Frequency Control of Isolated Power System with Wind Farm by Using Flywheel Energy Storage System

Transcription

1 Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem 4 Rion Takahahi Kitami Intitute of Technology Japan. Introduction For the recent expanion of renewable energy application, wind energy generation i receiving much interet all over the world. Many large wind farm have been intalled o far and recently huge offhore wind farm have alo been intalled. However, the frequency variation of power ytem due to wind generator output fluctuation i a eriou problem. If intallation of wind farm continue to increae, frequency control of power ytem by the main ource, that i, hydraulic and thermal power tation, will be difficult in the near future, epecially in an iolated power ytem like a mall iland which ha weak capability of power regulation. In uch a cae, the intallation may be retricted even though it i a mall wind farm. Though there i uch a difficulty, an introduction of the wind energy utilization i much effective in an iolated power ytem, becaue main power plant in a mall iland i motly a dieel engine driven generating plant and it ha no good effect on the environment. Hence, ome trategie are neceary to improve the tability of wind farm output. According to uch ituation, an application of battery ytem for the output power moothing ha been invetigated o far, and ome experimental tudie uing practical facilitie are being performed. The battery ytem i uitable for power compenation with relatively long period like load leveling. However, ince rapid repone i neceary to compenate power variation in an iolated power ytem, the battery ytem may not be appropriate becaue charging or dicharging peed of the battery i not o fat due to it chemical proce. Moreover, the ame capacity of electronic power converter a that of the battery power rating i required. In addition life time of battery i, in general, not o long and thu frequent replacement of battery cell will be needed. Thee characteritic caue cot increae. On the other hand, the application of Flywheel Energy Storage Sytem (called 'FESS' hereinafter) for power compenation i very effective. Thi ytem ha characteritic of large energy torage capacity, long life, and rapid repone of power control. It ha a heavy weight rotating ma connected to an adjutable peed generator. Thi chapter adopt an adjutable peed generator with econdary AC excitation a a driving machine of rotating ma, becaue thi type of generator ha already been put into practice in pumped torage hydro power plant in Japan []. There are alo ome practical application of FESS to improve power ytem tability [2]. The adjutable peed generator with econdary AC excitation can control not only active power output but alo reactive power output rapidly

2 66 From Turbine to Wind Farm Technical Requirement and SpinOff Product and independently. Thu moothing of both output power and grid voltage fluctuation in wind farm i poible by intalling FESS with the adjutable peed generator. In addition, ince only mall capacity of electronic power converter i needed in thi ytem, the total cot can be decreaed. Therefore, the FESS can be effective on moothing of wind farm output fluctuation, reulting in the frequency tabilization of the power ytem. With thee point a background, thi chapter propoe a control trategy of FESS to reduce the frequency variation in an iolated power ytem including a wind farm. The main feature are a follow: ) Cooperation with the main power plant, i.e., output of the main power plant i adjuted in cooperation with the FESS depending on it energy charge level; 2) Direct frequency control. In the cae of large power ytem, generally, moothing of rapid change of wind farm output in hort period i performed by energy torage ytem, while low change in long term i aborbed by other power plant for frequency control. However in the iolated power ytem, ingle or a few main ource generator can hardly regulate low power fluctuation. Therefore direct frequency control by energy torage ytem i deirable. In order to evaluate the effectivene of the propoed method, computer imulation analye are performed by uing PSCAD/EMTDC [3]. 2. Example of model ytem Overview of FESS operation Fig. how an overview of FESS operation propoed in thi chapter. The iolated power ytem conit of main power upply, a conumer load and a wind farm. FESS i intalled near the wind farm. FESS detect the network frequency and tabilize it by upplying or aborbing active power to/from the network. FESS alo end a command to the main power upply to adjut it output o a to keep uitable tored energy level of FESS. Wind farm Load (Conumer) Frequency detection Iolated power ytem FESS Fig.. Overview of FESS operation Brief configuration of power ytem Power compenation Extend governing Main power upply Fig. 2 how the power ytem model ued in thi chapter. A Wind Farm (WF) i modeled by a ingle induction generator with a wind turbine operating almot at contant peed. The FESS i intalled to the grid point of wind farm. A Synchronou Generator (SG) a a main

