CN116632902A - Multi-port flexible direct current transmission system based on PWM current source converter - Google Patents
Multi-port flexible direct current transmission system based on PWM current source converter Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
- H02M5/02—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc
- H02M5/04—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters
- H02M5/22—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M5/275—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M5/293—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/4807—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode having a high frequency intermediate AC stage
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/66—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal
- H02M7/68—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters
- H02M7/72—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/79—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/797—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output with possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
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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
- 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
- H02J2300/28—The renewable source being wind energy
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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
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
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Abstract
The application discloses a multi-port flexible direct current transmission system based on a PWM current source type converter, and belongs to the technical field of power generation, transformation or power distribution. The system is formed by connecting four substations in parallel to form a ring network type structure, wherein two modularized high-frequency isolation current source type converter ports are connected with an open sea wind power plant, two PWM current source type converter ports are connected with land substations, and four ports are connected through a ring network. The application takes the high-frequency isolation current source type converter as a motor side topological structure, and exerts the advantages of high efficiency, high power density and high reliability; the modularized structure is adopted, so that the power grade, the voltage grade and the fault tolerance are improved; the PWM current source type converter is used as a power grid side topology, so that a direct current side capacitor is removed, and the technical requirement of a system on a high-voltage direct current breaker is reduced; and the multi-port direct current transmission is used as a background, the power flow control is realized by using master-slave control and voltage droop control, and the steady-state transmission power limit of the new energy station is improved.
Description
Technical Field
The application belongs to the technical field of wind power generation, and particularly relates to a multi-port flexible direct current transmission system based on a PWM current source type converter.
Background
The offshore wind energy has the advantages of rich wind resources, high wind speed, stable wind power and small influence on the environment. The offshore wind energy conversion system can adopt two power transmission modes of high-voltage direct current and high-voltage alternating current. Compared with alternating current transmission, the high-voltage direct current transmission system has no reactive power transmission, and the grounding current is lower, so that copper loss and voltage drop of a line are smaller.
The high-voltage direct-current transmission systems are divided into two main types, one is a flexible direct-current transmission system based on a voltage source converter, and the other is a flexible direct-current transmission system based on a current source converter. The voltage source type modularized multi-level high-voltage direct current transmission technology is based on a high-frequency device with self-turn-off capability and a pulse width modulation technology, takes a capacitor as a side energy storage element, can independently control active and reactive power, and has good harmonic characteristic and alternating current side fault resistance capability. However, the capacitive energy storage element is very sensitive to direct current side faults, and the short circuit faults generate extremely large short circuit currents and are difficult to control.
The current source-based converter comprises two kinds of high-voltage direct current transmission technologies, namely a power grid commutation converter and a PWM current source-based converter. The grid commutation converter uses a thyristor device and takes an inductor as an energy storage element, and has the advantages of low manufacturing cost, low operation loss and high pressure resistance; but the harmonic performance is poor, the reliability is poor, the harmonic is easily influenced by faults on the alternating current side, and reactive compensation equipment is needed. Meanwhile, when the trend direction is changed, the polarities of the voltages at the two ends of the direct current link need to be turned over. The PWM current source type converter high-voltage direct current transmission technology has the same control capability and harmonic characteristic as the voltage source type flexible direct current transmission technology, and takes an inductor as an energy storage element, has the same innate resistance and control capability to direct current side faults as the power grid commutation converter, has smaller fault current rising rate and can be controlled, thereby reducing the requirements on a direct current breaker. The PWM current source converter adopts a full-control device, so that black start can be realized, reactive power compensation can be realized, and meanwhile, current harmonic waves can be reduced.
For offshore wind farms, high power density wind energy conversion systems are required because of the expensive infrastructure of the offshore wind farms. In conventional wind energy conversion systems, line frequency transformers are commonly used for voltage scaling and galvanic isolation. But the lightweight concrete has large volume and large volume, and increases the construction burden of the offshore wind energy conversion system engineering. Therefore, high frequency transformer based converters, such as solid state transformers, are a promising solution for offshore wind energy conversion systems. Solid state transformers in wind energy conversion systems employ two-stage power conversion, including AC/DC and DC/DC two-stage power conversion. The AC/DC stage is responsible for controlling the torque and rotational speed of the generator, and the DC/DC stage provides voltage matching and electrical isolation functions. However, the two-stage converter requires a bulky electrolytic capacitor for power decoupling, reducing the high power density of the power electronic transformer. In addition, the electrolytic capacitor has the problems of poor reliability and narrow operating temperature range, and is extremely easy to damage in the severe offshore environment.
The application aims to provide a multi-port flexible direct current transmission system and a control method based on a PWM current source converter, which start from three layers of wind power new energy converter topology, new topology control strategy, cluster networking and control, and stabilize the fluctuation of wind power from a novel topological structure of the converter by taking multi-port direct current transmission as the background, and improve the steady-state transmission power limit of a new energy station.
Disclosure of Invention
Aiming at the defects of the prior art, the application aims to provide a multi-port flexible direct current transmission system and a control method based on a PWM current source type converter, wherein a modularized high-frequency isolation current source type converter is used as a motor side power topology, and the advantages of high efficiency, high power density and high reliability are exerted; the modularized structure is used for improving the power grade, the voltage grade and the fault tolerance performance at the same time, so that the collection and the power output of new energy sources are effectively improved; the PWM current source type converter is used as a power grid side topology, so that a direct current side capacitor is removed, and the technical requirement of a system on a high-voltage direct current breaker is reduced; based on multiport direct current transmission, current control in a current type transmission system is realized by master-slave control and voltage sag control, and fluctuation energy is output through wind power plants at different placesCounteracting each other 。
The aim of the application can be achieved by the following technical scheme:
a multi-port flexible direct current transmission system based on a PWM current source converter, the networking system comprising: a grid-side PWM current source type converter, a high-frequency transformer, a generator source-side current source type converter and a three-phase filter. The system topology is as follows: four ports are connected in parallel to form a ring network type structure, wherein two modularized high-frequency isolation current source type converter ports are connected with an open sea wind power plant, two PWM current source type converter ports are connected with a land transformer substation, and four ports are connected through a ring network.
Furthermore, the cascade wind power plant connected with the direct current port is formed by connecting a plurality of permanent magnet synchronous generators in series, and each generator is connected in parallel and in series through input and output of a modularized high-frequency isolation current source type converter which comprises a three-phase filter capacitor, a three-phase full-bridge circuit, a high-frequency transformer, a diode bridge rectifier and a direct current link inductor. The three-phase full-bridge circuit includes: an a-phase bridge arm formed by connecting a first bidirectional switch tube and a fourth bidirectional switch tube in series, a b-phase bridge arm formed by connecting a third bidirectional switch tube and a sixth bidirectional switch tube in series, and a c-phase bridge arm formed by connecting a fifth bidirectional switch tube and a second bidirectional switch tube in series, wherein a three-phase bridge arm is connected with a primary coil of a high-frequency transformer, and a three-phase filter capacitor is connected between a midpoint of a three-phase bridge arm of a three-phase full-bridge circuit and a motor. The secondary side of the high-frequency transformer is connected with a diode bridge rectifier and then connected with a direct current link inductor.
Further, the grid-side PWM current source type converter is installed on land, and the requirements for volume and weight are less stringent than those of the converter of the offshore wind farm, so the grid-side converter is selected from the viewpoint of cost as a non-isolated PWM current source type converter. The grid-side PWM current source converter includes: the three-phase full bridge circuit, the three-phase filter circuit, the power frequency transformer and the direct current bus inductor. The three-phase full-bridge circuit is the same as that in the high-frequency isolation current source type converter, a three-phase bridge arm is connected with a bus inductance, a three-phase filter circuit is connected between the midpoint of the three-phase bridge arm of the three-phase full-bridge circuit and the primary side of the power frequency transformer, and the secondary side of the power frequency transformer is connected with a power grid.
Further, the main problem faced by the control system of the high-frequency isolation current source type converter at the motor side is multi-objective control of the rotating speed of the generator and the inductance current of the bus. Based on this, the application proposes a multi-objective control scheme according to conservation of energy on the motor side and the direct current side, while considering motor speed and bus inductance current control. The control scheme consists of two parts: the first part is direct current side control, which comprises the adjustment of the rotating speed of the rotor and direct current; the second part is the control of the ac side, including filter capacitor current compensation and application of active damping to reduce lower harmonics.
Furthermore, the modularized high-frequency isolation current source type converter can improve the total power level of the converter and the reliability of wind power equipment. However, since switching elements, transformers, etc. in different modules cannot be kept completely uniform, power imbalance between the modules occurs in the long-term operation of the converter. Based on the above, the application provides a method for realizing module power balance control by using output voltage and modulation degree. The modularized high-frequency isolation current source type converter adopts a structure that the inputs are connected in parallel and the outputs are connected in series, and the output current of each module is the same, so that the average voltage output by the module is used as a feedback value, and the power balance is realized.
