Fault on-line monitoring method for high-capacity photovoltaic direct-current conversion equipment

Abstract

The invention discloses an online fault monitoring method for high-capacity photovoltaic direct-current conversion equipment. According to the method, voltage and current of a direct-current capacitor of a high-voltage side power unit are monitored, oscillation frequency and frequency multiplication components of the direct-current capacitor are analyzed through real-time Fourier transform, and potential faults are identified. By monitoring the ratio of the frequency component to the direct current component or the output current and comparing the ratio with the average value of other units, the abnormality of a single power unit is accurately positioned. And according to the heavy load and light load conditions of the equipment, respectively setting a delay protection threshold value and a locking protection threshold value, and when the threshold value and the delay time are exceeded, giving an alarm and locking the equipment. The method has the beneficial effects that online sensitive detection is realized, an alarm can be given out before the device is close to failure so as to arrange maintenance, or locking is carried out before the device is prevented from being completely invalid so as to avoid fault expansion, and the safe operation level of the system can be effectively improved.

Description

Technical Field

[0001] The invention relates to the field of electric power, and in particular to an online fault monitoring method for large-capacity photovoltaic direct current conversion equipment. Background Art

[0002] In high-power DC loads such as electrolytic aluminum, directly using new energy sources such as photovoltaics to generate electricity has become one of the important development directions in the energy field. Compared with traditional fossil energy, renewable energy sources such as photovoltaic power generation have the advantages of being clean and low-carbon. However, when using new energy power generation for DC loads, existing technologies still face many challenges.

[0003] At present, the way that renewable energy power generation supplies power to DC loads mainly relies on photovoltaic power generation. For some DC loads with lower rated voltages, the photovoltaic power generation power supply methods are usually divided into the following two types: DC / AC, AC / DC conversion: In this method, the high voltage (such as 1500V) generated by the photovoltaic power generation system is converted into 220V AC power through a grid-type inverter and connected to the power grid. The power grid then converts the AC power into DC power through an AC / DC converter. Finally, after rectification by a step-down rectifier, the DC load is supplied with power. Although this method can adapt to the existing power grid architecture, the overall energy conversion efficiency is low because the electric energy needs to go through multiple conversion links, including the conversion process from DC to AC and AC to DC. In addition, due to the existence of multiple conversion links, the system complexity increases and the energy loss is large. At the same time, this method usually requires expensive energy storage devices to provide support, which further increases the construction and operation costs of the system and is not economical.

[0004] DC conversion method: Compared with the AC-DC conversion method, the DC conversion method is more efficient. In this method, large-scale photovoltaic systems usually use a 1500V voltage level to collect energy, then use a DC converter to boost the DC voltage, and then directly connect to the DC load system through the step-down process. This method reduces the links of power conversion, improves the energy conversion efficiency of the system, makes the conversion link simpler, and reduces the equipment cost. In addition, the DC conversion method has relatively high stability and is suitable for the power supply needs of large power loads.

[0005] However, the core equipment of the DC conversion method is the power electronic DC transformer, also known as the DC power router. Its reliability is crucial to the stability of the entire power supply system. Since the equipment adopts a complex power electronic topology, contains multiple semiconductor devices and electromagnetic components, and has a large number of components, it is prone to failure in long-term operation, and reliability and stability are difficult to guarantee. Especially in high-power application scenarios, the loss and aging of components are accelerated, resulting in a significant increase in equipment maintenance costs and operating risks. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a method for online fault monitoring of large-capacity photovoltaic DC conversion equipment.

[0007] The present invention is achieved through the following technical solutions: The invention discloses an online fault monitoring method for a large-capacity photovoltaic DC conversion device. The large-capacity photovoltaic DC conversion device is a DC transformer with a low-voltage side parallel connection and a high-voltage side series connection structure, a low-voltage side voltage of 1250VDC, and a high-voltage side voltage of 20kVDC. The low-voltage side is composed of a plurality of low-voltage side power units connected in parallel, and the high-voltage side is composed of a plurality of high-voltage side power units connected in series. Each low-voltage side power unit is connected one-to-one with the high-voltage side power unit through a high-frequency transformer.

[0008] This method is aimed at large-capacity photovoltaic DC conversion equipment, and its core structure is a design in which low-voltage side power units are connected in parallel and high-voltage side power units are connected in series. The voltage on the low-voltage side is set to 1250VDC, which is usually the DC voltage generated by solar photovoltaic modules, while the voltage on the high-voltage side reaches 20kVDC, which is mainly used to increase the voltage to adapt to long-distance power transmission or access to high-voltage power grids. The low-voltage side works through multiple parallel power units, each unit processes a part of the power, ensuring that the equipment can provide redundancy and stability while operating efficiently. The power units on the high-voltage side are connected in series to ensure the high voltage of the final output. The high-frequency transformer connects each low-voltage side power unit to the high-voltage side power unit one by one, ensuring that the power is effectively transmitted during the conversion process, while reducing the size of the transformer through high-frequency operation.

