Transformers Level 2

Autotransformer

any transformer where the primary and secondary circuits have a portion of their two windings in common

Variable Transformer

continuously adjustable autotransformer consisting of a single layer of wire wound on a toroidal core and a carbon brush that traverses this winding

Buck-Boost Transformer

designed to buck or boost line voltage and typically used to adjust voltage at the end of a long line

Wound-Current Transformer

has separate primary and secondary windings wrapped around a laminated core

Potential Transformer

used for voltage meters, voltage relays and load shedding

Phase Shifting Transformer

a type of harmonic-mitigating transformer (HMT)

What percentage of voltages are buck boost transformers used for?

5% - 25%

How do you protect a control transformer?

Primary and Secondary side (fusing?)

What happens when a ground fault occurs on an ungrounded system?

High transient voltages

What are the methods for synchronizing generators?

Synchro scope, three lights out, one light out, and an auto synchronize relay

Displacement Power Factor

reduction of power factor

Distortion Power Factor

ratio of true power to apparent power

Total Demand Distortion (TDD)

the ratio of the current harmonics to the maximum load current

Total Harmonic Distortion (THD)

the amount of harmonics on a line given as a percentage

Sealed VA

the power that a transformer must deliver to a load for an extended period

How are variable transformers rated? (voltage)

120V or 240V

What are the voltage and current ratings of wye and delta transformers?

1. Wye is treated as series (ELine = EWinding x 1.73) (ILine = IWinding)

2. Delta is treated as parallel (ELine = EWinding) (ILine = IWinding x 1.73)

Where should you not use buck boost transformers?

1. Not used with fluctuating loads

2. Not used to create 120/240V single phase when fed by 208Y/120V 3 phase, 4-wire source (you get 2 neutrals)

3. Not used as 3 units to create 3 phase, 4-wire wye service when fed from a 3 phase, 3-wire delta

4. If primary fuse blows, can cause damage

What kind of loads do triplen harmonics generate?

Only single-phase loads generate triplen harmonics

What does the position of a tap changer do?

A tap changer determines how much of the high-voltage winding of a main transformer is in the circuit with the autotransformer

Why is it a problem if you have neutral current?

Neutral wires were additionally designed to be the same ampacity as phase conductors

What is an SMPS?

A power supply for electronic devices that includes an internal control circuit that quickly switches the load current ON and OFF in order to deliver a stable output voltage

What is winding current?

Current that flows in individual windings

Why can't autotransformers block transients?

They share part of the windings between the primary and secondary

What is the formula for figuring how much power is delivered to a 3 phase transformer?

(Square Root of 3 x V x I) / 1000