33. Electrical - Ijeeer -Power Quality Improvement - Sanjeev Kumar Bhalla

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International Journal of Electrical and

Electronics Engineering Research (IJEEER) ISSN(P): 2250-155X; ISSN(E): 2278-943X Vol. 4, Issue 2, Apr 2014, 289-298 © TJPRC Pvt. Ltd.

POWER QUALITY IMPROVEMENT OF GRID CONNECTED TO DFIG BY ACTIVE

SHUNT FILTER

SANJEEV KUMAR BHALLA1, SATNAM SINGH MATHARU

2 & R. K. JARIAL

3

1School of Electronics & Electrical Engineering, Lovely Professional University, Phagwara, Punjab, India

2Department of Electrical Engineering, CTIEMT, Jalandhar, Punjab, India

3Department of Electrical Engineering, NIT, Hamirpur, Himachal Pradesh, India

ABSTRACT

Due to increased proliferat ion of Non Linear Devices in the networks, the Power Quality of the Grid deteriorates.

With increasing dependence on Wind Power as a renewable source of energy by the utilities, a large no. of DFIGs are

connected to the Grid. The Non Linear Loads draw a significant amount of Harmonic Current which in turn generates

Voltage Harmonics also in the Grid on account of Grid impedance. The present paper focuses on mit igation of the

Harmonics generated by a Non Linear Load using Active Shunt Filter. This ensures that the DFIGs connected to the Grid

are safeguarded from the detrimental effects of Harmonics, thus enabling them to maintain the Power Quality Standards.

As shown in the simulation results, the proposed model developed in PSCAD/EMTDC is capable of suppressing the

Current Harmonics substantially from very high to low levels which are acceptable as per prescribed standards.

The proposed system thus ensures that the current impurity is not passed on to the grid and high power quality is

maintained.

KEYWORDS: Doubly Fed Induction Generator (DFIG), Power Quality (PQ), Active Shunt Filter (ASF), Wind Turbine

(WT), Total Harmonic Distortion (THD)

1. INTRODUCTION

The increasing proliferation of Non Linear Loads in the network is resulting in PQ issues such as Flicker,

Harmonics, and Voltage Sags & Swells. Poor PQ results in many problems like reduced life of Equipment, deteriorated

system performance, mal functioning of control equipments and protection devices. The PQ issues have gained more

significance off late on account of development of highly sensitive equipments, in terconnection of large networks and

increasing awareness of consumers towards PQ.

DFIGs connected to the grid are generally located at remote locations. The Harmonics generated on account of

Non Linear Loads can travel to the DFIG through Long Transmission Lines and can largely compound the PQ problems of

DFIG which is expected to meet the PQ standards for various wind speeds and turbulence.

A technique for Harmonic Compensation by using an Active Filter connected in series to a passive filter that leads

to a practical and economic solution has been earlier proposed [1]. Uses and advantages of applying Active Power Filters

to compensate Power Distribution System has been presented in [2]. In [3], a comprehensive review of active filter

configurations, control strategies, selection of components and other related economic and technical considerations and

their selections for specific applications has been summarized.

http://www.tjprc.org/

290 Sanjeev Kumar Bhalla, Satnam Singh Matharu & R. K. Jarial

Impact Factor (JCC): 5.9638 Index Copernicus Value (ICV): 3.0

An active shunt filter based on P-Q theory has been proposed using a standard 16-bit microcontroller which

allows dynamic power factor correction and both harmonics and zero sequence current compensation. [4]. With the use of

Artificial Neural Network A lgorithm, the functionalities of a Shunt Active Filter can be enhanced [5] and it can

compensate for balanced and unbalanced Non Linear Load Currents and correct the Power Factor of the supply near to

unity.

Power Quality Improvement using Matrix Converters with Series & Shunt Filters have been presented in [6].

By using Active Shunt Filter, the Current THD was reduced from 30% to 15%, resulting in saving of energy 1% of

maximum load capacity in text ile mills where variable frequency drives with 6 pulse rectifiers were used [7].

In the previous work, the authors of this paper have developed a system model of DFIG in the dedicated power

electronics and system simulat ion tool, PSCAD/EMTDC [8]. As per universally acceptable IEEE Standards, THD of line

current should be less than 5% for grids with short circuit ratio less than 20 at the PCC[9]; so the present paper focuses on

the mitigation of current harmonics generated by a Non Linear Load. This ensures that the DFIGs connected to the Grid are

safeguarded from the detrimental effects of Harmonics, thus enabling them to maintain the PQ st andards. The paper has

been organized in the following sections: Section II explains the proposed system model of grid connected DFIG.

In section III, the machine and converter control method of DFIG has been described. The schematic of ASF for Grid

connected DFIG and its control circuit has been presented in Section IV & V respectively. The results have been analyzed

and discussed in Section VI. The conclusions of the investigations have been summarized in Sect ion VI I.

I. SYSTEM DESCRIPTION

The System model for grid connected DFIG has been modeled in PSCAD/EMTDC as shown in Fig ure 1.

The stator of the wound rotor induction machine is connected to the low voltage balanced three -phase grid and the rotor

side is fed via two back-to-back PWM converters containing IGBTs with a common DC bus. The front–end converter

controls the power flow between the DC bus and the AC side and allows the system to be operated in sub -synchronous and

super synchronous speed.

Figure 1: Schematic Diagram of Gri d Connected DFIG for Wind Turbine Application

The DFIG is connected to the HV grid after stepping up its voltage from 0.69 kV to 11 kV through a transformer

at PCC. The equivalent impedance of the transmission line for the case considered is represented as a lumped parameter.

The grid voltage is 11 KV and its equivalent Thevenin impedance is Z th. The vector control strategy of the power converter

is based on the stator flux field oriented control. The active power is generated in regard to wind speed and wind turbine

characteristics while the react ive power command is set in regard to the utility demand. The proper rotor excitation is

provided by the rotor side power converter. Decoupled control of the active and reactive powers is implemented.

The continuous wind variation is simulated by addition of a noise component to the average wind speed.

The equivalent circuit of a DFIG is shown in Figure 2 fo llowed by the model equations:

Power Quality Improvement of Grid Connected to DFIG by Active Shunt Filter 291


Figure 2: Equivalent Circuit of a DFIG

= rs. + – . (1)

= rs. + – . (2)

= . + – . (3)

= . + – . (4)

Where:

and are the Voltages developed in the stator on α and β axis respectively

and are the Voltages developed in the rotor on α and β axis respectively

rs and


and are the α and β axis stator currents respectively

and are the α and β axis rotor currents respectively

, and ate the reference speeds, synchronous speeds and generator speeds respectively


and are the Voltages developed in the rotor on α and β axis respectively

where the flux linkage is expressed as:

= xs. + xh. (5)

= xs. + xh. (6)

= xh. + xr. (7)

= xh. + xr. (8)

Where xs, xr and xh are respectively the stator reactance, rotor reactance and mutual reactance between stator and

rotor.




The induction machine torque is given by:

J = t m + tel (9)

where the mechanical and electrical torque formula as Tm and Te can be calcu lated respectively by:

Tm= . . (10)

Te = Im . (11)

where all the equations above are described in stator side per unit system and J is the moment of inert ia

Two voltage fed PWM converters are inserted in the rotor circuit, with the supply -side PWM converter connected

to the stator supply. The voltage-transfer characteristics of the system, including the three-phase back-to-back PWM

converters, are calculated by:

Vs = m1 (12)

Vr = ± s. = (13)

Where n is the turns ratio of stator to rotor of the DFIG, s is the slip and m1 and m2 are the PWM modulation

depths of the stator and rotor side converters, respectively.

The general space vector equivalent circuit for DFIG at steady state for the fundamental voltage is shown in

Figure 3.

Figure 3: General S pace Vector Equivalent Circuit for DFIG

The rotor currents (ira,irb,irc) of the machine can be resolved into direct and quadrature components id and iq.

The component id produces a flux in the air gap which is aligned with the rotating flux vector linking the stator; whereas

the component iq produces flux at right angles to this vector. The torque in the machine is the vector cross product of these

two vectors, therefore, only the component iq contributes to the machine torque and power. The component id controls the

reactive power entering the machine. If id and iq are precisely controlled, the stator side active and reactive powers can be

controlled effectively.

The induction machine is controlled in a synchronous rotating dq axis frame, with the d -axis oriented along the

stator-flux vector. It allows a decoupled control between the electrical torque and the rotor excitat ion current. The rotor

side PWM converter provides the actuation, and the control requires the measurement of the stator and rotor currents,

stator voltage and the rotor position. As the stator is connected to the grid and the influence of the stator resistance is small,

the stator magnetizing current is considered to be constant. Under stator-flux orientation, the relationship between the

torque and the dq axis voltages, currents and fluxes per-phase values are computed as follows:



= = Lo.ims = Ls.ids + Lo.idr (14)

Vdr = Rr.idr + – (15)

= + .id (16)

Vqr = Rr.iqr + + (17)

= .iqr (18)

= – (19)

Lm = (20)

Te= iqr (21)

(22)

(23)

And the stator flux angle is given by:

= ).dt (24)

= ).dt (25)

Also

= (26)

Where defines the position of the stator flux

II. MACHINE AND CONVERTER CONTROL METHOD

In order to ensure that the correct values of id and iq flow in the rotor, corresponding phase currents references

ira_ref, irb_ref and irc_ref are generated. A suitable voltage source converter is used to force these currents into the rotor.

