INTERNATIONAL
OF ELECTRICAL
&–
International
Journal of JOURNAL
Electrical Engineering
and TechnologyENGINEERING
(IJEET), ISSN 0976
6545(Print), ISSN 0976 – 6553(Online)
Volume 3, Issue
2, July- September (2012), © IAEME
TECHNOLOGY
(IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 3, Issue 2, July – September (2012), pp. 164-173
© IAEME: www.iaeme.com/ijeet.html
Journal Impact Factor (2012): 3.2031 (Calculated by GISI)
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IJEET
©IAEME
SIMULATION ANALYSIS OF 100KW INTEGRATED SEGMENTED
ENERGY STORAGE FOR GRID CONNECTED PV SYSTEM
M.Sujith(1), R.Mohan(2), P.Sundravadivel(3)
(1)
Assistant professor, Vidyaa Vikas College of Engineering and
Technology,Tiruchengode-637214 Email ID: msujitheee@yahoo.co.in
(2)
Assistant professor, Vidyaa Vikas College of Engineering and
Technology,Tiruchengode-637214 Email ID: mohanraju@yahoo.co.in
(3)
Assistant professor, K.S.R. College of Engineering,Tiruchengode-637214
Email ID: sundarkmp@gmail.com
ABSTRACT
The present a single-phase photovoltaic (PV) system integrating segmented
energy storage (SES) using cascaded multilevel inverter. The system is designed to
coordinate power allocation among PV, SES, and utility grid, mitigate the overvoltage at
the Point of common point (PCC), and achieve wide range reactive power compensation.
The power allocation principle between PV and SES is described by a vector diagram.
An appropriate reactive power allocation coefficient (RPAC) is designed to avoid duty
cycle saturation and over modulation so that wide range reactive power compensation
and good power quality can be achieved simultaneously. The self-regulating power
allocation control system integrating the preferred RPAC and an advanced active power
control algorithm are developed to achieve the aforesaid objective. Simulation results are
provided to demonstrate the effectiveness of the proposed cascaded PV system
integrating SES.
Key Words : Photovolatic, Segmented Energy Storage, Reactive power Allocation
Coefficient, Point of common Point
I INTRODUCTION
Energy Storage (ES) elements such as batteries ES have been applied to gridconnected residential PV systems for peak power shavings and backup power. Recently,
it is being looked at as a possible solution for improvement of the power quality of the
grid. Research in proves that integration of small energy storage can effectively reduce
the overvoltage caused by reverse power flow. Moreover, battery-integrated PV systems
can improve grid quality by introducing reactive power compensation and harmonics
cancellation.
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July
July- September (2012), © IAEME
Traditionally, two kinds of system configurations have been used in batterybattery
integrated PV systems: ac-link
link system and dc
dc-link system. The ac-link
link system has
separate dc/ac converters for the PV array and battery. The dc-link
link system has a common
dc/ac converter for the PV array and battery. Although each configuration has its own
advantages, they both require two conversion stages, i.e., dc/dc and dc/ac stage, between
the battery and the grid. However, it is reported tthat
hat the efficiency of current power
conditioning system with ES is 8% lower than the traditional PV system without ES.
Another disadvantage is that high switching frequency must be implemented for all the
converters in order to achieve lower voltage total harmonic distortion (THD).
II PV-GRID
GRID CONNECTED SYSTEM
The configuration of a single phase grid connected PV system is illustrated in
Fig.1.. It consists of solar PV array, input capacitor, single phase inverter, and low pass
output filter and grid voltage source. The solar PV modules are connected in a seriesparallel configuration to match the required solar voltage and power rating. The direct
current (DC) link capacitor maintains the solar PV array voltage at a certain level for the
voltage sourcee inverter. The single phase inverter with the output filter converts the DC
input voltage into AC sinusoidal voltage by means of appropriate switch signals and then
the filter output pass through an isolation step up transformer to setup the filter output
voltage to 220 VRMS required by the electric utility grid and load. The system also
consists of a battery bank for supplying the electrical loads of the clinic in case of electric
grid failure.
Photovoltaic power systems are generally classified according to their functional
and operational requirements, their component configurations, and how the equipment is
connected to other power sources and electrical loads. The two principal classifications
are grid-connected or utility-interactive
interactive systems and stand
stand-alone systems.
Fig.1 Schematic Diagram of PV
PV-Grid System
(a) CIRCUIT OPERATION
The PV module is connected to the grid through an H
H-bridge
bridge inverter. The ES
devices are integrated through cascaded H
H-bridge
bridge cells. The proposed system can operate
in stand-alone and grid-connected
connected mode through a static transfer switch (STS). Although
the cascaded multilevel inverter is usually adopted for high
high-power
power and high-voltage
high
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6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
applications, this research revealed the following advantages of applying this topology.
First, the cascaded multilevel converter with separate dc source is ideal for connecting
PV and SES. The SES can be controlled and maintained individually which improves the
system reliability. Second, this topology integrates ES charge/discharge control and dc/ac
power conversion. Therefore, there is only one conversion stage from ES to grid, which
leads to higher efficiency, lower cost, and lighter weight. Third, the wide range reactive
power compensation and proper active power allocation can be achieved simultaneously
to improve power quality.
In the proposed topology, the power allocation strategy between PV and SES
plays the key role since the power allocation and output voltage generation are coupled
with each other. An RPAC is then selected by plot analysis under different conditions.
The self-regulating power allocation control system is developed to achieve active power
control between PV and SES, and wide range reactive power compensation.
(b) Battery Active Power Control Algorithm
The battery active power control algorithm includes battery active power
reference generation and active power control. Depending on the system operation
conditions, the active power dispatch among PV, load, grid, and batteries may come into
five operation states as follows.
Operation state 1:if “P_main<P_load” and “SOC >0.2,”no power will be delivered to
grid. Batteries will provide power to meet the load requirement. Each battery is
controlled to provide half of (P_load−P_main) power.
Operation state 2: if “P_main<P_load” and “SOC <0.2,”grid will provide power with
(P_load−P_main) to load. In this case, batteries are not allowed to release energy.
Operation state 3: if “P_main>P_load” and Vpcc is not over the upper limit Vpcc limit,
the excess active power from PV will be delivered to grid, that is (P_grid =
P_main−P_load). In this case, there is no batteries energy exchange. So, P_auxi1_ref
andP_auxi2_ref are zero.
Operation state 4: if “P_main>P_load,” “Vpcc>Vpcc limit” and “SOC <0.9,” the excess
active power from PV will be delivered to grid and batteries. P_grid is limited to the
upper power limit P_grid_limit. Each battery is controlled to absorb half of
[P_grid_limit− (P_main−P_load)] power.
Operation state 5: if “P_main>P_load,” “Vpcc>Vpcc limit,” but “SOC >0.9,” the MPPT
for PV module cannot be achieved. P_main is limited to the upper power limit
P_main_limit. P_grid is limited to the upper power limitP_grid_limit. Batteries are not
allowed to absorb power.
(c) Power Allocation Analysis
The flexible active and reactive power allocation among PV, SES (ES1 and ES2),
and utility grid. In this paper, a battery is used as SES. Due to the PV power variation
under different operation conditions, SES will be charged or discharged to meet the
load/grid requirement so as to improve power quality and maintain system stability. In
addition, the low-order harmonic voltages being included in the quasi-square-wave of the
main inverter output voltage can be cancelled by the equivalent negative harmonic
voltage generated from auxiliary inverters. The proposed PV system with SES is able to
operate in both stand-alone mode and grid-connected mode through an STS.
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July
July- September (2012), © IAEME
III SIMULATION ANALYSIS
It is a detailed model of a 100
100-kW array connected to a 25-kV
kV grid via a DC
DC-DC boost
converter and a three-phase
phase three
three-level
level Voltage Source Converter (VSC). Maximum Power Point
Tracking (MPPT) is implemented in the boost converter by means of a Simulink model using the
“Incremental Conductance + Integral Regulator” technique.
The detailed model contains:
• PV array delivering a maximum of 100 kW at 1000 W/m2 sun irradiance.
• 5-kHz
kHz boost converter (orange blocks) increasing voltage from PV natural voltage (272 V
DC at maximum power) to 500 V DC. Switching duty cycle is optimized by the MPPT
controller that uses the “Incremental Conductance + Integral Regulator” technique.
• 1980-Hz (33*60) 3-level
level 33-phase
phase VSC (blue blocks). The VSC converts the 500 V DC to
260 V AC and keeps unity power factor.
• 10-kvar
kvar capacitor bank filtering harmonics produced by VSC.
VSC
• 100-kVA
kVA 260V/25kV three
three-phase coupling transformer.
• Utility grid model (25-kV
kV distribution feeder + 120 kV equivalent transmission systems).
In the average model the boost and VSC converters are represented by equivalent voltage
sources generating the AC voltage averaged over one cycle of the switching frequency. Such a
model does not represent harmonics, but the dynamics resulting from control system and power
system interaction is preserved. This model allows using much larger time steps (50 microsecs),
resulting in a much faster simulation.
Note that in the average model the two PV-array
PV array models contain an algebraic loop. Algebraic
loops are required to get an iterative and accurate solution of the PV models when large sample
times are used. These algebraic loops are easily solved by Simulink.
(a) PV Array
The 100-kW
kW PV array of the detailed model uses 330 Sun Power modules (SPR
(SPR-305).
305). The array
consists of 66 strings of 5 series--connected
connected modules connected in parallel (66*5*305.2 W= 100.7
kW). Open the PV-array
array block menu and look at model parameters. Manufacturer
Manufacturer specifications
for one module are:
Number of series-connected
connected cells : 96
Open-circuit
circuit voltage: Voc= 64.2 V
Short-circuit
circuit current: Isc = 5.96 A
Voltage and current at maximum power: Vmp =54.7 V, Imp= 5.58 A
The PV array block menu allows you tto plot the I-V and P-V characteristics for one
module and for the whole array. The characteristics of the SunPower-SPR305
SunPower SPR305 array are
reproduced below.
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July
July- September (2012), © IAEME
Fig.2 I--V and P-V characteristics of PV array
Red dots on blue curves indicate module manufacturer specifications
specifications (Voc, Isc, Vmp,
Imp) under standard test conditions (25 degrees Celsius, 1000 W/m2).
(b) Boost converter
In the detailed model, the boost converter (orange blocks) boosts DC voltage from
273.5 V to 500V. This converter uses a MPPT system which automatically varies the
duty cycle in order to generate the required voltage to extract maximum power.
Look under the mask of the “Boost Converter Control” block to see how the MPPT
algorithm is implemented. For details on various MPPT techniques, refer to
to the following
paper:
Moacyr A. G. de Brito, Leonardo P. Sampaio, Luigi G. Jr., Guilherme A. e Melo,
Carlos A. Canesin “Comparative Analysis of MPPT Techniques for PV Applications”,
2011 International Conference on Clean Electrical Power (ICCEP).
(c) VSC converter
The three-level
level VSC (blue blocks) regulates DC bus voltage at 500 V and keeps
unity power factor. The control system uses two control loops: an external control loop
which regulates DC link voltage to +/
+/- 250 V and an internal control loop which regulates
reg
Id and Iq grid currents (active and reactive current components).
Id current reference is the output of the DC voltage external controller. Iq current
reference is set to zero in order to maintain unity power factor. Vd and Vq voltage
outputs of the current controller are converted to three modulating signals Uref_abc used
by the PWM three-level
level pulse generator.
The control system uses a sample time of 100 mss for voltage and current
controllers as well as for the PLL synchronization unit. In the detailed
detailed model, pulse
generators of Boost and VSC converters use a fast sample time of 1ms
1 s in order to get an
appropriate resolution of PWM waveforms.
1. Run the photo.mdl for 3 seconds and observe the following sequence of events on
Scopes.
• From t=0 sec to t= 0.05 sec, pulses to Boost and VSC converters are blocked. PV
voltage corresponds to open
open-circuit
circuit voltage (Nser*Voc=5*64.2=321 V, see V
trace on Scope Boost). The three-level
three level bridge operates as a diode rectifier and DC
link capacitors are charged above 500
500 V (see Vdc_meas trace on Scope VSC).
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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July
July- September (2012), © IAEME
•
•
•
•
At t=0.05 sec, Boost and VSC converters are de-blocked.
de blocked. DC link voltage is
regulated at Vdc=500V. Duty cycle of boost converter is fixed (D= 0.5 as shown
on Scope Boost) and sun irradiance is set to 1000 W/m2. Steady state is reached
at t=0.25 sec. Resulting PV voltage is therefore V_PV = (1-D)*Vdc=
(1 D)*Vdc= (1
(10.5)*500=250 V (see V trace on Scope Boost). The PV array output power is 96
kW (see Pmean trace on Scope Boost) whereas maximum power with a 1000
W/m2 irradiance is 10
100.7
0.7 kW. Observe on Scope Grid that phase a voltage and
current at 25 kV bus are in phase (unity power factor).
At t=0.4 sec MPPT is enabled. The MPPT regulator starts regulating PV voltage
by varying duty cycle in order to extract maximum power. Maximum power
po
(100.7 kW) is obtained when duty cycle is D=0.453. At t=0.6 sec, PV mean
voltage =274 V as expected from PV module specifications (Nser*Vmp=5*54.7=
273.5 V).
From t=0.7 sec to t=1.2 sec, sun irradiance is ramped down from 1000 W/m2 to
250 W/m2. MPPT continues
ntinues tracking maximum power. At t=1.2 sec when
irradiance has decreased to 250 W/m2, duty cycle is D=0.485. Corresponding PV
voltage and power are Vmean= 255 V and Pmean=22.6 kW. Note that the MMPT
continues tracking maximum power during this fast irrad
irradiance
iance change.
From t=1.5 sec to 3 sec various irradiance changes are applied in order to
illustrate the good performance of the MPPT controller.
Fig. 3 Simulation Diagram for 100KW Grid Connected PV Array
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Fig.4. Waveforms of Boost Converter
Fig.5. Waveform for Modulation Index and Inverter
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Fig.6. Response of Voltage Source Converter
Fig.7. Synchronized Grid Power
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Fig. 8. Grid Voltage and Current
IV CONCLUSION
This paper has addressed the development of the cascaded PV system integrating
SES. The proposed PV system can provide enhanced active power smoothing and
expanded reactive power compensation. A developed dual-stage DFT PLL method was
verified to be able to achieve the active and reactive power separation and improve the
dynamic performance of the PV system. A coordinated power allocation strategy based
on the proposed dual-stage DFT PLL can effectively allocate the active and reactive
power between PV and SES. An appropriate reactive power allocation coefficient k2 was
derived from RPAC analysis under different conditions to achieve wide range reactive
power compensation without degrading power quality. The particular battery active
power control algorithm was conducted to deduce the active power allocation coefficient
k1 and improve the system stability and reliability. Overvoltage of PCC caused by
reverse power flow is eliminated by appropriately dispatching PV power to SES. The
simulation results confirmed the validity of the proposed power allocation control.
V REFERENCES
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Carlos A. Canesin “Comparative Analysis of MPPT Techniques for PV
Applications”, 2011 International Conference on Clean Electrical Power (ICCEP).
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house, Second edition, pp.385-397.
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6545(Print), ISSN 0976 – 6553(Online) Volume 3, Issue 2, July- September (2012), © IAEME
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