Energy management of a wind farm

A.ABDELKAFI, A.MASMOUDI, L.KRICHEN and A.OUALI Department of Electrical Engineering, National School of Engineering, University of Sfax

BP 1173, 3038, Sfax, Tunisia

 Abstract‐‐ In this work, we are interested in the integration of a wind energy conversion system (WECS) based on a permanent magnet synchronous generator (PMSG) to the power electric system. The control of the active power developed by the wind turbine is ensured by pitch control. The reference active and reactive powers should be calculated according to load demand which defines the active and reactive powers and on the forecasted curve of the wind speed. In a second stage, we are studied a wind farm including three wind turbines. The main goal of this study is to contribute to the participation of the wind energy management to satisfy load power constraints. Key‐words: Wind Turbine, PMSG, Converters, Control, Energy Management  

1. Introduction

The world consumption of the electric power evolves and this evolution depends mainly on three factors which are the economic growth, the demographic trends and the massive industrialization of certain countries. These factors largely resulted in increasing the planetary requirements in electricity. It is for that, the tendency is currently directed towards the exploitation and the integration of the renewable resources to meet the energy needs on the one hand and to fight against polluting gas the massive outbursts [1] on the other hand.

The wind power belongs to the new means of

production of decentralized electricity. During the last decade, the wind power records the fastest increase in the world of about 30% per year of the total request in electricity [2][3] because this energy respects the environment it is nonpolluting [4].

Currently the wind turbine does not take part in the production monitoring. So, this state should change quickly, taking into account the more significant share of this turbine in the wind farms. Because of the very random character of the wind, a wind turbine alone cannot adapt the production to consumption. This adaptation must be done by the intervention of other wind turbines having a reserve of power allowing a fast adjustment of the production.

In order to ensure the balance production-consumption, we should establish a control strategy which varies according to the availability of the sources and the load. This enabled us to contribute to

the participation of the produced wind energy management to satisfy the power constraints in the connection node.

The impact analysis of a wind farm in the connection node, and in particular, the impact of balance between the production and consumption were the subject of this study. The objective is to produce a total active power imposed by the load dispatching center on the connection node. For that, an appropriate control must be developed to transmit the required power and to maintain the load voltage.

The control strategy is decomposed into two

levels: a local control according to each wind turbine sizing "Pitch control" [4][5] and a global control which depends on the produced power by the wind farm and the total required power. To do this, we have established an algorithm which demonstrates that the three wind turbines can work in independence or in interaction.

2. Detailed wind farm model

In order to increase the power generated by the wind turbine, we considered the example of a wind farm represented in Fig.1. It consists of 3 wind turbines spatially organized and inter-connected based on a variable speed synchronous generator. These three generators are connected to a common continuous bus by three PWM converters.

In fact, the association of wind generators around a

common continuous bus is currently considered for wind power stations.

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In this paragraph, the model is extended to the all

wind turbines. The control of the continuous bus voltage will be particularly studied in relation to the transit of power between the wind turbines and the connection node. Indeed, if the connection of several generators on a common continuous bus reduces the number of power converters, it makes the control of the continuous bus voltage more delicate. Moreover, the converter connected to the connection node must be dimensioned to be able to forward the total generated power.

2.1. Modeling of the wind turbine

The mathematical relation for the mechanical power extraction from the wind proportionally depends of the wind speed cubed, as to relation:

12 , 1

with P the power extracted from the wind , ρ is the air density 1.22 Kg m , R is the length of the blade m , is the wind speed ms and C λ , β is the power coefficient.

The power coefficient expresses the rotor aerodynamics as a non-linear function of both tip speed ratio λ and the pitch angle of the rotor blades β. The tip speed ratio is defined as the ratio between the linear blade tip speed and the wind speed, expressed as

Ω 2

where Ω is the rotational speed of the blades rad s

The power coefficient is a function of the tip speed ratio λ and, for the used wind turbine, the following form is used to approximate it as a function of λ and β[6]:

, 0.53151

0.58 0.002 .

10 exp18.4

3

witch

110.02

0.0031

4

PMSG

Conv 1

Gear box

Turbine 1

Turbine 2

 

Conv 2

 

Conv 4

Grid and

Load PMSG

 

Gear box

Turbine 3

Conv 3

PMSG

 

Gear box

C

Fig. 1. Structure of the studied wind farm

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Similarly, the aerodynamic torque on the turbine axis is given by

5

where is the torque coefficient, which is linked to the power coefficient by

,,

6

2.2. Modeling of the gearbox

The gearbox multiplies the slow speed of the blades to approach the generator speed around the synchronous one through the gear box ratio G.

Ω Ω 7

where Ω is the machine speed.

2.3. Modeling of the drive drain The mechanical system is modeled by the

simplified motion equation:

Ω T fΩ 8

where is the mechanical inertia of the wind turbine and the generator, is the mechanical torque, T the electromagnetic torque, and f is a friction coefficient.

