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Report | October 2014

Protection implementation in LV networks

When there are spikes in electrical circuits, it´s instrumentation control technology that can come into play to protect the instrument and avoid fire hazards.

In an 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, the result is frequent tripping of an MV circuit breaker even when a fault is on LV side downstream network. For switching of MV circuit breakers, some support will be required from the local utility company depending on the type of electrical connection at the site which leads to longer down time and production losses. The choice of the protection devices have to consider transient phenomena during which the current may reach values higher than rated current of the transformer and decays in few seconds.

The protection devices should also guarantee that the transformer cannot operate beyond 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 limited to 2 seconds.

The below diagram indicates rough profile of the transformer inrush current curve and corresponding point of withstand for a transformer.

With the above scenario, it is necessary that correct protection is required to be considered both at MV and LV network to avoid unwanted trips and ensure protection release curves are above the inrush current curve and below the withstand point.

To achieve the protection coordination ensuring both curves (MV and LV curves) to be between these two points and also establish a proper coordination becomes extremely important and critical. Only with protection relays and protection releases which have versatile functionality within can we ensure proper coordination.

The below curve provides clear indication on coordination of protection releases when only standard protection releases are considered 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).

A curve is developed with an incorporation of protection release which is having the additional feature of double short circuit setting in LV circuit breakers.

When protection release is selected with 2 settings of time delayed short circuit settings, it enables close coordination between MV protection relay and LV protection release. This enables avoidance of nuisance tripping due to transformer switching current and ensures protection of the transformer is assured by having curve below withstand point.

Ground Fault Protection Philosophy
Implementing optimised and cost-effective ground fault protection in the restricted zone is one of the key aspects. In many installations including package substations which are very common in use, RMU is used in primary side of transformers which will 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 becomes very important to protect the transformer and improve its life-cycle. In the diagram 1, the fault current is flowing inside LV breaker and protection release will sense the fault and clears the fault.

In the diagram 2, when fault occurs at downstream of 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 magnitude of current at primary side being low corresponding to secondary side, relay (if right type of relay is used) will take longer time to trip which will deteriorate insulation of transformer leading to premature failure.

It will be excellent if LV breaker has an intelligent protection release which will sense this fault and gives trip command to MV Breaker. This enhances system reliability and enhances transformer life cycle.

Protection against Arc Flash Hazards
Every day hundreds of people face serious injuries (sometimes fatal injuries) due to arc accidents. This is one of the highest risks all over the world, not only in countries with low safety standards. Safety is becoming more and more important as legal and regulatory requirements increase.

The risk of arc accidents can be reduced by the design of systems (mechanical and electrical) and the routines for working with electric equipment. The importance of safety has led 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 consequences.

Two types fault classification 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 cause of arc is sometime due to human errors. A poor connection will cause generation of heat which in the end leads to an arcing accident (The reason for this may for instance may be wrong tightening torque being used, hostile atmospheric condition, excessive vibration, etc). The arcing could be also due to entrance of vermin or small animals in LV switchgear which is likely to create short circuit with the arc.

The effect of arc depends on arcing current and time. The time is the most critical factor that needs to be taken into consideration. The below picture is an illustration of result of arc phenomena. The arc leads to a rapid buildup of pressure and heat. The arc temperature has been determined to be about 20,000C.

The arc phenomena can be divided into four phases:
Compression phase:
The air volume occupied by the arc is overheated due to arc energy. Convection and radiation warms up remaining air volume within the cubicle.

Expansion phase: Due to heat and expansion, the internal pressure increases and heat tries to escape through the weakest point (a hole is formed). The pressure reaches maximum and starts decreasing in this phase due to release of hot air.
Emission phase: Due to contribution of arc energy, the air inside the LV switchboard is forced out.
Thermal phase: In this phase the temperature inside the switchboard reaches almost the level of the arc. This is the final stage where all metal and insulating parts come in contact with the arc and undergo erosion.
The arc guard system employs speed of light as a technology with fibre optics for sensing the arc. The system can detect the large intensity of light within Switchboard and sends out a trip signal with in 2ms.The fiber optics is insensitive any interferences due to magnetic field.
The disconnection time of the signal depends on opening time of the circuit breaker and normally tripping time 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
Increased arc safety in Switchgear which saves lives and reduces damages.
Fast acting and reliable with SIL2 certification.
Point sensor design makes it simple fast 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
  • Enhances life cycle complete electrical systems by limiting let-through energy flowing in the connected cables and equipment.

IEC- 60947 standards for low voltage switchgear provides clear definition for selectivity ´coordination between operating characteristics of two or more protection devices, so that devices when an over current occurs within the established limits, the devices closed to the fault trips and others continue operate satisfactorily´.
Standards also defines total selectivity as ´when two circuit breakers are in series, the protection device on the load side carries out protection without making other device to trip.
In case of partial selectivity ´when two circuit breakers are in series, the protection device on load side carries out protection up to defined level ( predefined by manufacturer) without making the other device to trip.

There are different types of selectivity techniques implemented in low voltage network.
Typically they are:

  • Current selectivity
  • Time selectivity
  • Energy selectivity
  • Zone selectivity
  • 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 also defined.

Energy selectivity
This is a specific type of selectivity which exploits current limiting features of circuit breakers. Normally it will not possible for the end user to determine energy selectivity values. The manufacturers typically make tables or slide rules or even calculation programs for ease of selection. As a user one needs to ensure that the type of circuit breakers is selected 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 with affecting other zones. The breakers close to fault sends a locking signal to higher level breaker. Higher breaker also continuously checks for any locking signals from downstream breakers.

This article has been authored by Ramprasad Satyam, Assistant Vice President-Design, ABB India Ltd.

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