Building a Topology Estimator for Large Inter-regional Networks

Building a Topology Estimator for Large Inter-regional Networks ETREP02.pdf Robin Podmore *, Margaret Goodrich**, David Becker***, Peter Hirsch***, Stephen Lee ***, Marck Robinson**** robin@incsys.com, margaret@psycor.com, dbecker@epri.com, phirsch@epri.com, slee@epri.com, marck@powerdata.com *Incremental Systems Corporation, 194th Ave. S.E., Issaquah, WA, 98027 **Psycor International, P.O. Box 2106, Malakoff, Texas 75148 ***Electric Power Research Institute, 3412 Hillview Ave., Palo Alto, CA, 94304 ****PowerData Corporation, 15193 S.E. 54 th Place, Bellevue, WA 98006 Abstract The Common Information Model (CIM) [1] defines a utility industry standard object-model for the development and integration of applications used for electric power systems engineering, planning, management, operation and commerce. This paper describes the scope and technical basis for a project to develop a real-time Common Information Model (CIM) for the entire Eastern Interconnection. The project, which also features an automatic real-time network topology estimator, could have farreaching implications for on-line security assessment in North America. The project will be implemented using the ICCP, CIM and Common Data Access (CDA) Application Program Interface (API) standards sponsored by the Electric Power Research Institute. Wide Area Models Needed In the area of power grid operation, the energy industry in North America faces a variety of challenges. Foremost among these is the difficulty of continuing to ensure secure grid operation in an open access environment. Electric load continues to grow at a steady rate, yet North American transmission system additions have slowed to a crawl. These facts, combined with mandated open access to transmission systems, are forcing the industry to operate the power grid closer to its operating limits. Pushing the system to its limits for maximum economic benefit while maintaining adequate security margins requires on-line security assessment. This, in turn, requires real-time models for the entire interconnected system network. Currently, at the interconnection level, this responsibility is assigned to regional Security Coordinators as shown in Figure 1. Each Security Coordinator is responsible for monitoring the overall security of the system for their region. The Security Coordinator Computer Systems are based on Energy Management Systems from a variety of suppliers. These systems include Topology Processor, State Estimator, Contingency Analysis and Dispatcher Powerflow applications. The Security Coordinator Systems are connected using the Interregional Security Network (ISN) Nodes with the ICCP protocol. The nodes all connect to the NERC ISN Frame Relay Network. Control Area Computer Systems are those systems that perform data acquisition, supervisory control, automatic generation control and possibly network analysis functions. The Control Area Computer Systems have the 1 0-7695-0493-0/00 $10.00 (c) 2000 IEEE 1

connections to the field devices via their RTUs. The Control Area Computers are the primary source of the measurements for the ISN. The Control Area Computer Systems link to the Security Coordinators primarily using the ICCP protocol. The operation of the ISN requires that the participants exchange a vast amount of realtime, modeling and scheduling information. This exchange must take place between systems and applications that are supplied by a diverse range of vendors. In order for the systems to be implemented in a timely manner and maintained with a reasonable amount of manpower it is essential that open industry standards such as ICCP and CIM be used wherever possible. Some progress has been made in the area of wide area modeling. Real-time operational models have been built for control areas, and are now being built for Security Coordinators (SCs) and independent system operators (ISOs). An effort currently underway is converting such models to a Common Information Model (CIM) format so that they can be shared between different control areas, SCs, and ISOs. The SC models are being built by merging the models of the individual control areas. Yet, development of a real-time model for the entire Eastern or Western Interconnection has not yet been attempted. The real-time CIM will support FERC s recent NOPR guidelines on regional transmission organizations (RTOs). In short, a real-time CIM can act as the modeling foundation for an RTO. By implementing real-time CIMs at SCs, a new RTO that consolidates some of these SCs is able to more quickly implement real-time power applications. In fact, this approach is likely to save 1-2 years in construction of models that meet RTO needs. The alternative of not using the CIM means that RTO modeling cannot begin until a proprietary EMS vendor format is selected. Better Topology Estimation Required On-line security assessment also requires accurate state estimator (SE) results. An SE assumes that the network topology is known before it estimates the voltages and angles for the entire system. If the actual network topology is not as assumed, the results of the SE are incorrect, adversely impacting on-line security assessment. SE engineers typically make needed corrections to the topology assumptions manually at the control area level. However, when an extra large model is developed (such as an Interconnection-wide model), an automated method of identifying topology errors is essential. This paper describes the scope of a project to build a real-time CIM for the Eastern Interconnection, as well as a Topology Estimator that will automatically determine the accurate network topology in real time to support on-line security applications for the Eastern Interconnection. The objectives of this project include the following: To build a real-time operational model of the Eastern U.S. and Canada interconnected systems. To enable the NERC Interchange Distribution Calculator (IDC) models to be updated using real-time topology information. To provide tools to facilitate integration of the NERC Inter-Regional Security Network (ISN) data with operational models. To demonstrate the feasibility of using the Common Data Access (CDA) Application Program Interface (API) for integrating applications distributed at different SCs. Project Overview The project team would consist of several vendors and their respective software applications would be integrated using the CIM and the CDA API. Real-time CIM software would be installed at designated SCs. The Multiregional Modeling Working Group (MMWG) would be provided with the operational models of the SCs, enabling the group to reconcile differences between the SC energy management system (EMS) models and MMWG planning model. Then, the project would implement a topology estimator interface for the IDC. The project will convert proprietary EMS models into the industry standard CIM, enabling 0-7695-0493-0/00 $10.00 (c) 2000 IEEE 2 2

their accessibility using the CDA API. The project will also allow operators to view oneline displays of all major stations in the Eastern U.S. via a Web browser. When operational problems occur across SC boundaries, these displays will provide operators a common basis for analyzing problems and developing solutions. This project supports NERC s IDC. The CIM and CDA infrastructure will be established to support applications such as IDC that need realtime information and operational models from multiple SCs. The Anatomy of a Real-Time CIM Examining each key element of the real-time CIM shown in Figure 2 from left to right aids understanding of the system. EMS model import filters will enable import of the models for the existing SCs into the CIM and underlying database and messaging infrastructure. This process, like several others in this system, will make use of API adapters which accomplish two objectives: Ensuring that applications can be run with different database engines and messaging middle-ware using the exact same API. Insulating application developers from the complexity of the CIM. An Inter-Control Center Communication Protocol (ICCP) data link will support real-time information exchange between the CIM database and external systems (e.g., EMS) using the ICCP protocol. This data link allows bi-directional exchange of analog, status and accumulator data. These data are stored in the measurement table of the CIM so that they are accessible via the CDA API. If available, results of the SC topology processor and State Estimator will also be transferred over the ICCP interface. One of the key elements in this real-time CIM for an SC is the topology estimator. This software automates the process of identifying network topology errors. For example, it will verify that MW flows on open lines are zero, verifies that the summation of flows into a bus are zero, and verifies that the summation of angle differences around a closed loop are zero. In real-time, the topology estimator determines the accurate network topology to support on-line security applications. The system will also feature a CDA interface to other SCs and the IDC. The Common Data Access services are defined to meet the requirements that are encountered in building mission-critical, high-availability, real-time Energy Management Systems and Distribution Management Systems. The services that are important for an application developer in this environment include the following: Get object attributes Update object attributes Get attribute extent Update attribute extent Create object Delete object Create attribute Delete attribute Query to determine attributes Structure Changed Event (An object has been created or deleted) Value(s) Changed Event Data Source Lock and Unlock Query to determine services supported The Get/Update attribute extent, Structure Changed Event and Value(s) Changed Event have been specifically included to meet the real-time requirements of EMS systems. These services are not commonly supported by commercial database management systems. The CDA API supports the request-reply processing communication style that is typical for client server databases. A client requests information from the component. The client waits until the component replies with a response. This is a synchronous or pull model of communication. The CDA API also supports event driven or asynchronous or push models of communications. The client registers for a call back when a component event occurs. Upon receiving the call back, the client can query the component for additional information. This event driven feature is critical for real-time systems. If this feature is not available the clients have to constantly poll the component, asking if there have been any changes. This provides an excessive load on the component server and the network if the response times are to be kept small. Component developers will be able to implement to a subset of the CDA services and still provide a 0-7695-0493-0/00 $10.00 (c) 2000 IEEE 3 3

