CN111682560B - Method for restraining electromechanical oscillation of power grid based on rapid power support of photovoltaic power generation system - Google Patents

Method for restraining electromechanical oscillation of power grid based on rapid power support of photovoltaic power generation system Download PDF

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CN111682560B
CN111682560B CN202010562963.6A CN202010562963A CN111682560B CN 111682560 B CN111682560 B CN 111682560B CN 202010562963 A CN202010562963 A CN 202010562963A CN 111682560 B CN111682560 B CN 111682560B
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power
photovoltaic
power generation
generation system
module
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CN111682560A (en
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刘新元
唐震
郝捷
李明贤
郑惠萍
张一帆
王玮茹
程雪婷
薄利明
张颖
陈丹阳
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State Grid Electric Power Research Institute Of Sepc
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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
    • 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
    • H02J3/241—The oscillation concerning frequency
    • 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
    • H02J3/381—Dispersed generators
    • 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
    • H02J3/48—Controlling the sharing of the in-phase component
    • 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
    • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
    • H02J2300/20—The dispersed energy generation being of renewable origin
    • H02J2300/22—The renewable source being solar energy
    • 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
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/56—Power conversion systems, e.g. maximum power point trackers

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Eletrric Generators (AREA)

Abstract

The invention belongs to the field of power electronics, and particularly discloses a method for inhibiting electromechanical oscillation of a power grid based on rapid power support of a photovoltaic power generation system, which comprises the following steps: the system comprises a power grid voltage and current acquisition module, a synchronous rotation coordinate transformation module, a voltage phase acquisition module, a synchronous machine rotating speed acquisition module, a rotating speed deviation judgment module, a rotating speed sagging control module, a photovoltaic rapid power modulation module and a photovoltaic steady-state power generation module. The invention divides the working process of the photovoltaic power generation system for inhibiting the electromechanical oscillation into 4 typical working conditions, sets the switching conditions of different working conditions and corresponding power output instructions according to the rotating speed deviation and the change rate of the synchronous machine rotor in different oscillation periods, is a simple and effective practical method, and has good application value.

Description

Method for restraining electromechanical oscillation of power grid based on rapid power support of photovoltaic power generation system
Technical Field
The invention relates to the field of power electronics, in particular to a method for restraining electromechanical oscillation of a power grid based on rapid power support of a photovoltaic power generation system, which is suitable for rapidly restraining the rotation speed oscillation phenomenon of a synchronous machine rotor caused by power unbalance of the power system.
Background
The power system is often subjected to different power disturbances, which easily causes the synchronous generator to generate rotor rotational speed oscillation phenomenon (called electromechanical oscillation) related to electromechanical energy conversion, and is unfavorable for the stable operation of the power system. With the large-scale access of the new energy power generation device mainly comprising the photovoltaic power generation system to the power grid, the duty ratio of the traditional synchronous generator is gradually reduced, and the inherent damping capacity and the inherent inertia level of the power system are reduced. In addition, the photovoltaic grid-connected power generation system is generally considered to be low in inertia and weak in damping, so that the damping capacity and the inertia level of the power system containing the new energy power generation device are further weakened, and the problem of electromechanical oscillation of the power grid is difficult to effectively inhibit.
The essence of the electric network electromechanical oscillation phenomenon is that the active power of an electric power system is unbalanced, so that the rotor speed of the synchronous generator alternately accelerates or decelerates. Therefore, an effective power control strategy is added to the photovoltaic power generation system to compensate the power shortage in the system or absorb surplus power, so that the electromechanical oscillation phenomenon of the synchronous machine rotor can be quickly subsided, and the photovoltaic power generation system has important significance for the stable operation of the power system.
The existing power grid electromechanical oscillation suppression strategy based on the photovoltaic power generation system mainly comprises droop control and inertia control, and active power output of the photovoltaic power generation system is regulated and controlled generally according to the rotating speed deviation and the rotating speed change rate of a synchronous machine rotor; in addition, the method also comprises the step of operating the photovoltaic power generation system as a static reactive compensator in a period of time when the electromechanical oscillation phenomenon of the power grid occurs, and adjusting the power grid voltage to indirectly restrain unbalanced power of the power grid. However, none of the existing control strategies exploit the potential of photovoltaic power generation systems to suppress grid electromechanical oscillations to the maximum extent.
