Seattle City Light's (SCL; Seattle, Washington, U.S.) underground secondary network-distribution system is one of the larger ones in the United States, serving approximately 16,500 commercial, governmental and multi-family residential customers. The network-distribution system has a load density of more than 230 MW per square mile and encompasses three areas: downtown, First Hill and University District.
With present load growth, SCL Network Engineering believes that a new downtown substation will need to be on-line in less than 10 years. In the interim, SCL engineers are trying to maximize existing substation capacity by increasing the capacity of the network-distribution system. One method is using real-time voltage, current and power-factor data to model and calculate actual system capacity instead of estimating the capacity using Power Flow simulations. Most of the real-time values for distribution feeders are recorded at the substations. However, real-time data for a portion of a feeder, such as a lateral feeder section or for an underground portion of an overhead feeder, must be obtained from equipment located at the feeder site.
The First Hill network, with an estimated peak load of 47 MVA, consists of five feeders tapped from the 26-kV Wye overhead system to form an underground network-distribution system. For years, there was not any way of knowing the exact incoming voltages, currents and power factors for the network portions of those feeders unless SCL personnel physically went to the feeder sites to measure and record the values. Although there were existing current transformers (CTs) on the feeders inside the underground manholes, they were not used for metering power flows. With feeder loading increasing, the acquisition of actual load data from First Hill became a priority. Engineering decided that the First Hill network system, located east of the downtown network system, was going to be the test bed for real-time remote monitoring/recording of a portion of a feeder. The results may set a standard for future metering of feeder sections.
Installation of a Telemetering System
The project took almost two years to evolve and finalize. One stumbling block during the design process was determining the location of voltage transformers. The initial plan was to place two voltage transformers in a cabinet in an underground manhole. Due to size constraints of the manhole and working clearances, the voltage transformers were redesigned to be mounted on a power pole.
The final product is the first telemetering system of an underground cable for SCL's network. Not only does it involve three different organizational units (Technical Metering, Overhead Systems, Network Engineering and Services) within City Light, but it also uses cellular technology. The metering system's components include: a MAXsys 2510 type M5S9 meter with modem (2400 baud rate) and load profile options made by Siemens; a stand-alone weather hardened commercial cell phone from Global Data model CS-832; two ABB type VOG-12 voltage transformers; and the three Astra CTs already existing inside the underground manhole.
Results and Operational Experience
Once the installation is complete and the metering system goes on-line, the meter will collect and store data. At the primary 26-kV level, the meter provides, but is not limited to, readings such as voltages from two of the three phases, currents from each of the phases, kilowatt-hours delivered, kilovar-hours delivered and power factor. Currently, data capture occurs every 5 minutes, but it can be set at lower or higher time intervals as needed.
On a daily basis, MV90, a software made by Energy Information System (EIS), a wholly owned subsidiary of Itron (Spokane, Washington), calls the meter and downloads the history into an internal load profile Betrieve database. This data is then automatically exported to a central Oracle database for access by any authorized users. The raw data from the meter requires conversion factors based on the CT ratio, voltage transformer ratio and intervals per hour at which data is recorded to give the actual value.
When administrators extract data from the Betrieve database, an internal software automatically converts the raw data to actual values. However, this is not true for end users extracting data from the Oracle database. A macro or formula is needed to output the data to the actual values. By using MS Excel, graphs can be generated from the converted data showing trends and analysis as required.
In addition to the MV90, Maxtrac32 (by Siemens) is another software used to obtain readings from the meter. The difference between the MV90 and Maxtrac32 software is that Maxtrac32 is capable of outputting instantaneous values in either digital or analog format, and MV90 provides long-term historical data.
In order to get instantaneous values, Maxtrac32 first downloads the load profiles history since the last download. Then actual load values are updated and refreshed about every 30 seconds. Therefore, it's recommended that Maxtrac32 should call the meter on a daily basis. Otherwise, the fewer calls the meter receives, the longer the download time is on the load-profile history. The download time may become an issue when a situation arises where the actual values are needed immediately, instead of the minutes required to download the history profile. With only 31 days of buffer within the meter, the meter must be called before the 32nd day. Otherwise, data in the buffer will be lost with new data overwriting the old.
The history load profiles that Maxtrac32 downloads each time it calls the meter are stored into a database. The limits of this database are that it can only be read from Maxtrac32. Maxtrac32 is capable of producing demand reports in table or graphical format along with an export option.
Unfortunately, there is an error in trying to produce a graphical format of all three phases of the current of feeder 26C125. Siemens has been notified of the problem along with a couple of others and is currently working on a fix.
Thus far, three of the five telemetering systems have been installed. One installation presented a challenge since the power pole (where the voltage transformers are mounted) is made of steel rather than wood. Instead of welding the mounting bracket for the voltage transformer, an extra arm attachment was fabricated to make the installation cleaner and safer.
Conclusion and Recommendations
So far, the telemetering system has performed as expected. However, like normal cellular services, during peak hours of cellular service, there are times when the dial up to the meter gets disconnected. If the download isn't completed before the disconnection, the entire download will start over again from the last completed download. Thus, continuous disconnection not only can be time consuming but also frustrating if data is needed quickly. The download rate is set at 1200 baud rather than the 2400 baud of the meter because it's more reliable when using cellular service. Therefore, until cellular service becomes more reliable and a faster modem is available in the meter, the download will continue to be a bit slow and time consuming. With the new version of MaxTrac, however, the waiting period is to be reduced substantially. (As of press time, SCL was in the process of installing the new version.)
With the collected data, analyses are made to determine whether measures should be taken to correct power factors and low voltages. Currently, the telemetering system provides not only actual data but also past history showing the load profile of the network cables tapped from the overhead system. By using the recorded load profile data, CSCL engineers prepare the network power grid for future and current loads by reconductoring cables, sizing up transformers, and other necessary measures to provide continuous and reliable power to the customers.
Installing a telemetering system on primary feeders, similar to what SCL has done, is highly recommended especially if a utility does not have full-scale SCADA system for all of its feeders. If a utility does not have engineering tools such as Power Flow or State Estimate, then a real-time telemetering system is even more useful. During emergencies and power outages, dispatchers can use monitoring tools such as MaxTrac and receive the instantaneous status of a feeder (or a main section of a feeder).
This project would have not been completed without help and significant work from SCL Metering division along with the Network Services construction and electrical crews. Authors would like to recognize SCL employees Ron Bailey, Ted Allestad, Bob Clore, Desmond Chan and Patrick Gallagher. We would also like to thank SCL Dr. Betty Tobin and Siemens for their help.
Kyle Ho received the BSEE degree in 1996 from University of Washington. His employment experience includes Boeing and in the Seattle City Light Network Engineering division since 1999, most recently as an assistant electrical engineer.
Hamed Zadehgol received the BSEE and MSEE degrees in 1990 and 1992, both from University of Washington. His employment experience includes a research assistantship with University of Washington and various positions in the Seattle City Light Network Engineering division since 1990, most recently as an engineering supervisor.