Most Common Custom Transformer Options

At Southwest Electric Co, we are experts in building custom distribution class oil-filled transformers with options unique to a wide variety of applications. This article walks through our most popular custom transformer options, from transformer type and termination options to accessories.

Transformer Types: Pad Mount & Unit-substation

Before  deciding on custom transformer options like accessories, there are a few initial choices. First is the transformer type. Distribution class transformers typically come in a pad mount style and sit on a concrete pad with the wiring coming from the ground into the terminal cabinet, which sits on the concrete pad with an open bottom. Similarly, some transformers can have a terminal cabinet stop short of the ground with a solid bottom. The electrical connections typically are brought to the transformer from conduit to the terminal cabinet either from the bottom, side or top. These are considered unit-substation style transformers and can have terminal cabinets on different sides for high voltage and low voltage compartments.

Picture 1 shows a large transformer with two terminal cabinets on opposite sides of the transformer tank wall. In this transformer, the cabinet does not reach the concrete pad, therefore it would not be considered a full pad mount transformer but a unit-substation style transformer. Also note that the bottom of the terminal cabinet is solid. In a pad mount, the bottom is always open because the wiring comes up from the ground into the cabinet

Unite Substation Transformer

Unit Substation Transformer

Picture 1: Unit substation style terminal cabinet. Note that the high and low voltage terminal cabinets are on opposite sides and the cabinet has a solid bottom since it does not rest on the concrete pad.

In a pad mount, the terminals are always in one cabinet separated by a voltage barrier. As you can see in the picture, the bottom is open, which allows the wiring to come up from the concrete pad. These are very typical style transformers that will sit outside of commercial buildings to bring power to the facility. The barrier is solid and keeps the low voltage from the high voltage for safety.

Pad Mount Transformer

Pad Mount Transformer

Picture 2:  Typical Pad mount transformer. Note that the terminals are all on the same side and in the same cabinet separated by a barrier. Also, note that the floor of the cabinet is open to allow the wiring from the concrete pad to enter the chamber.

Termination Options

Once the transformer type is established, there are several options for termination. One of our most popular is the dead-front bushing for the high voltage side. This bushing is configured in one of two ways: loop feed or radial feed. A loop feed is designed to be on a branch circuit that loops in another transformer in the circuit. This decision is typically at the request of the end user and utility. Our low voltage bushings are typically determined by the voltage and amps required. The higher the current, the larger the spade terminal will be. These will come with the provision for cable supports to prevent strain on the bushing.

Radial feed Loop Bushings

Dead Front Bushings

Picture 3: Radial feed and Loop feed dead front bushings

Transformer Bushings

Transformer Bushings

Picture 4: High current spade bushings. Note that one bushing has a 12-hole Nema pattern for KCM cabling, whereas the other has a 4-hole Nema pattern for smaller current.

Voltage Tap Selector Switch Options

The next custom transformer options applies to voltage tap selector switches. These are typically established in a percentage from the nominal incoming voltage from utility or customer supplied power. These tap switches are usually 5-position or 7-position, depending on how may taps the end user requested.  The 5-position tap switch is the most common with tap ranges in ± 2.5% and ±5% voltage increments. These switches are always de-energized when making the change to avoid damage. There are tap switches that can be operated under load, but these are typically used on large power transformers where de-energizing can cause damage to the transformer.

These types of switches are standard on all utility distribution transformers. However, our most popular tap switch is used in our oil and gas industry. This switch allows the end-user to select up to 25 different voltages for their output either in a delta or wye configuration. The dual switch also allows for different voltage combinations on the transformer.

Transformer Tap Switch

Picture 5: Typical three phase 5 position tap switch. The dual switch is used on special outputs with a range of voltages 1100-3800Volts or other combinations of 25 taps.

Custom Transformer Accessories

The last custom transformer options are often referred to as accessories and are used to monitor and protect the transformer during operation. These are not required to operate the transformer, however, almost all transformers come with many of these devices.

  1. Pressure relief valve: This device relieves pressure build up during operation.  Some of the devices are mechanical in nature, but some are automatic.
  2. Liquid Level gauge: This device monitors the oil level in the tank.  It is critical that the oil doesn’t drop too low as it could cause the transformer to fail in temperature and voltage.
  3. Liquid Temperature gauge: This device monitors the temperature of the oil and/or coils of the transformers.  Temperature is the number one failure mechanism of transformers.  If the transformer overheats it can lead to the life expectancy to decrease.
  4. Pressure vacuum gauge: This monitors the positive and negative pressure in the tank.  It assists in determining if there is a leak or other possible issues with the transformer.
  5. Dissolved Gas Analysis monitor: This device monitors the oil for gasses.  It is used for early detection of potential failure mechanisms.
  6. Drain valve with sampler and oil valve. These are used to fill and/or drain oil.  The sampler is used to take a small amount of the oil from the transformer for detailed oil analysis that can be used to determine if the oil or transformer has seen any potential damaging issues.

All these devices except the drain valve can some with a contact switch that can be used for controls, such as an alarm for over temperature.  These are wired into a control box that the end user can access for their controls.  In some instances, the control box and components may be configured for a hazardous location as shown in the control box below.

Transformer Gauges

Transformer Box

Transformer Drain Valve

Transformer Gauges

Liquid Level and Temperature Vacuum Gauge

Temperature Gauge

Picture 6:  Various accessories that are typically found on oil filled transformers.


Give us a call or email us with any questions you have or to discuss your next custom transformer!