Rockwell Automation 865 Differential Protection Relay - Manual (Page 148)

8-6 Applications
865-UM001A-EN-P – July 2009
Equation 8.5
% 100
) (
1
2
0 ⋅
−
=
∫
P
T
P S N
C
I
dt i i K
T
ε
T = Cycle time
K
N
= Rated transformation ratio I
NPrimary
/I
Nsecondary
i
S
= Instantaneous secondary current
i
P
= Instantaneous primary current
I
P
= Rms value of primary current
NOTE: All current based protection functions of Allen-Bradley
relays, except arc protection, thermal protection and 2nd harmonic
blocking functions, are using the fundamental frequency component
of the measured current. The IEC formula includes an RMS value of
the current. That is why the composite error defined by IEC 60044-1
is not ideal for Allen-Bradley relays. However the difference is not
big enough to prevent rough estimation.
Standard accuracy classes
At rated frequency and with rated burden connected, the amplitude
error, phase error and composite error of a CT shall not exceed the
values given in the following table.
Table 8.1 – Values for Standard Accuracy Classes
Accuracy
class
Amplitude error at
rated primary
current
(%)
Phase displacement
at rated primary
current
(°)
Composite error C at
rated accuracy limit
primary current
(%)
5P ±1 ±1 5
10P ±3 - 10
Marking:
The accuracy class of a CT is written after the rated power.
E.g. 10 VA 5P10, 15 VA 10P10, 30 VA 5P20
Accuracy Limit Current I AL
Current transformers for protection must retain a reasonable
accuracy up to the largest relevant fault current. Rated accuracy
limit current is the value of primary current up to which the CT will
comply with the requirements for composite error ε
C
.