U U Beta U
H T H N H N3 3 3
0
_ _ _
+ − × =
(Equation 83)
The third harmonic voltages Ū
3H_T
and Ū
3H_N
are the phasor with its real and
imaginary parts. Ū
3H_T
is approximately in the opposite direction to that of the
Ū
3H_N
, however the actual no-fault angle between those two phasors depends on
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 U Beta U
H T H N H N3 3 3_ _ _
+ ≥ ×
(Equation 84)
Ū
3H_N
Neutral side third harmonic voltage phasor
Ū
3H_T
Terminal side third harmonic voltage phasor
Beta
Setting to achieve a required degree of security under healthy conditions
|Ū
3H_T
+Ū
3H_N
| Magnitude of the third harmonic differential voltage
Beta
·|Ū
3H_N
| Magnitude of the third harmonic bias (restrain) voltage
Factor
Beta
, which is a setting, can be determined from the condition.
(Equation 85)
K is the security factor, for example K = 1.5.
Equation 85
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 85
cannot work. In such case, the
Voltage selection
setting is set to "No Voltage" and H3EFPSEF operates as a simple
neutral side third harmonic undervoltage protection.
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.
Calculating Beta value
The setting
Beta
gives the proportion of the third harmonic voltage in the neutral
point of the generator to be used as bias quantity.
Beta
must be set so that there
Protection functions
1MRS759142 F
548 REX640
Technical Manual
To Next Page
Loading...
