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I INTRODUCTION. High power electrostatic precipitators are widely used in industrial emission control. For most precipitators, power supply with a high voltage.

HIGH-VOLTAGE HIGH-POWER RESONANT CONVERTER FOR ELECTROSTATIC PRECIPITATOR Sanbao Zheng and Darisuz Czarkowski Department of Electrical and Computer Engineering Polytechnic University 333 Jay St, Brooklyn NY 11201 USA

output voltage and control the sparking rate of the precipitator. The closed-loop system has been implemented and investigated.

Abstract: A phase-controlled series-parallel resonant converter for electrostatic precipitators is described. Compared with the traditional linefrequency power supplies, the converter is smaller in size, higher in efficiency, and provides a faster transient response. The converter is digitally controlled and the possibility of spark control using a DSP algorithm is explored. Operating behavior of the power supply with the nonlinear precipitator load is experimented and investigated. Keywords: Electrostatic precipitator, resonant converter, digital signal processor. I

II SYSTEM DESCRIPTION As shown in Fig. 1, a PC SPRC consists of two identical series-parallel inverters. Each inverter is composed of two switches with their anti-parallel diodes and a series-parallel resonant circuit. The two inverters share a parallel-connected capacitor 2CP. These two resonant circuits form an overall resonant tank, which is symmetrical with respect to 2Cp. The precipitator load RL in Fig. 1 is nonlinear, i.e., the resistance of the load changes with the V-I working point as well as the property of the dust load. RL is connected to the resonant tank through a high-voltage transformer, a rectifier, and an output filter. If the switching frequency is fixed at a value close to the resonant frequency, the output voltage across the load RL can be regulated by varying the phase angle between the firing signals to the two switching legs (leg A and B) of the inverter. The steady state operation of the PC SPRC with a resistance load had been described in detail in [3]. From [3] and [4], the relationship between the output and input voltages is:

INTRODUCTION

High power electrostatic precipitators are widely used in industrial emission control. For most precipitators, power supply with a high voltage output of 20 to 50 kV is required to obtain the best effect of dust removing [1]-[2]. A traditional power supply uses high voltage transformer working at the line-frequency to step up the voltage, and the rectified output dc voltage is generally controlled with power electronic switches such as thyristor at the primary side of the transformer. Low frequency operation of the transformer results in big size, high power losses, and a slow transient response of this kind of a power supply. To overcome these disadvantages, a high frequency Switch Mode Power Supply (SMPS) has been recently developed. As a scaled-down prototype of a 100 kW power supply, a 1 kW Phase-Controlled Series-Parallel Resonant Converter (PC SPRC) [3]-[4] is designed and built in this paper. Series-parallel topology is advantageous in many aspects [5], and it is anticipated that this topology is especially suitable for a precipitator load because of its superior shortcircuit characteristic. The converter switches at 20 kHz, and the high voltage output is obtained with a 1:100 ferrite core transformer. This converter is used to power a scaled-down precipitator at 30 kV and 33 mA. A DSP controller is designed to regulate the

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Vo =

2

2 V I cos(

2

π

where:

π

a

2

φ 2

)

,

(1)

b2 + 2 QL a = (1 +

C

p

Cs

)[ 1 − (

ω 2 ) ] ω0

π

,

2

× RL 2× Cp ω ω 8 , QL = , ω b = − 0 ω0 ω Cs + C p ω 0L

is the angular switching frequency, ω0 is the angular resonant frequency of the circuit, and QL is the load quality factor of the resonant tank.

1100

C VI

L

A

Cs

Cs

Start

L

B Shift the phase by the value calculated in the (k-1)th step

2CP D + Vo

Lf Io

Cf

RL

Read the sampled value of the (k)th step

2ip

If 100:1

-

sparking detected?

L=1.22mH; Cs=0.1µF; 2Cp=0.22µF; Lf=30 H; Cf=200 pF; RL=1e6 Ohms nominal.; V I =300v; IGBT switches.

eK

Controller

-

UK

Phase Shifter

A/D

Converter

caculate the new sparking rate SR

N N

PI controller

SR>SR_S?

Calculate the phase to be shifted in the (k+1)th step

adjust the reference input according to SR

Y

Fig. 1. A phase-controlled series-parallel resonant converter.

VRef

Y

YK

Save the data of the (k)th step

End

Prefilter

DSP

Fig. 2. Block diagram of a closed-loop converter system.

Fig. 3. Flowchart of the PI compensator and the sparking control.

This paper describes the application of this converter in a precipitator power supply. The circuit parameters are given in Fig. 1. The power supply is digitally controlled using DSP. A block diagram of the closed-loop system is shown in Fig. 2. A DSP evaluation board is used to implement the controller, the phase shifter, and the A/D converter. The controller regulates the output voltage of the power supply and controls the sparking rate of the precipitator. In normal operation (while there is no sparking), the controller is a PI compensator [4]:

uk = (Ts k I + k p )ek − k p ek −1 + u k −1 ,

III

SPARK CONTROL

The polluted air flowing through a precipitator provides a high resistance conductor between the two electrodes of the precipitator. Sparking occurs if the air resistance drops suddenly or the air dielectric breaks down under the high operating voltage. Frequent sparking in a precipitator affects the efficiency of the precipitator and damages the precipitator as well as the power supply. Since the working condition and the dust load of a precipitator vary, and the output voltage must be high for a reasonable dust removing efficiency, it is impossible to prevent the occurrence of sparking by keeping the output voltage at a low value.

(2)

where the subscript k represents the values in the k-th step, Ts is the sampling period, KI is the integral gain, and KP is the proportional gain. When a sparking is detected, the controller is switched to a sparking processing routine as described in the next section.

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Compared with the ideal operating waveforms of the converter [3, 6], it is evident that the parasitic components of the high voltage transformer introduced some high frequency resonant modes into the power supply. However, it is acceptable for this kind of high voltage application as long as the safe operation margin of the circuit components is ensured. Fig. 11 shows the inductor current while the inverter output is short-circuited (Vp=0). It can be seen that the current is limited by the resonant tank under this short-circuit condition. This is very important since the precipitator load is susceptible to sparking, which essentially provide a quick shortcircuit at the inverter output. Fig. 12 shows the response of the output voltage to the phase step from 90 degree to 135 degree. It can be seen that the output voltage dropped in less than 1ms. In Fig.13, after a spark was detected, the controller extinguished the spark. Then, the controller brought the voltage back with a new lower reference input to avoid sparking from occurring repeatedly.

To limit the sparking to an acceptable rate, the average number of sparks over a given time period is calculated. If the sparking rate is higher than that specified, the reference input of the closed-loop system is adjusted to a lower value and thus the output voltage is decreased. A flowchart of this spark control is shown in Fig. 3. The spark detection is done by measurements of the output voltage and current. Two conditions are used to identify a spark: △Vo≥△Vth

(3)

△Io≥△Ith

(4)

where △Vo is the output voltage drop between two consecutive samplings and △Io the current increase, △ Vth is the voltage drop threshold and △ Ith is the current increase threshold. If these two conditions are satisfied, a spark is detected. To calculate the sparking rate, a 16-bit word is used. The word is left-shifted for one bit every N sampling periods. This makes the word like a timing widow with a width of 16×N×Ts, where Ts is the sampling period. When a spark is detected, a “1” is written into the first non-zero bit closest to the Least Significant Bit (LSB) of the word. Thus, the sparking rate at the current sampling period is easily obtained by counting the non-zero bits in the word. For example, if in the k-th sampling period there are S non-zero bits in the word, then the sparking rate is: (5) SR=S/(16×N×Ts)

V CONCLUSIONS

If the new SR is bigger than the specified limit SR_S, the reference voltage input will be adjusted to a smaller value to bring the output voltage down. The output voltage can also be brought back by giving a higher reference input.

By application of a high frequency resonant converter, smaller size, higher efficiency, and faster transient response of power supply for precipitator can be achieved. A series-parallel topology is advantageous considering precipitator sparking behavior and absorption of circuit parasitics. DSP control provides the implementation flexibility for various control algorithms. High turns-ratio transformer introduces additional resonant modes to the converter which result in overshoots in the operating waveforms. Thus, fine-tuning of the controller parameters and proper selection of the circuit components are necessary to ensure safe operating margins.

IV EXPERIMENTAL RESULTS

ACKNOWLEDGEMENT

The PC SPRC of Fig. 1 was built and tested with a 30kV/33mA precipitator. The steady state operation waveforms are shown in Figs. 4 through 12.

This work was supported by New York State Energy Research and Development Authority (NYSERDA) and Beltran Associates, Inc.

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VCA

4.35A/V

VDA

Fig. 4. Voltages across the IGBT switches (×10 probe).

Fig. 5. Inductor current with zero phase shifting (voltage measured across a 0.23Ω resistor).

0.5A/V

Fig. 6. Voltage across the parallel capacitor with zero phase shifting (×10 probe).

Fig. 7. Inverter output current with zero phase shifting (voltage measured across a 2Ω resistor).

IL_B

100mA/V

IL_A 4.35A/V

Fig. 8. Precipitator current with zero phase shifting (voltage measured across a 10Ω resistor)

Fig. 9. Inductor currents in both resonant tanks with a 90° phase shifting (voltage measured across 0.23 Ω resistors).

1103

4.35A/V

Fig. 10. Precipitator voltage with a 90° phase shifting.

Fig. 11. Inductor current with the inverter output short-circuited (voltage measured across a 0.23Ω resistor)

Io 100mA/V

18kV

Fig.12. The precipitator voltage response to the phase shifting step change from 90° to 135°.

Vo

Fig. 13. The voltage and current response during a sparking (sparking is detected and extinguished by controller, and then voltage is brought back). [5] R. L. Steigerwald, “A comparison of half-bridge resonant converter topologies,” IEEE Transactions on Power Electronics, Vol. 3, No. 2, pp 174~182, April 1988. [6] M. K. Kazimierczuk and D. Czarkowski, “Resonant power converters,” John Wiley & Sons, 1995, chapter 15-21.

REFERENCES

[1] K. J. Mclean, “Electrostatic precipitator,” IEE proceedings, Vol. 135, No. 6, pp347-361, July 1988. [2] N. Grass, “Fuzzy logic-based power control system for multi field electrostatic precipitators,” Record of the IEEE Industry Applications Conference 2000, Vol. 1, pp563-568, Oct. 2000, Rome, Italy. [3] D. Czarkowski and M. K. Kazimierczuk, “Phase-controlled series-parallel resonant converter,” IEEE Transactions on Power Electronics, Vol. 8, No. 3, pp 309-319, July 1993. [4] S. Zheng and D. Czarkowski, “Dynamics of a phase-controlled series-parallel resonant converter,” Proceding of the IEEE International Symposium on Circuits and Systems 2002, Vol. 3, pp 819-822, May 2002.

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