Txs Connections

Question 1

In an Open delta what is the reduced VA percent ?

Answer 1

57.7%
Take total VA and divide 1.732

Question 2

What’s most common transformer connection ?

Answer 2

Delta/Wye

Question 3

Advantages of Delta/Wye connection ?

Answer 3

•Supply single and three phase( same panel)
•Balanced single phase=balanced three phase
•30 degree phase shift between primary/secondary line voltage

Question 4

NGR are for impending e grounding. What are the low and high voltage ranges?

Answer 4

Low voltage 30V - 750V
High voltage 750V+

Question 5

Ungrounded systems what happens ?

Answer 5

Code requires a device to be notified and if not corrected by second ground fault an overcurrent device and uncontrolled shutdown takes place

Question 6

NGR advantages?

Answer 6

Fast response to where fault happened and reduce and prevent overheating and mechanical stress on conductors

Question 7

Solidly grounded systems advantages?

Answer 7

•Fault to ground = breaker trips
•reduced over voltages
•improved system and equipment fault protection
•greater safety

Question 8

Solidly grounded is for high voltage or low voltage?

Answer 8

Lower voltages

Question 9

Impedance grounded systems characteristics

Answer 9

Primary connected in series to neutral and the resistor is connected secondary of grounding transformer

Question 10

Which grounding system uses a CT?

Answer 10

Impedance grounded systems

Question 11

Low resistance grounded neutral current limits ? And properties ?

Answer 11

•200-400A
•Can carry for 10seconds before overheating
•Commonly on high voltage systems

Question 12

High resistance grounded neutral current limits and properties?

Answer 12

•5-10A
•used on low voltage systems under 750v
Advantage - eliminate dangerous flashovers to ground ( can occur on solidly grounded)

Question 13

What voltage appears across NGR when single line to ground fault occurs on résistance system?

Answer 13

A voltage equal to the normal line to neutral voltage

Question 14

What is purpose of NGR?
What system are they used in ?

Answer 14

•Limit ground fault current to a safe level
•impedance grounding of industrial power systems

Question 15

Open delta reduced capacity percentages

Answer 15

Capacity reduced to 57.7%
Capacity compared to two txs 86.6%

Question 16

Delta/Delta connection has to be ? What are rules ?

Answer 16

Paralleled
•same %iz
•same polarity
•same taps
•same secondary no load voltage

Question 17

Wye/Wye connection problems ?

Answer 17

Third harmonic (180hz) can induce current into neutral causing heating in neutral

Question 18

Advantage of primary neutral in Wye/Wye connection?

Answer 18

Elimination of voltage balancing problems

Question 19

What is the phase displacement in Wye/Delta connection for primary and secondary voltage?

Answer 19

30degrees

Question 20

If two transformers are connected in open delta, their output capacity will be _% of sum of their ratings ?

Answer 20

86.6%