alleviation of power fluctuations in wecs by different...

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 4, Issue 1, January 2015

215

ISSN: 2278 – 7798 All Rights Reserved © 2015 IJSETR

Abstract—With worldwide increasing shares of electricity generated by wind energy conversion system (WECS) and their influence on the mains stability and contribution to reliable energy supply becomes more and more relevant. This paper compares the performance of different control methods for Wind energy converters, regarding their electrical power-output fluctuation at different wind conditions. The results shows the different types of mitigation methods for power fluctuations and their

corresponding results.

Index Terms— Bi-directional dc–dc converter,

Current source inverter, Dual inverter, Energy storage, Electric double-layer capacitor, Non-integer voltage ratio, Space vector modulation, Wind energy, Wind power generation.

I. INTRODUCTION

In an attempt to prevent the realization of fears about

the global warming, renewable energy sources has

experienced a huge face lift to generate the electricity.

However, the generated power from the renewable source

is always fluctuated due to an environmental status. In

order to prop up renewable energy and reimburse the

fluctuating power, an energy storage system is effective

one. Among that Wind power is the fastest growing

renewable energy source due to its improving

technologies and economical competitiveness. Generally,

the availability of wind energy supply cannot be

controlled like energy conversion from fossil fuels;

because, wind energy underlies a stochastic fluctuating

behavior. These fluctuations cause dynamic power

oscillations in the wind energy converter’s (WEC) power

drain. In particular, at locations with highly turbulent

Manuscript received Jan, 2015.

Anitha.N, Electrical and Electronics Engineering, Kumaraguru

College of Technology, (e-mail: [email protected]). Coimbatore, India.

Narmatha Lakshmi.S, Electrical and Electronics Engineering,

Kumaraguru College of Technology, Coimbatore, India, (e-mail: [email protected]).

Geethanjali.S, Electrical and Electronics Engineering, Kumaraguru

College of Technology, Coimbatore, India, (e-mail: [email protected]).

wind characteristics, critical load peaks might occur in the

WEC plant. They propagate from the wind rotor to the

connected electrical grid and cause premature damage in

mechanical components, thermal overloads in electrical

components, as well as voltage variations in the mains

power supply. Their grid impact increases with rising

connected power of single plants or wind parks [1].

Fig.1. General configuration of a variable-speed WEC

In principle, variable-speed WECs provide the

technical possibility to reduce the cumulative load in the

power drain and the influence on the mains power supply

grid. Therefore, specially designed automatic controls

and operation management are required to run the

appliance. The automatic controls are reference variable

controls, which set the system to the optimal operating

point. The main task of the operation management is to

force a power or rotor speed set point for the momentarily

operation condition. For the owner of WECs or wind

parks, the foremost interest is to sell as much energy as

possible regardless on their grid influence. At the same

time, the grid operator prefers a smooth power delivery to

keep the grid influence at a low level itself. Just in recent

years, more regulations have been passed to control the

grid influence from WEC’s.

In this paper two different methods are compared in

regard of their influence on the reduction of power

Alleviation of Power Fluctuations in WECS by

Different Control Methods

Anitha.N, Geethanjali.S , Narmatha Lakshmi.S


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fluctuation of the WEC’s. The control methods

considered in this investigation are direct integration of

battery energy storage systems , Electric double-layer

capacitor applied for energy storage system. These

control methods are described in Section III. Foremost,

the following section gives a short overview of the system

characteristics of WEC’s.

II. WIND ENERGY CONVERSION SYSTEM

The wind rotor converts the air mass’s kinetic energy

into mechanical (rotating) energy. The drive shaft

transfers this rotating energy to the electrical generator.

Most WECs system uses gear boxes to match the

operating speed of the wind rotor and the generator. The

electrical energy is then fed into the electrical grid. The

generator can be directly connected to the mains power

supply, or power converter systems can provide the

coupling of the generator and the electrical grid (Fig.

1)[1].

In addition, the application-specific requirements are

maximum energy yield, low influence on the grid, and

low strain/stress in the power drain. The air mass’s kinetic

energy has a specific power Pwind that is characterized

by the air mass density ρA, the surface area of the wind

flow AR, and the wind speed w. It can be described by the

following formula:

Pwind= ½ ρAAƦω3

(1)

Keep in mind the cubic influence of the wind speed on the

wind power.

To gain the maximum energy yield, the wind rotor has

to operate at highest efficiency. The aerodynamic

efficiency is expressed by the cP coefficient, which

determines the amount of power extracted (PRotor) from

the wind crossing the rotor area

CP= PRotor

PWind (2)

According to Betz, the maximum cP value is cP,Betz

=0.59. The efficiency of wind rotors strongly depends on

the airfoil angle of incidence. To take into account the

rotational speed of the wind rotor and the wind speed, the

resulting angle of incidence is indicated by λ. λ is called

the tip speed ratio and describes the relation of the wind

rotor tip speed to the wind Speed ,

λ=2ΠRnR

𝜔 (3)

where R is the rotor radius and nR is the rotor speed.

In order to gain advantage of this special

characteristic, the WEC must be able to operate at

variable speed [2]. This demands a special structure of the

generator grid coupling. The main task is the decoupling

of the generator rotational speed from the grid voltage

frequency. In Fig. 1, general configuration of a variable

speed WEC’s structure is shown, where a dc link

connects the generator to the grid.

III. CONTROL METHODS FOR WECs

A. Direct Integration of Battery Energy Storage Systems

(BESS) for WECs

A direct integration method, presents a new direct

integration scheme for BESS with the use of grid-side

inverter. It utilizes the popular dual inverter topology, as

shown in Fig.2, where two 2-level inverters are cascaded

through a coupling transformer. The two inverters are

named as the main inverter and the auxiliary inverter in

line with their modes of operation. A battery bank is

directly connected to the dc link of the auxiliary inverter

without interfacing dc–dc converter. Unlike the simple

direct connection topology, this system facilitates full

controllability over charging/ discharging currents and

voltage of the battery.

The main inverter operates at the fundamental

frequency producing square wave outputs. Harmonics of

the square wave output are compensated by the

high-speed auxiliary inverter. This particular frequency

splitting arrangement can reduce switching losses as well

as device ratings of the main inverter. Another advantage

of this system is its ability to produce up to 13 voltage

levels in the phase voltage waveform whereas the

traditional 2-level inverter can produce only five levels

[3] & [4].

Fig.2. Experimental setup used to verify the linear

relationship between main inverter dc-link voltage and

battery power.

Extensive research has been done on modulation and

control of the aforementioned dual inverter topology,

especially for motor drive applications [5]-[8]. They all

have considered cases where fixed-integer dc-link voltage

ratios are present. A pulse width modulation (PWM)

scheme for this dual inverter is explained in [9] for 1:1

and 1:2 voltage ratios. Although a power sharing

controller is proposed in for dynamically varying dc-link

voltages, it also assumes identical dc-link voltage

variations thus making the ratio to be 1:1. A hierarchical


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modulation method is proposed here to handle the

dynamic changes in the auxiliary inverter dc-link voltage.

The operation is illustrated in Fig. 3. where the auxiliary

inverter is purposely turned off until 20ms. During this

period, the only available output is the main inverter

square wave output voltage as shown in the first half of

the waveform in Fig. 3. The harmonic distortion of the

output voltage is significant under this operation. After

20ms, the auxiliary inverter is turned on, and

consequently, the output voltage becomes smooth with

low harmonic distortion as shown by the second half of

the waveform in Fig.3.

Fig.3.Square wave output of the main inverter and

smoothing effect of the auxiliary inverter.

The wind speed profile shown in Fig. 4(a) is used in the

simulation, which in turn produces a dc-link voltage

variation at the main inverter as shown in Fig. 4(b).

Corresponding wind power variation Pw and dispatch

power Pd are shown in Fig. 4(c). From this graph, it can

be concluded that the proposed system has the ability to

supply the demand amidst fluctuations present in the

input power. The surplus or deficit of power is supplied or

absorbed by the battery with a current profile as shown in

Fig. 4(d). The output current and voltage of the inverter

are shown in Fig. 4(e) and (f), respectively. Although the

inverter output voltage shows some fluctuations, once it is

passed through a low-pass filter, a smooth waveform can

be observed as shown in Fig. 4(g).

Fig.4.(a) Wind speed. (b) Main inverter dc-link voltage

Vdc and auxiliary inverter dc-link voltage Vdcx . (c) Wind

power Pw and dispatch power Pd . (d) Battery current Ib .

(e) Inverter output current ias . (f) Inverter output voltage

before filtering vas . (g) Inverter output voltage after

filtering, vas,f .

Additional switches and converters required to

integrate energy storage devices into distributed power

systems can be avoided if the grid-side inverter itself can

be used as the interface. Accordingly, a modified

topology of the popular dual inverter system has been

proposed to connect a battery bank directly to the

auxiliary inverter dc link. The challenge with this

topology is the uncorrelated and dynamic changes present

in dc-link voltages, which results in unevenly distributed

space vectors. A detailed analysis on the effects of such

variations is presented in this section. Furthermore, a

modified SVM method is proposed to produce desired

current waveforms even in the presence of unevenly

distributed space vectors. The above simulation results

shows the efficacy of the proposed modulation method

and battery charging/discharging process.