[0009] The fault online monitoring method comprises: Collect the voltage and current of the DC support capacitor of each high-voltage side power unit; The voltage and current are subjected to real-time Fourier transform analysis and decomposed into components at the oscillation frequency, 2 times the oscillation frequency, and 4 times the oscillation frequency; the core of online fault monitoring is to monitor the voltage and current of the DC support capacitor of each high-voltage side power unit in real time. The DC support capacitor is an important component in the power unit, which is used to smooth the flow of power and prevent voltage fluctuations and transient currents from impacting the equipment. In order to accurately monitor possible faults, the signal is decomposed into the frequency domain using Fourier transform, and the components of the oscillation frequency, 2 times the oscillation frequency, and 4 times the oscillation frequency are extracted. Through these frequency components, abnormal phenomena in power transmission, such as signs of capacitor aging or failure, can be identified, and real-time monitoring can be performed during voltage or current fluctuations.

[0010] The different frequency components of the DC capacitor voltage of the high-voltage side power unit are monitored, including the ratio of the component at the oscillation frequency to the DC component, the ratio of the component at 2 times the oscillation frequency to the DC component, and the ratio of the component at 4 times the oscillation frequency to the component at 2 times the oscillation frequency; the purpose of this step is to monitor the different frequency components of the DC capacitor voltage. By calculating the ratio of the frequency component to the DC component, abnormal behavior in the operation of the capacitor can be identified. The ratio of the component at the oscillation frequency to the DC component indicates whether the capacitor is operating in a normal state. If the component at 2 or 4 times the oscillation frequency increases significantly, it may indicate that faults in the capacitor or power unit are accumulating, which may be caused by capacitor aging, damage or power fluctuations. This method can detect impending failures of capacitors or power units in advance and avoid equipment failures under high load or critical working conditions.

[0011] The different frequency components of the DC capacitor current of the high-voltage side power unit are monitored, including the ratio of the component at the oscillation frequency to the output current, the ratio of the component at twice the oscillation frequency to the output current, and the ratio of the component at four times the oscillation frequency to the component at twice the oscillation frequency; this part of the method monitors the current frequency components of the DC capacitor, which is similar to the aforementioned voltage monitoring. The oscillation frequency components of the capacitor current reflect the non-stationary components and interference in the current. In DC systems, the presence of frequency components is usually undesirable, especially high-order harmonics. Monitoring of these components can help identify current anomalies in the power unit, such as harmonic interference caused by nonlinear loads or equipment failures. These abnormal frequency components in the current will be reflected through proportional monitoring. If the ratio exceeds the normal range, it means that the power unit may need inspection or maintenance.

[0012] Monitor the different frequency components of the DC capacitor voltage and current of a single power unit on the high-voltage side, compare them with the corresponding average values ​​of all power units, and monitor the corresponding ratios; this process focuses on comparing the voltage and current frequency components of a single power unit with the average values ​​of all units to find out if any specific unit exhibits abnormal deviations. Through this comparison, abnormal behavior of a single power unit can be quickly detected, helping to distinguish whether it is a problem with a single component or a potential problem with the entire system. If the voltage and current of a unit at different frequencies are significantly different from those of other units, then this unit may be faulty and needs further investigation.

[0013] It is defined that when the output current of the whole machine is greater than or equal to 50% of the rated output current, it is a heavy load condition, and when the output current of the whole machine is between 20%-50% of the rated output current, it is a light load condition; different working load conditions are defined here to help optimize the fault monitoring strategy. Heavy load conditions mean that the equipment is processing higher power output, and the system is in a high-load working state, requiring more stringent monitoring and protection measures. Light load conditions correspond to lower power output. Although the load is small, there may be a certain risk of failure, so different protection strategies are required accordingly. This classification makes the monitoring method more flexible and adaptable, and the monitoring frequency and threshold are adjusted according to the load conditions to avoid false alarms or over-protection.

[0014] A delay time is set for heavy load conditions and light load conditions respectively, and two lockout protection thresholds are set for each ratio. The two lockout protection thresholds are applicable to heavy load conditions and light load conditions respectively; different delay times and protection thresholds are designed for different load conditions. Under heavy load conditions, since the system is in a high load state, the equipment may be subject to greater power fluctuations, so the delay time and threshold will be more stringent to ensure that the equipment will not be mistakenly triggered by accidental short-term fluctuations. Under light load conditions, the equipment runs more smoothly, so the allowable fluctuation range may be larger and the protection mechanism is relatively loose. This design improves the reliability and flexibility of the equipment under different loads and avoids unnecessary downtime.

[0015] When the proportion of the components exceeds the corresponding lockout protection threshold and exceeds the corresponding delay time, an alarm will be sounded and the equipment will be locked.

[0016] Furthermore, the voltage and current of the DC support capacitor of each high-voltage side power unit are collected, including setting up a high-speed FPGA and a sampling chip in each high-voltage side power unit to perform real-time sampling of the voltage and current, and the sampling results are transmitted to the main controller via optical fiber, and the main controller performs protection calculations, judgments, and executes locking or alarm operations.

[0017] In this step, FPGA (field programmable gate array) and high-speed sampling chips are introduced to realize real-time acquisition of voltage and current changes. The use of FPGA can greatly improve the processing speed and ensure that the system can respond to faults in a timely and accurate manner. The advantages of optical fiber transmission are strong anti-interference and long transmission distance, ensuring that the sampling results can be efficiently and stably transmitted to the main controller. After receiving the data, the main controller will analyze it according to the set protection logic to determine whether it is necessary to alarm or lock the equipment to ensure system safety.

