VLF Technology

Technologies used by HV Diagnostics | VLF Technology

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 50-300 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
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.	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	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.

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 they 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 square wave outputs.
Output voltage	We have 30 kV, 60 kV and 90 kV (all peak voltage) devices.
Output power	Our max load capability is specified at the maximum rated voltage. We have tools available to match the units output waveforms to your loads.
Portability	Our units have mass from 19.5 Kg (portable) to 127 Kg (mobile) (HVA90)
Diagnostic tools	We provide some basic diagnostic information with the VLF units and have 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.

BA75 - Portable Breakdown Analyzer for Oil Testing | BA100 - 100kV High Voltage Oil Tester | HVA30 - VLF and DC Hipot Instrument | HVA 90 - 4 in 1 Universal High Voltage Test System | TD30 - Tan Delta Test System (33kV Peak) | TD60 - Tan Delta Test System (62kV Peak) | PD30/PD60 - PD Detection and location for MV Cables