CN112117784B - Operation control method of virtual transformer - Google Patents
Operation control method of virtual transformer Download PDFInfo
- Publication number
- CN112117784B CN112117784B CN202010886654.4A CN202010886654A CN112117784B CN 112117784 B CN112117784 B CN 112117784B CN 202010886654 A CN202010886654 A CN 202010886654A CN 112117784 B CN112117784 B CN 112117784B
- Authority
- CN
- China
- Prior art keywords
- voltage
- current
- transformer
- stage
- input
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Active
Links
Images
Classifications
-
- 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
- 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
- H02J3/50—Controlling the sharing of the out-of-phase component
-
- 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]
Abstract
The invention provides an operation control method of a virtual transformer, which comprises the following steps: a medium-voltage alternating current input stage control step: a dq decoupling control input stage rectifier is adopted, an inner current ring respectively controls active current and reactive current, and an outer voltage ring is connected with capacitor voltage and active current on a direct current side; isolation voltage transformation stage control: a single phase shift is adopted to control an isolation transformer stage; an inverter control step: the inverter is controlled by adopting a voltage outer ring and a current inner ring. The invention can realize the functions of input/output alternating current amplitude value equal proportion transformation and phase synchronization; by establishing the voltage relation between networks, the networks can provide voltage support for each other, and meanwhile normalization management is facilitated; the plug-and-play alternating current port can be provided, and the port voltage does not need to be additionally detected; the external characteristics of the alternating current side of the transformer are the same as those of the transformer, so that the transformer is convenient for engineering personnel to use and understand.
Description
Technical Field
The invention relates to the field of power distribution, in particular to an operation control method of a virtual transformer.
Background
At present, the power distribution network is developed towards intellectualization and multipotency. Distribution transformers are one of the main electrical devices that implement transformation and power transmission, and have the advantages of low cost and high reliability. However, the conventional distribution transformer has large no-load loss, large volume and weight, does not have a fault isolation function, and cannot actively manage the quality problem of electric energy. Meanwhile, the distribution transformer cannot effectively manage a large amount of distributed energy access and direct current distribution requirements. Therefore, a new generation of intelligent power distribution network needs novel intelligent and comprehensive electrical equipment based on power electronic technology.
With the introduction of energy internet and energy routing concepts, the architecture, theory, technology and application of the solid-state transformer, the next generation of intelligent power distribution equipment, become the main direction of research. Fig. 1 shows an application scenario of a solid-state transformer in an energy internet. In the energy internet which mainly uses electric energy and has various energy forms such as cold energy, heat energy, natural gas and the like, the solid-state transformer has important functions of allocating power, providing a load interface, isolating faults and the like. In a pure direct current energy sub-network, the solid-state transformer is represented as a direct current solid-state transformer; in an ac/dc energy sub-network, the solid-state transformer behaves as an ac/dc hybrid solid-state transformer.
Currently, three-stage solid-state transformers are researched and applied more, and fig. 2 shows a typical three-stage solid-state transformer structure. The medium-voltage alternating current input stage converts medium-voltage alternating current into medium-voltage direct current and has a power factor control function; the isolation transformer stage converts the medium-voltage direct current into low-voltage direct current, and realizes electrical isolation and voltage grade conversion through a high-frequency transformer; the low-voltage AC output stage converts the low-voltage DC power to low-voltage AC power.
The solid-state transformer plays a role of a power transmission junction in a power distribution network, and can be applied to interconnection among a source, a network and a load. In the future, large-scale distributed energy is connected to each isobaric grade bus, the volatility of each grade of power grid is improved, the stability is reduced, power support needs to be provided among the grids, and therefore certain requirements are provided for the control method of the solid-state transformer. Compared with the traditional distribution transformer, the mutual support of the voltage between the networks can be realized as long as the relation of the voltage between the networks is established, and the stability of the system is improved. At present, one end of a solid-state transformer interconnected between alternating-current networks is controlled by adopting constant voltage/frequency, so that the solid-state transformer can only provide voltage support on one side.
The existing literature provides a solid-state transformer comprehensive control technology based on energy balance, the solid-state transformer is formed by combining multiple stages of power electronic conversion units, and the transient performance of the direct-current bus voltage in the solid-state transformer can be improved by performing comprehensive control by utilizing the energy transfer relation among the units at all stages, so that the transient performance of the solid-state transformer is improved. Firstly, establishing an energy model of the solid-state transformer, then establishing two energy branches with different time scales, respectively designing energy balance controllers for controlling two-stage bus voltage according to the energy balance relation, and simultaneously designing a voltage-sharing controller for the cascade modules according to the energy relation among the cascade modules in the topology. And then, analyzing the influence of the parameter difference of the passive devices in the actual system on the control model, and finally providing a comprehensive control strategy for grid-connected operation of the solid-state transformer based on the energy balance relation. The simulation result proves the effectiveness of the energy balance control method, and analyzes the influence of the capacitance value change of the two-stage bus on the transient value of the bus voltage respectively, which shows that the design margin of the bus capacitor can be reduced by adopting the energy balance control. The experimental result verifies the correctness of theoretical analysis and simulation results.
