Reliability test system and method for working condition simulation of cascade type converter capacitor

Abstract

This invention provides a reliability testing system and method for simulating the operating conditions of a cascaded converter capacitor. The system includes: an operating current loader applying an operating voltage to the capacitor under test using a control signal generated by a current controller; an operating voltage loader applying an operating voltage to the capacitor under test using a control signal generated by a voltage controller; a current filter removing high-frequency harmonic components of the operating current; a temperature and humidity loader applying external ambient temperature and humidity conditions to the capacitor under test; and a pulse discharge circuit performing overcurrent discharge on the capacitor under test. This invention enables the application of electrothermal operating condition simulation tests based on a cascaded converter capacitor to the capacitor under test, as well as constant overvoltage and repetitive overcurrent tests as specified in current national standards. It also allows for online monitoring of the capacitance value and hot spot temperature of the capacitor under test, significantly improving the efficiency, accuracy, and economy of capacitor reliability testing.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and more specifically, to a reliability testing system and method for simulating the operating conditions of cascaded converter capacitors. Background Technology

[0002] The current accelerated aging test standards for capacitors include the following two methods:

[0003] 1. National Standard GB / T 12727.2 Self-healing parallel capacitors for AC power systems - Part 2: Aging test, self-healing test and destructive test. The test method is based on constant overvoltage stress and repeated overcurrent impacts, and the change in the characteristic parameters of the tested capacitor does not exceed the specified range of the initial value.

[0004] 2. The International Electrotechnical Commission standard IEC 61071, the method for aging tests of power electronic capacitors, is also based on constant overvoltage and repeated overcurrent impacts. The criterion for judgment is that the ratio of the change in the characteristic parameters of the tested capacitor to the initial value does not exceed the specified value.

[0005] The above testing standards for capacitors all use the following conditions: rated maximum temperature overvoltage operation for a fixed time followed by pulsed discharge current impact, with capacitance loss as the judgment criterion. However, with the rise of new power electronics applications such as high-voltage direct current transmission and new energy vehicles, the actual operating conditions that capacitors in power electronic systems generally endure are high-frequency, high-current, high-amplitude voltage fluctuations, and high ambient temperature and humidity. Traditional testing standards mainly focus on reliability testing of capacitors under extreme conditions of short-term overstress, which will lead to deviations between the test results obtained according to the standards and the actual application conditions of capacitors.

[0006] Therefore, a new and reliable testing system and method are needed that can accurately perform complex operating condition testing of the capacitor under test in actual power electronic systems, as well as perform extreme operating condition testing under overvoltage and overcurrent conditions, and monitor the relevant reliability indicators of the capacitor to improve testing efficiency and the reliability of test results. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a reliability testing system and method for simulating the operating conditions of cascaded converter capacitors.

[0008] In a first aspect, the present invention provides a reliability testing system for simulating the operating conditions of a cascaded converter capacitor, wherein the operating condition simulation includes current conditions and voltage conditions under steady-state operation and / or dynamic operation; the testing system includes:

[0009] The voltage controller generates control signals for the operating condition voltage loader.

[0010] The current controller generates control signals for the operating current loader.

[0011] Current filters are used to filter out high-frequency harmonic components of operating current.

[0012] The operating current loader receives the control signal from the current controller and generates the operating current flowing through the capacitor under test through the current filter;

[0013] The operating condition voltage loader receives the control signal from the voltage controller and applies the operating condition voltage to the capacitor under test.

[0014] A DC power supply device is connected in parallel with the DC terminal of the operating current loader to provide power to the test system.

[0015] Optionally, the operating condition simulation also includes external ambient temperature and / or external ambient humidity conditions; correspondingly, the test system further includes a temperature and humidity loader, wherein the capacitor under test is placed in the cavity of the temperature and humidity loader, and the capacitor under test is provided with operating temperature and / or operating humidity.

[0016] Optionally, the test system further includes a pulse discharge circuit connected to the capacitor under test, for performing repeated overcurrent discharge tests on the charged capacitor under test.

[0017] Optionally, the operating current loader is a variable-structure multi-stage cascaded bridge converter circuit. According to the DC power supply requirements and output level requirements of the operating current loader, the bridge converter can be switched to either a full-bridge circuit or a half-bridge circuit by changing the port connection method. At the same time, the multi-stage cascaded bridge converter can be composed of single-stage or multi-stage bridge converters to form a cascaded structure.

[0018] Optionally, the operating voltage loader is a single-stage bridge converter circuit with a variable structure. Depending on the specific circuit structure of the cascaded converter sub-module, the bridge converter can be switched to either a full-bridge circuit or a half-bridge circuit by changing the port connection method.

[0019] The current filter is any one of an L-type filter, an LC-type filter, or an LCL-type filter.

[0020] Optionally, the current controller calculates the current difference between the output operating current and the reference current based on the sampling result of the operating current at the output port of the operating current loader and the given reference current signal. The current difference is used to generate a reference value of the output voltage of the operating current loader through the current control loop. The reference value of the output voltage is then pulse-width modulated and used as the switching signal of the power semiconductor switching device of the operating current loader.

[0021] Optionally, the voltage controller calculates the voltage modulation signal of the operating condition voltage loader based on the voltage sampling results of the capacitor under test and the given reference voltage signal, wherein the given reference voltage includes the average voltage of the capacitor and the reference voltage of the submodule.

