Repetitive control and cascaded multilevel inverter with integrated hybrid active filter capability for wind energy conversion system

Engineering Science and Technology, an International Journal 22 (2019) 811–826

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Full Length Article

Repetitive control and cascaded multilevel inverter with integrated hybrid active filter capability for wind energy conversion system Buddhadeva Sahoo a, Sangram Keshari Routray b,⇑, Pravat Kumar Rout b a b

Department of Electrical Engineering, SOA University, Bhubaneswar, Odisha, India Department of Electrical and Electronics Engineering, SOA University, Bhubaneswar, Odisha, India

a r t i c l e

i n f o

Article history: Received 2 May 2018 Revised 10 December 2018 Accepted 2 January 2019 Available online 8 January 2019 Keywords: Doubly-fed induction generator (DFIG) 31-Level RSCI Repetitive controller (RC) Non-linear load Wind energy conversion system (WECS) Hybrid active filter

a b s t r a c t This manuscript presents a repetitive control (RC) method and 31-level reduced switch cascaded inverter topology (31-level RSCI) with an integrated hybrid active filter capability for a grid-connected doubly-fed induction generator (DFIG) based on wind energy conversion system (WECS). Repetitive control approach is considered for the inverter operation due to its better controllability and accuracy under periodic disturbance conditions. Further to enhance the system performance by supplying the desired reactive power to DFIG and harmonic reduction, a 31-level RSCI topology with a reduced number of unidirectional switch operations is proposed and implemented in the rotor side converter (RSC). This leads to benefit of highest power extraction and offer the required reactive power to DFIG. In addition to that an LC filter on the grid side converter (GSC) is added, to work as a hybrid active filter for harmonic cancellation produced by the nonlinear load and so behaves like a static compensator (STATCOM) even under shutdown condition of the wind turbine. Indirect current control and flux oriented reference frame control are implemented for grid and rotor side converter respectively. The proposed approach is validated with the simulated test results under both steady state and dynamic conditions. Ó 2019 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction In the present power system scenario, the requirement of energy increases exponentially due to the rise in population, industrialization and the corresponding load demand. The needs of integrating the renewable energy sources to the main grid as a solution to the above are for full filling the future energy demand, to shut out the operational intricacies and to find its remedial measures for smooth synchronized operation [1,2]. Among all of the possible generation, wind energy is accepted widely due to its comparatively lower cost, moderate efficiency, eco-friendliness, unlimited resource in nature and technical advancements. Variable speed wind turbines are generally preferred over fixed speed types, particularly due to the easy operation based on the control point view, less mechanical stress, and reduction of power fluctuation due to the capability of extraction of maximum power to enhance the efficiency of power conversion [3,4]. However, the need of power converter increases the components and makes the control complex in case of uneven speed wind generators. With a proper ⇑ Corresponding author. E-mail address: [email protected] (S.K. Routray). Peer review under responsibility of Karabuk University.

control strategy, operationally these converters facilitate the matching of wind turbine characteristics with the grid connection requirements, including frequency, voltage, control of active and reactive power, and harmonics, etc. [5,6]. This study investigates further on these issues with an objective to propose and find an optimal solution in case of uneven speed wind generators. The stator of the DFIG is connected directly to the utility grid, and the rotor is connected to the utility grid through two backto-back power electronic converters by sharing a common dc-link as illustrated in Fig. 1. By controlling the rotor side converter (RSC), the rotational speed of the generator can be varied by injecting variable frequency rotor current to cope up with the variable wind speed at the turbine end [7–12]. Two types of cascaded inverters such as synchronous and asynchronous types are used in the distributed generation based microgrid system. In synchronous type, the dc voltage sources are considered as equal and in asynchronous type, the dc voltage sources of all H-bridges are generally unequal. To design a multi-level inverter (MLI) for improving the performance, most of the researchers have chosen the bidirectional power electronic switches [12–16]. Therefore, to design MLI, the numbers of insulated gate bipolar transistors (IGBT) used are more and increases the cost [16,17]. These motivate for the technical and economic point of view, design an

https://doi.org/10.1016/j.jestch.2019.01.001 2215-0986/Ó 2019 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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r

Speed

Vabc, grid

Vabc,source

I abc,grid

PCC

I abc ,source DFIG

GRID

Rotor side Converter(RSC)

I abc ,rotor

Grid side Converter(GSC)

Lf

I abc , gsc

I abc ,load

Vdc

RSC pulses ref Vdc link

Vdc

+

link

GSC pulses

RC controller

-

+ I + -

abc dq sin,cos

I abc ,load

dq abc

ref Iabc ,grid

Non-linear Load

nd

sin,cos

ωt Vabc sin,cos

RSC pulses

ref gd

sin,cos

abc dq

Vabc, grid

Hystresis I abc,grid Band

ref Igd 0

link

I abc ,source

Passive filter

PWM Generator

Discrete 2 order filter

GSC Controller Speed controller

e

+ abc dq

Current Regulator

sin,cos

Speed, + ref MPPT Speed

abc dq sin,cos

I abc ,rotor -

RSC Controller

+

e

r

Fig. 1. System configuration and the proposed algorithm.

asymmetric cascaded MLI using less number of unidirectional switches and implemented in RSC side to generate more voltage levels. Due to the random variation of the wind speed, the power is extracted by the wind turbine varies. It is essential to extract optimum power from the available wind energy at the input of the turbine. In this test system, to compensate the low efficiency, maximum power point tracking (MPPT) algorithm is used. Recently, substantial research has been carried out to track the maximum power from the wind energy conversion system [18,19]. In this proposed system, for better power quality and to reduce the fluctuations in power output, incremental conductance (IC) MPPT algorithm is selected. The major functions of the DFIG based wind energy conversion system are to extract maximum power from the wind turbine to tackle the problem of power fluctuation, to regulate the reactive power management between the wind turbine and the grid for voltage and active power control, to suppress the harmonic generation from the nonlinear load by providing adequate filter, and to operate under various grid and isolated mode of operation under the constraints like frequency, voltage and stability limits [20]. It is indispensable to devote the research for an adequate solution to the above functional challenges and to achieve better system stability and performance.

