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ABB's UFES retrofit delivers ultra-fast arc-fault protection

16 April 2012

ABB’s new, ultra-fast, active arc fault protection system now enables the effective retrofitting of medium-voltage switchgear for greatly enhanced system availability and operator safety. In the event of an arc-fault, the extremely short switching time of less than 1.5ms, combined with the rapid and reliable detection of fault currents and light intensity, ensures that an arc fault is extinguished almost immediately after it arises.

In rare cases, a failure inside a medium-voltage switchgear cabinet, due to a defect, exceptional service condition or incorrect operation can cause an internal arc which presents a significant hazard. This is due to the instantaneous increase in temperatures at the fault location to around 20,000°C, well above the melting point of steel, copper and insulation materials. Internal components are vaporized and the sudden release of heat and plasma (ionized gas) creates an explosive blast.

Should an arc-fault occur, protecting any personnel in the vicinity is the main priority. It is much better though to take active measures to prevent such an event from happening in the first place. This not only eliminates any risk for personnel, it also offers protection against damage and even destruction of system components and therefore enhances the availability of the processes that receive their power supply via the switchgear. There is now a solution in the form of the ultra-fast earthing switch (UFES) internal arc protection system, which is a combination of devices consisting of detection and release electronics and the corresponding primary switching elements (PSEs) which initiate a 3-phase short-circuit to earth in the event of a fault.

Retrofitting with UFES can offer advantages for every air-insulated medium-voltage switchgear installation. Particularly for older, non IAC (internal arc classification) switchgear systems, it will offer maximum personnel and equipment protection.

Upgrading of arc proof qualified switchgear by the active arc fault protection system, as recommended in Clause 5.101 of IEC 62271-200 is also useful. These types of switchgear already meet the passive operator safety requirements. But the destructive effects of an internal arc fault in the switchgear, like extreme pressure and temperature rise, the burning/ vaporization of metal and insulating material, the release of gases can be reduced effectively by the UFES. In addition, the safety of maintenance personnel will be greatly increased.

Installing UFES can result in these advantages:
Greatly increased system and process availability by preventing serious damage in switchgear systems and the opportunity to restore service rapidly after a fault
Greatly increased operator safety
Drastic reduction in downtime and repair costs resulting from replacement of system components, and the reduction in significant losses caused by long production stoppages
Minimization of pressure rise and gases in the faulty compartment and surrounding switchgear building

UFES Technology
Primary switching units combined with an electronic quick release
The UFES contains three complete U1 primary switching elements and an electronic QRU (quick release unit). Each primary switching element is similar in dimensions (height 210 mm, diameter 137 mm), shape and fastening points to a 24 kV pin-type insulator, and consists of a two-part vacuum chamber embedded in epoxy resin for environmental protection.

From a dielectric point of view, the chamber actually constitutes two vacuum gaps separated by a membrane. One gap contains a contact pin at earth potential while the other accommodates a fixed contact at high-voltage. Each element also features an integrated ultra-fast micro gas generator (SMGG), which is comparable in type and functionality to the gas generators used in automotive airbags. The SMGG drives a piston that acts as a single-shot actuator. The electronic unit, based on durable and fast analog technology, is phase independent in structure, and ensures current and light detection and reliable tripping within the shortest possible time.

When an internal arc fault occurs, the electronic unit detects the fault current (supplied by a current transformer) and the arc light in the compartment (measured by optical sensors). At almost the same time, the gas generator is activated to drive the piston actuator. This piston slides into the first part of the vacuum chamber, causing the contact pin to penetrate the membrane and engage with the fixed contact, permanently and without bouncing, creating a solid metal short-circuit to earth. The arc fault is therefore short-circuited and extinguished in less than 4 ms after it is first detected. The entire sequence leads to the safe connection of the piston to earth potential via a moving contact system.

Information processing
The electronic unit has three input channels that enable continuous monitoring of the instantaneous current. The response level, the criterion for detection of a fault current, can be adjusted to suit a wide variety of protection conditions. With a low input burden of less than 1 VA, the current measurement system can simply be looped into the secondary wiring of the existing current protection transformers.

In addition to current monitoring, nine optical inputs are available for arc-fault detection. The status of the arc-fault protection system is indicated by LEDs and a 7-segment display on the front panel of the unit.

Various floating contacts are provided as interfaces to other units. These can be used:
To transmit the status of the electronics to a remote control room
To send commands to a circuit breaker feeding into the arc fault
As an interlock to block the reclosing of a circuit breaker directly after tripping.

Together with the electronic watchdog function, the functional capability of the SMGG-igniter is also continuously monitored. The electronic unit can be switched to test mode in which all the response criteria for the switchgear can be simulated and checked by the user, and the corresponding trips displayed but not transmitted to the SMGGs.

In combination with ABB’s TVOC-2 light detection system, up to 150 compartments in a switchgear system (three compartments per panel) can be monitored using one electronic unit. The TVOC extension modules, each of which contains 10 optical inputs and extension possibility to 30 inputs, can be directly connected to the five interfaces provided. Because detection of an arc fault by these modules is also monitored by the electronic unit, at least 30 panels in a switchgear system are provided with active protection. As each compartment is individually monitored, the location of the fault can be easily determined.

UFES withdrawable retrofit solutions
UFES is available in a withdrawable assembly or truck design that makes it simple to upgrade existing switchgear systems with active arc fault protection. For air-insulated switchgear systems based on a withdrawable assembly, a plug and play module can be directly inserted, without any need to modify the infrastructure.

This solution, rated at up to 24 kV up to 25 kA for 3s and 17.5 kV up to 31.5 kA for 3s, is designed for switchgear configurations with spare panels. Similar solutions are also available for other switchgear system types with trucks or withdrawable assemblies.

UFES Service-Box
The type-tested UFES Service-Box can be mounted directly on the switchgear system, taking account of short-circuit strength, and connected by copper bars to the busbar of the switchgear system. It is available in two ratings: 12 kV / 17.5 kV up to 50 kA for 3s and 24 kV up to 25 kA for 3s

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