Effective insulation of key components at a PV plant can mean lower losses and fewer failures in the future. Insulation testing can help PV plant owners to determine the quality of the installation and therefore whether the plant faces potential problems.
Partial Discharge (‘PD’) testing is one of the most effective ways of identifying potential insulation breakdowns in MV and HV equipment at a solar farm – HV switchgear, HV cables and transformers can all be subjected to this type of testing. Insulation breakdown can occur when the insulator, be it solid, liquid or gas, is consistently exposed to a high level of electrical stress over a period of time forcing small, localised electrical discharges within the structure of the insulating material.
Insulation testing can help PV plant owners to determine the quality of their installation
Various factors can determine whether PD will occur in a given piece of equipment, such as the manufacturing process of the materials, its installation, the operating conditions or simply imperfection in the structure of the material. Serious consequences can result from PD as small amounts of inner discharge could lead to voids in the material over time and create more severe and abrupt degradation of the insulating material leading to an eventual failure of the whole component. A failure of the component can in turn lead to potential risks to people operating the equipment as well as the surrounding equipment. Insulation breakdown is in fact the most common cause of major failures in HV and MV equipment, therefore it is critical to not only maintain the operating conditions of the assets in line with good industry practice but also diagnose their status regularly.
So how does PD testing work? PD can be detected through the emission of electromagnetic radiation (ranging from visible light to radio waves), acoustic (from sonic to ultrasonic) and chemical reactions caused by the release of nitrous oxide gases. There are several ways to trace PD but the method that is the safest whilst not compromising the accuracy of the results is the use of UltraTEV testers, which QE’s operation managers are equipped and trained to use.
UltraTEV testers use two core techniques to identify PD within the component being tested. The testers can both ‘hear’ the ultrasonic radiation of surface PD and can also detect the transient earth voltage in the inner parts of the components. Training is critical to be able to differentiate the correct audio indicators of the testers, and where there are any ‘grey’ areas, QE personnel are equipped with an advanced tester which shows the values of sound wave intensity and frequency thus allowing the diagnosis to be narrowed down. QE can offer key PD testing services on critical HV assets.
Electrical installations around the world are wired using a standard conductor like copper or aluminium which create an easy physical path for a current to traverse. As an exposed conductor under load would pose a serious threat to the system, its components and surrounding users, conductors are covered with an insulator. Insulation resistance – known as ‘Riso’ and represented in ‘ohms’ [Ω] – is a measure of an insulator’s ability to contain the electrical energy within the conductor and to prevent leakage.
Whilst in an ideal world, the sheath of an insulator covering the conductor would not conduct any electricity, the reality is that there will always be an amount of current leaking out into the environment. The level of insulation resistance of a conductor largely depends on the material and geometry (shape) of the insulation, which in practice is often reflected by its thickness. Therefore, cables with thicker or better-quality insulation would be less prone to wear and damage and hence retain their high electrical resistance.
Factors that contribute to insulation breakdown typically include rodent activity, mechanical wear due to friction, poor quality of installation, overheating of elements due to issues with the circuit or exposure and other circumstances that reduce insulation properties such as exposure to chemical elements. Once an insulator starts to degrade, a parallel short circuit path may form, due to an exposed conductor making contact with another, which may lead to arcing – this poses a health and safety risk and a risk of damage to equipment. As these failures will happen regardless of the quality of the purchased components, the only way to prevent them is by monitoring the integrity of the circuit via Riso testing.
String insulation resistance testing allows any potential insulation degradation to be analysed at string level thus covering string cables and MC4 connectors which are the most exposed and commonly affected components. These tests can also assure the quality of the circuit and the installed components, whilst identifying poor quality cable if the values are consistently low.
This testing can be a very useful tool to verify if the site has been constructed using good quality materials and if the workmanship has been undertaken in line with good industry practice – a common obligation under EPC contracts – and therefore testing this can be a useful exercise prior to Final Acceptance. String insulation resistance tests require the isolation of the combiner box or string inverter in question, the disconnection of the string cable from the arrays and then the testing of the string in reference to the required value and standard. QE offers string insulation resistance testing in compliance with accepted standards, including BS EN 62446-1 2016. QE’s operation managers are specifically trained and work with high quality equipment.
QE’s operation managers are trained to undertake Partial Discharge testing and String Insulation resistance testing, both of which can identify worrying insulation breakdowns. If you would like to know more about these services, then please contact our Head of Engineering, Irene Reyes, via the enquiries section of our website.
This article is written and edited by Shirine Azzi. She can be contacted at: firstname.lastname@example.org