7SR210 & 7SR220 Applications Guide
Page 30 of 48 ©2018 Siemens Protection Devices Limited
2.17 Power Protection
2.17.1 Power (32)
Parallel Busbar Feeder
If power is fed to a busbar through two parallel infeeds, then in the event of any fault on one of these infeeds it
should be selectively interrupted. This ensures a continued supply to the busbar through the remaining the
remaining infeed. For this purpose directional devises are needed which detect a shortcircuit current or a power
flow from the busbar in the direction of the infeed. Reverse-power protection can be set far below the rated power.
Motor Protection
An under power element protects against a loss of load condition by measuring the real power flow. This feature
provides an alternative to under current measurement as load loss may result in only a small change in current.
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.
General Calculation
Real, apparent or reactive power is set in the relay as a multiple of the relay (or secondary) nominal
power where: -
Nominal relay power (secondary) = relay nominal current x relay nominal voltage.
Example:
CT ratio = 400:1
VT ratio = 33000:110
Nominal secondary apparent power (S):
Nominal 3-phase VA (secondary) = √3 x V
L
x I
L
= √3 x 110 x 1
= 190.53 VA
= 1 xSn
When calculating the required operate value in primary quantities the CT and VT ratios must be
considered i.e. where the above corresponds to a nominal primary 3-phase apparent power (S) of:
Nominal 3-phase VA (primary) = √3 x V
L
x I
L
= √3 x 330000 x 400
= 22.863 MVA
= 1 xSn
The primary nominal VA calculated above may not match that of the protected primary plant and so
settings should be adjusted accordingly.
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