Determinación cualitativa y cuantitativa de metales

There are a number of works in the power network literature that propose models to generate scalable-size power grid test cases, such as: the use of ring-structured grids to study the propagation pattern of disturbances [18]; a tree-structured power grid model for detecting critical transitions in the transmission network that cause cascading failure blackouts [19]; the Watts–Strogatz small-world network [1]; or the formation of a power grid topology starting from a uniform or a Poisson distribution for the nodal location (emulating what is often done in modeling wireless networks) [20]. While these proposed models provide some good perspectives on power grid

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(b)

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NYISO: Empirical PDF of Z_pr and DPLN-clip fitting PDF

NYISO Z_pr

DPLN-clip

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

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Z_pr (p.u.) F (Zpr)

NYISO: Empirical CDF of Z_pr and DPLN-clip fitting CDF

NYISO Z_pr

DPLN-clip

FIGURE 7.6 Comparison between empirical PDF/CDF distributions and fitted distribu-tions: (a) IEEE 300; (b) NYISO 2935.

Power Grid Network Analysis for Smart Grid Applications 169

characteristics, the generated topology fails to correctly or fully reflect that of a realistic power system.

This section presents a random topology power grid model, RT-nested-Smallworld, which constructs a large-scale power grid in a hierarchical way: first forming nected subnetworks with size limited by the connectivity requirement; then con-necting the subnetworks through lattice connections; and, finally, generating the line impedances from some specific distribution and assigning them to the links in the topology network. The hierarchy in the model arises from observation of real-world power grids: usually a large-scale system consists of a number of smaller-size subsystems (e.g., control zones), which are interconnected by sparse and important tie-lines. The model contains three main components: (a) clusterSmallWorld subnet-work, (b) lattice connections, and (c) generation and assignment of line impedances.

These will be described in detail in the following subsections.

7.5.1 ^clusTers^MallW^orld s^ubneTWork

Electrical power grid topology has “small-world” characteristics; it is sparsely con-nected with a low average nodal degree that does not scale with the network size. On the other hand, in order for a small-world model to generate a connected topology, the network size has to be limited. In this proposed model, different mechanisms from those of the Watts–Strogatz small-world model have been adopted to form a power grid subnetwork in order to improve its resulting connectivity, as shown in the following paragraphs. Consequently, the connectivity limitation on the network size can be expanded from what is indicated by Equation 7.9. The experiments have shown that for 〈k〉 = 2 – 3, the network size should be limited to no greater than 30, and for 〈k〉 = 4 – 5, no greater than 300.

Therefore, the first step of this new model is to select the size of subnetworks according to the connectivity limitation. Then a topology is built up through a modi-fied small-world model, called clusterSmallWorld. This model is different from the Watts–Strogatz small-world model in two aspects: the link selection and the link rewires. That is, instead of selecting links to connect most immediate 〈k〉/2 neighbors to form a regular lattice, it selects a number k of links at random from a local neigh-borhood Nd0 with the distance threshold of d₀, where k comes from a geometric distri-bution. The local neighborhood is defined as the group of close-by nodes with mutual node number difference less than the threshold d₀, that is, Ndi j j i d

0 0

( )={ ; | − <| } for node i. It is worth noting that the clusterSmallWorld model adopts a geometric distri-bution with the expectation of 〈k〉 for the initial node degree settings (i.e., for the link selection). Experiments have shown that the process of link selection and link rewir-ing that follows will transform the initial node degree distribution and finally result in a distribution with good matching approximation to the observed distribution.

For the link rewiring, the Watts–Strogatz small-world model selects a small portion of the links to rewire to an arbitrary node chosen at random in the entire network to make it a small-world topology. The clusterSmallWorld model uses a Markov chain with transition probabilities of (α, β), as shown in Figure 7.7, to select clusters of nodes and therefore groups of links to be rewired. This mechanism is introduced in order to pro-duce a correlation among the rewired links. After running the above Markov transition

170 Smart Grids

n times (i.e., one for each node in the network), we get clusters of nodes with “1”s alter-nating with clusters of nodes with “0”s, where “1” means to rewire and “0” means not.

Then, by a specific rewiring probability qrw, some links are selected to rewire from all the links originating from each 1-cluster of nodes, and the corresponding local links are rewired to outside 1-clusters. The average cluster size for rewiring nodes is Kclst= 1/β, and the steady-state probabilities are p0= β/(α + β), p1= α/(α + β). Experiments are per-formed on the available real-world power grid data to estimate the parameters. The average rewiring cluster size Kclst, the rewiring probability of links qrw, and the ratio of nodes having rewire links p1 can be directly obtained from statistical estimates. Then the transition probability can be computed as β = 1/Kclst and α = βp1/(1 − p1).

7.5.2 laTTice connecTions

The lattice connections are selected at random from neighboring subnetworks to form a whole large-scale power grid network. The number of lattice connections between neighboring subnetworks is randomly chosen to be an integer with the mean of 〈k〉.

7.5.3 assignMenTof line iMpedances

At this step m line impedances are generated from a specified heavy-tailed distribu-tion, and then sorted by magnitude and grouped into local links, rewire links, and lattice-connection links according to their corresponding proportions, as shown in Figure 7.8. Finally, line impedances in each group are assigned at random to the cor-responding group of links in the topology.

α

1–α 0 1 1–β

β

FIGURE 7.7 Markov chain for selecting cluster of rewiring nodes (0: not rewire; 1: rewire).

m

Local Rewires

Lattice-conn Impedances (p.u.)

Link number 1

FIGURE 7.8 Grouping line impedances.

Power Grid Network Analysis for Smart Grid Applications 171

7.6 TOPOLOGICAL ANALYSIS FOR THE