Mircea M. Radulescu

Control and Performance Evaluation of a Flywheel Energy-Storage System Associated to a Variable-Speed Wind Generator

The flywheel energy-storage systems (FESSs) are suitable for improving the quality of the electric power delivered by the wind generators and for helping these generators to contribute to the ancillary services. Supervisors must be used for controlling the power flow from a variable-speed wind generator (VSWG) to the power grid or to an isolated load. This paper investigates the control method and the energetic performances of a low-speed FESS with a classical squirrel-cage induction machine in the view of its association to a VSWG. A test bench is developed, and experimental results are presented and discussed

Design and Control Strategies of an Induction-Machine-Based Flywheel Energy Storage System Associated to a Variable-Speed Wind Generator

Flywheel energy storage systems (FESSs) improve the quality of the electric power delivered by wind generators, and help these generators contributing to the ancillary services. Presently, FESSs containing a flux-oriented controlled induction machine (IM) are mainly considered for this kind of application. The paper proposes the direct torque control (DTC) for an IM-based FESS associated to a variable-speed wind generator, and proves through simulation and experimental results that it could be a better alternative. This DTC application entails two specific aspects: 1) the IM must operate in the flux-weakening region, and 2) it must shift quickly and repeatedly between motoring and generating operation modes. DTC improvement for increasing the FESS efficiency, when it operates at small power values, is discussed. Some aspects concerning the flywheel design and the choice of the filter used in the FESS supervisor are also addressed.

Multiphysics Approach to Numerical Modeling of a Permanent-Magnet Tubular Linear Motor

This paper presents a multiphysics modeling through finite-element (FE) coupled electromagnetic and thermal field analysis of a permanent-magnet tubular linear motor (PMTLM). Two-dimensional axial-symmetric FE steady-state and transient solutions are first obtained for the magnetic-flux-density distribution, cogging force, thrust, and losses of the PMTLM prototype. The FE magnetic field results are then used for the 3-D FE thermal simulation to get the PMTLM temperature distribution. This paper proves that the multiphysics numerical field analysis is a viable tool for the design and performance optimization of PMTLMs. The accuracy of the proposed study has been assessed through prior analytical and experimental results. Regarding the design aspects, some peculiarities in the thermal behavior of PMTLMs are emphasized. Generally, thermal models being not ready to develop, experimental and analytical solutions remain a preferred choice.

Grid connected or stand-alone real-time variable speed wind generator emulator associated to a flywheel energy storage system

The level of penetration increase of dispersed generation in the electrical networks is a major scientific aim: to increase the possibility to participate in the ancillary services, to determine the influence zone according to the network connection point,...etc. The development of experimental test benches is necessary to validate theoretical studies. In this paper, we present a 3 kW test bench, emulating a grid connected or stand-alone real-time variable speed wind generator emulator associated to a flywheel energy storage system

Experimental results on a variable-speed small hydro power station feeding isolated loads or connected to power grid

Following a series of articles presenting the modelling and simulation of an autonomous variable- speed hydropower station feeding an isolated load or connected to a power grid ([2], [3]), this paper is aiming to validate on the test bench the solutions proposed earlier. The studied system is composed of a doubly-fed induction generator (DFIG) linked mechanically and electrically (through back-to-back power electronic converters in the rotor circuit) to a permanent-magnet synchronous machine (PMSM) which may recover or supply the slip power. One of the advantages related to this system is that the PMSM and the power converters are designed for only about 25% of the plant nominal power.

Harmonic characteristics of an induction machine connected to a distribution network

This paper presents a complete study of the induction machine allowing the determination of its harmonic characteristics generated in the line current. The method used to analyse the induction machine behaviour is based on the circuit-oriented approach. The developed model facilitates the estimation of frequencies and maximum values of the stator current harmonic components. Using the EMTP/ATP electrical network simulation package, the obtained circuit-oriented model is simulated in normal conditions at the rated load and the results are compared with theoretical results and experimental measurements.

Axial-flux vs. radial-flux permanent-magnet synchronous generators for micro-wind turbine application

This paper presents a comparative study between the axial-flux and radial-flux permanent-magnet synchronous generators for a 3 kW wind turbine application. This study emphasizes the most cost-effective solution to be implemented by the industrial partner. There are three constructive types of electric generators considered in this work. All are based on permanent-magnet synchronous generators (PMSGs), but one has an axial-flux topology, the other one is a radial-flux outer-rotor machine and the third one is a radial-flux inner-rotor machine. Finite-element field analysis was made for the three PMSGs in order to compare their efficiency and active materials estimated cost. The results show that the axial-flux PMSG is the best solution for micro-wind turbine application.

On the dynamic model of a doubly-salient permanent-magnet motor

An analytical model has been developed allowing the dynamic performance prediction of a new doubly-salient permanent-magnet (DSPM) motor. The configuration and the operation mode are briefly explained, and the need for a dynamic model is justified. Nonlinear magnetic circuit analysis and finite element calculations are employed for determining the winding inductance and flux linkage. The developed nonlinear description of the DSPM motor dynamics is a basis for a future work on the energy-optimized control of adjustable-speed drives using DSPM motors.

Current control methods for grid-side three-phase PWM voltage-source inverter in distributed generation systems

A comparison between two current control methods for grid-side inverter, i.e. PI control and forward predictive control, is made. The PI current control is implemented in d-q synchronous frame, while the forward predictive current control is implemented in abc stationary frame. The paper presents both simulation and experimental results, which were made at a reduced scale (2.2 kW). The constant-power test showed that both methods have good results in respect to existing grid codes.

Direct active and reactive power control of variable-speed doubly-fed induction generator on micro-hydro energy conversion system

This paper proposes a direct power control technique applied to a variable-speed doubly-fed induction generator (DFIG) of a micro-hydro energy conversion system. This assembly consists of a permanent-magnet synchronous machine (PMSM) linked mechanically and electrically (through back-to-back power electronic converters) to the DFIG (on the rotor side). The PMSM has the role to recover or supply the slip power of the DFIG. The main advantages related to this system are that the energy conversion system is completely autonomous (can supply isolated load or can be connected to power grid) and also, the PMSM and the power converters are designed for only about 25 % of the hydro-plant rated power.