In 2002, Orange & Rockland Utilities Embarked on a Project to interface circuit analysis capability to its geographic information system (GIS). The intent was to have GIS provide component information for the analytical software to automatically build the integrated electrical system model. The model would automatically attach loads at their specific geographic location. Then circuit changes, device status and other updates captured by the GIS system would automatically update the system model. All of this would interface through to the customer information system.

THE SOFTWARE SELECTION

O&R (Pearl River, New York, U.S.) selected the Distribution Engineering Workstation (DEW) from Electrical Distribution Design (Blacksburg, Virginia, U.S.) as its software of choice. It is an open-architecture analytical tool with sophisticated per-phase circuit-analysis capability. Once the model is built, any analysis application can be attached to the model. Isolated models are no longer needed to solve specific problems.

Using a common model to solve all problems reverses the paradigm where 90% of the time was spent gathering data and 10% of the time was spent on analysis. For instance, the DEW power-flow application uses a patented solution technique that runs extremely fast, can be parallel-processed and is much less susceptible to numerical instability than matrix methods. This allows a detailed solution of a very large system in near real time. For example, the entire O&R integrated distribution system model, which contains more than 365,000 components, is solved by the DEW power flow in less than 30 seconds on a standard laptop computer. Distributed processing can be employed to achieve faster solution rates. A test on a blade server is planned for 2009, and it is anticipated that solution time will be reduced to less than 2 seconds.

SWITCH ORDERS

A Web services interface was developed that allows GIS updates to be sent to DEW on a daily basis through a batch process. Working with EDD, load-research statistics such as kilowatt-hour to kilowatt conversion factors, diversity curves and load profiles by customer class were developed from data supplied by field-installed interval recorders. Additional interval recorders were added to the load-research program to obtain more accurate customer load-research statistic models. Understanding the time varying load behavior of different classes of customers is essential for accurate load estimates.

Additional functionality exploiting both Web services and the fast analytical capability were developed. At O&R, distribution switch orders are developed in the GIS system. Once a switching sequence is planned, the operator can request that the switching moves be checked to insure that system parameters, such as loads and voltage levels, do not exceed preset limits. Once the GIS system submits the planned switching steps to DEW via the Web interface, DEW changes the switch states within the model and performs a power flow on the circuits affected and provides a report that displays on the GIS map viewer. The report details loads and voltages before and after the switching moves. Other statistics — such as the number of customers transferred, dropped or restored — are also displayed. Locations with low voltage and overloads are identified and reported as alarms. Areas left without power as a result of the proposed switching also are identified and alarmed.

DATA SOURCES

The DEW model of the O&R distribution system is automatically built from several external data sources, including:

  • Customer load measurements, including hourly kilowatt-hour and demand measurements

  • GIS data

  • Load research data

  • SCADA data.

O&R provided additional information during database table setup, which was used in model creation: data for equipment impedances, and ratings and settings for regulating transformers and capacitor controllers.

The major issue encountered in building the model was data accuracy from the above sources. Power-flow simulations were used extensively to discover inconsistent and/or inaccurate data. When data errors were discovered, the error was fixed in the data source and the model rebuilt. The data-cleansing effort took more than two years. Processes are now in place, which include continual use of the power-flow simulation, to discover any data errors or inconsistencies that might be introduced into the system.

The DEW implementation diagram shows three servers along with planning workstations.

VIRTUAL SCADA

The load-estimation methodology of DEW — coupled with load scaling and fast analytical capability — has allowed O&R to interface this tool to its energy management system (EMS) in real time. With SCADA on 95% of its distribution substations, O&R monitors individual circuit phase currents and bus voltages at the substation level. This real-time data is made available to DEW, which scales estimated loads to real-time metered load levels, while maintaining the relative time-varying characteristics of the individual loads.

Power-flow calculations are performed on circuits whenever a change in real-time current or voltage exceeds a preset threshold. Threshold setting minimizes the number of circuits needing analysis at any given time, thus making processing faster. The real-time power-flow calculation makes use of a year's kilowatt-hour and demand measurements. This provides more accurate predictions of voltages and currents throughout the circuits. Use of historical load data for every customer in conjunction with SCADA measurements to estimate the voltage and current at every customer is referred to as virtual SCADA. The output of virtual SCADA is made available to visualization engines so that calculated loads and voltages can be displayed on demand for any user-selected object. The results can be displayed in DEW or on the GIS map viewer in real time, which makes this data readily available across the entire enterprise.

REAL TIME NEXT

With the Web services interface complete, an automatically maintained full-system model available and the virtual SCADA interface to EMS established, the next logical development step is to allow real-time distribution system control.

O&R has begun a Smart Grid pilot project to prove out this concept. Two 13.2-kV distribution circuits will be set up in an auto-loop configuration using reclosers with real-time SCADA inputs to EMS. SCADA-operable, group-operated load-break switches for sectionalizing will be installed at strategic points. The local intelligence within the reclosers will be used for fault clearing, but the centralized logic will take in the real-time data from the substations and field devices, apply it to the system model and determine the fault location. DEW will then send a signal to EMS to operate the appropriate sectionalizing switches to isolate the fault before allowing tie reclosers to connect the alternate source.

Another part of this project will place the substation load tap changers and the switched capacitor banks under real-time control for loss minimization and capacity optimization.

DEW IMPLEMENTATION

The DEW implementation diagram shows three servers along with planning workstations.

The As-Is Circuit Server is the heart of the operation and hosts a model of the entire system that is maintained to match field conditions. Offline analysis workstations, such as the planning workstation, can access the circuit server to retrieve any or all of the system model with current SCADA measurements attached. Once the circuit model is downloaded to an analysis client, local analysis programs can be used on the model.

The Study-Mode Server accepts Web-based requests for analysis studies, which typically come from distribution operators. The circuit models for analysis are obtained from the circuit server. Each analysis request results in the creation of a new instance that runs in the background. Multiple, different analyses can run concurrently. Web messages are used to return analysis results that can be displayed by visualization engines. Weather data is an input to the Study Mode Server. The load research statistics used to estimate loading from kilowatt-hour measurements are a function of weather conditions.

The Real-Time Power-Flow Server can solve from the transmission level (if modeled) down to the distribution transformer level. The operator can request to review voltage and current values on every object in the system, including distribution transformers. The power-flow calculation is performed at least once every 30 seconds.


Charlie Scirbona (scirbonac@oru.com) is the department manager for Distribution Engineering Standards for Orange & Rockland Utilities and department manager of Non-network Systems and Unit Substations for Con Edison, O&R's parent company. Before entering distribution engineering, Scirbona held various positions of increasing responsibility at O&R, ranging from line supervisor to division superintendent. He also has had operating responsibilities in overhead distribution, substation, transmission, relay and field engineering.