Electrified railway traction substation negative sequence treatment method and system

Abstract

The invention relates to the technical field of alternating current electrified railway power supply, in particular to a negative sequence treatment method and system for an electrified railway traction substation, and the system comprises a step-down isolation transformer and an SVG compensation subsystem. The primary side of the step-down isolation transformer is used for being connected with the high-voltage wire inlet side of a three-phase power grid of a traction substation, and three-phase medium-voltage electricity is formed after step-down isolation and connected to a three-phase medium-voltage bus. The SVG compensation subsystem is used for being connected to a three-phase medium-voltage bus; the SVG compensation subsystem is used for outputting negative sequence compensation current; the direction of the negative-sequence compensation current is opposite to that of the negative-sequence current generated by the traction load, and the negative-sequence compensation current and the load current generated by the traction load counteract each other, so that the treatment of the negative-sequence current is realized.

Description

Technical Field

[0001] The invention relates to the technical field of AC electrified railway power supply, and in particular to a negative sequence control method and system for an electrified railway traction substation. Background Art

[0002] Electrified railways use single-phase industrial frequency AC system, and their power supply comes from the external power system. The power system uses three-phase AC power supply. In an ideal power system, the three-phase current or voltage is symmetrical, that is, the amplitude of the three-phase current or voltage is equal, and the phase difference is 120 degrees. However, in actual power systems, current or voltage asymmetry may occur due to uneven load distribution, equipment failure or other reasons. In this case, the current or voltage can be decomposed into three components: positive sequence, negative sequence and zero sequence.

[0003] Since the traction load of electrified railways is a single-phase, mobile load, it will inevitably generate negative sequence current in the system. In theory, the negative sequence current can be eliminated through a three-phase-two-phase balanced transformer and reasonable distribution of the load. However, due to the volatility and impact of the traction load, this method is difficult to achieve the purpose of reducing or even eliminating the negative sequence current in actual application.

[0004] Negative sequence current or voltage may cause various hazards to the power system and its equipment. It will not only shorten the service life of the motor, cause unbalanced torque in the motor, and cause wear or damage to mechanical parts; it may also cause dynamic instability of the system, interfere with the normal operation of protection devices, and cause malfunction or failure. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a negative sequence control method and system for an electrified railway traction substation in view of the deficiencies in the above-mentioned prior art, so as to solve the technical problem that negative sequence current in the current electrified railway system harms the system.

[0006] The objective of the present invention is achieved by the following technical solutions: In a first aspect, the present invention provides a negative sequence management system for an electrified railway traction substation, including a step-down isolation transformer and an SVG compensation subsystem; The primary side of the step-down isolation transformer is used to connect the three-phase high-voltage incoming line side of the traction substation, and after step-down isolation, the three-phase medium-voltage electricity is connected to the three-phase medium-voltage busbar; The SVG compensation subsystem is used to be connected to a three-phase medium voltage bus; the SVG compensation subsystem is used to output a negative sequence compensation current; the direction of the negative sequence compensation current is opposite to the direction of the negative sequence current generated by the traction load, and the negative sequence current generated by the traction load offsets each other to achieve negative sequence current control. As a further improvement of the present invention, the step-down isolation transformer is a three-phase transformer in a Y / △ connection form.

[0007] As a further improvement of the present invention, the primary voltage level of the step-down isolation transformer is consistent with the incoming line voltage of the traction substation.

[0008] As a further improvement of the present invention, the SVG compensation subsystem includes a first single-phase cascade SVG, a second single-phase cascade SVG and a third single-phase cascade SVG, wherein the first single-phase cascade SVG is used to connect to the AB phase line in the three-phase medium-voltage bus; the second single-phase cascade SVG is used to connect to the BC phase line in the three-phase medium-voltage bus; and the third single-phase cascade SVG is used to connect to the CA phase line in the three-phase medium-voltage bus.

[0009] As a further improvement of the present invention, the first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG all include a plurality of single-phase H-bridge power units, and the plurality of single-phase H-bridge power units are cascaded.

[0010] As a further improvement of the present invention, the system also includes a voltage transformer and a current transformer arranged on the high-voltage side of the traction substation, and the voltage transformer and the current transformer are respectively used to collect the voltage and current of the traction load on the high-voltage incoming line side of the three-phase power grid, and upload the collected data.

[0011] As a further improvement of the present invention, it also includes a control subsystem, which is used to control and adjust the negative sequence compensation current values ​​corresponding to the first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG according to the negative sequence current generated by the traction load.

[0012] As a further improvement of the present invention, the control strategy of the control subsystem is: Receive the current and voltage of the traction load uploaded by the voltage transformer and the current transformer; calculate the current and voltage of the traction load using the symmetrical component method to obtain the negative sequence current vector value of the traction load; According to the negative sequence current vector value generated by the traction load, the negative sequence compensation current values ​​of the first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG are obtained; according to the negative sequence compensation current value of each single-phase cascade SVG, a corresponding control instruction is generated, and the control instruction is used to control the SVG compensation subsystem to output a current in the opposite direction to the negative sequence current generated by the traction load; The control instructions are converted into corresponding driving signals of the single-phase cascade SVG after pulse width modulation.

[0013] As a further improvement of the present invention, the system further comprises a fault alarm module, which is used to exit the running state and send out an alarm signal when a fault occurs in the SVG compensation subsystem.

[0014] In a second aspect, the present invention provides a method for negative sequence control of an electrified railway traction substation, based on the above-mentioned negative sequence control system of an electrified railway traction substation, comprising: Obtain the incoming line current and voltage at any time of the traction substation; Calculate the negative sequence current vector value formed by the traction load on the three-phase power grid side at the current moment based on the incoming line current and voltage; Based on the output of the three-phase reactive current by the negative sequence control system, a negative sequence compensation current is formed according to the three-phase reactive current; the direction of the negative sequence compensation current is opposite to the direction of the negative sequence current generated by the traction load, and is used to eliminate the negative sequence current at the current moment and complete the negative sequence control at the current moment. The beneficial effects of the present invention are as follows: the present invention provides a negative sequence control system for an electrified railway traction substation. Through voltage reduction isolation, high voltage electricity is converted into three-phase medium voltage electricity and connected to a three-phase medium voltage bus. The voltage level conversion between the high voltage and medium voltage systems is realized, and the step-down isolation transformer converts the high voltage incoming line voltage into the medium voltage level voltage required by the SVG compensation subsystem, realizes the voltage level matching between the high voltage power grid and the SVG compensation subsystem, and ensures that the SVG compensation system can work normally. Moreover, the energy is transferred between the primary and secondary sides of the step-down isolation transformer through electromagnetic induction, thereby realizing electrical isolation. When a fault occurs in the SVG compensation subsystem or the medium voltage bus, the step-down isolation transformer can isolate the fault point to prevent the fault from expanding. The SVG compensation subsystem outputs a negative sequence compensation current, and its direction is opposite to that of the negative sequence current generated by the traction load. By offsetting the negative sequence current generated by the traction load, effective control of the negative sequence current is achieved. The impact of the negative sequence current on the power grid is effectively reduced, and the power quality of the power grid is improved. By compensating for the negative sequence current and reactive power, the voltage fluctuation and distortion of the power grid are reduced, and the power quality of the power grid is improved.

[0015] Furthermore, when the Y / △ connection is used, it is beneficial to suppress the third harmonic on the grid side. Because an important feature of the third harmonic is the same phase, the △ connection can provide a loop for the third harmonic current, so that it forms a circulating current on the triangle side, thereby effectively weakening the transmission of harmonics to the system, avoiding grid voltage waveform distortion, and ensuring power supply quality. When the △ connection is used on the low-voltage side, the phase current is 1 / √3 of the line current, the winding wire cross-sectional area is small, and the processing and manufacturing are easier, which can reduce the manufacturing cost of the winding.

