control of a scig wind farm connected to a single power converter

8/12/2019 Control of a SCIG Wind Farm Connected to a Single Power Converter

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Control of a squirrel cage induction generator wind

farm connected to a single power converter

Llus Trilla, Oriol Gomis-Bellmunt , Adria Junyent-Ferre, Agust Egea Alvarez and Antoni Sudria-Andreu

Catalonia Institute for Energy Research (IREC)

Josep Pla 2, B2 Planta Baixa - 08019 Barcelona (Spain).

Email: [email protected] dInnovacio Tecnologica en Convertidors Estatics i Accionaments (CITCEA-UPC)

Departament dEnginyeria Electrica. Universitat Politecnica de Catalunya - 08028 Barcelona (Spain).

AbstractThe aim of this paper is to analyze the control ofseveral wind power generators based on squirrel cage inductiongenerators with one common power converter. This configurationis especially useful for offshore wind farms with a VSC-HVDCtransmission. A comparison with a configuration where eachgenerator is connected to one power converter, is done focusingin the total power extraction. Scalar-controlled squirrel cageinduction generators have been implemented in both cases. Thecontrol scheme has been modified implementing an aggregatedmodel in order to allow the common converter to performappropriated control tasks. A fault ride-through control protectsthe DC bus from overvoltages, it varies the torque and slipreference to limit the total output power. Results have beenvalidated by means of simulation in both scenarios showing thedifferences in the total power obtained and the influence of thegenerator operating point. The performance of the system underonshore grid faults has been simulated.

I. INTRODUCTION

The increasing demand on clean energy is making the

development of wind power more interesting [9]. Offshore

options are becoming more appealing since the difficulty of

finding good onshore locations with strong, regular winds thatare also close to the grid is increasing. Offshore wind farms

situated further than 60 km from the shore can be connected to

the grid through DC links [1] [11] [12]. In this case an offshore

power converter has to perform the DC power conversion,

different connection topologies have been studied by other

authors [7] [10]. Transmission using HVDC requires a full

power converter onshore that adapts the power to the needs

of the grid. This transmission system has some advantages,

as it smooths the impact of the wind farm on grid stability,

especially in areas with weak AC systems.

In modern wind power plants, usually each generator is

controlled by one power converter, which keeps the machine

working at its optimum operating point while maximizingwind power capture. As more devices are working in the

wind farm the probability of failure of one of them becomes

higher. This is an important factor because of the difficulties

accessing an offshore park. In addition, each power converter

adds a percentage to the energy loss and implies a higher initial

investment.

This paper analyzes a topology and control scheme where

one power converter controls several machines at the same

time. This configuration allows the wind farms to be divided

into clusters, controlled with a reduced number of power

converters. In the proposed scheme the control of various

squirrel cage induction generators is performed by variation of

system frequency and voltage, keeping the flux in the generator

constant.

Control actions have to manage a set of wind power

generators. In order to calculate the control reference valuesan aggregated model has been implemented. These models

are used in simulations of large wind farms testing different

connection topologies [13]. In the aggregated model different

incoming wind speeds are considered for each turbine, and

the generator angular speed is measured with sensors, then

the mean speed of all the machines is used by the controller

to compute the frequency and the voltage.

An analysis of the reaction when a wind speed change

is done and a comparison between this configuration and a

classic topology is presented here. A full power converter is

necessary to convert the AC medium voltage to HVDC. As

will be shown, the use of this converter to do control tasks,

and the elimination of individual power converters leads toa poorer performance since the operating point of the park is

moved away from the optimum. On the other hand the number

of elements offshore is reduced and thus there is less energy

loss.

In order to keep the DC bus voltage constant the controller

modifies the torque reference signal limiting the total output

electrical power. The line fault ride through performance of

the implemented control scheme is shown via simulation and

the resultant effect of the torque control is analyzed.

This paper is organized as follows: the wind farm connec-

tion topology proposed is introduced in Section II, followed

in Section III by a description of the model used. The control

scheme is explained in Section IV and simulation results arepresented in Section V. Conclusions are in Section VI.

I I . CONNECTION TOPOLOGY

In order to test the capacity of controlling several machines

in different operating points a common converter configuration

is modelled. This topology consists of several wind power

generators interconnected to the wind farm grid and controlled

by one full power converter. The DC conversion for the trans-

mission to the onshore grid is done by this power converter.

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D. Wind park grid and HVDC transmission

A transformer is connected to each generator and a medium

voltage AC grid has been considered. A high voltage trans-

former delivers the total power to the offshore converter and

the HVDC transmission cables are implemented according to

the Cigre benchmark model [15].

IV. CONTROL SCHEME

A. Scalar Control

The widely used scalar control [2], [4] has been imple-

mented to drive multiple generators. The scalar-controlled

devices have been used for a long time in industry for single

machine control and are easy to implement, although they give

inferior performance then vector control schemes. In general,

the scalar control is applicable when the speed does not have

to change rapidly in the machine.

In this control scheme the ratio voltage/frequency

(V olt/Hz) is kept constant. Assuming that the voltage isproportional to the frequency, flux amplitude is maintained

constant = Use

, neglecting stator resistance Rs.

It is necessary to measure the generator speed and thus use acommunication link in order to send the data to the controller

and compute the torque and the optimum torque. The slip

frequency necessary to minimize the calculated error can be

obtained with a classic PI. The torque is kept proportional to

the slip while the machine is in the linear working area, which

is close to zero slip speed.

In the practical implementation a boost voltage U0 = Rsisis added in order to compensate the drop at the stator re-

sistance. The boost voltage becomes important at low speeds

because the stator resistance tends to absorb more voltage but

is negligible at high frequencies.

B. Aggregated ModelThere are several ways to simulate a wind farm [8], depend-

ing on the topology of the park, the needs of the control or

the variables of interest in the simulation.

In this case it is important to note that the control system

does not control each machine individually; instead, it obtains

information about the whole set of generators. Thus, its task

is to apply the proper control actions to all the machines at

the same time. In order to get the data of the set of generators,

it is worth aggregating the models at some point before the

controller reads it. Fig. 2 illustrates the control scheme used.

In this scheme the aggregation is performed once the rotor

speed is sensed in each machine. When the whole set of speed

of generators is computed the total torque can be calculated as = Pe

gusing the total electric power obtained at the power

converter.

It is worth noting that the total power is obtained from

the total voltage and current applied to the system. This

fact prevents the controller computing the individual torque

of each machine (even knowing their individual angular

speed) because the actual power of each separate generator

is unknown. In this scheme the mean speed (9) of all the

generators involved has been used and the control action is

computed as only one machine has to be controlled. Voltage

and frequency, applied by the converter, are the same for all the

generators because they are sharing the same grid. Other types

of aggregation (e.g. torque, speed square) can be considered

in order to improve the control performance but comparison

of these techniques is out of the scope of this paper.

g =

N

i=1gi

N (9)

Wheregi is the electrical angular speed of each generatorand N is the number of generators involved. The optimumtorque

=KCp 2

g (10)

wherepis the number of pole pairs and r is the mechanicalangular speed of the rotor. Once the slip is computed with the

PI controller the frequency can be calculated e = slip +g =slip+pr and be imposed to the system.

C. Line Fault Ride Through

During a voltage sag in the AC grid the capacity of energyevacuation of the onshore converter drops drastically. When

the DC bus voltage increases and exceeds a minimum level

the controller reduces the torque and the slip reference in order

to decrease the amount of power generated. This reduction is

proportional to the voltage level reached by the DC bus, if this

voltage goes beyond a maximum level the torque and the slip

reference are set to zero until the voltage is back to its nominal

value. In Fig. 2 can be seen a schematic representation of the

line fault ride through control.

