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PROTECTION FOR UNEXPECTED DELTA SOURCES
1
PROTECTION FOR UNEXPECTED DELTA SOURCES
Ken Behrendt
Schweitzer Engineering Laboratories, Inc.
New Berlin, WI USA
A
BSTRACT

Utilities commonly use delta-wye transformers to serve three-phase customer loads from their
transmission networks and distribution feeders. With the proliferation of distributed generation
installed at the customer load site, these delta-wye transformers become sources of fault current
that can be difficult to detect and isolate. Under worst-case scenarios, without proper protection,
the customer generation may keep the faulted utility circuit energized even after the utility source
breakers have opened.
This paper discusses various techniques used to detect faults from delta sources. The paper
reviews the nature of fault current contributions and voltage characteristics from ungrounded
delta sources through the use of sequence component analysis. The paper also discusses the
possible impact of distributed generator control systems and interconnection protection. Armed
with this information, utility engineers and their customers can determine what protection and
control schemes they need to ensure faults are cleared properly to avoid the hazards of uncleared
faults.
I
NTRODUCTION

Utilities use delta-wye transformers extensively in utility systems to connect transmission and
subtransmission systems to distribution systems and large three-phase customer loads. This
transformer configuration offers several beneficial effects, including improving load balance and
blocking zero-sequence current flow, which simplifies ground fault protection.
Figure 1 shows a simplified one-line diagram representing a typical connection from a utility
source substation to a customer load through a delta-wye transformer. The delta-wye transformer
could be located at a utility-owned distribution substation, or the delta-wye transformer could be
located at a customer-owned substation. The line could be radial, as shown in this figure, or it
could be networked to another source substation with the delta-wye transformer tapped on the
line between the two substations. The line may very likely supply other customer loads or utility
substations. Protective relaying, appropriate to detect line faults and trip the associated line
breaker, is installed at the utility source substation.
Figure 1 also shows the sequence component networks needed to analyze balanced and
unbalanced fault conditions that occur on the utility line. Note that the representation for the
delta-wye transformer in the zero-sequence network includes an open circuit between terminals H
and L. This open circuit prevents the flow of zero-sequence current through the transformer,
which is a characteristic of the delta-wye transformer. 2
The fundamental equations for deriving phase currents (IA, IB, and IC) and phase-to-neutral
voltages (VA, VB, and VC) are as follows:

IA = I1 + I2 + I0
IB = a
2
I1 + aI2 + I0
IC = aI1 + a
2
I2 + I0

VA = V1 + V2 + V0
VB = a
2
V1 + aV2 + V0
VC = aV1 + a
2
V2 + V0
Where unity operator a = 1
120 degrees.
Legend
Z1S: Positive-Sequence Source Impedance
Z1L: Positive-Sequence Line Impedance
Z1T: Positive-Sequence Transformer Impedance
Z1Load: Positive-Sequence Load Impedance
Z2S: Negative-Sequence Source Impedance
Z2L: Negative-Sequence Line Impedance
Z2T: Negative-Sequence Transformer Impedance
Z2Load: Negative-Sequence Load Impedance
Z0S: Zero-Sequence Source Impedance
Z0L: Zero-Sequence Line Impedance
Z0T: Zero-Sequence Transformer Impedance
Z0Load: Zero-Sequence Load Impedance
Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Z1 Load
Negative-Sequence Network
Z2S
Z2L
Z2T
Z2 Load
Zero-Sequence Network
Z0S
Z0L
Z0T
Z0 Load
S
H
L
S
H
L
S
H
L
S
H
L

Figure 1 Typical Utility Source to Customer Load Through a Delta-Wye Transformer
Figure 2, Figure 4, and Figure 6 show the simplified one-line diagrams and appropriate sequence
component network connections for three-phase, phase-to-phase, and phase-to-ground faults,
respectively, on the utility line. During the fault, the sequence current flowing through the load is
significantly smaller than the sequence current from the utility source to the fault for two reasons.
First, the fault collapses the positive-sequence voltage applied to the load, which reduces the
current through the passive load impedance, and secondly, the load impedance is significantly
higher than the utility source and line impedances. For all practical purposes, the load impedance
is considered infinite and ignored in the fault current calculation.
To simplify the discussion, this paper ignores a fourth type of fault, the phase-to-phase-to-ground
fault. This fault is a combination of phase-to-phase and phase-to-ground faults. Results from the
analysis of these individual fault types can generally be extrapolated to determine the effect of a
phase-to-phase-to-ground fault.
With only the utility providing a source of power to the customer load, faults on the line between
the utility-source substation and the delta-wye transformer are completely isolated when the
utility-source line breaker opens, as shown in Figure 3, Figure 5, and Figure 7, for each of the
fault types. Symmetrical component analysis for currents and voltages on the line and at the
customer load site readily shows that all currents and voltages on the isolated system are zero. 3
Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Negative-Sequence Network
Z2S
Z2L
Z2T
Z2 Load
Zero-Sequence Network
Z0S
Z0L
Z0T
Z0 Load
S
H
L
S
H
L
S
H
L
S
H
L
3 V1S
V1H
V1L
I1>0
I1
0
V2S
V2H
V2L
I2=0
I2=0
V0S
V0H
I0=0
V0L
I0=0
Z1 Load



Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Negative-Sequence Network
Z2S
Z2L
Z2T
Z2 Load
Zero-Sequence Network
Z0S
Z0L
Z0T
Z0 Load
S
H
L
S
H
L
S
H
L
S
H
L
3 V1S
V1H
V1L
I1=0
I1=0
V2S
V2H
V2L
I2=0
I2=0
V0S
V0H
I0=0
V0L
I0=0
Z1 Load



Figure 2 Three-Phase
Line
Fault
Figure 3 Three-Phase Line Fault With Utility
Source Breaker Open
Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Negative-Sequence Network
Z2S
Z2L
Z2T
Zero-Sequence Network
Z0S
Z0L
Z0T
Z0 Load
S
H
L
S
H
L
S
H
L
S
H
L

V1S
V1H
V1L
I1>0
I1
0
V2S
V2H
V2L
I2>0
I2
0
V0S
V0H
I0=0
V0L
I0=0
Z1 Load


Z2 Load



Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Negative-Sequence Network
Z2S
Z2L
Z2T
Zero-Sequence Network
Z0S
Z0L
Z0T
Z0 Load
S
H
L
S
H
L
S
H
L
S
H
L

V1S
V1H
V1L
I1=0
I1=0
V2S
V2H
V2L
I2=0
I2=0
V0S
V0H
I0=0
V0L
I0=0
Z1 Load


Z2 Load



Figure 4 Phase-to-Phase Line Fault
Figure 5 Phase-to-Phase Line Fault With
Utility Source Breaker Open 4
Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Negative-Sequence Network
Z2S
Z2L
Z2T
Zero-Sequence Network
Z0S
Z0L
Z0T
S
H
L
S
H
L
S
H
L
S
L
G
V1S
V1H
V1L
I1>0
I1
0
V2S
V2H
V2L
I2>0
I2
0
V0S
V0H
I0>0
V0L
I0
0
H
Z1 Load


Z2 Load


Z0 Load


Line
Transformer
Load
Utility Source
Substation
Symmetrical Component Diagrams
E1S
Positive-Sequence Network
Z1S
Z1L
Z1T
Negative-Sequence Network
Z2S
Z2L
Z2T
Zero-Sequence Network
Z0S
Z0L
Z0T
S
H
L
S
H
L
S
H
L
S
L
G
V1S
V1H
V1L
I1=0
I1=0
V2S
V2H
V2L
I2=0
I2=0
V0S
V0H
I0=0
V0L
I0=0
H
Z1 Load


Z2 Load


Z0 Load


Figure 6 Phase-to-Ground Line Fault
Figure 7 Phase-to-Ground Line Fault With
Utility Source Breaker Open
Isolation is an important concept that is used extensively on electric power systems to ensure safe
and reliable operation: safe from the standpoint that protective devices automatically de-energize
and isolate faulty equipment or downed conductors to protect the public and also to protect utility
operating and maintenance personnel who get called upon to make repairs, reliable from the
standpoint that customers expect, and regulatory bodies dictate, that electric power be delivered
within prescribed voltage limits to prevent damage and maintain proper operation of customer
utilization equipment. Power system faults in grounded systems cause significant voltage sags
and swells that violate these prescribed limits. Isolating and de-energizing the faulty power
system element restores proper power delivery to most customers and interrupts power delivery
to customers on the faulty circuit. In this case, a few customers with no power is preferable to
having many customers with inadequate power quality [6].
Another important issue is the relationship between power system design and operation.
Equipment installed on a radial circuit with a solidly grounded wye source, as in our example
system, operates at all times within the phase-to-ground voltages established during normal
balanced conditions. All fault types tend to decrease the phase-to-ground and phase-to-phase
voltage, but never increase the voltage. The system can use phase-to-ground connected
equipment designed for the nominal phase-to-ground voltage. Phase-to-phase connected
equipment can be designed for use at the nominal phase-to-phase voltage. The normal voltage
triangle shown in (d) of Figure 8 defines the nominal phase-to-ground and phase-to-phase
operating voltages f