An Interconnected Wind Driven SEIG System Using SVPWM Controlled TL Z-Source Inverter Strategy for Off-Shore WECS

This paper discuss about the interconnection of wind driven SEIG for drive applications by using TL Z-source inverter strategy .TL Z-source consists of two coupled inductors having turns ratio γTL and four diodes are used . The wind energy system uses a two Self Excited Induction generator (SEIG) connected parallel in order to increase the reliability. The proposed system components like wind turbine SEIG, rectifier, SVM Controlled TL Z-source inverter, are modeled by matlab Simulink. The maximum power can be extracted and supplied to the load efficiently by using TL Z-source inverter with a proper value of modulation index. The simulation output is analysed experimentally using 500 W experimental setup.


Introduction
Wind energy conversion system includes a wind turbine generator, interconnection apparatus and control systems.Horizontal axis type wind turbines are used.Wind turbine can be designed for a constant speed or variable speed operation.In this paper we have introduced interconnection of wind turbine with TL Z-source inverter [1].This technique mainly used in OFFSHORE WIND ENERGY CONVERSION.Offshore wind power refers to the construction of wind farm in bodies of water to generate electricity from wind.Better wind speeds are available offshore compared to on land, for lossless transmission DC transmission system is used.Offshore wind more economically viable due to reducing the weight of turbine materials.Eliminating problematic gearboxes.For offshore windmill.wemainly used self excited induction generator (SEIG) [1][2] because of following reasons.used in remote area because of low maintaince .togenerate the power from variable speed as.low unit cost, reduced maintenance, rugged and brushless rotor, absence of a separate d.c .In this paper we using new interconnection technique using TL Z-source inverter we can improve the efficiency.
While comparing TL Z-source inverter with other inverters like VSI and CSI [2].In VSI where the independently controlled ac output is a voltage waveform, it is used in many industrial applications, such as adjustable speed drives; it is a buck inverter for dc-to-ac power conversion.It is a boost converter for ac-to-dc power conversion.In CSI where the independently controlled ac output is a current waveform.These structures are still widely used in medium-voltage industrial applications, where high-quality voltage waveforms are required.
In both VSI and CSI some disadvantages are occurred they are either a boost or a buck converter and cannot be a buck-boost converter.In other words, neither the V-source converter main circuit can be used for the I-source converter.It produce EMI noise.To overcome the above problems traditional V-source and I-source converters, this paper we used an TL Zsource inverter to control PWM SVM [1][7] algorithm is used.It is most commonly used in inverters for creation of alternating waveform.From the given block diagram two turbines are connected to a two separate SEIG for generating AC power .SEIG [2] offers various advantages over the conventional synchronous generators such as reduced unit cost, easy maintenance, rugged and simple construction, brushless rotor (squirrel cage) and so on .AC power is given to the separate diode rectifier is used to convert variable magnitude, variable frequency voltage at the induction generator terminal into DC voltage .The converted DC voltage are transmitted to a load through DC transmission system to avoid transmission losses for long transmission .After that the converted DC voltage are fed to the TL Z-source inverter for AC output it passed to the given load .This method gives high efficiency for off shore.TL Z-source inverter can produce any desired output ac voltage .It provides ride-through capability during voltage sags, improve the power factor and reliability, and extends output voltage range.To control PWM SVM algorithm is used .It is most commonly used in inverters for creation of alternating waveform.This system is mainly used in off-shore wind farm.

Modelling of Seig
An induction generator offers various advantages over the conventional synchronous generators such as reduced unit cost, easy maintenance, rugged and simple construction, brushless rotor (squirrel cage) and so on.Three-phase induction machine can be made to work as a self-excited induction generator (SEIG) [1].The SEIG is the induction machine driven by prime mover with capacitor connected in stator terminals.The output power of SEIG depends upon the Wind velocity variations of the horizontal axis wind turbine

Diode Bridge Rectifier and DC Link
Three phase diode bridge rectifier is used to convert variable magnitude, variable frequency voltage at the induction generator terminal into DC voltage.The output voltage is expressed as Vr=(3√2∕ )(√3∕√2)*Vds*ni The series reactor (L) and shunt reactor (C) acts as an input filter.The current ripples and voltage ripples are reduced by using the above components.
DC link current was governed by Pidc=(1/Ldc)(VrViVdcidc) where RDC and LDC are the reactor resistance and inductor respectively.

