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Why Arc Detection is important to photovoltaic systems

 

- permanent flow of energy in a solar generator during light/sun exposure with high DC voltages up to 1000 volts and fairly high DC currents is critical to all parallel and series resistances in a string like cable connectors, junction-box connectors, bypass diodes, but also decreasing insulation resistance of solar cables, aging effects and many others
- permanent climatic influences like temperature, humidity and others mean a permanent stress (corrosion) to contact materials inside connectors and junction boxes, increasing the contact resistance
- on animal farms ammoniac gases could occur with accelerated corrosion of contacts forcing the emergence of arcs
- once an arc occurs, there is an extremely dangerous brightness of light and a very high temperature of the plasma of about 3000C causes many materials to catch fire immediately with consequential damages
- older solar generators are especially critical to arcs as their cabling and connector system lost insulation resistance (mechanical influences) or have increased contact resistance due to corrosion
- a detection of arcs can prevent fire and consequential damages by turning off/short-circuit the solar generator, preferably each module in the string as realized by “PV SAFETY BOX” procedure. www.newtos.com
- depending on the size of the PV generator and number of strings, there is need for integration of several arc detect circuitries in only one box

AM1 is offering a very inexpensive and space saving application for arc detection. A small box was developed to carry one to five single arc detect circuitries built as daughter boards to be plugged in a motherboard.

Depending on the number of strings, the user is free to choose the number of daughter boards fitting his needs. This modular concept is most cost effective and flexible. In addition to the yellow alarm LED of each daughter board, there is an option for acoustical indication of arc detection by a specific beeper sound independent of the channel that presents an arc. Anyway, the yellow LED indicates clearly which string is affected by an arc.


The new box contains a reset (green) and test switch (red) , a plug-in external AC/DC power supply (indoor use only) featuring a wide input voltage range (100-240VAC), and cable glands for up to five solar cables of positive or negative polarity. It also contains a motherboard and up to five daughter boards being plugged right-angled into the motherboard connectors. As an option there will be available a power supply for outdoor use. Also, a DIN Rail power supply as well as an inverter power supply or other sources can be used.
Each arc detection channel is preferably connected serially e.g. in the Plus-wire between the generator and the inverter, preferably close to the inverter. The user is free to connect each string to an inverter of his choice, there is no need for using the same inverter for the strings being monitored for arcs. On each daughter board there is a true galvanic isolation between the solar DC path and the detection circuitry. Each daughter board has a green LED indicating readiness for measurement and a yellow LED indicating an arc detection. The yellow LED remains on until a reset have been made, either by pushing the reset button of the box or by applying an external reset signal. Please note that when pushing the reset button of the box or applying an external reset signal, all six channels will be reset simultaneously.
On the motherboard you will find a variety of connectors for the potential-free relay contacts of each daughterboard, for a common external reset and test function and for the power supply of 24VDC from the plug-in AC/DC power supply or other sources. Each relay has a DPDS – CO contact arrangement. The relay contacts are universal, serving e.g. as TTL-signalling or switching lamps, motor switches, power relays or sirens up to the user´s wishes.

By using a motor switch or solar power relay you might be able to disconnect a string from the inverter or open up a string to automatically power down/ short circuit the solar panels equipped with “PV SAFETY BOX”, thus reliably extinguishing serial or parallel arcs in the solar generator.

BCables from outside the box to these connectors are led through cable glands featuring IP65 and up. Each relay is featuring a fail safe behaviour: even in case of a loss of power to the box, the actual status (alarm or reset) of the contacts remains
permanently. This could prevent a motor-switch from turning on the solar power again once the relay drops during loss of power.
Once power has been supplied to the box by the AC/DC power supply or other sources, all green LED will be on after a couple of seconds (self test time), the box is now ready for arc detection adjustment once all cabling of the solar DC path has been done: all solar cables need to be sticked through the cable glands and the toroid choke of the particular daughter board.
To do the adjustment and optimisation, please connect a voltmeter to the test points MP0 (black, equals GND) and MP2 (red) and adjust the voltage by turning the screw of potentiometer P1 until you get a reading of about 0,11VDC (adjust the system in the “best” time of insolation = optimal setting) in an arc-free system. This serves as a compensation of various noises in the string and should be performed for each string individually.

To test the functionality of the arc sensors according to UL1699B, please push the red “Test” -button of the box or apply an outside test signal by shortening clamp CC5 and CC6 for a couple of seconds. All yellow LED need to be on at this time and will be off once a reset signal (pushing the green “Reset” button of the box or applying an external reset signal by shortening clamp CC11 and CC12 for about 2s) is applied.

In case of arc detection, the yellow LED of the affected string will turn on. At same time the relay of that daughterboard will switch and, if activated, a beeper sound in one-second-intervals will be heard. There is arc detection of serial and parallel arcs as well.

A differentiation of both types is possible if the string voltage at the DC input of the inverter is monitored at the same time as arc detection occurs: if the DC voltage goes close to zero volts at the time of the arc, it will probably be a parallel arc.

LED, relay and sound stays on until the reset button (green) of the box is pushed for at least two seconds or an external reset signal is applied at least for two seconds. Test button (red).
The beeper can be switched on and off by a little switch (S1) on the motherboard.

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