Substation Transformer Explained

By Randolph W. Hurst, Publisher

Substation Transformer

A Substation Transformer is designed for installation on three-phase transmission and distribution systems. Substation transformers are constructed for in-door or outdoor mounting on a concrete pad with high and low voltage cables entering operating compartments through enclosed sidewall mounted bushings (unit-type transformers) or through cover or sidewall mounted bushings (opentype transformers).

Liquid-filled substation transformers are used in a wide variety of utility and industrial applications. These step-down transformers are usually manufactured in accordance with applicable IEEE standards. Substation transformers are most often offered with one of 3 different fluids: Mineral oil or silicone transformer oil. Primary voltages of a substation transformer usually ranges from 2.4 kV to 69 kV in 225 through 20,000 kVA sizes, but 600 volt through 35 kV secondary voltage ratings are also available.

A substation is a part of the installation that includes the terminations of the transmission or distribution lines and switchgear, and which may also include transformers The substation also typically includes all the necessary devices for control and protection. Depending on the function performed, it can be defined as a transforming, conversion, transmission or distribution substation. A MV/LV transformer substation consists, therefore, of the set of devices dedicated to the transformation of the voltage supplied by the distribution network at medium voltage (e.g. 20 kV), into voltage values suitable for the power supply of the low voltage lines (e.g. 400 V).

Substations can also be divided into public and private substations:


Public Substations

These belong to the electricity distribution company and supply private users in singlephase or three-phase alternating current (typical voltage values for the two types of power supply could be 230 V and 400 V). They are in turn divided into urban or rural substations, consisting of a single small power transformer. Urban substations are usually built in brick, whereas rural ones are often installed externally directly on the MV pylon.


Private Substations

these can often be considered as terminal type substations, i.e. substations where the MV line ends at the installation point of the substation itself. They are owned by the User and can supply both civil users (schools, hospitals, etc.) and industrial users with supply from the public MV grid. The user must make available to the distributing company a special room, accessible to the staff of the company, in which the equipment for which the distribution company is responsible will be installed. There can be various design solutions, although in recent times the use of prefabricated substations is increasingly widespread.


Specifications to consider for Underground Substation Transformers

There are several reasons why a power transformer needs to be operated underground. It can be due to space limitations or restrictions set in place to meet environmental requirements. Special considerations need to be taken into account:

  • Space limitations: units need to be transported and installed in the limited space available
  • Safety considerations: use of ester fluid and/or special capabilities such as short-circuit strength and high impedance might be necessary
  • Quality requirements: to minimize operational risks and maximize the economic benefit of underground units, nothing but the highest quality can be used


Substation Transformer Benefits

1. Advanced water-cooling system

Operation of a transformer in an underground cavern always increases the demand for its cooling capabilities. For these transformers, that withstand increased levels of voltage and load, traditional air-cooling was not sufficient. As a solution, an advanced water-cooling system was designed and built.


2. Powerful and compact go well together

When installing a transformer bank of three 500-kV autotransformers, size is always an issue. This is even more important when the transformer is located under the streets of a megacity. The overall size of the units, how - ever, is considerably compact, measuring just 9 x 5.6 x 7.3 meters (L x W x H) each.


3. Impedance reach losses

Not only were the specifications in terms of voltage level extraordinary, the requirements for short-circuit strengths were also more demanding than usual. Each transformer should withstand a certain amount of short-circuit load. However, the autotransformers for the underground substation in Shanghai can stand impedance of up to 22 percent. This means that the units will operate as usual even when exposed to a load 22 percent higher than their rated current.


4. Control of hot spot and excessive losses

High impedance presented challenges for hot spot and stray loss control. Fully supported by a 3D magnetic model tool, the shielding structure design, especially on the tank and clamping frame, increases overall efficiency.