Current transformers or CTs are indispensable wherever large currents are measured and processed. These transformers effectively reduce high-voltage currents and safely evaluate and monitor the actual current through the AC transmission line in a convenient way, relying on traditional ammeters. CT performs this task by generating alternating current in the secondary winding that is proportional to the current in the primary winding.
In fact, the fundamental purpose of current transformers is different from that of standard voltage transformers. Current transformers have only one to a few primary winding turns—in the form of a single flat turn, a magnetic core wrapped in a coil of heavy wire, or just a bus bar or conductor placed through a hole. Because of this circuit arrangement, it is not surprising that CT is also called a series transformer.
Compared with this ultra-simple primary winding structure, the secondary winding of CT has many coils wound on a laminated core of low-loss magnetic material. The laminated core has a larger cross-section, which minimizes the magnetic flux density-and relies on wires with a smaller cross-section. The exact geometry depends on the amount of current that must be reduced when the wire tries to output a constant current…regardless of the connected load.
During operation, the secondary winding sends current to a short circuit (such as an ammeter) or a resistive load-unless the voltage generated in the auxiliary winding is sufficient to immerse the magnetic core… or cause a malfunction due to voltage breakdown. Compared with the voltage transformer, the basic current of the current transformer does not depend on the secondary load current…it is managed by the external load.
The auxiliary current is usually evaluated with the standard 1 A or 5 A to obtain a higher rating. Current transformers can reduce the current level from thousands of amperes or down to a standard of a known ratio…for normal applications, down to 5 A or 1 A again. Current transformers can serve such sophisticated and high-precision components and control equipment, because the latter can effectively protect them from any nearby cables carrying high-voltage power.
Metering applications and other uses of current transformers abound. For example, current transformers work in power factor meters, watt meters, watt-hour meters, and protection relays. Current transformers can also be used as trip coils in magnetic circuit breakers or MCBs.
Compared with the voltage transformer, the voltage tap can be easily and safely connected to the circuit to measure the voltage of the existing electrical system. In a more advanced variant, the transformer voltage tap is also used as a regulating connection on the transformer coil, through which engineers can control the voltage. These voltage taps adjust the voltage to maintain the secondary value at a certain nominal value.
More specifically, tap connection adjustment can adjust the output voltage of the transformer to ensure full voltage output. When the line voltage is lower or higher than the rated voltage of the primary transformer, this difference will have a proportional effect on the secondary voltage… which in turn will cause the current and voltage output to be inaccurate. Using the voltage tap can change the voltage ratio of the transformer to keep its secondary voltage at its target output voltage. On large transformers, the tap on the primary compensates for inputs that are higher or lower than normal. This kind of voltage tap connection is usually set by the component supplier to set some set line voltage values. In the case of a unique facility or site voltage, the supplier can adjust the voltage tap accordingly before sending.
The voltage tap is directly connected to the transformer winding. The whole number of turns is required between the voltage tap leads, otherwise it will be on the wrong side of the transformer.
A voltage tap switch is usually included in the installation to allow actions that require the transformer to be shut down. The machine operator should first cut off the transformer and deploy a safety ground on the transformer terminals. Then he or she should move the tap changer from the current position to the appropriate position.
Post time: Nov-22-2021