The CTs according to class C must have a calculated rated equivalent limiting
secondary e.m.f. E
alANSI
that fulfills the following:
alANSI alreq
E max imum of E>
EQUATION1384 V1 EN (Equation 14)
A CT according to ANSI/IEEE is also specified by the knee-point voltage V
kneeANSI
that is graphically defined from an excitation curve. The knee-point voltage V
kneeANSI
normally has a lower value than the knee-point e.m.f. according to IEC and BS.
V
kneeANSI
can approximately be estimated to 75 % of the corresponding E
al
according
to IEC 60044 6. Therefore, the CTs according to ANSI/IEEE must have a knee-point
voltage V
kneeANSI
that fulfills the following:
E
kneeANSI
> 0.75 · (maximum of E
alreq
)
EQUATION2101 V1 EN (Equation 15)
2.2 Voltage transformer requirements
The performance of a protection function will depend on the quality of the measured
input signal. Transients caused by capacitive Coupled voltage transformers (CCVTs)
can affect some protection functions.
Magnetic or capacitive voltage transformers can be used.
The capacitive voltage transformers (CCVTs) should fulfill the requirements according
to the IEC 60044–5 standard regarding ferro-resonance and transients. The ferro-
resonance requirements of the CCVTs are specified in chapter 7.4 of the standard.
The transient responses for three different standard transient response classes, T1, T2
and T3 are specified in chapter 15.5 of the standard. CCVTs according to all classes
can be used.
The protection IED has effective filters for these transients, which gives secure and
correct operation with CCVTs.
2.3 SNTP server requirements
The SNTP server to be used is connected to the local network, that is not more than 4-5
switches or routers away from the IED. The SNTP server is dedicated for its task, or at
least equipped with a real-time operating system, that is not a PC with SNTP server
1MRK504116-UUS C Section 2
Requirements
29
Application manual
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