[0018] Furthermore, an independent alarm threshold is set for each ratio. When the voltage or current criterion value of the power unit is higher than the alarm threshold but lower than the lockout protection threshold, only an alarm is issued without performing a lockout operation. This part increases the flexibility of fault handling. When the abnormal voltage or current has not reached the level that requires the device to be locked, the system will issue an alarm without shutting down immediately. This design can effectively reduce unnecessary equipment shutdowns and avoid production losses, while giving maintenance personnel enough time to diagnose and handle faults. The flexibility of setting alarm thresholds enhances the fault tolerance of the system.

[0019] Furthermore, during the protection delay time, if the output current changes by more than 10% of the rated current, the timer will be recalculated. This function enhances the system's ability to adapt to load changes. When the output current of the device changes significantly (more than 10% of the rated current), the delay time will be recalculated to ensure that the system can adapt to the new load conditions. This design takes into account the load fluctuations that may occur during the operation of the device, avoids unnecessary protection actions caused by short-term current fluctuations, and improves the stability of the system.

[0020] Furthermore, the lockout protection threshold and alarm threshold of the high-voltage side power unit are set by software and can be adjusted according to different load conditions. This flexible software setting enables the system to adjust according to actual working conditions to adapt to different load conditions. Under different loads, the voltage and current of the power unit may have different performances. Through the adjustable lockout protection threshold and alarm threshold, the system can automatically adjust according to the changes in load, further enhancing the safety and operating efficiency of the equipment.

[0021] Furthermore, the main controller is connected to the debugging computer via the Ethernet communication interface, and the locking protection threshold, alarm threshold and delay time are set and adjusted through the debugging software. By connecting the debugging computer via the Ethernet communication interface, the parameter setting and adjustment of the system are more convenient. The debugging personnel can adjust the locking protection threshold, alarm threshold and delay time in real time through the computer software without the need for cumbersome physical adjustments to the hardware. This design not only improves the debugging efficiency, but also allows the protection parameters to be flexibly adjusted according to the actual situation during the operation of the equipment, thereby improving the adaptability of the equipment.

[0022] Furthermore, under the overload condition, when one of the following conditions is met and the duration exceeds the overload protection delay time, an alarm is issued and the equipment is locked: The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the first blocking protection threshold; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the second blocking protection threshold; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the third lockout protection threshold, and the ratio of its component at four times the oscillation frequency to its component at two times the oscillation frequency exceeds the fourth lockout protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit exceeds the fifth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit exceeds the sixth blocking protection threshold; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to the component at 2 times the oscillation frequency exceeds the seventh blocking protection threshold; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average DC capacitor voltage of each high-voltage side power unit to the average exceeds an eighth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 2 times the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at 2 times the oscillation frequency to the average value exceeds the ninth blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the tenth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the eleventh blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twelfth lockout protection threshold.

[0023] Here is a description of when the device needs to sound an alarm and perform a lockout operation under heavy load conditions. Each condition is judged by the proportional changes in the capacitor voltage and current of the high-voltage side power unit under different frequency components. This design can react sensitively to small changes in capacitor voltage and current, so that quick action can be taken when potential faults occur in the system to avoid damage to the equipment.

[0024] For example: the ratio of the effective value of the DC capacitor voltage component of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the first lockout protection threshold; this criterion mainly focuses on the fluctuation of the capacitor voltage at the oscillation frequency. If the fluctuation exceeds the set value, it may mean that some components in the circuit are unstable or are about to fail.

[0025] The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to its component at 2 times the oscillation frequency exceeds the seventh lockout protection threshold; this criterion monitors the current changes at different frequencies, and the change of the component at 4 times the oscillation frequency compared to the 2 times oscillation frequency can reveal the nonlinear failure trend in the current. When this ratio exceeds the set value, it indicates that the equipment may have high-frequency interference or current abnormality.

[0026] The setting of these conditions helps the system to perform accurate fault monitoring and protection under heavy load conditions, avoiding irreversible damage to the equipment when it is running at full load.

[0027] Furthermore, under the light load condition, when one of the following conditions is met and the duration exceeds the light load protection delay time, an alarm is issued and the equipment is locked: The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the thirteenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the fourteenth blocking protection threshold; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the fifteenth blocking protection threshold, and the ratio of its component at four times the oscillation frequency to its component at two times the oscillation frequency exceeds the sixteenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit exceeds the seventeenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit exceeds the eighteenth blocking protection threshold; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to the component at 2 times the oscillation frequency exceeds the nineteenth blocking protection threshold; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average DC capacitor voltage of each high-voltage side power unit to the average exceeds the twentieth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the twenty-first blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the twenty-second blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twenty-third blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twenty-fourth lockout protection threshold.

[0028] Under light load conditions, the judgment criteria for equipment failure are different from those under heavy load conditions. Under light load conditions, the load of the power unit is small, and the fluctuations of the capacitor voltage and current of the equipment may be more sensitive, so the protection threshold will be adjusted accordingly. For example, the lockout protection threshold under light load conditions is generally set higher to prevent frequent lockout of the equipment due to slight fluctuations. This design ensures that the equipment can be effectively protected under different loads, and the equipment will not be shut down due to misjudgment under light load, nor will the potential failure risk be ignored.