The scheme can realize the power balance and the fixed value control of the direct current bus voltage of the whole solid-state transformer system. However, the bus voltages are controlled by constant values, and the system is stable and the power control is premised on that the main power grid is strong enough, and when the voltage of the power grid drops, voltage support cannot be provided among the bus voltages.
The existing literature proposes a structure of a novel solid-state transformer and a control strategy thereof, and an energy router is used as a core device of an energy internet and is concerned about effective consumption of renewable energy and safe and reliable operation of a power grid. Aiming at the defects of the topological structure of the existing energy router, the novel energy router suitable for interconnection of multi-voltage-level alternating current and direct current power grids is provided. Firstly, the topological structure of the novel energy router is analyzed, and corresponding control methods are provided for different parts. The input stage is a Modular Multilevel Converter (MMC) structure, and a virtual synchronous motor control strategy is applied to the MMC structure, so that the inertia and the damping of the system are enhanced. The output stage power is flexibly adjusted, so that a lower-stage power grid can provide power support for a higher-stage power grid through the energy router and participate in primary frequency modulation of the higher-stage power grid. The isolation level is constructed by connecting an input-series output-series (ISOS) module and an input-series output-parallel (ISOP) module in series and parallel through a double-active-bridge (DAB) module, so that the domain interconnection and the electrical isolation of the alternating-current and direct-current power grids with different voltage levels are realized. And then, providing a power coordination control method, and ensuring the stable operation of the energy router while meeting the power requirement of the power grid connected with the output stage. And finally, a system simulation model is built based on PSCAD/EMTDC, and the reliability and effectiveness of the topological structure and the control strategy of the novel energy router are verified.
The scheme is suitable for the alternating current-direct current power grid with multiple voltage levels. The topology and the control method can realize interconnection and power balance of alternating current and direct current power grids with different voltage grades. However, the low-voltage alternating current output control is a constant-voltage and constant-frequency strategy, and the low-voltage side can not provide voltage support for other buses, so that the stability of the system is reduced.
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide an operation control method of a virtual transformer.
The operation control method of the virtual transformer provided by the invention comprises the following steps:
a medium-voltage alternating current input stage control step: a dq decoupling control input stage rectifier is adopted, an inner current ring respectively controls active current and reactive current, and an outer voltage ring is connected with capacitor voltage and active current on a direct current side;
isolation voltage transformation stage control: a single phase shift is adopted to control an isolation transformer stage;
an inverter control step: the inverter is controlled by adopting a voltage outer ring and a current inner ring.
Preferably, in the step of controlling the medium-voltage ac input stage, the decoupled open-loop transfer function of the current inner loop is:
wherein: k is a radical of pwm For medium voltage AC input stage equivalent gain, L 1 For the input inductance of the AC side, T s Is the current sampling constant.
Preferably, the medium voltage ac input stage is accelerated by a PI controller, wherein the PI parameter K of the PI controller pi 、K ii The calculation formula is as follows:
preferably, the open loop transfer function of the voltage loop without the PI controller is:
wherein, C 1 Is a medium voltage DC side capacitance value, T ev ＝τ v +3T s ，τ v Representing the voltage outer loop sampling inertia constant.
Preferably, in the step of controlling the isolated voltage transformation stage, the working condition of the left rectification stage in one period is analyzed to obtain an input-output current average value model:
wherein n is the turn ratio of the positive side and the secondary side of the transformer, U dc1 、U dc2 The voltage of the input side and the voltage of the output side of the isolated transformer stage respectively, d is the phase-shifting duty ratio, f s To the switching frequency, L k Is a filter inductor.
Preferably, the disturbance amount is added to the input and output current average value model:
eliminating direct current and second-order high-order quantity to obtain an equivalent small signal model of an isolation stage:
in the formula, R Le Is a load resistance, C 2 Is a low voltage DC side capacitor, f s Is the switching frequency.
Preferably, in the inverter control step, the voltage outer loop generates the current inner loop reference current i through the PI controller d *、i q The current inner loop generates u through a PI controller d *、u q And sampling the alternating voltage at the medium-voltage alternating current input side by a PLL (phase locked loop) to output a phase angle following signal, enabling an output voltage modulation signal to follow the frequency and the phase angle at the medium-voltage alternating current side, and generating a switching signal after the modulation signal passes through an SPWM (sinusoidal pulse width modulation) link.
Preferably, the current inner loop open loop transfer function before inverter control compensation is:
wherein k is pwm For low voltage AC output stage inverter equivalent gain, L f For outputting the filter inductance, T s Is the current sampling constant.
Preferably, the voltage loop open loop transfer function before compensation is
Preferably, the compensated system cross-over frequency is 95.8Hz with a phase angle margin of 64.5 degrees.
Compared with the prior art, the invention has the following beneficial effects:
1. the invention can realize the functions of input/output alternating current amplitude value equal-proportion transformation and phase synchronization;
2. according to the invention, by establishing the voltage relation between networks, the networks can provide voltage support mutually, and meanwhile, normalization management is facilitated;
3. the invention can provide a plug-and-play alternating current port without additionally detecting the voltage of the port;
4. the external characteristics of the alternating current side of the transformer are the same as those of the transformer, so that the transformer is convenient for engineering personnel to use and understand.
Drawings
Other features, objects and advantages of the invention will become more apparent upon reading of the detailed description of non-limiting embodiments with reference to the following drawings:
FIG. 1 is a schematic diagram of a solid-state transformer applied to an energy Internet;
FIG. 2 is a schematic diagram of a three-stage solid-state transformer;
FIG. 3 is a schematic diagram of a main circuit of a solid-state transformer;
FIG. 4 is a block diagram of a medium voltage AC input stage control;
FIG. 5 is a block diagram of a current inner loop of a medium voltage AC input stage;
FIG. 6 is a diagram of an inner loop of current for a medium voltage AC input stage before compensation;
FIG. 7 is a diagram of a compensated inner loop bode of a medium voltage AC input stage current;
FIG. 8 is a block diagram of the outer loop control of the voltage of the medium voltage AC input stage;
FIG. 9 is a diagram of an outer ring bode of the medium voltage AC input stage voltage (before compensation);
FIG. 10 is a diagram of an outer ring bode of the medium voltage AC input stage voltage (after compensation);
FIG. 11 is a schematic diagram of an isolated transformer stage topology;
FIG. 12 is a diagram of an isolated transformer stage small signal model;
FIG. 13 is an isolation transformer stage control block diagram;
FIG. 14 is a block diagram of a low voltage AC output stage control;
FIG. 15 is a low voltage AC output stage current inner loop block diagram;
FIG. 16 is a diagram of the inner loop of the low voltage AC output stage current (before compensation);
FIG. 17 is a diagram of the inner loop of the low voltage AC output stage current (after compensation);
FIG. 18 is a block diagram of the voltage outer loop of the low voltage AC output stage;
FIG. 19 is a diagram of the outer ring of the low voltage AC output stage voltage bode (before compensation);
FIG. 20 is a diagram of the outer ring of the low voltage AC output stage voltage bode (after compensation);
FIG. 21 is a diagram of DC bus voltage (25kW-50kW) with load power fluctuations;
FIG. 22 is a schematic diagram of AC side conditions (25kW-50kW) with load power fluctuations;
FIG. 23 is a graph of output voltage following when the input voltage is abruptly changed in magnitude.
Detailed Description
The present invention will be described in detail with reference to specific examples. The following examples will assist those skilled in the art in further understanding the invention, but are not intended to limit the invention in any way. It should be noted that it would be obvious to those skilled in the art that various changes and modifications can be made without departing from the spirit of the invention. All falling within the scope of the present invention.
As shown in fig. 3 to 23, the operation control method of the virtual transformer provided by the present invention is mainly a virtual transformer operation control strategy for a 10kV intelligent power distribution system. The scheme is a power electronic equipment operation control strategy based on a new concept of a virtual transformer, and a specific technical implementation scheme is explained by taking a three-stage virtual transformer topology shown in fig. 3 as an example without loss of generality.
When the ac input/output terminals of the solid-state transformer are synchronous and the transformation ratio is constant, the solid-state transformer in this operation mode is called a "virtual transformer" because the ac-side external characteristics are the same as those of the transformer.
When the solid-state transformer works in the mode, the relation between voltage and frequency exists between the alternating current input end and the output end, voltage support can be provided for power grids of different levels, and meanwhile normalization management is facilitated.
The operation control of the virtual transformer mainly comprises medium-voltage alternating-current input stage control, isolation transformer stage control and inverter control.
The control of the medium-voltage alternating-current input stage aims at voltage stabilization of a medium-voltage direct-current side and power factor control of an alternating-current side, and the design of a controller mainly comprises a control method, a structural design and a control parameter design, specifically:
1. control method and structure design
The input-stage rectifier is generally controlled by adopting a dq decoupling control method, and according to the control target, the active loop can control the stability of the voltage at the direct current side, and the reactive loop can control the power factor. A three-phase SVPWM control block diagram based on double closed-loop control in dq rotation coordinate system is shown in fig. 4. The current inner loop respectively controls active current and reactive current by adopting a feedforward decoupling control method, and the voltage outer loop links the direct current side capacitor voltage and the active current.
2. Control parameter design
According to the double-ring structure, the inner ring control parameters are firstly designed.
The decoupled current inner loop is shown in FIG. 5 (taking d-axis current as an example), and the open-loop transfer function of the current inner loop before uncompensation is
Wherein k is pwm For equivalent gain, L, of the medium voltage AC input stage 1 For the input inductance of the AC side, T s The inertia constant is sampled for the current inner loop. By substituting the circuit parameters, a bode plot of the open loop transfer function of the current electrical loop can be drawn, as shown in FIG. 6. Therefore, the system has low crossing frequency and low response speed, and the response speed needs to be accelerated by the PI controller.
PI parameter K pi 、K ii The calculation formula is as follows:
the compensated open loop transfer function bode diagram of the system is shown in FIG. 7, which crosses the frequency 591Hz and has a phase angle margin of 41.2 degrees.
The outer loop control parameters are again designed. In order to simplify the design of the outer loop, the current inner loop should have a fast response speed, and the switching frequency is high enough, under this premise, the current inner loop closed loop function can be equivalent to:
the voltage outer loop control block diagram is shown as 0, where T ev Is an integrated inertia constant
T ev ＝τ v +3T s (5)
K v 、T v -PI control parameters
τ v Voltage outer loop sampling small inertia constant
The open loop transfer function of the voltage loop without the PI regulator is
In the formula, C 1 Is the medium voltage dc side capacitance.
The bode graph is shown as 0, the crossing frequency is 211Hz, which is about 1/3 of the current loop crossing frequency, and the oscillation is easy to generate. If the crossing frequency of the compensated system is 100Hz and the phase angle margin is 60 degrees, the frequency of the compensated system is determined
Calculating to obtain a voltage loop PI regulator parameter K pv 、K iv As shown in equation (8), the compensated system bode graph is shown in fig. 10, where the crossing frequency is 102Hz and the phase angle margin is 60.4 degrees.
The isolation voltage transformation stage control design is divided into a small signal modeling analysis and control structure block diagram design, wherein:
1. isolation transformer small signal modeling analysis
The topological diagram of the isolated transformer stage is shown in fig. 11, the left-side rectifier stage is equivalent to a direct-current voltage source, and the working condition in one period is analyzed to obtain an input-output current average value model:
wherein n is the turn ratio of the positive side and the secondary side of the transformer, U dc1 、U dc2 The voltage of the input side and the voltage of the output side of the isolated transformer stage respectively, d is the phase-shifting duty ratio, f s To the switching frequency, L k Is a filter inductor.
Adding disturbance to the above formula
An equivalent small-signal model of the isolation stage obtained by eliminating the direct current and the second-order high-order quantity is shown in FIG. 12, and can be known from the small-signal model
In the formula, R Le Is a load resistance, C 2 Is a low voltage DC side capacitor, f s Is the switching frequency.
2. Control structure block diagram design
As can be seen from equation (12), the control of the isolated transformer stage can be performed by a single phase shift control method, and the control block diagram is shown as 0, where K m 1. And comparing the sampling value of the low-voltage direct-current side voltage with the reference value, and forming a duty ratio control signal through the PI controller to maintain the stability of the low-voltage direct-current side voltage.
Because the isolated voltage-variable secondary side in the actual circuit is connected with the AC load of the inverter, the isolated voltage-variable secondary side cannot be equivalent to a resistor R Le Therefore, the specific PI parameters are given in the application case after debugging.
The inverter is controlled to output an alternating current in phase with the input alternating voltage and in proportion to the input alternating voltage. Design of controller
The method mainly comprises the following steps of designing a control method and a structure, designing control parameters, wherein:
1. control method and structure design
The inverter adopts a double-loop control method, and a control block diagram is shown as 0. The voltage outer ring generates a current inner ring reference current i through a PI controller d *、i q Inner loop of currentGenerating u by PI controller d *、u q And sampling the alternating voltage at the medium-voltage alternating current input side by a PLL (phase locked loop) to output a phase angle following signal, enabling an output voltage modulation signal to follow the frequency and the phase angle of the medium-voltage alternating current side, and generating a switching signal after the modulation signal passes through an SPWM (sinusoidal pulse width modulation) link to realize the synchronization of the output voltage and the voltage at the medium-voltage alternating current side.
2. Control parameter design
Taking d-axis output as an example, firstly, designing inner loop control parameters, considering the delay link of signal sampling and the small inertia delay of PWM control, and temporarily not considering the output voltage u od The structure of the inner current loop of the decoupled three-phase voltage source inverter is shown as 0, wherein K pi 、K ii Is a PI control parameter.
Small time constant 0.5T s And T s And combining to obtain a current inner ring open loop transfer function before compensation as follows:
k pwm for low voltage AC output stage inverter equivalent gain, L f For outputting the filter inductance, T s Is the current sampling constant.
The open loop transfer function bode transfer function before compensation obtained by substituting the circuit parameters is shown as 0, and the graph shows that the inner loop crossing frequency of the current before compensation is too low, and the system rapidity index is poor. Let the crossover frequency of the compensated current inner loop system be 1/20 of the switching frequency, i.e. 500 Hz. Writing the PI-controller transfer function to pole-zero form, i.e.
Take tau is ＝L f R, pole zero cancellation, the switch transfer function can be simplified to
Calculated according to the characteristics of a typical second-order system
The compensated open loop transfer function bode graph of the system is shown as 0, the crossing frequency is 483Hz, the phase angle margin is 65.5 degrees, and the design requirement is met. Meanwhile, the current loop closed loop transfer function can be approximately equivalent to an inertia constant of 3T s The inertia element of (1), however, is premised on a high switching frequency, as shown in equation (17).
And designing voltage outer ring control parameters again, and considering a voltage signal sampling delay link, wherein a voltage outer ring control block diagram is shown as 0. Wherein K pv 、K iv For PI control parameters, G ci Is a current loop closed loop transfer function. The open loop transfer function of the voltage loop before compensation is
The bode plot of the voltage loop open loop transfer function obtained from equation (18) is shown in FIG. 0. The crossing frequency of the voltage outer ring is 2850Hz, and the phase angle margin is-50.2 degrees. The system has high crossing frequency and very good rapidity, but has low stability, and the PI controller is required to balance the rapidity and the stability of the system. Setting the compensated voltage outer ring crossing frequency at 1/5 of the current inner ring crossing frequency, namely 100Hz, and setting the turning frequency of the PI controller at 20Hz, namely satisfying the condition
Get it solved
The compensated voltage loop open loop transfer function bode graph is shown as 0, the system crossing frequency after compensation is 95.8Hz, the phase angle margin is 64.5 degrees, and the design requirement is met.
The invention is further described in detail with reference to the following drawings and specific embodiments.
Examples of implementation:
in order to verify the technical scheme proposed by the patent, a system model as shown in fig. 3 is established based on a MATLAB-Simulink environment. When system power parameters are designed, the power capacity redundancy of the rectifier and the direct-current bulkhead transformer is high, and the rated load power is set to be 50 kW. If the load power is expected to be improved, only a plurality of inverter interfaces are needed to be connected in parallel, and the working condition is explained by only 1 inverter interface in the patent. Specific power parameters are shown in Table 1
TABLE 1 System Power parameters
Rectifier parameter design
In order to improve the voltage utilization rate, the rectifier adopts bipolar SVPWM modulation, and the effective value of the three-phase current source phase voltage is 6 kV.
A. DC side voltage design
The dc voltage range under SVPWM control is:
common medium-voltage direct-current voltage grade U in patent design dc1 ＝20kV。
B. Rectifier inductor design
Alternating current side inductance L when output power index is satisfied 1 ：
Inductor L meeting requirements of fast current tracking and harmonic suppression 1 ：
In the formula (22), U dc1 -average value of the rectifier dc side voltage;
I m -ac side current peak value;
omega-power frequency voltage angular frequency;
E m -a grid voltage peak value;
T s -a PWM switching period;
Δi max the maximum allowable current ripple, typically 20% I m 。
Is calculated to
26.9mH≤L 1 ≤735.5mH (23)
Get L 1 ＝50mH。
C. Rectifier DC side capacitance design
The main functions of the direct current side capacitor are as follows: (1) buffering the energy exchange between the AC side and the DC side of the rectifier to stabilize the voltage of the DC side; (2) and the harmonic voltage on the direct current side is suppressed.
