Electricity: power supply or regulation systems – For reactive power control – Using converter
Reexamination Certificate
2000-09-28
2001-06-05
Riley, Shawn (Department: 2838)
Electricity: power supply or regulation systems
For reactive power control
Using converter
Reexamination Certificate
active
06242895
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a controller for controlling a reactive series compensator serially inserted at compensator terminals into a power transmission line. Typically, in such a transformerless reactive series compensator an inverter control is performed in order to control the line current and/or the voltage applied from the compensator to the transmission line. The voltage/current control enables control of the flow of power from one end to the other end of the transmission line and a power flow into the inverter of the compensator in order to charge a capacitor which provides a compensator terminal output voltage at the compensator terminals.
Typically, as will be explained below with more details, a current feedback control loop and a voltage feedback control loop are employed in order to respectively control the reactive and the active part of the line current. For doing so, the modulation signal based on which the PWM control of the inverter is performed, is a sinusoidal signal of a particular phase. Adjusting the amplitude and the phase of the modulation signal allows the power control.
Using such control loops, the output voltage of the compensator can be controlled by a modulation index with a constant DC voltage. Alternatively, the output voltage can be controlled by the DC voltage only using a constant modulation index, as the paper by A. Beer, H. Stemmler, H. Okayama, “A Hybrid Transformerless Reactive Series Compensators”, EPE 1999—Lausanne describes (see page 8 of this paper). However, using constant DC voltage, the injection of harmonics into the transmission line by the inverter cannot be reduced even if the fundamental component is smaller than the DC voltage. Furthermore, using constant modulation index control, the controllability of the DC voltage is degraded in zero or very low DC voltage region since the active voltage cannot be sufficiently injected into the transmission line. Therefore, this method is not applicable to a full range operation. Furthermore, with the aforementioned two different control methods it is also difficult to reduce DC voltage dependent losses such as the switching loss of switching elements of the inverter and leakage loss of the DC capacitor of the compensator.
The present invention in particular addresses the problem as to how the injection of harmonics by a compensator controlled by a controller including a current and voltage control loop can be reduced. The present invention also addresses the other aforementioned problems.
Hereinafter, first the general background of transformerless reactive series compensators and of controllers for controlling a compensator and comprising a current and voltage control loop will be described.
DESCRIPTION OF THE RELATED ART
Recently, power electronics equipments for flexible AC power transmission systems (FACTS) have been investigated and applied to practical systems. A transformerless reactive series compensator is one of these equipments and is effective to perform a power flow control as was explained above. Since the transformerless reactive series compensator does not comprise the transformer its size is small and it can be advantageously used.
FIGS. 1
a
and
1
b
respectively show a typical configuration of a power transmission system comprising two AC power systems
1
a
,
1
b
coupled to each other through power transmission lines 2a,
2
b
having a respective inductance L
AC
and resistance R
AC
. As indicated in
FIG. 1
a
and
FIG. 1
b,
the power transmission system may be a single phase system or a three-phase system. Whilst in the single phase system only one series compensator
3
need to be provided, in the three-phase system a plurality of series compensators
3
are respectively serially inserted as shown in
FIG. 1
b
. Reference numerals
3
a
,
3
b
respectively show the terminals at which the respective series compensator (or compensators) are serially inserted.
As shown in
FIG. 2
, a typical series compensator
3
comprises a starting switch
4
, a filter
12
, an inverter
7
, a DC capacitor C
DC
, a control means C, a saw-tooth generator
10
and a modulation signal generation means
11
. The inverter
7
comprises four thyristors
5
a
,
5
b
,
5
c
,
5
d
respectively controlled by a PWM control signal SW
5a
, SW
5b
, SW
5c
, SW
5d
output by the control means C.
Whilst the expression “thyristor” is usually a device whose turn-off is not controllable, in
FIG. 2
, since a PWM is used for the inverter, a gate turn-off type thyristor is employed. Since a GTO (gate turn-off thyristor), a GCT (gate commutated thyristor) and an IGBT (insulated gate bipolar transistor) are also generally be possible for operating as the kind of switching power device in
FIG. 2
, hereinafter it is assumed that the expression “thyristor” comprises all such switching power devices.
Each thyristor has connected anti-parallely thereto a diode
6
a
,
6
b
,
6
c
,
6
d
. The filter
12
comprises two reactors
9
b
,
9
a
and a capacitor
8
for filtering higher order harmonics which are generated by the PWM control of the inverter
7
. The filter terminals are connected to the respective interconnections of the thyristors
5
a
,
5
b
and of the diodes
6
a
,
6
b
and the thyristors
5
c
,
5
d
and the diodes
6
c
,
6
d
. The DC capacitor C
DC
is connected at the other terminals of the thyristors and the diodes.
The circuit configuration of the series compensator
3
is conventional and is for example described in the European patent applications No. 98 116 096.3, No. 98 106 780.4 and No. 99 124 851.9 by the same applicant. These patent applications in particular describe the start and stop control of the series compensator
3
and a decoupling control.
A PWM control of the inverter
7
is carried out as principally shown in the diagram of
FIG. 3. A
modulation signal generation means
11
in
FIG. 2
generates a sinusoidal modulation signal m and the saw-tooth generator
10
outputs two saw-tooth carrier signals cs
1
, cs
2
. A PWM control signal SW
5a
, SW
5d
is generated by comparing the modulation signal m with the respective carrier signal cs
1
, cs
2
. That is, when the modulation signal amplitude is larger than the carrier signal cs
1
amplitude then the PWM switching signal SW
5a
is on and it is off if the modulation signal amplitude is smaller. Similarly, if the amplitude of the modulation signal is larger than the inverted carrier signal cs
2
then the other PWM switching signal SW
5d
is switched from ON to OFF. The PWM signals SW
5a
, SW
5d
are used to trigger the thyristor
5
a
,
5
d
. It should be noted that of course a similar control applies to the thyristors
5
b
,
5
c
which is however not described here for simplicity.
Assuming that the DC capacitor C
DC
was charged to u
DC
the output voltage u
c
at the connection terminals
3
a,
3
d
will have a waveform as shown in
FIG. 3
in the bottom graph. It will be appreciated that by changing the respective amplitudes of the modulation signal and/or the carrier signal and/or by changing the phase of the modulation signal and/or the carrier signal, different waveforms of the output voltage u
c
(hereinafter also called the inverter terminal voltage or compensator output voltage) can be achieved. Comparing
FIG. 3
with
FIG. 2
it can be seen that essentially the output voltage u
c
is the voltage applied to the terminal
3
a
,
3
b.
Whilst from
FIG. 3
it only appears as if the terminal voltage u
c
changes due to the PWM control of the inverter
7
, of course the line current i would also change since current and voltage are linked through the coupling effects due to the line impedance L
AC
. The simultaneous effects of the PWM control on the line voltage and the line current will be explained now.
FIG. 4
a
shows a summary diagram of the essential parts of
FIG. 2
necessary for explaining the current and voltage control.
FIG. 4
b
shows the principle phasor diagram for FIG.
4
a.
As was the case in
FIG. 1
a,
also in
FIG. 4
a
the compensator
3
is ser
Beer Andreas
Fujii Toshiyuki
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
Riley Shawn
LandOfFree
Controller of adjustable DC voltage for a transformerless... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Controller of adjustable DC voltage for a transformerless..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Controller of adjustable DC voltage for a transformerless... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2475380