Method and system for determining resistance value of bleeder resistor of flexible direct current converter valve

Abstract

The invention relates to a method and system for determining the resistance value of a bleeder resistor of a flexible direct-current converter valve, and belongs to the technical field of direct-current power transmission. Comprising the following steps: taking the smaller value of a first upper limit value and a second upper limit value of a corresponding bleeder resistor obtained according to the requirement of a power transmission system on the bleeder power of a converter valve and the requirement of the power transmission system on the voltage rise amount of a capacitor as the actual upper limit value of the value of the bleeder resistor; taking a larger value in a first lower limit value and a second lower limit value of the corresponding bleeder resistor as an actual lower limit value of the value of the bleeder resistor, wherein the larger value is obtained according to the requirement of a power transmission system on the current of a converter valve bleeder branch and the requirement that the bleeder resistor of the converter valve only acts once in each power frequency electric period; and determining the value range of the bleeder resistor according to the actual upper limit value and the actual lower limit value. According to the method, the requirement for the discharge power, the requirement for the capacitor voltage rise amount, the requirement for the discharge branch current and the requirement for the action of each power frequency electric cycle of the discharge resistor are comprehensively considered, and the discharge resistor is accurately and efficiently determined.

Description

Technical Field

[0001] The invention relates to a method and a system for determining the resistance value of a discharge resistor of a flexible direct current converter valve, and belongs to the technical field of direct current power transmission. Background Art

[0002] Flexible DC transmission technology can provide AC power support for passive systems. It is an ideal solution for large-scale new energy bases in deserts, Gobi, and wasteland areas in my country, as well as large-scale wind farms in the deep sea. However, when the AC power grid or DC transmission line at the receiving end fails, the power transmission capacity of this type of transmission system is severely limited, and the power response speed of the new energy station is slow, resulting in serious power surplus in the flexible DC converter valve at the sending end or at both the sending and receiving ends. The power surplus will cause the flexible DC converter valve submodule to overvoltage quickly, which is very likely to trigger the overvoltage protection, resulting in the serious consequence of system shutdown. Therefore, AC and DC energy dissipation devices are often used in engineering to actively dissipate surplus power.

[0003] AC energy-consuming devices are usually installed on the AC side of the converter valve at the sending end of the system, occupying a large area. In addition, when the receiving-end AC system or the remote DC line fails, the sending end may be unable to respond in time due to slow fault perception due to transmission delay. Therefore, it is not applicable to offshore wind power and ultra-long-distance onshore new energy base flexible DC island transmission systems.

[0004] In current engineering applications, DC energy-consuming devices are usually constructed separately from flexible DC converter valves and arranged as an important device on the DC side of the receiving-end converter valve. Therefore, they have the disadvantages of large size, high cost and complex structure.

[0005] Therefore, some scholars have proposed a flexible DC converter valve with energy discharge function. An energy discharge branch consisting of a fully controlled power electronic switch and a resistor in series is integrated at both ends of the sub-module capacitor of the flexible DC converter valve. The sub-module voltage is autonomously adjusted downward as needed to avoid overvoltage lockout or damage, thereby making the processing of surplus power more direct and flexible, and the system structure is compact and economical.

[0006] Under the most severe working conditions, such as the three-phase grounding fault condition of the AC system on the grid side of the inverter station when operating at full power, the system power output capacity is completely lost, and the flexible DC converter valve with energy discharge function must discharge all the system power to effectively suppress the overvoltage of the converter valve submodule. However, the resistance value of the discharge resistor should not be too large or too small. When the resistance value of the discharge resistor is too large, the current of the discharge branch is small. When the bridge arm current is large, the discharge branch input cannot effectively limit the submodule voltage from continuing to rise; when the resistance value of the discharge resistor is too small, the discharge branch current is too large, which will bring two problems. First, the discharge branch needs to use power electronic switching devices with a higher current tolerance level, which increases the equipment cost; second, after the discharge resistor is put into use, the submodule voltage drops rapidly, which will lead to an increase in the number of discharge branch actions in a control cycle, increasing the switch loss of the energy discharge branch (discharge branch).

[0007] At present, when conducting theoretical design of the discharge resistor value of a flexible DC converter valve with energy discharge function, relevant research only considers the requirement of matching the discharge power with the system rated power. The constraint range is too broad, resulting in inaccurate determination of the discharge resistor value. The subsequent detailed determination of the resistance value depends on a large number of complicated system electromagnetic transient simulation verifications, and the determination efficiency is low. How to comprehensively consider multiple constraints such as discharge power matching, allowable submodule capacitor voltage rise, discharge branch current requirements, and number of actions, and accurately and efficiently determine the resistance value of the discharge resistor of a flexible DC converter valve with energy discharge function, has not yet been addressed by existing technologies. Summary of the invention

[0008] The object of the present invention is to provide a method and system for determining the resistance value of a discharge resistor of a flexible DC converter valve, so as to solve the problem of low efficiency in accurately determining the resistance value of the discharge resistor of the existing flexible DC converter valve.

