Power Protection; Power (32); Sensitive Power (32S); Power Factor (55) - Siemens 7SR17 Rho Manual

Chapter 7) 7SR17 Rho Applications Guide
2.10 P P
OWER ROTECTION
2.10.1

Power (32)

An under power element protects against a loss of load condition by measuring the real power flow to the motor in
the running condition. This provides an alternative to under current measurement as load loss may result in only a
small change in current (see section 2.3).
To prevent spurious trip operations when the relay is first energised or when a motor is disconnected, the
directional power element does not operate for currents below the motor stopped threshold I
The under power output is initiated by a voltage element operating after a definite time delay. The applied power
setting will typically be 10-20% below minimum load, the power and time delay settings must take into account:
Where rated power cannot be reached during starting (for example where the motor is started with no
connected load) it may be necessary to inhibit this function for a set time. This feature requires a 52a
circuit breaker auxiliary contact mapped to an opto input to get the information CB Closed/CB Open.
Directional power measurement may operate on occurrence of a system power supply fail or system
fault. Power flow into the motor will reverse since the motor will act as a generator due to the inertia of
the connected load.
2.10.2

Sensitive Power (32S)

The CT accuracy should be considered when for the application and setting of this function.
2.10.3

Power Factor (55)

Power factor is often a more sensitive measurement of underload conditions than current. Settings must take into
account:
The power factor is low during motor starting. Start/running conditions of a VSD or synchronous motor
can be detected. This function can be used instead of a tacho speed switch to block stall protection for
starting, when the stall withstand is within the motor start current profile.
loss of excitation on a synchronous motor. This can be enabled for service once the excitation is applied,
it can be disabled during motor starting e.g. by setting the DTL.
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