PRIOR ART Photovoltaic cells generate electrical power from incident light.

A number of photovoltaic cells arranged together form a photo- voltaic module. A standard photovoltaic module produces about 50-200 W and has a voltage output about 10-100 V. Photovoltaic modules are usually arranged in systems for generating electri- cal power of desired amount. If the modules in the system are connected in parallel the voltage output from the system is the same as the voltage output from the photovoltaic modules. Fur- thermore the current output is the sum of the current output from the modules. Thus a very high current level results giving high losses due to resistance and thermal generation in cables con-

necting the modules to an output terminal of the system. Alter- naively, cables with large cross-sections may be used, increas- ing the cost of the system.

In order to increase the voltage and overcome the high losses, the photovoltaic modules can be connected in series. However, such a system suffers from that the photovoltaic module in the system that generate the smallest current limits the current through the series as a whole and thus also limits the power output (compare with batteries). The systems according to the prior art, for example solar power plants or systems on rooftops or on facades, thus usually comprise a number of photovoltaic modules connected in series in a string and each system com- prises several such strings connected in parallel. By arranging several strings of photovoltaic modules connected in series, the total power loss of the system can be minimised.

One problem with such photovoltaic systems is that modules within the same string must be of the same kind and be sub- jected to the same level of incident light. Otherwise the modules will produce different powers, currents and voltages making the module producing the smallest current limiting the total power output of the string, as described above. Optimising the system in a satisfactory way is difficult, since optimisation only can be made for a string as a whole and not for every single module within the string. Furthermore, a broken module or cell will dis- rupt the power production of the whole string making the system very vulnerable.

The solar power plant according to the prior art also comprises electric converters transforming the voltage generated in the photovoltaic modules to a voltage suitable for use. A number of configurations with different converters, different lengths of the strings and different interconnections between the strings are known. One such configuration comprises a DC/AC converter connected to each photovoltaic module. The converter raises

the voltage from each module making it possible to connect the modules in parallel. The configuration however demands a com- plex control of the output alternating voltages from the convert- ers since the output voltages must be synchronised both with each other and with the voltage in any external electric network.

Furthermore such DC/AC converters are expensive and raise the cost of the system. The DC/AC converters must also be de- signed for a very large range of input voltages since the output voltage from the modules vary with the level of incident light.

Another configuration comprises several strings of photovoltaic modules, one DC/DC converter connected to each string, and a DC/AC converter connecting the DC/DC converters to an exter- nal network. This configuration suffers from the problem with strings as described above.

One problem, from which all configurations according to the prior art suffer, is that the photovoltaic cells generate power as soon as they are illuminated. This gives severe safety problems for example during installation of new photovoltaic modules, dur- ing service or in the event of an accident such as a fire. Fur- thermore in some countries legislation demands that there is a main switch for turning all electricity in a building off. Some countries legislation also demands that this switch is located outside the building. If a system of photovoltaic modules is in- stalled on the rooftop, this calls for lengthy cables from the roof- top to the outside, and then back into the building again. An- other known way to solve this problem is to append a mechani- cal device onto a switch on the roof and letting the mechanical device extend down to the ground. A person may then break the electricity by using the mechanical device.

SUMMARY OF THE INVENTION One aspect of the invention is to indicate an electric converter for a photovoltaic modular system giving increased safety and

making it possible to turn the power generation in the system off from a distance.

This object is achieved with the electric converter according to the preamble of claim 1, which is characterised by the charac- terising part of claim 1.

Thus as long as the converter receives the first safety enable signal the electric converter feeds the first network. Since the electric converter also receives electrical power from the at least one photovoltaic module the module is operational and gener- ates electrical power. If the first safety enable signal is not re- ceived the converter avoids feeding the first network. The first network will then not receive any electrical power from the con- verter and an electrician may work with the first network without any hazard. Further, the electric converter will no longer convert any electrical power from the photovoltaic module, and an elec- trical charge will build up in the photovoltaic cells in the module, blocking the power generation. By disrupting the transmission of the first safety enable signal to the converter the electricity in the first network can therefore easily be shut off, and it also in- creases the safety.

The first safety enable signal can be disrupted for example by turning a switch. In one embodiment the switch is provided with a lock so that the switch can be locked in an open position with a key. An electrician can then turn the switch off, lock the switch in an open position, and bring the key with him during the elec- trical work. Then there is no risk that some other person by mis- take turns the switch on, while the electrician is still working with the electric network. The safety enable signal can also be disrupted by making an external unit which sends the safety en- able signal to the converter to stop sending the safety enable signal. This can be done either at the location of the external unit or from a distance from the external unit by transmitting control signals to the external unit. Furthermore if the first safety

enable control signal is transmitted through a wire to the electric converter and the wire is disrupted for example due to an acci- dent such as a fire, an earthquake or the like, the signal is dis- rupted and electric converter will avoid feeding the first network so that no voltage will be present in the first network. This will increase the safety for, for example ambulance personnel or rescue personnel. A small leak current may still be present in the first network even if the electric converter avoids feeding the first network. This leak current is however not hazardous in re- spect of personal damage or for causing fires. The electric con- verters may also feed the first network with a weak current, preferably with a voltage below 50 V, in order to feed electronic control circuits associated with the system.

The signal received in the electric converter can either be a pe- riodic signal or a continuous signal. If the signal is periodic it is transmitted periodically with a predetermined interval between the peaks. The time between two peaks is preferably shorter than 60 seconds and longer than 0.3 seconds. Preferably the time between two peeks is less than thirty seconds, most pref- erably less than five seconds. The particular time interval is preferably set so that the signal is considered as not received if one or a few peaks of the periodic signal are not received. Pref- erably the particular time interval is shorter than 90 seconds, most preferably shorter than 30 seconds. By using a periodic safety enable control signal it is possible to transmit other com- munication signals to the electric converter through the same wire in the space between the peaks. If the first safety enable control signal is continuous, the time interval is preferably as long as to make small disturbances in the control signal not to turn the converter into the safety state. Preferably, the time pe- riod is less than one second if the first control signal is continu- ous but longer than 0.3 seconds.

According to one embodiment of the invention the electric con- verter is arranged to enter the operative state from the safety

state provided that the receiving circuit receives a run control signal in the receiving circuit. Preferably the electric converter is not able to begin the feeding of the first network without a spe- cific run control signal from an external unit or without some other kind of external input. This leads to an increased control and increased safety for a system comprising the electric con- verter.

