Resolving SINAMICS S120 DC Link Undervoltage Overvoltage Alarms

Problem Description

A SINAMICS S120 cabinet drive system installed as a marine shaft generator exhibits intermittent DC link undervoltage and DC link overvoltage alarm/fault events during the critical shaft-to-busbar synchronization window. The reported system topology is a CU320-2DP controlled cabinet containing an Active Line Module (ALM), Active Interface Module (AIM), Motor Module, Line Connection Module (LCM), a SIMATIC S7-300 PLC, a VSM10 Voltage Sensing Module, a TM31 Terminal Module, and a starter/scout commissioning tool. The drive is operated in PTI (Power Take-In) and PTO (Power Take-Out) mode, in which the induction machine transitions between motoring and generating states as the shaft generator synchronizes with the main engine bus.

The two failure modes observed (overvoltage and undervoltage) are not interchangeable; each points to a different energy flow condition and a different subset of components. A disciplined diagnostic approach must therefore be applied before any hardware is replaced. The field engineer must first capture the DC link voltage trend, then correlate it to the line-side voltage, the active current, the reactive current, the synchronization status, and the load angle of the active infeed.

System Topology and Power Flow in PTI/PTO Shaft Generators

Unlike land-based drives, a marine shaft generator is a true bidirectional energy converter. The propeller shaft is mechanically coupled to a synchronous or induction machine. When the ship is in transit at steady cruising speed, the shaft must be operated as a generator (PTO, Power Take-Out) to feed the main switchboard. When the ship is maneuvering, at anchor, or running in harbor, the shaft must be motor-driven (PTI, Power Take-In) from the auxiliary generators. The transition between these two states is the synchronization event that triggers the alarms in question.

The Active Line Module is the heart of the energy flow. It maintains the DC link at approximately 1.41 × V_line-to-line (rectified average) and uses an IGBT bridge to allow active power flow in either direction. When the shaft is being driven (PTI) the ALM draws active power from the main bus to maintain DC link voltage. When the shaft is driving (PTO) the ALM inverts that DC link energy back to the main bus. The DC link is therefore the energy buffer between two independent three-phase systems, and any imbalance between the two systems shows up as a DC link disturbance.

DC Link Voltage: Fundamentals and Alarm Thresholds

For a 690 V class SINAMICS S120 system, the nominal DC link voltage V_DC,nom is approximately 975 V DC. The internal firmware monitors the actual DC link voltage and triggers alarms and faults at predefined thresholds.

The exact trip levels are derived from the configured line supply voltage p0210 / p0211 and the internal scaling. For a 400 V class system, multiply the figures above by 0.58. For a 500 V class system, multiply by 0.73. For an 830 V (high-voltage marine) class, multiply by 1.20. The user is responsible for confirming p0210 and p0211 against the actual ship bus rating before any other diagnostic step.

PTI/PTO Operating Mode and Synchronization

Synchronization is the most stressful operating event in a shaft generator. The drive must match four parameters of the main bus before closing the LCM contactor:

1. Voltage magnitude (ΔV typically < 5 %)
2. Frequency (Δf typically < 0.2 Hz)
3. Phase angle (Δφ typically < 10°)
4. Phase sequence (always matched)

The VSM10 Voltage Sensing Module samples the main bus voltage and the shaft machine terminal voltage and feeds both to the CU320-2DP. The control unit runs the internal synchronization function (typically r5400–r5499, parameters p5400–p5499) and only commands LCM closure when all four conditions are within tolerance. During the closed-loop approach, the drive temporarily operates in regenerative mode and then in motoring mode as it pulls the shaft into phase. This transition is exactly the moment when the ALM is asked to absorb or deliver large transient currents while the DC link capacitance is the only energy buffer.

The diagram above is a typical waveform when the drive enters motoring mode abruptly. The DC link voltage sags as the active current steps up, and the Vdc_min controller responds. If the controller cannot hold the DC link above the warning threshold, A30003 will fire and may escalate to F30003 if the sags become severe. Conversely, when a load is suddenly dropped, regenerative energy momentarily has nowhere to go, the DC link rises, and the Vdc_max controller commands the ALM to absorb the energy. If the Vdc_max controller is disabled, mis-configured, or the chopper/braking circuit is absent, F30002 will trip.

Diagnostic r-Parameters for DC Link Analysis

Open SINAMICS STARTER / Startdrive Commissioning Tool online, connect to the CU320-2DP, navigate to the drive's expert list, and record the following parameters with the trace/function generator at 100 ms or faster sampling. All r-parameters are read-only and updated in real time by the firmware.

Configure the trace buffer to capture at least 10 seconds around each synchronization event. Use a pre-trigger of 60 % so the trace starts recording before the LCM closure command. Save the trace file (.trc) and the parameter snapshot (.dnxpar) for later analysis. The trace will show whether the disturbance is on the line side (r0222, r0072), the DC bus side (r0070, r0232), or the load side (r0080, r0073).

