System Parameters and Base Calculations
📌 The discussion focuses on setting overcurrent relays (IDMT - Inverse Definite Minimum Time) for the main substation connecting 150 kV to 20 kV.
⚙️ Base calculations were performed for $Z_{\text{base}}$ and $I_{\text{base}}$ for both 20 kV ($Z_{\text{base}} = 4 \ \Omega$, $I_{\text{base}} = 2887 \text{ A}$) and 150 kV sides ($Z_{\text{base}} = 225 \ \Omega$, $I_{\text{base}} = 384.9 \text{ A}$).
⚡ Transformer data included a rating of $30 \text{ MVA}$ with $12.5\%$ impedance, resulting in $X_T$ (positive sequence) of $0.417 \text{ pu}$ based on the system MVA.
📊 Current Transformer (CT) ratios were established: $150 \text{ kV}$ side ($150/1$), $20 \text{ kV}$ incoming side ($1000/1$), feeder side ($600/1$), and coupling substation ($600/1$).

Short Circuit Current Calculations
💡 Three-phase short circuit current ($I_{3\phi}$) at the $20 \text{ kV}$ bus (0 km) was calculated to be $6.585 \times 10^3 \text{ A}$.
🔌 Cable impedance calculations for a $240 \text{ mm}^2$ feeder resulted in a positive sequence impedance of $Z_{L1\text{ pu}} = 0.034 + j0.079i$ per unit.
📉 Two-phase short circuit current ($I_{2\phi}$) at the $20 \text{ kV}$ bus was found to be $5.709 \text{ kA}$, and at the $150 \text{ kV}$ bus, it was $455.728 \text{ A}$ (for a $5 \text{ km}$ fault location).

Overcurrent Relay (OCR) Setting - $20 \text{ kV}$ Side (Incoming)
🎯 The pickup current setting ($I_{\text{pickup}}$) for the $20 \text{ kV}$ inverse time relay was set at $1.2$ times the nominal transformer secondary current, resulting in $I_{\text{pickup}} = 1039 \text{ A}$ (secondary side).
⏱️ The maximum permissible Time Operating Relay setting ($T_{\text{OCR}}$) based on IEC standards for this relay is $\leq 2 \text{ seconds}$; a setting of $1 \text{ second}$ was chosen for safety.
✅ Using the IDMT formula with $T_{\text{set}} = 1 \text{ s}$ and $I_{\text{pickup}} = 1039 \text{ A}$, the Time Multiplier Setting ($\text{TMS}$) calculated to be $0.248 \text{ s}$.
⚡ A definite time high-set (instantaneous) setting was also defined for the $20 \text{ kV}$ side, tripping at $4075 \text{ A}$ in $0.4 \text{ seconds}$ to handle severe short circuits.

Grading and Coordination Settings
⬆️ The $150 \text{ kV}$ relay $\text{TMS}$ was set to $0.371 \text{ s}$, chosen to be $1.5 \text{ s}$ (greater than $1 \text{ s}$ plus the required grading margin of $0.3 \text{ s}$) to ensure downstream protection trips first.
🔻 The Feeder relay (downstream of $20 \text{ kV}$ incoming) was set with $T_{\text{OCR}} = 0.4 \text{ s}$ (faster than the $150 \text{ kV}$ relay's $1.5 \text{ s}$ coordination time) resulting in $\text{TMS}_{\text{feeder}} = 0.218 \text{ s}$ (Inverse Time).
🔥 The Coupling Substation relay utilized an instantaneous high-set trip ($0 \text{ s}$ definite time) for severe faults, built upon an IDMT base setting with $\text{TMS}_{\text{coupling}} = 0.112 \text{ s}$.

Key Points & Insights
➡️ The coordination check confirmed that relay trip curves do not overlap, ensuring proper sequence: Coupling Substation trips first, followed by the Feeder, then the $20 \text{ kV}$ incoming relay, and finally the $150 \text{ kV}$ relay.
➡️ The selection of $T_{\text{set}}$ for downstream relays must be faster than the next upstream relay, incorporating a minimum grading margin of $0.3 \text{ s}$.
➡️ Protection coordination involves setting both Inverse Time (IDMT) protection (for moderate faults) and Definite Time/High-Set protection (for severe, instantaneous clearing) at various levels.

📸 Video summarized with SummaryTube.com on Nov 19, 2025, 11:49 UTC