INTRODUCTION TO WIND ENERGY SYSTEM

CHAPTER-1

1.1.INTRODUCTION

The wind is a free, clean, and inexhaustible energy source. It has served

mankind well for many centuries by propelling ships and driving wind turbines to

grind grain and pump water. Interest in wind power lagged, however, when cheap and

plentiful petroleum products became available after World War II. The high capital

costs and the uncertainty of the wind placed wind power at an economic disadvantage.

Then in 1973, the Arab nations placed an embargo on petroleum. The days of cheap

and plentiful petroleum were drawing to an end. People began to realize that the

world’s oil supplies would not last forever and that remaining supplies should be

conserved for the petrochemical industry. The use of oil as a boiler fuel, for example,

would have to be eliminated. Other energy sources besides oil and natural gas must be

developed.

The two energy sources besides petroleum which have been assumed able to

supply the long term energy needs of the United States are coal and nuclear energy.

Many people think there is enough coal for several centuries at present rates of

consumption, and likewise for nuclear energy after the breeder reactor is fully

developed. These are proven resources in the sense that the technology is highly

developed, and large coal and nuclear powered electrical generating plants are in

operation and are delivering substantial blocks of energy to the consumer.

Unfortunately, both coal and nuclear present serious environmental problems.

Coal requires large scale mining operations, leaving land that is difficult or

impossible to restore to usefulness in many cases. The combustion of coal may upset

the planet’s heat balance. The production of carbon dioxide and sulfur dioxide may

affect the atmosphere and the ability of the planet to produce food for its people. Coal

is also a valuable petrochemical feedstock and many consider the burning of it as a

boiler fuel to be foolish.

Nuclear energy has several advantages over coal in that no carbon dioxide or

sulfur dioxide are produced, mining operations are smaller scale, and it has no other

major use besides supplying heat. The major difficulty is the problem of waste

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disposal, which, because of the fears of many, will probably never have a truly

satisfying solution.

Because of these problems, wind power and other forms of solar power are

being strongly encouraged. Wind power may become a major source of energy in

spite of slightly higher costs than coal or nuclear power because of the basically non-

economic or political problems of coal and nuclear power. This is not to say that wind

power will always be more expensive than coal or nuclear power, because

considerable progress is being made in making wind power less expensive. But even

without a clear cost advantage, wind power may become truly important in the world

energy picture.

1.2.WIND TURBINES

A wind turbine is a rotating machine which converts the kinetic energy in

wind into mechanical energy. If the mechanical energy is then converted to electricity,

the machine is called a wind generator, wind turbine, wind power unit (WPU), wind

energy converter (WEC), or aero generator. Wind turbines can be separated into two

types based by the axis in which the turbine rotates. Turbines that rotate around a

horizontal axis are more common. Vertical-axis turbines are less frequently used.

1.2.1.HORIZONTAL AXIS WIND TURBINES

The Federal Wind Energy Program had its beginning in 1972 when a joint

Solar Energy Panel of the National Science Foundation (NSF) and the National

Aeronautics and Space Administration (NASA) recommended that wind energy be

developed to broaden the Nation’s energy options for new energy sources.[9]

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and

electrical generator at the top of a tower, and must be pointed into the wind. Most

have a gearbox, which turns the slow rotation of the blades into a quicker rotation that

is more suitable to drive an electrical generator. Since a tower produces turbulence

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behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made

stiff to prevent the blades from being pushed into the tower by high winds.

Additionally, the blades are placed a considerable distance in front of the tower and

are sometimes tilted up a small amount.

Horizontal axis wind turbine

Downwind machines have been built, despite the problem of turbulence,

because they don't need an additional mechanism for keeping them in line with the

wind, and because in high winds the 3 blades can be allowed to bend which reduces

their swept area and thus their wind resistance. Since cyclic (that is repetitive)

turbulence may lead to fatigue failures most HAWTs are upwind machines.

HAWT ADVANTAGES

• Variable blade pitch, which gives the turbine blades the optimum angle of attack.

Allowing the angle of attack to be remotely adjusted gives greater control, so the

turbine collects the maximum amount of wind energy for the time of day and season.

• The tall tower base allows access to stronger wind in sites with wind shear. In some

wind shear sites, every ten meters up, the wind speed can increase by 20% and the

power output by 34%.

• High efficiency, since the blades always move perpendicularly to the wind,

receiving power through the whole rotation. In contrast, all vertical axis wind

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turbines, and most proposed airborne wind turbine designs, involve various types of

reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part

of the cycle. Backtracking against the wind leads to inherently lower efficiency.

HAWT DISADVANTAGES

• The tall towers and blades up to 90 meters long are difficult to transport.

Transportation can now cost 20% of equipment costs.

• Tall HAWTs are difficult to install, needing very tall and expensive cranes and

skilled operators.

• Massive tower construction is required to support the heavy blades, gearbox, and

generator

• Reflections from tall HAWTs may affect side lobes of radar installations creating

signal clutter, although filtering can suppress it.

• Downwind variants suffer from fatigue and structural failure caused by turbulence

when a blade passes through the tower's wind shadow (for this reason, the majority of

HAWTs use an upwind design, with the rotor facing the wind in front of the tower).

• HAWTs require an additional yaw control mechanism to turn the blades toward the

wind.

1.2.2.VERTICAL AXIS WIND TURBINES

Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged

vertically. Key advantages of this arrangement are that the turbine does not need to be

pointed into the wind to be effective. This is an advantage on sites where the wind

direction is highly variable. VAWTs can utilize winds from varying directions. With a

vertical axis, the generator and gearbox can be placed near the ground, so the tower

doesn't need to support it, and it is more accessible for maintenance. Drawbacks are

that some designs produce pulsating torque. Drag may be created when the blade

rotates into the wind.

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VAWT ADVANTAGES

• A massive tower structure is less frequently used, as VAWTs are more frequently

mounted with the lower bearing mounted near the ground.

• Designs without yaw mechanisms are possible with fixed pitch rotor designs.

• A VAWT can be located nearer the ground, making it easier to maintain the moving

parts.

• VAWTs have lower wind startup speeds than HAWTs. Typically, they start creating

electricity at 6 M.P.H. (10 km/h).

• VAWTs may have a lower noise signature.

VAWT DISADVANTAGES

• Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part

because of the additional drag that they have as their blades rotate into the wind.

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• While VAWTs' parts are located on the ground, they are also located under the

weight of the structure above it, which can make changing out parts nearly impossible

without dismantling the structure if not designed properly.

• Having rotors located close to the ground where wind speeds are lower due to wind

shear, VAWTs may not produce as much energy at a given site as a HAWT with the

same footprint or height.

• Because VAWTs are not commonly deployed due mainly to the serious

disadvantages mentioned above, they appear novel to those not familiar with the wind

industry. This has often made them the subject of wild claims and investment scams

over the last 50 years.

1.3.CONTROL OF WIND TURBINES

1.3.1.POWER CONTROL

A wind turbine is designed to produce a maximum of power at wide spectrum of wind

speeds. The wind turbines have three modes of operation:

• Below rated wind speed operation

• Around rated wind speed operation

• Above rated wind speed operation

If the rated wind speed is exceeded the power has to be limited. There are various

ways to achieve this.