Further, a dq axis rotation coordinate system is established through grid voltage directional control, wherein the electric angle and the angular speed of the grid are obtained by a phase-locked loop, and the filter capacitor voltage can obtain an AC-DC axis component of the capacitor voltage through coordinate transformation. d-axis closed-loop control active power, and obtaining an input bus inductance current reference value i by a tide control strategy dc * And a DC feedback value i dc Obtaining a d-axis current reference value i through closed-loop control d * The method comprises the steps of carrying out a first treatment on the surface of the The q-axis closed loop control reactive power can be set according to the power grid demand. And compensating the filter capacitor current in a steady state to obtain a current reference value of the inverter. Finally, the switching pulses required by the converter are generated by means of current-mode space vector modulation.
Furthermore, the fluctuation of the new energy has an important influence on the safe and stable operation of the power system, and the multiport system based on the current source realizes the inhibition of the fluctuation of the wind turbine energy from two angles of networking topology and networking control strategy. Firstly, the wind power generation system has the characteristic of high randomness in the networking topology, so that the fluctuation of wind power generation is restrained in situ by the serial-parallel operation of a plurality of fans inside a wind power plant.
Further, on the other hand, suppression of wind turbine energy fluctuation is achieved from the viewpoint of networking control strategy. In order to verify the effectiveness of the PWM current source type converter in the multi-port direct current transmission system, a master-slave control and voltage sag control scheme of the multi-port flexible direct current transmission system based on the PWM current source type converter is designed. The principle of master-slave control is as follows: and (3) setting one land transformer substation as a master station, setting the other transformer substations as slave stations, absorbing all power fluctuation by the master station with larger capacity, and fixing the active power of the slave station to be a constant value. For current source type hvdc transmission systems, the dc bus current is typically controlled to achieve power balance of the system. However, in a multi-port system with a parallel structure, the current of each connected branch circuit has a coupling effect, so that power oscillation is caused. The voltage source type flexible direct current transmission system controls the voltage of the direct current bus, eliminates the mutual influence of all ports and realizes decoupling operation. However, in the current source system, the dc voltage is chopped as a switch, and the ripple amplitude is large. Therefore, the present work has studied the average value of the dc voltage as a control target.
Further, voltage sag control differs from master-slave control in that the dc voltage is regulated jointly by each of the onshore power stations. By applying a DC voltage u dcL Filtered value and U dc The values are compared and multiplied by a droop coefficient k to obtain a current I dc Adding to obtain DC current I of the inverter dc * The final given value of the direct current proportional-integral controller is obtained gd *。
The application has the beneficial effects that:
(1) The multi-port flexible direct current transmission system based on the PWM current source converter disclosed by the application adopts multi-port direct current transmission, and compared with two ports, the multi-port direct current transmission system can realize multi-point power supply and multi-drop power reception, is flexible in operation mode, has higher reliability, and can effectively solve the problem of power system digestion in the prior wind power grid connection.
(2) The application discloses a multi-port flexible direct current transmission system based on a PWM current source type converter, wherein a generator source side adopts a modularized high-frequency isolation current source type converter topological structure, and a high-reliability inductor is used for replacing a low-reliability capacitor, so that the reliable operation capacity of the fan source side converter is improved. Meanwhile, the current source type converter can realize single-stage power conversion, an intermediate-stage energy storage element is not needed, and higher efficiency and power density are achieved. The power class of the converter is improved by the modular current source converter structure to cope with today's increasing single-machine capacity. And the structure of parallel input and serial output can improve wind power plant output voltage level, further get rid of marine boost substation, reduce the cost of whole system.
(3) The application discloses a multi-port flexible direct current transmission system based on a PWM current source type converter, which uses the PWM current source type converter for high-voltage direct current transmission, has the same control capability and harmonic characteristic as those of a voltage source type modularized multi-level high-voltage direct current transmission technology, uses an inductor as an energy storage element, has the same innate resistance and control capability to direct current side faults as the line commutation current source type high-voltage direct current transmission technology, has smaller fault current rising rate and can be controlled, thereby reducing the requirements on a direct current breaker.
(4) According to the multi-port flexible direct current transmission system based on the PWM current source converter, disclosed by the application, the power flow control in the current type transmission system is realized by utilizing master-slave control and voltage sag control, the fluctuation energy output by wind power stations at different places is counteracted, the fluctuation of wind power is stabilized by a novel topological structure of the converter, and the steady-state transmission power limit of a new energy station is improved.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described, and it will be obvious to those skilled in the art that other drawings can be obtained according to these drawings without inventive effort.
Fig. 1 is a topology structure diagram of a current source type converter of a parallel ring network type PWM according to the present application;
FIG. 2 is a topology diagram of an input parallel and output series modular high frequency isolated current source converter of the present application;
FIG. 3 is a topology diagram of a high frequency isolated current source converter of the present application;
FIG. 4 is a schematic diagram of a decoupling circuit of the modular high frequency isolated current source converter of the present disclosure;
FIG. 5 is a topology diagram of a grid-side PWM current source converter of the present application;
fig. 6 shows a bidirectional operation mode of the grid-side PWM current source converter according to the present application, and (a) shows an inversion mode.
(b) Is a rectification mode;
FIG. 7 is a block diagram of a control system for a motor-side high frequency isolation current source type inverter according to the present application;
FIG. 8 is a block diagram of a control system of a grid-side PWM current source converter according to the present disclosure;
FIG. 9 is a block diagram of a master-slave control system according to the present application. (a) Direct current voltage control for the master station, (b) active power control for the slave station;
fig. 10 is a characteristic diagram of the present application, in which fig. 10 (a) to 10 (d) are characteristic diagrams of a remote wind farm and a land-based substation under master-slave control.
FIG. 11 is a block diagram of a control system for voltage droop control of the present application;
FIG. 12 is a graph showing the power voltage characteristics of the offshore wind farm according to the present application, wherein FIGS. 12 (a) and 12 (b) are graphs showing the power voltage characteristics of the offshore wind farm;
fig. 13 is a characteristic curve of the voltage drop of the present application, wherein fig. 13 (a) and fig. 13 (b) are characteristic curves of the voltage drop of the on-circuit substation;
fig. 14 is a waveform diagram of the present application, in which fig. 14 (a) and 14 (b) are steady-state waveforms of the four-port dc power transmission system, (a) is a voltage and current waveform of the offshore wind farm 1, and (b) is a reactive compensation waveform of the land transformer substation 1;
fig. 15 is a waveform diagram of the present application, in which fig. 15 (a) to 15 (d) are a comparison of waveforms of an on-shore substation and an open-sea wind farm cut-in and cut-out in a master-slave control manner, (a) is an on-road substation 2 cut-in system waveform, (b) is an open-sea wind farm 2 cut-in system waveform, (c) is an open-sea wind farm 2 fault removal waveform, and (d) is an on-road substation 2 fault removal waveform;
FIG. 16 is a waveform diagram of the present application switching from master-slave control mode to voltage droop control mode;
fig. 17 is a graph of dynamic response waveforms under the voltage droop control of the present application.
Detailed Description
The following description of the embodiments of the present application will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present application, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.
A multi-port flexible DC power transmission system based on PWM current source converter and control method,
the topological structure diagram of the current source type converter high-voltage direct current transmission system of the parallel ring network type PWM is shown in figure 1, the system comprises four ports which are connected in parallel to form a ring network type structure, wherein two modularized high-frequency isolation current source type converter ports are connected with a far-sea wind power plant, two PWM current source type converter ports are connected with a land transformer substation, and the four ports are connected through a ring network. The topological structure is provided with redundant direct current lines, and energy can be transmitted through the redundant lines under line faults, so that the reliability is high. Meanwhile, the power grid is connected with a plurality of power grids, the inversion station or the power grid fails, and energy can still be transmitted to another power grid. The fan converters in the wind power plant are connected in series for output, so that the output voltage level is improved, and the offshore boosting cost is reduced.
The cascade wind power generation field connected with the direct current port is formed by connecting a plurality of permanent magnet synchronous generators in series, and each generator is connected in parallel with the input and the output in series through a modularized high-frequency isolation current source type converter in a connection mode shown in figure 2. The structure with parallel inputs and serial outputs directly raises the output voltage level of each wind power converter, so that an offshore transformer substation can be omitted, the offshore construction difficulty is greatly reduced, and the construction cost is reduced. In addition, the high-frequency transformer is adopted to replace the traditional power frequency transformer, the size of the transformer is reduced, the single-stage matrix transformer is adopted in the transformer, an intermediate-stage electrolytic capacitor is not needed, and the reliability and the efficiency of the transformer are further improved.
The high-frequency isolation current source type converter shown in fig. 3 can be equivalently decoupled into two three-phase current source converters which are connected in parallel as shown in fig. 4, and the two three-phase current source converters alternately work in the positive half part and the negative half part of the high-frequency transformer. The space vector modulation method of the current source type converter can be applied to a high frequency isolation current source type converter. In addition, the isolation function provides opportunities for modular design, expands the power capacity of the wind energy conversion system, is also suitable for industrial production and maintenance, and improves the system reliability.
As shown in fig. 5, the PWM current source converter topology requires constant dc link voltage polarity from multiple terminals, and thus bidirectional current conduction is performed by using bidirectional switches in the PWM current source converter. Thus, a bi-directional power semiconductor device connected at a common source pin is used. For simplicity, a PWM current source converter and a power grid are connected with an industrial frequency transformer to achieve electrical isolation and voltage matching. Since a bidirectional power semiconductor device is used in the PWM current source type converter, a structure in which the dc link voltage is not inverted to operate in an inversion mode and a rectification operation mode can be obtained as shown in fig. 6, in which fig. 6 (a) is the inversion mode and fig. 6 (b) is the rectification mode. In practical applications, the synchronous mode of mosfets can be used instead of diodes in the figures, thereby reducing conduction losses.
