By Kris Koellner, Salt River Project, and Ginni Stieva, Power Measurement
Salt River Project (SRP) is a nearly $2 billion water and electric utility serving more than 800,000 customers in Phoenix, Ariz. As competition in its area increases, SRP is always on the lookout for ways to offer customers better service and better power quality (PQ). While the company has had a PQ program in place for years, it is continually striving to excel in this area.
Recently, SRP decided to expand upon the common utility practice of documenting outage frequency and duration to also include a system to measure voltage sags. Utilities generally focus their efforts heavily on system reliability, but pay less attention to power quality issues like voltage sags because these events don’t cause widespread outages and only affect customers who are sensitive to voltage fluctuations.
Industry-standard reliability indices don’t accurately capture the impact of power quality events because these indices generally don’t apply to outages that last less than one minute (or, in some cases, even five minutes). Because a typical voltage sag lasts only a fraction of a second and usually doesn’t involve a complete loss of voltage, traditional reliability indices don’t accommodate these events.
Although voltage sags don’t usually have as severe an impact on an end-use customer as momentary or extended outages, they can still cause customer equipment to drop off-line, diminish product quality, and decrease productivity. And although their impact on an event-by-event basis may not be as severe as an extended outage, voltage sags tend to occur much more frequently than outages, so the total impact of multiple voltage sags over a given timeframe may actually be significantly greater than an outage. In fact, for some customers, power quality is a much more serious issue than reliability.
When their reliability indices couldn’t explain certain customer satisfaction findings, SRP realized that power quality and power reliability are unique and must be dealt with differently. They wanted to implement a system that would better meet the needs of their PQ-sensitive customers. They recognized that without accurate data to track the impact of voltage sag events, system improvements couldn’t be justified, customer damage claims couldn’t be substantiated, and the trending of power quality levels over time couldn’t be accomplished. To solve this issue, SRP created a new program to measure voltage sag indices at key sites, with the goal of obtaining system-wide PQ monitoring coverage and producing a set of system-wide voltage sag indices that could be used to effectively measure their PQ performance.
To produce a set of system-wide voltage sag indices, a power quality monitoring base was needed. SRP had to collect enough statistical data on events within their system to understand what sag events were happening within its system, as well as their location, frequency, duration and severity. Four key indices were created to measure and quantify sags:
The Sag Energy Index (SEI) provides a measure of the total severity of a collection of sag events, where the collection could be based on a period of time or a particular location. This index takes into account voltage sag depth and duration on all phases.
The Sag Count Index (SCI) measures how often sags occur within a given time period.
The Sag Severity Index (SSI) measures the average severity of a collection of sag events.
Kevin Kittredge, associate power quality engineer at Salt River Project, uses ION Enterprise to check on PQ system conditions.
The Severe Sag Count Index (SSCI) is similar to the Sag Count Index (SCI), but only measures sags with a Sag Energy (SE) greater than a specific, user-defined threshold.
The data is also filtered and normalized to prevent unintentional miscalculation or bias in the results and to allow for easy comparison from site to site and year to year.
ION 7700 meter installed at a Salt River Project facility.
Power quality monitors serve as a data collection system that “watches” for voltage sag events that may impact customers. Determining the location of PQ monitors for the establishment of voltage sag indices was critical, balancing the cost of additional monitors versus the amount of coverage that a given monitor location can provide. On SRP’s electric system, the 12-kV distribution bus (which normally connects to four 12-kV feeders) is the optimal location for PQ monitoring, offering a more affordable solution than monitoring each feeder independently, but providing adequate detail to measure and assess voltage sag events. A single PQ monitor located at the 12-kV distribution bus captures both sags due to higher voltage class faults, as well as sags due to downstream feeder faults.
SRP already had a number of PQ monitors located in its substations because of ongoing power quality program requirements. However, to create a system capable of comprehensive monitoring, SRP installed additional devices at substations on sub-transmission loops throughout its electric system. In total, the system now has 66 ION 7700 meters from Power Measurement functioning as power quality monitors at 37 substations, including some in substations dedicated to large industrial customers.
Currently about two-thirds of the meters are connected to a central workstation via Ethernet. Some original sites are connected using modems over dedicated leased lines, but SRP is actively working to upgrade all sites to Ethernet by installing its own fiber-optic backbone to all substations.
The power quality information collected by the meters is gathered and aggregated using ION Enterprise 4.5 software from Power Measurement. The software resides on a pair of Compaq servers located in SRP’s Power Quality Lab. The information is shared with other departments (including operations, system planning and system protection) using both the “Vista” client application and the “WebReach” Web access feature in ION Enterprise. The ION system also provides automatic e-mail/pager notification to key SRP personnel as well as a summary “PQ Alert” that is sent out if a certain number of PQ monitors report events within a given timeframe.
SRP plans to expand its PQ monitoring system, prioritizing new sites by a number of criteria, including area coverage, the number of large and sensitive customers fed from that substation, ease of installation, new substation transformer bay additions, and communications availability. As the system expands, SRP’s power quality indices will become increasingly accurate, as more data will improve its statistical validity (with an ultimate goal of 90 percent confidence level, 10 percent confidence interval).
Salt River Project has installed power quality monitors in several substations throughout its electric system.
Already, other indicators of the system’s benefits are clear. The new system’s data correlates closely with SRP large industrial customer satisfaction measurements and helps to explain PQ-sensitive customer feedback. SRP is using this data to identify and repair conditions within its network that have a negative impact on power quality, improving its service levels to key customers and raising its customer satisfaction levels. And, even though many customers may not see a clear distinction between power quality and reliability, with SRP’s innovative approach to monitoring and managing both, customers are receiving some of the best power quality available on the market today.
Kris Koellner is a senior power quality engineer with SRP. His chief responsibilities are in the areas of PQ monitoring system administration and PQ index reporting. Kris has worked at SRP since 1994. Prior to joining the power quality services department in 1997, Kris worked in distribution planning. While in system planning, his work focused on tracking SRP electric system disturbances across all T&D voltage levels. Kris received a B.S.E. degree in Electrical Engineering from Arizona State University and is registered as an Engineer In Training (EIT) in the state of Arizona.
Ginni Stieva is a professional writer for Power Measurement, specializing in energy management strategies and communications technology.