1) For descriptions and modes of operation of
3.1.1 Protective Equipment and Features
Low-voltage protective devices1) Low-voltage high-rupturing-capacity fuses
Low-voltage high-rupturing-capacity (LV HRC) fuses have a high breaking capacity. They fuse quickly to restrict the peak short-circuit current to the ut-most degree. The protective character-istic is determined by the selected uti-lization category of the LV HRC fuse (e. g. full-range fuse for overload and short-circuit protection, or partial range fuse for short-circuit protection only) and the rated current (Fig. 3/1).
Low-voltage circuit-breakers, IEC 60947-2
Circuit-breakers for power distribu-tion systems are distinguished ac-cording to their type design (open or compact design), mounting type (fixed mounting, plug-in, withdraw-able), rated current (maximum nomi-nal current of the switch) method of operation (current limiting: MCCB; or non-current-limiting: ACB), protective functions (see releases), communica-tion capability (capability to transmit data to and from the switch), utiliza-tion category (A or B, see IEC 60947-2).
Releases / protective functions The protective function of the circuit-breaker in the power distribution sys-tem is determined by the selection of
the appropriate release. Releases can be divided into thermo-magnetic re-leases (previously also called electro-mechanical releases) and electronic tripping units (ETU).
C Overload protection
Designation: “L” or earlier “a”
(“L” for long-time delay). Depend-ing on the type of release, inverse time-delay overload releases are also available with optional characteristic curves.
C Short-circuit protection, instanta-neous
Designation: “I “ (previously also called ”n” release), e.g. solenoid re-leases. Depending on the applica-tion, I-releases are also offered with a fixed settable or OFF function.
C Short-circuit protection, with delay Designation: “S”, previously also
“z” release (“S” for short-time delay). For a temporal adjustment of protective functions in series connections. Besides the standard curves and settings, there are also optional functions for special applications.
Definite-time-delay overcurrent re-leases: For this “standard S-func-tion,” the desired delay time tsdis set to a definite value when a set current value (limit-value Isd) is ex-ceeded (definite time; similar to the DMT function in medium voltage) Inverse-time-delay overcurrent re-lease: For this optional S-function applies I2t = constant. This function
is generally used to ensure a higher degree of selectivity (inverse time;
similar to the inverse-time delay function in medium voltage) C Ground fault protection
Designation: ”G” (previously also called ”g” release). Besides the standard function (definite-time), there is also an optional function (I2t = inverse-time delay).
C Fault current protection
Designation: RCD (= residual cur-rent device). To detect diffecur-rential fault currents up to 3 A, similar to the RCCB function for the protec-tion of persons (up to 500 mA).
In addition, electronic releases also permit new tripping criteria which are not possible with electromechanical releases.
Protective characteristics
The protective characteristic curve is determined by the rated circuit-breaker current as well as the setting and the operating values of the re-leases (see Table 3/5).
Low-voltage miniature circuit-breakers (MCB)
Miniature circuit-breakers are distin-guished according to their method of operation – either high or low current limiting. Their protective functions are determined by electromechanical releases:
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Definite time-delay
LV HRC fuse LV circuit-breaker with releases
curves and setting ranges
Fig. 3/1 Protective characteristic of LV HRC fuse and LV circuit-breaker with releases
Definite
HV HRC fuse
MV circuit-breaker with time-overcurrent protection Variable operating zones and setting ranges
Fig. 3/2 Protective characteristic of HV HRC fuse and MV time-overcurrent protection
Releases
C Overload protection by means of inverse time-delayed overload releases, e.g. bimetallic releases C Short-circuit protection by means
of instantaneous overload releases, e.g. solenoid releases.
Medium-voltage protection equipment
High-voltage high-breaking-capacity fuses
High-voltage high-breaking-capacity (HV HRC) fuses can only be used for short-circuit protection. They do not provide any overload protection. A minimum short-circuit current is therefore required for correct opera-tion. HV HRC fuses restrict the peak short-circuit current. The protective characteristic is determined by the selected rated current (Fig. 3/2).
Medium-voltage circuit-breakers Circuit-breakers can provide time-overcurrent protection (definite and inverse), time-overcurrent protection with additional directional function or differential protection. Distance pro-tection is rarely used in the distribu-tion systems described here.
Protective characteristics
Secondary relays, whose characteris-tic curves are also determined by the actual current transformation ratio, are normally used as protective de-vices in medium-voltage systems.
Static numerical protection devices are increasingly preferred.
