CN107196318B - V2G technology-based electric vehicle participation power grid frequency modulation control method - Google Patents
V2G technology-based electric vehicle participation power grid frequency modulation control method Download PDFInfo
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- CN107196318B CN107196318B CN201710248040.1A CN201710248040A CN107196318B CN 107196318 B CN107196318 B CN 107196318B CN 201710248040 A CN201710248040 A CN 201710248040A CN 107196318 B CN107196318 B CN 107196318B
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B60—VEHICLES IN GENERAL
- B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
- B60L55/00—Arrangements for supplying energy stored within a vehicle to a power network, i.e. vehicle-to-grid [V2G] arrangements
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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
- H02J3/28—Arrangements for balancing of the load in a network by storage of energy
- H02J3/32—Arrangements for balancing of the load in a network by storage of energy using batteries with converting means
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/60—Other road transportation technologies with climate change mitigation effect
- Y02T10/70—Energy storage systems for electromobility, e.g. batteries
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/60—Other road transportation technologies with climate change mitigation effect
- Y02T10/72—Electric energy management in electromobility
Abstract
The invention discloses a V2G technology-based electric vehicle participation power grid frequency modulation control method.A charging and discharging circuit is a two-stage converter circuit and comprises a PWM (pulse-width modulation) rectifying circuit, a matched LC (inductance-capacitance) filter and a Buck-Boost converting circuit, wherein the two circuits are connected through a direct-current bus capacitor, the PWM rectifying circuit rectifies the power grid voltage into 700V direct-current voltage, and an alternating-current side filters harmonic waves through the LC filter and is connected with a power grid through a grid-side inductor; the Buck-Boost conversion circuit converts 700V direct current voltage into 60V direct current voltage and is directly connected with the electric automobile; according to the method, an AC/DC control module and a DC/DC control module are used, a frequency modulation control module is embedded into the DC/DC control module, a charging and discharging power reference value of a power battery is given through the frequency modulation control module, and a converter responds to realize primary and secondary adjustment of the power grid frequency.
Description
Technical Field
The invention belongs to the technical field of electric automobiles and stable operation and control of a power system, and particularly relates to a V2G (Vehicle to Grid) technology-based electric automobile participation power Grid frequency modulation control method considering the problems of power Grid frequency operation stability and power electronic device inertia loss.
Background
The global energy crisis and environmental pollution have driven the development of the electric automobile industry in all countries around the world. The number of electric vehicles is increasing, which brings great challenges and opportunities for the construction and development of electric power systems. On one hand, a large number of electric automobiles are connected into a power grid as a new load to be charged in a centralized manner, and if the electric automobiles are not managed and guided, the power consumption peak of the power grid is increased, the load of the power grid is too heavy, the peak regulation difficulty of the power grid is increased, and huge pressure is brought to power grid planning and construction. On the other hand, the current power system is vigorously developed to generate new energy, the new energy has strong intermittence and randomness, and the generated energy is seriously influenced by the natural environment. The electric automobile can be used as a system standby capacity as a mobile energy storage device, energy interaction with a power Grid is realized based on a V2G technology, certain auxiliary service can be provided for the power Grid, new energy consumption is promoted, system frequency stability is enhanced, and Vehicle network integration (GIV) is realized.
With the development and use of renewable energy sources mainly comprising wind power and photovoltaic and electric vehicles, the application of power electronic equipment is gradually increased, the rotating reserve capacity and the rotating inertia in a power grid are relatively reduced, and certain influence is caused on the stability of the power grid. In order to effectively deal with the problem, a specific electric vehicle charging and discharging control strategy must be improved based on the V2G technology, so as to provide inertia and frequency support for a power grid while meeting the charging requirements of users. At present, students improve the charging control strategy of the electric automobile, and a Virtual Synchronous Machine (VSM) technology is used for controlling a bidirectional converter, so that the electric automobile has the same one-time frequency modulation external characteristic as a synchronous motor, autonomously participates in power grid frequency and voltage regulation, has the specific inertia characteristic of the synchronous motor, and overcomes the problems of inertia loss and the like caused by the fact that a large-scale electric automobile is connected into a power system. However, most of the existing control strategies only consider the electric power battery as a simple load and do not consider the actual charging and discharging processes of the power battery and the service life of the power battery; in addition, the addition of a secondary frequency modulation function is not considered, and the frequency can not be adjusted without difference. Few scholars also propose a control strategy for participating in secondary frequency modulation of a power grid by an electric automobile based on the V2G technology, but the frequency modulation needs to be realized by a communication system and needs the participation of an intermediate agent, so that the length of an information transmission chain and the complexity of realization are increased, and information interaction delay and cost are increased to a certain extent. Therefore, the invention aims to develop an effective electric vehicle charging and discharging control method based on the V2G technology, realize the aim of providing frequency, inertia and voltage support for a power grid on the basis of meeting the charging requirements of users, realize the adjustment of the power grid frequency without difference under the condition of no communication and intermediate agents, and ensure the safe and stable operation of the power grid to a great extent.
