A good engineering practice dictates that breakers can be loaded to up to 80% of their nominal ratings. For instance 960 A for a 1200 A frame breaker. However, in case of an emergency like having a failure of one incoming transformer, the incoming breaker can be loaded closer to its nameplate rating.
Since the maximum incoming current at 13.8 kV is expected to be 1675 A (40 MVA), we can choose 2000 A incoming breakers. The breaker maximum loading is calculated to be 84% of the expected maximum rated current. The switchgear bus will be of the same rating, as the incoming breakers.
The most common types of breakers at 4.16 and 13.8 kV are vacuum type breakers.
2.7.4 Switchgear Breaker Ratings
The breakers must be selected based on their continuous capability and short‐circuit interrupting duty. The interrupting rating of the switchgear breakers will be based on the short‐circuit fault contributions from the source and the plant motor load to a fault on its bus. For the calculations of fault contributions on the 13.8 kV side of the main transformers, we will use conservative values and ignore the system source impedance, i.e. we will assume it to be zero, as noted earlier.
Let us assume the impedance of the main transformers is 9%.
For the selection of the switchgear breakers it is required to determine the following:
Voltage: It must be the most economical voltage, at which we can transmit power throughout the plant.
BIL level: It goes with the selected voltage and the method of system grounding as a standard. For 13.8 kV switchgear it is 95 kV peak for indoors and 110 kV peak for outdoors.
Breaker continuous 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