12551 Descriptions of the Devices

As an automatic synchronizing device, Dev. 25 will control the output of the DG unit to cause the frequency and voltage of the DG to match the grid and then reduce the phase angle between the DG and grid voltage wave. Minimizing this phase angle reduces synchronizing current and the associated torque at the instant of connection of the power sources. In the synchronizing check mode, this device would not control voltage and frequency.

Dev. 27 is an undervoltage relay. In the permissive role, this device would prevent an operation until the monitored voltage achieves some acceptable value. In the protective role, it can serve as a backup to other relays to initiate the disconnect of a power source. This device typically has adjustable trip and reset values and an adjustable operating time delay.

Dev. 32 acts in a protective mode. It monitors power flow in level and direction. It would typically operate to separate circuits when the power flow in a specific direction exceeds a preset value.

Dev. 40 monitors a synchronous generator. It acts to disconnect the generator from a bus if it should lose excitation.*

Dev. 46 provides protection to the generator. Current unbalance and negative sequence currents can result in excessive heating of the generator iron, resulting in damage. The unacceptable unbalance or negative sequence current can be the result of harmonics or single-phase faults on the power system. Since the generator can tolerate this condition for a short time, this device usually acts as a backup device to overcurrent relays.

* Because DG machines of the rating under consideration here are typically brushless generators, actual measurement of excitation is not practical. Therefore, excess reverse VARS is usually monitored to indicate loss of excitation. Accordingly, a directional power relay connected to measure VARS is typically used in the Dev. 40 role.

Dev. 47 provides protection against negative sequence voltages. Again, because the power system can tolerate negative sequence voltages for a short time, this relay can also act as a backup protective device.

Dev. 50 operates when the current in the circuit exceeds a preset value. This is an instantaneous operating relay typically used to protect circuits against fault currents. It is usually set to disconnect the protected circuit for currents in excess of six or more times normal full-load current in the circuit.

Dev. 51 serves to disconnect the protected circuit for currents in excess of a preset value. However, it usually operates on a defined time versus current curve. The higher the current flow, the faster the relay will operate. This device is set to operate at current levels in excess of 1.25 to 10 times normal full-load current in the circuit.

Dev. 52 is typically operated by the permissive and protective relays. It serves to connect or disconnect the protected circuit.

Dev. 59 is an overvoltage relay. In the permissive role, this device would prevent an operation until the monitored voltage achieves some acceptable value. In the protective role, it can serve as a backup to other relays to initiate the disconnect of a power source. This device typically has adjustable trip and reset values and an adjustable operating time delay.

Dev. 67 monitors current flow for magnitude and direction. When the current flow in a circuit exceeds a preset value in a defined direction, the relay operates to disconnect the circuit. Depending on the application, this device can be either instantaneous or time-versus-current delayed.

Dev. 81 is an over-, under-, or over- and underfrequency relay. In the permissive role, it acts to initiate an operation when the frequency of the monitored circuit achieves an acceptable value. In the protective role, it can serve as a backup to other relays to initiate the disconnect of a power source. This device typically has adjustable trip and reset values and an adjustable operating time delay.

Dev. 87 is a zone protection device used to compare the flow of current into a circuit zone with the current out of the zone. When these currents differ by a preset value, it can be concluded that an unacceptable condition exists and the protected zone needs to be disconnected. This can serve as a primary and/or backup protective device. It is typically used to limit damage to the windings of a generator resulting from part-winding faults.

Solar Stirling Engine Basics Explained

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