Transformers
Faraday’s Law of Induction
60:1
30:1
26:1
15:1
7200 V
120 V
240 V
277 v
480 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
Phase
BBPhase
C Phase
A Phase
B Phase
C Phase
A Phase
B Phase
C Phase
Distribution Systems
Typical Substation
Transformer
A BCN
7200V 7200V
7200V
24
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
Grounded
Wye
Ungrounded
Wye Transformer
Transformer
4 Wire Multigrounded
Wye
Ungrounded
– Delta Transformer
4 Wire Wye
Multigrounded
Wye
Ungrounded Wye – Delta Transformer
4 Open
Wire Multigrounded
Wye
Primary Phase
Grounded Wye –Delta Transformer
4 Wire Multigrounded Wye
Grounding Bank
4 Wire Multigrounded Wye
Delta Connected Transformer
4 Wire Multigrounded Wye
Delta Connected Transformer
Open Wye-Open
Delta Transformer
4 Wire Multigrounded
Wye 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
LOAD
BOX
CURRENT
+
RPRI
REMOTE
Vp
_
RFARM
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
LOAD
BOX
CURRENT
+
RPRI
REMOTE
Vp
_
RFARM
• 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
OR
Rp =
Parallel Combination of
Δ Vp / Δ Ipneutral
In Each Direction
I(net)
LOAD
BOX
CURRENT
+
RPRI
REMOTE
Vp
_
RFARM
• 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
Itotal
Ifarm
A Phase
B Phase
C Phase
Single phase
20A
20A
Substation
90
3 Wire Ungrounded Delta