CN108964488B - Hierarchical control method and system for cascaded rectifier - Google Patents

Hierarchical control method and system for cascaded rectifier Download PDF

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CN108964488B
CN108964488B CN201810923110.3A CN201810923110A CN108964488B CN 108964488 B CN108964488 B CN 108964488B CN 201810923110 A CN201810923110 A CN 201810923110A CN 108964488 B CN108964488 B CN 108964488B
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voltage
input
rectifier
reference value
grid
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CN108964488A (en
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孙尧
侯小超
韩华
粟梅
王辉
刘永露
熊文静
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Central South University
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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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/36—Means for starting or stopping converters
    • 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
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc power output without 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
    • H02M7/217—Conversion of ac power input into dc power output without 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
    • H02M7/219—Conversion of ac power input into dc power output without 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 in a bridge configuration
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/32—Means for protecting converters other than automatic disconnection
    • 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
    • H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
    • H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
    • H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H02M7/21—Conversion of ac power input into dc power output without 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
    • H02M7/217—Conversion of ac power input into dc power output without 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
    • H02M7/25—Conversion of ac power input into dc power output without 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 arranged for operation in series, e.g. for multiplication of voltage
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/007—Plural converter units in cascade
    • 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
    • H02M1/00—Details of apparatus for conversion
    • H02M1/0067—Converter structures employing plural converter units, other than for parallel operation of the units on a single load
    • H02M1/0074—Plural converter units whose inputs are connected in series

Abstract

The invention discloses a hierarchical control method for a cascade rectifier, which comprises the following steps: generating an initial set of parameters in an upper level controller to start the cascaded rectifier system; transmitting, by the upper controller, the initial set of parameters to respective lower controllers for controlling respective sub-modules in a cascaded rectifier system via a communication link; in the respective underlying controllers: generating PWM initial modulation signals based on the received initial parameter group and sending the PWM initial modulation signals to each submodule as a control instruction in real time so as to start the cascade rectifier system to work, and collecting the voltage and the current input at the front end and output at the rear end after each submodule is started; and further carrying out control calculation on the basis of the values of the voltage and the current detected in real time to obtain a PWM real-time modulation signal, and controlling the voltage input of the correspondingly connected sub-modules by adopting the PWM real-time modulation signal.

Description

Hierarchical control method and system for cascaded rectifier
Technical Field
The invention relates to the field of control of power electronic systems, is applied to the industries of energy Internet, high-voltage distribution network and electrified railway traction, and particularly relates to a hierarchical control method and system for a cascade rectifier.
Background
The cascade rectifier is used as a front-end rectification link of a power electronic transformer and an energy router, is widely applied to traction occasions of electrified railways and the energy internet industry, is a main device for medium-high voltage energy conversion in the prior art, and has the advantages of modularization, easiness in expansion, simple structure and the like compared with the traditional clamping type multi-level converter.
In medium and high voltage cascaded rectifiers, coordinated control between sub-modules is crucial. In the existing research, two control methods, namely unified modulation and model prediction, mainly exist, and all control targets are completed based on a centralized framework. Under the condition, a centralized controller is needed to collect output voltage and current signals of all modules, capacitance and voltage signals of a direct current side and voltage signals of a large power grid of a grid-connected side, process and give reference signals, so that capacitance and voltage balance of all modules, active power balance among modules and power grid voltage frequency synchronization are realized. Since the transmission signal is an ac periodic signal, high bandwidth communication is required to accomplish this.
In addition, the amount of the acquired information is too large, and particularly under the condition that the number of some extra-high voltage cascade modules is very large, the centralized controller is required to have very strong processing capacity, so that the application scale of the existing cascade rectifier is limited. Meanwhile, since one centralized controller manages all the sub-modules, when a single sub-module has communication packet loss or delay, the whole cascaded rectifier is prone to abnormal operation. The reliability of the whole system is greatly influenced due to the existence of the single-point fault problem.
