Fault Ride-Through Analysis of Doubly Excited Induction Generator During Voltage Dip

Fault Ride-Through Analysis of Doubly Excited Induction Generator During Voltage Dip

Malini Sahu, Satyadharma Bharti

Department of Electrical Engineering, Rungta College of Engineering and Technology Kohka-Kurud Road Bhilai (C.G.) Pin-490020

Abstract

This paper presents an overview of the effect of voltage dip on the doubly fed induction generator (DFIG) connected to utility grid. The main concern in the paper is to the control of the DFIG wind turbine and of its power converter with the ability to protect itself without disconnection during grid fault conditions. After setting up the basic machine, converters and grid-filter equations, parameters of controllers are derived. Regulator parameters are also discussed. The investigated DFIG system is simulated using MATLAB/Simulink to implement the derived model and its dynamic response to normal and faulty condition. The dynamic model of DFIG wind turbine includes models for both mechanical components as well as for all electrical components and controllers. The viewpoint of the paper is to carry out different simulations and hence to provide insight and understanding of the grid fault impact on both DFIG wind turbines and on the power system itself.

Index TermsWind Turbine, DFIG, Converters, LVRT, RSC, GSC, WECS.

1. Introduction

   Power generation from the fossil fuel has many adverse effects on the environment. Being non-polluting in nature and with the advancement in the arena of power electronics, the non- conventional energy resources, especially wind energy has become most promising alternative for fossil fuel in power generation. Wind generation now meets a significant percentage of electrical demand worldwide. In the first half of 2012, the world added about 16.5GW of wind generation, a 7.04% increase to total more than 254GW. This is enough to cover about 3% of the worlds electricity demand.[1]

   In WECS with full power handling capabilities, the power converter is in series with the synchronous or induction generator, in order to transform the variable amplitude/frequency voltages into constant amplitude/frequency voltages. In a partial power handling WECS or DFIG, the converter processes maximum twenty five to thirty per cent of the total generated power (e.g. slip power) which possess an advantage in terms of the reduced converter cost and increased efficiency of the system. [2]

   In doubly fed induction generator (DFIG), back to back converters is used to decoupled control of active and reactive

   power to utility grid. The function of rotor side converter (RSC) is to control the active and reactive power of generator by means of controlling the rotor current components. [3][4]The grid side converter (GSC) is to convert variable-frequency, variable- voltage power from a generator into constant-frequency constant- voltage power, and to regulate the output power of the WPGS. Traditionally a gearbox is used to couple a low speed wind turbine rotor with a high speed generator in a WPGS. . Based on the principles of field oriented control (FOC) for electrical machine drives, the converter switching states were selected from an optimal switching based on the instantaneous errors between the angular position of the estimated converter terminal voltage vector and the reference and estimated values of active and reactive power.

   Variable speed wind turbine DFIG Power electronics converter control system Wind power generation system	Utility grid
   			Electrical loads

   Figure 1. Typical wind power generation system connected to a utility grid.

   Voltage sag is a serious problem for DFIG because of low rating of RSC converter, approximately 30% of generator rating. The cause of voltage sag is sudden load changes, starting of heavy loads; drop a lightning bolt near transmission line etc. For reliable and safe grid operation, resulted in the power system operators revising nowadays the grid codes in many countries. [5][6]

   Basically, for wind power these grid codes require an operational behaviour with several control tasks similar to those of conventional power plants. One of these control tasks is the fault ride-through capability of the wind turbines, which addresses primarily the design of the wind turbine controller and protection in such a way that the wind turbine is able to remain connected to the network during grid faults (e.g., short circuit fault). [7][8] There is serious concern about the influence of the wind power on the power system stability, especially during grid

   faults. It is therefore necessary to carry out investigations of the dynamic interaction between power system and large wind farms, with suitable models and accurate transient simulations. [9][10]

   This paper proposes a FOC control strategy for a DFIG based wind energy generation system. The control method is based on the assumption of stator voltage vector on d-axis and the magnetic saturation where electro-magnetic transients and other nonlinear factors are neglected. During faulty condition, the magnitude variation of transmission line voltage is affects the

   output parameters of wind power generation system (WPGS).

