Category Archive for: «Transformers»

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Transformer Routine Test – Partial Discharge Measurement

Transformer insulation structure This routine test aims to measure the partial discharges which may occur in the transformer insulation structure during test. Partial-discharges are electrical arks which form the surges between electrodes of any area of the insulating media of a transformer between the conductors. These discharges may occur in air bubbles left in the […]

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Mineral oil-filled distribution transformer as one of the main part of electrical supply system

Proven Reliability As you already know, this is the most common type of distribution transformer. It is one of the most important part and can be found on electrical supply systems in every country of the world. Although the insulating oil is a flammable liquid, the reliability of the oil-immersed transformer has been proven over […]

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Maintenance and inspection of the transformer bushings

Six different inspections There are six different inspections to be performed on the bushings of the power transformer: Routine inspection Regular inspection (Once every two years) Inspection due to excessive partial heatings Local damages inspection (fissures) on the bushings Inspection for oil leaks Storage 1. Routine inspection Excessive local heating Pay attention to the clamping […]

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Testing of Power Transformer – Measurement of Zero-Sequence Impedance

Earth-fault protection Purpose of the measurement The zero-sequence impedance is usually measured for all star-connected windings of the transformer. The measurement is carried out by supplying a current of rated frequency between the parallell connected phase terminals and the neutral terminal. The zero-sequence impedance per phase is three times the impedance measured in this way. […]

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Protection Against Overexcitation Of a Transformer

Increase of the magnetizing current Overexcitation of a transformer means that the magnetic flux in the core is increased above the normal design level. This will cause an increase of the magnetizing current and the transformer can be damaged if this situation isn’t taken care of. For step-up transformers connected to generators during start-up, overexcitation […]

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An Example of Transformer Tap-changer Correct Adjustment

Example A 13800V/4160 V transformer has five taps on the primary winding giving -5%, -2 1/2 %, nominal, +2 1/2 % and +5 % turns. If, on-load, the secondary voltage reduces to 4050 V then, which tap, should be used to maintain 4160 V on-load (assuming the supply voltage remains constant)? The following answer results: […]

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A transformer is only as strong as its weakest link

Transformer rating Capacity (or rating) of a transformer is limited by the temperature that the insulation can tolerate. Ratings can be increased by reducing core and copper losses, by increasing the rate of heat dissipation (better cooling), or by improving transformer insulation so it will withstand higher temperatures. A physically larger transformer can dissipate more […]

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Instrument transformers (CTs, VTs) in the system

Three main tasks of CTs and VTs The three main tasks of instrument transformers are: To transform currents or voltages from a usually high value to a value easy to handle for relays and instruments. To insulate the metering circuit from the primary high voltage system. To provide possibilities of standardizing the instruments and relays to […]

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The most common accessories you can find on oil filled transformer

Accessories Every oil filled transformer has a variety of accessories for different purposes. Here, I’ll try to name the most common of them, together with picture and description. Note that pictures can be different depending on manufacturer. Also, you will notice an absence of transformer protection relay. I have written about it few times about […]

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Earthing (grounding) transformer – Voltages during a ground fault

Protection from fault current Earthing Transformer or grounding transformer is the neutral grounding transformer - star connected on the primary and has an open delta on secondary. Open Delta has two terminals. A resistor is connected across these two terminals. This voltage drop will be sensed by the earthing transformer connected across this resistor. At the […]

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