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
Background
Power quality events may be caused by the utility supplying the customer or by equipment operations within a customer’s or a neighboring customer’s plant. Comprehensive monitoring is required to pinpoint the source.
Description
In an industrial neighborhood in a small city in the Midwest, a crow flew into medium- voltage switchgear at a utility substation. The event caused a fault from the utility line to ground. Voltage sags and momentary loss of utility voltage resulted on the grid for several miles around the substation and were felt by more than 200 customers.
Four I-Sense monitors distributed throughout the neighborhood recorded the effects of this power quality event, as shown in Figure 1. Accurate time stamps permitted post-processing to cluster the recorded data and present them as one physical event (Figure 2). By clustering the data in this fashion, we hypothesized that the event was propagated on the distribution grid – that is, that this was a “grid” event rather than a set of uncorrelated events, each initiated from within a distinct customer’s premises.
The hypothesis that the recorded data all represented a single, utility-caused event was confirmed when utility company records revealed a relay operation on a parallel feeder with the same time stamp as the power quality events recorded by the sensors. Analysis of the waveforms clearly indicates a single-line-to-ground fault, which is the most common type of utility system fault. (Note that monitor #1 recorded line-line voltage, and the other monitors recorded line-neutral voltage.)
A customer at one monitored location experienced a 13-hour process shutdown as a result of this event.
Discussion
The ability to discriminate between grid and internal events is vital. An event originating from within a customer’s premises is the responsibility of the individual customer. An event originating from the utility, which affects multiple customers, is the utility’s responsibility.10 This case study validates the assumption that grid events are experienced by all utility customers in a geographical region and that every single customer does not need to be monitored to assess the power quality for a region. Grid-wide power quality and reliability monitoring would require deployment of sensors for only a small percentage of customer facilities.11
The exact nature of this responsibility is dictated by the conditions of service offered by the utility as determined through regulatory (or other) oversight of the utility’s operations.11 Knowledge of the topology of the distribution grid could further enhance the effectiveness of information from a network of monitors.
Figure 1. Single-line Drawing for Case #2
Waveforms from this event show that commercially available voltage sag mitigation equipment would have protected customer equipment from this event in all four monitored locations.
Figure 2. CASE #2 Event Detail Pages
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