This application is a continuation of International Application number PCT/EP2011/058026 filed on May 18, 2011, which claims priority to European Application number 10163130.7 filed on May 18, 2010, and European Application number 10163133.1 filed on May 18, 2010.
The invention relates to a method and an apparatus for diagnosis, in particular monitoring of contacts, of a photovoltaic system. In particular, the invention relates to a method for monitoring of contacts of a photovoltaic system, which has one or more photovoltaic modules, in order to identify the occurrence of events which adversely affect correct operation of the photovoltaic system.
A photovoltaic system uses photovoltaics to provide electrical energy.
During operation of photovoltaic systems, high electric currents can occur, which in some circumstances, and in conjunction with defective and/or damaged components in the photovoltaic system, can lead to considerable power losses. This relates in particular to contact resistances of contacts of junction points between modules, and to electrical line connections. Contact faults are evident, inter alia, by an increase in the contact resistance of the relevant electrical connection.
DE 10 2006 052 295 B3 describes a method and a circuit arrangement for monitoring of a photovoltaic generator, indicating a fundamental principle for generator diagnosis with signal injection and measurement between a PV generator (=photovoltaic generator) and an inverter. The method is restricted to the night-time hours without solar radiation, in which the inverter does not feed power into the grid system, and there is therefore no flow of current in the direct-current lines of the PV generator.
So far, no satisfactory method and no satisfactory apparatus are known for monitoring contacts of a photovoltaic system.
In one embodiment a generator impedance of the photovoltaic system is determined, independently of operating states of the photovoltaic system, for example, by means of a test signal with different frequencies injected into the photovoltaic system. Conclusions relating to the contacts are drawn by modelling of an alternating-current response of the photovoltaic system, based on the generator impedance determined by the test signal.
For this purpose, a method is proposed comprising injecting a test signal, which comprises a plurality of frequencies, into the photovoltaic system, and determining a generator impedance of the photovoltaic system by means of an evaluation of a response signal associated with the test signal. The method further comprises monitoring of contacts of the photovoltaic system independently of operating states of the photovoltaic system by modelling of an alternating-current response of the photovoltaic system, based on the determined generator impedance, wherein the modelling is specific to at least two different operating states of the photovoltaic system.
By considering at least two different operating states in the process of the modelling, it is possible to monitor the photovoltaic system at any time, independently of its operating states. In this case, the operating states may, inter alia, comprise: solar radiation during the daytime, low solar radiation (for example in twilight), no solar radiation during the night-time hours, low and considerable shadowing, full-load, partial-load and no-load states, switched-on and switched-off states, and the like.
The particular advantage in this case is that faults can be identified as soon as they occur, and not only in the night time, when there is no longer any solar radiation.
In one embodiment, the modelling is based on a magnitude and a phase information relating to the determined generator impedance. The phase information relating to the determined generator impedance can be determined from a real part of the generator impedance and an imaginary part of the generator impedance.
The alternating-current response of the photovoltaic system may be modelled using an equivalent circuit. The analytically designed equivalent circuit in this case specifies a circuit which describes the alternating-current response approximately or virtually identically. The equivalent circuit is representative for a functional relationship of the frequency-dependent generator impedance which can be matched to the measured values. Furthermore, it is possible to determine an alternating-current response of the photovoltaic generator by calculation using the characteristic variables of the individual components of the equivalent circuit (resistance, inductance and capacitance values). The photovoltaic system can be monitored based on the characteristic variables determined in this way (or a subset of these characteristic variables), for example with respect to the level of a contact resistance. If the equivalent circuit is chosen skillfully, it is in this case possible for at least one of the characteristic variables of the equivalent circuit to have a value which is substantially independent of operating states of the photovoltaic system. When using a characteristic variable such as this, the monitoring can be carried out reliably and independently of the operating state of the photovoltaic system.
If the supply line is very long, then this can be modelled for high frequencies by adding to the equivalent circuit a further supply-line inductance, a further supply-line resistance and, possibly, a supply-line capacitance arranged between the supply lines.
In this context, it should explicitly be mentioned that the values of the supply-line inductance, of the supply-line resistance and of the supply-line capacitance are not necessarily associated exclusively with the supply line itself, but that the generator, in particular the electrical connections within the generator, can also make a contribution to their values.
The modelling of the alternating-current response of the photovoltaic generator by means of an equivalent circuit can be further improved by the equivalent circuit comprising a combination of a plurality of partial equivalent circuits, with each partial equivalent circuit modelling a part of the photovoltaic system.
For example, a first partial equivalent circuit can model a part of the photovoltaic system which is in a first operating state, and a second partial equivalent circuit can model a second part of the photovoltaic system which is in a second operating state.
By way of example, a temperature influence can be taken into account by at least one partial equivalent circuit comprising a corresponding temperature-dependent component. By way of example, the temperature can additionally be determined by a measurement. Alternatively, the temperature can also be deduced from the alternating-current response, for instance from the characteristic variables which result from the modelling of the response.
Furthermore, when monitoring contacts of the photovoltaic system, an evaluation can be carried out based on expert knowledge, in which case a large number of already known events and their characteristics can contribute to rapid identification of fault states. For example, the expert knowledge may be in the form of a set of rules, in which case the rules can be stored, for example, in a data processing system or its program code.
An apparatus for monitoring of contacts of a photovoltaic system comprises a function generator for generating a test signal with a definable number of partial signals at different frequencies, and an injection device coupled to the function generator for injection of the test signal into the photovoltaic system. The system further comprises a device for determining a frequency-dependent generator impedance of the photovoltaic system from a response signal associated with the test signal, and at least one processing device for identification of parameters, and for monitoring of contacts of the photovoltaic system independent of operating states of the photovoltaic system by modelling of the frequency-dependent generator impedance of the photovoltaic system by performing a method as described above, and comparison with previously defined or previously identified reference values.
The at least one processing device may have an evaluation device for characterization of at least one property which, for example, can be associated with ageing of components and/or degradation of contacts of the photovoltaic system.
In one embodiment, the apparatus is integrated in an inverter in the photovoltaic system, thus resulting in a compact design with simple structure and reliable operation.
The alternating-current response of the photovoltaic system can therefore be described approximately by an equivalent circuit.
In this case, this response is calculated or modelled by determining the associated characteristic variables of the equivalent circuit. The characteristic variables are determined from a test signal injected into the photovoltaic system. In this case, the test signal comprises a plurality of frequencies, thus allowing a frequency response of the photovoltaic system and its generator impedance to be recorded. The information required for modelling, also including any necessary phase information, can be determined from the magnitude, the real part and the imaginary part of this generator impedance.
It is therefore easily possible to obtain all the parameters required for modelling. In this case, the photovoltaic system can be coupled to a grid system in the feed mode, or can be decoupled from it, can be operated on partial load or full load, with solar radiation or shadowed.
In particular, the monitoring is also possible independent of the operating state of the photovoltaic system. Constraints on the photovoltaic system, for example different cell types, operating states, line lengths and the like, can be combined in a simple manner by means of combined partial equivalent circuits to form equivalent circuits, in order to simulate the alternating-current response of the photovoltaic system. This knowledge allows the instantaneous response to be compared with known values, to diagnose the operating state of the system, and thus to identify faults immediately when they occur.
According to one advantageous variant of the method, it is also possible to produce and/or to store and to evaluate recordings of the determined impedance values or characteristic variables over relatively long time periods, in order in this way, for example, to allow degradations and wear or ageing to be identified on the basis of a long-term behaviour.
In one advantageous refinement of the invention, the apparatus including a signal generator and a control device can be integrated in the housing of the inverter, although it is likewise feasible for these components to be arranged entirely or partially outside the housing of the inverter.