A current reference PWM technique is implemented. The crucial step is to obtain the instantaneous position of the rotating

flux vector in space in order to obtain the rotating reference frame. This is achieved by subtracting the resistive drop of t he

rotor from the stator voltage. This is equivalent to the derivative of the s tator flux linkage per phase given as:

Va – ia.ra= (27)

The rotor side converter requires a DC power supply. The DC voltage is generated using another voltage source

converter connected to the AC grid at the generator stator terminals. A DC capacitor is used in order to remove ripple and

keep the DC bus voltage relatively smooth. The grid PWM converter is operated in such a way to keep the DC voltage on

the capacitor constant, therefore, the stator side converter is supplying the real power demands of the rotor side converter.




The mechanical power and torque ext racted from the wind energy are expressed as:

Pm = . (28)

Tm= . (29)

where ρ is the air density, R is the radius of turbine blade, Vw is the wind speed and Cp(λ,β) is the aerodynamic

efficiency of the turbine blade. The output energy of wind turbine depends on the method of tracking the peak power points

on the turbine characteristics due to fluctuating wind conditions. Optimal power point tracking to capture maximum energy

of wind is derived from the power-speed characteristics of the turbine.

Popt.=Kopt. Kopt.=0.5.Cp. (30)

The Popt defines the maximum energy captured.

After compensation for transmission friction losses, Popt. is given as:

= Kopt. = – (31)

The variable i*qr-active is imposed on the control method of the rotor side converter. When the output power of

DFIG falls below the minimum power corresponding to the maximum power point at minimum wind velocity V1, the

system goes to speed mode control. If the power of turbine is greater than Pmin, the optimum power point tracking shifts to

current mode control.

III. CURRENT HARMONICS MITIGATION BY ACTIVE SHUNT FILTER

The harmonic mitigation strategy proposed incorporates an active filter connected in shunt at the Point of

Common Coupling (PCC) to the common bus bar at which the DFIG is connected as shown in Figure 4. The proposed

ASF has also been modeled in PSCAD on the basis of work proposed earlier by H.Fugita & H.Akagi [1]. It consists of

ASF connected in parallel with the non linear load connected to the grid through a step down transformer. A 6 pulse

rectifier has been used as a source of harmonics. The non linear load injects harmonic impurity on the grid to which it is

connected. On account of grid impedance, the current harmonics would also result in voltage harmonics in the grid.

The harmonics generated in the grid would travel to the connected DFIG because it is substantially smaller in its capacity

as compared to the grid strength. This would deteriorate the performance of the DFIG whose PQ is always challenged on

account of persistent wind variation and turbulence. The ASF installed near to the non linear load has the potential to

mitigate the harmonics generated substantially to within tolerance limits. It primarily consists of a 12 pulse PWM inverter

along with a DC source. The PQ of the grid as well as the DFIG is thus improved and the DFIG is safeguarded from the

harmful effects of harmonics.



Figure 4: Schematic Diagram of Grid Connected DFIG with Non Linear Load and AS F

IV. CONTROL CIRCUIT OF ASF

The ASF works on the principle of active & reactive power theory. The technique incorporates a combined

system with a passive filter and a small-rated AF, both connected in series with each other. The passive filter removes load

produced harmonics just as a conventional filter does. The instantaneous bus voltages & currents are converted from three

axis to two axis using Park’s transformat ion. The equivalent transformat ion in PSCAD is as below:

Figure 5: Transformation of Bus Voltages from 3 Axis to 2 Axis

After filtering of instantaneous real and reactive powers based on alpha beta Quantities, only AC Component of

instantaneous real and reactive powers are left as shown is Figure 6.

Figure 6: Calculation of Instantaneous Power

The reference currents for the ASF is calcu lated in the PSCAD/EMTDC model as per the following:




Figure 7: Control Scheme for AS F

The references Currents are then compared with the actual currents of PWM inverter o f ASF to generate the firing

pulses of the GTOs. Correct ion currents are thus injected on the grid so as to mitigate the current harmonics.

The proposed system i.e ASF as discussed above has been installed at the common bus bar connected to the

DFIG. The system has been successfully simulated so as to mitigate the current harmonics of the bus connected to the grid

through which Power is Fed by the DFIG as shown in the simulation results.

V. RESULTS & DISCUSSIONS

The average wind speed (with noise component) was varied and corresponding variations in THD of stator current

were recorded. The DFIG was made to shift from speed control mode to torque control mode after 0.5 sec. Figure 7 shows

the effectiveness of modeled ASF in mitigating the current harmonics of DFIG for wind speed of 12 m/s.

Figure 8 (a): Current Waveforms at Vw 12m/s

Figure 8 (b): THD Recorded (Grid Current) (i) Without ASF (ii) With AS F

Table 1: (Observations of THD-Grid Current & Grid Voltage)

S.

No.

Wind S peed

(m/s)

THD (Grid

Current) without

Filter

THD (Grid

Current) with

Filter

THD

(Grid

Voltage)

1. 3 20.28 3.82 0.53

2. 4 20.29 3.37 0.54



Table 1: Contd.,

3. 5 20.30 3.42 0.57

4. 6 20.31 3.35 0.61

5. 7 20.29 3.36 0.65

6. 8 20.29 3.39 0.57

7. 9 20.27 3.59 0.82

8. 10 20.28 3.98 0.88

9. 11 20.26 4.08 1.21

10. 12 20.28 3.32 1.24

Figure 9: THD (Grid Current with and without AS F for Various Wind S peeds)

VI. CONCLUSIONS

The proposed model is capable of mitigating the harmonic impurity caused on account of non linear load, thus

smoothening the current waveform. The behavior of the grid to which DFIG is connected for various wind speeds has been

analyzed. The current harmonics were found to be in the high range of 20% for all wind speeds. The ASF effectively

suppressed the current harmonics to less than 4% for all wind speeds.

DFIG is not only protected from harmonic impurit ies caused by non linear loads but the proposed model is also

capable of suppressing the harmonics generated by the DFIG for various wind speeds. The proposed system thus ensures

successful mit igation of current harmonics yielding high power quality.

VII. REFERENCES

1. H. Fugita & H. Akagi, “A practical approach to Harmonic Compensation in Power System-Series Connection of

Passive & Active Filters”, IEEE Trans. On Industrial Applicat ions, Vol. 27, No.6, Nov-Dec 1991, pp. 1020-1025.

2. Luis A. Moran, Juan W. Dixon, Jose R Espinoza, Rogel R. Wallace, “Using Active Power Filters to improve

Power Quality”, 5th

Brazilian Power Electronics Conference, COBEP, 1999.

3. Bhim Singh, Kamal Al-Haddad and Amrish Chandra, “A review of Active Filters for Power Quality

Improvement”, IEEE Transactions on Industrial Electronics , Vol. 46, No.5, pp. 960-971, October 1999.

4. Joao Afonso, Mauricio Aredes, Edson Watanabe & Julio Martins, “Shunt Active Filter for Power Quality

Improvement”, International Conference UIE2000- Electricity for Sustainable Urban Development,

1-4 November, 2000 pp. 683-691.

5. L. H. Tey, P.L. So & Y C Chu, “Improvement of Power Quality using Adaptive Shunt Active Filter”, IEEE

Transactions on Power Electronics, Vol. 20, No.2, April, 2005.




6. Shubra Goel, Arif Khan & Omveer Singh, “Shunt and Series Active Filters based Power Quality Conditioners for

Matrix Converter”, International Journal of Scientific Engineering & Technology, Vol. 1, Issue 5,

pp. 209-217, Nov. 2012.

7. Anil Kumar & Jatinder Singh, “Harmonic Mit igation & Power Quality Improvement using Shunt Active Power

Filter”, International Journal of Electrical, Electronics & Mechanical Controls, Vol. 2, Issue 2, May 2013.

8. Satnam S. Matharu & Sanjeev Kumar Bhalla ”Modeling & Analysis of DFIG( WECS)”. International Journal of

Advanced

9. Innovative Research (IJAIR), Vol. 2, Issue 1, January 2013, pp. 115-120, ISSN-2278-7844

10. IEEE Standard 519 Applicability to Adjustable Frequency Controllers Bulletin C- 883 Page 10 Tab le 4.

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33. Electrical - Ijeeer -Power Quality Improvement - Sanjeev Kumar Bhalla