Power and Distribution Transformers Sizing Calculations – Part One


Power and Distribution Transformers Sizing Calculations – part one

 
Today, we will start explaining the Power and Distribution Transformers sizing calculations in details and I’d like to from all of you to review our course “EP-3: Electrical Procurement – Transformers Courseto be more familiar with the contents of our new articles about the Power and Distribution Transformers sizing calculations.

 
The contents of our articles for Power and Distribution Transformers sizing calculations will include the following points:

 
  • Glossary of Sizing Power and Distribution Transformers,
  • Power and distribution transformer components,
  • Power and distribution transformer classification: construction and application,
  • Three-phase power and distribution transformer connections,
  • Power and Distribution Transformers sizing calculations.

 

The following points were explained before ( or will be explained) in our course “EP-3: Electrical Procurement – Transformers Course” :

  • Power and distribution transformer components,
  • Power and distribution transformer classification: construction and application,
  • Three-phase power and distribution transformer connections.

 So, we will not go through these points here, we will focus only on the following two points:

 
  • Glossary of Sizing Power and Distribution Transformers,
  • Power and Distribution Transformers sizing calculations.

 

Now, we will start with the first point; Glossary of Sizing Power and Distribution Transformers.

 

 
 
 
Glossary of Sizing Power and Distribution Transformers
 

 

 


 

Term
Explanation
AA
Ventilated, self-cooled rating of a dry-type transformer.
 
AFA
Ventilated, forced-air-cooled rating of a dry-type transformer.
 
Additive polarity
The polarity designation of a transformer that has the primary and secondary terminals of corresponding polarity directly opposite of each other.
 
ANSI
American National Standards Institute
 
BIL
Basic Impulse Level
 
Bushing
A device that permits a conductor to pass through an enclosure, yet to be isolated from the enclosure. Typically the conductor is insulated from the enclosure by a porcelain or epoxy cylinder.
 
Copper losses
Transformer power losses that are caused by the resistance of the windings. Copper losses are also called I2R losses.
 
Core
A permeable metal frame around which transformer windings are wound. Usually made from silicon steel.
 
Current transformer (CT)
 
An instrument transformer that is intended to have its primary winding connected in shunt with a power supply circuit, the voltage of which is to be measured or controlled.
 
Dielectric strength
The insulation quality of a material or oil that is measured in kilovolts (kV).
 
Distribution transformer
 
A small transformer that is used to supply low voltage residential loads. Pole-mount and pad-mount transformers fall into this category. A distribution transformer usually has a kVA rating less than 500 kVA.
 
Distribution transformer installation
 
Installations that contain transformers 500 kVA and less are designated distribution transformer installations.
 
Dry-type transformer
A transformer in which the windings are immersed in air or some other dry gas, e.g., nitrogen. For Saudi Aramco purposes, a dry-type transformer in which neither of the windings is cast in resin is called a conventional dry-type transformer.
 
Eddy current loss
One component of transformer iron losses. Eddy current loss is the loss caused by the currents that are induced in the iron core of a transformer.
 
Efficiency
The efficiency of a transformer is the ratio of real power output to real power input, and it is usually expressed in percent.
 
Electromagnetic induction
 
The process by which an electromotive force is produced in a conductor when there is relative motion between the conductor and a magnetic field.
 
Excitation current
Excitation current is the current that flows in any winding used to excite the transformer when all other windings are opened circuited, and it is usually expressed in percent of
the rated current of the winding in which it is measured.
 
FA
Forced-air-cooled rating of a liquid-filled transformer
Hysteresis loss
One component of transformer iron losses. Hysteresis loss is the amount of energy required to overcome residual magnetism of the transformer core.
 
I2R losses
Transformer power losses that are caused by resistance of the windings. I2R losses are also called copper losses.
 
IEEE
Institute of Electrical and Electronics Engineers
 
Impedance voltage
The voltage at rated frequency that is applied to the line terminals of one of the windings of a two winding transformer to cause the rated current to flow through these terminals when the terminals of the other winding are short circuited. The applied voltage is measured while the windings are at the same specified winding temperature.
 
Inert gas
A gas that does not chemically react with other substances.
 
Instrument transformer
 
A transformer that is intended to reproduce in its secondary circuit, in a definite and known proportion suitable for utilization in measurement, control of protective devices, the voltage or current of its primary circuit with its phase relationships substantially preserved.
 
Iron losses
The combination of eddy current and hysteresis losses.
Load losses
The load or copper losses of a transformer are those losses incidental to the carrying of load.
Load tap changer (LTC)
 
A tap changer designed so that the transformer’s turns ratio can be changed without interrupting the power source.
 
MRD
Mechanical Relief Device
 
No-load losses
No-load losses are the losses in a transformer that is excited at rated voltage and frequency but which is not supplying load. Note: No-load losses include core loss, dielectric loss, and the loss in the windings due to exciting current.
 
No-load tap changer
(NLTC)
 
A tap changer that can only be operated when the transformer is completely de-energized from the power source.
 
OA
Self-cooled rating of a liquid-filled transformer
Pad-mounted transformer
 
An outdoor distribution-type transformer that is used as part of an underground distribution system, with enclosed compartments for high voltage and low voltage cables entering from below, and that is mounted on a foundation pad.
 
Pole-mounted transformer
 
A distribution-type transformer that is suitable for mounting on a pole or platform on overhead installations up to 69 kV, with ratings of 1 kVA to 167 kVA single-phase and 15 kVA to 500 kVA three-phase. Sometimes called an overhead transformer.
 
Power transformer
A large transformer that is used to deliver large amounts of power to industrial loads or to transmission and distribution systems. A power transformer usually has a kVA rating greater than 500 kVA.
 
Primary rated current
The primary rated current should be equal to the secondary current divided by the turns ratio. Note: This definition of primary rated current ignores magnetizing current.
 
Primary rated voltage
 
The primary rated voltage designated to be applied to the input terminals should be equal to the secondary rated voltage times the turns ratio. Note: Primary rated voltage is not necessarily equal to nominal system voltage.
 
Primary winding
The primary winding is the winding on the energy input side of the transformer. The primary winding is usually the high voltage winding on a power transformer.
 
Principal tap
The tap to which operating and performance characteristics of a transformer are referred.
 
Rated KVA tap
A tap through which the transformer can deliver its rated kVA output without exceeding the specified temperature rise. Sometimes called the fully-rated tap.
Rated power (single phase transformer)
 
Rated power is equal to the product of rated voltage and rated current of the same winding of a transformer.
Rated power (three phase transformer)
 
Rated power of a three-phase winding is equal to the product of rated voltage, rated current, and the 3. Both windings of a two-winding transformer have the same rated power, which by definition is the rated power of the transformer.
 
Reduced KVA tap
A tap through which the transformer can deliver only an output less than rated kVA without exceeding the specified temperature rise.
 
 
Regulation
The full-load regulation of a transformer is the arithmetic difference between the secondary no-load and full-load voltages of a winding, divided by the secondary no-load voltage, with rated voltage applied to the primary winding at rated frequency and with the winding at specified temperature.
 
Reliability (substation)
 
The reliability of a substation is defined as its ability to serve the intended function without failure, although the complete elimination of failures is impossible to achieve.
 
Secondary full-load voltage
 
The secondary full-load voltage should be equal to the full load rms voltage available at the output terminals of the transformer, at rated frequency and current, and the required power factor, at principal tap.
 
Secondary rated current
 
The secondary rated current shall be equal to the full load rms current deliverable at the output terminals of the transformer, at principal tap, without exceeding the specified temperature rise
Secondary rated voltage
 
The secondary rated voltage should be the equivalent no load voltage corresponding to the full-load condition.
Secondary winding
The secondary winding is the winding on the energy output side of the transformer. The secondary winding is usually the low voltage winding on a power transformer.
 
Short circuit impedance
 
The short circuit equivalent star-connection impedance of a two winding transformer, expressed in ohms per phase, measured between the terminals of a winding when the other winding is short circuited. The impedance is based on rated frequency and the specified winding temperature.
 
SPR
Sudden Pressure Relay
 
Step-down transformer
 
A transformer in which the energy transfer is from a high voltage circuit to a low voltage circuit.
 
Step-up transformer
A transformer in which the energy transfer is from a low voltage circuit to a high voltage circuit.
 
Substation
Installations that contain power transformers (501 kVA and above) are designated substations.
 
Substation (industrial)
 
A substation is defined as an industrial substation if its main function is to deliver power to one or more major industrial plants. Substations serving a hospital load or any other critical load facility and substations supplying industrial facilities are considered industrial substations. An industrial substation facility includes the transformer switching devices, the transformer(s), the secondary or low voltage bus, circuit breakers and load feeder cables together with all controls, metering, relaying, and auxiliary equipment.
 
Substation (residential)
 
A substation is defined as a residential substation if its main function is to deliver power to residential areas.
Subtractive polarity
The polarity designation of a transformer that has primary and secondary terminals of corresponding polarity diagonally opposite of each other.
 
Tap
A tap is a connection brought out of a winding at some point between its extremities to permit changing the voltage or current ratio of a transformer or impedance of a reactor.
 
Tap changer
A tap changer is a switching device that is used to change the voltage ratio of a transformer by means of taps brought out from between the extremities of the windings. Tap changer operating can be arranged for either de-energized operation, where the transformer must be disconnected from its supply before the tap switch or link can be moved; or loaded operation, where the load tap changer (LTC) is designed to operate, in conjunction with a transition impedance, while the transformer is carrying load.
 
Temperature rise
The increase in operating temperature that is above an ambient temperature of a winding or insulating fluid.
Turns ratio
The turns ratio of a transformer is the ratio of the effective number of turns in the high voltage winding to that in the low voltage winding. Note: In the case of a transformer having taps for changing its voltage ratio, the turns ratio is based on the principal tap.
 
Underground-type transformer
 
A distribution-type transformer that is designed for location in an underground enclosure.
Voltage ratio
The voltage ratio of a transformer is the ratio of the rated voltage of the high voltage winding to the rated voltage of the low voltage winding, and is equal to the turns ratio.
 
Voltage transformer (VT)
 
An instrument transformer that is intended to have its primary winding connected in shunt with a power supply circuit, the voltage of which is to be measured or controlled.
 

 

In the next article, we will focus on the second point; Power and Distribution Transformers sizing calculations. So, please keep following.

 
 

 

 

 

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