General Background

Different voltage levels are used because no single voltage is best for every part of the system: generators are limited to a moderate voltage, bulk power must be sent far at very high voltage to keep losses low, and consumers need a low, safe voltage.

Reasons

1. Generation limit: Generator insulation in the stator slots limits the voltage to about 11–25 kV (Nepal: 6.6/11 kV).
2. Low transmission loss: For a given power, $I = P/(\sqrt3 V\cos\phi)$, so loss $3I^2R \propto 1/V^2$. Raising voltage from 11 kV to 132 kV cuts the loss by $(132/11)^2 = 144$ times.
3. Less conductor material: Conductor volume for the same loss is $\propto 1/V^2$, so high voltage saves copper/aluminium.
4. Better regulation and capacity: % voltage drop $\propto 1/V^2$; power capacity of a line $\propto V^2$ (e.g. $P_{max} = V_sV_r/X$).
5. Distance and power: Longer lines and larger blocks of power need higher voltage (400 kV for hundreds of km, 33 kV for tens of km).
6. Safety and cost at the user end: Very high voltage needs large clearances and costly insulation; so in towns it is stepped down to 11 kV and finally 400/230 V for safe use.

Interconnected system with voltage levels

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Transformers make these levels possible economically; this is one of the main reasons AC is used for power systems.

Conventional resources are fossil and nuclear fuels (coal, oil, natural gas, uranium) that are finite and are used mainly in large thermal/nuclear plants. Renewable resources are naturally replenished (water, sun, wind, biomass, geothermal) and do not run out.

Nepal context

Nepal has no significant fossil fuel, so almost all its grid electricity comes from hydropower (a renewable source), with growing solar. Diesel plants are only used for backup. Conventional sources would mean importing fuel, so renewable generation is both cheaper in the long run and more secure for Nepal.

Three-phase AC delivers power with three voltages displaced by 120°. It is preferred over single-phase for generation, transmission and distribution because:

1. Constant power: Total instantaneous power in a balanced three-phase system is constant ($p = 3V_{ph}I_{ph}\cos\phi$), while single-phase power pulsates at twice the supply frequency. This gives smooth torque and less vibration in machines.
2. Less conductor material: For the same power, voltage, distance and loss, a 3-phase 3-wire line needs only 75 % of the conductor of a single-phase 2-wire line.
3. Higher machine output: A 3-phase generator or motor of the same frame size gives about 1.5 times more output than a single-phase machine, and has higher efficiency and p.f.
4. Self-starting motors: 3-phase induction motors produce a rotating magnetic field and start by themselves; single-phase motors need auxiliary windings or capacitors.
5. Cheaper transformers and switchgear per kVA.
6. Parallel operation of 3-phase alternators is easier and steadier.
7. Flexibility: A 3-phase 4-wire system gives both 400 V (3-phase loads) and 230 V (single-phase loads) from the same supply.
8. Better voltage regulation and lower losses for the same power transfer.

In the "war of currents" (1880s–1890s), Edison's DC system lost to the AC system of Tesla and Westinghouse. AC prevailed because of the following reasons:

1. Transformer: AC voltage can be stepped up and down easily and efficiently (98–99 %) with a static transformer. DC had no such device; converting DC voltage needed rotating machines.
2. High-voltage transmission: Using transformers, AC could be sent at high voltage and low current, giving low $I^2R$ loss and allowing power to travel tens of km. DC at low (110 V) voltage could only supply customers within about 1–2 km of the station, needing many small plants.
3. Simpler, cheaper generators: AC alternators have no commutator, so they can be built for higher voltage and larger ratings, with less maintenance.
4. Induction motor: Tesla's polyphase induction motor (1888) was robust, cheap, self-starting and had no commutator—ideal for industry.
5. Easy interruption: AC current passes through zero every half cycle, so circuit breakers can interrupt it easily; DC arcs are hard to extinguish.
6. Large central stations: High-voltage AC allowed big, efficient plants at remote sites (e.g. Niagara Falls hydro plant, 1896, sending power to Buffalo ~40 km away).
7. Polyphase systems: 3-phase AC gave constant power and lower conductor cost.

DC returned later as HVDC, only after mercury-arc and thyristor converters became available.

A power station converts primary energy into electrical energy and delivers it to the grid. Its major electrical components are:

1. Alternator (synchronous generator): Converts mechanical energy from the turbine into 3-phase electrical energy, usually at 6.6–11 kV (up to 25 kV in large plants).
2. Excitation system: Supplies DC to the rotor field (static or brushless exciter) and, with the AVR, controls the terminal voltage and reactive power.
3. Governor (control): Adjusts the turbine input to hold speed/frequency and share active power.
4. Generator step-up transformer: Raises the voltage from generator level to transmission level (e.g. 11/132 kV).
5. Busbars: Conductors that collect power from generators and distribute it to outgoing feeders.
6. Switchgear: Circuit breakers (SF₆/vacuum), isolators and earthing switches to connect, disconnect and protect equipment.
7. Protective relays and instrument transformers: CTs and PTs feed relays (differential, overcurrent, distance) and meters.
8. Lightning arresters: Protect equipment from lightning and switching surges.
9. Station auxiliary transformer and auxiliaries: Supply pumps, fans, lighting, cooling inside the plant.
10. Control room, metering and SCADA: Monitor and control the plant; energy meters record export.
11. Battery bank and DC system: Supplies relays and breaker trip coils even during AC failure.
12. Earthing system: Safe path for fault currents and personnel safety.

