Specifying Current Transformers Realistically
Current transformer is an important link in power system that helps us to measure, monitor & protect the power system. As such, the correct selection and specification of a current transformer also becomes a very important task. Unfortunately, not much attention is paid to correctly specify the CTs.
- K Sivakumar
This article aims to look into the significance of some of the CT Specifications and the effects of wrong specification – particularly with respect to application in Switchboards.
CT Specifications: Some of the vital CT Specifications are:
a. Rated Primary Current
b. Rated Secondary Current
- Accuracy Class
- Accuracy Limit Factor
Rated Primary Current: Vide Clause 11.2 (Table-11) of IEC 60044-1:
Vided the above, according to Standards, the declared accuracy of a CT can be guaranteed only between 100% to 120% of Rated Primary Current. If the actual primary current is less than 100%, the error will increase. So much so that at 20% of primary current, the error doubles and at 5% or primary current, it quadruples for a Class 0.1 Measurement CT.
And, vide Clause 11.2 (Table-12) of IEC 60044-1:
Note: Even with the special Class 0.2S & Class 0.5S CTs, the declared accuracy of the CT can be guaranteed only between 20% to 120% of Rated Primary Current. If the primary current is less than 20%, the error will increase. So much so that at 5% of primary current, the error doubles approximately and at 1% or primary current, it quadruples for a Class 0.2S Measurement CT.
This is of particular importance is CTs connected to Tariff Metering Applications.
In the case of Protection CTs too, the increased errors at reduced primary currents would greatly affect the performance of the protection schemes, thereby compromising on the very purpose of protection.
When selecting the rated primary current of the CT, the above fact shall be kept in mind. The CT Primary Rated Current shall be selected as close as possible to the actual load current anticipated in the location, where the CT is installed. If the CT primary current is not properly chosen and the actual primary current on the CT is a very low percentage of the rated primary current, then it would increase the error of the CT. If the present load of the installation is low, with anticipation of increased load in future, one can always opt for multi-ratio CTs, so that the CT tapping can be varied to choose the appropriate ratio, based on the requirements.
Rated Secondary Current: For the same ratio, accuracy class & burden, the size & cost of a CT with a 1A Secondary will be higher than a CT with a 5A Secondary. For switchboard applications, where space is a constraint and also as the lead burdens are not very large, CTs with 5A Secondary will serve the purpose. Only where long lead lengths are involved (e.g.) CTs installed in outdoor switchyards with long secondary leads for loads located in distant control rooms, should one opt for CTs with 1A Secondary.
Accuracy Class: The required accuracy class also shall be chosen judiciously. Going for a finer accuracy class when not needed, would only increase the cost unnecessarily. Also, the accuracy class of the CT shall perfectly match with the accuracy class of the meters connected. For example, there is no point in going for a Class 0.2S Measurement CT, if the meter connected is also not of Class 0.2S.
Burden: Vide Clause 11.2 of IEC 60044-1, “for classes 0.1, 0.2, 0.5 and 1, the current error and phase displacement at rated frequency shall not exceed the values given in table 11 when the secondary burden is any value from 25 % to 100 % of the rated burden”. “For classes 0.2 S and 0.5 S the current error and phase displacement at the rated frequency shall not exceed the values given in table 12 when the secondary burden is any value from 25 % and 100 % of the rated burden”.
Thus, as per Standards, the declared accuracy is guaranteed only when the actual secondary connected burden is between 25% and 100% of the rated burden of the CT. If the actual connected burden is lesser than 25% of the rated burden, the error will increase. In earlier days, 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 Secondary 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. Say, for a tariff metering application with a digital Trivectormeter, if one chooses a CT with a rated burden of, say, 20VA, then the actual burden connected on the CT will only be a very low fraction – about 2.5% - of the rated burden of the CT. Thus, as per Standards, at such a low burden connected to the CT Secondary, the error will multiply many fold and the tariff meter will not record the measurements accurately. Even when one goes for an accurate & costly Class 0.2S Meter, it will still measure inaccurately only.
In case of protection CTs, apart from affecting the relay operations due to increased errors, connecting a lesser burden to a CT with a higher rated burden, would increase the actual Accuracy Limiting Factor of the protection CT. The ALF of a CT is inversely proportional to the actual burden connected. For example, if one has a protection CT of Class 5P10 & a rated burden of 15VA, the actual ALF of the CT will be 10, only when 15VA burden is connected to the CT Secondary. If the actual burden connected is, say, only 5VA, then the ALF will increase to about 17. This will not only affect the operating performance of the protective relays connected, but also might damage the CT Secondary Winding Insulation and the equipment connected to the CT Secondary.
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.
Also, specifying a CT with a higher rated burden than the actual connected burden will increase the size & cost of the CT.
Contrarily, 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. And, reference is drawn to IS 4201:1983 (The Indian Standard Application Guide for Current Transformer), wherein it is stated that:
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."
Accuracy Limit Factor
The accuracy limit factors recommended in the Standards is 5, 10, 15, 20 & 30. The ALF of a protection CT shall be chosen realistically, duly considering the maximum anticipated fault current at the location of the CT. Selecting a CT with a higher than necessary AL F, will increase the size & cost of the CT. Alternately, selecting a CT with a lesser than necessary ALF, will affect the performance of the CT and might even cause irreparable damage to the CT.
Going by the above few paragraphs, it is prudent to select and specify current transformers judiciously, particularly for switchboard applications. Apart from optimizing the performance of the CT and of the connected equipment, such a measure would also optimize the size & cost of the CT as well as that of the switchboard as a whole.
Author is from Megawin Switchgear P. Ltd, Salem.