Root Cause Analysis: DC Link Undervoltage

Undervoltage is a net energy deficit. The DC link is being drained faster than the ALM can refill it. In a PTI/PTO shaft generator this is the more common of the two alarms and is usually load-driven. Investigate the following causes in order:

Root Cause Analysis: DC Link Overvoltage

Overvoltage is a net energy surplus. Energy is being pushed into the DC link faster than the ALM can extract it. In PTO mode, this typically happens when the main engine suddenly accelerates (e.g. during a sea state change) and the shaft machine momentarily acts as an un-loaded generator. In PTI mode, an overvoltage on a load drop is unusual but possible if the Vdc_max controller is misconfigured.

Step-by-Step Diagnostic Procedure

Apply the following sequence before opening the cabinet. Each step builds on the previous one and prevents the most common false diagnoses.

1. Verify the drive is in PTI/PTO mode. Read r5400 area or the operating mode display on the AOP30. Confirm that the active operating mode is p5401 (configuration) and that the function is enabled.
2. Capture a DC link trace. In STARTER, configure a 4-channel trace: r0070, r0072, r0073, r0080 at 100 ms sample time. Record 30 seconds of normal operation and then trigger a controlled synchronization. Save the trace.
3. Capture a line-side trace. Add r0222, r0223, and r3405 to the trace buffer. If the line frequency or voltage moves, the cause is on the supply side.
4. Capture the synchronization status. Read r5499 (synchronization status word). Confirm that the LCM closure command coincides with the synchronization OK bit. A premature closure will slam the DC link.
5. Read the fault history. Open r0947. The 64-event ring buffer will reveal whether the same fault (F30002 or F30003) is recurring, or whether different faults alternate.
6. Inspect p0210 / p0211. Compare to the actual ship bus. A common commissioning error is leaving p0210 at the factory default of 400 V when the bus is 690 V, which desensitizes the undervoltage trip.
7. Inspect p1250 and p1245. The Vdc_max and Vdc_min controllers must be enabled. If either is disabled, the drive has no active defense against a DC link excursion.
8. Measure the actual line voltage with a calibrated meter at the cabinet incoming terminals. Do not trust the VSM10 reading; it can be skewed by VT ratio errors.
9. Measure the actual DC link voltage with a high-voltage probe and oscilloscope at the DC bus test points on the cabinet. Compare to r0070 reported by the drive. A discrepancy of more than 5 % points to a measurement path issue.
10. Perform a controlled synchronization under no-load sea conditions. Repeat the trace. If the fault disappears, the root cause is a load-induced transient and the cure is operational (sea state, sequencing) plus controller tuning.

Fault and Alarm Code Reference

The following codes are the ones most likely to appear on a shaft generator application. The CU320-2DP displays the code in plain text on the AOP30 and stores the value in r0947 (fault) and r2122 (alarm). Each entry shows the firmware default reaction; the field engineer must verify the actual reaction in the project file because shipyards frequently customize the responses.

For the full SINAMICS S120 fault list refer to the SINAMICS S120/S150 List Manual. The List Manual is the canonical reference for fault value r0949, alarm value r2124, and the cause-and-remedy text for every code; always cross-check against the firmware version actually installed on the CU320-2DP.

Controller Tuning: Vdc_max and Vdc_min

If the trace shows a brief excursion that the Vdc controller could absorb if it were enabled or faster, retune the relevant parameters. The defaults are conservative and may not keep up with the synchronization transient on a ship.

Verification and Acceptance Test

After the corrective action, perform a full acceptance test. The test is a contract deliverable in most shipyards and the only way to prove that the fault is closed.

1. Perform three no-load synchronizations in calm water. Verify no A30002, A30003, F30002, or F30003 are recorded.
2. Perform three loaded synchronizations at 25 %, 50 %, and 75 % of nominal shaft power. Verify the DC link trace stays within ±10 % of nominal.
3. Trip the main bus (simulated by opening the LCM contactor). Verify the drive goes to OFF2 within 200 ms and that no F30002 is raised from the regen of the spinning shaft.
4. Drop 50 % load step in PTO mode. Verify the Vdc_max controller brings the DC link back to nominal within 500 ms.
5. Apply 50 % load step in PTI mode. Verify the Vdc_min controller brings the DC link back to nominal within 500 ms.
6. Capture the final trace and parameter snapshot. Save with the project's documentation set.

Preventive Maintenance Recommendations

• Clean the cabinet air filters every 6 months. The ALM is a regenerative inverter and its IGBT modules are most reliable with low inlet-air temperature; every 10 °C rise roughly halves the electrolytic capacitor life.
• Measure the DC link capacitance annually with a portable capacitance meter. Replace the capacitor bank when the value falls below 80 % of nameplate.
• Inspect DRIVE-CLiQ connectors for proper seating and strain relief at every class survey. A loose connector will intermittently drop ALM feedback and cause unpredictable overvoltage trips.
• Check the VSM10 fuse and VT secondary wiring at every dry-dock. A blown VT fuse on the bus side makes the drive think the bus is dead and may close the LCM contactor at the wrong moment.
• Update the S120 firmware to the latest service pack released by Siemens for the CU320-2DP. Firmware updates often include improved Vdc controller dynamics and ship-specific fault suppression. Always back up the project before a firmware change and validate the change on shore power first.