DELTA4000 series 12 kV power factor/tan delta testers

A variable frequency PF/TD test is an expansion of a traditional PF/TD test, whereby PF/TD tests are performed on every insulation component (e.g., CH, CHL, and CL) at multiple frequencies (e.g., between 1 to 500 Hz) including the line frequency measurement. The preferred name of the test is a narrowband dielectric frequency response (NB DFR).

• Provides earlier indication of a problem in the dielectric than a line frequency power factor/tan delta (PF/TD) test does.
• Distinguishes between ‘power factor lookalikes’. NB DFR can differentiate the case, for example, whereby a 0.3 % line frequency PF/TD is truly acceptable (representing 0.5 % water content in a service-aged transformer) from the case whereby a 0.3 % line frequency PF/TD hides a condition of  increasing moisture (representing 2.0 % water content in an ‘as-new’ transformer). For two transformers in different stages of life, the same PF/TD means different things. And it is impractical to know conclusively whether a transformer is in ‘as-new’ condition or ‘aged’, much less which stage of ‘aged’ therein, as the chronological age of a transformer gives no accurate measure. Thankfully, NB DFR testing removes the need to know and simply tells if the insulation system is acceptable or not.
• Allows the possibility of determining the temperature-corrected PF/TD at 20 °C for the insulation system based on its actual condition (via ITC, individual temperature correction) and not from standard tables.

While NB DFR testing takes only a couple of minutes more to complete than line frequency PF/TD testing does, when multiple insulation systems must be tested, the cumulative additional time required to perform NB DFR testing may be deemed an inconvenience. PF/TD tests performed at 1 Hz is a smart compromise. Adding one test measurement to a line frequency PF/TD test requires less than a minute more of test time. Yet performing PF/TD testing with this relatively slow-varying applied voltage waveform (i.e., 1 Hz), provides an abundance of insight into the health of a dielectric/insulation that cannot be obtained at line frequency.

Absolutely. In fact, when PF/TD testing a transformer, the HV and LV windings are short-circuited. Therefore, the insulation between each winding turn is not stressed/not assessed. Excitation current tests are performed without short-circuiting the windings and, therefore, assess the condition of the winding(s) turns insulation. Beyond assessing this turn-to-turn insulation for full or partial breakdown, an excitation current test can detect winding to ground short-circuited conditions and tracking problems in the insulation, such as from one phase winding to an adjacent phase winding. Beyond insulation testing capabilities, an exciting current test is often recognised for its ability to detect problems with a transformer’s core and the test’s diagnostic reach with regards to tap changers, both de-energised (DETC) and on-load (OLTC) is impressive.

For years, the industry has relied upon a couple of curves to correct for the temperature dependency of all transformers: whether new, service-aged, lightly loaded, over-loaded, clean, or contaminated, etc. But, while generic correction factors were available in IEEE standard C57.12.90-2006, section 10.10.5, they were subsequently removed in C57.12.90-2010 with the following note: “Note 3.b) Experience has shown that the variation in power factor with temperature is substantial and erratic so that no single correction curve will fit all cases.” The bottom line is that new insulation and aged insulation have different sensitivities to temperature, as do contaminated insulation systems versus dry and ‘clean’ insulation systems. Temperature correction curves and temperature correction tables do not account for these differences.

A NB DFR test allows for the determination of an insulation system’s unique, or individual temperature correction (ITC). This is significant as testing has revealed that not only does every transformer exhibit unique sensitivity to temperature and require individual temperature compensation, but, over its life, the temperature dependency of a transformer can change. Generally, as insulation deteriorates, an increase in temperature causes power factor/tan delta (PF/TD) to increase dramatically. Also of interest is that a transformer’s individual insulation components (CH, CHL, and CL) may each exhibit differing temperature dependencies.

The ITC method is based on the fact that the shape of the dielectric response (PF/TD versus frequency) for a large group of solid dielectric materials does not change drastically with temperature . Further, as temperature changes, the response shifts with respect to frequency while remaining intact. A PF/DF value measured at 60 Hz and 20 °C will occur at a different frequency if the temperature changes. Therefore, if testing at a non-20 °C insulation temperature, one can locate the 20 °C equivalent 60 Hz power factor somewhere along the measured response if the frequency at which it occurs at that temperature is known. This frequency is determined through the application of the Arrhenius equation.

Not exactly. The DELTA4000 measures the harmonic content of the signal in every test and, based on this information, calculates a Voltage Dependence Factor (VDF). If this value is too high (default > 0.5), the number turns red, indicating a voltage dependence of the test object. In this situation, a tip-up (step voltage) test should be performed to verify and quantify the voltage dependence. This feature gives reassurance that a voltage-sensitive dielectric problem doesn’t go missed, particularly in an asset that doesn’t have a proclivity for developing voltage-sensitive dielectric problems and, therefore, is not routinely subjected to tip-up tests.

While a line frequency power factor/tan delta (PF/TD) test is not acutely sensitive to an emerging dielectric problem, it is sensitive to temperature. For example, it is expected that a PF/TD measurement at a top oil temperature of 30 °C will be higher than a PF/TD measurement on the same insulation component at 25 °C simply because of the influence of temperature. Therefore, it is important to compensate for any variances in temperature between tests if one is to trend test data and trust that a change in PF/TD is truly due to a change in the insulation system’s condition. This temperature dependency variable is removed by correcting all PF/TD test results, including those measured at non-line frequencies, to their equivalent 20 °C values.