In an AC grid, the line frequency is critical to the quality of the supply provided. Deviations from the target frequency must be tolerated only in a very narrow range. Otherwise, certain consumers will no longer function properly or will even be damaged. In a heating power plant, it must be ensured that the steam turbine always has a constant rotational speed.
Especially in non-rigid networks, the generator rotational speed can be influenced by, e. g., the switching on and off of essential consumers. On-board generators on ships are typical for such applications. Much higher fluctuations in consumption can occur here than with the public power supply. It is essential to respond to speed fluctuations immediately to keep the specified line frequency constant.
Continuous monitoring of the generator rotational speed is an indispensable protective function. As soon as the rotational speed deviates from the tolerance range, a suitable safety function must trigger targeted measures. In the event of overspeed, this can mean that the power is reduced. In the event of underspeed, the response can be to activate additional generators, or to switch off loads or entire network segments.
Pepperl+Fuchs offers rotation speed monitors in the DWB series for continuously monitoring the rotational speed of generators. These are isolated barriers (for signals from hazardous areas) or signal conditioners (for signals from the safe area) that respond when a certain rotational speed is exceeded or not met. All devices are designed for SIL 2 applications according to IEC 61508/IEC 61511. These interface components are used for monitoring a discrete signal, a NAMUR sensor, or a mechanical contact. Its functionality is based on the premise that the input frequency is constantly compared with a reference frequency defined by the user.
If the DWB rotation speed monitor detects that the reference frequency is exceeded or not met, integrated relay outputs are triggered. A red LED indicates that this line fault detection has been triggered. An integrated start-up override sets the relay outputs to the user-selected value of up to 1000 s. The device can be easily programmed using the operating push buttons on the front of the device.