TRANSFORMER FUSING FACTOR BASIC AND TUTORIALS

TRANSFORMER FUSING FACTOR BASIC INFORMATION
What Is Transformer Fusing Factor?


The "fusing factor" is used to determine the K, or T fuse link rating that will strike a suitable balance between operation on secondary fault currents and operation on expected overload currents, such as motor starting currents.

It is obtained by using a rule of thumb such as one of the following: (The current obtained by the selected rule of thumb becomes the "fusing factor.")

1. 1.5 times the rated full-load current of the transformer (Generally used on transformers 25 kva and larger where motor starting currents are not the controlling factor)

2. 2.0 times the rated full-load transformer current

3. 2.4 times the rated full-load transformer current (This rule is frequently expressed as, “1 ampere per kva rating of transformers at 2400 volts, ½ ampere per kva at 4800 volts, and 1/3 ampere per kva at 6900 to 7600 volts.”)

4. 3.0 (or above) times the rated full-load transformer current.

Example:
If the selected rule of thumb is 2.4 times rated full-load current, the system voltage is 4800 volts and the transformer is rated 50 kva, what fuse link should be used?

Rated full-load current = 50,000 / 4800 = 10.4 amperes (see “Load Current Tables” on pages 98 and 99).
2.4X10.4 = 24.9 amperes. Use a fusing factor of 25

Suggested fuse link from table: 15K or 15T.

TRANSFORMER CORE EARTHING BASICS AND TUTORIALS

TRANSFORMER CORE EARTHING BASIC INFORMATION
What Is Transformer Core Earthing?


Core earthing
Before concluding the description of core construction, mention should be made of the subject of core earthing. Any conducting metal parts of a transformer, unless solidly bonded to earth, will acquire a potential in operation which depends on their location relative to the electric field within which they lie.

In theory, the designer could insulate them from earthed metal but, in practice, it is easier and more convenient to bond them to earth. However, in adopting this alternative, there are two important requirements:

ž The bonding must ensure good electrical contact and remain secure throughout the transformer life.

ž No conducting loops must be formed, otherwise circulating currents will result, creating increased losses and/or localised overheating.

Metalwork which becomes inadequately bonded, possibly due to shrinkage or vibration, creates arcing which will cause breakdown of insulation and oil and will produce gases which may lead to Buchholz relay operation, where fitted, or cause confusion of routine gas-in-oil monitoring results y masking other more serious internal faults, and can thus be very troublesome in service.

The core and its framework represent the largest bulk of metalwork requiring to be bonded to earth. On large, important transformers, connections to core and frames can be individually brought outside the tank via 3.3 kV bushings and then connected to earth externally.

This enables the earth connection to be readily accessed at the time of initial installation on site and during subsequent maintenance without lowering the oil level for removal of inspection covers so that core insulation resistance checks can be carried out.

In order to comply with the above requirement to avoid circulating currents, the core and frames will need to be effectively insulated from the tank and from each other, nevertheless it is necessary for the core to be very positively located within the tank particularly so as to avoid movement and possible damage during transport.

It is usual to incorporate location brackets within the base of the tank in order to meet this requirement. Because of the large weight of the core and windings these locating devices and the insulation between them and the core and frames will need to be physically very substantial, although the relevant test voltage may be modest.