According to one embodiment of the invention the electric con- verter is arranged to enter the safety state from the operative state in response to a stop control signal received in the receiv- ing circuit. Thus a converter can be turned off into the safety state even if the electric converter still receives the safety en- able control signal. This makes it possible to turn off the electric converter faster than by merely stopping the safety enable con- trol signal since there is no need to wait for the particular time interval. When switching the system off, preferably the stop con- trol signal is first transmitted and then the transmitting of the safety enable signal is disrupted. Furthermore, it is possible to turn a specific electric converter off even if the safety enable control signal still is transmitted. For example a broken con- verter or an electric converter associated with a broken photo- voltaic module, or an electric converter, from which the electrical power is not needed, can be turned off. When the electric con- verter is turned off the photovoltaic modules associated to the electric converter will not generate any power, which will de- crease the heat generation in the modules and increase their lifetimes, since a high temperature is degrading for photovoltaic cells. This leads both to increased security and increased avail- ability of the system.

According to a further embodiment of the invention the electric converter is arranged to enter the safety state in response to a fault detected by the electric converter. The electric converter is much faster in discovering faults than an external unit. Thus when the electric converter discovers a fault it automatically en-

ters the safety state and cannot go from the safety state to an operative state by itself. This ensures that no broken electric converters or photovoltaic modules are used to produce electri- cal power. This increases the safety of the system further.

According to a further embodiment of the invention the electric converter comprises a conversion control circuit arranged to control the voltage conversion ratio of the electric converter and that the conversion control circuit, in a protective operative sub- state of the operative state of the electric converter, is arranged to limit the maximum output voltage and/or output power from the electric converter, with which the electric converter feeds the first electric network. By limiting the output power and/or voltage from the electric converter it is assured that the system does not generate more power than can be consumed. By limiting the conversion ratio in the electric converter the impedance experi- enced by the photovoltaic modules will increase and the photo- voltaic modules will no longer operate in an optimal state. The current and power from the photovoltaic modules will thus be reduced. Then the produced power will match the amount of power needed. Furthermore, if the voltage in the first network becomes too high the electric converters and other electrical equipments connected to the first network can be damaged or at least be disturbed in their function. By limiting the voltage output the risk for damages due to high voltage is decreased substan- tially.

According to a further embodiment of the invention the electric converter enters the protective operative substate in response to a protective control signal received in the receiving circuit. Thus the electric converter can be controlled to limit the power and/or voltage output from the electric converter. Preferably the protec- tive control signal also comprises information about the desired maximum voltage and the electric converter is arranged to limit the voltage output to this maximum voltage. Thus the voltage in

the first network can be easily controlled according to given needs.

According to a further embodiment of the invention the electric converter comprises a maximum voltage protection circuit ar- ranged to turn the electric converter into the protective operative substate in response to the voltage in the first network surpass- ing a threshold voltage. Such a maximum voltage protection cir- cuit is very fast if a sudden increase in the voltage in the first network occurs. If, for example, the electric load suddenly dis- appears a sharp increase in voltage in the first network will oc- cur. Using an external unit for transmitting a signal to the elec- tric converter to limit the voltage will be too slow for full protec- tion of the electric converter and other equipments connected to the first network.

According to a further embodiment of the invention the electric converter is arranged to be turned into the safety state in re- sponse to the voltage in the first network surpassing the thresh- old voltage for a time period longer than a threshold time. If the voltage in the first network does not decrease when the maxi- mum voltage protection circuit controls the electrical converter to limit the maximum voltage, even after some time, it is possi- ble that there is a fault within the electric converter or within some other electric converter. In order not to damage any other electric converters or simply to protect the electric converter it- self, it is advantageous to turn the electric converter into the safety state after a time period with too high voltage.

According to a further embodiment of the invention the electric converter comprises an optimisation circuit and the electric con- verter is arranged to enter a normal operative substate of the first operative state from the protective operative substate pro- vided that the receiving circuit receives an optimise control sig- nal, in which normal operative substate the optimisation circuit is arranged to perform maximum power point tracking for the

photovoltaic module or string of modules. The normal operative substate is the state in which the electric converter usually works during normal operation of the system. Maximum power point tracking is used to maximise the generated power from the photovoltaic modules. There are several known algorithms for performing the maximum power point tracking, it is immaterial to the invention which one is used. In order to increase the safety the electric converter is arranged to turn into the normal opera- tive substate from the protective substate, and to perform the maximum power point tracking, provided that the electric con- verter receives the optimise control signal from an external unit.

Thus the electric converter cannot turn into the normal operative substate by itself. Naturally, the electric converter may also en- ter the normal protective operative substate from the second safety state directly.

Another aspect of the invention is to indicate a photovoltaic module for a power generating system giving increased safety and making it possible to turn the power generation in the sys- tem off in a simple way.

This object is achieved with the photovoltaic module according to the preamble of claim 14, which is characterised in that the photovoltaic module comprise a receiving circuit arranged to re- ceive control signals from an external unit and the photovoltaic module being arranged to output the electrical power or voltage in response to a safety enable control signal received in the re- ceiving circuit, and to avoid output of the electrical power or voltage in the absence of the safety enable control signal during at least a particular time interval. This module shows similar ad- vantages as discussed in connection with the electric converter above. Preferably the photovoltaic module comprises an electric converter according to any of the claims 1-11 integrated with the photovoltaic module.

Another aspect of the invention is to indicate a master node for a photovoltaic modular system giving increased safety and mak- ing it possible to turn the power generation in the system off in a simple way.

This object is achieved with the master node according to claim 17 which is characterised by the characterising part of claim 17.

Such a master node shows the advantages as described previ- ously in connection with the photovoltaic module and electric converter according to the invention. Preferably the electrical power feeding device is arranged to feed the first network with electrical power generated from incident light from at least one photovoltaic module. Providing a master node to control such an electrical power feeding device is advantageous since the feed- ing device otherwise would feed the first network as soon as the photovoltaic cell is illuminated. Preferably the electrical power feeding device is either the electric converter or the photovoltaic module according to the invention.

By disrupting the transmission of the first safety enable control signal from the master node to the electrical power feeding de- vice, the output voltage and/or output power from the electrical power feeding devices can easily be turned off. An electrician may then easily undertake work with the electrical system and the first network without risk for personal damage. The turning off of the transmission can be carried out either manually by for example disconnecting the master node or by disrupting a wire conducting the signal, for example by the use of a switch as de- scribed previously. The master node can in itself be controlled either directly, for example by pressing push buttons located on the master node, or indirectly, for example by sending control signals to the master node. This can be done from anywhere appropriate, for example from a location outside a building. The disruption of the signal will turn all the electrical power feeding devices connected to the master node and arranged according to claim 1 or photovoltaic modules according to claim 15 off, se-

curing that the first network will be free from electricity as long as no other sources are feeding the network. If such is the case, these sources can also be provided with the same security as the electric converter and the photovoltaic module according to the invention.

According to one embodiment of the invention the master node is arranged to transmit a run control signal to the electrical power feeding device turning the electrical power feeding device from the safety state, in which the electric converter is arranged to avoid feeding the first network, to the operative state, in which the electric converter is arranged to feed the first network.

Thus the master node controls the turning on of the electrical power feeding device. This gives the advantages as described previously in combination with the electric converter and photo- voltaic module. Preferably the master node is arranged to transmit the run control signal provided that the master node re- ceives information that the electrical power feeding device have access to electrical power from at least one photovoltaic cell, that there is an appropriate load for the electric power, and/or in the absence of any detected faults in the electrical power feed- ing device. Preferably the master node transmits said second run control signal if the master node receives information that the electrical power feeding device also can supply sufficient electrical power. The master node can also transmit said second run control signal if the master node receives information that a number of electrical power feeding devices can supply sufficient electrical power together. Preferably the master node is also ar- ranged to transmit said second run control signal if the master node receives information that a number of electrical power feeding devices are already feeding the first network and a fur- ther electrical power feeding device is needed to supply suffi- cient electrical power to the first network.

According to a further embodiment of the invention the master node is arranged to transmit a stop control signal to the receiv-

ing circuit turning the electric power feeding device from the op- erative state into the safety state. The master node can thus block electrical converters which are not functioning properly or when only some of the electrical converters are needed to sup- ply the electrical power. By blocking a photovoltaic module or an electrical converter having associated photovoltaic modules, the photovoltaic modules will not generate any electrical power. The modules will then at the same time generate less thermal en- ergy, which lowers the temperature of the modules and conse- quently increases their lifetimes. Thus the availability of the sys- tem increases since the modules will last longer. This feature also gives a quick and reliable way to turn all the electrical con- verters off, for example before electrical work. According to a further preferred embodiment of the invention the master node is arranged to transmit the stop signal in response to receiving in- formation that there is a fault in one of the electrical power feed- ing device, the photovoltaic module, the first network, or in the second network, or if ordered to transmit the stop signal.

According to a further embodiment the master node is arranged to transmit an inquiry signal to a reply end device, the reply end device being arranged at an opposite end of a communication bus relative to the master node, and being arranged to transmit a response signal to the master node in response to said inquiry signal, the communication bus connecting the master node and the reply end device, the master node being further arranged to receive said response signal and to transmit said stop signal to at least one electric power feeding device connected to the mas- ter node via the communication bus in absence of said response signal. Thus, if the master node does not receive a reply signal from the reply end device in response to a transmitted inquiry signal, the master node determines that there is a fault, such as a disconnection or short circuit, within the communication bus.

The electric feeding devices connected to the master node through the communication bus beyond the disconnection will not receive the safety enable signal and will automatically return

to the safety state. The electric feeding devices connected to the master node through the communication bus before the dis- connection will however continue to receive the safety enable signal. It is therefore an advantage to the safety of the system that the master node transmits a stop signal to these electric feeding devices. The master node may also be arranged to initi- ate an alarm or store or transmit information about the fault in- stead of or in combination with transmitting the stop signal.

According to a further embodiment the master node comprises a sensing circuit for sensing the voltage in the first network and the master node is arranged to transmit the stop control signal to the electrical power feeding device if the voltage in the first network is below a threshold voltage. If the voltage in the first network during operation suddenly falls, it is most likely due to a short circuit or some other similar fault. A system with a short circuit may produce large amounts of energy in the form of heat, which may damage the system and/or cause fires. Hence, it is advantageous for the safety of the system to turn the electrical power feeding devices off when the voltage in the first network quickly drops below the threshold voltage.

According to a further embodiment of the invention the master node is arranged to transmit a protective control signal to the receiving circuit turning the electrical power feeding device into a protective operative substate of the operative state, in which protective operative substate the electrical power feeding device is arranged to limit the output voltage and/or output power, with which the electrical power feeding device feeds the first net- work. Preferably the master node is arranged to transmit said protective control signal when the need for electrical power is smaller than the maximum power available from the electrical power feeding device. Preferably the master node comprises a sensing circuit for sensing the voltage in the first network and that the master node is arranged to transmit the protective con- trol signal to the electrical power feeding device if the voltage in

the first network is higher than a threshold voltage or if the elec- tric load in the first network is below a threshold resistance. Lim- iting the power generation in this way is slower but more exact than using a maximum voltage protection circuit. Preferably the master node transmits a parameter specifying the desired maximum voltage together with the protective control signal.

Transmitting this signal from the master node also gives the possibility to limit the electrical power output if the power needed is limited or if the voltage somewhere else far away and unnoticeable by the electric converter is too high. An optimise control signal can be transmitted to electrical converters in the second safety state turning the electrical converters into the normal operative substate.

According to a further embodiment of the invention the master node is arranged to transmit both general control signals con- trolling all electrical power feeding devices connected to and controlled by the master node at the same time, and individual control signals controlling one or a limited number of the con- nected and controlled electrical power feeding devices. Thus the master node can control the system more easily. Electrical power feeding devices having the same performance or strings of electric power feeding devices can be controlled together as a group. A single electrical power feeding device can also be con- trolled individually, for example a feeding device with a fault can be blocked without affecting the other electrical power feeding devices. General control signals can be given to quickly affect the whole system, for example a general signal to turn the whole system off. Preferably the master node directs a control signal to one or some of the electrical power feeding devices by speci- fying the identities of the selected electrical power feeding de- vice in the control signal. All electrical power feeding devices will receive all of the signals and each device is arranged to check the identity part of the signal to decide whether the signal applies to the feeding device itself. Thus there is no need to in- clude electronic devices in the communication network for di-

recting signals to different destinations within the network, which simplifies the design of the network, making the network less expensive, and simplifies any later expansion of the network.

Preferably the master node also comprises an identity library specifying the identities and the number of electrical power feeding devices controlled by the master node.

According to a further embodiment of the invention the master node is arranged to receive information from the electric con- verters connected to and controlled by the master node pertain- ing parameters for the individual electrical power feeding de- vices and any photovoltaic modules connected to the individual electrical power feeding devices. Thus the master node can eas- ily discover faults, store statistics, discover poor efficiency and in other ways check the condition of the electrical power feeding devices and any associated photovoltaic modules. Preferably the master node is also arranged to communicate with external devices relating to the system. Preferably the master node is also arranged to supply the information gathered through an in- terface suitable for reading by humans. Preferably the master node comprises a storage device arranged to store the informa- tion received from the electrical power feeding devices.

