Micom P721, Technical Manual

Page: 185 / 334

Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 49/52
13. CT REQUIREMENTS
The P72x CT requirements are indicated with respect to the Measurement Filter setting and
the application. The following CT requirements are for busbar and REF/BEF applications
considering the three measurement filter modes.
If the Measurement Filter is set to Fourier mode, the relay is tuned to operate with the
fundamental component of the differential current.
If the Measurement Filter is set to Sample mode, the relay operates when
setting
2 2
I
peak peak
diff
>
×
−
.
If the Measurement Filter is set to Fast sample mode, the relay operates when
setting 2 2 I
eous tan tan ins
diff
× × >
.
The general stability voltage requirement is described by , which
expresses the required stability voltage setting (V
( )
CT L F S
R R 2 I K V
+ × ≥
S
) in terms of an external fault (I
F
), burden
(2R
L
+ R
CT
) and a stability factor (K).
The assumption that one CT is completely saturated for an external fault does not describe
what actually happens when asymmetric CT saturation occurs. The CT that saturates only
saturates during parts of each current wave form cycle. This means that the spill current
waveform seen by the differential element is highly non-sinusoidal. The sensitivity of the
relay element to non-sinusoidal spill waveforms for through faults is a function of the
Measurement Filter setting (Fourier, Sample and Fast sample mode), its operating speed,
the differential voltage or current settings (Threshold V Diff or Threshold I Diff) and the
wave shapes.
The Measurement Filter and the operating speed are factors that are inherent to the relay
design. Spill current wave shapes are related to the ratio of the CT kneepoint voltage (Vk) to
relay circuit impedance. The stability voltage setting,
ST s S
R I V
=
, is determined by the
current setting and the stabilizing resistor. The stability of the high impedance differential
relay during through faults is determined by the ratio V
K
/V
S
. Where V
K
is the CT knee point
voltage and V
S
is the stability voltage setting.
The relationship between the ratio V
K
/V
S
and the required stability factor K has been found to
be a general form for various relay designs that have undergone conjunctive testing by
Schneider Electric. It is the absolute values of V
K
/V
S
and K that vary in the relationship for
different relay designs. The relationship that has been found for the P72x, taking into
account the application and the Measurement Filter setting, are shown in the following
sections.
Once stability has been considered, the next performance factor to take into account is the
operating time for internal faults. The CT kneepoint voltage, as a multiple of the protection
stability voltage setting (V
K
/V
S
), governs the operating time of a differential relay element for
heavy internal faults with transiently offset fault current waveforms. Using the operating time
curves derived for the P72x, it is possible to identify the ratio V
K
/V
S
that is required to
achieve a desired average operating speed for internal faults.
The approach with older electromechanical high impedance relays was to use a universally
safe K factor of 1.0, but the older relays operated quickly with a lower Vk/Vs ratio
(Vk/Vs = 2.0). With more modern relays it is desirable to identify the optimum K factor for
stability, so that the required Vk/Vs ratio for stability and operating speed will not make CT
kneepoint voltage requirements worse than traditional requirements.