Electricity: motive power systems – Synchronous motor systems
Reexamination Certificate
2000-06-16
2001-11-27
Fletcher, Marlon T. (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
C318S714000, C323S205000, C323S301000
Reexamination Certificate
active
06323618
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an excitation controller and an excitation control method for stabilizing voltage in an electric power system.
2. Description of the Prior Art
Prior art excitation controllers detect a high-side voltage of a transformer connected to a synchronous machine or generator and then control the synchronous machine according to the detected voltage such that the high-side voltage has a constant value, in order to improve the stability of the electric power system, as disclosed in Japanese patent application publication (TOKKAIHEI) No.4-79798, for example.
Referring next to
FIG. 23
, there is illustrated a block diagram showing the structure of a prior art excitation controller as disclosed in Japanese patent application publication (TOKKAIHEI) No.4-79798. As shown in the figure, a first potential transformer or PT
102
detects an output terminal voltage V
G
of a synchronous machine
101
, and a subtracter
104
subtracts the detected output terminal voltage V
G
from a reference voltage r
G
set by a voltage setter
103
. A reduced gain circuit
105
then multiplies the result of the subtraction from the subtracter
104
by a gain &bgr;.
A second potential transformer or PT
109
detects a transmission voltage V
H
on a transmission bus
106
(i.e., a high-side voltage of a transformer
108
connected, by way of a breaker
107
, to the transmission bus
106
), and a second subtracter
111
subtracts the detected transmission voltage V
H
from a transmission reference voltage r
H
set by a high-side voltage setter
110
. A high-voltage gain circuit
112
then multiplies the result of the subtraction from the second subtracter
111
by a gain K
H
.
Then an adder
113
adds the result of the multiplication from the reduced gain circuit
105
and the result of the multiplication from the high-voltage gain circuit
112
together. An automatic voltage regulator or AVR
114
generates a timing signal for controlling the rectifying timing of an exciter
115
based on the result of the addition from the adder
113
. The exciter
115
receives the timing signal and then supplies a field current to a field winding
116
of the synchronous machine
101
according to the timing signal.
In this way, the prior art excitation controller detects the high-side voltage of the transformer
108
, and then controls the synchronous machine such that the transmission voltage on the transmission bus
106
remains constant.
A problem with a prior art excitation controller constructed as above is that although it can keep the high-side voltage of a transformer connected to a synchronous machine constant, in addition to an excitation control panel on which an AVR and an adder are mounted, another control panel is needed for mounting a high-side voltage setter, a potential transformer, a reference value correction circuit, and so on thereon, thus increasing the manufacturing cost of the electric power system.
Another problem is that since a transmission bus is located at a considerable distance from an electric power plant building for housing the synchronous machine, the transformer, and so on, a long cable susceptible to noise is needed to connect an excitation control panel located in the electric power plant building to a PT located in the vicinity of the transmission bus, thus decreasing the reliability of the electric power system.
A further problem is that an expensive PT is needed to detect the transmission voltage on the transmission bus and hence the manufacturing cost of the electric power system is increased.
A still further problem is that since when a transformer equipped with a tap changer for changing taps under load conditions is provided, the tap-changer-equipped transformer including an automatic tap control function, changing taps changes the transformation ratio, it is difficult to maintain the output terminal voltage of the generator at a nominal value and to keep the transmission voltage on the transmission bus constant. Another problem is that since the reactance X
t
of the tap-changer-equipped transformer changes with a change in taps of the tap-changer-equipped transformer as this results in a change in the ratio of a cross-current prevention reactance X
DR
for preventing any cross current from flowing between the synchronous machine and another synchronous machine, to the reactance X
t
of the tap-changer-equipped transformer, a cross current can flow among a plurality of synchronous machines including the above-mentioned synchronous machines, which operate in parallel with one another, if the plurality of synchronous machines have different transformation ratios.
SUMMARY OF THE INVENTION
The present invention is proposed to solve the above problems. It is therefore an object of the present invention to provide an excitation controller and an excitation control method capable of maintaining the output terminal voltage of a synchronous machine or generator, at a nominal value and keeping the transmission voltage on a transmission bus constant without having to detect the transmission voltage on the transmission bus.
In accordance with one aspect of the present invention, there is provided an excitation controller comprising: a reactive current detector for detecting a reactive current output from a synchronous machine connected, by way of a tap-changer-equipped transformer equipped with a tap changer for changing taps under load conditions, to a transmission system; a tap controller for setting a tap ratio of the tap-changer-equipped transformer according to an output terminal voltage of the synchronous machine; a voltage setter for setting a synchronous machine's-side reference voltage of the tap-changer-equipped transformer on the basis of a transmission system's-side reference voltage of the tap-changer-equipped transformer, the tap ratio set by the tap controller, and the reactive current detected by the reactive current detector; and a controlling unit for controlling an excitation system for exciting the synchronous machine according to the reference voltage set by the voltage setter.
In accordance with a preferred embodiment of the present invention, the voltage setter obtains the synchronous machine's-side reference voltage by dividing the transmission system's-side reference voltage of the tap-changer-equipped transformer by the tap ratio, and by adding a voltage drop in the tap-changer-equipped transformer to a result of the division, the voltage drop being obtained from the reactive current. Preferably, another synchronous machine can be connected to the transmission system, and the voltage setter can obtain the voltage drop on the basis of a reactance of the tap-changer-equipped transformer and a cross-current prevention reactance for preventing any cross current from flowing between the synchronous machine and the other synchronous machine.
In accordance with another preferred embodiment of the present invention, the voltage setter obtains the synchronous machine's-side reference voltage by dividing the transmission system's-side reference voltage of the tap-changer-equipped transformer by a step-up ratio corresponding to the tap ratio, and by adding a voltage drop in the tap-changer-equipped transformer to a result of the division, the voltage drop being obtained from a reactance change ratio corresponding to the tap ratio. Preferably, another synchronous machine can be connected to the transmission system, and the voltage setter can obtain the voltage drop on the basis of the step-up ratio, the reactance change ratio, and a cross-current prevention reactance for preventing any cross current from flowing between the synchronous machine and the other synchronous machine.
In accordance with another preferred embodiment of the present invention, when the reactive current has a predetermined reference value other than zero, the voltage setter sets the synchronous machine's-side reference voltage of the tap-changer-equipped transformer in such a manner
Kitamura Hitomi
Shimomura Masaru
Tanaka Seiichi
Fletcher Marlon T.
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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