chapter 3 design of a pv-upqc system for voltage...
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CHAPTER 3
DESIGN OF A PV-UPQC SYSTEM FOR VOLTAGE SAG AND
SWELL COMPENSATION
INTRODUCTION
The recent increase in the use of non-linear loads creates many
power quality problems such as voltage sag, swell and current
disturbances. Voltage sag is one of the major power quality problems,
which may cause equipment tripping, malfunction or shut down of
domestic and industrial equipments. These effects could be expensive for
both customers and utilities as discussed by Eldany et al (2001). Hirofumi
et al (1998) has proposed a UPQC system to mitigate the sag. Wong et al
(2004) uses a parallel active filter to compensate the imbalance, reactive
power, neutral current and harmonics of the source.
Photovoltaic energy has great potential to provide power supply
with minimum impact on the environment, since it is clean and pollution
free as discussed by Kuo Liang et al (2001). When the power supply crisis
started in the recent past, PV generation became a very popular alternative
for fossil fuel electrical power generation. A large number of solar cells
connected in series and parallel constitute solar arrays. One way of using
photovoltaic energy is, in a distributed energy system, as a peaking power
source.
There are many control strategies reported in the literature to
determine the reference values of the voltage and the current of UPQC. The
Fuzzy Logic Control (FLC) for the control of UPQC method has been
developed by Singh et al (1998). Bulent Irmak et al (2009) presented
Power Quality Distributed-Voltage (PQD-V) using ANN controller. The
drawback of the existing research work is that they address only current
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and voltage harmonics problems. But, there is very meagre discussion of
other prominent power quality problems like deep sag, swell and
interruption.
This chapter presents the design and development of the proposed
PV-UPQC system. By using instantaneous p-q control theory techniques
long with PI and hysteresis band controller, the mitigation of voltage sag
and swell under different balance and unbalanced load conditions are
simulated. The use of a PV array for maintaining constant DC link voltage
is another distinguishing feature of the PV-UPQC system. With these
functions, the PV-UPQC is suitable for connecting at the PCC.
3.1 PV-UPQC POWER CIRCUIT AND ITS FUNCTIONS
The PV-UPQC power circuit structure is shown in figure 3.1. It
consists of major components such as shunt and series inverters, DC link
capacitor with PV array, the shunt and series inverter controllers, shunt and
series filters and series and shunt coupling transformers.
The basic function of the system is the compensation of voltage and
current disturbances. The function of the series inverter is to compensate
the voltage, when there are supply side disturbances, such as voltage sag,
swell and unbalance. The series part of the PV-UPQC consists of a series
inverter connected on the DC side to the energy storage capacitor with PV
array. On the AC side, it is connected in series with the feeder through the
series Low Pass Filter (LPF) and coupling transformers. The series LPF
prevents the switching frequency harmonics produced by the series VSI
entering the system. The coupling transformers connected in series provide
voltage matching and isolation between the network and the series inverter.
The series inverter injects compensation voltages in series with the
supply voltages, such that the load voltages are balanced and undistorted
and their magnitudes are maintained at the desired level.
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Figure 3.1 Proposed power circuit diagram of PV-UPQC system
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Based on the measured supply and load voltages, the control scheme
of the controller generates appropriate switching signals for the series
inverter switches. The series inverter is controlled in voltage-control mode,
using the PWM switching technique. In order to produce the injected
voltage of desired magnitude, phase shift and frequency, the desired signal
is compared with a triangular signal of higher frequency and appropriate
switching signals are generated. The DC link capacitor is alternately
connected to the inverter outputs with positive and negative polarity. The
switching harmonics present in the output voltages of the series inverter are
filtered out by the series low pass filter. The amplitude, phase shift,
frequency and harmonic contents of injected voltages are controllable.
The function of shunt inverter is to compensate load current
harmonics, reactive power and regulate the DC-link voltage between both
inverters. The shunt part of the PV-UPQC consists of a shunt inverter
connected to the common DC storage capacitor with PV array on the DC
side. On the AC side, it is connected in parallel with the load through the
shunt interface inductor and shunt coupling transformer. The shunt
interface inductors along with the shunt filter capacitor are used to filter out
the switching frequency harmonics produced by the shunt inverter. The
shunt coupling transformer is used for matching the shunt inverter voltage
and network voltage.
In order to achieve compensation objective, the shunt inverter filter
injects currents at the PCC so that the reactive and harmonic components
of the load currents are cancelled and the load current unbalance is
eliminated. The injection current is provided by the DC link capacitor.
Based on the measured current and voltage, the control scheme generates
the appropriate switching signals for the shunt inverter switches.
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The particular currents and voltages to be measured depend on the
applied control strategy and the shunt inverter is controlled in current
control mode. The appropriate inverter switches are turned on and off at
certain time instances so that the currents injected by the shunt filter tracks
some reference currents within a fixed hysteresis band according to the
compensation objectives. This band refers to the constant bandwidth of the
upper and lower hysteresis current.
