Medium-voltage transformers: fundamentals of medium-voltage transformers

A medium-voltage distribution transformer or service transformer is a transformer type that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer.

Practical levels of voltage are often referred to as medium voltage, meaning the incoming voltage to the transformer is on the order of 5 kV to 35 kV. Some distribution voltages may exceed 35 kV and would be considered high voltage, but most of the distribution system is within the medium-voltage range. Modern distribution transformers are manufactured in accordance with many standards, most notably by IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission).

In the United States, the features and functionality of most distribution transformers fall under IEEE standard C57.12.00 (Standard General Requirements for Liquid-Immersed Distribution Power and Regulating Transformers), however, there are a multitude of standards that apply to specific types and applications of transformers including substation type, compartmental padmount type, single-phase pole mounted transformers, generator duty transformers, high-temperature transformers, and many more.

Substation transformer

The substation transformer is the heart of the electrical substation. This transformer changes the relationship between the incoming voltage and current and the outgoing voltage and current. Substation transformers are rated by their primary and secondary voltage relationship and their power carrying capability. For example, a typical substation transformer would be rated 15 kV, 25 kV, 35 kV or 46 kV on the primary at a power rating of about 5-20 MVA. The secondary or low voltage can be 15 kV down to 5 kV or even less than 600 V. Substation-style transformer design and functionality is dictated by IEEE standards C57.12.00 and C57.12.36. These type transformers consist of a core and coils immersed in oil or dielectric fluid in a steel tank. The oil or fluid serves both as an insulator and as a coolant to keep the core at reliable operating temperatures. Substation units are easily identified by their exposed bushings, gauges, panels or monitoring equipment and are typically located behind a fence or with a restricted area.

Three-phase pad-mounted transformer

A three-phase pad-mounted transformer is a ground-mounted electric power distribution transformer in a locked steel cabinet mounted on a concrete pad. These types of transformers are generally smaller (45 – 5000 kVA) but can be produced at larger station sizes as well (up to 10 MVA). These are installed in locations near or within public areas. The compartmental, tamper-resistant pad-mounted transformer design makes it ideal for applications where public safety is imperative. The overall design and functionality is dictated by IEEE standard C57.12.34, while the tamper-resistant nature of the pad-mount transformer is dictated by IEEE standard C57.12.28 or C57.12.29 for coastal areas. 

Single-phase pad-mounted transformer

Single-phase pad-mounted transformers are commonly installed in residential areas and are intended for power distribution through underground systems. Features and layout of single-phase pad-mounted transformers can be found in IEEE standard C57.12.38. These transformers are typically rated 10-167 kVA (up to 250 kVA per scope of IEEE standards) and up to 35 kV on the primary. 

Single-phase pole-mounted transformer

Single-phase pole-mounted transformers are frequently installed in residential areas but can also be common for small businesses requiring three-phase power from a bank. These transformers can vary in size from as small as 5 kVA to as large as 500 kVA, with voltages up to 35 kV line-to-line. Pole-mounted transformer banks allow three single phase units to be connected to a three-phase system to distribute through overhead lines. Winding connection styles, mounting standards, and overall layout and functionality is held to IEEE standard C57.12.20.

Ventilated dry-type transformer

Ventilated dry-type transformers are voltage changing (step-up or step-down) or isolation devices that are air-cooled rather than liquid-cooled. The transformer case is ventilated to allow air to flow and cool the coil(s). For outdoor operations, a dry-type transformer enclosure will usually have louvers for ventilation. Dry-type transformers adhere to IEEE standards C57.12.01 and C57.12.91, with ratings from 15 kVA to 30 MVA, and primary voltages above 601 V.

• First letter: Internal cooling medium in contact with the windings
  • O Insulating liquid with fire point ≤ 300 ^oC (see ASTM D92)¹²
  • K Insulating liquid with fire point > 300 ^oC
  • L Insulating liquid with no measurable fire point
• Second letter: Circulation mechanism for internal cooling medium
  • N Natural convection flow through cooling equipment and in windings
  • F Forced circulation through cooling equipment (i.e., coolant pumps), natural convection flow in windings (also called non-directed flow)
  • D Forced circulation through cooling equipment, directed from the cooling equipment into at least the main windings (also called directed flow)

• Third letter: External cooling medium
  • A Air
  • W Water
• Fourth letter: Circulation mechanism for external cooling medium
  • N Natural convection
  • F Forced circulation [fans (air cooling), pumps (water cooling)]
• Basic Insulation Level (BIL): The amount of insulation built into the unit to withstand an impulse voltage
• Impedance: The vector sum of the inherent resistance and reactance of the transformer. Transformer impedance determines how much available fault current is present at the transformer secondary terminals
• Frequency: Typically, 50 or 60 Hertz. The transformer will be designed for the system frequency

The arrangement of a transformer is defined by the type of components such as live-front vs deadfront, location of components (loop vs radial feed system), indoors vs outdoors, throat vs flange (substations). All of them are based on the application and following IEEE standards such as C57.12.36 (substations), C57.12.34 (three-phase compartmental type), C57.12.38 (single-phase padmount), and C57.12.20 (single-phase overhead type). Many features of modern distribution transformers are centered around the form and fit of the unit within a facility, specific area or region, or by application. The design and layout of the transformer can link to particular feature sets or requirements such as the tamper-resistant nature of a compartmental-type pad-mount transformer.

