The most devastating fault type
An arc fault in medium voltage or low voltage switchgear is one of the most devastating fault types in power systems. Term ‘arc flash explosion’ is a good characterization of the fault type. It causes a serious burn hazard to personnel along with several other safety hazards. An arc fault may also lead to significant economic losses directly, by damaging the equipment and indirectly, through power supply outages and production process interruptions.
Several methods have been introduced to prevent arc faults and mitigate their impacts. In this research a comprehensive overview of the methods has been given, starting from the design of equipment until the extinction of the fault arc.
Development directions for arc protection have been investigated and suggested. Part of arc faults develop gradually and it is possible to construct systems for detecting such faults. In this research, mechanism and phenomena related to developing faults have been investigated.
Moreover, an online monitoring system enabling preemptive protection has been outlined, and suitable sensors have been tested in a laboratory.
The dissertation suggests standardization of already existing, effective and proven protection methods, especially protection based on optical detection. In minimizing the arc duration, efforts should be directed towards development of circuit breaker technology. In most critical sites, short-circuit devices can be applied.
Design, education and maintenance
Design of switchgear is a key issue in arc prevention, and it is supported by IEC and IEEE standardization. In fact IEC 62271-200 states that if switchgear is designed and manufactured satisfying the requirements of the standard, internal arc faults should, in principle, be prevented.
However, internal arc faults still occur for a number of reasons.
IEC 62271-200 presents a list of locations where internal arc faults are most likely to occur in metal-enclosed switchgear and controlgear:
- Connection compartments
- Disconnectors, switches, grounding switches
- Bolted connections and contacts
- Instrument transformers
- Circuit breakers
According to experience, another typical location of arc faults is cable termination. By focusing special design attention on the locations listed above, the probability of arc faults can be decreased.
Insulation of buses provides means to reduce the probability of arc faults caused by e.g. falling objects or vermin. Insulation can also prevent single-phase faults from escalating to high-power multi-phase faults. Another advantage of an insulated bus is that the insulation may help extinguish the arc.
High-resistance grounding (HRG) is another design related technology aiming at reducing the probability of an arc fault. HRG system has a resistance sufficiently high enough connected between the earth and point of connection on the system that there is a minimal current that flows during an earth fault. However, HRG system for arc fault mitigation is only effective in earth faults.
The human factor is often the direct cause of an arc fault, especially in cases with casualties. Systematic education of personnel, delivering information on the equipment and related safety hazards, is an efficient way to increase safety and reduce the number of accidents.
The same kind of systematic approach also applies to maintenance practices that ensure adequate condition of the equipment and help in identification of possible risks. Preventive maintenance, e.g. visual inspection, thermal imaging, partial discharge (PD) testing, and time-based testing of protection devices are examples of common preventive maintenance actions.
Protection systems based on the detection of light
The detection of light is the fastest arc detection method, applied by a number of manufacturers and by an increasing number of end-users. This is why the following is written from the light-based arc detection point of view, assuming that detection of light is at least one of the operation conditions of the arc protection relay.
Special attention should be paid to maintenance of circuit breakers. If a CB fails, the performance of other components of the protection system is insignificant, with the exception of the important breaker failure protection.
In limited applications where no selectivity is required, simple stand-alone devices can be utilized. The operation criteria may be “light only” or a combination of light and overcurrent or pressure. Stand-alone protection can be applied in e.g. wind power and small switchgear applications.
Figure 2 presents an example of a switchgear application with “light only” condition. The dashed lines depict the division of the switchgear into several compartments, each one equipped with a light sensor.
The protection system consists of light sensors with associated cabling (grey lines), stand-alone arc protection relay, and the circuit breaker. The protection relay consists of a power module, light sensor input channels, microprocessor, and the I/O module including the trip relay.
Arc protection integrated into protection relays
Numerical protection relays can be equipped with an arc protection option, including sensor inputs to light sensors and a high-speed overcurrent protection option. Communication between the relays is needed for selective protection, i.e. tripping of appropriate circuit breakers.
Figure 3 presents a scheme of an MV application enabling selective tripping of outgoing feeders, in case an arc fault occurs in outgoing cable compartment.
Dedicated arc protection systems
For complex systems, dedicated arc protection equipment can be applied. While numerical relays are multi-function relays, dedicated arc protection relays are committed to arc protection, and they are the key components of the arc protection system.
Figure 4 presents a simplified example of a dedicated arc protection system of MV switchgear.
The system is composed of one central unit, one current I/O unit, and four light I/O units. Current is measured by the CTs of the incoming feeders, connected to the central unit and the current I/O unit. Three light I/O units collect information from point type light sensors (two units, VAM 12LD, for outgoing feeders on the left side, and VAM 12L for the incoming feeder on the right), enabling selective protection in case of faults in cable terminations.
Fibre type light sensors are connected to one of the light I/O units (VAM 3L). The system is divided into a number of protection zones, and it includes circuit breaker failure protection, tripping the upper voltage level CB in case of CB failure.
|Title:||IEC 61850 based GOOSE messaging in arc protection of LV/MV switchgear – Doctoral thesis by Lauri Kumpulainen at University of Vaasa, Faculty of Technology, Department of Electrical Engineering and Energy Technology|
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