Voltage-reactive power control apparatus

A voltage-reactive power control apparatus provided together with VAR (volt-ampere reactive) supply equipment at principal points of the power system, such as load area substations. This voltage-reactive power control apparatus monitors the power-flow and load voltage at the load area. The apparatus quickly controls VAR supply equipment etc. upon detecting of voltage abnormalities, information that otherwise would be sent to the control point at a higher level station in the power system upon making decision to provide necessary countermeasures for wider range operation of the power system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a voltage-reactive power control apparatus 
incorporated to reactive power supplying equipment for adequately 
sustaining load side voltages in the power system. 
2. Description of the Prior Art 
The voltage of a power system always fluctuates, as is well known, in 
accordance with change of demand and supply. Large voltage fluctuations 
affect not only normal application or operation life of a variety of 
electrical consumer appliances connected to the system, but also the 
insulation design of various power apparatus to be installed in the power 
system, and moreover interferes with stable and efficient operation of the 
system including a power generator from the viewpoint of operation of the 
power system. Namely, the voltage-reactive power control is essential for 
all electrical appliances and power apparatus connected to the power 
system to ensure normal operations and for realizing stable and efficient 
operation of the system. 
Moreover, the increase in demand of power and difficulties in obtaining 
power plant sites caused by environmental concerns have resulted in power 
generation plants located in remote and localized, areas realization of 
large scale power generation base through utilization of a large capacity 
generator unit and long distance and high voltage transmission and heavier 
power flow in the power transmission facilities. 
Of various problems to be overcome for safe and economical running of a 
higher density, larger capacity power system which is expanding and 
becoming more complicated year after year, following are considered as the 
essential problems, although a voltage problem may not easily be evaluated 
directly from the economical viewpoint in its effect. 
(1) Voltage drop under a heavy load 
(2) Abnormal rise of voltage under light load 
(3) Voltage drop during system failure 
(4) Improvement in performance of voltage-reactive power control during 
system operation. 
The items (1), (2) will be described hereunder in more detail. 
In general, a voltage of power system is maintained and adjusted to a 
preset reference value through the voltage-reactive power control of the 
generator and phase modifying equipment. However, if the large supply 
power of VAR (volt-ampore Reactive) supply equipments falls as load 
increases suddenly or demand and supply of the reactive power is 
unbalanced, voltage of the primary system may abnormally fall or rise 
under the situation where the power system is large in scale. 
Such a system operating situation is reported in detail in the Technical 
Report of Japan Electrical Engineering Society (Part II), No. 238, P60. 
However, such voltage failure or unstable phenomenon is not eliminated. 
When a cause is once set up, a local and minor failure is first generated 
and it is then developed to the wider and major failure. Therefore, 
detection at early stage and quick countermeasures are required. 
The voltage-reactive power control apparatus of the prior art has mainly 
assigned monitoring and operation for such voltage variation or voltage 
abnormalities to the load dispatching center which manages operation of 
the whole system. Moreover the information coming from the load end is 
delayed, and synchronized information is not obtained. Therefore this 
information is inaccurate and cannot be used. 
SUMMARY OF THE INVENTION 
The present invention has been proposed to solve such problems and 
therefore it is an object of the present invention to provide a 
voltage-reactive power control apparatus which takes a measurement to 
detect a local failure and then spreads the range for measurement in case 
a wider area measure is required. 
The voltage reactive-power adjusting equipment of the present invention is 
installed at the principal point of the load area in the power system. 
The equipment first detects the power flow into the load and the voltage at 
the load side with the control and monitoring device installed within the 
equipment. 
Next the rate of change of the power-flow and that of the voltage are 
calculaed from the measured value obtained as above. When each of the rate 
of change exceed their own limit value, the margin value is calculated by 
the following system data to determine the control signal to be applied to 
VAR supply equipment with proper time delay, where the system data 
includes short circuit capacity of the power system, the power flow, the 
load-side voltage, and the voltage stability limit of the system 
calculated with the load being assumed a constant-power load. 
Especially when the margin for the above rate-of-change of voltage is 
small, the control signal to the VAR supply equipment has a time delay 
which depends on the ratio of voltage change to VAR supply change. 
The voltage-reactive power control apparatus of the present invention also 
forecasts voltage collapse by detecting a voltage drop which is often 
generated in local load area of the system and quickly operates the 
near-by apparatus, and also sends information to the upper level 
dispatching stations controlling the wider part of system. Moreover, if a 
countermeasure for wider range of system is required, the apparatus of the 
present invention takes preventive control action which may be done 
properly for the wider area of system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the present invention will be explained hereunder 
with reference to the accompanying drawings. 
