Relay Setting Calculator & Theory

System Voltage (kV)

Consumer Demand (kVA)

Number of Transformers Connected

Transformer 1

Rating (kVA)

Impedance (Z%)

% Overload Margin Allowed

CT Ratio (Primary / 1)

Individual Transformer Parameters

Transformer	Rating (kVA)	Z (%)	Rated Current	SC Current	Inrush (12x)

System Totals

Consumer Load

Total HV Rated Current

Total Short Circuit

Total Inrush (12x)

Calculated Ideal Primary Values

Setting	I> (O/C)	I>> (Hi-Set)	I>>> (Inst)	Ie> (E/F)	Ie>> (Hi-E/F)
Primary Amps
Logic	Load + Margin	Depends on Time*	90% of SC	20% Rated	100% Rated

Ashida (Fast) Micom P122 ABB REF615

Time-Current Characteristic Curve

O/C (Phase) Settings

Pickup (Primary Amps) TMS / Time Dial Curve Type

E/F (Earth) Settings

Pickup (Primary Amps) TMS / Time Dial Curve Type

Slide to simulate Fault Current: 100 A

O/C Trip: --

E/F Trip: --

📚 Theory & Formulas Used

The base currents are derived from the transformer's kVA rating and impedance (Z%).

I_rated =

kVA × 1000√3 × V_LL

I_SC =

I_rated × 100Z%

I_inrush ≈ 12 × I_rated

Overcurrent Pickup (I>): Load Current × 1.10 (Allows 10% overload margin).
Earth Fault Pickup (Ie>): Rated Current × 0.20 (Sensitive to 20% unbalance).
High-Set (I>>):
- If Time Delay < 100ms: 1.25 × Inrush (Prevents tripping on transformer energization).
- If Time Delay ≥ 100ms: 0.60 × Short Circuit (Ensures discrimination with downstream relays).
Instantaneous (I>>>): 0.90 × Short Circuit (Clear close-in faults immediately).

The trip time (t) is calculated using the international standard formula:

t = TMS ×

k((PSM)^α - 1)

Where PSM = Fault Current / Pickup Current

Curve Constants (k, α) determine the steepness of the response:

Curve Type	k	α (Alpha)	Characteristics
Standard Inverse (SI)	0.14	0.02	General protection
Very Inverse (VI)	13.5	1.0	Steeper, for feeder lines
Extremely Inverse (EI)	80.0	2.0	Fastest for high faults (fuse coordination)
Long Time Inverse (LTI)	120.0	1.0	Motor overload protection