Infinite bus and advantages of HVDC

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 ($Z_{th}\to0$, 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.

HVDC transmission single line diagram

 AC sys 1                           AC sys 2
    |                                  |
 [Conv Tx]                         [Conv Tx]
    |                                  |
 [Rectifier]==+==== DC line ====+==[Inverter]
    |      [Smoothing L]  [Smoothing L]  |
 [AC filters]                     [AC filters]

The rectifier converts AC to DC, power flows over the DC line, and the inverter converts it back to AC at the receiving end.

Advantages of HVDC over HVAC

1. Fewer conductors: Only 1 (monopole with earth return) or 2 (bipole) conductors; lighter, cheaper towers and narrower right-of-way.
2. Lower line losses: No skin effect, no reactive current; corona loss is smaller.
3. No charging current: Ideal for long submarine and underground cables.
4. No stability limit: No power angle, so very long lines can be used without stability problems.
5. Asynchronous interconnection: Can link 50 Hz and 60 Hz systems or unsynchronised grids.
6. Fast power control: Converter firing angle controls power quickly and precisely; can damp oscillations.
7. No increase in fault level of the connected AC systems.
8. Less insulation: Peak voltage equals working voltage (AC peak is $\sqrt2$ times RMS).
9. No reactive compensation needed along the line.

Limitations: costly converter stations, harmonics needing filters, reactive power required by converters, difficulty in DC circuit breaking, and no direct voltage transformation.