WO2024040876A1 - Transformer-lessgeneralized unified power flow controller, method, and system - Google Patents

Abstract

Disclosed are a transformer-less generalized unified power flow controller, a method, and a system. The controller comprises: a reactive power compensation module and a power flow adjustment module. The reactive power compensation module comprises a cascaded bridge inverter, and the power flow adjustment module comprises a multi-port single-phase MMC, the multi-port single-phase MMC being connected in series with the cascaded bridge inverter. The reactive power compensation module is a voltage source converter, which has a bidirectional reactive power compensation function, can absorb reactive power from the system, and also can provide reactive power compensation for the system; the power flow adjustment module implements decoupling control of active power and reactive power of a transmission line. Compared with an existing unified power flow controller, the present invention omits an expensive and heavy power frequency isolation transformer, which not only reduces the area occupied by an apparatus, but also decreases the installation and operation costs of the apparatus; in addition, the transformer-less design causes the apparatus to have a faster response speed and lower loss.

Description

A transformerless generalized unified power flow controller, method and system Technical field

The invention relates to the technical fields of AC power grid power flow control and power electronics, and in particular to a transformerless generalized unified power flow controller with multi-port active power flow control capability suitable for AC power grids and its control method and system.

Background technique

With load growth and the integration of large-scale renewable energy, the grid structure is becoming increasingly complex. On the one hand, modern power systems have developed into large-scale interconnected networks. There are imbalances in generator output and load demand, leading to uneven power distribution in the power grid, overloading of lines, increased losses, and reduced transmission capacity. On the other hand, the access to a high proportion of renewable energy and the continuous development of loads, as well as the diverse changes in power system operation modes caused by source-load interaction, will lead to extremely complex power flow distribution in the power system, which will in turn bring problems to the reliable operation of the power system. a huge challenge.

In order to solve the above problems, there are usually two options: The first option is to upgrade existing transmission lines or add transmission lines to enhance the grid structure. This option is limited by factors such as the lack of transmission corridors, the difficulty of land acquisition for new substations, and economic issues. It is difficult to realize due to restrictions; the second solution is to realize active power flow control of transmission lines through flexible AC transmission devices. This solution can control all electrical parameters that affect power transmission through power electronic devices, making the power grid flexible and controllable. Not only does it not need to change the structure of the existing power grid, it also has the advantages of flexible power flow adjustment, dynamic compensation, and oscillation suppression. At the same time, we make full use of the real-time and rapidity of power electronic device control to achieve rapid and dynamic adjustment of power flow on transmission lines, thereby optimizing power flow distribution.

The existing power flow controller with the most comprehensive power flow regulation function is mainly the generalized unified power flow controller. The typical topology of the existing generalized unified power flow controller consists of a parallel voltage source converter and multiple series voltage source converters. The converters are connected back-to-back, and the converters are connected to the transmission lines through power frequency isolation transformers. By adjusting the amplitude and phase angle of the series-side converter voltage, the active power and reactive power of multiple transmission lines can be adjusted independently. However, this topology requires the use of expensive and bulky power frequency isolation transformers, which results in a large overall volume of the device and high installation, operation and maintenance costs. At the same time, when expanding the device connection port, a series power frequency isolation transformer and three-phase voltages need to be added. Source-type converters have high port expansion costs.

Contents of the invention

The purpose of this section is to outline some aspects of implementation examples of the invention and to briefly introduce some Best practices. Some simplifications or omissions may be made in this section, the abstract and the title of the invention to avoid obscuring the purpose of this section, the abstract and the title of the invention, and such simplifications or omissions cannot be used to limit the scope of the invention.

In view of the problems existing in the prior art, the present invention is proposed.

Therefore, the technical problem solved by the present invention is: in view of the existing power flow controller equipment at the current stage, the overall volume is large, the operation and maintenance costs are high, and the equipment volume and operation and maintenance costs are reduced and it is difficult to achieve the functions of the power flow controller. To solve the trade-off problem, a new structure of a transformer-less power flow controller was designed by fully considering the portability, economy and practicality of the power flow controller.

In order to solve the above technical problems, the present invention provides the following technical solutions:

Transformerless generalized unified power flow controller includes,

A reactive power compensation module, the reactive power compensation module includes a cascade bridge inverter;

A power flow adjustment module, the power flow adjustment module includes a multi-port single-phase MMC;

Wherein, the multi-port single-phase MMC is connected in series with the cascade bridge inverter;

The reactive power compensation module is a voltage source converter with a reactive power two-way compensation function, which can absorb reactive power from the system and provide reactive power compensation for the system;

The power flow regulation module realizes decoupling control of the active power and reactive power of the transmission line.

As the transformerless generalized unified power flow controller of the present invention, the cascade bridge inverter is composed of multiple sub-modules cascaded.

As the transformerless generalized unified power flow controller of the present invention, wherein: the multi-port single-phase MMC includes a power flow regulating multi-port single-phase MMC and an energy balancing multi-port single-phase MMC;

The energy balancing multi-port single-phase MMC and the power flow regulating multi-port single-phase MMC are connected in parallel;

The cascade bridge inverter is connected to the AC output port of the energy-balanced multi-port single-phase MMC to realize the series connection of the cascade bridge inverter and the multi-port single-phase MMC;

The AC output port of the power flow regulating multi-port single-phase MMC is connected in series with the transmission line. By adjusting the amplitude and phase of the AC output port voltage of the multi-port single-phase MMC connected in series on the transmission line, the active power and reactive power of the transmission line are realized. Decoupled control.

As the control method of the transformerless generalized unified power flow controller of the present invention, the control target of the line power flow control loop is:

Obtain the reference value of the active power of the power flow control transmission line and reactive power reference values

Set line power flow control loop output

in, Reference value for the AC component of the output voltage of the AC output port of a power flow regulated multi-port single-phase MMC connected to a power flow regulated balanced transmission line, is the reference value of the AC component of the output voltage of the power flow regulating multi-port single-phase MMC AC output port connected to the power flow control transmission line. The subscript i indicates that the i-th transmission line is a power flow regulating balanced transmission line, and the subscript j indicates the j-th transmission line. A power flow control transmission line.

As the control method of the transformerless generalized unified power flow controller of the present invention, the cascaded bridge inverter control loop includes:

Voltage control outer loop, reactive power control outer loop and current control inner loop;

The control goal of the cascade bridge inverter control loop is to regulate the power flow and balance the reactive power of the transmission line to reach a reference value. The sum of the three-phase capacitor voltages of the cascade bridge inverter is stable as the reference value The output of the cascade bridge inverter control loop is the reference value of the AC component of the voltage at the AC output port of the cascade bridge inverter.

The reactive power control outer loop adjusts the reactive power reference value of the balanced transmission line according to the power flow. Calculate the reference value of the q-axis component of the current in the balanced transmission line for power flow regulation

The current control inner loop is controlled in the dq coordinate system, and uses a proportional integral controller to control the d-axis component and q-axis component of the power flow adjustment balance transmission line current respectively. The mathematical equation is:

in, and After multiplying by the Park inverse transformation matrix and then multiplying by -1 for inversion, the output reference voltage of the cascade bridge inverter control loop in the abc coordinate system is obtained. That is, V _pa , V _pb , V _pc ;

Among them, V _Cid and V _Ciq are the d-axis component and q-axis component of the AC component of the AC output port voltage of the power flow regulating multi-port single-phase MMC connected to the power flow regulating balanced transmission line, V _SMd and V _SMq are related to the cascade bridge type The d-axis component and q-axis component of the AC component of the energy-balanced multi-port single-phase MMC AC output port voltage connected to the inverter, k _p2 is the proportional link gain coefficient of the proportional integral controller, k _i2 is the integral link gain of the proportional integral controller Coefficients, V _id , V _iq , V _Cid , V _Ciq , V _SMd , and V _SMq are feedforward terms. Their function is to enhance the anti-interference ability of the control loop and speed up the response speed of the control loop. It is a decoupling term, and its function is to realize the decoupling control of d-axis and q-axis.

