enhanced performance for electrostatic precipitators fuzyy logic control

8/9/2019 Enhanced Performance for Electrostatic Precipitators Fuzyy Logic Control

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Enhanced Performance for Electrostatic Precipitators

by Means of Conventional and Fuzzy Logic Control

Norbert GrassInstitute for Electronic Systems

Ohm University

Nuernberg, Germany

[email protected]

Andreas ZintlDep. Electrostatic PrecipitatorsSiemens AG

Erlangen, Germany

[email protected]

Enrico HoffmannDep. Electrostatic PrecipitatorsSiemens AG

Erlangen, Germany

[email protected]

AbstractModern high voltage power supplies based on fast

switching devices like IGBTs offer the performance to adjustvoltage and current regarding the process conditions. Optimumperformance can be achieved if the control makes use of the

enhanced properties of the power supply. In critical precipitation

processes the control needs to be high dynamical and accurate.Furthermore, with IGBT inverter type high voltage powersupplies it is possible to increase the electrical powersignificantly, in many applications a factor 2 can be achieved,which leads to a significant improvement of the dedusting.Unfortunately, then the energy consumption of the precipitatorincreases leading to a higher amount of electricity to be generated

in the power station and finally this results in higher CO2emissions of the power station. With central and decentralizedcontrol algorithms the total energy consumption of anelectrostatic precipitator can be significantly reduced bycontinuous adaption of the control to the process conditions.Retrieving measured data from inside an electrostaticprecipitator is always difficult and unreliable due to the hightemperatures, dust and high voltage. Therefore, a rule based

control system has been developed which can operate with a fewinput signals only. Mainly the measured values of voltage andcurrent are sufficient for the control system and additional forenergy savings the stack emission signal is required.

Keywords; Power supplies for electrostatic precipitators, fuzzylogic control, energy efficient electrostatic precipitator,decentralized control.

I. INTRODUCTION

In comparison to SCR (silicon controlled rectifier) basedconventional high voltage power supplies IGBT inverters offermuch higher dynamic behavior. Voltage and current can becontrolled much more precise and much faster.

The application of modern power electronics with IGBT-switching devices (Insulated Gate Bipolar Transistors) andfully digital micro controller units can significantly increase theefficiency of an electrostatic precipitator.

The IGBT inverter (Fig. 1) has a 3-phase mains input

rectifier with a power factor (cos ) near unity and nocompensation is required. A voltage link capacitor is used as alow inductance current buffer for the H-bridge IGBT-inverter.The switching frequency is typically 10kHz. The current

inversion frequency has been selected at 500 Hz, thereforeaccurate and fast adjustment of current and voltage is possible.

Mains infeedInverter

HV transformer

HV rectifier Electrostatic

L1L2L3

precipitator

Voltage

linkMains infeed

Inverter

HV transformer

HV rectifier Electrostatic

L1L2L3

precipitator

Voltage

link

Figure 1: IGBT inverter circuit diagram

Compared to a standard SCR the average precipitatorcurrent can be increased significantly due to the flat V/I (Fig.2) characteristics of a precipitator. A small voltage increaseresults in a large current increase. In practice it was possible to

increase the electrical power by up to a factor 2 or 3 times inmany applications.

Figure 2: Emission versus electical power

Fig. 3 shows the dependency of the Emission on the coronapower. In respect to Fig 2 the higher corona power due to theconstant DC voltage leads to a significant reduction of theemissions. When a thyristor based mains controlled powersupply is used, the electrical power is limited by the flashovervoltage of the precipitator. Flashing usually occurs at the peak

978-1-4244-2279-1/08/$25.00 2008 IEEE 1


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voltage, while the de-dusting depends on the average voltageapplied to the precipitator. Each time a flashover has occurredthe voltage drops down and the current has to be turned off fordeionization of the flashover space. Due to the high spacecharge generated during the flashover in case of a SCR powersupply the deionisation period needs to be in the range of sometens of milliseconds (e.g. 50 ms). During this period of time

there is no current flow in the precipitator and therefore, no gasis being cleaning. IGBT inverters can deliver a DC voltagewithout a ripple and the current can be turned off immediatelyafter the occurrence of a flashover. Only a small space chargeremains in the area of the flashover and the deionization periodcan be much shorter. The result is a significantly higheraverage voltage and power which leads to a better operation ofthe precipitator, but higher energy consumption.