3.1.2 Low-Voltage Protection Equipment Assemblies
Protection equipment assemblies With series-connected distribution boards, it is possible to arrange the fol-lowing protective devices in series
(rel-C (rel-Circuit-breaker with downstream miniature circuit-breaker
C Circuit-breaker with downstream fuse
C Fuse with downstream circuit-breaker C Fuse with downstream miniature
circuit-breaker
C Several parallel infeeds with or without coupler units with stream circuit-breaker or down-stream fuse
Current selectivity must be verified in the case of meshed LV systems.
The high- and low-voltage protection for the transformers feeding power to the LV system must be harmo-nized and adjusted to the additional protection of the secondary power system. Appropriate checks must be carried out to determine the effects on the primary MV system.
In MV systems, HV HRC fuses are normally only installed upstream of the transformers in the LV infeed.
With the upstream circuit-breakers, only time-overcurrent protection de-vices with different characteristics are usually connected in series. Dif-ferential protection does not affect, or only slightly influences the grading of the other protective devices.
3.1.3 Selectivity Criteria
In addition to factors such as rated current and rated switching capacity, a further criterion to be considered when implementing a protection de-vice is selectivity. Selectivity is im-portant because it ensures optimum supply reliability. The following crite-ria can be applied for selective opera-tion of series-connected protecopera-tion devices:
C Time difference for clearance (time grading)
C Combination of time and current grading (inverse time grading) Power direction (directional tion), impedance (distance protec-tion) and current difference (differen-tial protection) are also used.
Requirements for the selective be-havior of protective devices Protective devices can only behave selectively if both the highest and the lowest short-circuit currents for the relevant system points are known at the project planning stage.
As a result:
C The highest short-circuit current determines the required rated short-circuit switching capacity Icu/Icsof the circuit-breaker.
Criterion: Icu/Ics > IKmax
C The lowest short-circuit current is important for setting the overcur-rent release; the operating value of this release must be less than the lowest short-circuit current at the end of the line to be protected, since only this setting of Id/Isd guarantees that the instantaneous overcurrent release can carry out its personnel and system protec-tion funcprotec-tions.
Note: With these settings, the ad-missible tolerance limit of ± 20%
must be observed!
Criterion: Isd≤ IKmin – 20 % C The observance of specified
trip-ping conditions determines the maximum conductor lengths or their cross sections.
C Selective current grading is only possible if the short-circuit currents are known.
C In addition to current grading, par-tial selectivity can be achieved us-ing combinations of carefully
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C The highest short-circuit current can be both the three-phase and the single-phase short-circuit current.
C With infeed into LV power sys-tems, the single-phase fault current will be greater than the three-phase fault current if transformers with the Dy connection are used.
C The single-phase short-circuit cur-rent will be the lowest fault curcur-rent if the damping zero phase-se-quence impedance of the LV cable is active.
With large installations, it is advisable to determine all short-circuit currents using a special computer program.
Here, our SIMARIS design® planning software comes as the optimum so-lution (see Chapter 12).
Grading the operating currents with time grading
Grading of the operating currents is also taken into consideration with time grading, i.e. the operating value of the overcurrent release of the upstream circuit-breaker must be at least 1.25 times the operating value of the downstream circuit-breaker.
Scattering of operating currents in definite-time-delay overcurrent releases (S) is thus compensated (≤±10%).
Plotting the tripping characteristics of the graded protective devices in a grading diagram will help to verify and visualize selectivity.
Time sequence for circuit-breakers When grading the operating currents, the time sequence of the breaking operation of the circuit-breakers must also be taken into consideration.
Fig. 3/3 illustrates the individual time-related terms using two graded LV circuit-breakers as an example.
Grading time, delay time The grading time tsdis the interval required between the tripping charac-teristics of two series-connected protection devices to ensure correct operation of the protective device immediately upstream of the fault.
The delay time to be set at the cir-cuit-breaker tsdis obtained from the sum of the grading times.
L and magnitude of short-circuit current
tg1 Safety margin ta1 te1 tL1
to1
ta1 Operating time of breaker Q1 ta2 Operating time of breaker Q2 te1 Disengaging time of breaker Q1 te2 Disengaging time of breake Q2 td2 Delay time of breaker Q2
≈grading time tst2
to1 Opening time of breaker Q1 to2 Opening time of breaker Q2 tL1 Arcing time of breaker Q1 tL2 Arcing time of breaker Q2 tg1 Total clearance time of breaker Q1 tg2 Total clearance time of breaker Q2
(tg= to+tL)
Fig. 3/3 Time sequence for the breaking operation of two graded LV circuit-breakers in the event of a short circuit TIP_Kap3_E 11.08.2005 18:43 Uhr Seite 5
3.1.4 Preparation of Current-Time Diagrams (Grading Di-agrams)
Manual preparation General notes
When characteristic tripping curves are entered on log-log graph paper, the following must be observed:
C To ensure positive selectivity, the tripping curves must neither cross nor touch.