Another aspect of the invention is to indicate a system for gen- erating electrical power from incident light, which gives in- creased safety and makes it possible to easily turn the power generation in the system off.

This object is achieved with the system according to claim 30.

This system shows the advantages as discussed in connection to the electric converter, photovoltaic module and the master node above. Furthermore the system provides a method for safely turning all the electrical power feeding devices in the sys- tem, such as electrical converters and/or photovoltaic modules, off, so that no electricity or voltage remains in the first network.

Preferably at least one of the electrical power feeding devices is an electric converter arranged according to any of the claims 1- 11. It is much more efficient to provide an electric converter as- sociated with a photovoltaic module with the safety feature as described, since it is also possible to use the master node to control other aspects of the performance of the electric con- verter, than to provide a photovoltaic module with the safety fea- ture directly. Furthermore it is easy to implement the control cir- cuits needed to achieve the security features in the electric con- verter since it is easy to simply turn the conversion in the elec- tric converter off.

Preferably the system also comprises a master node arranged according to any of the claims 14-26. Such a master node has the advantages as described above.

According to a further embodiment the system comprises at least one communication bus connecting the master node with at least one power feeding device, and at least one reply end device arranged at an opposite end of the communication bus relative the master node, the master node being arranged to transmit an inquiry signal to the reply end device, and the reply end device being arranged to transmit a response signal to the master node in response to said inquiry signal, the master node being further arranged to receive said response signal and to transmit said stop signal to the at least one electric power feed- ing device connected to the master node via the communication bus in absence of said response signal. If the communication bus is forked, the system preferably comprises several reply end devices, one reply end device arranged at the end of each branch of the communication bus. Preferably the master node is arranged to transmit the stop signal only to those electric feed- ing devices that are connected to the branch with a non- answering reply end device.

Preferably said electric converters are adapted for receiving electrical power from at the most one string of photovoltaic mod- ules. Thus a string can be turned off without disturbing the other strings with photovoltaic modules. This increases the availability of the system since a broken string or photovoltaic module will not block the production of electrical power within the other modules or strings. Preferably the electric converters are direct- ly and closely connected to the photovoltaic modules or string of modules. Since the photovoltaic modules will still build up a voltage even though the electrical converters are turned off there will be a voltage in the connection between the photo- voltaic modules and the electric converters. By limiting the length of this connection between the photovoltaic modules and electrical converters the safety is increased since the length of the cables that are still supplied with voltage is limited. Prefera- bly said electric converter is integrated with said photovoltaic module into a single unit. Thus there will be no electrical output at all from the unit with the photovoltaic module.

Preferably said electric converter is directly connected to only one photovoltaic module. By associating only one photovoltaic module to said converters the failure of one photovoltaic module will not disturb the electrical power generation of any other photovoltaic module. Furthermore the photovoltaic module may be optimised in full since no other modules must be considered.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a system for generating electrical power from in- cident light according to one preferred embodiment of the invention.

Fig. 2 shows a block diagram showing different states of one of the electric converters in fig 1 and the transitions be- tween the states.

Fig. 3a shows a circuit diagram of an electric converter accord- ing to one embodiment of the invention.

Fig 3c shows a schematic circuit diagram of a master node ac- cording to one embodiment of the invention.

Fig. 4 shows a photovoltaic module according to one embodi- ment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In Fig. 1 a system for generating electrical power according to one preferred configuration of the invention is schematically shown. The system 1 comprises a set of photovoltaic modules 3 arranged to generate electrical power from incident light. The system 1 also comprises electric DC/DC converters 5 associated to and integrated with each photovoltaic module 3. The electric converters 5 are arranged to receive the electrical power gener- ated by said photovoltaic modules 3, to transform a voltage and/or a current produced by said photovoltaic modules 3, and to feed a first electric network 7 with said electrical power from the photovoltaic modules 3.

The electric converters 5 are arranged to convert the low volt- ages received from the photovoltaic modules 3 into higher volt- ages. The electric converters 5 are designed to feed the first network 7 with substantially the same higher voltage. Thus dif- ferent power generation levels in the photovoltaic modules 3 will lead to different currents drawn from the electric converters 5 to the first network 7. Since the electric converters 5 transform the voltage received from the photovoltaic module into a higher voltage, and since the electric converters 5 still output the same amount of power, the current output from the electric converters 5 is lower, which reduces the power loss in the first network 7.

The system 1 also comprises a central electric converter 9 ar- ranged to receive electrical power from the first network 7, to transform a voltage and/or a current in the first network 7 and to feed a second external electric network 11, with the electrical power from the first network 7. The second electrical network 11 can for example be the power grid or a supply electrical network in a building. In this example the central converter 9 is a DC/AC converter. Thus the central converter 9 feeds the second net- work with alternating current. The central converter 9 also com- prises control circuits for synchronising the output alternating current with any alternating current in the second network 11.

The DC/AC converter is also arranged to have a variable con- version ratio. By changing the ratio the voltage in the first net- work 7 will be changed as well. Thus the central converter 9 is also arranged to control the voltage level in the first network 7.

The system 1 also comprises a master node 13 integrated with the central DC/AC converter 9, and arranged to periodically, with a time interval of 1 second between the signals, transmit a safety enable control signal to the electric converters 5. The electric converters 5 each comprise a receiving circuit 14 ar- ranged to receive control signals from the master node 13. The electric converters 5 are further arranged to feed the first net- work 7 with the electrical power in an operative state 15, (see fig 2) in response to the received safety enable control signal. The electric converters 5 are further arranged to enter a safety state, in which the electric converters 5 are arranged to avoid feeding the first network 7, in the absence of the safety enable control signal during at least a particular time interval. In this example the particular time interval is 1.2 seconds. Thus if one signal of the periodic safety enable signal is not received, the electric converters 5 will at once enter the safety state and will not feed the first network 7 with electrical power.