3 Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem 67 ource generator which i driven by a dieel engine i connected to the grid point through a tranmiion line, and reitive load are connected to the both end of the line. WF Dieel Power Plant Induction Generator 0MVA, 0.69kV, H=.5 FESS Grid connection 0.05 j0.3 (30MVA bae) Synchronou Generator 30MVA, 6.6kV, H=2.5 N S 6.6kV ACDCAC DoublyFed Induction Machine 7MVA, 6.6kV, 422.5MJ(max) L 3.5MW Reitive Load L 5MW Fig. 2. Model ytem of an iolated power ytem Configuration of FESS Fig. 3 how a model configuration of FESS. The FESS conit of the adjutable peed generator, the flywheel ma for kinetic energy torage, and econdary excitation circuit for adjutable peed control [4]. The adjutable peed generator ha baically the ame contruction a that of a wound rotor induction machine. The econdary excitation power i upplied from the terminal of FESS, and converted to DC power by the converter, then again converted to low frequency AC power by the inverter and upplied to the rotor. Thu, the rotor can rotate at aynchronou peed. The inverter control active and reactive power output (P T and Q T ) of the generator, and the converter control DC link voltage E DC and reactive power Q L flowing into the econdary excitation circuit. Thee electronic power converter are modeled a 6 forcecommutated power witche connected in a bridge configuration a hown in Fig. 4. A inuoidal PWM operation i carried out and witching ignal are generated by applying triangular carrier wave comparion. Conventional PI controller are ued for the inverter and the converter control a hown in Fig. 5 and 6 repectively. Parameter of the FESS generator are hown in Table II. A method of frequency tabilization by uing FESS The main purpoe of thi tudy i to reduce the network frequency variation by uing FESS. The configuration of the control ytem for the frequency tabilization i hown in Fig. 7. Reference of active power output of FESS, P T(ref), i determined according to the deviation of network frequency, which i detected by PLL at the terminal of FESS. When the frequency i decreaed, FESS upplie active power to the network. When the frequency i increaed, FESS aborb active power from the network. Thee control cheme correpond to block (A) in Fig. 7. At the ame time, P T(ref) i modified to prevent a hortage or an exce of the

4 68 From Turbine to Wind Farm Technical Requirement and SpinOff Product DoublyFed Induction Machine (7MVA, 6.6kV, H=50.0) Rotor current I 2D, I 2Q abcdq Active power P T Grid voltage V T Inverter DC AC abcdq 4.0kV V 2D, V 2Q Inverter Controller DC link capacitor * DC voltage E DC P T(ref) Network frequency F Reference Signal Regulator Converte AC DC abcdq (A) 2.2kV / 6.6kV j0.08pu 0.005j0.pu 30% capacity of the ytem Converter Controller V CD, V CQ (B) abcdq Line current I CD, I CQ Reactive power Q L Rotor peed W R_FESS Fig. 3. FESS circuit configuration * : Stored energy (J) i the rated power of the machine (W) 0.02 (). tored energy of FESS. In thi tudy, the maximum and the minimum rotor peed of FESS are pecified 30% (.3pu) and 70% (0.7pu) of the rated peed repectively. Conidering thee boundary peed, the value of P T(ref) i modified to a lower (or a higher) value when the rotor peed i under (or over).044pu, at which the tored energy become a half of the maximum torage energy. Thee control cheme correpond to block (B) in Fig. 7. Fig. 7 alo include a rule of FESS control to avoid operating under 0.7pu or over.3pu rotor peed a hown in Table I. a b c Fig. 4. Model of power converter Frequency < 50 Frequency > 50 W R_FESS > > W R_FESS > > W R_FESS 0 Table I. Rule of FESS control

5 Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem 69 I 2D I 2Q P T V T V T(ref) W R_FESS F Fig. 5. Output power controller of FESS P T(ref) Reference Signal Regulator Phae compenator V 2D V 2Q I CD I CQ E DC E DC(ref) Q L Q L(ref) Phae compenator V CD V CQ Fig. 6. Excitation power controller of FESS (A) Bae frequency 50.0 derivative dead band (0.05 to 0.05) F filter P T(prim) Half of Storage Energy W R_FESS (B) P T(ref) Table I 0 or Fig. 7. Reference ignal controller for frequency tabilization

6 70 From Turbine to Wind Farm Technical Requirement and SpinOff Product Wind farm model The wind farm conit of an induction generator and a wind turbine. An aerodynamic characteritic of the turbine blade expreed by eq.( 2) and ( 3) i adopted [5]. The captured power i expreed by eq.( ). Since the induction generator i operated at almot contant peed (approx..0 to.0 pu), the output power change widely with repect to wind peed variation. Generally, a wind turbine i equipped with a pitch angle controller. The conventional pitch controller hown in Fig. 8, that maintain the output of the generator to be the rated power when the wind peed i over the rated peed, i alo conidered in thi tudy. Parameter of the wind generator (IG) are hown in Table II. p 2 3 PM = ρcp( λ) πr VW[ W] () Γ C ( λ) = 0.5( Γ 0.0.2β 5.6) e (2) R 3600 Γ= (3) λ 609 Active Power PI controller Fig. 8. Pitch angle controller of wind turbine 5 Pitch actuator Rate limiter (Max ±0/ec) Pitch Angle β IG FESS Stator reitance (pu) Stator leakage reactance (pu) Magnetizing reactance (pu) Rotor reitance (pu) Rotor leakage reactance (pu) Table II. Parameter of induction machine. Synchronou generator model A Synchronou Generator (SG) i conidered a a main power upply unit in the network in thi tudy, which i aumed to be a dieel engine driven power plant. The characteritic of the dieel engine and it governor ytem in [6] are conidered. The governor control fuel upply to maintain the engine peed at the ynchronou peed. It block diagram i hown in Fig. 9, and it parameter are hown in Table III.

7 Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem 7 If FESS regulate the network frequency by it power compenation, the output of SG may not change, becaue the network frequency i controlled to be contant. Conequently, there i a poibility that FESS perform all of the network frequency control intead of SG. In uch cae, when the tored energy in FESS become full or empty, the power balance of the network cannot be maintained and thu the network frequency can deviate ignificantly. To avoid uch ituation, the output of SG alo need to be regulated according to the tored energy of FESS. In thi chapter, a cooperative control i propoed, in which the output of SG i increaed (or decreaed) when the rotor peed of FESS i below (or over).044pu which correpond to a half of the maximum torage energy of FESS. But if the additional command to the main ource generator change fat, it output will alo vary widely, and then it uffer large mechanical tre. Therefore a control gain i et for the additional command to change lowly a hown in Fig. 0. The governor of SG in thi tudy ha been deigned to control only engine peed, and thu the output of SG can be changed by modifying a monitored ignal of the engine peed to the governor. Thee control ytem are hown in Fig. 0. In addition, a imple AVR model hown in Fig. i ued in SG model. Parameter of the ynchronou generator (SG) are hown in Table IV. w ref (Synchronou peed) Controller K P K I Actuator T A Dead time of Engine e T w (Rotor peed) w ex (Additional ignal for output adjutment) Output torque Fig. 9. Governor model of the dieel engine.044 W R_FESS w ex To SG governor Fig. 0. Additional ignal controller for output adjutment of the dieel engine

8 72 From Turbine to Wind Farm Technical Requirement and SpinOff Product Terminal Voltage filter 0. V T0 (Reference) regulator 0 0. E fd0 (Initial value) 5 5 Field voltage E fd Fig.. AVR model of the ynchronou generator Proportional Gain of K P 8.0 Integral Gain K I 2.0 Pilot ervo time contant T A Dead time of engine T Table III. Parameter of the dieel engine governor Armature reitance (pu) Stator leakage reactance (pu) 0.4 Field reitance (pu) Field leakage reactance (pu) 0.2 Daxi Qaxi Magnetizing reactance (pu) Damper reitance (pu) Damper leakage reactance (pu) Table IV. Parameter of ynchronou generator.

9 Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem Simulation example A. Condition A determination of the energy torage capacity i very important for deigning energy torage ytem. In thi chapter, the energy torage capacity of FESS i determined from a point of view of adequate frequency control ability but reducing it a mall a poible. The power rating of FESS i decided a 70% of that of the wind farm ince intantaneou output change of the wind farm can hardly reach it power rating in normal operation. Comparative tudy between the propoed frequency control method (hown in Fig. 7 and Table I) and a power moothing method (hown in Fig. 2 and Table V) which i generally conidered in a wind farm connected to large power ytem, i performed in the imulation analyi here. In conventional power moothing method, an energy torage ytem only moothe wind farm output fluctuation, and low change of wind farm output i aborbed by everal thermal and hydraulic power plant intalled a main generator in large power ytem. However, ince the total power rating and the number of main power generator are limited in the cae of an iolated power ytem, power regulation may become difficult even when wind farm output fluctuation i mall. Moreover, the output of main power generator hould be adjuted alo to maintain the amount of reidual energy of torage ytem. If the tored energy i not regulated uitably, power balance of the iolated power ytem cannot be kept when the tored energy reache full or empty level. Therefore, it can be aid that the frequency tabilization in the cae of an iolated power ytem cannot be achieved only by the conventional power moothing cheme. Output of wind generator P T(prim) T D Low Pa Filter (order delay) Reference of FESS output power Fig. 2. Reference ignal regulator of the FESS for power moothing control P ref < 0 P ref > 0 W R_FESS > > W R_FESS > > W R_FESS 0 Table V. Rule of FESS control for power moothing

10 74 From Turbine to Wind Farm Technical Requirement and SpinOff Product 6 Wind peed (m/) Time () Frequency of the power ytem (Hz) Wind generator output (pu) Wind farm output (pu) Power output of the FESS (pu) Stored energy of the FESS (MJ) (Min:22.5, Mid:250, Max:422.5) Mechanichal torque of the dieel engine (pu) Time () Wind generator output Wind farm output (frequency control method).0 wind farm output (power moothing method) Power moothing method Frequency control method Time () Power moothing method Frequency control method Time () Power moothing method Frequency control method Time () Power moothing method Frequency control method Time () Fig. 3. Sytem repone under the frequency control method and power moothing method

11 Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem 75 In the comparative tudy, imulation by the conventional output power moothing method have alo been performed, in which the reference output from the WF to the grid i determined by inputting the net WF output into a firt order delay tranfer function and FESS upply the difference between the reference power and the net output to follow the WFtogrid output to the reference value a hown in Fig. 2. Therefore block A in Fig. 7 i replaced by Fig. 2 and the rule of FESS control hown in Table I i alo replaced by Table V. In the conventional method, the cooperation control with the main power plant i impoible. The power rating of FESS i choen to be 7MVA, ame a that in the frequency control method, and the time contant of the firt order delay i et to 30. B. Reult Fig. 3 how ytem repone under the propoed frequency control method and the conventional power moothing method. The frequency deviation reache about 0.3Hz at the maximum in the cae of the power moothing method, but it i regulated within about 0.05Hz in the cae of the frequency control method. The tored energy of FESS i remained well between the maximum and the minimum level, from which there may be a poibility that the energy torage capacity of FESS can be reduced. Repone of the prime mover output (dieel engine output) are almot the ame in both method. 4. Summary Thi chapter ha propoed a new method of network frequency regulation by uing Flywheel Energy Storage Sytem (FESS) for an iolated power ytem including a wind farm, and the validity of the propoed method ha been evaluated by computer imulation. From the comparative tudy between the propoed method and the conventional output moothing control of wind farm, it ha been hown that the propoed method i very effective on the tabilization of network frequency in an iolated mall power ytem. The propoed method can be applied baically not only to a FESS ytem but alo other type of energy torage ytem. Therefore the propoed method can contribute to expand wind energy utilization into iolated power ytem like a mall iland. 5. Reference [] T. Kuwabara, A. Shibuya, H. Furuta, E. Kita, and K. Mituhahi : "Deign and Dynamic Repone Characteritic of 400 MW Adjutable Speed Pumped Storage Unit for Ohkawachi Power Station," IEEE Tranaction on Energy Converion, Vol., No. 2, pp , June 996. [2] M. Kazuma, U. Yuuetu : "Hydroelectric Power Technologie Contributing to Power Sytem Quality Improvement", TOSHIBA REVIEW, VOL.58, NO.7, [3] Manitoba HVDC Reearch Centre ( [4] R.Takahahi, J.Tamura, Y.Tada, A.Kurita: "Model Derivation of an Adjutable Speed Generator and It Excitation Control Sytem", Proc. of 4th Power Sytem Computation Conference, Seion06, paper4, June [5] O. Waynczuk, D. T. Man, J. P. Sullivan : "Dynamic Behavior of a Cla of Wind Turbine Generator During Random Wind Fluctuation", Tran. of IEEE on Power Apparatu and Sytem, Vol. PAS00, No.6, pp , June 98.

12 76 From Turbine to Wind Farm Technical Requirement and SpinOff Product [6] Sanjoy Roy, O.P.Malik, G.S.Hope : "A kstep Predictive Scheme for Speed Control of Dieel Driven Power Plant", IEEE Tranaction on Indutry Application, Vol. 29, No. 2, pp , March/April 993.

13 Off Product Edited by Dr. Geche Kraue ISBN Hard cover, 28 page Publiher InTech Publihed online 04, April, 20 Publihed in print edition April, 20 Thi book i a timely compilation of the different apect of wind energy power ytem. It combine everal cientific dicipline to cover the multidimenional apect of thi yet young emerging reearch field. It bring together finding from natural and ocial cience and epecially from the extenive field of numerical modelling. How to reference In order to correctly reference thi cholarly work, feel free to copy and pate the following: From Turbine to Wind Farm Technical Requirement and Spin Rion Takahahi (20). Frequency Control of Iolated Power Sytem with Wind Farm by Uing Flywheel Energy Storage Sytem, From Turbine to Wind Farm Technical Requirement and SpinOff Product, Dr. Geche Kraue (Ed.), ISBN: , InTech, Available from: InTech Europe Univerity Campu STeP Ri Slavka Krautzeka 83/A 5000 Rijeka, Croatia Phone: 385 (5) Fax: 385 (5) InTech China Unit 405, Office Block, Hotel Equatorial Shanghai No.65, Yan An Road (Wet), Shanghai, , China Phone: Fax: