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Procedure for Accessing and Working in the Valve Halls and on associated Apparatus at Hunterston

HVDC Converter Station

OPSAF-11-091 Issue No. 1

© SP Power Systems Limited of 29 MSP 2.17

1. SCOPE

This document identifies the procedures to be adopted to provide Safety from the System when accessing or working within the Valve Halls and on the converter transformers and the 400kV AC transformer connections at Hunterston HVDC Converter Station. It also defines the working at height procedure for accessing the ceiling-mounted Valves.

2. ISSUE RECORD

This is a Reference document. The current version is held on the EN Document Library. It is your responsibility to ensure you work to the current version.

Issue Date Issue No. Author Amendment Details

May 2017 1 W. Falconer D. Naylor N. Parry

Initial Issue

3. ISSUE AUTHORITY

Author Owner Issue Authority

Name: William Falconer Title: Senior Project Engineer Transmission Operational Support

Name: Gary Evans Title: Operational Compliance Manager

Name: Wilma Kelly Title: Health & Safety Director

Date: ....1/6/17.........................

4. REVIEW

This is a Reference document which has a 5 year retention period after which a reminder will be issued to review and extend retention or archive.

The proposed revision date can be viewed in the Management Safety Procedures Document Index, DOC-00-238.

DISTRIBUTION

This document is not part of a manual maintained by Document Control and does not have a maintained distribution list.





5. CONTENTS

1. SCOPE ....................................................................................................................................... 1

2. ISSUE RECORD ........................................................................................................................ 1

3. ISSUE AUTHORITY .................................................................................................................. 1

4. REVIEW ..................................................................................................................................... 1

DISTRIBUTION ................................................................................................................................ 1

5. CONTENTS ............................................................................................................................... 2

6. REFERENCE AND RELATED DOCUMENTS .......................................................................... 3

6.1 Manufacturer’s Manuals ................................................................................................... 3

6.2 ScottishPower Documents ............................................................................................... 3

7. DEFINITIONS ............................................................................................................................ 3

8. INTRODUCTION ...................................................................................................................... 10

9. AUTHORISATION ................................................................................................................... 10

10. HANDLING STORAGE AND TRANSPORTATION OF CAPACITOR UNITS FOR THYRISTOR MODULES ................................................................................................................ 11

11. ACCESS TO A VALVE HALL ................................................................................................. 11

12. WORKING ON CONVERTER TRANSFORMERS .................................................................. 11

13. GENERAL REQUIREMENTS FOR ALL WORK WITHIN A VALVE HALL ........................... 11

14. PROCEDURE FOR WORK ON VALVE TOWERS WITHIN VALVE HALL ........................... 17

15. PROCEDURE FOR WORKING AT HEIGHT ON VALVE TOWERS THAT REQUIRES A PERSON(S) TO LEAVE MOBILE ELEVATED WORKING PLATFORM. .................................... 20

16. RESCUE PROCEDURE TO BE UNDERTAKEN IN THE EVENT OF AN EMERGENCY WHEN WORKING AT HEIGHT ON THE THYRISTOR MODULE PLATFORM ........................... 23

17. PRECAUTIONS FOR WORKING ON VALVE REACTORS ON THYRISTOR MODULE ..... 25

18. ADDITIONAL PRECAUTIONS FOR WORKING ON RESISTORS ON A THRYISTOR MODULE ........................................................................................................................................ 25

19. ADDITIONAL PRECAUTIONS FOR WORKING ON VALVE COOLING EQUIPMENT WITHIN THE VALVE HALL ........................................................................................................... 25

20. ADDITIONAL PRECAUTIONS WHEN WORKING ON THYRISTOR FIRING ELECTRONICS ................................................................................................................................................. 25

21. ADDITIONAL PRECAUTIONS FOR WORKING ON HV BUSHINGS WITHIN THE VALVE HALL .............................................................................................................................................. 26

22. PERFORMING CAPACITOR MEASUREMENTS .................................................................. 28

23. PROCEDURE TO BE APPLIED WHEN A CAPACITOR UNIT HAS DISRUPTIVELY FAILED ................................................................................................................................................. 28





6. REFERENCE AND RELATED DOCUMENTS

6.1 Manufacturer’s Manuals

Siemens Thyristor Valves and Associated Equipment Installation and Pre-Commissioning Manual P-010221_E_F1_0111_HF-0 Siemens Thyristor Valves and Associated Equipment Maintenance Manual

P-010221_E_B4_0111_HF_B Siemens Valve Cooling System Equipment Manual

P-010221_E_B4_0121_HF_E Siemens Portable Earthing Manual P-010221_E_B4_0571_H_001_B Siemens Thyristor Valve Towers Emergency Rescue from Height

P-010221_E_B4_0111_HF_ER

6.2 ScottishPower Documents

OPSAF-10-006 (PSSI 6) Demarcation of Work Areas in Substations OPSAF-10-009 (PSSI 9) Testing High Voltage Apparatus OPSAF-10-010 (PSSI 10) Plant and Apparatus Containing Sulphur Hexafluoride (SF6) OPSAF-10-011 (PSSI 11) High Voltage Static Capacitors OPSAF-10-027 (PSSI 27) Work or Testing on or near to High Voltage Direct Current Plant and Apparatus UKHS-GSP-SMS3029 - Electromagnetic Fields Procedure Health Safety and Environmental Handbook

7. DEFINITIONS

Terms printed in bold type are as defined in the ScottishPower Safety Rules (Electrical and Mechanical). Terms printed in italic type are as defined in OPSAF-10-027 (PSSI 27) or in OPSAF-11-002 - Management Safety Procedures Definitions or are listed below: Pole The Hunterston end of the Western Link contains two identical circuits which convert AC to DC or from DC to AC dependent upon the running direction of the DC link. Each of these circuits is termed as a Pole and contains the following Apparatus. • 400kV AC GIS switchgear • Power Line Carrier Filter (PLCF) AC equipment • Converter transformer • Valve Hall (includes all AC/DC converting Apparatus.) • DC Hall (including DC switchgear, reactors, capacitors, DC Current Measuring Devices and

DC Voltage Dividers) • DC cable





Figure 1. Valve Hall showing converter transformer bushings Valve Tower (Fig 2) An arrangement of twelve Thyristor Modules which are hung from the Valve Hall ceiling in two banks of six per phase which, when connected provide four complete valves in the 12-pulse Thyristor bridge. There are six Valve Towers in each Valve Hall.

Figure 2. Valve Tower and arrangement within Valve Hall





Thyristor Module (Fig 3 and Fig 4) Each Thyristor Module contains 28 Thyristors connected in series and also contains the RC Snubbers, Grading Capacitors and Valve Reactors along with Valve triggering electronics and cooling pipework.

Figure 3. Thyristor Module Component Identification

Figure 4.Thyristor Module

Valve Reactors

Thyristors

Grading Capacitors

RC Snubbers





Thyristor (Fig 5) High power electrical device that converts AC to DC. There are 28 series-connected high-power Thyristors in each Thyristor Module.

Figure 5. Thyristor RC Snubber (Fig 6) An RC Snubber, consists of a series circuit of a water cooled resistor and a capacitor is connected in parallel with each Thyristor. In interaction with the Valve Reactors, this arrangement limits the electrical stresses on the Thyristors to below the specified design levels.

Figure 6. RC Snubber





Grading Capacitor (Fig 7) The Grading Capacitors linearize the voltage distribution among the four Valves (in a quadruple-Valve tower) and within the Valves in the event of steep surge voltages. Each Thyristor Module includes two Valve sections connected in series (14 Thyristors each). In each Valve section the Grading Capacitor is shunting the series connection of Thyristor stack and Valve Reactors. A complete Valve is made up from three Thyristor Modules.

Figure 7. Grading Capacitor Valve Reactor (Fig 8) Each Thyristor module contains four series-connected Valve Reactors. These are non-linear reactors with a pronounced saturation characteristic of the iron core. The reactors limit the inrush current of the Thyristors (discharge of stray capacitance) and limit the voltage level and rate of voltage rise at the Thyristors in the event of surge voltage phenomena.

Figure 8. Valve Reactor Valve Cooling System (Fig 9) The Valve Cooling System is a two stage cooling system to ensure that the Thyristors do not overheat when in operation. The system contains a deionised water open header section which runs within the Valve Hall to the cooling plant where it is heat exchanged with a Glycol sealed system that runs from the cooling plant to cooling radiators located outside of the Valve Hall.





Figure 9 Valve Cooling System Layout Discharge Stick (Fig 10) An Approved device for the purpose of discharging a RC Snubber Capacitor or Grading Capacitor which may be charged. Note various Approved types exist which each fulfil the same purpose i.e. two insulated sticks with an electrical contact connected to a common earth clamp.

Figure 10. Discharge Stick

A: Outdoor Cooler B: Cooler Supply and return lines C: Control Cabinet Unit D: Cooling Unit E: Valve Hall Piping F: Primary Circuit Expansion Tank





Thyristor Module Rack Earth (Fig 11) Portable Earthing Device provided for the purpose of earthing the Thyristor Modules prior to accessing them for work or testing.

Figure 11. Thyristor Module Rack Earths Short-Circuiting Lead (Figs 12 and 13) An Approved lead used for short-circuiting an individual Capacitor Unit. This can be a clip-on short used during the disconnection of a Capacitor Unit or a bolt-on short used during removal transportation and storage.

Figure 12. Short-Circuiting Lead (example)

Figure 13. Bolted Short-Circuiting Lead (example)





Thyristor Module Earth Spigot (Fig 14) Earthing point positioned on the Thyristor Module for the purpose of connecting Thyristor Module Rack Earths. The positions of these spigots are arranged such that they can be applied safely from below the Thyristor Module.

Figure 14. Thyristor Module Earth Spigots.

8. INTRODUCTION

There are two identical Valve Halls at Hunterston HVDC Converter Station, one for each Pole. Each Valve Hall may be Isolated and Earthed separately from the other – i.e. both need not be out of service simultaneously. The Valve Halls are the areas within the Converter Station that contain the Apparatus which undertakes the conversion process of AC to DC and from DC to AC, depending upon running configuration. This Apparatus includes the through bushings from the converter transformers, Thyristor Valve assemblies, earth switches, cooling equipment and HVDC (600kV and neutral) busbars which lead to the DC Hall. Each Valve Hall contains six Valve Towers suspended from the ceiling. There are two outdoor converter transformers- again one for each Pole, which transform from 251kV to 400kV. The 251kV bushings protrude through the wall of the Valve Hall where conductors connect to the Valves. The 400kV bushings are connected to the 400kV GIS switchgear by overhead connections to the PLCF building and then cable onto the switchgear.

9. AUTHORISATION

The Valve Halls contain both High Voltage AC and DC Apparatus. Access to and work on Apparatus within the Valve Halls requires Persons to hold appropriate 275/400kV AC and HVDC authorisations. Despite the Valve AC voltage being 251kV the HV AC Apparatus between the Valves and the converter transformers shall be considered 275/400kV Apparatus for the purpose of authorisations since the transformers ultimately connect to the 400kV System.

Thyristor Module Earth Spigots





In addition, although the converter transformers are entirely HV AC Apparatus, isolation at HVDC is required and the transformer 251kV bushings protrude into the Valve Hall. Therefore appropriate HVDC authorisations are required to access or work on them. See also section 21. Persons who do not hold appropriate authorisation may receive standby Supervision from an appropriately Authorised Person, in accordance with OPSAF-11-035 (MSP 5.5) and OPSAF-13-001 (MSP 5.1). Similarly work on the 400kV AC connections between the converter transformers and the PLCF Apparatus requires both 275/400kV AC and HVDC authorisations.

10. HANDLING STORAGE AND TRANSPORTATION OF CAPACITOR UNITS FOR THYRISTOR MODULES

10.1 Capacitors that are to be disconnected from the Thyristor Modules shall be discharged using a Discharge Stick before being short-circuited using a Short-Circuiting Lead and shall remain short-circuited during transportation and storage. 10.2 Where there is a potential for RC Snubber Capacitors or Grading Capacitors to become charged they shall not be handled unless they are discharged using a Discharge Stick before being short-circuited using a Short-Circuiting Lead. 10.3 If a clip-on Short-Circuiting Lead has been applied, then before a capacitor is removed from a Thyristor Modules the clip-on lead shall where practicable be replaced by a bolt-on short lead.

11. ACCESS TO A VALVE HALL

The Valve Apparatus emits electromagnetic fields which pose Danger when in service. Therefore the Valve Hall is designed to prevent electromagnetic emissions from penetrating beyond its boundary and no access is permitted within the Valve Hall when the Pole is in operation or in a position to be made Live. In addition the HVDC 600kV conductors are at the lowest level of the Valve Towers and controlling access and entry routes ensures that Safety Distance is maintained. The Valve Hall entry doors and emergency exit doors are interlocked in such a way that all Earthing Devices associated with the circuit must be applied in addition to specific software interlocks that will ensure that the Pole is blocked from restarting. Section 13 describes the procedure to be followed to gain access to the Valve Hall where work has to be carried out.

12. WORKING ON CONVERTER TRANSFORMERS

The converter transformers have outdoor 400kV bushings and through-wall 251kV bushings into the Valve Hall. Access to the bushings that are within the Valve Hall is governed by the same conditions laid out in section 11. The converter transformer shall be Isolated and Earthed and a Safety Document shall be issued prior to carrying out work that requires encroachment of Safety Distance. Isolation is required at 400kV AC and on the DC side of the appropriate Valve Hall, therefore a minimum of a single Pole outage is required.

13. GENERAL REQUIREMENTS FOR ALL WORK WITHIN A VALVE HALL

13.1 Before taking steps to establish Safety from the System the HVDC link shall be re-configured in accordance with DC Link Operating Procedures to ensure safe shutdown conditions are observed.





13.2 The Valve Hall to be worked in shall be Isolated and Primary Earths applied using Earthing Devices located outside the Valve Hall (e.g. GIS Switchroom and DC Hall). The earth switches within the Valve Hall are not appropriate for use as Primary Earths because it is not practicable for them to be Locked before the Safety Document is issued giving access to the Valve Hall. 13.3 The earth switches within the Valve Hall shall be closed remotely and where practicable a visual inspection of the earth switches shall be carried out. The earth switches are identified in figures 15 to 18 below and shown in their closed positions. 13.4 The Control Person shall ensure that the time delay between closing of the earth switches within the Valve Hall and the first Portable Primary Earth being applied is a minimum of 20 minutes. This is to allow sufficient time for the Grading Capacitors and RC Snubber capacitors to discharge.

Figure 15. AC side star bushing earth switch (1T1A/2T1A)

Figure 16. AC side delta bushing earth switch (1T1B/2T1B)





Figure 17. DC side HV bushing earth switch (D101A2/D201A2)

Figure 18. DC side neutral bushing earth switch (D101B3/D201B3)

13.5 The combination of DC Link operating condition, and Apparatus isolation and earthing will permit the release of key interlock devices to enable access to the Valve Hall. 13.6 Demarcation shall be established at the entry points into the Valve Hall to be accessed in accordance with OPSAF-11-006 (PSSI 6). 13.7 Danger Notices shall be affixed to all possible points of entry to the Valve Hall not to be accessed. 13.8 A Permit for Work shall be issued to allow entry into the Valve Hall for the purpose of inspecting and securing the earth switches and the application of a Drain Earth to the neutral busbar of the star connected converter transformer bushings. The Permit for Work shall define the route to be taken through the Valve Hall while inspecting the earth switches and shall be issued with an Earthing Schedule. At no time shall Safety Distance be infringed during inspection of earth switches or application of Drain Earths. 13.9 The Person carrying out visual inspection shall remain on the DC Hall side of the Valve Hall as indicated in figures 19 and 20.





Figure 19. Path to take for visual inspection of the Valve Hall and the DC side earth switches for Valve

Hall 1

Figure 20. Path to take for visual inspection of the Valve Hall and the DC side earth switches for Valve Hall 2

13.10 The earth switches within the Valve Hall shall be confirmed closed and Locked onto hand control in the mechanism boxes adjacent to each of the DC earth switches.





Figure 21. Picture of typical earth switch mechanism box arrangement (Master and Slave). 13.11 The AC earth switches shall be placed onto hand control and Locked - access to the master mechanism boxes on the AC side of the Valve Hall shall be by the path indicated in the figures 22 and 23.

Figure 22 Path to master mechanism boxes on Valve Hall 1 AC side





Figure 23. Path to master mechanism boxes on Valve Hall 2 AC side

13.12 If an earth switch is found not to be closed properly, the switch shall be rendered inoperative, a new Permit For Work issued and a Portable Drain Earth shall be attached adjacent to the faulty earth switch. 13.13 A Portable Drain Earth shall be applied using an earthing rod to the star neutral earthing trapeze, which is indicated in figure 24, to tie the star point to earth. This shall be applied in accordance with the Earthing Schedule issued with the Safety Document.

Figure 24. Drain Earth trapeze on star neutral busbar

13.14 The Permit for Work shall be cancelled. Another Safety Document and, where necessary Drain Earths and an Earthing Schedule shall be issued before further work or testing proceeds.





13.15 Where maintenance is to be carried out an earth switch within the Valve Hall, Drain Earths shall be applied adjacent to the earth switch. The Drain Earths shall be quoted on an Earthing Schedule.

14. PROCEDURE FOR WORK ON VALVE TOWERS WITHIN VALVE HALL

Access to the Valve Towers is gained using a scissor Mobile Elevated Work Platform (MEWP) retained on site at Hunterston HVDC Converter Station. The MEWP shall be earthed using a Field Equipment Earth. Refer to section 15 for the working at height procedures for accessing the Valve Towers. In addition to the completion of section 13 above the following shall be carried out to each specific Valve Tower to be worked on. Valve Towers not to be worked on shall be identified at ground level as being unavailable for work using barriers and Danger Notices in accordance with OPSAF-10-006 (PSSI 6) section 12. 14.1 Consideration shall be given to any Danger presented by the Valve cooling system and appropriate precautions shall be taken if necessary – refer to section 19. A Permit for Work or Sanction for Test shall be issued for the work or testing to be carried out along with an Earthing Schedule. 14.2 The Earthing Schedule shall state where the Drain Earths and Thyristor Module Rack Earths are to be applied and where the Discharge Stick is to be used. These points of earthing can include:

(i) On the Valve Tower to be worked on all Thyristor Module Earth Spigots from ground level to the one above the point of work. See figure 25.

Figure 25. Picture of Thyristor Module Rack Earth arrangement from earthing manual

(ii) Additional points where a disconnection is planned to ensure both sides of the disconnection point remains connected to earth.





(iii) Any other point as deemed necessary by the Senior Authorised Person.

14.3 A distance of 0.8 metres shall be observed at all times during the application of Drain Earths and Thyristor Module Rack Earths. 14.4 A Field Equipment Earth shall be applied to the MEWP prior to accessing the Valve Tower. A Thyristor Module Rack Earth shall be applied using an Approved earthing rod to the bottommost Thyristor Module from either ground level or from a MEWP positioned adjacent to the Valve Tower and below the corona shield onto the Thyristor Module Earth Spigot. Where a MEWP is to be used to apply the Thyristor Module Rack Earth, the MEWP shall be at its lowest position that enables a Person standing on the MEWP to maintain the 0.8m distance to the corona shield prior to the application of the lowest-most Thyristor Module Rack Earth. The corona shield at the bottom of the Valve Tower is directly connected to the 600kV HVDC busbar which will have been confirmed as being in the closed position prior to accessing the Valve Tower for earthing purposes. 14.5 Before performing any work on the Valve Towers the recipient of the Safety Document shall ensure that:

(i) Thyristor Modules are inspected at ground level for signs of leaks from the cooling pipework on the Valve Tower to be worked on.

(ii) Thyristor Module Rack Earth is applied to the spigot of the bottommost thyristor module before raising the MEWP to that height

(iii) Grading Capacitors and RC Snubber Capacitors are inspected at a distance greater than 0.8 metres in order to identify any abnormal Capacitor Unit(s) due to their positioning on the valve module. (iv) Drain Earths and Thyristor Module Rack Earths are applied in accordance with the issued Safety Document and Earthing Schedule where applicable.

At no time shall a distance of 0.8m be encroached to any High Voltage Apparatus not already connected to earth until earths are applied and capacitors discharged. 14.6 Where testing requires the removal of earths and special requirements to allow subsequent access to Thyristor Modules, these actions shall be carried out by, or under the Personal Supervision of, a Senior Authorised Person under a Sanction for Test.

14.7 The RC Snubber capacitors and the Grading Capacitors on Thyristor Modules where Persons are to work shall be discharged. The process to discharge is as follows: (i) The Discharge Stick earth clamp shall be attached as close as is practicable to the

earthing spigots on the underside of the Thyristor Module to be worked on. An example position is shown in figure 26 where the clamp is attached to the clamp of the earth.





Figure 26. Application of Discharge Stick earth end to middle part of the Thyristor Module frame.

(ii) All of the RC Snubber capacitors shall be discharged by placing the one of the probes of the Discharge Stick onto each terminal of the RC Snubber capacitors as shown in figure 27. Care shall be taken to ensure the Discharge Stick does not interfere with any fibre optic cabling. An alternative point of contact for discharge is across the Thyristor by making contact to the aluminium spacers that are either side of each Thyristor.

Figure 27. Discharge Sticks applied to the RC Snubber capacitors and at Thyristor spacers. (iii) The Grading Capacitors shall then be discharged by placing the one of the Discharge

Stick probes onto each side of the Grading Capacitors. 14.8 Apply a Short-Circuiting Lead to all capacitors to be worked on. When any Person is in contact with a capacitor on a Thyristor Module the capacitor shall remain shorted by the Short-Circuiting Lead unless testing requires the Short-Circuiting Lead to be removed. When testing requires the removal of earths and special requirements to allow subsequent access to capacitors, these actions shall be carried out by, or under the Personal Supervision of a Senior Authorised Person under a Sanction for Test. 14.9 If testing has been carried out, capacitors on the Thyristor Module shall be discharged using a Discharge Stick by a Senior Authorised Person or under the Personal Supervision of a Senior Authorised Person and all Short-Circuiting Lead(s) re-applied.

Thyristor spacer discharge point

RC Snubber capacitor discharge

point





15. PROCEDURE FOR WORKING AT HEIGHT ON VALVE TOWERS THAT REQUIRES A PERSON(S) TO LEAVE MOBILE ELEVATED WORKING PLATFORM.

Access to the Valve Towers is gained by using a scissor MEWP retained on site at Hunterston HVDC Converter Station. In addition to precautions described in section 14 above, when undertaking work at height, the following safe system of work shall be applied when it is required to leave the MEWP and work on the Thyristor Module. 15.1 A minimum of three persons who are competent in the operation of the MEWP and the use of this procedure are required.

1. One person to travel in and operate the MEWP (MEWP operator). This person shall remain within the MEWP during normal work conditions and if required, undertake working at height rescue procedures.

2. A person to travel in the MEWP, who may also leave the MEWP to undertake the work on the Thyristor Module.

3. A person to manage the work area at ground level (ground-based person), which shall include

operating and lowering the MEWP in an emergency situation such as vehicle power loss or damage.

15.2 In addition to equipment for discharging and earthing, the following tools, personal protective equipment (PPE) and rescue equipment are required. Where appropriate these shall be registered and within their current test / inspection date.

2 x full body harnesses 3 x permanent attachment slings – approx 1 meter in length 3 x fall arrest lanyards 1 x rescue kit – including descender facility and a telescopic rescue rod 1 x sliding board 4 x access boards SWL 150kg lock knife Depending on the type of MEWP in use – especially if the platform may be rotated to a

position where the rescue described in part 16.2.4 cannot proceed, a second MEWP and additional person to operate it will be required to effect a rescue.

15.3 The MEWP to be used to access the Thyristor Module shall be suitable for the task, and shall have a gate facility that can allow safe access / egress to and from the Thyristor Module. Note: Climbing over the protective guard rails or using the guard rails as a platform is not permitted and shall not be carried out at any time. 15.4 A risk assessment for the working at height activities shall be completed – including any likely requirement for a person to leave the MEWP. 15.5 Persons shall not carry tools, working at height PPE or work equipment whilst climbing the access ladder into the MEWP platform. 15.6 During working at height operations, an exclusion zone shall be established and maintained around the point of work, and no persons shall enter the zone unless communication has been established with and agreement gained from both the MEWP operator and ground-based person. 15.7 The MEWP vehicle shall be positioned below and adjacent to the Thyristor Module. With two persons aboard and the access gate closed, the platform shall then be raised to a height where the





top guard rail of the MEWP platform is level with the top of the Thyristor Module to be accessed. The sliding board shall be laid across to the Thyristor Module (taking care not to damage any components). See figure 28 below. Note: The sliding board is not rated for load bearing activities and is only designed for sliding and positioning the access boards which are rated for load bearing.

Figure 28. Laying of sliding board on Thyristor Module. 15.8 Fit the first access board across the Thyristor Module, in a position where it will be in line with the MEWP platform gate and if required, additional access boards can be slid into position around the area where the work is to be carried out. See figure 29.

Figure 29. Positioning of access boards.





15.9: After the required number of access boards have been positioned, the MEWP shall be raised until the standing platform of the MEWP is level with the access boards on the Thyristor Module. See figure 30 – note the access boards are at the same height as and in line with the MEWP platform gate.

Figure 30. Alignment of MEWP gate to access boards. 15.10 Before opening the access gate the person who is to leave the MEWP shall attach a permanent attachment sling on to the designated anchor point situated above the Thyristor Module to be worked on (see figure 31 below) and attach their fall arrest lanyard to the sling.

Figure 31. Support insulator shackle being utilised as a permanent attachment point.





15.11 Now permanently attached, the person may open the access gate and safely access the Thyristor Module. 15.12 If work is required across a large area on the Thyristor Module, for example on the busbar connections between two Thyristor Module, then a second attachment sling and fall arrest lanyard may need to be applied at a more convenient point to give a greater range of movement. This shall be applied to the person’s harness before the first is removed. At least one point of permanent attachment shall be applied and maintained at all times and after risk assessment this operation shall only be undertaken if it does compromise the safety of the persons involved.

Figure 32. Access onto Thyristor Module from MEWP 15.13 On completion of work all persons shall return to the MEWP platform and after the access gate is closed and locked, detach the sling and fall arrest equipment from the anchorage. The access board(s) and sliding board shall be returned to the MEWP floor.

16. RESCUE PROCEDURE TO BE UNDERTAKEN IN THE EVENT OF AN EMERGENCY WHEN WORKING AT HEIGHT ON THE THYRISTOR MODULE PLATFORM

16.1 A working at height rescue shall only be carried out by a competent member of the Working Party, who holds a valid rescue training certificate.





16.2 If the person working on the Thyristor Module platform receives an injury or falls from height, the MEWP operator shall communicate with the ground based person to raise the alarm and then carry out one of the following rescue techniques; 16.2.1 If the injured person is within arm’s reaching distance of the MEWP platform, the MEWP operator shall support or pull the injured person back towards and into the MEWP platform. Note: If this operation means opening the access gate, then the MEWP operator undertaking the rescue shall apply their harness and permanent attachment first using a fall arrest lanyard to the designated anchor point with an attachment sling before undertaking the rescue. 16.2.2 If the injured person is not within arm’s reach of the MEWP platform, then the MEWP operator, while remaining within the MEWP platform, shall use the telescopic rescue rod to attach the rescue kit to the injured person’s harness. Once correctly attached, the MEWP operator shall pull the rescue rope or wind the operating wheel to winch the injured person towards the MEWP and back into the platform. Note: If this operation means opening the access gate, then the MEWP operator undertaking a rescue shall apply their harness and permanent attachment first using a fall arrest lanyard) to the designated anchor point with an attachment sling before undertaking the rescue. 16.2.3 If the person working on the Thyristor Module platform slips or falls and is hanging suspended from the Thyristor Module platform, the MEWP operator will need to exit the MEWP platform to perform a rescue. This requires the MEWP operator to assess the situation and shall not compromise his own safety when carrying out this operation. The MEWP operator shall apply his harness and permanent attachment using a fall arrest lanyard to the designated anchor point with an attachment sling. The MEWP operator shall open the rescue kit and safely position the contents onto the Thyristor Module. The MEWP operator shall open the MEWP access gate and leave the MEWP platform in order to assess the position of the causality. The MEWP operator shall attach the rescue kit wheel to a designated anchor point, using a permanent attachment sling, and attach the rescue kit to the casualty. Ensuring his own safety is not compromised, the MEWP operator shall position himself at a safe distance from the Thyristor Module edge and use the telescopic rod to attach the rescue kit rope to the harness of the casualty. Once the rope is securely attached, the MEWP operator shall wind up the operating wheel to remove the tension from the casualty’s fall arrest lanyard. When the tension has been adequately removed, the MEWP operator shall cut free or unbuckle the casualty’s fall arrest lanyard, and begin to lower the casualty to ground level. Once the casualty has been lowered to ground, the MEWP operator shall return back into the MEWP platform, close the access gate, remove the permanent attachment from the designated anchor point and return to ground level. 16.2.4 If the MEWP power or controls fail, then all work at height activities shall stop, the MEWP operator shall communicate with the ground-based person and all persons at height shall return back into the MEWP platform, ensuring their permanent attachment devices are removed from the Thyristor Module. All tools, PPE and equipment shall be securely placed on the floor of the MEWP platform and not hung or stored on or over the outside of the MEWP safety rails. Consideration shall be given to the position that the MEWP and platform are in and if it is feasible for the MEWP to descend from that position. Some MEWPs may be designed to allow the working platform to rotate when at the working position (e.g; by rotating the platform, to allow it to fit between





Valve Towers). Situations like this may preclude the use of this rescue technique if the path of the descending platform might be blocked. With the access gate closed (and locked) the MEWP operator shall instruct the person on the ground to undertake a rescue from the ground and begin to lower the MEWP back to ground level.

17. PRECAUTIONS FOR WORKING ON VALVE REACTORS ON THYRISTOR MODULE

The requirements of section 13 and 14 above shall first be completed. It shall be noted that the Valve Reactors contain water cooled sections. Prior to work beginning on the Valve Reactor, consideration shall be given to the application of safety precautions on the cooling water system as described in section 19 below.

18. ADDITIONAL PRECAUTIONS FOR WORKING ON RESISTORS ON A THRYISTOR MODULE

The requirements of section 13 and 14 above shall first be completed. The resistors on the RC snubber circuit are water cooled and prior to working on them precautions shall be taken on the Valve Cooling System to prevent Danger. Section 19 below describes precautions to be taken on the Valve Cooling System prior to working on water cooled Apparatus.

19. ADDITIONAL PRECAUTIONS FOR WORKING ON VALVE COOLING EQUIPMENT WITHIN THE VALVE HALL

19.1 Where work is to be carried out on the Valve Cooling Equipment within the Thyristor Modules, the precautions described in sections 13, and 14 shall be carried out. 19.2 Appropriate actions to prevent Danger shall be carried out in accordance with the Scottish Power Safety Rules. These precautions include but are not limited to closing of inlet valves, opening of vent and drain valves where applicable.

20. ADDITIONAL PRECAUTIONS WHEN WORKING ON THYRISTOR FIRING ELECTRONICS

20.1 When in normal operation and configuration the lasers are of Class 1 type meaning that there is no exposed laser light emanating from the valve based electronics. Where work is not required on the valve based electronics then it is not necessary to isolate as the control system will be in Earthed position which blocks the firing pulses. During normal operation of the link the DC control system only sends firing signals to the Thyristors when the relevant system is in a deblocked position. While the DC system is in Blocked, Standby, Stopped or Earthed status then the control system automatically disables the laser firing of the control system. 20.2 If there are broken and/or disconnected fibres from the valve based electronics system the equipment falls into Class 4, requiring precautions to be taken. If there is a requirement to replace or repair a fibre in the valve based electronics the supply to the fibres shall be Isolated and a Safety Document issued prior to accessing the Thyristor Valves. This isolation is achieved by the Switching off of the Valve based electronics supply switches that are located within the relevant cubicle – see figure 33.





Figure 33. Valve based electronics isolation Appropriate PPE such as Approved laser protection goggles shall be used to ensure safety of persons from potential of re-energisation of the laser electronics.

21. ADDITIONAL PRECAUTIONS FOR WORKING ON HV BUSHINGS WITHIN THE VALVE HALL

21.1 Where work is to be undertaken that requires the disconnection of busbars from the through-wall bushings from the converter transformer or from the DC Hall, in addition to ensuring that the Apparatus is Isolated and Earthed, the pressure within the SF6 filled bushings shall be reduced to a safe pressure before a Safety Document is issued for removal of conductors. 21.2 The bushings that contain SF6 within the Valve Hall are:

Star-connected bushings on each phase of the converter transformers (6 in total).

High Voltage side DC bushing leading to the DC Hall.





21.3 Locations for the connections to insert and remove SF6 gas are located in the DC Hall for the DC wall bushings and on the turrets of the converter transformers outside of the Valve Hall for the AC bushings.

Figure 34. DC Bushing SF6 gauge and top up point.





Figure 35. Location of AC Star Bushing SF6 monitoring and top-up points. 21.4 Where the conductors onto the bushings are to be removed, extra Drain Earths shall be applied to maintain earth continuity during the work.

22. PERFORMING CAPACITOR MEASUREMENTS

In addition to the completion of sections 13 and 14 above the following shall be carried out: 22.1 Before and after performing capacitance measurements, the Capacitor Unit(s) in a parallel group shall be discharged using a Discharge Stick by a Senior Authorised Person or under the Personal Supervision of a Senior Authorised Person and all Short Circuiting Lead(s) re-applied.

23. PROCEDURE TO BE APPLIED WHEN A CAPACITOR UNIT HAS DISRUPTIVELY FAILED

Industry experience of Capacitor Unit(s) suggest that there may not be visible signs of disruption on the Capacitor Unit case or bushings. Abnormal capacitor failures are Capacitor Units showing signs of excessive bulging, major leaks or broken bushings. On this type of failure there could be an open circuit on the bushing connection lead internally within the Capacitor Unit. In this situation a dangerous voltage may be present on the lead or if the lead was to come into contact with the casing it could make the casing Live at some point during removal or transportation.





Where there is no cause for concern that an open circuit exists the Capacitor Unit shall be removed using the procedure described in section 13 and 14 above. Where there is evidence to suggest that the bushing has been damaged and an open circuit may exist in the Capacitor Unit casing then an Approved procedure shall be written. A person with specialist knowledge of capacitors, their construction, failure modes and removal from service shall be consulted.