SINAMICS S120 F30027 Precharge Fault: Root Cause and Field Repair

1. Problem Overview

A common field scenario on SINAMICS S120 chassis-format drive systems is the spontaneous appearance of fault F30027 on one converter inside a multi-drive lineup, while the remaining converters on the same transformer secondary continue to run without any fault. In the documented case, three S120 chassis units rated at 710 kW each are fed from a 690 V AC common transformer secondary. All three are started simultaneously as part of a single coordinated process. Two converters complete their DC-link precharge and run normally; the third trips with F30027 on every start attempt. This article reconstructs the diagnostic path, parameter cross-checks, and hardware inspection steps that isolate the root cause and restore the unit to service.

2. F30027 Fault Definition

F30027 — Power unit: Precharging monitoring time expired (German: Leistungsteil: Vorlade-Überwachungszeit abgelaufen).

During the precharge sequence, the Line Module closes its line contactor and charges the DC link through dedicated precharge resistors (and, on Active Line Modules, through the IGBT bridge in diode-rectifier mode for the first millisecond window). The firmware starts a timer; if the DC-link voltage has not reached the configured precharge completion threshold within the configured monitoring window, the firmware trips F30027 and opens the line contactor.

3. System Context and Suspect Topology

The reported installation has the following electrical signature:

• 3 × SINAMICS S120 chassis converters, 710 kW each
• Common 690 V AC transformer secondary feeding all three line sides
• Simultaneous start of all three drives
• Two drives complete precharge; one drive trips F30027

Three-phase apparent power per drive at 690 V line-to-line and a typical industrial power factor of 0.86 is:

kVA = sqrt(3) × V_LL × I_line / 1000

Rearranging for line current: I_line = P / (sqrt(3) × V_LL × pf) = 710 000 / (1.732 × 690 × 0.86) ≈ 692 A per drive. This is consistent with an S120 chassis frame size FX or GX (air-cooled) or the equivalent liquid-cooled block. Three units starting simultaneously draw a combined inrush through the precharge resistors; if the transformer impedance is high, the dip on the common bus can briefly drop below the threshold the failing unit's precharge monitor expects.

4. Root Cause Catalog

The SINAMICS S120 firmware lists the following eleven distinct root causes for F30027. They are presented in the order Siemens recommends they be ruled out.

1. No mains voltage present at the line terminals of the affected Line Module when the contactor close command is issued.
2. Line contactor / circuit breaker not closed — mechanical or coil failure, auxiliary contact wiring fault, missing 24 V to the contactor coil.
3. Mains voltage too low — brown-out or excessive transformer impedance under simultaneous precharge.
4. Incorrect mains voltage setting in p0210. On 690 V units, p0210 must reflect the actual line-to-line voltage. A setting that is lower than reality does not directly cause F30027, but a setting that is higher than reality shortens the effective charging time available.
5. Overheated precharge resistors due to too many precharge cycles per unit time. The resistors are sized for a finite number of cold starts per hour; back-to-back OFF/ON commands accumulate thermal energy and increase resistance (positive temperature coefficient).
6. Overheated precharge resistors because the DC-link capacitance is too high. Adding Motor Modules or capacitor modules to the DC bus increases stored energy beyond the original sizing.
7. Overheated precharge resistors because power was drawn from the DC link while Ready-to-run (r0863.0) was missing. A Motor Module may be commanded to enable before the Line Module has flagged ready, dumping energy back through the precharge path.
8. Overheated precharge resistors because the line contactor was reclosed during rapid discharge (e.g., via a Braking Module dumping energy into the DC link). The simultaneous discharge and precharge path produces destructive inrush.
9. Ground fault or short circuit in the DC-link bus. A shorted IGBT module, a contaminated busbar, or a pinched cable can prevent the DC link from ever reaching its target voltage.
10. Precharging circuit possibly faulty (chassis units only). This is the precharge contactor, precharge resistor, and the gate-trigger wiring on the IGD board.
11. Faulty power supply and/or tripped fuse in Motor Modules (Booksize units only). For chassis, verify the 24 V and 5 V rails on the CU320-2 and any active interface modules.

5. Precharge Circuit Anatomy

Locating the precharge components is the first physical step. On a chassis-format S120, the precharge path consists of:

• Line contactor — bolted to the lower busbar assembly, with a 24 V (or 110/230 V) coil. Auxiliary contact feeds back to X9 on the Line Module.
• Precharge resistors — visually identifiable as green ceramic cylinders with metallic caps on both ends, typically mounted between busbars adjacent to the IGD board (the PCB carrying the IGBT gate drivers and current sensors). The ceramic body is the resistor element; the end caps are the bolted connections.
• IGD board — the gate-driver PCB on the side of the power block, with ribbon cables to the CU320-2 control unit and to the IGBT modules.
• DC-link busbars — DCP and DCN copper bars between Line Module and downstream Motor Modules.

6. Parameter Reference

The following parameters govern precharge behavior. Always cross-check with the unit-specific parameter list in the SINAMICS S120/S150 List Manual before changing values; defaults vary between Line Module types (Basic, Smart, Active) and frame sizes.

7. Pre-Work Safety

Working inside an S120 chassis requires a documented safe-isolation procedure:

1. Open the upstream breaker / line contactor and lock it out.
2. Wait the full 5 minutes for the DC link to self-discharge below 60 V. Confirm with a CAT IV 1000 V meter on the DCP/DCN terminals.
3. Earth the DC link through the chassis earthing studs if work involves busbar removal.
4. Verify the 24 V control supply is off to the CU320-2.
5. Use only insulated tools rated to 1000 V when re-energizing.

8. Step-by-Step Diagnostic Procedure

The following sequence rules in or out each root cause in order, matching the firmware's own diagnostic logic.

Step 8.1 — Confirm mains is present and at correct level

With the breaker closed and the converter commanded to precharge, measure line-to-line voltage at the input terminals of the affected Line Module (L1, L2, L3). All three phases must be within ±10 % of 690 V. Record the result. A reading below ~620 V indicates either a transformer tap issue, a high-impedance feeder, or an instantaneous dip caused by parallel precharge.

Step 8.2 — Verify p0210 against measured voltage

Connect to the converter with STARTER or Startdrive (TIA Portal) and read p0210. Compare with the measured line-to-line voltage. If the drive is set to 400 V but is actually running on 690 V, the precharge controller will close the contactor at the wrong moment and the DC-link threshold check will fail.

Step 8.3 — Verify line contactor closure

Listen for the contactor pull-in at command time. Check the 24 V coil voltage at the contactor terminals. Read the auxiliary contact feedback on the Line Module terminal X9 — most chassis units wire the contactor's normally-open aux contact back to a digital input that must read high within p0861 milliseconds of the close command.

Step 8.4 — Read r0863.0 status

With the fault active, navigate to the Line Module's status words and read r0863.0. If it never transitions to 1, the DC link never reached the ready threshold, confirming a precharge-stage failure rather than a downstream Motor Module issue.

Step 8.5 — Inspect the precharge resistors visually

With the unit isolated, locate the green ceramic precharge resistors near the IGD board. Inspect each for:

• Black burn spots, especially at the end-cap / lead-wire interface.
• Cracked or chipped ceramic body.
• Discolored or melted terminal hardware.
• Loose mounting bolts (visible arcing at the busbar interface).

Step 8.6 — Measure each precharge resistor

Disconnect at least one end of each resistor from its busbar so the reading is not paralleled by other paths. Use a digital multimeter on the ohms range.

• Compare the measured value to the value printed (or stamped) on the resistor body.
• A reading significantly below the nominal value indicates internal shorting — replace the resistor.
• A reading open or infinite indicates a failed element — replace the resistor.
• A reading within ±10 % of nominal at room temperature is acceptable for service, but remember that resistance rises with temperature; a hot resistor will read high.

Step 8.7 — Check the DC-link bus for insulation breakdown

With the Line Module disconnected from the mains and from downstream Motor Modules, use a 1 kV insulation tester (megger) to measure between:

• DCP and PE
• DCN and PE
• DCP and DCN (must show capacitance, not a short)

Any reading below 1 MΩ indicates contamination, moisture, or a shorted IGBT module. Isolate each Motor Module in turn to localize the fault.

Step 8.8 — Inspect the IGD board and gate-driver signals

Look for burnt components, popped capacitors, or discoloration on the IGD PCB. Verify the ribbon cables to the IGBT modules are fully seated. If a chassis spare IGD board is available, swap and re-test — this is a known failure mode on aging S120 chassis units.

9. Parameter Remediation as a Diagnostic Tool

The on-site engineer extended p0857 from 6000 ms to 10 000 ms and p0861 from 100 ms to 200 ms.

What this does: p0857 is the firmware's precharge monitoring window. Lengthening it gives the DC link more time to reach its target voltage. p0861 governs the line contactor hold time after OFF1; doubling it stabilizes a chattering contactor.

What this does not do: It does not fix a failed precharge resistor, a grounded DC bus, or a weak mains supply. If the underlying root cause is a 30 % under-rated precharge resistor or a 200 mΩ contact bounce, the longer window may merely shift the fault from "every start" to "every third start." Treat the parameter change as a diagnostic aid only, then repair the hardware root cause.

10. Hardware Repair — Precharge Resistor Replacement

If the visual inspection or ohmic measurement identifies a failed precharge resistor, replacement is the definitive repair:

1. Order by the Siemens spare-part number printed on the resistor or by the Line Module's BOM. Common references for the 690 V chassis family include wirewound ceramic-encased resistors in the 5–20 Ω range, sized for the unit's peak precharge current.
2. De-isolate the converter following Section 7.
3. Remove the busbar bolts holding the resistor in place. Note the orientation and any spacer washers.
4. Fit the replacement. Torque the bolts to the value listed on the busbar label (typically 15–25 Nm for M8, depending on the hardware).
5. Re-apply the torque-mark paint or witness marks.
6. Re-insulate and re-energize following the commissioning sequence in Section 11.

When a single resistor in a parallel set has failed, Siemens generally recommends replacing the complete set. Mixed old/new resistors in a parallel precharge path unbalance the current sharing and shorten the life of the surviving units.

11. Verification and Commissioning

After repair, perform the following to confirm F30027 no longer trips:

1. With the unit in the de-energized state, perform a final insulation test (Step 8.7) and torque-check on all busbar bolts.
2. Re-apply control voltage and verify the CU320-2 boots cleanly with no pre-existing faults.
3. Re-apply line voltage. The line contactor should close within 100–200 ms and the DC link should charge to ~970 V (the rectified peak of 690 V line) within the configured p0857 window.
4. Read r0863.0 — it must transition to 1.
5. Command the drive to enable via the same sequence used in production. Run the motor through the operating speed range and monitor the DC-link voltage on the AOP30 operator panel or in STARTER trace recording.
6. Repeat the start sequence three times in succession, with at least 5 minutes between starts, to confirm the precharge resistors are not thermally marginal.

12. Preventive Measures

• Stagger the start sequence across the three drives. A 1–2 second offset between precharge commands prevents the cumulative inrush from sagging the common 690 V bus below the precharge threshold of the most marginal unit.
• Annual thermography of the precharge resistor and busbar area with the unit running. Rising thermal signature precedes resistance drift by months.
• Periodic resistor ohmmeter check during scheduled shutdowns. Track the trend; a resistor that climbs from 10 Ω to 12 Ω over three years is approaching its thermal limit even though it still passes the ±10 % test.
• Document all parameter changes in the project parameter change log so future F30027 events can be correlated with commissioning modifications.
• Verify r0863.0 sequencing before any Motor Module enable. Motor Modules must not enable before the Line Module has flagged ready.

13. SINAMICS S120 Integration Context

The SINAMICS S120 drive system is configured in the TIA Portal / SIMATIC environment via a GSD file and is supported as a PROFINET or PROFIBUS node in the S7-1500 / ET 200MP automation family. The same F30027 fault mechanism applies whether the drive is controlled by a SIMATIC S7-1500 over PROFINET, by a third-party controller over PROFIdrive, or run standalone from a CU320-2 control unit. The integration path through TIA Portal is documented in the SINAMICS S120 configuration topic of the S7-1500 manual collection. Confirm that the drive object's fault handling (p0840, p0844, p0845, r0863) is consistent with the higher-level PLC's stop sequencing; misalignment between PLC stop and Line Module precharge is a common source of repeated F30027 trips on coordinated lines.

14. Frequently Asked Questions