Distribution Systems:Safety is the key with switchgear technology

ABB has completed the development of AX1, a new medium voltage switchgear product. The technology has unique construction characteristics and incorporates several design concepts.

Maria Jansson, ABB Power Technologies, Ludvika, Sweden.

The new air insulated medium voltage switchgear from ABB, AX1, is based on a safety philosophy that keeps the complete high voltage (HV) space for several incoming and feeder cubicles as one common metal enclosure, with no access to man under service conditions. In order to enhance safety against human hazard and to reduce the restoration time in case of failures, a device called the “Arc Eliminator” has been developed, type tested and applied. Other innovative features like complete monitoring and joints with coil spring contacts, also increase the safety of AX1.

The AX1 production is concentrated in Ludvika, which has the world’s biggest site for high voltage apparatus production, which makes it ideal for further development of AX1. The factory for AX1 is an integrated factory where production and office are next to each other. This allows quick, and reliable information exchange between administration and production.

Small spaces, high security

Thanks to its high safety and small size, installation of AX1 panels is perfect for small areas that require very high security. Due to AX1’s active arc protection, dangerous pressure increases never have a chance to build up, and harmful arc gases cannot arise if an internal open arc occurs within the switchgear. Therefore a decompression area for fire and gas is not needed, because the extremely fast arc eliminator in AX1 prevents any possible breakdown caused by damages or loss. Another feature that minimizes AX1 space requirements is its sliding doors which do not take up any space in the operating aisle.


AX1 is perfect for small places that need high security like in a mine
Click here to enlarge image

null

Tegner, in the city of Vàƒ¤steràƒ¥s, Sweden, is a good example of how AX1 can be placed in very small spaces. In the basement of the building, there are several installations of AX1 switchgear. One of these consists of 6+1 AX1 panels placed in a cubbyhole no bigger than 6 m2. Due to the lack of space, the support panel had to be placed on the wall opposite the AX1 panels. This example demonstrates the advantage of AX1 in small areas, i.e. its compact construction and the fact that no decompression area is needed.

Another example of AX1’s unique features is demonstrated at an installation in the town of Malmberget, north Sweden. The installation is located 1000 m below surface level in an environment demanding for both mining workers and for mechanical and electrical equipment involved in the ore mining. In a mine, there is a need for continuous drilling further and further into the rock. Sooner or later, the cable run becomes too long, and the switchgear has to be moved. This is very costly and causes disturbances. A mobile solution is therefore needed, enabling the switchgear to be moved quickly and easily in coordination with the mining process. It is here that the compact design proves useful. Since the cubicle door of AX1 is pushed down vertically when opened, no additional space is needed in front of the panels, and demands for maneuvering space are thus fulfilled. Another crucial feature of AX1 is the arc eliminator, which makes a pressure release system unnecessary, a very important thing for the safety of the workers in a mine.

Other applications

The compact design also makes AX1 ideal on oil and gas platforms. Furthermore, the AX1 panel is largely made of aluminium, making it light. Equipment used for marine applications is exposed to special stresses that do not occur with normal use on land, such as powerful wave motions, vibrations and corrosion due to a salt-laden atmosphere. To ensure trouble-free operation even under the toughest conditions, AX1 is rigorously tested and in compliance with IEC and UL demands.


Its compact design makes AX1 the market’s smallest switchgear in its class
Click here to enlarge image

null

Installations of AX1 can be found in London’s Canary Wharf in the Docklands. In the 42-storey banking office for HSBC, three AX1 switchgears are installed. Two are located at the bottom of the building, each holding 22 AX1 panels. The third one, having eight panels, is placed on the seventh floor. This is not a problem since the arc eliminators mean there is no need for any pressure relief system for evacuating the overpressure and gases out from the building.

AX1 design

Due to the tubular bus bar design, the panel dimensions have shrunk dramatically. Currently, AX1 has the market’s smallest size compared with other equivalently rated switchgear. Its small size means that AX1 can be easily integrated into a 7.3 m standard container. Apart from saving space, this also simplifies installation since the switchgear is fully assembled in the container on delivery. The only on-site activity remaining is to connect the power cables.

