ABSTRACT:
Most power quality problems in distribution systems are
related to voltage sags. Therefore, different solutions have been examined to
compensate these sags to avoid production losses at sensitive loads. Dynamic
Voltage Restorers (DVRs) have been proposed to provide higher power quality.
Currently, a system wide integration of DVRs is hampered because of their high
cost, in particular, due to the expensive DC-link energy storage devices. The cost
of these DC-link capacitors remains high because the DVR requires a minimum
DC-link voltage to be able to operate and to compensate a sag. As a result,
only a small fraction of the energy stored in the DC-link capacitor is used,
which makes it impractical for DVRs to compensate relatively long voltage sags.
Present control strategies are only able to minimize the distortions at the load
or to allow a better utilization of the storage system by minimizing the needed
voltage amplitude. To avoid this drawback, an optimized control strategy is
presented in this paper, which is able to reduce the needed injection voltage
of the DVR and concurrently to mitigate the transient distortions at the load
side. In the following paper, a brief introduction of the basic DVR principle
will be given. Next, three standard
control strategies will be compared and an optimized control strategy is
developed in this paper. Finally, experimental results using a medium-voltage
10-kV DVR setup will be shown to verify and prove the functionality of the
presented control strategy in both symmetrical and asymmetrical voltage sag
conditions.
KEYWORDS:
1.
Asymmetrical
voltage sag
2.
Dynamic
voltage restorer (DVR)
3.
In-phase
compensation
4.
Optimized
compensation
5.
Pre-sag
compensation
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig.
1. Basic concept of a DVR.
Fig.
3. Measured voltages during a long, unbalanced sag.
CONCLUSION:
Voltage sags are a major problem in power systems
due to the increased integration of sensitive loads. DVR systems are able to
compensate these short voltage sags. The control and the design of these systems are critical. Present
control strategies are able either to minimize load distortions or the needed
voltage amplitude. Both requirements are of utmost importance, especially the
needed voltage amplitude for compensating a voltage sag leads to a strict
limitation of the range of operation without oversizing the converter
significantly.
In this paper, the basic concept of an optimized
solution is presented. Based on a combination of the pre-sag and in-phase compensation
methods, the proposed optimized DVR control strategy can react to a short
voltage sag avoiding disturbances to the protected load. While for a long
voltage sag, the proposed method is still able to generate the appropriate
voltage without over modulation (or oversized DC-link capacitor) and with
minimized load voltage transient distortions. Furthermore, medium voltage level
experimental results are presented to verify the feasibility of this control
strategy in both balanced and unbalanced voltage sag situations. Although, the
effect of the control strategy has only been shown for long but shallow sags,
similar results occur for deep sags or large phase jumps.
In this study, it was found that the required
voltage amplitude of the DVR with the proposed optimized control strategy was reduced
by 25%, compared to the pre-sag controller. In other words, the maximum
compensation time is increased by approximately the same amount. Taking into
consideration that a phase jump of 12 is not extremely high and that the
advantages increases with larger phase jumps, an even higher gain is possible in practical systems. Summarizing
all advantages up, it can be stated that the compensation time of existing DVR
systems under pre-sag control can be significantly improved when applying the
proposed optimized strategy. In newly designed DVRs, the DC-link capacitance
can be decreased without reducing the range of operation.
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