Protection Elements; Thermal Protection; Unbalance Biasing - GE MM300 Instruction Manual

PROTECTION ELEMENTS

Protection elements

Thermal protection

Unbalance biasing

5–34
The primary protective function of the MM300 is the thermal model. The MM300 integrates
stator and rotor heating into a single model. The rate of motor heating is gauged by
measuring the terminal currents. The present value of the accumulated motor heating is
maintained in the Thermal Capacity Used
overload, the motor temperature and thermal capacity used will rise. A trip occurs when
the thermal capacity used reaches 100%. When the motor is stopped and is cooling to
ambient, the thermal capacity used decays to zero. If the motor is running normally, the
motor temperature will eventually stabilize at some steady state temperature, and the
thermal capacity used increases or decreases to some corresponding intermediate value,
which accounts for the reduced amount of thermal capacity left to accommodate
transient overloads.
The thermal model consists of six key elements.
• Unbalance current biasing that accounts for negative-sequence heating.
• Hot/cold biasing that accounts for normal temperature rise.
• RTD biasing that accounts for ambient variation and cooling problems
• An overload curve that accounts for the rapid heating that occurs during stall,
acceleration, and overload.
• Cooling rate that accounts for heat dissipation.
• Thermal protection reset that controls recovery from thermal trips and lockouts.
Each of these categories are described in the following sub-sections.
Unbalanced phase currents (that is, negative-sequence currents) cause rotor heating in
addition to the normal heating caused by positive-sequence currents. When the motor is
running, the rotor rotates in the direction of the positive-sequence MMF wave at near
synchronous speed. The induced rotor currents are at a frequency determined by the
difference between synchronous speed and rotor speed, typically 2 to 4 Hz. At these low
frequencies the current flows equally in all parts of the rotor bars, right down to the inside
portion of the bars at the bottom of the slots. On the other hand, negative-sequence stator
current causes an MMF wave with a rotation opposite to rotor rotation, which induces rotor
current with a frequency approximately two times the line frequency (100 Hz for a 50 Hz
system or 120 Hz for a 60 Hz system.) The skin effect at this frequency restricts the rotor
current to the outside portion of the bars at the top of the slots, causing a significant
increase in rotor resistance and therefore significant additional rotor heating. This extra
heating is not accounted for in the thermal limit curves supplied by the motor
manufacturer, as these curves assume only positive-sequence currents from a perfectly
balanced supply and balanced motor construction.
To account for this additional heating, the MM300 allows for the thermal model to be
biased with negative-sequence current. This biasing is accomplished by using an
equivalent motor heating current rather than the simple motor terminal current (I
equivalent current is calculated as shown in the following equation.
In the above equation:
• I represents the equivalent motor heating current in per-unit values on an FLA base.
eq
• I represents the average RMS current at each motor terminals in per-unit values on
avg
an FLA base.
actual value register. When the motor is in
2
æ ö
I
2
I = I ´ 1 + k ´ ç ç ÷ ÷
eq avg
I
è ø
1
MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
). This
avg
Eq. 1