P.Kaseem1, M.Ramasekhara Reddy2 PG Scholar, Power and Industrial Drives,Dept. of EEE, JNTUA college of Engg., Ananthapuramu, AndhraPradesh, India1Assistant Professor,Dept. of EEE, JNTUA College of Engg.
, Ananthapuramu, Andhra Pradesh, India2AbstractThispaper proposes a detailed control strategy for multiple parallel connectedconverter units integrated with wind turbine driving PMSG. A model of multiplerectifiers in parallel with common dc link and zero sequence current dynamicsare derived and analyzed. The structure of parallel back to back pulse widthmodulation converters are adopted for multi megawatt high power generationsystem. The fuzzy based controller is developed to restrain circulating currentsflows between the power modules caused by power device discrepancy and asynchronousoperation of the parallel units. The control driving signals are generated by individualcurrent control and produced by carrier phase shifting synchronously. The effectiveness of the proposed controlstrategy is verified through MATLAB simulations.
KEYWORDS: PMSG, Zero sequencecurrent, parallel operating controllers, Fuzzy logic controllerIntroduction Windis one of the most abundant renewable sources of energy in nature. Wind energycan be harnessed by a wind energy conversion system (WECS) 4-5 composed ofa wind turbine, an electric generator, a power electronic converter and thecorresponding control system. Based on the types of components used, differentWECS structures can be realized to convert the wind energy at varying windspeeds to electric power at the grid frequency. The most advanced generatortype is perhaps the permanent magnet synchronous generator (PMSG). This machineoffers, compared at the same power level and machine size, the best efficiencyamong all types of machines with high robustness and easy maintenance due toslip ring?lessand exciter?lessfeatures. The inherent benefit of permanent magnet which supplies rotor flux insynchronous machines without excitation loss supports the wind power generationdevelopment.
This thus results in the increasing use of PMSG 3.Therectifiers in parallel connected to the PMSG have the advantage of higherreliability, high efficiency, and lower grid side harmonics The parallelconfigurations 2–9 can be classified as parallel voltage source converters(VSC)with separate direct voltage links and parallel VS converters(VSC) with a common direct voltage link. Formultiple converters in parallel with a common dc bus, when discontinuous Space-VectorModulation (SVM)7 is used, due to different switching characteristics andimpedance discrepancy of individual converter, even if synchronized control ofeach converter is applied, the switching status of the converters in parallelwill differ from each other.
This creates currents that flow among powerswitching devices and will flow in a circular loop between the power convertersand not affect the net current in either the generator or the power grid. Thecirculating currents load switching devices and other components heavily,distort waveforms, and might damage converters. Further, these currents maycause a direct error in the measurement of ground fault currents of that loop,thereby making fault detection more difficult. Large common mode inductors arerequired to limit the amount of circulating currents between the converters.The particular discontinuous SVM modulation scheme was proposed without usingzero vectors.
However, since it was not attempted to reject zero sequencedisturbance, any mismatches between the parallel converters can still causezero sequence current even without using zero vectors. New modulation schemes8 introduce a new control variable to adjust the duration of zero vectorsinstead of eliminating the zero vectors. This method can effectively inhibitthe zero sequence circulation.Asshown in Fig.1, The structure of parallel back to back PWM converters isadopted for multi megawatt high-powergeneration system Fig.1.High-power direct-drive variable-speed PMSG wind generator system connected tothe power grid.
Fig.2.Multiple three-phase PWM rectifiers with parallel connection.Thismodel provides separate control of the generator side converter and grid-sideconverter. The interconnection of the power converters in this manner canaccommodate not only multiphase but also three phase generators for WTs.
Inthis paper, zero-sequence circulation mathematical model is derived andanalyzed. An improved space vector SVPWM parallel control strategy is presentedto repress the zero-sequence circulating current. Independent currentregulation is implemented for each branch power module. II. CIRCULATING CURRENT CONTROLLER Theterm circulating current has been generally used to depict streams that streamamong the converter units in parallel 1. In rehearse; a circling current isthought to be a present that goes amiss from the sought burden current leveljust as shared by the paralleled units. A.Expression of the Circulating Current at Generator Side Fig.
2 shows n rectifiers in parallel; all the active switches are assumed to be idealswitches, and the equivalent series resistances of inductors are alsoconsidered. Fig.3. Circulating current among multi converters.
ApplyingKirchhoff’s voltage law, the following equations can be obtained corresponding toa-phase, respectively: (1) is the voltage between the negative end of and neutral point. Where Zj = d/dt + Rj,j?{1, 2, . .. , n}, the general phase current expression for any individual branch unit inparallel connection.
(2)Wherek ?{a, b, c}; is the k-phase line current of the jthconverter, is the source voltage. The circulatingcurrents for n paralleled three-phase converters in Fig. 3 can bedefined as follows. Considering the circulating current for the firstbranch unit B1 as displayed in the Fig.
