4.4
SEL-749M Relay Instruction Manual Date Code 20080918
Protection and Logic Functions
Basic Motor Protection
EXAMPLE 4.2 Phase VT Ratio Setting Calculations
Consider a 4000 V motor application where 4200:120 V rated voltage
transformers (connected in open delta) are used.
Set PTR := 4200/120 := 35 and DELTA_Y := DELTA
When phase-to-phase potentials are connected to the relay, set DELTA_Y to
DELTA. When phase-to-neutral potentials are connected to the relay, set
DELTA_Y to WYE.
Relays that are not equipped with voltage inputs hide these settings and
disable voltage-based protection and metering functions.
VNOM/PTR Range
Check
The relay performs a range check for the PTR and VNOM settings that
depends upon the voltage-input delta or wye configuration. When setting
DELTA_Y is DELTA, then the allowed range of the quotient VNOM/PTR is
100–250 V (l-l). When setting DELTA_Y is WYE, then the allowed range of
VNOM/PTR is 100–440 V (l-l).
Note that the relay always uses line-to-line voltages, even when set for a wye
configuration. You should be careful to use a solidly-grounded wye system for
VNOM/PTR inputs greater than 250 V (l-l) to avoid a 1.73 increase in
terminal voltages from a line-to-ground fault.
Basic Motor Protection
Thermal
Overload Element
The SEL-749M motor thermal element provides integrated protection for all
of the following motor operating conditions:
➤ Locked Rotor Starts
➤ Running Overload
➤ Imbalance Current/Negative-Sequence Current Heating
➤ Repeated or Frequent Starting
The Thermal Method setting (SETMETH) selects the thermal element
algorithm that will be used in the SEL-749M. The Thermal Method setting
offers the following two options:
Rating Thermal Method. When selected, the relay configures a thermal
curve based on the motor Full Load Amps, Service Factor, Run State
Time Constant, Locked Rotor Amps, Hot Locked Rotor Time, and
Acceleration Factor (Locked Rotor Trip Time Dial) settings.
Curves Thermal Method. When selected, the relay offers 45 standard
motor thermal limit curves that you can select by curve number.
Both thermal element setting methods can provide outstanding motor
protection. In each case the relay operates a thermal model with a trip value
defined by the relay settings and a present heat estimate that varies with time
and changing motor current. The relay expresses the present motor thermal
estimate as a % Thermal Capacity Used. When the % Thermal Capacity
reaches 100 percent, the relay trips. You can see the present % Thermal
Capacity value by using the relay front-panel
Meter > Thermal function or
the serial port METER T command. See Appendix F: Motor Thermal
Element for more detail on the thermal model.
NOTE: When you set Run State Time
Constant (RTC1 or RTC2) to Auto, the
relay automatically configures the
overload curves so that the hot stator
and cold rotor limits are the same at
locked rotor current. If known, the
RTC1 or RTC 2 setting can be set to a
specific value to further optimize the
overload protection.
Courtesy of NationalSwitchgear.com
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