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First EMS Experience Building, Validating and Maintaining a Network Model Using CIM E. Margalejo, P.-A. Löf, X. Liu – NSTAR, Boston, Massachusetts, United States, P. Picard – SNC-Lavalin T&D, Montreal, Quebec, Canada
Abstract — NSTAR is an investor owned utility transmitting and delivering electricity and gas to customers in Eastern and Central Massachusetts. In the process of becoming a Local Control Center, which allows NSTAR to work more closely with the Independent System Operator – New England (ISO-NE), NSTAR upgraded its SCADA system and added an integrated Energy Management System (EMS) in 2007. In anticipation of future extensions, to save time and effort, and to adhere to best practices, NSTAR constructed its EMS transmission network model by expanding the ISO-NE EMS operational model. This process utilized the IEC 61970 series of international standards for EMS control center application program interfaces. The objective of this paper is to share practical experience of building, validating and maintaining an EMS network model using the IEC Common Information Model (CIM), to highlight integration challenges, to discuss CIM Model issues and to identify potential future model management improvements. Index Terms — IEC 61970, CIM, SCADA, EMS, Utility, Model Driven Integration, Power System Modeling, Data Management
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I. INTRODUCTION
his paper describes NSTAR’s strategy, methodology, and lessons learned during development and maintenance of its first Energy Management System (EMS) transmission network model utilizing IEC 61970 Common Information Model (CIM) model data exchange standards. The paper first summarizes the genesis of the NSTAR EMS model and its evolution to the current operational model, and then presents the CIM-based process used by NSTAR to create and maintain its EMS database. Finally, the paper highlights limitations encountered with the CIM Version 10, release 7, and introduces possible EMS data management process improvements based on its available CIM-compliant EMS network model tools. NSTAR is the largest Massachusetts-based, investor-owned electric and gas utility. The company transmits and delivers electricity and gas to 1.4 million electric customers and nearly 300,000 gas customers in Eastern and Central Massachusetts, including the metro Boston region. The Independent System Operator in New England (ISONE) is the not-for-profit independent corporation appointed E. Margalejo, P.-A. Löf and X. Liu are with NSTAR, One NSTAR Way, Westwood, MA 02090, USA (e-mail:
[email protected]). P. Picard is with SNC-Lavalin Energy Control Systems Inc., a division of SNC-Lavalin T&D, 2425 Pitfield Blvd., Montreal, Quebec H4S 1W8, Canada
by the Federal Energy Regulatory Commission (FERC) as the Regional Transmission Organization (RTO) for New England. The ISO-NE region has a total installed supply capacity of more than 32,000 MW and set an all time peak-demand record of 28,130 MW in August 2006. NSTAR became a Local Control Center (LCC) operating under ISO-NE in December of 2007. In the process of becoming an LCC, NSTAR deployed an updated Supervisory Control and Data Acquisition (SCADA) system as well as configured and installed a new Energy Management System. The SCADA system, which includes customized functionality, handles both distribution and transmission system assets. The EMS package includes State Estimation (SE), Dispatcher Power Flow (DPF) and Contingency Analysis (CA) packages. Strategic decisions made early on were proven to be key enablers for the overall LCC and EMS deployment success, namely: (1) importance of the (ISO-NE) external model for the NSTAR EMS transmission network model; (2) key role of the CIM for model data exchange; (3) need for a CIMcompliant tool to manage network model development; (4) adequate selection of measurements in the external network model and (5) ensuring vendor participation in Electric Power Research Institute (EPRI) CIM Interoperability tests. ISO-NE and NSTAR agreed to exchange EMS transmission network model data. For NSTAR, access to the RTO’s EMS data was a key element for achieving a shorter EMS network model development cycle, to minimize equivalents, and to start from an operationally proven network model. For the RTO, it was a foray into exchanging model data with a Transmission Operator based on CIM standards. Initial reservations regarding the impact of maintaining a very large external model have been counter-balanced by increased security operators’ situational awareness and ease of model maintenance. A drawback of the chosen strategy is the need for increased operator and support staff awareness and notification of outage/topology changes in the external model. II. MODEL BUILD AND MAINTENANCE PROCESS A. Standards for Application Integration EPRI began looking at the problem of integrating electric utility system operation and planning over 10 years ago [1]. The integration scope was for system operations and planning