[0022] Secondly, the present invention provides a reliability testing method for simulating the operating conditions of a cascaded converter capacitor, which employs the aforementioned reliability testing system for simulating the operating conditions of a cascaded converter capacitor, specifically selecting one of the following two testing methods:

[0023] Method 1: Simulation test of electrothermal operating conditions of capacitors based on cascaded converters, including:

[0024] The reliability test system for cascaded converter capacitor operating condition simulation calculates reference signals based on the cascaded converter model. It applies operating condition voltage and operating condition current to the capacitor under test independently or in combination through operating condition current loaders, operating condition voltage loaders, and temperature and humidity loaders, as well as optional external environmental operating temperature and / or external environmental operating humidity. The system simulates the electrothermal conditions and control behavior of the capacitor under steady-state operation and / or dynamic operation of the actual cascaded converter. The electrothermal conditions under dynamic operation are the cascaded converter power step condition and AC system fault condition.

[0025] Meanwhile, the reliability testing system for cascaded converter capacitor operating condition simulation is based on the acquisition of real-time electrothermal stress signals of the capacitor under test, realizing online monitoring of key state parameters of the capacitor under test; the online monitoring of key state parameters of the capacitor under test includes online monitoring of capacitance and hot spot temperature.

[0026] Method 2: Constant overvoltage and repetitive overcurrent tests on capacitors based on national standards, including:

[0027] The reliability testing system for cascaded converter capacitor operating condition simulation changes the control mode of the current controller and voltage controller, so that the operating condition current loader and operating condition voltage loader operate in voltage boost mode. The operating condition voltage loader applies overvoltage to the capacitor under test, and the pulse discharge circuit performs pulse discharge on the charged capacitor under test, realizing the constant overvoltage and repeated overcurrent test specified in the current national standard GB 17702.2.

[0028] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0029] The testing system and method provided by this invention can simulate the electrothermal stress of submodule capacitors under actual operating conditions of various cascaded converters (including modular multilevel converters and cascaded full-bridge converters) on the capacitor under test. It can also perform constant overvoltage and repeated overcurrent tests as specified in current national standards. At the same time, it can realize online monitoring of the capacitance value and hot spot temperature of the capacitor under test, which greatly improves the efficiency, accuracy and economy of capacitor reliability testing. Attached Figure Description

[0030] Figure 1 This is a block diagram of a reliability test system for simulating the operating conditions of a cascaded converter capacitor in one embodiment of the present invention;

[0031] Figure 2 shows three selectable circuit structures of the current filter in a reliability test system for simulating the operating conditions of a cascaded converter capacitor in one embodiment of the present invention.

[0032] Figure 3 shows the circuit structure of the reliability test system for cascaded converter capacitor operating condition simulation in one embodiment of the present invention, where both the operating condition current loader and the operating condition voltage loader adopt a full-bridge circuit.

[0033] Figure 4 shows the circuit structure of the reliability test system for cascaded converter capacitor operating condition simulation in one embodiment of the present invention when both the operating condition current loader and the operating condition voltage loader adopt a half-bridge circuit.

[0034] Figure 5 This is a control block diagram of the current controller in test method one of the reliability test system for simulating the operating conditions of a cascaded converter capacitor in one embodiment of the present invention.

[0035] Figure 6 This is a control block diagram of the voltage controller in test method one of the reliability test system for simulating the operating conditions of a cascaded converter capacitor in one embodiment of the present invention.

[0036] Figure 7 This is a schematic diagram of two circuit operating states in test method two of the reliability test system for simulating the operating conditions of a cascaded converter capacitor in one embodiment of the present invention.

[0037] In the diagram: 1-current loader; 2-DC power supply device; 3-voltage loader; 4-current filter; 5-voltage controller; 6-current controller; 7-capacitor under test; 8-pulse discharge circuit; 9-temperature and humidity loader. Detailed Implementation

[0038] The embodiments of the present invention will be further described below with reference to the accompanying drawings. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

[0039] Figure 1 This is a block diagram of a reliability test system for simulating the operating conditions of a cascaded converter capacitor in one embodiment of the present invention. (Refer to...) Figure 1 As shown, the test system in this embodiment includes a current loader 1 and its DC power supply device 2, a voltage loader 3, a current filter 4, a current controller 6, and a voltage controller 5; and optional pulse discharge circuit 8 and temperature and humidity loader 9. The voltage controller 5 generates a control signal for the voltage loader; the current controller 6 generates a control signal for the voltage loader; the current filter 4 is used to filter out high-frequency harmonic components of the current; the current loader 1 receives the control signal from the current controller 6 and generates a current flowing through the capacitor under test 7 via the current filter 4; the voltage loader 3 receives the control signal from the voltage controller 5 and applies a voltage to the capacitor under test 7; the DC power supply device 2 is connected in parallel with the DC terminal of the current loader 1 to provide power to the test system. Furthermore, the capacitor under test 7 is placed inside the cavity of the temperature and humidity loader 9, providing the capacitor under test with operating temperature and / or operating humidity. Furthermore, the pulse discharge circuit 8 is connected to the capacitor under test 7 to perform repeated overcurrent discharge tests on the charged capacitor under test 7.

[0040] This embodiment can simulate the electrothermal stress of submodule capacitors under various cascaded converter operating conditions on the capacitor under test, as well as the constant overvoltage and repetitive overcurrent tests specified in the current national standards.

