Kp2417961805

S.Md.Ishaq, W.V.Jahnavi, T.Devaraju / International Journal of Engineering Research and
Applications (IJERA) ISSN: 2248-9622 www.ijera.com
Vol. 2, Issue4, July-August 2012, pp.1796-1805
Unified Power Quality Conditioner in a Grid Connected Photo-
voltaic System
S.MD.ISHAQ, M.Tech, W.V.JAHNAVI, M.Tech, T.DEVARAJU, M.E, Ph.D,
Student, Asst.Professor (S.L), Professor, HOD,
Electrical Power Systems, E.E.E.Department, E.E.E.Department,
Sree Vidyanikethan Engineering Sree Vidyanikethan Engineering Sree Vidyanikethan Engineering
College. College. College.

ABSTRACT
In this paper the design of combined op- voltage is not pure sinusoidal. As these devices are
eration of UPQC and PV-ARRAY is proposed. the most important cause of harmonics, inter harmon-
The proposed system is composed of series and ics, notches and neutral currents, the power quality
shunt inverters connected back to back by a dc- should be improved. The solution to PQ problem can
link to which pv-array is connected. This system is be achieved by adding auxiliary individual device
able to compensate voltage and current related with energy storage at its dc-link by PV-array.
problems both in inter-connected mode and This auxiliary equipment has the general
islanding mode by injecting active power to grid. name of power conditioners and are mainly charac-
The fundamental aspect is that the power elec- terised by the amount of stored energy or stand alone
tronic devices (PE) and sensitive equipments (SE) supply time. That auxiliary equipment having both
are normally designed to work in non-polluted “shunt” and “series” inverter connected back to back
power system, so they would suffer from mal- by a dc-link is called the “unified power quality con-
functions when supply voltage is not pure sinusoi- ditioner” (UPQC).
dal. Thus this proposed operating strategy with UPQC with PV-ARRAY is greatly studied
flexible operation mode improves the power qual- by several researchers as a basic device to control the
ity of the grid system combining photovoltaic ar- power quality. The duty of UPQC is reducing pertur-
ray with a control of UNIFIED POWER bations which affect on the operation of sensitive
QUALITY CONDITIONER. loads. UPQC is able to compensate voltage sag,
swell, voltage and current harmonics using shunt and
NOMENCLATURE series inverters.
IL load current In spite of this issue, UPQC is able to com-
VL load voltage pensate voltage interruption and active power injec-
CF filter capacitance tion to grid because in its dc-link there is energy stor-
IF filter current age known as distributed generating (D.G) source.
VDC Dc voltage The attention to distributed generating (DG)
IS source current sources is increasing day by day. The reason is their
VS source voltage important roll they will likely play in the future of
Vf filter voltage power systems (Blaabjerg et al, 2004), (Barker and
Cs source capacitance deMello, 2000).Recently, several studies are accom-
Ls source inductance plished in the field of connecting DGs to grid using
Iabc 3-phase instantaneous load current power electronic converters. Here, grid's interface
Vlabc 3-phase instantaneous load voltages shunt inverters are considered more where the reason
Vsabc 3-phase source voltages is low sensitiveness of DGs to grid's parameters and
Ildq Two axis currents DG power transferring facility using this approach.
∆idc dc link capacitor voltage Although DG needs more controls to reduce the
Ic compensating current problems like grid power quality and reliability, PV
PLL phase locked loop energy is one of the distributed generation sources
LPF low pass filter which provides a part of human required energy
PI proportional plus integral controller nowadays and will provide in the future. The greatest
PWM pulse width modulation share of applying this kind of energy in the future
MPPT maximum power point tracking will be its usage in interconnected systems. Nowa-
days, European countries, has caused interconnected
I.INTRODUCTION systems development in their countries by choosing
One of the important aspects is that, power supporting policies. In this paper, UPQC and PV
electronic devices and sensitive equipments are de- combined system has been presented. UPQC intro-
signed to work in non-polluted power systems. So, duced in has the ability to compensate voltage sag
they would suffer from malfunctions when the supply and swell, harmonics and reactive power.

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A. Shunt inverter control

In fig. 1 the general structure of grid con- B. Series inverter control
nected PV systems is shown. The advantage of pro- C. DC/DC converter
posed combined system is voltage interruption com- Controlling strategy is designed and applied
pensation and active power injection to grid in addi- for two interconnected and islanding modes. In inter-
tion to the mentioned abilities. Also, this proposed connected mode, source and PV provide the load
system has higher efficiency and functioning ability power together while in islanding mode; PV transfers
in compare with other common PVs and causes re- the power to the load lonely. By removing voltage
duction in system's total cost. Simulation results, us- interruption, system returns to interconnected mode.
ing “MATLAB” software show that this proposed
system operates correctly. A. Shunt inverter control:
In this paper, shunt inverter undertakes two
II.SYSTEM DESCRIPTION main duties. First is compensating both current har-
UPQC has two shunt and series voltage monics generated by nonlinear load and reactive
source inverters which are as 3-phase 3-wire or 3- power, second is injecting active power generated by
phase 4-wire. Shunt inverter is connected to point of PV system. The shunt inverter controlling system
common coupling (PCC) by shunt transformer and should be designed in a way that it would provide the
series inverter stands between source and coupling ability of undertaking two above duties.
point by series transformer. Shunt inverter operates Shunt inverter control calculates the com-
as current source and series inverter operates as volt- pensation current for current harmonics and reactive
age source. Power when PV is out of the grid.

Fig.2: shunt inverter control block

The power loss caused by inverter operation
Fig. 1: Configuration of proposed UPQC with PV should be considered in this calculation. It has the
ability of stabilizing DC-link voltage during shunt
UPQC is able to compensate current's har- inverter operation to compensate voltage distortions.
monics, to compensate reactive power, voltage dis- The stabilization is maintained by DC-link capacitor
tortions and can compensate voltage interruption be- voltage controlling loop in which P.I. controller is
cause of having PV-array as a source. Common inter- applied.
connected PV systems structure is as fig. 1 which is Shunt inverter control consists of the control
composed of PV array, DC/DC and DC/AC convert- circuit as shown in figure below:
ers.
In this paper a new structure is proposed for
UPQC, where PV is connected to DC link in UPQC
as energy source.
In this case, UPQC finds the ability of in-
jecting power using PV to sensitive load during
source voltage interruption. Fig. 1 shows the configu-
ration of proposed system. In this proposed system,
two Operational modes are studied as follow:
A. Interconnected mode: where PV transfers power
to load and source.
B. Islanding mode: where the source voltage is in-
terrupted and PV provides a part of load power sepa- Fig.3: control block diagram of shunt inverter
rately.
SYSTEM DESIGN: A.1. Shunt inverter control in Interconnected
The controlling design of proposed system is mode:
composed of three following parts:
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Mode-1 of fig.3 shows UPQC shunt voltage
source inverter controlling block diagram to which

Synchronous reference frame theory then is applied Harmonic currents can be eliminated by low
by extracting the sensitive load currents as ILA, ILB, pass filter. In this case injected by
ILC. shunt inverter.
When unbalance condition occurs in each The compensation reference currents are
phase the stationary values are changed as +ve, − ve
& „0‟ sequences. So exact analysis is difficult to considered as:
achieve.
So the measured load currents applying syn- As it is shown in fig. 3, the reference cur-
chronous reference frame conversion method (dq0) rents will transfer to ABC frame by reverse convert-
or parks transformation are transferred to dq0 frame ing the Synchronous reference frame. Resulted refer-
using sinusoidal functions, i.e., frequency and phase ence currents will be compared with shunt inverter
angle of the converted load currents are determined. Output currents (Ifa, Ifb, Ifc) in a PWM current con-
troller (hysteresis type) and required controlling
This function is obtained by PLL by taking pulses are generated. Applying these signals to shunt
source side voltages VSA, VSB, VSC as reference, inverter power switches gate, required compensation
which provides to maintain the synchronism with current is generated by inverter.
supply voltage and current. NOTE: In machine analysis, for the development of
ABC-DQ0 transformation: electric torque Te, requires the interaction between
two axis currents which must be in quadrature.
Hence, in this transformation it is important to con-
sider this condition.

A.2. shunt inverter control in islanding mode:
In addition to previous duties, shunt inverter
Fig.3 (a): abc-dq phasor Fig.3 (b): abc-dq block control should inject active power of PV system to
This transformation can be thought of in control when PV is operating. If the voltage interrup-
geometric terms: As the projection of the three phase tion occurs at load, that exceeds a threshold value, the
separate sinusoidal phase quantities on two axes, inverter operation will switch from interconnected
called the direct (d) and quadrature (q) axis. mode to islanding mode. PV system provides re-
The direct & quadrature axis currents are: quired active power to stabilize load voltage.

(1)
General form of transformation is replacing
old variables by a new set of variables:
[New variables] = [transformation matrix] [old Vari-
ables]
Where,
(2)
(3)
(4)
Fig.4: shunt-inverter in islanding mode

In this case, shunt inverter controls output
(5)
voltage and current in order to inject to load using PI
controller as shown below:
2π 2π
cos θ cos(θ − ) cos(θ + )
3 3 Ia
2 2π 2π
= sin θ sin(θ − ) sin(θ + ) Ib
3 3 3
1 1 1 Ic
2 2 2
(6)
The d-axis and q-axis currents are con-
stant and instantaneous values considered as:
(7)

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Fig. 4(a): Controlling block diagram of shunt inverter distortions and helps the voltage of load voltage stay
in islanding mode (balanced sinusoidal 3-phase).

This reduces steady state error and maintains constant
power. In voltage control current is considered as
leading (capacitive load) and corresponding d-q cur-
rents are obtained as output as shown below:
(8)
(9)
Are inputs to current controller.
In this control considering current lagging (inductive
load), the corresponding DQ-voltages are obtained as
output as shown below: Fig.5 (a): control block diagram of series inverter

In order to reach this aim synchronous refer-
(10) ence frame theory is applied. In this method only d-
axis component is being controlled i.e., active power
(11)
component. By compensating the active power com-
ponent, reactive power component will be compen-
Double times the calculation is performed to
sated automatically.
maintain„s‟ constant(S = P+JQ), from this maximum
distortions are removed and compensating output
In this method the desired value of load
value is obtained. From the figure.4 these voltages
phase voltage is replaced in d and q-axis. The load
are again transformed to A-B-C sequence and com-
voltage should be kept sinusoidal with constant am-
pared with angular waveform in PWM control, and
plitude even if the voltage on system side is dis-
corresponding control pulses are generated. Applying
turbed.
these signals to shunt inverter switch, the correspond-
At interruption mode the system voltages
ing voltage is given to load.
are detected and then transformed to
B.Series inverter controlling: dq0 reference frame and this set of relations in be-
tween the 3- variables and dq variables is known
The duty of the series inverter is to compen- as park‟s transformation as shown below:
sate the voltage disturbance in the source side, grid Park’s:
which is due to the fault in the distribution line. Se-
ries inverter control calculates the voltage reference
values which are injected to grid by series inverter. In
order to control series inverter of UPQC, load sinu-
soidal voltage controlling strategy is proposed as
shown in figure below:
Fig.6: phasor of abc-dq

(11)

(12)
Fig.5: series inverter block diagram

(13)

This means d-axis of load reference voltage
In this condition UPQC, series inverter would be equals “VM”, while q-axis and zero axis of load refer-
controlled in a way that it compensates the whole ence voltage equals zero. Where Vm is desired peak

1799 | P a g e

value of load voltage and (θ) is voltage phase angle
which is calculated by phase locked loop (PLL). By

subtracting the desired value of d-axis phase voltage SOLAR-ARRAY:
(V*Ld) from VSd, all distortions VLd in d-axis are ob- The circuit of PV-Array is shown below:
tained. Also, the desired value of load phase voltage
in q-axis is zero. In other words, VSq represents total
q-axis distortions. So series compensation reference
voltage is resulted by relation (14):

(14)

The above equation of compensation refer-
ence voltage is then inversely transformed into a-b-c
reference frame as: V*Fabc
𝐚𝐛𝐜
𝐕 ∗ 𝐅𝐚𝐛𝐜 = [𝐓 𝐝𝐪𝟎 ]−𝟏 [𝐕 𝐝𝐪𝟎 ] (14.1)
𝟏 Fig.7: circuit of pv-array
𝐜𝐨𝐬 𝛉 𝐬𝐢𝐧 𝛉
𝐕𝐚 𝟑 𝐕𝐝 A solar cell basically is a p-n semi-
𝟐𝛑 𝟐𝛑 𝟏
𝐕𝐛 = 𝐜𝐨𝐬( 𝛉 −
𝟑
) 𝐬𝐢𝐧( 𝛉 −
𝟑
)
𝟑
× 𝐕𝐪 conductor junction which is made of several poly-
𝐕𝐜 𝟐𝛑 𝟐𝛑 𝟏 𝐕𝟎 crystalline silicon cells. At junction electrons from
𝐜𝐨𝐬( 𝛉 + ) 𝐬𝐢𝐧( 𝛉 + ) the sides mix and form a barrier, making it hard for
𝟑 𝟑 𝟑
(14.2) electrons on the N-side to cross to the P-side. Even-
These voltages are compared with an angu- tually equilibrium is reached, and an electric field
lar waveform in PWM controller and required con- separates the sides.
trolling pulses (g1... g6) are generated to be applied When photon (sun light) hits a solar cell, its
to series voltage source inverter switches. This cor- energy frees electron-hole pairs. The electric field
rected method is programmable with a low cost. The will send the free electron to the N-side and hole to
other advantage is that the controlling System‟s cal- the P-side. This cause further disruption of electrical
culation time is shortened and so controlling system's neutrality, and if an external current path is provided.
response is faster.
Note: electrons will flow through the path to their
Note: In order to improve series inverter operation, original side (p-side) to unite with holes that the elec-
SPWM method is used where the resulted value of tric field sent here, doing work for us along the way.
subtracting V*Fabc from VFabc is multiplied to a con- The electron flow provides the current, and
stant coefficient and the obtained value is added to cell electric field causes a voltage.
V*Fabc . Applying this method distinctively improves
operation of series inverter. Equivalent circuit of PV-array:

C. DC-DC Converter controlling to obtain the
Maximum Point Of PV-Array:
At, present Pv systems are not very efficient
with only about 12-20% efficiency in their ability to
convert sunlight to electrical power.
The efficiency can drop further due to other Fig.7 (a): equivalent pv circuit
factors such as: An equivalent circuit used together with the
(i) Solar panel temperature. following set of circuit equations to express a typical
(ii) Load conditions. current – voltage (I-V) characteristic of pv modules
and arrays as shown below:
In order to maintain the power derived from
the solar panel it is important to operate the panel at
its optimal power point. An important consideration
in achieving high efficiency in PV power system is to (15)
match “ PV source and load impedance properly for
any weather conditions, thus obtaining maximum
power generation”.
(16)
So in order to increase the efficiency, as
much power as possible should be extracted from the
array. The possible technique for this is known as (17)
“Maximum Power Point Tracking” (MPPT).
1800 | P a g e

The algorithm reads the value of current and
voltage from pv-array module. Power is calculated
from the measured voltage and current.

(18) The slope of the power can be calcu-
lated by consecutive output voltages and output cur-
rents and can be expressed as:
(19)

S → solar insolation (w/m2)
Tref → reference temperature (298 k)
T → cell temperature →(21)
Eg → band gap enargy of the cell semi conductor
K → boltzman constant
RS → series resistance
Rsh → shutn resistance
N →diode emission factor
IDO → diode reversal current
ISho → short circuit current at reference state
JO → temperature coefficient

From equation (18), Eg and IDO are not independent
of each other. Fig.5: The operating region on the curve

Thus, Eg is calculated using: So through this maximising the cyclic proc-
ess, correspondingly i/p and o/p pulses are generated
(20) by the control circuit and given to switch „s‟, this
m→ no cells in series switch provides the required voltage and also if there
The method to obtain optimum power point is any ripples they are eliminated and pure DC is
at solar array is algorithm of perturbation and obser- given to dc link.
vation (P & O) in Mppt.
Hence at every instant of PV-array we are
determining MPP and correspondingly capacitor-DC
PERTURBATION AND OBSERVATION:
link‟s voltage is charged.
This method is used to achieve the maxi-
mum power point. According to the structure of
The advantage of P&O method is its simple
MPPT, the required parameters are only voltage (v)
application and its disadvantage is its capability to
and current (I) of pv-modules.
determine the correct MPP during sudden and fast
variation in environmental conditions. Of course, the
probability of sudden variations in environmental
conditions is very low. As it is seen in fig. 8, the duty
cycle of converter switch is determined by measuring
voltage and array's current values in specific periods
of time and applying maximum power tracking algo-
rithm.

Finally, by applying this type of control with
PV-array generation the power quality of the System
is improved or will be made more reliable.
RESULTS AND DISCUSSION

In simulation part, power system is mod-
elled as a 3-wired 3-phase system by an RC load with
uncontrolled diode rectifier. The PV model applied in
Fig.7(b): MPPT algorithm flow chart simulation is as fig. 6 whose parameters are regulated
for normal condition (25°c temp & sun radiation)
Circuit parameters used in simulation are located in
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table 1. The maximum simulation time is regulated
on 600msec. Shunt inverter starts to operate at
100msec and series inverter starts at 200msec.

Table 1: Grid parameters
Source phase voltage (r m s) 220v/50Hz
Dc link voltage
Shunt inverter rating
Series inverter rating Fig. 10: Current of shunt inverter injected to grid
Shunt inverter inductance(Lf)
Shunt inverter capacitance(Cf)
Switching frequency 20KHz
Series inverter inductance(Ls)
Series inverter capacitance(Cs)
Series inverter Resistance(Rs) 12
Fig.11: source current
PV Array Rating

Nonlinear load's current is illustrated in fig. 8.

Fig. 12: source current and voltage

Fig. 11 shows the source current and fig. 12
presents the source voltage. At 0.45sec when PV out-
Fig. 8: Nonlinear load's current ages, source current returns to sinusoidal mode after
passing he transient state. With respect to fig. 12 it
Active and reactive powers consumed by can understood that, before PV outages, voltage has
load are shown in fig. 9. Simulated load consumes 180° phase difference with its current and PV injects
17kW active power and 8kVAr reactive power. In this current to source in addition to providing load. After
part of simulation, it is assumed that the PV array is PV outages, it is seen that, voltage and current are in
interconnected to grid and outages after 0.45s of op- same phase and UPQC compensates current harmon-
eration. The current, injected by shunt inverter is ics and power factor. Fig. 13 shows the total har-
shown in fig. 10. As it is shown in fig. 10 at the pres- monic distortions (THD) of source and load currents.
ence of PV, shunt branch injects a high current to Shunt inverter has been able to make current's wave
grid, a part of which is consumed to feed the load and form sinusoidal and reduce the THD of load current
else is injected to grid. When PV outages, the shunt from 22% to 3%.
branch undertakes the duty of compensating current
harmonics and current's reactive power.

Fig.13 : Load and source current THD

Fig. 9: Active and reactive power consumed by
Nonlinear load
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In fig. 14 active and reactive powers injected sag compensation increases and a part of active
by shunt inverter are shown. When PV outages, in- power injected by series inverter decreases.
jective reactive power doesn't vary distinctively,
while injective active power decreases to a negative
value from 37.5kW. In other words, shunt inverter is
not able to inject active power after PV outages and
required active power of series inverter is provided
through shunt inverter from the grid.

Fig. 16: voltage sag in source voltage

Fig. 14: Active and reactive power injected by
shunt inverter Fig. 17: load voltage and source current
DC link voltage is shown in fig.15 for the
above cases. When PV outages, the voltage of DC
link decreases and returns to its desired value by cor-
rect operation of DC controlling loop. In this part of
simulation, by applying voltage sag to the grid side,
series inverter‟s operation is studied to improve volt-
age sag and it is investigated that how series and
shunt inverters inject power to the grid at the pres-
ence of PV. Fig.16 source side voltage in which volt-
age sag has occurred during 0.25 to 0.35 seconds is
shown. Fig. 18: voltage injection by series inverter

Fig. 15: DC link voltage Fig. 19: Active power injected by series and shunt
inverters
In the third part of simulation, voltage interruption is
Fig. 17 shows the load side voltage and source cur- applied in the source and the operation of PV system
rent. Load voltage is completely compensated and in providing the power which is required by load is
current has 180° phase difference with load voltage. studied. In order to do this, a voltage interruption
This issue shows the complete compensation of load's occurs at 0.5 seconds and immediately the power
reactive power too. The voltage injected by series switches parallel with series transformer are closed
inverter to compensate the voltage sag is shown in and so other switches allocated between shunt and
fig. 18. The active power injected by series and shunt series inverters are opened and so isolate shunt in-
inverters during voltage sag occurrence is shown in verter from the grid. Controlling algorithm of parallel
fig. 19. When voltage sag occurred, share of series inverter, switches from interconnected mode to
inverter in PV's active power transferring and voltage islanding mode. In fig. 20 the source side voltage is
shown during voltage interruption occurrence. Fig.
21 shows load current. When interruption occurs,
1803 | P a g e

load current is provided by shunt inverter and PV. method is used to achieve the maximum power point
The active power injected by shunt inverter to load is of PV array. This proposed system's operation is ana-
shown in fig. 22. As it is seen, during interruption lyzed using (MATLAB) software and simulation
occurrence, the power injected by PV is the function results confirm that the proposed system operates
of the load's required power in PV power range off correctly.
course.
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