Siemens Masterdrive F027 Fault: Blue-Phase Current Variation Diagnostics on 2100 kW Parallel Drives

Fault F027 on a Siemens SIMOVERT MASTER DRIVES (6SE70) chassis represents an overcurrent / current-sensing asymmetry event at the AC output stage. On a 2100 kW master/slave paralleled unit, a single F027 trip that does not return after the IGD (Impulse Generator, Gating Distribution) and IVI (Isolated Voltage/Current Interface) boards are swapped, and after the blue-phase current transformer (CT) is renewed, almost always points to either (a) a partially failed IGBT module in that phase leg, (b) a contaminated gate-emitter interface, (c) a damaged fiber-optic link between the IVI and the IGD, or (d) a residual fault on the CUVC (Control Unit, Voltage-Closed-loop, the controller board) analog input path for that phase. This reference walks the field engineer through the complete diagnostic and isolation procedure, including how to interpret a >20 % two-phase imbalance, why the cumulative (Σ) current reading can still be normal, and which IGBT order numbers are correct for the 6SE70 water-cooled chassis in this rating.

1. Symptom Profile Recorded in the Field

• Drive rating: 2100 kW, 6-pulse rectifier, water-cooled MASTER DRIVES 6SE70 in master/slave (parallel) topology.
• Trigger event: spontaneous F027 during normal operation.
• First-line corrective actions already performed by site:
  • Replaced IGD board.
  • Replaced IVI board.
  • Replaced the blue-phase current transformer on the drive output.
• Drive returned to service, but the blue phase output current swings 400-500 A while the red and yellow phases remain stable.
• Phenomenon becomes prominent at speed reference ≥ 94 %. Below that threshold the variation is hidden inside normal load ripple.
• Master stack clamp-meter snapshot (single instant): L1 = 227 A, L2 = 312 A, L3 = 205 A.
• Slave stack clamp-meter snapshot (same instant): L1 = 218 A, L2 = 313 A, L3 = 214 A.
• Diagnostic parameter asymmetry on the controller: phase L1 reports -38 % to +70 %, phase L3 reports -17 % to +32 % deviation in the current-sensing channel.
• Σ (cumulative) output current remains rock-stable at 1600 A. This is the most important clue in the entire data set.
• Raising speed above 94 % caused the upstream Vacuum Circuit Breaker (VCB) to trip on a two-phase current difference > 20 % protection stage.
• Incoming mains current and voltage at the VCB were stable during the trip, ruling out grid-side disturbance.
• Drive has been running continuously at 94 % reference for five days without further trip - the fault is not yet a hard short, but is degrading.

2. What Fault F027 Actually Means on MASTER DRIVES 6SE70

F027 belongs to the overcurrent / converter-protection family. On a 6SE70 chassis, F027 is raised when the controller detects a phase-to-phase or phase-to-DC-link overcurrent condition through the IVI current-sensing path, or when the internal current imbalance between the three output phases exceeds a firmware-defined limit for a defined window. The firmware thresholds and exact condition text vary between firmware versions of the CUVC (typically V2.x through V4.x for the 6SE70 family), so always cross-check the active fault buffer against the manual for the exact firmware load on the unit.

3. The Master/Slave Topology and Why a Single Phase is Suspect

A 2100 kW MASTER DRIVES is normally delivered as two 1050 kW (or 1200 kW depending on the exact MLFB) chassis connected in parallel on the DC bus, with synchronized gating. The slave receives its gate commands optically from the master through the IGD-to-IGD fiber ring, and its IVI board reports currents back to the master's CUVC over the paralleling bus. The CTs for the blue phase on each chassis are independent. If a fault is reported on blue phase only across both stacks, the cause is almost always:

1. Common-mode fault - shared CT wiring, shared IVI fiber, or shared CUVC input.
2. An IGBT module on the same physical phase in both stacks (same color-coded gate driver) - rare but possible if both stacks came from the same manufacturing batch and the same phase-driver IGD channel is degraded.
3. Power-side asymmetry (e.g., DC-link capacitor in the blue phase dropping) - usually visible on r029 voltage diagnostics first.

