2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion
A wireless controlled circuit for PV panel disconnection in case of fire P. Guerriero, F. Di Napoli, V. d’Alessandro, S. Daliento Department of Electrical Engineering and Information Technology(DIETI), University of Naples Federico II, via Claudio 21, 80125 Naples, Italy. Phone: +39-081-7683122; e-mail:
[email protected]
Abstract— This paper propose a new system to short circuit a photovoltaic panel, embedded in a photovoltaic string, with the aim to ensure safe operation of firemen in case of fire. The circuit is activated by a wireless command and doesn’t require additional wiring. Moreover it is self powered by the solar panel itself and can be either turned on or turned off in a fully reversible way. Keywords—fire protection, reconfigurable systems
I.
arc
detection,
Recently, a more reliable approach has been proposed [3], it consists of a remote controlled relay which is mounted in parallel to each module and can be activate by pressing a devised main switch. The system is really effective but requires separate wiring schemes providing power supply and data communication. Thus, the number of wires in the PV plant increases. Active by pass devices [4,5], which are three terminal electronic devices that can be used instead of the conventional two terminal by-pass diodes, to reduce dissipated power, could be used as well but require special driving schemes. In a recent paper [6] features which should be accomplished by a reliable firefighter safety system have been revised and discussed according to the German Association for Electrical,
monitoring,
INTRODUCTION
During the last decade photovoltaic technology reached full commercial maturity and its worldwide diffusion increased dramatically. Despite that, an unsolved, potentially harmful, security risk still exists. It comes from the topology of photovoltaic (PV) systems which require series connection of PV modules, thus forming PV strings, in order to reach the DC output voltage suitable for DC/AC converters. A high voltage of several hundred Volts is usually present at the string terminals; such a voltage is generated by the photovoltaic effect and cannot be switched off as long as sunlight reaches the PV string. There is a twofold drawback of this fact. The first drawback concerns the possible occurrence of voltaic arc, whose activation is the first cause of fire setting in PV plants. PV plants should be provided of reliable arc detection systems able to immediately and automatically extinguish the voltaic arc by resetting the string voltage in order to prevent fire propagation. However, even if the voltaic arc could be correctly detected, the reset of dangerous voltages could be achieved, for each point of the PV field, only if all solar modules were individually short circuited. The second drawback arises when, unfortunately, fire takes place. In this case the effective action of firefighters to tackle the blaze is prevented by the presence of dangerous voltages. There are few solutions right now. The Italian standard CEI 8225 V1 [1] says that “it is impossible to keep the system with no voltage under sunlight” and only requires the presence of information plaques at a distance of five meters each others to advise for potential risks. The above statement is not completely true. Some companies developed highly sticky foams [2] indeed which should be spread over PV panels to obscure them; unfortunately they are difficult to remove and may bring to the irreversible damaging of PV panels. In any case too much time is required to keep safe conditions.
978-1-4799-4749-2/14/$31.00 ©2014 IEEE
Electronic & Information Technologies (VDE) application guide “VDE-AR-E 2100-712:2013-05 [7]. The following criteria for a
reliable system were individuated: 1. 2. 3. 4. 5.
Immunity to over-currents due to dc capacitor Safe PV module voltage after disabling. Low conduction losses. Possibility of individual module bypassing Cost efficiency.
In [6] it was concluded that none of the existing solutions fulfills all criteria. In this paper we propose an innovative single panel shortcircuiting system which both fulfills all above requirements and adds further new properties. The system exploits a parallel relay which is activated when a wireless command opens a MOSFET which is connected in series to the solar panel, thus the panel supplies both zero voltage and zero current. The system doesn’t require new wiring and can be easily installed on existing plants; morever it is fully self powered by a supercapacitor harvesting stage. It is fully reversible as well, in the sense that each module can be individually either disconnected or reconnected. The paper is organized as follows. In Section II the operating principle is described through circuit level simulations performed in the PSpice environment. In Section III the harvesting stage is described and experimentally tested on a prototype board. In Section IV experiments performed on a test PV field are shown. Conclusions are drawn in Section V.
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II.
