Skip to main content

Infinite bus and HVDC vs HVAC comparison

Short answer

An infinite bus has constant voltage and frequency, zero impedance, infinite inertia; a large grid approximates it. HVDC uses 1–2 conductors, no reactive power or stability limit, but expensive converters; economical beyond ~600 km. HVAC uses 3 conductors, uses transformers, but has charging current and stability limits, cheaper terminals for shorter lines.

Describe the concept of infinite bus in a power system. Compare between HVDC and HVAC systems for power delivery.

Infinite bus

An infinite bus is an ideal bus whose voltage magnitude and frequency remain constant, whatever real or reactive power is drawn from or injected into it. It behaves like an ideal voltage source with zero internal impedance and infinite inertia.

Properties:

  • Voltage and frequency are fixed; the phase angle is the reference.
  • Thevenin impedance seen from the bus is zero (Zth→0Z_{th}\to0, fault level →∞\to\infty).
  • Any single machine connected to it cannot change its V or f; the machine only changes how much P and Q it exchanges.

In practice, a large interconnected grid is treated as an infinite bus when the machine or load connected to it is small compared with the total system capacity.

 [G small]--X--+ Infinite bus (V, f const)
               | = very large grid

HVDC vs HVAC

BasisHVDCHVAC
Conductors1 or 2 (monopole/bipole)3 per circuit
Line costLower (towers, conductors)Higher
Terminal costHigh (converters, filters)Low (transformers)
Break-even distanceEconomical beyond ~600–800 km overhead, ~50 km cableEconomical for shorter lines
Reactive power, charging currentNonePresent; needs compensation
Skin effect, coronaNone / lowerPresent / higher
Stability limitNo stability limit on distanceLimited by P=VsVrsin⁡δ/XP = V_sV_r\sin\delta/X
Asynchronous linkCan connect 50 Hz to 60 HzNeeds same frequency
Power flow controlFast and preciseDetermined by impedances
Fault levelNot increased by linkIncreases on interconnection
Voltage transformationNot possible directlyEasy with transformer
Circuit breakingDifficult (no current zero)Easy
HarmonicsGenerated; need filtersNot a major issue

HVAC is used for most transmission and all distribution; HVDC is chosen for very long bulk transmission, submarine cables and linking asynchronous grids.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Power System Analysis I chapter 1 questions →