ABB's UniGear ZS1 range was launched in 2004, claimed to be the world's first ‘one size fits all' platform for primary MV (medium voltage) air-insulated switchgear (AIS) in the 12 to 24 kV range. Since then, we have seen a significant change in the UK market. Now, instead of the traditional withdrawable circuit breaker panel, a growing number of customers are calling for the simplicity, lower cost and smaller installation footprint offered by a fixed circuit breaker panel. Malcolm Cork of ABB outlines the testing programme behind the company's new fixed circuit breaker panel
ABB's response was the launch, in 2008, of the new UniGear 500 R fitted with the Vmax/F vacuum circuit breaker. It is just 500 mm wide - a significant space saving compared with the standard 650 mm panel, especially in typical applications of banks of 10 or more panels. It is ideally suited for various market segments requiring containerized solutions. Before it could be brought to market it had to undergo a rigorous type testing programme to ensure compliance with the relevant specifications. In particular we had to meet the Energy Networks Association Technical Specification ENATS 41-36 which covers distribution switchgear up to 36kV for the utilities. This brings together IEC 60694, IEC 62271-200 and 62271-100 with enhancements, clarifications and additional testing to meet the requirements of UK DNOs (Distribution Network Operators).
The starting point for the test programme was IEC 62271-200, High voltage switchgear and controlgear - Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV. Issued in November 2003, this international standard was a further development of the previous standard IEC 60298 of 1990. A key aim of the new standard was to focus more on functional characteristics than on design and construction. From February 2007, all new metal-enclosed switchgear must comply with IEC 62271-200 while pre-existing switchgear can continue in operation to IEC 60298.
The new standard sets a number of new requirements: Firming up of test conditions for the switching devices (making and breaking capacity);Changed sequence for dielectric testing; Introduction of new partition classes; Introduction of internal arc classified (IAC) qualifications
ÏThe type test programme simulated situations which occur very rarely in real-life. For example, a short-circuit at the maximum current level for which the installation has been designed is rather unrealistic because of the presence of current-limiting components (such as the cables) and because the power available is normally lower than the rated one.
Short-time and peak withstand current
This test showed that the main power and the earthing circuits can resist the stresses caused by the passage of the short-circuit current without any damage.
The temperature rise test was carried out at the rated current value of the switchgear unit and shows that the temperature does not become excessive inside of it. During the test, both the switchgear and the apparatus it might be fitted with was checked (circuit-breakers, contactors and switch-disconnectors).
These tests checked the switchgear has sufficient capability to withstand the lightning impulse and the power frequency voltage. The power frequency withstand voltage test is carried out as a type test, but is also routine on every switchgear unit manufactured.
Apparatus making and breaking capacity
All the apparatus (circuit-breakers, contactors and switch-disconnectors) was subjected to the rated current and short-circuit current breaking tests. Furthermore, they were also subjected to the opening and closing of capacitive and inductive loads, capacitor banks and cable lines.
Earthing switch making capacity
The earthing switch of the UniGear 500 R can be closed under short-circuit. In actual fact, the earthing switch is normally interlocked to avoid being operated on circuits which are still live. However, should this happen for any one of several reasons, safety of the personnel operating the installation would be fully safeguarded.
The mechanical life tests on all the operating parts highlight the reliability of the apparatus. General experience shows mechanical faults are one of the most common causes of a fault in an installation. The switchgear and apparatus it contains have been tested by carrying out a high number of operations - higher than those which are normally carried out in installations in service. Moreover, the switchgear components are part of a quality programme and are regularly sampled from the production lines and subjected to mechanical life tests to verify that the quality is identical to that of the components subjected to the type tests.
When developing any type of electrical equipment, personnel safety must take first place. So the equipment should be designed and tested to withstand an internal arc due to a short-circuit current of the same level as the maximum short-time withstand level.
The tests showed the metal housing is able to protect personnel working near the switchgear in the case of a fault which evolves as far as striking an internal arc.
An internal arc is among the most unlikely of faults, although it can theoretically be caused by various factors, such as:
- Insulation defects due to deterioration of the components such as caused by environmental conditions and pollution.
- Overvoltages of atmospheric origin or generated by operation of a component.
- Incorrect operations due to not following procedures or inadequate training.
- Breakage or tampering of the safety interlocks.
- Overheating of the contact area, due to the presence of corrosive agents or when the
connections are not sufficiently tightened.
- Entry of small animals in the switchgear.
- Material left behind inside the switchgear during maintenance operations.
Careful design can significantly reduce the possibility of these incidents but not all of them can be eliminated completely.
The energy produced by the internal arc causes the following phenomena:
- Increase in the internal pressure.
- Increase in temperature.
- Visual and acoustic effects.
- Mechanical stresses on the switchgear structure.
- Melting, decomposition and evaporation of materials.
Unless suitably controlled, these can have very serious consequences for the operators, such as physical harm (due to the shock wave, flying parts and the doors opening) and burns (due to emission of hot gases).
The tests checked the compartment doors remain closed and that no components are ejected from the switchgear even when subjected to very high pressures, and that no flames or incandescent gases escaped, thereby ensuring the physical integrity of the personnel operating near the switchgear. It also checked that no holes were produced in the external freely accessible parts of the housing and finally, that all the connections to the earthing circuit remained intact, guaranteeing the safety of personnel who may access the switchgear after the fault.
Typical areas where the international standards are enhanced by the ENA Technical Specifications relate to operational procedures, interlocking and consequently operator health and safety. ENA specifications also consider quality procedures, low voltage controls and auxiliary component requirements.
A good example of where ENA specifications have impacted on design is in how the exhaust gases are relieved. The standard UniGear 500 R is internal arc classified IAC AFLR up to 25kA x 1 second according to the IEC 62271-200 Annex A, with exhaust gas relief through the top via the main gas duct channel. The ENATS 41-36 version is classified IAC AFL up to 25kA x 1 second but with exhaust gas relief from the rear.
ENA approval lasts for three years, but it is very much a dynamic process that enables manufacturers to receive direct feedback of field experience.. For example, there have recently been reports of arc tracking occurring on fuse clips on equipment that is now over 30 years old. So we are looking at new designs to prevent this.
Following ENA approval, the equipment has already been installed in a number of applications that require a compact, space-saving, low-maintenance solution such as data centres and wind farms. The fixed circuit breaker can be replaced in less than 90 minutes. But there are some installations that will require the higher level of availability and ease of maintenance made possible by a withdrawable circuit breaker, especially on crucial circuits. The advantage of the UniGear 500 is it coordinates with the complete UniGear ZS1 portfolio. This makes it possible to specify on the same busbar, a UniGear ZS1 with withdrawable incomer, a fixed circuit breaker outgoing and additional starter switchgear.
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