Low-cost bidirectional overload flexible interconnect device and control method thereof

Abstract

The application discloses a low-cost bidirectional overload flexible interconnection device and a control method thereof. The device comprises a first module, a second module, a three-phase full-bridge topology and a DC bus. The first module and the second module are of the same structure and each comprises a three-phase TNPC topology. The three-phase full-bridge topology is connected with each TNPC topology in a back-to-back mode and shares a DC bus. The first module and the second module are connected with two transformer substations through the output ends of the TNPC topologies respectively, and are used for cooperating to perform power flow scheduling in a specified direction. The output end of the three-phase full-bridge topology is connected with the output end of the TNPC topology of the first module or the second module, and is used for assisting the first module or the second module in an inverting mode to realize active overload.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electric power, and particularly relates to a low-cost bidirectional overload flexible interconnection device and a control method thereof. BACKGROUND

[0002] With the rapid growth of electric vehicles, the number of charging facilities has increased significantly, and the maximum power has reached the MW level. The huge charging demand brings great challenges to the planning and operation of distribution networks, making the distribution network electric vehicle carrying capacity problem particularly prominent. During the peak charging period, transformer overload in distribution areas is prone to occur. The traditional way of transformer expansion, line reconstruction and other ways to improve the carrying capacity faces the problems of long implementation period, low asset utilization rate and single application scenario, which is difficult to adapt to the rapid growth of charging demand.

[0003] The flexible interconnection device of the distribution area has the advantages of flexible and controllable power flow, power interconnection capacity and fast response characteristics, and has become an important technical path to improve the carrying capacity limit of the distribution network electric vehicle. The flexible interconnection device is usually connected by two or more converters through a common DC bus, and the AC side is connected to different distribution area transformers to realize load balancing and power interconnection between different distribution areas, and to alleviate the overload problem of distribution network transformers caused by the rapid increase of electric vehicle charging load in some distribution areas. In the topology selection of single-port flexible interconnection device, the TNPC (T-type neutral point clamped) topology has the advantages of simple structure and control method and high efficiency, and is suitable for scenarios with high power density and high reliability. However, when the overload of the distribution area is serious and the flexible interconnection device needs to interconnect in excess capacity, the switching devices in the device may be damaged due to temperature exceeding the limit, which may cause the device to be off the grid, resulting in the loss of power interconnection capacity between distribution areas, and further exacerbating the negative impact of transformer overload in the distribution area. In order to improve the overload capacity of the flexible interconnection device, the existing research mainly focuses on the following aspects:

[0004] 1. Control parameter adjustment: The junction temperature of the device in the converter is related to the control parameters. By adjusting the switching frequency, duty cycle and other control parameters set by the control system, the junction temperature control can be realized, but the adjustment effect of this method is limited by the thermal characteristics of the device. For different devices, the control method has different effects, and in complex working conditions, the parameter adjustment needs to be considered in detail, otherwise there may be a risk of control instability.

[0005] 2. Optimization of heat dissipation design: The junction temperature of the device in the converter is related to the design of the heat dissipation system. By changing the heat dissipation material, such as using phase change material; using different heat dissipation system architecture, such as using copper pipe heat dissipation, using liquid cooling heat dissipation, etc., the heat dissipation efficiency can be improved, and the maximum junction temperature can be reduced under the same working condition, but this method is difficult to deal with the instantaneous temperature rise caused by the change of converter working condition.

[0006] 3. Hardware overcapacity: for devices with overload requirements, the engineering often uses overcapacity to make the device have overload capacity, that is, to increase the capacity of the cold standby device. This method will lead to increased device cost, land cost, and reduced system utilization.

[0007] Therefore, there is an urgent need for a new technical solution to solve the technical problem of how to improve the overload capacity of a flexible interconnection device. SUMMARY

[0008] The present application provides a low-cost bidirectional overload flexible interconnection device and a control method thereof to solve the technical problem of how to improve the overload capacity of a flexible interconnection device.

[0009] To achieve the above-mentioned purpose, the present application provides a low-cost bidirectional overload flexible interconnection device, comprising a first module, a second module, a three-phase full-bridge topology and a DC bus; the first module and the second module are structurally identical and each comprises a three-phase TNPC topology;

[0010] The three-phase full-bridge topology and each TNPC topology are connected in back-to-back with the DC bus; the first module and the second module are connected to two area transformers through the output ends of the TNPC topologies, respectively, for cooperating to perform power flow scheduling in a specified direction; the output end of the three-phase full-bridge topology is connected to the output end of the TNPC topology of the first module or the second module, for assisting the first module or the second module in inverting mode to realize active overload.

