Chapter 5 OPERATION CONTROL STRATEGY AND SIMULATION …

Chapter 5

OPERATION CONTROL STRATEGY AND SIMULATION OF WTG CONNECTED WITH UG

5-1 INTRODUCTION

The performance of WES interconnected with UG can be improved through

an application of advanced control method. This chapter introduces an appli-

cation of an artificial neural network on the operation control of the WES/UG

to improve system efficiency and reliability. The generated power from WTG

has been calculated by a computer program under known wind speed. The

computer program which proposed here and applied to carry out these calcu-

lations is based on the minimization of the energy purchase from UG. This

chapter focus on a hybrid system consists of WES accompanied with or with-

out battery storage interconnected with electric utility taking into account the

variation of wind speed and load demand during the day. Different feed for-

ward NN architectures are trained and tested with data containing a variety of

operation patterns. A simulation is carried out over one year using the hourly

data of the load demand, wind speed in Zafarâna site, Egypt as a case study.

This chapter introduces also a computer modeling, simulation, analysis of a

Control Strategy and Simulation of WTG Connected With UG

130

variable speed WTG interconnected with UG. The proposed computer simula-

tion program uses the instantaneous reactive power theory, IRPT. A computer

simulation program has been designed to simulate phase voltage, line voltage

of the inverter leg and current in each IGBT's. It also simulates AC output

current from the inverter that injected to the load/grid, load current, grid cur-

rent, power output from the inverter, power delivered to or from grid and fi-

nally power factor of the inverter and grid. The computer simulation program

is confirmed on a realistic circuit model which implemented in the Simulink

environment of Matlab and works as if on line.

5-2 METHODOLOGY OF CONTROL STRATEGY

5-2-1 Control Strategy Issue of WES Connected with UG without BS

WES connected with UG are designed to operate in parallel with UG as

shown in Fig. 5-1. A bi-directional interface is made between the WES AC

output circuit and the UG, typically at an on-site distribution panel or service

entrance. This allows the AC power produced by the WES to either supply

on-site electrical loads, or to back feed the UG when the WES output is

greater than the on-site load demand. At low speed or during other periods

when the electrical loads are greater than the WES output, the balance of en-

ergy required by the loads is received from the UG. When the UG is down,

these systems automatically shut down and disconnect from the grid. This

safety issue is required in all grid-connected WES, and ensures that the WES

will not continue to operate and feed back onto the utility grid when the grid

is down for service or repair. This safety issue has been done by NN.

Power flows in Fig. 5-1 must satisfy the following equations.

(t)LP(t)gP(t)WESP =± (5-1)

)t(outWTG,P*wON(t)WESP = (5-2)

Where;

Chapter 5

131

PWES(t) : The power from WES system, kW.

Pg(t) : The power to/from UG, kW.

PL(t) : The Load demand, kW.

ONw :The optimum umber of WTG's.

PWTG,out(t) :The hourly generated power from one WTG (see chapter 4).

t : The hourly time over one year.

S3

S2

S1

UG

Filter

Load

InputOutput

Step-upTransformer

Bus bar

Bus bar

~Step-down

Transformer

G. B. I.G.

Win

d Sp

eed

DC/ACAC/DC

Vdcw

Fig. 5-1 Single-Line Diagram for the Control Strategy of WTG/UG System.

The NN that makes the decision of connecting WTG with UG occurs only

when the output power from WTG lay in the allowed range of the hourly gen-

erated power from WTG as explained in chapter 4. The NN detects the value

of the PWTG,out(t) from WTG then it sends an ON, 1 or OFF, 0 trip signal to the

S1. The NN will send an ON-trip signal to switches only if the following

condition is realized:


132

(t))out,WTGPmax((t)out,WTGP (t))out,WTGPmin( << (5-3)

Where;

(t))out,WTGPmin( : The value of the output power from WTG at cut-in

wind speed.

(t))out,WTGPmax( : The value of the output power from WTG at cut-off

wind speed.

Four modes can be considered as follows:

Mode 1: If the wind speed is low and the WES can't connected to the UG,

then switch S1 is OFF, S2 is ON and S3 is OFF.

