Download Phase-Matching Considerations with Open-Delta PTs

The Official Publication of the InterNational Electrical Testing Association
Fall 2005
Feature
Phase-Matching Considerations
with Open-Delta PTs
W
henever a power system is supplied by multiple sources, such
as a main-tie-main configuration, special consideration should
be given as to how the sources are phased or matched to each
other. Obviously, both sources will have to be connected with the same
phase rotation. In a traditional open-transition system, matched rotation
of the sources may be sufficient. However, if a system is closed transition (the two sources may be tied together for some duration of time)
it is critical that the two sources be phase matched. This means that the
phase voltages on either side of the open point are in phase with each
other. Additionally, high-speed open transition type systems may also
need to be phase matched. With high-speed systems, it is possible that the
collapsing magnetic fields of motor loads may maintain the bus voltage
through the duration of the transfer. If this voltage is present and out of
phase, closing the second source may cause a large spike of an apparent short-circuit current, resulting in possible nuisance tripping or other
complications. Once any multisource system is initially energized, the
phase matching must be verified.
Why Use Potential Transformers?
The absolute best method to make certain that the phases are matched
is to directly measure the voltages across the open point. This eliminates
any possible wiring problems with the potential transformer (PT) cirNETA WORLD Fall 2005
by Kevin Miller, PE
Electrical Equipment
Upgrading
cuits. Traditionally, this measurement has been accomplished by
using a handheld voltmeter or
high voltage tester, depending
on the system voltages. However,
due to various concerns such as
additional exposure to arc-flash
hazards, it may not be practical
or safe to measure the voltages
directly. Determining that it is not
practical to measure the voltages
directly makes it necessary to verify the phasing by making voltage
measurements on the secondary of
PT circuits.
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Complications
with Open-Delta PTs
If the system is utilizing wyeconnected PTs, the voltages
measured are in phase with the
individual phase voltages. These
measurements can be treated just
as measuring the voltages directly.
However, a slightly more complicated approach is required when
dealing with open-delta PTs.
A typical open-delta PT connection is shown in Figure 1. Notice
that the B-phase secondary is
grounded. Therfore a voltage measurement from A-phase-to-ground
is in phase with the primary VAB,
and C-phase-to-ground is in phase
with VCB. Since the secondary Bphase connections are effectively
coupled, any measurements between B-phase on the two sets of
PTs are meaningless, leaving only
A- and C-phase measurements.
With this B-phase coupling, there
are only four meaningful voltage
measurements to be made: VA1VA2; VA1 -V C2 ; V C1 -VA2 ; V C1 -V C2 .
The following discussions only
apply if the B-phase secondaries
are grounded and the PTs on both
sources are connected open-delta,
as it is not possible to utilize two
different PT connection types when
making these phase comparisons.
Figure 2
Verifying the PT Connections
Since PTs may be the only method of verifying phasing, it is vital that
both sets of PTs be connected identically to each other. Although this can
be achieved by inspecting and testing the PTs, the most positive method
of verifying that the connections are correct and matched is to energize
both sets of PTs from the same source. Considering Figure 2, this can be
accomplished by closing one of the main breakers and the tie breaker,
leaving the other main open. Once supplied by a single source, the voltages between the two sets of PTs (based on a nominal voltage of 115 V)
should be measured as follows: VA1-VA2=0, VA1-VC2=115 (1.0x), VC1-VA2=115
(1.0x), VC1-VC2=0. If the measured voltages differ from these values, the
PT connections are not matched to each other and must be corrected.
Performing Measurements
Once the PT connections have been verified, the next step is to proceed
with energizing the PTs by the two different sources. This is accomplished
by opening the tie breaker and closing the other main. Until the phasing
has been verified as being correct, all of these transfers must be opentransition, taking care not to tie the two sources together.
For simplicity, source #1 will be set as the reference, with any suggested wiring changes being made to the source #2 connections. When
dealing with three cables or conductors, there are six possible orders to
connect them to the equipment, with only one being correct. Assuming
that the two sources can be matched, the following table lists the voltages
measured for the six possible connections of source #2. If dealing with a
system that has a nominal secondary value other than 115 V, the number
listed in parentheses is the multiple of nominal. It is worth noting, with
one exception, that all of the incorrect connections yield the same voltage
measurements, just in different orders: 1.0x; √3x; √3x; 2.0x
Figure 1
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NETA WORLD Fall 2005
Conn.
VA1-VA2
1
0V
2
VA1-VC2
VC1-VA2
VC1-VC2
0
Changes to Source 2
Connections
115V (1.0x)
115V (1.0x)
The sources are
correctly matched.
115V (1.0x)
199V
199V
230V (2.0x)
Exchange B and CØ
3
230V (2.0x)
199V
199V
115V (1.0x)
Exchange AØ and BØ
4
199V
115V (1.0x)
230V (2.0x)
199V
Move AØ to B, BØ to
C, and CØ to A
5
199V
230V (2.0x)
115V (1.0x)
199V
Move AØ to C, BØ to
A, and CØ to B
6
115V (1.0x)
0V
0V
115V (1.0x)
Exchange AØ and CØ
After necessary measurements have been made, the system should be
de-energized to make the necessary corrections. Once the system has been
corrected, the system should be re-energized and the phasing verified.
Systems That Can Not Be Matched
As stated, the previous table applies only if the two sources can be
matched, which is not always the case. There are several possible ways
to connect source transformers that will produce phase shifts between
secondary voltages such that there is no reconnection that can match the
voltages. The transformers must be compared to verify that they have the
same phasor relationships. Even with matched transformers, there are
still connection issues that can prevent the secondary voltages from being
matched. The most common example occurs when delta-wye transformers are used. These transformers must be connected with the same phase
rotation on the primary. If one of the transformers is connected with a
different primary rotation, there will be a 60° phase shift between the
secondary voltages, rendering them unmatchable. The following table
lists the six possible measurements that exist if the two sources are 60°
apart. The only solution to this problem is to correct the primary rotation
and then recheck the phasing to determine if other corrective action is
required.
Conn.
VA1-VA2
VA1-VC2
115V (1.0x)
VC1-VA2
1
0V
2
115V (1.0x)
199V
3
230V (2.0x)
199V
199V
4
199V
115V (1.0x)
115V (1.0x)
5
199V
230V (2.0x)
230V (2.0x)
199V
6
115V (1.0x)
199V
115V (1.0x)
0V
115V (1.0x)
VC1-VC2
0V
199V
115V (1.0x)
230V (2.0x)
0V
There are several other transformer connections, some of which
will not produce secondary voltages that can be matched. If any
measurements are made that are
not consistent with the previously
listed tables, the sources and transformers should be investigated to
determine what can be done to permit phasing of the secondaries.
Conclusions
Whenever possible and safe, it
is best to verify phase matching
by measuring the voltages to be
phased directly. If PTs are to be relied on to determine phasing, the
following must be verified:
• Both sets of PTs must be connected to the bus with the same
configuration and with the same
ratio and polarity.
• For open-delta connections, the
B-phase secondary on each PT
is grounded.
• Prior to paralleling the two
sources, measure the various
combinations of voltages to assure that phasing is correct.
The addition of the PTs adds
more potential for human error.
Anyone performing this type of test
must be competent and methodical
in verifying the system.
Kevin Miller received his Bachelor of
Electrical Engineering from The Georgia
Institute of Technology. He works as a field
service engineer with Electrical Equipment Upgrading, a NETA Full Member
Company, in Savannah, Georgia. His
primary responsibilities revolve around
project management. Kevin is a Registered Professional Engineer with the state
of Georgia and is a NETA Certified Level
IV Technician.
Article available for reprint with permission from NETA. Please inquire at neta@netaworld.org.
NETA WORLD Fall 2005
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