stray voltage testing transformers winding configurations load box
TRANSCRIPT
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