ABB 615 Series - Page 437

grounding. For example, the angle is about 145 degrees for a high-resistance-grounded
unit generator.
The equation defines the "operate" and "restrain" regions of the protection. The third
harmonic differential protection operates according to the following equation.
V V V
3 3 3H T H N H N
Beta
_ _ _
+ ≥ ⋅
GUID-6F60D909-1976-4A51-A8F5-97370554C78A V1 EN (Equation 51)
V
3H N_
Neutral side third harmonic voltage phasor
V
3H T_
Terminal side third harmonic voltage phasor
Beta Setting to achieve a required degree of security under healthy conditions
V V
3 3H T H N_ _
+
Magnitude of the third harmonic differential voltage
Beta
H N
⋅ V
3 _
Magnitude of the third harmonic bias (restrain) voltage
Factor Beta, which is a setting, can be determined from the condition.
Beta
K
H N
H T H N
⋅
+
=
V
V V
3
3 3
_
_ _
GUID-57727225-84D4-4B41-B618-05351CE22A7E V1 EN (Equation 52)
K is the security factor, for example K = 1.5. Equation 52 must be satisfied in the normal,
healthy condition of the protected generator, with a high enough value for K, so that no
unwanted operation of the protection should be expected, regardless the load on the
generator.
To assure a reliable functioning of the protection, it is necessary that the
generator produces third harmonic voltage which is at least 1% of the
generator rated voltage.
As phase-to-phase voltages do not contain a third harmonic component, in situations
where the VTs on the terminal side are connected between phase-to-phase, the differential
protection in Equation 52 cannot work. In such case, the Voltage selection setting is set to
"No Voltage" and 27/59THN operates as a simple neutral side third harmonic
undervoltage protection.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 431
Technical Manual