Frequently Asked Questions
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Partial discharges typically occur at 2,000V or greater and are measured in units of charge called picoCoulombs (pC). Partial discharges are small electrical sparks that occur within the insulation of high voltage electrical assets. Each discrete partial discharge is the result of an electrical breakdown of an air pocket within the insulation. These discharges erode insulation and eventually result in insulation failure. In the power industry this is a major risk to people, assets and the bottom line. It is important to know the condition of all high voltage equipment to ensure critical failure won’t happen.
Dielectric Loss Angle (DLA) measurement is an effective, non-destructive test method for electrical insulation testing. Dielectric loss is the time rate at which electric energy is transformed into heat in a dielectric when it is subjected to an electric field. Dielectric loss is associated with real component (watts) losses in a dielectric. Dielectric dissipation factor is the tangent of the loss angle and its commonly referred to as tan delta.
Yes. Here at NPD we have many alliances in the power industry and can provide you with a full maintenance package comprising of switchgear maintenance, relay maintenance and full diagnostic testing and condition assessments of your HV switchgear and buses. If its high voltage then we have the solution for you.
Yes. Here at NPD we are proud to be Associate Graduates of The Institution of Engineers Australia.
We are also members of the Electrical Contractors Association.
Yes. We adopt the SafetyConnect safety management system that complies with the
Electrical Safety Act, AS/NZ 4801 and the new WHS Act 2012 (Harmonization).
Yes. NPD have the required insurances to undertake this specialised type of testing works.
- 20 Million in Public and Products liability insurance
- 1 Million in Professional Indemnity insurance
- Insurance for our specialised PD and DLA testing equipment
- Full comprehensive insurance on our work vans
- Work cover insurance for our employees
Yes. We are based in Brisbane but NPD is very portable and have performed onsite testing all over Australia.
VLF stands for Very Low Frequency. VLF is generally considered to be 0.1 Hz and lower.
Many people assign too much mystery to VLF testing. A VLF hipot, and VLF testing, is very simple. It’s just an AC hipot but with a lower frequency output. We have all used AC hipots before to test various types of equipment. Now they can be used for field testing cable and rotating machinery.
The IEEE/EPRI/CEA and other world engineering bodies recommended test level for solid dielectric cable is up to three times (3Vo) line to ground voltage for 15 + minutes. For a 11kV cable, which ordinarily carries 6.3kV line-ground voltage, the test is usually performed at 19kV. A 33kV system is tested up to 57kV and a 66kV cable system is VLF tested up to 114kV.
Not much. DC can no longer be used. It damages cable insulation and tells little about the cable insulation quality. Recognizing the problems associated with DC testing, many utilities have reduced their test voltages 15 kVDC on 15 kV cable. 15 kVDC is barely above the peak AC stress a cable experiences in service and far below the twice-normal voltage occasionally experienced due to transients. The test is not meaningful.
Some have tried to develop partial discharge detection methods for field-testing cable insulation, with the goal being to determine the insulation quality and location of defects without risking failure during the test. None of the methods employed to date have proven very effective. PD testing works well on substation apparatus but it has severe limitations for cable testing. Experience has shown that PD testing lacks reliability, with results often times unrepeatable due to the many variables involved with cable testing: temperature, moisture, number and types of accessories, various cable types spliced together, and operator interpretation. The results are highly interpretive. Also, the equipment and/or the service is very expensive. Many utilities have wasted hundreds of thousands of dollars hiring PD testing services, only to replace good cables and splices while leaving bad cable in the ground to soon fail.
Tan Delta, or dissipation factor, testing (analogous to power factor) is performed, where a VLF unit is used to energize the cable and loss readings are measured. This method works well, so long as there are few splices, joints, etc. along the cable run and there is only one type of cable under test, which is usually not the case since many cable runs are of mixed cable types, all with a different loss factor. This method is easy to perform with minimal training and allows users to grade their cables’ level of deterioration. If done properly, it is a non-destructive diagnostic test.
Yes. The wave shape is sinusoidal and capacitance doesn’t change with frequency.
The only way to field test high capacitance loads with AC voltage, like cables and motors/generators, is to use a VLF AC hipot. The lower the frequency, the less current and power needed to test high capacitance loads.
VLF testing is used for any application requiring AC testing of high capacitance loads. The major application is for testing solid dielectric cable (per IEEE400.2), followed by testing large rotating machinery (per IEEE433-1974), and occasionally for testing large insulators, arrestors, and the like.
One of the best reasons to use VLF is to check installation quality and accessories, like splices. Many failures are due to damage during installation and/or defective workmanship. At the very least, every newly installed or repaired cable should be VLF tested before re-energizing.
VLF hipoting is not destructive to good insulation and does not lead to premature failures like with DC voltage testing. Using VLF does not cause degradation of the insulation. It does cause existing cable defects, like water trees and splice defects, to break through during the test. If a cable can’t hold 2 – 3 times normal voltage, it is not a good situation. Cable is factory tested with AC voltage at levels higher than the field test voltage. When people talk of VLF testing being destructive, it is only destructive in that a defective cable, joint, or splice may fail under test, which is the intent of the test.
Commission testing new or refurbished HV electrical plant is important so the operator can safely energise the equipment. The goal of these tests is the same as in the factory test, namely to have any weak components of the HV system fail in a controlled manner. Commissioning should be performed to meet the relevant standards.
VLF stands for "Very Low Frequency". VLF cable testing is a technique for testing of medium voltage cables. The VLF test can be used to measure insulation loss in cable, or can be used as a proof test to detect faults in an installed cable system. In the past DC voltages were used for cable testing, which sometimes was actually damaging to the cable insulation. A recent study (Cable Diagnostic Focused Initiative Project-NEETRAC- Georgia Tech) has shown that withstand tests are among the most routinely employed diagnostic tests in the USA; this study has also shown that the most preferred withstand tests use Very Low Frequency (VLF: 0.02 to 0.1 Hz) AC methods.
It is important to understand the integrity of your high voltage insulation. Over
time HV insulation degrades. The major environmental and operating
mechanisms which cause insulation to deteriorate over its working life include
excessive voltage, overheating, internal arcing, material ageing, moisture
ingress, mechanical stress and physical damage.
A predictive maintenance program is a window into the future of your
equipment’s performance. By performing offline diagnostic tests it is possible to
identify when a failure is likely to occur so you can plan an outage instead of
dealing with an unexpected failure. This data is essential in making the best use
of your maintenance dollars. Because Partial Discharge activity is often present
well in advance of insulation failure, it is possible to observe its development
over time and assist asset managers with strategic decisions regarding the
repair or replacement of the equipment.
We can provide you with an in-depth maintenance plan to assist you in making
these important decisions. The New Power Directors have a large amount of
experience in this area with a combined 20 years in the power industry working
for both Energex and Ergon Energy. We can ensure your equipment will be
tested to the same high standards that the Queensland power authorities
Next time you are planning a shutdown give us a call. The price of condition
based maintenance versus time based maintenance will save you thousands
in wasted dollars and unplanned outages due to costly equipment failures.