As the control method of the transformerless generalized unified power flow controller of the present invention, the control target of the common DC bus voltage balance control loop is to stabilize the common DC bus voltage to a reference value. The output of the common DC bus voltage balance control loop is the common DC bus balance reference voltage

The control of the common DC bus voltage balance control loop is carried out in the abc coordinate system. The proportional integral controller is used to control the abc three-phase public DC bus voltage. The mathematical equation is:

Among them, V _linka , V _linkb , and V _linkc are the three-phase public DC bus voltages, I _pa , I _pb , and I _pc are the three-phase currents of the cascade bridge inverter branch. is the feedforward three-phase reference voltage, k _p3 is the gain coefficient of the proportional link of the proportional-integral controller, and k _i3 is the gain coefficient of the integral link of the proportional-integral controller.

As the control method of the transformerless generalized unified power flow controller of the present invention, the control target of the circulation suppression control is to suppress the circulation between the multi-port single-phase MMCs of each phase, and the output of the circulation suppression control is to generate The double frequency voltage of the circulating current can be suppressed by compensating the sub-module modulation signal;

The circulation suppression control is carried out in the abc coordinate system, and a quasi-proportional resonance controller is used to suppress the intra-phase circulation of the abc three-phase. The mathematical equation is:

Among them, G _QPR (s) is the transfer function of the quasi-PR controller, ω is the resonant frequency, ω _c is the cut-off frequency that mainly affects the system bandwidth, k _p4 is the gain coefficient of the proportional link of the quasi-proportional resonant controller, k _r is the quasi-proportional resonance Controller resonance link gain coefficient, is the circulation compensation voltage of the power flow-regulated multi-port single-phase MMC submodule connected to the k-th transmission line, s is the model variable under Laplace change, is the energy balancing multi-port single-phase MMC sub-module circulating current compensation voltage, i _lkp and i _lkn are the power flow regulation multi-port single-phase MMC upper bridge arm current and lower bridge arm current connected to the kth transmission line, and the subscript l is l phase parameters.

As the control method of the transformerless generalized unified power flow controller of the present invention, the bridge arm voltage balance control includes two implementation methods: bridge arm voltage DC component adjustment and injection fundamental frequency circulating current adjustment.

As the control method of the transformerless generalized unified power flow controller according to the present invention, wherein: The adjustment method of the DC component of the bridge arm voltage includes adjusting the DC components of the upper and lower bridge arms to control the balance of the upper and lower bridge arm capacitor voltages, and outputting the multi-port single-phase MMC upper bridge arm and lower bridge arm sub-module voltage DC component reference values;

The bridge arm voltage equalization control uses a PI controller to control the DC component of the bridge arm voltage of the abc three-phase multi-port single-phase MMC. The mathematical equation is:

in, is the reference value of the DC component of the upper arm voltage, is the reference value of the DC component of the lower arm voltage, is the reference value of the average voltage of the upper and lower arms, V _average,lkp and V _average,klp are the average values of the voltages of the upper and lower arms, V _link,l is the DC bus voltage, and the subscript k is the kth transmission line The parameters of the connected power flow regulating multi-port single-phase MMC, the subscript l is the parameter of l phase, k _p5 is the gain coefficient of the proportional link of the proportional integral controller, k _i5 is the gain coefficient of the integral link of the proportional integral controller.

As the control method of the transformerless generalized unified power flow controller of the present invention, the adjustment method of injecting fundamental frequency circulation adjustment includes:

The bridge arm voltage balance control of the fundamental frequency circulation injection adjustment achieves the balance of the upper and lower bridge arm voltages by injecting the fundamental frequency circulation with the same phase as the voltage. The output of the injection fundamental frequency circulation adjustment is the reference value of the injected fundamental frequency circulation. Mathematics The equation is:

in, is the reference value of the fundamental frequency circulation injected into the power flow regulating multi-port single-phase MMC connected to the k-th transmission line, V _average,lkp and V _average,lkn are the upper and lower bridge arm voltage averages of the power flow regulating multi-port single-phase MMC, θ _k Adjust the phase of the fundamental frequency component of the multi-port single-phase MMC output voltage for the power flow connected to the k-th transmission line, k _p6 is the gain coefficient of the proportional link of the proportional-integral controller, and k _i6 is the gain coefficient of the integral link of the proportional-integral controller.

As the distribution method of the transformerless generalized unified power flow controller of the present invention, the basic equation is as follows:

If the first transmission line is a power flow regulating balanced transmission line, Real is the real part, for the transformerless The vector expression of the AC component of the series compensation voltage of the device-type generalized unified power flow controller on the kth transmission line, In order to achieve the target power flow on the k-th transmission line, the series compensation voltage vector expression required in series between the first transmission line and the k-th transmission line is, is the vector expression of the AC component of the energy-balanced multi-port single-phase MMC AC output port voltage in the multi-port single-phase MMC power flow regulation module connected to the cascade bridge inverter, is the conjugate vector expression of the alternating current on the k-th transmission line, is the conjugate vector expression of the AC current of the cascade bridge inverter branch, and n is the number of transmission lines interconnected by the transformerless generalized unified power flow controller.

As the control system of the transformerless generalized unified power flow controller of the present invention, wherein: the core equipment of the control system is the transformerless generalized unified power flow controller, AC transmission lines and substations;

The transformerless generalized unified power flow controller is installed at the convergence of multiple AC transmission lines;

The AC transmission line is connected to a transformerless generalized unified power flow controller. The AC transmission line can be connected to a high-voltage system via a step-up transformer, and can be connected to a low-voltage AC system via a step-down transformer. The low-voltage AC system can be connected to a low-voltage AC load, Energy storage equipment and electric vehicle fast charging stations.

As the equipment protection method of the transformerless generalized unified power flow controller of the present invention, wherein: the overvoltage protection of the voltage connected in series on the transmission line includes a protection device,

The protection device is connected in parallel with the AC output port of the power flow adjustment multi-port single-phase MMC in the multi-port single-phase MMC power flow adjustment module to protect the overvoltage of the voltage connected in series on the transmission line;

The protection device is composed of a metal oxide voltage limiter and a thyristor bypass switch connected in parallel;

Among them, the thyristor bypass switch is an anti-parallel thyristor, a resistor-capacitor loop, a static resistor in parallel and then in series with a saturated reactor;

The metal oxide voltage limiter limits the voltage to a safe voltage while the thyristor bypass switch achieves overvoltage protection by bypassing the AC output port of the power flow regulating multi-port single phase MMC.

As the equipment protection method of the transformerless generalized unified power flow controller of the present invention, wherein:

The AC output port of the multi-port single-phase MMC sub-module is connected in parallel with the thyristor bypass switch, so that when the multi-port single-phase MMC sub-module fails, the faulty sub-module can be quickly bypassed through the thyristor bypass switch, and redundant sub-modules can be put in at the same time. module, so that the failure of the sub-module does not affect the overall operation of the equipment;

The DC side of the multi-port single-phase MMC sub-module is equipped with a DC unloading circuit for capacitive energy release to prevent overvoltage from damaging power devices.

As the equipment protection method of the transformerless generalized unified power flow controller of the present invention, wherein: The AC port of the cascade bridge inverter sub-module is connected in parallel with a fast mechanical switch, so that when the cascade bridge inverter sub-module fails, the faulty module can be quickly removed;

The DC side of the cascaded bridge inverter sub-module is equipped with a DC unloading circuit that releases capacitive energy to prevent overvoltage from damaging the power devices.

Beneficial effects of the present invention: The existing cascade bridge inverter only has the reactive power compensation function and does not have the multi-AC transmission line interconnection and transmission line active power flow decoupling control functions. However, the present invention introduces multi-port single-phase The MMC power flow regulation module provides multiple AC interconnection ports to realize the interconnection of multiple AC transmission lines. By adjusting the amplitude and phase of the series compensation voltage connected in series on the transmission lines, it can realize active power decoupling of the active power and reactive power of the transmission lines. Control, compared with the existing unified power flow controller, the expensive and bulky power frequency isolation transformer is eliminated, which not only reduces the area of the device and the cost of installation and operation of the device, but also speeds up the response speed of the device and reduces the loss. lower. And when expanding the power flow control port, the traditional generalized unified power flow controller needs to add a three-phase inverter and a power frequency isolation transformer, and the port expansion cost is very high. However, the transformerless generalized unified power flow controller in the present invention , only need to increase the number of single-phase small-capacity MMC modules, and the expansion of the power flow control port can be realized quickly and economically.

Another beneficial effect of the present invention: the control method based on the controller of the present invention can realize the basic functions of the controller and ensure normal operation of the control.

Another beneficial effect of the present invention: the AC transmission line of the control system of the present invention can not only be connected to the high-voltage system via the step-up transformer, but also can be connected to the low-voltage AC system via the step-down transformer. The low-voltage AC system can be connected to the low-voltage AC load, Energy storage equipment and electric vehicle fast charging stations.

Another beneficial effect of the present invention: the equipment protection method based on the controller of the present invention can avoid serious damage to the internal components of the converter under fault conditions.

Description of drawings

In order to explain the technical solutions of the embodiments of the present invention more clearly, the drawings needed to be used in the description of the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention. Those of ordinary skill in the art can also obtain other drawings based on these drawings without exerting any creative effort. in:

Figure 1 is a schematic diagram of the topological structure of the transformerless generalized unified power flow controller and the system of interconnecting multiple transmission lines according to the present invention.

Figure 2 is a schematic diagram of the system of the transformerless generalized unified power flow controller without the energy-balanced multi-port single-phase MMC connected to the cascaded bridge inverter and its system of interconnecting multiple transmission lines according to the present invention. picture.

Figure 3 is a schematic diagram showing a typical topology example of the cascaded bridge inverter and the multi-port single-phase MMC described in the transformerless generalized unified power flow controller of the present invention.

Figure 4 is a schematic diagram of the topology of the multi-port single-phase MMC power flow adjustment module described in the transformerless generalized unified power flow controller of the present invention.

Figure 5 shows the control method of the transformerless generalized unified power flow controller of the present invention. The multi-port single-phase MMC power flow adjustment module adopts an MMC with two-level single-phase half-bridge type sub-modules in parallel, and the cascade bridge type inverter adopts Block diagram of the control method of the dual-port power flow control device in the cascaded full-bridge topology.

Figure 6 is a control system diagram of the transformerless generalized unified power flow controller of the present invention.

Figure 7 shows the control system of the transformerless generalized unified power flow controller of the present invention. The multi-port single-phase MMC power flow adjustment module adopts a two-level half-bridge type sub-module, and the cascade bridge-type inverter adopts a double-level cascade full-bridge topology. The topological structure of the port power flow control device and the schematic diagram of the system for interconnecting two transmission lines.

Figure 8 is a wiring diagram of the voltage overvoltage protection method and a schematic diagram of the protection device of the equipment protection method of the transformerless generalized unified power flow controller of the present invention.

Figure 9 shows the control system of the transformerless generalized unified power flow controller of the present invention. The multi-port single-phase MMC power flow adjustment module adopts a two-level half-bridge type sub-module, and the cascaded bridge-type inverter adopts a cascaded full-bridge topology. The topological structure of the port power flow control device and the system schematic diagram of the interconnection of three transmission lines.

Figure 10 shows the control method of the transformerless generalized unified power flow controller of the present invention. The multi-port single-phase MMC power flow adjustment module adopts an MMC with parallel two-level single-phase half-bridge type sub-modules, and the cascade bridge-type inverter adopts a stage Block diagram of the control method of the three-port power flow control device with full-bridge topology.

Figure 11 is a simulated power flow of each transmission line and voltage waveforms of each capacitor in the device under three working conditions of the transformerless generalized unified power flow controller, method and system of the present invention.

Detailed ways

In order to make the above objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. It is obvious that the described embodiments are part of the embodiments of the present invention, not all of them. Example. Based on the embodiments of the present invention, all other embodiments obtained by ordinary people in the art without creative efforts should fall within the protection scope of the present invention.

Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described here. Those skilled in the art can do so without departing from the connotation of the present invention. Similar generalizations are made, and therefore the present invention is not limited to the specific embodiments disclosed below.

Second, reference herein to "one embodiment" or "an embodiment" refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. "In one embodiment" appearing in different places in this specification does not all refer to the same embodiment, nor is it a separate or selective embodiment that is mutually exclusive with other embodiments.

The present invention will be described in detail with reference to schematic diagrams. When describing the embodiments of the present invention in detail, for the convenience of explanation, the cross-sectional diagrams showing the device structure will be partially enlarged according to the general scale. Moreover, the schematic diagrams are only examples and shall not limit the present invention. scope of protection. In addition, the three-dimensional dimensions of length, width and depth should be included in actual production.

At the same time, in the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper, lower, inner and outer" are based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present invention. The invention and simplified description are not intended to indicate or imply that the devices or elements referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore are not to be construed as limitations of the invention. Furthermore, the terms "first, second or third" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

Unless otherwise clearly stated and limited in the present invention, the terms "installation, connection, and connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integrated connection; it can also be a mechanical connection, an electrical connection, or a direct connection. A connection can also be indirectly connected through an intermediary, or it can be an internal connection between two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood on a case-by-case basis.

Example 1

Referring to Figures 1 to 4, an embodiment of the present invention is provided, which provides a transformerless generalized unified power flow controller, including a reactive power compensation module, with a reactive power two-way compensation function, which can absorb reactive power from the system, or Provide reactive power compensation for the system; the power flow adjustment module realizes decoupling control of the active power and reactive power of the transmission line.

Specifically, the reactive power compensation module is a voltage source converter with a reactive power bidirectional compensation function. It can absorb reactive power from the system and provide reactive power compensation for the system. The reactive power compensation module includes a cascade bridge type inverter. Inverter, cascade bridge inverter is composed of multiple sub-modules cascaded, as shown in Figure 3. Schematic diagram of typical topology examples of the cascade bridge inverter and multi-port single-phase MMC in the invented controller. The sub-modules of the cascade bridge inverter can be three-level or five-level. Yes, it can also be other multi-level sub-modules.

Specifically, the power flow adjustment module realizes decoupling control of the active power and reactive power of the transmission line. The power flow adjustment module includes a multi-port single-phase MMC, and the multi-port single-phase MMC is connected in series with a cascade bridge inverter.

Preferably, Figure 1 is a schematic diagram of the topology of the controller of the present invention and its system of interconnecting multiple transmission lines. The multi-port single-phase MMC includes a power flow regulating multi-port single-phase MMC and an energy balancing multi-port single-phase MMC. Energy balance The multi-port single-phase MMC is connected in parallel with the power flow regulating multi-port single-phase MMC. The cascade bridge inverter is connected to the AC output port of the energy-balanced multi-port single-phase MMC to realize the cascade bridge inverter and the multi-port single-phase MMC. The AC output port of the multi-port single-phase MMC for power flow regulation is connected in series with the transmission line. By adjusting the amplitude and phase of the AC output port voltage of the multi-port single-phase MMC connected in series on the transmission line, multi-port single-phase MMC power flow regulation is achieved. Common DC bus voltage stabilization of modules and decoupled control of active and reactive power in transmission lines.

Preferably, Figure 2 is a schematic diagram of the system in which the controller of the present invention omits the energy-balanced multi-port single-phase MMC connected to the cascade bridge inverter and its interconnected multi-transmission lines. The multi-port single-phase MMC The power flow regulation module includes multiple power flow regulation multi-port single-phase MMCs that share the same common DC bus and are connected in parallel with each other. The AC output port of the power flow regulation multi-port single-phase MMC is connected in series with the transmission line. By adjusting the power flow in series on the transmission line The amplitude and phase of the multi-port single-phase MMC AC output port voltage realizes active control of the active power and reactive power of the transmission line.

Further, Figure 4 is a schematic diagram of the topology of the multi-port single-phase MMC power flow adjustment module of the controller of the present invention. When the multi-port single-phase MMC power flow adjustment module only includes a power flow adjustment multi-port single-phase MMC, the cascade bridge type The inverter is directly connected to the positive or negative pole of the public DC bus, which can realize the series connection of the cascade bridge inverter and the multi-port single-phase MMC power flow regulation module. When the multi-port single-phase MMC power flow regulation module also includes a power flow regulation multi-port For single-phase MMC and energy-balanced multi-port single-phase MMC, the AC output port of the cascade bridge inverter can be connected to the AC output port of the energy-balanced multi-port single-phase MMC, and the cascade bridge inverter can also be connected to Series connection of multi-port single-phase MMC power flow regulation modules.

Furthermore, the control system uses a transformerless generalized unified power flow controller to realize the device topology and system connection of the power flow control of the transmission lines at both ends. The transformerless generalized unified power flow controller includes a cascaded bridge type with a three-phase cascaded full-bridge topology. Inverter and multi-port single phase in series with it MMC power flow regulation module.

Specifically, the multi-port single-phase MMC power flow regulation module includes three MMCs of single-phase half-bridge sub-modules that share the same common DC bus. The MMCs of the three single-phase half-bridge sub-modules are connected to two AC transmission lines and stages respectively. The bridge-type inverters are connected one by one, and by adjusting the series compensation voltage connected in series on the transmission line, the AC component of the MMC AC output port voltage of the single-phase half-bridge type sub-module connected in series with the cascaded bridge-type inverter, The amplitude and phase of the AC component of the AC output port voltage of the cascade bridge type inverter, where the series compensation voltage is the AC component of the AC output port voltage of the power flow regulating multi-port single-phase MMC connected in series on the transmission line. On the one hand, The internal energy balance of the transformerless generalized unified power flow controller, on the other hand, realizes the active control of the active power and reactive power on the AC transmission line, that is, the decoupling control of the line power flow.

Example 2

Referring to Figure 5, an embodiment of the present invention provides a control method based on a transformerless generalized unified power flow controller, including:

Line power flow control loop, cascade bridge inverter control loop, common DC bus voltage balance control loop, circulating current suppression control and bridge arm voltage balance control.

Further, the line power flow control loop is the main control including:

The control goal of the line power flow control loop is for the active power of the power flow control transmission line to reach the reference value and the reactive power to reach the reference value.

Preferably, set the line power flow control loop output to

in, Reference value for the AC component of the output voltage of the AC output port of a power flow regulated multi-port single-phase MMC connected to a power flow regulated balanced transmission line, is the reference value of the AC component of the output voltage of the power flow regulating multi-port single-phase MMC AC output port connected to the power flow control transmission line. The subscript i indicates that the i-th transmission line is a power flow regulating balanced transmission line, and the subscript j indicates the j-th transmission line. A power flow control transmission line.

Specifically, the line power flow control loop first calculates the d-axis component reference value of the power flow control transmission line current based on the active power reference value and reactive power reference value of the power flow control transmission line. and q-axis component reference value The calculation method is to solve the following system of equations:

Furthermore, the line power flow control loop is carried out in the dq coordinate system and controlled by a proportional integral controller. The mathematical equation is:

in, and Multiply by Park's inverse transformation matrix to obtain the output reference voltage of the line power flow control loop in the abc coordinate system Right now V represents the node voltage of the transmission line, I represents the current of the transmission line, ω represents the AC frequency of the transmission line, L represents the equivalent inductance value of the transmission line, L _PFCM represents the equivalent inductance value of the multi-port single-phase MMC bridge arm , R represents the equivalent resistance value of the transmission line, the subscript i of V, I, L and R represents the parameters of the power flow adjustment and balancing transmission line, the subscript j represents the parameter of the jth power flow control transmission line, the subscript d represents the d-axis component, the subscript q represents the q-axis component, the superscript * represents the reference value, k _p is the gain coefficient of the proportional link of the proportional integral controller, k _i is the gain coefficient of the integral link of the proportional integral controller, V _id , V _iq , V _jd , V _jq , are feedforward terms, which function to enhance the anti-interference ability of the control loop and speed up the response speed of the control loop.

in, It is a decoupling term, and its function is to realize the decoupling control of d-axis and q-axis.

Further, the cascade bridge inverter control loop is the main control including:

Voltage control outer loop, reactive power control outer loop and current control inner loop.

Specifically, the control goal of the cascade bridge inverter control loop is to regulate the power flow and balance the reactive power of the transmission line to reach the reference value. The sum of the three-phase capacitor voltages of the cascade bridge inverter is stable as the reference value And output is the reference value of the AC component of the voltage at the AC output port of the cascade bridge inverter.

Preferably, the reactive power control outer loop adjusts the reactive power reference value of the balanced transmission line according to the power flow. Calculate the reference value of the q-axis component of the current in the balanced transmission line for power flow regulation The calculation formula is:

Furthermore, the current control inner loop is controlled in the dq coordinate system, and the proportional integral controller is used to control the d-axis component and q-axis component of the power flow adjustment balance transmission line current respectively. The mathematical equation is:

in, and After multiplying by the Park inverse transformation matrix and then multiplying by -1 for inversion, the output reference voltage of the cascade bridge inverter control loop in the abc coordinate system is obtained. That is, V _pa , V _pb , V _pc , V _Cid , V _Ciq are the d-axis component and q-axis component of the AC component of the power flow regulating multi-port single-phase MMC AC output port voltage connected to the power flow regulating balanced transmission line, V _SMd and V _SMq is the d-axis component and q-axis component of the AC component of the energy-balanced multi-port single-phase MMC AC output port voltage connected to the cascade bridge inverter, k _p2 is the proportional link gain coefficient of the proportional integral controller, k _i2 is the gain coefficient of the integral link of the proportional integral controller, and V _id , V _iq , V _Cid , V _Ciq , V _SMd , and V _SMq are feedforward terms. Their function is to enhance the anti-interference ability of the control loop and speed up the response speed of the control loop.

in, It is a decoupling term, and its function is to realize the decoupling control of d-axis and q-axis.

Further, the common DC bus voltage balance control loop is the main control including:

The control goal of the common DC bus voltage balance control loop is to stabilize the common DC bus voltage to a reference value. And the output is the common DC bus balanced reference voltage

Preferably, the control of the common DC bus voltage balance control loop is carried out in the abc coordinate system, and the proportional integral controller is used to control the abc three-phase public DC bus voltage. The mathematical equation is:

Among them, V _linka , V _linkb , and V _linkc are the three-phase public DC bus voltages, I _pa , I _pb , and I _pc are the three-phase currents of the cascade bridge inverter branch. is the feedforward three-phase reference voltage, k _p3 is the gain coefficient of the proportional link of the proportional-integral controller, and k _i3 is the gain coefficient of the integral link of the proportional-integral controller.