Furthermore, the master node 13 is arranged to communicate with the electric converters 5. Correspondingly, each electric

converter comprises a microprocessor 16 arranged for commu- nicating with the master node 13. To this end, the system 1 also comprises a communication bus 12 connecting the master node 13 with the electric converters 5. The communication bus 12 comprises in this example electric conductors in the form of wires. The communication bus 12 is also used for transmitting the safety enable signal. Since the safety enable signal is transmitted in intervals, other communication signals may be transmitted in between the safety enable signals. The master node 13 is arranged to communicate either generally with all electric converters 5 at the same time, group wise with a group of the electric converters 5, or individually with only one electric converter 5 at a time. The communication bus may be consti- tuted of any communication bus known in the art. For example the communication bus can be wire-less communication, the internet, several separate communication buses, or even the first network 7 having the control signals superimposed on the DC current in the network 7.

The system 1 further comprises a reply end device 14 arranged at one end of the communication bus 12. The master node 13 is arranged to transmit an inquiry signal to the reply end device 14 demanding a reply signal. The reply end device 14 is corre- spondingly arranged to transmit a reply signal to the master node 13 in response to receiving the inquiry signal. The master node 13 thus determines upon reception of the reply signal that there are no interruptions or short circuits within the communica- tion bus 12. In another example of the invention a similar reply end device may be provided at one end of the first network. It is then possible for the master node to determine whether there is a fault in the first network.

In the following the different states of the electric converters 5 in fig 1 and the different transitions between the states will be de- scribed with reference to fig 2, which shows a block diagram representing the different states and substates of the electric

converters 5. First the different states shall be described in de- tail and thereafter the transitions between the states. The elec- tric converters 5 can each be controlled individually by the mas- ter node 13 and the electric converters 5 can thus enter and be in different states and substates independently of each other, even though the master node 13 coordinates the operation of the electric converters 5 in order to optimise the performance of the system 1 as a whole.

In a first operative state 15, the electric converter 5 is arranged to feed the first network 7 with electrical power as described above. The electric converter 5 is further arranged to feed the first network 7 only if the electric converter 5 receives the safety enable control signal to the receiving circuit 14.

The power generation in a photovoltaic module depends on the magnitude of the incident light, and the output current and the output voltage. For a given magnitude of incident light a photo- voltaic module has an optimum power output depending on the output voltage and the current drawn from the photovoltaic mod- ule. In a normal operative substate 17 of said operative state 15, an optimisation circuit 18, comprised in the microprocessor 16, is arranged to perform an algorithm for maximum power point tracking for the photovoltaic module 3. The electric con- verter 5 further comprises a conversion control circuit 20 ar- ranged to change conversion ratio of the electric converter 5, that is the ratio between input voltage and output voltage, which changes the electric load experienced by the photovoltaic mod- ule 3, which in turn changes the current drawn from the photo- voltaic module 3. The conversion control circuit 20 is controlled by the optimisation circuit 18 to change the conversion ratio in accordance with the maximum power point tracking algorithm.

The maximum power point tracking algorithm is performed con- stantly in the normal operative substate 17 for finding the maxi- mum power generation in the module 3. Several different algo-

rithms for maximum power point tracking are known in the prior art.

In a protective operative substate 19 of said operative state 15, the electric converter 5 is arranged to limit the maximum output voltage, with which the electric converter 5 feeds the first net- work 7. The electric converter 5 is arranged to limit the output voltage by the conversion control circuit 20 changing the con- version ratio of the converter 5. In the protective operative sub- state 19 the photovoltaic module 3 is not optimised for maximum power generation. Rather, by limiting the voltage, only a part of the power generated in the photovoltaic module 3 is fed into the first network 7 through the electric converter 5. Thus charge will build up in the photovoltaic module 3, which will inhibit the power generation in the module 3 even further.

In a safety state 21, the electric converter 5 is arranged to avoid feeding the first network 7 with electrical power, even if the electric converter 5 receives the safety enable control signal from the master node 13. Further the electric converter 5 is ar- ranged to remain in the safety state 21, unless the electric con- verter 5 receives an external input. In this example the electric converter 5 still feed the first network 7 with a very small leak current in the safety state 21. In another example the electric converter may feed the first network with a small test current during a limited period in order to test whether the first network exists and to test whether there is sufficient power generation in the photovoltaic module. If no power is drawn from a module 3 the output voltage from the module 3 may still be high even if the power generation is low due to for example low light. Thus information of the voltage is not sufficient to determine if the modules produces sufficient electrical power for feeding the first network 7.

In a normal safety substate 23 of the safety state 21, the electric converter 5 receives sufficient electrical power from the photo-

voltaic module 3 associated with the electric converter in order to feed the internal control circuits 14,16, 18, 20 of the electric converter 5.

In a sleep safety substate 25 of the safety state 21 the electric converter 5 do not receive sufficient electrical power from the photovoltaic module 3 in order to feed the internal control cir- cuits 14,16, 18,20 of the electric converter 5. The electric con- verter 5 can therefore not react to any external signals in the sleep safety substate 25.

In the following the different transitions between the states and substates of the electrical converter 5 shall be described. The transitions are marked with arrows in fig. 2.

The electric converter 5 is arranged to enter the sleep safety substate 25 when the magnitude of incident light is so small that the photovoltaic module 3 do not produce sufficient electrical power, as shown with arrow 27. The converter 5 is in this state for example during night, and can enter the sleep substate 25 from any other state.

At the coming of dawn the level of light increases and the photo- voltaic module 3 begins to generate electrical power. When the electrical power received by the electric converter 5 is sufficient to supply the control circuits in the electric converter 5, the elec- tric converter 5 automatically enters the normal safety substate 25, as shown with arrow 29.

When the electric converter first enters the normal safety state 23 from the sleep safety state 25, the electric converter 5 trans- mits a message to the master node 13 indicating that the elec- tric converter 5 is in the normal safety substate 23. The master node 13 in turn transmits a control signal to the electric con- verter 5 ordering the electric converter 5 to test whether the electrical power received from the photovoltaic module 3 is suf-

ficient to allowing feeding the first network 7. The electric con- verter 5 is arranged to perform the test and, if sufficient electri- cal power is received, the electric converter 5 communicates this to the master node 13. If sufficient power is not received, the electric converter 5 continues to perform the test periodically until the received electrical power is sufficient. The electric con- verter 5 also performs an internal functioning test for finding faults in the converter 5. The electric converter 5 also communi- cates the result of the test to the master node 13. In another ex- ample the master node is arranged to transmit a question signal to the electric converters every 15 minutes, asking whether the electric converters are in the normal safety mode.