Confine to the present work, it is an attempt to enhance the performance through the converter control strategy under both steady state and dynamic conditions. Many authors in recent time have proposed on converter control to achieve smooth operation of grid-connected DFIG based microgrid distribution system. To integrate the distributed generation based microgrid to the grid, it is mandatory to follow the grid requirements to have a synchronized and normal operation related to voltage, frequency, power quality, and reactive power etc. [21]. Many authors suggested on mitigation of the harmonics with reactive power management through flexible transmission system devices like a static compensator (STATCOM), energy storage devices like fly-wheel and battery etc. [21–25]. However, integrating these devices with DFIG is not an attractive solution due to the extra cost involved on compared to the benefits achieved with regards to efficiency and performance. Due to that many researchers focus on the control of harmonic generation using RSC and GSC control strategy. Even though the RSC based control able to mitigate the lower order harmonics by reactive power control, it has the major limitations like noise creation, higher losses and a higher rating of the converter used. Unlike RSC, the GSC control does not pass harmonics to the rotor windings. However, the need for extra passive filter integration with the GSC makes a system behave as a

B. Sahoo et al. / Engineering Science and Technology, an International Journal 22 (2019) 811–826

hybrid active filter to mitigate the lower order harmonics [19,26– 30]. With this idea, the present work motivates to design a GSC based robust control strategy with a hybrid active filter capability. Due to its easy implementation and cost-effectiveness, the PI controllers are extensively accepted in real-time systems, even in the DG integrated microgrid system. However, the working of controllers totally depends upon the constant gain parameters. Due to the application of constant parameters, these controllers lag to prove their best performance with different operating conditions [31,32]. This motivates to go further to design a better controller instead of PI controllers to rectify the above demerits by using constant controller parameters. Along with power control, it is also essential for controlling the multi-level inverter operation to mitigate harmonic distortion. To resolve the above problems, repetitive control (RC) approach plays an important role by tracking or eliminating the periodic signals, mainly in closed loop systems. Due to its better error cancellation characteristics, RC based controller is effectively eliminating any periodic signals, including any order harmonics [33–37]. In this study, a new control algorithm based on the repetitive controller is proposed for GSC with an objective to mitigate the harmonic signals produced by the non-linear load. Furthermore to enhance the performance of the proposed system, the indirect vector control concept is applied for the operation of the grid side converter (GSC). The control strategy of RSC based on flux oriented vector reference frame is designed with an objective of reactive power and speed control of the DFIG. The proposed approach for DFIG based system leads to the benefit that it operates as a hybrid active filter even under shutdown condition of the wind turbine. Secondly, it suppresses the load reactive power and harmonics at different conditions. Furthermore, to improve the system performance a 31-level RSCI is designed by using low dc voltage magnitude and getting more stepped voltage levels to reduce the voltage variations. The performance of the proposed DFIG controller is demonstrated through simulation under both steady state and dynamic conditions. The remaining part of the paper is organized as follows. Section 2 is devoted to the system configuration and operating principle of WECS. The proposed system design such as dc link voltage of converter selection, converter rating selection, and design of interfacing inductor is described in Section 3. In addition to that, the 31level RSCI topology is presented. Section 4 is devoted to the system control topologies by showing the rotor side and grid side converter control is explained. Section 5 is devoted to the simulation results under various conditions. The conclusion and findings of the proposed work are presented in Section 6.

2. System design and operating principle DFIG based WECS having hybrid active filter capabilities with the proposed algorithm of RSC and GSC control is demonstrated in Fig. 1. In addition to the control algorithm, an extra LC filter is added to perform the system as a hybrid active filter. In DFIG, the stator side is directly linked with the grid and the rotor side is linked with the grid through two back to back converters. Non-linear loads are connected at the point of common coupling (PCC). The DFIG with the proposed control approach acts as a hybrid active filter along with the normal active power generation. Under the influence of the non-linear loads, the PCC voltage may be distorted by the harmonic injection. The inbuilt GSC control functions to mitigate these harmonics from the stator/source end grid currents. The GSC control strategy is based on the synchronous reference frame (SRF) approach and is utilized for the extraction of the fundamental component of load current. At RSC the proposed control depends on flux oriented reference frame operation at

813

unity power factor and also responsible to achieve maximum power point tracking. In the proposed approach, 31-level cascaded voltage source converter is used to increase the system performance. 3. System design In this section a brief description about the DFIG based WECS is discussed. For the successful operation of WECS, the selection of converter rating, design of interfacing inductor, and selection of dc link voltage are essential. In the proposed scheme, the design of the 31-level RSCI topology and its operating conditions are presented. The related system design parameter values are provided in the Appendix. 3.1. DFIG based WECS The mechanical torque ‘Tm’ of the wind turbine can be represented as:

Tm ¼ 0:5qC p AR

V 2x k

ð1Þ

where ‘Cp’ is the wind turbine power coefficient, ‘A’ is the blade swept area (m2), ‘R’ is the radius of the rotor (m), V x is the rated speed of wind (m/s), and q is the air density (kg/m3) respectively. The tip speed ratio TSR (k) is calculated as:

k¼

Rxm Vx

ð2Þ

where xm denotes the angular mechanical wind speed (rad/sec). The maximum captured wind power is dependent on TSR value. 3.1.1. DC-link voltage of converter selection: Generally, twice the peak of maximum phase voltage is nearly equal to or less than the dc-link voltage of multi- level inverter. To select the optimal dc-link voltage of the inverter, the voltage at the rotor side and grid side are necessary to measure. The RSC voltage is calculated by the product of slip and stator voltage. In DFIG, the turns ratio of the stator and rotor is 2:1 and the operating slip is around 0:3 during normal operation. Therefore, comparatively the rotor side voltage is always smaller than the grid voltage at PCC (VPCC). Therefore the selection of dc link voltage is computed by considering only the VPCC. In this design, the VPCC at the GSC end is taken as 230 V. The dc-link voltage (V dclink ) of a 31-level RSCI is considered as [8]:

V dclink P

pffiffiffi 2 2  V PCC pffiffiffi P 375v 3m

ð3Þ

where ‘m’ is the modulation index generally vary within the range from 0 to 1. In this study, the maximum modulation index is chosen as one for linear operation. 3.1.2. Converter rating selection: To build the desired air gap voltage, DFIG draws a lagging voltampere reactive (VAR) for its excitation. It can be computed from the machine parameters with a lagging VAR of 2 kVAR ‘Qstator’ need for motoring mode of operation of the machine. However, in the DFIG case, the operating speed range under normal wind speed variation is around 0.7 p.u to 1.3 p.u and the corresponding maximum slip ‘Smax’ is around 0.3. Reactive power of 600 VAR (Smax* Qstator = 0.3*2 kVAR) is necessary for the rotor side ‘Qrotor’ for maintaining unity power factor operation on the stator side. In this condition, the existing maximum rotor active power (Protor = Smax * Prated) is around 1.5 kW, where Prated denotes as the

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rating of DFIG taken as 5 kW. The rating of the inverter ‘INVrated’ used in RSC is calculated as:

INV rated ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P2rotor þ Q 2rotor ¼ ð0:3  5Þ2 þ ð0:3  2Þ ¼ 1:615KVA ð4Þ

