UNIT I: INTRODUCTIONS

PART-A

1. List various renewable energy resources.

Renewable energy technologies that have the potential to provide future energy

supplies includes:

Solar energy(photovoltaic & solar thermal systems)

Wind energy

Energy from biomass and biogas

Hydroelectric systems

Ocean thermal energy conversion

Tidal energy

Geothermal energy

Hydrogen energy

Fuel cells

2. State the significance of renewable energy.

The conventional energy sources will not be sufficient to meet the growing demand.

Limited quantity of fossil fuels

Conventional energy sources produce pollution and increase the global temperature.

Renewable energy is available in nature at free of cost and inexhaustible. They produce

less pollution.

3. What are the limitations of solar power?

The major limitations of solar energy include an inability to generate power at night,

an inability to ramp up power production to meet demand, and cost of solar panels.

4. State the impact of Wind power penetration in power grid.

The impacts of Wind power in the electricity systems depend to a large extent on

the level of wind power penetration, Grid size and Generation mix of electricity in the

system.

5. What are the advantages and limitation of Tidal power generation?

Advantages:

1. Tidal power is completely independent of the precipitation (rain) and its uncertainty

besides being inexhaustible.

2. Large area of valuable land is not required.

3. When a tidal power plant works in combination with thermal or hydro-electric system

peak power demand can be effectively met with.

4. Tidal power generation is free from pollution.

Limitations:

1. Due to variation in tidal range the output is not uniform.

2. Since the turbines have to work on a wide range of head variation (due to variable tidal

range) the plant efficiency is affected.

3. There is a fear of machinery being corroded due to corrosive sea water.

4. It is difficult to carry out construction in sea.

5. As compared to other sources of energy, the tidal power plant is costly.

6. Sedimentation and silteration of basins are the problems associated with tidal power

plants.

7. The power transmission cost is high because the tidal power plants are located away

from load centres.

6. Which are the major contributors for global warming?

Fossil fuel combustion especially that is based on oil and coal is the major

contributor to increasing carbon dioxide concentration in the atmosphere, thereby

contributing to probable global warming.

7. Why renewable energy is better than coal burning?

Fossil fuels produce around 21.3 billion tons of CO2 each year that contributes to

global warming, causing surface temperature of the Earth to rise.

8. What is GHG emission?

Greenhouse gas is any gaseous compound in the atmosphere that is capable of

absorbing infrared radiation, thereby trapping and holding heat in the atmosphere. By

increasing the heat in the atmosphere, greenhouse gases are responsible for the

greenhouse effect, which ultimately leads to global warming.

9. List three major factors influencing the amount of GHG emissions.

Particulate matter

CO2

SOX

NOX

CO

10. What are the types of fuel cell?

Phosphoric acid fuel cell

Alkaline fuel cell

Polymer electrolyte membrane

Molten carbonate fuel cell

Solid oxide fuel cell

11. List the applications of fuel cells.

Cogeneration, fuel cell electric vehicles, providing power for base stations or

sites, distributed, emergency and solar hydrogen fuel cell water heating.

12. Write the advantages and disadvantages of fuel cell?

Advantages:

1. Conversion efficiencies are very high.

2. Require little attention and less maintenance.

3. Can be installed near the use point, thus reducing electrical transmission requirements

and accompanying losses.

4. Fuel cell does not make any noise.

5. A little time is needed to go into operation.

6. Space requirement considerably less in comparison to conventional power plants.

Disadvantages:

1.High initial cost

2.Low service life

13. What are the harmful effects of burning bio-mass?

The Burning of bio-mass is environmentally more polluting than gas but less

polluting than coal. Biomass combustion releases more than 100 different chemical

pollutants into the atmosphere. Wood smoke is reported to contain pollutants known to

cause bronchitis, emphysema, and other illnesses. These pollutants include up to 14

carcinogens, 4 cocarcinogens, 6 toxic that damage cilia, and additional mucus-

coagulating agents.

14. What are the merits of hydrogen fuel?

Although the hydrogen storage vessel is large, hydrogen burns 1.33 times more

efficiency than gasoline in automobiles.

15. What are the drawbacks of wind farms to humans?

Under certain circumstances shadow flicker has caused irritation, disorientation,

and seizures in humans. Future widespread development may be constrained because

local people feel that wind farms diminish the aesthetics of the area. Some communities

have even passed legislation to prevent wind turbines from being installed in residential

areas.

