S3 - Distribution Transformers

Question 1

The winding of the transformer which is connected to the source of power.

Answer 1

Primary Winding

Question 2

The winding of the transformer which delivers power to the load

Answer 2

Secondary Winding

Question 3

The magnetic circuit upon which the windings are wound

Answer 3

Core

Question 4

One which is rated for the higher voltage

Answer 4

High-tension Winding

Question 5

One which is rated for the lower voltage

Answer 5

Low-tension Winding

Question 6

A constant-voltage transformer so connected that the delivered voltage is greater than the supplied voltage

Answer 6

Step-Up Transformer

Question 7

A constant-voltage transformer so connected that the delivered voltage is less than that supplied

Answer 7

Step-Down Transformer

Question 8

Consists of separate insulated electric windings for the different phases, wound upon a single core structure, certain portions of which are common to the different phases

Answer 8

Polyphase Transformer

Question 9

Consists of several one-phase transformers with separate electric circuits but having certain magnetic circuits in common

Answer 9

Polyphase Transformer

Question 10

The ratio of primary to the secondary voltage, depends primarily upon the ratio of the primary to the secondary turns

Answer 10

Voltage Ratio

Question 11

The inverse ratio of the turns in the two windings

Answer 11

Current Ratio

Question 12

The change in secondary voltage from no load to full load

Answer 12

Regulation of a Transformer

Question 13

The ratio of the output to input or, in other words, the ratio of the output to the output plus the losses

Answer 13

Transformer Efficiency

Question 14

Determined by the resistances of the high-tension and low-tension windings and of the leads

Answer 14

Copper Losses

Question 15

Used by electric utility companies to step-down the distribution primary voltage to a secondary service voltage to be used by the customer

Answer 15

Distribution Transformer

Question 16

This is equal to the sum of the losses in the iron core. These losses consist of eddy-current losses and hysteresis losses

Answer 16

Iron Losses

Question 17

Can a “Quick Check Tester” be used on an energized transformer?

Answer 17

No - the unit should never be used on an energized transformer

Question 18

Can the CSP secondary breaker be used as a clearance point?

Answer 18

No - there is not a visible open point

Question 19

What is the maximum current rating of a single 4/0 Aluminum quadraplex service?

Answer 19

258 Amps

Question 20

What is the recommended pot-lead size for a 3-pot, 277/480 V transformer bank with three 167 kVA transformers?

Answer 20

500 KCM Copper

Question 21

What is the power in kW if you have a heater pulling 420 Amps on a 1-phase 240 volt circuit?

Answer 21

P = EI = 420 * 240 = 100,800 Watts = 100.8 kW

Question 22

What should the fuse size be for a 3-pot, 4kV where all 3 transformers are 100 kVA units?

Answer 22

65T

Question 23

What is the EPB standard primary connection for 120/208 V, 3-pot bank?

Answer 23

Wye Connected ( TC-10)

Question 24

A Troubleman is asking whether the primary common connection of a 4W, 3P, 120/240 V, 3-pot wye-delta transformer bank should be grounded, what is your answer?

Answer 24

No – the Wye common connection should be floated (TC-2, note 1; TC-6, note 1)

Question 25

What is the maximum summer loading in secondary amps of a 240 Volt, 3 Wire, 100 kVA Transformer?

Answer 25

521 Amps

Question 26

On a 3P, 4W 120/240 V, open delta transformer bank the common primary connection is floating, is this OK?

Answer 26

No, the transformer tank ground MUST be connected to the system neutral (TC-4, note 3)

Question 27

A single-tank, 3P, 4W, 120/208 V, 300 kVA transformer has (1) 500 MCM Copper transformer lead per phase, is this correct?

Answer 27

No, the transformer should have (2) 500 MCM Cu leads per phase (Drwg No: A-4599, sheet 2 of 2)