Tdworld 20468 Powerfactor1
Tdworld 20468 Powerfactor1
Tdworld 20468 Powerfactor1
Tdworld 20468 Powerfactor1
Tdworld 20468 Powerfactor1

Benefits of Power Factor Sweep Measurements on Bushings

Nov. 13, 2019
Power factor measurements are invaluable in identifying insulation defects in a power transformer

The power factor measurement has long been accepted as an invaluable tool for identifying insulation defects, such as ageing, deterioration, moisture ingress and localized failures, involving the winding and bushing insulation of a power transformer. Historically, power factor measurements are performed at a single test-frequency (typically 60 Hz) and at a single test-voltage (typically 10 kV). However, modern test instruments can perform power factor measurements at several different test frequencies and at several different test-voltages, with minimal additional time and effort.

Based on experience, performing power factor frequency sweep and voltage sweep measurements can help identify and confirm

  • Compromised insulation
  • User error
  • Test environment influencing a power factor measurement

In other words, the power factor sweep measurements help

  1. To better assess the condition of an insulation system
  2. To determine if the power factor measurements are even valid

Unfortunately, the power factor measurement is highly sensitive to the test environment (for example, sensitive to moisture on the surfaces of the bushings during the time of the test), to the test connections and to the test specimen’s earth-ground connection, among other things, so a simple tool that allows a user to better detect a “bad” power factor measurement is a useful tool.

Although power factor sweep measurements can be performed on the “overall” winding insulation of a power transformer, this paper focuses on applying the sweep measurements to the C1 insulation of bushings. The benefits of performing power factor sweep measurements on bushing insulation are discussed and demonstrated using several field case studies.

Finally, to be clear, the power factor sweep measurements are not the silver bullet or the holy grail of power factor testing, but they can be beneficial for the maintenance testing industry.

Who Can Benefit from Performing the Power Factor Sweep Measurements?

The Test Equipment Operator

Since the power factor measurement is highly sensitive, obtaining the correct power factor measurements in the field is challenging. In many cases, a questionable power factor measurement is not caused by compromised insulation but by either user error or the influence of the test environment.

Unfortunately, a bad measurement is often not identified until the user leaves the job site, places the transformer back into service and returns to the office. Often, the user only has a short window of time to test a power transformer, and therefore, there is only “one shot” to obtain the correct measurements.

With a power factor measurement at one test-voltage and at one test-frequency, it is difficult for the user to determine if a power factor measurement is even valid. However, invalid measurements often become obvious when the power factor sweep measurements are performed and analyzed.  Therefore, the test equipment operator should use the power factor sweep measurements as a tool to quickly identify and correct bad power factor measurements before they leave the job site with the incorrect test results.

Based on my experience from working directly with test equipment operators, I feel that there are many bad power factor measurements that go undetected, and unfortunately, a bad power factor measurement is a significant waste of a company’s resources.

The Engineer

In most cases, the test equipment operator obtains the power factor measurements in the field and distributes the test results to the engineer(s) via a test report. Then, based on the results, the engineer is responsible for assessing the condition of the insulation system to determine the appropriate course of action.

Often, there is a disconnect between the test equipment operator, who performs the measurements in the field, and the engineer, who assesses the test results in the office. In most cases, the engineer is not on-site when the measurements are performed, and therefore, it is difficult for the engineer to be confident that the measurements are even valid. However, if the engineer has the power factor sweep results close at hand, then they can better identify invalid measurements, which helps prevent an incorrect condition assessment.

Moreover, it is widely known that the best way to assess a power factor measurement is to compare the most recent measurement to a series of previous baseline measurements that were obtained at consistent test intervals. Unfortunately, many asset owners do not have a collection of reliable previous test results for their transformer fleet, which makes assessing the condition of an insulation system and determining the appropriate course of action a challenge. Fortunately, the power factor sweep measurements can be used to better assess the condition of an insulation system at a given point in time, especially when there are no historical test results to compare to.

Power Factor Sweep Test Analysis

The analysis of both the voltage sweep and frequency sweep measurements is performed visually. The power factor measurements are plotted versus the applied test-voltage and versus the applied test-frequency, and the condition of the insulation is assessed based on the shape of the plots (the traces). Although this paper focuses on applying the power factor sweep measurements to the C1 insulation system of bushings, the analysis strategies discussed are valid for assessing the power factor sweep measurements performed on the “overall” winding insulation of a power transformer.

