reactive power control of doubly fed ... vii...reactive power control of doubly fed induction...

B.Vaikundaselvan et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945

Int J Adv Engg Tech/Vol. VII/Issue I/Jan.-March,2016/841-850

Research Paper REACTIVE POWER CONTROL OF DOUBLY FED INDUCTION

GENERATOR USING DIRECT POWER CONTROL 1B.Vaikundaselvan, 2 M.Kannan

Address for Correspondence 1, 2 Professor, Department of EEE, Kathir College of Engineering, Coimbatore, Tamil Nadu, India

ABSTRACT Active and reactive power control of DFIG using back to back converter, operation is reviewed and the mathematical model for real and reactive powers are derived from its equivalent circuit. The control circuit for Grid Side Converter (GSC) and Rotor Side Converter (RSC) have been prepared and implemented by using DPC scheme. Direct power control technique based on grid voltage orientation has been proposed. The DPC gets the same control performance as vector control, but has better robustness and simple structure. This DPC approaches the Instantaneous Reactive Power (IRP) p-q theory, which is based on the Clarke transform of voltages and currents in three-phase systems into α and β orthogonal coordinates. In this proposed control, DPC is used to control both active and reactive power under steady state condition. The simulation of a 1hp DFIG based GCWECS with PI controller is carried out using MATLAB/Simulink. The steady state behavior of system was analyzed for synchronous (157 rad/sec), sub synchronous (135 rad/sec) and super synchronous speed (179 rad/sec). The reactive power supplied by DFIG for three modes of operations (synchronous, sub synchronous and super synchronous speeds) are zero, and has been validated by the simulation results. The hardware results also confirmed the reactive power as zero for all the three conditions. KEYWORDS— Renewable energy, DFIG, Back to back converter, Direct power control.

I. INTRODUCTION Owing to rapid decrease in fossil fuels and increase in global warming, our attention is diverted towards the importance of locally available natural resources. The natural resources will provide an alternative energy source with less cost and also be helpful in maintaining a pure and healthy atmosphere. Increased use of renewable energy sources such as wind energy, bio-gas, solar, and hydro potential has become essential to adopt a low - cost generating system, which is feasible for operating in remote areas. Out of all renewable energy sources, wind energy seems to be prominent and quite promising for electric power generation. Wind Energy Conversion (WEC) has been found to be economic compared to the cost of fossil fuels which are rising at a much faster rate. Therefore, the study of Wind Energy Conversion System (WECS) has regained importance, as they are particularly suitable for wind power plants. Quite recently the developments in wind turbine technology have been taking place consistently. India possesses a long coast line of about 7500 km and interestingly it is estimated that it has a wind energy capacity of 48000 MW at 60 Meters height from the ground. However, the installed capacity is only about 14000 MW (Indian Wind Energy Association 2011). The wind turbine installation commenced between the late fifties and early sixties. Since then, the numbers of wind turbine installations have increased in different parts of the country, mostly due to government encouragement. There is still a large potential, which is still untapped. The reasons for lack of entrepreneurs’ interest may be due to lower conversion efficiencies, higher cost of renewable energy, alternative energy availability and removal of government subsidies. The lower conversion efficiencies enhance the wind energy extraction cost, making uneconomical. The price for energy generation with wind turbine is several times higher than energy obtained from conventional sources. Hence, research studies are undertaken to determine ways and means for enhancing the conversion efficiency. Wind energy is conversion of kinetic energy (i.e. energy of motion of the wind) into mechanical energy that can be utilized to generate electricity. The wind blows against the blades and they rotate about

the axis. The rotational motion is converted to energy by wind turbines because wind turbines produce rotational motion. Wind-energy is readily converted into electrical energy by converting the turbine into an electrical generator. Wind energy generation has attracted much interest during the last few years. Large wind farms have been planned and installed in various locations around the world. Many of these wind farms are based on the Doubly Fed Induction Generator (DFIG) technology with converter ratings around 30 percent of the generator ratings. DFIG have more advantages over synchronous and induction generators when used in wind farms, such as robustness, reliability, low price, variable speed operation, active and reactive power control, relatively high efficiency, and lower converter cost. DPC technique based on grid voltage orientation has been proposed in this chapter. The DPC gets the same control performances as vector control, but has better robustness and simple structure. This DPC approaches the Instantaneous Reactive Power (IRP) p-q theory, which is based on the Clarke transform of voltages and currents in three-phase systems into α and β orthogonal coordinates. In this proposed control, DPC is used to control both active power and reactive power under steady state condition. Simulation is accomplished using MATLAB/Simulink software. The simulations are verified with laboratory hardware setup. II. OPERATION OF DFIG Fig. 1 show the basic configuration of DFIG. Based on the operation such as sub synchronous, synchronous and super synchronous speed of the DFIG, the power flow is from rotor to grid or grid to rotor. The total power supplied to grid is sum of the power from stator and rotor power.

