Current Transformer (CT)

Current Transformer (CT) 

A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). It produces a current in its secondary which is proportional to the current in its primary.

Current transformers, along with voltage or potential transformers, are instrument transformers. Instrument transformers scale the large values of voltage or current to small, standardized values that are easy to handle for measuring instruments and protective relays. The instrument transformers isolate measurement or protection circuits from the high voltage of the primary system. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary.

Current transformers are used extensively for measuring current and monitoring the operation of the power grid. Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on much larger commercial and industrial supplies.

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from the ground. Some CT configurations slip around the bushing of a high-voltage transformer or circuit breaker, which automatically centers the conductor inside the CT window.

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer. Sometimes a section of a bus bar can be removed to replace a current transformer.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.

The burden (load) impedance should not exceed the specified maximum value to avoid the secondary voltage exceeding the limits for the current transformer. The primary current rating of a current transformer should not be exceeded or the core may enter its non-linear region and ultimately saturate. This would occur near the end of the first half of each half (positive and negative) of the AC sine wave in the primary and would compromise the accuracy

Accuracy.

Accuracy classes for various types of measurement and at standard loads in the secondary circuit (burdens) are defined in IEC 61869-1 as classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important, especially in power measuring circuits. Each class has an allowable maximum phase error for a specified load impedance.

Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. A CT with a rating of 2.5L400 specifies with output from its the secondary winding of twenty times its rated secondary current (usually 5 A × 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent.

Burden.

The secondary load of a current transformer is termed the "burden" to distinguish it from the primary load.

The burden in a CT a metering electrical network is a largely resistive impedance presented to its secondary winding. Typical burden ratings for IEC CTs are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA, and 60 VA. ANSI/IEEE burden ratings are B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with a burden rating of B-0.2 will maintain its stated accuracy with up to 0.2 Ω on the secondary circuit. These specification diagrams show accuracy parallelograms on a grid incorporating magnitude and phase angle error scales at the CT's rated burden. Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate conductors. The most common cause of excess burden impedance is the conductor between the meter and the CT. When substation meters are located far from the meter cabinets, the excessive length of cable creates a large resistance. This problem can be reduced by using thicker cables and CTs with lower secondary currents (1 A), both of which will produce less voltage drop between the CT and its metering devices.

Knee-point core-saturation voltage.

The knee-point a voltage of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. In testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage until the knee point is reached. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. For voltages greater than the knee point, the magnetizing current increases considerably even for small increments in voltage across the secondary terminals. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much higher but constrained within a very small range of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times rated current.