ÇUKUROVA UNIVERSITY INSTITUTE OF NATURAL AND APPLIED ...

4. MODELING OF PROPOSED DVR Mustafa İNCİ
Depending on the phase angle of the grid voltage during the sag, the DVR has
to deliver a higher voltage amplitude to restore the correct voltage magnitude,
because the phase jump of the grid has also to be compensated by the DVR.
Therefore, the system has to be designed for a higher maximum voltage. In addition,
less energy from the DC-link can be extracted (Meyer at al., 2008). Figure 4.22
shows simulation results for 30% sag with 12 o angle jump starts at 0.3s with a
duration of 0.1s. The amplitude of V
error, presag
is higher than V
error, inphase
.
4.3.3. Voltage Control Strategy: Closed Loop
In the case of voltage compensated APFs, output filters nand inverters are the
main components. The inverter output voltage passes through the output filters to get
switching-ripple-free compensation voltages. However, the output filters bring in
time delay and resonance problems on the compensation voltage also. Thus, proper
control methods are required to get the output compensation voltage according to a
reference value (Kim et al., 2005). The accuracy and dynamic operation of dynamic
voltage restores is an important issue. Basically, there are two voltage control
strategies used in the dynamic voltage restorer: open loop and closed-loop.
Open Loop method is uncontrolled and poor dynamic performance due to
lack of its simple structure. Another disadvantage of using such open-loop control
scheme is that the steady-state load voltage may not be compensated to the desired
value owing to voltage drop across the transformer series impedance and the filter.
This becomes particularly important if the load is nonlinear as nonsinusoidal currents
drawn by such a load can distort the load voltage(Vilathgamuwa et al., 2002). In
proposed method, Closed loop method is preferred due to its strong dynamic
behavior. For the DVR injection voltage control, a multiloop control scheme is
implemented as illustrated in Figure 4.22.
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