FACTS and HVDC for Nepal's power industry

FACTS devices

FACTS (Flexible AC Transmission Systems) are power-electronic controllers that control voltage, line reactance and phase angle of AC lines in real time. Since $P = \frac{V_sV_r}{X}\sin\delta$, changing these lets lines carry more power with good stability. Main devices: SVC and STATCOM (shunt, reactive power and voltage control), TCSC and SSSC (series, line reactance control), and UPFC (combined, controls real and reactive power).

HVDC transmission

HVDC converts AC to DC with a rectifier, transmits DC over a two-conductor line or cable, and converts back to AC with an inverter. It suits long-distance bulk power, long cables and asynchronous interconnections, has lower line losses, and gives fast power control. Types: monopolar, bipolar, homopolar, back-to-back and multi-terminal.

Use in upgrading Nepal's power industry

FACTS:

• Nepal's network has long radial 132 kV lines with poor voltage at the tail end (e.g. far-west, hilly areas). STATCOM/SVC can hold voltage and cut losses.
• During the wet season lines are heavily loaded with hydropower export; TCSC can raise the power transfer of existing 220/400 kV lines instead of building new ones in difficult terrain.
• In the dry season, lightly loaded 220/400 kV lines cause overvoltage (Ferranti effect); shunt FACTS can absorb VArs.
• They improve stability of the weak grid when it is linked to the large Indian grid, and damp oscillations.

HVDC:

• Large export of surplus hydropower (thousands of MW from Karnali, Koshi, Arun basins) to distant Indian load centres or Bangladesh can be done by HVDC with lower losses and narrower ROW.
• A back-to-back HVDC station at the border allows asynchronous, controlled power trade, protecting Nepal's grid from disturbances in India's grid, and would be needed for any link with China.
• VSC-HVDC can give voltage support and black start to the weak Nepali grid.

Together, FACTS improve the use of the existing AC grid, while HVDC opens bulk export and regional trading, helping Nepal reduce spill energy and earn revenue.