Conference Record of the 2004 IEEE International Symposium on Electrical Insulation, Indianapolis, IN USA, 19-22 September 2004

Principals and Field Experience with the O.1Hz VLF Method regarding the Test of Medium Voltage Distribution Cables

Henning Oetjen HDW Electronics, Inc., SebaKMT Group

89 S . Commerce Dr., Suite 940 Bethlehem, PA 18017, USA hoetjen@hdwelectronics.com

Abstract: The IEEE Standards Group is currently addressing the topic of field testing methods for underground power cables. The first step has been accomplished by issuing the IEEE 400 document, which is also referred to a the Omnibus document and serves as the umbrella for aU field test methods to be covered under individual standards or user guides within the 400 series of

, documents. The IEEE 400.1 document will become the new test document covering all field DC test methods. The IEEE 400.2 wil l address Very Low Frequency AC field test methods, also referred to as VLF test methods. VLF testing is being considered as the safe alternative to DC Hipot testing of XLPE and mixed cables, which will cause premature failures in this type of cable because of,the known problem of induced space charges The paper compares different VLF technologies and discusses the

, correlation between field test data and test parameters.

INTRODUCTION

' Deregulation is having a big impact on the utility industry in the United States in many ways. Increasing the System

~ reliability has become one of the key objectives in the current environment, With regard to underground distribution cable systems consideration must be given to three components since they will affect the number of outages and their duration (SAIDI, Standard Average Interruption Duration Index / SAIFI, Standard Average Interruption Frequency Index)) These three components are restoration and specifically the employed fault locating techniques, preventive maintenance and predictive maintenance test methods. Fault locating

, techniques should be included in the considerations since they not only impact the SAIDI but can impact the data of any preceding or following preventive and predictive maintenance tests.

Preventive Maintenance Testing

DC Hipot testing as a proof or withstand test was initially used to test oil filled transformers and w e d out to be a reliable and meaningful test for PILL cables. However when applied to XLPE cables numerous studies have established beyond any doubt that DC Hipot testing will cause premature failure in this type of cable due to the formation of space charges. Because of the continuing replacement and substitution of PILC cables by XLPE cables the 0.1 Hz AC VLF proof test method was developed in the late 80's to replace the DC Hipot proof test. This change in test philosophy is also documented

by the fact that in the US as well as in Europe new test specifications are generated by the respective Standard Committees (IEEE and IEC), which eliminate the use of DC Hipot testing for XLPE cables.

Any preventive maintenance test is primarily a proof or voltage withstand test with a Go / No Go character. Initially the 0.1 Hz VLF proof test was no exception in this regard. The DC Hipot test fits the same characterization, however it offers in addition also some limited diagnostic information with regard to the cable insulation by means of its leakage current. Today's newest proof test VLF instruments can provide the same information. The 0.1 Hz cosine rectangular VLF technology bas the capability to measure directly the leakage current like in the DC technology. This feature is of particular interest since it allows correlation of previously gathered DC Hipot leakage data with new VLF test leakage data.

VLF Technologies

The two most widely used VLF technologies differ in the waveshape of the O.1Hz AC voltage. The 0.1 Hz VLF sinusoidal technology features a continuous sinusoidal wave shape with a half period time of 5 seconds. The cosine rectangular 0. IHz VLF technology represents the combination of a power frequency sinusoidal wave shape with a half period time of 2 to 8 milliseconds and a plateau phase of almost 5 seconds. This resembles a 50/60 Hz wave shape with a plateau phase of 5 seconds in between each polarity reversal.

Aside from this difference there are also common aspects to them. Both show a significantly accelerated growth rate for electrical trees when compared to that at power frequency (see figure 1). VLF testers using any 0.1 Hz AC frequency are substantially smaller in weight and size when compared to 60 Hz equipment. The ratio for the required input power for a given test voltage and cable capacitance between the 60 Hz and the O.1Hz cosine rectangular is 20 to 1 and 5 to 1 between the 0.1 Hz sinusoidal and the 0.1 Hz cosine rectangular wave shapes.

