Electrical transmission or interconnection systems – With harmonic filter or neutralizer
Reexamination Certificate
2000-12-15
2003-05-06
Sircus, Brian (Department: 2836)
Electrical transmission or interconnection systems
With harmonic filter or neutralizer
C307S102000
Reexamination Certificate
active
06559561
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for damping power oscillations in transmission lines, and to a device for carrying out the method.
The device comprises means for forming a damping signal in dependence on the amplitude of an estimated power quantity and with an eligible phase shift in relation to the phase position thereof, and an actuator to be influenced in dependence on the damping signal and hence to influence the power transmitted in the transmission line
BACKGROUND ART
In transmission lines, which connect two separate power networks or which connect two parts in one and the same power network, a constant phase-angle difference is maintained, during steady state at a certain transmitted power, between the voltages at the end points of the transmission line. Each change of the transmitted power entails a change of this angular difference. Because of the moments of inertia of the generators in the power network(s), each such change of the angular difference occurs in an oscillating manner with natural frequencies typically in the interval of 0.1 to 2 Hz. The internal damping of these power oscillations is often very small, and, in addition, decreases with increasing amplitude of the oscillation. If the amplitude of the oscillation is sufficiently great, the internal damping may even become negative, in which case the oscillation amplitude grows in an uncontrolled manner such that the transmission of power via the transmission line has to be interrupted.
Especially great power oscillations may occur upon a rapid disconnection of generators or in connection with lines in the power system being disconnected, for example in connection with short circuits on the transmission line or in some of the connected power systems.
FIG. 1
shows a typical appearance of a disturbance in the active power in a transmission line included in a power system, for example in case of a loss of a generator which is connected to and feeds power into the power system. The time t is plotted on the horizontal axis and the instantaneous active power p(t) is plotted on the vertical axis. In a given time interval, the disturbance may be characterized by a mean power P
av
and an oscillating component &Dgr;p(t), the latter having an angular frequency &OHgr;=2&pgr;f. As mentioned above, the frequency f usually lies within the interval 0.1 to 2 Hz.
The damping of the power oscillations may be improved by influencing the power transmitted by the transmission line. In a known way, this influence may, for example, be achieved:
by influencing the terminal voltage of a generator connected to the power network(s) by means of a so-called Power System Stabilizer (PSS), which influences the magnetization equipment for the generator and hence the terminal voltage thereof,
by influencing the total reactance of the transmission line by means of a controllable series capacitor connected into the line, a so-called Thyristor Controlled Series Capacitor (TCSC), in which case thus the total reactance of the transmission line consists of the line reactance plus the reactance of the series capacitor, or
by supplying/consuming reactive power at some point on the transmission line by means of a so-called reactive-power compensator (Static Var Compensator, SVC), which influences the voltage at that point on the fine where the compensator is connected and hence also the power flow in the transmission line.
The generator, the controllable series capacitor, and the reactive-power compensator, respectively, constitute actuators which modulate each of the above-mentioned quantities, the terminal voltage of the generator, the total reactance of the transmission line, the voltage at a certain point along the line, such that, in addition to the original power oscillation, an additional controlled power variation is achieved. If this controlled power variation is carried out with the same frequency as the original oscillation and with a phase position which deviates 90° from the phase position thereof, a damping of the original oscillation is obtained.
In order not to burden the representation with distinctions which are self-explanatory to the person skilled in the art, in the following description the same designations are generally used for quantities which occur in the installation as for the measured values and signals/calculating values, corresponding to these quantities, which are supplied to and processed in the control equipment which will be described in the following.
FIG. 2
schematically shows a known embodiment of damping equipment by means of a Power System Stabilizer (PSS). A generator
1
is connected, via a power transformer T
1
, to a transmission line
2
, which in turn is connected to a power network N
2
with an additional line
3
(only roughly indicated). The generator has magnetization equipment
1
a
. The voltage V and the current I through the transmission line are sensed by means of a voltage transformer T
2
and a current-measuring device IM, respectively. A voltage controller
4
, only symbolically shown, is supplied with a voltage-reference signal V
REF
and a measured value V
SVAR
of the actual value of the voltage V, which measured value is obtained via the voltage transformer T
2
. The output signal from the voltage controller is supplied to the magnetization equipment of the generator and influences its excitation current in such a way that the measured value V
SVAR
approaches the voltage-reference signal V
REF
to correspond thereto at least under steady-state conditions.
A power-calculating member
5
is supplied with the measured value V
SVAR
and with a measured value i(t) of the actual value of the current I and calculates therefrom a calculating value p(t) of the active power delivered to the power network N
2
by the generator. This calculating value is supplied to an identification member
6
for identification of the amplitude and the phase position of a power oscillation, if any. The identification member forms from the calculating value p(t) a control signal &Dgr;V
PSS
which is supplied to the voltage controller of the generator as an addition in addition to the normal voltage reference V
REF
. Since the power oscillation in the transmission line also occurs in the power delivered by the generator, in this way also a damping of the power oscillation in the transmission line may be achieved.
A known embodiment of the identification member
6
is illustrated in FIG.
4
. The calculating value p(t) is supplied to a so-called washout filter
61
with a transfer function
sT
w
1
+
sT
w
,
where s is the Laplace operator. The filter separates the constant or slowly varying component P
av
of the calculating value p(t) but forwards the oscillating part thereof. The filter has a cutoff frequency
1
2
π
T
w
chosen with a sufficient distance from the frequency of the oscillation which is to be damped.
The above-mentioned desired phase shift of 90° of the oscillating part of the calculating value p(t) is achieved with the aid of one or more lead-lag filters, in this embodiment by means of two cascade-connected filters
62
and
63
with the transfer functions
1
+
sT
1
1
+
sT
2
and
1
+
sT
3
1
+
sT
4
,
respectively.
The output signal D(t) from the lead-lag filter
63
constitutes a damping signal which, after a necessary adaptation (not shown in the figure) of the signal level to constitute the control signal &Dgr;V
PSS
, is utilized for modulating the terminal voltage of the generator, thus achieving the desired controlled power variation.
Because of limitations of the available control range of the actuators (limited by the maximum stresses which the apparatus may endure), limitations (only roughly indicated in the figure) of the output signals from the lead-lag filters are introduced.
These limitations have an adverse effect on the efficiency of the damping equipment in that the effective amplification at large signals is reduced below the nominal amplification at small oscillating amplitudes when the limitations are not active.
E
ABB AB
Polk Sharon A.
Sircus Brian
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