3.1.3. Design of interfacing inductor (Lf): The interfacing inductor ‘Lf’ is presented in between GSC and PCC. The value of parameter ‘Lf’ depends upon the switching frequency of GSC, the dc-link voltage of the inverter, and the maximum allowable GSC current (Igsc). By using the maximum power of the rotor side and line voltage of GSC, the maximum line current ‘Igsc’ can be calculated as [8–10]:

P rotor Igsc ¼ pffiffiffi ¼ 3:765A 3  V PCC

ð5Þ

In this study, the peak ripple current ‘DIgsc ’ is taken as 25% of rated GSC current. By using the above computed parameters, the inductor value is calculated as [8–10]:

Lf ¼

pffiffiffi pffiffiffi 3m  V dclink 3  1  375 ¼ ¼ 3:8mH 12af m DIgsc 12  1:5  10; 000  0:94

ð6Þ

3.2. Inverter topology Two seven-level inverter topologies with the same potential and different potential are shown in Fig. 2. Both the topologies differ with regards to different voltage polarity. The two topologies are comprised of two voltage sources (VL1, VR1) and six unidirectional switches out of which two on the left side (TL1, TL2), two on the right side (TR1, TR2), and other two in between (Ta, Tb) respectively. Simultaneous turned on condition of both left switches and right switches are avoided due to the occurrence of voltage short circuit condition.

LEFT (L)

All the voltage levels and switching conditions of the designed seven-level inverter are shown in Table 1. By analyzing the above table, it is clearly understood that if the dc source voltages are same, then the levels of voltage decrease to three. Therefore, different values of dc source voltages are considered to produce higher voltage levels (both odd and even) keeping the number of power electronic switches and dc voltage sources almost same as before. In Fig. 2(a) the dc voltages VL1 and VL2 are taken as 3 p.u and 1 p.u. respectively, and in Fig. 2(b) the dc voltages VL1 and VL2 are considered as 2 p.u and 1 p.u, respectively to generate the same voltage levels in both the cases. On the basis of total cost and lower magnitude dc voltage source requirement, Fig. 2(b) is preferred over Fig. 2(a) topology. The 31-level reduced switch RSCI is designed based on this topology and implemented in this study. Fig. 3 represents the circuit diagram of the 31-level RSCI [17], which consists of 10 unidirectional switch and four dc voltage sources. According to seven-level cascaded inverter concept, if the unidirectional power switches (TL1, TL2), (TL3, TL4), (TR1, TR2), and (TR3, TR4) are turned on simultaneously, the dc voltage sources of VL1, VL2, VR1, and VR2 are going to be short-circuited, respectively. This is the reason why the simultaneous turn-on condition of these power electronic switches is avoided. With the similar logic, the operating switches (Ta, Tb) should not be simultaneously turn-on. Following the similar guidelines as described for seven and 31level inverter, a general topology to develop higher voltage level inverter can be presented as follows.

Number of dc  voltage sources ¼ 2n

ð7Þ

Number of unidirectional power switch ¼ 4n þ 2

ð8Þ

Number of output voltage level ¼ 22nþ1  1

ð9Þ

Output voltage ¼ VL;n þ VR;n

ð10Þ

where n denotes the number of dc voltage sources on each leg. The total cost of the multilevel inverter also depended on the range of dc

LEFT (L)

RIGHT (R)

RIGHT (R) Ta

Ta TL1

TL1

TR1

VL1

VL1

VR1

VR1 TR2

TL2

TR2

TL2

TR1 + Vo -

+Vo-

Tb

Tb

(b) with different potential

(a)with the same potential Fig. 2. Seven-level inverter topology.

Table 1 Output voltages of seven-level cascaded inverter. NO.

TL1

TL2

TR1

TR2

Ta

Tb

Voutput [Fig. 2(a)]

Voutput [Fig. 2(b)]

1 2 3 4

U U U U ✗ ✗ ✗ ✗

✗ ✗ ✗ ✗ U U U U

✗ ✗ U U ✗ U U ✗

U U ✗ ✗ U ✗ ✗ U

✗ U ✗ U ✗ U ✗ U

U ✗ U ✗ U ✗ U ✗

VL1 VR1 VL1  VR1 0

VL1 VR1 VL1 + VR1 0

VL1 VR1  (VL1  VR1)

VL1 VR1  (VL1 + VR1)

5 6 7 *

‘U’ and ‘✗’ stands for the inverter switch ON and OFF condition respectively.

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voltage (Vvariety) source and the range of the blocking voltage of the power electronic switches [19]. The range of voltage can be expressed in terms of the number of switches in each leg as:

Therefore, the total blocking voltage (Vb,2) of the 31-level inverter can be expressed as:

Vvariety ¼ 2n

Vb;2 ¼ VTL1 þ VTL2 þ VTL3 þ VTL4 þ VTR1 þ VTR2 þ VTR3 þ VTR4

ð11Þ

The maximum blocking voltages for both sides of the cascaded seven-level inverter unidirectional switches (TL1, TL2), (TR1, TR2), and (Ta, Tb) are VTL1 = VTL2 = VL1, VTR1 = VTR2 = VR1, and VTa = VTb = VL1 + VR1, respectively. Therefore, the total blocking voltage (Vb,1) of the seven-level inverter can be expressed as:

Vb;1 ¼ VTL1 þ VTL2 þ VTR1 þ VTR2 þ VTa þ VTb ¼ 4ðVL1 þ VR1 Þ ð12Þ Similarly, the blocking voltage for the 31-level RSCI switches (TL1, TL2), (TL3, TL4), (TR1, TR2), (TR3, TR4), and (Ta,Tb) are VTR1 = VTR2 = VR1, VTR3 = VTR4 = VR2-VR1, VTL1 = VTL2 = VL1, VTL3 = VTL4 = VL2-VL1, and VTa = VTb = VL2 + VR2, respectively. Ta LEFT (L)

RIGHT(R)

TR 3

TL 3 VL 2

VL1

TL 1

+ Vo -

V

VR 2

R 1

TR 2

TL 2

TL 4

TR 1

þ VTa þ VTb ¼ 4ðVL2 þ VR2 Þ

ð13Þ

Similarly, the generalized expression for the blocking voltage (Vb,n) of the inverter can be expressed as:

Vb;n ¼ 4ðVLn þ VRn Þ

ð14Þ

The dc voltage sources magnitude VL1, VR1, VL2, and VR2 of the 31-level inverter are Vdc, 2Vdc, 5Vdc, and 10Vdc, respectively. The switching states of the 31-level RSCIs are given in Table. 2.

