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GE Multilin 489 Generator Management Relay 4-57
4 SETPOINTS 4.10 S9 THERMAL MODEL
4
The current measured at the output CTs is used for the thermal model. The thermal model consists of five key elements:
the overload curve and overload pickup level, the unbalance biasing of the generator current while the machine is running,
the cooling time constants, and the biasing of the thermal model based on hot/cold generator information and measured
stator temperature. Each of these elements are described in detail in the sections that follow.
The generator FLA is calculated as: (EQ 4.26)
The 489 integrates both stator and rotor heating into one model. Machine heating is reflected in a register called Thermal
Capacity Used. If the machine has been stopped for a long period of time, it will be at ambient temperature and thermal
capacity used should be zero. If the machine is in overload, once the thermal capacity used reaches 100%, a trip will occur.
The overload curve accounts for machine heating during stall, acceleration, and running in both the stator and the rotor.
The Overload Pickup setpoint defines where the running overload curve begins as the generator enters an overload condi-
tion. This is useful to accommodate a service factor. The curve is effectively cut off at current values below this pickup.
Generator thermal limits consist of three distinct parts based on the three conditions of operation, locked rotor or stall,
acceleration, and running overload. Each of these curves may be provided for both a hot and cold machine. A hot machine
is defined as one that has been running for a period of time at full load such that the stator and rotor temperatures have set-
tled at their rated temperature. A cold machine is defined as a machine that has been stopped for a period of time such that
the stator and rotor temperatures have settled at ambient temperature. For most machines, the distinct characteristics of
the thermal limits are formed into one smooth homogeneous curve. Sometimes only a safe stall time is provided. This is
acceptable if the machine has been designed conservatively and can easily perform its required duty without infringing on
the thermal limit. In this case, the protection can be conservative. If the machine has been designed very close to its ther-
mal limits when operated as required, then the distinct characteristics of the thermal limits become important.
The 489 overload curve can take one of three formats, Standard, Custom Curve, or Voltage Dependent. Regardless of
which curve style is selected, the 489 will retain thermal memory in the form of a register called Thermal Capacity Used.
This register is updated every 50 ms using the following equation:
(EQ 4.27)
where: time to trip = time taken from the overload curve at Ieq as a function of FLA.
The overload protection curve should always be set slightly lower than the thermal limits provided by the manufacturer. This
will ensure that the machine is tripped before the thermal limit is reached. If the starting times are well within the safe stall
times, it is recommended that the 489 Standard Overload Curve be used. The standard overload curves are a series of 15
curves with a common curve shape based on typical generator thermal limit curves (see the following figure and table).
When the generator trips offline due to overload the generator will be locked out (the trip relay will stay latched) until gener-
ator thermal capacity reaches below 15%.
ACCEL. INTERSECT @
100% VOLT: 5.00 x FLA
Range: 2.00 to STALL CURRENT @ 100% VOLTAGE in
steps of 0.01. Seen only if SELECT CURVE
STYLE is "Voltage Dependent"
MESSAGE
ESCAPE
NOTE
Generator Rated MVA
3 Rated Generator Phase-Phase Voltage×
----------------------------------------------------------------------------------------------------------------------
TC
used t
TC
used t 50ms–
50 ms
time to trip
--------------------------- -
100%×+=