Distribution analysis in a GIS environment
Arthur Fedoruk Salt River Project, 1521 N. Project Drive Tempe, AZ 85281-1298 Robert W. de Mello, William E. Schwinghammer Power Technologies, Inc. P.O. Box 1058, Schenectady, NY 12301 Introduction The Salt River Project (SRP) is one of Arizona's largest electric utilities and water providers. SRP serves more than 700,000 residential, business and industrial customers with electric power throughout a 2,900-square-mile area in central Arizona. The electric side of SRP is a political subdivision of the state of Arizona. In response to an increasingly competitive business environment SRP is moving toward integration of data and management functions through the use of Geographic Information Systems (GIS). SRP, as are many other electric utilities, is placing increased emphasis on controlling operating costs. The functions typically supported by GIS systems, work management, outage management, inventory and asset control reduce operating costs by providing personnel with timely information that results in increased accuracy, efficiency, productivity, and communication. Taking this philosophy one step further, SRP has implemented an integrated solution of distribution analysis that addresses the most common concerns of distribution operations and planning namely voltage, loss, and overload. Future extensions will address concerns in the areas of quality, protection, and reliability. For many years, analytical tools have been used for planning, and to some extent operation, of distribution systems. These tools include load flow calculation, short circuit calculation, and various optimizations that assist in determining network configuration, equipment settings, or equipment siting. The increasing use of GIS has provided a common database from which these analytical tools can automatically extract the information they need. Thus, the analytical tools have evolved from stand-alone programs with manually collected data to stand alone systems with automatic links to an enterprise wide database. This paper describes a new approach that more fully integrates the analysis functions themselves, rather than only links to data, into the enterprise wide systems from which data is obtained. This further evolution of analytical functions goes beyond making tools readily available to directly addressing engineering concerns. The tools can be relegated to a less prominent role by automating frequently performed tasks and streamlining complex tasks, thereby shifting focus from the tools themselves to the problem at hand and its solution. The reasons for doing this are twofold. First, this approach makes maximum use of network data that has been painstakingly collected and verified. Second, the approach allows staff to improve productivity by permitting them to recognize and solve problems within the familiar framework of their GIS rather than forcing them to master numerous independent applications. Analytical functions and optimizations The load flow (a.k.a. power flow) is the most basic of all analytical calculations performed on an electric distribution network. The load flow provides a snapshot of a network operating state. Given a set of loads, the load flow calculation determines voltage at all points of a network and current, power and loss for each branch in the network. Instances of high or low voltage and/or equipment overloading can be readily determined from load flow results, as can losses in the network. A state-of-the-art load flow algorithm must accommodate a variety of load types including constant power, constant current and constant impedance loads in addition to induction and synchronous machines. The ability to handle looped, or even meshed, networks is important since subtransmission (looped) and/or urban secondary systems (meshed) are often modeled. While distribution networks are typically operated in a radial configuration with a single source of power at the substation, the ability to include multiple sources is important as distributed generation plays an ever-increasing role in the distribution system. Finally, the load flow algorithm must faithfully handle unbalanced systems, that is, unbalanced loads, unbalanced topology, and unbalanced devices such as open-delta autoregulators. Two variations supplement the load flow: the short circuit calculation and the motor starting calculation. The short circuit calculation considers abnormal (faulted) situations. Motor starting considers the impact of starting large motors. Like the load flow, both short circuit and motor starting calculations determine voltage at all points of a network and current and power for each branch in the network. An additional set of analysis functions deals with various optimizations of the network. Most of the optimization functions in turn make use of the load flow algorithm. Optimizations include:
The PSS/Engines The PSS/Engines are a unique suite of power system simulation libraries. The PSS/Engines encapsulate the algorithms for load flow, short circuit, network optimizations, and many other functions that are needed for engineering analysis of distribution networks. Since they are libraries and not stand-alone programs, the PSS/Engines must be embedded in another system and the GIS is the logical target. The PSS/Engines are the building blocks from which a truly integrated analysis capability, or simply an embedded tool, is built. Direct users of the PSS/Engines are programmers who must integrate analysis capabilities into a GIS. In the case of the GIS implemented at SRP both Smallworld and GeoData were involved in the integration. The PSS/Engines provide state-or-the-art algorithms and optimizations without requiring that the programmers themselves become experts in the algorithms and optimizations. SRP, the user of the GIS and the one who benefits from the integrated analysis, is only an indirect user of the PSS/Engines. Indeed SRP need not be aware of the existence of the PSS/Engines or the particular algorithms employed to solve problems. SRP also benefits from robust and thoroughly tested algorithms and optimizations but only to the extent that their problems are solved faster and more easily. Since the PSS/Engines are function libraries, the interface to the PSS/Engines is accomplished at the programming level with function calls. There are no intermediate files to maintain. Each engine has a well-defined and stable application program interface (API). Interface with the user of the GIS is entirely under control of the programmers doing the integration and should retain the look and feel of the GIS environment. The PSS/Engines are written in C++ to take maximum advantage of object-oriented software technology but the functions in API of the PSS/Engines follow C programming conventions. This permits the PSS/Engines to be used with programs written in C, C++, or even Visual Basic on Windows computers. The individual engines within the PSS/Engines are described below. Base Engine The base engine provides a number of support services used to build and maintain an electric network and the components that make up the electric network. An electric network (also called a power system) is a collection of producers of electric power (sources and generators), consumers of electric power (loads, motors, etc.), devices to transfer electric power from producers to consumers (lines, transformers, switches, etc.) and miscellaneous other items (shunt capacitors, controllers, etc.). The base engine's support services consist of functions to manipulate these items; that is add, delete, and modify. The base engine also provides functions that perform loadflow, short-circuit, and motor-starting calculations. Capacitor Placement The capacitor placement engine is used to find the best sites in a network to place capacitors. Here "best" refers to the locations with the highest financial return considering the initial cost of the capacitor, annual maintenance cost of the capacitor, and cost of real and reactive power losses. Several load levels can be considered at the same time. Result of the optimization is the set of locations where capacitors should be placed; which capacitor(s) should be placed at each site; and whether or not a switched capacitor is needed at the site because of voltage constraints. Where switched capacitors are required, a switching schedule is produced. Loss, cost of loss, and information that quantifies financial return is also calculated. Predictive Reliability The distribution reliability analysis engine implements predictive reliability calculation methods for a radial network. It uses equipment outage-frequency and repair-time statistics to calculate customer-specific and/or system-wide loss of service statistics. The calculations in the predictive reliability engine can be used to evaluate the performance of a protection scheme or compare the reliability of alternate protection schemes. Fault Decrement The fault decrement engine is used to estimate currents in a network during the first few cycles following a fault. Both time-decaying DC offset and machine contribution to fault currents are considered. The fault decrement engine calculates make and break duty as well as symmetrical and asymmetrical rms currents and can be used for breaker sizing as well as relay coordination. Harmonics Engine The harmonics engine is used to put harmonic analysis into an application. Harmonic analysis investigates the behavior of an electric network at frequencies higher than the fundamental frequency. Most electric networks operate at 50 or 60 Hz, called the fundamental frequency. Some devices on these networks cause significant noise at higher frequencies. This noise, or harmonic pollution, corrupts the normal voltage waveform and can cause unwanted heating in transformers, misoperation of protective devices, and excitation of resonance. The harmonics engine provides tools to investigate these phenomena including functions to calculate standard measures of harmonic distortion such as total harmonic distortion (THD), telephone influence factor (TIF) in addition to functions that determine harmonic spectra and find resonance frequencies. Load Estimation The load estimation engine is used to scale or estimate loads in a portion of an electric network. Often, because of insufficient instrumentation, only the location and rough characteristic of loads are known. The exact magnitude of the load is unmeasured and therefore unknown. When an "upstream" measurement of power or current is available, it can be used to estimate the magnitude of "downstream" loads. The load estimation engine adjusts the magnitude of selected loads so that power or current in a selected line or transformer matches a specified value. In this way, an estimate of load magnitudes is obtained. Loss Minimization The loss minimization engine is used to determine the settings of transformer and regulator taps and the state of switched capacitors in a radial network that result in minimum loss while satisfying voltage constraints. Given a radial network and a set of transformers and regulators whose taps may be adjusted, and a set of capacitors that may be switched into or out of service, the loss minimization engine decides how to adjust tap and capacitors. Result of the adjustment is a new set of transformer tap positions and capacitor in-service/out-of-service states that lowers loss while satisfying voltage limits. Radial Reconfiguration The radial reconfiguration engine finds the best (that is, lowest loss) configuration of a radial network. Given an initial radial network and a set of tie switches that can be opened and closed to form new radial configurations, the radial reconfiguration engine finds the radial configuration that minimizes losses. Several load levels can be considered simultaneously, each with a relative duration. The radial reconfiguration engine will find the one radial configuration that minimizes overall losses when all load levels are considered. Voltage Trimming The voltage trimming engine is used to adjust the tap position of transformers and regulators in a radial network so that voltages fall between specified upper and lower limits. Given the set of transformers and regulators that can be adjusted, and upper and lower limits for voltage in the network the voltage trimming engine finds a set of tap positions to achieve acceptable voltages. If two or more sets are found the voltage trimming engine uses a minimum-loss criterion to pick the best set. Regulator Placement The regulator placement engine is used to find the best sites in a radial network to place voltage regulators. Regulators are sometimes used to maintain voltage within acceptable limits. In addition to finding sites for new regulators, the regulator placement engine can find better sites for existing regulators. Several load levels can be considered at the same time. Result of the optimization is the set of locations where regulators should be placed; regulator size; and whether or not automatic tap control is needed. Integrated analysis Simply stated, the primary concern of any electric distribution utility including SRP is to provide reliable, high-quality power in an economic manner. High quality means that voltage is kept within limits, the frequency of dips and sags is low, and harmonic distortion is low. These concerns impact both planning and operations. In the sections below the application of the PSS/Engines to real problems is discussed. In most cases a complete