Typically used on mines and in industrial plants using electrical equipment such as motors and other electrical machinery, medium voltage (MV) switchgear is primarily used to isolate sections of the electrical network and to interrupt current to downstream equipment in the event of a fault, so as to ensure the safety of people and to protect electrical assets and infrastructure. “MV switchgear panels are used in substations to interrupt the electrical supply feeding into a mine, plant or factory, either automatically in the event of a fault, or manually when any electrical work on the system needs to be done,“ begins ACTOM’s Rhett Kelly.
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The new GELPAG SIS solution adds to ACTOM’s local capability to develop, engineer and manufacture switchgear for African and the global markets.
A medium voltage network is generally in the range from 3.3 kV up to 33 kV, he says. Connecting and disconnecting at these voltages and currents involves controlled arcing between the contacts of the interrupting device used in the switchgear, which erodes the contact material. When an uncontrolled arc occurs due an insulation failure within the switchgear, this can cause the air or gas pressure to rapidly increase, potentially resulting in a high energy explosion. By using internal arc-tested and classified switchgear, the hot gases and plasma produced in the switchgear during such a fault can be safely released in a manner that ensures the safety of people.
Key to preventing arcing from happening is the integrity and type of internal insulation used. Describing the insulation options typically used to insulate the live conductors inside the switchgear, Kelly says that there are three traditional options for switchgear, oil-insulated switchgear (OIS), air-insulated switchgear (AIS), and gas-insulated switchgear (GIS). These systems are used to provide the required dielectric insulation between the energised conductors and the sheet metal of the earthed enclosure.
“Our new GELPAG metal-enclosed MV-switchgear solution uses a fourth insulation medium, namely solid dielectric insulation, which encapsulates the live conductors of the main circuit with an epoxy resin. Switchgear using such insulation is referred to as solid dielectric insulated switchgear (SIS). In the case of ACTOM’s GELPAG product, a conductive screen layer is added to the outside of the epoxy resin, which is earthed during normal operations, making the functioning of the switchgear immune to harsh environmental effects such as moisture and pollution, since none of the insulation surfaces within the switchgear is exposed to ambient air.
“The main current interrupting device is still a vacuum interrupter, embedded inside the epoxy resin,” he points out.
“Since we are not filling the whole of the enclosed area inside the panel with a gas such as SF6 – which has a very high global warming potential – or any other insulating fluid, there is no requirement for having gas or fluid monitoring devices fitted to the switchgear. In addition, the earthed screen layer on the epoxy resin eliminates the electrical stress in the air between phases, which allows the overall footprint of the switchgear to be reduced. This enables the overall volume of the whole substation building to be significantly reduced.
“We can typically achieve the same level of compactness as GIS, potentially even more, because the dielectric strength of epoxy is very high and independent of pressure,” Kelly explains.
MV Switchgear designed and manufactured in accordance with SANS 1885, the South African front end standard for the IEC 62271 MV switchgear standard, has to be metal enclosed by design, he explains, “In the case of GIS, the whole ‘tank’ enclosing the high voltage components of the switchgear is sealed for life, with the gas at a positive pressure above atmosphere.
The GELPAG SIS solution is manufactured and supplied in accordance with SANS 1885 does not require a sealed pressure system and associated monitoring systems to maintain the insulation integrity. GELPAG also falls under the category of metal enclosed switchgear, which means all the high voltage components are housed within a metal enclosed panel rather than a gas compartment,” he says.
“One of the big advantages of the earth screening system is that it maintains the integrity of the epoxy resin insulation without the need for a sealed pressure system, because moisture and pollution settling on the insulation surfaces will not result in any leakage currents, which typically give rise to erosion or tracking phenomena on the insulating surfaces. Unless the pollution deposits are periodically cleaned, tracking and/or erosion on unscreened systems degrades the quality of the insulation over time and are often the root cause of an internal arc fault as described above,” he adds.
“By screening our insulation system, we have effectively taken out the effects of environmental factors,” Kelly points out. “This panel has actually been tested underwater for 96 hours, and while it is obviously not intended to be operated underwater, this clearly shows that in flooded conditions in a mine or when underground dusty or humid conditions are high, the insulation system is well protected,” Kelly tells MCA.
