CN106998074B - Control method and system for modular multi-level flexible direct current converter station - Google Patents
Control method and system for modular multi-level flexible direct current converter station Download PDFInfo
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- H—ELECTRICITY
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Abstract
The invention provides a control method and a system for a modular multi-level flexible direct current converter station, wherein the control method comprises the following steps: the three-phase alternating current information obtained by sampling passes through a virtual synchronous motor controller and a circulating current controller respectively to obtain induced electromotive force and voltage regulation voltage; obtaining a three-phase voltage reference value according to the induced electromotive force and the voltage regulation; and a three-phase voltage reference value is modulated through PWM, and a switching signal is output, so that the control of the modular multilevel converter station is realized. The control method provided by the invention can automatically adjust and output active power and reactive power according to the frequency and voltage variation of the alternating current system, thereby effectively reducing the frequency and voltage fluctuation of the alternating current system.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a virtual synchronous motor control method and system for a modular multilevel flexible direct current converter station.
Background
In the face of threats of fossil energy crisis, global climate change and environmental deterioration all over the world, countries in the world have fully recognized that energy development and utilization must be shifted from traditional fossil energy to green renewable clean energy. Wind power, photovoltaic power generation, and the like have become the main forms of energy supply as important renewable energy sources. The permeability of distributed energy resources is increased, the installation proportion of the traditional synchronous generator is reduced, the rotating reserve capacity and the rotary inertia of a power system are relatively reduced, and in addition, due to the combined action of the volatility and the intermittency of the distributed energy resources, the problem that the safe and stable operation of a power grid is influenced by grid source coordination and the like occurs in the system.
The direct current transmission technology has obvious advantages in the aspects of reactive power demand, system stability, large-scale access to clean energy, large-capacity remote transmission and the like, particularly has obvious advantages in the aspects of dynamic reactive power compensation, power flow reversal, system control and the like at present when the flexible direct current transmission technology is mature day by day, so that the direct current transmission technology becomes one of the best means for accessing renewable energy, and provides a brand new technical scheme for the construction of a future transmission network.
However, the control mode of the traditional flexible direct current transmission is mainly based on a vector control technology, and on the basis of the control strategy, the flexible direct current transmission system can realize dynamic voltage support and active and reactive independent decoupling, and provides a stable and reliable way for the transmission of active power. In a traditional flexible direct current power transmission system, space orientation vector decoupling is easily affected by system control parameter change, mismatching and the like. In addition, the grid-connected converter under the traditional vector control strategy is difficult to provide proper damping in a low frequency band, so that subsynchronous resonance of an alternating current system and large-scale grid disconnection of a grid-connected converter station are easily caused, the stability of the voltage and the frequency of a power system is seriously threatened, particularly, during the transient fault period of the power system, the grid-connected converter station cannot provide enough inertia, and the instability of the system frequency is easily caused.
Therefore, there is a need for a control technique that can improve the delivery of renewable energy sources and improve the operational safety, stability, and reliability of the power system.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a control method and a control system for a modular multi-level flexible direct current converter station, wherein the control method comprises the following steps: the three-phase alternating current information obtained by sampling passes through a virtual synchronous motor controller and a circulating current controller respectively to obtain induced electromotive force and voltage regulation voltage; obtaining a three-phase voltage reference value according to the induced electromotive force and the voltage regulation; and a three-phase voltage reference value is modulated through PWM, and a switching signal is output, so that the control of the modular multilevel converter station is realized.
After three-phase alternating current information obtained by sampling passes through the virtual synchronous motor controller and the circulating current controller respectively, induced electromotive force and voltage regulation voltage are obtained, and the method comprises the following steps: the motion equation of the mechanical motion unit rotor in the virtual synchronous motor controller is as follows:
in the formula: j is rotor moment of inertia, TmIs mechanical torque, TeIs electromagnetic torque, DpDamping coefficient, omega is rated frequency, omega is actual frequency of the power grid;
the electrical equation for the stator windings of the electromagnetic units in the virtual synchronous machine controller is as follows:
in the formula: u. ofabcFor the output terminal voltage, i, of the stator windingabcIs the output end current of the stator winding, e is the induced electromotive force, RsIs the winding loss, LsIs a synchronous reactance.
The calculation formula of the induced electromotive force is as follows:
in the formula: e is induced electromotive force, theta is electrical angle, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current,
The difference value between the rated frequency omega and the actual frequency omega of the power grid is obtained;
the damping coefficient is the relation between active power and frequency change, and the definition formula is as follows:
wherein, Delta P is an active power set value PsetAnd the difference between the active power P and delta theta is the change in electrical angle.
The expression of the output active power is as follows:
wherein the content of the first and second substances,
is iabcAnd
the inner product of the two,
At a rated frequencyωDifference value of actual frequency omega of power grid, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current.
The control system comprises: the control module is used for obtaining induced electromotive force and voltage regulation voltage after the three-phase alternating current information obtained by sampling passes through the virtual synchronous motor controller and the circulating current controller respectively; the voltage synthesizer is used for outputting a three-phase voltage reference value after receiving the induced electromotive force and the voltage regulating instruction; and the modulation module modulates the three-phase voltage reference value through PWM and outputs a switching signal to realize the control of the modular multilevel converter station.
The virtual synchronous motor controller comprises a mechanical motion unit and an electromagnetic motion unit;
the equation of motion of the rotor in the mechanical movement unit is as follows:
in the formula: j is rotor moment of inertia, TmIs mechanical torque, TeIs electromagnetic torque, DpDamping coefficient, omega is rated frequency, omega is actual frequency of the power grid;
the electrical equation for the stator windings in the electromagnetic unit is as follows:
in the formula: u. ofabcIs the output voltage of the stator winding、iabcIs the output end current of the stator winding, e is the induced electromotive force, RsIs the winding loss, LsIs a synchronous reactance.
The calculation formula of the induced electromotive force is as follows:
in the formula: e is induced electromotive force, theta is electrical angle, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current,
The difference value between the rated frequency omega and the actual frequency omega of the power grid is obtained;
the damping coefficient is the relation between active power and frequency change, and the definition formula is as follows:
wherein, Delta P is an active power set value PsetAnd the difference between the active power P and delta theta is the change in electrical angle.
The expression of the output active power is as follows:
wherein the content of the first and second substances,
is iabcAnd
the inner product of the two,
Is the difference value of rated frequency omega and actual frequency omega of the power grid, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current.
Compared with the closest prior art, the technical scheme provided by the invention has the following beneficial effects:
1. according to the control method provided by the invention, the swing equation and the electromagnetic equation of the traditional synchronous generator are introduced into the controller, and the converter station simulates the traditional synchronous generator on the external characteristics and the operation mechanism, so that the converter station has the functions of voltage regulation and frequency modulation, a certain voltage and frequency support is provided for an alternating current system, and the stability and the reliability of the system are improved.
2. The control method provided by the invention can automatically adjust and output active power and reactive power according to the frequency and voltage variation of the alternating current system, thereby effectively reducing the frequency and voltage fluctuation of the alternating current system.
3. The invention provides inertia and damping for the system, so that the controllability of the whole flexible direct current system has better robustness, and the large-scale grid connection of distributed energy is realized while the stable operation capacity of the system is improved.
Drawings
FIG. 1 is a block diagram of the overall control method of the present invention;
fig. 2 is a block diagram of a control strategy of the virtual synchronous motor controller of the present invention.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the drawings in the specification.
The invention aims to design a control method of a modular multilevel converter station.A control system of a flexible direct current converter station consists of five parts, namely a sampling module, a control module, a voltage synthesizer and a modulation module.
The sampling module provides three-phase voltage and current input information for a control module comprising a virtual synchronous machine controller and a circulating current controller, and the output voltage of the control module is synthesized in a voltage synthesizer and then used as the input of a modulation module, so that the purpose of controlling the output voltage of the modular multilevel converter station is achieved.
The control method provided by the invention introduces a swing equation and an electromagnetic equation of the traditional synchronous generator into the controller, and the converter station simulates the traditional synchronous generator on the external characteristics and the operation mechanism, so that the converter station has the functions of regulating voltage and frequency, provides certain voltage and frequency support for an alternating current system, and improves the stability and reliability of the system.
