International Journal of Electrical and Electronics Engineering Research (IJEEER) ISSN 2250-155X Vol.2, Issue 3 Sep 2012 75-83 ÂŠ TJPRC Pvt. Ltd.,

HYBRID ACTIVE FILTER FOR TWELVE PULSE CONVERTER OPERATING UNDER ASYMMETRICAL OPERATION USING P-Q THEORY PRAMOD S MODI& S. K. JOSHI Department of Electrical Engineering, M. S. University of Baroda, Vadodara, India

ABSTRACT This work deals with the investigation of harmonics and mitigation of harmonics for the 12pulse converter operation under asymmetrical firing angle mode. The performance of converter is analyzed in unequal

firing angle mode and different harmonic patterns are studied. It was found that

under equal firing angle the both converter â€“ each six pulse is operating as a twelve pulse converter and the harmonic pattern is as per twelve pulse converter, but if they operate in unequal firing angle mode , the harmonic pattern is as per six pulse converter. To mitigate the harmonics the performance of the converter with passive filter and Hybrid active filter using P-Q based algorithm is simulated and the results obtained are as per the IEEE standard.

KEYWORDS: Twelve Pulse Converter, Unequal Firing Angle, Harmonics, Combined Hybrid Active Filter, THD.

INTRODUCTION Many large industrial loads such as used in Steel Plants, Mining etc. need converters (MW rating) feeding DC motor drives controlling to and fro motion of the mechanical load.[1] Under such drive environment there are wide fluctuations in the Var demand by the converter. Those apart, frequent harmonic related problems are also encountered. Passive filters have been considered good for harmonic current and displacement power factor compensation [5]. They also are capable of drawing fixed lead Var at fundamental frequency. When there is a variation of the Var demand by the non-linear load connected at PCC, as in the case of a phase controlled converter feeding DC motor drive, the passive filters may either over compensate or under compensate reactive power. Thus the power system continues to be affected by the fluctuations in Var demand of the load.

ACTIVE/REACTIVE POWER In this section the operation of 12-pulse converter has been analyzed in terms of active and reactive power drawn by the converter with considering the source inductance effect which also includes the transformer leakage inductance and constant dc current. For such an investigation a composite P-Q

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Pramod S Modi & S. K. Joshi

diagram has been derived for the 12-pulse converter from the P-Q diagram of the constituent 6-pulse converters under constant current control. The active and reactive power required by the converter is the total power required by two six pulse converter and it is depends on the individual firing angles[3][4]. The P vs. Q locus of each 6 pulse converter bridge traces a semicircle with radius of Vdo and Id where Vdo is the maximum dc voltage and Id is load current. Depending on the positioning of the point on the semicircular loci of the two converter bridges, there can be a number of operating modes in a 12 pulse thyristor bridge. Fig 1 shows the equal firing angle mode with the source inductance. The radius of the semi circle is depends on the current through the load.

α =0

Fig.1 Active/Reactive power locus for symmetrical firing angle operation

The complete semicircular range of the P-Q locus is not achievable due to the equivalent commutating reactance which represents the commutation overlap. The commutation overlap depends mainly on the leakage reactance of the transformer connecting the converter to the system bus and source inductance. The power locus of a 12-pulse converter can be obtained based on theory of converter operation according to which, the voltage due to each constituent 6-pulse converter with considering the source inductance is given by,

Vd i = Vd o Cosα i − X c I d

(1)

Vd i = DC side voltage

Vd o = No load maximum dc voltage of 6-pulse converter

α i = Converter delay angle Where subscript i identifies the 6-pulse converter number 1 or 2.

XC =

3ωLS

π

The commutation angle is given by 2ωLS I d µ = cos −1 cos α − 2V LL

(2)

How ever, the line commutated converters have to be operated between the

α

to end-stop.

The end –stop is defied as the maximum firing angle that can be applied to the converter so that the converter can be operated without commutation failure. The power locus obtained in the above manner can be plotted in fig. 1 for equal firing angle of both converters. Fig. 2 shows the Active power Vs. Reactive power profile at different values of source inductance.

Hybrid Active Filter for Twelve Pulse Converter Operating Under Asymmetrical Operation Using P-Q Theory

77

Fig 2

Fig.3

shows composite 3-Dimensional plot of the per-unit (p.u) active power against

simultaneous variation of firing angles of both the converters. Here x, y, z axes represents firing angles

α 1 , α 2 and net pu Active power drawn by the 12-pulse converter. The net Active power required by the converter is 1.p.u. at ( α 1 , α 2 ) equal to (0,0) and zero at

(α1 , α 2 ) equal (0,180), (90,90) and (180,0)

and -1 p.u. at (180,180) without source inductance. .The net active power drawn with source inductance at α 1 , α 2 equal to zero is less than 1. pu. Similarly at ( α 1 , α 2 ) equal to end-stop, the net active power drawn by the converter is less than -1p.u. As the source inductance is increased the operating region is decreased and the end stop value of the firing angle of the each converter is reduced.

Fig 3 3-D Plot of Active power with simultaneous variation in converter 1 and converter 2 firing angles without source inductance

Fig.4 shows composite 3-Dimensional plot of the per-unit (p.u) reactive power against simultaneous variation of firing angles of both the converters. The net reactive power drawn by the converter is zero at ( α 1 , α 2 ) equal to (0, 0), (0,180), (180, 0) and (180,180) and 1p.u. at ( α 1 , α 2 ) equal to (90, 90) without source inductance. .The net reactive power drawn with source inductance at α 1 , α 2 equal to (0, 0) is higher than zero. Similarly at ( α 1 , α 2 ) equal to end stop, the net reactive power drawn by the converter is higher than zero. As the source inductance is increased the operating region is decreased and the end stop value of the firing angle of the individual converter is also reduced.

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Pramod S Modi & S. K. Joshi

Fig. 4 3-D Plot of Reactive power with simultaneous variation in converter 1 and converter 2 firing angles

C0NSTANT REACTIVE POWER OPERATION The12-pulse rectifier operation in constant Var mode can be explained using fig.5. For a constant Var operation if a line parallel to x-axis be drawn at height equal to the required Var, any point on the line could be achieved using combinations of different firing angles

Îą1

and Îą 2 .[3]

Fig. 5

represents the range of firing angles at which the two converters need to work for different constant Var operating points. This figure depicts both the regions of operation. As the value of the source inductance is varied, the region of operation and range of

qset is also varied.

Fig. 5 Range of firing angles for constant reactive power operation

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Hybrid Active Filter for Twelve Pulse Converter Operating Under Asymmetrical Operation Using P-Q Theory

CONSTANT ACTIVE POWER OPERATION

α =0

Fig. 5 Limiting Firing angle operation of 12- pulse converter for pset value For a 12-pulse converter, the Var drawn from the source for given active power demand will be maximum under symmetrical mode. Therefore for given active power demand, only the minimum Var condition has to be known. The 12-pulse converter operation in constant active power mode can be explained using fig.6. In this case, when the converter is operated in asymmetrical mode, there will be a range of firing angles within which both the converters must operated to generate constant active power. Under certain operating condition the desired net active power pset can be expressed in per unit (p.u.) values as sum of the active power drawn by the each converter.

∴ p set ( p.u.) = (cos φ1 + cos φ 2 ) ∗ 0.5

pset ( p.u.) =

At

Vdo I d 2I 2 X (cosα1 + cosα2 ) − d c Smax Smax

αi = 0

I X Pmax ( p.u.) = 1 − d C Vd o

I X Pmin ( p.u.) = cos(end − stop ) − d C Vd o So the

Pset

is varied between

Pmin and Pmax

From fig.9 it is observed that, when one of the converters is operated at converter must operate at the firing angle

pset + 2I d2 X c − cosα1 S max

α 2 = cos−1 2

α1 =00,

the other

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Pramod S Modi & S. K. Joshi

At any value of active power transfer, the operating point of

α 1 ,α 2

may lie at any point on the

corresponding curve. The reactive power consumption depends on the positioning of curve. It is observed that the freedom of choice of

α 1 ,α 2

α 1 ,α 2

on that

reduces with the increase of active power

transfer. Fig 10 shows the variation of Converter 1 firing angle with converter 2 firing angle for a

p set

value. As the source inductance is increased this range of

p set

is also smaller and end stop limit for

the both converter is also decreasing. The value of the overlap angle is also depends on the load current. Fig. 6 shows the variation the range of active power (p.u.) as the load current Id is varied.

Fig. 6 Range of firing angles for constant Active power

SIMULATION OF HAF USING P-Q

I Sa I Sb I Sc

abc

∑

to αβ

∑

αβ

I Sfa

to abc

I Sfc

I Sfb

Va

Vb Vc

abc to αβ

∑

÷ ÷

Fig. 7 P - Q algorithm

Following equations are used to develop simulation model using P-Q theory based simulation.

Hybrid Active Filter for Twelve Pulse Converter Operating Under Asymmetrical Operation Using P-Q Theory

1 1 − i Sα 2 2 i = 3 Sβ 0 − 3 2

p Vα q = - V β

1 i Sa 2 i 3 Sb − i 2 Sc −

Vβ i Sα Vα i Sβ

SIMULATION WAVEFORM FOR HAF FOR EQUAL FIRING ANGLE MODE Hybrid active filter is simulated with equal firing and results are shown in fig. 8 Converter 1 Firing Angle = 0 degree and Converter 2 = 0 degree

Fig. 8 Variation of 5th ,7th, 11th, and 13th harmonic current

SIMULATION WAVEFORM FOR HAF FOR UNEQUAL FIRING ANGLE MODE Hybrid active filter is simulated with unequal firing and results are shown in fig. 9

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Pramod S Modi & S. K. Joshi

Fig. 9 Variation of 5th ,7th, 11th, and 13th harmonic current, output dc voltage and load current

CONCLUSIONS A new concept of operation of 12- pulse converter under asymmetrical mode is proposed here. It has been analyzed that the source impedance has a great effect of the performance of the converter for operation under constant active power and constant reactive power operation. If such a system is connected between source and a load , it can be used to maintain the constant active power / reactive power constant by using the combination of different firing angles. A hybrid filter is simulated using MATLAB. The performance of the supply current harmonics and supply voltage harmonics and THD at converter input terminal is as per IEEE standard.

REFERENCES [1] W. Mc Murray, A Study of Asymmetrical Gating Phase Controlled Converters, IEEE Transactions on Industry Applications, 8(3), 1972,289-295. [2] A.K. Gaja, J.K. Chatterjee, and P.S. Modi, "Constant Var Operation of Asymmetrical Firing Angle Controlled 12-Pulse Converter" Proceedings of the IASTED International Conference, Energy and Power Systems, Aprill8-20, 2005, Krabi, Thailand. [3] Faisal M Ahsan, J.K. Chatterjee, and Anupam Das, â€œOperation of 12 pulse converter in closed loop for controlled p q operationâ€? Power Electronics, Drives and Energy Systems, 2006. PEDES

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'06. International Conference on12-15 Dec. 2006 Page(s):1 - 6 Digital Object Identifier 10.1109/PEDES.2006.344236 [4] Alexander Kusko, Syed M Peeran, “Application of 12-pulse converter to reduce Electrical Interference and Audible Noise from DC motor Drives” IEEE transaction on Industry Application, vol.29, no.1 Jan-Feb 1993. [5] Yue Wang, Zhaoan Wang, Jun Yang, Jinjun Liu, Zhiping Fu, Yong Duan and Yahan Hua, A Novel Comprehensive Compensator for Electrified Railway System, IEEE Annual Power Electronics Specialist Conference,3, 2003, 1032 – 1037. [6]Maswood, A.I.; Shen Wei; “Harmonic

propagation in high power converter under

unbalanced and distorted input voltages” Power Engineering Society General meeting 2003, IEEE, 1491 [7]P.S.Modi and S.K. Joshi, “New Combined Hybrid Active Filter for Twelve Pulse Converter Operating under asymmetrical operation” AUPEC – 2011 at Brisbane Australia on sept. 25-29, 2012.