Furthermore, if the transformerless generalized unified power flow controller topology includes an energy-balanced multi-port single-phase MMC connected to the cascade bridge inverter, the output reference voltage of the line power flow control loop can be obtained Finally, the reference voltage of the AC output port of the power flow regulating multi-port single-phase MMC is:

in, AC multiport single-phase MMC for power flow regulation connected to power flow regulation balanced transmission lines The reference voltage of the output port, The reference voltage of the AC output port of the multi-port single-phase MMC for the power flow connected to the jth power flow control transmission line, is the output voltage of the power flow control loop, It is the power flow adjustment multi-port single-phase MMC AC output port connected to the power flow adjustment balanced transmission line calculated according to any one of the series compensation voltage distribution methods in the distribution method based on the transformerless generalized power flow controller provided by the present invention. The AC component of the reference voltage, the feedforward three-phase reference voltage in the common DC bus voltage balance control loop, Equal to zero, the reference voltage of the energy-balanced multi-port single-phase MMC AC output port is:

Specifically, the reference voltage of the AC output port of the cascade bridge inverter is:

Furthermore, if the transformerless generalized unified power flow controller topology does not include an energy-balanced multi-port single-phase MMC connected to the cascade bridge inverter, the output reference voltage of the line power flow control loop is obtained Finally, the reference voltage of the AC output port of the power flow regulating multi-port single-phase MMC is:

in, Reference voltage for the AC output port of the power flow regulated multi-port single phase MMC connected to the power flow regulated balanced transmission line, The reference voltage for the power flow regulating multiport single-phase MMC AC output port connected to the jth power flow controlled transmission line, the feedforward three-phase reference voltage in the common DC bus voltage balance control loop, The power flow adjustment multi-port single-phase MMC AC output port connected to the power flow adjustment balanced transmission line calculated by any one of the series compensation voltage allocation methods in the distribution method based on the transformerless generalized power flow controller provided by the present invention. The AC component of the reference voltage, the feedforward terms V _SMd and V _SMq in the current control inner loop of the cascade bridge inverter control loop are equal to zero, and the reference voltage of the AC output port of the cascade bridge inverter is:

Specifically, a plus sign is taken when the AC output port of the cascade bridge inverter is connected to the negative pole of the common DC bus, and a minus sign is taken when the AC output port of the cascade bridge inverter is connected to the positive pole of the public DC bus.

Further, circulation suppression control includes:

The control goal of the circulating current suppression control is to suppress the circulating current between the multi-port single-phase MMCs of each phase. The output of the circulating current suppressing control is the double frequency voltage that generates the circulating current. By compensating the sub-module modulation signal, the circulating current is suppressed.

Preferably, the circulation suppression control is carried out in the abc coordinate system, and a quasi-proportional resonance controller is used to suppress the intra-phase circulation of the abc three-phase. The mathematical equation is:

Among them, G _QPR (s) is the transfer function of the quasi-PR controller, ω is the resonant frequency, ω _c is the cut-off frequency that mainly affects the system bandwidth, k _p4 is the gain coefficient of the proportional link of the quasi-proportional resonant controller, k _r is the quasi-proportional resonance Controller resonance link gain coefficient, is the circulation compensation voltage of the power flow-regulated multi-port single-phase MMC submodule connected to the k-th transmission line, s is the model variable under Laplace change, is the energy balancing multi-port single-phase MMC sub-module circulating current compensation voltage, i _lkp and i _lkn are the power flow regulation multi-port single-phase MMC upper bridge arm current and lower bridge arm current connected to the kth transmission line, and the subscript l is l Phase parameters, the phase-locked loop locks the power flow and adjusts the node three-phase voltage of the balanced transmission line to obtain the phase of the locked line. The phase angle output by the phase-locked loop provides the angle for the Park transformation matrix from the abc coordinate system to the dq coordinate system.

Further, bridge arm voltage equalization control includes:

The bridge arm voltage balance control is realized in two ways: the DC component adjustment of the bridge arm voltage and the injection fundamental frequency circulating current adjustment.

Preferably, the method of adjusting the DC component of the bridge arm voltage includes adjusting the DC components of the upper and lower bridge arms to control the balance of the capacitor voltages of the upper and lower bridge arms, and outputting it as a multi-port single-phase MMC upper bridge arm and lower bridge arm sub-module voltage DC component reference value , the bridge arm voltage balance control uses the PI controller to control the DC component of the bridge arm voltage of the abc three-phase multi-port single-phase MMC. The mathematical equation is:

in, is the reference value of the DC component of the upper arm voltage, is the reference value of the DC component of the lower arm voltage, is the reference value of the average voltage of the upper and lower arms, V _average,lkp and V _averagelkp are the average values of the voltages of the upper and lower arms, V _link,l is the DC bus voltage, and the subscript k is connected to the kth transmission line The parameters of the power flow regulating multi-port single-phase MMC, the subscript l is the parameter of the l phase, k _p5 is the gain coefficient of the proportional link of the proportional integral controller, k _i5 is the gain coefficient of the integral link of the proportional integral controller.

Preferably, the adjustment method of injecting fundamental frequency circulating current includes adjusting the bridge arm voltage by injecting fundamental frequency circulating current. Balance control achieves the balance of upper and lower bridge arm voltages by injecting fundamental frequency circulating current with the same phase as the voltage. Its output is the reference value of the injected fundamental frequency circulating current. The mathematical equation is:

in, is the reference value of the fundamental frequency circulation injected into the power flow regulating multi-port single-phase MMC connected to the k-th transmission line, V _average,lkp and V _average,lkn are the upper and lower bridge arm voltage averages of the power flow regulating multi-port single-phase MMC, θ _k Adjust the phase of the fundamental frequency component of the multi-port single-phase MMC output voltage for the power flow connected to the k-th transmission line, k _p6 is the gain coefficient of the proportional link of the proportional-integral controller, and k _i6 is the gain coefficient of the integral link of the proportional-integral controller.

Example 3

This embodiment is the third embodiment of the present invention, which provides a distribution method based on a transformerless generalized unified power flow controller, including:

The allocation method of the controller is any group that satisfies the basic condition equations solution.

Specifically, the basic condition equation is:

Among them, it is assumed that the first transmission line is a power flow regulation balanced transmission line, Real is the real part, is the vector expression of the AC component of the series compensation voltage on the kth transmission line by the transformerless generalized unified power flow controller, In order to achieve the target power flow on the k-th transmission line, the series compensation voltage vector expression required in series between the first transmission line and the k-th transmission line is, is the vector expression of the AC component of the energy balance multi-port single-phase MMC AC output port voltage in the multi-port single-phase MMC power flow regulation module connected to the cascade bridge inverter, is the conjugate vector expression of the alternating current on the k-th transmission line, is the conjugate vector expression of the AC current of the cascade bridge inverter branch, and n is the number of transmission lines interconnected through the transformerless generalized unified power flow controller.

Preferably, a transformerless generalized unified power flow controller with active power flow control capability suitable for AC power grids is used to distribute the compensation voltage in series on the distribution network transmission lines. It is characterized by simplicity.

Preferably, another method of allocating the series compensation voltage is Right now The choice satisfies Therefore, the energy-balanced multi-port single-phase MMC connected to the cascade bridge inverter can be topologically omitted.

Preferably, another method of allocating the series compensation voltage is The choice satisfies To obtain the minimum value, the AC component amplitude of the required output voltage of the single-phase converter can be minimized.

Preferably, the distribution method of the series compensation voltage can also be any selection method that satisfies the basic condition equation.

Preferably, the voltage component of the AC output port of the single-phase converter in the multi-port single-phase MMC power flow regulation module contains different AC components required for control.

Example 4

Referring to Figures 6 to 7, an embodiment of the present invention is provided, which provides a control system based on a transformerless generalized unified power flow controller, including:

As shown in Figure 6 is a control system diagram of the present invention, the core equipment of the AC power grid multi-transmission line power flow control system is a transformerless generalized unified power flow controller, and the other components are AC transmission lines and substations. It should be noted:

The transformerless generalized unified power flow controller is installed at the gathering point of multiple AC transmission lines. The AC transmission lines are connected to the transformerless generalized unified power flow controller. The AC transmission lines can be connected to the high-voltage system through a step-up transformer, and can also be connected to the high-voltage system through a step-down transformer. The transformer is connected to the low-voltage AC system, and the low-voltage AC system can be connected to low-voltage AC loads or energy storage equipment or electric vehicle fast charging stations.