The multi-port flexible direct current transmission system based on the PWM current source type converter provided by the application has the advantages that the bus inductance current of each converter station needs to be controlled by itself, and the multi-objective control on the rotation speed of a generator and the bus inductance current is a problem faced by the high-frequency isolation current source type converter. Based on this multi-objective control problem, the control strategy adopted by the high frequency isolated current source converter is shown in fig. 7. The control scheme consists of two parts, wherein the first part is a direct current side controller and comprises the adjustment of the rotating speed of a rotor and the direct current. The second part is an ac side controller, including filter capacitor current compensation and active damping to reduce low order current harmonics.
In the DC-side controller, ω is referenced to rotational speed e * And actual rotor speed omega e The difference value of (2) is passed through proportional integral controller to obtain DC bus power reference value P dc * . The power reference is then divided by the filtered DC voltage u dcL Obtaining the DC reference value i dc * At the same time as the measured DC bus current i dc The difference value of (a) is passed through a proportional-integral controller to obtain a voltage difference reference value delta u of two ends of the direct-current bus inductor L * And u dcL The reference value u of the average output voltage at the two ends of the diode bridge can be obtained by adding d * . Ac measured active power Pac can be obtained by multiplying ud by idc. The q-axis reference current iq is obtained according to equation (1).
In the ac side controller, magnetic field orientation control is employed, in which the d-axis current reference id is set to zero. For filter capacitor voltage u abc Sampling, and calculating dq axis voltage u by Park and Clarke transformation dq . The steady-state dq axis voltage u is obtained by a low-pass filter dL And u qL . Then, the steady-state capacitance current is calculated from equation (2) and compensated in the control loop. Similarly, the high frequency voltage harmonic u dH And u qH Obtained by a high pass filter. By increasing the virtual damping resistance R H Active damping of the current source converter can be realized, and suppression is realizedLow current harmonics caused by a reduced modulation index. The delay angle alpha and the amplitude i of the reference current can be obtained through coordinate system transformation dcr . By means of direct current i dc And electric rotor angle theta e The angle thetar of the space vector modulation and the modulation index mr can be obtained. Finally, a switching pulse for a current source converter is realized.
The closed-loop control of the PWM current source converter at the power grid port is shown in fig. 8, and a dq axis rotation coordinate system is established through power grid voltage directional control, wherein the electric angle and the angular speed of the power grid are obtained through a phase-locked loop, and the filter capacitor voltage can obtain the AC-DC axis component of the capacitor voltage through coordinate transformation. d-axis closed-loop control active power, and obtaining an input bus inductance current reference value i by a tide control strategy dc * And a DC feedback value i dc Obtaining a d-axis current reference value i through closed-loop control d * The method comprises the steps of carrying out a first treatment on the surface of the Q-axis closed-loop control reactive power, and a Q-axis current reference i is obtained by using reactive power reference Q q *。u dH /R H And u qH /R H Is a damping current for LC resonance mitigation, where R H Is a virtual damping resistor. And compensating the filter capacitor current in a steady state to obtain a current reference value of the converter, wherein the current reference value is shown in a formula (3) (4), the formula (3) shows the current reference value in an inversion state, and the formula (4) shows the current reference value in a rectification state. Finally, the switching pulses required by the converter are generated by means of current-mode space vector modulation.
The main idea of master-slave control is to set one land transformer station as a master station, the other transformer stations as slave stations, and the master station with larger capacity absorbs all power fluctuation and fixes the active power of the slave station to be a constant value. For current source type hvdc transmission systems, the dc bus current is typically controlled to achieve power balance of the system. However, in a multi-port system with a parallel structure, the current of each connected branch circuit has a coupling effect, so that power oscillation is caused.
The voltage source type flexible direct current transmission system controls the direct current bus voltage, eliminates the mutual influence of all ports and realizes decoupling operation. However, in the current source system, the dc voltage is chopped as a switch, and the ripple amplitude is large. Therefore, the present application has been studied with the average value of the dc voltage as a control target. DC bus voltage control based on PWM current source type converter multi-terminal DC power transmission system as shown in FIG. 9 (a), reference DC bus voltage u will be made dc * And the filtered direct current voltage u dcL After the difference is made, a direct current reference value i of the inverter is output through the PI controller dc * Then the d-axis current reference value i is output through direct current proportional-integral (PI) control d * . As shown in fig. 9 (b), after the actual and the given value of the active power are differed, the PI controller outputs the dc reference value i of the inverter dc * Then the d-axis current reference value i is output through the control of the direct current PI d * . Since PWM current source type converters must implement dc current control, the d-axis current reference is generated by the dc current controller.
