A Novel Fuzzy Control Approach for Modified C- Dump Converter Based BLDC Machine Used In Flywheel Energy Storage System

Transcription

1 A Novel Fuzzy Control Approach for Modified C- Dump Converter Based BLDC Machine Used In Flywheel Energy Storage System B.CHARAN KUMAR 1, K.SHANKER 2 1 P.G. scholar, Dept of EEE, St. MARTIN S ENGG. college, Hyderabad 2 Assistant Professor, Dept of EEE, St. MARTIN S ENGG. college, Hyderabad ABSTRACT-This paper presents a novel fuzzy control approach for modified C-dump converter based Brushless DC (BLDC) machine used in the flywheel energy storage system. The converter can realize the energy bidirectional flowing and has the capability to recover the energy extracted from the turnoff phase of the BLDC machine. The inputs to the fuzzy logic controller are the linguistic variables of speed error and change of speed error, while the output is change in switching control frequency of the voltage source inverter.the principle of operation, modeling, and control strategy of the system has been investigated in this paper and the simulation results of the proposed system are also presented and discussed. I.INTRODUCTION Brushless DC motors have been used in various industrial and domestic applications. Due to overweighing merits of this motor, there is continuing trend to propose improved control schemes to enhance the performance of the motor. For analysis of the BLDC motor drives system under various conditions [3]. The permanent magnet brushless DC machine (BLDCM) is one of the suitable motors for the FESS [3]. The flywheel energy storage system (FESS) is an attractive option for temporary energy storage in high power utility applications and hybrid electric systems [1,2]. The common half-bridge topology for high-speed BLDCM is shown in Fig. 1. It includes a buck chopper and a half-bridge converter. Compared with the full-bridge converter, the half bridge converter has half the number of switches and avoids the short circuit across the phase leg in the full-bridge converter. However, this halfbridge topology has two disadvantages for the FESS. 1) The energy unidirectional flow, and 2) the energy of the turnoff phase are consumed on the resistance which means the waste of energy. In order to overcome these drawbacks, a FLC based modified C-dump converter for highspeed BLDCM used in the FESS is presented in this paper. The principle of operation and the analysis of the proposed converter are developed. II.PRINCIPLE AND DESCRIPTION OF THE DRIVE SYSTEM Fig. 2 shows the modified C-dump converter for BLDCM used in the FESS. The proposed converter includes a half-bridge converter (switches T a, T b, T c ), an energy recovery chopper (switch T r ; diodes D 1, D 2, D 3, D r ; inductance L r and capacitor C 0 ), a bidirectional DC DC converter (switches T1, T 2 ; inductance L 1 and capacitors C 3 ), and a DC filter (inductance L 1 and capacitors C 1, C 2 ). U 1 stands for the source and R 1 stands for the load. The modified converter has two working modes: the FESS charging mode and the FESS discharging mode. In the FESS charging mode, the source supplies energy to the flywheel, therefore S 1 is on and S 2 is off. In this mode, the half-bridge converter works in the motor operation. T a, T b and T c are operated with the duration of 120 electrical degrees. Fig.1. Common half-bridge topology for high-speed BLDCM. Fig. 2. Modified C-dump converter for the FESS. IJPRES 102

2 flywheel decreasing, the output voltage drops. In order to keep the output voltage stable, the bidirectional DC DC converter works in boost operation mode (T 2 works in PWM operation mode and T 1 is off). Fig. 4 illustrates the modified converter for the FESS working in the discharging mode. III.MODELING AND CONTROL STRATEGY The modeling and analysis of the proposed converter are presented in this part. Fig. 3. Modified converter working in the charging mode. Works in the pulse width modulation (PWM) operation mode and recovers the energy of the turnoff phase to the source [4]. The bidirectional DC DC converter works in buck operation mode (T 1 works in PWM operation mode and T 2 is off.) to control the motor speed. Fig. 3 illustrates the modified converter for the FESS working in the charging mode. Fig.4. Modified converter working in the discharging mode. A. Dynamic Model Four distinct modes of operation can be identified for the proposed converter in the charging mode. The equivalent circuits of the converter in its switching operation are shown in Fig. 5. The voltage drop of the switch and the diode, the resistance of the inductance, and the mutual inductance of the motor phases are ignored. T s considers as T a, or T b, or T c V dc is the bus voltage (voltage of the capacitor C 3 ), e s is the back-electromotive force (back-emf) of the motor, R s is the motor phase resistance, L s is the motor phase inductance, i s is the motor phase current, V c0 is the capacitor C 0 voltage, V in is the source input voltage (voltage of the capacitor C 1 ), L r is the energy recovery circuit inductance, is the current of the energy recovery inductance, L r and K 0 is the buck factor. 1) T s on, T r on + e + R i (1) V = V L = i (3) C V = k V (4) 2) T s on, T r off (2) Fig.5. The equivalent circuits of the converter in its switching operation.(a)t s on, T r on; (b) T s on, T r off; (c) T s off, T r on; (d) T s off, T r off. In the FESS discharging mode, the BLDCM (with flywheel) acts as a generator to discharge the kinetic energy of the flywheel into the load, therefore is off and is on. In this mode, the half-bridge converter acts as a diode rectifier to convert the highfrequency AC to the DC. T a, T b, T c, T r are all off and D a, D b, D c form a diode rectifier. With the speed of + e + R i (5) V = L ( if i > 0) (6) V = Constant (7) V = k V (8) 3) T s off, T r on + e + R i + V (9) V = V L (10) C = i i (11) V = k V (12) IJPRES 103

3 WhereV c0 is the voltage variation of the capacitor C 0. The voltage V c0 should be higher than V dc +e s. TABLE I RATINGS AND PARAMETERS OF BLDCM Rated voltage (V) 100 Rated stator current(a) 20 Rated power (kw) 2 Frequency (Hz) 5333 Phase inductance (mh) 0.06 Phase resistance (Ω) 0.1 Fig.6. Control structure of the converter in the charging mode. 4) T s off, T r off + e + R i + V (13) V = L C = i (15) V = k V (16) ( if i > 0) (14) B. Design of the Main Parameter The main parameters of the proposed converter are derived as follows. 1) Energy Extracted from the Turnoff Phase: The system works in steady state and the switching loss is ignored. The energy extracted from the turnoff phase can be described as W = L i (17) Where W LS is the energy extracted from the turnoff phase. i smax is the motor phase current in commutation moment; it can be obtained from (1). The power extracted from the turnoff phase is P = = 3 L i = L i Where n is the speed of the motor and is the pairs of poles. (18) 2) Energy recovery capacitor: The energy extracted from the turnoff phase is delivered to the energy recovery capacitor. Therefore W = L i C = ( Δ ) = C [(V + ΔV ) V ] (19) (20) 3) Energy Recovery Inductance: According to energy conservation, the energy recovered to source can be described as 1 2 L i = 1 2 C [(U + ΔU ) U ] = L i L i (21) Where i rmax (i rmin ) is the maximum (minimum) current of the inductance L r. In order to keep the energy recovery fast, the L r should not be too large. Therefore, it is better for the L r to working discontinuous conduction mode i MIN = 0 L i = L i (22) L = (23) C. Control Strategy The control structure of the modified converter working in the charging mode is shown in Fig. 6. It includes the motor speed control and the recovery capacitor voltage control. The motor speed control includes double loops: the inner current loop and the outer speed loop. The commutation of phases is decided based on the output of three Hall Effect sensors. The motor phases are protected against over current. The proportional integral (PI) control combined with the hysteresis control is used in capacitor C 0 voltage control. It is recommended for the converter due to its small voltage fluctuation of the energy recovery capacitor and current ripple of the motor. The disadvantage of PI controller is its inability to react to abrupt changes in the error signal, ε, because it is only capable of determining the IJPRES 104

4 instantaneous value of the error signal without considering the change of the rise and fall of the error, which in mathematical terms is the derivative of the error denoted as Δε. To solve this problem by we are using Fuzzy logic controller. D.Fuzzy logic controller: FLC contains three basic parts: Fuzzification, Base rule, and Defuzzification [5&6]. FLC has two inputs which are: error and the change in error, and one output. The Fuzzy Controller structure is represented in fig.7. The role of each block is the following: Fig 9. Membership function of voltage error Fig 7: The general structure of Fuzzy Logic Controller Fuzzifier converts a numerical variable into a linguistic label.. In a closed loop control system, the error (e) between the reference voltage and the output voltage and the rate of change of error (del e) can be labeled as zero (ZE), positive small (PS), negative small (NS), etc. In the real world, measured quantities are real numbers (crisp). The FLC takes two inputs, i.e., the error and the rate of change of error. Based on these inputs, The FLC takes an intelligent decision on the amount of field voltage to be applied which is taken as the output and applied directly to the field winding of generator. Triangular membership functions were used for the controller. Fig 10. Membership function of output field voltage Rule base stores the data that defines the input and the output fuzzy sets, as well as the fuzzy rules that describe the control strategy. Mamdani method is used in this paper. Seven membership functions were used leading to 49 rules in the rule base. Inference engine applies the fuzzy rules to the input fuzzy variables to obtain the output values. Defuzzifier achieves output signals based on the output fuzzy sets obtained as the result of fuzzy reasoning. Centroid defuzzifier is used here. Table II: Rule base for fuzzy controller NB NM NS ZE PS PM PB NB PB PB PM PM PS PS ZE NM PB PM PM PS PS ZE NS NS PM PM PS PS ZE NS NS ZE PM PS PS ZE NS NS NM PS PS PS ZE NS NS NM NM Fig 8. Membership function of voltage PM PS ZE NS NS NM NM NB PB ZE NS NS NM NM NB NB IJPRES 105

5 Fig. 11. Simulation diagram of proposed system Fig.12. Simulation results of the converter working in the charging mode. (a) Recovery current i r. (b) Voltage of the capacitor C 0. (c) Voltage between the drain and the source of the MOSFET (phase A). (d) Current of the phase A. IV.SIMULATION RESULTS To verify the performance of the proposed converter, simulation results have been performed. The ratings and parameters of the BLDCM are presented in Table I. Parameters of the converter are shown in Table III. TABLE III PARAMETERS OF THE CONVERTER Capacitor Value(μF) Inductance Value(mH) C 33 L 0.24 C 470 L 0.2 C 470 L 0.4 C 1000 Fig.12 shows the simulation results of the proposed converter working in the charging mode. At this range, the peak value of the phase current i is about 21 A, as shown in Fig. 12(d). Fig. 12(a) shows the recovery current i which is limited to a peak value of 9 A. Fig. 12(b) shows the voltage of energy recovery capacitorc which stays around 210 V, and increases to 213 V during commutation when the capacitor starts to discharge into the source. Fig. 12(c) shows the voltage (V ) between the drain and the source of the metal oxide semiconductor fieldeffect transistor (MOSFET), which equals to the phase terminal voltage plus the bus voltage (V ). Fig. 13. Simulation results of the converter working in the discharging mode. (a) Current of inductance L 2. (b) Output voltage of the converter. (c) Output voltage of the BLDCM rectified by the diode rectifier. Fig. 13 shows the simulation results of the proposed converter working in the discharging mode. Fig.13 (a) shows the current of inductancel.fig. 13(b) shows the output voltage of the converter (voltage of the capacitorc ) which stays around 100 V when the flywheel speed decreases. Fig. 13(c) shows the output voltage of the BLDCM rectified by the diode rectifier (voltage of the capacitor C ). V.CONCLUSION This paper has presented a modified C-dump converter for BLDCM used in the FESS based on FLC. The proposed converter can realize the bidirectional energy flowing and has the capability to recover the energy extracted from the turnoff phase which is useful for the motor driver system especially for the FESS. The ability of fuzzy logic to handle rough and unpredictable real world data made it suitable for a wide variety of applications, especially. IJPRES 106

6 The principle of operation, modeling, and control strategy of the system has been presented. Simulation results are validating the theoretical results and demonstrate the good performance of the converter. The study indicates that the converter is suitable for the FESS applications. REFERENCES [1] R. S. Weissbach, G. G. Karady, and R. G. Farmer, Dynamic voltagecompensation on distribution feeders using flywheel energy storage, IEEE Trans. Power Delivery, vol. 14, no. 2, pp , Apr [2] M. M. Flynn, P. Mcmullen, and O. Solis, Saving energy using flywheels, IEEE Ind. Appl. Mag., vol. 14, no. 6, pp , Nov./Dec [3] C. W. Lu, Torque controller for brushless DC motors, IEEE Trans.Ind. Electron., vol. 46, no. 2, pp , Apr [4] R. Krishnan and S. Lee, PM Brushless dc motor drive with a newpower converter topology, IEEE Trans. Ind. Appl., vol. 33, pp , July/Aug [5].M.Bhanu Siva, M.R.P Reddy, Ch.Rambabu "Power Quality Improvement of Three-phase fourwire DSTATCOM with Fuzzy logic Controller" ICSIT.,vol.2,pp ,2011. [6].Jain S. K., Agrawal P. and Gupta H. O., Fuzzy logic controlled shunt active power filter for power quality improvement, IEE Proc. Electr. Power Appl., vol. 149, no. 5, pp , IJPRES 107