Best practices to diagnosing power transformer faults

Introduction

Transformer is a static device that can convert low voltage into high voltage and high voltage into low voltage without changing frequency and power. Transformer is very important component of power system. Without it power system is incomplete. The role of transformer in power system to ensuring efficient power transmission across the power system grid. It is was important to ensure transformer operation properly and diagnosing faults of transformer on time to prevent costly failures. 

Power transformer common faults

There are following types of faults in power transformer.

1-Electrical faults

•  Internal and external short circuit faults

•  Phase to phase and turn to turn winding faults

•  Insulation failure faults 

2- Thermal faults

• Overheating due to run on overload and poor cooling system

• Hot spots in winding and core

3- Mechanical faults

• Core displacement faults and winding deformation faults due to short circuits

• HV and LV side bushing failure or damaged faults

• Main tank deformation or leak faults

• Tap changer malfunctioning faults

4- Oil related faults

• Moisture ingress in oil and contamination particles 

• Gases generation due to insulation breakdown and short circuits

• Oil leakages from different spots

Diagnostics techniques for power transformer faults

There are following techniques to diagnose power transformer faults. 

1-DGA (Dissolved gas analysis)

DGA is very powerful tool for detecting early errors in power transformer. It analyzes the gases in power transformer which dissolved in power transformer oil such as hydrogen, methane, acetylene, and ethylene. IEC-60599 is an international standard which that provide guidance to understand dissolved and free gases analysis in oil filled electrical equipment's like power transformers, reactors to identify their conditions. Through this test we detect early signs of arcing. overheating and partial discharge.  

2- Insulation resistance 

These tests are measure the quality of insulation integrity between transformer windings and earth. This test are done to ensure individual winding to winding and individual winding to ground insulation integrity. Mostly insulation resistance tests are measured in megaohm. Transformer insulation resistance are performed in three ways HV to LV, HV to ground, LV to ground. For 11kv transformer HV side test voltage is up to 5kv and LV side test voltage is up to 1000V.  For a good transformer the minimum test value should be between 450 megaohm to 500 megaohm. There are many such factors that affect the insulation resistance value of transformer like bad surface condition at bushing terminal, the quality of the oil being used may not be good, due to poor quality of winding insulation, due to the high temperature of oil used and also affected by moisture.

3- Winding resistance

A very important and special test for transformer also know as winding resistance. In this test both primary and secondary sides windings are checked by winding resistance measurement device. For this purpose a DC current are applied to the transformer winding and checked the voltage drop across the transformer windings.  

4- Partial discharge

Partial discharge is very important diagnostic method for power transformer to ensured its insulation health and integrity. PD can detects early insulation defects and also identified weak spots in solid insulation i.e. paper and oil. And also can prevent danger failures by detecting early faults. There are 2 types of PD testing. one is offline testing and second is online testing. Offline PD testing is performed when transformer is de energized. and online PD testing is performed when transformer is in service. Online PD monitoring is better then offline PD testing because in online PD monitoring it is very useful for trend monitoring and early fault detection. There are many benefits of PD testing such are enhance transformer reliability, extends service life of transformer, Unplanned outages can be reduced. 

5- Insulating oil analysis

Oil analysis is very important diagnostic method to identified the transformer insulation oil condition. In this method dielectric breakdown voltages are checked of insulating oil and also ensured moisture content in insulating oil. This test help us to understanding the insulation level and how much it can cool the transformer windings. If the oil changes color it indicates that it has either degraded or has accumulated too many contamination particles. As per OEM recommendation transformer oil analysis can help us to prevent unexpected outages and costly repairing.  

6- Sweep frequency response analysis (SFRA)

This diagnostic method used to identify mechanical issues in transformer like windings displacement and core displacement. In this method SFRA device sends a swept frequency signals into transformer winding after that this device measure the voltage response at different frequencies. The measured frequency response compared with previous measurement values to identified issues that indicates mechanical or electrical faults. SFRA test is performed when transformer is de-energized. SFRA test does not required any thing open in the transformer.   

7- Transformer turn ratio test (TTR)

This test is used to identified the ratio of turns in primary and secondary windings of transformer. This test also helps us to identified the potential issues like shorted turns, wrong connections, manufacturing issues within the transformer for reliability and long service life. In this test voltage are applied to primary winding and measuring the resultant voltage from secondary winding after that compared the resulting ratio with expected ratio written at transformer nameplate. This test is performed when transformer is de-energized.