Source: Deepak Divan, William Brumsickle and Joseph Eto, A New Approach to Power Quality and Electricity Reliability Monitoring – Case Study Illustrations of the Capabilities of the I-Grid™ System, Ernest Orlando Lawrence Berkeley National Laboratory, April 2003
For many customers who require highly reliable power, utilities provide dual feeds; i.e., service to a facility comes from two independent transmission lines. Under normal conditions, the plant load is shared by the two transmission lines. When a problem arises on one of the lines, plant loads are switched nearly instantaneously to the other line. In other words, the two lines provide redundant paths to ensure near continuous electric service to a customer.
A large automobile assembly plant (3,200 workers) is supplied from a dedicated substation that is fed from two separate transmission lines. An I-Sense monitor was located on each transmission line. See Figure 1.
A grid event occurred during which the monitor on one line recorded a voltage sag of 4.8 cycles (0.09 seconds) followed immediately by a complete interruption that lasted 9.8 seconds; the other monitor recorded a similar voltage sag followed by return to normal voltage and no subsequent interruptions. These data indicate that a fault occurred on the first transmission line, and a voltage sag was propagated from one line to the other. See Figure 2.
The physical cause of the event was later reported: a windstorm caused a line-line fault at the point of entry of Transmission Line #1 into the substation. The fault was initially fed by both transmission lines, causing a voltage sag on all downstream load buses. Circuit breakers subsequently disconnected the faulted transmission line, leaving all plant loads connected to the remaining Transmission Line #2.
This case study demonstrates the operation of a dual feed when a fault occurs: an automatic transfer is initiated from the faulted feed to the unfaulted feed. This strategy allows automatic restoration of power to the facility, permitting a restart of interrupted processes. The dual transmission-feed infrastructure meant that this company experienced only a four-cycle (0.07second) voltage sag rather than a several-hour interruption of service. Nevertheless, the voltage sag was sufficiently severe to cause some process interruptions. Commercially available voltage sag mitigation equipment would have kept all critical processes running during this event; 68 percent of voltage remained during the sag, and sag correctors can compensate down to 50 percent remaining voltage.
This example shows how highly reliable power can be provided with two utility feeds but also that use of dual feeds does not eliminate the short-duration voltage sags that can also cause process downtime.
Figure 1. Line Diagram for Case #1
Events recorded on Feeder #1: A two-line sag for four cycles, followed by nominal voltage for one cycle, followed by complete interruption for 9.8 seconds, followed by return to normal.
Events recorded on Feeder #2: A two-line sag for four cycles, with no subsequent interruption.
Event waveforms are shown below. Note that the plots of events 263(#1) and 251(#2) are of
exactly the same total duration. Note also that the beginning of event 264 is visible in the plot of event 263.
Figure 2. Case #1 Event Waveforms
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