Your questions answered: How can medium-voltage applications benefit power control systems?

When specifying switchgear, electrical engineers must understand the nuances of applying them to medium-voltage electrical systems. A May 1 webcast on the topic helped answer some key questions.

By Consulting-Specifying Engineer May 24, 2024

For electrical engineers, designing and specifying switchgear for medium-voltage applications can be complex. Medium-voltage switchgear is typically designed to provide control and protection of medium-voltage power equipment and circuits in buildings involving generators, motors, distribution and feeder circuits and utility interconnection.

During a webcast on May 1, presenters from ASCO Power Technologies provided information on the following learning objectives:

  • Learn about different types of medium-voltage switchgear.

  • Understand medium-voltage standards including ANSI 37.20.2, ANSI 37.20.3, ANSI 37.20.7.

  • Learn about medium-voltage circuit breakers, protective relays, types of enclosures and switchgear controls and configurations.

  • Understand medium-voltage switchgear components, options and accessories.

Additional questions were answered by Peter Rossomando, product manager, Power Control Systems, ASCO Power Technologies.

What are some of the standards that apply to medium voltage switchgear?

Answer: IEEE/ANSI 37.20.2 (metal clad), 37.20.3 (metal enclosed), 37.20.7 (arc resistant), 37.20.9 (gas insulated).

What are some medium voltage applications, and where is medium voltage switchgear used in electrical systems?

Answer: Water and wastewater, data centers, large health care campuses, industrial facilities and oil and gas applications. Medium voltage gear can be used at substations, primary and secondary distribution and for emergency/standby systems.

Does Schneider plan to use Rogowoski Coils instead of CTs for medium voltage relays and power metering?

Answer: Currently, we are not planning to use Rogowoski Coils.

Why are medium voltage VFD’s (or ASD’s) so much more expensive compared to the equipment they drive when low voltage VFD’s are comparatively cheap?

Answer: I’m not that familiar with the price differential, however, medium voltage VFDs have larger, more expensive internal contactors. They’re also physically larger, which also adds to increased material and manufacturing costs.

Do you have the 5kV voltage class?

Answer: Yes, 5kV, 15kV and 27kV class air insulated and gas insulated at higher voltages.

What does air/gas insulated mean?

Answer: All live parts in air insulated switchgear are exposed to open air within their enclosure, therefore air is the insulation medium. All live parts in gas insulated switchgear are encapsulated in gas filled chambers with SF6 gas as the insulation medium.

Any additional electrical room considerations for GIS equipment? e.g., Any mitigation for gas leaks?

Answer: There are leak detection systems that can monitor gas density in the GIS gear and alert the user that a leak has occurred. Most GIS gear is provided with gas monitoring systems.

What kind of maintenance is required in AIS that is not in GIS?

Answer: Maintenance typically associated with environmental conditions such as cleaning contacts because AIS is exposed to open air versus encapsulated GIS. Also ingress of dust, dirt, etc.

In Europe, the MV gear has an arc flash mitigation technique for venting/ducts from the gear to the outside. I don’t see this arc flash venting technique with U.S. gear. Is there a reason?

Answer: Yes, venting/exhaust ducts are provided with arc resistant switchgear. There is a slide in the presentation that illustrates this. A PDF of the deck will be provided.

How do you choose the BIL rating?

Answer: BIL rating is listed in a table in the ANSI standards and is fixed at 60kV for 5kV class, 95kV for 15kV class, 125kV for 27kV class and 150kV for 38kV class.

Please expand on what remote controls to use for on-off operation of circuit breakers.

Answer: Remote controls can be used to operate circuit breakers outside of the arc flash boundary. This can include remote circuit breaker control switches, a remote operator interface panel or completely separate control sections.

Does MV gear no longer use asymmetrical versus symmetrical ratings?

Answer: Most specifications reference symmetrical ratings but asymmetrical is still sometimes used, and MV breakers typically have asymmetrical ratings.

Is there an interlock on MV breakers that prevents racking in or out if the breaker has load and is closed?

Answer: Yes, there is an interlock system so the breaker cannot be racked out when energized/closed.

For a vacuum circuit breaker control switch, if there is a fault, does the switch reset the fault condition so that the breaker can be closed?

Answer: The circuit breaker protective relay will identify the fault and open the circuit breaker. Simultaneously, the relay’s associated lockout relay will trip and will not allow the breaker to be reclosed until the fault is cleared.

Is trapped charge a real problem when the circuits are opened?  Do the PTs and CPTs take a hit from switching transients and trapped charge?

Answer: Trapped charge is something that is important to account for in MV systems and higher voltage systems when circuit breakers are open.

What short circuit current should be used to compare with KAIC rating of MV circuit breaker: 1/2 cycle short circuit or 3-4 cycle short circuit?

Answer: The KAIC short circuit rating is 3 cycles.

Do you still have to use a charging handle prior to using a remote breaker control switch?

Answer: No, the manual charging handle does not have to be used. The breaker will be automatically charged from a battery system in these types of applications.

For Redundant MTTM medium voltage gears, are there (2) CPTs to ensure control power availability?

Answer: Yes, typically there are.

How was that breaker removed in the compartments?

Answer: The breaker must be open and then is moved to the disconnected position utilizing the breaker racking handle with the door closed. When the breaker is in the disconnected position (there’s an indication for this), the door can be opened and the breaker rolled out. A breaker in an upper compartment is rolled out with a rail system integral to the switchgear.

You said that compression lugs are required. I have seen mechanical lugs with the torque off bolts to make splices (for MV). Are they not allowed?

Answer: My comment was based on our current standard practice, which is to use compression-type lugs. The standards do not define a particular type of lug.

What is the power use (in VA or kVA) for a current transformer?

Answer: VA burden on a CT is dependent on the type of CT used, the devices connected to it and the distance from the connected devices. IEEE 57.13 table 10 defines some standards for 5A secondary CT burden.

Do you recommend concrete housekeeping pads for draw out switchgear?

Answer: Yes, MV gear is designed to be installed on a concrete foundation/pad.

Are all these CTs 600V or 480V rated?

Answer: The CTs are 600V rated. These can be used at medium voltage because they are placed on insulated bushings in the switchgear.

How do you mitigate space issues when high accuracy is required?

Answer: Depending on the application, in some cases meters and protective relays can utilize the same CT. In some applications, the CTs can be mounted in the cable compartment and the cables can be run through each CT. That requires additional hardware, etc., for the mountings.

Any more info on what is internally different between the quality grades of transformers? Other than accuracy, how do the outputs differ? Less harmonic content? Waveform shape?

Answer: The core of the CT determines the maximum amount of flux that can be developed in the CT. The higher the accuracy class of the CT, the more flux that needs to be supported by the CT, and the larger the core will be increasing the size of the CT

With GIS gear, what happens if there is a SF6 gas leak?

Answer: There are leak detection systems that can monitor gas density in the GIS gear and alert the user that a leak has occurred. Most GIS gear is provided with gas monitoring systems. If SF6 gas leaks, that will cause a decrease in the insulation capability and dielectric strength of the gear as well as in the ability to limit arcing events.

Would gas-insulated switchgear be advantageous over air-insulated gear, metal-enclosed or metal-clad at high altitudes?

Answer: GIS switchgear typically does not have an altitude correction factor. Air insulated does typically starting at 4000 feet.

If an arc event occurs, where does the arc flash energy go? Does it release to the top of the switchgear, or is it contained inside?

Answer: The energy is redirected out the top of the switchgear and into exhaust ducts, which vent into a secured location. There’s an illustration of this in the slides.

With GIS gear, is the actual make/break contact in SF6 or in a vacuum bottle inside of the SF6 enclosure?

Answer: There are some designs that use vacuum interrupting bottles and some that use SF6 gas interrupting bottles.

Does Schneider have more business in the metal clad type of MV switchgear versus the insulated gas switchgear for 15kV rated equipment?

Answer: Most 15kV and below switchgear is predominantly specified as air insulated metal clad gear, especially in North America. At that voltage level, the price is significantly less than GIS.

What is the procurement lead time for each type of MV switchgear?

Answer: That varies by manufacturers, complexity, features required and type of enclosure.

Can you discuss typical MV switchgear control power (120VDC, 48VDC? Dedicated battery system and charger?)

Answer: Reliable control power is critical in these applications for both breaker operation, protective relays and any additional controls such as PLCs. For that reason, separate dedicated DC battery systems are typical for these applications. 48VDC is common for 5kV class. 125VDC is common above 5kV class but can be used at 5kV. Battery systems should be sized to handle the steady state load of the controls along with the load required to operate the circuit breakers in the switchgear. IEEE 485 defines the recommended sizing for lead acid battery systems. IEEE 1115 defines recommended sizing for NiCad systems. We have found these systems to be very reliable in our experience. In critical applications, redundant battery systems are specified, which is common in data center and water/wastewater applications.

What ANSI device numbers do you think are underutilized?

Answer: In my opinion, 51N/51G for ground fault, which requires the zero sequence CT.

What is the maximum breaker amp rating and AIC available for 34.5kV GIS and 69kV GIS?

Answer: 2500A at 35kV. Typically, 1250A at 69kV.

Is there a special type of breaker that can be used with these protective relays, or do all medium voltage breakers have inputs for protective relays?

Answer: All medium voltage breakers require protective relays. No special type is required.

Does the Schneider MV switchgear also use the Arc Blok arc isolation, similar to the LV switchgear?

Answer: Arc Block is used in LV switchgear only.

What temperature is NEMA 3R rated down to?

Answer: NEMA 3R is always provided with strip heaters and thermostats. The temperature rating is dependent on the control components. For example, protective relays are typically rated to -40F and PLCs can range from -25 to -40F. For temperature conditions that are typically below zero, I would recommend an outdoor conditioned house type enclosure versus 3R.

Can arc resistant-rated MV switchgear be located in NEMA3R walk-in enclosures?

Answer: Arc resistant gear is available in NEMA 1 and would require an outdoor house type enclosure.

How much concern should there be for outdoor non-walk-in MV equipment, with relays and electronics in hot environments (Arizona as an example)? Is there enough concern with the heat being detrimental?

Answer: In this type of environment, I would recommend a conditioned house.

Is arc flash risk increased with using outdoor switchgear with sheltered aisles?

Answer: Not in my opinion.

What is your recommendation on installing NEMA 3R outdoor non-walk MV switchgear on an elevated concrete pad to protect against local flooding but at the same time be able to use a breaker dolly to draw out the breaker when needed to maintain it?

Answer: MV NEMA 3R gear is designed for mounting on a concrete pad. There’s a 4-inch base at the bottom of the gear, as well. The breaker lifting truck has a cradle that can be raised or lowered with a cranking mechanism so the cradle can be raised to the height required to remove the breaker.

What is your opinion about RC snubbers as a surge protectors?

Answer: RC snubbers are good options to protect transformers from switching transients whereas surge arrestors protect against voltage spikes. I think surge arrestors are appropriate for switchgear applications.

Is there a relationship between surge arrestors and BIL?

Answer: There isn’t a relationship between gear/breaker BIL rating and the type of surge arrestor. Distribution, intermediate or station class arrestors can be provided in switchgear regardless of the switchgear BIL rating.

Is there any standard on requirements for surge arrestors? Any required clearances?

Answer: The IEEE standard for surge arrestors is IEEE C62.11. There are recommended clearances between phases that can vary based kV and manufacturer.

Even with the metal clad switchgear, are the remote operation and racking devices used?

Answer: The remote racking device is a popular accessory. End user feedback is that they are used often for maintenance purposes.

Can you rack out a breaker if it is closed?

Answer: No, breakers must be open in order to be racked out.

Comparing gas insulated switchgear versus air insulated switchgear, is one more reliable when operating at the same voltage?

Answer: In my opinion, these are both very reliable if installed and maintained properly.

Why is 120VAC not used for circuit breaker control?

Answer: Generally, it is because the control power transformers required are physically large and would significantly increase dimensions and cost. In some applications, end users have provided a dedicated 120V feed to the gear for breaker operations.

How do you size the battery and charger?

Answer: These are sized based on the steady state load of control components and the number of circuit breakers in the gear.

What’s the required clearance at the back of the metal clad switchgear?

Answer: 36-inch minimum clearance.

What type of controls are placed inside a MV switchgear section between the upper and lower VCB compartments?

Answer: Typically, these controls include control relays to open and close the circuit breaker and terminal blocks for wiring connections. When required and if space allows some electronic components (ex: ethernet switches) are included. Single high section can house more control components including PLCs and Synchronization devices

When using arc quenching gear with ducted venting, does Schneider provide that duct work or is it by others? If by others, is there a standard that has to be followed to maintain system integrity?

Answer: ASCO and Schnieder provide a plenum that mounts on the top of the gear and the ductwork. These items are shipped separately and are installed after the equipment is placed. The standard length of exhaust duct provided is 80 inches. Additional duct can be provided.