A ComEd crew in the process of connecting the taps as part of placing into service one of the S&C IntelliRupters ComEd is installing on the distribution system.
Beyond Loop Schemes
Despite this success, ComEd has found loop schemes have significant limitations when it comes to being able to break large-customer-count feeders into smaller sections. ComEd has found that maintaining proper time-overcurrent coordination between traditional reclosers in a loop scheme containing more than three reclosers is extremely difficult to consistently achieve. Also, the utility has found that repeatedly closing into faults can cause damage to distribution equipment (I2t damage).
Additionally, in some instances, customers fed by different feeders served from the common substation bus have complained about voltage depression when LS reclosers have closed into faults while working in a fault-hunting fashion to isolate the faulted section of circuit. When a recloser is added to an existing loop scheme in an attempt to further segment the feeder into smaller segments, the new recloser must coordinate with the existing recloser and substation circuit breaker, as well as downstream fuses. This is difficult to achieve because of the small available space between the existing LS time-current curves in which to fit another traditional recloser time-current curve while maintaining proper coordination.
Over the last two years, ComEd has deployed S&C’s IntelliRupter PulseClosers in several loop-scheme configurations to test its compatibility with the ComEd distribution system. One of the features of the IntelliRupter is the device’s tighter tolerances for coordination as compared with traditional reclosers. Basically, the accuracy of the integrated mechanism, control and sensing provide tighter coordination tolerances. The tighter coordination tolerances allow more curves to be stacked together, which should allow more devices to be installed in series and still coordinate properly.
ComEd approached S&C about trying to use these tight tolerances to help overcome the coordination issues limiting further segmentation of existing loop schemes. Time-overcurrent coordination studies proved this should be possible in most instances. The much skinnier curves provided by the IntelliRupter can be shoehorned between existing recloser curves to fit into the existing ComEd loop schemes.
Another advantage of this advanced technology that became apparent as ComEd and S&C worked together is how the pulse-closing feature might be brought into play as a backup should perfect coordination not be achieved at certain fault magnitudes.
A pulse close is a lower-energy test of the line. Basically, if the condition arose where both the IntelliRupter and the traditional recloser would trip because of a fault, the recloser would still be able to reclose and hold because the pulse-finding feature of the IntelliRupter would force it open when the upstream recloser opened. After the recloser reclosed and held, the IntelliRupter would use pulse closing to test for a fault instead of testing for a fault like a traditional recloser.
Pulse closing is a fast close-open operation at a precise point on the sine wave. This produces a pulse of current though the line, which the IntelliRupter can then analyze to determine whether the line is still faulted. This small pulse of current is not enough to cause the upstream recloser to trip, thus proper segmentation is achieved even in instances where initial coordination cannot be achieved.
Another advanced feature of the IntelliRupter critical to the success of the project was its ability to support bidirectional profiles at the same time. Basically, the device needed to have the ability to look in the forward direction down the line with time-coordination curves that are entirely different than what it would use when looking in the backward direction. This is required so coordination is maintained even when the scheme is reconfiguring during an event. ComEd especially appreciated that the device has this capability without requiring any sort of control reboot to change profiles.
In addition, ComEd is seeking to quantify whether the pulse closing can assist with reducing equipment damage to its distribution system resulting from additional fault current flowing when the traditional DA devices reclose to test for faults. Since the pulse-closing interval is for such a short time, very little energy is passed through the distribution system while testing to find the fault section. A pulse close contains less than 2% of the energy typically let through during a traditional reclosing. As a result, ComEd hopes to see a reduction in related equipment damage and customer complaint issues.