Disclosure of Invention
Aiming at the defect that the existing photovoltaic power generation system control strategy is insufficient in power grid electromechanical oscillation inhibition capability, the invention provides a method for inhibiting power grid electromechanical oscillation based on a photovoltaic power generation system rapid power support technology.
The invention is realized by the following technical scheme:
the method for restraining the electromechanical oscillation of the power grid based on the rapid power support of the photovoltaic power generation system comprises the steps of firstly setting an operation interval of direct-current voltage in the photovoltaic power generation system between voltage corresponding to a maximum power output point and open-circuit voltage, and reserving half of frequency modulation standby capacity, namely setting the power generation power of the photovoltaic power generation system to be half of the maximum output power. The invention combines the advantages of a rapid power support and sagging control strategy, divides the working process of the photovoltaic power generation system for inhibiting electromechanical oscillation into 4 typical working conditions, sets the switching conditions of different working conditions and corresponding power output instructions according to the rotating speed deviation and the change rate of the synchronous machine rotor in different oscillation periods, is a simple and effective practical method, and has good application value. The technology disclosed by the invention has the advantages that the implementation process is simple, the capacity of restraining the electromechanical oscillation of the power grid by using the photovoltaic power generation system can be developed and utilized to the maximum extent, the rotor rotation speed oscillation phenomenon caused by the electromechanical oscillation can be quickly subsided, and the frequency stability of the power system is improved.
A method for suppressing grid electromechanical oscillations based on fast power support of a photovoltaic power generation system, comprising: the system comprises a power grid voltage and current acquisition module, a synchronous rotation coordinate transformation module, a voltage phase acquisition module, a synchronous machine rotating speed acquisition module, a rotating speed deviation judgment module, a rotating speed sagging control module, a photovoltaic rapid power modulation module and a photovoltaic steady-state power generation module.
The power grid voltage and current acquisition module acquires three-phase signals of power grid voltage and current;
the synchronous rotation coordinate transformation module converts the collected three-phase voltage and current signals into voltage and current direct current components under a rotation coordinate system;
the voltage phase acquisition module acquires phase signals of the power grid voltage in real time and outputs the phase signals to the synchronous rotation coordinate transformation module;
the synchronous machine rotating speed acquisition module acquires a real-time rotating speed signal of a rotor in the synchronous generator;
the rotating speed deviation judging module judges the detected rotating speed deviation and the change rate of the rotor of the synchronous generator, and the rotating speed deviation and the change rate of the rotor of the synchronous generator are used as switching conditions of the operation working conditions of the photovoltaic power generation system;
the rotating speed droop control module performs droop control on the output power of the photovoltaic power generation system according to the judging instruction obtained by the rotating speed deviation judging module;
the photovoltaic rapid power modulation module enables the output power of the photovoltaic power generation system to operate in a rapid power mode according to the judging instruction obtained by the rotating speed deviation judging module, namely, the photovoltaic power generation system only has two working conditions of outputting maximum power and minimum power of 0;
and the photovoltaic steady-state power generation module enables the photovoltaic power generation system to run in a steady-state power generation state according to the judging instruction obtained by the rotating speed deviation judging module, and no controllable active power is output any more to inhibit the electromechanical oscillation of the power grid.
(1) Under the action of a droop control strategy (proportional controller) or an inertia control strategy (differential controller), the photovoltaic power generation system outputs active current I to the synchronous electromechanical network d Can be described as:
I d ＝K p (ω 0 -ω)＝K p Δω
I d ＝sK d (ω 0 -ω)＝sK d Δω
wherein ω is the actual rotational speed of the synchronous generator rotor, ω 0 K is the synchronous rotation speed of the rotor p 、K d The gains of a proportional controller and a differential controller in the rotating speed control loop are respectively, and s is a differential operator.
(2) If the output power P injected into the power grid by the photovoltaic power generation system is considered PV Damping coefficient T of the power system D And the inertia coefficient T J Can be described as:
wherein D, H is the inherent damping coefficient and inertia coefficient of the system, K e The control coefficient of the photovoltaic power generation system characterizes the influence of the photovoltaic power generation system on the dynamic characteristics of the power grid under the action of a control strategy.
Obviously, the controller gain K p The larger the damping coefficient T of the system D The larger the damping torque provided by the photovoltaic power generation system to the power grid is, the larger the damping torque is, and the rotation speed deviation delta omega can be effectively restrained; similarly, K d The larger the inertia coefficient T of the system J The larger the inertia supporting effect provided for the power grid is, and the capability of the photovoltaic power generation system for inhibiting the rotating speed change rate dω/dt can be improved. Obviously, adjusting the controller gain can equivalently change the process of rotor speed oscillation.
