This dissertation proposes a wind energy conversion system is composed of a wind turbine PMSG, a rectifier, and an inverter. The wind turbine PMSG transforms the mechanical power from the wind into the electrical power, while the rectifier converts the AC power into DC power and controls the speed of the PMSG. The controllable inverter helps in converting the DC power to variable frequency and magnitude AC power. With the voltage oriented control, the inverter also possesses the ability to control the active and reactive powers injected into the grid. Multilevel inerter is used to step up the voltage and to reduce the THD. Here nine level and eleven level inverter are used and the voltage increases and THD reduces from 12.87 % to 7.46 %. Active and reactive power is controlled dc stabilization and the reactive power is near to unity Here PI controller is used to control the inverter output rms voltage and LC filter is used to remove the harmonics available in the system.

1. IJSRD - International Journal for Scientific Research & Development| Vol. 2, Issue 07, 2014 | ISSN (online): 2321-0613
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Grid‐Connection Control and Simulation of PMSG Wind Power System
Based on Multi‐Level NPC Converter
Sandeep Kumar Gautam1 K P Singh2
1
M.Tech Student 2
Associate Professor
1,2
Department of Electrical Engineering
Abstract— This dissertation proposes a wind energy
conversion system is composed of a wind turbine PMSG, a
rectifier, and an inverter. The wind turbine PMSG
transforms the mechanical power from the wind into the
electrical power, while the rectifier converts the AC power
into DC power and controls the speed of the PMSG. The
controllable inverter helps in converting the DC power to
variable frequency and magnitude AC power. With the
voltage oriented control, the inverter also possesses the
ability to control the active and reactive powers injected into
the grid. Multilevel inerter is used to step up the voltage and
to reduce the THD. Here nine level and eleven level inverter
are used and the voltage increases and THD reduces from
12.87 % to 7.46 %. Active and reactive power is controlled
dc stabilization and the reactive power is near to unity Here
PI controller is used to control the inverter output rms
voltage and LC filter is used to remove the harmonics
available in the system.
Key words: Multilevel inverter, Voltage source inverter,
matlab Simulink, rectifier
I. INTRODUCTION
Many industrial applications have begun to use high power
apparatus in recent year. Medium power motor drives and
utilities require medium voltage and higher power level. In a
medium voltage grid, connecting only one power
semiconductor switch directly will create problem. To
overcome this problem, a multi level inverter topology has
been introduced as an alternative in medium voltage and
high power situations. A multilevel inverter use renewable
energy as source and can achieve high power rating. So,
renewable energy sources such as solar, fuel cells and wind
can be easily interfaced to a multilevel inverter structure for
a high power application. The multilevel inverter concept
has been used since past three decades. The multilevel
inverter begins with a three-level inverter. Thereafter, many
multilevel inverter topologies have been developed.
However, the main concept of a multilevel inverter is to
achieve high power with use of many power semiconductor
switches and numerous low voltage dc sources to obtain the
power conversion that lookalike a staircase voltage
waveform. The dc voltage sources for multilevel inverter are
given by battery, renewable energy and capacitor voltage
sources. The proper switching of the power switches
combines these multiple dc sources to achieve high power
output voltage. The voltage rating of the power
semiconductor devices depends only upon the total peak
value of the dc voltage source that is connected to the
device. Three major classification of multilevel inverter
structures [1-3] are cascaded H-bridge inverter with separate
dc source, diode clamped (neutral-clamped), and flying
capacitor (capacitor clamped).
A. Modeling Of Permanent Magnet Synchronous Machines
Permanent magnet synchronous machines/generators
(PMSM/PMSG) direct-drive wind power generation system
term for transforming the mechanical power into electrical
power. A rigorous mathematical modeling of the PMSG is
the
Fig. 1: Cross-section view of the PMSM
II. POWER AND TORQUE ANALYSIS OF A PMSM
For any PMSM, the electrical power input can be expressed
in the abc reference frame as follows:
𝑃𝑎𝑏𝑐 = 𝑣 𝑎𝑠 𝑖 𝑎𝑠 + 𝑣 𝑏𝑠 𝑖 𝑏𝑠 + 𝑣𝑐𝑠 𝑖 𝑐𝑠 (1)
or in the dq-axes reference frame as follows:
𝑃𝑑𝑞 =
3
2
(𝑣 𝑑𝑠 𝑖 𝑑𝑠 + 𝑣𝑞𝑠 𝑖 𝑞𝑠) (2)
As a part of the input power, in the motoring mode,
the active power is the power that is transformed to
mechanical power by the machine, which can be expressed
as follows:
𝑃𝑒𝑚 =
3
2
(𝑒 𝑑 𝑖 𝑑𝑠 + 𝑒 𝑞 𝑖 𝑞𝑠) (3)
Where 𝑒 𝑞 = 𝑤𝑒 𝐿 𝑑 𝑖 𝑑𝑠 + 𝑤𝑒∅ 𝑟 = 𝑤𝑒∅ 𝑑 (4)
𝑒 𝑑 = −𝑤𝑒 𝐿 𝑞 𝑖 𝑞𝑠 = −𝑤𝑒∅ 𝑞 (5)
Here,𝑒 𝑑 and 𝑒 𝑞 , are the back EMFs in the dq-axes
reference frame, and ∅ 𝑑 and ∅ 𝑞 are the dq-axes flux
linkages. Substituting expressions (4) and (5) into (3), the
active power can be re-expressed as follows:
𝑃𝑒𝑚 =
3
2
𝑤𝑒(∅ 𝑑 𝑖 𝑞𝑠 − ∅ 𝑞 𝑖 𝑑𝑠) (5)
Hence, the electromagnetic torque developed by a PMSM
can be deduced as follows:
𝑇𝑒 =
𝑃𝑒𝑚
𝑤 𝑒
𝑝
2
=
3
2
(
𝑝
2
) (∅ 𝑑 𝑖 𝑞𝑠 − ∅ 𝑞 𝑖 𝑑𝑠) (6)

2. Grid‐ Connection Control and Simulation of PMSG Wind Power System Based on Multi‐ Level NPC Converter
(IJSRD/Vol. 2/Issue 07/2014/021)
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III. MULTILEVEL INVERTER
Multilevel inverter is best suited in power sector. It is used
as to control the active and reactive power of grid .In the
grid control system multilevel inverter is used to
(1) The stability of dc voltage.
(2) Inverter power factor is unity.
(3) Input current contain low harmonics [4].
The general function of multilevel inverter is to
synthesize the dc voltage source. Multilevel inverter have
capability to increase the high step up voltage without the
use of transformer [5]-[9].
By choosing appropriate angle of multilevel
inverter we can eliminate the harmonics in a specified
output waveform. The necessary conduction angle can be
calculated by choosing appropriate phase voltage the output
voltage Va0 can be given as [10]-[13]:-
𝑉𝑎𝑜 = 𝑉𝑎1 + 𝑉𝑎2 +V𝑎3+V𝑎4+… (7)
Since the waveform is symmetrical to X-axis hence
the Fourier coefficient An and Ao become zero. Hence only
the analysis of Bn is perform:
The standard Fourier series equation are given below
𝑓(𝑥) = 𝑎0 + ∑ (𝑎 𝑛 cos
𝑛𝜋𝑥
𝐿
+ 𝑏 𝑛 sin
𝑛𝜋𝑥
𝐿
)
∞
𝑛=1
… (8)
in term of Bn the equation can be written as:
𝐵𝑛 = (
4𝑉𝑑𝑐
𝑛𝜋
) ∑ 𝐶𝑂𝑆(𝑛𝛼 𝜅)∞
𝑗=1 … (9)
j = no. Of dc source
n= no of odd harmonics
𝛼 𝜅 = switching angle of level k
By solving above equation (3)
cos 𝛼1 + cos 𝛼2 + ⋯ … … . cos 𝛼 𝑘 =
𝑗𝑀
4
cos 3𝛼1 + cos 3𝛼2 + ⋯ + cos 3𝛼 𝐾 = 0
cos 5𝛼1 + cos 5𝛼2 + ⋯ + cos 5𝛼 𝐾 = 0
The above equation can be solved by using Newton
Raphsonmethod to find switching angle.
Fig. 2: Simulink model of full circuit
A. Simulink Model of Wecs
Fig. 3: Simulink model of wind energy conversion system
Fig. 4: Simulink model of wind turbine
Fig. 5: wind speed at 12 m/sec
Fig. 6: waveform of wind turbine at 12 m/sec wind speed

3. Grid‐ Connection Control and Simulation of PMSG Wind Power System Based on Multi‐ Level NPC Converter
(IJSRD/Vol. 2/Issue 07/2014/021)
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Fig. 7: Stator current of PMSG
Fig. 8: Simulink model of generator terminal
Fig. 9: Dc link Voltage
Fig. 10: Simulink model of nine level inverter
B. Simulation Results Of Nine Level Inverter
Fig. 12: phase voltage waveform of nine level inverter

4. Grid‐ Connection Control and Simulation of PMSG Wind Power System Based on Multi‐ Level NPC Converter
(IJSRD/Vol. 2/Issue 07/2014/021)
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Fig. 14: phase voltage waveform
Fig. 15: line voltage of nine level inverter
Fig. 16: line voltage waveform
Fig. 17: FFT analysis of nine level inverter
Fig. 18: Simulink model of eleven level inverter
C. Simulation Result Of Eleven Level Inverter
Fig. 19: phase voltage waveform

5. Grid‐ Connection Control and Simulation of PMSG Wind Power System Based on Multi‐ Level NPC Converter
(IJSRD/Vol. 2/Issue 07/2014/021)
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Fig. 20: phase voltage waveform
Fig. 21: line voltage of eleven level inverter
Fig. 22: line voltage of eleven level inverter
Fig. 23: FFT Analysis for eleven level inverter
Fig. 24: wind and Load active& reactive power with 9 level
Fig. 25: wind and Load active& reactive power with 11
level
IV. CONCLUSION
Active and reactive power is controlled dc stabilization and
the reactive power is near to unity Here PI controller is used
to control the inverter output rms voltage and LC filter is
used to remove the harmonics available in the system.
As the wind speed varies the voltage and current
also varies these affects the grid power as speed are going to
decreasing the active and reactive power also decreasing the
pitch angle controller is used to control the blade angle at
𝜃 = 0. The MPPT is used to track the maximum power
From the above we can conclude that as the level of
inverterIncreases line and phase voltage waveform increase
and the harmonic induced is decreases. Using eleven levels
neutral point clamped converter and nine levels NPC the
harmonics reduces significantly. This is the advantage of
multilevel inverter. From the FFT analysis of nine level
NPC inverter and eleven level NPC inverter the THD of
nine level is 12.87% which has more than eleven level NPC
inverter is 7.46% i.e. as the level of inverter increases THD
decreases.
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6. Grid‐ Connection Control and Simulation of PMSG Wind Power System Based on Multi‐ Level NPC Converter
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