Isolated power systems with WTG

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The control of isolated power systems with

wind generation

Cristian Cristea,IEEE Student Member, Joo Peas Lopes,IEEE Senior Member,

Mircea Eremia,IEEE Senior Member, Lucian Toma,IEEE Student Member

Abstract The present work investigates the dynamicbehaviour of a mixed system consisting of a wind farm and

a diesel group supplying a load, under different

disturbances. In this regard, dynamic models and control

systems that enable the generation part to support the grid

are needed. The objective of this work has been also the

implementation of the diesel group model and its afferent

control system into Matlab-Simulink and to demonstrate

their use by evaluating the response in time of the electric

parameters.

Index Terms wind generators, diesel engine, systemstability

I. INTRODUCTION

ustainable development and environment conservation is

the reasonability of every human being and therefore

common actions should be taken to limit the greenhouse

effect gases exhausted by the fossil fuelled power plants. In

this support the engineers have to develop technologies to

exploit, as efficiently as possible, clean energy sources. The

technology with the greatest impact in this area is the windpower production. However, their operation is a great

challenge for the responsible operators of the power systems

since they could introduce several frequency and voltage

perturbations into the system due to the random variation of

the wind, which lead to variations of the output power.

The wind-diesel system is one of the most appropriate

hybrid systems utilized in weak or isolated power systems. A

wind-diesel system is very reliable because the diesel group

acts to correct the power imbalances due to variations in wind

speed, by always providing appropriate amount of active

power equal to load power minus the output power of the

wind farm. The control of the voltage and frequency of a

weak autonomous wind-diesel system is more challengingthan in large grids [7]. Wind-diesel hybrid systems are

generally used for remote power supply. These systems are

often classed as weak grid systems as they have limited

reactive support. Power fluctuation problems are experienced

when the wind generator system uses an induction generator

for energy conversion. This problem could be due also to the

reactive power drawn by these induction generators from the

power system, when appropriate reactive sources are not

incorporated within the wind system. Power quality and

reliability are also some of the major concerns in a wind-

diesel hybrid system.

Cristian Cristea, Mircea Eremia and Lucian Toma are with University

Politehnica of Bucharest, Power Engineering Faculty, 313 Spl.

Independenei, 060042 Bucharest, Romania (phone: +40720008956; fax:

+40214029446; e-mail: [email protected]).

Joo Peas Lopes is with University of Porto, Department of Electrical

Engineering, Faculty of Engineering; (e-mail: [email protected]).

II. MODEL DESCRIPTIONThe dynamic analysis of a wind-diesel hybrid system has

been performed in this paper to study the effect of some

disturbances such as random wind variation and network

disturbances such as short-circuits. Voltage and power

fluctuations resulting from random wind velocity and short-

circuits can be a problem for the power system. Remote area

power supplies are characterized by low inertia, low damping

and poor reactive power support.

McGowan [8] developed a dynamic model for a no-storage

wind-diesel system and validated its components by

comparing the simulation results with experimental data.

Uhlen et al. [9, 10] implemented and compared two robust

controllers of a Norwegian wind-diesel prototype system.

Papathanassiou and Papadopoulos [11] studied the dynamics

of a small autonomous wind-diesel system using simplified

models and classic control theory techniques.

Diesel Engine

SynchronousGenerator

Bus bar20 kV

WindTurbine

Load

SG

IG

IG

IG

Diesel Group

Wind Farm

InductionGenerator

Transformer

690V/20kV

Transformer2.4kV/20kV

Fig. 1. Wind -diesel power system.

To simulate an isolated power system we consider a wind

farm consisting of three wind generators and a diesel group to

S


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maintain the frequency at constant value, which supply a

consumption area (Fig. 1).

Normal operation and response to a short-circuit are

analyzed.

A. Diesel Group

The diesel engine model gives a description of the fuel

consumption rate as a function of speed and mechanical

power at the output of the engine. The diesel engine is usuallymodelled by a simple first order model relating the fuel

consumption (fuel rack position) to the engine mechanical

power. Notice that the transfer function of a reciprocating

engine involves a small but significant time delay associated

with the mean time between firing [5].The efficiency of the combustion is the ratio of the

effective horsepower developed by the engine and available

on its crankshaft to the heat consumed during the same time,

that is [4]:

uB

i

Hm

vzW'

&

= (1)

Incomplete combustion is the main reason for which theindicated efficiency is lower than the ideal efficiency. The

mean effective pressure of the engine is defined as [4]:ip

h

ii

V

Wp = (2)

By solving (1) with respect to and substituting into (2)

we get:

iW

== '1'

BB

h

ui mCm

vzV

Hp && (3)

where Cis the appropriate proportionality constant. Note thatfor normal or stable power system operation is almost

constant and its value is imposed in order to keep the system

frequency constant at 50 Hz.

v

The mean pressure of mechanical losses is taken in a first

approximation proportional to the mean piston speed ,

, where since the piston travels a

distance of twice the stroke per revolution. Thus, can be

generally written as valid for any

engine with appropriate constant C.

dU

df Up md SfU 2=

fp

= 3Cpf )2( mpf=

The real mean effective pressure of the engine must be:kp

(4)fik ppp =

The real mechanical power of the diesel engine is

given by the equation:

DmP

km

HkHkhDm pK

VvpVvpzVP

=== (5)

The mechanical torque of the engine is given by the

following relation, in the p.u.:

kk

b

H

bm

DmDm pCp

KT

V

T

PT 2=

=

= (6)

The combustion efficiency of the engine depends on the

combustion quality as it has been mentioned before. Since

detailed representation of the fuel combustion model is not the

scope of this paper, the combustion efficiency is represented

as a function of the air-fuel ratio

B

L

m

mas follows:

=

B

L

m

mf (7)

Analytic expressions of

B

L

m

mf for a specific engine can

be found in [12]. At normal operation of the engine will beconsidered constant during simulations.

The block diagram of the diesel engine derived using the

above considerations is presented in Figure 2. Typical values

for the parameters of the model can be found in [12].

Figure 2 illustrates the block diagram of the diesel enginetogether with its governor. This model was implemented in

Simulink and was attached to an existing model of a

synchrounous machine (6).

DmT

minDmT

maxDmT

0

0

ref

0

-+

R

1

s

K12

2

1 s

K

+

DsH+2

1

+

- cP

2C

3C

DeT

+

-

Engine

Governor

1se

1

C

Bm&

Fig. 2. Diesel group control system.

B. Wind turbine

The wind turbine model is based on the steady-state power

characteristics of the turbine. The stiffness of the drive train is

infinite and the friction factor and the inertia of the turbine

must be combined with those of the generator coupled to the

turbine. The output power of the turbine is given by the

following equation:

3

2),( windpm v

AcP

= (8)

The mechanical power as a function of generator speed,

for different wind speeds and for blade pitch angle

mP

= 0 , is

illustrated in figure 3. This figure is obtained with the default

parameters (base wind speed = 12 m/s, maximum power at

base wind speed = 0.73 p.u. ( = 0.73) and base rotational

speed = 1.2 p.u.).

pk


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Fig. 3 Turbine power characteristics.

Figure 4 presents the structure of the wind turbine and the

induction generator. The stator winding of the wind generator

is connected directly to the grid and the rotor is driven by the

wind turbine. The wind energy is converted by the wind

turbine into electrical power with the help of the induction

generator and is transmitted to the grid through the statorwinding. The pitch angle is controlled in order to limit the

generator output power to its nominal value for high wind

speeds. The reactive power absorbed by the induction

generator is provided by the grid or by some devices like

capacitor banks.

Fig. 4. Structure of wind turbine with induction generator.

The electrical part of the machine is represented by a

fourth-order state-space model and the mechanical part by a

second-order system. All electrical variables and parameters

are referred to the stator. All stator and rotor quantities are in

the arbitrary two-axis reference frame (dq frame).

