FIELD OF THE INVENTION
The present invention relates to a control system of electrostatic precipitator
used for air pollution control. More particularly, the invention relates to a method
and apparatus for controlling energization to improve the efficiency of
electrostatic precipitator. Further, the invention relates to an advanced Back
Corona control system for controlling energization of electrostatic precipitator.
BACKGROUND OF THE INVENTION
Electrostatic precipitation is one of the most effective ways to control air
pollution generated by industrial emissions. This technique, which has proven
highly effective in controlling air pollution, is used for removal of undesirable
particulate matter from a gas stream by electrostatic precipitation. Electrostatic
precipitator (hereafter referred to as ESP) is an air pollution control device
designed to electrically charge and collect particulates generated from industrial
processes such as those occurring in power plants, cement plants, pulp and
paper mills and utilities. The electrically charged particles are attracted towards
electrode plates, fro example discharge electrode and collection plate. ESP is
divided into a plurality of fields depending on the dust load. During continuous
operation of an electrostatic precipitator, the dust from collector plates and
discharge electrodes must be periodically removed for further conveying of the
collected dust.
In an ESP, the electrostatic precipitation is achieved by applying High Voltage
Negative DC power between a Collection Electrode (CE) and an Emitting
Electrode (EE). The High Voltage Power supply comes in different versions viz.,
Linear Power Supply System and Switch-Mode Power Supply System. Depending
on specific collection efficiency. Power supply system is chosen for an Electric

Field.
There are various parameters, which are affecting the collection efficiency of the
ESP viz., particulate size, resistivity of dust particles, operating temperature,
specific collecting area and Back Corona, other parameter controls are primarily
addressed during sizing of the ESP.
Back-Corona is an effect of back ionization occurring within the dust layer at the
Collection Electrode and acts against the forward Corona generated at the
Emitting Electrode. Back-lonization is created by a potential drop caused by
resistivity of the dust particles collected in the collecting electrode. The Back
Corona neutralizes the ionic charges produced by the Forward Corona.
Therefore, for improving ESP Efficiency, it is essential to restrict or to eliminate
Back Corona during operation.
In the prior art, mechanisms exist to detect Back Corona by various methods.
However, for controlling Back Corona, TIME-DELAY between current pulses is the
most used method. TIME-DELAY causes the potential build-up.
In the prior art, mechanism exists to vary the TIME-DELAY using Half-Cycle
skipping (Intermittent Charging) and firing angle adjustment for a Linear Power
Supply configuration, where thyristor is the power switching component.
However, for thyristor configuration, it is not possible to achieve TIME-DELAY
precisely to the required level, as half-cycle skipping restricts the delay into half-
cycle bands.
In the prior art, mechanism exists to adjust the TIME-DELAY to a required level
using the half-cycle skipping. However, this is restricted to a constant ratio.

In the prior art, mechanism does exist to allow positive half cycle first and to
allow negative half cycle after the required TIME-DELAY, which causes the
transformer core to saturate if TIME-DELAY is considerably high in terms of
seconds.
In the prior art, mechanism does not exist, where the TIME-DELAY is dynamically
linked to RAPPING frequency.
In the prior art, mechanism also does not exist, where Higher intermittent
charging factor is modulated with smaller intermittent charging factor for
controlling back-corona in ESP.
Mechanism also does not exist in prior art, for controlling TIME-DELAY between
charge pulses, continuous multiple negative or positive half-cycles used
intelligently for precise control of TIME-DELAY.
EP184922A2 1986-06-18 teaches a method of controlling intermittent voltage
supply to an electrostatic precipitator.
US4311491A 1982-01-19 discloses an Electrostatic precipitator control for high
resistivity particulate.
US4987839A 1991-01-29 describes a method for removal of particulate matter
from combustion gas streams.
W09842444A1 1998-10-01 describes a method to Control current supply to an
Electrostatic Precipitator.
WO9420218A1 1994-09-15 describes a method to Controlling the supply for

conditioning agent to an Electrostatic Precipitator.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an advanced back-corona
control system to dynamically control a permissible discharge time using an
improved intermittent energization method providing precise TIME-DELAY
between pulses.
Another object of the invention is to propose an advanced back-corona control
system to dynamically control a permissible discharge time using an improved
intermittent energization method providing precise TIME-DELAY between pulses,
which eliminates saturation.
A further object of the invention is to propose an advanced back-corona control
system to dynamically control a permissible discharge time using an improved
intermittent energization method providing precise TIME-DELAY between pulses,
which is enabled to dynamically alter the TIME-DELAY by linking Rapping interval
of the ESP field.
A still further object of the invention is to propose an advanced back-corona
control system to dynamically control a permissible discharge time using an
improved intermittent energization method providing precise TIME-DELAY
between pulses, which modifies the time delay dynamically corresponding to the
loading factor of dust.
SUMMARY OF THE INVENTION
The present invention provides an advanced back-corona control system for ESP

energization by precisely controlling the TIME-DELAY between energization
linking, avoiding transformer core saturation, and linking the TIME-DELAY with
RAPPER INTERVAL.
The control logic, which is implemented according to the invention, uses a
microprocessor based Electronic Controller (EC), which continuously monitors the
number of positive and negative half cycles fired from a reference point,
representing an end of rapping activity for a field. The required TIME-DELAY is
continuously adjusted before the next rapping, thereby estimating the dust
thickness formed at the electrode surface. The required TIME-DELAY to be
achieved is implemented by the control logic by combining positive and negative
half cycles with firing angle control energization.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Hardware block diagram of a Micro processor based Electronic
Controller (EC).
Figure 2 - Shows TIME-DELAY implementation using half cycle and firing
angle control
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Figure 1 shows a hardware block diagram of the advanced back-corona control
system for efficiently operating an Electrostatic precipitator.
In Figure 1, block (1) is a data memory block, where the operating data during
execution of improved method is stored and retrieved. Block (2) is a Central

Processing Unit (CPU) of the system, where processing of operational data
occurs. Block (3) is a permanent memory, where the program which implements
the improved method is stored permanently. Block (4) is an output block, where
required pulses for activating Modified Charge Ratios are activated. Block (5),
represents an user input device, through which required parameters from the
user are received. Block (6) is a display device, which indicates the present
operating condition of the system.
In figure 2, block (7) represents the mains voltage waveform, switching at typical
frequency of 50/60Hz. The wave form is more or less sinusoidal in shape. Block
(8) represents a Modulated Intermittent Charge (MIC) factor of 1:3:8. Shaded
half-cycles represent the half-cycles, which are allowed through a switch device
for energizing the ESP. partly shaded half-cycles represent precise angle control
for achieving desired delay. Block (9) represents another example of MIC factor
1:3:20. Block (10) represents yet another example of MIC factor of 1:5:26.

WE CLAIM
1. An advanced back-corona control system to improve efficiency of
Electrostatic Precipitators, the system is configured to:
- implement a time-delay process in which the time to disallow flow
of current through dust layer in collecting electrodes of the ESP is
modified so as to allow discharge of the built-up charge in the dust
layer, wherein the combination of half-cycles and conduction angle
control of the pulses fired from a reference point is used for
energisation;
- achieve time-delays between 0 to 10 seconds by intermittent
energization according to a control logic incorporated in the
system; and
- eliminate transformer core saturation by using a modulated-
intermitent-halfcycle-ratios.

2. The system as claimed in claim 1, wherein the modulated-intermittent-
halfcycle charge factor 1:N:M defines the required half cycles to be
energized. The combination of delays produced by N and M factors is used
by the apparatus intelligently to avoid back-corona.
3. The system as claimed in claim 2, wherein introducing delay between
positive and negative half cycles specified as 1:N:M where 1 denotes the
first half cycle, N denotes the Nth half cycle and M denotes Mth halfcycle,
the delay between 1st and Nth achieved being (M-N-l)*half cycle time, M is
always an even number.

4. The system as claimed in claim 1, wherein the TIME-DELAY is estimated
based on the RAPPING interval, dust resistivity and ESP Field position
from the First field.
5. The system as claimed in claim 4, wherein the RAPPING interval means
the time interval between two successive rapping actions.

ABSTRACT

The invention relates to an advanced back-corona control system to improve
efficiency of Electrostatic Precipitators, the system is configured to: implement a
time-delay process in which the time to disallow flow of current through dust
layer in collecting electrodes of the ESP is modified so as to allow discharge of
the built-up charge in the dust layer, wherein the combination of half-cycles and
conduction angle control of the pulses fired from a reference point is used for
energisation; achieve time-delays between 0 to 10 seconds by intermittent
energization according to a control logic incorporated in the system; and
eliminate transformer core saturation by using a modulated-intermitent-halfcycle-
ratios.