selection of optimal controller for active and reactive ...

2018 International Conference on Computing, Mathematics and Engineering Technologies – iCoMET 2018

SELECTION OF OPTIMAL CONTROLLER FOR ACTIVE AND REACTIVE POWER CONTROL OF DOUBLY FED INDUCTION GENERATOR (DFIG) Darya khan Bhutto ,Jamshed Ahmed Ansari , Faheem Chachar,Sunny Katyara ,Jahangeer Soomro Department of Electrical Engineering Sukkur IBA University Abstract— The world is facing the challenge to generate and store electrical energy in an effective manner so the concentration of wind farm is increasing day by day. All these natural resources possess stochastic behavior so the variation in output waveforms at all these nonlinear power supply units are pre-default. Integration of these standalone, isolated dispatch able units with grid leads instability and imbalance in active and reactive power. In order to control active and reactive power in variable speed wind generators, different control schemes are adopted to mitigate voltage and current fluctuations. In this research work mathematical model of variable speed, a wind turbine with their control schemes is implemented on MATLAB software. In order to improve transient performance of DFIG, a conventional controller is compared with Proportional, Integral, and Differential (PID) controller to get optimal power output. Analysis of results clearly depict the robustness of PID controller over a conventional controller.

megawatts [1],[10][11]. These turbines are based on DFIG generator. Variable speed turbine can store some of the power fluctuations due to turbulence by increasing rotor speed, pitching the rotor blade turbine can control the power output at any given wind speed[12].Fig.1 below shows DFIG based wind turbine.

Keywords—DFIG; Active power; Reactive power; PID;Wind Turbine.

Fig.1 DFIG based wind turbine [1]

I. INTRODUCTION Power system plays a major role in the economy of the country[1] .The major concern for Engineers and researcher are to reduce losses, improvement in performance and decrease in per-unit cost of electricity[1],[2]. Conventional means of generating electric power are based on oil, coil, and gas which are scarce resources [1], [3]. Electricity coming from these resources is quite expensive and The residual of these ingredients not only leave a harmful effect on health but also pollute the environment[4]. A better option is to switch on renewable resources like solar, biomass and wind turbines[3], [4]. Wind farms are considered to be a potential source of electrical energy in near future [5].the wind turbine is subjected to sessional as well as the stochastic behavior of wind[2],[3],[5],[6]. In normal conditions, the control strategies work better but challenge arrives when the wind speed is above in cut-in speed so that power is contributed into grid[3][7][8]. Control schemes must have some mechanisms to limit power capture at high speeds to prevent overloading[3],[5],[7][9].Variable speed turbines are more controllable and having ability to generate power in

There are two main control technique: the vector control and the direct torque control use for enhancing the performance of induction machine [2]. Direct torque control (DTC) is a nonlinear type of control that operates in a hysteresis manner. It identifies the state of certain variables such as flux and torque and makes a decision about what rotor voltage to apply to drive the state to the desired value [1][8][9][13]. Vector control is a linear control structure based on simple single input, single output type controllers (proportional integral type controllers) or more advanced state space theory, a rotating frame is used to decompose complicated three phase relationships into orthogonal components , this research work is shaded under the umbrella of vector control schemes[1], [3], [14],[9],[15]. II. METHODOLOGY A. Background This research is focused on the simulation of a wind turbine based on Doubly-fed Induction generator to achieve maximum power using optimal power control strategy replacing conventional power control with PID controller

978-1-5386-1370-2/18/$31.00 ©2018 IEEE

2018 International Conference on Computing, Mathematics and Engineering Technologies – iCoMET 2018 [3].One of the frequent rotor side control strategies is using bidirectional back to back Ac-Ac power converters to maintain a constant frequency of rotor power[1], [5], [10], [12], [14], [16].Vector control technique can be applied to control rotor currents to achieve fast and decoupled control of the generator[17]. Associated work is categorized under linearized control technique so a rotating complex three phase parameters (voltage and currents) are transformed into equivalent stationary DQ frames. In the synchronous d-q reference frame rotating at ωs speed, the model of DFIG is given by the following equations: Stator voltage components:[1]

d  Vds = RsIds + dt ψ ds − ω sψ qs  Vqs = RsIqs + d ψ qs + ω sψ ds  dt

(1)

