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Shedding New Light on Solar PV Electrical Testing
- Published: Thursday, 01 March 2012 11:40
Jim Wallace, product and technology manager at Seaward Solar answers your questions on solar PV electrical testing
Q. How does Solar PV electrical testing differ from conventional installation and what are the risks involved?
Solar 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 voltage, which is not always commonly used by electricians in their normal 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.
Undetected faults may also develop into a fire hazard over time. 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.
Special measures must therefore be taken during installation of PV systems to eliminate the risks of dangerous working and latest electrical problems.
The consideration of panel operation 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.
Q. Should electrical contractors undergo any specialist training for testing of solar PV systems?
Grid connected PV system installation is quickly becoming a mainstream electrical contracting activity. Alongside MCS accreditation‚ the installation process itself is unlikely to be too difficult for a qualified electrician‚ although there are significant differences from the usual installation wiring technology that they are likely to be working with on a day to day basis.
Q. Are there any additional test requirements above and beyond those outlined in the IEE 17th Edition Wiring Regulations?
In general terms the installation of domestic grid connected PV systems also falls with the scope of Part P of the Building Regulations and it the responsibility of installer to ensure that systems are installed according to the existing BS7671 electrical installation standard – the 17th Edition IEE Wiring Regulations.
However‚ the inspection and testing of DC circuits associated with PV arrays requires special considerations. The IEE Guidance Note 7 Special Locations provides guidance on solar photovoltaic (PV) power systems.
The fundamental requirement for all Installers of PV systems in the UK is to satisfy the MCS requirement as detailed in the DTI’s guide Photovoltaics in Buildings which is aligned to the IEC 62446 standard. IEC 62446: 2009 ‘Grid connected PV systems’, sets out the minimum requirements for PV system documentation, commissioning tests, and inspection.
In short the standard sets out measures to ensure that:
• The PV panels and electrical supply connections are wired up correctly
• That the electrical insulation is good
• There has been no damage to cables during installation
• The protective earth connection is as it should be
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
Q. Are there any guidance notes or documents that you would recommend?
The DTI produce a guide entitled Photovoltaic in Buildings, Guide to the installation of PV systems 2nd edition. This document is freely available on the internet although it is likely to be updated in the near future.
Q. Is there likely to be further legislation concerning solar PV testing?
The Standard IEC62446 is under review and will have modifications in the future. It should be recognised that the PV industry has developed at such a rate that it has moved faster than the Standards making organisations can move and the result is a potential for a continual development of standards.
Q. What are the common mistakes made when testing these systems and how can they be avoided?
Simply because the installation of PV systems may be new to many contractors, there is the potential for an increased number of mistakes being made.
As yet there is no documented evidence detailing the mistakes made in the installation of UK PV systems. However surveys conducted in France and Australia have suggested that up to 60% of all installations have one or more defects.
As with any new process, training is absolutely critical to ensure effective inspection and testing which is the ultimate control for quality of an installation.
Q. What are the key warning signs that a system may not be performing as it should?
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 were installation has been provided free of charge in return for receipt of the FIT payments.
Although metering will always give an indication of system performance, effective electrical testing is also vital not only to prove the safe installation of a new system but also to verify ongoing functional performance.
Q. Is periodic inspection and testing necessary? If so, why?
The installation of PV system by householders is clearly only undertaken after careful consideration of the costs involved and potential return on investment provided by lower energy bills and FIT payments.
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.
Q. What equipment is required for testing a solar PV installation?
The absolute minimum testing that needs to be undertaken involves continuity measurements, open circuit voltage, short circuit current, insulation and irradiance.
Other tests involving the use of I-V curve tracers, power analysers and thermal imaging cameras are not mandatory but may be regarded as useful to carry out certain diagnostic testing or to assess different performance parameters of the solar PV system
It is therefore up to the installer to decide whether he wishes to purchase individual items of equipment or select one or possibly two instruments which provide a combination of tests to enable measurements to be taken in a fast, safe and efficient fashion.
Q. Is there a good selection of test equipment on the market?
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.
This sort of consideration has led to the introduction of a new generation of integrated testers capable of performing all of the tests required by IEC 62446.
The Seaward PV100 is the first dedicated multi-function electrical tester of its type and is capable of carrying out all electrical tests required by IEC 62446 on grid connected PV systems.
With the push of a single button the new combination tester carries out the required sequence of electrical tests in a safe and controlled manner, avoiding the risk of contact with exposed live DC conductors.
Q. What are the key considerations that should be addressed prior to selecting a piece of test equipment?
There are many instruments available on the market that are sold under the title of Solar Testing so it is vital to ensure that the instruments selected are capable of performing all of the tests required by the MCS.
The nature of PV testing is such that it can expose the installer to high voltages, so the selection of an instrument which is capable of automatically performing tests, greatly improves efficiency and safeguards the installer.
The availability of new multi-purpose solar PV test instrumentation also means that the functions of a number of individual test instruments can be combined in a single tool – with consequent savings in cost and improved practical considerations.
Q. Are manufacturers also able to provide test equipment for other renewable technologies such as heat pumps and solar thermal systems?
Certain instruments designed for the Solar PV market also have a place in other renewable technologies. For example, some irradiance meters can be used by those working with photovoltaic systems and also solar thermal heating systems.
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