Jim Wallace of Seaward Solar explains the importance of PV solar panel testing
The installation process of photovoltaic (PV) panels is one that shouldn’t pose too many problems for a qualified electrician. However, there are some significant differences from installing the usual wiring technology that is used on a day to day basis.
PV arrays produce a DC voltage when exposed to sunlight. In the wiring system associated with panel installation‚ the DC current generated by the solar array is converted to AC by means of an inverter which then feeds into the AC mains supply of a building.
The safe operation of the panels under both normal and fault conditions is essential in the design stage to ensure the required level of safety. It is then important to ensure that the long term safety of the system is not compromised by a poor installation or subsequent poor maintenance.
Much of this comes down to the quality of the installation and the system inspection and testing regime.
PV systems are unusual in that the energy source cannot be switched off. If there is daylight falling on a PV panel it will produce electricity and it is possible for a relatively small array of only a few panels to deliver a lethal shock.
Because of the current limiting properties of PV cells‚ they are incapable of producing sufficient fault currents to operate over-current protection devices such as fuses. Once established, a fault may remain undetected, not only posing a fire hazard but also wasting energy generated by the PV system.
Therefore, special measures must be taken during installation of PV systems to eliminate the risks of dangerous working and latent electrical problems.
These include completing the DC wiring before connection is made to the panels and avoiding working with both positive and negative conductors simultaneously. This will allow the effective isolation of the DC system (via a DC isolating switch and module cable connectors) while the array is installed, and the effective isolation of the PV array while the inverter is installed.
The requirement is that grid connected PV solar systems are tested according to 17th Edition Electrical Wiring Regulations but there are also additional requirements. For example‚ the testing of DC circuits in PV arrays is also covered in IEE Guidance Note 7 for special locations.
Engineering Recommendation G83/1 is the installation commissioning confirmation form for the connection of Small Scale Embedded Generators‚ such as PV arrays‚ of up to 16A per phase with public low voltage distribution networks. Installers are required to complete G83/1 with information on various tests‚ system details and a range of supporting information to satisfy the requirements of the Distribution Network Operator.
In addition, compliance with IEC 62446: 2009 – Grid connected PV systems, is needed as this sets out the minimum requirements for system documentation, commissioning tests and inspection.
In short the standard sets out measures to ensure that the PV panels and electrical supply connections have been wired up correctly, that the electrical insulation is good, the protective earth connection is as it should be, and that there has been no damage to cables during installation.
Under electrical tests the standard sets out specific requirements for earth continuity of array frame to earth and connection to main earthing terminal, polarity of all DC cables, PV string open circuit voltage test, PV string short circuit current test, PV array insulation test, operational test – PV string current, functional test, and irradiance.
IEC62446 also requires inverter details to be recorded and reported‚ with the Microgeneration Certification Scheme (MCS) also requiring that records are kept.
Between them‚ the various installation requirements for PV systems are designed to ensure the electrical safety of the installation‚ the electrical safety of installation personnel and the verification of performance/power output of the system.
In terms of test instrumentation‚ different PV electrical tests currently require the use of different testers – typically including an earth continuity and insulation resistance tester‚ a multimeter and DC clampmeter.
Using such an array of instruments can be cumbersome and time consuming – considerations which have led to the introduction of a new generation of integrated testers capable of performing all of the tests required by IEC 62446.
The installation of PV systems is clearly only undertaken after consideration of the costs involved and potential return on investment provided by lower energy bills and FiTs payments. The verification of system performance and energy output from the panels is therefore particularly important and a major reason why periodic verification and testing of the system is essential.
In many cases, simple electrical faults or wiring failures can cause a serious inefficiency in the ability of the panel to produce power. This is particularly important for installers working on ‘roof rental’ schemes where installation has been provided free of charge in return for receipt of the FiTs payments.
In such circumstances proper metering will always give an indication of system performance but effective electrical testing is vital not only to prove the safety of a solar system, but also to determine ongoing functional performance over extended periods.