FIELD OF INVENTION
The present invention relates in general to detection of failed thyristor and
protection of power supply unit in Electrostatic Precipitator (ESP). More
particularly, the invention relates to an apparatus for detection of failed
thyristor, caused by both open circuit and short circuit faults. The
invention further relates to a method for detection of failed thristor and
protection of power supply unit in electrostatic precipitator.
BACKGROUND OF 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, has been 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
etc. The electrically charged particles are attracted towards electrode
plates, viz., 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 & discharge
electrodes must be periodically removed for further conveying of the
collected dust.
In the ESP, the electrostatic precipitation is achieved by controlling the
transformer rectifier set by a dedicated ESP controller. An ESP installation
consists of a plurality of fields, each field being controlled by a dedicated
ESP controller. The power input to transformer rectifier set is regulated by
back-to-back thyristor as per the control signal to gate from dedicated ESP
controller.

Failure of thyristor in ESP power supply unit results in poor performance of
the ESP i.e. poor dust collection. Thyristor failure generally results in
unbalanced power supply to the transformer rectifier set. Prolonged
operation with unbalanced power supply leads to core saturation of the
transformer rectifier set. This lowers the high peak voltage and affects the
average voltage formation, both of which are essential for better dust
collection in ESP.
Therefore, there is a need to propose an apparatus end method to quickly
detect a failed thyristor during operation of the ESP and protect the
transformer rectifier unit.
US Patent No. 6,211,792 B1, entitled, "Methods and Apparatus for
detecting a failed Thyristor" in which both the open and short failures is
determined based on absolute value of the voltage across the thyristor
compared with threshold value.

European Patent No. 1 210 613 B1, entitled "Methods and Apparatus for
Detecting a failed Thyristor" which discloses that open fault of thyristors, is
determined by comparing peak and average power taken by the load. The
peak and average power are derived from instantaneous voltage and
current by the processing system.
The prior art discloses methods and apparatus for detection of failed
thyristors, wherein, the power supply schemes provide continuous power
input to the load. However, the methods and apparatus for detection of a
failed thyristor, disclosed in the prior art discussed herein, are not
appropriate for the schemes involving discontinuous, intermittent power
supply to the load. For example, applications may involve skipping of one
or more half cycles of the alternating voltage fed to the load.

OBJECTS OF THE INVENTION
It is therefore an object of invention to propose an apparatus for the
detection of failed thyristor and protection of power supply unit in
electrostatic precipitator, which enables continuous health monitoring of
the power supply unit, consisting of transformer rectifier unit and back-to-
back thyristors, to enhance the dust collection in ESP and thus achieve
optimum ESP operation.
Another object of the invention is to propose an apparatus for the
detection of failed thyristor and protection of power supply unit in
electrostatic precipitator, which is enabled to prevent any damage to the
power supply unit due to core saturation of the transformer rectifier unit
and avoid any unbalance in the input power supply.

Yet another object of the invention is to propose an apparatus for the
detection of failed thyristor and protection of power supply unit in
electrostatic precipitator, which enables fault detection in the back-to-back
thyristors, including regulation of the power to the ESP.
A further object of the invention is to propose an apparatus for the
detection of failed thyristor and protection of power supply unit in
electrostatic precipitator, which is enabled to detect both open and fault
short circuit faults are detected for protecting the transformer rectifier unit.
A further object of the invention is to propose a method for the detection
of a failed thyristor and protection of power supply unit in electrostatic
precipitator.

SUMMARY OF THE INVENTION
Method and apparatus consistent with this invention detect a failed shorted
thyristor in power supply unit for delivering power to ESP. The apparatus
consistent with this invention comprises of a voltage signal and current
signal threshold determination circuit. The threshold determination circuit
consists of hall-effect voltage sensor across the thyristor, hall-effect current
sensor in series with thyristor, operational amplifier and resistor for
threshold value determination. The conduction period observer and fault
identification circuit comprises of resistors, capacitors, diodes, operational
amplifiers, gate logics for detecting failed short thyristor, gate logics for
observing conduction period ,logic for determining actual current flow,
logic for determining voltage drop across thyristor and logic for comparing
pre-determined conduction duration with the actual conduction duration.
The method and apparatus also consists of an error annunciation circuit
comprising of opto-couplers and light emitting diodes for indicating error

condition to the embedded controller for prevention of damage to power
supply unit.
The summary and following detailed description should not restrict the
scope of the claimed invention. The accompanying drawings, which form
part of the detailed description, show an exemplary embodiment of the
invention and, together with the description, explain the principles of the
invention. There can however be other embodiments of the same
invention, all of which are deemed covered by this description.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The accompanying drawings, which are incorporated in and constitute a
part of this specification, illustrate embodiments of the invention and
together with the description, serve to explain the principles of invention.
In these drawings,

Figure 1 is an overall system block diagram, consistent with this invention,
of overall system consisting of line-to-line power supply, back-to-back
thyristor, transformer rectifier unit, ESP, comparator device, firing card and
ESP controller.
Figure 2 is hardware block diagram of comparator device consisting of
threshold determination, conduction period observer and fault identification
circuit.
Figure 3 is a flow chart, consistent with this invention, of a method for
detecting failed thyristor and protecting power supply.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE
INVENTION
Fig. 1 is an overall system block diagram, consistent with this invention.
The overall system consists of line-to-line power supply, back-to-back

thyristor, transformer rectifier unit, ESP, comparator device, firing card
and ESP controller. In the overall system block diagram, block (9) is the
line-to-line ac power supply. The back-to-back thyristor (3) is connected in
series with the primary of transformer rectifier unit (7) and regulates the
power input, thus regulating the power to ESP (8). The transformer
rectifier unit (7) consists of step-up transformer and bridge rectifier to
supply high voltage rectified DC to ESP (8). The hall-effect current sensor
(1) senses the current to transformer rectifier unit (7). Likewise, the hall-
effect voltage sensor (2) senses the voltage across the thyristor (3). The
ESP controller (6) reads the feedback, i.e. secondary voltage and current,
and arrives at the next conduction duration for current flow to ESP. The
firing card (5) energizes the gate signal to thyristor (3) for the given
conduction duration and opens up power flow to the transformer rectifier
unit.

In the overall system block diagram, block (4) represents the proposed
invention i.e. comparator device. The comparator device receives
predefined conduction duration, current signal through thyristor, voltage
across thyristor; identifies fault condition if any and indicates the fault
status to the ESP controller for further actions.
Figure 2 illustrates the hardware block diagram of comparator device (4)
consisting of predetermined conduction period observer circuit (10),
Voltage signal threshold circuit (11), Current signal threshold circuit (12),
Actual voltage drop detection circuit (13), Actual current conduction
detection circuit (14) and Error indication circuit (15).
The conduction period observer circuit (10) observes the conduction
duration received from the ESP controller (6) and compares it with the
actual duration of conduction in thyristor (3). The hall-effect voltage sensor
(2) indicates the voltage across the thyristor to voltage signal threshold
circuit (11) for comparing with the threshold value. Actual voltage drop

detection circuit (13) finds out the voltage drop duration. Likewise, the
hall-effect current sensor (1) indicates the current through the thyristor to
current signal threshold circuit (12) for comparing with the threshold value.
Actual current conduction duration detection circuit (14) finds out the
current flow duration. The error indication circuit (15) analyses the voltage
conduction, current conduction and the conduction duration and determine
the fault condition. The error condition is indicated to the ESP controller
(6).
Figure 3 depicts the flow chart, consistent with this invention, of a process
for detecting failed thyristor and protecting power supply unit of ESP. The
detected fault either fault short and fault open in the comparison device
are read through the ESP controller (6) for shutting down the power input
to transformer rectifier unit (7).

As shown in Figure 1, a line-to-line power supply (9) outputs sinusoidal
voltage on the line that have a positive half and a negative half. In each
half one thyristor of back-to-back thyristor conducts for given duration and
current flow to the ESP (8) through the transformer rectifier unit (7). The
duration for conduction and that of triggering the thyristor varies during
normal operation. The ESP controller(6) calculates the conduction duration
and commands triggering instantly based on the feedback parameters i.e.
current and voltage from the ESP, and a current is set through the firing
card (5), which generates pulses during said conduction period. The
triggering of the thyristor starts within a minimum duration and gradually
increase to the set value. In case of a spark exihibited in ESP, the
triggering stop for one cycle.
The hall-effect current sensor (1) measures the current including direction
of the current flow in the thyristor and provides a current signal to the
comparator card (4). It varies from 0 to 5 volts, which is proportional to
current value and direction. Similarly the hall-effect voltage sensor (2)

measures the voltage across the back-to-back thyristor (3) and provides
the voltage signal proportional to voltage across the thyristor to the
comparator card (4). Both the current signal and voltage signal are used to
re-construct a conduction duration signal which is compared with the
original conduction duration signal for fault detection.
During the operation, when one thyristor fail short, there is a heavy current
flow in one half of power supply. The re-constructed conduction signal
becomes equal to half cycle. Method and apparatus consistent with this
invention detects the current through the thyristor and voltage across the
thyristor, re-construct the conduction duration signal, compare with the
original conduction duration and indicates the failed short of thyristor.

During operation, when one thyristor fail open, there is no current flow in
one half of power supply. The re-constructed conduction signal becomes
zero. Method and apparatus consistent with this invention detects the
current through the thyristor and voltage across the thyristor, re-construct
the conduction duration signal, compare with the original conduction
duration and indicates the failed open of thyristor.

WE CLAIM :
1. An apparatus for detecting a failed shorted thyristor in power
supply unit for delivering power to a load, said apparatus
comprising :
a circulatory device for determining voltage signal and current
signal threshold, the device comprising a hall effect voltage sensor
disposed in series with a resistor installed across the thyristor; an
hall-effect current sensor placed in series with at least one
thyristor, an operational amplifier, and a resistor for threshold
value determination;
a conducdtion period monitor including a fault identification circuit
comprising resistors, capacitors, diodes, operational amplifiers, a
first gate logic for detecting failed short thyristor, a second gate
logic for monitoring conduction period provided by an embedded
controller, a first logic for determining the actual current flow
duration and voltage drop duration across the at least one and a
second logic for comparing pre determined conduction duration
with actual conduction duration;

an error annunciation circuit comprising opto isolators and light
emitting diodes for indicating error condition to the embedded
controller, the embedded controller being coupled to the error
annunciation circuit for prevention of damage to power supply
unit.
2. The apparatus as claimed in claim 1, wherein the load constitutes
the transformer rectifier unit and the Electrostatic Precipitator.
3. The apparatus as claimed in claim 1, wherein the predetermined
conduction duration constitutes an allowed conduction duration
provided by the embedded controller, the current being allowed to
flow through the thyristor to the ESP during said conduction
duration period.
4. A method of detecting failed shorted thyristor in power supply unit
for delivering power to a load, said method comprising the steps
of:
- detecting a threshold value for the voltage signal and voltage drop
duration using the operational amplifiers, the resistors and the hall
effect voltage sensor placed in series with a resistor and disposed
across the thyristor;

- detecting a threshold value for the current signal and current flow
duration using the operational amplifier, and a hall effect current
sensor in series with the thyristor;
- monitoring predetermined conduction duration provided by the
embedded controller and identifying fault failed short thyristor when
current flow duration and voltage drop duration across the thyristor
exceeds said predetermined conduction duration;
- Indicating error condition to the embedded controller when the error
is identified; and
- Preventing damage to the power supply unit by shutting down the
input power based on the command from the embedded controller,
in the event of an error condition.

5. The method as claimed in claim 4, wherein the load constitutes
the transformer rectifier unit and the Electrostatic Precipitator.
6. The method as claimed in claim 4, wherein the predetermined
conduction duration constitutes an allowed conduction duration

provided by the embedded controller, the current being allowed to
flow through the thyristor during said conduction period.
7. An apparatus for detecting a failed open thyristor in power supply
unit for delivering power to a load, said apparatus comprising:
- a circuitoy device for threshold value determination of a voltage
signal and current signal threshold, the device comprising a hall
effect voltage sensor in series with a resistor disposed across the
thyristor, a hall effect current sensor in series with the thyristor, an
operational amplifier, and a resistor for threshold value
determination;
- a conduction period monitor and fault identification circuit comprising
resistors, capacitors, diodes, operational amplifiers, a first gate logic
for detecting failed short thyristror, a second gate logic for
monitoring conduction period provided by the embedded controller, a
first logic for determining the actual current flow duration and voltage
drop duration across the thyristor, and a second logic for comparing
the pre determined conduction duration with actual conduction
duration;

- an error annunciation circuit comprising opto isolators and light
emitting diodes for indicating error condition to the embedded
controller;
the embedded controller being coupled to the error annunciation
circuit for prevention of damage to power supply unit.
8. The apparatus as claimed in claim 7, wherein the load constitutes
a transformer rectifier unit and the Electrostatic Precipitator.
9. The apparatus as claimed in claim 7, wherein the predetermined
conduction duration is the allowed conduction duration provided
by the embedded controller, the current being allowed to flow
through thyristor during said duration period.
10. A method of detecting failed open thyristor in power supply unit
for delivering power to a load, said method comprising :
- detecting voltage signal threshold value and voltage drop duration
using operational amplifiers, resistors and hall effect voltage sensor
(in series with a resistor) placed across the thyristor;

- detecting current signal threshold value and current flow duration
using operational amplifier and hall effect current sensor in series
with thyristor;
- monitoring a predetermined conduction duration provided by the
embedded controller and identifying fault failed open thyristor when
current flow duration and voltage drop duration across the thyristor
exceeds said predetermined conduction duration;
- Indicating an error condition to embedded controller when error is
identified; and
- shutting down the input power by means of the embedded controller,
in the event of an error condition which preventing the damage to
the power supply.

11. The method as claimed in claim 10, wherein the load constitutes
the transformer rectifier unit and the Electrostatic Precipitator.
12. The method as claimed in claim 10, wherein the predetermined
conduction duration is the allowed conduction duration provided
by the embedded controller, the current is allowed to flow through
thyristor during said allowed conduction period.

The invention relates to an An apparatus for detecting a failed shorted
thyristor in power supply unit for delivering power to a load, said apparatus
comprising : a circulatory device for determining voltage signal and current
signal threshold, the device comprising a hall effect voltage sensor
disposed in series with a resistor installed across the thyristor; an hall-
effect current sensor placed in series with at least one thyristor, an
operational amplifier, and a resistor for threshold value determination; a
conduction period monitor including a fault identification circuit comprising
resistors, capacitors, diodes, operational amplifiers, a first gate logic for
detecting failed short thyristor, a second gate logic for monitoring
conduction period provided by an embedded controller, a first logic for
determining the actual current flow duration and voltage drop duration
across the at least one and a second logic for comparing pre determined
conduction duration with actual conduction duration; an error annunciation
circuit comprising opto isolators and light emitting diodes for indicating
error condition to the embedded controller, the embedded controller being
coupled to the error annunciation circuit for prevention of damage to
power supply unit.