On one hand, in order to meet the voltage following performance index of the direct current side of the rectifier, the capacitance should be as small as possible. On the other hand, the capacitance C is used for meeting the load power fluctuation disturbance resistance index 1 Need to satisfy
Wherein, P L For rated transmission power, Δ U dcmax Is taken as the voltage anti-interference index and is 0.5 percent U dc1 Finally, take C 1 ＝500μF。
DAB parameter design
DAB inductance parameter design
DAB inductance L k Need to satisfy powerTransmission index, DAB transmissible power
To ensure DAB power transmission capability redundancy, take L k ＝0.1mH。
B.DAB secondary side capacitor design
Secondary side capacitor C 2 The main functions of the filter and the reduction of the influence of load disturbance on the voltage are satisfied
In the formula I o -the output current of the direct current is,
ΔU 2max the anti-interference index is generally 5% of the direct current voltage when the load of the converter is disturbed.
Calculated by substitution
C 2 ≥937.5μF (28)
In an actual circuit, an alternating current interface is required to be provided on a direct current output side, the capacitance filtering and voltage stabilizing capacity is required to be stronger, and C is selected 2 ＝40mF。
Inverter parameter design
For design convenience, the front-stage circuit is equivalent to an 800V direct current source U dc2 And the effective value of the AC side line voltage is 380V, when SPMW modulation is adopted, the modulation ratio m satisfies the following formula:
U ref -reference phase voltage peak value.
A. Inverter filter inductor design
Three-phase inverter filter inductor L f The design must satisfy two requirements of system rapidity and harmonic suppression, and L is calculated f The following inequalities need to be satisfied:
T s -inverter switching period
I m Peak value of the rated current flowing through the inductor
Δi max Maximum allowed current ripple, typically 20% I m
Substituting the system parameters into the calculation result to obtain L which is more than or equal to 0.6mH f Less than or equal to 15.9mH, taking L f The filter inductance equivalent resistance is typically relatively small, taken as 0.01 Ω, at 5 mH.
B. Inverter filter capacitor design
Frequency f of LC filter to avoid resonance of LC filter rcs Greater than 10 times the fundamental frequency and less than 1/5 times the switching frequency, i.e.
Take f rcs 1000Hz, the capacitance C can be selected according to the resonance formula f Value of (A)
At the same time, it is considered that 5% of the rated capacity of the inverter should not be less than the reactive power consumed on the filter capacitor, i.e. that:
ωC f U ref 2 ≤5％S (33)
is calculated to
C f ≤54μF (34)
And the reactive power consumption requirement is met.
Each switch tube in the circuit is an ideal model, and the parameters of the rest circuits are shown in table 2.
TABLE 2 virtual Transformer principal parameters
Control parameters and simulation results
after the design of control parameters and the experimental debugging in the embodiment, the PI parameters of each part are collated as shown in table 3.
TABLE 3 System control parameters
to verify the feasibility and the load disturbance resistance of the solid-state transformer-based virtual transformer operation strategy, the method is started at the load of 25 kW. The load was stepped to 50kW at 0.3 s.
As shown in 0, the direct current side voltage tends to be stable about 0.05 second after starting, the direct current side voltage of the medium-voltage alternating current input stage is stable at 20kV, and the fluctuation value is less than 0.5 percent U dc1 The secondary side direct current voltage of the isolated voltage transformation stage is stabilized at 800V, and the fluctuation quantity is less than 1 percent U dc2 . After the load power jump is carried out for 0.3s, the voltage on the direct current side of the medium-voltage alternating-current input stage and the voltage on the secondary side of the isolation transformation stage fluctuate to a certain extent, the voltage tends to be stable at 0.35s, and the voltage fluctuation quantity meets the expectation.
In 0, the input current signal is reduced by 20 times for the purpose of comparing the input and output ac voltages. After starting, the output rapidity of the inverter is good, and the phase is synchronous with the input voltage. The load power is doubled within 0.3s, the output voltage is basically unchanged, and the output current is doubled. The expected effect is met.
In order to verify the output voltage following performance of the operation strategy, as shown in 0, the effective value of the input three-phase voltage is changed from 10kV to 11kV at 0.1s, the output alternating voltage is followed and amplified, the voltage transformation ratio is kept constant in the process, and the phase synchronization is good.
The foregoing description of specific embodiments of the present invention has been presented. It is to be understood that the present invention is not limited to the specific embodiments described above, and that various changes or modifications may be made by one skilled in the art within the scope of the appended claims without departing from the spirit of the invention. The embodiments and features of the embodiments of the present application may be combined with each other arbitrarily without conflict.
