Open-circuit test method and device for direct-current power transmission system

Abstract

The invention discloses an open-circuit test method and device for a direct-current power transmission system, and belongs to the technical field of power electronic transverters, and the method comprises the steps: switching a control mode of the direct-current power transmission system to a no-load pressurization test mode, switching a direct-current field switch to a no-load pressurization test state, and carrying out the open-circuit test of the direct-current power transmission system; the outer loop control in the no-load pressurization test mode is switched into the sub-module average voltage control; closing a bypass switch on the direct current side of the to-be-tested converter valve; the converter valve to be tested comprises at least one of a first converter valve and a second converter valve; closing an inlet wire switch at the alternating current side of the converter valve to be tested so as to enable the converter valve to be tested to enter a short-circuit charging state; after charging of the to-be-tested converter valve is completed, the to-be-tested converter valve is unlocked according to a first preset voltage; turning off a bypass switch on the direct current side of the converter valve to be tested; and raising the direct-current voltage of the to-be-tested converter valve according to a preset target. The system can meet the requirements of the open-circuit test of the extra-high-voltage flexible direct-current power transmission system.

Description

Technical Field

[0001] This application relates to the field of power electronic converter technology, and in particular to an open-circuit test method and apparatus for a DC transmission system. Background Technology

[0002] With the rapid development and widespread application of new energy power generation technologies, the proportion of new energy power generation in the power system is gradually increasing. However, new energy power generation typically relies on power electronic equipment to connect to the grid, and these devices themselves are relatively weak in terms of disturbance resistance and support capabilities. As the proportion of new energy power generation continues to increase, the low inertia characteristic of the power system becomes more pronounced, posing a greater challenge to the stability of the power system and becoming a significant factor hindering the large-scale development of new energy under the goal of carbon reduction and energy conservation. Flexible DC transmission technology based on modular multilevel converters (MMC) has advantages such as independent control of active and reactive power and the ability to supply power to passive systems. It can stably connect high-proportion new energy or 100% pure new energy, becoming a key technology for solving the problem of large-scale new energy grid connection and transmission.

[0003] Ultra-high voltage flexible direct current transmission systems employ multiple voltage source converters in series, which can multiply the transmission capacity. With the continuous increase in the scale of newly built new energy power plants, the demand for transmitting new energy power using ultra-high voltage flexible direct current is becoming increasingly urgent. At present, the application research of ultra-high voltage flexible direct current is still in its initial stage, and some control modes and control methods need to be explored.

[0004] Open Line Tests (OLT) are a system commissioning test item in UHVDC transmission projects. Their main purpose is to test the insulation level of the DC poles after a prolonged shutdown or maintenance, to prevent the DC poles from starting when the line has a permanent ground fault. OLT is an important means of verifying the withstand voltage capability of various equipment in the DC field, the normality of the control and protection system, and the integrity of the DC transmission circuit. Unlike grid-commutated converters in conventional UHVDC transmission systems, the OLT testing methods for voltage source converters are not yet mature. In particular, the DC side voltage of voltage source converters rises after AC charging, making it impossible to conduct an open-line voltage test starting from 0 voltage as in conventional UHVDC systems.

[0005] Therefore, there is an urgent need to propose an open-circuit test method for ultra-high voltage flexible DC transmission systems, which can enable the DC port voltage of the flexible DC system to rise from zero voltage to a predetermined voltage at a predetermined rate during the open-circuit test. Summary of the Invention

[0006] A method and apparatus for open-circuit testing of a DC transmission system are provided, which can realize the DC port voltage of the flexible DC system to rise from zero voltage to a predetermined voltage at a predetermined rate during open-circuit testing, thus meeting the requirements of open-circuit testing of ultra-high voltage flexible DC transmission systems.

[0007] In a first aspect, embodiments of this application provide an open-circuit test method for a DC transmission system, the DC transmission system including a first converter valve, a second converter valve, and a DC field switch; the method includes: Switch the control mode of the DC transmission system to the no-load pressurization test mode, switch the DC field switch to the no-load pressurization test state, and switch the outer loop control in the no-load pressurization test mode to the sub-module average voltage control. Close the bypass switch on the DC side of the converter valve under test; wherein the converter valve under test includes at least one of a first converter valve and a second converter valve; Close the AC side incoming switch of the converter valve under test to put the converter valve under test into a short-circuit charging state; After the converter valve under test has been charged, the converter valve under test is unlocked according to the first preset voltage; Disconnect the bypass switch on the DC side of the converter valve under test; The DC voltage of the converter valve under test is increased according to the preset target.

[0008] In some embodiments, the DC transmission system further includes pole line separators; a method for switching the DC field switch to an unloaded pressurized test state includes: Close the DC-side outgoing cut-off switch of the converter valve under test, and close the DC-side bypass cut-off switch of the converter valve not under test; Close the switch and isolator connecting the converter valve under test to the polarity; Close the pole line separator when the converter valve under test has a DC line, and open the pole line separator when the converter valve under test does not have a DC line.

