CT Burden

I am not aware of this aspect.

The burden is the maximum load the CT can take while maintainin the true ratio. Anything more and the CT ratio will not be maintained (the load voltage created will start countering and affecting the CT secondary viktage.

Burden does not mean what CT is supplying, it is max what it can supply.

In continuation with my query on CT burden , I could find an answer and the link is given below.

http://www.google.co.in/#hl=en&q=Instrument+transformer+dimensioning+past+and+future+by+Jaeger&meta=&aq=f&oq=Instrument+transformer+dimensioning+past+and+future+by+Jaeger&fp=3c5f05f17abf6d93

Please download the paper from Cigre International conference proceedings.

CURRENT TRANSFORMERS – RATED BURDEN – IS IT A BURDEN?

by: K.Sivakumar, Manager-Training, Larsen & Toubro Limited, Switchgear Training Centre, Coonoor (T.N.)

Introduction:

Current transformers are of prime utility value in any electrical network. They are used for measurement as well as for protection purposes. As with any other equipment, specifications play a vital role in the performance of current transformers too. One such important specification is the rated burden of the current transformer. Unfortunately, not much attention is paid to correctly specify the burden of the CTs. This article aims to look into the significance of CT Burden and the effects of wrong specification.

Functioning of CT: Contrary to whatever is suggested by the name, a CT produces only voltage at its secondary terminals when a current flows through its primary winding. For, whenever a current flows thro the primary winding of a CT, a flux is set up in the core of the CT. This flux, when it cuts the secondary winding of the CT – as per the famous Faraday's Laws of Electromagnetic Induction – an e.m.f. is induced in the secondary winding. The magnitude of this e.m.f. is:

e = 4.44 Ф f N₂ volts

where,

e = e.m.f. induced in the CT Secondary winding, in Volts

Ф = Flux, in Webers

f = System Frequency, in Hz

N₂ = Number of turns in the CT Secondary winding

Only when a ohmic load is connected across the CT secondary terminals, this secondary e.m.f. circulates a secondary current, proportional to the primary current, through the connected load.

Class of Accuracy: In an ideal CT, the secondary current will be in exact proportion to the primary current truly following the design transformation ratio. But, in practice, the secondary current may or may not be truly following the primary current as decided by the turns ratio or the design transformation ratio. There will be errors, either on the positive side (plus error) or on the negative side (minus error).

Standards too acknowledge this fact and have assigned various accuracy classes for Measurement as well as Protection Class CTs.

As per IS 2705, the Accuracy Classes for CTs are as below:

Measurement CTs:

Class of Accuracy +/- % Ratio Error @ Rated Primary Current

0.1 0.1

0.2 0.2

0.5 0.5

1.0 1.0

3.0 3.0

5.0 5.0

Note: For Class 0.1 to 1.0, the error shall not exceed the values given above, when the secondary burden is any value between 25% to 100% of the rated burden and for Class 3.0 & Class 5.0, the secondary burden shall be between 50% and 100% of the rated burden.

Protection CTs:

Class of Accuracy +/- % Ratio Error +/-Composite Error

@ Rated Primary @ Rated Accuracy

Current Limit Primary Current

5P 1.0 5.0

10P 3.0 10.0

15P 5.0 15.0

Burden: It is the ohmic load, connected to the CT Secondary terminals.

This is normally specified in VA.

As per IS 4201:1983 (Application Guide for Current Transformer)

Cl. 6.1 (Measuring CTs): "…. the rated output should be as near to in value but not less than to the actual output at which the CT is to operate. Ordering a CT with a rated output considerably in excess of required output may result in increased errors…."

Cl. 9.5 (Protection CTs): "Normally, the standard VA rating nearest to the burden computed shall be used…"

As understood from the above, as far as measuring CTs are concerned, wrong specification of burden would impose increased errors and would affect the revenue, when the CT is used in Power/Energy measurements. Whereas, proper specification of burden is very much imperative in Protection Class CTs, as it would affect the protective system operation and thus, the system security.

Contrary to measuring CTs – which have to maintain their accuracy only over their measuring range, protective CTs will have to remain accurate for currents many times in excess of their rated current, for, only then the protection system would read the primary conditions exactly and would react accordingly. Hence, a protection CT must remain stable – it must not saturate, for currents that are many multiples of its rated current. This level of saturation in protection CTs is denoted by a term called ALF – Accuracy Limiting Factor. Typical ALFs are 5, 10, 15, 20 & 30.

