Current balancing control of high power parallel-connected AFE with small current ripples

Abstract

Parallel connected Active Front End (AFE) voltage converters are widely employed in high-power grid interfaces for renewable energy applications. With parallel connected two level power converters, the power rating of the system can be easily increased with higher stability and less control efforts than multilevel power converter based solutions. However, due to unmatched modules, unbalanced currents inevitably happen to different modules of the same phase, which seriously impacts the efficiency and performances of the converter. This paper presents an improved current balancing control methods for a high power (600 kVA) parallel connected AFE for wind energy applications. The proposed current balancing method is integrated within a well-known Voltage Oriented Control Scheme with few implementation efforts. The simulation results show that with the proposed strategy the current ripples of each parallelconnected converter leg decrease evidently and the THDs are therefore decreased. Moreover, the effectiveness of the proposed method is also confirmed by experimental results with a grid-code compatible testing set-up.

Active Front-End converter applied for the THD reduction in power systems

“…This THD is much higher than the one reported in this research. In [10] the authors present analytically and experimentally the control method for the current balance in an AFE power converter of 600kVA, however, the authors make the AFE converter analysis connecting only two VSCs in parallel, generating: a THD of 4.32% (this is higher than in our research work with THD of 2.2%).…”

“…g ,k (10) where the subscript  is the time taken by the closed loop of the first order transfer function to act.…”

THD Reduction in Wind Energy System Using Type-4 Wind Turbine/PMSG Applying the Active Front-End Converter Parallel Operation

“…The correct operation of the type-4 WT control depends on the precise design of the AFE converter parameters; thus, the element's values of the MSC are obtained from the WT-PMSG nominal power, P WT-PMSG , that is: the current is i MSC = (2/3)(P WT-PMSG /v MSC ); the machine-side impedance is Z MSCt = v MSC /i MSC , thus, the MSC works with 15% of the total WT-PMSG impedance, i.e., Z MSC = (0.15)Z MSCt ; from the WT-PMSG characteristics the following parameters are taken: L MSC , R MSG , D, H. The element's values of the GSC are obtained from the WES nominal power, but to achieve P WES = P WT-PMSG i GSC is generated using i MSC = (2/3)(P WES /v GSC ); the grid-side impedance is Z GSCt =v GSC /i GSC the GSC works with 15% of the total WES impedance, i.e.,: Z GSC = (0.15)Z GSCt ; therefore, L GSC is calculated with L GSC = Z GSC / ω 0 , the R GSC value varies according to the transferred power, in a range from 0.1 Ω to 0.5 Ω; the base WES capacitance C WES is calculated with C WES = 1/(Z GSC ω 0 ). Then, a better time response in the WES feedback is achieved, since the L MSC and R MSC values are used in (10), H and D values are used in (17), L GSC and R GSC values are used in (26), to obtain the system feedback gains. It is important to establish that from the generated active power by the GSC, v WES is kept constant in the presence of any perturbation; for which, it is essential to calculate the correct capacitance value that maintains the DC-link compensation.…”

“…As evidence, in [3] the authors describe the principal WT manufacturers, those in low voltage and medium voltage technologies are classified, generating power ratings of >3 MVA and <3 MVA, respectively. In the open literature there exists some research works that address the AFE converter topology applied to WES; for example, in [17] the authors analytically and experimentally present the control method for the current balance in an AFE power converter of 600 kVA, this is a very important topic in the parallel connection of power converters, however, the authors make the AFE converter analysis connecting only two VSCs in parallel, generating: A THD of 4.32% (three times higher than in our research work with THD of 1.23%); in addition, they use the space vector modulation for the switching of VSCs, which generates a more complex control if pVSC in parallel are connected. The main goal of this work is the AFE converter topology application for the THD reduction in a WES and the increase of power transfer between the WT and the AC grid; generating greater capability, efficiency, and reliability in the energy conversion at the WES.…”

THD Reduction in Wind Energy System Using Type-4 Wind Turbine/PMSG Applying the Active Front-End Converter Parallel Operation

“…As evidence, in [3] the authors describe the principal WT manufacturers, these in low voltage (LV) and medium voltage (MV) technologies are classified, generating powers ratings of >3MVA and <3MVA, respectively. In the open literature exists some research works that address the AFE converter topology applied to WES; for example, in [17] the authors present analytically and experimentally the control method for the current balance in an AFE power converter of 600kVA, this is a very important topic in the parallel connection of power converters, however, the authors make the AFE converter analysis connecting only two VSCs in parallel, generating: a THD of 4.32% (three times higher than in our research work with THD of 1.23%); in addition, they use the space vector modulation for the switching of VSCs, which it generates a more complex control if pVSC in parallel are connected.…”