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within one entity or in between multiple entities. Two key technologies were identified to meet these objectives: the Common Information Model (CIM) and Control Center Application Program Interface (CCAPI), as well as the InterControl Center Protocol (ICCP). The purpose of the Common Information Model is to serve as an interface between multiple applications and proprietary information models. The CIM is a conceptual model maintained in Unified Modeling Language (UML). CIM data exchange is implemented using the eXtensible Markup Language (XML) and the World Wide Web Consortium (W3C) Resource Description Framework (RDF). NERC endorsed the CIM and defined Common Power System Model requirements (CPSM) for EMS applications. The International Electrotechnical Commission (IEC) has adopted the CIM for EMS as standard IEC 61970 [2], CPSM profile for EMS as IEC 61970 part 452, and CIM for Distribution as IEC 61968. In collaboration with EPRI, and leveraging contributions from many utility projects, the CIM continues to be extended by Working Groups 13 and 14 of the IEC Technical Committee 57. B. NSTAR Transmission Network Model A cornerstone of the NSTAR EMS model development and maintenance strategy is the Network Model Management System (NMMS). This is where the initial assembly and maintenance of the NSTAR EMS network model is performed. The NMMS facility is an integrated set of modules for the representation and analysis of electrical power systems, sharing a common CIM-compliant Data Repository. The tool converts incoming ISO-NE EMS network model data (in CIM XML or other native EMS file formats), performs validations, and stores data in a CIMcompliant relational database. The model data can then be edited, merged with various other models, and exported in CIM format for processing by a CIM import process into the NSTAR EMS. The NMMS facility consists of a graphical user interface, data editing and manipulation tools, and a variety of data conversion utilities within open database architecture. The NSTAR EMS transmission network model consists of two major areas: one internal region covering the NSTAR service territory (or NSTAR Internal Network); and one external region covering the remainder of ISO-NE, including equivalents for New York, New Brunswick and Quebec (the NSTAR External Network). Figure 1 shows New England with its utility companies and tie lines to neighboring areas; where NSTAR’s service area is marked with a rectangle. The complete EMS model at NSTAR consists of over 2,100 buses, 800 substations, 1,500 lines, 800 transformers and close to 450 generators in four interchange areas, namely: New England, New York, New Brunswick and Quebec. There are more than 14,000 analog and digital points fed to this model through utility-owned Remote Terminal Units (RTU) or via the Inter-Control Center Communication Protocol (ICCP).
The NSTAR external model is fundamentally the same model as the ISO-NE EMS operational model, and is imported from ISO-NE into the Network Model Management System. Although the preferred import method is via CIM/XML, the current process relies on raw data from ISONE’s EMS. Within NMMS, modifications to the New York and New Brunswick equivalents are then performed and maintained by NSTAR. A more detailed replacement of the NSTAR portion of the ISO-NE model was created and is maintained in NMMS. This Internal Model is modified and expanded to include models for NSTAR–specific 115 kV substation power equipment and configurations, including supply points to the NSTAR distribution system. Coupled with model change tracking and historical model rollback, NMMS provides a complete CIM-compliant repository of co-existing ISO-NE External, NSTAR Internal and merged EMS model releases.
Fig. 1. NSTAR – ISO NE Transmission Context [1]
C. EMS Database Build and Data Maintenance at NSTAR At this time, NSTAR keeps its EMS External model current by keeping pace with the quarterly ISO-NE EMS’s releases, which are then merged with the Internal model into one overall CIM XML model suitable for processing within the NSTAR EMS. This process was first proven via a pilot CIM model data exchange and validation in December 2004. The participants included ISO-NE, NSTAR, and the NSTAR SCADA/EMS and NMMS vendors. The NMMS was deployed early in 2005. EMS data maintenance consists of keeping the integrated EMS/SCADA relational databases in sync with the NMMS through CIM/XML data exchange, while maintaining other data locally. Data conversion tools such as the NSTAR CIM Converter are utilized to facilitate the preparation of EMS data. The NSTAR CIM Converter is a command line utility that converts CIM/XML files exported from the NMMS into a set of source files that can be processed by the EMS relational database generator. It is equipped with a convenient
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conversion rule package to address CIM Model limitations. Various data elements are required for building the operational NSTAR EMS database. These data components include: EMS Displays, Network Model in CIM XML, EMS Network Hierarchy, Default EMS Network State (or power system resource status information), EMS Load Profiles, EMS Network Measurement Definitions, SCADA Point Definitions and Values, Limits, Contingency Definitions, EMS Application Parameters, and Device Parameters. Figure 2 depicts the process of assembling and generating the above information into the EMS database. This process includes: • Importing, merging and maintaining the various models into the NSTAR overall Network Model in NMMS; • Exporting the NSTAR Network Model in CIM/XML format for generating the EMS database; and • Performing data processing and entering other required EMS data. Building and maintaining of the EMS displays is discussed separately (Subsection D below).
the need for entire EMS database uploads. Consequently, each time the entire NSTAR NMMS network model is exported and converted to EMS format (e.g., when ISO-NE installs a new release of its network model), a complete regeneration of the EMS database is required. Installing the new database onto the online system does not affect the security functions. In the meantime, NSTAR is pursuing a more efficient and granular CIM-based network model update process (refer to Section IV, subsection B). D. EMS Displays Graphic Data Exchange NSTAR opted to generate EMS-specific operator displays in NMMS, instead of modifying existing SCADA displays. This solution required a graphic data exchange from NMMS to the EMS. This NSTAR choice was based on expected cost savings for display generation and maintenance, and improved change management derived from creating and maintaining EMS displays on the network model tool of choice. The data transfer process was executed outside the realm of CIM, but was simplified by the NSTAR vendor’s adherence to the CIM. NSTAR standardized its EMS displays on ISO-NE practices to facilitate security operator and outage coordinator daily communications. It is worth mentioning that the CIM-based Graphic Exchange (SVG) is not yet widely available and its functionality has never been tested in EPRI CIM interoperability tests. III. NETWORK MODEL DEVELOPMENT LESSONS LEARNED
Fig. 2. EMS Database Build and Maintenance Process
From a change management perspective, NSTAR EMS data changes can be grouped into two major categories: • ISO-NE External Network model releases and NSTAR Internal Network changes; and • SCADA and ICCP point definition changes, EMS tuning changes, power system device changes (e.g., limits), and contingency case changes. The first group makes up the bulk of the NSTAR EMS data changes. These are handled via the NMMS import/export process and the NSTAR CIM Converter. The second group results from operational considerations, and the related data changes are performed via the EMS editor and/or batch text file import utilities. The preferred NSTAR EMS data maintenance method for the first group was to pursue an incremental model update solution based on CIM XML partial/incremental data changes. However, lack of a definitive vendor and industry agreement on CIM XML incremental updates has resulted in
A. NMMS-EMS CIM Model Exchange Validation Validation of the NSTAR CIM XML model data exchanges was executed in two phases: first as an initial proof-of-concept Pilot Project, and then throughout the EMS Deployment Project with development of the EMS CIM Converter. The Pilot Project was used to confirm the feasibility of the NMMS-to-EMS CIM model data export and conversion. Using a collaborative approach, the scope included the verification of CIM Interoperability [4] between NMMS and EMS, as well as the feasibility of running the state estimator using the IEC 61970 - part 452 specification [5]. The syntax of the exchanged CIM/XML models was validated using a third party tool. It confirmed that instance CIM/XML network models matched the CIM10r7 RDF schema. The exchanged models were further validated using EMS database validation tools, which allowed for identification of connectivity errors, model consistency errors, and integrity errors. Execution of Power Flow (PF) applications revealed other model errors with regard to branch impedances, load model parameters and voltage control targets. The pilot project allowed for early identification of many data exchange issues and CIM model limitations. It was a key step towards a successful integration.
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B. Independent EMS Model Validation A separate track was used to validate the EMS model and the EMS solution results, both for the power flow and state estimator solutions. It relies on three data sources: (1) CIM XML model exports from NMMS; (2) the EMS database; and (3) power flow files from the RTO EMS loaded into a third party PF application. In the validation process, the above data sources are parsed and loaded into a typical personal computer database management application. Two complementary validation activities are used: one consisting of model data crosschecks, and another utilizing PF result comparisons. NMMS and EMS data are crosschecked to identify possible errors in the CIM export and import process. Further, the RTO EMS PF data is loaded into a third party PF application to render device names and real time state estimator solutions for generator outputs, loads and switch statuses, which are compared against the NSTAR EMS. The comparison is important to validate real time network topologies (especially nontelemetered switch positions), and to tune the NSTAR EMS online model, for example, for signs of flow measurements, for the effect of NSTAR changes in network equivalences, and for temporary limit overrides. C. EMS Tuning and State Estimator Data Quality Improvements In the initial setup of the online model, EMS control parameters (e.g., thresholds for measurement rejection) and generator Mvar enforcement had to be relaxed. Substations were assigned in or out of the Region-of-Interest (ROI) for the SE based on convergence and engineering judgment. The decision of whether a substation is inside or outside the ROI is based on observability considerations. Currently, the observable bus ratio of the NSTAR Internal network model is over 96%, and the same ratio in the External network is close to 90%. As a result of periodic EMS database re-builds upon RTO EMS releases, NSTAR developed solutions to continuously improve, store and maintain key SE and PF tuning parameters. For example, signs and standard deviations of flow measurements, switch normal positions and generator AVR control flags are maintained in spreadsheets and mapped to the EMS database through script files. Through use of techniques such as pseudo-measurements and tap estimations, the quality of SE solutions has steadily improved, and the convergence rate is today at better than 98.5%, with SE and CA running on average every 2.1 minutes and 2.7 minutes, respectively. The sum of normalized residuals (i.e. performance index) for the entire network is below 1000, and over 76% of estimation errors are below 3% of telemetry values. A test environment using real-time SCADA online telemetry is available to validate new model releases and network equivalences before putting them on-line. The test environment is also used to validate changes such as contingency definitions and new alarm settings.
D. Unresolved CIM Standard Limitations The NSTAR NMMS/EMS deployment was carried out based on CPSMv1.8 and CIM version CIM10r7. This section highlights CIM standard limitations that were encountered during the NSTAR CIM model exchange and CIM Converter development phase, and for which there is no solution in the latest CIM version (CIM13r12). (1) The CPSM profile does not define sequence impedance data required to perform short circuit analysis. This EMS function is currently not used and has been disabled. (2) A HVDC Link model is not included in IEC 61970 part 452. At NSTAR, each HVDC pole is modeled as one synchronous generator (on the normally sending end), one synchronous condenser and a load (on the normally receiving end) coupled together with a zero impedance branch, which may connect nodes at different voltage levels. (3) State estimator data requirements include measurement accuracies, commonly defined using measurement standard deviations, but are not available in the CIM. These have been added using conversion rules based on device type, unit capacity, voltage level, etc. (4) There is no mechanism in the CIM model to define the boundary between transmission and distribution network models. For the purpose of running distribution automation applications, an extraction had to be designed from a complete CIM model, including both transmission and distribution substations, for a Distribution Management System (DMS) connectivity model. This was achieved by using a fixed voltage level criterion of 115 kV and a set of conversion rules to perform filtering on relevant distribution substations. The CIM Converter was used to replace the transmission network model with an equivalent injection source in the DMS network model. (5) There is no mechanism in CIM to distinguish internal control areas from external control areas. The CIM Converter rule package was used to address this deficiency and complete the EMS network model. (6) Equipment static thermal limits are defined using one of several limit sets. NSTAR had to use specific keywords in order to identify seasonal limit sets. Dynamic limits are currently not supported by CIM. (7) Relays, calculated control logics and backup telemetries are not defined in CIM. (8) EMS configuration data is not addressed in CIM (e.g., contingency definitions, optimization controls). (9) The correctness of PF solutions could not be verified directly between NSTAR’s NMMS and EMS because the CIM XML data exchange includes only static data, unbalanced load and generation schedules and no power flow solutions. (10) Impedance correction tables as well as non-linear tap ratios of transformers and phase shifters were built independent of CIM. (11) There are different ways to identify Power System Resources (PSR) in a full CIM/XML data exchange, e.g. RDF
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IDs and NERC Master Resource Identifiers (MRID). The current approach for persistent and unique identifiers is similar to MRIDs, e.g. name-based resource identifiers. A comprehensive naming convention using predefined rules was established for control areas, substations, equipments, etc. It was implemented in the NMMS to ensure consistent equipment identifiers in both Graphic and CIM/XML files. E. CIM Standard Limitations Addressed in Later CIM Releases This section highlights CIM standard limitations that were encountered during the NSTAR CIM model exchange and CIM Converter development phase, which are addressed in the latest CIM release (CIM13r12). (1) In an energy control center, it is essential to configure control and alarm monitoring responsibilities for the different combination of operators, consoles and areas of jurisdiction. The CIM release used by NSTAR did not define equipment owners. In this case, conversion rules were used to define the area of jurisdiction for each piece of equipment based on their device type, control area, voltage level, etc. CIM13 has a new class PSROwner that can be used for this purpose. (2) Standard keywords for measurement units and measurement types were not available, which created confusion. Valid keywords have been defined in CPSMv6. (3) CIM Equipment limit sets are associated to equipments through measurement definitions. This created conversion issues because the CIM was not canonical and allowed multiple implementations without new functionality. A new Operational Limit package was developed in CIM13. (4) Branch groups were not modelled in earlier CIM versions. These are required to allow EMS applications to enforce and/or monitor limits on transmission corridors. A BranchGroup model has been included in CIM13. (5) The Interchange area class and interchange schedules were not defined in the CIM release used by NSTAR. The new CIM ControlArea packages intend to fix these issues. (6) The load model was incompatible with the EMS load model. The CIM Load model package has been completely reworked in CIM12. (7) The network model includes AC Line Segments of zero impedance connecting different control areas and different voltage levels. This required changes to the EMS proprietary format and to the Interchange Scheduling applications. The most recent CIM version now defines those line segments explicitly as equivalent branches. IV. POSSIBLE MODEL MANAGEMENT IMPROVEMENTS A. Improved Data Exchange with ISO-NE A significant improvement to the current model data exchange process would be to implement the CIM Model Authority Set (MAS) [6] data exchange feature. This new way of performing data exchange was introduced for the first time at the 2008 EPRI Interoperability Tests. It allows for separation of network model maintenance responsibilities
between multiple parties. It thus enables each model authority to maintain an accurate and reliable representation of their external networks as recommended by the NERC Best Modeling Practices [7]. The theory is that each model authority is responsible for publishing a self-contained representation of their network model in CIM/XML format and to agree on a common boundary set with their neighboring model authorities. This allows neighboring parties to merge selected models to update their external network, e.g., it allows the RTO to delegate model maintenance responsibilities to an LCC and it enables partial model updates between NMMS and EMS. B. NSTAR Incremental EMS Database Update The objective for model updates should be to import each new release or change of the CIM Model data from NMMS to EMS with as little disturbance of the on-line SCADA and EMS systems as possible. This would mean minimum production downtime (if any), as low a risk as can be achieved of introducing changes that might cause real-time application unavailability, such as failure of the SE, and a simplified work process. Experience has shown that the NMMS does not guarantee the resource identifier (RDF ID) persistence required for CIM incremental updates. In addition, standard use cases indicate that CIM incremental updates are meant for small model updates in between two full model synchronizations. Therefore, CIM incremental updates do not meet the objective. A suggested solution at NSTAR (refer to Figure 3) is to merge each new CIM Model release with the online EMS model using an EMS database comparison utility that detects selective model differences between two EMS, using namebased persistence and locally unique identifiers. The proposed utility generates human readable incremental EMS updates that can be automatically propagated to the on-line system after being validated on a development environment with telemetry. The proposed update process could avoid downtime as well as increase productivity and reliability. It could also eliminate the need to rely on CIM incremental updates and full database replacement, and could be integrated with the CIM conversion utility as part of a full model exchange or a CIM model authority set data exchange. C. CIM Model Extensions CIM model extensions would allow exchanging more information between NMMS and EMS and it would resolve identified CIM limitations. It would further reduce model edits done directly in EMS, thus minimizing differences with NMMS. CIM Extensions can be designed with freely available open source tools [8]. D. Operations and Planning Views of the Transmission Network Model The NSTAR NMMS could be a key enabler for achieving a common network model data repository for both planning and operations as described in [9]. This objective would be in line
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with NERC initiatives such as NERC MOD-012. For example, NMMS could import power flow and state estimator case data from the EMS through either a CIM-compliant or a PSS/E data exchange. Main challenges to an Operation and Planning model data exchange would be the mapping of power flow bus numbers and ownerships between operation and planning, the manipulation of sequence data, the integration of dynamic ratings and temporary limit overrides and the management of network dynamic data.