3, denotes the circulating current betweenB1 and B2. denotes the circulating current between B1and B3. Similarly, the circulating current between B1 and Bnis denoted as CCk1n. Therefore, the k-phasecirculating current of the first converter CCk1 consists of n circulating-currentcomponents as follows: (3)Theexpression for the k-phase circulating current of the jthconverter can be derived 9 as follows (4)Wherei ? j and and are the k-phase line current of the jth andith converter respectively.
The general expression of the circling current ofthe jth converter is determined as (5) It is shows that both the output dc voltageand the three ac phase voltages contribute to the generation of the circulatingcurrents. The impedance of the kth converter in a particular circulatingpath also influences the magnitude of its circulating current.B. Circulating-Current Model of Three-PhaseParallel Converters in abc Coordinate Aphase-leg-averaged model for a single two-level three phase rectifier is shownin Fig.
4 8. Fig.4. Phase-leg-averaged model of a single two-level three-phase rectifier.ApplyingKirchhoff’s voltage law and the current law to node n ( at ) results in a set ofdifferential and algebraic equations (6)Thesystem equation can be written as (7)ApplyingKirchhoff’s voltage law to loops that are formed among the converters resultsin 3n ? 1 algebraic equations (8)Wherek ? (a, b, c) and Rs and Ls are the equivalentresistance and inductance of the PMSG, respectively ApplyingKirchhoff’s current law to node n (at ) results in onealgebraic equation (9)Differentiating(9), we get, (10)Assuming = = · · · = Ln and = = · · · = Furtherarranges the set of equations as the state-space form (11) Where T Is the State vectorU= , , , , , , .
. . . , , , T Iis the input vector. Y is the output vector. The state matrix isA= Andthe input matrix isB= Theoutput matrix C is the identity matrix with the dimensions of 3n × 3nT= I is the identity matrixwith the dimensions of 3× 3. The transfer function matrix is calculatedin the Laplace domain based on the state-space model.G(s)= C(sI-A)-1B = (12)Wherethe first terms represent the circulating-current part and the second termsrepresent the currents that flow from the PMSG to the converters C.
Model of Three-Phase Parallel Converters in d?q?0 Rotating Coordinate Assumingthat the voltage udc and current are continuous and with small ripples,the phase voltage expression is ukj = dkjudc. Compared with theinductance, the resistance of each power module is small. Neglectingresistance, the state-space equations for the two converters in parallelconnection are (13) (14)Bytransforming (13) and (14) from the stationary reference frame into the synchronousd?q?0 rotating reference frame 6 (15) (16) (17)Where? is the ac line frequency. Ud?uq?u0 is the d?q?0 axiscomponents of the ac voltage in the d?q?0 reference frame, respectively.id1?iq1?i01 and id2?iq2?i02 are the d?q?0components for the first converter and second converter, respectively.
dd1?dq1?d01and dd2?dq2?d02 are the d?q?0 components of theduty cycles for the first converter and second converter, respectively. u0 = usa + usb + usc andd0 = da +db + dc. The equivalent circuit ofthree-phase parallel rectifiers in the d?q?0 rotating reference frame isshown in Fig. 5.
It is noted that a zero-sequence current occurs in the 0-axisand plays a significant role in the paralleled multiple rectifiers.D. Zero-Sequence CurrentControl Scheme Fig. 6.
Shows the designed control scheme for zerosequence dynamics developed according to the equivalent circuits of three phaseparallel Converters in d-q-o rotating referenceframe. . Fig.5. Equivalent circuit of three-phase parallel rectifiers in d?q?0rotating Reference frame. Fig.
6.Zero-sequencecurrent control scheme. A modified SVPWM control strategy is proposedfor parallel converters. Individual branch unit uses a separate currentregulator.
The controlling algorithm can be summarized as follows: First, thezero sequence current is suppressed by using a fuzzy controller on the 0-axiswhich produces the output zero-sequence voltage . The reference voltagevectors and aretransformed into the stator coordinate by coordinate transformation, accordingto the sector in which the reference vector stays by using SVPWM modulation,and duty cycles are calculated. Second, the zero sequence output voltage isnormalized and superposed with modulation duty cycles. Finally, the resultingduty cycle will be compared with the modulating carrier wave, and the switchingfunction is obtained. Fromthe Fig. 5, the two parallel rectifiers contain a zero sequence current path ind?q?0 reference frame due to the discrepancy of 0-axis duty cycle components. From(15) and (16), The dynamics of zero-sequence current are expressed The second term on the right canbe expected as a disturbance. Thefuzzy controller can be cascaded with the plant to achieve closed-loop currentregulation.
The bandwidth of the control can be designed to be high, and astrong current regulation that suppresses the zero sequence current can beachieved. For n number of rectifiers in parallel, the sum of zerosequence currents is equal to zero, i.e., + + · · · + =0. Due to the interaction among the n currents, the number ofindependent zero-sequence currents is n ? 1. The number of zero sequencecurrent controllers should be n ? 1 for n parallel rectifiers.
