WO1984002807A1

WO1984002807A1 - Direct current cable protection system

Info

Publication number: WO1984002807A1
Application number: PCT/GB1984/000008
Authority: WO
Inventors: Nehemiah Lekaile Diseko; Michael James Boden; Brian Anthony Rowe; Bjarne Reinholdt Andersen
Original assignee: Ass Elect Ind
Priority date: 1983-01-14
Filing date: 1984-01-13
Publication date: 1984-07-19
Also published as: EP0132266A1; GB8300991D0; GB8400612D0; IT1196677B; IT8467041A0; GB2133939A

Abstract

An H.V.D.C. cable protection system in which a transmission cablehas a normal polarity with respect to earth determined by a normal power flow direction. In the event of a fault, a reflected voltage surge on the line can in some cases give a 100% reverse polarity voltage on the line, with the result that the cable can be damaged unless it is designed (at significant extra cost) to withstand such voltage reversal. According to the invention a diode rectifier is connected between the lines at each end of the cable in such direction as to be reverse biased in normal operation. A reverse voltage on the line is thus limited to the forward drop of the diode. Since this would produce a very large diode fault current a resistance is incorporated in series with the diode to limit the current even though at a sacrifice of a small reverse voltage.

Description

Direct Current Cable Protection System

This invention relates to a high voltage direct current (h.v.d.c.) cable protection system in which the cable polarity .is not required to change suddenly in normal conditions. This may be because the direction of power flow is constantly in one direction or because measures are taken to maintain or reverse gradually the cable polarity in normal conditions in a bidirectional power system. The former case arises where, for example, the h.v.d.c. cable has a dedicated function of supplying a load centre from a generating plant. In such circumstances the cable polarity need never reverse in normal conditions.

The possibility of having to withstand substantial reverse voltage is a significant factor in the cost of an underground or submarine cable. Consequently it would be very desirable to be able to assume that the voltage polarity on the cable would not reverse, or at least would not suddenly reach any significant reverse magnitude.

Unfortunately, fault conditions can arise which almost instantaneously lead to an almost 100% reverse polarity voltage.

It is therefore an object of the present invention to provide a cable protection system which will prevent, or at least limit the extent of, any sudden polarity reversal on an h.v.d.c. cable. According to the present invention a high voltage direct current power transmission ^'system comprises a high voltage cable having at least one conductor connected between converters at respective terminal stations, characterised in that unidirectional current-conducting means are connected to a said conductor at each terminal station in a direction such as to be reverse-biassed by the normal cable voltage polarity, each current-conducting means incorporating a resistive component and the arrange- ment being such that in a fault condition, reverse voltage_, on the cable is limited by the voltage drop across the unidirectional current-conducting means.

The unidirectional current-conducting means may comprise a diode rectifier or a gated thyristor, which may be connected in series with a resistor.

Said cable may be a polar cable, in which case the unidirectional current-conducting means is suitably connected between the cable core and earth and is arranged to limit any reverse fault voltage to below the rated reverse voltage of the cable. By a polar cable is meant any cable which can withstand a significantly higher voltage of normal polarity than of reverse polarity.

However, the invention is also applicable to a bidirectional power transmission system employing a non- polar cable. In such a case the polarity of the unidirectional current-conducting means may be reversed so as to maintain its reverse bias irrespective of the relative polarity of the cable conductors, so as to allow the direction of power transmission to be reversed (accompanied by a slow and therefore harmless voltage reversal) but to quench any sudden voltage reversal caused by a fault condition.

In a high voltage direct current power transmission system comprising a plurality of such systems connected in parallel, the total resistance of each unidirectional current-conducting means is preferably equal to that of each of the others. An h.v.d.c. cable protection system will now be described, by way of example, with reference to the accompanying drawings, of which:

Figure 1 is a diagram of an h.v.d.c. scheme includin terminal stations;

Figure 2 is a diagram of one of the terminal stations;

Figure 3 is a current/time diagram showing fault currents with different resistive components; Figure 4 is a diagram of a monopolar terminal statio adapted to accommodate the invention in a bidirectional powe system;

Figure 5 is a ^'similar diagram in respect of a bipola terminal station, and Figure 6 is a diagram showing two similar bipolar terminal stations employing a non-polar cable.

Referring to Figure 1 , generating plant at station A is required to supply power to a load centre connected to station B. The plant at station A includes a rectifier r supplied from a 3-phase a.c. system. The rectifier feeds the d.c. cable C through a large reactor Lr the cable core being positive with respect to the cable sheath, which is earthed. At station B the cable is again connected by way of a large reactor L. , to an inverter i. The lead centre is then connected to the a.c. output of the inverter. At each station there is a surge arrester S connected between the cable and earth to limit the over- voltage (irrespective of polarity) that the equipment may have to sustain. As so far described, the system is conventional. In the event of a severe fault, where, for example, there is a failure of the cable termination due to excessive voltage, e.g. at the surge arrester protective level, the cable voltage collapses at the fault point and a negative voltage surge of the same amplitude travels outwards to the terminal -it- stations. The wavefront encounters the reactors Lr and

L. and is partially reflected. If the cable surge impedance is Z and the effective impedance of the reactors is Z. , the reflected voltage V in terms of the incident voltage V. is given as follows:

The effective d.c. reactor impedance Z. is normally much greater than the cable surge impedance Z so that the reflected voltag-e surge is comparable with the incident voltage surge in magnitude, and is of the same polarity. From the initial positive voltage prior to the fault the cable termination would be subjected to an almost instantaneous polarity reversal. Significant damage could then ensue unless the cable has been specifically designed and constructed (at increased cost) to withstand this duty. In the present embodiment according to the invention, however, a diode D is connected between the core and earth in such a direction as to be non-conductive in normal circumstances, i.e. with the cable core positive with respect to earth. The diode is connected where the reverse voltage would first occur, i.e. at the junction of the cable and the reactor Lr.

A diode connected in this way would achieve the object of limiting the reverse voltage applied to the cable but would leave a very large current flowing through the cable, the fault and the diode. This fault current is given by the initial voltage and the surge impedance and could typically be 1000 kV720 ohms, i.e. 50 kA. Since there are normally very few losses in such an arrangement a substantial part of the initial energy stored in the cable would be dissipated in the diode. Using a standard diode therefore, it is preferable to remove the energy dissipation to a relatively robust and cheap resistor R connected in series with the diode. This resistor may have a resistance in the range 0.5 to 5 ohms (preferably about 1.5 ohms), be capable of carrying 50 - 100 kA, and dissipating energy of typically 25 MJ. The use of a resistor in this way does mean, of course, that the cable will be subjected to a limited voltage polarity reversal to^" the extent of perhaps 10% of the pre-fault voltage. This will generally be tolerable.

Instead of a linear resistor R a non-linear resistor, of zinc-oxide for example, can be used. The resistance of such a resistor increases with decreasing current and tends ^• to suppress the fault current much more quickly. Figure 3 illustrates the effect in the circuit of Figure 2 of three different resistances, namely zero (for the diode alone), 1.5 ohms for the linear resistor, and, in broken line, the non-linear zinc-oxide resistor.

Instead of a diode a thyristor may be used. The thyristor may be continuously gated by a constant d.c. or may be supplied with gating current in response to a detected voltage reversal arising from a fault condition.

In some cases parallel cables are used to share the load current. In such cases each cable may have its own diode/resistor termination, so that in the event of a fault, the terminations carry currents determined by their resistor values. The current sharing and hence the distribution of the energy to be absorbed in such conditions can therefore be controlled by proper matching of the total resistance in each termination.

On the other hand a single diode or diode/resistor termination may be used for a number of parallel cables although this has the disadvantage of disabling all of the parallel cables if the termination fails.

In the above arrangement the direction of power flow is constant, always from station A to station B. If the system were required to transmit power in the opposite direction the rectifier at station A would be required to invert, and the inverter at station B to rectify, this being achieved by changing the firing angles .of the semi¬ conductor devices of the two converters from, say, = to 180 - «. The effect of such a change would be that the d.c. current would flow in the same direction but the voltag polarity at the two stations, and thus on the cable, would reverse.

Figure 4 illustrates how the Invention can still be incorporated in such a system. A reversing switch is connected in the output of the rectifier r (and similarly in the Input to the inverter at station B not shown) and is operated in synchronism with the direction of power flow. Thus, as the voltage polarity at the rectifier reverses, the switch is operated and the cable polarity remains constant.

Figure 5 shows a comparable arrangement for a bipolar converter system in which the converter at station A comprises two rectifier groups connected in series and the centre connection earthed. The cable conductors are then normally balanced about earth potential, being connected to the positive and negative rectifier outputs respectively.

In a unidirectional power flow system these connections would be permanent and the diode/resistor elements D/R would be connected as shown, i.e. so as to be reverse biassed by the normal conductor polarity. In a bidirectional power flow system a reversing switch is Incorporated, as in Figure 4, and is operated in dependence upon the direction of power flow. Again, therefore, the output polarity of the rectifier groups reverses in the inverting mode but the cable polarity remains constant.

In both Figures 4 and 5, the diode/resistor combinations limit the reversal of cable polarity in the event of a fault in the same manner as in the earlier arrangement. Figure 6a shows a terminal station connected to a bipolar cable C. The polarity of the core of the cable is dependent on the direction of power transmission (which in turn depends on the firing angle of converter r). Accordingly diode D is mounted on a two-position mechanicall rotatable arm which engates fixed terminals U1 and U2. The arm is controlled by an electromagnetic actuator indicated schematically as A, in dependence upon a polarity signal from converter R, so as to maintain diode D in a nominally reverse biassed condition. Any sudden voltage reversal caused by a fault condition will be quenched by diode D and resistor R.

Figure 6b shows a similar arrangement in which two reverse parallel-connected thyristors controlled by a gating circuit L replace the mechanical arrangement of Figure 6a. When the core of cable C is negative thyristor T1 is switched on and thyristor T2 is switched off in accordance with a polarity signal from converter R to gating circuit L. When the core of cable C is positive, T1 is switched on and T2 is switched off.

It will be appreciated that the invention is applicable to transmission systems employing two conductors in a common cable, the conductors carrying the forward and return currents respectively. In such a case the uni¬ directional current-conducting means is suitably connected directly between the two conductors.

OMPI SNATi

Claims

1. A high voltage direct current power transmission system comprising a high voltage cable (C) having at least one conductor connected between converters (r, i) at respective terminal stations (A, B), characterised in that unidirectional current-conducting means (D, R, Tl , T2) are connected to a said conductor at each terminal station (A, B) in a direction such as to be reverse biassed by the normal cable voltage polarity, each current-conducting means incorporating a resistive component (R) and the arrangement being such that in a fault condition, reverse voltage on the cable is limited by the voltage drop across . the unidirectional current-conducting means.

2. A power transmission system as claimed in Claim 1 wherein said cable (C) is a polar D.C. cable and said unidirectional current-conducting means (D, R, T1 , T2) is connected between the cable core and earth and is effective to limit any reverse fault voltage to below the rated reverse voltage of the cable.

3. A bidirectional power transmission system as claimed in Claim 2 incorporating reverse switching means for maintaining the relative polarity of the cable conductors irrespective of the direction of power transmission.

4. A bidirectional power transmission system as claimed in Claim 1 or Claim 2 wherein said cable (C) is non-polar and incorporating means for reversing the polarity of said unidirectional current-conducting means (D, R, T1 , T2) according to the nominal relative polarity of the cable conductors, so as to enable gradual reversal of said relative polarity consequent on changes in the direction of power transmission but to quench any sudden voltage reversal consequent on a fault condition.

5. A system as claimed in any preceding Claim wherein said unidirectional current-conducting means comprises one or more diode rectifiers (D).

6. A system as claimed in any of Claims 1 to 5 wherein .said unidirectional current-conducting means (D, comprises one or more gated thyristors (T1-, T2) .

7. A system as claimed in any preceding Claim wherein said unidirectional current-conducting means (D, R, T1 , T2) comprises a linear resistor (R).

8. A system as claimed in Claim 7 wherein the resistanc of said resistor (R) is between 0.5 and 5 ohms.

9. A system as claimed in Claim 8 wherein the resistanc of said resistor (R) is between 1 and 2 ohms.

10. A system as claimed in any of Claims 1 to 6 wherein said unidirectional current-conducting means (D, R, T1 , T2) comprises a non-linear resistor, the resistance of which resistor decreases with increasing current.

11. A system as claimed in any preceding Claim wherein said unidirectional current-conducting means (D, R, T1 , T2) has a resistance which is approximately 10% of the surge inpedance of said cable.

12. A bidirectional power transmission system as claimed in any preceding Claim comprising two high voltage polar D.C. cables (C) balanced with respect to earth.

13- A system as claimed in any preceding Claim wherein one said conductor is earthed and the other said conductor is connected to earth via said unidirectional current- conducting means (D, R, T1 , T2).

14. A power transmission system as claimed in Claim 4 wherein said unidirectional current-conducting means (D, R, T1 , T2) comprises two or more' thyristors (T1, T2) connected in reverse parallel and said switching means comprises controlled gating means (L) for switching on only those thyristors reverse biassed by said nominal relative polarity.

15- A high voltage D.C. system substantially as described hereinabove with reference to Figures 1 to 3 of the accompanying drawings.

16. A high voltage D.C. sys.tem substantially as described hereinabove with reference to Figure 4 or Figure 5 or Figure 6 of the accompanying drawings.

OMPI