Electrical Power Systems Flashcards

Electrical Supply System

• Electrical power is generated in power stations, typically located remotely where resources are available.
• Bulk power is transmitted to load centers at high voltages.
• Power is then distributed to consumers at normal voltage.
• The network between power stations and consumers is divided into transmission and distribution systems.
• Combined, these systems are known as the electric supply system or electric power system.

Power Generation

• Electrical power is typically generated at either $11 kV$ or $6.6 kV$ by multiple generators in parallel.
• Generating at higher voltages could lead to technical challenges.
• Power generated at $11 kV$ is stepped up to $110 kV$, $230 kV$, $400 kV$, $765 kV$, or even higher voltages using power transformers.

Transmission System - AC

• Power generated at generating stations is transmitted to main load centers at high voltages. This is known as the primary transmission system.
• Voltage levels can be $765 kV$, $400 kV$, or $220 kV$. The highest transmission voltage in India is $765 kV$.
• Advantages of transmitting power at high voltages:
  • Reduced conductor material volume.
  • Reduced current, leading to lower line losses and higher transmission efficiency.
  • Reduced voltage drop, improving line regulation.
• Power from main load centers is transmitted to sub-load centers at voltages of $33 kV$, $66 kV$, or $110 kV$. This is known as the secondary transmission system.
• Primary transmission systems usually use overhead lines.
• Secondary transmission can use either overhead lines or underground cables, with rural areas using overhead lines and city areas using underground cables.

Distribution System

• Power is received at sub-load centers at $33 kV$, $66 kV$, or $110 kV$.
• The voltage is stepped down to $11 kV$ at the sub-load centers.
• Power is distributed through $11 kV$ lines along main roads, which is referred to as the primary distribution system.
• $11 kV/415 V$ transformers are connected en route on the $11 kV$ lines.
• Power is then distributed through distribution lines along streets, known as distributors.
• Power is distributed using 3-phase, 4-wire lines/cables with voltage levels of 1-phase 230V and 3-phase 415V.
• Lines feeding power from the distribution transformer to the distributors are called feeders.
• Feeders, distributors, and service lines combinedly form the secondary distribution system.
• Distributors can be radial or ring.
• Secondary distribution of power is exclusively AC because most loads are designed to operate on AC, which is more efficient and cost-effective.

Distributors
*Examples: AB, BC, CD, DE, EF, and FG are distributor sections.

Main Components of Transmission and Distribution Systems

• Conductors carry electrical power.
• Supports (RCC poles, MS, tubular poles, or towers) maintain conductor height.
• Cross arms support conductors on poles or towers.
• Insulators isolate conductors from the ground.
• Miscellaneous items include lightning arresters, ground wires, and stay wires.

Comparison of Overhead (OH) and Underground (UG) Systems

• Secondary distribution in city areas often uses UG cable systems; primary distribution sometimes does as well.
• Comparison factors:
  • Public Safety: UG cables are preferred due to reduced hazards.
  • Initial Cost: UG cable systems are about 10 times more expensive than OH line systems, making OH lines preferable for lower initial investment.
  • Flexibility: OH line systems are more flexible and easier to modify.
  • Faults: UG cable systems have fewer faults compared to OH lines because they are not exposed to the atmosphere.
  • Appearance: UG cable systems have a better aesthetic appearance.
  • Fault Location and Repair: Fault location and rectification are typically easier in OH line systems.
  • Useful Life: OH line systems have about 50% of the useful life of UG cable systems.
  • Maintenance Cost: UG cable systems have lower maintenance costs due to fewer faults.
  • Interference with Communication Circuits: UG cable systems have less interference than OH line systems.
• While both systems have advantages and limitations, cities prioritize safety, making UG cable systems preferred despite economic considerations.

Simple Layout of Generation, Transmission, and Distribution of Power

A simple layout of the electrical power system includes the following:

*   Power station
*   Step-up power transformers
*   Primary transmission line
*   Step-down secondary transformers
*   Secondary transmission lines
*   Primary distribution
*   Distribution transformers
*   Secondary distribution

• $11 kV$ generators connect to an $11 kV$ bus.
• The $11 kV$ is stepped up to $230 kV$ via a transformer and connects to a $230 kV$ bus.
• The voltage is then stepped down from $230 kV$ to $33 kV$ and connects to a $33 kV$ bus.
• Next, voltage is stepped down from $33 kV$ to $11 kV$.
• The $11 kV$ bus supplies bulk consumers (industries) and distribution transformers.
• Distribution transformers step down voltage from $11 kV$ to $415 V$ for small consumers (domestic).

Transmission System - DC

• DC transmission has several advantages over AC transmission:
  • Less insulation is required.
  • Smaller towers and cross arms are needed.
  • Improved system stability.
  • Unity power factor.
  • No charging current.
  • No skin effect (skin effect increases the effective resistance of AC line conductors).
• Main disadvantages of HVDC transmission:
  • Lack of transformers for voltage step-up or step-down (must be done on the AC side).
  • Power cannot be generated at higher voltages due to commutation problems.
  • Limitations with HVDC switching devices and circuit breakers.
• HVDC transmission is more economical when the transmission distance exceeds 600 km for OH lines.
• DC transmission systems require more components in a substation.
• Capital cost is higher for transmission lines less than $600 km$, and less if they exceed $600 km$.

Types of DC Links

• HVDC links are used to connect two networks or systems. They are classified into three types:

  • Monopolar Link: Uses a single conductor of negative polarity with earth or sea for the return path.
    • Two converters are placed at the end of each pole.
    • Earthling is done using earth electrodes placed 15 to 55 km away from terminals.
    • Disadvantages include using earth as a return path.
    • Not commonly used today.
  • Bipolar Link: Has two conductors, one positive and one negative to earth.
    • Converter stations at each end are earthed through electrodes at the midpoint.
    • Electrode voltage is half the conductor voltage.
    • Advantage: If one link stops operating, it converts to monopolar mode, and half the system continues to supply power.
    • Commonly used in HVDC systems.
  • Homopolar Link: Has two conductors of the same (usually negative) polarity.
    • Operates with earth or metallic return.
    • Poles operate in parallel, reducing insulation costs.
    • Not presently used.

Equipment in a Transformer Sub-Station

The equipment required depends on the sub-station type, service requirements, and desired protection level. Generally, a transformer sub-station includes:

*   **Bus-bars**: Common electrical components that directly connect lines operating at the same voltage.
    *   Made of copper or aluminum bars (typically rectangular).
    *   Maintain constant voltage.
    *   Incoming and outgoing lines connect to the bus-bars.
    *   Common arrangements: single bus-bar, single bus-bar with sectionalization, and double bus-bar.
*   **Insulators**: Support conductors or bus-bars and confine current to the conductors.
    *   Porcelain is the most commonly used material.
    *   Types include pin, suspension, and post insulators.
    *   Post insulators are used for bus-bars; they consist of a porcelain body, cast iron cap, and flanged cast iron base.
*   **Isolating switches (Isolators)**: Disconnect parts of the system for maintenance and repairs.
    *   Essentially knife switches that open circuits under no load.
    *   Operated only when lines carry no current.
*   **Circuit breaker**: Opens or closes a circuit under normal and fault conditions.
    *   Operated manually (or remotely) under normal conditions and automatically under fault conditions (using a relay circuit).
    *   Bulk oil circuit breakers are used for voltages up to $66kV$, low oil circuit breakers for high voltages (>66 kV), and air-blast, vacuum, or SF6 circuit breakers for still higher voltages.
*   **Power Transformers**: Used to step-up or step-down voltage.
    *   Step-down transformers are used at subsequent sub-stations to reduce voltage to utilization levels.
    *   Modern practice uses 3-phase transformers (instead of 3 single-phase banks) for simpler installation and a single load-tap changing mechanism.
    *   Installed on rails fixed on concrete slabs with foundations 1 to 1.5 m deep.
    *   Naturally cooled, oil-immersed transformers are used for ratings up to 10 MVA; air blast cooled transformers are used for higher ratings.
*   **Instrument transformers**: Transfer voltages or currents in power lines to values suitable for measuring instruments and relays.
    *   Lines operate at high voltages and currents.
    *   Measuring instruments and protective devices are designed for low voltages (typically 110 V) and currents (about 5 A).
    *   Types: Current Transformer (CT) and Potential Transformer (PT).
        *   **Current Transformer (CT)**: A step-up transformer that steps down the current to a known ratio.
            *   Primary consists of one or more turns of thick wire in series with the line.
            *   Secondary consists of a large number of turns of fine wire.
            *   For example, a CT rated at 100/5 A will step down a line current of 100 A to 5 A.
        *   **Potential Transformer (PT)**: A step-down transformer that steps down the voltage to a known ratio.
            *   Primary consists of a large number of turns of fine wire across the line.
            *   Secondary consists of a few turns.
            *   For example, a PT rated at 66kV/110V will step down a line voltage of 66kV to 110V.
*   **Metering and Indicating Instruments**: Ammeters, voltmeters, energy meters, etc., are installed to monitor circuit quantities; instrument transformers are used with them.
*   **Miscellaneous Equipment**: Fuses, carrier-current equipment, and sub-station auxiliary supplies.

Key Diagram of 11 kV/400 V Indoor Sub-Station

The key diagram of this 11 kV/400 V sub-station can be explained as under:

*   The 3-phase, 3-wire $11 kV$ line is tapped and brought to the gang operating switch (G.O. switch) near the sub-station consisting of isolators in each phase.
*   From the G.O. switch, the $11 kV$ line is brought to the indoor sub-station as an underground cable and fed to the H.T. side of the transformer ($11 kV/400 V$) via the $11 kV$ O.C.B. The transformer steps down the voltage to $400 V$, 3-phase, 4-wire.
*   The transformer's secondary supplies the bus-bars via the main O.C.B. From the busbars, $400 V$, 3-phase, 4-wire supply is given to various consumers via $400 V$ O.C.B. The voltage between any two phases is $400 V$, and between any phase and neutral, it is $230 V$. Single-phase residential loads are connected between one phase and neutral, whereas 3-phase, $400 V$ motor loads are connected across 3-phase lines directly.
*   CTs are located at suitable places in the sub-station circuit to supply metering and indicating instruments and relay circuits.

Introduction to Smart Grid

• Traditional Power Grid:
  • One-way flow of electricity.
  • Centralized, bulk generation.
  • Heavy reliance on coal and oil.
  • Limited automation and situational awareness.
  • Consumers lack data to manage energy usage.
• Smart Grid:
  • Two-way flow of electricity and information.
  • Communications infrastructure.

Safety Precautions when Working with Electricity

1. Never touch or repair electrical equipment or circuits with wet hands.
2. Never use equipment with damaged insulation or broken plugs.
3. Turn off the mains when working on electrical sockets.
4. Always use insulated tools.
5. Be observant of warning signs indicating “Shock Risk” and exposed energized parts.
6. Always use appropriate insulated rubber gloves and goggles.
7. Never repair energized equipment; use a tester to ensure it's de-energized first.
8. Know the wire code of your country.
9. Always use a circuit breaker or fuse with the appropriate current rating.

Electrical Safety Devices

• Fuse: Electrical safety device that protects circuits from excessive current by opening the circuit when the current exceeds a maximum value.
• Circuit breaker: Devices that protect circuits from overload current conditions; they are not destroyed when activated.

Earthing

• The potential of the earth is considered zero for all practical purposes.
• Earthing connects electrical equipment to the earth with a very low resistance wire, making it attain earth's potential.
• Ensures safe discharge of electric energy due to insulation failure or contact with the casing.
• Earthing brings the potential of the equipment's body to ZERO, protecting personnel against electrical shock.

Importance of Earthing

• Case I: Healthy insulation, apparatus not earthed

  • Insulation is healthy ($R_{insulation} = \infty$).
  • Supply current flows only through the resistance of the apparatus ($R_{apparatus}$).
  • No current flows through the body resistance ($I_{body} = 0$).
  • The person is safe even if the apparatus is not earthed.

• Case II: Defective insulation, apparatus not earthed

  • Insulation is bad ($R_{insulation} = 0$).
  • Supply current divides into $I<em>{apparatus}$ and $I</em>{body}$.
  • Part of the supply current flows through the body to the ground.
  • The person experiences a shock as the apparatus is not earthed.

• Case III: Defective insulation, apparatus earthed

  • Insulation is bad ($R_{insulation} = 0$).
  • Supply current divides into $I<em>{apparatus}$, $I</em>{body}$, and $I_{RE}$.
  • Part of the supply current ($I_{body}$) flows through the body to the ground.
  • $I<em>{body}$ is very less compared to $I</em>{RE}$, the current flowing through the wire of negligible resistance connecting the apparatus' metal case to ground ($I<em>{body} << I</em>{RE}$).
  • The person does not experience any shock as the metal casing is earthed.

Earthing and its Necessity

• Earthing is connecting to the mass of the earth to ensure a safe discharge of electric current due to leakages and faults.
• All metallic parts of electrical installations (conduit, cable sheathing, panels, frames, switches, appliances, motors, gears, transformers, regulators, etc.) must be earthed using one continuous bus (bare wire).
• If one earth bus is not feasible, multiple earthing systems should be used, dividing equipment and appliances into sub-groups connected to different earth buses.
• Earthing conductors taken outdoors should be encased in a pipe, securely supported and continued up to at least 0.3 meters below ground level; no joints are permitted in an earth bus.
• The earthing of a lightning conductor system should not be bonded to the earthing of the electrical installation.
• Before energizing electric supply lines or apparatus, the earthing system must be tested for electrical resistance to ensure it is not more than two ohms (including the ohmic value of the earth electrode).

Types of Earthing

1. Plate Earthing
2. Pipe Earthing

• Earthing through a G.I. Pipe

  • A G.I. pipe is used as an earth electrode; the size depends on the current to be carried and the soil type.
  • For ordinary soils, the G.I. pipe used is 2 m long and 38 mm in diameter, or 1.37 m long and 51 mm in diameter.
  • For dry and rocky soils, increase the length to about 2.75 meters and 1.85 meters, respectively.
  • The pipe is placed vertically, buried to a depth of at least 2 meters in as moist a place as possible, near a water tap, water pipe, or water drain, and at least 0.6 meters away from building foundations.
  • The pipe is completely covered by 80 mm of Charcoal with a layer of common salt 30 mm all around it to decrease earth resistance.

• Earthing through a Plate

  • A G.I. or copper plate is used as an earth electrode.
  • If a G.I. plate is used, it should be 0.3 m x 0.3 m and 6.35 mm thick; if a copper plate is used, it should be 0.3 m x 0.3 m and 3.2 mm thick.
  • The plate is buried to a depth of at least 2 m in as moist a place as possible, near a water tap, water pipe, or water drain, and at least 0.6 m away from building foundations.
  • The plate is completely covered by 80 mm of charcoal with a layer of common salt of 30 mm all around it, keeping the faces vertical.

Renewable Energy

• Renewable energy refers to energy forms naturally obtained from the environment and replenished.
• Includes solar, wind, geothermal, hydropower, tidal, and biomass energy.
• Advantages:
  • Less maintenance cost due to few or no moving parts.
  • Economical; cuts costs on fossil fuels.
  • Little or no waste emitted.
  • Renewable energy sources do not deplete.

Solar Photovoltaic System

• A solar cell (photovoltaic cell or PV cell) converts light energy into electrical energy through the photovoltaic effect.
• A solar cell is basically a p-n junction diode and a form of photoelectric cell, whose electrical characteristics vary when exposed to light.
• Solar cells are also known as photovoltaic cells because of their photo-voltaic effect using various potential barriers.
• Made of semiconductor material like silicon.
• A solar cell works in three steps:

1. Photons in the sunlight hit the solar cell and are absorbed by the semiconductor material.
   Negative charged electrons are knocked off from their atoms and start flowing in the same direction to produce electric current.

• The output of a single solar cell is small, so they are interconnected to form a solar module, and modules are combined into panels, and panels into an array to get the required power output.
• When solar cells are connected in series, their voltage increases; when connected in parallel, the current increases.

Solar Photovoltaic system contains the following

*   Solar Panels
*   Hybrid Inverter
*   Battery
 *   Grid connects the solar panels to home appliances

Introduction to Energy Storage

• Energy storage systems retain anything (physical or virtual) for future use.
• Quantitative property held by an object that can be consumed to perform work or convert the form of energy.
• Used for diverse applications like portable electronics, Uninterruptible Power Supplies (UPS), and energy offset for renewable energy.

Energy Storage contains the following: Radiation, Sound, Thermal, Mechanical, Electric, Magnetic, Gravitational, Nuclear, Chemical, Ionization.

Energy Storage Systems

• The transition from non-renewable to renewable sources of energy will not happen overnight because the available green technologies do not generate enough energy to meet the demand.
• Developing new and improving the existing energy storage devices and mediums to reduce energy loss to the minimum, lower the costs of energy storage and maintain a reliable power supply.

Why is Energy Stored

• Energy storage uses various methods to store excess energy to be used at a later time, allowing energy providers to balance demand and supply.
• Selection of devices and media depends on the source of energy and the use.

Types of Energy Storage

• Chemical energy storage. e.g vanadium; it is used in vanadium redox batteries
• Electrochemical energy storage
• Battery: chargeable and rechargeable batteries
• Fuel cell
• Electrical energy storage
• Capacitor and superCapacitor
• Thermal energy storage

Overview of Battery

• Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit.
• All batteries are made up of three basic components: an anode (the '+' side), a cathode (the '-' side), and some kind of electrolyte (a substance that chemically reacts with the anode and cathode).
• Types:
  1. Lead acid
  2. Nickel cadmium
  3. Nickel metal hydride

1. Lithium ion
   5. Sodium sulphur
   6. Redox flow

Fuel Cell Technologies

• Fuel cell technology is a clean technology with low chemical pollution that electrochemically converts chemical energy of hydrogen and oxygen into electricity and heat, producing only water as a byproduct.

Working of a Fuel Cell

• A fuel cell consists of two electrodes (anode and cathode) separated by an electrolyte membrane.
• Organic fuels (hydrogen, methane, ethane, ethanol, etc.) undergo combustion and release heat.
• Reactions produce water and carbon-di-oxide as by-products and are redox reactions.
• Redox reactions involve the transfer of electrons that leads to the conversion of chemical energy into electrical energy.
• Catalyst present at the anode breaks the hydrogen molecule into protons and electrons.
• Electrons reach the cathode following an external path (producing current), while protons travel through the electrolyte membrane and combine with oxygen and electrons to produce water and heat.

Advantages of a Fuel Cell

1.  Does not require recharging; can reproduce energy until fuel is supplied.
2.  If hydrogen is used, byproducts are water, heat, and electricity, producing electrical energy efficiently without toxic substances.
3.  Highly efficient as they directly convert chemical energy into electrical energy (60% more efficient compared to alternatives).
4.  Does not contribute to air pollution.
5.  Not hazardous and does not lead to health problems as the working of fuel cells does not lead to the formation of smoke or smog.
6.  Does not have any mechanical part; therefore, they are noiseless.

Disadvantages of a Fuel Cell

1.  $Fuel cells are expensive in nature.
2.  Fuel cells are difficult to store as the fuel used in the cells require a particular temperature and pressure level to be maintained.
3.  Fuel cells are comparatively less durable.
4.  The average lifespan of fuel cells is not quite high.

Applications of a Fuel Cell

*   Fuel cells are widely used in transportation vehicles such as buses, trucks, cars, etc.
*   Fuel cells are prominently employed in material handling equipment to ease the process of transporting heavy goods from one place to another.
*   A number of backup power generation systems make use of fuel cells for their operation.Stationary fuel cells are a crucial element of the uninterrupted power supply devices installed in hospitals, residential buildings, industries, offices, etc.

• Hydrogen fuel cells provide a versatile option to power various electronic gadgets and communication devices such as mobile phones, laptops, etc.

Types of EVs

• (i) BEV (Battery Electric Vehicle)

  • Battery Electric Vehicle (BEV)
  • EVs run on battery power without an internal combustion engine's assistance,
    they can run much faster on a single charge than hybrid vehicles.

• (ii) HEV (Hybrid Electric Vehicle)

  • HEVS run on both an internal combustion engine and an electric motor that uses energy stored in a battery.
    *Batteries via regenerative braking stores the kinetic energy used to stop the car to charge its battery and help the internal combustion engine accelerate the vehicle.

• (iii) PHEV (Plug in Hybrid Electric Vehicle)

  • PHEVS expand on the concept of the standard
    hybrid vehicle. They have both an internal
    combustion engine and a battery-powered
    electric motor.
    *PHEVS can travel up to 40 miles on electric power alone, rather than a couple of
    miles with a standard hybrid vehicle.

EVS

• An electric vehicle (EV) is a vehicle that uses one or more electric motors for propulsion.
• EVs include road and rail vehicles, surface and underwater vessels, electric aircraft, and electric spacecraft.
• EVs first came into existence in the late 19th century.
• Government incentives to increase adoption were first introduced in the late 2000s, leading to a growing market for vehicles in the 2010s.
• As of July 2022, the global EV market size was $280 billion and was expected to grow to $1 trillion by 2026.

EV Charging Station

• EV charging points are primarily defined by the power (in kW) they can produce and therefore what speed they are capable of charging an EV.
  • (i) Slow charging (up to 3kW)
  • (ii) Fast charging (7-22kW)
  • (iii) Rapid chargers (43-50kW)
    4 Types of Connectors

CHAdeMO 'Charge de Move' Nissan, Toyota, and Mitsubishi

Level 2 Charging

CCS
Combined ChargingSystem (all EVS except Tesla)

Tesla Only Tesla

Exam Questions

8 MARKS

1. Draw the simple layout of generation, transmission and distribution system and explain. OR Draw the simple layout of electrical power system and explain.
2. List out the advantages of DC transmission system over AC transmission system. Also explain various types of DC transmission system.
3. Explain various components in a substation.
4. Draw the one line diagram of 11kV/400 V indoor substation and explain.
5. What is the need for earthing? Explain any two types of earthing in detail.
6. Explain in detail about fuel cells. List out its advantages, disadvantages and applications.
7. Explain HEVs, PHEVS and EVs in detail.

4 MARKS

1. Write short notes on smart grid. Compare it with traditional grid.
2. Compare fuse and circuit breaker.
3. List out the safety precautions when working with electricity. OR List out the safety precautions to avoid electric shock when working with electricity.
4. Write short notes on Solar Photovoltaic system.
5. Write short notes on energy storage systems.
6. Write short notes on EV Charging station.