The invention provides a control method of a virtual synchronous motor of a modular multilevel converter, which comprises the following steps: and (3) the external characteristics of the flexible direct current converter station are equivalent to the mathematical model, the motion equation, the electromagnetic equation (transient state or steady state) and the working characteristics of the synchronous motor.
In terms of control strategy, a flexible direct current converter and a control method of a traditional synchronous motor need to be used for reference. The control algorithm of the virtual synchronous motor needs to use variables such as voltage, power and the like, so that the stability and accuracy of the output of the converter station are ensured, and the voltage and current double-loop control of the converter station is adopted on a control structure. For a conventional synchronous machine, the prime mover and the speed regulator are responsible for providing mechanical power and adjusting the output frequency, and the excitation control system is responsible for adjusting the excitation voltage.
In a virtual synchronous machine, the control function of the excitation regulator simulating the excitation control system in the synchronous machine is replaced by a plurality of units with similar functions, and the control function of the speed regulator is simulated by the power and frequency regulator. The control system collects output voltage and current of the converter valve, a power and frequency regulator obtains a mechanical power instruction of the virtual synchronous motor, an excitation regulator obtains an excitation voltage instruction of the virtual synchronous motor, a reference value of system frequency and voltage is obtained after calculation of a virtual synchronous motor control algorithm, and a driving signal is generated after the calculation of the virtual synchronous motor control algorithm and the calculation of a double-ring control system and a modulation system to control the on-off of a switching tube, so that the whole closed-loop system is completed.
The core of the control method provided by the invention is the design of a virtual synchronous motor controller, and the circular current controller, PWM modulation and vector control of the traditional flexible direct current converter have no essential difference, are not the core part of the invention and are not discussed in detail.
The virtual synchronous motor controller is to realize the function tasks of automatically adjusting the output active power and reactive power according to the frequency and voltage variation of the alternating current system and providing frequency and voltage support for the alternating current system, so that the control method of the traditional synchronous generator can be used for processing.
As shown in fig. 2, by using a PI controller and combining equations (3) and (6), a virtual synchronous motor control strategy block diagram proposed by the present invention can be obtained, which includes: the active frequency modulation function module and the reactive voltage regulation function module can realize that the converter adjusts the output power according to the requirement of the alternating current system and provide frequency and voltage support for the system.
The classical second-order synchronous generator model is taken as a main point, and in order to enable the converter to better simulate and realize the performance of the synchronous motor, the relation between the mechanical motion and the electromagnetic motion of the synchronous motor is considered at the same time.
The mechanical motion of the synchronous generator can cause the converter station to have inertia in the dynamic relationship of power and frequency due to the existence of the rotational inertia and the damping coefficient, and damp the power oscillation, and the motion equation of the rotor is as follows:
wherein J is the rotor moment of inertia; t ismAnd TeThe mechanical torque and the electromagnetic torque of the generator are respectively; dpIs a damping coefficient; ω x and ω are dividedThe nominal (reference) angular frequency and the actual angular frequency of the grid.
The power grid frequency obtained by solving the above formula realizes the active regulation capability of the current converter, and the speed regulator is simulated in the control of the virtual synchronous machine.
Electromagnetic partial modeling of synchronous generators prototypes on the stator winding electrical equation, i.e.
In the formula uabcAnd iabcThe voltage and the current of the three-phase output end of the stator are respectively; e is three-phase induced electromotive force; rsAnd LsStator armature winding losses and synchronous reactance, respectively. The modeling only emphasizes the voltage-current relation characteristic of the stator, is simple and does not consider the inherent electromagnetic characteristic of the stator.
In order to make the control model algorithm more mechanistically simulate the synchronous generator, the flux linkage relation between the rotor and the stator is considered and can be deduced
Wherein θ is an electrical angle integrated by angular frequency ω; mfMutual inductance between the stator and the rotor; i.e. ifIs the rotor winding current;
meanwhile, an expression of inner product derived power is respectively made for the output active power P and the reactive power Q of the converter, as follows:
wherein the content of the first and second substances,
is iabcAnd
the inner product of the two is obtained,
is iabcAnd
the inner product of the two.
(1) The damping coefficient in the formula is substantially characterized by the relationship between active power and frequency change, and is defined as follows:
wherein, Delta P is an active power set value PsetAnd the calculated value P, and delta theta is the change of the electrical angle. The setting of the damping coefficient can be set according to the system, such as the power change is 100%, and the frequency fluctuation is 0.5%.
