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Easy retrofit delivers reliable arc-fault protection

01 June 2012

A new, ultra-fast, active arc fault protection system, based on ABB’s vast experience of vacuum interrupter and IS-Limiter technology, is now available as an effective retrofit for medium-voltage (MV) switchgear to ensure greatly enhanced system availability and operator safety.

In rare cases with MV switchgear - possibly due to a defect, exceptional service condition or incorrect operation - an internal arc may result, posing a significant hazard to personnel in close vicinity. The temperature at the fault location increases rapidly to around 20,000oC, vaporising internal components, while the sudden release of heat and plasma (ionised gas) creates a destructive explosive blast.

In any arc-fault occurrence, personnel protection is paramount, but a far better approach is 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 or even total destruction of system components ensuring continuity of supply to the processes served by the switchgear.

The solution offered by ABB is an ultra-fast earthing switch (UFES) internal arc protection system, combining 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. Rapid and reliable detection of fault current and light intensity, coupled with the device’s extremely short switching time of less than 1.5ms, ensures that an arc fault is extinguished almost immediately following its initiation. Retrofitting the UFES can offer advantages for every air-insulated MV switchgear installation, particularly for older, non-internal arc classification switchgear.

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 can be reduced effectively by the UFES, at the same time greatly improving the safety of maintenance personnel.

UFES technology
The UFES contains three complete U1 primary switching elements and an electronic quick release unit. Each primary switching element is similar in dimensions (height 210mm, diameter 137mm), shape and fastening points to a 24kV 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 analogue 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 4ms 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 1VA, 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, and 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 switchgear compartments (three per panel) can be monitored using one electronic unit. The TVOC extension modules, each of which contains ten 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. And as each compartment is individually monitored, the location of the fault can be easily determined.

An easy retrofit
UFES is available in a withdrawable assembly or truck design that facilitates easy upgrade of 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 method is suitable for switchgear configurations with spare panels. Similar approaches are also available for other switchgear system types with trucks or withdrawable assemblies.
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.

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