[0016] Furthermore, the cascade structure can reduce the switching frequency of each power unit while achieving the same equivalent switching frequency. This is because the combination of multiple units can be used to achieve the required output waveform modulation. The lower switching frequency can reduce switching losses, thereby improving the efficiency of the entire SVG (static var generator) system.

[0017] Furthermore, the present invention does not consume or generate active current, does not affect the active current of the traction power supply system and the power system, does not generate active power loss, and does not change the conventional traction transformer connection type of the traction substation, and can be applied to the management and transformation of existing traction substations.

[0018] Furthermore, the present invention has high reliability and strong independence, and will not affect the normal power supply of the traction power supply system when the negative sequence compensation system fails to exit. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 It is a structural schematic diagram of a negative sequence control system for an electrified railway traction substation according to an embodiment of the present invention; Figure 2 This is a main circuit diagram of a single-phase cascaded SVG in an embodiment of the present invention; Figure 3 It is a schematic flow chart of a negative sequence compensation method in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose and technical solution of the present invention clearer and easier to understand, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0022] Terminology explanation: SVG: Static VAR generator.

[0023] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments, wherein the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0024] Example 1 like Figure 1-Figure 3As shown, this embodiment provides a negative sequence control system for electrified railway traction substation, including a step-down isolation transformer and an SVG compensation subsystem. Without affecting the normal power supply of the traction power supply system, three sets of single-phase cascaded SVG can be used to output appropriate negative sequence compensation current to control the negative sequence current generated by the traction load, so that it meets the standard or even completely eliminates the negative sequence current. The following is a specific implementation method.

[0025] The primary side of the step-down isolation transformer is used to connect the high-voltage incoming line side of the three-phase power grid of the traction substation, and after step-down isolation, the three-phase medium voltage is connected to the three-phase medium voltage bus. In this embodiment, the step-down isolation transformer is a three-phase transformer with Y / △ connection.

[0026] The primary voltage level of the step-down isolation transformer is consistent with the incoming voltage of the traction substation. Specifically, the primary voltage level can be 110kV, 220kV or 330kV, and the secondary voltage level can be 35kV or 10kV.

[0027] The SVG compensation subsystem is used to be connected to the three-phase medium voltage busbar, and the SVG compensation subsystem is used to output a negative sequence compensation current; the direction of the negative sequence compensation current is opposite to the direction of the negative sequence current generated by the traction load, and the negative sequence current generated by the traction load offsets each other to achieve negative sequence current control.

[0028] Specifically, the SVG compensation subsystem includes three single-phase cascade SVGs, namely, a first single-phase cascade SVG1, a second single-phase cascade SVG2, and a third single-phase cascade SVG3. The first single-phase cascade SVG is used to connect to the AB phase line in the three-phase medium voltage bus; the second single-phase cascade SVG is used to connect to the BC phase line in the three-phase medium voltage bus; and the third single-phase cascade SVG is used to connect to the CA phase line in the three-phase medium voltage bus. Figure 2 As shown, the first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG all include n single-phase H-bridge power units, and the n single-phase H-bridge power units are cascaded, where n>1.

[0029] The single-phase H-bridge power unit is mainly composed of four power switching devices (usually insulated gate bipolar transistors IGBTs) and anti-parallel diodes. These four switching devices form an "H"-shaped bridge structure. In addition, it also includes a DC side capacitor for storing electrical energy and providing DC voltage support. When a forward voltage is required to be output, the two switching devices located at the top of the "H" bridge (such as IGBT1 and IGBT3) are turned on, while the two switching devices at the bottom (such as IGBT2 and IGBT4) are turned off. At this time, the positive end of the DC side capacitor is connected to the AC output terminal through the turned-on IGBT1, and the negative end of the DC side capacitor is connected to the other pole of the AC output terminal through the turned-on IGBT3, thus generating a positive voltage at the AC output terminal.

