MV/LV Distribution Transformer: Connection and Testing

Short answer

The distribution transformer steps medium voltage down to the end-use low voltage in networks, buildings, industries and solar plants. It can be oil-immersed or dry-type, is usually connected Dyn11 or Dyn5 with a low-voltage neutral, has taps that are changed with the unit de-energized, and is protected with fuses or with a breaker and relay depending on its size.

What it is and where it is located

The distribution transformer is the last step of transformation before the end user: it receives medium voltage from a feeder and delivers low voltage to a building, an industry, a neighborhood or the station service of a substation or a solar plant. It is mounted on a pole, on a pad outdoors, inside a compact substation or in an electrical room next to the medium-voltage switchgear and the low-voltage panel.

It is the same principle as the power transformer, but with lower ratings, medium voltage on the primary and a construction designed to operate unattended for decades. For which transformer tappings are full-power and which are reduced-power, see full-power and reduced-power tappings.

Main parts

Core of grain-oriented electrical steel or, in low-loss designs, amorphous metal, depending on the manufacturer.
Windings of copper or aluminum: the medium-voltage one with its taps and the low-voltage one, often made of foil.
Insulating and cooling medium: mineral oil or ester in liquid-immersed units; air and resin in dry-type units.
Tank, hermetically sealed (fully filled) or with a conservator; in dry-type units, an enclosure with the required ingress protection rating.
No-load tap changer, operated from the cover or from a panel.
Bushings on the medium- and low-voltage sides; on pad-mounted units, separable connectors for cable.
Accessories: level indicator and thermometer, pressure relief valve, integral protection device on hermetic units, temperature sensors in the windings of dry-type units, nameplate.

How it works

The medium voltage applied to the primary creates an alternating flux in the core that induces the secondary voltage according to the turns ratio. With a Dyn connection, the primary is delta and the secondary is wye with an accessible neutral. The delta lets zero-sequence and third-harmonic currents circulate, so unbalanced low-voltage loads do not shift the neutral. The clock number gives the phase shift between primary and secondary in multiples of 30°: Dyn11 means the low voltage leads the medium voltage by 30°. IEC 60076-1 defines this notation.

The taps change the number of turns of the medium-voltage winding to adjust the low voltage to the actual system voltage at that point. A common range, which appears in manufacturer specifications, is ±2 × 2.5 %, but the final range is set by the project specification. On how ambient temperature and tap position enter the loading calculation, read ambient temperature and tap changer in transformer loading. For the correction of losses to the reference temperature, see transformer losses and correction to 75 °C.

The heat from the losses is removed by natural convection: in liquid-immersed units the most common cooling class is ONAN (oil natural, air natural), and in dry-type units AN, with optional fans (AF) for overload.

Variants and differences with other voltage levels

Oil-immersed: the most common one on overhead networks and outdoors. Single-phase or three-phase on a pole, or three-phase pad-mounted with medium- and low-voltage compartments and dead-front design.
Cast-resin dry-type (IEC 60076-11): for indoor use, buildings, hospitals and industries where oil is to be avoided. The standard defines environmental, climatic and fire behavior classes that must be specified.
Substation power transformers: when stepping down from high to medium voltage it is no longer a distribution transformer but a power transformer with an on-load tap changer; see the high-voltage power transformer and the on-load tap changer.
Solar plants: step-up transformers of the inverter stations, sometimes with two low-voltage secondaries; they are described in solar plant step-up transformer.

Ratings and nameplate

The nameplate shows the rated power, the rated primary and secondary voltages and the tap range. It also shows the currents, the frequency, the vector group, the short-circuit impedance and the cooling class. It adds the insulation levels, the total mass and the mass of the insulating liquid (in liquid-immersed units), the manufacturing standard, the serial number and the year. On dry-type units the thermal insulation class and the environmental, climatic and fire classes are added. For how a transformer tapping range is written, see transformer tapping range and its notation.

No-load and load losses are guaranteed in the test report. IEC TS 60076-20 proposes methods to express energy efficiency, and many countries set maximum loss values by regulation. To dig deeper into uncertainty when measuring transformer losses, read transformer loss measurement and uncertainty.

Integration with protection and control

Medium-voltage fuses: the most widely used protection in distribution. They can be expulsion or current-limiting, alone or combined with a switch-disconnector; their selection and coordination are covered in the medium-voltage fuse guide.
Breaker and relay: for larger ratings or in industry, with phase and ground overcurrent 50/51 and 50N/51N fed by current transformers (CT).
Built-in protection: temperature relay with sensors in the windings of dry-type units; on hermetic units, an integral device for gas, pressure, level and temperature, depending on the manufacturer; one example is the DGPT2 relay.
Surge arresters next to the medium-voltage terminals on overhead networks.

In a digital switchgear panel, the relay publishes over IEC 61850 its overcurrent protection (PTOC), the trip (PTRC) and the breaker (XCBR); the transformer is modeled as YPTR when its status and temperatures are integrated into the system.

Commissioning and maintenance tests

The applicable transformer tests are:

Turns ratio and vector group on all taps.
Winding resistance, which also reveals loose contacts in the tap changer.
Insulation resistance between windings and to ground.
Dielectric strength and oil analysis on liquid-immersed units, when the design allows sampling.
Excitation current as a reference for later comparison.
Functional tests: temperature relay, built-in protection devices and trip to the breaker.
Thermographic inspection of connections under load.

Common failures and what to check first

Low voltage out of range: wrong tap, or different taps between phases after maintenance.
Overheating: overload, harmonics from electronic loads or loose low-voltage connections.
Blown fuse with no evident fault: inrush current, a wrongly selected fuse or an incipient winding fault.
Damage from lightning overvoltages when arresters are missing or too far away.
Moisture or leaks in tanks with deteriorated gaskets.
Dry-type units: dust and moisture on the coils, which cause surface discharges.

Testing and commissioning with Tiza Group

At Tiza Group we test and commission oil-immersed and dry-type distribution transformers for utility networks, industry, buildings and solar plants, together with their switchgear and protection, in Colombia or elsewhere on request. Contact us and we will propose the scope.

References

IEC 60076-1, Power transformers: general.
IEC 60076-2, Temperature rise for liquid-immersed transformers.
IEC 60076-11, Dry-type transformers.
IEC TS 60076-20, Energy efficiency of transformers.
IEEE C57.12.00, General requirements for liquid-immersed transformers.
IEEE C57.12.90, Test code for liquid-immersed transformers.

Frequently asked questions

Can the tap of an energized distribution transformer be moved?

No. The tap changer of a distribution transformer is, as a rule, designed to be operated de-energized. Take the unit out of service, move the tap on all three phases to the same position and verify the ratio and winding resistance before energizing again.

Why is the Dyn11 connection used in distribution?

The delta on the medium-voltage side gives a path to zero-sequence and third-harmonic currents, and the wye with a neutral on the low-voltage side allows loads to be fed between phase and neutral. The clock number (11, 5 or other) must match between transformers that will operate in parallel.

Which losses matter most in a distribution transformer?

No-load losses are present 24 hours a day while the unit is energized, even with no load; load losses grow with the square of the current. That is why the economic evaluation considers both according to the expected load profile.