A NEUTRAL POINT CLAMPED MULTILEVEL RECTIFIER ... - LabPlan

XVIII Congresso Brasileiro de Automática / 12 a 16-setembro-2010, Bonito-MS
A NEUTRAL POINT CLAMPED MULTILEVEL RECTIFIER FOR GRID CONNECTED WIND
ENERGY SYSTEM
FRANCISCA L. C. PIRES, FERNANDO L. M. ANTUNES, DEMERCIL S. OLIVEIRA JR.
Grupo de Processamento de Energia e Controle, Depto. de Engenharia Elétrica, Universidade Federal do
Ceará
Caixa Postal 6001, 60455-760, Campus do Pici, Fortaleza, CE, Brasil
E-mails: francisca_livia@yahoo.com.br, fantunes@dee.ufc.br,
demercil@dee.ufc.br
Abstract⎯ In this paper a neutral point clamped converter is used as a machine side converter in a wind energy conversion system.
This converter was used as a rectifier. For the rectifier operation was developed a maximum power point tracker control, without
speed sensors, associated to a power factor control, using a one cycle control technique. Simulation results are presented to access the
performance of the NPC rectifier along with the PFC and MPPT strategies.
Keywords⎯ Rectifier, neutral point clamped, wind energy system, one cycle control, maximum power point tracking.
Resumo⎯ Neste artigo um conversor do tipo neutral point clamped é utilizado como conversor no lado da máquina em um sistema
de conversão de energia eólica. Este conversor foi implementado como um retificador. Para a operação do retificador foi desenvolvido
um controle rastreador do ponto de máxima potência, sem a necessidade de sensores de velocidade, associado a um controle de correção
de fator de potência utilizando a técnica de controle de um ciclo. Resultados de simulação são apresentados para comprovar o desempenho
do retificador NPC com as estratégias de PFC e MPPT escolhidas.
Palavras-chave⎯ Retificador, ponto neutro grampeado, sistema de energia eólica, controle de um ciclo, seguidor de máxima potência.
1 Introduction
The development of renewable energy sources
has attracted considerable interest in recent years.
Several techniques have been used for connecting
these sources to the electrical grid.
Currently, obtaining electricity from the wind offers
the cheapest economic perspectives of renewable
energy sources. Wind generators integrated with
power electronic interfaces are becoming popular
due to their capability of extracting optimal energy
capture (Raju et. al., 2003).
The wind conversion systems could use fixed or variable
speed turbines and different kinds of generators
(squirrel cage induction, wound rotor induction,
doubly-fed induction and permanent magnet synchronous
generator). Also could present several converter
topologies and different ways of doing the
interconnection, depending on generator and turbine
types (Blaabjerg et. al., 2007).
For allowing the interconnection many control techniques
for the AC/AC converters are applied. Basically
it is necessary a rectifier with a power factor
correction capability using a PLL or another method
of control and a maximum power point tracker
(MPPT) to allow the maximum power extraction
from the turbines, a DC link which voltage can be
controlled by the rectifier or the inverter, a DC-AC
converter with the control of active and reactive
power injected in the grid.
1583
Multilevel converters have become popular in the
last years due to advantages at medium and high
voltage applications. Three-level converters are the
most popular because of their simplicity and efficiency.
Advantages of multilevel converters include good
power quality, good electromagnetic compatibility
(EMC), low switching losses, and high voltage capability.
The main disadvantages of this topology
are that a larger number of semiconductor switches
are required and the voltage on the DC side must be
supplied by a capacitor bank with several capacitors
or by several isolated voltage sources (Corzine,
2003).
This paper presents a three-level neutral point
clamped converter (NPC) in a back-to-back conversion
system for wind generators connected to the
grid. The NPC converter was designed to operate as
the machine side converter. For efficient operation is
required for the converter a power factor control
(PFC) and a MPPT.
For extracting the optimum power from the wind, a
simple MPPT strategy is used without the knowledge
of the turbine parameters (Reis et. al., 2007).
The power factor correction uses the principle of on
one-cycle control with a simple sine-triangle modulation
(Bento, 2009). Simulation results are presented
to access the performance of the NPC rectifier
along with the PFC and MPPT strategies.

XVIII Congresso Brasileiro de Automática / 12 a 16-setembro-2010, Bonito-MS
2 Wind Energy Conversion System
The configuration chosen for interconnection
with the grid is shown on Figure 1. This configuration
corresponds to a generator directly connected to
the grid through a back-to-back AC/AC converter
(Blaabjerg et. al., 2007). In this concept the generator
is completely decoupled from the grid (Pereira,
2008). The energy from the generator is rectified to
a DC link, and then it is converted to a suitable AC
energy for the grid (Carrasco et. al., 2006).
Figure 1. Wind energy system chosen
Each component showed in Figure 1 has its main
characteristics as following described (Blaabjerg et.
al., 2007e Carrasco et. al., 2006):
• Wind turbine capture power from the wind
and convert it to rotating mechanical power.
• The gear box can be used to convert the
low-speed, high-torque power to electrical
power, but it is not necessarily used.
• The generator can be a wound rotor synchronous
generator or a permanent magnet
synchronous generator.
• A multilevel power converter topology is
used to connect the stator windings to the
grid. For this system was considered a Neutral
Point Clamped (NPC) topology, which
presents a voltage with three levels.
The AC-AC converter is shown on Figure 2, in
which two NPC converters form the back-to-back
system, connected through a DC-link using two capacitor
banks with the same voltage. In this work is
described only the AC-DC side and its control methods
for this application. The control for the inverter
side was not implemented. For this application, it
has been considered a permanent magnet synchronous
generator with a rated power of 6kW.
3 Multilevel Power Converter
In order to decrease the cost per MW and to increase
the efficiency of wind energy conversion, nominal
power of wind turbines has been continuously
growing in last years (Carrasco et. al., 2006). So the
interest for multilevel converters implemented in
wind energy systems has been growing. This occurs
because of the low voltage stress across the switches.
1584
Therefore, multilevel converters are very useful in
medium and high voltage systems.
Figure 2. NPC in a back-to-back conversion system.
The different proposed multilevel converter topologies
can be classified in the following five categories,
shown in Figure 3 (Blaabjerg et. al., 2007):
• Diode clamped;
• Bi-directional switch interconnection;
• Flying capacitors;
• Multiple three-phase converters;
• Cascaded single phase H-bridge converters.
Figure 3. Multilevel topologies: a) one leg of a three-level diode
clamped converter; b) one leg of a three-level converter with bidirectional
switch interconnection; c) one leg of a three-level flying capacitor
converter; d) three-level converter using three two-level converters
and e) one leg of a three-level H-bridge cascaded converter.
As the ratings of the components increases and the
switching and conducting properties improve, the
advantages of applying multilevel converters become
more and more evident. The reduced content of
harmonics in the input and output voltage is highlighted,
together with the reduced EMI. Moreover,
the multilevel converters have the lowest demands
for the input filters or alternatively reduced number
of switching. For the same harmonic performance as
a two level converter, the switching frequency of a
multilevel converter can be reduced to 25% that results
in the reduction of the switching losses. Even
though the conducting losses are higher in the multilevel
converter, the overall efficiency depends on the
ratio between the switching and the conducting
losses (Carrasco et. al., 2006).
4 Diode Clamped Converter
One of the multilevel structures that is widely
used is the Diode clamped converter, presented on
Figure 3(a) An n-level Diode Clamped Converter
consists of (n-1) capacitors on the DC bus, 2(n-1)
switching devices per phase and 2(n-2) clamping
diodes per phase. The DC bus voltage is split into n
levels by using (n-1) DC capacitors. Each capacitor

XVIII Congresso Brasileiro de Automática / 12 a 16-setembro-2010, Bonito-MS
has Vdc/n volts and each voltage stress will be limited
to one capacitor level through clamping diodes.
Figure 4 show a three-level diode clamped converter.
The diode clamped converter has the following advantages
(Pereira, 2008):
• The switch must support a voltage of only
Vdc/(n-1) for a n-levels converter;
• The number of capacitors in DC-link is
smaller than in others multilevel topologies;
• In most cases it is not necessary a transformer;
• Switching losses and interferences are lower
than in two level converters.
While the disadvantages are (Pereira, 2008):
• Clamped diodes must be fast recovery and
support the rated current of the converter;
• Above three-levels, clamped diodes don´t
support the same voltage levels. Therefore,
are used series association of diodes that increase
the number of diodes;
• The voltage on the capacitors must be balanced,
that causes an additional problem to
the control.
