Jemima Widforss, Jan-Peter Antin ABB Electrification Vasteras, Sweden, jemima.widforss@se.abb.com, jan-peter.antin@se.abb.com; Michal Bures ABB Electrification Brno, Czech Republic, michal.bures@cz.abb.com
With temperatures of over 10,000 °C, current densities of up to 100 A/cm² and violent pressure waves, an electric arc flash in an electrical enclosure is an extremely dangerous and undesirable event.
Most arcing accidents in electrical equipment are down to human error (eg, when an operator is working on the equipment), faulty connections, or inquisitive animals. Most often, the accident occurs during switchgear maintenance or installation when the cabinet door is open. With the door open, the frontline defense of arc-proofed switchgear design – strong doors – is absent.
Accidents are rare but serious when they occur. Injuries can be severe or even fatal and damage to equipment extensive. Replacing and repairing the damaged equipment can lead to lengthy downtimes.
It is clear, then, that arc flashes are events to be avoided at all costs. If they cannot be avoided, their effects must be minimized. An arc guard system is, therefore, a necessary part of a modern switchgear design.
Much work has been conducted to mitigate the effects of arc flashes and ABB has had effective countermeasure products on the market for some decades. ABB’s TVOC-2 Arc Guard System™ – the new version of a well-established arc guard system that has been protecting people and electrical equipment from dangerous electrical arcs for over 35 years – is a device that uses optical sensors to detect an electric arc →02. The TVOC-2 interfaces perfectly with ABB Emax 2 circuit breakers, for instance, to provide a fast-response active protection system that limits the damaging effects of internal arcing.
TVOC-2 Arc Guard System
The TVOC-2 Arc Guard System employs optical detectors that sense the light flux associated with the electric arc phenomenon. Upon sensing extraordinary light levels, the TVOC-2 sends a tripping signal to the circuit breaker. Current interruption stops the local power grid from feeding further electrical energy into the arc. The reaction time of the detection is less than 1 ms. →03 shows example locations for the optical detectors. Strategic detector positioning will eliminate interference between zones of detection. Sensors are calibrated to have equal light sensitivity and their fish-eye lens design lets them observe a large solid angle, so orientation is not critical.
Electromagnetic interference (EMI) immunity is conferred by the use of fiber-optic cables, which are not only impervious to the EMI that will certainly accompany an arc fault but also faster at transmitting signals; every microsecond counts during an arcing event.
To avoid false tripping due to camera flashes, nearby welding activity, sunlight, etc., the arc mitigation device can be combined with a current sensing unit and set to activate only when an overcurrent is also registered. This is where the Rogowski sensor comes in.
ABB Rogowski coil current sensors
Over the past few decades, ABB has developed Rogowski coil current sensors for use in electrical equipment such as switchgear. Now, that same technology provides a basis for the new ABB CSU-2 current sensing unit for the TVOC-2. The CSU-2 detects the fast current rise that accompanies faults associated with arc flashes. To eliminate nuisance tripping, the TVOC-2 will only trip the circuit breaker if it “sees” a flash and, at the same time, the CSU-2 current sensing unit detects an overcurrent.
The Rogowski coil is placed around the conductors. The current flowing through the conductors creates a magnetic field in the sensor’s winding and thus induces a voltage in the winding. This voltage is proportional to the first derivative of the primary current and is presented as an output on the secondary terminals. The sensor response is linear over the whole measuring range, up to short-circuit currents, which means the sensor covers a much wider range than the current transformer conventionally used.
The Rogowski coil current sensor has several advantages over more traditional approaches.
Benefits of ABB Rogowski coil current sensors
Sensors based on alternative principles, such as the Rogowski sensor, are replacing conventional instruments in switchgear and other electrical installations as part of a strategy to significantly reduce equipment size, increase safety, offer greater rating standardization and provide a wider functionality range.
A variable clamping system for new installations and a split-core solution with an open loop for retrofits make Rogowski coil installation quick, easy, safe and reliable. The current sensor is connected to the CSU-2 by an RJ45 connector. A green indicator light on top of the CSU-2 flags that this connection is secure. The CSU-2 current sensing unit continuously supervises the Rogowski coils’ functionality to ensure fast and reliable detection of overcurrent.
One current sensor can be used for nominal currents from 100 A to 4 kA and still measure peak currents of more than 150 kA without saturating. This wide measurement range allows the same current sensor to be used in applications with quite different loads – for example, incoming and outgoing feeders. This aspect of the sensor has further advantages: panel builders can now operate with a smaller inventory and interchangeability of parts is enhanced. Furthermore, the same sensors can be used if a substation were to be upgraded for higher primary currents.
Rogowski coil sensors offer not only significant space savings compared to standard current transformers but also easier handling and storage, due to their light weight. Also, the absence of a ferromagnetic core means the power losses experienced by conventional solutions are eliminated and, therefore, the sensors exhibit extremely low energy consumption. Lower energy consumption improves both the economic and environmental credentials of the solution. Air cores are not susceptible to saturation, a major problem in current transformers with ferromagnetic cores →04 – 05.
05a Medium-voltage version with an openable coil. 
05b Medium-voltage version with a fixed coil. 
05c Low-voltage version with a small-diameter coil. 
05d Low-voltage version with a large-diameter coil.
05 Rogowski coil sensor elements. The sensing coil surrounds the conductor.
The CSU-2 uses dedicated and factory-calibrated Rogowski coil current sensors in order to ensure fast and safe tripping at low as well as high current levels.
Guarding the future of electrical installations
Arc protection systems are becoming an essential element of electrical cabinet design. Because such systems reduce damage and downtime costs, some insurance companies encourage their use by reducing the cost of cover for sites where they are installed. Protection against arcing events is also becoming enshrined in legislation. For example, the Low Voltage Directive of the European Union stipulates that measures to prevent damage by excessive heat, caused by arc flashes, for example, are to be taken.
The combination of the TVOC-2 Arc Guard System™ and the CSU-2 current sensing unit ensures an electrical installation adequately addresses all aspects relating to arc flashes. The duo ensures continuous operation, safety and protection. The Rogowski current sensor is easy to install or retrofit and will provide a current measurement with a longer life time and reduced maintenance requirements.
With a design based on ABB’s decades of experience in the arc mitigation field, the TVOC-2 and CSU-2 ensure the safety of personnel even when the cabinet door is open and provides all-around, comprehensive arc protection. The combination is one of the most effective and reliable arc mitigation products available.
