Parallel Operation of Permanent Magnet Synchronous Generator Based Windmills Connected to HVDC-VSC Link

Transcription

1 Parallel Operation of Permanent Magnet Synchronous Generator Base Winmills Connecte to HVDC-VSC Link R. Vijaya Priya S. Kishore Rey M.P. Selvan P. Raja Hybri Electrical Systems Laboratory Department of Electrical an Electronics Engineering National Institute of Technology Tiruchirappalli Abstract Development an implementation of a control scheme for win farms base on Permanent Magnet Synchronous Generators (PMSGs connecte to a single HVDC-VSC converter is the main objective of this paper. The presente scheme is base on computing the optimal frequency at which the power converter must work to extract the maximum power from parallel operate PMSG base winmills. The power extraction from win turbines connecte to PMSGs an its control, gri-sie converter control which is connecte to AC gri are iscusse in etail. The control scheme is valiate by means of case stuies using PSCAD/EMTDC simulation software on a win farm consisting of two win turbines. Keywors Permanent Magnet synchronous generator; Maximum power extraction; High Voltage Direct Current Voltage Source Converter. I. INTRODUCTION Large scale win farms consisting of win turbines of huge size in MW couple to ifferent types of electrical generators like Squirrel Cage Inuction Generators (SCIGs, Doubly Fe Inuction Generators (DFIGs, Synchronous Generators (SGs an Permanent Magnet Synchronous Generators (PMSGs. The aggregate of power output from these win farms will be in few hunres or sometimes in few thousans of MW. Due to the non-availability of lan space an presence of high win velocity at the mile of the ocean, offshore win farms are more popular nowaays for not only generating electrical power but also proviing ancillary services []. The interconnection of these win farms offers lot of technical challenges because of the location of the units an the stochastic nature of prouce power []. Most of the offshore win turbines are irect rive win turbines couple with PMSGs elivering power through full scale power converters []. In orer to transfer the power generate from remote offshore win farms to onshore AC gri, High Voltage AC (HVAC or High Voltage DC (HVDC transmission system is essential. HVDC technology has proven its superiority over HVAC for long istance transmission lines an submarine applications [], [4]. In general HVDC /4/$ IEEE technology is base on either Line Commutate Converters (HVDC-LCC or Voltage Source Converters (HVDC-VSC [], [], [4]. In literature, centralize power converter base HVDC power transmission has been investigate for offshore win farms with SCIGs [5] an SGs [6]. Another topology, which has a central power converter at gri sie an iniviual converter for each win turbine, has been propose for power transfer from DFIG base offshore win farms [7]. In this paper, a controlle scheme for PMSG base offshore win farms connecte to a centralize HVDC-VSC converter has been investigate. In the presente technique, the sening en HVDC-VSC is responsible for maximum power point tracking, which is achieve by making a variable frequency offshore gri. The tip spee ratio an power coefficient of iniviual win turbines are use to fin the optimal frequency at which the offshore gir nees to operate to extract the maximum power [6]. The Fiel Oriente Control (FOC is employe at win farm power converter an conventional -q current control is use for gri sie converter. The proceure explaine in [8] has been use for tuning the PI controllers of both win farm an gri sie converters. In this work, parallel operation of PMSG base win turbines connecte to AC gri through a HVDC-VSC link is simulate using PSCAD/EMTDC software. Different case stuies with machines of same an ifferent rating uner normal an abnormal gri conitions have been performe an the results are presente. II. WIND TURBINE SYSTEM A. Win power, P w The power available in the win can be written as Pw = ρ Avw ( where Pw is the air stream kinetic power (W, ρ is the air ensity (kg/m, A is the swept area of the rotor (m, v w is the win spee (m/s. B. Win turbine power generation, P wt Power generate in a single win turbine is given as

