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When an interruption of electrical power occurs, an automatic transfer switch shifts loads between utility and generator power. During these transitions, transfer switch timing and sequence is critical to ensure proper system operation. Consulting engineers must understand transfer switch types, timing requirements, ratings, and the effects that the characteristic of each emergency load has on generator operation. They must also understand how a facility’s electrical system (available fault current, number of generators, paralleling configuration) affects transfer switch choices.
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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
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