Disclosure of Invention
The invention aims to develop an effective electric vehicle charging and discharging control method based on a V2G technology, provide frequency, inertia and voltage support for a power grid on the basis of meeting the charging requirements of users, realize no-difference adjustment of the power grid frequency under the condition of no communication and no intermediate agent, and ensure safe and stable operation of the power grid.
The technical problem to be solved by the invention is that aiming at the problems of inertia loss and unstable frequency of a power system caused by large-scale network access of the electric automobile and large-scale application of power electronic devices, the functions of participating in power grid frequency modulation of the electric automobile, including primary frequency modulation and secondary frequency modulation under the condition of no communication, are realized by combining the advanced virtual synchronous machine control algorithm at the present stage.
In order to solve the problems, the invention provides a bidirectional charge and discharge control method of an electric vehicle with a V2G function by applying a virtual synchronous machine technology, which has a primary frequency modulation function and a secondary frequency modulation function under the condition of no communication. The charging and discharging circuit adopted by the control method is a two-stage converter circuit, and comprises a PWM (pulse-width modulation) rectifying circuit, a matched LC (inductance-capacitance) filter and a Buck-Boost converting circuit, wherein the two circuits are connected through a direct-current bus capacitor. The PWM rectification circuit rectifies the power grid voltage into 700V direct-current voltage, and the alternating-current side filters out harmonic waves through an LC filter and is connected with a power grid through a grid-side inductor; the Buck-Boost conversion circuit converts 700V direct current voltage into 60V direct current voltage and is directly connected with the electric automobile.
Corresponding to the main circuit of the bidirectional charge and discharge motor, the control method can be divided into two modules: the AC/DC control module is responsible for controlling the voltage of a direct current bus to be maintained at a constant value of 700V, introducing virtual inertia and damping and accurately responding to DC/DC power conversion; the DC/DC control module performs constant-voltage, constant-current or constant-power charging and discharging control on the power battery of the electric automobile, and the charging and discharging modes of the DC/DC control module are flexibly switched according to the battery state. And meanwhile, a frequency modulation control module is embedded into the DC/DC control module, a charging and discharging power reference value of the power battery is given through the frequency modulation control module, and the converter responds to realize primary and secondary regulation of the power grid frequency.
The AC/DC control module adopts a virtual synchronous machine control technology and comprises three sub-modules: the device comprises an inertial damping module, a power calculation module and a reactive-voltage control module.
The inertia damping module is based on the motion equation of the synchronous motor
Design is carried out, J is virtual inertia, Te is electromagnetic torque, Tm is mechanical rotationMoment and Kd are damping coefficients; and the difference between the electromagnetic torque and the mechanical torque and the damping torque is compared with the inertia constant, the virtual angular velocity omega of the virtual synchronous machine can be obtained through an integration link, and the virtual angular velocity is integrated to obtain the virtual phase theta of the alternating-current side voltage of the virtual synchronous machine. The mechanical torque is output by a direct current bus voltage PI regulator:
KP and KI are respectively proportional and integral coefficients of the PI controller, Vdc is a voltage reference value (700V) of the direct current bus, Vdc is an actual value of the direct current bus voltage, response to the requirement of the rear-stage DC/DC power is realized through a direct current bus voltage control loop, and a power reference value is provided for virtual synchronous machine control.
The power calculation module is mainly used for calculating electromagnetic torque and reactive power generated at the alternating current side of the synchronous converter and three-phase voltage output at the alternating current side, and the calculation formula is as follows:
e＝Mfifωsinθ
Te＝Mfif<i,sinθ>
Q＝-Mfifω<i,cosθ>
wherein:<·,·>represents a dot product operation, e ═ ea，eb，ec]TFor virtually synchronizing the electromechanical potentials, MfFor mutual inductance between stator and rotor of virtual synchronous machine ifFor virtual exciting current, theta is virtual synchronous machine power angle, i ═ ia，ib，ic]TAnd inputting current for the virtual synchronous machine, and Q is the reactive output of the virtual synchronous machine.
The reactive-voltage control module adopts improved reactive droop control, and when the alternating current side is adoptedWhen the voltage amplitude has an error with its reference value, i.e. Δ V ═ Vn-V ≠ 0, varying the amount of reactive power issued/absorbed by the virtual synchronous machine, calculating
VnThe reference voltage amplitude, V, delta Q, Kq and Kqi are proportional and integral coefficients. A reactive reference value QsetAnd the sum of the delta Q and the actual reactive value are subtracted, the virtual excitation Mfif of the virtual synchronous machine is obtained through the calculation of an integral link with the gain of 1/K, and the voltage of the alternating current side is adjusted.
