Voltage Transformer Requirements - ABB REL650 Applications Manual

Section 18
Requirements
18.2
498
The CTs according to class C must have a calculated rated equivalent limiting secondary
e.m.f. E that fulfils the following:
alANSI
>
E max imum of E
alANSI
EQUATION1384 V1 EN
A CT according to ANSI/IEEE is also specified by the knee point voltage V
is graphically defined from an excitation curve. The knee point voltage V
normally has a lower value than the knee-point e.m.f. according to IEC and BS. V
can approximately be estimated to 75 % of the corresponding E
61869-2. Therefore, the CTs according to ANSI/IEEE must have a knee point voltage
V that fulfills the following:
kneeANSI
> 0.75 · (maximum of E
E
kneeANSI
EQUATION2101 V1 EN
The following guide may also be referred for some more application aspects of ANSI class
CTs: IEEE C37.110 (2007), IEEE Guide for the Application of Current Transformers
Used for Protective Relaying Purposes.

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 61869-5 standard regarding ferro-resonance and transients. The ferro-resonance
requirements of the CCVTs are specified in chapter 6.502 of the standard.
The transient responses for three different standard transient response classes, T1, T2 and
T3 are specified in chapter 6.503 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.
alreq
)
alreq
1MRK 506 334-UUS A
(Equation 188)
that
kneeANSI
kneeANSI
kneeANSI
according to IEC
al
(Equation 189)
Application manual