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B. Electric double-layer capacitor applied for energy

storage system

In recent years, an energy capacitor system (ECaSS)

connected an electric double-layer capacitor (EDLC) with

power electronics devices have been developed as energy

storage system [10]-[12] and applied in power

system[13]-[15]. An EDLC is a safer one and has a longer

service life than the secondary battery, and requires

virtually no maintenance, but having the following

disadvantages: the dielectric voltage-withstand level of a

EDLC-cell is 3 V or lower and high internal-resistive loss

is directly proportional to squared current. To overcome

these issues, though parallel-monitor limiting the

charging voltage is attached to each EDLC-cell, the price

brings on accordingly expensive. Also, a voltage-source

inverter (VSI) has been widely used for ECaSS. However,

VSI becomes increasingly difficult to regulate an output

power due to the voltage drop at terminal of EDLC-bank.

In this section the current-source ECaSS (CS-ECS) using

a current-source inverter (CSI) is used to resolve an issues

of ECaSS using VSI. CS-ECS has the several merits. For

example, the output regulation is possible even if a

decrease or a fluctuation in dc-voltage arises as stored

power discharges. Also, high internal-resistive loss of

EDLC-cell can be reduced by a bi-directional dc-dc

converter, regulating dc current, since EDLC-bank is

connected to ac-feeder through bi-directional dc-dc

converter.

Fig.5.(a) Circuit configuration of current source ECS. (b)

Pulse gate signals.

The circuit configuration and pulse gate signals for

CS-ECS is shown in Fig. 5. The CS-ECS consists of a

multilevel CSI (connected to two full-bridge inverter),

bi-directional dc-dc converter (four-quadrant dc-dc

converter: FQ dc-dc converter), and EDLC-bank. The CSI

is utilized to reduce harmonic components of ac-current.

Pulse gate signals (12 pulses) for the CSI are show in Fig.

5(b). In order to balance the shunt currents (idc1 and idc2)

of dc-current idc, pulse gate signals in

the period of 120◦ are alternately supplied to the

respectively arm devices (e.g., U-arm upper devices Up1

and Up2). As shown in Fig. 5(b), this procedure is

repeated periodically. Referring concurrently to Figs. 1(a)

and 2, EDLC-bank is stored with an electric energy

(charging mode) when a polarity of dc-voltage vdc is

positive, and discharging mode when the polarity is

negative, because the polarity of dc-current idc is always

positive. Fig. 6(a) shows a diagram of operating principle

for FQ dc–dc converter.

Fig.6(a).Operating quadrant of current-source inverter.


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Fig.6(b). Conceptual diagram of active and reactive power

control.

Conceptual diagram for active and reactive power control

can be expressed by d-q axes reference coordinate frame,

as shown in Fig. . In Fig.6(b). α is the phase and In is the

root mean square of ac-current. Since dc-current idc is

proportional to In in the Fig.6(b). the active and reactive

power (pe and qe) are controlled by α and idc.

Fig. 7. Simulation results of wind turbulence and EDLC-cell failure. (a) Wind speed, mechanical input torque and rotor speed.

(b) Tie-line power and system frequency. (c) WTG terminal d-q axes voltage. (d) Active and reactive output power of

CS-ECS. (e) DC-side quantities and stored power of CS-ECS.


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Fig.7. shows simulation results of wind turbulence and

EDLC-cell failure, where solid line and dashed line show the

simulation results with CS-ECS and without CS-ECS,

respectively. As shown in Fig. 7(a), the nominal wind speed

for simulation is 9 m/s and the sine fluctuation is ±2 m/s. As

can be seen from Fig. 7(b) and (c) that the fluctuation of

WTG terminal voltage and tie-line power are effectively

suppressed by CS-ECS, and the tie-line power converges to

its steady state value due to the effect of filter. As can also be

seen from Fig.7.(e), although EDLC-cell break down at t =

7.0 s, CSECS continue to compensate only by 4-parallel

EDLC-bank. Also, shunt currents (idc1 and idc2) are

balanced by supplying pulse gate signals of the period of 120◦

in the respectively arm devices, alternately. Fig.8.Shows the

active and reactive power of CS-ECS.

Fig.8.Active and reactive power of CS-ECS.

Here the CS-ECS is used as an energy storage system

using current-source inverter to resolve the issues of ECaSS

using voltage-source inverter. This proposed system is

applicable to promote renewable energies, i.e., wind power

generation or solar power generation. The control system for

the active and reactive power control of CS-ECS is also

shown.

IV. CONCLUSION

The investigations have shown that the control methods of

WEC have great influence on their characteristic behavior.

The fluctuation of the power output of two different control

methods have been evaluated. In the direct integration

scheme the variable voltage ratio, power sharing and

maximum power point tracking and battery charging and

discharging are considered in the reduction of power

fluctuations. In Electric double layer capacitor system, the

output regulation is possible even if a decrease or a

fluctuation in dc-voltage arises as stored power discharges.

Also, high internal-resistive loss of EDLC-cell can be

reduced by a bi-directional dc-dc converter, regulating dc

current, since EDLC-bank is connected to ac-feeder through

bi-directional dc-dc converter. The results have shown a great

difference between the performances of the control methods.

Therefore the two kinds of control methods are studied in

mitigating power fluctuations under wind energy conversion

system.

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