Method for resisting commutation failure by switching nonlinear resistance auxiliary flexible LCC

Abstract

The application discloses a method for resisting commutation failure of a switching nonlinear resistance auxiliary flexible LCC, and comprises the following steps: connecting a controllable voltage source and a nonlinear resistance module in series on the valve side of a converter transformer on the inverter side of high-voltage direct-current power transmission; the controllable voltage source outputs capacitive voltage in a carrier phase-shifted modulation mode; and the nonlinear resistance module is controlled to be turned on and turned off by an IGBT group to switch a lightning arrester in a line on the valve side of the converter transformer. In the application, the nonlinear resistance module composed of the lightning arrester provides a direct-current voltage increment for the converter, makes up for the drop of the direct-current voltage on the inverter side caused by power grid failure, effectively suppresses the occurrence of the commutation failure problem, and further improves the ability of the flexible LCC to resist the commutation failure.

Description

Technical Field

[0001] This invention belongs to the technical field of suppressing high-voltage direct current commutation failure, and specifically relates to a method for switching nonlinear resistors to assist flexible LCC in resisting commutation failure. Background Technology

[0002] High Voltage Direct Current (HVDC) transmission has been widely used in long-distance power transmission and asynchronous grid interconnection due to its advantages such as long transmission distance, large transmission capacity, and low transmission loss. Among them, the Line Commuted Converter (LCC) technology is one of the most common conversion methods in HVDC at present. However, its inverter side is prone to commutation failure when faults such as voltage drop or short circuit occur, resulting in a sudden increase in DC current and a sharp drop in DC voltage. In severe cases, it may even cause DC blocking, which seriously impacts the safe and stable operation of the power grid.

[0003] Commutation failures are frequently reported. For example, the State Grid Corporation of China recorded a commutation failure at a converter station in 2013, which resulted in a power loss of 4530 MW and a significant decrease in the frequency of the AC system on the inverter side. To reduce the probability of commutation failures, scholars have conducted extensive research. Many studies have analyzed and proposed schemes for LCCs to mitigate commutation failures, which can be mainly divided into three categories: The first category involves installing reactive power compensation devices at the AC bus to improve the stability of the bus voltage. This type of scheme is less effective at suppressing commutation failures in the early stages of a fault. Furthermore, as the DC transmission capacity increases, the required reactive power compensation capacity becomes larger, reducing the economic efficiency of system operation. The second category involves modifying the converter's control strategy. The common approach is to adjust the control system setpoint in a timely manner according to the fault conditions, increasing the thyristor arc-extinguishing margin, thereby reducing the probability of commutation failures. Common solutions include commutation failure prevention (CFPREV) and voltage-dependent current order limiters (VDCOLs) that reduce the rectifier-side command current based on DC voltage disturbances. Due to the limited adjustment speed and effectiveness of the control system, these solutions primarily address continuous commutation failures after a fault occurs. Furthermore, these solutions often increase the extinction angle of the converter valve by advancing the firing angle, which increases the reactive power demand of the converter, further exacerbating AC-side faults and impacting normal commutation. A third approach involves modifying the converter topology by adding auxiliary commutation devices on the AC side. These devices alter the sinusoidal commutation voltage, increasing the extinction angle and ensuring sufficient blocking capability of the thyristors. These solutions often improve commutation failure performance but also suffer from high equipment costs.

[0004] In conclusion, it is urgent to propose a solution that can effectively suppress LCC commutation failure and avoid the above-mentioned drawbacks. Summary of the Invention

[0005] The purpose of this invention is to provide a method for switching nonlinear resistors to assist flexible LCC in resisting commutation failure, so as to solve the commutation failure problem that easily occurs in LCC systems.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A method for using a switching nonlinear resistor to assist a flexible LCC in resisting commutation failure includes the following steps:

[0008] S1. A controllable voltage source input terminal is connected in series at the AC terminal of the grid-commutated converter on the inverter side of the high-voltage direct current transmission. The output terminal of the controllable voltage source is connected in series with the input terminal of the nonlinear resistor module. The output terminal of the nonlinear resistor module is connected to the valve side of the inverter-side converter transformer. The grid side of the inverter-side converter transformer is connected to the AC bus. The nonlinear resistor module includes a series sub-module, and the sub-module includes a parallel IGBT group and a nonlinear resistor.

[0009] S2, assuming thyristor VT x To thyristor VT y Commutation;

[0010] Within a 60-degree range before commutation, n nonlinear resistors R1 are connected to the thyristor VT. x The converter transformer valve side line of the phase is already connected and generates the DC compensation voltage for that phase.

[0011] S3, during commutation, thyristor VT y Upon receiving the trigger signal, n nonlinear resistors R2 are simultaneously connected to the valve side circuit of the converter transformer in the third phase where thyristor commutation does not occur. At this time, nonlinear resistors R1 and R2 exist simultaneously in the valve side circuit of the converter transformer, jointly generating a DC compensation voltage until the commutation ends. The third phase where thyristor commutation does not occur is the non-commutation phase.

[0012] S4. After the commutation is completed, the n nonlinear resistors R1 are immediately removed from the line. At this time, only the nonlinear resistor R2 remains in the line, generating a DC compensation voltage within the 60-degree range.

[0013] Furthermore, in step S1, the nonlinear resistor is a surge arrester.

[0014] Furthermore, in step S1, the controllable voltage source outputs capacitive voltage using carrier phase-shift modulation.

[0015] Furthermore, in step S1, the IGBT group includes two IGBTs connected in reverse series.

[0016] Furthermore, the IGBT is equipped with an anti-parallel diode, and the simultaneous on or off of the two IGBTs controls the switching of the nonlinear resistor in the valve-side circuit of the converter transformer.

[0017] Furthermore, in steps S2-S4, the number of sub-modules in the nonlinear resistor module that are switched on or off in the line is selected by looking up a table according to the range of AC voltage drop on the inverter side during a fault, based on a pre-established table.

[0018] Furthermore, the total DC voltage increment generated by the switching of the nonlinear resistor module in steps S2-S4 is calculated through the following steps:

[0019] SA1. Calculate the average DC voltage increment ΔU caused by the continuous operation of a single-phase surge arrester. d0 ;

[0020] ΔU d0 =U arr

[0021] In the formula: U arr This is the voltage value at which the surge arrester functions;

[0022] SA2, based on the commutation overlap angle of the thyristor and the voltage value U when the surge arrester is in operation. arr Calculate the average DC voltage increment ΔU generated by the off-phase surge arrester during the simultaneous activation of two-phase surge arresters. d1 ;

[0023] SA3, based on the average DC voltage increment ΔU generated by the continuous operation of a single-phase surge arrester. d0 The average DC voltage increment ΔU generated by the off-phase surge arrester during the simultaneous activation of two-phase surge arresters. d1 Calculate the total DC voltage increment ΔU generated by the surge arrester. d ;

[0024] SA4. Calculate the total DC voltage increment ΔU generated by the nonlinear resistor module according to the following formula. dtotal :

[0025] ΔU dtotal =nΔU d ;

[0026] In the formula: n is the number of nonlinear resistor submodules put into operation in a phase.

[0027] Furthermore, in step SA2, the average DC voltage increment ΔU generated by the off-phase surge arrester during the simultaneous activation of both phases is calculated according to the following formula. d1 :

[0028]

[0029] In the formula: μ is the commutation overlap angle of the thyristor commutation.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] This invention provides a DC voltage increment to the converter through a nonlinear resistor module composed of surge arresters, compensating for the DC voltage drop on the inverter side caused by grid faults and suppressing the surge in DC current after a fault. In conjunction with a controllable voltage source to assist converter commutation, it can effectively reduce the converter's commutation time, increase the arc-extinguishing angle, improve the ability of the flexible LCC to withstand commutation failure, and prevent commutation failure from occurring. It has the advantages of simple operation and high reliability.

[0032] As a nonlinear resistor, a surge arrester exhibits high resistance under normal voltage conditions and abruptly switches to low resistance under overvoltage conditions. Compared to traditional resistors where the voltage across the arrester changes linearly with current flow, the voltage across the arrester in the low-resistance state is essentially equal to the residual voltage. This provides a stable DC voltage increment for the converter. By calculating and selecting appropriate arrester parameters, sufficient compensation voltage can be provided during voltage dip faults, assisting commutation and reducing the risk of commutation failure. Furthermore, the cost of a series nonlinear resistor composed of IGBTs and zinc oxide arresters is lower than that of auxiliary commutation devices, reducing equipment costs. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art are briefly introduced below; obviously, the drawings described below are some embodiments of the present invention, and those skilled in the art can obtain other drawings based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the inverter topology of a high-voltage direct current transmission line;

[0035] Figure 2a This is a schematic diagram of the working principle of the nonlinear resistor module before commutation;

[0036] Figure 2b This is a schematic diagram of the working principle of the nonlinear resistor module during commutation;

[0037] Figure 2c This is a schematic diagram of the working principle of the nonlinear resistor module after commutation;

[0038] Figure 3a These are the three-phase AC voltage waveforms before and after the nonlinear resistor module is switched on and off;

[0039] Figure 3b This is a waveform diagram of the DC terminal voltage on the inverter side before and after the nonlinear resistor module is switched on and off.

[0040] Figure 2a , Figure 2b and Figure 2c In the diagram, gray branches represent branches through which no current flows. Detailed Implementation

[0041] To make the objectives, technical effects, and technical solutions of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention. Based on the embodiments disclosed in the present invention, other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0042] Please see Figure 1 , Figure 1 This is a schematic diagram of the inverter topology of a high-voltage direct current transmission line. A controllable voltage source (CVS) input is connected in series at the AC terminal of the grid-commutated converter on the inverter side. The output of the CVS is connected in series with the input of a nonlinear resistor module (NLR). The output of the NLR is connected to the valve side of the inverter-side converter transformer. The grid side of the inverter-side converter transformer is connected to the AC bus. The CVS is composed of a series full-bridge sub-module, and the NLR is composed of a sub-module consisting of a surge arrester and a series IGBT group connected in series.

[0043] The controllable voltage source and the grid-commutated converter form a flexible LCC, and the controllable voltage source outputs capacitive voltage using carrier phase-shift modulation.

[0044] In the nonlinear resistor submodule, the series IGBT group consists of two identical IGBTs connected in reverse series, each with an anti-parallel diode. The series IGBT group and the surge arrester form a parallel structure. The two IGBTs are simultaneously turned on or off to control the switching of the surge arrester in the valve-side circuit of the converter transformer. When VR1 and VR2 in the series IGBT group in the nonlinear resistor submodule are simultaneously turned off, the surge arrester is connected to the valve-side circuit of the converter transformer; when VR1 and VR2 are simultaneously turned on, the surge arrester is disconnected from the valve-side circuit of the converter transformer.

[0045] Figures 2a to 2c In the middle, R a R represents the surge arrester in the nonlinear resistor module of phase a. b R represents the surge arrester in the nonlinear resistor module of phase b. c This refers to the surge arrester in the nonlinear resistor module of phase c.

[0046] Please see Figure 2a Taking the commutation process from valve arm 4 to valve arm 6 of a 6-pulse inverter as an example, this invention illustrates the method provided by the present invention for using a switching nonlinear resistor-assisted flexible LCC to resist commutation failure. When a voltage drop occurs on the AC bus, the commutation voltage also drops accordingly. In the 60-degree range before commutation from valve arm 4 to valve arm 6, the surge arrester R... a (The actual number of sub-modules n switched on in the nonlinear resistor module is determined by the voltage drop, and here n is assumed to be 1) It is in the connected state in the valve side line of phase A of the converter transformer, generating the DC compensation voltage of phase A, increasing the positive potential of the DC terminal, and increasing the DC voltage.

[0047] Please see Figure 2b When the commutation from valve arm 4 to valve arm 6 begins, that is, when thyristor VT6 receives the trigger conduction signal, the surge arrester R... cSynchronously connected to the C-phase valve side line of the converter transformer, at this time the surge arrester R a and R c Both exist simultaneously in the valve-side circuit of the converter transformer, and both generate DC compensation voltage. Additionally, the surge arrester R... a It also generates additional auxiliary commutation voltage to promote thyristor commutation until commutation is complete.

[0048] Calculate the potential U at the midpoint of the two phases AB of the bridge arm, i.e., the positive terminal of the DC end. dp for:

[0049]

[0050] Among them, u a Let u be the voltage of phase A of the power grid. b This refers to the voltage of phase B of the power grid.

[0051] It can be seen that the DC terminal positive potential increased by U compared to the voltage without a surge arrester. arr / 2. Therefore, in R a and R c During the same period, the surge arrester R of the off phase is activated. a The average increment of DC voltage ΔU d1 for:

[0052]

[0053] In the formula: μ is the commutation overlap angle of the thyristor commutation.

[0054] Please see Figure 2c After the commutation of valve arm 4 to valve arm 6 is completed, the control IGBT group will control the surge arrester R. a Immediately disconnect it from the line; at this point, only surge arrester R remains in the line. c It continues to generate the C-phase DC compensation voltage U over the subsequent 60-degree period. arr This reduces the negative potential of the DC terminal, thereby increasing the DC voltage. This is triggered by thyristor VT6 and R is simultaneously applied. c Until R c During the disconnection period, single-phase surge arrester R c The average increase in DC voltage ΔU generated by continuous input d0 for:

[0055] ΔU d0 =U arr (3)

[0056] In the formula: U arr The voltage value at which the surge arrester functions.

[0057] Combining the voltage increments from equations (1) and (3), calculate the total DC voltage increment ΔU generated by the surge arrester.d for:

[0058]

[0059] In summary, all valve arm commutation processes can be deduced by analogy. The switching of the nonlinear resistor module in the line only occurs during the first power frequency cycle after an AC voltage drop fault occurs on the inverter side. Its role in the line is to generate DC compensation voltage and also to generate auxiliary commutation voltage in the off-phase to prevent valve arm commutation failure.

[0060] The total DC voltage increment ΔU generated by the nonlinear resistor module dtotal for:

[0061] ΔU dtotal =nΔU d .

[0062] Where n represents the number of nonlinear resistor submodules in operation in a single phase. In the control system, this value is selected using a lookup table based on a pre-established table, according to the range of AC voltage drop on the inverter side during a fault.

[0063] The three-phase AC voltage waveforms before and after switching the nonlinear resistor module are as follows: Figure 3a As shown, u a u b u c The AC three-phase voltage during normal commutation operation, u' a 、u' b 、u' c This represents the AC three-phase voltage after the nonlinear resistor module is connected. The nonlinear resistor module is connected to the line in the latter 60-degree range of the 120-degree conduction range of a single valve arm phase voltage. It increases the phase voltage of the valve arm, improves the voltage drop caused by grid faults, and assists in normal phase commutation between valve arms.

[0064] The DC voltage waveforms on the inverter side before and after switching the linear resistor module are as follows: Figure 3b As shown, u ab u ac u bc u ba u ca u cb This is the AC line voltage during normal commutation operation, v d This is the DC terminal voltage on the inverter side during normal commutation operation; u' ab 、u' ac 、u' bc 、u' ba 、u' ca 、u' cb v' is the AC line voltage after the nonlinear resistor module is connected. dThis represents the DC terminal voltage on the inverter side after the nonlinear resistor module is connected. Over one power frequency cycle, the DC terminal voltage on the inverter side with the nonlinear resistor module connected increases by ΔU compared to the traditional inverter side without the nonlinear resistor module connected. d .

[0065] This invention can be applied to high-voltage direct current power systems. By adopting the nonlinear resistor switching scheme proposed in this invention, combined with topology modification and controllable voltage source control, the commutation failure caused by voltage drop in the system can be solved. This is of great significance for preventing inverter failure, improving the operational reliability of DC transmission systems, and ensuring the good economic benefits of power systems.

[0066] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still make modifications or equivalent substitutions to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are within the protection scope of the claims of the present invention pending approval.

Claims

1. A method for using a nonlinear resistor-assisted flexible LCC to resist commutation failure, characterized in that, Includes the following steps: S1. A controllable voltage source input terminal is connected in series at the AC terminal of the grid-commutated converter on the inverter side of the high-voltage direct current transmission. The output terminal of the controllable voltage source is connected in series with the input terminal of the nonlinear resistor module. The output terminal of the nonlinear resistor module is connected to the valve side of the inverter-side converter transformer. The grid side of the inverter-side converter transformer is connected to the AC bus. The nonlinear resistor module includes a series sub-module, and the sub-module includes a parallel IGBT group and a nonlinear resistor. S2, assuming thyristor VT x To thyristor VT y Commutation; Within a 60-degree range before commutation, n nonlinear resistors R1 are connected to the thyristor VT. x The converter transformer valve side line of the phase is already connected and generates the DC compensation voltage for that phase. S3, during commutation, thyristor VT y Upon receiving the trigger signal, n nonlinear resistors R2 are simultaneously connected to the valve side circuit of the converter transformer in the third phase where thyristor commutation does not occur. At this time, nonlinear resistors R1 and R2 exist simultaneously in the valve side circuit of the converter transformer, jointly generating a DC compensation voltage until the commutation ends. The third phase where thyristor commutation does not occur is the non-commutation phase. S4. After the commutation is completed, the n nonlinear resistors R1 are immediately removed from the line. At this time, only the nonlinear resistor R2 remains in the line, generating a DC compensation voltage within the 60-degree range.

2. The method for switching a nonlinear resistor to assist a flexible LCC in resisting commutation failure according to claim 1, wherein in step S1, the nonlinear resistor is a surge arrester.

3. In the method for switching nonlinear resistors to assist flexible LCC in resisting commutation failure according to claim 1, in step S1, the controllable voltage source outputs capacitive voltage using carrier phase-shift modulation.

4. The method for switching nonlinear resistors to assist flexible LCC in resisting commutation failure according to claim 1, wherein in step S1, the IGBT group includes two IGBTs connected in reverse series.

5. The method for switching nonlinear resistors to assist flexible LCC in resisting commutation failure according to claim 4, wherein the IGBT is equipped with an anti-parallel diode, and the two IGBTs simultaneously turn on or off to control the switching of the nonlinear resistor in the valve side line of the converter transformer.

6. The method for resisting commutation failure using a nonlinear resistor-assisted flexible LCC according to claim 1, characterized in that, In steps S2-S4, the number of sub-modules in the nonlinear resistor module that are switched on or off in the line is selected by looking up a table according to the range of AC voltage drop on the inverter side during a fault, based on a pre-established table.

7. The method for resisting commutation failure using a nonlinear resistor-assisted flexible LCC according to claim 1, characterized in that, The total DC voltage increment generated by the switching of the nonlinear resistor module in steps S2-S4 is calculated through the following steps: SA1. Calculate the average DC voltage increment ΔU caused by the continuous operation of a single-phase surge arrester. d0 ; DU d0 = U arr In the formula: U arr This is the voltage value at which the surge arrester functions; SA2, based on the commutation overlap angle of the thyristor and the voltage value U when the surge arrester is in operation. arr Calculate the average DC voltage increment ΔU generated by the off-phase surge arrester during the simultaneous activation of two-phase surge arresters. d1 ; SA3, based on the average DC voltage increment ΔU generated by the continuous operation of a single-phase surge arrester. d0 The average DC voltage increment ΔU generated by the off-phase surge arrester during the simultaneous activation of two-phase surge arresters. d1 Calculate the total DC voltage increment ΔU generated by the surge arrester. d ; SA4. Calculate the total DC voltage increment ΔU generated by the nonlinear resistor module according to the following formula. dtotal : ΔU dtotal =nΔU d ; In the formula: n is the number of nonlinear resistor submodules put into operation in a phase.

8. The method for resisting commutation failure using a nonlinear resistor-assisted flexible LCC according to claim 7, characterized in that, In step SA2, the average DC voltage increment ΔU generated by the off-phase surge arrester during the simultaneous activation of both phases is calculated according to the following formula. d1 : In the formula: μ is the commutation overlap angle of the thyristor commutation.

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