Burden On CT

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 ISec. Rated x (ZCT + ZExt.)
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.
ZCT = Internal Impedance of the CT Secondary winding, in
Ohms
Z Ext. = External connected burden impedance, including
Connecting lead impedance, in Ohms
Consequently,
ALF = {(ALV) / [ISec. Rated x (ZCT + ZExt.)]}
Here, ALV, Isec. & ZCT 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.

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CURRENT TRANSFORMERS – RATED BURDEN – IS IT A BURDEN?

by: K.Sivakumar, Manager-Training, Larsen & Toubro Limited, Switchgear Training

Functioning of CT: Contrary to whatever is suggested by the name, a CT produces

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

N2 = Number of turns in the CT Secondary winding

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

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

@ Rated Primary @ Rated Accuracy

Current Limit Primary Current

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)

ALV = ALF x ISec. Rated x (ZCT + ZExt.)

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.

ZCT = Internal Impedance of the CT Secondary winding, in

Z Ext. = External connected burden impedance, including

Connecting lead impedance, in Ohms

ALF = {(ALV) / [ISec. Rated x (ZCT + ZExt.)]}

Earlier, when measuring instruments and protective relays were of

Conclusion: Hence, it is suggested that customers, designers, specifiers as well as

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