Energy and reliability benefits of wind energy conversion systems
Introduction
Wind power is considered to be a very promising and encouraging alternative for power generation due to its tremendous environmental and social benefits, together with public support and government incentives. The utilization of a large amount of energy from the wind is being considered by many utilities. In North America, many US states and Canadian provinces have agreed to generate between 5% and 25% of their electrical power by 2010–2015 from renewable energy resources, most of which will come from wind [1].
Wind power additions to electric power systems provide a range of benefits including electrical energy production and reliability improvements. The electrical power output from a wind energy conversion system (WECS) at a specific site depends on many factors [2], [3], [4], [5], [6], [7], including the wind regime at the site, and the characteristics of the wind turbine generator (WTG), such as the cut-in, rated and cut-out wind speed parameters. It is important to select a matching WTG for a specific wind site from the candidate WTG in order to obtain maximum energy and reliability benefits. The power generation of a WECS does not vary linearly with the wind speed due to the nonlinear relationship between the WTG parameters and the wind speeds. It is therefore important to consider a site-matching WTG from the candidate WTG.
References [2], [3], [4], [5], [6], [7] illustrate some of the impacts of the above factors on the power system reliability. Case studies in references [2], [3], [4], [5] show that the WTG cut-in and rated wind speeds can have a significant effect on the reliability of a power system and the cut-out wind speed has almost no effect. The power system reliability indices of Loss of Load Expectation (LOLE) and Loss of Energy Expectation (LOEE) decreases somewhat exponentially with the number of WTG units added to the system, but tend to saturate when wind speeds continue to increase. The forced outage rate of the WTG has relatively little impact on the reliability performance of a power system [5], [6]. Reference [7] indicates that wind speed correlation between two wind regimes can have a considerable impact on the system reliability. There has, however, been relatively little research on the impacts of the above factors on the electrical energy production of a WECS.
Reference [3] introduces two risk-based indices designated as the load carrying capacity benefit ratio (LCCBR) and the equivalent capacity ratio (ECR) to determine the optimum site-matching WTG for a specific wind site. The optimum WTG can be determined through a comparison of the LCCBR and ECR indices. The process, however, does not include electrical energy production. The derated adjusted forced outage rate (DAFOR) [8] of a WECS can be evaluated from the multi-state generating unit output model [9]. This paper presents an index of ECR to determine the electrical energy production, reliability benefit of a WECS and the degree of wind site matching with a WTG.
Section snippets
Wind speed model
Analysis of electrical energy production and reliability benefits require an accurate wind speed simulating technique for each specific wind site. Many methods, such as auto-regressive and moving average (ARMA) models [10], require historical wind speed data collected over a number years to determine the necessary parameters of the wind speed model for a specific site. The wind speed time series can usually be modeled by many distributions, including Weibull distribution and normal distribution
Generation power model
There is a nonlinear relationship between the power output of a WTG and the wind speed [6], [11]. A WTG is designed to start generating at the cut-in speed Vci and is shut down for safety reasons if the wind velocity is higher than the cut-out speed Vco. In both cases, the power output is zero. The power output of a WTG unit increases with the wind speed between the cut-in speed and the rated speed Vr; after that the power output remains constant at the rated power Pr. Fig. 1 shows a typical
Equivalent capacity model for a WTG
The power output P(s) of a WTG is related to the wind speed s, which is described by a ND. There is only one power output P(s) corresponding to a specified wind speed s. Hence, the probability density function f(s) of the wind speed can be directly linked to the density function of the power output of a WTG at a specific wind site due to the same wind speed parameter s.
The equivalent capacity of a WTG denoted as EC can therefore be calculated using the definition of expectation for a random
Accuracy analysis of the proposed model
The wind speed time series at Lauwersoog wind site in 2001–2007 [13] were used to verify the calculation accuracy of the proposed model. The mean and standard deviation of the statistical wind speeds are 6.23 and 3.05 m/s, respectively. A WTG with a rated power output of 1 MW is assumed to be located at a site with the Lauwersoog wind regime. The cut-in, rated and cut-out wind speeds for the WTG are 3, 15 and 22 m/s, respectively.
Annual energy production using the proposed ND, Weibull distribution
Conclusion
The contributions in terms of energy production and reliability benefits of including wind energy conversion systems (WECS) in power systems are dependent on a wide range of factors including the wind speed characteristics and the WTG design parameters, such as the cut-in, rated and cut-out wind speed. The degree of the wind site matching with the WTG has a significant impact on the electrical energy produced by a WECS and the power system reliability benefits.
The equivalent capacity ratio
Acknowledgements
This work was supported in part by the National Natural Science Foundation of China (No. 51077135), Natural Science Foundation Project of CQ CSTC (No. CSTC2010BA3006) and Scientific Research Foundation of Stage Key Lab. of Power Transmission Equipment and System Security (No. 2007DA10512709103).
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