TLTopology
The TL Z-source inverter is shown in Figure 4 [11], where two coupled inductors having turns ratio γTL and four diodes are used.The TL topology is only applicable to the voltage-type Z-source inverter, and can produce the same gain and capacitor voltage as the generalized SL topology.Its governing expressions are also represented by (3) after substituting N with γTL.The number of inductor turns needed by both topologies to produce.The same gain is, therefore, roughly the same.The TL topology however uses lesser diodes, but with higher blocking voltages than the SL topology.This can clearly be illustrated by writing down their respective blocking voltages, which are found to be independent of the source positions.

TL Inverter
The last two expressions in ( 8) are clearly γTL times larger than those in (7).In other words, the TL topology reduces its diode count to four merely by making them blocked larger stresses otherwise distributed among N (=γTL) diodes in the SL topology.Other places of concentrated stresses are at the L11 and L21 windings of the TL inverter.This happens when in the shoot-through state, during which energies from the L12 and L22 windings are transferred to the L11 and L21 windings because of the blocking of diodes DTL3 and DTL4.That causes the instantaneous currents through L11 and L21 to surge greatly, which will not happen with those inductors of the generalized SL topology.

SVM Control Statergy
In this paper we used SVM control technique .byusing this technique we can eliminate the conditions (000) and (111).It represents as homopolar component and results in a unique correspondence between space vector in the complex plane and a 3phase system.The below diagram shows the switching pattern of TL Z-source inverter.

Simulation and Results
The z-source inverter with wind driven self -excited induction generator fed WECS is simulated using MAT LAB/SIMULINK and the results are presented.The minimum and maximum value of the self-excitation Capacitance requirement is previously the self-excited induction generator is used to understand the all characteristics behaviour of the generator system.The self-excited induction generator can be simulated using Mat lab/ simulink to study the dynamic performance of the machine.Self-excitation process is initiated at t = 0s without any load at the stator terminals.It is observed that voltage build up reaches the first steady state value at t = 5.5s.As the load on the generator increases, the stator voltage decreases with an increase in the stator current.At t = 20s, a capacitor with capacitance is increased to 60 μf to compensate the voltage drop.The generated voltage is expressed as an instantaneous value and rms value.

Conclusion
In this paper the performance analysis and simulation results of interconnection of SEIG fed ZSI for wind energy conversion system have been described.We combine two wind turbines driven SEIG and used diode rectifier for converting AC voltage to DC voltage produced by SEIG .Phase voltage produced by SEIG 580 V AC and Phase current of SEIG is 7A.In off shore wind farm for lossless transmission DC transmission is used.The converted DC voltage was fed to Z-source inverter.The Z-source inverter system provides ride-through capability during voltage sags, improves the power factor and reliability, and extends output voltage range.we can do either buck or boost .PWM SVM control technique was used for control.The output voltage of AC motor load is 634V AC.The dynamic voltage, current, rectifier voltage waveforms are developed and analysed.

Figure 1 .
Figure 1.Block diagram for interconnected wind driven SEIG system

Figure 2 .
Figure 2. Equivalent circuit of SEIG in d-q reference frame

Figure 4 .
Figure 4. Voltage space vectors with shoot through States for TL Z-source inverter.

Figure 5 .
Figure 5. Traditional switching pattern for TL Z-source inverter.Source Inverter

Figure 7 a
Figure 7 a.Simulink model of the self-excited induction generator

Figure 12 .Figure 13 .
Figure 12.Photo graph of the D.C separately excited motor fed SEIG with R load
To control PWM SVM algorithm is used.It is most commonly used in inverters for creation of alternating waveform.There are various types of SVM.It results in different quality and computational requirements.Traditional approaches on SVM are mainly based on five level or seven level inverters.