(3) Taking the photovoltaic power generation system operating on the sagging control strategy as an example, it is known from the steps (1) and (2) that K is increased p Active current I of photovoltaic power generation system can be increased d I.e. increasing the photovoltaic output power P PV Thereby enhancing the damping torque effect of the synchronous machine rotor. But K is p When the voltage is increased to the limit, the photovoltaic inverter outputs the maximum output power P of the photovoltaic according to the positive and negative polarities of the rotating speed deviation in the whole period mppt Or the minimum power 0 is used as a reference to continuously and alternately output. During this period, the photovoltaic power generation system's ability to suppress the grid's electromechanical oscillations will reach a limit. The analysis is as follows:
wherein, the equation of motion of the synchronous machine rotor:
wherein P is m Is the mechanical power of the prime mover, P e For electromagnetic power of synchronous machines, it is assumed that the grid load suddenly decreases by Δp m And ΔP e When the output power of the photovoltaic is changed, the electromagnetic power P e Is a variable amount of (a).
When the photovoltaic power generation system is runningIn a linear inertia control strategy, i.e. constant output maximum power P in a period when the output power of the photovoltaic power generation system is no longer local mppt Control gain K of differential controller d The following setting conditions should be satisfied:
considering that the rotor speed change rate reaches the maximum value at the initial moment of the electromechanical oscillation of the power grid, and the photovoltaic power generation system does not output controllable active power to participate in inhibition at the moment, the rotor speed change rate at the moment is as follows:
it can be seen that the gain K of the differential controller at this time d The following should be satisfied:
substituting it into the rotor motion equation of the synchronous machine can obtain:
at this time, the change rate of the rotating speed of the rotor of the synchronous machine is as follows:
when the photovoltaic power generation system operates in a rapid power support mode, the change rate of the rotating speed of the rotor of the synchronous machine is as follows:
the speed change rate of the synchronous machine is used as a standard to be compared to obtain the following steps:
(4) As can be seen from the comparison result of the rotation speed change rate in the step (3), the inhibition capability of the fast power support mode to dω/dt is obviously stronger than that of linear inertia control; similarly, the fast power support mode also has significantly greater damping capability for Δω than linear damping control. The physical mechanism is as follows:
the root causes of the electromechanical oscillations are: the disturbance power causes temporary power unbalance of the power grid, surplus or lack of power can promote the acceleration or deceleration of the rotor, and the more serious the power unbalance is, the larger the acceleration and deceleration energy is, the more serious the oscillation phenomenon is, namely, the greater the dω/dt and Δω indexes are, and the greater the probability of triggering the action of the relay protection device is. The rapid power support mode can fully utilize the power resource of the photovoltaic power generation system to enable the photovoltaic power generation system to rapidly emit or absorb energy with maximum capacity so as to effectively compensate the power shortage of the system or absorb the surplus energy of the system, and the acceleration and deceleration energy which causes excessive dω/dt and Δω is weakened from the source.
(5) The switching threshold values of the 4 operation conditions corresponding to the control strategy of the photovoltaic power generation system and the corresponding power output instructions are shown in table 1.
Wherein: omega th1 Synchronous machine rotor rotation speed deviation threshold value omega for executing sagging control strategy for photovoltaic power generation system th2 And R is R B And respectively executing a threshold value of the rotation speed deviation and the rotation speed change rate of the rapid power support strategy for the photovoltaic power generation system.
Table 1 switching commands for different conditions and corresponding current output commands
When the indexes such as dω/dt, Δω and the like are within the safe interval, i.e., ω 0 -ω th1 <ω<ω 0 +ω th1 And |dω/dt|is less than or equal to R B Indicating that the system does not generate electromechanical oscillation or the oscillation process is completely finished, the photovoltaic power generation system should operate in a steady-state power generation mode, at which point P PV ＝P 0 Wherein P is PV Represents the output power of photovoltaic, P 0 The photovoltaic steady state output power is shown, which corresponds to condition i in table 1. If the rotation speed index meets omega th1 <|Δω|≤ω th2 And |dω/dt|is less than or equal to R B At the moment, the photovoltaic power generation system can be switched to a rotation speed droop mode, and the rotation speed droop mode corresponds to a working condition IV. The output power of the photovoltaic can be obtained according to the rotation speed deviation: 1.5V d K p Δω+P 0 Wherein V is d The voltage direct current component of the d axis is the voltage of the power grid under the action of the synchronous rotation coordinate transformation module. The photovoltaic sagging control strategy can provide necessary damping torque for the power grid, so that the electromechanical oscillation is forced to be further weakened, and the frequency quality of the power grid is improved.
