Frank Okafor

Decoupled Direct Control of Natural and Power Variables of Doubly Fed Induction Generator for Extended Wind Speed Range Using Feedback Linearization

The electromagnetic torque, stator power, and the grid side converter power outputs of a doubly fed induction generator (DFIG) are differentiated to obtain a feedback linear relationship with the rotor voltage vectors serving as the control inputs. The derived model is such that can allow for a transition of DFIG to operate at low wind speed by shorting its stator terminal to one another. The torque and power quantities are the state variables of the system, which are insensitive to coordinate system orientation. Steady-state models are used to obtain optimal operating regimes with respect to minimal electrical losses. Decoupled torque control is developed by field orientation, while decoupled power control is derived from voltage orientation of the DFIG. Transients originating from transitioning into low wind speed operation are investigated. Robustness of each of the decoupled controllers against parameter variations is investigated by locating the poles of its closed-loop transfer function. The results for a 5-Hp machine are presented.

Determination of Steady-State and Dynamic Control Laws of Doubly Fed Induction Generator Using Natural and Power Variables

A doubly fed induction generator model is presented whereby the natural and power variables are the state variables. The natural variables are the electromagnetic torque (Te), the reactive torque (Tr), the magnitude of the rotor flux linkage (λr), the magnitude of the stator flux linkage (λs), and the rotor speed (ωr). The power variables are the real power (Pf) and reactive power (Qf) generated/absorbed by the grid-side converter into/from the grid. Simulation of the dynamic natural variable model of the induction machine is compared with a vector variable simulation. Steady-state operating regions are established for various power factor operations. The optimal stator power factor operation is estimated. A direct control of torque and power variables is developed. The robustness of the developed control against rotor parameter variation is investigated using small signal analysis and is compared with vector control. Results are shown for a 5-hp machine.

Efficiency optimization of doubly-fed induction generator transitioning into shorted-stator mode for extended low wind speed application

The rotor voltage of a doubly-fed induction generator (DFIG) increases with decrease in wind speed. Hence, much decrease in wind speed can result in the rotor voltage exceeding its rated value. In order to extend the operation of the DFIG for low wind speed application, the stator terminals of DFIG are shorted to one another whereby the stator loses its synchronous angular frequency characteristics. Transitioning into a shorted-stator mode ensures an operation whereby the rotor voltage decreases with decrease in wind speed. Since the stator of the shorted-stator mode is disconnected from the grid, the voltage of the grid no longer contributes to the development of the stator flux linkage. Therefore, for an effective transitioning, a control scheme is developed to ensure that the resulting transients and stator flux linkage are regulated within permissible limits with the objective of minimizing copper and magnetization losses. Some results are presented for a 5Hp machine.

Shorted stator induction generator for low wind speed power application

Shorted stator induction generator (IG) is presented for low wind speed power application. An analytical model is derived for predicting electrical performance of the generator while operating in regions 1 and 2 of turbine power versus wind speed curve. This paper determines an explicit optimal stator electrical speed of operation since the stator is disconnected from synchronism with the grid. A vector control scheme is developed for the IG. Simulation and experimental results are presented for a 5hp machine.

Determination of steady state control laws of doubly - fed induction generator using natural and power variables

A doubly - fed induction generator (DFIG) system is presented in terms of its natural and power variables as the state variables. The natural variables are the electromagnetic torque (T<inf>ℯ</inf>), the reactive torque (T<inf>r</inf>), the magnitude of rotor flux linkage (λ<inf>r</inf>), the magnitude of stator flux linkage (λ<inf>s</inf>), and the rotor speed (ω). These are used in modelling the induction machine and the machine side converter (MSC) of the system. The real power (P<inf>ƒ</inf>) and reactive power (Q<inf>ƒ</inf>) that the grid side converter (GSC) exchanges with the grid make up the power variables of the system and are used to model the GSC. Simulation of the dynamic natural variable model of the induction machine is carried out and the results are compared with a conventional induction machine simulation technique. Steady state analysis is presented for various power factor operations. A method of predicting an optimal stator power factor operation is proposed.

Decoupled control of natural and power variables of doubly-fed induction generator using feedback linearization

The electromagnetic torque, stator power, and the grid side converter (GSC) power outputs of a double-fed induction generator (DFIG) are differentiated to obtain a feedback linear relationship with the rotor voltage vectors serving as the control inputs. The derived model is such that the torque and power quantities are the state variables of the system, which are insensitive to coordinate system orientation. The steady-state equations from the derived model are used to obtain the optimal operating region with respect to minimal electrical losses. Decoupled torque control is developed by field orientation, while decoupled power control is derived from voltage orientation of the DFIG. Robustness of the decoupled controllers against parameter variation is investigated by locating the poles of its closed-loop transfer function. Results for a 5Hp machine are presented.

Simulink model for planning a new power transmission investment

This paper proposes an Investment Planning Model (IPM) that can be employed in determining the optimal period to inject new investments on some critical sections of any transmission network in the Wholesale Electricity Market in order to sustain its reliability and availability. The mathematical models for realization of the IPM are obtained by capturing the salient effect of increasing Existing Transmission Commitment on Available Transfer Capability. These mathematical models are implemented and executed with the help of embedded Matlab function blocks in the Matlab environment. The input units of the IPM were obtained via the PSAT 2.1.2 software package. In the course of simulations, the proposed IPM was used to predict the investment periods of all the transmission paths of a 14-bus IEEE test system as quickly as possible.

Evaluation of electrical load growth rate in a wholesale electricity market

This paper presents an electricity growth rate (eLGr) model for a given wholesale electricity market (WEM) over a period of time. It combines the existing growth models with other statistical and analytical test tools to develop the proposed model; which is then tested in Matlab environment and performed excellently well. The accuracy of the results obtained from it, compare very well with those obtained elsewhere. Furthermore, the proposed model is faster and specifies the appropriate function required.

Shorted-stator mode control of doubly-fed induction generator connected to a weak grid

A doubly-fed induction generator connected to a weak grid is presented. The grid is characterized by unbalanced voltage conditions, which introduces a negative sequence voltage to the stator windings. The negative sequence voltage conditions generate pulsations in the torque and power quantities of the generator, which affects its stability. Proportional plus resonant controllers are introduced into direct control of torque and power schemes to mitigate the pulsations. In order to extend the speed range of operation of the DFIG, the shorted-stator mode operation is considered so that at low wind speed some corresponding power are still generated into the PCC. The control of the shorted-stator mode shows more robustness against the unbalanced voltage conditions at the PCC with no pulsations in the electromagnetic torque. Some results are presented for a 5hp machine.

Sensitivity analysis of symmetrical direct control of torque and power variables of doubly-fed induction generator

The torque and power outputs of a doubly-fed induction generator (DFIG) are differentiated once to obtain a linear relationship with the rotor voltage vectors, which are the control input quantities. The derived model is used to develop direct control of the torque and power of the DFIG whereby the rotor voltage quantities are generated at a constant switching frequency. Decoupled control of torque and power is obtained by field orientation and voltage alignment, respectively. Sensitivity of the direct control schemes to system parameter variation is investigated using small signal analysis. Results for a 5 hp machine indicate that the vector control is more affected by system parameter mismatch.