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Bulletin #9
Automatic Transfer Switch Engineering Brief - Neutral
Position Delay vs. In-Phase Monitor
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When transferring large motors
and/or transformers between two sources of power which may not be in synchronism (i.e. the
normal power source and the emergency power source), consideration must be given to the
elimination of the "bump" that is felt when the electrical load is suddenly
disconnected from the first power source and immediately connected to the second power
source.
When a motor that has been running on line
is suddenly disconnected from its power source, the residual voltage produced by the motor
(which acts as a generator under these circumstances) will decrease in amplitude and
frequency as the motor slows down. Although the motor may take a long time to actually
stop, the voltage will decay very quickly to safe levels. Similarly, when a transformer is
disconnected from the line, time is required for the magnetic field to collapse.
The "bumps" are caused by the
momentary flow of extremely high line current because of an out-of-phase condition during
motor transfer, and because of induced voltage transients during transformer transfer. The
high current flow can exceed the instantaneous trip settings of protective devices in the
system and can be severe enough to trip circuit breakers, cause damage to shafts,
couplings, etc. This condition is especially pronounced in the case of a fast-operating
transfer switch, such as a solenoid operated type with all contacts on a common shaft.
Two methods are commonly utilized to
prevent the "bump" from occurring. One is the Neutral Position Delay (T.T.I.
option code NDT). The other is the In-Phase Monitor method. The following is an
explanation of each of these methods, along with the relative advantages and
disadvantages.
I. IN-PHASE MONITOR
The in-phase monitor inhibits load transfer
until the two power sources are in synchronism. The monitor is adjusted to signal the
transfer switch to operate when the incoming power source is within approximately ten
electrical degrees of the connected power source. Depending upon the difference in
frequency, the phase angle between the two power sources, and in the contact-to-contact
transition time, the transfer is made at or near synchronism.
A. ADVANTAGES
1. Transfer of motor loads is
accomplished without an appreciable power dip when the system is adjusted properly, when
heavy transformer loads are not included.
B. DISADVANTAGES
1. Successful transfer is totally dependent
upon the selection and adjustment of the governor in the emergency power source. A
governor which is faulty, maladjusted, or has too much "droop" may prevent
transfer. If the frequency is more than two cycles out of synchronism, or the connected
power source is unstable, the transfer switch will remain in the emergency position
indefinitely, or until the frequency is corrected.
2. The in-phase monitor detracts from the
reliability of the system because a complex electronic component is added to an otherwise
simple, straightforward control system.
3. The in-phase monitor does not have
control over the amount of slippage that is experienced from the time a motor is
de-energized until the transfer switch closes to the incoming power source. A heavily
loaded motor can go out of synchronism between the time it is de-energized until the time
it is re-energized, particularly in larger sizes of transfer switches which have longer
contact-to-contact transition times.
4. The in-phase monitor does nothing
to prevent the "bump" that is felt when switching transformers at high speed
because the sinusoidal line voltage wave form is not maintained after the transformer is
disconnected. The amount of the "bump" is determined essentially by the amount
of time required for the transformer voltage to decay. This, in turn, is dependent upon
the type of network supplied by the transformer. Therefore, a high speed transfer switch
with an in-phase monitor does nothing to eliminate the "bump" that is
felt when switching transformers.
5. The in-phase monitor is totally
ineffective during manual transfer under load. In fact a snap action transfer switch with
all contacts on a single operating shaft could present problems during manual transfer if
the need for in-phase monitor had been identified, since a manual transfer would probably
be out of phase.
6. The in-phase monitor is totally
ineffective if transfer to the generator source is necessary due to a failing utility
source (single phase or brown-out condition). If the transfer switch logic senses a
partial failure of a source, the in-phase monitor must be bypassed to allow transfer. In
this case, an instantaneous out-of-phase transfer cannot be avoided and may very well trip
the circuit breaker that feeds the only good power source. Then all power is lost to
the critical load.
7. If the generator fails while it is
powering the load while utility source is available (during test or the time delay before
returning to normal), there is no protection against an out-of-phase transfer. This could
also cause loss of the only remaining good source of power needlessly (in this instance,
the faster the switch transfer, the more likely there is to be a problem since there is
less time for the motor field to decay).
II NEUTRAL POSITION DELAY "NDT"
The NDT design eliminates the high current
surge by deliberately introducing off-time during load transfer, thereby allowing the
disconnected electrical loads to de-energize before reconnecting them to the alternate
source of power. This is accomplished by introducing a time delay between the opening of
the closed contacts and the closing of the open contacts. In fact, the inherent time it
takes to complete a normal transfer with a T.T.I. motor operated switch (approximately 1
second) is sufficient to eliminate the "bump" or current surge without even
adding the extra time delay of NDT.
A. Advantages
1. Foolproof operation under all conditions
of transfer.
2. Successful operation totally independent
of the frequency of the two power sources.
3. Flexibility. For instance, when utilized
in conjunction with a multiple-engine, generator control switchboard, the NDT design lends
itself to load "dumping" by switching the main contacts to an "off"
position, thereby eliminating the need for separate load dumping devices.
4. Cost is considerably lower than in-phase
monitor.
5. NDT is more reliable and much easier to
service than an in-phase monitor.
B. Disadvantages
1. A momentary dip in power when
transferring loads between two live sources (less than 60 cycles unless extended by time
delay relay).
CONCLUSION
In summary, the Neutral Position Delayed
transfer switch is by far the most reliable method of switching large motors and
transformers because it is flexible, simple, and foolproof. Upon transfer, the user
experiences only in-rush current rather than excessive line currents which may approach
short circuit values.
POINTS OF INTEREST
1. In a lengthy technical working paper
presented by I.E.E.E. entitled "Source Transfer and Reclosing Transients in
Motors" it was stated:
"The following are the two basic
approaches to preventing damaging currents and torques caused by interruption-reconnection
incidents:
(1) Delayed reclosing or transfer, which
allows time for the residual voltage of the motor(s) to decay to a level which is safe
regardless of reclosing angle.
(2) In-phase transfer or reclosing, in
which the transfer or reclosure is timed to occur when the residual and oncoming system
voltages are close enough in phase relation to avoid damaging transients, regardless of
residual voltage magnitude.
While both methods work quite well and
are widely used, they both have advantages and disadvantages. In many applications,
delayed reclosing has the virtues of simplicity, reliability, and economy; on the other
hand the relatively long open interval which is sometimes required to permit the motor
voltage to decay may be unacceptable. In-phase transfer provides the minimum service
interruption, but depends for safe operation on accurately sensing the phase relation
between the two voltages. This requires sensitive solid state or electromagnetic relaying
and control logic which adds cost and complexity."
The paper further states that:
"For typical systems composed of
*relatively small integral-horsepower induction motors and lagging-power-factor static
loads, an open circuit interval of 1 second is virtually always sufficient."
*Up to 300 HP.
The committee which developed this
paper comprised several prominent industry members, including two from Westinghouse and
two from Asco.
2. As a rule of thumb, neutral position
delay may be applied as follows:
a. For motors up to 100HP - no delay
required.
b. For motors 100HP-300HP - specify NDT set
at one second.
c. For motors over 300HP - specify NDT set
at two seconds.
If power factor correction capacitors are
applied at the motor terminals (as opposed to centrally on a system), further
consideration may be required for delay times - consult T.T.I.
Wound-rotor motors are not suitable for either in-phase or
neutral delayed transfer. They should be isolated and restarted.
NOTICE This information is the property of
Thomson Technology Inc. Permission is hereby given to reproduce this document in
this exact form only, without modification of any kind.
© THOMSON TECHNOLOGY INC.
Davidson Sales Company represents Thomson Technology, Inc.,
a worldwide leader in power generation controls. For questions regarding specialized
applications of automatic transfer switches, please contact
Davidson Sales Company. If you would like to download specifications for Thomson
Technology TS850 Series Automatic Transfer Switches, please visit our
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