The function constantly monitors the instant values of phase TRMS currents (including harmonics up to
31
st
) and calculates the set thermal replica status in 5 ms cycles. The function includes a total memory
function of the load current conditions according to IEC 60255-8.
The function is based on a thermal replica which represents the protected object's or cable's thermal
loading in relation to the current going through the object. The thermal replica includes the calculated
thermal capacity that the "memory" uses; it is an integral function which tells this function apart from a
normal overcurrent function and its operating principle for overload protection applications.
The thermal image for the function is calculated according to the equation described below:
Where:
• θ
t%
= Thermal image status, percentage of the maximum available thermal capacity
• θ
t-1
= Thermal image status, previous calculation cycle (the memory of the function)
• I
max
= Measured maximum of the three TRMS phase currents
• I
N
= Current for the 100 % thermal capacity to be used (pick-up current in p.u., t
max
achieved
in τ x 5)
• k
SF
= Loading factor (service factor), maximum allowed load current (in p.u.) value,
dependent on the protected object or cable/line installation
• k
amb
= Temperature correction factor, either from a linear approximation or from a settable
ten-point thermal capacity curve
• t = Calculation time step (0.005 s)
• e = Euler's number
• τ
1
= Thermal heating time constant of the protected object (in minutes)
• τ
2
= Thermal heating time constant of the protected object (in minutes)
The basic operating principle of the thermal replica is based on the nominal temperature rise, which is
achieved when the protected object is loaded with a nominal load in a nominal ambient temperature.
When the object is loaded with a nominal load for a time equal to its heating constant tau (τ), 63% of
the nominal thermal capacity is used. When the loading continues until five times this given constant,
the used thermal capacity approaches 100 % indefinitely but never exceeds it. With a single time
constant model the cooling of the object follows this same behavior, the reverse of the heating when
the current feeding is zero.
A
AQ
Q-M257
-M257
Instruction manual
Version: 2.07
© Arcteq Relays Ltd
IM00021
273
Summary of Contents for AQ-M257
Page 1: ...AQ M257 Motor protection IED Instruction manual...
Page 2: ......
