ABB 615 series Technical Manual page 437

1MAC059074-MB A
615 series ANSI
Technical Manual
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 Beta
+ ≥
3 H T _ 3 H N _
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Neutral side third harmonic voltage phasor
V
3H N _
Terminal side third harmonic voltage phasor
V
3H T _
Beta
Setting to achieve a required degree of security under healthy conditions
Magnitude of the third harmonic differential voltage
V V
+
3 H T _ 3 H N _
Magnitude of the third harmonic bias (restrain) voltage
Beta ⋅ V
3 _ H N
Factor Beta, which is a setting, can be determined from the condition.
Beta V
⋅
3 H N _
K
=
V V
+
3 H T _ 3 H N _
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K is the security factor, for example K = 1.5.
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
"No Voltage" and 27/59THN operates as a simple neutral side third harmonic
undervoltage protection.
V
⋅
3 H N _
Equation 52
cannot work. In such case, the Voltage selection setting is set to
Section 4
Protection functions
(Equation 51)
(Equation 52)
must be satisfied in the normal,
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