1.3.2.STALL CONTROL

Stalling works by increasing the angle at which the relative wind strikes the blades (angle of attack), and it reduces the induced drag (drag associated with lift). Stalling is simple because it can be made to happen passively (it increases automatically when the winds speed up), but it increases the cross-section of the blade face-

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on to the wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind.

A fixed-speed HAWT inherently increases its angle of attack at higher wind speed as the blades speed up. A natural strategy, then, is to allow the blade to stall when the wind speed increases. This technique was successfully used on many early HAWTs. However, on some of these blade sets, it was observed that the degree of blade pitch tended to increase audible noise levels.

1.3.3.PITCH CONTROL

Furling works by decreasing the angle of attack, which reduces the induced drag from the lift of the rotor, as well as the cross-section. One major problem in designing wind turbines is getting the blades to stall or furl quickly enough should a gust of wind cause sudden acceleration. A fully furled turbine blade, when stopped, has the edge of the blade facing into the wind.

Standard modern turbines all pitch the blades in high winds. Since pitching requires acting against the torque on the blade, it requires some form of pitch angle control. Many turbines use hydraulic systems. These systems are usually spring loaded, so that if hydraulic power fails, the blades automatically furl. Other turbines use an electric servomotor for every rotor blade. They have a small battery-reserve in case of an electric-grid breakdown. Small wind turbines (under 50 kW) with variable-pitching generally use systems operated by centrifugal force, either by flyweights or geometric design, and employ no electric or hydraulic controls.1.3.4.OTHER CONTROLS

Yawing

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Modern large wind turbines are typically actively controlled to face the wind direction measured by a wind vane situated on the back of the nacelle. By minimizing the yaw angle (the misalignment between wind and turbine pointing direction), the power output is maximized and non-symmetrical loads minimized. However, since the wind direction varies quickly the turbine will not strictly follow the direction and will have a small yaw angle on average. The power output losses can simplified be approximated to fall with cos3(yaw angle).

Electrical braking

Braking of a small wind turbine can also be done by dumping energy from the generator into a resistor bank, converting the kinetic energy of the turbine rotation into heat. This method is useful if the kinetic load on the generator is suddenly reduced or is too small to keep the turbine speed within its allowed limit.

Cyclically braking causes the blades to slow down, which increases the stalling effect, reducing the efficiency of the blades. This way, the turbine's rotation can be kept at a safe speed in faster winds while maintaining (nominal) power output. This method is usually not applied on large grid-connected wind turbines.

Mechanical braking

A mechanical drum brake or disk brake is used to hold the turbine at rest for maintenance. Suchbrakes are usually applied only after blade furling and electromagnetic braking have reduced the turbine speed, as the mechanical brakes would wear quickly if used to stop the turbine from full speed. There can also be a stick brake.

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1.4.GENERATORS

A generator is a device which converts mechanical energy into electrical energy. Wind generators have traditionally been wind turbines, i.e. a propeller attached to an electric generator attached to appropriate electronics to attach it to the electrical grid. Generators can be classified broadly into two categories:

a) Synchronous Generatorsb) Asynchronous Generators

The basis of this categorization is the speed at which the generators are run. Synchronous generators are run at synchronous speed (1500 rpm for a 4 pole machine at 50Hz frequency) while asynchronous generators run at a speed more than the synchronous speed.

1.4.1.SYNCHRONOUS GENERATOR

Synchronous generators are doubly fed machines which generate electricity by the principle when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

1.4.2.ASYNCHRONOUS GENERATOR

Asynchronous generators or Induction generators are singly excited a.c. machine. Its stator winding is directly connected to the

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ac source whereas its rotor winding receives its energy from stator by means of induction. Balanced currents produce constant amplitude rotating mmf wave. The stator produced mmf and rotor produced mmf wave, both rotate in the air gap in the same direction at synchronous speed. These two mmf s combine to give the resultant air-gap flux density wave of constant amplitude and rotating at synchronous speed. This flux induces currents in the rotor and an electromagnetic torque is produced which rotates the rotor.

Asynchronous generators are mostly used as wind turbines as they can be operated at variable speed unlike synchronous generator. Two kinds of asynchronous generators are used namely

a) Squirrel cage induction generator (SCIG)b) Doubly fed induction generator (DFIG)

1.4.3.SQUIRREL CAGE INDUCTION GENERATOR

A squirrel cage rotor is the rotating part. In overall shape it is a cylinder

mounted on a shaft. Internally it contains longitudinal conductive bars (usually made

of aluminum or copper) set into grooves and connected together at both ends by

shorting rings forming a cage-like shape. The core of the rotor is built of a stack of

iron laminations. The field windings in the stator of an induction motor set up a

rotating magnetic field around the rotor. The relative motion between this field and

the rotation of the rotor induces electric current in the conductive bars. In turn these

currents lengthwise in the conductors react with the magnetic field of the motor to

produce force acting at a tangent to the rotor, resulting in torque to turn the shaft. In

effect the rotor is carried around with the magnetic field but at a slightly slower rate of

rotation. The difference in speed is called slip and increases with load.

1.4.4.DOUBLY FED INDUCTION GENERATOR

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DFIG is 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 access to the rotor windings.

1.5.PRINCIPLE OF A DOUBLE FED INDUCTION GENERATOR

CONNECTED TO A WIND TURBINE

The principle of the DFIG is that rotor windings are connected to the grid via slip rings and back to- back voltage source converter that controls both the rotor and the grid currents. Thus rotor frequency can freely differ from the grid frequency (50 or 60 Hz).

By controlling the rotor currents by the converter it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generators turning speed. The control principle used is either the two-axis current vector control or direct torque control (DTC). DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator

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INTRODUCTION TO WIND-DIESEL HYBRID SYSTEM

The doubly-fed generator rotors are typically wound with from 2 to 3 times

the number of turns of the stator. This means that the rotor voltages will be higher and

currents respectively lower. Thus in the typical ± 30 % operational speed range

around the synchronous speed the rated current of the converter is accordingly lower

leading to a low cost of the converter.

CHAPTER-2

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2.1.INTRODUCTION

A Wind Diesel Hybrid System (WDHS) is any autonomous electricity

generating system using Wind Turbine Generators(s) (WTG)with Diesel Generator(s)

(DG) to obtain a maximum contribution by the intermittent wind resource to the total

produced power, while providing continuous high quality electric power [1]. The

main goal with these systems is to reduce fuel consumption and in this way to reduce

system operating costs and environmental impact. If the WDHS is capable of shutting

down the Diesel Generators during periods of high wind availability, the WDHS is

classified as high wind penetration. High penetration (HP) WDHS have three

operation modes: Diesel Only (DO), Wind Diesel (WD) and Wind Only (WO)

In DO mode the Diesel Generators supply the active and reactive power

demanded by the consumer load (WTGs are disconnected). In WD mode, in addition

to DG(s), WTG(s) also supply active power. In WO mode the Diesel Generators are

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not running, only the wind turbines are supplying active power, so that no fuel is

consumed in this mode. Several papers have been published on the subject of WDHS

dynamic simulation.

In the interaction between one DG and a constant/ variable speed WTG is

studied. In a no-storage WDHS is simulated against several perturbations, among

them the connection of a WTG to the DG isolated grid (DO to WD transition). In a

previous work [5] a HP-WDHS with a BESS is simulated in WO mode, but the

battery is modeled by a simple constant voltage source. In [6] the modeled HP-WDHS

has a DG with a locked-disengaged simplified clutch model and it is simulated the

mandatory transition from WO to WD when the active power generated is less than

consumed.

During this type of WO to WD transition the power system is without control

until the DE is added to the system. In the present article the WO mode is also

simulated, but a more elaborated model for a Ni-Cd battery is used and the main

battery variables: current, voltage and state of charge are presented during the

simulation. Additionally, in the present article a more realistic clutch model is also

used to transition from WO to WD, but in this case the transition simulated is

controlled and it is done in order to substitute a supplying BESS by the DE.

The different isolated hybrid power systems considered in the paper are multi-

wind/single diesel, and single wind/multi- diesel. Multi-wind/single diesel systems

attenuate the effect of power fluctuations produced by the turbulence of the wind. In

fact, it is estimated that the variability of the power should decrease by the square root

of the number of wind turbines[1]. Thus the need for short-term storage would

decrease when the wind generation capacity is made up of more than one machine.

Single wind/ multi-diesel systems allow a variety of possible operation and control

strategies.

Typically the most efficient diesel generators are allowed to run at their rated

output. Load fluctuations are supplied by one of the less efficient diesel generators,

which are allowed to cycle up and down. Diesel grid of Block Island, USA [1] is such

type of an example. First the system state equations are derived from real and reactive

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power balance equations of the system. A voltage deviation signal is used by

STATCOM controller to eliminate the reactive power mismatch in the system. Also

the voltage deviation signal is used by excitation system of the synchronous generator

to eliminate the voltage deviation.

2.2.OVERVIEW OF WIND DIESEL HYBRID SYSTEM

Given the significant economic (and environmental) incentives to add wind

energy to remote diesel generator systems. the technical aspects must also be

addressed. The peak demand loads of remote communities in Canada typically nm

from 45 kW to 6.7 MW .The installed capacity in the generation plant. typically

comprising three to five diesel generation. is usually large enough (approximately

twice the peak dernand) to allow for future demand load growth, and to accommodate

at least one machine king out of service. in moa modem installations, the sizes of the

diesel generator are staggered such that the appropriately sized generator(s) can be

engaged to most efficiently meet the demand load at that the. Diesel engine

manufacturers advise a minimum operating load of approximately 20 to 40 per cent in

order to maintain high fuel efficiency and to avoid excessive fouling and Wear.

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[ FIGURE 1: GENERAL WIND DIESEL SYSTEM]

Wind is a random and highly variable source of energy. The scales of wind

variation range term to long term short seasonal to hourly fluctuation. down to short-

term fluctuation in the order of minutes to seconds due to local turbulence (gusts).

Long-term wind trends cm be for- from weather patterns and disperse monitoring

stations.

Demand loads also exhibit short-term and long-term frequency trends. but

unlike the wind they are relatively easy to predict (both in frequency and amplitude)

since they follow the community's habits. With the knowledge of the demand pattern

throughout the &y, the load can be anti cipateci, and the appropriate diesel generation

capacity can be brought on line to provide a disable and optimally efficient supply.

With a significant mount of wind energy in the power mix, it is the short-term

fluctuation of the wind that is the greatest impediment to wind-diesel integration.

Diesel generators respond rapidly to load variations in order to keep the system's

frequency and voltage within an acceptable range. This makes them well suited for

operation with wind energy while the wind power penetration remains low.

With higher levels of penetration. additional controls will be needed to

maintain the stability and power quality of the system. A general wind-diesel system

is shown in Figure 1 The initial diesel system comprises the diesel generators

supplying the village load. With the addition of the wind turbines, a dump load will

most likely be needed to absorb the excess energy whenever the generated wind

power exceeds the darned. A clutch may be added between the engines and the

generators of some, or ail, of the diesel generator sets to allow the synchronous

generators to operate as synchronous condensers when it is desirable to nin

exclusively from wind power. In this system the synchronous generators regulate the

grid voltage and provide

2.3. THE ISOLATED POWER SYSTEM (WIND -DIESEL HYBRID SYSTAM ARCHITECTURE)

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The high penetration WDHS of Fig. 2 comprises one DG and one WTG. The

DG consists of a Diesel Engine (DE), a Synchronous Machine (SM), and a friction

clutch. The SM generates the voltage waveform of the isolated grid and its automatic

voltage regulator controls the system voltage to be within the prescribed levels during

the three modes of operation. For this reason the SM must be always running close to

its rated speed. The DE provides mechanical power to the SM and its speed governor

(speed regulator+ actuator) controls the DE speed. In this article the DE speed control

is isochronous, so the diesel speed governor will command the necessary fuelling rate

to make the DE run at constant speed.

The DE is needed to supply active power and regulate the system frequency in

the DO and WD modes. The clutch has three states: engaged, locked and disengaged .

If the clutch is disengaged, the frictional surfaces are not in contact and no torque is

transferred from the DE to the SM, so that if CT is closed the operation mode is WO.

In WO mode since the DE and SM axes are independent, the DE must not be running

in order to save fuel, but in this paper it will run at slow speed as it is explained later

on. With the clutch engaged, the frictional surfaces slip past one another and kinetic

friction torque is transferred to the SM. Finally, if the clutch is locked, the frictional

surfaces are locked together without slipping and static friction torque is transferred to

the SM. With the clutch locked the DE and SM turn at the same speed and the WDHS

is in the DO/WD mode if the WTG circuit breaker CT is opened/closed respectively.

Several real HP-WDHS include a clutch to transition from WO to WD modes and

vice versa

The WTG consists of a Wind Turbine (WT) driving an Induction Generator

(IG) directly connected to the autonomous grid conforming a constant speed stall-

controlled WTG (no pitch control). The WTG produced active power PT depends

among other factors on the cube of the wind speed and since the WT used has no

pitch control, there is no way to control the WTG active power, so it behaves as an

uncontrolled source of active power. The IG consumes reactive power so a capacitor

bank has been added to compensate the power factor. The Dump Load (DL) consists

of a set of semiconductor power switches and a binary bank of resistors. By

closing/opening these power switches, the dl consumed active power can be

controlled behaving as a controlled sink of active power. the battery based energy

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storage system (bess) consists of a battery bank and a power converter that interfaces

the battery bank to the autonomous grid. the bess can store or retrieve power as

needed, so it behaves as a controlled sink/source of active power.

The system frequency is regulated by maintaining an instantaneous balance of

the active power consumed and produced. in do and wd modes the de speed governor

modulates the de active power in order to accomplish this balance, so the de behaves

as a controlled source of active power. in wo mode the clutch is disengaged and the

active power consumed by the load (pl) is produced only by the wtg (pt).

