Testing as the key to safe and efficient solar PV
Daylight falling on a solar PV panel will inevitably produce electricity, which is just one reason why those involved with solar panels must be aware of installation and safety standards, which are also crucial for system performance‚ Jim Wallace guides you through the subject.
Despite the recent uncertainty over changes in the Feed-in Tariff, there remains considerable optimism for future of the solar PV sector. Grid-connected solar PV system installation has already largely become a mainstream activity for the building-services and electrical-contracting industries, and this has been supported with the introduction of new solar PV test equipment onto the market.
Although the system installation process itself is unlikely to be too difficult for a qualified electrician‚ 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.
Another important point is that PV panels generate DC, which is not always commonly used by electricians in their work. In addition‚ 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 hazard for an extended period but also wasting energy generated by the PV system and potentially developing into a fire hazard.
Without fuse protection against such faults, elimination of a fire risk can only be achieved by both good system design‚ and careful installation with appropriate inspection and testing.
PV arrays produce direct voltage when exposed to sunlight. In the wiring system associated with PV panel installation‚ the DC generated by the solar array is converted to AC by an inverter which then feeds into the AC mains supply of a building. It follows that electrical work on PV panel installation involves working on both the DC and AC sides of the circuit.
From the outset, therefore, the designer and installer of a PV system must consider the potential hazards carefully and systematically devise methods to minimise the risks.
Between them‚ the various installation requirements for PV installations are designed to ensure the electrical safety of the system and installation personnel as well as the verification of performance and power output of the system.
In general terms the installation of domestic grid-connected PV systems falls within the scope of Part P of the Building Regulations. and it is the responsibility of installer to ensure that systems are installed according to the existing BS 7671 electrical installation standard — the 17th Edition IEE Wiring Regulations.
However‚ the inspection and testing of DC circuits associated with PV arrays also requires some special considerations, as included in IEE Guidance Note 7 Special Locations on solar photo-voltaic power systems.
|All the electrical tests required for a solar PV installation can be carried out using this pre-programmed instrument.|
Also required is completion of Engineering Recommendation G83/1 which is the installation commissioning confirmation form for the connection of small-scale embedded generators‚ such as PV arrays‚ of up to 16 A per phase with public low-voltage distribution networks.
‘IEC 62446: 2009 Grid connected PV systems — minimum requirements for system documentation, commissioning tests, and inspection’ specifies the minimum requirements for solar-system documentation‚ commissioning tests and inspections.
Building or electrical works in the vicinity of a PV array are also likely, and the ownership of a system may change. As a result‚ the standard recognises that only by the provision of adequate documentation at the outset can the long-term performance and safety of the PV system be ensured.
The standard therefore sets out the information and documentation that should be provided to the customer following the installation of a solar-panel system and also the initial (and periodic) electrical inspection and testing required.
In short the standard sets out measures to ensure that:
• the PV panels and electrical supply connections have been wired up correctly;
• the electrical insulation is good;
• the protective earth connection is as it should be;
• there has been no damage to cables during installation.
IEC62446 also requires inverter details to be recorded and MCS (Microgeneration Certification Scheme) requires that records are kept.
The installation of PV systems by property owners is clearly only undertaken after careful consideration of the costs involved and potential return on investment provided by lower energy bills and the Feed-in Tariff.
As a result, the verification of system performance and energy output from the panels is particularly important and a major reason why periodic verification and testing of the system can also be very important — as well as being essential to comply with warranty and PV system guarantees.
In many cases, simple electrical faults or wiring failures can cause a serious inefficiency in the ability of the panel to produce power.
In such circumstances proper metering will give an indication of system performance, but effective periodic electrical testing is vital — not only to prove the safe installation of a new system but also to verify ongoing functional performance over extended periods.
In terms of test instrumentation‚ different PV system electrical tests currently require the use of different testers — typically including a tester for earth continuity and insulation resistance‚ a multimeter and DC clampmeter.
However, the danger with such ‘homemade kits’ is that not all the tests required by IEC 62446 will be covered and, with different PV system electrical tests potentially requiring the use of different testers, using such an array of instruments can be cumbersome and time consuming.
Such considerations have led to the introduction of a new generation of dedicated multi-function solar PV electrical testers, such as the Seaward Solar PV100, that are capable of carrying out all electrical tests required by IEC 62446 on grid-connected PV systems.
With the push of a single button, the combination tester automatically carries out the required sequence of electrical tests in a safe and controlled manner. Testing can be conducted quickly and easily, with the PV100 being pre-programmed to run a test sequence of required tests, using specially designed PV test leads which quickly connect and disconnect from the installation circuit.
For complete MCS compliance, alongside electrical testing, an irradiance meter is also required to measure of how much solar power is available at any particular location.
The most accurate solar readings are produced from irradiance meters with sensors that are similar to the technology utilised in the panels themselves. The ideal solution is to utilise an irradiance meter with a photovoltaic cell as its sensor rather than a PIN diode, which will not necessarily have the same response to sunlight as the PV cell itself.
Jim Wallace is product and technology manager with Seaward Solar.