Rotor components:[1-3]

d  Vdr = RrIdr + dt ψ dr − (ω s − ω r )ψ qr  Vqr = RrIqr + d ψ qr − (ω s − ω r )ψ dr  dt

(2)

Stator flux components:[1]

ψ ds = LsIds + LmIdr  ψ qs = LsIqs + LmIqr

3 Lm p (ψ dsIqr −ψ qsIdr ) 2 Lr

(6)

Generator active and reactive and reactive powers at the stator side are given by the expression:[1]

3   Ps = 2 (VdsIds + VqsIqs )  Qs = 3 (VqsIds − VdsIqs )  2

(7)

(9)

ψ s Lm   Ids = Ls − Lr Idr   Iqs = − Lm Iqr  Ls 3   Ps = 2 VsIqs  Qs = 3 VsIds  2

(10)

(11)

(4)

(5)

Ωr + f Ωr dt

ψ s = LsIds + LmIdr   0 = LsIqs + LmIqr

3 Lm   Ps = − 2 Ls VsIqr  Qs = 3 Vs (ψ s − Lm Idr )  Ls Lr 2

Let us note that this torque represents a disturbance for a wind turbine and takes a negative value[1].

Tt = Tem + J

(8)

Replacing the stator currents by their expressions given in (11), the equation below are expressed:

DFIG electromagnetic torque[1]:

T =−

Vds = 0   Vqs = Vs = ω s.ψ s

(3)

Rotor flux components:[1]

ψ dr = LrIdr + LmIds  ψ qr = LrIqr + LmIqs

B. The Simplified model of DFIG[1] The rotor side converter is controlled in synchronously rotating d-q axis frame with d-axis oriented along the stator flux vector position so to achieve decoupled control of stator active and reactive power. Model can be simplified by neglecting stator resistance and keeping stator flux constant as the stator is connected to the grid.

(12)

The electromagnetic torque is as follows:

T (ψ sIqr ) = −

3 Lm p 2 Ls

(13)

Rotor voltages can be expressed by:

 Lm2 Vdr = RrIdr − gω s( Lr − ) Iqr  Ls  (14)  2 Vqr = RrIdr − gω s  Lr − Lm  Idr + g LmVs    Ls  Ls  III. SIMULATION MODEL

978-1-5386-1370-2/18/$31.00 ©2018 IEEE Fig.2 of DFIG Sub system block simulation

2018 International Conference on Computing, Mathematics and Engineering Technologies – iCoMET 2018

Fig.3 of DFIG Sub system block simulation [1]

This section discusses the simulation graphs and the readings obtained by analysis of simulation model and scope readings. The simulation of 9 MW wind form based on doubly fed induction generator is simulated on Simulink. The simulation time for all the parameters is 5 seconds.Fig.4 below shows the stator active power.

Graph witnesses the inverted wave of active power with reactive power .the simulation time is 5 seconds, the rise time is 1.01 second, overshoot is 0.38 per unit and settling time is 4.44 second. Fig.6 represent Rotor active power. Rotor active power with conventional controller 0.4 0.3 0.2 Power in per unit P.U

In Fig.2 and Fig.3 expressed models are the software implantation of a simplified mathematical model of DFIG in MATLAB Simulink tool under the consideration of synchronized d-q reference frame. This model represents overall system components (parameters) needed to run simulation. IV. RESULTS AND DISCUSSION

0.1 0 -0.1 -0.2 -0.3 -0.4 0

0.5

1

1.5

2

2.5 Time in second

Stator active power with conventional controller

3

3.5

4

4.5

5

0.4

Fig.6 Rotor active power with PI controller

0.2

Power in per unit P.U

0

Fig.6 represents the graph of rotor active power with a conventional PI controller connected with DFIG. The simulation time is 5 seconds, the rise time is 1.01 second, overshoot is 0.37 per unit and settling time is 4.25 second. Fig.7 represent Rotor reactive power.

