Speed-sensorless Voltage & Frequency Control in ...

Australasian Universities Power Engineering Conference, AUPEC 2014, Curtin University, Perth, Australia, 28 September – 1 October 2014

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Speed-sensorless Voltage & Frequency Control in Autonomous DFIG based Wind Energy Systems Rishabh Dev Shukla1, Student Member, IEEE

Dr. Ramesh Kumer Tripathi2, Senior Member, IEEE

Department of Electrical Engineering Motilal Nehru National Institute of Technology Allahabad-211004, India E-mail: [email protected]

Department of Electrical Engineering Motilal Nehru National Institute of Technology Allahabad-211004, India E-mail: [email protected]

Abstract— This paper investigates the application of the hysteresis current control technique to control the voltage and frequency of a variable speed-constant frequency autonomous DFIG based wind energy system by using Direct Voltage Control method. The DFIG feeds an isolated R-L load. A diode bridge rectifier and a power electronics converter (called rotor side converter) having common dc link is connected between the stator and rotor of the DFIG. The control strategy uses the speed-sensorless control for the rotor side converter designed for low-to-medium wind speeds. To regulate the voltage and frequency at stator terminals, the error between the actual rotor currents and the reference rotor currents is given to the hysteresis controller. The reference currents are obtained by the direct voltage control technique. The control pulses for the rotor side converter are supplied by the hysteresis controller which is operated on the error signal. A 2 MVA Wind energy system is designed by using DFIG prototype in MATLab/Simulink. Simulation outcome obtained from the 2 MVA system are presented and discussed in this paper. Index Terms— DFIG, direct voltage control, rotor side converter, wind energy systems.

I.

INTRODUCTION

Nowadays, the wind energy systems (WESs) are more fashionable because they are almost developed enough in terms of technology for instance operation and control, less installation cost and ability to produce significant amount of electrical power. Additional, such WESs may prove to be cost-effective for supplying isolated loads in remote locations [1]. In present time, the Doubly Fed Induction Generator (DFIG) based wind energy systems (WESs) are widely used in connection with the grid. The grid connected operation of DFIG based WES is quite matured and comparatively older. It shows the potential to supply power at constant voltage and frequency with variable rotor speed. Recently the issues related to stand-alone operation of DFIG based WES is becoming the main concern for researchers and students working in this area. The autonomous wind energy systems are more useful for off-shore installations and to supply the isolated or remote areas. An autonomous energy generating

system must be able to supply the users with constant/controlled voltage and frequency [5]. The key issues [5], which need to be addressed by a autonomous WESs, are; development of a the local grid with regulated output voltage & frequency; unity power factor operation of the machine; sub-synchronous and super-synchronous speed operation with the rated torque; speed-sensorless operation; load harmonic compensation; initial excitation technique of the generator/machine. In reference to WESs, DFIG has numerous advantages for variable speed operation at medium to high power applications. It requires little maintenance and is robust. The power converter, connected to the rotor side, is rated at only a fraction of the generator nominal power [2]– [4]. The speed-sensorless operation of autonomous DFIG based WESs is desirable, because a speed-sensor/encoder has some drawbacks in terms of robustness, cost, cabling, and maintenance. A brief state-of-the-art review on mechanical position/speed sensorless control techniques for autonomous DFIG based WESs is presented in [5]. There are two fundamental methods of autonomous DFIG voltage and frequency control: direct voltage control (DVC) & stator flux oriented control [6]-[8]. The direct voltage control DVC is much simpler than the stator flux oriented control In the DVC control, any information from mechanical sensors/encoder for the rotor speed or position angle is not required. In [6], [7], the authors proposed dq0 transformation to regulate output or stator voltages and sine PWM technique is used for converter switching. This paper presents a hysteresis current control technique to control the voltage and frequency of a variable speed-constant frequency autonomous DFIG based WES by using Direct Voltage Control (DVC) method. The DFIG using a power electronics converter as the rotor side converter and a diode rectifier connected between the stator and the rotor and feeds an isolated R-L load. Hysteresis Current Control (HCC) is widely used due to its simplicity in implementation, fast and accurate transient response, direct limiting of device peak current and practical insensitivity to dc link voltage ripple (i.e. require small filter capacitor). Another advantage is that it does not need any knowledge of