In order to overcome the problem of the centralized control framework, reduce the requirement of the cascaded rectifier on the communication bandwidth, reduce the excessive requirement on the processing capability of the centralized processor, improve the reliability of the overall system caused by the communication failure, and the like, a novel control framework needs to be researched to improve the flexibility and reliability of the cascaded rectifier, so as to promote the application scale of the cascaded rectifier and further reduce the application cost of the cascaded rectifier.
Disclosure of Invention
In view of the above-mentioned technical problem, the present invention provides a hierarchical control method for a cascaded rectifier system, the control method comprising the steps of:
generating an initial set of parameters in an upper level controller to start the cascaded rectifier system;
transmitting, by the upper controller, the initial parameters to respective lower controllers for controlling respective sub-modules in the cascaded rectifier system via a communication link, wherein the respective lower controllers are correspondingly connected to the respective sub-modules;
in the respective underlying controllers: generating PWM initial modulation signals based on the received initial parameter group and sending the PWM initial modulation signals to each submodule as a control instruction in real time so as to start the cascade rectifier system to work, and collecting the voltage and the current of the front-end input and the rear-end output of each submodule after the submodule is started; further carrying out control calculation on the basis of the voltage and current values detected in real time to obtain a PWM real-time modulation signal, and controlling the voltage input of the correspondingly connected sub-modules by adopting the PWM real-time modulation signal; when the voltage and the current in the sub-module are detected to be abnormal, the connection between the sub-module and the system is automatically cut off, and a fault report message is sent to the upper controller;
the upper level controller receives the fault report message, regenerates the initial set of parameters and transmits the initial set of parameters to the respective lower level controllers via the communication link to redistribute the voltage input from the grid to the remaining respective rectifier sub-modules in the cascaded rectifier system.
According to an embodiment of the hierarchical control method for a cascaded rectifier according to the present invention, it is preferable that the initial parameter set includes an initial start-up voltage value, an initial start-up phase angle value, and a rated active power reference value.
According to an embodiment of the present invention, in generating the initial parameter set for starting the cascaded rectifier system in the upper controller, the method further comprises the following sub-steps:
generating the initial starting voltage value and initial starting phase angle value based on the acquired voltage amplitude value and phase angle value on the power grid;
a nominal inactive power reference value is derived based on the load demand distribution.
According to an embodiment of the present invention, in the step of generating the PWM real-time modulation signal, the following sub-steps are further included:
calculating the current active power of the submodule according to the voltage and the current input and collected from the front end of the submodule to determine a phase angle reference value of the output voltage;
determining an amplitude reference value of the input voltage according to the grid-connected power factor value;
synthesizing the phase angle reference value and the amplitude reference value to obtain a voltage reference value for controlling the submodule to input from a power grid;
and obtaining a PWM real-time modulation signal to be sent out by the bottom layer controller based on the voltage reference value.
According to the hierarchical control method for the cascaded rectifier of the present invention, it is preferable that, in the step of determining the amplitude reference value of the input voltage according to the grid-connected power factor value,
obtaining an input voltage amplitude reference value of the sub-module to be controlled according to the following formula:
Figure GDA0002581641330000031
in the starting state of the device, the device is started,
Figure GDA0002581641330000032
in the running state of the device, the device is in a non-running state,
Figure GDA0002581641330000033
wherein, ViExpressed as the input voltage amplitude reference, V, of the i-th rectifier module0Is the initial starting voltage value provided by the upper layer controller, and is set to different values V respectively during starting and running0The value V can be set according to the adjustable grid-connected power factor in the running stategIs the real-time grid voltage amplitude, VgIs the grid voltage amplitude in the nominal state, NfewRepresenting the number of modules participating in compensating the voltage fluctuation of the power grid, generally taking NfewAnd the angle is approximately equal to 10% N-20% N, N represents the cascade number of the rectifier modules, and delta is the phase angle difference value between the steady-state lower-cascade rectifier and the power grid.
According to the hierarchical control method for the cascade rectifier, in the step of calculating the current active power of the submodule according to the voltage and the current collected from the front-end input of the submodule to determine the phase angle reference value of the input voltage,
obtaining an output phase angle reference value and an input frequency of the sub-module to be controlled according to:
ωi＝ω*+k·(Pi-Pi *)
δi＝∫ωidt
wherein, ω isiDenoted as the input voltage angular frequency reference of the i-th rectifier module, ω x denotes the nominal angular frequency of the grid, k is a positive control gain, typically PiIs an input power reference determined by the load power, P is the input power reference to ensure the voltage balance of the DC capacitor at the load sideiIs designed as
Figure GDA0002581641330000034
Wherein, VdciIs the load-side capacitor voltage, V, of the i-th rectifier moduledcIs the nominal reference value, P, of the capacitor voltage0Is a nominal active power reference, kpIs the proportional control coefficient, kIIs the integral control coefficient and s is the laplacian operator.
In an embodiment of the present invention, it is preferable that virtual inductors or actual inductors are used to reshape connection impedances between each sub-module of the cascaded rectifier and the power grid into inductive characteristics, where input power characteristics of each rectifier module under resistive grid-connected impedance are represented as:
Figure GDA0002581641330000041
Figure GDA0002581641330000042
wherein, PiAnd QiRepresents the input active and reactive power of the ith rectifier module, | ZlineI is the module value of the grid-connected impedance, VgAnd deltagIs the voltage amplitude and phase angle of the grid.
According to another aspect of the present invention, there is also provided a system for controlling a cascaded rectifier, comprising:
the upper layer controller is used for generating an initial parameter group for starting the cascade rectifier system and transmitting the initial parameter group in a communication mode;
a plurality of bottom controllers communicatively coupled to the top controller and each hardwired to a respective sub-module of the cascaded rectifier, for:
generating PWM initial modulation signals based on the received initial parameter group and sending the PWM initial modulation signals to each submodule as a control instruction in real time so as to start the cascade rectifier system to work, and collecting the voltage and the current of the front-end input and the rear-end output of each submodule after the submodule is started; further carrying out control calculation on the basis of the voltage and current values detected in real time to obtain a PWM real-time modulation signal, and controlling the voltage input of the correspondingly connected sub-modules by adopting the PWM real-time modulation signal; when the voltage and the current in the sub-module are detected to be abnormal, the connection between the sub-module and the system is automatically cut off, and a fault report message is sent to the upper controller;
wherein the upper level controller further comprises a fault handling unit configured to receive the fault report message, regenerate an initial set of parameters, and transmit the regenerated initial set of parameters to the respective lower level controller via the communication link to redistribute the voltage inputs to the remaining respective rectifier sub-modules in the cascaded rectifier system.
According to the system for controlling the cascaded rectifier of the present invention, it is preferable that the bottom controller includes:
the active frequency control unit is used for calculating the current inactive power of the submodule according to the voltage and the current input and collected from the front end of the submodule so as to determine a phase angle reference value of the output voltage;
the reactive voltage control unit is used for determining an amplitude reference value of the input voltage according to the grid-connected power factor value;
a synthesis unit for synthesizing the phase angle reference value and the amplitude reference value to obtain a voltage reference value for controlling the output of the sub-module;
a PWM modulation signal output unit used for obtaining a PWM real-time modulation signal to be sent out by the bottom layer controller based on the voltage reference value.