[0009] To achieve the above object, the solution of the present invention includes:

[0010] A method for determining the resistance value of a discharge resistor of a flexible DC converter valve according to the present invention comprises the following steps:

[0011] The first upper limit value and the second upper limit value of the corresponding discharge resistor value are calculated according to the transmission system's requirement on the discharge power of the flexible DC converter valve with an energy discharge function and the requirement on the capacitor voltage rise amount of the corresponding converter valve submodule allowed after the discharge resistor is put into use;

[0012] The smaller value of the first upper limit value and the second upper limit value is taken as the actual upper limit value of the discharge resistance value;

[0013] The first lower limit value and the second lower limit value of the corresponding discharge resistance value are calculated according to the transmission system's requirement on the discharge branch current of the converter valve and the requirement that the discharge resistance of the converter valve only operates once per power frequency power cycle;

[0014] The larger value of the first lower limit value and the second lower limit value is taken as the actual lower limit value of the discharge resistance value;

[0015] Determine the value range of the bleeder resistor based on the actual upper limit and the actual lower limit.

[0016] Furthermore, the calculation formula of the first upper limit value is as follows:

[0017]

[0018] In the formula, R max1 is the first upper limit, U dc is the rated DC voltage of the converter valve, P is the rated active power of the converter valve, N sm is the rated number of submodules of the converter valve bridge arm.

[0019] Furthermore, the calculation formula of the second upper limit is as follows:

[0020]

[0021] In the formula, R max2 is the second upper limit, U in is the voltage setting value of the discharge resistor, ΔU is the maximum increase of the capacitor voltage of the submodule allowed after the discharge resistor is put into use, i arm is the bridge arm current, ΔT1 is the time required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use, is the rated power factor angle of the converter valve.

[0022] Furthermore, the calculation formula of the first lower limit is as follows:

[0023]

[0024] In the formula, R min1 is the first lower limit, k is the margin coefficient, U in is the voltage setting value of the bleeder resistor, ΔU is the maximum increase in the submodule capacitor voltage allowed after the bleeder resistor is put into use, I max It is the maximum current allowed in the discharge branch.

[0025] Furthermore, k∈[1.05,1.1].

[0026] Furthermore, the calculation formula of the second lower limit is as follows:

[0027]

[0028] In the formula, R min2 is the second lower limit, U R is the average value of the discharge resistor input and output voltage setting, E is the energy that a single discharge resistor needs to absorb in one power frequency power cycle, ΔT3 is the time from the discharge resistor action input moment to the exit moment, which is constrained by the requirement that the discharge resistor only acts once per power frequency power cycle, as the second time, is the rated power factor angle of the converter valve.

[0029] A system for determining the resistance value of a discharge resistor of a flexible DC converter valve of the present invention comprises a processor, and the processor is used to execute a computer program to implement the steps of the method for determining the resistance value of a discharge resistor of a flexible DC converter valve as described above.

[0030] Beneficial effects of the present invention:

[0031] The present invention is a pioneering invention, which provides a method and system for determining the resistance value of a discharge resistor of a flexible DC converter valve, which comprehensively considers the requirements of a power transmission system on the discharge power of a flexible DC converter valve with an energy discharge function, the requirements of the power transmission system on the amount of capacitor voltage rise of a corresponding converter valve submodule allowed after the discharge resistor is put into operation, the requirements of the power transmission system on the discharge branch current of the converter valve, and the requirement of the power transmission system that the discharge resistor of the converter valve only operates once per power frequency power cycle, accurately and efficiently determines the value range of the discharge resistor, reduces the dependence on the system electromagnetic transient simulation verification, and reduces the workload of electromagnetic transient simulation comparison.

[0032] The present invention specifically calculates a first upper limit value of the discharge resistor according to the power discharge requirement of the power transmission system for the flexible DC converter valve with an energy discharge function, calculates a second upper limit value of the discharge resistor according to the power transmission system's requirement for the capacitor voltage rise of the corresponding converter valve submodule allowed after the discharge resistor is put into use, and takes the smaller value of the first upper limit value and the second upper limit value as the actual upper limit value of the discharge resistor resistance value; further obtains a first lower limit value of the discharge resistor according to the power transmission system's requirement for the discharge branch current of the converter valve, calculates a corresponding second lower limit value of the discharge resistor resistance according to the power transmission system's requirement for the discharge resistor of the converter valve to act only once per power frequency power cycle, and takes the larger value of the first lower limit value and the second lower limit value as the actual lower limit value of the discharge resistor resistance value; finally, the value range of the discharge resistor resistance is determined according to the actual upper limit value and the actual lower limit value. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a topological structure diagram of a flexible DC converter valve with energy discharge function;

[0034] Figure 2It is a flow chart for determining the value range of the discharge resistance of the flexible DC converter valve with energy discharge function;

[0035] Figure 3 is a flow chart for determining a second upper limit value;

[0036] Figure 4 It is a simulation schematic diagram of the capacitor voltage change of the corresponding converter valve submodule allowed after the discharge resistor is put into use;

[0037] Figure 5 is a flow chart for determining the second lower limit value;

[0038] Figure 6 It is a simulation diagram of the change in capacitor voltage when the discharge resistor is critically operated twice in one power frequency electrical cycle. DETAILED DESCRIPTION

[0039] In order to solve the problems in the background technology, the present invention comprehensively considers the requirements of the transmission system on the discharge power of the flexible DC converter valve with energy discharge function, the requirements of the transmission system on the corresponding converter valve sub-module capacitor voltage rise allowed after the discharge resistor is put into use, the requirements of the transmission system on the discharge branch current of the converter valve and the requirement of the transmission system that the discharge resistor of the converter valve only operates once per power frequency power cycle, so as to accurately and efficiently determine the value range of the discharge resistor.

[0040] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0041] An embodiment of a method for determining the resistance value of a discharge resistor of a flexible DC converter valve:

[0042] A method for determining the resistance value of a discharge resistor of a flexible DC converter valve comprises the following steps:

[0043] A first upper limit value of the discharge resistor is calculated according to the power transmission system's requirement for the discharge power of the flexible DC converter valve with an energy discharge function, and a second upper limit value of the discharge resistor is calculated according to the power transmission system's requirement for the amount of capacitor voltage rise of the corresponding converter valve submodule allowed after the discharge resistor is put into use, and the smaller value of the first upper limit value and the second upper limit value is taken as the actual upper limit value of the discharge resistor;

[0044] A first lower limit value of the discharge resistance is obtained according to the power transmission system's requirement for the discharge branch current of the converter valve, and a corresponding second lower limit value of the discharge resistance is calculated according to the power transmission system's requirement that the discharge resistance of the converter valve only operates once per power frequency power cycle, and a larger value of the first lower limit value and the second lower limit value is taken as the actual lower limit value of the discharge resistance value;

[0045] Finally, the value range of the discharge resistor is determined according to the actual upper limit and the actual lower limit.

[0046] Specifically, Figure 2 As shown, the determination method includes the following steps:

[0047] S1: Calculate the first upper limit value R of the discharge resistor according to the system discharge power requirement max1 ;

[0048] S2: Calculate the second upper limit value R of the bleeder resistor according to the allowable submodule capacitor voltage rise requirement after the bleeder resistor is put into use max2 ;

[0049] S3: Take R max1 and R max2 The smaller value is used as the actual upper limit of the discharge resistor (energy dissipation resistor) R max ;

[0050] S4: Calculate the first lower limit value R of the discharge resistor according to the current requirement allowed by the discharge branch min1 ;

[0051] S5: Calculate the second lower limit value R of the bleeder resistor based on the requirement that the bleeder resistor only operates once per power frequency cycle. min2 ;

[0052] S6: Take R min1 and R min2 The larger value is taken as the actual lower limit of the discharge resistance R min ;

[0053] S7: Finally, according to the actual upper limit and lower limit of the bleeder resistor, the value range of the bleeder resistor is [R min , R max ].

[0054] Among them, the system discharge power requirement refers to the requirement of the transmission system for the discharge power of the flexible DC converter valve with energy discharge function. Specifically, according to the requirements of the actual project for the flexible DC converter valve with energy discharge function, the rated DC voltage of the converter valve, the rated active power of the converter valve and the rated number of sub-modules of the converter valve bridge arm as the system discharge power requirement are determined, and then the first upper limit value is obtained according to the rated DC voltage of the converter valve, the rated active power of the converter valve and the rated number of sub-modules of the converter valve bridge arm.

[0055] The first upper limit value R of S1 max1 The calculation formula is as follows:

[0056]

[0057] In the formula, Rmax1 is the first upper limit of the discharge resistance, U dc is the rated DC voltage of the converter valve, P is the rated active power of the converter valve, N sm is the rated number of submodules of the converter valve bridge arm.

