Oncor, the regulated energy-delivery business within TXU Corp. (Dallas, Texas, U.S.), is the first company in the United States to rely on high-speed Ethernet to transmit energy-production data from merchant power plants to the independent system operator (ISO). Oncor uses the information for transaction settlement and billing, and Ethernet provides a more cost-effective, reliable and secure solution than intermittent modem links.

Data Critical to Settlements

This important innovation was initiated when the Texas Electric Restructuring Act was signed into law in 1999 to introduce a competitive state energy market on Jan. 1, 2002. The act obligated all transmission utilities to provide power plant settlement meter data to the Electric Reliability Council of Texas (ERCOT) ISO. Under the act, ERCOT operates a Meter Data Acquisition System (MDAS), which collects generation and consumption energy data on a 15-minute-interval basis from all transmission utility companies in the state.

For Oncor, the act presented a significant challenge as more than 100 generating units scattered over one-third of Texas are connected to the Oncor transmission grid. Of these generating units, TXU owns 59 units and the remaining units are merchant/IPP/cogen facilities. Oncor installed and maintained more than 260 new revenue-accurate meters in its data-collection system, and added the necessary communications links to make the data available daily to ERCOT.

Another challenge was that many of the power plants required high-accuracy wide dynamic current metering, especially the independent power producers (IPPs) whose peaking plants consume little energy when dormant but deliver huge amounts of current when producing. The range in measurements — from 5 kW to 500 MW — requires special metering capabilities.

The Oncor transmission and pipeline business faced new market rules requiring it to ensure that the meters would be compatible and deliver the necessary information to both Oncor and ERCOT in a timely manner. Oncor needed a system that would allow it and ERCOT to gather data from the meters independently and quickly. The solution is an innovative combination of advanced metering devices using Ethernet, serial and dial-up communications links.

The yearlong conversion to ERCOT Polled Settlement (EPS) meters was a daunting task for Oncor because of its large number of geographically dispersed generating units. Oncor owns and maintains the EPS meters for all 102 generating units and is responsible for installing, controlling and maintaining the meters, recorders, instrument transformers, wiring, communications and other associated equipment needed to measure energy generation and internal plant consumption. It is also responsible for installing and maintaining backup metering at each EPS meter location for resources, auxiliary netting and bidirectional meter points. The final meter count at all the plants for output metering, load consumption metering and backup metering totals 262 meters.

Serial Communications Key

Oncor decided to use a hierarchical meter structure, linking several meters through RS-485 serial communication links to a central meter that acts as an Ethernet gateway — a definite advantage because RS-485 has a much longer range than Ethernet and is generally less expensive. About 40 gateway meters are connected to the existing Ethernet local area network (LAN) and wide area network (WAN). Through these networks, both Oncor and ERCOT can communicate with all 262 meters in the system individually. For some of meters (less than 20 in the overall system) located outside the range of a direct Ethernet connection, a modem connection to the gateway meter suffices. Generally the meters are located between the power plants and transmission switchyard to maintain the most beneficial separation between generation and transmission.

Oncor chose ION 8400_™ meters, supplied by ABB and manufactured by Power Measurement, as both the serial and gateway metering devices because of their direct Ethernet links, wide dynamic range and support for multiple simultaneous communications channels. At the time of the installation, ION_® meters were the only meters on the market with built-in Ethernet that supported MV-90, a common billing collection package used in the utility environment, including by both Oncor and ERCOT.

The meters are read daily over Oncor's WAN by three independent MV-90 systems, two located in Dallas and one located at ERCOT's facility near Austin, providing billing and status information instantly to Oncor and ERCOT. When either organization's MV-90 billing package initiates the process to collect meter data, the request is channeled through the Ethernet router to a gateway meter, and then passed through the RS-485 connections to the individual meters. The meters have rolling logs — with the oldest information dropping out of the log when new information is added — so the latest relevant data is always available regardless of when the meters are polled. And because of the reliability and speed of the Ethernet connections, the data collection process takes only a fraction of the time that would be required for dial-up connections, as well as being more cost effective, more reliable and more secure.

The ION 8400 meters also provided the wide dynamic current metering required to accurately monitor IPPs connected to Oncor's system. When an IPP facility is generating power, up to 500 MW can be flowing onto the grid. When the facility is not generating power, it can draw about 5 kW from the grid. These locations pose a challenge for the metering system because of the wide voltage range that must be accurately measured. The ION 8400 meters are specially designed to monitor these ranges, offering revenue certification accuracy — another reason the meters were chosen for all metering points within the system.

Other communications channels were used to pass information to the Resource's Energy Management group. The group can access the meters directly using DNP protocol through secondary RS-485 connections, polling the meters frequently (typically every 2 to 4 seconds), using the real-time data for dispatching and predictive planning.

Making Changes on the Fly

In addition to the technical complexities, the project faced significant administrative challenges. The technical specifications of the project changed as the project progressed, with the ERCOT protocols revised nine times and the ERCOT operating guides revised as recently as October 2001. These changes impacted the design phase of the project, making design changes necessary to ensure that all phases of the project were compatible and met ERCOT's standards outlined in the Texas restructuring law. Oncor worked with MSE Power Systems as the consulting engineering firm and prime contractor, Ernest P. Breaux Electrical Inc. as the construction contractor, and ABB and Power Measurement as the meter supplier to ensure that no details were overlooked.

The metering elements of the ERCOT deregulation model are crucial for the settlement process in an open market. A well-calibrated and maintained meter system is essential for the market to function efficiently. Oncor is secure in the knowledge that it has a system in place that can function in a deregulated market and meet its obligations to ERCOT and the public.

Ken Swift is Oncor's transmission metering manager, responsible for all activities relating to EPS metering. Swift, who has been with Oncor for 17 years, received the BSEE degree from the University of Texas at Arlington and is a registered professional engineer.