WO2002027890A1

WO2002027890A1 - Reduction of sheath losses on high voltage cables

Info

Publication number: WO2002027890A1
Application number: PCT/DK2000/000527
Authority: WO
Inventors: Hugh Allison Boyd
Original assignee: Nesa A/S
Priority date: 2000-09-27
Filing date: 2000-09-27
Publication date: 2002-04-04
Also published as: AU2000274044A1

Abstract

This invention consists of a method, and a device using the principle of electromagnetic induction, to reduce the circulating current, and the losses in the metallic sheath loops of single-phase high voltage transmission cables, where the sheaths are grounded or bonded together at both ends of the cable run. The method involves inducing locally an emf into each sheath loop, essentially equal and opposite to that induced by the flux of the load current in each conductor acting along the whole cable length. The method is applicable to both single-phase and multiphase cable systems. The device consists of a current transformer at a sealing end of each single-phase cable, connected in series with a voltage transformer in the grounding or bonding connection of each sheath at the same cable end. The primary winding of each current transformer is the phase conductor, and the secondary winding of each voltage transformer is a sheath loop. For each sheath loop the device satisfies the condition that the emf induced locally in the loop by the device is nominally equal and opposite to the emf induced in the loop by the cable load currents acing along the cable length. The circulating sheath loop current and the losses are then nominally zero.

Description

Reduction of sheath losses on high voltage cables

This invention consists of a method, and a device using the principle of electromagnetic induction, to reduce the circulating current, and the losses in the metallic sheath and armouring (hereafter called sheath) of high voltage AC transmission cables, where each insulated sheath is grounded (and /or bonded to others) at both ends of the cable run. With single-phase cables the circulating sheath current can be almost as large as the cable's load current. This leads to considerable I R losses in the sheath, - up to about 1 -2% of rating for lead sheaths. Depending - ., on the cable lay configuration the thermal rating is lowered by about 10 - 30 %. The sheaths of single-phase high voltage transmission cables, forming for example a three-phase system, are normally bonded together at both ends to avoid excessive voltages between the sheaths. The sheaths are then preferably grounded at each end to avoid any excessive voltage to ground, - especially under fault conditions, and to protect personnel during cable maintenance operations.

Bonding the sheath ends, either directly or through the grounding, however introduces large conducting loops. The cables are normally laid in flat, or triangular configuration in. a single trench. A small spacing is usually maintained between the cables to allow utilisation of the cable's full thermal rating. Irrespective of the cable lay, electromotive forces (emfs) are induced along the sheaths by the magnetic flux of the conductor load currents. Currents then circulate in the closed sheath loops. The sheath currents form a three-phase system similar to the cable load currents. This induction process is illustrated in Fig 1, for a simple 'go-and-return' two cable system carrying a current i^" in both directions. An induced current Is circulates in the sheath loop abed.

The flux Ψ_A forms a mutual inductance MA between the conductor A and the sheath loop abed. M_B is the corresponding mutual inductance between conductor B and the sheath loop, given by : -

M_A = M_B = 2.10^"7.ln— Henries/m - (1).

Where D is the distance between sheath centres and r is the mean sheath radius.

This formula also applies for a three-phase system with the cables laid in equilateral configuration. For a flat configuration the cable lay is no longer symmetrical, and the mutual inductance is a little different for the outer and inner conductors, and depends weakly on the spacing between the cables.

Present day methods to avoid circulating sheath currents in single core three-phase cable systems include: -

- operating with each sheath isolated at one end and grounded through a surge arrester, the other end.being directly grounded, and, operating with the sheaths sectionalised into equal lengths (typically about 1 km), and the sheath sections cyclically transposed along the cable length. The method is termed cross bonding. The sheaths are grounded at both ends, and at each third sheath transposition. The vector sum of the induced voltages between the grounding points, and along each complete sheath is nominally zero.

US patent 3,857,071 describes a cross bonding system including series saturable reactors to reduce the sheath current.

Operating with sheath ends grounded through surge arresters is at best only suitable for short cable runs, up to about a kilometre. The method requires special safety precautions to be taken by maintenance personnel, because of the sheath to ground voltage.

The method of sheath cross bonding has the following disadvantages: -

- transposing the sheaths is a complicated, and expensive technical procedure. On a 145 kN cable the cost of a straight joint is about 20 k$, compared to about 80 k$ for a transposition joint.

- the number of sheath sections can be quite large, short sections of about 1 km are necessary to avoid excessive inter sheath, and sheath to ground voltages when fault currents occur in the transmission system.

- although the cable sheath is systematically grounded at each third transposition, the intermediate sheath ends are essentially open circuit to the induced emfs. There is therefore a steady voltage build up along each section, and a voltage across the sheath insulation to ground.

- between grounding points the sheath voltage is a hazard to maintenance personnel, requiring special safety precautions.

surge arresters are necessary at the sheath transposition joints. any failure of the sheath to ground insulation results in circulating sheath currents. These currents are difficult to detect along the length of the cable.

The present invention involves inducing locally an emf into each sheath loop, essentially equal and opposite to that induced by the flux of the conductor load current Ic acting along the whole cable length. The device consists of an instruement type current transformer at a sealing end of each single-phase cable, connected in series with a second instrument type voltage transformer located in the grounding, or bonding connection, of each sheath at the same cable end. The principle of the method is illustrated in Fig 2 for a single-phase cable where the sheath ground loop abed includes the ground return path ad. The dot notation (•) indicates the sense of the windings, and the mutual couplings, Mcr for transformer 1, Mγγ for transformer 2, and Mcs between conductor C and sheath S. The primary winding of transformer 1, is the extended phase conductor, and the primary current is the cable conductor load current Ic only. The secondary winding of transformer 2 is the closed sheath ground loop abed. The method is applicable to both single-phase, and multiphase cable systems.

Transformer 1, mounted around the cable conductor, utilises the extended cable conductor loop as the primary winding, and produces a secondary voltage proportional to the conductor current Ic.

Transformer 2 utilises the extended cable sheath ground loop abed as the low voltage secondary winding. According to Faraday's Law of induction, the emf Et induced in the ground loop is given by : -

Et = e -dl - ÷dφldt

where e is the electromotive force (emf) induced in a length dl of the sheath ground loop abed, and φ is the flux in the transformer core linking the loop. By Faraday's Law the total emf Et round the loop is independent of both the size and shape of the loop, and the position of the flux φ within the loop.

Faraday's Law of induction also applies to the emf Ec in the sheath ground loop abed from the conductor load current Ic, given by : -

Ec = ix-dl = ÷dψ/dt

where e ' is the electromotive force induced in a length dl of the sheath ground loop abed, and Ψ is the flux from the conductor current linking the loop. By Faraday's Law the total emf Ec round the loop is independent of both the size and shape of the loop, and the position of the flux Ψ within the loop.

By a suitable choice of the windings of transformer 1, and transformer 2, the flux φ is arranged to be essentially equal and opposite with the flux Ψ . Thus both the driving emf ( = Et + Ec ), and the circulating current Is in the sheath ground loop abed are essentially zero. The current transformer loading is then the small magnetising current of the voltage transformer only. The impedance of the sheath ground loop, and the transformers have negligible influence on the induced emf Et, as the associated voltage drops in the circuit are negligible. It has been chosen nominally to term transformer 1, a current transformer (CT) as there is a current in the primary winding, and to term transformer 2 a voltage transformer (VT) as essentially there is no load current in the transformer, but a voltage in both windings.

German patent Patentschrifϊ- Nr 510932 (October 1930) describes, a method using a current transformer in each phase of a high voltage overhead line to neutralise the magnetically induced interference in a nearby low voltage telephone line, when an unbalance occurs due to a ground fault on a phase of the over headline. The current transformers are connected in series and the circuit is closed through a voltage transformer introduced in series with the telephone line. Under normal balanced steady state operating conditions the sum of the out put voltage from the current transformers is zero, and thus the protective system is passive, and no emf is introduced into the telephone line. The protective system functions first when the high voltage ground fault occurs on the overhead line. A transient current then circulates in the closed 'delta' of the four transformers in response to the appearance of a transient zero sequence fault current on the overhead line. During the fault it is intended that the voltage transformer introduces a series voltage in the telephone line, equal and opposite over the frequency range to the transient interference voltage coupled magnetically from the overhead line. This then allows transmission of essentially undisturbed telephone signals.

This is entirely different from the present invention, h the three-phase version as shown in Fig 3. Fig illustrates the three-phase system using single-phase cables, with three sets of CTs and VTs set up for cancelling the normally circulating sheath currents Isr, Iss and 1st. The three current transformers are clearly not connected in series, as the device is designed to operate continuously in the steady state at power frequency, on high voltage single-phase cables with a metal sheath. Each cable conductor load current is used to introduce a continuous power frequency emf into its own sheath circuit via the VT, such that the normal circulating sheath current in a sheath ground loop, or sheath loop between phases is neutralised. All the signals used in the sheath current neutralising system are of power frequency, and are derived from, and applied to the cable system only. No external signals or secondary circuits are involved.

The criterion for cancelling the normally induced circulating current Is in the sheath ground loop can easily be derived referring to Fig 2 for the single-phase case. Mcs is the mutual inductance between the conductor C and the sheath loop (abed). cris the mutual inductance of the current transformer.

M_VT is the mutual inductance of the voltage transformer.

N:l is the turns ratio of the voltage transformer. With the winding senses shown the criterion that Is is zero is that the emf induced in the sheath loop from the flux oflc, and the emf induced in the sheath loop by the voltage transformer VT are equal and opposite.

Neglecting the leakage reactance of the transformers the approximate relationship holds : -

IcjωMcs = IcjωMcrAN giving MCT = NM_Cs. - (2)

For a three-phase system with single-phase cables in equilateral configuration the value of Mcs can be calculated from equation (1). For different cable types and configurations of laying, accurate expressions for the mutual inductance Mcs or individual conductor to sheath loop can be found in the literature, for example 'Underground Systems Reference Book' Edison Electric Institute Publication No 55- 16, 1957.

The following example illustrates practical specifications for the current, and voltage transformers: -

A 10 km long 145 kN three-phase cable system, with three single core cables laid in equilateral configuration, with the mean sheath radius r = 4.0 cm., and the distance between sheath centres D = 9.0 cm.

From (1), the mutual inductance between conductor and sheath Mcs. = 1.6 mH, and

ωMcs = 0.5\ Ω.

With a load current Ic = 1 kA, the voltage induced in each sheath = 510 V.

The turns ratio of the voltage transformer is taken as N= 5.

From (2), the mutual inductance of the current transformer MCT — 8 mH., and

ωMcr = 2.5 Ω. The voltage on the secondary side of the current transformer is Ic ωMcr =2.5 kN.

The current transformer secondary winding is assumed to be toroidal.

The number of secondary turns required on the current transformer can be calculated from: -

MCT = 4JΓ.10 ^~' . μ _rN_CτA /l where Ncr is the number of turns, A is the area of a turn, and / the mean winding length of the current transformer secondary winding. μ_r is the effective permeability of the magnetic core including the effect of any air gaps introduced to ensure a degree of linearity of the magnetic circuit.

For μ_r = 200 ,A = 0.2x0.2 = 0.04 m² , and /= 1.3 m, thenN= 1030 turns.

The values of A and / depend on the physical design of the current transformer. It can be located either on the sealing end porcelain, or directly below on the cable riser. This influences the radial and longitudinal dimensions. In either location it must be arranged that the primary current is the load current Ic only.

The magnetic core of the current transformer should have a sensibly linear magnetic characteristic, to avoid saturation up to about the maximum emergency current rating of the cable. This is readily achieved by choice of the core material, and/or introducing radial air gaps in the magnetic circuit. Saturation should also be avoided in the voltage transformer.

Fig 3, shows a three-phase version of the principle, and the device for cancelling the circulating sheath currents Isr, Iss and 1st on a three-phase, single-phase cable system. Three sets of current and voltage transformers are mounted at one end of the cable system as shown. The primary of each current transformer is a phase conductor, and the secondary of each voltage transformer is a sheath loop. In the case of a particularly long cable run, - 20 km or more it can be convenient to install the sheath current cancelling device at both ends of the cable system.

The new with the present invention is that by means of suitably arranged current and voltage transformers at a cable end, the conductor current Ic in each single-phase cable is made to induce an emf round each sheath ground loop, equal and opposite to the emf normally occurring due to the longitudinally distributed magnetic coupling between the conductor current Ic and sheath. The normally circulating sheath current Is is then cancelled out, and the sheath losses minimised. The incremental inductance distribution around the loop is independent of the direction of current flow. Consequently there is no build up of voltage to ground along the sheath, and therefore no hazard to personnel. Exact equality between the opposing emfs is not necessary for the method to be effective, as the sheath losses are proportional to Is² (where Is is the circulating sheath current). Even with Is reduced by only 50 %, the losses are reduced by 75 %.

The method has the following advantages: the resultant emf around each sheath loop is nominally zero. Thus there is no hazard to maintenance personnel.

the voltage between sheaths, and between sheath and ground is nominally zero, thus possible sheath insulation failures are avoided.

the number of cable joints can be minimised. The cables can be laid in their full length without joints if the route and fabrication process so allows. the method can be applied to cables which are already laid. Sectioning of the sheaths is unnecessary.

- circulating sheath currents arising due to sheath insulation failure at any location on the sheath can be readily detected, as the secondary current in the current transformer is otherwise nominally zero.

- under network fault conditions when heavy currents can circulate in the sheath loops there is no risk of sheath insulation failure. The temporary high emfs are directed around the sheath loops and there are no corresponding voltages across the insulation.

the method is passive and adjusts automatically to the prevailing load current on the cable.

Claims

Patent claim.

1. A method to greatly reduce the circulating currents, and the associated I²R losses, in the loops formed by the sheaths of single-phase high voltage cables forming an AC cable system (multiphase or single-phase), where the sheaths are grounded or bonded together at both ends, caused by induction along the cable length due to the flux of the load current in each cable conductor characterised by inducing an equal and opposite emf locally into each sheath loop using the flux of each associated conductor load current as the inducing medium.

2. A device to greatly reduce the circulating current, and the associated I R losses, in the loops formed by the sheaths of single-phase high voltage cables forming an AC cable system (multiphase or single-phase), where the sheaths are grounded and/or bonded together at both ends, caused by induction due to the flux of each conductor load current according to patent claim (1) characterised by a current transformer at a sealing end of each single-phase cable, connected in series with a voltage transformer in the sheath grounding or bonding connection of each sheath at the same cable end. The primary winding of each current transformer is the phase conductor, and the secondary winding of each voltage transformer is a sheath loop. For each sheath loop the device satisfies the condition that the emf induced locally in the loop by the device is nominally equal and opposite to the emf induced in the loop by the cable load currents.