Active Power Changes Research Articles

Direct Active and Reactive Power Control of DFIG for Wind Energy Generation

This paper presents a new direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind energy generation system. The strategy is based on the direct control of stator active and reactive power by selecting appropriate voltage vectors on the rotor side. It is found that the initial rotor flux has no impact on the changes of the stator active and reactive power. The proposed method only utilizes the estimated stator flux so as to remove the difficulties associated with rotor flux estimation. The principles of this method are described in detail in this paper. The only machine parameter required by the proposed DPC method is the stator resistance whose impact on the system performance is found to be negligible. Simulation results on a 2 MW DFIG system are provided to demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, rotor speed, machine parameters, and converter dc link voltage.

A method of tracking the peak power points for a variable speed wind energy conversion system

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-/spl omega/ curve correspond to dP/d/spl omega/=0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PWM) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a DC motor fed from a commercial DC drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.

A Method of Tracking the Peak Power Points for a Variable Speed Wind Energy Conversion System

In this paper, a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P -ω curve correspond to dP / dω = 0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back PWM converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a dc motor fed from a commercial dc drive. All the control loops are executed by a single-chip DSP controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method.

Fast and accurate technique for power system state estimation

The paper presents a fast and accurate technique for power system state estimation. The proposed technique is based on the exact linearisation of the Cartesian co-ordinate formulation of nodal load flow equations. The complete Taylor series expansion is used to solve these equations while retaining the nonlinearity. In this technique, the Jacobian and Hessian matrices are kept constant, and hence need to be computed once only, at the beginning of the solution, and then stored in the computer memory using the sparsity storage procedure. Because, in this technique the active and reactive power problems are attempted simultaneously, not sequentially, there is no need to use decomposition into two subproblems, namely P–Θ and Q–V. The linear programming (LP) technique is used to solve the state estimation problem without any approximation for the load flow equations. For small changes in active and reactive power, the right-hand side of the LP is compensated only when the state estimation solution of the real component of the bus voltages is obtained with a minimum number of iterations compared with the fast decoupled state estimation (FDSE). For large band-data points, however, the imaginary components of the bus voltages must be computed, in addition to the real components. The proposed technique is more suitable than FDSE for solving ill-conditioned networks which have large bad-data points. An extensive study was carried out on a standard 23-bus test system to demonstrate the capability of the proposed technique for solving the state estimation problems.

The no-gain theorem and localized response for the decoupled P -<tex>heta</tex>power network with active power losses included

In this paper a no-gain theorem for electric power systems is presented using the S- E model based decoupled active power-phase angle P - \theta network, in which each transmission element consists of two branches, representing transmitted power and loss power. The theorem gives necessary and sufficient conditions such that if a power system is perturbed from its normal operating point then the magnitude of the change in active power through either branch of a given transmission line S- E graph is not greater than the sum of the magnitudes of the changes in active power through the sources. These conditions are given as a region of allowable perturbations, which are derived as a set of inequalities. Based on this result we present a proof, valid for systems with losses, of localized steady state response. Specifically, we prove that the effects of a single fault in a given power system diminish as one moves away from the fault, subject only to the constraint that the operating point of the system remains within the allowable regions given by the no-gain theorem. This result is often assumed without proof to be valid in the entire operating region.