The master node 13 is arranged to transmit a run control signal to the electric converter 5 upon reception of communications that the electric converter 5 has no faults and receives sufficient electrical power to feed the first network 7, and reception of in- formation that there is a need for electrical power in the first network 7. For example the master node may test whether there exists a second electrical network 11 or whether there is a too low voltage in the first network 7. The master node 13 is also arranged to begin transmitting the safety enable control signal periodically to the electric converters 5. The electric converter 5 is arranged to enter the operative state 15 in response to the run control signal in the receiving circuit 14, as shown with ar- row 34. In this example the electric converter 5 at once enters the normal operative state 17. In another example the electric converter 5 may enter the protective operative state 19 instead, depending on circumstances.

The electric converter 5 is arranged to turn into the operative state 15 in response to receiving the run control signal. In this example the converter 5 turns into the normal operative substate 17 of the operative state 15. The electric converter 5 is further arranged to monitor the voltage in the first network 7. In this ex- ample the electric converter 5 comprises a maximum voltage

protection circuit 30 arranged to turn the electric converter 5 into the protective operative substate 19 in response to the volt- age in the first network 7 surpassing a threshold voltage, as re- flected by the arrow 31 in fig 2. In response to the voltage in the first network 7 remaining above the threshold voltage for a time period longer than a threshold time the electric converter 5 is arranged to turn into the safety state 21, as indicated by the ar- row 32.

The electric converter 5 is also arranged to turn into the second protective operative substate 19 in response to a protective con- trol signal received in the receiving circuit 14 from the master node 13, as reflected by the arrow 33. The master node 13 is arranged to transmit the protective control signal upon reception of information either that the voltage in the first network 7 is too high, or if the electrical load as seen by the electric converters 5 is too small, for example if the external network 11 is discon- nected from the central converter 9. The master node 13 also transmits a desired value for the voltage in the first network 7 and the electric converter 5 is arranged to feed the first network 7 with a voltage no higher than the desired limit voltage set by the master node 13.

The electric converter 5 is arranged to remain in the protective operative substate 19 unless the electric converter 5 receives an external input. In this example the electric converter 5 only turns into the normal operative substate 17 from the protective sub- state 19 in response to an optimise control signal from the mas- ter node 13, as reflected by the arrow 35. The master node 13 is arranged to transmit the optimise control signal 35 for example in response to the master node 13 detecting that the voltage in the first network is decreasing or in response to the external second network 11 becoming reconnected.

The electric converter 5 is arranged to turn into the safety state 21 in response to that the safety enable signal is not received in

the receiving circuit 14 within a particular time period, as al- ready described above. The transition from the operative state 15 to the safety state 21 is represented by the arrow 37.

The electric converter 5 is also arranged to turn into the safety state 21 in response to the electric converter 5 receiving a stop control signal from the master node 13 in the receiving circuit 14. The master node 13 is arranged to transmit the stop control signal in response to the master node 13 receiving information that there is a fault detected either in the electric converter 5 or in the photovoltaic module 3 associated with the electric con- verter 5. In this way the master node 13 can block electric con- verters 5 which have faults or which are not producing sufficient electric power for being economical. The master node 13 can also block groups of electric converters 5 or all electric convert- ers 5, for example upon reception of an external order to stop the power generation.

The electric converter 5 is also arranged to turn into the safety state 21 if the electric converter 5 itself detects a fault. For ex- ample if any circuits in the electric converter 5 are broken or if the photovoltaic module 3 associated with the electric converter 5 shows abnormal behaviour. The electric converter 5 is also arranged to turn into the safety state 21 if the electrical power received from the photovoltaic modules is too low to be fed into the first network 7.

Eventually the level of light decreases as evening comes and the electric converters will be turned into the sleep safety sub- state 25.

The communication between the master node 13 and the electric converters 5 through the communication bus 12 comprises in this example also information about parameters concerning the power generated in the photovoltaic modules 3, the voltage transformed in the electric converters 5 and fed into the first

network 7. Such parameters can for example be the tempera- ture, the current, voltage and/or power generated in the photo- voltaic module 3, the conversion ratio of the electric converters 5, the current, voltage and/or power output from the electric con- verters 5 into the first network 7 and the power available for feeding the first network 7 generated by the photovoltaic mod- ules 3. The communication also comprises information about the state 15,17, 19,21, 23,25 of the electric converters 5 and the identity of the electric converters 5. It can also contain informa- tion about the state of internal circuits in the electric converters 5 and faults either in the converters 5 or in the modules 3.

Now returning to Fig. 1, the configuration of the system 1 has many advantages in combination with the invention. The electric converters 5 are each integrated with only one photovoltaic module 3. Thus it is impossible for the photovoltaic modules 3 to output voltage and electric power without the converters 5 re- ceiving the safety enable signal. Furthermore each photovoltaic module 3 is optimised individually, thus the power output from the modules 3 is increased. Since the electric DC/DC converters 5 transform the voltage from photovoltaic modules 3 from a low level to a higher level, the problem with high losses due to resis- tance in the system decreases.

The voltage in the first network 7 is, at least in part, determined by the DC/AC-central converter 9, since the voltage in the sec- ond network is relatively fixed and the conversion ratio of the DC/AC-central converter 9 in combination with the conversion ratio of the DC/DC-converters 5 changes the resistance experi- enced by the photovoltaic modules 3. This fact simplifies the op- timisation of each photovoltaic module 3 by the optimisation cir- cuits in the electric converters 5. Furthermore, since the electric converters 5 can raise the output voltage from the electric con- verters 5, irrespective of the magnitude of illumination of the photovoltaic modules 3, the central electric DC/AC-converter 9 can be designed for a narrower voltage range, decreasing the

cost and increasing the efficiency of the DC/AC-central electric converter 9.

Since the master node 13 is integrated with the DC/AC- converter 9 the master node 13 can also easily control the cen- tral converter 9, and the central converter 9 can also be turned off if the central converter 9 do not receive said safety enable signal. The master node 13 is furthermore, in this example, con- nected to the first network 7, arranged to measure the voltage in the first network 7, and control the central converter 9 in re- sponse to the voltage in the first network 7. The master node 13 is also arranged to turn the central converter 9 off if the voltage in the first network 7 is below a threshold value, since there is a risk for the electrical power to flow backwards into the first net- work 7 if the voltage in the first network 7 is too low.

Now referring to Fig. 3a, 3b, and 3c a preferred embodiment of an electric converter and a master node will be shown in greater detail. The embodiments shown shall be considered only as ex- amples since a man skilled in the art very easily can implement the circuits using other configurations.

In Fig. 3a a circuit diagram is schematically shown for an elec- tric converter 38 according to the invention. The electric con- verter 38 comprises two electric power receiving wires 40 ar- ranged to receive electrical power in the form of a voltage and a current from a photovoltaic module or a string of photovoltaic modules. The electricity received is in the form of a direct cur- rent.

