ABB RELION REX640 Technical Manual page 548

Protection functions
U
3 H T
The third harmonic voltages Ū
imaginary parts. Ū
Ū
3H_N
the type of generator earthing. For example, the angle is about 145 degrees for a
high-resistance-earthed unit generator.
The equation defines the "operate" and "restrain" regions of the protection. The
third harmonic differential protection operates according to the following equation.
U
3 H T
Ū
3H_N
Ū
3H_T
Beta
|Ū
3H_T
Beta ·|Ū
Factor
K is the security factor, for example K = 1.5.
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.
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
Voltage selection setting is set to "No Voltage" and H3EFPSEF operates as a simple
neutral side third harmonic undervoltage protection.
Calculating Beta value
The setting
point of the generator to be used as bias quantity.
548
U Beta U
+ − ×
_ 3 H N _
is approximately in the opposite direction to that of the
3H_T
, however the actual no-fault angle between those two phasors depends on
U Beta U
+ ≥ ×
_ 3 H N _
Neutral side third harmonic voltage phasor
Terminal side third harmonic voltage phasor
Setting to achieve a required degree of security under healthy conditions
+Ū | Magnitude of the third harmonic differential voltage
3H_N
| Magnitude of the third harmonic bias (restrain) voltage
3H_N
Beta , which is a setting, can be determined from the condition.
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.
When H3EFPSEF is reduced to function as only a third harmonic neutral
point undervoltage protection, it is necessary to block the function
during start-up and shutdown of generator and also when there is no
sufficient voltage.
Beta gives the proportion of the third harmonic voltage in the neutral
0
=
3 H N _
and Ū are the phasor with its real and
3H_T 3H_N
3 H N _
Equation 85
Equation 85
cannot work. In such case, the
1MRS759142 F
(Equation 83)
(Equation 84)
(Equation 85)
must be satisfied in the
Beta must be set so that there
Technical Manual
REX640