Live-front vs dead-front primary bushings

There are two distinct choices for high-voltage connection of a transformers.

Live-front (Porcelain type): The voltage-carrying parts are exposed. Livefront terminations have high voltage connectors, arresters, or fuses are exposed to the operator after cabinet has been opened.

Dead-front (separable insulated connector type): Often referred to as deadfront bushings. Safety is enhanced as there are no current-carrying parts exposed to the operator.

Deadfront connectors fall into two main categories: loadbreak and deadbreak.

• Types of dead-front connectors
  • IEEE 386 – Standard for separable insulated connector systems
  • 200 A load-break
    • 15 and 25 kV
  • 35 kV (2 standards)
    • Large Interface
    • Small Interface
  • 600 A and 900 A dead-break
    • 15, 25 and 35 kV
• Connectors and configurations
  • Configurations (bushing layout)
    
    • Radial feed
    • Loop feed
    • Bushing layout options
      • ANSI/IEEE C57.12.34 defines bushing spacing, bushing height, termination compartment size
        • Minimum
        • Specific

Radial system

A radial system employs one dedicated transformer to serve a customer or group of customers. The drawback is the fact that if there is a failure in the distribution line all of the customers downstream from the problem are without power. In the below example, the transformers are shown with fuses, which would isolate them from the distribution line in the event of an overload or transformer failure. Radial systems are mostly used in rural or remote areas. Radial feed systems require transformers to have only one high-voltage bushing per phase line being connected (one for grounded wye applications and two for delta and wye applications), since no current is being looped through any transformer.

Loop system

A loop system is fed by two different feeders – one connected to each end of the loop. Theoretically, the entire loop of transformers could be fed by either source, or the transformers can be split up between the two sources, depending on where an “open point” is created. In the event of a failed underground cable, this open point is “moved” (by using load-break switches or elbows), so that no customers are out of power while the cable is repaired. In the below example, the transformers are shown with fuses, which would isolate a failed transformer, limiting the outages to those customers fed by the failed transformer. Loop feed transformers have two high-voltage bushings per phase line being connected, so current can loop through the transformer, downstream to the next one.

Segments on a substation transformer

The plan view shows the ANSI segments used to identify the location of both the high-voltage and low-voltage bushings.

Segment 1 (front): Nameplate, gauges, valves, etc.

Segment 2: Standard for wall-mounted bushings (optional segment 4)

Segment 3: Standard for cover-mounted bushing

Segment 4: Standard for wall-mounted bushings (optional segment 2)

Cooling types

• Radiator panel style
  • Welded
  • Bolted (removable)
  • With fan(s)
• Corrugate style
  • Corrugate without fan(s)
  • Corrugate with fan(s)

Overcurrent protection

Overcurrent protection from fault conditions and overloading can be accomplished with various types of fuses and breakers. Sometimes these will be used in combination together and they often need to be coordinated together as a system to get a proper protection and functionality. 

• Fusing: One of the common types of overcurrent protection for transformers is fusing. Transformers use several types of fuses and there are benefits and disadvantages to each. These fusing philosophies are used to create fuse selection and coordination tables for fuses used in pad-mounted transformer applications.
  • Expulsion: Expulsion fuses are inexpensive protection and available in a few different types. The most common for medium-voltage distribution transformers is the Bay-O-Net style fuse. Once the fault condition is repaired, the expulsion fuse is easily replaced from the outside of the transformer tank. For proper protection, it needs to be used in-series with an isolating link or with partial range current limiting fuses. Available up to 38 kV class
  • Cartridge: Cartridge fuses are similar to Bay-O-Net fuses, but they do not have handles for external removal. They are completely inside the tank but would be accessible from the access cover on top. The advantage of this style is that there are slightly higher amperage ratings available than currently offered in the Bay-O-Net fuses.
  • Current limiting (full range vs partial): Current limiting fuses are another type of protection. They can limit the amperage that occurs during a fault event by forcing the current to go to zero. These are typically low-impedance faults in which the windings of the transformers are faulted and there is limited or no impedance for the fault current to flow through the transformer, which can cause damage to other equipment in the system if there are no current limiting fuses in place. 
  • Expulsion + partial range current limiting fuse in-series: One of the most common types of protection for distribution transformers is a combination of an expulsion fuse and a partial range current limiting. In this arrangement, secondary faults and overload currents are cleared by the Bay-O-Net fuse, and high-level faults are cleared by the current-limiting fuse. The two fuses are connected in-series, and are coordinated so that the current-limiting fuse operates only upon internal equipment failure

