Method for detecting the failure of switch-off devices in a photovoltaic system

Abstract

In order to detect the failure of switch-off devices which are designed to be able to reduce the DC voltages within a PV generator below a certain limit value in an emergency, the DC voltage at a DC input of the inverter of the PV system is continuously varied. Jumps in the current or in the power of the PV generator are detected depending on the DC voltage, and an actual number of switched-on or switched-off switch-off devices is determined from the absolute levels and the number of jumps, and is compared with a target value.

Description

[0001] Method for failure detection of shutdown devices in a photovoltaic system

[0002] Description

[0003] The invention relates to a method for detecting failures of shutdown devices in a photovoltaic system and a corresponding photovoltaic system which is configured to carry out such a method.

[0004] In some countries, there are standards that require a photovoltaic generator to have shutdown devices that can reduce the voltage below a certain limit in the event of an emergency.

[0005] Like all electronic devices, these shutdown devices can also fail. One possible consequence of a failure is a permanent shutdown of the connected photovoltaic modules, which can lead to significant yield losses.

[0006] Therefore, there is great interest in monitoring the correct operation of the shutdown devices and the connected photovoltaic modules. For this purpose, communication between the shutdown devices and other system components is sometimes implemented, but in some cases this is omitted entirely for cost reasons.

[0007] The object of the present invention is therefore to provide a method of failure detection of shutdown devices, which requires additional hardware and

[0008] System implementation is avoided. Furthermore, it is an object of the invention to provide a PV system configured for the method with such a shutdown device.

[0009] The object of the invention is achieved by a method for detecting failures of shutdown devices of a PV system with the features of independent patent claim 1, as well as a PV system according to independent patent claim 15. The dependent claims are directed to preferred embodiments of the method.

[0010] A method according to the invention for detecting failures of shutdown devices in a PV system is designed for implementation in a PV system comprising a PV generator, a plurality of shutdown devices, and an inverter. Each shutdown device is connected to a PV module or to a string of multiple PV modules of the PV generator and is designed to reduce the DC voltages within this PV generator below a certain limit in an emergency. The electrical supply to the shutdown devices is provided via the PV modules of the PV generator connected to the shutdown devices. The inverter is designed to vary the DC voltage at its DC input. The emergency situations that can or should lead to the shutdown devices being triggered can be determined from the corresponding normative requirements. In particular, such functionality is often required in the event of a fire.

[0011] The failure detection procedure is characterized by the following steps:

[0012] - continuous variation of the DC voltage,

[0013] - Detection of jumps in the current or power of the PV generator depending on the DC voltage,

[0014] - Determination of absolute heights and number of jumps,

[0015] - Deriving the actual number of switched on or off shutdown devices from the determined number and heights of the jumps,

[0016] - Comparison of the determined actual number of shutdown devices switched on or off with a target value,

[0017] - Alarm if the comparison shows a difference between the actual number and the target value.

[0018] The method eliminates the need for separate communication between the shutdown devices and other system components, such as the inverter, while at the same time ensuring reliable failure detection of the shutdown devices.

[0019] The method takes advantage of the fact that the electrical supply to the shutdown devices is provided on the generator side and can therefore be significantly influenced by the inverter without the need for a separate power supply. The electrical supply to the shutdown device can usually be provided from the DC voltage of the generator. For example, it is also possible to supply it via the ripple of the generator's DC current. The inverter of the PV system is designed to be able to influence the DC voltage within the PV generator. The inverter is able to increase or decrease the DC voltage at its DC input for the PV strings within the scope of its own technical capabilities and the technical capabilities of the PV generator. In normal operation, this happens, for example, as part of what is known as "MPP tracking" or "IV curve tracking".Failure detection is based on an extension of the operational management, specifically the DC control of the connected inverter, with the goal of generating an effect on the shutdown devices. The shutdown devices require a minimum operating voltage to operate. If the voltage of the PV modules drops below this minimum voltage, the shutdown devices shut down and disconnect the connected PV module or module group from the PV string. Conversely, if the voltage of the PV modules reaches the minimum operating voltage, the shutdown devices reconnect the connected PV modules. Due to general tolerances of the technical components, the minimum operating voltages vary across the majority of shutdown devices.