5 Neutral Point Clamped Converter
The topology chosen was the three-level diode
clamped converter also known as Neutral Point
Clamped converter or NPC converter. This structure
was first proposed by Nabae et. al. in 1980. Figure 4
shows this topology. The NPC topology is shown on
Figure 4.
The DC bus voltage is split into 3 levels by using 2
DC capacitors, C1 and C2. Each capacitor has Vdc/2
volts and each voltage stress will be limited to one
capacitor level through clamping diodes. The phase
voltage, VAN has three states as given in Table 1
(Mailah et. al., 2009). However, the line voltage VAB
presents five-level voltage.
The mainly advantage for the NPC converter is the
reduced number of levels and semiconductors for a
good performance and low levels EMI and THD.
Figure 4. Neutral Point Clamped topology.
This paper describes a NPC converter operating as a
rectifier for a back-to-back system. For this application
was developed a maximum power point tracker
1585
and a power factor control described at the next sections.
Table 1 - Three-level NPC converter voltage levels and their switching
states for phase A
Voltage Level (VAN) Switches ON
-Vdc/2 Sa3, Sa4
0 Sa2, Sa3
Vdc/2 Sa1, Sa2
6 Maximum Power Point Tracker
Variations of wind on the turbine cause different
values of power generated. When this power is low
some technique must be used to take advantage of
the maximum power extracted from the turbine.
This can be done measuring the rotational speed of
the turbine and adjusting a reference signal or measuring
the output rectifier power, without need
knowing any parameter of the turbine.
The method chosen for the MPPT uses the principle
of perturbation and observation. The dc-link voltage
Vdc is considered constant. The output average current
Idc is measured and the power Pdc is compared
with the previous value, so a reference amplitude
signal (Vm) is changed. This signal Vm is used to
correct the switching pulses of the rectifier.
Figure 5 shows the MPPT control program flowchart
used (Reis et. al., 2007).
Figure 5. – MPPT control program.
The input current in each phase, IA, IB, and IC are
measured and compared to the variable carriers.
This MPPT calculates the output power of the generator
by measuring DC link current and then perturbs
the operating point by increasing/decreasing
the reference Vm shown on Figures 6 and 8.The new
value of power is then compared with the previous

XVIII Congresso Brasileiro de Automática / 12 a 16-setembro-2010, Bonito-MS
value and depending upon the error, Vm magnitude
is further increased or decreased.
7 Power Factor Control
Power factor correction is performed using the
principle of one cycle control (OCC). Figure 6 shows
the schematic of the OCC control for a closed loop
operation (Bento, 2009) in one phase. The variable
Vm, in the output of the voltage regulator, define the
magnitude of the current reference and the shape
current is obtained through a principle of voltage
tracking applied, in a convenient way, to a switching
period.
Figure 6. Schematic of OCC control.
Different from a conventional control with current
loop, this method needs only three sensors for the
three-phase input current and one for the DC output
voltage. There is not current control loop neither
input voltage sensor to form a current reference.
The value of the reference signal Vm on an open loop
operation is defined by:
V
m
R ⋅V
=
2⋅
R
S dc
e
(1)
where Rs represents the sensor gain and Re is the
emulated resistance of the rectifier:
R
e
V
=
I
g
g
(2)
To apply the OCC technique shown on Figure 6 in
NPC rectifier it is necessary three phase reference
currents, measured on the input rectifier that are the
modulators for the Pulse Width Modulation (PWM)
and compare these modulators with variable carriers.
It was chosen the simplest modulation strategy,
SPWM (Sinusoidal Pulse Width Modulation), in
which there is no zero sequence injection and it is
equivalent to connect the load neutral and the source
neutral. Two triangular carriers are necessary: one
positive and other negative. Both are compared with
the measured sinusoidal currents. This SPWM strategy
scheme is shown on Figure 7 (Bento, 2009).
On figure 8, a complete scheme of this strategy is
shown, considering a fixed voltage on DC-link.
Considering a fixed DC voltage, it is not required
the voltage regulator from Figure 6. The reference
1586
sign Vm is controlled by the MPPT control. Two triangular
carriers are obtained using the Vm magnitude.
This carries are compared to three-phase reference
currents measured on rectifier input. As result,
twelve control pulses are obtained.
Figure 7. Basic scheme of SPWM strategy for a NPC.
Figure 8. Complete scheme for SPWM in the NPC rectifier.
8 Simulation Results
The NPC multilevel rectifier was simulated using
the PSIM software, with the parameters of Table
2. In this simulation was considered a fixed output
voltage at the DC-link.
The power generated in a wind system is variable
and depends on the wind. Therefore, simulations
were made considering the rated value of input power
and lower values. To obtain different values of
power were used variances on the three-phase input
voltage.
At simulation the rectifier behavior was observed in
these different values of input voltage and current
for applying the MPPT control.
Table 2 – Considerations for the NPC design
Generator rated power 6 kVA
Generator rated voltage 220 V
Rectifier output voltage 800 V
Rectifier efficiency 0.97
Switching frequency 10 kHz
Input current ripple 1.82 A
Output voltage ripple 20 V

XVIII Congresso Brasileiro de Automática / 12 a 16-setembro-2010, Bonito-MS
For the simulation with rated values, the converter
presented a total harmonic distortion (THD) equal to
3.92% and a power factor equal to 0.9976.
Figure 9 shows the results for rated conditions.
Three-phase current assume the expected peak value
of 12.85 A. Input voltage and current in each phase
presents almost the same angle. The power factor
was calculated for the software and is also showed
on Figure 9.
Figure 10 shows the rectifier output average current
and the input current in one phase after a step on the
input voltage (from 220 to 110V). Without the
MPPT method input current and average output current
decrease to almost a half of their rated values.
Using a MPPT method these currents increase to
values next to the rated ones.
Figure 9. Line currents in a, b and c phases (above) and voltage and
current in one phase (below).
Figure 10. Current of phase A and average output current without
MPPT (above) and with MPPT (below).
9 Conclusion
This paper has presented a multilevel rectifier for
wind generation systems connected to the grid. This
rectifier was simulated with MPPT and PFC control.
The MPPT was implemented only by measuring
turbine output power, without the knowledge of
wind turbine parameters, and the PFC control without
a current loop. The results showed the effectiveness
of the converter topology and control strategy
1587
used. The simulation considered variable voltage
and frequency source, obtaining a maximum extracted
power with a unity power factor and a low
THD value. The laboratory prototype is still under
developed and the experimental results will be obtained
soon to confirm the simulation results.
Acknowledgement
The authors thank to CNPq for the financial support
and incentive to scientific research, the GPEC for
the technical support and all the professors and students
who contributed with this work.
References
Bento, A. A. M (2009). A Técnica de Controle de
um Ciclo Aplicada à Correção do Fator de
Potência com Retificadores Boost. Doctorate
thesis in electrical engineering, Federal
University of Campina Grande.
Blaabjerg, F. and Iov F (2007). Wind Power - a
Power Source Now Enabled by Power
Electronics. 9 th Brazilian Power Electronics
Conference, 2007, Blumenau.
Carrasco, J. M; Galvin, E; Portillo, R; Franquelo, L.
G. and Bialasiewicz, J. T (2006). Power
Electronic Systems for the Grid Integration of
Wind Turbines. IECON 2006 - 32nd Annual
Conference on IEEE Industrial Electronics,
ISSN 1553-572X , pp 4182 – 4188.
Corzine, K (2003). Operation and Design of
Multilevel Inverters. University of Missouri –
Rolla. Developed for the Office of Naval
Research.
Raju, A. B; Fernandes, B. G. and Chatterjee, K
(2003). A Simple Maximum Power Point
Tracker for Grid connected Variable Speed
Wind Energy Conversion System with Reduced
Switch Count Power Converters. Power
Electronics Specialist Conference. PESC '03.
IEEE 34th Annual, Vol. 2, pp. 748-753.
Mailah, N. F; Bashi, S. M; Aris, I; Mariun, N
(2009) Neutral-Point-Clamped Multilevel
Inverter Using Space-Vector Modulation.
European Journal of Scientific Research, ISSN
1450-216X, Vol.28, No.1, 2009, pp.82-91.
Pereira, I. F. B. F (2008). Projectar, Simular e
Implementar um Inversor Multinível. Master’s
degree dissertation in electrical engineering,
University of Porto.
Reis, M. M; Nascimento, R. M.G; Soares, B. L;
Fava, S. A. M; Freitas, E. M; Silva, C. E. A.
and Oliveira, D. S (2007). A Grid Connected
Variable Speed Wind Energy Conversion
System. 9 th Brazilian Power Electronics
Conference, 2007, Blumenau.