5
vacon • 33
BASIC VARIANTS
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This will give some idea what could be expected to work, but because the battery properties are not
taken into account in the design, it is not possible to guarantee a safe operation. If the filter does
not operate as expected, the capacitance value can be increased for example by adding another
capacitor in parallel. Without interleaving, the capacitor must be a lot bigger (for example ten times
bigger) if the target is to be in the same level as with the interleaving control and chokes only. Note,
however, that this especially depends a lot on the battery.
With a real LCL-filter structure, dependence of the battery properties is minimized and the design
is more robust against resonances. The requirements for an additional battery side choke are quite
simple because the ripple is very small. The voltage rating of the filter capacitor should be similar
to the DC/DC converter DC-link capacitors.
The proposed rule to dimension an LCL filter is based on a safe resonance frequency. In this case
it is estimated that half of the switching frequency would be small enough with interleaving. In
addition, the battery side choke L2 is determined as 1/6 of L1 which corresponds to the typical
inductance ratio of chokes in standard inverter applications. With these assumptions, the required
capacitance (minimum value) can be calculated as:
Without testing, the interleaving capacitor must be bigger. At least double the size is recommended.
Crucial tests to verify filter applicability are:
1.
Thermal tests: The worst case scenario for the filter in a thermal point of view is an
operation point where continuous current is maximum, DC-link voltage as high as
possible and duty cycle d = 1/2. At this point current ripple is the biggest.
2.
Current tests: The worst operation point for current (both choke and output) is same as
in the thermal tests. With interleaving, the output performance can be verified also with
other peak and valley points of the duty cycle curve. If a capacitor is used, also the
capacitor current should be measured.
3.
Voltage tests: The voltage at the battery terminals should be measured. If the cable to
the battery is long, the voltage at the filter output can also be measured. The voltage
against ground is also interesting.
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