The distribution network operated by Tenaga Nasional Berhad (TNB) in Malaysia currently includes some 5,300 km (3,293 miles) of 33-kV, 22-kV and 11-kV medium-voltage bare-conductor overhead (MV BOH) lines and about 2,700 km (1,678 miles) of 33-kV and 11-kV medium-voltage aerial-bundled cable (MV ABC) systems. The MV BOH lines were constructed in the 1940s to supply power from primary substations over long distances to the load centers usually located in suburban and rural areas.
Malaysia has an above-average isokeraunic level of 200 thunderstorm days per year; hence, all overhead lines and the MV BOH line system experience demanding reliability challenges. Additionally, small animals — mainly rodents and squirrels — and vegetation are responsible for a significant number of interruptions to the MV BOH line system.
TNB started using MV ABC systems in the 1990s as a more economical alternative for distribution systems because there is no requirement to obtain right-of-way consents and it is easier to construct. As a result, the number of interruptions caused by lightning, animals and vegetation were reduced. However, the circuit length for MV ABC systems is normally shorter than that for MV BOH line circuits.
The New Design
At first, the MV ABC systems were constructed by emulating other utilities. They exhibited problems because some operational, climatic and environmental issues specific to TNB had not been properly addressed, which resulted in a number of circuit failures. To establish a more reliable, efficient and cost-effective system design, TNB had to re-evaluate quality and technical specification issues to adapt the MV ABC systems to its distribution network standards.
The TNB distribution division faced these challenges and improved the design of the earliest MV ABC systems. By April 2004, MV ABC systems had become standard for the distribution networks supplying suburban and rural areas. TNB started giving them priority over underground cable and bare overhead lines.
MV BOH vs. MV ABC
As Malaysia has an isokeraunic level of 200 thunderstorm days per year, TNB faces reliability issues with its 33-kV, 22-kV and 11-kV MV BOH lines. From 2001/2002 to 2006/2007, MV BOH had a higher number of recorded failures compared with MV ABC. The 33-kV, 22-kV and 11-kV overhead lines also experience weather-related interruptions and are subject to other causes of failures such as vegetation growth and animals. Aging overhead lines also contribute to failures; the majority of TNB's MV BOH lines are more than 40 years old, having been constructed from 1940 to 1970.
MV ABC systems offer improved safety features, are more reliable when exposed to the environment such as weather, animals and vegetation, and cost less overall than MV BOH lines. Technically, as the reactance of the MV ABC systems are less than that for MV BOH lines, the voltage regulation also is improved.
Initially, the MV ABC systems of the early 1990s failed to deliver the benefits anticipated by TNB. Certain characteristics such as climate conditions, the degree of exposure to ultraviolet rays and solar radiation temperature were the cause of inferior fault rates. TNB's original design had a single-layer copper screen with a high-density polyethylene (HDPE) outer sheath.
There were several issues with the original MV ABC:
Stress cracking of the outer sheath of the cable
Damage to the outer sheath caused by animals
Burnt outer sheath
Damaged copper screen
Dislodged straight through joint.
The stress crack found on the outer sheath of the MV ABC cable was addressed by replacing the HDPE outer sheath material with polyvinyl chloride (PVC) grade ST-2. Another interesting characteristic of HDPE sheath material, it emits a fruity-like smell that attracts animals such as rodents and squirrels. Damage caused by these animals, namely chewing on the outer sheath of the MV ABC, can cause further exposure of the copper screen to the environment. Stress cracks on the outer sheath of the MV ABC can expose the copper screen to the environment, causing damage and increasing the probability of a fault outage.
With the copper screen damaged, the capability of the MV ABC to carry fault current would be compromised. In the event of a fault on the MV ABC feeder, there would not be a return path for the fault current to flow to the source. Thus, the fault current would overheat the outer sheath and, subsequently, damage the MV ABC. Also, the original design of ABC had only a single layer of copper screen that, if damaged, would be exposed to further damage if a fault occurred.
TNB redesigned the copper screen so it has two layers, with a 15% overlap between the layers. The improved design not only ensures the robustness of the copper screen, but it also improves the sheath current-carrying capacity of the MV ABC, enhancing the life cycle of the insulation.
Another area of improvement TNB focused on was the construction standards. Failure to employ approved installation methods can result in stress on the MV ABC cables, joints and terminations. Looping of an ABC joint not only causes mechanical stress on the joint, but it also causes the spun-concrete pole to experience overloading. Thus, additional accessories were introduced to ensure satisfactory installation of the ABC system for a reliable, safe and economical network.
The redesigned MV ABC improved the performance of the MV ABC system significantly. The failure rate of the new 33-kV and 11-kV MV ABC system is more than five times lower than that of the original-design MV ABC in the Malaysian state of Perak.
The design and construction standards of the current MV ABC systems have shown tremendous improvement in terms of overall system reliability performance compared to the original MV ABC systems. Although the performance results of the new MV ABC only come from one state (Perak), the trend shows the new MV ABC design has been proven in terms of reliability, safety and financial benefits.
Although the use of PVC grade ST-2 as the outer sheath has mitigated the stress crack issues, care must continue to be taken when stringing the ABC cable onto the support poles because PVC is softer than HDPE. Improper stringing of the PVC-sheathed ABC will result in damage to the outer sheath.
The increasing cost of metals will increase the unit price of the double-layer copper screen. However, with the improved reliability and extended service life of the redesigned ABC, the overall cost still remains economical.
Mohd Faris Ariffin ([email protected]) holds a BSEE degree from Northern Arizona University and a MSEE degree from UNITEN in Malaysia. He is a senior technical expert (in overhead line system design and performance) with the asset management department, distribution division at TNB, a government-owned utility in Malaysia. He has also held a number of engineering positions in his 19-year career with TNB, with responsibility for transmission lines and substations, shift operations, supervisory control and data acquisition, and telecontrol, and is currently the network services manager. Ariffin is a registered professional engineer with the Board of Engineers in Malaysia.
MV ABC Component Failures
|Fiscal year||ABC conductor||ABC joint||ABC termination||Total|
TNB | www.tnb.com.my