VLF Testing

Introduction

What is VLF?

1. VLF stands for Very Low Frequency

2. VLF is generally considered to be 0.1Hz or lower.

3. VLF technology is used in TNB cable for cable testing

When should the VLF testing to be conducted?

1. The major application is for testing XLPE cable. It is an excellent tool for testing

cable insulation.

2. According to the Arahan Kejuruteraan Bil.9 Tahun 2000, the VLF test should be

conducted at site.

3. A VLF set should be used for the following:

a. Commisioning Test (Ujian Mula Tugas)

b. Proof Test (Ujian Selepas Membaiki Kerosakan)

c. Integrity Test (Ujian membuktikan Keadaan daya harap kabel)

Why VLF?

Because DC testing can be harmful to cables, especially XLPE cables.

Why is DC testing harmful to cables?

1. DC testing is inadequate in determining the ac breakdown strength of power

cables with thermosetting insulation.

2. Due to space charge build up around defects in the XLPE cables, DC testing can

deleteriously program and cause faults within thermosetting insulation of cables.

3. Not only it is damaging to thermosetting cable insulation, DC leakage current

readings tell little about the cable quality.

What are the benefits of VLF testing?

1. A VLF test can expose cable defects at a much lower voltage than a DC Hipot.

2. Voltage transients are also much smaller with a VLF AC Hipot test during cable

failure or flashover than with a DC Hipot test.

3. Other benefits:

a. Smallest, lightest weight AC-VLF Hipot available with the largest test

load ratings

b. Two-piece portability

c. Simple and easy operation

d. The VLF-0.1 Hz AC Hipot does NOT generate harmful voids in the

insulation through treeing

e. Harmful space charges are not injected into the dielectric during testing

f. No traveling waves are generated during testing

Is VLF test destructive?

1. VLF hipoting is not destructive to the cable like DC voltage testing is, where

actual damage to the insulation occurs during the test, causing future failure.

2. Using VLF does not cause degradation of the insulation. It is AC voltage, which

is what the cable is designed for and experiences during service.

3. It is only destructive in that a defective cable, joint, or splice may fail under test,

which is exactly what we want to occur. That is precisely the purpose of VLF

testing. It is not a diagnostic test. It is an AC stress test. A cable either holds the

test voltage or fails. If a cable has defects and can’t hold 2-3 times normal

voltage, it’s not going to last long.

Equipment

The equipment used by TNBD for VLF testing is the AC Hipot Series Model 6022 by

High Voltage INC. USA.

AC Hipot Series Model 6022

Page 6-10 of the operator manual (Appendix 1) shows the function of each control panel

indicator on the VLF-60 series front panel.

Procedures

Section 2 of the operator manual (Appendix 1) includes the correct procedures for setting

up the equipment as well as operating the equipment.

Setting up the Equipment

1. Be sure that all the controls are off, in their de-energized or fully counterclockwise

position.

2. Secure the interconnect cable to high voltage tank.

3. Secure a ground test lead to the panel.

4. Secure a ground test lead to HV tank.

5. Connect the ground stick to a solid earth ground.

6. Insert the coaxial output cable into the high voltage output connector on the tank.

7. Install the output limit resistor on the output cable.

8. Insert the EXT INTLK plug into the socket on the panel.

Operating the equipment for AC testing of High Voltage Cables

1. Ensure that all the steps listed in setting up the equipment have been accomplished.

2. Make sure that all insulators, stress cones, and pot heads are clean and free of

moisture. This will prevent flashover or minimize leakage.

3. Isolate the far end of the conductors under test for test voltage; that may mean

separating some of the conductors in a multi-conductor cable from each other and

their shields.

4. Any conductors or wires in the cable to the vicinity not being tested must be

grounded to avoid a buildup charge and possible shock hazard.

5. Voltage must be applied according to ‘jadual 1’ of ‘Arahan Kejuruteraan Bil 9 Tahun

2000’(Appendix 2).

Voltan Sistem

(Uo/U kV)

Voltan Ujian

(UT kV)

Tempoh

(Min)

6.35/11 20 60

12/22 40 60

19/33 60 60

Jadual 1

6. Prior to connecting anything to the test sample, be sure that the test sample is

identified, de-energised and grounded until ready to test.

7. First, the cable capacitance must be measured to determine the best operating

frequency. Place the control near the cable being tested.

8. Connect the input power terminal block to a grounded 50/60Hz source. A Generator

is an acceptable power source.

9. Turn on the main circuit breaker. The main power light will illuminate at both the

power section and the control.

10. Operate the METER MODE to uF position. Adjust the CAP ZERO for full scale

deflection (ZERO) on the meter.

11. Connect the HOT post to the center conductor of the test cable. Connect the COM

post to the grounded shield of the test cable. Read the capacitance scale ( x10).

12. If the reading is below 6.0uF depress the x1 push button, then the following table

provides operating frequency versus capacitance. A minimum load capacitance of

0.01uF is required to achieve full output of the VLF unit.

Capacitance Frequency

1.1 uF or less .1Hz

2.2 uF or less .05Hz

5.5 uF or less .02Hz

13. An oscilloscope can be connected to the scope output on the control for wave shape

monitoring. The oscilloscope should be properly grounded and the input should be set

to 1 volt/div, the time base should be 5 sec/div and the trigger should be set to roll

display to view the wave shape. An oscilloscope with signal memory display is best

used for this application. A BNC to BNC shielded jumper should be used for

connection between the SCOPE OUTPUT and the oscilloscope. A digital meter can

be used to monitor this connector. The voltage feedback calibration for the connector

is 1 volt for 10kV of output.

14. Connect the output lead to the test sample through the external energy dissipation

resistor. Be sure that there is enough clearance to grounded objects for the expected

test voltage. The minimum clearance in air is 10kV ac/inch.

15. Operate the current METER MODE switch to mA position to observe cable charging

and discharging currents.

16. With the output control at zero(zero start interlock engaged), depress the HV ON

push Button. The HV ON light will glow. At this time the pump and the fan on the

high voltage section will also energize. If this is the first test at this location, leave the

output control at zero and allow the pump to circulate oil for 10 minutes prior to

raising the output voltage and starting the test. This will purge air from the cooling

system. Increase the output by operating the OUTPUT control slowly clockwise until

the desired output voltage is reached. Raising the output too fast with large capacitive

loads may trip the output overload. Observe the Kilovolt meter to set voltage.

17. Maintain the output voltage for the test time specified in you standard procedures.

18. After the test is complete, rotate the output control to zero, allowing the load to return

to zero and until the unit to cycle for about 60 more seconds prior to depressing the

HV OFF push button. Allowing the unit to cycle for some time allows for the

complete discharge of the load and avoiding the normal self recharge that capacitive

loads will exhibit.

19. If the test sample fails during the test, the overload circuit will de-energized the high

voltage.

20. Prior to removing the output cable from the load, observe that the output voltmeter is

at zero, then use a ground stick to positively ground the test sample.

21. The result of the test would have to be recorded in the VLF high Voltage test

certificate (Sijil Ujian Voltan Tinggi VLF). A sample of the certificate is attached in

Appendix 2.

Safety Precautions

1. Do not operate the VLF Hipot set if the high voltage tank is 5o or more from level.

2. If the unit is operated out of level, overheating and internal arching may occur.

3. Do not store or transport VLF high voltage section on it’s side.

4. Before making any cable connections, ensure that the cable being tested has been

properly identified, de-energized, and grounded.

5. The shields of all cables must be securely tied to ground at the nearest end of the

cable.