Resolving -32768 Power Values from SIPROTEC 7SJ6021 via Profibus

Problem Summary

An SIPROTEC 7SJ6021 (MLFB 7SJ60211-4EB90-1FA0/CC) is connected to a SIMATIC S7-400 CPU over PROFIBUS-DP using the SIMATIC NET protocol profile. The cyclic measurement image is read successfully — current (I), voltage, frequency, and the binary status words are present and reflect the primary values correctly. However, the power measurands (active power P, reactive power Q, apparent power S, and power factor cos φ) are returned as the constant decimal value -32768 (hex 0x8000) regardless of primary loading, while the diagnostic buffer of the relay and the I-slave status word report no fault.

Hardware Architecture of the 7SJ60 Family

The 7SJ60 series is part of the SIPROTEC 3 generation of numerical protection relays, used for overcurrent, earth-fault, and basic load-side protection. Unlike the later SIPROTEC 4 (7SJ62/63/64) units, the 7SJ60 does not include analog voltage measurement inputs as a standard interface; voltage is only handled internally as a derived quantity for synchronism-check options, not for power calculation. A converter module that accepts 100 V / 110 V / 230 V VT secondary cannot be retrofitted on the 7SJ6021 board — the input stage and DSP firmware revision both lack the channel.

This structural difference is the key constraint. The 7SJ60 is fundamentally a current-only device from a measurement standpoint: it computes I_L1, I_L2, I_L3, I_E, derived 3I₀, and binary annunciators, but not P, Q, S, or cos φ.

MLFB Position 7 Decoding

The Siemens MLFB (Maschinenlesbare Fabrikatebezeichnung) encodes the analog input configuration in the 7th digit. For the 7SJ602:

For MLFB 7SJ60211-4EB90-1FA0/CC, digit 7 = 1, confirming a current-only configuration. The voltage terminals that exist on the terminal block (X3/X4 on early units) are present on the layout but are not bonded to any ADC channel; the firmware V3.5 (and V3.6) simply does not poll them. The SIPROTEC 4 generation (7SJ62/63/64) fixed-voltage emulation feature — where the user enters a fixed U_n value and the relay computes P/Q for SCADA display — does not exist in the 7SJ60 firmware tree.

PROFIBUS-DP Data Map and the -32768 Sentinel

The 7SJ602 supports the standard SIPROTEC 3 PROFIBUS-DP slave profile (GSD file SIEM8070.GSD, or device-specific SIExxxx.GSD). Two protocol variants exist:

• Standard mapping (V3.1+): 32-byte or 64-byte cyclic input image; measurement slots packed as 16-bit signed integers scaled by the configured primary nominal.
• Extended / DNP3-style mapping: The 7SJ602 Profibus manual specifies that energy and power measurands begin at byte offset 8 from the configured base address of the established cyclic I/O image and each measurand occupies 4 bytes (signed DInt, little-endian, scaled to 0.1 units per bit).

The measurand slot allocation is:

The S7-400 receives these as raw DInt values in the input process image (E-area / PIW) or in a data block filled via SFC14 "DPRD_DAT". When the conversion is limited to a 16-bit window by the GSD slot configuration, 0x8000 = -32768 is the value you observe. This is the expected, normal return for any 7SJ6021 — it is not a Profibus fault, not a GSD mismatch, and not a wiring problem.

Root Cause Confirmation

Three checks confirm that the -32768 is hardware-driven, not configuration-driven:

1. MLFB verification. Read the printed MLFB on the device front plate. Digit 7 = 1 means current-only. There is no firmware or parameter setting that converts this to a power-capable device.
2. DIGSI parameter tree. In DIGSI 4.x, navigate to Settings → Measurement → Power. The power-related parameters (e.g. address 0201 "Active Power Sign", 0202 "Reactive Power Sign", 0220 "Power Factor cos φ") are not present in the offline parameter set for MLFB digit 7 = 1. They only appear for digit 7 = 2 or 6.
3. Front-panel HMI. On the relay LCD, navigate to Measured Values → Power. The display will either show "—" or the menu entry will be absent. This mirrors what is being broadcast on PROFIBUS.

Solution A — External Voltage Transducer (Simeas P)

When the existing 7SJ6021 protection function must be retained, the standard mitigation is to add an external voltage and power transducer on the same primary VT secondary, then map its PROFIBUS output into the S7-400 alongside the 7SJ6021. The Siemens product for this is the SIMEAS P digital power transducer.

SIMEAS P key parameters

• Inputs: 3 × voltage (100/110/230/400 V) and 3 × current (1 A or 5 A), isolated
• Computed quantities: P, Q, S, cos φ, frequency, energy (kWh, kvarh), harmonics to the 40th
• Outputs: PROFIBUS-DP slave (DPV0/DPV1), Modbus RTU/TCP, IEC 61850, 4–20 mA, Ethernet
• Accuracy class: 0.2 / 0.5 selectable per IEC 60688
• Update rate: 200 ms typical for the PROFIBUS process image
• Power supply: 24–250 V DC / 100–230 V AC wide-range

The SIMEAS P GSD is loaded as a second DP slave on the same PROFIBUS segment (different station address, e.g. relay = 3, transducer = 17). The S7-400 configures both slaves in HW Config with the matching GSD; the SIMEAS P occupies its own 32-byte input slot. Power values from the transducer are read in parallel with current values from the 7SJ6021.

Solution B — Migration to SIPROTEC 4 (7SJ62 / 7SJ63 / 7SJ64)

For new installations or full retrofit, the modern equivalent of the 7SJ6021 is the 7SJ62 (basic), 7SJ63 (extended with directional elements and synchrocheck), or 7SJ64 (full motor + transformer differential). The 7SJ62/63/64 family natively supports 3 voltage inputs and computes P/Q/S/cos φ without external hardware.

7SJ62/63/64 voltage / power features (firmware V4.6)

• VT inputs: 4 (UL1, UL2, UL3, UN) — 100/110/230/400 V range
• Power calculation: P, Q, S, cos φ, 3I₀, 3U₀ computed in DSP at the protection-cycle rate (typically every 20 samples / 50 Hz cycle, decimated to 200 ms SCADA update)
• Fixed voltage emulation mode: When the VTs are physically absent, the user can configure a fixed U_primary value in the parameter tree (address 0211 "Voltage transformer connection", with sub-parameter 0211 "Fixed U_L1L2 = 110 kV") and the relay will compute the directional element and power value from I × U_fixed. This is a SIPROTEC 4 feature only — it does not exist on the 7SJ60 firmware.
• PROFIBUS-DP: Identical slot structure to 7SJ60, with the same "-32768 / 0x80000000" sentinel for unavailable values, but all four power measurands are populated when the firmware detects valid voltage samples.

Migration is typically a panel-cutout drop-in (same housing), but the GSD must be re-imported (SIExxxx.GSD for the new model), the DIGSI 4 project must be recreated, and the S7-400 HW Config must be re-compiled because the device's PROFIBUS ident number changes.

Solution C — SCADA-Side Compensation (Last Resort)

If neither hardware change is acceptable, the S7-400 can synthesise apparent power from current and a fixed U_LL value entered in the controller. This is a display-only approximation and must not be used for protection, billing, or tariff metering:

This approach gives operators a non-zero display on WinCC / PCS 7 faceplates but introduces a fixed voltage assumption — the value will not track VT tap changes, source transfers, or capacitor-bank switching.

S7-400 PROFIBUS Readback Example (SCL / STL)

Regardless of which solution is selected, the S7-400 reads cyclic I/O from the DP slave via SFC14 / SFC15. The following STL excerpt reads 32 bytes from the 7SJ6021 base address and unpacks the four power measurands. Replace the base address with the configured slot in HW Config:

// S7-400 STL — read 7SJ6021 power measurands
// DB100 = power data block, 32 bytes from SFC14
CALL SFC14
      LADDR  := W#16#100          // DP slave I-base address (config-dependent)
      RET_VAL:= MW 200            // return code
      RECORD := P#DB100.DBX0.0 BYTE 32

// Check sentinel — 0x80000000 = -2147483648 in DInt
L     DB100.DBD   8               // byte offset 8  = Active Power P (DInt)
L     L#-2147483648              // 0x80000000
==I
JC    pwr_invalid

// Valid — scale: 0.01 MW per LSB
L     DB100.DBD   8
ITD
DTR
L     1.000000e+004              // 0.01 MW → W
*R
T     DB110.DBD   0              // P in watts

L     DB100.DBD  12              // Reactive Q
ITD
DTR
L     1.000000e+004
*R
T     DB110.DBD   4              // Q in var

L     DB100.DBD  16              // Apparent S
ITD
DTR
L     1.000000e+004
*R
T     DB110.DBD   8              // S in VA

L     DB100.DBD  20              // cos phi
ITD
DTR
L     1.000000e-003              // 0.001 / LSB
*R
T     DB110.DBD  12              // cos phi (signed)

pwr_invalid: NOP 0

Verification Procedure

1. Check slot byte-0 of the input image with the PROFIBUS diagnostic repeater or STEP7 "Monitor/Modify". Bytes 0–7 should be the three phase currents and a padding word; bytes 8–23 should now contain non-zero DInt values for P/Q/S/cos φ.
2. Inject a known load. Apply a balanced 3-phase test source of e.g. 100 A × 400 V × √3 = 69.3 kVA at cos φ = 0.90. The P reading should be 62.4 kW ± 1 %.
3. Cross-check the front-panel HMI of the relay (if migrated to 7SJ62) or the SIMEAS P display (if using Solution A). The PROFIBUS value and the local display must agree within one decimal place.
4. WinCC / PCS 7 faceplate test. Force a tag update and confirm that the power faceplate no longer displays the yellow "invalid value" overlay.
5. Sign off the change in the plant change-management system, including the GSD version, DIGSI parameter export, and the S7-400 source/ST revision.

Solution Comparison Matrix

Related Standards and Reference Documents

• SIPROTEC 4 7SJ62 / 7SJ63 / 7SJ64 Manual, firmware V4.6 — the canonical reference for the modern replacement family, including the fixed-voltage emulation mode and the PROFIBUS-DP measurand map.
• Siemens Industry Online Support — 7SJ62 product information — entry point for the latest manuals, GSD files, and firmware release notes.
• IEC 60688:2012 — Electrical measuring transducers for converting AC electrical quantities to analogue or digital signals — accuracy-class basis for the SIMEAS P.
• IEC 60255-1 — Measuring relays and protection equipment: Common requirements — power-measurand accuracy framework for the 7SJ62/63/64.
• PROFIBUS-DP profile for protection equipment (PNO order No. 3.302) — slot and module definitions for SIPROTEC slaves.

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