the system parameters. The load/stator side converter control is outside from the scope of this paper and only a 6-pulse diode rectifier is used of the purpose. A dc battery is connected to the dc link for short duration of time to give the required initial energy excitation for the generator. This paper is organized as follows: Section II discusses the basics of grid connected and autonomous operating modes of DFIG based WES. Section III discusses the direct voltage control via hysteresis current controller. Section IV presents the simulation results with their discussions. Section V presents the conclusion. II.

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diagram of an autonomous DFIG based WES supplied to an isolated load is given in Fig. 2.

DFIG BASED WES: GRID CONNECTED AND AUTONOMOUS OPERATIONS

A schematic diagram of a DFIG based wind energy conversion system connected to the grid is illustrated in Fig.1. Generally, it consists of a wind turbine, a gearbox, a doublyfed induction generator (DFIG) a Grid Side Converter (GSC) and a Rotor Side Converter (RSC). By regulating the RSC and GSC, the DFIG characteristics can be accustomed so as to obtain maximum of effective power translation or capturing ability for a wind turbine and to control its power generation with a less fluctuation. Generally, power converters are controlled via vector control techniques, which give decoupled control of both active and reactive power.

Figure 2. Autonomous DFIG Based WES

The rotor is connected to the load via back-to-back power electronics converters with a common capacitive dc link for facilitating the bidirectional power flow ability. The converters are known as the rotor side converter and load/stator side converter. The fundamental equations for describing the DFIG system in synchronous reference frame are as follows [2], [3]: (1) (2) (3) (4)

Figure 1 Diagram of grid connected DFIG Based WES

The active and reactive powers delivered from the DFIG to the grid are controlled by means of controlling the rotor currents of the DFIG. The regulation of rotor currents is done by the RSC. The function of the GSC is to maintain the DC link voltage constant whatever be the direction of the rotor power flow. So as to maintain the DC link voltage constant, a bidirectional converter is mandatory to implement in the rotor side circuit. This converter work as a rectifier for the subsynchronous speed and for super-synchronous speed this converter works as an inverter to deliver all generated power to the grid. In grid connected mode, active & reactive powers are controlled, which results in failure to control output voltage and frequency in stand-alone mode. It makes the DFIG based WES useless after the grid outage. The autonomous and grid connected operation of the DFIG based WES is very different to each other and needs different controllers. In autonomous operating mode; stator is not connected to the utility grid but supplies the isolated load. The block

Where stator voltage, rotor voltage are vs, vr; stator flux, rotor flux are λs, λr; & stator current, rotor current are is, ir, and stator & rotor resistances & inductances are rs, rr and Ls, Lr respectively. The mathematical model of an autonomous DFIG system is based on the same equations as the grid connected system [eqn (1)-(4)]. In case of autonomous DFIG system, stator voltage is obtained as a consequence of the excited machine loaded on the stator side. The voltage at stator terminal during a resistive load supply is given as: vs=-Rois

(5)

Where Ro is the load resistance and the is stator or load current. In a fundamental model, DFIG can be taken as a machine fed from a current source from the rotor side. The rotor is fed from a current controlled voltage source converter, which can be treated as current source, thus the equations (3) & (5) can be ignored. From equation (2) & (4) and with a resistive load and neglected stator resistance the dependence of the rotor current on the stator voltage represented in dq frame can be described by (9) [29]: (6) Where Ro is the load resistance, stator resistance is neglected and Zs is the stator side impedance . For an unloaded system, we have; (7)


III.

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DIRECT VOLTAGE CONTROL TECHNIQUE

DVC [6], [7] is based upon the representation stator voltage vector in a rotating polar coordinate system by voltage element V and its location regarding angle θ i.e. (V, θ). The reference coordinate system rotates at a reference synchronous speed ωs* that corresponds the reference frequency of the generated stator voltage. The control technique ensures that the fixed stator voltage vector magnitude

vs

and the position

angle θ s , associated to the d-axis provide the constant voltage magnitude and frequency. The magnitude of rotor current and the frequency are directly given by the stator voltage PI controllers. The three phase sinusoidal stator voltages are represented in the rotating reference frame [d,jq] as a vector Vs:

V s = v sdp + jv sqp

(8)

When the amplitude & frequency of the three phase sinusoidal stator voltage are constant, the stator voltage vector components in (d, jq) coordinates (vsd vsq) are also constant, that gives constant value of the voltage vector magnitude

vs

and position angle θ s , referred to the d-axis of the rotating coordinate system (d, jq). In this method, the components (vsd vsq) of the stator voltage vector Vs is used to decide the voltage vector amplitude 2 sd

vs

Vs = v + v

and angle

θs ,as:

Figure 3. Direct voltage contol using hysteresis current control

The polar (A, θ) coordinates to Cartesian (a b c) coordinates referred to the rotor gives the three phase

2 sq

⎛v θ s = a tan ⎜⎜ sq ⎝ vsd

*

⎞ ⎟⎟ ⎠

The components of stator voltage vector in (d, jq) coordinate are given on the basis of the following equations: 2⎛ 2 ⎞ 2 ⎞⎞ ⎛ ⎛ vsd = ⎜⎜ vsa cos ωs*t + vsb cos⎜ωs*t − π ⎟ + vsc cos⎜ ωs*t + π ⎟ ⎟⎟ 3⎝ 3 ⎠ 3 ⎠ ⎠ (10) ⎝ ⎝ 2⎛ 2 ⎞ 2 ⎞⎞ ⎛ ⎛ vsq = ⎜⎜ vsa sin ωs*t + vsb sin⎜ ωs*t − π ⎟ + vsc sin⎜ ωs*t + π ⎟ ⎟⎟ 3⎝ 3 3 ⎠⎠ ⎝ ⎠ ⎝

( )

vs

*

*

vs and actual

stator voltage vector amplitude and second PI controller

(i.e. is frequency regulator) produces the reference rotor *

current angular speed ωir operated on error between the reference

θ s * (i.e.

zero) and the actual

θs stator

position angle. The integration of output signal ωir reference rotor current vector position angle to rotor (Fig.3).

*

*

(11)

*

vs is coincided with

the d-axis in the steady state. In the case of transient state, the

The method uses the two PI controllers. The one PI controller regulates the rotor current vector amplitude ir

⎡ira* ⎤ ⎡θ ir* ⎤ ⎢*⎥ ⎢ * ⎥ * ⎢irb ⎥ = ir Cos ⎢θ ir − 2π 3⎥ ⎢i * ⎥ ⎢θ * + 2π 3⎥ ⎣ rc ⎦ ⎣ ir ⎦ The reference stator voltage vector

( )

based on error calculate between the reference

*

reference rotor current signals i ra _ ref , i rb _ ref , i rc _ ref for hysteresis current controllers.

(9)

voltage

actual stator voltage vector reference location current vector

ir

vs

is displaced from the

*

vs by an angle θs , since the actual rotor

ir* by

is displaced from its reference position

an angle δ ir (Fig. 3). Based on the actual position angle θs of the stator voltage vector Vs, the PI controller (frequency regulator) regulates the rotor current angular speed

ωir

regarding the rotor current vector acceleration or deceleration. It makes sure that position angle of the rotor current vector is *

gives the

θir* with respect

*

changed and the vector achieves its reference position ir and consequently the reference position of the stator voltage is achieved and the θs dislocation angle is omitted.


IV.

