Variable Frequency Drives - GE 869 Instruction Manual

SYSTEM
Variable Frequency
Drives
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Setpoints > System > Motor > VFD
Some Variable Frequency Drives (VFD), for example pulse width modulated drives,
generate significant distortion in voltages introducing harmonics. However, distortion due
to these harmonics is not as significant in currents as in voltages. The functionality of
various 869 protection elements is made adaptive to the VFD motor applications
depending on the system configurations. Figure 1 shows two typical VFD motor
applications, the possible system configurations can be: (a) motor start and run through
the VFD only, (b) VFD with Bypass (BP) Switch i.e., motor run through the bypass switch
without VFD but the VFD is required for starting.
The VFD Function must be set to "Enabled" in order to ensure proper performance of the
869 relay for motor applications with VFD. In the motor application when VFD can be
bypassed via the Bypass Switch as shown in Figure 1(B), status of the bypass switch must
be configured as a selected input under setpoint "Bypass Switch".
If the VFD Function is "Enabled" and the Bypass Switch operand is not asserted (i.e., bypass
switch is "open") then 869 algorithms adopt the following changes:
• Frequency tracking source is switched from three-phase voltages to three-phase
currents. For the case where currents are not available or system frequency cannot
be measured from the available ac signals then power system Nominal Frequency is
used as a default. All elements will function properly for frequency range of 3Hz to
72Hz.
• Thermal Model Voltage Dependent (VD) function is blocked automatically.
• VFD Not Bypassed operand is asserted, which could be used to block the voltage
elements via Block setting of the elements.
• All motor current functions except Short Circuit, Ground Fault and Differential
elements are using the Motor Load Averaging Filter of length setpoint "Motor Load
Filter Interval" set under System Setup/Motor in order to mitigate oscillations.
• However, when VFD configuration support "Function" is enabled and Bypass Switch
operand is now asserted (i.e. bypass switch is "close") then frequency tracking source
will be switched from currents to voltages, all voltages elements will work as normal,
VFD Not Bypassed operand will be de-asserted while VFD Bypass operand will be
asserted and all motor functions will then be using the normal RMS currents.
Voltage inputs to the 869 motor protection relay are normally measured at the busbar side
of the VFD that are substantially sinusoidal, however; the output phase voltages from a
VFD are not sinusoidal and are distorted due to harmonics generated by the VFD.
Therefore, it is recommended to block the voltage elements via Block setting of the
elements using VFD Not Bypassed operand. On the other hand, when voltages are
measured at the motor side of the VFD, voltages at the motor terminals and relay inputs
are the same, hence, blocking of the voltage element is not required. Secondly, significant
distortion in the voltage waveforms is not always the case and depends on the VFD type. If
the input voltages are substantially sinusoidal, which can be verified from 869 metering,
oscillography and data logger, then blocking of the voltage elements is not required.
With the VFD motor application, the 869 motor protection relay uses the running average
technique (described in detail under setpoint "Motor Load Filter Interval" (in section
Setpoints/System Setup/Motor) in order to smooth out the phasor's ripple due to the
distortion generated by the VFD. When setpoint Motor Load Filter Interval is set equal to
non-zero cycles, Motor Load Averaging Filter can increase Trip/Alarm times for the
following protection elements: Acceleration Time, Current Unbalance, Mechanical Jam,
Overload, Thermal Model, Reduced Voltage Start, Undercurrent, Power Factor, Three-Phase
Apparent Power, Three-Phase Reactive Power, Three-Phase Real Power, Under Power. No
other elements are affected.
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS