Automatic Transfer Switch (ATS)The Key to High Reliability Source Switching

2

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

4

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)

5

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

6

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

7

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

8

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

9

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

10

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)

11

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)

12

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

13

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

14

• 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?

16

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

17

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

18

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

19

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

21

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.

22