The invention will be described in more detail in the following text with reference to the attached drawings, in which:
a-7d show example illustrations, in the form of diagrams, of measured values and modelled values of a generator impedance as a function of a frequency in various operating states;
a shows an example of a block diagram of an electrical system having a photovoltaic system, with a further example embodiment of an apparatus according to the invention;
b shows an example of a further equivalent circuit;
The photovoltaic module is connected to an inverter 7 via electrical lines 3, 4, 5, 6. The term PV generator, which is used in the following text, refers to all of the photovoltaic elements of the photovoltaic system 1, which convert radiation to electrical energy, as well as their supply line. In the present
In order to monitor the direct-current circuit of the photovoltaic system 1, a test signal which has a number of partial signals at a different frequency is produced by the function generator 8 and is fed into the direct-current circuit via the injection device 11. During a measurement cycle, the frequency of the partial signals is increased in steps or continuously, for example in the range from about 10 to 1000 kHz, thus producing a test signal with a number of, for example, sinusoidal oscillation excitations, whose frequency increases or decreases in steps. Starting from an oscillation excitation at a minimum frequency, the instantaneous value of a measured voltage 13, which is present at the PV generator, and a measured current 14 flowing in the direct-current circuit (in this case, the measured voltage 13 and the measured current 14 are each components of a response signal from the photo-voltaic system 1 associated with the test signal) are measured and stored for each frequency step by means of a measurement and evaluation device 15. Furthermore, the frequency of the test signal is also detected and stored for each voltage and current measurement point. The frequency range covered is, of course, matched to the properties of the photovoltaic system 1 to be monitored. The measurement and evaluation device 15 uses the stored voltage and current values for each frequency, which is likewise stored, of the test signal to calculate or model a complex-value generator impedance ZPV. The complex-value generator impedance ZPV is in this case determined using methods known from the prior art. This therefore results in a magnitude of the generator impedance ZPV associated with the respective input frequency f. In this context,
An equivalent circuit in the form of a series resonant circuit (a series circuit comprising a resistance R, a coil L and a generator capacitor C) is used to calculate the resistance R (which forms a characteristic variable for monitoring of the direct-current circuit) within the generator impedance ZPV. The values for R, L and C for the chosen equivalent circuit can now be determined from three measured values for the magnitude of the generator impedance |Z| 16, 17, 18 and the associated frequency values. The constraints required for this purpose and the calculation rules are known by those skilled in the art, and will therefore not be explained in any more detail.
The described test signal is applied continually, possibly at specific time intervals, to the photovoltaic system 1. During the process, the profile of the variable R determined using the described procedure is observed. If R increases above a specific limit value, then it is deduced that an excessively high contact resistance has occurred.
It should also be noted that the circular data points in
Furthermore, the profile, as illustrated in
An equivalent circuit of the photovoltaic system 1, which is used as the basis for evaluation, is therefore matched to a number of type-dependent factors and/or to a number of factors which are dependent on the operating mode.
Type-dependent factors of the photovoltaic system 1 in the following text mean, inter alia: a supply-line length, a module type of a photovoltaic module 2, a cell type of a photovoltaic module 2, a number of cells in a photovoltaic module 2, a type of circuitry, a number of photovoltaic modules in a string, or a number of strings in a PV generator.
Factors which are dependent on the operating mode in the following text mean, inter alia, solar radiation onto a PV generator or onto a part of a PV generator, a temperature of a PV generator or a temperature of a part of a PV generator, or an operating point of a PV generator or of a part of a PV generator.
It should be noted that, in the present context, equivalent circuits are used to model the alternating-current response (that is to say the response when stimulated with an alternating-current test signal) of a PV generator or of a part of a PV generator. One or more characteristic values are then determined from the chosen equivalent circuit, by means of suitable calculation and evaluation methods, from the detected measured values, in which case a characteristic value of an equivalent circuit means a value of a component, for example of a resistor R. The determined characteristic value or values is or are then used to identify the occurrence of an event which disadvantageously affects correct operation of a photovoltaic system 1. In consequence, the functional relationship of the frequency-dependent impedance can be modelled mathematically exactly, corresponding to the equivalent circuit, thus making it possible to determine all the characteristic variables in the equivalent circuit (resistances, inductances, capacitances). However, alternatively, an approximation formula, which is sufficiently accurate for the frequency range used in the measurement, can also be used, by means of which, if required, it is possible to determine explicitly only some of the characteristic variables in the equivalent circuit, for example only the characteristic variables which are relevant for monitoring of the PV generator, such as a resistance value. This makes it possible to considerably reduce the computational complexity for determination of the characteristic variables.