• Breakers: Breakers are a type of resettable overcurrent protection. They come in a variety of amperage and voltage categories.
  • High-voltage breakers are typically standalone devices and are found in an open substation. Medium- and low-voltage breakers can be standalone devices or can be grouped and assembled into a switchgear lineup or a mounted panelboard. 
    
• VFI (interrupters): A type of breaker that is unique to Eaton transformers is the Vacuum Fault Interrupter (VFI). This device is taken directly from Eaton’s pad-mounted switchgear equipment and integrated directly into the transformer as an electronically tripped, resettable primary protection device. In addition to the protection, this device can also be used as a loadbreak switch (on-off switch). The electronic controls for this device can allow for external control schemes.

This technical document discusses how a transformer equipped with an integral vacuum fault interrupter (VFI) can be an alternative to a non-VFI transformer in combination with stand-alone switchgear.

• Magnex: Eaton’s Cooper Power series Magnex interrupter is a resettable overcurrent protection device that can be used to protect lower primary amperage transformers from damaging overloads and faults. Like the VFI, it can be used as a loadbreak device.

Overvoltage

• Arresters: Distribution, intermediate and station class
  • Livefront
  • Riser pole
  • Deadfront
  • Under-oil
  • Low-voltage

 

• Switches: There are two main categories of transformer switches.
  • Load-break (energized operation) switch: A load-break switch has the capability to operate while the transformer has current flowing through it and may be used for turning a unit on/off, changing incoming feeds or sectionalizing the incoming loop.
    • Break-Before-Make (BBM): This type of loadbreak switch is constructed with conductive blades that are momentarily separated between switching operations, briefly deenergizing the circuit while switching. 
    • Make-Before-Break (MBB): This type of loadbreak switch has conductive blades that are constructed such that the circuit is never fully open, and energy continues to flow throughout the switching operation.
  • De-energized (non-load-break) switch: A de-energized switch cannot be operated while a transformer is under load or has current flowing through it. Common de-energized switches include tap-changers, dual voltage switches or delta-wye switches.

• Gauges: Transformers have a multitude of gauges that allow for easy monitoring or status checks of condition. Some of the most common types include liquid or winding temperature gauges, liquid level gauge and pressure/vacuum gauges. Most gauges come in one of two styles. 
  • Analog, non-contact type gauge: Used for local monitoring only of a transformer diagnostic
  • Contact gauge, or gauge with contacts: Includes internal conductive contacts capable of closing or opening when a given condition arises, relaying an electrical signal to an annunciator or control room as an alarm signal 

• Sampling valve: Sampling valves are most often incorporated into a transformer drain valve. The sampling valve is a crucial component of a liquid-filled transformer as it gives the operator the ability to pull a portion of the transformer fluid to run dissolved gas analysis, or other fluid testing, that may help determine the overall health of the unit. Eaton and other manufacturers now offer the fluid sample valve externally for safe access while the transformer is still energized.
  

• Visible break: Visible break is a term referring to a switch that allows visible isolation of a single-phase or three-phase circuit. The purpose of including the visible break switch is to allow an operator to quickly and effectively determine if a transformer or line is truly de-energized. Eaton transformers can be provided with external visible break on padmount transformers to allow safe de-energization without the need to enter the primary or secondary cabinets.

• Relays/schemes: System protection is the art and science of detecting problems with power system components and then isolating these components. Protective relays, associated communication systems, voltage and current sensing devices, station batteries and DC control circuitry make up the gambit of apparatuses in a protection system. Ultimately these protective devices and sensors help keep equipment such as transformers, reactors, generators, capacitors, buses and transmission lines protected from the dangers of surges, faults and overcurrent events.
  

Routine tests

Ratio, polarity and phase-relation test

1. The purpose is to verify the correct high-voltage and low-voltage turns ratio (at all tap positions and voltage settings)
   
2. This test verifies the unit is checked for open circuit conditions, short circuit (turn-to-turn) conditions and proper polarity and phase relationship (start vs. finish leads)
3. Test results are per IEEE standards C57.12.00 (+/-0.5% tolerance on test value from design value). Results are reported as either pass or fail

Resistance test

1. The purpose is to verify coils resistance and wire size is as expected when compared to design values, to obtain a resistance value to upgrade (correct) winding losses and to perform heat run calculations
2. Procedure for substation units is to test on rated tap and tap extremes. Padmount units are tested in nominal tap position only. In general, all readings are taken line-to-line, and high-voltage and low-voltage windings are connected in-series
3. Test results for reporting are not required by IEEE standard C57.12.00 but are available upon request. When used for heat run testing, the measurement is used for calculating I2R values and separating stray losses from measured winding losses

Insulation power factor test

1. The purpose is to determine the ratio of the power dissipated in the insulation in watts to the product of the effective voltage and current. (Is the insulation dry enough?)
   
2. The procedure for power factor testing is to short all high-voltage terminals together, and likewise with the low-voltage terminals. A 60 Hz, 120 V signal is applied to the unit by the measurement bridge. The first measurement is taken high-voltage to ground, the second high-voltage to low-voltage, and the final low-voltage to ground.
3. Test results are pass or fail: Test results are compared to an empirically derived curve of insulation factor power vs. temperature (typical pass value for a distribution transformer would be 1.0% or less).

Quality Control (QC) impulse test

1. The purpose is to verify the insulation integrity and BIL rating of units
   
2. The procedure consists of applying one reduced wave (approximately 50% of rated BIL) and one full wave (rated BIL) to each terminal. Non-impulsed terminals are grounded. First an operator would monitor the voltage and current waves on a DIMS 5 oscilloscope and then examine voltage and current waves for mismatch between reduced and full waves.

Core loss (No Load Loss) and percent exciting current test

1. The purpose of the test is to check accuracy of design calculations, check workmanship and materials and collect actual measured values for customer use and total ownership calculations. The results are also used in heat run calculations
   
2. The procedure consists of applying voltage to the low-voltage winding. Substations are tested on rated tap at 100% and 110% of rated voltage, while padmount units are tested at 100% only
3. The test results are compared to customer guarantees or design values for a pass / fail status

Induced potential (OX) test

1. Induced potential (also called low-frequency or overexcitation “OX” testing) is performed on all units prior to winding loss and impedance testing
2. The purpose of this test is to check turn-to-turn and layer-to-layer insulation
3. The procedure consists of applying twice-rated voltage to the low-voltage side of the unit, at 180 Hz for 40 seconds, or at 400 Hz for 18 seconds (the IEEE standard dictates the unit must see 7200 cycles).
4. Test results are reported as performed and passed when customer request certified test data, otherwise the test is considered a pass or fail

Winding loss (Load Loss) and percent impedance test

1. Winding loss (or load loss) testing is performed on all units during final testing
2. The purpose of the test is to check the test values against the design calculations, check workmanship and materials and collect actual measured values for customer use. Results may also be used in heat run calculations
3. The procedure for substation units is to test on rated tap and tap extremes. Padmount units are tested at the nominal position only. The operator must short-circuit the low-voltage winding and circulate rated current in the high-voltage winding in order to measure the losses
4. Test results are compared to customer guarantees or design values for a pass / fail status

Optional tests

IEEE Impulse Test (reduced wave, two chopped waves, full wave)

1. The IEEE impulse test is performed (60 kV BIL and above) upon request
2. This design test is performed because the application of two chopped waves applies different stresses to the winding than the full wave and puts the unit under similar stresses as a lightning strike or bushing flashover event. The test is meant to simulate the violent surges a transformer may see throughout its lifetime
3. The procedure for this test is to apply in sequence one reduced wave, two chopped waves and one full wave. The crest value of wave and time to chop is per IEEE standard C57.12.00, Table 4, unless otherwise specified
4. Test results are examined, matching reduced wave, first full waves and second full waves for variations in wave shape. Waveforms should overlay with minimal deviations. Matching before and after traces will result in a pass for the test

Sound level test

1. Audible sound level testing is performed when purchased or for design verification
2. The purpose of this test is to determine the amount of audible noise generated by the transformer
3. The specific procedure, including distance and sound sensor (microphone) placement is dictated in IEEE standard C57.12.90. In order to conduct the test, the unit to be tested is placed in a sound chamber and then energized at rated voltage. Sound levels are measured at prescribed intervals around the perimeter of the unit. The readings are then averaged to obtain the transformer sound level
4. Test results are reported and held per NEMA standards, specifically NEMA TR-1

Temperature (Heat Run) test (refer to IEEE standard C57.12.90)

1. Temperature rise (or Heat Run) testing is performed for two reasons: Customer request or design verification
2. The purpose is to evaluate thermal characteristics of the transformer (specifically its ability to stay cool during operation)
3. Test results are provided to determine if unit meets guarantee and/or design values