The DC/DC control module comprises two sub-modules: the converter control submodule and the frequency modulation control submodule.
The converter control submodule has three control modes: constant voltage charging, constant current charging and constant power charging, when normally charging, three kinds of control mode carry out nimble switching according to electric automobile's battery state: if the battery is in a low-power state, the battery power is rapidly increased by adopting constant-power charging, when the charging current reaches a specified value, the battery voltage is switched to a constant-current charging mode, and at the moment, the battery voltage continuously increases, and when the battery voltage reaches the specified value, the battery voltage is switched to constant-voltage charging. In addition, the DC/DC converter is more flexible to control, and advanced control methods such as negative pulse control and the like can be adopted when the DC/DC converter is in a normal charging state. When the electric automobile is allowed to participate in power grid frequency modulation, the DC/DC part adopts a constant power control mode to effectively track a power reference value given by the frequency modulation control submodule.
The frequency modulation control submodule is divided into a primary frequency modulation part and a secondary frequency modulation part.
The primary frequency modulation adopts a droop control strategy, when the frequency of the power grid is reduced, the charging power is reduced or the discharging power is improved, and when the frequency of the power grid is increased, the discharging power is reduced or the charging power is improved. A dead zone link is added in the droop control ring after the frequency difference is calculated, and if the frequency deviation of the power grid is larger than a dead zone value, namely | fn-f|>fdeathWhen let Δ f be f-fn，ΔP1＝KpfΔ f, wherein fnFor frequency nominal value, f is frequency actualValue,. DELTA.f is the frequency difference, fdeathFor the first order frequency modulation response dead zone value, KpfAs sag factor, Δ P1The power change value is output by the primary frequency modulation link, the positive value indicates that the charging power is increased, and the negative value indicates that the charging power is reduced. By Δ P1And changing the charging and discharging power of the electric automobile to stabilize the frequency of the power grid to a certain value. When the frequency fluctuation range is in a very small range, the primary frequency modulation control link does not play a role, and the problem that the service life of the battery is shortened due to frequent change of the charging and discharging state of the battery caused by small frequency disturbance is solved.
The secondary frequency modulation control is mainly used for calculating the power instruction correction quantity of the electric automobile according to the power grid frequency deviation after the primary frequency modulation is stable, and further adjusting the charge-discharge power reference value of the electric automobile. Because the secondary frequency modulation is adjusted on the basis of the primary frequency modulation, the secondary frequency modulation has a longer period compared with the primary frequency modulation.
According to the active power-frequency operation curve comprising the electric automobile and the power grid active power-frequency operation curve, when the frequency is greatly changed and exceeds the allowable fluctuation range (0.2Hz) of the normal frequency due to sudden drop or impact of power generation of the power grid and sudden increase/decrease of intermittent load, namely, | fn-f|>0.2Hz, and calculating the power reference correction quantity delta P of the electric automobile2Changing a charge-discharge power reference value of the electric automobile, enabling an active-frequency operation curve of the electric automobile to translate, controlling the power grid frequency within an allowable error range, stabilizing the power grid frequency at f 'after secondary frequency modulation, and requiring a frequency characteristic curve moving condition to ensure that the power battery does not exceed an adjustable power limit in a subsequent primary frequency modulation process by using a selection principle of f' as follows: that is to say, when operating at a minimum frequency, the electric vehicle power reaches a maximum discharge value. The maximum boundary value of the abscissa of the intersection of the corresponding load operating curve and generator operating curve is f ', where the frequency is recorded as f ' max, and f ' is selected to satisfy the interval fmin, fmax]And [ fmin, f' max ]]And (4) the following steps. When f' max<f 'max is selected when fn, and f' max>In the case of fn, f' may be selected as fn. F 'is thus selected'The principle of (1) is as follows:
the invention considers the electric automobile as a special load, considers other traditional loads in the system and the active power-frequency running characteristic of the electric automobile, and the power instruction correction quantity of the electric automobile is as follows:
when f' is epsilon [ fmin,fn-fdeath]When the temperature of the water is higher than the set temperature,
ΔP2＝(Kpf+KG+KL)(f-f')
when f' is epsilon [ fn-fdeath,fn+fdeath]When the temperature of the water is higher than the set temperature,
ΔP2＝KG(f-f')+(Kpf+KL)(f-fn+fdeath)
+KL(fn-fdeath-f')
when f' is epsilon [ fn+fdeath,fmax]When the temperature of the water is higher than the set temperature,
ΔP2＝KG(f-f')+2KLfdeath
+(KL+Kpf)(f-f'+2fdeath)
in the formula, f' death is a primary frequency modulation response dead zone value, KG is a power regulation rate of a generator, KL is a conventional load power regulation rate in a system, and Kpf is a droop coefficient of the electric automobile. In order to accurately control the frequency at f', the invention uses delta P2As Δ P2Corresponding to a two-stage adjustment of the secondary frequency modulation, Δ P2At Δ P2Further fine-tuning is performed on the basis of the adjustment. F ' is differed with the actual measuring frequency to obtain a ' correction compensation quantity ' delta P through an integral controller2Therefore, the adjusted charge/discharge power command is: p'set＝Pset+ΔP2+ΔP2'。
The secondary frequency modulation is used for further adjusting the charging power and the discharging power on the basis of the primary frequency modulation, the secondary frequency modulation is longer than the primary frequency modulation period, the power instruction correction amount is calculated for multiple times in each secondary frequency modulation period, and finally the average value is taken as the electric vehicle power reference correction amount.
