Power cable systems: The ideal underground power solution

July 31st, 2014, Published in Articles: Energize

 

As South Africa develops, more and more electric power is needed to electrify domestic homes as well as to meet the ever-increasing demand for more power by the mining, industrial and commercial sectors in existing and newly developed areas around the country’s cities.

This demand requires not just more power stations, but also more reliable and less visible power systems, resulting in a new era for power cable systems. Underground cables have become the preferred technology solution because these systems also provide property developers with significant cost saving solutions such as no need for overhead power line servitudes.

Latest developments in Eskom on power cable systems

Recent major power cable system projects for Eskom include:

  • An increased base of installed 11 and 22 kV cable system networks in newly developed and existing city areas
  • Large scale 88 and 132 kV power cable system strengthening projects in Gauteng
  • The Ingula Pumped Storage Scheme peaking power station 400 kV power cable system in KwaZulu-Natal, to connect the newly built underground generation station with the above ground level transmission network

The Ingula 400 kV power system will be the first extra high voltage (EHV) cable installation in Africa.

Definitions and South African technical requirement documentation for power cable systems

So what is a power cable system and what are the typical specifications and project requirements to consider from a utility perspective? Power cable systems can be defined as a power cable used in combination with cable joints, cable terminations and additional ancillary equipment where applicable to ensure a complete power system connection from one substation to another; or a substation to a customer’s point of supply. It is important to note that a cable is an electrically insulated and screened conductor for voltages exceeding 3,3 kV and not bare overhead line conductor as is used for overhead power lines. Specific power cable type descriptions can also be found in the relevant national and international standards which will be referred to later in this article.

For anyone new to power cable systems it is important to always understand that South Africa owns and maintains well developed South African National Standards (SANS) for cables, joints and terminations at all voltage levels. The list of documents that are used by utilities in South Africa for new power cable system installation projects are:

  • SANS 1507 Electric cables with extruded solid dielectric insulation for fixed installations (300/500 to 1900/3300 V) for low voltage (LV) power cables.
  • SANS 1339 Electric cables – Cross-linked polyethylene (XLPE) insulated cables for rated voltages 3,8/6,6 kV to 19/33 kV for medium voltage (MV) XLPE power cables.
  • SANS 1332 Accessories for medium-voltage XLPE and impregnated paper-insulated power cables (3,8/6,6 kV to 19/33 kV) for MV joints and terminations.
  • SANS 10198 The selection, handling and installation of electric power cables of rating not exceeding 33 kV.
  • SANS/IEC 60840 Power cables with extruded insulation and their accessories for rated voltages above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) – Test methods and requirements for High voltage (HV) power cable systems (i.e. cables, joints and terminations).
  • SANS/IEC 62067 Power cables with extruded insulation and their accessories for rated voltages above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) – Test methods and requirements for EHV power cable systems (i.e. ables, joints and terminations).

These SANS documents will refer to or adopt International Electrotechnical Commission (IEC) maintained standard requirements to ensure South Africa remains aligned with international best practices. SANS 1507 and SANS 1339 have been made compulsory in South Africa. For projects where impregnated paper-insulated metal-sheathed cables may be required, it is important to note that SANS 97 Electric cables – Impregnated paper-insulated metal-sheathed cables for rated voltages 3,3/3,3 kV to 19/33 kV (excluding pressure assisted cables) will apply and is also a compulsory standard in South Africa.

There are also National Rationalised Specifications (NRS) such as NRS 077 for XLPE-insulated cables and accessories for systems with nominal voltages of 44, 66, 88 and 132 kV to promote uniform requirements for HV XLPE-insulated cables and accessories in South Africa.

User or utility specific cable requirements

As a user or utility it is important to state or specify your preferences from a cable construction design perspective as part of any tender enquiry documentation, for utilities these may include:

  • For LV power cable designs: SANS 1507; 600/1000 V rated voltage, copper-stranded conductors; PVC insulation; number of cores and core identification (red, yellow, blue and black); PVC bedding; aluminium wire armouring for single core or galvanized steel wire armouring for multicore; ultraviolet (UV) stabilised PVC outer sheathing; and where applicable with reduced halogen emission properties.
  • For single or three core MV power cable designs:  SANS 1339 rated voltage, type A construction in accordance with SANS 1339, longitudinally water blocking for both the conductor and cable, copper or aluminium stranded conductors; triple extruded cross linked polyethylene (XLPE) insulation with both a semi conductive conductor screen and semi conductives trippable core screen; core identification (1, 2 and 3); PVC bedding under armouring; aluminium wire armouring for single core or galvanized steel wire armouring; UV stabilised PE outer sheathing for direct buried applications or fire retardant PVC outer sheathing with reduced or zero halogen emission properties.
  • For single core HV and EHV cables: SANS 60840 for HV or SANS 62067 for EHV; longitudinally water blocking for both the conductor and cable, copper or aluminium stranded conductors;  triple extruded cross linked polyethylene (XLPE) insulation with both a semiconductive conductor screen and semiconductive core screen; corrugated aluminium metal sheath; and PE outer sheathing. For tunnelled or above ground level installations a copper wire screen with a laminated aluminium/polyethylene sheath may also be considered that are zero halogen low smoke rated.
MV XPLE cable

Fig. 1: HV XPLE Cable construction

Older technology solutions which may still be used by some utilities includes MV impregnated paper-insulated metal-sheathed cables also referred to as PILC cables and HV self-contained fluid filled cables or also called oil filled cables.

These are no longer used as preferred options on new projects due to:

  • The proven performance of XLPE cable insulation designs (more than 30 years installation history in utilities)
  • The much lower costs of MV and HV XLPE cables compared to MV PILC or HV oil filled cables
  • The non-availability or limited availability of spare cables for HV oil filled cable repairs
  • The improved design for fully water blocked type tested MV, HV and EHV XLPE cable options available from both local and foreign cable manufacturers

The power cable manufacturing industry in South Africa is well established and also recently experienced the introduction of a manufacturing line for up to 132 kV 2500 mm2 Cu or Al XLPE cable.

User or utility specific cable accessory requirements

For LV and MV cable joints and terminations there are various technologies available on the market. Heat shrink technologies remain the preferred option for MV power cable joints and terminations but future trends will also include an increased use of cold applied technologies.

For HV and EHV cable joints and terminations it is crucial to note that it is preferred to buy a complete type tested system for your installation. This will mean that the cable, joints and terminations supplied to you were type tested as one system to proof electrical and mechanical compatibility as allowed for by the relevant type tests successfully performed. It is important to note that pre-qualification tests are additional tests required for only EHV cable systems.

User or utility specific type testing requirements

Type tests and where applicable pre-qualification tests are performed to proof the applicable voltage range, conductor size range, 3 core or 1 core cable construction design, raw materials used for manufacturing, extrusion line process, manufacturing process lay up, etc., for the cable system manufacturing. Any change to these parameters may require re-type testing.

EHV and HV cable testing - compressed

Fig. 2: Testing of EHV cable systems

Project tender and procurement process

For the procurement process of any power cable system, it will be crucial that project specific requirements are provided as part of the tender. These project specific requirements are normally documented in the form of a utility or user specification document which will include the following key aspects: Technical schedules A and B, reference to relevant SANS and/or IEC required type test requirements, the current rating requirement for normal operating conditions and emergency operating conditions, fault current rating requirements, cable system component preferences,  installation conditions, and if applicable the earthing and bonding design requirements for sheath standing voltage and circulating current considerations. Sufficient detail regarding the cable route preferences, joint bay locations, termination locations, and road or river crossing constraints need also to be included in the project specific requirement documents.

Once a tender has been issued it is best practice to allow for a site clarification meeting where prospect suppliers have an opportunity to familiarise themselves with the technical specifications and installation conditions for the project.

The evaluation of any power cable system for projects are based on the following key requirements:

  • Type test compliance
  • Technical schedules compliance
  • Calculated cable ampacity ratings for the specified trench or installation conditions as was provided in the project specific requirements
  • Component drawings, if applicable
  • Sheath standing voltage or circulating current calculations
  • Routine testing and factory acceptance testing
  • Commissioning requirements
  • Guarantees
  • Installation team competence and experience
  • Installation tool requirements
  • Project time lines
  • Quality inspection test plans during installation
  • Spares requirements
  • Maintenance requirements
  • Operating requirements, if applicable
  • Condition monitoring
  • etc.

A good practice will be to ensure the suppliers receive a full list of evaluation criteria as part of the tender to ensure they can meet your every project need and indicate upfront their possible shortcomings and strengths. For HV and EHV cable systems the user or utility may also request (as part of the tender) the installation of a distributed temperature sensing system and a dynamic cable rating system based on the real time thermal behaviour of the cable system. This will aid the utility or user to optimise the cable system current rating under heavy loading scenarios.

Fig 3

Fig. 3: Standard trench requirements for HV cables

 

Trench

Fig. 4: Actual cable trench

Project installation and commissioning

Once the contract has been awarded normal project management principles need to apply and it is recommended to ensure the relevant technical team members from the utility, supplier and installation contractor are present at all site meetings.

Prior to the equipment delivery, factory acceptance testing (FAT) may be required and the timelines for these need to be established as part of the contract signing with the supplier and installation contractor, especially where overseas travel are required to witness testing for HV and EHV cable system components. These FATs will consist of routine tests performed on maximum 10% of each ordering batch or at least two components of each type.

Packaging for delivery to site also need to ensure no damage occurs to your precious cargo during the transportation of the cable system components. The trenching, shoring, bedding, cable laying, blanketing, backfill, concrete cover slabs and warning tape requirements shall be performed as per the project specific requirements.

Some helpful tips during instruction will be to keep a list available with all road agencies and municipal or telecommunication service providers for those unplanned damages to roads, water and telecommunication services. Also ensure that compaction testing is executed to prevent later disputes where paving and road collapsing can occur.

Directional drilling and pipe jacking are also well accepted methods employed to concur those busy road intersections and river crossings. For cable jointing and termination it is important to note that very specific skill competencies are required for MV, HV and EHV cable systems. Jointers must always have proof of qualifications and experience and it is preferable to confirm the skills competencies during the tender evaluation process.

XLPE cable jointing - compressed

Fig. 5: HV XPLE cable jointing

Once the cable system is installed, commissioning tests will be performed in accordance with the following relevant standards:

  • SANS 10198-13 for MV and LV cable systems
  • SANS 60840 for HV cable systems
  • SANS 62067 for EHV cable systems

For the commissioning voltage testing it is preferable to ensure onerous testing requirements are mandated to ensure test voltages in excess of the supply voltage are used to test the system integrity. For MV cable systems Very Low Frequency test waveform sets are normally used, however for HV and EHV power cable systems it is preferred to make use of mobile variable frequency series resonant test equipment with frequencies between 20 to 300 Hz. A partial discharge test in combination with the voltage test may assist to identify eminent joint, termination or cable pre-failure conditions. Where applicable tan delta testing will also aid to provide the installed cable system benchmark tan delta values for comparison when maintenance testing are done on the cable system during its intended operational life.

Once the commissioning tests are completed, all QITP’s signed, the utility handover documentation signed and received, it’s then safe to say “energise”.

Contact Thinus du Plessis, Eskom, Tel 011 871-3444, duplestp@eskom.co.za

 

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