(6) If the rotation speed evaluation index dω/dt, Δω satisfies the threshold set by the condition II, i.e., Δω>ω th2 And |dω/dt|>R B Indicating that the power of the power system is seriously absent at the moment, the photovoltaic power generation system should timely increase the active power output so as to fill the absent power in the power system. Its current command I d * Should be immediately increased to the forward current limit, i.e. the current I corresponding to the maximum output power of the photovoltaic mppt Continuously providing positive maximum power support to the grid, at which point P PV ＝P mppt To significantly inhibit Δω, dω/dt, corresponding to condition ii. Conversely, if Deltaω<-ω th2 Or |dω/dt|>R B Indicating that the power of the power system is seriously excessive, the photovoltaic power generation system should immediately reduce the output of active power to the maximum extent, and the current command I d * Should be immediately reduced to 0 in reverse direction to make the photovoltaic output power P PV Decreasing 0 provides reverse rapid power support to the grid to reduce power surplus, corresponding to condition iii. Compared with the prior art, the invention has the following advantages:
compared with the conventional droop control and inertia control strategy of the photovoltaic power generation system, the electromechanical oscillation suppression strategy provided by the invention does not need a complicated controller parameter setting process, can maximally utilize the suppression capability of the photovoltaic power generation system for suppressing electromechanical oscillation of the power grid by setting the switching threshold values of 4 working conditions according to the power grid operation standard, quickly subsides the rotor oscillation phenomenon caused by power disturbance, does not need to increase other oscillation suppression equipment, and has good industrial application value and low cost.
According to the technical scheme, when the synchronous machine power grid is subjected to power disturbance to generate rotor rotation speed oscillation, the method can quickly subside the electromechanical oscillation phenomenon of the power grid without adjusting parameters of a controller in a sagging and inertia control strategy in real time, and is a simple and effective practical photovoltaic power generation system control strategy.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required in the embodiments will be briefly described below, and it will be apparent that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a non-linear operating diagram of the output power of a photovoltaic power generation system;
FIG. 2 is a schematic diagram of the operation of the photovoltaic power generation system of the present invention to suppress the electromechanical oscillations of the power grid;
FIG. 3 is a schematic diagram of the operation of the grid voltage and current acquisition module and the synchronous machine speed acquisition module of the present invention;
FIG. 4 is a control schematic diagram of the DC output current of the photovoltaic power generation system of the present invention;
FIG. 5 is a graph showing the comparison of the output power of the photovoltaic power generation system at different proportional controller gains according to the present invention;
FIG. 6 is a comparison of photovoltaic power generation system output power at different differential controller gains of the present invention;
FIG. 7 is a schematic illustration of 4 operating conditions of the photovoltaic power generation system of the present invention;
FIG. 8 is a schematic diagram of power output for 4 operating conditions of the photovoltaic power generation system of the present invention;
FIG. 9 is a flow chart of a control strategy proposed by the present invention based on a fast power support technique;
FIG. 10 is a comparison of the rotational speed suppression effect of the proposed strategy of the present invention with conventional control;
FIG. 11 is a comparison of the rate of change of rotational speed suppression effect of the proposed strategy versus a conventional control strategy;
FIG. 12 is a comparison of the output current of the power generation system of the proposed strategy and the conventional control strategy.
Detailed Description
The invention is further described below with reference to the accompanying drawings.
Examples
A method for suppressing grid electromechanical oscillations based on fast power support of a photovoltaic power generation system, comprising: the system comprises a power grid voltage and current acquisition module, a synchronous rotation coordinate transformation module, a voltage phase acquisition module, a synchronous machine rotating speed acquisition module, a rotating speed deviation judgment module, a rotating speed sagging control module, a photovoltaic rapid power modulation module and a photovoltaic steady-state power generation module.