[0030] If a negative voltage is to be output, the two lower switch devices are turned on and the two upper switch devices are turned off. In this way, the positive end of the DC side capacitor is connected to the negative pole of the AC output terminal through IGBT4, and the negative end of the DC side capacitor is connected to the positive pole of the AC output terminal through IGBT2, thereby generating a negative voltage at the AC output terminal. Similarly, the amplitude of this negative voltage can be adjusted through PWM technology.

[0031] When IGBT1 and IGBT4 are turned on at the same time, or IGBT2 and IGBT3 are turned on at the same time, the AC output terminal will be short-circuited, and the output voltage is zero. This zero voltage output mode is also used in some control strategies, such as when fine adjustment of the output waveform is required or soft switching is realized.

[0032] In the SVG compensation subsystem, multiple single-phase H-bridge power units are cascaded. The output of each unit is connected in sequence, for example, the output of the first unit is connected to the input of the second unit. For the total output voltage after cascading, it is equal to the algebraic sum of the output voltages of each single-phase H-bridge power unit. By reasonably controlling the output voltage of each single-phase H-bridge power unit (using pulse width modulation technology to control the on and off time of the switching device), it is possible to achieve precise control of the output voltage and current of the cascaded SVG to output the negative sequence compensation current that meets the requirements. This cascade structure can not only improve the output voltage level, but also improve the output waveform quality, reduce the switching frequency of a single power unit, and enhance the redundancy of the system to a certain extent.

[0033] This cascade structure has significant advantages. First, the cascade of multiple single-phase H-bridge power units can greatly increase the output voltage level. The output voltage amplitude of a single H-bridge power unit is limited. Through cascading, the output voltage is the sum of the output voltages of each unit, so that it can adapt to application scenarios of different voltage levels and meet the high-voltage requirements of electrified railway traction substations. Secondly, while achieving the same equivalent switching frequency, the switching frequency of each power unit can be reduced. The lower switching frequency effectively reduces switching losses and improves the operating efficiency of the entire SVG system. Furthermore, the cascade structure helps to improve the waveform quality. With the help of advanced modulation strategies such as carrier phase shift modulation (CPS-SPWM), as the number of cascaded units increases, the equivalent switching frequency of the output waveform increases, which can more accurately approach the sine wave, significantly reduce the harmonic content of the output, and greatly improve the power quality of the power grid.

[0034] The system also includes voltage transformers and current transformers installed on the high-voltage side of the traction substation. The voltage transformers and current transformers are used to collect the voltage and current of the traction load on the high-voltage incoming line side of the three-phase power grid, and upload the collected data to the SVG compensation subsystem.

[0035] The SVG compensation subsystem also includes a control subsystem (i.e., the negative-sequence management control subsystem in the figure), which is used to control and adjust the negative-sequence compensation current values ​​corresponding to the first single-phase cascade SVG, the second single-phase cascade SVG, and the third single-phase cascade SVG according to the negative-sequence current generated by the traction load.

[0036] Among them, the control strategy of the control subsystem is: Receive the current and voltage of the traction load uploaded by the voltage transformer and the current transformer; calculate the current and voltage of the traction load to obtain the negative sequence current vector value of the traction load.

[0037] The negative sequence current vector value generated by the traction load is calculated by using a symmetrical component method or a fast Fourier transform (FFT) algorithm. In this embodiment, the negative sequence current vector value generated by the traction load is calculated by using a symmetrical component method.