Various embodiments for adaptation of an equivalent circuit will be explained in the following text.
In the two equivalent circuits shown in
Investigations for the purposes of the present invention have shown that the alternating-current response of the first cell group can be modelled by a first partial equivalent circuit 33, and that of the second cell group can be modelled by a second partial equivalent circuit 35, with the partial equivalent circuits being connected in series, and each corresponding to one of the equivalent circuits as described in
Furthermore, the combined equivalent circuit 36 can be further simplified to an equivalent circuit as shown in
In this case, when two or more partial equivalent circuits are combined to form a combined equivalent circuit, the values of the individual components in the individual partial equivalent circuits must be adapted.
At the same time, in one application of the method according to the invention, it is possible to diagnose the state of the photovoltaic system 1 based on the decision as to whether two or more partial equivalent circuits, one combined equivalent circuit, or one equivalent circuit as shown in
At this point, it should be noted that the splitting of the cells into cell groups may be not only a result of the operating conditions, but may also be dependent on the design type. For example, if a PV module in a photovoltaic generator 30 is replaced by a new PV module which is different from the other modules, it may also be necessary to split the photovoltaic generator 30 into cell groups with associated partial equivalent circuits, in order to model the alternating-current response as accurately as possible. In this situation, it is normally impossible to combine the partial equivalent circuits themselves in identical operating conditions.
The effect of the matching of an equivalent circuit to the accuracy of determined values is illustrated in
The figures show the profile of the magnitude of the impedance |Z|, of the phase φ, the real part Re{Z} of the generator impedance ZPV and the imaginary part Im{Z} of the generator impedance ZPV over a frequency f, in each case without solar radiation (left-side of the figures—moon symbol) and with solar radiation (right-hand side of the figures—sun symbol). The figures also show the comparison of profiles which were each determined from measured values (circular measurement points) of two fundamental models, which will be described in the following text.
The illustration in
Since the resistance value RD will fall drastically during the daytime, based on previous experience, the real response during the daytime can in this case no longer be modelled by a simple RLC approach, and it is impossible to monitor the generator by means of characteristic variables of the basic equivalent circuit. In contrast, if the generator resistance RD 24 is considered within an extended model (identified by the solid lines in
In order to identify the model parameters which are used for the modelling and calculation as described above, it is first of all necessary to measure the complex-value generator impedance ZPV. DE 10 2006 052 295 B3 discloses a circuit arrangement which is suitable for this purpose. In this context, in order to identify the parameters of the equivalent circuits described above,
The majority of
In one embodiment phase information is employed in addition to the magnitude of the generator impedance ZPV in order to calculate the model parameters. However, alternatively, it is also possible to measure the real part Re{Z} and/or the imaginary part Im{Z} of the generator impedance ZPV (which likewise include the phase information), or any desired combinations. By way of example, in order to identify contact ageing, the model approach in the example of the second equivalent circuit shown in
The invention is not restricted to the described exemplary embodiments, and can be modified in many ways. In particular, it is possible to embody the features in combinations other than those mentioned.
Relevant characteristic values for an equivalent circuit can, of course, be determined not only as described in accordance with the known method, but also using further methods.
For example, the values of the magnitude of the generator impedance ZPV and φ as well as Re{Z} and Im{Z}, as well as the corresponding frequency values determined or calculated by means of the measurement and evaluation device 15, can be processed further using expert knowledge 55, by means of the processing device 56, which is designed to process expert knowledge 55, and taking account of an equivalent circuit, and can be used to determine characteristic values.