[0029] Furthermore, the overload protection delay time is 3 seconds; the first lockout protection threshold to the twelfth lockout protection threshold are 5%, 10%, 8%, 60%, 30%, 80%, 70%, 8%, 15%, 20%, 30%, and 45%, respectively, and the corresponding non-lockout warning thresholds are 3%, 5%, 4%, 40%, 20%, 60%, 50%, 5%, 12%, 15%, 20%, and 30%, respectively.

[0030] Furthermore, the light load protection delay time is 10 seconds; the thirteenth lockout protection threshold to the twenty-fourth lockout protection threshold are 4%, 8%, 6%, 80%, 20%, 50%, 90%, 10%, 30%, 40%, 50%, 65%, and the corresponding non-lockout warning thresholds are 3%, 5%, 4%, 50%, 10%, 30%, 60%, 5%, 15%, 25%, 30%, and 45%, respectively.

[0031] The beneficial effects of the present invention are: realizing online sensitive detection, and being able to send out an alarm before the device is close to failure so as to arrange maintenance, or locking the device before it completely fails to avoid the expansion of the fault, which can effectively improve the safe operation level of the system. DETAILED DESCRIPTION

[0032] The present invention will be further described below in conjunction with specific embodiments: Embodiment: A method for online fault monitoring of a large-capacity photovoltaic DC conversion device, wherein the large-capacity photovoltaic DC conversion device is a DC transformer with a low-voltage side parallel connection and a high-voltage side series connection structure, a low-voltage side voltage of 1250VDC, and a high-voltage side voltage of 20kVDC, wherein the low-voltage side is composed of a plurality of low-voltage side power units in parallel, and the high-voltage side is composed of a plurality of high-voltage side power units in series, and each low-voltage side power unit is connected one-to-one with the high-voltage side power unit through a high-frequency transformer.

[0033] This method is aimed at large-capacity photovoltaic DC conversion equipment, and its core structure is a design in which low-voltage side power units are connected in parallel and high-voltage side power units are connected in series. The voltage on the low-voltage side is set to 1250VDC, which is usually the DC voltage generated by solar photovoltaic modules, while the voltage on the high-voltage side reaches 20kVDC, which is mainly used to increase the voltage to adapt to long-distance power transmission or access to high-voltage power grids. The low-voltage side works through multiple parallel power units, each unit processes a part of the power, ensuring that the equipment can provide redundancy and stability while operating efficiently. The power units on the high-voltage side are connected in series to ensure the high voltage of the final output. The high-frequency transformer connects each low-voltage side power unit to the high-voltage side power unit one by one, ensuring that the power is effectively transmitted during the conversion process, while reducing the size of the transformer through high-frequency operation.

[0034] The fault online monitoring method comprises: Collect the voltage and current of the DC support capacitor of each high-voltage side power unit; The voltage and current are subjected to real-time Fourier transform analysis and decomposed into components at the oscillation frequency, 2 times the oscillation frequency, and 4 times the oscillation frequency; the core of online fault monitoring is to monitor the voltage and current of the DC support capacitor of each high-voltage side power unit in real time. The DC support capacitor is an important component in the power unit, which is used to smooth the flow of power and prevent voltage fluctuations and transient currents from impacting the equipment. In order to accurately monitor possible faults, the signal is decomposed into the frequency domain using Fourier transform, and the components of the oscillation frequency, 2 times the oscillation frequency, and 4 times the oscillation frequency are extracted. Through these frequency components, abnormal phenomena in power transmission, such as signs of capacitor aging or failure, can be identified, and real-time monitoring can be performed during voltage or current fluctuations.

[0035] The different frequency components of the DC capacitor voltage of the high-voltage side power unit are monitored, including the ratio of the component at the oscillation frequency to the DC component, the ratio of the component at 2 times the oscillation frequency to the DC component, and the ratio of the component at 4 times the oscillation frequency to the component at 2 times the oscillation frequency; the purpose of this step is to monitor the different frequency components of the DC capacitor voltage. By calculating the ratio of the frequency component to the DC component, abnormal behavior in the operation of the capacitor can be identified. The ratio of the component at the oscillation frequency to the DC component indicates whether the capacitor is operating in a normal state. If the component at 2 or 4 times the oscillation frequency increases significantly, it may indicate that faults in the capacitor or power unit are accumulating, which may be caused by capacitor aging, damage or power fluctuations. This method can detect impending failures of capacitors or power units in advance and avoid equipment failures under high load or critical working conditions.

[0036] The different frequency components of the DC capacitor current of the high-voltage side power unit are monitored, including the ratio of the component at the oscillation frequency to the output current, the ratio of the component at twice the oscillation frequency to the output current, and the ratio of the component at four times the oscillation frequency to the component at twice the oscillation frequency; this part of the method monitors the current frequency components of the DC capacitor, which is similar to the aforementioned voltage monitoring. The oscillation frequency components of the capacitor current reflect the non-stationary components and interference in the current. In DC systems, the presence of frequency components is usually undesirable, especially high-order harmonics. Monitoring of these components can help identify current anomalies in the power unit, such as harmonic interference caused by nonlinear loads or equipment failures. These abnormal frequency components in the current will be reflected through proportional monitoring. If the ratio exceeds the normal range, it means that the power unit may need inspection or maintenance.

[0037] Monitor the different frequency components of the DC capacitor voltage and current of a single power unit on the high-voltage side, compare them with the corresponding average values ​​of all power units, and monitor the corresponding ratios; this process focuses on comparing the voltage and current frequency components of a single power unit with the average values ​​of all units to find out if any specific unit exhibits abnormal deviations. Through this comparison, abnormal behavior of a single power unit can be quickly detected, helping to distinguish whether it is a problem with a single component or a potential problem with the entire system. If the voltage and current of a unit at different frequencies are significantly different from those of other units, then this unit may be faulty and needs further investigation.

[0038] It is defined that when the output current of the whole machine is greater than or equal to 50% of the rated output current, it is a heavy load condition, and when the output current of the whole machine is between 20%-50% of the rated output current, it is a light load condition; different working load conditions are defined here to help optimize the fault monitoring strategy. Heavy load conditions mean that the equipment is processing higher power output, and the system is in a high-load working state, requiring more stringent monitoring and protection measures. Light load conditions correspond to lower power output. Although the load is small, there may be a certain risk of failure, so different protection strategies are required accordingly. This classification makes the monitoring method more flexible and adaptable, and the monitoring frequency and threshold are adjusted according to the load conditions to avoid false alarms or over-protection.

[0039] A delay time is set for heavy load conditions and light load conditions respectively, and two lockout protection thresholds are set for each ratio. The two lockout protection thresholds are applicable to heavy load conditions and light load conditions respectively; different delay times and protection thresholds are designed for different load conditions. Under heavy load conditions, since the system is in a high load state, the equipment may be subject to greater power fluctuations, so the delay time and threshold will be more stringent to ensure that the equipment will not be mistakenly triggered by accidental short-term fluctuations. Under light load conditions, the equipment runs more smoothly, so the allowable fluctuation range may be larger and the protection mechanism is relatively loose. This design improves the reliability and flexibility of the equipment under different loads and avoids unnecessary downtime.

[0040] When the proportion of the components exceeds the corresponding lockout protection threshold and exceeds the corresponding delay time, an alarm will be sounded and the equipment will be locked.

[0041] Furthermore, the voltage and current of the DC support capacitor of each high-voltage side power unit are collected, including setting up a high-speed FPGA and a sampling chip in each high-voltage side power unit to perform real-time sampling of the voltage and current, and the sampling results are transmitted to the main controller via optical fiber, and the main controller performs protection calculations, judgments, and executes locking or alarm operations.

[0042] In this step, FPGA (field programmable gate array) and high-speed sampling chips are introduced to realize real-time acquisition of voltage and current changes. The use of FPGA can greatly improve the processing speed and ensure that the system can respond to faults in a timely and accurate manner. The advantages of optical fiber transmission are strong anti-interference and long transmission distance, ensuring that the sampling results can be efficiently and stably transmitted to the main controller. After receiving the data, the main controller will analyze it according to the set protection logic to determine whether it is necessary to alarm or lock the equipment to ensure system safety.

[0043] Specifically, the method for sampling the voltage and current of the power unit is as follows: First, a sampling system consisting of a high-speed FPGA and a dedicated sampling chip is set up inside each power unit to perform high-precision sampling of voltage and current in real time. The collected voltage and current data are transmitted to the main controller of the DC converter via high-speed optical fiber. The main controller is not only responsible for receiving data, but also has powerful computing capabilities, capable of performing protection calculations, judging abnormal situations and taking corresponding measures.

[0044] At each sampling moment, the main controller will actively send a synchronous sampling command to the high-speed FPGA of each power unit. After receiving the command, the high-speed FPGA starts sampling the key parameters inside the power unit, such as capacitor voltage and current. In order to ensure the reliability of data transmission, the sampling results will be processed by serial encoding and verification encoding before transmission, and then transmitted to the main controller at high speed via optical fiber.

[0045] After the main controller collects the sampling results from all power units, it first saves these data into a data buffer. The data buffer adopts an advanced first-in-first-out (FIFO) queue structure. Each time a sample is taken, the system automatically removes the earliest sample value and stores the latest sample data in. Through this mechanism, the data buffer always saves the latest continuous data sequence required for Fourier analysis, ensuring the accuracy and real-time nature of the analysis results.

[0046] After the data collection is completed, the main controller will perform Fourier analysis on the data sequence stored in the data buffer. Through Fourier transform, the system can decompose the voltage and current signals into various frequency components and accurately calculate the voltage and current components at each frequency point. Then, these components will be compared with the preset protection threshold to determine whether the system is in an abnormal state.

[0047] In order to improve the robustness of the system and prevent the spectrum analysis results from being invalid due to large and rapid fluctuations in the load, the system has designed a protection delay mechanism. When it is detected that the output current changes by more than 10% of the rated current in a short period of time, the main controller will reset the time to ensure that more accurate protection measures are taken within the protection delay.

[0048] In addition, the main controller has good debuggability. Through the Ethernet communication interface, the main controller can be connected to an external debugging computer. Technicians can use the dedicated debugging software on the debugging computer to easily configure the parameters of the main controller, set various protection thresholds and protection delay times in the system in real time, and ensure that the system is always in the best operating state.

