GB2090482A

GB2090482A - Method for limiting voltage across series-connected thyristors of high-voltage rectifier

Info

Publication number: GB2090482A
Application number: GB8130419A
Authority: GB
Original assignee: V Elektrotech I V I
Current assignee: V Elektrotech I V I
Priority date: 1980-12-26
Filing date: 1981-10-08
Publication date: 1982-07-07
Also published as: SU982505A1; FR2497417A1; CA1175097A; SE8107664L; GB2090482B; FR2497417B1; CH663119A5; IT8141693A0; JPS57110075A; IT8141693A1; SE446920B; DE3146555A1; IT1168460B

Abstract

The method limits voltage across series-connected thyristors 11, 12, etc of a high-voltage rectifier as a voltage surge is applied to the latter and consists in limiting voltage across a group of the series- connected thyristors with a specified number of thyristors in a group, which number is selected to suit the voltage surge parameters. For this purpose, the voltage is measured across the thyristor 11 next to the point where the voltage surge is applied, and then across the following thyristors. As the voltage across the first thyristor reaches a specified level, total voltage across the thyristors is measured, starting from the first thyristor, until it reaches a level equal to the ratio between the voltage surge amplitude and the total number of series-connected thyristors of the rectifier, multiplied by the number of thyristors giving the specified total voltage, corresponding to the number of thyristors in the group. The voltage across the group is then limited to this level, e.g. by switching a specified number of voltage limiters 61, 62 etc across the thyristors in the group. <IMAGE>

Description

SPECIFICATION Method for limiting voltage across seriesconnected thyristors of high-voltage rectifier The invention relates to converter equipment and, more particularly, to a method for limiting voltage across series-connected thyristors of a high-voltage rectifier.

The invention is applicable to the designing of high-voltage thyristor rectifiers for d.c.

transmission lines.

What is required is a method for limiting voltage across series-connected thyristors of a high-voltage rectifier, wherein the relationship between the number of thyristors in a group and the voltage limitation level would make it possible to improve the reliability of the rectifier and to reduce the number of thyristors, while maintaining the overvoltage protection level of the rectifier.

The invention provides a method for limiting voltage across series-connected thyristors of a high-voltage rectifier as a voltage surge is applied thereto, whereby voltage is limited across groups of series-connected thyristors with a specified number of thyristors in each group, the method being characterized, according to the invention, in that the number of thyristors in a group is selected to suit the voltage surge parameters, for which purpose voltage is measured across the thyristor next to the point where the voltage surge is applied and when voltage across this first thyristor raises to a specified level, total voltage across the thyristors is successively measured, starting from the first thyristor, until it reaches a level equal to the ratio between the voltage surge amplitude and the total number of series-conected thyristors of the rectifier, multiplied by the number of thyristors giving the specified total voltage corresponding to the number of thyristors in the group, the voltage across the group being limited to this level.

The method according to the invention for limiting voltage across series-connected thyristors of a high-voltage rectifier makes it possible to reduce the number of thyristors and improves the reliabilty of rectifiers.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in wherein: Figure 1 is a time plot of voltages across a group of thyristors and each individual thyristor at a moment when the high-voltage rectifier is acted upon by a voltage surge, which plot also shows the level to which voltage across the group is limited through the use of the method according to the invention; Figure 2 is a functional diagram of a device for carrying out the method according to the invention for limiting voltage across sectiedconnected thyristors of a high-voltage rectifier; Figure 3 is a plot of a pulse wave distribution among groups of thyristors of a highvoltage rectifier when voltage across the thyristors is limited by the method according to the invention.

The method according to the invention for limiting voltage across series-connected thyristors of a high-voltage rectifier consists in limiting voltage across groups of series-connected thyristors with a specified number of thyristors in each group. The number of thyristors in a group is selected to suit voltage surge parameters. For this purpose, voltage is measured across the thyristor next to the point where a voltage surge is applied. When voltage across this first thyristor raises to a specified level, total voltage across the thyristors is successively measured, starting from the first thyristor, until it reaches a level equal to the ratio between the voltage surge amplitude and the total number of series-connected thyristors of the rectifier, multiplied by the number of thyristors giving the specified total voltage corresponding to the number of thyristors in the group.Voltage across the group is then limited to this level.

In most cases voltage e, which acts upon the rectifier, may be represented to a certain degree of approximation by a wave with a known amplitude E (curve I of Fig. 1) and a duration T of its oblique-angled edge. Curve II represents voltage across a group of N thyristors. Curves will, IV . . . N represent voltages across individual thytistors of the group.

A specified voltage U, across the heaviestloaded thyristor is reached at an instant to (curve (III). As pointed out above, at this instant one calculates the sum total of voltages across the thyristors, starting from the first one.

where n is the total number of thyristors in the rectifier, voltage is limited across a group of N thyristors. The limitation level is equal to the wave amplitude E divided by the n number - of the groups. This rules out a N situation when

i.e. when voltage across the heaviest-loaded thyristor is above a permissible level. In addition, the total limitation level guarantees a rectifier protection level equal to the pulse wave amplitude.

The following description refers to a preferred embodiment of the method according to the invention, applied to a rectifier intended for d.c. transmission n series-connected thyris tors it, 12 . . ., 1N, each connected in parallel with a respective damping RC circuit 2,-3,, 22-32, . . ., 2N-3N. An anode reactor 4 is connected in series with the rectifier. The circuit containing the rectifier and reactor 4 is an arm of a converter bridge. A non-iine arrester 5, which determines the protection level of the rectifier, is connected in parallel with the above-mentioned bridge arm. Connected in parallel with the rectifier is an arrester which in the simplest case comprises limiters 6, 62, . .. , 6N. The operation threshold of each of the limiters 6, through 6N is selected to be equal to E/n Outputs of the limiters 61 through 6N are connected to (n-1) inputs of a switching unit 7 which in the general case may have (n-1) outputs. According to the invention, the number of thyristors in a group and the level to which voltage across the group is limited are determined as follows.

Voltmeters 8" 82 . . ' 8N measure voltages across the thyristors 11, 12 1 N and send signals to a switching unit 9 which connects outputs of the voltrneters 8t, 82, 8N to inputs of a computing unit 10 when voltage across the heaviest-loaded thyristor 1, raises to a specified level.

The computing unit 10 calculates the number N of thyristors, the sum total S of voltages across the thyristors and

it also compares these values. If these values are equal (a certain specified degree of error is not excluded), a signal is produced which is proportional to the number N of thyristors. The signal is transmitted to the switching unit 7 which connects outputs of groups of N limiters to those of the groups of N thyristors. The voltmeters 8" 82, . . . , 8N, the switching units 7 and 9 and the computing unit 10 may be of any type known to those skilled in the art.

The method according to the invention makes it possible to divide a rectifier into M groups of thyristors. Fig. 3 shows a distribution of a pulse wave e among groups of thyristors. The pulse wave e is applied on the side opposite to the reactor 4 (Fig. 2). The reactor 4 is remote from the point to which the pulse wave is applied, so its effect on the wave distribution is insignificant. The pulsed voltage is represented by an oblique-angled wave with an amplitude E (Fig. 3) and edge duration T. During the time interval O . , -: voltage across several groups of thyristors, for example, U, and U2, reaches the limitation level E/M.According ta Fig. 3, the limiters of the first and second groups are actuated at instants t1, and tJ. Operation of the limiters lasts for only a fraction of T. Within the time interval 0 . T, voltage across other remote groups, such as UM (Fig. 3), does not reach the limitation level. From instant T, voltages U, and U2 drop below the E/M level which is reached again during the time interval t'M-t,'.

Without the limiters 61 (Fig. 2), 62, . . , 6N, voltage across the groups would change as shown by the broken lines in Fig. 3. At instant T, voltages U1 and U2 would be above the limitation level. The increase in voltage could only be compensated by increasing the number of thyristors. A single E-level limiter connected in parallel with the rectifier as a whole cannot reduce U, and U2, because at instant 7 voltage UB across the rectifier is below the limitation level E. Such a limiter only eliminates an increase of voltage across the rectifier due to readjustments in the L-RC loop. A limitation level below E is inpermissible in long-term operation.

The method according to the invention for limiting voltage across series-connected thyristors of a high-voltage rectifier of a d.c. trans misson line, makes it possible to reduce the number of thyristors in a rectifier complete with control, protection, signalling and cooling systems. The method improves the reliability of rectifiers, because voltage across the thyristors can never rise above a permissible. level.

In addition, the method according to the invention for limiting voltage across groups of thyristors is such that a rectifier can never be switched on at a voltage surge amplitude, which would otherwise affect normal switching of a converter's rectifiers.

Claims

1. A method for limiting voltage across series-connected thyristors of a high-voltage rectifier as a voltage surge is applied to the latter, consisting in limiting voltage across groups of series-connected thyristors with a specified number of thyristors in each group, which number is selected to suit the voltage surge parameter, for which purpose voltage is measured across the thyristor next to the point where the voltage surge is applied and then across the following thyristors, whereupon, as voltage across the first thyristor reaches a specified level, total voltage across the thyristors is successively measured, starting from the first thyristor, until it reaches a level equal to the ratio between the voltage surge ampliture and the total number of series-connected thyristors of the rectifier, multiplied by the number of thyristors giving the specified total voltage level, corresponding to the number of thyristors in the group, voltage across the group being limited to this level.

2. A method for limiting voltage across series-connected thyristors of a high-voltage rectifier, substantially as hereinbefore described with reference to the accompanying drawings.