This is directly related to the fact that the full charging energy for each half wave must be provided respectively dissipated twice within each cycle. The 0.1 Hz cosine rectangular

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technology utilizes a patented energy recovery system (L C resonance circuit), which recuperates approx. 90 % of the required charging energy between half waves. The energy recovery is essential to accomplish a reversal in polarity within the same time period as under 60 Hz.

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Ted Voltqe V I Vo (RMS] --+ Figure 1 - Electrical tree growth rate ("h) as a function of test

voltage (RMS) and four different testing frequencies

The difference in wave shape has a significant effect on the rate of change of the potential (dvidt) during the transition from one to the other polarity. For 60 Hz power frequency this transition happens within 8 milliseconds. The sinusoidal 0.1 Hz wave takes 5 seconds or approximately 1000 times longer to change the polarity, which translates into a significantly lower rate of change of the electrical stress.. In comparison the cosine rectangular O.1Hz wave accomplishes the same transition in only 5 milliseconds (average), thus matching the 8 milliseconds of the 60 Hz power frequency very closely. The 0.1 Hz cosine rectangular technology exhibits a lower PD inception voltage for some defects because it replicates the rate of change of electrical stress at operating frequency (compared to 0.1 Hz sinusoidal, see Figure 2) while at the same time taking advantage of the accelerated growth of electrical trees at 0.1 Hz (see Figure I ) .

Because the sinusoidal technology cannot recovery the charging energy it is common to offer sinusoidal test sets with lower than 0.1 Hz test frequencies, i.e. .02 Hz, .05 Hz and even .01 Hz in order to be able to test cables with a large capacitance. When this solution is applied the duration of the test must be increased inversely proportional to the reduction in frequency, because the growth rate of the electrical trees is frequency dependent. Published test results show a significantly slower (IO times, see Figure 1) growth rate for O.OIHz compared to 0.1 Hz. The fact that the PD inception voltage is higher for lower frequencies still; makes it difficult to compare test results between 0.1Hz and 0.05 /O .02 / 0.01 Hz. No statistical field test data have been published to validate the equivalence of these lower frequencies in terms

meaningful test results when compared to 0.1Hz. The European Draft Standard provides only for the 0.1Hz frequency, the IEEE Draft Standard mentions lower frequencies, but provides no related test parameters in conjunction with them.

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Figure 2 -Needle fault and water tree- damage vs. inception voltage (RMS test voltage levels)

Another difference must be taken into account when comparing the test voltages for both technologies. In the European Draft Standard the test voltages are specified in RMS values (typical for AC testing) for both wave shapes. The newest IEEE draft, based on similar peak breakdown voltages of aged cables for the two waveforms, suggests peak values for the sinusoidal and RMS (= peak) for the cosine rectangular technology. In this aspect the opinions differ, it is undisputed that the PD or tree inception voltage depends on the peak voltage and its frequency, however the rate of the tree growth (see Figure 1) depends on the frequency and the available energy (energies at identical voltages RMS versus Peak are different by a factor of 1.96). The peak value of the equivalent test voltage for the sinusoidal 0.1 Hz wave equals between 2.8 & 4.2 times VO. In case of old cables this 40 % higher test voltage will canse PD inception in the cable system, which might not have existed before the test was started.

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Test Parameters

It is important to recognize that the test parameters for duration and voltage are not arbitrary chosen, but are the result

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from the experience with a large field data base in combination with a number of laboratory tests. Due to the nature of the failure mechanism in XLPE cables a voltage of a specific level (2 to 3 V,) should be applied for a test time of 60 minutes in order to convert critical sue water trees into electrical trees (voltage dependent) and have them grow and fail during the test time (time dependent). By maintaining a 60 minutes test time future in-service failures can he statistically avoided. Many times the cosine rectangular technology permits all 3 phases of a cable to be tested simultaneously, which compares very favorable with a 15 minutes Hipot test for each phase.