The inverter switches alternately connect the DC link capacitor to
the system, either in the positive or negative sequence. When the DC
capacitor voltage is connected in the positive sequence, it is added to the
supply voltage and the inverter current increases. When DC capacitor is
connected in the negative sequence, it is observed that the voltage is
opposite to the supply voltage which reduces the inverter current.
Alternately increasing and decreasing the current within the hysteresis band
results in generating the reference current. The DC side capacitor serves
two main purposes. Primarily, it maintains the DC voltage with a small
ripple in the steady state. Secondly, it serves as an energy storage element
to supply the difference between the active load and source power during
the transient period. The average voltage across the DC link capacitor is
maintained constant so that the shunt inverter filter can draw a leading
current. This voltage has to be higher than the peak of the supply voltage.
The proposed system can switch over to islanding mode operation during
interruption and supply the power to the load.
3.2 DESIGN OF PV-UPQC POWER CIRCUIT
The design of PV-UPQC power circuit includes the following three
main parameters:
Shunt interface inductors
DC link reference voltage
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DC link capacitor
The design of the shunt interface inductor and the DC link reference
voltage are based on the following criteria. The first criterion is the limiting
of the high frequency components of the injected currents. The second
criterion is that the instantaneous rate of change of current generated by the
shunt filter should be greater than the rate of change of current of the
harmonic component of the load. This ensures the proper harmonic
cancellation. On the other hand, a higher value inductance is preferable for
a better harmonic cancellation and reactive power compensation. However,
very high value will result in slow dynamic response of the shunt
compensator. In this situation, an effective solution has to be explored.
A higher DC link reference voltage results in a higher rate of change
of the shunt compensator current, better dynamic response and reactive
power compensation performance. However, it increases the stress in the
inverter switching devices. Again, an efficient solution has to be adopted.
The DC link capacitor size is selected to restrict the DC voltage ripple
within reasonable limits. The DC voltage ripple is determined by both the
reactive power to be compensated and the active power supplied by DC
capacitor during the interruption.
3.2.1 Design of PV-UPQC Volt Ampere rating of Shunt and Series
Inverters
Volt Ampere (VA) rating of series and shunt inverters of PV-UPQC
determines the size of the PV-UPQC. The power loss is related to the VA
loading of the PV-UPQC system. Figure 3.1(a) shows the loading
calculation of shunt and series inverters of PV-UPQC with the presence of
PV at its DC link. The linear load is deigned based on fundamental
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frequency.
Figure 3.1(a) VA rating phasor diagram of PV-UPQC
where,
Vs1 and Is1 = supply voltage and supply current
Vs2 and Is2= supply voltage and supply current during interruption
VL1and IL1 = load voltage and load current
VL2 and IL2 = load voltage and load current during interruption
IC1and IC2 = compensating current of reactive components of inverters
Io - output current, ZSHI –shunt inverter impedance
The load voltage is to be kept constant at Vo Per unit (pu), irrespective of
the supply voltage variation:
V =V =V =V =V pu (3.1)
The load current is assumed to be constant at the rated value:
I =I =I =I pu (3. 2)
Assuming that the PV-UPQC system to be lossless, the active power
demand in the load remains constant and is drawn from the source:
From phasor diagram 3.1(a)
V I =V I cos (3. 3)
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where
V I = Input power of inverter (Dc link power)
V I cos = output power of inverter (AC)
In case of an interruption:
V = (1-x)Vs1
= V (1-x) pu (3.4)
where, ‘x’ is per-unit sag.
To maintain constant active power under the voltage sag condition
I =( )
= pu (3.5)
Therefore series inverter VA (Sseinv) rating equals to
x-1)cos(xIV
IVS oos2injseinv pu (3.6)
Injected voltage through shunt inverter is V = V V
= V V (1 x) (3.7)
Injected current through shunt inverter is
I = I + I 2I I cos (3.8)
The injected current in terms of I is given by
I = I + ( ) 2I( )
= I +( )
2I( )
= I 1 +( )
2( )
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= I ( ) ( )( )
= I ( ) { ( )}( )
(3.9)
The load voltage VL2 is given by V = V + V
= V (1-x) + V V (1 x)
= V (1-x) + V (V V x)
= V (1-x) + V (V 2V x + V x )
= V (1-x) + V 1 (1 x)
= V [(1-x) + 1 (1 x) ] (3.10)
Shunt inverter VA (S ) rating equals to
S = V I + PLOSS
= V I + I
Subsisting the values of VL2 and IC2 above equation and simplify
S = V [(1-x)+ 1 (1 x) ]× I0[ 1 x 2+cos2 1 2 1 x1 ]
+ I
=( )
{[(1-x)+ 1 – (1 x) ]
× [ (1 x) + cos {1 2(1 x)}] } +(I × )
Simplify the above equation
= ( )
{ 1 ×[(1-x)+ cos {1 2(1 x)]}+ (I × )
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S =( )
(1 x) + cos {1 2(1 x)}
+ ZXx)-(1
)x1(21(cosx)1(2I SHI2
222o pu (3.11)
The proposed system highlights the efficient and imperative design
of power factor and VA loading of inverter. The VA loading of inverters is
calculated for the occurrence of supply voltage sag from 10 to 90% and
power factor variations from 0.6 lagging to unity. The range of supply
voltage sag has been chosen so that most practical cases can be evaluated
in this range.
