Reactive power compensator

Details Reactive power compensator Reactive power compensator

2002-07-22

2004-01-20

Riley, Shawn (Department: 2838)

Electricity: power supply or regulation systems

For reactive power control

Using converter

C323S255000

Reexamination Certificate

active

06680602

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved reactive power compensator effective in stabilizing voltage of a system.
2. Description of Related Art
From the viewpoint of stably supplying electric power from a generator or a transformer (as well as a power plant or a substation, hereinafter collectively referred to as a power unit) to a load, it is extremely important to keep the voltage supplied to the load within a predetermined range. It is a matter of course that any output voltage of the power unit is controllable in most cases. However, there may be a voltage drop due to distribution line and transformer provided on the midway. In particular, when a system is operated in cooperation with another system, it is not always possible to regulate the voltage from the viewpoint of the cooperation with another system, and consequently, the voltage supplied to the load fluctuates in some case.
Hitherto, equipment for regulating the voltage of the system has been used in the form of being interposed midway in the system (hereinafter referred to as voltage regulator for convenience of explanation) in order to minimize such voltage fluctuation. Typical equipment capable of discontinuously changing the voltage (regulating the voltage to a discrete value with a difference between one value and another by a predetermined width) such as tap-changing transformer has been already proposed as such equipment.
FIG. 7
is a flow diagram for explaining constitution and operation of a system including such a conventional voltage regulator In the drawing, reference numeral
15
is a power unit of the system. Specifically, the power unit can be a single generator or an output transformer of a substation. Numeral
16
is a system impedance of the power unit
15
, a distribution line connected to the power unit
15
etc., and the impedance is shown as lumped impedance for convenience of explanation. Numeral
17
is a tap-changing transformer with a tap control
17
a
, which is an example of the voltage regulator, added on a secondary side. Numeral
20
is a reactive power compensator connected to the secondary side of the tap-changing transformer
17
, and numeral
21
is a load.
On the supposition that:
Vo is a voltage of the power unit
15
,
Is is a power supply current flowing through the impedance
16
,
Vs is a voltage on the primary side of the tap-changing transformer
17
,
Vt is a voltage on the secondary side of the tap-changing transformer
17
(or a voltage of the load
21
in some cases),
1 to k is a transformation ratio (when a reference voltage is outputted) of the tap-changing transformer
17
,
I is an electric current flowing through the load
3
, and
Iq is a compensation current of the reactive power compensator
20
;
the electric current Is of the power supply flowing through the impedance
16
is a value obtained by subtracting the compensation current Iq of the reactive power compensator
20
from the load current I flowing through the load
21
and multiplying the remainder value by the tap ratio k of the tap-changing transformer
17
.
That is,
Is=
(
I−Iq
)·
k
  (1)
The load voltage Vt is obtained as follows:
Vt=
(
Vo−X·Is
)·
k
 ={
Vo−k
(
I−Iq
)}·
k
  (2)
Now, voltage stabilization operation of the system in
FIG. 7
is hereinafter described. To simplify the explanation, in the description, it is supposed that the load
21
is of a complete inductive load.
For better understanding, the systematic diagram in
FIG. 7
is shown in
FIG. 8
in the form of a block diagram showing the relation between the load terminal voltage Vt and the load current I etc. The operation of the tap control
17
a
is publicly known and detailed description of the operation is omitted herein. Fundamentally, as shown in
FIG. 9
, the operation includes decrease in voltage when the load voltage Vt is exceeding a voltage Vmax that was set at a value higher than a reference voltage Vref, or increase in voltage when the load voltage Vt is lower than Vmin set on the lower side. At this time, the tap voltage of the tap-changing transformer
17
is preliminarily set so that both of the value after the voltage increase and the value after the voltage decrease may come within the range of Vmax to Vmin. Vmax and Vmin are referred to as boundary voltage, and the zone between Vmax and Vmin is referred to as dead zone.
FIG. 10
is a characteristic graph showing the relation between the load current I and the terminal voltage Vt of the load
21
. For better understanding, first, operation of the system is hereinafter described on the supposition that the reactive power compensator
20
is not connected (i.e., Iq=0).
It is herein supposed that the reference voltage Vref of the tap change operation of the tap control
17
a
is equal to the voltage Vo of the power supply
15
and a point A in
FIG. 10
(when the load current I=0 and Vt=Vref) is an initial condition. When the load current I is 0, the load voltage Vt is Vo equal to the foregoing reference value of the tap change operation. Therefore, when increasing the load current I, the primary side voltage Vs of the tap-changing transformer
17
drops from the power supply voltage Vo by X·Is, and the load voltage Vt also drops on the right side region of the point A (on the side region where the load current I increases) as shown in FIG.
10
. At this point, if k=1, the dropped voltage is obtained based on the foregoing expression (2) as follows:
Vt=Vo−X·Is
  (3)
Then, if Vt continues to further drop to be lower than the voltage Vmin which is lower than Vref by the width VD of the dead zone possessed by the tap control
17
a
as its characteristic (for example, when a quantity S obtained by time integration of a quantity deviated from Vmin comes to reach a predetermined quantity Sref, as described later in detail), the tap control
17
a
changes the tap position of the tap-changing transformer
17
by one stage toward the voltage increase side. Thus, as indicated by the point B in
FIG. 10
, Vt increases within a range not reaching Vmax.
When I increases further, Vt drops to reach Vmin again, the tap position is changed again, and Vt increases again. Such an operation will be repeated to the limit of the tap position.
Though not describing in detail, in the case that the load current I flows in the reverse direction (in power regeneration direction) and the load voltage Vt increases, the fundamental operation is the same. There is a difference only in the aspect that the operating voltage of the tap control
17
a
is changed to the Vmax side and the voltage comes to drop at the operating point. In this case, the voltage is dropped within the range not reaching Vmin as a matter of course.
Described hereinafter is the case in which the reactive power compensator
20
is connected. To simplify the explanation, in the following description, it is supposed that I is 0 and Vt=Vref=Vo under the initial condition in the same manner as in the foregoing description of the case without the reactive power compensator
20
.
When K=1, the power supply current Is becomes (I−Iq), and therefore Is is small as compared with the case without the reactive power compensator
20
by a compensation current of the reactive power compensator
20
. Accordingly, the voltage drop (X·Is) caused by the systematic impedance X becomes small, and drop in Vs is not so large, and the drop of Vt is also small as much. In the case that the reactive power compensator
20
is used in order to stabilize the voltage, this principle is used to keep the load voltage Vt.
In general, as shown in
FIG. 11
, the output current Iq of the reactive power compensator
20
is a value obtained by multiplying a difference between an operating reference voltage Vtref set in the reactive power compensator
20
and the load voltage Vt by a gain G. Under the initial condition, supposing that the voltage command value

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