   Vpf

   Voltage (kV)

   Voltage (kV)

   Vf

   0

   Must not trip

   Must not trip

   T 3000

   Time (ms)

   These results on a 2MW DFIG generation system are presented to illustrate the performance of the proposed control strategy during the variations of input parameters.

2. Indian wind grid codes (IWGC)

   Grid connection codes define the requirements for the connection of generation and loads to an electrical network which ensure efficient, safe and economic operation of the transmission and/or distribution systems. [8][10] With the growth of wind power; the interaction between WECS and gird will cause new problems about the safe and reliable operation of systems.

   1. Voltage at the grid connection point

      The wind turbines are required to operate within typical grid voltage variations. For safe and reliable operation of grid, the approximately range of variation of voltage is in between +10% to -10%.

   2. Frequency of operation for wind farms

      For the operating range of frequencies between 47.5 Hz to 51.1 Hz, the WTGs shall operate according to the frequency response curve given by authority.

   3. Active power and power factor

      The grid connected wind farm shall be capable of applying the active power between the limits of 0.95 power factor lagging to

      0.95 power factor leading at the grid connection point.0

      1. Reactive power and voltage control

         Figure 2. Fault ride through characteristics

         Where,

         Vf = 15% of nominal system voltage

         Vpf = Minimum voltage mentioned in table

         The fault clearing time for various system nominal voltage levels is given in Table 1.

         Table 1. Fault clearing time and voltage limits

         Nominal system voltage (kV)	Fault clearing time, T(ms)	Vpf (kV)	Vf (kV)
         400	100	360	60.0
         200	160	200	33.0
         132	160	120	19.8
         110	160	96.25	16.5
         66	300	60	9.9

         With increasing penetration, wind farms will have major impact in India power system. So, the behaviour of wind farms should tend to be same as conventional power plants. Staying connected during system fault is that step towards that direction.

3. Mathematical modelling of DFIG

   The mathematical model of the DFIG is represented using the synchronously rotating reference frame (dq-frame) as shown in Figure 3.

   The wind farm shall have provision for VAr compensation

   /support such that they do not draw reactive power from the grid.

   1. Low voltage ride through capability

   During fault ride through, the WTGs in the wind farm shall have the capability to meet the following requirement [11]:

   1. Shall minimize the reactive power drawl from the grid.

   2. The wind turbine generators shall provide active power in

      +

      Vsd

      –

      Isd Rs Ls Lr Rr

      Lm

      – – )

      Ird

      +

      –

      Vrd

      proportion to retained grid voltage as soon as the fault is cleared.

      -sqs

      – +

      ( s r sq

      + –

      1. d-axis equivalent circuit

         +

         Vsq

         –

         Isq Rs Ls Lr Rr

         Lm

         -sds (s-r)sd

         Irq

         +

         –

         Vrq

         Qs 1.5(Vsq Isd Vsd Isq ) Pr 1.5(Vrd Ird Vrq Irq ) Qr 1.5(Vrq Ird Vrd Irq )

4. Control of rotor side converter (RSC)

   (10)

   (11)

   (12)

   – + + –

   1. q-axis equivalent circuit

   Figure 3. Equivalent circuit of DFIG in the synchronous

   The active and reactive powers which are delivered from the DFIG to the grid are controlled by means of controlling the rotor currents of the DFIG. The operation of rotor side controller is shown in Figure 4. [12][13]

   V R I

   reference frame

   • d

     (1)

     P*

     Ird

     *

     *

     PI PI

     *

     V

     V

     rd

     ds s ds e qs dt ds

     d

     Power

     control

     P

     Rotor current

     Icontrol

     Vqs Rs Iqs e ds dt qs

     (2)

     rd rq (s r )

     V R I ( )

   • d

     (3)

     I * V *

     dr r dr e r qr

     dt dr

     *

     Q

     Q

     rq rq

     PI PI

     V R I

     (

     )

   • d

     (4)

     qr r qr e r dr dt qr

     Slower

     Faster

     where V , V , V , V are the d and q-axis stator and rotor

     Qcontrol loop

     control loop

     ( )

     ds qs

     dr qr

     Irq

     rd s r

     voltages, respectively.

     Ids ,

     Iqs ,

     Idr , Iqr

     are the d and q-axis

     Figure 4. DFIG Rotor side converter control structure

     stator and rotor currents, respectively. ds , qs , dr , qr

     are the d and q-axis stator and rotor fluxes, respectively. e is the angular velocity of the synchronously rotating reference

     In Stator Voltage Orientation (SVO), neglecting the stator resistive voltage drop, the active and reactive powers of the stator and rotor are expressed as,

     frame.

     is rotor angular velocity, R and R are the stator

     P 1.5 Lm V I

     (13)

     1. s r

     2. L sd rd

     and rotor resistances, respectively. The flux linkage equations are given as:

     s

     Q 1.5 Vs Vs Lm I

     (14)

     ds Ls Ids Lm Idr

     (5) s

     L

     L rq

     qs Ls Iqs Lm Iqr

     (6)

     s s s

     P 1.5(V I V I )

     (15)

     r rd rd rq rq

     dr Lr Idr Lm Ids

     (7)

     Q 1.5(V I V I )

     (16)

     qr Lr Iqr Lm Iqs (8)

     WWhherere,e, LLss ,, LLrr anandd Lm are the stator, rotor, and mutual

     r rq rd rd rq

     From the above equations, it is obvious that power fed to the grid can be controlled by controlling the rotor currents components. [14][15] The rotor current components can be controlled by the vector control techniques.

     inductances, respectively, with

     Ls Lls Lm and

     L L

   • L :

   L being the self-inductance of stator and

5. Control of grid side converter (GSC)

   r lr m ls

   Llr being the self-inductance of rotor. The reactive and active power of the stator and rotor are expressed in d and q reference frame as follows:

   The purpose of the grid-side converter is to keep the constant DC link voltage irrespective of the direction of the rotor power flow. In order to maintain the DC link voltage constant, a bidirectional converter is required to implement in the circuit

   Ps 1.5(Vsd Isd Vsq Isq )

   (9)

   connected rotor side. [15][16] This converter work as a rectifier below the synchronous speed and above synchronous speed this

   converter works as an inverter to supply all generated power to the grid at a constant DC link voltage.

   V * I *

   Case I Simulated DFIG during normal condition

   *

   PI

   PI

   DC

   gd

   PI

   Vgd

   In this case, the dynamic behavior of the variable speed wind turbine (VSWT) is analyzed under normal condition. This

   VDC

   *

   DC voltage controller

   Igd

   Converter current controller

   V

   V

   *

   gq

   study of the results proves that the control strategy developed in this technique is well performed. The nominal converter dc link voltage was set at 1380 V, and the switching frequencies for both converters were 50MHz. The main objective of the grid side converter is to control the dc link voltage, and it has been

   controlled using a similar method as for the dc voltage controller in a voltage source converter (VSC) transmission system.

   Igq

   0

   Igq

   PI

   Fast control loop

   During the simulation, the grid side converter was triggered first to regulate the converter dc-link voltage. The DFIG stator was then energized with the rotor rotating at a fixed speed and the rotor side converter was disabled. When RSC is enabled, it has controlled the active and reactive power of generator using SVO strategy.

   Figure 5. DFIG Grid side converter control structure

   Sinusoidal pulse width modulation technique is used to generate the switching pulses for back to back converters. Also the main function of GSC is to compensate the reactive power with respect to their reference value.[17]

6. Simulation Results with Discussion

The proposed model is used to simulate under the three phase symmetrical fault condition. When three phase fault occurs at 132KV Bus, the voltage sag at 690V will depend on the percentage impedance drop of DFIG.

2.5

2

1.5

1

0.5

0

-0.5

-1

-1.5

-2

-2.5

Vabc B132V

phase a phase b phase c

Wind turbine

DFIG

Bus 690V

690V/

132kV

10 km line

Bus 132kV

10 km line

132/

264kV

264kV

1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 7. Three phase voltage at bus 132 kV in per unit

Iabc B132V

phase a phase b phase c

phase a phase b phase c

2.5

2

1.5

1

0.5

0

-0.5

Power electronics converter

RSC and GSC controller

2070V/

690V

Three phase fault

-1

-1.5

-2

-2.5

1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 8. Three phase current at bus 132 kV in per unit

Figure 6 Proposed DFIG model connected to faulty grid

Using the software package MATLAB/Simulink is used to simulate the WPGS under normal and faulty condition as shown in Figure 6. The DFIG is rated at 2 MW, and its parameters are given in the Appendix.

2.5

2

1.5

1

0.5

0

<>-0.5

-1

-1.5

-2

-2.5

Vabc B690V

1.4

1.35

1.3

1.25

1.2

1.15

1.1

1.05

1

Rotor speed (pu)

Wr

phase a phase b phase c

phase a phase b phase c

1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 9. Three phase voltage in per unit at bus 690 V

Iabc B690V

phase a phase b phase c

phase a phase b phase c

2.5

2

1.5

1

0.5

0

-0.5

-1

-1.5

-2

-2.5

1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 10. Three phase current in per unit at bus 690V

Grid side converter current (pu)

phase a

phase b

phase c

phase a

phase b

phase c

0.8

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 13. Rotor speed in per unit

DC link voltage (V)

1600

Vdc

1500

1400

1300

1200

11001 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 14. DC-link voltage during normal condition in volt

Active power (pu)

1.5

P

0.6

0.4

0.2

0

1

-0.2

-0.4

-0.6

-0.8

1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 11. Three phase grid side converter current in per unit fed to grid

Electromagnetic torque(pu)

0

Tem

-0.5

-1

-1.5

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 12. Electromagnetic torque during normal condition in per unit

0.5

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 15. Total active power during normal condition in per unit

Reactive power (pu)

Q

Q

1

0.5

0

0.5

-1

1 1.5 2 2.5 3 3.5 4 4.5 5

Time (sec)

Figure 16. Total reactive power during normal condition in per unit

A DFIG system connected to a grid using 132kV transmission line with 20km length. The three phase voltage and current of transmission line is shown in Figure 7 and Figure 8. Also the three phase voltage and current waveform (Figure 9 and Figure 10) at bus 690V is shows that, WPGS is worked properly in normal condition. In Figure 5, it can be seen that the DC

voltage control loop is compensating the error between reference and actual value accordingly (Figure 14). Figure 15 and Figure 16 shows the active and reactive power generated by DFIG, and hence proves that the wind power generation system works as a unity power factor.

Case II Simulated DFIG during three phase symmetrical fault condition

A three phase fault block is connected in centre of the

transmission line with 30 ohm fault resistance. It generates the 60% dip on three phase voltage waveform at 132kV bus (Figure

Iabc B132V (pu)

phase a phase b phase c

phase a phase b phase c

4

3

2

1

0

-1

-2

-3

-4

17).

It can be seen that the effect of voltage dip on

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 18. Three phase current at bus 132kV during faulty

transmission line affects the three phase current at 132 kV bus and the voltage at 690V bus (Figure 18 and Figure 19).

The reference value for GSC is made to change suddenly; as a result, it has been tracked by the actual parameters of generator. Hence, the GSC started injecting reactive power soon after a fault is initiated in the network (Figure 26).

The DC link transient is also slightly increased, due to the reactive power prioritization of the GSC as shown in Figure 24.

The fault duration inserted in transmission line is 0.3 sec as shown in results, which is removed in 1 sec. According to IWGC, the proposed model is able to withstand during faulty

2.5

2

1.5

1

0.5

0

-0.5

-1

-1.5

-2

-2.5

condition

phase a phase b phase c

phase a phase b phase c

Vabc B690V (pu)

duration. Hence the proposed control strategy works properly in both normal and three phase fault condition.

During faulty condition, the sudden increment of rotor current Figure 21 affects the back to back converters of lower rating, hence some protection system is needed to handle this situation.

.

Vabc B132V (pu)

phase a phase b phase c

phase a phase b phase c

2.5

2

1.5

1

0.5

0

-0.5

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 19. Three phase voltage at bus 690V during faulty condition

Iabc B690V (pu)

phase a phase b phase c

phase a phase b phase c

4

3

2

1

0

-1

-2

-3

-1

-1.5

-2

-2.5

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time (sec)

Figure 17. Three phase voltage at bus 132 kV during faulty condition

-4

1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2

Time (sec)

Figure 20. Three phase current at bus 690V during faulty condition

1

0.5

0

-0.5

Grid side converter current (pu)

2400

2200

2000

1800

1600

1400

DC link votage (V)

Vdc

phase a phase b phase c

phase a phase b phase c

-1

1 1.2 1.4 1.6 1.8 2 2.2

Time (sec)

Figure 21. Three phase grid converter current during faulty condition

1200

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 24. DC link voltage during faulty condition

Active power (pu)

1

0.5

0

-0.5

-1

-1.5

-2

Electromagnetic torque (pu)

Tem

1.5

P

P

1

0.5

-2.5

								2.8	3	0 1	1.2	1.4	1.6	1.8	2 2.2	2.4	2.6	2.8	3
					Time (sec)										Time (sec)

								2.8	3	0 1	1.2	1.4	1.6	1.8	2 2.2	2.4	2.6	2.8	3
					Time (sec)										Time (sec)

-3

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Figure 22. Electromagnetic torque during faulty condition

Rator speed (pu)

Figure 25. Total active power during faulty condition

Reactive power (pu)

Wr

Wr

1.23

1.22

1.21

1.2

1.19

1.18

1.17

1.16

1.15

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 23. Rotor speed during faulty condition

1

Q

Q

0.5

0

-0.5

-1

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Time (sec)

Figure 26. Total reactive power during faulty condition

1. Conclusion

   The response of DFIG wind turbine under normal and its dynamic response to voltage sag have been shown. The MATLAB/SIMULINK software package is used to simulate the proposed WPGS system. The results of normal condition are presented, and prove the decoupled control of active and reactive power of generator using field oriented control concept. The SVO based RSC and GSC controller is performed well, and provides the unity power factor. It has been observed that under normal

   condition, 20-30% power is flowing through the rotor circuit of DFIG.

2. References

In view of the voltage dip on transmission line, the developed DFIG model can be applied to ivestigate the dynamic behavior of generator voltage and currents and the variation in the total active/reactive powers as well as electromagnetic torque. After 1sec, the three phase fault causing the voltage dip on the 690V bus bar is cleared then the wind turbine is operated under the normal condition and produces the nominal active power and the reactive power which is maintained to be zero. It concludes that SVO strategy increases the low voltage ride through capability of machine and is acceptable for IWGC without tripping. This has also been discussed that under normal condition the converter power rating will be around twenty five percent to rated power and ensures the unity power factor operation of DFIG.

