Transformer Accuracy

Transformer Accuracy Principles

Measuring instruments, such as ammeters, voltmeters, kilowatt-hour meters, etc , whether electromechanical or electronic, meet insuperable design problems if faced with high voltages or high currents commonly used in power systems.

Furthermore, the range of currents employed throughout is such that it would not be practical to manufacture instruments on a mass production scale to meet the wide variety of current ranges required.

Current transformers are therefore used with the measuring instruments to:

(a)  Isolate the instruments from the power circuits.

(b) Standardise the instruments, usually at 5 amps or 1 amp.

(The scale of the instrument (according to the CT ratio), then becomes the only non-standard feature of the instrument)

Accuracy classes for various types of measurement are set out in the relevant IEEE(ANSI), CAN/CSA, AS or in our case BSEN /IEC 61869.

It will be seen that the class designation is an approximate measure of the accuracy, eg. Class 1 current transformers have ratio error within 1% of rated current. The phase difference is important when power measurements are involved, i.e. when using wattmeter's, kilowatt‑hour meters, VAr meters and Power Factor meters.

 

Importance of Accuracy

Transformer Accuracy

The table below details the limits of error for current transformers for special applications and having a secondary current of 5A.

 

Design Considerations:

As in all transformers, errors arise due to a proportion of the primary input current being used to magnetise the core and not transferred to the secondary winding. The proportion of the primary current used for this purpose determines the amount of error.

The essence of good design of measuring current transformers is to ensure that the magnetising current is low enough to ensure that the error specified for the accuracy class is not exceeded. This is achieved by selecting suitable core materials and the appropriate cross-sectional area of the core. Frequently in measuring currents of 50A and upwards, it is convenient and technically sound for the primary winding of a CT to have one turn only.

In these most common cases the CT is supplied with a secondary winding only, the primary being the cable or busbar of the main conductor which is passed through the CT aperture in the case of ring CTs  (i .e. single primary turn) it should be noted that the lower the rated primary current the more difficult it is (and the more expensive it is) to achieve a given accuracy.

Considering a core of certain fixed dimensions and magnetic materials with a secondary winding of say 200 turns (current ratio 200/1 turns ratio 1/200) and say it takes 2 amperes of the 200A primary current to magnetise the core, the error is therefore only 1% approximately. However, considering a 50/1 CT with 50 secondary turns on the same core it still takes 2 amperes to magnetise to the core. The error is then 4% approximately, to obtain a 1% accuracy on the 50/1 ring CT a much larger core and/or expensive core material is required.

Still not sure & need help? Feel free to reach out to us with your enquiry where we will be happy to help.

 

CT12-100 MV CT Launch

New 12kV Medium Voltage CT by ITL

ITL proud to announce the launch of its updated medium voltage current transformer CT12-100. Always listening to customer feedback, an area of their frustration was the desire was to allow for changes in the primary ratio rather than only on the secondary. With many such customers requesting the same the die was cast for ITL’s R & D team to improve on existing models to meet their customer's yearning.

In any development, it is important to weigh up the costs and benefit whilst getting the maximum performance from the equipment and at the same time achieve improvements in manufacturing productivity. ITL’s highly skilled engineering & production teams have achieved both in the CT12-100 range.

“The CT12-100 is the first in an updated line of indoor MV current & voltage transformers in development at ITL” Paul Munro (ITL’s Marketing Director) said.

With the Highest System Voltage 12kV, primary ratios from 100A to 1200A designed with both tariff measurement & protection class accuracy in mind ensures that ITL continues to provide their global customer base with the right product for the right job and at the right price.

Got Questions?

Drop our technical team an email to discuss or jump over to our contact form and complete, we would love to hear from you.
New MV Current Transformer

CT12-100

Defining CT Characteristics

Current Transformer Terminology

Current transformers convert an alternating current usually high into a proportional lower one, depending on their use. Measurement type CTs are required to transform the primary current, at various classes of accuracy, as specified by the class designation, over a current range from 1 to 120 per cent of its rated primary ratio.

