22-12-2012, 06:05 PM
Partial Power Conversion Device Without Large Electrolytic Capacitors for Power Flow Control and Voltage Compensation
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Abstract
A novel partial power conversion device (PPCD) is
proposed to realize power flow control and voltage compensation
in a three-phase power distribution system in this paper. The
PPCD circuit, which is derived from the conventional push–pull
forward converter, can achieve arbitrary voltage output without
any large electrolytic capacitors. Thus, the system reliability can
be enhanced. Furthermore, the converter has no full-rated components,
which reduces the cost. In this paper, an injection model for
power flow control is derived. A minimum power transfer control
strategy is proposed to minimize the power losses during the operation.
A closed-loop control method employing the synchronous
reference frame theory for voltage compensation is also developed
to enable the precise control. The systems with PPCD are simulated
byMATLAB/Simulink to verify the functions. The experiments for
voltage compensation are carried out based on a 30-kW prototype,
which shows the effectiveness.
INTRODUCTION
AS the price of the fossil fuel keeps on growing in the last
decades, the government and public people show great
interest to the renewable energy applications. The fossil-fuelbased
generation system is shifting to the renewable energybased
generation one in the electric grid, which is promoted by
the government in many countries [1]. The increasing penetration
of renewable energy, the growing demand of the electrical
power, and the aging of networks make it desirable to control
the power flow in power-transmission systems fast and reliably
[2]. On the other hand, as the rapid development of the
industrial economy, soaring installation of the nonlinear loads
greatly degrade the power quality of the grid.
PPCD CIRCUIT DEMONSTRATION AND ANALYSIS
The proposed PPCD circuit is derived from the push–pull
forward dc-dc converter [24]. Fig. 2(a) shows the schematic
of the ac–ac push–pull forward converter where the input and
output voltage is ac and the MOSFET S1 and S2 are replaced
by two bidirectional switches. The transformer T1 is a threewinding
transformer where the turns of two primary side and
secondary side are nP 1 , nP 2 , and ns , respectively. The turn
ratio N of T1 is expressed as N = ns /np where nP = nP 1 = nP 2 .
The coupling reference is pointed by “∗.” The advantage of the
topology is that the energy in the leakage inductances Lk 1 and
Lk 2 of T1 can be absorbed and recycled by adding a clamping
capacitor CS . As a result, the voltage stress on S1 and S2 is
limited.
Power Flow Control Model Analysis
A well-developed model for the UPFC device is described in
[25]. In this paper, the PPCD for power flow control is modeled
similarwith themodeling processmentioned in [25]. The system
can be first modeled as Fig. 4(a) shows. The PPCD is represented
by an ideal series voltage source VC in series with the line
impedance XL . The voltage on BUS#1 and BUS#2 is given in
(3) and (4), and the output voltage VOUT satisfies (5)
VOLTAGE COMPENSATION METHOD DEVELOPMENT AND
CLOSED-LOOP CONTROL ANALYSIS
The proposed PPCD is also designed to handle the voltage
problems in the power distribution system. In this section, the
system configuration for voltage compensation is given. Under
this configuration, a simple closed-loop control method is realized
with the control block diagram displayed, which shows
that the system is easy to handle by applying the conventional
proportional-integral-derivative (PID) controller.
Fig. 7 shows the system configuration for voltage compensation.
Vin is the voltage on point of common coupling (PCC),
which is distorted by other disturbance sources.
Closed-loop Control Realization
A control architecture, which is based on the parallel form
of a DVQS-applied converter, is proposed in [4] and [20] to
realize closed-loop control for power factor correction and current
harmonic filtering. SRF control theory is employed in the
architecture to extract the fundamental component and also the
harmonic components in the current waveform. In a similar
way, the closed-loop control for the PPCD is proposed to realize
fundamental voltage regulation and harmonics elimination
function. The control architecture is shown in Fig. 8.
CONCLUSION
In this paper, a novel PPCD is proposed to realize power
flow control and voltage compensation in the three-phase power
distribution system. The PPCD circuit is attributed to a direct
ac–ac converter, which is derived from a push–pull forward dc–
dc converter. It can achieve arbitrary voltage output without any
large electrolytic capacitors. Thus, the system reliability can be
enhanced. Furthermore, the converter has no full-rated components,
which reduces the cost. In this paper, the basic PPCD
circuit is introduced first. Next, an injection model along with
a minimum power transfer control strategy for power flow control
is developed. As a result, the circuit will have the minimum
power loss during the operation. Then, the system configuration
and the closed-loop control method employing the SRF theory
for voltage compensation are developed to enable the precise
control. In order to verify the functions, both of the systems
with PPCD are simulated by MATLAB/Simulink. Finally, the
experiments for voltage compensation are carried out based on
a 30-kW prototype, which shows the effectiveness.