Mode 2: If the wind speed is high and the generated power lower than the

load demand then the load demand will be supplied from WES and the deficit

energy will be taken from the UG. (S1=ON, S2=ON, S3=OFF)

Mode 3: If the wind speed is high and the generated power greater than the

load demand then the load demand will be supplied from WES and the

surplus energy will be sent to the UG. (S1=ON, S2=OFF, S3=ON).

Mode 4: If there is no wind speed and there is a problem in UG then the criti-

cal load demand will be not supplied. (S1=OFF, S2=OFF, S3=OFF). The

operation of the three switches shown in Fig. 5-1 can be summarized as

shown in Table (5-1).

Table (5-1) Operational modes of WES interconnected with UG Mode S1 S2 S3 Generated power vs. Load demand

1 OFF ON OFF PWES ~=0 2 ON ON OFF PWES < PL 3 ON OFF ON PWES > PL 4 OFF OFF OFF PWES =0 and grid has a problem

Chapter 5

133

5-2-2 Control Strategy Issue for WES/BS Connected to UG.

In this type of intertie system, the load demand supplied from WES, UG and

BS as shown in Fig. 5-2. If the UG power fails, power will be drawn instantly

from the backup batteries to support the critical load demand. Power flows

from the system shown in Fig. 5-2 must satisfy (5-4). The size of BS can be

determined from Eqn.(3-8) in chapter 3.

(t)LP(t)batP(t)gP(t)WESP =±± (5-4)

The four switches shown in Fig. 5-2, are shifted according to the generated

power from WES. When the wind speed is low, then the load will be supplied

from UG and the energy from WES due to small wind speed will be send to

the batteries, i.e., switch S1 and S3 will be OFF, switch S4 will be on state of

charging in the position 1 and switch S2 will be ON. If the wind speed be-

comes high, the load will supply from WES. If the generated power is greater

than the load demand, the surplus energy will be sent to the batteries until

reached fully charged i.e., S1 will be ON and switch S2 will be OFF and S4

will be on the state of charging in the position 1 and switch S3 will be OFF.

The four modes are summarized below:

Mode 1: If the wind speed is low then the wind energy will be sent to the bat-

teries (S1=OFF, S2=ON, S3=OFF, S4= on state of charge on position 1).


load demand then the load demand will be supplied from WES and the sur-

plus energy will be sent to the batteries (S1=ON, S2=OFF, S3=OFF, S4 =ON

state of charge on position 1).


load demand then the load demand will be supplied from WES and the sur-

plus energy will be sent to the batteries until reached full charge. The remain-

ing power will be sent to the UG. (S1=ON, S2=OFF, S3=ON, S4= ON state

of charge on position 1).


134

S3

S2

S1

UG

Filter

Load

Step-upTransformer

Bus bar

Bus bar

~Step-down

Transformer

InputOutput

NN for WTG/EUaccompanied with BS

G. B. I.G.

Win

d Sp

eed

DC/ACAC/DC

Battery Storage(BS)

S4

Fig. 5-2 Single-line Diagram for the control strategy of WES/UG/BS system

Mode 4: If there is no wind speed and there is a failure in UG then the load

demand will be supplied from batteries (S1=ON, S2=OFF, S3=OFF, S4 =ON

state of discharge on position 0). The operation of the four switches shown in

Fig. 5-2 can be summarized as shown in Table 5-2.

Table (5-2) Operational Modes of WES Connected to UG Accompanied

With BS

Mode S1 S2 S3 S4 Generated power vs. Load de-mand

1 OFF ON OFF 1 PWES < PL i.e wind speed is very low

2 ON OFF OFF 1 PWES > PL i.e. wind speed is high

3 ON OFF ON 1 PWES > PL i.e. wind speed is very high

4 ON OFF OFF 0 PWES =0

Chapter 5

135

5-3 METHODOLOGY OF WES OPERATION ISSUE WITH UG

5-3-1 Modeling of WTG Interconnected With UG

A cage IG is normally used together with a back-to-back converter because it

must be magnetized by a reactive stator current. The back-to-back converter

allows a fast torque control but it is much more expensive than the diode rec-

tifier. The back-to-back converter is usually made with insulated gate bipolar

transistor, IGBT's. The back-to back converter is a bi-directional power con-

verter consisting of two conventional PWM voltage source converters and a

common dc-bus as shown in Fig. 5-3. Two main disadvantages of the back-to

back PWM converter are:

• Higher price. The grid connected PWM converter is rather rare on the

market today and the converter requires more semiconductor components,

more sensors and more complicated control system. An increased cost of

about 40-50 % compared to the six pulse diode rectifier/PWM inverter is

possible.

• Higher losses as compared to a six pulse diode rectifier/PWM inverter.

The higher losses of a PWM converter are because both switching and

conduction losses.

Advantages of back to back converter

• Utility side harmonics are greatly reduced,

• Continuous power conversion from WTG for varying wind speed,

• Bi-directional power flow is possible,

• The output power factor can be regulated (lagging or leading) and

• The approach results in a high performance WTG system [39], [117].

The back-to-back converter consists of, as shown in Fig. 5-3, rectifier and

DC/AC inverter. In the following point these will be discussed in details.


136

Ls Ls

Ls

Ea

EbEc

~ ∼

~

Vconva

Vconvb

VconvcC

Lo

Utility Grid

Vinva

Vinvb

Vinvc

Rotor

Variable frequencyPWM boost Rectifier

DCLink

Constant FrequencyPWM InverterStator Circuit

G. B.

Win

d Sp

eed Pm

Pwind

nt n

PWTG,out PdcPinv

Fig. 5-3 WTG Equipped with Back-to-Back Converter

(a) Generators and Rectifiers

The complete generator system and its main components are shown in Fig. 5-

3. The turbine is described by its power Pwind and speed nt. The speed is raised

to the generator speed, n, via a G.B. Pm is the input power to the generator

shaft. The output electrical power from the generator is denoted by PWTG,out.

The rectifier creates a dc voltage Vdcw and a dc current Idcw. The power of the

DC link Pdcw is the mean value of the dc power, equal to Idcw *Vdcw. For the

proposed rectifier Fig. 5-3, let the source three-phase generated voltages from

WTGs are [117]:

120)tiSin(ωiV2CE

120)tiSin(ωiV2BE

t)iSin(ωiV2AE

−=

+=

=

(5-5)

And the converter reflected input voltages are:

)iδ120tiSin(ωiV2convcV

)iδ120tiSin(ωiV2convbV

)iδtiSin(ωiV2convaV

−−=

−+=

−=

(5-6)

Where; Vi :The amplitude of the generated phase voltage of WTGs at each wind

speed.

Chapter 5

137

δi :The phase shift between the generated voltage EA and the terminal

voltage at converter legs, Vconva.

The phase shift, δi is accomplished in PWM control by adjusting phase shift

angle δ between the control voltage, Vcontrol, with respect to the source voltage

Ea since Vcontrol is related to Vconva by the following equation:

)iδ120tiSin(ω*2

ma*dcwV2convcV

)iδ120tiSin(ω*2

ma*dcwV2convbV

)iδtiSin(ω*2

ma*dcwV2convaV

−−=

−+=

−=

(5-7)

Where;

ma : The amplitude modulation ratio.

Power flow in the PWM converter is controlled by adjusting the phase shift

angle δ between the source voltage Ea, Eb, Ec and the respective converter re-

flected input voltage Vconva, Vconvb, and Vconvc as shown in Fig. 5-4a,b for the

per phase equivalent circuit and phasor diagram. When source voltage Ea, Eb,

and Ec leads Vconva, Vconvb, and Vconvc respectively the real power flows from

the source to the converter. Conversely, if Ea, Eb, and Ec lags Vconva, Vconvb,

and Vconvc respectively real power flows from the converter’s dc side into the

ac source (inverter mode).

