Transformer Mistake a critical error during testing

 

Transformer Mistake #1: 

This Testing Error Will Fail You! 

Category: Transformer Testing & Commissioning

Author: Aaryan Gupta | Senior Electrical Engineer
Tags: Transformer Testing, Power Transformer, Electrical Testing Mistakes, Tan Delta, HV Test, Transformer Commissioning

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🔍 Introduction

Transformer testing is the backbone of electrical safety and system reliability. Yet, one simple mistake during testing can lead to test failure, project delays, or even equipment damage.

In this blog post, we’re uncovering Transformer Mistake #1 – a critical error during testing that most engineers overlook. 

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⚡️ Real Scenario: What Went Wrong?

During the routine High Voltage (HV) test, a 33/0.415 kV, 1000 kVA transformer was sent for testing. Everything seemed perfect:

• Setup ✅

• Earthing ✅

• Test equipment calibration ✅

But as the test began, the insulation breakdown occurred within seconds. Shocked?

👉 The mistake: Improper connection of the Bushing Tap (or not isolating the tap lead properly).

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🔍 Mistake Explained: Unconnected Bushing Tap or Tap Changer

Many engineers ignore the tap lead terminals during insulation testing or apply test voltage without isolating the tap changer. This results in:

• Stress concentration at winding tap points

• Internal arcing or insulation failure

• Immediate test failure or misleading results

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🧪 Common Transformer Testing Mistakes That Cause Failure

1. Not isolating tertiary winding or neutral terminal

2. Earthing not done properly before HV application

3. Testing without oil filtration/drying

4. Incorrect connections during Tan Delta / Capacitance Test

5. Voltage ramping done too quickly in HV testing

6. Skipping core insulation checks before HV test

7. Bushing tap not grounded/left floating

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💡 Correct Procedure for HV Test (as per IS/IEC 60076-3)

Voltage Class	HV Test Voltage (AC)	Test Duration
Up to 11 kV	28 kV	60 seconds
33 kV	70 kV	60 seconds
66 kV	140 kV	60 seconds

👉 Before the test:

• Isolate neutral and tertiary windings

• Ground all non-tested terminals

• Connect bushing tap lead to ground (if not under test)

• Start ramping test voltage slowly (5% per second)

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📉 Consequences of Testing Mistakes

• Equipment damage (coil burn, flashover)

• Transformer rejection in factory tests

• Costly delays in project commissioning

• Loss of customer trust

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✅ Best Practices to Avoid Transformer Testing Failures

Mistake	Best Practice
Tap lead floating	Always ground or isolate it properly
No core insulation check	Use Megger before HV test
Fast voltage rise	Ramp voltage gradually
Incomplete earthing	Use copper strip with proper clamps
Incorrect Tan Delta setup	Ensure test object isolation and shield grounding

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📘 Bonus Tip: Always Refer to Manufacturer’s Manual

Before testing, refer to:

• Transformer test circuit diagram

• Tap changer wiring guide

• Terminal layout drawing

It prevents errors and ensures safe, standard-compliant testing.

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🎯 Final Words

This one mistake – leaving bushing tap unconnected or floating during HV testing – has caused countless transformer failures.

Avoid it by following standard procedures and maintaining attention to detail. As engineers, your testing skill defines the reliability of the power system.

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📽 Watch This Error Explained

▶️ YouTube Short: “ये गलती पास नहीं होने देगी…” – Transformer Mistake #1
📌 Don’t forget to Like, Share, and Subscribe for more real-world testing insights.

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🔗 Related Blog Posts

• ✅ Complete Checklist for Transformer Testing

• ✅ How to Perform Tan Delta Testing – Step-by-Step Guide

• ✅ Transformer Commissioning: What You Should Never Skip

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📩 Want to Learn More?

📘 Join our Online Transformer Testing Course – with videos, PDFs, and real project files.
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