Step-Up/Down AC Voltage Regulator Using Transformer ... - IEEE Xplore

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 45, NO. 6, DECEMBER 1998

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Step-Up/Down AC Voltage Regulator Using Transformer with Tap Changer and PWM AC Chopper Do-Hyun Jang, Member, IEEE, and Gyu-Ha Choe, Senior Member, IEEE

Abstract—A step-up/down ac voltage regulator is proposed, in which the transformer with tap changer and the pulsewidth modulation (PWM) ac chopper are combined. The proposed regulator can step up or down the output voltage to input voltage. Also, the proposed regulator restrains more harmonics of output voltage compared to the conventional PWM regulator. The input current flows continuously in the proposed regulator, while it flows discontinuously in the conventional PWM regulator. Through digital simulation, several characteristics are investigated theoretically and then compared with those of the conventional ones. Practical verification of the theoretical predictions is presented to confirm the capabilities of the proposed regulator. Index Terms— Pulsewidth modulation ac chopper, step-up/ down ac voltage regulator, transformer with tap changer.

I. INTRODUCTION

T

O OBTAIN variable ac voltage from a fixed ac source, ac voltage regulators or ac choppers are widely used. The phase angle control (PAC) of regulators is extensively employed in many applications, such as industrial heating, lighting control, and starting and speed control of induction motors. This technique offers the advantages of simplicity and ability of controlling a large amount of power economically. However, delayed firing angle causes discontinuity and significant harmonics in load current, and a lagging power factor also occurs at the ac side, even though the load is completely resistive [1]. These problems can be solved by the introduction of more advanced control methods, such as symmetrical angle control (SAC) [2], asymmetrical angle control (AAC) [3], and time ratio control of high frequency (TRC) [4], or solved by modification of the power circuit with freewheeling path, etc. [5]–[8]. In the development of power semiconductor devices, pulsewidth modulation (PWM) techniques are increasingly being encouraged and will be sophisticated further. In conventional PWM methods, one of the main functions is to eliminate the harmonic contents of the output voltage by adjusting the number of pulses per half cycle. However, ac filters should be implemented to improve total harmonic distortion at the supply side, since the input current flows discontinuously [7], [8]. Manuscript received June 1, 1997; revised April 20, 1998. Abstract published on the Internet August 25, 1998. D.-H. Jang is with the Department of Electrical Engineering, Hoseo University, Chungnam, 337-795 Korea (e-mail: [email protected]). G.-H. Choe is with the Department of Electrical Engineering, Kon-Kuk University, Seoul, 133-701 Korea. Publisher Item Identifier S 0278-0046(98)08466-4.

Such a voltage regulator which uses only power semiconductor devices is the step-down type, and cannot step up the output voltage to input voltage source. Transformers with tap changer using thyristors or triacs are normally used to step up or down the output voltage. However, such regulators have the problems of low response and need the forced commutation. In this paper, a step-up/down ac voltage regulator, in which the transformer with tap changer and PWM ac chopper are combined, is proposed. The proposed regulator can step up or down the output voltage by the tap changer of transformer and offers accurate output voltage to load by adjusting the PWM ac chopper. Also, the input current flows continuously, while in the conventional PWM regulator, it flows discontinuously. Through digital simulation, several characteristics, such as changes of harmonics of output voltage and total harmonic distortion at supply and load side are investigated. Finally, the experimental results are presented to verify the theoretical predictions.

II. DESCRIPTION OF AC VOLTAGE REGULATORS A. Conventional AC Voltage Regulator Fig. 1(a) shows the power circuit of PAC which does not require the freewheeling path. Fig. 1(b) shows the power circuit of the PWM voltage regulator, which is composed of two pairs of inverse parallel power switches, one connected in series and the other in parallel with load. The series-connected transistors regulate the power delivered to the load, and the parallel ones provide the freewheeling path to discharge the stored energy when the series ones are turned off. Fig. 3(a) shows the waveforms of the output voltage and current in the conventional PWM ac voltage regulator. In order to obtain the PWM waveform, a switching signal is determined by the intersecting instants of the fixed triangular . carrier wave and the constant switching function By the proper choice of the PWM switching function, the output voltage has lower harmonic contents and, for the case of – loads, produces an approximately sinusoidal load current. However, total harmonic distortion is higher, since the input current is discontinuous [7], [8]. Also, PWM regulators have two significant disadvantages: 1) the output voltage cannot be stepped up and 2) the switching loss is produced in power devices.

