Voltage transformers (VTs) play a key role in the electrical power system. As such, it is vital to regularly test them to ensure their ability to function properly and avoid failure. There are three types of voltage transformers. The most common types are potential transformers which reduce the primary and secondary voltages from a higher to a lower potential voltage; control transformers which supply power to a circuit breaker or a motor starter; and voltage transformers which are used in high-voltage applications.
Voltage transformers are used in the energy grid 24 hours a day and could last for 40 years. During their life span, they are subjected to aging, network issues, and external problems. Utility companies perform many tests during commissioning, maintenance, and diagnosis to determine if any irregularities or problems might have been caused in transportation, installation or daily use.
Types of VT Testing
Testing occurs at three points in the life of a transformer: during commissioning, during routine screening, and for diagnostic purposes. The range of tests differs for each of these three stages. Commissioning and diagnostics require a thorough analysis of the various workings of the transformers while regular maintenance is limited to nine basic tests.
The four common tests shared by these three procedures are dielectric frequency response, exciting current, winding resistance, and dielectric breakdown on an oil sample. However, beyond these four tests are many other specific tests that businesses can conduct, depending on their need:
- Commissioning: transformer turns ratio, bushing CT ratio, tan delta power factor, dissipation factor, variable frequency power factor, dielectric frequency response, surge arrester loss, leakage reactance, sweep frequency response analysis, frequency response of stray losses, dynamic winding resistance, and core ground
- Routine screening: transformer turns ratio, brushing CT ratio, power factor/dissipation factor, variable frequency power factor/dissipation factor, and surge arrestor loss and current
- Diagnostic testing: tan delta/power factor/dissipation factor, variable frequency power factor/ dissipation factor, tan delta/power factor/dissipation factor tip-up test, leakage reactance, frequency response of stray loss, dynamic winding loss, sweep frequency response analysis, DC insulation resistance, partial discharge, and transformer turn ratio
Selecting the appropriate tests for the situation could save a company time and money.
Common Types of Transformer Failure
Transformer winding failures are the most common cause of transformer faults. A transformer winding is made up of paper-insulated, current-carrying material wrapped around sections of a core. Leads connect one winding to another winding, tap changer or bushing. All these parts must be properly insulated, supported, and cooled.
Over time, winding failure can deteriorate due to a variety of factors: dampness, surges, getting too hot, vibration and mechanical stress. Regular VT tests that target this area are strong advised. The following tests will determine the integrity of the windings, leads, and connectors:
- Transformer turns ratio
- Exciting current
- DC winding resistance
- Leakage reactance/short circuit impedance
- Frequency response of stray losses
- Sweep frequency response analysis
- Power/tan delta/dissipation factor/capacitance
- Variable frequency power factor/dissipation factor
- Dielectric frequency response
Issues with Modern-day Transformers
Today, not all transformers are created equal. With modern manufacturing pressures, some transformers are not constructed to the high standard as they have been in the past. Considering the cost to replace a faulty device, it is imperative to buy quality products to avoid costly maintenance.
“Plant engineers, facilities managers, general contractors, and specifying electrical engineers can learn a lot from the ‘post-mortem’ experiences of a CSI (crime scene investigation) technician such as (Jeff) Jones,” according to a report in Machine Design. “In most cases, the premature failures of transformers could have been avoided, and the culprit is often an inadequately designed or constructed unit.”
Jones himself explains, “Probably the most influential factor in transformer longevity is the level of craftsmanship, the attention to detail in the manufacturing process, and the quality control.”
At times, poor construction leads to faults. At other times, inferior materials are the cause of failure. VT testing is a critical step in diagnosing any issues before a catastrophic failure takes place.
Any piece of electrical equipment can be dangerous. VT testing requires adherence to safety guidelines, especially if the voltage is high. Indeed, if the transformer functioning in an altered or odd manner that defies safety standards, it is imperative to remove it from your system immediately. The statistics underline a cautious approach.
According to the Australian government, “between 2003– 2015:
- 142 workers died as a result of contact with electricity (an average of 11 workers each year)
- 87% (123) of these deaths happened when installing electrical infrastructure
- almost half of worker deaths occurred in the construction industry.”
Here are some guidelines to help your company maintain a high safety rating at all times and provide your employees with a safe environment when installing, inspecting or maintaining transformers:
- Turn off the power source before inspecting the transformer. Guard against any electrical leakage.
- Visually inspect the transformer for any damaged cords or improper installation and note if there is any burnt odor.
- Use personal protective equipment that is correctly rated for the voltage with which you are dealing with.
- Understand the maximum voltage of your transformer. Watch for a system overload.
- Remove any nuts and bolts from your immediate work area.
- Check the ground for the transformer to avoid static electricity and to protect the coils.
- Keep the work area dry.
- Set up caution signs to warn other staff about the possible danger.
VT testing is an exacting but necessary part of ensuring that the power supply remains reliable. By remaining vigilant in high-risk situations, your company will avoid any disastrous accidents. It is critical to select a well-trained employee, team or professional service provider to take care of your needs in commissioning, maintaining and diagnosing voltage transformers.
Are you looking for a reliable and experienced company to help you conduct VT testing? Contact us today!