A simulation method for AC / DC bidirectional converter suitable for power flow simulation calculation

Abstract

The present invention provides a method for simulating an AC / DC bidirectional converter suitable for power flow simulation calculation, comprising the following steps: establishing a main circuit model, the main circuit model consisting of an unconnected DC circuit and an AC circuit; the DC circuit has two branches, one of which consists of a switch element K 1 , resistor element R , controlled DC voltage source U con The second branch is formed by the switch element K 2 , controlled DC current source I con The AC circuit has three branches, each containing a controlled AC current source. I a , controlled AC current source I b , controlled AC current source I c ; Establish a control system calculation unit to calculate each simulation step. The present invention simulates the complex electrical external characteristics of the AC / DC bidirectional converter by establishing a main circuit model and a control system calculation unit. There is no need to build a converter circuit containing a large number of power electronic devices, which not only meets the engineering accuracy requirements but also ensures the simulation speed.

Description

Technical Field

[0001] The present invention belongs to the technical field of power flow simulation calculation of rail transit power supply system, and in particular relates to an AC / DC bidirectional converter simulation method suitable for power flow simulation calculation. Background Art

[0002] At present, most urban rail transit power supply systems use rectifier units to convert electric energy from three-phase AC to DC to power trains. Since the diode rectifier circuit of the rectifier unit can only flow energy in one direction, the large amount of regenerative braking energy generated by the train during frequent braking will not be able to return to the upper AC network of the power supply system through the rectifier unit. If the regenerative braking energy generated by the train cannot be absorbed by the adjacent train, it will be consumed in the braking resistor or brake shoe, causing energy waste and causing tunnel temperature rise. Urban rail transit power supply systems using rectifier units are such as Figure 1 shown.

[0003] To solve the above problems, an AC / DC bidirectional converter can be installed between the AC network and the DC network of urban rail transit to replace the existing rectifier unit, such as Figure 2 As shown. The AC / DC bidirectional converter generally includes the converter, transformer, voltage sensor, current sensor, control system and other main components. The AC / DC bidirectional converter can realize and control the bidirectional energy flow between the AC network and the DC network of the rail transit power supply system. When the DC network train starts traction, the AC / DC bidirectional converter is in the rectification state, controlling the energy flow from the AC network to the DC network; when the DC network train regenerates and brakes, the AC / DC bidirectional converter is in the inverter state, controlling the energy return from the DC network to the AC network. The topology of the converter can be selected from one of the topologies such as the two-level converter topology, the three-level converter topology, the multi-level converter topology, the cascaded multi-level converter topology or the MC structure. The converter can be implemented by various power electronic devices such as IGBT, IGCT, SIC, etc. The voltage sensor collects the voltage on the DC network side and the AC network side of the bidirectional converter in real time. The current sensor collects the current on the DC network side and the AC network side of the bidirectional converter in real time. Based on the above collected voltage and current, the control system calculates and outputs the control signal for controlling the conduction or disconnection of each switch tube of the converter.

[0004] The main purpose of installing AC / DC bidirectional converters in rail transit power supply systems is to achieve controllable bidirectional flow functions of rectification and inversion. At present, the most commonly used control strategy for AC / DC bidirectional converters is to control and switch the rectification, inversion or standby state of the AC / DC bidirectional converter based on the collected DC grid voltage, AC grid voltage, DC grid current, AC grid voltage, and the set rated power, rectification voltage threshold, inversion voltage threshold, rectification efficiency, inversion efficiency, equivalent internal resistance and other parameters. The control principle is as follows:

[0005] (1) When the DC grid voltage is greater than the rectifier voltage threshold and less than the inverter voltage threshold

[0006] The AC / DC bidirectional converter is in standby mode, neither rectifying nor inverting.

[0007] (2) When the DC grid voltage is not greater than the rectifier voltage threshold

[0008] 1) If the DC grid output power does not reach the rated power

[0009] The AC / DC bidirectional converter is in a constant voltage rectification state, controlling the energy flow from the AC grid side to the DC grid side, and stabilizing the DC grid side voltage at the rectification voltage threshold.

[0010] 2) If the DC grid output power reaches the rated power

[0011] The AC / DC bidirectional converter is in a constant power rectification state, controlling the energy flow from the AC grid side to the DC grid side, and rectifying at rated power, and is unable to stabilize the DC grid side voltage at the rectification voltage threshold.

[0012] (2) When the DC grid voltage is not less than the inverter voltage threshold

[0013] 1) If the DC grid-side inverter feedback power does not reach the rated power

[0014] The AC / DC bidirectional converter is in a constant voltage inverter state, controlling the energy flow from the DC grid side to the AC grid side and stabilizing the DC grid side voltage at the inverter voltage threshold.

[0015] 2) If the DC grid-side inverter feedback power reaches the rated power

[0016] The AC / DC bidirectional converter is in constant power inverter state, controlling the energy flow from the DC grid side to the AC grid side, and inverting at rated power, and is unable to stabilize the DC grid side voltage at the inverter voltage threshold.

[0017] The rail transit power supply system may install multiple sets of AC / DC bidirectional converters at multiple locations, and may have different rated powers and control parameters. The dynamic characteristics of the AC / DC bidirectional converter will inevitably affect the dynamic characteristics of the entire power supply system. In the power flow simulation calculation of the urban rail transit power supply system, the simulation methods of each device and device affect the speed and accuracy of the simulation calculation. The AC / DC bidirectional converter was applied to the rail transit power supply system relatively late, and its circuit structure is more complex than other devices in the power supply system. The converter, power electronic devices and their control systems have complex time-varying nonlinear characteristics, which greatly limits the calculation speed of the AC / DC bidirectional converter simulation. The power flow simulation calculation of the rail transit power supply system needs to consider a large number of electrical equipment, conductors and loads. The computing resources allocated to the AC / DC bidirectional converter are limited, and there are computing speed requirements. It is unrealistic to establish a very detailed model for the AC / DC bidirectional converter, but the use of an overly simplified model lacks accuracy and omits important dynamic characteristics. Therefore, the study of the generalized load equivalent model of AC / DC bidirectional converter suitable for power flow simulation calculation of rail transit power supply system not only meets the engineering accuracy requirements but also ensures the simulation speed, which is of great significance for improving the power flow simulation calculation of urban rail transit power supply system.

[0018] At present, for the power flow simulation calculation of urban rail transit power supply system, for the generalized load of AC / DC bidirectional converter, the simulation method basically adopts the model of voltage source plus internal resistance or the model of pure current source for equivalent, which cannot accurately simulate the complex electrical external characteristics of AC / DC bidirectional converter. Summary of the invention

[0019] The present invention aims at solving the technical problems in the prior art and provides a method for simulating an AC / DC bidirectional converter suitable for power flow simulation calculation. The method is based on the electrical characteristics of the AC / DC bidirectional converter and the constant parameters set before the simulation and the DC grid side voltage of the bidirectional converter collected in real time during the simulation. u dc , DC grid side current i dc , three-phase voltage on the AC grid side u a , u b , u c Carry out model building.

[0020] The technical solution adopted by the present invention is: a method for simulating an AC / DC bidirectional converter suitable for power flow simulation calculation, comprising the following steps:

[0021] S1: Establish the main circuit model, which consists of an unconnected DC circuit and an AC circuit; the DC circuit has two branches, one of which consists of a switch element. K1 , resistor element R , controlled DC voltage source U con The second branch is formed by the switch element K 2 , controlled DC current source I con The AC circuit has three branches, each containing a controlled AC current source. I a , controlled AC current source I b , controlled AC current source I c .

[0022] S2: Establish the control system calculation unit, which is based on the constant parameters set before the simulation and the DC grid voltage of the main circuit model collected in the simulation. u dc , DC grid side input current i dc And the three-phase voltage on the AC grid side u a , u b , u c , and calculate each simulation step. The output signal of the control system calculation unit is g 1 , r , u con , g 2 , i con , i a , i b , i c ， in g 1 Used to determine the switching elements of the main circuit model at the next simulation step K 1 Is it on or off? r Used to determine the resistance element in the next simulation step R The resistance value, u con Used to determine the controlled DC voltage source at the next simulation step U con The voltage value, g 2 Used to decide on the switching element at the next simulation step K 2Is it on or off? i con Used to determine the controlled DC current source at the next simulation step I con The current value, i a Used to determine the controlled AC current source at the next simulation step I a The effective value of the current, i b Used to determine the controlled AC current source at the next simulation step I b The effective value of the current, i c Used to determine the controlled AC current source at the next simulation step I c The effective value of the current.

[0023] Among them, the output signal g 1 and g 2 The value can only be 0 or 1; the output signal r , u con , i con , i a , i b , i c The value of can be any value.

[0024] g 1 When the value is 0, the switch element K 1 The state at the next simulation step is disconnected; g 1 When the value is 1, the switch element K 1 The state at the next simulation step is on;

[0025] g 2 When the value is 0, the switch element K 2 The state at the next simulation step is disconnected; g 2 When the value is 1, the switch element K 2 The state at the next simulation step is on;

[0026] r The value of the resistor element is the next simulation stepR The resistance value;

[0027] u con The value of the controlled DC voltage source is the next simulation step U con Voltage value;

[0028] i con The value of the next simulation step is the controlled DC current source I con The current value;

[0029] i a The value of the next simulation step is the controlled AC current source I a The effective value of current;

[0030] i b The value of the controlled AC current source is I b The effective value of current;

[0031] i c The value of the controlled AC current source is I c The effective value of the current.

[0032] The constant parameters set by the control system computing unit before simulation include the rated power P r , Rectification voltage threshold U rec , Inverter voltage threshold U inv , Rectification efficiency η rec , Inverter efficiency η inv , equivalent internal resistance R eq .

[0033] The calculation process of the control system calculation unit at each simulation step includes the following steps:

[0034] Step 1: Judgement Is it established?

[0035] If it is established, the main circuit model is in the standby state (state 1) in the next simulation step, and the control system calculation unit outputs: , the simulation step length calculation is completed;

[0036] If not, go to step 2;

[0037] Step 2: Judgement Is it established?

[0038] If established, proceed to step 3;

[0039] If not, go to step 4;

[0040] Step 3: Judgement Is it established?

[0041] If it is established, the main circuit model will be in constant voltage inverter state (state 2) in the next simulation step, and the control system calculation unit outputs: , the simulation step length calculation is completed;

[0042] If not, the main circuit model is in constant power inverter state (state 3) in the next simulation step, and the control system calculation unit outputs: , the simulation step length calculation is completed;

[0043] Step 4: Judgement Is it established?

[0044] If it is established, the main circuit model will be in constant voltage rectification state (state 4) in the next simulation step, and the control system calculation unit outputs: , , the simulation step length calculation is completed;

[0045] If not, the main circuit model is in constant power rectification state (state 5) in the next simulation step, and the control system calculation unit outputs: , the simulation step calculation is completed.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] 1. The AC / DC bidirectional converter simulation method proposed in the present invention simulates the complex electrical characteristics of the AC / DC bidirectional converter by establishing a universal main circuit model and a control system calculation unit. There is no need to build a converter circuit containing a large number of power electronic devices, which not only meets the engineering accuracy requirements but also ensures the simulation speed.

[0048] 2. The main circuit model, control system calculation unit and simulation calculation process adopted in the present invention are highly versatile, simple to implement and have high calculation accuracy. They can equivalently simulate AC / DC bidirectional converters of various topological structures, and the simulation method is also suitable for implementation on various simulation software (such as Matlab, Pscad, RT_Lab, C++, etc.).

[0049] 3. The present invention can be further expanded as needed. For example, the specific working principle, steps, input parameters and output parameters of the control system computing unit can be further expanded as needed to supplement some more detailed dynamic characteristics and output parameters of the AC / DC bidirectional converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic diagram of an urban rail transit power supply system using a rectifier unit in the prior art;

[0051] Figure 2 It is a schematic diagram of an urban rail transit power supply system using an AC / DC bidirectional converter in the prior art;

[0052] Figure 3 It is a schematic diagram of the main circuit model of the AC / DC bidirectional converter of the present invention;

[0053] Figure 4 It is a control system calculation unit of the AC / DC bidirectional converter of the present invention;

[0054] Figure 5 It is a control relationship diagram between the input and output parameters of the control system calculation unit of the present invention and the components of the main circuit model;

[0055] Figure 6 It is a schematic diagram of the simulation step length calculation of the control system calculation unit of the present invention;

[0056] Figure 7 It is a curve diagram of DC grid voltage variation of the AC / DC bidirectional converter of the present invention;

[0057] Figure 8 It is a power variation curve diagram of the AC / DC bidirectional converter of the present invention;

[0058] Fig. 9 It is a curve diagram of the effective value change of the A-phase current on the AC grid side of the AC / DC bidirectional converter of the present invention. DETAILED DESCRIPTION

[0059] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0060] An embodiment of the present invention provides an AC / DC bidirectional converter simulation method suitable for power flow simulation calculation, comprising the following steps:

[0061] S1: Establish the main circuit model, such as Figure 3 As shown in the figure, the main circuit model consists of an unconnected DC circuit and an AC circuit; the DC circuit has two branches, one of which is composed of a switching element. K 1 , resistor element R , controlled DC voltage sourceU con The second branch is formed by the switch element K 2 , controlled DC current source I con The AC circuit has three branches, each containing a controlled AC current source. I a , controlled AC current source I b , controlled AC current source I c ; A , B , C , D , E The five connection ports of the main circuit model are used in the simulation calculation of the power supply system of rail transit. A , B The two ports are connected to the DC grid side of the rail transit power supply system. C , D , E The three ports are connected to the AC grid side of the rail transit power supply system.

[0062] S2: Establish a control system calculation unit. The control system calculation unit calculates the DC grid-side voltage of the main circuit model collected in the simulation based on the constant parameters set before the simulation. u dc , DC grid side input current i dc And the three-phase voltage on the AC grid side u a , u b , u c , and calculate each simulation step. The output signal of the control system calculation unit is g 1 , r , u con , g 2 , i con , i a , i b , i c ， Output signal g 1 and g 2 The value can only be 0 or 1; the output signal r , ucon , i con , i a , i b , i c The value of can be any value.

[0063] g 1 When the value is 0, the switch element K 1 The state at the next simulation step is disconnected; g 1 When the value is 1, the switch element K 1 The state at the next simulation step is on;

[0064] g 2 When the value is 0, the switch element K 2 The state at the next simulation step is disconnected; g 2 When the value is 1, the switch element K 2 The state at the next simulation step is on;

[0065] r The value of the resistor element is the next simulation step R The resistance value;

[0066] u con The value of the controlled DC voltage source is the next simulation step U con Voltage value;

[0067] i con The value of the next simulation step is the controlled DC current source I con The current value;

[0068] i a The value of the next simulation step is the controlled AC current source I a The effective value of current;

[0069] i b The value of the controlled AC current source is I b The effective value of current;

[0070] i c The value of the controlled AC current source is I c The effective value of the current.

[0071] The constant parameters set by the control system computing unit before simulation include the rated power P r , Rectification voltage threshold U rec , Inverter voltage threshold U inv , Rectification efficiency η rec , Inverter efficiency η inv And the equivalent internal resistance R eq .

[0072] like Figure 5 As shown, the control relationship between the input and output parameters of the control system calculation unit and the components of the main circuit model in the simulation is given by the dotted line connection.

[0073] The calculation process of the control system calculation unit at each simulation step is as follows: Figure 6 As shown, the following steps are included:

[0074] Step 1: Judgement Is it established?

[0075] If it is established, the main circuit model is in the standby state (state 1) in the next simulation step, and the control system calculation unit outputs: , the simulation step length calculation is completed;

[0076] If not, go to step 2;

[0077] Step 2: Judgement Is it established?

[0078] If established, proceed to step 3;

[0079] If not, go to step 4;

[0080] Step 3: Judgement Is it established?

[0081] If it is established, the main circuit model will be in constant voltage inverter state (state 2) in the next simulation step, and the control system calculation unit outputs: , the simulation step length calculation is completed;

[0082] If not, the main circuit model is in constant power inverter state (state 3) in the next simulation step, and the control system calculation unit outputs: , the simulation step length calculation is completed;

[0083] Step 4: Judgement Is it established?

[0084] If it is established, the main circuit model will be in constant voltage rectification state (state 4) in the next simulation step, and the control system calculation unit outputs: , , the simulation step length calculation is completed;

[0085] If not, the main circuit model is in constant power rectification state (state 5) in the next simulation step, and the control system calculation unit outputs: , the simulation step calculation is completed.

[0086] This embodiment is simulated in the simulation software Matlab / Simulink, and the steps are as follows:

[0087] Step 1: Establish a general main circuit model and control system calculation unit. The main circuit model is built using switches, resistors, controlled DC voltage sources, controlled DC current sources, controlled AC current sources, voltage acquisition meters, current acquisition meters and other components in the Matlab / Simulink built-in module library. The control system calculation unit is built using embedded function modules in the Matlab / Simulink built-in module library. According to the control relationship between the input and output parameters of the control system calculation unit and the components of the main circuit model, the input and output parameters of the control system calculation unit and the components of the main circuit model are connected.

[0088] Step 2: Use the software's own programming language to program in the control system calculation unit built by Matlab / Simulink to implement the simulation step calculation principle of the control system calculation unit.

[0089] Step 3: Place the main circuit model A , B The two ports are connected to the DC grid side of the rail transit power supply system model. C , D , E3 The ports are connected to the three-phase AC grid side of the rail transit power supply system model.

[0090] Based on the main circuit model and control system calculation unit, the simulation is performed and the constant parameters of the AC / DC bidirectional converter simulation are set as follows: Rated power P r 1.5MW, rectifier voltage threshold U rec 1540V, inverter voltage thresholdU inv 1700V, rectification efficiency η d 0.95, inverter efficiency η c 0.95, equivalent internal resistance R eq The rated line voltage of the three-phase AC network is 35kV.

[0091] The simulation results are as follows Figure 7-Figure 9 As shown in the figure, some moments in the simulation results are analyzed below.

[0092] When the simulation time is 40s, the DC grid voltage u dc is 1587V, so The AC / DC bidirectional converter model is in state 1 (standby state) according to the working principle of the control system calculation unit, and the power of the AC / DC bidirectional converter model is 0;

[0093] When the simulation time is 20s, the DC grid voltage u dc 1700V, bidirectional converter power is 1.124MW, Not true. Established, The AC / DC bidirectional converter model is in state 2 (constant voltage inverter state) according to the working principle of the control system calculation unit. Therefore, the AC / DC bidirectional converter model maintains the DC grid side voltage at U inv , and the power is positive (i.e., inverting state);

[0094] When the simulation time is 18s, the DC grid voltage u dc 1732V, bidirectional converter power 1.5MW, Not true. Established, Not true. According to the working principle of the control system calculation unit, the AC / DC bidirectional converter model is in state 3 (constant power inversion state). Therefore, the AC / DC bidirectional converter model inverts at a constant rated power, but cannot maintain the DC grid voltage at U inv .

[0095] When the simulation time is 30s, the DC grid voltage u dc 1540V, bidirectional converter power is -0.153MW, Not true. Not true. The AC / DC bidirectional converter model is in state 4 (constant voltage rectification state) according to the working principle of the control system calculation unit. Therefore, the AC / DC bidirectional converter model maintains the DC grid voltage at U rec , and the power is negative (i.e., it is in rectification state);

[0096] When the simulation time is 10s, the DC grid voltage u dc 1480V, bidirectional converter power -1.5MW, Not true. Not true. Not true. According to the working principle of the control system calculation unit, the AC / DC bidirectional converter model is in state 5 (constant power rectification state). Therefore, the AC / DC bidirectional converter model rectifies at a constant rated power, but cannot maintain the DC grid voltage at U rec .

[0097] It can be seen from the simulation results that the AC / DC bidirectional converter simulation model constructed in this embodiment is simple to implement and has high calculation accuracy. It can equivalently simulate the complex electrical external characteristics of the AC / DC bidirectional converter and meet the engineering accuracy requirements.

[0098] The present invention has been described in detail above through embodiments, but the contents described are only exemplary embodiments of the present invention and cannot be considered to limit the scope of implementation of the present invention. The protection scope of the present invention is defined by the claims. Anyone who utilizes the technical solution described in the present invention, or a technician in the field, inspired by the technical solution of the present invention, designs a similar technical solution within the essence and protection scope of the present invention to achieve the above technical effects, or makes equal changes and improvements to the scope of application, etc., shall still fall within the scope of protection covered by the patent of the present invention.

Claims

1. A method for simulating an AC / DC bidirectional converter suitable for power flow simulation calculation, characterized in that: The steps include: S1: Establish the main circuit model, which consists of an unconnected DC circuit and an AC circuit; the DC circuit has two branches, one of which consists of a switch element. K 1 , resistor element R , controlled DC voltage source U con The second branch is formed by the switch element K 2 , controlled DC current source I con The AC circuit has three branches, each containing a controlled AC current source. I a , controlled AC current source I b , controlled AC current source I c ; S2: Establish the control system calculation unit, which is based on the constant parameters set before the simulation and the DC grid voltage of the main circuit model collected in the simulation u dc , DC grid side input current i dc And the three-phase voltage on the AC grid side u a , u b , u c , calculate each simulation step; the output signal of the control system calculation unit is g 1 , r , u con , g 2 , i con , i a , i b , i c ，g 1 Used to determine the switching elements of the main circuit model at the next simulation step K 1 Is it on or off? r Used to determine the resistance element in the next simulation step R The resistance value, u con Used to determine the controlled DC voltage source at the next simulation step U con The voltage value, g 2 Used to decide on the switching element at the next simulation step K 2 Is it on or off? i con Used to determine the controlled DC current source at the next simulation step I con The current value, i a Used to determine the controlled AC current source at the next simulation step I a The effective value of the current, i b Used to determine the controlled AC current source at the next simulation step I b The effective value of the current, i c Used to determine the controlled AC current source at the next simulation step I c The effective value of current; g 1 When the value is 0, the switch element K 1 The state at the next simulation step is disconnected; g 1 When the value is 1, the switch element K 1 The state at the next simulation step is on; g 2 When the value is 0, the switch element K 2 The state at the next simulation step is disconnected; g 2 When the value is 1, the switch element K 2 The state at the next simulation step is on; r The value of the resistor element is the next simulation step R The resistance value; u con The value of the controlled DC voltage source is the next simulation step U con Voltage value; i con The value of the next simulation step is the controlled DC current source I con The current value; i a The value of the next simulation step is the controlled AC current source I a The effective value of current; i b The value of the controlled AC current source is I b The effective value of current; i c The value of the controlled AC current source is I c The effective value of current; Constant parameters include rated power P r , Rectification voltage threshold U rec , Inverter voltage threshold U inv , Rectification efficiency η rec , Inverter efficiency η inv And the equivalent internal resistance R eq； The calculation process of the control system calculation unit in the simulation step length includes the following steps: Step 1: Judgement Is it established? If it is established, the main circuit model is in a standby state in the next simulation step, and the standby state is state 1, and the control system calculation unit outputs: , the simulation step length calculation is completed; If not, go to step 2; Step 2: Judgement Is it established? If established, proceed to step 3; If not, go to step 4; Step 3: Judgement Is it established? If it is established, the main circuit model is in a constant voltage inverter state in the next simulation step, and the constant voltage inverter state is state 2, and the control system calculation unit outputs: , the simulation step length calculation is completed; If not, the main circuit model is in constant power inverter state in the next simulation step, and the constant power inverter state is state 3, and the control system calculation unit outputs: , the simulation step length calculation is completed; Step 4: Judgement Is it established? If it is established, the main circuit model is in a constant voltage rectification state in the next simulation step, and the constant voltage rectification state is state 4, and the control system calculation unit outputs: , the simulation step length calculation is completed; If not, the main circuit model is in constant power rectification state in the next simulation step, and the constant power rectification state is state 5, and the control system calculation unit outputs: , the simulation step calculation is completed.

Images