-0.2 -0.4 -0.6 -0.8 -1

Rotor reactive power with conventional controller -1.2 0

0.5

1

1.5

2

2.5 Time in second

3

3.5

4

4.5

0.4

5

0.3

Fig.4 Stator active power with PI controller Fig. 4 represents the graph Stator active power with a conventional PI controller connected with DFIG simulation time is 5 seconds, the rise time is 1.01second, overshoot is 0.379 per unit and settling time is 4.36 second. Fig.5 represent Stator reactive power. Fig.5 represents the graph Stator reactive power with a conventional PI controller based DFIG.

Power in per unit P.U

0.2 0.1 0 -0.1 -0.2 -0.3 -0.4 0

0.5

1

Stator reactive power with conventional controller

1.5

2

2.5 Time in second

3

3.5

4

4.5

5

0.4

Fig.7 Rotor reactive power with PI controller

0.3

Power in per unit P.U

0.2

Fig.7 represents the graph rotor reactive power with a conventional PI controller connected with DFIG. Graph

0.1 0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6 0

0.5

1

1.5

2

2.5 Time in second

3

3.5

4

4.5

5

Fig.5 Stator reactive power with PI controller 978-1-5386-1370-2/18/$31.00 ©2018 IEEE

2018 International Conference on Computing, Mathematics and Engineering Technologies – iCoMET 2018 witnesses the inverted wave of active power with reactive power the simulation time is 5 seconds, the rise time is 1.02 second, overshoot is 0.33 per unit and settling time is 4.48 second. Fig.8 represent stator active power of DFIG based on PID controller. 0.4 0.2 0 Power in per unit P.U

Rotor reactive power with PID controller 0.4 0.3 0.2 Power in per unit P.U

Stator active power with PID controller

overshoot is 0.30 per unit and settling time is 4.3 second. Proceeding Fig.11 represent Rotor reactive power.

-0.2

0.1 0 -0.1 -0.2

-0.4

-0.3

-0.6

-0.4 0

0.5

1

1.5

2

2.5 Time in second

3

3.5

4

4.5

5

-0.8

Fig.10 Rotor reactive power with PID controller

-1 -1.2 0

0.5

1

1.5

2

2.5 Time in second

3

3.5

4

4.5

5

Fig.8 Stator active power with PID controller Fig.8 represents the graph of stator active power with PID controller connected with DFIG simulation time is 5 seconds, the rise time is 1.01 second, overshoot is 0.26 per unit and settling time is 4.3 second. Fig.9 represent Stator reactive power with PID.

Fig.10 represents the graph of rotor side reactive power with PID controller connected with DFIG. The simulation time is 5 seconds, the rise time is 1.02 second, overshoot is 0.33 per unit and settling time is 4.44 second. Proceeding Fig.11 represents the response Active power of DFIG with both controllers. Comparisiom of rotor active power with PI and PID controller 0.4 PID PI

0.3

Stator reactive power with PID controller 0.3

Power in per unit P.U

0.2

0.2

Power in per unit P.U

0.1 0 -0.1

0.1 0 -0.1 -0.2

-0.2 -0.3

-0.3

-0.4

-0.4 0

0.5

1

1.5

2

-0.5 -0.6 0

0.5

1

1.5

2

2.5 Time in second

3

3.5

4

4.5

5

Fig.9 Stator reactive power with PID controller Fig.9 represents the graph reactive power at stator with PID controller connected with DFIG. Graph witnesses the inverted wave of active power with reactive power .the simulation time is 5 seconds, the rise time is 1.01 second, overshoot is 0.25 per unit and settling time is 4.41 second. Fig.10 represent Rotor active power. Rotor active power with PID controller 0.4 0.3

