Critical power: Transfer switches and switchgear
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Today’s Webcast Sponsors:
Danna Jensen, PE, LEED AP BD+C,ccrd partners,Dallas, TX.
Ken Lovorn, PE, Lovorn Engineering Associates,Pittsburgh, PA.
Moderator: Jack Smith, Consulting-Specifying Engineer and Pure Power, CFE Media, LLC
Presenters:
Danna Jensen, PE, LEED AP BD+C,ccrd partners,Dallas, TX.
Ken Lovorn, PE, Lovorn Engineering Associates,
Pittsburgh, PA.
Understanding the code requirements for transfer switches and properly applying them in an emergency power design
Critical power: Transfer switches and switchgear
Topics
• Applicable codes and requirements• Open and closed transition switches• Applying transfer switches and switchgear in
emergency power system design
• Transfer switch timing and sequencing.
Applicable codes
• NFPA 70:National Electrical Code (2014)
• NFPA 110: Standard for Emergency and Standby Power Systems (2013)
• NFPA 99:Health Care Facilities Code (2012).
Applicable codes
• NFPA 70 Articles:– 517, 695, 700, 701, 702, and 708– 700: Emergency systems
• NFPA 110, chapter 6– Transfer switch equipment
• NFPA 99, chapter 6– Electrical systems.
Transfer switch requirements
• Prevent interconnection of two sources• Electrically operated/mechanically held• Listed for emergency system use • Supply only emergency loads• Suitable for operation of all functions intended to
supply.
Transfer switch requirements
• Generator exercising timers• Protection (selective coordination)• Motor load transfer provisions• Isolation of neutral conductor provisions• Include source monitoring and time delays.
Signaling/monitoring requirements
• Source monitoring:– Undervoltage sensing – Frequency sensing.
• Audible and visual annunciation – Switch position– When “not-in-automatic” mode– Not functioning– Ground fault.
Required time delays
• Engine start• Transfer to EPS• Retransfer to utility• Bypass delay• Engine shutdown.
Additional (optional) time delays
• Load priorities• Programmed transition• Elevator pre-transfer.
Switch types
• Automatic• Nonautomatic• Open or delayed
transition• Closed transition• Bypass isolation.
Open transition transfer switches
• Open transition means the load is disconnected from source one prior to being connected to source two
• Maximum isolation of the two sources• Power interruption to the load.
Closed transition transfer switches
• Closed transition means that the load is connected to source two prior to being disconnected from source one
• The two sources must be synchronized to be able to use closed transition
• As long as both sources are available, there is no power interruption to the load
• May have control issues if source one is dead, because source two cannot synchronize with a dead source.
Bypass transfer switches
• In the bypass mode, the transfer switch is isolated from both the normal and emergency sources so its mechanism may be maintained without a power interruption
• Applications• Drawbacks.
Switchgear mounted transfer switches
• Locating transfer switches in the switchgear lineup can:– Save installation time– Cause problems with adequate isolation between
switches and other components– Simplify control wiring when a number of switches
need to be coordinated– Potentially reduce electrical space requirements.
Transfer switch timing
• All loads at the same time• Separate loads into two or more steps• Delayed operation of transfer switches.
Single-step load assumption
• Worst-case starting condition• Possible generator failure• Severe voltage and frequency dip• Voltage may dip so low that control relays
could drop out.
Multiple-step load transfer
• May allow a reduced generator size• Mitigates major voltage dips• Allows more load without increasing
the generator size.
Delayed transfer applications
• High inertia loads• Elevator drive motors• Refrigeration compressors• Sources that are not in phase.
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Applying transfer switches in emergency power system design
Design considerations
• The specifics of a facility’s electrical system affects the transfer switch choice
• Available fault current, number of generators, paralleling configuration, etc.
Application considerations• Location
– Available space– Minimize damage– Separate from utility
service equipment– Qualified personnel– Electrical point of
interconnection.
MAIN SERVICEENTRANCE
ATS
Application considerations
• Load analysis– Critical loads – Inductive loads– Nonlinear loads– Solid state loads
(VFD).
Application considerations
• Priority selection– Automatic– Nonautomatic– Bypass-isolation– Open or delayed transition– Closed transition.
Application considerations
• 3-pole versus 4-pole
Equipment rating
• Current rating to support total load• Withstand and closing rating (UL 1008)
– Any breaker– Specific breaker– Short time– 3-cycle versus 30-cycle.
Switchgear considerations
• Number of generators• Paralleling
configuration• Proximity to ATS• ATS controls.
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Transfer switch timing application
Sample load list
• 50-kW lighting load• 30-ton air conditioning • 40-hp air handling unit• 40-hp air handling unit• 100-hp fire pump• 250-kW UPS• 60-hp elevator.
Single-step load transfer
• Lighting load• Air conditioning • Air handling unit• Air handling unit• Fire pump• UPS• Elevator.
Two-step load transfer, alt 1
• Step 1: UPS
• Step 2:– Lighting load– Air conditioning – Air handling unit– Air handling unit– Fire pump– Elevator.
Two-step load transfer, alt 2
• Step 1:– Lighting load– Air conditioning – Air handling unit– Air handling unit– Fire pump– Elevator.
• Step 2: UPS
Three-step load transfer, alt 1
• Step 1: UPS• Step 2:
– Lighting load– Air conditioning – Air handling unit
• Step 3: – Air handling unit– Fire pump– Elevator.
Three-step load transfer, alt 2• Step 1:
– Lighting load– Air conditioning – Air handling unit
• Step 2:– Air handling unit– Fire pump– Elevator
• Step 3: UPS.
Timing comparison
With 6-pulse UPS• Single step: 1,750 kW• Two step, alt 1: 1,750 kW• Two step, alt 2: 1,750 kW• Three step, alt 1: 1,750 kW• Three step, alt 2: 1,750 kW.
With 12-pulse UPS• Single step: 1,750 kW• Two step, alt 1: 1,250 kW• Two step, alt 2: 1,250 kW• Three step, alt 1: 800 kW• Three step, alt 2: 800 kW.
Conclusions
• Unfiltered 6-pulse UPS systems can dictate the size of the generator regardless of timing
• Sequential timing of transfer switches can permit smaller generator sizes
• Dividing the load into more steps can reduce the generator size.
Danna Jensen, PE, LEED AP BD+C,ccrd partners,Dallas, TX.
Ken Lovorn, PE, Lovorn Engineering Associates,Pittsburgh, PA.
Moderator: Jack Smith, Consulting-Specifying Engineer and Pure Power, CFE Media, LLC
Presenters:
Thanks to Today’s Webcast Sponsors:
Research and resources
• 2014 Consulting-Specifying Engineer Electrical and Power Study
• Electrical systems webcast: Designing electrical rooms
• Critical power webcast: Generators and generator system design
• Critical power webcast: NFPA 110: Standard for emergency and standby power systems
Critical power: Transfer switches and switchgear
Sponsored by: