Reconciliation of BCCS and WSM with

Integrative Node-Breaker Method

By

Node-Breaker Subgroup of the

Modeling SPS and Relays Ad-Hoc Task Force (MSRATF)

Western Electricity Coordinating Council

August 20, 2014

1Introduction

Technical Studies Subcommittee (TSS) formed the Modeling SPS and Relays Ad-hoc Task Force (MSRATF) in August, 2012 in response to findings and recommendations from the Southwest 2011 disturbance, and to achieve broad improvements in transparency between Transmission Owners (TOs), Generator Owners (GOs), Generator Operators (GOPs), Transmission Operators (TOPs), Transmission Planners (TPs) And Planning Coordinators (PCs), and Reliability Coordinators (RCs).

To aid in theeffort of improving transparencymembers of the Western Electricity Coordinating Council (WECC) and Peak Reliability would like to reconcile the WECC base cases with the West-wide System Model (WSM) that is used by Peak Reliability. The WSM is an Energy Management System (EMS) model of the form typically used in real-time operations. The WECC base cases are power-flow models of the form typically used in system planning. Since both models represent much of the same physical equipment, reconciliation of the models should streamline the ability to makecomparisons of WECC base cases and WSM cases, and allow validation of both cases by ensuring that a wider audience is using and examining the models.Improvements that can be derived from comparing theWECC base cases with the WSM include; (1) understanding and resolving differences between cases, and (2) avoiding duplication of effort by mapping the current transient stability models from WECC base cases into WSM snapshots.However, since each model includes details incompatible with the assumptions of the other model, some debate exists as to whether the models can be reconciled without loss of functionality in one model or the other (Figure 1).

Figure 1: WMS and WECC Base Case data intersections

The node-breaker modeling detail in the WSM, typical of EMS, is perceived as the significant barrier to reconciling the model. Refer to section 1 of the appendix for a detailed description of node-breaker modeling. An equallysignificant challenge is presented by the different naming used by the two models. In this paper, when referencing node-breaker modeling, “name(s)” refer to invariant identifiers of elements and are used to link the objects in the model to external functions, including references in operating procedures; and asset data and automated contingency files used in planning (Figure 2). Refer to section 2 of the appendix for additional details on “names”.

Figure 2: Links between intersecting data, and related non-intersecting data

The purpose of this paper is to outline a method that could be used to move the WECC base cases toward the node-breaker model. The method of incorporating node-breaker connectivity models in WECC base cases being proposed by MSRATF in this paper is what was being called the hybrid method, and now is more accurately called the integrative method. This method transitions from the current bus-branch model toa node-breakerrepresentation by enhancing sections of the bus-branch model (i.e.adding node-breaker modeling detail to bus-branch representation).Inserting the breaker-node modelsin a substation by substation approach will take a significant amount of time to fully transition the entire WECC base cases into a node-breaker model. This proposal is a transition from the suggestion made in the previous paper (“Node-Breaker White Paper”, January 22, 2014”).

The Node Breaker white paperoriginally proposed reconciling the two models by replacing the base topology for WECC base cases with node-breaker topology exported from the WSM.This paper presents a refinement on that proposal, using an integrative approach (called integrative method in this paper) which imports the node-breaker modeling used in the WSM into the WECC base cases while preserving the existing WECC base case naming which is essential to existing dynamic data files and automation files.

2Why UseAn IntegrativeMethod

Several shortcomings of the full-replacement approach were identified with further analysis, related to both loss of functionality (Figure 3) and procedural limitations.

Figure 3: Planning loss-of-functionality from replacement approach

The major shortcomings driving the recommended integrative approach are:

  • Data sharing concerns;the WSM data is restrictedunder a non-disclosure agreement that many users of WECC base cases are not able to sign;it would impose untenablelimits on what WECC base cases could be used for and to whom they could be distributed.
  • Switching to anintegrative method puts the responsibility of updating and maintaining the models in the hands of the data owners. It is the responsibility of the data owners to update and maintain their models in accordance with the Data Preparation Manual. A full or partialinsertion of node-breaker (connectivity) data in the system can be done to the extent that the data owners agree to the change; however, the owners will continue to be responsible for the accuracy of the models of their equipment.
  • Data that is important to the planning community and maintained in the bus-branch models would be lost through a full replacement with the WSM model. This includes the topological data currently in the bus-branch representation. The integrativemethod allows TOs to keep their topological representation of substation buses while inserting detailed connectivity data for each bus, such as bus arrangementandbus segments, branch connection, breakers and switches.
  • Descriptions of contingencies can be given in bus-branch format, or as a label (a branch label, bus segment label, etc.), or by specifying the breakers to open. The system simulation software will need to support this by retaining bus-branch data and linking breaker-node data to it. The desire to retain static identifiers for topological nodes (or buses) may be a transitional desire. In the long term we would recommend that engineers reference elements using a label of the element itself instead of using static bus identifiers as an identifier of other elements. This is described in further detail below, and in section 3 of the appendix.
  • Intentional modeling differences such as systems that are equivalenced in WSM cases would need to be identified and expandedif the WSM modelwere used as the starting point.

For similar reasons, the approach of reconciling the two models by overwriting WSM data with the WECC base case model is also not recommended. The desired solution is one that preserves the detail from both models, and maintains the familiarity and the functionality of both models for the uses to which both planning staff and real-time staff will put them. This is an "integrative" approach to implementing a common model, rather than a "replacement" approach.

3The IntegrativeMethod

3.1Topological Nodes and Connectivity Nodes

Critical to an integrative approach is the realization that what the planning tools call "buses" are a fundamentally different class of object from what the EMS tools call "buses" or "nodes". The common information model (CIM) recognizes this and refers to the two different objects as "TopologicalNodes" and "ConnectivityNode" respectively.

EMS systems that use the IEC Common Information Model standard 61970 Edition 3already distinguish Connectivity Nodes from Topological Nodes, and support a containment relationship between them, where any TopologicalNode must contain one or more ConnectivityNodes; and a TopologicalNode is a type of “IdentifiedObject” with a permanent non-volatile name and description. ConnectivityNodes may be contained only by zero or one TopologicalNode. An example of how Topology Nodes and Connectivity Nodes are used can be found in section 4 of the appendix.

Even legacy EMS systems already recognize the distinction between TopologicalNodes and ConnectivityNodes, and may include among their network applications a "topology processor" capable of merging all the nodes that are joined by closed zero-impedance devices into a single Topological Node equivalent to a planning "bus". A shortcoming of legacy EMS systems is that they apply arbitrary busnumbers to the topology that they produce,and these busnumbers change since switching devices that are in their non-normal state create new TopologicalNode arrangements. That shortcoming is overcome by more recent Topology processors that assign a non-volatile name to any TopologicalNode that is in its normal configuration, and use a specified range of unassigned identifiers for TopologicalNodes resulting from abnormal switching states.Vendors of EMS software must, if they have not already done so, implement the ability to specify a non-volatile persistent association between connectivity nodes and those topological nodes represented under normal system conditions.

Vendors of planning tool software must also implement Connectivity Nodes as a separate class of object from “buses”. The technique of flagging zero-impedance branches to represent breakers and switches, and creatingbus numbers as ConnectivityNodes (grouping buses together implicitly into TopologicalNode groupings), does not meet the all desiresof integrative modeling as described in section 2. The primary desire which is not met is the use of static topology node identifiers as keys of other elements (such as generators, transformers, etc.).

For this reason, to aid in the transition to these new models, Powerflow software vendors are asked toprovide a mechanism of maintaining static identifiers for Topological Nodes. In addition some mechanism for designating the difference between a transmission element (line, transformer, or series reactor/capacitor) and a switching devices (breaker, disconnect, fuse, load break disconnect, jumper, etc.) will be necessary. Also we recommend that software vendors provide the ability to view system diagrams using either ConnectivityNode detail, or the higher-level TopologicalNode layout. In addition in the short term the ability to continue defining contingencies using syntax that uses topology node’s static identifiers (such as bus numbers) should be maintained. In the long-term though we highly encourage the syntax is switched to using element labels instead.

3.2Building Models With An IntegrativeMethod

The integrative method continues the status quo of having twomodels maintained, the WECC base case model and the WSM. However, the WECC base case model can be built to include the one-to-many containment relationship of Topological Nodes and Connectivity Nodes. In the immediate term, this allowsusers of the WECC base case to continue using the syntax that uses topology node identifiers (such as a bus numbers) to refer to elements in the system (such as generators, transformers, etc.). The WECC base case will however be augmented to include the labels used in the WSM model as well thus enabling the engineering community to start using these identifiers themselves or at the very least pass data back to the maintainers of the WSM model using the identifiers which are uses in the WSM model. With these software requirements implemented, the integrative model-reconciliation process would comprise the following actions:

  1. Owners of planning data who submit updates to the WECC BCCS will begin to enter ConnectivityNodes, switches, and their configurations into their planning models, using connectivity-node names, switch-names, and node-breaker topology from the WSM. These data will be entered gradually over time, consistent with a realistic schedule to be defined between PEAK and WECC, recognizing the other demands on planning and staff's time, with the most significant system buses prioritized. No changes to the existing bus-branch topology or functionality are intended by simply adding these additional data.
  2. WSM data submitters, and WECC BCCS data submitters who submit modeling data for the same physical elements, shall identify one another and collaborate to create business-to-business procedures to maintain consistency between the two models on a go-forward basis. Such procedures must address: adding new planned facilities, updating simple planned configurations with detailed design configurations as work proceeds, reflecting the impact of schedule changes, removing retiredfacilities, and coordinating corrections to the modeling of existing equipment. The mapping of information between these two systems however should following the element label identifiers used in the WSM model.
  3. In any situations where the discrepancies between the real-time model as previously submitted to the WSM and the planning model as previously submitted to the WECC BCCS cannot be resolved by the simple addition of additional ConnectivityNodes and switches to the WECC Base case model, then one or both modes should be changed using the coordinated model-correction process established between the two responsible data-submitters. Changes should be driven by the principle that the model should support the highest-detail representation of the actual real-world equipment required by either of the parties, along with creating spurious models of equipment that does not really exist ("modeling artifacts") that may be required to work-around various software limitations.

The WSM case data comes from a very consistent source but planning casestended to have consistency problems because they started from various cases. To address this and other issuesWECC is implementing the Base Case Coordination System (BCCS). The BCCS will use a single base topology for all planning base cases. In order to alter the base topology for the desired base case files are stored in the data base to change load and generation profiles, and add planned projects to the case. This structure allowsdata owners to load a single file into the BCCS that will consistentlyenhance the bus-branch topology at the desired location with a node-breaker representation in every case built afterwards.

For the transition from bus-branch to node-breaker to be effective consistency must be ensured between the WSM and WECC base cases. This will require a common data source for the models and consistent namesin both sets of node-breaker models to allow the WSM and base cases to be compared and validated. Names used in the WSM models should beincorporated into the base case node-breaker connectivity models.

Although data owners can build their models by hand, amethod of incorporating consistent models is for the data owners to obtain their node-breaker models from their Peak RC representative.The key aspect of this process is that the incorporated models will be compared to the WSM models and any differences will need to be rectified.

3.3Maintaining Models With TheIntegrative Method

The process to maintain the BCCS integrative case consistent with the WSM case can be seen in Figure 4 below. The key aspect of this process is the comparison report. This report would be based on a comparison of the node-breaker data in both cases based on common facility labels. Differences in this data would be relayed back to the TOs, GOs, TPs, and PCs for investigation and corrections. Errors would then need to be corrected in the data base with the identified error (EMS/WSM or BCCS).This process would be ongoing in order to maintain the quality of the models.

Process of Maintaining Cases

Figure 4: Process of Maintaining Cases

Adoption of the process shown above (Figure 4) would allow for a consistent data set. The process would also allow both the WSM and the base case models to improve in quality and accuracy because people would get the opportunity to compare the BCCS and WSM models and identify problems and inconsistencies.

The recommendation of using the integrative method approach is a way to allow data owner’s full control of and responsibility for their data. It will also help to eliminateunintended differences between the planning models and the operations model.

Appendix

1Node-Breaker Modeling

Currently the majority of off-line planning studies are using bus-branch models to represent power system networks. These models typically represent each substation with a single bus at each nominal voltage level (topological modeling). Without knowledge obtained outside the represented bus-branch model it is not possible to determine the substation breaker configuration and how it operates during contingencies. The lack of breaker information requires that a large number of contingencies must be manually created to replicate bus contingencies and breaker failure contingencies. This manual effort often presents an opportunity for human errors to occur and takes a lot of time to reconfigure buses for study work.

A solution to this is to insert node-breaker (connectivity) models to amendthe simplified bus models in the bus-branch configuration. Node-breaker models can provide a fully built out substation representation (i.e. elements such as breakers, switches, branches, shunts, etc. are modeled individually and connected via connectivity nodes). These models provide several advantages over the simplified bus-branch models including but not limited to:

  • Providing improved visibility of substation configurations and equipment
  • Showing case specific switch and breaker statuses
  • Providing enough information for power flow programs to fully automate the creation and processing of contingencies. An example would be to enable automatic inclusion of stuck breaker contingencies.
  • Simplifying the modeling and simulation of protection system operations
  • Allowing the use of compatible data sets to be used for operational studies and planning studies when representing existing topology
  • Assisting in model validation from PMU data and system disturbances
  • Allowing for easy and smooth representation of topology changes within substations by changing the status of switching devices (e.g. bus splitting)

Common tower and common corridor outages would continue to need to be inserted manually.