An industrial substation steps incoming grid voltage down to plant distribution voltage, and it comes in three build types. AIS, air insulated, uses ambient air as the insulation between live parts and is erected on site. GIS, gas insulated, uses an insulating gas inside a sealed earthed enclosure, covered above 52 kV by IEC 62271-203. A containerised secondary substation, called a mini substation in Southern Africa, arrives as a factory-built, factory-tested MV/LV package to IEC 62271-202. Land cost, air quality, altitude and how fast you need power decide which one you buy.

TL;DR

• AIS insulates with air. GIS insulates with gas in a sealed enclosure. Containerised is a prefabricated MV/LV package with transformer, MV switchgear and LV board already inside.

• GIS starts making economic sense from roughly 66 kV to 132 kV upward, though the real crossover is set by land cost, not voltage.

• Altitude penalises AIS and leaves GIS alone. Only external insulation in atmospheric air needs altitude correction.

• IEC 61330 is withdrawn. It was replaced by IEC 62271-202 on 13 June 2006, now in its third edition, published 2022.

• NIEL has erected more than 40 substations up to 230 kV class, at 33, 66, 110, 132 and 220 kV, exporting to 45 countries.

What do AIS, GIS and CSS actually stand for?

AIS is an air insulated substation, where the insulation between phases and to earth is the surrounding atmospheric air. GIS is a gas insulated substation, sometimes written gas insulated switchgear, where live parts sit inside an earthed metal enclosure filled with insulating gas above atmospheric pressure. CSS is a containerised secondary substation, a prefabricated enclosure holding a distribution transformer, MV switchgear, an LV switchboard and the interconnections, wired and tested at the factory.

The three are the same equipment set in different packaging. I tell young engineers that a medium voltage panel is a substation in a package. Explode it out and you get the same breaker, CT, PT, isolator and relay arrangement at 132 kV.

The insulating medium is the one variable that changes everything downstream. Stand in front of an air insulated panel line-up and look at the R, Y and B phase conductors. The only thing separating them is ambient air. Fill that space with an insulating gas and the same assembly becomes gas insulated. The vacuum inside a vacuum circuit breaker does not make a panel AIS or GIS. The medium between the phases does.

AIS versus GIS: what tips the decision, and at what voltage?

AIS is cheaper and simpler to maintain wherever land is not constrained and the air is reasonably clean. GIS earns its premium in three conditions: when footprint or land cost dominates, when pollution or coastal salt would degrade air insulation faster than the maintenance cycle can manage, or at altitude where air clearances grow disproportionately.

The altitude argument is usually waved through without a number, so here is the mechanism. Above 1,000 m, external insulation withstand has to rise because air density falls, and the correction applies only to insulation distances sitting in atmospheric air. Inside a sealed gas enclosure the dielectric characteristics are identical at any altitude and no correction is needed.

That single asymmetry is the real argument. An AIS switchyard at 1,800 m in the Kenyan highlands or 1,300 m on the Zambian Copperbelt has to grow its clearances and therefore its land take. A GIS bay at the same site does not. Everything else about altitude follows from that, including the equipment derating we apply as standard: above roughly 1,000 m, reduced cooling air density requires derating even at unchanged ambient temperature, and weaker supply quality argues for more insulation margin and more conservative protection settings than an equivalent Indian specification.

Indian practice still leans heavily to AIS. Inter-state transmission scope documents routinely specify conventional AIS conforming to the CEA (Technical Standards for Construction of Electrical Plants and Electric Lines) Regulations, 2022. For a greenfield industrial plant in Chhattisgarh or Karnataka, AIS is the default and the burden of proof sits with GIS.

Mini substations and CSS: what you get, and what you give up

A containerised secondary substation, known as a mini substation across South Africa and much of Southern Africa, transforms MV to LV inside one prefabricated lockable enclosure. Three situations make the case: expanding or temporary sites, remote installations where fabrication-ready labour is not available locally, and projects where a factory-tested pre-wired unit takes commissioning risk off the critical path. We are building a container substation project in Africa for exactly the third reason.

The trade-off is flexibility, and buyers regret this one. The unit is sized and configured at the factory, so later capacity changes or non-standard modifications are harder than with a site-built substation designed with expansion bays from the outset. If phase two is on the drawing board, a CSS is the wrong answer unless you buy the phase two capacity now.

Note what the 1,600 kVA ceiling is. It is the exact transformer power limit written into the scope of IEC 61330, the withdrawn standard. It is a sensible commercial ceiling for secondary distribution, but it is a legacy of the old envelope rather than a physical limit. Any enquiry above it moves you to a site-built arrangement with a separate power transformer and an MV line-up.

Which standards apply, and which one is dead?

Four IEC documents cover the ground and one commonly cited standard is withdrawn.

IEC 62271-200 covers AC metal-enclosed switchgear above 1 kV up to 52 kV. Our indoor and outdoor MV panels are built and type tested to it. IEC 62271-203 covers gas-insulated metal-enclosed switchgear above 52 kV, third edition 2022. IEC 62271-202 covers prefabricated substations above 1 kV up to 52 kV for outdoor installation in publicly accessible locations, third edition published 22 June 2022. IEC 61936-1 gives general rules for design and erection of HV installations that are not prefabricated.

Then the dead one. IEC 61330:1995 was withdrawn on 13 June 2006 and replaced by IEC 62271-202. Its scope covered cable-connected prefabricated substations above 1 kV up to 52 kV with a transformer of maximum power 1,600 kVA. A great many CSS datasheets in the Indian and African market still cite it. If your specification template calls for IEC 61330, that line needs updating before the next enquiry goes out, because a bidder can comply with a dead requirement while delivering a unit that would fail verification under the standard actually in force.

The current edition moved three things that matter on an Indian or African site. Rated power is now a three-parameter rated value rather than a single kVA number, which makes the enclosure’s thermal envelope explicit. Minimum dimensions were introduced for access doors and operation aisle height. And a new informative Annex G gives a procedure for evaluating the impact of solar radiation on internal temperatures. That last one is not academic. A steel enclosure in direct sun in Nagpur or Lusaka is a different thermal problem from the same enclosure in Frankfurt, and it is now a testable one.

For Indian projects, Chapter IV of the CEA Regulations 2022, notified under section 73(b) of the Electricity Act 2003, covers substations and switchyards at 66 kV and above, and substations at 33/11 kV, 33/22 kV and 22/11 kV.

What does the SF6 phase-out mean for a substation you are specifying now?

Vacuum is our default up to 36 kV. There is no gas to monitor, top up or leak, which matters most on dusty, coastal, high-altitude and remote sites where maintenance skill is thin. Our MV range runs 11 kV to 36 kV, 630 A to 3,150 A, up to 40 kA, and is vacuum based for that reason. We also build SF6 variants where a customer specification demands them.

Regulation is settling the question independently of preference. Under Regulation (EU) 2024/573, in force from 11 March 2024, Article 13(9) prohibits putting into operation switchgear using fluorinated greenhouse gases as insulating or breaking medium: from 1 January 2026 for MV switchgear up to 24 kV, from 1 January 2028 for 52 kV to 145 kV, from 1 January 2030 for MV above 24 kV up to 52 kV, and from 1 January 2032 above 145 kV. Orders placed before 11 March 2024 are treated separately under Article 13(14).

The driver is the gas. SF6 carries a 100-year global warming potential of 24,300 in the IPCC Sixth Assessment Report, with an atmospheric lifetime measured in thousands of years and no natural sink. The tightening has already reached the equipment standard: IEC 62271-203:2022 introduced two gas classes, GWP up to 1,000 and above 1,000, and cut the type test tightness requirement for the high-GWP group from 0.5 per cent to 0.1 per cent per year per gas compartment. A fivefold tightening of permitted leakage is the standards body saying, in engineering language, that high-GWP gas is on notice.

India has no equivalent ban today. Two things still reach an Indian buyer. Export customers increasingly will not accept it. And as global OEMs move portfolios off SF6, spares and gas handling support for a unit bought in 2026 will thin out well before end of life.

How do you specify a substation so every bidder quotes the same thing?

Most enquiries give a voltage, an MVA and a feeder schedule, then stop. Nine lines are what a builder actually needs.

1. System particulars. Nominal and highest system voltage, frequency, phases, neutral earthing method, and prospective fault level in kA. Without the fault level every bidder assumes something different and the cheapest assumption wins.

2. Build type as a decision, not a preference. AIS, GIS or containerised, with the land constraint, pollution class or altitude that justifies it recorded in the document.

3. Standard and edition by name and year. IEC 62271-200, 62271-203, 62271-202, 61936-1, and CEA Regulations 2022 for India.

4. Site conditions in numbers. Altitude in metres, maximum ambient at the equipment rather than the met station, pollution severity class, seismic zone.

5. Icw with its duration, and Ipk. A panel rated 31.5 kA for 1 second and one rated 31.5 kA for 3 seconds are not the same panel.

6. Busbar rating held through the full line-up. The commonest way to cheapen an MV quotation is to average the busbar across outgoing feeders. We do not average. If the incomer is 3,500 A, everything is 3,500 A.

7. Insulation level. Power frequency and lightning impulse withstand in kV, corrected for altitude.

8. Degree of protection and internal arc class, stated with accessibility type and duration.

9. Protection, automation and interlocks. Relay make and model, IEC 61850 where SCADA integration is required, arc flash sensing, and mechanical interlocks. Interlocks are not a line item to negotiate. If somebody can rack a breaker out under supply, the value assigned to human life on that project is zero. Our control and relay panels cover up to 230 kV.

Then verify with four documents: the type test certificate at your exact rating, the routine test report against your serial numbers, the general arrangement drawing with busbar cross-sections marked, and the bill of materials with makes and models. A certificate is tied to the design and rating tested, not to a calendar expiry, and one issued for a different rating does not cover a different panel.

One thing no specification captures. When we replaced a 66 kV substation and switchyard for an integrated steel plant in Karnataka, the constraint was not the equipment. The old substation had to keep running until the new one took over, and the changeover had to happen inside one day. Design that sequence at the inquiry stage, not at commissioning. On a separate 66 kV package for a Nepalese rolling mill we supplied the transformer, the 66 kV control relay panel, the breaker and the outdoor substation equipment together, which removes the interface disputes that a multi-vendor switchyard generates.

Also Read:
https://newindiaelectricals.com/blog/medium-voltage-panels-how-to-specify-an-11kv-22kv-or-33kv-mv-switchgear-panel/