Device for exciting synchronous machine

A device for exciting a synchronous machine, comprises an exciter electrically connected to an exciting winding of the synchronous machine, a starting unit connected in parallel with the exciter and comprising at least one branch having a resistor and a thyristor switch placed in series. The control circuit of the thyristor switch has a threshold element. A protective thyristor is placed parallel to the branch of the starting unit and its control electrode is connected via the commutator of a current sensor of the starting unit branch to the control circuit of the protective thyristor. A safety device is inserted in series into the starting unit branch circuit.

FIELD OF THE INVENTION 
The present invention relates to electrical engineering and, in particular, 
to devices for exciting synchronous machines. 
The invention can be employed for excitation of synchronous machines and in 
starting systems, circuits for overvoltage protection of the exciting 
winding, and field discharge units of synchronous machines. 
DESCRIPTION OF THE PRIOR ART 
Known in the art are devices for excitation of synchronous machines (cf., 
for example, P. P. Vershinin and L. Ya. Hashper "Employment of Synchronous 
Electric Drives in Metallurgy"/in Russian/, Moscow, Metallurgy Publishers, 
1974, p. 130), comprising an exciter electrically connected to the 
exciting winding. Placed parallel to the output of the exciter is a 
starting unit comprising a resistor and a thyristor switch connected in 
series. The starting unit may be made up two or more parallel branches, 
each having a resistor and a thyristor switch. As a rule the thyristor 
switch control circuit comprises a threshold element which is voltage 
operated, for example, a voltage regulator tube. 
As a synchronous machine, for example a synchronous motor, is started, the 
thyristor switch connects the resistor parallel to the motor exciting 
winding. Here much better starting characteristics of the motor are 
obtained as well as better protection of the exciting winding and the 
excitor against overvoltages. 
In case overvoltages occur in the exciting winding of the synchronous 
machine (for example, during asynchronous operation), a control pulse 
passes through the threshold element placed in the control circuit of the 
thyristor switch. The thyristor switch is driven into conduction and 
connects the resistor parallel to the exciting winding thus damping the 
overvoltages. 
The resistance of such a resistor normally exceeds that of the exciting 
winding by a factor of from 2 to 10 in order to ensure an adequate 
starting torque during motor starting, as well as to restrict 
overvoltages. 
The magnitude of current flowing through the resistor and thyristor switch 
during the motor starting is comparable to the nominal exciting current. 
The duration of such current depends upon the starting process and does 
not usually exceed several seconds. At the end of the starting period 
(motor pulling into step) the thyristor switch cuts off the resistor and 
in normal operation, therefore, no current flows through the resistor. 
In case the thyristor switch is damaged, however, there might be a flow of 
current through the resistor in the process of normal motor operation. As 
a result, losses grow and the thermal balance of the equipments upset in 
the exciting system. As no information is available on the fact that 
current flows through the resistor for a long time, an accident prone 
situation might develop and the emergency cutoff procedure is used to 
disconnect the motor from the power source. 
Known in the art is a device for exciting a synchronous motor (cf., for 
example, "Exciters TE 8-320/5", Catalogue 05.03.145-77, Informelectro), 
which comprises an exciter, a starting unit connected parallel to the 
output of the exciter and composed of a resistor and a thyristor switch 
placed in series and a current sensor of the starting unit, having a 
delayed-action commutator. 
In the known device the current sensor comprises a current relay, an 
electronic time relay and an output relay. The current relay is inserted 
into the starting unit circuit, whereas the contacts of the current relay 
are connected into the control circuit of the electronic time relay. 
As the contacts of the current relay are closed, the electronic time relay 
switches on the output relay with a specific time delay. 
In this way the current sensor of the starting unit ensures information on 
any fault in the operation of the starting unit, which permits prevention 
of further damage. But this is possible only by disconnecting the exciting 
system and, consequently, the synchronous motor from the mains. 
In the known device, therefore, any fault of the thyristor switch can 
disable the exciting device and the motor. 
SUMMARY OF THE INVENTION 
The object of the invention is to improve the reliability of operation of a 
device for exciting of a synchronous machine. 
Another object of the invention is further improvement of reliability of 
operation of a device for exciting a synchronous machine by preventing 
overvoltages in the exciting winding of the synchronous machine. 
