⚡Fault Analysis Part 2 - Per Unit System

⚡ Welcome to Part 2 of our Fault Analysis series. Before we can calculate any fault current, there's one foundational tool every power systems engineer relies on: the Per-Unit (p.u.) System. Skip this, and every fault calculation in the rest of the series will feel confusing — so let's build it properly.

Why Do We Need a Per-Unit System?

Power systems have multiple voltage levels connected through transformers — generation at 11 kV, transmission at 220 kV, distribution at 11 kV or 400V, and so on. Doing calculations directly in actual (ohmic) values across all these levels is messy and error-prone.

The per-unit system solves this by expressing all quantities as a fraction of a chosen base value, so:

• Transformer turns ratio disappears from calculations — no need to refer impedances across transformers manually
• Equipment of different sizes becomes easy to compare (a per-unit impedance of 0.1 means the same relative thing whether the machine is 10 MVA or 500 MVA)
• Calculations become simpler and less error-prone across multi-voltage networks

The Basic Formula

Per-unit value of any quantity is defined as:

Per-Unit Value = Actual Value / Base Value

Choosing Base Quantities

We typically choose two independent base quantities, and the rest are derived from them:

Quantity	Relationship
Base Power (S_base)	Chosen directly — usually same across the whole system (e.g. 100 MVA)
Base Voltage (V_base)	Chosen per voltage level (changes across transformers)
Base Current (I_base)	S_base / (√3 × V_base) — for 3-phase systems
Base Impedance (Z_base)	V_base² / S_base

Changing Base — The Conversion Formula

Equipment nameplates give impedance in per-unit on their own rated base — but your system study might use a different common base. Convert using:

Z_pu(new) = Z_pu(old) × (S_base,new / S_base,old) × (V_base,old / V_base,new)²

This single formula is one of the most frequently tested numericals across SSC JE, GATE, and ESE — memorize it, and practice a few variations.

Why It Simplifies Transformers Specifically

In actual units, a transformer's impedance looks completely different depending on whether you measure it from the primary or secondary side (turns ratio squared difference). In per-unit, if base voltages on each side are chosen in the same ratio as the transformer's turns ratio, the per-unit impedance is identical on both sides. This is exactly why per-unit is the default language of power system studies — transformers effectively "disappear" from the math.

Worked Example

A transformer is rated 20 MVA, 11 kV, with 8% impedance on its own rating. Find its per-unit impedance on a system base of 100 MVA, 11 kV.

Z_pu(new) = 0.08 × (100/20) × (11/11)²
Z_pu(new) = 0.08 × 5 × 1 = 0.4 p.u.

Quick Revision Points

• Per-unit value = Actual value ÷ Base value
• Choose S_base and V_base independently; I_base and Z_base are derived
• Z_base = V_base² / S_base
• Base conversion: multiply by (new S/old S) and (old V/new V)²
• Per-unit impedance is same on both sides of a transformer if base voltage ratio = turns ratio

Practice MCQs

Q1. Base impedance is calculated as:
a) V_base / I_base
b) V_base² / S_base
c) S_base / V_base
d) I_base × V_base

Q2. A transformer's per-unit impedance stays the same on both sides when:
a) The turns ratio is 1:1
b) The base voltage ratio equals the transformer's turns ratio
c) The transformer is star-star connected
d) The transformer is unloaded

Q3. If S_base is doubled (with V_base unchanged), the new per-unit impedance:
a) Halves
b) Doubles
c) Stays the same
d) Becomes zero

Q4. Which quantity is typically chosen directly as a base rather than derived?
a) Base impedance
b) Base current
c) Base power
d) Base admittance

(Answers: 1-b, 2-b, 3-b, 4-c)

Coming up in Part 3: Symmetrical (Three-Phase) Fault Analysis — the most severe fault type, and the simplest to calculate since it only needs the positive sequence network.

Save this post for revision before your next mock test!

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⚡ Fault Analysis Series — Part 1: Introduction to Faults in Power Systems

⚡ If you're prepping for SSC JE, APGENCO/APTRANSCO AEE, or GATE Electrical, Fault Analysis is one topic you cannot afford to skip — it shows up in almost every power systems paper, and the concepts here carry forward into Protection, Switchgear, and Circuit Breaker questions too.

This is Part 1 of our Fault Analysis series. We're building this step by step — starting with fundamentals here, then moving into symmetrical components, sequence networks, and each fault type in the posts that follow.

What is a Fault?

A fault is any abnormal condition in a power system that causes current to flow through an unintended path — usually due to insulation breakdown, equipment failure, or external causes like lightning or falling trees on transmission lines.

When a fault occurs:

• Current in the faulted phase(s) rises sharply (often 10–20x normal current)
• Voltage at the fault point drops significantly
• If not cleared quickly, equipment can suffer thermal and mechanical damage

This is exactly why protection systems (relays + circuit breakers) exist — to detect and isolate faults within milliseconds.

Types of Faults

Faults are broadly divided into two categories:

1. Symmetrical Faults (Balanced)

• Three-phase fault (L-L-L), with or without ground
• All three phases affected equally — system remains balanced
• Least common (roughly 2–5% of all faults)
• Most severe — produces the highest fault current
• Analyzed using only the positive sequence network

2. Unsymmetrical Faults (Unbalanced)

These make up the vast majority of real-world faults — roughly 95%:

Fault Type	Approx. Frequency	Description
Line-to-Ground (L-G)	~70–80%	Single phase touches ground
Line-to-Line (L-L)	~10–15%	Two phases short together, no ground
Double Line-to-Ground (L-L-G)	~10%	Two phases short together AND to ground

Unsymmetrical faults break the system's natural balance, so we can't analyze them with simple per-phase methods — this is where symmetrical components (Part 4 of this series) become essential.

Common Causes of Faults

• Lightning strikes on overhead lines
• Insulation failure due to aging or moisture
• Falling trees or branches on conductors
• Birds/animals causing line-to-line or line-to-ground contact
• Equipment failure (transformer winding faults, cable insulation breakdown)
• Human error during switching operations

Why Fault Analysis Matters (Exam Angle)

Fault analysis isn't just theory — it directly decides:

• Circuit breaker ratings (breakers must handle worst-case fault current)
• Relay settings for protection coordination
• Conductor and equipment sizing to withstand fault-level stresses
• System stability — a fault that isn't cleared fast enough can cause cascading failures

This is why nearly every competitive exam (SSC JE, ESE, GATE) tests both the conceptual understanding and the numerical calculation of fault currents.

Quick Revision Points

• Faults are either symmetrical (all 3 phases, rare but severe) or unsymmetrical (1–2 phases, common)
• L-G fault is the most frequently occurring fault in real transmission/distribution systems
• Symmetrical faults need only positive sequence impedance for analysis
• Unsymmetrical faults need positive + negative (+ zero, if grounded) sequence networks
• Fault analysis determines circuit breaker ratings and relay protection settings

Practice MCQs

Q1. Which type of fault occurs most frequently in a power system?
a) Three-phase fault
b) Line-to-line fault
c) Line-to-ground fault
d) Double line-to-ground fault

Q2. Which fault produces the maximum fault current?
a) Line-to-ground fault
b) Line-to-line fault
c) Three-phase fault
d) Double line-to-ground fault

Q3. Symmetrical fault analysis requires which sequence network(s)?
a) Positive sequence only
b) Positive and negative sequence
c) Positive, negative, and zero sequence
d) Zero sequence only

Q4. Zero-sequence currents can only flow if:
a) The fault involves ground
b) The system is balanced
c) The fault is three-phase
d) The transformer is delta-connected on both sides

(Answers: 1-c, 2-c, 3-a, 4-a)

Coming up in Part 2: The Per-Unit System — why we use it, how to convert between bases, and why it's the essential first step before any fault current calculation.

Share this with your prep group if you found it useful!

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APGENCO & APTRANSCO AEE 2026 — Notification Summary + Prep Plan

⚡ APGENCO & APTRANSCO AEE 2026 — Notification Summary + Prep Plan

👋 Hey everyone — big recruitment news for AP power sector aspirants. APGENCO and APTRANSCO have both released their Assistant Executive Engineer (AEE) 2026 notifications, and since they share a common exam, this is one of the biggest combined opportunities we've seen in a while. Here's the full breakdown plus how to actually use the next month to prepare.

⏰ Heads up: The application window closed on 20 July 2026. If you already applied, this post is for you — focus on the CBT prep section below. If you missed it, keep an eye on this blog for the next AP Vidyut recruitment cycle.

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🔥 CRACK - SSC JE ELECTRICAL EXAM 🏆

I built a practice app for SSC JE Electrical — here's why, and what's in it

If you've been prepping for SSC JE Electrical for a while, you've probably run into the same problem I did. Most of the "practice apps" floating around are just old PDFs dumped into a quiz format, or generic templates that could be for any exam — same recycled questions, no real structure, nothing tailored to how this exam actually works.

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🔌DC Machines Quiz🔌

📘 Summary – DC Machines MCQs (Q1–100)

This complete set of 100 MCQs covers every major concept of DC machines, ensuring thorough preparation for PSU and competitive exams.

• Electromechanical Energy Conversion – torque, co‑energy, stored energy, and electromagnetic force principles.
• Losses – copper, iron, stray, mechanical, windage, and their dependence on speed, load, and frequency.
• Constructional Features – yoke, armature, commutator, brushes, laminations, slot wedges, pole shoes, and ventilation ducts.
• Armature Windings – lap, wave, simplex, duplex, winding pitches, equalizer rings, commutator segments, and coil spans.
• Generator Characteristics – shunt, series, compound types, OCC curves, residual magnetism, voltage build‑up, and regulation.
• Parallel Operation – equalizer bars, load sharing, compounding methods, and stability conditions.
• DC Motors – back emf, torque production, current relations, speed‑torque characteristics, and energy conversion principles.
• Applications – welding generators, exciters, boosters, traction loads, and industrial uses.

✅ By practicing all 100 questions with explanations, you’ll master the fundamentals of DC machines and be well prepared for SSC JE, GENCO, TRANSCO, NTPC, BHEL, and other PSU exams.

🔌⚡ DC Machines Quiz – Part 1 (Q1–10) ⚡🔌

👋 This is Part 1 of the DC Machines quiz. It covers electromechanical energy conversion, torque, losses, and basic construction concepts. Each question includes a detailed explanation.

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⚡🔌Transformers Quiz 🔌⚡

This quiz series is built around the most important transformer concepts that regularly appear in SSC JE, GENCO, TRANSCO, NTPC, BHEL, and other PSU exams. Here’s what you’ll practice:

• Voltage & Current Ratios – step‑up/step‑down relations, ampere‑turn balance.
• Magnetic Flux Behavior – why flux stays nearly constant, how V/f ratio affects it.
• Core Losses – hysteresis and eddy current losses, their dependence on frequency.
• Frequency Effects – what happens when voltage or frequency changes.
• Open‑Circuit & Short‑Circuit Tests – purpose and what they measure.
• Equivalent Circuit Representation – modeling core losses and leakage reactance.
• Phasor Diagrams – understanding current components and their phase relations.
• Voltage Regulation – conditions for zero or negative regulation at different power factors.

⚡ Transformer Quiz (Q1–12) ⚡

👋 This is Part 1 of the transformer quiz. It covers voltage/current relations, flux, losses, and frequency effects. Practice these 12 questions before moving to Part 2!

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🎯 CIL MT Exam Quiz — FREE Practice App

CIL MT Electrical Quiz - Practice Before the Exam

Hi friends, a lot of you have been asking for practice material for the CIL Management Trainee exam, so here's something that should actually help. Below is a free quiz covering the Electrical Engineering syllabus - use it to test where you stand before the real exam.

If you don't know much about CIL MT recruitment yet - Coal India Limited hires Management Trainees every year across engineering branches including Electrical, and the written exam is the first big hurdle. It's not just core electrical subjects either, they mix in aptitude, reasoning and English too, so don't only stick to circuit theory and ignore the rest.

(Always check Coal India's official site for the latest notification, dates and exact pattern - syllabus and cutoffs keep changing year to year, so don't rely only on this page for that.)

What's in the quiz

Machines, Power Systems, Circuit Theory, Control Systems, plus Quant, Reasoning, English and some CIL-specific GK. There's a timer built in and it tells you instantly whether you got it right, so it's actually decent for building exam speed and not just testing knowledge.

🎯 CIL MT Exam Quiz — 718 Questions FREE Practice App

🔥 Start Practice Now 🏆

Free, no login needed, works fine on mobile

Try it topic-wise first if you're weak somewhere, then switch to the mixed/timed mode once you're comfortable. Retake it a few times over the coming weeks - most people notice a real jump in speed after the third or fourth attempt.

One thing to be clear about - this quiz isn't put out by Coal India, it's just something built for practice by fellow aspirants/bloggers. For official notifications and syllabus always go to Coal India's site directly.

If you get stuck on any topic or want specific previous papers, drop a comment below and I'll try to put something together.

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