A method for monitoring the state of DC bus capacitor of wind power converter

Abstract

The present invention proposes a state monitoring method for the DC bus capacitor of a wind power converter based on a variable discharge network. The method mainly includes determining whether the converter is shut down by performing cyclic sampling detection on the bus voltage, using a variable discharge network to change the discharge path of the capacitor in the converter voltage drop stage, performing two different state discharges of the capacitor, drawing a discharge voltage curve, and using a CPU to calculate the size of the capacitor and the capacitor equivalent resistance to determine whether the capacitor is invalid. When the converter is working stably, a fault occurs and the power is off, and the voltage drops rapidly, the fault waveform is recorded. Its structure includes: a control board, a protection circuit and a control circuit, a current signal acquisition and conversion circuit, a clock circuit, and a variable discharge network circuit.

Description

Technical Field

[0001] The present invention relates to the technical field of online monitoring of the state of a bus capacitor of a wind turbine converter, and in particular to a state monitoring method of a DC bus capacitor of a wind power converter based on a variable discharge network. Background Art

[0002] In the new energy power generation system, the converter is its core component. Converter failure is one of the main faults encountered in the power generation process. The components that are most likely to fail in the converter are power semiconductor devices and DC-Link capacitors. The DC-link capacitor mainly plays the role of balancing power, connecting two-stage converters, and smoothing voltage fluctuations in the converter. It not only directly affects the utilization rate of wind energy by wind turbines, but also has a significant impact on grid connection. In power electronic systems, capacitors are one of the key components, but their failure rate in industrial applications has always been high, which seriously affects the reliability of power electronic systems. Around the system reliability design, scholars have made many innovative attempts. Capacitors are widely used in modern industry, such as power capacitors, DC-Link capacitors, bypass capacitors, rectifier filter capacitors, etc. DC-Link capacitors refer to the capacitors in the DC link after rectification, generally including inverter DC bus capacitors and inverter DC bus capacitors. DC-link capacitors are most widely used in industries such as renewable energy power generation and electric vehicles. For example, DC bus capacitors in wind power converters, DC bus capacitors in high-voltage inverters, and support capacitors in photovoltaic inverters are all DC-link capacitors.

[0003] Currently, how to accurately and quickly monitor the status of DC-link capacitors online is still a technical problem that needs to be solved. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention aims to study a state monitoring method for the DC bus capacitor of a wind power converter based on a variable discharge network. The method changes the structure of the variable discharge network, records and analyzes the voltage waveform of the capacitor, calculates the capacitance value and the equivalent resistance value in real time, and realizes accurate online monitoring of the fault state of the DC bus capacitor of the converter.

[0005] The technical solution adopted by the present invention is:

[0006] A method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network, the method comprising planning a variable discharge network, using the variable discharge network to change the access mode of the variable discharge network when the converter stops working and the bus voltage drops, and at the same time, collecting voltage through a voltage sensor and AD conversion to draw a voltage drop curve of the bus capacitor to monitor the bus capacitor online.

[0007] Furthermore, the variable discharge network includes a voltage-equalizing resistor of a three-phase converter, a relay, a small resistor connected in series with the relay, and a large balancing resistor. The relay and the small resistor are connected in series to form a small network connected in parallel at both ends of the voltage-equalizing resistor, and the large balancing resistor is connected between the DC bus bars.

[0008] Furthermore, the voltage drop curve includes two drop stages. In the first stage, the bus capacitor is quickly discharged only through the equalizing resistor and the balancing large resistor. In this discharge stage, the capacitance value is calculated using the equalizing resistor and the balancing large resistor. In the second stage, there are two discharge situations. One is that when the three-phase capacitor is balanced, the discharge is only performed through the equalizing resistor. Only the resistance of one phase of the capacitor is calculated using this discharge curve to obtain the equivalent series resistance of the remaining capacitors. When the three-phase capacitor is unbalanced, three small resistors in series are connected in parallel for discharge, and the equivalent series resistance of the three-phase capacitor is calculated using their respective voltage drop curves.

[0009] Furthermore, the method for determining whether the three-phase capacitors are balanced is to use the bus capacitor voltages collected by the three voltage collection channels and the capacitance values ​​calculated in the first stage to perform comparative analysis of electrical signals in the control board to determine whether they are balanced.

