EMC & Circuit Protection
Current transformer specification – a guide for the designer
By Alex Deakin, business development manager, SIGA (Electronics) C
urrent transformers are a low cost and effective method of current measurement as well as providing galvanic separation between the current carrying
conductor and the measuring device. They are also an effective means of detecting and measuring over currents, and as such they are increasingly widely used in green energy applications. Although their principle of operation is the same as that of power transformers, these devices have some specific design characteristics that need to be borne in mind in specification.
What is a current transformer? A current transformer (CT) is an instrument transformer in which the secondary current, in normal conditions of use, is proportional to the primary current and differs in phase from it by an angle which is approximately zero. In an ideal CT, the current in
the secondary winding will reflect the actual primary current without current ratio error or phase displacement. However, under normal conditions there will be current ratio error and phase displacement between primary and secondary currents.
The basic operating principle of CTs is the same as that of power transformers. The CT has a primary and a secondary winding. An alternating current flowing in the primary winding induces an alternating current in the secondary winding. But, unlike voltage or power transformers, a CT provides just one or very few turns as its primary winding, depending on the transformation ratio required. The primary winding can be either a single flat turn, a coil of heavy-duty wire wrapped around the core or simply a conductor or bus bar placed through a central hole. The secondary winding is terminated on the rated burden resistor, the value on which the accuracy requirements of the CT is based.
36 September 2021 Specification
When specifying a CT, the most important points to be considered are the ratio, the burden and the accuracy class. The most significant spec-point is the ratio, which typically ranges from 1000 to five, through 1000 to one, then 5000 to five and 5000 to one. Another important factor is the voltage × current value, or “burden”. Typically, 5VA is the standard burden required, and manufacturers like SIGA (Electronics) can accommodate any burden from 1A up. A further consideration is accuracy class of the device. There is always some difference between the expected value and actual value of output of an instrument transformer current error and phase angle error count in CTs. This is because the primary current of the CT must contribute the excitation component of a CT core. The accuracy class of CTs is a measure of the highest permissible percentage composite error at rated current. Standard accuracy classes of CT, per IS-2705 standards, are 0.1, 0.2, 0.5, 1, 3 and 5 for metering CTs. Accuracy class of 0.1 means the maximum permissible limit of error is 0.1%. IS-2705 categorises standard accuracy classes for the protection CT, such as 5P (5%), 10P (10%) or 15P (15%), where 'P' stands for protection. The broad portfolio available offers ring type CTs suitable for primary currents from 50A-10000A along with rectangular CTs with primary currents from 60A to 6300A having 1A or 5A secondary. These meet accuracy classes from 0.2 for metering purposes and class PX for protection. Fitted in applications with rated system volts 0.72/3kV, they can be built to meet a 3kV insulation level for up to 1 minute. SIGA (Electronics) boasts capabilities going down to class 0.2 all the way up to Class 5 in metering. From the protection side, capabilities extend to Class 6 or Class PX.
For the protection side, further factors need to be considered, for example where the secondary resistance is at 75°C. Normally specified by the customer, this provides an idea of where the design needs to be. Also, some customers want a CT that can handle a specific secondary fault current, and this parameter is also specified by the customer.
Standards
CTs need to conform to IEC regulations from design, through manufacturing to final test. Standards including IEC 61869 are addressed by specific conformance testing for CTs. The design and testing of CTs is governed by standard IEC 61869-2:2012 (replaces IEC 60044-1:1996). Further specific test requirements are needed to verify that they meet the IEC 61869 prototype manufacturing stage. These include power frequency withstand measurements. This involves on-site checks for accuracy and ratio error, together with phase angle and winding resistance. Another key measurement is the saturation point, usually performed using a CT analyser in the manufacturer’s test department. This needs to be calibrated on an annual basis. Further, a primary injection tester allows manufacturers to make measurements at 3000, 4000 or 5000 amps. The test rig can produce the required current. Testing the transformer secondary side at 1A and 5A will confirm that the solution is fit for purpose. Whilst most safety hazards are mitigated by device protection, others are alleviated through the method in which CTs are wound. Resin, or tape-wound versions require the correct amount of space around the windings.
Components in Electronics
For example, a resin block typically provides 10mm of resin around the outside of the component. In the case of tape-wound components, protection depends on the ability of the CT at temperatures where it is going to be installed. Normally class H materials are used, which are safe up to around 180°C.
Styles and finishes
Manufacturers offer full block or split core style CTs, finished with UL-recognised semi rigid cast resin or an IP-rated plastic box. There is no right or wrong answer as to which style or finish to specify, there are different advantages of each style depending on the application.
Applications
Uses of CTs include current measurement and fault detection in systems. Examples of the latter include the use of CTs installed in switchgear fitted on London Underground, electrical data centres and electrical substations. They are also the basis of Parasense units used in supermarket refrigeration and in the rapidly growing data centre industry. Other customers include providers of switchgear to aerospace, automotive, power generation, MOD applications and rail transport. Many CTs for switchgear are used in the renewable energy and green project markets, so where their units are becoming more efficient, they are being designed with minimal losses. These versatile devices have wide applications, and hopefully this article will give design engineers unfamiliar with their characteristics the confidence to specify and use them.
www.sigatransformers.co.uk www.cieonline.co.uk
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