The secondary frequency modulation step responds to the frequency change of the power grid by changing the charging and discharging power of the battery of the electric automobile, controls the frequency within an error allowable range, changes the charging and discharging power while considering the upper and lower limits of the charging and discharging power of the electric automobile, and charges and discharges according to the boundary power when the charging and discharging power exceeds the reasonable range of the electric automobile.
The invention has the advantages that the future large-scale electric automobile can be promoted to be used as mobile energy storage equipment to ensure the frequency stability of the power grid. The inertial support, the frequency support and the power support are provided for a power grid, the service life of a power battery and other problems are considered, and the charging and discharging control of the electric automobile is realized based on the V2G technology. The front-stage AC/DC control strategy adopts a virtual synchronous machine control strategy, and virtual inertia and a damping link are introduced, so that the problems of over-quick response and inertia loss of power electronic devices are solved, and inertia support is provided for a power grid; and introducing reactive droop control to provide reactive support for the power grid. The later-stage DC/DC control method considers the service life problem of the power battery, selects the charging and discharging mode according to the running state of the power battery, simultaneously introduces primary frequency modulation and secondary frequency modulation control, adjusts the charging and discharging state of the electric automobile according to the frequency of the power grid, controls the frequency of the power grid within an allowable error range, and can also realize the non-difference control of the frequency.
Drawings
Fig. 1 is a diagram of a hardware power circuit employed in the present invention.
FIG. 2 is a diagram of an AC/DC control module for use with the present invention.
FIG. 3 is a diagram of a DC/DC control module for use with the present invention.
Fig. 4 is a graph of the active power-frequency operating characteristics of the electric vehicle.
Fig. 5 is a graph of the active-frequency operation of the generator.
FIG. 6 is a graph of a power reference correction compensation loop.
FIG. 7 is a diagram of the construction of a simulation platform based on MATLAB/SIMULINK.
Fig. 8 is a graph of grid frequency in a simulated system and power flow in the system and tie-line.
Fig. 9 is a graph of power battery charge and discharge power.
Fig. 10 is a diagram of power battery SOC state and charging current voltage.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
The invention provides a control method for participating in power grid frequency regulation of electric vehicle bidirectional charging and discharging based on a V2G technology, aiming at the problems of inertia loss and unstable operation frequency in a power grid caused by large-scale network access and large-scale power electronic device application of the electric vehicle, and the control method can provide inertia and frequency support for the power grid and realize frequency no-difference regulation.
The hardware power circuit adopted by the invention is shown in figure 1, the two-stage power conversion circuit comprises a preceding-stage PWM (pulse-width modulation) rectification circuit, a matched LC (inductance-capacitance) filter and a Buck-Boost direct current conversion circuit, and the two circuits are connected through a direct current bus capacitor. The PWM rectification circuit rectifies the power grid voltage into 700V direct-current voltage, and the alternating-current side filters out harmonic waves through an LC filter and is connected with a power grid through a grid-side inductor; the Buck-Boost conversion circuit converts 700V direct current voltage into 60V direct current voltage and is directly connected with the electric automobile.
The invention mainly comprises two modules: the AC/DC control module shown in fig. 2 and the DC/DC control module shown in fig. 3, wherein the DC/DC control module can be divided into two parts: frequency modulation control part and converter charge mode control part. The frequency modulation control is divided into primary frequency modulation and secondary frequency modulation, the primary frequency modulation is realized through droop control, however, in consideration of the service life of the power battery, a dead zone link is passed after the frequency difference is calculated, and the droop control can only play a role in regulation when the frequency difference exceeds a set value of the dead zone, and fig. 4 is an active power-frequency operation characteristic curve of the electric automobile. Fig. 5 shows a system load and a generator active-frequency operation curve with the electric vehicle as a special load, and the secondary frequency modulation is realized by modifying the charging power reference value of the electric vehicle by calculating a power reference correction according to the curve. The AC/DC control module controls the voltage of the direct current bus to be maintained at a constant value of 700V, controls the voltage of the alternating current side at a reference value, introduces virtual inertia into the control and responds to power conversion at DC/DC; the DC/DC control part controls the constant voltage, the constant current or the constant power of the power battery of the electric automobile, and the charging mode of the DC/DC control part is switched according to the state of the battery.