 Turbine--[G]--[CB]--[GT 11/132 kV]--[CB]--Bus--Lines
           |                                 |
        Exciter/AVR                     LA, CT, PT

Differences

Conventional sources (coal, oil, gas, nuclear, large hydro in many texts) are long-established, mostly finite, and used in large central plants. Non-conventional sources (solar, wind, small/micro hydro, biomass, geothermal, tidal) are newer, mostly renewable and often small and decentralised.

Schematic: grid-connected solar PV power plant

  Sunlight
     |
 [PV array] --DC--> [DC combiner/
  (modules)          MPPT charge]
                          |
                     [Inverter DC->AC]
                          |  400 V AC
                   [Step-up Tx 0.4/11 kV]
                          |
                     [CB / meter]
                          |
                     11 kV grid
  (optional: battery bank on DC side)

Working:

1. PV modules convert sunlight directly into DC electricity (photovoltaic effect).
2. Strings of modules are combined; the MPPT controller keeps them at maximum power point.
3. The inverter converts DC to grid-synchronised 3-phase AC.
4. A transformer steps up the voltage and the power is fed into the grid through protection and metering. An off-grid plant uses a battery bank to store energy for night.

Example: Nuvakot 25 MW solar plant in Nepal feeds the national grid this way.

A power system network is the chain of equipment that generates electrical energy, transmits it over long distances and distributes it to consumers.

 GENERATION        TRANSMISSION       SUB-TRANS.
 [G]11kV-[GT]==== 220/132 kV ====[S/S]== 66/33 kV
 [G]11kV-[GT]====      ||           |
                  (other grids)     |
 DISTRIBUTION                       v
               [33/11 kV substation]
                         |
            11 kV primary feeders
               |                 |
         [11/0.4 kV DT]   [11 kV industry]
               |
     400/230 V LT lines -> houses, shops

Components and their functions

Generation produces power, transmission moves bulk power efficiently over long distances at high voltage, and distribution delivers it safely at usable voltage to each consumer.

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.

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

HVDC vs HVAC

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

Single line diagram with voltage levels

An interconnected power system links several generating stations and load centres through a common high-voltage transmission network (grid).

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Essential parts: generators (11 kV), step-up transformers, transmission lines (400/220/132 kV), grid substations, sub-transmission (66/33 kV), distribution substations (33/11 kV), primary feeders (11 kV), distribution transformers (11/0.4 kV) and LT network (400/230 V), with breakers and protection at every stage.

Isolated vs interconnected

Advantages of interconnected system

1. Higher reliability: If a plant fails, others supply the load.
2. Less reserve capacity: Spinning reserve is shared, so installed capacity per area falls.
3. Economic operation: Cheapest plants (e.g. run-of-river hydro) are loaded first; base and peak load plants are used optimally.
4. Use of diverse sources: Wet-season hydro surplus can be exported and dry-season deficit imported (Nepal–India exchange).
5. Load diversity: Peaks of different areas occur at different times, reducing total required capacity.
6. Better voltage and frequency stability.
7. Larger, more efficient units can be installed.
8. Exchange of energy between utilities and countries.

Disadvantages to note: higher fault levels and the possibility of a disturbance spreading (cascade failure) unless protection and control are good.

Single line diagram of an interconnected power system

Several generating stations and load centres are tied together through a high-voltage transmission grid so that any plant can supply any load.

 [G1]11kV-[11/132]--+===132 kV===+--[11/132]-[G2]
                    |            |
               Grid S/S A   Grid S/S B
                    |            |
                 [132/33]     [132/33]---->Tie to
                    |            |       neighbour grid
                 33 kV        33 kV
                    |            |
                 [33/11]      [33/11]
                    |            |
                 11 kV feeders  11 kV feeders
                    |            |
                 [11/0.4]     [11/0.4]
                    |            |
                 400/230 V    400/230 V loads

Typical levels in Nepal: generation 11 kV; transmission 400/220/132 kV; sub-transmission 66/33 kV; distribution 11 kV and 400/230 V.

Isolated vs interconnected

An isolated system is a single plant (or small group) supplying only its own local load with no link to other systems, e.g. a micro-hydro feeding a village. An interconnected system shares generation and load across a grid.

Thus an interconnected system gives higher reliability, lower reserve and cheaper operation, while an isolated system is simpler but less reliable and less economical.

Renewable energy sources are sources that are naturally replenished on a human time scale and do not get exhausted by use. They cause little pollution and their "fuel" is free.

Examples

Least suitable for Nepal

Tidal (and wave/ocean) energy is least suitable — Nepal is a landlocked country with no sea coast, so it cannot be used at all.

Among sources that physically exist in Nepal, geothermal (and large-scale wind) are the least suitable:

• Geothermal: Nepal has hot springs (Tatopani, Jumla, Myagdi), but their temperatures are low (below about 75 °C) and capacity is small, suitable only for bathing or heating, not for power.
• Wind: Wind speeds are good only in a few remote high-altitude places (e.g. Mustang) far from load and grid, with difficult transport and turbulent mountain wind.

By contrast, Nepal's steep rivers give huge hydropower potential (about 83,000 MW theoretical, ~43,000 MW economically feasible) and about 300 sunny days a year favour solar. So hydro and solar are the most suitable, while tidal (absent) and geothermal are the least.

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.

Explanation

For a grid with Thevenin voltage $E_{th}$ and impedance $Z_{th}$, the bus voltage is

$$V = E_{th} - I Z_{th}$$

If the system is very large, $Z_{th}$ is very small (short-circuit MVA very high), so a change in current $I$ caused by one machine hardly changes $V$. Likewise, the total inertia $H$ of thousands of MW of rotating machines is so large that the power change from one machine does not change the frequency noticeably.

 E_th --[Z_th -> 0]--+---- Infinite bus
  (H -> infinity)    |     V, f constant
                     +--X_s--(G) small machine

Example: A 5 MW micro-hydro plant synchronised to the 3000+ MW Indian–Nepal interconnected grid sees an infinite bus. Raising its excitation only increases its reactive output; raising its turbine input only increases its active power; the grid V and f stay the same.

The concept simplifies analysis of a single machine connected to a large system (stability studies, V-curves).

Single line diagram

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Function of each system

1. Generation system

• Converts primary energy (water, fuel, sun) into electrical energy in power stations at 6.6–11 kV.
• Includes turbines, alternators, exciters, governors and step-up transformers.
• Controls frequency (through governors) and voltage (through AVR).

2. Transmission system

• Carries bulk power from generating stations to major load centres at extra-high voltage (400, 220, 132 kV in Nepal).
• Interconnects power stations and neighbouring grids, allowing power exchange, sharing of reserve and economic dispatch.
• Includes towers, conductors, insulators, grid substations and protection.
• High voltage reduces current, losses and conductor cost.

Sub-transmission (66/33 kV) links grid substations to distribution substations in cities and districts.

3. Distribution system

• Delivers power from distribution substations (33/11 kV) to individual consumers.
• Primary distribution at 11 kV feeds distribution transformers and bulk consumers.
• Secondary distribution at 400 V (3-phase) / 230 V (1-phase) supplies houses, shops and small industries.
• Includes feeders, distributors, service mains, distribution transformers and meters.

An infinite bus is a bus of a very large system (ideally zero impedance and infinite inertia), so a load change at one point is negligible compared with its capacity.

• Voltage constant: Bus voltage $V = E_{th} - IZ_{th}$. For a huge interconnected system, $Z_{th}\approx0$ (very high fault level), so a change in load current produces almost no voltage drop; also many generators with AVRs share the reactive demand.
• Frequency constant: Frequency changes when the power balance changes, at a rate set by system inertia ($df/dt \propto \Delta P/H$). The total inertia of all machines is extremely large, and many governors share any load change, so $\Delta f \approx 0$.

Hence a load change that is tiny relative to the total generation leaves the voltage and frequency practically unchanged.

Significance of voltage level

For power $P$ sent at voltage $V$ and p.f. $\cos\phi$:

$$I = \frac{P}{\sqrt3V\cos\phi},\qquad P_{loss} = 3I^2R = \frac{P^2R}{V^2\cos^2\phi}$$

• Loss and % voltage drop $\propto 1/V^2$; conductor volume for the same loss $\propto 1/V^2$.
• Power-carrying capacity $\propto V^2$ (SIL $= V^2/Z_c$; $P_{max} = V_sV_r/X$).
• But insulation, clearances, tower size and substation cost rise with $V$.

So the voltage level is chosen to balance line losses against insulation cost, depending on power and distance (e.g. Still's formula $V = 5.5\sqrt{l/1.6 + P/100}$ kV, $l$ in km, $P$ in kW).

132 kV vs 400 kV for the same power and distance

Technically feasible: yes, and performance improves:

• Current falls by $400/132 = 3.03$ times, so loss falls by about $3.03^2 \approx 9.2$ times.
• Voltage regulation improves and stability margin rises (SIL of 400 kV ≈ 9 times that of 132 kV).
• Fewer circuits or smaller conductor area may be needed.

Economically: feasible only if the power is large. A 400 kV line needs bigger towers, longer insulator strings, wider right-of-way and costly 400 kV substations; corona and charging current also increase. If the power is small (suitable for 132 kV, e.g. 50–100 MW), the line would be lightly loaded far below its SIL (≈ 500–600 MW), the saving in loss would not pay for the extra cost, and at light load the Ferranti effect would need reactors.

Conclusion: It is technically possible and more efficient, but it is economical only when the transmitted power (now or in future) is large enough to justify the 400 kV investment.

An interconnected power system is one in which several generating stations, possibly of different types and owners, are connected together through a transmission network (grid) so that they jointly supply all the loads connected to it.

