Category: Eaton

How the ATC-900 evaluates closed or in-phase transitions

The controller software will confirm that the closed transition (CT) or

in-phase transition is enabled through the setpoints. When source

1 and source 2 are both available within voltage and frequency set

point tolerances:

• If S1 and S2 setpoints (UV/OV, UF/OF) are identical, there can be

a difference in voltage and/or frequency when transition occurs.

Also, if there is a difference in setpoints, the actual voltage and/or

frequency may be identical when the transition occurs.

• It confirms that sources are within closed or in-phase transitions

set point tolerances when:

• Voltage difference for closed transition = +/- 1 to 5%,

• There is no voltage difference setpoint for in-phase. As long

as both voltages are within voltage limits, any difference is

acceptable

• Freq. difference = +/- 0 to 0.3 Hz for closed transition

• Freq. difference = +/- 0 to 3 Hz for in-phase

• Phase angle, when the number of generators = 0. the phase

angle difference is fixed at +/- 8 degrees

• Phase angle, when the number of generators = 1 or 2. the

phase angle difference is fixed at +/- 5 degrees

• The controller attempts a closed or in-phase transition until the

sync timer (adjustable from 1 to 60 min) expires.

Synchroscope data for in-phase & closed transitions

Load voltage decay operation (open transition)

Load voltage decay utilizes the load voltage measurements to sense

back EMF that is generated when the transfer switch is in the

neutral position (electrical load disconnected). It provides a delay

in transfer in either direction if an unacceptable level is sensed

as established by a customer programmed level. The transfer will

not take place until the back EMF decays below the acceptable

programmed level. This feature has a separate setting of enabling or

disabling the operation. If disabled, the transfer switch will not delay

in the neutral position and will transfer between the sources as fast

as possible.

Time delay neutral operation (open transition)

Time delay neutral provides a time delay in the transfer switch

neutral position when both breakers/contactors are open. This delay

takes place when the load is transferred in either direction to prevent

excessive in-rush currents due to out of phase switching of large

motor loads.

Optional feature 29G: Type of operation (selectable automatic or

non-automatic)

This feature provides two door mounted selector switches marked

auto/manual and S1-trip-S2 which permits the selection of automatic

or manual operation. In manual operation, the user can manually initi

ate and electrically operate between S1. trip, or S2. On the contactor

bypass’s, 29G uses the top switch and the bottom switch. The newer

contactor bypasses (some starting in 2018) have 29G integrated and

standard for either the ATS or the bypass contactors. See the switch

instruction booklet for operating  instructions. The controller is in

monitor mode during the manual operation meaning the controller

will not attempt to change the switch position but will monitor the

voltage availability and the switch positions. The ATC-900’s display

will show “Monitor Mode”.

Several options for closed transition

Once an ATS is in the field, other uses or changes may occur that

could require changes from the original configuration. For example,

an application may initially require that only a closed transition should

be used and if it fails to sync, then the transfer switch will not switch

to the other source and will show an alarm. If the requirements

change  and the transfer switch needs to transfer using open transi

tion if closed does not occur, instead of sending the controller back

to the factory for reprogramming, the user can easily change the

transition setpoints to accomplish this.

If the controller is ordered with closed-transition enabled, the

user will have access to options 47C, D, E, F and G, which can be

changed by the setpoints to disable or enable the functions (see

below). Screen 3 of 5 in the system setup menu (See section 3.4.1)

is where the user can set the closed transition options as shown below:

47C = Closed/in-phase/load voltage decay

47D = Closed only (alarm on sync fail)

47E = Closed/in-phase/time delay neutral

47F = Closed/load voltage decay

47G = Closed/time delay neutral

Closed transition operation

Closed transition is a feature that will temporarily parallel two live

sources in a make-before-break scheme when performing a transfer.

This achieves a transfer between sources with no power interrup

tion. Both sources must be synchronized in frequency, phase, and

voltage before the transfer will take place. Closed transition voltage

and frequency limits are adjustable in the system setpoints  (3 of 5)

screen. Maximum phase difference is fixed at 0.3 degrees.

The user can also disable closed transition and just use the following:

In-phase > TDN

In-phase > LVD

Also with closed transition and in-phase disabled: TDN or LVD

The user setpoints for closed transition are as follows: Disabled (closed transition not used)

Alarm on synchronization fail (will not switch and will show an alarm)

Fallback to open on synchronization fail (to in-phase, TDN, and/or LVD)

For in-phase and closed transfers, the phase difference between

sources is displayed above the ATC-900’s sync time countdown message.

Emergency source

The emergency source is defined as the source that is not preferred.

If NONE is chosen for the preferred source setting, the emergency

source will be the source that is presently not attached to the load.

Therefore, in this condition after a transfer, the normal and emer

gency sources will switch between source 1 and 2.

Features

The ATC-900 has many features that are available to the user. These

features are standard and are available depending on the type of

transfer switch used (i.e. contactor, power frame switch/breaker,

or molded case switch/breaker). Appendix A has a list of all of the

features including any acronyms used along with a brief description.

See Table 8 “ATC-900 features” for a compact list of features, includ

ing a full list of available I/O. Consult Section 5 and Appendix B for

I/O descriptions. For some transfer switch configurations, standard

control input(s) and/or standard relay output(s) will come prepro

grammed from the factory. An example of this is service entrance

which requires the go to neutral input.

The ATC-900 is configured as either open-transition or closed-transi

tion. Closed-transition operation is covered in section 1.5.3.

Some ATC-900 programmable features are only available on specific

transfer switch construction types. By example, a contactor type

transfer switch (2-position) does not allow the electrical load to be

disconnected for a programmed time delay period therefore the

TDN (time delay neutral) feature is not available. A molded case type

transfer switch is mechanically interlocked to prevent paralleling of

the power sources therefore the closed transition feature is not available.

Normal source

The normal source is defined as the source that is preferred. The

preferred source setting allows the operator to select source 1.

source 2 or NONE as the preferred source. If NONE is chosen, the

preferred source or the normal source will be the source that is

presently attached to the load. The default is set as being source 1

as the preferred and normal source.

In-phase operation

The in-phase operation of an automatic transfer switch is an open

transition transfer. It will allow a transfer between two available

sources if the phase angle has a difference of five degrees or less.

See section 1.5.4 for more details.

As shown in the feature list of Appendix A, there are three scenarios

for in-phase:

32C= In-phase default to load voltage decay

32D= In-phase default to time delay neutral

32F= In-phase

The user setpoints for in-phase transition are as follows:

• Disabled (in-phase not used)

• Alarm on synchronization fail (will not switch and will show an alarm)

• Fallback to open on synchronization fail (To TDN or LVD)

For example, with the setpoint at fallback, using a two position

contactor, the switch will transfer to the other source if no synchroni

zation occurs, whereas a three position contactor will allow the use

of TDN or load voltage decay.

Glossary of terms and transition types

Available

A source is defined as available when it is within its undervoltage/

overvoltage/underfrequency/overfrequency/unbalance(if applicable)

setpoint ranges for the nominal voltage and frequency setting.

Unavailable

A source is defined as failed when it is outside of its undervoltage/

overvoltage/underfrequency/overfrequency/unbalance (if applicable)

setpoint ranges for the nominal voltage and frequency setting.

Normal source

The normal source is defined as the source that is preferred. The

preferred source setting allows the operator to select source 1.

source 2 or NONE as the preferred source. If NONE is chosen, the

preferred source or the normal source will be the source that is

presently attached to the load. The default is set as being source 1

as the preferred and normal source.

Operational simplicity

From installation, to programming, to usage, the ATC-900 was

designed with operational simplicity in mind. Only two styles need

to be considered, regardless of input/output requirements or system

voltages and frequencies. The ATC-900 provides the functionality of

numerous other devices combined in one package that mounts in

less than 7 by 11 inches of panel space.

The user-friendly front panel interface simplifies routine operation,

setpoint programming, and data presentation. A large color display

provides flexibility and ease of use. Front panel membrane pushbut

tons are used to navigate the intuitive menus and screens. A single

unit status LED at the top of the faceplate provides an immediate

indication as to the device’s operational mode. Pressing the Help

button provides immediate user assistance messages on the front

panel display.

The ATC-900 is communications ready, including Modbus RTU,

Ethernet (via external communication module), and USB for thumb

drives (memory sticks).

Functions/features/options

The primary function of the ATC-900 is to accurately monitor power

sources and provide the necessary intelligence to operate a transfer

switch in an appropriate and timely manner. In addition, the ATC-900

provides useful real-time and historical data, reliable two-way

communications, and programming through the device’s faceplate

or Modbus RTU. The ATC-900 features digital signal processor (DSP)

technology to provide and maintain superior precision and versatility

during both programming and data access.

Standard and optional features

A variety of programmable features are available to meet a wide

array of application requirements. Individual features or feature

combinations provide the information required to tailor switches to

individual needs.

Unlike earlier controllers, the ATC-900 comes with standard features

that are ready to use, with the exception of closed transition, current

metering, and Ethernet. Another advancement is that there are four

(4) standard inputs and four (4) standard outputs that the operator

can easily program by choosing from a wide array of predefined

functions. Additional inputs and outputs can be added in groups of

four (4) up to sixteen (16) for a maximum of twenty (20) in total.

Consult section 4.2.1 for I/O specifications.