CN112468086B

CN112468086B - Arc monitoring system and method applied to photovoltaic energy

Info

Publication number: CN112468086B
Application number: CN202011264540.2A
Authority: CN
Inventors: 许峰; 郭伟栋; 张耀; 丁辉; 张永
Original assignee: State Power Investment Group Hangzhou New Energy Production And Operation Co ltd; FONRICH NEW ENERGY TECHNOLOGY Ltd SHANGHAI
Current assignee: State Power Investment Group Hangzhou New Energy Production And Operation Co ltd; FONRICH NEW ENERGY TECHNOLOGY Ltd SHANGHAI
Priority date: 2020-11-13
Filing date: 2020-11-13
Publication date: 2022-01-25
Anticipated expiration: 2040-11-13
Also published as: CN112468086A

Abstract

The invention relates to an arc monitoring system and an arc monitoring method applied to photovoltaic energy. The data acquisition module collects the cascade current and the cascade voltage provided by each group of strings. The current detection module is used for detecting the first and second arc characteristics of the total current after the cascade currents of the groups of strings are gathered. Analyzing the cascade currents and the cascade voltages of all parallel strings when the total current conforms to one or more first type arc characteristics and the total current conforms to one or more second type arc characteristics: when the cascade voltage is reduced and the cascade current is increased in a plurality of strings, the rest strings with reduced cascade voltage and reduced cascade current are regarded as the strings with direct current arc faults.

Description

Arc monitoring system and method applied to photovoltaic energy

Technical Field

The invention mainly relates to the field of photovoltaic energy, in particular to an arc monitoring system and an arc monitoring method which are applied to a photovoltaic power generation system and used for detecting a direct current arc phenomenon.

Background

With the shortage of chemical energy and the development of electric power technology, photovoltaic is concerned more and more widely, and the photovoltaic energy needs to meet safety regulations in electric power application. Arcing is a gas discharge phenomenon, and sparks generated by current flowing through an insulating medium such as air are a manifestation of the gas discharge. Detecting arcs and actively taking countermeasures are key elements in maintaining photovoltaic energy systems under safety regulations. Although the industry is struggling to find the regularity and commonality of arcing phenomena to find accurate detection means for arcing, it is difficult to avoid the fact that it is difficult to provide a reasonable and strict detection mechanism for arcing and to design a corresponding accurate detection instrument. There are a few volume-production-type arc detection products on the market that can play a role in actual detection, and a real and effective direct current arc detection product faces nearly a blank market.

The accidents of arcing and firing caused by poor contact, aging, short circuit and the like in the photovoltaic system are more and more frequent, and the detection of the visible direct current arc fault is increasingly important in the photovoltaic system. Once a photovoltaic system has a direct-current arc fault, the fault arc of the system has a stable combustion environment due to no zero-crossing point protection and continuous energy generated by a photovoltaic module under the irradiation of sunlight. If measures are not taken timely and effectively, the phenomenon of high temperature over thousands of degrees can be generated, fire is caused, and some substances are melted and even evaporated to generate a large amount of toxic gas, so that the life safety of people is endangered, and the economy of the society is greatly lost.

Dividing the arc by current properties can be roughly divided into direct current arcs and alternating current arcs. The well-known alternating current application time is earlier, and alternating current fault arcs exist mature detection methods and commercial products, however, the starting time of a photovoltaic system is later, and the nature characteristics of a direct current arc are different from that of the alternating current, and a typical direct current has no zero-crossing point characteristics like the alternating current, so that the detection means of the alternating current arc cannot be applied to photovoltaic occasions. The variables affecting the electrical properties of the direct current arc are various originally, and the arc is more complicated due to different photovoltaic use environments. It is generally recognized in the industry that it is difficult to establish a mathematical model of a dc arc, and although some arc models are mentioned, these simplified models are usually studied based on some single characteristics or several very limited characteristics of an arc, and in fact, noise inevitably existing in a photovoltaic environment and accidental interference of a power system are very likely to mislead arc detection, which causes erroneous detection results, and dynamically changing illumination intensity and ambient temperature, and a great amount of switching noise are interference sources for misjudgment and missing judgment. The objective of the present application is to detect real dc arc faults existing in a photovoltaic system to avoid accidents such as fire caused by fault arcs.

Disclosure of Invention

The application relates to an arc monitoring system applied to photovoltaic energy, which is used for monitoring whether direct current arc faults exist in a plurality of groups of parallel strings, and each group of strings comprises a plurality of photovoltaic modules which are connected in series, and comprises:

the data acquisition module is used for collecting cascade current and cascade voltage provided by each group of strings;

the current detection module is used for detecting the first and second arc characteristics of the total current after the cascade currents of each group of strings are collected;

defining a first type of arc characteristic includes:

in the selected time period range, if the actual size of the peak-valley difference between the peak value and the valley value of the total current exceeds a preset peak-valley difference value, judging that the total current conforms to the first-type arc characteristics;

if the actual magnitude of the transient variation generated by the average value of the total current exceeds the preset current variation value, judging that the total current conforms to the first type arc characteristic;

if the transient change rate of the total current exceeds a preset change rate, judging that the total current accords with the first-class arc characteristics;

defining a second type of arc characteristic includes:

judging that the total current conforms to the second type of arc characteristics if the current increase of the high-frequency component appearing in the total current exceeds a prescribed current increase value within a specified frequency band range;

in a specified frequency band range, if the ratio of the high-frequency component to the direct-current component in the total current is changed sharply and the ratio of the high-frequency component to the direct-current component exceeds a specified ratio, judging that the total current conforms to the second type of arc characteristics;

when the total current conforms to one or more first type arc characteristics and conforms to one or more second type arc characteristics, analyzing the cascade current and the cascade voltage of all the parallel strings:

when the cascade voltage is reduced and the cascade current is increased in a plurality of strings, the rest strings with reduced cascade voltage and reduced cascade current are regarded as the strings with direct current arc faults.

The arc monitoring system applied to the photovoltaic energy comprises: the output power of each string is transmitted to an inverter which converts the dc power to ac power.

The arc monitoring system applied to the photovoltaic energy comprises: the inverter has a maximum power point tracking function, and the inverter performs maximum power point tracking on each group of strings.

The arc monitoring system applied to the photovoltaic energy comprises: and under the condition that the cascade current of the group strings with reduced cascade voltage and increased cascade current is larger than the cascade current of the rest other group strings with reduced cascade voltage and reduced cascade current:

the string with both the cascade voltage and the cascade current reduced is regarded as the string with the dc arc fault.

The arc monitoring system applied to the photovoltaic energy comprises: only the following are satisfied: the strings with the decreased cascade voltage and the increased cascade current are separated from the maximum power point, and the cascade currents of the strings are increased relative to the current level corresponding to the maximum power point; and

the rest other groups of strings with reduced cascade voltage and cascade current are separated from the maximum power point, and the cascade current of the rest groups of strings is reduced relative to the current level corresponding to the maximum power point;

The application relates to an arc monitoring method applied to photovoltaic energy, which is used for monitoring whether direct current arc faults exist in a plurality of groups of parallel strings, and each group of strings comprises a plurality of photovoltaic modules which are connected in series, and comprises the following steps:

detecting first and second arc characteristics of total current after the cascade currents of each group of strings are converged;

the first type of arc characteristic defined comprises:

a second type of defined arc characteristic includes:

collecting the cascade current and the cascade voltage provided by each group of strings;

on the premise that the total current is judged to accord with one or more first-class arc characteristics and one or more second-class arc characteristics, analyzing the cascade current and the cascade voltage of all parallel strings:

The method comprises the following steps: the output power of each string is transmitted to an inverter which converts the direct current into alternating current.

The method comprises the following steps: the inverter has a maximum power point tracking function to perform maximum power point tracking on each group of strings.

The method comprises the following steps: the cascade current of the group strings with reduced cascade voltage and increased cascade current is larger than that of the rest other group strings with reduced cascade voltage and reduced cascade current:

The method comprises the following steps: only the following are satisfied: the strings with the decreased cascade voltage and the increased cascade current are separated from the maximum power point, and the cascade currents of the strings are increased relative to the current level corresponding to the maximum power point; and

The hazard and the type of the direct current arc fault are explained in detail, the arc detection method is provided by combining the characteristics of the direct current arc and the attributes of the photovoltaic power station, and a reasonable and effective detection algorithm is clarified for the direct current arc fault.

Drawings

To make the above objects, features and advantages more comprehensible, embodiments accompanied with figures are described in detail below, and features and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following figures.

Fig. 1 is a photovoltaic power generation system in which photovoltaic modules are connected in series and then connected in parallel to supply power to an inverter that performs inversion.

