P-Q Theory Based UPQC for Reactive Power Compensation with UCAP

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395 -0056
Volume: 04 Issue: 06 | June -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 434
P-Q THEORY BASED UPQC FOR REACTIVE POWER COMPENSATION
WITH UCAP
K.C.SANDHYA1, B. SUGAD SINGH2
1II YEAR, POWER ELECTRONICS AND DRIVES, Bethlahem Institute of Engineering,Karungal-629157.
2Assistant professor, Department of Electrical and Electronics Engineering. Bethlahem Institute of
Engineering,Karungal-629157.
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Abstract - This paper proposes a p-q theory based control
of ulltracapacitor integrated unified power quality
conditioner (UCAP-UPQC-S). TheUltracapacitors(UCAP)have
low-energy density and high-power density ideal
characteristics for compensation. The fundamentalfrequency
positive sequence (FFPS) voltages are extracted using
generalized cascadeddelaysignalcancellation(GCDSC)which
is used in p- q theory based control to generate reference grid
currents for the shunt compensator. TThheeunifiedpowerquality
controller (UPQC) operating in voltage control mode. The
reference stator current is generated by using P-Q theory. The
ultracapacitor (UCAP) integrated at the DC-bus of the UPQC,
provides a part of active load power. The series VSC operates
such that it shares a part of the reactive power of the load
even under nominal grid conditions. The dynamic
performance of proposed system is verified by simulating it in
MatIab-Simulink using a combinationoflinearandnon-linear
load.
Key Words: Power quality, UPQC, UCAP, p-q theory,
Series compensation, Shunt compensation.
1. INTRODUCTION
There is an increasing need for renewable energy
systems (RES) with ancillary features particularly in low
voltage distribution systems. Ancillary features include
harmonic compensation, reactive power compensation,low
voltage ride through capability etc. This is due to the fact
that there is increased penetration of nonlinear power
electronics based loads[ 1]. These loads inject harmonic
currents into grid which can cause distortion at point of
conunon coupling (PCC) particularly in weak grid systems.
Moreover, due to the intermittent nature ofthecleanenergy
sources such as wind and solar energy, their increased
penetration lead grid voltage fluctuations depending upon
power generation and demand. These voltage fluctuations
can affect sensitive power electronic loads such as
adjustable speed drives, lighting systems etc which can lead
to frequent tripping, maloperation and thus leading to
increased maintenance costs. Renewableenergyintegration
with power quality enhancing sytems such as dynamic
voltage restorer (DVR), unified power quality conditioner
(UPQC) and distribution static compensator (DSTATCOM)
provides an ideal solution by combining benefits of clean
energy with power quality enhancement.DSTATCOM[2] isa
shunt VSC which for load power quality issues such as
current harmonics, load reactive power, unbalance etc.
DVR[3] is a series VSC which protects sensitiveloadsagainst
grid voltage disturbances such as sags/swells, flicker
interruption etc. UPQC isa versatiledeviceasitcompensates
for both load side and grid side power quality problems. A
detailed review of various UPQC configurations and control
has been given in [4]. The series VSC of UPQC comes into
operation under grid voltage sags/swells, flicker and
unbalance which are short duration variations. Compared
with shunt VSC which compensator, the series VSC
utilization is much lesser. Two major trends in UPQC are to
increase the utilization of series inverter[5] and integration
of distribution generation system particularly UCAP at the
DC-bus of UPQC[6]. The most commonlyusedalgorithmsfor
reference signal generation are based in timedomain. These
include pq theory [7], d-q theory [3] and instantaneous
symmetrical components theory. Some other advanced
control techniques for reference signal generation include
using adaptive filters such as adaptive notch filter [8],
ADALINE [9] etc. However, these methods require
calculations for each phase currents and voltages and are
more complex compared to methods based on p-q or d-q
theory which are inherently three-phase based techniques.
Though the classical p-q theory involves only simple
calculations, it doesn't produce accurate results under
conditions of voltage distortions or unbalance [10]. This
drawback can be overcome by using fundamental frequency
positive sequence (FFPS) voltages for generating reference
currents using p-q theory. Modified p-q theory using phase
locked loop (PLL) was proposed in [11]. The various other
methods to extract fundamental frequency positive
sequence voltages are using notch filters [12], generalized
cascaded delay signal cancellation (GCDSC) based methods
[l3] etc This paper proposes control of UCAP-UPQC by
modified p-q theory based technique wherein the
fundamental positive sequence voltages are extractedusing
GCDSC method. The shunt VSC compensates for part of load
reactive power and also injects real power obtained from
the SPY array into grid. The reference voltage for the DC bus
is obtained from maximum power point tracking (MPPT)
algorithm [14]. The series converter operates such that a
part of reactive load power is shared by the series converter
under sag and normal operatingconditionsthusreducing VA

loading on the shunt VSc. The system is simulated using
Matlab-Simulink and its dynamic performance is tested
under conditions of irradiation variation, voltage
sags/swells, distortions etc.
II.CONFIGURATION OF UCAP-UPQC
The topology of a UCAP-UPQC is presented in Fig.I.
The major parts of the system are a series VSC and shunt
VSC connected back to back through a common DC-bus. The
VSCs are connected to grid using interfacing inductors.
Ripple filters are used to filter out switching harmonics of
the VSCs. The series VSC injects voltage through a series
injection transformer. The SPY array isconnecteddirectlyat
the DC bus of UPQC through a reverse blocking diode.
Fig.1. One line diagram of UCAP-UPQC.
In this paper, UCAP-based energystorageintegration
through a power conditioner into the distribution grid is
proposed, and the following applicationareasareaddressed.
1) Integration of the UCAP with power conditioner
system gives the system active power capability.
2) Active power capability is necessary for
independently compensating voltage sags/swells and to
provide active/reactive power support and intermittency
smoothing to the grid.
3) Experimental validation of the UCAP, dc–dc
converter, inverter their interface, and control.
4) Development of inverter and dc–dc converter
controls to provide sag/swell compensation and
active/reactive support to the distribution grid.
5) Hardware integration and performance
validation of the integrated UCAP-PC system.
III.THREE-PHASE SERIES INVERTER
A.POWER STAGE
The one-line diagram of the systemisshowninFig.1.
The power stage is a three-phase voltage source inverter,
which is connected in series to the gridandisresponsiblefor
compensating the voltage sags and swells; the model of the
series DVR and its controller is shown in Fig. 2. The inverter
system consists of an insulated gate bipolar transistor
(IGBT) module, its gate-driver, LC ﬁlter, and an isolation
transformer. The dc-link voltageVdcisregulatedat260Vfor
optimum performance of the converter and the line–line
voltage Vab is 208 V; based on these,themodulationindexm
of the inverter is given by
m=2√2√3Vdc∗nVab(rms). (1)
where n is the turns ratio of the isolation
transformer. Substituting n as 2.5 in (1), the required
modulation index is calculated as 0.52.Therefore,theoutput
of the dc–dc converter should be regulated at 260 V for
providing accurate voltage compensation. The objective of
the integrated UCAPDVR system with active power
capability is to compensate for temporary voltage sag (0.1–
0.9 p.u.) and voltage swell (1.1–1.2 p.u.), which last from 3 s
to 1 min [15].
B.CONTROLLER IMPLEMENTATION
There are various methods to control the series
inverter to provide dynamic voltage restoration and most of
them rely on injecting a voltagein quadraturewithadvanced
phase, so that reactive power is utilized in voltage
restoration [3]. Phase advanced voltage restoration
techniques are complex in implementation, but the primary
reason for using these techniques is to minimize the active
power support and thereby the amount of energy storage
requirement at the dc-link in order to minimize the cost of
energy storage. However,thecostofenergystoragehasbeen
declining and with the availability ofactivepowersupportat
the dc-link, complicated phase-advanced techniques can be
avoided and voltages can be injected in-phase with the
system voltage during a voltage sag or a swell event. The
control method requires the use of a PLL to ﬁnd the rotating
angle. As discussed previously, the goal of this project is to
use the active power capability of the UCAP-DVRsystemand
compensate temporary voltage sags and swells.
Fig.2. Model of three-phase series inverter (DVR) and its
controller with integrated higher order controller.

IV. UCAP AND BIDIRECTIONAL DC–DC CONVERTER
A.UCAP BANK HARDWARE SETUP
UCAPs can deliver very high power in a short time
span; they have higher power density and lower energy
density when compared with Li-ion batteries [18], [19]. The
major advantage UCAPs have over batteries is their power
density characteristics, high number of charge–discharge
cycles over their lifetime, and higher terminal voltage per
module [5], [18]. These are ideal characteristics for
providing active/reactive power support and intermittency
smoothing to the distribution grid on a short-term basis. In
[20], it is proposed that UCAPs are currently viable as short-
term energy storage for bridging power in kilowatt range in
the seconds to few minutes timescale. The choice of the
number of UCAPs necessary for providing grid support
depends on the amount of support needed, terminal voltage
of the UCAP, dc-link voltage, and distribution grid voltages.
For a 260-V dc-link voltage, it is practical and cost-effective
to use three modules in the UCAP bank. Therefore, in this
paper, the experimental setup consists of three 48 V, 165 F
UCAPs (BMOD0165P048) manufactured by Maxwell
Technologies, which are connected in series.
B.BIRECTIONAL DC-DC CONVERTER
A bidirectional dc–dc converter is required as an
interface between the UCAP and the dc-link, since the UCAP
voltage varies with the amount of energy discharged, while
the dc-link voltage has to be stiff. The model of the
bidirectional dc–dc converter and itscontrollerareshownin
Fig. 4(a). The dc–dc converter should operate in Discharge
mode, while providing active/reactive power support and
voltage sag compensation.
Fig.3. Controller block diagram for DVR and APF.
The dc–dc converter should also be able to operate
in bidirectional mode to be able to charge or absorb
additional power from the grid during intermittency
smoothing. In this paper, the bidirectional dc–dc converter
acts as a boost converter, while discharging power from the
UCAP and acts as a buck converter while charging the UCAP
from the grid. Average currentmodecontrol,whichiswidely
explored in literature [19], is used to regulate the output
voltage of the bidirectional dc–dcconverterinbothBuck and
Boost modes while charging and dischargingtheUCAPbank.
This method tends to be more stable when compared with
other methods like voltage mode control and peak current
mode control. Average current mode controller is shown in
Fig. 3, where the actual output voltage Vout is compared
with the reference voltage Vref and the error is passed
through the voltage compensator C1 (s) that generates the
average reference current Iucref.
C. CONTROLLER IMPLEMENTATION
Average current mode control is used to regulate
the output voltage of the bidirectional dc–dc converter in
both Buck and Boost modes, while charging and discharging
the UCAP bank. While the UCAP-APF system is discharging
power, the dc-link voltage Vout tends to be less than Vref,
which causes the reference current Iucref to be positive,
thereby operating the dc–dc converter in Boost mode.Along
similar lines, when theUCAP-APFsystemisabsorbingpower
from the grid, the dc-link voltage Vout tends to be greater
than Vref, which causes the reference current Iucref to be
negative and thereby operating the dc–dc converter in Buck
mode. Average current mode control technique is widely
explored in the literature [19], and it was found as the ideal
method for UCAP-APF integration as it tends to be more
stable when compared with other methods like voltage
mode control and peak current mode control. Thisisa major
advantage in the present topology, where the stability of the
dc–dc converter has to be ensured over a wide operating
range and in both Buck and Boost modes of operation.
Average current mode controller and the higher level
integrated controller are showninFig.4(a),wheretheactual
output voltage Vout is compared with the reference voltage
Vref and the error is passed through the voltage
compensator C1 (s), which generates the average reference
current Iucref. This is then compared with the actual UCAP
current (which is also the inductor current) Iuc, and the
error is then passed through thecurrentcompensatorC2(s).
Fig.4. (a) Model of the bidirectional dc–dc converter and
its controller.

D. HIGHER LEVEL INTEGRATED CONTROLLER
The higher level integrated controllerisdesignedto
make system level decisions on the inverter and dc–dc
converter controllers. Based on various system parameters
like Pload, Qload, Pgrid, Qgrid, Vucap, Vdc, Idclnk, andIucap,
the higher level integrated controller will decide on
operating in one of the following modes: active power
support mode, reactive power support mode, renewable
intermittency smoothing mode, sag/swell compensation
mode, and UCAP chargemode.Inactivepowersupport mode
and renewable intermittency smoothing mode,theUCAP-PC
system must provide active power to the grid.Therefore,the
active power capability of the UCAP-PC system must be
assessed by the higher level integrated controller. Based on
the Pgrid and Pload values, the reference Prefiscalculatedin
the higher level integrated controller, and it will decideifthe
UCAP has enough energy to respond to the Pref command
based on the UCAP state of charge. If the UCAP has enough
capacity to respond to the request, then the dc–dc converter
controller is operated in grid support mode; otherwise, it is
operated in charge mode, where the UCAP is recharged and
the power request is met at a later time. In grid support
mode, the dc–dc converter will operate in a bidirectional
fashion in both Buck and Boost modes to respond to the
active power requests and regulate the dc-link voltage in a
stable fashion, while the inverter controller should respond
such that the commanded Pref is supplied by the inverter
through current control. In reactive power support mode,
the UCAP-PC system must provide reactive power to the
grid. In this mode, the UCAP-PC does not provide any active
power to the grid and even the PC losses are supplied by the
grid. Based on the Qgrid and Qload values,thereferenceQref
is calculated in the higher level integrated controller. In this
mode, the dc–dc converter controller can be programmedto
operate in grid support mode directly because the active
power requirement for operating in this mode is minimal.
Therefore, the goal of the dc–dc converter controller is to
regulate the dc-link voltage in a stable fashion, while the
inverter controllershouldrespondsuchthatthecommanded
Qref is supplied by the inverter through current control. In
sag/swell compensation mode, the UCAP-PC system is
programmed to prevent sensitive loads from disturbances
on the supply-side like voltage sag or voltage swell. These
disturbances require short-term energy storage, and in this
mode, the dc–dc converter controller can be programmedto
operate in grid support mode. Therefore, the goal of the dc–
dc converter controller is to regulate the dc-link voltage in a
stable fashion during both sag/swell events. It is also
required that the dc–dc converter be able to discharge and
meet the active power requirements during a voltage sag
and to be able absorb active power in a stable fashionduring
a voltage swell event. In charge mode, the UCAPisrecharged
by absorbing active power from the grid when the UCAP
state of charge falls below 50%. The rate at which the UCAP
can be charged is assessed by the higher level integrated
controller based on the Pgrid and Pload values and the
reference Pref is calculated. Then the dc–dc converter
controller is commandedto operateinchargemode, wherein
the dc–dc converter will operate in Buck Mode to absorbthe
power from the grid and the inverter controller must
respond to supply commanded Pref.
IV.SIMULATION RESULTS
The simulation of the proposed UCAP integrated
power conditioner system is carried out in Matlab fora 208-
V, 60-Hz system, where 208 V is 1 p.u. The system response
for a three-phase voltage sag which lasts for 0.1 s and has a
depth of 0.64 p.u. It can be observed that during voltage sag,
the source voltage Vsrms is reduced to 0.36 p.u., while the
load voltage VLrms is maintained constant at around 1.01
p.u. due to voltages injected in-phase by the series inverter.
This can also be observed from the plots of the line–line
source voltages (Vsab, Vsbc, and Vsca) ,the line–line load
voltages (VLab, VLbc, and VLca) ,and the line–neutral
injected voltages of the series inverter (Vinj2a, Vinj2b, and
Vinj2c) . The active power deﬁcit of the grid is met by the
DVR power Pdvr, which is almost equal totheinputpowerto
the inverter Pdc in available from theUCAP.Therefore,itcan
be concluded from the plots that the active power deﬁcit
between the grid and load during the voltage sag event is
being met by theUCAP-basedenergystoragesystemthrough
bidirectional dc–dc converter and the inverter. It canalsobe
noticed that the grid reactive power Qgrid reduces during
the voltage sag while Qdvr increases to compensate for the
reactive power loss in the system. Similar analysis can also
be carried out for voltage sags that occurinoneofthephases
(A, B, or C) or in two of the phases (AB, BC, or CA); however,
three-phase voltage sag case requires the maximum active
power support and is presented here. The proposed UCAP
integrated power conditioner system’s performance is then
simulated for the active and reactive power support case.
Fig.5. MATLAB/SIMULINK UPQC with UCAP.

Fig.6. Source voltage.
Fig.7. Source current.
Fig.8. DVR voltage.
Fig.9. Load voltage.
Fig.10. Load current.
VI. CONCLUSION
In this paper, the conceptofintegratingUCAP-based
rechargeable energy storage to a power conditioner system
to improve the power quality of the distribution grid is
presented. With this integration, the DVR portion of the
power conditioner will beabletoindependentlycompensate
voltage sags and swells and the APF portion of the power
conditioner will be able to provide active/reactive power
support and renewable intermittency smoothing to the
distribution grid. UCAP integration through a bidirectional
dc–dc converter at the dc-link of the power conditioner is
proposed. The control strategyoftheseriesinverter(DVR)is
based on inphase compensation and the control strategy of
the shunt inverter (APF) is based on id −iq method. Designs
of major components in the power stage of the bidirectional
dc–dc converter are discussed. Average current mode
control is used to regulate the output voltage of the dc–dc
converterdue to its inherently stable characteristic.Ahigher
level integrated controller that takes decisions based on the
system parameters provides inputs to the inverters and dc–
dc converter controllers to carry out their control actions.
The simulation of the integrated UCAP-PC system which
consists of the UCAP, bidirectional dc–dc converter, and the
series and shunt inverters is carried out using PSCAD. The
simulation of the UCAP-PC system is carried out using
PSCAD. Hardware experimental setup of the integrated
system is presented and the ability to provide temporary
voltage sag compensation and active/reactive power
support and renewable intermittency smoothing to the
distribution grid is tested. Results from simulation and
experiment agree well with each other thereby verifyingthe
concepts introduced in this paper. Similar UCAPbased
energy storages can be deployed in the future in a microgrid
or a low-voltage distribution grid to respond to dynamic
changes in the voltage proﬁles and power proﬁles on the
distribution grid.
ACKNOWLEDGEMENT
The authors would like to express a gratitude especially to
Ms.K.Christal saji.,Associate professor and Head of the
Department for the invaluable advice and support that she
has given to the authors.

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K.C.SANDHYA received her B.E degree in
Electrical and electronics Engineering in
2015 from Sun college of Engineering and
Technology, Now studying M.E Power
Electronics and drives in Bethlahem
Institute of Engineering.
B.SUGAD SINGH received his B.E
degree in Electrical and Electronics
Engineering and M.E degree in
Power Electronics and Drives from
anna university, Chennai. Atpresent
working as an assistant professor in
the department of Electrical and
Electronics Engineering, Bethlahem
institute of Engineering. His area of
interest includes Machines and
Power Electronics.

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