[0009] In some embodiments, closing the bypass switch on the DC side of the converter valve under test includes: When the converter valve under test is the first converter valve and the converter valve not under test is the second converter valve, close the bypass switch on the DC side of the first converter valve and the bypass deflector on the DC side of the second converter valve; when the converter valve under test is the second converter valve and the converter valve not under test is the first converter valve, close the bypass switch on the DC side of the second converter valve and the bypass deflector on the DC side of the first converter valve. When the converter valve under test is the first converter valve and the second converter valve, close the bypass switch on the DC side of the first converter valve and the bypass switch on the DC side of the second converter valve.

[0010] In some embodiments, switching the outer loop control in the no-load pressurization test mode to submodule average voltage control includes: The target control object and target reference value are determined through outer loop control. The difference between the target controlled object and the target reference value is used to output the target output value through the proportional-integral circuit of the outer loop control. The target output value is used as the command value for the inner loop current control of the average voltage control of the submodule.

[0011] In some embodiments, the target control object is the real-time value of the overall average voltage of all normal submodules of all bridge arms of the converter valve under test.

[0012] In some embodiments, the DC transmission system further includes a converter valve control system; the target reference value is the calculated voltage value of the submodule of the converter valve control system.

[0013] In some embodiments, the method for calculating the submodule voltage value is as follows: When calculating the number of submodules that are turned on using the first rated voltage, the first rated voltage is used as the calculated voltage value of the submodule. When calculating the number of conducting submodules using the second rated voltage, the calculated voltage of the converter valve control system is adjusted based on the number of submodules that have failed in the bridge arm with the most failures of the converter valve under test, and the adjusted calculated voltage of the converter valve control system is used as the calculated value of the submodule voltage.

[0014] In some embodiments, the converter valve under test is a full-bridge hybrid topology converter valve; the short-circuit charging state is a charging state of the full-bridge hybrid topology converter valve when the DC side is short-circuited.

[0015] In some embodiments, the converter valve under test includes a full-bridge module and a half-bridge module; the control method for short-circuit charging of the converter valve under test includes: With all modules of the converter valve under test locked, the capacitors of the full-bridge modules in the bridge arm of the converter valve under test are charged through the parallel diodes of the converter valve under test. If the full-bridge module of the converter valve under test is successfully powered, the full-bridge module is gradually disconnected until the charging voltage of the half-bridge module is the same as that of the full-bridge module. If all submodules are successfully powered, the N submodules with the highest voltage are disconnected, and the number of N is slowly increased; where N is a positive integer.

[0016] In some embodiments, the first preset voltage is zero DC voltage; unlocking the converter valve under test according to the first preset voltage includes: Set the DC bias voltage of the reference wave of the output bridge arm of the DC transmission system to zero.

[0017] In some embodiments, raising the DC voltage of the converter valve under test according to a preset target includes: The DC bias voltage of the reference wave of the output arm of the control DC transmission system is raised to the target voltage at a preset speed and half of the second preset voltage.

[0018] Secondly, embodiments of this application also provide an open-circuit test apparatus for a DC transmission system, the DC transmission system including a first converter valve, a second converter valve, and a DC field switch; the apparatus includes: The switching module is used to switch the control mode of the DC transmission system to the no-load pressurization test mode, switch the DC field switch to the no-load pressurization test state, and switch the outer loop control in the no-load pressurization test mode to the sub-module average voltage control. The first closing module is used to close the bypass switch on the DC side of the converter valve under test; wherein the converter valve under test includes at least one of the first converter valve and the second converter valve. The second closing module is used to close the incoming switch on the AC side of the converter valve under test, so that the converter valve under test enters the short-circuit charging state. The unlocking module is used to unlock the converter valve under test according to the first preset voltage after the converter valve under test has been charged. The disconnect module is used to disconnect the bypass switch on the DC side of the converter valve under test. The lifting module is used to raise the DC voltage of the converter valve under test according to a preset target.

[0019] Beneficial Effects: This application provides an open-circuit test method and apparatus for a DC transmission system. The open-circuit test method for the DC transmission system includes: switching the control mode of the DC transmission system to an unloaded pressurization test mode, switching the DC field switch to an unloaded pressurization test state, and switching the outer loop control in the unloaded pressurization test mode to submodule average voltage control; closing the bypass switch on the DC side of the converter valve under test; wherein the converter valve under test includes at least one of a first converter valve and a second converter valve; closing the incoming line switch on the AC side of the converter valve under test to allow the converter valve under test to enter a short-circuit charging state; after the converter valve under test has completed charging, unlocking the converter valve under test according to a first preset voltage; disconnecting the bypass switch on the DC side of the converter valve under test; and raising the DC voltage of the converter valve under test according to a preset target. The open-circuit test method for DC transmission systems provided in this application controls the incoming line switch of the converter valve under test to enable short-circuit charging of the converter valve under test. After charging is completed, the converter valve under test is unlocked and its bypass switch is disconnected to raise the DC voltage of the converter valve under test according to a preset target. This enables the DC port voltage of the flexible DC system to rise from zero voltage to a predetermined voltage at a predetermined rate during open-circuit testing, thus meeting the requirements of open-circuit testing of UHV flexible DC transmission systems. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] To gain a more complete understanding of this application and its beneficial effects, the following description will be provided in conjunction with the accompanying drawings, wherein the same reference numerals in the following description denote the same parts.

[0022] Figure 1 This is a flowchart of an open-circuit test method for a DC transmission system provided in the embodiments of this application; Figure 2 This is a schematic diagram of a single-pole topology of an ultra-high voltage flexible DC transmission system provided in the embodiments of this application; Figure 3 This is a control block diagram of the open-circuit test method for an ultra-high voltage flexible DC transmission system provided in the embodiments of this application; Figure 4 This is an overall flowchart of the open-circuit test method for the UHV flexible DC transmission system provided in the embodiments of this application; Figure 5 This is a schematic diagram of the principle of an open-circuit test device for a DC transmission system provided in the embodiments of this application.

[0023] Figure label: 10-First converter valve; 20-Second converter valve; 201-First DC output cutter; 202-Second DC output cutter; 203-Third DC output cutter; 204-Fourth DC output cutter; 205-First bypass switch; 206-First bypass cutter; 207-Second bypass switch; 208-Second bypass cutter; 209-Pole cutter; 210-Pole switch; 211-Pole-to-ground cutter; 300-Open-circuit test device for DC transmission system; 301-Switching module; 302-First closing module; 303-Second closing module; 304-Unlocking module; 305-Disconnecting module; 306-Lifting module. Detailed Implementation

[0024] The technical solutions of the embodiments of this application 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 this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the protection scope of this application.

[0025] In the embodiments of this application, "at least one" refers to one or more; "multiple" refers to two or more. In the description of this application, the terms "first," "second," "third," etc., are used only for the purpose of distinguishing descriptions and should not be construed as indicating or implying relative importance, nor should they be construed as indicating or implying order.

[0026] References such as “one embodiment” or “some embodiments” described in this specification mean that one or more embodiments of this application include a specific feature, structure, or characteristic described in connection with that embodiment. Therefore, the terms “comprising,” “including,” “having,” and variations thereof, in this specification, mean “including but not limited to,” unless otherwise specifically emphasized.

[0027] It should be noted that in the embodiments of this application, "and / or" describes the relationship between associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. In addition, the character " / ", unless otherwise specified, generally indicates that the associated objects before and after it are in an "or" relationship.

[0028] It should be noted that in the embodiments of this application, "connection" can be understood as electrical connection. The connection between two electrical components can be a direct or indirect connection between the two electrical components. For example, the connection between A and B can be a direct connection between A and B, or an indirect connection between A and B through one or more other electrical components.

[0029] It should be noted that the DC transmission system in this application embodiment is an ultra-high voltage flexible DC transmission system. For example, this application embodiment uses an open-circuit test of an ultra-high voltage flexible DC transmission system as an example for illustration, and will not be repeated below.

[0030] Figure 1 This is a flowchart illustrating an open-circuit test method for a DC transmission system provided in this application embodiment. This application embodiment provides an open-circuit test method for a DC transmission system; please refer to [link / reference]. Figure 1 The open-circuit test method for this DC transmission system includes the following steps: Step 110: Switch the control mode of the DC transmission system to the no-load pressurization test (OLT) mode, switch the DC field switch to the no-load pressurization test state, and switch the outer loop control in the no-load pressurization test mode to the sub-module average voltage control.

[0031] Among them, the no-load pressurization test (OLT) mode is the open-circuit test mode.

[0032] Figure 2 This is a schematic diagram of a single-pole topology of an ultra-high voltage flexible direct current transmission system provided in an embodiment of this application. For example, see [link to relevant documentation]. Figure 2The ultra-high voltage flexible direct current transmission system includes a first converter valve 10, a second converter valve 20, and a DC field switch. For example, the first converter valve 10 is a high-voltage valve, and the second converter valve 20 is a low-voltage valve.

[0033] Among them, DC field switches refer to various switches in ultra-high voltage flexible DC transmission systems, such as... Figure 2 The first DC output separator 201, the second DC output separator 202, the third DC output separator 203, ..., the polar switch 210, and the polar separator 211 are shown.

[0034] In some embodiments, see Figure 2 The DC transmission system also includes pole line separators 209; a method for switching the DC field switch to the no-load pressurized test state includes: closing the outgoing line separator on the DC side of the converter valve under test, and closing the bypass separator on the DC side of the converter valve not under test; closing the switch and separator connecting the converter valve under test to the pole; closing the pole line separator when the converter valve under test has a DC line, and opening the pole line separator when the converter valve under test does not have a DC line.

[0035] Assuming the converter valve under test is the first converter valve 10, then close the first DC output barrier 201 and the second DC output barrier 202 on the DC side of the first converter valve 10, and close the second bypass barrier 208 for the converter valve under test (i.e., the second converter valve 20); close the polar switch 210 and the polar barrier 211 connecting the first converter valve 10 to the polarity; close the polar barrier 209 when the converter valve under test has a DC line, and open the polar barrier 209 when the converter valve under test does not have a DC line (in other words, determine whether to close the polar barrier 209; if the converter valve under test has a DC line, close the polar barrier 209; if the converter valve under test does not have a DC line, open the polar barrier 209).