[0058] Among them, the requirement for the sub-module capacitor voltage rise allowed after the discharge resistor is put into operation refers to the requirement of the transmission system for the capacitor voltage rise of the sub-module of the corresponding converter valve allowed after the discharge resistor is put into operation. Specifically, according to the requirements of the actual project for the flexible DC converter valve with energy discharge function, the discharge resistor input voltage set value, the maximum sub-module capacitor voltage rise allowed after the discharge resistor is put into operation, the bridge arm current, the time constrained by the sub-module capacitor voltage rise requirement after the discharge resistor is put into operation, and the rated power factor angle of the converter valve are determined as the requirement for the sub-module capacitor voltage rise allowed after the discharge resistor is put into operation. Then, the second upper limit value is obtained according to the discharge resistor input voltage set value, the maximum sub-module capacitor voltage rise allowed after the discharge resistor is put into operation, the bridge arm current, the time constrained by the sub-module capacitor voltage rise requirement after the discharge resistor is put into operation, and the rated power factor angle of the converter valve.

[0059] The second upper limit value R of S2 max2 The calculation formula is as follows:

[0060]

[0061] In the formula, R max2 For The second upper limit of the discharge resistance calculated within the range, U in is the voltage setting value of the discharge resistor, ΔU is the maximum increase of the submodule capacitor voltage allowed after the discharge resistor is put into use, i arm is the bridge arm current, ΔT1 is the time required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use, is the rated power factor angle of the converter valve.

[0062] Among them, Within the range, the calculation formula for the time ΔT1 required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use is as follows:

[0063]

[0064] Where ΔT1 is the time required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use, i arm is the bridge arm current, u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, U inis the voltage setting value of the bleeder resistor, ΔU is the maximum allowable rise in the submodule capacitor voltage after the bleeder resistor is put into operation, and C is the submodule capacitor value.

[0065] Among them, the bridge arm current i arm The calculation formula is as follows:

[0066]

[0067] In the formula, i arm is the bridge arm current, P is the rated active power of the converter valve, Q is the rated reactive power of the converter valve, and U acV is the rated value of the AC outlet voltage of the converter valve, and ω is the system angular frequency.

[0068] Among them, the bridge arm voltage u arm The calculation formula is as follows:

[0069]

[0070] In the formula,

[0071]

[0072] u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, U acV is the rated value of the AC outlet voltage of the converter valve, ω is the system angular frequency, is the power factor angle of the converter valve, is the rated power factor angle of the converter valve.

[0073] Among them, the allowable current requirement of the discharge branch refers to the requirement of the transmission system for the discharge branch current of the converter valve. Specifically, the margin coefficient of the allowable current requirement of the discharge branch, the discharge resistor input voltage setting value, the maximum allowable sub-module capacitor voltage increase after the discharge resistor is put into operation, and the maximum allowable current of the discharge branch are determined according to the requirements of the actual project for the flexible DC converter valve with energy discharge function. Then, the first lower limit value is obtained according to the margin coefficient, the discharge resistor input voltage setting value, the maximum allowable sub-module capacitor voltage increase after the discharge resistor is put into operation, and the maximum allowable current of the discharge branch.

[0074] The first lower limit value R of S4 min1 The calculation formula is as follows:

[0075]

[0076] In the formula, R min1 is the first lower limit of the discharge resistance, k is the margin coefficient, and the recommended value range is 1.05~1.1, U inis the voltage setting value of the bleeder resistor, ΔU is the maximum increase in the submodule capacitor voltage allowed after the bleeder resistor is put into use, I max It is the maximum current allowed in the discharge branch.

[0077] Among them, the requirement that the discharge resistor only operates once per power frequency power cycle refers to the requirement of the transmission system that the discharge resistor of the converter valve only operates once per power frequency power cycle. Specifically, according to the requirements of the actual project for the flexible DC converter valve with energy discharge function, the average value of the discharge resistor input voltage setting and the exit voltage setting as the requirement that the discharge resistor only operates once per power frequency power cycle, the energy that a single discharge resistor needs to absorb within one power frequency power cycle, and the time from the discharge resistor operation input time to the exit time constrained by the requirement that the discharge resistor only operates once per power frequency power cycle are determined as the second time and the rated power factor angle of the converter valve. Then, according to the average value of the discharge resistor input and exit voltage setting, the energy that a single discharge resistor needs to absorb within one power frequency power cycle, and the time from the discharge resistor operation input time to the exit time constrained by the requirement that the discharge resistor only operates once per power frequency power cycle as the second time and the rated power factor angle of the converter valve, the second lower limit value is obtained.