The AC/DC control module adopts a virtual synchronous machine control technology and comprises three sub-modules: inertia, damping module, power calculation module, reactive-voltage control module.
The inertia damping module is based on the motion equation of the synchronous motorDesigning, wherein J is virtual inertia, Te is electromagnetic torque, Tm is mechanical torque, and Kd is a damping coefficient; and the difference between the electromagnetic torque and the mechanical torque and the damping torque is compared with the inertia constant, the virtual angular velocity omega of the virtual synchronous machine can be obtained through an integration link, and the virtual angular velocity is integrated to obtain the virtual phase theta of the alternating-current side voltage of the virtual synchronous machine. The mechanical torque is output by a direct current bus voltage PI regulator:
KP and KI are respectively proportional and integral coefficients of the PI controller, Vdc is a voltage reference value (700V) of the direct current bus, Vdc is an actual value of the direct current bus voltage, response to the requirement of the rear-stage DC/DC power is realized through a direct current bus voltage control loop, and a power reference value is provided for virtual synchronous machine control.
The power calculation module is mainly used for calculating electromagnetic torque and reactive power generated at the alternating current side of the synchronous converter and three-phase voltage output at the alternating current side, and the calculation formula is as follows:
e＝Mfifωsinθ
Te＝Mfif<i,sinθ>
Q＝-Mfifω<i,cosθ>
wherein:<·,·>represents a dot product operation, e ═ ea，eb，ec]TFor virtually synchronizing the electromechanical potentials, MfFor mutual inductance between stator and rotor of virtual synchronous machine ifFor virtual exciting current, theta is virtual synchronous machine power angle, i ═ ia，ib，ic]TAnd inputting current for the virtual synchronous machine, and Q is the reactive output of the virtual synchronous machine.
The reactive-voltage control module adopts improved reactive droop control, and when the voltage amplitude of the alternating-current side has an error with a reference value thereof, namely delta V is equal to Vn-V ≠ 0, varying the amount of reactive power issued/absorbed by the virtual synchronous machine, calculating
VnThe reference voltage amplitude, V, delta Q, Kq and Kqi are proportional and integral coefficients. A reactive reference value QsetAnd the sum of the delta Q and the actual reactive value are subtracted, virtual excitation Mfif of the virtual synchronous machine is obtained through the calculation of an integral link, and the voltage of the alternating current side is adjusted.
The DC/DC control module comprises two sub-modules: the converter control submodule and the frequency modulation control submodule.
The converter word control module has three control modes: constant voltage, constant current and constant power control, when normal charge, three kinds of control mode carry out nimble switching according to electric automobile's battery state: if the battery is in a low-power state, the battery power is rapidly increased by adopting constant-power charging, when the charging current reaches a specified value, the battery voltage is switched to a constant-current charging mode, and at the moment, the battery voltage continuously increases, and when the battery voltage reaches the specified value, the battery voltage is switched to constant-voltage charging. In addition, the DC/DC converter is more flexible to control, and advanced control methods such as negative pulse control and the like can be adopted when the DC/DC converter is in a normal charging state. When the electric automobile is allowed to participate in power grid frequency modulation, the DC/DC part adopts a constant power control mode to effectively track a power reference value given by the frequency modulation control submodule.
The frequency modulation control module is divided into primary frequency modulation and secondary frequency modulation, the primary frequency modulation adopts a droop control strategy, the charging power is reduced or the discharging power is improved when the frequency of the power grid is reduced, and the discharging power is reduced or the charging power is improved when the frequency of the power grid is increased. A dead zone link is added in the droop control ring after the frequency difference is calculated, and if the frequency deviation of the power grid is larger than a dead zone value, namely | fn-f|>fdeathWhen let Δ f be f-fn，ΔP1＝KpfΔ f, wherein fnFor the frequency nominal value, f is the frequency actual value, Δ f is the frequency difference, fdeathFor the first order frequency modulation response dead zone value, KpfAs sag factor, Δ P1The power change value is output by the primary frequency modulation link, the positive value indicates that the charging power is increased, and the negative value indicates that the charging power is reduced. By Δ P1And changing the charging and discharging power of the electric automobile to stabilize the frequency of the power grid to a certain value. When the frequency fluctuation range is in a very small range, the primary frequency modulation control link does not play a role, and the problem that the service life of the battery is shortened due to frequent change of the charging and discharging state of the battery caused by small frequency disturbance is solved.