16. How biomass conversion takes place?

Biomass is a renewable energy resources derived from the carbonaceous waste of

various human and natural activities. Biomass is the storage of solar energy in chemical

form in plant and animal materials.

17. What is meant by anaerobic digestion?

Anaerobic digestion is the process whereby bacteria break down organic material

in the absence of air, yielding a biogas containing methane.

18. List various types of bio gas plants.

1. Batch type

2. Continuous type

Balloon plants

Fixed dome plants

Floating drum plants

19. What is the principle of power generation using tides?

The periodic rise and fall of the water level of sea which are carried by the action

of sun and moon on water of the earth is called “tide”. The difference in potential energy

during high tide-aid during low-tide is called tidal energy.

PART-B

1. Explain the impact of renewable energy based power generation on environment issues in

detail. APRIL/MAY 2017

2. How does environment get affected by the use of the renewable energy? And also discuss

GHG emissions from the various energy sources.

3. Discuss the influence of different renewable energy sources with special reference to the

global warming context.

4. Describe the consequences of greenhouse effect.

5. Compare and contrast the conventional energy source and renewable energy source in the

environment aspect.

6. List out the available renewable energy sources. Explain how solar PV system and wind

energy sources plays significant role of power extraction. APRIL/MAY 2017

7. Explain the construction, working and different characteristics of solar array in detail.

8. Describe various types of wave energy conversion device and explain how to generate

electrical power from waves.

9. Describe the principle of generation of Bio gas and mention the factors affecting its

generation.

10. Explain the design and principle of operation of general Fuel cell and Fossil Fuel cell.

11. Explain the principle of operation of Ocean.

12. Explain the principle of operation of Biomass energy.

13. What are the types of ocean thermal energy conversion power plants? Describe in detail the

Anderson OTEC cycle.

14. What is Hydrogen energy? Explain the operation of Hydrogen energy system with a neat

schematic. APRIL/MAY 2017

15. Explain the operation of Wind energy conversion system with neat schematic.

16. Explain the operation of Ocean energy conversion system with neat schematic diagram.

17. Explain the working principle of various types of concentrating solar collectors with neat

sketch.

UNIT II: ELECTRICAL MACHINES FOR RENEWANLE ENERGY CONVERSION

PART-A

1. Name any four types of generators used in wind energy conversion systems.

APRIL/MAY 2017

Squirrel cage induction generator

Permanent Magnet Synchronous generator

Wound rotor induction generator

Doubly fed induction generator

2. What is the basic principle of wind energy conversion?

A wind energy conversion system (WECS) is composed of blades, an electric

generator, a power electronic converter, and a control system, as shown in Fig. The

WECS can be classified in different types, but the functional objective of these systems is

the same: converting the wind kinetic energy into electric power and injecting this

electric power into the electrical load or the utility grid.

3. What are the basic requirements for generation in WECS?

High torque

High power density

High efficiency

Lower maintenance

More robustness

4. Why are induction generators preferred over dc generators in WECS?

Simple and robust construction

Can run independently

Inexpensive minimal maintenance

Inherent overload protection

At high speed, reduces size and weight of machine (roughly 0.33m long, 0.5m

diameter) and filter components

5. What are the advantages of using WECS?

Wind energy is an inexhaustible source of energy and is virtually a limitless

resource

Energy is generated without polluting environment

This source of energy has tremendous potential to generate energy on large scale

Wind energy can be used directly as mechanical energy

In remote areas, wind turbines can be used as great source to generate energy

Land around wind turbines can be used for other uses, e.g. farming

6. What are the advantages of IG used in WECS?

Asynchronous operation of induction generators is considered an advantage for

application in wind turbine systems, because it provides some degree of flexibility

when the wind speed is fluctuating.

It is less expensive and more readily available than a synchronous generator

It does not require a DC field excitation voltage

It automatically synchronizes with the power system, so its control is simpler and

less expensive.

7. What are the disadvantages of induction generators?

It is not suitable for separate, isolated operation

It consumes rather than supplies magnetizing KVAR

It cannot contribute to the maintenance of the system voltage levels

Lower efficiency

8. Mention the advantages of PMSG.

Use of PM for the excitation system consumes no extra electrical power. So copper loss

of the exciter does not exist and absence of mechanical commutator and brushes or slip

rings means low mechanical friction losses.

PMSG is compactness

No need of rotor winding

These in turn allow the generator to be of small, light & rugged structure. As there is no

current circulation in the rotor to create a magnetic field, the rotor of a PMSG generator

does not heat up.

9. What is principle of operation of induction generators?

AC supply is connected to the stator terminals of an induction machine. Rotating

magnetic field produced in the stator pulls the rotor to run behind it (the machine

is acting as motor)

Now, if the rotor is accelerated to the synchronous speed by means of a prime

mover, the slip will be zero and hence the net torque will be zero. The rotor

current will become zero when the rotor is running at synchronous speed.

If the rotor is made to rotate at a speed more the synchronous speed, the slip

becomes negative. A rotor current is generated in the opposite direction, due to

the rotor conductors cutting stator magnetic field.

This generated rotor current produces a rotating magnetic field in the rotor which

pushes (forces in opposite way) onto the stator winding against the applied

voltage. Thus, the machine is now working as an induction generator

(asynchronous generator).

10. What is the range of slip used in induction generator?

In generation mode slip would be negative. Higher motor slip induces higher current in

the rotor and higher power conversion. The value of slip is generally a few to several

percentages.

11. State the difference between induction generator and synchronous generator.

In a synchronous generator, the waveform of generated voltage is synchronized

with (directly corresponds to) the rotor speed. The frequency of output can be

given as f=N*P/120 Hz. Where N is speed of thr rotor in rpm and P is number of

poles.

In case of induction generator, the output voltage frequency is regulated by the power

system to which the induction generator is connected. If induction generator is supplying

a standalone load, the output frequency will be slightly lower (by 2 or 3%) that calculated

from the formula f=N*P/120.

Separate DC excitation system is required in an alternator (synchronous

generator).

Induction generator takes reactive power from the power system for field excitation. If an

induction generator is meant to supply a standalone load, a capacitor bank needs to be

connected to supply reactive power.

Construction of induction generator is less complicated as it does not require

brushes and slip ring arrangement. Brushes are required in synchronous generator

to supply DC voltage to the rotor for excitation.

12. What are the disadvantages of synchronous generators in wind energy conversion

system?

Relatively high cost and maintenance requirements of synchronous generators

The power conversion system takes the full power generation

13. What are the merits of squirrel cage induction generators for wind energy conversion?

Ability to make the best use of available wind power

It eliminates the need for a capacitor bank

Variable speed operation can be achieved

14. What are the constructional differences between SCIG and DFIG?

SCIG: The SCIG makes an excellent generator when it is driven above its

synchronous speed. The same features that makes this motor desirable over the other

types of generators, namely the inherent ruggedness of the squirrel cage design and the

simplicity of the control systems.

The induction motor becomes a generator when it is connected to an electrical power

system and then driven above its synchronous speed by some prime mover. The

prime mover may be a turbine, an engine, a windmill or anything that is capable of

supplying the torque and speed needed to drive the motor into the over-speed

condition.

DFIG: DFIG for double fed induction generator, a generating principle widely used

in wind turbines. It is based on an induction generator with a multiphase wound rotor and

a multiphase slip ring assembly with brushes for accesses to the rotor windings. It is

possible to avoid the multiphase slip ring assembly but there are problems with

efficiency, cost and size. A better alternative is a brushless wound-rotor double fed

electrical machine.

15. List the merits and demerits of capacitor excited induction generator.

It is simple and robust

Easy and cheap for mass production

It enables stall regulated machines to operate at a constant speed when it is connected to a

large grid, which provides a stable control frequency

It doesn’t have current harmonics since it has no frequency conversion

Disadvantages:

The speed is not controllable and variable only over a very narrow range, in which only

speeds higher than the synchronous speed are possible for generator operation

It has less efficiency

Wind speeds can vary to a large extent, therefore the turbine speed cannot be adjusted

with the wind speed to obtain aerodynamic efficiency

There is a very big problem with gear box maintenance

It is highly noisy

16. Draw slip-torque characteristics of Squirrel cage induction generator.

17. State the principle of PMSG.

In PMSM, the rotor field is created by permanent magnets rather than dc current passing

through a coil. The principle of operation of PMSM is the rotor has a pair of magnetic poles.

The stator windings also create two magnetic fields that rotate with time and hence, the

rotor’s magnetic poles chase the stator’s magnetic field and in the process electromagnetic

torque is produced. The speed of rotation of rotor depends on the speed with which the

stator’s field rotates and hence, these machines are known as permanent magnet synchronous

generator.

