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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.
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.
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.
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.
"The following are the two basic approaches to preventing damaging currents and torques caused by interruption-reconnection incidents:
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.
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.
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