Fig. 10 (a) to (d) show characteristics of a far-sea wind farm and a land-based substation in a master-slave controlled multi-terminal dc power transmission system. The far-sea wind farm 1 and the far-sea wind farm 2 are set to constant power for capturing the maximum power of the wind. The land transformer station 1 is set as a master station to control direct current voltage, and the land transformer station 2 is set as a slave station to control power. In fig. 10, point a is set as the initial steady-state point for each station in the system. When the wind power captured by the far-sea wind farm 1 increases to point B in fig. 10 (a), the power of the primary station, i.e. the onshore substation 1, increases accordingly to keep the transmission system dc link voltage constant. As shown in fig. 10 (c), the power of the land transformer station 1 is increased to P 2B . On the other hand, the power of the open sea wind farm 2 and the onshore substation 2 stations in fig. 10 (b) and 10 (d) is maintained at P 3A And P 4A Is unchanged. Likewise, when the power of the wind farm is reduced to point C, the power of the primary station of the land substation 1 is reduced in response to keep the voltage of the direct current link of the transmission system constant.
And voltage droop control is adopted, and direct-current voltage is regulated by all power stations on land. Unlike master-slave control, the dc link voltage of voltage drop control varies within a certain range. Power voltage drop control is theoretically equivalent to voltage current drop control. Since the dc bus current of PWM current source converters and high frequency isolated current source converters has been controlled in converter stage control, current-voltage droop control is employed herein in place of PWM current source converters. Fig. 11 is a control block diagram of voltage droop control. By applying a DC voltage u dcL Filtered value and U dc The values are compared and multiplied by a droop coefficient k, with the current I dc Adding to obtain DC current i of the inverter dc * As shown in the formula (5), the d-axis current reference value i can be obtained through the direct current PI controller gd * . Wherein U is dc And I dc Is a stable operating point determined by system parameters.
In fig. 12, point a is set as the initial steady-state point of the system. Power increase to P for the far sea wind farm 2 3B Since the dc link current reference values of the onshore substation 1, the onshore substation 2 and the offshore wind farm 1 remain unchanged, the voltage in the dc power transmission network will rise. Thus, the operating point of the far-sea wind farm 2 moves to point B in fig. 10 (B). Then, in the voltage sag characteristic curve of fig. 13, the operating points of the land transformer substation 1 and the land transformer substation 2 move toward point B with an increase in the dc voltage. Thus, their dc link current increases. Since the far-sea wind farm 1 does not change the wind power generation, the power is unchanged when the dc link voltage increases. Thus, the direct link current in the far-sea wind farm 1 is reduced. Conversely, when the land-based substation power is reduced, the voltage of the dc grid may be reduced. In fig. 12 and 13, electricityThe station's operating point will move to point C. Similar to a voltage source converter based multi-terminal dc transmission system, the sag factor can be designed for land-based power stations of different power capacities.
First, in order to verify the steady-state performance of the modular high-frequency isolated current source type converter and the reactive compensation capability of the PWM current source type converter, the onshore substation 1 and the offshore wind farm 1 are operated back-to-back, and the offshore wind farm 1 and the two high-frequency isolated current source type converters are connected in parallel with inputs and in series with outputs. Fig. 14 (a) shows the primary side voltage current and the input/output current of the high-frequency transformer in the open sea wind farm 1, and it can be seen that the primary side current of the high-frequency transformer in a steady state approximates to a square wave, the efficiency is high, the input current is sinusoidal, the total harmonic distortion is 4.0%, the output current is constant, and the requirement of the wind farm is met. Fig. 14 (b) shows a phase a input voltage and current of the land transformer station 1, and the current source type converter operates with an advanced 0.9 power factor, a unit power factor and a lagging 0.9 power factor respectively, so that the voltage of the power grid is stable, the current is sinusoidal, and the reactive compensation capability of the PWM current source type converter is verified.
