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 |
| BIL, indoors/outdoors | 95 kV/110 kV | 170 kV |
| Phasing | 3 ph, 3 w | 3 ph, 3 w |
| Interrupting capacity | 40 kA r.m.s. symmetrical | 31 kA r.m.s. symmetrical |
| Construction | Metal‐clad | Metal‐clad or GIS |
| Incoming and tie breakers | 2000 A | 2000 A |
| Normal operation, incomers | 37.6 MVA at 80% | 72 MVA at 80% rating |
| Maximum loading, incomers | 47 MVA at 100% | 92 MVA at 100% rating |
| Feeder breakers | 1200 A | 800, 1200 A |
The breaker 2000 A rating for the incoming and tie breakers at the 13.8 kV switchgear is more than adequate for both normal and emergency duty. The branch feeders on the switchgear are usually positioned in such a way to minimize the power flow across the bus tie breaker.
Evidently, 13.8 kV voltage can be used for the plants up to 50 MVA. Certainly, 20 or 33 kV voltage can be equally applied and used for this 40 MVA plant as well as in even larger industrial facilities as the primary plant distribution voltage. The economies of these options would have to be worked out during the design phase of the project, when the largest plant loads are confirmed.
2.7.7 Transformer Connections: Cable, Cable Bus, or Bus Duct?
Let us now return to our 30/40 MVA plant. We confirm that the primary distribution will be at 13.8 kV, secondary at 4.16 kV, and LV at 480 V. Therefore, the primary 13.8 kV circuits will be brought into each process building: crushing, milling, floatation, and processing. Additional feeders will be needed for remote tailings and camp.
230 kV incoming to main 30/40 MVA transformers: The transformers will be located outdoors as part of the main switchyard. The HV connections to the transformer bushings will be by bare conductor drops from the switchyard overhead buses.
13.8 kV connections from the main transformers to 13.8 kV switchgear: The plant must be capable of working with a single transformer outage on its ONAF rating at 40 MVA → 1675 A. The transformers will be placed adjacent to the switchgear building with the LV side facing the 13.8 kV switchgear. This will be a short straight run by a three‐phase 2000 A cable bus into the transformer top bushings. Use 133% rated cables for ungrounded systems. A cable bus installation is far simpler, more flexible, and less costly than a rigid three‐phase bus duct.
HV connection to 12/15 MVA, 13.8 to 4.16 kV transformers: The HV connection will be rated at 650 A for 15 MVA ONAF rating. This can be accomplished by a drop from a 15 kV overhead line pole as a buried cable connection of 2–500 kcmil (2–250 mm2) per phase, 133% cable insulation for ungrounded systems, into the transformer HV cable boxes. Lightning arresters are required at transition and the overhead line to cables.
LV connection to 12/15 MVA, 13.8 to 4.16 kV transformers: A cable bus from the transformers to the switchgear: 2100 A, 3 ph for 15 MVA, 133% cable insulation for ungrounded systems, with top entry from the transformer bushings into 4.16 kV switchgear, also as top entry.
HV connection to 2/3 MVA, load center transformers at 13.8 kV: 150 A cable drop from overhead line, 133% cable insulation for ungrounded systems to the HV load interrupter on the dry type transformers. Lightning arresters are required at the cable/line transitions.
At 4.16 kV: 400 A cable from 4.16 kV switchgear, 133% cable insulation for ungrounded systems. Load interrupter is not required if the load center is in the same room as the 4.16 kV switchgear. Lightning arresters may not be required. Those arrester used on the transformer HV side are considered also effective for the LV side. However, arresters are inexpensive at this voltage level. It will not hurt to install them in here.
LV connection at 480 V from 2/3 MVA transformers to LV Switchgear: This connection is assumed to be butted directly from the transformers to the LV (480 V) switchgear buses.
Overcurrent Settings for the Incomers: An electrical inspector in USA will likely verify and insure that the interrupting device (breaker) always has a lower setting than the cable it is protecting. In the case of the 13.8 kV incomers, even though the cable bus may have a rating of 2000 A and it connects to a 2000 A frame breaker, the breaker would likely have the protective relay overcurrent trip set lower than the cable bus rating, for instance 1700 A.
Or to be even more selective, the relay settings of the incomers could be arranged to have two groups of settings: 900 and 1700 A. The Group 1 setting of 900 A is for the two incomer breaker operation and as soon as one transformer fails and the full 13.8 kV load is transferred to one breaker only, the relay setting change would be initiated to Group 2 (1700 A) for the 40 MVA transformer rating by the breaker status interlock of the failed breaker. Similarly, the same can be arranged for the 4.16 kV incomers.
2.7.8 Medium Voltage Switchgear and Controllers (4.16 kV)
The 4.16 kV connected load is estimated to be around 15 MW at 0.8 pf (18.7 MVA). The 4.16 kV loads are critical loads for the plant production, requiring adequate availability of power supply. To establish the highest level of availability of the plant, the MV load board will be supplied from two 13.8 kV feeders from the main substation. The MV (5 kV) control assembly will be used to feed the plant MV motors rated over 200–2000 kW and 2/3 MVA, 4.16 kV to 480 V unit substations (Figure 2.9). The 4.16 kV loads will be controlled by the MV controllers, employing MV‐fused 400 A contactors.
The MV motor control assembly will be located close to the plant. It will have two incoming 4.16 kV switchgear feeder cells and a tie breaker. Each incomer breaker will be rated 2500 A to be able to carry the full MV load from a single incoming transformer of 12/15 MVA transformers.
The 4.16 kV motor control board is a metal enclosed assembly, NEMA class E2 (fused) with integral