Method and a device for communication in a high voltage converter station

An apparatus and method for communication between low potential level and a valve of a high voltage converter station located on high voltage potential level is provided. The apparatus comprises means adapted to send serial messages on light conductors between a valve control unit and a first control unit for controlling semiconductor components of the valve of turn-on type on high voltage potential level in periods of time when the respective light conductor is free from signals associated with a change in the conducting state of the semiconductor component, such as turn-on and/or indications signals.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and a device for communication 
between a low potential level and a valve located at a high voltage 
potential level in a high voltage converter station. The valve has a 
plurality of valve units each having at least one semiconductor component 
of turn-on type and, for each semiconductor component, a first control 
unit located on a high voltage potential level and controlling the 
component. The first control units are connected to a valve control unit 
located on low potential level by light conductors. The light conductors 
carry signals associated with a change of the conducting state of the 
respective semiconductor component between the valve control unit and the 
respective first control unit, while separating them galvanically. 
The above mentioned "signals associated with a change of the conducting 
state of the respective semiconductor component" encompasses all types of 
signals that may be sent between the valve control unit and the first 
control units in connection with a change in the conducting state of the 
respective semiconductor components. In the case of thyristors, this 
includes firing or indication signals sent in connection with turning a 
thyristor on, and for semiconductor components in the form of IGBT's, it 
is firing signals and indication signals sent in connection with turning 
off or short-circuiting. 
Such high voltage converter stations may, for example, be stations in 
plants for transmitting electric power through High Voltage Direct Current 
(HVDC) for converting the direct voltage into alternating voltage and 
conversely. However, the invention is not restricted thereto, but is 
directed to all types of high voltage converter stations. High voltage 
here typically means voltages within the range of 10-500 kV. Each valve 
unit usually has a plurality of semiconductor components of turn-on type 
connected in series, such as thyristors, IGBT's or the like, which are 
controlled simultaneously so that they act as a single switch. The voltage 
to be held by the valve unit in a turned-off state of the semiconductor 
components is distributed among the semiconductor components connected in 
series, since each can normally only hold 1-5 kV. 
Although it is described below that the valves have first control units, 
the invention also comprises the case of a directly light controlled 
turning-on of the semiconductor components in which the first control 
units then principally only conducts light pulses on the semiconductor 
component in question. 
In the known devices in high voltage converter stations described in the 
introduction, two light conductors, extending between the valve control 
unit and the respective first control unit, are used only for 
communication between the electronic components on a low potential level 
and the individual semiconductor components, such as thyristors and 
IGBT's, for turning on and possibly re-turning on of thyristors and 
"turning on" and short circuit indication, respectively, of the IGBT's. 
There is no additional communication between the low potential level and 
the valve located on a high voltage potential level of the high voltage 
converter stations. 
However, it is desired to extend the communication between the high voltage 
potential level and the low potential level in the valves, but such 
communication is complicated as a consequence of the high potential level 
on which the valve is located. The more knowledge that is available about 
the state of the valve, the greater will be the possibility of keeping 
down the high costs caused by a shut down of parts of a high voltage 
converter station, or the entire station during a certain period of time. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a device and method of 
the type described in the introduction, which makes it possible to improve 
information exchange between the low potential level and the high voltage 
potential level of a high voltage converter station by a simple means. 
This object, according to the invention, is obtained by providing a method 
and a device of the type described in the introduction. which comprises 
means adapted to send serial messages on the light conductors between the 
valve and the low potential level during periods of time in which the 
respective light conductor is free from signals associated with turning-on 
the semiconductor components. 
Thus, the invention is based on the understanding that the light conductors 
already existing between the low potential level and the high voltage 
potential level of a high voltage converter station are utilized for 
sending serial messages therebetween in the periods of time during which 
no other signals such as different types of indication pulses and firing 
pulses, are sent on the light conductors. In this way, it is possible to 
easily and reliably communicate between the low potential level and the 
high voltage potential level with a low energy consumption, with very 
simple means, and at low cost. 
According to a preferred embodiment of the invention, the device comprises 
members adapted to determine when the light conductors are free from 
signals associated with the change of conducting states and to provide the 
means with information in this regard. This ensures that no messages, or 
parts of messages, are sent simultaneously with the signals associated 
with a change in the conducting state, so that disturbances in the control 
of the semiconductor components may be safely avoided. 
According to another preferred embodiment of the invention, the respective 
first control units are adapted to send an indication signal to the valve 
control unit on the light conductors connected thereto which states that 
the semiconductor component is ready for turning-on. The members are 
adapted to determine the point of time for sending such indication signals 
and to ensure the synchronizing of the sending of serial messages by the 
means with the sending of such indication signals by controlling the 
members to send the serial messages, or parts thereof, in the period of 
time immediately after the sending of an indication signal. By 
synchronizing the sending of the serial messages from the high voltage 
potential level to the low potential level with the indication signals in 
this way, it is ensured that no part of a message is sent simultaneously 
with an indication signal and disturbs the sending thereof. This 
synchronizing is important, since high voltage converter stations are 
usually connected to alternating voltage networks, which are asynchronous, 
i.e., the net frequency varies. This also results in an asynchronous 
turning-on of the different semiconductor components of the converter 
station, so that some type of coupling between the signals associated with 
a change of the conducting state and the signals forming the serial 
message is necessary for ensuring that the light conductor in question is 
really free for transfer of information when there is a need to send the 
serial message. By sending this message or parts thereof in the time 
period closely following the sending of an indication signal, the period 
of time between two consecutive indication signals may be utilized in the 
best possible way for sending a message or a part thereof. 
According to another preferred embodiment of the invention, the means are 
adapted to send a serial start and stop code, respectively, on the light 
conductors before and after each message sent thereby in periods of time 
during which the light conductors are free from signals associated with a 
change of the conducting state of the semiconductor component. By this, 
the receiver of the message is given reliable information that a message 
is starting and the message is ending. 
According to another preferred embodiment of the invention, the members are 
adapted to determine when the signal intended for the turning-on of the 
respective semiconductor component is sent from the valve control unit to 
the respective first control unit and to provide the means with 
information thereabout for synchronizing the sending of the serial message 
with the turn-on signal. This makes it possible to send serial messages 
without any mixing of the firing signal and the serial message from low 
potential level to high voltage potential level. By synchronizing the 
sending of the serial message in this direction exactly with the turn-on 
or firing signals and not only with the indication signals, it is ensured 
that a part of the message sent will not interfere with the basic function 
of the light conductors, i.e., the firing and the possible re-firing of 
the semiconductor component in question. 
According to another preferred embodiment of the invention, the means are 
adapted to send one bit at a time on the light conductor in question in 
each interval between the sending of two consecutive signals associated 
with a change of the conducting state of the semiconductor component in 
question on this light conductor. By sending one bit of information at a 
time between two such signals, i.e., per period, maximum reliability is 
achieved in the sense that there is no risk of conflicting with the normal 
firing function of the semiconductor component. Another important 
advantage of sending one bit at a time is that the power consumption gets 
very low when sending messages. It is not a problem that the data 
transmission is not particularly fast, since there are no particularly 
high demands on response times for this type of data transmission, 
primarily between the different positions in the valve and the electronic 
components on ground. 
According to another preferred embodiment of the invention, the means are 
adapted to send the serial messages in the form of signals of a lower 
level than the signals associated with the change of the conducting state 
of the respective semiconductor component for checking if the light margin 
of the latter is sufficient. By sending the message in this way, it can be 
determined that the light margin of the signals associated with a change 
of conducting states is large enough as long as the data transmission 
functions correctly and conversely, that the light margin is too small if 
there is any problem with the data transmission. 