The secondary frequency modulation control is mainly used for calculating the power reference correction quantity of the electric automobile according to the frequency change of the power grid after the primary frequency modulation is stable, and changing the charge-discharge power reference value of the electric automobile. Because the secondary frequency modulation is adjusted on the basis of the primary frequency modulation, and the secondary frequency modulation period is longer than the primary frequency modulation period, the secondary frequency modulation period is set to be 20 times of the primary frequency modulation control period, the secondary frequency modulation power reference correction is calculated for 20 times in each control period, and finally, the average value is taken as the electric automobile power reference correction.
The active power-frequency operation curve of the electric vehicle is shown in fig. 4, wherein the electric vehicle is used as a special load, and when the power of the electric vehicle is negative, the electric vehicle is in a discharging operation state, wherein when the frequency is in a dead zone region fn-fdeath，fn+fdeath]The charging and discharging power is not changed and is limited by the battery, the charging and discharging power is limited, and the droop characteristic is shown when the frequency exceeds the dead zone interval. When a conventional load is present in the system, the active power-frequency operating curve of the load of the system is shown in fig. 5, and is represented by a rated frequency fnThe reason why the slope of the curve is first increased and then decreased is that the electric vehicle does not exhibit a droop characteristic in the dead zone interval and the slope is decreased when the electric vehicle reaches the power limit value.
According to the active power-frequency operation curve comprising the electric automobile and the power grid active power-frequency operation curve, when the frequency is greatly changed and exceeds the allowable fluctuation range (0.2Hz) of the normal frequency due to sudden drop or impact of power generation of the power grid and sudden increase/decrease of intermittent load, namely, | fn-f|>0.2Hz, and calculating the power reference correction quantity delta P of the electric automobile2Changing a charge-discharge power reference value of the electric automobile, enabling an active-frequency operation curve of the electric automobile to translate, controlling the power grid frequency within an allowable error range, stabilizing the power grid frequency at f 'after secondary frequency modulation, and requiring a frequency characteristic curve moving condition to ensure that the power battery does not exceed an adjustable power limit in a subsequent primary frequency modulation process by using a selection principle of f' as follows: that is, when operating at the minimum frequency, the electric vehicle power just reaches the maximum discharge value, as shown in fig. 5. The maximum boundary value of the abscissa of the intersection of the corresponding load operating curve and generator operating curve is f ', where the frequency is recorded as f ' max, and f ' is selected to satisfy the interval fmin, fmax]And [ fmin, f' max ]]And (4) the following steps. When f' max<f 'max is selected when fn, and f' max>In the case of fn, f' may be selected as fn. The principle of choosing f' is therefore:
in fig. 5, the electric vehicle is initially and stably operated at point a for normal charging, the grid generated power suddenly drops at a certain time, the generated operating curve is reduced from PG1 to PG2, the electric vehicle operation stable point moves from point a to point B, the frequency error exceeds the allowable maximum range, and f' is selected as the final operating frequency. The variable quantity of the power set value of the electric automobile is as follows:
when f' is epsilon [ fmin,fn-fdeath]When the temperature of the water is higher than the set temperature,
ΔP2＝(Kpf+KG+KL)(f-f')
when f' is epsilon [ fn-fdeath,fn+fdeath]When the temperature of the water is higher than the set temperature,
ΔP2＝KG(f-f')+(Kpf+KL)(f-fn+fdeath)
+KL(fn-fdeath-f')
when f' is epsilon [ fn+fdeath,fmax]When the temperature of the water is higher than the set temperature,
ΔP2＝KG(f-f')+2KLfdeath
+(KL+Kpf)(f-f'+2fdeath)
wherein, KG is the power regulation coefficient of the generator. Meanwhile, the power reference correction quantity compensation loop in FIG. 6 calculates the "correction compensation quantity" Δ P2', the new charge and discharge power reference value of the electric automobile is as follows: p'set＝Pset+ΔP2+ΔP2'so that the system frequency can be accurately stabilized at f'. And judging whether the new charge-discharge power reference value is in a normal range of the electric automobile operation, comparing the new reference value with the upper limit and the lower limit of the charge-discharge power, and adjusting according to the comparison condition.