According to technology development specialist, Johan Jordaan, these features make ACTOM’s new GELPAG 12 kV SIS metal-enclosed MV-switchgear panel ideal for mining or even coastal environments. The safety features are arguably at a level above traditional switchgear. With no exposed conductive or live surfaces, it reduces the risk to personnel operating and maintaining the equipment – particularly when operating procedures are not correctly followed. Furthermore, the switchgear has an extremely low probability of an internal arc developing due to the earth screen layer on the epoxy resin. This solution poses far less risk for operators than traditional switchgear designs,” he continues, adding that switchgear could rupture explosively in the event of an internal arc due to the high energy, temperature and pressure that develops due to the vaporising the conductors.
Kelly adds that ACTOM’s SIS system has nevertheless been fully tested and classified on all sides for internal arcing up to 40 kA for 1.0 s. “Even though we know we are unlikely to ever get an internal arc, we still had to do the internal arc classification test to meet the requirements of the South African and International standards.”
“By using earth-screened epoxy resin insulation along with vacuum interrupter technology, we have practically eliminated the chance of an internal arc ever occurring,” he assures, before going on to demonstrating the gas venting pathways included for this very unlikely event.
“The device that interrupts the load or fault current we call the vacuum interrupter. A vacuum interrupter is a hermitically sealed system that keeps the area around the main switching contacts under vacuum, so that the interruption of any arc current is assisted by the dielectric properties of a vacuum,” Kelly explains.
“Because vacuum is such a brilliant insulator and arc quenching medium, when the contacts separate, the arc is extinguished after the first zero crossing, within a half cycle of the 50 Hz ac current wave. And an open gap of just six to eight millimetres is all that is needed between the contacts to maintain an open circuit voltage at 12 kV,” he says, adding that the unit’s electrical insulation capacity passed withstand-level tests for a 95 kV lightning impulse.
The switchgear has a three-position switch, with a closed position, an open position and an earth position. “The mechanism that drives this and the circuit-breaker vacuum interrupters incorporate a proprietary designed system using bellows to keep the contacts of the three-position switch and the circuit-breaker drive insulator hermetically sealed at all times.
“Another feature that is really noteworthy is that, because this is a fixed-pattern switchgear solution in terms of the open (isolated), closed and earth positions, we have included a mirror-based system to enable an operator to visually check the exact position of the moving contact at any time, therefore confirming its safety status,” he adds.
As is the case of most MV switchgear, the operating mechanism used to open and close the current interrupting device is a mechanical device that has been tested for extended mechanical endurance in accordance with the relevant IEC standards. To break the circuit, a conventional circuit breaker mechanism is used, with the energy stored in a system of springs, because at any time, the switchgear must be ready and able to rapidly open the breaker. “The energy is derived from the main closing spring, which is automatically charged using a small motor. On closing and latching the contacts, some of the closing spring’s energy is transferred to charge the contact pressure and opening springs,” says Kelly. If there is a fault such as an electrical short circuit, a protection relay, used in conjunction with current transformers, will automatically trigger an electromagnetic coil to release the stored mechanical energy in the mechanism to open the breaker. Alternatively, a simple push switch can be used to do the same thing manually.
To complete the picture, he says that earthing the circuit– e.g. the connected cables – is achieved by moving the three position switch into the earth position and closing the circuit breaker.
In terms of successes in the field for ACTOM’s 12 kV SIS MV-switchgear, Johan Jordaan says that ACTOM has sold some 160 of these units to a copper mine in the DRC for underground applications where there is extreme humidity. “We have also recently coupled a three-panel switchboard fitted inside an outdoor enclosure to a battery energy storage system (BESS) for a shopping centre. The compact nature of our GELPAG switchgear makes them ideal for use in e-house applications,” he says, adding that a 36 (40.5) kV version of the same product is planned for launching in 2025.”
Concluding, Rhett Kelly says: “The development of the new ACTOM GELPAG SIS solution adds to our local capability to develop, engineer and manufacture switchgear for African and the global markets, be it for indoor switchgear, containerised switchgear, compact substations, renewable energy applications, minisubs, outdoor kiosks or bulk metering units.”