Claims (9)
1. A method for controlling the operation of a virtual transformer,
the operation of the virtual transformer is a working mode for simulating the synchronization of the alternating current input and output ends of the transformer and the constant transformation ratio, in the working mode, the characteristics of the alternating current input and output two ports of the solid-state transformer are the same as those of the alternating current transformer, the characteristics comprise input and output alternating current amplitude equal proportion transformation and phase synchronization, voltage support can be provided between networks by establishing a voltage relation between the networks, the normalization management is convenient, meanwhile, a plug-and-play alternating current port is provided, and the voltage of the port does not need to be additionally detected;
the method comprises the following steps:
a medium-voltage alternating current input stage control step: a dq decoupling control input stage rectifier is adopted, an inner current ring respectively controls active current and reactive current, and an outer voltage ring is connected with capacitor voltage and active current on a direct current side;
isolation voltage transformation stage control: a single phase shift is adopted to control an isolation transformer stage;
an inverter control step: a voltage outer ring and a current inner ring are adopted to control the inverter;
in the step of controlling the medium-voltage alternating-current input stage, the decoupled current inner loop open-loop transfer function is as follows:
wherein: k is a radical of pwm For medium voltage AC input stage equivalent gain, L 1 For the input inductance of the AC side, T s Is a current sampling constant;
in the inverter control step, the output voltage modulation signal is made to follow the medium voltage AC side frequency and phase angle, and the input side voltage u is sampled iabc Sampling the input side voltage u as the output AC voltage phase via a phase locked loop iabc The dq conversion is carried out to obtain the dq axis voltage u id 、u iq ，u id Multiplying by a virtual transformer transformation ratio coefficient 1/k, and sending to the solid-state transformer as an output side voltage reference instruction value u d * Realizing the output side voltage u oabc And the input side voltage u iabc Virtual transformer function of synchronous regulation.
2. The operation control method of the virtual transformer according to claim 1, wherein the medium voltage ac input stage is accelerated by a PI controller, wherein a PI parameter K of the PI controller pi 、K ii The calculation formula is as follows:
3. the operation control method of the virtual transformer according to claim 2, wherein the voltage loop open loop transfer function without the PI controller is:
wherein, C 1 Is a medium voltage DC side capacitance value, T ev ＝τ v +3T s ，τ v Representing the voltage outer loop sampling inertia constant.
4. The operation control method of the virtual transformer according to claim 1, wherein in the step of controlling the isolation transformer stage, the operation condition of the left rectifying stage in one period is analyzed to obtain an input-output current average value model:
wherein n is the turn ratio of the positive side and the secondary side of the transformer, U dc1 、U dc2 Respectively the input side voltage and the output side voltage of the isolated transformer stage, d is the phase-shift duty ratio, f s To the switching frequency, L k Is a filter inductor.
5. The operation control method of the virtual transformer according to claim 4, wherein a disturbance amount is added to the input and output current average value model:
eliminating direct current and second-order high-order quantity to obtain an equivalent small signal model of an isolation stage:
in the formula, R Le Is a load resistance, C 2 Is a low voltage DC side capacitor, f s Is the switching frequency.
6. The operation control method of the virtual transformer according to claim 1, wherein in the inverter control step, the voltage outer loop generates the current inner loop reference current i through a PI controller d *、i q The current inner loop generates u through a PI controller d *、u q And sampling the alternating voltage at the medium-voltage alternating current input side by a PLL (phase locked loop) to output a phase angle following signal, enabling an output voltage modulation signal to follow the frequency and the phase angle at the medium-voltage alternating current side, and generating a switching signal after the modulation signal passes through an SPWM (sinusoidal pulse width modulation) link.
7. The operation control method of the virtual transformer according to claim 6, wherein the current inner loop open loop transfer function before inverter control compensation is:
wherein k is pwm For low voltage AC output stage inverter equivalent gain, L f For outputting the filter inductance, T s Is the current sampling constant, and r is the equivalent resistance of the inductor.
8. The method of claim 7, wherein the open loop transfer function of the voltage loop before compensation is
Wherein C is f Is a filter capacitor.
