Electricity + Control September 2018

DRIVES + MOTORS + SWITCHGEAR

Next generation medium voltage switchgear

Joe Richard, Schneider Electric

H owever, the electrical distribution industry is experi- encing workforce, financial and regulatory pressures that require new designs to meet modern applica- tions. The industry’s workforce is changing as experienced en- gineers and electricians retire and are not being replaced with individuals with the same experience level or an equiva- lent number of workers. Furthermore, the average man-hour costs are increasing across the industry and capital and op- erational expenditures are under more scrutiny. In order to deal with a tougher economy, budgets are being tightened, putting equipment space at a premium in facilities across many industries that require higher power density from their limited floor space. Added to this, both industrial and reg- ulatory standards for safety are increasing as the effects of electrical hazards are better understood and discussed. For these and other reasons, it is time to re-evaluate me- dium voltage switchgear applications. Traditionally, medium voltage switchgear has been Air Insulated Switchgear (AIS), meaning that the principal meth- od for achieving dielectric stability is sufficient air gaps be- tween phase conductors. In metal-enclosed fused switches, the cabinets are completely open and rely on an air gap to separate the phases and prevent cross-over to the grounded enclosure sheet metal. In metal-clad switchgear, the requirement for an insulating cover around each conductor was introduced. However, the widespread use of bus differential protection schemes shows the design community’s lack of trust in the current insulation system’s ability to prevent phase-to-phase faults. There has been some investigation of Gas Insulated Switchgear (GIS) types in the ANSI market over the past decade to address dielectric concerns and explore the bene- fits of reduced footprint and maintenance. But concerns sur- rounding regulatory oversight of SF6 gas have made many designers hesitant to use GIS in lower voltage ranges (5 kV and 15 kV), where there are several alternatives. Therefore, with fewer options at these lower medium voltage levels, it is challenging to address the need for increased equipment Medium voltage switchgear designs have seen only minor changes since the establishment of metal-clad switchgear in the 1970s.

Take Note!

The SIS system uses moulded epoxy insulation as the principal source of dielectric strength. SIS allows for a de- creased air gap between the phase conductors, which reduces equip- ment size. It can also eliminate exposed live parts when used throughout the switchgear’s interior.

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12 Electricity + Control

SEPTEMBER 2018