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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