investigation and solution of the problem area requires the use of two or more engines. Voltage Voltage must be maintained within limits, usually within five percent of nominal. Regardless of how uniform the voltage at the source (substation) may be throughout the day, the voltage at any point in the feeder will change as load on the feeder increases and decreases from hour to hour. Voltage drop from the substation usually increases as feeder loading increases. Usually there is little information about the loads on a feeder. The magnitude of demand under high and low load scenarios must be estimated from measurement made at the substation. Once loads are determined, voltage regulators, and sometimes capacitors, are applied to correct voltage that has drifted too far from nominal. Load estimation is used to assign values to individual loads under peak and/or light load situations. Thereafter, the load flow is used to detect instances of high or low voltage. Attempts to alleviate voltage problems may involve the adjustment of existing transformers and regulators, moving existing regulators, or locating sites for new regulators. A complete engineering solution must also identify a control strategy for new regulators. The algorithms needed to investigate and solve voltage problems reside in four of the PSS/Engines: the load estimation engine, the load flow function of the base engine, the loss minimization engine, and the regulator placement engine. Loss Line and transformer losses have long been an important consideration in the construction and operation of distribution networks. Indeed, despite the deregulation of the electric industry there continues to be regulatory pressure to minimize loss in the distribution network. The load flow is used to detect instances of high loss, probably best characterized by loss per unit length (e.g., mile) of lines and cables. Attempts to reduce loss may involve the adjustment of existing transformers and regulators, adding capacitors to the network, or reconfiguring the network to reduce feeder length. Application of capacitors can lead to a resonance condition at some harmonic frequency. Hence, harmonic analysis is an important step in capacitor siting. The algorithms needed to investigate and reduce losses reside in five of the PSS/Engines: the base engine, the loss minimization engine, the capacitor placement engine, the harmonic analysis engine, and the radial reconfiguration engine. Quality Broadly defined, power quality includes items considered under the topics of voltage and reliability. Because these aspects of quality are considered elsewhere, this discussion is restricted to harmonic distortion and voltage sags. Although the source of harmonic pollution is generally with a utility's customer and responsibility for mitigating harmonic distortion also belongs to the customer, the utility may be involved in assuring that the harmonics produced by one customer do not travel back to the power system and affect other customers. Similarly, voltage sags, lasting a few seconds or less, are often caused by a utility's customer starting a motor or other device that has a large initial current. Ultimately it is the responsibility of the customer to mitigate the impact of equipment starts but the utility may be involved in assuring that the actions of one customer do not negatively impact another customer. The ability to simulate and predict problems with quality resides in two of the PSS/Engines: the harmonics engine and the motor starting functions in the base engine. Protection Overcurrent protection of the distribution system usually relies on the system's radial design. On a radial system, protection is coordinated on the premise that fault current decreases as distance from the substation increases. Estimating the magnitude of fault current is important to both properly size protective devices and properly coordinate their operation. The short circuit function of the base engine and the fault decrement engine provide this ability. Reliability A great number of service interruptions experienced by a utility's customers are caused by problems on the distribution system. Overhead lines are primarily affected by meteorological conditions such as wind and lightning but succumb to tree contact and animals as well. Underground cables experience fewer outages but, when they occur, service restoration will generally take a long time. Reliability is evaluated both in terms of outage rate and outage duration. Thus, the frequency of outages must be considered simultaneously with protection and the location of switches and tie switches that may facilitate restoration. In short, the design of the network also matters. The ability to predict the reliability of a network is important in evaluating the merits of one design against the merits of another design. The predictive reliability engine provides exactly this capability. Conclusions The transition to GIS has focused primarily on the elimination of tedious paper databases and multiple analysis programs as well as centralization of management functions. The extension of these systems to support engineering analysis can provide additional benefits. At present, most implementations of distribution analysis tools have focused on the use of the tool's analysis capabilities as simply one step in the solution of recurring problems - voltage, loss, quality, protection, and reliability. This approach can be tedious and time consuming. This approach also requires a high level of expertise in the use of the tool. The approach of integrating analysis functions, as has been done at SRP, goes one step beyond the simple embedding of analysis tools. The suggested approach de-emphasizes the use of analysis tools in favor of a focus on problem solving. Detailed knowledge of the use of the analysis tool is not required. Rather staff can be directed to a problem area and offered a choice of actions (solutions) which will reduce or eliminate the problem. When integrated like this, analysis functions become available to a broader range of staff, increasing efficiency and productivity, but more importantly, permitting better decisions to be made by more staff. Problems are identified and solutions quickly offered. Implementation of a solution is more rapid and the operation efficiency of the utility is enhanced. This paper has presented a set of function libraries that encapsulate the algorithms needed for the analysis and optimization of distribution systems. The availability of these algorithms with a welldefined application program interface enables system integrators and other solution providers to truly integrate distribution analysis into the GIS environment. | ||
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