Besides locating faults, the most useful applications of the substation monitoring system involve real paybacks in the area of predictive maintenance and improved system reliability.
More and more utilities are using online substation monitoring to better understand system performance, solve problems quickly, and continually assess the health of equipment. This month I'll address the use of a substation monitoring system to analyze voltage sag events. As you know, voltage sags are among the most common power quality disturbances. Utilities like United Illuminating, New Haven, Conn., use substation sag performance as a yardstick for prioritizing system maintenance and improvements. Read on to find out some of the other benefits associated with this type of analysis.
Voltage sags are caused by faults. A fault on the transmission system can cause a voltage sag over a wide part of the system, affecting customers on many different distribution systems. Faults on the distribution system not only affect customers on the particular faulted circuit, they result in voltage sags on all the circuits supplied from a common substation bus. Only a small number of customers experience an interruption when a fault occurs on the power system, but many customers experience voltage sags (see Fig. 1). A typical distribution customer will experience 15 to 50 disruptive voltage sags per year, compared to less than five interruptions.
Substation Monitoring System
Fig. 2, on page 74, illustrates the important components of a substation monitoring system. A substation data manager (InfoNode) collects monitoring information from individual data collection devices (DataNodes) at important locations within the system. Monitoring at each substation bus characterizes the voltage.
The current in the step-down transformer provides important information to help determine if a transmission fault caused the voltage sag. The feeder currents determine fault locations on the feeder circuits, proper operation of protective devices, and possible fault causes. The primary location for system monitoring is the distribution substation. This is the point of common coupling for all voltage sags caused by faults on parallel feeder circuits and the transmission system. You can apply many intelligent applications to a substation monitoring system to improve system operation efficiency and evaluate overall system performance. We will discuss a few of these applications here.
Summarizing Sag Performance
Usually, voltage sags are characterized by magnitude and duration (see Fig. 3) using magnitude/duration curves. This practice is convenient for comparing the severity of a voltage sag with the ride-through characteristics of a facility's equipment. Examples of magnitude/duration curves include the CBEMA (Computer and Business Equipment Manufacturers Association) curve, the ITIC (Information Technology Industry Council — CBEMA's new name) curve, and the SEMI curve, which semiconductor manufacturers developed. You can see voltage sag events plotted against the ITIC curve in Fig. 4.
When plotting these events, it is important to consider the concept of aggregation. You don't want to count events on different phases as separate events. You may not even want to count subsequent events that happen within a short period (because of reclosing, for instance) as separate events.
Voltage sag performance indices developed for an Electric Power Research Institute (EPRI) power quality project recommend an aggregation period of one minute, which is a good rule. This means you should count all events within a one-minute period as a single event, based on the worst magnitude and duration within that period.
Once you've characterized and aggregated the sags, you can develop summary charts that describe sag performance, similar to the histogram shown in Fig. 5. Summarizing sag performance provides facility management with the information they need to decide whether or not to invest in power conditioning equipment. You also can chart the performance over a longer period of time to look for trends that may indicate the need for system maintenance or updates (see Fig. 6).
The acronym SARFI in Fig. 6 stands for System Average RMS Variation Frequency Index. It is an index that indicates the number of sag events that occur when the minimum retained voltage is below some threshold.
You can develop sag performance summaries in the substation's InfoNode or by uploading the data to an Enterprise System application, like PQView, which performs the necessary calculations.
As a result of deregulation in the power industry, different companies are often responsible for the transmission system and distribution system. If, for example, an independent system operator (ISO) oversees the transmission system, then the distribution company bears no responsibility for voltage sags caused by transmission-system faults.
The substation monitoring system, however, should identify the cause of the voltage sag (as a transmission or distribution fault), allowing separate tracking for distribution-system performance. All you need to do is evaluate the voltage and current waveforms at the transformer secondary.
You also can use the monitoring system to identify the location of the fault. For example, combine the information about the circuit impedances with analysis of the fault current waveforms to estimate the fault impedance and the distance to the fault.
Posting the fault location information in real time over the Internet or the corporate Intranet offers even greater benefits. Utilities can dispatch crews to the fault location more efficiently, reducing restoration times and improving reliability statistics for the system. Fig. 7, on page 76, illustrates how you can set up the substation monitoring system to automatically calculate the fault location and report it over the Web interface.
Evaluating Protective Devices
The fault currents at the substation and the resulting voltage sags provide excellent information for evaluating the operation of protective devices. Most protective devices operate on a time-current curve. If you provide the substation monitoring system with this information, then you can evaluate protection performance directly and report any problems immediately.
You also must consider the coordination of protective devices on the distribution system. For instance, engineers often coordinate fuses on branch circuits with substation breakers to prevent interruptions to the entire feeder circuit for a fault on a branch circuit. Figs. 8a and 8b, on page 78, illustrate a case with a lack of coordination between the branch fuse and the substation breaker. The fuse on a branch circuit successfully cleared the fault, but the substation breaker already picked up on the event as a fault and interrupted the whole feeder circuit unnecessarily.
You can flag this kind of coordination problem with the monitoring system and have an engineer resolve the problem before it impacts customers again.
Besides locating faults, the most useful applications of the substation monitoring system involve real paybacks in the area of predictive maintenance and improved system reliability. These advanced applications are currently under development.
Waveform signatures and other signals on the substation monitoring system will enable you to identify breaker performance, capacitor bank problems, transformer loading, regulator operation, arrester problems, and cable problems. If you have ideas or examples of other intelligent monitoring system applications, drop me an e-mail (firstname.lastname@example.org) with some information, and we'll consider it for a future column.
Mark McGranaghan directs power quality projects and product development at Electrotek Concepts. He also works on numerous power quality standard committees and helped EPRI to develop their power quality program. McGranaghan has BS and MS degrees in electrical engineering from the University of Toledo and an MBA from the University of Pittsburgh.