3.3 CONTROL STRATEGY OF THE PV-UPQC SYSTEM
There are many control strategies reported in the literature to
determine the reference values of the voltage and the current of UPQC. The
concept of instantaneous active power (p) and reactive power (q) and its
application in shunt filter reference current generation was introduced by
Akagi et al (1998).The p-q theory introduced by Tan Zhili et al (2006), has
been modified into a single-phase p-q theory by Khadkikar et al (2009).
The synchronous reference frame theory has been discussed by
Guorong et al (2007), symmetrical component transformation has been
presented by Ghosh et al (2004) and Unit Vector Template (UVT)
technique by Singh et al (2010). The Fuzzy Logic Control (FLC) for the
control of UPQC method has been developed by Singh et al (1998). Based
on the above discussion, p-q theory with hysteresis current control mode is
suitable for parallel mode operation of PV-UPQC system and p-q theory
with PWM voltage control mode is suitable for interruption mode
operation. The hysteresis control method is simple to implement and it has
enhanced system stability, increased reliability and mitigates power quality
problems.
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The proposed PV-UPQC consists of two main controllers as
follows:
sag and swell compensation using series inverter controller
Shunt inverter controller
Controller strategies are designed based on two operation modes,
interconnected (parallel mode) and islanding modes. In the interconnected
mode, the source and the PV array jointly supply power to the load. While
in the islanding mode, the PV arrays alone supply power to the load.
3.4 SAG AND SWELL COMPENSATION USING SERIES
INVERTER CONTROLLER
The series inverter control component of the controller injects the
appropriate voltage to the load during voltage sag and swell, such that the
load voltage becomes balanced, distortion free and have the desired
magnitude. Theoretically, the injected voltage can be of any arbitrary
magnitude and phase. However, the power flow and device rating are
important issues that have to be considered when determining the
magnitude and the phase of the injected voltage.
The PV-UPQC system ensures that the phase of the injected voltage
is maintained 90° in advance with respect to the supply current, so that the
series compensator consumes no active power in steady state. In the second
case, the PV-UPQC injected voltage is in phase with both the supply
voltage and current, so that the series compensator consumes only the
active power, which is delivered by the shunt compensator through the DC
link.
In the case of quadrature voltage injection, the series compensator
requires additional power capacity, since the shunt compensator Volt
Ampere (VA) rating is reduced as the active power consumption of the
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series compensator is minimized. It compensates part of the reactive power
demand by the load. The series compensator does not compensate for any
part of the reactive power demand of the load and it has to be entirely
compensated by the shunt compensator. The shunt compensator must
provide the active power injected by the series compensator.
It can be concluded that PV-UPQC system is the optimum solution
for the active power supply, phase angle matching and supply of reactive
power. An approximate sub-optimal control strategy for the PV-UPQC
system to minimize the losses in operation has been proposed. This
approach of generating the reference voltage is based on the analysis of the
fundamental frequency. The load voltage (VL) is equal to the sum of the
source voltages ( SV ) and the voltage injected by the series filter (Vf).
jbaVVV fSL (3.12)
where,
a + j b - Vector of unity magnitude of the supply current
To compensate for supply voltage sag, swell and distortion, some
additional component has to be added. The reference voltage (Vref) value is
calculated using reference filter voltage (Vf*), which is obtained by
subtracting the positive sequence fundamental component from the
disturbed source voltage.
The function of the series inverter is to compensate the voltage
disturbances like sag and swell on the source side, which are due to the
fault in the distribution line. The series inverter control calculates the
reference voltage to be injected by the series inverter. This compares the
positive-sequence component with the disturbed source voltage. The
reference voltage for PWM switching of the series inverter is obtained
from the proportional control and the feed forward control. The control
equation needed to calculate the reference voltage is given in (3.13). Figure
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3.2 shows the configuration of the series inverter control, which is based on
this equation.
Figure 3.2 Block diagram of the series inverter control
Vref = [(Vs* -Vs) –Vf] x k + Vf* (3.13)
where
Vref - Reference voltage, Vs - Source voltage, Vf - Injected voltage
Vf* - Reference injected voltage Vs* - Positive-sequence voltage
In this case, when the PV- UPQC control strategy is applied, the
injected voltage is in phase with the supply voltage. Hence, the load
voltage is in phase with the supply voltage and there is no need for
calculating the angle of the reference load voltage. Thus, the reference load
voltage is determined by multiplying the reference magnitude (which is
constant) with the sinusoidal template phase-locked to the supply voltage.
