ABB 615 Series ANSI, Техническое руководство

Страница: 301 / 713

Section 4
1MAC050144-MB C
Protection functions
294 615 series ANSI
Technical Manual
Figure 141: Functional module diagram. I_A, I_B and I_C represent phase currents.
Max current selector
The sampled analog phase currents are pre-processed and the RMS value of each phase
current is derived for each phase current. These phase current values are fed to the
function.
The max current selector of the function continuously checks the highest phase current
value. The selector reports the highest value to the temperature estimator.
Temperature estimator
From the largest of the three-phase currents, a final temperature is calculated according to
the expression:
(Equation 13)
I the largest phase current
I
ref
set Current reference
T
ref
set Temperature rise
The ambient temperature is added to the calculated final temperature estimation, and the
ambient temperature value used in the calculation is also available in the monitored data
as
TEMP_AMB
. If the final temperature estimation is larger than the set Maximum
temperature , the
PICKUP
output is activated.
Current reference and Temperature rise setting values are used in the final temperature
estimation together with the ambient temperature. It is suggested to set these values to the
maximum steady state current allowed for the line or cable under emergency operation for
a few hours per years. Current values with the corresponding conductor temperatures are
given in cable manuals. These values are given for conditions such as ground
temperatures, ambient air temperature, the way of cable laying and ground thermal
resistivity.
Thermal counter
The actual temperature at the actual execution cycle is calculated as:
Temperature
estimator
Thermal
counter
ENA_MULT
ALARM
BLK_CLOSE
TRIP
BLK_OPR
PICKUP Max
current
selector
I_A
I_B
I_C
AMB_TEMP
Θ
final
ref
ref
I
I
T
=
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟ ⋅
2