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An Improved Electronic Load Controller for Self-. Excited Induction Generator in Micro-Hydel. Applications. Bhim Singh, S. S. Murthy and Sushma Gupta.
An Improved Electronic Load Controller for SelfExcited Induction Generator in Micro-Hydel Applications Bhim Singh, S. S. Murthy and Sushma Gupta Abstract—This paper describes the mathematical modelling of selfexcited induction generators (SEIGs) with are improved electronic load controller (IELC) for microhydel applications supplying variety of loads. In small hydro plants, governor unit of turbine can be eliminated using IELC, which is simple and cost effective. The improved electronic load controller is a combination of a three-phase insulated gate bipolar transistor (IGBT) based current controlled voltage source inverter (CC-VSI) and a high frequency DC chopper which keeps the generated voltage and frequency constant in spite of change of balanced/unbalanced loads. A dynamic model of the SEIGIELC suppling different types of loads using stationary d-q axes reference frame is developed for predicting the behavior of the system under transient conditions. The simulation is carried out for compensation of balanced/unbalanced loading conditions. The simulated results show that generated frequency and voltage remain constant with change in load. The proposed IELC acts as reactive power compensator, harmonic eliminator, load balancer and load controller. Key Words: Self-excited induction generator, improved electronic load controller, Microhydel, Voltage and Frequency Regulation.

I. INTRODUCTION In hilly and isolated areas plenty of hydro potential is available. These hydro potentials can be used to drive hydro turbine to generate the electricity. However, induction machine can be used as a generator provided its reactive power requirement is fulfilled by capacitor banks, is called self-excited induction generator (SEIG). The SEIG has advantages like simplicity, low cost, rugged, maintenance free, absence of DC, brushless etc. as compared to the conventional synchronous generator. The analysis of the SEIG is complicated because its operation depends on the prime-mover speed, capacitor and load. Capacitance requirement with load and speed for the SEIG is reported in the literature [1-3]. Considerable literature is also reported on the transient analysis of the SEIG under balanced/unbalanced resistive, reactive and motor loads. In the literature [4-6], d-q axes modeling are reported for the transient analysis of SEIG under balanced and unbalanced excitation system. Jain et al. [7] have given a generalized model for the transient analysis of SEIG under symmetrical and unsymmetrical conditions. In hydro plants, a turbine is used with governor to control power generation. In micro hydel application, water is available free of cost then a turbine without governor can be used as prime mover and capacitors are connected across the Bhim Singh (e-mail: bsingh(aee.iitd,ac.in*l. S.S. Murthy (e-mail: ssmurthv(g!ee.iitd.ac.in1 and Sushma Gupta (e-maih sush [email protected]) are with the Department of Electrical Engineering, Indian Institute of Technology, Delhi, Huaz Khas New Delhi-16, INDIA

SEIG according to the rated power and the constant voltage can be maintained by electronic load controller (ELC) [8-14]. Thus electronic load controller (ELC) keeps the load constant on the SEIG under balanced and unity pf load. But in case of unbalanced loads, SEIG currents and voltage are unbalanced and at lagging PF loads SEIG voltage drops down because SEIG and load demands the reactive power, which is not fulfilled by the ELC. Most of the reported electronic load controllers are based on controlled and uncontrolled rectifier with DC chopper, which injects the harmonics in the SEIG. Due to harmonics injection, SEIG is derated and voltage and current of SEIG are non-sinusoidal. In case of unbalanced load, SEIG is further derated due to presence of positive and negative sequence component. The current controlled voltage source inverter with self-supporting DC bus employed as static compensator (STATCOM) can be used for filtering the harmonics and balancing the load. In the reported literature [15-19] STATCOM acts as a voltage regulator to maintain constant voltage for the SEIG. Larsen et al [15] have mentioned the advantage of the STATCOM. Marra and Pomilio [19] have given the VS-PWM bi-directional converter for SEIG, which can regulate the frequency and voltage in case of balanced and linear load. However, there is hardly any attempts on the voltage and frequency regulation under unbalanced and non-linear loads. In this paper, an improved electronic controller (IELC) is presented which is the combination of CC-VSI and DC chopper. The IELC consists of current controlled voltage source inverter, which acts as a voltage regulator, and a DC chopper at DC bus of VSI keeps the rated power on the SEIG. A control technique is developed such that SEIG generates the constant power. In microhydel applications, turbine speed is kept constant and for a constant value excitation capacitor SEIG generates constant voltage, frequency and power, which is known as single point operation. Connecting the capacitor across the SEIG according to the balanced and unity PF power can reduce the rating of the CC-VSI. In this case, load balancing, reactive power compensation and harmonic elimination should be provided for the load by the CC-VSI. A mathematical model is developed for the transient analysis of IELC under the resistive, reactive and nonlinear loads with balanced/unbalanced conditions. The improved electronic load controller acts as a voltage and frequency regulator, harmonic eliminator, and load balancer. II. SYSTEM CONFIGURATION AND CONTROL SCHEME The schematic diagram of SEIG with excitation capacitor, improved electronic load controller ((CC-VSI)+DC chopper),

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consumer load and control scheme is shown in Fig.l. Excitation capacitors are selected to generate the rated voltage of SEJG at no load. The reactive power requirement of SEIG and load is fulfilled by the CC-VSI with self-supporting DC bus. The SEIG generates constant power and when consumer power changes then DC chopper of IELC dumps the difference power (generated - consumed) by consumers in the IELC. Thus, generated voltage and frequency are not affected by the application and removal of the consumer load. The IELC consists of a three-phase IGBT based current controlled voltage source inverter, DC bus capacitor, DC chopper and AC inductors. The output of the inverter is connected through the AC filtering inductor to the SEIG terminals. The DC bus capacitor is used as an energy storage device and provides self-supporting DC bus. DC Chopper is used to control dump power in IELC due to change in the consumer load. The control technique to regulate the terminal voltage, load balancing, and harmonic elimination of the SEIG is based on the controlling of source currents (have two components inphase and quadrature with AC voltage). The in-phase unit vectors (u,, n, and u.) are three-phase sinusoidal functions, computed by dividing the AC voltages v,, vb and vc by their amplitude V,. Another set of quadrature unit vectors (wa, wb and wc) is sinusoidal function obtained from in-phase vectors (ua, ub and u j . To regulate AC terminal voltage (V,) it is sensed and compared with the reference voltage. The voltage error is processed in the PI controller. The output of the PI controller (I*™,) for AC voltage control loop decides the amplitude of reactive current to be generated by the CC-VSI. Multiplication of quadrature unit vectors (w,, wb and wc) with the output of PI based AC voltage controller (rsraa) yields the quadrature component of the source reference currents (i*saq, i*sbq and i V ) . For self-supporting DC bus of CC-VSI, its DC bus voltage is sensed and compared with DC reference voltage. The error voltage is processed in another PI controller. The output of the PI controller (I*md) decides the amplitude of active current. Multiplication of in-phase unit vectors (us, Ut, and Uc) with output of Ft controller (I*,md) yields the in-phase component of the source reference currents (i*sa(i, i*Sbd and i*SCd). The sum of quadrature and in-phase components is the total source reference currents (i*M, i*Sb and i*sc), which are compared with the source line current (ira, i!b and iK). These current error signals are amplified and compared with the triangular carrier wave. If the amplified current error signal is equal to or greater than the triangular carrier wave, lower device of the inverter phase lag is turned on and upper device turned off. If the amplified current error signal is equal to or less than the triangular carrier wave lower device of the inverter phase is turned off and upper device is turned on. The generated power by the SEIG is maintained constant by the third PI controller. The generated power is compared with the reference rated power. The PI controller processes output error of the comparator. The output of the PI controller is compared with triangular wave. If output of PI controller is more than the triangular wave, gate pulse of chopper switch (IGBT) is made high and its current increases through chopper

switch so that SEIG experience same load. If controller output is less than PWM carrier triangle wave, gate pulse of IGBT is low and chopper switch is made open. HI. MODELLING OF SEIG-IELC SYSTEM The schematic diagram is shown in Fig. 1, which consists of SEIG, IELC, its control scheme and loads. The mathematical modelling of each component is as follows.

A, Modelling of control scheme of IELC Three-phase voltages at the SEIG terminals (va, vb and vc) are considered sinusoidal and hence their amplitude is computed as:

Fig. 1 Schematic and power diagram of the improved SEIG-IELC system

V,= {(2/3)(v. 2 +v b '+v £ ! )} ! ' 2 (1) The unit vector in phase with va, vb and vcare derived as: u , = vyv,; ub = v l /V 1 ; u,= v,/V. . . (2) The unit vectors in quadrature with v,, vb and vc may be derived using a quadrature transformation of the in-phase unit vectors Ua, U*. and Uc [17] as:

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V

V

(3)

(4) (5) 1) Quadrature Component of Source Reference Currents: The AC voltage error Ve[ at the n"1 sampling instant is: V er(0) =V,, rf(nr V 1(n) (6) Where V m , (n) is the amplitude of reference AC terminal voltage and V w ,is the amplitude of the sensed three-phase AC voltage at the SEIG terminals at n"1 instant. The output of the PI controller (I*™,,,,,) for maintaining AC terminal voltage constant at the nth sampling instant is expressed as: I'™** = I W , , + Kni { Vcnn, - VtI(n,,} + K;, VcrW (7) Where Kp, and Kia are the proportional and integral gain constants of the proportional integral (PI) controller, VH(n) and Vt,,,,.,} are the voltage errors in n"1 and (n-1)"1 instant and I'™,^.,, is the amplitude of quadrature component of the source reference current at (n-1)"1 instant. The quadrature components of the source reference currents are estimated as: 1 saq

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2) In-Phase Component of Source Reference Currents: The DC bus voltage error Vdctr at n'h sampling instant is: y _ y _ y^ (g\ Where Vdm, is the reference DC voltage and Vdc(0) is the sensed DC link voltage of the CC-VSI. The output of the PI controller for maintaining DC bus voltage of the CC-VSI at the n1* sampling instant, is expressed as: T* = T* + K i V. —V i+K.V (101 * smd(n)

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B. Modelling of DC bus chopper The generated power of the SEIG is calculated by transforming three-phase quantity (a-b-c) into two-phase quantity (a-P axes) as follows [20]: ^=^2/3(^^2^/2) (18) vp =V2/3 (V3/2 v, -V3/2 vc) (19) i a = V2/3(h-.ib£-i,/2) (20) iP =V2/3 (V3/2 ib -V3/2ic) • (21) The generated instantaneous power of SEIG can be defined as: PK» = va ia + vp ip (22) Power (PjJ is compared with reference power according to rated power of generator (Pref) as: Perln) = Pref — Pgsn(n)

(23)

Power error is processed in the PI controller to maintain the constant generated power at the SEIG at the n* sampling instant, is expressed as: v w , = V m l .,> + K^ } Per(n) - P*,,.,)} + Kp, Ptrtn) (24) Kpp and K^ are the proportional and integral gain constants of the power controller. The PI controller output (V'™,,)) is compared with the triangular carrier (Vm) waveform and output is fed to the gate of the chopper switch (IGBT). when V*^,,.^ VBi, SD = 1 and when V m n , < VBi, SD = 0 (25) The SD is the switching function used for generating the gating pulse of IGBT of the chopper of ELC.

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Modelling of CC-VSI The CC-VSI is a current controlled VSI and modeled as follows: The derivative of its DC bus voltage is defined as: pvdt = (SA ia + SB i* + SC iK - SD VJR$ C^ (26) Where SA, SB and SC are the switching functions for the ON/OFF positions of the VSI bridge switches S r S 6 and SD is the switching function of chopper. J sa 1 sa(| ' sad V,' **} The DC bus voltage reflects at the output of the inverter in the form of the three-phase PWM AC voltage ea, eb and ec. i*u = i% +i*scd (14) These voltages may be expressed as: 4) PWM current controller: e, = v* (2 SA- SB- SC) / 3 (27) The total reference currents (i*a, i*sb and i*K) a r e compared eB = Vfc (- SA+ 2 SB- SC) / 3 (28) with the sensed source currents (iB, isb and i,c). The ON/OFF ec = vdc (- SA- SB + 2 SC) / 3 (29) switching patterns of the gate drive signals to the IGBTs are The (CC-VSI) line voltages are given as: generated from the PWM current controller. The current errors eab = e, - eb; efc = eb - ec; sa = ec- e, (30) are computed as: The volt-amp equations of the output of voltage source i««,, = i* B -i» (15) inverter (CC-VSI) are as: W = i*Sb — iSb (16) v, = R r i ta + Upi,,, + eab - Rf id, - Lf picb (31) w=i*.-i. (17) vb = Rf i* + Lf picb + e^ - R, icc - Lf pire (32) These current error signals are amplified and then io, + u + i« = 0 (33) compared with the triangular carrier wave. If the amplified Value of iK from eqn (33) is substituted in to eqn. (32) which current error signal corresponding to phase a (i« n ) is greater results in: than the triangular wave signal switch S4 (lower device) is ON vb = R, icb + Lf picb + etc + rf ia + L, pica + Rr isb + Lf p u (34) and switch S, (upper device) is OFF, and the value of Rearranging the eqn. (31) and eqn. (34) it results in: switching function SA is set to 0. If the amplified current error Lrpio, - Lr pu = va - e,b - Rr ia + Rr i* (35) signal corresponding to isaoT is less than the triangular wave Lrpi a + 2 Lfpid, = v b - e b c - R ( i i a - 2 Rfid, (36) signal switch S, is ON and switch S4 is OFF, and the value of Hence, the CC-VSI current derivatives are obtained by solving SA is set to I. Similar logic applies to other phases. the eqn (35) and (36) as: pica = {( vb - e*) + 2 (v. - ert) - 3 R( i a } /(3Lf) (37) picb = {(vb - Ob.) - (v. - elb) - 3 Rr i»}/(3Lf) (38)

I*siad(ni is considered as the amplitude of active source current. Kpd and Kid are the proportional and integral gain constants of the DC bus PI voltage controller. In-phase components of source reference currents are estimated as: i*s.j = I*.md ua; i'sw = I*,mj ub; i*^ = I*,« u, (11) 3) Total Source Reference Currents: Total source reference currents are sum of in-phase and quadrature components of the source reference currents as:

C.

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D. Modelling of SEIG The dynamic model of the three-phase SEIG is developed using stationary d-q axes references frame, whose voltageampere equations are [17]: •R] [i] + [L] P [i] + H>r [G] [i] (39) From which, current derivatives can be expressed as p[i] = [L] ' { [v] - [R] [i] - co^G] [i] (40) Where [v] = [Vd3 v,s vd, v, t ] T; [R] = diag [ Rs Rs Rr Rr]

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0 L s + L m 0 L m r -i 0 0 0 0 L m 0 L r + L m 0 [Uj- 0 - L m O L r + L m L m O"L r + L o 0 Lm 0 L r + L m m

Fig.2 shows the transient waveforms of 3-phase generator voltages (vsatc), generator currents (iabe), three-phase resistive load currents (iia, i)b and i,c), three-phase IELC currents (ic, i* and iK), generated power and its reference (Psen) amplitude of SEIG terminal voltage and its reference (V,/Vlltf), DC bus voltage and its reference (VJ Vdcref) and generator speed (wE) demonstrating the response of IELC for regulating the SEIG terminal voltage supplying with pure resistive load (7.5 kW). At 5.1-sec. one and 5.2-sec. two-phase of the load are disconnected from the SEIG. Consequently IELC currents increase to balance the SEIG system and chopper current increases to maintain the constant power on the SEIG. At 5.3sec. one phase and at 5.4-sec. two-phase of load is reconnected

(41) The electromagnetic torque balance equation of SEIG is defined as: TM=Tc+J(2/P)p(A (42) The derivative of rotor speed of the SEIG from eqn. (42) is: p(H={P/(2J)}(T s M -T c ) (43) where the developed electromagnetic torque of the SEIG is expressed as [17]: Te = (3P/4) U, (i,. i* - i* v) (44) In microhdel system, prime mover have drooping characteristic and may be expressed as: T*.* = (3370-10w r ) (45)' The SEIG operates in the saturation region and its magenetizing characteristics is non-linear in nature. Therefore the magnetizing current should be calculated in each step of integration in terms of stator and rotor currents as: IB = V(id,+idr)J + (iip + i,) 1 (46) Magnetizing inductance is calculated from the magnetizing characteristics between Lm and Im. Relation between Lm and Ira is obtained by synchronous speed test [17] and can be written as: Lm= 0.1407 + 0.0014 L- 0.0012 L2 + 0.000048 Im3 (47)

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= {(i, - ilc - ia) +2 (ib - i,, - ice)} / (3 C) (49) v, +vb + vc = 0. (50) where i,, ib and ic are SEIG stator line currents, i,a, in, and ilc are 3-phase load currents and ia, i* and i« are CC-VSI currents. C is per phase no load excitation capacitor value connected parallel to SEIG.

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