CN114079393A

CN114079393A - Conversion system and control method

Info

Publication number: CN114079393A
Application number: CN202110584137.6A
Authority: CN
Inventors: 刘红; 陶勇; 陆城
Original assignee: Delta Electronics Shanghai Co Ltd
Current assignee: Delta Electronics Shanghai Co Ltd
Priority date: 2020-08-14
Filing date: 2021-05-27
Publication date: 2022-02-22

Abstract

The application provides a conversion system and a control method, which comprises N power converters and N controllers, wherein the N controllers correspond to the N power converters one by one, each controller can receive a first end current and a second end voltage of the corresponding power converter, and can also receive a neighbor direct current voltage signal only reflecting the second end voltage of other M power converters in the conversion system, and carry out voltage control on the corresponding power converter according to the received signal. The application adopts full distributed control without setting an integrated controller, when part of controllers break down, the rest controllers can continue to work, and the reliability is higher. In addition, each controller only receives the voltage signals of the adjacent power converters, so that the number of connecting lines between the controllers can be greatly reduced, and the reliability of the conversion system is further improved. In addition, an integrated controller is not arranged, the modular design is realized, and the expansion of a conversion system is facilitated.

Description

Conversion system and control method

The present application claims priority from chinese patent application filed on 14/08/2020, having application number 202010816482.3 and entitled "conversion system and control method", which is incorporated herein by reference in its entirety.

Technical Field

The present application relates to the field of power electronics technologies, and in particular, to a conversion system and a control method.

Background

A power electronic converter is a device for converting electric energy by using a power electronic device, and a chain-link converter includes a plurality of power electronic converters (hereinafter, referred to as power converters) with input terminals connected in series, and is widely used in power devices due to its high degree of modularity and good expandability, for example: solid-state Transformer (EPT), Static Var Compensator (SVC), and medium-high voltage high-Power inverter.

The chain-type conversion system generally adopts centralized control or local centralized control, in which a central controller collects input end signals and output end signals of each power converter, performs calculation, outputs Pulse Width Modulation (PWM) signals to each power converter, and controls each power converter to work. The local centralized control is provided with a central controller and a plurality of local controllers, one local controller controls one power converter, the central controller collects input end signals and output end signals of all the power converters, the operation is carried out, intermediate control quantity is output, and each local controller generates PWM signals according to the intermediate control variable and the output end signals of the corresponding power converter.

However, whether centralized control or local centralized control is performed, central controller control is provided, and if the central controller fails, the chain-link converter cannot continue to operate, which results in low reliability.

Disclosure of Invention

The present application provides a transformation system and a control method, which aim to solve the above problems in the prior art.

In a first aspect, the present application provides a transformation system comprising:

n power converters, each power converter including a first terminal and a second terminal, the first terminals of the N power converters being electrically coupled in series, an

N controllers corresponding to the N power converters one by one, each controller receiving the first terminal current and the second terminal voltage of the corresponding power converter,

each controller of at least (N-1) controllers receives a neighbor direct-current voltage signal only reflecting the voltage of the second end of other M power converters in the conversion system, and generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighbor direct-current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter, wherein N is more than or equal to 3, M is more than or equal to 1 and less than or equal to (N-2), and M and N are integers.

In a second aspect, the present application provides a control method for controlling a transform system, the transform system comprising:

The N controllers correspond to the N power converters one by one, and each controller receives the current of the first end and the voltage of the second end of the corresponding power converter;

the control method comprises the following steps:

s1, each controller of at least (N-1) controllers receives a neighbor direct current voltage signal which only reflects the voltages of the second ends of the other M power converters in the conversion system; and

s2, each controller in at least (N-1) controllers generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighbor direct current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end,

wherein N is more than or equal to 3, M is more than or equal to 1 and less than or equal to (N-2), and M and N are integers.

The application provides a conversion system and a control method, which comprises N power converters and N controllers, wherein the N controllers correspond to the N power converters one by one, each controller can receive a first end current and a second end voltage of the corresponding power converter, and can also receive a neighbor direct current voltage signal only reflecting the second end voltage of other M power converters in the conversion system, and carry out voltage control on the corresponding power converter according to the received signal. Compared with the prior art, the method and the device have the advantages that the full-distributed control is adopted, the corresponding controller is arranged for each power converter, the centralized controller is not needed, when part of the controllers break down, the rest controllers can continue to work, and the reliability is higher. In addition, each controller only receives the voltage signals of the adjacent power converters, so that the number of connecting lines between the controllers can be greatly reduced, and the reliability of the conversion system is further improved. In addition, an integrated controller is not arranged, the modular design is realized, and the expansion of a conversion system is facilitated.

Drawings

Fig. 1 is a schematic diagram of a chain-link converter employing centralized control in the prior art;

FIG. 2 is a schematic diagram of a prior art chain-link converter with localized centralized control;

FIG. 3A is a schematic diagram of a main circuit structure of the transform system provided in the present application;

FIG. 3B is a schematic diagram of another main circuit structure of the transformation system provided in the present application;

fig. 4 is a schematic structural diagram of a transformation system according to an embodiment of the present application;

FIG. 5 is a schematic structural diagram of a transformation system with a unidirectional ring network information transmission structure according to a second embodiment of the present application;

fig. 6 is a schematic structural diagram of a transformation system with a bidirectional ring type information transmission structure according to a second embodiment of the present application;

fig. 7 is a schematic structural diagram of a transformation system having a chain spanning tree information transmission structure according to a third embodiment of the present application;

fig. 8 is a schematic structural diagram of a transformation system having a broadcast spanning tree information transmission structure according to a third embodiment of the present application;

fig. 9 is a schematic control diagram of an instruction generating unit according to a fourth embodiment of the present disclosure;

fig. 10 is a schematic control diagram of another instruction generating unit according to a fourth embodiment of the present disclosure;

fig. 11 is a schematic diagram of a partial control principle of another instruction generating unit according to a fifth embodiment of the present application;

fig. 12 is a schematic diagram illustrating a control principle of a differential voltage ring according to a fifth embodiment of the present application;

fig. 13 is a schematic diagram illustrating a control principle of another differential voltage ring according to a fifth embodiment of the present application;

fig. 14 is a schematic diagram illustrating a control principle of another differential voltage ring according to a fifth embodiment of the present application;

fig. 15 is a schematic flowchart of a control method according to a sixth embodiment of the present application;

FIG. 16 is a schematic diagram of leg voltages, voltages of DC link capacitors, and first terminal currents of respective power converters in a conversion system provided herein;

fig. 17 is a schematic diagram of differential mode components of bridge arm voltages and differential mode components of voltages of dc link capacitors of respective power converters in the conversion system provided by the present application;

FIG. 18 is a schematic view showing a pressure equalizing effect of the comparative example;

FIG. 19 is a schematic diagram illustrating a pressure equalizing effect according to an embodiment of the present application;

FIG. 20 is a schematic diagram of PWM carrier waves of each controller in the embodiment of the present application;

fig. 21 is a schematic diagram of a partial control principle of another instruction generating unit according to a fifth embodiment of the present application.