2 Pwt = Cp * Pw = Cpρ Avw ( where C p is the power coefficient which is the percentage of power in the win that is converte in to mechanical power. C. Power coefficient an Tip spee ratio Let C p be the power coefficient which can be written as c5 Cp ( λθ, = c( c cθ c4θ c6 e Λ where efine as, θ c7 Λ ( is the angle an λ is the tip spee ratio ω R t λ = (4 vw c Λ = λ+ c θ + θ 8 9 (5 Equation (7 can be rewritten as, N P wt = ρ A C i piv (7 = wi N P wt = Cp A v i wi ρ (8 = For instance, if two win turbines are subjecte to ifferent win spees then the power generate by the iniviual win turbine an the aggregate power of both the win turbines at ifferent electrical frequency of stator voltage can be plotte as given in Fig.. [ c,..., c ] are characteristic constants for each win where 9 turbine. Fig. Generate power Vs. frequency In orer to extract the maximum power from the win farm, the operating frequency of the win farm converter has to be set at the optimal frequency corresponing to the peak point of the aggregate power (P as inicate in Fig.. E. Win farm electrical analysis Fig. power coefficient Vs tip spee ratio of typical win turbine C p λ curve is shown in Fig.. The A typical C p λ curve has a maximum value which correspons to the optimum operating point of the win turbine to extract maximum power at the corresponing win velocity. D. Win farm power analysis The schematic of the electrical system consiere for investigation in this work is shown in Fig.. It has the win farms with two win turbines. Each win turbine is couple with a PMSG, whose stator terminals are connecte to the win farm transformer. The win farm transformer is feeing the whole win farm power to the gri through HVDC-VSC link. The total power generate by a win farm compose of N wt number of win turbines can be written as P N wt = P (6 i= wti If all the win turbines are of equal rating then all the constants will have the same value. Fig. Offshore win farm connecte to the main AC gri through HVDC- VSC link

3 (i Win farm electrical gri: Referring all the electrical quantities to the generator sie an consiering all the voltages an currents with respect to the synchronously rotating reference frame, the win farm voltage can be written as: v = v + Ri Lwi + L i v = v + Lwi + Ri + L i sq cq l sq l e s l sq s c l e sq l s l s where v c is the converter sie voltage, v s is the win farm gri voltage an i s, i sq are the total win farm currents in rotating reference frame. The win farm power converter transformer is moele as RL series branch with resistance R l an inuctance L l. Here, sub-inexes, q stan for irect an quarature axis quantities in the rotating reference frame. The active power an reactive power provie by the win farm power converter can be written as Pc = ( vsis + vsqisq Qc = ( vsqis vsisq (9 (0 (ii Permanent magnet synchronous generator: The PMSG stator voltage with respect to the rotating reference frame can be written as: vgq = ri s gq + ωrlqigq + Lq igq + λmωr ( III. CONTROL STARTEGY In the win turbine PMSG system iscusse in section II, there are three control variables as follows: the optimal power generate by the PMSGs at ifferent win spee, the active an reactive power injecte into the gri an the DC link voltage of the HVDC-VSC link. In this system, the win farm sie converter regulates the spee of the PMSGs to implement the MPPT control. Meanwhile, the gri-sie converter controls the active an reactive power injecte into the gri an the DC link voltage. A.Generator-sie converter Generator electromagnetic torque can be expresse as Te = p ( λmigq + ( L Lq igqig (4 In a surface mounte PM machine which is consiere in this case stuy, the -axis an q-axis inuctances are equal (L = L q. Thus, the torque expression can be simplifie as Te p ( λmigq = (5 From the above expression, it is clear that there is a linear relationship between the electromagnetic torque an q-axis current i q, such that the electromagnetic torque is easily controlle by regulating the q-axis current. The control scheme of the win farm sie converter is shown in Fig. 4. vg = ri s g ωrlig + L ig ( where i q, i gq are the generator currents an v g, v gq are generator voltages in rotating reference frame. The generator λ stator wining resistance is r s, m is the flux ue to permanent magnet, an L an L q are the generator inuctances. The generator torque can be expresse as Tm = p ( λmigq + ( L Lq igqig ( where, p is the number of pole pairs of PMSG. The FOC plays a significant role in the control of PMSGs, which makes it possible that PMSGs can be controlle as easily as DC machines. In FOC approach, the flux proucing current component, i an the torque proucing current component, i q are along the -axis an q-axis respectively. Thus, the q-axes currents can be controlle inepenently by two close loop controls, which inirectly controls the spee an the torque of the PMSGs. Fig. 4 Win farm sie converter control scheme As state earlier, the FOC approach couple to the optimal tip spee ratio base MPPT control strategy is applie here as the control algorithm for the win farm-sie power converter. As seen in Fig. 4, there are three feeback loops in the control system: spee control loop, q-axis current control loop an - axis current control loop. In the spee control loop, at every sampling time, the actual spee of the generator is sense an compare with its reference value, which is generate by the optimal tip spee ratio control. Then the error in spee is sent to a PI controller which provies the reference q-axis current (i q *. The reference -axis current, i *, is always set at zero. The q-axis reference current obtaine from the spee control loop an -axis reference current are use in the respective current control loop. In the current control, three-phase stator currents are sense an transforme into the q-axes reference frame using Park's transformation. The PI controllers present in the current control loop provie reference stator voltages in qaxis reference frame. The q-axis reference voltages are transforme into abc frame by inverse Park s transformation. Sinusoial Pulse Wih Moulation (SPWM approach is