[0041] In this embodiment, the operating conditions of the capacitor in a cascaded converter are simulated. Specifically, the cascaded converter can be either a modular multilevel converter or a cascaded full-bridge converter. The simulated operating conditions include one or more of the following: current conditions, voltage conditions, external ambient temperature conditions, and external ambient humidity conditions under steady-state and dynamic operation.

[0042] In some preferred embodiments, the main components of the operating current loader 1 include a DC power supply device 2 and a bridge converter circuit with selectable number of single or multiple series stages, used to provide an operating current including DC and AC components to the capacitor under test. Specifically, in the operating current loader 1, the DC power supply device 2 provides a DC voltage to each cascaded unit converter, and its positive and negative terminals are connected in parallel with the DC positive and negative terminals of the bridge circuit of each cascaded unit converter; the positive output terminal of the operating current loader 1 is connected to the input terminal of the current filter 4, and the negative output terminal of the operating current loader 1 is connected to the negative output terminal of the operating voltage loader 3; the operating current flows out from the positive output terminal of the operating current loader 1 and flows in from the negative output terminal of the operating current loader 1.

[0043] In some preferred embodiments, the bridge converter circuit may include either a half-bridge circuit or a full-bridge circuit.

[0044] In some preferred embodiments, the DC power supply device 2 may include an independent DC voltage source and a transformer multi-tap rectifier circuit.

[0045] In some preferred embodiments, the operating voltage loader 3 is an optional bridge converter circuit, including either a half-bridge circuit or a full-bridge circuit, used to provide an operating voltage including a DC voltage component and AC voltage fluctuations to the capacitor under test. Specifically, the positive and negative DC terminals of the operating voltage loader 3 are connected in parallel with the capacitor under test, the positive output terminal of the operating voltage loader 3 is connected to the output terminal of the current filter 4, and the negative output terminal of the operating voltage loader 3 is connected to the negative output terminal of the operating current loader 1.

[0046] In some preferred embodiments, the main components of the current filter 4 include a filter inductor and a filter capacitor, used to filter out high-frequency harmonic components of the operating current and improve current quality. Specifically, the input terminal of the current filter 4 is connected to the positive output terminal of the operating current loader 1, and the output terminal of the current filter 4 is connected to the positive output terminal of the operating voltage loader 3. Specifically, the circuit structure of the current filter 4 can adopt any of the following: L-type filter; LC-type filter; LCL-type filter. The appropriate type is selected based on the specific testing requirements.

[0047] In some preferred embodiments, the main components of the pulse discharge circuit 8 include a discharge reactor, used to perform repeated overcurrent discharge tests on the charged capacitor under test. Specifically, the discharge reactor is connected to the test system in parallel with the capacitor under test 7.

[0048] In some preferred embodiments, the optional temperature and humidity loader 9 is used for adjustable control of the external ambient temperature and humidity of the capacitor under test 7. Specifically, the capacitor under test is placed inside the cavity of the temperature and humidity loader 9, providing the capacitor with operating temperature and humidity. The temperature and humidity loader 9 has no electrical connection to the capacitor under test. Optionally, the temperature and humidity adjustment range of the temperature and humidity loader 9 is as follows: maximum temperature, not lower than the maximum operating temperature of the capacitor under test as indicated by the capacitor manufacturer; minimum temperature, not higher than the minimum operating temperature of the capacitor under test as indicated by the capacitor manufacturer; maximum humidity, the relative saturation humidity at the set temperature; minimum humidity, not higher than 20% relative humidity at the set temperature. Of course, in other embodiments, the temperature adjustment can be made according to specific experimental requirements and is not limited to the above selections.

[0049] In the above embodiments, the current controller 6 is used to adjust the operating current loader 1 according to the error between the operating current output by the operating current loader 1 and the given reference current, so that the operating current is the same as the given reference current; the voltage controller 5 is used to adjust the operating voltage loader 3 according to the error between the voltage of the capacitor under test and the given reference voltage, so that the operating voltage is the same as the given reference voltage; the reliability test system for cascaded converter capacitor operating condition simulation can calculate the reference signal according to the cascaded converter model, and apply operating voltage, operating current, external environmental operating temperature and external environmental operating humidity independently or in combination to the capacitor under test through the operating current loader 1, the operating voltage loader 3 and the temperature and humidity loader 9 to realize the electrothermal stress operating condition simulation test.

[0050] In some preferred embodiments, the operating current loader 1, the operating voltage loader 3, and the current filter 4 are all variable circuit structures.

[0051] Specifically, the operating current loader 1 is a variable-structure multi-stage cascaded bridge converter circuit. This multi-stage cascaded bridge converter circuit includes a full-bridge circuit and a half-bridge circuit. The DC terminal of each bridge converter circuit is connected in parallel with the DC power supply device 2, and the output ports of each bridge converter circuit are connected in series. The variable structure specifically means that, according to the DC power supply requirements and output level requirements of the operating current loader 1, the bridge converter can be switched to either a full-bridge circuit or a half-bridge circuit by changing the port connection method. Simultaneously, single-stage or multi-stage bridge converters can be selected to form a cascaded structure. In some embodiments, refer to... Figure 3 , 4 As shown, the specific port connection method is as follows:

[0052] - Single-stage or multi-stage full-bridge structure operating current loader 1: The negative output terminals of each stage of the bridge converter inside the operating current loader 1 are connected to the positive output terminals of the next stage bridge converter. The positive output terminal of the operating current loader 1 is connected to the input terminal of the current filter 4. Depending on the structure of the operating voltage loader 3, the negative output terminal of the operating current loader 1 is connected to the negative output terminal of the full-bridge structure operating voltage loader 3, or to the negative DC terminal of the half-bridge structure operating voltage loader 3. For example... Figure 3 As shown in the image.