[0010] Furthermore, it also includes fault judgment of the converter: communicating with the converter main control board, the converter main control board constantly monitors whether the PWM drive signal of the converter exists, and when it is detected that the PWM signal is normal but the voltage suddenly drops rapidly, it is recorded as a fault, and the bus voltage and bus capacitance waveforms at this time are recorded to provide data for subsequent monitoring;

[0011] The control board is an electronic circuit board with a DSP chip, including a signal acquisition circuit, a control circuit protection circuit and a power supply circuit;

[0012] The variable discharge network is composed of a capacitor, a large balancing resistor, a relay, and a small resistor, and can change the capacitor discharge path.

[0013] Furthermore, the clock circuit is used in conjunction with the control board CPU to record and broadcast the fault. At the moment of the fault, the bus capacitor voltage drops rapidly. Through the recording of the voltage sensor and the analysis of the main control CPU, it is determined whether the capacitor is in a fault state. If so, the fault waveform of the voltage signal in the subsequent time is recorded to provide data support for subsequent analysis.

[0014] Furthermore, the method specifically comprises the following steps:

[0015] Step 1, detecting the bus voltage of the DC bus capacitor by a voltage sensor, converting the bus voltage signal by an AD conversion circuit, and then collecting the signal into the control board CPU; executing step 2;

[0016] Step 2: determine whether the working state of the converter is in the shutdown state, if not, execute step 1, if yes, execute step 3;

[0017] Step 3: Continue sampling and wait until the bus voltage drops to 800V, close the relay to connect the variable discharge network to the circuit; execute step 4;

[0018] Step 4, during the discharge process of the DC bus capacitor, record the time domain curve of the discharge voltage change of the DC bus capacitor through voltage detection; execute step 5;

[0019] Step 5. When the bus voltage drops to 100V, the voltage sensor is used to detect whether the voltages of the three-phase capacitors are equal to determine the balance of the three-phase capacitors: if balanced, close the relay of one-phase parallel branch, record the discharge curve of one-phase capacitor, and obtain the equivalent series resistance of the remaining capacitors by calculating only one-phase resistance; if unbalanced, close the relays of the three-phase branches at the same time, and record the discharge curves of the three-phase capacitors respectively, and calculate the equivalent series resistance of the three capacitors respectively by connecting three resistors in parallel.

[0020] Furthermore, in step 5, the method for calculating the capacitor voltage and the equivalent resistance includes:

[0021] ,

[0022] ,

[0023] In the above formula, is the total discharge resistance in the first stage, is the voltage across the capacitor, and ESR is the equivalent series resistance; by differential solution, we can get:

[0024] ,

[0025] in Represents the bus voltage, due to the voltage balancing resistor And parallel discharge balance large resistor The value of is much larger than the equivalent series resistance of the bus capacitor, that is, , In the calculation process, the series resistance of the capacitor is ignored and its total discharge resistance is considered to be is the total resistance of the two parallel resistors, that is , then calculate the discharge time constant , we can get the capacitance , thereby realizing the calculation of the capacitance value.

[0026] Furthermore, in step 5, the test for determining the balance of the three-phase capacitor specifically includes: when the voltages at both ends of the three-phase capacitor are equal, the three-phase capacitor is considered balanced, otherwise, it is considered unbalanced; when the three-phase capacitor is balanced, only one-phase bus capacitance needs to be measured and calculated, and the capacitances of the other two phases are the same as the capacitance of this phase; when the three-phase capacitor is unbalanced, the variable network is connected for measurement and calculation respectively, and during the second stage of discharge, the average resistance and the small resistance are equal. In parallel, the total discharge resistance in the second stage is:

[0027] ,

[0028] in is a small resistance in series with the relay, which is a known quantity; is the voltage-equalizing resistor, whose resistance is also known. The capacitance can be obtained from the above process. The value can be seen from the voltage drop curve of the second stage: , after finding the total discharge resistance of the second stage, the equivalent series resistance of the capacitor can be solved, that is:

[0029] ,

[0030] Advantages and beneficial effects of the present invention:

[0031] The present invention monitors the DC bus voltage drop curve during the shutdown process of power equipment such as three-phase frequency converters, and accesses different variable discharge networks in two different stages, and uses DSP main control to accurately calculate the C and ESR of the DC bus capacitor to achieve non-intrusive online monitoring of the DC bus capacitor of the three-phase AC / DC / AC system. The present invention is suitable for online monitoring of aluminum electrolytic capacitors, with simple methods, accurate monitoring, and low cost. It can be applied not only to three-phase AC / DC / AC systems of wind power converters, but also to other three-phase AC / DC / AC systems of photovoltaics, three-phase aviation power supply systems, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 : Schematic diagram of the detection process;

[0033] Figure 2 :Monitoring flow chart of the whole process;

[0034] Figure 3 : Schematic diagram of three-phase converter system;

[0035] Figure 4 : A local enlarged view of the variable discharge network;

[0036] Figure 5: Schematic diagram of the overall system structure;

[0037] Figure 6 : Design diagram of voltage acquisition circuit;

[0038] Figure 7 : Schematic diagram of variable discharge network structure;

[0039] Figure 8 : Schematic diagram of relay drive circuit. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the embodiments of the present invention more clear, the technical solution of the present invention will be clearly described below in conjunction with the accompanying drawings of the present invention.

[0041] Figure 1 The schematic diagram of the present invention. As shown in the figure, the method includes: planning a variable discharge network, using the variable discharge network to change the access mode of the variable discharge network when the converter stops working and the bus voltage drops, collecting voltage through a voltage sensor and AD conversion, drawing the voltage drop curve of the bus capacitor, and monitoring the bus capacitor online. The bus voltage is detected by a voltage sensor, and the AD conversion circuit converts the signal, which is collected in the control board CPU to judge the working state of the converter. When the voltage drops to 800V, the relay is closed to connect the variable discharge network to the circuit. The variable discharge network includes a voltage-equalizing resistor, a relay, a small resistor connected in series with the relay, and a large balancing resistor of the three-phase converter. The relay and the small resistor are connected in series to form a small network connected in parallel at both ends of the voltage-equalizing resistor, and the balancing resistor is connected between the DC buses. The voltage drop curve has two drop stages. In the first stage, the bus capacitor is only discharged quickly through the voltage-equalizing resistor and the large balancing resistor. In this discharge stage, the capacitance value is calculated using the voltage-equalizing resistor and the large balancing resistor; in the second stage, two discharge conditions are considered. When the three-phase capacitor is balanced, it is discharged only through the voltage-equalizing resistor. This discharge curve is used to calculate the resistance of only one phase of the capacitor to obtain the equivalent series resistance of the remaining capacitors; when the three-phase capacitor is unbalanced, three small resistors in series are connected in parallel for discharge, and the equivalent series resistance of the three-phase capacitor is calculated using their respective voltage drop curves. When the converter is detected to be working normally and the voltage suddenly drops rapidly, the grid voltage and bus capacitor waveforms at the time of the fault are recorded to provide data for subsequent monitoring. The clock circuit is used to cooperate with the control board CPU for fault recording and broadcasting. At the moment of the fault, the bus capacitor voltage will drop rapidly. Through the recording of the voltage sensor and the analysis of the main control CPU, it is determined whether the capacitor is in a fault state. If so, the fault waveform of the voltage signal in the subsequent time is recorded to provide data support for subsequent analysis.

[0042] Figure 2The whole monitoring process is shown in the figure, and the steps include:

[0043] Step 1: Detect the bus voltage of the DC bus capacitor through a voltage sensor, perform cyclic sampling detection, convert the bus voltage signal through an AD conversion circuit, and then collect it into the control board CPU; execute step 2;

[0044] Step 2: determine whether the working state of the converter is in the shutdown state, if not, execute step 1, if yes, execute step 3;

[0045] Step 3: Continue sampling and wait until the bus voltage drops to 800V, close relay 0 to connect the balancing large resistor of the variable discharge network to the circuit; execute step 4;

[0046] Step 4: After the large balancing resistor is connected, the discharge speed of the bus capacitor can be improved, which is regarded as the first stage of discharge. During the discharge of the DC bus capacitor, the voltage detection is performed to record the time domain curve of the DC bus capacitor discharge voltage change for subsequent analysis and calculation; execute step 5;

[0047] Step 5, continuous sampling detection, when the bus voltage drops to 100V, use the voltage sensor to detect whether the voltages of the three-phase capacitors are equal, and judge the balance of the three-phase capacitors: if balanced, close the relay of one-phase parallel branch, record the discharge curve of one-phase capacitor, and obtain the equivalent series resistance of the remaining capacitors by calculating only one-phase resistance; if unbalanced, close the relays of the three-phase branches at the same time, and record the discharge curves of the three-phase capacitors respectively, and calculate the equivalent series resistance of the three capacitors respectively by connecting three resistors in parallel.

