Parallel Operation of Wind 

Turbine, Fuel Cell, and Diesel 

Generation Sources

Preprint 

June 2004 • NREL/CP-500-35353 

E. Muljadi 

C. Wang 

M.H. Nehrir

To be presented at the IEEE-Power Engineering Society

General Meeting 

Denver, Colorado 

June 6 – June 10, 2004 

National Renewable Energy Laboratory 

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1

Abstract—We investigated a small isolated hybrid power system 

that used a parallel combination of dispatchable and nondispatchable power generation sources. The non-dispatchable 

generation came from a nature-dependent wind turbine, and the 

dispatchable generations were a fuel cell and a diesel generator. 

On the load side, the non-dispatchable portion was the village 

load, and the dispatchable portion was the energy storage, which 

could be in many different forms (e.g., space/water heater, 

electrolysis, battery charger, etc.) The interaction among 

different generation sources and the loads was investigated. 

Simulation results showed the effect of the proposed system on 

voltage and frequency fluctuations. 

Index Terms—wind turbine, diesel, fuel cell, power generation, 

parallel operation, hybrid generation 

I. INTRODUCTION

ince ancient history, wind-turbine technology has been 

used to improve the quality of life. For example, people 

have used wind power to pump water or mill grains. More 

recently, wind turbines have been used for water or oil 

pumping [1], battery charging, or utility power generation. An 

important aspect of wind turbine power generation is that it 

does not pollute air or water. Wind power is also well suited 

for remote areas with low population, where there is little 

incentive for a utility to build transmission lines. 

Hybrid power systems have been used very successfully to 

reduce pollution and to conserve diesel fuel consumption [2-

3]. For this paper, we analyzed a hybrid power system 

consisting of a 225-kW wind turbine, a 400-kW diesel 

generator, and a 75-kW fuel cell array to provide power to a 

village. A dispatchable load was used to help balance the 

power in the system. An example of dispatchable load to 

regulate system frequency can be found in reference [4]. 

In the following sections, we describe the overall system 

and each individual generation source, as well as the 

interaction among the generation sources and loads. 

II. SYSTEM CONFIGURATION 

A block diagram of the system is shown in Fig. 1. The 

wind turbine was operated with a constant frequency generator 

(induction generator) with a capacity of 225 kW about 1200 

rpm. It was a stall-regulated wind turbine. The diesel engine 

had a rated capacity of 400 kW. The fuel cell was rated at 75 

kW, and the local loads were mostly residential and water 

 1

 National Renewable Energy Laboratory, Golden, CO 80401 

2

 Montana State University, Department of Electrical & Computer 

Engineering, Bozeman, MT 59717 3

 This article is offered by the Midwest Research Institute (MRI) employee 

under U.S. Government Contract No. DE-AC36-99GO10337. Government 

and MRI retain non-exclusive, royalty-free license to publish or reproduce 

published articles or allow others to do so for Government purposes. 

pump. A dispatchable load was available to balance out the 

real power in the system. 

The diesel generator provided a smooth generation, 

whereas the output power of the wind turbine generation 

depended on the wind velocity. The wind velocity was 

reflected in the power generation; for example, if the wind 

varied smoothly, the output also changed smoothly. On the 

other hand, the output power fluctuated if there was wind 

turbulence. In the wind farm setting, the collective output 

power fluctuation was minimal [5]; however, in a small 

isolated operation with a weak power network, the impact of 

power fluctuation on power quality can be significant. In this 

paper, we do not discuss power quality. Interested readers 

should refer to the available standard on this issue [6]. 

The fuel cell was installed to help regulate the frequency in 

the system. The fuel cell was controlled to produce a unity 

power factor; thus, the voltage regulation was completely 

dependent upon excitation of the generator and the capacitors 

available in the power grid. 

Diesel Generator 

Wind Turbine Generator 

c

o

n

v Fuel Cell 

Other 

Loads 

Electrolyzer & other 

dispatchable loads 

Fig. 1. Physical diagram of the analyzed power system 

The load could be divided into three different types. The 

first was the residential type of load for lighting, heater, or 

small appliances. The nature of this load was very tolerant to 

disturbance (i.e., it was immune to low power quality power, 

fluctuating voltage, and fluctuating frequency). The second 

type of load was the industrial or heavy load, such as 

compressors, water pumps, and intermittent large loads. 

Although insensitive to power quality, this type of load can be 

the cause of voltage and frequency fluctuations. Another type 

of load was the dispatchable load, which may consist of an 

E. Muljadi1

, C. Wang2, M.H. Nehrir2 3

Parallel Operation of Wind Turbine, Fuel Cell, and 

Diesel Generation Sources

S
 

2

electric heater, a battery charger, or an electrolyzer to provide 

hydrogen to the fuel cell. 

III. COMPONENTS OF THE POWER SYSTEM 

The following sections of this paper describe each of the 

three different power sources: diesel generator, wind-turbine 

generator, fuel cell. 

A. Diesel Generator 

From an electrical system point of view, a diesel generator 

can be represented as a prime mover and generator. Ideally, 

the prime mover has the capability to supply any power 

demand up to rated power at constant frequency. The 

synchronous generator connected to it must be able to keep the 

voltage constant at any load condition. 

Figure 2 is a block diagram of the diesel generator. The 

diesel engine kept the frequency constant by maintaining the 

rotor speed. When power demand fluctuates the diesel 

generator could vary its output via fuel regulation to its 

governor. The synchronous generator must control its output 

voltage by controlling the excitation current. Thus, the diesel 

generating system, as a unit, must be able to control its 

frequency and its output voltage. The ability of the diesel 

generator to respond to frequency changes was affected by the 

inertia of the diesel gen-set, the sensitivity of the governor, 

and the power capability of the diesel engine. The ability of 

the synchronous generator to control its voltage was affected 

by the field-winding time constant, the availability of the DC 

power to supply the field winding, and the response of voltage 

control regulation. 

Governor

f *

f 

Diesel Engine Synchronous 

Generator 

V *

V 

fuel

+ 

_ 

exciter 

_ 

+ 

To power network 

f*: reference frequency 

V*: reference voltage 

Fig. 2. Diesel generator Ccntrol block diagram 

B. Wind-Turbine Generator 

The power generated by the wind turbine was defined as 

follows: 

3 P = 0.5ρACpV (1) 

where: 

ρ = density of air 

A = swept area of the blade 

Cp = performance coefficient 

V = wind speed. 

The torque generated by the turbine was represented as: 

s

P T

ω = (2) 

where: 

T = mechanical torque at the turbine side 

P = output power of the turbine 

ωs = rotor speed of the wind turbine. 

A typical Cp curve characteristic defined Cp as a function 

of the tip-speed ratio (TSR) given by equation 3: 

V

R TSR ω s = (3) 

where R was the radius of the wind turbine rotor. A typical 

power versus wind speed characteristic of the wind turbine is 

illustrated in Fig. 3. 

Wind Turbine Output

Wind Speed (m/s)

5 7.5 10 12.5 15 17.5 20

Power (kW)

0

50

100

150

200

250

300

Fig. 3. A typical wind turbine output versus wind speed. 

Because this turbine was constant pitch, the wind turbine 

relied on the blade-stall condition in the high wind speed 

region to limit the output power. The generator used was an 

induction generator. Thus, the rotor speed varied within a 

very small range or was practically constant. In the high 

speed, the tip speed ratio was expected to be small because the 

wind speed increased at constant rotor rpm, thus the power 

coefficient, Cp, was also very small. The output power was 

self-regulated up to 225 kW. Note that in the higher wind 

speeds (>15 m/s), the output power of the wind turbine was 

very flat. In the lower wind speeds, the output wind turbine 

was almost linear. 

C. Fuel Cell 

A group of proton exchange membrane (PEM) fuel-cell 

stacks were applied to enhance the performance of the hybrid 

system. The operation principle of PEM fuel cell can be found 

in the literature, [e.g., 7,8]. 

A MATLAB/SIMULINK-based dynamic model for the 

PEM fuel cell [9] was adapted to the Vissim packet [10] for 

this study. 
 

3

A block diagram of the fuel-cell power-generation system 

that we used is given in Fig. 4. Fuel cell stacks were connected 

in series/parallel combination to achieve the rating desired. 

The output of the fuel cell array was connected to a DC bus 

through a DC/DC converter. The DC bus voltage was kept 

constant via a DC bus voltage controller. The DC bus voltage 

was then interfaced with the utility power grid or a custom 

load through a DC/AC inverter, together with its voltage and 

frequency regulators. 