Detailed Description

To make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the present application will be clearly and completely described below with reference to the drawings in the present application, and it is obvious that the described embodiments are some, but not all embodiments of the present application. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

A power electronic converter is a device for converting electric energy by using a power electronic device, and a chain-type conversion system includes a plurality of power electronic converters (hereinafter, referred to as power converters) having input terminals connected in series. The chain-type conversion system has high modularity and good expandability, and therefore, the chain-type conversion system is widely applied to power devices, such as: solid-state Transformer (EPT), Static Var Compensator (SVC), and medium-high voltage high-Power inverter.

As shown in fig. 1 and 2, in the prior art, the chain-type conversion system generally adopts centralized control or local centralized control, and the centralized control collects the input end signal (input voltage v) of the chain-type conversion system by a central controller_gAnd an input current i_g) And the output signals (v) of the individual power converters_dc1,v_dc2,…,v_dcn) The operation is performed, and a Pulse Width Modulation (short for: PWM) for modulating the input voltage (v) of each power converter_b1,v_b2,…,v_bn). The local centralized control is provided with a central controller and a plurality of local controllers, one local controller controls one power converter, the central controller collects input end signals and output end signals of all the power converters, the operation is carried out, and intermediate control quantity is output, each unit controller generates PWM signals according to the intermediate control variable and the output end signals of the corresponding power converters, and the PWM signals are used for controlling all the power converters to work.

In order to solve the above problems in the prior art, the present application provides a transformation system based on complete distribution and a control method thereof. The conversion system does not need to use an integrated controller, only a corresponding controller is configured for each power converter, and each controller only needs to receive the output end direct-current voltage of other parts of power converters. Therefore, the number of connecting wires among the power converters is reduced, and the reliability of the chain type conversion system can be improved.

Fig. 4 is a schematic structural diagram of a transformation system according to an embodiment of the present application. The conversion system in fig. 4 includes N power converters and N controllers.

Each power converter comprises a first end and a second end, and the first ends of the N power converters are electrically coupled in series to form a main circuit of the conversion system. N is not less than 3 and is a positive integer.

One of the main circuit structures of the chain-link converter system is shown in fig. 3A, and the chain-link converter system includes a plurality of power converters with their input terminals connected in series, and the output and load of each power converter are shown as R₁To R_NShown) connected. Each power converter comprises an H bridge and a capacitor, the capacitor is connected in parallel to the direct current end of the H bridge, the alternating current side of the H bridge is used as the alternating current side of the power converter, and the direct current side of the H bridge is used as the direct current side of the power converter. A port formed by the middle points of the two bridge arms in the H bridge is a first end of the power converter, and a port formed by the parallel connection of the bridge arms and the capacitor is a second end of the power converter. When the chain-link converter having this structure is used for connection with an alternating-current power supply, the first terminal of the power converter serves as an input terminal, and the second terminal of the power converter serves as an output terminal. When the chain-link converter is used for connecting with a direct current power supply, the first end of the power converter is used as an output end, and the second end of the power converter is used as an input end.

Another main circuit structure of the chain-type conversion system is shown in fig. 3B, each power converter includes a single bridge arm and a capacitor, the capacitor is connected in parallel with the bridge arm, a midpoint of the bridge arm and one end of the bridge arm form a first end of the power converter, and two ends of the bridge arm form a second end of the power converter. The chain-link conversion system of this structure can be used only for a direct-current power supply. When the first terminal of the power converter is used as the input terminal, the second terminal of the power converter is used as the output terminal. When the second terminal of the power converter is used as the input terminal, the first terminal of the power converter is used as the output terminal.

The N controllers correspond to the N power converters one by one, and each controller receives the first-end current and the second-end voltage of the corresponding power converter. Each of the at least (N-1) controllers receives a neighbor DC voltage signal reflecting only the voltages at the second terminals of the other M power converters in the conversion system, M is greater than or equal to 1 and less than or equal to (N-2), and M is a positive integer.

That is, there are two cases: in the first case, each controller is capable of receiving a neighbor dc voltage signal and receiving the first terminal current and the second terminal voltage of the corresponding power converter. That is, in the first case, each controller can only obtain the second terminal voltages (i.e., the neighboring dc voltage signals) of some of the remaining N-1 power converters, and cannot obtain the second terminal voltages of all of the remaining N-1 power converters.

Accordingly, the working principle of each controller is as follows: and generating a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighboring direct-current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter, thereby realizing the voltage control of each power converter.

The second case is that each of the (N-1) controllers receives the neighbor DC voltage signal and receives the first terminal current and the second terminal voltage of the corresponding power converter. The remaining one of the controllers receives only the first terminal current and the second terminal voltage of the corresponding power converter.

In the second case, of the (N-1) controllers, each controller can obtain only the second terminal voltages (i.e., the neighbor DC voltage signals) of a part of the power converters in the remaining (N-1) power converters, and can not obtain the second terminal voltages of all the power converters in the remaining (N-1) power converters.

Accordingly, the operating principle of each of the (N-1) controllers is: and generating a control signal according to the neighbor direct current voltage signal, the first end current and the second end voltage of the corresponding power converter, wherein the control signal is used for controlling the first end voltage of the corresponding power converter. The remaining 1 st controller generates a control signal for controlling the voltage at the first terminal of the corresponding power converter according to the current at the first terminal and the voltage at the second terminal of the corresponding power converter.

In the conversion system provided by the embodiment of the application, full-distributed control is adopted, a corresponding controller is arranged for each power converter, an integrated controller is not required to be arranged, when part of controllers have faults, the rest controllers can continue to work, and the reliability is higher. In addition, each controller only receives the voltage signals of the adjacent power converters, and the voltage signals of all other power converters are not required to be obtained, so that the number of connecting lines between the controllers and the number of data between the controllers can be greatly reduced, and the reliability of the conversion system is further improved.

The following continues to describe the transformation system provided in the second embodiment of the present application, and the second embodiment describes the structure of the transformation system according to the first case.

In the second embodiment, the information transmission structure between the N controllers is a balanced graph, such as a unidirectional ring network or a bidirectional ring network. That is, the controllers communicate in a leaderless mode. Each controller is used as a node in the balanced graph, and the 1 st controller to the Nth controller respectively correspond to the nodes 1 to the nodes N. For each node, the number of information flowing into the node is equal to the number of information flowing out of the node.

In fig. 5, the information transmission structure between N controllers is a unidirectional ring network, and the N controllers are connected in sequence to form a ring. Signals between the controllers flow in one direction. Node 1 receives the second terminal voltage of the nth power converter from node N and also transmits the second terminal voltage of the 1 st power converter to node 2. The node 1 only contains the voltage signal of the nth power converter in the received neighboring direct current voltage signal, and the transmitted signal only contains the voltage signal of the 1 st power converter. Therefore, the number of the information flowing into the node 1 and the number of the information flowing out of the node 1 are both 1, so that the node 1 is a balanced node, and so on, and each node in the unidirectional ring type network is a balanced node.

In fig. 6, the information transmission structure between N controllers is a bidirectional ring network, and the N controllers are connected in sequence to form a ring. Signals between the controllers flow in both directions. Node 1 receives the second terminal voltage of the nth power converter transmitted by node N and the second terminal voltage of the 2 nd power converter transmitted by node 2, and transmits the second terminal voltage of the 1 st power converter to node 2 and node N. The number of the information flowing into the node 1 and the number of the information flowing out of the node 1 are both 2, the node 1 is a balance node, and so on, and each node in the bidirectional ring type network is a balance node.

