Distribution System Fundamentals and Theory
Foundation
Build a mental model of the distribution system: substation bus, feeder breaker, feeder main, laterals, distribution transformers, secondaries, and customer service. Know the common primary voltage classes and why higher voltage reduces line losses for a given power. Understand three-phase fundamentals - wye versus delta, line versus phase quantities, and the √3 relationship between them. Power factor, reactive power, and the effect of capacitors on voltage and losses are recurring topics.
Concepts to master
Per-unit analysis simplifies calculations across transformers. Voltage drop depends on conductor impedance, load current, and power factor; it is not simply proportional to current. Load diversity and demand factors explain why transformer sizing differs from the sum of connected loads. Know the difference between a radial and a looped system and how each affects reliability and fault current. Symmetrical components explain unbalanced faults, but distribution questions usually stay at the level of sequence of events and protective coordination.
Traps
Questions often conflate line and phase voltage, or assume adding capacitors always raises voltage everywhere on the feeder. Another trap is treating connected load as demand.
Rule of thumb
In a balanced wye, line voltage equals √3 times phase voltage, and line current equals phase current; in a delta, line current equals √3 times phase current.
Worked example
A 12.47 kV wye feeder has a phase-to-neutral voltage of about 7.2 kV; the phase-to-phase value is 12.47 kV. A capacitor bank reduces reactive current, lowering losses and typically raising voltage near the bank.