The consequences of an arcing short-circuit or earth-fault inside a low voltage or medium voltage switchgear panel can be very disastrous. The extremely hot electric arc can destroy valuable equipment causing prolonged and costly distribution downtimes. Moreover, an arc fault poses a serious threat to the switchgear operating and maintenance staff.
An arc fault can arise from, for example, insulation faults and weaknesses, maloperation of a switchgear device, improper (loose) busbar or cable joints, overvoltage, corrosion, pollution, moisture, ferro resonance (instrument transformers) and even ageing under electrical stress. Most of these fault reasons can be prevented by appropriate maintenance. Despite of all precautions, arc faults may be caused by human error, or by animals entering the switchgear panel.
Time is a critical parameter for the detection and minimization of the effects of arc faults as the arc incident energy rapidly increases by time. An arc fault lasting 500ms may cause severe damage to the installation. If the arc lasts for less than 100ms the damage is often restricted, but if the arc is extinguished in less than, for example, 4ms the damage is insignificant.
1. REA arc fault protection system
The REA system is a fast and flexible arc fault protection system for air-insulated low voltage and medium voltage switchgears to protect human lives and prevent or reduce asset damage.
The function of the REA arc fault protection system is based on detecting the intense light of an electric arc alone or on a detection of light and simultaneous phase or neutral overcurrent. The REA arc fault protection system uses two types of sensors for detecting light: a non shielded, bare-fiber sensor that detects light along its entire length and light collecting lens-type sensors with typically one sensor installed per switchgear compartment. On detection of an arc fault, the REA arc fault protection system delivers trip commands in less than 2.5ms to all circuit breakers that feed the fault zone. Furthermore, the operation indicators of the REA arc fault protection system help localizing faults by selectively guiding the maintenance staff to the fault zone, that was identified by the arc fault protection system.
2. REA arc fault protection system modules
The actual REA arc fault protection system consists of one or more arc fault protection main modules type REA 101 and a necessary number of extension modules type REA 103, REA 105 and REA 107. The main module REA 101 can operate as a stand-alone device or cooperate with other REA 101 modules, and with the REA 103, REA 105 or REA 107 extension modules. The extension modules REA 103 and REA 107 allow the number of sensor fibers and/or lens-type sensors to be increased to extend the area of protection. The use of the extension module REA 105 including fast trip outputs will allow protection schemes with increased selectivity to be created.
♦ REA 101 arc fault protection main module:
The arc fault protection module REA 101 is the main module of the REA arc fault protection system. The REA 101 module can operate as a stand-alone device or in combination with other REA 101 modules and with the extension modules REA 103, REA 105 or REA 107. The REA 101 module can utilize one loop or radial type fiber sensor, or a lens-type sensor for light detection.
Working principle: The main module REA 101 detects three, two or single phase fault currents. The REA 101 module is activated when an arc fault occurs between the phases of the busbar system. Using a loop-type fiber sensor, REA 101 provides fast tripping of the LV infeeder circuit breaker. Moreover, either a fast tripping of the HV side circuit breaker or the circuit-breaker failure protection (CBPF) can be applied for enhancing protection redundancy.
♦ REA 103 arc fault protection extension module:
REA 103 is an extension module to be used together with the main arc fault protection module REA 101. The REA 103 extension module is operating as a light detecting module in the REA arc fault protection system. REA 103 detects light and provides the operator with information about the location of the fault. For light detection REA 103 can use two loop-type or radial sensor fibers.
The built-in self-supervision system of the REA 103 module continuously monitors the operating voltages of the module and the continuity of the sensor fiber loops. Any fault or malfunction detected will be used for alerting the operator via the REA 101 main module.
Working principle: The main module REA 101 is used for detecting phase or neutral (residual) overcurrent. The REA 101 module is activated when an arc fault occurs between phase conductors or between a phase conductor and the grounded frame of the switchgear system. The REA 101 module is provided with a loop-type fiber sensor for detecting arc faults appearing between the power transformer and the busbar system. The arc fault protection system has been enlarged with a REA 103 extension module. The REA 103 module is equipped with a loop-type fiber sensor for detecting possible arc faults occurring in the busbar system. The REA 103 module transfers a LIGHT message to the REA 101 module, which issues a trip signal when the simultaneous current condition for tripping is met.
