Selectivity techniques

The different selectivity techniques, also from the operational point of view

Selectivity (discrimination) is achieved by automatic protective devices when a fault is cleared by the protection device installed immediately upstream of it, while all the other protection devices remain unaffected.

Two areas can be defined, depending on the current ranges and circuit-breaker trip curves:

In < I < 8÷10 x In

Overload zone

In which the thermal protection for thermal-magnetic trip units and protection L for electronic trip units are normally called upon to trip.

I ≥ 8÷10 x In

Short-circuit zone

In which the magnetic protection for thermal-magnetic trip units or protections S, D and I for solid-state trip units are normally called upon to trip.

Different selectivity techniques are associated with these two zones.

Time-current selectivity

Current selectivity

Time selectivity

Energy selectivity

Zone selectivity

Time-current selectivity

Introduction

Operatively speaking

The protections against overload generally have a definite time characteristic, whether they are realized by means of a thermal trip unit or by means of function L of an electronic trip unit.
By definite time characteristic we mean a trip characteristic for which, as the current increases, the trip time of the circuit-breaker decreases.

When there are protections with this type of characteristics, the selectivity technique used is time-current selectivity.
Time-current selectivity realizes trip selectivity by adjusting the protections so that the load-side protection, for any possible overcurrent value, trips more rapidly than the supply-side circuit-breaker.

When the trip times of the two circuit-breakers are analyzed, it is necessary to consider:

The tolerances over the thresholds and trip times;
The real currents circulating in the circuit-breakers.

With regard to tolerances, ABB SACE makes the trip curves of its trip units available in the technical catalogues and in DOCWin software. In particular, in the curve module of DOCWin, the curves of both the electronic and thermomagnetic trip units include the tolerance values. Thus, a trip of the protection unit is represented by two curves, one showing the highest trip times (top curve), and the other one indicating the most rapid trip times (bottom curve).

For a correct analysis of selectivity, the worst conditions must be considered, i.e.:

the supply-side circuit-breaker trips according to its own bottom curve.

the load-side circuit-breaker trips according to its own top curve.

As regards the real currents circulating in the circuit-breakers:

if the two circuit-breakers are passed through by the same current, it is sufficient that there is no overlapping between the curve of the supply-side circuit-breaker and of the load-side circuit-breaker;

if the two circuit-breakers are passed through by different currents, it is necessary to select a series of significant points on the time current curves and check that the trip times of the supply-side protection are always above the corresponding times of the load side protection.

In particular, in the case of circuit-breakers equipped with electronic trip units, since the trend of the curves is at I²t=const, to verify them properly, it is sufficient to examine two current values:

1.05 x I1¹ of the supply-side circuit-breaker (value below which the supply-side protection never trips)

1.20XI3 (or I2)² of the load-side circuit-breaker (value above which the load-side protection certainly trips with the protections against short-circuit).

1.05 x I1¹ of the supply-side circuit-breaker
By assuming I_A =1.05xI1, and with reference to what has been said about the real currents circulating inside the circuit-breakers, the IB current on the load side is obtained.

The trip times of the two devices are obtained from the time-current curves.

If the following is true for both the points considered:
tA > tB
then selectivity in the overload zone is guaranteed.

In the figure on the side, an absorption of current from other loads has been assumed.

1.20XI3 (or I2) of the load-side circuit-breaker
By assuming IB = 1.20XI3 (or I2), the IA current on the supply side is obtained in the same way and, from the time-current curves, the trip times of the two devices are obtained.

1.05 is the value for minimum definite non-intervention defined in the Standard (IEC60947-2). For some types of circuit-breakers this value could vary
(see the technical catalogue for further information).
1.2 is the value for maximum definite intervention for protection against short-circuit defined in the Standard (IEC60947-2). For some types of circuit-breakers, this value might be lower
(see the technical catalogue for further information).

Current selectivity

Introduction

Operatively speaking

This type of selectivity is based on the observation that the closer the fault point to the power supply source of the installation, the higher the short-circuit current. Therefore, it is possible to discriminate the zone where the fault occurs, by setting the instantaneous protections to different current values.

