IEEE ISIE 2006, July 9-12, 2006, Montreal, Quebec, Canada
Conceptual Study of Unified Power Quality
Conditioner
(UPQC)
V. Khadkikar' Student Member, IEEE, A. Chandra' Senior Member, IEEE, A.
0.
Barry2 and T. D. Nguyen3
'Departement de genie electrique, Ecole de technologie superieure, Montreal, CANADA.
2Laboratoire de simulation de Reseaux, IREQ, Varenne, CANADA.
3Secteur Industriel et qualite du service electrique, Hydro Quebec, Montreal, CANADA.
e-mail: v khadkikargyahoo.com, chandragele.etsmtl.ca
Abstract- This paper deals with conceptual study of unified
power quality conditioner (UPQC) during voltage sag and swell
on the power distribution network. One of the peculiarities of
UPQC is that it can inject voltage from 00 to 3600. Based on this
injected voltage phase angle with respect to the utility or PCC
voltage phase angle, lPQC can absorb or inject active power.
Thus the UPQC can work in zero active power consumption
mode, active power absorption mode and active power delivering
mode. The series active power filter (APF) part of UPQC works in
active power delivering mode and absorption mode during voltage
sag and swell condition, respectively. The shunt APF part of
UPQC during these conditions helps series APF by maintaining
dc link voltage at constant level. In addition to this the shunt APF
also compensates the reactive power required by the load and
harmonics generated by them. The MATLAB / SIMULINK
results are provided in order to verify the analysis.
cause of voltage sag and swell is sudden change of line current
flowing through the source impedance.
This paper is based on the steady state analysis of UPQC
during different operating conditions. The purpose is to
maintain sinusoidal source current with unity power factor
operation along with load bus voltage regulation. The major
concern is the flow of active and reactive power during these
conditions, which decide to amount of current flowing through
the active filters and through the supply. This analysis can be
useful for selection of device ratings.
The steady state power flow analysis is discussed in the
section II. The SIMPOWERSYSTEM (SPS) Matlab / Simulink
based simulation results are discussed in section III and finally
section IV concludes the paper.
Key words-- Active Power Filter (APF), UPQC, Voltage
Harmonics, Voltage Sag, Voltage Swell, Power Quality, SPS
MA TLAB /SIMULINK.
II. STEADY - STATE POWER FLOW ANALYSIS
I.
INTRODUCTION
In today's complex electronics environment many problems
can occur because of poor quality of power. Therefore, it has
become necessary to provide a dynamic solution with greater
degree of accuracy as well as with fast speed of response. The
active power filtering has proven to be one of the best solutions
for mitigation of major power quality problems [1-2]. The
shunt APF's are utilized to overcome all current related
problems, such as current harmonics, reactive current and
current unbalance. Whereas, all voltage related problems, such
as voltage harmonics, voltage sag and swell, and voltage
unbalance, are handled by using the series APF.
With the advancement in the field of power electronic
devices and control application, it is very common now these
days to come across the situation where compensation of both
current as well voltage related problems is must. The UPQC,
an integration of shunt and series APF is one of the most
suitable as well as effective device in this concern [1-6], [812]. A UPQC tackles both current as well as voltage related
power quality problems simultaneously.
Recently more attention is being paid on mitigation of
voltage sags and swells using UPQC [8-12]. The common
1-4244-0497-5/06/$20.00 © 2006 IEEE
The powers due to harmonics quantities are negligible as
compared to the power at fundamental component, therefore,
the harmonic power is neglected and the steady state operating
analysis is done on the basis of fundamental frequency
component only. The UPQC is controlled in such a way that
the voltage at load bus is always sinusoidal and at desired
magnitude. Therefore the voltage injected by series APF must
be equal to the difference between the supply voltage and the
ideal load voltage. Thus the series APF acts as controlled
voltage source. The function of shunt APF is to maintain the dc
link voltage at constant level. In addition to this the shunt APF
provides the var required by the load, such that the input power
factor will be unity and only fundamental active power will be
supplied by the source.
The voltage injected by series APF can vary from 00 to 3600.
Depending on the voltage injected by series APF, there can be
a phase angle difference between the load voltage and the
source voltage. However, in changing the voltage phase angle
of series APF, the amplitude of voltage injected can increase,
thus increasing the required kVA rating of series APF [7].
In the following analysis the load voltage is assumed to be in
phase with terminal voltage even during voltage sag and swell
condition. This is done by injecting the series voltage in phase
or out of phase with respective to the source voltage during
voltage sag and swell condition respectively. This suggests the
1 088
real power flow through the series APF. The voltage injected
by series APF could be positive or negative, depending on the
source voltage magnitude, absorbing or supplying the real
power. In this particular condition, the series APF could not
handle reactive power and the load reactive power is supplied
by shunt APF alone. The equivalent circuit of a phase for
UPQC is shown in the Fig. 1. The source voltage, terminal
voltage at PCC and load voltage are denoted by vs, vt and vL
respectively. The source and load currents are denoted by is
and iL respectively. The voltage injected by series APF is
denoted by vSr, where as the current injected by shunt APF is
denoted by iSh.
-+V
R
s
+
tVI
L
PCC
iSh(t
LOAD
BUS
0
DA
=
QSr
IL
=
iL
(2)
(3)
OL
-
Vt = VL(1+ k) Z 0°
Where factor k represents the fluctuation of source voltage,
defined as,
Vt VL
k
(4)
VL
The voltage injected by series APF must be equal to,
-kVL /o0
(5)
The UPQC is assumed to be lossless and therefore, the active
power demanded by the load is equal to the active power input
at PCC. The UPQC provides a nearly unity power factor
source current, therefore, for a given load condition the input
active power at PCC can be expressed by the following
equations,
(6)
Pt PL
VSr
vt.iS
VL
=
VL
=
-
Vt
=
VL.iL*COS/L
(1 + k)-iS
=
VL.iL.COS1L
k
I1+ k .COSQL
VSr
iSh
=
iS - iL
ISh
=
isZO°
iSh
=
is
ISh
=
(is
Psh
VLZO°
(12)
(13)
k.vL *is IC 0 SOS
is *Siinos
(14)
pSr vSr JS =
L is
Q r 0
(15)
The complex apparent power absorbed by the shunt APF can
be expressed as,
S Sh = VL JiSh
(16)
The current provided by the shunt APF, is the difference
between the input source current and the load current, which
includes the load harmonics current and the reactive current.
Therefore, we can write;
Taking the load voltage, VL, as a reference phasor and
suppose the lagging power factor of the load is Cos(L, we can
write;
VL
-
Os = 0, since UPQC is maintaining unity power factor
Fig. 1 Equivalent Circuit for a UPQC
=
(1 1)
VSr*iSI C OSi5S
pSr
=
=
QSh
=
-
(17)
-
iL
z
-
OL
(iL .Cos5L
iL CosL) +
(18)
jiL .SinfL)
(19)
jiL.SintL
(20)
VL *Sh *CosS05h
VL (iS iL -COSOL)
VL 'iSh 'SinOsh
(21)
VL.iL.SinfL
(24)
(22)
(23)
Based on the above analysis the different modes of operation
are discussed below:
CASE I:
The reactive power flow during the normal working
condition when UPQC is not connected in the circuit is shown
in the Fig. 2 (a). In this condition the reactive power required
by the load is completely supplied by the source only. When
the UPQC is connected in the network and the shunt APF is
put into the operation, the reactive power required by the load
is now provided by the shunt APF alone; such that no reactive
power burden is put on the mains. So as long as the shunt APF
is ON, it is handling all the reactive power even during voltage
sag, voltage swell and current harmonic compensation
(7)
(8)
a) No UPQC
(9)
b) With Shunt APF
Fig. 2 a) - b) Reactive Power Flow
The above equation suggests that the source current is
depends on the factor k, since OL and iL are load characteristics
and are constant for a particular type of load.
The complex apparent power absorbed by the series APF can
be expressed as,
(10)
S Sr
V Sr is
condition. As discussed above the series APF is not taking any
active part in supplying the load reactive power. The reactive
power flow during the entire operation of UPQC is shown in
the Fig. 2 (b). In this case no active power transfer takes place
via UPQC, termed as Zero Active Power Consumption Mode.
1089
CASE II:
If k < 0, i.e. vt < VL, then from (4) and (12), Ps, will be
positive, means series APF supplies the active power to the
load. This condition is possible during the utility voltage sag
condition. From (9), is will be more than the normal rated
current. Thus we can say that the required active power is
taken from the utility itself by taking more current so as to
maintain the power balance in the network and to keep the dc
link voltage at desired level. This active power flows from the
source to shunt APF, from shunt APF to series APF via dc link
and finally from series APF to the load. Thus the load would
get the desired power even during voltage sag condition.
Therefore in such cases the active power absorbed by shunt
APF from the source is equal to the active power supplied by
the series APF to the load. Since series APF supplies active
power, termed as Active Power Delivering Mode. The overall
active power flow is shown in Fig. 3.
IPs
I
< >PL >
DC LINKSupy
Ps'= Power Supplied by the source to the load during voltage sag condition
Psr'= Power Injected by Series APF in such way that sum Psr'+Ps' will be
the required load power during normal working condition i.e. PL
NWh= Power absorbed by shunt APF during voltage sag condition
Psr'=Psh'
Fig. 3 Active Power Flow during Voltage Sag Condition
CASE IV:
If k = 0, i.e. Vt = VL, then there will not be any real power
exchange though UPQC. This is the normal operating
condition. The overall active power flow is shown in Fig. 5.
UPQC
DC LINK
Fig. 5. Active Power Flow during Normal Working Condition
CASE V:
If the terminal voltage is distorted one containing several
harmonics, in such cases the series APF injects voltage equal
to the sum of the harmonics voltage at PCC but in opposite
direction. Thus the sum of voltage injected by series APF and
distorted voltage at PCC will get cancelled out. During this
voltage harmonic compensation mode of operation the series
APF does not consume any real power from sources since it
injects only harmonics voltage. Here UPQC works in zero
active power consumption mode.
CASE VI:
CASE III:
If k > O, i.e. Vt > vL, then by (4) and (12), Ps, will be
means series A-PF is absorbing the extra real
power from the source. This is possible during the voltage
swell condition. Again by (9), is will be less than the normal
rated current. Since vs is increased, the dc link voltage can
increase. To maintain the dc link voltage at constant level the
shunt APF controller reduces the current drawn from the
supply. In other words we can say that the UPQC feeds back
the extra power to the supply system. Since series APF absorbs
active power, termed as Active Power Absorption Mode. The
overall active power flow is shown in Fig. 4.
negative, this
If the load is a non linear one producing harmonics, in such
cases the shunt APF injects current equals to the sum of
harmonics current but in opposite direction, thus canceling out
any current harmonics generated by non linear load. During
this current harmonics compensation mode of operation the
shunt APF does not consume any real power from the source
since it injected only harmonics currents. Here UPQC works in
zero active power consumption mode.
The phasor representations ofthe above discussed conditions
- (d) for
are shown in the Fig. 7 (a) - (d) and Fig. 8 (a)
inductive and capacitive type of load respectively.
1,
vvL
.sI1
41
Figa)\
I,I
SutA I L
Shunt APF ON
Normal working Condition,
without any compensation
Ig.
o4
Fig.c)
Ps"= Power Supplied by the source to the load during voltage swell condition
Psr"= Power Injected by Series APF in such way that sum Ps"-Psr" will be
the required load power during normal working condition
Psh"= Power delivered by shunt APF during voltage sag condition
Psr"=Psh"
Fig.4 Active Power Flow during Voltage Swell
1090
+1
\
L
I
Vi
Fig.d)
1
UPQC ON, Voltage Sag
UPQC ON, Voltage Swell
Fig. 7 a)- d) Phasor Representation: Inductive Load
V. Vt
L'
Phasor 7 (a) represents the normal working condition,
considering load voltage VL as a reference phasor. oL is lagging
power factor angle of the load. During this condition is will be
exactly equal to the iL since no compensation is provided.
When shunt APF is put into the operation, it supplies the
required load vars by injecting a 900 leading current such that
the source current will be in phase with the terminal voltage.
The phasor representing this capacitive effect is shown in Fig.
7 (b). The phasor representations during voltage sag and
voltage swell condition on the system are shown in the Fig. 7
(c) and Fig. 7 (d) respectively. The deviation of shunt
compensating current phasor from quadrature relationship with
load voltage suggests that there is some active power flow
through the shunt APF during these conditions.
depend on the vars to be compensated. This condition is
represented by zone II. Now, if the load is capacitive one,
theoretically, the load would draw leading current from the
source, i.e. load generates vars. This load generated vars are
compensated by shunt APF by injecting 900 lagging current.
The magnitude of compensating current depends on the vars to
be cancelled out, represented by zone III. During the operation
of UPQC in zone II and III larger the var compensation more
would be the compensating current magnitude.
vt
Fig.a)
I.
J
Figb)
Normal working Condition,
without any compensation
1:
7)
ZONE VI
i
iA
ZONE VII
I--,
Voltage Swell _
Capacitive Load
90 < 0, <180
(Lagging)
Is
UPQC ON, Voltage Sag
UPQC ON, Voltage Swell
Voltage Sag
Inductive Load
° <0S < 90
(Leading)
ZONE I
Load
*'Resistive
A-
ZONE V
Voltage Sag
Capacitive Load
O<C <90O
(Lagging)
-
-
ZONE III
Capacitive Load
Fig. 9 Operating Zones of UPQC based on
Compensation
Im
l
ZONE IV
/
Fig.d)
90°
\
Angle QS LOcUS
Fig.c)
=
\k
VJLjt
Shunt APF ON
If
Inductive Load
(Leading)
Voltage Swell
Inductive Load
90 < Sh < 180
(Leading)
!I
ZONE II
-.
(5 = 90°
(Lagging)
oSh Variation for Sag/Swell
Zone IV and zone V represents the operating region of
UPQC
during the voltage sag on the system for inductive and
Fig. 8 a) - d) Phasor Representation: Capacitive Load
capacitive type of the loads respectively. During the voltage
Phasor in Fig. 8 (a) represents the normal working condition, sag as discussed previously, shunt APF draws the required
considering leading power factor angle of the load. During this active power from the source by taking extra current from the
condition is will be exactly equal to the itL When shunt APF is source. In order to have real power exchange between source,
put into the operation, it cancels out the vars generated by load UPQC and load, the angle OSh should not be 900. For inductive
by injecting a 900 lagging current such that the source current type ofthe load, this angle could be anything between 00 to 900
will be in phase with the terminal voltage. The phasor leading and for capacitive type of the load, between 00 to 900
representing this inductive effect is shown in Fig. 8 (b). The lagging. This angle variation mainly depends on the 00 of sag
phasor representations during voltage sag and voltage swell need to be compensated and load var requirement.
condition on the system are shown in the Fig. 8 (c) and Fig. 8
Zone VI and zone VII represents the operating region of
(d) respectively.
UPQC during the voltage swell on the system for inductive and
Fig. 9 shows variation of angle oSh during different modes of capacitive type of the loads respectively. During the voltage
operations of UPQC, represented by zones. Figure consists of swell as discussed previously, shunt APF feeds back the extra
seven zones of operations. The x axis represents the reference active power from the source by taking reduced current from
load voltage whereas the shunt APF compensating current can the source. In order to achieve this angle ,Sh would be between
vary from 00 to 3600. Zone I, II and III represents the case of 900 to 1800 leading and between 900 to 1800 lagging for
pure resistive, inductive and capacitive load respectively. If the inductive and capacitive type of load respectively.
load is pure resistive, shunt APF does not inject any
compensating current since there is no reactive power demand
III. SIMULATION RESULTS
from the load, this condition is represented by zone I.
In order to show the impact of 00 sag and 00 swell variation
Considering the case of inductive load, the load var
requirement is supplied by shunt APF by injecting 900 leading on the angle OSh' shunt compensating current and source
current. The magnitude of the compensating current would current, a MATLAB/Simulink based simulation is carried out.
1091
>~ 4I
Table I gives the values of OSh, iSh and is for different load var
requirements. Here, the load active power demand is kept
constant, 8000 watts. With these conditions the input voltage is
reduced up to 50 00 of the rated value.
Fig. 10 a) shows the variation in angle oSh with 00 of sag
variation for three different load var demands. As 00 of sag
increases, the angle oSh reduces from 900 and reaches towards
lower value, suggests more and more active power is being
transfer through shunt APF. Shunt APF is now playing an
important role of handling extra load active power demand in
addition to the load var support. The aforementioned task is
achieved by taking fundamental current component from the
source, as 0O of sag increase, shunt APF draws more and more
fundamental component, easily seen from Fig. 10 b). The
impact of variation in both angle oSh and compensating current
iSh can be seen on the source current, which increases as °O of
sag increases, as shown in the Fig. 10 c). This is necessary to
maintain the active power balance in the network and to
maintain the dc link voltage at constant level.
80D0 W{constant)
Active Pover =
Case :6000 Vars
qbsh
100
CaseII :8000 Vars
Case IIIl :10O ars
90
80_
70-
60
Table I: Impact of % Sag & Load Var Variation
OSh
%Swell
No UPQC
-0%
90o00
10% T96.0
20%
101.5u
30%
1 05.5
40%
108.5°
50%
110Ti a
0%/.
.10%
30%/.
20%
%Sag
(a) Angle
8060
_
40
30
20I
0%
10%/0
20%
30%o
%Sag
40%
is
110
100
0%/.
10%
20%
30%
% Sag
40%
Is
74.0 A
53.0 A
49.16A
45.64 A
42.36 A
40.05 A
38.29 A
--
52.2 A
53.8A
54.4 A
55.5 A
57.0 A
58.5 A
:60 Vars
100
90
8070
60
60%
0%
20%
10%
30%
%S11I
(a) Angle
80-
III
40%
50%
oSh Variation
-
70
11
60%
Actve Power =80DO W(constant)
Case :6000 Vars
Case ll :8000 Vars
Case III :10000 Mars
is}'h
60
so
40
11
*
50 ri
*
30
20
60%o
50%
50%
Ish
Vars
Case:II 8000
Case ia
o10000 ars
I
0J%
10%
50°/
40/0
30°/0
20%
6.0%
% Sv1
(b) Shunt Compensating Current Variation
Active Power = 8000 W(constant)
CawI :6000 Vars
Case ll :8000 Vars
Cas III: 1000DO ars
8000 Vars
110
50%
4
70
50
Case
oSh Variation
Acfive Power = 8000 W(constant)
Case :6000 Vars
Case Il :8000 Vars
Case II :10000 Vars
fh
90
40%
-39.8 A
40.4A
41.5 A
43.0 A
44.5 A
46.5 A
Is
lASh
-65.8 A
53.0 A l900°
48.78A l94.0
45.08 A
98.5a
42.27 A 101.5a
40.15 A 104.0°
39.23 A 106.Ou
Active Power = 8000 W(constant)
osh
120
4
-4
30
20
10
0
Ish
Table II gives the values of OSh' iSh and is for different load
var requirements with a swell on the system. Fig. 11 a) shows
the variation in angle OSh with 00 of swell variation for three
different load var demands. As 00 of swell increases, the angle
oSh increases from 900, suggests more and more active power
is being fed back through shunt APF. The shunt compensating
current magnitude increases as shown in Fig. 11 b), to achieve
the aforementioned task. The increase in angle oSh and
-III
4-- I
50
40
6000 Vars
(b) Shunt Compensating Current Variation
Active Po\er = 8000 W(constant)
i
CawI
Case
Case
60
:6000 Mars
Mars
:, :8000
:10000 Mars
i
55
50
4 ~
~
~
~
~
~
~
~
40
35
30
0%
60%
10%
20%
30%
40/0
50
60%
%1SAI
(c) Source Current Variation
(c) Source Current Variation
Fig. 10 Variation in System Parameters due to % of Sag Variation
Fig.
1092
11
Variation in System Parameters due to % of Swell Variation
compensating current results in reduction in source current
magnitude which decreases as %0 swell increases, as shown in
Fig. t I c).
Table II: Impact of % Swell & Load Var Variation
%Swell
NoUPQC
0%
10%
20%
30%
40%
50%
oS5
6000 Vars
Ish
--
--
90.0°
96.0°
101.5
39.8 A
40.4 A
41.5 A
43.0 A
44.5 A
46.5 A
105.50
108.5
110.0°
Is
65.8 A
53.0 A
48.78 A
45.08 A
42.27 A
40.15 A
39.23 A
8000 Vars
Ish
--
90.0°
94.0°
98.5°
101.50
104.0°
106.0°
--
52.2 A
53.8 A
54.4 A
55.5 A
57.0 A
58.5 A
ACKNOWLEDGMENT
The authors would like to thank Natural Sciences and
Engineering Research Council of Canada (NSERC) and IREQ
/ Hydro-Quebec for providing financial support for this
research work.
REFERENCES
Is
74.0 A
53.0 A
49.16 A
45.64 A
42.36 A
40.05 A
38.29 A
From Table I and II, when the UPQC is not connected in the
circuit, the source supplies both active as well as reactive
power demanded by the load. But, as soon as UPQC is put into
operation, source supplies only active part of load power
demand. During this var compensation mode of operation the
source current is almost constant i.e. 53 A, independently on
the load var requirements, as seen from Table I and II for 0 °0
sag and swell.
In normal operating condition, the shunt APF
provides the load vars, whereas, series APF handles no active
or reactive power, so in this case the rating of series APF
would be small fraction of load rating. The shunt APF rating
mainly depends on the compensating current provided by it,
which depends on the load power factor or load var
requirement. Higher the load var requirement higher would be
the shunt APF rating. From (4), (9) and (12), the series APF
rating depends on two factors, source current is and factor k.
The current is increases during voltage sag condition where as
decreases during voltage swell condition. Therefore the rating
of series APF is considerably affected by the °0 of sag need to
be compensated. A compromise needs to be made while
considering the series APF ratings, which directly affects the
sag compensation capability of UPQC. Depending on the load
requirement an optimisation can be done on these two issues.
The shunt APF rating would increase slightly while
considering °0 sag and °0 swell compensation since it has to
maintain the dc link voltage.
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[12] Ghosh A, Jindal A.K. and Joshi A., "A unified power quality conditioner
for voltage regulation of critical load bus", Power Engineering Society General
Meeting, 2004. IEEE 6-10 June 2004, pp. 471 - 476.
IV. CONCLUSION
A conceptual analysis of UPQC has been carried out during
voltage sag and swell condition on the system. The series APF
injects in phase voltage during voltage sag condition and out of
phase voltage during voltage swell condition and maintains the
voltage at load bus at desired constant level. The shunt APF
helps series APF during voltage sag and swell condition by
maintaining the dc link voltage at set constant level, such that
series APF could effectively supply or absorb the active power.
In addition to this shunt APF also provides the required load
vars and thus making the input power factor close to unity.
This analysis is very useful in selection of kVA ratings of both
series and shunt APFs depending on the %0 of sag and swell
needed to be compensated.
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