P+
OPERATING PRINCIPLE AND NUMERICAL ANALYSIS
The schematic of the disconnecting circuit we propose is shown in Fig.1. As can be seen it consists of a power MOSFET connected in series to the solar panel and a relays connected in parallel to the series of the solar panel plus MOSFET. The MOSFET is driven by a microcontroller which receives remote commands sent via a WiFi network. During normal operation the MOSFET is in the on state and offers a series resistance of only few milli Ohms [4], thus conduction losses are fully negligible. When an alarm signal is sent the MOSFET is pushed in an open circuit condition while the relays is short circuited.
R SuperCap
P
-
Fig. 2. Circuit scheme of the harvesting stage
system and, after that, the system could be automatically recovered. Moreover, the energy harvesting stage guarantees that the system is fully self powered, this is an enormous advantage over other systems, as they require either on board batteries or additional wiring to ensure power supply. The harvesting stage should not affect the operating point of the solar panel; to this end the special schematic shown in Fig. 2 has been implemented [9]. As can be seen the energy storage element is a supercapacitor which is charged by the source current of a power MOSFET. Problems could arise when the capacitor starts to draw current because it could appear as a short circuit parallel to the solar panel. In order to prevent this occurrence the resistor R acts as a negative feedback which stabilizes the charging current. In the circuit of Fig.2 the Gate to Source voltage is indeed reversely affected by an increase of the source current. The correct operation of the harvesting stage is crucial for the reliability of the whole circuit because a stable power supply is a prerequisite for assuring both the functionality of the microcontroller and enough energy for wireless communication and relay driving, the latter being particularly onerous. In order to limit energy requirements the bistable configuration shown in Fig. 3 has been adopted. Proper sizing has been achieved by accurately evaluating currents needed for the relay to switch. The circuit has been simulated in the PSpice environment according to the schematic of Fig.3.
Fig. 1. Schematic of the disconnecting circuit
In this condition the solar panel supply true zero current while the voltage at the external terminals of the solar panel (S+ and S- in Fig.1) is zero. It is important to note that the solar panel is not short circuited, the voltage between the terminals P+ and P- (which are only accessible inside the junction box), is the open circuit voltage of the solar panel indeed. Hence, the energy harvesting stage is always powered as far as the module is sunny. This solution allows the system to be continuously controllable so that the panel can be reconnected when needed. This last feature could be particularly interesting if associated with an arc detection system as the arc could be extinguished by nulling the voltage at each point of the PV
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III.
PROTOTYPE FABRICATION AND TEST
The main elements of the circuit described above are shown in Fig. 4 which shows a picture of the realized board.
Fig. 3. Driving circuit for the relay.
As can be seen, a double coil relay has been used. A1 and A2, are the driving terminals, used to respectively turn on (short circuit) or turn off (open circuit) the device. The supply voltage is applied to A3. It should be underlined the role of the capacitance C0 which is slowly charged by the harvesting stage (through Rs) and supplies the whole switching current when needed. This solution prevents fluctuations of the supply voltage during the switching which could cause the shutdown of the microcontroller. The switching of the relays is driven by two power MOSFETs whose gates receive the switch command sent by the microcontroller. The correct operation of the circuit has been checked by monitoring the currents flowing through the two coils during the switching. As an example Fig.4 shows the currents flowing into the relays inductances when MOSFET M1 is turned on. As can be seen the current flowing through L2 (red curve) has a negligible spike of only few nano Amperes while L1 absorbs about 50 milli Amperes after 1 ms, which is the requested switching current. As already said above the capacitance C0 is chosen to supply the whole energy with an acceptable voltage ripple. The correct operation of this stage is evidenced in the next section where experiments on a realized prototype are shown. 1,5
Fig. 5. Prototype of the disconnecting circuit.
In the lower left corner we can see the Wi-Fi antenna, on the right side there is the relay and, in the center, the microcontroller and the power MOSFET. It should be noticed that the MOSFET, whose circuit role is shown in Fig.1, allows temporary disconnection of the solar panel for an arbitrary time length, this time can be exploited to perform measurements of the working parameters of the solar panel as described in [8,9]. The board of Fig.5 appears more complex than it could be because it embeds the measurement circuitry for single panel monitoring and diagnostic. Hence our system, in the present form, is an effective monitoring system which, in case of fire or maintenance, assures safe intervention. Thus the cost efficiency of the whole system is extremely favorable as the cost of the augmented board is almost the same of the former monitoring system. A further important feature of the system, also suggested in [5], is that it can be easily set to transmit a “heartbeat” signal which, eventually, allows automatic neutralization of PV panels. Before mounting to PV panels, the circuit has been tested to verify both the correct operation of the harvesting stage, in order to ensure that the relays always has enough energy to switch. Fig. 6 shows the voltages supplied by the harvesting stage during the switching of the relay. Two DC voltages are needed for the correct operation of the board: 12 V are supplied to the relay while 3.3 V are supplied to the microcontroller.
50
1,0
40
30 0,0 20
-0,5 10
-1,0 -1,5 0,0
I(L1) [mA]
I(L2) [nA]
0,5
0,2
0,4
t [ms]
0,6
0,8
0 1,0
Fig. 4. Coils currents during relay switching
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4,0
3,8
12,0
Vsupply 3.3V [V]
Vsupply 12V [V]
12,5
the open circuit voltage of the solar panel, thus ensuring continuity of power supply to the electronic board. The behavior of the system has been checked by exploiting the functionality of the monitoring section which allows the measurement of both open circuit voltage Voc and operating voltage of the solar panel. Fig.8 evidences the expected features. The red curve is the voltage measured between the terminals P+ and P- of Fig.1 and coincides with Voc, the black curve, Vpan, is measured between S+ and S-. The relay is alternatively closed and opened for about one hour. The figure shows that the external voltage is effectively set to zero when the relay is switched on, at the same time the internal voltage is fully unaffected so that the board is always powered and the relay can be switched off when needed. It should be noticed that, when the relay is open, the two voltages shown in Fig.8 are not actually simultaneously measured. It is, indeed, obvious that the panel can not supply the open circuit voltage and the operating voltage in the same time. Hence, those voltage are alternatively measured by commutating the series MOSFET to keep the panel in the two states; the curves appear as continuous because measurement time is only few milliseconds so that the switching is not perceptible in the time scale used in Fig.8. A second important experiment has been performed to verify that the disconnection and short circuiting of a solar panel doesn’t affect the operation of other panels of the string. To this end both the string voltage and the string current were monitored during a whole day and compared with the panel voltage and panel current respectively. Operating voltages are reported in Fig.9 where the red curve refers to the string voltage while the blue curve is the panel voltage already shown in Fig.8.
3,6 11,5
3,4 11,0
3,2
10,5 10,0 -5
-4
-3
-2
-1
0 t [s]
1
2
3
4
5
3,0
Fig. 6. Supply voltages for the relay (red) and for the microcontroller (blue)
Switching occurs at time t=0. As can be seen only a small ripple on the lower voltage is observable thus confirming that both the relay and the microcontroller are always powered. Wireless communication section is based on the MiWi protocol (IEEE 802.15.4 compliant, 2.4 GHz frequency), and was implemented through the Microchip MRF24J40MA transceiver, which guarantees very low power consumption. IV.
EXPERIMENTS
Vpan_10 [V]
The disconnecting board has been mounted on the back side of two PV panels, embedded in a ten panels string, as shown in Fig.7.
Fig. 7. Picture of mounted disconnection boards.
25
25
20
20
15
Vpan Voc 15
10
10
5
5
0
0 12:00
As can be seen the box containing the board has four terminals, two of them are directly connected to the terminals of the PV panel while the others provide series connection with other panels. An improved version embeds low consumption power by pass diodes [10] as well, and substitutes the junction box. The correct operation of the system requires that each panel can be individually “switched off” in the sense that the voltage measurable at its external terminals must be zero. On the other hand the “internal” voltage, only measurable inside the junction box, should be
13:00
14:00 t [h]
15:00
16:00
Fig. 8. Comparison between solar panel external (S+ and S- of Fig.1) and internal (P+ and P- of Fig.1) voltages.