[0038] According to the negative sequence current vector value generated by the traction load, the negative sequence compensation current values ​​of the first single-phase cascade SVG1, the second single-phase cascade SVG2 and the third single-phase cascade SVG3 are obtained; according to the negative sequence compensation current value of each single-phase cascade SVG, a corresponding control instruction is generated, and the control instruction is used to control the SVG compensation subsystem to output a current in the opposite direction to the negative sequence current generated by the traction load; After pulse width modulation, the control instructions are converted into corresponding single-phase cascaded SVG drive signals. In the SVG compensation subsystem, pulse width modulation can accurately control the on and off time of the switching devices (such as IGBT, etc.) in the single-phase H-bridge power unit. When the pulse width modulation signal acts on the switching devices of the upper and lower bridge arms, the output voltage can be controlled by changing the ratio of the upper bridge arm on time to the cycle (i.e., duty cycle). In the case of multiple single-phase H-bridge power units cascaded, the output voltage and current of the entire SVG can be accurately controlled by pulse width modulation control of each unit.

[0039] The current control of the SVG compensation subsystem adopts a closed-loop control strategy, such as proportional-integral (PI) control or proportional-integral-differential (PID) control. According to the compensation current command, the output current is adjusted in real time to track the change of the command current. By controlling the switching state of the H-bridge power unit, the SVG output current is accurately controlled. Taking PI control as an example, it adjusts the output current according to the deviation between the compensation current command and the actual output current by adjusting the control parameters (such as the coefficient and the integral time constant), so that the output current can closely track the change of the command current. This closed-loop control method can effectively improve the stability and control accuracy of the system, and ensure that the negative sequence compensation current output by the SVG can accurately offset the negative sequence current generated by the traction load.

[0040] By controlling the switching state of the H-bridge power unit, the output current of the SVG can be precisely controlled.

[0041] This system can be applied to existing traction substations or newly built traction substations without changing the connection type of the traction transformer of the traction substation. It is suitable for various traction transformer connection types, including three-phase Vv, Vx type transformers, three-phase-two-phase balanced transformers, single-phase transformers, YNd type, etc.

[0042] In addition, the negative sequence management system also includes a fault alarm module, which is used to exit the operating state and send out an alarm signal when a fault occurs in the SVG compensation subsystem. The negative sequence management system of the electrified railway traction substation has the characteristics of high reliability and strong independence. When the negative sequence management system fails and exits operation, it is independent of the traction power supply system and does not affect the normal power supply of the traction power supply system.

[0043] By accurately calculating the negative sequence current vector value generated by the traction load and generating and controlling the corresponding negative sequence compensation current accordingly, the negative sequence current can be effectively controlled within the standard range, and even completely eliminated in an ideal situation. This is of vital importance to improving the power quality of the power grid and reducing various adverse effects caused by negative sequence current.

[0044] Example 2 like Figure 3 As shown, based on the negative-sequence control system for electrified railway traction substation in Example 1, this embodiment provides a negative-sequence control method for electrified railway traction substation, and the following is a specific implementation method.

[0045] Obtain the incoming line current and voltage at any time of the traction substation. In this embodiment, the negative sequence management control system calculates the negative sequence current vector value formed by the traction load on the three-phase power grid side at time a by monitoring the incoming line current and voltage at time a of the traction substation. Wherein, time a can be a certain moment in history or the current moment.

[0046] The negative-sequence current vector value formed by the traction load on the three-phase grid side at the current moment is calculated based on the incoming line current and voltage; the negative-sequence management control system controls the three single-phase cascaded SVGs to emit a suitable reactive current combination at time a, so that the negative-sequence current formed by the three-phase reactive current combination is opposite in direction to the negative-sequence current generated by the traction load.

[0047] Based on the output of three-phase reactive current by the negative sequence control system, a negative sequence compensation current is formed according to the three-phase reactive current; the direction of the negative sequence compensation current is opposite to the direction of the negative sequence current generated by the traction load, and is used to eliminate the negative sequence current at the current moment and complete the negative sequence control at the current moment. The negative sequence currents at moment a offset each other, completing the negative sequence control at the current moment, and the negative sequence control system enters the next moment of monitoring and controlling the three single-phase cascaded SVGs to output appropriate compensation negative sequence currents.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A negative sequence control system for an electrified railway traction substation, characterized in that: Includes step-down isolation transformer and SVG compensation subsystem; The primary side of the step-down isolation transformer is used to connect the high-voltage incoming line side of the three-phase power grid of the traction substation, and after step-down isolation, the three-phase medium voltage electricity is connected to the three-phase medium voltage busbar; The SVG compensation subsystem is used to be connected to a three-phase medium voltage bus; the SVG compensation subsystem is used to output a negative sequence compensation current; the direction of the negative sequence compensation current is opposite to the direction of the negative sequence current generated by the traction load, and the negative sequence current generated by the traction load offsets each other to achieve negative sequence current control.