Fig. 2 is a diagram showing a plurality of photovoltaic modules connected in series in each string and thereby providing a high level of string voltage.

Fig. 3 is a string with both the cascade voltage and the cascade current reduced as a string with a dc arc fault.

Fig. 4 is a string with the voltage of the cascade reduced and the current of the cascade reversed as a string of a dc arc fault.

Detailed Description

The present invention will be described more fully hereinafter with reference to the accompanying examples, which are intended to illustrate and not to limit the invention, but to cover all those embodiments, which may be learned by those skilled in the art without undue experimentation.

Referring to fig. 1, a photovoltaic module array is the basis for the conversion of light energy to electrical energy in a photovoltaic power generation system. The illustrated photovoltaic module array has strings of cells mounted therein. Regarding the battery string: each battery string is formed by connecting a plurality of photovoltaic modules which are mutually connected in series, and the photovoltaic modules can be replaced by direct current power supplies such as fuel cells or chemical batteries. A plurality of different battery strings are connected in parallel: although each battery string is composed of a plurality of photovoltaic modules and the plurality of photovoltaic modules inside are connected in series, a plurality of different battery strings are connected in parallel with each other and collectively supply electric energy to an energy collecting device such as a photovoltaic inverter INVT. In a certain battery pack string, the application takes the series-connected multi-stage photovoltaic modules PV1-PVN as an example, the output voltages of the series-connected multi-stage photovoltaic modules PV1-PVN are mutually superposed to provide the total cascade voltage with higher potential to the inverter INVT, and the inverter INVT collects the output power of the series-connected multi-stage photovoltaic modules and then carries out direct current to alternating current inversion. The battery strings such as ST1-STK are connected in parallel and the total current of the series current of each battery string is taken as the input current of the inverter. K and N are positive integers greater than 1.

Referring to fig. 1, the first of the two current methods for detecting an arc fault on the dc side of a photovoltaic power generation system is a detection method based on a voltage-current waveform change. The current across the arc changes instantaneously and the voltage across the arc also changes instantaneously when an arc fault occurs. Such a method has advantages in that the principle of the detection method is easily understood, and voltage and current are objects that can be easily detected and measured, and thus are generally adopted schemes. However, the photovoltaic power generation system is greatly influenced by factors such as illumination intensity and ambient temperature, the amplitude of the output current and voltage is naturally unstable, for example, instantaneous changes of current and voltage are generated due to shadow shielding or sudden and sudden illumination, and the inherent current pulsation of the input side caused by the alternating current output by the inverter also changes the output characteristics of the photovoltaic module. A disadvantage of such methods is therefore that it is difficult to distinguish whether the changes in current and voltage are due to environmental causes or changes due to arc faults.

Referring to fig. 1, the second of the two current methods for detecting an arc fault on the dc side of a photovoltaic power generation system is a detection method based on frequency characteristics. The arc is accompanied by high-frequency clutter signals and embodies arc characteristics, and the high-frequency clutter signals cannot appear under normal working conditions. The presence of these signals therefore indicates a dc arc fault. Some vendors have produced specialized dc arc fault detectors based on the second category of methods. The detection is carried out at the photovoltaic module and the junction box or the inverter end, and is detection of the arc fault at the direct current side of the whole photovoltaic system instead of detection at the module level. The conflagration hidden danger can appear when the electric arc fault appears, and current scheme can't fix a position the fault point fast, needs the fortune dimension personnel to investigate all photovoltaic module and cables once more, and work load is huge and inefficiency, and the potential safety hazard is great. The time for eliminating the fault arc leads to the shutdown of the whole photovoltaic system, so that the early warning processing and the event response are difficult to achieve accurately and quickly in time, and the loss of the power generation yield of the power station is further caused. The biggest defect of the traditional arc fault detection scheme is that the judgment is missed and the judgment is mistaken, and the photovoltaic system has a large amount of switching noise and environmental factors which can cause interference on the real arc detection. It is therefore important and most difficult to implement string-level arc detection, i.e., to detect the specific string in which an arc is occurring.

Referring to fig. 1, current photovoltaic arc fault techniques all employ passive detection techniques. Specifically, the high-frequency characteristics of the current or voltage of the photovoltaic string are detected and analyzed to distinguish whether an arc fault exists in the system. There are three major factors in photovoltaic systems that make this approach very difficult to implement: the first is that there are many sources of interference in the photovoltaic system, especially interference from the inverter, which is in different operating conditions, and which interferes with the current and voltage on the string side of the dc string differently, and this interference is also related to the ac side of the inverter. Such uncertain disturbances present great difficulties for arc detection. The second is that in many cases the dc arc is very stable and does not change very significantly in current or voltage, thus increasing the difficulty of identifying the arc by current or voltage characteristics, one of the objectives of this application being to overcome this concern. Thirdly, different photovoltaic power stations have different field wiring, different operating environments and the like, and a set of unified arc identification method is difficult to find for different power stations.

Referring to fig. 1, a dc arc is a gas discharge phenomenon, which generates a high intensity instantaneous current in an insulating case. Unlike the ac arc, the dc arc has no zero crossing, meaning that if a dc arc fault occurs, the trigger portion will remain stable burning for a significant period of time without extinguishing. The lack of tightening of cable joints in photovoltaic power stations can lead to poor contact and reliability problems with connectors or certain switches, long-term degradation of the insulation, damage to the insulation due to external forces, and the like, which can cause dc arcing. As the plant runtime increases, the probability of dc arcs occurring also increases. Regardless of the other contacts and insulation, there are over 80000 optical contacts in a 10MW substation and the possibility of dc arcing at all times. Even though only 1/1000 contact points may have dc arcing during 25 years of plant operation, the plant will have 80 dc arcing events with a very high probability of fire.

Referring to fig. 2, the first photovoltaic module PV1 has an output voltage V_O1The output voltage of the second photovoltaic module PV2 is denoted as V_O2And so on, the output voltage of the Nth photovoltaic module PVN is V_ON: so that the total string level voltage on the first string, i.e., the left string set ST1, is approximately V by calculation_O1+V_O2+…V_ON=V₁. Different groups of battery packs are connected in series and in parallel and supply power for the inverter. The multi-stage photovoltaic modules PV1 to PVN are connected in series, the respective output voltages of the multi-stage photovoltaic modules being superimposed on a transmission line. The transmission line voltage is much higher than a single photovoltaic module and as shown the inverter inverts the transmission line voltage from direct current on the transmission line to alternating current, which is a conventional solution. Photovoltaic modules are connected in series to form a string, and the inverter can try to make the string work at the maximum working point。

Referring to fig. 2, the above description is explained with the first string ST1 as an alternative example. Such as with an optional set of strings STK: the output voltage of the first photovoltaic module PV1 is V_O1The output voltage of the second photovoltaic module PV2 is denoted as V_O2And so on, the output voltage of the Nth photovoltaic module PVN is V_ON. So that the total string level voltage on the kth string, i.e., the right group string STK, is approximately V by calculation_O1+V_O2+…V_ON=V_K。

With reference to fig. 2, a concern in distributed or centralized photovoltaic power plants is: shadow occlusion causes mismatches among numerous photovoltaic modules. Problems are also found in: the battery output characteristics of the photovoltaic module are shown in the fact that the output voltage and the output current are closely related to external factors such as light intensity and ambient temperature, and due to uncertainty of the external factors, the corresponding voltage of the maximum output power and the maximum power point changes along with the change of the external factors. For example, the power output by the photovoltaic module has randomness and severe fluctuation, and the random uncontrollable characteristic has high probability of causing great impact on the power grid and may also cause negative influence on the operation of some important loads. Based on these doubts, achieving maximum power point tracking of photovoltaic modules in consideration of external factors is a core goal of the industry to maximize energy and revenue.

