A Very Low Frequency (VLF) Withstand Test is an AC Withstand Test usually carried out at a frequency between 0.1 Hz and 0.01 Hz. This kind of test is suited for testing high capacitance loads such as cable and rotating machinery. This is a pass fail test i.e. bad cable will fail during testing rather than in service.

The Theory

Basic electrical theory: Xc = 1/(2*pi*f*C)

Where:

Xc = Capacitive reactance (Ω)

f = Frequency (Hz)

C = Capacitance (F)

It can be seen that capacitive reactance, which is the resistance across the power supply output, is inversely proportional to frequency. By reducing the frequency the capacitive reactance is increased.

More basic electrical theory: I = V/R

Where:

I = Current (A)

V = Voltage (V)

R = Resistance (Ω)

The lower the frequency, the higher the capacitive reactance (Xc). The higher the Xc the lower the current and power needed to apply a voltage.

 Example:

A length of power cable with capacitance of 1 µF needs to be tested at 34 kV peak.

Power frequency testing (50 Hz):

Using the above formulae it can be calculated that the capacitive reactance would be 3.183 kΩ so the required current would be 10.68 A. Therefore, to test a 1 µF cable at a frequency of 50 Hz the test set would have to be able to give 363 kVA. When you consider that a 30 kV, 40 kVA AC test set can greatly exceed 620 kg it puts into perspective the size an AC test set would have to be to produce 363 kVA.

VLF Testing (0.1 Hz)

Testing the same length of cable at 0.1 Hz would generate 1.59 MΩ of capacitive reactance and the required current would be 21 mA. Therefore testing the same length of cable at 0.1 Hz would require 0.714 kVA, which is 500 times less than at a power frequency of 50 Hz.

Most test sets have a frequency range of 0.1 Hz to 0.01 Hz and the most desirable test sets will select the optimum frequency based upon the capacitance of the cable. The capacitance of the cable is dependent upon its construction materials and its length. As cable length increases so does the capacitance so dropping the frequency allows kilometres of cable to be tested.

Does this difference in test frequency range have any effect on test results?

 This was reported on in the National Electric Energy Testing, Research and Applications Center (NEETRAC) report “Estimating the Impact of VLF Frequency on Effectiveness of VLF Withstand Diagnostics” by N. Hampton et al (2014). This report concluded “… there is no distinguishable difference between failure rates on test for the common VLF test frequencies of 0.05 Hz and 0.1 Hz, from data obtained through laboratory and field tests, and all insulation types.”