Yokogawa 106 Series AB40 Synchroscopes

Synchroscopes measure and display the frequency difference and phase angle between two power systems. Only when these two quantities are zero is it safe to connect the two systems together. 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. Synchroscopes are used in any power plant that connects to an outside power grid and also in isolated plants containing more than one generator. Each generator must be synchronized with the others before being connected to the plant bus. If line voltages are unequal when they are connected, a heavy current will flow as each line will attempt to equalize the other, causing damage in the process.