The electric converter 38 further comprises a pulse width modu- lator 42 and a transformer 44. The pulse width modulator 42 is arranged to generate two quasi-square waves from the direct current received. The two square waves are phase displaced relative each other such that their pulses are opposite. Each square wave is transmitted separately to the transformer 44 in a

wire 43a, 43b. The two square waves thus correspond to an al- ternating current. The transformer 44 comprises two windings and transforms the alternating current into a new alternating cur- rent with a different voltage depending on the design of the transformer 44. This transformer action is well known in the prior art. The transformed alternating current is further transmitted to a rectifier circuit 46, turning the alternating current into a direct current. This is achieved with diodes in a configuration, which is also well known in the prior art. The rectifier circuit 46 also comprises a low pass filter in order to remove high frequency parts of the direct current. The direct current is then further transmitted as the output voltage from the electric converter and feeds a first network 48.

The electric converter 38 also comprises a microprocessor 50 arranged to control the pulse width modulator 42. Referring to fig 3b, the microprocessor 50 is arranged to control the pulse width modulator 42 to generate a variable pulse width for at least one of the square waves. By decreasing the pulse width of the square waves the conversion ratio will be decreased. For maximum conversion the pulse width of the two square waves therefore should be maximal. Thus the microprocessor 50 is ar- ranged to control the conversion ratio of the electric converter 38 by controlling the pulse width modulator 42.

The microprocessor 50 is further arranged to perform, if the electric converter 38 is in the normal operative substate, a maximum power point tracking algorithm for the photovoltaic module or modules connected to the converter 38. The micro- processor 50 is arranged to change the conversion ratio of the converter 38, which will draw different currents from the photo- voltaic module since the resistance experienced by the photo- voltaic module will change with the pulse width and thus the conversion ratio. By changing the conversion ratio the photo- voltaic module can be made to work on different points on its

working curve, and thus different amounts of electric power will be generated in the module.

The microprocessor 50 further receives information about the power generated in the photovoltaic module. The microproces- sor 50 controls the pulse width modulator 42 to change the pulse width slightly, and thus changes the power generation in the module. The microprocessor 50 is arranged to assess the change in the power generated and controls the pulse width modulator 42 to change the pulse width in a direction for in- creasing the power generated in the module. The microproces- sor 50 is thus arranged to continuously seek the maximum power generation work point of the photovoltaic module. There are several known algorithms for carrying out the maximum power point tracking. It is immaterial to the invention concept, which algorithm is chosen.

The processor 50 is further arranged to limit the voltage output from the electric converter 38, when the electric converter 38 is in the protective operative substate. The processor 50 then con- trols the pulse width modulator 42 to generate a square wave having a pulse width, which will limit the conversion ratio and the output voltage from the electric converter 38. The micro- processor 50 is further arranged to continuously or periodically receive information about the output voltage from the electric converter 38, and to control the pulse width modulator 42 ac- cordingly, in order for the output voltage to correspond to a de- sired maximum voltage limit.

The microprocessor 50 is arranged to receive information about the voltage, current and power received from the photovoltaic module and the voltage, current and power output to the first network 48. The electric converter 38 also comprises a current measuring device 52 arranged to measure the current through the input side of the transformer 44. The current measuring de- vice 52 is connected to ground to facilitate the measurements.

The current measuring device 52 transmits information about the current in the input side of the transformer 44 both to the pulse width modulator 42 and to the microprocessor 50. The micro- processor 50 is in this example arranged to calculate the output current of the electric converter 38 departing from the informa- tion from the current measuring device 52. The microprocessor 50, in this example, also calculates the output voltage departing from the information about the input voltage and the conversion ratio. In another example of the invention the electric converter instead comprises a voltage measuring device measuring the voltage at the output side of the electric converter. The micro- processor is then instead arranged to receive information about the voltage at the output side from the voltage measuring de- vice. The pulse width modulator 42 is arranged to turn the pulse generation off if the measured current is above a threshold value.

The electric converter 38 also comprises a receiving circuit 54 arranged to receive control signals from an external unit, in this example from a master node. The receiving circuit 54 comprises terminals for connecting to the external unit and writeable and readable registers. The registers are readable and writeable both by the microprocessor 50 and the external unit, in order to allow communication between the external unit and the micro- processor 50. The receiving circuit 54 receives control and communication signals from the external unit in the form of digi- tal signals, which are written into the registers. The microproc- essor 50 reads the signals written in the registers, and thus re- ceives control signals and communication signals. Correspond- ingly, the microprocessor 50 may write communication signals to the external unit in the registers.

The control and communication signals comprise in this example two parts, one identification part and one information part. The identity part comprises the identity of the processor 50, and thus also the identity of the electric converter 38. Alternatively, if the

signal is transmitted from the external unit, the identity part can also comprise an identity corresponding to a general signal, or an identity corresponding to a group signal. The microprocessor 50 is arranged only to act on signals directed to the microproc- essor 50, such as a signal directed to a group to which the mi- croprocessor belong. The microprocessor 50 always transmits a signal with a code identifying the microprocessor 50 specifically.

The information part comprises either a control signal or a com- munication signal. A communication signal comprises for exam- ple questions and information from the external unit to the elec- tric converter 38, or corresponding answers and information from the electric converter 38 to the external unit.

In this example, the communication between the microprocessor 50 and the external unit relates to questions such as: what state the electric converter is in, which is the latest control signal transmitted to the converter, what is the magnitude of the elec- tric power, current or voltage received by the electric converter from the photovoltaic module, is there enough power received from the photovoltaic module in order to feed the first network, or what is the magnitude of the electric power, voltage or current output from the electric converter to the first network.

The control signals transmitted to the electric converter can for example be control signals turning the electric converter from one state to another state. If the control signal transmitted is to turn the electric converter into the protective operative substate, the control signal can also contain information about the desired maximum voltage limit.

The electric converter 38 further comprises a power testing cir- cuit 56 arranged to test the amount of electrical power received from the photovoltaic module connected to the electric converter 38. If no electrical power and no current is drawn from the mod- ule, the voltage received from the photovoltaic module can be high even if the power generation in the module is low. Thus it is

not sufficient to control the activation of the converter 50 only departing from information on the voltage received from the module. By testing the power generation in the photovoltaic module, for example across an internal resistive element, the real power generation in the photovoltaic module can be as- sessed.

