1255 Protective Relaying

Protective relay schemes are designed to respond to changes in the parameters they monitor to initiate an appropriate control sequence when any of those parameters exceed a preset limit. While such relay schemes are referred to as protective, they are also permissive. That is, the relays will prevent a control sequence until the monitored parameters satisfy preset conditions. The IEEE and ANSI have established a shorthand method of defining protective relay functions. The method assigns unique numbers to relay functions. Table 12.1 is a partial list of the assigned numbers and their functions for the population of protective relays likely to be found in the DG environment.

The relay functions are selected as functions of their location in a circuit and the characteristics of the equipment at that location. For example, many grid operators will require a directional power relay at the point of interconnection of the DG unit and the grid. The relay will likely be set to trip at a specific power level flowing back into the grid. The device usually called for is Dev. 32. In this scenario, the relay would initiate a designed control response whenever power flow from the DG unit into the grid exceeds a preset value. Thus, it would operate as a protective device. Similarly, grid operators may require Dev. 27 at the point of interconnect. This would act as a permissive device. It could also be used as a protective device in a backup role to Dev. 32. It is useful to provide some commentary on each of these devices and the roles they would likely play in the DG environment.

TABLE 12.1

ANSI and IEEE Device Protective Relay Functions

TABLE 12.1

ANSI and IEEE Device Protective Relay Functions

ANSI Device


Device Function


Automatic synchronizing or synchronism check




Directional or overpower


Loss of excitation


Negative sequence and unbalanced current


Negative sequence voltage


Instantaneous overcurrent


Time-delayed overcurrent


Circuit breaker




Directional overcurrent


Frequency, over- or underfrequency, or both


Differential overcurrent

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