Referring to fig. 2, as environmental and conventional energy problems become more severe, the photovoltaic power generation technology has been emphasized by more and more countries and regions and is regarded as a priority development object, and the photovoltaic power generation is one of the most mature and most developed scale power generation modes in the new energy power generation technology. Solar photovoltaic modules are divided into monocrystalline silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells and the like in the current mainstream technology direction, and the service life required by the silicon cells is generally as long as more than twenty years, so that the solar photovoltaic modules are essential for long-term and durable control of the solar photovoltaic modules. It is a well-known problem that many factors cause a reduction in the power generation efficiency of the photovoltaic module, for example, manufacturing differences, installation differences or shading or maximum power tracking adaptation among the photovoltaic modules themselves cause inefficiency. Taking shadow blocking as an example, if some photovoltaic modules are blocked by clouds, buildings, tree shadows, dirt, and the like, some photovoltaic modules become loads from the power supply and no longer generate electric energy and consume the output power of other photovoltaic modules. For example, when the same string of battery plates cannot normally generate electricity due to poor product consistency or shading, the efficiency loss of the whole string of battery packs is serious and the number of battery plate arrays accessed by inverters, especially centralized inverters, is large, the battery plates of each string of battery packs cannot operate at the maximum power point of the battery plates, which are the inducement of the loss of electric energy and generated energy. Because the local temperature of the photovoltaic module at a place with a serious hot spot effect may be higher, some of the photovoltaic modules even exceed 150 ℃, the photovoltaic module is burnt or forms dark spots, welding spots are melted, packaging materials are aged, glass is burst, welding strips are corroded and other permanent damages are caused, and the potential hazards to the safety and the reliability of the photovoltaic module are caused. The photovoltaic system has to solve the problems of real-time management and control of photovoltaic modules and management of the photovoltaic modules, and the specific requirements are that the working state and working parameters of each mounted photovoltaic cell panel can be managed and controlled in real time, the voltage abnormity, current abnormity, temperature abnormity and other abnormal conditions of the photovoltaic modules can be reliably pre-warned, and some countermeasures are taken, so that the adoption of module-level active safety shutdown or other emergency power-off measures for the abnormal battery modules is very significant and necessary.

Referring to fig. 2, the photovoltaic inverter INVT has a maximum power point tracking MPPT function. Photovoltaic power generation is greatly influenced by temperature and irradiance, and in order to obtain more electric energy under the same condition and improve the operation efficiency of a system, the tracking of the maximum power point of a photovoltaic cell becomes a long-standing problem in the development of the photovoltaic industry. Early researches on the maximum power point tracking technology of a photovoltaic array mainly comprise a constant voltage tracking method, a photovoltaic array combination method and an actual measurement method. The constant voltage tracking method is actually equivalent to voltage stabilization control, and does not achieve the purpose of maximum power point tracking. The photovoltaic array combination method is used for adjusting the number of series-parallel connection of photovoltaic arrays according to different loads, and has no real-time property. The actual measurement method is to use an additional photovoltaic array module to establish a reference model of the photovoltaic array at a certain sunshine amount and temperature, and the method does not consider the real-time shading condition and the difference of each solar panel. At present, the maximum power tracking method of the photovoltaic array is mainly divided into a method based on a mathematical model, a self-optimizing method based on disturbance and a method based on an intelligent technology. The method based on the mathematical model is based on establishing an optimized mathematical model as a starting point to construct a solving method and a photovoltaic array characteristic curve so as to obtain the maximum power output of the photovoltaic array, so that the equivalent circuit model of the photovoltaic cell and the correctness of various parameters need to be considered emphatically.

Referring to fig. 2, the principle and features of a conventional MPPT method for power optimization: for example, in the early output power control for photovoltaic modules, a Voltage feedback method Constant Voltage Tracking is mainly used, and the Tracking method ignores the influence of temperature on the open-circuit Voltage of the solar cell, so that an open-circuit Voltage method and a short-circuit current method are proposed, and the common property of the open-circuit Voltage method and the short-circuit current method is basically very similar to the maximum power point. In order to more accurately capture the maximum power point, a disturbance observation method, a duty ratio disturbance method, a conductance increment method and the like are proposed. The disturbance observation method is characterized in that the current array power is measured, then a small voltage component disturbance is added to the original output voltage, the output power is changed, the changed power is measured, the power before and after the change is compared, the power change direction can be known, if the power is increased, the original disturbance is continuously used, and if the power is reduced, the original disturbance direction is changed. The duty ratio disturbance working principle is as follows: the interface between the photovoltaic array and the load usually adopts a voltage converter controlled by a pulse width modulation signal, so that the input and output relationship of the converter can be adjusted by adjusting the duty ratio of the pulse width modulation signal, and the function of impedance matching is realized, and therefore, the magnitude of the duty ratio substantially determines the magnitude of the output power of the photovoltaic cell. The incremental conductance method is a special way to the disturbance observation method, the biggest difference is only in the logical judgment formula and the measurement parameters, although the incremental conductance method still changes the output voltage of the photovoltaic cell to reach the maximum power point, the logical judgment formula is modified to reduce the oscillation phenomenon near the maximum power point, so that the incremental conductance method is suitable for the climate with instantaneous change of the sunlight intensity and the temperature. The actual measurement method, the fuzzy logic method, the power mathematical model, the intermittent scanning tracking method, the optimal gradient method, the three-point gravity center comparison method and the like belong to the most common maximum power point tracking method. Therefore, the MPPT algorithm used in the photovoltaic energy industry is diversified, and repeated description is omitted in the application.

Referring to fig. 2, a photovoltaic module array is the basis for the conversion of light energy to electrical energy in a photovoltaic power generation system. The photovoltaic module array is provided with battery strings, and each battery string is formed by serially connecting photovoltaic modules PV1-PVN in series. The total electrical energy provided by the array of photovoltaic modules is transmitted by a dc transmission line to an energy harvesting device or energy harvesting device comprising an inverter INVT as shown for inverting dc power to ac power or a charger for charging a battery. Usually, a bypass diode connected in parallel with the photovoltaic module is connected between the positive electrode and the negative electrode of each photovoltaic module, so that when the output power of the photovoltaic module is reduced, the photovoltaic module can be bypassed by the bypass diode matched with the photovoltaic module, rather than the photovoltaic module with reduced output power entering a negative pressure region, which would otherwise cause extremely high power dissipation at the two ends of the photovoltaic module, and even cause combustion.

Referring to fig. 2, in the power-voltage curve of each group string, each group string has a unique maximum output power point under the same environmental conditions, and the output power of the photovoltaic module on the left side of the maximum power point shows a rising trend as the output voltage of the photovoltaic module rises. After the maximum power point is reached, the output power of the photovoltaic group string is rapidly reduced, and the reduction speed is far greater than the increase speed, namely the output power of the photovoltaic component on the right side of the maximum power point shows a reduction trend along with the increase of the output voltage of the photovoltaic component. The output voltage corresponding to the maximum power point of the string is about equal to 78-80% of the open circuit voltage.

Referring to fig. 2, each group string is also equipped with a data acquisition module SENS. For example, the second string ST2 is provided with a data acquisition module SENS that collects the cascade current I2 and the cascade voltage V2 of the second string ST 2. The data acquisition module is usually configured with a processor and additional peripheral hardware for acquiring various target parameter data of the dc power supply such as the cascade current and the cascade voltage, and even the temperature. It is very meaningful for the data acquisition module to be able to retrieve these target parameter data, such as calculating the total current of the transmission line and adjusting the voltage on the transmission line based on the total power of each battery string, and transmitting various target data to the cloud server for backup or calling. In an alternative example, the data acquisition module may acquire a series of related target parameter data such as voltage and current, power, temperature and power generation of the photovoltaic module and the string, for example, the voltage parameter is acquired by a voltage sensor, the current parameter is acquired by a current sensor, the temperature parameter is acquired by external hardware such as a temperature sensor, and the illumination radiation intensity is acquired by an illumination sensor. It is easily understood that the more kinds of peripheral hardware, the more kinds of parameters related to the photovoltaic module can be obtained by the data acquisition module, but the cost is increased, so that a compromise is needed. In some specific embodiments, the various types of target parameter data further include ambient environmental factor data of the photovoltaic module detected by the environmental monitor: the environmental monitor is also regarded as a data collector with high integration level due to the environmental temperature, humidity, wind speed, illumination intensity, air pressure and the like. The data acquisition modules in different groups of serial configurations can mutually transmit data through communication modes such as wireless communication or carrier communication, or the data acquisition module SENS and the inverter INVT can mutually transmit data through communication modes such as wireless communication or carrier communication.

Referring to fig. 2, the first string ST1 is also provided with a data acquisition module SENS. The first string ST1 is equipped with a data acquisition module SENS that collects the cascode current I1 and the cascode voltage V1 of the first string ST 1. Other strings are also equipped with data collectors: the third string ST3 is also equipped with a data acquisition module SENS. The third string ST3 is equipped with a data acquisition module SENS that collects the cascade current I3 and the cascade voltage V3 of the third string ST 3. A representative group string also has a data collector: the kth string STK is equipped with a data acquisition module SENS. The kth string STK is provided with a data acquisition module SENS that collects the string current IK and the string voltage VK of the kth string STK.

Referring to fig. 2, the total current IB obtained by summing the cascade currents I1-IK of the strings ST1-STK is regarded as the input current of the inverter INVT, which is the power conversion device, and the dc power generated by the parallel strings ST1-STK is supplied to the inverter to perform the dc-ac inversion conversion. The preliminary survey of the arc is monitored and the results of the preliminary survey are taken at the total current IB of the busbar, not directly at each cluster.

