Summary:A three phase meter current transformer is used to convert a primary current to a secondary current....
A three phase meter current transformer is used to convert a primary current to a secondary current.
They can be found in various shapes and sizes, and have a wide range of amperage ratings. These are often used for power metering applications.
Commonly referred to as a current transformer or CT, these devices are used to turn the main circuit current (primary) into a secondary current. These devices typically come in a standard form with a nameplate that shows the relationship between the primary and secondary currents. The relationship is typically expressed as a ratio such as 100/5 or 500/5. The higher the number, the larger the secondary current that is produced.
These devices are widely used for a variety of applications including power metering, motor current monitoring and variable-speed drive monitoring. In addition, many of these devices are designed to be tamper-resistant, which makes them popular choices for industrial applications.
Choosing the Right Current Transformer
The first thing to consider when choosing a current transformer is the output of the meter it will be used with. This will help determine the size of the device to purchase, as it is necessary to factor in the input power factor (cos(phi)) and minimum voltage requirements of the meter.
Accuracy & Measurement
Most current transformers have an accuracy rating that is determined at a full rated load. This includes the impedance of the secondary winding itself, the leads to and from the current transformer, and the load it is connected to.
There are various classes of accuracy and measurement for current transformers, with Class 0.1 being the lowest and Class 3 the highest. Each classification has its own accuracy and measurement error parameters that are included in the IEC 61869-1 standards.
The most common measurement error for a current transformer is the difference between the primary and secondary currents. This is a result of the magnetic core being saturated by the current flowing through it.
This can be an issue with CTs that are used in metering applications as the meter will be operating when there is a high current flowing through it, which could cause the meter to be inaccurate when it is turned on or off. This is not an issue with CTs that are used for relay protection, however, as this is when their performance is of interest.
Another issue is that a CT can become saturated by external dc magnetic fields. This can be a potential problem in metering environments where dc currents are present.
In addition, the dc current can be at a very low level for a short time during faults on the power grid. This can affect the meter's accuracy, as it may measure several times its normal value for a very short period of time.
To avoid this problem, a current transformer should never be operated with no load attached when the main current is flowing through it. This can be done by placing a short circuit across the secondary terminals before disconnecting the ammeter or load from the current transformer.