Your questions answered: Understanding the new generation of medium-voltage switchgear
Webcast presentation on October 18, 2016, by Joe Richard, U.S. launch manager, Schneider Electric, participated in the question and answer session.
Joe Richard addressed unanswered questions from the Oct. 18, 2016, webcast on innovation and the new generation of medium-voltage switchgear.
Q: What will provisions for potential transformers (PTs) and current transformers (CTs) look like in the new switchgear? No more PT drawers?
Joe Richard: Instrument transformer technology is changing and moving forward along with the rest of medium-voltage (MV) switchgear design. High-burden classes required for accuracy on older electromechanical relays are no longer a requirement since digital relays and meters require miniscule load values in comparison. In both cases, we are seeing movement away from traditional, iron core instrument transformers and development toward sensor technology such as low-power CTs like Rogowski coils (air-core CTs) and low-power voltage transformers like resistive or capacitive dividers. These devices bring benefits in reliability, safety, and space savings.
Q: What is the potential to add controls to the new design style of MV gear such as generator paralleling and SCADA controls? Can you elaborate? Is there space allotted for additional controls?
Richard: As with all MV breaker designs, the breakers have an on and off position. There is not a trip position as in some low voltage breaker designs. The brains of the MV switchgear still relies on MV protection relays and control systems. This new switchgear technology will still take advantage of traditional controls and automation systems for applications like generator paralleling, load shedding, and auto-transfer schemes. Much of this technology advanced in recent years with the advent of digital control devices. Space allotment is going to be a limiting factor. Small footprint switchgear means we have to trade off space elsewhere in the gear. Modern digital relays are basically small computers. We are now able to perform more automation and more complex protection with less discrete devices. When the amount of devices exceeds the amount of space available in low-voltage switchgear, we proceed with what is usually done, which is to implement a remote relay cabinet.
Q: Is there any cost reduction, initial and/or maintenance, associated with the reduced dimension switchgear?
Richard: Yes, as part of the design of this new generation of MV switchgear, total cost of ownership is heavily considered. By reducing footprint, we can save cost in material for switchgear and square footage required in electrical rooms. Due to longer maintenance cycles and reduced-maintenance switchgear, long-term costs of maintaining MV power distribution networks are reduced.
Q: What switchgear options are available to mitigate arc flash levels?
Richard: Some traditional methods such as arc-resistant enclosures, relay arc mitigation, and energy- reducing maintenance switches are still available in these new styles of switchgear. However, the real benefit comes from advancement in dielectric designs that greatly reduce the probability of arc flash events over the lifetime of the switchgear.
Q: Can you explain in a bit more detail the reduced access pieces and reduced live exposure?
Richard: One of the main goals of the new generation of MV switchgear is to increase safety. Dielectric systems with ground shielded phases throughout the gear aim to have no exposed live parts anywhere interior to the switchgear, especially when the switchgear is installed and live. Safety interlocking of breakers, grounding switches, and cable compartment doors look to prevent inadvertent access to live environments by requiring compartments to be grounded before mechanical access is granted. Finally, the addition of items like live line indicators provide multiple ways to check for live voltage in addition to metering options before any MV compartments are opened.
Q: With a smaller footprint, getting the power and control cables into the switchgear and terminating them can be a problem in maintaining the bending radius. Has that been looked at?
Richard: Yes, this is a chief consideration in small footprint switchgear. This is helped by three things. First, modular switchgear allows for several options of bases and door depths to allow for more cable bending, spreading, and mounting space. Secondly, lighter switchgear means hitting and mounting conduit stub ups will be an easier task. Third, by moving cable connections to the edges of switchgear cubicles, whether front- or rear-connected, we make it easier for installers to work with the cables by not requiring them to crawl into large cable compartments to reach cable entry points.
Q: Please explain the definition and characteristics of "sharps."
Richard: For air-insulated switchgear, the insulation works in concert with the air between bus bars and other electrical components. Corona discharge can be limited, or even avoided altogether, by the careful design of component insulation, like epoxy coatings. The so-called “sharps” are rough places on either the conductive bus or the grounded structure near the bus that tend to focus the electrical field lines (called equipotential lines) around the bus—in a non-uniform way. It is the concentration of these field lines at the “sharps” that cause localized discharges of the corona, so they should be designed out whenever possible to extend the service life of the insulation.