It should be noted that the information transmission structure referred to herein does not necessarily coincide with the hardware communication wiring diagram, and in the following description, taking the case that the hardware wiring structure is a unidirectional ring network, the node N sends its own information to the node 1, meanwhile, the node N forwards the information sent to the node N-1 to the node 1, so that the number of the information received by the node 1 is 2, at this time, node 1 must also send its own information to two different nodes to form a balanced graph, for example, one to node 2 and the other to node 3 via node 2, i.e. when the hardwired fabric is a unidirectional ring network, the current node may receive the second terminal voltages of the power converters corresponding to all other nodes, or may only receive the second terminal voltages of the power converters corresponding to the adjacent nodes, and is not limited by the hardware wiring structure of the controller.

When the information transmission structure among the N controllers is a balanced graph, each controller can receive the neighbor direct-current voltage signal and receive the first terminal current and the second terminal voltage of the corresponding power converter. And each controller generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighboring direct current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter.

The working principle of the embodiment of the present application is explained as follows: the information transmission structure among the N controllers is a balanced diagram, and each controller can receive a neighbor direct-current voltage signal and receive the first terminal current and the second terminal voltage of the corresponding power converter. And generating a control signal according to the received information, and controlling the voltage of the first end of each power converter to realize the normal work of the conversion system under the completely distributed control.

In the conversion system provided by the embodiment of the application, the information transmission structure among the N controllers is a balanced graph, each controller can receive the neighboring direct-current voltage signal reflecting the voltage of other controllers, so that enough voltage information of the controllers can be received, and meanwhile, the connection relation among the controllers is simplified.

The following continues to describe the transformation system provided in the third embodiment of the present application, and the third embodiment describes the structure of the transformation system according to the second case described above.

The difference from the second embodiment is that the information transmission structure between the N controllers is a spanning tree. That is, the controllers communicate in a leader-follower mode. Each controller is used as a node in the spanning tree, and the first controller to the Nth controller respectively correspond to the nodes 1 to the nodes N. In the spanning tree, all the remaining nodes can be communicated from the root node, the root node of the spanning tree is set as a Leader (English expression Leader) node, and other nodes are set as Follower (English expression follow) nodes.

When the information transmission structure between the N controllers is a spanning tree, the information transmission structure between the N controllers may be a chain type or a broadcast type.

In fig. 7, the information transmission structure between N controllers satisfies a chain spanning tree, the N controllers are connected in sequence without forming a ring, and signals between the controllers flow in a single direction, that is, the signals flow from the root node to the downstream node. The first node to which no signal flows is taken as the root node. Since no signal is flowing in, the controller corresponding to the root node receives only the first terminal current and the second terminal voltage of the corresponding power converter. The controllers corresponding to the remaining nodes can receive the neighboring direct-current voltage signals and receive the first terminal current and the second terminal voltage of the corresponding power converter. The neighboring dc voltage signal is only reflective of the second terminal voltage of the power converter corresponding to the controller at the upstream node.

In fig. 8, the information transmission structure between N controllers is a broadcast spanning tree, the controller corresponding to the root node is connected to the remaining (N-1) controllers, the signal flow direction between the controller corresponding to the root node and the other controllers is unidirectional, and the controller corresponding to the root node only receives the first terminal current and the second terminal voltage of the corresponding power converter. The control corresponding to the remaining nodes can receive the neighboring direct current voltage signal and the corresponding first terminal current and second terminal voltage of the power converter. The neighboring direct current voltage signal can only reflect the second terminal voltage of the power converter corresponding to the root node.

Correspondingly, the controller corresponding to the root node generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the current of the first end and the voltage of the second end of the corresponding power converter. And the remaining nodes generate control signals for controlling the voltage of the first end of the corresponding power converter according to the neighboring direct current voltage signals, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter.

In the embodiment of the application, the information transmission structure among the N controllers is a spanning tree, and the controllers except the root node can receive the neighbor direct-current voltage signals and receive the first terminal current and the second terminal voltage of the corresponding power converter. And generating a control signal according to the received information to control the voltage of the first end of each power converter. And the root node generates control information according to the first end current and the second end voltage of the corresponding power converter, controls the first end voltage of the power converter corresponding to the root node and realizes the normal work of the conversion system under the completely distributed control.

In the transformation system provided by the embodiment of the application, the information transmission structure among the N controllers is a spanning tree, and each node receives a neighboring direct-current voltage signal sent by an upstream node, so that the data transmission quantity among the controllers can be reduced, and the connection relation among the controllers is simplified.

The following continues to describe a conversion system 200 provided in the fourth embodiment of the present application, which includes N power converters 201 and N controllers 202.

The stability problem and the voltage equalization problem of the transform system are analyzed separately below.

With continued reference to fig. 3A, each H-bridge is a power converter. In the figure, the first terminal of the power converter is taken as the input terminal, and the second terminal of the power converter is taken as the output terminal. i.e. i_gIs the grid current, i.e. the first terminal current, v, of the power converter_gIs the network voltage, L is the filter inductance, C_iIs the direct current link (dc-link) capacitance of the ith power converter_dciIs the DC link voltage of the ith power converter, i.e. the second terminal voltage of the ith power converter, v_biIs the bridge arm voltage of the ith power converter, i.e. the first terminal voltage of the ith power converter, i_dciIs the DC output current of the ith power converter, i.e. the second terminal current, P, of the ith power converter_iThe output power of the i-th power converter is the dc side output power, i.e. the second terminal power of the i-th power converter.

When the output power of each power converter is consistent, the capacitor voltage is consistent, and the bridge arm voltage is consistent, the input of each power converter equally shares the power grid voltage.

The following cell power balance equation (1) is known from the circuit:

wherein: e_iIs the energy of the dc link capacitance of the ith power converter,

the common mode and differential mode decomposition can be performed on equation (1) as follows, where the above N equations are summed and divided by N to obtain a common mode power balance equation

Wherein,

is the average energy of the direct current chain,

is the common-mode component of the bridge arm voltages,

is the output power common mode component.

Subtracting the common mode equations from the N equations in the formula (1) to obtain the following differential mode power balance equation (3):

wherein E is_difi＝E_i-E_comIs the direct current chain differential mode energy, v_bdifi＝v_bi-v_bcomIs the differential mode component of the first terminal voltage, P_difi＝P_i-P_comIs the output power differential mode component.

The common mode circuit equation including the current state of the grid side is shown in the following formula (4):

the conversion system can normally work on the premise that the above equation can reach a steady state, namely that the common mode power absorbed by the bridge arm is balanced with the output common mode power, and the differential mode power absorbed by the bridge arm is balanced with the output differential mode power.

According to the above formula (4), using v_bcomCan control the current of the power grid, thereby controlling the flowTotal or common mode power into all power converters. From the formula (3), it can be seen that v is used_bdifiThe differential mode power can be controlled. Because the energy of the direct current link capacitor is reflected by the direct current link voltage, the stable work of the circuit can be realized through the closed-loop feedback control of the direct current link voltage.

The cascaded H bridge adopts power converters to be connected in series to deal with medium-high voltage occasions, the voltage stress of each power converter device is determined by respective direct current link voltage, in order to prevent overvoltage of units, the N direct current link voltages need to be controlled to be consistent, and the problem of direct current link voltage equalizing is the main challenge of control of the cascaded H bridge. Using v_bdifiThe differential mode power can be controlled, and the direct current link voltage balance is realized by combining the closed loop feedback of the direct current link voltage.

Based on the above analysis of the power stability problem and the voltage balancing problem, the control principle of the controller is introduced in combination with the two cases in the first embodiment.

For the first case, the N controllers are labeled as (N-1) and 1 st controller, and the specific control principles of the (N-1) and 1 st controllers are described below, respectively.

Each of the (N-1) controllers includes a calculation unit, an instruction generation unit, and a modulation unit. The computing unit is used for computing to obtain a local common mode signal according to the neighbor direct current voltage signal and the voltage of the second end of the corresponding power converter. The instruction generating unit is used for generating a control instruction according to the local common-mode signal, the corresponding first end current and the second end voltage of the power converter, and the modulating unit is used for modulating the control instruction to generate a control signal.

Preferably, the modulation unit is configured to perform carrier phase shift modulation on the control instruction to generate a control signal, and when the control signal is generated by performing carrier phase shift modulation on the control instruction, load delay in each power converter can be reduced by using fully distributed control, so as to improve a control bandwidth.

The computing unit performs weighted average on the second terminal voltage of the corresponding power converter and the second terminal voltages of the M other power converters to obtain a local common mode signal. Since each controller can obtain the second terminal voltages of only a part of the power converters in the remaining (N-1) power converters, the second terminal voltages of all the power converters in the remaining (N-1) power converters cannot be obtained. Compared with the common mode signal calculated according to the second terminal voltages of the N power converters, the local common mode signal is calculated according to the second terminal voltages of (N-1) power converters at most. Because the local common-mode signal is adopted, the voltages of other all power converters are not required to be obtained, the central controller is not required to collect the voltages of the second ends of all power converters, and the common-mode signal is calculated according to the voltages of the second ends of all power converters, so that the converter provided by the application can adopt completely distributed control.

More specifically, the local common mode signal is calculated according to the following formula:

wherein v is_dccomiAnd Neb (i) represents the neighbor of the node i, that is, the node corresponding to the voltage analyzed from the neighbor direct current voltage signal received by the node i, that is, if the node i receives the information of the node j, the node j is called as the neighbor of the node i.

In the formula (5), ρ_iAs a weighting coefficient, p_i＞0，ρ_j＞0，

Preference is given to

N_iThe number of neighbors.

The process of obtaining the local common mode signal is described below by taking the transmission structure of the chain spanning tree and the transmission structure of the broadcast spanning tree as examples.

In the transmission structure of the chain spanning tree, node 1 is used as a root node, and nodes 2 to N are used as following nodes. The immediately upstream node (i-1) of each following node is a neighbor node, and for the 2 nd to nth controllers, the local common mode signal is calculated according to the following formula:

in the transmission structure of the broadcast spanning tree, node 1 is used as a root node, and nodes 2 to N are used as following nodes. The node 1 is used as a neighbor node from the node 2 to the node N, and for a following node, a local common-mode signal is calculated according to the following formula:

in the transmission structure of the chain spanning tree and the broadcast spanning tree, the 1 st controller includes an instruction generating unit and a modulating unit. The 1 st controller corresponds to the following node. The instruction generating unit in the 1 st controller is used for generating a control instruction according to the first end current and the second end voltage of the corresponding power converter. The modulation unit is used for modulating the control instruction to generate a control signal.

The specific control principle of the instruction generating unit in the (N-1) controllers and the 1 st controller, respectively, will be described below. When the transmission structure between N controllers is a spanning tree, the control schematic of the following node refers to fig. 9. The control schematic of the root node refers to fig. 10.

More specifically, as shown in fig. 9, the instruction generating unit in the (N-1) controllers includes a common-mode voltage loop, a current loop, and a differential-mode voltage loop. The common-mode voltage loop is used for generating a local common-mode signal v according to the local common-mode signal v_dccomiAnd a preset voltage reference value v_dcrefGenerating a given signal I_dref. The current loop is used for setting a current reference value I_qrefGiven a signal I_drefAnd corresponding first terminal current i of the power converter_gGenerating a common mode command v_bcomi. The differential voltage loop is used for generating a local common-mode signal v_dccomiSecond terminal voltage v of corresponding power converter_dciAnd a first terminal current i_gGenerating a differential mode instruction v_bdifi. The instruction synthesis unit is used for synthesizing a common-mode instruction v_bcomiSum-difference mode instruction v_bdifiPerforms an operation and generates a control command v_bi。

As shown in fig. 10, the instruction generating unit in the 1 st controller also includes a common-mode voltage loop, a current loop, and a differential-mode voltage loop. The common-mode voltage loop is used for being used for outputting the second end voltage v of the corresponding power converter_dciAnd a preset voltage reference value v_dcrefGenerating a given signal I_dref. The current loop is used for setting a current reference value I_qrefGiven a signal I_drefAnd corresponding first terminal current i of the power converter_gGenerating a common mode command v_bcomi. The differential mode voltage loop is used for converting the second end voltage v of the corresponding power converter_dciAnd a first terminal current i_gGenerating a differential mode instruction v_bdifi. The instruction synthesis unit is used for synthesizing a common-mode instruction v_bcomiSum-difference mode instruction v_bdifiPerforms an operation and generates a control command v_bi。

The specific control principle of each controller for the second case is the same as that of the (N-1) controllers in the first case, and reference may be made to fig. 9.

The following describes a process of obtaining a local common mode signal by taking two balanced diagrams, i.e., a unidirectional ring and a bidirectional ring as an example of an information transmission structure.

The information transmission structure among the N controllers is a unidirectional ring type, namely, no root node exists in the nodes. The immediately upstream node (i-1) of each node is a neighbor node, and the local common-mode signal is calculated according to the following formula:

the information transmission structure among N controllers is a bidirectional ring type, an immediately adjacent upstream node (i-1) and a downstream node (i +1) of each node are neighbor nodes, and then local common-mode signals are calculated according to the following formula:

in the embodiment of the application, each controller calculates a local common-mode voltage signal according to the second-end voltage of the corresponding power converter and the second-end voltages of no more than (N-2) other power converters, and generates a corresponding control instruction according to the local common-mode voltage signal, the second-end voltages of the corresponding power converters and the first-end currents, and a central controller does not need to collect the second-end voltages of all the power converters to obtain the common-mode voltage signal, so that the conversion system provided by the embodiment of the application can realize completely distributed control.

The following continues to describe the transformation system 200 provided in the fifth embodiment of the present application. The system includes N power converters 201 and N controllers 202.

Fifth embodiment a detailed control principle of the instruction generating unit is further described based on the fourth embodiment. As shown in FIG. 11, the common mode voltage loop depends on the local common mode signal v_dccomiA preset voltage reference value v_dcrefGenerating a given signal I with a corresponding active current feed-forward quantity at a first terminal of the power converter_dref. And a given signal is generated according to the active current feedforward quantity, so that the dynamic response of the load during sudden change can be improved. The current loop is used for setting a current reference value I_qrefGiven a signal I_drefAnd corresponding first terminal current i of the power converter_gGenerating a common mode command v_bcomi。

The common mode voltage loop comprises a first subtracter, a proportion unit and a first adder. The first subtracter is used for converting a preset voltage reference value v_dcrefSubtracting a local common-mode signal v_dccomiA fourth intermediate variable is obtained. And the proportion unit is used for carrying out proportion control on the fourth intermediate variable to obtain a fifth intermediate variable. The first adder is used for adding the fifth intermediate variable and the corresponding active current feedforward quantity I of the power converter_fAdding to obtain a preset current reference value I_dref。

The current loop comprises a first multiplier, a second subtracter, a third subtracter, a fourth subtracter and a proportion quasi-resonance controller. First, theA multiplier for multiplying the given signal I_drefAnd the cosine of the phase of the power grid voltage is multiplied to obtain a sixth intermediate variable. The second multiplier is used for multiplying the preset current reference value I_qrefAnd the sine of the phase of the power grid voltage is multiplied to obtain a seventh intermediate variable. The second subtracter is used for subtracting the seventh intermediate variable from the sixth intermediate variable to obtain an eighth intermediate variable i_ref. The third subtracter is used for converting the first end current i of the corresponding power converter_gMinus an eighth intermediate variable i_refTo obtain a ninth intermediate variable. And the proportional quasi-resonance controller is used for carrying out proportional quasi-resonance control on the ninth intermediate variable to obtain a tenth intermediate variable. The fourth subtracter is also used for feeding the grid voltage forward quantity v_fgSubtracting the tenth intermediate variable to obtain a common-mode command v_bcomi。

Because each controller adopts a current loop and only one current of the circuit can not enable each controller to realize no-static-error control, the current loop adopts a Proportional Quasi-Resonant controller (PQR for short).

Wherein, the active current feed-forward quantity of the first end of the power converter is obtained according to the average power of the direct current side (second end) of the power converter and the voltage amplitude of the alternating current side (first end), that is, the active current feed-forward quantity is calculated according to the following formula (10).

Wherein, I_fRepresenting the active current feed-forward quantity, i_dccomiRepresenting the common-mode component of the current of the DC-link capacitor, v_dciVoltage, v, representing the capacitance of the DC link_gMRepresenting the magnitude of the ac voltage supplied by the grid.

The power grid voltage feedforward quantity is calculated according to the following formula:

wherein M represents receptionNumber of direct voltages to the neighbor, v_gRepresenting the ac voltage supplied by the grid.

As shown in fig. 21, in an embodiment, a coordination integration element may be further added on the basis of the common-mode voltage loop shown in fig. 11. More specifically, the common-mode voltage loop is dependent on the local common-mode signal v_dccomiA preset voltage reference value v_dcrefGenerating a given signal I by the local integral signal and the corresponding active current feedforward quantity at the first end of the power converter_dref. The current loop is used for setting a current reference value I_qrefGiven a signal I_drefAnd corresponding first terminal current i of the power converter_gGenerating a common mode command v_bcomi。

The common-mode voltage loop comprises a first subtracter, a first proportion unit, a second proportion unit, a sixth subtracter, a third proportion unit, a first adder, a third adder and an integration unit. The first subtracter is used for converting a preset voltage reference value v_dcrefSubtracting a local common-mode signal v_dccomiA fourth intermediate variable is obtained. And the first proportion unit is used for carrying out proportion control on the fourth intermediate variable according to the first proportion coefficient to obtain a fifth intermediate variable. And the second proportion unit is used for performing proportion control on the fourth intermediate variable according to the second proportion coefficient to obtain an eleventh intermediate variable. The sixth subtracter is used for integrating the local common mode signal Int_comiSubtracting the local integral signal Int_iAn integral error is obtained. And the third proportion unit is used for carrying out proportion control on the integral error according to a third proportion coefficient to obtain a twelfth intermediate variable. And the third adder is used for adding the twelfth intermediate variable and the eleventh intermediate variable to obtain a thirteenth intermediate variable. The integration unit is used for performing integration control on the thirteenth intermediate variable to obtain a local integration signal Int_i. And the first adder is used for adding the fifth intermediate variable, the corresponding active current feedforward quantity of the power converter and the local integral signal to obtain a preset current reference value.

The local common-mode integrated signal is obtained by performing weighted average on a local integrated signal generated by a common-mode voltage loop in the other M controllers and a local integrated signal generated by a common-mode voltage loop in the current controller, and the other M controllers are neighbor controllers of the current controller.

The structure of the current loop is the same as the common mode voltage loop shown in fig. 11, and the description thereof is omitted.

By adding the coordination integral link to the common-mode voltage ring, the steady-state error of the common-mode voltage ring can be eliminated, so that the control accuracy of the controller is improved, and the performance of a conversion system is improved.

The following describes a process of obtaining a local common mode integrated signal by taking different transmission structures as examples.

In the transmission structure of the chain spanning tree, node 1 is used as a root node, and nodes 2 to N are used as following nodes. The immediately upstream node (i-1) of each following node is a neighbor node, and for the 2 nd to nth controllers, the local common mode integrated signal is calculated according to the following formula:

in the transmission structure of the broadcast spanning tree, node 1 is used as a root node, and nodes 2 to N are used as following nodes. The node 1 is used as a neighbor node from the node 2 to the node N, and for a following node, a local common mode integral signal is calculated according to the following formula:

in the transmission structure of the balanced diagram, the transmission structure between the N controllers is a unidirectional ring, that is, there is no root node in the nodes. The immediately upstream node (i-1) of each node is a neighbor node, and each node calculates a local common-mode integral signal according to the following formula:

in the transmission structure of the balanced diagram, the information transmission structure among the N controllers is a bidirectional ring type, the immediately adjacent upstream node (i-1) and the immediately adjacent downstream node (i +1) of each node are neighbor nodes, and each node calculates a local common mode integral signal according to the following formula:

as shown in FIG. 12, the differential mode voltage loop depends on the local common mode signal v_dccomiSecond terminal voltage v of corresponding power converter_dciFirst terminal current i_gAnd the differential mode voltage feedforward quantity v of the corresponding power converter_fGenerating a differential mode instruction v_bdifi. Wherein, differential mode voltage ring specifically includes: the current direction judging unit judges whether the current is in the first direction or not according to the current direction.

The fifth subtracter is used for dividing the local common-mode signal v_dccomiMinus the second terminal voltage v of the corresponding power converter_dciAnd obtains a first intermediate variable. A proportional unit for converting the first intermediate variable and a proportional coefficient k_dMultiply and get the second intermediate variable. The current direction judging unit is used for judging the current i of the first end of the corresponding power converter_gAnd multiplying the intermediate value by the second intermediate variable to obtain a third intermediate variable. And the second adder is used for adding the third intermediate variable and the corresponding differential mode voltage feedforward quantity of the power converter to obtain a differential mode instruction.

It should be noted that the third intermediate variable may be directly output as the differential mode command without considering the differential mode voltage feedforward amount.

The differential mode voltage feedforward quantity is obtained by calculation according to the differential mode power of a direct current side (second end) and the current amplitude of an alternating current side (first end), and is specifically calculated according to the following formula:

wherein v is_fRepresenting a differential mode voltage feedforward quantity, i_dcdifRepresenting the current of the dc-link capacitanceDifferential mode component, i_dcd_ifi＝i_dci-i_dccomi，i_dciRepresenting the current of the DC-link capacitor in the ith power converter, i_dccomiIs a common mode DC output current calculated in the same manner as the common mode voltage I_gMRepresenting the current magnitude at the first terminal of the corresponding power converter.

