Evolution of Power System

HVDC (High Voltage Direct Current) transmission sends bulk power as DC between two converter stations. Compared with HVAC it is cheaper and better for long lines and cables, but its converter stations are costly and complex.

Advantages of HVDC over AC

• Fewer conductors: a bipolar line needs only 2 conductors (3 for AC), so towers are lighter, the right-of-way is narrower and the line costs less.
• No stability limit on distance: there is no reactance or power angle on DC, so power transfer does not fall with length. Any distance can be used.
• No charging current: long cables (submarine or underground) can be used, while AC cables are limited to about 40–60 km.
• Lower losses: no skin effect, no reactive current, and less corona loss for the same voltage.
• Asynchronous link: it joins systems of different frequency (50/60 Hz) or unsynchronised grids, and does not raise their fault level.
• Fast power control: power flow is set by the converter firing angle in milliseconds, which helps damp oscillations.
• Lower insulation level: for the same power, the peak voltage on DC is lower than the AC peak ($\sqrt{2}$ times rms).

Disadvantages

• Costly converter stations (thyristor/IGBT valves, transformers, filters), so HVDC pays only beyond the break-even distance.
• Harmonics are produced; AC and DC filters are needed.
• Reactive power demand: line-commutated converters absorb 50–60% of the DC power as reactive power.
• DC circuit breakers are difficult to build because DC current has no natural zero.
• No transformers on DC: voltage cannot be stepped up or down easily, and tapping power along the route (multi-terminal) is hard.
• Converters have a low overload capacity.

Power electronics is the use of semiconductor switches (diodes, thyristors, IGBTs, GTOs, IGCTs, SiC MOSFETs) to convert and control electric power. It is vital today because almost every new part of the power system, from generation to the load, needs fast and controlled conversion of voltage, frequency or power flow.

Why it is vital

1. HVDC transmission: rectifiers and inverters (LCC with thyristors, VSC with IGBTs) make long-distance bulk transmission, long submarine cables and asynchronous interconnections possible.
2. FACTS devices: SVC, STATCOM, TCSC, SSSC and UPFC use power electronics to control voltage, impedance and phase angle in real time. They raise the power transfer capacity of existing lines and improve stability.
3. Renewable energy integration: solar PV gives DC and wind turbines run at variable speed, so inverters and back-to-back converters are needed to connect them to the 50 Hz grid. Grid-forming inverters now also provide synthetic inertia and voltage support.
4. Energy storage: batteries need bidirectional converters to charge and discharge into the AC grid.
5. Power quality: active filters, dynamic voltage restorers (DVR) and D-STATCOMs remove harmonics, voltage sags and flicker for sensitive loads.
6. Efficient loads: variable-frequency drives for motors save 20–50% of energy in pumps and fans; LED drivers, SMPS and induction heating are all power-electronic.
7. Electric vehicles: chargers and traction inverters are power converters; vehicle-to-grid (V2G) needs bidirectional control.
8. Microgrids and DC grids: converters let DC microgrids, data centres and multi-terminal DC grids connect with the AC network.
9. Protection and control speed: converters respond in milliseconds, much faster than mechanical tap changers or breakers, which helps fault ride-through and stability.

Benefits gained

• Better controllability of power flow (the grid becomes "flexible").
• Higher efficiency and lower losses.
• Use of existing lines closer to their thermal limit.
• Easy integration of distributed and renewable generation.

Challenges

Harmonic generation, lower system inertia, cost of devices, and need for good cooling and control. These are being reduced by wide-bandgap (SiC, GaN) devices and modular multilevel converters (MMC).

Because the modern grid is moving towards renewable, distributed and controllable power, power electronics has become the "enabling technology" of the modern power system.

Back-to-back DC

A back-to-back (B2B) HVDC link is an HVDC system in which the rectifier and the inverter are in the same station, with no DC transmission line between them. The DC link is only a few metres of bus with a smoothing reactor. It is used to connect two AC systems, not to carry power over distance.

 AC grid A          B2B station          AC grid B
 (50 Hz)   +---------------------------+  (50/60 Hz)
 ===|======|[Rectifier]==Ld==[Inverter]|=====|=====
           +---------------------------+
                 DC link (no line)

Because there is no line, the DC voltage is chosen low (tens of kV) and the current high, which makes the valves cheaper.

Advantages

• Asynchronous interconnection: links two grids of different frequency (50 Hz and 60 Hz, e.g. Japan) or same frequency but not in synchronism (e.g. India's regional grids earlier).
• Controlled power exchange: power flow is set exactly by the converter control, so the link can be used for power trading.
• No transfer of faults or disturbances: a fault in one grid does not raise the fault level of the other, and oscillations are not passed across.
• Stability support: fast power modulation can damp low-frequency oscillations.
• Low cost and losses: no DC line, low DC voltage and simple insulation.
• Voltage support: a VSC-type B2B can also supply reactive power to both sides.

EHV (Extra High Voltage) systems work at about 300–765 kV AC, and UHV above that. Worldwide, power systems are changing because of the need to carry large amounts of renewable power over long distances, to control power flow, and to make the grid smart and reliable.

New trends in EHV systems worldwide

1. Higher voltages (UHV AC and DC): China uses 1000 kV AC and ±800 to ±1100 kV DC lines carrying 8–12 GW over 2000–3000 km. India has 765 kV AC and ±800 kV DC.
2. HVDC and VSC-HVDC: voltage-source converters (MMC) allow multi-terminal DC grids, offshore wind connection and black-start ability.
3. FACTS devices: SVC, STATCOM, TCSC, UPFC increase line loading, control voltage and damp oscillations.
4. Compact and high-capacity lines: bundled conductors, compact towers, HTLS (high temperature low sag) conductors for re-conductoring, and multi-circuit towers to save right-of-way.
5. Gas-insulated substations (GIS) and GIL: SF6 or SF6-free (clean-air, g³) insulation in small space; very useful in hilly or urban areas.
6. Underground and submarine XLPE cables at 400–525 kV.
7. Smart grid and digital substations: IEC 61850, PMU-based wide-area monitoring (WAMS), SCADA/EMS, dynamic line rating, digital protection.
8. Renewable integration and storage: large solar/wind parks, pumped storage and battery storage connected through EHV networks.
9. Regional interconnections and power trading: cross-border grids (e.g. Europe, SAARC/BBIN) and electricity markets.

How Nepal can adopt them

• 400 kV backbone: complete the east–west 400 kV corridor (Inaruwa–Dhalkebar–Hetauda–Butwal and beyond) and north–south 400 kV corridors from hydropower areas (Koshi, Kaligandaki, Karnali) to load centres and border points.
• Cross-border links: strengthen 400 kV links with India (Dhalkebar–Muzaffarpur, New Butwal–Gorakhpur) and plan links with China and Bangladesh to export surplus wet-season hydropower.
• HVDC for export: for large storage and export projects in the far west or east (several GW over long distance to Indian load centres), an HVDC or HVDC back-to-back link will allow controlled trading and asynchronous operation.
• FACTS: STATCOM/SVC at weak substations to hold voltage on long radial lines and lightly loaded 220/400 kV lines in the dry season.
• GIS substations in narrow valleys and Kathmandu, where land is scarce.
• HTLS re-conductoring of old 132 kV lines to raise capacity without new right-of-way.
• Smart grid: expand SCADA/load dispatch, PMUs, smart meters and automation in NEA's network to cut losses and outages.
• Storage and solar: pumped-storage and reservoir projects plus solar to balance the run-of-river surplus/deficit.
• Capacity building: training of engineers, national grid code, and private/foreign investment through clear policy.

By adopting these trends, Nepal can evacuate its hydropower reliably, reduce spill in the wet season and earn from regional power trade.

HVAC transmits power as three-phase alternating current, while HVDC converts AC to DC at a rectifier, sends it on a DC line and converts back at an inverter. HVAC is cheaper for short and medium distances and easy to tap; HVDC is better for long-distance bulk power, cables and asynchronous links.

Comparison with justification

So HVAC is preferred when distance is below the break-even distance (about 500–800 km for overhead lines, 30–50 km for cables) or when many tappings are needed; HVDC is preferred beyond it.

