Electrical Protection (Current & Potential Transformer)
The current transformer is well established but is
generally regarded merely as a device, which reproduces a primary current at a
reduced level. A current transformer is designed for measuring purposes
operates over a range of current up to a specific rated value, which usually
corresponds to the circuit normal rating, and has specified errors at that
value. On the other hand, a protection current transformer is required to
operate over a range of current many times the' circuit rating and is
frequently subjected to conditions greatly exceeding those which it would be
subjected to as <i measuring current transformer. Under such conditions the
flux density corresponds to advanced saturation and the response during this
and the initial transient period of short-circuit current is important.
It will be appreciated, therefore, that the method of specification of current transformers for
measurement purposes is not necessarily satisfactory for those for protection.
In addition an intimate knowledge of the operation current transformers is
required in order to predict the performance of the protection. Current transformers
have two important qualities:
1. They produce the primary current conditions at a much lower level
so that the
current can be carried by
the small cross-sectional area cables associated with panel wiring and relays.
2. They provide an insulating barrier so that relays which are
being used to protect high voltage equipment need only be insulated for a
nominal 600V.
Potential
transformers
establish a known ratio between the potential difference across the equipment and the potential difference
across a voltmeter, For example,
suppose a potential transformer establishes a ratio of 500 to one. If a
voltmeter connected across the secondary terminals reads 132 volts, you know
the potential difference across the primary terminals is 66,000 volts.
Current
Transformer Design
A current transformer can
take one of two forms. One type is wound like an ordinary transformer, with
primary and secondary windings round a common core. As a CT steps current down,
it steps voltage up. The primary winding, though connected in the system's
high-voltage system, is in fact the LV (high current) winding as far as the
transformer is concerned, and the secondary is the HV (low current) winding. Wound-primary CTs are used where the primary current is low
and where it is necessary to have several primary turns to achieve enough
ampere-turns in the CT. The examples shown in Figure I (a) and (b) are
typical; burdens are in the range 5 to 30VA per phase. Wound-primary CTs must
be able to withstand the full voltage and fault current of the main system on
their primary'
windings.
