31-01-2013, 02:32 PM
The Electrical Grid
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INTRODUCTION
The electrical grid is a network that allows for the distribution of electricity from suppliers to consumers.
Grid Operation
Operationally the conventional electrical grid starts at power generating systems such as power stations (see Power Stations article this web site) that generate 3 phase alternating current (AC) electricity. The 3 phase AC current is passed through a transmission substation that uses transformers to step up (increase) the voltage from thousands of volts to hundreds of thousands of volts. Increasing the voltage allows for efficient transmission of electricity over long distances. After being converted to high voltage, the 3 phase electricity is sent over long distance transmission lines through three lines, one for each phase. Before it can be distributed to end users, the electricity must pass through a power substation that steps down (decreases) the voltage with transformers so that it can be distributed to communities and used in homes and businesses at the correct voltage.
A 3 phase current is used because electricity is generated in a sine wave that has peaks and troughs, meaning that power strength for a single phase fluctuates between weaker and stronger moments. By generating three phases and offsetting them by 120 degrees, the moment of peak power is evenly distributed between the three phases, allowing for more consistent peak power output. Having consistent peak power output is important mainly for industrial purposes, e.g., industrial 3 phase motors.
Alternating current is used because it is easier to change voltages with it than with DC, and a very high voltage is fundamental to long distance electrical transmission because it reduces energy loss by lowering resistance in the wires. To find out more about this, see the following article:
Grid Distribution System
As a distribution system the electrical grid can be organized in several ways. How an electrical grid is set up depends on factors such as topology, geography and the level of grid interaction for an area.
Electrical grids systems include:
• Radial
• Mesh
• Looped (Parallel path flow)
Radial
A radial type grid is the simplest setup. Known as a radial network, it involves a series of networks and sub-networks organized as radial trees that begin with a power source and distribute electricity through networks with progressively lower voltages, eventually ending with communities, homes and businesses.
Mesh
A mesh network involves the radial structure but includes redundant lines, which are in addition to the main lines and organized as backups for the purpose of rerouting power in the event of failure to a main line.
Looped (Parallel path flow)
Looped or parallel path flow systems involve the way different grid networks are connected to each other. Having one network connected to another allows networks to share and balance the flow of electricity as required, where one network can act as either a backup or additional supplier. In this setup each network can run its own transmission line to the same (often high demand) distribution point via parallel transmission lines. It is also possible for electricity to flow from one network to the other and then back again to the original, i.e., a looped flow. In a looped/parallel flow setup issues can arise with controlling the flow of electricity at the point(s) of network contact.
Grid Networks
Electrical grids are composed of many smaller electrical networks that are linked together into a larger network called a Wide Area Synchronous Grid, also known as an “interconnection”. A Wide Area Synchronous Grid allows all the independent electrical networks in a particular area to be connected by synchronizing the electrical frequency between them. North American interconnections are synchronized at 60Hz and European ones at 50Hz (see images below).
Distributed Generation
Distributed generation, also known as “micro grid”, takes advantage of advances in electrical generating systems, e.g., solar panels, wind turbines and cogeneration, which allow for creating and distributing electrical power outside the traditional grid system. As such these systems are smaller, more locally focused and can act in addition to or separate from the traditional grid supply. Because distributed generation systems transmit electricity over short distances, i.e., they are local, they reduce the amount of energy loss compared to the grid. On the other hand, because they rely on alternative energy technologies, they can have a large initial cost. Distributed generation systems can generate between 3kW – 10,000kW of electricity.