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inverse-, very inverse-, and extremely inverse-time characteristics to fit the requirements of the
particular application. There are also definite minimum-time overcurrent relays having an
operating time that is practically independent of the magnitude of current after a certain current
value is reached. Induction overcurrent relays have a provision for variation of the time
adjustment and permit change of operating time for a given current. This adjustment is called
the time lever or time dial setting of the relay. It is possible to adjust the operating time of relays
to selectively trip circuit breakers which operate in series on the same circuit. Overcurrent Relays with Voltage Restraint or Voltage Control. A short circuit on
an electric system is always accompanied by a corresponding large voltage dip, whereas an
overload will cause only a moderate voltage drop. A voltage-restrained overcurrent relay is
designed to operate at lower current values when the system voltage drops below its nominal
value. A voltage-controlled relay will not operate until the system voltage drops below a
predetermined setting, as would occur during a short circuit.
4.3.3 Directional Relays. Directional Overcurrent Relays. Directional overcurrent relays consist of a typical
overcurrent unit and a directional unit combined to operate together for a predetermined
phase-angle and magnitude of current. The current in one coil is compared in phase-angle
position with a voltage or current in another coil of that unit. The reference current or voltage is
called the polarization. The relay operates only for current flow in one direction and will be
insensitive to current flow in the opposite direction. Directional Ground Relays. The grounded-neutral industrial power system may
use directional ground relays, constructed much the same as the directional overcurrent relays. In
order to properly sense the direction of fault current flow, directional ground relays require a
polarizing source which may be either potential or current, as the situation requires. Directional Power Relays. The directional power relay is, in principle, a
single-phase or three-phase contact-making wattmeter and operates at a predetermined value of
power. It is often used as a directional overpower relay set to operate if excess energy flows out
of an industrial power system into the utility power system. Under certain conditions it may also
be useful as an underpower relay to separate the two systems if the power flow drops below a
predetermined value. It is also used to disconnect a generator operating in parallel with a larger
generator or a utility, should the prime mover's fuel supply be interrupted.
4.3.4 Differential Relays. All the previously described relays have the common characteristic
of adjustable settings to operate at a given value of some electrical quantity such as current,
voltage, or power. There are other fault-protection relays which function by virtue of continually
comparing two or more currents. Certain fault conditions will cause a difference in these
compared values and the resulting differential current can be used to operate the relay. Current


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