2.4. Modeling of the PMSG

The model generally used of the PMSG is the Park model. By considering only the fundamental harmonic of the flux distribution in the air-gap of the machine and by neglecting the homopolar component, the theory of the space vector gives the dynamic equations of the stator voltages as follows:

L L I 9

L L 10

where is the phase resistance of the stator winding Ω , L stator inductance , is the flux of the permanent magnetic , and are the

– components of the stator voltages , respectively, and are the – components of the stator currents , respectively, ω is the rotor electric speed ) and is the number of pairs poles.

The electromagnetic torque is given by:

T I 11

2.5. Modeling of the converters

The modeling of the converters is made by using the concept of instantaneous average value [10]. The

converter consists of three legs of which its two ideal switches are in complementary states. A switching function is defined for each power switch [2]. It represents the ideal commutation orders and takes the value 1 when the switch is closed and 0 when it is opened :

f f 1 12

V V V

= 2 1 1 1 2 1 1 1 2

13

where V is the simple voltage modulated by PWM converter.

3. Maximum delivered power

The main control objective of variable speed wind turbine generator is power efficiency maximization. Fig.2 illustrates the aerodynamic power according to the PMSG speed for various values of the wind speed. The PMSG output generated power must follow the maximum delivered power characteristic for different wind speeds to optimize the turbine operating, in other words to keep the constant Ω ⁄ .

Fig. 2. Aerodynamic power versus PMSG speed.

For this reason, the measured turbine speed is

chosen to determine the PMSG reference power. is the maximum power which must be extracted

from the wind:

12

Ω 14

where is the maximum power coefficient and is the optimum speed ratio.

A reference power value can be imposed as

follows: - if < , then we can provide ,

adaptation of the device that ensures the maximum point of power .

- if > , then we cannot provide , beyond the nominal output of the turbine the

0 50 100 150 200 250 3000

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Rotational Speed (rad/s)

Aer

odyn

amic

Pow

er (

W) 6m/s

8m/s10m/s12m/s14m/s

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maximum power is extracted, the pitch control is activated and the PMSG works at constant power. For strong speeds, the aerodynamic brake is operated.

The block diagram of the whole pitch controller is

shown in Fig. 3. When the wind speed is higher than the nominal value, the blades are pitched in order to limit the power extracted from the wind. From Fig. 4 it can be concluded that the optimal pitch angle is approximately zero below the nominal wind speed and from this speed, the pitch angle increases with the wind speed. This observation greatly facilitates pitch control [4]. The study of the dynamics of this actuator is very complex [5-9]. The regulation of the pitch control is modeled by a regulator with proportional action P generating a reference pitch variation. This reference is limited by a first order transfer function with a time constant τ , gives the dynamic behavior of the speed of pitch variation. The operating curve of the studied wind turbine is illustrated in Fig. 4. The pitch angle deviation is necessary to limit the power to the nominal value.

4. Management of the assessment: production - consumption

According to the specialized literature, the control of the wind turbines is done preferably by the aerodynamic mechanics systems. However, following the transformation energy principles of the wind it appears that another form to do the regulation of the produced power by the wind turbine is to act on its rotor speed.

Balance between the electric production and

consumption is reached when the sum of power injections is equal to the sum of load demands and

taking into account the core losses in the lines. Balance is essential from the safety point of view. Indeed, an imbalance between consumption and the production cause:

- Modifications of power flow. - Frequency deviation with respect to its main value.

The control strategy of the wind farm in this context must take account of the power demand in the connection node and the available renewable energy. Indeed, at a given moment, the load demand can be lower or higher than the available energy. Thus, the wind generators can take part in the ancillary services in the total management of electric power and specially the smoothing of the active power.

The control strategy is decomposed into two

levels: a local control according to the sizing of each wind turbine "Pitch control" and a global control according to the power requested from the connection node and the available the wind power. The objective consists in increasing or decreasing of the pitch angle to obtain the equality between the power generated and the required one.

The three wind turbines considered in this study

are subjected to three different winds v , v and v . The generated powers of the three turbines P , P et P are then distinct. In this study we supposed that the total active power requested from the node of connection is P . The reference power for each wind turbine is P P 3⁄ .

In the case where, the power delivered by the wind

turbine i is higher than the requested power, it is necessary to increment the pitch angle β to adapt the production to consumption. In the contrary case, we decrease the pitch angle β . This type of adjustment is called independent adjustment between the three wind generators.

