Deregulated distribution companies (DCs) continue to face severe technical and business challenges, and considerable management efforts are necessary to reorganize utilities for this new environment. Seeking to reduce costs and improve efficiency, DCs are using software systems to automate processes wherever possible. Software tools that include a technical database with supervisory control and data acquisition (SCADA) form the backbone of an advanced automation system known as a distribution management system (DMS).
For many years, the Serbian utility Elektromorava Power Distribution Co. (EDPC; Pozarevac, Serbia) has been using a DMS successfully for the management of its distribution network in the city of Pozarevac. One 110/35-kV substation and five 35/10-kV substations supply the medium-voltage 10-kV distribution network of Pozarevac. The low-voltage distribution network (0.4 kV) is supplied by 170 distribution substations 10/0.4 kV, with a total installed transformer capacity of 126 MVA. The 10-kV network is 242 km (150 miles) in length with 409 branches supplying a winter peak load of 47 MW. The 10-kV network power loss is 1.96 MW (4.1%) at peak loading conditions — a value that excludes the low-voltage network losses.
The DMS provides a high-quality realization of all electrical engineering tasks in a DC, including monitoring, operation management and planning, optimization and incidents analyses, control and network development planning. SCADA was one of the first systems developed to enhance distribution automation. It consists of computer software, hardware and telecommunications equipment. SCADA is used for remote monitoring, centralized acquisition of substation data (measuring, alarms and statuses of switches), remote operation of switchgear and remote setting of local control devices (relay protection and voltage/reactive power control).
DMS software consists of DataBase, a DMS server that integrates all components, user interfaces and DMS analytical functions. Many DCs in Serbia and throughout the world now use DMS software produced by the DMS Group Ltd. (Serbia and Montenegro, email@example.com.). The system encompasses power analytical calculations and optimization, providing the tools necessary for efficient monitoring, management and design of distribution systems.
The DataBase is a software system necessary for operation of all other DMS software components, as well as for several other DC technical tasks, including maintenance and trouble call management. DataBase is implemented as a Relational Data Base Management System (RDBMS) and consists of three basic parts:
Static data (electric parameters, history of the load, state and network topology).
Slowly varying data (changes of switchgear statuses, control devices settings).
Periodically varying data (values of state variable — measurements, alarms).
DMS Server is implemented as an Object Linking and Embedding (OLE) for Process Control (OPC) server. It integrates a real-time OPC server, an historical OPC server and an ODBC (Open DataBase Connectivity) gateway for DataBase access. The DMS Server provides multi-user operation for a large number of users over the same set of real-time data or DataBase data; open architecture of the system that offers full connectivity with different SCADA systems and DataBases; enhanced support for real-time operation and fully automated control room operations.
User interfaces are the software tools necessary for the application of DMS Software. The most important user interfaces are:
Distribution Network Builder (DNB) is the main front-end application for DB editing. This editor enables efficient editing and overview of all data about the distribution network. It provides a multi-user and user-friendly, fully graphical environment for DB browsing and editing. DNB is capable of editing alphanumeric as well as graphic data — schematic and geographic schemes.
Dynamic Mimic Diagram (DMD) is a multi-user graphical (schematic and geographical) user interface for visualization of the distribution system, and efficient managing and monitoring of the distribution system state. It is a modern replacement for the old wall-diagram schemes. DMD clients are provided with views of distribution network, SCADA view of all substations and technical data on every element.
DMS Analytical Functions System is the most sophisticated component of the DMS. Its foundation is the mathematical model of the distribution network, Network Model. It transforms the distribution network elements (transformers, lines and switchgear) into scaled, normalized equivalent circuits (mathematical models of elements) and graphs. These graphs provide efficient processing of the topology, state and performances of the distribution network. Network Model provides the basic platform for integration and running of all other DMS analytical functions. DMS analytical functions are classified into four groups:
Preparatory analytical functions (Network Model, Topology Analyzer, Load Calibration, Load Forecasting).
Analytical functions (Load Flow, Fault Calculation, Reliability Analysis, State Estimation, Circuit Breakers/Fuses Capacity, Performance Indices, Motor Start).
Basic analytical functions (Under-load Switching, Voltage Control, Relay Protection, Supply Restoration, Network Reconfiguration, Capacitor Placement, Energy Losses, Volt/VAR Control, Switching Sequence Management, Security Assessment, Maintenance Scheduling, Load Management, RTU Placement, Network Reinforcement, Network Development).
Composite analytical functions (Operation Improvement, Fault Management, Network Scanner, Dispatcher Training Simulator).
DMS Software Application Benefits
Deregulated power industry customers now expect a reliable, high-quality supply of electrical energy from a high-quality distribution network. This means a network with a small number of faults and outages, good voltage profile and small power losses. A reliable, high-quality supply can be provided by extensive investments on expensive power equipment (transformers and lines) and with application of automation equipment (SCADA and computer-based DMS). However, as investment budgets are normally restricted, they are usually insufficient to satisfy customers' expectations in terms of network reliability (regulated utilities) or to provide the maximum profit (deregulated utilities). Therefore, when the optimal compromise between investment on power equipment and the investment into automation equipment is required, DMS often offers the best solution.
The benefits from the application of automation equipment and DMS in DCs include:
Network visualization. DMD provides clear and user-friendly manmachine interface for visualization of any size section of the distribution network (schematic and geographical), topology analyses with coloring of the network (energization, feeders area and voltage levels), location of any element of the network (find function) and clear insight in technical data of all elements of the network. Thus, DMD is an effective substitution for the “static” mimic boards usually wall-mounted in the DC control center.
Monitoring of the normal state and assessment of the faulted states of the network. DMD is used for presentation of the actual or any simulated state of the network (saved or forecasted cases enabled by state-estimation function). Voltages, currents and loads are calculated (Load Flow function) and can represent the state of the entire network or any parts of the network. Monitoring of the network includes alarms (violation of technical limits for voltages, currents and temperatures), assessment of the reliability, balance of loads on feeders and transformers, and calculation of power losses. Using the DMS Fault Calculation function provides optimal solutions for actual faults and the high-quality insight into potential and simulated faulted states (short-circuit calculations, thermal and mechanical stress calculations, as well as breaking capability of the circuit breakers).
Coordination of network automation equipment. DMS software provides an efficient use of the automation equipment, simple visualization and management of the network automation equipment (circuit breakers, reclosers, relays, fault detectors and transformer tap changer positions) and integration of different SCADA systems installed on the same distribution network. The DMS Relay Protection function provides an overview of relay locations, settings of relays, selectivity and sensitivity analysis, and relay setting.
Minimum of outage duration (minimum of non-supplied electric energy). DMS software offers an optimal and efficient action plan for the restoration of supply in the event of an outage, optimizing the switching operations and alternative source of supply and minimizing non-supplied electric energy (DMS functions Supply Restoration and Large Area Restoration).
Voltage quality and reactive power control. DMS provides optimal planning of the (re) installation and operation of Volt/VAR control devices (under-load and off-voltage tap changing transformers, fixed or switchable capacitor banks and reactors, voltage regulators, synchronous motors or generators). Application of the DMS functions Capacitor Placement and Volt/VAR Control achieves the following objectives:
Network voltages and currents are maintained within predetermined limits.
Reactive power demand from the transmission network is minimized.
Minimization of active power loss.
Optimization of the voltage profile (maintaining voltage levels compatible with the appliances connected to the network).
Dynamic control of total active power consumption for profit maximization of the DC, by changing of voltages inside limits.
Minimum power and energy losses. The power and energy losses for a day, week, season or year are analyzed and minimized by the DMS. In addition to the application of the Volt/VAR Control function, the power and energy losses could be minimized by application of Optimal Reconfiguration function.
Minimum damage caused by incorrect switching operations. The DMS enables an efficient study and “what/if” analysis regarding switching operations on the distribution network. Users can simulate the effects and consequences of each switching operation in advance and determine the optimum solution, thereby minimizing the damage due to incorrect switching operations. (Use of the Under-Load Switching function can solve these situations).
Load management enables control of the peak load on the distribution network, minimizing the demand cost from the purchase side and increasing the DC's profit.
