Sizing of Current Transformer

8/10/2019 Sizing of Current Transformer

1/6

Sizing of Current Transformers

Dilson Subedi(1)

, Tandin Dendup(1)

, Deepen Sharma(1)

(1)Centre of Excellence in Control & Protection, Tala Hydropower Plant, Druk Green Power Corporation Limited

Abstract: Current transformers (CTs) are used to transform large primary currents to a small secondary current.Since most standard equipments are not designed to handle large primary currents, the CTs have an importantrole in any electrical system for the purpose of Metering and Protection, both of which are integral part in any powersystem. The proper sizing of CTs (also called Dimensioning of CTs) therefore becomes very important for thesystem designers as it will play an integral role in metering (which is money) as well as protection (which isreliability) of the equipment.

This paper outlines the basic parameters to be considered for a CT when designing a protection system andmetering Circuit. It will also outline the basic nature of the Protective Class (P) and Special Protective Class (PS) ofCT's. The trend these days is to upgrade the protection system from electromechanical relays to Numerical relayswithout usually changing the Current transformers. The electromechanical relays thus having high impedance ascompared to it equivalent numerical relays therefore drastically reduces the overall burden on the CT's; The effectof under burdening of CTs owing to such an up-gradation from electromechanical to numerical relaying will also bediscussed, along with available methods for burden compensation and their disadvantages

1 Introduction

As the development of digital measurement and protection equipment has progressed over the last

years, the criteria used for sizing the necessary instrument transformers have changed as well.

Whereas in the past, due to the high burden of electromechanical relays, it was the rated power of the

current transformers (CT) and potential transformers (PT) that was the crucial parameter. Nowadays, it

is the transient performance of instrument transformers that has gradually become the over-riding

influence within the digital world of relays, measuring and controlling devices.

Firstly, due to paradigm change in the technology of the power system substations the tradit ional usage

of high VA-rated instrument transformers can become even dangerous both for themselves and for the

secondary circuits and equipment connected to them. Secondly, the reduction of the switchgear

dimensions, especially Gas Insulated Switchgear (GIS), leads to a reduction of the available instrumenttransformer compartments. That is the reason why the volumes of the instrument transformers have to

be optimized and adapted to the actual needs of modern measurement and protection equipment

connected to them. Since most of the generating stations in Bhutan have reached a stage where the

up-gradation of its protection equipment is of utmost importance keeping in view the growing power

system infrastructure. Therefore knowing the usage of dimensions of CT will be essential information

while planning such up-gradation.

2 Current Transformer Specifications

While ordering a Current Transformer from a manufacturer we must provide them with certain system

data as to enable them to design. Apart from the nominal system data like primary current (IP) &

secondary current (IS), it is also advisable to specify the maximum short circuit current ( IPSC, MAX) at theCurrent Transformer location, the desired nominal over current factor (KSCCN) and the Burden that would

be connected to the Current Transformer (SBN). The over current factor is also called as Accuracy

Limiting Factor (ALF) for Protective class Current Transformers and Instrument Security Factor (ISF) for

Metering class Current Transformer. The importance of specifying the above will be discussed in the

following sections.

In turn the manufacturer will give the Knee point voltage (VKNEE), Secondary Coil resistance (RCT) and

the nominal burden to be connected to the Current Transformer in its name plate details. These details

will be essential when we think about upgrading the instrument connected to the Current Transformer.

2.1 Calculation of Current Transformer Specifications

For the purpose of keeping a mathematical basis for specifying the Current Transformer in the system

we must first find out the nominal primary current IP. Once this is known we must decide on the

secondary current IS. Generally there are two options for standard secondary currents viz. 5 amperes


2/6

and 1 ampere. In olden days when electromechanical relays were used higher currents were required to

operate the relay and therefore 5 ampere CT was used, now a days with the development of digital

technology all of the protection functions can be achieved by 1 ampere. This will give us our CT ratio.

Secondly we must know the type of instrument that will be connected to the CT i.e. protection

instrument or the metering equipment, since this will affect the over current factor of the CT.

Once these values are known we can now proceed towards designing a CT.

First we must ensure that the CT should not saturate during normal operation;

Therefore;

VKNEE> KSCCN.IFS, MAX(RCT + RBN), (1)

Where;

IFS, MAX:is the maximum secondary fault current;

KSCCN= IPSC, MAX/ IPN, (2)

RBN= Rated burden in ohms.

The over current factor will be different depending upon the application for which the CT is used.

For metering application, the CT should operate with minimum error in the nominal region and should

saturate on fault conditions so that the connected metering equipment is not damaged, hence the value

of KSCCNfor metering CT also called Instrument Security Factor is very low.

