01-06-2012, 04:39 PM
Seven-Level Shunt Active Power Filter for High-Power Drive Systems
Seven-Level Shunt Active Power Filter for High-Power Drive Systems.pdf (Size: 261.21 KB / Downloads: 67)
INTRODUCTION
ADJUSTABLE-SPEED motor drives (ASDs) have found
extensive application in a variety of high-power systems.
One example is the electric propulsion system used in modern
naval ships, the power ratings of which can be tens of
megawatts. Typically, the front-ends of such ASDs employ a
diode or a thyristor rectifier. In spite of their simple control and
robust operation, these devices can generate voltage and current
harmonics that might affect the operation of other devices in
the same ac system. Conventionally, passive LC filters are used
to mitigate harmonic-related problems. However, due to their
large size and inflexibility, passive filters are gradually being
replaced by active filters that utilize power electronic inverters
to provide compensation for harmonics [1].
ACTIVE FILTER TOPOLOGY
The proposed active filter topology is shown in Fig. 1. It consists
of an H-bridge configuration made from three-level flying
capacitor branches. Essentially, it is a voltage-source inverter
(VSI) with capacitive energy storage (Cdc) shared by all three
phases.Atotal of eight switching devices are used in each phase.
A tapped reactor is used to connect the two legs of the Hbridge.
Typically, the reactor is wound to be center tapped,
making the output line-to-ground voltages (vag for example) the
average of the voltages from each side of the H-bridge. Then,
the line-to-ground voltages will have five distinct voltage levels
[9]–[12]. However, with this topology, the tap is set at 1/3. This
results in seven distinct output voltages, and therefore, improves
the power quality. The switching operation is described next,
wherein all seven levels are clearly illustrated.
ACTIVE FILTER CONTROL
To effectively compensate the load harmonic currents, the
active filter controller should be designed to meet the following
three goals:
1) extract and inject load harmonic currents;
2) maintain a constant dc capacitor voltage;
3) avoid generating or absorbing reactive power with fundamental
frequency components.
Harmonic Current Regulator
A current regulator is needed to generate the commanded
compensation current. Generally, a hysteresis control provides
fast response and is suitable for nonsinusoidal current tracking.
However, it suffers from some serious disadvantages such
as variable switching frequency and phase interaction problems
[1]. In addition, to fully take advantage of the benefits of a
multilevel converter, a current regulator that uses a voltagesource
pulsewidth modulation (PWM) is desirable. Several
frequency-selective harmonic current regulators were proposed
in [14]–[17] that achieve zero steady-state error for the dominant
harmonics. Nonetheless, they all have to target specific
frequencies and require a significant amount of computation
time.
Magnetizing Current Minimization
The current through the reactor consists of two components.
One is the compensating current that flows out of the tap terminal
and is shared by the two parts of the reactor. The other is the
magnetizing current that is generated when an average voltage
is applied across the reactor input terminals. The magnetizing
current does not contribute to the filtering function and should
be minimized to reduce current ratings of the switching devices
and avoid reactor saturation. Ideally, the magnetizing current
has a zero average component. In practice, however, the magnetization
current tends to drift away from zero if uncontrolled
because of the differences in component parameters and controller
errors.
CONCLUSION
A new type of power converter has been introduced in this
paper. The converter is based on parallel connection of phase
legs through an interphase reactor. However, the reactor has
an off-center tap at one-third resulting in an increased number
of voltage levels. Specifically, two three-level flying capacitor
phase legs are paralleled in this way to form a seven-level power
converter. The converter is utilized in an active filter application.
The details of the high-level control as well as the switching
control have been presented. The control ensures reactor current
sharing aswell as flying capacitor voltage balance. The proposed
active filter has been validated for a naval ship board power
system using detailed simulation and RTDS hardware.