Postponement of the investments. Investments in expensive power equipment and facilities can be postponed by DMS software application. For example, after a supply transformer fault, its load is transferred to the next transformer in the same substation or to alternative source substations by MV feeders. If insufficient capacity exists, a new transformer has to be installed. This expensive solution can be avoided if a sufficient number of low-cost remotely controlled tie-switches are installed in selected locations on the MV network (connections to alternate-source substations). They can be used for a fast, remote-controlled restoration of the supply (Fault Management DMS function) and for other DMS applications.
Development planning of distribution networks. The user can easily simulate and evaluate any development scenario from a set of pre-specified development scenarios of the considered distribution network. These scenarios could consist of simple network reinforcements (MV section with a distribution transformer and corresponding LV network) or significantly large power installations (supply substation with several feeders and many transformers). The installation of any new network element can be easily simulated by “building” it in the DMD of the existing network. Load forecast for a specified time horizon is then calculated. Planning engineers can easily perform all steady-state calculations for selected time cuts (running the corresponding DMS power functions in study mode for forecasted states). This gives the opportunity to evaluate and rank scenarios both technically and economically; using cost/benefit analysis enables the planning engineer to select the optimal solution.
Training simulator. DMS software offers high-quality training methods in the management and use of the distribution network.
Application of DMS Software
Figure 2 shows the schematic DMD of distribution network of Pozarevac together with results of ONR application — yellow-tagged switches denoting the network's optimal open-tie switch locations. Currently open switches, shown with a white square or circle, should remain open if they have a yellow mark, otherwise they should be closed. Currently closed switches, shown with a dark square or circle, should be open if they have a yellow mark, otherwise they should stay closed.
The optimal switching scenario is listed and, normally, only 10% to 20% of switching points should be changed — low-cost operations that could be completed within a day. The average reduction in network power loss is 10 to 20%, which represents a 0.5% reduction in the network's annual energy consumption. Because the cost of network switching is negligible, the 0.5% reduction in the DC's annual energy purchase cost can be regarded as a net increase in the utility's profit.
The voltage control in distribution networks is established as an optimal control of voltages and feeder losses, centralized in the DC Control Center. OVC application improves voltage profile and increases consumption, with a reduction in network losses. The on-load tap changers on supply transformers are continuously remote controlled or seasonally controlled with manual settings of optimal “control flow.” The off-load tap changers on distribution transformers are set seasonally at optimal positions. The annual DC's profit is evaluated up to 2% of annual power purchase cost.
Hence, the total value of annual profit of ONR and OVC application is around 2.5% of DC's annual cost of energy purchases. However, in practice, the total benefits are higher than evaluated due to indeterminate savings (for example, postponement of investments and reduction of operational costs). Therefore, the total benefit of DMS software application and profit increase is estimated at up to 5% of DC's annual cost of energy purchases.
Investments in DMS software tools are relatively modest and include standard PC computers, software and installation costs, training and integration interfaces with SCADA and DB. They are estimated to be $1000/MW of network peak load or around 0.5% of the value of DC's annual cost of energy purchase. The balance between investment and profit of DMS software application is presented in Fig. 3.
The business benefit of DMS software application could be high, with a payback period of less than one year and an increase in profit up to 10 times greater than the investment. In the competitive environment, the revenue increase will be the critical issue for DC. Every investment opportunity offering this level of return is likely to be approved.
DMS software is a powerful set of dynamic mimic diagrams (manmachine interface); state estimation and load flow tools; topology analysis and incident handling tools; optimization algorithms; and tools for engineering analyses. DMS software is used for visualization, monitoring and control (remote or manual) of distribution networks, in addition to different operation and optimization analyses. The software is designed to reduce costs (workforce and maintenance optimization, postpone investments, power losses reduction) and increase revenues (less interruptions and undelivered electric energy, more efficient use of facilities and increasing consumption). A relatively modest investment in a DMS application can be a profitable and cost-effective alternative to large investments in distribution network construction and development.
Miodrag Ristic graduated in 1986 and then worked as a protection engineer in Kostolac Thermal Power Plant for three years. He joined the electric power distribution company Elektromorava Pozarevac in 1989, where he served as engineer for protection, planning and development. Currently, he is a deputy manager of company with responsibilities in analysis and control of the distribution network.