First, the principle will be explained. FIG. 1 is an equivalent circuit of 
the system for explaining the principle of the present invention. In this 
figure, a load L (power at the load end is designated as P+jQ) is 
connected to the power source side of the system through an impedance Z in 
the side of power source. V.sub.R designates load voltage and V.sub.S, 
voltage at the power source side. 
With the short-circuit capacity of the system designated as S, the 
following standardization can be carried out. 
##EQU1## 
Where, W: load power 
w: standardized load power (P. U) 
V.sub.R : standardized load voltage (P. U) 
Using w, v.sub.R, following relationship can be set. 
EQU v.sub.R.sup.4 +{2wcos(.rho.-.theta.)-1}v.sub.R.sup.2 +w.sup.2 =0(1.1) 
Here, 
.rho. is power factor angle of power source side impedance Z, 
tan.rho.=X/R 
Moreover, 
.theta. is power factor angle of load side impedance Z.sub.L, 
tan.theta.=X.sub.L /R.sub.L 
For the load power w, 
p=w cos.theta. 
q=w sin.theta. 
Where, since p=active power and q=reactive power, from the above formula, 
##EQU2## 
moreover, 
##EQU3## 
where as, 
##EQU4## 
Then, as can be seen easily 
##EQU5## 
Namely, 
##EQU6## 
Where, S is assumed to be constant during this calculation period. 
Using these relationships, a value of X/R is first obtained. 
Next, from the formula (1.1), 
EQU w=-v.sub.R.sup.2 cos (.rho.-.theta.)+.sqroot.v.sub.R.sup.2 -v.sub.R.sup.4 
sin.sup.2 (.rho.-.theta.) (1.3) 
From this relationship, 
##EQU7## 
Namely, when the reference value of V.sub.S is predetermined, the 
short-circuit capacity S of the power system can be calculated from the 
above formula using the values of .rho., .theta. and v.sub.R as known 
values. 
In addition, it is known that the formula (1.1) shows the stable limit 
point when the load is constant power load, with the relationships as 
##EQU8## 
Accordingly, the values of w.sub.m, v.sub.Rm can be obtained using the 
values of .rho., .theta. and S. 
Where, 
w.sub.m is the maximum value of load power w 
v.sub.Rm is the maximum value of voltage in the power receiving end when 
the load power becomes the maximum 
From this result, a surplus of the current load flow and voltage for the 
stable limit can be calculated by the following calculation. 
Namely, 
##EQU9## 
where m.sub.w : margin of the present load flow for voltage stability 
limit 
Next, 
##EQU10## 
m.sub.v : margin of load voltage for voltage stability limit 
Finally the present margin of power flow and voltage for the stability 
limit can be monitored constantly by the calculations of formulae (1.7), 
(1.8). 
Finally, using a value S, values of p and q may be obtained as follow, 
##EQU11## 
and thereby, following calculations may be executed. 
##EQU12## 
In summary of the above, power flow for stable voltage limit, margin of 
active power flow and voltage for the stable limit and sensitivity 
coefficient of voltage for change of power flow can be calculated and 
monitored by measuring the effective power P, reactive power Q and load 
side voltage V.sub.R at the load end and detecting changes of these 
values. 
When voltage tends to clip and a countermeasure is required after detection 
and discrimination of the current condition, VAR supply equipment is to be 
switched. 
If it is supposed that the VAR supply equipment having admittance Y.sub.c 
is switched, following relationship can be obtained. 
EQU v.sub.R.sup.2 =v.sub.R.sup.4 (1-2ZY.sub.c sin .rho.+Z.sup.2 Y.sub.c.sup.2) 
+2v.sub.R.sup.2 w{cos (.rho.-.theta.)-ZY.sub.c cos.theta.}+w.sup.2(1.11) 
Namely, the relationship between voltage power flow when the phase VAR 
supply equipment is turned on is expressed as follow with reference to 
V.sub.R of FIG. 1, 
##EQU13## 
This formula can be written as follows through comparison of absolute 
values in the right and left sides. 
##EQU14## 
Here, using the relationships, 
##EQU15## 
above formula can be standardized as follows. 
EQU v.sub.R.sup.2 =v.sub.R.sup.4 (1-2ZY.sub.c sin.rho.+Z.sup.2 Y.sub.c.sup.2) 
+2v.sub.R.sup.2 w{cos (.rho.-.theta.)-ZY.sub.c cos .theta.}+w.sup.2 
This is similar to the formula (1.11). 
It is understood from the following explanation, load power in satisfying 
the above relationship becomes larger than the value in satisfying the 
formula (1.11) for the same load voltage V.sub.R. 
Namely, improvement effect of the voltage stability limit value by the VAR 
supply equipment can be expressed as follows: 
First from the formula (1.11) 
EQU v.sub.R.sup.2 =v.sub.R.sup.4 (1-2ZY.sub.c sin .rho.+Z.sup.2 Y.sub.c.sup.2) 
+2v.sub.R.sup.2 w{cos (.rho.-.theta.)-ZY.sub.c cos.theta.}+w.sup.2 
When this formula is differentiated for w as indicated below. 
##EQU16## 
At a stability limit, the above value becomes infinite. 
In this case, 
##EQU17## 
or 
EQU v'.sub.om =w'.sub.om 
A value of load power w for the same load voltage is expressed as follows. 
EQU w.sup.2 +2v.sub.R.sup.2 cos (.rho.-.theta.)w+(v.sub.R.sup.4 
-v.sub.R.sup.2)=0 
In case a shunt capacitor is inserted, 
EQU w.sup.2 +2v.sub.R.sup.2 {cos (.rho.-.theta.)-ZY.sub.c cos.theta.}w 
+v.sub.R.sup.4 (1-2ZY.sub.c sin.rho.+Z.sup.2 Y.sub.c.sup.2)-v.sub.R.sup.2 
=0 
Here, under the following preambles, 
##EQU18## 
following relationship can be obtained. 
EQU w.sup.2 +2v.sub.R.sup.2 .multidot.cos (.rho.-.theta.)aw+(b.sup.2 
v.sub.R.sup.4 -v.sub.R.sup.2)=0 (1.14) 
The formula (1.12) can be solved for w. 
##EQU19## 
Moreover, the formula (1.14) can also be solved for w. 
##EQU20## 
Next, the sign of the second item in the radical sign of the second item in 
the formula (1.16) is checked. 
##EQU21## 
From the formula (1.16), 
EQU .thrfore.b.sup.2 -a.sup.2 &gt;0.thrfore.b&gt;a 
Comparison between the value in the radical sign of the second items of the 
formulae (1.14) and (1.15) results in the following relationship. 
##EQU22## 
In the same way, comparison between the first items results in the 
following relationship. 
EQU .DELTA..sub.1 =-v.sub.R.sup.2 cos (.rho.-.theta.)-{-v.sub.R.sup.2 acos 
(.rho.-.theta.)}=v.sub.R.sup.2 cos (.rho.-.theta.).multidot.(a-1)&lt;0 
clearly from the above 
EQU .DELTA..sub.1 +.DELTA..sub.2 &lt;0 
Therefore 
##EQU23## 
On the contrary, a pair of values of v.sub.R can be obtained for w by 
solving the formula (1.11) for v.sub.R. In the case of inserting the shunt 
capacitor, the maximum power flow value w.sub.m can be proved in the 
following relationship by the calculation conducted below. 
##EQU24## 
Inversely, the stability limit of power flow can be improved remarkably 
for the case where the shunt capacitor is not inserted. By the way, the 
value of the stability limit improvement can also be foreseen because the 
values of Z, Y.sub.c, S, W, .rho., .theta. can be known. 
Meanwhile, using the time reference adequately controlled, such stability 
limit can also be obtained from P, Q, V.sub.R obtained by measuring 
##EQU25## 
and the value S calculated. 
Direction and magnitude of change for P, Q, V.sub.R can be known by 
supervising such values from time to time. 
Namely, it can be determined whether values of p, q, v are changing 
suddenly or gradually and in the safe or risky direction by checking 
relationship of 
##EQU26## 
A system configuration indicating the embodiment of the present invention 
will now be explained. 
In FIG. 2, 2 and 3 designate the primary substations being connected to the 
power source 1 through the transmission lines 4, 5. The primary 
substations 2 and 3 are linked through the line 6. 
11, 12 . . . are the secondary substations located near the load areas from 
the primary substations. The primary substation 2 and secondary substation 
11 are linked through the line 13 and the primary substation 2 and 
secondary substation 12, through the line 14. 
The load feeders 11A, 11B, 11C, . . . are connected to the low voltage side 
of bank 110 of the secondary substation. Moreover, it is also assumed that 
the tertiary side of bank 110 has VAR supply equipment on bus 15. 
With such system configuration, the power flows from 1 to the load through 
the transmission line 4, banks 10, 110 and feeders 11A, 11B, 11C. Also 
from the bank 120 which is in parallel with the bank 110 of the secondary 
substation 11, the power flows to a load through the feeders 12A, 12B, 
12C. 
In such a power system, the control and monitoring devices 17, 18 are 
respectively provided to the banks 110, 120 of the secondary substations 
11, 12. Since the feeders 11A, 11B, 11C, 12A, 12B, 12C viewed from the 
banks 110, 120 are all in the load area, the direction of the power flow 
is uniform. 
That is, the power system as seen from the control and monitoring device 
17, 18 location can be regarded as the power source 1, and the feeders 
11A, 11B, 12A, 12B, 12C as the load. 
Besides the reverse power flow does not occur. 
In such a system configuration and layout of apparatus and devices, the 
balance between the load and power source, and the variation of load are 
measured, detected and monitored near the banks 110, 120 and the VAR 
supply equipments 11Z (shunt reactor 11ZL, shunt capacitor 11ZC) are 
adjusted and controlled as required. The system as an object of the 
present invention has the configuration and layout described above and the 
power flow of that system flows in a uniform direction. 
Namely, the bus of load voltage V.sub.R, described in the explanation about 
the principle, corresponds to a high voltage bus from the secondary 
substations 11, 12, the banks 110, 120 and others to a load side and a 
part of the phase modifying equipments 11ZL, 11ZC, 12ZL, 12ZC connected to 
the buses 15, 16 of the phase modifying equipment to a shunt capacitor 
(SC). 
A practical example of the control and monitoring devices connected to the 
system will be explained. 
In FIG. 3, the same elements as that in FIG. 2 are given like numerals and 
the same explanation will not be repeated. 
In that figure, 20 designates a primary circuit breaker of the bank 110; 
21, potential transformer (PT) for high voltage bus; 22, current 
transformer (CT) for primary current of bank 110; 23, transformer for 
measuring voltage of intermediate voltage bus; 24, current transformer for 
secondary current of the bank 110. Outputs of these PTs and CTs are input 
to the on-load tap changing control panel 19 and the control and 
supervisory apparatus 17. 25 designates current transformer for tertiary 
current of the bank 110 and the output thereof is also input to the 
control and supervisory apparatus 17. 19 is existing on-load tap changer 
control panel (hereinafter referred to as LRA panel) having the 
voltage-reactive power regulating function. The control and supervisory 
apparatus 17 is provided for controlling such LRA panel 19. 
Next, the control and supervisory apparatus 17 will be explained hereunder 
in detail. 
181 denotes converter circuit which receives four signals being input to 
the LRA panel 19 and the signal sent directly from the transformer 25 for 
measuring tertiary current of the bank 110. With these inputs, the 
effective power P, reactive power Q, primary voltage V.sub.R of bank, and 
secondary voltage of bank, tertiary capacitance Q.sub.c of bank of the 
bank 110 as a whole are measured. 
182 denotes sample and hold amplifier which holds analog value of output 
voltage of converting circuit 181. 
183 denotes multiplexer which receives five signals whole effective power 
P, reactive power Q, primary voltage V.sub.R of bank, secondary voltage of 
bank, and tertiary capacitance Q.sub.c of bank of the bank 110 from the 
output side of sample hold amplifier 182 and switches them over by the 
control command from the microcontroller 185. 
184 denotes A/D converter connected to the multiplexer 183 for converting 
an input analog voltage into a digital value. 
185 designates microcontroller connected to the A/D converter 184. This 
microcontroller sequentially receives inputs of such analog measured 
values for the functional operation to be described hereafter, operating 
the circuit breakers 11Z1, 11Z2 through the LRA apparatus 19 as a result 
of such functional operation measured values and also transmitting the 
result to the electrical stations for operating and controlling the 
primary side of the secondary substation. 
Next, operations of the control and supervisory devices 17 will then be 
explained using a flowchart of FIG. 4. 
First, a load voltage V.sub.R at the principal point in the system 
connecting the apparatus of the present invention and a power flow W 
(=P+jQ) flowing into the load passing through such point are read in the 
step ST11. 
