IMPROVED RIDE-THROUGH OF PMSG WIND TURBINE DURING SYMMETRICAL VOLTAGE DIP USING A MAGNETIC AMPLIFIER

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1 IMPROVED RIDETHROUGH OF PMSG WIND TURBINE DURING SYMMETRICAL VOLTAGE DIP USING A MAGNETIC AMPLIFIER R. A. Irahim, M.S. Hama, Khale H. Ahme, Y. G. Dessouky, an B.W. Williams Ara Acaemy for Science an Technology, Egypt, rania_assem@alexsees.com Alexanria University, Egypt Strathclye University, UK Keywors: Low voltage rie through, permanent magnet synchronous generator, magnetic amplifier, voltage ip. Astract Directrive win turines ase on permanent magnet synchronous generator with full scale ack to ack converters are ecoming very promising ue to their high power ensity, gearless structure an flexile control. With the remarkale growth of win energy capacity connecte to the utility gris, strict gri interconnection reuirements of power plants comprising permanent magnet synchronous generators technology are essential to improve the control of electrical power system, oth in steaystate an transient operation. Low voltage rie through capaility enhancement is one of the gri reuirements which ensures the safe operation of the win farm uring the network isturances an avois its shutown. This paper proposes an improve topology ase on magnetic amplifier in the oost converter circuit to enhance the rie through capaility of permanent magnet synchronous generators ase win energy systems. Nomenclature C p f i g,g i a,,i c i, i, i s i c I inv I rec L f L,L P w P Gri,Q Gri P Gen, Q Gen R a R T e Performance coeff. for win turine Supply freuency, (Hz) Boost converter inuctor current (A) PMSG stator current components, (A) Three phase gri currents, (A) axis components of gri currents, (A) reference gri current components, (A) PMSG supply current, (A) Magnetic amplifier control wining current, (A) Inverter IGBT current, (A) Rectifier ioe current, (A) Booster inuctance, (H) PMSG axis inuctances, (H) Power asore y winmill Gri active & reactive power, (Watt, VAR) PMSG active & reactive power, (Watt, VAR) Stator armature resistance, (Ω) winmill lae raius (m) Electromagnetic torue, (N.m) V V inv V V c V rec v a,, v c P Q P gri Q C V c β δ φ f φ, φ θ λ ρ ω 1 Introuction Win velocity (m/sec) Inverter IGBT voltage, (V) Input capacitor voltage of oost converter, (V) DC link capacitor voltage, (V) Rectifier ioes reverse voltage, (V) Three phase gri voltages, (A) Generator output increment (W) Gri power increment (W) Capacitor store energy increment (J) Incremental change in DC voltage Blae pitch angle (BPA) Loa angle, etween axis an phase a voltage, (ra) Permanent magnet flux linkage, (W) axis fluxlinkages, (W) Phase angle of gri voltage, (egree) Tipspee ratio (TSR) Air ensity Electrical rotor angular spee, (ra/s) Win energy is consiere as the largest proucer of renewale energy. Accoring to the Annual Energy Outlook, winpowere generating capacity has grown over the past ecae, from 18GW of installe capacity in 2000 to an estimate 179 GW in 2010 [1]. This large amount of variale generate power will ring issues such as power system operation; ynamic an steay state staility forcing the nee for national regulatory gri coes. Within these regulations, low voltage rie through (LVRT) reuirement for win power plants have gaine a great importance where the win power system shoul stay connecte to the gri for the gri fault conitions, contriuting to keep network voltage an freuency stale y elivering active an reactive power to the gri [2]. In power systems where the win power generation is of a major portion, the gri will experience huge power outage if the win farms trip off [3], leaing to instaility in the power gri as well as estructive effects to the win power converters. Variale spee win energy conversion systems (WECS) employing permanent magnet synchronous generators (PMSG) are one of the most popular win energy configurations owing to their extene spee operating range,

