In
urban rail transit, there is a large number of harmonics brought by diode
rectifiers, and shunt
active
power filters (APFs) are an effective method of harmonics rejection.
Traditional shunt APFs work with a dedicated DC link leading to complexity,
while those with a common DC bus but using non-isolated topologies bring
serious problems of zero-sequence circulating current (ZSCC), which introduce
losses and provoke poor quality. Consequently, this paper first analyzes the
limitations of traditional non-isolated APFs on modulation radio. Based on the
analysis, this paper put forward a novel isolated APF with a common DC link
based on existing diode rectifiers in urban rail transit, which realizes
braking energy recovery as an additional function. To this end, harmonics
brought by diode rectifiers are reduced while rejecting ZSCC. Meanwhile,
braking energy can feedback to the city grid with lower harmonics. Finally,
simulation and experiment using a 1-kW prototype converter verify the
feasibility and validity of the proposed converter on harmonics suppression and
braking energy recovery.
KEYWORDS:
1. Active
power filter (APF)
2. LLC series resonant converter
3. Soft
switching
4. Braking
energy recovery
5. Zero
sequence circulating current (ZSCC)
SOFTWARE:
MATLAB/SIMULINK
CIRCUIT DIAGRAM:
Figure 1. Topology of the shunt isolated APF with
common DC link integrating braking energy recovery based on HFPET and LLC converter.
Figure 2. Load current iL(A), grid current ig(A)
and circulating current icc(A). ZSCC of shunt APF directly connected
to 3-phase grid without HFPET icc_nonisolated (A) and ZSCC of
shunt isolated APF with HFPET icc_isolated (A).
Figure 3. Simulation results of current on side of
grid iga(A), igb(A), igc (A) and voltage of DC
traction grid Udc (V) under periods of without APF (running process
of train), with APF (running process of train), mode switching period and
braking energy recovery.
Figure 4. Simulation waves of non-linear load
current iLa(A) and
feedback
current ifa (A).
CONCLUSION:
In
urban rail transit, harmonics need to be suppressed and braking energy also
should be utilized to reduce losses. In this paper, focuses on the problems of
harmonics in urban rail transit while braking energy recovery is also included.
Based on traditional shunt non-isolated APFs, it is shown that ZCSS would occur without an isolated HFPET, the
amplitude of which is 23% of the load current which brings high losses and increases
current stress of the switches. Two types of potential loops of ZCSS are analyzed
and equivalent circuits are built for accurate analysis, which shows that the
modulation radio of the inverter would be constrained at the upper limit of π/6
to guarantee no ZSCC. Consequently, a shunt isolated APF with a common DC link integrating braking
energy recovery is put forward to realize suppression of harmonics and
elimination of ZSCC together with energy feedback. In this topology, the LLC
series resonant converter operates in soft-switching condition of ZVS and ZCS,
bringing high efficiency and wide voltage range. Closed-loop control strategy
of voltage and current during period of APFs and braking energy recovery are
designed respectively. During the process of starting and running of the train,
the voltage of the DC bus is lower than the reference voltage, the converter is operating as the APF without ZSCC. In the
process of braking, the voltage of the DC bus rises so that APF is paused and
braking energy recovery is put into use. Simulation based on MATLAB/Simulink
and PLECS has been done to verify analysis on characteristics of soft-switching
and eliminating harmonics. A 1-kW prototype based on DSP and CPLD has been
built to realize compensation of harmonics and energy feedback. All the needs
are well satisfied and this novel converter combine two functions together and
restrain ZCSS to realize an high DC voltage utilization with a smaller area and volume compared to traditional methods of
achieving the APF and braking energy recovery.
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