9. The method of claim 8, wherein the compensated system cross-over frequency is 95.8Hz, and the phase angle margin is 64.5 degrees.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202010886654.4A CN112117784B (en) | 2020-08-28 | 2020-08-28 | Operation control method of virtual transformer |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CN202010886654.4A CN112117784B (en) | 2020-08-28 | 2020-08-28 | Operation control method of virtual transformer |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CN112117784A CN112117784A (en) | 2020-12-22 |
| CN112117784B true CN112117784B (en) | 2022-08-09 |
Family
ID=73804988
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN202010886654.4A Active CN112117784B (en) | 2020-08-28 | 2020-08-28 | Operation control method of virtual transformer |
Country Status (1)
| Country | Link |
|---|---|
| CN (1) | CN112117784B (en) |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105553304A (en) * | 2016-01-15 | 2016-05-04 | 湖南大学 | Novel modular multi-level solid-state transformer and internal model control method thereof |
| CN109067193A (en) * | 2018-08-17 | 2018-12-21 | 燕山大学 | A kind of cascade connection type electric power electric transformer and its imbalance compensation control method |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN203827197U (en) * | 2014-05-05 | 2014-09-10 | 国家电网公司 | Modular H bridge cascaded multilevel mutually-balanced electric power electronic transformer |
| CN110460238A (en) * | 2019-08-09 | 2019-11-15 | 华中科技大学 | A kind of decoupling control method and device of the DAB converter of input series and output parallel |
| CN110690731A (en) * | 2019-08-20 | 2020-01-14 | 东南大学 | Power electronic transformer applicable to hybrid micro-grid and coordination control and mode switching method thereof |
-
2020
- 2020-08-28 CN CN202010886654.4A patent/CN112117784B/en active Active
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN105553304A (en) * | 2016-01-15 | 2016-05-04 | 湖南大学 | Novel modular multi-level solid-state transformer and internal model control method thereof |
| CN109067193A (en) * | 2018-08-17 | 2018-12-21 | 燕山大学 | A kind of cascade connection type electric power electric transformer and its imbalance compensation control method |
Non-Patent Citations (1)
| Title |
|---|
| 基于虚拟同步机的新型固态变压器控制;朱明磊等;《电器与能效管理技术》;20170731(第7期);第20-26页 * |
Also Published As
| Publication number | Publication date |
|---|---|
| CN112117784A (en) | 2020-12-22 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| CN105553304A (en) | Novel modular multi-level solid-state transformer and internal model control method thereof | |
| Meng et al. | Optimal input and output power quality control of single-phase AC–DC–DC converter with significant DC-link voltage ripple | |
| CN108418231B (en) | Hybrid multi-port railway power converter and power coordination control method thereof | |
| CN205389177U (en) | Novel many level of modularization type solid -state transformer | |
| Zorig et al. | Control of three-level T-type inverter based grid connected PV system | |
| Lu et al. | A distributed feedforward control method for power electronic transformers | |
| CN112436508B (en) | Solid-state transformer capable of continuously running under fault working condition and regulation and control method thereof | |
| Feng et al. | Combined DC voltage control scheme for three-port energy router based on instantaneous energy balance | |
| He et al. | Advanced traction power supply system based on modular multilevel converters | |
| CN112117784B (en) | Operation control method of virtual transformer | |
| Tong et al. | Flexible substation and its control for AC and DC hybrid power distribution | |
| CN111952995B (en) | Direct-current capacitor voltage balance control method under single-phase fault of power grid | |
| CN114069633A (en) | Hybrid intelligent distribution transformer containing high-frequency isolated back-to-back converter | |
| Lei et al. | Exploration of a Modular Multilevel Converter for Direct AC-AC Conversion | |
| CN106099937A (en) | A kind of Research on Unified Power Quality Conditioner and control method thereof | |
| Lai et al. | Transient analysis on transfer process of hybrid soft open point under power grid outage | |
| Wang et al. | An improved current feedforward control strategy for multi-port power electronic transformer | |
| Li et al. | Study on the control strategy of VSG-based hydrogen production unit grid-connected for weak grid | |
| Gao et al. | Design of backstepping controller for T-type network inverter | |
| Zhang et al. | Multi-loop Feedback Analysis of Distributed Power Inverters in Microgrids | |
| Xie et al. | Study on control strategy of power electronic transformer based on modular multilevel converter | |
| Helali et al. | Voltage Balance Control of Five-Level Cascaded H-Bridge Rectifier-Based Smart Transformer | |
| Ma et al. | The DC bus voltage control based on virtual inertia for SST | |
| Qilong | A Power Balance Control Method of Input Independent Output Paralleled DCT for Offshore Wind Energy Systems | |
| Xu et al. | Impedance Model and Stability Analysis of Three-phase Grid-connected Inverter in Vehicle-to-grid |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PB01 | Publication | ||
| PB01 | Publication | ||
| SE01 | Entry into force of request for substantive examination | ||
| SE01 | Entry into force of request for substantive examination | ||
| GR01 | Patent grant | ||
| GR01 | Patent grant |