The use and specification of Current Transformers

Current transformers are used to measure high currents; higher than 5A. So the most important parameter in defining a CT is indeed the Ratio that gives us the Magnetude of primary current and the secondary current. But for the following specifications of the current transformer, the purpose of the CT is needed since measuring CT's and Protection CT's require different specifications. Indeed, there will be two mayor groups of Current Transformers:

Protection current transformers Measurement current transformers

Regarding specification, different standards have different ways in specifying CT's but it all comes down to specifying core property's (saturation point or knee-point) and secondary wire property's (RCT) although it may look a totally different.

Protection CT's

Figure 6. The ratio of a protection CT as a function of the current

Protection CT's:

are meant to protect an elektrical installation in case of overcurrent or short circuit and their operating current range is above nominal current In or more specific from In to ALF times In. It is important for the good functionning of the protection relays that the CT's are NOT saturated at ALF times rated current. Where ALF is the ratio of the expected maximum fault current over the rated current. It is thus important that the core material has a high saturation induction.

their accuracy is not very high but most important is that the accuracy in fault conditions is high enough. This can only be the case when the core is not saturated in case of a fault current. Therefore their accuracy is best described with an Accuracy Limit and an Accuracy Limit Factor (ALF).E.g. a 5P20 CT has an Accuracy limit of 5% at 20 times rated current (Accuracy Limit Factor). The accuracy of this CT at rated current is 1%.

They will be connected to one or more protection relays according the application, they can be defined in a few ways:

o The standard IEC protection class CT's are of class "P" that only takes the AC behaviour into account in IEC 60044-1

o Class PX CT's are defined by the position of the knee-point (saturation point or knee-point voltage and magnetising current) and the secondary wire resistance RCT.

o Class PR CT's are defined like the PX CT's but they have a low remanence; less than 10%. Note that remanence in CT's can be 60-80% that may cause quick saturation in case of a fault-current DC offset in the remanent direction. A class PX CT can't have that problem.

o CT's for transient response class "TP" are defined by their connected load R B, time constant TS and their overcurrent figure KSSC. These linearised CT's have air-gaps in the core to obtain extreme high saturation voltage and current.

Figure 7. The error limits of a protection CT per IEC 60044-1

Ex. A 5P10 CT at 10 times rated current has a maximum error of 5% and only 1% at nominal current. A 10P15 CT at 15 times rated current has a maximum error of 10% and 3% at nominal current.

Measurement CT's

Figure 8. The ratio of a measurement CT as a function of the current

Are aimed to measure accurately within their normal operating range of 0 to In. Therefore, the core material must have a high permeability (µ-metal) so that the magnetising current is low.

Measurement CT's are often being used for billing of electrical power consumption and their accuracy is determinent for a lot of money.

For the protection of the measuring instruments in case of a fault current, it is favorable that for currents far above rated current In, the core is saturated and the output lowers so that the fault-current trough the meter is only a part of the expected current trough the meter. This is expressed by the Instrument Security Factor SF. Of course, the dilemma is that the CT must be accurate at In (and 1,2 x In) but at f.i. 5 times rated current ( ) the CT may be saturated for at least 10%.

The accuracy of a measurement CT is given by it's accuracy class that corresponds to the error% at rated current and at 1.2 times rated current In. The standard accuracy classes

according IEC are class 0.2, 0.5, 1, 3 en 5. For classes 3 and 5, no angle error is specified. The classes 0.2S and 0.5S have their accuracy shifted toward the lower currents. This means that they have 5 measuring points instead of 4 (or 2 for class 3 & 5).

Figure 9. The error for measurment CT's according IEC 60044-1

The accuracy of the CT must be within these limits at the given currents and with rated load and at 1/4 of the rated load. A measurement CT that is not loaded is therefore not necessary accurate! Ratio turn correction may have been applied to get the CT ratings witthin spec and then not loading gives a higher error.

Kesimpulan

a. Perbandingan tingkat kelas ketelitian untuk CT 0.5S adalah mampu mengukur beban rendah sampai 1% sedangkan CT 0.5 tidak dapat mengukur. 

b. Pada beban rendah 1%, 5% dan 20% pergeseran sudut dan waktu untuk CT0.5S lebih kecil dibandingkan CT 0.5(lihat table 1 & 2)

c. Presentase batas kelas ketelitian pengukuran untuk CT 0.5S sama dengan persentase batas kelas ketelitian meter pada beban 20%.

d. Kerugian akibat selisih pengukuran energi listrik karena perbedaan kelasketelitian kWH Meter 0.5S dan CT 0.5 terpasang dapat ditekan bilamenggunakan CT 0.5S.