Linear Approximations to AC Power Flow in Rectangular Coordinates

This paper explores solutions to linearized powerflow equations with bus-voltage phasors represented in rectangular coordinates. The key idea is to solve for complex-valued perturbations around a nominal voltage profile from a set of linear equations that are obtained by neglecting quadratic terms in the original nonlinear power-flow equations. We prove that for lossless networks, the voltage profile where the real part of the perturbation is suppressed satisfies active-power balance in the original nonlinear system of equations. This result motivates the development of approximate solutions that improve over conventional DC power-flow approximations, since the model includes ZIP loads. For distribution networks that only contain ZIP loads in addition to a slack bus, we recover a linear relationship between the approximate voltage profile and the constant-current component of the loads and the nodal active and reactive-power injections

Integrating reliability into performance-oriented design of fault-tolerant switch-mode DC-DC converters for photovoltaic energy-conversion applications

This work bridges the disconnect between two consequential design concerns in switch-mode power converters deployed in photovoltaic energy-processing applications: steady-state performance and system reliability. A general framework for fault-tolerant design is presented in the context of a multiphase, interleaved boost converter. A unified, system-level, steady-state description for this topology is proposed. The theoretical derivations are validated against detailed numerical simulations, and their applicability over a wide range of ambient conditions is demonstrated. The steady-state characterization of the converter is then employed to specify the failure rates of circuit components and establish the effects of ambient temperature, insolation, number of phases, and device ratings on system reliability. A Markov reliability model is derived to assess the reliability of a general N-phase converter. The proposed analytical tools provide a methodical framework for design of fault-tolerant, multiphase converters employed in a wide range of photovoltaic systems

Uncovering Droop Control Laws Embedded Within the Nonlinear Dynamics of Van der Pol Oscillators

This paper examines the dynamics of power-electronic inverters in islanded microgrids that are controlled to emulate the dynamics of Van der Pol oscillators. The general strategy of controlling inverters to emulate the behavior of nonlinear oscillators presents a compelling time-domain alternative to ubiquitous droop control methods which presume the existence of a quasi-stationary sinusoidal steady state and operate on phasor quantities. We present two main results in this work. First, by leveraging the method of periodic averaging, we demonstrate that droop laws are intrinsically embedded within a slower time scale in the nonlinear dynamics of Van der Pol oscillators. Second, we establish the global convergence of amplitude and phase dynamics in a resistive network interconnecting inverters controlled as Van der Pol oscillators. Furthermore, under a set of non-restrictive decoupling approximations, we derive sufficient conditions for local exponential stability of desirable equilibria of the linearized amplitude and phase dynamics

Generation of analogs having potent antimicrobial and hemolytic activities with minimal changes from an inactive 16-residue peptide corresponding to the helical region of Staphylococcus aureus δ-toxin

The δ-toxin is a 26-residue peptide from Staphylococcus aureus with the sequence formyl-MAQDIISTIGDLVKWIIDTVNKFTKK. NMR studies indicate that the segment IISTIGDLVKWIIDTV occurs in an a-helical conformationin the toxin. A synthetic peptide corresponding to this segment, although helical, did not exhibit hemolytic activity. Since charged residues like D and K are likely to modulate cytolytic activity, analogs of the 16-residue peptide were synthesized where D was systematically replaced by K. Analogs in which the first D and both Ds were replaced by K showed potent antimicrobial and hemolytic activities. The analog in which the second D was replaced by K was relatively less active. However, all the peptides showed an a-helical structure with similar helical content. The activities of the peptides were found to correlate directly with their ability to permeabilize model membranes. Thus, by minimal judicious replacement of charged amino acids, it should be possible to generate cytolytic peptides from short segments of peptide toxins