IMM 8001/8002 – Digital Motor Protection Relay
ICE / CEE
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IMU307-1 GB Ind. B
33
The response time-related inaccuracies come from :
the trip moment with respect to the current sine wave,
the inaccuracy of the starting and time counter stop system, especially when the measured
times are very short,
the inaccuracy related to the response times of the relay.
Due to this, the tolerances related to the operating levels and time-delays
will be doubled
with
respect to those specified in the technical characteristics.
5.3. PROTECTION AGAINST THERMAL OVERLOADS [49]
Function
The IMM 8000 relays generate a thermal image of the machine which takes into account the
influence of the positive and the negative phase sequence currents. This thermal image models the
machine as a single time constant system (Electrical equivalent : RC circuit). This time constant is
liable to have two values :
τ
during normal operation, and
τ
c on cooling.
The equivalent thermal current Ith is calculated with the following formulae :
Ith² =
I Pos² + Ki² I Neg²
for I Pos
≤
3.2 In
Ith² =
3.2 I Pos + Ki² I Neg²
for I Pos > 3.2 In
I Pos : positive phase sequence current flowing through the machine
I Neg : negative phase sequence current flowing through the machine
In
: machine rated current
Ki²
: weighing factor for the negative phase sequence current heating effect
The coefficient Ki² is automatically adjusted by the value of the unbalance unit operating level [46]
INeg> :
Ki² = 3 for INeg>
equal to or greater than 30 % In
Ki² = 9 for INeg>
less than 30 % In
To operate with a constant current equal to 1,05 In machine (current allowed by IEC Standard 34 to
provide machine rated power with 95% of the nominal voltage), and keep a sufficient margin for
accuracy, the operating level for the thermal unit is set to 1.07 In.
Thermal unit operating level correction with 3 stator RTDs : IMM 8002 only
Account is taken of the absorbed positive and negative phase sequence current values only in the
thermal image : all the mechanical overloads or electrical unbalances (particularly affecting the rotor)
are effectively detected.