Typical diagram

 Hydro G1         Thermal/Solar G2      Hydro G3
   |11kV             |11kV                |11kV
 [GT]              [GT]                 [GT]
   |                 |                    |
 ==+=====220/132 kV==+=========132 kV=====+==
   |                 |                    |
 [S/S]            [S/S]---tie line--->Neighbour grid
   |33 kV            |33 kV
 [33/11]          [33/11]
   |11 kV            |11 kV
 [11/0.4]--loads  [11/0.4]--loads

Advantages over an isolated system

1. Higher reliability: If a plant fails, others supply the load.
2. Less reserve capacity: Spinning reserve is shared, so installed capacity per area falls.
3. Economic operation: Cheapest plants (e.g. run-of-river hydro) are loaded first; base and peak load plants are used optimally.
4. Use of diverse sources: Wet-season hydro surplus can be exported and dry-season deficit imported (Nepal–India exchange).
5. Load diversity: Peaks of different areas occur at different times, reducing total required capacity.
6. Better voltage and frequency stability.
7. Larger, more efficient units can be installed.
8. Exchange of energy between utilities and countries.

Disadvantage: fault level increases and a disturbance can spread unless protection is well coordinated.

A hydropower plant converts the potential energy of water stored at a height into kinetic energy, then into mechanical energy in a turbine, and finally into electrical energy in a generator. Power available:

$$P = \eta\rho g Q H\quad(\text{W})$$

where $Q$ = discharge (m³/s), $H$ = net head (m), $\eta$ = overall efficiency.

Block diagram

 [Reservoir/Intake/Dam]
          |
   [Headrace / tunnel]
          |
 [Forebay / surge tank]
          |
      [Penstock]
          |
   [Turbine] --shaft--> [Generator] -> [Step-up Tx]
          |                 |              |
     [Tailrace]       [Exciter, AVR]   [Switchyard]
          |                                |
        River                           Grid

Working

1. Dam/intake: Stores or diverts river water and creates head.
2. Headrace and forebay/surge tank: Carry water to the penstock; the surge tank absorbs pressure changes (water hammer).
3. Penstock: Pressure pipe that brings water down to the powerhouse, converting potential energy to pressure/kinetic energy.
4. Turbine: Pelton (high head), Francis (medium head) or Kaplan (low head) turns the water energy into shaft rotation; governor controls flow to keep speed constant.
5. Generator: Synchronous generator coupled to the turbine produces 3-phase AC at 11 kV.
6. Transformer and switchyard: Step up the voltage (e.g. 11/132 kV) and connect to the grid.
7. Tailrace: Returns water to the river.

Example: Kali Gandaki "A" (144 MW) in Nepal uses this arrangement with Francis turbines.

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.

DC transmission (HVDC) carries power as direct current between converter stations. Compared with AC lines it has the following merits and demerits.

Advantages

1. Fewer conductors and cheaper line: Two conductors (bipole) or one (monopole) instead of three; smaller towers and right-of-way.
2. Lower losses: No skin effect, no reactive current flow, lower corona loss.
3. Less insulation: Insulation is designed for the DC voltage, while AC needs it for the peak ($\sqrt2V_{rms}$).
4. No charging current: Suitable for long submarine and underground cables.
5. No stability limit: No phase angle between ends, so distance is not limited by stability.
6. Asynchronous link: Connects grids of different frequency or not in synchronism.
7. Fast, accurate power flow control by converter firing angle.
8. No increase of fault level of the interconnected AC systems.
9. Better voltage regulation: No reactive drop ($IX$) on the line; no Ferranti effect.

Limitations

1. Costly terminal equipment: Converter transformers, thyristor valves, filters and smoothing reactors.
2. Harmonics: Converters produce harmonics needing AC and DC filters.
3. Reactive power demand: Converters absorb 50–60 % of rated power as reactive power, supplied by capacitors/filters.
4. Voltage cannot be transformed directly; no economical way to tap power along the line.
5. DC circuit breaking is difficult because there is no natural current zero.
6. Economical only beyond break-even distance (~600–800 km overhead, ~50 km cable).
7. Complex control and maintenance; converters have low overload capacity.

For a given power, distance and allowed loss, the conductor volume needed is inversely proportional to $V^2\cos^2\phi$.

Derivation (3-phase, 3-wire line)

Let power $P$ be sent over length $l$ at line voltage $V$ and power factor $\cos\phi$; conductor resistivity $\rho$, cross-section $a$.

$$\begin{aligned} I &= \frac{P}{\sqrt3V\cos\phi}, \qquad R = \frac{\rho l}{a}\\ W &= 3I^2R = 3\cdot\frac{P^2}{3V^2\cos^2\phi}\cdot\frac{\rho l}{a} = \frac{P^2\rho l}{aV^2\cos^2\phi} \end{aligned}$$

For a fixed permitted loss $W$:

$$a = \frac{P^2\rho l}{WV^2\cos^2\phi}$$

Volume of conductor (3 conductors):

$$\text{Vol} = 3al = \frac{3P^2\rho l^2}{WV^2\cos^2\phi} \quad\Rightarrow\quad \text{Vol}\propto\frac{1}{V^2\cos^2\phi}$$

Explanation

• Higher voltage: For the same power, current falls as $1/V$, so the loss $I^2R$ falls as $1/V^2$. To keep the same loss, a higher resistance (thinner conductor) is acceptable. Doubling the voltage cuts the conductor volume to one-quarter.
• Higher power factor: For the same real power, a low p.f. needs more current ($I\propto1/\cos\phi$), carrying useless reactive current that still causes $I^2R$ loss. Raising the p.f. from 0.7 to 1.0 reduces the required volume to $0.7^2 = 49\ \%$.

This is why power is transmitted at high voltage and utilities insist on good consumer power factor.

Single line diagram with typical voltage levels

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Why different voltages are used

Generation (6.6–25 kV): Generator voltage is limited by the insulation that fits in the stator slots and by machine cost. Higher generation voltage is not practical.

Transmission (132–400 kV, up to 765 kV): Bulk power must travel long distances. For a given power:

• current $\propto 1/V$, so loss $\propto 1/V^2$ and % drop $\propto 1/V^2$,
• conductor volume for the same loss $\propto 1/V^2$,
• line capacity $\propto V^2$.

So the voltage is raised as high as insulation economics allow.

Distribution (11 kV and 400/230 V): Near consumers, very high voltage would be unsafe and need large clearances in towns. Distances are short, so 11 kV primary and 400/230 V secondary are used for safety and cheap equipment, matching appliance ratings.

Why 3-phase AC instead of single-phase

1. Less conductor: A 3-phase 3-wire line needs only 75 % of the conductor of a 1-phase line for the same power, voltage and loss.
2. Constant power: Balanced 3-phase power is constant, giving smooth torque; 1-phase power pulsates at $2f$.
3. Bigger, more efficient machines: A 3-phase generator gives about 1.5 times the output of a 1-phase one of the same size.
4. Self-starting induction motors on 3-phase supply.
5. Both 400 V and 230 V available from a 3-phase 4-wire system.
6. Cheaper transformers and switchgear per kVA and easier parallel operation.

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.

National grid as an infinite bus

A national grid (e.g. Nepal's INPS connected with the Indian grid) links many generating stations and loads, with total capacity of thousands to hundreds of thousands of MW.

• Very low Thevenin impedance: Many generators and lines in parallel make $Z_{th}$ very small (fault level of several thousand MVA). So $V = E_{th} - IZ_{th}$ hardly changes when one load or generator changes its current.
• Very large inertia: The stored kinetic energy of all synchronised rotors is huge. Frequency changes as $df/dt \propto \Delta P/H_{total}$, so a change of a few MW causes negligible frequency change.
• Many controllers: Hundreds of governors and AVRs share any change in demand, so V and f are held near nominal.

Example: A 10 MW hydro plant connected to a 300,000 MW grid is only about 0.003 % of total capacity. Changing its excitation alters only its reactive output; changing its gate opening alters only its MW output; grid voltage and 50 Hz frequency remain practically constant. Hence, for that machine, the national grid acts as an infinite bus.

Conventional sources of energy are the long-established sources that have been used on a large scale for electricity generation, mainly fossil fuels (coal, oil, natural gas), nuclear fuel and (in many textbooks) large hydropower. Fossil and nuclear fuels are finite and are being used up faster than nature can replace them.

Features of a steam (coal-fired) thermal power station

 Coal -> [Boiler] -> steam -> [Turbine]--[Alt]-> Grid
            ^                     |
            |                [Condenser]
       [Feed pump] <-- water -----+

1. Principle: Coal is burned in a boiler; heat converts water into high-pressure superheated steam, which drives a steam turbine coupled to an alternator (Rankine cycle).
2. Location: Near coal mines or a railway line and a large water source for the condenser and cooling.
3. Low initial cost per kW compared with hydro, and shorter construction time.
4. High running cost: Continuous purchase and transport of coal.
5. Low overall efficiency: About 30–40 %, because much heat is rejected in the condenser.
6. Pollution: Emits CO₂, SO₂, NOx, fly ash; needs ash handling, electrostatic precipitators and tall chimneys.
7. Large space for coal storage and ash disposal.
8. Slow start: Takes hours to start and load up; best used as a base-load plant.
9. Large unit sizes (200–1000 MW) possible; output is steady and independent of weather.
10. Water requirement is large for the condenser and boiler make-up.

Nepal has no such coal plants; its conventional backup is small diesel plants, while hydropower supplies the grid.

Major components of an electric power station

1. Prime mover (turbine) with governor
2. Alternator (synchronous generator)
3. Excitation system and AVR
4. Generator step-up transformer
5. Busbars
6. Switchgear: circuit breakers, isolators, earth switches
7. Protective relays, CTs and PTs
8. Lightning arresters
9. Station auxiliary transformer and auxiliaries
10. Control room, metering, SCADA and battery (DC) system
11. Earthing system

Functions of four major components

1. Alternator

• Converts mechanical energy from the turbine into 3-phase AC electrical energy, usually at 6.6–11 kV.
• Its speed fixes the frequency: $f = PN/120$.

2. Excitation system with AVR

• Supplies DC current to the rotor field winding.
• The automatic voltage regulator varies this current to hold terminal voltage and to control reactive power output.

3. Step-up transformer

• Raises generator voltage to transmission level (e.g. 11/132 kV), reducing current and line losses.
• Isolates generator from the transmission network.

4. Circuit breakers with protective relays

• Relays (fed by CTs and PTs) sense abnormal conditions such as short circuit, overload, earth fault, loss of excitation.
• The breaker automatically opens to isolate the faulty part, protecting equipment and keeping the healthy system running; it is also used for normal switching.

 Turbine--[G]--[CB]--[11/132 kV Tx]--[CB]--[Bus]-->Lines
            |                            |
         Exciter/AVR                 CT, PT, LA

A power system is the network that generates electrical energy, transmits it over long distances and distributes it to consumers, with protection and control at every stage.

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Parts of the structure

1. Generating stations

• Hydro, thermal, nuclear, solar or wind plants generate 3-phase AC at 6.6–25 kV (Nepal: mostly hydro at 11 kV).
• A step-up transformer raises the voltage for transmission.

2. Transmission system

• Extra-high voltage lines (132, 220, 400 kV in Nepal; up to 765 kV and above elsewhere) carry bulk power to load centres.
• Lines are interconnected to form a grid, allowing power exchange and shared reserve.

3. Sub-transmission system

• At grid substations the voltage is reduced to 66 or 33 kV and carried to distribution substations in towns and districts.
• Large industries may be supplied directly at this level.

4. Primary distribution

• Distribution substations (33/11 kV) feed 11 kV feeders through the load area.
• Medium industries take supply at 11 kV.

5. Secondary distribution

• Distribution transformers (11/0.4 kV) supply LT distributors at 400 V 3-phase and 230 V single-phase for homes, shops and small industries.

6. Protection and control

• Circuit breakers, relays, lightning arresters at all substations; load dispatch centre (SCADA) monitors and controls the whole system, keeping frequency and voltage within limits.

Typical voltage levels

In Nepal: generation 11 kV; transmission 400/220/132 kV; sub-transmission 66/33 kV; distribution 11 kV and 400/230 V.

Why three-phase is preferred in distribution

1. Less conductor material: For the same power, voltage and loss, 3-phase needs only 75 % of the conductor of single-phase.
2. Constant power: Balanced 3-phase power does not pulsate, so motors run smoothly.
3. Self-starting 3-phase induction motors for industry, pumps and mills.
4. Two voltages from one system: 3-phase 4-wire gives 400 V for motors and 230 V for lighting and appliances.
5. Smaller, cheaper transformers and switchgear per kVA, and better voltage regulation.

The power system grew from small isolated DC stations in the 1880s to today's huge interconnected AC/HVDC grids. Its evolution can be followed through these milestones.

Major milestones

Trends seen in the evolution

1. DC to AC: The transformer and induction motor made AC dominant.
2. Isolated to interconnected: Small plants merged into regional, national and international grids for reliability and economy.
3. Rising voltages: From 110 V to 765 kV AC and ±1100 kV DC to carry more power over longer distances.
4. Larger units: Generator ratings rose from kW to over 1000 MW.
5. Return of DC: HVDC for long-distance and asynchronous links.
6. Automation and renewables: Computer control, smart meters, distributed solar/wind.

In Nepal, the system grew from Pharping (1911) to the Integrated Nepal Power System (INPS) at 132/220/400 kV, now linked to India for power trade.

An interconnected power system links several generating stations through a transmission grid; sub-transmission and primary distribution lines then carry power to load areas.

 G1 11kV          G2 11kV          G3 11kV
   |                |                |
 [11/220]        [11/132]         [11/132]
   |                |                |
 B1 220 kV          |                |
   |                |                |
 [220/132]          |                |
   |                |                |
 B2 132 kV ====== B3 132 kV ====== B4 132 kV
   |                                 |
 [132/33]                        [132/66]
   |                                 |
 B5 33 kV (sub-trans.)           B6 66 kV
   |                                 |
 [33/11]                         [66/11]
   |                                 |
 B7 11 kV feeders                B8 11 kV
   |                                 |
 [11/0.4] -> 400/230 V           [11/0.4] -> LT

Bus voltages

• Generating stations (hydro, thermal) feed the grid through step-up transformers.
• 220 kV and 132 kV lines interconnect the stations and grid substations, forming loops for reliability.
• 66/33 kV lines carry power to distribution substations.
• 11 kV primary feeders carry power to distribution transformers and bulk consumers.

These are the levels used in Nepal's INPS (with 400 kV now added for cross-border lines).

For the same power, distance, line loss and maximum voltage, a 3-phase system needs less conductor material and so costs less.

Single-phase 2-wire line

Power $P$, voltage $V$ (between conductors), p.f. $\cos\phi$, length $l$, area $a_1$:

$$\begin{aligned} I_1 &= \frac{P}{V\cos\phi},\qquad W = 2I_1^2\frac{\rho l}{a_1} = \frac{2P^2\rho l}{a_1V^2\cos^2\phi}\\ a_1 &= \frac{2P^2\rho l}{WV^2\cos^2\phi},\qquad \text{Vol}_1 = 2a_1l = \frac{4P^2\rho l^2}{WV^2\cos^2\phi} \end{aligned}$$

Three-phase 3-wire line (line voltage $V$)

$$\begin{aligned} I_3 &= \frac{P}{\sqrt3V\cos\phi},\qquad W = 3I_3^2\frac{\rho l}{a_3} = \frac{P^2\rho l}{a_3V^2\cos^2\phi}\\ a_3 &= \frac{P^2\rho l}{WV^2\cos^2\phi},\qquad \text{Vol}_3 = 3a_3l = \frac{3P^2\rho l^2}{WV^2\cos^2\phi} \end{aligned}$$

Comparison

$$\frac{\text{Vol}_3}{\text{Vol}_1} = \frac{3}{4} = 0.75$$

So 3-phase 3-wire needs only 75 % of the conductor of 1-phase 2-wire (with the same voltage between conductors).

Other economic points

Hence 3-phase delivery is more economical for generation, transmission and distribution; 1-phase is only used for small loads tapped from the 3-phase system.

Diagram of a typical power system

 [G] 11 kV       Step-up       Transmission
  |---->[11/220 kV]====== 220/400 kV ======+
                                            |
 [G] 11 kV                          [220/132 kV]
  |---->[11/132 kV]====== 132 kV ====== Grid S/S
                                            |
                              Sub-transmission 66/33 kV
                                            |
                                    [33/11 kV] S/S
                                            |
                              Primary distribution 11 kV
                                            |
                                    [11/0.4 kV] DT
                                            |
                         Secondary 400/230 V consumers

Generation

• Power stations (hydro, thermal, nuclear, solar, wind) convert primary energy into electrical energy.
• Generators produce 3-phase AC at 6.6–25 kV (Nepal: 11 kV); generator voltage is limited by insulation.
• Components: turbine, alternator, exciter/AVR, governor, step-up transformer, switchyard.

Transmission

• Carries bulk power over long distances at high voltage: 132, 220, 400 kV (higher in large countries).
• High voltage reduces current, so loss $\propto 1/V^2$ and conductor cost fall.
• Components: towers, conductors (ACSR), insulator strings, earth wire, grid substations with transformers, breakers and protection.
• The transmission network interconnects stations to form a grid.
• Sub-transmission (66/33 kV) carries power from grid substations to distribution substations.

Distribution

• Delivers power to consumers.
• Primary distribution: 33/11 kV substations and 11 kV feeders supplying distribution transformers and medium industries.
• Secondary distribution: 11/0.4 kV transformers feed 400 V 3-phase and 230 V single-phase LT lines to houses and shops.
• Components: feeders, distributors, service mains, distribution transformers, fuses, meters.

Renewable energy sources are naturally replenished and do not run out. The main types used for electricity are:

1. Hydropower

• Potential energy of water at a height drives a turbine: $P = \eta\rho gQH$.
• Types: storage, run-of-river, pumped storage; large, small, micro (<100 kW).
• High efficiency (85–90 %), dispatchable (storage type), long life; but high initial cost and seasonal flow variation.
• Nepal's main source (e.g. Upper Tamakoshi 456 MW).

2. Solar energy

• Solar PV: Cells convert sunlight directly into DC, inverted to AC.
• Solar thermal (CSP): Mirrors concentrate sunlight to raise steam for a turbine.
• No fuel, modular, little maintenance; but intermittent (only daytime) and needs land or storage.

3. Wind energy

• Wind turns rotor blades coupled to a generator: $P = \frac12\rho A v^3 C_p$.
• Clean and cheap per kWh at good sites; output varies with wind speed, noise and visual impact.

4. Biomass and biogas

• Burning wood, agricultural residue, bagasse, or using biogas from digesters to run engines or boilers.
• Uses waste; carbon-neutral if replanted; low energy density and fuel collection problem.

5. Geothermal energy

• Heat from the earth's interior produces steam to drive turbines.
• Continuous (base load); only at volcanic/hot-spring sites.

6. Tidal and wave energy

• Rise and fall of tides or ocean waves drive turbines.
• Predictable, but only on coastlines; high cost.

7. Others

• Fuel cells using green hydrogen, ocean thermal energy conversion (OTEC).

Hydropower

Principle: Water stored at a height has potential energy. It flows through a penstock to a turbine, which turns an alternator. Power:

$$P = \eta\rho gQH\ \text{W}$$

where $Q$ = flow (m³/s), $H$ = net head (m), $\eta$ = overall efficiency.

 Dam/Intake -> Headrace -> Surge tank -> Penstock
     -> Turbine --[Generator]--> Tx -> Grid
     -> Tailrace -> River

Major features

• Fuel is free; very low running cost; renewable and non-polluting.
• High efficiency (85–90 %) and long life (50+ years).
• Quick start and load change: useful for peak load and frequency control.
• High initial cost and long construction time; site depends on geography; often far from load centres.
• Output varies with season (run-of-river plants drop in dry season).
• Examples: Kali Gandaki A (144 MW), Upper Tamakoshi (456 MW).