3

3.5

4

4.5

5

Fig.11 Rotor active power with PID and conventional controller Fig.11 represents the graph of the active power at rotor with PI and PID controller connected with DFIG. Simulation time is 5 seconds and graph clearly indicates the response of both controllers operated on same parameters. The settling time and the overshoot of PI are 4.25 seconds and 0.37 per unit whereas for PID controller the settling time is 4.3 and overshoot per unit is 0.3 p.u. As the wind speed is crossing the threshold value the rotor of DFIG stops to prevent overcurrent in the rotor as well as to hold on stator flux. Proceeding Fig12 represent the response of both controllers subjecting rotor comparision of rotor reactive power with PI and PID controller

0.1

0.4

0

0.3

-0.1

0.2

-0.2 -0.3 -0.4 0

0.5

1

1.5

2

2.5 Time in second

3

3.5

4

4.5

5

Fig.10 Rotor active power with PID controller

Power in per unit P.U

Power in per per unit P.U

0.2

2.5 Time in second

PID PI

0.1 0 -0.1 -0.2 -0.3

Fig.10 represents the graph of rotor active power with conventional PID controller connected with DFIG. the simulation time is 5 seconds, the rise time is 1.01 second,

-0.4 0

0.5

1

1.5

2

2.5 Time in second

3

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4

4.5

5

Fig.12 Rotor reactive power with PID and conventional controller

978-1-5386-1370-2/18/$31.00 ©2018 IEEE

2018 International Conference on Computing, Mathematics and Engineering Technologies – iCoMET 2018 Fig.12 represents the graph of reactive power of rotor with PI and PID controller connected with DFIG. Simulation time is 5 seconds and graph clearly indicates the response of both controllers operated on same parameters .The settling time and rise time in seconds for PI controller are 4.4 and 1.02 second and the overshoot of PI is 0.33p.u whereas PID controller possesses 1.01 second rise time,4.2 second settling time with 0.2 P.U overshoot value. V. ANALYSIS OF RESULTS

Active power Reactive power

Active power Reactive power

[1] [2]

[3] [4]

This section discusses the results of PI and PID controllers connected with doubly fed induction generator. Table 1.1 represents the results of active and reactive power at stator and rotor side. It compares the response of conventional rotor side power controller with PID controller and results prove quick and better transient response of PID controller over Conventional PI controller. Table 1.1:Power at stator and rotor side Power at stator side Parameters PI Rise time(sec) 1.01 Settling time(sec) 4.36 Overshoot(p.u) 0.379 Rise time(sec) 1.01 Settling time(sec) 4.44 Overshoot(p.u) 0.38 Power at rotor side Rise time(sec) 1.01 Settling time(sec) 4.25 Overshoot(p.u) 0.37 Rise time(sec) 1.02 Settling time(sec) 4.4 Overshoot(p.u) 0.33

REFERENCES

[5]

[6] [7]

PID 0.01 4.3 0.26 1.01 4.4 0.25 0.01 4.3 0.3 1.01 4.2 0.2

[8] [9] [10] [11] [12] [13]

[14]

VI. CONCLUSION The DFIG is nowadays a popular choice for wind energy conversion systems. This popularity is mostly due to its ability for large variable speed drive. In this paper the vector control strategy has been implemented on MATLAB; this technique has been used for reference tracking and decoupling of active and reactive power exchanged between the stator and the grid. Due to varying nature of wind system is subjected to power perturbations. In order to mitigate these unwanted power oscillations suitable controllers with appropriate power gains are required; so for this purpose conventional (lead-lag) power controller is compared with PID feedback controller. The proposed power controller for doubly fed induction generator had been successfully implemented as a part of vector control strategy to estimate rotor side and grid side power gains. The results the robust performance of suggested control schemes. Results recommend the PID controller as it presents batter response for controlling oscillations of active and reactive power at stator as well as rotor side.

[15]

[16]

[17]

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978-1-5386-1370-2/18/$31.00 ©2018 IEEE