Patent Application
20110227584
This nonprovisional application claims priority under 35 U.S.C. §119(a) to German Patent Application No. DE 10 2010 011 476.6, which was filed in Germany on Mar. 16, 2010, and which is herein incorporated by reference.
1. Field of the Invention
The invention relates to a method for testing the insulation of a photovoltaic system from ground with the aid of a test pulse transmitted to the connecting lines of the photovoltaic system.
2. Description of the Background Art
A method for protecting a PV system is known from WO95/25374. This method reacts once damage has already occurred, in that an attempt is made to limit the effects of the damage on the photovoltaic system by the means that the electromagnetic radiation accompanying the short-circuit arc is detected and the affected system components are isolated from the short-circuit.
Known from the document DE 10 2004 018918 is an insulation fault localization method in the field of alternating current, in which each subnetwork that can be connected is provided with its own test generator, its own insulation monitoring device, and its own differential current transformer.
U.S. Pat. No. 5,155,441 describes an AC system in which a single insulation tester is used sequentially to monitor multiple motors, which must be deenergized and stationary then.
Lastly, it is known from DE 69213626 to supply multiple AC subnetworks through associated circuit breakers. Coupling switches serve to establish a predefinable network configuration. Each network section then has a separate overall insulation monitor associated with it, and each branch of each network section has a local insulation monitor.
The method mentioned at the outset is customary in photovoltaic systems for early detection of a ground fault or an impending insulation weakness. To this end, an insulation monitor is attached to the connecting lines; said insulation monitor generates the test pulse and transmits it to the connecting lines. In this design, the test pulse is transmitted at the input of the inverter, which converts the photovoltaically generated direct current into alternating current for feeding into a supply grid. Today, inverters of up to a MW are available as a result of advances in semiconductor technology for power transistors. In the associated large-scale systems, the use of the classical insulation monitor is not successful, since the size of the wiring system that is present results in excessively high capacitances that damp the test pulse such that no reliable statement can be made about the state of the insulation. To date, modifications to the insulation monitors have not provided a satisfactory solution.
It is therefore an object of the present invention to provide a remedy here, and also to be able to check photovoltaic systems of any desired size using a standard test device.
This object is attained in accordance with a first embodiment of the invention in that the photovoltaic system is subdivided through circuit designs into multiple subsystems that are electrically insulated from one another, and the test pulse is transmitted to the connecting line associated with the applicable subsystem in sequential order.
Thus, this method does not take the obvious route of further developing the tester, but instead pursues the course of changing the photovoltaic system, or rather making it more easily subdivided, in such a way that standard testers can be used. This can involve higher device costs, but at an acceptable level.
The breakdown into subsystems by circuit design means should be accomplished in such a manner that each subsystem comprises multiple photovoltaic arrays, namely a sufficient number that their line lengths can be managed by the pulse tester used. The lines here can be connected to a bus bar, which itself is routed to the input of an inverter.
The connection to the subsystems, if applicable to the individual PV arrays, should be connected through a two-pole switching means to the output of an insulation monitor that generates the test pulse. For this purpose, a multiplexer may be located in the insulation monitor, which sequentially transmits a test signal to the lines to the relevant subsystems connected to the output of the multiplexer. Alternatively, the two-pole switching means can comprise a plurality of electronic switches that connect the relevant pair of connecting lines leading to the subsystems to a test pulse bus bar or isolates them therefrom, with the test pulse being transmitted on said test pulse bus bar and distributed from there via the switching means to the individual subsystems.
According to a second embodiment of the invention, the object is attained in that the behavior of the current of the test pulse through the connecting lines is sensed by current sensors at suitable locations. Here, as well, modifications are made to the system, requiring a one-time increased use of material and installation effort; however, this is compensated for by the advantages of the use of standard equipment for insulation monitoring.
It is advantageous to generate a first series of measurement pulses at a point in time close to the installation of the system and to document their behavior, branching, and/or distribution in the network of the connecting lines to the one or more PV arrays. In this way, a reference is generated, e.g., immediately after installation of the photovoltaic system, as to what the insulation should look like in the ideal case without the occurrence of degradation from contamination, aging, increases in contact resistance, etc. After a selectable period of time has elapsed, the behavior of the test pulse is compared with the corresponding behavior at the earlier point in time. Conclusions concerning insulation deficiencies that have arisen in the meantime can then be drawn from the changes.
The current sensors can be provided at the feed lines leading to individual arrays of the photovoltaic system proceeding from a bus bar. This is especially advantageous when additional switching means are provided that connect the relevant connecting lines leading to the individual arrays to the bus bar or isolate said connecting lines therefrom. The bus bar is connected to the input of the inverter here.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:
Shown in
In currently available large-scale PV systems, for example, n=20 of these subsystems 3 are connected directly to two bus bars 7,7′ through feed lines 6,6′, which are connected to the respective plus and minus inputs 9 of an inverter 11. Provided in the power lines 6,6′ are current transformers 10,10′, of which it is preferable for one 10′ to be provided in the line 6′ leading to the positive pole 9′ of the PV system and one 10 to be provided in the line 6 leading to the negative pole 9.
From the bus bars 7,7′ connected to an inverter 11, the feed lines 6,6′ lead to the subsystems 3 through a 2-pole disconnect switch 13. Because of the high current to be switched, the disconnect switch 13 is a mechanical switch 13, which draws a considerable arc during the actual switching process, resulting in wear of the switch contacts. Switching activities should be managed in a correspondingly sparing manner.
This is permitted by the instant first embodiment in that the insulation monitor 5 transmits its test pulse 15 directly to the bus bars 7,7′ without needing to have actuated the disconnect switches 13. For example, this can be done at night, when no solar-generated voltage is present. With suitably high-resistance insulation of the insulation monitor 5, the test pulse 15 can also be modulated onto the bus bars 7,7′ in the daytime during ongoing operation of the photovoltaic system 1.
If the feed lines 6,6′ to all subsystems 3, as well as the subsystems 3 themselves, are in a properly insulated state, then the test pulse 15 transmitted on the positive bus bar 7′ would be distributed more or less uniformly over the subsystems 3 in accordance with the particular line lengths present, and the ammeters 10′ would indicate approximately the same value. The ammeters 10 measuring the return current likewise indicate the same current value except for the damping losses that are to be expected.
In
Immediately following the installation of the PV system 1, a series of test pulses 15 can be transmitted to the feed lines 6,6′ for the first time. Assuming that all insulation weaknesses identified during the setup phase have been remedied, a reference distribution of the currents, which reflects how the test pulse 15 propagates within the system 1, is thus provided. The measured currents from all current transformers 10,10′ that are present are documented so that they are available at a later comparison measurement. The analysis unit 14 then determines how the current distribution has changed, and issues a warning signal in the event of an unacceptably high change of, e.g., plus/minus 10% deviation from the original measured value.
In the second embodiment of the invention shown in
If the first subsystem PV1 is to be tested for insulation weaknesses, then all other switches S2 to Sn of the subsystems PV2 to PVn are opened, and only the switch S1, which connects the feed lines 6,6′ of the first subsystem PV1 to the test pulse bus bars 17,17′, is closed. In this way it is made possible, even for the large-scale system 1, to test the insulation with a conventional insulation monitor 5 in the accustomed manner.
In this way, all subsystems PVn are gradually connected to the insulation monitor 5, by the means that only the relevant switch S that is associated with the subsystem PV to be tested is closed, while all other switches S remain open. This subdivision of the overall system 1 into subsystems 3, each of which is connected to the insulation monitor 5 via the switches S1 to Sn, is to be understood as division as defined in the claims.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE102010011476.6 | Mar 2010 | DE | national |