According to another preferred embodiment of the invention, the means are 
located in the respective first control unit and the means are adapted to 
receive function parameters of the valve measured on high voltage 
potential level and after processing thereof, to send information 
thereabout in the form of the serial message to a valve supervising 
arrangement located on low potential level. By this, the device may be 
utilized to improve the information on ground level about the state of the 
valve by receiving information directly from high voltage potential level, 
so that after the occurrence of different types of faults any measures 
required may be taken quickly, and therefore costs may be saved. 
The advantages of a method according to the invention and the different 
embodiments thereof defined in the appended method claims appear clearly 
from the above discussion relating to a device according to the invention. 
Additional advantages as well as advantageous features of the invention 
will appear from the following description and the other dependent claims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The general construction of a valve of a high voltage converter station, 
for example, for converting High Voltage Direct Current (HVDC) into 
alternating current and conversely, and a valve control unit associated 
therewith are schematically illustrated in FIG. 1. The valve control unit 
1 is in the converter stations adapted to communicate through light 
conductors 2, i.e., fiber-optic cables, with control units 3 for 
controlling semiconductor components 4 of turn-on type belonging to valve 
units, such as thyristors, in which the control only relates to firing, 
and IGBT's, in which the control then also relates to turn-off. A greater 
number of semiconductor components than is shown are usually connected in 
series within one single valve unit and each are controlled simultaneously 
through a control unit 3. This is indicated through the dashed line at 5. 
A valve consists of a plurality of such valve units. For simplicity in the 
description, assume that the semiconductor components consist of 
thyristors which may not be turned off, although the invention is not in 
any way restricted thereto. 
The valve control unit 1 is located on low voltage potential level, i.e., 
on ground, and it receives, in a conventional way, a control pulse through 
an input 6 to a logic circuit 7 for normal turning-on or a firing. In 
turn, the logic circuitry 7 sends a firing signal FP to different light 
emitting diodes 8 to make the light conductors 2 send firing signals to 
all the control units 3 comprising part of the valve unit. The control 
units 3 have photo diodes 9 adapted to receive the firing signals and, 
through a firing circuit 10, act upon the gate of the thyristor 4 for 
turning-on the thyristor. The control unit 3 has, in a conventional way, a 
circuit 11 for sending an indication pulse IP (See FIG. 2) to light 
emitting diodes 12, and further through light conductors 13, to photo 
diodes 14 of the valve control unit so as to indicate that the voltage 
across the thyristor in question in the forward biasing direction thereof 
is sufficient for a turn-on to take place rapidly and efficiently with low 
power losses during the firing step. 
The indication pulses continue through a logic circuit 15, the function of 
which will be explained further below, to the circuit 7 to control the 
sending of a firing pulse thereby. The valve control unit also has a logic 
circuit 16 for supervising the different positions of the valve and for 
sending the different positions of the valve from the valve control unit 
through a supervising bus 17. 
Except for a particular arrangement of the circuits 15 and 16, the features 
described above are part of the prior art. The characterizing features of 
the invention will now be described. 
Members 18, 19 adapted to detect function parameters of the valve on high 
voltage potential level and send values detected thereby to the thyristor 
control units 3 are arranged in the vicinity of each of the thyristors. 
Furthermore, means 20, in the form of a microcomputer consuming a weak 
current, is arranged in each thyristor control unit and is adapted to 
process the values of the function parameters detected by the members 18 
and 19. Means 20 is connected with the light emitting diodes 12 so as to 
send information resulting from the processing through the light 
conductors 13. The sending of the data is synchronized with the sending of 
firing and/or indication pulses, which are indicated by the arrows 21, 22, 
and will be explained more in detail below with reference to FIG. 2. The 
signals coming from the light conductors 13 to the valve control unit are 
filtered in the logic circuit 15 to separate the indication pulses, so 
that the indication pulses are sent on to the circuit 7 while the data 
emanating from the means 20 are sent to the supervising arrangement 16 for 
a suitable treatment therein. 
The supervising arrangement also has an output 23 connected with the light 
conductors 2 through the light emitting diodes 8 in order to send messages 
to the control units 3, such as, for example, additional firing pulses to 
check the function of the different thyristors. 
Since the signals sent with the change of the conducting state of the 
thyristor on the light conductors 2, 13, i.e., firing and indication 
signals, are comprised of short pulses, the two light conductors between 
the valve control unit and each thyristor control unit are normally both 
under utilized. They are also comparatively difficult to utilize for 
another communication as a consequence of the asynchronous networks 
usually connected to the converter station. However, these light 
conductors are utilized in the present invention for sending serial 
messages between high voltage potential level and low potential level. 
This is done by using the time immediately after the sending of an 
indication pulse by the circuit 11 and the light emitting diode 12 towards 
low potential and after sending the firing pulse through the circuit 7 and 
the light emitting diode 8 towards the thyristors, respectively, to send 
serial messages between the individual positions in the valve and the 
electronic system on ground. The positions of the valve, the function 
parameters of which may be detected by the members 18 and 19, may, for 
example, be the temperature of the thyristor or a medium for cooling 
thereof, the status of voltage dividers arranged close to the thyristor, 
leakage of the cooling medium in a system for cooling the respective 
thyristor and the presence of smoke. 
In the top graph in FIG. 2, it is illustrated how a firing pulse FP is sent 
on the light conductor 2 and, in the time period closely thereafter, a 
data bit 24, which has a lower level than the firing pulse and is 
eventually sent only for checking if the firing pulse has a sufficient 
light margin. The reception of this data bit 24 by the control unit 3 may 
result in the sending back of suitable data through the means 20 to the 
supervising arrangement 16 on low potential. 
The middle graph of FIG. 2 illustrates how an indication pulse IP is sent 
on a light conductor 13 and, immediately thereafter, a data bit, in which 
during three consecutive periods the binary data bits 101 are sent. Thus, 
a firing pulse and an indication pulse, respectively, are each a "start 
pulse" for a message. It is emphasized that the graphs shown for the 
firing pulse and the indication pulse are not connected with each other, 
but the sending of the indication pulse and the firing pulse in practice, 
follow each other. The advantages of sending a bit of data per period have 
thoroughly been discussed above. The so-called one-bit system functions 
just as well for conventional line-commutated thyristor valves as for VSC 
valves with IGBT's. The difference is that there will, in conventional 
valves, be a bit frequency of 50-60 Hz and for VSC valves a bit frequency 
as high as the PWM (Pulse Width Modulation)--frequency, which, for 
example, may be about 2 kHz. The protocol of the one bit system is 
suitably constructed according to the NRZI-coding well known in field bus 
context, with the start and stop bit code being 01111110. To avoid 
confusion of the start/stop bit during data transmission, a zero is 
preferably put into a message when there are more than five consecutive 
ones. 
The one-bit system has a very low power consumption and is usually 
sufficiently fast, but should the demands on transfer speed increase, it 
is of course possible to send more data bits per period, even potentially 
the entire message. This is offset by an increase in power consumption, 
more expensive fiber optic components, and the risk of conflict with the 
basic function of the fiber optic system, i.e., the firing and the 
possible re-firing of the valve. However, the fiber-optic component 
already in high voltage converter stations may be used for data transfer 
when the one bit system is used. The case of a transmission of four data 
bits per period synchronized with sending an indication pulse IP is 
illustrated in the bottom graph of FIG. 2, in which the data sent over the 
three periods illustrated is the following: 110110001001. 
For supervising the firing channel when the valve is connected to voltage, 
but blocked, the method described in PCT/SE93/00662 is used for 
communicating with and firing a semiconductor component at the time. A 
complete control of the valve in the blocked and voltage connected 
position is obtained in this way. 
The invention is of course not in any way restricted to the embodiments 
described above, but many possible modifications thereof will be apparent 
to one skilled in the art, without departing from the basic idea of the 
invention as defined in the claims. For example, it may be that the 
invention can be directly applied on semiconductor components fired by 
light in which messages are sent from a high voltage potential level to a 
low potential level. In addition to the alternatives mentioned above, the 
converter station on which the invention is used can be in the form of a 
Static Var Compensator (SVC).