4. Control technique The vector control techniques adopted for the RSC and GSC side are presented. The control schematics along with the test system structure are demonstrated in Fig. 1. For the smooth operation of the DFIG, the converter controls are framed with the objective to regulate the active power (or torque), reactive power (or voltage), speed and dc link voltage, etc., by computing the optimal modulation index and firing angle according to the error of the system.

4.1. Control of RSC The entire control of the RSC can be categorized into three parts such as outer control, inner control and maximum power control. The outer control function is to achieve the requisite active and reactive power for the system stability and the inner control regulates the rotor current to avoid any fluctuation of power output.

TR 4 Tb

Fig. 3. 31-level reduced switch cascaded inverter (RSCI) circuit diagram.

Table 2 Output voltages of 31 level RSCI and its switching states: NO.

TL1

TL2

TL3

TL4

TR1

TR2

TR3

TR4

Ta

Tb

Voutput (Vdc)

01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16

U U ✗ ✗ U U ✗ ✗ U U ✗ ✗ U U ✗ U ✗ U ✗ ✗ U U ✗ ✗ U U ✗ ✗ U U ✗ ✗

✗ ✗ U U ✗ ✗ U U ✗ ✗ U U ✗ ✗ U ✗ U ✗ U U ✗ ✗ U U ✗ ✗ U U ✗ ✗ U U

U U U U U U U U ✗ ✗ ✗ ✗ ✗ ✗ ✗ U ✗ U U U U U U U ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗

✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ U U U U U U U

U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U U ✗ ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U ✗

✗ U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ ✗ U U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U ✗ U

U U U U ✗ ✗ ✗ ✗ U U U U ✗ ✗ ✗ U ✗ U U U ✗ ✗ ✗ ✗ U U U U ✗ ✗ ✗ ✗

✗ ✗ ✗ ✗ U U U U ✗ ✗ ✗ ✗ U U U ✗ U ✗ ✗ ✗ U U U U ✗ ✗ ✗ ✗ U U U U

✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ U ✗ U U U U U U U U U U U U U U U

U U U U U U U U U U U U U U U ✗ U ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗ ✗

VL2 + VR2 = 15 VL2 + VR2  VL1 = 14 VL2 + VR2  VR1 = 13 VL2 + VR2  VR1  VL1 = 12 VL1 + VR2 = 11 VR2 = 10 VL1  VR1 + VR2 = 9 VR2  VR1 = 8 VL2 + VR1 = 7 VL2 + VR1  VL1 = 6 VL2 = 5 VL2  VR1 = 4 VL1 + VR1 = 3 VR1 = 2 VL1 = 1 0

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

U ✗ ✗ ✗ ✗ ✗ ✗ ✗ U U U U U U U U

‘U’ and ‘✗’ stands for the inverter switch ON and OFF condition respectively.

(VL1 = 1) (VR1 = 2) (VL1 + VR1 = 3) (VL2  VR1 = 4) (VL2 = 5) (VL2 + VR1  VL1 = 6) (VL2 + VR1 = 7) (VR2  VR1 = 8) (VL1  VR1 + VR2 = 9) (VR2 = 10) (VL1 + VR2 = 11) (VL2 + VR2  VR1  VL1 = 12) (VL2 + VR2  VR1 = 13) (VL2 + VR2  VL1 = 14) (VL2 + VR2 = 15)

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e

R stator

ds

Lsm

+

-

Lrm

(

e

W¼

R rotor

dr

s

+

Ls Wm  Lsm ir Lm

ð22Þ

Torque can be calculated as:

i qr

qr

qs

)

-

i qs

Vqs

r

Vqr

Lm

Te ¼

3 P Lm 3P ðw idr  wds iqr Þ ¼  W iqr 2 2 Ls qs 22 s

ð23Þ

From Eq. (23), active power Ps and reactive power Qs can be calculated as:

(a) e

R stator -

Ps ¼ xe T e ¼

qs

+

Lsm

Lrm

(

ds

Lm

r

)

dr

qr

-

+

i ds

Vds

e

R rotor

i dr

3P 3 xe Ws ir ¼ xs Ws ir 22 2

ð24Þ

The reactive power Qs can be calculated as:

    3P 3P Qs ¼  xe W is ¼  xe W i s s s 22 22

Vdr

ð25Þ

where xs denotes stator angular frequency and related to xe as

xs ¼ 2p xe . At constant stator flux (Ws ), increase or decrease in (idr) results in increase or decrease in (ids) which in turn regulates Q s . Stator side real and reactive power output is controlled by iqr and idr respectively. The maximum power or reference d-axis current can be reached by operating the generator rotor speed for a constant wind speed conditions. This control loop is used as a speed

(b) Fig. 4. Equivalent circuit of machine (a) q-axis (b) d-axis.

ref

4.1.1. Outer control The two-axis synchronous frame representation of the stator and rotor circuits is illustrated in Fig. 4(a) and (b) respectively. From the equivalent circuits as shown in Fig. 4(a) and (b),

V ds

dWds ¼ Rstator ids þ  xe Wqs dt

V qs

dWqs ¼ Rstator iqs þ þ xe Wds dt

controller for getting the reference d-axis rotor current idr ðnÞ. ref

ref

idr ðnÞ ¼ idr ðn  1Þ þ K pd fxerror ðnÞ  xerror ðn  1Þg þ K id xerror ðnÞ ð26Þ ref

ð15Þ

where xerror ¼ xr xr

ð16Þ

The ‘xr ’ and ‘xr ’ denote as the sensed speed of the rotor and reference speed of rotor respectively. K pd and K id denote respectively the proportional and integral control parameters. xerror ðnÞ and xerror ðn  1Þ represent the speed errors at nth and (n-1)th instant

ref

where V ds , V qs and ids , iqs are represented as d-q axis voltage (V) and current (A) components at stator side respectively. Wds and Wqs are represented as d q-axis stator flux linkages respectively. The end terms xe Wqs and xe Wds of Eqs. (15) and (16) are known as speed emf depends on the rotation of d-q axis. The terms xe and Rstator denote as synchronous mechanical speed (rpm) and stator resistance (O) respectively. At xe = 0, the system tends to its stationary frame of representation. The flux linkage equations can be represented as:

Wds ¼ Lsm ids þ Lm ðids þ idr Þ

ð17Þ

Wqs ¼ Lsm iqs þ Lm ðiqs þ iqr Þ

ð18Þ

ref

ref

respectively. ‘idr ðnÞ’ and ‘idr ðn  1Þ’ denotes the reference d-axis current at nth and (n-1)th instant respectively. By Tip speed ratio (TSR) control the reference speed of the rotor is calculated at a fixed wind speed. Normally, to make the stator reactive power ‘Qs’ to zero, the reference q-axis current is to be set appropriately. ref

Therefore, the reference q-axis current ‘iqr ’ is chosen according to the injection of reactive power into the DFIG. 4.1.2. Inner control The inner current control loops function to control the d-q axis act

Wdm ¼ Lm ðids þ idr Þ Wqm ¼ Lm ðiqs þ iqr Þ

ð19Þ ð20Þ

where Lsm = Ls – Lm, Lrm = Lr – Lm‘Lm’ denotes as the mutual inductance in between stator and rotor side. ‘Lsm’ and ‘Lrm’ denote as the mutual inductance at stator and rotor side respectively. In this study, the stator resistance Rstator is neglected and the stator flux is assumed to be constant and aligned with the d-axis. Therefore

Wds ¼ W, and Wqs ¼ 0. The vector representation of current d-q axis

ref

and Wqm is denoted as Wm .

