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MV Transformer Protection Settings (50/51/50N/51N)

Reference pickups in ABB REF615 format; reference only, not for commissioning.

EstimatePreliminary result, not a design verification.

Before you calculate

Reference pickups for MV transformer protection

REFERENCE VALUES ONLY — NOT FOR COMMISSIONING. Illustrative 50/51/50N/51N pickups from editable demonstration multipliers, converted to the ABB REF615 Start value format. Real settings come from a protection-coordination study and the grid provider requirements.

Learn more — complete guide →

REFERENCE VALUES ONLY — NOT FOR COMMISSIONING. Illustrative pickups from editable demonstration multipliers, shown in ABB REF615 5.0 FP1 IEC format. Final settings need a protection-coordination study, the real fault levels, CT and relay verification, inrush data, the upstream and downstream devices and the grid provider requirements.

Step 1

Transformer and CTs

Transformer rated power (kVA)

MV voltage, line-to-line (kV)

Short-circuit impedance uk (%)From the nameplate; 6 % is not a universal value.

Phase CT primary (A)

Phase CT secondary

51 curve

Earth-fault current

Advanced: demonstration multipliers, curves, delays, Start value Mult, test currents

The multipliers are ILLUSTRATIVE demonstration values, not IEC settings and not recommendations.

51: × rated MV current

50: × LV short-circuit current at MV

51N: × earth CT primary

50N: × earth CT primary

51N curve

51 Time multiplier (0.05 to 15.00)

51N Time multiplier (0.05 to 15.00)

51 operate delay if DT (ms)

50 operate delay (ms, hypothetical)

51N operate delay if DT (ms)

50N operate delay (ms, hypothetical)

51: Start value Mult (0.8 to 10.0, step 0.1)

51: simulate ENA_MULT active

50: Start value Mult (0.8 to 10.0, step 0.1)

50: simulate ENA_MULT active

51N: Start value Mult (0.8 to 10.0, step 0.1)

51N: simulate ENA_MULT active

50N: Start value Mult (0.8 to 10.0, step 0.1)

50N: simulate ENA_MULT active

51 Minimum operate time (ms, 20 to 60000)

51N Minimum operate time (ms, 20 to 60000)

IDMT Sat point (× pickup, 10 to 50)Above it the inverse time no longer decreases.

Phase test current for the 51 time (A primary)

Earth test current for the 51N time (A primary)

Step 2

Reference results

Check before relying on this result

REFERENCE VALUES ONLY — NOT FOR COMMISSIONING. Final settings need a protection-coordination study with the real fault levels, CT and relay data, inrush, the upstream and downstream devices and the grid provider requirements.
The 50 pickup is 10.0 × the CT rated secondary current: CT burden, accuracy limit, saturation and the relay input limits must be verified before such a value means anything.

Rated MV current46.19 ACT secondary at rated load: 2.309 A

Estimated MV current during an LV short circuit769.8 ATheoretical MV-side symmetrical current for a three-phase fault at the transformer LV terminals, considering transformer impedance only and an ideal stiff MV supply.

51 (PHLPTOC) reference pickup0.58 × InTheoretical 57.74 A (0.57735 × In) → 58 A primary / 2.9 A secondary · CT 100/5 AIEC Normal (Standard) Inverse, code 9 · raw IEC t ≈ 0.318 s · ABB-adjusted reference ≈ 0.318 s · ABB-adjusted reference time; the raw IEC equation gives 0.3180 s. Excludes relay tolerance, outputs and the breaker. ENA_MULT inactive: base pickup.REFERENCE VALUES ONLY — NOT FOR COMMISSIONING.

50 (PHIPTOC) reference pickup10.01 × InTheoretical 1000.74 A (10.0074 × In) → 1001 A primary / 50.05 A secondary · CT 100/5 ADefinite time · No pickup from this element at the test current. ENA_MULT inactive: base pickup.REFERENCE VALUES ONLY — NOT FOR COMMISSIONING.

51N (EFLPTOC) reference pickup0.200 × InTheoretical 20.00 A (0.2 × In) → 20 A primary / 1 A secondary · CT 100/5 AIEC Normal (Standard) Inverse, code 9 · raw IEC t ≈ 0.428 s · ABB-adjusted reference ≈ 0.428 s · ABB-adjusted reference time; the raw IEC equation gives 0.4280 s. Excludes relay tolerance, outputs and the breaker. ENA_MULT inactive: base pickup.REFERENCE VALUES ONLY — NOT FOR COMMISSIONING.

50N (EFIPTOC) reference pickup1.00 × InTheoretical 100.00 A (1 × In) → 100 A primary / 5 A secondary · CT 100/5 ADefinite time · No pickup from this element at the test current. ENA_MULT inactive: base pickup.REFERENCE VALUES ONLY — NOT FOR COMMISSIONING.

ABB parameters per function block (REF615 5.0 FP1 IEC) — reference only; check the real firmware and PCM600 configuration
Function block	Start value	Start value Mult	ENA_MULT (simulated)	Operating curve type	Time multiplier	Operate delay time	Minimum operate time	Io signal Sel
PHLPTOC (51)	0.58 × In	1	Inactive	9 = IEC Normal (Standard) Inverse	0.1	—	20 ms	—
PHIPTOC (50)	10.01 × In	1	Inactive	15 = IEC Definite Time	—	50 ms	—	—
EFLPTOC (51N)	0.200 × In	1	Inactive	9 = IEC Normal (Standard) Inverse	0.1	—	20 ms	2 = Calculated Io
EFIPTOC (50N)	1.00 × In	1	Inactive	15 = IEC Definite Time	—	50 ms	—	2 = Calculated Io

About these parameters: each block has its own Start value Mult, and it acts only while that block's ENA_MULT input is active (simulated here). General setting IDMT Sat point = 50. Not every REF615 configuration has every block or a sensitive Io input. Source: ABB 615 series Technical Manual 1MRS756887 rev Q (2023-03-06), product version 5.0 FP1; Stage ranges and steps, curve codes and IEC coefficients were compared with the PDF (1MRS756887 rev Q) by the owner's independent reviewer (v0.1.72 validation report); the TMS, Start value Mult and CT-primary steps, Minimum operate time 20 to 60000 ms and IDMT Sat point 10 to 50 come from the same review (pp. 48-49, 55, 220, 231-233, 311, 316-319, 1132-1136). KiloVolt has not itself read the PDF.

Not established here: the real maximum and minimum MV fault currents, the inrush waveform, 50 sensitivity and selectivity, earth-fault settings in isolated, resonant or impedance-earthed networks, CT accuracy and saturation, coordination with the grid provider fuses or relays. Earth-fault currents cannot be inferred from kVA and uk.

1. Rated MV current

From the nameplate power and the MV line-to-line voltage.

I_{n,MV}=\frac{S_n}{\sqrt{3}\,U_{MV}}

I_{n,MV}=\frac{1600}{\sqrt{3}\times 20}=46.188\ \text{A}

2. Estimated MV current during an LV short circuit

Transformer impedance only, ideal stiff MV supply: not the network short-circuit level.

I_{k,MV}\approx\frac{I_{n,MV}}{u_k/100}

I_{k,MV}\approx\frac{46.188}{0.06}=769.8\ \text{A}

3. ABB Start value from a primary pickup

Divide by the CT primary, round to the function step; never clamped.

\text{Start value}=\mathrm{round}\!\left(\frac{I_{pickup}}{I_{CT,pri}},\ \text{step}\right)\times I_n

\frac{57.735}{100}=0.57735\ \rightarrow\ 0.58\times I_n

4. 51 inverse time at the test current

IEC 60255-151 curve; excludes relay tolerance, outputs and the breaker.

t=TMS\,\frac{k}{M^{\alpha}-1},\quad M=\frac{I}{I_{pickup}}

t=0.1\times\frac{0.14}{(500/58)^{0.02}-1}=0.318\ \text{s}

Calculations use full precision internally; the displayed numbers are rounded for readability.