The UPS750 continually monitors internal systems and the incoming utility power. The UPS will automatically change between modes as required. The changes are accomplished without operator intervention. The detection and switching logic inside the UPS ensures that the changes in operating mode are automatic and transparent to the critical load. The operating modes are listed below:
- On-line mode
- Discharge mode
- Bypass mode
- Automatic voltage regulation (AVR) mode
- Shutdown mode
A description of each of the UPS750 flywheel power unit (FPU) subassemblies follows:
The line static switch in the SIO is a continuous-duty rated, phase-controlled, full-wave, three-phase, line-commutated electronic switch using Silicon Controlled Rectifiers (SCRs) as switching devices.
The line static switch turns off within one-half cycle of detecting an input voltage or frequency outside of preset limits.
Phasing of the switch during normal operation inhibits reverse power flow from the output of the switch to the UPS input.
Connection to the AC input through the switch is initiated when the phase difference between the input and output is within a few degrees. The connection is initiated then to reduce inrush currents when reconnecting to input power after a discharge.
Flywheel Energy Storage Subsystem (FES)
The FES supplies ride-through energy for the UPS system is supplied by a FES in the FPU. The FES stores energy in the inertia of a rotating mass. The energy is immediately accessible, to support the critical load, without interruption.
The FES consists of a rotating flywheel, with a stator, contained in a single housing all kept under vacuum.
During normal operation, about 75 percent of the FES rotor weight is relieved on the flywheel bearings. The weight relief happens through magnetic suspension, provided by field coils mounted inside the FES housing.
Normally, the flywheel rotor spins at 7,700 RPM. The complete FES can withstand the following fault conditions:
- Sudden loss of vacuum
- AC short-circuits
- DC short-circuits
- Catastrophic bearing failure
- Over speed testing to 120% of rated speed
The 6 IGBT inverter and LC filter is directly connected to the load through the output contactor.
The utility inverter and LC filter continuously provides a regulated low THD, high quality, 3-phase voltage source for the critical loads.
When the UPS is online, the utility inverter keeps both the UPS output voltage and DC bus voltage at constant levels.
There is a 6 IGBT flywheel inverter for each 3-phase armature.
The flywheel inverters put power in the FES from the DC bus when the UPS is online to motor the flywheel and keep it at full speed.
The flywheel inverters take power out of the FES when the UPS is in discharge to regulate the DC bus at a constant voltage.
The static bypass switch consists of two SCRs per phase, arranged in a back-to-back configuration.
The UPS automatically transfers to bypass. Transfers to and from bypass are synchronized and operate without interruption of load voltage or current.
If input power is available but synchronization has not been achieved when the FES becomes fully discharged, the UPS may affect a break-before-make transfer.
Manual transfers to bypass commanded from the UPS control panel by turning the key switch to the bypass position. Turning the key switch from the bypass position to the online position will manually return the UPS from bypass to normal operation.
The operator can command the system to enter the online or bypass modes via the key switch on the system cabinet of an UPS system. When the EPO button on the front of the system is pressed, the system removes the output power, and drops the load.
When the system is in the online mode, both the input and output contactors are closed, and the bypass contactor is OFF. Power flows through the system via the closed static switch that is in the ON position and through the line inductor. The utility inverter is also ON at this time.
There are four conditions to the on-line mode:
On Line - When operating in the online mode, the system is ready to protect the load. When online, output voltage is being regulated to within +/-2% of the nominal set-point. Online mode is the standard operating mode for the system. When the system is in this mode, the load is protected and the system can discharge, to support the load.
Motoring - The UPS is providing a constant voltage to the load. The UPS is drawing power from the input to motor the flywheels up to charging speed (4000 rpm). The UPS is voltage regulating and, when the flywheels reach 4000 rpm, the UPS can discharge.
Charging - The system enters this state when the flywheel reaches 4000 rpm and there is enough stored energy in the flywheels to support the load. The flywheels have not yet reached standby speed. The system can sustain discharge in this state.
Standby - When all system flywheels reach nominal speed, the system is in the online (standby) state. The UPS is regulating voltage and ready to discharge the flywheels to protect the load. The flywheel draws about 1 kW from the input source to maintain standby speed.
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| Illustration 1 | g03639625 |
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System online mode power flow - standby and motoring states | |
The system enters this state when the unit is supplying power to the load. The system automatically enters this state when the input voltage is disqualified. Refer to Illustration 2 that shows the UPS power flow when in the discharging state.
The input contactor is open, the static switch is OFF, and power is flowing out of the FPU. The flywheel inverters regulate the voltage on the DC bus. The utility inverter converts the DC bus voltage to 3-phase AC voltage, which is supplied to the load.
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| Illustration 2 | g03640604 |
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System power flow during discharge | |
There are four conditions that initiate the discharge mode:
Discharge - The UPS has disconnected from the input and is delivering power to the load from the flywheel.
Phase Match - The UPS is still delivering power to the load, has qualified the input voltage, and will match output (frequency and phase) to input source.
Walk-in - Once a phase match is achieved, the UPS connects to the source and transfers the load from the flywheel to the source. At the end of walk-in, the UPS enters the online (motoring) state.
