ABB 615 Series - Page 439

% Generator load
25
50 75 100
Differential voltage
Bias voltage
If Beta = 1.00, bias voltage
is same as V
3H_N
Magnitude of
3
rd
Harmonic
voltage in
secondary volts
2
4
6
8
If beta = 1.00 then
Security factor K:
25.1
2.1
5.100.1
_3_3
_3






K
VV
VBeta
K
NHTH
NH
Stability margin is 25%.
These should be atleast
30-50%.
Beta = 1.2 recommended
1.5
1.2
GUID-C6CF4531-2EEE-4EE2-AABF-5B177579383E V1 EN
Figure 208: Typical example of variation of the bias voltage and differential voltage
with a change in the active generated load (the angle between the third
harmonic terminal and neutral voltage is 150°)
Calculating CB Open factor
One of the factors, though not major, governing the magnitude of the generated third
harmonic voltage is the generator terminal capacitance. If there is no generator breaker,
the capacitive coupling to ground is the same under all operating conditions. However, the
generator breaker normally exists between the protected generator and its power
transformer.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 433
Technical Manual