510

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 43, NO. 4, AUGUST 1996

odified Approach of Feeding Regenerative Energy to the

Subrata Saha, Aditi Vasant Dandekar, and Victor Prince Sundersingh

Abstract- A modified approach of injecting the regenerated energy from a 3 - q ac squirrel cage induction motor drive 5 system, during its braking operation, sinusoidally into the 3 - Q supply using a feedback inverter between the thyristor rectifier bridge and the 3 - 4 inverter block is described. The validity of the modified scheme is verified by simulation and experiment.

is fired immediately. The voltage across the inductor L1 reverses and the current starts flowing through the loop JKABCDLJ to charge the capacitor Co. The loop equation neglecting thyristor drop is given by
Th

I. INTRODUCTION

EGENERATIVE braking of a 3 - q5 ac squirrel cage induction motor, driven by a voltage source inverter is a widely accepted concept, especially in applications like railway tractions, lifts, CNC machines, etc. All these applications have frequent starts and stops and, hence a substantial amount diL diL of energy can be regenerated to the supply during the braking L 2 2 L 1 2 V,sinwt = 0. (3) dt dt operation resulting in energy saving. Several ac drive systems with regenerative braking facility have been reported in the Here, diL,/dt is the rate of change of current flowing past few years [1]-[7]. Except for the work reported in [6], through the inductor L2. Now, the change in the capacitor all the drive systems suffer from a single major drawback voltage AV, during this time is given by of sending a square-wave regenerated current into the mains, i thereby injecting undesired harmonics into the supply. In this A V - 2 dt. (4) - CO paper, a modified technique and control strategy is discussed, for the feedback inverter proposed by K. Matsui et al. [ 5 ] , to Here, i, is the capacitor current and is expressed as i, = send a sinusoidal regenerated current to the mains. Z L ~- i ~During .the above T, OFF period the current flowing ~ through the inductor L1 starts decreasing as the energy stored 11. PROPOSED SCHEME in it is used to feed power to the mains as well as to charge the capacitor COand inductor L2. At a certain instant of time, the The feedback inverter configuration with only one phase (415 V) of the 3 - 4 SCR bridge is shown in Fig. 1. After the current flowing through the inductor L2 will be greater than in motoring dc link current becomes zero the feedback inverter L1 and the capacitor current i, will become negative. Now, in configuration is enabled by first charging the capacitor COto addition to loop JIHRYGFEDLJ, loop FEDCBAKJIHRYGF the dc link voltage El by external means. Then, when phase comes into action and the capacitor COstarts discharging. The R becomes positive with respect to phase Y, just thyristors corresponding loop equation is given by S3 and S2 are turned on and made to conduct for 180.At the diL L22dt - V El V,sinwt = 0. , same time, transistor T, is turned ON and a finite amount of energy is stored in inductor L1. During this time, the equation The ON and OFF durations of the transistor T, should be for the active loop ABCLJKA neglecting T, drop is given by designed such that the entire energy stored in the inductor L1 diL during its ON period is totally used during its OFF period. L 1 2 =E dt Hence,the current flowing through the inductor L1, should where diLl l d t is the rate of change of current flowing through become zero at a particular time. At this instant, the current the inductor L1. Next, when T, is turned OFF, the thyristor loop JIHRYGFEDLJ will be out of action and only the loop FEDCBAKJIHRYGF exists for sending regenerated current to Manuscript received June 5, 1995; revised September 30, 1995. S . Saha is with the Department of Energy Systems Engineering, I.I.T. the mains. This is again followed by the ON period of T, and Powai, Bombay 400 076 India. during this ON period, loop FEDCBAKJIHRYGF will still be A. V. Dandekar and V. P. Sundersingh are with the Department of Electrical in action for returning the regenerated current to the mains. Engineering, I.I.T. Powai, Bombay 400 076 India. To ensure that the current flowing through the inductor L2 is Publisher Item Identifier S 0278-0046(96)04151-2.

