A method for online monitoring of capacitance value of MMC power module

Abstract

The present invention discloses an online monitoring method for the capacitance value of an MMC power module, and relates to the field of direct current transmission technology. At present, due to the existence of communication delay, transient characteristics of IGBT disconnection, sampling error, etc. in the actual system, the actual calculation of the flexible direct current engineering valve control will have certain errors. The method continuously samples the characteristic parameters of the MMC power module in the time interval t in the peak area where the bridge arm current is relatively large. The sampling time period t includes multiple valve control cycles. The characteristic parameters include the bridge arm current value I of the flexible direct current converter valve. arm The capacitor voltage U of the power module is sampled and calculated with delay correction, and then the capacitance value C is calculated according to the switching instruction state of the submodule. sm ; Finally, within the sampling time period t, select the interval where the module voltage changes greatly, and the output capacitor value C sm It can accurately calculate the capacitance value of MMC power modules and reduce the calculation error of online monitoring of capacitance value.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current power transmission, and in particular to an online monitoring method for the capacitance value of an MMC power module. Background Art

[0002] Capacitors are important components for the normal operation of flexible DC transmission projects (MMC). If valve control has the ability to monitor the capacitance of capacitors in real time, it can detect submodule failures in advance. Given the large number of submodules, the calculation process needs to be as simple and efficient as possible. The existing methods currently use additional monitoring peripherals, and some calculation processes are too complicated and are not feasible.

[0003] At present, the available data for the valve control of flexible direct current projects include bridge arm current, submodule voltage, and submodule switching instructions. The bridge arm current will charge and discharge the submodule in the switched-on state, thereby causing the module voltage to change. The capacitance value can be calculated through the change relationship of the module voltage. Due to the communication delay, transient characteristics of IGBT disconnection, sampling error, etc. in the actual system, there will be certain errors in the actual calculation. Summary of the invention

[0004] The technical problem to be solved and the technical task proposed by the present invention are to improve and perfect the existing technical solutions, and provide an online monitoring method for the capacitance value of an MMC power module, so as to reduce the calculation error of the online monitoring of the capacitance value. To this end, the present invention adopts the following technical solutions.

[0005] An online monitoring method for the capacitance value of an MMC power module comprises the following steps:

[0006] 1) When the flexible DC converter valve is running, in the peak area of ​​the bridge arm current, the characteristic parameters of the MMC power module are continuously sampled in the time interval t. The sampling time interval t includes multiple valve control cycles. The characteristic parameters include the bridge arm current value I arm and the capacitor voltage U of the power module;

[0007] 2) According to the switching instruction status of the submodule, use the formula Calculate the capacitance C sm , where U1 and U2 are the voltages at the start and end of the sampling time period t, respectively; s represents the switching instruction of the submodule; 0 represents the removal of the switching instruction of the submodule; 1 represents the input of the switching instruction of the submodule; the right side of the equal sign is the current integral variable I in the time period integral , calculate the current integral variable for each valve control cycle, and calculate the capacitance value at the end of the sampling time period t;

[0008] 3) In the sampling time period t, select the interval where the module voltage changes greatly, and the output capacitance Csm .

[0009] The valve control cycle is short and the frequency is high. A single control cycle is only tens of microseconds. The change of the sub-module voltage in a single cycle is extremely small, generally <2V. In addition, the sampling error of the capacitor voltage, the sampling error of the current and the transient switching characteristics of the IGBT make the calculation accuracy of the capacitor value in a single cycle low. Therefore, a sampling time period t including multiple valve control cycles is used for continuous sampling and calculation. In the peak area where the bridge arm current is large, the module voltage changes greatly, which makes it easier to calculate the capacitance. The capacitance of the MMC power module can be accurately calculated, reducing the calculation error of the online monitoring of the capacitance value.

[0010] As a preferred technical means: in step 1), the sampling time period t is 1 / 4 of the bridge arm current cycle. The main component of the bridge arm current is the fundamental current superimposed with the DC component, and the peak area of ​​the bridge arm current is larger, accounting for about 1 / 4 of the bridge arm current cycle. The module voltage changes greatly, which is more convenient for calculating the capacitance.

[0011] As a preferred technical means: in step 1), in the peak area of ​​the bridge arm current, the sending end selects the area where the bridge arm current is <0, and the receiving end selects the area where the bridge arm current is >0. This can better meet the needs of capacitance calculation. If the current crosses zero in the reverse direction, the calculation is stopped and the next current that meets the conditions appears.

[0012] As a preferred technical means: the sampling time period t is delayed by Δt after the bridge arm current is reversed and then starts to be calculated, so as to intercept the interval with large voltage change.

[0013] As the preferred technical means: valve control communication link delay, the communication delay from valve control generating instructions to the sub-module central control board receiving the instructions requires t1, the sub-module central control board sampling circuit hardware delay t2, the sub-module central control board software sampling cycle t3, the time for the capacitor voltage to be uploaded to the valve control device is t4, and the communication delay ttotal = t1+t2+t3+t4. The calculation of communication delay is realized.

[0014] As a preferred technical means: for a control period of t x The module voltage in this cycle is based on n = ttotal / t x Calculate the control instruction from the previous cycle. Correct the submodule voltage and control instruction delay and current sampling time. Correct the communication delay to make the sampling and calculation accurate and reliable.

[0015] As a preferred technical means: during a sampling timing period, according to the corrected sub-module control instruction, the module's input state is judged, and the current value after the optimized sampling time is used to integrate and calculate the current integral variable I in the input state integral =I integral +I arm *tx . Realize the current integral variable calculation.

[0016] As a preferred technical means: record the voltages U1 and U2 at the start and end of the corrected sampling time period t, and then calculate C sm =I integral / (U1-U2). This is used to calculate the capacitance value.

[0017] Beneficial effects: This method can accurately calculate the capacitance value of the MMC power module and reduce the calculation error of the online monitoring of the capacitance value. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is the topology diagram of the converter valve of the flexible DC transmission system.

[0019] Figure 2 It is a schematic diagram of valve control communication link delay.

[0020] Figure 3 It is a graph showing the changing relationship between the module capacitor voltage, bridge arm current and control instructions.

[0021] Figure 4 It is a schematic diagram of an example process of the present invention. DETAILED DESCRIPTION

[0022] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0023] like Figure 1 As shown in FIG. 1 , the flexible DC transmission system, the MMC topology structure consists of 6 bridge arms, each bridge arm has the same number of power modules and a bridge arm reactor. The power module adopts a full-bridge or half-bridge structure.

[0024] The data available for the flexible direct current engineering valve control include bridge arm current, submodule voltage, and submodule switching instructions. Based on these existing signals, the capacitance value can be calculated to achieve online monitoring. The bridge arm current will charge and discharge the submodule in the switched-on state, thereby causing the module voltage to change. The capacitance value can be calculated through the change relationship of the module voltage.

[0025] The valve control cycle is short and the frequency is high. A single control cycle is only tens of microseconds. The change of the submodule voltage in a single cycle is extremely small (<2V). In addition, the sampling error of the capacitor voltage, the sampling error of the current and the transient switching characteristics of the IGBT make the calculation accuracy of the capacitor value in a single cycle low. In order to make the calculation result more accurate, this method continuously samples for a period of time and then calculates according to the formula.

[0026] An online monitoring method for the capacitance value of an MMC power module comprises the following steps:

[0027] S1) When the flexible DC converter valve is running, in the peak area of ​​the bridge arm current, the characteristic parameters of the MMC power module are continuously sampled in the time interval t. The sampling time interval t includes multiple valve control cycles. The characteristic parameters include the bridge arm current value I arm and the capacitor voltage U of the power module;

[0028] S2) According to the switching instruction status of the submodule, use the formula Calculate the capacitance C sm , where U1 and U2 are the voltages at the start and end of the sampling time period t, respectively; s represents the switching instruction of the submodule; 0 represents the removal of the switching instruction of the submodule; 1 represents the input of the switching instruction of the submodule; the right side of the equal sign is the current integral variable I in the time period integral , calculate the current integral variable for each valve control cycle, and calculate the capacitance value at the end of the sampling time period t;

[0029] S3) In the sampling time period t, select the interval where the module voltage changes greatly, and the output capacitance value C sm .

[0030] In order to make it easier to calculate the capacitance: the sampling time period t is 1 / 4 of the bridge arm current cycle. The main component of the bridge arm current is the fundamental current with the DC component superimposed, and the peak area of ​​the bridge arm current is about 1 / 4 of the bridge arm current cycle. The module voltage changes greatly, which makes it easier to calculate the capacitance.

[0031] In order to better meet the needs of capacitance calculation, in step 1), in the peak area of ​​the bridge arm current, the sending end selects the area of ​​bridge arm current < 0, and the receiving end selects the area of ​​bridge arm current > 0. It can better meet the needs of capacitance calculation. If the current crosses zero in the reverse direction, the calculation is stopped and the next current that meets the conditions appears.

[0032] In order to use the interval data with large module voltage changes, the sampling time period t is delayed by Δt after the bridge arm current flips and then starts to calculate, intercepting the interval with large voltage changes.

[0033] In order to realize the calculation of communication delay, such as Figure 2 The valve control communication link delay shown in the figure is t1, the communication delay from the valve control generating command to the submodule central control board receiving the command, the hardware delay of the sampling circuit of the submodule central control board is t2, the software sampling period of the submodule central control board is t3, the time for the capacitor voltage to be uploaded to the valve control device is t4, and the communication delay ttotal = t1+t2+t3+t4. The calculation of communication delay is realized.

[0034] In order to calculate the module voltage in this cycle, for a control period of t x The module voltage in this cycle is based on n = ttotal / tx The control instruction of the previous cycle is used to calculate the module voltage of this cycle.

[0035] In order to realize the current integral variable calculation, during a sampling timing period, the module's input state is judged according to the corrected sub-module control instruction, and the current integral variable I is calculated by integrating the current value after the optimized sampling time in the input state. integral =I integral +I arm *t x . Realize the current integral variable calculation.

[0036] In order to calculate the capacitance value, record the voltages U1 and U2 at the start and end of the corrected sampling period t, and then calculate C sm =I integral / (U1-U2). This is used to calculate the capacitance value.

[0037] like Figure 3 The figure shows the charging and discharging relationship between the module capacitor voltage and the bridge arm current in this example. In the figure, I arm Refers to the bridge arm current, V sm Refers to the submodule voltage value, PWM refers to the submodule control instruction, 1 is input and 0 is cut off. The main component of the bridge arm current is the fundamental current superimposed with the DC component. The main cycle is 20ms, the valve control cycle is 50us, and the current sampling time length t is 5ms. Figure 4 The overall process description of the example is shown below:

[0038] a1) Select the peak area I with larger bridge arm current arm >x% rated current, the sending end selects the area where the bridge arm current is <0, and the receiving end selects the area where the bridge arm current is >0; x is a preset value, ranging from 10 to 100;

[0039] a2) Determine the current value I arm Is it reverse? If I arm >0, that is, no reverse, execute the next step. If the current crosses zero and reverses, stop the calculation;

[0040] a3) After the current changes, the delay Δt starts to calculate;

[0041] a4) Determine the sampling time length to be 5ms;

[0042] a5)5ms timer count starts timing;

[0043] a6) Correction of submodule voltage U1 delay;

[0044] a7) Record the corrected starting point voltage U1;

[0045] a8) Determine whether the 5ms timer has finished timing, if not, proceed to the next step; if yes, go to step a14)

[0046] a9) 5ms timer count continues to count;

[0047] a10) Correction submodule control instruction delay;

[0048] a11) Determine the module's input status, if yes, proceed to the next step

[0049] a12) Correction of current sampling delay;

[0050] a13) Perform current integration calculation, current integration variable I integral =I integral +I arm *t x , go to step a18);

[0051] a14) Correction of submodule voltage U2 delay;

[0052] a15) Record the endpoint voltage U2;

[0053] a16)5ms timer count is cleared;

[0054] a17) Current integral is reset;

[0055] a18) In the 5ms sampling period, use the interval data with large module voltage changes to calculate Csm=I integral / (U1-U2), output the calculation result.

[0056] The valve control cycle is short and the frequency is high. A single control cycle is only tens of microseconds. The change of the sub-module voltage in a single cycle is extremely small, generally <2V. In addition, the sampling error of the capacitor voltage, the sampling error of the current and the transient switching characteristics of the IGBT make the accuracy of calculating the capacitance value in a single cycle low. Therefore, a sampling time period t including multiple valve control cycles is used for continuous sampling and calculation. In the peak area where the bridge arm current is large, the module voltage changes greatly, which makes it easier to calculate the capacitance. The capacitance value of the MMC power module can be accurately calculated, reducing the calculation error of the online monitoring of the capacitance value. The interval data with large module voltage changes is used to output the calculation results, which can reduce the output amount of data.

[0057] above Figure 1-4 The online monitoring method for the capacitance value of an MMC power module shown is a specific embodiment of the present invention, which has embodied the outstanding substantial features and significant progress of the present invention. According to actual use needs and under the guidance of the present invention, equivalent modifications in shape, structure, etc. can be made to the method, which are all within the protection scope of this scheme.

Claims

1. A method for online monitoring of the capacitance value of an MMC power module, characterized in that The following steps are involved: 1) When the flexible DC converter valve is in operation, in the peak area of ​​the bridge arm current, the characteristic parameters of the MMC power module are continuously sampled in the time period t. The sampling time period t includes multiple valve control cycles. The characteristic parameters include the bridge arm current value I arm and the capacitor voltage U of the power module; 2) According to the switching instruction status of the submodule, use the formula: Calculate the capacitance C sm , where U1 and U2 are the capacitor voltages at the start time t1 and the end time t2 of the sampling time period t, respectively. s represents the switching instruction of the submodule, 0 represents the removal of the switching instruction of the submodule, and 1 represents the input of the switching instruction of the submodule. The right side of the equal sign of the formula is the current integral variable I in this time period. integral , calculate the current integral variable for each valve control cycle, and calculate the capacitance value at the end time t2 of the sampling time period t; 3) In the sampling time period t, select the interval where the capacitor voltage of the power module changes greatly, and output the capacitor value Csm; In step 1), the sampling time period t is 1 / 4 of the bridge arm current cycle; In step 1), the peak area of ​​the bridge arm current is larger, and the area where the bridge arm current is <0 is selected at the sending end, and the area where the bridge arm current is >0 is selected at the receiving end; The sampling time period t is delayed after the bridge arm current flips tStart the calculation again and intercept the interval with large voltage change; The communication delay of the valve control communication link is ttotal. The communication delay from the valve control generating the command to the submodule central control board receiving the command requires t1, the hardware delay of the sampling circuit of the submodule central control board is t2, the software sampling cycle of the submodule central control board is t3, and the time for the capacitor voltage to be uploaded to the valve control device is t4. The communication delay ttotal = t1+t2+t3+t4; For a control period of t x The capacitor voltage of the power module in this cycle is based on n=ttotal / t x Calculate the control instruction from cycles ago; During a sampling timing period, the module's input state is determined according to the corrected submodule control instruction, and the current value after the optimized sampling time is used to integrate and calculate the current integral variable I in the input state. integral =I integral +I arm t x ; Record the voltages U1 and U2 at the start time t1 and the end time t2 of the corrected sampling period t, and then calculate C sm =I integral / (U1-U2).

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