And secondly, verifying the power fluctuation suppression capability of the system for switching in and switching out the new transformer substation. Fig. 15 (a) is an experimental waveform of a system switching into the land-based substation 2, wherein the networking control adopts a master-slave control method, the land-based substation 1 controls the dc voltage to be constant at 60V, the land-based substation 2 is provided with an active power reference value of 300W, and after switching into the system, the dc current i of the land-based substation 2 dc4 Increase, while the direct current i of the land-based substation 1 dc2 Decreasing to keep the dc link voltage constant. Fig. 15 (b) is a waveform diagram of the system switching into the far-sea wind farm 2, during which the dc link voltage fluctuates due to the power increase. Thus, the direct current i of the far-sea wind farm 1 dc1 There is a fluctuation. The direct current voltage of the power transmission network is unchanged, even if the input power fluctuates by one time, the power of the land transformer substation 2 still remains unchanged, and the voltage station consumes the newly-added active power. The substation may lose power transmission capacity due to certain fault conditions, such as a grid three-phase short circuit fault. Thus, in case of failure, a safe shutdown of the plant is crucial. FIG. 15 (c) shows a system cutA waveform diagram of the far-sea wind power plant 2 is generated, and at the moment, the direct current i of the far-sea wind power plant 2 dc3 To zero, the voltage across the offshore wind farm 2 will also decrease due to the change in power, resulting in voltage fluctuations of the offshore wind farm 1. In order to ensure that the dc link voltage is unchanged, the current of the land-based substation 1 is correspondingly reduced. Fig. 15 (d) is a waveform diagram of the system cut-out of the land-based substation 2, the dc current i of the land-based substation 1 dc2 Increase the DC current i dc4 Reduced to zero. Due to the small variation of the average direct voltage, the direct current i of the open sea wind farm 1 dc1 There are also fluctuations.
Fig. 16 is a graph of transient waveforms measured during a transition from master-slave control to voltage droop control mode. Before conversion, set I in formula (5) dc The average current for the direct current links of the land transformer substation 1 and the land transformer substation 2 is 7.4A. U (U) dc The actual dc voltage was set to 60V. In the master-slave control process, the land transformer substation 2 works at a slave station, the active power is 300w, and the land transformer substation 1 works at a master station. After conversion to voltage sag control, the direct currents of the land-based substation 1 and the land-based substation 2 are 7.4A. The direct-current voltage is kept stable in the conversion process, so that the influence on the far-sea wind power plant is small. The dynamic response diagram of the voltage sag control is shown in fig. 17, the power of the open sea wind farm 2 is increased by 1.2 times, and the direct current and the voltage of the land transformer substation are increased. Since the droop coefficient of the land transformer substation 1 is set to be 0.5 and is higher than 0.2 of the land transformer substation 2, the active power transmitted by the land transformer substation 1 is more. Thus, after a dynamic response, the direct current i of the land-based substation 1 dc2 Up to 8.4A, while i of the land-based substation 2 dc4 Only up to 7.8A. Since the active power of the far-sea wind farm 1 is constant, its DC current i dc1 Decreasing with increasing dc voltage.
In the description of the present specification, the descriptions of the terms "one embodiment," "example," "specific example," and the like, mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present application. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiments or examples. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The foregoing has shown and described the basic principles, principal features and advantages of the application. It will be understood by those skilled in the art that the present application is not limited to the embodiments described above, and that the above embodiments and descriptions are merely illustrative of the principles of the present application, and various changes and modifications may be made without departing from the spirit and scope of the application, which is defined in the appended claims.
Claims (10)
1. A multi-port flexible direct current transmission system based on a PWM current source type converter is characterized in that the system is of a ring network type structure formed by connecting four ports in parallel, wherein the ports of two modularized high-frequency isolation current source type converters are connected with an open sea wind power plant, the ports of the two PWM current source type converters are connected with a land transformer substation, the four ports are connected through a ring network, and power flow control is achieved through master-slave control and voltage droop control.
2. A multi-port flexible direct current transmission system based on a PWM current source type converter according to claim 1, wherein said modular high frequency isolated current source type converter comprises: the three-phase full-bridge circuit comprises a three-phase filter capacitor, a three-phase full-bridge circuit, a high-frequency transformer, a diode bridge rectifier and a direct current bus inductor;
the three-phase full-bridge circuit includes: an a-phase bridge arm formed by connecting a first bidirectional switch tube and a fourth bidirectional switch tube in series, a b-phase bridge arm formed by connecting a third bidirectional switch tube and a sixth bidirectional switch tube in series, and a c-phase bridge arm formed by connecting a fifth bidirectional switch tube and a second bidirectional switch tube in series are connected with a primary coil of a high-frequency transformer, a three-phase filter capacitor is connected between the midpoint of the three-phase bridge arm of a three-phase full-bridge circuit and a motor, and a secondary side of the high-frequency transformer is connected with a diode bridge rectifier and then connected with a direct-current bus inductor.
3. A multi-port flexible direct current transmission system based on a PWM current source converter according to claim 1, wherein the system comprises a grid side PWM current source converter comprising: the three-phase full-bridge circuit is the same as that in the modularized high-frequency isolation current source type converter, a three-phase bridge arm is connected with the bus inductance, the three-phase filter circuit is connected between the midpoint of the three-phase bridge arm of the three-phase full-bridge circuit and the primary side of the power frequency transformer, and the secondary side of the power frequency transformer is connected with a power grid.
