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2.7.2
Over Voltage
Motors can operate on moderate steady state overvoltage within the motor tolerance. Overvoltage causes an
increase in magnetisation (no load) current due to an increase in iron loss in the machine. At a given shaft load,
the overvoltage also causes a decrease in load current. In general the resulting total current will be less than the
motor current at rated voltage. Smaller motors have a relatively high magnetising current and therefore
overvoltages will cause the motor operating temperature to increase.
2.8
Negative Phase Sequence (NPS) Overvoltage (47)
Voltage unbalance can be caused by faulty contactors, transformer/OLTC faults or unbalanced loading of the
three phases. Very high levels of NPS Voltage indicate a reversed phase sequence due to an incorrect
connection. NPS Voltage level is an indicator of the system supply quality.
Unbalanced voltages produce unbalanced currents, see section 2.1.4. The motor NPS impedance is lower than
the Positive Phase Sequence (PPS) impedance and therefore the ratio of NPS to PPS Current is higher than the
equivalent ratio of NPS to PPS Voltage. A 1% voltage unbalance typically translates into a 6% current unbalance.
NPS Voltage DTL elements can be used as Alarms to indicate that the level of NPS voltage has reached
abnormal levels.
2.9
Frequency (81)
At decreased frequency without a corresponding voltage reduction the flux density within the motor core is
increased thus increasing the hysteresis and eddy current losses and heating.
Under-frequency elements can be used to provide an alarm.
2.10 Power Protection
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.
©2018 Siemens Protection Devices
7SR105 Rho and 7SR17 Rho Applications Guide
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