The current direction judging unit obtains an intermediate value according to the following formula:

the current direction judging unit obtains a third intermediate variable according to the following formula:

v_Z＝-k_dv_dcdifisign(i_g) (18)

wherein v is_ZDenotes a third intermediate variable, v_dcdifi＝v_dci-v_dccomi，v_dcdifiRepresenting the differential-mode component, k, of the voltage of the DC-link capacitor in the ith power converter_dIndicating the scaling factor.

The local differential mode voltage loop is an important link in a controller and relates to voltage balance of each direct current link capacitor in a conversion system. The following describes a design method of proportional control parameters in a local differential mode voltage ring by taking a leaderless mode of one-way ring communication as an example.

Firstly, a mathematical model of pressure equalization is established

Wherein L is_LLIs a Laplace matrix, related to the communication architecture between controllers, I_gMIs the amplitude of the AC side current, v_dc0C is the voltage rating of the dc link capacitor and C is the capacitance value of the dc link capacitor.

Then, a Laplace matrix L is obtained_LLThe characteristic values of (A) are as follows:

the transformation system equalizing ring stability is equivalent to the unit negative feedback coefficient stability corresponding to the following N open-loop transfer functions.

Then, a scaling factor k is obtained_dThe stability ranges of (a) are as follows:

wherein k is_dDenotes a scaling factor, tau denotes a communication time interval,

I_gMrepresenting the magnitude of the current, v, at the first terminal of the corresponding power converter_dc0And C represents the capacitance of the capacitor in the corresponding power converter. Preferably, the first and second electrodes are formed of a metal,

as can be seen from the above formula, the larger the communication delay, the smaller the stability range, the larger the number of power converters, and the smaller the stability range, but the change with the increase in the number of power converters is not obvious. k is a radical of_dShould be in the stability range, specifically set by the dynamic response speed and stability compromiseAnd (6) counting.

As shown in fig. 13, the differential mode voltage loop can also adopt the following control principle: the subtracter converts the local common-mode signal v_dccomiMinus the second terminal voltage v of the corresponding power converter_dciAnd obtains a first intermediate variable. The proportional unit combines the first intermediate variable and a proportional coefficient k_dMultiply and get the second intermediate variable. Then the adder adds the second intermediate variable and the differential mode voltage feedforward quantity v of the corresponding power converter_fAfter addition, the differential mode instruction is obtained through a rear-stage proportion unit again. Wherein, the differential mode voltage feedforward quantity v_fCan be calculated according to equation (16). The proportionality coefficient of the latter-stage proportional unit varies with the current at the first end of the power converter, and is specifically shown in formula (25):

wherein k is_tIndicating the scaling factor of the latter scale unit.

Alternatively, as shown in fig. 14, the differential mode voltage loop may also employ the following control principle: the subtracter converts the local common-mode signal v_dccomiMinus the second terminal voltage v of the corresponding power converter_dciAnd obtains a first intermediate variable. The proportional unit combines the first intermediate variable and a proportional coefficient k_dMultiply and get the second intermediate variable. The second intermediate variable passes through the rear-stage proportion unit again and then is compared with the corresponding differential mode voltage feedforward quantity v of the power converter_fAnd adding to obtain the differential mode instruction. The scaling factor of the latter stage scaling unit is determined according to equation (25).

Wherein the differential mode voltage feedforward amount v shown in FIG. 14_fCan be calculated according to the following equation (26).

In the conversion system provided by the embodiment of the application, in order to solve the problem that the current of each converter cannot be independently and freely controlled, on one hand, local common-mode voltage feedback is adopted in a local common-mode voltage ring instead of the voltage of each converter, so that the output of each local common-mode voltage ring is basically consistent to a certain extent. On the other hand, the current loop employs a controller having a static error, such as a proportional control, a proportional quasi-resonant controller, or the like. A controller for completely eliminating the static error such as proportional integral, proportional resonance and the like is not adopted. The current setting and the current feedback can have certain errors so as to ensure that the current control of each converter does not conflict when the output of the local common-mode voltage loop of each converter is slightly different.

As shown in fig. 15, a sixth embodiment of the present application further provides a control method for controlling a transformation system, where the transformation control system structure is described in the foregoing embodiments. The control method comprises the following steps:

s1, each of the at least (N-1) controllers receiving a neighbor dc voltage signal in the converter system reflecting only the voltages at the second terminals of the other M power converters.

Wherein N is more than or equal to 3, M is more than or equal to 1 and less than or equal to (N-2), and M and N are integers. That is, each of the at least (N-1) controllers can only obtain the second terminal voltages of a part of the remaining (N-1) power converters, and can not obtain the second terminal voltages of all of the remaining (N-1) power converters.

And S2, each controller of at least (N-1) controllers generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighbor direct-current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter.

Each controller of the at least (N-1) controllers analyzes the voltages of the second ends of the other M power converters from the neighboring direct-current voltage signals, and generates a control signal according to the voltages of the second ends of the other M power converters, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter, wherein the control signal is used for controlling the voltage of the first end of the corresponding power converter.

In the control method provided by the embodiment of the application, each controller does not need to obtain the voltage of all the rest power converters, so that the number of connecting lines between the controllers and the number of data between the controllers can be greatly reduced, and the reliability of the conversion system is further improved.

The seventh embodiment of the present application further provides a control method for controlling the transformation system, wherein the transformation control system structure is described with reference to the foregoing embodiments. The control method comprises the following steps:

s10, each of the at least (N-1) controllers receiving a neighbor dc voltage signal in the converter system reflecting only the voltages at the second terminals of the other M power converters.

Here, this step has already been described in detail in S1, and is not described here again.

And S20, each controller of at least (N-1) controllers generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighbor direct-current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter.

The process of generating the control signal specifically includes S11 to S13.

And S11, calculating to obtain a local common mode signal according to the neighbor direct current voltage signal and the corresponding second terminal voltage of the power converter.

And carrying out weighted average on the second terminal voltages of the M other power converters and the second terminal voltages of the corresponding power converters to obtain a local common mode signal. Preferably, the weighted value of each second terminal voltage is equal and is 1/N.

Each controller can only obtain the second terminal voltage of a part of the power converters in the remaining (N-1) power converters, and can not obtain the second terminal voltages of all the power converters in the remaining N-1 power converters. Compared with the common mode signal calculated according to the second terminal voltages of the N power converters, the local common mode signal is calculated according to the second terminal voltages of (N-1) power converters at most.

And S12, generating a control command according to the local common mode signal, the corresponding first terminal current and the second terminal voltage of the power converter.

The process of generating the control command specifically includes S121 to S124.

And S121, generating a given signal according to the local common mode signal and a preset voltage reference value.

The method comprises the steps of subtracting a local common mode signal from a preset voltage reference value, carrying out proportional control, and superposing a proportional control result and an active current feedforward quantity of a first end of a corresponding power converter to obtain a given signal I_dref。

In one embodiment, S21 specifically includes the following steps. And subtracting the local common mode signal from the preset voltage reference value to obtain a fourth intermediate variable. And carrying out proportional control on the fourth intermediate variable according to the first proportional coefficient to obtain a fifth intermediate variable. And carrying out proportional control on the fourth intermediate variable according to the second proportional coefficient to obtain an eleventh intermediate variable. And subtracting the local integral signal from the local common mode integral signal to obtain an integral error. And carrying out proportional control on the integral error according to the third proportional coefficient to obtain a twelfth intermediate variable. And adding the twelfth intermediate variable and the eleventh intermediate variable to obtain a thirteenth intermediate variable. And performing integral control on the thirteenth intermediate variable to obtain a local integral signal. Adding the fifth intermediate variable, the corresponding active current feedforward quantity of the power converter and the local integral signal to obtain a given signal I_dref. The local common-mode integrated signal is obtained by performing weighted average on a local integrated signal generated by a common-mode voltage loop in the other M controllers and a local integrated signal generated by a common-mode voltage loop in the current controller, and the other M controllers are neighbor controllers of the current controller.

And S122, generating a differential mode command according to the local common mode signal, the second terminal voltage and the first terminal current of the corresponding power converter.

Wherein a local common-mode signal v is converted into a local common-mode signal_dccomiMinus the second terminal voltage v of the corresponding power converter_dciThen, proportional control is carried out, and the result after proportional control and the corresponding first end current i of the power converter are obtained_gAnd multiplying the coefficients obtained by the directions, and adding the multiplied result and the corresponding differential mode voltage feedforward quantity of the power converter to obtain a differential mode instruction.

And S123, generating a common mode instruction according to the preset current reference value, the given signal and the corresponding first end current of the power converter.

The generating of the common mode command specifically includes: according to a preset current reference value I_qrefAnd given signal I_drefObtaining a current reference value i_refReference current to value i_refAnd subtracting the current of the first end of the corresponding power converter, performing proportional quasi-resonance control, and subtracting the result of the proportional quasi-resonance control from 1/M of the voltage of the first end of the corresponding power converter to obtain a common-mode instruction.

And S124, operating the common mode instruction and the differential mode instruction and generating a control instruction.

And S13, modulating the control command to generate a control signal.

And carrying out carrier phase shift modulation on the control command to generate a control signal.

In the control method provided by the embodiment of the application, due to the fact that the local common-mode signal is adopted, the voltage of other power converters does not need to be obtained, the voltage of all the power converters does not need to be collected by the central controller, and then the common-mode signal is calculated according to the voltage of all the power converters, so that the converters provided by the application can adopt completely distributed control.

The following description focuses on advantageous effects of the transformation system and the control method provided by the embodiments of the present application with reference to specific embodiments.

Taking the information transmission structure as a unidirectional ring network as an example, 3 power converters are cascaded. As shown in fig. 16 and 17, the power converter load currents are 3A before 0.2s, and after 0.2s, the 2 nd power converter load becomes 2A and the 3 rd power converter load current becomes 4A. Waveforms of the dc link capacitor voltage (the second terminal voltage of the power converter), the bridge arm voltage (the first terminal voltage of the power converter), and the grid current (the first terminal current of the power converter) obtained by the simulation are shown in fig. 16, respectively. The waveform of the bridge arm voltage and the direct-current link capacitor voltage and current satisfy the corresponding relation of the differential mode control equation, for example, in the time period between 0.2s and 0.21s, the current sign is positive, and the differential mode component of the bridge arm voltage and the direct-current link capacitor voltage differential mode component satisfy the proportional relation.

In a partially distributed conversion system with 3 cascaded power converters, a centralized controller sends a common-mode voltage signal to each unit controller, and communication delay affects control bandwidth. When the communication delay becomes longer, the control bandwidth can only be reduced, the control performance is reduced, and the response is slowed. As shown in fig. 18, the dc link capacitor voltage waveform adopts a control period of 100 μ s, and the dc link capacitor voltage set value is 1580V. When the communication is delayed for 200 mus, the dynamic response of the partially distributed voltage is slowed, the convergence time is longer than 0.03s, the static difference is increased, and the deviation is very large at 1580V.

In the completely distributed conversion system provided by the application, the local common-mode voltage ring, the current ring and the local differential-mode voltage ring are all realized in the controller corresponding to each power converter, and are not influenced by communication delay, and the communication delay only can influence the control speed of the differential-mode voltage-sharing ring. As shown in fig. 19, the dc link capacitor voltage waveform has a smaller deviation 1580V and a convergence time of less than 0.025s, and obviously has a better dynamic response and a smaller static error compared with fig. 18.

The transformation system usually adopts a carrier phase shifting mode to form higher equivalent switching frequency, and then generates a control signal according to a control instruction. For example: the switching frequency of each power converter is f, and the equivalent switching frequency of the N power converters can reach 2 Nf. If centralized control is adopted, the centralized controller needs to control the frequency to be 2Nf to achieve the effect that each power converter is controlled by the control frequency f. As the switching frequency of power converters increases or the number of power converters increases, centralized control requires shorter and shorter control cycles. The ideal control period Teq of the centralized controller is 1/(2N) of the switching period of the power converter, and it is difficult to meet the demand for higher frequency.

As shown in fig. 20, each triangular wave represents a loading signal corresponding to a power converter, each power converter can only load a control output at a carrier zero crossing point thereof, if centralized control is adopted, only the 1 st power converter can be loaded immediately, other power converters can be loaded at respective carrier zero crossings, the average control delay of the centralized control reaches Tc2+ Tf/2, Tc2 is the control calculation time of the common-mode voltage loop, and Tf is the carrier period. If completely distributed control is adopted, each power converter calculates the control quantity before the carrier zero crossing point of the power converter, reloads and modulates the power converter, the control delay of a current loop can be reduced, the distributed control delay is only Tc1+ Tf/2, and Tc1 is the control calculation time of the current loop. The bandwidth can be increased by reducing the current loop control delay, for example: when Tc1 is Tf/2, the control bandwidth can be increased by more than 30% due to the reduction of delay.

In a word, the invention provides a conversion system with completely distributed control, a high-performance and high-cost centralized controller is not needed, so that the conversion system is controlled to be completely modularized, the number of power converters in the conversion system can be flexibly configured to deal with different voltage classes, and the expansibility is good. Compared with a centralized control scheme, the single-point failure risk of the controller is reduced. Compared with the existing centralized control or partial distributed control scheme, the distributed control reduces the requirement on communication delay and can reduce the communication cost. The distributed control adopts local control, greatly reduces the requirement on the operational capability of the chip, and can greatly improve the response speed of a current loop and a common mode voltage loop particularly when being applied to a carrier phase-shifting mode. In addition, the invention adopts local information to replace global information, and can control only by receiving the voltage of the neighbor power converter without receiving the voltages of all other power converters, thereby reducing the requirement on communication bandwidth.

Finally, it should be noted that: the above embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A transformation system, comprising:

n power converters, each of the power converters including a first end and a second end, the first ends of the N power converters being electrically coupled in series, an

N controllers in one-to-one correspondence with the N power converters, each controller receiving a first terminal current and a second terminal voltage of the corresponding power converter,

each of at least (N-1) controllers receives a neighbor direct-current voltage signal only reflecting the voltage of the second end of other M power converters in the conversion system, and generates a control signal for controlling the voltage of the first end of the corresponding power converter according to the neighbor direct-current voltage signal, the current of the first end of the corresponding power converter and the voltage of the second end of the corresponding power converter, wherein N is more than or equal to 3, M is more than or equal to 1 and less than or equal to (N-2), and M and N are integers.

2. The transformation system according to claim 1, wherein the information transmission structure between the N controllers is a balanced graph.

3. Transformation system according to claim 2, characterized in that said balanced graph is a unidirectional ring network or a bidirectional ring network.

4. The transformation system according to claim 1, wherein the information transmission structure between the N controllers is a spanning tree.

5. Transformation system according to claim 4, characterized in that said spanning tree is chained or broadcast.

6. The transformation system of claim 4, wherein the N controllers comprise (N-1) controllers and a 1 st controller,

each of the (N-1) controllers comprises:

the computing unit is used for computing to obtain a local common mode signal according to the neighbor direct current voltage signal and the second end voltage of the corresponding power converter;

the instruction generating unit is used for generating a control instruction according to the local common-mode signal, the first end current and the second end voltage of the corresponding power converter; and

a modulation unit, configured to perform modulation processing on the control instruction to generate the control signal,

the 1 st controller includes:

the command generation unit is used for generating a control command according to the first end current and the second end voltage of the corresponding power converter; and

and the modulation unit is used for modulating the control instruction to generate the control signal.

7. The transformation system according to claim 6,

the instruction generating unit in the (N-1) controllers includes:

the common-mode voltage ring is used for generating a given signal according to the local common-mode signal and a preset voltage reference value;

the current loop is used for generating a common-mode instruction according to a preset current reference value, the given signal and the first end current of the corresponding power converter;

the differential mode voltage loop is used for generating a differential mode instruction according to the local common mode signal, the second end voltage and the first end current of the corresponding power converter; and

an instruction synthesis unit for operating the common mode instruction and the differential mode instruction and generating the control instruction,

the instruction generating unit in the 1 st control includes:

the common-mode voltage loop is used for generating a given signal according to the second end voltage of the corresponding power converter and a preset voltage reference value;

the differential mode voltage loop is used for generating a differential mode instruction according to the second end voltage and the first end current of the corresponding power converter; and

and the instruction synthesis unit is used for calculating the common mode instruction and the differential mode instruction and generating the control instruction.

8. The transformation system of claim 2, wherein each of the N controllers comprises:

9. The transformation system according to claim 8, wherein the instruction generation unit comprises:

10. The conversion system according to claim 7 or 9, wherein the common-mode voltage loop generates the given signal based on the local common-mode signal, a preset voltage reference and an active current feed-forward amount of the corresponding first end of the power converter.

11. The conversion system of claim 10, wherein the common-mode voltage loop comprises:

the first subtracter is used for subtracting the local common-mode signal from the preset voltage reference value to obtain a fourth intermediate variable;

the first proportion unit is used for carrying out proportion control on the fourth intermediate variable according to a first proportion coefficient to obtain a fifth intermediate variable;

the second proportion unit is used for carrying out proportion control on the fourth intermediate variable according to a second proportion coefficient to obtain an eleventh intermediate variable;

the sixth subtracter is used for subtracting the local integral signal from the local common-mode integral signal to obtain an integral error;

the third proportion unit is used for carrying out proportion control on the integral error according to a third proportion coefficient to obtain a twelfth intermediate variable;

a third adder, configured to add the twelfth intermediate variable and the eleventh intermediate variable to obtain a thirteenth intermediate variable;

the integration unit is used for carrying out integration control on the thirteenth intermediate variable to obtain the local integration signal;

a first adder, configured to add the fifth intermediate variable, the active current feed-forward quantity at the first end of the corresponding power converter, and the local integral signal to obtain the given signal;

the local common-mode integrated signal is obtained by performing weighted average on the local integrated signal generated by the common-mode voltage loop in the other M controllers and the local integrated signal generated by the common-mode voltage loop in the current controller, and the other M controllers are neighbor controllers of the current controller.

12. The conversion system according to claim 7 or 9, wherein the differential mode voltage loop generates a differential mode command according to the local common mode signal, the second terminal voltage of the corresponding power converter, the first terminal current, and a differential mode voltage feed forward quantity of the corresponding power converter.

13. The conversion system of claim 12, wherein the differential mode voltage loop comprises:

the subtracter is used for subtracting the second end voltage of the corresponding power converter from the local common-mode signal to obtain a first intermediate variable;

the proportion unit is used for multiplying the first intermediate variable and a proportion coefficient to obtain a second intermediate variable;

the current direction judging unit is used for obtaining an intermediate numerical value according to the current direction of the first end of the corresponding power converter and multiplying the intermediate numerical value by the second intermediate variable to obtain a third intermediate variable;

and the adder is used for adding the third intermediate variable and the differential mode voltage feedforward quantity to obtain the differential mode instruction.

14. The transformation system according to claim 13, wherein the scaling factor satisfies the following condition:

I_gMrepresenting the current amplitude, v, of the first terminal of the corresponding power converter_dc0And C represents the capacitance of a capacitor in the corresponding power converter.

15. The transformation system according to claim 14,

16. the conversion system according to claim 6 or 8, wherein the computing unit performs a weighted average of the second terminal voltages of the M other power converters and the second terminal voltages of the corresponding power converters to obtain the local common mode signal.

17. The conversion system according to claim 6 or 8, wherein the modulation unit is configured to perform carrier phase shift modulation on the control command to generate the control signal.

18. A control method for controlling a transform system, the transform system comprising:

The N controllers correspond to the N power converters one by one, and each controller receives the first end current and the second end voltage of the corresponding power converter;

the control method comprises the following steps:

s1, each controller of at least (N-1) controllers receiving neighbor direct current voltage signals of the conversion system only reflecting the voltages of the second ends of the other M power converters; and

s2, each controller of at least (N-1) controllers generates a control signal for controlling the first end voltage of the corresponding power converter according to the neighbor direct current voltage signal, the first end current and the second end voltage of the corresponding power converter,

19. The control method according to claim 18, wherein the S2 includes:

s11, calculating to obtain a local common mode signal according to the neighbor direct current voltage signal and the second end voltage of the corresponding power converter;

s12, generating a control command according to the local common mode signal, the first end current and the second end voltage of the corresponding power converter; and

and S13, modulating the control command to generate the control signal.

20. The control method according to claim 19, wherein the S12 includes:

s121, generating a given signal according to the local common mode signal and a preset voltage reference value;

s122, generating a differential mode command according to the local common mode signal, the second terminal voltage and the first terminal current of the corresponding power converter;

s123, generating a common mode instruction according to a preset current reference value, the given signal and the first end current of the corresponding power converter; and

and S124, operating the common mode instruction and the differential mode instruction and generating the control instruction.

21. The control method according to claim 20, wherein the S121 includes:

subtracting the local common-mode signal from the preset voltage reference value to obtain a fourth intermediate variable;

carrying out proportional control on the fourth intermediate variable according to a first proportional coefficient to obtain a fifth intermediate variable;

carrying out proportional control on the fourth intermediate variable according to a second proportional coefficient to obtain an eleventh intermediate variable;

subtracting the local integral signal from the local common-mode integral signal to obtain an integral error;

carrying out proportional control on the integral error according to a third proportional coefficient to obtain a twelfth intermediate variable;

adding the twelfth intermediate variable and the eleventh intermediate variable to obtain a thirteenth intermediate variable;

performing integral control on the thirteenth intermediate variable to obtain the local integral signal;

adding the fifth intermediate variable, the active current feedforward quantity of the corresponding power converter and the local integral signal to obtain the given signal;

22. The control method according to claim 19, wherein the S11 includes:

and carrying out weighted average on the second terminal voltages of the M other power converters and the second terminal voltage of the corresponding power converter to obtain the local common mode signal.

23. The control method according to claim 19, wherein the S13 includes:

and carrying out carrier phase shift modulation on the control command to generate the control signal.