What it is and when you need it
Power transformer tests are a set of electrical measurements that show the condition of the windings, the core, the insulation, the bushings and the tap changer without opening the unit. The transformer is usually the most expensive asset in a substation or an industrial plant, and an internal failure takes it out of service for months. That is why it is worth measuring at three moments:
- Commissioning, after transport and installation, to confirm that it arrived as it left the factory and that the connections are those of the project.
- Maintenance, periodically, to follow the trend of the insulation and the windings.
- After an event: a nearby short circuit, a differential or Buchholz relay trip, a lightning strike or a relocation.
We test power, grounding and auxiliary service transformers, at medium, high and extra-high voltage. It is part of our primary equipment testing service.
What is included
The scope is tailored to each case, but a complete set of tests includes:
- Turns ratio and vector group at all the agreed tap changer positions, with the deviation from the nameplate.
- Winding resistance per phase and per tap, which reveals loose connections, worn tap changer contacts or open turns.
- Excitation current per phase, sensitive to core problems and shorted turns.
- Short-circuit impedance and leakage reactance, which detects winding deformation. It is compared with the nameplate %Z.
- Power factor (tan δ) and capacitance of the insulation between windings and to ground, and of bushings with a test tap.
- Insulation resistance, polarization index (PI) and dielectric absorption ratio (DAR).
- Verification of ratios and polarities of the bushing CTs when they are part of the scope.
How we work
- Planning. We review the nameplate data, factory reports and earlier measurements. We define the test sequence, the taps to measure and the safety conditions with your operating team.
- Preparation on site. We verify the lockout, disconnect the bushings from busbars or cables, ground what is required and record oil temperature, ambient temperature and humidity, because several measurements depend on them.
- Execution. We use calibrated multifunction test sets (for example the Omicron CPC 100 with CP TD1 for tan δ and capacitance). The order matters: DC winding resistance leaves residual magnetism in the core, so excitation current is measured first, or the core is demagnetized after the DC test.
- Analysis. We correct for temperature where applicable and compare between phases, against the nameplate, against the factory values and against the historical trend. If a value does not comply, we repeat the measurement and rule out external causes, such as dirty connections or induction from neighboring lines, before concluding.
- Report. We deliver a report per test with values, conditions and result, and a summary with observations and recommendations.
Criteria and standards
Acceptance criteria come from IEC 60076-1, from IEEE C57.12.90 (test methods) and from the IEEE C57.152 guide for field diagnostic tests. In practice:
- Ratio: the deviation from the nameplate must stay within the tolerance set by the manufacturing standard for the corresponding tap.
- Winding resistance: it is corrected to the reference temperature and compared with the factory values and between phases; a marked difference on a single phase or a single tap points to a localized problem.
- Excitation current: in a three-phase three-limb core transformer, the normal pattern is two similar phases and one different; what matters is that the pattern repeats between tests.
- Short-circuit impedance: it is compared with the nameplate and with the previous measurement; a small but sustained change can indicate winding movement.
- Power factor and capacitance: they are judged against the factory values and the trend. For bushings, the reference is the bushing nameplate values.
When acceptance depends on factory or manufacturer values, we say so in the report and note where the comparison value came from. A single measurement says little; the trend over time is what best shows deterioration.
What you receive
- A report per test with measured values, measurement conditions, the criterion applied and the result.
- Ratio and resistance tables per tap, with the deviation of each.
- Comparison against factory values and against earlier measurements when you have them.
- Clear observations and recommendations: energize, repeat a test or investigate further.
- The test files for the unit, so the next measurement can be compared with this one.
Why Tiza Group
- One team accountable from start to finish: it plans, measures, analyzes and explains the results.
- Experience in substations and industry, from medium to extra-high voltage.
- Field judgment from design to commissioning: tests are interpreted with a view to how the transformer will operate and how its protection sees it.
- Calibrated test equipment, for repeatable measurements you can compare with confidence.
Field work is done in Colombia, or elsewhere on request. Contact us to plan your transformer tests.