In order to solve the problem of a centralized control framework of the cascade rectifier, the invention designs a hierarchical control framework based on a multi-time scale concept, an upper auxiliary control layer is responsible for the services of whole system starting, rated power giving, fault management and the like, and a single sub-module unit controlled at the bottom layer can realize natural balance of capacitance and voltage and self-synchronization of power grid frequency. The advantages of the invention are summarized as follows:
1) the designed hierarchical control can decouple and separate the control from the time scale, the upper layer is responsible for auxiliary service of the slow scale, the lower layer realizes the control of a single sub-module of the fast scale, the control level is clear, and the design is convenient;
2) the proposed bottom layer control method can autonomously realize capacitor voltage equalization and frequency self-synchronization without the need of scheduling by an integrated controller;
3) the physical cascade structure and the bottom layer control of the cascade rectifier adopt modular design, thereby being convenient for flexible expansion;
4) the layered control method greatly reduces the communication traffic between the upper control and the bottom control, improves the system reliability and reduces the system communication cost;
5) the hierarchical control method can promote the large-scale application of the cascade rectifier in the extra-high voltage/extra-high voltage power system.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention. In the drawings:
FIG. 1 shows a block diagram of a system architecture of a cascaded rectifier according to one embodiment of the present invention; and
FIG. 2 is a block diagram of the internal structure of a submodule in a cascaded rectifier according to an embodiment of the present invention;
fig. 3 shows waveforms of grid-connected current, input ac voltage, load-side dc capacitor voltage, operating frequency and transmission power angle of the module 1 in steady state for four sub-modules;
fig. 4 shows a graph of the input active and reactive power of four sub-modules.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in further detail below with reference to the accompanying drawings.
Referring to fig. 1, a block diagram of a hierarchical control architecture of a cascaded rectifier system in accordance with one embodiment of the present invention is shown.
The upper layer controller is respectively connected with the plurality of bottom layer controllers through communication links. Each underlying controller is locally hardwired to a respective sub-module in the cascaded rectifier system. The voltage input of each submodule is controlled by a correspondingly connected underlying controller. A start-up command is then sent by the upper level controller to cause the entire cascaded rectifier system to be incorporated into the grid of the power system.
As described above, the control system of the present invention is divided into two layers from the time scale of the control response: one is a control layer with a slow time scale; the other layer is a control layer with a fast time scale. There is low bandwidth information communication between the upper level controller of the slow time scale and the lower level controller of the fast time scale. The content of the low-bandwidth information communication is mainly that the upper-layer controller transmits a system starting instruction to the lower-layer controller, and the lower-layer controller returns a fault report message when detecting that the rectifier submodule connected correspondingly fails. That is to say, the upper layer controller is responsible for services such as whole system starting, power compensation giving and fault management, and the bottom layer controller controls respective modules independently respectively, and realizes the capacitance-voltage natural balance, the power grid frequency self-synchronization and the given active power output of a single submodule.
Specifically, in the upper level controller, an initial set of parameters to activate the cascaded rectifier system is generated by calculation and load demand allocation. The initial set of parameters includes: an initial start-up phase angle value and an initial start-up voltage value and the active power required for powering the load. The upper layer controller generates an initial starting phase angle value delta for starting the cascade rectifier system according to the amplitude value and the phase angle information of the power grid acquired by the phase-locked loop0And an initial starting voltage value V0And the non-impact grid connection of the cascade rectifier system is realized. The reference value Q of the active power to be supplied to the load is obtained by the load demand distribution.
Then, the initial parameter set is transmitted to each bottom layer controller through a communication link between the upper layer controller and each bottom layer controller.
And the plurality of bottom controllers generate PWM initial modulation signals based on the received initial parameter group and send the PWM initial modulation signals as control instructions to each correspondingly connected submodule in real time so as to start the cascade rectifier system to work. Meanwhile, the bottom layer controller collects the voltage and current output by each submodule after being started. And further carrying out control calculation on the basis of the values of the voltage and the current detected in real time to obtain a PWM real-time modulation signal, and controlling the voltage input of the correspondingly connected sub-modules by adopting the PWM real-time modulation signal.
In order to ensure the synchronization of control voltage input, when generating a PWM real-time modulation signal, the invention is completely different from the prior art in that a bottom layer controller directly acquires the voltage on a rear-end direct-current capacitor of a rectifier submodule to calculate the control quantity.
When the bottom controller detects that the voltage and the current in the sub-modules are abnormal, the connection between the sub-modules and the system is automatically cut off, the sub-modules with faults are short-circuited through the bypass switch, and then fault report messages are sent to the upper controller through the communication link.
A fault handling unit in the upper level controller receives the fault report message, regenerates the initial set of parameters and transmits to each lower level controller over the communication link to redistribute the voltage inputs assumed by each remaining rectifier sub-module in the cascaded rectifier system.
Redistributed initial voltage value V0Can be calculated as follows:
Figure GDA0002581641330000071
wherein N represents the total number of rectifier modules, NnorRepresents the number of rectifier modules capable of working normally, and V is the input voltage amplitude of the rated working state of each module obtained according to steady state analysis.
In the event of a fault, the upper level controller need only recalculate the magnitude of the initial voltage to be redistributed without recalculating the initial phase angle and the value of the active power supplied to the load.
As shown in fig. 2, there is shown a block diagram of the internal structure of the ith underlying controller connected to the ith sub-module. In fig. 2, the bottom layer controller i further includes an active frequency control unit, a reactive voltage control unit, a synthesizing unit, and a PWM modulation signal output unit. The active frequency control unit is used for calculating the current active power of the submodule according to the voltage and the current output and collected from the rear end of the submodule so as to determine the phase angle reference value of the input voltage.
In fig. 2, the active power actual output Q of the submoduleiThe active power calculation unit calculates the active power. By means of the rated active power value Q transmitted by the upper controller and the actual input of active power QiAnd angular frequency nominal value omega*Calculating as input the output to be controlledReference value omega of input voltage angular frequencyiAnd further obtaining a phase angle reference value through transformation.
Specifically, the voltage angular frequency reference value ω is obtained according to the following formulaiAnd input frequency:
ωi＝ω*+k·(Pi-Pi *) (2)
δi＝∫ωidt
wherein, ω isiExpressed as the input voltage angular frequency reference of the i-th rectifier module, ω ×, expressed as the nominal angular frequency of the grid, k is a positive control gain, PiIs an input power reference determined by the load power, P is the input power reference to ensure the voltage balance of the DC capacitor at the load sideiIs designed as
Figure GDA0002581641330000072
Wherein, VdciIs the load-side capacitor voltage, V, of the i-th rectifier moduledcIs the nominal reference value, P, of the capacitor voltage0Is a nominal active power reference, kpIs the proportional control coefficient, kIIs the integral control coefficient and s is the laplacian operator.
On the other hand, as shown in fig. 2, the reactive voltage control unit determines the amplitude reference value V of the output voltage according to the grid-connected power factor value and the likei：
Figure GDA0002581641330000081
In the starting state of the device, the device is started,
Figure GDA0002581641330000082
in the running state of the device, the device is in a non-running state,
Figure GDA0002581641330000083
wherein, ViExpressed as the ith rectifierInput voltage amplitude reference, V, of a module0Is the initial starting voltage value provided by the upper layer controller, and is set to different values V respectively during starting and running0The value V can be set according to the adjustable grid-connected power factor in the running stategIs the real-time grid voltage amplitude, VgIs the grid voltage amplitude in the nominal state, NfewRepresenting the number of modules participating in compensating the voltage fluctuation of the power grid, generally taking NfewAnd the angle is approximately equal to 10% N-20% N, N represents the cascade number of the rectifier modules, and delta is the phase angle difference value between the steady-state lower-cascade rectifier and the power grid.
The amplitude reference value and the angular frequency reference value of the input voltage calculated above are then fed to a synthesis unit (not explicitly shown in fig. 2) which synthesizes the phase angle reference value with the amplitude reference value to obtain a voltage reference value to be input to control the corresponding sub-module, as shown in fig. 2.