Then, the reference series filter voltage is obtained using the expression
(3.13). Using the optimum of the different techniques for calculating the
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reference voltage of the series compensator, the voltage rating of the series
compensator is considerably reduced.
3.5 SHUNT INVERTER CONTROLLER
The effectiveness of the shunt inverter basically depends on the design
characteristics of the current controller. The method implemented to
generate the reference current and reference voltages is the use of
instantaneous p-q control theory techniques. The control scheme of a shunt
inverter calculate the current reference waveform for each phase of the
inverter, maintain the DC link voltage constant and generate the inverter
gating signals. The current reference circuit generates the reference
currents required to compensate the load current harmonics and reactive
power.
The p-q control theory, firstly, transformation of the voltage and
currents from the abc to 0 coordinates. The Clarke transformation and its
inverse of three-phase generic voltages are given by
vv
0v=
2/32/12/1
2/302/112/12/1
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cvbvav
(3.14)
cvbvav
=2/3
2/30
2/12/12/12/1
12/1
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vv
0v
(3.15)
Similarly, three-phase generic instantaneous line currents, ia, ib and ic can
be transformed to the 0 axes.
One advantage of applying the 0 transformation is to separate
zero-sequence components from the abc-phase components. The and -
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axes do not contribute to zero sequence components. No zero-sequence
current exists in a three-phase three-wire system, so that zero sequence
current (i0) can be eliminated from the above equations resulting in
simplification. If the three-phase voltages are balanced in a three-phase
three-wire system, no zero-sequence voltage is present, so that zero
sequence voltage (vo) can be eliminated. However, when zero-sequence
voltage and current components are present, the complete transformation
has to be considered.
The p-q theory defines three instantaneous powers in three phase
systems, with or without neutral conductor
qpp0
=v
v0
v0v000V
iii0
(3.16)
where,
po - instantaneous zero-sequence power,
p - instantaneous real power,
q - instantaneous imaginary power
There are no zero-sequence current components in three phase three-
wire systems, that is, io= 0. In this case, only the instantaneous powers
defined on the -axes exist, because the product voio is zero. Hence, in
three-phase three wire systems, the instantaneous real power p represents
the total energy flow per unit time, in terms of components. The
instantaneous active and reactive powers for a three-phase three-wire
system are defined as:
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ii
vv-
vv
qp
(3.17)
where,
p –Active power, q -Reactive power
v and v -Transformation of voltages
The three-phase voltages are transformed from a-b-c to frame
and vice versa using the following transformation relations:
cvbvav
2/32/302/12/11
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vv
(3.18)
cvbvav
=2/32/1
2/32/101
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vv
(3.19)
The same transformation matrices are used for the transformation of
currents, from equation (3.19), the current i and i are expressed as:
ii
=2v2v
1vv-
vv
qp
(3.20)
where, i and i -transformation of currents
both active power( p) and reactive power (q) defined above are composed
of two components.
qq~qpp~p
(3.21)
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where, and represent the oscillating and average parts active
power and represent the oscillating and average parts reactive power
The p-q theory has a prominent merit of allowing complete analysis
and real time calculation of various powers and respective currents
involved in a three-phase circuit. Further, knowing the values of
undesirable currents in a circuit in real time allows us to eliminate them.
For instance, if the oscillating powers were undesirable, by compensating
the currents ia and ia of the load and their corresponding currents in b
and c phases the compensated current drawn from the network would
become sinusoidal. It can be easily shown that, ia- (ia +ia ) produces a
purely sinusoidal waveform. This is one of the basic ideas of shunt inverter
filtering.
The shunt inverter of PV-UPQC can be controlled in two ways:
The shunt inverter reference current is tracked. The shunt inverter
current is used as feedback control variable and compared with the
load current and the shunt compensator reference current is
calculated from it. The reference current is determined by
calculating the active fundamental component of the load current
and subtracting it from the load current. This control technique
involves both the shunt inverter and load current measurements.
The shunt inverter can also be controlled by tracking the supply
current, when it is used as the feedback variable. In this case, the
shunt active filter ensures that the supply reference current is
tracked. Thus, the supply reference current is calculated rather than
the current injected by the shunt active filter. The supply current is
often required to be sinusoidal and in phase with the supply voltage.
Since the waveform and phase of the supply current is only known,
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its amplitude needs to be determined. The hysteresis current control
technique involves only the supply current measurement. Therefore,
it has been used in the PV-UPQC simulation model.
Figure 3.3 shows the configuration of shunt inverter control, which
includes the current control for harmonic compensation and the output
voltage control in source voltage interruption mode. In normal operation,
the shunt control calculates the reference value of the compensating current
from the harmonic current and the reactive power, considering the power
loss (Ploss) due to the system and inverter operation. This loss should be
compensated to maintain the DC link voltage during operation of the series
inverter. The reference value of the compensating current is derived using
equation (3.22).
ii
*
*
=2v2v
1vv
v-v
qpp~ loos (3.22)
where,
V’ ,V’ - Transformed reference voltages
i* , i* - compensating currents
Here, the reference voltage is determined using (3.23) and (3.24).