W ¼ Ls is þLm ir ¼ Ls ðis þ ir Þ Lsm ir s

Substituting Eq. (19), in Eq. (21) and rearranging

ð21Þ

ref

rotor current ‘idr , iqr ’ for power control. The vector model d-q axis component of the rotor voltage in terms of rotor flux linkages is expressed as follows:

V dr ¼ Rrotor idr þ

dWdr  ðxe  xr ÞWqr dt

ð27Þ

V qr ¼ Rrotor iqr þ

dWqr þ ðxe  xr ÞWdr dt

ð28Þ

s

components, (ids , iqs ) and (idr , iqr ) are represented as is and ir respectively. Similarly, the vector representation of flux components Wdm

act

rotor current component ‘idr , iqr ’ closed to the d-q axis reference

The generalized equation can be written from Eqs. (27) and (28) as follows:

V r ¼ Rrotor ir þ

d Wr  ðxe  xr Þ Wr dt

ð29Þ

The corresponding expressions for rotor flux can be expressed as:

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wqr ¼ Lrm iqr þ Lm ðiqs þ iqr Þ

ð30Þ

wdr ¼ Lrm idr þ Lm ðids þ idr Þ

ð31Þ

Substituting Eqs. (17) and (18) in Eqs. (30) and (31) respectively, the rotor flux component can be written as:

wqr ¼

! L2m ir ¼ rLr iqr Lr  Ls 

wdr ¼ Lr idr þ Lm

wds  Lm idr Ls

ð32Þ 

¼ rLr idr þ



Lm w Ls s

ð33Þ

Applying Eqs. (32) and (33), the rotor voltage components as expressed in Eq. (27) and Eq. (28) can be written as:

didr  xsl rLr iqr dt   diqr Lm  ¼ Rrotor iqr þ rLr þ xsl rLr idr þ ws dt Ls

V dr ¼ Rrotor idr þ rLr

ð34Þ

V qr

ð35Þ

Introducing the two virtual variables, the reference rotor voltage components are represented as [7]: ref

V dr ¼ V dr þ xsl rLr iqr ref

V qr ¼ V qr  xsl



L  rLr idr þ m ws Ls

ð36Þ  ð37Þ

By using Park’s transformation the d-q axis rotor current comact

act

" ¼

sinhs coshs

iqr

4.2. Control of GSC The control of GSC has been implemented in this proposed approach to mitigate the harmonics generated by the non-linear load in the test system. Fig. 1 depicts the block diagram of GSC control. As demonstrated in Fig. 1, the real power component of GSC current is controlled to realize power balance of the converters by ensuring constant dc voltage through RC controller. The control of q axis component of GSC is designed to regulate the desired reactive power supply to the non-linear load. However, the q axis component of grid reference current (Igq,ref) is considered as zero for the idea not to absorb any reactive power from the grid. In addition to that, in the grid side an interfacing inductor (Lf) is used to mitigate the harmonics produced by the non-linear load. 4.2.1. Outer control The reference q-axis frame is assumed to align with the PCC ^g. voltage e (i.e. ed = 0). The GSC voltage can be represented as V The grid side real ‘Pgsc ’ and reactive power ‘Q gsc ’ can be calculated as:

Pgsc ¼

3 3 ðeq iq;gsc þ ed id;gsc Þ ¼ eq iq;gsc 2 2

ð44Þ

Q gsc ¼

3 3 ðeq id;gsc  ed iq;gsc Þ ¼ eq id;gsc 2 2

ð45Þ

act

ponents ‘idr , iqr ’ are calculated as [8].

2 act 3 4 idr 5

4.1.3. Maximum power control By adjusting the power or torque, maximum power point tracking (MPPT) can be achieved in the RSC control. Based on this, the maximum output power is generated according to the wind speed variation as input to the inner control loop.

2

3

    # ia;rotor sin hs  23P sin hs þ 23P 6 7      i 4 b;rotor 5 cos hs  23P cos hs þ 23P ic;rotor

ð38Þ

where the rotor slip angle ‘hs ’ is computed as:

hs ¼ he  hr

ð39Þ

where he and hr denote the angle between rotor current and voltage axis calculated from phase locked loop (PLL), and rotor position respectively. The rotor voltage d-q component can be expressed in terms of rotor current error and PI control parameters as follows:

V dr ðnÞ ¼ V dr ðn  1Þ þ K pd fidr;error ðnÞ  idr;error ðn  1Þg þ K id idr;error ðnÞ

ð40Þ

V dclink ¼

V qr ðnÞ ¼ V qr ðn  1Þ þ K pq fiqr;error ðnÞ  iqr;error ðn  1Þg þ K iq iqr;error ðnÞ

ð41Þ

ref

where id;error ¼ idr idr ref

id;error ¼ idr idr

ð42Þ

where K pd , K id , K pd , and K iq denotes the PI control gains. The threeref

ref

P gsc and Q gsc can be controlled linearly related to the dq-axis current respectively, if the PCC voltage is kept constant. Here in this controlled approach both GSC and RSC control act in a coordinated manner with an objective to control the stator real and reactive power by RSC control and dc-link voltage by GSC control. The convention of PRSC depends upon the rotor current, where power is taken positive in case injected towards the rotor circuit. Similarly, the convention of PGSC depends upon the GSC current, where the power flows from GSC to the utility grid is considered positive. The dynamics of dc-link voltage (Vdc-link) variation in terms of the power from RSC (P RSC ) and power leaving the GSC side to grid (PGSC ) is represented as:

ref

ð46Þ

The block diagram of the closed loop system without RC controller is illustrated in Fig. 5. The E(s) represents the dc-link voltage error signal; D(s) represents the disturbances introduced by the nonlinear load; Gc(s) denotes the closed loop controller, and Gp(s) represents the reduced switch cascaded inverter. The tracking error E0(s) of the closed loop system is computed as follows:

E0 ðsÞ ¼

phase reference rotor voltages ‘V ra , V rb , V rc ’ are computed by inverse Park’s transformation as [8]:

3 ref 2 32 3 ref sinhs coshs 6 V ra 7     76 V dr 7 6 ref 7 6 2 P 2 P 7 6 6 V rb 7 ¼ 4 sinhs  3  coshs  3  54 ref 5 5 4 sin hs þ 23P cos hs þ 23P V qr ref V rc

1 dV 2dc c ¼ PRSC  PGSC 2 dt

EðsÞ  DðsÞ 1 þ Gc ðsÞGp ðsÞ

ð47Þ

2

*

ð43Þ

The three-phase reference rotor voltages are compared with the triangular wave by fixing the switching frequency for producing pulse width modulation (PWM) signal of the rotor side converter.