valuable facility. An application or user can query the Component to determine what services are supported. At the bottom of Figure 2, three graphical user interface applications will enable users to access one-line diagrams and tabular displays from real-time CIMs using a Web browser. These diagrams and displays will be dynamically updated whenever real-time values change. The one-line viewer offers graphical schematics, updated in real-time. The one-line editor enables operators to arrange these schematics, and the one-line auto builder enables automatic construction of schematics from the CIM database. The applications at an individual SC will be tightly coupled. Each application will need to be coordinated to work with a single consistent database. For example, if a new database modification is being installed, the Topology Estimator should not be updating this with a new solution. Applications will need to read from and write to the CIM. Figure 3 shows how various real-time CIMs at the SCs will be able to communicate via the NERC ISN. The real time CIMs at the different SCs will be loosely integrated. Database configuration changes can be made at each of the SCs independently. Each SC will publish an event when its database is updated and the other SCs will be able to receive the required modifications and then adjust their external models. The IDC, control area, and MMWG users will also be plugged into the network. The IDC estimates the flow impact of each interchange transaction on the flow gates of the Eastern Interconnection by computing power transfer distribution factors (PTDF) and outage transfer distribution factors (OTDF). The latter models the effect of a limited number of critical contingencies for certain flow gates. Upon the request from a SC to relieve congestion on a flow gate, the IDC computes a list of transactions that can be curtailed in a priority order. The IDC depends upon accurate information on the status of the lines and the bus splits in the entire interconnection. Currently, this information is input to the IDC manually. The real-time CIMs and the Topology Estimator will allow this information to be updated automatically in real-time. The IDC will subscribe to the results published by each of the Topology Estimators at the SCs. The IDC will selectively register to receive information concerning the high voltage network. Summary This paper has described a practical approach for implementing a Topology Estimator for large interconnected networks such as the Eastern U.S. and Canada. The approach is based upon a distributed network in which calculations are performed at each of the SCs. Each SC has responsibility for check out and maintenance of the topological model for its own area. With the distributed approach it is not necessary to build a single interconnection wide database that has details of all the switches and breakers in the entire interconnection. An application such as the IDC that requires information on the entire interconnection subscribes to receive the line energization status and bus split information from each SC. This information supports building an accurate realtime bus branch model for the entire interconnection. By using industry standards, such as ICCP and CIM and off the shelf software components, multiple vendors can participate in delivery of the various subsystems. The system will establish an architecture and the industry standard interfaces so that other applications that require data to be shared between SCs can be integrated. The approach takes advantage of the existing MMWG model. In future with data warehousing in place at the SCs it will be possible to automatically generate an MMWG model for the entire interconnection based on actual recorded data. 0-7695-0493-0/00 $10.00 (c) 2000 IEEE 4 4

MARITIMES HQ BPA MAPP OH NYISO ISONE MECS WAPA MAIN AEP ECAR APS VP PJM CAISO ENTERGY SPP TVA DUKE SCS ERCOT SECURITY COORDINATORS ISN NODES FP&L Figure 1: NERC Security Coordinators and Inter-regional Security Network (ISN) Nodes 0-7695-0493-0/00 $10.00 (c) 2000 IEEE 6 6

EMS Data Base EMS Import Filter ICCP Data Link Topology Estimator Remote CDA Interface to other SCs and IDC CDA API EPRI Common Information Model Database and Messaging Infrastructure CDA API One-line Viewer One-line Editor One-line Auto Builder Figure 2: Real-time Common Information Model for Security Coordinators SC#! SC#2 SC#3 SC#4 SC#5 NERC ISN IDC Control Areas SC, CA and MMWG users Figure 3: Integration of Real Time CIMs with the NERC ISN 0-7695-0493-0/00 $10.00 (c) 2000 IEEE 7 7