[0078] The second lower limit value R of S5 min2 The calculation formula is as follows:

[0079]

[0080] In the formula, R min2 For The second lower limit of the bleeder resistor value calculated within the range, U R is the average value of the discharge resistor input and output voltage setting, E is the energy that a single discharge resistor needs to absorb in one power frequency power cycle, ΔT3 is the time from the discharge resistor action input moment to the exit moment, which is constrained by the requirement that the discharge resistor only acts once per power frequency power cycle, as the second time, is the rated power factor angle of the converter valve.

[0081] The calculation formula for the energy E that a single discharge resistor needs to absorb in one power frequency power cycle is as follows:

[0082]

[0083] Where, E is the energy that a single discharge resistor needs to absorb in one power frequency power cycle, P is the rated active power of the converter valve, N sm is the rated number of submodules of the converter valve bridge arm.

[0084] Among them, Within the range, the discharge resistor is required to act only once per power frequency power cycle. The time from the time when the discharge resistor is put into action to the time when it is withdrawn is used as the calculation formula of the second time ΔT3 as follows:

[0085]

[0086] In the formula, ΔT2 is the time from the moment when the discharge resistor is withdrawn to the moment when the bridge arm current crosses zero, which is the first time. ΔT3 is the time from the moment when the discharge resistor is put into operation to the moment when the discharge resistor is withdrawn, which is the second time. i arm is the bridge arm current, u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, C is the capacitance of the submodule, and ΔU2 is the difference between the input voltage setting value and the exit voltage setting value of the discharge resistor.

[0087] Among them, Within the range, the discharge resistor only operates once per power frequency power cycle. The time from the discharge resistor action exit moment to the bridge arm current positively crossing zero moment is taken as the calculation formula of the first time ΔT2 as follows:

[0088]

[0089] In the formula, ΔT2 is the time from the moment when the discharge resistor is constrained to exit the action to the moment when the bridge arm current crosses zero positively, which is the first time. arm is the bridge arm current, u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, C is the capacitance of the submodule, and ΔU2 is the difference between the input voltage setting value and the exit voltage setting value of the discharge resistor.

[0090] The discharge power matching condition and the allowable submodule capacitor voltage rise in the four limiting conditions comprehensively considered by the present invention are based on the perspective of ensuring equipment safety, the discharge branch current requirement is based on the perspective of economy, and the number of actions is based on the perspective of engineering implementation experience.

[0091] Among them, the discharge power matching condition: under the most serious fault of the transmission system, the ability to transmit power to the outside is completely lost. Therefore, the discharge power through the discharge resistor matches the transmission power of the transmission system, that is, the minimum requirement of the discharge power is equal to the rated active power of the transmission system. This is the basic condition to ensure that the flexible DC converter valve does not stop operating due to overvoltage due to power surplus, thereby achieving fault ride-through.

[0092] Among them, the allowable submodule capacitor voltage rise: the input of the discharge resistor obviously needs to set the input value, but considering that the flexible DC converter valve itself has a certain overload operation capacity, the input value cannot be selected to be too low, otherwise a minor fault will cause the discharge resistor to operate, causing unnecessary heating to the converter valve; therefore, the discharge resistor input value needs to be selected to be slightly higher. At this time, after the discharge resistor is operated, since the submodule charging current may still be greater than the discharge branch current in some intervals, the submodule capacitor voltage will undergo a process of first rising and then falling. The larger the discharge resistor, the higher the maximum value of the submodule capacitor voltage will be in this process. However, the maximum value of the submodule capacitor voltage cannot exceed the overvoltage closing value set by the converter valve. Therefore, it is also necessary to limit the upper limit of the discharge resistor value.

[0093] Among them, the discharge branch current requirement is: the smaller the discharge resistance, the more significant the effect of suppressing the voltage increase of the sub-module capacitor will be, but this will cause the discharge branch current to increase. The larger the current, the greater the current-carrying capacity of the discharge branch power electronic switch will be, and the corresponding device procurement cost will become higher. Therefore, it is necessary to limit the discharge branch current from an economic perspective.

[0094] Among them, the number of actions: usually in the flexible DC transmission system, in order to reduce the switching loss of the power electronic switch and increase its service life, the number of actions of the power electronic switch is usually controlled to be 1 to 2 times within one power frequency power cycle (0.02s). In order to achieve the limitation of the sub-module capacitor voltage, the loss of the discharge branch power electronic switch is reduced as much as possible and its service life is increased, and the restriction condition is that it only acts once in one power frequency power cycle (0.02s).

