What is VLF - High Voltage Inc

VERY LOW FREQUENCY (VLF) AC TECHNOLOGY
VLF Overview & Answers to Frequently Asked Questions
VLF-30CM
0-30 kV @ 0.4 uF
VLF-12011CMF
0-120 kV @ 5.5 uF
VLF-6022CM
0-62 kV @ 5.5 uF
VLF-34E & VLF-65E with TD accessories
Solid State, Programmable, Wireless
0-34 kVac and 0 – 65 kVac
Many more models are available from 30 kVac – 200 kVac.
VLF FAQ
This paper is written to answer the many questions concerning Very Low Frequency AC Hipots and their applications. It is
not written to make the argument for VLF AC cable testing versus DC. That case has been made for over 20 years, with near
unanimous worldwide consensus that DC testing is not only damaging to solid dielectric cable insulation but is also an
ineffective means of determining the insulation quality of a cable. There is good reason why most utilities and
industrial/commercial applications worldwide have stopped DC testing and instead perform VLF AC Withstand, VLF Partial
Discharge, and/or VLF Tan Delta testing.
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What is VLF?
VLF is the abbreviation, and commonly used name, for Very Low Frequency. VLF AC hipots produce a frequency output of
0.1 Hz and lower rather than the conventional 50 Hz or 60 Hz. VLF technology was developed for a specific reason, to more
easily high voltage field test certain loads of high capacitance normally requiring very high current and power to test at the
traditional power frequencies of 50 Hz. or 60 Hz.
Why 0.1 Hz?
When using AC high voltage to test a load, the lower the frequency of the applied voltage the lower the current and power
required. The easiest and most economical way to AC high voltage field test high capacitance loads, like cables and
motors/generators, is to use a VLF AC hipot. A frequency of 0.1 Hz. was selected long ago as the standard frequency to be
used for VLF testing. World standards permit, and most VLF instruments deliver, output frequencies of 0.1 Hz. - 0.01 Hz.
For cable testing, 0.1 Hz – 0.01 Hz. is permitted, although 0.1 Hz. and 0.05 Hz. are preferred for use. For motor and
generator coil testing, 0.1 Hz. is required. For some other testing using VLF, explained later 0.1 Hz. is required.
The Math behind VLF Testing
There is no mystery to VLF technology. It conforms to basic physics and established principles of electricity. The most
elemental equation governing the laws of electricity is Ohm’s Law: I = V/R. The higher the denominator R (resistance), the
lower the I (current) when a V (voltage) is applied. In AC circuits, the load is mostly capacitive. That means that the R of a
circuit is directly proportional to its operating frequency. R is the Capacitive Reactance, Xc, and calculated using 1/C,
 = 2f and C is the fixed capacitance of the load. Xc (capacitive reactance) = 1/2πfC.
Example: A 15 kV cable 5,000’ (1500 m) long has approximately 0.5 uF of capacitance. At 60 Hz. the capacitive reactance is
5300 ohms. At a common test voltage of 22 kVac rms, it would require a power supply rated for 4.2 amps, or 91 kVA. A
very large and expensive test set; obviously not very practical for field use.
At 0.1 Hz. the capacitive reactance is 3.2 megohms. The same 22 kV rms test would draw 7 mA, or .154 kVA, 600 times less
than at 60 Hz. At 0.01 Hz, a cable 6000 times longer can be tested than at 60 Hz.
Put another way, at 60 Hz. a cable must be charged to its test voltage every 4.2 milliseconds: 0 – 90° (the peak) of the
waveform. At 0.1 Hz, 0 – 90° takes 2.5 seconds, permitting 600x more time to charge the cable, requiring a far less powerful
voltage source.
Is 0.1 Hz still AC?
Yes. The wave shape of the HVI VLF design is sinusoidal with polarity reversals every half cycle, only at a slower rate than
60 Hz. Frequencies as low as 0.01 Hz are recognized in the IEEE 400.2-2013 standard as useable for cable testing, although
typically 0.1 Hz. – 0.05 Hz. are preferred. VLF is not DC, where a monopolar negative polarity is applied to the load for long
periods of time, which causes space charges to develop within the insulation through a polarizing dipole effect on the
molecules and a stored energy to develop within the load. Again, VLF is an AC (alternating current) voltage, sinusoidal with
conventional polarity reversals every half cycle.
Where is VLF used?
VLF testing is principally used for two applications: AC field testing medium and high voltage cable and testing rotating
machinery - motor and generator coils. These two applications are defined and sanctioned by several world standards,
including IEEE 400.2-2013 for cable testing and IEEE 433-2012 for testing rotating machinery. VLF can be used for testing
other high capacitance loads like large insulators, arrestors, bus duct, etc. but no recognized standards exist.
One of the best applications for the use of VLF is to check installation quality of cable and accessories, like splices and
terminations. Many in service failures are due to damage to cable during its installation, improper workmanship, faulty
materials, etc. These in service failures can be prevented through VLF Acceptance testing after installation and after every
cable fault repaired. For motor and generator coil testing, producers of these products must factory test their coils with
50/60 Hz. power frequency AC Dielectric test sets, or hipots. However, rewind and repair shops and field maintenance
testing can use VLF and should due to the size, weight, and price advantage over the use of prohibitively large, heavy,
expensive, and difficult to set up 50/60 Hz. hipots.
What VLF hipots are available?
High Voltage, Inc. produces VLF hipots that produce from 30 kVac up to 200 kVac with load ratings from 0.4 uF to 50 uF,
the equivalent capacitance ratings of cable approximately 3000’ (914 m) to 40 miles (64 km) in length and can test the
largest of generator coils. Both manual controlled, conventional oil cooled designs with analog controls (knobs, meters,
switches, etc.), and fully programmable, automatic, wireless, PC controlled solid state designs are available.
How do you do the test?
The test is very simple. With the cable to be tested isolated from any voltage source, connect the high voltage output lead
of the VLF to the conductor and a common ground to the shield. Like any hipot, apply the test voltage for the required
duration. A basic withstand test is that easy. Other diagnostic tests, yet to be described, are a little more complicated.
What’s the test voltage and for how long?
The IEEE 400.2-2013 standard offers precise test voltages for medium voltage and some high voltage cable. (Generally
however, the test voltages are approximately 1.7 - 3 times (1.7 Uo - 3 Uo) the normal line-to-ground voltage for 30 – 60
minutes, with the multiple number depending on the voltage rating and thickness of the cable insulaiton.) A chart of the
test voltages is below. The European standard mandates 3 Uo (3 times the normal line-to–ground voltage) for 60 minutes
for any cable. Other countries have also written standards for VLF cable testing.
For a 15 kV cable, the Maintenance test is usually performed at 22 kVac peak and the Acceptance test voltage is 30 kVac
peak. A 35 kV cable is Maintenance tested at 47 kVac peak and Acceptance tested at 62 kVac. The standard includes cables
rated to 69 kV, although 200 kVac peak VLF units are available for testing cables rated up to 150 kVac.
IEEE Std. 400.2-2013
Cable Rating
phase to phase
kV rms voltage
5
8
15
25
35
46
69
VLF Test Voltage for Sinusoidal Waveform
Installation
phase to ground
peak voltage
13
16
27
41
55
72
106
Acceptance
phase to ground
peak voltage
14
18
30
45
62
81
119
Maintenance
phase to ground
peak voltage
10
14
22
34
47
61
89
Different VLF units output different waveforms. What’s best?
All HVI VLF units, and most others, produce a sine wave output. The original German designs, which are still offered, do not
produce a sine wave output. They produce a trapezoidal, or cosine-rectangular, waveform. The cosine-rectangular
waveform works well to VLF hipot cable; however, it is not as usable as a sinusoidal design as a voltage source for Tan Delta
and Partial Discharge testing. For a VLF unit to be used for diagnostic testing, either Tan Delta of Partial Discharge, it should
produce a sine wave. The IEEE recognizes the sine wave output as advantageous and mandates it when VLF is used for
testing rotating machinery. Stick with a sine wave design to keep your future diagnostic testing options open.
Is the VLF test destructive?
VLF hipoting is not destructive to good insulation and does not lead to premature failures like DC voltage testing. Using
VLF does not cause degradation of good insulation nor aggravate defects too small to be triggered into PD under the test
voltage. It does cause existing cable defects that are severe enough to be triggered into partial discharge under the test
voltage, to break through, or fail, during the test. Minor defects that are not triggered into PD from the test voltage are
unaffected. If a cable can’t hold 2 – 3 times normal operating voltage, it is not a reliable cable. Cause failure at the defect
location during a controlled outage or prior to energizing newly installed or repaired cables, find the fault, make the repair,
and be left with a good cable. It is AC voltage; what the cable is designed for and experiences during service. Cable is
factory tested with AC voltage at levels far higher than the field test.
But my cable might fail during the test
Precisely, that is the point of AC Withstand testing. It is not a diagnostic test. It is an AC stress test, or proof or pressure
test. There are no leakage current readings to measure. A cable either holds the test voltage or fails. If this method of
testing is not acceptable, there are diagnostic tests that can be performed that nearly eliminate the chance of cable failure
during the test. These tests allow the user to learn something of the cable insulation rather than possibly cause a failure
during the test. See the Tan Delta and Partial Discharge sections of this FAQ.
Who endorses VLF?
Nearly every applicable engineering body in the world, cable producers, and the hundreds of utilities worldwide that use
the over 5000 VLF units shipped by HVI and others over the past 20 years. EPRI, IEEE, IEC, CEA, VDE, other countries
engineering organizations, nearly every cable manufacturer, and many utilities throughout the world have embraced the
effectiveness of VLF testing. German VLF test standards (DIN-VDE Standard 0276-620 & 0276-1001) have existed for many
years and the IEEE has released an updated VLF specific cable testing standard - IEEE 400.2-2013. IEEE 433-2012 for VLF
testing of rotating machinery has existed for over 40 years, originally released in 1974.
What are the alternatives to VLF Withstand testing cables?
Not many, when you consider the available technologies and weigh the costs, effectiveness, ease of use, and other factors.
A 50/60 Hz. power frequency AC Withstand test is not usually an option, as described earlier. Certainly DC should no longer
be used since it leads to future cable failures and tells little about the cables insulation and accessories quality. One can
reduce the DC test voltage, but then the test is even less meaningful. A 5 kVdc megohmmeter IR test on a 15 kV cable that
operates at over 10 kVac peak stress when in service reveals little or nothing about a cable’s quality, nor does a 24 hour no
load on-line soak test. There are several “diagnostic” tests possible but many are experimental, esoteric in their theory and
design, and not economically feasible; except for off-line Tan Delta and Partial Discharge testing, described below.
VLF Diagnostic testing methods
There are times when a go/no-go, or pass/fail Withstand test is not suitable. We would rather learn something about the
health of the cable system without risk of failure during the test; a test that measures the quality, or integrity, of the
insulation and its expected life is preferred. A VLF hipot can be, and often is, used as a variable voltage source to apply an
overvoltage to the test object while various measurements and observations are made about the insulation quality.
Two common methods of using VLF technology to perform off-line elevated voltage diagnostic, or non-destructive, testing
on cables and rotating machinery are Tangent Delta and Partial Discharge. Both use a VLF instrument to apply a variable
voltage to the load while diagnostic measurements are taken. Based on these measurements, the voltage can be raised up
to perhaps 1.7 Uo to 2.0 Uo the normal operating voltage, but only for very brief periods, perhaps only seconds. From the
data gathered, judgments can be made as to the integrity and expected life of the insulation.
Tan Delta, or Tan δ, Dissipation Factor (analogous to power factor), testing is performed to provide an overall assessment
of the integrity of the insulation, usually compared to the ideal or to many other tests to prioritize where replacement,
repair, or rejuvenation money should be spent. This method works well, is easy to perform with minimal training, and is
relatively easy to interpret results. TD testing is a very common test performed by many.
Partial Discharge testing exposes specific locations and severity of troublesome electrical discharges along a cable path or
the overall electrical noise (PD) within a coil. PD testing is more difficult to perform and interpret than Tan Delta and is
more expensive. However, both are valid but reveal very different data sets. PD testing is commonly performed on cables,
substation apparatus, and other gear but at 50/60 Hz. PD testing cables and generator coils using 0.1 Hz. is a newer
technology but no less possible or reliable than at power frequency.
The surest way to weed out bad cables and accessories is to perform a simple, over voltage AC hipot test, just like we do
with vacuum bottles, arrestors, hot sticks, switchgear bus, insulators, etc. Yes the cable may fail under test if it has a severe
defect, but that’s the point of the test. If a cable can’t withstand 2 - 3 times its normal voltage for 30+ minutes, it’s bound
to soon fail. Cause failure when convenient to repair, rather than waiting for an in-service failure to occur at the worst
possible time. If preferred, VLF voltage sources can be used to perform non-destructive diagnostic testing, like the Tan
Delta/Power Factor and Partial Discharge methods described earlier.
Summary
Very Low Frequency technology is readily available and has been used for decades to AC voltage test cables and rotating
machinery in the field that was never before practical, nor even possible in many cases. IEEE, IEC, and other Standards exist
to define the testing and interpretation of results. Overvoltage AC Withstand and Diagnostic testing methods are readily
available in voltages up to 200 kVac peak from several vendors worldwide.
Please consult High Voltage, Inc. with any questions.
www.hvinc.com
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