Since pt (also called wind power) and pl are uncontrolled the DL + BESS

must perform the instantaneous balance of the active power. being pd the power

consumed by the dl, ps the power consumed/supplied by the BESS, j the SM inertia

and x the SM shaft speed, the power equation of the SM in wo mode if no losses are

taking into account is

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[ Fig. 2. Layout of the isolated WDHS ]

Where PT is considered positive if produced and PL, PD and PS are

considered positive if consumed. In Eq. (1) the SM shaft speed x is in rad/s ant it is

related with the system frequency (frequency of the voltage waveform) f by x = 2pf/p,

with p the number of pole pairs of SM. Eq. (2) shows that to obtain a synchronous

shaft speed constant (dx/dt = 0), the DL + BESS combination must consume power

when PT exceeds PL and the BESS must generate power when PT is less than PL

The situation where the BESS supplies power is temporary, so that if this

situation persists the control system of the WDHS must order to start the DE and

when the speed difference between DE and SM is small enough engaged the clutch,

changing to the WD mode. With the clutch locked the DE will supply the necessary

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active power PDE to keep the system frequency at rated value. Finally to obtain Eq.

(1) in WD mode, PDE (positive when produced)must be added to the left side of (1)

and J must be the sum of the DE + SM inertia

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THE CONTROL MECHANISM OF WIND-DIESEL HYBRID SYSTEM

CHAPTER-3

3.1. OPERATING STATES OF WIND DIESEL HYBRID

SYSTEM There are three devices subject to the direct control of the wind-diesel

controller: the rotary converter AC machine, the rotary converter DC machine, and

the secondary load controller (which actually consists of multiple distributed load

controllers). Each of these devices has several different control modes associated with

it. For example, the AC machine can be controlled to achieve any of the following:

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· Match voltage with the AC bus (prior to synchronization)

· Share reactive power with the diesel generators

· Deliver a specified amount of reactive power to the grid

· Regulate AC bus voltage

The power flow management algorithm determines the appropriate control

mode for each of these three devices depending on the operating state of the power

system.

The Wales wind-diesel hybrid power system involves multiple diesels and

multiple wind turbines. In addition there is a power converter consisting of two

separate rotating machines and a secondary load that is divided into “local dump

load” and “remote dump load”. Because each of these components may or voltage

source inverter, the frequency is typically set by a crystal oscillator and does not vary.

However, a similar situation exists in that any power imbalance then typically shows

up as an increase or decrease in voltage on the AC and/or DC side of the inverter. The

problem then becomes one of voltage control rather than frequency control.

To develop a power flow management algorithm flexible enough to handle all

possible operating states, one must identify a minimum set of key state variables that

provide sufficient information to determine the appropriate control mode for each

device.

Our top level state variable is the diesel status, because it has the biggest

impact on how voltage and frequency is regulated. “Diesel ON” refers to the state

where one or more diesel generators is connected to the bus and loaded (i.e. not in

load or unload ramp). Conversely, “Diesel OFF” refers to the state in which all diesel

generators are either disconnected from the bus or connected but not fully loaded. The

other state is “Diesel Wind” state where both the diesel and wind turbine gives the

required load demand.

3.1.1.DIESEL ON STATE

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The stand-alone diesel generator is designed to regulate the voltage and

frequency on an isolated power bus. In a multiple diesel configuration equipped with

automatic load sharing controls, the diesels collectively regulate frequency and share

both the real and reactive power load in proportion to their respective ratings. Diesel

gensets do an excellent job of frequency and voltage control provided that the real and

reactive power load on them remains within their rated capacity and they are not

subject to large reverse power transients. In the Diesel ON state, we allow the diesel

generator(s) to perform the i intended function of frequency and voltage control, and

we control the rotary converter and/or secondary loads to maintain the diesel loading

in a comfortable range. In summary, in Diesel ON state,

· The diesel generator(s) assume both frequency and voltage control

· Power flow to the secondary loads and/or energy storage is controlled to maintain

diesel loading within a comfortable range

· The rotary converter ac machine is used to assist the diesel generators in meeting the

var load, as necessary.

3.1.2.DIESEL OFF STATE

In the Diesel OFF state, the only synchronous machine left on the system is

the AC machine of the rotary converter. The rotational speed of the rotary converter

will establish the grid frequency. As with the diesel generator, the voltage regulator

on the rotary converter AC machine controls the field current so as to maintain the

desired AC bus voltage. Frequency is controlled by modulating power flow to the

secondary load or battery, depending on factors to be discussed below.

Is there instantaneous excess wind power?

In the case where there is excess wind power, secondary (or “dump”) load

may be used to provide frequency control. As long as there is excess wind

power, this works fine, but suppose the wind suddenly drops, resulting in a

power deficit. As wind power drops, secondary load will be rapidly removed

in an attempt to maintain grid frequency. Once it has all been removed, the

ability to control frequency is lost. The system must switch immediately to

frequency control by the DC machine.

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Is the battery “full”?

This question refers to whether the present level of current into the battery can

be sustained. It is actually several questions rolled into one. With a “yes”

answer to any one of them, the battery is considered “full”.

Is the battery at a high state of charge (i.e., actually full)?

3.2.WIND-DIESEL ELECTRICAL NETWORK

A wind-diesel electrical network in its simplest form comprises a single

electrical bus, or node, to which all the generators and loads are connected A more

complex wind diesel electrical network comprises a grid of several buses

interconnected by impedance transmission lines or transformers.

For the purposes of this study the village and dump loads have been represented as

passive components. Thus, the loads can be incorporated into one algebraic mode1 to

represent the entire electrical network. The algebraic network equations can be solved

directly, thus an iterative solution to the network voltages and currents. commonly

found in other wind-diesel models, is avoided.

3.2.1.VILLAGE LOAD

In classical power system analysis, the demand load is represented by a

constant, passive impedance, hence the village load power is

The impedance of the village load, ZL , is assumed constant with respect to voltage

and frequency, thus PL - V' and QL a v2, and both PL and QL are independent of

frequency. This assumption is often on the pessimistic side.

3.2.2.DUMP LOAD

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At low levels of instantaneous wind power penetration wind turbine is seen as

a negative load. At this level the diesel generator maintains the network frequency

under fluctuating wind power and demand load. As the instantaneous wind power

penetration increases, the diesel generator loading will be reduced dom to its

minimum loading. Ideally, beyond this point the dump load, controlled by the dump

load controller. begins to dump excess power from the system to maintain the system

frequency and minimum diesel loading. Hence, the dump load must be variable and

able to absorb the maximum potential surplus power.

The dump load consists of eight three phase resistors connected in series with

GTO switches. The resistors values follow an 8 bit binary progression so that the

power consumed by the DL, provided that the voltage in the isolated grid is nominal,

can be expressed in the form:

Equation (5) means that the power can be varied discretely from 0 to 255.

PSTEP, where PSTEP is the power corresponding to the least significant bit and IJ is

‘‘1’’ when the associated GTO is turned on and ‘‘0’’ when the GTO is turned off. For

this article PSTEP = 1.4 Kw and then PD-NOM = 357 kW, which is a 30% greater than

PT-NOM, so that the WDHS can be controlled in WO mode even in the case of no

consumer load and BESS fully charged/failure.