18. What are the advantages of DFIG with WECS?

Ability to produce more output than its rated power without being overheated

Ability of transferring maximum power both in sub-synchronous and super synchronous

modes

Converter is connected to rotor, so its power rating is reduced and the whole power flows

through the stator.

Generation of electrical power at lower wind speeds.

Control of power factor is possible.

19. What are the merits of PMSG for WECS?

Light weight and small size in construction

Low losses and high efficiency

No need of external excitation current

No need of gearbox

PART-B 1. Draw the schematic of Permanent Magnet Synchronous generator and explain the construction

and principle of operation in detail. Also discuss the characteristics and issues briefly.

2. Explain the steady state equivalent circuit model and performance characteristics of squirrel

cage induction generator in detail.

3. Explain construction, principle of working and characteristics of IG with neat sketches.

4. Explain the principle of operation and constructional features of DFIG used for renewable

energy conversion.

5. Explain construction, principle of working and characteristics of PMSG with neat sketches

6. Draw the circuit model of PMSG and explain the methods used for steady state analysis. List

the merits and demerits of PMSG for wind energy conversion system. APRIL/MAY 2017.

7. Explain DFIG based wind power generation. Illustrate the independent dq control strategy

adopted for dq control.

8. Explain construction, principle of working and characteristics of SCIG coupled with wind

turbine. Analyze the merits and demerits of the above APRIL/MAY 2017

9. Explain the analysis of Induction Generator used for Wind Energy Conversion System.

10. Show the relative merits and demerits of wind energy conversion system with PMSG, SCIG,

DFIG. APRIL/MAY 2017.

11. Define reference theory. Also explain about the fundamentals of reference theory in detail.

UNIT III : POWER CONVERTERS

PART-A

1. Draw the block diagram of solar photovoltaic system.

2. What are the factors for battery sizing?

Battery duty cycle- the load (including duration) the battery

Cell size-rated capacity of the battery

Equalizing charge – prolonged charge

Full float operation – operation with batteries and load connected in parallel

Period-time during which load is expected to be constant during sizing

calculations

Rated capacity

Valve

Vented battery – battery in which the products of electrolysis and evaporator

are allowed to escape freely to the atmosphere.

3. What is line commutated converters? And what is inversion mode of operation of line

commutated inverter?

In a controlled rectifier, Control over the output dc voltage is obtained by controlling the

conduction interval of each thyristor. This method is known as phase control and converters are

also called “phase controlled converters”. Since thyristors can block voltage in both directions it

is possible to reverse the polarity of the output dc voltage and hence feed power back to the ac

supply from the dc side. Under such condition the converter is said to be operating in the

“inverting mode”. The thyristors in the converter circuit are commutated with the help of the

supply voltage in the rectifying mode of operation and are known as “Line commutated

converter”.

4. Draw the schematic diagram of line commutated converter.

5. Explain inversion mode of operation of line commutated inverter.

If α is made larger than 90° the direction of power flow through the converter will

reverse provided there exists a power source in the dc side of suitable polarity. The

converter in that case is said to be operating in the inverter mode.

For a phase controlled rectifier to operate in inverter mode (quadrant 4) is not enough

to change the firing angle value corresponding to an inverter angle αinv,>90o . Besides this

condition, it is required that the active load to be connected in such a manner at the output

terminals of the rectifier so that its internal voltage E to become the source that maintains

the I0 current flow in a positive direction.

6. Define buck–boost converter.

The buck–boost converter is a type of DC-to-DC converter that has an output voltage

magnitude that is either greater than or less than the input voltage magnitude. It is a switched-

mode power supply with a similar circuit topology to the boost converter and the buck converter.

The output voltage is adjustable based on the duty cycle of the switching transistor.

7. Draw the schematic diagram of buck boost converter.

8. List the advantages of boost and buck-boost converter.

Boost converter:

This converter is able to step up the voltage at lowest component count

possible. The input current is continuous which is very desirable for sources

like PV or battery.

The switch used here has the common ground with the source which makes

the drive circuit and control circuit arrangement easier.

The output voltage is positive as opposed to the buck-boost converter which

makes the control is easy.

Buck-boost converter:

1.Step-up and step-down of voltage is possible with minimum component count.

(Cuk, Sepic, Zeta uses almost double component count)

2.Less costly compared to most of the other converters if compromised performance

is desired for a low cost.