[0053] - Single-stage or multi-stage half-bridge structure operating current loader 1: The negative DC terminals of each stage of the bridge converter inside the operating current loader 1 are connected to the positive output terminal of the next stage bridge converter. The positive output terminal of the operating current loader is connected to the input terminal of the current filter 4. Depending on the structure of the operating voltage loader 3, the negative DC terminal of the operating current loader 1 is connected to the negative output terminal of the full-bridge structure operating voltage loader 3, or to the negative DC terminal of the half-bridge structure operating voltage loader 3. For example... Figure 4 As shown in the image.

[0054] The bridge converter can be switched to either a full-bridge circuit or a half-bridge circuit by changing the port connection method, which can be achieved by reconnecting the lines.

[0055] Specifically, the operating voltage loader 3 is a single-stage bridge converter circuit with a variable structure. The variable structure specifically refers to the ability to switch the bridge converter to either a full-bridge circuit or a half-bridge circuit by changing the port connection method, depending on the specific circuit structure of the cascaded converter submodule. In some embodiments, refer to... Figure 3 , 4 As shown, the specific port connection method is as follows:

[0056] - Full-bridge circuit structure operating voltage loader 3: The positive output terminal of operating voltage loader 3 is connected to the output terminal of current filter 4. Depending on the structure of operating current loader 1, the negative output terminal of operating voltage loader 3 is connected to the negative output terminal of operating current loader 1 (full-bridge circuit structure) or to the negative DC terminal of operating current loader 1 (half-bridge structure). For example... Figure 4 As shown in the image.

[0057] - Half-bridge circuit structure operating voltage loader 3: The positive output terminal of operating voltage loader 3 is connected to the output terminal of current filter 4, and the negative output terminal of operating voltage loader 3 is left floating. Depending on the structure of operating current loader 1, the negative DC terminal of operating voltage loader 3 is connected to the negative output terminal of operating current loader 1 (full-bridge circuit structure) or to the negative DC terminal of operating current loader 1 (half-bridge structure). For example... Figure 4 As shown in the image.

[0058] In some embodiments, the current filter 4 can be any one of an L-type filter, an LC-type filter, or an LCL-type filter, such as... Figure 2 As shown, the specific port connection methods for each type of current filter are as follows:

[0059] L-type filter: It is a two-port circuit. The filter input is connected to the positive output of the current loader 1, and the filter output is connected to the positive output of the voltage loader 3.

[0060] LC type / LCL type filter: It is a three-port circuit. The filter input terminal is connected to the positive output terminal of the operating current loader 1, the filter output terminal is connected to the positive output terminal of the operating voltage loader 3, and the filter bypass capacitor terminal is connected to the negative output terminal of the operating current loader 1 of the full-bridge circuit structure or to the negative DC terminal of the operating current loader 1 of the half-bridge structure.

[0061] Furthermore, the main components of the bridge converter circuit are power semiconductor switching devices, including but not limited to: insulated gate bipolar transistor modules or single transistors, and metal-oxide-semiconductor field-effect transistor modules or single transistors.

[0062] Based on the aforementioned reliability testing system for simulating the operating conditions of cascaded converter capacitors, another embodiment of the present invention also provides a reliability testing method for simulating the operating conditions of cascaded converter capacitors, specifically including two optional testing methods:

[0063] Method 1: A reliability testing system for cascaded converter capacitor operating condition simulation. This system calculates reference signals based on a cascaded converter model and applies operating voltage, operating current, external ambient temperature, and external ambient humidity independently or in combination to the capacitor under test via operating current loader 1, operating voltage loader 3, and temperature and humidity loader 9, thus simulating electrothermal stress conditions. Simultaneously, based on the real-time acquisition of electrothermal stress signals from the capacitor under test, online monitoring of key state parameters is achieved. This online monitoring includes capacitance and hotspot temperature. See details... Figure 5 As shown.

[0064] Method 2: The reliability test system for cascaded converter capacitor operating condition simulation operates in voltage boost mode according to the circuit operation state control mode. The operating current loader 1 and the operating voltage loader 3 are able to apply overvoltage to the capacitor under test through the operating voltage loader 3, and pulse discharge circuit 8 is used to pulse discharge the charged capacitor under test, so as to realize the constant overvoltage and repeated overcurrent test specified in the current national standard GB 17702.2.

[0065] Specifically, the circuit operation state control method mentioned above, for example, when an L-type filter is selected for the current filter, the test system operates in voltage boost mode, which includes two periodically switching circuit operation states:

[0066] State 1: The upper bridge arm of the bridge converter circuit of the working current loader is turned on at the positive output terminal, and a positive voltage is output; the lower bridge arm of the positive output terminal of the working voltage loader is turned on, and the L-type filter stores energy.

