CT Specifications – Gold Report

CT Specifications – Gold Report

Good old technical conference papers are gold strips. 

New-age conferences are networking and business trips! – Sesha

Preamble

In my ‘About This Site’ blog, I said that a good, two-page technical report is worth more than its weight in gold! This blog is one of them.  This is slightly longer because I have included a detailed explanation. If you are a non-technical person, you can still read the Introduction, The Problem, The Fireworks, and the Site Recommendations.  You will enjoy it! 😊

Introduction

Current Transformers (CT) are used in power systems to step down large currents.  Stepped down currents are used for measurement (as inputs to Ammeters) and protection (as inputs to relays).  This blog is about a major site issue due to an incorrect CT specification for differential protection of a large utility grid transformer. 

Alpha Generating Company was carrying out a major refurbishment of Station Auxiliaries.  Delta Projects were the project managers.  It was a major upgrade which included the replacement of the outdoor 22kV Current Transformer (CT).  This CT was a part of differential protection for the 22kV/132kV High Voltage (HV) grid transformer. The grid transformer is owned by a separate company, let us say Beta Transmission Company.  In addition, Edison Consultants were the specialist consultants.

Edison Consultants specified 22kV, 600/5A, 2.5P100 F20 Current Transformer, as per the old AS 1675-1986 Australian standard.  This was based on the drawings available at the client site. The CT specification was approved by the client and was sent to Delta Projects.

The Australian standard for CTs at that time was AS 60044.1-2003.  This standard is based on the International standard IEC 60044.1. Australia now adapts International Electrotechnical Commission (IEC) standards, except where local conditions require a separate standard.  Accordingly, Delta Projects was required to specify the CT as per AS 60044.1. The technical difference between the two standards are as below:

• AS 1675 specifies the ‘Maximum allowable voltage’ at the secondary terminals – not the Volt-Ampere (VA) rating of the CT.
• AS 60044.1 specifies the ‘CT rating in VA’ – not the ‘Maximum allowable voltage’.

Fortunately, they had a young engineer who knew Ohm’s law.  So, it was left to him to convert the CT specification from AS 1675 to AS 60044.1.  Ohm’s law is given below.!!

        Resistance (ohms) = Voltage (Volts)  / Current (Amps).

Conversion from AS 1675 to AS 60044.1 

AS 1675 specification 2.5P100 F20 is interpreted as below:

• 2.5P        – Accuracy 2.5%  (at 20 times rated current).
• 100 F20 – Maximum permissible voltage at CT secondary terminals is 100V at 20 times rated current. 

Protection CT must maintain the specified accuracy under short circuit conditions.  Short circuit currents are generally less than 20 times the rated current.  So, a factor of 20 is popular for specifying protection CTs.

The young engineer at Delta Projects studied AS 1675 & AS 60044.1 and calculated VA rating of the CT as below.

                  Irated =   5A (Secondary side)

                 Vsec-max =   100V @ 20 x Irated 

                ∴ Rated load or burden (Rrated) = 100V / (20x5A) = 1 ohm

               ∴ VA rating = (Irated)^2 * Rrated = 5 x 5 x 1 = 25 VA        …  (Q.E.D)

AS 60044.1 standard specifies that the accuracy class for protection CT is 5P or 10P, that is, 5% or 10%.  This is specified in Table 14 of the standard.  Accordingly, the young engineer arrived at the new specification as below:  

• New CT Specs:  22kV, 600/5A, 5P20, 25VA

As per AS 60044.1, 5P20 specifies 5% accuracy at 20 times rated current.

Did you notice the problem with the new specification?  Look again!

The young engineer was very pleased with himself!  The Purchase Order was released in favour of a CT manufacturer in Europe.  The work site is in Australia.  The delivery period was a few months—no problems!  There was plenty of other refurbishment work to be done at the site.

The Problem

A CT with an accuracy of 5% has been ordered instead of the required accuracy of 2.5%.  This oversight went unnoticed!! 

AS 60044.1 does not mention any specific accuracy requirement for differential protection—it is left to the user’s discretion.  Many large industrial sites specify 5P20 for differential protection for their main transformers.  Such industrial sites are the bread and butter business for project managers, such as Delta Projects.  However, large electric utility transformers, as in this case, are a different kettle of fish.  Utility transformers operate at a much higher level of availability.  Hence, their differential protection requires a higher accuracy to avoid nuisance or false trips. 

IEC standards deliberately leave a lot of discretion to the users.  This is because it is an International Standard.  It gives leeway to each country to make more specific recommendations to suit their requirements.  It has its pros and cons, but it is confusing to a novice engineer.

To be fair, AS 60044.1 does include specifications for a more accurate CT, for example 2.5%.  It is called the PX type specification. Even in that section, AS 60044.1 does not mention differential protection!

N.B.  PX type specification is very different.  I will write a separate blog for PX type CT specifications.  

The Fireworks!

The CT arrived as per schedule and was installed at the site.  The problem was noticed by Beta Transmission Company during commissioning.  The CT is meant to protect ‘their’ transformer.  The CT did not meet the accuracy specifications, so they rejected the CT!  The project could not continue without their approval.

The delivery period for the replacement CT was unacceptable to the client, namely, Alpha Generating Company.  Their generators and refurbished station auxiliaries were ready to go.  Idle generators mean loss of revenue on a daily basis.  The fireworks started.  Emails were flying and it landed on Sesha’s desk!  

Investigation of CT Design Principles

I did what I usually do—I started reading the literature on the design principles.  With a bit of luck, I came across the following Schneider Electric publication: 

Cahier Technique No 195 – “Current Transformers: Specification errors and solutions” – Paola FONTI.  

This 20-page publication is worth its weight in gold— but I have a pdf copy on my disk drive!  It has everything one needs to know about CT design principles and practical applications. The relevant information for us is as below:

        PX is designed for a maximum flux density of 1.4Tesla CT core

       5P is designed for a maximum flux density of 1.6Tesla CT core

       10P is designed for a maximum flux density of 1.9Tesla CT core

As mentioned before, 2.5P is not a standard specification.  However, for this analysis, we will assume 2.5P as equivalent to PX class.  We need to find the PX equivalent for the 5P20, 25VA CT.

As given above, 5P CT is designed for 1.6 Tesla.  For the PX equivalent, the flux density in the CT core must be limited to 1.4 Tesla.  How can we do that?

Flux density in the core is determined by two factors:

1. Primary current – we have no control over this.
2. Rated Load or Burden – we can control this!

We have already calculated the rated burden of 5P20 25VA CT.

         Rb(5P)  = 1 ohm         (Rated Burden for 5P) 

We need to reduce the burden in proportion to the required flux density!

       ∴ Rb(2.5P)  = 1.4/1.6 * 1  = 0.875 ohm

Eureka!  We have the answer!  We can achieve 2.5P20 specification by restricting the burden to 0.875 ohm.   In other words, reduce the load or burden by 0.125 ohm.  

The new VA rating of the transformer is:

       VA(2.5P) = (Irated)^2 * Rb(2.5P)  = 5 * 5 * 0.875 = 21.875 VA 

Hence, 5P20, 25 VA CT is equivalent to 2.5P20, 21.875 VA CT!

Site Recommendations

To achieve 2.5% accuracy, we need to restrict the load (burden) to 0.875 ohm.  The main burden of a CT is the cable wiring from the relay to the CT secondary terminals.  The CT cabling details at the site are as below:

• Existing CT circuit is 75m of 4sqmm cable
• Additional CT cable length is 30m after refurbishment

Hence, the whole of CT wiring should be replaced by 2 x 4sqmm cable in parallel, instead of the existing 1 x 4sqmm cable.  This will reduce the CT burden by about 0.3 ohm. This is more than adequate to meet the 2.5% accuracy requirement.

Epilogue

This is a real case study.  After this report was sent, the project manager was ecstatic!  He rang me up and said that he will use 3 x 4sqmm cable instead of the recommended 2 x 4sqmm cable!!  I am yet to hear from Beta Transmission Company.  One reason may be that many engineers in that company are my students.  They probably thought, Sesha can’t be wrong, can he?

The project was completed on time!  Only a few extra dollars were spent on the CT cable.  This 2-page technical report is definitely worth more than its weight in gold!  I hope you agree with me?  Please do send me your comments.  

Acknowledgement

This blog is based on the technical report prepared by me when I was working with Welcon Technologies in Gladstone, Queensland, Australia. 

Many Thanks to Sapna Prasad for edits and suggestions.