System for controlling AC excited synchronous machine

A control system for an AC excited synchronous machine for use in an electricity generator/motor system. The AC excited synchronous machine can be driven not only in a variable-speed operation based on 2-axis current control but also in a constant exciting frequency operation based on only direct-axis current component control. A phase signal is switched to drive stably the AC excited synchronoius machine in a self-excited operation or in a rotary phase modifying operation. Further, when it is desired to start pumping-up water, a synchronizing power is provided to keep constant the rotational speed of the machine at the time of establishing a desired water pressure. Because of the switching arrangement of the phase signal, the AC excited synchronous machine can be operated as an ordinary synchronous machine exhibiting ordinary synchronous characteristics, that is, self-excited operation characteristics, rotary phase modifying operation characteristics and pumping-up start characteristics. Even when the synchronous machine is cutoff from an AC power system and the voltage of the synchronous machine is abruptly changed, the stable self-excited operation of the machine can be realized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a variable-speed generator/motor 
system using an AC excited synchronous machine and, more particularly, to 
a system for controlling an AC excited synchronous machine which can be 
suitably used as a load adjuster/rotary phase modifier in order to improve 
the stability of an AC power system.

Description of the Prior Art 
There has been suggested a prior art generator/motor system using an AC 
excited synchronous machine which makes the most use of a function of 
being able to quickly adjust active and reactive power which is a feature 
of such an AC excited type and when the system is cut off from an AC power 
system, suppresses an exciting frequency to below a preset level, to 
thereby exhibit a self-excited operation function competitive with a DC 
excited synchronous machine. Such a prior art system will be explained 
below by referring to FIG. 1. 
In the drawing, an AC excited synchronous machine 5 has an armature winding 
5a which is connected to an AC system 1 through a synchronous circuit 
breaker 2b, a main transformer 3 and a system circuit breaker 2a. 
Directly coupled to the AC excited synchronous machine 5 is a pump/water 
wheel 4 which acts as a water wheel in an electricity generation mode and 
acts as a pump in a water pumping-up mode. 
A frequency conversion unit 6 includes 3-phase exciting transformers 7A, 7B 
and 7C connected to the AC system 1, and 3-phase thyristor power 
converters 8 for converting AC system frequency powers transformed by the 
transformers 7A, 7B and 7C into low frequency AC power respectively, these 
transformers being independently provided with respect to the different 
phases of an exciting winding 5b of the AC excited synchronous machine 5. 
A phase detection unit 9 includes a voltage transformer 10 for detecting a 
system voltage phase .theta..sub.v, a voltage phase operator 11, a 
resolver 12 for detecting a rotary phase .theta..sub.r expressed in terms 
of an electrical angle of the AC excited synchronous machine 5, and a slip 
phase operator 13. 
An exciting current controller 14 functions to adjust the amplitude and 
phase of 3 phase AC current commands rotating together with the slip phase 
.theta..sub.s on the basis of current commands I.sub.q and I.sub.d of two 
mutually perpendicular axes, i.e., a quadrature axis and a direct axis, 
received therefrom, and to supply to automatic pulse phase shifters 16A, 
16B and 16C firing angle signals 15A, 15B and 15C which are then supplied 
to associated thyristors of the thyristor power converters 8 as main 
switching elements, so that the exciting winding currents of the AC 
excited synchronous machine 5 coincide with the 3 phase AC current 
commands. As a result, automatic pulse phase shifters 16A, 16B and 16C 
output firing pulse signals 17A, 17B and 17C respectively. 
In this case, the current commands adjusted by the exciting current 
controller 14 may be provided in such a method as described, for example, 
in JP-B-53-7628 and JP-B-57-60645. 
That is, when this method is employed, the current command I.sub.q with 
respect to a current component in phase with the slip phase .theta..sub.s 
is adjusted to control an effective power output, while the current 
command I.sub.d with respect to a current component shifted by 90 degrees 
from the slip phase .theta..sub.s is adjusted to control a voltage. 
The effective power output and voltage are obtained from a current 
transformer 18 and the voltage transformer 10, converted into 
predetermined DC signals at a P, V sensor 19 and then applied to an 
automatic effective-power regulator (APR) 20 and an automatic voltage 
regulator (AVR) 21, respectively. 
Meanwhile, a slip phase signal issued from the slip phase operator 13 is 
supplied to a voltage/frequency converter (F/V) 22 to be converted into a 
slip frequency signal f.sub.s therein. The slip frequency signal f.sub.s 
is converted into an output correction command .DELTA.P.sub.o through a 
dead zone circuit 23 for generating a signal only when the slip frequency 
signal f.sub.s is out of a range (-f.sub.m .ltoreq.f.sub.s 
.ltoreq.+f.sub.m, where the frequency f.sub.m is determined by the output 
voltage limit of the frequency conversion unit 6) and through a 
first-order time lag element 24 for moderating its output variation. The 
output correction command .DELTA.P.sub.o is further added at an adder 20a 
to an output command P.sub.o to be inputted to the automatic 
effective-power regulator 20. In this case, these commands .DELTA.P.sub.o 
and P.sub.o are defined as positive when the AC excited synchronous 
machine 5 is operated as a motors and driven in its accelerating 
rotational direction in the water pumping-up mode. 
The aforementioned prior art has had such a problem that no consideration 
is paid to the rotary phase modifying operation in which even the AC 
exciting operation has substantially no effect on the ordinate-axis 
current component control, whereby the operation control system is 
complicated. 
The prior art also has another problem that no consideration is paid to the 
fact that the adjustment of the ordinate-axis current command for the 
purpose of suppressing the slip frequency in the self-excited operation 
mode causes the generator voltage to be varied, so that when the voltage 
is abruptly changed such as when the system is cut off, the 
quadrature-axis current command causes the deterioration of the voltage 
control characteristics, thus resulting in that the self-excited operation 
sustaining ability of the system is inferior to that of a synchronous 
machine. 
SUMMARY OF THE INVENTION 
It is a major object of the present invention to provide a system for 
controlling an AC excited synchronous machine which can suppress an abrupt 
change in a voltage when cut off from an AC system and also can easily 
realize a stable self-excited operation. 
Another object of the invention is to realize a control system in a 
variable-speed pumped-storage hydroelectric plant which can smoothly 
switch between a rotary phase modifying operation and a load operation in 
both rotational directions as a water wheel and a pump. 
A further object of the invention is to provide a control system which can 
stably maintain a predetermined rotational speed even when the load of a 
pump is abruptly changed at the time of starting pumping-up water. 
In accordance with one aspect of the present invention, the above first 
object is attained by providing such a control system that can realize not 
only a variable-speed operation of an AC excited synchronous machine based 
on 2-axis current control but also a synchronous operation of a constant 
exciting frequency based on an only-direct-axis current component control. 
In accordance with another aspect of the present invention, the above 
second object is attained by providing a control system that can switch 
between phase signals to thereby provide stable self-excited and rotary 
phase modifying operations. 
In accordance with a further aspect of the present invention, the above 
third object is attained by a control system which produces a 
synchronizing power when a water pressure is established in a water 
pumping-up start mode. 
The switching of the phase signal enables the AC excited synchronous 
machine to be used to exhibit its ordinary synchronous machine 
characteristics. As a result, the control system can show self-excited, 
rotary phase modifying and pumping-up start characteristics competitive 
with those of a synchronous machine, whereby stable operating 
characteristics can be obtained. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be detailed by referring to preferred 
embodiments shown in attached drawings. 
Referring first to FIG. 2, there is shown a first embodiment of the present 
invention, in which the same parts as those in the prior art of FIG. 1 are 
denoted by the same reference numerals and explanation thereof is omitted. 
In FIG. 2, a slip phase operator 25 performs substantially the same 
operation as the slip phase operator 13 in the prior art, but when a make 
contact switch X 26 is closed the operator 25 is arranged to hold slip 
phase outputs sin .theta..sub.s and cos .theta..sub.s. Thus, when the make 
contact switch X is closed, the system enters into a DC exciting 
operation. 
An ordinate-axis current command switch 27 acts, when a make contact switch 
Z 28 is closed, to change over a switch 28a, whereby the ordinate-axis 
current command I.sub.q is made zero and then applied to the exciting 
current controller 14. 
A 2-phase oscillator 29 functions to output a sinusoidal wave of a set slip 
frequency value F.sub.so. 
A slip phase switch 30, when a make contact switch Y 31 is closed, acts to 
change over a switch 30a, whereby an output of the 2-phase oscillator 29 
is connected to the exciting current controller 14. 
Accordingly, when the make contact switches X and Y are opened and closed, 
the slip frequency is made to be F.sub.so or zero (DC), which results in 
that the AC excited machine 5 can enter into its synchronous operation. 
When the make contact switch Z is closed, this causes the system to be put 
in such a mode that only a direct-axis current is controlled as in an 
ordinary synchronous machine. 
FIGS. 3 and 4 are circuit diagrams of different examples for performing the 
phase signal switching operation of the first embodiment of FIG. 2. 
Explanation will first be made as to the control operation of the contact 
switches X, Y and Z in connection with FIG. 3. First, when a DC exciting 
command is inputted, this causes the make contact switch X 26 to be closed 
so that an auxiliary relay XA 32 is excited. When a slip constant command 
is inputted, on the other hand, this causes a make contact switch 33 to be 
closed so that a relay W 34 is excited. At this time, if a break contact 
switch XA 35 is not opened by the DC exciting command, then a relay 
Y.sub.1 37 is excited through the break contact switch XA 35 and a make 
contact switch W 36. 
A tuning detection contact switch 38 is closed when the output of the 
2-phase oscillator 29 is synchronized with the output of the slip phase 
operator 25 within an allowable range. 
When synchronization between the two sorts of phase signals is confirmed 
under a condition that the slip constant command causes the make contact 
switch Y.sub.1 39 to be closed, the tuning detection contact switch SY 38 
is closed so that a relay Y 40 is excited and a make contact switch Y 31 
is closed, whereby the system enters into a constant rotational speed 
operation of the slip frequency set value F.sub.so. 
When the slip frequency is fixed at the set value F.sub. so or DC 
excitation is set, the make contact switches 31 and 41 are closed to 
excite a relay Z 42, whereby the make contact switch Z 28 is closed and 
the command switch 27 causes the I.sub.q command to be set at zero. 
In accordance with the present embodiment, when the system is shifted to a 
constant slip frequency operation, confirmation is made as to 
synchronization between the two sorts of phase signals, i.e., between the 
output signals of the slip phase operator 25 and 2-phase oscillator 29. As 
a result, the control mode of the system can be smoothly switched and the 
system can be suitably shifted, in particular, to the rotary phase 
modifying operation. 
In the event where the set value F.sub.so of the slip frequency is 
positive, the AC excited synchronous machine 5 is driven at a constant 
speed below its synchronous speed and thus can be advantageously reduced 
in its mechanical loss during its rotary phase modifying operation. 
When the set value F.sub.so of the slip frequency is set to be the output 
f.sub.s of the voltage/frequency converter F/V 22, the switching of the 
control mode causes no speed change, which is suitable in the case where 
the synchronous machine has a large inertial moment in its rotary part. 
Shown in FIG. 4 is another example of the phase signal switching operation 
of FIG. 2, wherein, when the absolute value of the effective power command 
P.sub.o exceeds a set value, a make contact switch 43 is closed. While the 
frequency conversion unit 6 is operated, an excitation make-contact switch 
45 is closed to thereby excite a relay RJ 46. 
So long as the frequency conversion unit 6 is in operation and the slip 
phase operator 25 is used to provide a variable-speed operation, a time 
limit make contact switch 51 is closed. 
Under this condition, when the synchronous circuit breaker 2b (refer to 
FIG. 2) is closed and inserted in parallel and thereafter the effective 
power command increases, a make contact switch 43 is closed to excite a 
relay PX 44. The relay PX 44 keeps its contacts until the frequency 
conversion unit 6 is stopped to open a break contact switch 48 or until 
the synchronous machine enters into the self-excited operation to open the 
time limit break-contact switch 51. 
A make contact switch 49 is closed when a bus current detected at the 
current transformer 18 (see FIG. 2) becomes below a set value. 
When the AC excited synchronous machine 5 is in the load operation, a make 
contact switch 47 is in its closed state so that, when the synchronous 
machine 5 is cut off from the AC system 1, the make contact switch 49 is 
closed to excite a relay XB 50. This results in that a manual reset 
make-contact switch 53 is closed to excite a relay X 52, whereby the make 
contact switch X 26 is closed so that the output of the slip phase 
operator 25 is kept and the AC excited synchronous machine 5 enters into 
the self-excited operation based on DC excitation. The relay XB 50 is 
deexcited by the time limit make-contact switch 51. 
Next, explanation will be made as to the procedure when the synchronous 
machine is returned from the self-excited operation to the ordinary 
operation. 
That is, the system circuit breaker 2a is released, the rotational speed of 
the AC excited synchronous machine 5 is adjusted at its synchronous speed, 
the synchronization and voltage at both sides of the system circuit 
breaker 2a are checked and then closed and inserted in parallel. 
Thereafter, the manual reset make-contact switch 53 is opened to again 
release the hold state of the slip phase operator 25. 
According to the present embodiment, even when the detected current value 
becomes zero immediately after the parallel connection of the synchronous 
machine through the 2b or system circuit breaker 2a, the cut-off detection 
contact switch will not be erroneously operated, thus enabling the set 
value of the detected current to be increased and preventing any erroneous 
operation due to a detection error. 
Further, since a current signal is used to detect the cut-off condition, 
such a situation that a power signal becomes zero in a transient 
phenomenon after the occurrence of an abnormality in the power supply 
system side can be eliminated and thus any erroneous operation can be 
advantageously removed. 
There is shown in FIG. 5 a second embodiment in which the present invention 
is applied to a variable-speed water wheel electricity generation system. 
In the drawing, a guide vane driver 54 for a reversible pump/water wheel 4 
is operated under the control of a guide vane opening command. 
A command operator 55 calculates an effective power command P.sub.o on the 
basis of an effective-power variation command .DELTA.P.sub.o received from 
an external device. A water wheel characteristic function generator 57 
computes an optimum rotational speed command N.sub.o. A water wheel speed 
regulator 59 adjusts the guide vane opening command Y so that a difference 
between the command speed N.sub.o received from the water wheel 
characteristic function generator 57 and the speed signal N received from 
a rotational speed detector 32 becomes zero. 
When the guide vane opening of the water wheel exceeds its start set 
opening and the synchronous circuit breaker 2b is connected in parallel, 
the pump/water wheel 4 functions as a prime mover to drivingly rotate the 
AC excited synchronous machine 5. When the synchronous machine 5 is 
desired to be shifted to the rotary phase modifying mode under this 
operational conditions, a contact switch 56 is first operated to reduce 
the effective power command P.sub.o down to zero and at the same time, a 
contact switch 58 is operated so that the rotational speed command N.sub.o 
is switched to a value N.sub.so corresponding to the set value F.sub.so of 
the slip frequency. After the rotational speed is set, the contact switch 
33 is closed to put the synchronous machine in a constant speed operation 
of the exciting frequency F.sub.so. At this stage, a contact switch 60 is 
operated under a condition that the slip change-over contact switch 31 is 
in its closed state, thereby fully closing the guide vane opening Y. As a 
result, the AC excited synchronous machine 5 is put in the motor operation 
under the influence of the synchronizing power with the AC system 1. 
Conversely, when the synchronous machine is desired to be shifted from the 
rotary phase modifying operation to the water wheel operation, the contact 
switch 60 is again operated to open the guide vane to its start opening. 
Then, under a condition that the command value to the water wheel speed 
regulator 59 is N.sub.so, the regulator 59 is operated simultaneously with 
the opening of the contact switch 31, with the result that the pump/water 
wheel 4 acts again as a prime mover. 
