Design and Simulation of a Stand-alone Wind-Diesel Generator with a Flywheel

Energy Storage System to Supply the Required Active and Reactive Power.

I.J. Iglesias, L. García-Tabarés and A. Agudo.

Centro de Estudios y Experimentación

de Obras Públicas (CEDEX).

C/Alfonso XII,3 28014 Madrid.SPAIN

I. Cruz and L. Arribas

Centro de Investigaciones Enérgéticas, Medioambientales

y Tecnológicas ( Ciemat). Madrid. SPAIN.

Abstract-This paper presents the design and simulation of a
stand-alone generation plant, which combines a wind-diesel
generator with a flywheel energy storage unit. Without any
storage system, the diesel engine has to be continuously
operating, resulting in high fuel consumption. The flywheel is
designed to supply the rated power during 1.8 minutes. This
time is enough to compensate regular wind oscillations and
therefore to avoid a high number of motor diesel starting
operations. The kinetic energy storage unit is based on a carbon-
fibber composite flywheel rotating at 30.000 rpm and storing 4.5
MJ. The rotating system is driven by a Switched Reluctance
Motor and connected to the network through a double Voltage
Source Inverter. The design and simulation with SABER

 of all

the complete system is presented in this paper

I. I

NTRODUCTION

This paper presents the design and simulation of a

Wind-Diesel generation plant with a kinetic energy storage
unit. This generation plant is conceived to supply electric
power to an isolated load not connected to the electrical
network. The main power generation system consists on a
wind asynchronous squirrel-cage generator. The reactive
power is supplied initially by a Diesel-Synchronous
Generator unit, which maintain the voltage supplied to the
isolated load. In the first designs of this kind of systems [1],
the Diesel motor supplies the active power demanded by the
load during a decreasing of the input wind power. This
implies that the diesel motor has to operate in a continuous
mode, resulting in high fuel consumption. The main objective
of this project consists on reducing the fuel consumption by
including a Flywheel or kinetic storage system, which
permits the switching-off of the diesel motor. This storage
system is capable of supplying the necessary active (and
reactive) power demanded by the load when the
asynchronous wind generator is not able to produce it. The
storage unit can act as a damper of the wind power
oscillations and allows the plant to be in operation without
the extra generation of the diesel motor. The rated power of
the first prototype will be 50 kW. For this power rate, the
kinetic storage unit to be implemented will store an energy of
4.5 MJ (1.25 kWh), based on a Flywheel rotating at 30.000
r.p.m., which is able to supply the rated power during 1.8
minutes. This is in principle enough to compensate regular
wind oscillations and therefore to avoid a high number of
motor diesel starting operations [2,3].

The kinetic storage unit is driven by a Switched

Reluctance Motor (SRM) of three phases and two poles. This
motor has been chosen due to its simplicity and robustness,
very appropriate for this kind of high-speed application. A

double VSI converter connects the motor to the network:
First, an asymmetrical half-bridge topology has been selected
to drive the SRM, with two IGBTs and two diodes per phase.
Although this is the most expensive option, this is the most
flexible, and allows exploiting the full potential of the SRM.
A standard Voltage Source Inverter with variable hysteresis
current bands connects the drive to the network injecting the
required active and/or reactive power.

This paper presents the design and simulation of the

whole system by means of SABER



simulator. First, the

kinetic storage system has been simulated by modelling both
the motor and the network side converters, the SRM (with a
template written in MAST, the HDL language provided with
SABER), the rotating flywheel and the control system based
on maintaining the DC capacitor voltage. Second, the
complete wind-diesel generator has been simulated with all
the elements working together: the asynchronous wind
generator, the diesel-synchronous generator group, the kinetic
storage unit and the complete control system which maintain
both the frequency and the voltage of the isolated network.

II. O

VERALL SYSTEM CONFIGURATION

Figure 1 shows the overall system configuration with

the asynchronous directly coupled wind-generator, the
Diesel-Synchronous generator unit, the Flywheel Storage
System and the load. A set of discharge resistors are also
included to dissipate the extra input energy when the storage
unit is fulfilled and therefore it is not able to store more
energy.

Asynchronous Generator

Diesel Motor

Synchronous Generator

Load

Double Power Converter

Flywheel

Switched
Reluctance
Machine

Discharge
Resistors

Figure 1. Overall System Configuration

Figure 2. Flywheel FEM simulation

II K

INETIC

E

NERGY

S

TORAGE

S

YSTEM

A. Flywheel

As a first step, a flywheel prototype has been designed in

order to test its mechanical behaviour [4,5]. The main
characteristics of this flywheel are the following: Stored
energy: 0.6 kWh. Dimensions: r=154 mm, R = 290 mm, L =
290 mm. Carbon Fibber mass: 12 Kg. Rated speed: 30.000
rpm

This flywheel rim has been designed with different

materials depending on the requirements of specific parts:

•

Isotropic material as steel used to make the cylinder-
drum, both lads and hub.

•

Anisotropy material (extremely unidirectional and high
strength material), as composites winding on the
cylinder-drum. This is a hybrid design with two layers or
rings of different composite material in order to achieve
higher radial strength. Inner ring: glass fibber and
polyester. Outer ring: carbon fibber and epoxy resin.

This flywheel is proposed as a prototype in order to make the
spin test on it. Figure 2 represents the output of the rim
dynamic analysis based on FEM simulation. It represents the
stress map in the flywheel, simulated spinning over 30,000
r.p.m. It is possible appreciate the strain suffered by the
flywheel at high rotational speed. (It is remarkable
deliberately that the strains are exaggerated, in order to get a
better visual understanding).

B. Switched Reluctance Motor

This kind of motor has been selected mainly for two

reasons: First, the high speed and the difficulty to extract the
generated heat in the rotating parts (rotor will be in a vacuum
atmosphere), discard those motors with induced rotor
currents or with brushes. Second, the cost of a SRM is
smaller than a Permanent Magnet Machine, and in this
project the cost impact of the kinetic unit over the complete
plant cost can be unacceptable and therefore has to be
carefully analysed. Both kind of machines can accomplish
with the required specifications, but the simplicity, robustness
and less cost of the Switched Reluctance Motor have been the
main reasons to be selected.

This motor has been specifically designed for this

application and it is now under construction at CIEMAT
facilities. In order to achieve high speed, a configuration with

three stator poles and two rotor poles was selected, a
compromise to obtain high rotating speed with a relatively
low switching frequency and moderate torque ripple [6].

A new winding technique has been used based on the

concept of double pancake coils. This procedure uses a
special winding machine with two spools rotating in opposite
directions allowing a fast and precise manufacturing of the
coils which can also be wound with spatial curvature (instead
of being flat) so that they can almost fill the space between
stator poles, thus allowing a very high filling factor and also a
high power density.

The electromagnetic design of the motor was carried out

using a standard finite element code, which basically
provided the polar inductance as a function of the position,
taking into account iron saturation. This dependence was
introduced into the MAST model to simulate the machine
with the SABER simulator.

Main parameters of the machine are shown in table I:

Maximum Power

50 kW

Maximum Torque

15.9 Nm

Maximum R.P.M

30000 rpm

Maximum Current (RMS)

102 A

D.C. Voltage

750 V

Stator/ Rotor pole number

3/2

Outer radius

73 mm

Active length

100 mm

Number of turns per pole

40

Table I. Main Parameters of the SRM

Previous to this prototype, a 2 kW unit was built and

tested to validate some of the concepts involved in the design
of this type of machine, specially, those related to the
winding procedure. Figure 3 depicts the winding machine
while winding a coil of the demonstrator.

C. Power Converter. Design and simulation.

Figure 4 shows the schematic simulated in Saber with

the SRM model, and the power electronics converter. This
converter is composed by two parts: The motor side converter
presents a half-bridge topology with two IGBTs and two
diodes per phase, this topology allows the SRM to operate as
a motor and a generator with a total symmetry between these
two modes of operation. The network side converter is a

Figure 3. Winding machine for the SRM.

conventional hysteresis-bands current controlled VSI capable
of injecting or absorbing both active and reactive power into
the network.
C-1. Switched Reluctance Motor Side Converter

Figure 5 and 6 show some results of the SRM

simulation. Figure 5 represents the current supplied to one
phase by the motor side converter in the motor operation.
This pulsed current generates a motor torque given by
equation:

2

).

.(

2

1

I

L

T

θ

∂

∂

=

(1)

To maintain constant the current a PWM technique is

applied to the IGBTs [7]. Two different modulations are
possible: the “hard switching” and the “soft-switching”. In
both types to increase the phase current all the DC voltage +
V

dc

is applied to the motor phase by switching-on both upper

and lower IGBTs. However, to decrease the phase current
there are two possibilities: 1) Switching-off both IGBTs
causes the current to flow through the power diodes and
applying -V

dc

volts to the motor phase (hard-switching). 2)

Switching-off only one IGBT causes the current to flow
through the other IGBT and its corresponding power diodes
and applying 0 volts to the motor phase (soft-switching). This
second strategy has been selected because the current

decreasing is slower and permits a lower switching frequency
to maintain the current close to the reference value.

The same soft-switching technique can be applied in the

generator operation. Firing-off both IGBTs –V

dc

volts are

applied to decrease the phase current, and firing-on only one
IGBT 0 volts are applied causing a phase current increasing.

The pulse current has to be maintained during the

increasing ramp inductance (60º) for the motor operation and
during the decreasing ramp (60º) for the generator operation.
On and off angles have to be calculated to obtaining the
maximum torque for a given current pulse amplitude.
Simulations with Saber have been very useful to calculate
such angles, because an analytical calculation of these is very
complex. By varying the firing time in several simulations
the optimum on and off angles have been obtained. These
values, which have been adjusted by a polynomial function of
the rotor speed, will be stored in a DSP to control the SRM
inverter. Figure 6 shows the total torque and the one-phase
torque when the machine is working as a motor. It can be
observed that in the decreasing current ramping a negative
torque (generator) appears, reducing the total average torque.
By this reason is very important a carefully adjustment of the
off angle for the motor operation, and in the same way, an
adjustment of the on angle for the generator operation [8].

Fig. 4. Saber Schematic of the SRM Drive.

Funcionamiento como motor (15000 rpm)

t(s) = 2.25m

(3)swt_1sup

(3)swt_1inf

0

0

-10

20

50

80

110

140

170

200

(A)

0

500u

1m

1.5m

2m

2.5m

3m

3.5m

(H)

t(s)

500u

1m

1.5m

2m

2.5m

3m

3.5m

4m

4.5m

5m

5.5m

6m

6.5m

7m

(A)

: t(s)

(3)i(r.r1)

(H)

: t(s)

(3)...1(msrv2_6_2.msrv)

Figure5. SRM phase motor current. Motor operation.

Soft-switching technique

Detalle del par total y el par de u n a fase para una potencia de 30 kW.

( N.m)

: t(s)

(70)...l(msrv2_6_2.msrv)

(70)par1(msrv2_6_2.msrv)

24m

24.5m

25m

25.5m

26m

26.5m

27m

27.5m

28m

28.5m

29m

29.5m

t(s)

- 2

0

2

4

6

8

10

12

14

16

18

20

22

24

(N.m)

Figure 6. Total torque (red) and one-phase torque (green)

The SRM inverter is controlled in a very simple way:

First, the switching angles are stored in a Digital Signal
Processor to generate the switching times according with the
rotor speed. Second, the current pulse amplitude is obtained
from a PI control loop where the main objective is to
maintain the DC voltage close to the nominal value. A
decreasing of this voltage in the motor operation will cause a
decreasing of the current amplitude reference, however a
decreasing of DC voltage, in the generator operation, will
cause an increasing of the current amplitude reference. The
change between the two modes of operation (motor and
generator) is based on a hysteresis band around the rated DC
voltage. For example, if the SRM is working as a motor and
the DC voltage decreases below the lower limit band, it will
change to the generator mode to recover the DC voltage
inside the hysteresis band values. Thus the DC voltage is
used to balance the power between the network inverter and
the SRM inverter. Figure 7 shows the block scheme of this
control.

C-2. Network Side Converter

The network side converter is a hysteresis-bands

current controlled VSI as shown in figure 1. This converter is
capable of injecting the active and reactive power demanded
by the control system of the whole plant. The connection
between the power injected by this converter and the power
supplied to the Flywheel is done by means of controlling the
DC voltage into a band around its rated value as explained
before. Figure 8 shows the current supplied by the inverter
during a change from motor to generator mode.

III. E

XPERIMENTAL RESULTS OF THE

F

IRST

SRM P

ROTOTYPE

.

A first prototype of the Switched Reluctance Motor

Drive has been developed and constructed at CEDEX Labs.
Figure 9 shows a photograph of the motor and the IGBTs
converter. This figure also shows the system developed for
measuring the speed and position [9], .in order to avoid the
implementation of a digital encoder capable to work up to
30.000 rpm. This system is based on a disk with three slots,
three photodiodes and three phototransistors to detect the
change between each 60º sector. A high frequency internal
clock serves to determine the exact position, by counting the
pulses after each change of sector and with the assumption
that in a high inertia system, the speed variation between one
sector and the following is very small.

The main characteristics of this prototype are shown in

table II.

Converter Topology:

Input Rectifier:

Diodes Bridge

Output inverter:

Half-bridge IGBT.

Inverter IGBTs:

SKM 100 GAL 123 D and
SKM 100 GAR 123 D
I

C

=90 A. V

CES

= 1200v.

IGBT DC Chopper:

Operating to maintain DC
voltage below 150 V.

Switched Reluctance Motor

Stator/ Rotor pole number

3/2

Stator outer radius

76 mm

Stator inner radius

50 mm

Minimum airgap

0.3 mm

Rotor length

50 mm

Number of turns per pole

20

Nominal Current (rms)

20 A.

Nominal Power

2 kW.

Table II. SRM Prototype Characteristics.

First tests of this prototype have been developed at

CEDEX, to check the behaviour of all the electronics
systems. Figure 10 shows the current supplied by the power

mode

+

-

PI

OPERATION

MODE

Limiter

Imáx

ON and OFF

ANGLES

GENERATOR

speed

Imáx

mode

FIRING PULSES

GENERATOR

I

1

I

2

I

3

ad_on

ad_off

Firing pulses

Measured

DC Voltage

Reference

DC Voltage

Measured

Currents

theta

Fig. 7. Control of the SRM inverter.

Line voltage and Current. Input Inverter. Iline THD=3%

(A)

: t(s)

(1)i(l.la)

(-)

: t(s)

(1)vina

0

5m

10m

15m

20m

25m

30m

35m

t(s)

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

300

350

(A)

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

300

350

(-)

Fig. 8. Current supplied by the Hysteresis-band current controlled input

inverter (red). Line voltage (black). Current THD= 3%.

Fig 9. First Lab SRM Prototype.

converter to the SR motor while this prototype is running at
3100 rpm.

IV. S

IMULATION OF THE

C

OMPLETE

S

YSTEM

.

In order to test the behaviour of the complete system

and the ability of the Kinetic Storage Unit (KSU) to
compensate wind oscillations, a complete simulation have
been carried out with Saber



. Figure 11 shows the schematic

of all the complete system including not only the kinetic
storage unit but also the whole wind-generation system: the
wind asynchronous generator, the self-excited synchronous
generator driven by the diesel-motor, the isolated network
and the discharge resistors. Both the synchronous and

asynchronous generators have been simulated by means of
two models written in MAST language and based on the d-q
model of the electrical machines.

The developed simulation consists mainly on a starting

process in the following manner: 1) Diesel-Synchronous
Generator unit connection to create the isolated network. 2)
Connection of the asynchronous wind generator. 3)
Disconnection of the Diesel Engine and connection of the
KSU to maintain both the frequency and the voltage of the
network. 4) Sudden increase of the load to test the ability of
the KSU to supply the required power necessary to maintain
the network parameters. 5) Sudden increase of the input
power to check the ability of the KSU to store the extra
power, or in the case when the KSU is full and it is not able
to store more energy, the extra energy will be dissipated in
the discharge resistors.

Figure 12 depicts the first case of a variation of

frequency when the asynchronous wind generator is
connected at 200ms (first spike in the curve) and when an
increment of the load occurs at 1.5 s (second spike in the
curve). Before this time, there is no any external load and the
wind turbine is generating a power of 30 kW, therefore the
kinetic unit is storing energy. At 1.5 s a 50 kW external load
is connected, while the input wind power remains constant at
30 kW. In this case the kinetic unit has to supply the
additional power and it begins to work as a generator. It can
be observed that such high increment of the load only gives a
variation of the frequency up to 49.85 Hz. This small
decrease is due to the fast response of the kinetic unit.

Fig. 11. Complete System Simulation. Saber Schematic.

Fig 10. First Lab SRM Prototype: Current supplied to

three phases of the motor and optical sensor signal.

Figure 13 shows a detail of the network voltage and the

current supplied by the KSU in the same case. It can be
observed as at 1.5 s. power factor varies from 1 to –1
showing the motor to generator transition of the SRM.
Current change its phase from 0º to 180º in one cycle.

Finally, figure 14 shows the frequency when a sudden

increase of the input wind power occurs and the KSU is not
able to store more energy. In this case the extra power has to

be dissipated into the discharge resistors and two different
strategies can be adopted: The classical one consists on
limiting the maximum frequency value and connecting in a
discrete form the resistors by means of solid-state relays. The
second option, only possible in the case when a KSU is
implemented consists on a chopper connected to the DC link
of the SRM inverter. In this case, the power can be
continuously dissipated to the discharge resistors and
therefore the frequency can be continuously regulated around
its nominal value as it is shown in figure 14.

V. C

ONCLUSIONS

•

A complete simulation of a wind-diesel generation
plant with a flywheel energy storage unit has been
developed showing the ability of the Kinetic Storage
Unit (KSU) to compensate the wind power
oscillations.

•

The KSU is able to supply both active and reactive
power to compensate both the frequency and the
voltage of the network. This unit is designed to
supply the total power during a period of 1.8 minutes.

•

A 50 kW. Prototype is now under construction
including the Flywheel, the Switched Reluctance
Motor and the Power Converter.

•

Simulation and design of the power converter have
been carried out and demonstrate the good behaviour
of this system in the following aspects: 1) Low
harmonics level injected into the network, 2) Very
fast response time to compensate transient
phenomena and 3) Ability to dissipate the extra
energy into the discharge resistors by means of a DC
chopper, keeping the network frequency very close to
its nominal value.

R

EFERENCES

1.

Ray Hunter & George Elliot (1994) “Wind Diesel Systems. A Guide to
the Technology and its Implementation”. Edited by Cambridge
University Press.

2.

Bakis, C.E. (1998) “Batteries for the 21

st

Century: Composite

Flywheel” Engineered Materials and Systems, Pensilvania State
University.

3.

Grudkowski, T.W., Dennis, A.J. Meyer, T.G. & Wawrzonek, P.H.
(1996) “ Flywheel for energy storage” SAMPE Journal Vol. 32 Nº 1, p.
65-69.

4.

Ha, S.K..,Choi, S.K., Kim, D.J. & Chin, L.J. (1999) “Stress Analysis of
a Hybrid Flywheel Rotor Using a Modified Generalized Stress
Assumption” XII ICCM, Paris.

5.

Ha, S.K..,Kim, D.J. & Choi, S.K. (1999) “ Optimal Design of an
Hybrid Composite Flywheel Rotor Using Finite Element Methods” 44

th

International SAMPE Symposium, p. 2119-2169.

6.

Cossar C. & Miller T.J.E. (1992) Electromagnetic Testing of Switched
Reluctance Motors. International Conference on Electrical Machines.
Manchester, September 15-17, pp470-474

7.

Miller, T.J.E. (1993) “Switched Reluctance Motors and their control”.
Magna Physics Publishing & Clarendon Presss. Oxford.

8.

Bass ,T.J. Bass, Ehsani, M & Miller, T.J.E. “Robust Torque Control of
Switched Reluctancer Motors without a shaft position sensor”, IEEE
Trans. on Ind. Electron. Vol IE-33, nº3 pp 212-216. Aug. 1986

9.

Ehsani, M, Husain,I, Kulkarni,A.B., “Elimination of Discrete Position
Sensor and Current Sensor in Switched Reluctance Motor Drives”
IEEE Trans. on Indus. Applic. Vol 28, Jan 1992, pp 128-135

Graph 2

(-)

: t(s)

(1)f

0

500m

1

1.5

2

2.5

3

3.5

4

t(s)

49.1

49.2

49.3

49.4

49.5

49.6

49.7

49.8

49.9

50

50.1

50.2

(-)

Fig. 12. Frequency control for a network

load (30 kW) sudden variation

.

Graph 1

(V)

: t(s)

(1)va

(A)

: t(s)

(1)i(l.la)

1.43

1.44

1.45

1.46

1.47

1.48

1.49

1.5

1.51

1.52

1.53

1.54

1.55

1.56

1.57

t(s)

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

300

350

(V)

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

140

(A)

Fig. 13. Phase variation of the kinetic unit current.

Variation from storing to giving energy.

Frecuencia ante un exceso de potencia. Las resistencias de descarga entran por freq(6) o por Vdc(9,11,12)

(-)

: t(s)

(6)f

(9)f

(12)f

(11)f

0

250m

500m

750m

1

1.25

1.5

1.75

2

2.25

2.5

2.75

3

t(s)

49

49.2

49.4

49.6

49.8

50

50.2

50.4

50.6

50.8

51

51.2

(-)

Fig. 14. Frequency variation in a sudden increase of the input wind

power. .External discharge resistors (blue) or internal discharge

resistors (red).