Referring to fig. 2, the current detection module DETC detects a first type arc characteristic and a second type arc characteristic of the summed total currents I1-IK of the sets of strings ST 1-STK. The current sensing module is generally configured with a processor and additional peripheral hardware for sensing current, such as current data collected by a current sensor or the like, and the processor can be used to analyze the first type and second type of arc characteristics of the total current with the current data being transferred to the processor. Equivalent devices with the same function as the processor: a logic device, a control device, a state machine, a controller, a chip, a software driver, or a plurality of microprocessors or gate arrays. The first and second types of arc characteristics of the total current IB will be explained further below. The current detection module can be integrated directly into the inverter, since the summed total current itself is to be fed to the inverter. Of course, the current detection module can also be a stand-alone module and allow it to establish a wired or wireless communication relationship with the inverter.

Referring to fig. 2, one of the characteristics of the first type of arc characteristic with respect to the aforementioned merged total current IB: and in the selected time period range, if the actual size of the peak-valley difference between the peak value and the valley value of the total current IB exceeds a preset peak-valley difference value, judging that the total current IB conforms to the first type of arc characteristics. It is assumed that the total current IB corresponds to the first type of arc characteristic when the actual magnitude of the peak-to-valley difference between the peak and valley values of the total current, e.g., 2.5 amps, exceeds a predetermined peak-to-valley difference value, e.g., 1.8 amps, over a selected time period in the order of milliseconds. For example, within a selected time interval of 8 milliseconds, the total current IB meets the first type of arc characteristic when the actual magnitude of the peak-to-valley difference between the peak and valley values of the total current, e.g., 1.9 amps, exceeds a predetermined peak-to-valley difference value, e.g., 1.7 amps. For example, within a selected 15 millisecond period, the total current IB meets the first type of arc characteristic when the actual magnitude of the peak-to-valley difference between the peak and valley values of the total current, for example 2.1 amps, exceeds a predetermined peak-to-valley difference value, for example 2.0 amps. Therefore, the range value of the selected time period range can be set according to the actual requirement, and the preset peak-valley difference value as the threshold value can also be set according to the actual requirement, and they can be burned into the microprocessor in advance or the value stored by the microprocessor can be modified in an online programming manner, and it should be noted that the peak value and the valley value are the current values obtained by the current detection module DETC through actual measurement.

Referring to fig. 2, a second characteristic of the first type of arc characteristic with respect to the total current IB after the aforementioned confluence is: if the actual magnitude of the transient variation generated by the average value of the total current exceeds the preset current variation value, the total current is judged to be in accordance with the first type arc characteristic. Assuming that the actual magnitude of the transient variation occurring in the average of the total current IB, for example, 0.7 a, exceeds a predetermined current variation value, for example, 0.4 a, the total current is determined to conform to the first type of arc characteristic. If the actual magnitude of the transient variation occurring in the average of the total current IB exceeds a predetermined current variation value, for example, 0.6 a, it can be determined that the total current is in accordance with the first type arc characteristic. The average value of the total current is an average value obtained by averaging the total current value actually measured by the current detection module DETC by a microprocessor or the like. The transient variation of the mean is a degree of change before and after the mean calculated by a microprocessor or the like when the mean encounters a transient variation event, for example, a reduction variation of the mean that becomes smaller when the mean encounters a transient variation event. The preset current change value can be set according to actual requirements, and can be burnt into the microprocessor in advance or the value stored by the microprocessor can be modified in an online programming mode.

Referring to fig. 2, the third characteristic of the first type of arc characteristic with respect to the total current IB after the aforementioned confluence: and if the transient change rate of the total current exceeds a preset change rate, judging that the total current accords with the first type arc characteristic. If the transient rate of change of the total current IB, for example 85 milliamps per millisecond, exceeds a predetermined rate of change, for example 80 milliamps per millisecond, the total current is determined to be in accordance with the first type of arc characteristic. And for example, if the transient change rate of the total current IB, for example 98 milliamps per millisecond, exceeds a preset change rate, for example 95 milliamps per millisecond, it is determined that the total current is in accordance with the first type of arc characteristic. The transient change rate of the total current is calculated by a microprocessor and the like on the basis of measuring the total current, the transient change rate of the current is set according to actual requirements, and the transient change rate can be burnt into the microprocessor in advance or modified online.

Referring to fig. 2, one of the characteristics of the second type of arc characteristic with respect to the aforementioned merged total current IB: if the current increase amount of the high-frequency component occurring in the total current exceeds a predetermined current increase value within a predetermined frequency band, the total current is judged to be in accordance with the second type arc characteristic. If the current increase amount of the high-frequency component occurring in the total current, for example, 0.5 ampere, exceeds a predetermined current increase value, for example, 0.1 ampere, within a predetermined frequency band, for example, 0.1KHZ to 100KHZ, it is judged that the total current satisfies the second type arc characteristic. If the current increase amount of the high-frequency component occurring in the total current, for example, 0.2 ampere, exceeds a prescribed current increase value, for example, 0.05 ampere, within a specified frequency band, for example, 1.0KHZ to 3KHZ, it is judged that the total current satisfies the second type arc characteristic. The band-pass filter can be used for screening out high-frequency components in the total current. The current increase amount of the high-frequency component is measured by a microprocessor or the like based on the measurement of the total current. The specified current increase value can be set according to actual requirements, and can be burnt into the microprocessor in advance or modified online.

Referring to fig. 2, a second characteristic of the second type of arc characteristic with respect to the total current IB after the aforementioned confluence is: and judging that the total current conforms to the second type arc characteristic if the ratio of the high-frequency component to the direct-current component in the total current is changed sharply and exceeds a specified ratio within a specified frequency band range. Assuming that within a specified frequency band range, for example, some frequency band ranges 0.1KHZ-100KHZ, the total current is judged to conform to the second type of arc characteristic if the ratio of the high frequency component to the dc component in the total current IB sharply changes and the ratio of the high frequency component to the dc component exceeds a prescribed ratio, for example, 2. Assuming that the total current IB is within a specified frequency band, for example, within a certain frequency band range of 1.0KHZ-3KHZ, the total current is determined to satisfy the second type of arc characteristics if the ratio of the high frequency component to the DC component in the total current IB sharply changes and the ratio of the high frequency component to the DC component exceeds a prescribed ratio of 1.2. The high-frequency component in the total current can be discriminated by using the band-pass filter, and the direct-current component is a signal component which is irrelevant to time in the total current signal. The quantity and the direct current component of the high-frequency component are measured by a microprocessor and the like on the basis of measuring the total current, the microprocessor also needs to calculate the ratio of the high-frequency component to the direct current component, and the ratio is usually increased sharply but the increasing degree needs to be carefully screened when the arc occurs. The specified ratio of the high-frequency component to the direct-current component can be set according to actual requirements, and can be burnt into a microprocessor in advance or modified online.

Referring to fig. 2, emphasis is placed on the first type of arc characteristics: the peak-to-valley difference and the preset peak-to-valley difference between the peak value and the valley value of the total current, the actual magnitude of the transient variation of the average value of the total current and the preset current variation value, the transient variation rate and the preset variation rate of the total current, and the like are merely examples and are not limited. The second type of arc characteristic is to be emphasized: the specific parameters mentioned above, such as the current increase of the high frequency component, the predetermined current increase value, the ratio of the high frequency component to the dc component, and the predetermined ratio, are only examples and are not intended to be limiting. This is because these parameters are closely related to the number of strings of the string, to the number of photovoltaic modules in each string, to the field wiring and operating environment of each photovoltaic power plant, etc., so the specific parameters listed are by way of example and not of limitation, and in practice allow adaptive online modification of some of the specific parameters mentioned above according to the current status of each photovoltaic power plant.

Referring to fig. 2, the total current is analyzed to obtain preliminary information of the arc, and the dc fault arc is preliminarily determined when the total current meets one or more first type arc characteristics and meets one or more second type arc characteristics. If the total current satisfies one or two or three of the first type of arc characteristics and the total current also satisfies one or two of the second type of arc characteristics: the total current is deemed to be suspected of having a dc fault arc and attention is paid to only the suspected arc and not the determined dc fault arc.

Referring to fig. 2, one of the first type of arc characteristics is satisfied with respect to the total current: for example, the total current may satisfy only one of the characteristics of the first type of arc characteristic, only satisfy two of the characteristics of the first type of arc characteristic, or satisfy only three of the characteristics of the first type of arc characteristic. Two terms of the first type arc characteristic are satisfied with respect to the total current: for example, the total current satisfies one of the characteristics of the first arc characteristic and also satisfies the second characteristic of the first arc characteristic, or the total current satisfies one of the characteristics of the first arc characteristic and also satisfies the third characteristic of the first arc characteristic, or the total current satisfies the second characteristic of the first arc characteristic and also satisfies the third characteristic of the first arc characteristic. And three terms satisfying the first type arc characteristic with respect to the total current are that one of the characteristics and two and three of the characteristics are all satisfied simultaneously.

Referring to fig. 2, one of the second type of arc characteristics is satisfied with respect to the total current: for example, the total current satisfies only one of the characteristics of the second type of arc characteristic or only two of the characteristics of the second type of arc characteristic. While two terms that satisfy the second type of arc characteristic with respect to the total current are satisfied in which one of the characteristics and both of the characteristics are satisfied simultaneously.

Referring to fig. 3, the total current is preliminarily determined to be a dc fault arc when it meets one or more first type arc characteristics and one or more second type arc characteristics. The total current is only suspected to be the dc fault arc, and further confirmation is needed to determine whether the dc fault arc is true. How to further confirm whether the suspected arc is a real dc fault arc requires analysis of the cascade voltages V1-VK and the cascade currents I1-IK of all the parallel strings ST 1-STK. Note that the conditions for the preliminary determination of a dc fault arc are: the total current of the three first type arc characteristics at least satisfies a first type arc characteristic and the total current of the two second type arc characteristics at least satisfies a second type arc characteristic. And the conditions for further judging as the direct current fault arc are as follows: when the cascade voltage of a plurality of strings is reduced and the cascade current is increased, the rest strings with reduced cascade voltage and reduced cascade current can be regarded as the strings with direct current arc faults.

Referring to fig. 3, the string voltages V1-VK and the string currents I1-IK of the strings ST1-STK are analyzed on the premise that the total current at least meets a first type of arc characteristic and the total current at least meets a second type of arc characteristic: for example, the string currents I2-IK of the string sets ST2-STK, respectively, of all those string sets connected in parallel are increased, while the string voltages V2-VK of the string sets ST2-STK, respectively, of all those string sets connected in parallel, for example, are decreased; the rest of the other string with reduced cascade voltage and reduced cascade current, such as the string ST1 with reduced cascade voltage V1 and reduced cascade current I1, is regarded as the string with real direct current arc fault. Wherein the failure of either condition is not considered a true dc arc fault. Because the battery output characteristics of the photovoltaic module are shown in the fact that the output voltage and the output current are closely related to external factors such as whether the module is shielded or not, the shielding degree, the illumination intensity and the ambient temperature, the output voltage and the output current of the photovoltaic module can change along with the change of the external factors due to the uncertainty of the external factors, the output characteristics of the photovoltaic module can also be changed due to the inherent current pulsation of the input side of the photovoltaic module caused by the alternating current output by the inverter, and the output characteristics of the photovoltaic module have randomness and uncontrollable fluctuation so that whether an arc event is caused by a real direct current arc or the external factors cannot be judged.

Referring to fig. 3, an arc event is considered to have occurred when the total current meets at least one first type of arc characteristic and the total current meets at least one second type of arc characteristic. Note that an arc event need not be a dc arc fault. Actions such as plugging and unplugging a switch or rotating a motor can cause an electric arc to occur in a power system, but the electric arc does not exist continuously but is transient and does not affect the normal operation of the system and equipment, so the electric arc is called a good arc, namely a normal arc. In addition to normal arcs, arcs which are caused by short circuit of lines, aging of insulation, poor contact of lines and the like, can be continuously combusted, and are easy to ignite surrounding inflammable substances are called bad arcs, namely direct-current fault arcs. The key point in discriminating whether an arc event is a normal arc or a dc fault arc is to analyze the conditions of the cascade voltage V1-VK and the cascade current I1-IK of the string ST 1-STK.

Referring to fig. 3, when an arc event is monitored, that is, when the total current is monitored to conform to one or more first-type arc characteristics and to conform to one or more second-type arc characteristics, the cascade voltage of each string after the arc event is compared with the cascade voltage before the arc event, and at the same time, the cascade current of each string after the arc event is compared with the cascade current before the arc event. Reference numeral 101 represents a rough schematic of the cascade voltage and the cascade current when no arcing event occurs in the string ST1-STK, and reference numeral 102 represents a rough schematic of the cascade voltage and the cascade current after arcing event occurs in the string ST 1-STK. When the cascade voltage and the cascade current of a plurality of strings are reduced and increased after an arc event occurs, and the other strings are reduced after the arc event occurs, so that the strings with the other reduced cascade voltage and the other reduced cascade current can be regarded as the strings with a direct current arc fault, namely, the previously preliminarily judged arc event (suspected direct current fault arc) is further confirmed as a real direct current fault arc, otherwise, the strings are not the direct current fault arc. The main technical scheme is that an arc event is judged to be generated according to the fact that the monitored total current conforms to one or more first-type arc characteristics and conforms to one or more second-type arc characteristics, and then the cascade currents and the cascade voltages of all parallel-connected strings are analyzed to further judge whether the arc event is good arc or bad arc.

Referring to FIG. 3, the cascade voltage V2-VK (reference numeral 102) of the string ST2-STK after an arc event is reduced compared to the cascade voltage V2-VK (reference numeral 101) before the arc event occurred.

Referring to fig. 3, the string currents I2-IK (reference numeral 102) of the string ST2-STK after an arc event are increased compared to the string currents I2-IK (reference numeral 101) before the arc event occurs.

Referring to fig. 3, the cascade voltage V1 (reference numeral 102) of string ST1 after the arc event is reduced compared to the cascade voltage V1 (reference numeral 101) before the arc event occurs.

Referring to fig. 3, the string current I1 (reference numeral 102) of the string ST1 after the arc event is reduced compared to the string current I1 (reference numeral 101) before the arc event occurs.

Referring to fig. 3, in the case where a plurality of string sets such as ST2-STK have a reduction in the string level voltage and an increase in the string level current, the remaining string sets having a reduced string level voltage and a reduced string level current such as ST1 may be regarded as the string set having an arc fault. Considering a simple traditional arc judgment method, the method is easily interfered by other periodic signals such as electromagnetic interference, white noise and the like of various power devices and inverters of a photovoltaic power station. Moreover, the output of the photovoltaic power station is influenced by the change of temperature and sunlight intensity, the topological structure of the photovoltaic inverter and the radiation of electrical equipment, and the like, so that the detection of the fault electric arc is easily seriously interfered. For example, the content of various harmonics in the total current increases, so that signal increments of some non-fault arcs appearing in the arc characteristic values are often misjudged as a dc arc fault, i.e., a bad arc, and therefore it is necessary to improve the reliability and accuracy of arc detection and suppress the misjudgment as much as possible, which is the object of the arc monitoring scheme described in this embodiment. The respective strings ST1-STK still perform maximum power point tracking by the inverter at this stage and cause the variations of the string-level voltages V1-VK and the string-level currents I1-IK to become apparent.

Referring to fig. 3, preventing misjudgment is one of the important issues of arc fault protection technology, such as misjudgment conditions including a series of factors including normal working arc, inrush current, non-sinusoidal waveform, various loads, cross interference, etc. If misjudgment occurs in the stage of judging the fault arc, the normal operation of other electrical equipment can be influenced, and obviously, the protection significance is lost. The low false-operation-rate fault arc judgment scheme described herein can meet the demand.

Referring to fig. 3, the mismatch of the photovoltaic modules is hidden, and many solar power generation systems may ignore or not be aware of the mismatch problem of the photovoltaic modules, resulting in energy waste. The reasons for the mismatch are manifold, the main mechanism is caused by the mismatch of the combination of voltage and current, the cloud is shielded and fluttered by local foreign objects, the shielding or surface contamination of nearby objects, different installation inclination angles and installation orientations, aging and temperature variation, and other factors, and the mismatch of the photovoltaic module directly induces the unbalanced power loss of the photovoltaic module. The photovoltaic inverter INVT has a maximum power point tracking function.

Referring to fig. 3, when an arc event is detected, even if the plurality of strings have the aforementioned conditions of reduction of the cascade voltage and increase of the cascade current, the cascade voltage and the cascade current of the remaining strings are reduced, and it cannot be directly determined that the string with the reduced cascade voltage and the reduced cascade current is the string with the dc arc fault. More stringent conditions: the cascade current of the group of strings with the reduced cascade voltage and the increased cascade current is only larger than the cascade current of the rest other groups of strings with the reduced cascade voltage and reduced cascade current: the string with reduced cascade voltage and reduced cascade current can be really regarded as the string with the direct current arc fault. For example, when an arc event is detected, the string currents I2-IK of the strings with the decreased string voltage and the increased string current, such as string ST2-STK, are larger than the string currents I1 of the rest strings with the decreased string voltage and current, such as string ST1, and the strings with the decreased string voltage and current, such as string ST1, are really regarded as the strings with the direct current arc fault, otherwise, the strings are false fault arcs. Note that at this time, when the magnitudes of the so-called cascade current I2-IK (reference numeral 102) and the cascade current I1 (reference numeral 102) are compared, these cascade currents are current values after the occurrence of an arc event, and not current values before the occurrence of an arc event.

Referring to fig. 3, when an arc event is detected, even if the plurality of strings have the aforementioned conditions of reduction of the cascade voltage and increase of the cascade current, the cascade voltage and the cascade current of the remaining strings are reduced, and it cannot be directly determined that the string with the reduced cascade voltage and the reduced cascade current is the string with the dc arc fault. More stringent conditions: those strings whose cascade voltage decreases and whose cascade current increases deviate from the maximum power point and whose cascade current increases relative to the current level corresponding to the maximum power point; and the rest other groups of strings with reduced cascade voltage and cascade current are separated from the maximum power point, and the cascade current of the rest other groups of strings is reduced relative to the current level corresponding to the maximum power point; the string with both the cascade voltage and the cascade current reduced is regarded as the string with the dc arc fault. For example, those strings in which the cascade voltage decreases and the cascade current increases after an arcing event is detected, such as ST2-STK, are out of the maximum power point, and ST2-STK have their cascade currents I2-IK (reference numeral 102) increased after the arcing event relative to the current levels corresponding to ST2-STK operating at the maximum power point; the rest of the strings with reduced cascade voltage and reduced cascade current, such as ST1, are deviated from the maximum power point, and the current levels of the strings, such as ST1, and the cascade current I1 (reference numeral 102) are basically reduced relative to the current levels corresponding to the operation of ST1 at the maximum power point; the string with the reduced cascade voltage V1 and the reduced cascade current I1, such as ST1, is considered to be a string with a real dc arc fault, otherwise a false fault arc. When comparing the current levels of the cascade currents I1-IK with the maximum power point of the string, the cascade currents I1-IK used at this time are all current values after the occurrence of the arc event and not current values before the occurrence of the arc event. The current level corresponding to the maximum power point of the string is the current level of the cascade current output by the string when the string operates at the maximum power point under the maximum power point tracking function of the inverter.

Referring to fig. 4, the string voltages V1-VK and the string currents I1-IK of the strings ST1-STK are analyzed on the premise that the total current at least meets a first type of arc characteristic and the total current at least meets a second type of arc characteristic: for example, the string currents I3-IK of the string sets ST3-STK, respectively, of all those string sets connected in parallel are increased, while the string voltages V3-VK of the string sets ST3-STK, respectively, of all those string sets connected in parallel, for example, are decreased; and at the same time the cascade current I1 of the group string ST1 among those group strings connected in parallel increases and the cascade voltage V1 decreases. The remaining strings with reduced cascade voltage and reversed cascade current, such as string ST2, are considered to be the strings with arc faults. Either condition is not met and is not considered a true arc fault. Because the output characteristics of the photovoltaic module are shown in the fact that the output voltage and the output current are closely related to whether the module is shielded or not and external factors such as shielding degree, illumination intensity and ambient temperature, uncertainty of the external factors causes the output voltage and the output current of the photovoltaic module to change along with changes of the external factors, current pulsation of an input side of the photovoltaic module caused by alternating current output by an inverter also changes the output characteristics of the photovoltaic module, and the output characteristics of the photovoltaic module have randomness and uncontrollable fluctuation so that the cause of an arc event cannot be judged.

Referring to fig. 4, an arc event is considered to have occurred when the total current meets at least one first type of arc characteristic and the total current meets at least one second type of arc characteristic. Note that an arc event need not be a dc arc fault. Actions such as plugging and unplugging a switch or rotating a motor can cause an electric arc to occur in a power system, but the electric arc does not exist continuously but is transient and does not affect the normal operation of the system and equipment, so the electric arc is called a good arc, namely a normal arc. In addition to normal arcs, arcs which are caused by short circuit of lines, aging of insulation, poor contact of lines and the like, can be continuously combusted, and are easy to ignite surrounding inflammable substances are called bad arcs, namely direct-current fault arcs. The key point in discriminating whether an arc event is a normal arc or a dc fault arc is to analyze the conditions of the cascade voltage V1-VK and the cascade current I1-IK of the string ST 1-STK.

Referring to fig. 4, when an arc event is monitored, that is, when the total current is monitored to conform to one or more first-type arc characteristics and to conform to one or more second-type arc characteristics, the cascade voltage of each string after the arc event is compared with the cascade voltage before the arc event, and at the same time, the cascade current of each string after the arc event is compared with the cascade current before the arc event. Reference numeral 201 represents a rough schematic of the cascade voltage and the cascade current when no arcing event occurs in the string ST1-STK, and reference numeral 202 represents a rough schematic of the cascade voltage and the cascade current after arcing event occurs in the string ST 1-STK. When cascade voltage reduction and cascade current increase exist in a plurality of strings after an arc event occurs, and cascade voltage reduction and cascade current reversal exist in other strings after the arc event occurs, so that the strings with other remaining cascade voltage reduction and cascade current reversal can be regarded as the strings with a direct current arc fault, namely, the previously preliminarily judged arc event (suspected direct current fault arc) is further confirmed as a real direct current fault arc, otherwise, the strings are not the direct current fault arc. The main technical scheme is that an arc event is judged to be generated according to the fact that the monitored total current conforms to one or more first-type arc characteristics and conforms to one or more second-type arc characteristics, and then the cascade currents and the cascade voltages of all parallel-connected strings are analyzed to further judge whether the arc event is good arc or bad arc.

Referring to fig. 4, the cascade voltages V1, V3-VK (reference numeral 202) of ST1, ST3-STK after an arc event are reduced compared to the cascade voltages V1, V3-VK (reference numeral 201) before the arc event occurs.

Referring to fig. 4, the cascade currents I1, I2-IK (reference numeral 202) of ST1, ST3-STK after an arc event are increased compared to the cascade currents I1, I3-IK (reference numeral 201) before the arc event occurred.

Referring to fig. 4, the cascade voltage V2 (reference numeral 202) of string ST2 after the arc event is reduced compared to the cascade voltage V2 (reference numeral 201) before the arc event occurs.

Referring to fig. 4, the cascade current I2 (reference numeral 202) of string ST2 after an arc event is reversed compared to the cascade current I2 (reference numeral 201) before the arc event occurred. For example, the cascade current I2 of string ST2 is the normal output current and the current is positive before the arc event, whereas the cascade current I2 of string ST2 is backward-flowing after the arc event, i.e., the current cannot be normally output and the current is in the reverse direction.

Referring to fig. 4, if the string voltage is decreased and the string current is increased in ST1 and ST3-STK, the rest strings with decreased string voltage and reversed string current, such as ST2, can be regarded as the strings with arc faults. Note that the inverter is still performing maximum power point tracking at this stage for each ST 1-STK.

Referring to fig. 4, when an arc event is detected, even if there are the aforementioned conditions of the cascade voltage reduction and the cascade current increase in a plurality of strings, the cascade voltage reduction and the cascade current inversion of the remaining strings cannot be directly determined that the string with the cascade voltage reduction and the cascade current inversion is a string with a dc arc fault. More stringent conditions: only strings that satisfy the reduction of the cascade voltage and the increase of the cascade current leave the maximum power point and their cascade currents are increased with respect to the current level corresponding to the maximum power point: the string with the cascade voltage reduction and the cascade current reversal can be really the string with the direct current arc fault. For example, when an arc event is detected, those strings whose cascade voltage decreases and whose cascade current increases, such as strings ST1, ST3-STK, are only moved away from the maximum power point and strings ST1, ST3-STK have their cascade currents increased relative to the current level corresponding to the maximum power point of strings ST1, ST3-STK, the strings whose cascade voltage decreases and whose cascade currents reverse, such as strings ST2, are considered as those in which a dc arc fault has occurred. At this time, when comparing the current levels corresponding to the maximum power points of the string, the used string currents are all the current values after the occurrence of the arc event but not the current values before the occurrence of the arc event. The current level corresponding to the maximum power point of the string is the current level of the cascade current of the string when the string operates at the maximum power point under the maximum power point tracking function of the inverter.

Referring to fig. 4, when an arc event is detected, even if there are the aforementioned conditions of the cascade voltage reduction and the cascade current increase in a plurality of strings, the cascade voltage reduction and the cascade current inversion of the remaining strings cannot be directly determined that the string with the cascade voltage reduction and the cascade current inversion is a string with a dc arc fault. More stringent conditions: only those strings that satisfy the reduction in the cascade voltage and the reversal of the cascade current are free from the maximum power point and their cascade voltage is reduced relative to the voltage level corresponding to the maximum power point: the string with the cascade voltage reduction and the cascade current reversal can be really the string with the direct current arc fault. For example, when an arc event is detected, those strings in which the cascade voltage decreases and the cascade current reverses, such as string ST2, are only moved away from the maximum power point and the voltage level of its cascade voltage relative to the maximum power point of string ST2 is decreased in string ST2, and strings in which the cascade voltage decreases and the cascade current reverses, such as ST2, are considered to be strings in which a dc arc fault occurs, otherwise not to be a real fault arc. At this time, when comparing the voltage levels corresponding to the maximum power points of the string, the used string voltages are all the voltage values after the occurrence of the arc event but not the voltage values before the occurrence of the arc event. The voltage level corresponding to the maximum power point of the string is the voltage level of the cascade voltage of the string when the string operates at the maximum power point under the maximum power point tracking function of the inverter.

Referring to fig. 3, in an alternative embodiment, the cascade currents and the cascade voltages of all parallel strings are analyzed when the total current meets one or more first type arc characteristics and meets one or more second type arc characteristics: when there are a plurality of string sets with decreased cascade voltage and increased cascade current, such as string set ST2-STK, the remaining string sets with decreased cascade voltage and decreased cascade current, such as string set ST1, are regarded as the string sets with DC arc fault, and it is noted that the cascade current of the remaining string sets does not flow reversely, such as the cascade current of string set ST 1.

Referring to fig. 4, in an alternative embodiment, the cascade currents and the cascade voltages of all parallel strings are analyzed when the total current meets one or more first type arc characteristics and meets one or more second type arc characteristics: when there are a plurality of string sets with a decreased string voltage and an increased string current, such as string sets ST1 and ST3-STK, the string set ST2 with the rest of the string sets with decreased string voltage and reversed string current is regarded as the string set with dc arc fault, and it is noted that the string current of the rest of the string sets with reversed current, such as the string set ST2, flows in the reverse direction.

Referring to fig. 3, in an alternative embodiment, the inverter INVT has a maximum power point tracking function, which is explained in detail in the foregoing. In the process of performing maximum power point tracking, the input current of the inverter and the input voltage must be periodically adjusted to find the maximum power point. The input current of the inverter is the total current after the cascade current of each group of strings is collected, and the input voltage of the inverter is the cascade voltage of each group of strings. In an alternative embodiment, when an arc event occurs, i.e., when the total current is monitored to meet one or more first type arc characteristics and one or more second type arc characteristics, the inverter INVT may be adjusted to decrease the first period value T1 by periodically adjusting the magnitude of the input current and the magnitude of the input voltage of the inverter by performing maximum power point tracking. In an alternative example, for example, the original first period value T1 is adjusted to the second period value T2, and the second period value T2 is lower than the first period value T1. The inverter periodically adjusts the magnitude of the input current and the magnitude of the input voltage of the inverter to perform the maximum power point tracking at the first period value T1 before the arc event occurs, and periodically adjusts the magnitude of the input current and the magnitude of the input voltage of the inverter to perform the maximum power point tracking at the second period value T2 after the arc event occurs. The reason is that the photovoltaic module is greatly influenced by the illumination intensity and environmental factors: the output current and voltage of the photovoltaic module naturally have instability and transient change characteristics, and particularly, due to the influence of a large amount of switching noise radiation and the like of the photovoltaic inverter, the content of harmonic waves in the total current is increased, so that signal increment of certain non-fault arcs appearing in the arc characteristic value is often misjudged as direct-current arc faults, namely broken arcs. If it is detected that an arc event occurs, that is, when the total current is detected to conform to one or more first-type arc characteristics and one or more second-type arc characteristics, it is asserted that the inverter performs maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter to reduce the period value: if the arcing event is a true DC arc fault, the string-level voltage and current of one of the groups of strings will decrease rapidly with decreasing period value and the string-level current will increase rapidly with decreasing period value, while the string-level voltage and current of the other group of strings will decrease rapidly with decreasing period value; on the contrary, if the arc event is not a real dc arc fault but a good arc, the situation that the value of the cascade voltage of one part of the string is rapidly decreased following the decrease of the period value and the value of the cascade current is rapidly increased following the decrease of the period value will not occur naturally, and the situation that the value of the cascade voltage and the value of the cascade current following the period of the other part of the string is rapidly decreased following the decrease of the period value will not occur naturally when the operation of decreasing the period value is performed. In addition, the period value is adjusted on the premise that the arc event is a real direct current arc fault, the total current after the cascade currents of all the groups of strings are gathered can be quickly increased to avoid the serious waveform distortion of sine waves output by the inverter caused by the excessively low input current of the inverter, and the fault arc is quickly exposed in time to avoid the fire hazard caused by the arc, and the arc intensity can be weakened at any time to cause the missed detection. Thus, when the total current is monitored to meet one or more first type arc characteristics and one or more second type arc characteristics, in an alternative embodiment, the inverter may be adjusted to perform maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter, and the cascade currents and the cascade voltages of all parallel strings are analyzed: and when the period value is adjusted to be small and the cascade voltage is synchronously reduced and the cascade current is synchronously increased in a plurality of the group strings, regarding the rest group strings with synchronously reduced cascade voltage and cascade current as the group strings with the direct current arc faults.

Referring to fig. 4, in an alternative embodiment, the inverter INVT has a maximum power point tracking function, and in the alternative embodiment, when an arc event occurs, that is, when the total current is monitored to conform to one or more first type arc characteristics and to conform to one or more second type arc characteristics, the inverter INVT may perform maximum power point tracking to periodically adjust the magnitude of the input current and the first period value T1 of the input voltage of the inverter to be decreased. In an alternative example, for example, the original first period value T1 is adjusted to the second period value T2, and the second period value T2 is lower than the first period value T1. The inverter periodically adjusts the magnitude of the input current and the magnitude of the input voltage of the inverter to perform the maximum power point tracking at the first period value T1 before the arc event occurs, and periodically adjusts the magnitude of the input current and the magnitude of the input voltage of the inverter to perform the maximum power point tracking at the second period value T2 after the arc event occurs. The reason is that the photovoltaic module is greatly influenced by the illumination intensity and environmental factors: the output current and voltage of the photovoltaic module naturally have instability and transient change characteristics, and particularly, due to the influence of a large amount of switching noise radiation and the like of the photovoltaic inverter, the content of harmonic waves in the total current is increased, so that signal increment of certain non-fault arcs appearing in the arc characteristic value is often misjudged as direct-current arc faults, namely broken arcs. If it is detected that an arc event occurs, that is, when the total current is detected to conform to one or more first-type arc characteristics and one or more second-type arc characteristics, it is asserted that the inverter performs maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter to reduce the period value: if the arcing event is a true DC arc fault, the string-level voltage of one portion of the string will decrease rapidly with decreasing period value and the string-level current will increase rapidly with decreasing period value, while the string-level voltage of the other portion of the string will decrease rapidly with decreasing period value but the string-level current will reverse rapidly with decreasing period value; on the contrary, if the arc event is not a real dc arc fault but a so-called good arc, the cycle value is reduced, the voltage of the string in one part of the string will not decrease rapidly with the reduction of the cycle value and the current of the string will not increase rapidly with the reduction of the cycle value, and the voltage of the string in the other part of the string will not decrease rapidly with the reduction of the cycle value and the current of the string will not decrease rapidly with the reduction of the cycle value. The period value is adjusted on the premise that the arc event is an arc fault, the total current after the cascade currents of all the groups of strings are gathered can be quickly increased to avoid the input current of the inverter from being too low to cause severe waveform distortion of sine waves output by the inverter, and the fault arc can be quickly exposed in time to avoid the fire caused by the arc and prevent the arc intensity from weakening at any time and missing detection. Thus, when the total current is monitored to meet one or more first type arc characteristics and one or more second type arc characteristics, in an alternative embodiment, the inverter may be adjusted to perform maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter, and the cascade currents and the cascade voltages of all the parallel strings are analyzed: and when the period value is adjusted to be small and the cascade voltage synchronous reduction and the cascade current synchronous increase exist in a plurality of the group strings, regarding the rest group strings with the cascade voltage synchronous reduction and the cascade current synchronous reverse as the group strings with the direct current arc fault. The arc detection accuracy is improved and erroneous judgment is suppressed as much as possible.

Referring to fig. 4, drawbacks of arc events are screened: it is difficult to distinguish whether the change in total current with respect to the first and second types of arc characteristics is due to environmental factors or due to an arc fault. In an alternative embodiment, when the total current is detected to meet one or more first type arc characteristics and one or more second type arc characteristics, the inverter may be adjusted to perform maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter by a first period value, for example, the original first period value T1 is adjusted to a second period value T2, and the second period value T2 is lower than the first period value T1 according to design requirements. Considering that the total current IB inherently pulsates at twice the power frequency of the ac output from the ac side of the inverter, the power frequency of the ac will vary slightly in different countries and regions. The total current ripples at twice power frequency can cause the fluctuation of the cascade current of each group of strings, however, when an arc event occurs, the photovoltaic module can generate instantaneous changes of current and voltage along with shadow shielding or light intensity fluctuation, and in addition, the total current ripples at twice power frequency, so that the change inducement of the total current is more difficult to distinguish. In an alternative embodiment, the second period T2 may be designed to be lower than the reciprocal value of 1/f of the power frequency f. Even the second period T2 can be designed in an alternative embodiment to be lower than half the reciprocal value 1/f of the power frequency f, i.e. 0.5 x 1/f. In an alternative embodiment, the first period value T1 is allowed to be much greater than the reciprocal 1/f of the power frequency f. The arc misjudgment caused by the fluctuation of the cascade current of each group of strings caused by the total current pulsation with the frequency twice as high as the power frequency is prevented. It is therefore necessary to improve the reliability and accuracy of arc detection and to suppress erroneous determination as much as possible, as is the case with the present example. When the total current is monitored to conform to one or more first type arc characteristics and one or more second type arc characteristics, the inverter can be adjusted to a first period value by performing maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter, adjusted to a second period value, and the cascade current and the cascade voltage of all the strings are analyzed: when the first period value is adjusted to be small and the cascade voltage of a plurality of the group strings is synchronously reduced and the cascade current is synchronously increased, the rest group strings with the cascade voltage synchronously reduced and the cascade current synchronously reversed are regarded as the group strings with the direct current arc fault. In alternative embodiments, the second period value is lower than the reciprocal value of the power frequency, and even lower than half the reciprocal value of the power frequency. Preferably, the second period value is lower than one fourth of the reciprocal value of the power frequency, that is, 0.25 x 1/f, and then the maximum power point tracking is performed to adjust the two adjusting actions before and after the dc-side input current and the input voltage of the inverter, so that the maximum power point tracking is performed, the maximum power point tracking is performed with a high probability of just falling within a half pulse period of the total current pulsating at twice the power frequency, and the pulse waveform of the total current within the half pulse period of the total current has no phase switching or no phase inversion, so that the determination of the arc fault is more accurate.

Referring to fig. 3, in an alternative embodiment, when the total current is monitored to meet one or more first arc characteristics and one or more second arc characteristics, the inverter may be adjusted to perform maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter to be smaller by adjusting a first period value T1, such as the previous first period value T1 to the second period value T2, and the second period value T2 is lower than the first period value T1 according to design requirements. Considering that the total current IB inherently pulsates at twice the power frequency of the ac output from the ac side of the inverter, the power frequency of the ac will vary slightly in different countries and regions. The total current ripples at twice power frequency can cause the fluctuation of the cascade current of each group of strings, however, when an arc event occurs, the photovoltaic module can generate instantaneous changes of current and voltage along with shadow shielding or light intensity fluctuation, and in addition, the total current ripples at twice power frequency, so that the change inducement of the total current is more difficult to distinguish. In an alternative embodiment, the second period T2 may be designed to be lower than the reciprocal value of 1/f of the power frequency f. Even the second period T2 can be designed in an alternative embodiment to be lower than half the reciprocal value 1/f of the power frequency f, i.e. 0.5 x 1/f. In an alternative embodiment, the first period value T1 is allowed to be much greater than the reciprocal 1/f of the power frequency f. The arc misjudgment caused by the fluctuation of the cascade current of each group of strings caused by the total current pulsation with the frequency twice as high as the power frequency is prevented. It is therefore necessary to improve the reliability and accuracy of arc detection and to suppress erroneous determination as much as possible, as is the case with the present example. When the total current is monitored to conform to one or more first type arc characteristics and one or more second type arc characteristics, the inverter can be adjusted to a first period value by performing maximum power point tracking to periodically adjust the magnitude of the input current and the magnitude of the input voltage of the inverter, adjusted to a second period value, and the cascade current and the cascade voltage of all the strings are analyzed: when the first period value is adjusted to be small and the cascade voltage of a plurality of strings is synchronously reduced and the cascade current is synchronously increased, the rest strings with the cascade voltage synchronously reduced and the cascade current synchronously reduced are regarded as the strings with the direct current arc faults. In alternative embodiments, the second period value is lower than the reciprocal value of the power frequency, and even lower than half the reciprocal value of the power frequency. Preferably, the second period value is lower than one fourth of the reciprocal value of the power frequency, that is, 0.25 x 1/f, and then the maximum power point tracking is performed to adjust the two adjusting actions before and after the dc-side input current and the input voltage of the inverter, so that the maximum power point tracking is performed, the maximum power point tracking is performed with a high probability of just falling within a half pulse period of the total current pulsating at twice the power frequency, and the pulse waveform of the total current within the half pulse period of the total current has no phase switching or no phase inversion, so that the determination of the arc fault is more accurate.

Referring to fig. 4, an arc generated in a photovoltaic energy system can be classified into a normal arc and an abnormal arc. An arc caused by an operation such as normal shutdown of the circuit breaker is a normal arc, and an arc caused by a fault such as wire aging or poor contact is an abnormal arc, which means that the arc detection is to correctly distinguish a good arc from a bad arc. Because such complex factors often cause great challenges to the detection of the fault arc, and simultaneously, higher requirements are put on a detection algorithm. The fault arc detection method comprises the steps that at the initial stage of arc generation, various parameter changes of the arc on the total current and on the cascade voltage and the cascade current are detected through various sensors, whether the arc is generated is judged through analysis, and not only can the good arc and the bad arc be accurately identified, but also the good arc and the bad arc in series connection and the good arc and the bad arc in parallel connection can be identified.

While the present invention has been described with reference to the preferred embodiments and illustrative embodiments, it is to be understood that the invention as described is not limited to the disclosed embodiments. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above description. It is therefore intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention. Any and all equivalent ranges and contents within the scope of the claims should be considered to be within the intent and scope of the present invention.

Claims

1. An arc monitoring system for a photovoltaic energy source for monitoring the presence of a dc arc fault in a plurality of strings connected in parallel, each string including a plurality of photovoltaic modules connected in series, comprising:

defining a first type of arc characteristic includes:

defining a second type of arc characteristic includes:

the output power of each group of strings is transmitted to an inverter for converting direct current into alternating current, the inverter has a maximum power point tracking function, and the inverter performs maximum power point tracking on each group of strings;

when the total current accords with one or more first-type arc characteristics and one or more second-type arc characteristics, the inverter is adjusted to be smaller by periodically adjusting the input current and the input voltage of the inverter by performing maximum power point tracking, and the cascade current and the cascade voltage of all parallel strings are analyzed:

when the period value is adjusted to be small, and when the cascade voltage synchronously decreases and the cascade current synchronously increases in a plurality of the group strings, the rest group strings with synchronously decreased cascade voltage and cascade current are regarded as the group strings with the direct current arc faults.

2. Arc monitoring system for photovoltaic energy application according to claim 1, characterized in that:

and under the condition that the cascade current of the group strings with reduced cascade voltage and increased cascade current is larger than the cascade current of the rest other group strings with reduced cascade voltage and reduced cascade current:

3. Arc monitoring system for photovoltaic energy application according to claim 1, characterized in that:

only the following are satisfied: the strings with the decreased cascade voltage and the increased cascade current are separated from the maximum power point, and the cascade currents of the strings are increased relative to the current level corresponding to the maximum power point; and

4. An arc monitoring method applied to photovoltaic energy sources, which is used for monitoring whether direct current arc faults exist in a plurality of groups of strings connected in parallel, wherein each group of strings comprises a plurality of photovoltaic modules connected in series, and the method is characterized by comprising the following steps:

the first type of arc characteristic defined comprises:

a second type of defined arc characteristic includes:

transmitting the output power of each group of strings to an inverter for converting direct current into alternating current, wherein the inverter has a maximum power point tracking function and executes maximum power point tracking on each group of strings;

firstly, on the premise that the total current accords with one or more first-type arc characteristics and one or more second-type arc characteristics, the inverter is adjusted to be small by carrying out maximum power point tracking to periodically adjust the input current and the input voltage of the inverter, and then the cascade current and the cascade voltage of all parallel strings are analyzed:

5. The method of claim 4, wherein:

the cascade current of the group strings with reduced cascade voltage and increased cascade current is larger than that of the rest other group strings with reduced cascade voltage and reduced cascade current:

6. The method of claim 4, wherein: