Bus and bay current ratings

The design gives a continuous rated current to the high-voltage (HV) buses, the HV bays, the medium-voltage (MV) incomers and the MV bus sections. It takes the HV currents per voltage level and sets the MV ratings from the transformer's low-voltage current.

1HV buses and baysCopy link to HV buses and bays

The design takes a rated current for the HV main buses and one for the HV bays per level, from its level table. In the IEC + CEA / CBIP set these are the design's values per level. They are not calculated from the power flow of the project. The HV current transformers take the bay rated current as their primary current when CT primary current is on Auto.

In the ANSI / IEEE set, the bus and bay currents are continuous currents that IEEE C37.04-2018 lists on the line of the level's rated short-circuit current. That is Table 12 at 69 kV, below 100 kV, and Table 16 at the higher levels. The currents of both sets are in the table.

The first rows are the IEC + CEA / CBIP set's levels, from 33 kV to 765 kV. The last rows are the ANSI / IEEE set's, from 69 kV to 345 kV.

2MV incomers and busesCopy link to MV incomers and buses

An MV incomer carries the full current of one transformer low-voltage winding. The design calculates that current from the winding's rating $S$ and the collector bus voltage $U$ as $I_r=S/(\sqrt{3}U)$. A transformer with split low-voltage windings, at 400 kV in the IEC + CEA / CBIP set and at 345 kV in the ANSI / IEEE set, has two low-voltage windings. Each has half the transformer's rating and its own MV section.

The incomer's rating is the next standard rated current at or above $I_r$. In the IEC + CEA / CBIP set the design's list of standard currents is 630 A, 800 A, 1250 A, 1600 A, 2000 A, 2500 A, 3150 A, 4000 A, 5000 A and 6300 A. In the ANSI / IEEE set it is the smallest continuous current IEEE C37.04-2018 lists at the collector bus's rated maximum voltage, for the breaker's class. Indoor switchgear takes the breakers for cable systems, and an outdoor yard those for line systems. A current above the largest listed one takes the largest, and a warning on Transformer rating asks for a smaller transformer or more transformers.

An MV bus section takes the incomer's rating, since one transformer's full current can flow along its section to reach its incomer. The design takes 4000 A as the top of the indoor switchgear ranges at the collector bus. An incomer above it gives a warning on Switchgear on the MV system page, which names outdoor switchgear, a lower transformer rating or more transformers.

3Short-time withstandCopy link to Short-time withstand

The rated currents above are continuous currents. The short-time withstand of the buses and bays is a separate rating, the design fault level of the table for its duration. The short-circuit study checks it against the thermal equivalent short-circuit current $I_{\mathrm{th}}$ (Peak current and thermal equivalent current).

The design does not calculate the conductors' ampacity or the buses' thermal and mechanical withstand from these currents.