EME 810
Solar Resource Assessment and Economics

6.5 The Power Grid System

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Reading Assignment

  • J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 9: Solar Economics (focus on Managing the Grid)
The main form of energy that we think of in society is power from electricity. As a society, we typically deliver electric power though a complex distribution system called the power grid.

Power Grid 101

One of the highly visible SECS technologies is photovoltaics, which delivers (generation) electricity to the client, and now pushes excess electricity onto the electricity power grid. The electricity power grid is the physical system that delivers (transmission) electricity from the place where it is generated to the site where it is used (end-use, demand).The electricity leaving the generating station enters a sub-station with a step-up transformer that raises the voltage extremely high for long-distance transmission.

Reminder: Power is Voltage times Current ( W=V×I )

When electricity travels through wires (a conductor), some energy is lost, but less energy is lost when the electricity is transmitted at a higher voltage. At a high voltage, the same amount of power can be transmitted, but using a lower current. The amount of energy lost from the conductor is called line loss, and line losses are directly proportional to the current. By reducing current, we reduce losses for the same power transmitted. Typically in the U.S., line losses between generation and end-use are in the 6% to 8% range.

The high-voltage electricity is carried over transmission lines to local substations where a step-down transformer reduces the voltage to levels suitable for customer loads. Distribution lines carry the lower-voltage electricity from the local substations to customer sites.

A schematic showing the components of the electricity power grid from generating station to the site where it is used. Further description in text preceeding image.
Figure 6.2 The Electricity Power Grid
Click Here for a text version of The Electricity Power Grid Schematic

On the left, there is a Generating Station with a line going into the “Generator Step Up Transformer”. This is the Generation portion of the diagram.

From there, it goes to the Transmission Customer (138kV or 230kV) and to Transmission Lines (765, 500, 354, 230, and 138kV). This is the transmission portion of the diagram.

From the Transmission Customer and Transmission lines, it goes into a Substation Step Down Transformer for distribution. This is the Distribution portion of the diagram.

From there, it goes to the Subtransmission Customer (26kV and 69kV), the Primary Customer (13kV and 4kV), and the Secondary Customer (120V and 240 V).

Credit: U.S. Department of Energy (Public Domain)
A photograph of a power plant's substation, where voltage is increased to aid transmission.
Figure 6.3 Typical substation at a power plant (steps up voltage for transmission)
A photograph of a substation that reduces voltage after transmission.
Figure 6.4 Typical substation that steps down voltage from transmission

Try This!

The Power Grid is a simulation created by the Cyber Resilient Energy Delivery Consortium for education.

  • Access the Power Grid animation.
  • Examine the "Quick-Start Guide" as a helpful resource.
  • Play with the simulation to understand the relationships.

Grid Energy Storage

A photograph of the Raccoon Mountain reservoir, which just looks like a big lake.
Figure 6.5 - The Raccoon Mountain project is TVA’s (Tennessee Valley Authority) largest hydroelectric facility. Water is pumped to the reservoir on top of the mountain and then used to generate electricity when additional power is needed by the TVA system. The Raccoon Mountain Pumped-Storage Plant is located in southeast Tennessee on a site that overlooks the Tennessee River near Chattanooga. The plant works like a large storage battery. During periods of low demand, water is pumped from Nickajack Reservoir at the base of the mountain to the reservoir built at the top. It takes 28 hours to fill the upper reservoir. When demand is high, water is released via a tunnel drilled through the center of the mountain to drive generators in the mountain’s underground power plant.

Primitive as it may seem, the energy storage technology that is "grid-tied" and having the largest capacity is accessed by simply pumping water up to a higher elevation, and storing it as potential energy. Called pumped storage, or pumped storage hydroelectricity, the energy is recovered when the water from the higher elevation is used to drive turbines for hydroelectric power conversion.

The Energy Storage Association reports, "Pumped storage hydropower can provide energy-balancing, stability, storage capacity, and ancillary grid services such as network frequency control and reserves." While the US has 20 GW of installed capacity, worldwide over 100 GW of capacity exist. The US figure accounts for roughly 2% of the country's generating capacity, while other areas' figures are as high as 10%.

All in all, however, this process uses more electricity than it produces. So, why do it? When a power plant has extra capacity, it generates electricity used to pump water uphill. Then, when the plant is stretched to capacity and electricity is at its highest price, this pumped storage can be used to generate low-cost hydroelectricity.


Reference:

Modified from Vera Cole, Power Grid, EGEE 401. Accessed October 2013.