Real-time power management is a plant manager's secret weapon
Sequence of event playback
Recovering from a system disturbance depends on the time it takes to establish the cause of the problem and take remedial action. This requires a fast and complete review and analysis of the sequence of events prior to the disturbance.
Power management software should assist one's operational and engineering staff to quickly identify the cause of an operating problem and determine where energy costs can be reduced. The software should also be able to reconstruct exact system conditions to check for operator actions and probe for alternative actions.
Besides reducing losses and improving data-gathering capability, an application such as this should assist in increased plant reliability and help control costs. The event playback feature is especially useful for root cause and effect investigations, improvement of system operations, exploration of alternative actions, and replay of hypothetical scenarios.
An event-playback capability can translate into savings. The savings for a typical 50 MW plant is illustrated in Figure 2. A conservative estimate of a 10% reduction in downtime for an outage that lasts an hour, for example, would yield an estimated $33,000 in savings.
An advanced power management system should provide options for full remote control to system elements such as motors, generators, breakers, load tap changers, and other protection devices.
In addition, the software should provide user-definable actions that can be added or superimposed on the existing system for automating system control. This is similar to adding PC-based processors and/or controllers or simple breaker interlocks to any part of the system.
Supervisory and advisory controls
State-of-the-art supervisory and advisory control capabilities should be used to control and optimize real-time parameters throughout the system. Through using optimization algorithms, the user could program the power management system. For energy producers in particular, this type of energy management system could minimize generation fuel costs, and optimize system operation.
Demand response or management
Another significant cost component of operations is the demand charge of an energy bill. The demand charge is 40% to 60% of the bill for sites without peak shaving generation. A single unmanaged demand charge can produce a very large hike in the power bill each month, and with "ratcheting" demand charges, the effect can linger.
An intelligent power management system could provide the current and predicted demand for each day, thus managing peak demands on a continuous basis. Loads can be shed manually or automatically, peak-shaving generators can be started, and load startup can be postponed or sequenced.
In a study performed for a large industrial facility (150MVA), advanced optimization algorithms native to the energy management system were used to reduce real and reactive power losses. Assuming a conservative power loss reduction of only 0.1% at an average electrical energy cost of $0.13/kWh, an energy management system would yield savings of more than $135,000 per year and would essentially pay for itself through the immediate realization of savings in operating and maintenance costs.
Intelligent proactive load shedding
A major disturbance in an electrical power system may result in certain areas becoming isolated and experiencing low frequency and voltage, which can result in an unstable operation. A model-based power management system can easily determine the electrical subsystems, including islanding detection. The system could then provide optimal "fast load shedding" based on the actual operating condition of the system including type and location of the disturbances.
A response time of less than 40ms can be achieved through high-speed load shedding. The longer it takes to shed a load during a disturbance, the more a load must ultimately be shed. Through adding intelligence and proactive calculations, fast response times can be achieved. Additionally, through speeding up the load shedding process, the actual amount of load that is shed will be far less than that of using the conventional methods such as frequency relay and PLC-based.
Power management systems should have the intelligence to initiate load shedding based on a user-defined Load Priority Table (LPT) and a pre-constructed Stability Knowledge Base (SKB) in response to an electrical or mechanical disturbance. Load shedding schemes through conventional frequency relays are generally a static control with fixed frequency settings. But a power management system would be able to adapt to all real-time situations and provide a true dynamic load shedding control.
The Intelligent Load Shedding built into a model-based solution also offers logic verification. Logic verification is a necessary step during a Factory Acceptance Test (FAT) as well as Site Acceptance Test (SAT). It is performed during system commissioning. With an integrated model-based solution, it is easy to capture operating data and evaluate results of Intelligent Load Shedding using dynamic stability.
An Intelligent Load Shedding component also offers an analytical and instantaneous visible confirmation that the load shedding logic will perform as expected. Every scenario can be checked by using the live system data and a transient stability program. Since the model may be expanded to accommodate additional substations or manufacturing trains, the load shedding model can be updated in the process. The entire system can be retested in a fraction of the time it would take for a conventional hardware-based system (See figure 2).
Real-time power management
Through extending the power monitoring system by equipping it with an appropriate electrical system context, simulation modules, and playback routines, the system operator and engineer will have a powerful new set of tools. These tools can help a user accurately predict the behavior of an electrical system. A standard power management system, on the other hand, evaluates collected data in a non-electrical system environment without recognizing the interdependencies of equipment.
The playback of recorded message logs into the simulator-equipped monitoring system provides the operator with an invaluable means of exploring the effects of alternative actions during historical events. Simulation techniques readily extend into power system control and can be used to perform system automation and load shedding functions.
Each of these capabilities should be included in one application. For a plant to upgrade and grow, flexibility and compatibility are essential. Real-time power management is a plant manager's secret weapon in assuring reliable plant operations.
Shervin Shokooh is a senior principal electrical engineer and Tanuj Khandelwal is a principal electrical engineer for ETAP.