SIMULATION, RESULTS & DISCUSSION

The simulation model of the autonomous DFIG based WES [shown in Figure 4(a)] is built using MATLAB/SIMULINK. The simulation parameters and rating of power electronics converters [shown in Figure 4(b)] are given in Table I. In this paper we have taken three cases. Case1 shows the steady state operation i.e. constant load and constant wind speed operation of the system; case2 shows the loads switching operation of the system when wind speed is constant; and case3 shows the system performance when wind speed is variable and load is constant. A dc battery is connected to the dc link for time period 0 to 0.1 sec to give the required initial energy excitation for the DFIG system. The reference for the generated stator voltage is equal to 690 Vrms line-to-line (or 976 V_peak) i.e 398.38Vrms line-to-ground (or 563.38 Vph_peak), and result shows that generated stator voltage follows the reference value and stays at the specified value (i.e. 976V_peak or 563.38 Vph_peak) and reference frequency is equal to 50 Hz in all the cases. Table I. Data Specification Table [10] S. N 1. 2. 3. 4. 5. 6. 7. 8 9. 10 . 11 . 12 . 13 .

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DFIG Parameter

Value

Nominal Rated Power Rated Line-to-line Stator Voltage (Vrms) Rated Line-to-line rotor Voltage (Vrms) Stator and rotor connection Nominal Frequency Stator Resistance & leakage inductance Rotor Resistance & leakage inductance Magnetization Inductance No. of pole pair Matching transformer rating Sampling time

2 MVA 690 V 2070 V Star and Star 50 Hz 2.6mΩ&0.087 mH 26.1 mΩ & 0.783 mH 2.5 mH 2 1 MVA, 690/2070 Vrms 10 μsec

Power Electronics Converter data IGBT switch (6no.)

Value 3300 V, 1200 A

Power Diode (6no.)

2000 Vrms, 1000 Arms

(a)

(b) Figure 4. (a) Simulation Block diagram of system; (b) Detailed diagram dc-link converter.

Case 1: In this case, we show a steady state operation of the system. A load of 1 MW and 24 kVAr is connected throughout the simulation, i.e. 0 – 6 s. The results are shown at end of the this section. In Fig. 5 (a) load or stator voltage is constant at their reference value i.e. 398.38 Vrms (or 563.39 V peak) and also the stator/load current and frequency at stator/load terminals shown in Fig. 5 (b) is constant due to constant load and wind speed. Both, the active power and re active power, shown in Fig. 5 (c), are constant due to constant load and wind speed. The active power is equal to 1 MW and reactive power is equal to 24 kVAr. Fig. 5 (d) represents the generator speed, torque and the rotor currents. The speed is constant due to the constant wind speed turbine. The constant load operation makes the torque constant. Case 2: Shows the loads switching operation of the system when wind speed is constant. In this case, a load of 0.5 MW and 12 kVAR is connected throughout the simulation i.e. 0 – 6s. Another load ( i.e. 0.5 MW and 12 kVAR) of the same rating is connected for the period of 2 – 4 s. The results are shown at end of the this section. In Fig. 6 (a), stator/load voltage ( vabc) is constant at their reference value i.e. 398.38 Vrms (or 563.39 V peak) and the current value is changing due to the variable load. The current is increased with increase in load and vice-versa. Frequency at Stator/load is shown in Fig. 6 (b) and constant at reference value of 50 Hz with very slight variation at the load switching points. In Fig. 6 (c), the active and reactive power variation is presented. Up to 2 s the active and reactive power is equal to 5x105W (i.e. 0.5MW) and 1.2x104 (i.e. 12kVAr). The -ve sign shows that the quantity (valid for both active and reactive powers) is delivered from generator (induction motor works as a generator) to the load. Another load is switched ON for a period of 2– 4 s, thus the active is changed to 1 MW from 0.5 MW and reactive power is changed to 24kVAr from 12kVAr. After 4 s, the additional load is disconnected so that active and reactive power retained to its pervious value i.e. 0.5 MW and 12kVAr. Fig. 6 (d) represents the generator speed, torque and rotor currents variation respectively. The generator speed is constant due to constant wind speed. There is some variation in torque and rotor currents amplitude due to variable load. -ve sign in generator torque shows that it is generator mode and follows the torque supplied by the wind turbine.

