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Current harmonics mitigation from grid connected variable speed wind turbine due

to nonlinear loads using shunt active power filter

Abstract: Quality of power now-a-days is of high concern due to increasing demand of

supply and energy resources are limited; another cause is amplified penetration of nonlinear

loads on the power system. To overcome energy mandate, technologists are shifting to the

renewable energies' side; wind or solar, and so on. For mitigating effect of nonlinear loads

various techniques have been used by researchers in past decades. Scholars used shunt

active power filter (APF) to mitigate current harmonics from the fixed wind turbine

generator (WTG), in this paper shunt APF has been applied with variable-speed wind

turbine generator using synchronous reference frame (SRF) for the extraction of

compensation signal for APF. Gate driver signals are generated from Bang-Bang Controller

(Hysteresis Band Current Controller HBCC). Model had been developed using

MATLAB/SIMULINK, simulation results portraying decreased THD levels within the

system and clearly reveal the effectiveness of Shunt APF in meeting the IEEE-519 standard

recommended for harmonic levels.

Keywords: Wind Turbine Generator, Shunt Active Power Filter, Synchronous Reference

Frame, THD

1. Introduction

The added utilization of power electronics controlled devices and loads with nonlinear

characteristics in the power system has increased distortion level in current and voltage

waveforms in shape of ‘Harmonics' [1]

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Various problems caused by harmonics in the power system and also cause an effect on

consumer products, such as distorted voltage waveforms, equipment overheating, blown

capacitor fuses, transformer overheating, excessive neutral currents, low power factor, etc.

[2-5]. Different techniques have been employed by the researchers in the last decades to

mitigate the issue of harmonics and to provide consistent and sinusoidal power to the

consumers who does not damage to their equipment. Furthermore, generation of current

harmonics induces more losses in the generator and thereby producing more heat, which in

turn reduces efficiency and life of the generator [6]. Like is the case with grid tied wind

turbine generator who is, likewise, regarded by these current harmonics and supply

distorted waveform at the level of common coupling (PCC).

Passive and Active filters have been enforced to reduce harmonic distortion. Passive filters

have the drawback that it only provides over or under compensated for harmonics,

whenever a change in load occurs, and they have bulky size, component ageing, resonance

problem and fixed compensation performance [7]. Whereas, active filter recognized as an

active power filter (APF) is preferred over the passive filter. Active Power Filter offers a

benevolent solution to the problems of power quality, which allow the compensation of

current harmonics and together with reactive power compensation.

The SAPF operates by injecting the reactive, unbalanced, and harmonic load current

components into the utility system with the same magnitudes as the non-active load

currents demanded by a given nonlinear load but with opposite phases [8-11]. The method

for extraction of harmonic compensation signal and determination of reference current play

a central role as these are related to accuracy and speed of Shunt APF response and also

matters the design and application of Shunt APF [12]. The reference current extraction

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methods are categorized into two groups, Time domain and frequency-domain [13, 14].

Time-domain methods include d–q transformation (or SRF), p–q transformation (or

instantaneous reactive power), symmetrical component transformation, etc., are based upon

the measurements and transformation of three-phase quantities [9]. Time domain as

compared with the frequency domain has the fast response whereas frequency domain,

based on Fourier's transformation has an accurate response [12]. The compensation method

used within this paper is synchronous reference frame (SRF) technique based upon time

domain control type in which all harmonic load current components are targeted and

compensated. The control strategy and the current injection will depend on reference signal

extraction method The performance of Shunt APF with the grid-connected variable-speed

wind turbine is then analyzed by calculating total harmonic distortion of compensated

quantities using MATLAB/SIMULINK software.

2. Topology and Operational Principle of Shunt Active Power Filter

Shunt Active Power Filter widely used among all the filters [15-22]. Shunt APF induces the

compensation current in the system which is equal in magnitude but in anti-phase to

measured harmonic load current to mitigate the effect of harmonics from supply side and

deliver almost sinusoidal power to consumers. Figure 1, depicting shunt APF with the grid-

connected wind turbine generator. Shunt APF has two main parts reference current

extraction method and gate driver signal for inverter. Reference current been extracted from

the load current measured at PCC and accordingly APF will supply/draw current at PCC.

The effectiveness of SAPF depends on gate driver signal control [23]. For application of

APF various PWM current control techniques have been proposed by many researchers.

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But in terms of quick current controllability and easy implementation, HCC has highest

rating among other methods such as sinusoidal- PWM and triangular current controller [24,

25]. Line inductance used here in order to remove ripples generated by filter. For

maintaining and regulation of DC voltage dc link capacitor is used with conventional PI

controller. A full bridge diode rectifier also connected at the point of common coupling

(PCC). Combination of R, L and C loads is connected across the bridge rectifier (source of

harmonics generation). Principal operation of SAPF, the current controlled compensation to

draw/supply current from/to the power system (PCC). This compensation of current enables

the active power filter to cancel and compensate reactive power drawn by nonlinear loads.

3. Harmonic Current Extraction Technique

Synchronous Reference Frame method to extract the harmonic current was developed in

MATLAB/SIMULINK for SAPF under the unbalance condition due to variable wind

speed. Under SRF method distorted 3-phase load current is transferred from it’s a-b-c

stationary reference frame to d-q reference frame [26] using equation (1).

[ id

iq]=[ sin θ sin(θ−2 π3 ) sin(θ+

2π3 )

cosθ cos(θ−2π3 ) cos (θ+ 2 π

3 )]∗[ia

ib

ic] (1 )

The components of current Id and Iq extracted from the load current represent active and

reactive power components of current and can be decomposed using equation (2 and 3).

id=id+~id (2 )

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iq=iq+~iq (3 )

For filtering harmonic contents extracted id-iq current passed through low-pass filter (LPF)

and allowed only the fundamental frequency components. Function of PI controller is to

regulate capacitor dc link voltage. The dc capacitor voltage is sensed and compared with

some set reference voltage to calculate error voltage. This voltage error is involved with PI

gain (KP=0.1 and KI-1) to regulate capacitance in dynamic conditions. Further the output

of PI controller is subtracted from d-axis current to eradicate steady-state error [27]. The

procedure is then developed to extract the reference signal in d-q rotating frame, which is

converted back to a-b-c stationery frame using inverse Park’s transformation utilizing

equation (4,5 and 6). Figure 2 depicting Simulink model for SRF method.

isa¿ =id sin (ωt )+cos ( ωt ) ( 4 )

isb¿ =id sin(ωt−2 π

3 )+cos(ωt−2 π3 ) (5 )

isc¿ =id sin(ωt+ 2π

3 )+cos(ωt+ 2 π3 ) (6 )

4. Gating Signal Generator for APF

The different approaches for generating gate driver signals, such as hysteresis-based current

control (HBCC), PWM current or voltage control, deadbeat control, sliding mode of current

control, PI controller, fuzzy-based current control, and Neural network-based current

control, can be instigated to obtain the switching signal for APF. HBCC is considered for

this paper (Simulink model shown in Figure 3) because of its characteristics by having

unconditioned stability, quick response, and good accuracy.

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5. System Parameters and Working

The 1.5 MVA 575 V variable speed PMSG based wind turbine is connected with 120kV,

60 Hz grid through 30km transmission line to analyze the effect of nonlinear loads. At the

point of common coupling, nonlinear load is attached, which is generating current

harmonics and affecting utility supply, furthermore, affecting current supplied by WT.

Therefore, increase in total harmonic distortion in current and voltage at PCC. Proposed

system for mitigating these harmonics is connected at PCC. Simulation results presenting a

decrease in THD level within the utility system and at PMSG.

6. Simulation Results

Simulations of the system without compensation are shown in Figure 4, THD level of

PMSG current for Phase A, Phase B and Phase C are 5.76%, 4.62% and 4.83%

respectively. The PCC current THD level is 7.42%, 6.75% and 5.70% for Phase A, B and C

respectively. PCC voltage THD level for its corresponding Phase A, B and C are 8.05%,

7.30% and 6.76%.

The simulation results of the system with Shunt APF is shown in Figure 5 with reduced

THD levels. THD level of PMSG current reduced to 1.15%, 1.24% and 2.19% from 5.76%,

4.62% and 4.83% for their respective phase A, B and C. whereas, PCC current THD level

decreased from 7.42%, 6.75% and 5.70% to 0.18%, 019% and 0.19% of their

corresponding Phase A, B and C. Voltage THD at PCC decreased to 0.80%, 1.01% and

0.55% from 8.05%, 7.30% and 6.76% of their Phase A, B and C respectively. The

compensated current injected by Shunt APF to mitigate the effect of harmonics is depicted

in Figure 6.

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7. Summary of Simulation Results

Figure 7 below elaborating the percentage magnitude of total harmonic distortion in each

phase of PMSG current, PCC current and PCC voltage without compensation.

Consequence giving the picture that system is not running under the harmonic standard

(IEEE-519) specified by IEEE [28].

After the application of Shunt APF along with synchronous reference frame method for the

extraction of harmonic components under the variable speed of wind, system having

decreased level of THD magnitude as specified in Figure 8.

Summarizing chart for Figure-7 and 8 is displayed in table below, where the phase to phase

comparison with the system without compensation and with SAPF is shown. Results

showing that using SAPF system THD level has reduced to limits stated in IEEE-519.

8. Conclusion

Application of Shunt Active Power Filter with grid-connected variable-speed PMSG based

with turbine generator was studied in this paper to mitigate the effect of harmonics due to

nonlinear loads. Simulation results gave evident in reduction of current and voltage

harmonics level within the system. With the use of this technique, drop in harmonics will

decrease heat caused and therefore, will not affect life span of the generator. On the other

hand, with the application of SAPF voltage supplied to consumers also having the

sinusoidal waveform which in turn will not damage their appliances and utilities.

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Figure Captions

Figure 1. Configuration of Shunt APF

Figure 2. Simulink Model for SRF Method

Figure 3. HBCC Simulink Model

Figure 4. PMSG Current, PCC Current and PCC Voltage with their THD Levels without

SAPF

Figure 5. PMSG Current, PCC Current and PCC Voltage with their THD Levels with

SAPF

Figure 6. Compensation Current injected by SAPF

Figure 7. THD Magnitude with compensation

Figure 8. THD Magnitude with SAPF

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Figure 2. Simulink Model for SRF Method

Figure 1. Configuration of Shunt APF

Vdc_ref

PLL

abc-dq0

LPF

LPF

dq0-abc

dq0

sin_cosabc

abc

sin_cosdq0

[Vabc]

Supply Voltage

[Iref]

Reference Signal

[Iabc]

Measured Current

Fo=200Hz

Fo=200Hz

[Vdc]

Inverter Capacitor Voltage

PI

1100

Vabc (pu)

Freq

wt

Sin_Cos

VdcABC

ABC

inductor

gABC

+

-

SAPF

A

B

C

PMSG Wind Turbine

A

B

C

a

b

c

PCC

Non-Linear Load

[Pulses]

N

A

B

C

120 kV Grid

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23456789

101112131415161718192021222324252627282930313233343536373839404142

Figure 3. HBCC Simulink Model

6S6

5S5

4S4

3S3

2S2

1S1

Relay2

Relay1

Relay

Memory5

Memory4

Memory3

Memory2

Memory1

Memory

AND

LogicalOperator8

AND

LogicalOperator7

NOT

LogicalOperator6

NOT

LogicalOperator5

NOT

LogicalOperator4

AND

LogicalOperator3

AND

LogicalOperator2

AND

LogicalOperator1

AND

LogicalOperator

[Iref]

1time

123456789

10111213

15

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18

19202122

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32

PMSG Current PCC Current PCC Voltage

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Phase A Phase A Phase A

Phase B Phase B Phase B

Phase C Phase C Phase C


Phase A Phase A Phase A

Figure 4. PMSG Current, PCC Current and PCC Voltage with their THD Levels without SAPF

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4

Phase B Phase B Phase B

Phase C Phase C Phase C

Figure 5. PMSG Current, PCC Current and PCC Voltage with their THD Levels with SAPF

Figure 6. Compensation Current injected by SAPF

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Compensation Method


Phase A Phase B Phase C Phase A Phase B Phase C Phase A Phase B Phase C

PMSG Current PCC Current PCC Voltage0.00%

1.00%2.00%

3.00%

4.00%

5.00%6.00%

7.00%

8.00%

9.00%

5.79%

7.42%8.05%

4.62%

6.75%7.30%

4.83%5.70%

6.76%

Phase APhase BPhase C

Total Harmonic Distortion

Mag

nitu

de o

f Har

mon

icsFigure 7. THD Magnitude with compensation

PMSG Current PCC Current PCC Voltage0.00%

0.50%

1.00%

1.50%

2.00%

2.50%

1.15%

0.18%

0.80%

1.24%

0.19%

1.01%

2.19%

0.19%

0.55%

Phase APhase BPhase C

Total Harmonic Distortion

Mag

nitu

de o

f Har

mon

ics (%

)

Figure 8. THD Magnitude with SAPF

3

Without any Compensation

5.79% 4.62% 4.83% 7.42% 6.75% 5.70% 8.05% 7.30% 6.76%

With SAPF 1.15% 1.24% 2.19% 0.18% 0.19% 0.19% 0.80% 1.01% 0.55%Table. Summary of THD Levels

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