The invention consist of a device for exciting a synchronous machine, 
comprising an exciter electrically connected to an exciting winding of the 
synchronous machine, a starting unit connected parallel to the exciter and 
comprising at least one branch composed of a resistor and a thyristor 
switch placed in series, the control circuit of said thyristor switch 
having a voltage actuated threshold element, and a current sensor of the 
starting unit branch, which is equipped with a delayed-action commutator. 
According to the invention, a safety device is connected in series into 
the circuit of the starting unit branch, a protective thyristor is 
connected parallel to the starting unit branch and a protective thyristor 
control unit is connected to the control electrode of said thyristor via 
the delayed-action commutator of the current sensor of the starting unit 
branch. 
It is desirable that in a device for exciting a synchronous machine, 
according to the invention, the control unit of the protective thyristor 
is the resistor of the starting unit branch in order to simplify the 
control unit of the protective thyristor and to insure self-contained 
operation of the device. 
It is preferable in a device for exciting a synchronous machine, according 
to the invention, for a field discharge contactor is to be included into 
the exciting winding circuit. One output is connected to the safety device 
of one branch, and the other output is connected to the adjoining branch 
of the starting unit in order to expand functional capabilities of this 
starting unit.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a schematic diagram of a device for exciting a 
synchronous machine. 
A device for exciting a synchronous machine comprises an exciter 1 usually 
composed of a thyristor converter 2 and a group of transformers 3. The 
exciter 1 is intended for feeding an exciting winding 4 of a synchronous 
machine 5 with an automatically adjusted direct current. A starting unit 6 
is connected parallel to the input of the exciter 1, which is intended for 
starting the synchronous machine 5 and protecting the exciting winding 4 
and the exciter 1 from overvoltages. The starting unit 6 has at least one 
branch 6' made up of two series-connected elements: a resistor 7 and a 
thyristor switch 8 composed of two thyristors 9 connected in antiparallel. 
The control circuit of each thyristor 9 has a threshold element 10 which 
is voltage actuated, for example, a VR tube. In order to make the 
thyristor 9 nonconductive in the normal operating conditions of the 
synchronous machine 5 it is necessary that the voltage setting (that is 
the actuating voltage of the VR tube) be fixed higher than the supply 
voltage. A safety device 11 is inserted in series in the circuit of the 
branch 6' of the starting unit 6 and a protective thyristor 12 is placed 
parallel to the branch 6' of the starting unit 6. The protective thyristor 
12 and the safety device 11 are designed for cutting the starting unit 6 
off in case of any fault therein. A control circuit 13 is inserted in the 
control circuit of the protective thyristor 12 and connected to the 
control electrode of the protective thyristor 12 via a delayed-action 
commutator 14 of a current sensor 15 of the branch of the starting unit 6 
(in this case the commuator is a contact). 
In the abovementioned diagram the current sensor 15 of the branch of the 
starting unit 6 and the commutator 14 are made as one device--a thermal 
time relay comprising a thermal contactor equipped with a thermal delay 
device, which is placed upon the resistor 7 and controls the heat 
generated when current flows through the branch 6' of the starting unit 6. 
Other embodiments of the current sensor 15 are possible. For example, there 
may be used such principles as direct control of current in the branch 6' 
of the starting unit 6, or control of the magnetic field produced by such 
current, or control of the voltage drop in some element of the branch 6' 
of the starting unit 6, or the principle of photoeffect, or others. The 
commutator 14 can be any device insuring the flow of current from the 
control circuit 13 towards the control electrode of the protective 
thyristor 12, for example, a relay contactor, a transistor, a thyristor 
and the like. 
Time delay can be obtained by using a time relay, a capacitor charge or 
discharge effect, the thermal inertia effect and the like. 
The control circuit 13 of the protective thyristor 12 can be embodied in 
any way as soon as it insures the flow of current of a specific shape and 
power along the circuit "control electrode--cathode" of the protective 
thyristor 12 when the commutator 14 of the current sensor 15 operates. 
It should be noted, however, that the starting unit 6 may comprise several 
parallel branches 6' composed of series-connected resistors 7 and 
thyristor switches 8. In this case each parallel branch 6' should have a 
safety device 11 and a protective thyristor 12. The control circuit 13 can 
be both individual for each branch and common for all branches. The 
current sensor 15 in this embodiment should be provided for each branch 
6'. 
The embodiment of the control circuit of the protective thyristor 12 
illustrated in FIG. 2 differs from the embodiment of FIG. 1 in that here 
the control circuit 13 of the thyristor 12 is the resistor 7 of the branch 
6' of the starting unit 6, which insures voltage in the control circuit of 
the protective thyristor 12 when current flows in said branch 6' of the 
starting unit 6 through the closed commutator 14. 
The embodiment of a device for exciting a synchronous machine of FIG. 3 
differs from the embodiments of FIGS. 1 and 2 in that here a field 
discharge contactor 16 is inserted into the circuit of the exciting 
winding 4. One output 17 of said contactor 16 is connected to the safety 
device 11 of one branch 6', whereas the other output 18 is connected to 
the safety device 11 of an adjacent branch 6' of the starting unit 6. It 
should be pointed out that there might be several such branches 6' 
depending on the power of the starting device and criteria of reliability. 
In this case protection is provided even if one of the additional branches 
is disconnected. 
The contactor 16 for field discharge is intended for intensified discharge 
of the field of the synchronous machine 5 in case of internal short 
circuiting in the synchronous machine 5 by maintaining a permanent arc 
voltage on its opened contacts. 
In this case the extreme right branch 6' of the starting unit 6 is intended 
for protection of the exciting winding 4 of the synchronous machine 5 from 
overvoltages in case of discontinuity of the arc of the contactor 16 and 
presence of some residual electromagnetic energy in the exciting winding 
4. The threshold elements 10 (the same VR tube in this case) of the 
thyristors 9 of said branch 6' have the same voltage actuation setting as 
the voltage actuation setting of the threshold elements 10 of the other 
branches 6' of the starting unit 6 and said branch 6', therefore, operates 
in starting conditions and all other aforementioned conditions together 
with other branches 6' of the starting unit 6, the field discharge 
contactor 16 being closed. 
The device for exciting a synchronous machine operates as follows. 
The synchronous machine 5 (FIG. 1) is brought up to a nominal speed of 
rotation in accordance with a specific program which is commonly known and 
is not explained here. Then current from the exciter 1 is supplied to the 
exciting winding 4, its magnitude depending upon the load of the 
synchronous machine 5. 
In this mode of operation of the synchronous machine 5 the voltage at the 
output of the exciter 1 is less than the actuating voltage of the 
threshold element 10 and no control pulse is generated in the thyristors 
9. The thyristors 9 of the thyristor switch 8 are nonconductive. And the 
starting unit 6 is cut off. 
During the transient operating conditions of the synchronous machine 5, 
involving overvoltages in the exciting winding 4 of the synchronous 
machine 5 (for example, asynchronous operation, starting of the 
synchronous machine, connection of the generator to the mains by 
self-synchronization method and others), if voltage in the exciting 
winding 4 or at the outputs of the exciter 1 becomes higher than the 
setting of the threshold element 10, current flows in the control circuit 
of the thyristor 9. The thyristor 9 becomes conductive and connects the 
resistor 7 parallel to the exciting winding 4, thus reducing the voltage 
in the exciting winding 4 to the value determined by the parameters of the 
synchronous machine 5 and of the resistor 7. 
If two or more parallel branches 6' of the starting unit 6 are switched on 
according to the embodiment of FIG. 3, all parallel branches 6' of the 
starting unit 6 are connected to the exciting winding 4 in the above 
mentioned transient operating conditions, thus lowering the overvoltage. 
The starting unit 6 is cut off by the known method, for example, through 
the control system of the exciter 1. 
During emergency conditions, when the synchronous machine 5 has an exciting 
system as in FIG. 3, the contactor 16 contributes to intensified discharge 
of the field of the synchronous machine 5, which is necessary to curb 
further development of the accident. 
This is achieved by maintaining a permanent arc voltage on the opened 
contacts of the contactor 16. 
In some cases, for example incomplete disconnection of phases during cutoff 
of the synchronous machine from the mains, arc extinction may occur before 
the electromagnetic energy of the field of the synchronous machine 5 is 
discharged fully. The winding 4 in this case is subjected to severe 
overvoltage. When such overvoltage is larger than the actuating voltage 
setting of the threshold element 10 of the thyristor switch 8, the 
thyristors 9 of the extreme right branch 6' are turned on as described 
above and connect the resistor 7 of the additional branch 19 parallel to 
the exciting winding, thus reducing the voltage in said winding. 