9. Draw the schematic of boost converter.

10. What is Boost Converter?

The boost converter, also known as the step-up converter, is another switching converter

that has the same components as the buck converter, but this converter produces an output

voltage greater than the source. The ideal boost converter has the five basic components, namely

a power semiconductor switch, a diode, an inductor, a capacitor and a PWM controller.

11. What are matrix converters? APRIL/MAY 2017

A matrix converter is an array of controlled semiconductor switches that directly connect

each input phase to each output phase, without any intermediate dc link. The matrix converter

offers possible “all silicon” solution for AC-AC conversion removing the need for reactive

energy storage components used in a conventional converter system.

It provides sinusoidal input and output waveforms, with minimal higher order harmonics

and no sub harmonics; it has inherent bi-directional energy flow capability; the input power

factor can be fully controlled. Last but not least, it has minimal energy storage requirements,

which allows to get rid of bulky and lifetime limited energy-storing capacitors.

12. List out the applications of matrix converters.

Standard: wind/ water force machines (blowers, boilers, pumps & general industrial

machines)

Specific applications: compact or integrated motor drives, aircrafts, submarines, AC/AC

power convertions.

13. What is a grid interactive inverter? State its significance.

A grid-tie inverter converts direct current (DC) into an alternating current (AC)

suitable for injecting into an electrical power grid, normally 230V RMS at 50 Hz.

Grid-tie inverters are used between local electrical power generators: solar

panel, wind turbine, hydro-electric, and the grid.

In order to inject electrical power efficiently and safely into the grid, grid-tie

inverters must accurately match the frequency, voltage and phase of the grid sine

wave AC waveform. This may also called synchronous inverters. Grid interactive

inverter cannot be used in standalone application where utility power is not

available and also it is provided with anti-island protection.

14. List the limitations in the operation of matrix converter.

The matrix converter has also some disadvantages. First of all it has a maximum input

output voltage transfer ratio limited to 87 % for sinusoidal input and output waveforms. It

requires more semiconductor devices than a conventional AC-AC indirect power frequency

converter, since no monolithic bi-directional switches exist and consequently discrete

unidirectional devices, variously arranged, have to be used for each bi-directional switch.

Finally, it is particularly sensitive to the disturbances of the input voltage system.

15. Draw the circuit diagram of matrix converter.

16. What are the advantages of dc link inverters?

No increase in the dc link voltage when compared with conventional hard switched

inverter. That is the dc link voltage is 1.0 per unit.

The zero voltage condition can be created at any time.

Well established PWM techniques can be employed.

Power devices of standard voltage ratings can be used.

17. What are the disadvantages of AC voltage controller?

It introduce harmonics in the supply current and the load voltage waveforms particularly

at low output voltages.

18. Define state of charge.

State of charge (SOC) % - an expression of the present battery capacity as a percentage of

maximum capacity. SOC is generally calculated using current integration to determine the

change in battery capacity over time.

19. What is the role of capacitor and the minimum value required for the boost converter?

Capacitors filters the ripple current of the inductor. So as to minimize ripple present at output

terminal of the converter. Also , output capacitor required to maximize the output voltage of

boost converter.

20. Define the term fill factor of a solar cell.

The Fill Factor (FF) is essentially a measure of quality of the solar cell. It is calculated

by comparing the maximum power to the theoretical power (PT) that would be output at both the

open circuit voltage and short circuit current together.

FF=𝑃𝑚

𝐼𝑆𝐶∗𝑉𝑂𝐶

21. What are the factors to be considered for the selection of inverter and batteries for solar

energy conversion?

AC voltage

DC voltage

Maximum power

Type of use

Type of battery

Size of battery

PART-B

1. Draw the block diagram of solar photovoltaic system. And explain the principle of

operation in detail.

2. Describe the grid interactive inverters in detail. APRIL/MAY 2017

3. Draw the schematic diagram of Buck-Boost converter and explain the operation in detail.

Also derive the expressions.

4. Explain the following in detail: i) AC voltage controller & write limitations of AC

voltage controller ii) Voltage control in PWM inverters.

5. Describe working of AC-DC-AC converter with circuit and wave form for wind energy

conversion. APRIL/MAY 2017

6. Draw the schematic diagram of standalone solar PV system. What are the main

components used in it? Explain their functions. APRIL/MAY 2017

7. Explain the operation of line commutated converter under inversion mode with the help

of a neat circuit diagram and necessary waveforms.

8. Describe how a three phase line commutated converter is operated as an inverter.

9. Write short notes on grid interactive inverter.

10. Explain with neat diagram the philosophy of operation of a solar source fed boost

converter. Also derive the expressions.

