08-10-2012, 02:57 PM
A New Nine-Level Active NPC (ANPC) Converter for Grid Connection of Large Wind Turbines for Distributed Generation
A New Nine-Level Active NPC.pdf (Size: 2.09 MB / Downloads: 41)
Abstract
Wind power is one of the most promising emerging
renewable energy technologies for distributed generation (DG).
In this paper, a new nine-level active neutral-point-clamped (9L
ANPC) converter is proposed for the grid connection of large wind
turbines (WTs) to improve the waveform quality of the converter
output voltage and current. Therefore, the bulky passive grid filters
can be reduced or even removed. The topology, operating
principles, control schemes, and main features, as well as semiconductor
device selection of the proposed converter are presented in
detail. The floating capacitor voltage control based on redundant
switching states and capacitor prioritization is detailed. A comparison
between the new topology and other existing 9L topologies is
presented to illustrate the characteristics and performance of the
new converter. The proposed 9L ANPC converter is studied in the
case of the grid connection of a 6-MW WT without using passive
grid filters in DG systems. Simulation and experiment results are
presented to validate the proposed converter topology and control
schemes. The proper operation and the compliance to the harmonic
limit standards of the filterless grid-connected WT system
are verified by simulation results.
INTRODUCTION
AT PRESENT, a large number of distributed generation
(DG) units based on renewable energy sources (wind,
photovoltaic, geothermal, and hydro) are being developed and
installed, increasing energy conversion efficiencies, reducing
dependence on fossil fuels, and reducing CO2 emission [1]–[5].
Wind power is one of the most promising renewable energy
technologies for DG. Over the past few decades, the capacity
of wind turbine (WT) units has increased from a few tens of
kilowatts to today’s multi-megawatt level [6]. For high-power
WTs, medium voltage (MV) technology can be effectively applied
in the electrical energy conversion to reduce the copper losses of the system components and decrease the cable cost
by increasing the voltage and thus reducing the current [7].
Multilevel converters, which are one of the most popular MV
converter topologies, are very attractive for large WT applications
[8], [9]. Besides the capability to handle higher voltage
with lower voltage-rating devices, other features of multilevel
converters are more advantageous, such as lower harmonic distortion
of the output voltage, lower switching losses, as well as
reduced electromagnetic interference (EMI) [10], [11]. Among
various multilevel converters, the most well-known topologies
are the neutral-point-clamped (NPC), flying capacitor (FC), and
cascaded H-bridge (CHB) converters.
REVIEW OF THE 9L ANPC CONVERTERS
A five-level ANPC (ANPC5L) converter, shown in Fig. 2,
can generate five voltage levels at the output [14]. In [15], it
was applied on the grid side for a 5–6MW WT system, which
allows a filterless grid connection to meet the harmonic limits of
the IEEE519 Standard. However, it still requires a tuned passive
filter for the compliance with the more stringent VDEW standard.
Further improvement in the converter output waveform
quality is necessary for the reduction or even complete removal
of the passive grid filter. Optimizing converter topologies and
generating more number of voltage levels are more reasonable
and practical solutions forMWWT applications. Previous work
implied that 9L voltagewaveform shows a global compliance on
almost the whole range with VDEW Standard [16]. It is obvious
that, along with a higher number of voltage levels, the multilevel
converters can generate better output waveform quality.
However, this upgrade requires additional components, such as
semiconductor devices and capacitors, which increase the complexity,
cost, and reliability of the wind power converters and
the DG systems. Therefore, in this paper, we mainly focus on
9L converter topologies. In this section, the existing 9L ANPC
converters are reviewed and their characteristics are discussed
and summarized.
VERIFICATION OF THE PROPOSED 9L ANPC CONVERTER
To verify the proposed 9L ANPC converter topology and
control scheme, simulation and experiment are employed. The
test circuit diagram is shown in Fig. 12. Since we use redundant
switching states to control the floating capacitor voltage, there
is no interference between different phases of the converter.
Therefore, one phase of the new 9L ANPC PEBB is sufficient to
prove the correctness of the proposed topology and the capacitor
voltage control scheme. In this paper, the dc-link neutral-point
voltage control is not addressed in the controller design because
this control target is much easier to be implemented in the frontend
converter in the back-to-back converter configuration. Two
constant dc voltage sources are connected to the dc link. For
both simulation and experiment, the main parameters are: half
dc-link voltage Vdc = 60 V; fundamental frequency f = 60
Hz; carrier frequency fc = 2 kHz; and floating capacitors C1 =
2.7 mF and C2 = 5.4 mF. Load is 12-Ω resistor in series with
2.5-mH inductor.
CONCLUSION
In this paper, a new9LANPCconverter, named ANPC5L plus
HBBB converter, has been proposed to improve the waveform
and power quality, and thus reduce the size of the bulky passive
grid filters or even achieve filterless grid connection of large
WTs in DG systems.