Control system for variable speed water-wheel generator apparatus

A control system for a variable speed water turbine generator apparatus includes a first function generator that is responsive to an output command signal. The first function generator produces a rotation number command signal. A speed adjuster produces an output signal in accordance with a different signal between the first function generator output and an actual rotation number. An adder adds the generator output command signal to the speed adjuster output signal to determine an overall output command signal. An output aduster produces an output signal in accordance with a difference between the adder output and an actual power output of the generator for controlling the firing angles of the frequency converter. A second function generator, responsive to the generator output command signal, determines an opening command signal for the variable opening vanes. An opening adjuster produces an output signal in accordance with a difference signal between the second function generator output and an actual opening of the variable opening vanes to control the variable opening vanes. The first and second function generators increase command values for the rotation number and the variable opening vanes, respectively, as the output command signal increases. The frequency converter is further controlled to maintain the rotation number within a predetermined operation range.

BACKGROUND OF THE INVENTION 
This invention relates to a control system for a variable speed water 
turbine generator apparatus. 
A water turbine generator apparatus, conventionally well known and adopted, 
uses a synchronous machine as a generator and therefore the frequency of 
generation output has a proportional relationship with the rotation number 
of the generator. Conversely, in a variable speed generator, the rotation 
number can be controlled to a value which is separate from and independent 
of the frequency of generation output and so, the rotation number of a 
water turbine can advantageously be controlled to a value at which 
efficiency of the water turbine is maximized while maintaining the 
generation output frequency at a frequency of an electric power system. 
A control system for this type of variable speed water turbine generator 
apparatus, such as illustrated in FIG. 1 of the present application, has 
been proposed in, for example, JP-A-57-182920. 
Referring to FIG. 1, a wound-rotor induction generator 1 is driven for 
rotation by means of a water turbine 2 directly connected to the rotor of 
the generator. The generator 1 is driven at variable speeds while a 
secondary winding 1b of the generator 1 being supplied with an AC exciting 
current which is so adjusted, by means of a cyclo-converter 3, as to have 
a predetermined internal phase difference angle in accordance with a 
rotation speed of the generator 1, so that AC power of a constant 
frequency equal to a rated frequency of an electric power system 4 may be 
generated from a primary winding 1a of the generator 1. A water turbine 
characteristic function generator 5 is supplied with a rotation speed 
signal N, a generation output command Po applied externally and a 
water-level detection signal H and generates an optimum rotation speed 
command Na and an optimum guide vane opening command Ya which are used for 
operating the generator apparatus at maximum efficiency. An induction 
machine 7 for slip phase detection has a rotor directly coupled to the 
generator 1 and a primary winding 7a connected to the output of the 
generator 1 and it delivers a slip phase signal Sp through a secondary 
winding 7b. The slip phase signal Sp and optimum rotation speed command Na 
are applied to a control unit (not shown) included in the cyclo-converter 
3 in order for the cycloconverter 3 to control the frequency and internal 
phase difference angle of the AC exciting current supplied to the 
secondary winding 1b of the generator 1, in the manner described above. 
The optimum guide vane opening command Ya is applied to a guide vane 
driver 8 which in turn controls the opening of guide vanes 9 such that a 
waterwheel output P.sub.T can be optimized. 
In the variable water turbine generator apparatus, it is required that the 
generation output be rendered coincident to a generation output command 
issued from, far example, a central load-dispatching office, and the 
rotation speed of the water turbine and the guide vane opening be 
controlled to proper values under that generation output, whereby the 
water turbine can be operated at maximum efficiency under that generation 
output. To this end, two operation terminals are adjustable which are 
represented by the cyclo-converter 3 operative to effect excitation 
control, such as frequency and internal phase difference angle control, 
for the rotor and by the guide vane operating of the water wheel. 
Importantly, it should therefore be decided what control items are to be 
shared by respective operation terminals in realizing a control system. 
The known reference, however, fails to provide sufficient disclosure in 
this regard. Especially, where a generation output control mode is 
provided independently of the aforementioned optimum rotation speed 
control mode and optimum guide vane control mode so that three control 
modes are involved, the prior art has absolutely failed to clarify a way 
of allotting the three control modes to the two operation terminals and 
compatibly applying thereto those control modes in combination. 
Further, in the known reference, the response speed in the rotation speed 
optimizing control is retarded relative to the response speed in the guide 
vane optimizing control to prevent the water wheel from transiently coming 
into a specified bad operation or running condition range. However, on the 
other hand, there is a possibility that the slower response speed in the 
rotation speed optimizing control causes the rotation speed to overshoot 
and transiently go beyond a predetermined permissible variable speed band. 
This means that the possibility of step-out is disadvantageously 
increased. 
SUMMARY OF THE INVENTION 
In view of the above, the present invention intends to solve the problems 
encountered in the prior art and has for its object to provide a variable 
speed generator apparatus which can rotationally achieve the generation 
output control, rotation speed optimizing control and guide vane opening 
optimizing control at a time and can ensure stable operations within a 
permissible variable speed band. 