[0067] In state two, the upper bridge arm of the bridge converter circuit of the working condition current loader is turned on at the positive output terminal, outputting a positive voltage. The upper bridge arm of the positive output terminal of the working condition voltage loader is turned on, the measured capacitor stores energy, and the voltage of the measured capacitor is pumped up to the reference overvoltage.

[0068] Specifically, in a preferred embodiment, the optional pulse discharge circuit 8 is applied to constant overvoltage and repetitive overcurrent tests, wherein the steps of a single overcurrent test are as follows:

[0069] Step 1: The operating current loader 1 and the operating voltage loader 3 operate in voltage boost mode. The operating voltage loader 3 pumps the voltage of the DC power supply device 2 to twice the rated voltage of the capacitor under test.

[0070] Step 2: The power semiconductor switching devices of the operating current loader 1 and the operating voltage loader 3 are turned off.

[0071] Step 3: The pulse discharge circuit 8 closes the discharge switch, and the charged capacitor under test undergoes overcurrent discharge through the discharge reactor to complete one discharge process; specifically, the inductance value of the discharge reactor is selected in the same way as in the national standard GB 17702.2-17.2.b).

[0072] In the above embodiment, the current controller 6 calculates the current difference between the output operating current and the reference current based on the sampling result of the operating current at the output port of the operating current loader 1 and the given reference current signal. This current difference is then used by the current control loop to generate a reference value for the output voltage of the operating current loader 1. This reference value, after pulse width modulation, serves as the switching signal for the power semiconductor switching devices in the full-bridge circuit of the operating current loader 1. Optionally, the current control loop includes, but is not limited to, proportional-integral-resonant control and hysteresis comparator control.

[0073] Optionally, the voltage controller 5 is specifically used to: calculate the voltage modulation signal of the operating condition voltage loader 3 based on the voltage sampling results of the capacitor under test and a given reference voltage signal. Specifically, the given reference voltage includes the average capacitor voltage and the submodule reference voltage.

[0074] The voltage modulation signal of the operating voltage loader 3, after passing through the switching signal modulation stage, serves as the switching signal for the power semiconductor switching device of the operating current loader 1. Optionally, the implementation of the switching signal modulation stage depends on the modulation method used in the cascaded converter, including but not limited to: carrier phase-shift modulation; carrier stacking modulation; and nearest-level approximation modulation.

[0075] In the above embodiments, test method one calculates voltage and current reference signals based on the cascaded converter model, simulating the electrothermal and control behaviors under actual cascaded converter operating conditions, including the voltage, current, external ambient temperature, and external ambient humidity conditions of the capacitor under test. Specifically, the voltage and current conditions include the steady-state voltage and current conditions simulating the cascaded converter submodule in a stable operating state, and the dynamic voltage and current conditions simulating the power electronic system containing the capacitor under test in a dynamic operating state, including but not limited to power step fluctuations, AC system fault dynamic operating states, and the dynamic voltage and current conditions of the capacitor under test.

[0076] In the above embodiments, in test method two, the optional current filter 4 circuit structure uses an L-type filter, and the test system operates in voltage boost mode, which includes two periodically switching circuit operating states:

[0077] In state one, the upper bridge arm of the bridge converter circuit of the current loader 1 is turned on and outputs a positive voltage, and the lower bridge arm of the positive output of the voltage loader 3 is turned on and the L-type filter stores energy.

[0078] In state two, the upper bridge arm of the bridge converter circuit at the positive output terminal of the current loader 1 is turned on, outputting a positive voltage. The upper bridge arm of the positive output terminal of the voltage loader 3 is turned on, the capacitor under test stores energy, and the voltage of the capacitor under test is pumped up to the reference overvoltage.

[0079] Specifically, in one embodiment, the complete test steps of test method two are as follows:

[0080] Step 1: The capacitor under test is subjected to 1.25 times its rated voltage for 750 hours;

[0081] Step 2: The capacitor under test is charged to twice the rated voltage and then discharged through pulse discharge circuit 8, 1000 times.

[0082] Step 3: Repeat the steps in Step 1.

[0083] Step four, test complete.

[0084] In the above embodiments, the state parameters of the capacitor under test 7 include its capacitance and hot spot temperature. Specifically, the signals required for monitoring the capacitance of the capacitor under test are the operating current signal and the operating voltage signal; the calculation methods for the capacitance of the capacitor under test include, but are not limited to: voltage-current integration method; voltage-current bandpass filter method. Specifically, the hot spot temperature monitoring method for the capacitor under test is a thermocouple-based temperature measurement method.

[0085] To better illustrate the above technical solutions, the implementation details are described in detail below with reference to specific embodiments. However, it should be understood that the present invention is not limited to the following embodiments.

[0086] like Figure 1 As shown, a preferred embodiment of the present invention provides a reliability testing system for simulating the operating conditions of a cascaded converter capacitor, including its circuit structure and control block diagram. The circuit structure includes: an operating condition current loader 1 composed of a single-stage or multi-stage full-bridge circuit and its DC power supply device 2; an operating condition voltage loader 3 composed of a single-stage full-bridge circuit; an L-type current filter 4; a capacitor under test 7 composed of the capacitor under test; an optional pulse discharge circuit 8; and an optional temperature and humidity loader 9. The control section consists of a voltage controller 5 and a current controller 6. The current controller 6 receives a reference current signal. I ref and the measured operating current signal I L Voltage controller 5 receives reference voltage value signal. V c_ref and the measured positive and negative voltage signals of the capacitor under test V c .

