“International Journal for Science and Emerging
ISSN No. (Online):2250-3641
Technologies with Latest Trends” 9(1): 27-32 (2013)
ISSN No. (Print): 2277-8136
Mani Bansal* and Navneet Singh Bhangu**
*, ** Department of Electrical Engineering, Guru Nanak Dev Engineering College
Ludhiana, INDIA
(Received 27 June 2013 Accepted 3 July 2013)
-
Abstract—Power quality is a major concern for electrical engineers and researchers now days. Various power quality problems are voltage sag, swell, interruptions, harmonics etc. This paper discusses the problem of harmonics and its reduction in distribution network against the induction motor drive load. Harmonics distortion is reduced using Distribution static compensator
(DSTATCOM) modeling in the MATLAB/ Simulink environment. The control technique used is Instantaneous power theory which calculates the required current injected into the power system. The simulated results show the effectiveness of DSTATCOM in reducing harmonics distortion.
Keywords- Power quality, harmonics, DSTATCOM, Instantaneous power theory, MATLAB.
The intensive use of power electronic converters and non linear loads resulted in the deterioration of power quality which ultimately causes economical losses. Non linear loads pose harmonics into the power system which deviate the sinusoidal voltage and current waveform to non sinusoidal one. This distortion in the waveform is measured in term of index known as
Total harmonics Distortion (THD).THD may be defined as the ratio of the square root of the sum of squares of the rms value of harmonic component to the rms value of the fundamental component [2]. supported by short term energy stored in a dc capacitor [3]. The DSTATCOM proposed in this paper is employed to provide harmonics compensation. The control technique used for the
DSTATCOM control is Instantaneous reactive power theory (IRP) [1].
The DSTATCOM is a voltage source converter based static compensator that is used for the correction of line currents [8]. It is connected in shunt to the distribution network via coupling transformer [6].
I
THD
=
According to IEEE 519 standard, The THD level for current harmonics should be less than 5%. In this paper, the effort is made to reduce the harmonics as per standards. In order to achieve this, a custom power device called Distribution
Static Compensator (DSTATCOM) is modeled using MATLAB/Simulink. DSTATCOM is a voltage source converter (VSC) based power electronic device [8]. Usually, this device is Fig.1 Schematic Diagram of DSTATCOM

28 Bansal* and Bhangu**
Fig.1 shows the schematic configuration of
DSTATCOM. DSTATCOM system consists of a standard three-phase Insulated Gate Bipolar
Transistor (IGBT) based three legs VSC bridge with the input ac inductors and a dc energy storage device to obtain a self-supporting dc bus
[5].The DSTATCOM is capable of generating continuously variable inductive or capacitive shunt compensation at a level up its maximum
MVA rating [10]. The DSTATCOM continuously checks the line waveform with respect to a reference ac signal, and therefore, it can provide correct amount of leading or lagging reactive current compensation to reduce the amount of voltage fluctuations [10]. The
DSTATCOM has been utilized for voltage regulation, correction of power factor and elimination of current harmonics [11]. In this paper, the performance of DSTATCOM is analysed for elimination of current harmonics.
A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle [7].
A basic VSC structure is shown in Fig.2 where Rs and Ls represent the resistance and inductance between the converter ac voltage
(V
C
) and the ac system voltage (V) and i s
is the current injected into the grid. A dc capacitor is connected on the dc side to produce a smooth dc voltage. The switches in the circuit represent controllable semiconductors, such as IGBT or power transistors [7, 8].
Depending on the converter rating, series-connected IGBT valves are arranged in either a three-phase two-level or three-level bridge. Each IGBT position is individually controlled and equipped with integrated antiparallel diodes [8]. For converter the most important part is the sequences of operation of the IGBTs. The IGBTs signals are referred to the Pulse Width Modulation (PWM) that will generate the pulses for the firing of the
IGBTs. IGBTs are used in this simulation because it is easy to control the switch on and off of their gates and suitable for the DSTATCOM
[9].
Fig.2 Voltage source converter [5]
Fig.3 MATLAB based model of voltage source converter (DSTATCOM)
The control technique investigated in this paper is Instantaneous Power Theory which is based on instantaneous values in three phase power systems with or without neutral wire. This is valid for steady state or transitory operations as well as for generic voltage and current waveforms. It consists of an algebraic transformation of the three phase voltages in the a-b-c coordinates to the α-β-0 coordinates. This transformation is called Clarke transformation. It is then followed by the calculation of the p-q theory instantaneous power components [12].
The mathematical computation of the power components are as shown below:
(1)
Where V a
, V b
, V c
are phase voltages. Identical relations hold for line currents i a
, i b
and i c
. The instantaneous three phase power is given by:
p
3Φ
(t) = v a i a
+ v b i b
+ v c i c
= v
α i
α
+ v
β i
β
+ v
0 i
0
= p a
(t) + p b
(t) + p c
(t)

