IDENTIFYING FAULTY, LOW YIELDING, SOLAR PHOTOVOLTAIC (PV) SYSTEMS
The purpose of this article is aimed at providing the information needed to make a sensible and informed decision as to whether a Solar PV system is operating as it should by identifying faulty, low yielding, solar PV system causes.
Experience tells us that most system performance issues come down to unrealistic expectations from comparisons against incomparable systems. Before delving into a suspected problem, it must be noted that financial yields and system performances will vary significantly between manufacturers, panel orientations/pitches and site conditions. Solar PV arrays can seldom be compared at the side of one another for these reasons.
The first thing I would recommend anybody to do, beyond the obvious, would be to check the system against the forecast you should have received pre-installation from your installer. If the system is yielding well below the forecast then you should contact your installer for an explanation. If they are not forthcoming then use the steps below to see if there is something less obviously wrong.
By writing this article I am not encouraging anybody to go reaching for the toolbox as these systems can be extremely dangerous. Look, but don’t touch!
SO, HOW DO I KNOW IF I HAVE A FAULT WITH MY SOLAR PV SYSTEM?
Here is a list of some common reasons why you may experience poor yield from a solar PV system. A more detailed explanation of each item is given further in the document.
Solar Panel Failure
Solar PV Inverter Failure
Solar PV Inverter Mismatch
Solar Panel Shading Problems
Solar Panel External Influences
Solar Panel Installation Problems
Solar PV Over Voltage Problems
Solar PV System Under Voltage Problems
Solar Panel Temperature Related Problems
Solar PV Inverter Overheating Problems
Electrical Supply Fault to Solar PV Inverter
Solar Panel Cell Degradation
Solar Panel Failure
If you have a solar panel failure it will routinely show itself up in one of two ways:
– physical damage – This can usually be seen through a visual inspection of the array such as broken glass, water ingress and mould growth.
– electrical failures – These can range from a bypass diode failure in the back of a panel, cell failure or damaged cables and connectors. These faults will usually show up as a low voltage or open circuit. Whilst electrical failure is harder to spot than physical damage, you can carry out some simple maths to give you a clue. If you cannot see anything wrong, then carry out the following simple steps.
If there is a suspected electrical failure, (low voltage), then this can be identified by counting up the amount of panels on a string and multiplying that by the MPP voltage, (Vmpp), from the panel data sheet. Do not get the MPP voltage mixed up with the open circuit voltage, (Voc), on the data sheet as this is much higher than the MPP voltage and will lead to mis-diagnosis. On a sunny day the voltage should be within approximately 5% of that calculated on a system up to around 5 years old. Please note where there are multiple strings on a system you will also require the system schematic diagram as there may be different numbers of panels on each string.
The measurement of the DC voltage can usually be taken from the display on the front of many inverters or the manufacturers desktop monitor if you have one. If this is not possible then you will need to contact a suitably qualified engineer to carry out the checks. Under NO circumstances should an untrained person attempt to access the electrical components of such a system as they can be highly dangerous.
Solar PV Inverter Failure
Almost all inverters have some sort of on board diagnostics and show a fault code or illuminate a series LED lights to indicate there is a problem with the system. You should consult your inverter manufacturer’s literature to find out what the fault code means, as this will vary between different manufacturers. As I am sure you can appreciate, there are far too numerous reasons for inverter failure to discuss in this article.
Solar PV Inverter Mismatch
Inverter mismatch can result in very poor performance and yields. Inverters have a minimum/maximum voltage/current window that they will operate in efficiently and constantly track the voltage and current received from the solar array. This is called the tracking point. Inverters operate at their most efficient when at the Maximum Power Point (MPP). If an inverter is oversized for an array then the voltage from the panels will be too low for the inverter to operate efficiently. If the inverter is undersized then conversely the Voltage would be too high for the inverter and it will shut down to protect its self or worse still can be irreparably damaged.
Matching the inverter to the array is one of the fundamental design requirements in a solar PV system and if mis-matched it will show up as poor performance, an oversized inverter would be late to start up in a morning due to low voltage and early to shut down in an evening. In extreme cases it may also close down unnecessarily in the day under cloudy conditions and thus miss large parts of the solar day.
If an undersized inverter is used then the unit may shut down at the highest yielding times of the solar calendar due to overheating and over voltage to protect itself from harm.
It can be beneficial to undersize an inverter, in particular on a partially shaded or off south array as they will perform better the closer they are to their maximum tracking point (MPPT).
The only real way to check this accurately is by using a sizing tool from the inverter manufacturer or by 3rd party solar PV design software. It is very simple to do and can be carried out quickly from the office with some basic information from site.
Solar Panel Shading Problems
Shading of an array can be disastrous. However the effects can be reduced by good design and the use of inverters with multiple tracking points or micro inverters. Significant differences in shading losses can be made by simple panel orientation changes. Shading is an obvious problem and can be seen very easily by monitoring shade on the array throughout the year at different times of the day.
Ideally the array should remain un-shaded between 10am and 14pm April to September to yield good returns in the UK without the need to employ secondary measures to counter the effects. This will however vary between system layouts and can only be accurately monitored through a site survey.
By employing shading tools or some good site-specific information from a survey, the site can be quickly mapped out on a 3D model using design software. In turn the whole of the solar year can be simulated and the effects compared against an un-shaded array with the same characteristics. By employing this approach it will quickly tell the assessor if the shading is the issue, if the site is unsuitable or the system wiring is not optimised for the layout/shade.
Solar Panel External Influences
Weather – External influences such as bad/freak weather events; in particular a bad summer with exceptional cloud cover can skew figures and look like a faulty system.
