0，cosδ0＝1；
2) the coupling of an active ring and a reactive ring is not considered, and a closed-loop transfer function formula of the active power ring is obtained through a small-signal model of the output power of the microgrid bidirectional converter as follows:
wherein G(s) is an active power loop closed loop transfer function,andthe active power disturbance quantity output by the microgrid bidirectional converter and the virtual machine output by the microgrid bidirectional converter under the complex frequency domain respectivelyWork power disturbance variable, XsThe total output reactance of the microgrid bidirectional converter after the virtual impedance is introduced, and s represents a Laplace operator; k is a proportionality constant,ωnand ξ, which respectively represent the natural oscillation angular frequency and the damping ratio of the closed-loop transfer function of the active power loop, are obtained by the following formula:
3) according to the closed-loop transfer function of the active power loop and the alternating-current sub-network frequency formula, the open-loop transfer function of the micro-grid bidirectional converter under the alternating-current sub-network frequency is obtained and expressed as follows:
in the formula, Gf(s) is an open loop transfer function, k, of the microgrid bidirectional converter under the condition that the control target is the frequency of the alternating current sub-networkfIs a frequency adjustment factor.
When the transmission power of the microgrid bidirectional converter is negative, the active power transmitted by the microgrid bidirectional converter is as follows:
in the formula, ReqIs an equivalent resistance, U, of the DC sub-networkdcIs a DC bus voltage, Pe1The active power output when the transmission power of the microgrid bidirectional converter is negative is transmitted, C is a direct-current side capacitor, and d is a differential operator. Considering small signal interference, the active power/dc voltage transfer function is expressed as follows:
in the formula, GP-U(s) is an active power/direct-current voltage transfer function when the transmission power of the microgrid bidirectional converter is negative,andthe direct-current bus voltage disturbance quantity and the active power disturbance quantity output when the transmission power of the microgrid bidirectional converter is negative under the condition of steady-state operation are respectively.
According to the closed-loop transfer function of the active power loop, the direct-current bus voltage formula and the active power/direct-current voltage transfer function when the transmission power of the microgrid bidirectional converter is negative, the lower open-loop transfer function of the microgrid bidirectional converter control target which is the direct-current bus voltage is obtained and expressed as follows:
wherein: gU(s) is an open loop transfer function, k, of the microgrid bidirectional converter under the condition that the control target is the direct-current bus voltagepuAnd kpiAnd respectively representing a proportional coefficient and an integral coefficient of the direct current bus voltage PI regulator.
4) And performing stability analysis on the lower open-loop transfer function of the frequency of the alternating-current sub-network and the voltage of the direct-current bus according to the control target of the micro-grid bidirectional converter.
6. The stability analysis method for the virtual synchronous machine-based microgrid bidirectional converter control method according to claim 5, characterized in that the analysis of the step 4) includes:
drawing parameters J, D, K by using a root locus method according to the open-loop transfer function of the microgrid bidirectional converter control target, namely the frequency of the alternating-current sub-network and the voltage of the direct-current busiWhen changing, the changing track of the closed loop transfer function pole on the s plane is divided intoThe method comprises the following steps:
the larger the rotary inertia J is, the closer the pole of the closed-loop transfer function is to the virtual axis, the more unstable the system is, the larger the rotary inertia is, the larger the overshoot and the adjustment time are caused, so that the power oscillation is caused, and the more difficult the system is to be stable; along with the increase of the damping coefficient, the stability of the system is enhanced; along with the increase of the integral coefficient of the integral link, the pole of the closed loop is far away from the gradually real axis, the overshoot is increased, but the pole is always kept on the left side of the virtual axis, and the system stability is unchanged.
When the control target of the microgrid bidirectional converter is direct-current bus voltage, the open-loop transfer function is composed of a proportional link, a first-order differential link, an integral link, a second-order oscillation link and a first-order inertia link. Cut-off frequency f for keeping the system stable and having good dynamic characteristicscIs located in the middle frequency band and satisfies kpi/kpu＜fc＜ωn。
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