Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 80

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

Transformerless STATCOM Topology Based onExtended Modular Multilevel Converters Using

Fuzzy Logic Controller1. Swetha Nallala, swethanallala0@gmail.com , 2. C.Balachandra Reddy, cbcredyy202@gmail.com,

3. Dr. B. Ravindranath Reddy, Bumanapalli_brreddy@yahoo.co.in

Abstract —This paper presents a new control scheme tocontrol a transformless STATCOM topology based onExtended Modular Multilevel Converter (EMMC) in order tomanage more accurate compensation for high-powerapplications. EMMC is composed of parallel connection ofmultiple MMC per phase in order to deal with large-currentrequirements. These converters can be controlled for variouspurposes such as reactive power and unbalance compensation,voltage regulation, and harmonic cancellation. Moreover,related control strategies are also suggested for both the MMCand the EMMC to ensure that the source-end three-phasecurrents are sinusoidal and balanced. The proposed model isdeveloped and analyzed using MATLAB/Simulink Softwareand the simulation results obtained justify the accuracy ofproposed control technique.Index Terms—Extended modular multilevel converters,Modular multilevel converters, unbalanced compensation, PIcontroller, Fuzzy controller.

I-INTRODUCTION

In recent years, the need for high power apparatushas been derived by numerous industrial applications withminimum cost. Medium voltage motor drives and utilityapplications are some examples, since they require mediumvoltage and megawatt power level. As a result, severalmultilevel power converter structures have been introducedas an alternative in high power and medium voltageapplications. Multilevel converters not only achieve highpower ratings, but also enable the use of renewable energysources. Photovoltaic and wind energy sources as well asfuel cells can be easily interfaced to a multilevel convertersystem for a high power application.

Usually transformers in transformer-based systemsare expensive and take up a considerable amount of space.This is especially true in the appliance market, where thecost and size of the components surrounding the powersupply may be significantly less than the cost of the powersupply alone. Hence a new Transformerless power supply issuggested as an alternative to transformer-based systems.

During the last years, several multilevel convertertopologies have been developed. The elementary concept ofa multilevel converter was to achieve higher power by usinga series of power semiconductor switches with several lowervoltage DC sources to perform power conversion .

MMC had overcome the all restrictions of othertopologies and the idea of modular multilevel converters(MMC) was first introduced by Marquardt for medium-

voltage applications. Modular multilevel converters (MMC)have recently been proposed as an alternative toconventional multilevel converters in medium voltageapplications. They provide a viable approach to constructinga reliable and cost effective STATCOM, with an increasednumber of levels capable of eliminating the couplingtransformer and replacing it with cheap reactors to allow apower exchange with the power system. Further, anextended MMC (EMMC) is also proposed in order tomanage more accurate compensation for high-powerapplications.

Here the EMMC is proposed that is composed ofparallel connection of multiple MMC per phase in order todeal with large-current requirements.. The EMMC produceshigher quality waveforms for the network compared to theMMC, reducing the conductive electromagnetic interferenceconcerns in large-current applications. Also, parallelingmultiple MMC improves reliability, performance, andefficiency of the overall system, making it more flexible.

This paper is initially focusing on description of theEMMC-based STATCOM, both the proposed power circuitand control algorithms. Then, the modulating technique isrelated to the controller such that capacitor voltagebalancing is working properly even under unbalancedconditions for the converter. Both original and extendedtopologies were simulated separately in the presence of thesame network, where the simulations are comparedaccordingly.

