Arc flash protection: tripping the light fantastic

Ultra-fast clearing of arc flash faults in medium voltage (MV) switchgear panels is essential in controlling arc flash hazards. Reducing the arcing time through faster detection is the most practical way of reducing incident energy levels and improving workplace safety.

REA 10 Arc Protection System Speed and Efficiency - Long fibre principle
ABB’s innovative REA fast arc flash detection system uses fibre-optic sensors to cut the typical relay operating time to less than 2.5 ms – far faster than conventional relay technology.

Arc flash is not a new phenomenon, but interest and concern about the dangers of arc flash events have increased dramatically over recent years. This is largely due to new guidelines and standards put forward by the various international trade and safety bodies such as the Institute of Electrical and Electronic Engineers (IEEE) and the US Occupational Safety and Health Administration (OSHA).

The reduction of arc flash incident energy levels is a priority, and one way to do this is to detect and trip for an arc flash event in the shortest time possible.

Protection at the speed of light
In modern protection systems, the need to operate in a few milliseconds is typically met by detecting the light from an arc flash and initiating tripping action via solid-state tripping elements. This approach is recognized in the IEC standard 62271-200.

The intensity of light instantaneously released by an arcing fault can be thousands of times higher than normal ambient light, and it is this phenomenon that is used in arc flash detection relays to achieve faster operating times than is possible with conventional relaying. Optical sensors detect the sudden increase in light intensity. Instantaneous over-current elements are used as fault detectors to supervise the optical system for security.

First-generation arc flash protection, dating from the early 1990s, uses only single-point light receptors called ‘lens sensors’. In this type of system, lens sensors are typically located in each cubicle where an arc flash might occur. Each lens sensor is individually targeted, for more precise location of the arc flash fault, and radially connected to electronics via a clad fibre.

From around 2000 onwards, ABB has been rolling out its new generation REA series of arc flash detection relays. In addition to the traditional lens sensors, this system accommodates a radically different type of light sensor: a long unclad fibre-optic sensor that can absorb light throughout
its length.

Fibre-optic sensor benefits
There are several advantages to the REA’s fibre-optic sensor technology. First, the unclad fibre enables light to enter through its exposed surface, and this propagates back to the electronics – effectively making the entire fibre a sensor. This dramatically reduces the cost of installation. A single optical fibre sensor can be as long as 60 metres, typically covering the same protection zone at much lower cost than lens sensors alone. Second, any concerns about shadows from internal structures that might block the direct exposure to an arc flash are eliminated. Third, if the fibre sensor is configured in a loop, the system
can provide regular self-checking of the sensor’s integrity and continuity, and generate an alarm if a problem is detected.

For additional security, the REA includes a set of conventional 1 A/5 A current transformer inputs. These are typically connected to the current transformers located on the source side of the main breakers to drive instantaneous phase and ground over-current elements, which act as fault detectors to supervise the optical flash detector. They utilize peak-to-peak waveform detectors in order to eliminate delays associated with conventional root-mean-square (RMS) calculations.

In normal operation, both light and over-current must be present simultaneously for tripping to occur. Detection of an intense light alone will not result in a trip unless the system is intentionally set to operate that way (which is not recommended).

Over-current settings enable different current threshold levels for phase and ground fault currents. High-speed insulated gate bipolar transistors (IGBTs) are used to provide two fully trip-rated outputs rather than relatively slow conventional dry contacts. The typical operating time is less than 2.5 ms from arc detection to trip initiation.

Over 3,500 REA systems utilizing long fibre-optic sensor technology have been installed in 36 countries around the world. There have been at least two documented cases to date where an arc flash accident occurred in MV switchgear protected by arc flash relays utilizing the long fibre sensor technology.
Optical arc flash relays are ideally suited to modern vacuum and SF6 breaker technologies where the fault interruption takes place inside a sealed container. However, with proper precautions, an optical relay may also be applied to air magnetic breakers.

In addition to the installation of dedicated arc flash relaying, arc flash mitigation strategies should also include replacing older air magnetic breakers (typical operating time of five cycles) with modern vacuum and SF6 technology (typical operating time of three cycles or less).

REA 10 Arc Protection System
The REA optical arc flash relaying system is the fastest protection currently available, and the development of long-fibre light sensors has made this protection practical for both new and retrofit applications.

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