Because the external control input characteristic of each submodule is not the traditional current source type any more, but the voltage source type is presented, the frequency synchronization between the submodule and the power grid can be realized independently, the real-time collection of the power grid frequency information is not needed, and the communication traffic of the controller is greatly reduced. In addition, the voltage input of the single rectifier submodule can be controlled according to the capacitance voltage of the single submodule, and the balance between the active loss and the absorbed active power of the submodule is reasonably maintained, so that the self-balance of the local direct current capacitance voltage can be achieved.
Further, in order to perform PWM control on the sub-module, the obtained voltage reference value is sent to the PWM modulation signal output unit. And obtaining a PWM real-time modulation signal to be sent by the bottom layer controller by the PWM modulation signal output unit based on the voltage reference value.
The method comprises the following steps of reshaping the connection impedance between each submodule of the cascaded rectifier and the power grid into a resistance-inductance characteristic by adopting a virtual inductor or adding an actual inductor, wherein the input power characteristic of each rectifier module under the resistance grid-connected impedance is represented as follows:
Figure GDA0002581641330000084
Figure GDA0002581641330000091
wherein, PiAnd QiRepresents the input active and reactive power of the ith rectifier module, | ZlineI is the module value of the grid-connected impedance, VgAnd deltagIs the voltage amplitude and phase angle of the grid.
Under the rated power grid state, because the load power required by each module in a steady state is the same, each module has the same input amplitude and phase angle, and the input active power and reactive power of the ith rectifier module in the steady state are simplified to
Figure GDA0002581641330000092
Figure GDA0002581641330000093
According to the above formula, it can be obtained,
Figure GDA0002581641330000094
expression combined with grid-connected power factor
Figure GDA0002581641330000095
When the grid-connected power factor is set to be a fixed value, the operating voltage V under the steady state can be obtained according to the formula0
In order to verify the feasibility of the control scheme proposed by the invention, a low-voltage system consisting of four cascaded modules is also realized here based on the real-time HIL test of the OPAL-RT platform. The results of the HIL test are shown in fig. 3-4.
Wherein fig. 3 shows the grid current i from top to bottomgFour modules of input AC powerVoltage V1-V4, capacitor voltage V on DC sidedc1At operating frequency f1And the waveform of the phase angle. It can be seen that the four cascaded modules have the same input voltage amplitude, phase angle and frequency, thereby achieving voltage equalization between the modules. Meanwhile, the direct current side capacitor voltage is maintained at the reference value of 200v, and the operation frequency of the module 1 is 50Hz synchronous with the power grid, so that the frequency self-synchronization with the power grid and the capacitor voltage self-equalization are realized.
In addition, as shown in fig. 4, in a rated grid voltage state, four modules absorb active power of 4kW, and maintain the consumption of the back-end load, so that the balance between the absorbed power and the load loss is realized; in addition, the four modules absorb reactive power to compensate for 0.54kVar under a steady state, and the grid-connected power factor of the cascade rectifier system can reach 0.992 by calculating.
It is to be understood that the disclosed embodiments of the invention are not limited to the particular structures, process steps, or materials disclosed herein but are extended to equivalents thereof as would be understood by those ordinarily skilled in the relevant arts. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.
Reference in the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase "one embodiment" or "an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment.
