Resolving Sinamics S120 F30001 F30002 Faults on Thruster Drives

Overview

Simultaneous tripping of a Siemens Sinamics S120 drive on F30001 (overcurrent) and F30002 (DC link overvoltage) within the same deceleration or load-reversal event is a classic signature of a drive regenerating into a network that cannot absorb the returned energy, compounded by harmonic resonance between the supply inductance and the DC link capacitance. On a ship thruster driven by a 1050 kW, 630 V, 1160 A induction motor in V/f mode from a 1250 A Sinamics S120, both faults occurring at the same operating point (around 650 rpm on the propeller) point to a weak-supply resonance rather than a motor parameter or braking-resistor sizing issue.

This article documents the field case, the diagnostic reasoning path, the harmonic and DC link interpretation, and the corrective action that allowed precise manoeuvring of the vessel without further drive trips. The procedure applies to any large Sinamics S120 (or comparable voltage-source inverter) installation fed from a generator or weak transformer, particularly marine thrusters, winches, and pitch-control drives where load reversals are frequent.

System Description and Fault Symptoms

The reported installation is a bow or stern thruster on a working vessel. Key nameplate and configuration data from the field case:

Observed symptom on the trace: the drive was operating at approximately 650 rpm, the DC link voltage was rising while phase current was simultaneously spiking, and the inverter tripped on F30001 (overcurrent, power unit) and F30002 (DC link overvoltage) within the same 50 ms to 100 ms window. A clear periodic component with a period of approximately 45 ms (about 22 Hz, with a 60 ms / 16.67 Hz component reported on earlier traces) was visible on the current waveform, indicating a sub-synchronous oscillation rather than a pure 50/60 Hz waveform distortion.

Root Cause Analysis: Why Both Faults Trigger Together

Two distinct physical mechanisms converge to produce the F30001 + F30002 combination:

1. Regenerative Energy With Nowhere To Go

When a propeller is driven by tidal flow or by inertia of the vessel while the drive enters a deceleration or reverse sequence, the motor becomes a generator. Energy flows from the motor through the IGBT free-wheel diodes into the DC link capacitors. The DC link voltage rises at a rate determined by the regenerative current and the total bus capacitance. If the drive has no braking resistor and no Active Line Module configured for grid regeneration, the only path for that energy is the line-side rectifier, and the line-side rectifier can only pass energy to the network as long as the line voltage minus the rectifier drop is below the DC link voltage. When the DC link voltage exceeds the rectified peak of the line, the rectifier diodes reverse-bias and the energy has nowhere to go. The bus voltage then rises until F30002 trips.

The general capacitor energy equation describes the rise:

dVDC/dt = Pregen / (VDC × CDC)

where Pregen is the regenerative power in watts, VDC the instantaneous bus voltage, and CDC the total DC link capacitance of the S120 modules. A weak network with low short-circuit power raises the line impedance and reduces the ability of the line to absorb regenerated energy before the bus voltage reaches the F30002 threshold.

2. Harmonic Resonance Between Line Inductance and DC Link Capacitance

The reported 60 ms oscillation period on the trace corresponds to approximately 16.67 Hz, which is exactly one third of 50 Hz. This is the characteristic signature of a third-harmonic LC resonance between the supply inductance (line reactor, transformer leakage, generator sub-transient inductance) and the drive's DC link capacitance. The 45 ms component observed at the 650 rpm operating point (around 22 Hz) suggests a similar mechanism shifting with the line impedance as the generator load changes.

The resonant frequency of a series L-C network is:

fres = 1 / (2π × √(Lline × CDC))

When the rectifier's switching action excites this resonance, the DC link sees a high-Q oscillation superimposed on the rectified waveform. The peak of the oscillation drives the bus voltage past the F30002 threshold on every cycle, and the steep dv/dt at the resonant peak causes very high di/dt into the IGBTs on the next switching event, tripping F30001. The simultaneous occurrence of both faults is therefore not coincidence but a coupled phenomenon: the resonance provides the voltage transient, and the inverter protection provides the overcurrent response.

A general description of the DC link overvoltage mechanism in variable-frequency drives is provided in the AutomationDirect DC Bus Overvoltage White Paper, which confirms that any DC bus overvoltage fault is an instantaneous event triggered when the bus voltage exceeds the drive's threshold, regardless of whether the energy source is the rectifier, the line, or the motor acting as a generator.

3. Operating Point Amplification

At 650 rpm on a thruster, the propeller is at a low advance ratio where the hydraulic load on the blade is light and the slip is high. Any small oscillation in motor torque, combined with the low mechanical damping of water at low advance ratios, can excite torsional and electrical resonance simultaneously. This is why the drive tripped consistently at the same speed band but not at full power ahead.

Diagnostic Procedure

Before changing hardware, gather the following data. Each item is non-negotiable for a confident diagnosis.

1. Capture a high-resolution trace of phase current (at least two phases), DC link voltage, motor speed, and drive output frequency at the moment of trip. Use the S120 internal trace function (STARTER / Startdrive trace) at the maximum sample rate, typically 0.125 ms for current and 0.25 ms for the DC link. A scope trace from the line side is even more valuable.
2. Run a Fast Fourier Transform (FFT) on the line current and on the DC link ripple. Look for dominant components at 50 Hz, 150 Hz, 250 Hz (characteristic 6-pulse rectifier harmonics) and at the sub-synchronous 16.67 Hz or 22 Hz region. The presence of a strong 16.67 Hz peak confirms a third-harmonic LC resonance, not just classical supply harmonics.
3. Record the supply data: generator kVA, sub-transient reactance X''d, transformer impedance Z% (if fitted), primary and secondary voltage, and the prospective short-circuit current at the drive bus. A rule of thumb is that the supply short-circuit power should be at least 50 to 100 times the drive rating for a clean DC bus; below that, a line reactor is mandatory.
4. Verify motor parameters in p0300 to p0350: rated voltage, current, power, cos φ, efficiency, stator resistance, leakage reactance, magnetising current. V/f mode is forgiving, but a wrong rated voltage causes the drive to over-flux at low speed and saturate the motor, which can look like overcurrent.
5. Inspect the DC link: confirm the actual DC link capacitance for the S120 module fitted and verify that no capacitor section has failed. A failing capacitor lowers the bus capacitance, raises the resonant frequency, and amplifies the oscillation.
6. Check for any existing harmonic trap or filter on the bus. The 400 V harmonic filter present in this case was originally intended for a different supply configuration; it was still physically in the circuit but had been bypassed.
7. Confirm absence of a braking resistor. If regeneration is expected, a braking module (Sinamics S120 Braking Module plus Braking Resistor) or a Smart Line Module configured for regenerative operation must be present. Without one, F30002 will be the inevitable result of any net regenerating interval.

Power Quality and Network Sizing

The 690 V generator bus on a vessel is typically powered by a diesel-generator set with a sub-transient reactance in the 12% to 20% range. Cable runs are short but the generator internal impedance dominates. The single most useful diagnostic quantity is the short-circuit ratio at the drive terminals:

SCR = Isc / Idrive rated

For SCR below 20, harmonic filtering and a line reactor are mandatory for any six-pulse rectifier load above approximately 100 kW. For SCR below 10, an Active Line Module is recommended over a Basic Line Module. The 1050 kW / 1160 A thruster on a small auxiliary generator is well into the territory where SCR is below 20 and the DC link is vulnerable to commutation notches and resonance.

Three-phase apparent power sizing, for reference when a transformer is added to stiffen the supply:

S3φ = √3 × VLL × Iline / 1000 [kVA]

A 1000 kVA transformer at 690 V secondary delivers about 837 A of rated current; a 1500 kVA unit delivers about 1256 A and provides a 6% to 8% impedance that limits the fault current and damps the bus oscillation. If the motor current is single-phase rather than line current, divide the apparent power by √3 when comparing to a three-phase transformer rating.

