Transformer testing encompasses a range of tests and procedures aimed at evaluating the performance, safety, and reliability of transformers. These tests are conducted during manufacturing, first time installation/charging, and regular maintenance to detect potential issues and ensure compliance with industry standards.
Different types of tests for Transformer Testing
During pre-commissioning stage of a transformers, broadly two types of tests are conducted : Transformer Oil test and Standard electrical tests.
Transformer testing involves conducting some standard tests such as voltage ratio test , winding resistance test, insulation resistance test, tan delta test etc . These tests provide insights into the transformer’s efficiency, load capacity, and insulation integrity.
1.Turns Ratio Test
This test is conducted using a turn ratio kit. Leads from the kit are connected in LV and HV side & readings are taken on different taps. The turns ratio test evaluates the transformation ratio between the primary and secondary windings of a transformer. Value of ratio obtained on different taps is matched with transformer factory report. Discrepancies in the turns ratio can indicate winding faults or manufacturing defects. Sample report :
| TAP NO. | 1UN/2UN | 1VN/2VN | 1WN/2WN |
| 1 | 1.824 | 1.828 | 1.824 |
| 2 | 1.805 | 1.809 | 1.804 |
| 3 | 1.783 | 1.787 | 1.783 |
2.Insulation Resistance Test
The insulation resistance (IR) test measures the resistance between the transformer windings and the ground. IR test is conducted using a IR tester which injects DC voltage in winding of transformer and measures its resistance. Higher is the IR value better is the insulation , this test helps identify insulation deterioration and potential insulation breakdown.
PI is Polarizing index and it is the ratio of IR value taken for 10 minutes (600 sec) to the IR value taken for 1 minute (60 sec) ,ideally its value should be more than 1.5. It is to be noted that high polarization index of an insulator implies healthiness of insulator. Following readings are taken:
| 60sec | 600sec | P.I | |
| HV/IV-EARTH | |||
| HV-LV | |||
| LV-E |
3.Winding resistance test
The winding resistance test of a transformer is a test used to measure the resistance of the transformer windings. This test helps assess the condition of the transformer’s insulation and connections, ensuring its reliability and safety. This test is also conducted using a winding resistance test kit and usually 10 A current is injected and corresponding winding resistance is measured. This test is also conducted at all taps. Following readings are taken ,a sample report is shown below:
| Tap No. | H.V winding (Ohm) | IV winding (m-Ohm) | LV winding (m-Ohm) | ||||||
| 1U2U | 1V2V | 1W2W | 2UN | 2VN | 2WN | 3U3V | 3V3W | 3U3W | |
| 1 | |||||||||
4.Core/magnetic balance Test
| TAP NO. | 1U-N(V) | 1V-N(V) | 1W-N(V) | 2U-N(V) | 2V-N(V) | 2W-N(V) |
| 1 | 250.9 | 226.3 | 25.6 | 137.8 | 123.0 | 14.3 |
| 94.8 | 244.4 | 149.3 | 49.5 | 132.9 | 85.4 | |
| 16.18 | 221.7 | 235.6 | 8.86 | 120.2 | 128.5 |
R-N=Y-N+B-N , r-n=y-n+b-n
5.Short circuit test
HV side of the Transformer is kept open and LV side is shorted with each other ,then current readings are taken as follows.
Short Circuit Test-:HV open and LV side short.
| TAP | Measured Current hv side | Measured Current lv side | ||||
| 9N | 1U | 1V | 1W | 2U | 2V | 2W |
| 6.84A | 6.83A | 7.0A | 11.38A | 11.34A | 11.71A | |
7.Tan-delta test
Tan-delta test is conducted to assess insulation status of transformer bushings and its windings. Detailed procedure has been explained here.
8.Stability test
3 Ph ,440V AC supply is given in the primary side field C.T of the transformer with its secondary side shorted after its C.T. Current readings are taken in both secondary sides-LV and HV of the transformer and also at REF terminals and it is ensured that neutral current in C.T sec is ideally Zero .Ratio of CT can also be confirmed by this test as both primary and secondary current readings can be taken using a clampmeter.
9.System IR
It is to be noted that all above routine testing is done on transformer by removing it from the system , it means that jumpers in HV and LV side are opened which isolates the transformer completely from the system and then all tests are performed. System IR means taking value of IR after jumpering in HV and LV side of the transformer has been done .
After jumpering is done, L.A, C.T, some BPIs and line insulator come in the circuit of system IR test and if any of the equipment’s insulation is poor the overall IR value becomes poor. Thus it checks that the complete system has good IR value before charging. This value is usually less compared to that taken without jumpers connected in the transformer.
Oil test
After report of above routine tests are obtained and they are found to be in order ,it is also seen that oil test reports are also in order. Different kinds of tests are done on oil which have been explained here.
When both oil report and routine test reports are found satisfactory the transformer is charged on No load and it is kept in observation for some time .Temperature rise in winding and Oil is recorded and if no abnormality is found during no load situation it is charged on load.
Importance of Comprehensive Testing
Comprehensive transformer testing is essential for several reasons:
- Ensuring Operational Efficiency: By identifying potential faults and inefficiencies, testing helps optimize transformer performance and minimize energy losses.
- Preventing Downtime: Early detection of issues through testing allows for proactive maintenance, reducing the risk of unexpected downtime and costly repairs.
- Ensuring Safety: Testing helps identify safety hazards such as insulation breakdowns or overheating, mitigating the risk of electrical accidents and fires.
- Compliance with Standards: Adhering to established testing standards ensures that transformers meet regulatory requirements and operate safely within specified parameters.
Frequently Asked Questions (FAQs)
- How often should transformer testing be conducted? Transformer testing frequency depends on factors such as the transformer’s age, load, and operating conditions. Generally, testing is conducted during commissioning, after major maintenance, and periodically throughout the transformer’s lifespan.
- Can transformer testing predict failures? While transformer testing can detect potential issues and assess the transformer’s condition, it cannot predict failures with absolute certainty. However, proactive testing can significantly reduce the risk of unexpected failures.
- Are there non-invasive testing methods available? Yes, advancements in technology have led to the development of non-invasive testing methods such as frequency response analysis and online monitoring systems, which allow for testing without disrupting transformer operation.
- What safety precautions should be taken during transformer testing? Safety is paramount during transformer testing. Ensure proper grounding, use personal protective equipment (PPE), and follow established safety protocols to minimize the risk of electrical accidents.
- How long does transformer testing typically take? The duration of transformer testing varies depending on the scope and complexity of the tests involved. Simple tests like turns ratio or insulation resistance may take a few hours, while comprehensive diagnostic testing may require several days.
- Can transformer testing be outsourced? Yes, many companies offer transformer testing services, including on-site testing and laboratory analysis. Outsourcing testing can be beneficial for organizations lacking the necessary expertise or equipment.
Conclusion
Transformer testing is a critical aspect of ensuring the reliability, safety, and efficiency of electrical distribution systems. By employing comprehensive testing methodologies and adhering to best practices, organizations can mitigate the risk of unexpected failures, optimize transformer performance, and maintain uninterrupted power supply.
Prioritizing regular testing and staying informed about advancements in diagnostic techniques are essential steps in safeguarding transformer assets and enhancing overall system reliability.