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Voc_10 [V]
13,0
3,0
50
2,5
40
2,0
Vpan_10 [V]
Vstring [V]
180
60
30 160
I [A]
200
I string I panel
1,5
20
1,0
10
0,5
0
0,0
140 12:00
12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30 t [h] Fig. 9. Comparison between solar panel external (S+ and S- of Fig.1) voltage (blue) and string voltage (red).
Fig. 10.
As can be seen, during short-circuiting, the panel voltage is effectively subtracted from the string voltage. As the operating voltage of the controlled panel is almost constant (when not shorted) variations in the string voltage, which is higher in the late afternoon, should be attributed to some solar panels which are exiting a shaded condition In a complete firefighter protection system all PV panels should be equipped with their own disconnection board thus allowing the reset of all voltages in the solar plant. It is interesting to note that the possibility of arbitrary disconnection and reconnection of individual solar panels suggests that, in principle, the same circuit could be adopted to build up reconfigurable systems; experiments on this subject are being performed. Lastly, figure [10] compares the string current (blue) with the panel current (red); it is evident that the string current is not affected by short circuiting the solar panel. As the string is a series connected system indeed, the short circuit over the controlled panel offers a flowing path for the current and the string works like a “shorter” system. At the same time the current supplied by the disconnected panel is abruptly set to zero. True zero current coming from the disconnected panel strongly relaxes the first requirement recalled in section I. Over currents are strongly reduced indeed. On the other hand, in case of fire, disconnection of all panels can be sequentially driven for each panel of the string, thus avoiding sudden short circuiting of the whole string. V.
13:00
14:00
t [h]
15:00
16:00
Comparison between solar panel current and string current
REFERENCES [1] [2] [3] [4]
[5]
[6]
[7]
[8]
CEI 82-25-V1, “Guide for design and installation of photovoltaic (PV systems connected to MV and LV netwoks)”. “Prevento Solar”, technical datasheet, available on line: http://www.febbex.com. “SolteQ-BFA”, technical datasheet, available on line: http://www.solteq.eu. V. d’Alessandro, P. Guerriero and S. Daliento, “A Simple Bipolar Transistor-BasedBypass approach for Photovoltaic Modules,” IEEE Journal of Photovoltaics, vol. 4, no. 1, pp. 505-513, 2014. DOI: 10.1109/JPHOTOV.2013.2282736. H. Schmidt, B. Burger, and H. Häberlin, “A novel diodeless bypass technology for high performance PV modules,” Proc. European Photovoltaic Solar Energy Conference, 2007, pp. 2688-2694. S. Spataru, D. Sera, L. Mathe, T. Kerekes, “Firefighter safety for PV systems: overview of future requirements and protection systems”, IEEE Energy Conversion Congress and Exposition, 2013, pp. 4468-4475 "VDE-AR-E 2100-712:2013-05 Measures for the DC range of al PV installation for the maintenance of safety in the case of fire fighting or technical assistance,” VDE, 2013.
M. Gargiulo, P. Guerriero, S. Daliento, V. d’Alessandro, M.Crisci,“A novel wireless self-powered microcontroller-based monitoring circuit for photovoltaic panels in grid-connected systems,” Proc. of IEEE 20th International Symphosium on Power Electronics, Electrial Drivers, Automation, and Motion (Speedam), 2010, pp. 164-168. [9] V. d’Alessandro, P. Guerriero, S. Daliento, M. Gargiulo, “A straightforward method to extract the shunt resistance of photovoltaic cells from current–voltage characteristics of mounted arrays,” SolidState Electronics, vol. 63, no. 1, pp. 130-136. 2011. DOI: 10.1016/j.sse.2011.05.018 [10] S. Daliento, L. Mele, P. Spirito, R. Carta, L. Merlin, “Experimental study on power consumption in lifetime engineered power diodes”, IEEE Transaction on Electron Devices, vol. 56, no. 11, pp. 2819-2824, 2009. DOI: 10.1109/TED.2009.2031005
CONCLUSIONS
In this paper we have proposed a wireless remote controlled circuit that ensures safe workers operation in illuminated solar systems. The circuit combines both opening of each solar panel, thanks to a series connected MOSFET, and shortening, by means of a parallel relay. Furthermore it is fully selfpowered and doesn’t require additional wiring.
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