UL Requirements for Pad-Mounted Distribution Transformers

A pad-mounted transformer is an electrical transformer installed on a concrete pad at ground level. It is used to step down high-voltage electrical power to a lower voltage level appropriate for homes and businesses. The pad-mounted transformer is typically sealed in a weather-resistant metal enclosure to protect it from the elements, and it may be installed in a residential, commercial, or industrial setting.

Figure. Pad-Mounted Distribution Transformer:

The purpose of the pad-mounted transformer is to provide a safe and convenient way to distribute electrical power to users. By lowering the voltage of the electrical power, the pad-mounted transformer reduces the risk of electrical shock and makes it possible to use the power in various applications. The concrete pad provides a sturdy and stable base for the transformer, and the metal enclosure protects it from damage.

Applications of Pad-Mounted Transformers

Pad-mounted transformers are commonly used in various applications, including:

1. Residential areas : Pad-mounted transformers are often used to distribute electrical power to homes and small businesses. They are typically installed near the curb or at the edge of a property and provide a safe and convenient way to access electrical power.
2. Commercial and industrial areas : Pad-mounted transformers are also used in commercial and industrial settings, where they help to distribute electrical power to large buildings, shopping centers, factories, and other types of facilities.
3. Underground power distribution: Pad-mounted transformers are often used in underground power distribution systems, which help step down high-voltage power to a lower voltage suitable for homes and businesses.
4. Renewable energy systems: Pad-mounted transformers are used in renewable energy systems, such as wind and solar power systems, to step down the high voltage generated by these systems to a level that can be used by homes and businesses.
5. Electric vehicles: Pad-mounted transformers are used to provide electrical power for charging electric vehicles, and they are typically installed at public charging stations or in private garages.

In each of these applications, pad-mounted transformers play a crucial role in ensuring the safe and efficient distribution of electrical power. They are designed to be durable, reliable, and easy to use, and they are an essential component of modern electrical power systems.

UL Requirements for Pad-Mounted Transformers

UL, or Underwriters Laboratories, is a global safety science company that provides safety certification for a wide range of electrical and electronic products. For pad-mounted transformers, UL has established a number of requirements that must be met for the product to receive UL certification. Some of the essential UL requirements for pad-mounted transformers include the following:

• The pad-mounted transformer of the compartmental type shall comprise a tank with compartments for terminating high and low-voltage cables. The transformer tank and compartments must be combined into a single unit for installation on a pad. There must be no visible screws, bolts, or other external removable fastening devices. There should not be any gaps through which foreign objects such as sticks, rods, or wires could come into contact with live parts. The design must include a way to lock the compartment door(s) with a padlock. The construction should prevent water (except in cases of flooding) from entering the compartment to ensure the proper functioning of the transformer.
• Access to the high-voltage (incoming) compartment can only be gained after opening the door to the low-voltage (outgoing) compartment. The doors and compartment hood must be removable to simplify connections and allow for cable pulling.
• The nameplate must be located in the low-voltage section of the compartment and must be readable even with the cables in place. If the nameplate is attached to a removable part, the manufacturer’s name and the serial number of the transformer must be permanently fixed to a non-removable part.
• The transformer tank must be of sealed-tank design with a welded main cover. A bolted and tamper-resistant handhole must be included in the tank cover to provide access to internal connections. Both the high-voltage and low-voltage compartments must have provisions for grounding the tank.
• The tap changer control can only be operated when de-energized and must be externally operable using a hot stick and required a minimum of two operator actions to change taps. The primary compartment is the preferred location for the control.
• The average temperature rise of the windings above ambient temperature, when evaluated at the transformer’s rated capacity, must not exceed 65°C or 55°C at the base rating of the transformer and, when tested at 112% of the base rating, must not exceed 65°C.
• The core and coil must undergo vacuum processing to maximize the penetration of insulating fluid into the coil insulation system. During the vacuum process, the windings will be energized to heat the coils and remove moisture. The core must be made of grain-oriented silicon steel and must be precisely stacked to eliminate gaps in the corner joints. The coils can either be made of aluminum or copper.
• The dielectric coolant must be listed as less-flammable oil and comply with the specifications outlined in Section 450-23 of the National Electrical Code and Section 15 of the National Electrical Safety Code (IEEE Std C2™-2002).
• The high-voltage overcurrent protection system that comes with the transformer shall be a load break expulsion Bay-O-Net fuse assembly that is externally removable. The bayonet fuses must be in a series with backup current-limiting fuses (CLFs) to ensure comprehensive protection.
• All gauges and measuring instruments included in the transformer should be placed outside the cable compartments so that access does not involve exposure to any active circuits. These devices should be located inside a separate NEMA® 4 rated enclosure, either on the tank’s low- or high-voltage side. The devices shall include a liquid level gauge, a dial-type thermometer, a pressure/vacuum gauge, a pressure relief valve, a half-inch fluid sampling valve, and a winding temperature indicator.
• All transformers shall be tested for the following:
  • No-Load losses at rated current
  • Total losses at rated current
  • Percent Impedance at rated current
  • Excitation current (100% voltage) test
  • Winding resistance measurement tests
  • Ratio tests using all tap settings
  • Polarity and phase relation tests
  • Induced potential tests
  • Full wave and reduced wave impulse test
• The transformers must adhere to the efficiency levels for liquid-immersed distribution transformers, as outlined in the Department of Energy ruling “10 CFR Part 431 Energy Conservation Program: Energy Conservation Standards for Distribution Transformers.”

Conclusion

In conclusion, pad-mounted transformers are essential to modern electrical power systems. They provide a safe and convenient way to distribute electrical power to homes and businesses and play a crucial role in reducing the risk of electrical shock and fire. To ensure that pad-mounted transformers are safe, reliable, and of high quality, they must meet a number of requirements established by organizations such as UL. By meeting these requirements, pad-mounted transformers can receive UL certification, which provides assurance to customers that the product is safe, reliable, and meets industry standards for performance and quality.