One of the most important advantages of a well- designed UD system is greater reliability for con- sumers compared to an overhead system. UD lines and equipment are located where they are not vulnerable to most of the common hazards that cause outages on overhead facilities, such as trees, weather, some animals, and vehicles. However, material or design defects in a UD sys- tem may reverse the reliability advantage of un- derground distribution. In fact, many early UD systems installed by cooperatives and other utili- ties turned out to be less reliable than comparable
overhead systems. These experiences have made it clear that reliability engineering is a necessary part of UD system design.
MEASUREMENT OF RELIABILITY
Reliability is usually measured in two ways. The first is the frequency of interruptions occurring at a particular point on a system, referred to as the interruption rate or outage rate. Outage rates are measured in outages per year. The second measure is the average duration of an interrup- tion, also referred to as the restoration time.
Outage duration is usually measured in hours. A combination of these two measurements yields the percentage of availability for a particular lo- cation on a distribution system. A simple index of reliability used by many utilities is hours of outage per year, per consumer.
For this discussion, outages are considered to be sustained interruptions. Reliability calculations of this type usually do not consider momentary interruptions that are successfully cleared by au- tomatic circuit reclosing operations. This analysis considers only those outages that require man- ual intervention to restore service. Furthermore, almost all faults attributable to underground sys- tem components are permanent.
System reliability undeniably affects many as- pects of a cooperative’s service. Although the order of importance may vary with individual situations, the results of distribution system out- ages include the following:
• Consumer dissatisfaction;
• Consumer financial losses resulting from interrupted production, equipment damage, or other causes;
• Impairment of other cooperative facilities;
• Costs to the cooperative of service restora- tion; and
• Lost cooperative revenue.
All these factors have a serious impact on satis- factory cooperative system operation. Engineers, therefore, must be aware of the basic principles of reliability assessment so they can achieve satisfactory but economical UD system designs. Appendix Aprovides a method for calculating UD system reliability.
Comprehensive reliability analysis also con- siders the number of consumers or kVA of load each outage affects. Thus, facilities serving many consumers (or kVA) may need to be designed for higher reliability than should facilities serving few consumers (or kVA). The analysis presented in this manual, however, does not consider this parameter because most cooperative UD sys- tems are fairly uniform in design and consumer concentration. There is generally no need to dis- criminate in design quality between some parts of the system and others.
CABLE FAILURE RATES
In the mid-1980s, the failure rates for common- ly used UD primary cables were unacceptable. The failure rates for cross-linked polyethylene (XLPE) and high-molecular-weight polyethylene (HMWPE) cables were approaching 0.02 and 0.08 per mile per year, respectively. Further- more, studies revealed that these failure rates were continuing to worsen as the cables aged. The most common causes of failure were elec- trochemical treeing of the insulation layer and corrosion of the exposed neutral conductors.
In December 1987, the Rural Electrification Administration (REA), currently called Rural Util- ity Services (RUS), responded to the cable failure problem by issuing a revision of Bulletin 50-70 (U-1), REA Specification for 15-kV and 25-kV
Primary Underground Power Cable. The main
specification changes were the following:
• Removing all HMWPE cable from approval,
• Increasing minimum insulation thickness to 220 mils for 15-kV cable and to 345 mils for 25-kV cable, and
• Requiring cable to be jacketed.
At that time, RUS did not disapprove the use of XLPE cable. Nevertheless, concerns about XLPE were raised in studies, leading to the bul- letin’s revision.
As a consequence of these experiences in the 1980s, cooperatives should procure new cable with the requirement that the revised RUS specifi- cations be met. Any XLPE cable acquired should also be tree retardant (TR-XLPE). As a result of recent vastly improved quality control in cable manufacturing processes, both TR-XLPE- and ethylene propylene rubber- (EPR) insulated ca- bles provide improved reliability. Industry tests are continuing to develop information on the expected failure rates for different insulation sys- tems. RUS is currently preparing an even further refined U-1 specification to reflect these continu- ing cable insulation improvements.Section 2 discusses cable selection in detail.
LOOP-FEED DESIGN
The time spent to locate an underground cable fault, excavate to the point of its failure, and
install a UD cable repair joint is typically much longer than that required to perform a compara- ble repair on an overhead line. Therefore, if the overhead type of radial distribution system con- figuration were used for UD, the restoration time for most UD outages would be much longer than is typical on overhead systems.
This difficulty is overcome by using loop-feed design for UD systems. Under loop-feed design, each cable run serving several pad-mounted transformers is connected with a power supply point on both ends (see Figure 1.15). This formed loop is opened at some point to allow use of radial overcurrent protection methods and to prevent unwanted power transfers through the cable. If the cable fails, a repair crew can disconnect both ends of the failed cable section and close the circuit at the normal open point (see Figure 1.16). These actions promptly restore service to all consumers on the cable run. The damaged cable can then be repaired or replaced later without causing additional outage time.
It must be noted that it is vitally important for loop-feed UD systems to be fed from two sources of the same feeder circuit out of a sub- station, with no switching or sectionalizing de- vices in between. Having the two sources fed from different feeder circuits could cause unex- pected high-power flow through the UD system if the sources were tied together during switch- ing operations on the UD loop. These high cur- rent levels could result in exceeding cable and/or termination current-carrying ratings, or could create outages on source fusing devices. Furthermore, on single-phase UD looped sys- tems, it is vitally important that both sources be connected to the same phase for safe operation.
UD SYSTEM RELIABILITY STUDY
Well-designed UD systems can provide improved reliability relative to overhead systems. However, to achieve high reliability, the cooperative needs to apply the specialized engineering knowledge gained from many years of experience with under- ground power distribution. This knowledge covers the field performance records of different types of cables, the proper application of surge arresters, appropriate sectionalizing, and loop-feed de- signs, all of which are treated by this manual.
FIGURE 1.15: Loop-Feed Design of UD System Under Normal Conditions.
Riser Pole Riser Pole Transformer T4 Parking Stand Surge Arresters To Ground Rod Copper Ground Conductor To T3 X3 X1 X2 N.O. T6 T5 T4 T1 T2 T3 Legend
N.O. Normally Open Point Single-Phase, Pad-Mounted Transformer
To T5
Riser Pole
Damaged Cable Section Riser Pole Transformer T5 Parking Stand Surge Arresters To Ground Rod Copper Ground Conductor To T4 X3 X1 X2 Transformer T6
Front View Showing Isolated, Damaged Cable Section
Parking Stand Surge Arresters To Ground Rod To Riser Plate X3 X1 X2 T6 T5 T4 T1 T2 T3 Legend Single-Phase, Pad-Mounted Transformer Cable Fault
FIGURE 1.16: Loop-Feed Design of UD System with Damaged Cable Section.
The cooperative’s involvement with a UD system does not end after installation; the cooperative must operate and maintain the system through- out its life. Because many components of a UD system are difficult to access, operation and main- tenance of the system can also be difficult. For example, it is difficult to access a pad-mounted transformer that is surrounded by shrubbery or located too close to fences or buildings. Like- wise, it is difficult to repair a faulted cable that is buried beneath landscaped areas or utility build- ings. The engineer needs to be aware of these problems when considering whether to place facilities along the front or rear property line and also must consider the effect of joint-use trench on operation and maintenance activities.
FRONT VERSUS REAR PROPERTY LINE PLACEMENT
One of the fundamental choices in UD system design is whether to locate facilities along the front property line or along the rear property line. Usually, this is a joint decision between the utility and the consumer or developer. Consumers or developers will have some authority because they must normally give the utility an easement that allows the installation of underground facilities. Often the consumers or developers believe that pad-mounted equipment detracts from the