Synchroscope
Connecting two unsynchronized AC power systems together is likely to cause high currents to flow, which will severely damage any equipment not protected by fuses or circuit breakers.
However, the accuracy of this approach is low since it is difficult to discern slight phase differences, and the lamps do not show the relative speeds of the two systems.
Synchroscopes are instruments that show the relative frequency (speed) difference and the phase angle between the machine to be synchronized and the system voltage.
A two-phase rotor winding is connected to the incoming machine source by a phase-shifting network through brushes and slip rings.
[2] In a Weston pattern synchroscope, the moving element is not free to rotate continuously and oscillates back and forth slowly as the two sources are brought into synchronism.
The moving pointer is illuminated by a pilot lamp connected to a three-winding transformer fed by both sources.
Cross-coil type instruments draw a relatively large amount of power from the systems and are intended for only brief operation.
The moving-iron and polarized-vane instruments put less burden on the system and can operate for a longer time without overheating.
If the generator is running faster than the grid, the pointer rotates continually in the opposite direction, marked "fast" or "lead".
If two electrical networks operating at two different phase angles were to be connected to each other, a fault similar to a short circuit would occur, presenting a hazard of damage to the generator or other equipment.
The angle between the systems is zero when the synchroscope pointer is directly on the line in between the "slow" and "fast" markings on the dial.
Depending on the application and the circuit design, the operator closes the breaker when the synchroscope pointer is passing through approximately "11 o'clock", while traveling slowly in the fast direction.
The slightly faster frequency is intended to prevent the generator paralleling onto the grid as a motor load, potentially damaging the prime mover (steam turbine or reciprocating internal combustion engine).
This helps reduce the noise and vibration due to slight differences, for example, in the speeds of two propellers on an aircraft.
