Capicitor Application Issues
Capacitors must be built to tolerate voltages and currents in excess of their ratings according to standards. The applicable standard for power capacitors is IEEE Std 18-2002, IEEE Standard for Shunt Power Capacitors.
Heat as one of most common cause of motor failure
This slide speaks about that how motor operation fails due to heat. how heat affect motors?
Tuesday, 13 May 2014
Restricted Earth Fault Protection
The windings of many medium and small sized transformers
are protected by restricted earth fault (REF) systems. The illustration shows
the principal of REF protection.
Basic Operation
Under normal conditions and by application of Kirchhoff’s laws
the sum of currents in both current transformers (CTs) equals zero. If there is
an earth fault between the CTs then some current will bypass the CT's and the
sum of currents will not be zero. By measuring this current imbalance faults
between the CTs can be easily identified and quickly cleared.
Fault detection is confined to the zone between the two CTs
hence the name 'Restricted Earth Fault'.
REF protection is fast and can isolate winding faults extremely
quickly, thereby limiting damage and consequent repair costs. If CTs are
located on the transformer terminals only the winding is protected. However,
quite often the secondary CT is placed in the distribution switchboard, thereby
extending the protection zone to include the main cable.
Without REF, faults in the transformer star secondary winding
need to be detected on the primary of the transformer by the reflected current.
As the winding fault position moves towards the neutral, the magnitude of the
current seen on the primary rapidly decreases and could potentially not be
detected (limiting the amount of winding which can be protected). As the
magnitude of the currents remain relatively large on the secondary
(particularly if solidly earthed), nearly the entire winding can be protected
using REF.
It
should be remembered that the protection as illustrated covers only the
secondary of the transformer. Sometimes REF protection is added to the primary
as well (although if primary protection is required I would prefer to consider
full differential protection).
As
it is essential that the current in the CTs be balanced during normal
conditions (and through faults), historically REF has been implemented using
High Impedance Relays. CT's have also been specified as matched pairs and the
impedance of leads/wires and interconnecting cables has had a large influence
on the functioning of the relay. Measurement errors associated with these
issues have been responsible for nuisance tripping and the system can be
difficult to commission. This may be the reason some people avoid the use of
REF. Recent advances in numerical relay technology have all but eliminated
these issues, making the implementation of REF relatively easy, ensuring no
nuisance tripping and simplifying commissioning.
ANSI Designation
Often 87N or 64REF are used to denote restricted earth fault
protection. The intent is that both mean the same thing.
Which is the correct or better to use may be open to
interpretation.
To me, 87N is more appropriate as REF is a differential type
protection (ANSI 87 – differential protection relay). I think 64 is strictly
more relevant to detecting earth/ground faults due to breakdown of insulation.
What is restricted earth fault relay protection and How is it provided?
A restricted Earth Fault (REF) means an earth fault from a
restricted/localized zone of a circuit. The term “REF protection method” means
not to sense any earth faults outside this restricted zone. REF is a type of “Unit
Protection“ applied to transformers or generators and is more sensitive than
the method known as differential protection. An REF relay works by measuring
the actual current flowing to earth from the frame of the unit. If that current
exceeds a certain present maximum value of milliamps (mA) then the relay will
trip to cut off the power supply to the unit. Differential protection can also
be used to protect the windings of a transformer by comparing the current in
the power supply’s neutral wire with the current in the phase wire: if the
currents are equal then the differential protection relay will not operate; if
there is a current imbalance then the differential protection relay operates.
However, REF protection is also applied to transformers in order to detect
ground faults on a given winding much more sensitively than differential
protection can do.
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