Micom P721, Техническое руководство

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 11/52
V
k
/V
s
, where V
k
is the Knee point voltage of the CT. The K factor is clearly indicated in the
CT requirements section at the end of this chapter.
To obtain a high speed operation for internal faults, the Knee point voltage V
k
of the CT must
be significantly higher than the stability voltage V
s
. A ratio of 4 or 5 would be appropriate.
The operating times and K factors for V
K
/V
S
≤
16 are indicated in the CT requirements
section at the end of this chapter.
NOTE: The maximum internal fault level for the single phase CT input in the
P721 and P723 must not exceed 20In.
The current transformers used in a high impedance scheme should be of equal ratio, of
similar magnetizing characteristics and of low reactance construction. In cases where low
reactance current transformers are not available and high reactance ones must be used, it is
essential to use the reactance of the current transformer in the calculations for the voltage
setting. Thus, the current transformer impedance is expressed as a complex number in the
form R
CT
+ jX
CT
. It is also necessary to ensure that the exciting impedance of the current
transformer is large in comparison with its secondary ohmic impedance at the relay setting
voltage.
Once the setting is determined, the scheme sensitivity can be calculated by considering the
relay operating current reflected to the primary plus the excitation current of the CTs
connected in parallel, whether carrying primary current or not.
FIGURE 9: HIGH IMPEDANCE DIFFERENTIAL SCHEME SENSITIVITY
The primary operating current for the scheme can be calculated as follows:
( )
e S P
nI I N I
+ × =
Where:
I
P
= Primary operating current
N = CT ratio
I
S
= relay operating current at setting
I
e
= excitation current at setting voltage (obtained from the CT magnetizing curve). The
setting voltage is calculated as V = Is
×
(R
ST
+ R
r
). R
ST
is the stabilizing resistor and R
r
is the
relay burden which is neglected in the P72x.
N = number of CTs connected in parallel
If the relay setting voltage is high, the CT magnetizing current at the setting voltage summed
for all the CTs can be significant.
In REF/BEF applications, typical setting values are chosen to provide a primary operating
current, I
P
, at 10 to 25% of the minimum earth fault level for a resistance earthed system.
Also in REF/BEF applications, for a solidly earthed system, the typical setting provides a
primary operating current of between 10 and 60 % of the winding rated current. In busbar
applications it is desirable that the primary operating current does not exceed 30% of the
minimum fault level. In busbar applications, it is also common to set the relay so that the
primary operating current is 110 to 130% of the maximum feeder circuit loading.