The reference voltage (V*1) is expressed by the sum of the source voltage
(V s) and the filter current difference IPF calculated by the PI controller.
PFPFPF I*II (3.23)
pk*1V dtIIkI PFPF (3.24)
If the shunt inverter is assumed to generate the reference voltage for
each period of power frequency, the transfer function of the filter current
can be derived as in (3.25).
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)/L(ks}L/R)k{(s)/L(ks)L/k(
II
PFIPFP2
PFIPFP*PF
PF (3.25)
where, IPF, I*PF - filter currents
kp, kI- proportional constant
The voltage reference determined by using positive-sequence
detector extracts the positive-sequence component from the disturbed
three-phase source voltage in figure3.4. This detector derives the
transformation of reference voltages V' , and V'S , based on the - -0
transformation. The measured source voltage passes through the Phase-
Locked Loop (PLL) and the sine wave generator to calculate the
fundamental component of the transformation currents i' = sin( 1t) and i'
= cos ( 1t).
The calculated active power ps' and reactive power qs' includes the
positive-sequence fundamental component of the source voltage VS. So,
the instantaneous value of the positive-sequence component is calculated as
given in equation (3.26).
'sq'sp
'i-'i
'i'i
'2i'2i
1'V
'V(3.26)
The two functions of the shunt inverter are to compensate the current
harmonics and to supply the active power to the load during voltage
interruption.
When the voltage interruption occurs, the operation mode is
changed from normal compensation mode to interruption mode. The PV
array provides the active power to maintain the load voltage constant. The
shunt inverter starts to perform the voltage and current control using the PI
controller.
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Figure 3.3 Block diagram of shunt inverter control
Figure 3.4 Block diagram of positive sequence voltage detector
3.6 AVERAGE DC LINK VOLTAGE REGULATION
The regulation of DC link voltage (Vdc) is one of the tasks in PV-
UPQC, since the injecting voltage (Vinj) depends on the regulated voltage
of DC link capacitor. The proposed PI controller regulates the DC link
voltage and reduces the harmonics in the inverter. The PI approach is used
to supply reference current determination and it is based on the fact that the
magnitude of the supply current depends on power balance between the
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supply and the load. The DC link capacitor serves as energy storage
element. If the shunt active filter losses are neglected, in steady-state, the
power supplied by the system has to be equal to the real power demand of
the load and no real power flows into the DC link capacitor. The average
DC capacitor voltage is thus maintained at reference voltage level. If the
power unbalance caused by a load change occurs, the DC link capacitor
must supply the power difference between the supply and load that will
result in reducing the DC capacitor voltage.
To restore the average DC capacitor voltage to the reference level,
some active power has to be supplied to the DC capacitor. For this propose,
the supply current has to be increased. When the average DC capacitor
voltage increases, the magnitude of the supply current has to be decreased.
The amplitude of the supply current is automatically controlled by
controlling the average voltage across the DC link capacitor. The DC
voltage regulation is achieved by using a PI controller. The capacitor
voltage is compared with some reference value and a PI controller
processes the voltage error. The output of the PI controller is the magnitude
of the reference supply current and it is constant in steady-state. To get the
source reference current, a sinusoidal template that is in phase with the
supply voltage is multiplied by this magnitude. Applying this concept, the
numbers of current sensors are reduced resulting in the simplification of
the control circuit. Therefore, this control technique has been chosen to be
used in the PV-UPQC simulation model.
3.6.1 DC Link Voltage Regulation Using Fuzzy Logic Control
The voltage regulation in the UPQC DC link using fuzzy logic
control has been presented by Singh et al (1998). The structure of a
complete fuzzy control system is composed from the blocks, namely,
fuzzification, knowledge base, inference engine and defuzzification. The
fuzzification module converts the crisp values of the control inputs into
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fuzzy values. Inputs to the fuzzy controller are categorized as various
linguistic variables with their corresponding membership values such as
low, medium and high, where each is defined by a gradually varying
membership function. In fuzzy set terminology, all the possible values that
a variable can assume are named universe of discourse and the fuzzy sets
cover whole universe of discourse. The shape of fuzzy sets can be
triangular and trapezoidal. A fuzzy controller converts a linguistic control
strategy into an automatic control strategy and fuzzy rules are constructed
by knowledge database. Initially, measured DC link voltage Vdc and the
input reference voltage Vdc-ref are the input variables of the fuzzy logic
controller. Then, the output variable of the fuzzy logic controller is
presented by the control current.
The control scheme consists of three phase sine wave generator for
reference current generation and generation of switching signals. The peak
value of reference currents are estimated by regulating the DC link voltage.
