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JWG B4/B1/C4.73 Surge and extended overvoltage testing of HVDC Cable Systems

Background :

In 1991 a CIGRE Joint Working Group was established including members of the study committees SC33 Insulation Coordination, SC21 HV Cables, SC 14 DC Links and Power electronic equipment. Scope of the JWG was the evaluation of transient overvoltages on DC cable insulation and study of applicability of overvoltage limiting devices. Technical Brochure 86 was produced in 1994.

During this time the only systems considered where paper type cables (mass, oil, gas impregnation) and line commuted converter technology. Internal and external types of overvoltage were considered.

The meanwhile increasing role of extruded cables in DC transmission is addressed in TB496 and subsequent standardization efforts, which includes extruded cables up to 500 kV and respects LCC as well as VSC converter types.

However, TB 496 considers basically internal converter failures impact on the cable system by specifying a SIWL testing procedure. LIWL is considered optional and a specific test level or a method to achieve a test level for a specific project is not stated. Instead the necessity for a LIWL test is left for customer-supplier negotiations. If LIWL is required for a project a fixed peak voltage UP1 = 2.1 U0 being opposite polarity to the DC pre-stress is specified.

Recently a couple of new demands or developments have penetrated the commercial projects,

  • Overhead line – cable mixed system for extruded cable systems
  • New converter generations, and topologies, such as e.g. half-bridge / full-bridge VSC type
  • Multi-terminal HVDC systems and/or DC/AC mixed grid system.

Related to those demands and the above mentioned available recommendations from TB86 and TB496 a gap is observed, which is respectively

  • For a different mixed OHL – cable system a large fluctuation in the demands of LIWL for the cable system has been anticipated depending on system layout (cable length, OHL tower design, grounding conditions) and regional parameters (lightning occurrence). It is currently left to supplier and customer to agree on lightning impulse testing levels. A recommendation on a method how to approach proper LI test levels based on project specific parameters would be desired in order to support customer and supplier.
  • The new converter changes might lead to changed requirements on the cable system performance and therefore on the required testing as currently mentioned in TB496. This could be e.g. the case for VSC based recommended programs. An investigations about the relevance to update overvoltage programs, or. E.g. impulse shapes, in VSC based cable systems.
  • Similar as converter topology, new system connection types beyond a pure point to point connection (multi-terminal, mixed AC/DC) can generate overvoltage signals, which are altered to what is currently respected in TB496. An investigation about the relevance to update overvoltage programs, or e.g. impulse shapes.

It is therefore suggested to start a JWG B4/B1/C4, which is looking into surges and extended over-voltages for HVDC cable systems with a goal to output recommendations on

  • Method for LI level determination in mixed OHL-cable systems based on project specific parameters
  • Impulse and overvoltage shapes revisited, e.g. long impulse requirements, due to converter topology developments
  • Impulse and overvoltage shapes revisited due to Impact of multi-terminal and DC/AC mixed grid systems.

Scope :

Looking into surges and extended over-voltages for HVDC cable systems with a goal to produce recommendations on

  1. Method for LI level determination in mixed OHL-cable systems based on project specific parameters and make recommendations for testing
  2. Impulse and overvoltage shapes revisited, e.g. long impulse requirements, due to converter topology developments
  3. Impulse and overvoltage shapes revisited due to Impact of multi-terminal and DC/AC mixed grid systems.

Convener:M. Saltzer (Sweden)

Progress Report 2016 (pptx, 98kB)