D. Onshore converter

The VSC converter controls independently active and re-

active power. This converter has to keep constant the HVDCvoltage adjusting the active power delivered to the grid and

reactive power use to support the grid voltage when a line

fault is detected as is implemented by [16]. There is no

communication between offshore and onshore controllers and

they act separately. When the system is facing a voltage dip the

onshore converter has a limited rank of action, if the HVDC

voltage control is lost the offshore controller starts to operate.

V. SIMULATION RESULTS

Topology and the control scheme have been tested in sim-

ulation using Matlab/SimulinkR, modelling three turbines of

2MW each one. Reactions when facing wind speed variations

and their adaption to these changes for the whole set ofgenerators and each individual machine have been checked.

In the model some simplifications have been made, en-

ergy losses have been considered in cables, transformers and

converters but harmonic distortion in power converter and

transformer saturation are not considered. The parameters used

to simulate the model of generator can be found in [5] and in

Table I.

Two different systems have been compared using the same

wind series, the wind steps used can be seen at Fig. 3.


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Fig. 2. Control scheme

TABLE IPARAMETERS OF THE GENERATOR IN P.U. 50HZ, 2MW, 690V

Rs 0.048 R

r 0.018 Xm 3.800Xs 0.075 X

r 0.120

5 10 15 20 25 30 35 408

8.5

9

9.5

10

10.5

11

11.5

12

time(s)

windspeed(m/s)

Incoming wind speed

Fig. 3. Incoming wind speed

The system has a common power converter and three wind

power generators. The controller performs the control tasks

using the aggregated model. In order to show a comparison

with classical topology another system is modelled connecting

one power converter to each generator. The control schemeused in this case is a classic scalar control.

The total power generated is shown in Fig. 4. Solid line

represents the power generated with the common converter

configuration, and the dashed line is the power generated

with the individual topology. The results show variation in

the power extracted when the incoming winds are different

(the common power converter scheme generates less power

because not all the machines are operating at their optimum

point). In fact as the difference in the incoming winds received

by the turbines increases, its optimum becomes further from

the operating point, and thus less power is generated. However,

when incoming winds are equal for the three turbines the

power generated reaches its maximum level and becomes

the same with both topologies, because all the turbines are

working at their optimum level.

10 15 20 25 30 35 400.52

0.525

0.53

0.535

0.54

0.545

0.55

time(s)

Power(p.u.)

Total electric power

Common converter

Individual converter

Fig. 4. Total electrical power extracted

The individual power generated by each machine is plotted

in Fig. 5 where the horizontal lines mark the limit of the energy

extraction (this limit is reached with the individual converter

topology). As can be seen turbine number 2 is operating closeto its optimum because its operating point is in the middle of

the other two turbines.

The maximum energy extraction is reached when the Cp co-efficient reaches its optimum, in Fig 6. is shown the evolution

of this parameter.

In order to check the performance of the LFRT control a

voltage sag to the 0.1 p.u. has been simulated, Fig. 7. Wind

speed is kept constant but different for each turbine. The

evolution of the torques (solid lines) of the machines and the


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10 11 12 13 14 15 16 17 18 19 200.9

0.95

1

1.05

1.1

1.15

time(s)

Voltage

(p.u

)

DC voltage

Fig. 9. HVDC voltage

10 11 12 13 14 15 16 17 18 19 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

time(s)

Power(p.u.)

Total electrical power

Electric power

Fig. 10. Total power

without strong transient states.

APPENDIX

The parameters used to model the turbine can be find in

Table II.

TABLE II

PARAMETERS OF TURBINE

Inertia 16 105 kg m2 Gear ratio 65.27 c2 116 c7 21Radius 37.5 m c1 0.5 c6 5 c9 0.035

ACKNOWLEDGMENT

This work was supported by the Ministerio de Ciencia e

Innovacion under the project ENE2009-08555.

10 11 12 13 14 15 16 17 18 19 20134

136

138

140

142

144

146

148

time(s)

An

gularspeed(rad/s)

Angular speed

Generator1

Generator2

Generator3

Electrical angular speed

Fig. 11. Mechanical and electrical speeds

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control of a scig wind farm connected to a single power converter

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