Abstract
Abstract-Variable wind speed leads to variable wind power generation, voltage fluctuations, and frequency deviations, which are the main problems related to wind energy integration into a grid. These problems become more evident in weak grids. In addition, wind farms have to take the grid problems into consideration and have to provide support during grid instability and transients.
Abstract
In this paper, a Permanent Magnet Synchronous Generator 
I.

I NTRODUCTION
Wind energy is available and clean source of energy that has been used to generate electrical power. The focus on electrical power generated from wind energy has been noticeably increased due to the environmental problems that fossil fuels make. Global warming and green house emissions are the main harmful results of fossil fuel consumption. United States wind power has been quickly spread and wind generation installed capacity for 2012 reached approximately 50 (GW) of power according to the U.S Department of Energy (DOE) and the American Wind Energy Association (A WEA). According to the report prepared by the U.S. Department of Energy with contributions from the National Renewable Energy Laboratory (NREL), the wind energy installed capacity by 2030 will be around 20% of the total capacity. Wind power integration to the grid has limitations and standards that have to be followed. Standards Wind power integration to the grid has different difficulties and problems due to the variation of the wind speed which results in fluctuation of the wind generated power that leads to fluctuations in voltage and frequency. Grid faults also one of the problems that wind turbine control should take into consideration. Wind turbines should be properly controlled to overcome the grid faults that lead to voltage and frequency drop. It has to have a fault right through capabilities to protect the turbine generator form accelerating and damaging mechanical part due to instability that increases vibration and stress on the mechanical parts like the speed conversion system (gear box). Generator acceleration also leads to over current and overvoltage in the DC bus of the electrical conversion system that makes the mechanical system unstable and might damage the electrical converter [3] , [2].
II. ADAPTED Low VOLTAGE RIDE THROUGH
Wind turbine power plants has to produce the same voltage required by conventional power system and it has to keep that voltage constant and be able to operate with the required grid voltage variation [ 4] , [5] .
II. ADAPTED Low VOLTAGE RIDE THROUGH
Low voltage ride through for wind turbine power plants are imposed in order to avoid the power produced by the plant during the grid faults. Due to the rapid increase of wind power generation into the grid, low voltage ride through will help the system to avoid some of the frequency and voltage control that might happened du to disconnection of wind turbine generation. Voltage ride through addresses that wind turbines are able to remain connected to the grid during the grid faults and support reactive power to help the voltage recover [2], [4] , [6] .
II. ADAPTED Low VOLTAGE RIDE THROUGH
Wind turbine grid voltage support is becoming standards for all grid code that are created by power generation agencies. Three phase, two and single phase faults voltage ride through became common requirements for voltage support [7] , [5] , [4] .
II. ADAPTED Low VOLTAGE RIDE THROUGH
In this paper, figure (1) shows the E.ON Germany voltage code which is followed as a standard. Figure ( 2) shows the reactive current support during normal and low voltage operations. LICs have to store the wind energy during the fault period, it has to store the active power and support up to 2 (pu) per unit of the reactive power. On the other hand, it has to support 90% of the available power in 1 second after the grid voltage recovers to 90%. 
700
1500 3000 Figure I . E.ON (Gennany) voltage code during faults [8] . 
III. SYSTEM STRUCTURE AND MODELING.
The system structure consists of wind turbine, electrical generator, power conversion components, and controllers. The most common wind turbines are three blades turbines that rotate and transmit the rotation to an electrical generator through a gear box velocity conversion system. The generator and the power conversion components are responsible for the conversion of unregulated AC power coming from the turbine generator to DC power through energy storage system and then from Dc power to a regulated Ac source to meet the load current and voltage requirements.
Grid TrausfOlUlef
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Grid TrausfOlUlef
AeroiKiuelic Power Figure 3 . Wind power conversion system general components [7] .
Grid TrausfOlUlef
A.
Wind turbine modeling
This model is a steady state model combined with a drive train. The stiffness of the drive train is infinite and the friction factor and the inertia constant are combined with the electrical generator.
Wind turbine modeling
The mechanical output power of this turbine can be found as :
Wind turbine modeling
(1) Where : p is air density kg/m3; Ab is the wept are of the wind turbine blades; v is the wind speed in m/s; Cp (-t,p) is coefficient factor as a function of pitch angle and tip speed ratio; f3 is pitch angle of the blades; -t is tip speed ratio; W is rotational speed of the blades; r is radius of the blades; wr is the speed of the tip of the blade. Tip speed ratio can be found as :
Wind turbine modeling
Matlab Simulink synchronous generator with a fixed field excitation is used in this system and to have the same electrical concept of a PMSG. The generator is represented in rotationally rotor reference frame (dq frame) and all the electrical quantities are seen from the stator.
Wind turbine modeling
C.
Converter modedling.
Back to back converter is conslstmg of a rectifier (AC/DC) which is on the wind turbine generator side, DC energy storage and inverter (DC/ AC) which is on the grid side. The rectifier rectifies the AC power source coming from the wind turbine generator to a DC power that is stored in the DC bus energy storage. Inverter converts the DC power coming from the DC bus to AC with specific voltage and frequency to meet the grid requirements. The rectifier and the inverter are mad up of three arm bridges with an RC snubber circuit connect in parallel with each switch which are an IGBT diodes. Generator side converter control is done to satisfy a maximum wind power point tracking (MPPT). Maximum power point tracking allow the wind turbine to extract the maximum power from the wind.
B. Inverter (G rid side converter) control
Gird side converter is a DC/ AC converter. The control is done in three different ways, the first is for normal operation without power smoothing to keep the DC bus constant, so that whatever power coming from the wind will be delivered to the grid. The second one is normal operation with power smoothing so higher quality smoothed power is delivered to the grid, and this will result the DC bus to vary within a small gab depending on how smoothed is the delivered power. The third way is control for low voltage ride through (LVRT) support.
C.

Ultracapacitor Energy storage level for L VRT
The proposed control is an ultracapacitors energy storage on the DC bus of the Back to back converter and it is used either to store or support energy. It is used to store the energy coming from the wind during the LVRT period since the active power deliver to the grid during LVRT is zero. The incoming power is equal to the wind turbine generated power and the delivered is almost zero so that the power will be stored in the ultracapacitors and lead to increases the voltage.
Ultracapacitor Energy storage level for L VRT
According to equation (3), the available stored energy in the ultracapacitor is a function of the capacity of the capacitor and the square of the capacitor voltage.
Ultracapacitor Energy storage level for L VRT
Since the voltage of the capacitor is controlled to be around 1200 volt in normal operation, the voltage band of the capacitor becomes from (1200-1400) v and the capacitor value used is 5.86 farad. Using Equation (3) and According to this voltage lower and upper band, the energy storage level will be as calculated below.
(4)
According to the wind turbine power characteristics used, the maximum power at the base wind speed which is 12m1s is 0.73 pu of the electrical generator power which is (1.5/0.9) MJ and according to the E.ON voltage code , the maximum time the required the grid to recover after low voltage is 1500ms. If we assume that the wind turbine is running at bass wind speed the energy produced within LVRT according to the equation (4) will be :
(4)
After the voltage recovers to 90%, the active power will ramp up to 90% of the generated wind turbine within 1 second. So if we also assume the wind turbine is running at bass wind speed, the generated power will be 0.73 pu of the electrical generator power. And the energy storage level during the ramp up period will be according to the following equations. E = 0.9* i1,m, *t = 0.9*0.73*( 1. 5 / 0.9 ) *t = 0. 55 t Mj 
(4)
and for t=1 sec, the energy stored during the ramp up is
(4)
Maximum total energy =0.55Mj+ l.825 Mj=2.378Mj which is more than the total power capabilities of the ultracapacitor within the voltage band of (1200-1400)v but it is almost the (1000-1400) v storage capability which is 2.812 Mj. These calculation shows that the DC bus voltage will go above the 1400 v.
(4)
In the simulation, the wind speed was 9.6 mls and According to the wind turbine power characteristics, the maximum power at the wind speed of 9.6m1s is 0.37 pu and energy can be found using equations (4) Maximum total energy =0.28MJ+0.925MJ= 1.205Mj which is less than the total power capabilities of the ultracapacitor within the voltage band of (1200-1400)v. In simulation; figure (18) the DC bus voltage will go above the 1400v because the DC bus voltage starts form 260v -1400v which makes its capability 1.09MJ.
IV. SIMULATION RESULTS

A. Normal operation andfixed DC bus voltage.
Figure (7) shows the mechanical generator speed per unit and it follows the reference speed that achieves MPPT. Generator speed varies according the variable wind speed profile shows in figure (6).
A. Normal operation andfixed DC bus voltage.
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-Genera t or SI)ccd -Reference speed
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Tlme(s) Figure 7 . Generator speed and generator reference speed for variable wind speed.
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Figure (8) shows the DC bus voltage. It stays constant according to the Grid side converter control. Constant DC bus control deliver whatever power comes from wind to the grid as shown in figure (9) .
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12. I 1200. 8 1200. :E 1200.
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;1200. Figure 8 . DC bus voltage for variable wind speed.
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• Frigure (17) and (18) show the active power delivered to the grid and genertaed form the wind and DC bus voltage. The delivered active power goes to zero during the fault period which raises the DC bus voltage. When the grid voltage recovers to 90 % of the normal voltage, the delivered active power ramps up to 90% of the avialbe power within 1 second. 1 o. 5
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• " w. V. CONCLUSION Double conversion wind energy system was simulated in Matlab Simulink and achieved the desired results through different control techniques. MPPT control was done showing that the generator speed runs at the fixed and variable reference speed that achieves MPPT for the generator. Active and reactive powers delivered to the grid were controlled to maintain constant DC bus voltage or using an adaptive filter to smooth the power. The adaptive filter technique makes the DC bus voltage fluctuate within a desirable band related to the filter design.
�2�-
Under low voltage ride through (L VRT), active and reactive powers were controlled according to E.ON grid code of Germany. Wind turbine system was integrated with a grid voltage profile that will dip to zero and recover to 0.9 pu within the none trip zone of the E.ON voltage code. During the low voltage period, simulation shows good results in active and reactive power control. Active power goes to zero when voltage dips and reactive power support should take a place to support the grid voltage. Reactive current was controlled to be 1 pu unit or up to 2 per unit during the LVRT. After the voltage recovers to 0.9, active power was controlled to achieve a ramp rate of 90% of the available wind generated power in 1 second.

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