Fig. 1Basic configuration of DFIG wind turbine

DFIG supplied the power to grid from its stator When Ps > 0. Rotor received the power from grid when ωr<



ωs, it means that Pr < 0. Rotor also supplied power to grid when Pr > ωs, it means that Pr > 0. Here ωr, ωs, Pr, Ps are rotor speed, stator speed, power from rotor and power from stator respectively. Hence by operating the DFIG in sub synchronous, synchronous and super synchronous modes and the outputs in terms of rotor powers are Pr < 0, Pr = 0 and Pr > 0 respectively. A three-phase wound-rotor induction machine can be setup as a doubly-fed induction motor. In this case, the machine operates like asynchronous motor whose synchronous speed can be varied by adjusting the frequency of the ac currents fed into the rotor windings. The same wound-rotor induction machine setup can also serve as a doubly-fed induction generator. In this case, mechanical power at the machine shaft is converted into electrical power supplied to the ac power network via both the stator and rotor windings. Furthermore, the machine operates like a synchronous generator whose synchronous speed (speed at which the generator shaft must rotate to generate power at the AC power network frequency) can be varied by adjusting the frequency of the AC currents fed into the rotor windings. In a conventional three-phase synchronous generator, when an external source of mechanical power (prime mover) makes the rotor of the generator rotate, the static magnetic field created by the dc current fed into the generator rotor winding rotates at the same speed as the rotor. As a result, a continually changing magnetic flux passes through the stator windings as the rotor magnetic field rotates, inducing an alternating voltage across the stator windings. Mechanical power applied to the generator shaft by the prime mover is thus converted to electrical power that is available at the stator windings. In conventional (singly-fed) induction generators, the relationship between the frequency of the ac voltages induced across the stator windings of the generator and the rotor speed is expressed using the Equation (1).

f� =ω ��

�� (1)

f�- Frequency of the ac voltages induced across the stator windings � − Speed of rotor in rps Ps – Number of poles in the DFIG per phase From the above Equation (1), it is very clear that, when the speed of the generator rotor is equal to the generator synchronous speed, the frequency of the AC voltages induced across the stator windings of the generator is equal to the frequency of the ac power network. The same operating principles apply in a doubly-fed induction generator as in a conventional (singly-fed) induction generator. The only difference is that the magnetic field created in the rotor is not static (not static means, it is created using three-phase AC current instead of dc current), but rather rotates at a speed proportional to the frequency of the ac currents fed into the generator rotor windings. This means that the rotating magnetic field passing through the generator stator windings not only rotates due to the rotation of the generator rotor, but also due to the rotational effect produced by the ac currents fed into the generator rotor windings. Therefore, in a DFIG both the rotation speed of the rotor and the frequency of the AC currents fed into the rotor windings determine the speed of the rotating magnetic field

passing through the stator windings, and thus, the frequency of the alternating voltage is induced across the stator windings. Taking into account the principles of operation of doubly-fed induction generators, it can thus be determined that, when the magnetic field at the rotor rotates in the same direction as the generator rotor, the rotor speed and the speed of the rotor magnetic field add up. The frequency of the voltages induced across the stator windings of the generator can thus be calculated using the Equation (2).

f� =ω ��

�� + f� (2)

f�- Frequency of rotor current fed into the rotor of DFIG Conversely, when the magnetic field at the rotor rotates in the direction opposite to that of the generator rotor, the rotor speed and the speed of the rotor magnetic field are subtracted from each other. The frequency of the voltages induced across the stator windings of the generator can thus be calculated using the Equation (3).

f� =ω ��

�� − f� (3)

