FIELD OF THE INVENTION
The present invention relates to an automatic tracking method in multi-channel
excitation control system for synchronous machines. More particularly, the
present invention relates to a method for selecting and routing control voltage of
regulation channels in multi-channel excitation system for synchronous
machines.
BACKGROUND OF THE INVENTION
For reliable and uninterrupted operation of a synchronous machine excitation
equipment plays a vital role. Excitation Equipment has Power circuits, Regulation
circuits, Logical Control Circuits and Monitoring circuits. The power circuit using
thyristor bridges provides the DC power required to the field of the synchronous
machine, by converting the AC source voltage to a controlled DC voltage.
The Regulation circuit provides the control voltage, the magnitude of which
decides the pulse position for converter control circuits. The logical control
circuits execute the control for, operator commands, regulation circuits and field
breaker circuit depending upon the system condition. Monitoring circuits monitor
the validity of the controlled variables like Synchronous machine voltage and the
field current. Reliable operation of power circuits can be ensured by adequate
design margin in the rating of components. In the case of Regulation circuits, as
they are electronics circuits, such a design margin is not possible and hence
redundant circuits are employed. When redundant Regulation circuits are
employed, follow up control is essential for ensuring smooth switch over of
control between them, without significant change in the Synchronous machine
parameters like, Field current, Terminal voltage or Reactive power.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method for selecting and
routing control voltage of regulation channels in multi-channel excitation system
for synchronous machines.
Another object of the invention is to propose a method for determining the
difference between the controller output voltage of operating channel and any of
the standby channels, independently for display at the location of excitation
panel and at a remote location like control room.
SUMMARY OF THE INVENTION
The invention provides a method for selection of control voltage of Regulation
channels in excitation system of synchronous machine. The control voltage so
selected is extended to the follow up control circuit available in each channel.
With this, every stand by channel follows the operating channel, while on
standby.
The present invention describes the implementation of logical control of
redundant regulation circuits for follow up, based on the Controller output
voltage of the operating channel, to ensure smooth change over of control
between them without change in field current, terminal voltage or reactive power
of the synchronous machine.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Fig.l Block diagram of Excitation system
Fig. 2 Formation of signal "Controller output voltage of channel in operation" in
logic controller.
Fig.3 Balance Voltage Indication (Method of displaying balance voltage between
operating channel and any stand by channel using only one voltmeter and one
selector switch).
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 shows the block diagram of Excitation system. Voltage available from the
source of excitation power is rectified in thyristor bridges (TY1 and TY2) and fed
to the field of, Main exciter in the case of brushless machines or Generator in the
case of Machines with slip rings through Field Circuit Breaker FCB. Current
transformers T4 and T5 sense the field current from AC side of thyristor bridges.
Tl and T2 are Voltage transformers for sensing the synchronous machine
voltage. T3 is current transformer for sensing synchronous machine current. CHI
to CH4 are Regulation channels. CHI and CH2 are Automatic Voltage Regulator
channels (AVR1 and AVR2), regulating the terminal voltage of the machine. CH3
and CH4 are Field Current Regulator channels (FCR1 and FCR2), regulating the
Generator / Main Exciter field current. REF5, REF6, REF7 and REF8 are Reference
value Generation circuits for CHI to CH4 respectively. FC1 to FC4 are Firing pulse
generation circuits together with any intermediate pulse stage as applicable for
each one of the four channels.

Out of FC1 to FC4, only one will be outputting pulses to pulse bus depending
upon the channel in operation. GCU1 and GCU2 are thyristor Gate Control Units
including pulse transformers. They pick up the pulses from pulse bus for
conduction of thyristor devices and control the output voltage from the thyristor
bridges for control of field current.
The follow up control blocks Dl to D4 are built as part of the Firing pulse
Generation circuits FC1 to FC4 respectively. When the no. of channels is only two
a connecting cable can ensure that the Controller output voltage of the operating
channel (say channel-1) is available for comparison and error detection in the
follow up control circuit of the stand by channel i.e., channel-2 and vice versa.
The task ahead is to ensure, when there are three or more channels and when
one of the channels is in operation, follow up control circuits of all the three
stand by channels get the controller output voltage of the channel in operation,
for comparison, error detection and raise / lower control of reference values.
Formation of Signal Controller output voltage of channel in operation :
Fig. 2. provides the details of the how the signal, Controller output voltage of
channel in operation, has been formed and routed.
1 to 4 are the Controller output voltages (analog signal) of regulation channels 1
to 4. One out of CHI to CH4 will be True / High depending upon the channel in
operation. A combination of Analog switches and adder does the work to form
the signal "Controller output voltage of channel in operation".

For example, analog switch 5 will output UC1 when Channel 1 is in operation and
all other analog switches i.e., 6 to 8 will output a value of zero. Thus at the
output of summing amplifier / adder gate 9, only the controller output voltage of
channel 1 will be available. Similarly, if any other channel is in operation, output
of corresponding analog switch among 6 to 8 will alone be having the controller
output voltage of corresponding channel and the output of gate 9 is always the
Controller output voltage of Channel in operation.
Selective routing of the signal "Controller output voltage of Channel in operation"
is carried out by analog switches 10 to 13. For example when channel-1 is in
operation, excepting analog switch 10, other switches 11 to 13 will output the
signal for follow up by respective channels and analog switch 10 will output zero,
as control voltage of channel-1 (i.e., channel in operation) shall not be disturbed
by follow up action.
The balance voltage indication:
When there are only two channels one voltmeter can be used for indication of
difference between the Controller output voltages of the two channels. When the
no. of channels is four, balance voltmeters for indicating the difference signals
will increase to 6 i.e., difference voltage between various combinations of
operating and stand by channel increases the number.(CHl - CH2, CHI - CH3,
CHI - CH4, CH2 - CH3, CH2 - CH4 and CH3 - CH4). To address this
requirement, circuit shown in Fig.3 has been used.
To one end of voltmeter LVM, provided for difference / balance voltage
indication, on Local of Excitation panel, Controller output voltage of channel in
operation is connected. The other end of the meter gets the Controller output

voltage of channel selected by operator for changeover. Thus one meter and,
one switch which has same no. of poles as the no. of channels, can be used for
checking the balance between operating channel and any other stand by
channel.
For, Control room / remote checking of the difference / balance voltage, RVM is
used. This meter gets the balance voltage signal from the transducer(U)
(15V...0...+15V / 4mA...12...20mA). Conversion of Voltage signal to current signal
is done considering the possible voltage drop across the cable between Excitation
panel and the control room.
When the Remote Selector Switch is selected for say Channel-3 / position-3,
relay RL3 picks up. The contact of the relay RL3 routes the controller output
voltage of channel-3 to one of the terminals of transducer. The other terminal of
transducer always gets the controller output voltage of channel in operation. For
example, when channel 1 is in operation, the Remote Voltmeter RVM can display
the balance voltage between Channel-1 and Channel-3 (RSW switch selection).
The local voltmeter can display the balance voltage between Channel-1 and
Channel-2 (LSW switch selection).

WE CLAIM :
1. A method for selecting control voltage of regulation channels in multi-channel
excitation system for synchronous machines, the synchronous machine
comprises voltage transformers (T1, T2) for sensing the machine voltage, a
current transformer (T3) for sensing machine current, the excitation system
having three or more regulation channels. Circuit considered here has four
channels (CHI to CH4), two of which (CH1, CH2) constitute automatic voltage
regulator channels regulating the terminal voltage of the machine, the other two
channels (CH3,CH4) constituting field current regulator channels, regulating the
exciter's field current; at least one each reference value generation circuits
(REF5,REF6,REF7,REF8) for each of said regulator channels (CH1 to CH4); at
least four firing pulse generation circuits (FC1 to FC4), wherein the follow-up
control blocks (D1 to D4) are available as a part of the firing pulse generation
circuits (FC1 to FC4), along with a separate programmable logic controller for
implementing the method and other control functions of excitation, the method
comprising the steps of :-
- forming a signal representing controller output voltage of channel in
operation based on four controller output voltages of, said four regulation
channels;
- carrying-out selective routing of signals via analog switches, when only
three switches outputting the signal for follow-up by the respective
channel excepting one analog switch, outputting zero as the control
voltage of this respective channel;
- determining difference or balance voltage between the Controller output
voltages of the operating channel and any one of the stand by channels
using at least one voltmeter and one selector switch for indication at local
of excitation panel; and

determining independently difference or balance voltage between the
controller output voltage of channel in operation and any standby channel
using at least one voltmeter, one selector switch, one relay unit and one
voltage to current transducer for indication at any remote location like
control room.

ABSTRACT

The invention provides a method for selection of control voltage of Regulation
channels in excitation system of synchronous machine. The control voltage so
selected is extended to the follow up control circuit available in each channel.
With this, every stand by channel follows the operating channel, while on
standby.