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Automatic Transfer Switch (ATS)The Key to High Reliability Source Switching
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ATS – The Key to High Reliability Source Switching
Various technologies are used to switch critical loads from a primary to a backup power source. When nothing goes wrong in the facility (overloads, short circuits, etc.) most technologies perform adequately. When problems occur, certain ATS technologies will keep the power on…others will not. Knowing the differences and correctly applying ATS in your facility is key to providing high reliability source switching. This presentation will include:
• A definition of ATS • An overview of typical ATS applications, constructions, and types• A review of the IEC codes and testing procedures for ATS• A comparison of the performance of each technology under IEC testing – under normal and abnormal circumstances• Application guidelines for selectively coordinated, high reliability, fault- tolerant source switching.• Overall recommendations for a Power Quality installations.
Presentation Overview
Section 1 – ATS Basics
What is an ATS ?What are typical applications of
ATS?Common constructions of ATSCommon types of ATS
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G
What is an Automatic Transfer Switch (ATS) ?An ATS is a device designed to Automatically switch (transfer) between (2) sources
of power to improve the reliability of the electrical supply to a connected load.
Primary Source
(typically a Utility
Supply or Utility Bus)(Normally Energized)
Backup Source
(typically a Generator
Supply (Normally
De-Energized)
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Typical Applications of ATS
Utility and Engine Generator>Single unit standby applications
are very common
Utility and Utility>Dual utility applications are
becoming more common
Generator and Generator>Prime power /
standby application
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Types of ATS construction
KnifeBladeType
KnifeBladeType
TwinContactor
Type
TwinContactor
Type
TwinMCCBType
TwinMCCBType
PowerContactor
Type
PowerContactor
Type
StaticTransfer
Type
StaticTransfer
Type
GE components GE Power Quality
Market Trend
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Typical ATS Constructions Knife Blade
Typically Non-Load breaking Usually Manually Operated (and slow) Sometimes motor operated (still slow) Low withstand ratings – no arc chutes or vents Low endurance ratings = limited life
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Typical ATS Constructions Twin Contactor
Switching device having 1 position of rest (IEC def’n) Returns to position of rest @ loss power (un-latched type) or add’l
latching arrangement to hold position on loss of power Built using 1 of up to 12 types of contactors under IEC 60947-4-1 Code,
from basic contactor to high-capacity motor starters. Often very basic controller Mechanical/safety interlock often not available or optional Variety of operators, from motor to electromagnets Not a True double throw device - possible to close into both Sources
IEC 60947-4-1
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Typical ATS Constructions Twin MCCB (Molded Case Circuit Breaker)
2 Position device, typically operated from add-on motors Built using 1 of up to 2 types of breaker under IEC 60947-2 Code, from basic
instantaneous-trip type to short-time rated devices. Mechanical/safety interlock is an add-on, sometimes optional Motor operators usually slow, often unreliable for high duty. Wide range of manf-assigned withstand ratings (low, med, or high) Not a True double throw device - possible to close into both Sources
IEC 60947-2
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Typical ATS Constructions Power Contactor Type (GE Zenith Std.)
UL 1008
IEC 60947-6-1
Std. Double Throw mechanism - Inherent mechanical interlock against closing to both sources
No add-on interlocks required Fewer moving parts High Withstand Current ratings to allow fault clearing coordination Integrated Arc quenching components Solenoid Operated (very fast)
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Power Contactor ATS - Composition • Arc quenching grids & enclosed
arc chutes• Movable contact assembly has 2
pieces, arcing contact and main current carrying contact.
• Arcing contacts make first, break last [See Video].
• Silver alloy contacts Resists welding Enhanced withstand ratings
• “Over center” switching principal to achieve a mechanically locked position in either Source 1 or 2
• High speed solenoid actuated drive assures contact transfer in 30-50 msec
(Click to see video)
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Typical ATS Constructions Static
Two live sources required. Solid-state switching >> no moving parts Extremely FAST operation, transfer in less than ¼ cycle. SCR technology Typically used on critical loads where 2 reliable and independent power
supplies available, or switching 2 x UPS Outputs. Both sources must be in sync Often cost prohibitive for Standby applications
N E
1 2
L
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StandardTransition
(No Center Position)
Delayed Transition
(With Center OFF Position)NOTE: “OFF” Position
indicatesNeither Source connected
Closed Transition(Make-Before-Break Operation)
NOTE: “P” Position indicates both sources paralleled for 5
cycles
Standard Transfer Switch: Types
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• Essentially (2) ATS in Parallel, (1) Automatic Switch (ATS) & (1) Manual Switch (MTS).
• ATS is withdrawable, similar to withdrawable CB’s
• (2) redundant paths from each source to load.
• ATS & MTS are mechanically and electrically interlocked
– To prevents accidental closure of both sources
• Bypassing power from Source to Load permits testing or maintenance of ATS without interruption to load.
• Available in Open, Delayed, and Closed Transition
• If power fails while bypassed, Genset is auto started to permit fast transfer using MTS.
Bypass Isolation ATS
MECHANICALINTERLOCKS
TWO PATHS FROM SOURCE TO LOAD
WITHDRAWABLEATS
FIXED MOUNT MTS
Section 2 - Codes & Standards for equipment applied as ATS
What are the Internationally- recognized codes?
What is the Performance difference
of devices tested to these codes?
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Devices arranged for Source Switching, but not IEC rated as
ATS
Contactors & Motor
Starters
CircuitBreakers
Contactors & Motor
Starters
CircuitBreakers
Power Contactors
Devices arranged for Source Switching, IEC rated as ATS
Internationally-recognized ATS Codes and Standards
IEC 60947-4-1UL 508
IEC 60947-2UL 489
IEC 60947-6-1UL 1008
Devices may be arranged to function as an ATS, but not be rated as ATS by a Recognized body
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Twin Contactors Twin CB’s CB: Breaker type PC/CC: Contactor
types.
IEC CodeCircuit Breaker
60947-2Contactor60947-4-1 Transfer Switching Equipment 60947-
6-1(IEC ‘A’)(IEC ‘AC-1’)Util. Cat.
IEC Performance Testing of ATS - Overview
(AC-33A, 33B)(AC-31A, 31B)
Operational Performanc
e
Overload or Low current
short circuit
Performance
High/Short Circuit
Performance
High Operations @Rated current
Low Operations @ majority No
Current
31A: High Operations @ Rated Current33A: High Operations @ 2 x Rated Current31B: Low Operations @ Rated Current33B: Low Operations @ 2 x Rated Current
Low Operations @6x Rated Current, High pf, for up to 630A Rating. (N/A
>630A)
Low Operations @1.5x Rated
Current, High pf
31A: High Operations @ 1.5 Rated Current33A: High Operations @ 6x Rated Current31B: Low Operations @ 1.5 Rated Current33B: Low Operations @ 6x Rated Current
GE ATS Ratings
No Requireme
nts
Break SC: Yes Make SC: Yes Make & Hold: No Withstand SC: No
CC/PC Type: Break SC: NoMake SC: YesMake & Hold: YesWithstand SC: Yes
CB Type: Break SC: YesMake SC: YesMake & Hold: NoWithstand SC: No
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Short Circuit Testing of PC-Type ATS
• PC-type ATS are tested to:>Withstand fault currents
(remain closed), for a set period of time, permitting downstream devices to trip and isolate the fault.
>Close into faults, to permit the Backup supply fault current necessary to clear the fault.
>For this reason, PC-type ATS have Withstand and Close Ratings (WCR) vs. Traditional kAIC Fault rating (as in a CB).
FaultTesting (click to see video)
• Contactors and Circuit Breakers have Fault current ratings, expressed in kAIC
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Performance of ATS Technologies - Summary Twin
CB’sCB: Breaker type PC/CC: Contactor
types.Twin Contactors
Low Operation switching of Rated
Load
Building Event
OK
Switching Motors w/ 6-10X inrush @
start
OK (up to 630A)>630A Not OK
31A, 31B33A, 33B
OK
High Duty Cycle/Switching OK Not OK
31A, 31B33A, 33B
Switching into a short circuit on load side
of ATS.
Short on load side of ATS Need
upstream SCPD to trip
Big problem
Inst. Trip. Selectivity w/ downstream SCPD ???
Same as above
Designed to withstand & permit downstream SCPDto clear
Not OK
“Real life” performance of
ATS in your facility
Same as CB
Designed to close into and withstand to permit downstream SCPD to clear
Section 3 - Application Guidelines for PQ Installations
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PQ Application Recommendations for ATS
Specify ATS certified As an ATS to IEC 60947-6-1 or UL 1008 Un-certified ATS have not been tested for the purposes of source switching. Specify an ATS with adequate duty cycle rating
“A” or “B” IEC Utilization Catagories, based on application
Specify an ATS with adequate overload capability for switching inductive loads
“31” or “33” IEC Utilization Catagories, based on application
Specify PC Type of ATS for critical load application … Specify PC-type where fault withstand, vs, tripping under faults, is
required for proper system Selective Coordination for faults below ATS.
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