III. DFIG MODELLING The DFIG has two sets of three-phase windings that display self and mutual inductances. Mutual inductances change as the machine turns and the angle between stator and rotor circuits varies with time, which ultimately leads to a time-varying mathematical model of the machine. This angle dependency in DFIG’s model and the associated complexities can be surmounted by making a transformation from three-phase magnitudes to two-axis magnitudes (Clark’s transformation) and transforming the magnitudes into direct and quadrature components referred to a synchronously rotating reference frame (Park’s transformation). The DFIG model in the synchronous reference frame can be expressed as Equations (4) to (12), where the model corresponds to the angular speed of the rotating reference frame. v�� = R�i�� + sψ

��+ ω�ψ

�� (4)

v�� = R�i�� + sψ��

− ω�ψ��

(5)

v��′ = R�

′ i��′ + sψ

��′ + (ω� − ω�)ψ

�� ′ (6)

v��′ = R�

′ i��′ + sψ

��′ − (ω� − ω�)ψ

��′ (7)

ψ��

= L�i�� + L��′ (8)

ψ��

= L�i�� + L��′ (9)

ψ��′ = L�

′ i��′ + L�i�� (10)

ψ��′ = L�

′ i��′ + L�i�� (11)

T� =�

�P�(ψ

��i�� − ψ

��i��) (12)

Where,L� = L�� + L� ; L�′ = L��

′ + L� ; L� = 3 L�� 2⁄ A. Mathematical Representation of DFIG The mathematical model is similar to the squirrel cage induction machine; the only difference is that the rotor voltage is non-zero in DFIG. In order to simulate the Wind Energy Conversion System a proven method is needed to represent the characteristics of a DFIG. The rest of this section is devoted to the mathematical equations that describe a DFIG. The voltage equations to represent a 3-phase induction machine can be expressed as follows, [V��] = [r�][i��] + ρ�ψ

�� (13)

[��] = [��][��] + ρ�ψ��

� (14)

�� = �� + �� (15) Q� = �V��i�� − V��i�� (16)



Where ν, r, i, and ψ respectively refer to the voltage, resistance, current, and flux linkage of the phase windings. The subscripts a, b, and c refer to their phase component. The subscripts r and s refer to the stator and rotor windings. The term ρ represents the derivative (d/dt). The flux linkages shown in Fig. 2 for a linear magnetically coupled circuit are expressed as Equation (17) to (20).

Fig. 2 Equivalent circuit of induction machine in d-q

reference frame

v��⃗ = R�ı��⃗ +�ψ��⃗

��+ jω�ψ

��⃗ (17)

v��⃗ = R�ı��⃗ +�ψ��⃗

��+ j(ω� − ω�)ψ

��⃗ (18)

ψ�

��⃗ = L�ı��⃗ + Lmı��⃗ (19)

ψ�

��⃗ = L�ı��⃗ + L�ı��⃗ (20)

Where �� and �� are rotor and stator voltages �� and �� are rotor and stator fluxes R�, R�,L�, and L�, are rotor and stator resistance & inductance. ��= Mutual inductance ω� and ω� are Rotor & Synchronous angular speed In rotating �� reference frame the machine model is

v�� = R�i�� − ω�ψ��

+�ψ��

�� (21)

v�� = R�i�� − ω�ψd� +�ψ��

�� (22)

v�� = R�i�� + (ω� − ω�)ψ��

+�ψ��

�� (23)

v�� = R�i�� + (ω� − ω�)ψ��

+�ψ��

�� (24)

ψ��

= L�i�� + L�i�� (25)

ψ��

= L�i�� + L�i�� (26)

ψ��

= L�i�� + L�i�� (27)

ψ��

= L�i�� + L�i�� (28)

Torque produced is T� =

�

�p(ψd�iq� − ψq�id�) =

�

�pL�(id�iq� − iq�id�)

(29) The mechanical part of the model is �ω�

��=

�

�(T� − T�) (30) (6.30)

The active and reactive powers inputs from the network can be calculated as P� = (v��i�� + v�� i�� (31)

Q� = (v��i�� − v�� i��) (32)

P� = �V��i�� + V��i�� (33)

Q� = �V��i�� − V��i�� (34)