11. Explain in detail about (i) sizing the solar arrays (ii) selection of inverters

12. Discuss the three phase uncontrolled rectifiers in details.

13. Describe principle of operation of PWM inverter and describe how it is used for wind

energy conversion.

14. Discuss the operation of Matrix Converter.

15. Explain the converter topologies used for solar energy conversion.

16. Consider a buck boost converter of input voltage Edc =14V. The duty cycle α=0.6 and

the switching frequency is 25kHZ. The inductance L=180μH and the filter capacitance

C=220μF. The average load current Io =1.5A. Compute the average output voltage and

peak current of the device.

17. A single phase full bridge inverter has a resistive load of R=3Ω and the DC input voltage

is Edc =50V. Compute the RMS output voltage at the fundamental frequency, the output

power, the average and peak currents of each thyristor and peak reverse blocking voltage

of each thyristor.

18. Analyze the principle of working of buck-boost converter with time ratio and current

limit control. Draw the circuit and necessary waveforms.

UNIT IV : ANALISIS OF WIND AND PV SYSTEMS

PART-A

1. What is meant by fault ride through capability?

LVRT (Low Voltage Ride Through – also known as FRT - Fault Ride Through)

has become a crucial feature of the wind turbine control system.

The LVRT-term is capturing the ability of a wind turbine to stay connected to the

grid throughout a short mains voltage drop (a brownout) or a mains failure (a

blackout).

When the voltage of the grid is dropping it is essential that a wind park stay online

in order to prevent major blackouts. It is not only essential that the park stays

online - it is equally essential that the park is working actively to compensate for

the faulty grid condition.

Similar requirements for critical loads such as computer systems and industrial

processes are often handled through the use of an UPS or capacitor bank to supply

make-up power during these events.

2. What is meant by energy payback period?

It is defined as the time to produce the amount of energy required of production and

installation, a wind turbine with a working life of 20 years will offer a net benefit within five to

eight months of being brought online.

3. What will happen if no load is connected to a solar pv system?

If no load is connected, the electric circuit is incomplete/open. Thus there is no transfer of

electrons through the circuit thus no diffusion of charges inside the PV system, as no current is

produced in the external circuit is seen. Thus no power is produced in the circuit. There will only

be heating of PV panel by convection & radiation.

4. Draw the basic block diagram of WECS.

5. Why pitch angle control is used for WECS?

When a generator reaches rated power, the turbines must limit the mechanical power

delivered to the generator. This is valid because the generator reaches the rated power at for

instance 15 m/s while the maximum speed is typically 25 m/s for a wind turbine.

The pitch angle is controlled to keep the generator power at rated power by reducing the

angle of the blades. By changing the angle of the blade, the pitch control changes the impact of

the wind on the rotor in such a way that it remains at the desired number of revolutions.

6. What is stand-alone operation of fixed speed WECS?

Fixed-speed WECS operate at constant speed. That means that, regardless of

the wind speed, the wind turbine rotor speed is fixed and determined by the

grid frequency. Fixed-speed WECS are typically equipped with squirrel-cage

induction generators (SCIG), soft starter and capacitor bank and they are

connected directly to the load.

Especially in areas with relatively weak grids this system is employed.

The advantages of this scheme are that mechanical oscillations in the drive

train are absent as it is in fixed speed systems.

7. What are the advantages of stand-alone operation of WECS?

Reliability

Low maintenance

High efficiency

8. Differentiate between fixed and variable speed wind energy conversion systems.

APRIL/MAY 2017

Fixed speed WECS: Fixed speed systems are the simplest and most widely used

arrangement. They operate at constant speeds. It is also called as constant speed constant

frequency mode of operation.

Variable speed WECS: The Variable -speed system therefore incorporates a generator

control system that can operate with variable speed. In this arrangement the Variable voltage

Variable frequency (VVVF) power generated by the machine is converter to fixed frequency and

fixed voltage power by the use of back to back power converter.

9. List the merits of variable speed wind energy conversion system.

Variable speed wind energy systems have several advantages compared with fixed speed

wind energy systems. Such as:

Yielding maximum power output, developing low amount of mechanical stress,

improving efficiency and power quality. Power electronics devices with a variable speed system

are very important, where AC–DC converter is used to convert AC voltage with variable

amplitude and frequency at the generator side to DC voltage at the DC-link voltage. The DC

voltage is converted again to AC voltage with constant amplitude and frequency at the load side

for electrical utilization.

