Series connection of current transformer [CT] is used when accuracy improvement is desired without increasing CT ratio. Series connection of two CT improves accuracy by splitting burden voltage drop between two CTs. Each CT hence will only have half the excitation voltage leading to large reduction in excitation current (error causing current) thus improving accuracy. Series connection of CT requires careful consideration as misapplication can cause CT open circuit which is dangerous or could lead to severe CT saturation.
When two CTs with same ratio are connected in series, secondary excitation voltage developed across a given burden is halved. For example, if there is a 150:5 CT with connected burden of 1Ω, then for a primary current of 3,000A the secondary current will be 100A. 150:5 CT must be capable of producing 1Ω*100A=100V secondary without causing CT to saturate. When two 150:5 CTs are connected in series with 3,000A primary, 100A secondary will flow through both the CTs but each CT now only need to develop 50V across the burden. By halving secondary excitation voltage, secondary excitation current is considerably reduced resulting in large improvement in accuracy.
The most common CT connection is a simple radial connection from one CT to one or more meters or relays through intermediate CT shorting blocks. For certain applications such as transformer differential or bus differential relaying etc. CTs are connected in parallel. Parallel CT connection is discussed in this article.
Read: Current transformer DC saturation and time to saturate
Low ratio CTs with ‘low’ primary current have inherently low accuracy class. For example, primary current of a 5MVA, 138kV/12.47kV transformer is only 21A. If a 50:5 CT is selected, ANSI accuracy class for this low ratio CT might be C10. It is difficult to find 50:5 CT with high accuracy such as C200 or C400. Problem of CT accuracy usually show up when this 50:5 CT need to be connected differentially to the secondary (12.47kV) side 300:5 CT which can easily have ANSI C200 or better accuracy.
Read: Potential transformer accuracy
With the advent of modern digital relays with very low burden, higher tolerance for CT saturation, saturation detection and avoidance, adjustable trip curves etc., series connection has lost most of its application relevance. There are some limitations and precautions to be considered if series connection of CT is desired and these are discussed below.
Read: Zero sequence current transformer
Application considerations for series connection of current transformer
When CTs are connected in series it is recommended to have CTs over a single bus that is continuous, un-splitable between the mounting positions of CTs that are in series unless other suitable precautions are taken. CTs that are in series need to have same CT ratio, accuracy class. CT polarity of series connected CT need to face same direction (otherwise the secondary current would just cancel each other).
CT Installation
It is recommended that series connected CTs are installed over a common bus that cannot be separated. An example of this would be two bushing CTs on a HV circuit breaker or transformer.
If bus between two series connected CTs is opened, for example with a tie breaker, then one or both CT secondary can become open circuited. If two CTs are connected in series and current on CT2 primary is zero, then it will not allow secondary current from healthy CT1 to pass. See figure 3. This will appear as high impedance to CT1 and will quickly move in to saturation.
Another possibility for open circuit is accidental disconnection or damage of CT secondary leads. Open circuit CT can lead to CT damage, wire damage and is a fire hazard. It is recommended to install CT open circuit protectors to prevent damage when CTs are connected in series.
Read: How to select taps for multi ratio current transformer?
CT Ratio
Current transformers that are connected in series need to have same ratio and preferably of same accuracy. It is also ideal if CTs are from same manufacturer so that core characteristics are exactly similar.
If two CTs of different ratio are connected in series, then the larger CT will block current flowing in from smaller CT. As an example, consider a 50:5 CT1 in series with 1600:5 CT2. Ideally if CTs were independent, for a primary current of 50A, CT1 will produce 5A secondary and CT2 will produce 0.156A. When series connected with 50A primary current on both, CT2 can only pass 0.156A and will block rest of CT1 current. This will appear as a high impedance and will quickly drive CT1 in to saturation. This extreme case shows that CT ratios need to be similar.
Read: Adjusting current transformer ratio
References
[1] Protective relaying, Principles and applications; J Lewis Blackburn; Thomas J. Domin
[2] The art and science of protective relaying; C Russel Mason