The power grid voltage and current acquisition module acquires three-phase signals of power grid voltage and current;
the synchronous rotation coordinate transformation module converts the collected three-phase voltage and current signals into voltage and current direct current components under a rotation coordinate system;
the voltage phase acquisition module acquires phase signals of the power grid voltage in real time and outputs the phase signals to the synchronous rotation coordinate transformation module;
the synchronous machine rotating speed acquisition module acquires a real-time rotating speed signal of a rotor in the synchronous generator;
the rotating speed deviation judging module judges the detected rotating speed deviation and the change rate of the rotor of the synchronous generator, and the rotating speed deviation and the change rate of the rotor of the synchronous generator are used as switching conditions of the operation working conditions of the photovoltaic power generation system;
the rotating speed droop control module performs droop control on the output power of the photovoltaic power generation system according to the judging instruction obtained by the rotating speed deviation judging module;
the photovoltaic rapid power modulation module enables the output power of the photovoltaic power generation system to operate in a rapid power mode according to the judging instruction obtained by the rotating speed deviation judging module, namely, the photovoltaic power generation system only has two working conditions of outputting maximum power and minimum power of 0;
and the photovoltaic steady-state power generation module enables the photovoltaic power generation system to run in a steady-state power generation state according to the judging instruction obtained by the rotating speed deviation judging module, and no controllable active power is output any more to inhibit the electromechanical oscillation of the power grid.
(1) Under the action of a droop control strategy (proportional controller) or an inertia control strategy (differential controller), the photovoltaic power generation system outputs active current I to the synchronous electromechanical network d Can be described as:
I d ＝K p (ω 0 -ω)＝K p Δω
I d ＝sK d (ω 0 -ω)＝sK d Δω
wherein ω is the actual rotational speed of the synchronous generator rotor, ω 0 K is the synchronous rotation speed of the rotor p 、K d The gains of a proportional controller and a differential controller in the rotating speed control loop are respectively, and s is a differential operator.
(2) If the output power P injected into the power grid by the photovoltaic power generation system is considered PV Damping coefficient T of the power system D And the inertia coefficient T J Can be described as:
wherein D, H is the inherent damping coefficient and inertia coefficient of the system, K e The control coefficient of the photovoltaic power generation system characterizes the influence of the photovoltaic power generation system on the dynamic characteristics of the power grid under the action of a control strategy.
Obviously, the controller gain K p The larger the damping coefficient T of the system D The larger the damping torque provided by the photovoltaic power generation system to the power grid is, the larger the damping torque is, and the rotation speed deviation delta omega can be effectively restrained; similarly, K d The larger the inertia coefficient T of the system J The larger the inertia support provided to the power grid, the greater the light can be liftedThe ability of the photovoltaic power generation system to suppress the rate of change dω/dt of the rotational speed. Obviously, adjusting the controller gain can equivalently change the process of rotor speed oscillation.
(3) Taking the photovoltaic power generation system operating on the sagging control strategy as an example, it is known from the steps (1) and (2) that K is increased p Active current I of photovoltaic power generation system can be increased d I.e. increasing the photovoltaic output power P PV Thereby enhancing the damping torque effect of the synchronous machine rotor. But K is p When the voltage is increased to the limit, the photovoltaic inverter outputs the maximum output power P of the photovoltaic according to the positive and negative polarities of the rotating speed deviation in the whole period mppt Or the minimum power 0 is used as a reference to continuously and alternately output. During this period, the photovoltaic power generation system's ability to suppress the grid's electromechanical oscillations will reach a limit. The analysis is as follows:
wherein, the equation of motion of the synchronous machine rotor:
wherein P is m Is the mechanical power of the prime mover, P e For electromagnetic power of synchronous machines, it is assumed that the grid load suddenly decreases by Δp m And ΔP e When the output power of the photovoltaic is changed, the electromagnetic power P e Is a variable amount of (a).
When the photovoltaic power generation system operates in a linear inertia control strategy, namely the output power of the photovoltaic power generation system is not in a local period any more, the maximum power P is constantly output mppt Control gain K of differential controller d The following setting conditions should be satisfied:
considering that the rotor speed change rate reaches the maximum value at the initial moment of the electromechanical oscillation of the power grid, and the photovoltaic power generation system does not output controllable active power to participate in inhibition at the moment, the rotor speed change rate at the moment is as follows:
it can be seen that the gain K of the differential controller at this time d The following should be satisfied:
substituting it into the rotor motion equation of the synchronous machine can obtain:
at this time, the change rate of the rotating speed of the rotor of the synchronous machine is as follows:
when the photovoltaic power generation system operates in a rapid power support mode, the change rate of the rotating speed of the rotor of the synchronous machine is as follows:
the speed change rate of the synchronous machine is used as a standard to be compared to obtain the following steps:
(4) As can be seen from the comparison result of the rotation speed change rate in the step (3), the inhibition capability of the fast power support mode to dω/dt is obviously stronger than that of linear inertia control; similarly, the fast power support mode also has significantly greater damping capability for Δω than linear damping control. The physical mechanism is as follows:
the root causes of the electromechanical oscillations are: the disturbance power causes temporary power unbalance of the power grid, surplus or lack of power can promote the acceleration or deceleration of the rotor, and the more serious the power unbalance is, the larger the acceleration and deceleration energy is, the more serious the oscillation phenomenon is, namely, the greater the dω/dt and Δω indexes are, and the greater the probability of triggering the action of the relay protection device is. The rapid power support mode can fully utilize the power resource of the photovoltaic power generation system to enable the photovoltaic power generation system to rapidly emit or absorb energy with maximum capacity so as to effectively compensate the power shortage of the system or absorb the surplus energy of the system, and the acceleration and deceleration energy which causes excessive dω/dt and Δω is weakened from the source.