[0036] Assuming the converter valve under test is the second converter valve 20, then close the third DC output barrier 203 and the fourth DC output barrier 204 on the DC side of the second converter valve 20, and close the first bypass barrier 206 for the converter valve under test (i.e., the first converter valve 10); close the polar switch 210 and the polar barrier 211 connecting the second converter valve 20 to the polarity; close the polar barrier 209 when the converter valve under test has a DC line, and disconnect the polar barrier 209 when the converter valve under test does not have a DC line.

[0037] Assuming the converter valves under test are the first converter valve 10 and the second converter valve 20, then close the first DC output blade 201 and the second DC output blade 202 on the DC side of the first converter valve 10, and close the third DC output blade 203 and the fourth DC output blade 204 on the DC side of the second converter valve 20; close the polar switch 210 and the polar blade 211 connecting to the polar ground; close the polar blade 209 when the converter valve under test has a DC line, and open the polar blade 209 when the converter valve under test does not have a DC line.

[0038] The bypass switch on the DC side of the converter valve is located on the converter valve side, for example, see [reference needed]. Figure 2 The first bypass switch 205 is located on the side of the first converter valve 10, and the second bypass switch 207 is located on the side of the second converter valve 20.

[0039] Among them, the bypass switch on the DC side of the converter valve is located on the pole and ground wire side, for example... Figure 2 The first bypass separator 206 shown is located on the side between the pole line and the ground line, and the second bypass separator 208 is located on the side between the pole line and the ground line.

[0040] Among them, the two DC output line separators on the DC side of the converter valve under test are located between the bypass switch and the two sides of the bypass separator on the DC side of the converter valve, such as Figure 2 The first outgoing line separator 201 and the second outgoing line separator 202 shown are located between the first bypass switch 205 and the first bypass separator 206 of the first converter valve 10, and the third outgoing line separator 203 and the fourth outgoing line separator 204 are located between the second bypass switch 207 and the second bypass separator 207 of the second converter valve 20. Specifically, the first outgoing line separator 201 and the third outgoing line separator 203 are positive DC side outgoing line separators AI, and the second outgoing line separator 202 and the fourth outgoing line separator 204 are negative DC side outgoing line separators CI.

[0041] Among them, the switch and the isolator connecting the converter valve under test to the polarity are Figure 2 The polarity-connected switch 210 and polarity-connected spacer 211 are shown.

[0042] Specifically, closing the pole isolation switch when the converter valve under test has a DC line connected, and opening the pole isolation switch when the converter valve under test does not have a DC line connected, refers to the no-load pressurized DC field switch state: When the converter valve under test has a DC line connected, the neutral line switch of this station is closed (e.g., closing the switch and isolation switch connecting the converter valve under test to the pole), the pole switch is closed (closing the pole isolation switch), and the pole switch of the opposite station is open; when the converter valve under test does not have a DC line connected, the neutral line switch of this station is closed (e.g., closing the switch and isolation switch connecting the converter valve under test to the pole), and the pole switch is open (opening the pole isolation switch). In other words, when testing the insulation performance of the line, the positive pole of the converter valve is connected to a DC line and pressurized; when only testing the insulation performance of the converter valve equipment, no line is connected to avoid line interference.

[0043] In some embodiments, switching the outer loop control under no-load pressurization test mode to submodule average voltage control includes: determining the target control object and the target reference value through the outer loop control; outputting the target output value by passing the difference between the target control object and the target reference value through the proportional-integral circuit of the outer loop control; and using the target output value as the command value for the inner loop current control of the submodule average voltage control.

[0044] Figure 3 This is a control block diagram of the open-circuit test method for an ultra-high voltage flexible DC transmission system provided in the embodiments of this application. (See also...) Figure 3 The outer loop control (i.e., the outer loop d-axis control) is controlled by the submodule voltage outer loop controller, while the submodule average voltage control is controlled by the submodule current inner loop controller. The proportional-integral (PI) stage of the outer loop control is the PI controller within the submodule voltage outer loop controller. Specifically, the difference between the target controlled object and the target reference value is input to the PI controller in the submodule voltage outer loop controller, and the target output value is obtained after passing through the PI stage. Then, the target output value is used as the d-axis current command value for the inner loop current control of the submodule average voltage control.

[0045] In some embodiments, the target control object is the real-time value of the overall average voltage of all normal submodules of all bridge arms of the converter valve under test.

[0046] The target control object for the submodule average voltage control is the real-time value of the overall average voltage of all normal submodules in all arms of the converter valve. U avg Its value originates from the valve control system (i.e., the converter valve control system). Specifically, the calculation method is as follows: the valve control system obtains the real-time voltage values ​​of all normally operating submodules, sums them, and then divides the sum of the real-time voltage values ​​of all normally operating submodules by the number of normally operating submodules to obtain the average real-time voltage value of the submodule. U avg .

[0047] In some embodiments, the DC transmission system further includes a converter valve control system; the target reference value is the calculated voltage value of the submodule of the converter valve control system.

[0048] Among them, the target reference value for the average voltage control of the submodule U avgref Calculated voltage value for submodule U avgcalc Its value comes from the valve control system.

