Components of the grid have to undergo type testing for the benefit of the utilities who use them and their manufacturers alike. Energy consultancy KEMA of the Netherlands describes how its facilities independently verify the performance of such equipment.
P.G.A. Bus and S.A.M. Verhoeven, KEMA, The Netherlands
|The main hall at KEMA’s High-Voltage Laboratory in Arnhem, the Netherlands|
Societies today depend on their electricity supplies. Countries worldwide strive to connect their citizens to this source of energy to give them the basis for a certain quality of life. While for those people and businesses who are already connected to an electricity grid, maintaining the quality and availability of their power supply is vital to maintaining the quality of their lives and the health of their economies. It is T&D grids which transport this electrical power from generating plants in remote locations to end-users.
The reliability and availability of these T&D networks depend greatly on the reliable and safe operation of the components in them, such as circuit breakers, transformers, load-break switches, connectors, disconnectors, insulators, reactors, surge arrestors, cables and cable joints.
The companies which make these components operate in an international T&D market in which their aim is to export their products to countries across the continents. Utilities have to construct reliable power networks at the lowest possible cost and will come across components made by unknown manufacturers, so utilities face having to determine whether these products meet minimum standards.
For an unbiased report on the technical performance of these components it is essential that both manufacturers and utilities can rely on test reports from a fully independent testing company, one that both parties fully trust. KEMA’s facilities in the Netherlands can service its customers in all five of the categories of test developed by technical institute Short-circuit Testing Liaison (STL, see box on page 36).
While the STL guidelines and the standards created by institutes such as the IEC and ANSI (see box) form a basis for consistent and stringent testing of T&D components, they form only one of the three pillars on which quality verification of the performance of components stands.
The other two are:
- A suitable installation which can generate enough power for, say, short-circuit testing – which can be of the order of gigawatts – and the required voltage levels of the kind at which T&D components operate, which can be as high as 1100 kV.
- Competent and experienced people who have extensive knowledge of the test objects, who intimately know the capabilities of their test installations and who can calculate and set up the required testing circuit and installation.
KEMA has based its short-circuit laboratory and its recently built high-voltage laboratory on its 80-plus years of experience and development of testing installations.
The high-power laboratory is the most powerful of its kind in the world. It performs short-circuit and switching performance tests and uses four short-circuit generators with a total output of up to 10 GVA at 60 Hz.
The lab includes a highly sophisticated and reliable installation for the synthetic testing of ultra-high voltage circuit breakers. It upgraded this installation in 2008 to be able to test equipment with rated voltages of up to 1200 kV. The installation employs single and three-phase synthetic testing, as required by circuit breaker standards.
KEMA opened the lab in October 2009 after the old facility had become too small to allow the organization to take a modern approach to testing. The new lab measures 80 m in length by 50 m in width by 20 m in height and includes three production areas for medium, high and ultra-high voltage testing.
This new laboratory uses a novel approach to high-voltage testing. While the classic approach involves the assembly of component to be tested between test transformers that are rooted to one location in a test hall, it results in low utilization of the test transformers, only 30 per cent. The new approach has been to make all test transformers and other test equipment transportable.
Now the component is housed in an empty test bay. While the component is being prepared the required test transformer is brought to it.
On completion, the test equipment moves to another bay. This approach has doubled the utilization of the test transformers and has a far-reaching effect on how tests must be organized and performed. Two key elements in this approach are the presence of many test bays, which can operate independently, and the guarantee that transportation to and from the test bays does not intervene with other testing activities.
The scope of the high-voltage laboratory is to test all T&D components. It is imperative the laboratory can easily be transformed to meet the specific needs of the testing of cables, transformers, insulators, MV panels, circuit breakers and other equipment. This, combined with the lab’s novel approach, leads to the need for flexible test bays. Indeed the whole of the test facility is flexible.
Everything from test transformers, regulators, fencing, and control and safety systems are mobile and can be changed easily. The driving theme behind KEMA’s novel approach can be surmised in the following phrase: “Any test on any component can be made at any spot in the laboratory in a safe and efficient way.”
The building acts as a Faraday cage and is made of a steel structure with standard metal cladding. During the specification phase of the building we utilized standard building materials. The result is a radio frequency environment of extremely low noise that is suitable for the measurement of partial discharges as low as 1 pC.
Flex Power Grid Laboratory
The Flex Power Grid Laboratory is a separate laboratory that can create a multitude of electrical circuit configurations for delivering and absorbing power.
It can supply industrial medium voltages – those of up to 24 kV – at a continuous high-power rating of 1 MVA at the same time as creating complex and realistic conditions that mimic a grid of poor power quality in which the electricity frequencies range from DC up to 75 Hz AC in four-quadrant operation. Also, distortion is pre-programmable and can comprise harmonics of up to 2.4 kHz or dynamic network phenomena.
This laboratory tests high-power electronics, such as converters for batteries, PV-systems and active harmonic filters. These converters are used in low- and medium-voltage networks for embedded generation and as a side effect improve the power quality of networks.
The software and hardware in these converters determine how well they work in real life. The power network is very dynamic, especially when the penetration of embedded generation high. The laboratory can test how well a converter works under realistic conditions.
The need for testing
The evaluation of the results of 16 years of type testing at KEMA’s High Voltage Laboratory shows that 20-50 per cent of all type tests on cables and cable accessories result in a change in design or scrapping of the type test.
These results show manufacturers the necessity for thorough testing of new designs of cables and accessories. For the users of cable systems, these results indicate the value of buying type-tested components or even type-tested systems.
|Figure 1: Distribution tests per component|
Although improvements in materials and production techniques continue they are not causing failure rates to decrease noticeably. Figures 1, 2 and 3 on power transformers show the need for continued testing of new T&D components. They show statistics that have been collected since 1996.
|Figure 2: Failure rate per component|
Figure 3 breaks down transformers according to their MVA ratings. It shows that the highest initial failure rate, close to 50 per cent, is among the largest transformers. Much effort goes into testing the performance and safety of components of the kind that can be used in a reliable T&D network, effort that gives consumers of electric energy an undisturbed power supply of quality.
|Figure 3: Transformer tests per power rating|
Tests carried out over the years on T&D equipment show a definite need for independent verification of the performance of components. Standards, which define minimum requirements, play an important role here, as do independent laboratories.
These laboratories must have the facilities, the competence and the reputation to convince utilities and manufacturers that equipment meets their requirements.
International testing standards
International standards committees from institutes such as the IEC and ANSI have created standards by which to test the behaviour of T&D network equipment under normal and extreme conditions.
The test procedures that these standards define reflect real-life scenarios, such as short circuits in overhead-lines, lightning and impulse discharges, switching operations and the extreme environmental or mechanical stresses that T&D components should be able to withstand.
All stakeholders are involved in defining the tests, which prove whether T&D components meet required performance and safety characteristics. The standards committees employ experts from manufacturers, electricity utilities, testing institutes and universities, and end-users of electric energy to arrive at realistic standards.
Short-circuit Testing Liaison
On top of the standards for testing that institutes such as the IEC and ANSI have created are the guidelines by Short-circuit Testing Liaison (STL). These guidelines are practical and ensure that the interpretation of these standards is uniform across the world. STL consists of testing laboratories and testing organizations and is a purely technical institute. It creates its guidelines by relying on the competence and expertise of its members. This voluntary society also defines test report templates to ensure that end users can easily compare results from different laboratories. The society also defines rules of conduct to assure the quality of the results.
STL has created five categories of test. These verify:
- the switching performance of switchgear
- the short-circuit performance of T&D apparatus
- dielectric performance
- temperature performance
- mechanical performance.