Field of the invention
This invention relates to an isolated wind energy conversion system. More particularly, this invention relates to a control system for controlling the voltage and frequency of an autonomous wind energy conversion system (AWECS).
Background of the invention
Isolated wind energy conversion system (WECS) employing asynchronous generator for electricity generation, is having a great challenge of voltage and frequency control under the conditions of varying wind speed and varying consumer loads. Mohan et al have proposed a voltage regulator for such conditions for feeding frequency insensitive heating loads. Nayar et al have proposed a constant voltage and frequency controller for WECS through a diode rectifier and line commuted thyristor inverter. However, such types of traditional power conversion schemes are having poor line power factor and harmonic distortion in line and machine currents. Later on Wekhande et al have used double sided pulse width modulated (PWM) converter system to overcome some of the above problems. Erickson et al have reported the concept of matrix converter for maintaining fixed voltage and frequency from variable voltage and frequency of the generator and achieves the high efficiency at low wind speed. However, such concept for an isolated system is not justified because of complex control and special training and handling of matrix converter along with this, controllers are not capable to sustain under various load dynamic conditions. In this invention a voltage and frequency controller is proposed to full fill all these requirements under the conditions of varying consumer loads (balanced/unbalanced, linear/non-linear, single phase and three phase) and varying wind speed through maintaining the generated voltage and frequency at fixed value.

Object and summary of the invention
It is an object of the present invention is to provide an isolated system in which voltage and frequency remained constant under the condition of varying consumer loads and wind speed.
It is another object of the present invention to provide an isolated system which is having capability to feed the balanced/unbalanced, linear/non-linear; single phase/three phase loads.
To meet the above mentioned objective, the present invention provides an
isolated wind energy conversion system comprising:
a generator for generating voltage and frequency,
a load optionally having a neutral point,
a controller for neutral current compensation, harmonic elimination, load
balancing and for regulating said isolated system under different electrical and
mechanical dynamic conditions,
a point of common coupling for connecting the generator, the controller and the
load.
Brief description of the drawings
These and other aspects of the present invention will be apparent from the
following detailed description when read in conjunction with the accompanying
drawings in which like designations are used to designate substantially identical
elements.
Fig 1 shows an isolated wind energy conversion system according to the present
invention.

Fig 2 demonstrates the block diagram of a controller according to the instant invention.
Fig 3 demonstrates the performance of the controller under the condition of 3-phase 4-wire non-linear consumer loads.
Fig 4 demonstrates the performance of the controller under the conditions of varying wind speed.
Fig 5 demonstrates the harmonic spectra of the generator voltage, generator current and consumer load current
Detailed description of the drawings
Fig 1 shows the isolated wind energy conversion system and brief description of each component of the said system. In the isolated wind energy conversion system, an asynchronous generator is used to generate voltage and frequency. The asynchronous generator comprises stator winding 11 a, lib, and lie. The wind turbine lid is connected to the rotor 12 of the asynchronous generator through gear box 13. 12a, 12b and 12c show the star connected excitation capacitor bank with neutral 12d. Three single phase voltage source converter (VSC) 17a, 17b and 17c are connected to the point of common coupling (PCC) 15 through three single phase transformers 18a, 18b and 18c. Each single phase voltage source converter is made of Insulated Gate Bipolar Junction Transistors (IGBTs) 14a 14b 14c 14d for phase A, 15a, 15b, 15c, 15d for phase B and 16a, 16b, 16c and 16d for phase C. A battery energy storage system is represented by Thevenin equivalent model where 2la, 21 b show the parallel combination of resistance and capacitance for self discharging of battery and 22a shows internal open circuit voltage of the battery and its internal resistance, 23. A filter

capacitance 24 is positioned to smoothen the battery output voltage. The system also includes a black box of consumer load 25 which includes a neutral point. In the point of common coupling (15), stator terminal of the asynchronous generator, three terminals of primary winding of transformers and three terminals of the consumer loads are connected. The neutral point for the consumer load is created through the neutral point of the excitation capacitor and remaining three terminals of primary winding of the transformers.
Fig 2 demonstrates the block diagram of a controller according to the instant invention. The controller comprises a voltage sensor 55 and a current sensor to sense the frequency and amplitude of the voltage and current. The controller also comprises a means 55a coupled to said voltage sensor for separating the amplitude and frequency of the generated voltage. The means used for frequency measurement is a phase locked loop (PLL). An amplitude of voltage is used in the PI (Proportional- integral) controller 55b, to estimate the reactive component 66a, of source current. A frequency PI controller 55c is used to estimate active component 66b of source current. Summation block 66c is used to provide the addition of active and reactiv e component of source currents and gives total reference source currents. These reference source currents are compared with actual sensed source current sensed by a current sensor 66 to generate the unipolar PWM switching signals by a comparator 66d for IGBTs of the VSCs.
Fig 3 demonstrates the performance of the controller under the condition of 3-phase 4-wire non-linear consumer loads. The waveforms of the generator voltage (vabc), generator current (iabc), capacitor current (icca), load current (i|abc), compensator current (icabc), neutral currents of source (isn) load (i|n) and controller (icn), terminal voltage (vt), frequency (f), speed of the wind turbine(i)w), battery current (ibb), battery voltage (vbb), and variation in power

(Pioad, Phat. Pgen) are shown during different dynamic conditions. Three single-phase diode bridge rectifiers with L-C filter based non-linear load is applied between each phase and neutral at 2.1 s and after opening of one phase at 2.25 s and another phase at 2.4 s, the load becomes unbalanced. In both of these cases it is observed that the voltage and frequency of the system remain constant and the neutral current of the source is compensated to zero value. At 2.55 s the load is fully removed from the system and it is observed that the controller responds in desirable manner to regulate the voltage and frequency along-with additional mentioned features of load balancing and harmonic elimination.
Fig 4 demonstrates the performance of the controller under the conditions of varying wind speed. At 2 s the wind speed is 8m/s and unbalanced consumer load (one phase is open) of 14kW is applied. It is observed that due to insufficient power generation at low wind speed an additional power required by the load is supplied by the battery to regulate the frequency. At 2.1 s as the wind speed is increased from 8m/s to 9m/s, an output power of the generator (Pgen) is increased at particular load, now the power supplied by the battery (Phat) is reduced and it starts charging because now the load demand is met by the generator itself and having the availability of enough wind power. For maintaining the speed of the generator for constant frequency operation it is shown that tip speed ratio (TSR) is also reduced in same proportion as the wind speed is increased. At 2.3 s, the wind speed is reduced from 9m/s to 7m/s then it is observed that the battery again starts discharging to meet the demand of the consumer loads. At 2.5 s when the load is fully removed it is shown that the battery starts charging to store the total generated power. In this manner, the controller provides the load leveling and regulation of the frequency.
Fig 5 demonstrates the harmonic spectra of the generator voltage, generator current and consumer load current under the condition of non-linear load current

and it is observed that total harmonic distortion at the generator terminal is under the limit of IEEE-519 standards.
All documents cited in the description are incorporated herein by reference. The present invention is not to be limited in scope by the specific embodiments and examples, which are intended as illustrations of a number of aspects of the invention and any embodiments which are functionally equivalent are within the scope of this invention. Those skilled in the art will know, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. These and all other equivalents are intended to be encompassed by the following claims.

We Claim:
1. An isolated wind energy conversion system comprising:
a generator for generating voltage and frequency,
a load optionally having a neutral point,
a controller for neutral current compensation, harmonic elimination, load
balancing and for regulating said isolated system under different electrical and
mechanical dynamic conditions,
a point of common coupling for connecting the generator, the controller and the
load.
2. An isolated wind energy conversion system as claimed in claim 1,
wherein the controller comprising:
a voltage sensor for sensing the frequency and amplitude of the source voltage,
a current sensor for sensing the frequency and amplitude of the source current,
a means (55a) coupled to said voltage sensor for separating the amplitude and
frequency of the generated voltage,
a frequency controller coupled with said means for regulating and calculating
the active component of the source current,
a voltage controller with said means for regulating and calculating the reactive
component of the source current,
a summer coupled to said frequency controller and voltage controller for
providing the resultant reference source current,
a comparator for comparing the resultant reference source current with the
sensed source current and generating a signal for regulating said isolated system
under different electrical and mechanical dynamic conditions.
3. An isolated wind energy conversion system as claimed in claim 1,
wherein the generator is an asynchronous generator.

4. An isolated wind energy conversion system as claimed in claim 1,
wherein said generator comprising:
stator winding of the generator,
a wind turbine coupled to a rotor of the generator through a gear box,
an excitation capacitor bank with neutral point.
5. An isolated wind energy conversion system as claimed in claim 1,
wherein said controller comprising plurality of voltage source converter
connected to a point of common coupling through a plurality of winding of
transformers.
6. An isolated wind energy conversion system as claimed in claim 5,
wherein said voltage source converter comprises a plurality of IGBTs (Insulated
Gate Bipolar Junction Transistors) for each phase.
7. An isolated wind energy conversion system, wherein said controller
further comprising a battery energy storage system.
8. An isolated wind energy conversion system as claimed in claim 2,
wherein the frequency controller comprises a mean(55c) for frequency control
loop and a means (66b) for estimation of active component of current.
9. An isolated wind energy conversion system as claimed in claim 2,
wherein the voltage controller comprises a means (55b) for voltage control loop
and a means (66a) for estimation of reactive component of current.
10. An isolated wind energy conversion system as claimed in claim 1,
wherein the load is balanced or unbalanced, linear or non-linear and single
phase or three phase loads or 3- Phase 4-Wire Loads.

11. An isolated wind energy conversion system as claimed in claim 1,
wherein the neutral point of the load is created though the neutral point of the
generating system which is made of neutral point of capacitor bank and
transformers of the controller.
12. An isolated wind energy conversion system as claimed in claim 1,
wherein the operation of the controller is based on the optimum voltage rating
of the battery storage system for selecting different transformer turns ratio.
13. An isolated wind energy conversion system substantially as herein
described with reference to the accompanying drawings.