3.3.THE CONTROL SYSTEM

Our proposal to control the DL and BESS in the presented power system is by

means of a Distributed Control System (DCS). A DCS comprises several CPU based

electronic control units (also called nodes) physically distributed and linked by a

communication network (also called communication bus). As it can be seen in Fig. ,

the presented DCS consists of three nodes: a SM and DE shaft speeds measurement

sensor node NW and two actuators nodes: the DL converter ND and the BESS

converter NS. The type of control that the sensor node NW applies depends on the

operation mode. As commented in the previous section, in WD mode the DE

26

performs the isochronous speed control, so NW calculates a PD frequency regulator

whose input is the frequency error (difference between the current frequency and the

power system nominal frequency 50/60 Hz) and whose output is the reference power

PREF needed to be absorbed (PREF > 0) by the DL + BESS combination or to be

supplied (PREF < 0) by the BESS to balance the active power of the system. In WO

mode, there is no DE controlling the system frequency, so to calculate PREF, the

sensor node NW applies a PID regulator to the frequency error to control the power

system frequency. The integral part of the PID eliminates the steady state frequency

error, performing an isochronous speed control. The node NW also calculates the

power sharing between DL and BESS when PREF > 0 by computing the reference

power to be dumped by DL PD-REF and the reference power to be stored/retrieved

PS-REF by BESS, so that:

(3) is simplified to PREF = PD-REF if battery is fully charged/failure and (4)

means that the DL can only consume power up to its rated power (PD-NOM) and

BESS can consume/supply power up to its rated power (PS-NOM). On the other

hand, the nodes of a DCS exchange information between them through message

passing. In order to coordinate DL and BESS actuators when PREF > 0, the sensor

node NW shall communicate with the message shown in Fig. 1 the current reference

powers PS-REF and PD-REF through the network to the actuator nodes ND and NS.

This message is periodic and guarantees that both actuators receive its reference

power at the same time.

3.4.FREQUENCY AND VOLTAGE CONTROL

A fundamental requirement of a wind-diesel system is that the addition of

wind power to the autonomous diesel generator system should not degrade the power

quality (frequency and voltage performance) of the system from that of the diesel

generator(s) operating alone. This is based on the prernise that the system was

27

originally an autonomous diesel generator system, and that the power quality of the

diesel generator(s) is satisfactory.

The diesel generator of the existing system will have been specified to meet

the requirements of the village load. In a HPNSWD system, the addition of wind

power to the system can greatly increase the active and reactive power fluctuations in

the system. This increased variability may require that the frequency and voltage

control systems of the diesel generator(s) be augmented In addition, to maximize

diesel fuel savings, the HPNSWD system must be able to operate solely from wind

power with the diesel engine(s) shut down. This mode of operation requires a

frequency control system independent of the diesel engine..

3.4.1.VOLTAGE CONTROL

The voltage of the network is most effectively regulated by controlling the

reactive power in the system. The AVR of the diesel generator regulates the network

voltage by controlling the amount of reactive power produced (or absorbed) by the

diesel's synchronous generator. The synchronous generator remains operational at all

times. When the diesel engine is off the synchronous generator functions as a

synchronous condenser.

A village load normally consists of many small loads. Therefore the

synchronous generator typically sees relatively small reactive and active power

fluctuations. The wind turbine, on the other hand, is an electric machine with a

capacity comparable to, if not larger than, the capacity of the synchronous generator.

The induction generator of the wind turbine also requires a large amount of reactive

power for excitation. The start-up and shutdown of the wind turbine cause sudden

changes in the reactive and active power in the system. In addition, both the reactive

and active power of the turbine can fluctuate widely in turbulent wind conditions.

These rapid power fluctuations can cause voltage variations that are detrimental to the

operation of voltage sensitive loads and/or result in objectionable disturbances to

lighting levels.

3.4.2.VOLTAGE REGULATION

28

Analogously, regulating the AC voltage of the power system is a problem of

maintaining an equilibrium between the source and sinks of reactive power (VARs) in

the system. The induction generators of the wind turbines, transformers in the

distribution system, and induction motors in the consumer load are all reactive power

sinks. Power factor correction capacitors on the wind turbines or the distribution

system are sources of reactive power. Synchronous generators, both on the diesel

gensets and on the rotary converter, can either be sources or sinks, but generally they

are supplying the reactive power demanded by the sinks.

Unlike the case of real power, where an imbalance can be absorbed by the

system as a change in stored kinetic energy, there is no storage mechanism for

“reactive energy”, which only actually exists as a mathematical construct. The

reactive power supplied by the sources is inherently equal to the reactive power

absorbed by the sinks. This is expressed in Equation 2, in which the reactive power

flows for each component are expressed as functions of voltage.

Where Q = reactive power (KVAR)

AC bus voltage

If the reactive power sources are unable to deliver the reactive power

demanded by the sinks, the bus voltage will fall such that the equilibrium is

maintained. With reactive power, the issue is not so much ensuring that equilibrium is

maintained (which is automatic), but that the equilibrium occurs at the desired voltage

level. On a synchronous machine, the function of the voltage regulator is actually to

control the generator excitation such that the generator delivers the reactive power

demanded by the load at the desired voltage.

3.4.3.FREQUENCY CONTROL

The frequency of an autonomous power system remains constant as long as

the power supplied and the power demanded remains balanced. A power imbalance

will accelerate or decelerate the rotational velocities of the system, causing the system

29

frequency to increase or decrease, respectively. The diesel generator regulates the

system frequency by controlling the amount of power developed by the diesel engine.

The AOC 15/50 wind turbine used in this study is a fixed pitch, constant

speed, passive yaw turbine. Therefore there is no control mechanism to regrate the

power output of the wind turbine. The odd method to maintain a constant frequency in

the system is to absorb the surplus power produced by the wind turbine. The diesel

engine is capable of being back-driving to absorbing some excess power (up to 30%

of rated power for short periods). But the unidirectional coupling between the engine

and the synchronous generator prevents the diesel engine from being used in this way.

In addition, the significant capacity of the wind turbine. Relative to the diesel

generator and the sluggish response of the diesel's speed governor make back-driving

the diesel an undesirable method for dynamic frequency control. A more desirable

approach is to use a fast acting programmable dump load to absorb any surplus wind

power in the system, thus the power balance and a constant system frequency. The

controller which controls the dump load and redacts the frequency under conditions of

surplus wind power is referred to here as the dump load controller.

As stated in Chapter 2, a WDHS system has three distinct modes of operation

which are determined by the relative levels of the wind power and the demand

(village) load. Refining the list from Chapter 1, these three operational modes are:

1. The instantaneous demand load minus the instantaneous wind power is greater than

the diesel's minimum loading level (wind power seen as a negative load). The diesel

generator regulates the system frequency.

2. The instantaneous demand load minus the instantaneous wind power is less than the

diesel minimum loading level. The diesel engine continues to operate. The diesel

generator and the dump load controller work in parallel to regulate the system

frequency and to maintain a minimum load on the diesel.

3. The wind power exceeds the demand load and the wind conditions are sufficient to

allow the diesel engine to be safely shut-down. The dump load is now solely

responsible for regulating the system frequency.