[0011] Preferably, the first module and the second module further comprise a three-phase auxiliary diode group;

[0012] The auxiliary diode group comprises positive bridge arm diodes and negative bridge arm diodes; in each phase, the cathodes of the positive bridge arm diodes are connected to the positive pole of the DC bus, the anodes of the negative bridge arm diodes are connected to the negative pole of the DC bus, and the anodes of the positive bridge arm diodes and the cathodes of the negative bridge arm diodes are connected to the output end of the TNPC topology; the three-phase auxiliary diode group is used to shunt the three-phase TNPC topology in rectifying mode.

[0013] Preferably, the assisting the first module or the second module in inverting mode to realize active overload comprises:

[0014] When the device receives a power flow scheduling instruction indicating that short-time overload is required, the three-phase full-bridge topology is connected to the output end of the TNPC topology of the first module or the second module in inverting mode through the output end, for shunting the first module or the second module in inverting mode.

[0015] Preferably, the positive bridge arm diodes and the negative bridge arm diodes are selected from Schottky diodes or fast recovery diodes, and the forward voltage of the positive bridge arm diodes and the negative bridge arm diodes is less than the forward voltage drop of the anti-parallel diodes of the switching devices in the TNPC topology.

[0016] Preferably, the heat dissipation module is further included;

[0017] In the first module and the second module, each phase TNPC topology and each phase auxiliary diode group are separately cooled by the heat dissipation module, and each phase is separately cooled by the heat dissipation module;

[0018] In the three-phase full-bridge topology, each phase switching device is separately cooled by the heat dissipation module.

[0019] The application further provides a control method of the low-cost bidirectional overload flexible interconnection device, based on the device of the application, the method comprising:

[0020] According to the power flow scheduling instruction, the working modes of the first module and the second module are set, the module working in the rectification mode is defined as a rectification port, and the module working in the inversion mode is defined as an inversion port; when the device works in the normal working condition, the rectification port adopts the grid-following type constant voltage control, and the inversion port adopts the grid-following type constant power control; the rectification port and the inversion port both adopt the three-level modulation method for driving signal modulation;

[0021] When the device needs to be overloaded for a short time, the output end of the three-phase full-bridge topology is connected to the inversion port; the three-phase full-bridge topology is controlled by using the grid-following type constant power control, and the power instruction in the three-phase full-bridge topology control is set as the difference between the overload power instruction and the total rated power of the first module and the second module; when the driving signal is modulated, the two-level modulation method is adopted; in the rectification port control, the direct current voltage reference value is adjusted to the ratio of the modulation wave peak value to the carrier peak value being 1, and the modulation method is changed from the three-level modulation method to the two-three-level switching modulation method; the overload power instruction is obtained from the power flow scheduling instruction.

[0022] Preferably, when the device needs to start the short-time overload, if the power instructions of the first module and the second module are less than the rated power of the modules, the power instructions of the first module and the second module are adjusted to the rated power of the modules.

[0023] Preferably, the two-three-level switching modulation method comprises:

[0024] According to the relationship between the rectification port grid voltage phase and the first interval, the two-level modulation method and the three-level modulation method are switched:

[0025] If the rectification port grid voltage phase is located in the first interval, the two-level modulation is adopted, otherwise the three-level modulation is adopted; the first interval comprises , wherein The adjustment factor is a decimal between 0 and 1, indicating the proportion of the two-level modulation in one grid cycle.

[0026] Preferably, when the power flow scheduling instruction contains the requirement of changing the power flow direction:

[0027] The power instruction in the rectifier port and inverter port control is reduced to 0; the three-phase full-bridge topology is disconnected with the inverter port; the working mode of the first module and the second module is interchanged to obtain a new rectifier port and inverter port; and whether normal working condition control or short-time overload control needs to be performed is determined according to the power grid flow scheduling instruction.

[0028] The present application has the following beneficial effects:

[0029] The low-cost bidirectional overload flexible interconnection device has the following beneficial effects: when the single-port converter works in the rectification mode, the current partially flows through the antiparallel diode of the device module, and the low-cost auxiliary diode in parallel can be shunted; when the single-port converter works in the inverter mode, the three-phase full-bridge topology is shunted from the converter stage by using a multiplexing scheme, so that the power loss can be reduced. Compared with the super-configured converter scheme in engineering applications, the device of the present application can realize bidirectional overload capacity improvement by adding only a few devices, thereby reducing the device cost and volume cost and further improving the dynamic capacity limit of the transformer substation.

[0030] The control method of the low-cost bidirectional overload flexible interconnection device has the same beneficial effects as the device of the present application. In addition, the method of the present application can improve the overload capacity of the flexible interconnection device.

[0031] In addition to the purposes, features and advantages described above, the present application has other purposes, features and advantages. The present application will be further described in detail below with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the present application, and the illustrative embodiments of the present application and their description serve to explain the present application. The accompanying drawings do not constitute an inappropriate limitation on the present application. In the drawings:

[0033] Figure 1 is a low-cost bidirectional overload flexible interconnection device structure schematic diagram of the preferred embodiment of the present application;

[0034] Figure 2 is a second module structure schematic diagram of the preferred embodiment of the present application;

[0035] Figure 3 is a three-phase full-bridge topology structure schematic diagram of the preferred embodiment of the present application;

[0036] Figure 4 is a rectifier port different device driving signal schematic diagram of the preferred embodiment of the present application in one modulation period;

[0037] Figure 5This is a schematic diagram of voltage and current waveforms during active power mutual assistance according to a preferred embodiment of the present invention. Figure 5 (e1) shows the voltage and current waveforms of phase A at the rectifier port. Figure 5 (f1) shows the voltage and current waveforms of phase A at the inverter port. Figure 5 (g1) shows the voltage and current waveforms of phase A of the auxiliary module;

[0038] Figure 6 This is a schematic diagram of the current loop of the rectifier port under different output states in a preferred embodiment of the present invention. Figure 6 (a1) represents the positive voltage and positive current output state. Figure 6 (b1) represents the zero-voltage positive current output state. Figure 6 (c1) represents the zero-voltage, negative-current output state. Figure 6 (d1) represents a negative voltage and negative current output state;

[0039] Figure 7 This is a schematic diagram showing the ratio of conduction loss to switching loss of each device in a preferred embodiment of the present invention when no overload measures are adopted;

[0040] Figure 8 This is a schematic diagram showing the ratio of conduction loss to switching loss of each device when only overload modulation is used in a preferred embodiment of the present invention;

[0041] Figure 9 This is a schematic diagram showing the ratio of conduction loss to switching loss of each device when only the auxiliary diode is connected in parallel according to a preferred embodiment of the present invention;

[0042] Figure 10 This is a schematic diagram showing the proportion of conduction loss and switching loss of each device when only the DC voltage is reduced in a preferred embodiment of the present invention;

[0043] Figure 11 This is a schematic diagram showing the proportion of conduction loss and switching loss of each device when three overload measures are comprehensively adopted in a preferred embodiment of the present invention.

[0044] Figure 12 This is a schematic diagram showing the percentage of each device in the rectifier port relative to the total loss of the rectifier port in a preferred embodiment of the present invention;

[0045] Figure 13 This is a schematic diagram of the highest instantaneous temperature of each device at the rectifier port under different overload measures and different power levels according to a preferred embodiment of the present invention.

[0046] Figure 14 This is a schematic diagram showing the highest instantaneous temperature of each device at the inverter port under different overload measures and different power levels according to a preferred embodiment of the present invention.

[0047] Figure 15 This is a schematic diagram showing the highest instantaneous temperature of the auxiliary diode and auxiliary module components under different overload measures and different power levels according to a preferred embodiment of the present invention. Detailed Implementation

[0048] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

[0049] See Figure 1 In a preferred embodiment of the present invention, a low-cost bidirectional overload flexible interconnection device is provided, comprising a first module, a second module, a three-phase full-bridge topology, and a DC bus.

[0050] See Figure 2 The first and second modules have the same structure, both including a three-phase TNPC topology and a three-phase auxiliary diode group.

[0051] In a preferred embodiment of the present invention, see [reference needed]. Figure 2 Taking the second module as an example, the auxiliary diode group includes the positive bridge arm diodes. and negative bridge arm diode In each phase, the positive bridge arm diode The cathode is connected to the positive terminal of the DC bus, and the negative bridge arm diode is connected to the negative bridge arm diode. The anode is connected to the negative terminal of the DC bus, and the positive bridge arm diode... Anode and negative bridge arm diodes The cathode is connected to the output of the TNPC topology; the three-phase auxiliary diode group is used to shunt the three-phase TNPC topology in rectification mode. , represents the three phases a, b, and c, and the subscript port indicates the port name.

[0052] In a preferred embodiment of the present invention, the positive bridge arm diode and the negative bridge arm diode are selected as Schottky diodes or fast recovery diodes, and the forward voltage of the positive bridge arm diode and the negative bridge arm diode is less than the forward voltage drop of the anti-parallel diode of the switching device in the TNPC topology.

[0053] In a preferred embodiment of the present invention, see [reference needed]. Figure 1 The DC bus includes DC-side capacitors, and the DC-side capacitors include the upper capacitor. With lower capacitor The positive terminal of the upper capacitor is connected to the positive terminal of the DC bus, and the negative terminal of the lower capacitor is connected to the negative terminal of the DC bus. The negative terminal of the upper capacitor and the positive terminal of the lower capacitor are connected together to the neutral line of the three-phase TNPC topology.

[0054] In a preferred embodiment of the present invention, see [reference needed]. Figure 2 Taking the second module as an example, each phase of the three-phase TNPC topology consists of four switching devices. Each switching device contains a switching transistor, and a diode is connected in anti-parallel to the switching transistor. The four switching devices are the first switching device. Second switching device Third switching device Fourth switching device First switching device The collector is connected to the positive terminal of the DC bus; the third switching device The collector is connected to the neutral point O, and the emitter is connected to the second switching device. The emitter; the fourth switching device The emitter is connected to the negative terminal of the DC bus; the first switching device emitter, second switching device collector, fourth switching device The collector of phase x is connected to a single point, namely the output terminal of phase x bridge arm; , represents the three phases a, b, and c, and the subscript port indicates the port name.

[0055] In a preferred embodiment of the present invention, see [reference needed]. Figure 3 In a three-phase full-bridge topology, each phase consists of two switching devices, each containing a switching transistor connected in anti-parallel to a diode. These two switching devices are considered the fifth switching devices. Sixth switching device Fifth switching device The collector is connected to the positive terminal of the DC bus, and the emitter is connected to the sixth switching device. collector; sixth switching device collector connection of the fifth switching device The emitter is connected to the negative terminal of the DC bus; the fifth switching device. The emitter and the sixth switching device The connection point of the collector is the x-phase output terminal; , representing the three phases a, b, and c.

[0056] In a preferred embodiment of the present invention, the three-phase full-bridge topology and each TNPC topology are connected back-to-back to each other via a common DC bus, i.e., the connection is made according to the existing known connection methods between the three-phase full-bridge topology and the TNPC topology and the DC bus. For specific connection methods, please refer to the aforementioned section on the structure of the three-phase full-bridge topology and the TNPC topology. The first module and the second module are respectively connected to two distribution transformers through AC circuit breakers via the output terminals of the TNPC topology (see...). Figure 1 Transformer 1 and transformer 2 in the three-phase full-bridge topology are used to coordinate power flow scheduling in a specified direction; the output of the three-phase full-bridge topology is connected to the TNPC topology output of the first or second module through an AC circuit breaker, and is used as an auxiliary module to assist the first or second module in inverter mode to achieve active overload.

[0057] In a preferred embodiment of the present invention, the method for assisting the first or second module in inverter mode to achieve active power overload includes:

[0058] When the device receives a power flow dispatch command indicating a short-term overload is required, the three-phase full-bridge topology connects to the TNPC topology output of the first or second module in inverter mode via its output terminal. Specifically, this is achieved by connecting to the first or second module in inverter mode via a closing AC circuit breaker. This serves to shunt current to the first or second module in inverter mode, reducing power loss and enabling the first or second module in inverter mode to have active power overload capability. During device operation, one of the first and second modules must be in inverter mode, and the other in rectification mode. The specific mode selection depends on the direction of power flow dispatch.

[0059] In a preferred embodiment of the present invention, when the device is operating under normal conditions, the three-phase full-bridge topology is in standby mode, and only the first module and the second module transmit active power. When the device is operating in overload mode, the three-phase full-bridge topology is connected in parallel to the first module or the second module in inverter mode through the closing of the AC circuit breaker, so as to improve the overload capacity of the inverter port through the shunt at the smaller capacity module level.

[0060] In a preferred embodiment of the present invention, all switching transistors are IGBTs or all are MOSFETs; the diodes are connected in anti-parallel configuration with the anode of the diode connected to the emitter of the switching transistor, and the cathode of the diode connected to the collector of the switching transistor. In a preferred embodiment of the present invention, all switching transistors are IGBTs.

[0061] In a preferred embodiment of the present invention, the device further includes a heat dissipation module. In the first and second modules, each phase of the TNPC topology and each phase of the auxiliary diode group are separated and cooled individually by the heat dissipation module, and each phase is separated and cooled individually by the heat dissipation module; in the three-phase full-bridge topology, each phase switching device is separated and cooled individually by the heat dissipation module.

[0062] In a preferred embodiment of the present invention, the two auxiliary diodes of each phase are placed on the same heat dissipation module to draw out the power loss of the anti-parallel diode and reduce the thermal coupling effect between the switching transistor and the anti-parallel diode.

[0063] The low-cost bidirectional overload flexible interconnection device of this invention, when the single-port converter operates in rectification mode, allows the current portion to flow through the anti-parallel diodes of the device module, with the parallel low-cost auxiliary diodes providing current shunting. When the single-port converter operates in inverter mode, the three-phase full-bridge topology uses a multiplexing scheme for current shunting from the converter stage, reducing power loss. Compared to the over-configured converter solution used in engineering applications, this invention's device achieves improved bidirectional overload capacity with only a few additional components, reducing component and volume costs, and further enhancing the dynamic capacity expansion limit of the transformer substation.

[0064] In a preferred embodiment of the present invention, a control method for a low-cost bidirectional overload flexible interconnect device is also provided. Based on the device of the present invention, the method includes:

[0065] The operating modes of the first and second modules are set according to the power flow dispatching instructions. The module operating in rectification mode is defined as the rectification port, and the module operating in inverter mode is defined as the inverter port.

[0066] When the device is operating under normal conditions, the rectifier port adopts grid-type constant voltage control, and the inverter port adopts grid-type constant power control; both the rectifier port and the inverter port use three-level modulation method to modulate the drive signal.

[0067] In a preferred embodiment of the present invention, grid-connected constant voltage control and grid-connected constant power control include:

[0068] (1) Network-type constant voltage control includes:

[0069] Sampling is performed at the beginning of each sampling period to collect the three-phase voltage, three-phase current, and DC bus voltage at the target port; the three-phase voltage and three-phase current at the target port are input into the phase-locked loop to obtain the grid voltage phase at the target port.

[0070] The dq components of the three-phase voltage and three-phase current at the target port are obtained by performing dq transformation on the phase of the grid voltage at the target port.

[0071] The difference between the DC voltage reference value and the DC bus voltage is input to the PI controller to obtain the d-axis current command value. The difference between the d-axis current command value and the q-axis current command value (0) and the dq components of the three-phase current at the target port is then input to two independent PI controllers. The outputs of the two independent PI controllers are simultaneously superimposed in a first and second superposition to obtain the dq-axis components of the reference voltage at the target port. The first superposition includes the outputs of the two independent PI controllers corresponding to the superimposed q-axis and d-axis components of the three-phase current after decoupling calculation. The second superposition includes the outputs of the two independent PI controllers corresponding to the superimposed d-axis and q-axis components of the three-phase voltage at the target port.

[0072] The reference voltage dq axis component of the target port is subjected to inverse dq transformation, then normalized and limited to obtain the three-phase modulation wave of the target port; the switching device drive signal is modulated according to the three-phase modulation wave; the target port module is driven according to the switching device drive signal.

[0073] (2) Network-based constant power control includes:

[0074] Sampling is performed at the beginning of each sampling cycle to collect the three-phase voltage, three-phase current, and DC bus voltage at the target port; the three-phase voltage and three-phase current at the target port are input into the phase-locked loop to obtain the grid voltage phase at the target port.

[0075] The dq transformation is performed on the three-phase voltage, three-phase current and grid voltage phase at the target port to obtain the dq components of the three-phase voltage and three-phase current at the target port.

[0076] The dq-axis current command is obtained based on the power command provided by the power grid power flow dispatch command; the difference between the dq-axis current command and the dq component of the three-phase current at the target port is calculated and then input into two independent PI controllers. The outputs of the two independent PI controllers are simultaneously superimposed in the first and second superpositions to obtain the reference voltage dq-axis component at the target port.

[0077] The reference voltage dq axis component of the target port is subjected to inverse dq transformation, then normalized and limited to obtain the three-phase modulation wave of the target port; the switching device drive signal is modulated according to the three-phase modulation wave; the target port module is driven according to the switching device drive signal.

[0078] In a preferred embodiment of the present invention, when the device needs to perform a short-term overload, the output terminal of the three-phase full-bridge topology is connected to the inverter port; grid-following constant power control is used to control the three-phase full-bridge topology, and the power command in the three-phase full-bridge topology control is set to the difference between the overload power command and the total constant power of the first and second modules. Two-level modulation is used when modulating the drive signal; in the rectifier port control, the DC voltage reference value is adjusted to the ratio of the modulation peak value to the carrier peak value is 1, and the modulation method is changed from three-level modulation to two-three-level switching modulation; the overload power command is obtained from the power grid power flow dispatch command.

[0079] In a preferred embodiment of the present invention, when the device needs to start short-term overload, if the power command of the first module and the second module is less than the rated power of the module itself, the power command of the first module and the second module is adjusted to the rated power of the module itself.

[0080] In a preferred embodiment of the present invention, the two- or three-level switching modulation method includes:

[0081] The switching between two-level modulation and three-level modulation methods is performed based on the relationship between the phase of the grid voltage at the rectifier port and the first interval.

[0082] If the grid voltage phase at the rectifier port is within the first interval, two-level modulation is used; otherwise, three-level modulation is used. The first interval includes... ,in The adjustment factor is a decimal between 0 and 1, representing the proportion of two-level modulation within one power grid cycle.

[0083] Zero-level bridge arm in Within the phase interval, the average current flowing through is at its maximum, which affects the switching factor. By selecting appropriate values ​​and switching the modulation strategy to two levels within this range, the junction temperature of the positive and negative bridge arms and the zero bridge arm can be balanced without disrupting the voltage balance at the midpoint of the capacitor.

[0084] In a preferred embodiment of the present invention, two-level and three-level switching modulation is performed based on the STM32F28335 DSP digital signal processor, and a two-level modulation flag is set in the STM32F28335 control system. Three-level modulation flag bit These controls the on / off states of the two modulation modes, where 1 indicates on and 0 indicates off. and The states within a power grid cycle include:

[0085] ;

[0086] In a preferred embodiment of the present invention, when calculating the drive signal, three-level modulation is set in the EPWM module. S x1 and S x3 , S x2 and S x4 When the switching states are complementary and the two-level modulation is applied... S x2 , S x3 Setting the switch state to 0 yields the switch state table shown in Table 1. Here, P3 represents the state where the output voltage is positive under three-level modulation, O represents the state where the output voltage is zero under three-level modulation, N3 represents the state where the output voltage is negative under three-level modulation, P2 represents the state where the output voltage is positive under two-level modulation, and N2 represents the state where the output voltage is negative under three-level modulation.

[0087] Table 1 TNPC Two / Three-Level Switch Status Table

[0088]

[0089] In a preferred embodiment of the present invention, when the power flow dispatching instruction includes a requirement to change the power flow direction:

[0090] Reduce the power command in the rectifier port and inverter port control to 0; disconnect the three-phase full-bridge topology from the inverter port; interchange the working modes of the first module and the second module to obtain new rectifier ports and inverter ports; determine whether the device needs to perform normal operating condition control or short-term overload control according to the power grid power flow dispatch command.

[0091] The control method for the low-cost bidirectional overload flexible interconnect device of the present invention, based on the device of the present invention, has the same beneficial effects as the device of the present invention. Furthermore, the method of the present invention can improve the overload capacity of the flexible interconnect device.

[0092] Verification section:

[0093] See Figures 4 to 6 Specifically, the overload capacity of the low-cost bidirectional overload flexible interconnection device topology and its control method is analyzed. The positive and negative arms of the rectifier-side TNPC topology are connected in parallel with diodes, employing a two- or three-level switching modulation method. Taking phase A as an example, the IGBT and diode conduction time within one power grid cycle is calculated as follows:

[0094] When the device is operating under normal conditions, the DC side voltage is When the device operates under overload conditions, the DC side voltage drops to Adjustment system It can be calculated using the following formula:

[0095] ;

[0096] Where Udc represents the DC bus voltage; U m If the peak value of the modulation wave is not normalized, then the normalized phase voltage can be expressed as:

[0097] ;

[0098] If only three-level modulation is used, let the initial phase of phase A of the grid voltage be... When it is 0, At that time, its normalized phase voltage By integrating, the conduction time of each state of phase A bridge arm can be obtained. expression:

[0099] ;

[0100] When in Two-level modulation is used, and the conduction time of each state of phase A bridge arm is... The expression is:

[0101] ;

[0102] in, This indicates the time of the P state output of phase A under three-level modulation; This indicates the time when phase A outputs the 0 state under three-level modulation; This indicates the time when phase A outputs state N under three-level modulation; This represents the normalized phase voltage of phase A; Indicates the phase of the rectifier-side power grid; Indicates the time of one modulation cycle; This indicates the time of the P state output of phase A under two-level modulation; This indicates the time when phase A outputs state N under two-level modulation.

[0103] Taking the inter-transformer power exchange scenario as an example, this paper analyzes the current flow path of the rectifier port topology of the flexible interconnection device under different operating modes. In the inter-transformer power exchange scenario, the flexible interconnection device only transmits active power. For the A-phase topology, when it is in the rectifier state, the voltage zero-crossing point is not considered. Taking the AC measured DC side as the current reference direction, the output voltage and current at the grid connection point can be divided into the following two types: (1) positive voltage and positive current; (2) negative voltage and negative current. The current flow path and loss of the device when the rectifier port switches between two and three levels will be analyzed in detail below.

[0104] Switching devices in TNPC topology Including switching transistors and In this context, the subscript 'x' indicates phase x, 'i' indicates the device number, and 'port' indicates the port name ('rec' for rectifier port and 'inv' for inverter port); similarly, three-phase full-bridge topology switching devices... Including switching transistors and In this context, the subscript 'x' indicates phase x, 'i' represents the device designation, and 'ass' indicates that the device is used for auxiliary current shunt. When the grid connection point outputs positive voltage and positive current, the grid phase... At this time, the voltage output state of the bridge arm switches between P and O, corresponding to the bridge arm S a1 , S a2 and S a2 , S a3 Alternating conduction, due to the forward voltage drop of the auxiliary diode. Less than the threshold voltage of the two IGBT anti-parallel diodes Current will flow only through the auxiliary diode. ; power grid phase hour, S a2 , S a3The EPWM signal is 0, keeping the transistor off, and current flows only through the upper bridge arm auxiliary diode. Therefore, during the period when the grid connection point outputs positive voltage and positive current, Switching and conduction losses will occur for a period of time, but there will be no losses at other times. D a3_rec It will only generate conduction loss for a period of time, and no loss at other times; It will only generate conduction loss.

[0105] Similarly, when the grid connection point outputs negative voltage and negative current, the grid phase... At this time, the voltage output state of the bridge arm switches between N and O, corresponding to the bridge arm S a3 , S a4 and S a2 , S a3 Alternating conduction, due to the forward voltage drop of the auxiliary diode. Less than the threshold voltage of the two IGBT anti-parallel diodes Current will flow only through the auxiliary diode. ; power grid phase hour, S a2 , S a3 The EPWM signal is 0, keeping the transistor off, and current flows only through the upper bridge arm auxiliary diode. Therefore, during the period of negative voltage and negative current output at the grid connection point, Switching and conduction losses will occur for a period of time, but there will be no losses at other times. It will only generate conduction loss for a period of time, and no loss at other times; Only conduction losses will occur. Through the combination of topology and modulation mode, heat transfer and heat equalization of the TNPC bridge arms are achieved, optimizing the loss distribution.

[0106] Furthermore, the effectiveness of the method of the present invention was verified using Simulink / Plecs software.

[0107] according to Figures 1 to 3 A circuit model was built in the simulation software. The selected components and parameters are shown in Table 2. The thermal simulation parameters of the components were provided by Infineon datasheets and official website simulation models. IGBTs were selected as the switching transistors.

[0108] Table 2. Simulation Parameters of Low-Cost High Reactive Power Overload Energy Storage Converter Topology

[0109]

[0110] Among them, FF450R12ME4 and FF200R12KE4 have a half-bridge structure inside, serving as the positive and negative bridge arms of the TNPC topology and the single-phase bridge arm of the three-phase full-bridge topology, respectively. FF400R12KT3P_E has a common collector structure inside, serving as the zero-level bridge arm of the TNPC topology.

[0111] See Figure 7 and Figure 12 Taking phase A as an example, Figures 7 to 11 The percentages of conduction losses and switching losses for each device under different cooling strategies are given when the device operates at its rated power of 250kW. Figure 12 The percentage of total device losses to total A-phase device losses under different cooling strategies when the device operates at its rated power of 250kW is given. By comparison, the following conclusions can be drawn: Under single power factor rectification conditions, the proposed modulation method (where, This invention can improve the losses of the positive and negative bridge arms of the TNPC and reduce the losses of the zero-level bridge arm; the device of this invention can extract the losses of the positive and negative bridge arms, giving the modulation strategy a larger temperature adjustment range; the method of this invention can reduce the switching losses of individual devices in the positive and negative bridge arms, while giving the TPNC bridge arms higher current overload capacity.

[0112] See Figures 13 to 15 Taking phase A as an example, the highest instantaneous junction temperatures of each device are given when the device reaches steady-state temperature under the rated 250kW operating conditions without overload measures and with the method of this invention. The PLECS simulation results show that when the device operates at the rated 250kW, the device with the highest instantaneous junction temperature is... It reached 123.9℃, while It also reached 120.2℃. After using the device and method of this invention, the device with the highest instantaneous junction temperature was... The temperature reached 90.3℃, and the temperature of each device in the TNPC topology was relatively uniform.

[0113] Based on this operating condition, the transmission power command was further increased until the junction temperature of any device reached its limit (125℃ for IGBT and 150℃ for the auxiliary diode) when the temperature reached steady state, and the maximum active current overload multiple was tested. See also... Figures 13 to 15 Taking phase A as an example, the highest instantaneous junction temperatures of each device are presented when the device operates under a 1.6 times active current overload condition and employs the overload method of this invention, reaching a steady-state temperature. The PLECS simulation results show that when the device operates under a 1.6 times current overload condition, the device with the highest instantaneous junction temperature is... The temperature reached 147.5℃, nearing the junction temperature limit, while the instantaneous highest junction temperature of other devices still had sufficient safety margin from their own junction temperature limits. Therefore, using this method, the flexible interconnect device can have an active current overload capacity of 1.6 times.

[0114] The embodiments of the present invention achieve an active current overload capacity of 1.6 times without significantly increasing the cost of converter components. Taking the component prices on websites such as Infineon's official website (www.infineon.com) and LCSC's online marketplace (www.szlcsc.com) as examples, the parameters and prices of each component are shown in Table 3.

[0115] Table 3 Device Parameters and Prices

[0116]

[0117] Taking the devices selected in the PLECS simulation of this embodiment as an example, the oversized TNPC topology flexible interconnect device requires a total of 24 IPMs (Intelligent Power Modules, i.e., heat dissipation modules), with a total device cost of 21,787.68 yuan, and can achieve bidirectional double overload. After adding the auxiliary diode group, the total device cost of the device in this method is 15,704.52 yuan, and it can achieve bidirectional 1.6 times overload. After conversion to the same overload multiple, the device cost of the device in this method is only 90.1% of that of the oversized scheme under the premise of the same active current overload multiple (1.6 times). In addition, the use of this device and method can also save on land costs and avoid reducing the system utilization efficiency.

[0118] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-cost bidirectional overload flexible interconnection device, characterized in that, It includes a first module, a second module, a three-phase full-bridge topology, and a DC bus; the first module and the second module have the same structure, both including a three-phase TNPC topology; The three-phase full-bridge topology is connected back-to-back with each TNPC topology via a common DC bus; the first module and the second module are respectively connected to two distribution transformers through the TNPC topology output terminal for collaborative power flow scheduling in a specified direction; the output terminal of the three-phase full-bridge topology is connected to the TNPC topology output terminal of the first module or the second module to assist the first module or the second module in inverter mode in achieving active power overload, including: When the device receives a power flow dispatching command indicating a short-term overload is required, the three-phase full-bridge topology is connected to the TNPC topology output terminal of the first or second module in inverter mode via its output terminal, for the purpose of shunting the first or second module in inverter mode.

2. The low-cost bidirectional overload flexible interconnection device according to claim 1, characterized in that, The first module and the second module also include a three-phase auxiliary diode group; The auxiliary diode group includes positive bridge arm diodes and negative bridge arm diodes; in each phase, the cathode of the positive bridge arm diode is connected to the positive terminal of the DC bus, the anode of the negative bridge arm diode is connected to the negative terminal of the DC bus, and the anode of the positive bridge arm diode and the cathode of the negative bridge arm diode are connected to the output terminal of the TNPC topology; the three-phase auxiliary diode group is used to shunt the three-phase TNPC topology in rectification mode.

3. The low-cost bidirectional overload flexible interconnection device according to claim 2, characterized in that, The positive and negative bridge arm diodes are selected as Schottky diodes or fast recovery diodes, and the forward voltage of the positive and negative bridge arm diodes is less than the forward voltage drop of the anti-parallel diodes of the switching devices in the TNPC topology.

4. The low-cost bidirectional overload flexible interconnection device according to claim 3, characterized in that, It also includes a heat dissipation module; In the first module and the second module, each phase TNPC topology and each phase auxiliary diode group are separated and cooled separately by the heat dissipation module, and each phase is separated and cooled separately by the heat dissipation module. In the three-phase full-bridge topology, each phase switching device is cooled separately through the heat dissipation module.

5. A control method for a low-cost bidirectional overload flexible interconnection device, based on the device according to any one of claims 2 to 4, characterized in that, The method includes: The operating modes of the first and second modules are set according to the power flow dispatching instructions of the power grid. The module operating in rectification mode is defined as the rectifier port, and the module operating in inverter mode is defined as the inverter port. When the device is operating under normal conditions, the rectifier port adopts grid-type constant voltage control, and the inverter port adopts grid-type constant power control. Both the rectifier port and the inverter port use three-level modulation method to modulate the drive signal. When the device needs to undergo short-term overload, the output terminal of the three-phase full-bridge topology is connected to the inverter port; grid-following constant power control is used to control the three-phase full-bridge topology, and the power command in the three-phase full-bridge topology control is set to the difference between the overload power command and the total constant power of the first and second modules. Two-level modulation is used when modulating the drive signal; in the rectifier port control, the DC voltage reference value is adjusted to the ratio of the modulation peak value to the carrier peak value is 1, and the modulation method is changed from three-level modulation to two-three-level switching modulation; the overload power command is obtained from the power grid power flow dispatch command.

6. The control method for the low-cost bidirectional overload flexible interconnection device according to claim 5, characterized in that, When the device needs to start short-term overload, if the power command of the first module and the second module is less than the rated power of the module itself, the power command of the first module and the second module is adjusted to the rated power of the module itself.

7. The control method for the low-cost bidirectional overload flexible interconnection device according to claim 6, characterized in that, The two- or three-level switching modulation method includes: The switching between two-level modulation and three-level modulation methods is performed based on the relationship between the phase of the grid voltage at the rectifier port and the first interval. If the phase of the mains voltage at the rectifier port is within the first interval, then two-level modulation is used; otherwise, three-level modulation is used. The first interval includes... ,in The adjustment factor is a decimal between 0 and 1, representing the proportion of two-level modulation within one power grid cycle.

8. The control method for the low-cost bidirectional overload flexible interconnection device according to claim 7, characterized in that, When a power flow dispatching instruction includes a requirement to change the direction of power flow: Reduce the power command in the control of the rectifier port and the inverter port to 0; disconnect the three-phase full-bridge topology from the inverter port; interchange the working modes of the first module and the second module to obtain a new rectifier port and inverter port; determine whether the device needs to perform normal operating condition control or short-term overload control according to the power grid power flow dispatch command.

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