Thermal (steam) power

Principle: Fuel (coal, oil, gas) is burned in a boiler; the heat turns water into high-pressure steam, which expands in a steam turbine coupled to an alternator, then is condensed and pumped back (Rankine cycle). Chemical energy → heat → mechanical → electrical energy.

 Fuel -> [Boiler] -> steam -> [Turbine]--[Gen]-> Grid
            ^                    |
       [Feed pump] <- [Condenser] <- cooling water

Major features

• Lower initial cost and shorter construction; can be built near loads or fuel source.
• High running cost due to fuel; overall efficiency only 30–40 %.
• Pollution: CO₂, SO₂, NOx, ash; needs chimneys, precipitators and ash disposal.
• Needs large amounts of cooling water.
• Slow start; best for base load.

A power grid is formed by interconnecting generating stations and load centres by high-voltage transmission lines so that all plants operate in parallel and supply the combined load. This interconnection is needed for the following reasons.

1. Reliability and continuity: If one generator or plant trips, the others in the grid supply its load; consumers do not lose supply.
2. Reduced reserve capacity: Each isolated station needs its own standby units. In a grid, the spinning reserve is shared, so total installed capacity required is lower.
3. Economic operation: The most efficient and cheapest plants (e.g. run-of-river hydro) are run at full load and costly plants only at peak, i.e. economic load dispatch.
4. Load diversity: Peak demands of different regions occur at different times. Interconnection lowers the combined peak, so less capacity is needed (higher diversity factor).
5. Using remote resources: Hydro sites in mountains can supply cities far away; seasonal surplus in one area can supply another.
6. Seasonal and cross-border exchange: Nepal exports surplus hydro power in the wet season and imports in the dry season through 132/220/400 kV links with India.
7. Larger, efficient units: Grid can absorb large generating units, which have lower cost per kW.
8. Stable voltage and frequency: A large system is "stiff"; load changes cause negligible voltage and frequency variation.
9. Better use of transmission: Power can flow over parallel paths, reducing overload.

Why high voltage lines

Interconnecting lines carry large power over long distances. Since loss $\propto P^2R/V^2$ and capacity $\propto V^2$, only high voltages (132–400 kV) make such transfer efficient and stable.

 Plant A ====132 kV==== Plant B
    \                   /
     \== Load centre ==/
          ||
      Tie to neighbour grid

Drawbacks such as higher fault levels and spread of disturbances are handled by good protection and control, so interconnection is fully justified.

Transmission voltage is high to reduce losses and conductor cost over long distances, but generation and distribution are kept at lower voltages for practical, safety and cost reasons.

Why generation voltage is low (6.6–25 kV)

• The stator winding insulation must fit in narrow slots; very high voltage would need thick insulation, increasing machine size and reducing slot space for copper.
• Rotating machines are subject to vibration and heating, so high-voltage insulation would be unreliable and costly.
• Hence generators are built for about 11–25 kV and a step-up transformer is used.

Why transmission voltage is high

• For power $P$: $I = P/(\sqrt3V\cos\phi)$, loss $= P^2R/(V^2\cos^2\phi)$.
• Raising $V$ reduces current, loss, voltage drop (all $\propto 1/V^2$ in %), and conductor volume ($\propto 1/V^2$), which matters on long lines.

Why distribution voltage is low (11 kV, 400/230 V)

• Safety: Distribution lines pass through towns and enter buildings; high voltage would be dangerous.
• Short distances: Over a few km, losses at 11 kV or 400 V are acceptable.
• Cheap equipment: Low-voltage insulation, clearances, poles and transformers are much cheaper and smaller.
• Utilisation: Appliances and motors are designed for 230/400 V.

Major sources of electricity worldwide (approximate shares, early 2020s)

So about 60 % of world electricity still comes from fossil fuels.

Why the present supply scenario is not sustainable

1. Depletion of fuels: Coal, oil and gas are finite; at current use, known reserves will last only decades (oil and gas) to about a century or two (coal).
2. Climate change: Burning fossil fuels emits large amounts of CO₂; the power sector is the largest single source of greenhouse gases, causing global warming.
3. Air pollution and health: SO₂, NOx, particulates and mercury from coal plants cause acid rain, smog and respiratory disease.
4. Low efficiency: Thermal plants convert only 30–40 % of fuel energy into electricity; the rest is wasted as heat.
5. Energy security and prices: Many countries depend on imported fuel; prices fluctuate with geopolitics (e.g. oil and gas price shocks).
6. Nuclear issues: Radioactive waste disposal, accident risk (Chernobyl, Fukushima) and high cost.
7. Unequal access: Many people (especially in rural areas) still lack reliable electricity.
8. Water and land use: Thermal plants need large quantities of cooling water; mining damages land.

A sustainable path needs more renewables (hydro, solar, wind), better efficiency, storage and smart grids. Nepal, with large hydro potential, can contribute by exporting clean hydropower.