JXs

Is

Vconv,iVi

Fig. 5-4a. Per Phase Equivalent Circuit


138

Vconv,i

Vi

Ii*XSδ i

Fig. 5-4b The per Phase Equivalent Circuit and Phasor Diagram

The active and reactive power output from WTG can be expressed as follows:

S

iiconvii X

SinVVP

δ**3 ,= (5-8)

S

iconviiconvii X

VVVQ

,2

, cos**3

−=

δ (5-9)

The output from WTGs is variable in voltage and frequency and to interface it

to electric power utility Vdcw must be constant value. Controlling ma can do

the control of Vdcw. The value of Vdcw also is function in power flow where

Vdcw increases when the input power is greater than the output power and visa

versa.

(b) Design Issue of a DC/AC Power Converter

The device for converting DC to AC is called an inverter. In Fig. 5-5, a ge-

neric three-phase inverter is shown.

The design targets of the three-phase Three-legged inverter are as follows:

• Output voltage: 220 V (line-to-neutral)

• Output frequency: 50 Hz

• Output voltage THD: < 3% (balanced linear load)

• Output power: 600 kW (three-phase)

• Maximum per-phase power: 200 kW

5-3-2 The Proposed Control System

Figure 5-6 shows an overview of the power circuit of the proposed WTG in-

terconnected with UG. It consists of three-phase bridge converter, LC-filter,

Chapter 5

139

CLf

Three-PhaseUG

Vinva

Vinvb

Vinvc

DCLink

Constant FrequencyPWM Inverter

PdcPinv

Cf

Vdcw

+

-

Fig. 5-5 Circuit Diagram of the Three-Phase Voltage Source Inverter.

three-phase transformer, three-phase load, UG and controller. The proposed

system control scheme for the system under study usually uses the Instanta-

neous Reactive Power Theory, IRPT [108]. The variables which will be

sensed for the controller are DC link voltage, Vdcw, inverter filter output cur-

rents Iinva, Iinvb, and Iinvc, load currents ILa, ILb, and ILc and load phase voltages

Va, Vb, and Vc. The DC link voltage, Vdcw must be controlled to be higher

than the peak line voltage of the UG.

The load currents and load voltages are sampled and transformed into the

two-axis αβ-coordinate system and then into the rotating dq-coordinate sys-

tem. IRPT uses the park transformation which given in Eqn. (5-10) to gener-

ate two orthogonal rotating vectors α and β from the three-phase vectors a, b

and c. This transformation is applied to the voltages and currents and so the

symbol x is used to represent volt or current. IRPT assumes balanced three-

phase loads and does not use the x0 term.


140

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

−−−=

cxbxax

2/3 2/3 0

2/1 2/1 12/1 2/1 2/1

32

βxαxox

[108] (5-10)

The instantaneous active and reactive powers Pw and Qw are calculated from

the transformed voltage and current.

Then the reference compensating currents have been determined as given in

Eqn. (5-11).

⎥⎥⎦

⎤

⎢⎢⎣

⎡

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡ −

+=

wQwP

αV βVβV αV

2βV2

αV

1*βi

*αi (5-11)

Where;

Pw : The WTG instantaneous real power, W.

Qw : The WTG instantaneous imaginary power, VAR.

Vα, Vβ: The Supply voltage in vector α and β.

iα*, iβ* : The reference compensating currents vector α and β.

In a balanced three-phase system with linear loads, the instantaneous real

power Pw and imaginary power Qw are constant and equal to the three-phase

conventional active power P3Φ and reactive power Q3Φ respectively.

So, the inverse park transformation is applied to iα* and iβ* and this gives the

output currents in standard three-phase form, as shown in Eqn. (5-12).

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢

⎣

⎡

−−

=*βi

*αi

/23- 2/1/23 2/1

0 1

32

*ci

*bi

*ai

(5-12)

Detail description of this item is similar to the simulation of PV/UG in item 3-

3-5 in chapter 3.

Chapter 5

141

Load380 v,50 Hz

S1S3

S2

UG

~

Pinv, Qinv

G.B.

I.G.

Win

d Sp

eed

Lf

Cf

Vinva

Vinvb

Vinvc

Constant FrequencyPWM Inverter

Vconva

Vconvb

VconvcC

Variable frequencyPWM boost Rectifier

DCLink

Iinva

Iinvb

Iinvc

Fig. 5-6 Schematic Diagram of the WTG Connected to the UG

There are two modes of operation:

• Mode 1: When the generated energy from WTG is lower than the load de-

mand then the deficit energy will be supplied from the UG. i.e. S1=ON,

S2=ON and S3=OFF as shown in Fig. 5-6.

• Mode 2: When the generated energy from WTG is greater than the load

demand then the surplus energy will be transmitted to the UG. i.e. S1=ON,

S2=OFF and S3=ON as shown in Fig. 5-6.

5-4 APPLICATIONS AND RESULTS

5-4-1 Operation Control of WES CONNECTED TO UG

Figure 5-7 shows the structure of the proposed three layers NN. X1, X2, X3

and t are the four-input training matrix and represent electrical power gener-

ated from WES, Electric utility power, load demand, and time respectively.

W(1) and W(2) are the weight matrices. The network consists of four input lay-

ers, five nodes in hidden layers and three nodes in output layer which sigmoid


142

transfer function. The network has been found after a series of tests and modi-

fications. Figure 5-8 shows the evaluation of the 4+5+3 NN errors. Table (5-

3) shows weights and biases for 4+5+3 NN.

Fig. 5-7 Structure of the proposed three layers NN used to Interconnect WES

with UG.

Fig. 5-8 Relation between Error and Epoch 4+5+3 NN

Chapter 5

143

Table (5-3a) Weights W1 and Biases for 3+5+3 NN for WES interconnected with UG

W1 bias -0.0572 0.0209 -8.0752 -8.4323

-0.0465 -8.3182 21.4211 5.9986 -19.0674 -33.4265 -8.4072 2.0607 -1.8458 -9.1831 -1.9314 -7.1331 -0.1006 -11.1134 10.9303 -1.6038

Table (5-3b) Weights W2 and Biases for 3+5+3 NN for WES interconnected with UG

W2 Bias -19.1372 7.6632 -0.1054 -11.5428 20.2439 16.7222 9.9625 -24.6260 4.8211 14.9381 -15.4927 12.7552 -2.6071 26.4931 -6.4171 -3.3530 3.5730 -7.5463

Figures 5-9 and 5-10 display the optimal operation of the WES/UG hour by

hour through the day which represents the months of January and July respec-

tively. From Figs. 5-9 and 5-10 it can be seen that the deficit energy has been

taken from UG. (i.e. The NN send a trip signal to switches S1 to turn ON, S2

to turn ON and S3 to turn OFF).On the other hand, the surplus energy has

been injected to UG through the day (i.e. The NN send a trip signal to

switches S1 to turn ON, S2 to turn OFF and S3 to turn ON). Figure 5-11

shows the difference between output from NN and the desired output for the

test data of 120 examples (Five months). These differences are displayed for

switches S1, S2 and S3. From this Figure it can be seen that the ANN of

4+5+3 operates with a high accuracy.


144

-80

-60

-40

-20

0

20

40

60

80

100

1 3 5 7 9 11 13 15 17 19 21 23

Time, Hour

Pow

er, M

WPgPLoadPwtg

Fig. 5-9 Optimal Operation of the WES/UG to Feed the Load Demand during

January (winter)

-80

-60

-40

-20

0

20

40

60

80

100

120

1 3 5 7 9 11 13 15 17 19 21 23Time, Hour

Pow

er, M

W

PgPLoadPwtg

Fig. 5-10 Optimal Operation of the WES/ UG to feed the Load Demand dur-

ing July (summer).

Chapter 5

145

Fig. 5-11 Relation between outputs and target for five months

Figures 5-12 and 5-13 display the output of the proposed NN of 4+5+3 for

month of January and July respectively using test data. This output may be 1

or 0 for each switch. From Figures 5-9 and 5-12 (January) it can be noticed

that the trip signal which produced from NN sent to switch S1 at hours 8, 9,

10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 22. This means that the WES feed

the load demand at these hours. On the other hand, switch S2 (for example)

equal to 1 at hours 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 19, 20, 21, 22, 23 and 24.

This means that the UG should supply the load demand at these hours. On

the other hand, the power injected to UG through switch S3 at hours 11, 12,

13, 15, 16, 17 and 18. From switch S1 and S2 it can be noticed that the WES

with UG feed the load demand at hours 8, 9, 13, 14, 19 and 22. From Figures

5-10 and 5-13 (July) it can be noticed that the trip signal which produced

from NN sent to switch S1 at hours 1, 2, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,

16, 17, 18, 19, 20, 21, 23 and 24.


146

Fig. 5-12 Outputs of Neural Network for Month of January.

Fig. 5-13 Outputs of Neural Network for month of July.

Chapter 5

147

This means that the WES feed the load demand at these hours. On the other

hand, switch S2 (for example) equal to 1 at hours 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,

12, 13, 14, 16, 17, 18, 21 22, 23 and 24. This means that the UG should sup-

ply the load demand at these hours. On the other hand, the power injected to

UG through switch S3 at hours 1, 15, 19 and 20. From switch S1 and S2 it

can be noticed that the WES with UG feed the load demand at hours 2, 5, 6,

7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 21, 23 and 24.

5-4-2 Operation Control of WES/BS Interconnected with UG

A new subroutine computer program has been proposed and written using

Matlab software to simulate the system shown in Fig. 5-2. The flowchart of

this program is shown in Fig. 5-14. This subroutine program has been

branched from the flowchart of the computer program shown in Fig. 4-3 (see

chapter 4). The output of this program has been used to be the input of NN.

The outputs of NN are four trip signals that sent to switches S1, S2, S3 and

S4. Through a series of tests and modifications it has been found that the

network consists of 4 input layers, 7 nodes in hidden layer and four nodes in

output layer which sigmoid transfer function. Figure 5-15 shows the structure

of the proposed three layers NN. (X1, X2, X3, X4 and t) are the five-input

training matrix and represent state of charge, electrical power generated from

WES, electrical power for UG, load demand and time respectively. W(1) and

W(2) are the weight matrices. The network consists of five input layers, seven

nodes in hidden layers and four nodes in output layer which sigmoid transfer

function. Figure 5-16 shows the evaluation of the 5+7+4 NN errors. Table (5-

4) shows weights and biases for 5+7+4 NN. Figures 5-17 and 5-18 display the

optimal operation of the WES/UG accompanied with BS hour by hour

through the day which represents the months of January and July respectively.


148

For time=1:1:24 hr

End

Batteries arecharged i.e. S4=1and the load is fed

from grid

If Wind speed(t)< Limit or

Inverter failure

yes

No

If wind (t) >Limit or P WES (t)

> P L(t)

Feed the loadS1=ON

If SOC (t) >0.8*size of

battery

Noyes

Surplus power send

to UG S3=ON

yes

No Surplus power sendto Battery S4=1,

S1=ON

If SOC (t) >0.2*size of

battery

yes

Load feed frombattery S1=ON,

S4=0

NoLoad feed fromUG;S1=OFF,

S2=ON

For i=1:1:12 month

From End ofProgram Fig. 4-3

Fig. 5-14 Flowchart of the Proposed Computer Program For

WES /UG accompanied with BS.

Chapter 5

149

Fig. 5-15 Structure of the proposed three layers NN used to Interconnect

WES/BS HEPS with UG.

Fig. 5-16 Relation between error and Epoch for 4+7+4 ANN


150

Table (5-4a) Weights W1 and Biases for 5+7+4 NN for WES interconnected with UG accompanied with BS

W1 bias

-0.1307 2.6104 -7.4984 5.8654 3.8497 -4.0149 0.4709 -1.7175 5.6230 -20.7827 -2.0382 -2.3321 0.7567 -6.4294 19.2424 -22.3204 -3.6784 2.3392 -0.0784 5.8238 - 4.6823 16.5393 0.4126 0.0815 1.4047 1.4595 0.9732 7.6773 1.2217 3.316 0.0023 0.0079 8.5436 -10.0777 -0.0756 8.3354 1.5108 -2.5815 -0.5392 0.9185 1.4640 -3.5416

Table (5-4b) Weights W2 and Biases for 5+7+4 NN for WES interconnected

with UG accompanied with BS

W2 bias -8.3771 -1.1730 -10.910 12.9445 6.3495 21.7662 2.3242 10.511918.0362 25.4048 21.3842 26.0004 -3.8761 6.5412 5.4306 -4.886417.5799 -1.2536 5.7630 -27.142 10.3634 -1.0848 -0.253 -0.1326-40.9033 7.5640 38.0838 -24.747 -13.406 -17.039 -16.92 -6.1142

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-80

-60

-40

-20

0

20

40

60

80

100

1 3 5 7 9 11 13 15 17 19 21 23Time, Hour

Pow

er, M

WPgPLoadPwtg

Fig. 5-17 Optimal Operation of the WES/UG Accompanied with BS to feed

the Load Demand during January (winter)

-80

-60

-40

-20

0

20

40

60

80

100

120

1 3 5 7 9 11 13 15 17 19 21 23Time, Hour

Pow

er, M

W

PgPLoadPwtg

Fig. 5-18 Optimal Operation of the WES/UG Accompanied with BS to feed

the Load Demand during July (summer)


152

Figure 5-19 reveals state of charge for BS which corresponding to the optimal

operation of the WES/UG accompanied with BS through the months of Janu-

ary and July respectively. Figure 5-20 shows the difference between output

from NN and the desired output for the test data of 120 examples (Five

months). These differences are displayed for switches S1, S2, S3 and S4.

From this Figure it can be seen that the ANN of 5+7+4 operates with a high

accuracy.

0

5

10

15

20

25

30

1 3 5 7 9 11 13 15 17 19 21 23

Time, hour

SOC

, MW

h

SOC for Jan.SOC for July

Fig. 5-19 State of charge of WES/BS HEPS with UG during January (winter)

and July (summer)

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Fig. 5-20 Relation between outputs and target for five months

Figures 5-21 and 5-22 display the output of the proposed NN of 5+7+4 for the

month of January and July respectively using test data. This output may be 1

or 0 for each switch. From Figures 5-17, 5-19 and 5-21 (January) it can be no-

ticed that the trip signal which produced from NN sent to switch S1 at hours

5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and 22. This means that the WES

and BS feed the load demand at these hours. On the other hand, switch S2

(for example) equal to 1 at hours 1, 2, 4, 5, 6, 7, 8, 9, 14, 20, 21, 23 and 24.

This means that the UG should supply the load demand at these hours. On the

other hand, the power injected to UG through switch S3 at hours 11, 12, 15,

16, 17 and 18. Finally, battery storage will be on state of charge through

switch S4 at hours of 1, 2, 3, 4, 6, 7, 8, 9, 10, 11, 12, 14, 15, 20, 21, 23 and

24. On the other hand, the BS will be discharged through the hours of 5, 12,

13, 16, 17, 18, 19 and 22. Then the BS can feed the load demand only during

these hours accompanied with WES.


154

Fig. 5-21 Outputs of Neural Network for January

Fig. 5-22 Outputs of Neural Network for July

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5-4-3 Simulation of WES Interconnected with UG

Figure 5-6 shows an overview of the power circuit and control circuit of the

proposed WTG interconnected with UG. The power circuit of Fig. 5-6 has

been simulated using Matlab/simulink as shown in Fig. 5-23. The parameters

of the simulated circuit are as follows: Three-phase line voltage 380 V, 50 Hz.

Output filter Lf=0.40 mH, Cf=70 µF. The total power load level is 200 kW

with 303.8 A per phase load current for a duration 0.2 sec. After 0.2 sec the

load have been changed from 200 kW to 575 kW with 873.62 A per phase

load current for a duration from 0.2 sec to 0.4 sec. Finally the load is suddenly

changed to 200 kW with 303.8 A per phase load current for a duration from

0.4 to 0.5 as shown in Fig. 4-24. The variation of the generated power from

WTG according to wind speed variation is also shown in Fig. 5-24.

Fig. 5-23 Overview of the Power Circuit and Control System of the Three-

Phase DC/AC Converter


156

Fig. 5-24 Simulated of the Generated Power from WTG, Load Demand and

Grid Power.

The following figures show simulation results of the proposed control strat-

egy. The proposed model has a purely sinusoidal controlled ideal voltage

source at the inverter terminals. Due to the small width of the hysteresis band

the voltage generated by the proposed model is nearly sinusoidal when seen at

this bus. Figure 5-25 shows the waveform of the current following in one

branch of IGBTs inverter. On the other hand, Fig. 5-26 shows waveform pf

the inverter line current injected by the WTG with total harmonic distortion

1.3 %. The load line current of the load demand is shown in Fig. 5-27. From

Fig. 5-24 it can be seen that there is a surplus power in the period of 0.3 sec

and in the period from 0.4 sec to 0.5, so the surplus power will be injected to

the UG for these periods. This, in turn, NN sent a trip signal to switches S1 to

turn ON, S2 to turn OFF and S3 to turn ON. On the other hand, there is a

deficit power in the period of 0.3 to 0.4 sec. So, the UG will supply the load

demand in cooperated with WTG for this period. i.e. NN sent a trip signal to

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switches S1 to turn ON, S2 to turn ON and S3 to turn OFF. These can be seen

in Fig. 5-28 and Fig. 5-29. Figure 5-28 shows waveform of grid line current

with total harmonic distortion of 3.7% that injected to or drawn from grid and

Fig. 5-29 displays the simulated power factor of the grid. Also, from these

Figures 5-28 and 5-29 it can be seen that the power factor is leading in the pe-

riod of surplus power and lagging in the period of the deficit power. The input

current iα(t) and iβ(t) and their corresponding load voltage vα(t) and vβ(t) are

in phase. Thus there is a guarantee to operate the inverter with a power factor

very close to one as shown in Fig. 5-30. From these figures it can be seen that

the proposed model is very excellent.

Fig. 5-25 Waveform of the Switch Current in IGBT's


158

Fig. 5-26 Waveform of the Inverter Line current to the Load/Grid

Fig. 5-27 waveform of the Load Line Current

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Fig. 5-28 Waveform of the Grid Line Current.

Fig. 5-29 Waveform of the Power Factor of the Grid


160

Fig. 5-30 Waveform of the Power Factor of the Inverter

5-5 CONCLUSIONS

This chapter presents a technique to design and control of WES intercon-

nected with UG accompanied with or without battery storage. This technique

uses energy balance to reduce the cost of electricity while meeting the load

demand. A controller that monitors the operation of interfaced system has

been designed. This controller indicates the available energy from WES and

electric utility in order to meet the load demand. From the results obtained

above, the following are the salient conclusions that can be drawn from this

chapter:

1- Control strategy of WES/UG accompanied with or without battery storage

system has been studied by using neural network.

2- A new computer program has been proposed for modeling and simula-

tion of any WTG interconnected to UG. By using this computer program

the interface between WTG and UG, which consists of inverter, control

circuit and LC filter, can be designed, modeled and simulated. This com-

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puter program has been applied on modeling and simulation of a specified

WTG interconnected with UG in Zafarâna power plant in Egypt as a case

study.

3- The total harmonic distortion at the local bus is within acceptable limits

and reached to 1.3 % for the inverter current and 3.7% for the grid cur-

rent.

4- Perform the necessary preliminary studies before investing and connect-

ing wind turbines to the grid where purchased and sold power from UG

have been calculated.

5- A novel technique based on NN has been proposed to achieve the optimal

operation of WES/UG without BS. This ANN operates the WES con-

nected with UG to feed the load demand.

6- A novel technique based on artificial neural network, ANN, is proposed

to achieve the optimal operation of WES/UG accompanied with battery

storage.

7- The 4+7+4 artificial neural network is suitable neural network for accu-

rate operation of WES/UG accompanied with battery storage.

8- This ANN has a very high accuracy and achieves the optimal hour by

hour operation for WES/UG accompanied with battery storage.

Chapter 5 OPERATION CONTROL STRATEGY AND SIMULATION …

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