18-09-2012, 11:01 AM
Design of D-STATCOM for Fast Load Compensation of Unbalanced Distribution Systems
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ABSTRACT
This paper introduces the design of distribution level
static compensator (D-STATCOM) for the load compensation
of unbalanced distribution systems. The D-STATCOM is
intended to replace the widely used static Var compensator
(SVC). For fast response requirement, the feedforward
compensation scheme is employed. This enables the DSTATCOM
to quickly balance the three-phase unbalanced load
and correct the power factor at the same time (called "load
compensation"). First, the compensation scheme of the DSTATCOM
is derived with the symmetrical component method.
Then, the system block diagram for simulation is built.
Accordingly, the simulation is performed with the program
Matlab/Simulink. Finally, The simulation results are given for
verifications.
INTRODUCTION
With the development of high technique industries, the
requirements to power quality have become strict. The
currently power quality problem mainly includes unbalance,
flicker, harmonics, sag, dip, swell, and interruption [I].
These power quality problems may cause abnormal
operations of facilities or even trip the protection devices.
Hence the maintenance and improvement of electrical power
quality have become an important scenario today.
The static Var compensators (SVCs) are presently used
in the power industry to effectively improve the flicker,
unbalance, and correct the load power factor [2]. But the
TCR of the SVC needs significant space demand. Due to the
natural limitation of communication of TCR, the SVC can't
response immediately with the change of load. This leads to
about 5 ms to a half-cycle of time delay. Hence the SVC has
merely a maximum compensation ability of about 65% to
flicker [3]. The distribution level static compensator (DSTATCOM)
[7-IO], which is one of the FACTS elements
[4-61, is a newly developed compensator. Comparing with
the SVC, the D-STATCOM has quicker response time and
compact structure. It is expected that the D-STATCOM will
replace the roles of SVC in the nearly future.
COMPENSATION SCHEME OF D-STATCOM
Fig.1 shows the single-phase equivalent circuit of DSTATCOM.
The D-STACOM employs an inverter to
convert the DC link voltage v& on the capacitor to a voltage
source of adjustable magnitude and phase. Therefore the DSTATCOM
can be treated as a voltage-controlled source. In
some applications, due to the Norton equivalent, the DSTATCOM
can also be seen as a current-controlled source.
In Fig. 1, the inductance L and resistance R represent the
equivalent circuit elements of the step-down transformer and
the inverter used for the D-STATCOM. The voltage is
the effective output voltage of D-STATCOM, 6 is the
power angle. The load power is 4 + iQ, , the source power is
F: + iQ., and the compensation power is F: + iQ. . The power
balance equations are shown in ( I ) and (2). The reactive
power output of the D-STATCOM is inductive or capacitive
can be controlled by the operation mode of the DSTATCOM,
as shown in Fig. 2.
CONCLUSION
Improvement of electrical power quality has become an
important scenario in recent years. Comparing with the
traditional SVC, the D-STATCOM has less space demand,
higher response time, and lower noise generated. The DSTATCOM
will gradually replace the widely used static var
compensator (SVC) in the nearly future. This paper
introduces the design of D-STATCOM for the fast load
compensation of three-phase three-wire distribution systems
with unbalanced load. For fast response requirement, the
feedfonvard compensation scheme is employed with
symmetrical components method. In this way, the modeling
and control of D-STATCOM is very similar with that of
SVC. Hence the existing understanding to SVC remains
useful. This makes the functions of D-STATCOM very easy
to be realized. Simulation results show that the proposed DSTATCOM
has very quick response time and compensation
effect. The experimental result