II-EMMC-BASED STATCOM

EMMC is composed of parallel connection ofmultiple MMC per phase in order to deal with large-currentrequirements. Each MMC is comprised of two polarizedstar-connected half-bridge cascaded converters (HBCC),which are connected to the network in parallel as shown infig 1. While one HBCC has a negative common link(NLHBCC), the other has a positive common link (PL-HBCC), and the negative and positive links are floatingpoints. Each leg of both of the HBCCs consists of a numberof series-connected half-bridge modules (HBM), and thelegs are connected in a star structure. An n-level HBCC isdefined by the available (n - 1) identical HBMs cascaded ineach leg of that HBCC. Thus, each HBCC can be directlyconnected to a medium-voltage network without a couplingtransformer. In four-wire load compensation, both of the

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 81

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

star-connected HBCCs have four similar legs. Tocompensate a three-wire unbalanced load, the converter canalso be composed of two three-leg polarized HBCCs. Boththe NL-HBCC and the PLHBCC have the same powerrating as well as the same current contribution to theSTATCOM. The reference voltages of the NL-HBCC legand the PL-HBCC leg against the pair-leg reference voltage,all for phase a is shown in fig2.

A simple model of the EMMC, having two MMC, isshown in Fig.3. While all MMC have the same electricalspecification, all NCP legs connected together, and the sameis done for all PCP legs to achieve a better voltageregulation for the capacitors. When the MMC providesunbalanced currents to compensate the nonlinear unbalancedload, one output phase of the MMC may produce activepower while the other phases consume active power. Thiscauses discharge of those capacitors supplying active power,and charge of those capacitors consuming active power inlegs. Meanwhile, unequal capacitor voltages of two pairlegs, connected to two different phases, cause a directbalancing current (IB ) flowing from the overcharged pair-leg toward the undercharged pair-leg . Under suchcircumstances, the total current of each leg of the NL-HBCCand the PL-HBCC is equal to (x = a, b, c,N)

(1)Hence, voltage regulation for the HBM capacitors is

managed without any additional connections or energytransfer circuits.

Fig 1 Circuit structure of the MMC based STATCOM andthe way of its connection to the network

Fig 2 Reference voltages of the NL-HBCC leg and the PL-HBCC leg against the pair-leg reference voltage, all for

phase a.

Here the suggested high power EMMC-basedSTATCOM comprises of two or more identical MMC allconnected in parallel as shown in fig5. The total outputcurrent of each phase is equally supplied by all legs coupledto that phase. For instance, each HBCC of the STATCOMshown in Fig.1, provides a quarter of the output current foreach phase. Subsequently, for m parallel MMC, the nominalcurrent rating for all HBM is specified by

iH max =max(ix )/2m (2)

for x=a,b,c,N

where max (ix ) is the maximum output current coming outof STATCOM. The balancing current magnitude for eachleg depends on the share of active power flowing into or outof that leg. It can be seen from the circuit structure that thesum of balancing current for each HBCC is zero, namely

IBa + IBb + IBc + IBN = 0 (3)

Therefore, considering (1) and (3), the stored energy of alllegs remains balanced, a necessary condition for thecapacitor voltage balancing purposes

Fig3 Structure of the EMMC that is composed of twoparallel MMC, connected to the network.

Considering the discussed current sharing for the EMMCbased STATCOM, (3) has to be rewritten in order tocalculate the instantaneous reference voltages for each leg(VNx and VP x) of the EMMC-based STATCOM containingm parallel MMC as follows:

(4)

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 82

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

The reference voltages obtained by (4) is identicalfor all corresponding legs of each MMC within the EMMC.A phase-shift PWM modulation technique, similar to that ofthe MMC-based STATCOM, can be applied to the EMMC.Therefore, the switching pattern for each HBM is obtainedby comparing the general reference voltage with the carriersignal of each HBM as shown in fig3.

Fig.4 Illustration of the EMMC switching patternmodulation based on the phase-shift PWM, assuming theEMMC includes two MMC (their carriers are shifted by180◦ to reduce the ripple of output current) and one HBMper leg.