T hree pha se T hree phase s ta to r /lo a d s ta to r /lo a d v o lt a g e c ur r e nts ( in v o lt , ( in a m p ) ph a se -to -g r o un d)

0 -500 0

1

2

3

4

5

6

0

0

1

2

3

4

5

6

0

1

2

3

4

5

6

1

2

3

4

5

6

55

Frequency at stator/load terminals (in Hz)

0

50

45

0

-1000

0

1

x 10

0

2

3

4

5

6

5

55

50

45

-5

-10 -15

1

x 10

2

3

4

5

6

5

-5

0 4 x 10

1

2

3

4

5

6

-10

0

-1 -2

0 4 x 10

1

2

3

4

5

6

0

1

2

3

4

5

6

0

-2

0

1

2

3

4

5

6

100

-4

G enera to r speed ( in r a d /s e c )

-3

0

0

pow er R e a c t i v e p o w e r A c t i(vi ne W ) ( in V A r )

A c tiv e po w e r ( in W )

0

1000

-1000

R e a c t iv e p o w e r ( in V A r )

500

-500

1000

G enera to r speed (in r a d /s e c )

5

respectively. The speed varies due to the variable wind speed and accordingly generator torque also varies.

500

Frequency at stator/load terminal (in Hz)

Three phase stator/lo ad currents (in amp)

Three phase stator/lo ad vo lta ges (in volt, phase-to-ground)


100

2

3

4

5

6

0

-1

0

1

2

3

4

5

6

2000 0

-2000

0

1

2

3 Time (in sec)

4

5

6

Figure 5. Results corresponding to case 1 (i.e. constant wind & load); (a) Three phase voltages and currents at stator/load terminals; (b) Frequency at stator/load terminals; (c) Active and reactive power at stator/load terminals; (d) speed, torque and three phase rotor currents of the generator respectively.

Case 3: In this case, wind speed is variable and load is constant. There is a abrupt decrease in wind speed to 8 m/s from 11 m/s at 2 sec and abrupt increase in wind speed to 12 m/s from 8 m/s at 4 sec. The results are shown at end of the this section. In Fig.8 (a) stator/load voltage is constant at their reference value i.e. 398.38Vrms (563.39 V peak) and also the stator/load current and frequency at stator/load terminals, Fig. 8(b), is constant due to constant load with the slight variations at the sudden speed changing points of wind. Both the active power and reactive powers, as given in Fig.8 (c) are constant due to constant load. The active power is equal to 1 MW and reactive power is equal to 24 kVAr. Fig. 8 (d) represents the generator speed, torque and rotor currents variation

0

G e n e r a to r to r q u e ( in N - m )

1

0 -2000 -4000 -6000 -8000

T hree pha se ro to r c u r r e n ts ( in a m p )

0 4 x 10

2000

0

1

2

3

4

5

6

0

1

2

3

4

5

6

0

1

2

3 Time (in sec)

4

5

6

0 -2000

Figure 6. Results corresponding to case 2 (i.e. variable load); (a) Three phase voltages and currents at stator/load terminals; (b) Frequency at stator/load terminals; (c) Active and reactive power at stator/load terminals; (d) speed, torque and three phase rotor currents of the generator respectively. Three phase load/stator terminal voltages (phase-to-ground, in volt)

1

500 0

-500 3

3.05

3.1

3.15

3.2

3.25

3.3

3.35

3.4

3.25

3.3

3.35

3.4

1000

Three phase load/stator terminal currents (in amp)

Three phase r o to r c ur r e nts G e ne r a to r to r que (in a m p ) (in N -m )

0

0

-1000 3

3.05

3.1

3.15

3.2 Time (in sec)

Figure 7. Enlarged view of three phase load/stator voltage and currents shown in fig.5(a), fig.6(a), and fig.8(a) in between 3 to 3.4 sec.


500 0 -500 0

1

2

0

1

2

1

2

3

4

5

6

3

4

5

6

3

4

1000 0 -1000

Frequency a t s ta to r/lo a d term ina l (in H z)

Thr ee pha se stator /load curre nts (in am p)

Three phase sta to r/loa d vo lta ges (in v olt, phase -to -ground)

V.

(a)

55

50

45

0

x 10

(b)

5

6

A c t iv e p o w e r ( in W )

0

5

-5

-10

R e a c t iv e p o w e r ( in V A r )

-15 0

0 4 x 10

1

2

3

4

5

6

0

1

2

3

4

5

6

-1 -2 -3

T hr e e pha s e r o to r c ur r e n ts (in a m p)

G e n e r a to r to r q ue (in N -m )

G e ne r a to r s pe e d (in r a d/s e c )

(c)

200 100 0

0

1

2

3

4

5

6

0

1

2

3

4

5

6

2

3 Time (in sec) (d)

4

5

6

0 -5000 -10000 2000 0 -2000 0

1

Figure 8. Results corresponding to case 3 (i.e. variable wind speed); (a) Three phase voltages and currents at stator/load terminals; (b) Frequency at stator/load terminals; (c) Active and rective power at stator/load terminals; (d) speed, torque and three phase rotor currents of the generator respectively.

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CONCLUSION

In this paper a hysteresis current control technique is used for rotor side converter to control the output voltage and frequency of a DFIG based WEG system has been studied and simulated. The current control technique uses the direct voltage control method for reference current generation. On basis of extensive simulation studies carried out using MATLAB/SIMULINK, it is observed that the performance of the controller both in transient as well as in steady state is quite satisfactory. A brief explanation of grid connected and autonomous operating modes of DFIG based WES is given. The performance of DFIG, in terms of Active power; Reactive power; terminal voltage, current and frequency; generator speed, torque & rotor currents, was observed when it supplies the isolated (R-L type) load. Future work will concern the laboratory tests of the proposed technique in the paper. In addition the proposed technique will be extended for nonlinear loads. REFERENCES [1] Vijayakumar, K.; Kumaresan, N.; Gounden, N.G.A, "Operation of inverter-assisted wind-driven slip-ring induction generator for stand-alone power supplies," Electric Power Applications, IET, vol.7, no.4, pp.256, 269, April 2013. [2] Muller S., Deicke M., and De Doncker R.W., “Doubly Fed Induction Generator System for Wind Turbines.” IEEE Industry Application Magazine vol. 8, pp. 26-33, 2002 [3] Rishabh Dev Shukla & Prof. R.K. Tripathi, “Dynamic Performance of DFIG based WECS under different Voltage Sag” in International Journal of Chemtech research, Vol.5, No.2, pp. 980-992, April-June 2013. [4] Rishabh Dev Shukla & Prof. R.K. Tripathi, “Maximum Power Extraction Schemes & Power control in Wind Energy Conversion System” in International Journal of Scientific & Engineering Research, Volume 3, Issue 6, June-2012. [5] Rishabh Dev Shukla, Ramesh Kumar Tripathi, A novel voltage and frequency controller for standalone DFIG based Wind Energy Conversion System, Renewable and Sustainable Energy Reviews, Volume 37, September 2014, Pages 69-89. [6] Iwanski G. and W. Koczara. “Sensorless direct voltage control method for stand-alone slip-ring induction generator,” in Proc. 11th EPE, Dresden, Germany,CD-ROM. 2005. [7] Grzegorz Iwanski and Wlodzimierz Koczara, “Sensorless Direct Voltage Control of the Stand-Alone Slip-Ring Induction Generator,” IEEE Transactions on Industrial Electronics, VOL. 54, NO. 2, APRIL 2007. [8] G. Iwanski, “DFIG based standalone power system operating at low load conditions,” 13th European Conference on Power Electronics and Applications, (EPE '09) year 2009. [9] Kazmierkowski, M.P.; Malesani, L., "Current control techniques for three-phase voltage-source PWM converters: a survey," Industrial Electronics, IEEE Transactions on , vol.45, no.5, pp.691,703, Oct 1998 doi: 10.1109/41.720325 [10] Gonzalo Abad, Jesus Lopez, Miguel Rodriguez, Luis Marroyo, & Grzegorz Iwanski, “ Double fed Induction Machine- Modeling and control for Wind Energy Generation,” IEEE Press Series on Power Engineering, Wiley Publication, 2011.