_drehfeldgenerator

7/30/2019 _drehfeldgenerator

1/6

OPTIMISED POWER FLOW ON WIND POWER PLANTS WITH THE

DOUBLY FED INDUCTION GENERATOR

Balduino Rabelo, M.Sc. Wilfried Hofmann, Prof. Dr.

Chemnitz Technical University

Faculty of Electrical Engineering

Department of Electrical Machines and Drives

Reichenhainerstrasse 7009127 Chemnitz Germany

Tel: +49 371 5313586/3323 Fax: +49 371 5313324

email: [email protected] [email protected]

http://www.tu-chemnitz.de

Abstract. Abstract In this work the efficiency of the electrical part of a wind generation system is

studied. A wound rotor induction machine is used with two back-to-back converters on the rotor

circuit in an slip power recovery scheme. The slip control results in an operating speed range that

suits the wind energy generation requirements. The vector-controlled doubly fed induction generator

(DFIG) enables the decoupling between active and reactive power as well as between torque and

power factor. The losses in the machine copper and those originating in the converters are calculated

and a loci of the optimal points inside the operation range is derived. Therefore, besides the optimal

speed tracking for maximal wind energy conversion, an optimal control strategy for the two

converters is developed. This strategy aims to minimise the electrical losses and is based on the

control of the reactive power flow of the system. The harmonic losses as well as those of the core and

the LC-filter have not been taken into consideration.

Keywords: Renewable energy systems, induction machines, SVC (static Var compensation),

optimisation.

1. INTRODUCTIONThe wound rotor induction machine presents some

advantages in its use as a generator in wind power plants as

described in [1]. From an economical point of view, in the

case of wind generation, the losses on the system play an

important role. In an induction machine the efficiency is

mainly influenced by the copper losses on the rotor,

modelled by the rotor resistance. That is, the smaller the

slip, the smaller these losses. In the case of the DFIG, the

slip can be controlled and it is of particular importance that

the static converters between the mains and the rotor circuit

can be dimensioned in such a way that only a small part of

the entire power, that is to say the slip power, flowsthrough the rotor. This also enables the improvement of the

overall efficiency, because of the substantial converter

losses. By means of a LC-filter one can produce

(capacitive) reactive power and, under certain operating

conditions, influence the mains voltage and the power

factor. The machine supplies active power to the network

(over-synchronous operation) or is fed from the network

(sub-synchronous operation) through the rotor [2], [3]. A

short overview of the system can be seen in the second part

of this work, and the control principles are derived in [4].

For larger systems, in the MW range, it is necessary to

minimise these losses specified above. A 600 KW DFIG

system was examined in [5] in relation to the minimal

converter losses operating condition, whereas the machinelosses remained unconsidered. Sub-optimal values for

reactive power division were implemented as a table for

different conditions of operation.

In this study the copper losses in the generator are

calculated and both sub-optimal procedures, converter and

machine minimal losses are compared. Additionally, the

system total losses for the different operating points are

calculated, as well as the optimal reactive power

distribution. The latter in order to achieve the minimal total

losses.

2. DESCRIPTION OF THE SYSTEMThe generator slip-rings are connected to a rotor-side

converter (RSC), which shares a DC-link with the mains-

side converter (MSC) in a so-called back-to-back

configuration. Both, RSC and MSC, can lead a third of the

machine rated power. The AC-side of the MSC is connected

through an LC-filter a three-phase capacitor bank and

series inductances to the network, while the stator of the

generator is connected to the network by a protective

switch. The system diagram is shown in fig. 1.


2/6

Fig. 1. System overview

The RSC controls the d and q-components of the rotor

current that is the electromagnetic torque and the power

factor. So the active and reactive power, which flow through

the rotor and stator, can be controlled. The d-component of

the MSC current feeds the DC-link voltage during under-

synchronous operation and during over-synchronous

operation energy is delivered to the mains. Additionally the

reactive power requirement of the system and of the LC-

filter is controlled by the q-component of the MSC current,and so the systems power factor can be influenced. The LC-

filter has the function of damping the harmonics in the line

current and supplying the capacitive reactive power

necessary for the magnetisation process of the generator.

This enables the system to operate at different power factors

and therefore to fulfil the supply needs.

3. MINIMAL COPPER LOSSESThe copper losses in the generator depend on the machine

coil resistances and on the stator and rotor currents, as

described in [6]

( ) ( )2222rqrdrsqsdsCu IIRIIRP +++= (1)

To calculate the minimal copper loss, first the stator

currents are represented as functions of the machine

inductances, rotor currents and rated flux as in (2) and (3).

rd

s

m

sd IL

LI

= (2)

rq

s

m

s

sq

sq IL

L

LI

= (3)

Then these expressions are properly substituted in equation

(1) and differentiated upon the q-component of the rotor

current, as follows

0= rqCu

IP (4)

With the previous computed value of the optimal q-

component of the rotor current, the optimal q-component of

stator current can be calculated, in a sense that both must

build the machine rated flux.

22

mssr

sqms

rqLRLR

LRI N

OPT +

= (5)

OPT

N

OPT rq

s

m

s

sq

sq IL

L

LI

= (6)

The machine parameters employed here are described in

table 1 at the end of the paper. The curves showing the

machine copper losses for different values of stator power

factor and in relation to the d-axis stator current are plotted

in fig. 2. It is noticeable here that for each value ofIsdor

active power, there is one optimal value of stator power

factor that minimises the generator losses. These optimal

values are normally inductive due to this natural

characteristic of the machine.


3/6

Fig. 2 Machine copper losses in relation to the d-axis

stator currentIsd.

The copper losses in percent related to the optimal values

for different stator power factors can be seen in fig. 3.

These curves are computed by expression (7).

optCuCu

CuCu

PP

PP

= (7)

Fig. 3 Copper losses in percent of the optimal losses

Fig. 4 and 5 show, respectively, the optimal stator power

factor curve and the generator efficiency in relation to

active power range orIsd.

Fig. 4 Optimal stator power factor versus currentIsd.

Fig. 5 Generator efficiency .

4. CONVERTERLOSSESIn order to calculate the converter losses, the followingmodel from [ 7] is used:

)()()( DPTPTPP VSVSVDV ++= , (8)

whereby Pv is the total loss on the converters, Pvd(T) and

Pvs(T) are the conducting and switching losses of the used

semiconductor, IGBT's in this case, respectively, and

Pvs(D) correspond to the switching losses of the diodes.

Further, expression (9) is so characterised:

++=2

2

12

6 aCEacaCEV IrEIfIUP (9)

whereby ,

)()()(2

1DoutToutTin eeeE ++= (10)

In these equations (9) and (10) UCE is the collector-emittervoltage of the IGBT's, fc corresponds to the switching

frequency, einand eoutthe turn on and off switching energy

of the semiconductors and Ia the converter rms current.These parameters were obtained from the SEMIKRON

Databook for semiconductors [8].

In fig.6 one sees the copper and MSC energy dissipation by

full load and for different power factors in relation to the

reactive power dividing factor .

,n

s

Q

Q= (11)

where Qs and Qn are the reactive power values on the stator

and on the network, respectively. The dashed line

corresponds to the minimal copper losses in the generator.

For each value of the power factor there is meets a certain

value of, which minimises these losses.


4/6

Fig. 6 Machine Copper Losses

The RSC losses have their minimal loss points near the

minimal copper losses line whilst the MSC minimal losses

occur on different points and are more significant than those

of the RSC and the generator, as it is shown in fig. 7.Therefore it is meaningful to employ an optimisation of

the MSC losses instead of the generator or the RSC. In

fig. 8 one sees that this procedure gives the closest results to

the minimal total losses.

Fig. 7 MSC losses

Fig. 8 Total losses

The optimal values of, which minimise the total losses for

different conditions, differ from the sub-optimal values,

which optimise the machine copper losses. The former are

closer to the sub-optimal values of, which minimise the

MSC losses. With this interpretation one can then project

the whole system.

5. OPTIMAL SYSTEMLOSSESThe maximal wind energy density is found somewhere

between the minimum and maximum power ratings. If the

system operates under different values of active power, it is

important to calculate the optimal values of for the whole

power operating range.

In fig. 9 to 11 one sees the surfaces representing the system

losses in relation to the active power and , for values of

slip and power factors. On these surfaces the locus of the

points of minimum losses for each active power value is

plotted. The diagrams show the copper, MSC converter

losses and their specific minimum values (indicated by the

thicker line), as well as the total minimum values. In

addition one can also say that minimising the MSC loss is

the procedure which most closely reaches the optimum for

the entire operating range.

Fig. 9 Total system losses in relation to the active power

and

Fig. 10 MSC losses in relation to the active power and


5/6

Fig. 11 Generator copper losses in relation to the active

power and .

The following diagram in fig. 12 illustrates the optimal

values of for a certain slip and different values of thepower factor.

Fig. 12 Locus of the optimal reactive power dividing

factor.

For small values of the active power the values of lead

against infinite, which is to be expected, if the LC-filter

supplies the necessary reactive power for magnetisation of

the generator. These values can be implemented as a look-up table, as it is described in [7] for a sub-optimal method.

These results were simulated for a 5 minutes time series of

measured wind data for the real plant from [2], for the

system working with 0.9 inductive power factor, and

compared with the sub-optimal method of dividing the

reactive power implemented in the generator control. This

latter is based on fixed values of for each power factor

and were derived from the optimal values for full load

condition. It is clear that the optimal method shown here

achieved better energy savings because it considers all load

conditions to which the windmill is submitted regarding to

the variation of wind.

Figure 13 shows the speed on the generator axis as well as

the developed electromagnetic torque and the input power

for this wind time series. The torque values were computed

from the speed versus torque characteristic of the

WINDTEC 600 windmill [9].

Fig. 13 Speed, electromagnetic torque and power input on

the generator axis.

The instantaneous gains on the system efficiency for

optimal and sub-optimal reactive power dividing are

compared on the figure 14.

Fig. 14 Achieved efficiency values for both methods

optimal and sub-optimal.

In average for the simulated real time the optimal method

efficiency was 98.8 % against 97.1 % of the sub-optimal

method. Furthermore, it seems that the improvements are

better for weak wind speeds. It is remarkable to say that the

best results on improving the system efficiency occur for

inductive operation of the plant and the optimal and sub-

optimal methods have the same results for unity power

factor.

These results will be published on further works, where the

annual energy production will be computed and so the

impact on the costs of energy production will be analysed.


6/6

6. CONCLUSIONIn order to minimise the losses, which occur in the machine

copper, there is an optimal stator power factor for each

active power value. The network reactive power

requirement is then fulfilled by the MSC converter and the

LC filter.

The fact that the minimisation of the copper losses

regarding the system total losses is not optimal is evident.

However, this is not negligible, if the life span of the

generator is taken into account.

The question arises as to whether it is or not worthwhile to

use the system total loss optimisation procedure instead of

the sub-optimal procedures. Furthermore, the expected

improvements and their expenditure also need to be

considered.

The loss models used here are parameter dependent, and

these parameters should correspond with reality, in order forthe optimisation to be successful. This calls for adaptive

control and/or artificial intelligence schemes.

In future work, the LC-filter losses as well as the harmonic

losses will be considered as well as its dimensioning

regarding not only the filtering characteristics but also

system losses. The dynamic behaviour of the wind power

plant will also be investigated. Thus the variation of the

system states will be determined during changes in the

operating conditions. This will allow for the development of

suitable control strategies.

7. REFERENCES[1] Leonhard. W.,Control of Electrical Drives, Springer-

Verlag, Berlin,1985.

[2] Hofmann,W., Thieme, A., Control of a Double-Fed

Induction Generator for Wind-Power Plants, Proceedings

of Power Quality, pp.275-282, May,1998.

[3] Dittrich,A., Hofmann,W., Stoev,A., Thieme, A., Design

and Control of a Wind Power Station with Double Fed

Induction Generator, Proceedings of EPE97,pp.2723-2728, Trondheim,1997.

[4] Quang,N.P., Dittrich,A., Thieme,A., Doubly-fed

induction machine as generator: control algorithms with

decoupling of torque and power factor, Electrical

Engineering 80, pp 325-335, 1997.

[5] Hofmann,W.,Optimal Reactive Power Splitting in Wind

Power Plants Controlled by Double-Fed Induction

Generator, Proceedings of AFRICON99, pp. 943-948,

Cape Town, Sept. 1999.

[6] Tang,Y., Xu,L.,A Flexible Active and Reactive PowerControl Strategy for a Variable Speed Constant Frequency

Generating System, IEEE Trans. on Industry

Applications,1993.

[7] Hofmann, W., 'Blindleistungsoptimierte Regelung von

Windkraftanlagen mit Doppeltgespeisten

Drehstromgeneratoren', Fachtagung Leistungselektronik u.

Intelligente Bewegungssteuerungen, LIBS99, pp.216-221,

Magdeburg, March, 1999.

[8] SEMIKRON 99 Power Electronics99

[9] Windtec 600 Windmill Technical Data;

http://www.windtec.tlk.co.at.co.at

Parameter Value

Power 600 KW

Number of Poles 4

Rs 3.6 m

Rr 4.3 m

Lls 0.680 mHLlr 0.865 mH

Lm 5.4 mH

Tab. 1 Machine Parameters

THE AUTHORS

Mr. Balduino Rabelo was born in

Guanhaes, Brazil and got his B.Sc. and

M.Sc. degrees in electrical engineering

from the Federal University of Minas

Gerais (UFMG) in 1992 and 1998,respectively. He is currently working

towards his Ph.D. degree at the Technical

University of Chemnitz. His research

interests are control of induction machines and converters,

parameter identification and renewable energy systems.

Prof. Wilfried Hofmann was born in

Dresden, Germany and got his Dipl.Ing.

and Dr. Ing. degrees in electrical

engineering at the TU Dresden in 1978

and 1984, respectively. He is the

Chairman of the Department of Electrical

Machines and Drives at the Technical

University of Chemnitz since 1992. His

research interests are control of induction machines and

converters, position control, magnetic bearings, hybrid

electromechanical vehicles and renewable energy systems.

ACKONWLEDGEMENT

We would like to thank for the financial and technical

support from the Deutsche Forschungsgemeinschaft (DFG)

and from the Department of Electrical Machines and

Drives of the Technical University of Chemnitz.

_drehfeldgenerator

Documents

Transcript of _drehfeldgenerator