The current transformer rated current ratio shall match the connected load circuit and secondary circuit requirements.
Current transformers shall be capable of withstanding without
damage the full load, peak and rated short time currents of their associated
equipment.
Where space within a current transformer chamber permits dedicated
current transformers shall be used for protection, instrumentation and
metering.
Current Transformer Class Section for Uses
Current transformers used for energizing indicating instruments
shall be of Class 1 accuracy and for energizing integrating meters of Class 0.5
accuracy in accordance with IEC
185. Current
transformers for generator low forward power interlock relays shall be Class
0.1 accuracy. Accuracy class of current transformers for tariff metering shall
be Class 0.2.
Current transformers for protective and protective/indication
purposes shall be designed to suit the particular requirements of the
associated protection, which in general shall be in accordance with the
recommendations given in BS
3938 or approved equivalent.
Class 5p current transformers shall be used for inverse time
over-current and/or earth fault protection. The rated accuracy limit current
shall be equivalent to the maximum symmetrical three phase fault current or
earth fault current of the protected circuit or equivalent to the switchgear
breaking capacity unless otherwise approved by the Engineer.
The current transformers shall be capable of meeting the 5p error classification at rated accuracy limit current over the full range of relay settings, unless otherwise approved by the Engineer.
The current transformers shall be capable of meeting the 5p error classification at rated accuracy limit current over the full range of relay settings, unless otherwise approved by the Engineer.
Current Transformer Using Purposes
Current transformers used for indication/metering purposes shall
be designed to saturate at a value of primary current sufficiently low to
protect the secondary circuit from damage at all possible values of primary
fault current up to the associated primary short time thermal rating.
Current transformers for combined purposes (e.g. protection relays
and indicating meters) shall have a dual Class 5p/Class 1 performance, and the
secondary circuit shall have an approved means (saturating reactor or
saturating interposing C.T.) of protecting the meters and reducing their burden
under system fault conditions.
Current transformers for protection using high impedance relays
shall be of the low reactance type and their performance shall be stated in
terms of the Class X parameters of BS 1938. (Low reactance current transformers
may be shown to be low reactance by virtue of their construction as defined in
Clause 4.4.2.21. of BS 3938.
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Current Transformer Test Required
If all the constructional requirements are not met then type tests
will be required to prove that the current transformers are low reactance; the
primary test current shall not be less than the through fault stability current
of the protection scheme.
The rated volt-amp output of each current transformer shall not be
less than 110% of the connected burden as installed in service, the burden of
cable connections being taken into account.
The secondary windings of each set of current transformers shall
be earthed at one point only via an accessible bolted disconnecting link,
preferably located within the relay cubicle.
Where double-ratio secondary windings are specified provided a
label shall be provided at the secondary terminals of the current transformer
indicating clearly the connections required for either tap. The connections and
the ratio in use shall be indicated on all connection diagrams.
Design magnetization curves and d.c resistance values shall be
submitted before manufacture for each current transformer used for protective
purposes and shall be subsequently verified by works routine tests and also by
site commissioning tests.
Current Transformer Installation
Where current transformers have to operate or be mounted on
apparatus provided under other contracts, the Contractor shall be responsible
for ensuring design and installation compatibility with other Contractors and
for keeping the Engineer informed.
Metal clad switchgear current transformers shall be located on the
non-bus-bar side of the circuit breaker except where current transformers are
provided on both sides of the circuit breaker for protection zone overlap. The
primary conductors shall be accessible for primary current injection treating
on site.
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