(5) The switching threshold values of the 4 operation conditions corresponding to the control strategy of the photovoltaic power generation system and the corresponding power output instructions are as follows:
wherein: omega th1 Synchronous machine rotor rotation speed deviation threshold value omega for executing sagging control strategy for photovoltaic power generation system th2 And R is R B And respectively executing a threshold value of the rotation speed deviation and the rotation speed change rate of the rapid power support strategy for the photovoltaic power generation system.
When the indexes such as dω/dt, Δω and the like are within the safe interval, i.e., ω 0 -ω th1 <ω<ω 0 +ω th1 And |dω/dt|is less than or equal to R B Indicating that the system does not generate electromechanical oscillation or the oscillation process is completely finished, the photovoltaic power generation system should operate in a steady-state power generation mode, at which point P PV ＝P 0 Wherein P is PV Represents the output power of photovoltaic, P 0 The photovoltaic steady state output power is shown, which corresponds to condition i in table 1. If the rotation speed index meets omega th1 <|Δω|≤ω th2 And |dω/dt|is less than or equal to R B At the moment, the photovoltaic power generation system can be switched to a rotation speed droop mode, and the rotation speed droop mode corresponds to a working condition IV. The output power of the photovoltaic can be obtained according to the rotation speed deviation: 1.5V d K p Δω+P 0 Wherein V is d The voltage direct current component of the d axis is the voltage of the power grid under the action of the synchronous rotation coordinate transformation module. The photovoltaic sagging control strategy can provide necessary damping torque for the power grid, so that the electromechanical oscillation is forced to be further weakened, and the frequency quality of the power grid is improved.
(6) If the rotation speed evaluation index dω/dt, Δω satisfies the threshold set by the condition II, i.e., Δω>ω th2 And |dω/dt|>R B Indicating that the power of the power system is seriously absent at the moment, the photovoltaic power generation system should timely increase the active power output so as to fill the absent power in the power system. Its current command I d * Should be immediately increased to the forward current limit, i.e. the current I corresponding to the maximum output power of the photovoltaic mppt Continuously providing positive maximum power support to the grid, at which point P PV ＝P mppt To significantly inhibit Δω, dω/dt, corresponding to condition ii. Conversely, if Deltaω<-ω th2 Or |dω/dt|>R B Indicating that the power of the power system is seriously excessive, the photovoltaic power generation system should immediately reduce the output of active power to the maximum extent, and the current command I d * Should be immediately reduced to 0 in reverse direction to make the photovoltaic output power P PV Decreasing 0 provides reverse rapid power support to the grid to reduce power surplus, corresponding to condition iii. FIG. 1 is a non-linear operation diagram of the output power of a photovoltaic power generation system, first illustrating a non-linear power output curve of a photovoltaic cell, wherein the abscissa is the DC side capacitance voltage value U dc Wherein U is dc U is the minimum inversion voltage of the photovoltaic inverter mmpt U is the direct-current side voltage corresponding to the maximum power output point of the photovoltaic oc Is the open circuit voltage of the photovoltaic cell. The ordinate is the output power P of the photovoltaic cell PV . As can be seen from FIG. 1, the photovoltaic output power is limited to [ U ] mmpt ，U oc ]Output power P PV The output shows monotonicity, and the power output interval of the photovoltaic can be 0 and P mmpt ]Namely global regulation, is beneficial to designing a negative feedback control system.
Fig. 2 is a schematic diagram of a photovoltaic power generation system accessing a synchronous machine power grid, and a steady-state power generation function of the photovoltaic power generation system and a power grid electromechanical oscillation suppression function can be realized by adding a conventional control strategy to a grid-connected inverter.