[0049] Through real-time data acquisition, accurate Fourier analysis and flexible protection mechanism, the accuracy of voltage and current monitoring and the reliability of system protection are effectively improved.

[0050] Furthermore, an independent alarm threshold is set for each ratio. When the voltage or current criterion value of the power unit is higher than the alarm threshold but lower than the lockout protection threshold, only an alarm is issued without performing a lockout operation. This part increases the flexibility of fault handling. When the abnormal voltage or current has not reached the level that requires the device to be locked, the system will issue an alarm without shutting down immediately. This design can effectively reduce unnecessary equipment shutdowns and avoid production losses, while giving maintenance personnel enough time to diagnose and handle faults. The flexibility of setting alarm thresholds enhances the fault tolerance of the system.

[0051] Furthermore, during the protection delay time, if the output current changes by more than 10% of the rated current, the timer will be recalculated. This function enhances the system's ability to adapt to load changes. When the output current of the device changes significantly (more than 10% of the rated current), the delay time will be recalculated to ensure that the system can adapt to the new load conditions. This design takes into account the load fluctuations that may occur during the operation of the device, avoids unnecessary protection actions caused by short-term current fluctuations, and improves the stability of the system.

[0052] Furthermore, the lockout protection threshold and alarm threshold of the high-voltage side power unit are set by software and can be adjusted according to different load conditions. This flexible software setting enables the system to adjust according to actual working conditions to adapt to different load conditions. Under different loads, the voltage and current of the power unit may have different performances. Through the adjustable lockout protection threshold and alarm threshold, the system can automatically adjust according to the changes in load, further enhancing the safety and operating efficiency of the equipment.

[0053] Furthermore, the main controller is connected to the debugging computer via the Ethernet communication interface, and the locking protection threshold, alarm threshold and delay time are set and adjusted through the debugging software. By connecting the debugging computer via the Ethernet communication interface, the parameter setting and adjustment of the system are more convenient. The debugging personnel can adjust the locking protection threshold, alarm threshold and delay time in real time through the computer software without the need for cumbersome physical adjustments to the hardware. This design not only improves the debugging efficiency, but also allows the protection parameters to be flexibly adjusted according to the actual situation during the operation of the equipment, thereby improving the adaptability of the equipment.

[0054] Furthermore, under the overload condition, when one of the following conditions is met and the duration exceeds the overload protection delay time, an alarm is issued and the equipment is locked: The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the first blocking protection threshold; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the second blocking protection threshold; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the third lockout protection threshold, and the ratio of its component at four times the oscillation frequency to its component at two times the oscillation frequency exceeds the fourth lockout protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit exceeds the fifth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit exceeds the sixth blocking protection threshold; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to the component at 2 times the oscillation frequency exceeds the seventh blocking protection threshold; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average DC capacitor voltage of each high-voltage side power unit to the average exceeds an eighth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 2 times the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at 2 times the oscillation frequency to the average value exceeds the ninth blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the tenth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the eleventh blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twelfth lockout protection threshold.

[0055] Here is a description of when the device needs to sound an alarm and perform a lockout operation under heavy load conditions. Each condition is judged by the proportional changes in the capacitor voltage and current of the high-voltage side power unit under different frequency components. This design can react sensitively to small changes in capacitor voltage and current, so that quick action can be taken when potential faults occur in the system to avoid damage to the equipment.

[0056] For example: the ratio of the effective value of the DC capacitor voltage component of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the first lockout protection threshold; this criterion mainly focuses on the fluctuation of the capacitor voltage at the oscillation frequency. If the fluctuation exceeds the set value, it may mean that some components in the circuit are unstable or are about to fail.

[0057] The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to its component at 2 times the oscillation frequency exceeds the seventh lockout protection threshold; this criterion monitors the current changes at different frequencies, and the change of the component at 4 times the oscillation frequency compared to the 2 times oscillation frequency can reveal the nonlinear failure trend in the current. When this ratio exceeds the set value, it indicates that the equipment may have high-frequency interference or current abnormality.

[0058] The setting of these conditions helps the system to perform accurate fault monitoring and protection under heavy load conditions, avoiding irreversible damage to the equipment when it is running at full load.

[0059] Furthermore, under the light load condition, when one of the following conditions is met and the duration exceeds the light load protection delay time, an alarm is issued and the equipment is locked: The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the thirteenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the fourteenth blocking protection threshold; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the fifteenth blocking protection threshold, and the ratio of its component at four times the oscillation frequency to its component at two times the oscillation frequency exceeds the sixteenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit exceeds the seventeenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit exceeds the eighteenth blocking protection threshold; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to the component at 2 times the oscillation frequency exceeds the nineteenth blocking protection threshold; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average DC capacitor voltage of each high-voltage side power unit to the average exceeds the twentieth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the twenty-first blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the twenty-second blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twenty-third blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twenty-fourth lockout protection threshold.

[0060] Under light load conditions, the judgment criteria for equipment failure are different from those under heavy load conditions. Under light load conditions, the load of the power unit is small, and the fluctuations of the capacitor voltage and current of the equipment may be more sensitive, so the protection threshold will be adjusted accordingly. For example, the lockout protection threshold under light load conditions is generally set higher to prevent frequent lockout of the equipment due to slight fluctuations. This design ensures that the equipment can be effectively protected under different loads, and the equipment will not be shut down due to misjudgment under light load, nor will the potential failure risk be ignored.

