11-10-2012, 05:19 PM
Control of Parallel Multiple Converters for Direct-Drive Permanent-Magnet Wind Power Generation Systems
Control of Parallel Multiple.doc (Size: 61 KB / Downloads: 36)
ABSTRACT:
This paper proposes control strategies for megawatt level direct-drive wind generation systems based on permanent magnet synchronous generators. In the paper, a circulating current model is derived and analyzed. The parallel-operation controllers are designed to restrain reactive power circulation and beat frequency circulation currents caused by discontinuous space vector modulation. The control schemes do not change the configurations of the system consisting of parallel multiple converters. They are easy to implement for modular designs and large impedance required to equalize the current sharing is not needed. To increase the system reliability, a robust adaptive sliding observer is designed to sense the rotor position of the wind power generator. The experimental results proved the effectiveness of the control strategies.
EXISTING SYSTEM:
The paralleling converters are more desirable than paralleling switching devices. Compared with the single-module solution, converters in parallel offer higher reliability, high efficiency, and lower grid-side harmonics. When discontinuous space-vector modulation is used in multiple Parallel converters, because of the different switching characteristics and impedance discrepancy of individual converter, even if synchronized control of each converter is applied, the switching status of the converters in parallel will differ from each other. This results in what is called circulating current in which currents that circulate among power switching devices will not flow into the generators and power grid. The existence of this circulation will increase the current flow through the power switching devices, increase the loss of converters, and perhaps damage converters.
EXISTING SYSTEM TECHNIQUE
Fukuda and Matsushita proposed a control of two parallel three phase converters as a controlled six-phase converter; the zero sequence circulation can be inhibited. But two parallel converters need to be controlled as one integrated converter; this is not suitable for modular design. A modulation scheme without using zero vectors was proposed to avoid the interaction caused by the particular discontinuous space-vector modulation (SVM). However, since rejection of the circulating disturbance was not attempted, any mismatches between the parallel converters can still cause circulating current even without using zero vectors.
PROPOSED SYSTEM:
The paper proposed a new method to control the acting duration of zero vectors by using a proportional integral (PI) regulator. This technique can effectively minimize the circulation. However, the method is merely developed based on an averaged control mode and its effectiveness reduces when the zero vectors shrink; it may not predict the peak of the current. The average current-mode control does not guarantee a low magnitude current peak, although the simulation and experimental results show the small current peak.
PROPOSED SYSTEM TECHNIQUE
The parallel back-to-back pulse width modulation (PWM) converters are used in megawatt-level high power generation systems. The generator-side converter and grid-side converter can be controlled separately. In this paper, a mathematical model for circulating current is derived and analyzed to reveal the mechanism of circulation for the multiple converters in parallel connection. An improved space vector (SV)-PWM parallel control strategy is presented to avoid the circulating current. Independent current regulation is implemented for each branch power module.