Specifically, as shown in Figure 7, the multi-port single-phase MMC power flow adjustment module of the control system of the present invention adopts a two-level half-bridge type sub-module, and the cascade bridge-type inverter adopts a dual-port power flow control device with a cascade full-bridge topology. Topology and schematic diagram of the system for interconnecting two transmission lines. The power flow control system of both ends of the transmission line implemented by the transformerless generalized unified power flow controller. The internal energy balance of the transformerless generalized unified power flow controller is expressed as the common DC bus. If the capacitor voltage remains stable and the capacitor voltage in the cascade bridge inverter remains stable, the active power flowing into the above capacitor is required to remain zero, that is:

Among them, the equation in the first row expresses that the active power flowing into the common DC bus capacitor is zero, and the equation in the second row expresses that the active power flowing into the cascade bridge inverter capacitor is zero, Represents the AC component vector expression of the voltage at the MMC AC output port of the single-phase half-bridge submodule connected to transmission line 1, Represents the AC component vector expression of the voltage at the MMC AC output port of the single-phase half-bridge submodule connected to the transmission line 2, Represents the AC component vector expression of the voltage at the MMC AC output port of the single-phase half-bridge submodule connected to the cascaded bridge inverter, Represents the AC component vector expression of the voltage at the AC output port of the cascade bridge inverter, Represents the conjugate vector expression of the current in transmission line 1, Represents the conjugate vector expression of the current in transmission line 2, Expresses the conjugate vector expression of the branch current of the cascade bridge inverter. by regulating and The amplitude and size of , make the above equation hold, that is, the internal energy balance of the transformerless generalized unified power flow controller is achieved.

Specifically, a transformerless generalized unified power flow controller is used to realize power flow control of three transmission lines. In this implementation case, the transformerless generalized unified power flow controller includes a three-phase cascaded full-bridge topology inverter and a multi-port single-phase MMC power flow regulation module connected in series. The multi-port single-phase MMC power flow regulation module The module contains four MMCs of single-phase half-bridge sub-modules that share the same common DC bus. The MMCs of the four single-phase half-bridge sub-modules are respectively connected to three AC transmission lines and cascaded H-bridge inverters. By adjusting the series compensation voltage connected in series on the feeder, the AC component of the AC output port voltage of the MMC of the single-phase half-bridge submodule connected in series with the cascade bridge inverter, the AC output port of the cascade bridge inverter The amplitude and phase of the AC component of the voltage, on the one hand, realizes the internal energy balance of the transformerless generalized unified power flow controller, and on the other hand, realizes the active control of the active power and reactive power on the AC feeder, that is, the decoupling of the line power flow. control.

Example 5

Referring to Figure 8, an embodiment of the present invention provides an equipment protection method for a controller, including overvoltage protection of voltages connected in series on transmission lines, multi-port single-phase MMC sub-module fault bypass protection and cascade bridge type Inverter sub-module protection.

Specifically, the overvoltage protection of the voltage connected in series on the transmission line is to protect the overvoltage of the voltage connected in series on the transmission line through a protection device.

Specifically, Figure 8 is a wiring diagram and a schematic diagram of the protection device of the voltage overvoltage protection method of the equipment protection method of the present invention. The protection device is composed of a metal oxide voltage limiter and a thyristor bypass switch connected in parallel. The metal oxide voltage limiter To limit the voltage to a safe voltage, the protection device is connected in parallel with the AC output port of the power flow regulating multi-port single-phase MMC in the multi-port single-phase MMC power flow regulating module. Protect against overvoltage of the voltage connected to the transmission lines.

Specifically, multi-port single-phase MMC sub-module fault bypass protection includes:

The multi-port single-phase MMC sub-module is connected in parallel with the thyristor bypass switch through the AC output port of the multi-port single-phase MMC sub-module, so that when the multi-port single-phase MMC sub-module fails, the failed sub-module can be quickly bypassed through the thyristor bypass switch. At the same time, redundant sub-modules are invested so that the failure of the sub-module does not affect the overall operation of the equipment. The DC side of the multi-port single-phase MMC sub-module is equipped with a DC unloading circuit for capacitive energy release to prevent overvoltage damage to power devices. , to protect multi-port single-phase MMC sub-module fault bypass.

Specifically, the thyristor bypass switch is an anti-parallel thyristor, a resistor-capacitor loop, and a static resistor connected in parallel and then in series with a saturated reactor. The thyristor bypass switch bypasses the AC output port of the power flow regulating multi-port single-phase MMC to achieve overflow. pressure protection.

Further, cascade bridge inverter sub-module protection includes:

The cascade bridge inverter sub-module is connected in parallel with the fast mechanical switch through the AC port of the cascade bridge inverter sub-module, and the DC side of the cascade bridge inverter is equipped with a DC unloading circuit that releases capacitive energy. The bridge type inverter sub-module is used for protection.

Cascade bridge inverter sub-module protection includes the AC port of the cascade bridge inverter sub-module being connected in parallel with a fast mechanical switch, so that when the cascade bridge inverter sub-module fails, the faulty module can be quickly removed.

Furthermore, when an external transmission line fails, the transformerless generalized unified power flow controller blocks all sub-modules and triggers the protection device and the thyristor bypass switch at the same time. When the protection device is triggered, the voltage in series on the transmission line is clamped. to about 0V, after the parallel cascade bridge inverter blocks all sub-modules, due to Then regardless of the direction of the current, the capacitor can be charged, where M is the number of sub-modules per phase of the cascade bridge inverter, V _C is the DC voltage of the sub-module, U _phase is the effective value of the grid phase voltage, and U _ab is the grid line Voltage rms value. Since the sum of the DC voltages of the capacitors in each phase is greater than the phase voltage amplitude at the grid-connected point, when STATCOM is blocked, the sum of the DC voltages of the capacitors in the charging circuit is greater than the two-point charging voltages, and the diodes are turned off due to the reverse voltage drop, and cannot charge the capacitors. Charging is carried out and the charging current is zero.

Example 6

Referring to Figures 9 to 11, another embodiment of the present invention is shown. Combined with the above implementation case, MATLAB/Simulink software is used to simulate and verify the system. The simulation parameters are as shown in the following table:

Specifically, the interconnection system of three transmission lines with flexible interconnection is implemented by a transformerless generalized unified power flow controller. Refer to Figure 9 for the connection schematic diagram. The multi-port single-phase MMC power flow adjustment module of the control system of the present invention adopts a two-level half-bridge. The topological structure of the three-port power flow control device using the cascaded full-bridge topology of the sub-module and cascade bridge inverter and its system diagram for interconnecting three transmission lines.

Specifically, the control method of the simulation example is shown in Figure 10. The multi-port single-phase MMC power flow adjustment module of the control method of the present invention adopts MMC and cascade bridge-type inverter with two-level single-phase half-bridge sub-modules in parallel. Block diagram of the control method of a three-port power flow control device using cascaded full-bridge topology. The transformerless generalized unified power flow controller contains four MMCs of single-phase half-bridge sub-modules, among which three single-phase half-bridges are connected to the AC transmission line. The MMC of the bridge sub-module controls the active power and reactive power on transmission line 2 and transmission line 3, and the corresponding control loop is the line power flow control loop.

Specifically, the MMC of the single-phase half-bridge submodule connected to the cascade bridge inverter controls the common DC bus voltage balance, and the corresponding control loop is the common DC bus voltage balance control loop. The cascade bridge inverter compensates the reactive power on the transmission line 1, and the corresponding control loop is the cascade bridge inverter control loop.

Specifically, the simulation example of the distribution method of the series compensation voltage of the transformerless generalized unified power flow controller on the distribution network transmission line considers the optimization target as the output required by the MMC of the single-phase half-bridge submodule. The AC component of the output voltage has the smallest amplitude, that is The choice satisfies Get the minimum value.

Furthermore, in order to verify the active power flow control capability of the transformerless generalized unified power flow controller, three operating conditions were set in the simulation:

Working condition 1: Node 1 emits 0.2p.u. reactive power, node 2 absorbs 0.5p.u. active power and emits 0.2p.u. reactive power, node 3 absorbs 0.5p.u. active power and emits 0.2p.u. reactive power.

Working condition 2: Node 1 emits 0.2p.u. reactive power, node 2 absorbs 0.2p.u. active power and emits 0.2p.u. reactive power, node 3 absorbs 0.8p.u. active power and emits 0.2p.u. reactive power.

