Electrification Infrastructure Reducing Whole Life Cost

Transcription

1 Electrification Infrastructure Reducing Whole Life Cost DC Stray Currents Martin Sigrist Thameslink Programme 28-Oct-14 / 1

2 Overview Objective How are Stray Currents produced Impact of Stray Current Controlling the effects of DC Stray Currents AC/DC interfaces and the types of Mitigation used What we are doing on Thameslink? 28-Oct-14 / 2

3 Objective To consider the mechanism of Stray Currents (DC) To under stand its effect on railway infrastructure and to third parties To consider the methods of mitigation or control To look at one area being investigated 28-Oct-14 / 3

4 How are Stray Currents Produced The DC Railway are NOT intentionally connected to earth Otherwise DC current would directly flow to earth and to any metal structure, pipework, other electrical earthing systems etc. Traction return load currents through the rails back to the Substation can provide an indirect return path via earth Rails can connect via ballast especially when wet or contaminated Return bonding defectives can cause alternative current paths Difference in supply voltages can cause a flow of stray current This has been the case on the UK DC railway in London as the system was historical graded from 750v to 660v (interface with LU) Load grabbing due to rectifier rating variations, outages or HV system voltage differences When DC Railways met AC Railways 28-Oct-14 / 4

5 DC Stray Current Outline Principle DC Train Bridge DC substation Positive Pipes Negative Return Stray Current Earth The amount of leakage is related to the traction load and the quality of track insulation (i.e. ballast height, condition of rail pads etc) 28-Oct-14 / 5

6 Why could DC Stray Current Occur? Direct Contact between the running rail and ballast Missing, damaged or ineffective track resilience pads (these are not primarily for insulation but do provide assistance) Ballast that is either continuously wet or contaminated lowers the effective resistance to earth Defective or incomplete traction return bonding Increases the preferred return path resistance Degraded conditions around structures, say steel bridges, pipes giving a more favourable return path Traction system feeding / outages causing voltage grading Also do not forget the conductor rail High ballast level around the conductor rail Failed insulators (either due to high ballast level or missing insulators causing low conductor rail) 28-Oct-14 / 6

7 Impact of DC Stray Currents How serious is this to the Railway and other parties Question? I0 Amps of DC current leaving steel and continuously flowing for one year will approximately remove? Answer Around 96 kilograms of metal 28-Oct-14 / 7

8 Impact of DC Stray Currents (2) What does it affect on the Railway (or Third Parties)? Basically it could affect any metallic component or structure that forms part of the DC Current conductive path, such as: Running rails, causing rail defects due to loss of strength; Steel Structures e.g. Bridges, Signal Gantries, OLE Structures; Reinforced concrete structures, loss of strength if the rebar conducts DC Current; Earth mats used for Electrical protection, giving hot sites; For Third Parties adjacent to a DC Railway, any metallic structure could be affected, such as pipes, building foundations or steelwork, earthing systems etc. 28-Oct-14 / 8

9 Examples 28-Oct-14 / 9

10 Controlling the effects of DC Currents (1) For DC Railways areas only, then at present the only cost effective solution is good housekeeping / maintenance / design, concentrating on: Making sure that high ballast areas around rails are removed That the ballast contamination is reduced (including wet spots, high level of fines etc) Return bonding is effective with damaged and missing bonding being replaced as soon as practical Areas with known metallic structures near the return system should have special attention When changes are made to the infrastructure DC Stray Currents are taken in to account and suitable separation is integrated into the infrastructure at the design stage Voltage grading of the traction system is minimised 28-Oct-14 / 10

11 Controlling the effects of DC Currents (2) For areas where DC and AC meet, then there are additional issues that need to be considered, such as: Separation of the traction return systems (one earthed, the other not) Ensuring that track circuit operation is maintained safety, limits of track circuit immunisation need to be considered Electrical Safety takes priority over DC Stray Currents Dual electrified areas require specialist bonding arrangements AC/DC interface equipment needs regular and enhanced maintenance to ensure correct operation Over the lifetime of the interface, any changes in the area should reinvestigate the interface to ensure that the interface operation is not degraded in anyway 28-Oct-14 / 11

12 Controlling the effects of DC Currents (3) Types of AC/DC interface in service today: Provision of AC Isolation Transformers Provision of DC Contactors Nothing e.g. no specific control measures What do the interfaces allow? To prevent DC stray currents propagation to earthed AC railway and outside party structures To assist in immunisation of vulnerable Signalling equipment in the dual system area and its vicinity (just as important for AC stray current and 50Hz track circuits) To permit dual traction system changeover for the Rolling Stock between the AC and DC systems 28-Oct-14 / 12

