TRANSFORMER TAP CHANGER DESIGN FOR MODERATE KVA AND CURRENT BASIC AND TUTORIALS

TRANSFORMER TAP CHANGER DESIGN FOR MODERATE KVA AND CURRENT BASIC INFORMATION
Notes On Transformer Tap Changer Design



Tap-Changer Designs for Moderate kVA and Current. In the smaller ratings, where both the voltage and the current are moderate, the energy to be ruptured in switching from tap to tap becomes relatively so small that light and simple equipments are feasible.

A variety of mechanical designs, together with special circuits, has been evolved with the purpose of providing simpler, smaller, and inherently less expensive equipments. The following may be noted:

1. Designing the tap changer so that it is capable of rupturing the current directly on the same switches which select the taps

2. Designing the circuit so that the tapped winding is reversed in going from maximum to minimum range, thereby securing a substantial reduction in the rating of core and coils for a given output

3. Using higher switching speed, by means of which the life of the arcing contacts is increased.


Tap Changers Designed to Interrupt Current. The contactors C (Fig. 10-27) operate to open the switching circuits so that there is no interrupting duty on the selector contacts which connect to the transformer taps.

When the rated current is moderate, it becomes possible to rupture the current directly on the tap-selector switches and thus obtain a major economy in the cost of the mechanical equipment.

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.