Ideally, transformer feeders should be protected by:
HV HRC fuses
High-voltage high-rupturing-capacity (HV HRC) fuses used in conjunction with switch-disconnectors for rated transformer outputs of up to approx.
1,250 kVA for low switching rates, or Circuit-breakers with protection Circuit-breakers with protection (see page 54) from approx. 800 kVA and for high switching rates; also when several circuit-breakers with S-re-leases are arranged in series on the low-voltage side and selectivity is not possible with upstream HV HRC fuses.
The anticipated selectivity ratios must, therefore, be checked before the protection scheme is chosen and dimensioned.
Protection by means of HV HRC fuses
Dimensioning HV HRC fuses The rated current of the HV HRC fuses specified by the manufacturers for the rated output of each trans-former should be used when dimen-sioning the HV HRC fuses. The low-est rated current is dictated by the rush currents generated when the transformers are energized and is 1.5 to 2 times the rated transformer currents.
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Minimum breaking current Ia min For the determination of the maxi-mum rated current it must be ob-served that, with short circuits on a transformer’s secondary side, the minimum breaking current Ia min of the fuse must be exceeded, affecting even the installation's busbar system.
Generally, the load on Ia min is 4 to 5 times that of the transformer’s rated current. Between these limit values, the fuse link can be chosen according to selectivity.
Back-up protection with transmission range
HV HRC fuses must ensure sufficient back-up protection in case of a possi-ble failure of the downstream protec-tive device. The required transmis-sion range can be seen in Fig. 3/38, illustrated for three circuit diagrams.
The working range of the back-up protection increases with the de-creasing protective rated current of the fuse.
Safety clearances between the melting current characteristic of HV HRC fuses and other protective devices
Rated currents of LV HRC fuses must be selected in such a way that, be-tween the established maximum short-circuit current near the low-volt-age side’s busbar system (converted to the medium-voltage side) and the minimum breaking current Ia min (cir-cle in the melting current characteris-tic), a minimum safety clearance of 25% is observed from Ia min to the transformer’s short-circuit current Ik (see Fig. 3/39 to 3/43).
7RM network master relay S
Fig. 3/38 Protection zones of HV HRC back-up fuses necessary for various protection devices used on the low-voltage side
1000 2000 3000 5000 7500 10000 20000 50000
40 80 120 200 400 800 2000
Further data on safety margins for gradings as shown in Fig. 3/38, case b and c, for example, can be found in the following sections.
Grading of HV with LV HRC fuses in infeed circuits
Grading with LV HRC fuses
Example of a transformer with a rated output of 400 kVA (Fig. 3/39): LV HRC
current 630 A) are mainly used for transformers with rated outputs of up to 400 kVA; circuit-breakers with overcurrent releases are used on the low-voltage side for rated outputs
≥500 kVA.
Tangent prearcing time/current characteristics F2 (LV HRC) and F3 (HV HRC) – referred to 0.4 kV – and possible tripping of the switch discon-nector on the medium-voltage side by the upstream HV HRC fuse are ac-ceptable, since both fuses protect the same system element and interrup-tion will occur in all cases (restricted selectivity). HV HRC fuses with higher rated currents (e.g. 160 A as shown in Fig. 3/40) would not be suitable here, since their minimum breaking current Ia minis 12 kA, i.e. well above the short-circuit current Ikwhich the transformer can carry (max. 9.5 kA).
Grading of HV HRC fuses with circuit-breakers for mesh-con-nected systems and downstream LV HRC fuses
Selecting the HV HRC fuse rating In meshed systems with several transformers and parallel system op-eration, the LV feeder circuit-breakers are not fitted with overcurrent re-leases (LS) but, instead, have sepa-rate 7RM19 network master relays which only respond to reverse cur-rents.
Given the absence of the LS-release as a grading element, the back-up protection range of the HV HRC fuse must be extended as shown in Fig.
3/38, case b. In Fig. 3/40, this is achieved by selecting the HV HRC fuse with the lower current rating.