FIG. 3 is a schematic diagram of a synchronous rotation coordinate transformation module and a voltage phase acquisition module for acquiring three-phase voltage V at grid-connected point abc And current signal I abc Converted into DC components of voltage and current in a rotating coordinate system, i.e. V d 、V q 、I d 、I q . And then the phase angle theta of the three-phase power grid voltage can be captured through the voltage phase acquisition module and is input into the rotary coordinate transformation module to complete the coordinate transformation function. Where f is the actual frequency of the grid. FIG. 4 is a control schematic of a photovoltaic grid-tied inverter, wherein I d * 、I q * 、U d * 、U q * Reference signals, U, of direct current and voltage components respectively abc * The active power output of the photovoltaic inverter can be controlled by controlling the reference signal of the direct current component by dividing the reference signal of the three-phase voltage.
FIG. 5 shows the gain K of the proportional controller after adding a droop control strategy to the photovoltaic inverter p And when the active power output of the photovoltaic power generation system changes regularly. As can be seen from FIG. 5, when the gain K p To a certain extent, the photovoltaic power generation system will have its maximum power P within a certain period of time mppt And continuous output, namely nonlinear droop control, wherein the damping capacity of the photovoltaic power generation system for inhibiting the electromechanical oscillation of the power grid can be developed and utilized to the maximum extent, so that the rotational speed deviation of the synchronous machine rotor can be inhibited rapidly.
FIG. 6 shows the gain K of the differential controller after adding an inertia control strategy to the photovoltaic inverter d And when the active power output of the photovoltaic power generation system changes regularly. As can be seen from FIG. 6, when the gain K d To a certain extent, the photovoltaic power generation system will have its maximum power P within a certain period of time mppt And continuous output, namely nonlinear inertia control, is realized, and the photovoltaic power generation system can provide inertial support for the synchronous machine power grid to the maximum extent, so that the change rate of the rotor rotating speed can be restrained rapidly.
Fig. 7 is a schematic diagram of a power grid electromechanical oscillation suppression strategy based on a rapid power support technology, and different inverter active current output working conditions are selected according to the judgment result of a rotation speed deviation judgment module, so that the damping and inertia capacities of a photovoltaic power generation system on the power grid electromechanical oscillation are developed and utilized to the maximum extent.
FIG. 8 is a power schematic diagram showing an oscillation suppression strategy according to the present invention. In the initial stage of oscillation, the rotor rotation speed oscillation phenomenon is most serious, and the photovoltaic active power output is between 0 and P mppt Switching back and forth, and suppressing unbalanced power to the maximum extent. In the later period of oscillation, when the rotor rotation speed index reaches the set interval range, the photovoltaic power generation system is switched to a sagging control strategy, the photovoltaic active power output is linearly regulated according to the rotation speed deviation, the rotor rotation speed deviation is further restrained, and the stability of the power system is improved.
FIG. 9 is a flow chart of the strategy according to the present invention, specifically, the decision selection flow criteria for 4 working conditions.
Fig. 10 is a graph showing the effect of the proposed strategy on the suppression of the electromechanical oscillations of the grid by the photovoltaic power generation system in comparison with the conventional droop control, nonlinear droop control and no-power control strategies. As can be seen from FIG. 10, the suppression strategy provided by the present invention has the most obvious effect of suppressing the oscillation phenomenon of the rotor speed, and can rapidly smooth the electromechanical oscillation phenomenon of the power grid.
Fig. 11 is a waveform showing the variation law of the absolute value of the variation rate dω/dt of the rotor rotational speed shown in fig. 10. Fig. 11 shows that the inhibition effect of the proposed strategy on the rotor speed change rate is also obviously stronger than that of the conventional control strategy, so that the triggering of the related frequency protection device can be effectively avoided, and the frequency stability of the power grid can be improved.
FIG. 12 is a schematic diagram showing the three-phase current I output by the photovoltaic inverter when the photovoltaic power generation system of FIGS. 10 and 11 is operating in the proposed strategy abc Waveform diagram. As can be seen from fig. 12, the photovoltaic power output is between 0 and P at the initial stage of the electromechanical oscillation of the power grid mppt And switching, wherein the current output shows linearization rule in the later period of oscillation.