A 5P10 CT means this is a protection CT with a composite error of +/- 5% and this error will be maintained upto 10 times the rated primary current of the CT. If the primary current is more than 10 times the rated primary current, then this CT will saturate and will not reproduce secondary current linearly with the primary current.

Now, for a given CT, the accuracy limit voltage (ALV) is fixed at the time of designing the CT. That is,

ALV = ALF x I_{Sec. Rated} x (Z_CT + Z_Ext.)

Where,

ALV = Accuracy Limit Voltage in Volts

ALF = Accuracy Limiting Factor for the protection core

I _{Sec. Rated} = Rated Secondary current of the protection core, in Amps.

Z_CT = Internal Impedance of the CT Secondary winding, in

Ohms

Z _Ext. = External connected burden impedance, including

Connecting lead impedance, in Ohms

Consequently,

ALF = {(ALV) / [I_{Sec. Rated} x (Z_CT + Z_Ext.)]}

Here, ALV, I_sec. & Z_CT are fixed at the time of designing the CT.

So, the actual ALF will be in inverse proportion to the external connected burden. Lower the connected burden, higher will be the ALF and vice versa.

If ALF increases, it means that the CT will not saturate at the desired level and will reproduce the primary for currents much beyond the design Accuracy Limit Primary Current. This may damage the relays and other devices, even the CT secondary winding itself, as the reproduced secondary current would be higher than the designed/desired value.

On the other hand, if the ALF reduces, this means that the CT will saturate much earlier. Here too, the CT will not reproduce the primary fault currents exactly. The primary side may see a higher fault current and the secondary connected protective relays will see a lesser fault current due to the earlier saturation of the CT core. As the current seen by the relay is lesser, the operating time of the relay will be higher (if an inverse time-current characteristic is chosen, as is usual with many power system protection schemes). This is also dangerous as higher fault current would flow through the system components for a longer time than desired.

So, it can be seen that connecting higher burden as well as connecting lesser burden than the rated burden, can both prove to be harmful to the system components. Better practice would be to correctly specify the rated burden of the CT Protection core as per actual connected burden.

Earlier, when measuring instruments and protective relays were of electromagnetic type, they imposed a huge burden on the CT cores. When a number of such devices were to be connected in series with a CT secondary, it was practical to specify CT Metering Cores as well as Protection Cores with rated burdens of 15VA or 20 VA or even 30VA. But, with the advent of digital meters and digital protective relays, the burden imposed by these devices on the CT cores is greatly reduced. For example, the burden of the current coil of a conventional analogue, electro-magnetic, energy meter was about 5VA. Compare this with the burden of the current coil of modern day digital trivector meter, which is less than 0.5VA. Similarly, the burden of an electro-mechanical over current relay is about 5VA, whereas the burden of a digital microprocessor based over current relay is less than 0.25VA.

More and more systems are updated with these sophisticated electronic measuring instruments as well as digital protective relays. But, unfortunately, while specifying the CT burden it is not paid due consideration. Customers specify CTs with the earlier 25VA or 30VA, perhaps thinking that as a factor of safety or cushion. But, as we have seen earlier in this article, such practice of over-specifying CT burdens will only be harmful to the system as well as the CT itself, thereby totally negating the factor of safety concept itself.

Moreover, CTs with lesser burden will also be smaller in size and also cheaper. So, customers can have the added benefit of precious space saving as well as economy. More importantly, operational hazards too are minimized.

Conclusion: Hence, it is suggested that customers, designers, specifiers as well as engineers in projects, operation & maintenance pay attention to this much ignored or over-looked area of CT burden specification and henceforth specify the CT burdens correctly as per actual requirements only.

Dear Mr. Sivakumar,

Your literature on CT is very useful and excellent.

I have one doubt on metering CT. My query is ..the CT is rated for 15VA and the connected load is 2.5VA only. In my case accuracy is not a matter. But the thermal rating matters. The meters offered by the vendor is only with 15In momentary thermal withstanding capability. How to calculate the saturation values of the CT what is the standard governing this factor. (Rct is 2.5Ohms, CTR 2500/1A)

Regards

N.Arul Govind

(I underwent u r training when i was in L&T)