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North American Network Operators Group

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Re: East Coast outage?

  • From: Randy Neals (ORION)
  • Date: Sun Aug 17 10:45:12 2003

> I wasn't aware that there are high voltage DC long-haul lines that then
> are converted to AC for local distribution.

Another use for HVDC is to isolate transmission networks.
Hydro Quebec uses Back-to-Back High Voltage DC conversion equipment at its
interconnection points with other transmission networks such as the New
York, Vermont and Mass. transmision networks.

The HVDC interconnection removes frequency synchronization as a concern at
the interconnect and allows much simpler protection and control
implementations as there are less electrical properties to
consider/monitor/manage at the HVDC interconnect point.

Perhaps the H-Q interconnect design is one of the reasons that H-Q was
unaffected by the blackout.

Conversely, the "Lake Erie Loop" is an example of a richly meshed ring with
multiple paths. when synchronized, flow on the mesh/ring is a function of
voltage. To import power into a grid you lower the voltage slightly, to
export power you raise the voltage slightly.

AC Syncronization across the interconnect can limit power transfer
capability. Out of sync condition causes the interconnect to be reactive
with current peaks leading or lagging voltage peaks. Phase Angle Regulators
(PARs) are transformers with phase shifting capabilities. They are often
used at AC interconnect points to manipulate the synchronization to optimize
power transfer.


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