Category Archive for: «Transformers»

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Do Not Energize Oil-Filled Transformer Without Performing These 15 Tests and Checks!

Obligatory Checklist The following tests and checks should be performed at a minimum to ensure that the transformer is ready to be energized. Do not energize the transformer without performing these tests and checks. If any of below explained tests fail and transformer is energized anyway, it may lead to the serious hazard, and that’s […]

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Basic Transformer Routine Test – Measurement of Winding Resistances

Purpose of the measurement The resistance between all pairs of phase terminals of each transformer winding are measured using direct current. Furthermore the corresponding winding temperature is measured. The measured resistances are needed in connection with the load loss measurement when the load losses are corrected to correspond to the reference temperature. The resistance measurement […]

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4 Essential Features of Transformer On-Load Tap Changer (OLTC)

Done on the HV winding On-load tap changers (or circuit tap changers), as the name suggests, permit tap changing and hence voltage regulation with the transformer on-load. Tap changing is usually done on the HV winding for two reasons: Because the currents are lower, the tap changer contacts, leads, etc., can be smaller. As the […]

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An Example Of Transformer Overload and Short-Circuit Protection

NEC Article 450 // Transformers Vaults Transformer protection consists of both overload protection and short-circuit protection. Overload protection is usually accomplished via proper selection of the secondary overcurrent protective device. NEC Article 450 gives specific primary and secondary overcurrent device ratings that may not be exceeded. These vary depending upon the accessibility of the transformer […]

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Why cooling is essential for long transformer life?

Pushing loads over design limitations As you already know, cooling is really essential for long transformer life. Most transformers are designed for 55°C or 65°C rise. Using newer insulations such as Nomex, temperatures of 95°C and higher can be achieved. These temperatures can only be maintained if the transformer operating conditions do not exceed the […]

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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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