The actual capacitor voltage is compared with a set reference value. The
error signal is then processed through a fuzzy controller, which contributes
to zero steady error in tracking the reference current signal.
3.7 VOLTAGE CONTROL IN DC LINK CAPACITOR
In voltage source inverter, the DC voltage has to be maintained at a
certain level to ensure the DC to AC power transfer. Because of the
switching and other power losses inside PV-UPQC system, the voltage
level of the DC capacitor will be reduced if it is system is interrupted.
Thus, the DC link voltage control unit is intended to keep the average DC
bus voltage constant and equal to a given reference value. The DC link
voltage control is achieved by adjusting the real power supply by the PV
array. This real power is adjusted by changing the amplitude of the
fundamental component of the reference current. The PV array source
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provides some active current to recharge the DC capacitor. Thus, in
addition to supplying the reference current, the shunt active filter has to
supply some amount of active current as compensating current. This active
compensating current flowing through the shunt active filter regulates the
DC capacitor voltage.
3.7.1 Voltage Control in DC Link Capacitor using ANN Controller
The DC link capacitor voltage control by using ANN controller in
Power Quality Distributed-Voltage (PQD-V) is discussed by Bulent Irmak
et al (2009). The detection of the disturbance signal with the reference
signal of the controller is the prime requirements for the desired
compensation in case of PQD-V. The ANN is made up of interconnecting
artificial neurons. It is essentially a cluster of suitably interconnected
nonlinear elements of very simple form that possess the ability to learn and
adapt. It resembles the human brain in two aspects:
Knowledge is acquired by the network through the learning process
Interneuron connection strengths are used to store the knowledge.
These networks are characterized by their topology, the way in which they
communicate with their environment, the manner in which they are trained
and their ability to process information. ANN is being used to solve AI
problems without necessarily creating a model of a real dynamic system.
For improving the performance of a PQD-V, a multilayer feed forward-
type ANN-based controller is designed. This network is designed with
three layers, the input layer with 2, the hidden layer with 21 and the output
layer with 1 neuron, respectively. The large data of the DC-link current
and intervals from the conventional method are collected. These data are
used for training the NN. The activation functions chosen are tan sigmoid
for input and hidden layers and pure linear in the output layer,
respectively. This multilayer feed forward-type NN works as a
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compensation signal generator. The ANN is shown in figure 3.5, the
training algorithm used is Levenberg–Marquardt back propagation
(LMBP).
Figure 3.5 Exploded diagram of the artificial neural network.
In order to sustain the constant frequency in the utility, Singh et al
(1998) have utilized the Fuzzy Logic Controller based constant frequency
UPFC. A Constant Frequency (CF) UPQC is composed of a UPQC and a
matrix converter based frequency changer. UPQC is a combination of
series and shunt active filters. The series and shunt active filters have been
employed to compensate the voltage, current imbalance and harmonics.
The Frequency Converter (matrix converter) has been used to control the
supply frequency when it exceeds the power quality limit.
To overcome above limitation, a PI controller is used for
determining the magnitude of this compensating current from the error
between the average voltage across the DC capacitor and the reference
voltage. The PI controller has a simple structure and fast response. A
simple linear control technique is applied to calculate the DC capacitor
average voltage error by using proportional gain control and the
proportional coefficient. Here, the expression used for calculation of the
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proportional coefficient is obtained through integration of a first-order
differential equation. However, the formula derivation for the proportional
coefficient is not that simple for a three-phase PV-UPQC, a residual
steady-state error occurs with a proportional controller only.
3.8 DEVELOPED PV-UPQC SYSTEM SIMULATION MODELS
This section discusses the proposed system model, which includes
the following:
• Series inverter and shunt inverter models
• Series inverter and shunt inverter control system models
• PV array with control model
Figure 3.6 shows the overall PV-UPQC models implemented using
Matlab /Simulink software. They consist of a series inverter and a shunt
inverter models and its control models and the PV array with control
model. The system is connected in distribution system.
In PV-UPQC model, three phase inverters are connected to the
common DC link capacitors with photovoltaic array. This voltage source is
an external source supplying DC voltage to the inverter for AC voltage
generation. The PV-UPQC is installed in the distribution system, as shown
in figure 3.6. The design considered for the development of PV-UPQC
system Simulink models include circuit topology, conversion efficiency,
maximum load power and power quality.
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47
3.9 SIMULATION PARAMETERS
The following circuit parameters used in the simulation is given in
Table 3.1
Table 3.1 PV-UPQC simulation parameters
Source Voltage 230Vrms(325Vpeak) 50Hz
Impedance R=0.04 , L=0.4mH
DC-Link Capacitor C1=2000µf
Reference Voltage 400V
Shunt Inverter Filter L, C 1.245 mH,20µF
Switching Frequency. 10kHz
Series Inverter Switching Frequency. 10kHz
Filter L, C 1.245 mH, 140µF
PV Array Power 10kW
3.10 RESULTS AND DISCUSSION
The performance of PV-UPQC is evaluated in terms of load
balancing, unbalancing and mitigation of voltage sag and swell under
different load conditions. The simulation results of the Simulink model
(from figure 3.6) are represented in figures 3.7 to 3.11, which indicate the
performance of the proposed system for seven different cases.