 Cost
  |          / AC total
  |        /
  |      /    __-- DC total
  |    / __--
  |  /--   (DC starts high:
  |/ |      converter cost)
  +--+---------------------> Distance
   break-even distance

Scope of HVDC in Nepal

1. Power export to India and Bangladesh: Nepal will have large wet-season surplus from big hydropower (Upper Karnali, Arun-3, West Seti, Pancheshwar, Koshi basin). Bulk transfer of several GW to distant Indian load centres (Delhi, Bihar) or to Bangladesh can be cheaper by HVDC.
2. Asynchronous link with India or China: a B2B HVDC station allows controlled power trading without joining the two grids in synchronism, so disturbances in the large Indian grid do not pass to Nepal's weak grid. A link with China (different grid) would need B2B HVDC.
3. Controlled power flow: power scheduling for trade can be set exactly, which suits cross-border contracts.
4. Narrow right-of-way in hills and forests, where land acquisition is hard.
5. Stability of a weak grid: VSC-HVDC can support voltage and give black-start ability.

Limitations in Nepal: high cost of converter stations, technical expertise, the need for large enough power (above about 1000 MW) to justify HVDC, and the fact that most domestic lines are short. So in the near term Nepal's internal grid will remain HVAC (132/220/400 kV), with HVDC most suitable for large export corridors and asynchronous cross-border links.

FACTS (Flexible AC Transmission Systems) are power-electronic based devices that control the voltage, impedance and phase angle of AC lines, to raise power transfer capacity and controllability.

Why FACTS devices are required

Power on a line is

$$P = \frac{V_s V_r}{X}\sin\delta$$

so it depends on $V$, $X$ and $\delta$. Conventional AC networks cannot change these quickly. FACTS are needed to:

• Load lines close to their thermal limit instead of being held low by stability limits.
• Control power flow along chosen paths and reduce loop flows.
• Hold voltage and supply fast reactive power.
• Improve transient stability and damp power oscillations.
• Avoid or delay building new lines (saves ROW and cost).

Major FACTS devices

1. SVC (Static VAR Compensator) – shunt; a thyristor-controlled reactor (TCR) in parallel with fixed or thyristor-switched capacitors (TSC). It varies its susceptance to absorb or supply reactive power and hold bus voltage.

   Bus ----+---------+--------
           |         |
          TCR       TSC/FC
           |         |
          ===       ===

2. STATCOM (Static Synchronous Compensator) – shunt; a voltage-source converter (VSC) with a DC capacitor, connected through a coupling transformer. If its output voltage is above bus voltage it supplies VArs, below it absorbs VArs. Faster than SVC and gives full current even at low voltage.

   Bus ---+------
          |
        [Xfmr]
          |
        [VSC]==||== DC cap

3. TCSC (Thyristor Controlled Series Capacitor) – series; a capacitor in parallel with a thyristor-controlled reactor, inserted in the line. Changes the effective line reactance $X$, so it controls power flow and damps oscillations.

  ---+---| |---+---  line
     |         |
     +--TCR----+

4. SSSC (Static Synchronous Series Compensator) – series; a VSC injects a voltage in quadrature with line current through a series transformer, acting like a variable series reactance.

5. UPFC (Unified Power Flow Controller) – a STATCOM and an SSSC sharing a common DC link. It can control voltage, impedance and phase angle together, so both real and reactive power flow are controlled.

  Vs --[series Xfmr]------- line --> Vr
          |
        [VSC2]
          ||  common DC link
        [VSC1]
          |
       [shunt Xfmr]
          |
         Vs bus

6. TCPAR/TCPST (phase-angle regulator) – changes the phase angle $\delta$ by injecting a quadrature voltage with thyristor tap changers.

HVDC transmission converts AC to DC at the sending end, sends it on a DC line or cable, and converts it back to AC at the receiving end.

Necessity of HVDC

• Long-distance bulk power: AC transfer is limited by line reactance and stability; DC has no such limit. Beyond about 500–800 km, HVDC is cheaper.
• Long cables: AC cables draw large charging current and become useless beyond 40–60 km; DC cables have no charging current (submarine links, city feeds).
• Asynchronous interconnection of grids with different frequency or not in step.
• Fast and exact control of power flow, useful for power trading and stability.
• Lower losses and cheaper line (two conductors, smaller towers and ROW).
• No increase in fault level when two systems are connected.
• Integration of remote renewables and offshore wind.

General configuration of an HVDC system

Main parts: converter transformers, converter valves (12-pulse thyristor bridges or VSC), smoothing reactors, AC filters and shunt capacitors, DC filters, DC line/cable, earth electrodes and control system.

 AC1 --[Xfmr]--[Rectifier]--Ld---- DC line ----Ld--[Inverter]--[Xfmr]-- AC2
   |                                                                |
 AC filters                                                    AC filters

Types of links:

1. Monopolar: one conductor (usually negative) with earth or sea return. Cheap; used for submarine cables.
2. Bipolar: two conductors, $+V_d$ and $-V_d$, with the neutral earthed at both ends. Most common; if one pole fails the other works as monopolar with earth return (half power).
3. Homopolar: two or more conductors of the same polarity (usually negative, less corona) with earth return.
4. Back-to-back: both converters in one station, no line.
5. Multi-terminal: three or more stations connected in series or parallel.

 Bipolar:
  [R+]======= +Vd line =======[I+]
   |                            |
  earth                        earth
   |                            |
  [R-]======= -Vd line =======[I-]

HVDC back-to-back system

In a back-to-back system the rectifier and inverter are in the same station and connected by a short DC bus with a smoothing reactor; there is no DC line. It is used to connect two asynchronous grids (e.g. 50 Hz and 60 Hz, or two unsynchronised 50 Hz grids) and to control power exchange between them. Since there is no line, a low DC voltage and high current are used, lowering valve cost. It prevents fault and disturbance transfer between the two grids.

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.

Pressure cables are used above about 66 kV, where solid (paper) cables fail because voids form in the insulation and cause ionisation (partial discharge). In pressure cables the voids are removed or suppressed by keeping oil or gas under pressure.

Oil-filled (oil pressure) cables

Low-viscosity oil is kept under pressure (about 1–3 atm or more) and fills the voids in the paper insulation. Oil ducts let the oil flow along the cable, and oil reservoirs (feeding tanks) at intervals take care of expansion and contraction with load.

• Conductor-channel type (single core): the oil channel is at the centre of a hollow stranded conductor.
• Sheath-channel type: channels are made in the metallic sheath.
• Three-core filler-space type: oil ducts are in the filler spaces between cores.

   Single-core oil-filled cable
      ___________________
     /  lead sheath      \
    |  paper insulation   |
    |   (oil impregnated) |
    |   ( conductor  )    |
    |   (  [oil duct] )   |
     \___________________/

Advantages: no voids, higher working stress, smaller size, faults can be found by oil leak. Disadvantage: costly, needs reservoirs and careful laying.

Gas pressure cables

The cable is placed in a steel pipe filled with an inert gas (nitrogen) at about 12–15 atm.

• External pressure cable: the cable has a thin triangular lead sheath that acts as a diaphragm; the gas pressure outside compresses the cable so any voids close.
• Internal (impregnated) gas pressure cable: gas at high pressure is inside the insulation, so the ionisation voltage of the small voids rises and discharge does not occur.

      Steel pipe (N2 at ~14 atm)
     _____________________
    /   o        o        \
   |       o              |   o = cores with
   |    (3 cores)         |       thin lead sheath
    \_____________________/

Advantages: higher current rating and voltage (up to 275 kV), no oil leak problem, better thermal performance. Disadvantage: high cost of steel pipe and gas system.

FACTS (Flexible AC Transmission Systems) are power-electronic controllers (SVC, STATCOM, TCSC, SSSC, UPFC, phase-angle regulators) that control the three parameters that decide AC power flow: voltage, line reactance and phase angle. Because they bring fast, electronic control to a network that was earlier controlled only slowly and mechanically, they are changing how power systems are planned and operated.

$$P = \frac{V_s V_r}{X}\sin\delta, \qquad Q \approx \frac{V_s(V_s - V_r\cos\delta)}{X}$$

Significant changes initiated by FACTS

1. Higher utilisation of existing lines: traditional lines are loaded well below their thermal limit because of stability and voltage limits. Series compensation (TCSC, SSSC) reduces $X$ and shunt devices (STATCOM) hold $V$, so the same line carries much more power. This delays new lines and saves right-of-way.
2. Controllable power flow: earlier, power flowed by Kirchhoff's laws according to impedance; loop flows and overloading of parallel paths were common. UPFC and TCSC let operators direct power along chosen paths, which is essential for deregulated markets and wheeling contracts.
3. Improved stability: fast control (a few cycles) increases transient stability margin, damps inter-area oscillations, and mitigates subsynchronous resonance.
4. Voltage control and reactive support: STATCOM/SVC give dynamic VAr support, preventing voltage collapse and correcting the Ferranti effect on light load.
5. Renewable energy integration: wind and solar output varies; FACTS smooth the voltage fluctuations and help meet grid-code fault ride-through requirements.
6. Power quality: custom-power devices (D-STATCOM, DVR, active filters) at distribution level reduce sags, flicker and harmonics.
7. Interconnection of grids: FACTS allow secure interconnection of neighbouring utilities and regions with controlled exchange.
8. Deregulation and markets: independent power producers need guaranteed transfer paths; FACTS provide the control that open access needs.

Acceleration of change

• Mechanical switching (tap changers, switched capacitors) works in seconds and wears out; thyristor/IGBT switching works in milliseconds with no wear.
• With wide-area monitoring (PMUs) and digital control, FACTS make the grid "smart" and adaptive.
• Lower cost of power semiconductors and modular converters (MMC) make FACTS cheaper and more common.

Example

A 400 kV line with $X = 100\ \Omega$ and $\delta = 30^\circ$ carries $P = \frac{400^2}{100}\sin 30^\circ = 800$ MW. With 40% series compensation, $X = 60\ \Omega$ and $P = 1333$ MW at the same angle, a 67% increase without a new line.

Limitations

High cost, harmonics, need for skilled operation and coordination among devices.

Thus FACTS convert the passive AC network into an actively controlled, flexible system, and so they initiate and accelerate major changes in power system planning, operation and markets.

In 25 years the electric power system is expected to be clean, digital, decentralised and highly controllable, with power electronics and renewable energy at its core.

Expected features

• Renewable-dominated generation: most energy from hydro, solar and wind; coal and oil plants mostly retired. Nepal's grid will be hydro plus solar, exporting surplus power.
• Large-scale storage: pumped storage, reservoir hydro, grid batteries and green hydrogen will balance variable renewables.
• UHV and HVDC super-grids: ±800 kV to ±1100 kV DC and 1000 kV AC lines, multi-terminal DC grids and cross-border/regional grids (e.g. South Asian or Asian super-grid) for power trading.
• Power electronics everywhere: grid-forming inverters, FACTS, solid-state transformers and DC circuit breakers; the grid will be "inverter-based" with low physical inertia.
• Smart grid: smart meters, PMUs, IoT sensors, AI-based forecasting and self-healing distribution networks; consumers become "prosumers" who sell rooftop solar power.
• Electrification of transport and cooking: electric vehicles, e-buses and induction cooking increase demand; vehicles act as mobile storage (V2G).
• Microgrids and DC distribution: local microgrids able to island during faults; DC networks for data centres, EV charging and buildings.
• Advanced insulation and equipment: SF6-free GIS, HTLS conductors, superconducting cables in cities, compact substations.
• Cyber-security and resilience become as important as electrical protection.
• Market-based operation: real-time and peer-to-peer energy trading.

Overall, the future grid will be greener, more reliable and more flexible, with power flowing in both directions and controlled digitally.

A belted cable is a three-core, paper-insulated cable used for voltages up to about 11 kV (sometimes 22 kV). It gets its name from an extra layer of paper insulation, the belt, wound around the three insulated cores together.

Constructional features

        lead sheath
     ____________________
    /  belt insulation   \
   |   ( C1 )    ( C2 )   |
   |     fillers (jute)   |
   |        ( C3 )        |
    \____________________/
     bedding, armour, serving

1. Conductors: three stranded copper or aluminium conductors, usually sector (D) shaped to reduce overall size.
2. Core insulation: each conductor is wrapped with impregnated paper.
3. Fillers: the gaps between the cores are filled with jute or paper fillers to give a round shape.
4. Belt: an impregnated paper belt is wound over the three cores and fillers together.
5. Lead sheath: keeps moisture out and protects insulation.
6. Bedding, armouring and serving: bedding of jute/paper, steel tape or wire armour for mechanical protection, and outer serving of jute.

Limitations (why not used above about 22 kV)

• The electric field is not purely radial; it has a tangential component along the paper layers. Paper is weak along its layers, so leakage current flows tangentially and heats the insulation.
• Heating and load cycles create voids in the insulation (between fillers and belt), leading to ionisation, partial discharge and breakdown.

For these reasons, screened (H-type, SL-type) and pressure cables are used for higher voltages.

For long-distance bulk power transmission, HVDC is preferred because beyond the break-even distance it is cheaper, has lower losses and has no stability limit.

Why HVDC for long-distance bulk power

1. Lower line cost: a bipolar DC line needs 2 conductors instead of 3, uses lighter and cheaper towers, and needs narrower right-of-way. For the same insulation, DC working voltage can be close to the AC peak, so more power per conductor.
2. No stability limit: AC power $P = \frac{V_sV_r}{X}\sin\delta$ falls as $X$ grows with length, and the line needs series compensation. DC has no reactance or power angle, so any distance is possible.
3. No reactive power or charging current in the line: there is no Ferranti effect and no need of shunt reactors along the line.
4. Lower losses: no skin effect, no reactive current, lower corona loss. For long lines the saving in losses pays for the converters.
5. Break-even distance: terminal cost is high for HVDC but line cost per km is lower. The total costs cross at about 500–800 km for overhead lines (30–50 km for cables); beyond this HVDC is cheaper.

 Cost |        / AC
      |      /
      |    /   ___-- DC
      |  / __--
      |/--
      +---|------------> Length
          break-even

6. Fast power control: converters change power in milliseconds, which helps stability of both AC systems and allows exact scheduling.
7. No increase in fault level of the connected AC systems.
8. Long cables possible: for routes with sea crossings.

Examples: Three Gorges–Shanghai (±500 kV, about 1000 km), Xiangjiaba–Shanghai (±800 kV, about 2000 km), Rihand–Delhi (±500 kV) in India.

Significance of back-to-back (B2B) HVDC

In a B2B link the rectifier and inverter are in the same station with no DC line. Its significance:

• Connects two asynchronous grids (different frequency, or not in step), e.g. 50/60 Hz systems in Japan, and earlier the regional grids of India.
• Gives controlled power exchange for trading.
• Prevents transfer of faults, frequency disturbances and oscillations between grids; does not raise fault level.
• Low DC voltage and high current lower the valve cost.

EHV (about 300–765 kV) and UHV (1000 kV AC and above, ±800 kV DC) lines are needed because the power a line can carry rises with the square of voltage, while losses and the cost per MW fall as voltage rises.

Justification

1. Power capacity rises as $V^2$:

$$P = \frac{V_sV_r}{X}\sin\delta \propto V^2$$

The natural (surge impedance) loading is $SIL = \frac{V^2}{Z_c}$. Taking $Z_c \approx 400\ \Omega$:

So one 765 kV line carries as much as about twelve 220 kV lines.

2. Lower losses: for power $P$ the current is $I = \frac{P}{\sqrt{3}V\cos\phi}$, so the loss $3I^2R \propto \frac{1}{V^2}$. Doubling the voltage cuts losses to a quarter.
3. Less right-of-way per MW: fewer lines are needed, which saves land, forest and resettlement cost, very important in hilly areas like Nepal.
4. Lower cost per MW-km: although towers and insulation cost more, the cost per unit of power carried is lower.
5. Better stability over long distance: a higher voltage gives a larger power margin for the same angle and, with bundled conductors, lower reactance.
6. Remote large generation: big hydro, coal or renewable plants are far from load centres (e.g. Himalayan hydro to Indian plains, Chinese west-to-east transfer), needing bulk transfer over hundreds of km.
7. Interconnection of grids and power trading need strong high-capacity links.

Costs to accept

Corona loss, radio and audible noise, switching overvoltage, larger clearances and insulation, and high field effects must be controlled using bundled conductors, closing resistors, arresters and proper design. Even with these, EHV/UHV is the economical way to move large blocks of power over long distances.

EHV-AC transmission (about 300–765 kV) carries large power efficiently, but it brings several technical problems that must be solved in design.

Main issues

1. Corona: high surface gradient causes corona loss, radio interference (RI), TV interference and audible noise. Bundled conductors are used to reduce it.
2. Switching overvoltages: energising, re-closing and fault clearing create surges of 2–3 pu, which govern the insulation design. Closing resistors and arresters are needed.
3. Lightning overvoltages and insulation coordination.
4. Ferranti effect and reactive power: large line charging causes high receiving-end voltage at light load; shunt reactors are required.
5. Stability limits: long lines need series compensation, which may cause subsynchronous resonance.
6. Electrostatic and magnetic field effects under the line on people, animals and vehicles; induction in nearby lines and pipelines.
7. Large clearances and right-of-way, heavy towers and big insulator strings.
8. Line design: conductor selection, bundle spacing, sag and tension, and environmental impact.

Reasons for the resurgence of HVDC

The first power systems (Edison) were DC, but AC won because of the transformer. HVDC has come back strongly since the 1950s because:

1. Mercury-arc valves, then thyristors and IGBTs made reliable high-power AC/DC conversion possible; modular multilevel VSC converters made it more flexible.
2. Long-distance bulk transmission: remote hydro, coal and renewable sources need transfer over 1000+ km, where HVDC is cheaper (beyond the break-even distance) and has no stability limit.
3. Submarine and underground cables: DC cables have no charging current, so long sea links (and offshore wind connections) are possible.
4. Asynchronous interconnection of grids with different frequency or for controlled power trade (back-to-back links).
5. Fast controllability helps damp oscillations and improves stability of AC systems.
6. Lower losses, two conductors, narrow ROW – important for environmental and land issues.
7. No increase of fault level when grids are joined.
8. Renewable energy and multi-terminal DC grid concepts have increased interest further.

Why AC is still inevitable

1. Transformers: AC voltage can be stepped up and down simply and efficiently; DC needs expensive converters.
2. Generation is AC: synchronous generators produce AC directly, and most loads (induction motors, lighting, appliances) use AC.
3. Easy tapping and meshed networks: AC substations can take power out at any point; HVDC tapping needs extra converters, so DC is mostly point-to-point.
4. Circuit breaking: AC current passes through zero twice per cycle, so breakers are simple; DC breakers are difficult and costly.
5. Lower terminal cost for short and medium distances (below break-even distance).
6. No harmonics or reactive demand from converters, and simpler operation and maintenance.
7. Existing infrastructure, standards and experience are all AC.

So HVDC is used for special purposes (long distance, cables, asynchronous links), while AC remains the backbone for generation, distribution and meshed transmission. Modern grids are hybrid AC/DC.

The supply frequency (50 Hz or 60 Hz) was chosen not because it is best for every purpose, but because it is an acceptable middle value between requirements that pull in opposite directions. Low frequency is good for some equipment and high frequency for others.

Requirements favouring a LOW frequency

• Transmission: line reactance $X = 2\pi fL$ is lower, so voltage drop is smaller, power transfer $P = \frac{V^2}{X}\sin\delta$ and stability limits are higher.
• Charging current and Ferranti effect ($I_c = 2\pi fCV$) are smaller in long lines and cables.
• Skin effect and eddy currents are lower, so conductor resistance is lower.
• Commutator machines (traction motors, rotary converters) commutate better at low frequency; that is why railways used 16⅔ Hz and 25 Hz.
• Low-speed hydro generators with few poles fit low frequency.

Requirements favouring a HIGH frequency

• Lighting flicker: incandescent and arc lamps flicker visibly below about 40–50 Hz. This was a major reason to reject 25 Hz.
• Size of transformers and machines: $E = 4.44fN\phi_m$, so for the same voltage a higher $f$ needs less flux and iron; transformers and motors become smaller and cheaper.
• Speed of machines: $N_s = \frac{120f}{P}$; higher frequency gives higher speed, suitable for steam turbines and small motors.
• Aircraft and ships use 400 Hz to save weight.

Limits on very high frequency

Iron losses (hysteresis $\propto f$, eddy $\propto f^2$), higher line reactance, larger inductive voltage drop and interference with telephone lines.

Result

Early systems used many frequencies (25, 40, 50, 60, 133 Hz). Around 1900–1920 the industry settled on 50 Hz (Europe, Asia, Nepal) and 60 Hz (America), which keep flicker invisible, give reasonably small machines and transformers, and still allow acceptable reactance and losses on transmission lines. So the standard frequency is truly a compromise between contradictory requirements.

In the late 1880s, Thomas Edison's company supplied low-voltage DC, while George Westinghouse (using Tesla's patents) promoted high-voltage AC, which could be transmitted far using transformers. To protect his DC business, Edison started a publicity campaign against AC, known as the "War of the Currents".