[0087] In this embodiment, the current controller 6 and the voltage controller 5 can respectively implement their functions through chips, computing circuits or computer software including digital signal processors (DSPs) or field programmable gate arrays (FPGAs).

[0088] In this embodiment, two optional test methods are provided for the reliability test system simulating the operating conditions of cascaded converter capacitors.

[0089] Method 1: Electrothermal stress simulation test. Operating condition voltage, operating condition current, external ambient temperature, and external ambient humidity are applied independently or in combination to the capacitor under test using operating condition current loader 1, operating condition voltage loader 3, and temperature and humidity loader 9 to achieve an electrothermal stress simulation test. Simultaneously, based on the real-time acquisition of the electrothermal stress signal of the capacitor under test 7, online monitoring of the key state parameters of the capacitor under test 7 can be achieved.

[0090] Specifically, the reference voltage and reference current signals are calculated based on the simulated cascaded converter model.

[0091] In this embodiment, the current controller 6 receives a reference current signal and a measured operating current signal, calculates the error signal where the operating current deviates from the reference current, and uses either proportional-integral resonant control or hysteresis comparison control to generate a control signal for the operating current loader 1. The operating current output by the operating current loader 1 is adjusted using this control signal so that the operating current signal is approximately the same as the reference current signal in the cascaded converter system parameter model. The block diagram of the optional proportional-integral resonant control for the current controller 6 is shown below. Figure 5 .

[0092] In this embodiment, the voltage controller 5 receives a reference voltage signal and the measured positive and negative voltage signals of the capacitor under test 7. The reference voltage includes the average reference voltage of the capacitor and the submodule reference voltage. The error signal between the voltage of the capacitor under test and the average reference voltage of the capacitor is calculated. The resulting error signal is processed by optional proportional-integral control or proportional control to generate a capacitor voltage balance signal. The capacitor voltage balance signal is added to the submodule reference voltage to generate a control signal for the operating condition voltage controller 5. The switching signal of the operating condition voltage loader 3 is obtained through the control signal and modulation method, ensuring that the voltage of the capacitor under test 7 remains balanced and approximately the same as the capacitor voltage in the simulated actual cascaded converter. The modulation methods of the operating condition voltage loader include carrier phase-shift modulation, carrier stacking modulation, and nearest-level approximation modulation used in actual cascaded converters. The optional proportional-integral control block diagram of the voltage controller 5 is shown below. Figure 6 .

[0093] In this embodiment, the temperature and humidity loader 9 is an optional structure. If it is necessary to simulate the ambient temperature and humidity conditions of the capacitor under test, the temperature and humidity loader 9 should be selected, and the capacitor should be placed inside the temperature and humidity loader cavity. The reference temperature and reference humidity should be set according to the specific experimental conditions.

[0094] Online monitoring of key state parameters of the capacitor under test (CUT) 7 includes online monitoring of capacitance and hot spot temperature. Specifically, online monitoring of the capacitance of CUT 7 can be achieved through algorithmic processing of the operating current signal, the voltage signal of CUT 7, and the switching signal of the operating voltage loader using chips, arithmetic circuits, or computer software, including digital signal processors (DSPs) or field-programmable gate arrays (FPGAs). Online monitoring of the hot spot temperature of CUT 7 can be achieved through temperature measurement techniques, including thermocouple temperature measurement.

[0095] Method 2: Constant Overvoltage and Repeated Overcurrent Test. The test circuit can be modified by changing the control mode of the current controller 6 and the voltage controller 5 to make the operating current loader 1 and the operating voltage loader 3 operate in voltage boost mode. This allows the operating voltage loader 3 to apply an overvoltage to the capacitor under test 7, and the pulse discharge circuit 8 to pulse discharge the charged capacitor under test 7. This method is compatible with the constant overvoltage and repeated overcurrent test specified in the current national standard GB 17702.2.

[0096] Specifically, in terms of the structure of the test system, this embodiment uses an L-type current filter and a pulse discharge circuit.

[0097] Specifically, in terms of testing methods, the operating current loader 1, operating voltage loader 3, current controller 6, and voltage controller 5 in test method two should operate in voltage boost mode.

[0098] In this embodiment, the operating current loader 1 adopts... N Series-connected structure, turn-on time of the upper arm at the positive output terminal T on With switching cycle T The ratio is the duty cycle. D The DC power supply voltage is V dc The rated capacitance of the capacitor under test is C The rated DC voltage is V c_rate The actual capacitor voltage is V c .

[0099] The voltage boost mode includes the following two periodically switching circuit operating states. The device switching states and current loops in each state are detailed below. Figure 7 .

[0100] State 1: Current controller 6 controls the upper bridge arm of the positive output terminal of the operating current loader to turn on, and voltage controller 5 controls the lower bridge arm of the positive output terminal of the operating voltage loader 3 to turn on. The turn-on time is... T on L-type current filter for energy storage;

[0101] State 2: Current controller 6 controls the upper bridge arm of the positive output terminal of the operating current loader 1 to turn on, and voltage controller 5 controls the upper bridge arm of the positive output terminal of the operating voltage loader 3 to turn on. The turn-on time is... TT on The energy of the L-type current filter is transferred to the capacitor under test 7, and the capacitor under test stores the energy.