Bansal* and Bhangu** 29
= p
α
(t) + p
β
(t) + p
0
(t) = p (t) + p
0
(t) p
αq
= v
α
i
αq
= v
α v
β
q
/ ∆
(2) (11) p
βp
= v
β
i
βp
= v
β
2
p / ∆
Where and p
0 p = p
α
+
(t) = v
0 p i
β
0
is instantaneous real power;
is the instantaneous zero
(12) p
βq
= v
β
i
βq
= v
α
v
β
q
/ ∆ sequence power. (13)
There is an advantage of using the transformation of α-β-0 is to separate the zero
Therefore, the three phase active power can be rewritten as sequence component of the system. p
3Φ
(t) = p
α
+ p
β
+ p
0
= p
αp
+ p
αq
+ p
βp
+ p
βq
+
The reactive power measurement can be given p
0
= p
αp
+ p
βp
+ p
0 by:
(14) q
(t) ≈ v
α i
β
– v
β i
α Thus from equations (11) and (13)
(3) p
αq
+ p
βq
= 0
(15)
Rewritten in terms of a-b-c components as q = - [( v a
v b
) i c
+ ( v b
v c
) i a
+ ( v c
v a
) i b
Thus, p
αp
= α axis instantaneous active power. p
βp
= β axis instantaneous active power.
The powers p and q can be rewritten as p
αq
= α axis instantaneous reactive power. p
βq
= β axis instantaneous reactive power.
From above equations, it is observed that the
(4)
From this matrix equation, ∆ = v
α
2
+ v
β
2 reactive power corresponds to the parts of instantaneous power, which is dependent on the instantaneous power q , in each independent phase and vanishes when added ( a two phase (α-β) system [12]. p
αq + p
βq
= 0), in
(5)
Instantaneous real power p, gives the net energy per second being transported from source to load
Separating the Active and Reactive parts and vice- versa at any time, which is dependent only on the voltage and currents in phases α and
β and has no zero sequence present.
(6) Firstly, the three phase voltages and current are
Where, the current components are transformed from a-b-c coordinates to the α-β-0
i
αp
= v
α
p / ∆, i
αq
= v
β
q / ∆
(7) power is calculated using α-β coordinates. i
βp
= v
β
p
/ ∆, i
βq
= v
α q
/ ∆
(8)
Power in phases α and β can be separated as
(9)
Where, the power components are
p
αp
= v
α
i
αp
= v
α
(10)
2
Fig. 4 MATLAB computation using Clarke transformation p / ∆ computed using above said theory as shown below in Fig.5.

30 Bansal* and Bhangu**
Fig.5 Computation of ∆
Next is the calculation of current injected into the system as per algorithm. The current computed in this way is the two coordinates.
This requires the transformation of two coordinates (α-β) into the a-b-c coordinates as the system will take three phase currents. This is accomplished by using inverse Clarke transformation. Then this current is injected into the power system with the help of pulse generators as the IGBTs accepted the signals in the form of pulses. The computation of current in α-β coordinates and a-b-c coordinates is shown in Fig. 6 & 7 respectively.
A power system model is developed in the
Simulink in which the load connected to the three phase source is the direct torque control
(DTC) Induction motor drive which keeps the torque and flux, hence speed of the motor within their tolerant bands by proper switching of the transistors as shown in Fig.8. These transistors are power electronic equipments which cause the harmonics in the system and deviates the system from sinusoidal currents and voltages. The harmonics produced in the system should be within the permissible limits otherwise the system operation will become malfunctions which results in the discontinuity of supply to the consumers. In this model, two parallel feeders are shown, one is connected to shunt connected compensation device and the other is left as it is. DSTATCOM is connected with six pulse generators which provided gating signals to six IGBT/Diode with a delay time .
Fig.6 Calculation of injection current
Fig.7 Inverse Clarke transformation
Fig.8 MATLAB based Proposed Test model

6.1 Parameters of the Test system
TABLE –I
Bansal* and Bhangu** 31 mitigated. This is clearly shown in Fig.11 & 12 below:
Fig.11 THD for Load current before compensation
6.2 Results and Discussion
Fig. 9 Load current before compensation
Fig. 10 Load current after compensation
A non linear industrial drive is considered in the proposed model and the distribution static compensator control is modeled in such a way that the harmonics distortion in the current is
Fig.12 THD for Load current after compensation
According to IEEE harmonics standards, for distribution systems (120V to 69000V), the
Total Harmonics Distortion should be less than
5%. In this case while considering non linear load as DTC induction motor drive, the harmonics distortion is 5.70% which violates the
IEEE standards. After the implementation of
DSTATCOM at the point of common coupling,
The THD obtained is less than 5% i.e. 4.39%.
In this paper, model of DSTATCOM is developed using Simulink in MATLAB. A comprehensive control technique is simulated which is based on the algebraic transformation of the three phase voltages and currents in the ab-c coordinates to the α-β-0 coordinates. Then, injection current is calculated as per algorithm and the simulated results showed the effectiveness of DSTATCOM (custom power device) in mitigating the current harmonics.

32 Bansal* and Bhangu**
[1] Bhim Singh and Jitendra Solanki, “A comparison of control algorithms for DSTATCOM,” IEEE Transactions on
Industrial Electronics, Vol.56, No.7, July 2009.
[2] Mahesh M. Swamy, “Understanding input harmonics and techniques to mitigate them,” Yasakawa Electric
America, pp. 01-50.
[3] Arindam Ghosh and Gerard Ledwich, “Load compensating
DSTATCOM in weak AC systems”, IEEE transactions
on Power Delivery, Vol. 18, No. 4, October 2003.
[4] C. Sankaran, “Power quality”, CRC Press, ISBN- 0-8493-
1040-7, 2002.
[5] S.Suresh, N.Devarajan, M.Geetha and V. Rajasekaran, “
Investigation on DSTATCOM operation for power quality
improvement in a three phase three wire distribution system with a new control strategy,” Control theory and
informatics, ISSN 2224-5774, Vol 1, No.2, pp. 06-13,
2011.
[6] Satyaveer Gupt, Ankit Dixit, Nikhil Mishra and S.P. Singh,
“Custom power devices for power quality improvement:
a review,” International Journal of Research in Engineering and Applied Sciences, ISSN- 2249-3905, Vol.2, Issue2,
pp.1646-1659, 2012.
[7] Y. Hu, member IEEE, Zhe Chen, senior member IEEE and
H.Mckenzie, “Volatge source converters in distributed
generation Systems,” DRPT 2008, Nanjing China.
[8] Rodda Shobha Rani and B. Jyothi, “VSC based
DSTATCOM & pulse-width modulation for power quality
improvement,” International Journal of Engineering
Trends and Technology- Vol.2, Issue2, pp. 38-41, 2011.
[9] Pradeep Kumar, Niranjan Kumar and A.K.Akella, “Review of DSTATCOM for stability analysis,” IOSR Journal of
Electrical and Electronics Engineering (IOSRJEEE), ISSN:
2278-1676 Vol.1, Issue 2, pp. 01-09, 2012.
[10] Michael D. Stump, Gerald J. Keane and Frederick K.S.
Leong, “ The role of custom power products in power
quality improvement at industrial facilities,” IEEE catalogue no. 98EX137, pp. 507-517,1998.
[11] R. Heera Singh, N. Ravi, H.O. Gupta, “Impacts of custom power device on the dynamic performance of power
network,”.
[12] Moinuddin K Syed and DR. BV Sanker Ram, “
Instantaneous power theory based active power filter: a
MATLAB/ SIMULINK approach”, Journal of Theoretical and
Applied Information Technology, © 2005 - 2008 JATIT.
Mani Bansal was born at Ludhiana on 02 October, 1987. He obtained the B.Tech degree in
Electrical Engineering from DAV institute of Engineering & Technology, Jalandhar, Punjab, India in
2010. Currently pursuing M.Tech degree in Power Engineering from Guru Nanak Dev Engineering
College, Ludhiana, Punjab, India. His interest area includes power quality improvement, custom power and robust control.
Navneet Singh Bhangu obtained B.E. degree in Electrical Engineering from Guru Nanak Dev
Engineering College, P.U., Chandigarh, India. He did his M.Tech in 2001. He has various publications in National and International conferences/ Journals. His interest area includes Power system, Energy management and reliability engineering.