rating: It is usually the minimum frame size that can carry the load with 20% spare.
Breaker interrupting kA capacity: It is based on the calculation of the bus fault for the worst‐case scenario, with at least 20% spare.
Bus continuous and interrupting rating: It is determined in a similar manner as for the breakers.
Let us proceed with the basic calculations:
We will calculate short‐circuit fault levels on the MVA basis and then convert it to kA (see Chapter 3 for the breaker ratings on kAic [kA interrupting capacity] basis).
The worst‐case short‐circuit scenario for the 13.8 kV switchgear bus is one with the plant operating with a single main transformer and the 13.8 kV bus tie breaker is closed. The interrupting rating for the 13.8 kV switchgear is available at 25, 40, 50, and 63 kAic r.m.s. symmetrical.
Contribution from the grid is based on infinite bus criteria (source impedance = 0). Therefore, the contribution through the transformer impedance is MVAsc = 30/0.09 = 333 MVA.
Assume 60% of the plant‐operating load is motor load (0.60 × 40 MVA = 24 MVA).
The motor impedance X″ = 17% (0.17 pu) on the motor kW base, as per ANSI standards.
Motor fault contribution can be then calculated as follows: 24/0.17 = 141 MVA. The cycloconverter is not taken into account as a short‐circuit fault contributor. It is considered a DC load. VFD‐operated loads are not included either. If this work were being done on a computer as a system study, the 4 kV motors would be entered individually, while the motor load on the 480 V buses would be entered as grouped loads, one for each 480 V bus.
Therefore, the total fault at the 13.8 kV busbar is 333 + 141 = 474 MVA.
Add a 25% margin to this figure: 1.25 × 474 MVA = 592 MVAsc, or 24.8 kAsc.
Select switchgear and breaker rating: 40 kAic r.m.s. symmetrical.
Based on the aforementioned, we conservatively determine, without taking into account the cables and line impedances, that 13.8 kV switchgear (breakers and bus) should have interrupting rating of 40 kAic.
Furthermore, we determine the 13.8 kV feeder breakers, which are delivering power to the plant can be sized either 800 or 1200 A continuous rating as applicable for the various plant load centers. In Europe and Asia, it is common to use a mixture of breaker size on the same bus. In North America, the breakers tend to be of the same frame size for better exchangeability and fewer spare parts.
We now wish to determine the interrupting rating of the 4.16 kV plant switchgear. The impedance of the plant transformers is assumed to be 7% on their 12 MVA basis. We will also include the impedance of the main (grid) transformers of 9% on a 30 MVA base. All the transformer impedances must be converted on a common base, in this case 30 MVA, as follows:
We calculate the interrupting rating of the 4.16 kV circuit breakers on 30 MVA basis:
Fault contribution from the source: MVAsc =
Fault contribution from 5 MVA motor load:
It totals to about 142 MVA, or 19.7 kAsc on the 4.16 kV bus. By adding a margin factor of 1.20–1.25 to the breakers and the 4.16 kV switchgear, we conservatively select 40 kAic r.m.s. symmetrical.
The BIL insulation level rating for this type 4.16 kV indoor switchgear is 60 kVpeak.
The preceding calculations were based on the fact that the main transformers were not operating parallelly. The transformer pairs are of equal design and construction and with approximately equal impedances. If we allow the parallel operation, the voltage profile would improve throughout the plant. Large plant motors would likely start without any difficulty. So why do not we operate the plant with the transformers in parallel?
If we allow a parallel operation, the fault contribution from the source to the 13.8 kV switchgear would come from both the main transformers, as follows:
Therefore, with the motor contribution added, the fault level would considerably increase. The interrupting rating required for our 13.8 kV switchgear for this application would be a step higher, which may be cost excessive for 13.8 kV. If you observe the one‐line diagram, motor contribution from the 480 V buses to faults on their own buses would be increased. However, the contribution to the other buses would be minor due to the cable, O/H line and transformer impedances between the 480 V bus and the fault located at any other bus.
The continuous ratings for the bus tie and the incomer breakers will be determined further in Section 2.7.6.
2.7.5 Single Incomer Substation for a Small Plant
Let us review a small plant having a 5/7 MVA, ONAN/ONAF, 138 to 4.16 kV transformer. A single incoming transformer would suffice. This plant may have some 4.16 kV and some 480 V loads. The 138 kV primary side may include a 600 A fused load interrupter with arcing horns to allow for switching the magnetizing current of the unloaded transformer. Except for switching the magnetizing current, while the LV side is kept open, the switch is not allowed to be operated. The fuse serves for short‐circuit protection only. The maximum incomer current on the transformer 4.16 kV secondary side is 970 A < 80% of 1200 A frame breaker for continuous rating.
A 1200 A rated breaker would be appropriate for this application. The incomer is fed to a single 4.16 kV switchgear 1200 A bus.
2.7.6 13.8 or 33 kV Switchgear for a Larger Plant
There is a correlation between the rating of the switchgear incoming breakers and the incoming transformers ratings.
For an even larger plant primary distribution system, the use of 33 kV would be likely, with the transformer incoming and bus tie breakers rated at 2000 A. These 33 kV breakers can accommodate 60/80 MVA, ONAN/ONAF incoming transformers at 80% of their breaker nameplate ratings. The switchgear bus can also be selected as 2000 A, while the plant feeders to the individual plant load centers will be any of those available: 1200, 1600, or 2000 A breakers. In this instance, we can standardize on a continuous rating of 1200 A, which can be loaded up to 960 A (55 MVA at 33 kV), i.e. 80% of the breaker nominal rating and 100% of the breaker nameplate rating for a temporary emergency usage.
Here is the switchgear for a large plant either at 13.8 or 33 kV, metal‐clad or 33 kV gas insulated switchgear (GIS) with vertical sections having single breaker positions. In summary, the two switchgear assemblies are rated as follows:
| Nominal voltage | 13.8 kV | 33 kV |
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BIL, indoors/outdoors
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