Transformers

Faraday’s Law of Induction

30:1

7200 V 240 V

60:1

120 V

15:1

480 v

26:1

277 v

• The only relationship between the

primary and secondary winding is the

ratio and polarity.

• Either winding can be left ungrounded,

or grounded at any point without

affecting the grounding or the voltage

of the other winding.

Transformers can be

interconnected together in a

variety of ways on both the

primary and secondary sides to

get a variety of three phase

secondary voltages.

A Phase

B Phase

C Phase

B Phase

A Phase

C Phase

B Phase

A Phase

B Phase

C Phase

B Phase

Distribution Systems

24

Typical Substation

Transformer C A N B

7200V

7200V 7200V

Multi-Grounded Wye Distribution – Most common Type

Neutral – Phase Voltage 7200V and 14,400V most common

Delta Distribution System

Various Transformer

Connections May Be Used

On Multi-Grounded

Distribution System.

Four Wire Multi-Grounded

System Allowable Transformer

Connections

• Ungrounded Wye Primary Transformer

Bank

• Grounded Wye Primary Transformer Bank

• Delta Primary Transformer Bank

• Open Wye (Primary) – Open Delta

(Secondary) Transformer Bank

4 Wire Multigrounded Wye Ungrounded Wye Transformer Grounded Wye Transformer

4 Wire Multigrounded Wye Ungrounded Wye – Delta Transformer

4 Wire Multigrounded Wye Ungrounded Wye – Delta Transformer

Open Primary Phase

4 Wire Multigrounded Wye Grounded Wye –Delta Transformer

Grounding Bank

4 Wire Multigrounded Wye Delta Connected Transformer

4 Wire Multigrounded Wye Delta Connected Transformer

4 Wire Multigrounded Wye Open Wye-Open Delta Transformer Bank

The secondary (low voltage) side of the

three phase banks can be interconnected in a

variety of manners, independently of the

connection on the primary (high voltage)

side.

Probability of Ferroresonance

From Westinghouse Distribution Transformer Guide.

Intended to examine the condition

of the distribution primary neutral

and the farm grounding system by

measuring the resistance of the farm

grounding system and primary

neutral. It also measures the cow

contact voltage in response to the

primary neutral voltage.

Inject a current into the

neutral system, measure

the voltage rise, measure

the current into the farm

and the primary, and

determine the resistance of

each with the formula

R = V / I

RPRI RFARM

LOAD

BOX

CURRENT

REMOTE

+

Vp

_

One Bushing Single Phase Transformer

Load

Box

3 Phase Delta - Delta 120/240 Volt

Load

Box

Load

Box

3 Phase Floating Wye - Delta 120/240 Volt (4 wire)

Load

Box

Load

Box

3 Phase Delta - Grd Wye 120/208 Volt

Load

Box

Load

Box

3 Phase Grd Wye - Grd Wye 120/208 Volt

Load

Box

Load

Box

2 Phase Open Wye - Open Delta 120/240 Volt (4w)

Load

Box

Load

Box

2 Phase Open Wye - Open Delta 240 Volt (3w)

Load

Box

Load

Box

Important Considerations are:

• A balanced three phase load on any system using three transformers will NOT cause any neutral current in the primary

• If the transformer bank has no primary winding connection to the primary neutral, the load (at the farm) CAN NOT cause a primary neutral current.

• On most systems, with the exception of open wye/open delta connection, a balanced three phase load box will do nothing to load up the primary neutral.

• Rf = Δ Vs / Δ Isec(farm)

• Rp = Δ Vp / Δ Ip ?

• Rt = Δ Vp / Δ Itotal ?

• K = Vcc (hi) / Vs(hi) x 100

• ΔCR = (IPrimary Neutral / ITotal)x 100 ?

Rupstream

Rdownstream

RPRI RFARM

LOAD

BOX

CURRENT

REMOTE

+

Vp

_

• Δ ITotal = Δ Iload box /transformer ratio

• Δ Inet = Δ I sec(farm)

• Δ Ipri neutral = Δ Itotal - Δ Inet

OR

Δ Ipri neutral =

Δ Ipri n upstream

+/-

Δ Ipri n downstream

• Rt = Δ Vp / Δ Itotal

• Rf = Δ Vs / Δ Isec(farm)

• Rp = Δ Vp / Δ Ipneutral

• K = Vcc (hi) / Vs(hi) x 100

• ΔCR = (IPrimary Neutral / ITotal)x 100

Δ Vp / Δ Ipneutral

In Each Direction

Rp =

Parallel Combination of

OR

I(net)

RPRI RFARM

LOAD

BOX

CURRENT

REMOTE

+

Vp

_

• Δ ITotal = Iload box /xfrmr ratio

• Δ Inet = Δ I sec(farm)

• Δ Ipri n = Δ Itotal - Δ Isec(farm)

5

• Rt = Δ Vtotal / Δ Ipri

• Rf = Δ Vs / Δ Isec(farm)

• Rp = Δ Vp / Δ Ipneutral

• K = Vcc (hi) / Vs(hi) x 100

• ΔCR = (IPrimary Neutral / IPhase)x 100

Ifarm

Itotal

A Phase

B Phase

C Phase

90

Single phase

20A

20A

Substation

3 Wire Ungrounded Delta