IEEE Transactions on Power Electronics, 2017
ABSTRACT:
This paper presents a two degrees of freedom (2DOF)
control scheme for voltage compensation in a dynamic voltage restorer (DVR). It
commences with the model of the DVR power circuit, which is the starting point
for the control design procedure. The control scheme is based on a 2DOF
structure implemented in a stationary reference frame (α−β), with two nested controllers used to obtain
a pass-band behavior of the closed-loop transfer function, and is capable of
achieving both a balanced and an unbalanced voltage sag compensation. The 2DOF
control has certain advantages with regard to traditional control methods, such
as the possibility of ensuring that all the poles of the closed-loop transfer
function are chosen without the need for observers and reducing the number of
variables to be measured. The use of the well-known double control- loop
schemes which employ feedback current controllers to reduce the resonance of
the plant is, therefore, unnecessary. A simple control methodology permits the
dynamic behavior of the system to be controlled and completely defines the
location of the poles. Furthermore, extensive simulations and experimental results
obtained using a 5 kW DVR
laboratory prototype show the good performance of the proposed control
strategy.
KEYWORDS:
1.
Power Quality
2.
Dynamic
Voltage Restorer (DVR)
3.
Control Design
4.
Resonant
Controller
5.
Stationary
Frame Controller
6.
Voltage Sag.
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.
1. Power system with a DVR included.
Figure
2. DVR simulation for a balanced voltage sag. (a) Line-to-neutral
three-phase
voltages at PCC, (b) line-to-neutral voltages generated by the
DVR,
(c) line-to-neutral load voltages, and (d) error signal in α − β (redblue).
Figure 3 DVR simulation for an
unbalanced voltage sag. (a) Line-to-neutral
three-phase voltages at PCC, (b)
line-to-neutral voltages generated by the
DVR, (c) line-to-neutral load voltages,
and (d) error signal in α − β (redblue).
Figure 4. DVR simulation for a 30 %
balanced voltage sag. (a) Line-toneutral
three-phase voltages at PCC, (b) error
signal in α − β (red-blue) for
the 2DOF-Resonant scheme, (c) error
signal in α − β (red-blue) for doubleloop
scheme, and (d) error signal in α−β
(red-blue) for the double-loop with
Posicast scheme.
Figure 5. DVR simulation for a 30 % type-E
unbalanced voltage sag. (a)
Line-to-neutral three-phase voltages at
PCC, (b) error signal in α − β (redblue)
for the 2DOF-Resonant scheme, (c) error
signal in α − β (red-blue)
for double-loop scheme, and (d) error
signal in α − β (red-blue) for the
double-loop with Posicast scheme.
This
paper presents a control scheme based on two nested controllers for voltage sag
compensation in a DVR. The nested regulators provide the control with two
degrees of freedom,
and
the control scheme is implemented in the stationary reference frame.
Furthermore, in order to accomplish the requirements for voltage sag
compensation, it is necessary to track the component at the fundamental
frequency. This is achieved using a resonant term in one of the controllers. The
proposed control design methodology is able to define all the poles of the
closed-loop system without observers and with a reduction in the number of
variables that must be measured, thus making it possible to avoid the use of the
traditional current loop employed in control schemes for the DVR. The structure
with the nested regulators achieves perfect zero tracking error at the nominal
frequency and blocks the DC offset, signifying that it has some advantages over
other control methods, such as double-loop schemes with proportional-resonant
regulators. Moreover, the design methodology is thoroughly explained when the
delay in the calculations is taken into account.
In this case, the design procedure allows the
dominant poles of the closed-loop system to be chosen. If the closed-loop poles
are chosen carefully, this control structure can also be applied to other
systems which require higher delays, e.g., power converter applications with a
reduced switching frequency. The design methodology can additionally be
extended to the discrete domain. Comprehensive simulated and experimental
results corroborate the performance of the 2DOF-Resonant control scheme for
balanced and unbalanced voltage sags. The proposed control scheme is able to
compensate both types of voltage sags with a very fast transient response and
an accurate tracking of the reference voltage, even when the different types of
loads and frequency deviations of the grid voltages are considered. Extended
comparisons with a PR controller using a double-loop scheme and a PR controller
in a double loop with a Posicast regulator have been carried out, demonstrating
that the performance of the 2DOF-Resonant controller is superior in all cases.
Moreover, the study of the stability as regards parameter variations for the
compared control schemes demonstrates the more robust behavior of the
2DOF-Resonant control scheme.
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