Design, Modelling and Simulation of Three-Phase ...

2013 NIRMA UNIVERSITY INTERNATIONAL CONFERENCE ON ENGINEERING, NUiCONE-2013, 28-30 NOVEMBER, 2013

1

Design, Modelling and Simulation of Three-Phase Front-End Converter for Unity Power Factor and Reduced Harmonics in Line Current Renju Mathew,NehaAgarwal, Manisha Shah, and P. N. Tekwani Abstract-- Low power factor and high harmonic content are the major drawbacks of conventional front-end rectifiers. Front-end converter is reported in this paper for three-phase system. A systematic design approach is presented for front-end converter and the modelling of the same is also described in detail in presented work. The control strategy for three phase front-end converter consists of inner current-loop and outer voltage-loop. The parameters for voltage control loop are derived from Unity Modulus (Magnitude Optimum) method. The power circuit of front-end converter with closed loop control is simulated in MATLAB for various steady state, transient and dynamic conditions. The proposed converter operates in forward power flow as well as in reverse power flow conditions. It is found that unity power factor is obtained for steady state as well as all dynamic and transient conditions. Irrespective of load conditions, constant dc output voltage is maintained demonstrating good voltage regulation and effectiveness of the proposed controller. It is also observed that %THD (Total Harmonic Distortion) ofthe supply current had drastically reduced in the reported system. Index Terms-- Front End Converter, Unity Modulus method, %THD. I.

T

A front-end converter is simulated for three-phase system.The proposed front-end converter works at unity power factor for steady state as well as all dynamic and transient conditions. Irrespective of any load conditions up to rated, it maintains constant output dc voltage. Also theTotal Harmonic Distortion (%THD) has drastically reduced at supply side. Even if one of the converters fails to work, the other healthy converter is capable of taking full-load current and is able to maintain desired constant voltage. It also exhibits reverse power flow capability. The converter with closed-loop control is simulated in MATLAB for various steady state, transient and dynamic condition. The control loop includes outer voltage-loop and inner current-loop. The design of the controller and calculation of the parameters used in the controller are derived from Unity Modulus method which is explained in following stions.

II.

Proposed Front-End Converter for ThreePhase System

INTRODUCTION

here-phase AC to DC converters are extensively used in many applications such as uninterruptible power supplies(UPS), battery chargers, adjustable speed drives, utility interface with nonconventional energy sources such as solar PVs, wind etc. The ac-dc converters can be realized using diode rectifiers or phase-controlled rectifiers. But, both of these behave as nonlinear loads on the power system. As a result, the current drawn by the rectifier is rich in harmonic contents which distorts supply main voltage andalso gives poor supply power factor [1]. Conventionally, output dc voltage can be changed by either using tap-changing transformer or auto-transformer with diode bridge rectifiers. But this scheme is costly, bulky and dynamic response is poor, due to the presence of transformer. SCR rectifiers can be used to eliminate the need of tappedtransformer as in the above scheme. Hence it overcomes the disadvantages of weight, size and cost, and at the same time, efficiency is high. But during lower output voltages, it injects lower order harmonics which causes the distortions in input sine wave.Also SCR rectifiers do not provide bidirectional power flow capability. Hence to overcome all the above said disadvantages, an IGBT based front-end converter is reported in this paper [2].

Fig. 1 Power circuit for three-phase front-end converter system

A. PWM switching scheme for three-phase system For both the converters, three sine waves which are 120o phase shifted are used as reference signals, as in [3], each associated with one leg of the converter. And one carrier wave is used for all the three legs of converter-1 and converter-2, but for converter-2 the carrier wave in 90º phase shifted. Now, when the magnitudeof sine wave is greater than that of carrier wave, the upper switch is turned ON in every leg. For each leg, the upper switch acts in complimentary of the lower switch i.e. if the upper switch is turned ON then the lower switch should be OFF. Same

2

2013 NIRMA UNIVERSITY INTERNATIONAL CONFERENCE ON ENGINEERING, NUiCONE-2013, 28-30 NOVEMBER, 2013

scheme is applicable for the converter-2. This results in lowering of THD value of the input current[4].

The minimum value of modulation index is decided by the fact that the minimum value of Vris equal to Vs[7]-[8]. mmin =

III. Design of Control Loops Now for per phase system, the phasor diagram for frontend converter is shown in fig.2, as in [5],

√2Vs

7

Vdc

And the value of Ls is selected in such a way that the maximum modulation index (m) operates near to unity but less than unity. Input source inductance Ls is calculated as follows, mVdc =Vr(pk) = V2s(pk) +ω2 I2s(pk) L2s V2r(pk) -V2s(pk)

Ls = Fig. 2. Phasor Diagram for Front-End Converter

If the dc link voltage is to remain constant, then the input power and the output power must remain balanced [6]. Now, Power = VsIscosΦ, Also, Power = VsVrsinδ/Xs

A. Design for three phase system The parameters for three phase system are (as in [5]) Input voltage Vs= 440 V. Output voltage Vdc=2800V. P=1400kW. f=60 Hz For three-phase system, as in [9] 3√3Vm(3-Φ) Vdc(3-Φ) = cos α π Here, Vm =Vr input voltage , Now, modulation index m = 0.8, hence 3√3 2

m=

Vdc

From the phasor diagram shown in fig.2 Is cos =

Xs

sinδ

(1)

Vr(3-

Is Xs sin =Vs -Vr cosδ Is sin =

Vs -Vr cosδ

(2)

Xs

From (3) and (4), V2s +(Is Xs )2

(5)

Hence, Vr = Vs 1+X2pu whereXpu =

Is Xs Vs

,

(6)

The above equation implies that Vris greater than Vs by a factor (1+Xpu2)1/2 , which is known as boost factor, as in [4]. Hence from (5) and(6), the range of Vr can be given as Vs