Aa you know when the voltage to the running electric motor suddenly decays, the electric motor begins to act as the generator for a short period of time. The duration of the generator action depends on the type of the motor, inertia of the driven equipment as well as the amount of the passive load on the same circuit. This phenomena causes the power systems engineers to consider motor contribution when they calculate the available fault current. During the open transition load transfer, the independent sources are likely not to be synchronized. Fast open transition motor load transfer, can cause connection of the generating motor to the power source out of synchronism. The consequences of this connection are similar to the closing the generator out of synchronism: high line currents (with possible operation of the over current devices) and mechanical stress to the equipment shafts and gears (with possible mechanical damage). Transformers are also known to cause high line currents during fast open transition load transfers, due to the stored magnetic field.
There are three most common ways to accomplish this transfer: non-delayed transition, in-phase transition, and time delayed transition.
In case of non-delayed transition, the normal source will be connected to the load as soon as the emergency source is disconnected from the load, without any intentional time delay. Advantages: the duration of the outage is minimized; lowest cost. Disadvantages: not suitable for transferring large inductive loads (motors, transformers, etc.).
In case of in-phase transition, the normal source will be connected to the load as soon as the emergency source is disconnected from the load, but the transfer will only take place when the two sources are in synchronism. Advantages: the duration of the outage is minimized. Disadvantages: relies on the generator to passively fall into synch with the utility, which is not always possible. In this case the transfer may not occur until the generator runs out of fuel. If the substantial passive load is connected to the motor during the transfer, the motor my fall out of synch faster than the transfer time of the ATS. The time of transfer is not predictable. Does not eliminate inrush during transfer of the heavy transformer loads.
In case of time delayed transition, the normal source will be connected to the load after the emergency source is disconnected from the load, and after an adjustable time delay. Advantages: the most reliable and flexible operation suitable for reliable transfer of any loads. Disadvantages: the duration of the outage during transfer is increased.
System operation of a typical open transition ATS is described below:
Upon sensing of the utility failure (utility voltage is not within under voltage setpoint) the Time Delay Engine Start timer shall start timing. If utility failure condition remains upon expiration of the Time Delay Engine Start timer the generator set shall be automatically started and brought up to speed and voltage.
At that time utility circuit breaker shall open and generator circuit breaker shall close (after adjustable time delay neutral). At this time the generator is supplying power to the site load. Upon sensing of utility return (utility voltage is within set undervoltage tolerance) the Time Delay Emergency to Normal timer shall start timing. If utility power remains healthy (utility voltage is within set undervoltage tolerance) upon expiration of the Time Delay Emergency to Normal timer the generator circuit breaker shall open and the utility circuit breaker shall close (after adjustable time delay neutral). At this time the generator set shall be put in the cooldown mode of operation.
Time Delay Neutral shall determine the minimum time period the load will be deenergized before the load is energized from the alternative source (normal or emergency).
If the generator set should fail (generator voltage is not within undervoltage setpoint) while Time Delay Emergency to Normal timer is timing, the Time Delay
Emergency to Normal timer shall be bypassed, generator circuit breaker shall open and the utility circuit breaker shall close (after adjustable time delay neutral).
During the closed transition transfer from the emergency source back to the returned normal source, the emergency source will be disconnected after the normal source is connected. This will not cause an outage to the load during the transfer. Typically the duration of the connection of the two sources is less than
100 mS. Since the incoming and the running power sources connect while in synchronism, any type of load can be reliably transferred in this manner.
The synchronizing of the sources for the closed transition transfer can be accomplished via active synchronizing and passive synchronizing. In both cases the paralleling of the two sources is supervised by a synchronizing check relay.
In case of passive synchronizing, the system control relies on the generator to randomly fall in to synchronism with the utility source. In case of passive synchronizing there is no way to be sure that the transfer will occur.
In case of active synchronizing, the system control monitors the voltage and frequency of both sources and provides a speed correction signal to the prime movers (engine, turbine, etc.) governing system to bring it in to synchronism with the running source (utility).
Reference: Advanced Power Technologies, Inc. www.aptinc.net