Test Parameters versus Meaning of Test Results

Since the late 80s when the 0.1 Hz cosine rectangular technology was first introdnced thousands of XLPE cables have been tested by applying a one hour test duration and a test voltage of 2 to 3 Vo. A cable, which passes this test protocol, will statistically not fail within the next 2-3 years. A change in test parameters (voltage and or time) will produce test results, which require a new interpretation, which itself will require a substantial and new database with field test results. Test methods that stress the cable in different ways (sinusoidal 0.1Hz vs. cosine rectangular 0.1 Hz) require their own individual database in order to provide a valid interpretation of the field test data related to them. This principle will stand even if the same or similar test parameters are applied.

10+ Years of Field Experience

In the last part of the paper a brief summary of the field test data is provided, which had been collected over a 15 year period, using the cosine rectangular 0.1 Hz VLF technology. Several hundred customers worldwide are using this technology since the late 1980s. The most significant results are as follows:

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During the 60 minute test @ 2 to 3V0 (RMs) 66% of all breakdowns occur during the first IO minutes, 75 % of all breakdowns occur during the Rrst 30 minutes Approx. 10% of all faults occur between 30 and 40 minutes Approx. 10% of all faults occur between 50 and 60 minutes Statistically no faults occur after 60 minutes, this also confirms, that no new faults in service aged cables are created because of the prescribed test parameters. PILC cables have an average fault rate of one per 20 miles and XLPE cables one per 14 miles All faulty joints fail regardless of their design within first 20 minutes (88% within fust 10 minutes) Faults in PLLC cables occur within the specified time in spite of a lower voltage level when compared to the level

in the DC Hipot test; this qualifies the cosine rectangular 0.1 Hz VLF test for mixed cable installations Existing water trees (XLPE cables) frequently fail shortly before the completion of the 60 minutes test; therefore it is important to adhere to the 60 minutes test duration, especially on older cables (Figure 3).

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Figure 3 -distribution of number of faults vs. test time

CONCLUSIONS

The cosine rectangular wave shape @own for over 15 years and described in the European Patent 0231459, November 21, 1986) combines 3 essential elements, which uniquely qualify it as a very effective Hipot test method for XLPE and PILC cables: - Low PD or tree inception voltage - An accelerated growth rate of electrical tree - Only VLF proof test technology to measure directly the approximate leakage current (fig.4)

Many tests by utility users have shown that a well structured VLF test program can reduce the occurrence of service failure significantly by failing local weak spots during test.

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and a good indication of the local condition of all cable types involved.

REFERENCES

1. Bach, R., m e r , W., Oldeboff, H. Spannungspriifungen ZUI Beurteilung von Mittelspannungskabelanlagen, Elektrizit2tswirtschaq Jg(92). 1993, Heft 17/18, S. 1068 ff. (also available in English)

2. Eager, Jr., G.S., Fryszczyn, B., Katz, C., ElBadaly, HA., Jean, A.R. Effect of D.C. Testing Water Tree Deteriorated Cable and a Preliminary Evaluation of V.L.F. as Alternative, IEEE Transactions on Power Delivery, Vol. 7, No. 3, pp. 1582-1591, July 1992.

3. Eager, G.S., Katz, C., Fryszczyn, B., Densley, J., Bemstein, B. S. High Voltage VLF Testing of Power Cables, E E E Transactions on Power Delivery, Vol. 12(2), pp. 565 - 670, 1997.

4. Koch, R., Neudert, E., Potzel, R. Wachstwn von TE-KanSlen in Kabelisolierungen bei Unterschiedlicben Spannungsbelastungen: Highvolt-Kolloquium 1999.

5. Neudert, E., Sturm, M. Characterization of Tree Process in XLPE by PD Measurement at 50 Hz and Very Low Frequencies, ICDI Budapest, 1997.

6. Schichler, U. Erfassung von Teilentladungen an Polymerisierten Kabeln bei der Vor-On-Priifung und im Netzbetrieb. Dissertation. Hannover 1996.

7. WeiOenberg, W., Goehlicb, L., Scharschmitt, J. Site tests of XLPE-insulated high-voltage cable systems with AC voltage, Elektrizit2tswirtscb&, Jg(96), 1997, Heft 9, S. 400 ff.

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