 

The design of this type of transformer requires a "steel" core and winding wire, typically copper which will when connected to its rated burden (load); perform within the limits of error as indicated by the standard’s specification e.g. IEC 61869 or IEEE C57.13. It is an advantage for a measurement type transformer to saturate above this range, which provides a protection against damage to instruments by limiting the secondary current when surge currents or faults appear in the primary circuit.

 

What Is:

Measuring transformer:

A current transformer intended to supply indicating instruments integrated meter, relay and similar apparatus.

Current transformer:

An instrument transformer in which the secondary current, in the normal condition of use, is substantially proportional to the primary current and differs in phase it by an angle which is approximately zero for an appropriate direction of connections.

Rated primary current:

The value of primary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

Rated secondary current:

- The value of secondary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

Rated transformation ratio:

The ratio of the rated primary current to the rated secondary current.

Current error (ratio error):

The error with a transformer introduces into the measurement of a current and which arises from the fact that actual transformation ratio is not equal to the rated transformer ratio. The current error expressed in percentage is given by the formula:
Current error, percent = (Ka.Is-Ip) x 100 / Ip
Where Ka= rated transformation ratio
Ip= actual primary current
Is= actual secondary current when Ip is flowing under the conditions of measurement

Phase displacement:

The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for the perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually expressed in minutes.

Accuracy class:

A designation assigned to a current transformer the errors of which remain within specified limit under prescribed conditions of use.

Burden:

The impedance of the secondary circuit in ohms and power factor.

Rated burden:

The impedance of the secondary circuit on which the accuracy requirements are based. It is usually expressed as apparent power (in VA), at the rated secondary current and at a specified power factor.

Rated output:

The value of the apparent power (in volt-amperes at a specified power (factor) which the current transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it.

Highest system voltage:

- The highest rms line to line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variations due to fault condition and the sudden disconnection of large loads.

Rated insulation level:

That combination of voltage values (power frequency and lightning impulse, or where applicable, lightning and switching impulse) which characterizes the insulation of a transformer with regard to its capability to withstand by dielectric stresses. For low voltage transformer the test voltage 4kV, at power-frequency, applied for 1 minute.

Rated short-time thermal current (Ith):

The rms value of the primary current which the
current transformer will withstand for a rated time, with their secondary winding shortcircuited
without suffering harmful effects.

Rated dynamic current (Idyn):

The peak value of the primary current which a current transformer will withstand, without being damaged electrically for mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited.

Rated continuous thermal current (Un):

The value of current which can be permitted to flow continuously in the primary winding, the secondary windings being connected to the rated burdens, without the temperature rise exceeding the specified values.

Instrument security factor (ISF or Fs):

The ratio of rated instrument limit primary current to the rated primary current. The times that the primary current must be higher than the rated value, for the composite error of a measuring current transformer to be equal to or greater than 10%, the secondary burden is equal to the rated burden. The lower this number is, the more protected the connected instrument is against.

Routine test:

Tests carried out on each current transformer to check requirements likely to vary during production.
Depending on which standard the transformer is to meet e.g. IEC or IEEE/ANSI. In general, the following tests apply to each individual transformer:
1. Verification of terminal markings
2. Power-frequency withstands test primary winding.
3. Partial discharge measurement.
4. Power-frequency withstand test on secondary windings.
5. Power-frequency withstand test, between sections.
6. Inter-turn overvoltage test
7. Determination of errors.
The order of the tests is not standardised, but the determination of error shall be performed after the other test.

Type test:

Tests carried out to prove the general qualities and design of a given type of current transformer in accordance with the requirements of the applicable standers Tests may be carried out on a prototype which may incorporate special arrangements for the measurements required by the applicable standard.
The following tests are type test:
1. short time current test
2. temperature rise test
3. lightning impulse test
4. switching impulse test
5. wet test for outdoor type transformer
6. determination of errors
7. radio interference voltage measurement (RIV) (As specified in IEC 61869-1 for 123kV and above)
All the dielectric type test should be carried out on the same transformer unless otherwise specified.

Special tests / optional tests:

Chargeable testing which may be in the nature of type tests or routine tests, and are carried out only by agreement between ITL & the customer.

Got Questions?

Drop our technical team an email to discuss or jump over to our contact form and complete, we would love to hear from you.
Defining a Current Transformer

Measurement CT

Agent Appointed – PAKISTAN

Following our international growth plan, ITL is proud to announce our new agency partner in Pakistan. Ittehad Energy Systems (IES). Based Karachi with a planned office in Lahore within the next 12 months this dynamic company already has a number of experienced Electrical Sales Engineers who be promoting & supplying Instrument Transformers Limited's wide range of current transformers and voltage transformers to the Industrial & Electrical Utility markets.

With our increased business activity in the region, now was the right time for us to formalise our position in the country. Ittehad was born out of former employees of our Saudi Arabian agent whom we have worked with for over 30 years. This previous experience was of particular interest to ITL and vastly reduces the product learning curve and accelerates market awareness making it a quick win for both companies.

Critical to success is a field support function. ITL has dedicated Engineering & Sales Team members who regularly engage with and provide assistance to our international agent network ensuring we understand the techno-commercial aspect of any project. In addition, regular sharing of market intelligence data allows ITL to adapt its marketing strategy or deploy extra resources to maximise local market opportunities.

Our product line is often hidden from view but is an essential element in our every day lives. Without electricity in some form, economies cannot grow beyond local bartering on what you can hand make. Electricity gives us industrialisation, innovation and the world we live in. Why wouldn't you want to contribute to that?

Next time you need a CT or PT and are based in Pakistan, then speak to Ittehad on +92-336-2337482 or email rubnawaz.khan@ittehadenergy.com where they will be more than happy to assist you in your project.

If you would like to be considered as an independent sales agent for Instrument Transformers Limited send an email to Office.ROW@itl-uk.com

Agent Certificate for Pakistan

Agent -Certificate

ITL Renews Russian Connection

ITL has renewed its connection with the Russian, Kazakhstan, Belarus, Armenia & Kyrgyzstan markets with an updated Current Transformer & Voltage Transformer product lines that comply with the EAC requirements of TR CU 020/2011 (Electromagnetic compatibility of technical means) & TR CU 004/2011 (Safety of low-voltage equipment). About 70% of products exported to Russia / the Customs Union require mandatory EAC certification system to pass the customs clearance. The EAC is replacing the former national GOST certification in Russia, Belarus, Kazakhstan, Armenia and Kyrgyzstan. At the moment, there are 36 Technical Regulations in effect and 20 more will be adopted in the future.

Ministry pre-qualification was undertaken and once accepted, any goods exported to Russia / The Customs Union must be clearly marked by the EAC logo. The logo has to be on each individual unit, packaging and any documentation and if possible, next to the manufacturer’s trademark.

The EAC conforming units are specially designed to work in environments down to -50 Degrees Celcius. These products have been manufactured to the most recent IEC standards for instrument current and voltage transformers, using the latest technology and a host of materials suited for low-temperature environments.

Paul Munro, Marketing Director for Instrument Transformers Limited said: "Always listening to customer feedback, there was a particular pain point experienced in the Eurasian market. Whilst ITL was not directly affected, we took the lead in assisting our customers to remove potential barriers to market by having a speciality product line developed to meet the newest TR CU requirements. Now completed, we are supporting a number of customers in this initiative, whilst actively rolling out an awareness program to our wider global customer base."

This is one of the many projects that Instrument Transformers Limited has undertaken to ensure that they timeously adapt to an ever-changing & challenging marketplace.

Need a CT or PT, then you need to be speaking to ITL at sales@itl-uk.com

EurAsEC Customs Union

EAC Certification Logo

Alive & Kicking after 28 years

At ITL we always promote the reliability and quality of our transformers, and from a marketing perspective, it can be seen as spin in front of the customer. It is great to have supporting feedback from the field and today this CT was part of a 12 off manufactured for Centre Switchgear (latterly amalgamated into ABB in the 90's to become ABB Centre Switchgear Limited). The project was for Bishopsgate Incoming Main Power and has been faithfully serving for 28 years. 

Normally in system upgrades the CT is removed and replaced, not this one, the customer is connecting upgraded monitoring equipment and we expect that this CT will continue to provide steadfast performance.