30

The power quality must be maintained within each mode, as well as during the

transfer between consecutive modes. This requires that the dump load controller be

stable when operating with the turbine alone, with the diesel generator and the wind

turbine operating in parallel, and with the diesel alone. The latter scenario is not a

requirement if the dump load is automatically disabled in the diesel only

configuration.

3.4.4.CONTROL SYSTEM FOR FREQUENCY CONTROL

Another way to see Eq. (1) is by means of the rightmost adder and block of the

diagram of Fig3. In this figure the frequency regulator receives as input the frequency

error (difference between the current frequency and the power system frequency

50/60 Hz) and outputs the reference power PREF to the ESS and DL actuator blocks.

Under normal conditions the ESS node takes up to its nominal power of the PREF

power as its reference power PS-REF and the DL takes the difference PREF-PS-NOM if

positive or zero in other case as its reference power PD-REF .

The previous positive limit PS-NOM can be decreased depending on the storage

charge level of the ESS. It can reach 0 if the storage is fully charged and then the DL

will have to dump the whole PREF power (PD-REF = PREF).With this variable ESS

positive power limit (PPL), the power sharing of Fig.3 guarantees t ha t t he D L

wi l l dump ju s t t he exces s w ind powe r t ha t ES S canno t s to r e . T o

make th i s   power sharing possible, both actuators must be coordinated and this

coordination implies that the DL and ESS controllers must know the current PREF with

the same precision and in the same instants.

31

[ F ig .3 Block diagram for frequency control ]

Our proposal to coordinate both DL and ESS actuators in WO mode is by

means of a d i s t r i bu t ed con t ro l s y s t em (DCS ) . A DCS i s c o m p o s e d

b y s e v e r a l C P U b a s e d e l e c t r o n i c c o n t r o l u n i t s ( a l s o c a l l e d

n o d e s ) p h y s i c a l l y d i s t r i b u t e d a n d l i n k e d b y a communication

network (also called communication bus). As it can be seen in Fig. 5, the DCS

defined for the frequency control in WO mode consists of three nodes: a SM shaft

spe ed me asu re men t se nso r ( s how n in F i g . 2 ) node w h ich a l s o

i nc l udes t he f r eque ncy regulator block of Fig. 2 and two actuators nodes: the

DL controller and the ESS controller embedded in DL and ESS converters,

respectively, and also shown in Fig. 2.

The nodes of a DCS exchange information between them through messages.

The sensor node NW shall communicate with a periodic message the current reference

power PREF through the network to the actuator nodes ND and NS. This broadcasted

message guarantees that both actuators receive the very same PREF data at the same

instants.

32

[ Fig. 4. Active power sharing between ESS and DL actuators]

In the DCS of Fig. 5 there is another node NGC, this is the WDHS general

controller, which by transmitting and receiving messages, supervises all the nodes of

the DCS, sets the mode of operation of the WDHS, modifies parameters of the

regulator block to improve the dynamics of the hybrid system, etc. This proposed

DCS has other advantages against a centralized control. Because it has two actuators,

flexibility can be improved by simple ‘failure messages’ from the actuator nodes or

from NGC. Using these type of messages three control actuations could be performed:

both actuators as it has been explained, dump load actuator alone (PS=0) (same

actuation as if the ESS is full and has the limitation of PT – PL >0) and storage system

actuator alone (PD=0) (with the limitation / PT - PL / < PS-NOM ) what gives a more

flexible structure in case of failure of any actuator. With PPL messages from NS, the

power sharing between ESS and DL actuators showed in Fig. 3can be accommodated

according to this new positive limit.

33

[ Fig. 5. frequency control in WO mode]

3.7.FREQUENCY REGULATION

The entire power system, including all its generators, distribution wiring, and

even motors present in the village load, can be thought of as one big

electromechanical entity, as shown in Figure 3. Power flows into this system as power

from the wind transferred to the wind turbine rotor, mechanical power developed in

the diesel engines as a result of combustion, and electric power drawn from the

battery.

Power flows out of the system to consumer resistive loads, to consumer

mechanical loads, to secondary loads, and as various mechanical and electrical losses.

At any given moment, if more power is flowing into the system than out of it, the

difference will be stored as an increase in kinetic energy of the rotating machines

within the system, both generators and motors that happen to be on-line at that time.

The effect of any power imbalance in the system is expressed in Equation 1.

34

where = active power (kW)

K.E. = kinetic energy of system

J = moment of inertia of rotating machine

w= angular velocity of rotating machine

This increase in kinetic energy is manifested as an increase in rotational speed

of the synchronous machines in the system and thus an increase in electrical

frequency. The task of frequency regulation is essentially a problem of maintaining an

instantaneous balance of the real power flowing into and out of the system

35

SIMULATION SCHEMATICS

CHAPTER-4

36

4.1.SIMULATION

The Matlab-Simulink [11] model of the WDHS of Fig. 2 is shown in Fig. 6.

Some of the components described next such as the IG, the SM and its voltage

regulator, the consumer load, etc. are blocks which belong to the SimPowerSystems

library for Simulink.

The SM has a rated power (PSM-NOM) of 300 kVA. An IEEE type 1 Voltage

regulator plus an exciter regulates the voltage in the SM terminals. The mechanical

parts of the DE and the SM and the friction clutch are modeled by using blocks of the

SimDriveLine library . The model inside the corresponding block of Fig. 6 includes

the inertia constants of the SM (HSM = 1s) and DE (HDE = 0.75 s) and the friction

clutch. The DE mechanical torque TD and the SM electric torque TS both in per unit

values (pu) are the inputs of the block. The shaft speeds of the DE and SM , both in

pu, are the outputs of the block.

The clutch state is set by the binary input signal CLUTCH, which controls the

clutch pressure actuator. When CLUTCH signal is high (clutch engaged/locked), the

pressure actuator is ordered to apply force normal to the surfaces. When CLUTCH

signal is low (clutch disengaged), the pressure actuator is ordered to relieve the force

normal to the surfaces. The actuator dynamics is modeled as a simple first order

system with 0.04 s time constant whose input is CLUTCH and whose output is the

normalized clutch pressure. The mechanical parts and Friction Clutch Block in Fig. 6

detailed schematic can be seen in[13]

The DE along with its actuator and speed regulator are included in the Diesel

Engine block of Fig. 6 and their modeling is justified in[14]. This block has the

current DE speed (pu) as input and outputs the mechanical torque (pu) to take the DE

speed to its speed reference. The DE has been simulated by means of a gain, relating

fuelling rate to torque (lower/upper torque limits are 0/1.1 pu) and a dead time. The

actuator has been simulated by a second order system and the speed regulator by a

PID control.

37

The constant speed stall-controlled WTG [15] consists of an Induction

Generator (IG) of 275 kW (WTG rated power PTNOM = 275 kW) directly connected to

the autonomous grid and the Wind Turbine (WT) block. This WT block contains the

wind turbine characteristic which defines the mechanical torque applied to the IG as a

function of the wind speed and the IG shaft speed. This WTG has no pitch control, so

there is no way to control the power it produces.

