Energy efficiency minimized costs and high system availability represent now three central aspects of plant management. To achieve them it is necessary to know when, where, and how the energy is consumed. This is why measuring and monitoring the principal electrical parameters of the network is becoming more important. The current transformer is one of the essential parts of electric power systems. All types of protection and control devices need current transformers. For this reason, all professionals should know its fundamentals. After reading this article you will have a basic knowledge about the current transformer.
What is a current transformer?
The current transformer is a special type of electrical equipment, that steps down high primary currents to low secondary currents. The primary winding is connected with the current being measured and the secondary winding to measuring instruments.
The primary winding of the current transformer consists of a few turns and is connected in series with the line carrying current. The secondary winding has a larger number of turns and is connected with the instruments.
The current transformer is used for measuring and protecting applications. By using a current transformer we can measure high currents easily. It is recommended to apply current transformers for currents of 40 A or higher.
Current transformers have two basic functions:
- Limiting and minimizing the current for metering and protection devices.
- Isolating power circuits from the metering and/or protection circuit.
Current transformer applications
The current transformer can be used in the following applications.
- Kilowatt-hour meters
- Power factor meters
- Control relays
- Measuring transducers
Construction of a current transformer
The current transformer consists of a primary winding, a secondary winding, a magnetic core, and an insulated body. The high-grade silicon steel core is annealed, varnished then insulated with polycarbonate core caps. The secondary winding is toroidally wound by high-precision semi-automatic machinery. For the tape wound ring-type current transformer, the PEW-coated windings are then covered with elephantine paper, varnished, and double-tapped with PVS tapes. For the encapsulated type current transformer, the windings are enclosed in a compact and heat-resistant split cap.
How does a current transformer work?
The current transformer works to transform or change the magnitude of AC (50…400 Hz) in a system, typically from a higher current value to a lower current value. The transformation, or amount of change, is dependent on the number of turns of both the primary and secondary conductors. CT consists of three main components: a primary winding, a core, and a secondary winding.
The relationship, or ratio, between the number of turns in the primary and secondary windings, is responsible for reducing or ‘stepping down’ the current in a system to a value that is usable to the current monitoring device, such as an overload relay or power monitoring product. The following formula demonstrates how the ratio between the windings can lower the current:
How is the current transformer ratio calculated?
The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a current transformer with a ratio of 300:5 is rated for 300 primary amps at full load and will produce 5 amps of secondary current when 300 amps flow through the primary.
If the primary current changes, the secondary current output will change proportionally. For example, if 150 amps flow through the 300 amp rated primary the secondary current output will be 2.5 amps (150:300 = 2.5:5)
Current transformer models
There are several different models of current transformers, each facilitating the step-down and metering of current, but the manner in which that is accomplished can be different. The following explains the characteristics of the three main models of current transformers.
Wound current transformer
The wound current transformer has a primary winding of more than one full turn wounded on the core. The primary and secondary winding of the wound current transformer is insulated from each other consisting of one or more turns encircling the core. Constructed as multi-ratio CTs by the use of taps on the secondary winding. The wound type provides excellent performance under a wide operating range.
Toroidal current transformer
The toroidal current transformer does not contain a primary winding. Instead, the line that carries the current flowing in the circuit passes through a window or hole in the toroidal transformer. Some current transformers have a “split-core”, which allows them to be opened, installed, and closed without disconnecting the circuit to which they are attached.
Bar-type current transformer
The bar-type current transformer uses the actual cable or busbar of the main circuit as the primary winding, which is equivalent to one turn. Bar types are available with higher insulation levels and are usually bolted to the current caring device.
Current transformer connection
Single ratio CT
Multi ratio CT
Current transformer selection
To select the current transformer correctly, the following points should be clarified:
- The application. (for measuring or protection)
- The features of the wording environment. (indoor or outdoor, operating temperature, air humidity, etc…)
- Operation voltage and frequency.
- Range of the primary current. (maximum and minimum of the current to be measured)
- The dimension of the cable or bus bar.
- Data of the overload.
- Short circuit current.
- Specification of the measuring device associated with the current.
- Transformer. (accuracy, rated current, consumption, etc…)
- The diameter and length of the cable. The cable is used to connect the current transformer and associated measuring device.
We recommend that you choose the ratio immediately higher than the maximum measured current (In). Example: In = 1103 A; ratio chosen = 1250/5.
- For small ratings: From 40/5 to 75/5 and for an application with digital devices, we recommend that you choose a higher rating, for example, 100/5. This is because small ratings are less accurate and the 40 A measurement, for example, will be more accurate with a 100/5 CT than with a 40/5 CT.
- The specific case of the motor starter: to measure motor starter current, you must choose a CT with primary current Ip = Id/2 (Id = motor starting current)
Accuracy of a current transformer
The current transformer accuracy is determined by its certified accuracy class that is stamped on the nameplate. For example, a CT accuracy class of 0.3 means that the CT is certified by the manufacturer to be accurate to within 0.3 percent of its rated ratio value for a primary current of 100 percent of rated the ratio.
A current transformer with a rated ratio of 200/ 5 with an accuracy class of 0.3 would operate within 0.45 percent of its rated ratio value for a primary current of 100 amps. To be more explicit, a primary current of 100A is certified to produce a secondary current between 2.489 amps and 2.511 amps.
The ratio of a current transformer
The current transformer ratio is specified with the assumption that the primary conductor passes through the window once, but it is possible to modify the ratio by looping the primary conductor through the opening additional times. Introducing two loops decreases the 300:5 ratio by a factor of two, yielding a 150:5 ratio, and three loops provide a decrease by a factor of three, or 100:5
The polarity of a current transformer
The polarity of a current transformer is determined by the direction the coils are wound around the core of the CT (clockwise or counterclockwise) and by the way the leads, if any, are brought out of the transformer case.
All current transformers are subtractive polarity and will have the following designations to guide proper installation:
(H1) primary current, line facing direction; (H2) primary current, load facing direction; and (X1) secondary current.
Taking care to observe proper polarity is important when installing and connecting current transformers to power metering and protective relays.
Current transformer failure reasons
The most common failures of the current transformer are:
- Mechanical deformation, floating core ground, magnetostriction.
- Short circuits, open circuits.
- Partial breakdown of capacitive layers.
- Short circuits of single turns.
- Partial discharge, moisture in solid insulation, aging, contamination of insulation fluids.
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