For protection application, the CT should have minimum error in faulted conditions for the operation of

the connected protection equipment as well as for event recording in case of faults. Therefore the KSCCN

value also called Accuracy Limiting Factor will be higher for protective CT. Although the KSCCNvalue is

higher which will allow higher current to flow in the CT secondary the CT will eventually saturate for very

high fault currents. This in turn will protect the connected equipment from very high fault currents.

3 Current Transformer Operation

A CT due to its physics always tries to draw such a secondary current ISthrough its secondary circuit

that equalizes the magnetic flux excited by the primary current IP. It means that each current transformer

is forced to introduce such a secondary current ISso that the secondary magnetic flux linked with itequalizes at every point of time with the primary flux.

Ideally there should be no magnetizing flux is inside of an ideal CT core, or, in other words, ideal

working-conditions for a CT are given when its core is fully balanced and no magnetic flux is present. In

reality, there are no ideal conditions as described above. There exists always some secondary burden

as resistance or impedance, e.g. at least the inner secondary winding burden, which causes a voltage

drop in the secondary circuit.

Practically, the current transformer during its duty of core-balancing by drawing the secondary current

through the secondary circuit always has to overcome a couple of burden. In other words it is forced to

magnetize itself to produce such a voltage (on the inductance L) that draws the secondary

ampere-turns current which equalizes the ampere-turns of primary current. Such burden for the CTsare internal impedance of the secondary winding and the total impedance that is connected to its

secondary clamps (i.e. wire and instrument burden). Thereby, the higher the burden or the higher the

primary current, the higher voltage must be induced to allow the secondary current flow.


3/6

The maximum detectable short-circuit current IPSC, MAXon the CT secondary side must not lead to a

voltage drop higher than the knee point voltage in order to avoid too high error of the secondary current

due to magnetic saturation. Furthermore, the limit of the magnetizing current may be required to ensure

enough fault sensitivity.

3.1 Current Transformer LimitationsThe most important issue is to keep in mind that the exactness specified at IPNcan only be guaranteed

by the CT manufacturer, when the CT is burdened with rated burden SBN.

1. Accuracy (current error and phase displacement) is specified for more working points that lie not only

at the nominal current IPNbut start below it up to slightly above the nominal current (e.g. at 1%, 5%, ... up

to 120% of IPN).

2. Instead of overcurrent factor, an instrument security factor (ISF) or an Accuracy Limiting Factor (ALF)

is specified, at which the composite error must be at least 10%. This is to protect the connected

measuring devices against too high over-currents. Thereby, the most important are the following

remarks:

a) The accuracy class of metering CT can be only maintained when the CT is burdened with operational

burden SBOthat lies between 25%... 100% of its nominal burden SBN, and

b) The nominal instrument security factor ISFNor nominal Accuracy Limiting Factor ALFNis defined at

nominal working conditions of the CT, i.e. when the CT is burdened with its nominal burden SBN.

Remembering the fact that the error limits for protective classes are maximum values, the actual errors

have to be lower. This is a minimum requirement for dimensioning of protective class CTs and it can be

also understood as forbidden area for the error of such protective-core CT.

For the instrument security factor ISF the error must be at least 10% in order to protect measuring

devices against too high over currents by limitation of the CT secondary current. This is a maximum

requirement for dimensioning of measuring class CTs.

Acc. to IEC 60044-1 standard the winding resistance RCT is usually not given on the rating plate

because this calculation is done by the CT manufacturer only.

3. For the performance the CT magnetizing curve should be always analyzed. So the usage of

resistances and voltages is the right way. On the contrary, the usage of power (burden) and overcurrent

factor can be misleading.

4 Under burdening of CT

Nowadays, modern digital protection devices work with 1A nominal current and have burdens lower

than 0.1 VA so only the burden of secondary wiring is the most significant one. Maximum operationtimes have been shortened to a few milliseconds up to a few cycles, so the DC component must be now

considered by an additional transient dimensioning factor KTD.

Therefore the over current factor is now calculated as:

KSCCN= KTD. IPSC, MAX/ IPN, (3)

However if a low burden is connected to a high rated burden CT, as is the case these days due to

up-gradation of connected equipment without changing the CT, then the over current factor KSCCNwill

not be valid for that circuit as it is dependent on the burden connected. The new operational overcurrent

factor KSCCOcan be calculated as:

VKNEE=KSCCN. IFS, MAX. (RCT+RBN)=KSCCO. IFS, MAX.(RCT+RBO), (4)

Where


4/6

RBO= Connected burden in ohms

Since knee point voltage VKNEEwill remain same on different connected burden.

