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For Distribution/Industrial Networks
In electrical installations the operating load kW and its average power factor (PF) can be ascertained from the electricity bill. Alternative it can also be easily evaluated by the formula
Average PF = kWh/kVAh
Operating load kW=kVA demand x Average PF
The average PF is considered as the initial PF and the final PF can be suitable assumed as required. In such cases required capacitor kVAr can be calculated as shown in the example
Example
Calculate the required kVAr compensation for 500 kW installation to improve the PF from 0.75 to 0.96.
kVAr = kW x multiplying factor from table 4.2
= 500 x 0.59
= 295 kVAr.
Note
The table (a) is based on following Formula:
KVAr required = kW (tan Ø 1– tan Ø 2)
Where Ø 1=Cos-1 (PF 1) and
Ø 1=Cos-1 (PF 2) and
PF1 and PF2 are initial and final
Power factors respectively
Notes
- It is considered uneconomical in industrial applications to improve power factor by individual compensation for motor ratings below 15hp.
- For motor ratings above 250hp the capacitor kVAr rating would be about 25% of the motor rating in hp.
- In all cases it should be ensured that the capacitor current at rated voltage is always less than 90% of the no load current of the motor. This is due to the fact that when capacitor current exceeds the no load magnetizing current of the motor, excessive voltage surges can occur due to self excitation in the event of an interruption in power supply which will prove harmful to both the motor as well as the capacitor.
- The capacitor kVAr values indicated in the above table are after taking into consideration the condition specified in the item-3 above and assuming motor loading of greater than 80%.
- If the motor is loaded less than 80% the capacitor kVAr required may be greater than the values indicated in the above table. In such case the capacitor should be connected upstream in group or central compensation mode.
Capacitor kVAr for Transformer
Power and distribution transformers, which work on the principle of electron-magnetic nduction, consume reactive power for their own needs even when its secondary is not onnected to any load. The power factor will be very low under such a situation. To improve the power factor it is required to connect a fixed capacitor or a capacitor bank at the LT side of the transformer. The table (b) gives the approximate kVAr of capacitors required.
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Initial PF
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Final PF
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0.9
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0.91
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0.92
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0.93
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0.94
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0.95
|
0.96
|
0.97
|
0.98
|
0.99
|
|
0.4
|
1.807
|
1.836
|
1.865
|
1.896
|
1.928
|
1.963
|
2
|
2.041
|
2.088
|
2.149
|
|
0.42
|
1.676
|
1.705
|
1.735
|
1.766
|
1.798
|
1.832
|
1.869
|
1.91
|
1.958
|
2.018
|
|
0.44
|
1.557
|
1.585
|
1.615
|
1.646
|
1.678
|
1.712
|
1.749
|
1.79
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1.838
|
1.898
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0.45
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1.5
|
1.529
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1.559
|
1.589
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1.622
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1.656
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1.693
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1.734
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1.781
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1.842
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|
0.46
|
1.446
|
1.475
|
1.504
|
1.535
|
1.567
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1.602
|
1.639
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1.68
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1.727
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1.788
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|
0.48
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1.343
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1.372
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1.402
|
1.432
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1.465
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1.499
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1.536
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1.577
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1.625
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1.685
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0.5
|
1.248
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1.276
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1.306
|
1.337
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1.369
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1.403
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1.44
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1.481
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1.529
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1.59
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0.52
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1.158
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1.187
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1.217
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1.247
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1.28
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1.314
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1.351
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1.392
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1.44
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1.5
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0.54
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1.074
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1.103
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1.133
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1.163
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1.196
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1.23
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1.267
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1.308
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1.356
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1.416
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0.55
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1.034
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1.063
|
1.092
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1.123
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1.156
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1.19
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1.227
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1.268
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1.315
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1.376
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|
0.56
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0.995
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1.024
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1.053
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1.084
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1.116
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1.151
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1.188
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1.229
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1.276
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1.337
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0.58
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0.92
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0.949
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0.979
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1.009
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1.042
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1.076
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1.113
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1.154
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1.201
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1.262
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0.6
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0.849
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0.878
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0.907
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0.938
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0.97
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1.005
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1.042
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1.083
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1.13
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1.191
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0.62
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0.781
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0.81
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0.839
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0.87
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0.903
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0.937
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0.974
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1.015
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1.062
|
1.123
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0.64
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0.716
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0.745
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0.775
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0.805
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0.838
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0.872
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0.909
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0.95
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0.998
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1.058
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0.65
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0.685
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0.714
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0.743
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0.774
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0.806
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0.84
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0.877
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0.919
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0.966
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1.027
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0.66
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0.654
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0.683
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0.712
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0.743
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0.775
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0.81
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0.847
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0.888
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0.935
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0.996
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|
0.68
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0.594
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0.623
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0.652
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0.683
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0.715
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0.75
|
0.787
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0.828
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0.875
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0.936
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|
0.7
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0.536
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0.565
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0.594
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0.625
|
0.657
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0.692
|
0.729
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0.77
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0.817
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0.878
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0.72
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0.48
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0.508
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0.538
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0.569
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0.601
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0.635
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0.672
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0.713
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0.761
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0.821
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0.74
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0.425
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0.453
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0.483
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0.514
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0.546
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0.58
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0.617
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0.658
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0.706
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0.766
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0.75
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0.398
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0.426
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0.456
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0.487
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0.519
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0.553
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0.59
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0.631
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0.679
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0.739
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0.76
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0.371
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0.4
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0.429
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0.46
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0.492
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0.526
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0.563
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0.605
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0.652
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0.713
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0.78
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0.318
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0.347
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0.376
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0.407
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0.439
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0.474
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0.511
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0.552
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0.599
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0.66
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0.8
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0.266
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0.294
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0.324
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0.355
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0.387
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0.421
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0.458
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0.499
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0.547
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0.608
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0.82
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0.214
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0.242
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0.272
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0.303
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0.335
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0.369
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0.406
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0.447
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0.495
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0.556
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|
0.84
|
0.162
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0.19
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0.22
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0.251
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0.283
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0.317
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0.354
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0.395
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0.443
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0.503
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0.85
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0.135
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0.164
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0.194
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0.225
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0.257
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0.291
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0.328
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0.369
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0.417
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0.477
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