Each rate of change of effective power P and reactive power Q is calculated 
from power flow (P,Q), load voltage V.sub.R and previously measured value 
of them and a phase angle .rho. of system impedance is estimated in the 
step ST12 in accordance with the formula (1.2). Within a very short 
period, the phase angle .rho. may be assumed as constant. Next, the phase 
angle .theta. of the load is measured in the step ST13. After obtaining 
the value of X/R, the normalized power flow w is calculated by the formula 
(1.3) using the phase angle .rho. of power source and the phase angle 
.theta. of load. Next, the short-circuit capacity S of the system is 
obtained in the step ST14 by dividing a value of power flow W with the 
normalized power flow w. 
Here, since the values of V.sub.R, P, Q, W, S, .rho., .theta. are already 
known, following values can be obtained for the adequate time interval 
.DELTA.t in the step ST15 from such values. 
##EQU27## 
From the values of p, q, w, v.sub.R and the current values of p, q, w, 
v.sub.R, the variation and the rate of the change (magnitude and 
direction) of the power flow and voltage normalized in current can be 
sensed in the step ST16. 
When the power flow and voltage values observed in these cases are near the 
reference limit value having been set by separate calculation on the off 
line basis, or in case the power flow and voltage are very far from the 
reference limit value but these rates of the change are large, sequential 
operation is shifted to the next step (step ST17) regarding such values to 
be within the area of the pattern of system dynamic change. 
Namely, the stability limit values of the system with constant power loads 
such as v.sub.Rm, w.sub.m of the formulae (1.5), (1.6) are obtained in the 
step ST18 using the values of .rho., .theta. (S, P, W, V.sub.R) obtained 
from the assumption method described above. 
Using these values, margin of power flow limit (formula 1.7) in the case 
where whole loads can be considered as the constant power load is obtained 
in the step ST19. 
##EQU28## 
A voltage margin m.sub.v for the voltage stability limit can be checked in 
the step ST20 using the formula (1.8). 
This value is recognized to be proper if the difference between this and 
the stability limit calculated separately by the off-line calculation is 
not significant. Then the power system operation can be safely continued 
(steps ST21-23). 
If this value is larger than a reference model value of the off-line 
calculation, sensitivity coefficient of the power flow-voltage is again 
calculated using the formulae (1.9) and (1.10). 
This sensitivity coefficient becomes large with the increase of power flow, 
but it is also known that not only the sensitivity coefficient of 
q-v.sub.R but also the sensitivity coefficient of p-v.sub.R become large 
at the value near voltage stability limit as causing voltage to be fallen 
rapidly. Particularly when the values of .delta.v.sub.R /.delta.p, 
.delta.v.sub.R /.delta.q exceed 1, it indicates that the system operation 
is in the dangerous area. Therefore, it is decided whether such values 
have exceeded or not the set value obtained by dividing such value with an 
adequate margin. When the value is in the safe zone, operation is 
continued, and otherwise, the system change operation starts (step ST22, 
23). 
For the change of the system operation, switching of VAR supply equipment 
is required. The required amount of VAR to be supplied from the equipment 
is calculated from the equation (1.11) giving the system voltage V.sub.R 
to be maintained for the present power flow. 
When a calculated VAR value is within VAR supply equipment capacity which 
may be obtained at the adjusting point, the required minimum equipment 
capacity is applied or when such value is larger, the maximum equipment 
capacity which may be available at present is applied (step ST25, 26). 
If the VAR supply equipment which may be operated is short in total, such 
information is transmitted to the electrical station which is capable of 
operating the systems of the wider range including such load system as the 
object of operation (step ST27), and, for example, possibility of similar 
voltage adjustment is decided for operation in the primary substation 2 
shown in FIG. 2. 
After a series of such operations proceeds and the VAR supply equipment is 
operated (or in some cases, that equipment is turned off), changes occur 
for the voltage-power flow of the system. Therefore, such changes are 
detected and measured in the step ST28. In the step ST29 the value before 
adjusting operation is compared with current value thereby, verifying the 
improved effect of system (voltage-power flow) operation. 
In case such effect is considered insufficient, a value being change of 
system condition generated by such turning on is used for deciding (step 
ST30) the short-circuit impedance Z and short-circuit capacity S. At the 
same time, it is stored as values of the new system constants which is 
used for deciding the next sequential operation after this operation. 
In addition, when improving effect of system operation is still 
insufficient, various values such as insufficient VAR supplying capacity 
and other system conditions are transmitted to the electrical stations of 
the higher voltage side, enabling the system operation viewed from the 
upper voltage side (step ST31).