L1-diLl dt where V is the voltage across the capacitor CO of the , feedback inverter. During this time, the loop JIHRYGFEDLJ which regenerates current to the mains will come into action only when (V, - E l ) 2 V sinwt, the source voltage. The , 2 corresponding loop equation neglecting S, and S drop is given by

+ +

02784046/96$05.00 0 1996 IEEE

SAHA et al.: A MODIFIED APPROACH OF FEEDING REGENERATIVE ENERGY TO THE MAINS

511

M A

I N

K
Fig. 1. Regenerative braking scheme using the feedback inverter.

sinusoidal, simulation of the feedback inverter is carried out by a step-by-step method with the help of (1) to (5). 111. SIMULATION RESULTS The step width chosen for the simulation is 1 ps. It is assumed, that the starting dc link voltage is 600 V during regeneration and the line-to-line voltage of the 3 - $ supply is 415 V. It is also assumed that the dc link voltage remains constant throughout the full cycle of 20 ms. While simulating the feedback inverter configuration, emphasis is given on two control statements to determine the ON and OFF periods of the transistor T, for successfully regenerating sinusoidal current into the mains. These are: 1) The energy stored in inductor L1 during the T, ON period should be fully dissipated during its OFF period. 2) The current flowing through L2 during the T,. OFF period should rise to a maximum value satisfying a sine wave 1 1 = 1,1 sin wt, whereas the current during its ON period should decrease to a minimum value satisfying , another sine wave I2 = Im2 sinwt, where I1 > Im2. The values chosen for simulation are L1 = 9 mH, LZ = 50 mH, CO= 2 pF, 1 = 5 A and I, = 4 A. The ON and OFF , l , periods of T, in a 10-ms half cycle are determined by a trialand-error method using numerical solution, so that both the boundary conditions are satisfied. The ON and OFF durations of T, are listed in Table I. It is seen that the switching pattern of T,. gives an unique pulse-width modulation (PWM) which is very near to sinusoidal PWM. Fig. 2(a) shows the current flowing in the inductor L1 during a half cycle. It is clear that the current i~~ goes to zero after every switching cycle satisfying the control statement (a). The simulated regenerated sine wave current waveform for a full cycle is shown in Fig. 3(b). It is seen from this figure that the inductor current i~~ , after the transistor T, ON period, goes below the lower set value I m 2 sin wt. The reason for this is as follows. According to the control statement (b), when the inductor current i~~ falls below Im2 sin wt, the transistor T, is switched OFF and the thyristor Th is turned ON. But, the current flowing in the inductor L1 does not instantly flow into the mains, because the voltage across the capacitor COminus the d.c. link voltage El at this instant can be much below the corresponding value

TABLE 1 SWITCHING PERIODS OF THE T A SS O R NIT R

T,

No.of the switching ON duration I OFF duration cycle in a half cycle 1 2 0.072 0 475 0.096 3

4 5 6
7

0,132 0.168
0.201

1
1

0.302
0.204

a
9 10 11 12 13 14 15 16 17 18

0.229 0.251 0.267 0.375 0.275 0.267 0.251 0.227 0.195 0.157 0.111
0.060

0.283 0n .o 0.263 0.282 0.37

0.m

0.302 0.332 0.373

0.418

0.480
0.565

1 loo .

of the mains voltage V, sin wt. Hence, initially, the current through L1 is used to charge the capacitor CO and when the voltage across it is sufficiently large, the inductor current i~~ flows into the mains and i~~ starts increasing. Until this condition is reached, current i~~ will be decreasing, resulting in it to go below the set value. It was observed that very critical changes in the ON-OFF period were necessary to prevent ih2 from falling below the set value Imzsinwt. Since the harmonic distortion as shown in Fig. 2(c) under this condition is well within the specified limits, further improvement was not attempted. The magnitude of harmonic distortion in the above regenerated current waveform is determined by the fast fourier transform (FFT)function defined in MATLAB 386 by feeding the current waveform discretized at 1 p s intervals. The plot

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Time (sec)

(a) Fig. 3 . DC link voltage and regenerated current fed back to the supply

0.002 0004 0.006 0008 0.01 0.012 0014 0016 0018 0.02
Time (sec)

(b)
4.5

xl ' 0

v)

' b

1.5

reduce the high-frequency harmonic component. On the other hand, if a square-wave inverter is used for the regenerative braking scheme, then atleast an additional L -- C filter will be necessary. Since, a square-wave inverter contains high third harmonic and fifth harmonic components equal to 33% and 20% of the fundamental frequency, respectively, large values of L and C will be necessary. Further, as the inductor has to carry the regenerated fundamental current and at the same time support the harmonic voltages, the voltage ampere (VA) rating of the inductor for good filtering may be as high as 50% of the VA rating of the inverter. In addition, the filter capacitor further increases the cost. Similarly, a single pulse PWM inverter will also call for an L - C filter because of the stringent rules laid down by the utility companies. However, in this case, the filter VA rating will be reduced because the harmonic distortion for a particular value of pulse width is less than that of a squarewave inverter [9]. Thus, the present scheme has certainly a cost advantage over a simple commonly used square-wave or single-pulse PWM inverter for the regenerating braking system. Finally, the size of the proposed inverter also becomes more compact because of the absence of bulky low-frequency filter components.

05 .
0

1
0
-

500

1000

---A

1500

2000

2500

Frequency (Hz)

I v . EFFICIENCY THE SYSTEM OF

To determine the efficiency of the regenerative braking scheme the losses in the different components are calculated as follows. The common power losses in the transistor T,. and the thyristor Th are the ON-state conduction losses, switching losses, and the losses resulting from the snubber. All the above mentioned losses in the power devices are calculated using the general approach [9] and using appropriate values for the turnon time, turn-off time, and the collector to emitter saturation voltage Vce(sat) transistor T, and turn-on time, turn-off of the time, reverse-recovery charge, and ON-state voltage drop of the thyristor Th. The turn-on losses in the transistor T,. is zero as it switches at zero current. Losses in the inductor L1 and L2 are determined by calculating their VA ratings and assuming their efficiency to be 95% which is quite conservative. All

(c)
Fig. 2.

Simulated current through nductor L1 for a half cycle. (b) Simulated regenerated current waveform. (c) Power spectral density of the simulated regenerated current waveform.
(a)

of power spectral density against frequency as generated by the package shows that the lower order odd harmonics are less than 2% of the fundamental. This is well within the desired percentage limit of harmonic distortion specified by the utility standards [8]. Harmonics distortion peaks found at 1850 Hz is due to the switching frequency of T,.. This may be automatically filtered by the combined effect of the line inductance and the capacitance. If required, an additional air core inductor in series with the line may be used to further

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513

TABLE I1 EFFTCIENCY CALCULATION

DEVICE

SPECIFICATION

l off Time = 8 pa Snubber Capacitor C = 0.047 pF , On State Voltage Drop = 1.5 V Turn On Time = 3 ps Thyristor li Turn off Time = 20 ps Reverse Recc~ery Charge = 15 pC at 35 A Snubber Capacitor c, = 0.OlpF Line Commutated On State Voltage Thyristors DIVD= 1.5 V i

SrmbberLcSs
39.15

Switching flun on Lcm hm off Snubber Loss Conduction Lose

12.06
12.48
20.57

9.54

Loss h Induct& Ll Loss in Indudor & Total Power Loss

Power regenerated to the mains

69.50 2.63 183.73 1320.50

these losses are listed in Table I1 and the efficiency of the system is found to be 87.78%.

v.

EXPERIMENTAL RESULTS

The proposed scheme is verified experimentally by sending the regenerated current of a 3 - q5 ac, 1-hp squirrel cage induction motor to one of the phases of the 3 - q5 supply with the help of the above feedback inverter configuration. The experimental drive system consists of a 3 - q5 bridge, a 3 - q5 inverter, and the feedback inverter configuration having the same simulated component values. A sinusoidal PWM signal with the same ON and OFF durations as listed in Table I is generated from a microcomputer to turn ON alternately the transistor T,. and thyristor Th of the converter. The waveforms of the dc link voltage and the current fed back to the supply in the regeneration mode are shown in Fig. 3. The current waveform is nearly sinusoidal, with a frequency of 50 Hz, similar to the above simulated waveform.
VI. CONCLUSION

same feedback inverter configuration,the above idea can easily be extended to feed regenerated energy, during braking, to all three phases of the supply. For this scheme, the switching strategy of the bridge thyristors has to be modified during the regeneration mode so that sinusoidal current is returned to the mains phase by phase, independently.
REFERENCES [1] D. H. Braun, T. P. Gilmore, and W. A. Maslowski, Regenerative converter for PWM AC drives, in IEEE IAS Annual Meeting Con$ Proc., 1991, pp. 862-868. [2] J. C. Clare, P. R. Mayes, and W. F. Ray, Bidirectional power converter for voltage fed inverter machine drives, in IEEE IAS Annual Meeting ConJ Pr&, 1992, pp. 189-194. [3] J. M. Liptak, F. C. Joyner, and J. C. Guyesha, Regenerative controller for a voltage source inverter drive, in IEEE IAS Annual Meeting Con& Proc., 1985, pp. 1672-1677. _141 K. Matsui. K. Tusboi, and S. Muto, Power regenerative controls by _ utilizing thyristor rectifier of voltage source inverter, in IEEE IAS Annual Meeting Con$ Proc., 1989, pp. 913-919. [5] K. Matsui, U. Mizuno, and Y . Murai, Improved power regeneration controls by using thyristor rectifier bridge of voltage source inverter and a switching transistor, IEEE Trans. Ind. Applicat., vol. 28, pp. 1010-1016, 1992. [6] B. T. Ooi, J. W. Dixon, A. B. Kulkami, and M. Nishimoto, An integrated AC drive system using a controlled current PWM rectifierhverter link, IEEE Trans. Power Electron., vol. 3, pp. 64-70, 1988. [7] C. Schauder, A regenerative two quadrant converter for DC link voltage source inverter in IEEE IAS Annu. Meeting Con$ Proc., 1990, pp. 954-960. [8] C. K. Duffey and R. P. Stratford, Update of harmonic standard IEEE519 IEEE recommended practices and requirements for harmonic control in electric power systems, in IEEELAS 35th Petroleum and Chemical Industry Conf, 1988, pp. 249-255. [9] M. H. Rashid, Power Electronics Circuits, Devices and Applications. Englewood Cliffs, NJ: Prentice-Hall, 1988, p. 240, pp. 420-423, p. 461, and p. 493.

A modified control scheme for sending a sine-wave regenerated current to the mains is proposed and a detailed analysis is carried out to determine harmonic distortion, losses, and efficiency of the system. Simulation results strongly prove the validity of the proposed scheme. These results are further confirmed experimentally by successfully regenerating a sinewave current from a 3 - q5 ac, 1-hp squirrel-cage induction motor to one of the phases of the 3 - q5 supply during its braking operation. Using the common 3 - q5 bridge and the

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 43, NO. 4, AUGUST 1996

Subrata Saha obtained the B.E. degree in electncal engineering from Bengal Engineenng College, Calcutta University, Calcutta, India, in July 1991 and the M.Tech. degree in energy systems engineenng from 1.1 T. Bombay, Bombay, India, in Apnl 1993. After that he joined the Department of Energy Systems Engineering as a Research Fellow and is currently working toward the Ph.D. degree. His fields of interest are energy-saving ac dnve systems and applications of power electronics in energy conservation and alternate energy sources.

Victor Prince Sundersingh has been a member of the faculty of the Department of Electncal Engineering at I.I.T. Bombay, Bombay, India, for the last 30 years. He is currently a Professor and his mam field of interest is power devices and their applications. He has nearly 50 papers to his credit in vanous national and international joumals.

Aditi Vasant Dandekar received the B.E. degree in electrical engineering from the College of Engineering Pune, Pune University, Pune, India, in June 1992. She secured the first position in her class and received the prestigious gold medal from the University. She obtained the M.Tech. degree in electrical engineering with specialization in power electronics from I.I.T. Bombay in February 1994. At present she is working as a research executive at Crompton Greaves Limited in research and development (Electricah). Her fields of interest are power converters and ac drives: