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📘 Hard Protection Coordination Case Study – Multi Relay Analysis

This case study explains coordination between feeder relay, transformer relay and incoming relay using PSM and time grading.

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🔹 System Description

• 11 kV Feeder → Relay R1
• 33/11 kV Transformer → Relay R2
• 33 kV Incoming Line → Relay R3

CT Ratios:

• R1: 400/1
• R2: 600/1
• R3: 800/1

Plug Settings:

• R1: 1 A
• R2: 1 A
• R3: 1 A

Fault at 11 kV feeder = 4000 A.

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🔹 Step 1: Calculate Secondary Fault Currents

R1 secondary = 4000 / 400 = 10 A R2 secondary = 4000 / 600 = 6.67 A R3 secondary = 4000 / 800 = 5 A

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🔹 Step 2: Calculate PSM

PSM1 = 10 / 1 = 10 PSM2 = 6.67 / 1 = 6.67 PSM3 = 5 / 1 = 5

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🔹 Step 3: Operating Time (Standard Inverse)

T = 0.14 / (PSM^0.02 − 1)

For PSM = 10 10^0.02 ≈ 1.047 T1 = 0.14 / (0.047) ≈ 2.98 sec

For PSM = 6.67 6.67^0.02 ≈ 1.039 T2 = 0.14 / (0.039) ≈ 3.59 sec

For PSM = 5 5^0.02 ≈ 1.032 T3 = 0.14 / (0.032) ≈ 4.37 sec

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🔹 Step 4: Apply Coordination Time Interval (CTI)

Required CTI = 0.3 sec

• R1 operates first (Primary)
• R2 = R1 + 0.3 sec
• R3 = R2 + 0.3 sec

Final coordinated times: R1 = 2.98 sec R2 ≈ 3.3 sec R3 ≈ 3.6 sec

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🔹 What Happens During Fault?

• Feeder fault → R1 trips first
• If R1 fails → R2 trips
• If R2 fails → R3 trips

This ensures selective isolation of faulty section.

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🎯 Engineering Understanding

• Primary relay must have highest PSM
• Backup relays must have time delay
• CT ratio affects PSM
• Coordination ensures system stability

Protection Coordination = Intelligence in Relay Setting