CHAPTER 4: SETPOINTS FLEXLOGIC
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 4–353
Simple V/Hz ratio detection for protected equipment
V/Hz
BASE
= 1.00High V/Hz ratios in the power system are harmful for the insulation of the
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
for switching-in/out transformer equipment. The setpoints of the FlexElement below reflect
detecting a 5°C degree temperature difference between the istalled RTDs measuring the
top and bottom oil temperatures:
• Input 1(+): RTD 1 (top-oil)
• Input 2(-): RTD 2 (bottom-oil)
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Over
• Pickup: 0.050 pu
• Hysteresis:
• 60.0%
With an RTD
BASE
= 100°C, the required temperature difference of 5°C, would be computed
as a pickup value of 5°C/100°C = 0.050 pu.
If it is required to drop out the operation of the FlexElement when the temperature
difference becomes 2°C, the hysteresis would be calculated as 5°C/100°C - 2°C/100°C =
3°C/100°C = 0.03 pu. Therefore:
Hysteresis = (0.03 pu *100)/0.05 = 60%
High Breaker Arcing current detection
High breaker arcing current can be detected by using a FlexElement during the opening of
a breaker. One or more FlexElements can be configured for detecting levels of maximum
arcing current during the tripping of a particular breaker, and give an indication for the
health of the breaker.
The base unit for the breaker arcing current is programmed in the relay as: BASE = 2000
kA
2
*cycle
• Input 1(+): Total Arcing Current
• Input 2(-): Off
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Over
• Pickup: 2.500 pu
• Hysteresis: 0.0 %
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