The electric converter 38 also comprises a maximum voltage protection circuit 58 arranged to turn the electric converter 38 into the protective operative substate, in response to the voltage in the first network surpassing a threshold voltage. In this exam- ple the maximum voltage protection circuit comprises zener di- odes arranged to allow a back current if the voltage over the zener diode is higher than a threshold voltage. The maximum voltage protection circuit also comprises a LED emitting light when current flows through the LED. The zener diodes and the LED are connected in series so that the LED emit light when the voltage in the first network is higher than the threshold voltage.

The maximum voltage protection circuit also comprises an opto transistor arranged to allow a current flow when illuminated by the LED. The current flow through the opto transistor is hence a measure of the voltage in the first network. The current flow through the opto transistor is directed to the pulse width modula- tor 42, which is arranged to decrease the pulse width, and thus to decrease the conversion ratio, in response to the current from the opto transistor. The fact that the zener diodes are not ideal leads to a smooth decrease of the voltage output.

The electric converter 38 also comprises a first linear voltage regulator 60 arranged to receive electrical power from the photovoltaic module, to decrease the voltage received from the photovoltaic module and feed the microprocessor 50 with the decreased voltage. The microprocessor 50 is adapted for five volts and will be damaged if fed by higher voltages. The linear voltage regulator 60 is arranged to feed the processor 50 with the appropriate voltage of five volts. The electric converter 38

also comprises a second linear voltage regulator 62 arranged to feed the pulse width modulator 42. In this example the pulse width modulator 42 is designed for a low voltage and will be damaged if fed with a high voltage. The second linear voltage regulator 62 is arranged to decrease the voltage received by the photovoltaic module in order to secure that the pulse width modulator 42 is not fed with too high voltage. In another exam- ple the pulse width modulator is adapted for higher voltages and will not risk to be damaged by the voltage from the photovoltaic module. Thus this electric converter will not comprise a second linear voltage regulator.

In Fig. 3c a schematic structure of a master node 70 is shown.

The master node 70 comprises a processor 72 arranged to transmit control signals and communicate with electrical power feeding devices connected to the master node 70 and feeding a first network 73 with electrical power generated from incident light, such as electric converters or photovoltaic modules. The electrical power feeding devices are connected to the master node 70 via a communication bus 74. The processor 72 is ar- ranged to transmit a safety enable signal periodically to the electrical power feeding devices via the communication bus 74.

The processor 72 is also arranged to control the electrical power feeding devices connected to the master node 70 in accordance with the invention as described above.

The master node 70 is further provided with a lockable switch 76 in connection to the communication bus 74. The switch 76 is ar- ranged to interrupt the communication bus 74, and thus any sig- nals sent to the electrical power feeding devices. The safety en- able signal can thus easily be disrupted by turning the switch 80 to an open position. The switch 76 is further arranged to be lockable in the open position so that a person turning the elec- trical power feeding devices and thus the electricity in the first network 73 off, can bring a key locking the switch 76 in the open

position with him during for example installation work, so that another person cannot turn the electricity on while he is still working.

The processor 76 is also arranged to communicate externally through a second communication bus 78. The external commu- nication can for example relate to statistics concerning the power output of the system or relating to faults discovered in the electrical power feeding devices connected and controlled by the master node. The processor 76 is also arranged to receive control signals from an operator through the external communi- cation bus 78. In this example the master node 70 is arranged to transmit controls signals to other parts of the system in re- sponse to the control signals received from the operator via the external communication bus 78. Thus the system can be con- trolled manually by an operator, for example via a computer with appropriate software. This software may also be designed to comprise the switch and lock feature as described above, for example by the use of a password, instead of a physical key. In one example of the invention the master node is integrated with a computer providing an interface for manual operation and con- trol.

The master node 70 further comprises an input terminal 80 con- necting the master node 70 to the first network 73. The master node 70 further comprises a DC/DC-converter 82 connected to the input terminal 72, and arranged to lower the input voltage from the first network 73 to five volts in order to feed electronic circuits in the master node 70, such as the processor 72. Thus the processor 72 is fed by the electrical power feeding devices controlled by the master node. The master node 70 also com- prises a battery 83 arranged to feed the master node 70, if for example, the power in the first network 73 is turned off.

The master node 70 also comprises two resistors 84 connected in series across the terminal input 80. The processor 72 is ar-

ranged to measure the voltage across one of the resistors 84 in order to assess the voltage in the first network 73.

Fig. 4 shows a schematic view of a photovoltaic module accord- ing to the invention. The photovoltaic module 90 comprises a number of photovoltaic cells 92 arranged to generate electrical power and/or voltage from incident light. In another example the photovoltaic module may comprise only one photovoltaic cell.

The photovoltaic module 90 also comprises a voltage output cir- cuit 94 arranged to regulate the electrical power output from the photovoltaic module, a microprocessor 96 arranged to control the voltage output circuit 94, and a receiving circuit 98 arranged to receive control signals from an external unit. The receiving circuit 98 is arranged to receive a safety enable control signal from an external unit and the microprocessor 96 is arranged to control the voltage output circuit 94 to output the electrical power or voltage in response to the safety enable control signal received in the receiving circuit 98. The microprocessor 96 is further arranged to control the voltage output circuit 94 to avoid output of the electrical power or voltage in the absence of the safety enable control signal in the receiving circuit 98 during at least a particular time interval.

The system, the electric feeding devices, the photovoltaic mod- ules and the master node as described above, may assume other states or substates in addition to those states described.

For example an initialisation state may follow a complete shut down of the system or a new installation of the system compris- ing self diagnostics, defining parameters and searching for elec- tric feeding devices and/or reply end devices. A test state may be entered at need comprising self diagnostic and in which the electric feeding devices feeds the first network with a voltage below, for example, 50 V in order to detect disconnections or short circuits in the first network. Every morning may be begun in a wake up state in which the electric feeding devices test the power produced in the photovoltaic modules.

The electric feeding devices may furthermore be arranged to transmit some signals by themselves and/or to transmit some or all signals only in response to receiving a signal from the master node. The signals can contain codes identifying the target elec- tric feeding device or the signals can be multiplexed or in other ways directed to a specific electric feeding device. The master node may comprise several separate units located in different parts of the system and a system may also comprise several master nodes. The safety enable signal may be transmitted through a communication bus on its own, while other signals are transmitted through another communication bus, etc. Further- more, the invention may be applied in all kinds of configurations and combinations of electric feeding devices, such as electrical converters, and photovoltaic modules, both configurations that are already known in the art and those that will be known in the future.

The term"comprises/comprising"when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, inte- gers, steps or components or groups thereof.

The invention is not limited to the embodiments shown but can be varied freely within the framework of the following claims.