The dump load [15] consists of eight three phase resistors connected in series

with GTO switches. The resistors values follow an 8 bit binary progression so that the

power consumed by the DL, provided that the voltage in the isolated grid is nominal,

can be expressed in the form:

It (eq-5) means that the power can be varied discretely from 0 to 255.PSTEP, where

PSTEP is the power corresponding to the least significant bit and IJ is ‘‘1’’ when the

associated GTO is turned on and ‘‘0’’ when the GTO is turned off. For this article

PSTEP = 1.4 Kw and then PD-NOM = 357 kW, which is a 30% greater than PT-NOM, so that

the WDHS can be controlled in WO mode even in the case of no consumer load and

BESS fully charged/failure [5].

The BESS is based on a Ni-Cd battery bank, a LC filter, an IGBT three-phase

bidirectional Current Controlled Inverter (CCI) of rated power PS-MOM = 150 kW and a

150 kVA elevating transformer.

The elevating transformer isolates the three phase power inverter and the

battery bank from the autonomous grid. Its rated line to line voltage in the

grid/inverter sides are 480/120 VAC.

A detailed description of the CCI block can be seen in bellow. The CCI

receives its active power reference PS-REF from the power sharing block. PS-REF can be

established for inverter mode operation (the CCI supplies power to the isolated grid

and discharges the battery), or rectifier mode operation (the CCI absorbs power from

38

the isolated grid and charges the battery). Although the CCI can control the reactive

power it consumes/produces its reactive reference power is set to 0.

The 240 V Ni-Cd battery [15] model consists of a DC voltage source function

of the state of charge (SOC), based on the discharge characteristic of the battery, and

an internal resistance of assumed constant value. The energy stored in the battery is

93.75 kWh, which is obtained from a storage energy need of 15 min for the 150 kW

CCI rated power and a Ni-Cd battery operating between 35% and 75% of its rated

capacity (150 kW_15 min/(0.4_60 min/h) = 93.75 kWh) . This 93.75 kWh

corresponds to a capacity C of 390.625 Ah (93.75 kWh/240 V = 390.625 Ah). The

Ni-Cd battery is tolerant to a current ripple of rms value up to 0.2 C with the only

effect of an increased water usage. Connecting the battery directly to the DC side of

the power converter would excess this ripple size, so a LC filter has been used for

smoothing the battery current.

The described short term BESS helps to cover the load during short term load

peaks or wind power deficits, minimizing the number of the DG start/stop cycles

needed. This start/stop cycles minimization reduces the fuel consumption and DG

wearing and the Sizing made in [6] is done according to these benefits. Also short

term BESS improves the frequency stability and the continuity of supply of the

isolated power system as it is shown in the simulation section. In addition to previous

mentioned benefits, long term energy storage allows to match a variable and

uncontrolled renewable energy production to a generally variable and hardly

predictable load demand. Long term storage allows storing excess renewable energy

to meet the load during days of higher than average load or lower than average

renewable energy availability. In the sizing of long term energy storage in order to

obtain the most cost efficiency configuration in an isolated power system is

addressed.

The WD/WO block of Fig. 6 sets the operation mode selected by the WDHS

DCS. Its output is ‘‘1’’ for WD mode and ‘‘0’’ for WO mode. If WD/WO_ = 1 or

CLUTCH = 1 the speed reference to the DE is 1 pu as the DE is needed to keep the

system frequency at rated value. In WO mode (WD/WO* = 0 and CLUTCH = 0) the

DE speed reference is 0.3 pu, so that the DE is kept running instead of being stopped

39

to simplify the simulation of the DE cranking system. When a DE is started the

cranking system is switched on until the DE reaches the firing speed, where the DE

internal combustion process starts, and then, the DE cranking system is switched off

and the DE speed controller is activated with a speed reference of 1 pu. For this article

the time the cranking system is switched on is supposed to be 0.5 s, and the firing

speed is 0.3 pu, which are standard values for a DE. So if the WDHS is in WO mode

and the WD/WO* changes from 0 to 1, the diesel speed reference is not changed to 1

pu until the 0.5 s delay of Fig. 6 elapses, to take into account that the DE is already

running at 0.3 pu firing speed.

The NW node of Fig. 2 is simulated by the Active Power Regulator (APR) and

the Power Sharing block of Fig. 6. The APR, whose schematic is shown in Fig. 7, has

the SM and DE shaft speeds and the binary WD/WO_ signal as inputs. Based on these

signals the APR calculates the adequate PREF directed toward the Power Sharing

block and orders to engage/disengage the clutch, by means of the CLUTCH signal.

The CLUTCH order is the output of the RS flip-flop of Fig. 7, which outputs ‘‘1’’ for

DO/WD modes and ‘‘0’’ for WO mode. The conditions to set it when its value is ‘‘0’’

(WO to WD transition) are that there must be a transition to WD mode order and the

DE-SM slip is less than 2 *10^-3 pu. This 2 *10^-3pu slip accuracy is available in many

commercial auto synchronizers. The calculation of PREF by using a PD control in

WD mode or a PID control in WO mode is also shown in Fig. 7. The CLUTCH signal

selects the proportional and derivative gains of the controls for WD mode (blocks in

grey color) when set and these gains for WO mode, besides of adding the integral part

when reset. When CLUTCH is set the integral part is removed and its output last

value is reduced to 0 by ramping down with a 150 kW/s slope. The gains for WD

mode are calculated to position the dominant pole pair of the WDHS liberalized

model to be a double pole in order to increase speed response and minimize the

over/under shooting. The WD derivative part is disabled during a 0.02 s interval after

the clutch is engaged to avoid high output from this part during the engagement

process. The values of the gains for WO mode, the WO derivative filter and the 2.5

ms sample time come from . This 2.5 ms sample time sets the 400 Hz transmission

frequency for the PREF Message of Fig. 2.

40

The BESS/DL sharing defined in (3) and (4) is performed within the Power

Sharing Block of Fig. 6, according to the following equation:

that means that the DL does not actuate unless the positive PREF needed is greater than

the BESS rated power PS-NOM, guaranteeing that the DL only will dump just the wind

power excess that the BESS cannot store. So when PREF > PS-NOM, this block assigns to

the DL the minimum integer number XD-REF which verifies XD-REF - PSTEP > PREF –

PS-NOM and after this, PS-REF is defined as PS-REF = PREF – XD-REF.PSTEP. With these

calculations (3) and (5) are always satisfied and the value of PS-REF is accommodated to

take into account the discrete nature of the DL used in this simulation.

41

[Fig .6. Matlab-Simulink model of the isolated WDHS ].

42

[ Fig.7. APR Simulink schematic ]

43

4.2.SIMULATION RESULTS

Time in sec

[ Fig. 8. System frequency per unit and diesel engine speed per unit.]

In the graphs presented below the system frequency/SM speed and the DE

speed are plotted in pu value in Figs. 8 and 9. The rms voltage in pu value is shown in

Fig. 10. The active powers for the WTG, SM, BESS and consumer load are plotted in

kW in Fig. 7. The DL does not actuate during the presented test as the positive PREF

calculated in APR block during the test is in the [0,PS-NOM] range, so that the DL

reference and consumed powers PD-REF and PD are cero during the test and therefore PD

is not plotted. Fig. 11 shows the active powers as being positive when produced and

negative when consumed, so that the sum of active powers in Fig. 11 is null whenever

the power system is in equilibrium.

44

At the starting point in t = 0, the WDHS is in WO mode (WD/ WO_ = 0), so

the DE speed is 0.3 pu (Fig. 8) as it has been explained, the clutch is disengaged

(CLUTCH = 0), the input torque to the SM is zero and the WO PID regulator controls

system frequency. The active powers (Fig. 11) in the load and BESS are -50 and -42

kW (consumed) respectively. The wind speed is 8 m/s and the active power produced

by the WTG is 92 kW, being the system in equilibrium. The battery initial SOC is

50%.

Time in sec [Fig. 9 Closer view of system frequency and diesel engine speed in per units.]

45

4.2.1.FREQUENCY REGULATION IN WO MODE

Starting at the initial state, a positive step of 100 kW in the load is applied in t

= 0.2 s. by closing the three phase breaker 3PB shown in Fig. 3, so that the total load

(175 kW) is greater than the WTG produced power. Fig. 11 shows the load step, how

the BESS changes from consuming -42 kW to supplying 58 kW and a transient in the

WTG active power with minimum and maximum during oscillations of 78 and 192

kW respectively. During the transient the minimum frequency pu is 0.9985 (Fig. 9)

and the RMS voltage pu minimum and maximum are 0.9921 and 1.0038 respectively

(Fig. 10). In the steady state reached at t = 1.727 s, the WTG produced active power

stays at the same initial value (92 kW) as the wind speed has not changed and the

BESS supplies the active power deficit to the consumer load.

Time in sec

[ Fig. 10. RMS voltage per unit.]

46

4.2.2.WO TO WD MODE TRANSITION

As this situation with the BESS producing power can not be permanent, in t =

2.2s. the WD/WO block in Fig. 5 changes to ‘‘1’’, ordering to the DE to reach rated

speed and to engage the clutch when conditions are met. The WD/WO_ signal is

delayed 0.5 s, so in t = 2.7 s the speed reference to the diesel speed controller changes

from 0.3 to 1 pu and since this instant the DE begins to accelerate. As it can be seen in

Fig. 8 the DE speed increases and when the DE-SM slip is less than 0.002 pu at t =

4.11 s, the CLUTCH output from the APR block changes to 1. With CLUTCH = 1 the

Friction Clutch Block changes from disengaged to engaged state, so the DE starts to

transfer torque to the SM and the APR changes the WO PID regulator to the WD PD

regulator, changing the proportional and derivative constants, the derivative filter and

ramping down the WO integral part with the 150 kW/s slope. The described sequence

of events is presented in Fig. 8, where it is plotted the DE speed and the SM

speed/system frequency until the clutch locks at t = 4.12 s and since this point the

system frequency response.

In Fig. 11 it is shown that the fpu over/undershoots with values of 1.0029 and

0.9995 respectively and reaches the steady state at t = 6.777 s. The VRMS pu

response in Fig. 10 has a 1.0114 and 0.9879 maximum and minimum peaks

respectively during the transient after the clutch engages. The WTG active power in

Fig. 11 presents a minimum and maximum during oscillations of 65 and 122 kW

respectively, but its value at steady state is 92 kW as the wind speed has not changed.

Also Fig. 11 shows that the consumer load active power overshoots due to the system

voltage variations as the consumer load is purely resistive. The SM runs with zero

input power until the clutch engaging instant so the active power supplied by the SM

is around 0 in that interval.

After the clutch locked instant the DE and SM runs behaving as if they were

forming one axis, with the DE transferring the necessary torque to the SM to take the

system frequency to 1 pu and the SM active power shows in that interval over/under

shooting with values of 75 and 56 kW respectively. Also in Fig. 11 the active power

produced/ consumed by the BESS shows an over/undershooting of 61/-9 kW

respectively. In the steady state the DE + SM is supplying the 58 kW that the BESS

47

was supplying previous to the WO to WD mode change order and the active power of

the BESS is 0 kW.

Time in secs

[ Fig. 11. WTG, BEES, SM (DG) and load active powers ]

Fig. 12 shows the Ni-Cd battery voltage (normalized to its 240 V rated

voltage), current (also normalized to its rated current 150 kW/240 V = 625 A) and

state of charge (SOC) in pu. The battery current is considered positive when

discharging and negative when charging. The battery current in Fig. 11 looks like a

scaled version of the BESS active power in Fig. 12. This is so because of the small

variations in the battery voltage during the simulation. Initially Ni-Cd battery absorbs

charging current of -0.265 pu storing the wind power excess. After the positive 100

kW load step in t = 0.2 s, Ni-Cd battery start to supply discharging current with a

48

maximum of +0.4784 pu, until steady state is reached in t = 1.727 s where discharging

current is +0.395 pu. Once the DE is engaged, the BESS does not have to supply

power to the system, so after the transient due to the engagement process, with current

peaks of +0.417 pu and _0.057 pu respectively, steady state is

Time in secs

[ Fig. 12. Normalized battery current, voltage and SOC. ]

reached in t = 6.777 s with 0 pu current. The battery SOC initially set at 50%, hardly

changes because of the relatively great battery capacity and the short simulation time.

The battery voltage is initially 1.05 pu. After the load step increasing, minimum

voltage is 1.02 pu. During DE engagement process minimum and maximum voltage

peaks are 1.022 pu and 1.041 pu respectively. These data agree with the employed

battery model, since SOC variations are negligible, voltage will vary little and will

follow the current variations due to the internal resistance.

49

FUTURE SCOPE

CHAPTER-5

5.FUTURE SCOPE50

From this project we concluded that it can give reliable power supply but due to

the diesel generation system it is quite expensive because of the amount of fuel input

to the diesel engine. In future if we add solar generation system with wind diesel

system then we can reduce the cost of generation. The solar energy generation system

will also give the power along with the wind and diesel system. So the number of

diesel engine need to start will be less. During the day time the solar system work

quite effectively to contribute the power. We can also add biomass energy generation

system along with wind diesel hybrid system to save the fuel cost and to get reliability

of power supply. Using the nonconventional energy generation system is eco friendly

too. we can also consider some of the other new hybrid systems are

1. Solar – Diesel system

2. Wind-Solar system

51

CONCLUSION

CHAPTER-6

CONCLUSION

52

A comparative analysis of different storage technologies currently in use was

effectuated according to several criteria such as cost, energy density, specific power,

contributing to reducing fuel consumption and GHG emissions, the lifetime and

efficiency of each technology. This analysis was served to determine the performance

index of each storage technology based on the nature of the project application. The

determination of the performance index of each technology represents, despite its

subjectivity resulting from the use of the decision matrix, a solution where we have

some difficulty to choose a technology and where the constraint of time does not

achieve a detailed modeling of the studied systems. This method showed that the

CAES answers to the choice criteria with a performance index approximately 82 %.

Other systems are also more or less effective but at the cost, simplicity, adaptability to

the WDHS, the contribution to reducing fuel consumption and GHG emissions and

duration of life that there is some difference. For these reasons, CAES technology was

adopted to associate with the wind-diesel hybrid system.

53

REFERENCE

CHAPTER-7

14.REFERENCE

54

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