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
BLOCK DIAGRAM:
Fig.
1 DVR strcuctures
a
Conventional DVR
b
CHB-based DVR
EXPECTED SIMULATION RESULTS
Fig.
2 Three-phase voltage sag
a
Network voltage
b
Injected voltage by the DVR
c
Load-side voltage
Fig.
3 Voltages of the DC link capacitors
Fig.
4 Unbalanced voltage sag (a 20% voltage sag on phase A)
a
Source voltage
b
Injected voltage by the DVR
c
Load-side voltage
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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3
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4
Bollen, M.H.J.: ‘Understanding power quality problems: voltage sags and interruptions’
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