Protection of Off Grid Solar PV Systems
A Discussion of Various Protection Practices in Off-Grid Solar PV Systems
For may people, the concept of solar power is a new and potentially exciting subject. Particularly given the rather dismal state of the national grid in South Africa.
But like any other form of energy, solar PV systems can be dangerous if not treated with respect.
This article discusses some of the problems that might arise and the methods used to prevent or reduce the harmful effects that can result.
Please note this article is meant to merely make people aware of the various issues that need to be addressed, as well as provide a brief overview of some of the methods used to improve safety. It is not meant to be an exhaustive study on the subject. It should certainly not be relied upon as an installation guide or set of instructions.
AC Grounding (Earthing) for Off-Grid Solar PV Systems
We discussed some of the important safety concepts in home electricity in another article.
In that article, we also mentioned that alternative power sources such as solar PV panels can be just as dangerous as normal Eskom power. So all the same safety procedures and practices should be applied to those systems.
One of the key safety mechanisms mentioned was the inclusion of an earth wire. As discussed, the earth wire offers an alternative path for the electricity in the event of a fault. The cartoon below illustrates the point quite well.
The Difference Between Off Grid (Island) and Normal Eskom Power Installations.
In a normal Eskom/council connected home, the earth wire in your home is connected to the neutral point of the transformer in the council substation where it is also physically connected to the ground, by means of a stake driven into the ground. The diagram below illustrates this.
The earth wire thus enables an alternative path for the electricity to flow safely back to the substation in the event that the live wire touches the shell of the appliance. In that event, the earth leakage breaker will detect that not all the electricity that flowed “in” via the live wire, flowed “out” via the neutral wire, which means that some sort of fault occurred where electricity leaked to earth. As soon as the earth leakage breaker detects this, it should trip and stop all electricity flow.
In the same way, the earth leakage breaker should also trip if you grab a live wire and the current flows through you to the ground you are standing on.
So as you can see, the combination of the earth leakage breaker, together with the earth wire that is connected to both the physical ground beneath your feet, as well as the neutral point at the source of power, provides a key safety mechanism that reduces the harmful effects of most faults caused by the live wire touching something it shouldn’t.
However, in a completely off-grid installation (also referred to as an island installation), there is no connection to the local substation, and thus no council supplied connection for your AC earth wire.
You need to establish the same infrastructure yourself.
You do this by connecting a suitable earth wire to the 220V AC neutral wire right where it connects to your inverter – this will be your main earth point, and all earth wires in your home should lead back to that single point. That point should also be connected to a suitable stake driven into the ground.
In this way, if the live wire touches the shell of the appliance, you will have provided an alternative path for the electricity to flow back safely to the inverter. In the event that happens, the earth leakage breaker should detect that not all the electricity that flowed “in” via the live wire, flowed “out” again through the neutral wire, and thus a fault must have occurred somewhere, so then the breaker should trip and break the circuit, stopping all AC electricity flow.
In fact with back-up systems where the system has to stand in for the council power when that fails, its a good idea to use the above approach as well. Because you cannot necessarily rely on the earth connection in the local substation being present during a power failure.
Grounding PV Arrays to Reduce Lightning Damage
Because your PV panels normally sit on your roof, it is possible for lightning to strike the panels directly, or indirectly if it strikes something sufficiently close by. Because there are cables running from your PV panels down to your MPPT and Inverter, it is thus possible for lightning to damage all those expensive components if it strikes your house.
The likelihood of your home being struck by lightning will depend on where you live.
So it may be prudent to add lightning protection to your PV system.
The basic objective is to provide a very short straight preferential path for the lightning to follow, and thus reduce the amount of lightning that will flow down any other wires or structures, and thereby reduce the damage to the solar array and other components as far as possible. One way of achieving this is by running a suitable earth wire from the PV array straight down to a spike secured in the ground.
Its important that this earthing wire is sufficiently thick and that it runs in a straight/direct path down to the earth spike. Lightning will always take the shortest path regardless, so if your conductor does not go straight down, lightning may find another more expensive path.
In order to try and further prevent lightning flowing down your PV cables to the MPPT/Inverter, you can add a DC surge protector (usually included in whats known as a “Combiner Box”). When the surge protector detects the voltage surge caused by lightning, it connects those PV cables directly to earth (preferably the same thick wire running from the array down to the spike in the ground), and thus offers the lightning a better path to follow down to ground, rather than through your expensive electronics. Obviously the surge protector has to react very quickly to be effective. You will find that the more expensive units will have faster reaction times.
For a more comprehensive discussion on lightning protection of large scale PV arrays, please read this interesting article on the subject recently published by Energize Magazine.
Protection of DC Circuits
In solar power systems, the photovoltaic panels generate direct current (DC), which is different from alternating current (AC) that your household appliances use.
Many people think that AC is more dangerous than DC and thus do not take much notice of the possible dangers inherent in DC circuits.
But in some ways, the DC side of your solar system can be more dangerous than the AC side. This is because of two main reasons:
- You can’t switch off the sun, so if a fault occurs, the sun will keep energising the system regardless. The same goes for the stored energy in your batteries. The energy stored in the battery bank will flow through the circuit regardless of any fault until the battery bank is completely flat, which can take days.
- DC current has the characteristic of drawing an arc when a contact opens. This mean that when a DC switch opens for example, if the gap between the terminals opens slowly, a high temperature arc will form between the two terminals which can melt the terminals or any nearby plastic and perhaps cause a fire. This is exactly what happens when you weld metal with an electric arc welder. This characteristic also means if a joint becomes loose, it may start arcing and cause a fire.
The last point above has the implication that DC switchgear is more complex and expensive because the terminals have to fly apart explosively to prevent an arc forming. Please note that for this reason you cannot use AC switchgear for DC applications.
The video below illustrates the problem of arcing in DC switchgear very well:
While these types of faults look very scary, fires caused by arc faults in solar PV systems seem to be very rare according to this article: “Research indicates that rooftop solar-caused fires are very rare. A German study found approximately 75 instances out of some 1.3 million installations, while a U.S study found only seven instances in the entire country“.
The video below discusses some of the faults that can occur on solar PV panels:
In South Africa there is no standard for PV system installations yet, although one is currently being drafted. Until that standard is published, you should consider taking at least these precautions on the DC side of your system:
- Install specialised PV fuses on the PV cables. If there are more than two strings in your array, each string should have its own set of fuses. The cable running back to the MPPT from the panels should also have a solar fuse installed. Please note solar fuses are different to normal DC fuses because they have been specifically designed to protect against the types of faults that occur in solar array’s. Combiner boxes are a good way of achieving this.
- Install suitable fuses on the battery bank, at least one fuse per parallel string of batteries.
- Ensure that you use the correct cables for each application.
- Ensure that you use the correct connectors for each application. Make sure that all connections are properly tightened to prevent loose joints that may start arcing or cause heat buildup. If you are using crimped lugs make sure you use a proper crimping tool.
- Ensure all cables are properly routed and secured so that they cannot work loose or get snagged and thus either be damaged or cause the connections to become loose.
- Ensure that you only use quality components throughout. Stay away from components whose prices seem too-good-to-be-true.
- Be aware of, and do not exceed the voltage ratings of the various components. Please note that solar panels connected together in strings can generate lethal voltages.
For some detailed information on some of the protection methods used in solar PV applications, download the Eaton Solar Circuit Protection Application Guide.
Disclaimer: Please note the above discussion was a simplified overview of the subject to explain the basic safety principles to laymen. It should not be relied upon in detail because certain aspects were simplified to enable the discussion for non-technical people. For further detailed information please consult the relevant codes and standards.