Although the embodiments of the present invention have been described above, the above description is only for the convenience of understanding the present invention, and is not intended to limit the present invention. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (5)

1. A hierarchical control method for a cascaded rectifier, the control method comprising the steps of:
generating, in an upper controller, an initial set of parameters to start the cascaded rectifier system, the initial set of parameters including an initial start voltage value, an initial start phase angle value, and a nominal active power reference value, wherein the initial start voltage value and the initial start phase angle value are generated based on the obtained voltage magnitude and phase angle value on the grid; obtaining a rated active power reference value based on load demand distribution;
transmitting, by the upper controller, the initial parameters to respective lower controllers for controlling respective sub-modules in the cascaded rectifier system via a communication link, wherein the respective lower controllers are correspondingly connected to the respective sub-modules;
in the respective underlying controllers: generating PWM initial modulation signals based on the received initial parameter group and sending the PWM initial modulation signals to each submodule as a control instruction in real time so as to start the cascade rectifier system to work, and collecting the voltage and the current of the front-end input and the rear-end output of each submodule after the submodule is started; further carrying out control calculation on the basis of the voltage and current values detected in real time to obtain a PWM real-time modulation signal, and controlling the voltage input of the correspondingly connected sub-modules by adopting the PWM real-time modulation signal; when the voltage and the current in the sub-module are detected to be abnormal, the connection between the sub-module and the system is automatically cut off, and a fault report message is sent to the upper controller;
the upper layer controller receives the fault report message, regenerates the initial parameter set and transmits the initial parameter set to each lower layer controller through a communication link so as to redistribute the voltage input borne by each rectifier submodule remained in the cascade rectifier system, wherein in the step of generating the PWM real-time modulation signal, the method further comprises the following substeps:
calculating the current active power of the submodule according to the voltage and the current input and collected from the front end of the submodule to determine a phase angle reference value of the input voltage;
determining an input voltage amplitude reference value according to a grid-connected power factor index;
synthesizing the phase angle reference value and the input voltage amplitude reference value to obtain a voltage reference value for controlling the submodule to input from a power grid;
obtaining a PWM real-time modulation signal to be sent out to the rectifier sub-module by the bottom layer controller based on the voltage reference value,
in the step of determining the input voltage amplitude reference value according to the grid-connected power factor index,
obtaining an input voltage amplitude reference value of the sub-module to be controlled according to the following formula:
Figure FDA0002774935160000021
in the starting state of the device, the device is started,
Figure FDA0002774935160000022
in the running state of the device, the device is in a non-running state,
Figure FDA0002774935160000023
wherein, ViExpressed as the input voltage amplitude reference, V, of the i-th rectifier module0Is the initial starting voltage value provided by the upper layer controller, and is set to different values V respectively during starting and running0Setting a value, V, according to an adjustable grid-connected power factor in an operating stategIs the real-time grid voltage amplitude, VgIs the grid voltage amplitude in the nominal state, NfewRepresenting the number of modules participating in compensating the voltage fluctuation of the power grid, and taking Nfew10% N to 20% N, N representing the number of cascades of the rectifier modules,
Figure FDA0002774935160000025
is the phase angle difference between the cascaded rectifier and the grid in a steady state.
2. The hierarchical control method for a cascaded rectifier of claim 1,
in the step of calculating the current active power of the submodule according to the voltage and current collected from the front-end input of the submodule to determine the phase angle reference value of the input voltage,
obtaining a phase angle reference value and an input frequency of the sub-module to be controlled according to the following formula:
ωi＝ω*+k·(Pi-Pi *)
δi＝∫ωidt
wherein, ω isiExpressed as the input voltage angular frequency reference, delta, of the i-th rectifier moduleiExpressed as the phase angle reference value of the i-th rectifier module, ω x expressed as the nominal angular frequency of the grid, k is a positive control gain, PiIs an input power reference, PiIs an input power reference determined by the load power, P is the input power reference to ensure the voltage balance of the DC capacitor at the load sideiIs designed as
Figure FDA0002774935160000024
Wherein, VdciIs the load-side capacitor voltage, V, of the i-th rectifier moduledcIs the nominal reference value, P, of the capacitor voltage0Is a nominal active power reference, kpIs the proportional control coefficient, kIIs the integral control coefficient and s is the laplacian operator.
