TRANSFORMER OIL CONTAINMENT BASICS AND TUTORIALS

TRANSFORMER OIL CONTAINMENT BASIC INFORMATION
How To Contain Transformer Oils?


Oil containment
Even where the more traditional system of chippings and sump is used as a base for the transformer compound, consideration will need to be given to the possibility of loss of all the oil from the transformer tank and its cooler. Suitable provision must be made to ensure that this will not enter drains or water courses.

Such provision will normally be by means of a bund wall surrounding the transformer and its cooler which together with any sump must be capable of containing the total oil quantity in addition to the maximum likely rainfall over the area.

Since the bunded area will under normal operating conditions need provision for storm water drainage, then suitable oil interception arrangements must be made for separation and holding any oil released.

Segregation and separation

Where it is not economic to consider the type of elaborate measures described above, then other design features must be incorporated to allow for the possibility of fire. Such features involve segregation or separation of equipment.

Separation involves locating the transformer at a safe distance from its standby, where one is provided, or any other plant and equipment which must be protected from the fire hazard. A distance of 10 metres is usually considered to be sufficient.

This means that not only must the transformer be a minimum of 10 metres from its standby, but all connections and auxiliary cabling and services must be separated by at least this distance.

On most sites such an arrangement will be considered too demanding of space, so this leads alternatively to the use of a system of segregation, which relies on the use of fire-resistant barriers between duty and standby plant and all their associated auxiliaries.

The integrity of the barrier must be maintained regardless of how severe the fire on one transformer or of how long the fire persists. In addition the barrier must not be breached by an explosion in one of the transformers, so it will normally be necessary to construct it from reinforced concrete and of such an extent that flying debris from one transformer cannot impinge on any equipment, including bushings, cables, cooler and cooler pipework or switchgear associated with its standby.

Generally for access reasons transformers should be at least 1 metre from any wall but this space may need to be increased to allow for cooling air.

DISTRIBUTION TRANSFORMERS HARMONICS AND DC EFFECTS BASIC AND TUTORIALS

DISTRIBUTION TRANSFORMERS HARMONICS AND DC EFFECTS BASIC INFORMATION
What Are The Harmonics And DC Effects Of Distribution Transformers?


Harmonics and DC Effects
Rectifier and discharge-lighting loads cause currents to flow in the distribution transformer that are not pure power-frequency sine waves. Using Fourier analysis, distorted load currents can be resolved into components that are integer multiples of the power frequency and thus are referred to as harmonics.

Distorted load currents are expected to be high in the 3rd, 5th, 7th, and sometimes the 11th and 13th harmonics, depending on the character of the load.


Odd-Ordered Harmonics
Load currents that contain the odd-numbered harmonics will increase both the eddy losses and other stray losses within a transformer. If the harmonics are substantial, then the transformer must be derated to prevent localized and general overheating.

ANSI standards suggest that any transformer with load current containing more than 5% total harmonic \ distortion should be loaded according to the appropriate ANSI guide (IEEE, 1998).


Even-Ordered Harmonics
Analysis of most harmonic currents will show very low amounts of even harmonics (2nd, 4th, 6th, etc.) Components that are even multiples of the fundamental frequency generally cause the waveform to be nonsymmetrical about the zero-current axis.

The current therefore has a zeroth harmonic or dc-offset component. The cause of a dc offset is usually found to be half-wave rectification due to a defective rectifier or other component.

The effect of a significant dc current offset is to drive the transformer core into saturation on alternate half-cycles. When the core saturates, exciting current can be extremely high, which can then burn out the primary winding in a very short time.

Transformers that are experiencing dc-offset problems are usually noticed because of objectionably loud noise coming from the core structure. Industry standards are not clear regarding the limits of dc offset on a transformer.

A recommended value is a dc current no larger than the normal exciting current, which is usually 1% or less of a winding’s rated current (Galloway, 1993).