[0049] In some embodiments, the calculation method for the submodule voltage is as follows: when the number of submodules turned on is calculated using a first rated voltage, the first rated voltage is used as the submodule voltage calculation value; when the number of submodules turned on is calculated using a second rated voltage, the calculation voltage of the converter valve control system is adjusted according to the number of submodules that have failed in the bridge arm with the most failures of the converter valve under test, and the adjusted calculation voltage of the converter valve control system is used as the submodule voltage calculation value.

[0050] The first rated voltage is a fixed rated voltage, and the second rated voltage is a dynamic rated voltage.

[0051] Specifically; calculated submodule voltage values U avgcalc This is determined by the method used to calculate the number of active submodules in the valve control system. For example, when calculating the number of active submodules using a fixed rated voltage, the fixed rated voltage is used as the calculated voltage value for the submodules in the valve control system. When calculating the number of active submodules using a variable rated voltage method, the calculated voltage of the valve control module (i.e., the valve control system) is dynamically adjusted based on the number of faulty submodules in the bridge arm where the converter valve experiences the most faults. The dynamically adjusted variable rated voltage of the valve control module is then determined as the calculated voltage value for the submodules of the valve control module. U avgcalc .

[0052] Step 120: Close the bypass switch on the DC side of the converter valve under test.

[0053] The switching valve under test includes at least one of the first switching valve 10 and the second switching valve 20. For example, if the switching valve under test is only the first switching valve 10, only the high-valve OLT is tested. For example, if the switching valve under test is only the second switching valve 20, only the low-valve OLT is tested. For example, if the switching valve under test is both the first switching valve 10 and the second switching valve 20, both the high-valve and low-valve OLTs are tested simultaneously, i.e., the side-pole OLTs.

[0054] In some embodiments, closing the bypass switch on the DC side of the converter valve under test includes: when the converter valve under test is the first converter valve 10 and the converter valve not being tested is the second converter valve 20, closing the bypass switch on the DC side of the first converter valve 10 and the bypass deflector on the DC side of the second converter valve 20; when the converter valve under test is the second converter valve 20 and the converter valve not being tested is the first converter valve 10, closing the bypass switch on the DC side of the second converter valve 20 and the bypass deflector on the DC side of the first converter valve 10; when the converter valve under test is both the first converter valve 10 and the second converter valve 20, closing the bypass switch on the DC side of the first converter valve 10 and the bypass switch on the DC side of the second converter valve 20.

[0055] Closing the bypass switch on the DC side of the converter valve under test means, for example, referring to... Figure 2 Assuming the converter valve under test is the first converter valve 10, i.e., only the high-valve OLT is being tested (the untested converter valve is the second converter valve 20), the first bypass switch 205 on the DC side of the first converter valve 10 and the second bypass deflector 208 on the DC side of the second converter valve 20 are closed to engage the first converter valve 10 and isolate the untested converter valve 20. Assuming the converter valve under test is the second converter valve 20, i.e., only the low-valve OLT is being tested (the untested converter valve is the first converter valve 10), the second bypass switch 207 on the DC side of the second converter valve 20 and the first bypass deflector 206 on the DC side of the first converter valve 10 are closed to engage the second converter valve 20 and isolate the untested converter valve 10. Assuming the converter valves under test are the first converter valve 10 and the second converter valve 20, that is, when the test pole OLT (i.e., the high and low valves are simultaneously OLT), the first bypass switch 205 on the DC side of the first converter valve 10 and the second bypass switch 207 on the DC side of the second converter valve 20 are closed to put the first converter valve 10 and the second converter valve 20 under test into operation.

[0056] Step 130: Close the AC side incoming switch of the converter valve under test to put the converter valve under test into a short-circuit charging state.

[0057] See Figure 2 When the converter valve under test is the first converter valve 10, the first incoming line switch 212 on the AC side of the first converter valve 10 is closed to put the first converter valve 10 into a short-circuit charging state. When the converter valve under test is the second converter valve 20, the second incoming line switch 213 on the AC side of the second converter valve 20 is closed to put the second converter valve 20 into a short-circuit charging state. When the converter valve under test is both the first converter valve 10 and the second converter valve 20, the first incoming line switch 212 on the AC side of the first converter valve 10 and the second incoming line switch 213 on the AC side of the second converter valve 20 are closed to put both the first converter valve 10 and the second converter valve 20 into a short-circuit charging state.

[0058] In some embodiments, the converter valve under test is a full-bridge hybrid topology converter valve; the short-circuit charging state is a charging state of the full-bridge hybrid topology converter valve when the DC side is short-circuited.

[0059] The short-circuit charging control of the converter valve under test is implemented in the valve control system.

[0060] In some embodiments, the converter valve under test includes a full-bridge module and a half-bridge module; the control method for short-circuit charging of the converter valve under test includes: when all modules of the converter valve under test are locked, charging the capacitor of the full-bridge module in the bridge arm of the converter valve under test through the parallel diode of the converter valve under test; when the full-bridge module of the converter valve under test is successfully powered, gradually disconnecting the full-bridge module until the charging voltage of the half-bridge module is the same as the charging voltage of the full-bridge module; when all sub-modules are successfully powered, disconnecting the N sub-modules with the highest voltage, and slowly increasing the number of N; where N is a positive integer.

[0061] Specifically, the short-circuit charging control process of the converter valve under test includes three steps, as follows: Step 1, Uncontrolled Charging: When all modules are locked, the AC system charges through the diode of the commutation valve under test (e.g., see [reference]). Figure 2 (Assuming the converter valve under test is the first converter valve 10, it corresponds to the first diode D1; assuming the converter valve under test is the second converter valve 20, it corresponds to the second diode D2; assuming the converter valve under test is the first converter valve 10 and the second converter valve 20, it corresponds to the first diode D1 and the second diode D2) The full-bridge submodule capacitor in the bridge arm is charged, but the half-bridge module cannot be charged.

[0062] Step 2, Controllable Charging 1: After the full-bridge module successfully draws power, gradually disconnect the full-bridge module until the voltage charging of the half-bridge module is close to (or basically the same as) that of the full-bridge module.

[0063] Step 3, Controlled Charging 2: After all submodules successfully draw power, select and disconnect the N submodules with the highest voltage, and slowly increase N to achieve the goal of controlling the average voltage of all submodules to be approximately equal to the calculated voltage value of each submodule. U avgcalc .

[0064] Step 140: After the converter valve under test has been charged, unlock the converter valve under test according to the first preset voltage.

[0065] The first preset voltage is zero DC voltage. In order to raise the DC voltage to the target value from zero DC voltage, it is necessary to unlock it with zero DC voltage. Therefore, after the converter valve under test is charged, the converter valve under test is unlocked according to the first preset voltage.

[0066] In some embodiments, the first preset voltage is a zero DC voltage; unlocking the converter valve under test according to the first preset voltage includes: setting the DC bias voltage of the output arm reference wave of the DC transmission system to zero.

[0067] Specifically, after charging is complete, the converter valve under test is unlocked with zero DC voltage. The method for unlocking the converter valve with zero DC voltage is as follows: the DC bias voltage of the bridge arm reference wave output by the control system (i.e., the UHV flexible DC transmission system) is... U ref_PZ The sum of the voltages of the upper and lower bridge arms of the converter valve is 0, thus achieving DC 0-voltage control.

[0068] Step 150: Disconnect the bypass switch on the DC side of the converter valve under test.

[0069] To raise the DC voltage to the target value starting from zero DC voltage, the bypass switch on the DC side of the converter valve under test is closed in step 120 to start from zero DC voltage (this is because, without the bypass switch open, normal charging would theoretically cause the DC side voltage to reach the peak value of the AC line voltage). After the converter valve under test has finished charging, the bypass switch is opened to prepare for boosting the voltage to the target value.

[0070] Step 160: Increase the DC voltage of the converter valve under test according to the preset target.

[0071] Among them, the preset target U REFDC The preset speed and the second preset voltage are specified. The preset speed is 1 kV / s, and the second preset voltage is the rated DC voltage. The specific values ​​of the preset speed and the second preset voltage can be set according to actual conditions and are not specifically limited here.

[0072] In some embodiments, raising the DC voltage of the converter valve under test according to a preset target includes: controlling the DC bias voltage of the output arm reference wave of the DC transmission system to rise to the target voltage at a preset rate and half of a second preset voltage.

[0073] The target voltage is the second preset voltage. The range of the second preset voltage is from 0 to the rated DC voltage.

[0074] Specifically, the method for raising the DC voltage of the converter valve under test according to the preset target is as follows: the DC bias voltage of the bridge arm reference wave output by the control system is... U ref_PZ According to the predetermined speed (i.e., the preset speed) U RAMP The second preset voltage is raised to half of the target voltage.

[0075] Figure 4This is an overall flowchart of the open-circuit test method for an ultra-high voltage flexible DC transmission system provided in the embodiments of this application. For example, see [link to relevant documentation]. Figure 4 The overall process of the open-circuit test method for this ultra-high voltage flexible DC transmission system is as follows: Step 1, such as Figure 4 As shown in S101, the control mode of the UHV flexible DC transmission system is switched to OLT mode, the DC field switch is switched to OLT state, and the outer loop d-axis control in OLT mode is switched to sub-module average voltage control.

[0076] Step Two, as follows Figure 4 As shown in S102, the DC side bypass switch (BPS) of the converter valve is closed.

[0077] Step 3, as follows Figure 4 As shown in S103, when the AC input switch ACB of the converter valve is closed, the converter valve enters the short-circuit charging state.

[0078] Step 4, as follows Figure 4 As shown in S104, after charging is complete, the converter valve is unlocked with zero DC voltage.

[0079] Step 5, as follows Figure 4 As shown in S105, the DC side bypass switch BPS of the split converter valve is used.

[0080] Step Six, as Figure 4 As shown in S106, the converter valve operates at a predetermined speed. U RAMP , predetermined goals U REFDC Increase DC voltage.

[0081] Figure 5 This is a schematic diagram of an open-circuit test device for a DC transmission system provided in an embodiment of this application. This application also provides an open-circuit test device for a DC transmission system, see below. Figure 5The open-circuit test device 300 for the DC transmission system includes: a switching module 301, used to switch the control mode of the DC transmission system to the no-load pressurization test mode, switch the DC field switch to the no-load pressurization test state, and switch the outer loop control in the no-load pressurization test mode to the sub-module average voltage control; a first closing module 302, used to close the bypass switch on the DC side of the converter valve under test; wherein the converter valve under test includes at least one of a first converter valve and a second converter valve; a second closing module 303, used to close the incoming line switch on the AC side of the converter valve under test, so that the converter valve under test enters the short-circuit charging state; an unlocking module 304, used to unlock the converter valve under test according to a first preset voltage after the converter valve under test has been charged; a disconnecting module 305, used to disconnect the bypass switch on the DC side of the converter valve under test; and a lifting module 306, used to lift the DC voltage of the converter valve under test according to a preset target.

[0082] The technical solution of this application provides an open-circuit test device for a DC transmission system. By controlling the incoming line switch of the converter valve under test, the converter valve under test is short-circuited and charged. After charging is completed, the converter valve under test is unlocked and its bypass switch is disconnected, so as to raise the DC voltage of the converter valve under test according to a preset target. This can realize that the DC port voltage of the flexible DC system open-circuit test rises from zero voltage to a predetermined voltage at a predetermined speed, meeting the requirements of the open-circuit test of the UHV flexible DC transmission system.

[0083] In some embodiments, the DC transmission system further includes pole separators; the switching module 301 is also used for: Close the DC-side outgoing cut-off switch of the converter valve under test, and close the DC-side bypass cut-off switch of the converter valve not under test; Close the switch and isolator connecting the converter valve under test to the polarity; Close the pole line separator when the converter valve under test has a DC line, and open the pole line separator when the converter valve under test does not have a DC line.

[0084] In some embodiments, the first closing module 302 is further configured to: When the converter valve under test is the first converter valve and the converter valve not under test is the second converter valve, close the bypass switch on the DC side of the first converter valve and the bypass deflector on the DC side of the second converter valve; when the converter valve under test is the second converter valve and the converter valve not under test is the first converter valve, close the bypass switch on the DC side of the second converter valve and the bypass deflector on the DC side of the first converter valve. When the converter valve under test is the first converter valve and the second converter valve, close the bypass switch on the DC side of the first converter valve and the bypass switch on the DC side of the second converter valve.

[0085] In some embodiments, the switching module 301 is further configured to: The target control object and target reference value are determined through outer loop control. The difference between the target controlled object and the target reference value is used to output the target output value through the proportional-integral circuit of the outer loop control. The target output value is used as the command value for the inner loop current control of the average voltage control of the submodule.

[0086] In some embodiments, the target control object is the real-time value of the overall average voltage of all normal submodules of all bridge arms of the converter valve under test.

[0087] In some embodiments, the DC transmission system further includes a converter valve control system; the target reference value is the calculated voltage value of the submodule of the converter valve control system.

[0088] In some embodiments, the method for calculating the submodule voltage value is as follows: When calculating the number of conducting submodules using the first rated voltage, the first rated voltage is used as the calculated voltage value of the submodule. When calculating the number of conducting submodules using the second rated voltage, the calculated voltage of the converter valve control system is adjusted based on the number of submodules that have failed in the bridge arm with the most failures of the converter valve under test, and the adjusted calculated voltage of the converter valve control system is used as the calculated value of the submodule voltage.

[0089] In some embodiments, the converter valve under test is a full-bridge hybrid topology converter valve; the short-circuit charging state is a charging state of the full-bridge hybrid topology converter valve when the DC side is short-circuited.

[0090] In some embodiments, the converter valve under test includes a full-bridge module and a half-bridge module; the control method for short-circuit charging of the converter valve under test includes: With all modules of the converter valve under test locked, the capacitors of the full-bridge modules in the bridge arm of the converter valve under test are charged through the parallel diodes of the converter valve under test. If the full-bridge module of the converter valve under test is successfully powered, the full-bridge module is gradually disconnected until the charging voltage of the half-bridge module is the same as that of the full-bridge module. If all submodules are successfully powered, the N submodules with the highest voltage are disconnected, and the number of N is slowly increased; where N is a positive integer.

[0091] In some embodiments, the first preset voltage is zero DC voltage; the unlocking module 304 is further configured to: Set the DC bias voltage of the reference wave of the output bridge arm of the DC transmission system to zero.

[0092] In some embodiments, the lifting module 306 is further configured to: The DC bias voltage of the reference wave of the output arm of the control DC transmission system is raised to the target voltage at a preset speed and half of the second preset voltage.

[0093] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0094] The above are merely preferred embodiments of this application and are not intended to limit this application in any way. Although this application has disclosed preferred embodiments as above, it is not intended to limit this application. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the technical solution of this application. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of this application without departing from the scope of the technical solution of this application shall still fall within the scope of the technical solution of this application.

