---
title: "T/F: interconnection decreases SCMVA"
subject: "Transmission and Distribution System Design"
chapter: 1
key: "semester_8/transmission_and_distribution_system_design:tf-interconnection-scmva"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-interconnection-scmva
site: BCT Engineering
---

# T/F: interconnection decreases SCMVA

Transmission and Distribution System Design · Chapter 1: Introduction

- Asked in: 2074 Bhadra, 1+3 marks; 2072 Magh, 3 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-interconnection-scmva
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-1

## Question

State and justify whether the following statement is true or false: Interconnected system decreases SCMVA of the connected bus.

## Short answer

False. Interconnection adds parallel fault-current paths, reducing Thevenin impedance, so short-circuit MVA increases. SCMVA = base MVA / Z_th pu, and lower Z_th raises fault level.

## Answer

**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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is short-circuit MVA at a bus related to the Thevenin impedance?
   A. SCMVA = MVA_base x Z_th
   B. SCMVA = MVA_base / Z_th (pu)
   C. SCMVA = MVA_base / Z_th^2
   D. SCMVA = Z_th / MVA_base
   Answer: B. SCMVA = MVA_base / Z_th in pu, so a smaller Z_th gives a larger SCMVA. (Key point: SCMVA relation)

2. Why does interconnecting a bus with another system change its SCMVA?
   A. The tie path is in parallel with the source impedance, lowering Z_th
   B. The tie line adds series impedance, raising Z_th
   C. The tie line removes the local generators
   D. The tie line only changes the base MVA
   Answer: A. Other generators feed the fault through the tie, a parallel path, so Z_th falls and SCMVA rises. (Key point: Parallel path lowers Z_th)

3. A bus has Z_th = 0.2 pu on 100 MVA base and a parallel 0.3 pu path is added. What is the new SCMVA?
   A. 500 MVA
   B. 333 MVA
   C. 833 MVA
   D. 1000 MVA
   Answer: C. Z_th = 0.2x0.3/0.5 = 0.12 pu, so SCMVA = 100/0.12 = 833 MVA. (Key point: Numerical example)

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