Accordingly, after the limitation of the guide vane opening is released, 
when the contact switch 56 is again operated to increase the effective 
power command P.sub.o, a predetermined electricity generating operation 
can be realized. 
According to the second embodiment of the present invention, since the 
command value to the water wheel speed regulator is arranged to coincide 
with the value in the rotary phase modifying operation at the time of 
switching between the rotary phase modifying operation and water wheel 
operation, smooth switching of the control mode can be realized. 
A third embodiment of the present invention will next be explained with 
reference to FIGS. 6 and 7. 
A control system of FIG. 6 includes a water pumping-up starter 61, a pump 
characteristic function generator 62 for calculating a rotary speed 
command N.sub.o on the basis of a water pumping-up input command received 
from an external device, and a water pumping-up mode speed controller 63 
for calculating an effective power command P.sub.1 so that a difference 
between a rotational speed command value N.sub.o received from the 
function generator 62 and a detected value N received from a rotational 
speed detector 32. When a make contact switch PT 64 is operated, the 
rotational speed command N.sub.o is switched to a value N.sub.so 
corresponding to a set value F.sub.so of the slip frequency. 
Explanation will be made as to the procedure until the variable-speed water 
pumping-up system having the above arrangement starts its water pumping-up 
operation by referring to FIG. 7. 
More specifically, at the same time when the system circuit breaker 2a is 
turned ON at a time t.sub.o, the make contact switches 26 and 28 (X and Z) 
are closed to switch the synchronous machine to the DC exciting mode. 
The operation of the frequency conversion unit 6 is started at a time 
t.sub.1 so that, when the starter 61 is started at a time t.sub.2, the 
acceleration of the rotational speed is started. 
As soon as the acceleration is finished and the starter 61 is stopped at a 
time t.sub.3, the make contact switch X 26 is opened to terminate the DC 
excitation, during which the rotational speed is gradually decreased. 
At a time t.sub.4, the voltage controller 21 is started to put the 
synchronous machine in the parallel operation. In this connection, the 
synchronization only requires, on principle, its confirmation, after which 
the synchronous circuit breaker 2b is immediately closed at a time t.sub.5 
to provide the parallel mode. 
At a time t.sub.6 after the voltage is established, the hold state of the 
ordinate-axis current command is released and also the speed controller 63 
and the automatic effective-power controller 20 are also operated. At this 
time, the contact switch 64 causes the rotational speed command N.sub.so 
to be fixed at N.sub.so. 
At a time t.sub.7, when the rotational speed is regulated at N.sub.so, the 
make contact switch Y is closed and the synchronous machine enters into 
the rotary phase modifying operation at the slip frequency F.sub.so. When 
the monotonous decrease of the rotational speed is taken into 
consideration, the rotational speed control operation in a period from the 
time t.sub.6 to the time t.sub.7 may be omitted. 
After water level depression is released at a time t.sub.8, the 
establishment of a priming water pressure at a time t.sub.9 can be 
confirmed by the water pressure of a runner chamber exceeding the set 
value P.sub.so. Between the times t.sub.8 and t.sub.9, the input of the 
pump/water wheel is abruptly changed but the rotational speed is kept 
constant under the influence of the synchronizing power. 
At a time t.sub.10, the opening of the guide vane exceeds its set value and 
when the control of the ordinate-axis component current command is started 
to start the water pumping-up operation, the starting operation is 
completed. 
In accordance with the third embodiment of the present invention, since 
acceleration is realized under the rotational speed control prior to the 
establishment of the water pressure, restart due to a failure in the 
parallel operation can be made unnecessary. 
In accordance with the present invention, since a water pumping-up 
electricity generation system using an AC excited synchronous machine can 
be used in substantially the same operational mode as an ordinary 
synchronous machine as necessary, when a self-excited operation, a rotary 
phase modifying operation or water pumping-up start operation is desired, 
the desired operation can be realized readily and smoothly with a high 
stability.