Single Phase Symmetra Drops the Load When Returning from Bypass

Issue

A single phase Symmetra UPS system turns off power to the attached load equipment when coming out of maintenance bypass.


Product Line

Symmetra Power Array
Symmetra RM
Symmetra LX


Environment

All serial numbers


Cause

When the user opens the maintenance bypass switch to take UPS out of bypass, the UPS will be in a logical, temporary bypass state while the system prepares itself to transition to an online state.

The UPS is designed to provide the best possible power to the load equipment in all modes of operation.  When in maintenance bypass, the load is physically connected to utility power and will receive whatever the utility is providing.  While the UPS is in the temporary bypass state, the logic will ensure that the load equipment is being provided quality output power.  

While in temporary bypass, if the utility power is not within a range deemed acceptable by the UPS, the output power will be turned off.


Resolution

The range of acceptable input voltage to operate in bypass This alarm state or on-battery operation are dictated by a user-configurable UPS setting. The settings are as follows:

Output Voltage Setting
The output voltage setting on single phase Symmetra systems is capable of being changed depending upon the user's requirements. This is generally changed via the PowerView display on the UPS. Best practices stipulate that the output voltage setting of the UPS should match the input voltage. This keeps the load from experiencing large voltage gradients when transferring to and from bypass operation.

Bypass Setting
The bypass setting on single phase Symmetra UPS systems determines how the UPS should operate in the event that the UPS fails and the line frequency or voltage is out of normal range. The user can select Go to Bypass or Drop Load. Going into the bypass mode of operation may subject the load to poor utility power. The user may not want to subject the load to this poor utility power and prefer to cut power to the load. This setting determines also when the UPS will be in an alarm state due to poor utility power.

If the UPS is set to Go to Bypass the system will operate online without alarm over an input voltage range of -20% to +10% of the nominal output line voltage setting. If the input voltage goes outside of this range, the system will indicate "Bypass Out of Range" and will drop the load if power module output happens to collapse (such as in the event of an overload). The UPS will also prohibit the user from commanding the system to bypass. However, the UPS can be forced to bypass when the maintenance bypass breaker is closed.

ex.
Bypass Setting: Go to Bypass
Input Voltage: Line I 132.7, Line II 132.7, LI-LII 230 VAC
Output Voltage Setting: 208 V
Message: Bypass out of range

If the UPS is set to Drop Load the system will operate online without an alarm over an input voltage range of -12% to +6% of the nominal output line voltage setting. If the input voltage goes outside of this range, the system will indicate "Bypass Out of Range" and prohibit transferring the UPS to bypass via an accessory including the PowerView. If the UPS is overloaded or the Power Modules are unable to normally power the load, the UPS will drop the load.

ex.
Bypass Setting: Drop Load
Input Voltage: Line I  127.5, Line II 127.5, LI-LII - 221 VAC
Output Voltage Setting: 208 V
Message: Bypass out of range

In the event that the UPS is already in a logical bypass state, such as when the UPS is being taken out of maintenance bypass, input voltage must be +/-25% of the nominal output line voltage or the UPS will drop the load. This is not a user configurable setting and does not depend upon the Bypass setting. The message, "Dropped load from bypass, Input/Freq Out of Range" will be displayed.  Pleaes note that this would not happen while in maintenance bypass since that is a physical connection; this would only apply if the input voltage was out of range when taking the UPS out of maintenance bypass.

ex.
Input Line Voltage: Line I 90V, Line II 90V, LI-LII 156 VAC
Output Voltage Setting: 208V
UPS Action: Drop load when returning from bypass operation

No matter which Bypass setting is selected, the UPS will always prohibit a command to bypass if the input voltage is outside the narrow range (-12% to +6%), or the UPS output is not in sync with the input. This behavior cannot be changed via configuration of the UPS. It is therefore possible, however, that with the "Go to Bypass" setting enabled, the UPS will safely go to bypass if necessary (ie due to overload) but will not allow a command to bypass.

The Bypass out of Range alarm will only be present when the UPS must drop the load in the event of an emergency.