Protective relays Reverse power

If the field excitation is also lost, the generator will run as an induction motor again driving the primer mover as above. As an induction motor, it will now operate at less than the synchronous speed and cause slip frequency current and slip losses in the rotor circuit, which may overheat the rotor and damage it, see also Section. 1.3 and equation (1.9). A reverse power relay under such a condition will disconnect the generator from the mains and protect the machine. 

Tb relaying is based on either undervoltage reduced power flow or reversed power flow to the motors plus normal motor protection relaying. 

Critical interlock While identifying the hazards it is important to categorize critical interlocks (for new plants, data/experience from similar plants may be helpful) for which sufficient backup may have to be arranged as a safety measure. In many of these cases, in addition to software interlock, hardware interlocks are also provided. In some cases, even mechanical protection is provided. A critical trip interlock pertinent to a turbine is an example. For turbine overspeed, condenser vacuum, etc., in addition to electronic interlock (with sufficient hardware/software redundancy), mechanical interlocks are provided so that the equipment is saved. Choke and kill in offshore is also an example of this. Regulations To meet the requirement of a regulatory board or company standards, measures are taken to enhance overall safety. Incorporation of these safety measures does not always mean that risk is adequately reduced. These measures could make the system acceptable for use, and a local regulatory board may require additional precautions. An example will elaborate the situation. Many turbine manufacturers give low forward power relays or reverse power... 

Anti-motoring protection Reverse-power relay... 

A directional G/F relay basically is a power-measuring device, and is operated by the residual voltage of the system in conjunction with the residual current detected by the three CTs used for non-directional protection, as shown in Figure 21.19. To provide directional protection, therefore, a residual VT is also essential, in addition to the three residual CTs. The voltage phasor is used as a reference to establish the relative displacement of the fault current. In healthy conditions, i.e. when the current flows in the right direction. = 0. (refer to. Section 15.4.3 for details), and the relay remains inoperative. The relay operates only when the current flows in the reverse direction. 

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