Distribution TransformersFadilla Putri IrintikaKenneth Keulana JudaElectrical Engineering 2013

0Transformers are electrical equipments that are used in order to stop DC electrical power to pass through one side to the other and to alter the current of an input alternating current (AC). They are utilizing electrical wire coiled around a core to transfer an AC signal into transient electromagnetic fields.

Distribution transformers

0A distribution transformer is a static device constructed with two or more windings used to transfer alternating current electric power by electromagnetic induction from one circuit to another at the same frequency but with different values of voltage and current.

0The purpose of a distribution transformer is to reduce the primary voltage of the electric distribution system to the utilization voltage serving the customer.

0Figure 1. shows distribution transformers in stock at an electric utility company service building. The distribution transformers available for use for various applications.

Differences

0Power Transformers: used in transmission network of higher voltages for step-up and step-down application (400 kV, 66 kV, 33kV) and generally rated above 200MVA

0Distribution Transformers: used for lower voltage distribution as a means to end user connectivity (11kV, 6.6 kV, 3.3 kV, 440 V, 230 V) and are generally rated less than 200 MVA

Pole Type Distribution

Pad Type Distribution

Distribution Transformers Construction

0Two types of construction are the core type and the shell type. In the core type.

0Transformers must be constructed with the proper windings for the primary-voltage system and the desired secondary voltage. Properly manufactured distribution transformers can be connected wye-delta if it is desired to obtain three-wire three-phase secondary voltages from a three-phase four-wire grounded neutral wye primary-voltage system.

Distribution Transformer Operation

Single-Phase Distribution Transformers

Single-Phase Distribution Transformers

Voltage V = volts per turn × number of turns N Voltage primary winding = VpVoltage secondary winding = VsVolts per turn = Vt

Number of turns of primary winding = Np Number of turns of secondary winding = Ns

Vp= Vt x Np

Vs= Vt x NsOR

Both of the above quantities are equal to Vt. If we multiply both sides of the equation by the same quantity Np , we obtain:

OR

Quantities equal to the same quantity are equal, so

When a load is connected to the secondary winding of the transformer, a current Is will flow in the secondary winding of the transformer. This current is equal to the secondary voltage divided by the impedance of the load Z .

With Lenz’ Law that any current that flows as a result of an induced voltage will flow in a direction to oppose the action that causes the voltage to be induced

Three-Phase Transformers

0A three-phase transformer is basically three single-phase transformers in a single tank in most cases. The windings of the three transformers are normally wound on a single mul- tilegged laminated steel core.

Distribution Transformer Polarity

0When the current flows in the same direction in the two adjacent primary and secondary terminals, the polarity of the transformer is said to be subtractive, and when current flows in opposite directions, the polarity is said to be additive.

Power Losses

0Transformers are inductive since the consume the power with lagging power factor. The key input for estimating distribution transformer energy loss is the transformers load that determines the power factor and energy consumed

Hysteresis Loss

0Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core

0Pn= Wn . ƒ=ŋ . ƒ . β1.6max

Eddy Current Loss

0 It is a complex function of the square of supply frequency and square inverse of the material thickness

0Can be reduced by making core a stack of plates electrically insulted from each other.

Paralleling Single Phase Distribution Transformers

0The conditions are:1. Voltage ratings are identical.2. Tap settings are identical.3. Frequency ratings are identical.

reference

0http://www.slideshare.net/automaticvoltage1/power-distribution-transformers