Session 1

Chairman: Ernst Gockenbach, Leibniz Univ. Hannover, Germany
Rapporteur: Gilbert Teyssèdre, Laplace, Toulouse Univ, France
Only keywords of interventions given; please refer to presentations….

Topic 1: Progress on the understanding of physical phenomena and on materials for use in DC

Presentation by Ernst Gockenbach Leibniz Universität Hannover, Germany

Outline of the topics:

• Introduction: development status, roadmap, material characterisation and criteria;
• Environmental impact – recyclability (of XLPE and oil-containing cables)
• Understanding of ageing phenomena under DC; polarity reversal effects… Important questions regarding both inversion and high fields. How far power law for lifetime holds.
• Electrical stresses at triple points
• Behaviour of DC cable operated under ac

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Presentation by V. Werle, Tennet, Germany

Most of type tests are made for ac cases – how far is it relevant for dc; Need to improve significance of tests and address routine testing. -

Comments:

• T. Kvarts, Dom Energy, Danmark, windfarm development; Concern to maintain the knowledge on what should be asked to manufacturers; Brochure TB496 quite general and to be updated?
• L. Colla, Prysmian, Italy. Brochures on development tests are very general; TSO need confidence.
• P. Mirebeau, Nexans, Rance – Concerns with stability of the system: material side and electrical. Want some crosslinking. All CB with thermoplastic has failed (CB moves).
• A. Bareggi, Pirelli: HDPE and LDPE widely used in France; be careful when refusing non-crosslinked solutions.
• E. Gockenbach: Presently, most project are with a limitation to 70°C in service temperature of cables. There are supposedly reasons behind…
• Ch Frohne, Nexans, Germany: Limitation to 70°C is governed by electrical conductivity rise (and exposes to thermal ruaway). Q: any possibility to gather failure modes from operators in order to refine type tests?
• V. Werle, Tennet, Germany: as utility, wishes to dispose from "uniformed" type tests irrespective of materials – easier to manage.

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Presentation by June-Ho Lee, Hoseo University, Korea

Fundamental study on electrical properties of thermoplastic olefins as recyclable material. EPR particles incorporated in PP matrix. Performance vs. XLPE regarding treeing an space charge

Comments:

• Ray Awad, Consultant, Canada – Question on availability of such products…
• J-H Lee: this in link with specific providers
• N. Hozumi, Toyohshi U., Japan: why differences on space charge behaviour depending on copolymers?
• J-H Lee: The smaller size of copolymer seems to bring better performances.
• E. Gockenbach:  is recycling an issue, a problem?
• SD Mikkelsen, Energinet, Denmark, Yes; this can be a positive argument
• A. Bareggi: confirms recycling is an additional benefit
• A Gustaffson, ABB, Sweden: Recycling of XLPE is effective. It depends on what we want to as recycling technique ad outputs

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Presentation by M. Taleb, RTE, France

Shows the availability of new materials as with by-product free crosslinked materials. Also shows that depending on interface materials, the charge build-up can be different… Charges can be accumulated at the interfaces between dielectrics.
Proposes to introduce space charge measurements within future standards.

Comments:

• E. Gockenbach: Reluctant to use Space charge tests at industrial scale (ok for lab-level measurement)
• J. Karlstrand, CableGrid Consulting, Sweden. Not easy to make SC measurement for manufacturers;
• U. Vercellotti, CESI Milano – SC: implementation and methods are very variable – not fit to a simple standard,
• Ch Frohne, Nexans: difficult to standardise SC, but it is already here for prequalification for full cables. More for the development side to understand what is going on.
• N. Hozumi: invented the measurement for full size. Recognizes that it is an heavy job to apply to full size.
• J-H Lee: the use of space charge data is not necessarily straightforward -to discuss about SC criteria

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Presentation by J. Karlstrand, CableGrid Consulting, Sweden

Presentation related to interfaces, terminations, field strengthening at triple points.
Conclusions: Concerns put forward –that could be raised in Jicable HVDC’17 can be split in 3 families

• New material There is on offer for new materials, meaning deviating from peroxide cross-linked polyethylene, candidate as insulation for HVDC. It can be interesting to provide an overview of those candidates and show how they behave vs. XLPE and where can be the properties to worry about. Testing procedures should encompass those materials
• Standards, in specific Brochure TB496 are quite contended. It may appear not enough specific, or the conditions proposed are not relevant for some cases. We could organize some discussion about the scientific relevance of related recommendations and build propositions of improvement.
• Interfaces and triple points. Junctions are a weak point of cable systems. Session on related challenges: Appropriate modelling of materials, data and knowledge on materials, material-dependent behaviour (insulation), appropriate design, and manufacturing…

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Session 2

Chairman: Ray Awad, Independent Consultant, Canada
Rapporteur: Christian Rémy, Prysmian PowerLink, France
Only keywords of interventions given; please refer to presentations.

Topic 2: Very high DC voltage: 500 kV extruded power insulated cables: reality? Current standardization, qualification extension

Introduction of the topic by Ray Awad, Independent Consultant, Canada

Ray Awad noticed that the subject is similar to the situation of the 60/70’s when PE insulated HVAC extruded cables have been introduced in France, and then XLPE extruded cables.
Yves Maugain clarified the times of introduction of extruded cables in France PE in the 60’s and XLPE the following decade

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Presentation by Søren Damsgaard Mikkelsen, Energinet.dk, Denmark

The presentation refers to the Viking Link Project between UK and Denmark, 1.4 GW, 700 km long. The qualification of polymeric cables is not started yet because of some pending questions:

• Is the PQ test representative of a 40 years life?
• Is more testing required?
• Availability and testing of transition joints between extruded and MI cables.

Comments:

• Pierre Argaut: SILECCABLE, France: There were the same questions in the 1980/90’s about 400 kV AC XLPE cables (for the Bewag Project).
  There was no specification, a Cigré WG has been created, resulting recommendations published and eventually an IEC specification for extruded cables till 500 kV AC issued. Another Cigré publication (TB 303) latterly defined an “Extension of Qualification” testing protocol.
  A XLPE/MI transition joint is available, made of back-to-back assembled terminations.
  Everything is ready for the 525 kV DC systems.

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Presentation by Christian Rémy, Prysmian PowerLink, France

The recent advancements in HVDC cables technologies are presented, either in terms of qualifications or in terms of system in operation. State of the Art.
The feasibility of 600 kV XLPE and 525 kV thermoplastic HVDC cable systems is demonstrated for VSC converters (TB 496).
The feasibility of 700 kV MI (PPL technology) HVDC cable systems is also demonstrated.

Comments:

• Luigi Colla remind that several projects with extruded 320 kV DC systems are in operation or under construction. To be notice also the Eurasian Interconnector project (EU’s 10 years plan), the deepest cable when realised.
• Ray Awad asked about the monitoring of long submarine cables, Pierre Argault remind that a Cigré WG is dealing with the fault location issue, monitoring could be added.
• Anders Gustaffson notices that PQ labs are too busy and jammed. He adds that manufacturer and customers have to agree on what is really needed (common understanding of the needs)
• Thomas Kvarts asked the introduction of operation parameters like temperature variation or thermal drop in the testing programme. He claims also the customer has to be aware of the development tests made by the manufacturer.
• Ken Barber made comments about the WG on long length AC cables: monitoring technologies are lacking for long submarine cables. We still have to think about what can be done for monitoring long AC or DC cables.
• Mohamed Mammeri: electrical stress is a main parameter, we still have margin between 320 kV and 525 kV (the gradient will not be increased by 525/320…), so we can stay confident for EHVDC cables.
• Shoji Mashio on PQ tests: there is no space charges trapping in FF cables so the PQ test is not an issue at all, but type test is important. His opinion is that there is no issue with extruded DC cables when homogeneous materials are used (insulation, factory joints), the problems are occurring when accessories are mounted on the cable. In this case a PQ test is mandatory.
• Yang XU reported that 3 DC systems are operated in China, with monitoring on short length only. He asked about which maintenance should be made on DC systems.

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Presentation by Hideo Tanaka, VISCAS Corporation, Japan

The developments in the field are driven by increasing installation of renewable generation. Japan has to develop off shore wind farms with extruded 500 kV HVDC mass power transportation requiring improved productivity and materials.
A 2500 Cu extruded 320 kV DC, with 90°C maximum conductor permissible temperature, land system has been successfully prequalified.
A 2500 Cu extruded 320 kV DC (90°C), submarine system (factory joint) prequalification test is under way.
The development of a 500 kV DC submarine system is under preparation.

Comments:

• Ernst Gockenbach reports that measurements on 9 mm thick plates are showing the space charge distribution is independent of time.

Topic 3: Corridor projects in Europe for underground HVDC links, hybrid possible systems

Presentation by Frédéric Lesur, RTE, France

Development of a MgB2 superconducting cable for very high DC power transmission the project is funded by EU in the frame of its TYNDP plan.
5 to 20 GW corridors are identified in Europe (eHighways 2050)
A 3 GW demonstrator (10 kA-320 kV DC) will be developed and tested (Sept 2018). It will validate the novel MgB2 superconducting material and provide guidance on technical aspects, economic viability, and environmental impact of the =innovative technology.

Comments:

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Session 3

Chairman: Shoji Mashio, Sumitomo Electric, Japan
Rapporteur: Jean Charvet, RTE, France
Only keywords of interventions given; please refer to presentations.

Topic 4 : Submarine HVDC links: State of the art of extruded and paper power insulated cables, power transmitted

Introduction of the topic by Shoji Mashio, Sumitomo Electric, Japan

Presentation by Ken Barber, NAN Electrical Cable, Australia
There are now multiple change in the direction of power flow per day, very short time between the change (5 min), and higher variability of HVDC flow.
Should the type testing requirements for HVDC cables operating in conjunction with LCC HVDC converters be reconsidered?

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Presentation by Søren Damsgaard Mikkelsen, Energinet.dk, Denmark

Quality assurance: how to ensure that the high quality accomplished for the cable lengths that have been tested is maintained for more than 1500 km of cable produced over several years?
Should more actions (like testing) be required to ensure a high level of quality through production of the whole cable length?
How to do fault location on very long DC cables?

Comments:

• Shoji Mashio, Sumitomo Electric, Japan: Traditionally, Type test concerns design verification, not checking the production capabilities. To be discussed indeed if additional tests are required for very long production length.

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Presentation by Christian Rémy, Prysmian, France

Presentation of state of the art submarine HVDC cable technologies from Prysmian, and projects

Comments:

• Benjamin Pailler, RTE, France : What are the limit concerning water depth ?
• Christian Rémy, Prysmian, France: it does not depend on the technology, but on the accessories to be used, and mechanical strength.
• June-Ho LEE, Hoseo University, Korea : When do you expect HVDC thermo-plastic cables to be installed for submarine applications ?
• Christian Rémy, Prysmian, France: Prysmian is in the final stage of qualification, waiting for a customer.

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Topic 5 : Converters: Technical coordination for the connection « cable / converters »

Introduction of the topic by Shoji Mashio, Sumitomo Electric, Japan

There are different topologies of HVDC converter systems and improvements ongoing, however only one test protocol for cables according to CIGRE TB496. One must consider new equipment that are being developed, such as DC GIS and also induction of AC overhead lines on parallel DC cables, use of LCC converters (big power) with extruded cables.

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Presentation by Søren Damsgaard Mikkelsen, Energinet.dk, Denmark

There is a lack of standardization over cable manufacturers about which voltage level shall be used for Type testing of DC cables. Also some are testing products at 525kV type test while others at 500kV PQ test.
This issue is studied by the joint CIGRE Working Group B1/B4. It is a very important topic for the customer.
Moreover, no standard DC GIS termination for cables are available. This is a problem especially for offshore DC installation.

Comments:

• Markus Salzer, ABB, Convenor of JWG B1/B4/C4, Sweden: The JWG has just started in March 2016, with 22 participants, notably on:
  Mixed systems with overhead lines and underground cable systems: lightning impulse test voltages, and how to deduce proper test voltages (lacks in TB496)
  New converter types and technologies that are being developed: new type of overvoltage that the cable will see.
  B4 Working Groups who are dealing with insulation coordination with new converter types, and guidelines for electromagnetic transient calculations are useful inputs for the JWG.
  JWG D1 B3.57 is dealing with testing of DC GIS terminations.
• Pierre Mirebeau, Nexans, France: in HVAC, although electrical stress vary depending on the protection devices, standardisation was made at the maximum level of electrical stress to be compatible with all substations. For HVDC, the electrical stress vary with converter topology. Would it be possible to have the same approach as HVAC by setting a standard value that would cover all converter typologies?
• Markus Salzer, ABB , Sweden: it requires some discussions, which are already on-going in the JWG.

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Presentation by Benjamin Pailler, RTE, France

Focus on the phenomenon of Transient Overvoltage (TOV) which happens on healthy pole during a pole to earth fault on a symmetrical monopole configuration, which is not covered by TB496.
Which tests are representative of such a wave form? Is there a need to introduce/create a new test to cover this waveform? Polarity + or - ?
How TOV can endanger and reduce life expectancy of cables.
In term of insulation co-ordination, is there a need to limit the duration and/or the peak value of this TOV? Does it impact the values set for LIPL and SIPL?

Comments:

• Shoji Mashio, Sumitomo Electric, Japan: This problem is especially important with HVDC cables used in combination with overhead lines. TB496 only defines Switching impulse level on the same polarity at 2.1p.u.
• Luigi COLLA, Prysmian, Italy: 2p.u for this kind of fault is a theoretical value. Practically, overvoltage are in the range of 1.5 to 1.6 p.u.
  Up to 1.5p.u of TOV, it does not affect LIPL and SIPL voltage levels. Beyond 1.5p.u. of TOV, LIPL and SIPL voltage levels rise, and cable needs to be overdesigned. It would be complicated at 500kV voltage level, however, it seems today that 500kV converters would not be symmetrical monopoles.
• Hani Saad, RTE, France: there is a need to standardize TOV, LIPL and SIPL for symmetrical monopoles. Choosing 1.5p.u. or less for cable TOV would induce a significant increase in price of converter station.
• Shoji Mashio, Sumitomo Electric, Japan: 1.5 p.u. is very challenging when dealing with cables connected to overhead lines because of lightning energy.

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Presentation by Shoji Mashio, Sumitomo Electric, Japan

M. Mashio recall the requirements of TB496 regarding switching and lightning impulse voltages for LCC and VSC systems and the basic understanding of behaviours after faults on symmetrical monopoles.
Presentation of the retrofitting of 250kV LCC-HVDC Link in Japan (submarine, land, and O/H). Some polarity reversals have happened but very short (0.2s) thanks to LCC.
For LCC system, opposite polarity superimposed LIWV test have been demonstrated to confirm dielectric strength against lightning strike as well as system surge for MI cable. Same analogy is applicable for DC extruded cable.
Since LCC has been used for most of hybrid systems, as far as LCC type test protocols is applied, hybrid system is not threatened in operation conditions.
For VSC system, current TB496 focuses on SIWV for the provision of abnormal voltage in symmetrical monopole system with underground cable, but does not fully cover operation conditions for hybrid system.
JWG B4/B1/C4 launched to set up new coordination

Comments:

• Markus Salzer, ABB, Convenor of joint WG B1/B4/C4, Sweden: Lightning impulse voltage level for TT should be considered as project specific, because it is very dependent on regional conditions.
• Shoji Mashio, Sumitomo Electric, Japan: Agree, and recommendations should be given for the level to be chosen.
• Mohammed Mammeri, General Cable, France: Do you think that the shape of the impulse given in TB496 cover the impulse of the TOV ?
• Shoji Mashio, Sumitomo Electric, Japan: No, we can only say that lightning impulse is more stringent compared to Switching impulse.
• Benjamin Pailler, RTE, France: Regarding discharge of the healthy pole presented by Mr. Mashio; is it for restarting the station or protecting the cable against long time overvoltage ?
• Shoji Mashio, Sumitomo Electric, Japan: For the cable itself, long time TOV should not be a problem, because it was tested in FAT at 1.85p.u. for 1h. It is usually necessary to quickly discharge in order to restart.

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Final words by Lucien Deschamps

A good job has been made for this workshop and it will be useful for the preparation of the next Jicable HVDC seminar in November 2017 at Dunkirk, France.
Lucien Deschamps thanks the 3 Chairmen, the Rapporteurs and all the participants of this workshop.