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What does the nameplate of the current transformer say?

The examination of current transformers starts with the nameplate. This nameplate contains vital information necessary for identification and application, detailing aspects like the total core count, classes for protection and metering, accessible taps, rated thermal current, as well as rated primary and secondary currents.


 the nameplate of the current transformer
What does the nameplate of the current transformer say?

Each current transformer (CT) comes with a nameplate detailing its specifications and characteristics. It is imperative for engineers to meticulously inspect and confirm the information on the CT nameplate during the commissioning process, ensuring it matches the provided schematics and documentation.


The CT nameplate acts as a detailed reference, providing essential information on different facets of the transformer.

Typical information found on a CT nameplate includes the manufacturer's name, type of CT, insulation level, serial number, manufacturing date, cooling class, number of phases, frequency, KVA or MVA rating, voltage ratings, tap voltages, rated temperature rise, polarity for single-phase transformers, phasor or vector diagram for polyphase transformers, percent impedance, basic lightning impulse insulation level (BIL), and approximate mass details.


Table of Contents:



1. Total Number of Cores

This refers to the number of distinct cores within the CT assembly. The presence of multiple cores is common when various functions or measurements are necessary.


The number of cores listed on a current transformer's nameplate denotes the cores used for various functions within the transformer. Typically, these cores are designated for measurement and protection roles. Measurement cores may be classified as 0.2, 0.2s, 0.5, or 0.5s, whereas protection cores might be labeled as 5P or 10P.


Each core possesses distinct attributes essential for the current transformer's designated function. Thus, the number of cores on the nameplate reflects the transformer's versatility.

For specific nameplate details of a current transformer, it is advisable to refer to the manufacturer's technical documents or user manuals, which vary based on the transformer's model and serial number.



2. Protection and Metering Classes

Current transformers (CTs) are crafted to serve various functions, including protection and metering. The nameplate details the classes applicable to each core, confirming the CT's appropriateness for its designated purpose.


The protection and measurement classes listed on current transformer nameplates denote the transformer's accuracy for various applications. Measurement classes indicate the precision of the secondary current from 5% to 125% of the transformer's primary current. Protection classes aim to guarantee the proper functioning of protective devices during faults, typically designated as 5P or 10P.


Measurement Classes:

  • The class is divided into various classes such as 0.1, 0.2, 0.5, 1, 3 and 5.

  • These classes define the maximum allowable percentage rate error at a given load.


Protection Classes:

  • 5P and 10P refer to the accuracy of protection current transformers.

  • Here "5" and "10" represent the permissible compound error of the CT, and "P" indicates the protection class.


These classifications dictate the applications in which transformers can be utilized and the levels of accuracy they can achieve. For instance, high-accuracy measurement current transformers (CTs) are employed in revenue metering that demands precise accuracy, whereas protective CTs are engineered to guarantee the proper functioning of protection devices during faults.


Figure 1 – Current transformer (CT) nameplate

 nameplate
Figure 1 – Current transformer (CT) nameplate

3. Taps Available

Certain current transformers (CTs) feature multiple tapping points on the primary winding to adjust the transformation ratio. The nameplate details the available tap settings, offering adaptability for various applications.


The taps indicated on the nameplates of current transformers are the adjustable connection points that allow the transformer to function at various voltage levels. These taps enable the adjustment of the transformer's secondary voltage in response to fluctuations in primary voltage or load conditions.


Typically, taps are denoted as a percentage of the nominal voltage, with standard tap configurations including +2.5%, +5%, -2.5%, and -5%. Some transformers may offer additional tap settings. The voltages for these taps are delineated by a slash (/) separating the individual taps on a transformer winding.


For example, the taps indicated on the nameplate of a transformer may be:

  • Nominal: 100%

  • Taps: +5% / +2.5% / -2.5% / -5%


This means that the transformer's nominal voltage can be adjusted up or down by 5%. Tap changers are crucial for optimizing transformer performance and accommodating varying grid conditions.


For detailed information on the tap settings of a specific current transformer, it is advisable to refer to the manufacturer's technical documentation or user manuals, which will vary based on the model and serial number of the transformer.



4. CT Secondary and Primary Terminal Details

The nameplate details the configuration and labeling of the CT's primary and secondary terminals, guiding the wiring connections.


The details of the primary and secondary terminals found on the nameplates of current transformers denote the configuration and polarity of the transformer's connection points. It is crucial to adhere to these details to guarantee the transformer's proper connection and functionality.


Primary Terminal Details:

  • Usually marked with letters such as "H1", "H2", or letters and numbers such as "P1", "P2".

  • When primary current enters terminal "H1", secondary current exits terminal "X1".


Secondary Terminal Details:

  • Secondary terminals are usually marked with letters such as "X1", "X2", "S1", "S2".

  • The polarity of the secondary current depends on the polarity of the primary current and this is important for the correct operation of the transformer.


The transformer's nameplate also lists the turns ratio, rated current and voltage, tap settings, and other crucial technical specifications. These details are vital for the proper selection, connection, and operation of the transformer.


For specific nameplate information of a particular current transformer, it may be necessary to refer to the manufacturer's technical documentation or user manuals, which vary based on the transformer's model and serial number.



5. Polarity Details

Polarity details, like dot notation or distinct markings, are often found on the nameplate. These features aid in the proper installation and alignment of the CT with its corresponding electrical circuits and protective devices.


The polarity details on the nameplates of current transformers signify the polarity at the connection points of the transformer's primary and secondary windings. These details are crucial for ensuring the transformer's proper connection and functioning.


Polarity is typically indicated by white paint dots (for positive) or plus-minus signs on the transformer terminals, along with symbols on the nameplate. These markers show the connections where the input and output voltages (and currents) have the same polarity at the same time.


For instance, the primary winding terminals might be labeled "H1" and "H2," while the secondary winding terminals could be labeled "X1" and "X2." If the current entering the primary winding at terminal "H1" is positive, then the current exiting from the secondary winding at terminal "X1" will also be positive and in proportion to the primary current.


This information is vital for the proper selection, connection, and utilization of the transformer. For specific polarity details of a current transformer, it may be necessary to refer to the manufacturer's technical documentation or user manuals, which vary based on the transformer's model and serial number.


Figure 2 – 110 kV current transformer nameplate


nameplate
Figure 2 – 110 kV current transformer nameplate

6. Rated Thermal Current

This refers to the highest continuous current a current transformer (CT) can carry without surpassing its temperature thresholds. It's a crucial factor in assessing the CT's appropriateness for a specific application.


The Rated Thermal Current, also known as Thermal Rating Current, displayed on current transformer nameplates, denotes the maximum current the transformer can sustain continuously without overheating. This figure represents the transformer's capacity to handle a certain amount of current over a set period without surpassing a designated temperature increase, typically reflecting its short-term overload capability.


For instance, a current transformer's standard rating stipulates that it can sustain 100% of its rated current at an ambient temperature of 30°C without the average winding temperature rising more than 55°C. Consequently, the highest allowable average winding temperature is 85°C.


Should you require further details on the thermal rating current for a specific current transformer, it is advisable to refer to the manufacturer's technical data or user manuals, which vary based on the transformer's model and serial number.



7. Rated Primary and Secondary Current

The nameplate displays the rated current values for the primary and secondary sides of the current transformer (CT). These values are crucial in determining the transformation ratio and aid in the selection of suitable protection and metering devices.


The rated primary and secondary current specified on the nameplates of current transformers are the nominal current values ​​​​designed for the primary and secondary windings of the transformer. These values ​​are critical to the correct operation of the transformer and are often expressed as:


  • Primary Current (Ipr): It refers to the maximum current passing through the primary winding of the transformer and is usually specified in amperes (A). Standard values ​​can be 10, 12.5, 15, 20, 25, 30, 40, 50, 60, 75 A and decimal multiples of these values.

  • Secondary Current (Isr): It refers to the current passing through the secondary winding of the transformer and is usually specified as 1 A or 5 A. With the proliferation of digital devices, CTs with a secondary current rating of 5 A have become less common.


For example, you may see a value such as "400:5" on the nameplate of a current transformer. This shows that the primary current is 400 A and the secondary current is 5 A. This ratio refers to the ratio at which the transformer converts its primary current into the secondary current, which ensures the correct operation of measuring or protection devices.


If you need more information about the rated primary and secondary current values ​​of a particular current transformer, you may need to consult the manufacturer's technical documentation or user manuals, depending on the transformer model and serial number.



8. Applicable Standards

The nameplate on a current transformer (CT) typically lists the applicable standards and specifications it adheres to. This confirms that the CT conforms to the necessary industry criteria for performance, precision, and safety. Commissioning engineers can ensure the CT's specifications are correct and that it integrates seamlessly into the electrical system by meticulously examining the nameplate details and cross-referencing them with the accompanying schematics or documentation.


This comprehensive verification process aids in error prevention, guarantees optimal performance, and ensures adherence to industry standards.

Figure 3 – Nameplate of the Current Transformer

nameplate
Figure 3 – Example of the nameplate of a current transformer with two secondaries

Standards listed on the nameplates of current transformers confirm compliance with the necessary industry guidelines for performance, accuracy, and safety. Typically established by international or local regulatory authorities, these standards outline the specifications to be adhered to throughout the transformer's design, manufacturing, and testing processes.


For example, a current transformer may comply with the following standards:

  • IEEE (Institute of Electrical and Electronics Engineers)

  • IEC (International Electrotechnical Commission)

  • ANSI (American National Standards Institute)


These standards outline the capabilities, limitations, and requirements essential for users, operators, and maintenance staff to guarantee the safe and efficient functioning of transformers. To determine if your transformer adheres to particular standards, you should review the details provided on the nameplate or refer to the manufacturer's technical documentation.


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