Method of taking a pole of a high-voltage d-c transmission station out of service

Method of taking a pole of high-voltage d-c transmission system (HVDCTS) out of service, having a rectifier station connected to a first three-phase network and an inverter station connected to a second three-phase network, two d-c lines connected from one station to the other, each station being divided into halves, the station halves of each station being connected together on one side to a station center grounding electrode, the other side of each station being connectible to the positive d-c line so as to form a positive pole, and to the negative d-c line so as to form a negative pole, and residual-current switching means for taking a pole out of service, which includes grounding the station pole through a pole grounding switch which is spaced from the station center grounding electrode, and subsequently commutating a partial fault current of other poles, occurring from the pole being taken out of service, into the station center grounding electrode by the residual-current switch in the form of a circuit breaker.

The invention relates to a method of taking a pole of a high-voltage d-c 
transmission system out of service, which includes a rectifier station 
connected to a first three-phase network and an inverter station connected 
to a second three-phase network and connected to the first station by two 
d-c lines. Each station is divided in half, the halves of each station 
being together connected to ground on one side. On the other side, each 
station half is connected to the positive d-c line, together forming the 
positive pole. Alternatively, the other side of each station half is 
connected to the negative d-c line, together forming the negative pole. 
The pole is taken out of service by means of a residual-current switch. 
The above described construction of a high-voltage d-c transmission system 
is generally known from ETZ-A, vol. 89 (1968), no. 8/9, and especially in 
accordance with the Special Publication of the Working Group HGUe, page 4, 
ETZ-A, vol. 97 (1976), no. 7, page 408. 
For maintenance or after a disturbance, a high-voltage d-c transmission 
system (HVDCTS) pole must be taken out of service without disturbing the 
operation of the other poles. To this end, the pole must be separated from 
the high-voltage line and from the common ground connection of all station 
poles. If the station pole which is to be taken out of service does not 
carry current, a disconnect switch can be used as the residual-current 
switch for cutting out the faulty pole. This condition of zero current in 
the HVDCTS pole at the instant of interruption, however, is not present in 
the case of disturbances which occur, for instance, in the ivent of a 
short to ground in the three-phase lead of a converter group near ground. 
If another operative pole is simultaneously not to be let down, even 
briefly, the latter will feed a partial fault current into the short to 
ground through the line of the pole which is to be disconnected. 
It is generally known to employ a conventional three-phase power breaker as 
the residual-current switch for cutting out the faulty station pole and 
commutating the partial fault current to the station center grounding 
electrode. However, this procedure is limited by two conditions. Firstly, 
the ground electrode line must not contain appreciable inductances, and 
secondly, the smoothing chokes of the converters must not be disposed in 
the ground line. The limit of applicability of a three-phase power circuit 
breaker as the residual-current switch is reached if a parallel ground 
return path exists by way of closed bypass switches of the converter 
groups through the smoothing chokes and the high-voltage line to another 
HVDCTS station. In that case, the three-phase power breaker cannot be 
opened directly in order to cut off the pole because of the large 
inductance of the circuit to be cut off. 
A similar problem exists if an existing ground connection is disconnected 
because of excessively large ground currents and the ground line of a pole 
which has failed is to be used as the return line. A HVDC load switch is 
conventionally used for solving this problem, as disclosed in: 
Specification of HVDC circuit breakers for different system applications, 
J. P. Bowles, L. Vanghan, N. G. Hingorani, CIGRE 1976, 13-09. 
Accordingly, a HVDC load breaker would be required as the residual-current 
switch for commutating a current from the high-voltage line to the ground 
electrode line. However, this solution is expensive and uneconomical. 
It is accordingly an object of the invention to avoid the 
hereinaforementioned shortcomings of devices of this general type, and to 
provide a simple and economically justifiable method of taking a HVDCTS 
station pole out of service without the need of using an expensive HVDC 
load breaker. 
With the foregoing and other objects in view, there is provided in 
accordance with the invention a method of taking a pole of a high-voltage 
d-c transmission system (HVDCTS) out of service, having a rectifier 
station connected to a first three-phase network and an inverter station 
connected to a second three-phase network, two d-c lines connected from 
one station to the other, each station being divided into halves, the 
station halves of each station being connected together on one side to a 
station center grounding electrode, the other side of each station being 
connectible to the positive d-c line so as to form a positive pole, and to 
the negative d-c line so as to form a negative pole, and residual-current 
switching means for taking a pole out of service, which comprises 
grounding the station pole through a pole grounding switch which is spaced 
from the station center grounding electrode, and subsequently commutating 
a partial fault current of other poles, occurring from the pole being 
taken out of service, into the station center grounding electrode by the 
residual-current switch in the form of a three-phase power circuit 
breaker. 
The advantages obtainable with this method invention are in particular that 
it becomes possible to use an ordinary three-phase power breaker. For 
instance, a compressed-air power circuit breaker as shown at page 5, line 
3 of British Patent No. 1,151,854 may be used as the residual-current 
switch instead of the uneconomical HVDC load breaker, and only a 
relatively simple, coordinated control of the pole grounding switches, the 
residual-current switches and the line disconnect switches need be used. 
In accordance with another mode of the invention, the method comprises 
grounding by means of pole grounding electrodes and residual-current 
switches at both ends of a pole, the rectifier and the inverter side, and 
subsequently disconnecting high voltage overhead lines or high voltage 
cables connecting the two ends, by disconnect switches. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in 
method of taking a pole of a high-voltage d-c transmission station out of 
service, it is nevertheless not intended to be limited to the details 
shown, since various modifications may be made therein without departing 
from the spirit of the invention and within the scope and range of 
equivalents of the claims. 
The invention, however, together with additional objects and advantages 
thereof will be best understood from the following description when read 
in connection with the accompanying drawing which is a circuit diagram for 
carrying out the method, of taking a pole of a HVDCTS out of service, of 
the invention.

The construction of such a HVDC system is known from ETZ-A, vol. 89, no. 
8/9, and particularly from the Special Publication of the Working Group 
HVDCT, page 4; the reference symbols and their meaning are well known and 
therefore are given here in tabular form: 
1--Converter Transformer 
2--Three-phase Leads 
3--Converter Group 
4--Converter Valve 
5--Converter Group 
6--Bypass Switch 
7--Bypass Switch 
8--Station Pole 
9--Station Center 
10--Grounding Electrode Line 
11--Station Center Grounding Electrode 
12--Smoothing Chokes 
13--Residual-Current Switch 
14--Station Pole 
15--Ground Return Line 
18--Line Disconnect Switch 
19--Rectifier Station 
20--Inverter Station 
21--High Voltage Line 
Referring now to the figure of the drawing, the method according to the 
invention provides for the use of a pole grounding switch 16, which is 
connected to an additional grounding electrode or station ground 17. 
The two HVDCT stations 19 and 20 each include several station poles which 
are each connected through a common ground electrode or grounding switch. 
In the present embodiment, for instance, two station poles 8 and 14 are 
used. In the HVDC bi-pole system shown, station 19 operates as a rectifier 
station and station 20 as an inverter station. Through the three-phase 
leads 2, the converter transformer 1 feeds the converter group 3 (only one 
bridge of the group being shown here), which together with the converter 
group 5 forms the station pole 8. If a short to ground occurs in the 
three-phase leads 2 of the converter group 3 which is close to ground, 
then the return current I.sub.2 of the still operative station pole 14 
flows not only through the electrode line 10 to the station center 
grounding electrode 11, which may be a grounding switch, but also to the 
fault location through the last-current-carrying converter valve 4 of the 
half close to ground of the converter group 3 afflicted with the fault. In 
order to interrupt this so-called residual current I.sub.1 by the valve 
control of the station pole 8 in question, the fault would have to be 
localized and the valves fired in a targeted manner. The apparatus 
required for this purpose is too expensive and unreliable, however, and it 
is therefore advisable to use a three-phase power circuit breaker as the 
residual current switch. Due to its arc voltage, the three-phase power 
circuit breaker is capable of cummutating the residual current I.sub.1 
from the fault location into the grounding electrode line 10. This method, 
which is known, is no longer applicable if there is no short to ground on 
the station side ahead of the smoothing choke 12 and if, furthermore, at 
the time of disconnecting the pole, the bypass switches 6 and 7 provided 
in the converter groups 3 and 5 are closed and therefore a path parallel 
to the ground return line 15 exists through the smoothing chokes 12 and 
the high-voltage line 21. Because of the high inductance of the circuit to 
be disconnected, a HVDC load breaker would have to be used as the 
residual-current switch 13 instead of the three-phase power circuit 
breaker. 
This uneconomical solution is circumvented in a surprisingly simple manner, 
according to the method invention, by first grounding the station pole 8 
separately by closing the pole grounding switch 16, before the residual 
current switch 13 is opened. It is assumed that the grounding electrode 
11, which is connected to the station center 9, is physically spaced or 
removed far enough from the station ground or ground electrode 17 used for 
grounding the pole, so that both are connected to each other by reference 
ground only. By grounding the pole, the condition which exists on the 
valve side in the station in the event of a short to ground is 
intentionally brought about before the residual-current switch is opened. 
Thereby, the condition for actuating the residual-current switch, namely 
low inductance in the commutation circuit, is brought about. To finally 
cut off the station pole 8, it must further be disconnected from the 
high-voltage line 21. For this purpose, disconnect switches 18 are 
provided on the line side ahead of the smoothing chokes 12; these 
disconnect switches must be switched at zero current only. However, as 
long as both sides of a pole (rectifier side and inverter side) are not 
grounded and separated by the residual-current switches from the 
respective station centers, there is the possibility that a considerable 
partial fault current I.sub.3 can still flow over the line. In a further 
embodiment of the invention, the procedure, after grounding has been 
effected, for disconnecting one pole side (e.g., the rectifier side 19) is 
similar for the other pole side (e.g., the inverter side 20) and only 
after the other station has reported back that that pole is grounded as 
well, and the residual-current switch there has been opened, is the 
release command for opening the line separators given.