In case 1, the system is in normal operation, three-phase voltages
and currents are sinusoidal and balanced. In case 2, three-phase balanced
voltage sag results in 30% decrease from the nominal value.
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While in case 3, single-phase sag results in 30% decrease from
nominal value. Sag event occurs in phase A. As the other phases are not
affected, both phases B and C keep their nominal value.
In case 4, the three-phase unbalanced sag in phase A is 30% and it is
20% in phase C resulting in the decrease from their nominal values. As the
phase B is left in normal operation, nominal value of the phase B remains
unchanged. In case 5, three-phase balanced voltage swell results in 15%
increase from nominal value. In case 6, single-phase swell occurs resulting
in 15 % increase from nominal value. While swell event occurs in phase A,
other phases are not affected and both phase B and C keep their nominal
value. In case 7, with the unbalanced swell mode, phase A voltage
increases by 15% and phase C voltage increases by 10% from their
nominal values. As the phase B is left in normal operation, nominal value
of the phase B remains change.
Case 1: Normal Operation Balanced Supply with Linear Loads
In case 1, normal operation mode is simulated. Under normal
conditions, three-phase voltages are sinusoidal under balanced condition
and the simulation result is illustrated. The peak load voltage measured is
325V and it is depicted in figure 3.7(a) and the load current is shown in
figure 3.7(b).
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Figure 3.7 (a) Load voltage during normal operation
Figure 3.7 (b) Load current during normal operation
Case 2: Balanced Voltage Sag
In case 2, the simulation is conducted considering balanced voltage
sag with and without PV-UPQC. The occurrence of a three-phase fault
results in 30% of sag and the voltage decreases from its nominal value
between the period 0.1s and 0.2s in all phases. The simulation is illustrated
through figures 3.8(a) to 3.8 (d). The output load voltage is kept constant
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(325V) by using PV-UPQC. Figure 3.8 (e) load voltage sag compensation
with PQD-V using ANN controller.
Figure 3.8 (a) Source voltage with sag
Figure 3.8 (b) Load voltage sag compensation with PV-UPQC using PI
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Figure 3.8 (c) source current with sag
Figure 3.8 (d) Load current with PV-UPQC
Figure 3.8 (e) Load voltage sag compensation with PQD-V using ANN
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Figure 3.8 (f) Load voltage sag compensation with UPQC using FLC
Case 3: Single Phase Voltage Sag
In case 3, the single-phase voltage sag is evaluated with and without
PV-UPQC. The occurrence of a single-phase fault results in 30% of sag
and the voltage decreases from its nominal value. Sag event occurs only in
phase A. Consequently, as other phases are not affected, both phase B and
C keep their nominal value, as show in figures 3.9 (a) to 3.9 (d). The output
load voltage is kept constant (325V) with PV-UPQC.
Figure 3.9 (a) Single-phase source voltage with sag
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Figure 3.9 (b) load voltage with PV-UPQC
Figure 3.9 (c) Single-phase source current with sag
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Figure 3.9 (d) Load current with PV-UPQC
Case 4: Unbalanced Voltage Sag
In case 4, the simulation is conducted considering three phases
unbalanced sag with and without PV-UPQC. In this case, the three phase
unbalanced sag in phase A is 30% and it is 20% in phase C resulting in the
decrease from their nominal values between the period 0.1s and 0.2s as
shown in figures 3.10 (a) to 3.10 (d).
Figure 3.10 (a) unbalanced sag source voltage
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Figure 3.10 (b) Load voltages with PV-UPQC
Figure 3.10 (c) Source current with unbalanced Sag
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Figure 3.10 (d) Load current with PV-UPQC
Case 5: Balanced Voltage Swells
In case 5, the simulation is conducted considering balanced voltage
swell with and without PV-UPQC. The occurrence of a three-phase fault
results in 15% of swell and the voltage increases from its nominal value
between the period 0.1s and 0.2s in all the phases. The simulation is
illustrated through figures 3.11 (a) to 3.11 (d). The output load voltage is
kept constant.
Figure 3.11 (a) Source voltage with swell
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Figure 3.11 (b) Load voltages with PV-UPQC
Figure 3.11 (c) Source current with swell
Figure 3.11 (d) Load current with PV-UPQC
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Case 6: Single Phase Voltage Swells
In case 6, the single-phase voltage swell is evaluated with and
without PV-UPQC. The occurrence of a single-phase fault results in 15%
of swell and the voltage increases from its nominal value. Swell event
occurs only in phase A. Consequently, as other phases are not affected,
both phase B and C keep their nominal value, as show in the figures
3.12(a) to 3.12(d). The output load voltage is kept constant.
Figure 3.12 (a) Single-phase source voltages with swell
Figure 3.12 (b) Load voltages with PV-UPQC
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Figure 3.12 (c) Single-phase source current with swell
Figure 3.12 (d) Load current with PV-UPQC
Case 7: Unbalanced voltages swell
In case 7, the simulation is conducted considering three phases
unbalanced swell with and without PV-UPQC. In this case, the three phase
unbalanced swell in phase A is 15% and it is 10% in phase C resulting in
the decrease from their nominal values between the period 0.1s and 0.2s as
shown in figures 3.13(a) to 3.13 (d). The output load voltage is kept
constant.
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Figure 3.13 (a) unbalanced source voltage with swell
Figure 3.13 (b) Load voltages with PV-UPQC
Figure 3.13 (c) Unbalanced source current with swell
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Figure 3.13 (d) Load current with PV-UPQC
Table 3.2 Comparison of percentage load voltage compensation for
Power Quality Distributed-Voltage (PQD-V) by Bulent Irmak et al
(2009) and proposed PV-UPQC.
Case
NoEvents
Percentage of load
voltage
compensation for
PQD-V with ANN
Percentage of load
voltage
compensation for
PV-UPQC with PI
1 30% sag 3 phase
with balanced
96.5 100
2 30% sag 3 phase
with unbalanced
96.5 100
3 15% swell 3 phase
with balanced
96.5 100
4 15% swell 3 phase
with un balanced
96.5 100
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For comparative analysis, 30% sag and 15% swell are considered
for both PQD-V with ANN system and PV-UPQC system. As shown in
table 3.2, the maximum percentage load voltage compensation achieved by
the PV-UPQC system is 3.5% higher than that of the PQD-V with ANN
system. It has been shown that the proposed system has better performance
in comparison with the PQD-V. The comparisons are also shown in figures
3.14 and 3.15.
Figure 3.14 Percentage load voltage compensation of PQD-V and PV-
UPQC with sag
Figure 3.15 Percentage of load voltage compensation of PQD-V and
PV-UPQC with swell
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Table 3.3 Comparison of source voltages and load voltages with and
without PV-UPQC
CaseNo
Events
With out PV-UPQC
source voltages [V rms](volts)
With PV-UPQC Load voltages [V rms](volts)
Phase A Phase B Phase C PhA
PhB
PhC
PhA
PhB
PhC
1 Normal Normal Normal 230 230 230 230 230 230 2 30% sag 30%sag 30% sag 161 161 161 230 230 230 3 30% sag Normal Normal 161 230 230 230 230 230 4 30% sag Normal 20% sag 161 230 181 230 230 230 5 15%swell 15%swell 15%swell 266 266 266 230 230 230 6 15%swell Normal Normal 266 230 230 230 230 230 7 15%swell Normal 10%swell 266 230 264 230 230 230
Table 3.3 shows voltage sag and swells with balanced and
unbalanced supply with and without PV-UPQC. The system restores the
load voltage to normal values. A comparison of the evaluation conducted
considering voltage sag and swells is shown with and without PV-UPQC.
In addition, figures 3.16 and 3.17 highlight the accomplished results of
enhancement of power quality by the proposed system.
Figure 3.16 Load voltages with and without PV-UPQC with sag
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Figure 3.17 Load voltages with and without PV-UPQC with swell
On the basis of the research work presented in this chapter, a paper
entitled ‘Improved Power Quality Using Photovoltaic Unified static
Compensation Techniques’ has been published in the Asian Power
Electronics Journal. Vol.4 No.2, pp. 59-63. Aug 2010.
On the basis of the research work presented in this chapter, a paper
entitled “Implementation of UPQC for Voltage Sag Mitigation”, -
International Journal of computer communication and information
system, Vol.2, No.1, pp.180-184, Dec 2010.
3.11 CONCLUSION
In this chapter, the power circuit structure, design considerations
and the controller design is discussed. It can be observed that the average
DC voltage regulation method turned out to be the most feasible control
solution that accomplishes two tasks simultaneously. It controls the voltage
across the dc link capacitor and also determines the amplitude of the supply
current. Among the all control techniques, the hysteresis current control
technique is the most preferable for the shunt inverter controller. The
hysteresis control method leads to enhanced system stability, compared to
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ANN and FLC controllers The PV-UPQC design helps in the development
of the simulation model and controllers. With this controller, the DC link
voltage will be maintained stiff by controlling PV array DC-DC converter.
The PV-UPQC system has been simulated in different sag and
swells conditions. The results obtained show that the proposed system has
the ability to compensate voltage sag and voltage swell. The effectiveness
of the simulation results has been verified under balanced and unbalanced
load conditions.
By comparing PQD-V system with respect to IEEE and IEC
standards the proposed PV-UPQC system gives much improved
performance. The simulation results show that the proposed system gives
more efficiency and better dynamic response. The main advantage of the
proposed system is that the control strategy enables it to produce balanced
voltage waveforms.