0278–0046/98$10.00  1998 IEEE

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(a)

(b) Fig. 2. Proposed power circuit of ac voltage regulator.

waves, precalculated switching patterns by Newton–Raphson method [6], [7] are used. Fig. 4 shows the output voltage waves and the harmonics analysis for two regulators when the precalculated switching patterns are used. It is observed that the proposed regulator restrains more harmonics of the output voltage compared to the conventional ones. (c)

III. CIRCUIT EQUATIONS

Fig. 1. Conventional power circuit of ac voltage regulator: (a) PAC, (b) PWM, and (c) tap changer control.

A. Mathematical Expressions of Input Waveforms Fig. 1(c) shows the representative ac voltage regulator with tap changers in which the secondary winding of transformers or autotransformers has several taps. Sequence control and synchronous tap-changing control can adjust the output voltage. Basically, the technique is tap changing of a transformer using static thyristor switches and continuous adjustment by the PAC between taps. Such type of regulators can step up or down the output voltage to the input voltage by using a tap changer. B. Proposed Power Circuit of AC Voltage Regulator The conventional ac voltage regulators of Fig. 1(a) and (b) have the main drawback of the step-down type of output voltage. In this paper, the PWM regulator of Fig. 1(b) and tap changer regulator of Fig. 1(c) join together to step up or down the output voltage. Fig. 2 shows a power circuit of the proposed step-up/down ac voltage regulator, in which the PWM ac chopper, power transformer with tap changer, and controlling transformer are combined. The turns ratios of power transformer and controlling transformer are 1 : , with sinusoidal wave and 1 : 1, respectively. Tap voltage is determined by the tap, which is connected with load. The PWM ac chopper, named the part voltage regulator, operates in order to perform accurate output voltage. The tap selection by microprocessor is feasible by using a microprocessor-based digital system. Triacs or solid-state relays (SSR’s) in small or medium systems and SCR’s in large systems are recommended as power devices to connect between taps and load. The waveforms for the switching signals, the output voltage, and current are shown in Fig. 3(b) when the proposed regulator steps up the input voltage. The output voltage at the load at the tap terminal terminal is the sum of the tap voltage at a part voltage regulator. Accordingly, and PWM voltage the load current approaches to sinusoidal wave, and input current is continuous. In this paper, instead of triangular carrier

Suppose that the ac input voltage and can be expressed as

remains unchanged (1)

The output voltage at the load of the proposed regulator is the at the tap terminal and PWM voltage sum of tap voltage at a part voltage regulator (2) Assuming that the turns ratio of power transformer is 1 : , , the tap and the total number of taps in transformer are of the selected tap ( ) can be voltage expressed as (3) where (4) The tap current

by tap voltage is expressed as (5)

where

, (6)

and are the resistance and inductance of the load, respectively. pulses The PWM wave by a part voltage regulator has per half cycle, the turn-on switching angles are designated , ( ) per half and the turn-off angles are cycle and (7)

JANG AND CHOE: STEP-UP/DOWN OF AC VOLTAGE REGULATOR

907

(a) (a)

(b) Fig. 4. Waveform of the output voltage and its harmonic analysis: (a) conventional regulator and (b) proposed regulator.

(b) Fig. 3. Waveform of the switching functions, the output voltages, and currents in two regulators: (a) PWM regulator and (b) step-up of the proposed regulator.

When the switch is turned on and the is turned off , voltage by a part voltage during the interval regulator is supplied to the controlling transformer with the turns ratio of 1 : 1 (8)

During the interval

,

is (10)

During [

], the differential equation formed by a part

voltage regulator is given by

where (9)

(11)

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PWM current


during [

] is obtained by solving (11)

Also, during [ ], input current can be represented by the voltage ratio of transformer or the turns ratio (21) (22)

(12) is the same with of (6), and is an initial where at . value of input current is turned off and the is turned on during When the ], the supply voltage is cut from the load. So the [ . stored energy discharges through the freewheeling path of ] is given by The differential equation during [

is the rms value of the th harmonic of Assuming that input current, the total harmonic distortion THD at supply side is defined as (23) B. Fourier Representation of Output Voltage and Load Current The output voltage at load of the proposed regulator is and PWM voltage and can be the sum of tap voltage represented by a Fourier series as

(13) is an initial value of the freewheeling Assuming that at , the freewheeling current during current ] is obtained by solving (13) [

(24) Consequently, the fundamental coefficient

is expressed as

(14) and

The following relations between at

at

are formed: (15) (16)

], tap voltage from a source transformer During [ from a part voltage regulator are applied and PWM voltage during [ ] can to load. Therefore, the load current be represented by summing the tap current of (5) and a part of (12) of PWM current

(25) is the number of taps, is the turns ratio of power where is the number of pulses per half cycle. transformer, and are expressed as For odd, the harmonic coefficients

(26) The load current

is derived from (24) as follows:

(17) ], input current During [ voltage ratio of transformer

can be represented by the

(27) . Assuming that is the rms where value of the th harmonic of load current, the total harmonic distortion THD at load side is defined as

(18) THD Inserting (4) and (9) into (18), turns ratio

(28)

can be represented by the IV. THEORETICAL CHARACTERISTICS

When a part voltage regulator is not connected to the load ], the load current is represented by during [ summing the tap current of (5) and the freewheeling current of (14)

A digital simulation is executed here to theoretically investigate the characteristics of the proposed regulator. It is assumed per half cycle is five, the total that the number of pulses in the power transformer is five, and the number of taps turns ratio is 1 : 2. When the input voltage, 100 V with 60 Hz, is supplied to the primary winding of power transformer, of the selected tap ( ) is the voltage expressed as

(20)

(29)

(19)


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(a)

(a)

(b) Fig. 5. Harmonic output voltage versus fundamental output voltage. (a) Conventional regulator and (b) proposed regulator.

(b) Fig. 6. Total harmonic distortion at input versus fundamental output voltage: (a) conventional regulator and (b) proposed regulator.

Input voltage of the part voltage regulator is determined to voltage source and taps and then expressed as (30) From the conditions, the rms value of output voltage in a model regulator is 200 V, and the rms value of output voltage in a part voltage regulator is 50 V. The rms value of input voltage in the conventional PWM regulator is assumed to be 200 V to compare with the proposed one. Fig. 5 shows the variation of the harmonic output voltage of two regulators when the fundamental value ( ) varies. From Fig. 5(a), the dominant orders of ) and ( ) [6], residual harmonics are the order ( [7]. However, from the proposed regulator of Fig. 5(b), the values of dominant harmonics are reduced inversely according to the total number of taps. Fig. 6 shows the variation of the total harmonic distortion THD of input current when the load power factor is fixed varies at 0.2, 0.5, and 0.8 and the fundamental voltage from 0 to 200 V. The conventional regulator in this paper is the PAC and PWM control one of Fig. 1(a) and (b). , but THD of the conventional regulator is large at low . However, THD of the proposed decreases with higher regulator is lower over most of the range compared to the conventional ones. Fig. 7 shows the variation of the total harmonic distortion THD of load current under the same condition of Fig. 6. THD of the conventional PWM or proposed regulator becomes much lower compared to THD of Fig. 6. Also, THD of the proposed regulator becomes lower compared to the PWM ones. From the theoretical calculations, it is predicted that the total harmonic distortion at the supply or load side becomes lower

(a)

(b) Fig. 7. Total harmonic distortion at load versus fundamental output voltage: (a) conventional regulator and (b) proposed regulator.

as the number of taps are increased. However, the number of taps should be determined properly according to the turns

910


(a)

(a)

(b) Fig. 8. Waveforms of input voltage, input current, output voltage, and load current of the conventional regulator: (a) input voltage and output voltage and (b) load current and input current.

(b) Fig. 10. Harmonic analysis of the output voltage: (a) conventional regulator and (b) proposed regulator. (a)

V. EXPERIMENTAL RESULTS

(b) Fig. 9. Waveforms of input voltage, input current, output voltage, and load current at step-up of the proposed regulator: (a) input voltage and output voltage and (b) load current and input current.

ratio, output voltage, and power in the regulator, because the control system of the regulator becomes complex and the manufacturing cost is then higher with many taps.

An experiment was performed in order to verify the feasibility of the proposed step-up/down ac voltage regulator. The software to perform the proposed switching pattern is stored in a ROM. Using a one-chip microprocessor 8051, one of the taps is selected, and the switching signal is read out in the lookup table to make the PWM waveform. Fig. 8 shows the waveforms of output voltage , load when the current , input voltage , and input current conventional PWM regulator operates. The rms value of input is 100 V and the frequency is 60 Hz. The power voltage factor of load is about 0.5, the duty cycle is 0.5, and the number of pulses per half cycle is five. It is observed that input current is discontinuous, and load current is continuous, but tough. Fig. 9 shows the waveforms of output voltage, load current, input voltage, and input current under the same condition of Fig. 8 when the proposed regulator operates as the step-up type. The rms value of output voltage is measured to be 125 V. Input current is continuous except for connecting


to Tap T , and load current approaches the sinusoidal wave. Fig. 10 shows the spectrum analysis of output voltages for two regulators. It is observed that the output voltage of the proposed regulator has a lower harmonics value compared to the conventional one. VI. CONCLUSION A step-up/down ac voltage regulator has been proposed, in which the transformer with tap changer and the PWM ac chopper are combined. The proposed regulator can step up or step down the output voltage to input voltage and restrains more harmonics of the output voltage compared to the conventional regulators. Input current of the proposed regulator flows continuously except for connecting Tap T , while in the conventional PWM, it flows discontinuously. Therefore, the total harmonic distortion at the supply or load side is decreased. Also, total harmonic distortion at input or load becomes lower inversely according to the number of taps. However, the proposed regulator presents heavier weight and higher volume because of transformer in the power circuit, and the manufacturing cost is then higher. REFERENCES [1] E. El-Bidweihy, K. Al-Badwaihy, M. S. Metwally, and M. El-Eedweihy, “Power factor of AC controllers for inductive loads,” IEEE Trans. Ind. Electron. Contr. Instrum., vol. IECI-27, pp. 210–212, June 1980. [2] G. N. Revankar and D. S. Trasi, “Symmetrical pulse width modulated ac chopper,” IEEE Trans. Ind. Electron. Contr. Instrum., vol. IECI-24, pp. 41–45, Feb. 1977. [3] B. W. Williams, “Asymmetrically modulated AC chopper,” IEEE Trans. Ind. Electron., vol. 29, pp. 181–185, June 1982. [4] A. Mozder, Jr. and B. K. Bose, “Three-phase ac power control using power transistor,” IEEE Trans. Ind. Applicat., vol. IA-12, pp. 499–505, Sept./Oct. 1976. [5] S. Iida and S. Miyairi, “Effects of PWM applied in single phase AC power control,” Trans. Inst. Elect. Eng. Jpn., vol. 103-B, no. 1, pp. 7–14, 1983.

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[6] G.-H. Choe, M.-H. Park, and A. Wallace, “An improved PWM technique for AC chopper,” IEEE Trans. Power Electron., vol. 4, pp. 496–505, July 1989. [7] D.-H. Jang, G.-H. Choe, and M. Ehsani, “Asymmetrical PWM technique with harmonic elimination and power factor control in ac chopper,” IEEE Trans. Power Electron., vol. 10, pp. 175–184, Mar. 1995. [8] D.-H. Jang and G.-H. Choe, “Improvement of input power factor in ac choppers using asymmetrical PWM technique,” IEEE Trans. Ind. Electron., vol. 42, pp. 179–185, Apr. 1995.

Do-Hyun Jang (S’86–M’90) was born in Chunan, Korea, in 1956. He received the B.S. degree from Hanyang University, Seoul, Korea, in 1980 and the M.S. and Ph.D. degrees from Seoul National University, Seoul, Korea, in 1982 and 1989, respectively. Since 1985, he has been with the Department of Electrical Engineering, Hoseo University, Chungnam, Korea, where he is currently a Professor. From 1993 to 1994, he was a Visiting Scholar in the Department of Electrical Engineering, Texas A&M University, College Station. His research interests are in the fields of ac voltage regulators, switched reluctance motor drives, and PWM control for inverters.

Gyu-Ha Choe (S’78–M’80–SM’96) was born in Pusan, Korea. He received the B.S., M.S., and Ph.D. degrees from Seoul National University, Seoul, Korea, in 1978, 1980, and 1986, respectively. Since 1980, he has been with the Department of Electrical Engineering, Kon-Kuk University, Seoul, Korea, where he is currently a Professor and Dean of Academic Research and Promotion Office. From 1987 to 1988, he was a Visiting Scholar in the Department of Electrical Engineering, Oregon State University, Covallis. His research interests are in the fields of active power filters, PWM control, ac voltage regulators, and inverter welding machines. Dr. Choe is a member of the Korean Institute of Power Electronics Engineers and a Registered Engineer Consultant in Korea.