The document provides an overview of power converters used in hybrid energy systems. It discusses: 1) The components of a grid-connected photovoltaic (PV) system including the PV array, DC-DC boost converter, three-phase inverter, LC filter, and connection to the utility grid. 2) Control techniques for the three-phase inverter such as maximum power point tracking (MPPT) and synchronous reference frame control. 3) Sizing considerations for PV panels and batteries based on daily energy usage. 4) Types of three-phase AC voltage controllers that use thyristors to regulate the output voltage.

1. Unit-III
Power Converters for Hybrid System
BY
R.ARULJOTHI
M.Tech( 2nd YEAR)
15EE302
Under the guidance of
DR. N.P. SUBRAMANIAM, M.E.,Ph.D.,
ASSISTANT PROFESSOR
DEPARTMENT OF ELECTRICALAND ELECTRONICS
ENGINEERING
PONDICHERRY ENGINEERING COLLEGE

2. INTRODUCTION
• For environmental concern and increase in peak power demand, PV solar
cells has become an alternative energy source for green and clean power
generation.
• With the growing demand for renewable sources of energy, the
manufacturing of solar cells and PV array has advanced in recent years.
• The word “photovoltaic” combines two terms – “photo” means light and
“voltaic” means voltage. A photovoltaic system in this discussion uses
photovoltaic cells to directly convert sunlight into electricity.
• The technology employed in photovoltaic (PV) systems is well-developed
and there are improvements and modifications occurring regularly,
primarily in production processes. The systems are quite reliable and have
been well tested in space and terrestrial applications.

3. • As the world electricity consumption rapidly increases with population
growth, new power generation capacities are required to cover that
demand. So, there is need for power generation using renewable sources.
Hence Photovoltaic System is implemented to tie with the power grid.
• In order to extract maximum power output from PV system efficiently,
MPPT technique is being used.
• A DC/DC converter used which tries to match the load impedance to the
ratio between voltage and current of the array at the maximum power point
(MPP).
• The converter used is a Voltage source inverter (VSI) which is controlled
using synchronous d-q reference frame to inject a controlled current into
the grid and Phase lock loop (PLL) is used in controller to lock grid
frequency and phase .

4. COMPONENTS OF GRID CONNECTED PV
SYSTEM
Several components are needed to construct a grid connected PV system to
perform the power generation and conversion functions.
• Photovoltaic system
• DC-DC converter and Three-phase inverter
• LC filter
• Transformer
• Utility Grid
Components of a Grid Connected PV System

5. PHOTOVOLTAIC SYSTEM
Modeling of PV cell
Equivalent circuit diagram of PV cell
• The equivalent circuit of the PV cell, formed by a current source Iph in
anti-parallel with diode driven by a current Id.

6. From Figure -2 applying KCL
Finally, from the equivalent circuit of PV cell Ipv and Vpv are
pv ph d RpI I I I  
( )
expexp 1 ( ) /
pv pv s
pv par ph par sat pv pv s p
s
q V I R
I N I N I V I R R
N AkT
   
        
  
lnln ph pv sat
pv Rs T
sat
I I I
V I AV
I
  
    
 

7. PV SYSTEM
CONVERTER
LOAD
+
-
Vmodule
moduleI
PV MODULE
+
-
ILOAD
V load
Block diagram of the PV system

8. DC-DC BOOST CONVERTER
• The boost DC converter is used to step up the input voltage by storing
energy in an inductor for a certain time period, and then uses this energy to
boost the input voltage to a higher value.
Circuit diagram of Boost Converter
The relationship between the input and output voltages is given by
( ) 0in on in out offV t V V t  
1
1
on offout
in off
t tV
V t d

 


9. THREE-PHASE INVERTER
• The three phase inverter is used to obtain a three-phase voltage output from
DC source. Three-phase voltage source inverter is a combination of three
single-phase bridge circuits.
Three-phase inverter
In grid connected PV system, the current output of the voltage source inverter will
be injected to the grid. The output of the inverter should be in phase and have an
identical frequency to the voltage of the grid.

10. SCHEMATIC DIAGRAM OF GRID
CONNECTED PV SYSTEM
Schematic diagram of grid connected PV system

11. CONTROL OF THREE PHASE GRID
CONNECTED PV SYSTEM
• The DC/DC boost converter is controlled using a maximum power point
tracking technique .
• For inverter control system dq transformation and SVPWM technique are
being used.
• Grid synchronizations plays important role for grid connected systems.
PLL technique is employed to synchronise the output frequency and phase
of grid voltage with inverter voltage using different transformation.

12. SIZING OF PV SYSTEM
• Different size PV panels will produce different amounts of power. The rated
output wattage of the panel is the amount of watts the panel will create in one
hour of direct sun .
• For our area, multiply the rated wattage by 5.1 to get the average amount
produced in one day.
• The 5.1 factor is the viable operating hours per day and accounts for the fact
that there will be more sun available in the summer and less in the winter.
For Example
• If a panel is rated at 48 watts, multiply that figure by 5.1 to get 245 watt-hours
per day. Use that figure divided into the “Daily Energy Use” that was
calculated above and the resulting number will be the number of panels of that
particular size you will need.
• If the “Daily Energy Use” figure above was 2,000 watts per day, 2,000 divided
by 245 gives us 8.16, rounded up to 9 panels. (Note that there are tracking
systems that will increase the effective hours of sunlight striking a PV panel
beyond 5.1.)Panel Rating (48) x Avg. operating time (5.1) = panel watt-hours
per day (245)
• Daily Energy Use (2,000) / Panel watt-hours (245)= number of panels (8.16),
round up to even number = 9

13. SIZING OF BATTERIES
• Batteries are the best method of storing energy from a PV system for the
periods when the sun is not shining. (This is for stand-alone or non -grid
connected systems.) The information from calculating the daily load will be
needed for determining the battery sizing.
Steps for sizing the battery bank:
• Divide the “Daily Energy Use” (derived from using the Chart on page 6) by
the voltage of the battery (typically 12 volts). The result is amp-hours
which is the common manner of measuring battery capacity. For example,
if the “Daily Energy Use” is 2,000 (watt-hours), divide 2,000 by 12 to get
167 (amp-hours).
• Multiply the daily amp-hours by the number of days that you want to have
power in storage in case the sun is not shining adequately. Three to five
days is recommended. For this example, we will choose four days.
Multiply 167 amp-hours per day times 4 days to get 668 amp-hours.
• Batteries should not be discharged excessively. A deep cycle lead-acid
battery (the main battery option) will last longest if it is discharged only
50%. By dividing the total amp-hours from Step 2 (668) by .50, the optimal
battery capacity is determined; 668/.50 = 1336 amp-hours at 12 volts.

14. THREE PHASE AC VOLTAGE CONTROLLERS
• There are many types of circuits used for the three-phase ac regulators(ac to
ac voltage converters).
• The three-phase loads (balanced) are connected in star or delta. Two
thyristors connected back to back, or a triac, is used for each phase in most
of the circuits as described below.
• The circuit of a three-phase, three-wire ac regulator (termed as ac to ac
voltage converter) with balanced resistive (star-connected) load. It may be
noted that the resistance connected in all three phases are equal. Two
thyristors connected back to back are used per phase, thus needing a total
of six thyristors. Please note the numbering scheme, which is same as that
used in a three-phase full-wave bridge converter or inverter, described in
module 2 or 5.
• The thyristors are fired in sequence , starting from 1 in ascending order,
with the angle between the triggering of thyristors 1 & 2 being 60° (one-
sixth of the time period (T) of a complete cycle). The line frequency is 50
Hz, with T=1/f=20ms. The thyristors are fired or triggered after a delay of a
from the natural commutation point.

15. • The natural commutation point is the starting of a cycle with period,
(60°=T/6) of output voltage waveform, if six thyristors are replaced by
diodes. Note that the output voltage is similar to phase-controlled
waveform for a converter, with the difference that it is an ac waveform in
this case.
• The current flow is bidirectional, with the current in one direction in the
positive half, and then, in other (opposite) direction in the negative half. So,
two thyristors connected back to back are needed in each phase. The
turning off of a thyristor occurs, if its current falls to zero. To turn the
thyristor on, the anode voltage must be higher that the cathode voltage, and
also, a triggering signal must be applied at its gate.

16. MATRIX CONVERTER
• Matrix converter is a device which converts AC input supply to the
required variable AC supply as output without any intermediate conversion
process whereas in case of Inverter which converts AC - DC - AC which
takes more extra components as diode rectifiers, filters, charge-up circuit
but not needed those in case of matrix converters.
Circuit scheme of a three phase to three phase matrix converter. a,b,c are at the
input terminals. A,B,C are at the output terminals

17. GRID-TIE INVERTER
• A grid-tie inverter is a power inverter that converts direct current (DC)
electricity into alternating current (AC) with an ability to synchronize to
interface with a utility line.
• Its applications are converting DC sources such as solar panels or small
wind turbines into AC for tying with the grid.
• Grid-tie inverters are also designed to quickly disconnect from the grid if
the utility grid goes down.
• Inverters take DC power and invert it to AC power so it can be fed into the
electric utility company grid. The grid tie inverter (GTI) must synchronize
its frequency with that of the grid (e.g. 50 or 60 Hz) using a local oscillator
and limit the voltage to no higher than the grid voltage.
Example of a large three-
phase inverter for
commercial and utility scale
grid-tied PV systems

18. Uncontrolled Rectifiers
• Bridge rectifiers are components which have every branch of a rectifier circuit in a single compact case.
Bridge rectifiers can be found that operate from a few amps to several hundred amps.
• The amount of AC voltage mixed with the rectifier's DC output is called ripple voltage. In most cases, pure
DC output is preferable, so minimizing ripple voltage is of importance. If the power levels are not too great,
filtering networks may be used to reduce the amount of ripple in the output voltage.
• Sometimes the method of rectification is referred to in terms of the number of DC pulses output for every
cycle of AC input. A single-phase, half-wave rectifier circuit, then, would be called a 1-pulse rectifier
because it produces a single pulse during the time of one complete cycle of the AC waveform.
• A single-phase, full-wave rectifier (regardless of design, center-tap or bridge) would be called a 2-
pulse rectifier because it outputs two pulses of DC during one cycle of AC input. A three-phase full-wave
rectifier would be called a 6-pulse unit. Three-phase rectification leads to less fluctuation of current and
voltage.
Single-Phase and
Three-Phase
Bridge Rectifier

19. MODELING USING SIMULATION
Rectifier

20. OUTPUT FOR RECTIFIER
Voltage
Current
Time
Time

21. • Boost

22. OUTPUT BOOST CONVERTER
Voltage
TimeCurrent
Time

23. • Inverter

24. OUTPUT OF INVERTER
• Voltage
Time