Farm Electrification: Power Transmission & Distribution

• Module 3 covers power transmission and distribution.

Topical Outline

• Power grid extension
• Transformers
• Electrical grounding

Intended Learning Outcomes

• Define power transmission and distribution.
• Define a power grid.
• Discuss different types of transformers

Farm Electrification

• Farm Electrification relates to the design, construction, installation, operation, and maintenance of electrical power sources, wiring, lighting, and systems for farm production and processing.
• Farm electrification includes off-grid systems using stand-alone power sources not connected to utility distribution networks.
• The process comprises generation, transmission, and distribution using renewable energy and hybrid systems,as stated in R.A 10915.

Farm Electrification Systems

• Farms source power from the grid and/or generate it on-site.
• Electrification can be interconnected or combined with generation, transmission, distribution, and farm operations.
• This Includes system design, construction, installation, operation and maintenance.
• Farm electrification should be safe, efficient, reliable, and up-to-date

Components of the Grid

• The grid consists of generation (red), transmission (blue), distribution (green), and customers (black).
• Transmission lines carry 765, 500, 345, 230, and 138 kV

Generation

• Generation "creates" electric energy
• Generation is fueled by coal, nuclear, wind, gas, biomass, solar, and hydro.

Load

• Load "consumes" electric energy
• Loads range from small devices like cell phone chargers (1 watt) to large industrial facilities (10's of millions of watts).

Distribution

• Distribution's primary purpose is to serve loads that are connected to a distribution system
• Distribution systems are generally radial (non-networked) in nature.
• Distribution systems are not used for interstate commerce.

Transmission

• Transmission is designed to carry large quantities of energy over relatively long distances between generating stations and main substations.
• Transmission moves power relatively long distances from generators to load with lower losses
• They are highly interconnected for enhanced reliability and form the interstate system for electricity.
• Transmission was traditionally built for reliability for vertically integrated utilities and is now a critical part of electric markets.

Transmission Lines

• Electric transmission systems operate at relatively high voltages compared to generator and distribution network voltages.
• Transmission lines carry energy more efficiently as voltage increases.
• Failure of a transmission line means energy cannot be supplied to one part of the region.
• To improve reliability, additional lines can be constructed between distribution substations.
• Losing one transmission line from the generating station increases energy flow over the remaining lines to reach substations.

Functions of Transmission Lines

• Transmit power from generating stations to load areas.
• Interconnect load areas, generating stations, and Individual systems to improve reliability
• Interconnect utilities to allow generation reserves and other benefits.

Distribution System

• The distribution system distributes electrical power locally and is located between the substation fed by the transmission system and the consumer meter.
• A distribution line consists of feeders, distributors, and service mains:

Feeders

• Feeders connect substations to the distribution area
• They feed electrical power from the generating station to the substation
• No tapings are taken from feeders, so the current remains constant throughout.
• The design focuses on the current carrying capacity

Distributers

• Distributers are conductors from which tapings are taken from pole-mounted transformers to consumers.
• The current through a distributor is not constant because sections of the circuit are tapped at various points along its length
• Voltage drop is a main point of consideration, with a limit of 6% variation for consumers

Service Mains

• Service mains are small cables connecting the distributor to the consumer's meter
• They form the connecting links between the distributor and the consumer terminals
• Distribution system voltage ranges from 120 V to 69 kV.
• Medium voltage distribution systems (2.4 kV to 69 kV) are used between substations and load centers.
• Low voltage distribution systems (120 V to 600 V) are used to provide power to residences, small industries, and commercial buildings.

Classification of Distribution Systems

• By type of current: AC or DC systems
• By type of construction: Overhead or underground systems
• By type of service: General lighting and power, industrial power, railway, or streetlight
• By number of wires: Two-wire, three-wire, or four-wire
• By scheme of connection: Radial, ring/loop, or interconnected distribution systems

AC Distribution System

• AC distribution system is the electrical system between the step-down substation and the consumers' meters.
• AC distribution system is classified into primary and secondary distribution systems
• Primary distribution systems have voltages somewhat higher than general utilization and handle large blocks of electrical energy.
• Common primary distribution voltages: 11 kV, 6.6 kV and 3.3 kV
• Electric power is transmitted from generating stations at high voltage to substations located in or near cities
• Voltage is stepped down to 11 kV at the substation using step-down transformers.

Primary Distribution System

• Power is supplied to various substations for distribution to big Consumers at this voltages.
• Secondary distribution systems include the range of voltages at which the ultimate consumer utilizes electrical energy.
• Secondary distribution employs 400/230 V, 3-phase, 4-wire system.

DC Distribution System

• DC system is required for variable speed machinery (i.e. DC motors), electrochemical work, and congested areas where storage battery reserves are necessary.
• AC power is converted into DC power at the substation using converting machinery, e.g., mercury arc rectifiers, and motor-generator sets. The DC supply from the substation may be obtained in the form of 2-wire or 3-wire for distribution.

Radial Distribution Systems

• Separate feeders radiate and feed the distributor at one end only.
• Only one path connects each customer and the substation.
• If interrupted it results in complete loss of power to the customer

Ring or Loop Distribution Systems

• Consist of two or more paths between power sources and the customer.
• The loop circuit starts from the substation bus-bars, loops through the area served, and returns to the substation.

Interconnected Distribution System

• Supplied by multiple feeders.
• Radial primary feeders can be tapped off from the interconnecting tie feeders.
• They can also be supplied directly from the substation.

Basic Transformer Components

• Source is applied to the primary winding
• The load is connected to the secondary winding
• The core provides a physical structure for placement of windings and a magnetic path to concentrate magnetic flux lines close to the coils.
• Typical core materials: air, ferrite, and iron
  • Air and ferrite cores are used at high frequencies
  • Iron cores are used for audio frequencies and power applications

Turns Ratio

• (n) is defined as the ratio of the number of turns in the secondary winding (Nsec) to the number of turns in the primary winding (Npri): n = Nsec/Npri
• The secondary voltage can be determined with the turns ratio.

Direction of Windings

• The direction of the windings determines the polarity of the voltage across the secondary winding with respect to the voltage across the primary
• Phase dots are used to indicate polarities

Step-Up Transformers

A transformer is called this when the secondary voltage is greater than the primary voltage

• The ratio of secondary voltage (Vsec) to primary voltage (Vpri) is equal to the ratio of the number of turns in the secondary winding (Nsec) to the number of turns in the primary winding (Npri)

Step-Down Transformer

• A transformer where the secondary voltage is less than the primary voltage.
• The amount by which the voltage is stepped down depends on the turns ratio.
• The turns ratio of a step-down transformer is always less than 1.

Transformers as Isolation Devices

• Transformers provide electrical isolation because there is no electrical connection between the two windings.
• Energy is transferred entirely by magnetic coupling.
• DC Isolation: A transformer is capable of blocking DC current thus preventing bias from affecting the next amplifier.
• The AC signal from the amplifier is transferred to the next stage

Power Line Isolation

• Electrical devices are isolated by the devices from AC power lines .

Current and Power

• A current result because of the voltage from connecting a load resistor to the secondary winding.

Primary Power Equals Load Power

• In an ideal transformer, power delivered in the secondary (Vseclsec) equals the power in the primary (Vprilpri). -If the voltage is stepped up in the secondary, the current is stepped down by the same amount. -If the voltage is stepped down in the secondary, the current is stepped up by the same amount.

Tapped Transformers

• Utility-pole transformer has a center tap is equal to half voltage across each secondary winding.
• Center-tap windings are used for rectifier supplies and impedance-matching transformers

Multiple-Winding Transformers

• Transformers with more than one winding on a common core used to operate on, or provide different operating voltages.

Autotransformers

• One winding serves as both the primary and secondary, tapped to achieve needed turns ratio for stepping up or down the voltage.

Electrical Grounding

• The purpose of grounding is for safety

System Grounding

• Grounding the portions which are current-carrying .
• This is due to a voltage between any conductor and ground rising above safe levels

Equipment Grounding

• Grounding the device to prevent electric shock
• The potential difference is different than the the ground in the earth

Polarity and Switching

• Using color code to cover the conductors
  • White wire is always the neutral wire.
  • Red or black wires are hot conductors
  • Green wire is the grounding wire.
  • White wires are connected to silver terminals, red or black to brass terminals.
  • All switches are in the hot conductors.
• Grounded neutral should never be interrupted by a fuse, circuit breaker, switch.

Short Circuit and Overcurrent Protection

• Electrical systems require safeguards to assure that safe levels of current are not exceeded.
• Prevent damage by cutting current flow when a short circuit occurs.
• Two basic classes of overcurrent devices: fuses and circuit breakers.
• Fuses are devices of which a portion is destroyed when interrupting the circuit. They use a low melting point metal calibrated to melt at a specific current rating. And all fuses have an inverse time characteristic.
• A circuit breaker opens a circuit automatically on a predetermined overload current without damage and have a bimetallic strip connected in series with the contacts.