Distribution management system
A distribution management system is a collection of applications designed to monitor and control the electric power distribution networks efficiently and reliably. It acts as a decision support system to assist the control room and field operating personnel with the monitoring and control of the electric distribution system. Improving the reliability and quality of service in terms of reducing power outages, minimizing outage time, maintaining acceptable frequency and voltage levels are the key deliverables of a DMS. Given the complexity of distribution grids, such systems may involve communication and coordination across multiple components. For example, the control of active loads may require a complex chain of communication through different components as described in
In recent years, utilization of electrical energy increased exponentially and customer requirement and quality definitions of power were changed enormously. As electric energy became an essential part of daily life, its optimal usage and reliability became important. Real-time network view and dynamic decisions have become instrumental for optimizing resources and managing demands, leading to the need for distribution management systems in large-scale electrical networks.
Overview
Most distribution utilities have been comprehensively using IT solutions through their Outage Management System that makes use of other systems like Customer Information System, Geographical Information System and Interactive Voice Response System. An outage management system has a network component/connectivity model of the distribution system. By combining the locations of outage calls from customers with knowledge of the locations of the protection devices on the network, a rule engine is used to predict the locations of outages. Based on this, restoration activities are charted out and the crew is dispatched for the same.In parallel with this, distribution utilities began to roll out Supervisory Control and Data Acquisition systems, initially only at their higher voltage substations. Over time, use of SCADA has progressively extended downwards to sites at lower voltage levels.
DMSs access real-time data and provide all information on a single console at the control centre in an integrated manner.
Their development varied across different geographic territories. In the US, for example, DMSs typically grew by taking
Outage Management Systems to the next level, automating the complete sequences and providing an end to end, integrated view of the entire distribution spectrum. In the UK, by contrast, the much denser and more meshed network topologies, combined with stronger Health & Safety regulation, had led to early centralisation of high-voltage switching operations, initially using paper records and schematic diagrams printed onto large wallboards which were 'dressed' with magnetic symbols to show the current running states. There, DMSs grew initially from SCADA systems as these were expanded to allow these centralised control and safety management procedures to be managed electronically. These DMSs required even more detailed component/connectivity models and schematics than those needed by early OMSs as every possible isolation and earthing point on the networks had to be included. In territories such as the UK, therefore, the network component/connectivity models were usually developed in the DMS first, whereas in the USA these were generally built in the GIS.
The typical data flow in a DMS has the SCADA system, the Information Storage & Retrieval system, Communication Servers, Front-End Processors & Field Remote Terminal Units.
Why DMS?
- Reduce the duration of outages
- Improve the speed and accuracy of outage predictions.
- Reduce crew patrol and drive times through improved outage locating.
- Improve the operational efficiency
- Determine the crew resources necessary to achieve restoration objectives.
- Effectively utilize resources between operating regions.
- Determine when best to schedule mutual aid crews.
- Increased customer satisfaction
- A DMS incorporates IVR and other mobile technologies, through which there is an improved outage communications for customer calls.
- Provide customers with more accurate estimated restoration times.
- Improve service reliability by tracking all customers affected by an outage, determining electrical configurations of every device on every feeder, and compiling details about each restoration process.
DMS Functions
- Network visualization & support tools
- Applications for Analytical & Remedial Action
- Utility Planning Tools
- System Protection Schemes
Network Connectivity Analysis (NCA)
Distribution network usually covers over a large area and catering power to different customers at different voltage levels. So locating required sources and loads on a larger GIS/Operator interface is often very difficult. Panning & zooming provided with normal SCADA system GUI does not cover the exact operational requirement. Network connectivity analysis is an operator specific functionality which helps the operator to identify or locate the preferred network or component very easily. NCA does the required analyses and provides display of the feed point of various network loads. Based on the status of all the switching devices such as circuit breaker, Ring Main Unit and/or isolators that affect the topology of the network modeled, the prevailing network topology is determined. The NCA further assists the operator to know operating state of the distribution network indicating radial mode, loops and parallels in the network.Switching Schedule & Safety Management
In territories such as the UK a core function of a DMS has always been to support safe switching and work on the networks. Control engineers prepare switching schedules to isolate and make safe a section of network before work is carried out, and the DMS validates these schedules using its network model. Switching schedules can combine telecontrolled and manual switching operations. When the required section has been made safe, the DMS allows a Permit To Work document to be issued. After its cancellation when the work has been finished, the switching schedule then facilitates restoration of the normal running arrangements. Switching components can also be tagged to reflect any Operational Restrictions that are in force.The network component/connectivity model, and associated diagrams, must always be kept absolutely up to date. The switching schedule facility therefore also allows 'patches' to the network model to be applied to the live version at the appropriate stage of the jobs. The term 'patch' is derived from the method previously used to maintain the wallboard diagrams.
State Estimation (SE)
The state estimator is an integral part of the overall monitoring and control systems for transmission networks. It is mainly aimed at providing a reliable estimate of the system voltages. This information from the state estimator flows to control centers and database servers across the network. The variables of interest are indicative of parameters like margins to operating limits, health of equipment and required operator action. State estimators allow the calculation of these variables of interest with high confidence despite the facts that the measurements may be corrupted by noise, or could be missing or inaccurate.Even though we may not be able to directly observe the state, it can be inferred from a scan of measurements which are assumed to be synchronized. The algorithms need to allow for the fact that presence of noise might skew the measurements. In a typical power system, the State is quasi-static. The time constants are sufficiently fast so that system dynamics decay away quickly. The system appears to be progressing through a sequence of static states that are driven by various parameters like changes in load profile. The inputs of the state estimator can be given to various applications like Load Flow Analysis, Contingency Analysis, and other applications.
Load Flow Applications (LFA)
Load flow study is an important tool involving numerical analysis applied to a power system. The load flow study usually uses simplified notations like a single-line diagram and focuses on various forms of AC power rather than voltage and current. It analyzes the power systems in normal steady-state operation. The goal of a power flow study is to obtain complete voltage angle and magnitude information for each bus in a power system for specified load and generator real power and voltage conditions. Once this information is known, real and reactive power flow on each branch as well as generator reactive power output can be analytically determined.Due to the nonlinear nature of this problem, numerical methods are employed to obtain a solution that is within an acceptable tolerance. The load model needs to automatically calculate loads to match telemeter or forecasted feeder currents. It utilises customer type, load profiles and other information to properly distribute the load to each individual distribution transformer. Load-flow or Power flow studies are important for planning future expansion of power systems as well as in determining the best operation of existing systems.
Volt-VAR Control (VVC)
Volt-VAR Control or VVC refers to the process of managing voltage levels and reactive power throughout the power distribution systems. These two quantities are related, because as reactive power flows over an inductive line that line sees a voltage drop. VVC encompasses devices that purposely inject reactive power into the grid to alter the size of that voltage drop, in addition to equipment that more directly controls voltage.In the legacy grid, there are three primary tools for carrying out voltage management: Load Tap Changers, voltage regulators, and capacitor banks. LTCs and voltage regulators refer to transformers with variable turns ratios that are placed at strategic points in a network and adjusted to raise or lower voltage as is necessary. Capacitor banks manage voltage by “generating” reactive power, and have thus far been the primary tools through which true Volt/VAR control is carried out. These large capacitors are connected to the grid in shunt configuration through switches which, when closed, allow the capacitors to generate VARs and boost voltage at the point of connection. In the future, further VVC might be carried out by smart inverters and other distributed generation resources, which can also inject reactive power into a distribution network. A VVC application helps the operator mitigate dangerously low or high voltage conditions by suggesting required action plans for all VVC equipment. The plan will give a required tap position and capacitor switching state to ensure the voltage stays close to its nominal value and thus optimize Volt-VAR control function for the utility.
Beyond maintaining a stable voltage profile, VVC has potential benefits for the ampacity of power lines. There could be loads that contain reactive components like capacitors and inductors that strain the grid. This is because the reactive portion of these loads causes them to draw more current than an otherwise comparable, purely resistive load would draw. The extra current can result in heating up of equipment like transformers, conductors, etc. which might then need resizing to carry the total current. An ideal power system needs to control current flow by carefully planning the production, absorption and flow of reactive power at all levels in the system.