Types of Current Transformers and Their Construction
Very high currents can be present on power lines and in high-power circuits. It can be difficult to measure these currents directly. A current transformer is a transformer used to step down the line current to make it easier to measure.
The total power in a transformer is the same on the primary and secondary sides. The only way to step down the current is by stepping up the voltage. Therefore, a current transformer is a modified step-up voltage transformer.
Current transformers are unique because they usually have only one winding (see Figure 1). The primary is connected to the line load in series. When the primary has a high current rating, the primary winding may be a straight conductor that runs through the magnetic circuit's center. This straight conductor indicates a one-turn winding.
Figure 1. A current transformer typically has one pass of a conductor as the primary and many turns of wire for the secondary.
When the primary has a low current rating, the primary winding may consist of several turns wrapped around the core. This provides the required flux on low-current applications or to compensate for line drop to a power meter.
The secondary consists of many turns of wire wrapped around a core. The number of turns is determined by the desired turns ratio of the current transformer. The primary current of a current transformer is not controlled by the secondary, as it would be in a two-winding potential transformer. The secondary of a current transformer cannot affect the current in the primary, as the load on the feeder determines the primary current.
When the primary circuit is energized, the secondary of a current transformer must never be left open. When the circuit is operational, the load on the secondary maintains low magnetizing currents and thus low turn-to-turn potentials. When the secondary becomes an open circuit, the magnetizing currents rise, and the current transformer acts as a step-up potential transformer. The voltage can rise to a destructive level and cause a short between the turns as the result of the degradation of the insulation. Therefore, a current transformer should always have its secondary shorted when not connected to an external load (see Figure 2.)
Figure 2. The secondary of a current transformer must be never left open.
All transformers have losses in power transfer from resistance, magnetizing current, hysteresis, and other factors. These factors must be compensated for in the design of the transformer in order to ensure an accurate measurement.
A current transformer is constructed of high-permeability steel at the flux density at which the transformer operates. The flux density is kept to a low value so that the magnetizing current is low. The circular coil of high-silicon steel provides the low-reluctance magnetic circuit needed to provide the necessary field strength for the secondary winding. The three types of current transformers in general use are the window, bar, and wound.
Window Current Transformers
A window current transformer is a transformer that consists of a secondary winding wrapped around a core and the primary sent through the opening in the core. After the secondary is wound around the core, the assembly is placed into a mold, and an insulating material is injected around the transformer. Taps are brought out from the winding (see Figure 3). A power line is passed through the window and acts as the primary. This completed assembly is referred to as a window current transformer.
Figure 3. A window current transformer has an open area in the centre for a power line to be passed through as the primary
Bar Current Transformers
A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. A bar current transformer can withstand the stresses of heavy overcurrent. To avoid magnetic stresses that could destroy the bus and damage the transformer, care must be taken to properly mount these transformers with respect to adjacent conductors. This type of transformer is typically found on installations where the potential is 25kV or less (see Figure 4).
Figure 4. A bar current transformer has a bar permanently placed in the window. Primary connections are made on the bar
Wound Current Transformers
A wound current transformer is a transformer with separate primary and secondary windings wrapped around a laminated core. A wound current transformer is designed so that the primary winding consists of one or more turns of large cross-section wire connected in series with the circuit to be measured. This kind of current transformer is located on the high-voltage side of substations and contains a primary conductor that carries the current and a wound current transformer for the output current (see Figure 5).
Figure 5. A wound current transformer has several turns of wire for the primary
The current Rating of the primary winding of a current transformer is determined by the maximum value of the load current to be measured. For example, if the current Rating is 400 A with a secondary rating of 5A, the ratio between the primary and secondary is 400:5, or 80:1. This means that the secondary winding has 80 times as many turns as the primary, and the current transformer can be used to measure a line load of 400A. The primary must be rated to withstand 400A.
Manufacturers often provide manuals and troubleshooting charts with their products to assist in locating the cause of a problem. These manuals should be consulted when a problem occurs because they provide detailed information on remedying a specific problem and finding a permanent solution. These charts and manuals are often found on the Internet.
The output of the secondary is current proportional to the primary current. The output is used to measure the primary current and used to provide power to the instruments used to make the measurement. The secondary of a current transformer is always rated at 5A, regardless of the current rating of the primary. This enables the production of standardized current devices rated at 5A. The nameplate would commonly have a rating like 400:5 to show that the secondary is designed to carry 5A.
Current transformers are used in many applications in the industry, both for metering and for feedback. All of the loads that could possibly be connected to the secondary are designed to accept a maximum current of 5A. Common applications of current transformers include power metering, motor current monitoring, and variable-speed-drive monitoring.
Current transformers are used with potential transformers for power metering from a utility to a customer. Power, or wattage, is found by multiplying the voltage and the current. A potential transformer provides a way to measure the voltage. A current transformer provides a way to measure the current (see Figure 6). The wattmeter is in series with the ammeter and must operate on 5A or less.
Figure 6. Current transformers are used with potential transformers to meter the power delivered from a utility to a customer.
Motor Current Monitoring
Large motor starters have current transformers on the lines to the starter to monitor the motor currents (see Figure 7). The outputs of the current transformers are connected to an overload relay. The maximum current on the secondary of the current transformer is 5A. The overload heater is in series with the current transformer and must operate on 5A or less.
Figure 7. Current transformers are used to feed an overload relay in motor starter applications.
A 200HP motor draws approximately 280A at full load on 480V. Current transformers rated at 300:5 (60:1) can be used to monitor the current. Overload heaters rated at 280A are not readily available. An overload relay can be used to monitor the output of the current transformer. Since the current transformer ratio is 60:1, the heaters can be reduced by the same ratio. The load current of 280A divided by 60 is 4.67A. Therefore, heaters rated at 4.67 A can be used.
Variable-speed drives use current transformers to monitor the incoming current. This is part of the instantaneous electronic trip (IET) circuit, which shuts down the drive if the current suddenly rises beyond the drive's Rating.
For current transformers, the accuracy rating is determined at a full-rated load. The full load includes the impedance of the secondary winding itself, the impedance of the leads from the transformer to the load, and the load itself. At lower loads, the accuracy may be only half the stated accuracy.