2 high power ensity, full ecoupling feature etween the generator an the gri, an maximum energy capture at ifferent win spee. It is thus essential for gri operators to apply the LVRT regulations an rules to ensure successful rie through of PMSG when connecte to the electrical utility gri. Methos for LVRT capaility enhancement can e ivie into ones reuiring aitional harware to the WECS an others reuiring change in the control system. Harware solutions can e summarize as ynamic raking resistance (active crowar), capacitor sizing, an energy storage systems [4, 5]. As for control techniues, LVRT capaility enhancement can e achieve y moifications in the control strategy of the pitch controller, generator sie, oost converter an gri sie controllers [6]. These methos are explaine thoroughly in [7] an has een extene to the farm level through the aitions of FACTS compensation techniues [8]. In previous attempt y authors, LVRT capaility enhancement was successful using a topology ase on magnetic amplifiers foun in [9]. The paper stuie the impact of the aition of magnetic amplifier in DC link voltage limit as well as reucing the voltage an current stresses on the power converter switches. In this paper, the authors further explore aitional potentials of the topology y moifying the configuration thus reucing the overall size an cost of the propose system. Simulation results can e foun for the new propose magnetic amplifier configuration. The new improve configuration shows promising results in limiting the DC link voltage rise, thus reucing the stresses on the power converters. 2 System Behaviour an Investigation The WECS employe for gri connection of PMSG win turine can e foun in figure 1 which consists of a rive train, PMSG, ioe rige rectifier, oost converter circuit an a voltage source inverter (VSI). A 16 kw, 690V PMSG gri connecte system moelling have een carrie out which inclues aeroynamic moel of the win turine using euations (1) to (4) an PMSG ynamic euations in frame using euations (5) to (7) [10] Pw = ρπ R V Cp (1) 2 ωr λ= (2) V λ1 Cp = β 0.2β 13.2 e λ1 λ 1 = λ 0.02β β + 1 v Rai L Li (3) (4) i = + ω (5) t i v = R i + L +ω L i (6) a f t +ωϕ (( ) ) 3 Te = ρ i L L i +ϕf (7) 2 Boost converter circuit acts as the generator sie converter an in this case is use for maximum power extraction from the win an controls the rectifier output current [11]. Maximum power point extraction of win for PMSG has een iscusse in literatures [1214]. Among these methos, optimal relationship ase (ORB) techniue has een use in this work as it lacks any win spee measurement as well as its fast acting response to win change. In this metho, an optimal relationship is otaine etween the oost inuctor current versus oost converter capacitor DC voltage thus extracting maximum power at any given win spee. Booster current is regulate using a controller followe y PWM moulator. Gri sie converter is ase on three phase, two level 6 IGBT with anti parallel ioe unit whose control is ase on maintaining a fixe DC link voltage as well as exporting all the extracte power to the gri. Stationary axis currents are controlle where the active power is use in the inner loop to control the clink capacitor voltage. The reactive power is set to zero for unity power factor operation an is varie accoring to the gri reuirements [15]. The system uner investigation in figure 1 have een simulate an sujecte to severe voltage ips (90% voltage ip for 140ms) as mentione in the EON gri coes reuirements [16] an simulation results are shown in figure 2 (a) (g). During the voltage ip inciance at 2.5 sec, the gri power has fallen in response to the voltage ip while the PMSG extracte power will remain unchange as seen from figures 2(f) an 2(g). Since the gri power has fallen to a very low value, the win extracte power will e transforme into store energy in the DC link capacitor causing rapi rise in its voltage (up to 1.2 pu) as well as high inrush currents (up to 7.5 pu) in the gri thus amaging the power converter units following the euation (8). P G P L = Q C = V c. I c = V c. C V c t In larger MW win turines, similar voltage ips can cause much severe voltage rise as well as higher inrush currents epening on the capacitor size, ip voltage uration an network effective impeance. PMSG i s Rectifier i C f Pulse Lookup Tale Boost Converter L f i = 0 i C i v v VSI PW M to Figure 1: Block iagram for WECS using PMSG i θ v (8) θ PLL to i a to i c v a v c Gri

3 (a) (f) () (g) Figure 2: WECS ehavior to 90% voltage ip without LVRT control. (a): Generator output voltage, (): Generator output current, (c): DC link voltage, (): DC link current, (e): Inverter current, (f): Generate an gri active power, (g): Generate an gri reactive power 3 Moifie System Configuration using Magnetic Amplifier (c) () (e) Magnetic amplifiers have een use in applications such as instrumentation, relays, position servo mechanism, an automatic attery chargers owing to their high efficiency, reliaility an ruggeness [1719]. As explaine in [9], magnetic amplifiers have the aility to control a large current signal (either AC or DC) flowing in its main wining through a small DC signal in the control wining y changing the magnetization characteristics of the magnetic core [20, 21]. In the first author s attempt, magnetic amplifiers were use in the configuration foun in figure 2 as a metho of series compensation when inserte etween the PMSG an the ioe rectifier (AC sie configuration). In this configuration, each phase is split into two each having two magnetically couple winings an a power ioe. All six cores are eing linke y a single iase DC controlle circuitry. During positive half cycle of the AC supply, the forwar iase coils conuct thus transferring power to the loa. Splitting each phase always prouce an MMF in the DC circuit that are always cancelle out. During normal gri operation (no voltage ips), if the DC control current is such that it correspons to the saturate part of the BH magnetization curve, then the mutual inuctance etween the AC an DC coil will e small causing a small voltage rop across the magnetic amplifier. On the other han if a gri voltage ip occurre, the DC current

4 ' ' ' will cause the magnetic amplifier to operate in the linear part of the BH curve, then the magnetic amplifier will experience large voltage rop on its terminals limiting the DC link voltage rise. Results for the use of magnetic amplifier using the AC sie configuration can e foun in figure 4 (a) to (f) which proves that magnetic amplifier coul e use in LVRT capaility enhancement. The use of six magnetic elements an six power ioes ae cost an size to the WECS which might e consiere as one of the rawacks of this approach. D a () D a i s Rectifier Boost Converter VSI Gri PMSG D Cf L f C D D c D c Magnetic Amplifier I c Lookup Tale i Pulse i i = 0 v i v PW M to θ v θ PLL to i a i to i c v a v c Figure 3: WECS with magnetic amplifier in AC Sie (e) (a) (f) Figure 4: WECS ehavior to 90% voltage ip with magnetic amplifier in the AC sie. (a): Electromagnetic torue, () DC link voltage, (c): DC link current, (): Inverter voltage, (e): Generate an gri active power, (f): Generate an gri reactive power (c) () Moifications have een mae to the topology as to minimize the numer of magnetic elements as much as possile yet still serving the same purpose for DC link voltage limit. The six magnetic amplifiers an six ioes where replace y a single magnetic amplifier circuit ae in the oost converter (DC sie) as seen in figure 5. The principle of operation is the same as the AC sie, when a fault occurs, the DC control current changes the operation of the magnetic amplifier from the saturate to the unsaturate state thus exhiiting larger voltage rop on its terminals. Simulation results for the DC sie magnetic amplifier can e foun in figure 6 (a) to (f). Comparing the simulation results of magnetic amplifier in the AC sie with the ones at the DC sie proves the effectiveness of the DC sie compensation. DC link voltage is eing controlle an the stresses on the power converter semiconuctor switches are eing limite as seen in figure 5

5 an figure 6. The DC voltage oscillations present in the AC sie configuration are eing eliminate in the DC sie one. Besies, magnetic amplifier aition in the DC link has eliminate the oscillations in the DC link current, which in turn will e reflecte on the DC link power. As for the PMSG electromagnetic torue oscillations, it is oserve that the PMSG exhiits stale operation with no effect on the electromagnetic torue at severe voltage ip. PMSG Rectifier Magnetic Amplifier C f Boost Converter C VSI Gri () Lookup Tale i Pulse i = 0 i i v v PWM to i θ v to θ i a i c PLL to Figure 5: WECS with magnetic amplifier in DC Sie v a v c (e) (a) (f) Figure 6: WECS ehavior to 90% voltage ip with magnetic amplifier in the DC sie. (a): Electromagnetic torue, () DC link voltage, (c): DC link current, (): Inverter voltage, (e): Generate an gri active power, (f): Generate an gri reactive power 4 Conclusion () (c) LVRT capaility enhancement is consiere one of the most important issues for gri connecting win turines to electrical gris. The prolem is emonstrate with special focus on the effects on PMSG win turine systems, such as DClink voltage rise as well as stresses on power electronic converters. Magnetic amplifiers have een suggeste y authors in previous work to enhance the riethrough capaility an this paper suggests moifications to the configuration reucing its size an overall cost. Simulation results are presente comparing oth topologies an valiating the iea.

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