Claims (1)

1. A method for suppressing electromechanical oscillations of a power grid based on rapid power support of a photovoltaic power generation system, comprising: the system comprises a power grid voltage and current acquisition module, a synchronous rotation coordinate conversion module, a voltage phase acquisition module, a synchronous machine rotating speed acquisition module, a rotating speed deviation judgment module, a rotating speed sagging control module, a photovoltaic rapid power modulation module and a photovoltaic steady-state power generation module;
(1) The power grid voltage and current acquisition module acquires three-phase signals of power grid voltage and current;
(2) The synchronous rotation coordinate transformation module converts the collected three-phase voltage and current signals into voltage and current direct current components under a rotation coordinate system;
(3) The voltage phase acquisition module acquires phase signals of the power grid voltage in real time and outputs the phase signals to the synchronous rotation coordinate transformation module;
(4) The synchronous machine rotating speed acquisition module acquires a real-time rotating speed signal of a rotor in the synchronous generator;
(5) The rotating speed deviation judging module judges the detected rotating speed deviation and the change rate of the rotor of the synchronous generator, and the rotating speed deviation and the change rate of the rotor of the synchronous generator are used as switching conditions of the operation working conditions of the photovoltaic power generation system;
(6) The rotating speed droop control module performs droop control on the output power of the photovoltaic power generation system according to the judging instruction obtained by the rotating speed deviation judging module;
(7) The photovoltaic rapid power modulation module enables the output power of the photovoltaic power generation system to operate in a rapid power mode according to the judging instruction obtained by the rotating speed deviation judging module, namely, the photovoltaic power generation system only has two working conditions of outputting maximum power and minimum power of 0;
(8) The photovoltaic steady-state power generation module enables the photovoltaic power generation system to run in a steady-state power generation state according to the judging instruction obtained by the rotating speed deviation judging module, and controllable active power is not output any more to inhibit electromechanical oscillation of the power grid;
the switching threshold values of the 4 operation conditions corresponding to the control strategy of the photovoltaic power generation system and the corresponding power output instruction are specifically as follows: wherein:ω th1 a synchronous machine rotor rotational speed deviation threshold value of a sagging control strategy module is executed for the photovoltaic power generation system,ω th2 and (3) withR B The method comprises the steps that a threshold value of a rotational speed deviation and a threshold value of a rotational speed change rate of a rapid power support strategy are respectively executed for a photovoltaic power generation system;
ωin order to synchronize the actual rotational speed of the generator rotor,ω 0 d is the synchronous rotation speed of the rotorω/dtSuppressing the rate of change of rotational speed for photovoltaic power generation systems，ΔωIn order to be a deviation of the rotational speed,K p is the gain of the proportional controller,P mppt is the maximum output power of the photovoltaic;
(a) When dω/dt、ΔωWhen the index is within the safe interval, i.e.ω 0 -ω th1 <ω<ω 0 +ω th1 And |dω/dt|≤R B Indicating that the system does not generate electromechanical oscillation or the oscillation process is completely finished, the photovoltaic power generation system should operate in a steady-state generation power mode at the momentP PV =P 0 WhereinP PV The output power of the photovoltaic is represented by,P 0 representing the photovoltaic steady-state output power, which corresponds to the working condition I;
(b) If the rotation speed index meetsω th1 <|Δω|≤ω th2 And |dω/dt|≤R B At the moment, the photovoltaic power generation system is switched to a rotation speed sagging mode, and the rotation speed sagging mode corresponds to a working condition IV; the output power of the photovoltaic is obtained according to the rotation speed deviation: 1.5V d K p Δω+P 0 Wherein V is d The method comprises the steps that a voltage direct current component of the power grid voltage on a d axis under the action of a synchronous rotation coordinate transformation module is used;
(c) If deltaω>ω th2 And |dω/dt|>R B Indicating that the power of the power system is seriously absent at the moment, the photovoltaic power generation system increases the active power output in time so as to fill the absent power in the power system; its current commandI d * Immediately increasing to the forward current limit, i.e. the current corresponding to the maximum output power of the photovoltaicI mppt Continuously providing positive maximum power support to the grid, at this timeP PV =P mppt To significantly inhibit deltaω、dω/dtCorresponding to a working condition II;
(d) If deltaω<-ω th2 Or |dω/dt|>R B Indicating that the power of the power system is seriously excessive, the photovoltaic power generation system immediately reduces the output of active power to the maximum extent,its current commandI d * Should be immediately reversely reduced to 0 so as to lead the output power of the photovoltaicP PV Decreasing 0 provides reverse rapid power support to the grid to reduce power surplus, corresponding to condition iii.
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