III. FUZZY LOGIC CONTROLLER Fuzzylogic controller, approaching the human reasoning that makes use of thetolerance, uncertainty, imprecision and fuzziness in the decision makingprocess and manage to propose a very satisfactory operation, without the needof a detailed mathematical model of the system, just by integrating the expert’sknowledge into fuzzy rules. In addition, it has essential abilities to dealwith noisy date or inaccurate, thus it has able to develop control capabilityeven to those operating conditions where linear control techniques fails i.e.,large parameters variations.
RuleBase: It consists of a number of If-Then rules. Then side of rules is calledthe consequence and If side is called antecedent. These rules are very similarto the human thoughts and then the computer uses the linguistic variables. Rulebase of FLC is listed in table 1TABLE 1.MEMBRSHIP FUNCTI0N TABLE FUZZY RULES E(n) NB NS ZE PS PB NB NS ZE PS PB ZE PB PB PS PS PS PS PS ZE ZE PS ZE ZE ZE NS ZE ZE NS NS NS NS NS NB NB ZE IV.CONTROL OF PMSG WITH MULTIPLE RECTIFIERSInwind turbine PMSG systems, three system variables need to be strictlycontrolled 6: (1) the optimal power generated by the PMSG at different windspeed levels; (2) the active and reactive power injected into the grid; (3) theDC bus voltage of the back to back converter. The proposed system contains adirect-drive wind turbine PMSG fed by a back-to-back converter. The use ofparallel converters compared with a solution with only one converter is higherreliability, higher efficiency, and the possibility of extremely low gridharmonics.
Inparallel connection, one converter unit functions as a master and the othersfunction as slaves. A serial communication bus is arranged between theconverter units in which each unit has its own modulation cycle counter and itis synchronized with each other on the basis of serial communication messages.In this manner, the modulation counters operate as simultaneously as possible. Fig.7. Overall structure for the control of parallel multi converters on themachine side Carrierphase-shifting modulation technique 10 has a great advantage for powerconverters in parallel. When a module fails to operate, the master controllerjust changes the corresponding carrier phase angle and limits the capacity ofthe system, other modules can continue to work, standby unit can also beactivated, and full-power operation can still be achieved.
The PMSG iscontrolled by two 750-kW generator-side converters connected in parallel in arotor rotating d?q axis frame, with the d-axis oriented along therotor-flux vector position. In this way, the d-axis current is held tozero to obtain maximum electromagnetic torque with minimum current. The optimumactive power setting or torque reference can be calculated according to maximumpower point tracking strategies. The two sets of PWM driving signals aregenerated by using separate current regulators and produced bycarrierphase-shifting synchronously. The rotor position is fed by the rotor positionobserver without any position sensor.
Each converter module is independent ofeach other identifying the rotor flux position. The currents of each module arebalanced and synchronized with respect to each other producing the optimaltotal generator torque. This arrangement will reduce the requirements for largeimpedance needed to equalize the current sharing and allow increasing the powerhandling capability for a converter with parallel connection. The zero-sequencecurrent fuzzy controller have been integrated with the control of parallelconverters.
V. Simulation Results Fig8.Tenkilowatt generator side circulating current with PI controller Fig.9.Ten kilowatt grid side circulating current with PI controller Fig.10.
Generatorcurrents of individual converter when generator operated at 1.5KW with PI controller. Fig.11.Three phase Generator currents of individual converter when generator operatedat 1.5KW with PI controller. Fig.
12.Tenkilowatt generator side circulating current with Fuzzy controller Fig.13.
Ten kilowatt grid side circulating current with Fuzzy controller Fig.14.Three phase Generator currents of individual converter when generator operatedat 1.
5KW with Fuzzy controller. Fig.15.Generatorcurrents of individual converter when generator operated at 1.5KW with Fuzzy controller OBSERVATION TABLE Circulating currents Total harmonic distortion (THD) With PI controller Fuzzy Logic Controller Generator(10KW)side circulating current 29.
20% 17.52% Grid side circulating current 33.14% 28.16% Generator(1.5MW) currents of individual converter 35.
86% 25.26% Three Phase Generator(1.5MW) currents of individual converter 34.13% 20.
20% V. CONCLUSION Thispaper has described the control schemes of a permanent magnet wind powergenerator connected to parallel converters with common dc link. A dynamic modelof zero-sequence currents has been derived and analyzed for a number of n three-phasePWM rectifiers in parallel connection.Thezero sequence currents are effectively controlled and suppressed by using themodel technique SVPWM with fuzzy logic controller. REFERENCES1 Zhuang Xu,Rui Li, and Dianguo Xu, “Control of Parallel Multirectifiers for aDirect-Drive Permanent-Magnet Wind Power Generator,” IEEE TransactionsOn Industry Applications, Vol. 49, No.
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