GE Multilin 850 Instruction Manual page 555

CHAPTER 4: SETPOINTS
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850 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL
% hysteresis = ((abs(-0.9)-abs(-0.7))/ PFBASE)*100 = 20%
The minimum pickup should not be less than 0.01 pu, as the measurement resolution for
the Power Factor is 0.01.
• Input 1(+): Pwr1 PF
• Input 2(-): Off
• Operating Mode: Signed
• Input Comparison Mode: Level
• Direction: Under
• Pickup: -0.700 pu
• Hysteresis: 20.0 %
Detecting high THD (Total Harmonic Distortion)
THD = 100%
BASE
A FlexElement can be programmed to detect excessive amount of harmonics in the
system, and Alarm, Trip, or switch-in/out an equipment to suppress the high amount of
harmonics. The Total Harmonic Distortion is an estimation of how the AC signals are
distorted and as shown above, it can be used as an input for the FlexElement.
For example if an operation from a FlexElement is desired when the THD for the phase A
voltage exceeds 20%, then having a base of 100%, the pickup setting should be set to
0.200 pu.
• Input 1(+): J2 Phase A THD
• Input 2(-): Off
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Over
• Pickup: 0.200 pu
The harmonics and THD values are measured as percentage of the fundamental signal,
and have resolution of 0.01%. However for the minimum pickup setting of 0.001 pu, this
would mean percentage step of 0.1%.
Simple V/Hz ratio detection for protected equipment
V/Hz = 1.00High V/Hz ratios in the power system are harmful for the insulation of the
BASE
protected equipment – transformer, generator, or elsewhere in the power system. If not
detected, it can lead to excessive heat and degradation of the insulation which will
damage the equipment. A FlexElement can be used for simple detection of V/Hz values,
and to issue an Alarm, or Trip, if detected above Pickup setting. Since the base unit for V/Hz
= 1.00, programming of the pickup setpoint is straight forward for the desired FlexElement
operation. For the example given here, a value of 1.200 pu has been selected.
• Input 1(+): Volts Per Hertz 1Input 2(-): OffOperating Mode: AbsoluteInput Comparison
Mode: LevelDirection: OverPickup: 1.200 puHysteresis: 8.3 %
Now, if the FlexElement is needed to drop down when the V/Hz ratio becomes equal to 1.1,
the hysteresis can be calculated as:1.2pu-1.1pu = 0.1 puHysteresis = (0.1*100)/1.2 = 8.3%
RTD inputs for Top-Bottom oil temperature on Transformer
The difference between the Top-oil and the Bottom-oil temperatures of a transformer tank
can be monitored, and if this difference exceeds a specified level, can be an indication of a
problem associated with oil pumps forcing the circulation of the oil, or problems
associated with the transformer cooling equipment. A FlexElement can be programmed to
detect the Top-Bottom oil difference, and issue an alarm, trip, or energize a contact used
FLEXLOGIC
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