GIS Cable and Transformer Interfaces (IEC 62271)

Short answer

A GIS (gas-insulated switchgear, above 52 kV) connects to the rest of the substation through interfaces: the power cable, the transformer and the busbars of a future extension. IEC 62271-203, 2011 edition, deals with interfaces in clause 5.107. For the cable it refers to IEC 62271-209 and for the direct connection to the transformer it refers to IEC 62271-211. Those two parts fix dimensions, pressures, forces and the split of supply between manufacturers, so that a termination or a transformer from one manufacturer fits the GIS of another.

What IEC 62271-203 says about interfaces

Clause 5.107 opens with a practical recommendation. To make testing easier, the design may include isolating facilities at each connected component, and that separation is preferable to dismantling. In the case of an air bushing, the high-voltage connection can be removed, preferably on the air side. Those isolating facilities must withstand the test voltages of the components.

It also provides for the interface for extensions: the user must state in the technical specification where future extensions might occur. If asked, the manufacturer supplies drawings to connect another GIS later. That interface must be limited to busbars or busducts, not to active equipment such as circuit breakers or disconnectors.

The pressures of the connection enclosures are explained in GIS gas compartments and pressure coordination. Cables and their testing are introduced in power cables, the transformer in power transformer and the passage of the conductor through the tank in high-voltage bushings.

To see how this fits with the rest of the IEC 62271 series, see the standards for primary equipment testing.

Cable connection: IEC 62271-209

The 2007 edition of this part covers the connection of fluid-filled and extruded-insulation cables to a GIS, in single-phase or three-phase arrangements. Its objective is to establish electrical and mechanical interchangeability between cable terminations and the GIS, and to determine the scope of supply limits. It does not cover directly immersed terminations, which are described in a CIGRE brochure.

Between the cable and the GIS there is always an insulating barrier that separates the cable insulation from the GIS gas. Depending on the termination, the standard distinguishes two types:

The values the standard defines for this interface are:

• Rated voltage. The lower of the cable and the GIS ratings, taken from the series 72.5 – 100 – 123 – 145 – 170 – 245 – 300 – 362 – 420 – 550 kV (clause 5.2).
• Normal current. The main circuit interface applies up to 3,150 A. Contact surfaces must be silver-plated, copper-plated or solid copper (clause 5.4).
• Temperature rise. For full interchangeability, the interface must be designed so that, at the rated cable current at 90 °C, there is no heat transfer from the GIS termination to the cable termination. If this cannot be achieved, the GIS manufacturer supplies the temperature-rise data of the termination and of the SF6 as a function of current.
• Gas pressure. The minimum functional pressure for insulation does not exceed 0.35 MPa absolute at 20 °C up to 300 kV, and 0.4 MPa above 300 kV. The filling pressure is assigned by the GIS manufacturer and cannot be lower than the minimum functional pressure (clause 5.6).
• External design pressure of the termination. It is 0.85 MPa absolute at 20 °C. The termination must withstand vacuum when the connection enclosure is evacuated during filling (clause 6.1).
• Forces. The maximum additional transverse force that the GIS applies to the interface in a three-phase connection does not exceed 5 kN. In a single-phase connection a total mechanical force of 2 kN is assumed. The GIS manufacturer guarantees that they are not exceeded (clause 6.2).

The standardized dimensions cover 72.5 kV to 550 kV with four sizes, both for fluid-filled terminations (Figure 3) and for dry-type ones (Figure 5). The standard warns that interchangeability for 245 kV to 300 kV is not maintained with respect to earlier editions, because of the reduction of the termination enclosure.

Direct connection to the transformer: IEC 62271-211

The 2014 edition applies to single-phase and three-phase direct connections between a GIS above 52 kV and the transformer. It fixes the electrical and mechanical interchangeability and the scope of supply limits of the connection. In the direct connection, one end of the bushing is immersed in the transformer oil or gas and the other in the GIS gas. The connection complies with IEC 62271-203 for the GIS, the IEC 60076 series for the transformer and IEC 60137 for fully immersed bushings.

The points the standard fixes are:

• Rated voltage. The same series from 72.5 kV to 550 kV.
• Continuous current. The interface dimensions allow up to 3,150 A.
• Pressure. The minimum functional pressure for insulation used in the bushing design does not exceed 0.35 MPa absolute at 20 °C with SF6. The maximum operating pressure used to calculate the mechanical strength of the bushing must be at least 0.85 MPa absolute at 20 °C (clause 5.2).
• Forces at the interface. Normal loads follow Table 1 of IEC 60137, level I, with a minimum of 2 kN. The selection of the bushing is the transformer manufacturer’s responsibility.
• Settlement. A maximum relative vertical settlement of 5 mm between the GIS and transformer foundations is considered (clause 5.6).
• Vibration. The GIS and transformer manufacturers must agree on how to take into account the vibration of the energized transformer (clause 5.7).

Circulating currents and insulating joint

The single-phase enclosures of a GIS carry return currents. To minimize those that circulate between the GIS and the transformer, the GIS manufacturer installs bonding connections between the enclosures. If circulating currents above 250 A are expected through the transformer connection enclosure, the standard requires an insulating joint between the GIS and the transformer (clause 5.1).

One note states that practical experience allows a load current of up to 1,250 A without additional precautions, because about 80 % of the load current flows through the GIS bonding. Another note states that the user should clarify with the transformer manufacturer the maximum current acceptable through its enclosure, to avoid the insulating joint if possible.

The insulating joint may also be needed to isolate the transformer tank from neighboring enclosures and for the GIS and transformer protection to operate properly. It must withstand 5 kV rms at power frequency for 1 min, and it may carry non-linear resistors in parallel to limit the very fast transient ground potential rises when a switching device operates. The standard accepts two locations, shown in Figure 1.

Cable and transformer tests with the GIS connected

IEC 62271-203 gives guidelines for dielectric tests:

• the parts of the GIS that remain connected to the cable must withstand the test voltages set by the cable standards for the same rated voltage;
• during the dielectric tests of the cable, the adjacent parts of the GIS are de-energized and grounded, unless special measures are taken against discharges from the cable onto energized parts;
• for extruded-insulation cables (IEC 60840 and IEC 62067), the after-installation tests use AC voltage, and the GIS near the termination may be subjected to that voltage;
• for oil-filled cables (IEC 60141-1), the after-installation tests use DC voltage. If it is not acceptable to apply it to the GIS, special means must be provided, such as disconnection or a higher gas density;
• to make transformer testing easier, an insulated grounding switch may be included in the bushing or in the GIS, and it is advisable to avoid opening the GIS for this.

To see how a cable is tested on site, see the guides on cable diagnostic testing and on-site withstand voltage testing.

What to agree between manufacturers

• The scope of supply limits of each interface, with the figure of the standard that applies.
• The current through the bonding and through the transformer enclosure, and whether an insulating joint is needed.
• The minimum functional and filling pressure in the connection enclosure.
• Who provides the test facilities: disconnection, cable test bushing or insulated grounding switch.

When to call us

At Tiza Group we define the GIS interface with cables and transformers together with you in the primary engineering design, and we check that the data from each supplier match. See our primary engineering service. Write to us and we will send you a quote.

Related guides

• GIS gas compartments and pressure coordination
• Power cables: parts, types, protection and testing
• Power transformer: parts, functions and testing
• High and extra-high voltage bushings: types, C1, C2 and testing
• Power cable diagnostic testing: which tests

References

• IEC 62271-203, High-voltage switchgear and controlgear - Part 203: Gas-insulated metal-enclosed switchgear for rated voltages above 52 kV, 2011 edition, clause 5.107.
• IEC 62271-209, Part 209: Cable connections for gas-insulated metal-enclosed switchgear for rated voltages above 52 kV - Fluid-filled and extruded insulation cables - Fluid-filled and dry-type cable-terminations, 2007 edition, clause 1 and clauses 4 to 8.
• IEC 62271-211, Part 211: Direct connection between power transformers and gas-insulated metal-enclosed switchgear for rated voltages above 52 kV, 2014 edition, clause 1 and clauses 4 and 5.

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