IV. CONTROL METHODS Different control methods are popular in the industry, such as scalar method (V/f), vector control, direct and indirect field oriented control, rotor and stator flux control, adaptive flux observer, stator flux orientation and field weakening control. All the said methods are complex, for reducing the complexity to choose the direct power control scheme to control the machine. The DFIG control level performs the control of the rotor side and the grid-side back-to-back converters. A vector control approach is adopted for the rotor controller, while two cross coupled controllers adjust

the speed and power of the system. The goals of such controllers are to track the optimum operation point, limit the power in the case of high wind speeds, and control the reactive power exchanged between the wind turbine generator and the grid. The control of the grid-side converter keeps a constant dc-link voltage while injecting the active power to the grid. Internal current loops in both converters are typically using proportional integral (PI) controllers. DTC and DPC schemes have been presented as alternative methods which directly control machine flux and torque via the selection of suitable voltage vectors. It has been shown that DPC is a more efficient approach compared to modified DTC. However, the DPC method also depends on the estimation of machine parameters and it requires a protection mechanism to avoid over current during a fault in the system. In induction wind generators, unbalanced three phase stator voltages cause a number of problems, including overheating and stress on the mechanical components from torque pulsations. Therefore, beyond a certain amount of imbalance (6%), induction wind generators are switched out of the network. In DFIG control of rotor currents allows for adjustable speed operation and reactive power control. In addition, it is possible to control the rotor currents to correct the problems caused by unbalanced stator voltages. This paper presents a novel controller design for a doubly-fed induction generator that provides adjustable speed and reactive power control while greatly reducing torque pulsations. B. Rotor Side converter The RSC supplies the voltage to the rotor windings of the DFIG. The purpose of the rotor-side converter is to control the rotor currents such that the rotor flux position is optimally oriented with respect to the stator flux in order that the desired torque is developed. The rotor-side converter uses a torque controller to regulate the wind turbine output power and the voltage (or reactive power) measured at the machine stator terminals. The power is controlled in order to follow a pre-defined turbine power-speed characteristic to track the maximum power point. The actual electrical output power from the generator terminals, added to the total power losses (mechanical and electrical) is compared with the reference power obtained from the wind turbine characteristic. Usually, a Proportional-Integral (PI) regulator is used at the outer control loop to reduce the power error (or rotor speed error) to zero. The output of this regulator is the reference rotor current irqref that must be injected in the rotor winding by rotor-side converter. This q-axis component controls the electromagnetic torque Te. The actual irq component of rotor current is compared with irqref and the error is reduced to zero by a current PI regulator at the inner control loop. Fig. 3 show the rotor side converter control scheme. The output of this current controller is the voltage vrq

generated by the rotor-side converter. With another similarly regulated ird and vrd component the required 3-phase voltages applied to the rotor winding are obtained. The generic power control loop is illustrated in the next section.



Fig. 3 Rotor side converter control scheme

The grid side converter is used to partly control the flow of real and reactive power from the turbine system to the grid. The grid-side converter feeds the grid through a set of interfacing inductors. The grid-side converter (a voltage source inverter) can generate a balanced set of three-phase voltages at the supply frequency and that the voltage (E) can have a controllable magnitude and phase. Load angle control is used to illustrate the basics of real and reactive power control, though in practice, a more sophisticated control is used which provides superior transient response. Essentially, load angle control uses the angle, δ, between the voltage generated by the grid-side converter, E, and the grid voltage, V, The steady-state equations relate to the real and reactive power flow from the grid-side converter to the grid are shown in Equation (35) and (36).

P =�� δ�

�� and Q =

��

��−

��

��cos δl (35)

P =��δ�

�� and Q =

��

��−

��

�� (36)

Showing that P can be controlled using load angle δl, and Q can be controlled using the magnitude of E. The combination of control and power electronics enables the grid-side converter to produce the necessary voltage magnitude, E, and load angle δ, in order to meet a required Ps and Qs demand set by the main system controller. The controller should be able to synchronize the grid frequency and phase, in order to connect and supply power. At any instance, the power exported by the GSC is determined by the state of the DC- link voltage. The grid-side converter controller monitors the DC- link voltage. If the DC- link voltage rises, the grid-side converter can export more real power by increasing the load angle in order that the DC- link voltage moves back towards it nominal value. If more power is being exported by the GSC than is currently being generated by the RSC, the DC link voltage will fall below its nominal value. The grid-side controller will then reduce the exported real power to allow the DC link voltage to recover to its nominal value. In this sense, the DC link voltage indicates power flow balance between the generated energy and the exported energy in the rotor side. If the input and output power to the dc link capacitor do not match, then the DC link voltage will change. The quality of the energy supplied to the network must

meet basic requirements and it will be set by the grid code in force at the PCC. The grid code specifies many performance indicators of the quality of the energy supplied by the grid-side converter, along with other important issues such as fault level. The relevant grid code(s) in operation must be determined prior to tendering for work on the turbine power electronics and control. The grid code has important implications on the control system of the turbine. If the generator-side controller continues to generate power, the DC link capacitance will be over charged. Therefore, a grid fault will require the generator to stop generating energy, which then means that there is no longer a restraining torque to control the blade speed. In a wind turbine, a loss of supply will cause an over speed condition, as the blade system will accelerate due to the aerodynamic torque produced by the blades. Shorting resistors, or a crowbar circuit, is often switched across the rotor circuit of the generator in order that the energy generated by the blade system can be absorbed and the high speed condition controlled to a safe and manageable level. In addition, there are often aerodynamic (pitch control) and mechanical braking mechanisms included in wind turbines as an additional over-speed safety measure. C. Grid Side Converter Different control strategies are used to perform the control of the grid side converter. They all are focused on the same topics, the control of the DC-link voltage, active and reactive power delivered to the grid, grid synchronization and to ensure high quality of the injected power .They can be classified depending on the reference frame used in the control structure. In this project the focus is on the synchronous and stationary reference frame control strategies. In both cases, the control strategy contains two cascaded loops. Fig. 4 shows the grid side converter control scheme. The inner loops control the grid currents and the outer loops control the DC-link voltage and the reactive power. The current loops are responsible for the power quality, thus harmonic compensation can be added to the action of the current controllers to improve it. The outer loops regulate the power flow of the system by controlling the active and reactive power delivered to the grid. The total current



delivered into the grid is the sum of the currents from stator side and grid side. Since the negative-sequence current of the DFIG stator is eliminated with the effective control of GSC.

Fig. 4 Grid side converter control scheme

D. Direc Power Control The DPC inherits most of its theoretical background from the DTC. DPC also exploits the geometrical relationship between stator and rotor fluxes and the fact that the rotor flux can be fully controlled through the RSC. This distinction change in the control approach has a significant impact on the robustness and simplicity of the DPC strategy, whose main features include: 1) Independence from machine parameters 2) Reduced number of electrical magnitudes to be measured and

3) No need for reference frame transformations V. SIMULATION RESULTS Simulations of the proposed control strategy for a DFIG based wind power generation system were carried out, using MATLAB/Simulink. Discrete models were used with a simulation time step of 1.5s. The DFIG is rated at 1.3 kW. The nominal converter DC- link voltage was set at 1200 V. The grid side converter has to maintain a constant DC-link voltage, and it is controlled by a method similar to the DC voltage controller in a PWM voltage source rectifier. During simulations, a sampling frequency of 10 kHz was used for the proposed control strategy. A high-frequency ac filter is connected to the stator side to absorb the switching harmonics generated by the two converters. The filter is a single-tuned one with inductance and resistance connected in parallel instead of series, which results in a wide band filter having impedance at high frequencies limited by the resistance. In a practical system, voltages and currents are sampled at the beginning of each sampling period. The required rotor control voltage for the sampling period is then calculated and passed to the SVM (Support Vector Machine) module. Impossible to avoid, there is a time delay between the instant sampling and SVM modulator’s receiving the required rotor control voltage and updating its register values.

Fig. 5 Grid side converter phase volt and current for a sub synchronous speed (135 rad/s)

Fig. 6 Grid side converter phase volt and current for synchronous speed (157 rad/s)

Rotor voltage calculation of the proposed DPC strategy is relatively simple, and the time delay should be adroit small. In spite of that, the calculated output rotor voltage is delayed by 4 ms (one sampling period) to closely represent a practical DPC control system. During the simulation, the grid side converter is enabled first, so that the converter DC-link voltage is regulated. The generator is then excited by rotor-

side converter with the rotor rotating at a fixed speed till the stator voltage matches with the network voltage, such that the DFIG system is switched into grid-connected operation. The Figures 5 to 7 shows the grid side converter phase volt and current for a sub synchronous, synchronous and super synchronous speed respectively.



Figure 7 Grid side converter phase volt and current for super synchronous speed (179 rad/s)

Fig. 8 Rotor side converter phase voltage and current for sub synchronous speed (135 rad/sec)

The Fig. 8 to 10 shows the rotor side converter phase volt and current for a sub synchronous, synchronous and super synchronous speed respectively. The Fig. 11 to 13 shows the stator phase volt and current for a sub synchronous, synchronous and super synchronous speed respectively.

Fig. 14 shows the Load voltage, load Current with different rotor angle changes and Fig. 15 shows the constant reactive power for different power generation.

Fig. 9 Rotor side converter phase voltage and current for synchronous speed (157 rad/sec)

Fig. 10 Rotor side converter phase voltage and current for super synchronous speed (179 rad/sec)



Fig. 11 Stator phase voltage and current for sub synchronous speed (135 rad/sec)

Fig. 12 Stator phase volt and current for synchronous speed (157 rad/sec)

Fig. 13 Stator phase volt and current for super synchronous speed (179 rad/sec)

Fig. 14 Load voltage, load current with rotor angle changes

E. Description of the Experimental Work-Bench As it can be seen in the picture shown in Fig. 16, the bench is composed of two controlled electrical drives one is the wind turbine emulator (DC motor) and the

other one is the electric generator (DFIG), coupled on the same shaft. The wind turbine emulator (WTE) consists of a DC motor driven by a Siemens AC/ DC converter that can be easily configurable.



Fig. 15 Constant reactive power

Fig. 16 DFIG and DC motor coupled on the same shaft

Fig. 17 Experimental test rig of DFIG

Fig. 17 shows a proposed grid connected DFIG with all supporting components like GSC, RSC, measuring units (Power quality analyser) and FPGA- controller. The specific components used in the presented proposed grid connected DFIG is: A 1.3 kW, 220 V, 1500 rpm, DC shunt

machine. It is mechanically coupled to the generator to transmit the desired torque.

An AC/DC Siemens converter for DC drives. It supplies the suitable voltage or current to the DC machine in order to achieve the specified torque reference.

A 1 hp, 1410 rpm, 4 poles, 415 V wound rotor asynchronous machine an AC/DC, 1200 V, 25 A three-phase controlled IGBT converter. When working below the synchronous speed, it rectifies the voltage or currents from the grid to feed the DC bus from which the generator converter will feed the wind generator rotor. Above that speed, it will control the DC bus voltage delivering power to the grid. At all times, the control system of this converter regulates the reactive power exchanged with the grid to which the generator’s rotor is connected. A DC/AC 1200 V, 25 A, three-phase controlled IGBT’s converter feeds the DFIG’s rotor with AC obtaining the energy from the DC bus. The control system calculates how the rotor phases must be fed so that the generator’s stator delivers the maximum electric power associated to different wind conditions to the grid. The control system also regulates the reactive power exchanged with the grid through the generator’s stator by means of this converter.

A FPGA - Spartan 6 based control system composed development board has been used

as controller. This hardware allows for the simultaneous controlling of the IGBT firing signals in the DC/AC generator side converter and those of the AC/DC grid side converter. The interface included with this system allows for the real-time tracking of the change of any variable in the entire generation system. A Hall Effect sensors box for adapting voltage and current measurements to the FPGA input channels.

F. System Operation The described system is designed to be operated in the following manner 1. The DC machine coupled with DFIG act as

wind turbine emulator and is initiated using a specific wind reference. The wind turbine emulator control system (Siemens control) reads the turbine speed from the generator control system (FPGA).

2. When the generator control system detects the minimum speed at which the DFIG must be started, both the grid converter and the generator converter are started in order to prepare the system for connection to the grid.

3. Once the DFIG stator generates three voltages equal to those of the grid to which it will be connected, the grid connection switch is closed.

4. When the generator stator is connected to the grid, the generator control system can operates the specified active and reactive power (zero).

5. At the same time, the grid converter control system maintains the DC bus voltage equivalent to the reference value.

6. From this point, the system works automatically and the only parameter that can be modified is the DC machine current or torque reference.

