Generator Assessment and Tan Delta Testing: Online Condition Monitoring Indonesia

Direct answer (AEO): Generator assessment combines insulation diagnostics, electromagnetic condition checks, and mechanical measurements to establish whether a generator can continue in service safely. Tan delta analysis — the measurement of insulation dissipation factor at increasing voltage — is the key insulation-health test, and it is complemented by partial discharge monitoring, end-winding vibration measurement, rotor and excitation system checks, and stator winding condition assessment. In an online condition monitoring Indonesia programme, generators are evaluated on two tracks: continuous monitoring while running, and offline tests such as tan delta performed during scheduled outages, with the two sets of results interpreted together.

Need a generator assessment partner in Indonesia? Contact Tiaravib — email info@tiaravib.com or WhatsApp +62 821-1378-3727.

What Tan Delta Analysis Reveals About Generator Insulation

An ideal insulating material behaves purely capacitively: current through it leads voltage by ninety degrees and no energy is dissipated. Real insulation is not ideal — it has losses from conduction, from polarisation of water and contaminant molecules, and from partial discharges within voids and at interfaces. Tan delta, also called the dissipation factor or insulation power factor, is the tangent of the angle by which the current’s phase departs from the ideal ninety degrees. It is a direct measure of how much energy the insulation is wasting, and therefore of how degraded it is.

The test is performed by applying an increasing AC voltage to the winding and measuring tan delta at each step. Because the losses change with voltage in a way that is characteristic of the defect, the shape of the tan delta curve is as informative as any single reading.

Reading the tan delta curve

A sound winding produces a low and essentially flat tan delta curve across the test voltage range. Three deviations carry diagnostic weight. A uniformly elevated value across the whole range indicates generalised deterioration — typically moisture, overall ageing, or widespread contamination. A curve that rises steeply as voltage increases, called a tip-up, points to ionisation beginning inside voids, which is a localised defect that will progress. A curve that is high even at low voltage and rises only gradually suggests moisture or surface contamination rather than internal voids. Separating these patterns determines whether the correct response is a drying and cleaning programme, a targeted repair, or rewinding.

Test conditions and comparability

Tan delta results are strongly influenced by temperature and humidity. A measurement taken on a humid morning with a cold winding is not comparable to one taken in dry conditions with a warm winding. For the trend to be meaningful, tests must be performed at comparable temperatures with temperature corrected, on insulation in the same moisture state, and with the same instrument and connection arrangement. The value of tan delta analysis rests almost entirely on this comparability; a single isolated measurement tells you very little.

Building a Complete Generator Assessment

Tan delta is one test among several, and generator conclusions should rest on the whole set. The assessment divides naturally into insulation, electrical, and mechanical domains.

Insulation condition

Beyond tan delta, insulation condition is assessed through insulation resistance and the polarisation index, which together indicate moisture and contamination; partial discharge measurement, which locates and characterises active degradation; and dielectric loss measurements across a range of voltages. For a generator that cannot be taken offline frequently, the offline tests are scheduled in the outage window and their results compared against the unit’s historical record.

Electrical and electromagnetic condition

A generator in service can be assessed continuously for electrical degradation. Partial discharge can be measured online using couplers on the stator winding, allowing active degradation to be observed under real operating voltage and temperature. Current signature analysis detects rotor winding problems, including broken rotor bars on induction machines and excitation anomalies on synchronous units. Stator winding temperature distribution compared against load reveals whether cooling is being distributed evenly, and flux or core monitoring identifies stator core faults. The generator assessment service combines these so that the electrical condition is characterised continuously rather than only at outages.

Mechanical condition

A generator is also a rotating machine, and its mechanical condition needs ongoing attention. End-winding vibration is measured because end windings experience electromagnetic forces at twice line frequency and are a known failure location, particularly on older windings or after a rewind. Stator core vibration indicates looseness or core problems. Rotor vibration, bearing condition, and shaft runout are covered by the same vibration analysis applied to any rotating machine, and where the generator is driven by a turbine, the rotor and coupling conditions require joint assessment with the turbine’s own condition data.

Comparing the Assessment Techniques

Online and Offline: Interpreting the Two Together

The strongest generator assessment combines continuous online data with periodic offline testing, using each to interpret the other. The relationship works in both directions.

Online data points to where offline attention is needed. A gradual rise in online partial discharge activity, or a change in the current signature, tells the engineer which winding or which part of the machine to examine most carefully during the next outage. Without that signal, an offline test round treats the whole machine uniformly and can miss a developing localised defect.

Offline tests calibrate the online interpretation. A tan delta result that shows moisture and a mild tip-up explains why the online PD activity is elevated at times of high ambient humidity. Knowing the insulation state means the online trend can be read as a moisture-driven variation rather than as a new defect, which prevents both a false alarm and a missed change.

This mutual interpretation is the reason generator monitoring is organised as a programme rather than as a series of tests. A tan delta value on its own, without an online baseline and a history, is an isolated number; the same value read against five years of trend data, online PD, and winding temperatures is an accurate statement about remaining insulation life.