F1
1000 2000 3000 5000 10000 20000 50000
40 80 120 200 400 800 2000
tk Command time for network master relay of circuit-breaker Q1 tvs Virtual prearcing time of fuses
Ik Short-circuit current with individual transformer operation
Q1 Tripping characteristic for network master relay set to 1.2 In transf. = 1,200 A Minimum breaking current Ia min of HV HRC fuse
Ia min≈25% Safety margin
Fig. 3/40 Example showing grading with HV HRC fuses – network master relay in the
infeed – and LV HRC fuses in the outgoing feeder; transformer rating 630 kVA
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The proximity of the characteristics F1 and F2 does not have a detrimen-tal effect on selectivity in this case, since the ring interconnections also function as infeeds in the event of a fault, which means that selectivity is improved as a result of the total short-circuit current Ik∑in the feeder being distributed among the infeeds (with the two ring interconnections in Fig. 3/40). This is possible because a higher short-circuit current (Ik∑) flows through the LV HRC fuse F1 than through the HV HRC fuse F2 (IkT).
Grading of HV HRC fuses with low-voltage circuit-breakers and down-stream LV HRC fuses using a 630 kVA transformer as an example Requirements
Selectivity is required between the protection devices of the feeders and those of the infeed, which form a functional unit; a safety margin of at least 100 ms is necessary between the characteristic of an LV HRC fuse and that of an S-release (Fig. 3/41).
Between LV HRC fuses and S-releases
Thus, selectivity is achieved with the 400 A LV HRC fuse-link used in the example. The setting and delay time tdmust be adjusted with the S-re-lease (6 kA setting).
In such cases, selectivity can be achieved more easily using downstream circuit-breakers, e.g. SENTRON WL (Fig. 3/43), or using a considerably more powerful transformer, the associ-ated circuit-breaker enables the S-re-lease to be set to a higher value.
F1 (optional 160 A) F2
1000 2000 3000 5000 10000 20000 50000
40 80 120 200 400 800 2000
tn Prearcing time for fuses td Delay time for S-release
Minimum breaking current Ia min of HV HRC fuse Ia min
≈25% Safety margin (requirement)
Safety margin 100 ms Safety margin 100 ms
(required)
Fig. 3/41 Example showing the grading of HV HRC fuses F2 with circuit-breaker Q1 and downstream LV HRC fuse F1 in the outgoing feeder
Between HV HRC fuses and S-releases
Since the protection devices in the in-feed form a functional unit, a restric-tion in selectivity in the upper short-circuit current range is accepted in the case of faults in the vicinity of the busbars (as indicated by the circle in the diagram) for the 100 A HV HRC fuse in Fig. 3/42.
Safety margin between HV HRC fuse and S-release
If, on the other hand, selectivity is re-quired, e.g. with different switching priorities at the two voltage levels or in order to avoid the medium-voltage switchgear having to be switched off, for example, when HV HRC fuses are replaced, there should be a safety margin of approximately 100 ms on the base line Ikbetween the charac-teristic curve of the S-release and the left-hand limit of the scatter band of the prearcing-time/current character-istic of the HV HRC fuse.
Scatter band of HV HRC fuses According to EN 60 282-1/ DIN VDE 0670-4, the scatter band width of HV HRC fuse-links can be ± 20%.
Siemens HV HRC fuse-links have a scatter band width of ± 10%.
Result:
When selecting circuit-breakers in-stead of low-voltage fuses, selectivity can easily be attained, overlapping of characteristics Q1 and F2 should be avoided, as it could result in erro-neous tripping. For such cases, HV HRC fuse-links with a higher rated current should be chosen.
F1
(optionally 160 A) F2 (choose Q1) 100 A
1000 2000 3000 5000 10000 20000 50000
40 80 120 200 400 800 2000
≈25% Safety margin (required)
More pronounced intersection of Q1 and F2 must be avoided if possible
td2=220 ms
Fig. 3/42 Example showing the grading of HV HRC fuses F2 with circuit-breaker Q2
and downstream circuit-breaker Q1 with LS-release in the outgoing feeder
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Protection by means of circuit-breakers with definite-time over-current protection (DMT)
Requirement
The two feeder circuit-breakers (in Fig. 3/43) form a functional unit and require selectivity with respect to the protection devices on the low-voltage side.