♦ REA 105 arc fault protection extension module:
REA 105 is an extension module to be used together with the main arc fault protection module REA 101. The REA 105 module detects light and carries out tripping, if the REA 101 module simultaneously provides an overcurrent signal. When the REA 105 extension module is used for interconnecting two REA 101 modules in the system, the REA 105 module also delivers information about the overcurrent status between them. Moreover, the REA 105 module carries out tripping simultaneously whenever the REA 101 module executes tripping. The REA 105 module can accommodate one looptype or radial-type sensor fiber for light detection.
The REA 105 module also provides circuit-breaker failure protection by delivering a delayed LIGHT signal to the REA 101 module, which then trips the upstream circuit breaker.
The built-in self-supervision system of REA 105 continuously monitors the operating voltages of the device and the state of the sensor fiber loop. Any fault or malfunction detected will be used for alerting the operator via the REA 101 module. A detected fault in the operating voltages will block the device to prevent incorrect operation.
Working principle: The main module REA 101 detects three, two or single phase fault currents. REA 101 is equipped with a loop-type fiber sensor for detecting arc faults occurring on one of the busbars of a double-busbar switchgear system. REA 101 also trips the bus coupler via the extension module REA 105. The REA 105 module also uses a loop-type fiber sensor for detection of arc faults on the other busbar. The REA 105 module trips the bus coupler on detection of light and reception of a CURRENT message from the REA 101 module. Simultaneously, the REA 101 module trips the infeeder circuit breaker on reception of a LIGHT message from the REA 105 module.
♦ REA 105 arc fault protection extension module:
REA 107 is an extension module to be used together with the main arc fault protection module REA 101. The REA 107 extension module is operating as a light detecting unit in the REA arc fault protection system. REA 107 allows connection of up to eight lens-type sensors. The sensor input specific LED indicators of REA 107 facilitate fault location by guiding the staff to inspect the area of coverage of the sensor(s) that detected the arc.
REA 107 is equipped with two alarm contact outputs, which enable the partition of the lens sensors into two groups. The built-in self-supervision system of the REA 107 module continuously monitors the operating voltages of the device. Any fault or malfunction detected will be used for alerting the operator via the REA 101 module. A detected fault will block the device to prevent incorrect operation.
Wroking principle: The main module REA 101 detects three, two or single phase fault currents. The REA 101 module is equipped with a lens-type sensor for detecting arc faults occurring in the infeeder cubicle. The system is extended with a REA 107 module equipped with eight lens-type sensors for detecting possible arc faults appearing in the busbar compartment, circuit-breaker feeder compartments and cable termination compartments. The REA 107 module sends a LIGHT message to the REA 101 module, which issues a trip signal when the simultaneous overcurrent condition for tripping is met.
♦ Attached document:
– Products catalogue: Arc fault protection solutions-ABB
– Products guide: Arc fault protection solutions REA_10-ABB
– Mitigation solutions: Arc Flash Mitigation Solutions-ABB
3. REA Solutions
The serious consequences of an arc fault hazard and the substantial damage can be mitigated with the fast operating REA arc fault protection system. In addition to arcing short circuits, even arcing earth faults with current levels below the normal load current can be detected and interrupted already before they escalate to two or three-phase short-circuits.
Overcurrent relays applied as busbar protection may be too slow to ensure safe fault clearance times at arc faults. For example, the operating time of the overcurrent relay tripping the incomer CB may have to be delayed hundreds of milliseconds for selectivity reasons. This delay can be avoided by using the REA arc fault protection system. The total fault clearance time can be reduced to 2.5ms or less plus the operating time of the circuit breaker. Moreover, autoreclose attempts doomed to fail at faults occurring in the cable compartment can be eliminated using arc fault protection.