A system model shown in fig. 7 is built based on an MATLAB/SIMULINK simulation platform, the bidirectional charge and discharge machine is controlled by the control strategy, no secondary frequency modulation control is added before 1s, and the dead zone of a primary frequency modulation control link is set to be +/-0.005 Hz. The load 4 is added into the system 1 at 0.3s, the secondary frequency modulation control link is added at 1s, the action dead zone value of the secondary frequency modulation control is set to be +/-0.02 Hz, the maximum charging power of the power battery is set to be 15kW, and the maximum discharging power is set to be 10 kW. Fig. 8 is a graph showing the grid frequency and the power flow in the system and the tie line in the simulation system, fig. 9 is a graph showing the charging and discharging power of the power battery (negative value shows charging, and positive value shows discharging), and fig. 10 is a graph showing the SOC state and the charging current and voltage of the power battery (negative value shows charging).
The load of the system 1 is increased when 0.3s is needed, the generated power is insufficient, the system frequency is reduced, the system frequency slowly stabilizes to a certain value due to the fact that a primary frequency modulation control link exists in the bidirectional charging and discharging machine control, the charging power of the power battery is reduced in the process, the charging voltage and the charging current are reduced to some extent, the SOC increasing speed is slightly slowed down, and finally the charging power is stabilized to about 4.3 kW. And (2) adding a secondary frequency modulation control link at 1s, wherein the secondary frequency modulation period is 20 times of the primary frequency modulation period, f' is 49.98Hz, the frequency of the system at the moment is 49.96Hz and exceeds the set value of the action dead zone of the secondary frequency modulation control by +/-0.02 Hz, the secondary frequency modulation control plays a role, the frequency of the system slowly rises and is stabilized within the error range of +/-0.02 Hz, the charging power of the power battery is reduced in the process, the charging voltage and current are all reduced to some extent, the SOC increase speed is obviously slowed down, and finally the charging power is stabilized at about 1.5 kW. The simulation proves that the electric automobile can effectively participate in the frequency modulation process of the power system as the reserve capacity of the power system, certain inertia exists in the frequency variation process, the defect that power electronic devices act too fast is overcome, and inertia and frequency support are provided for a power grid.
The control method provided by the invention considers the problems of power battery service life and power grid inertia loss, provides frequency, inertia and voltage support for the power grid while meeting the charging requirements of electric vehicle users, and can realize the no-difference adjustment of the power grid frequency under the condition that the adjustable capacity of the electric vehicle is enough.
The technical solution of the present invention is explained in detail above. It is obvious that the invention is not limited to what has been described. Based on the embodiments of the present invention, those skilled in the art can make various changes, but any changes equivalent or similar to the present invention are within the protection scope of the present invention.
Claims (5)
1. A V2G technology-based electric vehicle participation power grid frequency modulation control method is characterized in that a charging and discharging circuit is a two-stage converter circuit, the first-stage converter circuit comprises a PWM (pulse-width modulation) rectification circuit and an LC (inductance capacitance) filter matched with the PWM rectification circuit, the second-stage converter circuit comprises a Buck-Boost conversion circuit, the two-stage converter circuit is connected through a direct current bus capacitor, the PWM rectification circuit rectifies power grid voltage into 700V direct current voltage, the alternating current side filters harmonic waves through the LC filter, and the alternating current side is connected with a power grid through a grid side inductor; the Buck-Boost conversion circuit converts 700V direct current voltage into 60V direct current voltage and is directly connected with the electric automobile; according to the method, an AC/DC control module and a DC/DC control module are used, a frequency modulation control module is embedded into the DC/DC control module, a charging and discharging power reference value of a power battery is given through the frequency modulation control module, and a Buck-Boost conversion circuit responds to realize primary and secondary regulation of the power grid frequency; the AC/DC control module is responsible for controlling the voltage of the direct-current bus to be maintained at a constant value of 700V, introducing virtual inertia and damping and making an accurate response to DC/DC power conversion; the DC/DC control module performs constant-voltage, constant-current or constant-power charging and discharging control on the power battery of the electric automobile, and the charging and discharging modes of the DC/DC control module are flexibly switched according to the battery state.