Outgoing feeders with LV HRC fuses
If low-voltage fuses are connected downstream, selectivity with circuit-breakers with mechanical releases (3WF) can only be achieved up to a certain maximum fuse current rating;
in the example, Q2 with mechanical S-releases (setting range 3 to 6 kA)
≤400 A for F1. Larger LV HRC fuses are also selective if SENTRON WL circuit-breakers with an S-release range of 2 to 12 · Irare used.
Outgoing feeders with mixed components
If outgoing feeders with mixed com-ponents are used, the safety margin of at least 100 ms relative to the largest permissible LV HRC fuse-link for F1 is the crucial factor in determin-ing the settdetermin-ing for the S-release of Q2. In the case of mechanical S-re-leases with the highest current set-ting of 6 kA, this results in a delay time tdof 220 ms for the smallest permissible safety margin of 100 ms.
This determines the starting point for all subsequent upward and down-ward gradings in the diagram.
Outgoing feeders with circuit-breaker
Since selectivity cannot be achieved using LV HRC fuses with a higher current rating (see Fig. 3/41), circuit-breakers with time or, if possible, current grading should be used.
Q3 1,000 2,000 3,000 5,000 10,000 20,000 50.000
40 80 120 200 400 800 2,000
to Opening time of circuit-breaker (Q1) td Delay time of “S“ release (Q2) tvs Prearcing time of fuses F1
tc Command time of DMT protection (Q3) F1
400 A
S = 6 kA Q2
Base IkT <15 kA (individual operation)
A at 0.4 kV
Fig. 3/43 Example showing the grading of circuit-breaker with DMT protection (Q3), SENTRON WL circuit-breaker, 1000 A with LS-releases (Q2) and downstream outgoing feeders, e.g. 400 A LV HRC fuse (F1) and 630 A distribution circuit-breaker (Q1) in a 630 kVA transformer feeder
Based on the assumption that verifi-cation of the short-circuit currents would show that current grading would be possible, a 630 A (Q1) dis-tribution circuit-breaker with LI-re-leases was selected.
Intersection of the characteristics Q2 and Q3 in the middle short-circuit range is permissible because:
C the L-release of the low-voltage circuit-breaker Q1 (not shown in Fig. 3/43) protects the transformer against overloading, which only occurs in the range 1–1.3 times the rated current of the transformer;
C there is a safety margin of ≥150 ms (≈300 ms in the example shown in Fig. 3/43) between the I > tripping value of the DMT protection and the LV HRC fuse characteristic F1 and selectivity is, therefore, achieved.
Higher rated transformer outputs and broader setting ranges for the S-release of Q2 make it easier for the characteristic Q3 I > to be shifted to the left of the characteristic Q2 s.
This also provides a certain degree of back-up protection with respect to the L-release of circuit-breaker Q2.
DMT protection
Nowadays, digital devices are used to provide DMT protection in practically all applications. They have broader setting ranges, allow a choice be-tween definite-time and inverse-time overcurrent protection or overload protection, provide a greater and more consistent level of measuring accuracy and are self-monitoring.
Selecting current transformers for DMT protection
The following points should be ob-served when selecting current trans-formers for DMT protection (these considerations are applied in the ex-ample shown in Fig. 3/43):
Current transformers with a rating of 40 to 200 A could be selected for rated currents of 36.4 A on the high-voltage side of the 630 kVA transformer, with the characteristic Q3 I> at 200 A po-sitioned at the abscissa for 10 kV and with the broad setting ranges. Here, it is important to bear in mind the higher investment costs for current transformers with lower rated primary currents.
If, for example, 60/1 A current trans-formers are selected, the current sensors must be set as follows:
Setting the current sensors I>, I>>
and timing elements Current sensor I >:
The setting for a selected operating value of 200 A is as follows:
200 A ______
Ip=
60/1 = 3.3 A
Timing element for I > excitation:
ti> = 0.5 s
Current sensor I>>:
The current sensor I>> should only re-spond to faults on the high-voltage side (in the shortest possible time).
Operating current I>> approximately IkT· 1.20 Operating current (in secondary circuit) =
728 A ______
Ip=
60/1 ≈ 12.1 A