Total selectivity can normally be achieved in specific cases only where the fault current is not high and where there is a component with high impedance interposed between the two protections (transformer, very long cable or a cable with reduced cross-section, etc.), and consequently there is a great difference between the short-circuit current values.

Therefore, this type of coordination is used above all in final distribution (low rated current and short-circuit current values, and high impedance of the connection cables). The time-current trip curves of the devices are normally used for this study.
It is intrinsically fast (instantaneous), easy to realize and cost-effective.

However:

The ultimate selectivity current is usually low and therefore selectivity is often partial;
The setting level of the protections against overcurrent rises rapidly;
It is impossible to have redundancy of protections, which guarantees that the fault is cleared rapidly if one protection fails to operate.

This type of selectivity can also be realized between circuit-breakers of the same size not providing time-delayed overcurrent protection (S).

The protection against short-circuit of the supply-side circuit-breaker A shall be set to such a value that it does not trip for faults occurring on the load side of protection B.
(In the example in the figure I3_minA > 1kA)

The protection of the load-side circuit-breaker B shall be set to such a value that it trips for faults occurring on its load side.
(In the example in the figure I3_MaxB < 1kA)

Obviously, the setting of the protections must take into account the real currents circulating in the circuit-breakers.

Note

This selectivity limit, associated to the magnetic threshold of the supply-side circuit-breaker, is exceeded in all cases where energy type selectivity is realized. If the settings indicated for energy selectivity are respected, for the combinations of circuit-breakers with an energy selectivity value given in the coordination tables published by ABB, the selectivity limit to be taken into consideration is the one given in the tables and not the one obtained by using the above mentioned formula.

The ultimate selectivity value that can be obtained is equal to the instantaneous trip threshold of the supply-side protection minus any tolerance value.
Is = I3_minA

Time selectivity

Introduction

Operatively speaking

This type of selectivity is a development of the previous one. For this type of coordination, in addition to the current trip threshold, a trip time is also defined: a certain current value will make the protections trip after a defined time delay, such as to allow any protection placed closer to the fault to trip, excluding the area of the fault.
Thus, the setting strategy is to increase gradually the current thresholds and the trip delays by getting closer to the power supply sources (setting level directly correlated to the hierarchical level).
The delayed trip thresholds must take into account the tolerances of the two protection devices and the effective currents flowing through them.

The difference between the delays set for the protections in series must take into account the times of fault detection and clearance of the device on the load side and of the inertia time (over-shoot) of the device on the supply side (time interval during which the protection can trip even when the phenomenon is over). As in the case of current selectivity, the study is made by comparing the time-current trip curves of the protection devices.

Speaking generally, this type of coordination:

is easy to study and realize;
is not very costly as regards the protection system;
allows even high selectivity limit values to be obtained (if Icw is high);
permits redundancy of the protection functions.

However:

the trip times and energy levels let through by the protections, especially by those close to the sources, are high.

This type of selectivity can also be realized between circuit-breakers of the same size, equipped with electronic trip units providing time-delayed overcurrent protection.

The protections against short-circuit of the two circuit-breakers will be set:

With the I₂ trip thresholds against time-delayed short-circuit set so as not to create trip overlapping, taking into consideration the tolerances and the real currents circulating inside the circuit-breakers;
With t₂ trip times set so that the load-side circuit-breaker B clears the fault, whereas the supply-side circuit-breaker A, still in its timing phase, manages to "see" the extinction of the current and thus remains closed.

The ultimate selectivity limit obtained is equal:

To the instantaneous trip threshold of the supply-side protection, if this function is enabled, minus the tolerance (if any):
Is = I3_minA
To the Icw value for supply-side air circuit-breakers when the instantaneous protection function is set to OFF.

Note
These selectivity limits are exceeded in all the cases where energy type selectivity is realized.
If the settings indicated for energy selectivity are respected for the combinations of circuit-breakers with an energy selectivity value given in the coordination tables published by ABB, the selectivity limit to be taken into consideration is the one given in the tables, and not the one that can be obtained from the considerations in this paragraph.