Simplifying we get

KSCCO= KSCCN. (RCT+RBN)/ (RCT+RBO) (5)

The above equation shows that the new overcurrent factor will be very high. This high over current

factor will expose the connected equipment to high secondary current which is dangerous to both the

equipment and the operator.

Under burdening will also have the following effect:-

1. Using under-burdened, protective CT with turn- compensation and measuring CT, can exceed the

error at nominal current (the error curve is lower for lower burdens than nominal burden).

2. Using under-burdened CTs with small current ratio can damage the connected relays in case of

strong close-in fault (very high secondary current that flow on the CT secondary due to high KSCCO).

These two critical and very important facts have to be considered in the same way for the instrument

security factor ISF for measuring classes, too. The operating factor of security ISFO for the

under-burdened measuring CT can be calculated:

ISFO= ISFN. (RCT+RBN)/ (RCT+RBO), (6)

As already mentioned above, for measuring classes, the new error curve may exceed also the accuracy

limit for nominal currents.

5 Burden Compensation

According to IEC 60044-1 the error limits for CTs are required to be valid only for burden that is

between 25% and 100% of its nominal burden SBN. This requirement leads very often to additionally

installed resistors in order to compensate too low burdens, which is surely a dangerous practice of

introducing new elements into the secondary circuit of a CT and should be abandoned. Such additional

resistive elements introduce a risk as it may be damaged by over currents and lead to interruption of the

CT secondary circuit.

6 Case Study- Under Burdened CTs

220kV Switch Yard, Maintenance Division, BHP

CT TYPE: IMB 245; EQUIPMENT: Current Transformer

LOCATION: 220 kV Rurichhu-Tsirang Feeder

Table 1.NAME PLATE DATA

CORES I II III IV V

IPrimary 600-300-150 600-300

ISec 1 1 1 1


5/6

Accuracy Class 5P 5P 0.5 5P 5P

Rated Burden 20 20 30 20 20

Rated Burden 20 20 30 20 20

ALF/ISF 30 30


6/6

measurements are kept high for metering as well as protection functions. The use of burden

compensation methods for cost saving on current transformers is not advisable as it will raise a question

of integrity for the circuit during faulted condition when the ISis very high.

8 References

[1] Jaeger et al. D.T. 2008. Instrument Transformer Dimensioning: Past and Future. 4th GCC CIGREInternational Conference, Manama (Bahrain).

[2] Ibrahim et al. D.T. 2006. Saturation of current transformers and its impact on digital over current

relays. 2006 IEEE PES transmission and distribution conference and exposition, Latin America,

Venezuela.

[3] CT Test Report 220KV Rurichhu-Tsirang Feeder CT, CoECaP,2013.

9 Biographies

Dilson Subedi holds a Bachelors of Engineering in Electronics and Communication

Engineering from PSNA College of Engineering and Technology under Anna

University, Chennai in India. He joined Druk Green in 2010 and worked in ElectricalMaintenance Unit under Maintenance Division in Kurichhu Hydro Power Plant. He

currently works in the Centre of Excellence in Control and Protection (CoECaP). He is

in Network Protection and Automation work group under CoECaP which mainly deals in

study and testing of protection and automation in power system networks.

Tandin Dendup holds a Bachelors of Technology in Electronics and CommunicationEngineering from Jaypee Institute of Information Technology University in India. He

joined Druk Green in 2010 and worked in Control and Protection Unit underMaintenance Division in Chhukha Hydro Power Plant. He currently works in the Centreof Excellence in Control and Protection.

Deepen Sharmagraduated with Bachelors of Technology in Electrical Engineering fromthe National Institute of Technology (Former REC), Allahabad, India in 2003. He thenobtained his Masters of Technology in Power Electronics, Machines & Drives from theIndian Institute of Technology, New Delhi, India, in 2007. As a part of his Masters thesishe has co-authored few papers on switched mode power electronic converterspublished in various forums like IEEE International Conferences, PEDES06, PEDS07

& Journal of Power Electronics, 2007.He started his professional carrier in Ferro & Steel processing industries for few years before joiningDruk Green Power Corporation Ltd. (DGPCL), Bhutan in 2011. He is currently working as a practicingengineer in one of the Centre of Excellences (CoEs) under DGPCL, called the Centre of Excellence inControl & Protection (CoECaP), since January 2012.He is currently managing one of the two CoECaPswork groups, Control & System Analyses, the group that is broadly concerned with Study, Analyses &Testing of Power System Controls including the Governing & Excitation System, Primary Systems such

as Instrument Transformers, Circuit Breakers, Surge Arrestors & Power system earthing/groundingsystems.

Sizing of Current Transformer

Documents

Transcript of Sizing of Current Transformer