ABSTRACT
The Distributed Power Flow Controller (DPFC) is a
new device within the FACTS family. It is emerged from the UPFC and has
relatively low cost and a high reliability. The DPFC consists of two types of
converters that are in shunt and series connected to grids. The common dc link
between the shunt and the series converters is eliminated. The active power exchange
between the shunt and series converters that is through the common dc link in
the UPFC, is now though the transmission line at the 3rd harmonic frequency.
The redundancy of the series converters provides the high reliability of the
system. In this paper, the DPFC behavior during the failure of a single series converter
unit is considered. A control scheme to improve the DPFC performance during the
failure is proposed. The principle of the control is based on the facts that,
the failure of single series converter will lead to unsymmetrical current at
the fundamental frequency. By controlling the negative and zero sequence
current to zero, the failure of the series converter is compensated. In this
paper, the principle of the DPFC are firstly introduced, and followed by the
behavior of the DPFC during the failure of a single series converter. The
design of the control scheme and corresponding simulation are presented.
KEYWORDS
1.
Power Flow
Control
2.
Flexible AC
Transmission System
3.
Current
Control
4.
symmetrical
component
5.
Voltage Source
Converter
6.
Transmission
7.
Distributed
Power Flow Controller
8.
Unified Power
Flow Controller.
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM
CONCLUSION
This
paper analyzed the DPFC performance during a failure of a single series
converter unit. Series converters are protected by crowbar diodes to prevent
over-voltage at the secondary side of the single-turn transformer. Therefore
the failed series converter unit appears short-circuit to the transmission line
and the voltage injection is unbalanced between phases. Because of this
unbalance, the power network becomes asymmetric thereby resulting unsymmetrical
current at the fundamental frequency. Also, the 3rd harmonic current that used
to be zero sequence contains positive and negative components thereby leaking
to rest of networks. A supplementary control scheme is proposed to add at the
DPFC central control to improve the DPFC performance during series converter
failure. Its principle is to monitor the zero and negative sequence components
of the line current and control them to be zero. The control scheme has been
simulated in Matlab, and it proved that the asymmetric caused by the failure
can be totally compensated.
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