If necessary, ambiguities can be avoided and the parameter area can be restricted by skilful formulation of expert knowledge 55 into secondary conditions.
a shows a simplified electrical circuit diagram of an electrical system having a photovoltaic system with a further exemplary embodiment of an apparatus according to the invention. The photovoltaic system 101 (also referred to as DUT, Device Under Test) is monitored by means of a method according to the invention, which can be carried out by an apparatus 102 according to the invention.
The photovoltaic system 1 has a number of photovoltaic modules 103 . . . 105 (so-called strings), only three of which are shown here, and which are connected in accordance with existing requirements. The photovoltaic system 101 has line inductances LZ 106, 107 and line resistances RZ 108, 109.
A negative connecting terminal 110 of the photovoltaic system 101 is electrically connected via an electrical conductor 115 to a negative DC voltage input of an inverter 116. A positive connecting terminal 111 of the photovoltaic system 101 is correspondingly connected via electrical conductors 112, 113 and 114 to a positive DC voltage input of the inverter 116. A secondary winding 117 of a transformer T1 and a primary winding 118 of a transformer T2 are connected into the positive jump 111, 112, 113, 114). The windings are designed such that they do not significantly influence the method of operation of the photovoltaic system 101, in particular with regard to the losses which occur. The function of the transformers T1 and T2 will be explained in detail later. One of the two transformers T1, T2 or both can likewise be connected in the negative jump of the photovoltaic system 101.
The inverter 116 is connected by electrical conductors 120, 121 to an electrical grid system 119, for example to the public electricity grid system, in order to convert an electrical power, which has been produced in the form of a DC voltage by the photovoltaic system 101, in accordance with existing requirements, and to feed it into the electrical grid system 119.
An apparatus 102 is used to monitor the photovoltaic system 101 and has a signal generator 123 which can be driven by a control device 122 and feeds a test voltage uTEST(t) via a primary winding 124 into the direct-current circuit (101, 111, 112, 113, 114, 115, 110). The signal generator 123 has an internal impedance Zi 125 and a controllable source 126, which can be controlled by the control device 122 and which in this case is a voltage source.
For metrological detection of the reaction of the photovoltaic system 1 (DUT) to the test voltage uTEST(t), a voltage ui,DUT(t) 129 is output via a secondary winding 127 of the transformer T2 and via a resistor R 128 connected in parallel with it, which voltage allows metrological detection of the current iDUT(t) 129a, if the transfer function of the arrangement T2 and the resistor 128 is known. The voltage ui,DUT(t) 129 is passed to the control device 122 (dashed-dotted lines), where it is processed further. Furthermore, a voltage uu,DUT(t) 132 is output via a measurement element which is connected in parallel with the terminals 110 and 111, in this case an RC element which includes a resistor 130 and a capacitance 131, which voltage allows metrology detection of the voltage uDUT(t) 133 if the transfer function of the measurement element is known, in this case of the RC element which includes the resistor 130 and the capacitance 131. The voltage uu,DUT(t) 132 is likewise passed to the control device 122 (dashed-dotted line), where it is processed further. A radiation sensor 134 is furthermore optionally connected to the control device 122, providing the control device 122 with information as to whether it is currently daytime or night-time. Alternatively, this information can also be determined from a clock time or from the photocurrent from the photovoltaic system 101.
In one advantageous refinement of the invention, the apparatus 102 including the signal generator 123 and the control device 122 may be integrated in the housing of the inverter 116, or it is likewise feasible for these components to be arranged entirely or partially outside the housing of the inverter 116.
b illustrates a simplified equivalent circuit of a photovoltaic system 101 which was defined in the course of the development work relating to the present invention, specifically that an electrical response of the photovoltaic system 101 can be modelled by means of a circuit 135 comprising a resistance R 135a, an inductance L 315b and a capacitance C 135c. An arrangement such as this, which is annotated with the reference symbol 135, is referred to as a series resonant circuit. A series resonant circuit as described above can therefore be used as an electrical equivalent circuit of a photovoltaic system 101. The equivalent circuit then behaves—within certain limits—electrically identically to the photovoltaic system 101 being modelled by it. In particular, the electrical behaviour of a photovoltaic system 101 when it is dark can be modelled by means of a series resonant circuit 135, that is to say when the photovoltaic system 101 is not subject to any radiation from the sun.
