In electrical distribution network, protection coordination between LV circuit breaker located downstream of transformer and MV circuit breaker located on primary side of the transformer becomes extremely critical and complicated to ensure proper protection.
When a coordination of MV and LV circuit breakers is not carried properly it leads to frequent tripping of MV circuit breaker even when the fault is on the LV side, downstream of the network. For switching of MV circuit breakers, some time support will be required from local utility company, depending on the type electrical connection at site, which leads to longer down time and production losses.
The choice of the protection devices depends on the transient phenomena where the current may reach values higher than rated current for the transformer and decay in few seconds. The protection devices should also guarantee that the transformer cannot operate beyond the point of maximum thermal overload under short circuit conditions. In accordance to IEC 60076-5 the transformer is required to withstand value of short circuit current for not more than two seconds.
Fig 1 indicates rough profile of the transformer inrush current curve and corresponding withstand point for the given transformer.
In situations as shown in Fig 1, it is necessary that appropriate protective steps are considered both in the MV and LV network to avoid unwanted trips and to ensure that protection release curves are above the inrush current curve and below the withstand point.
To achieve that proper coordination becomes extremely important and critical. Protection relays and protection releases, which have versatile functionality, can ensure proper coordination.
As can be seen in following curve in Fig 2, there is good coordination between protection releases where only standard protection releases are applied on LV circuit breakers. In this case you will observe LV circuit breaker curve crosses over MV curve at approximately 7.5kA where both MV and LV circuit breakers trip (where maximum short circuit current is around 24kA).
The curve, as seen in Fig 3, developed with the incorporation of protection release, has additional feature of double short circuit setting in LV circuit breakers.
The protection releases in ABB circuit breakers can be offered with Double S protection function. This function enables two independent and simultaneously active protection thresholds that can be specified. With this feature, perfect selectivity is achieved in critical conditions.
It can be observed from Fig 4 that shows two possible settings for S protections, which are S and S2. When protection release is selected with two settings of time delayed short circuit settings, it enables close coordination between MV protection relay and LV protection release. This prevents nuisance tripping while transformer switches current and ensures its protection by getting curve below withstand point.
Ground fault protection philosophy
Implementing optimised cost effective ground fault protection in restricted zone is one of the key aspects. In many installations, including package substations, which are very common, RMU is used on the primary side of transformers. They normally have simple over-current relays and in most of the cases REF relays are not used. In this scenario the protection against fault in the restricted zone is critical to protect the transformer and improve its life cycle.
In Fig 5, the fault current is flowing inside LV breaker and protection release will sense the fault and clear the same. In the Fig 6, when fault occurs downstream the secondary side of transformer and upstream of circuit breaker, fault current does not flow through LV breaker. The only way to clear the fault is by tripping MV breaker. Due to the magnitude of current at primary side, which will be low corresponding to the relay on secondary side (if right type of relay is used), it will take longer time to trip, which will deteriorate the insulation of transformer leading premature failure. It will be much better if LV breaker has an intelligent protection release that can sense this fault and give trip command to MV breaker. This enhances system reliability and enhances transformer life cycle.
Protection against arc flash hazards
Every day hundreds of people are injured (sometimes with fatal injuries) due to arc flash related accidents. This is one of the highest risks all over the world. Safety is becoming more and more important as legal and regulatory requirements getting stringent.
The importance of safety has encouraged ABB to develop ?arc-proof? switchgears, where the mechanical design as well as the choice of electrical components reduces both the risk of an accident and its severity.
Types of fault in LV switchgear
- Bolted fault Two or more live parts at different potential come in contact (Phase-Phase, Phase-Earth).
- Arc Fault Occurs due to reduction in dielectric strength of insulating materials between two conductors.
The arc due to short circuit may occur due to various reasons in an LV switchboard. The arc may sometimes be a result of human errors. Poor connection will generate heat leading to an accident (This may be, for instance, due to hostile atmospheric conditions, excessive vibration, etc). The arc may also occur due to infestation of LV switchgear with insects creating a short circuit in the system.
The effect of arc varies based on arcing current and time. Time is the most critical factor that must be taken into account. The following infographic in Fig 6 shows the importance of responding in milliseconds.
The arc leads to a rapid build-up of pressure and heat. The arc temperature has been determined to be about 20,000?C. The extreme heat results in burning and melting of metals and release of toxic gases. This also leads to loss of production, damage to equipment and buildings.
The short circuit protecting devices within main distribution boards may not detect arc faults at times. This could happen due to:
1.In order to provide the required selectivity with downstream devices, the incoming circuit breakers are provided with intentional delay ranging from 150-200ms. The graph in Fig 6 shows that within this period the arc might cause major damage.
2.Due to the high inrush currents that arise during energisation of transformers, there is a high possibility of nuisance tripping of the circuit. As a result, the incoming circuit breaker protection release is set at higher value. Fault current responsible for arc formation might occur at a value lower than that set.
The arc formation phenomena can be divided into four phases
- 1.Compression phase: The air volume occupied by the arc is overheated due arc energy.
- 2.Expansion phase: Due to heat and expansion, the internal pressure increases and the hot air tries to escape through the weakest point.
- 3.Emission phase: The air inside LV switchboard is forced out.
- 4.Thermal phase: In this phase the temperature inside the switchboard becomes almost as hot as the arc. This is the final stage where all the metal and insulating parts come in contact with arc and undergo erosion.
Arc guard system uses with fibre optics that communicate at the speed of light for sensing the arc. The system can detect the intensity of light within the switchboard and it sends out a trip signal within 2ms. Fibre optics system is insensitive to any interferences from to magnetic field.
The disconnection time of the signal depends on opening time of the circuit breaker and the actual tripping time. It will be within 50ms. The arc guard system can be easily installed even in the existing LV Switchboard.
The arc guard system is designed and developed to ensure:
- 1.Increased arc safety in switchgear to saves lives and reduces damages
- 2.To be fast acting and reliable with SIL2 certification, and
- 3.Point sensor design makes it easy to locate the fault and restart the system.
Protection coordination
For a process plants, selection of protection devices becomes very critical for ensuring power supply reliability. The selected solutions should provide economical and functional service for the complete installations and avoid unwanted shut downs leading to huge production losses.
The protection devices selected shall ensure:
- Safety of the installation and people
- Rapid identification of faults and isolate the fault area without affecting areas which are healthy, and
- Enhance life cycle of complete electrical systems by limiting let-through energy flowing in the connected cables and equipment.
Current/time selectivity
In current selectivity, one can discriminate the fault zone by setting different values of short circuit protection. In time selectivity, apart from different values of current settings, even trip time is defined
Energy selectivity
This is specific type of selectivity, which exploits current limiting features of circuit breakers. As a user one needs to ensure that the type of circuit breakers to be selected are based on published chart.
Zone selectivity
In this case, dialogue is created between the circuit breakers in the network. When a current exceeds set threshold, the system allows only fault zone to be identified correctly and nearest breaker clears the fault without affecting other zones. The breaker close to the fault sends out a locking signal to the breaker at the higher level. Higher breaker also continuously checks for any locking signals from downstream breakers.
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