C With electronic inverse-time delay (long-time delay) overcurrent re-leases, there is only one tripping curve, as it is not affected by pre-loading. The selected characteristic curve must therefore be suitable for a motor or transformer at oper-ating temperature.
C With mechanical (thermal) inverse-time delay overload releases (L), the characteristic curves shown in the manufacturer catalog apply for cold releases. The opening times to are reduced by up to 25% at normal op-erating temperatures.
Tolerance range of tripping curves C The tripping curves of circuit-breakers
given in the manufacturer catalogs are usually only average values and must be extended to include tolerance ranges (explicitly shown in Fig. 3/4, 3/20 and 3/24 only).
C With overcurrent releases – instanta-neous (I) and definite-time delayed releases (S) – the tolerance may be
±20% of the current setting (accord-ing to EN 60947-2 / IEC 60947-2 / VDE 0660 Part 101).
Significant tripping times
For the sake of clarity, only the delay time (td) is plotted for circuit-breakers with definite-time-delay overcurrent releases (S), and only the opening time (t ) for circuit-breakers with
in-Grading principles
Delay times and operating currents are graded in the opposite direction to the flow of power, starting with the final circuit.
C Without fuses, for the load breaker with the highest current setting of the overcurrent release.
C With fuses, for the fused outgoing circuit from the busbars with the highest rated fuse-link current.
Circuit-breakers are preferred to fuses in cases where fuse links with high rated currents do not provide se-lectivity vis-à-vis the definite-time-de-lay overcurrent release (S) of the transformer feeder circuit-breaker, or only with very long delay times tsd (400 to 500 ms). Furthermore, circuit-breakers are used where high system availability is required as they help to clear faults faster and the circuit-breakers’ releases are not subject to aging – especially with consumers with very long infeed distances.
Procedure with two or more volt-age levels
In the case of selectivity involving two or more voltage levels (Fig. 3/39 ff.), all currents and tripping curves on the high-voltage side are converted and
Tools for preparing grading diagrams
C Standard forms with paired current values for commonly used volt-ages, e. g. 20/0.4 kV, 10/0.4 kV, 13.8/0.4 kV, etc.
C Templates for plotting the tripping curves
Fig. 3/4 shows a hand-drawn grading diagram with tripping curves for two series-connected circuit-breakers, not taking into account tolerances. The time sequence for the breaking oper-ation illustrated in Fig. 3/3 was used here (time selectivity). When the SIMARIS design planning software is used, a manual preparation of grading diagrams is no longer necessary.
td2≈180 ms
Current I (r.m.s. value) 101 2 3 4 6 102 2 3 4 6 103 2 3 4 6 104 2 3 4 6 105
Fig. 3/4 Grading diagram with tripping curves of the circuit-breakers Q1 and Q2 shown in Fig. 3/3
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Low-voltage time grading Grading and delay times
Only the grading time tgtand delay time tsdare relevant for time grading between several series-connected circuit-breakers or in conjunction with LV HRC fuses (Fig. 3/5).
The delay time tgt2of breaker Q2 can be equated approximately with the grading time tgt2 ; the delay time tgt3 of breaker Q3 is calculated from the sum of the grading times tgt2+ tgt3. The resulting inaccuracies are cor-rected by the calculated grading mar-gins. In the interests of simplicity, only the grading times are added.
Proven grading times tgt Series-connected circuit-breakers:
Those so-called "proven grading times"
are guiding values or rules of thumb.
Precise information must be obtained from the device manufacturer.
C Grading between two circuit-break-ers with electronic overcurrent releases (Q1 and Q2) should be
about 70-80 ms
C Grading between two circuit-break-ers with different release types (Q2 = ETU and Q3 = TM) should be about 100 ms
C For circuit-breakers with ZSI (zone-selective interlocking, i.e. short-time grading control) the grading di-stance has been defined as 50 ms Irrespective of the type of S-release (mechanical or electronic), a grading time of 70 ms to 100 ms is neces-sary between a circuit-breaker and a downstream LV HRC fuse.
Between an LV HRC fuse and a downstream circuit-breaker, a grad-ing time tgt(safety margin) of at least 1 s must be maintained from the prearcing-time/current characteristic of the LV HRC fuse to the point at which the tripping curves L and I or S intersect, in order to allow for the scatter band of the L-release (Fig.
3/6).
L S
Q3
Q2
M