Working condition 3: Node 1 emits 1p.u. reactive power, node 2 emits 0.2p.u. active power and absorbs 0.2p.u. reactive power, node 3 absorbs 0.5p.u. active power and emits 0.8p.u. reactive power.

Further, Figure 11 shows the simulation results of the three working conditions of the present invention, including a total of 11 waveform diagrams. From left to right and from top to bottom, they are: transmission line 1 active power P ₁ waveform diagram, transmission line 1 reactive power Power Q ₁ waveform diagram, transmission line 2 active power P ₂ waveform diagram, transmission line 2 reactive power Q ₂ waveform diagram, transmission line 3 active power P ₃ waveform diagram, transmission line 3 reactive power Q ₃ waveform diagram, transmission line Voltage waveform diagram of multi-port single-phase MMC capacitor connected to 1, voltage waveform diagram of multi-port single-phase MMC capacitor connected to transmission line 2, voltage waveform diagram of multi-port single-phase MMC capacitor connected to transmission line 3, and cascade bridge type inverter The multi-port single-phase MMC capacitor voltage waveform connected to the inverter and the cascade bridge inverter capacitor voltage waveform.

Furthermore, the simulation waveform results show that when three transmission lines are interconnected, the transformerless generalized unified power flow controller not only realizes active power flow control with active power and reactive power decoupling on the transmission lines, but also maintains the stability of the device. Internal internal energy balance, that is, stable capacitor voltage, and port expansion capabilities.

Furthermore, compared with the power flow controllers currently proposed, the topology of the present invention also has independent power flow adjustment capabilities. The specific details are compared as shown in the table below:

The traditional unified power flow controller contains a bulky power frequency transformer, the overall volume of the equipment is large, and the cost of operation and maintenance is high. When expanding the power flow control port, it is necessary to add a series converter and a power frequency transformer, which makes the port of the device Expansion costs are higher. Although the T-type transformerless power flow controller eliminates the bulky power frequency transformer and has advantages in size and cost, it needs to sacrifice a degree of control freedom to maintain the balance of the controller power, so it cannot control the reactive power at the source end, and The ports of the T-type transformerless power flow controller cannot be expanded. Both back-to-back voltage source converters and Hexverter-based power flow controllers are full-power topologies, which make the equipment larger and the cost of port expansion higher. The power flow controller proposed by the present invention has independent power flow control capability, small equipment size, low operation and installation costs, easy port expansion, and low expansion costs.

It should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solution of the present invention can be carried out. Modifications or equivalent substitutions without departing from the spirit and scope of the technical solution of the present invention shall be included in the scope of the claims of the present invention.

Claims (17)

1. A transformerless generalized unified power flow controller, which is characterized by including:

   A reactive power compensation module, the reactive power compensation module includes a cascade bridge inverter;

   A power flow adjustment module, the power flow adjustment module includes a multi-port single-phase MMC;

   Wherein, the multi-port single-phase MMC is connected in series with the cascade bridge inverter;

   The reactive power compensation module is a voltage source converter with a reactive power two-way compensation function, which can absorb reactive power from the system and provide reactive power compensation for the system;

   The power flow regulation module realizes decoupling control of the active power and reactive power of the transmission line.
2. A transformerless generalized unified power flow controller as claimed in claim 1, characterized in that: the cascaded bridge inverter is formed by cascading multiple sub-modules.
3. A transformerless generalized unified power flow controller as claimed in claim 1, characterized by:

   The multi-port single-phase MMC includes a power flow regulating multi-port single-phase MMC and an energy balancing multi-port single-phase MMC;

   The energy balancing multi-port single-phase MMC and the power flow regulating multi-port single-phase MMC are connected in parallel;

   The cascade bridge inverter is connected to the AC output port of the energy-balanced multi-port single-phase MMC to realize the series connection of the cascade bridge inverter and the multi-port single-phase MMC;

   The AC output port of the power flow regulating multi-port single-phase MMC is connected in series with the transmission line. By adjusting the amplitude and phase of the AC output port voltage of the multi-port single-phase MMC connected in series on the transmission line, the active power and reactive power of the transmission line are realized. Decoupled control.
4. A control method based on the transformerless generalized unified power flow controller according to any one of claims 1 to 3, characterized in that it includes:

   The main control modules include line power flow control loop, cascade bridge inverter control loop and public DC bus balance control loop, which realize the basic functions of the controller and ensure normal operation of the control;

   Circulating current suppression control and bridge arm voltage balancing control are used to suppress the phenomenon of circulating current and voltage imbalance in the power flow adjustment module and improve the performance of the controller;

   The phase-locked loop locks the power flow to adjust the three-phase voltage of the node of the balanced transmission line to obtain the phase of the locked line. The phase angle output by the phase-locked loop provides the angle for the Park transformation matrix from the abc coordinate system to the dq coordinate system.
5. The control method according to claim 4, characterized in that: the control target of the line power flow control loop is:

   Obtain the reference value of the active power of the power flow control transmission line and reactive power reference values

   Set line power flow control loop output

   in, Reference value for the AC component of the output voltage of the AC output port of a power flow regulated multi-port single-phase MMC connected to a power flow regulated balanced transmission line, is the reference value of the AC component of the output voltage of the power flow regulating multi-port single-phase MMC AC output port connected to the power flow control transmission line. The subscript i indicates that the i-th transmission line is a power flow regulating balanced transmission line, and the subscript j indicates the j-th transmission line. A power flow control transmission line.
6. The control method according to claim 4, characterized in that: the cascade bridge inverter control loop includes:

   Voltage control outer loop, reactive power control outer loop and current control inner loop;

   The control goal of the cascade bridge inverter control loop is to regulate the power flow and balance the reactive power of the transmission line to reach a reference value. The sum of the three-phase capacitor voltages of the cascade bridge inverter is stable as the reference value The output of the cascade bridge inverter control loop is the reference value of the AC component of the voltage at the AC output port of the cascade bridge inverter.

   The reactive power control outer loop adjusts the reactive power reference value of the balanced transmission line according to the power flow. Calculate the reference value of the q-axis component of the current in the balanced transmission line for power flow regulation

   The current control inner loop is controlled in the dq coordinate system, and uses a proportional integral controller to control the d-axis component and q-axis component of the power flow adjustment balance transmission line current respectively. The mathematical equation is:
   

   in, and After multiplying by the Park inverse transformation matrix and then multiplying by -1 for inversion, the output reference voltage of the cascade bridge inverter control loop in the abc coordinate system is obtained. That is, V _pa , V _pb , V _pc ;

   Among them, V _Cid and V _Ciq are the d-axis component and q-axis component of the AC component of the AC output port voltage of the power flow regulating multi-port single-phase MMC connected to the power flow regulating balanced transmission line, V _SMd and V _SMq are related to the cascade bridge type The d-axis component and q-axis component of the AC component of the energy-balanced multi-port single-phase MMC AC output port voltage connected to the inverter, k _p2 is the proportional link gain coefficient of the proportional integral controller, k _i2 is the integral link gain of the proportional integral controller Coefficients, V _id , V _iq , V _Cid , V _Ciq , V _SMd , and V _SMq are feedforward terms. Their function is to enhance the anti-interference ability of the control loop and speed up the response speed of the control loop. It is a decoupling term, and its function is to realize the decoupling control of d-axis and q-axis.
7. The control method according to claim 4, characterized in that: the control target of the common DC bus voltage balance control loop is to stabilize the common DC bus voltage to a reference value. Common DC bus voltage is flat The output of the balancing control loop is the common DC bus balancing reference voltage.