A Proportional-Integral (PI) controller (Fig. 5) is used to

control the blade pitch angle in order to limit the electric

output power to the nominal mechanical power. The pitch

angle is kept constant at zero degree when the measured

electric output power is under its nominal value. When itincreases above its nominal value the PI controller increases

the pitch angle to bring back the measured power to its

nominal value.

+

-

Pitch angle max.

Pel Pitch angleController (PI)

Pitch angle

0Pmec

Fig. 5 Pitch angle controller

III. MODELING IN SIMULINK

The diesel group has an important role in this work since its

responsibility is to regulate the frequency in the system. The

components of the block diagram of the diesel group

implemented in Simulink are shown in Figure 6. The diesel

group consists of three major components: the synchronous

generator, the diesel engine together with the governor and the

automatic voltage regulator (AVR).

The models of the synchronous machine and the automaticvoltage regulator (exciter) used in this work has been chosen

from the preset models available in SimPowerSystems library.

These models are in accordance with IEEE recommendations.

Fig. 6. The Simulink model of the diesel group.

Asynchronous generators are frequently used to convert the

mechanical force into electrical energy. The implementation in

Simulink/Matlab of the induction generator and wind turbine

with the pitch control can be seen in Figure 7.

Fig. 7. Implementation of the induction generator and wind turbine.

Fig. 8. Representation in Simulink of the power system.


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The input of the wind turbine and controller block are the

wind speed (m/s) and the speed (p.u.), while the outputs are

the mechanical power (p.u.) and the pitch angle (degrees).

IV. RESULTS

As mentioned before, the isolated system consists of a wind

farm, a diesel generator and a load. The wind farm consists of

three wind generators. They are connected to a common 20

kV busbar via a step-up transformer boosting the voltage from690 V. A diesel generator is connected to same 20 kV busbar,

through a 2.4kV/20kV, to correct the variations in wind power

generation. The diesel generator is provided with speed and

voltage control capabilities. The diesel group has an installed

power of 15 MW, the load is 7 MW and the wind generators

are of 1.5 MW each.

Two disturbances are considered is our first scenario. After

30 seconds from the starting of simulations, the connection of

a new wind turbine is considered, then after another 40

seconds, time in which all parameters stabilizes, a symmetrical

three-phase-to-ground fault occurs at one of the three wind

turbine.

Activepower[p.u.]

0 10 20 30 40 50 60 70 80 90-0.5

-0.25

0

0.25

0.5

0.75

1

1.25

Time (s)

a.

MechanicalPower[p.u.]

0 10 20 30 40 50 60 70 80 900

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Time (s)

b.

Frequency[p.u.]

0 10 20 30 40 50 60 70 80 900.95

0.96

0.97

0.98

0.99

1

1.01

1.02

1.03

1.04

Time (s)

c.

Fig. 9. Frequency related parameters.

Figure 9 shows the active power produced by the wind

farm, the mechanical power of the diesel group and the system

frequency, and Figure 10 presents the bus voltage on the 20

kV busbar, the reactive power drawn by the wind farm and the

field voltage of diesel group, al in the case of the above

mentioned disturbances.

Busvoltage[p.u.]

0 10 20 30 40 50 60 70 80 900

0.2

0.4

0.6

0.8

1

1.2

1.4

Time (s)

a.

Reactivepower[p.u.]

10 20 30 40 50 60 70 80 90

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

Time (s)

b.

Fieldvoltage

[p.u.]

0 10 20 30 40 50 60 70 80 900

1

2

3

4

5

6

Time (s)

c.

Fig. 10. Voltage related parameters.

As it can be seen, when some disturbances occurs into the

system such as connection of additional wind generator or

occurrence of a short circuit, the wind generator satisfy the

load voltage ride through capability requirements. High

currents are flowing during a voltage drop. Due to the highthermal capacity of the induction machine it can be expected

that these currents will cause no problem. More problematic

might be the high amount of reactive power required by the

turbine during voltage disturbances. When the dip lasts to

long this may lead to voltage collapse for the wind generator

and therefore it will be disconnected from the network.

In the second scenario a short-circuit on the 20 kV busbar

occurring after 60 seconds from the start of simulation is

considered. The simulation lasts for 120 seconds. The active

and reactive power is considered for entire wind farm.


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Activepower[p.u.]

20 30 40 50 60 70 80 90 100 110 120

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time (s)

a.

Frequency[p.u.]

0 20 40 60 80 100 120

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

Time (s)

b.

MechanicalPower[p.u.]

0 20 40 60 80 100 1200.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Time (s)

c.

Busvoltage[p.u.]

0 20 40 60 80 100 1200.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Time (s)

d.

Reactivepower[p.u.]

20 30 40 50 60 70 80 90 100 110 120

0.5

1

1.5

2

2.5

Time (s)

e.

Fig. 11. Response in the case of the second scenario.

V. CONCLUSIONS

A diesel group dynamic model and a wind farm have been

implemented based on individual mathematical models. The

models used to simulate the wind farm include aerodynamic

aspects and mechanical details of the turbines, the electrical

system of the turbine, the cable connections inside the farm

and the connection to the transformer. These models present a

powerful tool for the investigation of wind farm dynamics and

for the development of wind farm controllers.These models developed in Simulink are flexible since they

can be easily connected one to another forming an equivalent

electrical network including electrical machines and other

components, allowing the steady-state and dynamic analysis

of the power system, which is the purpose of the presented

methodology.

The wind farm models can be used to develop wind farm

control, investigate dynamic interaction within the farm and

between the wind farm and the diesel group as well as to study

the wind farm in response to wind speed increase/decrease

and systems faults.

Dynamic models of wind farms based on individual

turbines are large and complicated. The number of statevariables is high and some of the time constants are small,

leading to a relatively long simulation time. When

incorporating the dynamic models of wind farms in large

electric grids, the complexity of the wind farm models has to

be reduced. Aggregated wind farm models, in which all

turbines are represented by equivalent models, are more

suitable for this purpose. However, aggregated models of the

wind farms based on individual turbines are less accurate and

therefore less applicable.

MATLAB/Simulink appears to be less suitable for very

large models with many (thousand or more) state variables.

Computation of the steady state becomes high time consuming

and the simulation time for normal runs increases more thanproportional.

The malleability of the developed platform was confirmed

though the analysis of sudden loss of power generated by

renewable sources (wind power) as well as in scenarios where

the diesel group is no longer alone in the task of maintaining

the system frequency control.

VI. NOMENCLATURE

- diesel engine efficiency of the combustion;

uH - diesel engine heat value of the fuel (kJ / kg);

z - number of cylinders of the Diesel engine (operatingduring a combustion cycle);

v - diesel engine stroke cycles per second [ ]

where: K=2 for two-stroke engine or K=4 for a four-

stroke engine;

)/( Km =

iW - diesel engine means effective work (developed by

one piston during a combustion cycle) (kWh);

Bm& - diesel engine consumption rate (kg/sec);

hV - diesel engine one stroke volume [ where

D: cylinder diameter, S: stroke] (m

4/2 SD=3);


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m - diesel engine speed (rad/sec);

HV - diesel engine total stroke volume [ ] (mhzV=3);

bT - base toque for the per-unit transformation;

mP - mechanical output power of the turbine (W);

pc - performance coefficient of the turbine;

- air density (kg/m3);

A - turbine swept area (m2);

windv - wind speed (m/s);

- tip speed ratio of the rotor blade tip speed to wind

speed;

- blade pitch angle (deg);

VII. REFERENCES

[1] J. Peas Lopes, Integration of Dispersed Generation on Distribution

NetworksImpact Studies, Proc. of the IEEE Winter Meeting, N.Y.,

February 2002.

[2] P. Ledesma, Dynamic Analysis of Power Systems with Wind

Generation, Ph.D. dissertation, Univ. Carlos III de Madrid, Madrid,

Spain, 2001.[3] J. G. Slootweg, S. W. H. de Haan, H. Polinder and W. L. Kling,

Modelling Wind Turbine in Power System Dynamics Simulations,

2001 IEEE Power Engineering Society Summer Meeting, Vancouver,

Canada, July 15-19, 2001.

[4] G.S. Stavrakakis and G.N. Kariniotakis, A General Simulation

Algorithm for the Accurate Assessment of Isolated Diesel Wind

Turbine Systems Interaction. Part I: A General Multimachine Power

System Model, IEEE Transaction on Energy Conversion, vol. 10, pp.

577-583, 1995

[5] S. Roy, O.P. Malik. G.S. Hope. "An adaptive control scheme for speed

control of Diesel driven power plants". IEEE Trans. on Energy

Conversion, vol. 6. no. 4. December 1991.

[6] V. Akhmatov, Analysis of Dynamic Behavior of Electric Power System

with Large Amount of Wind Power, PhD Thesis, April 2003.

[7] F. Jurado, J. Saez, Neuro-fuzzy control in biomass-based wind-diesel

power system, 14thPSCC Sevilla, 24 28 June 2002.

[8] J. G. McGowan, W.Q. Jeffries and J. F. Manwell, Development of

Dynamic Models for no Storage Wind-Diesel Systems, Proceedings of

the 17th British Wind Energy Association Conference, UK, pp 111-116,

July 1995

[9] K. Uhlen, B. A. Foss and O. B. Gjosaerter, Robust Control and

Analysis of a Wind-Diesel Hybrid Power Plant, IEEE Trans. Energy

Conversion, Vol. 9, No. 4, 1994, pp 701-708

[10] K. Uhlen, Modeling and robust control of autonomous hybrid power

systems, Ph.D. thesis, The University of Trondheim, 1994.

[11] S. A. Papathanassiou and M.P. Papadopoulos, dynamic characteristicsof autonomous wind diesel systems, Renewable Energy, vol. 23, no. 2,

2001, pp 293-311.[12] V. L Maleev, Internal Combustion Engines, McGraw Hill (19th

edition), 1985.

Cristian Cristea (MS06) was born in Videle, Romania, in 1982. He received

the B.Sc (Hons.) degree in electrical engineering from the UniversityPolitehnicaof Bucharest in 2006. He is currently pursuing his Ph.D. degree

in power systems. His research interests include renewable energy, with

particular focus on wind energy, and application of FACTS devices in power

systems.

J. A. Peas Lopes (M80SM94) received the Electrical Engineering degree

(five-year course), the Ph.D. degree in electrical engineering, and the

Aggregation degree from the University of Porto, Porto, Portugal, in 1981,

1988, and 1996, respectively. He is an Associate Professor of aggregation

with the Department of Electrical Engineering, Faculty of Engineering,

University of Porto. In 1989, he joined the staff of Instituto de Engenharia de

Sistemas e Computadores do Porto (INESC) as a Senior Researcher, and he is

presently Co-coordinator of the Power Systems Unit of INESC.

Mircea Eremia (M98, SM02) received the B.S. and Ph.D. degree in

electrical engineering from the Polytechnic Institute of Bucharest in 1968 and

1977 respectively. He is currently Professor at the Electric Power Engineering

Department from University Politehnica of Bucharest. His area of research

includes transmission and distribution of electrical energy, power system

stability and FACTS applications in power systems.

Lucian Toma (MS04) was born in Romania in 1977. He received the

engineer degree and M.S. in power systems from University Politehnica of

Bucharest in 2002 and 2003, respectively. He is currently pursuing his Ph.D.degree in power systems. Mr. Toma is employed as assistant professor Electric

Power Engineering Department from University Politehnica of Bucharest.

Since 2002 he has been with the where he is now. His main fields of interests

are electricity markets, transmission and distribution of electrical energy and

FACTS applications in power systems.

Isolated power systems with WTG

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