Dae-Keun Choi, “LCL필터를 갖는 계통연계 인버터의 제어이득 조정기법,” 아주대학교 공학석사 학위 논문, 2010. > Paper

As the importance of the renewable energy increases and relevant technologies develop, the use of the distributed power generation

systems (DPGSs) such as a photovoltaic system and a wind turbine system has been increased. In such systems, power conversion

devices take up very important part and are continuously developing [1-3]. However, the harmonics caused by a low switching 

frequency during power conversion can make the systems instable. To attenuate the effects of the harmonics, inductance filters (L-

filters) are connected to the either side of the power conversion device. As a capacity of the system increases, the inductance of the

L-filter increases as well. And such increase in inductance may cause a rise in manufacturing cost or undesirable dynamic response

characteristics. The large inductance of the L-filter can be reduced by employing an LCL-filter [4]. The LCL-filter requires smaller

inductance than the L-filter used alone to effectively reduce the switching harmonics. Therefore, the manufacturing cost of the system

can be reduced. However, when filter parameters are not properly chosen, it is not possible to achieve effective reduction of the

harmonics and the additional poles caused by the added LC part can further cause resonance and, hence, system instability [11].

One possible solution for this situation is to use a passive damping method that can solve resonance phenomenon by simply adding

a resistance to the capacitor terminal of the LCL-filter. This method is applicable for a small capacity system, but as a capacity grows,

the efficiency goes down due to the additional damping resistance and requires additional heat sink. Therefore, active damping is

used for a large capacity. Conventional active damping method used lead-leg compensator, which is difficult to select time constant

for damping [5], [6]. Moreover, the methods to use virtual resistance and multi roof controller have to add current sensor in the

capacitor [7]. Furthermore, even after the system is stabilized, the grid impedance may change. This, in turn, will cause the reduction

in the bandwidth of the controller. If a mainly inductive variation of the grid impedance occurs in the grid, the bandwidth of the

controller decreases and the filter will not fulfill the initial design purpose [9], [10]. It is necessary that an active damping method

can be applicable when the grid impedance changes for a stable control of system. The proposed active damping method is,

compared to the conventional method, easy to set up controller parameter, simplifies calculation algorism and is no need for

additional sensor. Moreover, it is possible to control actively for the changing impedance when it is grid connected. The

objective of this paper is to improve stability of DPGSs with an LCL-filter by employing a gain scheduling method using grid

impedance estimation. The validity of the proposed method is proved by simulation and experimental results.