The guidelines used to visually assess the power factor sweep traces are discussed later. In general, the analysis involves determining if the shape of a trace is “normal” or “abnormal”. If either of the sweep measurements produces an abnormal trace, then the insulation system should be investigated, and/or tested more frequently in the future.

The Power Factor Voltage Sweep Test (Voltage Tip-Up Test)

Performing a power factor measurement at multiple test-voltages helps identify both compromised insulation and “bad” power factor measurements. At a minimum, an oil-and-paper insulation system should be tested at two test-voltages (for example, at 2 kV and 10 kV). In most cases, the power factor measurement performed on bushing insulation should not be voltage-sensitive. Therefore, regardless of the applied test-voltage, the measured power factor value should be the same.

If the measured power factor value is not reasonably similar when comparing the same power factor measurement at two different test-voltages, then the measurement is questionable and should be investigated. To investigate further, the power factor measurement can be repeated at four or five different test-voltages (for example, 2 kV, 4 kV, 6 kV, 8 kV and 10 kV) to establish a definitive “pattern”.

In many cases, when a power factor measurement is invalid, the power factor measurement becomes voltage-sensitive. Therefore, by simply performing a power factor measurement at two different test-voltages—first, at a relatively low test-voltage (at 1 kV or 2 kV, for instance) and second, at a relatively high test-voltage (at 10kV, for example)—“bad” power factor measurements can be better detected, relative to a single power factor measurement at one test-voltage.

When the insulation of a bushing begins to deteriorate, the C1 power factor measurement for that bushing often becomes voltage-sensitive. Therefore, at a minimum, a bushing C1 power factor measurement should be performed at two different test-voltages (for example, at 2 kV and 10 kV). If the power factor value obtained at the two different test-voltages is not reasonably similar, then the C1 insulation of the bushing is typically deemed questionable. Multiple cases provided in this paper demonstrate this “voltage sensitive” phenomenon. Note, however, that there are a few bushing types that produce C1 power factor measurements that are slightly voltage-sensitive, even when the C1 insulation system of the bushing is healthy.

The Power Factor Frequency Sweep Test

Performing a power factor measurement at multiple test frequencies helps to better identify both compromised insulation and bad measurements. The power factor frequency sweep test involves performing power factor measurements at a series of different test frequencies (for example, 15 Hz, 30 Hz, 45 Hz, 60 Hz, 150 Hz, 200 Hz, 300 Hz and 400 Hz). The general guidelines used to assess a frequency sweep trace are provided below. The guidelines are most appropriate for analyzing C1 power factor measurements performed with an oil temperature at or around 20°C.

  • In general, if an oil-and-paper insulation system is healthy, then the measured power factor value increases versus frequency (from left to right). In other words, the frequency sweep trace climbs uphill versus frequency.

  • As an oil-and-paper insulation system deteriorates, the frequency sweep trace typically becomes flat, or worse, decreases versus frequency (from left to right). If the trace decreases versus frequency throughout all or most of the frequency sweep, then the insulation system is typically considered questionable and is either investigated further and/or tested more frequently in the future.

  • Another characteristic of compromised insulation is a distinct “fish hook” in the lower frequency range of the sweep (that is, at frequencies below 60 Hz). If the frequency sweep trace produces a definitive fish hook in the lower frequency range, then the insulation system is typically considered questionable and is either investigated further and/or tested more frequently in the future.

  • One advantage of performing power factor frequency sweep measurements on the C1 insulation system of a bushing is that, in most cases, a bushing mounted on a power transformer has two or three similar unit bushings that can be tested and compared with each other. In general, the power factor sweep measurements should behave similarly when comparing similar unit bushings. Ideally, the traces for similar unit bushings “overlay” or “overlap” when plotted against each other. Most importantly, the shape of the frequency sweep traces should be reasonably similar, when comparing similar unit bushings. If the shape of the trace of one bushing is dissimilar relative to the shape of the traces of the other similar unit bushings, then the dissimilar bushing is either investigated further and/or tested more frequently in the future.

The C1 power factor frequency sweep measurements for four different sets of similar unit bushings are provided in Figure 1. Notice that the shape of the traces is similar when comparing the similar unit bushings. Also, for all the traces in Figure 1, the measured power factor increases versus frequency (from left to right), which is typically indicative of healthy insulation.