Dirt – Dirty panels will yield poorly; make sure the panels are kept clean and free from leaves, bird droppings, dust and the like. If leafs are allowed to collect on the panels, we have found pine needles to be a particular problem as they can gather in the corners of a panel around the raised edges and if not removed can allow the growth of an algae like fauna which left to its own devices could spread across the panel. In these situations it is good practice to use panels with extra deep boarders between the cells and panel edges. Low-pitched panels exaggerate collection of debris where the natural cleaning of panels by rainfall is not so good due to the low mounting angles. In these situations regular cleaning will be required to maintain yields.
Solar Panel Installation Problems
The quality of the installation can play a part in the yielding of a system although these problems quite often take time to show up.
Issues can arise from the simplest faults from connectors that have not been pushed home quite right and burn out/come undone due to expansion/contraction over the seasons or work loose through vibrations and the like from wind. Water ingress into these couplers/connectors can also build up to become a problem over time. A loose connection in an A/C isolator may mean the system keeps dropping out through under voltage. When an inverters A/C supply is isolated for only a split second it will take a minimum of three minutes to re-start under the rules of G83 and G59 that the systems have to comply with. As you can imagine this could cause significant yield loss over a period of time if every few minutes a contact makes and breaks through being loose. We have seen this problem first hand and seen the losses this can cause.
I find as an installer and having had over 25 years experience within the electrical contracting industry that by a simple visual inspection of any installation, Solar PV or otherwise, you get a good feeling of what to expect due to the amount of attention paid to the detail. Generally if an installation looks good it is and conversely the same are true in reverse. It must be said though that I don’t think a loose connection or connector not pushed home correctly can be construed as poor installation. As an electrician who prides himself in his work, I must have terminated thousands of cables throughout my years and I struggle to believe that all those terminations have been 100%, regardless of how much I would like to think they are. So don’t be too harsh on your installer if your problem turns out to be a simple loose connection. After all we are only human and we all make mistakes from time to time.
Solar PV Over Voltage Problems
Over voltage can be disastrous to the yield of a PV system. If the supply voltage is close to its upper limits before a PV system is installed then the energising of a PV system will take the voltage even closer to an over voltage state or even over the limit.
Solar PV inverters export energy by raising the potential 2 to 5 Volts higher than that of the grid voltage. In an installation with high incoming voltage, in particular commercial systems where the inverter tolerances are lower under G59 arrangements and supply fluctuation are generally greater; the inverter can trip out on over voltage. Each time this scenario is played out the inverter will take three minutes to re-start. As you can imagine, this can result in huge losses and can go undetected for long periods.
Contact your local District Network Operator (DNO) and make them aware of the problem. They will probably install a set of data loggers on your supply and monitor your voltage to check your claims. If it is found to be high enough to warrant changing then they will correct the problem free of charge. This may take quite a while and is not guaranteed of a free fix.
Solar PV System Under Voltage Problems
Under voltage will result in the same sort of losses you would expect from over-voltage for the same reasons. These faults are rare and if on the supply side of your meter can only be rectified by the DNO and should be reported immediately.
Solar Panel Temperature Related Problems
High panel temperatures in sunny conditions will result in lower performance due to the panel heat coefficients that reduce the efficiency of a panel the hotter it gets. These losses should not be excessive and can easily be calculated using the panel data sheets and the panel temperatures. I would not expect these losses to start a chase for answers to a poor yielding system, particularly in UK conditions. So for the purposes of seeking a problem of poor yielding can largely be disregarded.
Solar PV Inverter Overheating Problems
An inverters temperature, like almost all electronic devices, can be critical to its life expectancy and efficiency. The cooler the conditions they operate in general mean they will operate closer to their maximum life expectancy and more efficiently. Some inverters have a reduced power mode when high ambient temperatures are reached.
If the inverter is located in a hot environment such as an insulated loft space then high ambient temperatures can be reached in summer. The heat given off by the inverter under normal use and higher output in summer months compounds this.
If you suspect your inverter is cutting out due to excessive heat build up then firstly check the area surrounding the inverter is free from obstacles blocking the inverters cooling fan/fins. Check the manufacturer’s literature for clearances around the inverter and that they are met as air needs to be able to circulate around the unit to keep it cool. Check the inverter manufacturer’s literature for the maximum operating temperature and monitor the area where they are located. Periodically check the inverter output and display at high yielding times. If the inverter is in a reduced power mode due to overheating then this will be shown in some way on the inverters display.
Electrical Supply Fault to Solar PV Inverter
An electrical supply fault to the inverter could cause significant losses. If the supply is regularly tripping out then like over and under voltage the system will experience serious losses. This problem may not be related to the Solar PV system but some issue pre-installation which causes a supply to temporary fail such as nuisance tripping of RCD’s and the like. It may be that a faulty fridge is to blame for the consistent tripping of an electrical supply and later on in the same year it is noted that the Solar PV system has recorded poor yields, yet the fridge problem has been long forgotten. These problems can be easily overlooked leaving the contractor no real chance of finding the fault as it no longer exists.
Solar Panel Cell Degradation
This is not something you should be experiencing in the early years of a system but may play out in years to come. A typical solar panels output will degrade at approximately 1% year on year. This can be easily calculated using the age of the system and rate of degradation from the panel manufacturers data sheet.
If by reading this article you still believe there is something wrong with your system that you don’t understand or cannot resolve then please feel free to contact us either by phone 01509 341971
01509 341971, by email info@cgascoigne.co.uk or by using the “contact us” tab on our web site. We are here to help.
Please note the above guide has been formed through my own experiences within the industry and should be read with errors and omissions accepted.