GE 845 Instruction Manual page 439

CHAPTER 4: SETPOINTS
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845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
Pickup = 0.5MVA / 15.936 MVA = 0.0313 pu
FlexElement settings:
• Input 1(+): Pwr1 Apparent
• Input 2(-): Off
• Operating Mode: Absolute
• Input Comparison Mode: Level
• Direction: Under
• Pickup: 0.0313 pu
Power Factor Cap Bank Switch-In Example
PF = 1.00
BASE
FlexElement can be programmed to switch-in cap bank, if for example the measured 3Ph
Power Factor has negative value(lag), and drops below the pickup of -0.7 pu. Programming
the Hysteresis setpoint to the desired percentage can define the PF value at which the cap
bank can be switched off. For example, if the cap bank is required to be switched off at PF
value of -0.9, than the percent hysteresis is computed as:
% 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
FLEXLOGIC
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