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Fundamentals of Electric Distribution

Energy Sources

  • Energy is not free
  • You cannot create energy out of nothing
  • To get energy out, you have to put energy in
  • Our electric generator is actually an energy conversion device
  • It takes a kinetic energy input and converts it to electrical energy
  • The formula for Energy Input and output is Ein = Eout + losses
  • The mechanical energy in equals the electrical energy out minus some losses
  • Out generator relies on a mechanical hand crank for energy input
  • Obviously, power utilities don't use hand cranks to generate energy
  • So where do we get this input energy from?
  • Two main methods create the input needed to create electrical energy
  • The first method involves a two step conversion and the use of steam
  • Steam is created by using fossile fules , nuclear energy, biomass, or earth's own heat to boil water or some other liquid until it creates high pressure
  • This pressure is released through some sort of turbine
  • The steam expands as it reaches the turbine causing the turbine to spin
  • This creates the mecahnical input for the generator
  • The other method used to supply energy to the generator is to leverage a direct conversion of kinetic energy
  • Windmills capture the energy of moving air
  • The generator is generally connected directly tot he windmill blade assembly so that little energy is lost getting to the generator
  • Hyro electric plants work similarly to windmill s
  • Gravity forces water past a water turbine and makes it spin
  • The turbine is connected directly to a generator above it to create electrical energy
  • These generate electricity without the waste or emissions associated with steam generating methods
  • They do have their drawbacks however - calm days can stop windmills
  • Hydroelectric plants can be victims of drought

The Power Grid

  • Whatever their power generation method, power plants in mos tindustrialized countries are connected to a power grid
  • The grid provides multiple paths of energy to individual locations
  • When one power plant shuts down, power can be drawn from other plants on the grid to keep energy flowing where it is needed
  • Another advantage of a power grid is that it allows power sharing to address differences in load requirements
  • For example, suppose a utility has all the capacity it needs to provide power to its service area but their cost of producing energy is higher than a hydro electric plant hundreds of miles away
  • When the hydroelectric plant is producing more energy than its service area requires, it can sell this energy to higher cost producers benefitting producers, suppliers and cusotmers

Utility to End Users

  • Power plants typically generate power int he range of tens of thousands of kiloVolts
  • To reduce the amount of power lost during distribution the voltage is raised from 100kV to 500kV
  • When the electricity gets closer to where it will be used, an electric substation lowers the voltage into the thousands of volts range
  • Finally, transformers at a factory drop the voltage to the 480V typically used in inudstry
  • This is how utility power reaches an industrial end user
  • How does a transformer transform from low volts to high volts? and back again?
  • AC has a unique characteristic that makes it superior to DC when it comes to distribution
  • We can create a magnetic field with one coil and generate voltage and current in another coil
  • A changing magnetic field will induce voltage and current in a conductor
  • It is not possibel to change the votlage of a DC system with something as simple as a transformer
  • Air is the medium for the magnetic flux from one coil to create a voltage on the other coil
  • However, using a magnetixable material in the cores of both coils makes the system more efficient
  • An important feature of any transformer is the turns ratio
  • This is a ratio that compartes the number of loops on the input side of the transformer to the number of turns on the output side
  • We sometimes call the input side the primary and the output side the secondary side of the transformer
  • The truns ratio determines the change in voltage from the primary to the secondary
  • Why do we go to all the trouble of raising and lowering voltage to get it from the power plant to the factory?
  • The equation for power is power = volts x current
  • If we need to deliver 1 million watts (one megawatt) then any valid product of volts x current will get us there
  • We could transfer 1 million volts at one amp or 1 million amps at one volt
  • However, there are considerations that must be addressed at each extreme
  • To conduct on emillion amps, you would need an extremely large cbale
  • The other problem is that even good conductors have a little bit of resistance and the longer the conductor, the greater the resistance
  • This means there are power losses int he conductor
  • These losses are proportional to the amount of current flowing
  • The higher the current, the more power has to be generated to compensate the losses
  • The line losses turn out to be proportional to the square of the current
  • So you can understand why power companies prefer to transmit power at high voltage
  • There is also a downside to increasing the voltage but its not related to power losses
  • Higher voltages are more dangerous, are harder to work on, require more spacing between conductors, and may require very thick and expensive insulation on the wires
  • Wires on transmission lines and towers typically skirt that insulation cost
  • The power companies use air and the space between cables to insulate them from each other

  • the types of loads that use the most energy in industry are AC motors, electric heating, and lighting
  • Motors are found almost everywhere - conveyor lines, ventilation, air conditioning
  • Electric heating, or resistance heating, is the least efficient method of generating heat but it is still used in many places beacuse it is easier to install and control than many other methods
  • Lighting, especially incandesecent lighting, is used pervasively in industry
  • Incandescent lighting is an inefficient method of lighting
  • It is basically resistance heating, but designed to give off light
  • It is also inexpensive to installa nd easy to control

3 Types of Resistance

  • For resistance in an electrical circuit, we generally use the symbol for a common resistor
  • That can represent something like a simple light or heating element
  • However, AC circuits can see some special types of resistance
  • The three types of resistance are capacitance, inductance, resistance
  • All resistance falls into one of these three capacities

  • Suppose we have an electrical barrier
  • It is made of a lfexible material and designed so that no water can flow through it
  • When we apply pressure on one side,t he bladder stretches in the opposite direction

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