Digital simulation of a hybrid active filter-an active filter inseries with a shunt passive filter

Abstract

Active filters have long been in use for the filtering of power system load harmonics. In this paper, the digital simulation results of a hybrid active power filter system for a rectifier load are presented. The active filter is used for filtering higher order harmonics as the dominant harmonics are filtered by the passive filter. This reduces the rating of the active filter significantly. The DC capacitor voltage of the active filter is controlled using a PI controller

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Digital Simulation of a Hybrid Active Filter - An
Active F‘ilter in Series with a Shunt Pcwsive Filter
1. Mnhesh Sii;arain, I(.R.Padiyar, V.Ralnanar~lyaIli~11
Uepartiiieiit of Electrical Enginecriiig
lndiaxi Institute of Science, Bang-rl ore
hl,-t.rnct - Active filters have been in use for
lilt.rririg lonrl harmonics. In this paper, the dig-
i h 1 siriiulatioii results of a hybrid active filter
systmri fnr R rectifier load are presented. The
twtive filter is used for filtering higher order liar-
tritmirs as t.he doininant harmonics get filtered
il,:~ ilw passive filher. This reduces the rating of
the active filter significantly. The DC capacitor
voltage of the aclive filter is controlled using a
PI controller.
IN’lrRODUCTION
The iieed for efiicierit use of electrical power has
incicwed the areas of application of power elec-
troitics. h4my such power electronic converters
draw xion-simmid a1 current and inject harmonics
iiito the power distribution system. These har-
ixionics lead to higher losses in the system. Some
of tlw liru.ii101iic. procliiciiig loruls arc’ rwt ificrs,
ibrC-fiirllm:eS, cycloconver tCis I A U ~ tuljustablc sp~ccl
drives. IEEE 519 Harmonic Standards specify tlie
voltagc! harnionic limits at tlie point of co~nrnon
c-oiipling (I’CCI) anti the current harrrioiiic limits
:kt t l i ~
I’W I)etwcen the customer and the utility.
Tlic~ utility i s respoIaible for the voltage distor-
t ion aiitl tlic iiidividiial customer for the current,
(lis t~ )rt,ioxi.
1’iL’isivc filters l i d been used initially to compeii-
sate for tlie currcnt fiarmonics injected by large
iiitliistrial non-lixiuar loads. This was due to their
low coht, siinplirity mid higher efficiency. But
tliey arc susceptihle to series and parallel reso-
II~JZIICCS with the supply and the load, and their
tiltwing charcactei.istics are influenced by the sup
ply impcxlancc. Tuned passive filters are also sus-
wIit,iI)lv to load and line switcliirig transients, arid
tlrcy are always &-tuned. Hence active filtering
I )ec;l,ill(: Ilc!cessarJ:
Active filters citii be used for cornpensating liar-
Iil0iiic.s as well a s daxripixig resonaxice in a power
65
diatxibutioii system [l-21. The fiiiictioxis of tlic. ;w-
tivc: filter ciLi1 be extended to in
coiiipmsatioii, flicker coinpensatioii. voltago rc’gii-
lat ion a@llst sags ;~iicl swtdls, etc. to irnprovv tlic
power quality in a distribution system [3].
Active filtcrs can be of various types - shuirt, SP-
riw, liybrid active filters axid uuificcl power qii;1.1-
ity conditioner. Each in turn can be of a voltagc
source inverter (VSl) type or a currcnt soiirw iii-
verter (CSI) type. An active filter consisting of x
VSI with a capac-itor on tiit. DC side is prefcwcvl
over that of a CSI with an inductor 011 thc I . ) ( . ;
side, due to its lower iiiitial costs aid higlier cffi-
cicncy. This paper deals wit!i a hybrid activc filtvr
whidi i s a coinbination of ai active filter in scnriw
with a shinit passive fi1tc.r. The load consitlvi vtl
for cornpensation is a rectifier load. This config-
iiration (Fig. 1) is preferrod due to iinprovciiicwt
in hot11 tlie source current and the voltage at tlic
l’(‘(: fi)r f Iii* Iorul (*i~iirCicli*icvl. Also tlw l ~ r i ) ~ ~ v * t i o i i
~it~-twSiuy
I i c w ~ is siiiipk c.t)iiil)i~.r~d
iirations [45].
The liy hid act ivv filter (*I )iifigura t io11 1 mol )s(v 1
by Ehjita et a 1 [5] lias bccii c:hosc:ii for tlic stiitly.
The control scheine for tliis was Imted on tlie In-
stantaneous lteactivr: l’owvr (IlU’) theory [ti]. 1L.-
ceiit work iisirig tliis corifigiiration for nctivc fil-
tcriiig allti voltage rcgiilntioii, I J ~ w ~
theory, wim by villi Zyl t*t crl (31. Several pkwrs
have been reported on otlier confignratious, vzz.,
the sliriiit active filter and tlie hylnitl series ;u-tivt*
filtw. The coiitrol sc~w~rxic~
;ulopt(d in this ~);ij)(*i is
based on tlie Synchronous 1tefcrcnc.c~ Ekaranic (Sit 1 : )
theory proposed by Bliattwharya r d (11 [i’). Tlitt
details of these are presented later in this l>ii.lj(’r.
to o t Iwi v o i i t i K -
on t h 11t1’
PRINCIPLE OF OPERATION
Ail active filter requires sensixig of a siiital)la
signal for evaluating its reference. The sign,zls
that can be scractl for a hybrid active filter arv

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‘L -
4
is
“1
1
Acltve Filler
5
Fig. 1. Circuit configuratioa of the liyttrid active filter
system.
the source current (I,) and the voltage (Vtj at
the IYX, the inost suitable signal being is [I].
Therefore, the active filter is controlled as a volt-
age source
Va/ = K .ish
where isll is the! iiwmoilie component of the source
ciirrcrrt ant1 K is a constant that can have a corn-
plex Valw!.
Cousitler a iton-linear load which is connected
to the PCC. This load caii he iriodelled as a cur-
rent soiirce If,. A pmsivc filter of impedance Zj
is coitiiected t,o compensate for the harmonics in
il,. For ttminonic aridysis, a simplified rnodel of
the tlistributioii system is considered [fi]. Here the
source voltage (V,) and the source impedance (z$)
arc approxirnsted a s the Theveiziii equivdents as
seeu from the PCC. The cyiiivdent circuit for t,lie
hybrid active filtei systerri is showit in Fig.2(a).
The ltarmonic contents in V, and fs are regulated
accorcliikg to 1EEE 519 standards.
(1)
(a)
(b)
(e)
Fig. 2.
Equivalent circuit for the hybrid active
filter systciii, (a)Single-pliase equivalent circiiit,
(b)Harinoiiic erluivabnt circuit for the load cur-
rent, (c)HarincAc erpuivideilt circuit for the source
voltage.
The hsrmuiiic analysis of the circuit in Fig.2(a)
is c;wried 011 t rjshg tlie superyosi tion principle,
First consider the load alone to iiijcct Iiariiloiiic*s
(I,yt) irito the systmn. The source ciirreiit liar-
rrioxiic component (Ish) arid the harrnonic c:o~iipo-
neiit of the voltage (Vfi) at tlic I’CC are givw by
&%om eqris.(2) and (3), it i s SCCII that thtw is ; ~ i i
improvement in the filteriiig cfwacteristitu for tlic
load current harmonics for a large value of IC. For
a low iiarinonic content in source curre/it, the (:ou-
dition fK] >> muz(IZs). JZ,l) is to be ,satisfirti.
A purdy imaginary value for K might rrsiilt ill
rcsoxiance in t~he system, axid herrce the very 01)-
jective of using an active filter is last, whurew ii
purely real value for K will make the wtivc filtvr
act as a resistance at harmonic frequencies aiid
hips in damping resonance ill the system. lltiiw
K is selected to be real. The equivalent circuit foi
the load curreiit larxnouics is realized from c c p (2)
and as stiowu izi Fig.2(b).
Considering rlext, tlie source aloiic to irtjevt, h . 1 5 -
moiiics (V,,) irito the system; VsIlr f91, ancl vtli
related by
1
l s h =
r-
vsh
K 4 2, t L j
From equs.(4) and (5) it is seeit that evcwtIioiii;ii
is reduceti sigiiificwttly, Vt, wormis if K i
selwted sudr that K , % > I%, + X J } ~ lirnc-c. tl, s
roiifigiiratiun of a hybrid active filter is imt siiit-
ablr if tie sourre voltage itnrniwits arc to 1,c lli-
tered. itealizatioti of ~q11~.(4)
the equivalent circuit for the soiirce voltag(’ 11.1 1
mouics, shown ixi Fig.2(c).
A disadvantage of selecting K to be real is t li,ii
tlie active filter draws real liariiioriic power froit1
the system. This will lead tu variation in its U(:
capacitor voltage (Vd) (51. As the regulatioii of V,
is Qecessary for the successful operatioil of tlw w-
tive filter, a Pi controller is used here. Tfrc 1,loc.k
diagram for the DC capacitor voltage coiitroi i s
~ltowii in Fig.d. The iisc of the PI controller in-
creases the system type by one. Therefore, t h
stearfy state error of tilie system for a step input
in its refererice (V,,,,),
i s reduced to zero. Iicrri
and (5) rtwilts i t )
66

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K
~ kb
cos^
s i n i
Fig. 4. Control scheme for the SRF based compensator.
Fig. 3. Block diagram for the DC capacitor voltage
control.
7 4 t ) is a function of the modulation index of the
PWM inverter weld as the active filter. If Zd is
the output of the Pi controller, the reference to
the active filter is given by
?Ja/ 7 K ish + Z d is/ = 7n(t) Vd
(6)
where is/ and ish are the fundamental and the har-
irioriic components of the source current respec-
tively. Here 2,s and ish are extracted using the
SIW harmouic isolator. Thus, the real harmonic
power drawn by thle active filter is injected back
into the system at fundamental frequency.
CO"I!ROL SCHEME
Tlie performance of the active filter is greatly
influenced by the filtering algorithm employed for
the extraction of the fundamental and harmonic
corriponents of the source current. This extrac-
tion can be done using time-domain or frequency-
domain techniques, The time-domain techniques
are preferred due to their easy implementation,
fast response arid little computational burden.
Arnorig the time-domain techniques, the IRP a p
prod1 [GI and the !$IW approach [7] arc presently
the most popular ones. SRF approach is used here.
Fig.4 shows the control scheme for the proposed
SRF based compensator. Tlie three-phase sourcc
currents is,, 236 and z,,
are transformed from their
a - 6 - c coordinates into twephase Kron's D - CJ
coordinates using eqns.(7) and (8).
r
In the synchronously rotating D - Q reference
frame, the components at the fundamental fre-
quency w1 are transformed into DC quantities and
those at the harmonic frequencies into non-DC
quantities. They undergo a frequency shift iu the
spectrum. The DC components in D -Q reference
frame are extracted according to
r
(9)
where G(s) is the transfer function of the first or-
der LPF. The DC components are transformed
back into three-phase a-6-c coordinates to obtain
the fundamental and the harmonic components as
shown below:
. .
10
22
[ "1
2slrc
= [ : : I
3C - [ "1
Zsfc
(12)
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Page 4

TABLE I
FOR DIGITAL C~RCUIT
Supply phase voltage
Rectifier commutation inductance
Rectifier DC reactor
PARAMETERS SIMULATION
50.81kV
L,,,
129.61nH
Ldc
1H
v8
Line inductance
Line reaistance
L8
R8
14.6mH
0-458752
5th harmonic tuned filter inductor
5th harmonic tuned filter capacitor
Ls
Cb
60mH
6.7547pF
5th harmonic tuned filter quality factor Qs
Active filter DC capacitor
Active filter DC voltage reference
20
5mF
5okV
C d
vd-vef
From eqns.(ll) and (12), the reference to the
active filter (ref. eqn.(6)), can be computed as
[
= K [ : : E ] 4- z d [ ; I : ]
2s hc
(13)
va Jc
h f c
SIMULATION RESULTS
Digital simulation using MATLAB SIMULINK,
has beer1 carried out on the use of a hybrid active
filter shown in Fig.1 for a 3-phase, 50Hz, 88kV dis-
tribution system [3]. The non-linear load consid-
ered for compensation is a BOMVA, 6-pulse thyris-
tor rectifier. The supply voltage is assumed to be
sinusoidal. The short circuit ratio (SCR) of the
system at the PCC is 84. The source impedance
has an X,/R, ratio of 10. The passive filter con-
sists of only a 5th harmonic tuned filter. Table
1 sliows the circuit parameter values used in the
simulation. A commutation inductmce (Le-) is
designed to have a voltage drop of 5%, as this
will result in smaller notchm in the voltage at the
PCC. The switching frequency of the active filter
is 15kHz. K is chosen to be a real quantity of mag-
uitude 5 p.u. Therefore, the active filter inserts a
resistance of 5 p.u. at harmonic kquencies,
Fig.5 shows the simulation waveforms before
and after the active filter is switched on. The w-
tive filter is started at t=0.12 sec. The cut-off
frequency for the LPF used in the SRF based har-
monic isolator is 30 Hz. There is a significant im-
provement in the waveshape of a, after starting the
active filter.
Fig.6 shows the FFT of 6 , before the active fil-
ter is started. The IEEE 519 harmonic standards
limit the current distortion (for a 88 kV distri-
bution system of SCR=84) to 5.0% for h < 11,
68
0.w om
0 1
012
O M
o i a
O M 02 022
024
020
-2
OW
00
008
'"'"
01
012
014
016
018
0 2 022
024
028 I
x to'
a*
3 -2
-1
0.W
ZO
0.-
0.1
0.12
0.14
0.18
1 ('dC)
0.18
0 2
0.22
0.24
026
Fig. 5. Simulation waveforms for phase 'a'; the active
filter is started at 0.12 sec.
2.25% for 11 5 h < 17, 2.0% for 17 5 h < 23,
1.25% for 23 5 h < 35, 0.35% for 35 5 h, and il
total demand distortion (TDD) to 6%, where / I is
the harmonic no. It is seen that the current distor-
tion limits are exceeded in the source current for
harmonics of order 11 and above. Fig.7 shows tllc
FFT of 6 , after starting the active filter. The har-
monic content in 6 , is confined to within the limits
stated above. Figs.8 and 9 show the FFTs of ut
before and after the active filter is started respec-
tively. The IEEE 519 harmonic standards h i i t
the voltage distortion for this system to 1.5% for
individual harmonics and a total harmonic distor-

Page 5

Fig. 6. FFT of i, bidore the active filter is started for
(a) tlie fuudamerital frequeiicy, (b) the liarinoiiic
frequencies.
40
20
0
' " 7 -
...
...
..
..
..
..
..
..
. . . . . . . . . . .
....... .:,. ............... .: ........
,
.
.
.

.
I
1
.
.
I
.

.
........ j. ................... .......
0
Fig. 7. FFT of i . after the active filter is started for
(it) the Clud"ta1 hquency, (b) the liarinoiiic
frequaicics.
05
7
C 04
w
5
02
0.1
Fig. 8. FFT of v1 beforci the active filter is startcxl for
(a) the fundamnentttl frequency, (b) tlie liarlrionic:
frequeiicies.
laT
0
50
100
150
200
I:.
(4
Fig. 9 . FFT of ut after the
(a) tlie fuiidaineiital frequency, (b) tlte liaririoiiic
frcquencics.
' '
active filter i s startecl for
69