Decoupled Piecewise Linear Power Flow and Its Application to Under Voltage Load Shedding

Various optimizations in power systems based on the AC power flow model are inherently mixed-integer nonlinear programming(MINLP)problems.Piecewise linear power flow models can handle nonlinearities and meanwhile ensure a hi^h accuracy.Then,the MINLP problem can he turned into a tractable mixed-integer linear programming(MILP)problem.However,piecewise linearization also introduces a heavy computational burden because of the incorporation of a large number of binary variables especially for large systems.To achieve a better trade off between approximation accuracy and computational efficiency,this paper proposes a model called decoupled piecewise linear power flow(DPWLPF)for transmission systems.The P-Q decoupling characteristic is used to ease the evaluation of the piecewise cosine functions in the power flow equations.Therefore,in optimizations,the coupling between variables is reduced.Moreover,an under voltage load shedding(UVLS)approach based on DPWLPF is presented.Case studies are conducted for benchmark systems.The results show that the DPWLPF facilitates the solution of optimal power flow(OPF)and UVLS problems much better than conventional piecewise models.And DPWIJM^still enhances the approximation accuracy by usinj»the decoupled piecewise modeling.

Grid Voltage Sensor-less Protection Scheme for One Cycle-Controlled Single-phase Photovoltaic Inverter Systems

One-cycle-controlled(OCC)inverters are suitable for small single-phase photovoltaic distributed-generator systems because of their simplicity,phase-locked-loop free structure,grid voltage sensor-less operation,and cost-effectiveness.Grid voltage sensor-less control helps reduce cost and increases reliability in operation.However various sensors are used for implementation of a protection mechanism.In this paper,a grid voltage sensorless protection scheme for OCC based single-phase inverter systems is proposed.The estimated value of voltage at point of common coupling(VPCC)is used for protecting the system during over/under voltage conditions of the grid,implementing of voltage ride through conditions,and for disconnecting the grid during islanded conditions.The VPCC is estimated from the measured inverter current,switching pulses,and the measured dc-link voltage using a second-order filter.Simulation and experimental studies are performed to verify the efficacy of the proposed voltage sensor-less protection mechanism triggered using estimated VPCC.