Worked Example

Equipment Selection:

Relay: Siemens 7SS5

CT: 600/1 5P10 15 VA Ri = 4 Ω

CT Leads: 6 mm2 50 m (r=0.0179

Wmm2/m)

Iscc.max = 30 kA

CT Calculation:

From Siemens relay data:

Current Transformer Dimensioning

Posted @ 3/26/2012 6:54 AM by Steven | Files in Power Systems

Correct dimensioning of current transformers is required to ensure satisfactory operation of measuring

instruments and protection relays.

Measuring CTs

Measuring current transformers are normally class 0.5 or 1 for general use. Class 0.1 or 0.2 for precision and

tariff measurements. Typical accuracy limiting factors of 10 and burdens of 5, 10 and 15 VA are common. A

typical specification may be 0.5M10 15 VA.

Protection CTs

Protection CTs provide signals to protective relays to

enable correct operation under steady state and transient

conditions.

IEC 60044 & 60185

IEC 60044 specifies the requirements for protection CTs (in

addition to measuring CT's, VTs and electronic sensors).

Generally accuracy class 5P (1%) is specified with rated

accuracy limiting factors of 10 or 20 being common.

Typically burdens would be 5, 10, 15 or 20 VA. A typical

specification would be 5P10 15 VA.

To correctly select a CT the following dimensioning

formulae can be used:

with

where:

KALF is the rated CT accuracy factor •

K*ALF is the effective CT accuracy limiting

factor

•

RBN is the rated burden resistance •

RBC is the connected burden •

Ri is the internal CT resistance

(secondary winding)

•

Iscc.mzx is the maximum short circuit current •

IN is the rated primary CT current •

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The rated KALF of 10 is greater than 7.6,

therefore stability of the relay is

confirmed.

KOF is an over dimensioning factor •

The over dimensioning factor KOF is dependant on the type

of protection

relay and primary DC time constant. These values are

normally

available in the protection relay manual.

BS 3838

CTs can be defined by the knee point voltage UKN and the

internal secondary resistance Ri. To convert an IEC design

the following can be used:

where:

I2N is the nominal secondary current •

ANSI/IEEE C 57.13

Class C of the ANSI standard defines CTs by their secondary terminal voltage at 20 times nominal current (for

which the ratio error shall not exceed 10%). Standard classes are C100, C200, C400 and C800 for 5 A nominal

secondary current. This terminal voltage can be calculated from the IEC data as follows:

with

and

Connection Leads

The burden of connection leads can be estimated from:

where:

l is the connection lead length in m •

ρ is the resistivity in Ω mm2 m

-1 (=0.0179 for copper) •

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A is the cross sectional area in mm2 •

Related

For an introduction and guidance on how to apply CT sizing, refer the following post:

How to Size Current Transformers

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