[0061] Furthermore, the overload protection delay time is 3 seconds; the first lockout protection threshold to the twelfth lockout protection threshold are 5%, 10%, 8%, 60%, 30%, 80%, 70%, 8%, 15%, 20%, 30%, and 45%, respectively, and the corresponding non-lockout warning thresholds are 3%, 5%, 4%, 40%, 20%, 60%, 50%, 5%, 12%, 15%, 20%, and 30%, respectively.

[0062] Furthermore, the light load protection delay time is 10 seconds; the thirteenth lockout protection threshold to the twenty-fourth lockout protection threshold are 4%, 8%, 6%, 80%, 20%, 50%, 90%, 10%, 30%, 40%, 50%, 65%, and the corresponding non-lockout warning thresholds are 3%, 5%, 4%, 50%, 10%, 30%, 60%, 5%, 15%, 25%, 30%, and 45%, respectively.

[0063] The specific values ​​of the above-mentioned locking protection threshold and non-locking warning threshold are determined according to typical values ​​in actual conditions. When the present invention is used in other scenarios, appropriate locking protection threshold and non-locking warning threshold should be determined according to actual conditions.

[0064] Taking a typical 6MW, 1250V to 20kV DC transformer as an example, it uses 20 low-voltage side power units and 20 high-voltage side power units. One high-frequency transformer is set between each low-voltage side power unit and the high-voltage side power unit, and a total of 20 high-frequency transformers are set.

[0065] In each power unit, a DC capacitor is provided. In each low-voltage side power unit and high-voltage side power unit, an oscillating power electronic switch is provided, the switch device is an IGBT, the oscillation frequency is 10kHz, and the operating frequency of the isolation transformer is also 10kHz.

[0066] When working, the typical operating characteristics of DC capacitor voltage and current are: Under heavy load conditions (greater than 50% load): The ratio of the effective value of the DC capacitor voltage component at the oscillation frequency to the DC component of the high-voltage side power unit is approximately 1%; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component is about 3%; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at 4 times the oscillation frequency to its component at 2 times the oscillation frequency is approximately 25%; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit is about 10%; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit is about 40%; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to its component at 2 times the oscillation frequency is about 30%; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average value of the DC capacitor voltages of each high-voltage side power unit to the average value is approximately 3%; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at twice the oscillation frequency and the average value of the component of the DC capacitor voltage of all high-voltage side power units at twice the oscillation frequency to the average value is approximately 8%; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the component of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value is approximately 10%; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at 4 times the oscillation frequency to the average value is approximately 10%; The difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency is about 20% of the average value; At light load (20% - 50% load): The ratio of the effective value of the DC capacitor voltage component at the oscillation frequency to the DC component of the high-voltage side power unit is approximately 0.5%; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component is approximately 2%; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at 4 times the oscillation frequency to its component at 2 times the oscillation frequency is approximately 35%; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit is about 5%; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit is about 20%; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to its component at 2 times the oscillation frequency is approximately 40%; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average value of the DC capacitor voltages of each high-voltage side power unit to the average value is approximately 2%; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at twice the oscillation frequency to the average value is approximately 10%; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the component of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value is approximately 15%; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value is approximately 20%; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of the high-voltage side power units of all power units at 4 times the oscillation frequency to the average value is approximately 30%; Therefore, in order to protect in time while avoiding false alarms, the typical value of the heavy load protection delay time is 3 seconds, and the typical value of the light load protection delay time is 10 seconds. Taking into account the typical fault tolerance, avoiding false alarms and avoiding rejection, the above-mentioned blocking protection threshold and non-blocking warning threshold are set.

[0067] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device, wherein the large-capacity photovoltaic DC conversion device is a DC transformer with a low-voltage side parallel connection and a high-voltage side series connection structure, a low-voltage side voltage of 1250VDC, and a high-voltage side voltage of 20kVDC, wherein the low-voltage side is composed of a plurality of low-voltage side power units in parallel, and the high-voltage side is composed of a plurality of high-voltage side power units in series, and each low-voltage side power unit is connected one-to-one with the high-voltage side power unit through a high-frequency transformer, characterized in that: The fault online monitoring method comprises: Collect the voltage and current of the DC support capacitor of each high-voltage side power unit; Performing real-time Fourier transform analysis on the voltage and current to decompose them into components at the oscillation frequency, twice the oscillation frequency, and four times the oscillation frequency; Monitor different frequency components of the DC capacitor voltage of the high-voltage side power unit, including the ratio of the component at the oscillation frequency to the DC component, the ratio of the component at twice the oscillation frequency to the DC component, and the ratio of the component at four times the oscillation frequency to the component at twice the oscillation frequency; Monitor different frequency components of the DC capacitor current of the high-voltage side power unit, including the ratio of the component at the oscillation frequency to the output current, the ratio of the component at twice the oscillation frequency to the output current, and the ratio of the component at four times the oscillation frequency to the component at twice the oscillation frequency; Monitor the different frequency components of the DC capacitor voltage and current of a single power unit on the high-voltage side, compare them with the corresponding average values ​​of all power units, and monitor the corresponding ratio; It is defined that when the output current of the whole machine exceeds 50% of the rated output current, it is a heavy load condition, and when the output current of the whole machine is between 20%-50% of the rated output current, it is a light load condition; A delay time is set for heavy load condition and light load condition respectively, and two blocking protection thresholds are set for each ratio respectively. The two blocking protection thresholds are applicable to heavy load condition and light load condition respectively; When the proportion of the components exceeds the corresponding lockout protection threshold and exceeds the corresponding delay time, an alarm will be sounded and the equipment will be locked.

2. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 1, characterized in that: The voltage and current of the DC support capacitor of each high-voltage side power unit are collected, including setting a high-speed FPGA and a sampling chip in each high-voltage side power unit to perform real-time sampling of the voltage and current, and the sampling results are transmitted to the main controller via optical fiber, and the main controller performs protection calculations, judgments, and executes locking or alarm operations.

3. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 1, characterized in that: An independent alarm threshold is also set for each ratio. When the criterion value of the voltage or current of the power unit is higher than the alarm threshold but lower than the lockout protection threshold, only an alarm is issued without performing a lockout operation.

4. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 1, characterized in that: Further including: During the protection delay time, if the output current changes by more than 10% of the rated current, the timing will be restarted.

5. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 3, characterized in that: The blocking protection threshold and the alarm threshold of the high-voltage side power unit are both set by software and can be adjusted according to different load conditions.

6. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 5, characterized in that: The main controller is connected to a debugging computer via an Ethernet communication interface, and the locking protection threshold, alarm threshold and delay time are set and adjusted via debugging software.

7. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 3, characterized in that: Under the overload conditions, if one of the following conditions is met and the duration exceeds the overload protection delay time, an alarm will be issued and the equipment will be locked: The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the first blocking protection threshold; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the second blocking protection threshold; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the third lockout protection threshold, and the ratio of its component at four times the oscillation frequency to its component at two times the oscillation frequency exceeds the fourth lockout protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit exceeds the fifth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit exceeds the sixth blocking protection threshold; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to the component at 2 times the oscillation frequency exceeds the seventh blocking protection threshold; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average DC capacitor voltage of each high-voltage side power unit to the average exceeds an eighth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 2 times the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at 2 times the oscillation frequency to the average value exceeds the ninth blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the tenth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the eleventh blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twelfth lockout protection threshold.

8. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 3, characterized in that: Under the light load condition, if one of the following conditions is met and the duration exceeds the light load protection delay time, an alarm will be issued and the equipment will be locked: The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at the oscillation frequency to the DC component exceeds the thirteenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the fourteenth blocking protection threshold; The ratio of the component of the DC capacitor voltage of the high-voltage side power unit at twice the oscillation frequency to the DC component exceeds the fifteenth blocking protection threshold, and the ratio of its component at four times the oscillation frequency to its component at two times the oscillation frequency exceeds the sixteenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at the oscillation frequency to the output current of the power unit exceeds the seventeenth blocking protection threshold; The ratio of the effective value of the component of the DC capacitor current of the high-voltage side power unit at twice the oscillation frequency to the output current of the power unit exceeds the eighteenth blocking protection threshold; The ratio of the component of the DC capacitor current of the high-voltage side power unit at 4 times the oscillation frequency to the component at 2 times the oscillation frequency exceeds the nineteenth blocking protection threshold; The ratio of the difference between the DC capacitor voltage of a single high-voltage side power unit and the average DC capacitor voltage of each high-voltage side power unit to the average exceeds the twentieth blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor voltage of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the twenty-first blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at twice the oscillation frequency and the average value of the components of the DC capacitor current of the high-voltage side power units of all power units at twice the oscillation frequency to the average value exceeds the twenty-second blocking protection threshold; The ratio of the difference between the component of the DC capacitor voltage of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the components of the DC capacitor voltage of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twenty-third blocking protection threshold; The ratio of the difference between the component of the DC capacitor current of a single high-voltage side power unit at 4 times the oscillation frequency and the average value of the component of the DC capacitor current of all high-voltage side power units at 4 times the oscillation frequency to the average value exceeds the twenty-fourth lockout protection threshold.

9. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 7, characterized in that: The overload protection delay time is 3 seconds; the first lockout protection threshold to the twelfth lockout protection threshold are 5%, 10%, 8%, 60%, 30%, 80%, 70%, 8%, 15%, 20%, 30%, and 45% respectively, and the corresponding non-lockout warning thresholds are 3%, 5%, 4%, 40%, 20%, 60%, 50%, 5%, 12%, 15%, 20%, and 30% respectively.

10. A method for online fault monitoring of a large-capacity photovoltaic DC conversion device as claimed in claim 8, characterized in that: The light load protection delay time is 10 seconds; the thirteenth locking protection threshold to the twenty-fourth locking protection threshold are 4%, 8%, 6%, 80%, 20%, 50%, 90%, 10%, 30%, 40%, 50%, 65% respectively, and the corresponding non-locking warning thresholds are 3%, 5%, 4%, 50%, 10%, 30%, 60%, 5%, 15%, 25%, 30%, 45% respectively.