[0048] Furthermore, the method also includes:

[0049] Check whether the converter is working normally. When the converter is working normally, if there is a sudden rapid voltage drop, record it as a fault and the grid voltage and bus capacitance waveforms at that time to provide data for subsequent monitoring.

[0050] In step 5, the method for calculating capacitor voltage and equivalent resistance is:

[0051] ,

[0052] ,

[0053] In the above formula, is the total discharge resistance in the first stage, is the voltage across the capacitor, and ESR is the equivalent series resistance; by differential solution, we can get:

[0054] ,

[0055] in Represents the bus voltage, due to the voltage balancing resistor And parallel discharge balance large resistor The value of is much larger than the equivalent series resistance of the bus capacitor, that is, , , the capacitor equivalent series resistance can be ignored in the calculation process ( ), considering that its total discharge resistance is the total resistance of the two parallel resistors, that is , then calculate the discharge time constant , we can get the capacitance , thereby realizing the calculation of the capacitance value.

[0056] Furthermore, as described in step 5, the voltage balance needs to be checked before the second stage of discharge. When the voltages at both ends of the three-phase capacitors are equal, the three phases are considered balanced, otherwise, they are considered unbalanced. When the three phases are balanced, only one phase bus capacitance needs to be measured and calculated, and the capacitances of the other two phases are the same as this phase capacitance; when the three phases are unbalanced, the variable network needs to be connected for measurement and calculation respectively. In the second stage of discharge, the average resistance and the small resistance In parallel, the discharge resistance value of the second stage is:

[0057] ,

[0058] in is a small resistance in series with the relay, which is a known quantity; is a large average resistor, and its resistance is also a known quantity. The capacitance is obtained from the above process It is worthwhile to know from the capacitor discharge curve of the second stage: , after finding the total resistance of the second stage, the equivalent series resistance of the electrolytic capacitor can be solved, that is:

[0059] .

Claims

1. A method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network, characterized in that: The method includes: planning a variable discharge network, using the variable discharge network to change the access mode of the variable discharge network when the converter stops working and the bus voltage drops, and at the same time, collecting voltage through a voltage sensor and the voltage collected by AD conversion, drawing a voltage drop curve of the bus capacitor, and performing online monitoring of the bus capacitor; The variable discharge network includes three capacitors connected in series between the DC busbars of the three-phase converter, voltage-equalizing resistors respectively connected in parallel to both ends of the three capacitors, a relay, a small resistor connected in series with the relay, and a large balancing resistor. The relay and the small resistor are connected in series to form a small network, which is respectively connected in parallel to both ends of each voltage-equalizing resistor, and the large balancing resistor is connected between the DC busbars. The voltage drop curve includes two drop stages. In the first stage, the bus capacitor is quickly discharged only through the equalizing resistor and the large balancing resistor. In this discharge stage, the capacitance value is calculated using the equalizing resistor and the large balancing resistor. In the second stage, there are two discharge situations. One is that when the three-phase capacitor is balanced, the discharge is only performed through the equalizing resistor. This discharge curve is used to calculate only the resistance of one phase of the capacitor to obtain the equivalent series resistance of the remaining capacitors. When the three-phase capacitor is unbalanced, the relays connected in series with the small resistor are closed respectively, and the equalizing resistor is connected in parallel with the small resistor for discharge. The equivalent series resistance of the three-phase capacitor is calculated respectively using their respective voltage drop curves.

2. The method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network according to claim 1 is characterized in that: The method for judging whether the three-phase capacitors are balanced is to use the bus capacitor voltages collected by the three voltage collection channels and the capacitance values ​​calculated in the first stage to compare and analyze the electrical signals in the control board to judge whether they are balanced.

3. The method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network according to claim 2 is characterized in that: It also includes fault judgment of the converter: communicating with the converter main control board, the converter main control board constantly monitors whether the converter PWM drive signal exists, and when it is detected that the PWM signal is normal but the voltage suddenly drops rapidly, it is recorded as a fault, and the bus voltage and bus capacitance waveforms at this time are recorded to provide data for subsequent monitoring; The control board is an electronic circuit board with a DSP chip, including a signal acquisition circuit, a control circuit, a protection circuit and a power supply circuit; The variable discharge network is composed of a capacitor, a large balancing resistor, a relay, and a small resistor. The variable discharge network can change the capacitor discharge path.

4. The method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network according to claim 3 is characterized in that: Use the clock circuit to cooperate with the control board CPU to record and broadcast faults. At the moment of fault, the bus capacitor voltage will drop rapidly. Through the recording of the voltage sensor and the analysis of the main control CPU, it is determined whether the capacitor is in a fault state. If so, the fault waveform of the voltage signal in the subsequent time is recorded to provide data support for subsequent analysis.

5. The method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network according to claim 3 is characterized in that: The method specifically comprises the following steps: Step 1, detecting the bus voltage of the DC bus capacitor by a voltage sensor, converting the bus voltage signal by an AD conversion circuit, and then collecting the signal into the control board CPU; executing step 2; Step 2: determine whether the working state of the converter is in the shutdown state, if not, execute step 1, if yes, execute step 3; Step 3: Continue sampling and wait until the bus voltage drops to 800V, close the relay connected in series with the large balancing resistor to connect the variable discharge network to the circuit; execute step 4; Step 4, during the discharge process of the DC bus capacitor, record the time domain curve of the discharge voltage change of the DC bus capacitor through voltage detection; execute step 5; Step 5. When the bus voltage drops to 100V, the voltage sensor is used to detect whether the voltages of the three-phase capacitors are equal to determine the balance of the three-phase capacitors: if balanced, discharge only through the equalizing resistor, record the discharge curve of one-phase capacitor, and obtain the equivalent series resistance of the remaining capacitors by calculating only one-phase resistance; if unbalanced, close the relay connected in series with the small resistor, discharge after connecting the equalizing resistor and the small resistor in parallel, and record the discharge curves of the three-phase capacitors respectively, so as to calculate the equivalent series resistance of the three capacitors.

6. The method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network according to claim 5, characterized in that: In step 4, the method for calculating the capacitor voltage and equivalent resistance includes: In the above formula, is the total discharge resistance in the first stage, is the voltage across the capacitor, and ESR is the equivalent series resistance; by differential solution, we can get: in Represents the bus voltage, due to the voltage balancing resistor And parallel discharge balance large resistor The value of is much larger than the equivalent series resistance of the bus capacitor, that is, , In the calculation process, the series resistance of the capacitor is ignored, and its total discharge resistance R is considered to be the total resistance of the voltage-equalizing resistor and the balancing large resistor in parallel. Then the discharge time constant is calculated as , we can get the capacitance , thereby realizing the calculation of the capacitance value.

7. The method for monitoring the state of a DC bus capacitor of a wind power converter based on a variable discharge network according to claim 6, characterized in that: In step 5, the test for judging the balance of the three-phase capacitor specifically includes: when the voltages at both ends of the three-phase capacitor are equal, the three-phase capacitor is regarded as balanced, otherwise, it is regarded as unbalanced; when the three-phase capacitor is balanced, only one-phase bus capacitance needs to be measured and calculated, and the capacitances of the other two phases are the same as this phase capacitance; when the three-phase capacitor is unbalanced, the variable network is connected for measurement and calculation respectively, and the voltage-equalizing resistor is connected in parallel with the small resistor, and the total discharge resistance when the three-phase capacitor is unbalanced in the second stage is obtained as follows: in is a small resistance in series with the relay, which is a known quantity; is the voltage-equalizing resistor, whose resistance is also known. The capacitance can be obtained from the above process. The value of the discharge time constant can be seen from the voltage drop curve of the second stage. , after finding the total discharge resistance of the second stage, the equivalent series resistance of the capacitor can be solved, that is: 。

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