Faults on power systems are inevitable — a tree touches a line, insulation fails, lightning strikes. What matters is how quickly and selectively the faulty part is disconnected. That job belongs to protective relays and circuit breakers. Switchgear and protection is one of the highest-scoring areas in PGCB, BPDB, BREB/PBS, DPDC, DESCO, WZPDCL, NESCO and BCS (EEE) exams and is asked in every sub-station viva. This post explains the full chain — CT → relay → trip coil → breaker — with an animation, IDMT calculations and MCQs.
- Protection system = CT/PT (sense) + relay (decide) + trip circuit & DC battery (command) + circuit breaker (interrupt).
- Requirements: selectivity, speed, sensitivity, reliability, stability, simplicity and economy.
- Most common fault: single line-to-ground (≈ 70–80 %); most severe: three-phase symmetrical.
- IDMT relay (IEC standard inverse): t = TMS × 0.14 / (PSM0.02 − 1).
- Breaker types by arc quenching: air, oil, air-blast, vacuum (11–33 kV), SF₆ (132 kV and above).
- Why protection is needed
- Types of faults
- Elements of a protection scheme (animation)
- Qualities of a good protection system
- Zones of protection, primary and backup
- Types of relays
- IDMT overcurrent relay (animated curve)
- Circuit breakers
- Breaker ratings and arc terms
- Solved problems
- Practice MCQs
- বাংলায় দ্রুত রিভিশন
1. Why Protection Is Needed
A short circuit can make current rise to 10–20 times normal within milliseconds. Without fast disconnection this causes overheating and fire, mechanical damage from electromagnetic forces, voltage collapse affecting healthy customers, loss of stability of generators, and danger to people. Protection detects abnormal conditions and isolates only the faulty section so the rest of the system keeps running.
2. Types of Faults
| Fault | Type | Approximate occurrence | Severity |
|---|---|---|---|
| Single line-to-ground (LG) | Unsymmetrical | ≈ 70–80 % | Least severe (usually) |
| Line-to-line (LL) | Unsymmetrical | ≈ 10–15 % | Moderate |
| Double line-to-ground (LLG) | Unsymmetrical | ≈ 5–10 % | Severe |
| Three-phase (LLL / LLLG) | Symmetrical | ≈ 2–5 % | Most severe — used to rate breakers |
Symmetrical faults are analysed with per-phase methods; unsymmetrical faults need symmetrical components (positive, negative and zero sequence). Abnormal conditions also include over-voltage, under-frequency, overload and loss of excitation.
3. Elements of a Protection Scheme (Animation)
- Instrument transformers: the CT (current transformer, e.g. 400/1 A or 400/5 A) and PT/VT (e.g. 11 kV/110 V) step down quantities safely for the relay.
- Relay: compares measured quantities with settings and decides whether to trip.
- Trip circuit: powered from a station DC battery (e.g. 110 V or 220 V DC) so it works even when AC voltage collapses during a fault.
- Circuit breaker: its trip coil releases the mechanism, contacts separate, and the arc is extinguished at a current zero.
Total fault-clearing time = relay operating time + breaker interrupting time (modern HV breakers: 2–3 cycles, i.e. 40–60 ms at 50 Hz).
4. Qualities of a Good Protection System
- Selectivity (discrimination): trip only the breakers nearest the fault.
- Speed: clear faults quickly to limit damage and maintain stability.
- Sensitivity: detect even small fault currents (e.g. high-resistance earth faults).
- Reliability: operate whenever required (dependability) and not when not required (security).
- Stability: remain inoperative for faults outside the protected zone (through-faults) and for normal events such as transformer inrush or motor starting.
- Simplicity and economy.
5. Zones of Protection, Primary and Backup
The system is divided into overlapping zones — generator, transformer, busbar, line, motor — each with its own primary protection. Zones overlap around every breaker so that no part is left unprotected. If primary protection fails, backup protection (often time-delayed overcurrent in an adjacent zone) operates after a short delay.
6. Types of Relays
| By technology | Description |
|---|---|
| Electromagnetic | Attracted armature or induction disc (IDMT); rugged, no auxiliary supply |
| Static | Transistor/analog circuits; faster, no moving parts |
| Numerical (digital) | Microprocessor-based; many functions in one unit, event recording, communication (IEC 61850) — standard in modern sub-stations |
| By function | Operating principle | Typical use |
|---|---|---|
| Overcurrent (instantaneous, definite-time, IDMT) | Operates when current exceeds setting | Distribution feeders, motors, backup |
| Earth-fault | Residual / zero-sequence current | All feeders and transformers |
| Directional | Uses voltage reference to sense direction of power flow | Ring mains, parallel feeders |
| Differential (Merz-Price) | Compares current entering and leaving the zone | Transformers, generators, busbars |
| Distance (impedance, mho) | Measures V/I = impedance to fault | 132–400 kV transmission lines |
| Buchholz | Gas accumulation / oil surge | Oil-immersed transformers |
| Under-frequency / over-voltage | Frequency or voltage limits | Load shedding, generator protection |
7. IDMT Overcurrent Relay (Animated Curve)
An Inverse Definite Minimum Time (IDMT) relay operates faster for larger currents, but never faster than a definite minimum. Two settings determine its behaviour:
- Plug setting (PS) — pickup current as a percentage of CT secondary rating (e.g. 50–200 %).
- Time multiplier setting (TMS) — scales the curve vertically (0.05–1.0) for coordination.
Grading/coordination: successive relays toward the source are set with a time margin of about 0.3–0.4 s so that only the relay nearest the fault operates.
