Introduction
Outdoor distribution substations are a critical, but often overlooked portion of the power delivery system. It is not unusual for utilities to own hundreds of substations and large industrial plants can also have one or more substations themselves. Statistics indicate that there are 10,287 operational transmission substations in the U.S. today, so distribution substation quantities likely exceed 100,000.
Many utilities do not have the resources to maintain distribution substation equipment to the degree necessary to keep ahead of the ever-increasing number of failures and many facilities lack the in-house expertise. In fact, many facility personnel do not even venture inside the substation fences and rightfully so, as the open conductors present a serious hazard.
Architecture
Substation design can vary vastly, although many of the designs are similar (see Figure 1), and all use similar components including:
Structure – used to support the overhead lines, insulators and bus and to provide clearance from ground and other phases.
Primary disconnect – typically 69 kV, 115 kV or 138 kV overhead lines (sometimes cable) enter the substation. The disconnecting device may consist of an open-air fused switch, circuit switcher or, circuit breaker.
Transformer – typically a 5 to 20 MVA fluid- filled transformer is used to convert the primary voltage to 4,160 or several possible higher distribution voltages in the 12,000 volt range. Certain designs may also employ fluid-filled regulators or the transformer may be equipped with a load tap-changer (LTC).
Switchgear – Many secondary feeder protection options are available including a lineup of switchgear circuit breakers, stand alone substation-type circuit breakers, or reclosers.
Feeders – Secondary substation output most often consists of underground cables, overhead lines, or connection directly to the plant using metal enclosed bus duct.
Other items – Additional equipment often includes surge arrestors, instrument transformers, control power transformers, and power-factor correction capacitor banks. Protective relays also play an important role in reliability but are generally more accessible for preventive maintenance testing.
Difficulties
In most cases, it is very inconvenient, if not nearly impossible, to de-energize distribution substations in order to conduct traditional outage-based preventive maintenance testing. Often, distribution design does not incorporate redundant substations, and any outage results in a complete power loss to the entire secondary network which may affect multiple facilities. These serious shortcomings exemplify the critical role of distribution substations and bring to light the need for enhanced reliability.
No-Outage Technologies
Fortunately, there are several relatively new technologies that can be combined into an inexpensive and efficient program to identify many of the most common failure modes. Another advantage of applying the new program is that all tests are performed without an outage. The following equipment is compact, lightweight and battery operated so that the technician can easily place a couple of instrument cases in the back of his truck, drive up to the substation, and begin conducting surveys immediately.
Corona Camera – The days of headphones and parabolic dishes have given way to the extremely valuable technology of corona imaging. The camera detects the ultraviolet (UV) light associated with corona and superimposes the corona image onto the normal camera image (see Figure 2). Special filters eliminate the solar UV to allow operation during daytime. The integrity of all outdoor insulators, bushings, and conductors can be checked efficiently and accurately with this technology. Problems such as those discovered in Figure 3, no longer need to be an undiscovered failure-in-progress.
Infrared Camera – The value of this technology has proven itself time and time again. Infrared imaging will efficiently detect conductor problems related to loose connections and can also help prove transformer liquid levels or spot an LTC overheating problem.
Hand-Held Partial Discharge Detector – This versatile instrument shown testing switchgear in figure 4 efficiently detects electromagnetic signals associated with partial discharges or partial insulation failure in switchgear, cable terminations, bus duct, and other equipment.
Oil analysis – All equipment containing oil including transformers, LTCs, and oil circuit breakers should be sampled and analyzed in order to detect deterioration.
SF6 analysis – any SF6 circuit breakers should have the gas sampled and analyzed to determine if any anomalies exist within the tank.
Conclusion
The new technologies discussed above can be combined together in an efficient, cost-effective program for assessing outdoor substation equipment in order to significantly increase reliability. Together, the technologies will detect a high percentage of potential failures well before complete failure occurs.
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