The electromagnetic compatibility shall ensure an un- disturbed operation of other electronic equipment in the environment of PV plants. Regarding the DC side, most important is the interference via radiated disturbances, since the RF disturbance current on DC lines generates an electromagnetic field. Due to the long DC cabling radiation occurs even in the frequency range below 30 MHz. This means that limits of radiated electromagnetic fields must be respected in the frequency range 150 kHz – 30 MHz. Since the measurement of radiated disturbances at a PV generator is very expensive, measurements in the laboratory are preferable. Therefore the limits of the radiated field are transferred to limits of the disturbance voltage on DC lines. In order to evaluate the electromagnetic disturbances, comprehensive investigations of numerous different PV plants have been performed within two European projects
The limitation of RF disturbance signals on DC lines is important because numerous sensitive radio services are located in the frequency range where disturbances of PV systems may occur (AM broadcast, emergency calls, navi- gation, aircraft wireless service, amateur radio, etc.). Since PV inverters differ from any other electronic devices due to the long DC cabling, a special product standard for PV components would be necessary. The relevance of EMC at PV systems was already realised some years ago
Other electronic equipment may be influenced by means of galvanic coupling via AC mains or radiated coupling via electromagnetic fields. These effects are mostly covered by existing standards.
Normally, galvanic coupling manly occur in the fre- quency range of 150 kHz to 30 MHz, whereas radiated coupling appears beyond 30 MHz. In PV systems there is usually no electronic equipment connected to the DC lines, and the PV generator is in no way affected by disturbances generated in the inverter. So, why is it important to limit the disturbance voltage on DC lines in the Frequency range between 150 kHz and 30 MHz, if there is no galvanic interference path?
They propagate as voltages and currents respec- tively along the DC lines. Since the DC lines may extend for several 10 meters, they represent an inductance. The PV generator represents a capacitance due to the large area of the modules. Both together results in an resonant circuit for several frequencies. The longer the DC lines and the larger the PV generator, the lower is the resonance frequency. Thus the cable-generator configuration acts as an antenna and electromagnetic fields are generated. The original line conducted disturbances are “transformed” into radiated disturbances.
Due to the long DC cabling radiation occurs even in the frequency range below 30 MHz. This means that limits of radiated electromagnetic fields must be respected in the frequency range between 150 kHz and 30 MHz in order to make the PV system electromagnetically compatible and to allow an undisturbed operation of other electronic equip- ment in the environment of the PV system.
Inverters with a bad filter design on the DC side exceed the limit considerably. Whereas the emission of an inverter with proper design are in the range of the ambient noise or inherent noise of the spectrum analyser respectively. How- ever, this kind of on-site measurement is very expensive and time consuming. Thus, measurements in the laboratory are preferable. Disturbance voltage measurements are reproducible, practicable with great accuracy and well known in EMC measurement technology. Now, a way must be found in order to transform the on-site magnetic field measurements to voltage measurements in the labo- ratory.
As already mentioned above, the disturbance voltage on the DC lines is the source for the radiated electromag- netic field. Thus a relation between field (H) and voltage
(V) exists. This relation is called antenna factor. It may also transform the limits of the magnetic field to limits of the disturbance voltage.
Both simulation and measurement show the same be- haviour of the impedance characteristic: at lower frequen- cies the impedance of the generator-cable configuration is capacitive (capacity of the generator against ground). The decrease is 20dB per decade. At 2 or 3 MHz respectively a resonance occurs due to the inductance of the DC cable. With increasing frequencies series and parallel resonance alternate (recognisable at the numerous peaks). The height of the generator model in the simulation is taller than the measured generator. So the capacitance against ground is smaller and therefore the CM impedance is higher com- pared to the measurement result.
These principles are generally the same for every PV generator. The first resonance is moving in dependence on the DC cable length and the generator size due to the asso- ciated inductivities and capacities respectively. But almost any generator has a high impedance at frequencies of a few 100 kHz. This was checked at several generators by means of measurements and simulations
New limits for disturbance voltage on DC lines should reflect the project results on the one hand. On the other hand a concordance with limits in existing standards is desired. At least the limits for the magnetic field strength according to VDE 0878-1 must be respected
.
For certain frequencies the calculated limits are shown in .In order to obtain a best match to existing stan- dards the calculated limits (Quasi-Peak) are slightly cor- rected . Between 0.15 MHz and 0.5 MHz the limit is moved to 80 dBmV in order to agree with EN 55014-1. Since radio services are explicitly protected by the EMC act there is a discontinuity at 0.5 MHz (lower frequency of AM broadcast). The limit jumps down to 74 dBmV. Between 0.5 MHz and 2.5 MHz the limit decreases with 20 dB per decade according to the CM antenna factor . In contrast to the calculated values in Table 1 the limit is increased to 60 dBmV in the range 2.5 MHz to 30 MHz. Thus a match to the common limit on AC lines is achieved.
Inverters meeting these limits are likely not to cause harmful interference in practical operation.
In the disturbance voltage (peak values) of a trafoless inverter is compared to the new limits. Keeping in mind that peak values are usually some decibels higher than quasi peak values, this inverter complies quite well with the new limit. Other examples of DC side measure- ments are depicted in . It is shown that a compliance with these new limits is easily possible with reasonable technical effort.
The electromagnetic compatibility shall ensure an un- disturbed operation of other electronic equipment in the environment of PV plants. Regarding the DC side, most important is the interference via radiated disturbances, since the RF disturbance current on DC lines generates an electromagnetic field. Due to the long DC cabling radiation occurs even in the frequency range below 30 MHz. This means that limits of radiated electromagnetic fields must be respected in the frequency range 150 kHz – 30 MHz. Since the measurement of radiated disturbances at a PV generator is very expensive, measurements in the laboratory are preferable. Therefore the limits of the radiated field are transferred to limits of the disturbance voltage on DC lines. In order to evaluate the electromagnetic disturbances, comprehensive investigations of numerous different PV plants have been performed within two European projects
The limitation of RF disturbance signals on DC lines is important because numerous sensitive radio services are located in the frequency range where disturbances of PV systems may occur (AM broadcast, emergency calls, navi- gation, aircraft wireless service, amateur radio, etc.). Since PV inverters differ from any other electronic devices due to the long DC cabling, a special product standard for PV components would be necessary. The relevance of EMC at PV systems was already realised some years ago
Other electronic equipment may be influenced by means of galvanic coupling via AC mains or radiated coupling via electromagnetic fields. These effects are mostly covered by existing standards.
Normally, galvanic coupling manly occur in the fre- quency range of 150 kHz to 30 MHz, whereas radiated coupling appears beyond 30 MHz. In PV systems there is usually no electronic equipment connected to the DC lines, and the PV generator is in no way affected by disturbances generated in the inverter. So, why is it important to limit the disturbance voltage on DC lines in the Frequency range between 150 kHz and 30 MHz, if there is no galvanic interference path?
They propagate as voltages and currents respec- tively along the DC lines. Since the DC lines may extend for several 10 meters, they represent an inductance. The PV generator represents a capacitance due to the large area of the modules. Both together results in an resonant circuit for several frequencies. The longer the DC lines and the larger the PV generator, the lower is the resonance frequency. Thus the cable-generator configuration acts as an antenna and electromagnetic fields are generated. The original line conducted disturbances are “transformed” into radiated disturbances.
Due to the long DC cabling radiation occurs even in the frequency range below 30 MHz. This means that limits of radiated electromagnetic fields must be respected in the frequency range between 150 kHz and 30 MHz in order to make the PV system electromagnetically compatible and to allow an undisturbed operation of other electronic equip- ment in the environment of the PV system.
Inverters with a bad filter design on the DC side exceed the limit considerably. Whereas the emission of an inverter with proper design are in the range of the ambient noise or inherent noise of the spectrum analyser respectively. How- ever, this kind of on-site measurement is very expensive and time consuming. Thus, measurements in the laboratory are preferable. Disturbance voltage measurements are reproducible, practicable with great accuracy and well known in EMC measurement technology. Now, a way must be found in order to transform the on-site magnetic field measurements to voltage measurements in the labo- ratory.
As already mentioned above, the disturbance voltage on the DC lines is the source for the radiated electromag- netic field. Thus a relation between field (H) and voltage
(V) exists. This relation is called antenna factor. It may also transform the limits of the magnetic field to limits of the disturbance voltage.
Both simulation and measurement show the same be- haviour of the impedance characteristic: at lower frequen- cies the impedance of the generator-cable configuration is capacitive (capacity of the generator against ground). The decrease is 20dB per decade. At 2 or 3 MHz respectively a resonance occurs due to the inductance of the DC cable. With increasing frequencies series and parallel resonance alternate (recognisable at the numerous peaks). The height of the generator model in the simulation is taller than the measured generator. So the capacitance against ground is smaller and therefore the CM impedance is higher com- pared to the measurement result.
These principles are generally the same for every PV generator. The first resonance is moving in dependence on the DC cable length and the generator size due to the asso- ciated inductivities and capacities respectively. But almost any generator has a high impedance at frequencies of a few 100 kHz. This was checked at several generators by means of measurements and simulations
New limits for disturbance voltage on DC lines should reflect the project results on the one hand. On the other hand a concordance with limits in existing standards is desired. At least the limits for the magnetic field strength according to VDE 0878-1 must be respected
.
For certain frequencies the calculated limits are shown in .In order to obtain a best match to existing stan- dards the calculated limits (Quasi-Peak) are slightly cor- rected . Between 0.15 MHz and 0.5 MHz the limit is moved to 80 dBmV in order to agree with EN 55014-1. Since radio services are explicitly protected by the EMC act there is a discontinuity at 0.5 MHz (lower frequency of AM broadcast). The limit jumps down to 74 dBmV. Between 0.5 MHz and 2.5 MHz the limit decreases with 20 dB per decade according to the CM antenna factor . In contrast to the calculated values in Table 1 the limit is increased to 60 dBmV in the range 2.5 MHz to 30 MHz. Thus a match to the common limit on AC lines is achieved.
Inverters meeting these limits are likely not to cause harmful interference in practical operation.
In the disturbance voltage (peak values) of a trafoless inverter is compared to the new limits. Keeping in mind that peak values are usually some decibels higher than quasi peak values, this inverter complies quite well with the new limit. Other examples of DC side measure- ments are depicted in . It is shown that a compliance with these new limits is easily possible with reasonable technical effort.