[0020] By increasing or decreasing the voltage at the inverter's DC input, the shutdown devices can trigger a targeted connection or disconnection of the PV modules. In a first process step, the DC voltage is continuously varied from an upper voltage value Vi to a low voltage value V2, or increased from a low voltage value V2 to an upper voltage value V1. The range from the low voltage value V2 to the upper voltage value V1 can extend from a minimum voltage V2 of 0 volts to a maximum voltage V1, for example, the open-circuit voltage Voc of the PV generator.However, since the switching on or off of the shutdown devices does not usually extend over this entire maximum range, a smaller voltage range can also be selected to increase the efficiency of the process, as long as it is ensured that the range of the summed minimum voltages of the shutdown devices, including all deviations formed by the tolerances, is covered.

[0021] While the DC voltage is varying, the current or power of the PV generator is measured as a function of the DC voltage in a subsequent process step. The resources required for this are usually provided by the PV system's inverter, as these measured variables are already recorded and processed for operational control. The necessary measuring devices are therefore available. These can be external, integrated into the inverter, or even provided redundantly. A processing unit, which can also be integrated into the inverter, detects jumps in the voltage-dependent curve of the measured variable, illustrated below using the example of the generator power P(V).A typical ideal P(V) characteristic curve of a PV generator essentially increases continuously from a DC voltage of 0 volts up to a maximum value, the so-called MPP (Maximum Power Point), before then falling to zero at a voltage value that corresponds to the open circuit voltage Voc of the PV generator. Since the individual PV modules are only switched on above a minimum operating voltage, there is a jump in the generator output, or a short, steep increase, which corresponds to the switching on of a PV module or a group of modules by a shutdown device. In an ideal PV generator without technical tolerances, all shutdown devices would switch on simultaneously, or at the same switching voltage, so that only a jump equal to the output of the entire PV generator would occur. The recorded jumps are further processed in a subsequent process step.The absolute heights and the number of jumps are determined from the recorded jumps. From this, the actual number of switched on or off shutdown devices is derived. The height of a jump for a shutdown device is reproducible and characteristic and essentially depends on the connected PV module. A jump three times the height of a jump characteristic of a single shutdown device is counted as three shutdown devices, since these will then have switched on approximately simultaneously at the same DC voltage. The actual number is further processed in a subsequent process step by comparing it with a target value. This target value is predetermined and can, for example, be stored in a processing unit. Where exactly the processing takes place is irrelevant for the implementation of the process.For example, this can be done directly in an integrated inverter control system or separately in a processing unit, which can be located locally or remotely on the PV system, or in a central control unit that controls an entire PV power plant. If the comparison reveals a difference between the actual number and the target value, an alarm is triggered in a further process step. This alarm can be implemented in a suitable manner, for example, by notifying external responsible parties, or by means of a visual, textual, or acoustic notification.

[0022] In a preferred embodiment of the method, the steps are performed multiple times. In particular, the steps of the method that lead to the determination of the actual number of defeat devices are performed multiple times to achieve greater certainty in the determination.

[0023] Preferably, the steps are repeated in such a way that the DC voltage is alternately reduced and increased. This variant takes into account that different dynamics can occur when switching on and off the shutdown devices.

[0024] In a further preferred embodiment of the method, the method is carried out in two stages. In the step of continuously varying the DC voltage, the voltage range in which jumps occur is first determined by rapidly varying the DC voltage. Subsequently, the DC voltage is varied more slowly within this determined voltage range. The speed of the variation is to be selected depending on the typical voltage tolerances and edge steepness of the shutdown devices. The terms "speed," or "rapid variation" and "slow variation," can also mean, in the context of the voltage change, that the sampling rate is narrower.

[0025] In a further preferred embodiment of the method, the shutdown devices are configured to monitor the voltage of the respective connected PV modules and to switch them on or off at a predefined minimum voltage value. In this variant of the method, the dependence of the minimum voltage values ​​on the random technical tolerances of the individual components of the respective shutdown devices is reduced.

[0026] In a further preferred embodiment of the method, the shutdown devices are configured to switch with a predefined delay time. The delay time can be randomly selected by the shutdown device or specified for each shutdown device of the photovoltaic generator. This specification can be defined, for example, during commissioning of the photovoltaic generator. These measures increase the probability that not all shutdown devices switch simultaneously, but ideally all shutdown devices can be recognized as separate jumps in the course of the observed characteristic measured variables, in particular the power of the PV generator. This applies to both the shutdown threshold and the switch-on threshold. The shutdown threshold and the switch-on threshold of a shutdown device can be different from one another.In particular, the switch-on threshold can be at a higher voltage than the switch-off threshold.

[0027] In a preferred embodiment of the method, the shutdown devices are configured to detect the activation of other shutdown devices of the PV generator and to activate them according to a predefined sequence. This can be determined, for example, from the voltage / current measurement. From this assignment, the faulty shutdown device can be identified.

[0028] It is particularly preferred that the method be triggered cyclically. This can be achieved, for example, by fixed trigger times.

[0029] In an alternative or additional embodiment of the method, the triggering of the method depends on suitable technical conditions. Advantageous suitable conditions include, for example, a sufficiently high PV generator output, high voltage, or high current, which can occur at certain times of the day when the generator is free of shadows.

[0030] Accordingly, in another preferred embodiment, the method can only be triggered under suitable external conditions. In particular, under external conditions where the minimum voltages of the shutdown devices vary widely. It is particularly preferred if the triggering is temperature-dependent and only occurs within an optimal temperature range.

[0031] In another preferred variant of the method, the method is triggered by an external alarm indicating a faulty low power level. A photovoltaic system in operation with a photovoltaic generator configured to execute the connection / disconnection procedure generally has additional safety monitoring devices that arise from different requirements. For example, if another monitoring device in the photovoltaic system detects and issues an alarm about an unusually low generator power level, this external alarm can be used as a trigger signal to start the described procedure.

[0032] In one variant of the method, the inverter alternatively or additionally monitors the temporal progression of characteristic measured variables of the PV generator, in particular the generator current and the generator voltage. This variant of the method can be triggered in particular by the inverter sending a wake-up signal or operating signal to the shutdown devices. Depending on when and in which order the individual shutdown devices are activated and connected, a stepped connection pattern can be generated in the temporal progression of the monitored measured variables, from whose step height and number of steps conclusions can be drawn about the status of the shutdown devices. In particular, the method can be triggered as scheduled in the morning with the first wake-up signal generated by the inverter.This signal is typically generated when the inverter registers a generator voltage sufficient to operate the photovoltaic generator. This signal can be sent via a separate communication line or wirelessly from the inverter or its control unit to a shutdown device. This signal is particularly preferably sent from the inverter to the shutdown device via the DC lines using so-called PLC (Power Line Communication). The various trigger conditions for the procedure can also be used in combination, allowing the procedure to be initiated from a range of different trigger conditions.

[0033] A further aspect of the invention is directed to a photovoltaic system configured to carry out the method according to the invention. This comprises at least one PV generator, a plurality of shutdown devices, and an inverter, wherein the individual components are configured to carry out the method steps in their respective effective range. The invention is illustrated below with the aid of figures, of which

[0034] Fig. 1 shows a schematic diagram of an I(V) characteristic and a P(V) characteristic of a PV generator in general; and

[0035] Fig. 2 shows a schematic diagram of a P(V) characteristic of a PV generator for implementing the method.

[0036] Fig. 1 shows a schematic diagram of an I(V) characteristic and a P(V) characteristic of a photovoltaic generator in general. The DC voltage V of the PV generator is plotted on the x-axis. This can be significantly influenced by the inverter of a corresponding PV system. The inverter is designed to increase and decrease the DC voltage at its input for individual connected PV strings of the generator. This type of DC control is used, among other things, for MPP tracking (tracking the operating point with the maximum output power of the generator) and IV curve tracking. Starting with a short-circuit current value Isc, the curve of the generator current I(V) as a function of the generator voltage V is almost constant for low generator voltages. Accordingly, the output power P(V) increases almost linearly with increasing voltage V.With further increased voltage values, the generator current drops rapidly to zero at an open-circuit voltage value Voc. Accordingly, the generator power P(V) rises to a maximum value (MPP - Maximum Power Point) and then also drops to zero with a further increase in voltage up to the open-circuit voltage Voc. With MPP control, the PV system is operated in such a way that the current IMPP and voltage VMPP are selected to deliver the maximum power PMPP possible.

