Selectivity of electrical protections

Introduction:

The selectivity of the protections consists of cutting off the power supply to the faulty electrical installation while maintaining those which are healthy.

Depending on the design, power to a single device or set of circuits will be disconnected.

The selectivity of the protections is obtained by:

  • Smart network design and circuit subdivision
  • The choice of suitable protection devices
  • Protection settings

The basic principles of selectivity are applicable to both high voltage and low voltage. However, some solutions, given their implementation cost, are only used at high voltage.

For carrying out the selectivities, it is used:

Either

  • Ampermetric staging and chronometric staging
  • Possibly associated with blocking or logic “discrimination” (in high voltage)

Either

Line, machine and bus differential protections (in high voltage)

The stages of an electrical network:

When designing an electrical protection system, it is important to have a global view, but not necessarily in detail, of the network.

We produce a synthetic diagram representing on a single page from top to bottom the floors of the distribution:

  1. Sources and protections of arrivals
  2. Departures protections
  3. Protection of divisional departures
  4. Receiver protections

In this diagram, the devices not participating in the protection are not represented.

At least 2 separate diagrams are produced:

  • One intended for short-circuit protection
  • One intended for earth fault protection

For the particular case of a homopolar generator, it is considered as a receptor with respect to protection against short circuits and as a source for earth faults.

Stages of an electrical network

The ampermetric staging:

The basic principle is simple: it involves choosing a trip threshold for the downstream stage at a value at least 20% lower than the threshold of the upstream protection.

This value of 20% is not a mandatory rule. It can be reduced, but it does away with the trip tolerances of the protections.

When protection is provided by fuses, it is necessary to juxtapose the melting and tripping curves of the circuit breakers to check that the discrimination is respected.

In the case of a transformer, the upstream and downstream protections are considered to be part of the same protection stage because their tripping / melting leads to the same consequences.

Example: HV distribution and HV/LV transformer

Note: The values shown on LV breaker curve correspond to the ones seen from HV side.

The chronometric staging:

The basic principle is to allow the time necessary for the downstream protection to trip before requesting the protections located upstream.

In practice, it is considered that a difference in tripping time delay of 0.2 seconds is sufficient. This value is not obligatory but it makes it possible in particular to overcome the mechanical operating times of the circuit breakers, the breaking of arcs and the tolerances of the protection time delays.

The maximum time delays depend on the rules set by the managers of the distribution networks and on the resistance of equipment to short circuits.

It is common not to be able to have 0.2 seconds between floors because the timing of the head circuit breaker is too low. In this case, a device commonly known as “logical selectivity” described in a future article can be used.

Chronometric staging

Tle logical selectivity:

This system comes in addition to the chronometric staging of the protections when a difference of at least 0.2 s between stages cannot be respected.

Wiring or programmed logic blocks the operation of the normal upstream protection if one of the circuit breakers located downstream has detected a fault. This system ensures selectivity even if the time delay difference between the stages is of the order of 0.1 to 0.15 seconds.

However, if the downstream circuit breaker has not tripped – for example because of an operating fault – an emergency protection with a longer time delay on the upstream circuit breaker makes it possible to eliminate the fault.

The selectivity provided by differential protections on high voltage network:

The high voltage differential protection system has nothing to do with the differential devices used on low voltage networks.

The principle of differential protection is to locate the faulty part of the installation and to trip the circuit breaker (s) that supply it. The system is therefore intrinsically selective and the triggering is instantaneous.

The difficulty is to reliably locate the faulty area.

The same set of protection ensures the detection of short circuits and earth faults.

Example of “line” protection: opening of breakers on the line in default

There are differential protections for lines, busbars, motors and generators as well as for transformers.

Given its high cost linked to the number of current transformers and protection technology, this type of protection is reserved for essential parts of the distribution. The implementation, adjustment and testing of these protections requires specific skills and experience, particularly for transformers.

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