Case Studies

The following case studies demonstrate the value of performing power factor sweep tests on the C1 insulation of a bushing. The cases involve power factor sweep measurements that helped identify both compromised insulation and bad measurements.

Case Study #1: HAEFELY 115 kV Bushings (2000)

The first case involves the power factor measurements that were performed on three HAEFELY 115 kV bushings. The power factor measurements are provided in Figure 2.

Based on the results in Figure 2, the 10 kV power factor for H3 is higher than its nameplate value, whereas the 10 kV power factor for the other two bushings is below their respective nameplate values. Also, notice that the 2 kV and 10 kV power factor measurements for the H3 bushing are dissimilar. In contrast, the 2 kV and 10 kV power factor measurements for the H1 and H2 bushings are reasonably similar, which further suggests that the H3 measurements are abnormal.

When comparing the power factor frequency sweep traces amongst similar unit bushings, the shape of the traces should be reasonably similar. Clearly, the shape of the H3 trace is dissimilar relative to the traces of the other two bushings. Moreover, the H3 trace decreases versus frequency and has the distinctive fish hook in the lower frequency range, which is typically indicative of compromised insulation.

Interestingly, the 10kV power factor for H3 is below 1.5 to 2 times its nameplate value, which the industry generally considers to be acceptable[1]. However, the H3 bushing is clearly behaving differently than the other two similar bushings, which is a cause for concern. This case demonstrates that the power factor sweep measurements are more sensitive to compromised insulation than the conventional 10kV power factor measurement. The H3 bushing should be flagged as the weak link among the three bushings, and if returned to service, should be tested more frequently to ensure that its insulation does not rapidly fail.

Case Study #2: Lapp POC Series 2 115 kv Bushings (1998)  

The second case involves the power factor measurements performed on three Lapp POC Series 2 115 kV bushings. The power factor measurements are summarized in Figure 3

Note, the power factor frequency sweep trace for the H3 bushing is not plotted since its 10 kV power factor value is three times its nameplate value, which is already enough to condemn the H3 bushing. In other words, the power factor frequency sweep trace for the H3 bushing only confirms “what we already know”.  Additionally, the 2 kV and 10 kV power factor values for the H3 bushing differ significantly, which can be used as further evidence to condemn the H3 bushing.

Based on the results in Figure 3, the 10 kV power factor value for H1 is higher than its nameplate value, whereas the 10 kV power factor value for the H2 bushing is below its nameplate value. Also, the frequency sweep trace for the H1 bushing is clearly dissimilar relative to the frequency sweep trace for the H2 bushing. The H1 trace decreases versus frequency and has developed the distinctive fish hook in the low-frequency range, which again, is typically indicative of compromised insulation. Although the 10 kV power factor for H1 is well below 1.5 to 2 times its nameplate value, the H1 bushing is clearly behaving differently than the H2 bushing. As a result, the H1 bushing should be tested more frequently to ensure that its insulation does not rapidly fail.

Case Study #3: Lapp POC 138 kV Bushings (1998)

The third case involves the power factor measurements that were performed on three Lapp POC 138kV bushings. The power factor measurements are summarized in Figure 4.

Based on the results in Figure 4, the power factor values for all three bushings are higher than their respective nameplate values. Also, notice that the power factor measurement for the H2 bushing is clearly voltage-sensitive. Moreover, notice that the power factor frequency sweep traces for the three bushings are erratic and jagged. In general, jagged power factor frequency sweep traces are indicative of bad power factor measurements. Regardless of whether an insulation system is healthy or compromised, its power factor frequency sweep trace should be smooth.

By only analyzing the 10 kV power factor measurements, it is not obvious that the power factor measurements for the three bushings are invalid. However, with the voltage and frequency sweep test results, it is obvious that the power factor measurements are bad. In this case, the customer determined that the primary-side bushing terminals were not short-circuited together when the C1 power factor measurements were performed, and therefore, identified the cause of the abnormal power factor measurements.

Conclusion

With both the voltage and frequency sweep measurements close at hand, invalid measurements are easier to identify. The equipment operator can use the sweep measurements to quickly identify and correct invalid measurements in the field before the transformer is placed back into service. The engineer can use the sweep measurements to identify invalid power factor measurements, especially when they must rely on the data provided within a test report. Finally, power factor sweep measurements help to accurately assess the condition of a bushing’s insulation system. This helps the technician choose the best course of action.

References

1. IEEE C57.152-2013 "IEEE Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors"

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