4. The multi-port flexible direct current transmission system based on the PWM current source type converter according to claim 1, wherein the power grid side PWM current source type converter is topological, current commutation is achieved through a two-way switch, the PWM current source type converter inputs direct current side bus inductance, inductance current is modulated through a two-way switch device, an output alternating current side three-phase filter circuit filters high-frequency components of output PWM chopping current on one hand, on the other hand, two-way switch tube commutation is assisted, and electric isolation between a land power grid and a multi-port direct current network is achieved through a power frequency transformer.
5. A multi-port flexible direct current transmission system based on a PWM current source type converter according to claim 1, wherein the control system of the modular high frequency isolated current source type converter consists of two parts: the first part is direct current side control, which comprises the adjustment of the rotating speed of the rotor and direct current; the second part is AC side control, including filter capacitor current compensation and active damping to reduce low order harmonic wave;
in a direct current side controller controlled by a direct current side, double closed-loop control of direct current bus current and motor rotating speed is realized through a proportional-integral controller, wherein a rotating speed ring is an outer ring, and a current ring is an inner ring;
in an AC side controller for AC side control, a filter capacitor voltage is sampled by adopting magnetic field orientation control to carry out coordinatesAfter transformation, a steady-state capacitance current is obtained through a low-pass filter, and is compensated in a control loop, and a virtual damping resistor R is added H The active damping control of the high-frequency isolation current source type converter is realized, and low-current harmonic waves caused by the reduction of modulation indexes are restrained.
6. The multi-port flexible direct current transmission system based on the PWM current source type converter according to claim 1, wherein the modularized high-frequency isolation current source type converter adopts a mode of input parallel connection and output series connection.
7. A multi-port flexible direct current transmission system based on a PWM current source type converter according to claim 1, wherein the control system of the grid side PWM current source type converter comprises: the phase-locked loop has its input connected to the line voltage of the power network side and outputs the power network frequency omega g Grid phase theta g A grid voltage dq axis component;
the input end of the low-pass filter is connected with the power grid voltage dq axis component and the power grid frequency, and the power grid voltage dq axis component after the low-pass filtering is output;
the input end of the high-pass filter is connected with the power grid voltage dq axis component and the power grid frequency, and the power grid voltage dq axis component after the high-pass filtering is output;
the input end of the current set value correction module is connected with the d-axis component of the input direct current bus set current value, the actual current value and the filter capacitor steady-state current, the error of the input direct current bus set current value and the actual current value is processed by the proportional-integral controller, the d-axis component of the filter capacitor steady-state current is accumulated, and the final current set value is output;
the coordinate conversion module is used for carrying out coordinate conversion on the final current given value and outputting the given value of the direct current and a trigger delay angle;
and the space vector modulation module is connected with the given value of the direct current and the trigger delay angle at the input end, calculates the modulation ratio and the modulation angle and outputs the switching pulse of the matrix converter.
8. A control method of a multi-port flexible direct current transmission system based on a PWM current source type converter is characterized in that closed loop control is carried out on the average value of direct current bus voltage, the multi-port direct current transmission system cannot directly control direct current of the direct current bus due to the complexity of a topological structure, the analog voltage source type converter adopts the direct current bus voltage to eliminate the influence among power stations, and the average value of the direct current bus voltage is adopted to eliminate the fluctuation of the direct current bus voltage in the current source type converter, so that power balance among power stations is achieved.
9. The control method of a multi-port flexible direct current transmission system based on a PWM current source converter according to claim 8, wherein the control method comprises a master-slave control scheme comprising the steps of: the method is characterized in that a reference DC bus voltage is differed from a filtered DC voltage and then a DC current reference value of an inverter is output through a proportional-integral controller, and then a d-axis current reference value is output through DC proportional-integral control.
10. The method of controlling a multi-port flexible dc power transmission system based on a PWM current source converter according to claim 8, wherein the control method comprises a voltage droop control scheme comprising the steps of: the DC voltage is regulated by all power stations on land, the DC link voltage can be changed in a certain range, its specific idea is to filter the DC voltage and U dc The values are compared and multiplied by a droop coefficient and added with the direct current to obtain the converter direct currentFinal reference value of current flow, i.e.D-axis current reference value is obtained through a direct current proportional-integral controller, wherein U is as follows dc And I dc Is a stable operating point determined by system parameters.
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| CN117175920B (en) * | 2023-09-01 | 2024-03-01 | 国网经济技术研究院有限公司 | High-efficiency multiport direct current converter applied to wind power mismatch |
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