2. The method of claim 1, wherein the AC/DC control module employs a virtual synchronous machine control technique, which comprises three sub-modules: namely, an inertia damping module, a power calculation module and a reactive-voltage control module;
the inertia damping module is based on the motion equation of the synchronous motor
Design is made with J as virtual inertia, TeFor electromagnetic torque, TmAs mechanical torque, KdFor the damping coefficient, Δ ω is the virtual angular velocity ωA deviation of (a); the product of the electromagnetic torque and the mechanical torque and the damping coefficient and the virtual angular velocity deviation delta omega is subjected to difference and then is compared with an inertia constant, through an integration link, the virtual angular velocity omega of the virtual synchronous machine can be obtained, the virtual angular velocity is integrated to obtain the virtual phase theta of the voltage on the alternating current side of the virtual synchronous machine, and the mechanical torque is output by a direct current bus voltage PI regulator:
wherein, KPAnd KIProportional and integral coefficients, V, of the PI controller, respectivelydcVoltage reference value 700V, V of DC busdcFor the actual value of the direct-current bus voltage, the direct-current bus voltage control loop realizes the response to the subsequent DC/DC power demand and provides a power reference value for the virtual synchronous machine control;
the power calculation module is used for calculating electromagnetic torque and reactive power generated at the alternating current side of the PWM rectification circuit and three-phase voltage output at the alternating current side, and the calculation formula is as follows:
e＝Mfifωsinθ
Te＝Mfif<i,sinθ>
Q＝-Mfifω<i,cosθ>
wherein:<·,·>represents a dot product operation, e ═ ea，eb，ec]TFor virtually synchronizing the electromechanical potentials, MfFor mutual inductance between stator and rotor of virtual synchronous machine ifFor the virtual exciting current, theta is the virtual phase of the AC side voltage of the virtual synchronous machine, i ═ ia，ib，ic]TInputting current for the virtual synchronous machine, Q is the reactive output of the virtual synchronous machine,the a, b and c are phase sequence a, phase sequence b and phase sequence c respectively;
the reactive-voltage control module adopts improved reactive droop control, and when the voltage amplitude of the alternating-current side has an error with a reference value thereof, namely, the voltage amplitude is equal to Vn-V ≠ 0, varying the amount of reactive power issued/absorbed by the virtual synchronous machine, calculating
VnIs a reference voltage amplitude, V is an actual voltage amplitude, Δ Q is a reactive variation, Kq、KqiFor proportional and integral coefficient, the reactive reference value Q is setsetThe sum of the delta Q and the actual reactive value are subtracted, and the virtual excitation M of the virtual synchronous machine is obtained through the calculation of an integral link with the gain of 1/KfifAnd regulating the voltage on the alternating current side.
3. The method of claim 1, wherein the DC/DC control module comprises two sub-modules, namely a converter control sub-module and a frequency modulation control sub-module;
the converter control submodule has three control modes, namely constant voltage charging, constant current charging and constant power charging, if the battery is in a low-power state, the constant power charging is adopted to enable the electric quantity of the battery to quickly rise, when the charging current reaches a specified value, the constant current charging mode is switched, the battery voltage continuously rises, and when the battery voltage reaches the specified value, the constant voltage charging mode is switched;
the frequency modulation control submodule is divided into a primary frequency modulation part and a secondary frequency modulation part, the primary frequency modulation adopts droop control, the charging power is reduced or the discharging power is improved when the frequency of the power grid is reduced, and the discharging power is reduced or the charging power is improved when the frequency of the power grid is increased; the secondary frequency modulation is to calculate the power instruction correction quantity of the electric automobile according to the power grid frequency deviation after the primary frequency modulation is stable, and further adjust the charge and discharge power reference correction quantity of the electric automobile.
4. The method of claim 3, wherein the method further comprisesThe electric vehicle 'power instruction correction quantity' delta P2Comprises the following steps:
when f' is epsilon [ fmin,fn-fdeath]When the temperature of the water is higher than the set temperature,
ΔP2＝(Kpf+KG+KL)(f-f')
when f' is epsilon [ fn-fdeath,fn+fdeath]When the temperature of the water is higher than the set temperature,
ΔP2＝KG(f-f')+(Kpf+KL)(f-fn+fdeath)+KL(fn-fdeath-f')
when f' is epsilon [ fn+fdeath,fmax]When the temperature of the water is higher than the set temperature,
ΔP2＝KG(f-f')+2KLfdeath+(KL+Kpf)(f-f'+2fdeath)
in the formula (f)nF is the frequency nominal value, f is the frequency actual value, f' is the grid frequency after the secondary frequency modulation, fmaxIs the maximum boundary value of fminIs the minimum boundary value of fdeathFor the first order frequency modulation response dead zone value, KGFor the power regulation of the generator, KLFor the conventional load power regulation rate in the system, KpfThe droop coefficient of the electric automobile is obtained; the adjusted charge-discharge power instruction is as follows: p'set＝Pset+ΔP2+ΔP2', wherein PsetFor the charge-discharge power command before adjustment, Δ P2Is' Δ P2The compensation quantity of (a) is obtained by making a difference between f' and the actual measurement frequency through an integral controller.
5. The method of claim 1, wherein the frequency change is responded by changing the charging and discharging power of the battery of the electric vehicle, the frequency is controlled within an error allowable range, the charging and discharging power is changed by frequency modulation while considering the upper and lower limits of the charging and discharging power of the electric vehicle, and when the charging and discharging power exceeds a reasonable range of the electric vehicle, the charging and discharging are carried out according to the boundary power
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