10. Draw the schematic diagram of grid integrated SCIG based WECS.

11. What are the issues created in grid integrated PMSG based WECS?

Larger, more expensive converter

The losses in the converter are higher because all power is processed by the converter.

Decreasing in the efficiency of the system

12. Draw the schematic diagram of grid integrated PMSG based WECS.

13. Draw the I-V characteristics of solar cell.

14. Draw the schematic diagram of grid integrated solar system.

15. What are the grid connected issues?

Technical issues:

i. Power quality

a. Harmonics

b. Frequency and voltage fluctuation

ii. Power fluctuation

a. Small time Power fluctuation

b. Long time Power fluctuation

iii. Storage

iv. Protection issues

v. Optimal placement of RES

vi. Islanding

Non- Technical issues:

i. Due to scarcity of technical skilled workers

ii. Less availability of transmission line to accommodate RES

PART-B

1. Explain the stand alone operation of fixed speed wind energy conversion system.

2. Explain the stand alone operation of variable speed wind energy conversion system. Explain

the functions of components used. Mention the merits and demerits of variable speed wind

energy conversion.

3. Explain the operation of grid integrated SCIG based wind energy conversion system with a

nest the block diagram.

4. What is the need for grid integration of variable speed wind energy conversion system?

5. Explain the operation of grid integrated PMSG system with a neat block diagram.

6. Explain with the help of a neat block diagram the functions of various blocks of a WECS.

7. Discuss in detail the grid integrated solar system and stand alone operation of solar energy

conversion system with neat diagram. Discuss how a photovoltaic system is integrated with

utility grid.

8. Discuss in detail the grid system characteristics and explain with a neat diagram the stand

alone and grid integrated solar system.

9. Explain the circuit model of grid integrated solar system. Also explain how the insolation and

temperature affects the I-V characteristics of a solar cell.

10. Explain about various grid connection issues and its impact on system stability.

UNIT – V HYBRID RENEWABLE ENERGY SYSTEMS

PART-A

1. What is the need for maximum power point tracking?

The major principle of MPPT is to extract the maximum available power from PV

module by making them operate at the most efficient voltage (maximum power point).

MPPT checks output of PV module, compares it to battery voltage then fixes what is the

best power that PV module can produce to charge the battery and converts it to the best

voltage to get maximum current into battery. It can also supply power to DC load, which

is connected directly to the battery. MPPT varies the electrical operating point of the

modules and enables them to deliver maximum available power.

2. What is the need for hybrid energy systems?

There are severe power shortages and associated power quality problems.

Load shedding in many cities due to power shortage and faults is a major problem

for which there is no immediate remedy. Since the gap between the power

demand and supply is increasing every year. To meet the demand and for the sake

of continuity of power supply.

In addition to the technical considerations, cost benefit is a factor that has to be

incorporated into the process of optimizing a hybrid energy system.

3. What are the benefits of the hybrid energy systems? APRIL/MAY 2017

It provides continuous of power supply and meets the demand.

Hybrid energy systems (renewable coupled with conventional energy source) can

significantly reduce the total life cycle cost of a standalone power supplies in

many off-grid situations

Provides reliable supply of electricity using a combination of energy sources

4. What is the range of hybrid systems?

According to the connection to the distribution grid:

Grid connected- they must be synchronized with the distribution system

Stand alone - used for the supply of remote objects

According to the construction:

Serial structure of hybrid systems

Parallel structure hybrid systems

According to the presence of conventional sources:

Hybrid systems with conventional energy sources

Hybrid systems without conventional energy sources

5. List various types of hybrid energy systems. (write some of the hybrid systems used in

industry)

PV-Diesel

Wind/PV Hybrid System

PV/Hydro Hybrid System

PV-Wind-diesel

Wind-Diesel

Biomass-PV-Diesel Hybrid System

PV/Solar thermal/grid-connected hybrid System

6. Write the merits of wind-diesel hybrid system.

A wind-diesel hybrid system combines diesel generators and wind turbines. Wind cannot

produce continuous supply throughout the year, whenever there is a lag in the wind

energy the insufficient supply is managed by the diesel generator. As a result the

continuous supply is obtained.

9. What are hybrid systems?

Hybrid power system is used to describe any power system combine two or more energy

conversion devices, or two or more fuels for the same device, that when integrated, overcome

limitations inherent in either. Usually one of the energy sources is a conventional one (which

necessarily does not depend on renewable energy resource) powered by a diesel engine, while

the other(s) would be renewable viz. solar photovoltaic, wind or hydro.

