Distribution Systems: DMS, Volt/VAr control

Distribution Management System (DMS)

A DMS is a collection of applications designed to monitor & control the entire distribution network 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 outages, minimizing outage time, maintaining acceptable frequency and voltage levels are the key deliverables of a DMS. In order to support proper decision making and O&M activities, DMS shall have to support the following functions:

 

1) 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. 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 (CB), Ring Main Unit(RMU) and/or isolators that affect the topology of the network modelled, 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.

(2) State Estimation(SE)

Power system state estimation is a process whereby telemetered data from network measuring points to a central computer, can be formed into a set of reliable data for control and recording purposes. It allow the calculation of margins to operating limits, health of equipments and required operator actions with high confidence despite of measurements that are corrupted by noise or could be missing or inaccurate. In power networks, state estimation takes observable data from the field and derives a model of what is actually happening, by processing the data to identify bad readings or to estimate missing data. The quality of data telemetered from various points on the distribution network is typically imperfect. Problems in devices or in the telecommunications networks associated with those devices suggest that prior to conducting an analysis, the data must be pre-processed to eliminate bad data points, estimate non-telemetered points, and resolve any issues with “time skew” for unsynchronized telemetry systems.

(3) Load Flow Applications (LFA)

Load flow study is an important tool involving numerical method analysis applied to a power system. 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. 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. 

(4) Volt/ VAR Control (VVC)

Volt/VAR Control or VVC refers to the process of managing voltage levels and reactive power (VAR) throughout the power distribution systems. There could be loads that contain reactive components like capacitors and inductors (such as electric motors) that put additional strain on the grid. This is because the reactive portion of these loads causes them to draw more current than an otherwise comparable resistive load would draw. The erratic current results in both over-voltage/under-voltage violations as well as heating up of equipments like transformers, conductors, etc which might even need resizing to carry the total current. A power system needs to control it by scheming the production, absorption and flow of reactive power at all levels in the system. 

A VVC application shall help the operator to mitigate such conditions by suggesting required action plans. The plan will give the required tap position and capacitor switching to ensure the voltage to its limit and thus optimize Volt Var control function for the utility.

(5) Load Shed Application(LSA)

Power system by its characteristics have long stretches of transmission line and multiple injection points, hence instabilities which lead to critical failure or unpredicted system conditions are unavoidable. The instabilities usually arise from power system oscillations generated due to faults, peak deficit or protection failures. Distribution load shedding & restoration schemes plays a vital role in emergency operation & control in any utility. It detects the emergency situation and performs a predefined sets of control actions, like opening, closing of non critical feeders, reconfigure the downstream or sources of injections, or performs a  tap control at transformer. Usually distribution network is complex and covers larger area, the emergency actions taken at downstream reduces lots of burden on upstream network. In a non-automated system, system awareness and operators ability to respond to the situation plays key role in mitigation. If the decisions are not fast enough, the problem can grow exponentially and causes major catastrophic failure. 

DMS needs to provide a modular automated load shedding & restoration application which automates emergency operation & control requirements for any utility. The application should cover various activities like Under Frequency Load Shedding (UFLS), limit violation and time of day based load shedding schemes which are usually performed by the operator.  

(6) Fault Management & System Restoration (FMSR)

Reliability and quality of power supply are key parameters which need to be ensured by any utility. Reduced outage time duration to customer, shall improve over all utility reliability indices hence FMSR or automated switching applications plays an important role. The two main features required by a FMSR are: Switching management & Suggested switching plan

The DMS application receives faults information from the SCADA system and processes the same for identification of faults and on running switching management application; the results are converted to action plans by the applications. The action plan includes switching ON/OFF the automatic load break switches / RMUs/Sectionalizer .The action plan can be verified in study mode provided by the functionality .The switching management can be manual/automatic based on the configuration.

(7) Load Balancing via Feeder Reconfiguration (LBFR)

Load balancing via feeder reconfiguration is an essential application for utilities where they have multiple feeders feeding a load congested area. To balance the loads on a network, the operator re-roots the loads to other parts of the network. A Feeder Load Management (FLM) is necessary to allow you to manage energy delivery in the electric distribution system and identify problem areas. A Feeder Load Management monitors the vital signs of the distribution system and identifies areas of concern so that the distribution operator is forewarned and can efficiently focus attention where it is most needed. It allows for more rapid correction of existing problems and enables possibilities for problem avoidance, leading to both improved reliability and energy delivery performance.  

On a similar note, Feeder Reconfiguration is also used for loss minimization. Due to several network and operational constraints utility network may be operated to its maximum capability without knowing its consequences of losses occurring. The overall energy losses and revenue losses due to these operations shall be minimized for effective operation. The DMS application utilizes switching management application for this, the losses minimization problem is solved by the optimal power flow algorithm and switching plans are created similar to above function

(8) Distribution Load Forecasting

Distribution Load Forecasting (DLF) provides a structured interface for creating, managing and analyzing load forecasts. It should be designed to facilitate both “top-down” and “bottom-up” forecasting methodologies in the same environment without placing any restrictions on the types of models available and should support short-term, medium-term, as well as, long-term forecasting.

DLF provides data aggregation and forecasting capabilities that is configured to address today’s requirements and adapt to address future requirements and should have the capability to produce repeatable and accurate forecasts. 

 

 

 

 

 

 

 

Volt/VAR control

The concept of Voltage/VAR control is essential electrical utilities’ ability to deliver power within appropriate voltage limits ( plus or minus 5% of nominal voltage level ) so that consumers’ equipment operates properly, and to deliver power at an optimal power factor to minimize losses. Voltage regulation and VAR regulation are often referenced in combination (i.e. Volt/VAR control), they are perhaps easier to understand if described as two separate, but interrelated concepts.

Voltage Regulation. Feeder voltage regulation refers to the management of voltages on a feeder with varying load conditions. Electric utilities traditionally maintain distribution system voltage within the acceptable range using transformers with moveable taps that permit voltage adjustments under load. Voltage regulators located in substations and out on the lines, and substation transformers with Tap Changing Under Load are commonly used for voltage control purposes (Load Tap Changer or LTC). These transformers are equipped with a voltage regulating controller that determines whether to raise or lower the transformer tap settings or leave the tap setting unchanged based on “local” voltage and load measurements.

The optimal strategy for distribution feeder design and operation is to establish acceptable voltage conditions for all customers while being as efficient as possible. The voltage profile along the distribution feeder and the flow of reactive power (VARs) on the feeder are typically maintained by a combination of voltage regulators and switched capacitor banks installed at various locations on the feeder and in its associated substation. Each voltage regulator includes a controller that raises or lowers the voltage regulator tap position in response to local (at the device) current and voltage measurements. Similarly, each capacitor bank includes a controller that switches the bank on or off in response to its local measurements.

VAR Regulation. Nearly all power system loads require a combination of real power (watts) and reactive power (VARs). Real power must be supplied by a remote generator while reactive power can be supplied either by a remote generator or a local VAR supply, such as a capacitor. Delivery of reactive power from a remote VAR supply results in additional feeder voltage drop and losses due to increased current flow, so utilities prefer to deliver reactive power from a local source. Since demand for reactive power is higher during heavy load conditions than light load conditions, VAR supply on a distribution feeder is regulated or controlled by switching capacitors on during periods of high demand and off during periods of low demand.

Volt/VAR Regulation. Supplying VARs when and where demanded is inherent to operating an electric power system. But the flow of reactive power affects power system voltages just as the flow of real power does. The effects of real power flow nearly always have negative effects on voltage while the effects of reactive power flows are sometimes positive and sometimes negative. Experience has proven that overall costs and performance of operating a power system can be best managed if voltage control and reactive power control are well integrated.

Traditional Volt-VAR Control

Traditionally, feeder voltage regulators, Substation transformer load tap changers (LTCs) and switched capacitor banks are operated as completely independent (stand-alone) devices, with no direct coordination between the individual controllers. This time-honoured approach is effective for maintaining acceptable voltage and reactive power flow in the vicinity of the controllers, but typically does not produce optimal results for the entire feeder.

 

However, this approach has several key limitations:

• The system is not continuously monitored, so controller failures and malfunctions are not automatically detected. Line capacitors are particularly failure-prone. Without continuous monitoring, these devices may be switching on and off at the wrong time, or may be totally inoperative due to a blown fuse. This condition can go undetected until the problem deteriorates into a more serious and potentially unsafe problem.
• The system lacks flexibility to respond to changing conditions on the distribution feeders. Controller settings work well under normal circumstances. However, if the feeder is reconfigured for any reason (for example, while a faulted portion of the feeder is being repaired), the controller settings may not produce the desired results.
• The system cannot be used to respond to system emergencies. Occasionally, distribution utilities are called upon to place all switched capacitors in service as rapidly as possible to respond to power grid emergencies. Since the stand-alone controllers lack remote control capabilities, it is not possible to rapidly switch all capacitor banks on demand.

 

Integrated Volt VAR Control

Integrated Volt VAR Control (IVVC) is an advanced function that determines the best set of control actions for all voltage regulating devices and VAR control devices to achieve a one or more specified operating objectives without violating any of the fundamental operating constraints (high/low voltage limits, load limits, etc.). IVVC achieves following utility-specified objective functions:

• Minimize distribution system power loss
• Minimize power demand (sum of distribution power loss and customer demand)
• Maximize revenue (the difference between energy sales and energy prime cost)
• Weighted combination of the above

It is also possible to bias the recommended control actions to minimize the number of operations for specified load tap changers, regulators or capacitor banks that are nearing end of life or end of maintenance cycles.

 

IVVC uses an on-line power flow (OLPF) function and available real time measurements to compute the conditions that exist at any point on the feeder, total electrical losses, and other parameters that are not practical to monitor directly. The OLPF results are delivered to an “optimizing engine”, which is software designed to determine the correct set of control actions to achieve “optimal” conditions required by the electric utility. These control actions are then sent to the proper device controllers via SCADA.