How can lightning strikes harm PV installations, and what can system installers and owners do to protect their investments?
To start, we can examine exactly how lightning enters the equipment. Lightning enters a PV installation's wiring in common mode. In PV input circuits, the battery and AC components react the same way: Long wires connect the PV array to the controller. These wires act as an antenna. When a surge comes in the vicinity of the wires, it induces voltage. The closer the strike, the higher the voltage induced into the wires.
Because the surge from a lightning strike raises the voltage equally in the positive and negative wires, there is no danger to the low-voltage transistors in the unit until the surge finally finds a path to ground.
So, the voltage builds up on the PV-plus and PV-minus inputs or battery-plus and battery-minus components – and keeps on building higher and higher. The entire process all happens very quickly, of course.
All products listed to UL and ETL standards are required to pass a hipot test at the factory. This test is typically measured at 1,000 VAC – plus twice the peak working voltage.
This figure can then be converted to the equivalent DC voltage, which is 1.414 times higher than the AC requirement. There are sometimes advantages in using this higher DC voltage due to parasitic capacitance that can make the product fail when using AC.
Charge controller testing
For our charge controller, we are required to connect all four of the terminal block wires together: PV-plus and PV-minus, and battery-plus and battery-minus. All four of these connections are connected – at the same time – to one lead of the hipot machine. The other lead of the machine is attached to the casting, which represents the ground because all equipment enclosures are supposed to be connected to earth ground.
Once these two connections are made, we push the ‘go’ button on the hipot machine, which slowly ramps up from zero volts to 2,300 VDC. It holds the voltage at 2,300 V for one second.
The goal of this test is to check the insulation between all of the electrical circuits inside the controller to ground (the casting). If the unit withstands this 2,300 VDC for one second without drawing more than 3 milliamps of current, the test automatically terminates.
If the product under test does not withstand this voltage, the machine stops, records the voltage where the breakdown has occurred, and turns on loud buzzers and bright lights. Personnel must then find out what was built wrong, fix the problem and run the test again.
Fortunately, a failed hipot test does not happen very often, but depending on what went wrong, it can damage the product. Potential causes of hipot failures include points where insulation broke down or insufficient distance from a circuit board trace to a casting stand-off.
Hipot tests and lightning
When a charge controller leaves the factory and has a UL or ETL label applied, customers can be assured that the product has passed this test. Products that have not passed the test run the risk of containing live circuits breaking down to the enclosure case (the casting). This situation would be dangerous for anyone touching the case, which is supposed to be at ground potential.
How does lightning relate to the hipot test? We know products tested under standard hipot test conditions are assumed to withstand 2,300 VDC, in the case of a charge controller. During the hipot test, the positive and negative wires are tied together and a very high voltage is applied to the circuit.
Because the wires are tied together, no problems occur with the low-voltage parts in the product. Even though high voltage is applied, the difference between the leads of the individual components is zero – until there is a breakdown in the insulation or air gaps designed to withstand this high voltage.
When there is a breakdown, the hipot machine will only put out 3 milliamps – a level that usually does not hurt the product. In this case, the breakdown point is corrected and the product is tested again.
When a near lightning strike is induced into the wiring of a system, a very similar thing happens as during the hipot test. The lightning will continue to rise – sometimes to 100,000 V. Of course, there are no consumer products that can withstand that high of an induced voltage.
Rather, these products typically have approximately a 10% to 20% margin of safety over and above the 2,300 V requirement. It costs money and space to have a higher breakdown voltage, so design margins are typically slim.
Now that the lightning has been induced into the wiring, reaching 3,000 V or more, we can estimate that some place inside the controller will break down and arc over to the case, which is connected to earth ground. The lightning is not limited to only 3 milliamps of current; it can go well over 3,000 amps.
Additionally, as the circuits break down, the lightning will continue rising, causing the circuits to potentially break down in 100 different places at the same time. With thousands of amps flowing in 100 different places inside the unit's casing, there will be no way to rebuild a unit that has been subjected to this type of damage.
One step system designers and installers can take to help prevent this type of damage is to add a surge protection device (SPD) to the input of the controller. When lightning gets induced into the PV wires, the voltage starts rising, but when it reaches 385 V, the SPD begins conducting.
The SPD has thermal metal oxide varistors connected from PV-positive components to the ground and from PV-negative components to the ground. Depending on how close the strike is, there could be 100,000 A behind the surge. Some surge arrestors can protect at levels up to 115,000 A.
The higher the current, the higher the voltage will rise, even though the SPD is trying to clamp the voltage down. We have measured about 900 V of clamp with a 3,000 A surge. UL uses 3,000 A as its simulated lightning surge. If each controller is tested at the factory to be able to withstand at least 2,300 V, a 900 V/3,000 A surge will not hurt the unit.
However, if the lightning strike is a direct hit, there may be no saving the product or SPD. The damage may depend on where the lightning entered the wiring. The resistance of the wiring may help save the installation in this case, as the resistance offers impedance to the surge and gives the SPD a chance to act.
Robin Gudgel is the founder of Midnite Solar, a provider of electrical products for renewable energy systems. The company can be contacted at (360) 403-7207.