Note that in a steady state situation, the damping coefficient setting is generally selected such that the frequency change does not exceed 0.5 Hz. In addition, because the proposed frequency control has no delay link, the value of the rotational inertia should be set to be smaller, and generally the equation J is equal to DptcIs determined, where tcThe time constant is a system-dependent and generally takes a value of several milliseconds to several tens of milliseconds.
Besides the formula (7), the reactive voltage regulation control function of the control method also needs to obtain reactive input parameters through reactive voltage droop control, so that the control method can well track the voltage of an alternating current power grid. Similarly, the reactive droop coefficient reflects the variation of reactive power and voltage, and is defined as follows:
where Δ V represents the AC voltage set value VsetAnd the measured value VgI.e. represents the offset of the ac voltage from the reference value, deltaq represents the reactive power setpoint QsetAnd the difference between the calculated value Q, i.e., the amount of change in reactive power.
The reactive droop coefficient is set according to a system required value, and reactive power can be obtained through calculation according to the formula (7) and is used as negative feedback to act together with a reactive power set value and a reactive power value for droop control to form a reactive power voltage regulating part, so that the function of the excitation regulator is simulated.
Equation (3) is a relation for controlling the output voltage of the inverter, and the excitation winding current is generally a direct current and has almost no change, and the latter term of the equation can be ignored.
It is worth noting that: the design method can be used for simultaneously controlling the inverter station and the rectifier station, the change of the power flow direction can cause the change of formula symbols, and when the method is used for controlling the virtual synchronous machine of the rectifier station, the method proposes to add fixed direct-current voltage control so as to provide a reliable direct-current power supply for the inverter station.
Based on the same inventive concept, the invention also provides a system for reducing the risk of the hierarchical access direct current commutation failure, which is explained below.
The system provided by the invention can comprise:
the control module is used for receiving the three-phase sampling information and then outputting induced electromotive force and voltage regulation voltage; the voltage synthesizer is used for outputting a three-phase voltage reference value after receiving the induced electromotive force and the voltage regulating instruction; and the modulation module outputs a switching signal after the three-phase voltage reference value passes through the PWM module, so that the control of the modular multilevel converter station is realized.
The control module includes: a virtual synchronizer controller and a circulation controller; and after receiving the three-phase sampling information, the virtual synchronous machine controller and the circulation controller respectively output induced electromotive force and the voltage regulation voltage. The virtual synchronous motor controller includes a mechanical motion unit and an electromagnetic motion unit.
The equation of motion of the rotor in the mechanical movement unit is as follows:
in the formula: j is rotor moment of inertia, TmIs mechanical torque, TeIs electromagnetic torque, DpThe damping coefficient is, omega is rated frequency, omega is actual frequency of the power grid.
The electrical equation for the stator windings in the electromagnetic unit is as follows:
in the formula: u. ofabcFor the output terminal voltage, i, of the stator windingabcIs the output end current of the stator winding, e is the induced electromotive force, RsIs the winding loss, LsIs a synchronous reactance.
The flux linkage between the rotor and stator windings is as follows:
in the formula: e is induced electromotive force, theta is electrical angle, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current,
The difference value between the rated frequency omega and the actual frequency omega of the power grid is obtained;
the damping coefficient is the relation between active power and frequency change, and the definition formula is as follows:
wherein, Delta P is an active power set value PsetAnd the difference between the active power P and delta theta is the change in electrical angle.
The expression of the output active power is as follows:
wherein the content of the first and second substances,
is iabcAnd
the inner product of the two,
Is the difference value of rated frequency omega and actual frequency omega of the power grid, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person of ordinary skill in the art can make modifications or equivalents to the specific embodiments of the present invention with reference to the above embodiments, and such modifications or equivalents without departing from the spirit and scope of the present invention are within the scope of the claims of the present invention as set forth in the claims.
Claims (2)
1. A control method for a modular multilevel flexible dc converter station, characterized in that the control method comprises the steps of:
the three-phase alternating current information obtained by sampling passes through a virtual synchronous motor controller and a circulating current controller respectively to obtain induced electromotive force and voltage regulation voltage;
obtaining a three-phase voltage reference value according to the induced electromotive force and the voltage regulation voltage;
modulating the three-phase voltage reference value through PWM, and outputting a switching signal to realize the control of the modular multilevel converter station;
the three-phase alternating current information obtained by sampling respectively passes through the virtual synchronous motor controller and the circulating current controller to obtain induced electromotive force and voltage regulation voltage, and the method comprises the following steps:
the motion equation of the rotor of the mechanical motion unit in the virtual synchronous motor controller is as follows:
in the formula: j is rotor moment of inertia, TmIs mechanical torque, TeIs electromagnetic torque, DpDamping coefficient, omega is rated frequency, omega is actual frequency of the power grid;
the electrical equation of the stator winding of the electromagnetic unit in the virtual synchronous motor controller is as follows:
in the formula: u. ofabcFor the output terminal voltage, i, of the stator windingabcThe output end current of the stator winding, e is induced electromotive force, RsIs the winding loss, LsIs a synchronous reactance;
the calculation formula of the induced electromotive force is as follows:
in the formula: e is induced electromotive force, theta is electrical angle, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current,
The difference value between the rated frequency omega and the actual frequency omega of the power grid is obtained;
the damping coefficient is the relation between active power and frequency change, and the definition formula is as follows:
wherein, Delta P is an active power set value PsetAnd the difference between the active power P and the active power P, wherein delta theta is the change of the electrical angle;
the expression of the active power is as follows:
wherein P is active power, theta is electrical angle,
For the output current i of the stator windingabcAnd phase relation
The inner product of the two,
Calculating a difference between the rated frequency ω and the actual frequency ω of the grid, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current.
2. A control system for a modular multilevel flexible dc converter station, characterized in that the control system comprises:
the control module is used for obtaining induced electromotive force and voltage regulation voltage after the three-phase alternating current information obtained by sampling passes through the virtual synchronous motor controller and the circulating current controller respectively;
the voltage synthesizer is used for outputting a three-phase voltage reference value after receiving the induced electromotive force and the voltage regulating instruction;
the modulation module modulates the three-phase voltage reference value through PWM and outputs a switching signal to realize the control of the modular multilevel converter station;
the virtual synchronous motor controller comprises a mechanical motion unit and an electromagnetic motion unit;
the equation of motion of the rotor in the mechanical motion unit is as follows:
in the formula: j is rotor moment of inertia, TmIs mechanical torque, TeIs electromagnetic torque, DpDamping coefficient, omega is rated frequency, omega is actual frequency of the power grid;
the electrical equation of the stator winding in the electromagnetic motion unit is as follows:
in the formula: u. ofabcFor the output terminal voltage, i, of the stator windingabcThe output end current of the stator winding, e is induced electromotive force, RsIs the winding loss, LsIs a synchronous reactance;
the calculation formula of the induced electromotive force is as follows:
in the formula: e is induced electromotive force, theta is electrical angle, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current,
The difference value between the rated frequency omega and the actual frequency omega of the power grid is obtained;
the damping coefficient is the relation between active power and frequency change, and the definition formula is as follows:
wherein, Delta P is an active power set value PsetAnd the difference between the active power P and the active power P, wherein delta theta is the change of the electrical angle;
the expression of the active power is as follows:
wherein P is active power, theta is electrical angle,
For the output current i of the stator windingabcAnd phase relation
The inner product of the two,
Calculating a difference between the rated frequency ω and the actual frequency ω of the grid, MfFor mutual inductance between stator and rotor, ifIs the rotor winding current.
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| CN109672207B (en) * | 2018-12-04 | 2022-06-14 | 国网江苏省电力有限公司电力科学研究院 | Back-to-back system control method and system based on virtual synchronous machine |
| CN111654051B (en) * | 2020-05-21 | 2021-09-07 | 东南大学 | Direct current fault ride-through control method suitable for mixed MMC |
| CN111830431B (en) * | 2020-06-23 | 2023-07-04 | 中国电力科学研究院有限公司 | Method and system for carrying out fault discrimination on true bipolar flexible direct current transmission system |
| CN112600232B (en) * | 2020-12-04 | 2023-02-07 | 国网新疆电力有限公司 | Extra-high voltage direct current transmission system converter station control system based on virtual synchronous machine |
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