Siemens SIPROTEC 7SJ531 - Page 69

-ETHOD OF OPERATION3*
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 4HERMAL OVERLOAD PROTECTION
4HE THERMAL OVERLOAD PROTECTION PREVENTS THE PRO
TECTED OBJECT EG IN CASE OF CABLES OR MOTORS FROM
DAMAGE CAUSED BY THERMAL OVERLOADING
4HE UNIT COMPUTES THE TEMPERATURE RISE ACCORDING
TO A THERMAL SINGLE
BODY MODEL AS PER THE FOLLOWING
THERMAL DIFFERENTIAL EQUATION
D
DT


Ç
 

Ç
)

WITH  ACTUAL TEMPERATURE RISE REFERRED TO THE FI
NAL TEMPERATURE RISE FOR THE MAXIMUM PER
MISSIBLE CURRENT OF THE PROTECTED OBJECT
Kw)
.
Ç THERMAL TIME CONSTANT FOR HEATING UP OF
THE PROTECTED OBJECT
)
ACTUAL CURRENT OF THE PROTECTED OBJECT
RMS VALUE REFERRED TO THE MAXIMUM
PERMISSIBLE CURRENT OF THE PROTECTED OB
JECT )
MAX
Kw)
.
7HEN THE TEMPERATURE RISE REACHES THE FIRST SET
THRESHOLD A WARNING ALARM IS GIVEN IN ORDER TO REN
DER POSSIBLE AN EARLY LOAD REDUCTION )F THE SECOND
TEMPERATURE THRESHOLD IS REACHED THE PROTECTED OB
JECT CAN BE DISCONNECTED FROM THE NETWORK
4HE TEMPERATURE RISES ARE CALCULATED SEPARATELY FOR
EACH INDIVIDUAL PHASE ! CHOICE CAN BE MADE
WHETHER THE MAXIMUM CALCULATED TEMPERATURE RISE
OF THE THREE PHASES OR THE AVERAGE TEMPERATURE RISE
SHOULD BE DECISIVE ! TRUE RMS VALUE MEASUREMENT
IS PERFORMED IN ORDER TO INCLUDE FOR THE EFFECT OF HAR
MONIC CONTENT
4HE MAXIMUM PERMISSIBLE CONTINUOUS THERMAL OVER
LOAD CURRENT )
MAX
IS DESCRIBED AS A MULTIPLE OF THE
RATED CURRENT )
.

)
MAX
K

)
.
)N ADDITION TO THE K VALUE THE TIME CONSTANT Ç AS
WELL AS THE WARNING TEMPERATURE 
WARN
MUST BE EN
TERED INTO THE PROTECTION UNIT
!PART FROM THE TEMPERATURE
DEPENDENT WARNING
STAGE THE OVERLOAD PROTECTION ALSO INCLUDES A CUR
RENT
DEPENDENT WARNING STAGE 4HIS LATTER ALARM
STAGE CAN GIVE AN EARLY ANNUNCIATION OF AN IMPEND
ING OVERLOAD CURRENT EVEN WHEN THE TEMPERATURE RISE
HAS NOT YET REACHED THE WARNING OR TRIP TEMPERATURE
RISE VALUES
4HE TIME CONSTANT DURING STAND
STILL OF A MACHINE
MAY BE MUCH LONGER THAN DURING OPERATION IN CASE
OF SELF
VENTILATED MACHINES BECAUSE THE ROTOR DOES
NOT VENTILATE 4HAT IS WHY THE PROTECTION USES FOR THE
STANDING MACHINE THE COOLING
DOWN TIME CONSTANT
WHICH CAN BE SET AS A FACTOR OF THE HEATING
UP TIME
CONSTANT FACTOR K
Ç
 )N THIS ASPECT THE MOTOR IS AS
SUMED TO STAND STILL WHEN THE CURRENT CONSUMPTION
IS LESS THAN THE SETTABLE THRESHOLD ) "2+
#,/3%$
)N CASE OF FORCED
VENTILATED MACHINES TRANSFORM
ERS OR CABLES IS K
Ç

4HE THERMAL OVERLOAD PROTECTION IS SUPPLEMENTED BY
THE START
UP TIME MONITOR SEE 3ECTION  AND THE
RESTART LOCKOUT FUNCTION SEE 3ECTION  FOR USE ON
MOTORS $URING START
UP OF THE MOTOR THE OVERLOAD
PROTECTION DOES NOT OPERATE IE CHANGE THE CALCU
LATED TEMPERATURE RISE 3TART
UP CRITERION IS THE SET
TABLE CURRENT THRESHOLD ) 3424
$%4
&IGURE  SHOWS THE LOGIC DIAGRAM OF THE THERMAL
OVERLOAD PROTECTION
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