APPENDIX A

TABLE 2.PARAMETERS OF WIND TURBINE

Nominal Mechanical Output Power	1.8MW
Cut-in wind speed	6m/s
Base wind speed	12m/s
Cut-out wind speed	18m/s
Base rotational wind speed	1.2pu
Maximum power at base wind speed	1pu

TABLE 3. PARAMETERS OF THE DFIG SIMULATED

Rated Power	2MW
Stator Line-Line Voltage	690Vrms
No. of Pole pair	3
Operating frequency	50
Stator resistance, ( Rs )	0.0108pu
Stator leakage inductance,( Ls )	0.102pu
Rotor resistance, ( Rr )	0.0121pu
Rotor leakage inductance, ( Lr )	0.11pu
Mutual inductance, ( Lm )	3.362pu
Inertia constant , (H(s))	0.5
Friction factor, (F)	0.01pu

1. R. Chedid, F. Mrad, M. Basma, Intelligent Control of a Class of Wind Energy Conversion Systems, IEEE Transactions on Energy Conversion, Vol. 14, No. 4, December 1999.

2. G.W.E. Council. India wind 2012 report, Available at: www.gwec.net/publications/country/india-wind energy-outlook- 2012.

3. S. Muller, M. Deicke, Rik W. De Doncker, "Doubly Fed Induction Generator System for Wind Turbines", IEEE Industry Application Magazine, May/June 2002.

4. Jan Linders, Control by variable rotor speed of a Fixed-Pitch Wind Turbine Operating in a Wide Speed Range, IEEE Transactions on Energy Conversion, Vol. 8, No. 3, September 1993.

5. Hee-Sang Ko, Gi-Gab Yoon, Nam-Ho Kyunga,Won-Pyo Hong, Modeling and control of DFIG based variable speed wind turbine Electric Power Systems Research 78, 2008.

6. Peng Zhou, Yikang He, Dan Sun, Improved Direct Power Control of a DFIG-Based Wind Turbine During Network Unbalance IEEE Transactions on Power Electronics, Vol. 24, No. 11, November 2009.

7. Lie Xu, Yi Wang, Dynamic Modeling and Control of DFIG-Based Wind Turbines Under Unbalanced Network Conditions, IEEE Transaction on Power Systems, vol. 22, no. 1, February 2007.

8. Rishabh Dev Shukla, Ramesh Kumar Tripathi, Low Voltage Ride Through (LVRT) Ability of DFIG based Wind Energy Conversion System II, IEEE conference on Engineering and Systems, march 2012.

9. Lasantha Gunaruwan Meegahapola, Tim Littler, Damian Flynn, Decoupled-DFIG Fault Ride-Through Strategy for Enhanced Stability Performance During Grid Faults, IEEE TRANSACTIONS ON SUSTAINABLE ENERGY, VOL. 1, NO. 3, OCTOBER 2010.

10. Lie Xu, Coordinated Control of DFIGs Rotor and Grid Side Converters During Network Unbalance, IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 3, MAY 2008.

11. Centre of wind energy technology, Draft report on Indian wind grid code, Available at: http://www.cwet.tn.nic.in/Docu/Wind_grid_code_for_India%20, July 2009.

12. He Yikang, Hu Jiabing, Zhao Rende," Modeling and Control of Wind-Turbine Used DFIG under Network Fault Conditions", ICEMS 2005, Eighth International Conference on Electrical Machines and System, vol. 2, pp: 986 991, september 2005.

13. A. Bharathi Sankar, Performance Analysis of PWM Inverter For Wind Driven Doubly Fed Induction Generator, IEEE International Conference On Advances In Engineering, Science And Management (ICAESM -2012) March 30, 31, 2012.

14. R. Pena, J.C.Clare,Doubly fed induction generator uising back-to- back PWM converters and its application to variable speed wind- energy generation, IEE Proc. Electr. Power Appl., vol. 143, no 3, May 1996.

15. Mustafa Kaykci, Jovica V. Milanovic, Reactive Power Control Strategies for DFIG-Based Plants, IEEE Transactions on Energy Conversion, vol. 22, no. 2, June 2007.

16. Lie Xu,Direct Active and Reactive Power Control of DFIG for Wind Energy Generation, IEEE Transactions on Energy Conversion, vol. 21, no. 3, September 2006.

17. M. H. J. Bollen Understanding Power Quality Problems. Voltage sags and interruptions, IEEE Press Series on Power Engineering, 2000.