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It is milliseconds that make a difference

Jun 02, 2017

An internal arc fault is rare, but it is the most severe fault that can occur within a switchgear system.

As the demand for primary energy is growing, so are the technical requirements and economical challenges in the power distribution networks. One of the key challenges for our customers is to ensure reliable and efficient protection of the electrical infrastructure. This needs to be achieved with a low operational cost and without jeopardizing safety standards for the operating personnel, which is of the utmost importance.

Have you ever wondered how disastrous an internal arc fault in low- or medium-voltage switchgear (e.g. in not internal arc classified / arc resistant switchgear) can be?

An internal arc fault is rare, but it is the most severe fault within a switchgear system. The destructive impact of such an arc fault event can lead to severe injuries or even fatalities of operating personnel through to costly damages of the switchgear and long outages of the switchgear/power system. Relative to the loss of a human life or a severe injury, the possible subsequent investigations by the authorities or financial implications due to supply interruption as well as equipment replacement appear to be irrelevant.

Besides harsh environmental conditions and old equipment, human “interference” is one of the predominant reasons that may cause an internal arc with disastrous consequences. Even well trained and experienced engineers are by no means immune to making mistakes – dropping tools and making improper cable terminations are two main causes of an internal arc fault.

In the past arc protection could only have been achieved by conventional passive protective measures i.e. maintaining the IP code, safety procedures for dead working, protective clothing or integrated disconnector/earthing switches. Available active arc detection systems e.g. pressure sensors or fast relays such as ABB’s REA or TVOC-2 system can already significantly improve the protection by reducing the arcing time to 55 – 75 milliseconds (ms).

Today’s intelligent technologies like the Ultra-Fast Earthing Switch (UFES) immediately mitigate the effects of internal arc faults and raise safety standards to a whole new level. The UFES does not only act as an active protection for personnel, but also saves the equipment from heavy damages minimizing the necessary repair work and downtime.

The UFES is a smart combination of an arc detection system e.g. ABB´s REA or TVOC-2 systems, and the corresponding primary switching elements (PSE), which initiate a 3-phase short-circuit to earth in the event of an arc fault. The extremely short switching time of the primary switching element, less than 1.5 ms, in conjunction with the rapid and reliable detection of the fault, ensures that an arc fault is extinguished almost immediately after it arises. With a total extinguishing time of less than 4 ms after detection, an active protection concept with the UFES enables switchgear installations to achieve the highest possible level of protection for personnel and equipment.

Designed for rated voltages up to 40.5 kV and rated short-time currents up to 100 kA, the UFES can, in principle, be installed in any new or existing installation of medium- or low-voltage equipment. And there is already a wide range of type tested solutions for several applications available. Looking at ABB products, UFES is included in the accessory portfolio of various low- and medium-voltage AIS solutions e.g. UniGear ZS1, ZS8.4 or MNS. It is also available as an integral part of an enclosed RESIBLOC dry-type transformer solution. Outside ABB, also OEMs (original equipment manufacturers) benefit from the active protection approach.

Besides switchgear applications, manufacturers of capacitor banks or drives, also see a high potential to significantly increase safety of their equipment.

The phase-wise unit design with three single PSEs also offers versatile installation opportunities for retrofitting existing equipment. Fixed mounted in an external box or installed on a withdrawable truck unit, for the availability-focused operator, are just two examples of how to integrate the PSEs. A further advantage is the compatibility with external arc detection systems like ABB´s REA or TVOC-2. Thus, new or existing purely relay-based arc detection systems can be easily extended and improved to a high-end solution.

Successful internal arc type test 50kA / 100ms (IEC 62271-200) of a UniGear ZS1 without pressure relief duct / absorber with and without UFES.

Source: Andreas Peters (ABB)