The inductance LF in series with each leg cansubstantially lower down fluctuation of the output currents.The switching signals for each HBM, in a pair-leg, aregenerated by different Triangular carrier signals as shown inFig.4. Hence, because of asynchronous switching signals inthose legs, the presence of a circulating current betweenparallel MMC (IC ) would be unavoidable. The bigger theinductance LF , the lower will be this circulating currentamplitude. However, increasing LF causes some drawbacks:decreases the controller efficiency, affects the frequencyresponse, raises the inductor size, and increases the powerlosses.

A solution is suggested here by inserting a coupling inductorLC in series with LF as illustrated in Fig.5. Considering thepolarity marks, LC shows a zero inductance in conductingcommon mode, being activated in differential mode. In fact,the core magnetic flux of LC is virtually zero when all pair-legs connected to one phase share identical currents. Hence,the total series inductance remains almost unchanged.Practically, there is an insignificant inductance due to theleakage of imperfect magnetic coupling that is negligible. Itshould be noted that LC has no effect on the balancing dccurrent IB .

Fig 5 EMMC Based STATCOM

III-PI CONTROLLER

Proportional-Integral-Derivative controller (PIDcontroller) is a control loop feedback mechanism(controller) widely used in industrial control systems and itsblock diagaram is shown in fig 6. A PID controllercalculates an error value as the difference between ameasured process variable and a desired setpoint. Thecontroller attempts to minimize the error by adjusting theprocess through use of a manipulated variable.

The PID controller algorithm involves threeseparate constant parameters, and is accordingly sometimescalled three-term control: the proportional, the integral andderivative values, denoted P, I, and D.

Some applications may require using only one or twoactions to provide the appropriate system control. This isachieved by setting the other parameters to zero. A PIDcontroller will be called a PI, PD, P or I controller in theabsence of the respective control actions. PI controllers arefairly common, since derivative action is sensitive tomeasurement noise, whereas the absence of an integral termmay prevent the system from reaching its target value due tothe control action.

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 83

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

Fig 6 A block diagram of a PID controller in a feedbackloop

Most modern PID controllers in industry areimplemented in programmable logic controllers (PLCs) oras a panel-mounted digital controller. Softwareimplementations have the advantages that they are relativelycheap and are flexible with respect to the implementation ofthe PID algorithm. PID temperature controllers are appliedin industrial ovens, plastics injection machinery, hotstamping machines and packing industry.

The PID control scheme is named after its threecorrecting terms, whose sum constitutes the manipulatedvariable (MV). The proportional, integral, and derivativeterms are summed to calculate the output of the PID

controller. Defining as the controller output, the finalform of the PID algorithm is:

Where Kp: Proportional gain, a tuning parameterKi: Integral gain, a tuning parameterKd: Derivative gain, a tuning parameter

: Error =SP-PVt: Time or instantaneous time (the present)

: Variable of integration; takes on values from time 0 tothe present .

There are so many methods to calculate the values ofProportional gain (Kp),Integral gain(Ki),Derivative gain(Kd).One among them is described as follows:

Ziegler–Nichols method

Another heuristic tuning method is formally known as theZiegler–Nichols method, introduced by John G. Ziegler andNathaniel B. Nichols in the 1940s. As in the method above,the and gains are first set to zero. Theproportional gain is increased until it reaches the ultimategain, , at which the output of the loop starts to oscillate.

and the oscillation period are used to set the gainsas shown:

Ziegler–Nichols method

Control Type

P - -

PI -

PID

These gains apply to the ideal, parallel form of the PIDcontroller. When applied to the standard PID form, theintegral and derivative time parameters and are onlydependent on the oscillation period .

A PI Controller (proportional-integral controller) is aspecial case of the PID controller in which the derivative(D) of the error is not used as shown in fig 7.

The controller output is given by

where is the error or deviation of actual measured value(PV) from the setpoint (SP).

.

Fig 7 Basic block of a PI controller

A PI controller can be modeled easily in softwaresuch as Simulink or Xcos using a "flow chart" box involvingLaplace operators:

where

= proportional gain

= integral gain

Setting a value for is often a tradeoff betweendecreasing overshoot and increasing settling time.

The lack of derivative action may make the systemmore steady in the steady state in the case of noisy data.This is because derivative action is more sensitive to higher-frequency terms in the inputs. Hence the PI controllercontrols the voltage error between the reference voltage and

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 83

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

Fig 6 A block diagram of a PID controller in a feedbackloop

Most modern PID controllers in industry areimplemented in programmable logic controllers (PLCs) oras a panel-mounted digital controller. Softwareimplementations have the advantages that they are relativelycheap and are flexible with respect to the implementation ofthe PID algorithm. PID temperature controllers are appliedin industrial ovens, plastics injection machinery, hotstamping machines and packing industry.

The PID control scheme is named after its threecorrecting terms, whose sum constitutes the manipulatedvariable (MV). The proportional, integral, and derivativeterms are summed to calculate the output of the PID

controller. Defining as the controller output, the finalform of the PID algorithm is:

Where Kp: Proportional gain, a tuning parameterKi: Integral gain, a tuning parameterKd: Derivative gain, a tuning parameter

: Error =SP-PVt: Time or instantaneous time (the present)

: Variable of integration; takes on values from time 0 tothe present .

There are so many methods to calculate the values ofProportional gain (Kp),Integral gain(Ki),Derivative gain(Kd).One among them is described as follows:

Ziegler–Nichols method

Another heuristic tuning method is formally known as theZiegler–Nichols method, introduced by John G. Ziegler andNathaniel B. Nichols in the 1940s. As in the method above,the and gains are first set to zero. Theproportional gain is increased until it reaches the ultimategain, , at which the output of the loop starts to oscillate.

and the oscillation period are used to set the gainsas shown:

Ziegler–Nichols method

Control Type

P - -

PI -

PID

These gains apply to the ideal, parallel form of the PIDcontroller. When applied to the standard PID form, theintegral and derivative time parameters and are onlydependent on the oscillation period .

A PI Controller (proportional-integral controller) is aspecial case of the PID controller in which the derivative(D) of the error is not used as shown in fig 7.

The controller output is given by

where is the error or deviation of actual measured value(PV) from the setpoint (SP).

.

Fig 7 Basic block of a PI controller

A PI controller can be modeled easily in softwaresuch as Simulink or Xcos using a "flow chart" box involvingLaplace operators:

where

= proportional gain

= integral gain

Setting a value for is often a tradeoff betweendecreasing overshoot and increasing settling time.

The lack of derivative action may make the systemmore steady in the steady state in the case of noisy data.This is because derivative action is more sensitive to higher-frequency terms in the inputs. Hence the PI controllercontrols the voltage error between the reference voltage and

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 83

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

Fig 6 A block diagram of a PID controller in a feedbackloop

Most modern PID controllers in industry areimplemented in programmable logic controllers (PLCs) oras a panel-mounted digital controller. Softwareimplementations have the advantages that they are relativelycheap and are flexible with respect to the implementation ofthe PID algorithm. PID temperature controllers are appliedin industrial ovens, plastics injection machinery, hotstamping machines and packing industry.

The PID control scheme is named after its threecorrecting terms, whose sum constitutes the manipulatedvariable (MV). The proportional, integral, and derivativeterms are summed to calculate the output of the PID

controller. Defining as the controller output, the finalform of the PID algorithm is:

Where Kp: Proportional gain, a tuning parameterKi: Integral gain, a tuning parameterKd: Derivative gain, a tuning parameter

: Error =SP-PVt: Time or instantaneous time (the present)

: Variable of integration; takes on values from time 0 tothe present .

There are so many methods to calculate the values ofProportional gain (Kp),Integral gain(Ki),Derivative gain(Kd).One among them is described as follows:

Ziegler–Nichols method

Another heuristic tuning method is formally known as theZiegler–Nichols method, introduced by John G. Ziegler andNathaniel B. Nichols in the 1940s. As in the method above,the and gains are first set to zero. Theproportional gain is increased until it reaches the ultimategain, , at which the output of the loop starts to oscillate.

and the oscillation period are used to set the gainsas shown:

Ziegler–Nichols method

Control Type

P - -

PI -

PID

These gains apply to the ideal, parallel form of the PIDcontroller. When applied to the standard PID form, theintegral and derivative time parameters and are onlydependent on the oscillation period .

A PI Controller (proportional-integral controller) is aspecial case of the PID controller in which the derivative(D) of the error is not used as shown in fig 7.

The controller output is given by

where is the error or deviation of actual measured value(PV) from the setpoint (SP).

.

Fig 7 Basic block of a PI controller

A PI controller can be modeled easily in softwaresuch as Simulink or Xcos using a "flow chart" box involvingLaplace operators:

where

= proportional gain

= integral gain

Setting a value for is often a tradeoff betweendecreasing overshoot and increasing settling time.

The lack of derivative action may make the systemmore steady in the steady state in the case of noisy data.This is because derivative action is more sensitive to higher-frequency terms in the inputs. Hence the PI controllercontrols the voltage error between the reference voltage and

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 84

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

mean voltage of system by controlling the duty cycles ofsemiconductor devies.

IV-FUZZY LOGIC CONTROLLER

. The basic concept in FL, which plays a central role inmost of its applications, is that of a fuzzy if-then rule or,simply, fuzzy rule. Although rule-based systems have a longhistory of use in Artificial Intelligence (AI), what is missingin such systems is a mechanism for dealing with fuzzyconsequents and fuzzy antecedents. In fuzzy logic, thismechanism is provided by the calculus of fuzzy rules. Thecalculus of fuzzy rules serves as a basis for what might becalled the Fuzzy Dependency and Command Language(FDCL).. In most of the applications of fuzzy logic, a fuzzylogic solution is, in reality, a translation of a human solutioninto FDCL.First-generation simple fuzzy logic controllerscan generally be depicted by a block diagram in fig 8.

The knowledge-base module contains knowledgeabout all the input and output fuzzy partitions. It willinclude the term set and the corresponding membershipfunctions defining the input variables to the fuzzy rule-basesystem and the output variables, or control actions, to theplant under control.

The rule base consists of different type of rules based on thetype of membership function. In this paper the suggestedmembership function is triangular and the rules are made.The definition of a membership function: A graph thatdefines how each point in the input space is mapped tomembership value between 0 and 1. Input space is oftenreferred as the universe of discourse or universal set (u),which contain all the possible elements of concern in eachparticular application.

Fig.8 A Simple Fuzzy Logic Control System

Before we start defining different types ofmembership functions, let us consider a Fuzzy IF-THENrule for a car:

IF the speed of a car is high, THEN apply less force to theaccelerator

IF the speed is low, THEN apply more force to theaccelerator

Triangular: This is formed by the combination of straightlines. The function is name as “trimf” .We considers theabove case i.e. fuzzy set Z to represent the “number close tozero”. So mathematically we can also represent it as

0 if x<-1

μz(x) = x + 1 if -1 ≤ x <0 (1.4)

1 –x if 0 ≤ x <1

0 if 1≤ x

fig (9) called “triangular membership function”

Fig.9.Traingular Membership Function

By using this membership function , a group ofrules are obtained according to the inputs. These rules areprogrammed and stored as a .fis file, which imported to theworkplace to run.

V-MATLAB/SIMULINK CIRCUITS & RESULTS:

MATLAB is a high-performance language fortechnical computing. It integrates computation,visualization, and programming environment. Furthermore,MATLAB is a modern programming language environment:it has sophisticated data structures, contains built-in editingand debugging tools, and supports object-orientedprogramming. These factors make MATLAB an excellenttool for teaching and research. MATLAB has manyadvantages compared to conventional computer languages(e.g.,C, FORTRAN) for solving technical problems.MATLAB is an interactive system whose basic data elementis an array that does not require dimensioning. The softwarepackage has been commercially available since 1984 and isnow considered as a standard tool at most universities andindustries worldwide.

The four-leg MMC based STATCOM connected to athree-phase four-wire system is modeled and simulatedusing the MATLAB with its Simulink. The ripple filter isconnected to the STATCOM for filtering the ripple in thePCC voltage. The three-phase source, and the shunt-connected three-leg MMC are connected as shown in Fig.7.1

The control algorithm for the STATCOM is also modeled inMATLAB. The reference source currents are derived fromthe sensed PCC voltages (Vsa, Vsb, Vsc ), load currents (iLa,iLb, iLc), and the dc bus voltage of STATCOM (Vdc). A PWMcurrent controller is used over the reference and sensedsource currents to generate the gating signals for the IGBTsof the MMC of the STATCOM. The MATLAB-basedmodel of the three-phase four-wire STATCOM with linearloads and non-linear loads are shown in following sections.

Swetha Nallala, C.Balachandra Reddy and Dr. B. Ravindranath Reddy 85

International Journal of Emerging Trends in Electrical and Electronics (IJETEE – ISSN: 2320-9569) Vol. 10, Issue. 8, Sep. 2014

Simulations were carried out using PSIM, while the controlalgorithm was managed with MATLAB; then, they werelinked together using SIMCOUPLER of the PSIM. Assumethat a 25-kV network is supplying a distorted unbalancedload in an electrical railway application. The load is coupledacross two phases of the PCC.

In this work the performance of the four leg EMMC-basedthree-phase four-wire STATCOM is demonstrated alongwith harmonic reduction and the source-end three-phasecurrents are sinusoidal and balanced. The developed modelis analyzed under varying loads and the results are discussedshortly.

Fig.10.Simulation model of the three-leg EMMC basedSTATCOM connected with distorted and non linear loads.

Fig 11 simulation model of fuzzy logic controller

The supply voltage and current of the system is shown in fig12.

Fig 12 source voltage and current waveforms

It can be seen from Fig 13 (d) and (f) that while both MMCand EMMC balance the source-end currents, the EMMC-based STATCOM produce much smoother currents than

those of the MMC. Additionally, the source-end currents arein phase with the fundamentals of positive sequencevoltages, containing no reactive components. Overall FLCimproves the power quality and performance of system.

Fig. 13. Voltage and current waveforms: (a) voltages of thePCC, (b) load currents, (c) EMMC currents, (d) source-endcurrents after compensation by the EMMC-basedSTATCOM, (e) MMC currents, and (f) source-end currentsafter compensation by the MMC-based STATCOM

VI-CONCLUSION

The operation principle of a Extended ModularMultilevel Converter (MMC) was presented, describing theneed for a correct balancing algorithm in order to haveproper operation. This modular EMMC based STATCOMintroduces a transformerless design, employing severalisolated units composed of a number of cascaded HBM inorder to achieve higher efficiency, higher reliability, lowerweight and size, lower switching frequency and lowerratings for the switches. Simulations and experiments showproper operation of both proposals under unbalanced-distorted conditions. From the simulation results, it isobserved that MMC based STATCOM makes the sourceend currents more sinusoidal. Improvement in power qualityand performance is achieved. The proposed controller canbe replaced by a Artificial Neural networks(ANN).

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Her research interests include design, modeling and controlof high power electronics converters and their applicationsas FACTS controllers

Mr. C.Balachandra Reddy receivedM.Tech degree in Electrical Engineering from NITWarangal,now doing as a Ph.D Research Scholor at JNTUHyderabad, in His area of interests Power quality issues inwind power generation.

Dr.B.Ravindranath Reddy ,He working as a DeputyExcutive Engineer JNTU Hyaderad.