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Cleanroom Technology and Rigorous Testing – Must Haves for Medium Voltage Cable Quality
Key factors in guaranteeing long service life
After widespread experience dealing with the aftermath of electrical failures in underground cable in the 1970s and 1980s, most utilities understand the importance of keeping contamination out of materials used for cable insulation. But what’s the best approach for dealing with material cleanliness? And how do you then guarantee that the cable has in fact been manufactured to the proper quality standards? The answer lies in patented cleanroom technology using closed loop systems to eliminate potential contamination sources from the cable extrusion facility, along with a rigorous testing protocol that inspects every reel.
Contamination in cable insulation causes expensive and time consuming repairs
We tend to think of contamination in electrical insulation materials as the ultimate fly in the ointment – a small defect that spoils something valuable. Very apt, since the level of contamination found in electrical insulation materials is a major cause of reduced dielectric strength and insulation deterioration. Since repairs and replacement of underground cable are extremely expensive and time consuming, it is crucially important to keep contamination out.
We actually do not know the exact effects of specific contamination types, sizes, and shapes on cable life, but we do have a few clues. For example, particle size seems to play an important role.
A 10 mil metal contaminant located radially in a 175 mil insulation wall would have a 165 mil insulator in series with a 10 mil conductor, which reduces the amount of insulation below what is considered the targeted level. Similarly, we know that the shape of the contaminant is important, because evidence exists that there is greater voltage stress at a small radius point compared to a larger, smooth surface.
So, while we may not know precisely the factors involved in contamination, we do know it causes insulation material to work harder and that some localized deterioration of insulation can occur. If reasonable contamination control measures are not taken, premature aging and early failures are very likely.
Patented cleanroom technology guarantees cable manufacturing quality
Recognizing the urgency of contamination control, Hendrix Wire & Cable developed and patented the first bulk rail car cleanroom facility in 1999. While other contamination control systems have entered the market, including pellet inspection with optical scanning to cull out contaminants, the bulk car approach is a completely closed loop process. The rail car comes in sealed, and stays sealed, so no ambient air ever comes into contact with the insulation compound material. Although there are questions about contamination sources, it is logical to conclude that contaminants can be introduced from the ambient environment. Hendrix therefore believes keeping the compound segregated from ambient air is a necessary element to superior cable manufacturing and wholeheartedly endorses using clean room technology to that end.
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The sealed transport system includes dedicated compound containers and handling of materials in a cleanroom environment. A railcar serves as the primary container for the insulation. The only opening of the closed compound container and the closed compound transfer system occurs when the two are coupled together at Hendrix in a cleanroom environment. See Figure 1 .
Figure 1 – Sealed railcar in cleanroom
The rail car is positioned in a cleanroom facility located directly adjacent to the cable extrusion tower. A short length of stainless steel conveying tube provides a closed system to move the insulating compound from the railcar to the stainless steel extruder hopper with a dedicated conveying air system certified under Federal Standard 209E Airborne Particulate Cleanliness
Classes in Cleanrooms and Clean Zones .
Cleanrooms are classified according to the number and size of particles permitted per volume of air; class information refers to the number of particles of size 0.5 µm or larger permitted per cubic foot of air. The Hendrix railcar cleanroom is certified to Class 10,000 and the conveying air is certified to Class 100. A sealed silo system is available for contingency use.
The stainless steel extruder hoppers and a true triple head extruder (where the conductor shield, insulation, and insulation shield are formed in the same extrusion head) ensure that no compound is exposed to a non-controlled environment during the extrusion process.
An option exists to use the same system at a compound manufacturer’s plant – the compound manufacturer could load their material in clean, seal it and bring it to the Hendrix facility, making it a closed loop system all the way upstream.
By purchasing the cleanest compound, controlling contamination at the source, eliminating exposure to non-controlled environments, minimizing conveying distances, and establishing audit processes to ensure consistent performance, the Hendrix system (Patent #5,868,528 –
Issued 2/9/99) eliminates potential sources of contamination at the cable extrusion facility.
While modern cables eliminate the majority of water and significantly resist the growth of water trees (partially conductive, branching tree-like figures that form within the dielectric and can
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Quality cable manufacturing – the proof is in the testing
As the first cable company to promote a rigorous electrical testing protocol for all cable manufacturing, Hendrix Wire & Cable has been way ahead of the curve in terms of electrical test requirements. Hendrix has held a leadership position in the industry since 1976 when it first implemented its Hendrix Quality 200 (HQ200) partial discharge test protocol. The testing regimen was designed by Hendrix’s former director of technology in the 1970s and has been used ever since as an internal test protocol for all Hendrix underground cable.
The HQ 200 testing protocol was the first to require each reel of cable to be tested to 200 volts per mil with a partial discharge of less than 5 picocoulombs (PC). It includes a measure of PC discharge after the 5 minute test and no discharge over 5 PC to full test voltage. In 1994, HQ200 became the industry standard, when it was adopted virtually unchanged as an AEIC/ICEA standard.
With its long history of quality innovation, including closed loop compound handling with closed extrusion for true contamination-free cable manufacturing, and the promotion of electrical testing now used by the entire industry, Hendrix ultra-clean manufacturing produces quality medium voltage cable that stands up to the test of time.
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