4. Functional Description of the Suspect Boards

4.1 CUVC (Control Unit, Voltage-Closed-loop)

The CUVC is the central controller. It runs the closed-loop current regulator and reads the three-phase current feedback from the IVI. Fault F027 triggered at the CUVC level means the regulator saw a feedback signal that the modulator could not accept. The CUVC itself is rarely the root cause when the IVI, IGD, and CT have all been changed - but a corrupted analog front end on the CUVC remains a possibility and is the next thing to swap if the IGBTs are confirmed good.

4.2 IVI (Isolated Voltage/Current Interface)

The IVI converts the CT secondary currents into isolated analog signals and routes them to the CUVC, and it also decodes the fiber-optic commands from the IGD into gate-drive signals for the IGBT modules. The IVI was already replaced. Confirm that the new IVI is the correct variant for the firmware (the IVI has revision states; mismatched IVI/CUVC firmware combinations can also produce a current-feedback offset).

4.3 IGD (Impulse Generator, Gating Distribution)

The IGD distributes the gating pulses generated by the CUVC through fiber-optic links to each IGBT module. The IGD was already replaced. When a new IGD is installed the fiber-optic loop must be re-terminated and the receive-power trim on each leg verified - this is a frequent cause of intermittent F027 returning after the IGD swap.

4.4 CT (Current Transformer)

Output CTs on the 6SE70 chassis are toroidal, mounted around the phase busbar inside the cabinet. They are usually the LEM-type LF series, scaled to the drive rating. Replacing the CT does not, by itself, prove the new CT is correctly seated in the busbar window or that the burden resistor on the IVI matches the new CT ratio. Verify ratio (e.g., 2000:1 nominal for 6SE70 in the 1000-2000 kW class - confirm against the specific MLFB), and verify burden.

5. Reading the Diagnostic Parameters Correctly

The -38 % to +70 % reading on the current-imbalance diagnostic (typically r029 or r024 depending on firmware) for L1, and -17 % to +32 % for L3, indicates that the controller is seeing a fluctuating negative-sequence / zero-sequence current component. A healthy drive sits inside ±5 % on these readouts. Reading excursions of +70 % on a single phase, with stable cumulative current, tells you:

• The measured phase current for L1 is moving against the regulator's expected value.
• The regulator is compensating by adjusting the other two phases to hold ΣI constant.
• What you see on the clamp meter as "400-500 A swing on blue" is the regulator over-correcting the failing feedback channel.

This is consistent with a slowly failing IGBT module (gate leakage, rising Vce(sat)) or a failing gate-driver fiber.

6. Order Numbers for the 2100 kW MASTER DRIVES IGBT Stack

From the field data, the IGBT part numbers relevant to this chassis are:

7. Step-by-Step Diagnostic Procedure

1. Capture and archive the fault buffer. Read r947 (fault number), r949 (fault time), r951 (fault value list). Note operating point, speed reference, DC-link voltage, and motor torque at the moment of the trip. Save to a PMU/OP trace.
2. Re-verify the IGD fiber-optic budget. With the drive in standby, measure the optical receive power at each IGBT gate-driver board. The Siemens 6SE70 fiber system uses 660 nm plastic fiber; receive power must be inside the band stated on the IGD label (typically printed in dBm or as a green/red LED on the driver card). Any leg reading marginal or in the red zone - particularly the blue-phase leg - re-terminate that fiber.
3. Re-seat and re-burden the new blue-phase CT. Confirm the CT is fully closed around the busbar, the secondary is shorted during installation (do not open-circuit a CT with primary current flowing - lethal hazard), and the burden resistor on the IVI matches the new CT turns ratio.
4. Phase-rotation test (the diagnostic proposed in the source thread). With the drive inhibited and the line contactor open, interchange any two of the three incoming phases at the drive input terminals (L1, L2, L3). Do not interchange the motor leads. Apply a low-speed reference (≤10 %) and a no-load motor. Read the per-phase current on the display:
   • If the "blue-phase variation" follows the rotated phase identity (i.e., the swinging phase is now whichever phase you labelled as the one that previously sat at the blue terminal): the fault is upstream - CT, IVI, CUVC analog path.
   • If the swinging phase stays physically on the same power-stage leg regardless of which incoming phase you call "blue": the fault is downstream - IGBT module, gate driver, or IGD channel.
5. Static IGBT test (drive de-energised, DC link discharged, locked-out, and verified dead). Use a curve tracer or a megger/diode-test on each IGBT module. A healthy IGBT shows the body-diode forward drop (~0.4-0.7 V) on one direction and open in the other. A shorted module reads low resistance both ways. A leaking module reads < 5 MΩ in the reverse direction. Test every device in the suspected leg - including the four modules the site already replaced if any are spares still in the cabinet.
6. Gate-resistance test. With the module out of circuit, measure gate-emitter resistance. A healthy gate reads > 10 MΩ. Any reading below 1 MΩ indicates gate-oxide degradation. This is the most common failure mode that the diode test still passes - the module is not shorted, but the gate driver cannot pull it fully on, so R_DS(on) rises, conduction loss rises, current sharing between parallel devices in the leg breaks down, and the cumulative current reading stays nominally correct while the clamp meter sees a phase imbalance.
7. Check the inter-stack current sharing. On a paralleled 2100 kW unit, each chassis must contribute ~50 % of the phase current. If one chassis is contributing 70 % of the blue-phase current and the other 30 %, the CTs, IVI, and IGBTs on the under-contributing chassis are suspect. Use r026 or the paralleling diagnostic to confirm sharing.
8. Verify the VCB protection setting. A 20 % two-phase differential is a typical negative-sequence element. Confirm the pickup and the timer. If the VCB is set tighter than the drive can guarantee at high speed, nuisance trips will follow any IGBT degradation.

8. Verification After Repair

1. With the drive at standstill, run the no-load identification routine (P115 = 1 or as per firmware). All three current offsets should land inside ±2 counts of zero.
2. Run the motor uncoupled at 25 %, 50 %, 75 %, and 100 % speed. The per-phase currents on the display must match the clamp-meter readings within ±3 %.
3. Watch r029 / r024 (current-imbalance diagnostic) for 30 minutes at 100 % speed. Must stay inside ±5 %.
4. Run a load-step test: step from 50 % to 100 % torque several times. The drive must not raise F027 and the VCB must not see > 20 % phase imbalance.
5. Log the operating point, DC-link voltage, per-phase current, and the three phase temperatures on the IGBT heat-sink sensors. Walk away only when the temperature delta between the hottest and coolest phase is < 5 K at full load.

9. Decision Matrix - Where the Fault Most Likely Lives

10. Safety Considerations for the Diagnostic

11. Recommended Spare Inventory for This Drive Class

12. Summary of Recommended Action Plan

1. Schedule a controlled shutdown. Do not wait for the next F027 - the VCB trip has already proven the protection is working but the next event may damage the IGBTs.
2. During the shutdown perform the static and gate-resistance test on every IGBT module in the blue phase of both stacks.
3. Re-terminate the fiber-optic links on the IGD for the blue phase and re-verify the optical receive-power budget.
4. Re-verify the new blue-phase CT seating, ratio, and burden.
5. Run the phase-rotation test described in Section 7 step 4 to localise the fault between the power stage and the control stage.
6. Replace the failing IGBTs from the correct MLFB - do not mix 3AE0 and 4AE0.
7. Run the full verification procedure from Section 8 before returning the drive to production service.
8. Document the as-found condition and the repair for the site maintenance log - F027 events on master/slave 6SE70 chassis are recurrent when the root cause is not fully isolated.

FAQ