VLF

Very Low Frequency (< 1 Hz) test techniques have been proposed since the turn of the century by engineers in the field of motors and generators. Until recently the technology was not available to manufacture test equipment to produce the required VLF power signals at competitive costs.

VLF competes with DC test sets at one end of the spectrum and with 30-300 Hz ac resonant test sets at the other end of the spectrum. VLF offers an appropriate cost-effective compromise between the lower-cost, lower technology application of DC voltages and the considerably higher cost, larger sized 50 Hz alternatives.

The issues that are faced by a test engineer are :

	DC	VLF	Resonant Testers (30-300 Hz)
Operator Safety	Cable discharge required	Cable discharged on every cycle	Cable discharged on every cycle
Risk of damage to the cable	High unipolar fields can damage dielectric	Alternating field less likely to damage	Alternating field less likely to damage
Effectiveness of test	Test resistive components only	Test resistive and reactive components	Test resistive and reactive components
Relevance to actual conditions	Far removed	Correlated	Can be closest
Cost	Cheapest	Medium cost	Expensive
Portable	Yes	Yes	No
Diagnostic tools	Not available	Available	Available

VLF Test sets are able to apply a range of low-frequency test conditions to the device under test. In the most simple of tests, the ability of the device under test (DUT) to withstand the applied voltage for a predetermined period of time is tested. If the DUT breaks down at some point then this is detected by the test set as a current surge and the test may be stopped depending on the test settings. If the breakdown occurs before the timeout then this is interpreted as a failure.

Different standards specify different voltages and times to be used depending on the rating of the DUT and also depending on whether the test is performed at commissioning or during maintenance. Standards in use that refer to VLF include :

IEEE 400 and 400-2
IEC 60060-3
IEEE 433
VDE 0276
CENELEC HD 620 S1 and HD621
NEN 3620
SANAS 10198

The VLF test set must be capable of providing the current required to drive the capacitive load presented by the DUT at the required test voltage and the required test frequency.

For a given capacitive (typical of cables and other HV devices tested using VLF techniques) load the frequency of the applied test voltage can be reduced in order to reduce the loading that the DUT presents to the test set. The load presented is inversely proportional to the frequency of the test voltage.

There are a number of different VLF test sets available on the market and we compete on the following criteria :

Test waveforms	Our VLF is a pure sinewave (<3% THD) which facilitates diagnostic testing. Our units can also provide +-DC and also square wave outputs. Cheaper technologies produce mechanically generated alternating outputs and trapezoidal (cosine) outputs
Output voltage	We have 28, 34, 60, 90, 94 and 120 kV (all peak voltage) devices.
Output power	Our max load capability (often specified in uF as the equipment is designed to drive large capacitive loads) is specified at the maximum rated voltage. We have tools available to match the units output waveforms to your loads.
Portability	Our single-component units have mass from 14 Kg (portable) to 127 Kg (mobile) (HVA90). Alternative units are often heavier and may comprise more than one component.
Diagnostic tools	We provide some basic diagnostic information with the VLF units and have advanced Tan Delta and Partial Discharge test sets to compliment our equipment.
Features	Our unit is microprocessor controlled with an intuitive user interface allowing testing using the device screen and controls. Automated test sequences can be programmed and saved and test options such as burn-in are available.
Data storage and retrieval	This is easily achieved through a serial link or through USB memory sticks.
Quality of manufacture	Our equipment is manufactured in Europe in an ISO accredited facility.
Service and backup	We have a network of agents throughout the world ready to assist with any problems that you may encounter.
Cost	We compete very favourably on a cost-for feature basis.

TECHNOLOGIES