Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2001-01-17
2002-06-25
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S129000, C363S051000, C361S101000
Reexamination Certificate
active
06411529
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a plant for transmitting electric power comprising a semiconductor device of turn-off type connected in series with a conductor and a member connected in anti-parallel therewith able to assume a state in which it blocks a conduction of current therethrough when a voltage is applied over the power semiconductor device in the conducting direction thereof and a state in which it conducts a current therethrough when a voltage is applied over the power semiconductor device in the opposite direction.
“A plant for transmitting electric power” is to be given a very broad sense and is intended to comprise in principle all equipment and the like in which electric power is fed in a conductor.
However, the invention is particularly well applicable to the case of transmitting high electric powers through high voltage, in which high voltage is typically a voltage between 2 and 400 kV, and then primarily in connection with stations provided with converters for converting the voltage with respect to level and/or type (direct voltage, alternating voltage), and for that reason the particular case of a plant having a VSC-converter for converting direct voltage into alternating voltage or direct voltage and conversely will hereinafter be described for illuminating, but accordingly not in any way restricting, the invention and the problem forming the basis therefor.
A plant of the type defined in the introduction is schematically illustrated in appended
FIGS. 1 and 2
, said plant having a VSC (Voltage Source Converter)-converter, which here is constituted by a so called 6-pulse bridge having three phase legs
1
-
3
, each consisting of two current valves
4
-
9
connected in series, which in their turn are formed by a first power semiconductor device
10
of turn-off type and a rectifying member
11
in the form of a so called free-wheeling diode connected in anti-parallel therewith. Each valve comprises in practise a comparatively high number of power semiconductor devices of turn-off type connected in series and free-wheeling diodes for enabling the valve to take the high voltages it has to take in the blocking state. The midpoints between the two valves of the respective phase leg are at a phase output
12
connected through a phase reactor
13
to a phase each of a three phase alternating voltage network
14
. The phase legs are at the ends thereof connected to a pole conductor
15
,
16
each of a direct voltage network
17
, the voltage of which is defined through two capacitors
18
,
19
connected in series and having a grounded midpoint
20
.
The power semiconductor devices
10
of turn-off type are controlled in a way known per se through a control arrangement
21
according to a determined pulse width modulation pattern (PWM) for using the direct voltage over the DC-capacitors
18
,
19
to generate a voltage on the phase output
12
, the fundamental tone component of which constitutes an alternating voltage having a desired amplitude, frequency and phase position. The switching frequency of the power semiconductor devices
10
is then usually 1-3 kHz, but the frequency of the alternating voltage will be 50 or 60 Hz. The plant has also a breaker
33
adapted to close and open, respectively, the connection between the alternating voltage network and the converter, and which is closed when a voltage is supplied to the converter after interruption of the operation.
The principle of a VSC-converter already known and just described implies that the direct voltage is higher than the peak value of the alternating voltage, i.e. the diodes
11
connected in anti-parallel are reversed biased when the converter is blocked. When a ground fault occurs on the direct voltage side, which is schematically indicated at
22
, this means that the diodes
11
will be excerted to a short circuit current having a peak value being approximately 2,5 times higher than a symmetrical short circuit current until the breaker
33
therebehind has opened, which takes place after 2-3 periods. The blocking of the 6-pulse bridge does not prevent the fault current as schematically indicated at
23
. This means in the practise that the users require that the diodes have to be able to take said current therethrough during a considerably longer time, maybe 12-20 periods, for having a sufficient safety margin, which means that unreasonable demands are put on the diodes, which have to be heavily over-dimensioned for satisfying this and they will by that be very costly.
Another problem adhered to a converter of this type concerns the putting of the converter under voltage after an interruption of the operation. When the alternating voltage breaker in such a case is closed the DC-capacitors
18
,
19
are charged through the diodes
11
in the valves. Energy stored in the phase reactor
13
when the capacitors are charged will raise the direct voltage further, in the worst case to a duplicating of the direct voltage. This problem may be solved by using a so called switching-in-resistance, but that solution is costly and it is desired to improve the result thereof.
It is shown in
FIG. 2
how a so called quick disconnector
24
is arranged between the direct voltage network
17
and the converter for the respective pole conductor. The direct voltage network has in this case a so called meshed fashion with a system of a plurality of stations with converters connected in parallel to the direct voltage network. The quick disconnector
24
is formed by a power semiconductor device
25
of turn-off type and a diode
26
connected in anti-parallel therewith and has the object to isolate the converter at a ground fault on the direct voltage network
17
, so that the converter will not feed a large current directly into such a ground fault. However, the presence of the diode
26
means that the quick disconnector
24
will be conducting in one direction. This results in its turn in the fact that if a ground fault
27
schematically indicated occurs within the converter, all the converters connected to the system will feed current into the fault, such as indicated through the dashed lines
28
-
30
. If all the stations of the system, as is the case in
FIG. 2
, are provided with quick disconnectors of their own they will open as a consequence of the overcurrent and this means that all transmission on the direct voltage network will be interrupted as a consequence of an internal fault in one of the stations, which of course is unacceptable.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a plant for transmitting electric power of the type defined in the introduction, which has a function improved in certain respects with respect to such plants already known and enables a reduction of the inconveniences discussed above.
This object is according to the invention obtained by providing such a plant, in which said member is formed by a controllable second power semiconductor device having the conducting direction opposite to that of said first power semiconductor device of turn-off type.
Through this totally new approach to use a controllable power semiconductor device as a member connected in anti-parallel with a power semiconductor device of turn-off type a number of advantages in different operation situations are obtained. By arranging the controllable second power semiconductor device the flexibility is considerably increased, since it will be possible to design the unit formed by the two power semiconductor devices to either block when a voltage is applied over the power semiconductor device of turn-off type in the blocking direction or conduct by controlling the second power semiconductor device to conduct. Thus, an optimum adaption to the operation situations prevailing may be obtained.
According to a preferred embodiment of the invention, which is applicable to a plant comprising a VSC-converter for converting direct voltage into alternating voltage or direct voltage and conversely and which has at least one phase leg with two current val
ABB AB
Dykema Gossett PLLC
Riley Shawn
LandOfFree
Control for power plant does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Control for power plant, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Control for power plant will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2898431