Why CT Secondary Should Not Be Kept Open?

Why CT Secondary Should Not Be Kept Open?

The electrical power system load current always flows through current transformer primary; irrespective of whether the current transformer is open circuited or connected to burden at its secondary.
If CT secondary is open circuited, all the primary current will behave as excitation current, which ultimately produce huge voltage. Every current transformer has its won non-linear magnetizing curve, because of which secondary open circuit voltage should be limited by saturation of the core. If one can measure the rms voltage across the secondary terminals, he or she will get the value which may not appear to be dangerous. As the CT primary current is sinusoidal in nature, it zero 100 times per second.(As frequency of the current is 50 Hz). The rate of change of flux at every current zero is not limited by saturation and is high indeed. This develops extremely high peaks or pulses of voltage. This high peaks of voltage may not be measured by conventional voltmeter. But these high peaks of induced voltage may breakdown the CT insulation, and may case accident to personnel. The actual open-circuit voltage peak is difficult to measure accurately because of its very short peaks. That is why CT secondary should not be kept open.

Knee Point Voltage of Current Transformer

Knee Point Voltage of Current Transformer

This is the significance of saturation level of a CT core mainly used for protection purposes. The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%. The CT core is made of CRGO steel. It has its won saturation level. The EMF induced in the CT secondary windings is
E₂ = 4.44φfT₂ Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T₂ is the number of turns of the secondary winding. The flux in the core, is produced by excitation current I_e. We have a non-liner relationship between excitation current and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation current. This non-liner relation curve is also called B - H curve. Again from the equation above, it is found that, secondary voltage of a current transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage and excitation current as shown below. It is clear from the curve that, linear relation between V and I_e is maintained from point A and K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′.

Polarity test of CT

The Polarity of current transformers is extremely important. Just like a battery, a current transformer too has a polarity. The polarity determines the direction of the secondary current in relation to the primary current.
Wrong connection of the current transformers can cause false operation of the protection relays. Hence, it is vital to ensure that the current transformers are connected with the correct polarity.The figure shows a setup to test the polarity of a current transformers.A DC source is connected with the positive terminal to P1 and the negative terminal to P2. An analog voltmeter is connected to the secondary terminal of the CT. The positive terminal of the meter is connected to terminal S1 of the CT while the negative is connected to terminal S2.A contact is momentarily made through the switch. The contact is made for a second and broken. This is important as continuous contact can short-circuit the battery. The momentary make-break contact causes a deflection in the analog multimeter in the positive direction, if the polarity is correct.If the deflection is negative, it indicates that the polarity of the current transformer is reversed. The terminals S1 and S2 need to be reversed and the test can be carried out.

USE of CT

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 virtually every building with three-phase service and single-phase services greater than 200 amperes.

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from 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. Also, 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.

Function of CT

Function

Like any transformer, a current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. In principle, the only difference between a current transformer and a voltage transformer (normal type) is that the former is fed with a 'constant' current while the latter is fed with a 'constant' voltage, where 'constant' has the strict circuit theory meaning.

The alternating current in the primary produces an alternating magnetic field in the core, which then induces an alternating current in the secondary. The primary circuit is largely unaffected by the insertion of the CT. Accurate current transformers need close coupling between the primary and secondary to ensure that the secondary current is proportional to the primary current over a wide current range. The current in the secondary is the current in the primary (assuming a single turn primary) divided by the number of turns of the secondary. In the illustration on the right, 'I' is the current in the primary, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is an AC ammeter.

The most basic current transformer comprises the secondary wound around the primary conductor, but typically current transformers consist of a silicon steel ring core wound with many turns of copper wire as shown in the right illustration. The conductor carrying the primary current is then passed through the ring; the CT's primary therefore consists of a single 'turn'. The primary 'winding' may be a permanent part of the current transformer, with a heavy copper bar to carry current through the core. Window-type current transformers (aka zero sequence current transformers, or ZSCT) are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. To assist accuracy, the primary conductor should be central in aperture.

Current Transformer

A current transformer (CT) is an electric device that produces an alternating current (AC) in its secondary which is proportional to the AC in its primary. Current transformers, together with voltage transformers (VTs) or potential transformers (PTs), which are designed for measurement, are known as instrument transformers.

When a current is too high to measure directly or the voltage of the circuit is too high, a current transformer can be used to provide an isolated lower current in its secondary which is proportional to the current in the primary circuit. The induced secondary current is then suitable for measuring instruments or processing in electronic equipment. Current transformers also have little effect on the primary circuit. Often, in electronic equipment, the isolation between the primary and secondary circuit is the important characteristic.

Current transformers are used in electronic equipment and are widely used for metering andprotective relays in the electrical power industry

PARTS