Battery Based Voltage And Frequency Controller For Parallel Operated Isolated A Synchronous Generators

Battery Based Voltage and Frequency Controller for Parallel Operated Isolated Asynchronous Generators Bhim Singh, Senior Member IEEE and Gaurav Kasal

Ambrish Chandra, Senior Member IEEE and Kamal-Al-Haddad, Fellow IEEE

Department of Electrical Engineering Indian Institute of Technology, Delhi New Delhi-110016, India Email: [email protected] [email protected]

Ếcole de Technologie Superieure, Universite du Quebec, Montreal, QC H3C 1K3, Canada Email: [email protected] [email protected]

Abstract— This paper deals with a battery energy storage system (BESS) based voltage and frequency (VF) controller for parallel operated isolated asynchronous generators (IAGs) driven by uncontrolled pico hydro turbines used in constant power applications. The individual AC capacitor banks are used to generate the rated voltage at no load while additional demand of reactive power for the load and generators is met by the controller. The proposed controller is having capability of harmonic elimination, load balancing and load leveling along with voltage and frequency control. The proposed controller is realized using an IGBT (Insulated Gate Bipolar Transistor) based voltage source converter (VSC) and battery system at its DC link. Since the VSC allows bidirectional flow of active and reactive power; thus it regulates the voltage and frequency. The effectiveness of the proposed controller is demonstrated through the MATLAB based simulations using Simulink and PSB (Power System Block set) toolboxes and different aspects of the controller are studied for parallel operated isolated asynchronous generators. Key-words- Isolated Asynchronous Generator, Battery Energy Storage System and Parallel Operation.

I.

INTRODUCTION

Utilization of renewable energy is becoming important from the viewpoints of environmental conservation and depletion of fossil fuels. In recent years, stand alone asynchronous generators with their low maintenance, low cost and brushless construction [1] have emerged as possible solution for harnessing such type of renewable energy sources like small hydro and wind energy. Because of having such advantages these stand alone or isolated asynchronous generator may be operated in parallel to meet the increased electric load demand and for full utilization of the generated power. Parallel operation of isolated asynchronous generators is required for demand of increased loads and these generators are superior compared to other electric generators because there is no need of synchronization between two asynchronous generators. But after having certain advantages poor voltage and frequency regulation is the major bottleneck in their commercialization, so need of effective voltage and frequency controller without ignoring the power quality measures is required for parallel operated IAGs. A number of attempts have been made in the area of developing voltage and frequency controller for isolated asynchronous generators driven by uncontrolled pico, and micro hydro turbines or wind turbines in constant and variable

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power application [2-5] of isolated asynchronous generators. Research work have also been carried out to investigate the controllers [6], steady state analysis and modeling of parallel operation of isolated asynchronous generator [7-10]. However, an attempt is made here to investigate BESS based voltage and frequency controller for parallel operated IAGs. The proposed controller is using 3-leg VSC with battery at its DC link [11] for supplying the required reactive power, elimination of harmonic, load balancing and load leveling along with regulating the voltage and frequency of the proposed electrical system. II.

SYSTEM CONFIGURATION AND WORKING PRINCIPLE

Fig 1 shows the system configuration of proposed isolated electrical generating system along with its controller. The BESS based VF controller connected at the point of common coupling (PCC) acts as a source of leading or lagging reactive current The proposed controller consists of IGBT based current controlled voltage source converter along with a battery at its DC link. The output of the VSC is connected through the AC filtering inductor to the IAGs terminals. The delta connected individual capacitor banks are used to generate the rated voltage at no load while additional demand of reactive power to regulate the voltage is fulfilled by the controller because of having a capability of bidirectional flow of active and reactive powers of voltage source converter. The basic principle of regulating the frequency by the controller is that it maintains the constant output power at the generators terminal because input power from the turbine is constant so the frequency at the terminals remain constant. If the consumer becomes load is less than generated power then additional power is used to charge the battery, if consumer load becomes higher than generated power, the battery supplies the active power through discharging so the total load at the generators terminals remain constant at all time which in turn maintains the system frequency constant. III. CONTROL STRATEGY Fig. 2 shows the control scheme of BESS based VF controller to provide the single point operation (constant voltage and frequency along with constant excitation capacitor) of IAGs. The control scheme is based on the generation of reference

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Fig. 1. Schematic diagram of BESS based VF controller for parallel operated isolated asynchronous generators.