According to the invention, the above object can be accomplished by a 
control system for a variable speed water turbine generator apparatus 
comprising a generation output detector for detecting a generation output 
from a variable speed generator; a water turbine characteristic function 
generator for receiving a signal indicative of a water level which is 
detected as being applied to the water turbine and a generation output 
command signal and for producing an optimum rotation speed command and an 
optimum guide vane opening signal. Further the control system includes a 
unit for receiving a difference between the optimum rotation speed command 
signal and a signal indicative of a detected actual rotation speed and for 
operating to make the difference zero; a power control unit for comparing 
a composite signal of an output signal from the computing unit and the 
generation output command signal with an actual generation output signal 
detected by the generation output detector to produce a comparison 
difference and for controlling an AC exciting current supplied to the 
variable speed generator such that the comparison difference is made zero. 
Also, a rotation variation suppressing circuit is included with the 
control system for generating a signal in a direction in which a variation 
of the actual rotation speed is suppressed when the actual rotation speed 
tends to very beyond a predetermined permissible variation range, with the 
output signal from the suppressing circuit being applied to the composite 
signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 2, there is illustrated, in block form, a control 
system according to an embodiment of the invention. In FIG. 2, components 
like those in FIG. 1 are designated by like reference numerals. 
A water turbine characteristic function generator 5 receives a generation 
output command Po and a waterlevel detection signal H and generates an 
optimum rotation speed command Na and an optimum guide vane opening 
command Ya. A comparator 10 compares the optimum rotation speed command Na 
with an actual rotation speed signal N detected by a rotation speed 
detector 6 to produce a difference .DELTA.N (=Na - N) which in turn is 
applied to a computing unit 11. The computing unit 11 includes a 
proportional element K.sub.1, an integration element K.sub.2 /S and an 
adder 12 and produces a correction signal .DELTA.C which corrects the 
generation output command Po such that the difference .DELTA.N, unless 
null, is made zero. Thus, the correction signal .DELTA.C is added at an 
adder 13 to the externally supplied generation output command Po to 
produce an ultimate generation output command Po' that is supplied to a 
comparator 15. 
The optimum guide vane opening command Ya is applied to a guide vane driver 
8. The guide vane driver 8 includes an adder 14 and an integration element 
K.sub.4 S whose output signal is negatively fedback to the adder 14. 
The comparator 15 compares the ultimate generation output command Po' 
applied as one input thereof with an actual generation output signal 
P.sub.G detected by a generation output detector 16 and applied as the 
other input and produces a difference .DELTA.P (=Po'-P.sub.G) which in 
turn is supplied to a power control unit 17. The power control unit 17, 
comprised of a proportional element K.sub.5, an integration element 
K.sub.6 /S and an adder 18, produces an output signal Ep applied to a 
cyclo-converter 3. The cyclo-converter 3 includes an automatic pulse-phase 
shifter (not shown) operable under the application of a slip phase signal 
Sp extrcted from a secondary winding 7b of a slip-phase detection 
induction machine 7 and the output signal Ep from the power control unit 
17 and is operative to control firing angles of semiconductor devices such 
as thyristors so as to adjust electric power supplied to a secondary 
winding 1b of a wound-rotor induction generator 1. The remaining parts 
identical to those of the FIG. 1 system need not be described herein. 
The water turbine characteristic function generator 5 in FIG. 2 adapted to 
obtain the optimum rotation speed command Na and the optimum guide vane 
opening command Ya has characteristics as shown in FIGS. 3A and 3B. More 
particularly, FIG. 3B shows that for a constant head H, the optimum 
rotation speed command Na is increased to provide higher speeds as the 
generation output command Po increases but should be confined within a 
range defined by permissible upper and lower limits for variable speed 
operatins of the water turbine. FIG. 3A shows that for a constant head H, 
the optimum guide vane opening command Ya is increased to provide larger 
openings as the generation output command Po increases and that for a 
constant Po, the Ya is increased to provide larger openings as the head H 
decreases, where H.sub.1 &lt;H.sub.2 &lt;H.sub.3. The function generator 5 is 
supplied with both the generation output command Po and head H but in the 
case of a water power station in which the head will not vary by itself to 
a great extent, the function generator may conveniently determine an 
optimum guide vane opening command Ya and an optimum rotation speed 
command Na under the application of only a generation output command Po. 
When the generation output command Po is increased, the water turbine 
characteristic function generator 5 operates to increase the Ya and Na as 
described above. In this operation, responses occur in the control system 
of FIG. 2 as will be described below. 
Specifically, when the generation output command Po is increased stepwise 
as illustrated at section (a) in FIG. 4 in order to increase the 
generation output P.sub.G, for example, stepwise at time T.sub.o, the 
generation output P.sub.G of the generator 1 is caused to increase to 
follow a variation in the generation output command Po but in this phase, 
the ultimate generation output command Po' is smaller than the generation 
output command Po as will be clarified later and consequently an increased 
generation output P.sub.G is below the Po as illustrated at section (g) in 
FIG. 4. The ultimate generation output command Po' is processed by the 
integration element K.sub.6 /S in power control unit 17 cooperative with a 
negative feedback circuit comprised of the power control unit 17, 
cyclo-converter 3, generator 1, generation output detector 16 and 
comparator 15, so that the difference .DELTA.P (=Po'-P.sub.G) is gradually 
decreased to finally establish P.sub.G =Po'. On the other hand, the 
response of opening Y of the guide vane 9 shown at (d) in FIG. 4 as it 
changes to the optimum guide vane opening command Ya shown at (b) in FIG. 