Solution: Add Correct Line-Side Impedance

The cure in this case was not a braking resistor, not a motor parameter change, and not a drive firmware update. The cure was to re-insert the existing THD harmonic filter into the line feeding the drive, and to set the loading on the filter such that its inductance, in series with the network inductance, raised the LC resonant frequency with the DC link capacitance away from the 16.67 Hz / 22 Hz region. The filter had been previously bypassed because, on no-load or light-load conditions, it introduced a voltage rise of approximately 100 V on the 400 V bus, which made the motor controller saturate at low load.

The general rules that emerged from the field case and that apply to similar Sinamics S120 installations:

1. Match filter rating to actual load. A harmonic filter sized for a 1500 kVA load cannot be left open-circuited on a 200 kVA load without raising the bus voltage dangerously. Either de-tune the filter for the actual load, or use an Active Line Module with active harmonic compensation.
2. Add a line reactor sized to give approximately 3% to 5% impedance at the drive rated current. For 1250 A, this is a reactor in the 10 µH to 30 µH range, designed for the actual line-to-line voltage and a current rating of at least 1.5 × I_rated for thermal margin.
3. Verify the resonance has moved. After any filter or reactor change, re-capture the DC link ripple and the FFT. The new resonant peak must be at least one octave away from the rectifier switching frequency and below the drive's lowest switching-related harmonic.
4. Confirm DC link overvoltage threshold margin. For a 400 V class Sinamics S120 the F30002 threshold is typically 760 V; for a 690 V class it is typically 1500 V. Check the actual firmware version in r0018 and the relevant threshold parameter in the drive documentation. Always confirm the actual value shown in r0297 (DC link voltage) during a controlled deceleration to ensure at least 20% margin to the F30002 threshold.
5. Re-run the fault trace at the same 650 rpm operating point and confirm that the 45 ms / 22 Hz ripple is suppressed and that the drive holds through the manoeuvre.

Verification and Commissioning Checks

Sinamics S120 Fault Code Reference

All Sinamics S120 fault codes, their threshold parameters, and the recovery sequence are defined in the official Siemens Industry Online Support portal. Always cross-reference the specific firmware version of the Control Unit (CU320-2 PN or CU320-2 DP) and the power unit type (Blocksize, Chassis, or Cabinet Modules) against the latest SINAMICS S120 List Manual for the exact threshold value, the parameter that sets it, and the cause/remedy text.

Troubleshooting Matrix

Preventive Measures for Marine VFD Installations

1. Specify the line filter or Active Interface Module (AIM) at design stage. A 1050 kW Sinamics S120 on a shipboard 690 V generator bus must be ordered with the appropriate line filter or AIM for that supply. Do not retrofit a 400 V filter to a 690 V bus; the insulation class and component ratings are not equivalent.
2. Document the supply short-circuit level in the drive project. Store it as a comment in the project, and re-measure it every 5 years or after any change to the ship's electrical network (additional generator set, larger thruster, hotel load, etc.).
3. Enable trace recording on F30001 and F30002 so that the next event is captured automatically. Set p4950 to trigger the trace on the fault code.
4. Set p0290 (power unit overload reaction) and p2100 / p2101 (fault reaction configuration) deliberately. The default reactions are correct, but the warning thresholds in r0046 should be set to give at least one alarm window before a trip on a shipboard installation where maintenance windows are rare.
5. Add an Active Line Module (ALM) if regenerative energy is expected and if the ship's electrical network has the capacity to absorb it. For a 1050 kW thruster, the ALM provides four-quadrant operation, near-unity power factor, and a regulated DC link that suppresses the F30002 root cause entirely. It is more expensive than a Basic Line Module plus braking resistor, but it eliminates the overvoltage and harmonic issues at the same time.
6. Train the bridge and engine-room crew to recognise the difference between a true thruster failure (mechanical, hydraulic) and an electrical drive trip. A correctly behaving Sinamics S120 will indicate its state in plain text on the AOP30 or via the alarm buffer. A drive that simply stops without an alarm code is a different fault (line contactor, pre-charge, encoder cable).

Frequently Asked Questions