 Open Access
 Total Downloads : 148
 Authors : Rekha. K. G, Jaison Joy
 Paper ID : IJERTV6IS040681
 Volume & Issue : Volume 06, Issue 04 (April 2017)
 DOI : http://dx.doi.org/10.17577/IJERTV6IS040681
 Published (First Online): 27042017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Power Quality Enhancement Based on Multilevel STATCOM for High Power Applications
Rekha K. G1 and Jaison Joy2
P.G. Scholar1, Assistant Professor2 Department of Electrical & Electronics Engineering
Thejus Engineering College, Thrissur, Kerala
Abstract A multilevel static compensator (STATCOM) based on cascaded two level inverter is proposed in this paper. The main aim is reactive power compensation with improved power quality.The cascaded two level inverter based topology has better utilization of dc link voltage and low harmonic content. It includes two conventional three phase two level inverters connected in cascade through a three phase transformer. The cascaded inverter uses 12sided polygonal voltage space vectors. To realize 12sided polygonal voltage space vectors, the dc link voltage of inverter2 must be maintained at 0.366 times dc link voltage of inverter1. The dclink voltages of the inverters are regulated at asymmetrical levels to achieve fourlevel operation. In high power applications, VAR compensation is achieved using multilevel inverters. The intention of this study was to mitigate voltage sag problem using proposed STATCOM along two controlling methods namely FIS(Fuzzy inference system) and PI.Simulation studies are carried out to predict the system performance. A laboratory prototype is also developed to validate the results obtained by simulation.The performance of the scheme is analyzed through MATLAB/SIMULINK.
Keywords DCLink Voltage Balance, Multilevel Inverter, Power Quality (PQ), Static Compensator (STATCOM).

INTRODUCTION
Power Generation and Transmission is a complex procedure, requiring the working of many components of the power system in tandem to maximize the output. One of the main components to form a key part is the reactive power in the system. It is required to maintain the voltage to distribute the active power through the lines. Loads like motor loads and other loads require reactive power for their operation. To improve the performance of ac power systems, we have to control this reactive power in an efficient way and this is known as reactive power compensation. There are two aspects to the problem of reactive power compensation: load compensation and voltage support. Load compensation consists of improvement in power factor, balancing of real power drawn from the supply, better voltage regulation, etc. of large fluctuating loads. Voltage support consists of reduction of voltage fluctuation at a given terminal of the transmission line. Two types of compensation can be used: series and shunt compensation. These modify the parameters of the system to give enhanced reactive power compensation. In recent years, static VAR compensators like the STATCOM have been developed. These quite acceptably do the job of absorbing or generating reactive power with a faster time response and come under Flexible AC Transmission Systems (FACTS). This allows an increase in
transfer of apparent power through a transmission line, and much improved stability by the adjustment of parameters that rule the power system i.e. current, voltage, phase angle, frequency and impedance. When ac loads are fed through inverters it is required that the output voltage of desired magnitude and frequency should be achieved. A variable output voltage can be obtained by varying the input dc voltage and maintaining the gain of the inverter constant. Alternatively, if the dc input voltage is fixed and it is not controllable, a variable output voltage can be obtained by varying the gain of the inverter, which is normally accomplished by pulsewidthmodulation (PWM) technique within the inverter.
Fig. 1.Single line diagram representing STATCOM.
The inverters which produce which produce an output voltage or a current with levels either 0 or Â± are known astwo level inverters. In highpower and highvoltage applications these twolevel inverters however have some limitations in operating at high frequency mainly due to switching losses and constraints of device rating. This is where multilevel inverters are advantageous. Increasing the number of voltage levels in the inverter without requiring higher rating on individual devices can increase power rating. The unique structure of multilevel voltage source inverters allows them to reach high voltages with low harmonics without the use of transformers or seriesconnected synchronizedswitching devices. The harmonic content of the output voltage waveform decreases significantly.

CASCADED TWOLEVEL INVERTERBASED
MULTILEVEL STATCOM
The power system and STATCOM model is shown in Fig.2. This model represents the connection point of STATCOM in the power system
Fig. 2.Power system and the STATCOM model
The schematic diagram of cascaded twolevel inverter based multilevel STATCOM is shown in Fig.3. In which the low voltage (LV) side and high voltage (HV) side of a transformer is connected to inverter and transmission line respectively.
Fig. 4.Equivalent circuit of twolevel inverter based STATCOM

Phase Equivalent Circuit
Fig. 5.Equivalent circuit of phase a
Equivalent circuit of phase a is shown in Fig.5. In the figure, the RMS source voltage is represented as , total loss in the system is represented as R, the transformer winding leakage inductance is represented as L, the voltage across primary side of the transformer of inverter1 and inverter2 is(1 2).
Applying KVL to the loop
,
, + , + + (1 2) (1)
Similarly for b and c phases
,
, + , + + (1 2) = 0 (2)
,
, + , +
+ (
) (3)
1
2
Fig. 3.Cascaded twolevel inverter based STATCOM
From the Fig.3, the three phase RMS source voltages
By assuming resistances = = = R and inductances=
== L, the above can be written in mathematical model form as,
, , , and , are referred to the lowvoltage side of the
0 0 ,
, 1 2
(
)
transformer. The leakage inductances of lowvoltage side
, 1 , 1 2
windings of the transformer are
,
and respectively.
= 0
0 [] + [
(
)] (4)
,
, 1 2
,
(
)
[ ]The transformer losses are represented in terms of resistances are and respectively. The output voltages of inverter1
[ 0 0]
and inverter2 are 1, 1,1and 2, 2, 2. Finally leakage resistances of dclink capacitors 1and2are
1and2respectively.
The equation (4) is known as mathematical model in the stationary reference form of cascaded twolevel inverter based STATCOM. To control both the active and reactive currents independently, above stationary reference frame equations can be converted into rotating reference frame equations. The source voltage of qcomponent is set to be zero so that the source voltage of dcomponent can be align with the synchronously rotating reference frame.
The dynamic model in the synchronously rotating reference frame is given as
,
, 1 v, (
e )
[ ] = [] [ ] + [ d
1
d2 ] (5)
,
,
(eq1
e2)
Here , represents the direct (d)axis voltage component of ac
, ,
source and , represents daxis and qaxis current components of cascaded twolevel inverter.

Control Strategy
The block diagram of control circuit is shown in Fig. 5. The daxis and qaxis voltages can be controlled as follows
= 1 + , + , (6)
= , + , (7)
Fig. 6.Control circuit diagram
1
Where and represents daxis and q axis reference

DCLINK Balance Controller
voltage components of the inverter. The parameters 1 and
2 are known as control parameters and these can controlled as
2 = (1 + 1) ( + , ) (8)
The total dclink balance controller is used to provide magnitude and phase of resultant voltage supplied by the
cascaded inverter. The active power sharing between the inverter and grid is depends on angle .From the figure, the
,
reference voltage components of qaxis of the two inverters
, is obtained as
2 = (2 + 2) ( + ) (9)
1
2
Where is the direct (d)axis reference current and is given
by 1 = 2
(11)
= (4 + 4) ( + 2) (12)
= (3 + 3) [( + ) (1 2)] (10)
2
2
1
2
Where controls the inverter1 dclink voltage, controls
Where and represents the reference voltages of dclink
1
2
1
2
the inverter2 dclink voltage. The dclink voltage of inverter
capacitors of inverter 1 and inverter2. The reference reactive current component i.e qaxis component is obtained either from load, when used for load compensation or from voltage regulation loop when used in transmission lines.
Fig.6. Shows that the three phase voltages , , are
2 is controlled at 0.366 times dclink voltage of inverter 1, so four level operation is obtained and output voltage harmonic spectrum is improved. The inverter1 dclink voltage and inverter2 dclink voltage is expressed in terms of total dclink
() voltage.
given to phaselocked loop (PLL) to generate the unit signals
cos and sin . Phase lock loop or phase locked loop
1
= 0.732
(13)
(PLL) is a type of control system, which is used to generate output signal to match the phase of input signal. These unit
signals are used to transform the converter currents, , , ,
2 = 0.268 (14)
The power transfer to inverter1 is indirectly controlled and
,
for inverter2, power transfer is directly controlled. Therefore
into synchronously rotating reference frame currents. So that it is easy to control reactive and active current components. These currents consist of large switching frequency ripples and which are eliminated by using lowpass filters (LPF). The reference voltages to the converter are ,
are generated from controller using ( + )and .
inverter2 attain its reference value quickly when compared to inverter1. The control circuit uses the sinusoidal pulse width modulation (SPWM) technique to generate gate signals from the obtained reference voltages.
1
2
III. CASCADED HBRIDGE MULTILEVEL INVERTER
The inverter supplies desired reactive component of current
and draws active component of current , by considering
these reference current components. Which can be further used to regulate total dclink voltage ( + )of the
The cascaded Hbridge multilevel Inverter uses separate dc sources (SDCSs). The multilevel inverter using cascaded inverter with SDCSs synthesizes a desired voltage from
inverter.
1
2
several independent sources of dc voltages, which may be
obtained from either batteries, fuel cells, or solar cells. This configuration recently becomes very popular in ac power supply and adjustable speed drive applications. This new
inverter can avoid extra clamping diodes or voltage balancing capacitors. Again, the cascaded multilevel inverters are classified depending the type of DC sources used throughout the input. A singlephase structure of an mlevel cascaded inverter is each separate dc source (SDCS) is connected to a singlephase fullbridge, or Hbridge, inverter. Each inverter level can generate three different voltage outputs, +, 0, and by connecting the dc source to the ac output by different combinations of the four switches, 1, 2, 3, and
4. To obtain +, switches 1and 4are turned on, whereas
can be obtained by turning on switches 2and 3. By turning on 1and 2or 3and 4, the output voltage is 0. The ac outputs of each of the different fullbridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs as shown in Fig.6. One more alternative for a multilevel inverter is the cascaded multilevel inverter or series Hbridge inverter. The series Hbridge inverter appeared in 1975. Cascaded multilevel inverter was not fully realized until two researchers, Lai and Peng. They patented it and presented its various advantages in 1997. Since then, the CMI has been utilized in a wide range of applications. With its modularity and flexibility, the CMI shows superiority in highpower applications, especially shunt and series connected FACTS controllers.The CMI synthesizes its output nearly sinusoidal voltage waveforms by combining many isolated voltage levels. By adding more Hbridge converters, the amount of Var can simply increased without redesign the power stage, and buildin redundancy against individual Hbridge converter failure can be realized. A series of singlephase full bridges makes up a phase for the inverter.A threephase CMI topology is essentially composed of three identical phase legs of the serieschain of Hbridge converters, which can possibly generate different output voltage waveforms and offers the potential for AC system phasebalancing.This feature is impossible in other VSC topologies utilizing a common DC link. Since this topology consists of series power conversion cells, the voltage and power level may be easily scaled. The dc link supply for each full bridge
Fig.7. CHB inverter
converter is provided separately, and this is typically achieved using diode rectifiers fed from isolated secondary windings of a threephase transformer. Phaseshifted transformers can supply the cells in mediumvoltage systems in order to provide high power quality at the utility connection.

FUZZY LOGIC CONTROLLER
The control system is based on fuzzy logic. Fuzzy logic controller is a one type non linear controller and automatic. This type of the control approaching the human reasoning that makes the use of the acceptance, uncertainty, imprecision and fuzziness in the decisionmaking process, manages to offer a very satisfactory performance, without the need of a detailed mathematical model of the system, just by incorporating the experts knowledge into the fuzzy. Fig 5 shows the fuzzy logic controller block diagram. The fuzzy logic control system is based on the MAMDHANI fuzzy model. This system consists of four main parts. First, by using the input membership functions, inputs are fuzzified then based on rule bases and the interfacing system, outputs are produced and finally the fuzzy outputs are defuzzifiedand they are applied to the main control system. Error of inputs from their references and error deviations in any time interval are chosen as MATLAB. The output of fuzzy controller is the value that should be added to the prior output to produce new reference output as shown in Figs.8 to 9.
Fig.8. selection of input and output variables.
Fig.9. Input1 membership function
Fig.9. Input2 membership function.
Fig.10. Output mebership function
TABLE I SYSTEM PARAMETERS
PARAMETERS Values
Rated power 5MVA
Transformer voltage rating
AC supply frequency,f Inverter1 dc link voltage, Vdc1 Inverter2 dc link voltage, Vdc2 Transformer leakage reactance, Xl
Transformer resistance, R
DC link capacitances, C1, C2 Switching frequency
11KV/400
50Hz
659 V
241 V
15%
3%
50 mF
1200 Hz

SIMULATION RESULTS
Fig. 8. DClink voltages of two inverters
Fig. 9.Simulink model of Cascaded twolevel inverterbased multilevel
STATCOM
Fig. 10. Voltage and Current during sag condition.
Fig. 11.Voltage compensation and corresponding current.
Fig. 12.Five Level Output Voltage
Fig.12 shows the Five Level Output Voltage of 5Level Cascaded Based Multilevel Statcom.
Fig. 13. 7Level Output Voltage
Fig.13 shows the 7Level Output Voltage of 7 Level Cascaded Based Multilevel Statcom
TABLE II SUMMARY OF THD VALUES
S.No
THD
Without STATCOM
11.23%
With two Level STATCOM
4.73%
With 5Level STATCOM
2.75%
With 7Level STATCOM
151%

CONCLUSION

In this paper, a multilevel STATCOM based on cascadedtwo level inverter is proposed. The cascaded inverter uses12sided polygonal voltage space vectors. Using 12sided polygonal voltage space vectors, dc bus utilization is increasedand better THD is obtained at high values of modulationindex. The dc link voltage of inverter2 must be maintained at0.366 times dc link voltage of inverter1 to obtain polygonalvoltage space vectors. A simple control strategy for reactive power compensation as well as to maintain dc link voltagesat the required levels is proposed. This control strategy isverified with simulation studies. Experimental results are alsopresented to validate the simulation results.DClink
voltage balance is one of the major issues in cascaded inverterbased STATCOMs. In this paper, a simple var compensating scheme is proposed for a cascaded twolevel inverterbased multilevel inverter. The scheme ensures regulation of dclink voltages of inverters at asymmetrical levels and reactive power compensation. The performance of the scheme is validated by simulation and experimentations under balanced an unbalanced voltage conditions.
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