between an RTO and LCCs, and between applications within a utility, and to propose new business case solutions. VI. REFERENCES [1] [2] [3] [4]
[5] [6] [7]
[8]
[9]
S.T. Lee,”The EPRI Common Information Model for Operation and Planning”, Power Engineering Society Summer Meeting, 1999, IEEE Draft IEC 61970: Energy Management System Application Program Interface (EMS-API)-Part 301: Common information model (CIM) ISO-NE OASIS Map [Online] http://oasis.iso-ne.com/oasis EPRI, “Testing of the Generic Interface Definition (GID Standards and the Common Information Model (CIM) Extensible Markup Language (XML) Interoperability Test #5: The Power of the Common Information Model (CIM) and the Generic Interface Definition (GID) to Exchange Power System Data”, EPRI, Palo Alto, CA: 2004. 1009404. IEC 61970 Energy Management System Applications Program Interface (EMS-API) – Part 452: CIM Transmission Network Model Exchange Profile, Rev. 1.8, Apr 2002, Based on CIM10v7 J.P. Britton, “Designing Model Exchange Processes with CIM and ‘RMA sets’”, in Power Systems Conference and Exposition 2006, 2006 IEEE PES, Oct. 2006, pp. 487-489 NERC Real-Time Tools Best Practices Task Force, “Real-Time Tools Survey – Analysis And Recommendations”, NERC, 2008 [Online] http://www.nerc.com/filez/rtbptf.html L. King, “The Common Information Model for Distribution: An Introduction to the CIM for Integrating Distribution Applications and Systems”, EPRI, Palo Alto: CA, 2008. 1016058. D. Becker and T.L. Saxton, "CIM Standard for Model Exchange Between Planning and Operations’’, IEEE PES GM --- Conversion and Delivery of Electric Energy in the 21st Century, 2008, pp. 1-5
VII. BIOGRAPHIES Enrique Margalejo is a Senior Operating Support Engineer with NSTAR Electric. He previously worked at the University of São Paulo, Boston Edison, Fujitsu Consulting, Convergent Group, and Schlumberger. He graduated from the University of Asuncion, Paraguay, as an Ind./Civ. Eng., and received a M. Eng. degree (Electrical) from Purdue University. Per-Anders Löf (S’89, M’95) is a Senior Operating Support Engineer with NSTAR Electric. He has previously worked in Sweden at ABB Corporate Research and at Svenska Kraftnät (Swedish National Grid Company). He received his Civ.Ing., Tekn.Lic and Ph.D degrees from the Royal Institute of Technology, Stockholm, Sweden in 1987, 1991 and 1995, respectively.
Fig. 3. Possible Model Management Improvements
V. CONCLUSIONS This paper has highlighted the benefits of using IEC 61970 CIM standards for efficient integration of EMS applications in an electric utility, and for the implementation of efficient modeling practices. It also revealed relevant integration challenges and CIM model issues encountered during the EMS model data exchange processes. In NSTAR’s case, use of the CIM facilitated information exchange between different software vendors’ proprietary formats and allowed for use of one tool for staging, merging, exporting and maintaining the different portions of a large network model. NSTAR’s first EMS experience in building, validating and maintaining a network model enabled the identification of possible improvements in the model data exchange processes
Xiaodong Liu (M’98) is a Senior Operating Support Engineer with NSTAR Electric. He received his Ph.D. degree from Shanghai Jiaotong University, P.R. China in 1998. He was with EnerNex Co., U.S., Macquarie Gen., Australia, and Shanghai Jiaotong Univ. His fields of interest include network modeling, transients and dynamic simulations. Philippe Picard (M’08) is with SNC-Lavalin T&D. He received his B.Eng. and M.Eng. degrees (Electrical) from École Polytechnique de Montréal, Quebec, Canada, in 2003 and 2008, respectively. He participates in EPRI CIM interoperability tests and is an executive member of the recently created IEEE TF on the CIM. His interests are in EMS Real-time Reliability Tools and Power System Modeling.