This branch 6' of the starting unit 6 is disconnected only when the 
electromagnetic energy of the field of the synchronous machine 5 is 
completely discharged as current dies out in the circuit of the thyristors 
9 of the above mentioned branch 6'. Other branches 6.sup.1 of the starting 
unit 6 connected parallel to the exciter 1 protect this exciter 1 from 
overvoltage. Such overvoltage may occur in the exciter 1 in the above 
described conditions, particularly if the group of transformers 3 which 
supplies power to the converter 2 of the exciter 1 has current components. 
Other branches 6' of the starting unit 6 are turned on and off as 
described above. 
The above given modes of operation are in fact normal technological 
operational conditions of the synchronous machine. 
The device for exciting a synchronous machine will operate as follows in 
case of faulty operation of the thyristor switch 8 (FIG. 1) of the branch 
6' of the starting unit 6 (for example in case of breakdown of the 
thyristor 9). Current flowing through the resistor 7 is now determined by 
voltage in the exciter 1 and resistance of the resistor 7. The resistor 7 
warms up. The heat produced in the resistor 7 is monitored by the thermal 
contactor functioning as a sensor and within a period determined by the 
time setting of the thermal contactor its contact closes. A control pulse 
is sent from the control circuit 13 of the protective thyristor 12 via the 
commutator 14 to the protective thyristor 12. The protective thyristor 12 
becomes conductive and bypasses the branch 6' of the starting unit 6. The 
short circuit current of the exciter 1 passes through the thyristor 12 and 
the safety device 11. The safety device 11 blows and cuts the starting 
unit 6 and the protective thyristor 12 from the exciter 1. The synchronous 
machine 5 goes on working since the faulty branch 6' is cut off. If the 
starting unit 6 is composed of several parallel branches 6', disconnection 
of one such branch in no way impairs the efficiency of operation of the 
device for exciting a synchronous machine. Other serviceable parallel 
branches 6' take care of protection of the starting unit 6. 
When the control circuit 13 is made as in the embodiment of FIG. 2, the 
control voltage for the protective thyristor 12 is fed from the resistor 
7. 
This voltage is produced by the resistor 7 when current flows through the 
branch 6' of the starting unit 6. Consequently, in case of fault in the 
starting unit 6 the voltage of the required polarity is produced in the 
resistor 7 of the faulty branch 6'. This voltage is supplied, when the 
commutator of the contactor 14 closes, to the control electrode of the 
protective thyristor 12. The protective thyristor 12 is made conductive 
and the faulty branch of the starting unit 6 is cut off as described 
above. 
The proposed device for exciting a synchronous machine permits 
disconnection of the starting unit 6 or its branch 6' in case of faults in 
the circuits of the starting unit 6 and, consequently, prevention of 
accidents without cutting off the synchronous machine 5. 
When the starting unit 6 has several parallel branches 6', disconnection of 
a faulty parallel branch 6' of the starting unit 6 insures both 
uninterrupted operation of the synchronous machine 5 and protection of the 
exciting device by other branches 6'. 
The embodiment of FIG. 2 permits elimination of the control circuit 13 of 
the protective thyristor 12 and of any external power source for the 
protective thyristor control circuit. It also ensures fairly reliable 
separation of circuits connected to the exciting winding 4 from the 
circuits of the automatic equipment of the synchronous machine. 
Moreover, the proposed device permits protection of the exciting winding 4 
and the exciter 1 against overvoltages not only when the field discharge 
contactor 16 is closed (for example during starting, asynchronous 
operation and the like) but also when the field discharge contactor 16 is 
disconnected. No additional protective discharger is required in this 
case. It should be also noted that the power of the starting unit 6 is not 
increased, since the number of branches and the total volume of the 
starting unit 6 depend upon the starting operating conditions which 
account for most of energy produced in the circuits of the starting unit 
6, whereas the energy left in the exciting winding after extinction of the 
arc of the field discharge contactor 16 is significantly less and can be 
easily dissipated in the branches 6' of the starting unit 6 connected 
parallel to the exciting winding 4.