[0037] The P(V) characteristic curve of a PV generator for implementing the method, as shown in Fig. 2, differs from an ideal generator power curve in that the DC control creates an effect on the generator power curve that is related to the individual shutdown devices. By increasing or decreasing the voltage at the DC input of the inverter, a targeted connection or disconnection of the PV modules can be triggered by the shutdown devices. Fig. 2 shows a voltage reduction. In the first process step, the DC voltage is continuously reduced from an upper voltage value Vi to a low voltage value V2. The range from the low voltage value V2 to the upper voltage value V1 covers the range of the summed minimum voltages of the shutdown devices, including all deviations formed by the tolerances.

[0038] While the DC voltage is being reduced, the power of the PV generator is measured as a function of the DC voltage. The resources required for this are usually provided by the PV system's inverter, as these values ​​are already recorded and processed for operational control. The necessary measuring devices are therefore available. In the area of ​​the summed minimum voltages, the individual shutdown devices switch off during continuous voltage reduction, or the shutdown devices disconnect the individual PV modules from the power path. This creates jumps in the process, which are recorded.

[0039] In a further step, the absolute heights and the number of jumps are determined from the recorded jumps. From this, the actual number of shutdown devices switched off is derived. The height of a jump in a shutdown device is reproducible and characteristic and essentially depends on the connected PV module. As an example, five recorded jumps are shown here, with four jumps having approximately the same height and one jump having three times the height. This jump of triple height is then counted as three shutdown devices, since they then switched at approximately the same time, at the same DC voltage. The actual number is further processed in a next process step by comparing it with a target value. This target value is predetermined and can, for example, be stored in a processing unit.Deviations between the actual number and the target value can then be used to quantify faulty shutdown devices and identify them using the described refinements of the procedure. A corresponding error message can then be generated via a suitable alarm.

Claims

Patent claims:

1. A method for detecting failures of shutdown devices of a PV system, wherein the PV system comprises a PV generator, a plurality of shutdown devices and an inverter, wherein each shutdown device is connected to a PV module or to a string of several PV modules of the PV generator and is designed to be able to reduce the DC voltages within this PV generator below a certain limit in an emergency, wherein the electrical supply of the shutdown devices is provided via the PV modules of the PV generator connected to the shutdown devices, wherein the inverter is designed to vary the DC voltage at its DC input, characterized by the steps: - continuous variation of the DC voltage, - Detection of jumps in the current or power of the PV generator depending on the DC voltage, - Determination of absolute heights and number of jumps, - Deriving an actual number of switched on or off shutdown devices from the determined number and heights of the jumps, - Comparison of the actual number of shut-off devices switched on or off with a target value, - Alarm if the comparison shows a difference between the actual number and the target value.

2. The method according to claim 1, wherein the method is carried out several times.

3. The method of claim 2, wherein the method is repeated such that the DC voltage is alternately reduced and increased.

4. Method according to one of claims 1 to 3, wherein the method is carried out in two stages, wherein firstly the voltage range in which jumps occur is determined by rapidly varying the DC voltage and then the DC voltage is varied more slowly within this determined voltage range.

5. Method according to one of the preceding claims, wherein the shutdown devices are configured to monitor the voltage of the respective connected PV modules and to switch them on or off at a predefined minimum voltage value.

6. Method according to one of the preceding claims, wherein the shutdown devices are designed to switch on with a predefined delay time.

7. Method according to one of the preceding claims, wherein the shutdown devices are configured to detect the activation of the other shutdown devices of the PV system [description: from the voltage / current measurement] and to activate them according to a predefined sequence.

8. Method according to one of the preceding claims, wherein the method is triggered cyclically.

9. Method according to one of the preceding claims, wherein the method is only triggered under suitable technical conditions.

10. Method according to one of the preceding claims, wherein the method is only triggered under suitable external conditions.

11. The method according to claim 10, wherein the triggering of the method is temperature dependent.

12. Method according to one of the preceding claims, wherein the method is triggered by an external alarm indicating a faulty low generator power.

13. Method according to one of the preceding claims, wherein the method is triggered by a wake-up signal sent from the inverter to the shutdown devices.

14. Method according to one of the preceding claims, wherein the inverter monitors the temporal course of the characteristic measured variable of the PV generator.

15. A photovoltaic system configured to carry out the method according to any one of the preceding claims.

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