T/F: interconnection decreases SCMVA

FALSE. Interconnection increases the short-circuit MVA (SCMVA) of the buses that are connected.

Justification

• The short-circuit MVA at a bus is

$$SCMVA = \frac{MVA_{base}}{Z_{th}\ (\text{pu})}$$

where $Z_{th}$ is the Thevenin impedance seen from the faulted bus.

• When a bus is tied to another system, the second system's generators feed the fault through the tie line. The tie path is in parallel with the existing source impedance, so $Z_{th}$ becomes smaller.
• Smaller $Z_{th}$ means larger fault current and larger SCMVA.

Example: a bus with $Z_{th} = 0.2$ pu (100 MVA base) has SCMVA $= 500$ MVA. If an interconnection adds a parallel path of $0.3$ pu, then $Z_{th} = (0.2 \times 0.3)/(0.5) = 0.12$ pu and SCMVA $= 833$ MVA.

Consequences

• Circuit breakers and switchgear at the connected buses may need higher breaking capacity.
• A higher SCMVA is also good: the bus becomes a "stiffer" source, so voltage dips and flicker during load changes are smaller.
• Fault levels can be limited by series reactors, high-impedance transformers or HVDC links (which do not add fault current).