[0102] The steady-state voltage of the capacitor 7 under test can be expressed by the formula:

[0103] ;

[0104] The testing steps for test method two are as follows:

[0105] Step 1: Constant Overvoltage Test. Under this test, the capacitor under test 7 is subjected to an overvoltage of 1.25V. V c_rate The DC voltage and current controller 6 control the duty cycle of the operating current loader. D 1 for

[0106] ;

[0107] The capacitor under test, 7, withstands 1.25V. V c_rate Overvoltage for 500 hours;

[0108] Step Two: Repeat the overcurrent test. In this test, the capacitor under test 7 is subjected to an overcurrent of 2... V c_rate The DC voltage and current controller 6 control the duty cycle of the operating current loader. D 2 for

[0109] ;

[0110] The capacitor under test 7 was charged to 2 V c_rate After the DC voltage is applied, the current controller 6 controls the operating current loader to turn off all switching devices, and the voltage controller 5 controls the operating voltage loader 3 to turn off all switching devices, closing the discharge switch of the pulse discharge circuit 8. The capacitor under test 7 completes one overcurrent test through the pulse discharge circuit 8 (discharge reactor). The overcurrent test is repeated 1000 times.

[0111] Step 3: Repeat Step 1;

[0112] Step 4: Experiment complete.

[0113] The above embodiments of the present invention provide a novel and reliable testing system and method, which can accurately perform complex operating condition testing of the capacitor under test in actual power electronic systems, as well as perform extreme operating condition testing under overvoltage and overcurrent conditions, and monitor the relevant reliability indicators of the capacitor, thereby improving testing efficiency and the reliability of test results.

[0114] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A reliability testing system for simulating the operating conditions of a cascaded converter capacitor, characterized in that, The operating condition simulation includes the current and voltage conditions of the capacitors under steady-state and / or dynamic operation of the modular multilevel converter and / or cascaded full-bridge converter; the test system includes: The voltage controller generates control signals for the operating condition voltage loader. The current controller generates control signals for the operating current loader. Current filters are used to filter out high-frequency harmonic components of operating current. The operating current loader receives the control signal from the current controller and generates the operating current flowing through the capacitor under test through the current filter; The operating condition voltage loader receives the control signal from the voltage controller and applies the operating condition voltage to the capacitor under test. A DC power supply device is connected in parallel with the DC terminal of the operating current loader to provide power to the test system. The operating voltage loader is a single-stage bridge converter circuit with a variable structure. Depending on the specific circuit structure of the cascaded converter submodule, the bridge converter can be switched to either a full-bridge circuit or a half-bridge circuit by changing the port connection method. The specific port connection method is as follows: Full-bridge circuit structure operating voltage loader: The positive output terminal of the operating voltage loader is connected to the output terminal of the current filter. Depending on the structure of the operating current loader, the negative output terminal of the operating voltage loader is connected to the negative output terminal of the full-bridge circuit structure operating current loader or to the negative DC terminal of the half-bridge structure operating current loader. Half-bridge circuit structure operating voltage loader: The positive output terminal of the operating voltage loader is connected to the output terminal of the current filter, and the negative output terminal of the operating voltage loader is left floating. Depending on the structure of the operating current loader, the negative DC terminal of the operating voltage loader is connected to the negative output terminal of the operating current loader of the full-bridge circuit structure or to the negative DC terminal of the operating current loader of the half-bridge structure.

2. The reliability testing system for simulating the operating conditions of cascaded converter capacitors according to claim 1, characterized in that, The operating condition simulation also includes the external ambient temperature and / or external ambient humidity conditions of capacitors in modular multilevel converters and cascaded full-bridge converters. Correspondingly, the testing system also includes: A temperature and humidity loader, wherein a capacitor to be tested is placed inside the cavity of the temperature and humidity loader, and the capacitor to be tested is provided with operating temperature and / or operating humidity.

3. The reliability testing system for simulating the operating conditions of cascaded converter capacitors according to claim 1, characterized in that, Also includes: A pulse discharge circuit is connected in parallel with the capacitor under test to perform an overcurrent discharge test on the charged capacitor under test.

4. The reliability testing system for simulating the operating conditions of cascaded converter capacitors according to any one of claims 1-3, characterized in that, It also includes one or more options for the following operating conditions: current loaders and current filters. The operating current loader is a variable-structure multi-stage cascaded bridge converter circuit. This multi-stage cascaded bridge converter circuit includes a full-bridge circuit and a half-bridge circuit. Based on the DC power supply requirements and output level requirements of the operating current loader, the bridge converter can be switched to either a full-bridge circuit or a half-bridge circuit by changing the port connection method. Furthermore, the multi-stage cascaded bridge converter can be composed of single-stage or multi-stage bridge converters forming a cascaded structure. The specific port connection method is as follows: Single-stage or multi-stage full-bridge structure operating current loader: The negative output terminals of each stage of the bridge converter inside the operating current loader are connected to the positive output terminal of the next stage bridge converter. The positive output terminal of the operating current loader is connected to the input terminal of the current filter. Depending on the structure of the operating voltage loader, the negative output terminal of the operating current loader is connected to the negative output terminal of the operating voltage loader of the full-bridge structure, or to the negative DC terminal of the operating voltage loader of the half-bridge structure. Single-stage or multi-stage half-bridge structure operating current loader: The negative DC terminals of each stage of the bridge converter inside the operating current loader are connected to the positive output terminal of the next stage bridge converter. The positive output terminal of the operating current loader is connected to the input terminal of the current filter. Depending on the structure of the operating voltage loader, the negative DC terminal of the operating current loader is connected to the negative output terminal of the operating voltage loader of the full-bridge structure, or to the negative DC terminal of the operating voltage loader of the half-bridge structure. The current filter is any one of an L-type filter, an LC-type filter, or an LCL-type filter, and the specific port connection method is as follows: The L-type filter is a two-port circuit. The input terminal of the current filter is connected to the positive output terminal of the operating current loader, and the output terminal of the current filter is connected to the positive output terminal of the operating voltage loader. The LC / LCL type filter is a three-port circuit. The input terminal of the current filter is connected to the positive output terminal of the operating current loader, the output terminal of the current filter is connected to the positive output terminal of the operating voltage loader, and the bypass capacitor terminal of the current filter is connected to the negative output terminal of the operating current loader with a full-bridge circuit structure or to the negative DC terminal of the operating current loader with a half-bridge structure.

