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Green Blackouts?

Increasing renewable generation threatens reliability.

Fortnightly Magazine - August 2010

more seconds it takes to offset the shock, the bigger the shock grows by speeding up or slowing down all the generators on the grid to the blackout point where loads or transmission lines are cut by automated frequency or transmission relays 1, causing more shocks. Provided that doesn’t happen because rapid offset was provided, a few generators subsequently do provide such slow offset to permanently replace within about ten minutes the power or demand sudden loss of which triggered the shock. This slow recovery enables all the generators and loads on the grid to withdraw their rapid offsets over that time and stand ready rapidly to temporarily offset the next shock.

Understanding Grids’ Decline

How has the power grid’s ability to rapidly respond deteriorated and how can the deterioration be stopped? Can technology both decrease renewables’ instability and also increase their ability to provide stability to the grid? Why are very rapid changes a challenge unique to electric power systems?

An answer to the last question begins by recognizing that an electric power generating system traditionally has been a collection of rotating mass inside turbines with a quality called inertia. Inertia serves to make slower any sudden turbine slowdown due to a sudden torque force or counter-spin due to sudden loss of a power generator on the electric grid and due to the very rapid need to spread less power over the same user load base namely at lower electrical frequency ( i.e., at fewer cycles per second of rotating speed). Also, electric motors that are consuming electricity experience the same torque that slows them down and thereby reduces demand so as to permit the electric power generators to slow down somewhat less.

More important, a device called a governor triggers all the electric generators equipped with one to simultaneously and very rapidly produce their small share of the very swiftly produced counter-torque ( i.e. governor response) needed to rapidly neutralize the torque that’s causing the generators to slow down and needed basically to arrest the decline in turbine rotation speed. This change in speed of the turbines is measured by the change in frequency of alternating current electricity (which is normally 60 cycles per second, also known as 60 Hertz or Hz), which is everywhere the same on the grid. Arresting, and thereby minimizing, the decline in electrical frequency to an equilibrium point where supply and demand temporarily are rebalanced, is a capability that each power generator supplies mainly for itself, but it’s measured as a flow of power to the place where the original loss of a generator occurred (see Figure 2) .

Governors are gradually reset to their original positions as one or more generators take the time—about 10 minutes—required to ramp up replacement of the original amount of power loss, thereby generating new speed-up torque prompting the governors to reverse torque to their pre-shock positions. Frequency thereby returns from the post-shock extreme constrained by the governor response deployed, to its pre-shock position, from where the governors are ready to respond immediately to the next event.