Fig. 4. Operating curve of the wind turbine

5 10 150

5

10

15

Wind speed (m/s)

Bet

a__r

ef (°

)

5 10 150

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0.2

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Pow

er C

oeff

icie

nt

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Aer

odyn

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) Without pitchWith pitch

  Proportional Regulator

1 

 

Fig. 3. Pitch actuator modeling

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In the case where, the power delivered by the wind turbine i is lower than the power requested by the load and the blade angle β becomes negative, thus the turbine i cannot ensure its contribution of energy production, i.e. the one third of the requested power. This deficit of energy is caused by imbalance between the production caused by the wind speed, and consumption. To satisfy the load, we adjust the pitch angle β and β of the turbine j and k. Indices i, j and k are defined as follow:

If i 1 then j 2 and k 3 If i 2 then j 1 and k 3 If i 3 then j 1 and k 2

For example, when the wind turbine 1 is not able to provide energy enough, turbines 2 ou/et 3 must replace it as far as possible.

In this paragraph, we will study the case where one or two wind turbines can provide a power higher than the required power whereas the remainder of the wind turbines cannot provide its share of the required power.

In this case it is necessary to calculate the total generated power P P P P and to compare it with the total requested power P 3P . Si P P , it is necessary to increment the pitch angle β to adapt the production to consumption, and vice towards that.

The control device "pitch control" adjusts the blade angle β , β and β so that the power produced by the three turbines is equal to the consumed power in the connection node.

If the requested power remains higher than produced energy, and to solve this problem, a first solution consists in disconnecting the less priority load. A second solution uses the storage energy systems.

5. Simulation results

To validate the control strategy ensuring the total management of the wind power, we applied three profiles of wind represented by Fig.5 on duration of 500s. The power demand as reference power is given in Fig.6. We supposed in this study that the power-factor is fixed at 0.85.

The obtained simulation results prove the effectiveness of the applied control techniques to establish a balance between the production and consumption. But if the required power remains higher than the generated power, it is necessary to add another system to ensure the balance and to smooth the power.

Fig. 5. Wind speeds

Fig.6. Requested active power

Fig. 7. Local control “pitch angle”

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Fig. 8. Commande globale

Fig. 9.Commande totale de l’angle d’orientation des pales

Fig. 10. Fig. 11.

To protect the wind turbines against the mechanical failures in the event of over speed, we noticed on the fig. 7 that the aerodynamic behavior of each turbine can be controlled actively by modifying the blade angle β , β and β . This control system makes it possible to limit the wind power for strong winds.

Fig.8 shows the effect of the global control

strategy to adjust the blade angles. With this type of control, the three reference angles Δβ , Δβ and Δβ are regulated with a value to ensure the adaptation between the wind power and the consumption at the connection node. Therefore we increase or decrease the β (Fig.9) to obtain the balance between these three turbines and the power demand. β β Δβ

with: i 1,2,3

The consumed active and reactive powers are represented in figures 10 and 11. The power demand is perfectly satisfied during the interval 0,430s . This result shows the effectiveness of the control strategy to eliminate the imbalance between production - consumption. Between 430 and 500s the power demand remains higher than the produced energy. In this case it is necessary to add another system to ensure balancing and to smooth the power.

These results show the effectiveness of the adopted

control strategy based on the increasing or the decreasing of the pitch angles. The control law makes it possible to attenuate the fluctuations of the electric and mechanical quantities following the variations of the wind speed and the load. The effect of pitch control appears on the adjustment of the active and reactive powers.

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In the case where, the power delivered by the wind turbine i is lower than the power requested by the load and the blade angle β becomes negative, thus the turbine i cannot ensure its contribution of energy production, i.e. the one third of the requested power. This deficit of energy is caused by imbalance between the production caused by the wind speed, and consumption. To satisfy the load, we adjust the pitch angle β and β of the turbine j and k. Indices i, j and k are defined as follow:

If i 1 then j 2 and k 3 If i 2 then j 1 and k 3 If i 3 then j 1 and k 2

For example, when the wind turbine 1 is not able to provide energy enough, turbines 2 ou/et 3 must replace it as far as possible.

In this paragraph, we will study the case where one or two wind turbines can provide a power higher than the required power whereas the remainder of the wind turbines cannot provide its share of the required power.

In this case it is necessary to calculate the total generated power P P P P and to compare it with the total requested power P 3P . Si P P , it is necessary to increment the pitch angle β to adapt the production to consumption, and vice towards that.

The control device "pitch control" adjusts the blade angle β , β and β so that the power produced by the three turbines is equal to the consumed power in the connection node.

If the requested power remains higher than produced energy, and to solve this problem, a first solution consists in disconnecting the less priority load. A second solution uses the storage energy systems.

5. Simulation results

To validate the control strategy ensuring the total management of the wind power, we applied three profiles of wind represented by Fig.5 on duration of 500s. The power demand as reference power is given in Fig.6. We supposed in this study that the power-factor is fixed at 0.85.

The obtained simulation results prove the effectiveness of the applied control techniques to establish a balance between the production and consumption. But if the required power remains higher than the generated power, it is necessary to add another system to ensure the balance and to smooth the power.

Fig. 5. Wind speeds

Fig.6. Requested active power

Fig. 7. Local control “pitch angle”

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Win

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