594 research outputs found
Dynamics and stability of wind turbine generators
Synchronous and induction generators are considered. A comparison is made between wind turbines, steam, and hydro units. The unusual phenomena associated with wind turbines are emphasized. The general control requirements are discussed, as well as various schemes for torsional damping such as speed sensitive stabilizer and blade pitch control. Integration between adjacent wind turbines in a wind farm is also considered
MOD-2 wind turbine farm stability study
The dynamics of single and multiple 2.5 ME, Boeing MOD-2 wind turbine generators (WTGs) connected to utility power systems were investigated. The analysis was based on digital simulation. Both time response and frequency response methods were used. The dynamics of this type of WTG are characterized by two torsional modes, a low frequency 'shaft' mode below 1 Hz and an 'electrical' mode at 3-5 Hz. High turbine inertia and low torsional stiffness between turbine and generator are inherent features. Turbine control is based on electrical power, not turbine speed as in conventional utility turbine generators. Multi-machine dynamics differ very little from single machine dynamics
Control of large wind turbine generators connected to utility networks
This is an investigation of the control requirements for variable pitch wind turbine generators connected to electric power systems. The requirements include operation in very small as well as very large power systems. Control systems are developed for wind turbines with synchronous, induction, and doubly fed generators. Simulation results are presented. It is shown how wind turbines and power system controls can be integrated. A clear distinction is made between fast control of turbine torque, which is a peculiarity of wind turbines, and slow control of electric power, which is a traditional power system requirement
Binary spreading process with parity conservation
Recently there has been a debate concerning the universal properties of the
phase transition in the pair contact process with diffusion (PCPD) . Although some of the critical exponents seem to coincide with
those of the so-called parity-conserving universality class, it was suggested
that the PCPD might represent an independent class of phase transitions. This
point of view is motivated by the argument that the PCPD does not conserve
parity of the particle number. In the present work we pose the question what
happens if the parity conservation law is restored. To this end we consider the
the reaction-diffusion process . Surprisingly this
process displays the same type of critical behavior, leading to the conclusion
that the most important characteristics of the PCPD is the use of binary
reactions for spreading, regardless of whether parity is conserved or not.Comment: RevTex, 4pages, 4 eps figure
Phase transition of the one-dimensional coagulation-production process
Recently an exact solution has been found (M.Henkel and H.Hinrichsen,
cond-mat/0010062) for the 1d coagulation production process: 2A ->A, A0A->3A
with equal diffusion and coagulation rates. This model evolves into the
inactive phase independently of the production rate with density
decay law. Here I show that cluster mean-field approximations and Monte Carlo
simulations predict a continuous phase transition for higher
diffusion/coagulation rates as considered in cond-mat/0010062. Numerical
evidence is given that the phase transition universality agrees with that of
the annihilation-fission model with low diffusions.Comment: 4 pages, 4 figures include
The branching structure of diffusion-limited aggregates
I analyze the topological structures generated by diffusion-limited
aggregation (DLA), using the recently developed "branched growth model". The
computed bifurcation number B for DLA in two dimensions is B ~ 4.9, in good
agreement with the numerically obtained result of B ~ 5.2. In high dimensions,
B -> 3.12; the bifurcation ratio is thus a decreasing function of
dimensionality. This analysis also determines the scaling properties of the
ramification matrix, which describes the hierarchy of branches.Comment: 6 pages, 1 figure, Euro-LaTeX styl
Pair contact process with diffusion - A new type of nonequilibrium critical behavior?
Recently Carlon et. al. investigated the critical behavior of the pair
contact process with diffusion [cond-mat/9912347]. Using density matrix
renormalization group methods, they estimate the critical exponents, raising
the possibility that the transition might belong to the same universality class
as branching annihilating random walks with even numbers of offspring. This is
surprising since the model does not have an explicit parity-conserving
symmetry. In order to understand this contradiction, we estimate the critical
exponents by Monte Carlo simulations. The results suggest that the transition
might belong to a different universality class that has not been investigated
before.Comment: RevTeX, 3 pages, 2 eps figures, adapted to final version of
cond-mat/991234
Criticality and oscillatory behavior in non-Markovian Contact Process
A Non-Markovian generalization of one-dimensional Contact Process (CP) is
being introduced in which every particle has an age and will be annihilated at
its maximum age . There is an absorbing state phase transition which is
controlled by this parameter. The model can demonstrate oscillatory behavior in
its approach to the stationary state. These oscillations are also present in
the mean-field approximation which is a first-order differential equation with
time-delay. Studying dynamical critical exponents suggests that the model
belongs to the DP universlity class.Comment: 4 pages, 5 figures, to be published in Phys. Rev.
Renormalization of cellular automata and self-similarity
We study self-similarity in one-dimensional probabilistic cellular automata
(PCA) using the renormalization technique. We introduce a general framework for
algebraic construction of renormalization groups (RG) on cellular automata and
apply it to exhaustively search the rule space for automata displaying dynamic
criticality. Previous studies have shown that there exists several exactly
renormalizable deterministic automata. We show that the RG fixed points for
such self-similar CA are unstable in all directions under renormalization. This
implies that the large scale structure of self-similar deterministic elementary
cellular automata is destroyed by any finite error probability. As a second
result we show that the only non-trivial critical PCA are the different
versions of the well-studied phenomenon of directed percolation. We discuss how
the second result supports a conjecture regarding the universality class for
dynamic criticality defined by directed percolation.Comment: 14 pages, 4 figure
Novel universality class of absorbing transitions with continuously varying critical exponents
The well-established universality classes of absorbing critical phenomena are
directed percolation (DP) and directed Ising (DI) classes. Recently, the pair
contact process with diffusion (PCPD) has been investigated extensively and
claimed to exhibit a new type of critical phenomena distinct from both DP and
DI classes. Noticing that the PCPD possesses a long-term memory effect, we
introduce a generalized version of the PCPD (GPCPD) with a parameter
controlling the memory effect. The GPCPD connects the DP fixed point to the
PCPD point continuously. Monte Carlo simulations show that the GPCPD displays
novel type critical phenomena which are characterized by continuously varying
critical exponents. The same critical behaviors are also observed in models
where two species of particles are coupled cyclically. We suggest that the
long-term memory may serve as a marginal perturbation to the ordinary DP fixed
point.Comment: 13 pages + 10 figures (Full paper version
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