Figure 3: Emission versus electical power

With IGBT inverters and the fast flashover processing thesystem can be operated with significantly higher flashoverfrequency in order to increase the average power further. If theflashover rate is driven to high the precipitator responds with aavalanche of flashovers which drives down the electrical

power. Therefore, the power is to be kept below the occurrenceof the uncontrollable high local space charge to avoid thiscritical state of operation.

II. CONVENTIONAL CONTROL

A. Current and Voltage Control

The control of the electrical system certainly can berealized as conventional control. The inner loop is the currentcontrol part depending on the electrical parameters astransformer inductance and precipitator voltage. Currentcontrol needs to be very fast due to fast impedance changes ofthe load on the occurrence of flashovers. Then current controlis possible even during flashovers.

The voltage control is also dependent on electricalparameters as long as flashing is rarely happening.

As a general rule, if the parameters of a system are knownor can be measured conventional control or steady state controldelivers the optimum performance of the system.

B. Flashover Processing

The flashover processing is adjustable through thefollowing parameters:

Current drop after the first occurrence of aflashover

Deionization period [ms]

Current start value after deionization

Current upward gradient and limit

Fig. 4 shows the current control during flashoverprocessing.

IPrec

FO-

wait.time

FO-deion.

time

FO-

reduction

tFigure 4: Flashover processing

C. Flashover Voltage Detection

Conventional systems mainly used the flashover limit as themaximum possible voltage to be applied to the system.However, this voltage limit may change during operation of the

precipitator due to the process conditions. For instance inoxygen converters in steelworks or in some power stations aswell the flashover voltage changes very dynamically and

therefore conventional logic works not very efficient to adaptto the process. The process can be controlled stable if a voltagesetpoint below the flashover limit is selected. The maximum

possible power cannot be applied in that case and themisalignment becomes higher in case of dynamically changing

processes.

Certainly, there is always an improvement compared toSCR power supplies, but operation is below the optimum

possible performance.

III. R ULE BASED CONTROL (FUZZY LOGIC)

Due to the disadvantages of a conventional systemdescribed above, a rule based system has been developed to

calculate the voltage / current set-points and the parameters ofthe flashover processing.

Fuzzy Logic offers the advantage of implementing expertknowledge in a microcontroller. In case of extreme processfluctuations (e.g. frequent flashovers, batch processes, start-upsand shutdowns, etc.) fuzzy logic adapts the operating point forthe system without intervention from the operators. Especiallyfor systems impossible to describe in mathematical form, fuzzylogic offers improvements compared to the methods that have

been used in the past. This is particularly the case with

In cooperation with Siemens AG, Erlangen, Germany

0

2

4

6

8

10

0 2 4 6 8 10

Corona power (Us x Is) [%]

Emission

Increased EP Efficiency

limit for thyristor

controller

reducedemission

by PIC410F

increased power

by PIC410F

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electrostatic precipitators due to their large number of processparameters.

Input signals are assigned to member functions representingcharacteristic properties or ranges of the signal (i.e. high,medium, low, etc.).

Expertise can be placed in a context of "if.. then.." rules and

integrated into the control procedure. Since the control strategyis based on highly abstract and universal rules, a high level ofstability is achieved and even undefined operating conditionscan be controlled safely.

Finally, the results need to be defuzzyfied to obtain exactoutput values which can be used as setpoints for current controlor as parameters for the flashover processing.

As learned from the human behavior, the output signals ofdifferent rules can be combined employing the followingoptimization strategies:

Maximum corona discharge power for low andmedium resistance dusts with high flashover rates and fast-

changing process conditions Energy saving while still maintaining the specified

flue gas emission values.

These contrary goals could be integrated in one controlstructure and can be processed simultaneously. The highest

priority was given to the fulfillment of the emissionrequirements. If this could be achieved even with lowerelectrical power, costs can be saved. When process conditions(i.e. boiler load, production materials, humidity or temperature)change, the required power to achieve the requested emissionlevel will be found in practice between 10% and 100% of themaximum possible power. Only a few data have to be insertedduring commissioning the operation parameters are found by

the fuzzy logic system.The fuzzy maximum power point controller uses only input

signals which are available in the power supply unit, i.e.electrical signals. The output signals are generated by rules

processing the direct input signals and the changes in thesesignals. The current set-point and the flashover reaction are

processed by the fuzzy system. This leads to a very stablecontrol which is self adapting to various process conditions.

Because of the easy measurement of electrical values,control is based mainly on electrical data like voltage currentand flashover data. Especially the flashover behavior contains ahuge amount of information which is accumulated in a data

base.

IV. CONTROL OF LARGE INDUSTRIAL PRECIPITATORS

The controllers of the high voltage power supplies areequipped with communication features for networking theentire high voltage power supplies with an optimizing anddiagnostics computer. Additional features like energymanagement for huge precipitators are available. The emissionmonitoring signal from the stack is used as control signal.According to a three field precipitator model the corona powerof each electrical field is individually regulated. Typically,savings are up to 1MW on large precipitators.

Figure 5: Data communication for HV power supplies

The system detects the requested power for each field inorder to achieve the minimum power for all fields. In manyapplications good results were gained with high comparable

power in the inlet fields and low power in the middle and outletfields.

Special treatment is required during rapping of thecollecting electrodes when operating with decreased electrical

power. The centralized control additional synchronizes rappingof the entire fields to avoid simultaneous rapping. Actualcontrol of the rapping patterns is processed in the controller ofthe high voltage power supply. This two level processing wasintroduced for safety and reliability reasons. In case of networkor computer failure the rapping can still be performed and the

power can be increased automatically in each high voltagepower supply controller. In that case, of course, power savingsare not possible but the emissions have to be kept low as

highest priority.When low currents are applied to some of the fields the

effect of rapping can be seen in the emissions. Dust from thecollecting electrodes is released and restrained into the gasflow. The effect can be seen as peaks on the emission signal.Detecting the peaks and assigning it to the entire zone whichwas rapping enables the control to increase the power for theduration of the rapping and, thus avoiding the peaks in theemission signal.

In case of rapid process changes the emission signal mightbe very sensible and increase. This can be detected by thecentralized control system and power is to be increasedimmediately.

V. DUST EMISSIONS VERSUS CO2 EMISSIONS

With higher electrical power the dust emissions can bedecreased as shown above. Higher energy consumption causeshigher CO2 emissions due to the additional electrical energywhich needs to be generated and the additional amount of coalwhich will be burned in the power station. The emission curvein Fig. 3 also shows on the right side that improving theemissions slightly requires a high amount of additional power.That means if the emissions are already in the low range it isdifficult and very costly to improve further. It has to be taken

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into account that instead of reducing emissions by a smallamount a lot of energy and CO2emissions can be saved.

An example is given by the numbers shown in the tablebelow.

A calculation sheet is available to determine the possibleenergy and CO2savings. The data to be inserted is shown in thefields with a white background. The value of 830 kg ofgenerated CO2 is just an average value which can be adjustedeasily for any coal fired power station.

REFERENCES

[1] E. Kuffel, W.S. Zaengl: High-Voltage-Engineering Pergamon PressOxford, New York, Beijing, Frankfurt, Sao Paulo, Sydney, Tokyo,Toronto 1986 .

[2] D.A. Lloyd: Electrostatic Precipitator Handbook Bristol, 1988

[3] Harry J. White: Entstaubung industrieller Gase mit Elektrofiltern VEBDeutscher Verlag fr Grundstoffindustrie Leipzig, 1969

[4] Norbert Grass: Fuzzy Logic-Optimising IGBT Inverter for ElectrostaticPrecipitators, Conference Proceedings IEEE IAS 1999 annual meeting,Phoenix, 4-7. Oct. 1999

[5] Norbert Grass: Fuzzy-Logik-gesteuerter Spannungs-Zwischenkreis-

Umrichter fr Elektrofilter, Dissertation 1997, University of Erlangen,Germany

[6] Norbert Grass: Application of different types of high voltage supplies onindustrial electrostatics precipitators, IEEE Trans. Plasma Sci. Vol. 28,no. 5, 2000, pp. 1481-1485

[7] Norbert Grass: Fuzzy-Logic-Based Power Control System forMultifield Electrostatic Precipitators IEEE Transactions on IndustryApplications, Vol. 38, No 5, 2002

[8] Norbert Grass, Werner Hartmann, Michael Klckner: Application of

different types of high-voltage supplies for electrostatic precipitatorsIEEE Transactions on Industry Applications, Vol. 40, No 6, 2004

[9] Lucian Dascalescu, Michaela Mihailescu, Akira Mizuno, "Thebehaviour of conductive particles in pulsed corona fields", Journal ofPhysics, D:Applied Physics. 29 (1996), pp. 522-528G.

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enhanced performance for electrostatic precipitators fuzyy logic control

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