10. What are the merits of Hybrid RES over the isolated RES?

Utilization of the renewable energy sources is compulsory to minimize the energy crisis

around the world. Based on the alternative renewable energy sources, a number of

power generation systems have been designed to contribute for the management of

optimized energy supply systems.

The unpredictable nature of alternative energy sources is their common drawback, and

they do not harness usable power for some considerable duration of time over the

year. Therefore, trends are in progress to integrate two or more than two of these

alternative energy systems to meet the required electricity supply in

power generation operations. These systems are designated as hybrid systems and can be

grid connected or off-grid depending upon their purpose and mode of fabrication.

11. What are the applications of solar PV system?

Stand-Alone Systems

Lighting

Photovoltaic thermal hybrid solar collector

Water Pumping

Charging Vehicle Batteries

Refrigeration

Consumer Products

Public utilities

Communications and spacecraft applications

Electricity for remote areas

Disaster Relief

Signal Systems

12. Draw the schematic diagram of grid interactive solar PV system.

13. List various types of MPPT.

a. Perturb and Observe Algorithm

b. Incremental Conductance method

c. Voltage Tracking Method

d. Current Tracking Method

Other methods:

• Fuzzy Logic Method

• Neural Network Method

• Load voltage /current maximization

• Parasitic Capacitance method.

14. Give the merits and demerits of MPPT.

Advantages:

Improved Efficiency

Improved reliability

Sustained system, no frequent collapse of PV power.

Increased running time.

Disadvantages:

Dependency on specialized and accurate sensors (Voltage and/or Current sensors)

15. Enumerate the importance of MPPT in the operation of a photo voltaic system.

APRIL/MAY 2017

Solar irradiation that hits the photovoltaic modules has a variable character depending on

the latitude, orientation of the solar field, the season and hour of the day. During the course of a

day, a shadow may be cast on the cell that may be foreseen, as in the case of a building near the

solar field or unforeseeable as those created by clouds. Also the energy produced by each

photovoltaic cell depends on the irradiation and temperature. From these considerations, the

necessity to identify instant by instant that particular point on the V x I characteristic of the PV

generator in which there is the maximum amount of power transfer to the grid occurs.

16. What is MPPT?

Maximum Power Point Tracking, referred to as MPPT, is an electronic system that

operates the photovoltaic modules to produce maximum power. MPPT varies the electrical

operating point of the modules and enables them to deliver maximum available power.

Common Algorithms

• Voltage Tracking Method

• Current Tracking Method

• Perturb & Observe. (P&O).

• Incremental Conductance Method.

17. What are the types of pumps used for solar water pumping applications?

Submersible Diaphragm Pumps

Submersible Centrifugal Pumps

Submersible Helical Rotor Pumps

Surface Diaphragm Pumps

Surface Centrifugal Pumps

Floating Centrifugal Pumps

18. Draw the schematic diagram of PV-Diesel hybrid system.

16. What are the advantages of PV-Diesel hybrid system?

Improved reliability

Improved energy services

Reduced emissions and noise pollution

Continuous power

Increased operational life

Reduced cost

Efficient use of energy

PART-B 1. Show power electronic system used for hybrid solar PV and wind energy system and

explain its operation. Discuss the technical challenges associated in it.

2. Explain the various types of MPPT techniques in detail.

3. Is wind energy an excellent supplement to the PV? If so justify with a suitable case study.

4. What is MPPT? Discuss the types of MPPT with its merits and demerits. Explain the

incremental-conductance algorithm with a neat example.

5. Enumerate the importance of MPPT in the operation of a photo voltaic system.

6. Explain with case study how to get maximum power generation in wind energy

conversion system.

7. (i) Explain the need of hybrid system for renewable energy power generation.

(ii) With a neat schematic, explain the integration of Wind-PV hybrid system.

(iii) Also explain the issues and challenges in the operation of hybrid system.

8. With a neat sketch, explain the operation of various types of hybrid systems.

9. Derive an expression for the total cost of a hybrid system and three from deduce a simple

condition for the feasibility of the system.

10. Explain MPPT techniques for WECS.

11. Design a hybrid PV system which should be able to supply the power to the load of your

college foe 24 hours. It should be using solar, diesel and wind as the source of energy.

Explain with neat block diagram.