ABSTRACT:
This paper concentrates on
the LCL filter with damping resistance intended to connect the shunt active
power filter of an active DC-traction substation to the point of common coupling
with the transmission grid. In order to find design conditions and conceive a
design algorithm, attention is directed to the transfer functions related to
currents and the associated frequency response. The mathematical foundation of
the design method is based on the meeting the requirements related to the significant
attenuation of the high-frequency switching current, concurrently with the unalterated
flow of the current that needs to be compensated by active filtering. It is
pointed out that there are practical limitations and a compromise must be made between
the two requirements. To quantify the extent to which the harmonics to be
compensated are influenced by imposing the magnitude response at both highest
harmonic frequency to be compensated and switching frequency, a performance
indicator is defined. As an additional design criterion, the damping power losses
are taken into consideration. The validity and
effectiveness of the proposed method are proved by simulation results
and experimental tests on a laboratory test bench of small scale reproducing the specific
conditions of a DC-traction substation with six-pulse diode rectifier.
KEYWORDS:
1. DC-traction substations
2. LCL filter
3. Passive damping
4. Regeneration
5. Shunt active power filters
BLOCK DIAGRAM:
Fig. 1. Block diagram of the active
DC-traction substation.
Fig. 2. Voltages and currents in the TT’s primary in traction
regime.
Fig.
3. Harmonic spectrum of the current in the primary of TT.
Fig.
4. Bode magnitude diagram for Cf =10F, Rd =27; L2 =1.48mH.
Fig. 5. LCL filter input current.
Fig.
6. Current flowing through the capacitor of the interface filter.
Fig. 7. Harmonic spectra of the LCL filter input current (black
bars) and
output current (yellow bars) for harmonic order k[1, 37].
Fig.
8. Voltages and currents upstream of PCC during the operation in
traction
regime.
Fig.
9. Succesive traction (filtering) and braking (regeneration) regimes: (a)
phase
voltage (blue line) and supply current (green line); (b) DC-capacitor
voltage
(black line) and DC-line voltage (red line).
CONCLUSION:
A new design method of an LCL filter
with damping resistance intended to couple the three-phase VSI of an active DC-traction
substation to the power supply has been proposed in this paper. The following
elements of originality are outlined.
1) The theoretical substantiation is
based on the frequency response from transfer functions related to currents,
taking into account the existence of the series damping resistances.
2) The expressed amplitude response and
resonance frequency highlight their dependence on only pairs L2Cf and
RdCf, It is a very important finding for the conceived design algorithm.
3) The expression of the
power losses in the damping resistances is highlighted and an equivalent resistance
is introduced as a quantitative indicator of them.
4) By considering the
switching frequency as main parameter and taking into consideration the
frequency of the highest order harmonic to be compensated, the design algorithm
is based on the imposition of the associated attenuations.
5) In the substantiation
of the design algorithm, a detailed analysis is performed on the existence of
physical-sense solutions, providing the domain in which the values of the parameters
must be located.
6) As a large number of
parameters values sets can be obtained, a new performance indicator (MPI)
is proposed, to quantify the extent to which the harmonics to be compensated
are influenced.
The
analysis and the simulation results achieved for an active DC-traction substation
with six-pulse diode rectifier and LCL coupling filter have indicated that the
proposed method is valid and effective. The experimental tests conducted in a
laboratory test bench of small scale reproducing the specific conditions of a
DC-traction substation illustrate good performance of the system for active
filtering and regeneration connected to the power supply by the passive damped
LCL filter.
The
design proposal can be applied in any three-phase LCL-filter-based shunt active
power filter.
REFERENCES:
[1] A. Ghoshal and V. John, “Active
damping of LCL filter at low switching to resonance frequency ratio,” IET
Power Electron., vol. 8, no. 4, pp. 574–582, 2015.
[2] G. E. Mejia Ruiz, N. Munoz, and J.
B. Cano, “Modeling, analysis and design procedure of LCL filter for grid
connected converters,” in Proc. 2015 IEEE Workshop Power Electron. and Power
Quality Appl. (PEPQA), pp. 1–6.
[3] M. Hanif, V. Khadkikar, W. Xiao, and
J. L. Kirtley, “Two degrees of freedom active damping technique for
filter-based grid connected PV systems,” IEEE Trans. Ind. Electron.,
vol. 61, no. 6, pp. 2795–2803,June 2014.
[4] X. Wang, F. Blaabjerg, and P. C.
Loh, “Grid-current-feedback active damping for LCL resonance in grid-connected
voltage source converters,” IEEE Trans. Power Electron., vol. 31, pp.
213–223, 2016.
[5] W. Xia, J. Kang, “Stability of
LCL-filtered grid-connected inverters with capacitor current feedback active
damping considering controller time delays,” J. Mod. Power Syst. Clean
Energy, vol. 5, no. 4, pp. 584–598, July 2017.