4 is slower than the response of the generation output P.sub.G as it 
changes to the ultimate generation output command Po'. Accordingly, water 
turbine output P.sub.T is initially increased more slowly than the 
generation output P.sub.G as illustrated at (e) in FIG. 4, with the result 
that as shown at (f) in FIG. 4, the rotation speed N is temporarily 
decelerated after the abrupt change of the generation output command Po 
until it (N) thereafter stops decreasing at time t.sub.1 so that the water 
turbine output P.sub.T, which is increasing as the guide vane opening Y 
increases substantially reflects the generation output P.sub.G which has 
been raised earlier than the P.sub.T. Since at time t.sub.1 the actual 
rotation speed N is lower than the optimum rotation speed command Na shown 
at (c) in FIG. 4, providing a positive difference .DELTA.N (=Na-N) and 
consequently a positive correction signal .DELTA.C delivered out of the 
computing unit 11, an ultimate generation output command Po'=Po-.DELTA.C 
corrected by this positive correction signal .DELTA.C becomes smaller than 
the generation output command Po, thus producing an actual generation 
output P.sub.G which is slightly smaller than the generation output 
command Po. After time t.sub.1, the guide vane opening Y continues to 
increase until it reflects the generation output command Po and 
consequently, the difference between water-wheel output P.sub.T and 
generation output P.sub.G grows to increase the rotation speed N until it 
approaches the optimum rotation speed command Na along with concurrent 
approach of the correction signal .DELTA.C to zero, whereby eventually the 
ultimate generation output command coincides with the generation output 
command Po and the rotation speed N equals the optimum rotation speed 
command Na. Specifically, the difference .DELTA.N (=Na-N) is gradually 
decreased to reach the stationary condition of N =Na by means of the 
integration element K.sub.2 /S in computing unit 11 cooperative with a 
negative feedback circuit comprised of the computing unit 11, adder 13, 
comparator 15, power control unit 17, cyclo-converter 3, generator 1, 
rotation speed detector 6 and comparator 10. The opening Y of the guide 
vane 9 is controlled through a negative feedback circuit comprised of the 
integration element K.sub.4 /S and adder 14 included in the guide vane 
driver 8 such that Ya-Y=0 is eventually settled, which means that Y is 
brought into coincidence with Ya. 
In the above operation, it is preferred that the proportional gain K.sub.1 
and the integration gain K.sub.2 be chosen such that the response of the 
computing unit 11 to .DELTA.N (=optimum rotation speed command Na - 
rotation speed N) is much slower than the response of the generation 
output P.sub.G to the ultimate generation output command Po' and the 
response of the actual guide vane opening Y to the optimum guide vane 
opening command Ya. 
Incidentally, when a large generation output command is applied or when a 
generation output command is additionally changed under a condition that 
the rotation speed has already deviated considerably from an optimum value 
during the preceding transient phenomenon still persisting, the rotation 
speed transiently varies to a great extent due to the fact that the 
response speed in the rotation speed optimizing control is slower as 
described previously and in extremities, there is a possibility that the 
rotation speed goes beyond the upper limit or the lower limit of a 
permissible variable speed operation band admitted by the cyclo-converter. 
This problem can be dealt with by another embodiment of the invention as 
illustrated in FIG. 5. In this embodiment, there is additionally provided 
a power modifying signal function generator 21 adapted to produce a 
positive generation output modifying signal where the rotation speed N 
increases by .DELTA.N.sub.1 in excess of a synchronous speed corresponding 
to zero slip and conversely, a negative generation output modifying signal 
when the rotation speed N falls by .DELTA.N.sub.2 below the synchronous 
speed. The output signal, i.e., power modifying signal .DELTA.D is also 
combined with the correction signal .DELTA.C so as to be reflected in the 
ultimate generation output command Po'. 
Thus, when the rotation speed N tends to increase beyond the variable speed 
band, the Po' or the generation output P.sub.G is increased to decrease 
the rotation speed, thereby suppressing an excessive increase in the 
rotation speed. 
Conversely, when the rotation speed N tends to decrease beyond the variable 
speed band, the Po' is decreased, consequently, the generation output 
P.sub.G is decreased to increase the rotation speed, thereby suppressing 
an excessive decrease in the rotation speed. 
In this manner, the rotation speed N can steadily be confined within the 
variable speed band. 
As described above, according to the present invention, the generatio 
output control, rotation speed optimizing control and guide vane opening 
optimizing control can all be performed compatibly at a time and stable 
variable speed operations within the permissible variable speed band can 
be ensured.