Planning Generator Testing Around Outages

Generator insulation testing requires the machine to be offline, and the practical challenge is getting the test into an outage window without either compromising the test or compromising the outage plan.

Offline testing should be scheduled early in the outage, before other work disturbs the machine, and before any cleaning or drying operation that would change the insulation state being measured. Test conditions should be recorded — winding temperature, ambient temperature, and relative humidity — because without them the result cannot be compared with the unit’s history. Where a unit has a long testing history, it is worth ensuring that the same connection arrangement and instrument are used each time, since changes in the test setup can produce variations comparable to the deterioration being tracked.

Where a generator has no history at all, the first test establishes the baseline and no diagnosis should be drawn from it in isolation. A second measurement, taken under comparable conditions, is what makes the first one useful. Units with no test history are therefore a priority for the first assessment round, and the results become the reference for every subsequent test.

Standards and Safety

Generator insulation testing follows the established international framework. Tan delta and insulation power factor measurement is covered by IEC 60034-27 for the stator winding insulation of rotating electrical machines; partial discharge measurement on rotating machines is also addressed by that family of standards, with offline and online measurement defined separately; insulation resistance and polarisation index procedures follow IEEE 43; and vibration severity on the mechanical side follows ISO 20816 for the machine class. Applying test voltages to a large generator winding is a formal safety activity requiring isolation, earthing, and a permit regime, and no test should be attempted while any other work is in progress on the same machine. Test planning should start from the unit’s isolation arrangement and the site’s switching procedure.

The Institute of Electrical and Electronics Engineers publishes the insulation diagnostic standards, including the IEEE 43 and 286 series, that govern tan delta and insulation resistance testing.

International Electrotechnical Commission standards define the insulation test procedures and acceptance limits applied to rotating machine and power apparatus windings.

Key Takeaways

• Tan delta measures dielectric loss in stator insulation and reveals moisture, contamination, voids, and thermal ageing.
• The shape of the tan delta curve — flat, uniformly elevated, or showing tip-up — carries more diagnostic weight than a single value.
• Results are only comparable if temperature, humidity, and test setup are controlled and recorded.
• Generator assessment combines insulation, electromagnetic, and mechanical domains; no single test is sufficient.
• Online partial discharge and current signature data direct offline testing to the right places, and offline tan delta explains online trends.
• Offline tests belong early in a planned outage, before cleaning or drying changes the insulation state.

Frequently Asked Questions

What does tan delta tell you about a generator?

Tan delta measures the dielectric losses in the stator winding insulation, which increase as the insulation absorbs moisture, becomes contaminated, develops internal voids, or ages thermally. A low, flat curve indicates sound insulation, while an elevated or rising curve indicates deterioration. The most useful conclusion comes from comparing the current result with the unit’s own previous tests taken under comparable conditions, rather than from a single measured value.

Why is a tip-up in the tan delta curve significant?

A tip-up means tan delta rises steeply as test voltage increases, which indicates that ionisation is beginning inside voids in the insulation. Unlike general moisture or ageing, which raise the whole curve uniformly, tip-up points to a localised internal defect that will tend to grow. It usually justifies a closer examination, increased test frequency, and consideration of a targeted repair or rewind rather than simple drying.

How does tan delta relate to partial discharge testing?

They are complementary insulation diagnostics. Tan delta gives a whole-winding measure of dielectric loss and is sensitive to moisture and widespread ageing, while partial discharge detects and can locate active localised degradation such as voids and slot discharge. A generator with elevated tan delta but low PD activity typically has a moisture or contamination problem, while a unit with low tan delta but rising PD activity may have a localised developing defect that only detailed examination will reveal.

Can generator insulation condition be monitored online?

Partially. Online partial discharge monitoring with couplers on the stator winding, current signature analysis, winding temperature distribution, and mechanical vibration are all measured continuously while the generator runs. Tan delta, insulation resistance, and withstand tests require the machine offline. The generator assessment service therefore uses online data to watch for change during operation and reserves offline testing for scheduled outage windows.

How often should a generator be assessed?

Continuous online monitoring runs permanently on critical units. Offline insulation tests such as tan delta are performed during major outages, which for many generators means every few years, or more often on units with a known abnormal condition or a long service history without testing. After any finding, the interval shortens until the trend is clearly established as stable.

What causes high tan delta in generator stator insulation?

The common causes are moisture absorbed into the insulation, surface contamination from dust or oil, thermal ageing of the insulation system, and internal voids that produce ionisation losses under voltage. Moisture is frequently the dominant factor in tropical climates, which is why the result should be assessed together with the unit’s operating history and ambient conditions rather than compared to a single fixed threshold.

For a generator assessment or tan delta testing programme, contact Tiaravib via WhatsApp +62 821-1378-3727 or info@tiaravib.com.

Related reading on the Tiaravib site: switchgear monitoring, partial discharge analysis, Tiaravib project experience and the reliability articles archive. External references: the International Electrotechnical Commission and the Mobius Institute certification body for condition monitoring competence.