Restore the real-power and reactive-power sharing controls before reconnecting the 150 kVA unit to the vital-load busbar. A machine that accepts load, pulls most or all of it, then unloads and accelerates is showing unstable or missing load-sharing feedback—not a simple shortage of UPS capacity.
Control Approaches and Recommendation
Two separate controls govern parallel operation. Treating them as one loop leads to incorrect adjustments.
| Control function | Primary quantity | Typical fault indication | Required check |
|---|---|---|---|
| Excitation and voltage sharing | Reactive power and terminal voltage | High circulating current or unequal reactive load | Check voltage sensing, current-transformer feedback, regulator configuration, and the reactive-share interconnection between machines. |
| Prime-mover or real-power sharing | Real power and frequency | One unit pulls the connected load, sheds it, or accelerates after unloading | Check speed feedback, torque command, governor or drive control, droop settings, and the real-power share signal required by the installed architecture. |
The observed sequence points first to the real-power loop because the machine captures the load and subsequently runs at high frequency. Check the reactive-sharing circuit in parallel because an open, reversed, or misconfigured regulator interconnection can also create severe circulating current and trigger a protective trip. Before anything else, identify whether the rotary unit uses an electric-motor drive or a combustion-engine prime mover. An engine-driven machine requires governor or equivalent real-power load-sharing control; a motor-driven machine requires the corresponding torque or power-sharing function in its drive and rotary control system.
Fault Mechanism
Before breaker closure, the incoming source must match the live bus in phase sequence, voltage, frequency, and phase angle within the synchronizer's configured acceptance limits. Synchronism permits connection, but it does not guarantee stable sharing after connection.
Once paralleled, input torque determines real-power contribution. Increasing torque makes a synchronous source export more real power; reducing torque makes it export less. If the real-power feedback has the wrong polarity, is missing, or is biased, the controller can command the 150 kVA machine to assume most of the bus load. The resulting correction may drive its output down until protection opens the connection.
Excitation determines reactive-power contribution. Incorrect regulator sharing, voltage-sensing errors, or reversed current-transformer feedback can make two machines exchange reactive current even when the useful bus load has not changed. Current can therefore rise without a proportional increase in real power.
High frequency after load rejection is a diagnostic clue. Electrical load has disappeared, but the mechanical or drive input has not reduced at the required rate, or the speed feedback is incorrect. Treat the overspeed response as part of the real-power control investigation rather than automatically blaming the synchronizer.
Commissioning Prerequisites
- Separate synchronization from load sharing. Confirm which device authorizes breaker closure and which controllers regulate real and reactive power. Do not move on until the signal path is identified from sensing input to final torque and excitation command.
- Retrieve commissioned values. Record voltage, frequency, synchronizer limits, regulator mode, droop or load-share selections, controller gains, protection thresholds, and transformer-ratio entries from approved records or the applicable manufacturer documentation. Do not substitute assumed values.
- Confirm stable standalone operation. Run the affected machine disconnected from the vital-load busbar. Its voltage and frequency must settle at the site setpoints without hunting. Instability while isolated must be corrected before any parallel test.
- Check the healthy machine as the reference. Compare modes, selectors, wiring terminations, sensing values, and controller indications with the identical 150 kVA machine. Equal ratings do not prove equal configuration.
- Prepare synchronized trending. Capture breaker status, bus and incoming frequency, terminal voltage, real power, reactive power, current, speed, torque or governor command, and active protection indications. The reported load acceptance lasts about one second, so ordinary panel observation may miss the initiating signal.
Corrective Procedure
- Validate measurement polarity. With the unit isolated under an approved test condition, verify voltage-sensing phase order and each current transformer's phase assignment, ratio entry, and polarity. Never open an energized current-transformer secondary. A reversed power measurement makes a stable sharing loop act as positive feedback.
- Restore reactive-load sharing. Inspect the interconnection between the voltage regulators for continuity, correct terminals, shielding, and configured operating mode. Confirm that increasing excitation produces the expected reactive-power direction on the controller display.
- Restore real-power sharing. For an engine-driven unit, check governor mode, speed feedback, actuator direction, droop or isochronous selection, and the required load-share connection. For an electric-motor-driven unit, check the drive or rotary controller's torque command, real-power feedback, limiting states, and communication or hardwired sharing path.
- Remove stale or forced commands. Check for manual bias, maintenance overrides, fixed output commands, failed analog inputs, and retained controller modes left from previous testing. Confirm every permissive and mode indication matches the intended parallel configuration.
- Return tuning to the commissioned baseline. Correct wiring, ratios, modes, and feedback direction before changing gains. If tuning remains necessary, change one setting at a time using the manufacturer's procedure and record the original value.
- Perform a controlled parallel test. Match the live bus, wait for the synchronizer's valid condition, and close the breaker at the minimum operating condition allowed by the plant procedure. Stop the test if real power immediately ramps toward the full connected load, reactive current circulates sharply, frequency departs from the bus value, or protection starts a trip sequence.
Verification Sequence
- Repeat the standalone test and confirm stable voltage and frequency at the approved setpoints.
- Confirm correct phase sequence and verify that voltage, frequency, and phase angle enter the configured synchronizer window before breaker closure.
- Close onto the bus under controlled conditions and confirm the unit remains connected beyond the previous one-second failure interval.
- Add load in approved increments. At each step, compare real power, reactive power, current, frequency, and controller commands on both machines. Real and reactive load should divide according to the configured sharing scheme rather than transferring abruptly to the affected unit.
- Reduce load and observe the response. Frequency must remain controlled as electrical torque falls, with no load rejection or uncontrolled acceleration.
- Review the event log and captured trend for the first signal that changed. Accept the repair only when no protection, limiter, feedback-quality alarm, or breaker command precedes a stable load-sharing run.
Recurring Diagnostic Errors
| Error | Why it fails | Correct practice |
|---|---|---|
| Adjusting only the voltage regulator | The regulator primarily controls voltage and reactive power, not real-power division. | Diagnose the real-power and reactive-power loops separately. |
| Changing gains before checking wiring | Gain changes cannot correct reversed polarity, an open share line, or an incorrect ratio. | Prove every sensing and command direction first. |
| Using total current as the load-sharing measure | Current includes both real and reactive components. | Trend real power and reactive power independently. |
| Assuming successful synchronization proves readiness | The synchronizer controls breaker-closing conditions, while other loops govern operation after closure. | Verify post-closure torque, excitation, and feedback response. |
| Treating high frequency as the initiating fault | Frequency can rise after the machine drops electrical load while input power remains high. | Use time-aligned trends to identify whether load rejection, speed feedback, or torque command changed first. |
FAQ
How do I stop a HITEC rotary UPS from taking the entire bus load?
Check real-power feedback polarity, speed feedback, torque or governor command, operating mode, and the load-share connection. Correct those inputs before changing controller gains.
How do I tell a real-power sharing fault from a reactive-power fault?
Compare real power and reactive power rather than total current. Dominant real-power pickup points to torque or speed control; high current with disproportionate reactive power points to excitation, regulator sharing, or current-transformer feedback.
How do I test the voltage-regulator sharing connection?
With the equipment in an approved isolated test state, verify continuity, terminal assignment, configured regulator mode, current-transformer polarity, and the indicated reactive-power direction. Reconnect to the bus only after the two regulators show compatible sharing modes.
How do I diagnose a rotary UPS that takes load for one second and drops it?
Trend breaker status, real and reactive power, current, frequency, speed, torque or governor command, excitation command, and protection states from before closure through the trip. The first changing signal identifies whether control instability or protection initiated the unloading.
How do I verify the HITEC rotary UPS repair?
Synchronize within the configured acceptance window, close under controlled conditions, add and remove load in approved increments, and compare both machines' real and reactive power. The final verification is stable sharing with controlled frequency, no abrupt load transfer, and no protection or breaker command in the event record.