Claims

1. An open-circuit test method for a DC transmission system, characterized in that, The DC transmission system includes a first converter valve, a second converter valve, and a DC field switch; the method includes: The control mode of the DC transmission system is switched to the no-load pressurization test mode, the DC field switch is switched to the no-load pressurization test state, and the outer loop control under the no-load pressurization test mode is switched to the sub-module average voltage control; the bypass switch on the DC side of the converter valve under test is closed; wherein, the converter valve under test includes at least one of the first converter valve and the second converter valve; Close the AC side inlet switch of the converter valve under test to put the converter valve under test into a short-circuit charging state; After the converter valve under test has been charged, the converter valve under test is unlocked according to the first preset voltage; Disconnect the bypass switch on the DC side of the converter valve under test; The DC voltage of the converter valve under test is increased according to the preset target.

2. The method according to claim 1, characterized in that, The DC transmission system further includes pole wire separators; the method for switching the DC field switch to the no-load pressurized test state includes: Close the DC-side outgoing cut-off switch of the converter valve under test, and close the DC-side bypass cut-off switch of the converter valve not under test; Close the switch and disconnector connecting the converter valve under test to the polarity; When the converter valve under test has a DC line, the pole line separator is closed; when the converter valve under test does not have a DC line, the pole line separator is opened.

3. The method according to claim 1, characterized in that, The bypass switch on the DC side of the closed converter valve under test includes: When the converter valve under test is the first converter valve and the converter valve not under test is the second converter valve, close the bypass switch on the DC side of the first converter valve and the bypass deflector on the DC side of the second converter valve; when the converter valve under test is the second converter valve and the converter valve not under test is the first converter valve, close the bypass switch on the DC side of the second converter valve and the bypass deflector on the DC side of the first converter valve. When the converter valves under test are the first converter valve and the second converter valve, close the bypass switch on the DC side of the first converter valve and the bypass switch on the DC side of the second converter valve.

4. The method according to claim 1, characterized in that, The step of switching the outer loop control under the no-load pressurization test mode to submodule average voltage control includes: The target control object and target reference value are determined through the outer loop control. The difference between the target controlled object and the target reference value is passed through the proportional-integral circuit of the outer loop control to output the target output value. The target output value is used as the command value for the inner loop current control of the average voltage control of the submodule.

5. The method according to claim 4, characterized in that, The target control object is the real-time value of the overall average voltage of all normal sub-modules of all bridge arms of the converter valve under test.

6. The method according to claim 4, characterized in that, The DC transmission system also includes a converter valve control system; the target reference value is the voltage calculation value of the submodule of the converter valve control system.

7. The method according to claim 6, characterized in that, The calculation method for the submodule voltage value is as follows: When calculating the number of conducting submodules using the first rated voltage, the first rated voltage is used as the calculated voltage value of the submodule. When calculating the number of conducting submodules using the second rated voltage, the calculated voltage of the converter valve control system is adjusted based on the number of submodules that have failed in the bridge arm with the highest number of failures in the converter valve under test, and the adjusted calculated voltage of the converter valve control system is used as the calculated voltage value of the submodule.

8. The method according to claim 1, characterized in that, The converter valve under test is a full-bridge / half-bridge hybrid topology converter valve; the short-circuit charging state is a charging state of the full-bridge / half-bridge hybrid topology converter valve when the DC side is short-circuited.

9. The method according to claim 8, characterized in that, The converter valve under test includes a full-bridge module and a half-bridge module; the control method for short-circuit charging of the converter valve under test includes: When all modules of the converter valve under test are locked, the capacitor of the full-bridge module in the bridge arm of the converter valve under test is charged through the parallel diode of the converter valve under test. If the full-bridge module of the converter valve under test successfully draws power, the full-bridge module is gradually disconnected until the charging voltage of the half-bridge module is the same as that of the full-bridge module. If all submodules are successfully powered, the N submodules with the highest voltage are disconnected, and the number of N is slowly increased; where N is a positive integer.

10. The method according to claim 1, characterized in that, The first preset voltage is zero DC voltage; the step of unlocking the converter valve under test according to the first preset voltage includes: The DC bias voltage of the reference wave of the output bridge arm of the DC transmission system is set to zero.

11. The method according to claim 1, characterized in that, The step of raising the DC voltage of the converter valve under test according to the preset target includes: The DC bias voltage of the reference wave of the output bridge arm of the DC transmission system is raised to the target voltage at a preset speed and half of the second preset voltage.

12. An open-circuit test device for a DC transmission system, characterized in that, The DC transmission system includes a first converter valve, a second converter valve, and a DC field switch; the device includes: The switching module is used to switch the control mode of the DC transmission system to the no-load pressurization test mode, switch the DC field switch to the no-load pressurization test state, and switch the outer loop control of the no-load pressurization test mode to the sub-module average voltage control. The first closing module is used to close the bypass switch on the DC side of the converter valve under test; wherein the converter valve under test includes at least one of the first converter valve and the second converter valve; The second closing module is used to close the incoming switch on the AC side of the converter valve under test, so that the converter valve under test enters the short-circuit charging state. The unlocking module is used to unlock the converter valve under test according to a first preset voltage after the converter valve under test has been charged. The disconnect module is used to disconnect the bypass switch on the DC side of the converter valve under test; The lifting module is used to lift the DC voltage of the converter valve under test according to a preset target.

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