Fig. 2. Schematic diagram of control scheme for BESS based VF controller.

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source currents (have two components in-phase and quadrature with AC voltage). The in-phase unity amplitude templates (ua, ub and uc) are three-phase sinusoidal functions, which are derived by dividing the AC voltages va, vb and vc by their amplitude Vt. Another set of quadrature unity amplitude templates (wa, wb and wc) is sinusoidal function obtained from in-phase vectors (ua, ub and uc). To regulate AC terminal voltage (Vt), it is sensed and compared with the reference voltage (Vtref). The voltage error is processed in the PI voltage controller. The output of the PI controller for AC voltage control loop decides the amplitude of reactive current (Ismq*) to be generated by the BESS based VFC. Multiplication of quadrature unity amplitude templates (wa, wb and wc) with the output of PI based AC voltage controller (Ismq*) yields the quadrature component of the reference source currents (isaq*, isbq* and iscq*). For a constant power generation and load leveling, the active power component of the source current is fixed at a rated value, which is the amplitude of in-phase component of source current (Ismd*). Multiplication of in-phase unit amplitude templates (ua, ub and uc) with in phase component of source current (Ismd*) yields the in-phase component of the reference source currents (isad*, isbd* and iscd*). The sum of instantaneous quadrature and in-phase components of these currents is the reference source currents (isa*, isb* and isc*), and these are compared with the sensed source currents (isa, isb and isc). These current error signals are amplified and compared using PWM hystresis controller for generating the PWM signals for switching of the devices of CC-VSC. IV. CONTROL ALGORITHM Modeling of the control scheme for BESS based voltage and frequency controller is given as follows: A. In Phase Component of Reference Source Current For the constant power application, IAGs should generate constant active power. For the constant power, in-phase component of reference source currents is set equal to the rated amplitude of active power component of the current which is calculated as: *

Ismd = 2 (Prated)/ ( 3 Vrated) (1) where Prated and Vrated are total rated power of both the generators and rated voltage. The instantaneous line voltages at the IAGs terminals (va, vb and vc) are considered close to sinusoidal and their amplitude is computed as: Vt = {(2/3) (va2 +vb2 +vc2)}1/2 (2) The unity amplitude templates having instantaneous value in phase with instantaneous voltage (va, vb and vc), which are derived as: (3) ua = va/Vt ; ub = vb/Vt ; uc = vc/Vt Instantaneous values of in-phase components of reference source currents are estimated as: i*sad = I*smd ua; i*sbd = I*smd ub; i*scd = I*smd uc (4) B. Quadrature Component of Reference Source Current The AC voltage error Ver at the nth sampling instant is:

(5) Ver(n) = Vtref(n) – Vt(n) where Vtref(n) is an amplitude of the reference AC terminal voltage and Vt(n) is the amplitude of the sensed three-phase AC voltage at the IAGs terminals at nth instant. The output of the PI controller (I*smq(n)) for maintaining constant AC terminal voltage at the nth sampling instant is expressed as: I*smq(n) = I*smq(n-1) + Kpa { Ver(n) – Ver(n-1)} + Kia Ver(n) (6) where Kpa and Kia are the proportional and integral gain constants of the proportional integral (PI) controller (values are given in Appendix). Ver (n) and Ver(n-1) are the voltage errors in nth and (n-1)th instant and I*smq(n-1) is the amplitude of quadrature component of the reference source current at (n-1)th instant. The instantaneous quadrature components of the reference source currents are estimated as: i*saq = I*smq wa; i*sbq = I*smq wb; i*scq = I*smq wc (7) where wa, wb and wc are another set of unit vectors having a phase shift of 90º leading the corresponding unit vectors ua, ub and uc which are given as follows: (8) wa = -ub / √3 + uc / √3 wb = √3 ua / 2 + (ub – uc) / 2√3 (9) wc = -√3 ua / 2 + (ub – uc) / 2√3 (10) C. Reference SourceCcurrents Total reference source currents are sum of in-phase and quadrature components of the reference source currents as: i*sa = i*saq +i*sad (11) i*sb = i*sbq +i*sbd (12) i*sc = i*scq +i*scd (13) D. PWM Current Controller The reference source currents (i*sa, i*sb and i*sc) are compared with the sensed source currents (isa, isb and isc). The current errors are computed as: (14) isaerr = i*sa – isa isberr = i*sb – isb (15) iscerr = i*sc – isc (16) The ON/OFF switching patterns of the gate drive signals to the IGBTs are generated from the PWM hystresis current controller using above value of current errors. V. DESIGN OF THE CONTROLLER The BESS based controller consists of a CC-VSC with the battery at its DC link. In Fig 1, Thevenin equivalent circuit of the battery based model [11] is shown at DC link of controller. The terminal voltage of the equivalent battery is obtained as: Vbat =(2√2/√3)V (17) where V is the line rms voltage. Since the battery is an energy storage unit, its energy is represented in kWh when a battery is used to model the battery unit, the equivalent capacitance can be determined from : C1 = (kWh 3600 * 103)/0.5(vocmax2-vocmin2) (18) In the given Thevenins equivalent model R2 is the equivalent resistance (external + internal) of a parallel/series combination of the battery, which is usually a small value. The parallel circuit of R1 and C1 is used to describe the energy and voltage during charging or discharging. R1 in parallel with C1, represents self discharging of the battery, since the self

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discharging current of a battery is small, the value of resistance R1 is large. From these equations, different parameters of a battery (R1, C1,) are selected and given in Appendix. Here it is considered that the battery is having energy storage of 20 kW for 4 Hrs peaking capacity, and its variation in voltage of 845V-855V. VI. MATLAB BASED SIMULATION Fig 3 shows the MATLAB based simulation model of the parallel operated IAGs along with its controller. A 15kW, 415V, 50Hz, 4-pole and 7.5kW, 415V, 50Hz 4-ploe Yconnected asynchronous machines are used for isolated parallel operation and band of hystresis controller is considered 0.1. The data of saturation characteristics of the machines are obtained from synchronous speed test and given in Appendix, which are used in this model. Simulation is carried out at MATLAB version of 7.1 in discrete mode at 5e-6 step size with ode 23tb (stiff/TR-BDF-2) solver.

A. Performance of Parallel Operated IAGs with BESS Based VFC Feeding Linear Loads Fig. 4 shows the performance of the BESS based VF controller for parallel operated IAG with balanced/unbalance resistive loads. Before application of a consumer load, the battery consumes all generated active power. Delta connected three, single phase load each of 8 kW total of 24kW are applied at 2.5sec, and it is observed that due to high power requirement of consumer load, the battery supplies additional power required by consumer loads. The function of a battery is achieved for the load leveling and a constant power is maintained at generator terminals. At a 2.6 sec when opening of one phase and later on at 2.7 sec opening of second phase of load are performed and the load becomes unbalanced and it results in charging and discharging of the battery, which shows the load balancing aspect of BESS based VF controller system. After removal of the full load at 2.8 sec the battery again starts charging with total generated powers. So in this way the BESS based controller keeps the generated power constant at the generator terminals which in turn regulates the system frequency. B. Performance of Parallel Operated IAGs with BESS Based VFC Feeding Non- linear Loads The performance of the BESS based controller for parallel operated asynchronous generators feeding balanced/unbalanced non-linear loads is demonstrated in Fig. 5. On the application of three phase diode rectifier based nonlinear load at 2.6sec, the battery is discharged for giving additional power, while in steady state of a balanced load the battery starts charging because of low load at the terminal similar to linear load. At 2.8 sec one phase is opened of the load so the load becomes unbalanced while generators currents remain balanced and ripple in the battery voltage is observed which shows the load balancing aspects of the controller. At 2.9 sec the load is removed and from these waveforms it is observed that the battery starts charging and consumes total generated power which is not consumed by the consumer loads. In such a manner it regulates the active power and in turns makes the system frequency constant.

Fig. 3. MATLAB based simulation model of the parallel operated asynchronous generator with BESS based VFC.

VII. RESULTS AND DISCUSSION The performance of the proposed controller for a parallel operated isolated asynchronous generators is demonstrated with balanced/unbalanced linear and non-linear loads. Simulated and transient waveforms of the generator voltage (vabc), generator currents (iabc1 and iabc2), excitation capacitor currents (iccabc1 and iccabc2), consumer load currents (ilabc), controller currents (icabc), battery current (ib) and voltage (vb), terminal voltage (Vt) speed of the both generators (ω1,ω2), system frequency (f) and variation in generators power (Pgen1 and Pgen2), consumer load power (Pload) and a battery power (Pbat) at different dynamic conditions are shown in Figs 4 and 5 for linear and non-linear loads respectively. Parameters of the considered machines are also given in Appendix. Total harmonic distortion (THD) of the generator voltage (va), generators current (ia1, ia2), load current (ilc) are also given in Table I.

TABLE I TOTAL HARMONIC DISTORTION UNDER DIFFERENT NON-LINEAR LOAD CONDITIONS S.N

Condition of Load

1 2

Balanced Non-linear Load Un-Balanced Non-linear Load

% Total Harmonioc Distortion (THD) ia1 ia2 ilc va 0.27 0.73 0.44 23.40 0.29 0.67 0.42 38.02

VIII. CONCLUSION The performance of a battery energy storage system based voltage and frequency controller for parallel operated isolated asynchronous generators has been demonstrated for load leveling, voltage and frequency regulation. The proposed controller is having good capability for harmonic elimination, load balancing, load leveling and voltage and frequency control. MATLAB based simulation results have shown the satisfactory performance for the proposed electrical distribution system feeding linear and non-linear loads.

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Fig. 4. Transient waveforms of Parallel operated isolated asynchronous generators with BESS based VFC feeding linear loads.

IX. APPENDIX A. The parameters of 15kW, 415V, 50Hz, Y-connected, 4-pole asynchronous machine are given below. Rs = 0.58Ω, Rr =0.78Ω, Xlr= Xls= 2.52Ω, J = 0.23kg-m2, Im<2.8 Lm = 0.22 2.814.2 B. The parameters of 7.5kW, 415V, 50Hz, Y-connected, 4-pole induction machine are given below. Rs = 1Ω, Rr = 0.77Ω, Xlr= Xls= 1.5Ω, J = 0.1384 kg-m2 Lm = 0.134 (Im<3.16) Lm = 9e-5Im2 – 0.0087Im + 0.1643 (3.16
Lm = 0.068 (Im>12.72) C. BESS controller parameters Lf = 4mH, Rf = 0.1Ω, and Cdc = 6500µF, R1 = 10k, R2 = 0.1Ω and C1 = 15000F, Kpa = 0.17, Kia= 0.03 D. Consumer Loads Resistive load 8kW single phase loads Non-linear load 24kW with 3000µF capacitor and 3mH inductor at DC end of three phase diode rectifier E. Prime Mover Characteristics for 15kW Machine Tsh = K1-K2 ωr, K1 = 6500, K2 = 40 F. Prime Mover Characteristics for 7.5 kW Machine Tsh = K1-K2 ωr, K1 = 3300, K2 = 10

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Fig. 5. Transient waveforms of Parallel operated isolated asynchronous generators with BESS based VFC feeding non-linear loads.

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