Combined Operation of a VSC Based Grid
Interfaced Solar Photovoltaic Power Generation
System with Night Time Application
Arun Kumar Verma, Bhim Singh, Fellow, IEEE, D.T Shahani, Ambrish Chandra, Fellow, IEEE and Kamal Al-Haddad, Fellow, IEEE
for power quality improvement. At full sun, the system under
consideration can supply the load and simultaneously solve
the problems of harmonics, unbalanced loads and reactive
power in a 3-phase, 4 wire distribution system. The grid
interfaced SPV power generating system interfaced with AC
distribution system through a DC-DC boost converter and a 4leg VSC, tracks maximum power from the SPV array. The
MPPT is incorporated with a DC–DC converter. VSC is used
to interface SPV array with utility grid under both on-grid and
off-grid operation modes [3,4]. The VSC injects sinusoidal
currents into the grid, and can control the power factor.
For extracting reference grid currents, many control
algorithms are reported in the literature for control of grid
interfaced SPV power generating system such as PBT (Power
Balance Theory) in which reference grid currents are
estimated from active and reactive powers, SRFT
(Synchronous Reference frame Theory) [3]. The proposed
MISCT (Modified Instantaneous Symmetrical Component
Theory) based control algorithm uses an indirect current
control to compensate the phase delay due to the ripple filter
resulting in improved performance of the system [5-8]. The
performance of the proposed system is studied for the various
power quality aspects of ZVR (Zero Voltage Regulation), PFC
(Power Factor Correction), neutral current compensation and
harmonics elimination.
Abstract—A 4-leg VSC (Voltage Source Converter) based SPV
(Solar Photovoltaic) power generating system is integrated to a 4wire distribution system using MISCT (Modified Instantaneous
Symmetrical Component Theory) based control algorithm. This
MISCT is used to estimate reference currents to control 4-leg
VSC. The control algorithm is used to balance the loads and to
improve the power factor of ac mains. The PCC voltage is
maintained at reference value by required reactive power
compensation by VSC and its DC link voltage is regulated using a
PI (Proportional-Integral) controller. Performance of proposed
SPV generating system is simulated using MATLAB/simulink
and sim-power system toolboxes. Simulation results of the
proposed system are demonstrated for improved power quality of
a three phase 4-wire distribution system for load balancing,
harmonics elimination, neutral current compensation, voltage
regulation and power factor correction.
Index Terms— Solar Photovoltaic, Neutral
compensation, Power Quality, Voltage Source Converter
I.
current
INTRODUCTION
Everlasting increasing demand of electricity and rise of
greenhouse gases due to conventional sources of energy is one
of serious issues to the environmentalist. Solar photovoltaic
(SPV) energy is a solution to all these problems created by the
greenhouse gases. SPV power has great potential such as
environment friendly, clean, pollution- free and inexhaustible.
With recent research in the SPV materials, the price of SPV
module has drastically gone down and SPV power generating
system becomes more practical. Grid interfaced SPV power
generation is the recent advancement of SPV application [1,2].
Grid interfaced SPV power generating system is operational
only approximately 8 hours of a day and system remains
unused for rest of the day. This may lead to the grid instability
and mean time, an optimum use of the power converters
involved in the SPV system is also not possible. Therefore, in
order to use grid interfaced SPV system optimally, the grid
interfaced SPV system can be designed to provide the function
of power quality improvement and the system can be
simultaneously used to satisfy the peak load demand, which
needs the combined operation with VSC. It provides the PFC
(Power Factor Correction), load balancing and harmonics
mitigation with reactive power compensation and
simultaneously injects the maximum power available from the
SPV array in to the grid. When the solar intensity is reduced to
zero, the VSC(Voltage Source Converter) can still be utilized
II. SYSTEM CONFIGURATION
The proposed grid interfaced SPV power generating system
employing a SPV array of a 20 kW peaking power capacity, a
DC-DC boost converter and a 4- leg VSC is given in Fig.1 (a)
[9]. The DC bus is connected to a bi-directional eight-switch
current controlled VSC and a DC-DC boost converter. The
active power from the SPV array is fed to the three-phase
utility grid (415 V, 50 Hz) or the linear/ nonlinear, balanced/
unbalanced loads connected to a 4-wire distribution system. A
RC filter is used to filter the switching harmonics of VSC
output PWM voltage. To track maximum power during
varying solar intensity from SPV array a variable step size
INR (Incremental Resistance) method is used [10]. A MISCT
(Modified Instantaneous Symmetrical Component Theory)
based control algorithm has been used to control VSC in order
to generate reference grid currents. The proposed control
scheme of VSC regulates its DC bus voltage and manages the
active and reactive power exchanged with grid.
1
III. DESIGN OF PROPOSED SPV POWER GENERATING SYSTEM
V D(1−D)
V D(1−D) 700*0.5(1−0.5)
ORLb = dc
Δi1 = dc
=
=2.170mH ≈2.5mH
(2Lb fsw)
(2Δi1 fsw) (2*1.008*10000)
The design of proposed SPV power generating system is
given in terms of various stages as SPV array, interfacing
inductors, DC bus capacitor and the ripple filter is as follows.
where Δi1 is input current ripple of DC-DC boost converter,
and it is considered as 3 % ,thus i1 (= P/Vin) =33.613A, where
Vdc=Vin is the DC input voltage to the boost converter.
Corresponding to the maximum ripple condition, D =0.5[12],
Thus a calculated value of Δi1 is 1.008 A. Thus the calculated
value of inductance (Lb) from (3) is obtained as 2.91 mH and
it is selected as 3mH.
A.
Design of SPV Array
Proposed SPV power generating system has many
photovoltaic cells in series and parallel to feed a 20 kW peak
power at PCC. SPV array is a combination of several modules
assembled in series and parallel to achieve a peaking power
capacity of 20 kW. In order to achieve increased efficiency of
SPV array it must operate near MPP.
The generalized equation for an active power for SPV array
is as,
(1)
PmaxM = VmppM * ImppM
Maximum power, PmaxM = (ns*85% of Voc * np *85 % of Isc ) is
generally achieved under this condition As per [9-11], each
photovoltaic cell has an open circuit voltage, Voc of order of
0.62 V and short circuit current, Isc of 4 A thus Imppc is 3.4 A
and Vmppc is 0.527V of each cell.
The total estimated maximum power is given as,
(2)
PmaxM = VmppM * ImppM = 20 kW
With np parallel strings of PV cells and ns series connected
cells, and considering, VOCM =700 V, PmaxM = np* ns*0.85*
Voc*0.85* Isc =20,000W = np*0.85* 700V*0.85*4A. It results
in np = 9.886 = rounding off to 10 cells and VpmaxM=0.85*ns*
Voc =0.85*700V =595 V. It results in ns = 595V/ (0.62V*0.85)
= 1129 cells.
To achieve 20 kW peak power, 1129 cells are connected in
series and 10 are connected in parallel which lead to (VmppM)
of 595 V and maximum current (ImppM) of 51 A which
constitutes a PV array of 1129*15 cells, respectively.
PV
panel
Boost DC-DC
converter
Lb
S1
D
S3
S7
Vpv
I PV
SW
Vdc Cd
_
+
V*dc
iLa
P3
PL
isa
isb
isc
PCC
Vn
iLa iLb iLc iLn
Nonlinear
Loads
S8
V* sref
_
+
C. Design of DC Link Capacitor
The value of DC link capacitor is given as, [12],
Cb =
Ploss
PG
vsb vsc
Amplitude Vt
calculation
Thus, from eq. (4) the calculated value of Vdc is 677.60 V and
it is selected as 700 V.
DC Voltage
PI
Controller
PLdc +
LPF
+
Φ K= a,b,c
vsa
vsa
vsb
v sc
Vsk ILk
B. Selection of DC Bus Voltage
Proper compensation takes place if the DC link voltage is
greater than the twice of the line to line grid voltage. The
selection of the DC bus voltage is given in eq.(4) where the
value of m is 1, and VLL is the line voltage of VSC and taken
as 415V [13],
(4)
Vdc = (2 2VLL ) ( 3m) = (2 2*415) ( 3*1) = 677.60V ≈ 700V
iac Va
ibc Vb
icc Vc
La
Lb
Lc
Ln
LPF
MPPT
Controller
iLb
iLc
S6
S4
S2
Gating
pulse
Fig. 2 Calculation of the internal parameter of control algorithm
Ripple
Filter
S5
AC Voltage
PI
Controller δ
Isn isa isb isc
Estimation
of the i* sa
Reference i*sb
Source
Current
Current i* sc
Regulator
(3)
Id
28.571
=
= 2166.43μF ≈ 2500μF (5)
2*314*0.02*700
2*
*
ω
v
(
)
(
dcrip )
where Id is the DC link current (Pdc/Vdc = 28.57A), ω is
angular frequency and is % ripple voltage considered as 3%
of Vdc. Hence estimated value of Cb is 2166.43 μF and it is
selected as 2500 μF.
