Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2001-06-04
2002-06-04
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
active
06400585
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. §119 to European Patent Application No. 00201949.5-2207-, filed Jun. 2, 2000, the entire contents of which are incorporated herein by reference.
TECHNICAL FIELD
The invention relates to a method for control of a converter station having a voltage source converter coupled between a direct current link and an alternating current network in a high voltage direct current transmission system, the control system having means for control of active power flow between the direct current link and the alternating current network by influencing the phase displacement between a bus voltage in the alternating current network and a bridge voltage of the voltage source converter, and to a control system for carrying out the method.
BACKGROUND ART
For a general description of controls systems for voltage source converters reference is made to Anders Lindberg: PWM and Control of Two and Three Level High Power Voltage Source Converters. Royal Institute of Technology, Department of Electric Power Engineering. Stockholm 1995, in particular pages 1, 77-106, and appendix A.
The block diagrams to be described in the following can be regarded both as signal flow diagrams and block diagrams of control equipment for the transmission system. The functions to be performed by the blocks shown in the block diagrams may in applicable parts be implemented by means of analogue and/or digital technique in hard-wired circuits, but preferably as programs in a microprocessor. It shall also be understood that although the in the figures shown blocks are mentioned as members, filters, devices etc. they are, in particular where their functions are implemented as software for a microprocessor, to be interpreted as means for accomplishing the desired function. Thus, as the case may be, the expression “signal” can also be interpreted as a value generated by a computer program and appearing only as such. Only functional descriptions of the blocks are given below as these functions can be implemented in manners known per se by persons skilled in the art.
In order not to weigh the description with for the person skilled in the art obvious distinctions, usually the same designations are used for quantities that appear in the high voltage transmission system and for those measured values and signals/calculated values corresponding to these quantities that are supplied to and processed in the control equipment and control system described below.
Parts that are similar to each other and appear in more than one figure are given the same designation numbers in the various figures.
Connecting lines between sensed values and blocks have occasionally been omitted in order not to unnecessary weigh the figures. However, it is to be understood that the respective variables appearing at the inputs of some blocks are supplied from the sensed values respectively from the blocks where they are generated.
FIG. 1
shows in the form of a schematic single line and block diagram a high voltage direct current transmission system as known in the prior art. A first and a second converter station STN
1
and STN
2
respectively, are coupled to each other via a direct current link having two pole conductors W
1
and W
2
respectively. Typically, the pole conductors are cables but they may also at least to a part be in the form of overhead lines. Each converter station has a capacitor equipment, C
1
and C
2
, respectively, coupled between the pole conductors, and comprises a voltage source converter CON
1
and CON
2
, respectively. Each converter comprises two three-phase groups of semiconductor valves in six-pulse bridge connection. The semiconductor valves comprise, in a way known per se, branches of gate turn on/turn off semiconductor elements, for example power transistors of so-called IGBT-type, and diodes in anti-parallel connection with these elements.
Each converter is via a phase inductor PI
1
and PI
2
, respectively, coupled to a respective three-phase alternating current electric power network N
1
and N
2
. Although not shown in the figure, it is well known in the art that the converters may be coupled to the three-phase network via transformers, in which case the phase inductors in some cases may be omitted. Filter equipment F
1
and F
2
, respectively, are coupled in shunt connection at connection points between the phase inductors and the three-phase networks.
The ac-voltage of the alternating current network N
1
at the connection point of the filter F
1
is designated UL
1
and is sensed with a sensing device M
1
. This voltage is in the following called the bus voltage of the alternating current network N
1
. The ac-voltage set up by the converter CON
1
is designated UV
1
and is in the following called the bridge voltage of the converter CON
1
. The alternating current at the converter CON
1
is designated IV
1
and is sensed with a measuring device M
3
. Similarly the ac-voltage at the connection point of the filter F
2
is designated UL
2
and is sensed with a sensing device M
4
, and the alternating current at the converter CON
2
is designated IV
2
and is sensed with a measuring device M
6
. The ac-voltage at the connection point of the filter F
2
is in the following called the bus voltage of the alternating current network N
2
. The ac-voltage set up by the converter CON
2
is designated UV
2
and is in the following called the bridge voltage of the converter CON
2
.
The dc-voltage across the capacitor equipment C
1
is designated Ud
1
and is sensed with an only symbolically shown sensing device M
7
. The voltage across the capacitor equipment C
2
is designated Ud
2
and is sensed with an only symbolically shown sensing device M
8
.
The first converter station comprises control equipment CTRL
1
and the second converter station control equipment CTRL
2
, usually of similar kind as the control equipment CTRL
1
.
The converter stations can operate in four different modes, one of dc-voltage control and active power control and one of ac-voltage control and reactive power control. Usually, one of the converter stations, for example the first one, operates under dc-voltage control for voltage control of the direct current link, whereas the second converter station operates under active power control and under ac-voltage or reactive power control. The operation modes are set either manually by an operator, or, under certain conditions, automatically by a not shown sequential control system.
A known embodiment of the control equipment CTRL
1
is shown in
FIG. 2
, and illustrated for the case of operation under dc-voltage control for the purpose of voltage control of the direct current link. It comprises a dc-voltage controller UdREG, an ac-voltage controller UaREG, a selector means SW
21
, and a converter current control system IREG.
The dc-voltage controller is supplied with an actual value of the sensed dc-voltage Ud
1
across the capacitor equipment C
1
and a voltage reference value Ud
1
R thereof and forms in dependence on the deviation of the actual value and the reference value an output signal p
1
R.
The ac-voltage controller is supplied with an actual value of the sensed bus voltage UL
1
and a voltage reference value UL
1
R thereof and forms in dependence on the deviation of the actual value and the reference value an output signal DUL
1
.
The output signal DUL
1
and a reference value Q
1
R for the reactive power flow through the converter CON
1
are supplied to two different inputs on the selector means SW
21
.
In dependence on a mode signal MD
21
either of the output signal DUL
1
and the reference value Q
1
R is transferred and supplied to the converter current control system IREG in the form of a signal designated q
1
R.
An embodiment of the control equipment CTRL
2
as described in the European Patent Application No. 99112542.8 (to be published) is shown in
FIG. 3
, and illustrated for the case of operation under active power control and reactive power control.
The control equipment CTRL
2
comprises a dc-voltage contro
ABB AB
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Riley Shawn
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