   The control of the common DC bus voltage balance control loop is carried out in the abc coordinate system. The proportional integral controller is used to control the abc three-phase public DC bus voltage. The mathematical equation is:
   

   Among them, V _linka , V _linkb , and V _linkc are the three-phase public DC bus voltages, I _pa , I _pb , and I _pc are the three-phase currents of the cascade bridge inverter branch. is the feedforward three-phase reference voltage, k _p3 is the gain coefficient of the proportional link of the proportional-integral controller, and k _i3 is the gain coefficient of the integral link of the proportional-integral controller.
8. The control method according to claim 4, characterized in that: the control target of the circulating current suppression control is to suppress the circulating current between the multi-port single-phase MMCs of each phase, and the output of the circulating current suppressing control is the double frequency voltage that generates the circulating current. , by compensating the sub-module modulation signal, the suppression of circulating current is achieved;

   The circulation suppression control is carried out in the abc coordinate system, and a quasi-proportional resonance controller is used to suppress the intra-phase circulation of the abc three-phase. The mathematical equation is:
   

   Among them, G _QPR (s) is the transfer function of the quasi-PR controller, ω is the resonant frequency, ω _c is the cut-off frequency that mainly affects the system bandwidth, k _p4 is the gain coefficient of the proportional link of the quasi-proportional resonant controller, k _r is the quasi-proportional resonance Controller resonance link gain coefficient, The circulating current compensation voltage of the power flow-regulated multi-port single-phase MMC submodule connected to the k-th transmission line, s is the model variable under Laplace change, is the energy balancing multi-port single-phase MMC sub-module circulating current compensation voltage, i _lkp and i _lkn are the power flow regulating multi-port single-phase MMC upper bridge arm current and lower bridge arm current connected to the kth transmission line, and the subscript l is l phase parameters.
9. The control method as claimed in claim 4, characterized in that: the bridge arm voltage balance control includes two implementation methods: bridge arm voltage DC component adjustment and injection fundamental frequency circulating current adjustment.
10. The control method of the controller according to claim 9, characterized in that: the adjustment method of the DC component of the bridge arm voltage includes adjusting the DC component of the upper and lower bridge arms to control the balance of the upper and lower bridge arm capacitor voltages, and the output is a multi-port single-phase MMC. Reference values of the voltage DC components of the upper arm and lower arm submodules;

    The bridge arm voltage equalization control uses a PI controller to control the abc three-phase multi-port single-phase MMC bridge The DC component of the arm voltage is controlled, and the mathematical equation is:
    

    Among them, V ^* _{arm, lkp} is the reference value of the DC component of the upper arm voltage, is the reference value of the DC component of the lower arm voltage, is the reference value of the average voltage of the upper and lower arms, V _average,lkp and V _average,klp are the average values of the voltages of the upper and lower arms, V _link,l is the DC bus voltage, and the subscript k is the kth transmission line The parameters of the connected power flow regulating multi-port single-phase MMC, the subscript l is the parameter of l phase, k _p5 is the gain coefficient of the proportional link of the proportional integral controller, k _i5 is the gain coefficient of the integral link of the proportional integral controller.
11. The control method of the controller according to claim 9, characterized in that: the adjustment method of the injected fundamental frequency circulation adjustment includes:

    The bridge arm voltage balance control of the fundamental frequency circulation injection adjustment achieves the balance of the upper and lower bridge arm voltages by injecting the fundamental frequency circulation with the same phase as the voltage. The output of the injection fundamental frequency circulation adjustment is the reference value of the injected fundamental frequency circulation. Mathematics The equation is:
    

    in, is the reference value of the fundamental frequency circulation injected into the power flow regulating multi-port single-phase MMC connected to the k-th transmission line, V _average,lkp and V _average,lkn are the upper and lower bridge arm voltage averages of the power flow regulating multi-port single-phase MMC, θ _k Adjust the phase of the fundamental frequency component of the multi-port single-phase MMC output voltage for the power flow connected to the k-th transmission line, k _p6 is the gain coefficient of the proportional link of the proportional-integral controller, and k _i6 is the gain coefficient of the integral link of the proportional-integral controller.
12. A distribution method based on the transformerless generalized unified power flow controller as described in any one of claims 1 to 3, characterized by including:

    The allocation method of the controller is to satisfy any group of basic condition equations solution;

    Among them, the basic equation is as follows:
    

    If the first transmission line is a power flow regulating balanced transmission line, Real is the real part, Vector table of the AC component of the series compensation voltage on the kth transmission line for the transformerless generalized unified power flow controller expression, In order to achieve the target power flow on the k-th transmission line, the series compensation voltage vector expression required in series between the first transmission line and the k-th transmission line is, is the vector expression of the AC component of the energy-balanced multi-port single-phase MMC AC output port voltage in the multi-port single-phase MMC power flow regulation module connected to the cascade bridge inverter, is the conjugate vector expression of the alternating current on the k-th transmission line, is the conjugate vector expression of the AC current of the cascade bridge inverter branch, and n is the number of transmission lines interconnected by the transformerless generalized unified power flow controller.
13. A control system based on the transformerless generalized unified power flow controller as described in any one of claims 1 to 3, characterized by including:

    The core equipment of the control system is a transformerless generalized unified power flow controller, AC transmission lines and substations;

    The transformerless generalized unified power flow controller is installed at the convergence of multiple AC transmission lines;

    The AC transmission line is connected to a transformerless generalized unified power flow controller. The AC transmission line can be connected to a high-voltage system via a step-up transformer, and can be connected to a low-voltage AC system via a step-down transformer. The low-voltage AC system can be connected to a low-voltage AC load, Energy storage equipment and electric vehicle fast charging stations.
14. An equipment protection method based on the transformerless generalized unified power flow controller as described in any one of claims 1 to 3, characterized by including: overvoltage protection of voltages connected in series on transmission lines, multi-port single-phase MMC Sub-module fault bypass protection and cascade bridge inverter sub-module protection are designed to avoid serious damage to the internal components of the converter under fault conditions;

    The overvoltage protection of the voltage connected in series on the transmission line is protected by a protection device against the overvoltage of the voltage connected in series on the transmission line;

    The multi-port single-phase MMC sub-module is connected in parallel with the thyristor bypass switch through the AC output port of the multi-port single-phase MMC sub-module, and the DC side of the multi-port single-phase MMC sub-module is equipped with a DC unloading circuit for capacitor energy release , prevent overvoltage from damaging power devices, and protect multi-port single-phase MMC sub-module fault bypass;

    The cascade bridge inverter sub-module is connected in parallel with a fast mechanical switch through the AC port of the cascade bridge inverter sub-module, and the DC side of the cascade bridge inverter is equipped with a DC unloading circuit that releases capacitive energy. Protect the cascaded bridge inverter sub-modules.
15. The equipment protection method according to claim 14, characterized in that: the overvoltage protection of the voltage connected in series on the transmission line includes a protection device,

    The protection device is connected with the multi-port single-phase power flow adjustment module in the multi-port single-phase MMC power flow adjustment module. The AC output ports of the phase MMC are connected in parallel to protect the overvoltage of the voltage connected in series on the transmission line;

    The protection device is composed of a metal oxide voltage limiter and a thyristor bypass switch connected in parallel;

    Among them, the thyristor bypass switch is an anti-parallel thyristor, a resistor-capacitor loop, a static resistor in parallel and then in series with a saturated reactor;

    The metal oxide voltage limiter limits the voltage to a safe voltage while the thyristor bypass switch achieves overvoltage protection by bypassing the AC output port of the power flow regulating multi-port single phase MMC.
16. The equipment protection method as claimed in claim 14, characterized in that:

    The AC output port of the multi-port single-phase MMC sub-module is connected in parallel with the thyristor bypass switch, so that when the multi-port single-phase MMC sub-module fails, the faulty sub-module can be quickly bypassed through the thyristor bypass switch, and redundant sub-modules can be put in at the same time. module, so that the failure of the sub-module does not affect the overall operation of the equipment;

    The DC side of the multi-port single-phase MMC sub-module is equipped with a DC unloading circuit for capacitive energy release to prevent overvoltage from damaging power devices.
17. The equipment protection method according to claim 14, characterized in that: the AC port of the cascade bridge inverter sub-module is connected in parallel with a fast mechanical switch, so that the cascade bridge inverter sub-module can be quickly removed when it fails. Faulty module;

    The DC side of the cascaded bridge inverter sub-module is equipped with a DC unloading circuit that releases capacitive energy to prevent overvoltage from damaging the power devices.