S1
S8
0
D. Design of AC Inductor
The value of AC inductor (Lf) is estimated as follows [13],
Vsa Vsb Vsc
Fig. 1 Proposed SPV power generating system
Lf =
B. Design of DC-DC Boost Converter
The MPP is tracked through DC-DC boost converter and
fed the active power to the DC bus of VSC. The ripple current
for the boost converter is given as [12],
3mVdc
3*1*(700V )
=
= 2.905mH ≈ 3mH (6)
12hfsΔi 12*1.2*10000*(0.1*28.98A)
where m is modulation index, current ripple (Δi), DC bus
voltage (Vdc) and h is the overload factor, considering, ∆i =
2
With power factor consideration with fundamental voltage and
current component is φ then
10% of input current, fs = 10 kHz. The value of Lf from (6) is
calculated order of 2.90 mH. A value of Lf of 3 mH is selected
in this work.
*
*
*
∠ {vsa + avsb + a 2 vsc } = ∠ {iisa
+ aiisb
+ a 2iisc
} −φ
IV. MATLAB BASED MODELING
Or
tan −1 { x1 x2 } = tan −1 { x3 x4 } − φ
The configuration of the SPV power generating system is
modeled by using MATLAB/SIMULINK with sim-power
system tool boxes as shown in Fig 2. Fig. 2 explains the detail
control modeling and the reference current generation using
MISCT. It shows the estimation of the active power and
control angle δ, active power losses, unit template, Vt for the
reference current generation. Further the estimation of the
reference current and PWM switching signal generation are
achieved for the control of the combined operation of the VSC
based SPPV power generating system as shown in Fig.1.
where
*
*
*
x2 = iisa
+ (iisb
/ 2) + (iisc
/ 2) ,
tan φ
And δ =
3
dIpv
dI pv Vpv
dI pv
= Vpv + Ipv
dVpv
dI pv
*
*
*
vsa iisa
+ vsbiisb
+ vsc iisc
= pldc + ploss
{
where,
(15)
}
(16)
Vdcer = Vdc* − Vdc
{
where
}
*
Vder = Vtref
− Vt
3) Reference current generation
Reference currents are estimated as per eq. (10, 11, 12) are as,
*
iisa
= {vsa + ( vsb − vsc ) δ } ( pldc + ploss ) y
(8)
*
iisb
= {vsb + ( vsc − vsa ) δ } ( pldc + ploss ) y
(18)
*
iisc
= {vsc + ( vsa − vsb ) δ } ( pldc + ploss ) y
where y =
B. Control of Four- Leg VSC
The reference grid current for proposed SPV power generating
system is estimated by sensing grid voltages (vsa,vsb,vsc), grid
currents (iisa,iisb,iisc) , load currents (iLa,iLb,Ilc) and DC link
voltage Vdc as feedback signals. The amplitude for unit
templates of the grid voltage is given as,
y=Σ
∑
v 2i
i = sa , sb , sc
These grid reference currents are further used to estimate
PWM signal for 4-leg VSC.
4) Generation of PWM Current Controller
These reference grid currents (i*sa, i*sb and i*sc) are compared
with the sensed grid currents (isa, isb and isc) in the PWM
current controller. After amplification of the current errors, the
output of current amplifiers are compared with fixed
frequency triangular carrier wave (10 kHz) to generate gating
signals for IGBTs of VSC.
(9)
As per reference current generation method [5-8] the reference
grid current is considered as,
*
*
*
iisa
+ iisb
+ iisc
=0
(13)
2) Voltage regulation
For voltage regulation the phase difference between PCC
voltage and grid current is estimated in order to regulate the
PCC voltage which gives the reactive power component, this
reactive power component is given as,
δ (k ) = δ (k − 1) + K pa Ver ( k ) − Ver ( k − 1) + K iaVer ( k ) (17)
dVpv dI pv and accordingly MPP is tracked.
( 2 3) ( vsb2 + vsc2 + vsc2 )
(12)
where, Ploss( k) = Ploss( k−1) +Kpd Vdcer( k) −Vdcer( k−1) + KidVdcer( k)
Thus as in INR method of tracking, the instantaneous
resistance V pv I pv is compared with incremental resistance
Vt =
)
From eqs. (10, 11 and 12), it results in,
( vsb − vsc + δ vsa ) iisa* + ( vsc − vsa + δ vsb ) iisb* + ( vsa − vsb + δ vsc ) iisc* = 0 (14)
1) Power factor correction
From Eq.(14), if δ =0, grid currents are to be in phase with
grid voltages, the active power to the load is supplied by the
SPV array. The filtered active power and the active power
losses are given as,
V. CONTROL ALGORITHM
=
(
*
*
− iisc
x1 = 3 2 ( vsb − vsc ) , x2 = ( 3 2) vsa , x3 = 3 2 iisb
The control algorithm of a proposed SPV power generating
system consists of two stages. First MPPT control to track
peak power of SPV array using DC-DC boost converter and
the second stage is used to control a grid interfaced four-leg
VSC also operating as a shunt compensator. The detailed
control algorithm is explained as follows.
A. Maximum Power Point Tracking
The drawbacks of P&O and IC are overcome by using
variable step size INR method [14]. Variable step is usually
large when away from MPP and reduces to small as the
system approaches MPP which reduces oscillations around
MPP. The MPPT depicts as follows,
δ(n) = δ(n − 1) ± K(dp / dpv)
(7)
where δ is the duty ratio at nth sampling instant and δ(n −1) is
a duty ratio at (n − 1) th sampling instant and is a factor to
control step size.
Variable step INR method is based on a fact that slope of (
dVpv dI pv ) must be zero at MPP as,
dVpv
(11)
(10)
3
Fig 3 Performance of SPV power generating system under non-linear load with night time application
current. Here SPV array voltage Vpv, SPV array current and
power as Ipv and Ppv respectively and load neutral current and
source neutral current by iLn and iSn.
5) Neutral Current Compensation
The grid neutral current is compensated by using a fourth leg
of VSC. The grid neutral current (isn) is sensed and compared
with its reference value (i*sn = 0). The current error is used to
generate switching pulses for IGBT’s of fourth-leg of VSC.
A. Performance of Proposed System at Unbalanced Linear
Load for Voltage Regulation and Load Balancing
The system performance is shown under unbalanced nonlinear
load with solar intensity gradually reducing to zero at 0.5s in
Fig.3. The load unbalancing is occurred at 0.55s and at 0.65s,
and during load unbalancing grid currents are balanced and
sinusoidal. The solar intensity is 1000w/m2 initially and
reduced to zero gradually at 0.5s. At 0.5 s onwards the active
power is supplied by the grid and the direction of the active
power flow is reversed after 0.5 s. The grid neutral current is
almost negligible explains the load balancing operation of the
VI. RESULTS AND DISCUSSION
The system performance under linear load as well as nonlinear
load is demonstrated in Figs. 3-4. A nonlinear load is realized
by using a diode bridge rectifier load and load unbalancing is
done by removing one phase and another. The system
performances are depicted as grid voltages (vsa, vsb, vsc) grid
currents (iisa, iisb, iisc) load current (iLa, iLb, iLc) active power
from/to grid (P) and reactive power (Q) and (ii) is the VSC
4
and grid voltage are observed well within the acceptable limits
of an IEEE-519 standard. The VSC has been found to deliver
SPV power to grid with low THD in grid voltage and current.
The balance grid currents and constant DC link have verified
the effectiveness of the proposed controller.
system. The DC PI voltage regulator maintains DC link
voltage to its reference value. With decreasing solar intensity,
the SPV array current (Ipv) and SPV voltage (Vpv) and SPV
power (Ppv) are also reduced. The PCC voltage is maintained
to its reference value depicting the proper voltage regulation.
TABLE-I
PERFORMANCE OF THE SPV POWER GENERATING SYSTEM IN
DIFFERENT OPERATING MODE
Mode of Operation Parameters
Nonlinear RC load
Grid Voltage %THD
345.5V,1.33%
PFC
Grid Current %THD
48.61,4.60%
Load Current %THD
53.97,59.24%
Grid Voltage %THD
346.5, 1.67%
ZVR
Grid Current %THD
49.32,4.54%
Load Current %THD
54.61,60.02%
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
Fig.4 Performance of SPV power generating system under nonlinear load for
voltage regulation
B.
Performance of Proposed System at Unbalanced
Nonlinear Load for PFC and Load Balancing
The system performance is shown under unbalanced nonlinear
load with constant solar intensity. The load unbalancing is
occurring at 0.3s and at 0.45s. During load unbalancing, the
grid currents are balanced and sinusoidal. Both grid voltages
and grid currents are in phase which shows the unity power
factor operation of the proposed system. The load
compensation is provided by the VSC current. The DC PI
controller maintains the DC link to its reference value. Table-I
shows the THD (Total Harmonic Distortion) of the grid
current, grid voltage and load current which are 4.80%, 1.33%
and 59.24% respectively which are well within limit of IEEE519 standard [15].
[10]
[11]
[12]
[13]
[14]
[15]
VII. CONCLUSION
The performance of a four-leg VSC based SPV power
generation system has been simulated and effectiveness of the
control has been verified using the dynamics of the system.
The constant voltage at PCC and DC link of the self-supported
DC bus has been achieved and the THD of the grid current
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