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Monday, August 25, 2014

What's wrong with the electric grid?

We've done many shows on the potential benefits of reducing our dependence on large, centralized grids and looking at alternatives such as building microgrids.   This article does a great job of giving an overview of the problem inherent in the current structure.

It is fairly long and complicated (and thanks to our co-host and contributor, Seth Handy for sending this story along) so we'll run some of it and you can read the rest at:  http://scitation.aip.org/content/aip/magazine/physicstoday/news/10.1063/PT.5.5020

A massive blackout in August 2003 forced a reexamination of the mixture of physics, engineering, economics, and politics that attempts to keep the power flowing.
The warnings were certainly there. In 1998, former utility executive John Casazza predicted that "blackout risks will be increased" if plans for deregulating electric power went ahead. And the warnings continued to be heard from other energy experts and planners.
So it could not have been a great surprise to the electric-power industry when, on 14 August 2003, a blackout that covered much of the northeastern US dramatically confirmed these warnings. Experts widely agree that such failures of the power-transmission system are a nearly unavoidable product of a collision between the physics of the system and the economic rules that now regulate it. To avoid future incidents, the nation must either physically transform the system to accommodate the new rules, or change the rules to better mesh with the power grid's physical behavior.
Understanding the grid's problems starts with its physical behavior. The vast system of electricity generation, transmission, and distribution that covers the US and Canada is essentially a single machine— by many measures, the world's biggest machine. This single network is physically and administratively subdivided into three "interconnects"—the Eastern, covering the eastern two-thirds of the US and Canada; the Western, encompassing most of the rest of the two countries; and the Electric Reliability Council of Texas (ERCOT), covering most of Texas (see figure 1). Within each interconnect, power flows through AC lines, so all generators are tightly synchronized to the same 60-Hz cycle. The interconnects are joined to each other by DC links, so the coupling is much looser among the interconnects than within them. (The capacity of the transmission lines between the interconnects is also far less than the capacity of the links within them.)
Figure 1. Normal U.S. base electricity transfers and first-contingency incremental transfer capabilities, in MW. Credit: North American Electric Reliability Council
Figure 1. Normal U.S. base electricity transfers and first-contingency incremental transfer capabilities, in MW. Credit: North American Electric Reliability Council
Prior to deregulation, which began in the 1990s, regional and local electric utilities were regulated, vertical monopolies. A single company controlled electricity generation, transmission, and distribution in a given geographical area. Each utility generally maintained sufficient generation capacity to meet its customers' needs, and long-distance energy shipments were usually reserved for emergencies, such as unexpected generation outages. In essence, the long-range connections served as insurance against sudden loss of power. The main exception was the net flows of power out of the large hydropower generators in Quebec and Ontario.
This limited use of long-distance connections aided system reliability, because the physical complexities of power transmission rise rapidly as distance and the complexity of interconnections grow. Power in an electric network does not travel along a set path, as coal does, for example. When utility A agrees to send electricity to utility B, utility A increases the amount of power generated while utility B decreases production or has an increased demand. The power then flows from the "source" (A) to the "sink" (B) along all the paths that can connect them. This means that changes in generation and transmission at any point in the system will change loads on generators and transmission lines at every other point—often in ways not anticipated or easily controlled (see figure 2).
Figure 2. Electric power does not travel (a) just by the shortest route from source to sink, but also by parallel flow paths through other parts of the system. (b) Where the network jogs around large geographical obstacles, such as the Rocky Mountains in the West or the Great Lakes in the East, loop flows around the obstacle are set up that can drive as much as 1 GW of power in a circle, taking up transmission line capacity without delivering power to consumers.
Figure 2. Electric power does not travel (a) just by the shortest route from source to sink, but also by parallel flow paths through other parts of the system. (b) Where the network jogs around large geographical obstacles, such as the Rocky Mountains in the West or the Great Lakes in the East, loop flows around the obstacle are set up that can drive as much as 1 GW of power in a circle, taking up transmission line capacity without delivering power to consumers.
To avoid system failures, the amount of power flowing over each transmission line must remain below the line's capacity. Exceeding capacity generates too much heat in a line, which can cause the line to sag or break or can create power-supply instability such as phase and voltage fluctuations. Capacity limits vary, depending on the length of the line and the transmission voltage (see table 1). Longer lines have less capacity than shorter ones.
TABLE 1. CAPACITY LIMITS FOR ELECTRICAL TRANSMISSION LINES- Data from Transmission Planning for a Restructuring U.S. Electricity Industry, by Eric Hirst and Brendan Kirby, June 2001, prepared for Edison Electric Institute, Washington, DC.
Table 1. Data from Transmission Planning for a Restructuring U.S. Electricity Industry, by Eric Hirst and Brendan Kirby, June 2001, prepared for Edison Electric Institute, Washington, DC...

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