Self-Discharge (Service Mode Only) - The self-discharge state causes the flywheel to reduce speed to zero (0) RPM. The state is used to prepare the system to be moved, or to secure the system for maintenance.
Note: The online-self discharge mode can be entered once in a 24-hour time period. The system will not go into this state automatically.
When the UPS is "Online" and the input voltage is disqualified, the UPS will disconnect from the source and support the load with the flywheel energy.
There are four reasons for the flywheel to go into discharge mode:
RMS Voltage High or Low - If RMS voltage goes high or low by a certain percentage, as defined by user specified adjustable operating parameters, the UPS discharges. A message appears in the log-file.
Fast Voltage Transient - The UPS constantly monitors voltage coming from the utility once every 52 microseconds, which makes this one the most common reason for discharges. If voltage spikes above or below a certain user specified range, the flywheel discharges to correct the voltage. A message appears in the log-file.
Frequency High or Low - The UPS is constantly monitoring the frequency of the utility power. If the UPS determines the frequency is out of user specified range, the flywheel discharges to correct the frequency. A message appears in the log-file.
DC Bus Voltage High or Low - The utility inverter keeps AC voltage at 480 VAC (depending on location) and the DC bus at 750 VDC. When an input disturbance or a large step load occurs, the phase load changes, and the DC bus goes up or down.
Exhaustive Discharge Configurations
System Auto Power Load Configuration
"Auto Power Load" is a software configuration operating parameter. The "Auto Power Load" parameter is used to determine UPS behavior when power is first applied to the input. The parameter is used following an exhaustive discharge of stored electrical energy.
Note: Auto Load and Start configurations can only be changed by qualified service personnel.
| Auto Load and Start Settings | System Operation
(Key switch in "Online" position) |
| 0 | UPS remains in Shutdown Mode when power is first applied to the input. The load will not be automatically powered. Manual operation is required to power the load and bring the UPS online. |
| 1 (Default) | UPS automatically goes into Bypass Mode, then goes to On-line Mode when power is first applied to the input. Manual operation is not required to bring the UPS online. |
The bypass mode directly connects the incoming utility to the system load.
Note: The load is not protected when the system is in bypass mode and can be affected by a disruption of the incoming power.
The bypass circuit also provides a path for power directly from an alternate AC source.
The system enters bypass mode by four methods:
- At system start-up by an operator command via the display
- From maintenance monitoring software
- Failure to recover from a fault
- The key switch (manual)
Repeated errors that cause the system to move between the bypass mode and online mode can lock the system into the bypass mode. When the system is locked in the bypass mode, the user must change the mode of the system via the key switch or UPS View.
The UPS output contactor isolates the UPS750 module outputs. The system bypass circuit breaker works in parallel with the optional static bypass switch. The "Static Bypass" switch is a solid-state device that can instantaneously connect the alternate AC source to the load.
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| Illustration 3 | g03640658 |
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Bypass mode power flow | |
Automatic Voltage Regulation Mode
The AVR mode is an operating mode where output voltage is regulated to the nominal set point. The power factor and harmonic benefits of the UPS are also still present in this mode. However, the load is not protected when the system is in AVR mode.
The AVR mode can be entered by one of the following events:
- System start-up
- Flywheel errors
- Auto start
- Charging
- Verify signals
System start-up - When the UPS is transitioning from bypass to online, the UPS transfers from bypass to AVR mode, and then begins to motor (or preheat) the flywheels. The UPS will remain in AVR mode until there are enough flywheels above 4000 rpm to support the load connected to the UPS output.
Flywheel errors - There is not enough flywheel power available to support the load connected to the UPS output.
Auto Start - The system remains in AVR mode until the system is charged and can sustain discharge. During the AVR auto start state, the flywheel must achieve 45 rpm before changing to the AVR charging state.
Charging - The system enters the charging state when starting to charge the flywheel. When the flywheel has achieved nominal speed, the system enters the online stand-by state. Normally, the system enters the online charging state when the flywheel reaches 4000 rpm.
Verify Signals - In this state, the system verifies that the correct system telemetry is present. The verify signal state is used when starting the system and during recovery from flywheel errors.
High voltage may be present internally in the UPS for no less than five (5) minutes after an emergency power off (EPO) has occurred. A qualified electrical professional must restart the system after an EPO event.
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Personal injury or death can result from high voltage. When power generation equipment must be in operation to make tests and/or adjustments, high voltage and current are present. Improper test equipment can fail and present a high voltage shock hazard to its user. Make sure the testing equipment is designed for and correctly operated for high voltage and current tests being made. |
Note: Shutdown mode opens the input circuit breaker and removes power from the load.
In shutdown mode, there is no power flow through the system. The system is shut down, and no power is being supplied to the load. All contactors are open, and both the static switch, and the utility inverter are OFF.
Shutdown mode is entered into when an EPO event has occurred.
Note: An EPO can occur when you press the EPO button on the front of the controls cabinet, or a remote EPO button, if installed.
Shutdown mode can also be entered into when there are internal or external conditions that could lead to system failure.
Operator intervention is required to put the system back online, if the system was shut down. If an EPO has occurred, there is a reset switch located behind the door on the SIO cabinet that must be moved to the "I" position.
The EPO button is on the front of the system input/output SIO cabinet. Pressing the EPO button transitions the UPS to the shutdown mode.