POWER TRANSFER SCHEMES
PG-UPT
(Parallel Generation - Uninterrupted Power Transfer) &
CTTS (Closed Transition Transfer System)
ENGINEERING BRIEF
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9087A – 198th Street, Langley, BC Canada V1M 3B1 y Telephone (604) 888-0110
Telefax (604) 888-3381 y E-Mail: info@thomsontechnology.com y www.thomsontechnology.com
POWER TRANSFER SCHEMES
INTRODUCTION
There are many applications where power continuity is important for life safety or economic
reasons. These applications require two independent sources of power. In the event of a
failure or abnormality of the utility supply, the vital load is transferred to the alternate source.
Ideally, this transfer would cause no interruption to the load and would involve no major
transients. It is possible to accomplish such a transfer where the application requires absolute
continuous, “no-break” power at all times. To do so, requires that both sources of power be
continuously available.
Most standby or emergency power sources consist of on-site engine-generators that are
started upon failure of the normal source. There will always be a break in power to the load
until the alternate source becomes available. Typically, there will be a short break in power
anytime the load is switched, even if both power sources are available. These breaks normally
occur during testing of the system and upon re-transfer of the load to its utility supply when
power is restored. Since load interruptions are sometimes inconvenient, there is interest in
eliminating these interruptions during transfer when both power sources are available. Such
systems can be provided to do so safely.
Recently, systems consisting of modified transfer switches (closed transition transfer) or
transfer switches with paralleled contactors have been proposed as an economical way to
accomplish “no-break” switching during test and retransfer. As attractive as these schemes
may appear, they require an understanding of the serious safety and reliability issues. These
systems may cause increased power system voltage transients during the transfer of power.
Transfer switching schemes that have been modified to provide closed transition transfer may
also jeopardize the complete installation and should be reviewed carefully to ensure
compliance with applicable CSA/UL standards and local utility authorities.
FAST POWER RESTORATION
Most loads can tolerate a short break in power. Hospital emergency power systems, for
instance, are designed to handle short power breaks as long as power is restored to vital
circuits within ten seconds. Loads requiring fast restoration are supplied by engine-generator
sets that start upon failure of the normal supply and a transfer switch which transfers the vital
load from its utility supply to the alternate source when it becomes available. The key factor in
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these systems is maximum reliability. A momentary interruption is acceptable. A sustained or
prolonged interruption is not!
Maximum Reliability Requires:
• That the load must be quickly and reliably transferred if a power source is available.
• That the switching of the load must be done in such a manner as not to jeopardize the
source to which it is being transferred.
• That any system disturbance encountered must be predictable and reproducible so that all
equipment and personnel can be preprogrammed to restore normal operation quickly and
reliably after a transfer.
TRANSFER SYSTEM
Open Transition
The conventional transfer of a critical load between power sources is accomplished with a
double-throw transfer switch arrangement. Historically, transfer switches have been designed
with a positive mechanical interlock that absolutely prevents both sources being closed to the
load at the same time. Since the sources are un-synchronized, paralleling them could cause
serious equipment damage and failure of the system.
There will always be a break in power until the alternate source is available. All loads
connected to the system must be provided with automatic controls, to restart or reclose the
load to the power source when restored.
Consideration should be given to the manner in which the load is returned to the normal
source. In most instances, it is desirable to have a short time delay before retransferring the
load automatically in order to assure that the utility supply is going to remain available. There
will be a break in the power to the load during retransfer, but this will be minimal since both
sources are available. However, the break will always occur with the use of a conventional
transfer switch.
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Parallel Generation - Uninterrupted Power Transfer (PG-UPT)
Closed Transition Transfer Switch (CTTS)
When testing a system (no actual power loss) and when returning the load to its normal
source, both power sources are available, and it is possible to accomplish a “no-break”
transfer.
All “no-break” transfer schemes involve paralleling of the two sources for some period of time.
Since conventional transfer switches have mechanical interlocks to positively prevent
paralleling of the two sources, they cannot be used. It is necessary, therefore, to either:
• Replace the transfer switch with paralleling circuit breakers and controls (PG-UPT).
• Provide a modified transfer switch that has overlapping contacts some of the time (closed
transition transfer).
• Parallel the transfer switch contacts with contactors.
PG-UPT (Parallel Generation - Uninterrupted Power Transfer)
This requires replacing the transfer switch with two electrically interlocked circuit breakers,
synchronizing controls, some type of power transfer control, and a full complement of
protective relaying for both sources.
If only one power source is available, the circuit breakers operate in a “break-before-make”
configuration, the same as a conventional transfer switch (but without the positive mechanical
interlock). If both power sources are available (test and retransfer), the synchronizing controls
bring the alternate source engine-generator into synchronism with the normal utility source,
parallel them, and gradually shift the load from one source to the other.
Full protective relaying is necessary in order to protect both the utility source and the
generator source and to provide immediate disconnection of a failing source.
Utilities require permission to parallel with their lines and are generally quite strict in
demanding protective relays. Protective relaying is just as important to the engine-generator,
since the tremendous fault current available from the utility can destroy the engine-generator in
the event of an abnormality.
This type of system can provide a “no-break” transfer during test and return transfer to normal.
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CTTS (Closed Transition Transfer Switch)
The idea of using a closed transition transfer switch to accomplish “no-break” transfer is now
being actively promoted. In the event of a power failure, this transfer switch operates in a
conventional “break-before-make” mode. However, during test and retransfer, the sources are
paralleled (make-before-break) during the transfer operation. To accomplish this, the
mechanical interlock has been removed. Relays are provided to check the relative phase
relationships and the relative voltage and frequencies of the two sources. When the voltage
and frequency are within approximately 5% and the phase relationship within approximately
15%, a signal is given which causes both sets of contacts of the transfer switch to be closed to
the load at the same time, paralleling the two power sources. After a brief period, one side of
the switch is opened, leaving the load connected to the other source. The transfer thus occurs
with no apparent break in power. No active synchronizing controls or protective devices for
either source are provided.
Generally, the cost of such a system should be lower in comparison to true paralleling controls.
However, utility approval, system integrity and potential liability must be considered.
These systems do not employ any active synchronization, and there is no way to automatically
adjust the voltage or frequency of either source. There is no power transfer control to gradually
transfer the load between the two sources. The sudden application or removal of large block
loads from the engine-generators, as they are paralleled with a utility, can cause transients
between the two sources and system disturbances. The magnitude of these disturbances
depends upon the size of the load and the dynamic characteristics of the system, but they can
be detrimental to a sensitive load.
Since the positive mechanical interlock has been removed from the transfer switch, the
electrical controls must prevent paralleling the sources under some conditions (when one
source has failed or is failing) but also permit it under others.
Anytime on-site power generation is paralleled with a utility, approval of the utility must be
obtained. Proponents of Hybrid schemes suggest protective relaying is not necessary (since
the sources are only in parallel for a short duration, i.e.: 100 milliseconds).
Many utility engineers, believe paralleling with the utility, regardless of the period of time of the
interconnection, requires full utility protective relaying to prevent damage to their system. As a
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minimum, local utility approval will be for each application, CTTS or PG-UPT. We feel if a
utility were to agree to unprotected paralleling for as little as 100 milliseconds, they would hold
the owner of the equipment liable for all damage to their system if paralleling should occur for a
longer time.
In the event of a utility failure, an on-site power source, paralleled with the utility grid, even
momentarily, could energize utility lines and cause bodily harm or death to utility personnel.
Incorrect paralleling of an on-site generator with the utility can result in destruction of the
engine-generator itself.
Liability problems arise from the fact that no protective devices are provided to protect and
isolate the paralleled power sources in the event of an abnormality.
SUMMARY
Conventional open transition transfer switches offer a proven reliable method of
transferring between two power sources. The power interruption that occurs with
conventional transfer switches may not be acceptable in some applications. Two
options are available on systems that cannot tolerate any power interruption, PG-UPT &
CTTS. PG-UPT is an active synchronizing method with protective relaying, is CSA
certified and meets typical utility company approval requirements. CTTS is a lower
cost, passive synchronizing system that requires CSA certification and local utility
approval.
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BENEFITS OF TRANSFER SWITCH WITH PG-UPT SYSTEM:
“Soft” Loading and Unloading
Upon return of utility power, the PG-UPT System provides not only a “Bumpless” transfer, but
also provides a “Soft” (slow ramp down) unloading of the generator. This same “Soft” loading
feature is utilized in a test or Peak Shaving mode, thus eliminating undo stresses on the
engine-generator.
Active Synchronization
PG-UPT parallels the two incoming sources when they are accurately synchronized (both
frequency and phase angle), not just when the sources are approximately “in phase”.
Peak Shaving
PG-UPT can be used for continuous peak shaving, if desired.
Load Testing
PG-UPT allows testing the generator set at any level up to the building load, without any
interruption or switching transients.
More Serviceable
PG-UPT may use draw-out type stored energy air circuit breakers for switching, which may be
simply removed for servicing.
Higher Rating
PG-UPT has a time “interrupting” rating and excellent short-time rating, allowing proper system
coordination. Very high closing/with-stand ratings are available.
Protection
Both the generator and utility are protected by the relays incorporated into the system as
required by the utilities.
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