7. At any turbine speed, the DFIG will deliver the maximum active power established by a reference table, including the higher speed values at which the pitch angle must be changed. The generator control system operates to maintain zero reactive power interchanged with the grid.

8. According to the grid converter control system, it is important to remember that when the wind turbine operates below the synchronous speed, this converter (GSC) acts as a rectifier, allowing the energy to flow from the grid to the rotor. But when the turbine speed increases above the synchronous value, it works as an inverter, delivering power from the rotor to the grid. Moreover, this converter also collaborates in the global reactive power control.

G. Hardware Results To demonstrate the operating possibilities of this system, we present the results of a specific test consisting of the production of an increment of the wind speed while operating the system on manual reference mode. The test results are presented in Fig. 18 to 20. The intention of this test is that the wind increases carries an increment of the machine rotating speed which increases from 1400 r.p.m., under synchronous speed 1500 r.p.m, to a super-synchronous speed 1280 r.p.m.



Fig. 18 Stator voltage, stator current and rotor current for rotor frequency as 27.830 Hz

Fig.19 Stator voltage, stator current and rotor current for rotor frequency as 16.588 Hz

Fig. 18 to 20 show the evolution of the voltage amplitude, the frequency, and the three phase currents injected into the grid, as well as the value of the active (P) and reactive (Q) power interchanged with the grid through the stator for different rotor

frequencies, whose values follow at all times the constant references established for this test. Fig. 21 shows that, the indication of zero reactive power for the rotor frequency as 30.442 Hz.

Fig. 20 Stator voltage, stator current and rotor current for rotor frequency as 15.693 Hz



Fig. 21 Indication of zero reactive power for the rotor frequency as 30.442 Hz

VI. CONCLUSION The operation of DFIG was reviewed and the mathematical model for real and reactive powers are derived from its equivalent circuit. The control circuit for GSC and RSC have been prepared by using direct power control scheme and implemented. The simulation of a 1.3 kW DFIG based GCWECS with PI controller is carried out using MATLAB/simulink. The steady state behavior of system was analyzed for synchronous (157 rad/sec), sub synchronous (135 rad/sec) and super synchronous speed (179 rad/sec). The reactive power supplied by DFIG for three modes of operations (synchronous, sub synchronous and super synchronous speeds) is zero, and has been justified by the simulation results. The hardware results also confirmed the reactive power as zero for all the three conditions. REFERENCES

1. Baroudi, JA, Dinavahi, V & Knight, AM 2005, ‘A Review of Power Converter Topologies for Wind Generators’, IEEE International conference on Electric machines and drives, pp.458-465.

2. Buja, GS & Kazmierkowski, MP, 2004, ‘Direct Torque Control of PWM Inverter-Fed AC Motors, A Survey’, IEEE transactions on industrial electronics, vol. 51, no. 4, pp.744-757.

3. Engelhardt, S, Erlich, I, Feltes, C, Kretschmann, J, & Shewarega 2011, ‘Reactive Power Capability of Wind Turbines Based on Doubly Fed Induction Generators’, IEEE transactions on energy conversion, vol. 26, no. 1, pp.364-372.

4. Hailiang Xu, Jiabing Hu & Yikang He 2012, ‘Operation of Wind Turbine Driven DFIG Systems under Distorted Grid Voltage Conditions, Analysis and Experimental Validations’, IEEE transactions on power electronics, vol. 27, no. 5, pp.2354-2366.

5. Hughes, FM, Anaya-Lara, O, Jenkins, N, & Strbac, G 2005, ‘Control of DFIG-Based Wind Generation for Power Network Support’, IEEE transactions on power systems, vol. 20, no. 4, pp.1958- 1966.

6. Jiabing Hu, Hailiang Xu & Yikang He 2013, ‘Coordinated Control of DFIG’s RSC and GSC Under Generalized Unbalanced and Distorted Grid Voltage Conditions’, IEEE journal, vol. 60, no. 7, pp. 2808 – 2819.

7. Lingling Fan, Haiping Yin & Zhixin Miao 2011, ‘On Active Reactive Power Modulation of DFIG-Based Wind Generation for Inter area Oscillation Damping’, IEEE transactions on energy conversion, vol. 26, no. 2, pp.513-521.

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reactive power control of doubly fed ... vii...reactive power control of doubly fed induction...

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