AX1’s tubular bus bars ensure an optimum and robust panel design. There are uniform electric fields around the bus bars, making it possible to adopt short phase-to-phase clearances. Earth potential always exists between the phases; which minimizes the risk of current creepage. Insulation barriers are not needed between phases, or between phases and earth, and the busbars do not need to be insulated either. Furthermore, the bus bars with their unique coil spring contacts, simplify joining the panels together.

It is the tubular busbars in AX1 that has made it possible to use a connection technique with helicon springs, entirely without bolts. Coil spring contacts are used in all connection points in the main circuit, which can be disassembled in the busbar system between the panels, in the upper and lower connection of the switching device and in the cable connection device. The coil spring contact technique is also used in movable current transmission points inside the switching devices. However this technique is not unique for AX1. It is a technique based on a proven ABB patent and has been applied in high-voltage circuit-breakers and in GIS switchgear during the last 20 years.

The new measurement technique used in AX1 implies that current is measured by Rogowski coils. A Rogowski coil is an annular coil wound on a body of non-magnetic material, which is temperature-stable. A Rogowski coil can measure currents of a few amperes up to short-circuit current. This also makes it possible to accurately measure high fault currents as well as to have a simple way of checking the circuit breaker’s contact wear. Each sensor is jointly tested and the error, given in the form of a correction factor, is compensated for in the panel computer. Consequently the measurement accuracy is high.

Reliability


In this basement of the building Tegner in Vàƒ¤steràƒ¥s, Sweden, there is an AX1 installation of 6+1 panels in a cubbyhole no bigger than 6 m2
Click here to enlarge image

While it is impossible to put a value on the safety of switchgear in economic terms, reliability on the other hand has great economic importance, since an unplanned breakdown can cause huge costs or lost revenue.

Most errors occur in the cable connections usually due to incorrect assembly. In AX1, prefabricated cable connections outer cone type or plug-in type are used. Sources of error are mechanical parts in the operating devices. That is why, in AX1, the circuit breaker and the disconnector/earthing functions are integrated and permanently mounted in the main circuit enclosure. Furthermore, the operating devices can be removed without the panel having to be taken out of service. Another, less frequent error, is overheating in busbar connections. Since AX1 has a tubular busbar system with non-screw connection technique (the coil spring contacts) there is no risk of overheating due to insufficient tightening torque. The coil spring contacts are well protected with O-rings and rubber sealing glands to prevent influence from the outer environment.

The most serious error in switchgear is an internal arc fault. The extremely fast arc eliminator in AX1, means damages caused by arc fault are prevented. This in turn, makes it possible to immediately, after inspection, take the switchgear back into service again. Conventional switchgear takes weeks to be able to operate again after an arc fault.

Since the losses during operation are minimized, this provides large energy saving over the switchgear’s lifetime, which also reduces the environmental impact. Furthermore, recyclable material has been used throughout the design.

The arc eliminator

The arc eliminator is released by the integrated ABB optical arc guard detection system. Three-phase contacts close simultaneously in less than five milliseconds, measured from light pulse to contact. The pressure rise will be less than 50 per cent of what the maximum value would have been without an arc eliminator.

If an open arc occurs within the switchgear, all three phases are earthed so quickly that a dangerous pressure increase does not have time to build up, and hot and poisonous arc gases do not have time to form. The switchgear will consequently be subject to insignificant damage, and a person standing in the front of the switchgear will not be exposed to any risk of injury. After the cause of the fault has been established and suitable measures have been taken, operation of the switchgear can be immediately restored without anything needing to be repaired.


Ludvika has the world’s biggest site for high voltage apparatus production – ideal for AX1 development
Click here to enlarge image

Recently in Thailand, there was a live fault case where the arc eliminator saved the switchgear. It reacted like an airbag system in a car, released and prevented the switchgear from substantial damage. Later, after checking the actual fault location, it was completely clean and intact.

Self maintenance

AX1 is called “the intelligent switchgear”. Its intelligence can be found both in the panel computers, and in several supervision systems reporting failures. There is one included in every AX1 panel and each is served by a range of sensors that gather a variety of useful information. AX1 does not need traditional periodic inspection, since it is fully supervised by its own panel computers. They keep a watchful eye on the status of the switchgear and warn in plenty of time if inspection or service is required.

No posts to display