[0095] The topological structure of the flexible DC converter valve with energy discharge function of the present invention is as follows: Figure 1 As shown, the flexible DC converter valve includes 6 bridge arms, each bridge arm is composed of several sub-modules SM connected in series, and the sub-modules can all be half-bridge topology, all full-bridge topology or other sub-module topology, or a mixed series structure of half-bridge, full-bridge, or other topology sub-modules. The capacitors of each sub-module are connected in parallel with an energy discharge branch composed of a fully controlled power electronic switch and a resistor in series, which can actively discharge energy when the capacitor voltage is too high, thereby reducing the capacitor voltage and ensuring the safety of the sub-module.

[0096] For example, the rated active power of the flexible DC converter valve with energy discharge function is 2000MW (megawatts), the rated reactive power of the flexible DC converter valve is ±300Mvar (megavar), the rated voltage of the DC side is ±500kV (kilovolts), the rated voltage of the AC side is 520kV, the number of single bridge arm sub-modules is 454, the rated working voltage of the sub-module is 2203V (volts), and the capacitance of the sub-module is 9mF (millifarad).

[0097] The flow chart of determining the value range of the discharge resistor of the flexible DC converter valve with energy discharge function of the present invention is as follows: Figure 2 As shown, the following steps are included:

[0098] 1. Calculate the first upper limit value R of the discharge resistance according to the system discharge power requirements max1 .

[0099] The first upper limit value R max1 The calculation formula is as follows:

[0100]

[0101] In the formula, R max1 is the first upper limit of the discharge resistance, U dc is the rated DC voltage of the converter valve, P is the rated active power of the converter valve, N sm is the rated number of submodules of the converter valve bridge arm.

[0102] Among them, according to U dc =1000kV, P=2000MW, N sm =454, and R is calculated max1 =6.61Ω.

[0103] 2. Calculate the second upper limit value R of the bleeder resistor according to the allowable submodule capacitor voltage rise requirement after the bleeder resistor is put into use max2 .

[0104] The second upper limit value R max2 The calculation steps are as follows: Figure 3 As shown, including:

[0105] First, the bridge arm current is calculated, and the bridge arm current i arm The calculation formula is as follows:

[0106]

[0107] In the formula, i arm is the bridge arm current, P is the rated active power of the converter valve, Q is the rated reactive power of the converter valve, and U acV is the rated value of the AC outlet voltage of the converter valve, and ω is the system angular frequency.

[0108] Then calculate the bridge arm voltage, the bridge arm voltage u arm The calculation formula is as follows:

[0109]

[0110] In the formula,

[0111]

[0112] u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, U acV is the rated value of the AC outlet voltage of the converter valve, ω is the system angular frequency, is the power factor angle of the converter valve, is the rated power factor angle of the converter valve.

[0113] According to the bridge arm current and bridge arm voltage, we can get The calculation formula of the time ΔT1 required to constrain the voltage rise of the submodule capacitor after the discharge resistor within the range is put into use is as follows:

[0114]

[0115] Where ΔT1 is the time required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use, i arm is the bridge arm current, u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, U in is the voltage setting value of the bleeder resistor, ΔU is the maximum allowable rise in the submodule capacitor voltage after the bleeder resistor is put into operation, and C is the submodule capacitor value.

[0116] Finally, the second upper limit value is calculated, and the second upper limit value R max2 The calculation formula is as follows:

[0117]

[0118] In the formula, R max2 For The second upper limit of the discharge resistance calculated within the range, U in is the voltage setting value of the discharge resistor, ΔU is the maximum increase of the submodule capacitor voltage allowed after the discharge resistor is put into use, i arm is the bridge arm current, ΔT1 is the time required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use, is the rated power factor angle of the converter valve.

[0119] When the voltage rise interval after the discharge resistor is put into operation is just near the maximum value of the bridge arm current, the module voltage rise is the largest after the discharge branch is put into operation. Therefore, the submodule voltage rise calculation interval is selected to be just symmetrical to the left and right of the maximum value of the bridge arm current, such as Figure 4 As shown. In this range, the module voltage is in On the basis of , ΔU is increased, and at the end of the interval, the discharge resistor current is exactly equal to the bridge arm current.

[0120] in, Figure 4 The linear change waveform of the capacitor voltage is only for illustration, not the actual change waveform of the capacitor voltage, but it does not affect the theoretical analysis results.

[0121] Among them, ΔU=0.1kV, and the calculation results are:

[0122] i arm =1.59sin(ωt)

[0123]

[0124] R max2 =2.64Ω

[0125] 3. Take R max1 and R max2 The smaller value of max .

[0126] By comparison, R max1 >R max2 , take the upper limit value of the discharge resistance R max =2.64Ω.

[0127] 4. Calculate the first lower limit value R of the discharge resistance according to the current requirement allowed by the discharge branch min1 .

[0128] The first lower limit R min1 The calculation formula is as follows:

[0129]

[0130] In the formula, R min1 is the first lower limit of the discharge resistance, k is the margin coefficient, and the recommended value range is 1.05~1.1, U in is the voltage setting value of the bleeder resistor, ΔU is the maximum increase in the submodule capacitor voltage allowed after the bleeder resistor is put into use, I max It is the maximum current allowed in the discharge branch.

[0131] Among them, according to U in =2.8kV, I max =1.8kA, k=1.1, calculated R min1 =1.77Ω.

[0132] 5. Calculate the second lower limit value R of the bleeder resistor based on the requirement that the bleeder resistor only operates once per power frequency cycle. min2 .

[0133] The second lower limit value R min2 The calculation steps are as follows: Figure 5 As shown, including:

[0134] First, calculate the energy that a single bleeder resistor needs to absorb in one power frequency power cycle. The calculation formula for the energy E that a single bleeder resistor needs to absorb in one power frequency power cycle is as follows:

[0135]

[0136] Where, E is the energy that a single discharge resistor needs to absorb in one power frequency power cycle, P is the rated active power of the converter valve, N sm is the rated number of submodules of the converter valve bridge arm.

[0137] Secondly, calculate The discharge resistor in the range only acts once per power frequency cycle. The time from the time when the discharge resistor in the range is constrained to exit the action to the time when the bridge arm current crosses zero from positive to positive is taken as the first time. The discharge resistor only acts once per power frequency cycle. The time from the time when the discharge resistor in the range is constrained to exit the action to the time when the bridge arm current crosses zero from positive to positive is taken as the first time ΔT2. The calculation formula is as follows:

[0138]

[0139] In the formula, ΔT2 is the time from the moment when the discharge resistor is constrained to exit the action to the moment when the bridge arm current crosses zero positively, which is the first time. arm is the bridge arm current, u arm is the bridge arm voltage, U dc is the rated DC voltage of the converter valve, C is the capacitance of the submodule, and ΔU2 is the difference between the input voltage setting value and the exit voltage setting value of the discharge resistor.

[0140] Then calculate in The time from the time when the discharge resistor is put into action to the time when it is withdrawn is taken as the second time. The calculation formula of the second time ΔT3 is as follows:

[0141]

[0142] In the formula, ΔT2 is the time from the moment when the discharge resistor is withdrawn to the moment when the bridge arm current crosses zero, which is the first time. ΔT3 is the time from the moment when the discharge resistor is put into operation to the moment when the discharge resistor is withdrawn, which is the second time. i arm is the bridge arm current, u arm is the bridge arm voltage, U dcis the rated DC voltage of the converter valve, C is the capacitance of the submodule, and ΔU2 is the difference between the input voltage setting value and the exit voltage setting value of the discharge resistor.

[0143] Finally, the second lower limit value R is calculated. min2 The calculation formula is as follows:

[0144]

[0145] In the formula, R min2 For The second lower limit of the bleeder resistor value calculated within the range, U R is the average value of the discharge resistor input and output voltage setting, E is the energy that a single discharge resistor needs to absorb in one power frequency power cycle, ΔT3 is the time from the discharge resistor action input moment to the exit moment, which is constrained by the requirement that the discharge resistor only acts once per power frequency power cycle, as the second time, is the rated power factor angle of the converter valve.

[0146] If the energy discharge branch is limited to operate only once in a cycle, after the first operation, the module voltage is charged to the discharge resistor for the second time just when the bridge arm current crosses zero. Figure 6 shown.

[0147] in, Figure 6 The linear change waveform of the capacitor voltage is only for illustration purposes and is not the actual change waveform of the capacitor voltage, but it does not affect the theoretical analysis results.

[0148] Among them, according to the discharge resistance input voltage setting value of 2.8kV, the discharge resistance exit voltage setting value of 2.6kV, it can be obtained that ΔU2=0.2kV, U R =2.7kV, and according to E = 0.015MJ (megajoule), ΔT2 = 3.338ms (milliseconds), ΔT3 = 2.636ms (milliseconds), R is calculated min2 =1.31Ω.

[0149] 6. Take R min1 and R min2 The larger value of the discharge resistance is taken as the lower limit value R min .

[0150] By comparison, R min1 >R min2 , get the lower limit value of the discharge resistance R min1 =1.77Ω.

[0151] 7. Finally, the value range of the discharge resistance is [R min , R max ].

[0152] According to the upper limit value of the bleeder resistor R max =2.64Ω and the lower limit of the bleeder resistor R min1 =1.77Ω, and the value range of the bleeder resistor is [1.77Ω, 2.64Ω].

[0153] The present invention calculates the first upper limit value R of the discharge resistance according to the system discharge power requirement max1 , calculate the second upper limit value R of the bleeder resistor according to the allowed submodule capacitor voltage rise requirement after the bleeder resistor is put into use max2 , take R max1 and R max2 The smaller value of max ; Calculate the first lower limit value R of the discharge resistance according to the current requirement allowed by the discharge branch min1 , calculate the second lower limit value R of the bleeder resistor based on the requirement that the bleeder resistor only operates once per power frequency power cycle min2 , take R min1 and R min2 The larger value of the discharge resistance is taken as the lower limit value R min ; Finally, the value range of the discharge resistance is [R min , R max ].

[0154] The present invention can accurately calculate the value range of the discharge resistance of the flexible DC converter valve with energy discharge function, greatly reduce the workload of electromagnetic transient simulation comparison, and more efficiently guide the electrical design of the converter valve.

[0155] An embodiment of a system for determining the resistance value of a discharge resistor of a flexible DC converter valve:

[0156] A system for determining the resistance value of a discharge resistor of a flexible DC converter valve includes a processor, and the processor is used to execute a computer program to implement the steps of a method for determining the resistance value of a discharge resistor of a flexible DC converter valve. A method for determining the resistance value of a discharge resistor of a flexible DC converter valve has been described in detail in an embodiment of a method for determining the resistance value of a discharge resistor of a flexible DC converter valve, and will not be repeated here.

Claims

1. A method for determining the resistance value of a discharge resistor of a flexible DC converter valve, characterized in that: The steps include: The first upper limit value and the second upper limit value of the corresponding discharge resistor value are calculated according to the transmission system's requirement on the discharge power of the flexible DC converter valve with an energy discharge function and the requirement on the corresponding capacitor voltage rise of the converter valve submodule allowed after the discharge resistor is put into use; The smaller value of the first upper limit value and the second upper limit value is taken as the actual upper limit value of the discharge resistance value; The first lower limit value and the second lower limit value of the corresponding discharge resistance value are calculated according to the requirements of the power transmission system on the discharge branch current of the converter valve and the requirement that the discharge resistance of the converter valve only operates once per power frequency power cycle; The larger value of the first lower limit value and the second lower limit value is taken as the actual lower limit value of the discharge resistance value; Determine the range of the bleeder resistor's resistance value based on the actual upper limit and the actual lower limit.

2. The method for determining the resistance value of the discharge resistor of the flexible DC converter valve according to claim 1, characterized in that: The calculation formula of the first upper limit is as follows: In the formula, R max1 is the first upper limit, U dc is the rated DC voltage of the converter valve, P is the rated active power of the converter valve, N sm is the rated number of submodules of the converter valve bridge arm.

3. The method for determining the resistance value of the discharge resistor of a flexible DC converter valve according to claim 1, characterized in that: The calculation formula of the second upper limit is as follows: In the formula, R max2 is the second upper limit, U in is the voltage setting value of the discharge resistor, ΔU is the maximum increase of the capacitor voltage of the submodule allowed after the discharge resistor is put into use, i arm is the bridge arm current, ΔT1 is the time required to constrain the submodule capacitor voltage rise after the discharge resistor is put into use, is the rated power factor angle of the converter valve.

4. The method for determining the resistance value of the discharge resistor of a flexible DC converter valve according to claim 1, characterized in that: The calculation formula of the first lower limit is as follows: In the formula, R min1 is the first lower limit, k is the margin coefficient, U in is the voltage setting value of the discharge resistor, ΔU is the maximum increase of the submodule capacitor voltage allowed after the discharge resistor is put into use, I max It is the maximum current allowed in the discharge branch.

5. The method for determining the resistance value of the discharge resistor of the flexible DC converter valve according to claim 4, characterized in that: Said k∈[1.05,1.1].

6. The method for determining the resistance value of the discharge resistor of a flexible DC converter valve according to claim 1, characterized in that: The calculation formula of the second lower limit is as follows: In the formula, R min2 is the second lower limit, U R is the average value of the discharge resistor input and output voltage setting, E is the energy that a single discharge resistor needs to absorb in one power frequency power cycle, ΔT3 is the time from the discharge resistor action input moment to the exit moment, which is constrained by the requirement that the discharge resistor only acts once per power frequency power cycle, as the second time, is the rated power factor angle of the converter valve.

7. A system for determining the resistance value of a discharge resistor of a flexible DC converter valve, comprising a processor, characterized in that: The processor is used to execute a computer program to implement the steps of the method for determining the resistance value of a discharge resistor of a flexible DC converter valve according to any one of claims 1 to 6.

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