8. Circuit Breakers
A circuit breaker can make, carry and break normal currents and interrupt fault currents automatically. When contacts separate, an arc forms; the breaker’s job is to cool and de-ionise it so it does not re-strike after a natural current zero.
| Type | Arc-quenching medium | Typical voltage | Notes |
|---|---|---|---|
| MCB / MCCB / ACB | Air (arc chutes) | LV (≤ 1 kV) | Homes, buildings, LV panels |
| Bulk / minimum oil (OCB) | Transformer oil | 11–132 kV (older) | Fire risk, maintenance heavy |
| Air-blast | Compressed air | 132–400 kV (older) | Noisy, needs compressor |
| Vacuum (VCB) | Vacuum (no medium) | 11–33 kV | Compact, long life, little maintenance; common in distribution sub-stations |
| SF₆ | Sulphur hexafluoride gas | 66–765 kV | Excellent insulating and arc-quenching; used in GIS and transmission grid sub-stations |
Fuse vs circuit breaker: a fuse both detects and interrupts but must be replaced after operating; a breaker is reusable and works with relays for selective, remote and automatic operation. An isolator (disconnector) is operated only off-load for visible isolation during maintenance.
9. Breaker Ratings and Arc Terms
- Breaking capacity (symmetrical): √3 × V × Ibreak (MVA) or the rated short-circuit breaking current in kA.
- Making capacity = 2.55 × symmetrical breaking current (peak), allowing for the DC offset of the first cycle.
- Short-time rating: current the breaker can carry for 1 s or 3 s without damage.
- Restriking voltage: transient voltage across contacts after arc extinction; recovery voltage: normal-frequency voltage that follows; RRRV: rate of rise of restriking voltage (kV/µs).
- Current chopping: abrupt interruption before natural zero (vacuum breakers, small inductive currents) → over-voltages; controlled with surge arresters/RC snubbers.
10. Solved Problems
Problem 1. An IDMT relay (IEC standard inverse) has CT ratio 400/5 A, plug setting 125 % and TMS 0.2. Find the operating time for a fault current of 5000 A.
- Relay pickup (secondary) = 1.25 × 5 = 6.25 A ⇒ primary pickup = 6.25 × 80 = 500 A
- PSM = 5000 / 500 = 10
- t = 0.2 × 0.14 / (100.02 − 1) = 0.028 / 0.0471 ≈ 0.59 s
Problem 2. A 33 kV breaker has a rated symmetrical breaking current of 25 kA. Find its breaking capacity and making current.
- Breaking capacity = √3 × 33 × 25 ≈ 1429 MVA
- Making current = 2.55 × 25 = 63.75 kA (peak)
11. Practice MCQs (click to see answer)
1. The most frequently occurring fault in power systems is: (a) Three-phase fault (b) Line-to-line fault (c) Single line-to-ground fault (d) Double line-to-ground fault
2. The most severe fault is: (a) LG (b) LL (c) LLG (d) Three-phase symmetrical
3. Differential protection is mainly used for: (a) Feeders (b) Transformers and generators (c) Lighting circuits (d) Motors below 1 kW
4. Distance relays are used to protect: (a) Transmission lines (b) Batteries (c) Fuses (d) Lightning arresters
5. Trip circuits of breakers are supplied from: (a) AC mains (b) Station DC battery (c) CT secondary only (d) Generator terminals
6. Vacuum circuit breakers are generally used up to: (a) 1 kV (b) 33 kV (c) 400 kV (d) 765 kV
7. Making capacity of a breaker is ______ times symmetrical breaking current. (a) 1.8 (b) 2.55 (c) √2 (d) 3
8. The ability of protection to remain inoperative for external faults is called: (a) Sensitivity (b) Selectivity (c) Stability (d) Speed
9. An isolator is operated: (a) On load (b) Off load (c) During faults (d) Automatically by relay
10. TMS of an IDMT relay changes the: (a) Pickup current (b) Operating time scale (c) CT ratio (d) Frequency
11. SF₆ is preferred in HV breakers because it is: (a) Conductive (b) Electronegative with high dielectric strength and good arc quenching (c) Cheap and abundant (d) Flammable
12. বাংলায় দ্রুত রিভিশন
- সুরক্ষা ব্যবস্থা: CT/PT (পরিমাপ) → রিলে (সিদ্ধান্ত) → DC ব্যাটারিচালিত ট্রিপ সার্কিট → সার্কিট ব্রেকার (বিচ্ছিন্ন)।
- সবচেয়ে বেশি ঘটে সিঙ্গেল লাইন-টু-গ্রাউন্ড ফল্ট (≈৭০–৮০%); সবচেয়ে মারাত্মক তিন-ফেজ ফল্ট।
- ভালো সুরক্ষার গুণ: সিলেক্টিভিটি, দ্রুততা, সংবেদনশীলতা, নির্ভরযোগ্যতা, স্থিতিশীলতা।
- রিলে: ওভারকারেন্ট/IDMT (ফিডার), ডিফারেনশিয়াল (ট্রান্সফরমার, জেনারেটর), ডিসট্যান্স (ট্রান্সমিশন লাইন), বুখোলজ (তেলভর্তি ট্রান্সফরমার)।
- IDMT: t = TMS × 0.14/(PSM0.02 − 1)।
- ব্রেকার: ১১–৩৩ kV-এ ভ্যাকুয়াম (VCB), ১৩২ kV ও তার বেশিতে SF₆। মেকিং ক্যাপাসিটি = ২.৫৫ × ব্রেকিং কারেন্ট। আইসোলেটর লোডবিহীন অবস্থায় চালাতে হয়।
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References: Badri Ram & D. N. Vishwakarma, Power System Protection and Switchgear; V. K. Mehta, Principles of Power System; IEC 60255-151 (overcurrent relay characteristics). Images and animations: original work by Totalecer.
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