2. The negative sequence control system for electrified railway traction substation according to claim 1 is characterized in that: The step-down isolation transformer is a three-phase transformer with Y / △ connection.

3. The negative sequence control system for electrified railway traction substation according to claim 2 is characterized in that: The primary voltage level of the step-down isolation transformer is consistent with the incoming line voltage of the traction substation.

4. The negative sequence control system for electrified railway traction substation according to claim 1 is characterized in that: The SVG compensation subsystem includes a first single-phase cascade SVG, a second single-phase cascade SVG and a third single-phase cascade SVG. The first single-phase cascade SVG is used to connect to the AB phase line in the three-phase medium-voltage bus; the second single-phase cascade SVG is used to connect to the BC phase line in the three-phase medium-voltage bus; the third single-phase cascade SVG is used to connect to the CA phase line in the three-phase medium-voltage bus.

5. The negative sequence control system for electrified railway traction substation according to claim 4 is characterized in that: The first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG all include a plurality of single-phase H-bridge power units, and the plurality of single-phase H-bridge power units are cascaded.

6. The negative sequence control system for electrified railway traction substation according to claim 1 is characterized in that: The system also includes a voltage transformer and a current transformer arranged on the high-voltage side of the traction substation. The voltage transformer and the current transformer are respectively used to collect the voltage and current of the traction load on the high-voltage incoming line side of the three-phase power grid, and upload the collected data.

7. The negative sequence control system for electrified railway traction substation according to claim 4, characterized in that: The SVG compensation subsystem also includes a control subsystem, which is used to control and adjust the negative sequence compensation current values ​​corresponding to the first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG according to the negative sequence current generated by the traction load.

8. The negative sequence control system for electrified railway traction substation according to claim 7 is characterized in that: The control strategy of the control subsystem is: Receive the current and voltage of the traction load uploaded by the voltage transformer and the current transformer; calculate the current and voltage of the traction load using the symmetrical component method to obtain the negative sequence current vector value of the traction load; According to the negative sequence current vector value generated by the traction load, the negative sequence compensation current values ​​of the first single-phase cascade SVG, the second single-phase cascade SVG and the third single-phase cascade SVG are obtained; Generate corresponding control instructions according to the negative sequence compensation current value of each single-phase cascaded SVG, wherein the control instructions are used to control the SVG compensation subsystem to output a current in the opposite direction to the negative sequence current generated by the traction load; The control instructions are converted into corresponding driving signals of the single-phase cascade SVG after pulse width modulation.

9. The negative sequence control system for electrified railway traction substation according to claim 4, characterized in that: The system also includes a fault alarm module, which is used to exit the running state and send out an alarm signal when a fault occurs in the SVG compensation subsystem.

10. A method for negative sequence control of an electrified railway traction substation, based on the negative sequence control system of an electrified railway traction substation according to any one of claims 1 to 9, characterized in that: include: Obtain the incoming line current and voltage at any time of the traction substation; Calculate the negative sequence current vector value formed by the traction load on the three-phase power grid side at the current moment based on the incoming line current and voltage; Based on the negative-sequence control system outputting three-phase reactive current, a negative-sequence compensation current is formed according to the three-phase reactive current; the direction of the negative-sequence compensation current is opposite to the direction of the negative-sequence current generated by the traction load, and is used to eliminate the negative-sequence current at the current moment and complete the negative-sequence control at the current moment.

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