Main features of the campaign

• Danger message: Edison's company published warnings (the 1888 "red book" pamphlet) that high-voltage AC was deadly and unsafe for homes and streets.
• Public animal electrocutions: Harold P. Brown, an engineer supported by Edison's side, publicly killed dogs, calves and horses with AC to show that it was more dangerous than DC.
• Electric chair: Edison's side encouraged New York State to adopt the electric chair using a Westinghouse AC generator, so that AC would be linked with death. William Kemmler was the first person executed this way in 1890. Edison suggested the term "Westinghoused" for being electrocuted.
• Lobbying for voltage limits: they tried to get laws passed to limit transmission voltage to a few hundred volts, which would have made AC transmission useless.
• Newspaper articles and statements stressing accidents with overhead AC wires.

Outcome

The campaign failed. AC's advantages – transformers, long-distance transmission, and the induction motor – were too strong. Westinghouse lit the 1893 Chicago World's Fair and built the Niagara Falls AC power plant (1895–96). Edison's own company merged into General Electric (1892), which also adopted AC. AC then became the world standard, though DC has returned today as HVDC.

Classification of high voltages

Voltage levels are commonly classified as below (limits differ a little between standards and textbooks):

In Nepal, NEA uses 400 kV and 220 kV (EHV/HV backbone), 132 kV and 66 kV (HV), and 33 kV and 11 kV (MV) levels.

Need of high voltage for transmission

1. Less current, less loss: $I = \frac{P}{\sqrt{3}V\cos\phi}$, so loss $3I^2R \propto \frac{1}{V^2}$.
2. Higher power capacity: $P = \frac{V^2}{X}\sin\delta$ and $SIL = \frac{V^2}{Z_c}$ both rise with $V^2$.
3. Less conductor material: for the same loss, conductor volume $\propto \frac{1}{V^2}$.
4. Better voltage regulation and efficiency.
5. Fewer lines and less right-of-way for large power, so lower cost per MW-km over long distances.

The choice between HVDC and HVAC is mainly economic and depends on the cost of terminals and line, the power to be transmitted and the transmission length.

1. Cost

• HVAC: terminal cost (transformers, switchgear) is low, but the line costs more per km (3 conductors, heavier towers, wider ROW, compensation).
• HVDC: terminal cost (converter stations, filters, reactive compensation) is high, but the line is cheaper per km (2 conductors, lighter towers) and losses are lower.

 Cost |          / AC
      |        /
      |      /  ___-- DC
      |    /__--
      |  _/
      |/  |
      +---+-----------------> Length
        break-even distance

2. Transmission length

• The two cost lines cross at the break-even distance: about 500–800 km for overhead lines and about 30–50 km for underground/submarine cables.
• Below this length HVAC is cheaper; above it HVDC is cheaper.
• For cables, AC charging current is so large that AC is not practical beyond about 50 km.

3. Transmission power

• HVDC terminal cost is spread over the power, so it is economical only for large power (usually above about 500–1000 MW) over long distance.
• For small power and short lines, or where power must be tapped at many points, HVAC is chosen.
• Long AC lines need series compensation as power increases, because of stability limit; DC has no such limit.

Other factors

Asynchronous interconnection, need for fast control, ROW limits and environmental issues favour HVDC; meshed networks and multiple tappings favour HVAC.

FACTS (Flexible AC Transmission Systems) are power-electronic devices that control voltage, line reactance and phase angle of AC transmission lines to improve controllability and power transfer capability. Since

$$P = \frac{V_sV_r}{X}\sin\delta,$$

controlling $V$, $X$ or $\delta$ directly controls the power flow.

Role of FACTS devices

1. Increase power transfer capability: lines can be loaded near their thermal limit; series devices (TCSC, SSSC) reduce $X$.
2. Power flow control: UPFC, TCSC and phase shifters send power along desired paths and remove loop flows and overloads.
3. Voltage control: shunt devices (SVC, STATCOM) supply or absorb reactive power to keep bus voltage within limits and prevent voltage collapse.
4. Stability improvement: fast response increases transient stability margin and damps power oscillations; also reduces subsynchronous resonance.
5. Reduce losses and reactive flow in the network.
6. Delay new lines: better use of existing lines saves cost and right-of-way.
7. Renewable integration and power quality: smooth voltage fluctuations from wind and solar; custom power devices reduce sags and harmonics.
8. Support deregulated operation: secure, controllable transfer for power trading.

Main devices: SVC and STATCOM (shunt), TCSC and SSSC (series), UPFC and IPFC (combined).