Fuel Cell Array

Connected to the

utility power grid

or a custom load

DC

VFC

DC

AC

DC

VDC

VDC,ref

Controller 1

-

+

VAC,ref

Controller 2

-

+

VAC

Controller 3

f

ref

-

+

f

3-phase or

Single Phase

Controller 1: DC Bus Voltage Controller

Controller 2: AC Bus Voltage Controller

Controller 3: Frequency Controller

Fig 4. Block diagram of a fuel-cell power system 

D. Dispatchable Load 

We assumed that the dispatchable load was a simple dumpload capable of absorbing real power from the hybrid power 

system. The purpose of this dump-load was to quickly absorb 

any excess wind power, thus helping to control the system 

frequency. In this paper, the dump load was modeled as an 

adjustable power sink and not modeled as a specific load, such 

as an electric heater, electrolyzer, battery charger, or a 

combination of all. 

IV. CONTROL PHILOSOPHY 

The control philosophy in this system was based on 

maximizing the nature-dependent energy sources and reducing 

the diesel fuel consumption. Thus, the power generated by the 

wind turbine was dictated by the wind, and the diesel and fuel 

cell were operated to balance the offset between the source 

and the load consumption. The wind turbine was properly 

compensated to help with the voltage regulation. Because of 

the nature of the control of the diesel generator, the diesel 

generator was used to control the frequency (via its governor) 

and the output voltage at the power network (via its 

excitation). 

Dispatchable loads, such as an electrolyzer, battery charger, 

and heater, were deployed as a dumpload to ensure that the 

generator always carried minimum load, avoiding negative 

power output of the diesel generator (i.e. thus it is not possible 

for the wind turbine to drive the synchronous generator into 

synchronous motor).

A. No Fuel Cell Connected 

In this simulation, no fuel cell contributed to the power 

system. The power was generated by the diesel generator and 

the wind turbine. The wind speed varied and so did the power 

generated by the wind turbine (Fig. 5). The wind speed 

reached its peak at 14 s with output power of about 210 kW. 

The lowest output power was reached at about 34 s. The 

reactive power compensation was shown to be adequate. 

WTG kW and kVAR

Time (sec)

5 10 15 20 25 30 35 40 45 50

P and Q of WTG

-100

0

100

200

300 P-kW

Q-kVAR

Fig. 5. Real and reactive power output of WTG 

Fig. 6 shows the real and reactive power output of the 

diesel generator. The wind turbine output variation was 

reflected in the real and reactive power output. The total 

output of both the diesel generator and the wind turbine output 

were used to compensate for the load and the water pump. 

The start-up of the water pump was reflected by power spike 

at t = 10 s. P(kW) and Q(kVAR)

Diesel Power and Reactive Power

Time (sec)

5 10 15 20 25 30 35 40 45 50

0

100

200

300

400

500

Water pump starts 

P-kW

Q-kVAR

Fig. 6. Real and reactive power output of diesel gen-set 

The output power of the diesel generator also exhibited the 

power required by the village load shown in Fig. 7. The real 

and reactive power consumed by the village load varied with 

time, reaching maximum power at 500 kW. This load was 

modeled as a constant impedance load. Between t = 23 s and 

t = 25 s, the village load was under-served because of lower 

wind speed during that time frame; the wind turbine generated 

about 150 kW, the water pump drew 70 kW, and the diesel 

generator reached its rated power. 

The lack of generation in the time frame shown is also 

reflected by the frequency and voltage traces shown in Fig. 8. 

The frequency dip shows that there was a sufficient deficiency 

in power generation that could not be fulfilled by temporarily 
 

4

stored energy in the generator inertia, resulting in frequency 

decline. The frequency returned to normal when the load was 

reduced to 400 kW. The start-up of the water pump did not 

affect the frequency very much because the surge during the 

water pump start-up was very short. The voltage dip, 

however, was significant as a result of large reactive power 

demand during water pump start-up. When there was a 

sudden reduction in village load, both the excitation and the 

governor of the diesel could not react fast enough, resulting in 

an increased step in the voltage and frequency (around t = 25 

s). 

P (kW) and Q (kVAR)

Village Load

Time (sec)

5 10 15 20 25 30 35 40 45 50 -600

-500

-400

-300

-200

-100

0

100 P-kW

Q-kVAR

Underserved load 

Fig. 7. Village load versus time 

Time (sec)

Voltage and frequency in p.u.

5 10 15 20 25 30 35 40 45 50

V and F (in p.u.)

.90

.95

1.00

1.05

1.10

Voltage

Frequency 

Water pump starts Underserved load 

Load reduced 

Fig. 8. Voltage and the frequency of the systems

B. Fuel Cell Connected to Support the Load. 

When the fuel cell was connected to the network, the power 

source came from three different sources. The wind turbine, 

the diesel generator, and the fuel cell output were combined to 

supply the load. A comparison of figures 8 and 9 shows the 

difference in the voltage and the frequency before and after 

the fuel cell was installed in the system. 

During start-up of the water pump, the voltage dip was the 

same as the voltage dips without the fuel cell installed. The 

load was fully served during the entire observation. The 

voltage and frequency dip was previously seen when the load 

was underserved (between t = 23 s and t =25 s). The load 

reduction did not show a voltage and frequency jump between 

t = 25 s and t = 30 s) as shown previously when the load was 

underserved. 

Voltage and frequency in p.u.

Time (sec)

5 10 15 20 25 30 35 40 45 50

V and F (in p.u.)

.90

.95

1.00

1.05

1.10

P-kW

Q-kVAR

Water pump starts 

Load fully served 

Fig. 9. Voltage and the frequency of the systems with fuel cell installed 

As shown in Fig. 10, the village load was fully served as a 

result of the additional power provided by the fuel cell. It was 

expected that the fuel cell would compensate for the power dip 

resulting from the wind power reduction (t = 23 s). 

Village Load

Time (sec)

5 10 15 20 25 30 35 40 45 50

P (kW) and Q (kVAR) -600

-500

-400

-300

-200

-100

0

100

P-kW

Q-kVAR

Load is fully served

Fig. 10. Village load with fuel cell installed 

P-fuel-cell, P-load, P_w_pump

Time (sec)

0 5 10 15 20 25 30 35 40 45 50

P (kW) -300

-250

-200

-150

-100

-50

0

50

Fuel Cell

Pump

Fig. 11. Output power of fuel cell, water pump 

The fuel cell did not respond very well during the start-up 

of the water pump (Figure 11, sharp increase of load demand), 

but it did respond appropriately during low wind generation 

(between t = 23 s to t = 25 s) and when the village load was 
 

5

reduced (between t = 25 sto t = 30 s). During a very short 

period of time (t = 17 s to t = 23 s), the dump load (shown as 

negative power) was deployed to prevent the frequency 

drifting up (during high wind). As mentioned above, the 

dump load could be substituted with a battery charger or 

electrolyzer, but this is not modeled at present. 

C. Wind speed is reduced. 

When the wind speed decreased, the power deficit was 

divided between the two power sources (the diesel and the fuel 

cell). The fuel cell and the diesel had to generate higher 

power in order to maintain the power balance in the system. 

The wind speed presented in the previous sections was 

decreased by 3 m/s. Fig. 12 shows the comparison of the two 

wind speeds. The impact of reducing the wind speed could be 

very significant in the lower wind speed region because of the 

linearity of the wind turbine output in this region. In the 

higher wind speed, the wind turbine output was very flat (in 

the stall region); thus, the difference in the output power 

would not be very significant. 

In this scenario, the fuel cell contributed more power to the 

network. Figure 13 shows that the voltage and frequency 

regulation was very good. The voltage dip during water pump 

start-up did not change, and the frequency and voltage during 

low wind (between t = 23 s to t = 25 s) and during load 

reduction (between t = 25 s and t = 30 s) was very stable. 

 

Hybrid Project

Time (sec)

0 10 20 30 40 50

Wind Speed (m/s)

0

5

10

15

20

Fig. 12. Wind speed reduced by 3 m/s 

The output power of the wind turbine decreased 

significantly, especially in the lower wind speed region. The 

maximum output power of the wind turbine was 200 kW, and 

the minimum output power of the wind turbine was 50 kW. 

The contribution of the fuel cell was necessary to compensate 

for the lack of power generated from the wind turbine. Fig. 14 

shows the real and reactive power generated by the wind 

turbine. The reactive power was shown to be positive at some 

places. This was possible because the wind turbine was 

compensated by a fixed capacitor. At lower output (lower slip 

due to lower winds), the induction generator required less 

reactive power; thus, there was a small export of reactive 

power from the wind turbine when it generated lower power. 

Voltage and frequency in p.u.

Time (sec)

5 10 15 20 25 30 35 40 45 50

.90

.95

1.00

1.05

1.10 Voltage

Frequency

Fig. 13. Voltage and frequency at lower wind speed. 

 

WTG kW and kVAR

Time (sec)

5 10 15 20 25 30 35 40 45 50

P and Q of WTG

-100

0

100

200

300 P-kW

Q-kVAR

Fig. 14. Real and reactive power of the wind turbine at reduced wind. 

 

P-fuel-cell, P-load, P_w_pump

Time (sec)

5 10 15 20 25 30 35 40 45 50

P (kW) -250

-200

-150

-100

-50

0

50

100 Fuel Cell

Pump

Fig. 15. Fuel cell operation at higher power. 

Fig. 15 shows the output of the fuel cell. The fuel cell must 

operate at higher output power when the wind turbine 

generation is low. The fuel cell operated at about 60 kW in 

the lower wind speed region (t > 28 s). 

V. CONCLUSION 

This paper investigates the parallel operation of a wind 

turbine, a diesel generator, and a fuel cell array. The wind 

turbine contributed significantly to the load, thus reducing the 

fuel consumption of the diesel generator when the wind speed 

was available. 

In this case study, when the fuel cell was not included, the 
 

6

voltage and frequency deviations of the wind/diesel/fuel cell 

system could be significant, thus affecting the system power 

quality. The diesel generator had to operate near its rated 

power, thus limiting its capability to adjust the output 

frequency. With the fuel cell installed, the system voltage and 

frequency variations were minimized, and the system had 

good voltage and frequency regulation. 

VI. ACKNOWLEDGMENT

We acknowledge the support of the U.S. Department of 

Energy and the National Science Foundation through grant 

ECS-0135229. 

VII. REFERENCES

[1] Muljadi, E., Flowers, L., Green, J., Bergey, M., "Electrical Design of 

Wind-Electric Water Pumping," ASME Journal of Solar Energy 

Engineering, Nov. 1996, Vol. 118, pp. 246-252. 

[2] Muljadi, E., McKenna H.E., "Power Quality Issues in a Hybrid Power 

System," Proceedings of the IEEE Industry Applications Society 

Conference, Chicago (CD ROM version), September 30 - October 4, 

2001. 

[3] Tsitsovits, A.J. , Freris, L.L., Dynamics of an Isolated Power System 

Supplied from Diesel and Wind, Proc. IEEE, 130, Part A, No. 9, pp. 587-

595, 1983. 

[4] Kunisch, H.J., Kramer, K.G., Dominick, H., “Battery Energy Storage, 

Another Option for Load-Frequency-Control and Instantaneous 

Reserve,” IEEE Transactions on Energy Conversion, Vol. EC-1, No. 3, 

September 1986, pp. 41-54. 

[5] Muljadi, E., Butterfield, C.P., Gevorgian, V.,"The Impact of the Output 

Power Fluctuation of a Wind Farm on a Power Grid," Conference 

Record, Third International Workshop on Transmission Networks for 

Offshore Wind Farms, Royal Institute of Technology, Stockholm, 

Sweden, April 11 - 12, 2002. 

[6] IEC Standard 61 400-21 Measurement and Assessment of Power Quality 

of Grid Connected Wind Turbines. International Electrotechnical 

Commission. 

[7] Smith, J.A., Nehrir, M.H., Gerez, V., Shaw, S.R. “A broad look at the 

workings, types, and applications of fuel cells,” Proceedings of 2002 

IEEE Power Engineering Society Summer Meeting, Vol. 1, pp. 70-75, 

July 2002, Chicago IL. 

[8] Larminie, J.,Dicks, A., Fuel Cell Systems Explained, John Wiley & 

Sons, Ltd., 2001. 

[9] Wang, C.,Nehrir, M.H., “A Dynamic Model for PEM Fuel Cells Using 

Electrical Circuit,” Proceedings of 35th North American Power 

Symposium, pp. 30-35, Oct 2003, Rolla, MO. 

[10] VisSim/Simulink Translator User’s Guide-Version 5, Visual Solutions, 

Inc, 2002. 
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Parallel Operation of Wind Turbine, Fuel Cell, and Diesel 

Generation Sources: Preprint 

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14. ABSTRACT (Maximum 200 Words)

We investigated a small isolated hybrid power system that used a parallel combination of dispatchable and nondispatchable power generation sources. The non-dispatchable generation came from a nature-dependent wind 

turbine, and the dispatchable generations were a fuel cell and a diesel generator. On the load side, the nondispatchable portion was the village load, and the dispatchable portion was the energy storage, which could be in 

many different forms (e.g., space/water heater, electrolysis, battery charger, etc.) The interaction among different 

generation sources and the loads was investigated. Simulation results showed the effect of the proposed system on 

voltage and frequency fluctuations. 

15. SUBJECT TERMS

wind turbine, diesel, fuel cell, power generation, parallel operation, hybrid generation 

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