5. The reliability testing system for simulating the operating conditions of cascaded converter capacitors according to claim 2, characterized in that, The temperature and humidity loader has a temperature and humidity adjustment range of: The maximum temperature shall not be lower than the maximum operating temperature of the capacitor being tested as specified by the capacitor manufacturer. The minimum temperature shall not exceed the minimum operating temperature of the capacitor being tested as specified by the capacitor manufacturer. Maximum humidity is the relative saturation humidity at the set temperature; Minimum humidity: not exceeding 20% ​​relative humidity at the set temperature.

6. The reliability testing system for simulating the operating conditions of cascaded converter capacitors according to any one of claims 1-3, characterized in that, The current controller calculates the current difference between the output current and the reference current based on the sampling result of the operating current at the output port of the operating current loader and the given reference current signal. The current difference is used to generate a reference value for the output voltage of the operating current loader through the current control loop. The reference value of the output voltage is then pulse-width modulated and used as the switching signal for the power semiconductor switching device of the operating current loader.

7. The reliability testing system for simulating the operating conditions of cascaded converter capacitors according to any one of claims 1-3, characterized in that, The voltage controller calculates the target voltage signal of the capacitor under test based on the voltage sampling results of the capacitor under test and the given reference voltage signal, and generates the control signal of the operating condition voltage loader according to the selected modulation method. The given reference voltage includes the average voltage of the capacitor and the reference voltage of the submodule.

8. A reliability testing method for simulating the operating conditions of a cascaded converter capacitor, characterized in that, The reliability test is conducted using the cascaded converter capacitor operating condition simulation system described in any one of claims 1-7, specifically selecting any one of the following test methods: Method 1: Simulation test of electrothermal operating conditions of capacitors based on cascaded converters, including: The reliability test system for cascaded converter capacitor operating condition simulation calculates reference signals based on the cascaded converter model. It applies operating condition voltage and operating condition current to the capacitor under test independently or in combination through operating condition current loaders, operating condition voltage loaders, and temperature and humidity loaders, as well as optional external environmental operating temperature and / or external environmental operating humidity. The system simulates the electrothermal conditions and control behavior of the capacitor under steady-state operation and / or dynamic operation of the actual cascaded converter. The electrothermal conditions under dynamic operation are the cascaded converter power step condition and AC system fault condition. Meanwhile, the reliability testing system for cascaded converter capacitor operating condition simulation is based on the acquisition of real-time electrothermal stress signals of the capacitor under test, realizing online monitoring of key state parameters of the capacitor under test; the online monitoring of key state parameters of the capacitor under test includes online monitoring of capacitance and hot spot temperature. Method 2: Constant overvoltage and repetitive overcurrent tests on capacitors based on national standards, including: The reliability testing system for cascaded converter capacitor operating condition simulation changes the control mode of the current controller and voltage controller, so that the operating condition current loader and operating condition voltage loader operate in voltage boost mode. The operating condition voltage loader applies overvoltage to the capacitor under test, and the pulse discharge circuit performs pulse discharge on the charged capacitor under test, realizing the constant overvoltage and repeated overcurrent test specified in the current national standard GB 17702.

2.

9. The reliability test method for simulating the operating conditions of a cascaded converter capacitor according to claim 8, wherein in method one, the voltage controller selects any one of the following voltage modulation methods based on the voltage modulation method applicable to the cascaded converter to generate the switching signal of the power semiconductor switching device of the operating condition voltage loader: Nearest level approximation modulation; Carrier phase shift modulation; Carrier stacking modulation.

10. The reliability test method for simulating the operating conditions of a cascaded converter capacitor according to claim 8, characterized in that, In the second method, when an L-type filter is selected as the current filter, the test system operates in voltage boost mode, which includes two periodically switching circuit operating states: State 1: The upper bridge arm of the bridge converter circuit of the working current loader is turned on at the positive output terminal, and a positive voltage is output; the lower bridge arm of the positive output terminal of the working voltage loader is turned on, and the L-type filter stores energy. In state two, the upper bridge arm of the bridge converter circuit of the working condition current loader is turned on at the positive output terminal, outputting a positive voltage. The upper bridge arm of the positive output terminal of the working condition voltage loader is turned on, the measured capacitor stores energy, and the voltage of the measured capacitor is pumped up to the reference overvoltage.

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