AN3095 datasheet(21/55 Pages) STMICROELECTRONICS

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AN3095

DC-AC converter

Doc ID 16555 Rev 3

21/55

DC-AC converter

The DC-AC inverter is a standard single-phase full bridge based on IGBTs with ultrafast co-

pack diodes, as depicted in Figure 3. The connection to the grid is realized by means of

current control performed in DQ rotating reference frame. An LCL filter is placed between

the bridge and the grid in order to reduce the current harmonics generated by the unipolar

sinusoidal pulse-width modulation (USPWM) at 17 kHz. L filters or LC filters may also be

chosen for the application, but in the first case large values of inductance are required to

perform good high frequency noise damping and large currents through the capacitor may

arise in the second case together with high voltage harmonics. LCL filters show good

performance in terms of current harmonic reduction but they may lead to instability of the

control loop in the presence of large grid impedance. This instability is due to the presence

of extra poles introduced by the additional inductor. The problem may be solved with proper

filter design and by adding a damping resistor in series with the filter capacitor.

The value of Lf is designed in order to limit the current ripple to about 10 % of the nominal

current value according to:

Equation 44

The filter capacitor value is designed to limit the exchange of reactive power below 5 % of

nominal active power:

Equation 45

To avoid resonance problems for the filter, due to low and high order harmonics, its resonant

frequency, given by

, should be in a range between ten times the line

frequency and one half of the switching frequency:

Equation 46

In fact, if the resonant frequency is too small the filter resonance increases the low

frequency harmonics and, in the same way, if it is too high it increases the harmonics

multiple of the switching frequency.

With a filter capacitor value of 3.3 µF and a grid inductor value of 2 mH the resulting

resonant frequency is 2771 Hz, which is in the specified range.

() ()

17000

324

450

grid

BUS

−

⋅

−

352

grid

reactive

≤

≥

≤

res

kHz

500

res

grid

≤

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