4 employe as the moulation strategy in this work, because it generates less harmonic istortion in the output stator voltage/current an leas to more efficient use of the DC supply voltage than the conventional Pulse Wih Moulation (PWM. The output of the SPWM strategy are six PWM signals to control the ON/OFF state of the six IGBT switches in the win farm sie converter. B. Gri-sie converter As state earlier, the main objective of the gri-sie converter control is to regulate the active an reactive power fe to the gri an the c link voltage. The expressions of the active an reactive power injecte into the gri can be written as follows Pg = ( vgig + vgqigq Qg = ( vgqig vgigq (6 where, i g an i gq, are the gri currents, an v g an v gq, are the gri voltages in q-axes reference frame. From the above expressions, it can be unerstoo that the -axis an q-axis components of the gri currents an voltages are couple in cross-prouct fashion in the reactive power term, which makes the active power an reactive power control complex. In orer to tackle this cross coupling issue, the Voltage Oriente Control (VOC approach is selecte for the gri-sie converter control. In the VOC approach, the q-axis of the rotating reference frame is aligne with the rotating gri voltage space vector. Accoringly, the -axis component of the gri voltage space vector is equal to zero. Using VOC approach, the expressions of the active an reactive power can be written as follows P = ( v i = v i Qg = ( vgqig g gq gq c c (7 where, v c an i c, are the voltage an current of the DC link, respectively. Fig. 5 Gri-sie converter control From the above expressions, it can be observe that the active an reactive power can be controlle inepenently by the -axis an q-axis components of the gri currents, respectively. The control scheme of the gri-sie converter is shown in Fig. 5. IV. PARALLEL OPERATION OF PMSGS The major avantage of parallel operation of PMSGs connecte to single HVDC-VSC converter is that no requirement of iniviual power converter for each win turbine, which reuces the cost an increases the reliability of the system. A Optimal electrical frequency search for maximum power extraction In orer to generate the maximum possible power from the win farm, it is propose to operate the electrical win farm at the optimal frequency. To fin the optimum electrical frequency that maximizes the total power of the win farm, the expression for the total power is ifferentiate with respect to the angular electrical spee an equate to zero. The iniviual power in each win turbine is P ρac ( λ, θ v = (8 wti pi wi The total power in the win farm having n win turbines is written as P = Pwt+ Pwt Pwtn (9 P = A C v (0 n ρ pi ( λ, θ wi i= where C p is function of λ an it is function of angular spee an win spee. The optimal electrical frequency for maximum power extraction of win farm can be obtaine as follows P = 0 ( ωe B Simulation results an analysis The control scheme iscusse above is teste on a system with two PMSG base win turbines by means of simulations using PSCAD/EMTDC. The win farm fees power to the gri through HVDC-VSC link. The system is analyze with two ifferent case stuies as iscusse below. Performance stuy of two win turbines uner normal gri conition. Performance stuy of two win turbines with gri isturbance. Performance stuy of two win turbines uner normal gri conition Two.5 MW win turbines connecte to the AC gri through HVDC-VSC link are consiere in this case stuy. It is assume that both the win turbines are expose to same win spee. However, the win spee is assume to vary from 0

5 m/s to 8 m/s an then to m/s an 9 m/s as shown in Fig. 6. The Fig. 7 epicts that the machines couple with the win turbines are tracking the win spee. As the win spee changes, the mechanical power, which is proportional to win spee, also changes as shown in Fig. 8. While oing the parallel operation of the machines line-to-line voltages an frequency of both the machines have to be same else circulating current may exists between two machines. The results presente in Figs. 9- confirm the parallel operation of PMSGs. Figs. 4 an 5 show the power output of two PMSGs. The DC link voltage is maintaine constant by grisie converter control, which is shown in Fig.6. The gri voltage is set at constant magnitue of 690V. The gri currents, whose magnitue is proportional to win spee are shown in Fig. 7. The active power injecte into the gri is inicate in Fig. 8. Fig. 6 Win spee Fig. Frequency of both machines Fig. Stator current of PMSG- Fig. Stator current of PMSG- Fig. 7 Spee tracking of two win turbines Fig.4 Power output of PMSG- Fig. 8 Total power in the win (Mechanical power Fig.5 Power output of PMSG- Fig.6 DC link voltage Fig. 9 Output voltage of PMSG- Fig. 7 Gri current of the system Fig. 0 Output voltage of PMSG- Fig.8 Active power injecte into the gri

6 . Performance stuy of two win turbines with gri isturbance The performance of the system has been analyze with 0% sag for a small uration in gri voltage as shown in Fig. 9. During this stuy the win spee is assume to be constant. Since the win spee is assume to be constant at 0 m/s, the power extracte from the win by the win turbine is constant an hence the electrical power evelope by the machine is constant, which is shown in Fig. 0. However, ips in the DC link voltage can be observe at particular points as seen in Fig. because the DC link voltage is maintaine by gri-sie converter which is connecte to AC gri. As the power output at the win farm sie is constant, the power injecte to the gri shoul be maintaine constant. Here, as the voltage ecreases, the gri current increases to keep the injecte power constant. The waveform of gri currents an RMS value of gri current is shown in Fig. an. The Fig. 4 shows the active power injecte into the gri. Fig. 9 0% sag in gri voltage for 0. sec Fig. 4 Active power injecte into the gri V. CONCLUSION A control scheme for win farm base on permanent magnet synchronous generators (PMSGs connecte to the main gri through HVDC-VSC link has been presente. Parallel operation of PMSGs uner normal an abnormal gri conitions are stuie in this work. The optimal frequency at which maximum power can be extracte is compute an MPPT controller is implemente to control the win farm power converter. For a gri isturbance of voltage sag, the power, voltage an current at gri-sie is analyze. The stator currents an voltages of both PMSGs, DC link voltage, gri-sie voltage, current an power injecte into the gri for ifferent cases are presente an analyze. The performance of the investigate control scheme is satisfactory uner normal an abnormal gri conitions. Fig. 0 Mechanical Power an total power output of PMSGs Fig. DC link voltage Fig. Gri current at 0% sag in voltage Fig. RMS value of gri current at 0 % sag in voltage REFERENCES [] B. Wu, Y. Lang, N. Zargari, an S. Kouro, Power Conversion an Control of Win Energy Systems, IEEE Press Series on Power Engineering. [] T. Ackermann, Transmission Systems for Offshore Win Farms, IEEE Power Engineering Rev., Vol., no., pp. - 7, December 00. [] Ime Jlassi, Jorge O. Estima, Sejir Khojet El Khil, Najiba Mrabet Bellaaj, an Antonio J. Marques Caroso, Multiple Open-Circuit Faults Diagnosis in Back-to-Back Converters of PMSG Drives For Win Turbine Systems, IEEE Transactions on Power Electronics, Vol. 0, No. 5, May 05. [4] J. Arrillaga, Y. Liu, an N. Watson, Flexible Power Transmission: The HVDC Options, John Wiley an Sons, 007. [5] Lluis Trilla, Orial Gomis-Bellmunt, Aria Junyent-Ferre, Agusti Egea Alvarez an Antoni Suria-Anreu, Control of a Squirrel Cage Inuction Generator Win Farm Connecte to a Single Power Converter, at UPEC 00. [6] Oriol Gomis-Bellmunt, Aria Junyent-Ferre, Anreas Sumper, Joan Bergas-Jane, Control of Win Farm Base on Synchronous Generators with a Central HVDC-VSC Converter, IEEE Transactions on Power Systems, Vol.6, no., August 0. [7] L. Xu, L. Yao, an C. Sasse, Gri Integration of DFIG-base Win Farms Using VSC Transmission, IEEE Transactions on Power Systems, Vol., no., pp , August 007. Chanra Bajracharya, Marta Molinas, Jon Are Suul, Tore M Unelan, Unerstaning of Tuning Techniques of Converter Controllers for VSC-HVDC.