8/11/2019 Why PS Class CT is Used for Diffenential
Protection
1/3
Current transformers (CTs) in a fewwords
Posted MAR 18 2011 b y EDVARD in TRA NS FO RM ER S wi th 7
COMMENTS
Principle of Operation
cross-se ctional view of t he panel and m ultifunctional CT.A
current t ransformer is d ened a s a s a n instrument transformer i
n w hich the secondary curr ent issubstantially p roportional to
the primary c urrent (under normal conditions o f operation) and
differs i n phasefrom it by an angle which is a pproximately z ero
for an appropriate direction of the connections. Thishighlights t
he accuracy requirement of the current transformer but also
important is t he isolating function , which m eans n o m atter w
hat the system voltage t he secondary ci rcuit need to b e
insulated only for a l owvoltage.The current transformer works on
the principle of variable ux. In the ideal current transformer,
secondarycurrent would be exactly e qual (when multiplied by the
turns r atio) and opposite to the primary cu rrent.
But, as i n the voltage transformer, some of the primary c
urrent or the primary a mpere-turns a re utilized formagnetizing
the core, t hus l eaving less t han the actual primary a mpere
turns to be transformed into the
secondary ampere-turns. This naturally introduces a n error in
the transformation. The error i s classied intocurrent ratio error
and the phase error .
Top
Definitions
Typical terms u sed for sp ecifying current transformer are:
Rated primary cu rrentThe v alue o f current which is t o b e t
ransformed t o a lower value. In C T parallence, the load o f the C
T refersto the primary current.
Rated secondary currentThe current in the secondary ci rcuit and
on which the performance of the CT is b ased. Typical values o
fsecondary cur rent are 1 A or 5 A.
Rated burdenThe apparent power of the secondary circuit in
Volt-amperes expressed a t the rated secondary cu rrent andat a s
pecic p ower factor.
http://electrical-engineering-portal.com/current-transformers-in-a-few-wordshttp://electrical-engineering-portal.com/current-transformers-in-a-few-wordshttp://electrical-engineering-portal.com/2011/03http://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/current-transformers-in-a-few-words#commentshttp://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/current-transformers-errors-solutionshttp://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/current-transformers-errors-solutionshttp://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/current-transformers-how-to-specifyhttp://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/current-transformers-how-to-specifyhttp://electrical-engineering-portal.com/transformer-winding-faultshttp://electrical-engineering-portal.com/current-transformers-in-a-few-wordshttp://electrical-engineering-portal.com/current-transformers-in-a-few-wordshttp://electrical-engineering-portal.com/current-transformers-in-a-few-wordshttp://electrical-engineering-portal.com/2011/03http://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/current-transformers-in-a-few-words#commentshttp://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/current-transformers-errors-solutionshttp://electrical-engineering-portal.com/download-center/books-and-guides/schneider-electric/current-transformers-how-to-specifyhttp://electrical-engineering-portal.com/transformer-winding-faultshttp://electrical-engineering-portal.com/current-transformers-in-a-few-words
8/11/2019 Why PS Class CT is Used for Diffenential
Protection
2/3
Composite ErrorThe RMS value of the difference b etween the
instantaneous p rimary cur rent and the instantaneoussecondary
current multiplied by the turns ratio, under st eady state
conditions.
Accuracy limit factorThe value of primary cu rrent up to w hich
the CT compiles with c omposite error r equirements. This i
stypically 5 , 10 or 15, which means t hat the composite error of
the CT has t o be within specied limits a t 5,10 or 15 times the
rated primary cu rrent.
Short time ratingThe value of primary cu rrent (in kA) that the
CT should be able to withstand both thermally a nd
dynamicallywithout damage t o t he w indings w ith the s econdary
ci rcuit being s hort-circuited. The t ime s pecied isusually 1 or
3 s econds.
Class P S/ X CTIn balance systems o f protection, CT s w ith a
high degree of similarity in their characteristics are
required.These requirements ar e m et by Class P S (X) CT s.
Their performance is d ened in terms o f a knee-point voltage
(KPV), the magnetizing current (Image) at theknee point voltage or
1/2 or 1/ 4 the knee-point voltage, and the resistance of the CT
secondary w indingcorrected t o 75C. Accuracy is d ened in terms o
f the t urns r atio.
Knee point voltageThe point on the magnetizing curve where a n
increase o f 10% in the ux density (voltage) causes anincrease of
50% in the m agnetizing force (current).
Summation CTWhen the currents in a number of feeders ne ed not
be individually metered but summated to a single m eteror
instrument, a su mmation current transformer can be used.The
summation CT consists of two or moreprimary w indings w hich are c
onnected to the feeders t o b e su mmated, and a single se condary
winding,which feeds a c urrent proportional to the summated primary
curr ent.
A typical ratio would b e 5 +5+5/ 5A, which m eans t hat three
primary f eeders o f 5 a re t o b e su mmated to asingle 5 A
meter.
Core balance CT (CBCT)The CBCT, also k nown a s a zero se quence
CT, is us ed for ear th leakage a nd earth fault protection.
Theconcept is s imilar t o the RVT. In the CBCT, the three core
cable or t hree single cores of a three phasesystem pass t hrough
the inner di ameter of the CT.
When the system is f ault free, no current ows i n the secondary
of the CBCT. When t here is a n e arth fault,the residual current
(zero p hase sequence current) of the system ows t hrough the
secondary o f the CBCTand this o perates t he relay. In order to
design the CBCT, the inner diameter of the CT, the relay type,
therelay s etting a nd t he p rimary o perating c urrent need t o
be f urnished.
Interposing CTs ( ICTs)
