Electricity: motive power systems – Generator-fed motor systems having generator control
Reexamination Certificate
2000-06-19
2001-12-11
Masih, Karen (Department: 2837)
Electricity: motive power systems
Generator-fed motor systems having generator control
C318S568220, C318S700000
Reexamination Certificate
active
06329773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an excitation control apparatus of a synchronous generator used to improve the voltage stability of an electric power system.
2. Description of Related Art
FIG. 5
is a constitutional block diagram of a conventional excitation control apparatus of a synchronous generator.
In
FIG. 5
, a reference numeral
1
indicates a transmission voltage setting circuit for setting a transmission voltage of a power transmission system (not shown) to a reference value V
Href
. The transmission voltage of the power transmission system relates to a high voltage on a high voltage side of a transformer (not shown) arranged between a synchronous generator (not shown) and the power transmission system. A reference numeral
2
indicates a subtracting unit for subtracting a transmission voltage V
H
actually applied to the power transmission system from the reference value V
Href
of the transmission voltage set in the transmission voltage setting circuit
1
, a reference numeral
3
indicates a tap position detecting circuit for detecting a tap position of the transformer, and a reference numeral
4
indicates a voltage-up ratio calculating circuit for calculating the inverse value of a voltage-up ratio n of the transformer from the tap position of the transformer detected in the tap position detecting circuit
3
and multiplying the reference value V
Href
of the transmission voltage V
H
by the inverse
1
of the voltage-up ratio n.
A reference numeral
5
indicates a reactive current setting circuit for setting a reference value of a reactive current Iq of the synchronous generator to a value Iq0, a reference numeral
6
indicates a multiplier for multiplying the reference value Iq0 of the reactive current Iq set in the reactive current setting circuit
5
by a rated reactance value Xt of the transformer, a reference numeral
7
indicates an adding and subtracting unit for adding a multiplied result V
Href
of the voltage-up ratio calculating circuit
4
and a multiplied result Iq0*Xt of the multiplier
6
together and subtracting a reference value V
gref
of a terminal voltage Vg of the synchronous generator from an added result V
Href
+Iq0*Xt, a reference numeral
8
indicates a multiplier for multiplying an added-subtracted result V
Href
+Iq0*Xt−V
gref
of the adding and subtracting unit
7
by a divided result &bgr;/KH obtained by dividing a gain reducing coefficient &bgr; by a gain KH of a transmission voltage control, a reference numeral
9
indicates an adder for adding a subtracted result V
Href
−V
H
of the subtracting unit
2
and a multiplied result &bgr;/KH*(V
Href
+Iq0*Xt−V
gref
) of the multiplier
8
, a reference numeral
10
indicates a multiplier for multiplying an added result V
Href
−V
H
+&bgr;/KH*(V
Href
+Iq0*Xt−V
gref
) of the adder
9
by the gain KH of the transmission voltage control, and a reference numeral
11
indicates a signal producing unit for multiplying a deviation (V
gref
−Vg) between the reference value V
gref
of the terminal voltage Vg of the synchronous generator and the terminal voltage Vg by the gain reducing coefficient &bgr; to obtain a multiplied result &bgr;*(V
gref
−Vg) and adding this multiplied result &bgr;*(V
gref
−Vg) and a multiplied result KH(V
Href
−V
H
)+&bgr;*(V
Href
+Iq0*Xt−V
gref
) of the multiplier
10
to obtain an added result.
In the above configuration, an operation of the conventional excitation control apparatus is described.
In the example shown in
FIG. 5
, the added result of the signal producing unit
11
is output to an auto-voltage regulating unit AVR (not shown), and the transmission voltage V
H
of the power transmission system is controlled according to the added result to make the transmission voltage V
H
agree with the reference value V
Href.
In cases where a transmission voltage control is performed to control the transmission voltage V
H
of the power transmission system to a constant value, the transmission voltage V
H
is expressed according to an equation (1).
V
H
=V
Href
−n*&bgr;*Xt/(&bgr;+n*KH)*(Iq−Iq0) (1)
Here, in the reactive current setting circuit
5
, the reactive current Iq of the synchronous generator is set to the reference value Iq0 in cases where the transmission voltage V
H
agrees with the reference value V
Href.
Therefore, as is apparently indicated in the equation (1), in cases where the synchronous generator is operated on condition that the reactive current Iq of the synchronous generator almost agrees with the reference value Iq0, the transmission voltage V
H
of the power transmission system can be maintained to the reference value V
Href
or a value near to the reference value V
Href.
In contrast, in cases where the terminal voltage Vg of the synchronous generator is controlled to a constant value, the transmission voltage V
H
is expressed according to an equation (2).
V
H
=n*V
gref
−nr*Iq*Xt (2)
Here, the symbol nr denotes a reactance change ratio of the transformer.
As is apparently indicated by comparing the equations (1) and (2) with each other, in the transmission voltage control in which the transmission voltage V
H
of the power transmission system is controlled to a constant value, as compared with a terminal voltage control in which the terminal voltage Vg of the synchronous generator is controlled to a constant value, a possibility of the decrease of the transmission voltage caused by a reactance change of the transformer is low, and an adverse influence of the change of the reactive current Iq is low. Therefore, the transmission voltage V
H
of the power transmission system can be stabilized.
Also, in the terminal voltage control in which the terminal voltage Vg of the synchronous generator is controlled to a constant value, when the voltage of the power system is lowered, the reactive current Iq of the synchronous generator is increased, and the voltage V
H
on the high voltage side of the transformer is lowered in proportion to the system voltage. In contrast, in the transmission voltage control in which the transmission voltage V
H
of the power transmission system is controlled to a constant value, even though the system voltage of the power system is lowered, because a lowering degree of the reactance of the transformer is recovered by heightening the terminal voltage vg of the synchronous generator, the lowering of the voltage on the high voltage side of the transformer can be prevented.
Accordingly, because the conventional excitation control apparatus of the synchronous generator is constituted as is described above, the voltage stability of the power transmission system can be effectively improved.
However, in cases where the tap position of the transformer is changed, though the voltage-up ratio n of the transformer and the reactance value of the transformer are changed, because the transmission voltage V
H
is controlled by using the gain KH set to a constant value, there is a drawback that the transmission voltage V
H
becomes instable. In detail, there are following drawbacks.
First Drawback:
In cases where an accident of a wiring or a sharp increase of a load occurs, the transmission voltage V
H
of the power transmission system is sharply lowered, i.e., droops. In this case, it is required to set the terminal voltage Vg of the synchronous generator to a higher value for the purpose of maintaining the transmission voltage V
H
.
However, when the terminal voltage Vg of the synchronous generator exceeds its upper limit, an auto-tap-control function OLTC of the transformer is normally operated, and a tap value of the transformer is changed to a tap value of a loading operation. As a result, the voltage-up ratio n of the transformer is heightened, and the terminal voltage Vg of the synchronous generator is returned to its rated value or a value near to the rated value.
In this case, when the tap position of the transfor
Shimomura Masaru
Xia Yuou
Leydig , Voit & Mayer, Ltd.
Masih Karen
Mitsubishi Denki & Kabushiki Kaisha
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