Current Transformers: Operation, Types, and Polarity Considerations

Current transformers (CTs) primarily aim to change voltage values. A secondary result is that they also change current values.

 

Image used courtesy of Adobe Stock

The step-up transformer decreases the current in the secondary winding, and the step-down transformer increases the current in the secondary winding to keep the power flowing into the transformer the same as the power coming out of the transformer. The primary purpose of some types of transformers is to change current values; these transformers are called current transformers. In most cases, current transformers reduce large amounts of current for metering (for example, changing 500 amps of current at the distribution panel into 1 amp of current that can be fed into an ammeter).

 

Current Transformer Winding Ratios

According to the principle of transformer action:

\[\frac{V_{Pri}}{V_{Sec}}=\frac{Amps_{Sec}}{Amps_{Pri}}\]

To make the secondary current smaller than the primary current, the secondary voltage must be higher than the primary voltage, and the number of turns in the secondary winding will be more than the number of turns in the primary winding. Large primary currents, such as 500 amps, will make the primary windings prohibitively large if there are many turns, so current transformers use the primary conductor as the primary winding. With a single wire as the transformer primary, there is one turn in the primary winding. Because the primary of the current transformer is the supply conductor itself, the current transformer is only the secondary winding. The flux created by the current flowing in the supply conductor cuts across the larger number of secondary winding turns placed around the primary conductor. Figure 1 shows an example of a current transformer. The secondary winding is a large number of turns of small wire encapsulated in an epoxy or some other insulating material.

 

Figure 1. A donut, or solid core, current transformer that has been slipped around a supply conductor. Image used courtesy of Wikimedia Commons

 

Current Transformer Types

The three main types of current transformers are:

1. Solid-core current transformers have a secondary winding that forms a solid ring, or donut, that is slipped over the supply conductor before it is terminated.

2. Split-core current transformers have a secondary winding designed so the winding is split and may be separated and placed around a supply conductor that is already terminated. A clamp-on ammeter is a type of split-core current transformer; the jaws are opened and placed around the conductor to measure the amperage flowing in the conductor.

3. Bar-type current transformers already have the secondary winding around a bar. The supply conductor is bolted to each end of the bar, and current then flows through the bar.

 

Current Transformer Current and Voltage Ratios

The current transformer ratios reflect current values, not voltage. Most current transformers will have ratios such that the maximum secondary amperage is 5 amps; for example, 100:5 (100 amps to 5 amps), which makes the ratio of turns 1:20 because transformer current is inversely proportional to voltage. Therefore, 80 amps of current through the supply conductor will cause 4 amps to flow through an external ammeter. The ammeter or measuring device determines the secondary amperage rating that is required. In all cases, the magnetic flux from the primary conductor cuts across the larger number of turns in the secondary winding, inducing a large secondary voltage and proportionally smaller secondary current. For example, if the transformer ratio is 500:5, the turns ratio is 1:100, and if the primary current is 300 amps, the secondary current is:

\[\frac{1\,Turn_{Pri}}{100\,Turn_{Sec}}=\frac{Amps_{Sec}}{300\,Amps_{Pri}}=3\,amps\]

Ammeters that can measure 5 amps or less are easy to find. The scale of the ammeter only needs to be changed from 3 amps to 300 amps to reflect the actual amperage of the circuit.

The voltage ratio is the inverse of the current ratio—in our example, 5:500; if the primary voltage is 240 volts, the secondary voltage is:

\[\frac{5\,Turn_{Pri}}{500\,Turn_{Sec}}=\frac{240\,V_{Pri}}{V_{Sec}}=24,000V\]

As you can see, the secondary voltage level can be quite high. This high-voltage potential is different across the ammeter. As long as current flows in the secondary of the current transformer, there is sufficient CEMF to oppose the secondary voltage and keep it at much lower levels. This is exactly what happens to limit the exciting current on the primary of a regular transformer.

 

Dangers From Open Secondary Conductors

If the wires of the transformer secondary were no longer connected to a load, there would be no secondary current or CEMF, and the secondary voltage would reach extremely high levels, breaking down the insulation of the secondary conductors and causing a short, destroying the current transformer. Therefore, the secondary windings must always have current flowing through them if there is current through the primary.

 

Tech Tip

To avoid the destruction of the current transformer, the secondary winding must never be disconnected from its load while the current flows in the primary conductor. The secondary conductors must be shorted together before any load is disconnected or if there is no load connected to the secondary of the current transformer.

 

Current Transformer Polarity

Current transformers have polarity just as regular transformers. Current transformers should be mounted with the H1 side toward the power source. When the current entering the primary winding at H1 is positive, the current from the X1 secondary is also positive and proportional to the primary current. The H1 and X1 terminals are often marked with a dot to indicate the same polarity. Proper polarity is required for the meters to read correct values. The diagram in Figure 2 shows the symbol for a current transformer with a polarity mark. The supply conductor is connected to the same polarity terminal on the watt meter as the current transformer. When the same polarities are connected to the proper terminals, the voltage and current are in-phase and the watt meter will read correctly.

 

Figure 2. A watt meter connection diagram showing the current transformer and voltage connections for proper metering. Image used courtesy of Ahmed Sheikh

 

Current Transformer Takeaways

A comprehensive understanding of current transformers is vital for professionals in the electrical field. Proficiency in their functions, types, and operational principles is essential for ensuring accurate metering, maintaining equipment integrity, and preventing potential hazards. Proper knowledge and handling of current transformers are crucial, particularly in avoiding dangers associated with open-circuited secondary conductors. These can lead to extremely high voltages and insulation breakdown and pose significant risks to equipment and personnel. Therefore, understanding current transformers is essential for efficient power distribution and measurement and to ensure electrical system safety.