ransformer is a static electrical component with no moving parts that is used for stepping voltage up or down or isolating one circuit from another. Transformers have the ability to convert low-voltage, high-current AC to high-voltage, low-current AC, or vice versa, with minimal energy losses. Minimizing energy losses is critical in all power generation, transmission, and distribution systems. Transformers work only with AC in accordance with the physical laws of magnetic induction, and they are inherently low-loss components. The simplest low-voltage transformers can be made by winding separate coils of insulated wire around a ferromagnetic core, typically a stack of steel laminations.

When one coil or winding, called the primary or input coil, is energized, the core is magnetized so that the resulting magnetic flux induces a voltage in the second winding, called the secondary or output coil.The change in voltage (voltage ratio) between the primary and secondary coils depends on the number of turns in each winding.

Transformers are widely used in electrical power and lighting circuits as well as many low-voltage electronic products. The large transformers in power generation stations step up the output voltage of AC generators to higher values for more efficient transmission over transmission lines while also reducing the current values. Somewhat smaller transformers at electrical substations step the transmitted voltage down to the values more useful for regional and local distribution to customers while also stepping up the current.

Some of the smallest commercial transformers are found in the AC-to-DC converters that convert line AC voltages to the low DC voltages required for powering electronic products including cordless telephones, notebook computers, and cellular telephone battery chargers. However, transformers are the largest and heaviest components in the stand-alone linear power supplies for industrial, military, and commercial applications. The 60-Hz transformers built into TV sets and stereo systems are also large and heavy, but the high-frequency switching transformers in desktop and laptop computers are considerably smaller and lighter

Transformers can also isolate circuits, suppress harmonics, and regulate line voltage between distribution substations and consumers. Zigzag grounding transformers, for example, derive neutrals for grounding and a fourth wire from a three-phase neutral wire. They can be operated at voltages below their nameplate ratings, but they should not be operated at higher voltages unless they have taps intended for that purpose. However, when a transformer is operated below its nameplate rated voltage, its kVA output is reduced correspondingly.

Single-phase transformers rated 1 kVA and larger and three-phase transformers rated 15 kVA and larger can be reverse-connected without a loss of kVA rating. This is possible with high-power transformers because their turn ratios are the same as their voltage ratios. The turns ratio compensation on a low-voltage winding of a singlephase transformer rated below 1 kVA rules this out, because the low-voltage winding has a higher voltage than is indicated by the nameplate at no load. Although the transformer will not be harmed, its output voltage when reverse-connected will be lower than is stated on its nameplate.

TRANSFORMER CLASSIFICATION

Transformers are classified in many different ways: dry- or liquid-insulated, singlephase or polyphase, step-up or step-down, and single-winding or multiwinding. In addition, they are classified by application. For example, there are voltage or potential transformers (VTs) and current transformers (CTs) that are used step high voltage and current down to safe levels for the measurement of voltage, current, and power with conventional instruments.

• Autotransformers
• Auto zigzag grounding transformers
• Buck-boost autotransformers
• Current transformers
• Distribution transformers
• Substation transformers
• Voltage transformers

In addition to the many different kinds of transformers, there are many different ways to connect them. These include delta-to-delta, delta-to-wye, wye-to-delta, and T.

NEC TRANSFORMER REQUIREMENTS

NEC 2002 Article 450, Transformers and Transformer Vaults (Including Secondary Ties), states the requirements for the installation of transformers. It covers overcurrent protection for transformers rated for more or less than 600 V, autotransformers rated for 600 V or less, grounding autotransformers, secondary tiers, parallel operation, guarding ventilation, and grounding. Specific provisions apply to different types of transformers such as dry-type and liquid-insulated for both indoor and outdoor installation. Transformer vault location, construction, ventilation, and drainage requirements are also given.

The NEChas defined a secondary tieas a circuit operating at 600 V or less between phases that connect two power sources of power supply points, such as the secondaries of two transformers.

SUBSTATION TRANSFORMERS

Power is transmitted at high voltages because less current is required to transmit a given amount of energy at a higher than a lower voltage. Consequently, electrical energy can be transmitted with less I2R or line loss when high transmission voltages are used. Transmission voltages as high as 500 kV can only be obtained from step-up transformers at power stations, because AC generators cannot generate voltages at those values.

Step-down transformers in distribution systems reduce the high transmission voltages to the values needed by industrial, commercial, and residential consumers. Large power transformers rated at 35 kV or more can reach efficiencies of 99 percent at full load.

below Figure shows a primary or secondary open substation transformer that can transform transmission voltages to useful levels. Both three-phase and single-phase substation transformers can be constructed in this configuration. They are made from a wide range of steel cores and winding conductors to meet specific substation requirements.

The transformers can be either wound-core or shell-type . Their core and coil assemblies are placed in steel tanks that are filled with either electrical-grade mineral insulating oil or another suitable dielectric coolant. These transformers can be self-cooled or forced-air-cooled. In addition, they can be built specifically for indoor or outdoor installation.

Transformer Characteristics

below Figure is a diagram of a conventional (core-type) single-phase transformer. It shows two independent coils or windings, the primary and secondary, wound on a closed-ring core made from a stack of laminated sheet steel

If an AC voltage is applied to the primary winding of the transformer, an electromagnetic field or flux forms around the core and expands and contracts at the input frequency. This changing flux cuts the wires in the secondary winding and induces a voltage in it. Because the turns of both windings are cut by the same magnetic flux, the voltage induced in each turn of both windings is the same. Thus the voltages across the windings of a transformer are directly proportional to the turns in each winding:

Primary ampere-turns = secondary ampere-turns

From this expression, it can be determined that the ratio of the currents in a transformer is inversely proportional to the ratio of the turns. The voltage that appears across the secondary winding depends on the voltage at the primary winding and the ratio of turns in the primary and secondary windings.

Transformers obey the law of conservation of energy, meaning that the product of voltage and current (power) in the primary winding equals the product of voltage and current (power) in the secondary winding, except for losses. For example, if the voltage at the secondary terminals is twice the voltage at the primary terminals, the current at the secondary terminals must be half that at the primary terminals to keep the product of voltage and current constant.

A step-up transformer has more turns in its secondary winding than in its primary winding, so the voltage across the secondary winding will be higher than the voltage across the primary winding. Similarly, a step-down transformer, has fewer turns in its secondary winding than in its primary winding, so the secondary voltage will be lower than the primary voltage.

The letters identifying the input and output terminals in Above Figure follow the industry standard marking code. High-voltage winding terminals are marked H1, H2, etc., and low-voltage winding terminals are marked X1, X2, etc. Thus, the high-voltage winding is commonly called the H winding, the low-voltage winding is commonly called the X winding, and the numbers of turns of each winding are designated as Th and Tx

POWER TRANSFORMER RATING

Power transformer capacity is rated in kilovolt-amperes (kVA). The output rating for a transformer is determined by the maximum current that the transformer can withstand without exceeding its stated temperature limits. Power in an AC circuit depends on the power factor of the load and the current, so if any AC electrical equipment is rated in kilowatts, a power factor must be included to make its power rating meaningful. To avoid this, transformers and most AC machines are rated in kVA, a unit that is independent of power factor.

In addition to its kVA rating, the nameplates of transformers typically include the manufacturer’s type and serial number, the voltage ratings of both high- and low voltage windings, the rated frequency, and the impedance drop expressed as a percentage of rated voltage. Some nameplates also include an electrical connection diagram.

Power transformers are generally defined as those used to transform higher power levels than distribution transformers (usually over 500 kVA or more than 67 kV). The kVA terminal voltages and currents of power transformers, defined in ANSI C57.12.80, are all based on the rated winding voltages at no-load conditions. However, the actual primary voltage in service must be higher than the rated value by the amount of regulationif the transformer is to deliver the rated voltage to the load on the secondary.