V dc

D(s) link

E(s)

+_

Vdc

+_

E0(s) Controller Gc(s)

Inverter Gp(s)

+

+

link

Fig. 5. Block diagram of the closed loop system without RC controller.

Y(s)

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In practical cases, majorly the lower order harmonics are injected into the systems due to the non-linear load and system nonlinearity such as delay time. To achieve a better power quality with lesser THD and to enhance the system performance, a repetitive controller approach is considered in the proposed approach. The block diagram of the closed loop system with RC controller is illustrated in Fig. 6. Fig. 7 demonstrates the closed loop block diagram of the RC controller. A learning filter l(s) is integrated to compensate the transfer of error by the RC controller. A filter ‘Q(s)’ is used to limit the working of the RC controller within a specified frequency band and to reduce the error in between ‘l(s)’ and dc-link voltage ‘Ev(s)’. The phase delay of the filter ‘Q(s)’ is minimized by regulating the delay time ‘eST p ’ of the periodic signal. An approach of RC controller also functions to produce the output frequency with the multiples of an integer ‘k’ and periodic frequency ‘xp ’. The respective time period can be computed as ‘2xPp ’ [18]. To demonstrate the tracking response of the RC controller, the system tracking error Ev ðsÞ in terms of the tracking error E0(s) of the original system without RC is given by:

Ev ðsÞ ¼ E0 ðSÞ 

ST p

1  Q ðsÞe 1  ð½1  lðsÞ  Q ðsÞeST p Þ

ð48Þ

4.2.2. Inner current control The GSC side converter through inner current control with an interfacing inductor (Lf) is demonstrated in Fig. 1. The dynamics of GSC converter is represented as:

^ ^ gsc ¼ L dI þ jxLf ^I þ E ^ V dt

49Þ

The d-q axis component aligning with the reference frame qaxis along with the PCC voltage is represented as:

V q;gsc ¼ Lf

diq;gsc þ xLf id;gsc þ eq dt

ð50Þ

V d;gsc ¼ Lf

did;gsc  xLf iq;gsc dt

ð51Þ

In a similar manner, the stator current ‘Id, using abc to d-q transform as [30]:

stator’

2

Id;stator ¼

2 sinhe 3

Ia;stator

is computed

3

    6 7 sin he  23P sin he þ 23P 4 Ib;stator 5 Ic;stator

ð52Þ

The active load fundamental component current ‘^Ild ’ is achieved by implementing the SRF theory [32]. The load current in the syn

chronously rotating d-q frame ‘Ild ’ can be computed through instantaneous load current and the phase angle value is computed from PLL. In the synchronous rotating frame, fundamental three phase currents are transformed to dc quantities and harmonic components are changed to non-dc quantity. Low pass filter (LPF) is used to smooth the distortions and the dc values of ‘^Ild ’ in SRF

RC Controller

Z(s)

*

V dc

D(s)

E(s)

link

Vdc

Ev(s) +_

+_

+

+

Controller

Inverter

Gc(s)

Gp(s)

+

+

link

Fig. 6. Block diagram of the closed loop system with RC controller.

Y(s)

Ev(s)

+

Filter Q(s)

+

e

sTp

Learning filter l(s)

Z(s)

Fig. 7. Closed loop block diagram of the RC controller.

are also generated. To control the grid side converter, hysteresis current control is selected as a feedback current control where sensed current follows the reference current within the hysteresis band (HB). If the error current exceeds the positive half of the HB side then the below switch of the GSC converter is on, otherwise, the top switch of the converter is on.

5. Result analysis To justify the effectiveness of the proposed DFIG having active filter capabilities, simulated results are discussed in this section for validating the steady state and dynamic performances. The working of the GSC acts as an active filter for the WECS at stall condition is presented. The power direction is considered positive in the case flowing into the PCC through GSC. The entire result is demonstrated in two cases. In the first cases, the analysis has been done for DFIG based on conventional RSC and GSC with PI controller. The second case the result analysis has been done for DFIG based on 31-level RSCI in RSC and GSC with RC controller. From the findings, at last, a comparative analysis has been done to validate the enhanced performance achieved with the application of the proposed approach. All the simulated results are presented for a longer time period, to show the efficacy of the suggested approach. For clear understanding and to show the variability, all the simulation results starting time is fixed at 4 s and according to our requirements, the ending time is presented in the manuscript.

5.1. DFIG based with conventional rotor side and grid side converter with PI controller 5.1.1. Case-1: Under constant speed operation The simulation results of grid side current (Iabc, grid), source/stator side current (Iabc, source), load side current (Iabc, load) and grid side converter current (Iabc, gsc) of DFIG based WECS on fixed wind speed of 10.6 m/s are shown in Fig. 8(a–d) respectively. The time-varying variations of all these currents indicate a substantial harmonic content with a conventional PI controller. The THD calculation for the grid side and source side currents through FFT analysis are shown in Fig. 9(a–c) respectively. The results indicate an undesirable higher THD level in case of both grid side current and source side current up to 4.82% and 1.32%, respectively under the non-linear load current with THD value 30.52%. This indicates the need of a robust controller to enhance the accompanied power quality issues for WECS.

5.1.2. Case-2: Under stall condition The simulation results of grid current (Iabc, grid) of DFIG based WECS at stall condition (operated zero speed) is shown in Fig. 10 (a). The harmonic content is demonstrated in Fig. 10(b) by calculating the THD of Iabc, grid through FFT analysis. Even under this condition, a substantial growth of 3.65% THD value of grid side current is found out. For better performance, it is necessary to reduce the harmonic content through better controllability and filter capability of WECS.

B. Sahoo et al. / Engineering Science and Technology, an International Journal 22 (2019) 811–826

(a)

819

(b)

(c)

(d)

Fig. 8. Simulation results of (a) Grid current (Iabc,grid) (b) Source current (Iabc,source) (c) Load current (Iabc,load) (d) current of grid side converter (Iabc,gsc).

(a)

(b)

(c) Fig. 9. THD analysis of (a) grid current (Iabc,grid) (b) load current (Iabc,load) (c) source current (Iabc,source).

(a)

(b) Fig. 10. (a) Grid side current (Iabc,grid) (b) THD of grid side current (Iabc,grid).

5.1.3. Case-3: Under unbalanced load condition The simulated results of GSC current (Iabc, gsc) and grid side current (Iabc, grid) are shown for the DFIG based WECS under unbal-

anced load condition in Fig. 11(a–b) respectively. Harmonic analysis for both the current has been done through THD calculation as demonstrated in Fig. 12(a–b). The time variation of grid side

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B. Sahoo et al. / Engineering Science and Technology, an International Journal 22 (2019) 811–826

(a)

(b)

Fig. 11. (a) Grid side current (Ia,grid) (b) Grid side converter current (Ia,gsc).

(a)

(b)

Fig. 12. THD of the proposed system with sudden removal of the load (a) THD of grid side converter current (Ia,gsc) (b) THD of grid current (Ia,grid).

current and grid side converter current indicates a higher amount of harmonic content up to 8.39% and 4.57% THD value respectively. Like Case-1 and Case-2 under this condition also the conventional PI controllers fails to provide the better quality of power supply. 5.2. DFIG based with a 31-level cascaded multilevel inverter in rotor side and conventional grid side converter with RC controller Reduced switch 31-level multilevel inverter topology is used in RSC to produce the required voltage levels, by using the minimum number of unidirectional switches as shown in Fig. 13. In this section, the proposed approach is implemented and the improved results are shown for different conditions in comparison to conventional control as discussed in Section 5.1. 5.2.1. Case-1: Under constant speed operation The simulation results of the test system with the proposed control approach and topology are presented by considering a constant wind speed of 10.6 m/s as demonstrated in Fig. 14. The rotor reference speed of the system is taken as 1750 rpm as the proposed DFIG based WECS worked at maximum power point tracking (MPPT). To achieve the lower THD, a hybrid active filter is integrated in the test system. Implementation of repetitive controller (RC) approach to design the grid side converter control, injects the desired harmonic currents to the load for generating the sinusoidal grid side (Iabc,grid) and source side current (Iabc,source). On the

basis of harmonic, it is clearly indicated that due to the presence of non-linear load the current response of the load (Iabc,load) is nonlinear in nature. Compared to the conventional converter as discussed in Case-1 of Section 5.1, the proposed control strategy and MLI topology results significantly better harmonic free current and voltage responses in case of the grid current (Iabc,grid), gsc current (Iabc,gsc), source current (Iabc,source), and source voltage (Vabc,source) as shown in Fig. 14. In addition to that, the result also signifies a better current regulation within the operational limit. Fig. 14 indicates the steady-state stable power conditions as shown for source power (Psource), power at gsc (Pgsc), power at load (Pload), and power at grid (Pgrid). Above the synchronous speed, the power comes into the PCC through GSC and this direction of power flow at GSC (Pgsc) is marked as positive. The total power of the DFIG is calculated by adding the stator/source power (Psource) and GSC power (Pgsc). As indicated in Fig. 14, after supplying the desired amount of power to the load, the rest amount of power is supplied to the grid. Harmonic analysis by Fast Fourier Transform (FFT) analysis has been done for the confirmation of lesser THD value in comparison to conventional approach and to validate the result within the range of IEEE-519 standard. Fig. 15(a–d) show the results for (a) Current at grid side (Ia,grid), (b) Current at load side (Ia,load), (c) Current at source side (Ia,source), and (d) Voltage at source side (Va,source) respectively. The THD results of the nonlinear load current (Ia,load) is 30.52% as shown in Fig. 15(b). Due to the proposed control and

Fig. 13. Simulation results of 31 voltage level of the inverter.

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Fig. 14. Simulation results of wind energy conversion system at steady state wind speed of 10.6 m/s at maximum rotor speed 1750 rpm.

(a)

(b)

(c) Fig. 15. FFT analysis results of (a) Current at grid side (Ia,grid), (b) Current at load side (Ia,

(d) load),

(c) Current at source side (Ia,source), (d) Voltage at source side (Va,source).

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inverter, the THD is reduced to 2.16%, 0.78% and 0.18% in case of grid current (Ia,grid), source current (Ia,source), and source voltage (Va,source) respectively as shown in Fig. 15(a, c, d). The simulation results clearly indicate a better harmonic reduction performance and current regulation of the proposed approach to act the DFIG with self-active filter capability.

5.2.2. Case-2: Under stall condition The performance of the system under stall condition of DFIG and the corresponding FFT analysis for computing THD values are demonstrated in Figs. 16 and 17. This condition is also similar to the previous case, but the wind speed is zero. In this case, also due to the non-linear load, the load current (Iabc,load) contains harmonics, which is similar to the Case-1 of Section 5.2. By using the proposed indirect current control approach in GSC, the simulation results for grid current (Iabc,grid), gsc current (Iabc,gsc), source current (Iabc,source), load current (Iabc,load) and stator/source voltage (Vabc,source) are significantly better in comparison to conventional converter as discussed in Section 5.1 under stall conditions. The steady state stable active power conditions are shown in Fig. 16 for source power (Psource), power at gsc (Pgsc), power at load (Pload), and power at grid (Pgrid). In addition to that the steady-state stable reactive power conditions are shown in Fig. 16 for source

power (Qsource), power at gsc (Qgsc), power at load (Qload), and power at grid (Qgrid). At wind speed zero/null condition, the performance of the grid side converter behaves like a hybrid active filter by combining the conventional converter with LC filter. The corresponding simulated results are shown in Fig. 16. Due to zero wind speed, the system cannot generate power. Therefore, the stator/source currents are zero and the load power is fed from the grid side. Due to the power supplied from the grid to load; the grid power (Pgrid) becomes negative. The reactive power of grid (Qgrid) canceled out due to the grid side converter harmonic current and reactive power injection. Even load currents are nonlinear, due to the cancellation of the reactive power and harmonics, the grid currents are balanced and linear. By this process, a reduced harmonic free and better current regulation is achieved. Fig. 17(a–b) shows the FFT analysis results. It is found that even though the harmonics at the load side is very high value nearly 30.52%, the grid current case it is substantially reduced to 1.61% well within the limit of IEEE-519 standards and also gives a better result as compared to the conventional approach. 5.2.3. Case-3: Under reduced wind speed The performance of the proposed WECS operating at a reduced wind speed is shown in Fig. 18. As indicated in Fig. 18 for a time

Fig. 16. Results of the proposed system at null wind speed.

