Gasch R., Twele J. (Eds.) Wind Power Plants: Fundamentals, Design, Construction and Operation

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452 13.2 Wind turbines for stand-alone operation

Fig. 13-19 shows the schematic design of a pitch-controlled turbine feeding an in-

sular grid. The power demand P

grid

can be met at a wind speed of 5 m/s if the op-

timal angle of attack is used. For higher wind speeds (A), the angle of attack re-

duces gradually thanks to the pitch control. This leads to a constant power output

of the rotor at required synchronous speed.

Fig. 13-20 Wind turbine with stall control and consumer control system for stand-alone opera-

tion, a) Block diagram, b) Load curve

13 Concepts of electricity generation by wind turbines 453

Fig. 13-21 Variable-speed asynchronous generator with an intermediate DC circuit

If the wind speed becomes so small that, even with the optimum angle of attack,

the wind turbine cannot meet the demand (B), a control system has to decide

which consumers are to be temporarily disconnected. This will often be loads,

which serve a storage facility like pumps or refrigerators, etc.

Fig. 13-20 shows a stall-controlled turbine. It differs from the above system in

that an excess energy output (A) is fed into resistors. The advantages are that the

energy converted into heat by resistors can be utilized for heating, and that the

rotor construction is more robust. If the wind speed decreases below the point

where the rotor can supply all the power demanded by the grid, a consumer con-

trol system has to switch off lower priority loads (B), in this case too.

13.2.5 Asynchronous generator operating in an insular grid

If an asynchronous generator is connected to a grid, it draws reactive power

required for its operation from the grid. In a local grid, this is not possible. The

reactive power has to be supplied by capacitors or through fairly complex power

electronic.

Capacitors provide reactive power that is dependent on frequency and on the

square of the voltage. If one of these values changes the reactive power provided

by the capacitors changes, too. Consequently, the usable range of rotational speeds

is extremely limited. So, if an asynchronous generator is used in a wind turbine,

the reactive power has to be provided by a number of switchable capacitor stages.

A stall-controlled wind turbine is necessary (Fig. 13-22). Usually, a substantial

amount of auxiliary equipment is needed. This is the reason, why this concept did

not gain wider acceptance, in spite of inexpensive asynchronous machines.

Variable-speed wind turbines with an asynchronous machine often have an

intermediate DC circuit. The converter on the generator side provides the reactive

power for the asynchronous machine. If the start-up excitation is supplied by

capacitors, a smaller frequency converter will suffice. It is possible to manipulate

454 13.2 Wind turbines for stand-alone operation

active and reactive power separately (Fig. 13-21). The performance characteristics

of the rotor and the load curve correspond to the system shown in Fig. 13-9. For

the grid side, frequency and voltage are set by the inverter. The control effort is

high, but for the most parts standardised commercial equipment can be used.

Fig. 13-22 Asynchronous generator in a local stand-alone grid for heating; reactive power is

supplied by capacitor banks

13 Concepts of electricity generation by wind turbines 455

13.3 Wind turbines in isolated grids

Outside of the urbanised areas all over the world, there are small towns, villages

and single farms without a reliable electric power grid. The classical solution, a

diesel generator, is becoming less and less affordable due to rising oil prices. The

demand for wind-diesel systems and wind-photovoltaic systems is rising.

But due to the boom of grid-connected wind turbines in Western Europe since

1991 further development of the above systems was neglected. The concepts from

the 1980s are no longer up-to-date: power electronics now offers a large variety of

new possibilities.

Although there are several prototype plants in Greece and on Spanish and

Portuguese islands in the Atlantic Ocean, there are few field-tested ones on the

market:

Neither systems with the low power ratings desired for single farmsteads nor

those of a few MW suitable for smaller towns are readily available.

In principle, variable-speed wind turbines with a synchronous generator can be

used. However, the supervisory and control logic for a hybrid system is quite dif-

ferent from that of a grid-connected one.

Further, there is a lack of inexpensive storage systems which buffer the fluctua-

tions of the wind energy. Flywheels for short-term storage are slowly appearing on

the market (ENERCON [6], URENCO, etc.). Battery storage continues to be used

for long-term storage even though they are good for only a limited number of

charge / discharge cycles [4, 8, 14].

The diesel motor should not run in stand-by mode in parallel to the wind tur-

bine for long periods of time. During idling the injection nozzles of the diesel en-

gine collect soot and the fuel consumption is not negligible (5 to 10% of rated

consumption, which is around 190 g /kWh for large motors).

To avoid cold starts of the diesel engine the coolant may be preheated electri-

cally while the motor is turned off. This will reduce mechanical wear during start-up

substantially and lead to a significantly longer service life of the motor. However,

it does consume some energy.

A certain amount of voluntary load reduction can be achieved if LEDs are

installed on the meter of the customer:

green: cheap tariff, wind power,

yellow: medium tariff, wind and diesel power,

red: expensive tariff, diesel power.

This leaves it to the consumer to decide on his own, and at the same time he

develops a certain feel for the system.

Fig. 13-23 shows the typical diurnal curve of the wind and diesel power pro-

duction for a small grid on the Island of Brava (approx. 4000 inhabitants).

The wind turbine has a rated capacity of 120 kW. The “penetration” of the sys-

tem by wind power is relatively high, at times up to 50%.

456 13.2 Wind turbines for stand-alone operation

Wind

Diesel

250

200

150

100

14 15 16 17 18

20 21 22 23 24

Time of day in h

Power in kW

Fig. 13-23 Typical diurnal wind and diesel power production for a small local grid on the island

of Brava, Cape Verde [12]

Fig. 13-24 Instructions for the manual control of the pump for the storage of drinking water of a

small local grid according to the wind conditions [12]

However, manual load management is still used for these small systems. Fig. 13-24

shows the concept of the supervisory system. If the power demand of the

13 Concepts of electricity generation by wind turbines 457

inhabitants is sufficiently low, the storage reservoir (geodetic head of H = 435 m)

of the drinking water system will be filled [12].

The diesel generator on Brava is the master machine of the grid despite the high

wind power penetration since the wind turbine is of the classic Danish concept. A

wind-diesel system, in which the diesel can be turned off automatically, requires

substantially more control equipment.

13.3.1 Wind-diesel system with a flywheel storage

The wind turbine E-30 (250 kW) delivers power into a DC power line from which

the power is then fed via an inverter into the three-phase grid, Fig. 13-25. As long

as the supplied wind power is below the demand of the grid the diesel generator

runs in parallel and governs the grid and matches the shortfall. Neither the fly

wheel storage nor the master synchronous machine (MSM) is on-line.

If the available wind power exceeds the demand from the consumer, the fly

wheel storage starts up (1,5 t rotating mass, rotational speeds between 1500 and

3300 rpm). It can store up to 5 kWh and can deliver on demand up to 200 kW for

a short time. The grid is now governed by the master synchronous machine, and

the diesel generator is switched off. Short deficits in the wind power supply are

filled by the fly wheel storage and a battery bank if one exists. The power quality

is excellent (i.e. voltage and frequency are constant), [13], but the costs are high.

Battery bank

WEC Battery other energy- Fly wheel MSM

sources, e.g. solar storage

580V - 630V

Diesel



3-phases-supply

400V

50Hz



Battery bank

WEC Battery other energy- Fly wheel MSM

sources, e.g. solar storage

580V - 630V

DieselDiesel



3-phases-supply

400V

50Hz



Fig. 13-25 Stand-alone system with a master synchronous machine (MSM), acc. [13]

458 13.2 Wind turbines for stand-alone operation

13.3.2 Wind-diesel system with a common DC line

The system sketched in Fig. 13-26 was developed for an island in the Gulf of

Finland where there is a radar station and a homestead [10]. The synchronous gen-

erator is rated at 30 kW and uses permanent magnets for its field. It feeds – like

the diesel generator and the bank of batteries – into a common DC line. Finally, an

inverter converts from DC into 400 V three-phases AC for output.

The batteries are able to store approx. 100 kWh. The diesel generator may feed

the three-phase AC line directly via the bypass in case of long times without wind

or of a technical problem at the wind turbine or the batteries.

The rectifier will then be bypassed.

Rectifier

///



///



Diodes

Fuses

250 Ah / 408 V

Battery bank

30 kW

D = 14 m



400 Volt

Diesel

Permanent-

magnetic

generator

///



400 Volt

Radar

station

Bypass only at fault

Rectifier

///



///



Diodes

Fuses

250 Ah / 408 V

Battery bank

30 kW

D = 14 m



400 Volt

Diesel

Permanent-

magnetic

generator

///



400 Volt

Radar

station

Bypass only at fault

Fig. 13-26 Wind-diesel system for the island Osmussaare (radar station) [10]

13 Concepts of electricity generation by wind turbines 459

13.3.3 Wind-diesel-photovoltaic system (minimal grid)

A control system offered by the company SMA is able to coordinate the power

generation from various renewable energy sources. All these sources feed into a

common three-phase line, as shown in Fig. 13-27.

13.3.4 Final remark

The variety of available hybrid systems with ratings between several kW and

1000 kW is at present quite small. However, their number will be expected to

grow as the price of oil goes up and the hunger for electric power intensifies in

countries that have many small towns without connection to a power grid. Hybrid

systems will eventually reach a level of maturity comparable to that of today’s

grid-connected wind power plants.

CHP

48 kWh

48 V / 60 V

Battery bank

optional:

WEC with RF

e.g. 2.5 kW

14 kW

3~ 400 V

50 or 60 Hz

Diesel

Heat



optional:

PV-installation with RF

3 - 15 kWp

optional:

Mini-CHP

4.7 kW / 12 kW

Consumer

3-phases supply

for loads up to 27 kW

for an annual need of

6000 - 30000 kWh

…

CHP

48 kWh

48 V / 60 V

Battery bank

optional:

WEC with RF

e.g. 2.5 kW

14 kW

3~ 400 V

50 or 60 Hz

DieselDiesel

Heat



 



optional:

PV-installation with RF

3 - 15 kWp

optional:

Mini-CHP

4.7 kW / 12 kW

Consumer

3-phases supply

for loads up to 27 kW

for an annual need of

6000 - 30000 kWh

…

Fig. 13-27 Hybrid system with power generation from several energy sources in a small grid [7]

460 13.2 Wind turbines for stand-alone operation

References

[1] Dietrich, W.: Drehzahlvariables Generatorsystem für Windkraftanlagen mittlerer

Leistung (Variable-speed generator system for wind turbines of medium capacity),

PhD thesis. Technical University of Berlin, 1990

[2] Schörner, J. et al.: Stand und Entwicklungsrichtung des Antriebsstranges von Wind-

kraftanlagen (State of art and development tendencies of wind turbine drive trains),

Windkraftjournal, Vol. 6/2001, S. 38-48

[3] Alstom: Technische Information (Technical information)

[4] Quaschning, V.: Regenerative Energiesysteme(Renewable energy systems), 4th edi-

tion, Hanser-Verlag, München, 2006

[5] Heier, S.: Windkraftanlagen (Wind energy converters), 3

edition, B.G. Teubner

GmbH, Stuttgart/ Leipzig/ Wiesbaden, 2003

[6] ENERCON: Technische Information Inselsystem (Technical Information Stand-Alone

System), 2001

[7] SMA: Technische Information Sunny Island System Kit (Technical information on the

Sunny Island System Kit), 2003

[8] Cruz-Cruz, I.: Vortrag: Seminario Internacional de Energias Sustenables, Puerto

Natales, Chile, Okt. 2003

[9] Akkmatov, V., Nielsen, A.H. et al.: Variable-speed wind turbines with multi-pole

synchronous permanent magnet generators, Wind Engineering, Vol. 27, No. 6, 2003,

S. 531-548

[10] Ruin, S.: New Wind-Diesel System on Osmussaare, Wind Engineering, Vol. 27, No. 1,

2003, S. 53-58

[11] Bundesverband Windenergie: Windenergie 2002 - Marktübersicht BWE (Wind energy

2002 – Market survey by BWE), p. 122-123

[12] Jargstorf, B.: “High Penetration” Windprojekte - Eine Alternative zu Wind/ Diesel-

Sytemen („ High penetration“ wind energy projects – an alternative to wind/diesel

systems), DEWEK, 1996

[13] ENERCON: Technische Information Stand Alone System, April 2003

[14] ENERCON: Technische Information Battery test for stand alone application, April

2002

[15] Bundesverband Windenergie: Windenergie 2000 - Marktübersicht BWE (Wind energy

2000 – Market survey by BWE)

[16] Hacker, G.: Wind ins Netz – Netzeinspeisung und Akkuladung mit Kleinwindrädern,

(Wind into the grid – Grid-connection of and accumulator charging with small wind

turbines) ISBN 3-00-011545-5, 2003

R. Gasch and J. Twele (eds.), Wind Power Plants: Fundamentals, Design, Construction 461

14 Wind turbine operation at the interconnected

grid

On the one hand, the grid-connected operation of wind turbines places demands

on the wind turbines’ operational behaviour and its technical equipment for grid

connection. On the other hand, the operation of the interconnected grid is increas-

ingly influenced by wind power production. Some years ago, the effects were neg-

ligible because the installed wind power was small compared to the grid capacity.

But nowadays, at least in Germany, Denmark and Spain, the grid integration of

wind energy has become a technical and economical challenge.

The following chapter first describes these challenges from the viewpoint of

the grid. The interface between grid and wind turbine is then highlighted. Finally,

the different wind turbine concepts, described in the previous chapters, are ana-

lyzed with respect to their grid compatibility. This chapter reflects the basics on

the German interconnected grid in 2007, the year of publication of the 5

German

edition of this book. The German electricity market is currently being trans-

formed, among other because of regulations introduced by the EU; therefore, the

keen interested reader should search additional sources for the up-to-date informa-

tion on the electrical grid.

14.1 The interconnected electrical grid

14.1.1 Structure of the interconnected electrical grid

The interconnected grid of Germany is part of the synchronous European inter-

connected extra high voltage grid, Fig. 14-1, of the Union for the Coordination of

Transmission of Electricity (UCTE). Therefore, it has to comply with the

European UCTE guidelines. The structure and the technical standards of this

interconnected grid are mainly based on requirements drawn, decades ago, from

the structure of a distribution grid with only a few large-scale power generation

plants supplying the distributed consumers.

Nowadays, decentralized wind power often feeds into this grid at its endings in

weak and remote rural sub-grid sections, primarily in the coastal areas. Together

with the liberalised international electricity market, this implies the need to trans-

form the interconnected grid from a distribution grid to a transportation grid for

large-scale transfers of electrical energy (not only at the highest voltage level).

As long as wind turbines provided only a relatively small share of the power

generation capacity, they had very little impact on the grid operation. However,