5.4.17. Transformer thermal overload protection TT> (49TR)
Thermal overload function for transformers (TOLT) is used for power transformers thermal capacity
monitoring and protection.
TOLT function constantly monitors phase TRMS currents (including harmonics up to 31
st
) instant
values and calculates the set thermal replica status in 5 ms cycles. TOLT function includes total
memory function of the load-current conditions according to IEC 60255-8
TOLT function is based into thermal replica, which represents the protected object or cable thermal
loading in relation to the current going through the object. Thermal replica includes the calculated
thermal capacity used in the “memory” since it is integral function which tells apart this function from
normal overcurrent function operating principle for the overload protection applications.
Thermal image for the TOLT function is calculated according to equation described below:
, where
θ
t%
= Thermal image status in percent of the maximum thermal capacity available
θ
t-1
= Thermal image status in 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., with this current
t
max
will be achieved in time t x 5)
k
SF
= Loading factor (service factor) coef cient, maximum allowed load current in per unit value
depend of the protected object or cable/line installation
k
AMB
= Temperature correction factor either from linear approximation or settable 10 point
thermal capacity curve.
τ
1
= Thermal heating time constant of the protected object (in minutes)
τ
2
= Thermal heating time constant of the protected object (in minutes)
e = Euler’s number
t = Calculation time step in seconds (0.005s)
The basic operating principle of the thermal replica is based into that the nominal temperature rise is
achieved when the protected object is loaded with nominal load in nominal ambient temperature. When
the object is loaded with nominal load for time equal its heating constant tau (τ), 63% of the nominal
thermal capacity is used. When the loading continues until ve times this given constant the used
thermal capacity inde nitely approaches to 100% but never exceeds it. With a single time constant
model cooling of the object follows this same behavior reversible to the heating when the current
feeding is completely zero.
AQ-T257
Instruction manual
Version: 2.00
135
© Arcteq Relays Ltd