Hea-Gwang Jung, “영구자석형 풍력발전 시스템의 3 병렬 설계와 계통연계 기술,” 아주대학교 공학석사 학위 논문, 2010. > Paper

The demand of power conversion system for the purpose of a variable speed control is increasing in the wind power system. The

PMSG (Permanent Magnet Synchronous Generator) has advantages of a wide range of wind speed and an easier control scheme,

compared to a DFIG (Doubly Fed Induction Generator). Switches and filters, however, need to be selected regarding the overall

system rating because the power conversion part is directly connected between a generator and a grid. The rated current of the

power devices can be reduced to one third value under the three-parallel operation. However, it is required that the inductors of the

each leg are appropriately designed to avoid the circulating currents and the three-parallel operation is controlled by proper

controllers. The THD (Total Harmonics Distortion) reduction of output currents and power quality regulation are important in the

grid connection system. The inductance of the input or output circuits of the power conversion devices have conventionally been

used to reduce these harmonics. However, as the capacity of the systems have been increasing, high values of inductances are

needed, so that realizing practical filters has been becoming an ever more difficult due to the price rises and the poor dynamic

responses. These problems, caused by realizing practical L-filters in large-scale facilities can be solved by using LCL-filters [2-5]. This

is expected because an additional LC part can reduce the harmonics effectively in several hundreds of kVA. Moreover, an LCL-filter

realization is easy and effective, with little increase in overall system cost and without having to introduce additional sensors.

However, unless all of the filter’s parameters are properly selected, it is not possible to achieve effective reduction of the harmonics,

and the additional resonance poles caused by the second LC part can further raise the stability problems. There are two solutions to

this situation, namely, 1) introduce passive damping by adding a resistor connected to the parallel capacitor, and 2) introduce active

damping where no additional resistor is needed. Passive damping is simply a way to guarantee the stability of the system, but there

is additional loss caused by the added resistance. To avoid such losses, papers relating to active damping algorithms that are aimed

at resolve the stability problems without additional resistors, have been announced. But these algorisms require additional voltage

sensor, exact tuning of gains or filter parameters. This paper proposes the compensation method using power theory which has

implementional advantages because it is simple and realizable without the need for an additional sensor and exact tuning of gains

or filter parameters. In this paper, the system design and Grid-connection technique for high-power wind turbines using PMSG are

proposed. The simulation results verify the proposed system and methods.