B. Sahoo et al. / Engineering Science and Technology, an International Journal 22 (2019) 811–826

(a)

(b)

Fig. 17. FFT analysis results of (a) Current at grid side (Ia,

grid),

(b) Current at load side (Ia,

Fig. 18. Results of the proposed system at reduced in wind speed.

load).

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interval of 4 s to 6 s, the wind speed is reduced in a slope wise manner from 10.2 m/s. As per the reduction of wind speed the grid current is also increased, to meet the load demand. As shown in Fig. 18, when wind speed is fixed or maximum, the grid current is less to meet the load demand. But with decreasing the wind speed, the grid is totally responsible for the load demand, therefore the Iabc,grid current gradually increases. The simulation results for grid current (Iabc,grid), rotor current (Irotor), load current (Iabc,load) and stator/source voltage (Vabc,source) is shown in Fig. 18. Due to the speed changes from synchronous speed to subsynchronous speed, the phase sequence of the rotor current is affected as indicated in Fig. 18. The time variation of all the current indicates a stable and harmonic free characteristic. The speed of the wind, rotor reference speed, and the actual rotor speed is demonstrated in Fig. 18 to indicate the stable synchronized operation of the system. The active and reactive power conditions are illustrated in Fig. 18 for source power (Pstator), stator reactive power (Qsource), power at gsc (Pgsc), power at load (Pload), and power at grid (Pgrid). To achieve MPPT condition, the rotor reference speed is also reduced with the reduced wind speed. Due to the reduction of the reference rotor speed; the actual rotor speed is also decreased. In addition to that, the slip of the induction generator becomes negative to positive and the actual speed of the rotor is changed to super synchronous speed to subsynchronous speed. Due to the

decrease in wind speed, the direct axis rotor current is also decreasing, which affects the active power of source/stator (Pstator). The reduction in wind speed also affects the power flow of the rotor and it is reversed from its normal operation. At constant wind speed, the rotor transmitted the power to the grid through GSC. Due to the speed decreases to synchronous speed, the direction of rotor power is changed to the grid side to the rotor side and the power of the GSC (Pgsc) becomes negative. In this case, by taking the load power (Pload) constant, there are two types of wind conditions happen, one is at high wind speed and another is at low wind speed. At high wind speed after giving required power to load, the extra power is supplied to the grid side (Pgrid). In the case of low wind speed, the generated power of the WECS is not enough to handle the load. Therefore, the power is supplied from the grid side to load and the sign of the power becomes negative. A better current regulation with reduced harmonic free power supply is achieved. 5.2.4. Case-4: Under unbalanced load condition The load compensation facility of the proposed approach under unbalanced load condition is demonstrated in this section. The simulation results for grid current (Iabc,grid), source current(Isource), load current (Iabc,load), and stator/source voltage (Vabc) are shown in Fig. 19. The unbalanced load condition is achieved by the sudden

Fig. 19. Simulation of the proposed system with sudden removal of the load.

(a)

(b)

Fig. 20. THD of the proposed system with sudden removal of the load (a) THD of grid side converter current (Ia,gsc) (b) THD of grid current (Ia,grid).

B. Sahoo et al. / Engineering Science and Technology, an International Journal 22 (2019) 811–826

removal of one phase load (Ia, load) for a time interval of 4 s to 4.25 s. Due to that, the simulation results are presented for 4 s to 4.25 s. Even for unbalanced load condition, the stator/source and grid currents are observed to be balanced and sinusoidal in nature by compensating the removal phase current of load using grid side converter (GSC) current. Due to the GSC current supplied to balance the load current, the grid side current of phase ‘a’ (Ia,gsc) is increased suddenly. The total load current (Iabc, load) reduces with the removal of one phase, and the corresponding increase in grid side current (Iabc, grid) is shown in Fig. 19. The THD value of GSC current (Iabc,gsc), and grid current (Iabc,grid) are presented in Fig. 20 through FFT analysis. The reduced THD value of 5.11% and 2.08% for (Ia,gsc) and (Ia,grid) respectively as compared to Case-3 in Section 5.1 indicates an improved filter capability of the proposed approach for DFIG based microgrid system. By this process even under unbalance condition a better current regulation with substantially reduced harmonic content is achieved. 6. Conclusion The proposed approach improves the effectiveness of the operation of DFIG based WECS with an integrated hybrid active filter capability, repetitive control (RC) approach, and 31-level cascaded multi-level inverter. By using the minimum number of unidirectional switches and lower magnitude dc-voltage, the proposed 31-level cascaded inverter performs better according to both technical and economic aspects. Due to the presence of 31-level RSCI and the control algorithm in the RSC, the system is able to produce reduced harmonic voltages and also supply the required reactive power to the DFIG. GSC control algorithm suppresses the harmonics and supplies the reactive power for the loads in the proposed system. By using the RSC control algorithm in the proposed WECS, the decoupled control of real and reactive power performs better in comparison to the conventional approach. At zero speed of the wind, GSC works as an active filter to supply reactive power to mitigate the harmonics produced by the non-linear load and so the system behaves like a static compensator (STATCOM). By using an LC filter in addition with the GSC, the system behaves as a hybrid active filter. The paper shows the simulation results of the proposed system at the different conditions like steady-state performance at constant wind speed and dynamic performance by changing the wind speed. The steady state performance of the proposed DFIG based WECS has been simulated by a constant wind speed and zero wind speed condition. The dynamic performance of the system has been simulated under reduced wind speed condition and change in the load condition. Appendix DFIG/machine ratings: Rstator = 1.32 O, Lsm = 6.832 mH, Rrotor = 1.708 O, Lrm = 6.832 mH, Lm = 0.219H Pnominal = 3.7 kW, Vnominal(rms) = 230 V, Frequency = 50 Hz, Pmech = 3*1.5*103 W, Pbase = 3*1.5*103/0.9 VA, Wind speed = 12 m/s, rotational speed = 1.2p.u, L = 4e-3H, C = 1e-6F. References [1] D.M. Tagare, Electricity Power Generation: The Changing Dimensions , John Wiley & Sons, 2011 Sep 23. [2] G.J. Herbert, S. Iniyan, D. Amutha, A review of technical issues on the development of wind farms, Renew. Sustain. Energy Rev. 1 (32) (2014 Apr) 619–641. [3] I. Munteanu, A.I. Bratcu, N.A. Cutululis, E. Ceanga, Optimal control of wind energy systems: towards a global approach, Springer Sci. Bus. Media (2008) 5. [4] A.A. Zin, H.A. Mahmoud Pesaran, A.B. Khairuddin, L. Jahanshaloo, O. Shariati, An overview on doubly fed induction generators0 controls and contributions to wind based electricity generation, Renew. Sustain. Energy Rev. 1 (27) (2013 Nov) 692–708.

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