Voice of Experience: Fundamentals of Underground Padmount Transformers
In recent months Incident Prevention has received several questions about underground (UD) padmount transformers, so in this installment of “Voice of Experience,” I’d like to take the time to cover the general aspects of these types of transformers.
To begin, there are a few different types of single-phase and three-phase UD padmounts: live front with exposed live primary parts, 600-amp bolt-on elbows and loop feed with bushings and elbows. All of these transformers are available in several voltage ranges.
The proper PPE must be worn when an employee is opening, entering and working on energized transformers. This includes a rated hard hat, eye and face protection, rubber gloves, heavy leather boots and arc-rated FR clothing. Additionally, all PPE must be worn by any employee exposed to energized equipment and cables until the transformer has been de-energized and checked for the absence of voltage, and all exposed parts have been properly grounded.
When working with transformers, there is always a chance of equipment failure. This can expose employees to arcs, flashes or ground gradient step potentials while they are opening or closing doors or switching cables in padmounts. Fortunately, a variety of equipment is available to protect employees while performing these types of tasks. A switch stick at least 6 feet long should be used when switching cables, opening bayonet fuses, checking for the absence of voltage or grounding conductors or equipment. Minimum approach distances must be maintained at all times when unprotected parts of the body are exposed to energized equipment. With the revisions made to OSHA 29 CFR 1910.269, it is possible to use rubber gloves to apply system safety grounds to secondary spades operating at fewer than 600 volts after the absence of voltage has been verified with a properly rated voltage meter. All primary system safety grounds shall be applied with tested and approved hot-line shotgun sticks.
Modern day underground loop-feed transformers are safe, versatile and fairly easy to install and maintain. The National Electrical Safety Code (NESC) requires all transformers to be properly marked for the purposes of employee and public notification and protection. Insulated bushings and elbows that are properly installed, with bleed/drain wires from the bushings and elbows to the tank grounds or concentric neutral, are also required.
Both the industry and OSHA consider transformer cables to be insulated for voltage, which reduces the hazard of exposed live parts, but it’s important to realize there are safe work procedures required when handling cables. When the concentric neutral or LC shield has been removed to terminate cables, and there is a possibility of contact with any part of the cables or equipment not covered with the concentric neutral, workers must wear properly rated gloves and all other necessary PPE.
Primary cables and bushings are insulated and not considered exposed, but they must be considered in reach and employees must take precautions to avoid exposure to elbows’ and bushings’ hazardous voltages on surfaces. Hazardous amounts of static charge can be present on all energized cables, elbows and bushings when the concentric neutral or LC shield has been removed. Rubber gloves and all PPE are required for employee protection. Properly rated fiber covers and/or rubber blankets limit the chance of contact with energized parts and protect employees from primary components when working in transformers and on secondary bushings of single-phase transformers. Properly rated cover must be applied with hot-line tools. Cover may be applied to in-reach energized parts if the employee is wearing rubber gloves.
Exposed energized parts operating at greater than 4 kV cannot be worked on by an employee wearing all PPE. Instead, testing for voltage, phasing and grounding must be performed with hot-line tools while maintaining minimum approach distances.
Secondary bushings are close together and exposed, which requires properly insulated and rated spade covers to cover all energized, exposed and ungrounded parts not being worked on. If secondary is de-energized and grounded, the primary cables remain energized. Proper cover should be placed on energized primary cables and equipment parts that are in reach.
All equipment grounding terminations are extremely important for safety and reliability. There should be at least two tank ground lugs on all transformers because the high-side ground on the primary coil is an internal connection to the tank. The neutral spade has a copper strap to split the secondary coil. If two single-phase UD pads are used to provide a three-phase delta service, the copper strap must be removed from the neutral bushing and tank connection on the power transformer; this is done in order to put the transformer’s secondary coil to the series secondary coil of the lightning transformer to provide the “high leg” of delta service. With the internal primary coil ground, the tank grounds become vital to the safety of employees and the public. If the tank ground is left off or damaged, the tank itself could be energized at primary voltage.
Positioning and Labeling
Proper placement of all cables during construction setup – prior to installing the pad and transformer – is necessary for the safe switching of primary cables from elbows to parking stands or feed-through/grounding bushings. Secondary cables should be positioned in the left rear corner of the pad opening, and the primary conductors should be placed in the right front corner. When switching the primary cables, this will allow room to move them in front of the secondary cables. While installing additional cables after the transformer is in service, great care should be taken when digging around existing energized cables and selecting the best point of entry for primary and secondary cables.
The NESC requires the transformer to be properly labeled both inside and outside to notify employees and the public of hazards and dangers. The safety code also requires the transformer to be placed on the customer’s property no fewer than 10 feet from any buildings. If motorized vehicles may come near the transformer, it should be protected by bollards to prevent it from being struck, sliding on the pad and incurring damage. If the transformer slides on the pad, elbows may disconnect from bushings, creating a hazardous condition. All precautions, including de-energizing, should be considered when the transformer is in an abnormal position on the pad. Energized exposed primary cable terminations can present a danger if they are disconnected from bushings and not secured inside single- or three-phase transformers.
Three-phase loop-feed transformers share the same versatility and reliability as single-phase transformers. They are available in many high- and low-side voltages as well as tap changers to compensate for load demands and variable primary voltages. Tap changers should never be changed when a transformer is under load. Some transformers have internal breakers to disconnect secondary coils and drop customer load. Bayonets on large three-phase transformers will not break all load and attempting to do so will create a hazardous condition for employees.
Another hazard to consider is the creation of ferroresonance by switching three-phase circuits. Ferroresonance is likely to occur when the capacitive reactance of the cable is equal or almost equal to and in series with the inductive reactance of the transformer coils when the transformer secondaries are not loaded. This can occur at any primary voltage in underground but usually is not encountered in overhead except in higher distribution voltages. Switching a three-phase transformer one phase at a time puts cable current in series with the coil. To reduce the possibility of resonance hazards, best practices may include gang-switching using the transformer’s AB switch or removing the bayonets in the transformers, effectively isolating the coil until all three incoming phases are energized.
Penta-head bolts and company locks provide safety and security from unauthorized entry. The tagging of all primary and secondary cables in transformers is extremely important. There is no universal tagging method; it is left up to the company’s operating system. I encourage tagging individual cables in a way that indicates the location of the next termination. For instance, “Phase A to Pad #3” is a simple statement that is easy to understand when looking at the single line of a UD system. Keep in mind that a tag only indicates the configuration at that particular time, and operations, modifications and repairs can necessitate tagging updates to ensure accuracy. All cables should be verified as safe before being handled without protective equipment.
As I mentioned earlier, everything I’ve discussed about UD padmount transformers in this article is general information for you to consider. If you have more detailed questions about anything covered here, please email me at firstname.lastname@example.org or reach out to Kate Wade at email@example.com.
About the Author: Danny Raines, CUSP, safety consultant, distribution and transmission, retired from Georgia Power after 40 years of service and opened Raines Utility Safety Solutions LLC, providing compliance training, risk assessments and safety observation programs. He is also an affiliate instructor at Georgia Tech Research Center OSHA Outreach in Atlanta. For more information, visit www.electricutilitysafety.com.