ABSTRACT:
This study presents design and analysis
of a dynamic voltage restorer (DVR) which employs a cascaded multilevel
inverter with capacitors as energy sources. The multilevel inverter enables the
DVR to connect directly to the medium voltage networks, hence, eliminating the
series injection transformer. Using zero energy compensation method, the DVR
does not need active energy storage systems, such as batteries. Since the
energy storage system only includes capacitors, the control system will face
some additional challenges compared with other DVR systems. Controlling the
voltage of capacitors around a reference voltage and keeping the balance
between them, in standby and compensation period, is one of them. A control
scheme is presented in this study that overcomes the challenges. Additionally,
a fast three-phase estimation method is employed to minimise the delay of DVR
and to mitigate the voltage sags as fast as possible. Performance of the
control scheme and estimation method is assessed using several simulations in
PSCAD/EMTDC and MATLAB/SIMULINK environments, and experiments on a 7-level
cascaded H-bridge converter.
SOFTWARE: MATLAB/SIMULINK
Fig.
1 DVR strcuctures a Conventional DVR b
CHB-based DVR
Fig.
2 Three-phase voltage sag
a
Network voltage
b
Injected voltage by the DVR
c
Load-side voltage
Fig.
3 Unbalanced voltage sag (a 20% voltage sag on phase A)
a
Source voltage
b
Injected voltage by the DVR
c
Load-side voltage
Fig.
4 Voltages of the DC link capacitors
Fig.
5 Three-phase 20% voltage sag with voltage harmonics
a
Network voltage
b Injected
voltage by the DVR
c
Load-side voltage
CONCLUSION:
This paper presented design and
performance assessment of a DVR based on the voltage sag data collected from
MWPI. Using a multilevel converter, the proposed DVR was capable of direct connection
to the medium voltage-level network without a series injection transformer. In
addition, development of zero active power compensation technique helps to
achieve voltage restoration goal just by the capacitors as energy storages. Due
to internal losses of H-bridge cells and probable inaccuracies in measurements,
voltage of DC link capacitors may become unequal, which prevents proper operation
of the converter. A voltage control scheme, comprised of three separate
controllers, was proposed in this paper for keeping voltage balance among the
DC link capacitors within nominal range. A fast estimation method was also
employed for calculation of phase and magnitude terms in an unbalanced
three-phase system. This estimation method is able to recognise voltage sags in
approximately half a cycle. Several simulations were performed in PSCAD/EMTDC
environment to verify the performance of CHB-based DVR. Additionally, a
laboratory-scale prototype of the proposed DVR was built and tested. Results of
the experimental test also confirmed validity of the proposed control system.
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