13 Where are the AC/DC Interfaces AC Isolation transformers ECML/Moorgate, Drayton Park HS1 Freight connection, Dollands Moor WCML/NLL, Primrose Hill North London Line, various (4 interfaces) HS1/ Southern; Ashford, Ebbsfleet, Hook Green DC Contactors TLP Ludgate Cellars/Blackfriars No specific control measures WCML/Euston Watford Junction (4 interfaces) WCML/Merseyrail, Allerton Depot Non linear resistors NLL/ELL, Dalston to Highbury & Islington And in the future? Crossrail, HS2, AC to Southampton? 28-Oct-14 / 13

14 AC isolation transformers how they work AC feeder station DUAL ELECTRIFIED AREA AC train DC substation Isolation transformer 1 Isolation transformer 2 DC train Isolation transformer spacing is dependant on the Maximum length of train and the signalling design i.e. two trains should not bridge out the IRJs Can be used where the DEA has little or no outside party or station interfaces that could provide a DC stray current path No control interface to signalling, no moving parts 28-Oct-14 / 14

15 DC contactors how they work AC feeder station DUAL ELECTRIFIED AREA DC train Contactor 2 Normally open Contactor 1 Normally closed DC substation AC train Train Direction Contactor spacing is dependant on the Maximum length of train and the signalling design i.e. two trains should not bridge out the IRJs Control of stray current in the DEA is required, such as TLP LU/DNO interfaces at Farringdon Needs control system from signalling system to detect train position and operate the contactors 28-Oct-14 / 15

16 AC/DC Interfaces Considerations AC/DC Interfaces can add constraints to the operational railway : How are overlength trains dealt with? What happens to the service if equipment fails? Size of equipment, location requirements and cost Traction Power Equipment needs a large footprint The position of equipment is normally precisely tied to the operational needs The AC/DC interface arrangements are normal highly bespoke to the area installed and requires a high level of specialist knowledge and therefore sits outside normal railway traction cost profiles Operation & Maintenance Strategy This needs to be enhanced over the normal level as these are complex systems and equipment configurations which are over and above those for a normal railway 28-Oct-14 / 16

17 So what is Thameslink doing? During the early stages of the Programme the Dual Electrified Area (DEA) had to be extended, this meant: Existing AC/DC interface needed to move due to an extension of the 25kV Overhead Line system Due to the DEA extension, stray DC current issues between the Southern Area and the 25kV AC earthed area were questioned as this was a known issue in the area Safety Cases for TLP required a re-examination of the stray DC current issues in the area As part of the investigation a permanent monitoring system has been installed The purpose of the monitoring system to check for changes in the level of stray DC currents when either infrastructure or operational changes occur as part of the Programme In addition various ad-hoc monitoring is undertaken to confirm any further affects, for example to London Underground and to the Rolling Stock 28-Oct-14 / 17

18 So what are we monitoring? Thames River Southwark Substation Ludgate Cellars Substation Farringdon Substation V Blackfriars City Farringdon Thameslink Farringdon Farringdon St Pancras AC/DC Interface V V V To MML Limit of DC Electrification To ECML Limit of AC Electrification 28-Oct-14 / 18

19 AC/DC Interface in Normal Separated Systems Thames River Southwark Substation Ludgate Cellars Substation Farringdon Substation +/- 1A +/- 5A +10V V Blackfriars City Farringdon Thameslink Farringdon Farringdon St Pancras +/- 2/3A AC/DC Interface V Limit of AC Electrification V +/- 0.5V +/- 1V +/- 1.5V Limit of DC Electrification V To ECML +/- ma To MML 28-Oct-14 / 19

20 AC/DC Interface in Bypass Systems Connected Thames River Southwark Substation Ludgate Cellars Substation Farringdon Substation +20A +200A +15V V Blackfriars City Farringdon Thameslink Farringdon Farringdon St Pancras +/- 10A AC/DC Interface V +3V +3.5V +4V Limit of AC Electrification V Limit of DC Electrification V To ECML +1A To MML 28-Oct-14 / 20

21 Examples from the Monitoring System (1) 28-Oct-14 / 21

22 Examples from the Monitoring System (2) 28-Oct-14 / 22

23 Examples from the Monitoring System (3) Kentish Town St Pancras Low Level AC to DC Changeover at Farringdon 28-Oct-14 / 23

24 Summary considering whole life Stray Currents can vary depending on: Traction Load Condition of the infrastructure (Bonding, Track Pads etc) All Railway System infrastructure changes should consider DC Stray Currents at the design level Existing DC Railways need to consider DC Stray Currents in the Maintenance regimes to ensure good housekeeping Interfaces between AC and DC Railways need enhanced or special attention for any change or on-going maintenance While there are DC Railways, DC Stray Currents will exist but they need to be managed 28-Oct-14 / 24

25 Questions? 28-Oct-14 / 25