# Current Transformer And Potential Transformer Theory Pdf

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Current Transformers CT and Potential Transformers PT are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively. A CT has large number of turns on its secondary winding, but very few turns on its primary winding. The primary winding is connected in series with the load so that it carries full load current. A low voltage range ammeter A is connected across the secondary winding terminals.

## Difference Between Current Transformer (CT) & Potential Transformer (PT)

From these primary variables, we may determine impedance, reactance, resistance, as well as the reciprocals of those quantities admittance, susceptance, and conductance. Other sensors more common to general process measurements such as temperature, pressure, level, and flow are also used in electric power systems, but their coverage in other chapters of this book is sufficient to avoid repetition in this chapter. Two common types of electrical sensors used in the power industry are potential transformers PTs and current transformers CTs.

Electrical power systems typically operate at dangerously high voltage. It would be both impractical and unsafe to connect panel-mounted instruments directly to the conductors of a power system if the voltage of that power system exceeds several hundred volts. For this reason, we must use a special type of step-down transformer referred to as a potential transformer to reduce and isolate the high line voltage of a power system to levels safe for panel-mounted instruments to input.

Shown here is a simple diagram illustrating how the high phase and line voltages of a three-phase AC power system may be sensed by low-voltage voltmeters through the use of step-down potential transformers:. The following photograph shows a potential transformer sensing the phase-to-ground voltage on a three-phase power distribution system. The normal phase voltage in this system is 7. A panel-mounted voltmeter, for example, would have a scale registering volts when its actual input terminal voltage was only volts.

Here, the physical variable being sensed by the potential transformer is still voltage, just at a ratio greater than what the panel-mounted instrument receives. Like the mA DC analog signal standard so common in the process industries, or volts is the standard potential transformer output voltage used in the electrical industry to represent normal power system voltage.

This next photograph shows a set of three PTs used to measure the voltage on a Note how each of these PTs is equipped with two high-voltage insulated terminals to facilitate phase-to-phase line voltage measurements as well as phase-to-ground:.

Another photograph of potential transformers appears here, showing three large PTs used to precisely step the phase-to-ground voltages for each phase of a kV system kV line voltage, kV phase voltage all the way down to volts for the panel-mounted instruments to monitor:.

The secondary terminals of these PTs connect to two-wire shielded cables conveying the volt signals back to the control room where they terminate at various instruments. These shielded cables run through underground conduit for protection from weather. Just as with the previous PT, the standard output voltage of these large PTs is volts, equating to a transformer turns ratio of about A special form of instrument transformer used on very high-voltage systems is the capacitively-coupled voltage transformer , or CCVT.

These sensing devices employ a series-connected set of capacitors dividing the power line voltage down to a lesser quantity before it gets stepped down further by an electromagnetic transformer. For the same reasons necessitating the use of potential voltage instrument transformers, we also see the use of current transformers to reduce high current values and isolate high voltage values between the electrical power system conductors and panel-mounted instruments.

Shown here is a simple diagram illustrating how the line current of a three-phase AC power system may be sensed by a low-current ammeter through the use of a current transformer:. The secondary winding consists of multiple turns of wire wrapped around the toroidal magnetic core:.

The turns ratio of a CT is typically specified as a ratio of full line conductor current to 5 amps, which is a standard output current for power CTs. Therefore, a ratio CT outputs 5 amps when the power conductor carries amps.

The turns ratio of a current transformer suggests a danger worthy of note: if the secondary winding of an energized CT is ever open-circuited, it may develop an extremely high voltage as it attempts to force current through the air gap of that open circuit. An energized CT secondary winding acts as a current source, and like all current sources, it will develop as great potential voltage as it can when presented with an open circuit.

Given the high voltage capability of the power system being monitored by the CT, and the CT turns ratio with more turns in the secondary than in the primary, the ability for a CT to function as a voltage step-up transformer poses a significant hazard. Like any other current source, there is no harm in short-circuiting the output of a CT. Only an open circuit poses the risk of damage.

Later subsections will elaborate on this topic in greater detail. Current transformers are manufactured in a wide range of sizes, to accommodate different applications. The black and white wire pair exiting this CT carry the 0 to 5 amp AC current signal to any monitoring instrument scaled to that range. Both styles are commonly found in the electrical power industry, and they operate identically:.

The installed CTs appear as cylindrical bulges at the base of each insulator on the high-voltage circuit breaker. This particular photograph shows flexible conduit running to each bushing CT, carrying the low-current CT secondary signals to a terminal strip inside a panel on the right-hand end of the breaker:. Signals from the bushing CTs on a circuit breaker may be connected to protective relay devices to trip the breaker in the event of any abnormal condition.

Shown here is a set of three very large CTs, intended for installation at the bushings of a high-voltage power transformer. Each one has a current step-down ratio of to In this next photograph we see a tiny CT designed for low current measurements, clipped over a wire carrying only a few amps of current. This particular current transformer is constructed in such a way that it may be clipped around an existing wire for temporary test purposes, rather than being a solid toroid where the conductor must be threaded through it for more permanent installation:.

This last photograph shows a current transformer used to measure line current in a kV substation switchyard. The actual CT coil is located inside the red-colored housing at the top of the insulator, where the power conductor passes through. An important characteristic to identify for transformers in power systems — both power transformers and instrument transformers — is polarity. When multiple power transformers are interconnected in order to share the load, or to form a three-phase transformer array from three single-phase transformer units, it is critical that the phase relationships between the transformer windings be known and clearly marked.

Also, we need to know the phase relationship between the primary and secondary windings coils of an instrument transformer in order to properly connect it to a receiving instrument such as a protective relay.

For some instruments such as simple indicating meters, polarity phasing is unimportant. For other instruments comparing the phase relationships of two or more received signals from instrument transformers, proper polarity phasing is critical.

The marks should be interpreted in terms of voltage polarity , not current. Note how the secondary winding of the transformer develops the same polarity of voltage drop as is impressed across the primary winding by the DC pulse: for both the primary and secondary windings, the sides with the dots share the same positive potential.

If the battery were reversed and the test performed again, the side of each transformer winding with the dot would be negative:. A single 9-volt dry-cell battery works well given a sensitive meter. To emphasize this important point again: transformer polarity dots always refer to voltage, never current. The polarity of voltage across a transformer winding will always match the polarity of every other winding on that same transformer in relation to the dots. The direction of current through a transformer winding, however, depends on whether the winding in question is functioning as a source or a load.

This is why currents are seen to be in opposite directions into the dot, out of the dot from primary to secondary in all the previous examples shown while the voltage polarities all match the dots. Transformer polarity is very important in the electric power industry, and so terms have been coined for different polarity orientations of transformer windings.

If polarity dots for primary and secondary windings lie on the same physical side of the transformer it means the primary and secondary windings are wrapped in the same direction around the core, and this is called a subtractive transformer. If polarity dots lie on opposite sides of the transformer it means the primary and secondary windings are wrapped in opposite directions, and this is called an additive transformer.

The following examples show how voltages may either add or subtract depending on the phase relationships of primary and secondary transformer windings:. Instrument transformers PTs and CTs by convention are always subtractive. When three single-phase transformers are interconnected to form a three-phase transformer bank, the winding polarities must be properly oriented.

Windings in a delta network must be connected such that the polarity marks of no two windings are common to each other. Curved arrows are drawn next to each winding to emphasize the phase relationships:.

Windings in a wye network must be connected such that the polarity marks all face the same direction with respect to the center of the wye typically, the polarity marks are all facing away from the center :. Failure to heed these phase relationships in a power transformer bank may result in catastrophic failure as soon as the transformers are energized!

Note the solid black squares marking one side of each CT secondary winding as well as one side of each primary and secondary winding in this three-phase power transformer. Comparing placement of these black squares we can tell all CTs as well as the power transformer itself are wound as subtractive devices:. An example of the importance of polarity marks to the connection of instrument transformers may be seen here, where a pair of current transformers with equal turns ratios are connected in parallel to drive a common instrument which is supposed to measure the difference in current entering and exiting a load:.

Properly connected as shown above, the meter in the center of the circuit registers only the difference in current output by the two current transformers. If current into the load is precisely equal to current out of the load which it should be , and the two CTs are precisely matched in their turns ratio, the meter will receive zero net current.

If, however, a ground fault develops within the load causing more current to enter than to exit it, the imbalance in CT currents will be registered by the meter and thus indicate a fault condition in the load. Let us suppose, though, that a technician mistakenly connected one of these CT units backwards. If we examine the resulting circuit, we see that the meter now senses the sum of the line currents rather than the difference as it should:. The following schematic diagrams show how PTs and CTs should behave when sourcing their respective instruments:.

This is why you will never see fuses in the secondary circuit of a current transformer. Such a fuse, when blown open, would pose a greater hazard to life and property than a closed circuit with any amount of current the CT could muster. While the recommendation to never short-circuit the output of a PT makes perfect sense to any student of electricity or electronics who has been drilled never to short-circuit a battery or a laboratory power supply, the recommendation to never open -circuit a powered CT often requires some explanation.

That is to say, short-circuiting the secondary winding of a CT will not result in more current output by that CT than what it would output to any normal current-sensing instrument! The latent danger of a CT is underscored by an examination of its primary-to-secondary turns ratio. A single conductor passed through the aperture of a current transformer acts as a winding with one turn, while the multiple turns of wire wrapped around the toroidal core of a current transformer provide the ratio necessary to step down current from the power line to the receiving instrument.

However, as every student of transformers knows, while a secondary winding possessing more turns of wire than the primary steps current down , that same transformer conversely will step voltage up.

This means an open-circuited CT behaves as a voltage step-up transformer. Given the fact that the power line being measured usually has a dangerously high voltage to begin with, the prospect of an instrument transformer stepping that voltage up even higher is sobering indeed. In fact, the only way to ensure a CT will not output high voltage when powered by line current is to keep its secondary winding loaded with a low impedance. It is also imperative that all instrument transformer secondary windings be solidly grounded to prevent dangerously high voltages from developing at the instrument terminals via capacitive coupling with the power conductors.

Grounding should be done at only one point in each instrument transformer circuit to prevent ground loops from forming and potentially causing measurement errors. The preferable location of this grounding is at the first point of use, i. Connections made between instrument transformers and receiving instruments such as panel-mounted meters and relays must be occasionally broken in order to perform tests and other maintenance functions.

An accessory often seen in power instrument panels is a test switch bank , consisting of a series of knife switches. A photograph of a test switch bank manufactured by ABB is seen here:. Some of these knife switches serve to disconnect potential transformers PTs from receiving instruments mounted on this relay panel, while other knife switches in the same bank serve to disconnect current transformers CTs from receiving instruments mounted on the same panel.

For added security, covers may be installed on the switch bank to prevent accidental operation or electrical contact. Some test switch covers are even lock-able by padlock, for an added measure of access prevention. Test switches used to disconnect potential transformers PTs from voltage-sensing instruments are nothing more than simple single-pole, single-throw SPST knife switches, as shown in this diagram:.

There is no danger in open-circuiting a potential transformer circuit, and so nothing special is needed to disconnect a PT from a receiving instrument. A series of photographs showing the operation of one of these knife switches appears here, from closed in-service on the left to open disconnected on the right:. Test switches used to disconnect current transformers CTs from current-sensing instruments, however, must be specially designed to avoid opening the CT circuit when disconnecting, due to the high-voltage danger posed by open-circuited CT secondary windings.

This is done through the use of a special make-before-break knife switch:.

## Current transformer

Hence the secondary of current transformer is never left open. Accuracy class: Metering needs high accuracy at load current while Protection need not have that high accuracy. Safety Factor: It determines the multiplicity of load current at which the CT saturates. Metering CT should have lesser value so that it saturates before the meters can get damaged due to some abnormality. Protection should have high value since its main purpose desires the CT to withstand high fault currents to be sensed back to the relays. The primary is typically the main power line conductor, which passes directly through the toroidal core.

## Difference between Current Transformer and Potential Transformer

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.

Free download application of potential transformer pdf. Potential Transformer PT Definition — The potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watt-hour meters, etc. The potential transformers and accessories covered by this specification shall comply with the requirement of the latest edition of the following standards unless otherwise stated in this specification: IS: Part I-IV : Specification for Voltage Transformer. IS: Application guide for Voltage Transformers.

From these primary variables, we may determine impedance, reactance, resistance, as well as the reciprocals of those quantities admittance, susceptance, and conductance. Other sensors more common to general process measurements such as temperature, pressure, level, and flow are also used in electric power systems, but their coverage in other chapters of this book is sufficient to avoid repetition in this chapter. Two common types of electrical sensors used in the power industry are potential transformers PTs and current transformers CTs. Electrical power systems typically operate at dangerously high voltage.

Transformers Engr. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force EMF or "voltage" in the secondary winding. This effect is called mutual induction. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will be transferred from the primary circuit through the transformer to the load.

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