We stand by the quality & reliability of our products and are proud of our achievements over the past 45 years. Always welcoming new challenges from whatever our customers throw at us and whilst we may not be the cheapest in the market, without a doubt we can say that our customers definitely get a positive return on investment when installing an Instrument Transformers Current Transformers or Voltage Transformers.

If you are not using our transformers, firstly I would cheekily say WHY, but I'm sure you have your reasons. None the less if you haven't already joined our global customer base where our goal is to exceed your expectations and help you build your brand as a quality manufacturer of switchgear & electrical equipment then speak to us now.

Reach out today on using our chat page, email our sales & marketing teams or do it the old-fashioned way and give us a call on +44(0)1355236057 where one of our highly qualified engineers will assist with your requirements.

Main Incoming Current Transformer

Main Incoming CT

2016 in ITL

2016 has definitely been a year full of changes and events all around the World. There were actually so many it would take quite a while to list them all. The two obvious ones are the new American President and UK leaving the EU.  Brexit was the one which has influenced us the most and will probably even more in the future. We will talk about that in one of our future posts so stay tuned.

But what changes happened on our end in 2016? As every year, we have focused on improving our products and services for our customers. We are very well known in the market for our Low Voltage range of products but in the last couple of years there has been a spike in demand for our Medium Voltage units. To meet this increase, we have acted and invested in our Vacuum resin casting plant. There was a lot to learn and still is but the equipment is up and running.

2016 is also the birth-year of our Test Reports Repository, or TRR. Below you can read about why we did it, what feedback we got from our customers and why some of us have grey hair.

Resin casting Plant

The decision to invest into new resin casting plant was actually made at the end of 2015. For couple of years we have experienced a continuous increase in demand for ITL Medium Voltage products and the new plant was a logical step. It makes our life a bit easier, our products better and allows us to better control the quality of the finished product. There was a lot of research done by our engineers, we had to move things around on our production floor and it took us quite the while to find the product to suit our needs. But at the end we have opted for state-of-the-art German quality and as far as we can tell it was the right call. Bottom line, our resin casting capabilities drastically improved.

The TRR rises

Every single unit that is manufactured in our East Kilbride factory is tested on our premises (and like as a carpenter measure twice & cut once, we randomly test twice) before despatch. This way we ensure that every CT and VT coming out of our factory floor is fully compliant with required international standard. The outcome of the test is a Routine Test Report. Historically, we were sending paper copies with the goods.  Then, with the coming of electronic age we have started sending them in PDF format via e-mail. There is a slight shortcoming though. The buyer who receives the Report is not the person who actually needs it. It is the Manager spearheading the project or, QA Dept. or in some cases the actual commissioning team at site who needs to see the results. With all these touch points with a customer, we decided to simplify and improve the process by providing an online Test Report Repository (or TRR as we call it here) idea was born. 

Test Report Repository

Basically, the idea is to have a dedicated space which customers can access online and simply download the document without having to phone in or e-mail us. Please, don’t get us wrong, we love to talk to you. But we have customers all over the world and sometimes we might not be here to pick up the phone or e-mails can get lost. And let’s be honest, it saves time for everybody and we all know time is a precious commodity.

Great idea. But the road to make it happen was a difficult one. Our marketing guy Jerry sure has something to say.

Jerry – “It surely was a challenge. First, we had to rebuild our sites from scratch in-house as it gives us a more flexibility and control in what we want to do and the TRR was one of the major factors to go for it. There were so many things to consider. Security of the reports, login process, search criteria, actual documents storage, you name it. Both I and Paul have put a lot of time and effort into developing the platform and rolled out the beta version to a couple of customers and it got quite a bit of traction. Now we are up and running, sure, however as with any software platform there are still tweaks to be made but the main thing for us now is to educate our customers and roll it out on a large scale.”

ITL Resin casting plant

Resin cast plant

ITL Reaches 30,000!

Latest Current Transformer Solution

ITL 30000/1A current transformer

30000/1A Current Transformer from ITL

When testing current transformers it is always best to inject the appropriate rated primary current to ensure that the product truly can meet service conditions. For lower ratio transformers, there is a trend by customers for traditional transformer testing to be replaced by a CT Analyser which injects voltage and using some fancy algorithms provides test data to show compliance. Whilst helpful on the reporting side and field validation, ITL have elected not to use a CT Analyser for primary testing. Opting to continue traditional testing methods for high ratio current transformers by direct current injection to prove compliance and incorporating hybrid reporting format from a CT analyser.

Led by ITL’s Technical Director Greig McFarlane, the company has invested heavily in upgrading its High Current Test Dept. which allows current transformers to be tested to 30,000 Amps with a secondary ratio of 1 Amp. In keeping with our philosophy of exceed customers expectations of giving exactly what the customer asks for, these secondary ratios can be customised should this be necessary.

With such high ratio transformers, many of ITL’s competitors have chosen to loop multiple cable turns through the primary to achieve high current accuracy resolution and while this may be an acceptable method of testing for other organisations, it is not for ITL. This procedure can have the effect of introducing localised saturation which can influence test results and not reflect true operational performance. Greig said “especially when providing accuracy results at very high levels you want to ensure that any downstream risk is minimised.”

“By using state of the art high accuracy test bridges ensures that we continue surpass our customers’ expectations for accurate, reliable and long service transformers.”

As always, we stand ready to meet our customers’ ever demanding needs for CT /VT measurement & protection solutions, with enthusiasm and look forward to helping you soon.

Call us on +44 1355 236 057, or email sales@itl-uk.com

ITL – TransformingYourWorld.net

Our test reports repository

Making things easier for everyone!

Screenshot 2016-08-25 09.02.33

ITL Test Certificate Example

We are always striving to make our customers’ life as easy as possible. ITL have always been known as the "go to CT/VT guys" but also for a great customer service. Fast responses, knowledge, quick orders, you name it (as in life we have stumbled in our past but more importantly we learn from our mistakes to prevent recurrance). However, our other great asset is efficiency. And that is where Test Reports Repository comes in.

Test Reports, or Test Certificates, are very important part of our products. It may seem they are just a supporting document but they clearly state that our CTs and VTs meet international requirements which is an invaluable information for a buyer and particularly for an engineer.

Traditionally, we supply paper copies or electronic copies of test reports. Sometimes we have buyers, engineers etc. e-mailing/ phoning us asking for duplicate test reports for a particular unit (I'm sure you can relate to this scenario). Emails are not always perfect and get lost, sent to junk or just plain deleted in error and don’t get to the intended recipient for what ever reason. We are sure you know it can get messy. So we created TRR. It is an easy way you can access your current transformer or voltage transformer test report by just logging in and doing quick search. 

It makes life easier for both us and our customer. And that is what ITL is about.

Below you can see a short video of how it works. Or you can have a quick test run here. (Username: C110011; Password: TestCertsTest)

Want to sign up? Take a look here! 

Keep a lookout for our developments.

ITL – one of the Key Players on Transformers market

The instrument transformers market is always changing. The next four years look bright for the industry. According to this analysis the market will grow at a CAGR of 7.81% during the period 2016-2020. This is obviously great news for all the players in the industry. But first, let’s take a look at why.

Modernisation of the Grid

_L6H3487

One of ITL Split core units!

One of the main reasons for this growth is investments in the grids. There is an increased demand for new transformers as governments are funding schemes to improve power transmission and distribution. A great example is India with their APDRP scheme (Accelerated Power Development and Reforms Programme). The trend is to find more effective and reliable solutions for grids to minimise waste of energy during long range transmission and avoid outages as much as possible.

Transformers play a significant role to support this movement. We understand that and focus on providing the perfect product for every project. Our units are well known in the industry for their longevity (see image below) and our engineers have the knowledge to solve almost any obstacles. That is why we are the “go to” guys when an unusual solution is needed. Sometimes it tends to go like this: “I need the CT to do this and it needs to fit here, make it happen”. Luckily, custom transformers are our specialty and words like "Magicians" & "Wizards" are regularly referred to our talented transformer design & manufacturing teams.

IMG_0672

More than 20 years old. A bit weathered but still going strong!

All these points plus our dedication to provide the best service to our customers means we have been listed as one of the key players in the Transformers industry by RnR Market Research last month. You can read all about it here. It is obviously great to receive a recognition like this as it is a reflection on the attitude of our whole team.