Zark Bedalov - Practical Power Plant Engineering

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: MVA sc= = 113 MVA.

Fault contribution from 5 MVA motor load: = 29 MVA sc,

It totals to about 142 MVA, or 19.7 kA scon 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.

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.

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:

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.

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.