chitika ad

News

Discrimination with LV Power Circuit-Breakers

Discrimination Techniques


The purpose of this article  is to set out the discrimination techniques which apply specifically to low voltage power circuit-breakers. These devices are characterized by their high rating (800 A to 6300 A), and their location at the head of the LV installation, generally directly downstream of an MV/LV transformer. This location justifies the strict discrimination requirements which apply to them. 

This article begins with a resumé of discrimination techniques, followed by an explanation of the links between discrimination and general circuit-breaker characteristics. Finally, some practical examples will be provided on selection of devices to be installed.



Fig 1: Installation example (with 1600 kVA MV/LV transformers) with indication of the order in which discrimination is examined (Photo Credit Schneider Electric)




 Introduction 


In a radial feeder layout  the purpose of discrimination is to disconnect only the faulty load or feeder from the network and no others, while keeping as much as possible of the installation powered up. Safety can thus be combined with continuity of service, and the fault easily located. It is an especially important concept for high-power equipment, since this is generally located at the head of the installation and therefore has even greater consequences in the event of false tripping. Discrimination is said to be total if it is assured irrespective of the value of the fault current, up to the maximum value available in the installation. If this is not the case, it is said to be partial. The faults encountered in an installation are of different types: 


  • overload 
  • short-circuit as well as: 
  • earth fault 
  • voltage dip or momentary loss of supply


 Discrimination according to the type of fault 


The techniques for using discrimination have to be adapted to the phenomena involved, and therefore differ according to the type of fault. 


Overloads 


These are currents between 1 and 10 times the duty current. They should be eliminated within a period which is compatible with the thermal withstand of the conductors concerned. 

The trip time is generally inversely proportional to the square of the current (this is known as “inverse time” tripping). Circuit-breaker discrimination works by comparing the time/current curves for the long-time delay releases affected by the fault. It is effective if, for any overload current value, the time during which the upstream circuit-breaker does not trip is greater than the maximum breaking time for the downstream circuit-breaker  (including the arc suppression time). In practice, this condition is achieved if the ratio Ir1/Ir2 is greater than 1.6.

For each type of fault there is a specific corresponding protective device (protection against overload, short-circuit or earth fault currents, or against loss of voltage, etc). Each of these faults can cause a loss of discrimination if coordination of the protective devices has not been taken into account.


Short-circuits 


Because of the magnitude of short-circuit currents, and especially the presence of electrical arcs which generally accompany them, the circuits concerned should be interrupted almost instantly, in less than a few hundred milliseconds. Discrimination can work, to some extent, by comparing the time/current curves, provided that time tc is at least thirty or forty milliseconds. Below that time, these curves are not sufficiently precise to reach a verdict with certainty. Moreover, the time and current are not then the only discriminating criteria. 

Depending on the situation, it may be necessary to take account of the peak current, limiting, or a combination of time and current . It is then necessary to refer to the discrimination tables published by the relevant circuit-breaker manufacturer. Various techniques can be used to achieve discrimination in the event of a short-circuit between two circuit-breakers, and these are outlined in the document. 


Earth leakage currents 


Here too, discrimination has to be taken into account so as to prevent an insulation fault at some point in the installation leading to tripping of the main devices. There are 2 major protection “families” with regard to leakage currents. For low or very low current values (typically between 30 mA and 30 A), a sensor is used which surrounds all the live conductors. This sensor naturally adds up the total current, and provides a signal which is proportional to the fault current. The presence of an earth (or ground) fault current causes the sum of I1+I2+I3+In to be other than zero. This system is generally known as “residual protection” or “vigi”. For higher leakage current values, above 20% of the nominal current, one sensor is used per live conductor.

The system, which is called simply “ground fault protection”, adds up all the signals provided by each of these sensors. In both cases, discrimination works by differentiating between thresholds and time delays. It can be controlled by time/current curves . 


Voltage dips or loss of supply 


These phenomena can be generated by a short-circuit in the installation, or by a fault upstream of it, and lead to tripping of the main devices if they are equipped with an undervoltage trip unit. 

The solution consists of using time-delayed undervoltage trip units, with a reaction time which is longer than the short-circuit trip time of the equipment located downstream. Even without a time delay, undervoltage trip units should offer immunity against undervoltages lasting approximately ten milliseconds, in order that they are not affected during short-circuits eliminated by equipment located near the loads.

This article has been extracted from "Cahier technique no. 201 Discrimination with LV power circuit-breakers" by Schneider Electric


Download the Free Guide Here





No comments