The total impedance of the series resonant circuit 135 is the complex sum of the inductive reactance 135b, of the capacitive reactance 35c and of the resistance 135a. At resonance, that is to say when the series resonant circuit is at the resonant frequency, the capacitive and inductive reactances cancel one another out, leaving the resistance 135a. In summary, the invention proposes that the resistance 135a of the series resonant circuit 135 be determined at the resonant frequency, and that a statement relating to the state of the contacts of the photovoltaic system 101 then be made on the basis of the determined resistance 135a.
This will be explained in detail in the following text with reference to
The individual steps of the flowchart may be stored, for example in the form of a computer program, in a microcomputer device, which is not illustrated, for the control device 122 (cf.
The illustration shows the process for a measurement cycle. For the purposes of the present invention, a measurement cycle means the application of a test voltage uTEST(t) to the DUT, with the frequency of the test voltage uTEST(t) being increased in steps by a step width Δf up to a maximum frequency fMAX starting from a minimum frequency fMIN.
In a START act 150, the control device 122 starts a measurement cycle. In a further act 151, parameters are defined for the present measurement cycle, for example—depending on the type of photovoltaic system 101 to be monitored—being read from a look-up table in the control device 122. This relates in particular to the parameters fMIN, fMAX, Δf and an amplitude u of a test signal at a test voltage uTEST(t). Further parameters may be defined in this act, if required.
Reference will now be made to
Reference will now once again be made to
Reference will now be made to
If the comparison at 156 in
Reference will now be made to
An assembly 190 has an operational amplifier OP1 and associated circuitry R1 and R2. The assembly 190 represents a non-inverting amplifier for level matching of the input signal ue, and the AC voltage component of the output signal from this assembly is coupled via a capacitor C1 to a downstream assembly 191. The assembly 191, with an operational amplifier OP2 and its circuit R3, R4, R5, R6, V1 and V2, together with the assembly 192 and its circuitry R7, represents a rectifier. Averaging is then carried out by means of the low-pass filter R8 and C2 in order to smooth the signal. The level of the output signal ua is once again matched to a downstream device by means of the assembly 193 with an operational amplifier OP4 and its circuitry R9 and R10, for example, as already stated, with this level being matched to an analogue/digital converter which is not illustrated.
By way of example,
The determined resistance value for the impedance Z in the region of resonance of a photovoltaic system 101 (DUT, cf.
The embodiments described above are only by way of example and do not restrict the invention. It can be modified in many ways within the scope of the claims.
For example, the test signal may have a different oscillation form, for example a square-wave, a triangular-wave, or the like.
It is also feasible to be able to input and output the test signal by means of a single transformer.
The control device 122 may also have an evaluation device which can use the determined values over relatively long time periods to characterize further characteristics of the photovoltaic system 101, such as ageing of the components.
With regard to the above description of preferred exemplary embodiments, it should be noted that a number of preferred refinements are also described in detail in the following text, but that the invention is not restricted to these refinements but can be configured in a varied form as required within the scope of the claims. In particular, terms such as “top”, “bottom”, “front” or “rear” should not be understood as being restrictive, but relate only to the respectively described arrangement. Furthermore, when individual components are explained, these can in principle also be configured in many ways, unless stated to the contrary. Furthermore, the scope of protection also includes specialist modifications of the described arrangements and methods, as well as equivalent refinements.
Number | Date | Country | Kind |
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10163130.7 | May 2010 | EP | regional |
10163133.1 | May 2010 | EP | regional |
Number | Date | Country | |
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Parent | PCT/EP2011/058026 | May 2011 | US |
Child | 13677685 | US |