Energy selectivity

Introduction

Operatively speaking

The coordination of the energy type is a particular type of selectivity that exploits the current-limiting characteristics of moulded-case circuit-breakers. It is recalled that a current-limiting circuit-breaker is "a circuit-breaker with a break-time short enough to prevent the short-circuit current reaching its otherwise attainable peak value" (IEC 60947-2).
In fact, all the ABB SACE moulded-case circuit-breakers of the Tmax series, the miniature circuit-breakers and the air current-limiting circuit-breaker E2L and E3L have more or less marked current-limiting characteristics. Under short-circuit conditions, these circuit-breakers are extremely fast (trip times in the range of a few milliseconds) and open when there is a strong asymmetrical component. Thus, it is impossible to use the time-current trip curves of the circuit-breakers, obtained with symmetrical sinusoidal types of wave forms, for the coordination study.

These phenomena are mainly dynamic (therefore proportional to the square of the instantaneous current value) and are heavily dependent on the interaction between the two devices in series. Therefore, the end user cannot determine the energy selectivity values. The manufacturers provide tables, slide-rules and calculation programs giving the ultimate current selectivity values of I_s under short-circuit conditions for different combinations of circuit-breakers. These values are defined by theoretically integrating the results of tests carried out in compliance with the prescriptions in Annex A of Std. IEC 60947-2.

The short-circuit protections of the two circuit-breakers shall observe the conditions below.

Supply-side thermomagnetic trip unit

Supply-side electronic trip unit

I₃=OFF

The ultimate selectivity limit Is obtained is the one given in the tables ABB SACE makes available for its customers.

Zone selectivity

Introduction

Operatively speaking

This type of coordination is an evolution of time coordination.
In general, zone selectivity is realized by means of dialogue between the current measuring devices that, once the setting threshold has been detected as having been exceeded, allows the fault zone to be identified correctly and the power supply to the fault to be cut off.

It can be realized in two ways:

The measuring devices send the information on the current setting threshold having been exceeded to a supervision system and the latter identifies which protection has to intervene;
When there are current values exceeding their setting, each protection sends a lock signal by means of a direct connection or a bus to the hierarchically higher level protection (on the supply side in relation to the power flow direction) and, before intervening, checks that a similar lock signal has not arrived from the load-side protection. Thus, only the protection immediately on the supply side of the fault trips.

This second way guarantees without doubt shorter trip times. Compared with a coordination of the time type, the need to increase the intentional delay as one moves towards the power supply source is no longer necessary. The delay can be decreased to the time needed to exclude the presence of a possible lock signal coming from the load-side protection.
This type of selectivity is suitable for radial networks and, when associated with directional protection, is suitable for meshed networks too.

Compared with the coordination of time type, zone selectivity permits:

a reduction in the trip times (these can be lower than hundred milliseconds);
a reduction both in the damage caused by the fault and in the interferences in the power supply system;
a reduction in the thermal and dynamic stresses on the components of the installation;
a very high number of selectivity levels.

However:

it is more burdensome both in terms of cost and of complexity of the installation;
it requires an auxiliary supply.

As a result, this solution is mainly used in systems with high rated current and short-circuit current values and with binding requirements for safety and service continuity. In particular, examples of logical selectivity are often found in the primary distribution switchgear placed closely on the load side of transformers and generators.

This type of selectivity can be realized:

between Emax 2 air circuit-breakers equipped with Ekip Touch and Ekip Hi-Touch trip units.
The ultimate selectivity limit which can be obtained is equal to the Icw
Is = Icw

between Tmax T4L,T5L and T6L moulded-case circuit-breakers equipped with PR223 EF trip units.
The ultimate selectivity limit which can be obtained is 100kA
Is = 100kA

Then, by means of S51/P1 contact module, it is possible to make a chain of zone selectivity between Tmax and Emax 2. It is also possible to realize a selectivity chain including ABB MV protections.

Operating principle of zone selectivity between ABB circuit-breakers:

In the presence of current values higher than their setting, each protection sends a lock signal (by means of a direct connection or a bus) to the hierarchically higher level protection (on the supply side in relation to the power flow direction) and, before tripping, checks that an analogous lock signal has not arrived from the load-side protection. In this way, only the protection immediately on the supply side of the fault trips.

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