Michael Scholl, [email protected] (A paper written under the guidance of Prof. Raj Jain) DownloadAbstract:High availability is one of the major goals of smart grid systems. This paper examines the availability of wind turbines, a high voltage DC (HVDC)transmission system, and a supervisory control and data acquisition (SCADA)/outage management system (OMS)/distribution management system(DMS) control system as examples of electricity generation, transmission, and control systems in a smart grid. It also examines the sensitivity of eachsystem to improvements in component availability in order to determine where to focus availability improvements. The results show thatimprovements in supplier software on the front end protocol (FEP) of the control system and better backup sites for the control system provide thelargest increases in the availability of the entire system.Keywords: Smart Grid, Performance Analysis, Availability, Sensitivity AnalysisTable of Contents:1. Introduction2. Availability3. Wind Power Plant3.1 Parts of a Wind Turbine3.2 Availability Data3.3 Sensitivity Analysis4. HVDC Transmission System4.1 Parts of a HVDC Transmission System4.2 Availability Data4.3 Sensitivity Analysis5. Control System5.1 Parts of a SCADA/OMS/DMS Control System5.2 Availability Data5.3 Sensitivity Analysis6. Conclusion7. Conclusion7. Acronyms1. IntroductionAs technological advances are made there has been a push to update the electrical grid to take advantage of new technologies. Such a technologicallyadvanced electrical grid is often called a "smart grid" and is often more efficient, more secure, and more reliable. These smart grids, however, havemore components, all of which are capable of breaking down and causing the grid to go dark. One of the ways to analyze how often this happens is tolook at the availability of the system.The electrical grid is often considered to have four distinct operations: electricity generation, electric power transmission, electricity distribution, andelectricity control. There have been studies on the availability of smart grid generation [Ribrant07], transmission [Zadkhast10], and control[Jensen10], but there has not been a study on the sensitivity of the systems to improvements in the availability of different components. This paperexamines the data collected in these previous studies in order to determine which components to focus on in order to improve the overall availabilityof the smart grid.The grid examined in this paper is based on Figure 1. This model uses wind turbines as the generators, High Voltage DC (HVDC) transmission systemsfor power transmission, and a supervisory control and data acquisition (SCADA)/outage management system (OMS)/distribution management system(DMS) control system. The supplier software on the front end protocol (FEP) of the control system and better backup sites for the control system areshown to have the largest improvements on the availability of the smart grid.Availability and Sensitivity Analysis of Smart Grid Components http://www1.cse.wustl.edu/~jain/cse567-11/ftp/grid/index.html1 of 10 5/4/2011 4:48 PMFigure 1: Model of a Smart Grid System2. AvailabilityAvailability is important because it provides a metric for identifying the likelihood that a system is operational. This is done by comparing how long ittakes a component to break down to how long it takes to fix the component. If the time it takes a component to break is considered to be anexponentially distributed function it can be expressed as:In this case the average time for a component to fail is called the mean time to failure (MTTF) and is equal to 1/labmda. Similarly, the time it takes tofix a component can be modeled as an exponentially distributed function with the parameter µ. The average time it takes to fix a component is calledthe mean time to repair (MTTR) and is equal to 1/µ. The availability of the system, therefore, is defined as:For a system with components in series the overall availability is simply the product of each component‘s availability:Most systems, however, consist of a series of systems, each of which has ki redundant components that are in parallel. The overall availability of sucha system is 1 minus the product of 1 minus the availability raised to k for each subsystem [Rausand04]:3. Wind Power PlantAvailability and Sensitivity Analysis of Smart Grid Components http://www1.cse.wustl.edu/~jain/cse567-11/ftp/grid/index.html2 of 10 5/4/2011 4:48 PMOne of the benefits of having an intelligent electrical grid is the ability to incorporate renewable energy sources, such as wind power, which do notproduce a constant amount of power. Wind power is harnessed by wind turbines which spin when the wind blows, turning a shaft connected to agenerator, which converts the mechanical energy to electrical energy.3.1 Parts of a Wind Power PlantFor the purposes of collecting failure information, wind turbines were split into 12 parts as shown in Figure 2: the electrical system, sensors, the bladesand pitch system, hydraulic systems, the control system, the gearbox, the yaw system, the generator, the structure, mechanical brakes, the main shaftand bearings (drive train), and the hub [Besnard10]. A 13th category, the entire system, was added to account for other failures. If any of thesesystems fails then the whole turbine is considered to have failed.Figure 2: Model of a Wind Turbine3.2 Availability DataThe data collected in [Ribrant07] is compiled in Table 1. Because the parts are connected in series, the overall availability is the product of theindividual component availabilities. This comes out to 99.40%.Table 1: Collected Data and Calculated Availabilities of Wind Turbine ComponentsAvailability and Sensitivity Analysis of Smart Grid Components http://www1.cse.wustl.edu/~jain/cse567-11/ftp/grid/index.html3 of 10 5/4/2011 4:48 PM3.3 Sensitivity AnalysisIn order to determine which component improves the overall availability the most, a sensitivity analysis is performed. This is done by looking at howthe availability of the overall system changes when one component‘s availability is changed to 100%. For the wind turbine this analysis produces theresults in Table 2.Table 2: Sensitivity Analysis of Wind Turbine AvailabilityFrom this data it is shown that improving the availability of the gearbox has the largest effect on the availability of the wind turbine as a whole. This isexpected because the wind turbine is a collection of components connected in
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