3. The hierarchical control method for a cascaded rectifier of claim 2,
and remolding the connection impedance between each submodule of the cascaded rectifier and the power grid into inductive characteristics by adopting a virtual inductor or adding an actual inductor, wherein the input power characteristic of each rectifier module is represented as follows:
Figure FDA0002774935160000031
Figure FDA0002774935160000032
wherein, PiAnd QiRespectively representing the input active power and reactive power, | Z, of the ith rectifier modulelineI is the module value of the grid-connected impedance, VgAnd deltagRespectively the voltage amplitude and phase angle, V, of the networkjAnd deltajRespectively representing the input voltage magnitude and phase angle of the jth rectifier module.
4. A system for hierarchically controlling a cascaded rectifier, comprising:
an upper level controller to generate an initial set of parameters to start the cascaded rectifier system and to communicate the initial set of parameters, the initial set of parameters including an initial start voltage value, an initial start phase angle value, and a rated active power reference value, wherein the upper level controller generates the initial start voltage value and the initial start phase angle value based on the obtained voltage magnitude and phase angle values on the grid and derives the rated active power reference value based on a load demand allocation;
a plurality of bottom controllers communicatively coupled to the top controller and each hardwired to a respective sub-module of the cascaded rectifier, for:
generating PWM initial modulation signals based on the received initial parameter group and sending the PWM initial modulation signals to each submodule as a control instruction in real time so as to start the cascade rectifier system to work, and collecting the voltage and the current of the front-end input and the rear-end output of each submodule after the submodule is started; further carrying out control calculation on the basis of the voltage and current values detected in real time to obtain a PWM real-time modulation signal, and controlling the voltage input of the correspondingly connected sub-modules by adopting the PWM real-time modulation signal; when the voltage and the current in the sub-module are detected to be abnormal, the connection between the sub-module and the system is automatically cut off, and a fault report message is sent to the upper controller; wherein, when generating the PWM real-time modulation signal: calculating the current active power of the submodule according to the voltage and the current input and collected from the front end of the submodule to determine a phase angle reference value of the input voltage; determining an input voltage amplitude reference value according to a grid-connected power factor index; synthesizing the phase angle reference value and the input voltage amplitude reference value to obtain a voltage reference value for controlling the submodule to input from a power grid; obtaining a PWM real-time modulation signal to be sent to the rectifier sub-module by the bottom layer controller based on the voltage reference value, and obtaining an input voltage amplitude reference value of the sub-module to be controlled according to the following formula when determining the input voltage amplitude reference value according to a grid-connected power factor index:
Figure FDA0002774935160000041
in the starting state of the device, the device is started,
Figure FDA0002774935160000042
in the running state of the device, the device is in a non-running state,
Figure FDA0002774935160000043
wherein, ViExpressed as the input voltage amplitude reference, V, of the i-th rectifier module0Is the initial starting voltage value provided by the upper layer controller, and is set to different values V respectively during starting and running0Setting a value, V, according to an adjustable grid-connected power factor in an operating stategIs the real-time grid voltage amplitude, VgIs the grid voltage amplitude in the nominal state, NfewRepresenting the number of modules participating in compensating the voltage fluctuation of the power grid, and taking Nfew10% N to 20% N, N representing the number of cascades of the rectifier modules,
Figure FDA0002774935160000044
and a fault processing unit, which is used for receiving the fault report message, regenerating an initial parameter group and transmitting the initial parameter group to each lower layer controller through a communication link so as to redistribute the voltage input born by each rectifier submodule remained in the cascade rectifier system.
5. The system for hierarchically controlling a cascaded rectifier of claim 4, wherein the underlying controller comprises:
the active frequency control unit is used for calculating the current active power of the submodule according to the voltage and the current input and collected from the front end of the submodule so as to determine a phase angle reference value of the input voltage;
the reactive voltage control unit is used for determining an amplitude reference value of the input voltage according to the grid-connected power factor value;
a synthesis unit for synthesizing the phase angle reference value and the amplitude reference value to obtain a voltage reference value for controlling the input of the sub-module;
a PWM modulation signal output unit used for obtaining a PWM real-time modulation signal to be sent out by the bottom layer controller based on the voltage reference value.
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